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CLAY – GENERAL

Clay is the most widely known material, but, at the same time, it has remained a complicated subject all along. First of all, there is the unresolved question of whether it should be included in the list of rocks or in that of minerals, while in popular perception, strengthened by traditions and conventions through centuries and millennia, it is neither a rock nor a mineral. In the regimes of legislation and commerce, however, clay is a group of mineral commodities. But the fact remains that each of these mineral commodities is again an assemblage of more than one clay mineral. So needless to say, different authors treat them differently depending on the objective and on the limitations of testing facilities. The problem lies, essentially, in the tendency to use non-standard terms to describe different types of clay encountered in the field.

DEFINITION

Clay is a subset of the omnipresent material called “soil”. Soil is essentially the waste material originating from rocks due to weathering and decayed plant and animal matter. Modifications in the nature and composition of soil may be caused by a combination of physical, chemical and biological processes. It constitutes the top layer of the ground.

Clay is the finest grained fraction of the soil, and it comprises very fine minerals composed of hydrous silicates called “clay minerals”. But, defining clay has been a subject of research and controversy for a long time, and even now there seems to be no universal agreement on a standardized definition. According to Wentworth’s scale (1922), clay comprises particles of size below 1/256 mm (cf., silt: 1/256- 1/16 mm, sand: +1/16 – 2 mm, granular gravel: +2 – 4 mm, gravel: +4 mm). Twenhofel (1937) considered that in clay, the clay minerals must constitute at least one-fourth of the total matter. Bateman (1942) laid emphasis on chemical composition, colloidal nature, plasticity and fired properties. According to him, clay is an earthy substance consisting chiefly of hydrous aluminium silicates with colloidal material and specks of rock fragments, which generally become plastic when wet and stone-like when fired. However, as we see today, not all types of clay are plastic, and all of them are not necessarily fired before use. According to Pettijohn (1949), undue reliance on grain size may often be misleading. The US Bureau of Mines (USBM) and the US Geological Survey (USGS) have accepted a simplistic and general definition that

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clays are hydrous aluminium silicates of a great many mineral species, containing varying proportions of impurities.

All these controversies stem from the fact that no two clays are similar, and industries have never cared for definition of the clays that they use. Instead, they have all along relied on the trial tests and on the past records of performance of the clays drawn from historically well-known sources. Nevertheless, clay minerals form an important component of any clay. These are the nanometre-sized products of decomposition by weathering of primary silicate minerals. All clay minerals (some of which are similar enough to form broad groups) are essentially hydrous aluminium silicates, but they show variations in mineralogy, in chemical composition and in crystal form. The known clay minerals are:

A. Kaolinite group [(OH)8Al4Si4O10] or [Al2O3.2SiO2.2H2O]

1. Kaolinite 2. Nacrite 3. Dickite 4. Anauxite

B Smectite Group (earlier name Montmorillonite Group) : Composition uncertain, one of the suggestions is [(Al, Mg)8(Si4O10)3(OH)20.nH2O]. In addition, calcium, lithium or sodium may also be present.

1. Montmorillonite 2. Beidellite

3. Nontronite

4. Hectorite

C. Illite group: Contains iron, magnesium and potassium. D. Halloisite :- Contains more water than kaolinite.

1. Halloisite

2. Metahalloisite 3. Allophane

4. Endellite

E. Palygorskite : Contains magnesium instead of aluminium.

1. Sepiolite 2. Attapulgite

F. Chlorites : Magnesium-rich clay minerals.

This list should never be taken as exhaustive. With invention of more and more sophisticated microscopes, the chance of identifying newer and newer clay minerals remains always open. Clay contains one or more of the clay minerals. In addition, there are some so- called “impurities” found in clays. they include sericite, paragonite, quartz, chlorite,

Clay – General 55

serpentine, limonite, hematite, pyrite, calcite, dolomite, feldspars, zeolites, rutile and carbonaceous matter.

In industry, shale is often included within the broad meaning of clay. Petrologically, however, shale is indurated clay (claystone) which is generally found buried under the surface and which has developed lamination or fissility caused by the pressure of overburden.

There are 16 terms currently used in industrial circle ― china clay or kaolin, ball clay, fire clay, sodium bentonite, calcium bentonite or pascalite, attapulgite or fuller’s earth, brick clay, stoneware clay, pipe clay, roofing tile clay, pottery clay, terracotta clay, terracotta clay shale, brick-making clay shale, flint clay and pozzolanic clay.

HISTORY

Clays were probably the earliest mineral used by civilized men. The first archeological evidence of clay dated back almost to the beginning of the Copper Age. But by that time the techniques of using clay had already reached an advanced stage. Ancient men living as early as 10,000 years ago knew how to make not only fired potteries, but even painted ones, and also to make building bricks. Artifacts comprising potteries with attractive designs painted on them, have been dug out from the ruins of Mohenjo Daro and Harappa (Indus valley), Nile Valley, Hoang-Ho (China), Turkestan (Central Asia), Chaldea (Mesopotamia), Parsepolis (Iran) — the civilizations flourishing during the period 5000-3000 BC. So much evidence of such painted potteries belonging to the period 8000-4000 BC have been found, that some historians have described the civilization during that whole period as “Painted Pottery Civilization”. Apart from potteries, other ancient objects made of clay have also been found. For example, toys made of clay during the period of ‘Rig Vedas’ (scripture believed to be 6,000 years old) have been unearthed in Mohenjo Daro and Harappa; and clay tablets with hieroglyphics engraved in them, have been found in Sumeria (3000 BC). Extremely thin- walled terracotta vases have been unearthed in Beluchistan, Pakistan. Fuller’s earth , as we know, is used for dry-cleaning purpose. But its first recorded use for this purpose was in Cyprus Island in 5,000 BC, and this clay was then called “cymolean earth”. Technique of making clay tiles – often glazed and decorated, were known to the Muslims who later introduced them in India.

China clay is another clay with a long history of use in China. During the reign of Sui dynasty (581-617 AD), some glass made its way to China. Chinese craftsmen, while trying to imitate it with local clay, ended up in a kind of impure but first real “china”. When the reign of Tang dynasty (618-906 AD) was coming to an end and that of Sung dynasty was beginning (906 AD) i.e., around the first half of 10th century, porcelain (a product based on china clay) made its appearance. Throughout the rules of Sung dynasty and subsequent Ming dynasty the art of porcelain received royal patronage and encouragement, and highly artistic porcelain vases and other objects were made during this period. In 1191, tea-drinking was introduced for the first time in Japan by a Buddhist Monk, and this boosted demand for porcelain cups made in China. Later on, Japanese learnt the art from China. This porcelain grade clay became intimately associated with China and the outside world started calling this type of clay, wherever it was found, china clay. The original name ‘kaolin’ also owes its name to a Chinese word “kauling” meaning high ridge – the name of a hill where it was first mined.

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Compared to pottery clay, terracotta clay, tile clay, fuller’s earth and china clay, the history of bentonite is very recent. It started in 1830 with the discovery and naming of a new type of clay by Emille Pascal on a mountain in Wyoming, USA. In 1847 a similar clay was later found in a place called Montmorillon in France and christened montmorillonite. The name bentonite was popularized by an American geologist after he found the clay in Benton Formation in Rock Creek area of Wyoming in 1888.

GENERAL PROPERTIES OF CLAY

Although non-ceramic applications of some clays are not uncommon, clays are essentially ceramic materials. In fact, the word “ceramic” derived from Greek originally meant fired and fused common clays. But in modern usage it includes some other inorganic materials mixed with clays in different forms (common clay, china clay, ball clay, etc.) which are fired together and fused, but clay still remains the core component. One may come across in literature several physical properties that are required to be tested for determining the suitability of a clay for ceramic or other industrial uses. Some of these are tested in raw state and others in fired state. These are as follows.

1. Raw clay:

(1) Raw clay colour: The industrial colours is expressed in terms of what are called “LAB parameters”. ‘L’, ‘A’ and ‘B’ indicating different colours as:

(i) “L” means white

(ii) ‘A+’ means red

(iii) ‘A-’ means green

(iv) ‘B+’ means yellow

(v) ‘B-’ means blue

This is important only when the clay is to be used without burning (as in paper, paint

etc.). Otherwise, this is no indication as to the purity of a clay. Some of the darkest ball clays become perfectly white on burning.

(2) Raw clay brightness: Like raw clay colour, brightness is also important only when the clay is to be used without burning (as in paper, paint etc.). Brightness is measured as a percentage value of the reflectance of clay from blue light having wave length 457 microns (the range of wave length of visible light is 400-700 microns).

(3) Particle size: Coarse granular materials are usually analyzed for particle size distribution using sieving techniques. Dry sieving is normally used for materials in the size range down to 150 microns, and wet sieving is the standard procedure in the next finer range down to about 50 microns. The finer size ranges (clay particles may be as small as less than 2 microns) which have important effects on the physical properties of clays (e.g., plasticity, shrinkage) are normally measured by methods based on sedimentation techniques (Stokes’ decantation and Andreasen Pipette methods).

(4) Deflocculation behaviour: This property is particularly relevant to china clay and ball clay which are to be used in casting applications. The test involves dispersion of clay

Clay – General 57

in de-ionized water, partial deflocculation by addition of sodium silicate in

increments, and measurement of viscosity at each stage of increment.

(5) Plasticity and liquid limit: It is the ability of a clay to undergo non-reversible change of shape in response to applied shear stress. Although the exact mechanism of plasticity is not fully understood, it is believed that a combination of certain factors like grain size, binding power, tensile strength, extensibility, adsorption, texture and molecular constitution is responsible for it. Depending on the amount of water held, a clay can be liquid, plastic, or solid. Attenberg plasticity limit marks the passage of the plastic condition to the solid condition and is defined by the water holding of a small roller of soil which, on rolling, breaks in morsels when its diameter reduces to 3 mm. Plasticity of a clay is expressed either by the percentage of water at the plasticity limit or by what is called Attenberg number (also called plasticity index) which has been expressed in terms of a chart by taking the difference between the water content of a material at the point when it becomes liquid (liquid limit) and that at the point when it becomes plastic. But generally, at the preliminary level of testing at least, plasticity is judged by hand feel as low, medium and high or as fair, good and very

good.

(6) Impermeability: It is proportional to plasticity.

(7) Base exchanging power: Base exchange is the exchange of ions in solution for those

of a solid. Upon contact with a solid, the solution will undergo a change reciprocal to that of the solid. This is also known as cation exchange capacity, which means the quantity of positively charged ions (cations) that a clay mineral can accommodate on its negatively charged surface, and it is expressed as milli-equivalents per 100 gm (equivalent weight is the molecular weight of an element divided by its valency). The mechanism is not fully understood. Certain clays are believed to possess a power of selective adsorption by virtue of which they are able to exchange bases with other substances. Montmorillonite shows a large capacity, kaolinite has slight, illite has intermediate.

(8) Absorptive power: Absorption is related to porosity so far as pure water is concerned. Absorption limit of a clay corresponds to the percentage of water held at the point when no more water penetrates into the clay. It is measured by making water drops fall one by one on a homogeneous paste till they are absorbed in less than 30 seconds. When a drop is not absorbed in less than 30 seconds, then that is the absorption limit, and the water content at that limit denotes the “water of absorption”. It is expressed either in terms of percentage by weight of water absorbed or in terms of millilitres/100 gm.

(9) Swelling index: It denotes the degree of increase in volume of a clay on absorption of water. Swelling index is the ratio of the weight of water to the minimum weight of clay to produce a gel. It is only partially related to absorption, but the actual mechanism of swelling is still not fully understood. Various theories have been put forth. According to one theory, swelling is driven by capillary forces and/or by chemical gradients – particularly the latter in case of clay. Chemical gradient occurs due to positive difference between the concentration of certain species (sodium, potassium, etc.) in clay and that of the water in contact with clay. According to another theory, swelling is related to microstructure and organization of the pore spaces and water in clays. In clay, the clay minerals are in the form of platelets

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arranged in parallel and separated by micro-pores (it is these micro-pores in which the absorbed water resides). The inter-lamellar pores are filled up with single- molecule layers of water. This inter-lamellar water is higher in viscosity and density than free water. It does not flow even if subjected to high hydraulic gradient and, instead, deforms along with the solid part. Due to higher viscosity and density, it exerts outward osmotic pressure.

(10)Bonding power: The extent to which a clay can remain plastic when mixed with materials such as sand, etc., is its bonding power. It depends on the plasticity of the clay itself and on the size and nature of particles of the added material and it is also related to swelling due to which a clay is enabled to enter into the pores of a surrounding material and makes the latter a strong, solid and well-bonded mass.

(11)Strength:- The higher the plasticity, the greater the strength of a clay. It is evaluated in terms of modulus of rupture, which is the load required to break a specimen bar of standard length and cross section area.

(12)Hardness: Some types of clay are harder compared to others. For example, clay shales and flint clays are harder than bentonite, china clay, etc.

(13)pH value: It is the negative logarithm of the effective hydrogen-ion concentration or hydrogen ion activity in gramme equivalent per litre. It is used in expressing both acidity and alkalinity on a scale whose values run from 0 to14 with 7 representing neutrality. Numbers less than 7 indicate increasing acidity and those greater than 7 increasing alkalinity.

(14)Viscosity: Viscosity is that property of a liquid (or a semi-solid mass) which is a measure of its internal resistance to deform under shear stress, and it is measured by the stress in dynes/cm2 or Pascal (Pa) required to be applied to overcome that resistance and maintain a velocity of flow of one centimetre per second. This unit of measurement of viscosity is Poise which is 1 gm.cm.sec or 1 Pascal second (Pa.sec). It is often expressed in centipoise (cP). This property is often important for describing a clay when mixed with water.

(15)Chemical composition: A knowledge of the chemical composition of a clay provides a useful guide to the way in which it will behave in a ceramic product made from the clay. Clay is normally analyzed for the oxides of aluminium, silicon, titanium, calcium, magnesium, potassium and sodium and the loss on ignition (LOI). These analyses provide valuable guides as follows.

(i)

(ii)

(iii)

Al2O3 and SiO2: These may help in differentiation between china clay and ball clay. If a large number of samples are analyzed, then it is found that these constituents are fairly consistent in china clay, but widely variable in ball clay.

Fe2O3 and TiO2: These are the colouring oxides (pure TiO2 is very white when fresh, but over time it turns yellow), and their analyses, in general, can be used as guide to fired colour.

CaO and MgO: Analysis of these are of importance in case the types of clay are

bentonite and fuller’s earth.

(iv) K2O and Na2O: The alkali oxides derived from feldspars and micas affect

vitrification behaviour since they act as fluxes.

(v) LOI: Clays are hydrates containing varying number of water molecules. So LOI

generally comes from the loss of water on burning and it varies from clay to clay.

Clay – General 59

In case of lignitic bentonite, however, its value is much higher because of loss of both water and carbon.

Sometimes additional constituents like soluble salt, trace elements and carbon are done for certain clay.

(16) Mineralogical composition- The methods currently available for determining mineralogical composition of clay are not precise. This is largely because of the effects of particle orientation and shape which can lead to overestimation of some constituents in relation to others. But, nevertheless, certain clay minerals like kaolinite, montmorillonite, attapulgite are predominant in certain clays, and this analysis can provide useful indications to the type of clay.

2. Fired clay:

(1) Fired clay colour: This is important when the clay is used after firing. The whiter a clay burns, the higher its quality is supposed to be. But in potteries, etc., the pattern of colour is more important than whiteness. The fired colour depends on certain impurities present in clay.

(2) Shrinkage: It is due to the fact that as water is removed on drying, the solid particles approach closer to each other, the volume of the whole mass being thereby reduced. On further heating in a kiln, the particles partially vitrify and come further closer to each other thus causing further shrinkage. It is proportional to fineness of the particles. Shrinkage is non-reversible.

(3) Thermal expansion: Contrary to shrinkage, thermal expansion is reversible. Samples of clay which have already been fired, are re-heated to observe the reversible expansions that occur. This involves dilatometry, i.e. the study of size changes which occur as a material is heated. It can provide useful information about the firing behaviour of a clay-based ceramic product. Its most valuable role is in assessing glaze/body compatibility, i.e., differential expansion, if any, of glaze in relation to the glazed body.

(4) Fusibility: No single fusion point can be attributed to clays. They usually start fusing partially much before they are completely melted.

(5) Refractoriness: It denotes that a given clay is capable of retaining its shape at a given temperature under normal pressure. In other words, it is resistance of a clay to high temperature. In most of the applications, a single type of clay is seldom used, and batches consisting of mixtures of several types are fired in kilns. As such, fusion temperatures of the different types of clay do not provide any guide to the refractoriness of a composite mixture. For determining the refractoriness of a clay in relation to its actual behaviour in a kiln, therefore, standard pyrometric cones are used. A pyrometric cone is a pyramid with a triangular base and of a defined shape and size. It is shaped from a carefully proportioned and uniformly mixed batch of ceramic materials so that when it is heated under kiln conditions, it will bend due to softening, and at a definite temperature and after a definite time, its tip becomes level with the base. By varying the composition of the mix, a series of cones with increasing softening temperature, is made. A cone of identical shape and size made of any clay(s) is then heated and matched with one of the reference cones. The

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(6) (7)

(8)

softening temperature of the reference cone as read from a chart, can then give the softening temperature of the test cone, and the reference cone number called “pyrometric cone equivalent” or PCE is the expression of refractoriness of the test clay. Three standard series of pyrometric cones are in vogue – Orton in USA, Seger in Germany and Staffordshire in UK. Of these the Orton pyrometric cone equivalent (or simply Orton) is used in India as the measure of refractoriness of clays. This series starts with the coolest cone numbered 022 (equivalent to 5860C when a 1.75 inches high self-supporting cone is heated at the rate of 600C/hour) and the most resistant cone numbered 42 (equivalent to 20150C when a 15/16 inches high small cone is heated at a fast rate of 1500C/hour). In between these limits, the numbers are 021, 020, 019 … 02, 01, 1, 2, 3, … 39, 40, 41. A refractory material is one of which the PCE is not less than 21, i.e., which does not fuse at less than 15660C temperature. Vitrification: It signifies the degree of fusion that has occurred under certain conditions of heating. The extent of vitrification of a clay is proportional to the amount of fluxing material it contains, and to the duration and intensity of heating. Vitrification range: It is the difference between the temperature at which vitrification of a clay starts and that at which the clay melts completely. Higher proportion of small particles, and larger amounts of basic compounds and fluxes tend to shorten vitrification range.

Porosity: Porosity of raw clay is of little importance and that of fired clay can be regarded as the opposite of vitrification. In other words, vitrifiable clays burn denser.

On careful scrutiny of the above 16 properties, it will be revealed that many of them are interrelated. For example:

(1) Particle size, strength, binding power, swelling and impermeability are closely related to plasticity.

(2) Particle size and shrinkage are interrelated.

(3) Vitrification range is nothing but the difference between the fusion temperature and

the vitrification temperature, fusibility is nothing but the opposite of refractoriness.

(4) Absorption is measure of porosity (partially) which is just opposite of vitrification

(5) Absorption is also partly related to swelling index.

The relationship amongst the other properties is not known for certainty. However, these properties are for preliminary investigation of a clay for the purpose of fixing its type and possible industrial use. At the stage of actual use, even after knowing the type of clay, industries may need to investigate the quality of a particular clay and this may require more detailed testing. But more importantly, industries rely on experience and judgement and, as far as possible, try to stick to a known and time-tested source of supply.

Clay – General 61

CLASSIFICATION OF INDUSTRIAL CLAY

Amongst the clay minerals, kaolinite, montmorillonite and attapulgite are the most important from the point of view of industrial classification of clay, while amongst the impurities, carbonaceous matter, sodium oxide, calcium oxide and magnesium oxide render some clays their distinct identity. But, on the whole, classification of clay into different types and their names are based on a mix of criteria related to industrial use, physical nature, ceramic property, mineralogy and the name of type area. The names traditionally used to identify or designate a particular type of clay are also based on the distinctive criteria.

The classifications of clay proposed prior to 1912 were mainly on the basis of stratigraphical position or chemical composition or uses. From 1912 onwards, some serious attempts were made. These are described and analyzed here.

1. Searle’s classification (1912): This classification system was put forth with first the origin from a petrological point of view and then a limited number of physical properties and chemical composition as the basis. The properties considered were only refractoriness, fired colour and vitrification. The classification system (Source: A. B. Searle: The Natural History of Clay; Cambridge University, 1st Edition, 1912) is as follows.

(1) Primary clays:

(a) Claysproducedbyweatheringofsilicates(e.g.,somekaolins)

(b) Clays produced by lateritic action – very rich in alumina, some of which is

apparently in a free state.

Cornish china clay).

(2) Secondary clays:

(a) Refractoryclays(e.g.,fireclayandsomepipeclays).

(b) Pale-burning non-refractory clays (e.g., pottery clay, ball clay and some shales) (c) Vitrifiableclays(e.g.,stonewareclay,pavingbrickclay)

(d) Red-burning non-refractory clays (e.g., brick/terracotta clays/ shales).

(e) Calcareousclaysormarlscontainingmorethan5%calciumcarbonate.

(3) Residual clays: Clays formed by any of the foregoing processes and deposited along with calcareous or other matter which have later been removed leaving behind the clay (e.g., white clays of the Derbyshire hills).

This classification suffers from several inadequacies – particularly in the present context. These are:

(i) In the first level, origin of clay has been considered, but this is still not fully understood.

(ii) In the second level, no uniform basis is discernible.

(iii) The commonly used terms are not correlatable with the types of clay and as such it is

of no use to the user industries.

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2. Ries’ classification (1917): In this also, origin and mode of formation of clay were taken as the basis as follows.

A. Residual clays formed in locations of rock alteration due to various agents of either surface or deep-seated origin.

1. Those formed due to surface weathering by solution or disintegration or decomposition of silicates.

(a) Kaolin – white in colour, usually white burning. Parent rock: granite, pegmatite,

rhyolite, limestone, shale, feldspathic quartzite gneiss, schist etc. Shape:

blankets, tabular, steeply dipping masses, pockets or lenses.

(b) Ferruginousclaysderivedfromdifferentkindsofrock.

2. White residual clays formed by the action of ascending waters possibly of igneous origin.

(a) Formedbyrisingcarbonatedwaters

(b) Ferruginousclaysderivedfromdifferentkindsofrock.

3. Residual clays formed by the action of downward moving sulphate solution. 4. White residual clays formed by replacement due to action of waters.

B. Colluvial clays representing deposit formed by wash from the foregoing and of either refractory or non-refractory character.

C. Transported clays 1. Deposited in water

(a) Marine clay or shale deposits often of great extent. White-burning clays (ball clays, fireclays or shales); Buff-burning clays (impure clays or shales – calcareous or non-calcareous)

(b) Lacustrine clays, deposited in lakes and swamps. Fire clay or shale – red- burning, calcareous.

(d) Estuarine clay, deposited in estuaries. Mostly impure and finely laminated.

(e) Deltaclays

2. Glacial clays — often stony, found in the drift; may be either red- or cream-burning. 3. Wind-formed deposits, loess.

4. Chemical deposits, some flint clays.

This classification has also not been successful from the point of view of utilization. Besides, all the currently used terms are not accommodated here. Although a few names such as ball clay, fire clay, shale, flint clay, etc. find mention, their differences have not been brought out.

3. Parmelee’s classification (1921): This is probably the most elaborate classification proposed so far. In this classification (Source — C. W. Parmelee and C. R. Schroyer: Further Investigations of Illinois Fire Clays; State Geol. Survey, Illinois, 1921), ultimate types of clay number 28, and there are as many as four levels of classification as follows.

Clay – General 63

I. Clays burning white or cream coloured, not calcareous.

A. Open burning clays, i.e. still distinctly porous at cone 15. These are of value in the manufacture of pottery because of their good colour and strength. These may also possess high degree of refractoriness and of value in manufacturing refractories.

1. Low strength, e.g. residual kaolins such as those from North Carolina

2. Medium and high strength, e.g. secondary kaolins such as those from Florida and

Georgia

B. Clays burning dense, i.e., become nearly or completely nonporous between cones 10 and 15.

(a) Non-refractory clays

3. Good colour, medium to high strength, medium shrinkage. Uses: pottery, certain

whiteware, porcelain, stoneware.

4. Poor colour, medium to high strength, medium shrinkage, Uses: stoneware,

terracotta, abrasive wheel, zinc retort, face brick, saggars. (b) Refractory clays

5. Good colour, medium to high strength, medium shrinkage. Uses:

refractories, especially for glass (if do not over-burn seriously at 5 cones higher), and the uses stated under 3.

(C) Dense burning clays, that become nearly or completely non-porous between cones 5 and 10 and do not over-burn seriously at 5 cones higher than the temperature at which minimum porosity is reached.

(a) Non-refractory clays

6. Good colour, medium to high strength, medium shrinkage, usually reach minimum

porosity between cones 5 and 8. Type: ball clay. Uses: pottery, whiteware,

porcelain and stoneware.

7. Poor colour, medium to high strength, medium shrinkage. Uses: stoneware,

terracotta, abrasive wheel, zinc retort, face brick, saggars. (b) Refractory clays

8. Non-porous or practically so at cone 5, do not seriously over- burn at 12 cones higher, highly refractory (softening point at cone 31 or higher), bonding strength minimum 325 pounds per square inch. Uses: component in graphite crucibles for melting brass.

9. Non-porous at about 12750C (cone 8), not over-firing at 14000 C or higher, strength and softening point as above. Uses: glass pots.

10. Become dense at about 12750C (cone 8), do not over-burn below 14250C, bonding strength minimum 325 pounds per square inch or higher, softening point cone 29 or higher. Uses: glass pots.

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II. Buff-burning clays

A. Refractory clays

(a) Open burning, i.e. having porosity 5% or more at cone 15 or above; indurated, non-

plastic or slightly plastic unless it has been weathered. Type: flint clay.

11. Normally aluminous, maximum alumina 40 per cent. Use: refractories.

12. Highly aluminous, alumina exceeds 40 per cent. Use: refractories, abrasives.

(b) Open burning, i.e. having porosity 5% or more at cone 15 or above; plastic.

13. Normally siliceous, maximum silica 65 percent. Uses: fire bricks and other

efractories, terracotta, sanitary ware, glazed and enameled brick.

14. Siliceous, silica content above 65 percent. Type: many of the New Jersey fire

clays. Uses: fire brick and other refractories.

within that range.

15. Medium to high strength, do not over-burn for 5 cones higher than point of

minimum porosity. Uses: glass pots, fire brick, saggars, architectural terracotta,

sanitary ware, enameled and face brick.

(d) Dense burning attaining porosity of 5% or less at cone 10 or lower.

16. Non-porous or practically so at cone 5, do not seriously over-burn at 12 cones higher, highly refractory (softening point at cone 31 or higher), bonding strength minimum 325 pounds per square inch. Uses: component in graphite crucibles for melting brass, architectural terracotta, sanitary ware, enameled and face brick.

17. Non-porous at about 12750C (cone 8), not over-firing at 14000 C or higher, strength and softening point as above. Uses: glass pots, architectural terracotta, sanitary ware, enameled and face brick.

18. Become dense at about 12750C (cone 8), do not over-burn below 14250C, bonding strength minimum 325 pounds per square inch or higher, softening point cone 29 or higher. Uses: glass pots, architectural terracotta, sanitary ware, enameled and face brick.

B. Non-refractory clays

(a) Open burning, not attaining porosity of 5% or less at any cone lower than 10.

19. High or medium strength. Uses: architectural terracotta, stoneware, yellow ware, face brick, sanitary ware.

20. Low strength. Use: brick.

(b) Dense burning, attaining less than 5% porosity at cones lower than 10.

21. High or medium strength. Uses: architectural terracotta, stoneware, abrasive wheels, sanitary ware, face brick, paving brick.

III. Clays burning red, brown or other dark colours

A. Open burning clays, i.e. those which do not attain low porosity at any temperature short of actual fusion.

22. Medium or high strength. Uses: brick, drain tile, hollow blocks, flower pots, pencil clays, ballast.

Clay – General 65

23. Low strength. Use: brick.

B. Dense burning

(a) Having a long vitrification range (5 cones)

24. High or medium strength. Uses: conduits, sewer pipe, paving brick, floor tile, electrical porcelain, cooking ware, silo block, art ware face brick, architectural terracotta, roofing tile.

25. Low strength. Uses: as dust body in the manufacture of electrical porcelain, floor tile, building brick.

(b) Having a short vitrification range

26. High or medium strength. Uses: building brick, face brick, hollow block,

flower pot

27. Slip clays.

IV. Clays burning dirty white, cream white or yellowish white.

28. Contain calcium or magnesium carbonate or both, never reach very low

porosity, have a very short heat range. Use: common brick.

A very careful analysis of the classification system will bring to light many demerits as below.

(i) The classification is too complex for any practical application.

(ii) There are too many bases in the same level which will involve too many tests for

defining the position of an unknown clay.

(iii) The interrelationship of various parameters have not been taken into account

(e.g. porosity and vitrification range, strength and plasticity).

(iv) In some places, colour and plasticity of raw clay has formed the basis after the clay has already been classified on the basis of fired colour; this defies logic

because colour and plasticity of a clay are examined before it is fired.

(v) The classification applies to use of clays by firing only, but many clays are used

without firing.

(vi) Some common types of clays like shale, bentonite, fuller’s earth etc. cannot be

fitted in this system.

(vii) The interrelationship amongst the different types of clay like china clay, ball

clay, brick clay etc. have not been brought out.

(viii) The position of some of the clays (e.g., terracotta clay) is not unique, and they

find place in more than one position in the system.

3. Bateman’s classification (1942): This classification (Source – Allan M. Bateman: Economic Mineral Deposits, 1st edition, 1942) is as follows:

Kaulir Kisor Chatterjee

Type

1. Kaoline

(a) China clay (b) Paper clay

2. Ball clay

3. Fire clay:

(a) Flint clay

(b) Diasporic clay

4. Stoneware, paving and sewer pipe clays

5. Brick and tile clay 6. Bentonite

7. Fuller’s earth

Chief Use

White ware, porcelain Fillers in paper-makng

White ware, mixing Refractories

Stoneware, paving bricks, sewer pipes

Brick and tile

Iron and steel works, filtering

Filtering

Chief characteristics High grade, fine-grained, white burning

White burning High alumina

Dense burning Common clays

Absorptive qualities

In this system, all the terms in use today do not find a place. Moreover, no basis is discernible at all.

5. Rosenthal’s classification(1949): The main plank of this system is the mode of formation of clays. The classification system (Source — E. Rosenthal: Pottery and Ceramics; Harmondsworth, UK, 1st edition, 1949) can be schematically represented as follows.

Clay:

(1) Primary

(2) Secondary

(a) Refractoryclay (b) Vitrifiableclay (c) Fusibleclay

This is no doubt an oversimplification of the complexity of clays and the diversity of their usage. Only three properties have been taken into account, and there is no correlation of the terms used here with those currently used by industries. As such, it does not serve any practical purpose.

6. USBM’s classification (1970): The US Bureau of Mines developed a system of classification of clays, originally in 1956 and later expanded in 1970, which is schematically represented as follows.

Clay:

(1) China clay (Kaolin) (2) Ball clay

(3)

Fire clay (including stoneware clay) (a) Plastic

(b) Semi-plastic (c) Semi-flint (d) Flint

(4) Bentonite

(a) Swelling

(b) Non-swelling 5. Fuller’s earth

6. Miscellaneous clay

Clay – General 67

It is essentially a list of the names of some of the commonly used industrial clays without any basis or parameter to bring forth their interrelationships. Moreover, fire clay, stoneware clay and flint clay are distinctly different terms having distinctly different usage, and their combining together is not justifiable. The class “miscellaneous clays” is also a very general term.

7. Chatterjee’s classification (1978): K. K. Chatterjee (the present author) proposed this classification in 1978 (ref.: J. Metals and Minerals Review, vol XVII, no. 2, January, 1978) after studying the steps of testing of clays by the research institutes of the Council of Scientific and Industrial Research (CSIR), India. For testing clay samples, they are first crushed and ground, then plasticity is studied and finally they are fired to study the different fired properties such as refractoriness, fired colour, vitrifiability etc. (ref. Council of Scientific and Industrial Research: Indian Clays – their Occurrence and Characteristics – Samples examined; Pt-I and II, 1958). Since hardness is directly related to grindability and the economics of grinding, it has been taken as the first criterion for classification of clay. The next criteria are the properties that can be studied immediately after grinding and without firing, i.e. base exchanging power, plasticity and swelling characteristic. Only after clay has been classified to the maximum possible extent on the basis of these four criteria, the properties that can be studied only after firing, have been considered. These properties (refractoriness, fired colour and vitrifiability) have been used to complete the classification. Thus the criteria for classification in the successive levels follow a logical order and they have been selected so as to require the minimum possible testing for fitting any unknown clay sample in the system. Also, care has been taken that the position of none of the types of clay is interchangeable, and that the interrelationship amongst the different types is clearly visible at a glance. In the original system, 15 of the 16 currently used types of clay (china clay or kaolin, ball clay, fire clay, sodium bentonite, calcium bentonite or pascalite, attapulgite or fuller’s earth, brick clay, stoneware clay, pipe clay, roofing tile clay, pottery clay, terracotta clay, terracotta clay shale, brick-making clay shale, flint clay and pozzolanic clay) were accounted for. The type pozzolanic clay was at that time, not common – at least in India. Now that it has also become very common in literature, the original system has been expanded to include this type also. Besides, the name attapulgite has been gaining acceptance over fuller’s earth internationally, although in India, the name fuller’s earth continues to be popular. So, now both these names have been incorporated. The term “cement clay” has been deliberately omitted, because no natural clay can be universally marked as cement clay and any clay can be used after manipulating its composition by addition of bauxite, hematite, sand, laterite etc.

68 Kaulir Kisor Chatterjee

to suit the specification of the composite feed. The modified classification system is as follows.

Clay:

I. Hard

(pozzolanic clay) II. Soft

A. Clay with good base exchanging power 1. Plastic

(1) Swelling (sodium bentonite)

(2) Non-swelling (calcium bentonite or pascalite) 2. Non-plastic (attapulgite or fuller’s earth)

B. Clay with poor base exchanging power 1. Plastic

(1) Refractory

(a) Poorly vitrifiable (fire clay)

(b) Highly vitrifiable (pipe clay, stoneware clay) (2) Non-refractory

(a) White burning (ball clay) (b) Red, brown, dark burning

(i) Uniform pleasing fired colour (terracotta clay) (ii) Non-uniform fired colour

– Highly vitrifiable

(roofing tile clay, pottery clay)

– Poorly vitrifiable (brick clay) 2. Non-plastic (china clay or kaolin)

In USA, only sodium bentonite is regarded as bentonite proper, whereas calcium bentonite or pascalite (also called in both USA and UK as “calcium montmorillonite”) and attapulgite are referred to as two types of fuller’s earth. In Brazil, the nomenclature is similar to that in India – calcium bentonite as a type of bentonite and attapulgite or fuller’s earth, a separate entity.

The individual clays are dealt with in the subsequent chapters.

A. Plastic

1. Uniform and pleasing fired colour

(terracotta clay shale)

2. Non-uniform fired colour (brick-making clay shale)

B. Non-plastic

1. Poor reactivity with lime (flint clay)

2. Good reactivity with lime at ordinary temperature

(pozzolanic clay) II. Soft

A. Clay with good base exchanging power 1. Plastic

(1) Swelling (sodium bentonite)

(2) Non-swelling (calcium bentonite or pascalite) 2. Non-plastic (attapulgite or fuller’s earth)

B. Clay with poor base exchanging power 1. Plastic

(1) Refractory

(a) Poorly vitrifiable (fire clay)

(b) Highly vitrifiable (pipe clay, stoneware clay) (2) Non-refractory

(a) White burning (ball clay) (b) Red, brown, dark burning

(i) Uniform pleasing fired colour (terracotta clay) (ii) Non-uniform fired colour

– Highly vitrifiable

(roofing tile clay, pottery clay)

– Poorly vitrifiable (brick clay) 2. Non-plastic (china clay or kaolin)

The individual clays are dealt with in the subsequent chapters.

CLAY – KAOLIN (CHINA CLAY)

Kaolinite is the dominant clay mineral in china clay or kaolin along with some other minerals of the same group namely dickite and nacrite, as well as halloisite. Kaolinite originates from feldspar contained in different rocks, by weathering and the process is called kaolinization. The alteration of feldspar takes place in two stages–first to montmorillonite and then to kaolinite. Kaolin deposits may be either primary (in situ) or secondary (transported), but the in situ deposits are more common. Chemically, the composition of pure china clay is dominated by SiO2 in silicate form (47-50%), Al2O3 (34-37%) and water showing as loss on ignition (10-12.5%). The chemical composition shows remarkable consistency. In nature, small amounts of other constituents like free silica, mica, Fe2O3 , TiO2, CaO, MgO, K2O and Na2O are invariably present – to a greater extent in primary deposits, and to a lesser extent in secondary ones in which, part of these impurities are removed by natural sorting. Production of modern whiteware commenced in India in the middle of 19th century at Pathargatta in Bhagalpur district, Bihar.

BENEFICIATION

China clay is valued for its purity, and the impurities are required to be removed to the maximum possible extent to convert a crude china clay to a marketable one. Some of the uses specify such a high degree of purity that the best grade deposits have to be selected and then beneficiated. There are two processes, namely dry and wet, which are described here.

1. Dry process: It is a simple and relatively inexpensive process, but its scope is limited, and only part of the relatively coarser sized and heavier impurities can be removed leaving the finer and lighter clay particles purer. The crude clay is first crushed and dried to eliminate the natural moisture. Then pulverized to very fine size to liberate the clay particles from the impurities – an operation called “levigation”. Finally, the separation of the clay particles is done by air classification.

2. Wet process: This is a complex and widely practised process with some variations. It can refine crude china clays to high levels of purity. First the clay is made into a slurry (called “slip” in USA) and then the slurry is separated into different coarse fractions according to particle size, by treating with different reagents or by hydro-cyclones or by spiral classifiers. The coarse or the grit fraction containing (+) 45 microns sized particles is removed. The slurry containing (-) 45 micron sized particles is then separated into (-)2 micron sized and 2

70 Kaulir Kisor Chatterjee

micron (+) sized fractions by hydro-cyclones or de-flocculation. The former is required for certain special non-ceramic applications. But both these slurry fractions still contain TiO2, Fe2O3 (both colouring matters) and mica. The two colouring matters are removed by high intensity magnetic separation method, and, if all three are present, then ultra-flotation techniques are employed. Thus purified fractions of the clay slurry is bleached to improve brightness. Finally, the slurry fractions are filtered, dried (generally sun-dried in order to keep the cost low) and pulverized (levigated).

CRITERIA OF USE

The general properties of clays have been discussed in the chapter “Clays – General”. There are some specific properties which are important from the point of view of industrial

use of 1.

3. 4.

5. 6. 7.

8. 9. 10.

china clay. These properties of pure marketable grade china clay are as follows.

Whiteness: Both the raw and fired colours of china clay are white. The normal content of the colouring matter Fe2O3 is only 0.6-1.3 per cent. The colouring parameter ‘L’ value is as high as 90 -96 (cf., the value of fresh TiO2, which is the bench mark for whiteness, is 98-100).

Brightness: The brightness of calibrated china clay (i.e. even-sized fine grains) as measured in terms of reflectance from blue light (wave length 457 microns) is 80-87 (cf., the value of fresh TiO2, which is the bench mark for brightness, is 97-98). Fusion point of china clay is 17850C.

Dry shrinkage (at 110oC) is 14% (max), total fired shrinkage (at 1350oC) is 18% (max).

Its plasticity is very low (almost non-plastic).

Its specific gravity is 2.6.

It is soft and can be pulverized to a fine size of less than one micron. At this size, the particles quickly disperse in water and remain in suspension for a long time. Chemically, it is inert and non-toxic, but its oil absorption is fairly high (25-45%).

It is electrically non-conductor.

Its refractive index is 1.56.

These properties are standards in a marketable grade beneficial to china clay. There may, however, be variations in the contents of certain chemical impurities and in a few of the physical parameters, which are especially specified by user industries.

USES AND SPECIFICATIONS

For various purposes, china clay is used in either raw or processed form. The impurities in china clay are SiO2, Fe2O3, TiO2, CaO, MgO and Na2O which should be kept low as per the standard specifications. Apart from these, physical properties such as plasticity, brightness, colour, grit content, particle size, conductivity, etc. play an important role in deciding about the end use of china clay.

Clay – Kaolin (China Clay) 71

Very often, in certain uses – particularly those requiring firing, china clay is not fired alone, and is mixed with other materials. As a result, some of the properties of pure china clay become modified in the charge, and these adjustments are made carefully to suit the specifications of some user industries.

The important uses are:

1. Ceramics and glazes (non-refractory and refractory)

2. Bone china

3. Textile

4. Paper

5. Rubber

6. Plastic

7. Paint

8. Pharmaceuticals and cosmetics

9. Insecticide

10. White cement

11. Ink

12. Ultramarine

13. Synthetic zeolite

14. Catalyst

15. Soaps and detergents

16. Fiber glass

17. Explosives and pyrotechnics

18. Adhesives and sealants

19. Metakaolin

These uses are discussed as follows:

1. Ceramics and glazes

(a) Non-refractory: The word “ceramic” derived from Greek “keramos” originally meant fired and fused common clays. The original ceramic products (e.g., bricks, potteries) made only of clay were hard and resistant to heat and chemicals, but at the same time coloured, porous and brittle. Today, the product range has gone much beyond bricks and potteries and include white wares (porcelain-wares, table wares, sanitary wares), white tiles (both glazed and non-glazed) and electrically insulating porcelain. These high quality white products are not only hard and resistant to heat and chemicals but also nonporous and strong, and to make them, china clay (with some additional materials added to it) is used.

Manufacturing process: The principle of the manufacturing process, in essence, consists in mixing china clay, quartz or silica sand, other types of clay (e.g., ball clay), feldspar and some flux (soda) with 30-40% water. This mixture is ground, thoroughly agitated, filter pressed, moulded into the required shape, dried and then fired to a temperature ranging from 1200-15000C depending on the product hardness required, but usually at a temperature of 13000C. This fired product before glazing is called biscuit and the firing is called biscuit firing (cf., baking of biscuits). Different ceramic products can be prepared by varying the types of clay and their proportion in the mixture. A special type of ceramic product used for

72 Kaulir Kisor Chatterjee

making “water filter candles” contains micro-pores to arrest passage of particles suspended in water. To make these, some finely ground organic substance (wood powder) is added to the raw material mix. On firing, the organic matter burns out leaving the candle porous.

Glazing: If the product is to be a glazed one, then it is glazed before firing. The purpose of glazing is to provide a uniform firmly adhering coating on the surface of the ceramic body concealing defects such as pinholes, bubbles, etc. The glaze is made of the same ingredients but with predominance of quartz and feldspar. The ingredients are mixed, finely ground and mixed with water, and this mixture is the glaze. The moulded raw body of the product is dipped into the glaze and then fired to a temperature of about 14000C. Colours and decorative designing, if required, are painted after glazing and before firing.

Glaze may be “raw glaze” or “fritted glaze”. Raw glaze consists of insoluble material applied as such (soluble components crystallize in the mixture and cause blemishes on the treated surface), while fritted glaze is heated beforehand to cause chemical change in the components. Glaze should not only melt but also spread uniformly.

Specifications: Certain desirable properties like white fired colour, low shrinkage, softness and poor electrical conductivity are intrinsic in a marketable grade beneficial to china clay. But the most disadvantageous properties are low plasticity and higher-than-firing- temperature fusion point. The former will not make it amenable to moulding whereas the latter will unnecessarily raise the manufacturing cost. Certain other materials, are therefore added to improve the plasticity and to reduce the fusion temperature, and in the process, some other properties may have to be compromised. So a careful balancing of the proportions of different materials has to be done.

Optimum range of particle size is important. Too coarse a size increases the porosity of the product whereas very fine particles create problems in squeezing out of the water during filter pressing.

Colour of china clay after firing is very important and that should be as white as possible. Shrinkage should be as low as possible, because otherwise cracks may develop after firing. The clay must not fuse at or below the firing temperature, i.e. 13000C. Fe2O3 and TiO2 form low-melting iron-titanate glass causing blisters in the products and consequent increase in porosity. Besides, Fe2O3 makes the product coloured. TiO2 has also a high melting point and it will unnecessarily increase the firing temperature. Both these constituents are therefore objectionable.

Alkalis are deleterious. Sodium and potassium in the form of carbonates combine with silica at the firing temperature to form silicates which are water soluble. Presence of these silicates in the ceramic product, obviously, will not be desirable. Moreover, alkalis have fluxing property and they get into the raw material mix through the feldspar and soda. So their presence in the china clay may create problems of managing the chemistry of the mix.

MgO is highly refractory in nature and so is undesirable in white ware, because white wares, by definition, are non-refractory in nature. Besides, it is hygroscopic, absorbing 120% of its volume of water slowly over a period of time.

Grit is not very harmful, but still too much grit increases porosity of the product and also the grinding cost. Quartz or free silica (i.e., other than the silica present in the form of aluminium silicate) changes into different polymorphs with rise in temperature such as beta- quartz at 5740C, tridymite at 8700C and crystobalite at 14000C. Although, the final form crystobalite is thermally stable, it is most often not reached below the firing temperature. Each of the other forms tend to change over to low-temperature forms and each change is

Clay – Kaolin (China Clay) 73

accompanied with change in volume – expansion when the change is to high temperature forms and contraction when the change is to low temperature forms. Consequently, clays containing high percentage of quartz tend to expand on heating and contract on cooling, and the ceramic products made out of such clay tend to develop cracks (only flint is transformed to crystobalite readily, and hence this variety of quartz is preferred for mixing with china clay in the feed). The silica in the form of silicate is an integral constituent of all clays, and it is not harmful (on the contrary, it adds to the strength of the product ).

Alumina, which is also an integral constituent of the clay minerals, has some advantages. China clay consists of a number of clay minerals, and hence the percentage alumina in it may vary from one china clay to another. Besides, it gets into the mix through the feldspar also. As such, alumina has a high melting point, but at even below the firing temperature (13000C) it melts imperfectly to become a highly viscous fluid, facilitating a coherently bonded non- porous product. Alumina also gets added to the mix through feldspar.

Lime (CaO) is highly hygroscopic. So, if it is present in clay, the product will absorb water in course of time on exposure, and ultimately, crumble. Also, at 11000C (i.e. below the firing temperature), CaO reacts with alumina and silica and forms new compounds, mostly silicates. Some of these silicates lower the fusion point of clay. Also, if lime is present in the form of CaCO3 or CaSO4, then CO2 or SO3 is expelled on heating, and the ware is left more porous. Finally, lime makes the melt more fluid and it reduces the range between softening and flowing temperature. Sometimes this range may be as short as 400C only. The result is that it becomes difficult to control the temperature of the furnace to remain within this range. For these reasons, lime is very objectionable in china clay.

Loss on ignition or LOI may be on account of water, and the carbonate and sulphate of calcium. While the latter is not desirable, the water is necessary to render plasticity to the china clay (in fact, plasticity of the mix is enhanced by adding some plastic clay).

Amongst the clay minerals, montmorillonite is the most detrimental to the china clay. Montmorillonite has high water absorption and high swelling power, which increase the filter pressing time for the clay during mould preparation. Even 1% content of this mineral is not acceptable.

The Bureau of Indian Standards (BIS), in 1993, has considered three grades–(I, II and III depending on the quality of the ceramic product to be manufactured) and has prescribed specifications in terms of certain parameters, some of which are applicable only to the finished ceramic product and not to china clay. The important parameters that are relevant to china clay for use in non-refractory ceramics are: 60-70% of particles less than 2 micron size and the rest coarser up to 44 micron; 10.5-13.0% ( minimum) LOI; 32-37% (minimum) Al2O3; 1.8-0.5% (maximum) Fe2O3; 1.0 -0.7% (minimum) TiO2; 8-6% (minmum) dry shrinkage at 1100C; 18-16% (minimum) fired shrinkage at 13500C; 10-14 (minimum) Attenberg number for plasticity. Out of these parameters the last one is not very critical for pure china clay. The Attenberg number can be modified by adding some other plastic material.

(b) Refractory: Refractory materials are defined as those resistant to heat and having a melting temperature of not less than 15800C. The function of refractory lining on a furnace wall is not only to withstand high temperature, but also to withstand temperature fluctuation, and to resist penetration, abrasion, and erosion by hot gases and molten materials in the furnace, and over and above, it should not chemically react with those materials.

74 Kaulir Kisor Chatterjee

China clay has limited use in the refractory industry because the refractory products are not suitable for use under conditions where they are expected to withstand repeated thermal shocks, and its main application is in the production of insulation bricks in combination with talc. So far as the specifications are concerned, the higher the fusion temperature, the better. In this case, MgO is favourable on account of its high fusion temperature. The hygroscopic nature of the MgO does not come in the way, because the product is used under conditions of high temperature and is not supposed to be exposed to water. Silica (free silica as quartz) increases the refractoriness of the clay on account of its high melting point (17000C). The problem of differential expansion of quartz due to change in form with increase in temperature is not relevant, because, at about 15000C, the quartz grains fuse together in the form of cristobalite which is a stable phase of silica, and no further deformation takes place during exposure of the bricks to the conditions in the furnace. So, free silica content in the clay is not objectionable.

Fe2O3 melts at a relatively lower temperature, and if, in addition, TiO2 is also present then at the high temperature in furnaces where the refractory products are used, they form low- melting iron-titanate glass causing blisters in the refractory bricks and consequent increase in porosity. The Indian industries prefer less than 1% Fe2O3 in the china clay. Alkalis and lime lower the fusion temperature, and hence are deleterious.

Particle size is not of any critical consequence, and relatively coarse particles can be used because, on firing, they will fuse together to form a compact mass. Orton PCE should be 28- 34, but it can be modified by addition of some other refractory material (china clay does not possess such high values).

2. Bone china

Bone china is a special translucent variety of porcelain first introduced in England in 1794. It is a sophisticated fine product using tri-calcium phosphate [Ca3(PO)2] obtained by burning bone, as the most fundamental raw material. The industry prefers the ash of cattle bone because it is iron-free. The raw material mix consisting of 45-50% bone ash, 25-30% china clay, 25-30% feldspar (flux) and about 5% ball clay (plasticizer) is first mixed with water and ground; then the slurry is de-watered by filter-pressing, shaped according to the product desired and slowly dried, and finally biscuit fired at a temperature lying within a carefully controlled short range of 1250-13000C. The firing temperature is very critical in the case of bone china. Under-firing leaves open pores and cannot bring the all-important translucency, while over-firing produces blisters on the surface.

The biscuit is then glazed using colourless transparent glaze. The glazing material consists of borosilicate and white-burning china clay with some feldspar to act as a flux. China clay helps the glaze to remain in suspension. Glaze is applied to the biscuit by either dipping or spraying, and then fired at a temperature 1050-11000 C, i.e., lower than the biscuit firing temperature. This firing of a glazed product which has previously been fired at a higher temperature is called glost firing.

The bone ash in the right proportion acts as a flux, but when in excess, it increases refractoriness. Some of the lime of bone first reacts with china clay to form anorthite, while the P2O5 reacts with other compounds to form glass. For making bone ash, cattle bone is first crushed, washed and then subjected to heating in a highly oxidized condition by slowly increasing the temperature to 900-10000 C, whereby most of the organic matter is removed. The calcined bone is ground with water to very fine size, allowed to age for a few days, and

Clay – Kaolin (China Clay) 75

then dried to a moisture content of 10-15 per cent. This bone ash contains a little organic matter which, combined with fine size, gives it some plasticity.

So far as china clay is concerned, it should be white-burning, and for that iron content should be as low as possible. The industry generally accepts up to around 3-5% of Fe2O3.

3. Textile

China clay is used here as a filler to give weight and strength to the body of cloths. Its inertness, fine particle size, moderately high specific gravity, softness, high dispersion, white colour and high reflectivity are the properties utilized. Very fine particle size ranging from 2 to 5 micron is preferred because this ensures very high dispersion and uniform spread of the clay in the body of the textile. Freedom from grit is of critical importance because they cut the reeds and threads. Since whiteness is important, all colouring matters are objectionable. MgO and CaO, by virtue of their being hygroscopic, result in crumbling of the clay body of the textile product on exposure to water, and so both are objectionable.

The BIS, in 1995, has recommended a set of specifications for china clay to be used as a filler in different products which also applies to textile. According to those specifications, some of the more important parameters are: particle size below 53 micron and mostly below 10 micron; the colouring substances CuO, As2O3, MnO and Fe2O3 maximum 70 ppm, 10 ppm, 0.013% and 0.75% respectively; matter soluble in water 0.5% (max); matter soluble in HCl 2.5% (max).

4. Paper

In paper, china clay is used both as a filler and as a surface coating material.

(a) Filler: It is used in varieties of papers, cardboards, hard boards etc. to impart evenness to the surface by occupying the interstices between cellulose fibers. Particle size is the most critical parameter. Levigated china clay of less than 2 micron particle size that facilitate a very high degree of dispersion so as to spread uniformly and fill up the ultra-fine pores in the cellulose, is specified. Matter soluble in water should be 0.5% (max) and that in HCl 2.5% (max). Colouring matters are also undesirable. An

average daily news paper contains 10% of its weight of china clay.

(b) Coating: China clay is used as a coating material in high quality white paper to make the surface glazed. It is obvious that china clay should be as white as possible. Fe2O3 imparts colour and hence is objectionable. Grit is highly objectionable as it will spoil the smoothness of the surface of the paper. CaO will absorb water and will result in crumbling of clayey ingredient of the finished paper on exposure to air. So, both are objectionable. Grain size should be ultra-fine (for very high quality paper below 2 micron size is preferable), and more important is the uniformity of the size, because, otherwise, surface of the paper will not be smooth. Besides, fine particles provide a very large surface area which facilitates retention of the clay (i.e., the ratio of the clay added during manufacture to the clay which remains in the finished paper). The combination of fine particle size and moderate specific gravity gives to the china clay excellent rheological properties enabling it to be easily dispersed in water and produce slurries with low viscosity and high solids content–a factor of immense importance in the paper coating industry. If a glazed surface of the finished paper is

Kaulir Kisor Chatterjee

desired, then brightness of the clay will also be a critically important factor. Chemical inertness of china clay is also an advantage.

The BIS, in 1995, has recommended a set of specifications for china clay to be used as a paper-coating material. According to those specifications, some of the more important parameters are: particle size below 10 micron; Fe2O3 maximum 0.6%; matter soluble in HCl 1.0% (max); reflectance in blue light wave length 80.0-85.5; and loss on drying i.e. water absorption of finely ground china clay 2% (max).

5. Rubber

China clay is mixed with rubber as a filler before its casting. Rubber molecules consist principally of flat platelets, which accounts for its elasticity and hardness. The effect varies considerably with different clays. Hard clays reinforces the stiffness of rubber in a greater degree than soft clays. Both hard and soft clays improve acid resistance of rubber when used in acid tank lining. China clay has proved very suitable as a reinforcing and stiffening material, particularly where resistance to abrasion is an important requirement as in the case of footwear. Low shrinkage, fine size of particles which consist of flat platelets, light weight, high dispersion, and chemical inertness of china clay are the main criteria.

As the clay is intimately mixed with the molten rubber, high LOI (i.e. shrinkage) will result in cracking. Grit is objectionable because it will not facilitate intimate mixing. For the same reason, particle size of the clay should be very fine (preferably less than 10 micron) with good dispersion so that the lightweight particles can uniformly disperse in the medium of the molten rubber. Lime is objectionable because it is hygroscopic and will cause crumbling of the rubber product on exposure to air. Colour is not very critical, and off colour is acceptable.

6. Plastic

In the plastic industry, china clay is used as a functional extender for imparting strength, electrical resistance (important in PVC cable sheaths), gloss and low water absorption. Low specific gravity, fine particle size, high dispersion, poor electrical conductivity, high reflectivity, low water absorption (i.e. low shrinkage on drying) and inertness are the important criteria for this application of china clay. In polythene film, filler where it can enhance the absorption characteristics. During the curing of polyester resin the intensity of the exothermic reaction is to some extent contained by china clay by virtue of its water content.

7. Paint

China clay was known to the prehistoric man for its use as a white paint as has been discovered from the 8,000-year-old rock art found in the caves of Mirzapur district of Uttar Pradesh, India. Nowadays, it is not used much as a pigment in oil paints owing to its high oil absorption and its poor opacity in oil due to its refractive index (1.56) being very close to that of linseed oil. It is, however, used as an extender or suspending agent in oil paints, and as a pigment in water paints or in distempers. White colour, easy dispersion with anti-settling properties and chemical inertness are the main criteria for use of china clay in the paint industry.

When it is used as a suspending agent in oil paints, obviously, the colour should be white, and for this, Fe2O3 should be low. Grit is objectionable as it may abrade the surface on which the paint will be applied. The most important are good dispersion and anti-settling property,

Clay – Kaolin (China Clay) 77

because the purpose of using china clay is to keep the paint in suspension in oil medium and for this reason, grain size should be fine and uniform. Moreover, very fine sized particles can scatter both visible and ultraviolet light, thus increasing the value of the paint. However, relatively coarser china clay, which gives paint a matte finish, can also be used. MgO and volatile matter should be low. MgO is hygroscopic and slowly absorbs water resulting in volume increase (120%) and crumbling of the paint, while in case of the volatile matter its escape after applying the paint on a surface may leave pits and holes on the coating. Oil absorption should obviously be as low as possible.

Whether used as an extender or a pigment, matter soluble in water should be low, otherwise, the paint may easily be washed by water after application.

The BIS in 1997, has prescribed specifications of a few parameters, some of which are:

volatile matter 2% (max), particle size less than 63 micron(for coarse china clay) and less than 40 micron(for fine china clay), matter soluble in water 1.0% (max).

8. Pharmaceuticals and cosmetics

China clay is used for the manufacture of cosmetics and pharmaceutical products like face powder, talcum powder, toothpaste, tooth powder, adhesives, surgical plaster, lotion and ointment for external use and porcelain in the dental preparation. In the earlier times, china clay used to be prescribed by physicians of Europe and America for treatment of cholera and as an absorbent of toxins in the alimentary canal, but for this use very careful sterilization is necessary as china clay is generally heavily contaminated with pathogenic micro-organisms. It is also used in 53% concentration for making morphin poultice. Very fine size and good plasticity are obviously necessary as these will afford a good paste and will offer a fine poultice. Since china clay is non-plastic, addition of some other plastic material is necessary. Grit is undoubtedly not tolerable because it does not go with these basic criteria., and the china clay must be superfine. Besides, china clay for medicinal purpose must be free from the toxic substances like lead, arsenic and other heavy metals.

The BIS in 1995, has prescribed specifications of a few parameters, some of which are:

particle size less than 45 micron, matter soluble in HCl acid 2.0% (max), Pb 5% (max), As2O3 2% (max).

9. Insecticide

It is used as a distributing agent on account of its fineness, non-abrasive properties, dispersion and chemical inertness. Particle size should be mostly 10 micron. Substances toxic to plants, etc. (e.g., arsenic) are highly deleterious and should be very low (less than 10 ppm).

10. White cement

White cement is made of 65% CaO, 24% SiO2 and only 5.9% Al2O3 which form an aggregate generally consisting of 46% di-calcium silicate, 33% tri-calcium silicate and only 14% tri-calcium aluminate. White cement is manufactured by burning a mixture of high grade low-alumina limestone and pure white silica sand along with certain other materials in a kiln at 1400-15000 C to form “clinker” which is cooled, mixed with a little pure gypsum to slow down the rate of setting, and ground to ultra-fine size (300-350 nanometre or 40000 mesh). The main constituents in the raw material mix are lime and silica. Sometimes clay is added to balance the composition. While in the ordinary portland cement which is a comparatively low-value commodity, locally available low cost clay is used, in white cement pure and white

78 Kaulir Kisor Chatterjee

china clay is required. China clay must be free from any colouring substance like Fe2O3, MnO, CuO etc. and must be fine grained.

Some of the white cement manufacturers using raw china clay specify china clay containing 15 to 32% Al2O3 and 1.5% Fe2O3.

11. Ink

China clay is used here as an extender or suspending agent. Water-washed and chemically bleached china clay is used largely in printing inks. In letter press carbon inks, china clay with a particle size smaller than two microns is used. China clay for this application should be free from grit.

12. Ultramarine

The purer type of china clay forms the essential raw material for the manufacture of ultramarine. For making it, a mixture containing 80% china clay and the rest sulphur, silica and soda ash (Na2CO3) is heated and maintained at a temperature of 15000C for 24 hours. The ideal composition for manufacture of ultramarine is Al2O3-39.4%, SiO2-46.7% and H2O- 13.9%. It should be free from free silica and low in iron.

13. Synthetic zeolite

New uses of china clay are in the manufacture of synthetic zeolites. It is a major breakthrough in the industrial application of china clay. Calcined china clay is used as a source of alumina and silica to produce synthetic zeolites. Synthetic zeolites are used in the refineries and petrochemical industries as molecular sieves which are becoming widely used.

14. Catalyst

The relatively new field is the chemical modification of china clay to make it an effective catalyst for use in petroleum refining. China clay is treated with sulphuric acid and calcined to produce the catalyst.

15. Soaps and detergents

In the manufacture of soaps and detergents the role of china clay is as a dehydrating agent. The china clay should be in the form of free flowing powder with Al2O3 30 to 35% and Fe2O3 1 to 1.5%. Water absorption capacity should be 40 to 60 ml per 100 gm or approximately 40-60 percent.

16. Fiber glass

Dried china clay is used in the fiber glass manufacture as a filler. China clay should have high alumina and low iron content. It should be water-washed and spray dried to ensure that the moisture content is low, and then ground to a fine size. The typical grade of china clay for fiber glass consists of 44% SiO2, 37% Al2O3, 0.6% CaO, 2% (max) Na2O, 0.5% Fe2O3 and 1% H2O.

17. Explosives and pyrotechnics

An explosive mixture essentially contains an oxidizer and a fuel. Some pyrotechnic devices are meant to produce moderate heat in circumstances where building a fire would be

Clay – Kaolin (China Clay) 79

inconvenient (e.g., self-heating food cans). They need a quick, short-lived, moderate and small initial fire, like lighting a match, for activating the main device. For this initial firing, a sensitive mixture containing calcium silicide (CaSi2), iron oxide (Fe2O3), Pb3O4 and a little china clay is used. The role of china clay in this and other similar kinds of explosives and pyrotechnics is to act as a filler to moderate the intensity of the fire. The most critically deleterious constituents are grit and water. The former would hinder fine homogeneous mixing of the ingredients and also, on explosion, spatter and cause violence, while the latter would consume part of the heat energy. As per the specifications of the BIS in 1998, loss on drying should be 1.5% (max), loss on ignition should be 14% (max) and grit should be 0.001% (max), while the particle size should be less than 63 micron.

18. Adhesives and sealants

Adhesive is an organic or inorganic substance capable of bonding together other substances by surface attachment. On the other hand, sealant is an organic substance soft enough to pour or extrude and capable of subsequent hardening to form a permanent bond with the substance. One of the components in these is some pigment which is added for performing certain functions. China clay is the source of aluminium silicate pigment (ASP). ASP provides to an adhesive or sealant hiding ability, whiteness and thixotropy, increases viscosity and reduces drying time (due to higher solids). ASP-bearing adhesives are suitable for application in plywood because it does not cause wear and tear of sawblades.

19. Metakaolin

In the industrial circle it is referred to as “highly reactive metakaolin or HRM”. It is a powerful pozzolan manufactured from kaolin and used for making “white concrete”. Metakaolin literally means changed kaolin, and it is a change of kaolinite crystals into a disordered amorphous state. This is effected by dehydroxylization (calcining) i.e. removal of the water by application of heat over a defined period of time. At 100-2000C, most of the clay minerals lose their adsorbed water, but the temperature at which kaolinite loses water is 500- 10000C. Beyond this temperature, sintering takes place and finally dead-burnt mullite is formed which is not reactive. Mortar based on metakaolin has the advantages of high early strength, reduced permeability, greater durability, less efflorescence, resistance to degradation caused by alkali-silica reaction (ASM). But it also has the disadvantage of high heat of hydration with reduced workability. This necessitates use of some super-plasticizer and chilled water for preparing the admixture.

20. Other uses

(a) Audio and video cassettes: There is possible application of china clay for audio and video cassettes where it serves as a blocking agent.

(b) Electrical industry: It is used in high voltage insulation compounds for electrical wires.

CLAY – BALL CLAY

Ball clay is a transported type of secondary sedimentary clay containing mainly kaolinite, but differing from kaolin in the content of impurities that include higher contents of SiO2, TiO2, CaO and Na2O and lower content of alumina and also in higher plasticity. In addition to kaolinite, it may contain illite and montmorillonite. Besides, unlike the well developed crystals of kaolinite of china clay, those of ball clay are poorly developed. Most commercial deposits of ball clay are associated with lignite, and in that case it is characterized by organic matter as an impurity. It may also be non-lignitic, occurring as lenses associated with other clays. It owes its name to the general practice in English quarries of recovering it in the form of ball-shaped chunks weighing 30-35 lbs for the purpose of convenience of loading, and this practice was due to its high plasticity. But its identity is often blurred and there is often a tendency to confuse it with kaolin or other clays.

Analyses of non-lignitic and lignitic ball clays show some differences in chemical composition. Typical values of some chemical constituents in them are:

Constituent Non-lignitic

SiO2 49-75% Al2O3 16-34% Fe2O3 0.8-2.5% TiO2 0.9-1.6% CaO 0.2-0.3% MgO 0.3-0.5% K2O .2-3.3% Na2O 0.2-0.7% LOI 5-12%

Lignitic

42-52% 31-32% 1.1-1 0.7-1 0.2-0 0.2-0 1.0-2 0.1-0 12-23%

The high LOI in lignitic ball clay is largely due to presence of organic matter which, on firing, burns out as CO2.

82 Kaulir Kisor Chatterjee

CRITERIA OF USE

The most strikingly characteristic properties of ball clay that make it stand apart from other clays are:

(1) Plasticity: It is very high and due to this, shrinkage after firing is also very high. The very high plasticity renders ball clay very sticky and difficult to handle. The high plasticity is due to prolonged hydrolysis during formation and transportation, during which the colloidal particles bound together the coarser particles.

(2) White to light brown fired colour: Although the raw colour may be dark due to the presence of carbon, it becomes lighter on firing as the carbon burns out.

(3) Refractoriness: It is in the range of 26-32 Orton PCE, i.e., 1600-17500C. It is less than that of china clay (PCE 35, i.e., 17850C).

(4) Vitrifiability: The typical English ball clay starts partially fusing at 12000C. At this temperature, due to high shrinkage, it vitrifies to a very dense mass. This early start of fusion ensures a long vitrification range (1200-16000C or longer).

(5) Carbonaceous matter: High content of carbonaceous matter hinders flocculation around it when the ball clay is treated with water. As a result, inside the plastic mass of clay, deflocculated zones are formed. Along such zones, the plastic mass tends to slip when pressed during moulding creating planes of weakness in the product.

USES

1. Fired product:

(a) Blending with other clays: Ball clay is not used alone due, particularly, to its low refractoriness and unmanageably high plasticity. But it is invariably used as an additive to other types of clay like china clay, fire clay, etc. (which are nonplastic) to adjust the plasticity of the raw material mix so as to facilitate moulding into desired shapes. Besides, its acceptable fired colour and its ability to vitrify relatively easily into a dense mass add to the advantages if the use is for fired products. In this way, it may find application in the manufacturing of any of the fired products based primarily on these nonplastic clays—ceramics, porcelain, bone china, refractories, etc. The percentage of ball clay added is carefully controlled so as to achieve optimum properties—both physical and chemical―in the raw material mix. Among the deleterious constituents mica and free quartz or grit are particularly objectionable. So far as carbonaceous matter is concerned, the black colour is of no concern as it gets eliminated during firing. But too high a content creates problem during mould preparation because deflocculated slip planes give rise to zones of weakness. Generally 0.5% and more is undesirable. However, less than 0.5% carbonaceous matter is considered desirable because then the clay becomes thixotropic, and the slip planes, while aiding flow of the plastic mass and mouldability, do not affect the quality of the product.

(b) Earthenware wall tile: This is a product (perhaps the only one) in which the only type of clay used is ball clay. Glazed earthenware wall tiles are used for the surface of

Clay – Ball Clay 83

walls where cleanliness is an important factor as in hospitals, kitchens, bathrooms, chemical laboratories etc. The standard size of the tiles is [14.5 cm X 14.5 cm X 4.5 mm], but other sizes are also customized. To make such tiles, a mixture consisting of ball clay, dolomite, wollastonite, talc and slate pencil powder is wet ground to 120 mesh size, passed through a magnetic separator to remove iron particles, agitated and filter-pressed by which water is driven away and cakes are formed. The cakes are dried, powdered, mixed with some binder and then tile-pressed. The tiles are first biscuit fired (ceramic engineers’ term for firing before glazing like baking of biscuits), cooled, glazed on the top surface and glost fired (ceramic engineers’ term for post-glazing firing) up to 10500C temperature.

2. Cold uses: Ball clay is used as an additive to bentonite in animal feed pellets and for lining of toxic waste dumps. It is also used as a filler in rubber and plastic.

CLAY – FIRECLAY

Fireclay is that type of clay which can withstand fire without cracking or vitrifying. The “fire” is the fire of a furnace and the minimum temperature of this fire specified as the international standard in order that a clay can qualify to be fireclay is 15000C (Orton PCE 18). In practice, however, the fireclays used in industries are required to withstand not only high temperature once but also repeated heating and cooling, i.e., thermal shocks. As a cushion, therefore, their tolerance temperature specified is much higher, about 16000C or 26 PCE. It contains at least 18% and generally 30% or above Al2O3, and combines the properties of clay (fine grain size, softness, compactness, etc.) with those of alumina (resistance to high temperature). The product made of fireclays is called firebrick which constitutes one of the main planks on which many high-temperature-process industries stand. Worldwide, fireclay occurs as sedimentary beds associated with coal seams. Typical fireclay is plastic, but generally, the nonplastic/semiplastic flint clay is clubbed with it because both are used for making firebricks.

HISTORY

Industrialists recognized the importance of durable and refractory firebricks for carrying out various metallurgical processes as early as during the early stages of industrial revolution or even before. In India, till the mid-19th century, firebricks used to be imported. Although in 1859, Burn and Co. Ltd started making firebricks at its pottery factory in Raniganj, West Bengal, they did not become popular till 1875 when, for the first time, they were used in the blast furnace of Bengal Iron Works Co. with good results. Thereafter, the firebricks produced by this factory were in demand by other blast furnaces, foundries, railway workshops, etc. It was only after the establishment of Tata Iron and Steel Works at Jamshedpur that more number units came up (most of them in the Raniganj and Jharia coalfields of the Jharkhand- West Bengal Gondwana basin) for meeting the growing demand of firebricks. With the growth of firebrick industry grew mining of the fireclay beds occurring in these coalfields. In due course, both fireclay mining and firebrick manufacturing spread to other parts of the country — Madhya Pradesh, Orissa, etc. During the 3-year period 1944-1946, the average annual production in India as reported by the mines was about 82,000 tons which rose to 100,000 tons in 1947, to 125,000 tons in 1950, to 275,000 tons in 1960, to 584,000 tons in 1970, to 762,000 tons in 1980 and to 878,000 tons in 1982. After this consistent rise, the

86 Kaulir Kisor Chatterjee

production declined reaching 522,000 tons in 1990 and to 463,000 tons in the year ending March, 2003. Subsequently, it registered nominal increases and has been hovering around 600,000 tons. The downturn since 1982 has been due to the development of superior refractory bricks to meet the demands of new generation iron-making and other industries employing high temperature processes.

CRITERIA OF USE

Fireclay is made up mainly of kaolinite mixed with crude vegetable matter, pyrites, lime, magnesia, alkalis, titanium and ferric oxide, and it has widely varying colours — white, grey, brown, black. But the mineralogy or both the raw and fired colours are of little importance for

its uses. (1)

(2) (3)

(4) (5)

The most important properties that set it apart from other minerals are as follows:

Plasticity: Fireclay is generally plastic, unlike china clay, which is also predominantly kaolinitic. But the special variety of fireclay, i.e., flint clay is semiplastic to nonplastic. Shrinkage:Inspiteofhighplasticity,itsshrinkageondryingorfiringislow. Refractoriness: It is highly refractory. The Orton PCE may go up to beyond 33 (+17400C).

Vitrifiability: It does not vitrify even at very high temperatures.

Chemical composition: The beneficial constituents are Al2O3 (18-40%), SiO2 and MgO. The alumina content tends to be higher in flint clay. Also, the fluxes like the oxides of sodium, potassium and calcium are low. This is due to their leaching away in a reducing atmosphere that was prevalent in flowing water and under coal beds.

USES AND SPECIFICATIONS

(1) Firebrick: The combination of high refractoriness and poor vitrifiability make it a suitable raw material for refractory firebricks. Firebricks find application in linings of boilers, cement kilns, blast furnaces, glass-making furnaces, etc. The function of refractory lining on a furnace wall is not only to withstand high temperature, but also to withstand temperature fluctuation, and to resist penetration, abrasion, and erosion by hot gases and molten materials in the furnace, and over and above, it should not chemically react with those materials. The life of the refractory lining is increasingly becoming a critical parameter in the productivity of a furnace and economics of a plant, because each time the lining fails and needs replacement, the furnace has to be shut down.

For making fire brick, the fireclay mixed with aluminous materials like kyanite, sillimanite or bauxite is first crushed and ground. Water is added in suitable proportion, and the mass is left for aging. The aged mass is then extruded in the form of a dense cake which is moulded into shapes of brick, dried and fired at 1200-14000C temperature.

The low shrinkage helps the fired brick to retain its shape and also to resist high temperature without cracking. Plasticity is an important criterion inasmuch as the fireclay will not need much of any plastic clay (ball clay) to be blended with it. But flint clays will need

Clay – Fireclay 87

such blending. The poor vitrifiability enables the firebrick to effectively withstand the high furnace temperatures without partial fusion. For very high temperature applications, the alumina content of fireclay may be increased by adding some high-alumina material like bauxite to it, but in that case, the firing temperature will have to be higher.

Al2O3 is refractory, and higher its content, higher is the refractoriness. But high Al2O3 also means high firing temperature and high cost of manufacturing. So high-alumina fireclay is preferred for making firebricks for use under very high temperature conditions. MgO contributes to refractoriness on account of its high fusion temperature. The hygroscopic nature of the MgO does not come in the way, because the product is used under conditions of high temperature and is not supposed to be exposed to water.

Fe2O3 melts at a relatively lower temperature, and if, in addition, TiO2 is also present then at the high temperature in furnaces where the refractory products are used, they form low- melting iron-titanate glass causing blisters in the refractory bricks and consequent increase in porosity. The Indian industries prefer less than 1-3% Fe2O3 in the fireclay depending on the quality of firebrick to be produced. Alkalis and lime lower the fusion temperature, and hence are deleterious.

Particle size is not of any critical consequence, and relatively coarse particles can be used because, on firing, they will fuse together to form a compact mass.

(2) Cordierite saggars: A comparatively recent development is a material called “cordierite saggar”, made from artificial cordierite. Saggars are trays used as kiln furniture and shelves for firing powders or components in porcelain and ceramic manufacturing. Artificial cordierite has the composition Mg2Al4Si5O18. It has the same characteristics as natural cordierite, which is formed due to contact metamorphism. It crystallizes above 9500 C and remains stable over a considerable range of temperature, i.e., up to 17500C. It has low linear expansion, low coefficient of thermal expansion, excellent resistance to thermal shock, high mechanical strength, ability to work in both oxidizing and reducing atmospheres and to withstand rapid temperature changes without breakage. Saggars made of artificial cordierite have long lives of 10-15 firing cycles. Artificial cordierite is made from an admixture of fireclay, grog, bauxite powder and talc.

(3) Sialon: It is an advance material comprising a mixture of silicon, aluminium, oxygen, and nitrogen (i.e. Si-Al-O-N). Sialon is suitable for applications requiring high mechanical strength at elevated temperatures, high specific strength (for weight saving without sacrificing strength), high hardness and toughness, low coefficient of friction and good thermal shock resistance. Possible uses may include refractory brick or material for resisting molten metal, heat engines welding shrouds, gas turbine engines, metal cutting etc. Ordinary sialon can be made by reacting a mixture of fireclay and coal in a nitrogen atmosphere.

(4) Graphite bricks: During earlier times, graphite refractory bricks used to be made using a mix of graphite and plastic fireclay, mainly for use in areas of much heat and corrosion. These are no longer popular.

(5) Clay-bonded crucible: Compared to furnaces, crucibles are smaller in size. They are advantageous when mixing small quantities of different products requiring different alloys. It is also easier to change a damaged crucible, as opposed to a furnace lining. The main uses of crucibles are for foundry melting of steel, in nonferrous metallurgy (brass, aluminium) and in precious metal metallurgy. The life of crucible is highly sensitive to the type of metal involved. For example, each heat cycle of nonferrous metal melting takes days and each time,

88 Kaulir Kisor Chatterjee

the temperature rises from normal to as high as 15000C, and the crucible is frequently subjected to rigorous thermal shocks.

There are two types of graphite crucibles namely clay-bonded crucible and silicon carbide crucible. It is the clay-bonded type of crucible that fireclay finds use in. For making such crucibles, graphite (40-50%), plastic fireclay (20-30%), crucible scrap (25%) and sand (5%) are first mixed, then moulded into the desired shape and size, dried and finally fired in a reducing atmosphere. The graphite for this purpose should contain about 90% FC and flake size should be +150 micron.

However, in the early 1950s, consumer preference started shifting from the former towards the latter type.

CLAY – BENTONITE

Bentonite is essentially a clay with predominance (75-85%) of the clay mineral montmorillonite (a complex hydrate of aluminium, magnesium and silicon) and containing an exchangeable base — sodium or calcium. Depending on whether sodium or calcium is the dominant exchangeable base, bentonite is called sodium bentonite or calcium bentonite. The former is also called swelling type or (sometimes) true bentonite and the latter non-swelling type or pascalite or (sometimes) sub-bentonite. But transitional types between these two are also found in nature. Bentonite is believed to have been formed by the alteration of volcanic ash deposits mostly of Upper Cretaceous age.

In India, the record of the earliest mining of bentonite dates back to pre-independence period when it was mined in Kashmir. But now, the most important mining and processing centre is in the region comprising Kheda, Sabarkantha, Bharuch, Jamnagar, Amreli, Bhavnagar and Kutch districts of Gujarat from where 50,000-120,000 tons of bentonite are mined annually. Some mining is also carried out in the Barmer district of Rajasthan. Its occurrence is reported in Bhagalpur district of Bihar, but there is no significant mining/processing activity.

PROCESSING

Processing of bentonite involves simple techniques of removal of water (drying) and CO2 (if present), chemical treatment with soda ash for adjusting some properties, and pulverizing. Drying is done by leaving it under the sun for 2-4 weeks, and simultaneously, visible impurities like grits are removed by hand-picking. A solution of soda ash is then added to increase the swelling power depending on requirement. The norm of its addition generally varies in India from 5 to 15 kg per ton. Afterwards, the bentonite is pulverized to (-) 200 mesh size.

Sometimes, bentonite is activated for increasing the efficiency of its swelling and base- exchanging capabilities. There are three common processes:

1. In an earlier practised process in Germany, dried bentonite was mixed with soda ash. The product was known as toxoton or tonsil. Now, this process is followed in South Africa. Instead of soda ash various long-chain synthetic polymers like carboxy methyl cellulose (CMC), starch (poly-phosphates) etc. can also be added. But these

Kaulir Kisor Chatterjee

2. 3.

organic materials become ineffective under the high-temperature and high-stress conditions of drilling due to action of hard water and attack by bacteria.

Bentonite is dried at 1100 C, finely ground, then digested with 96% sulphuric acid for several hours, and finally washed and dried.

Bentonite is sun-dried, crushed to 100 mesh size, heated with a 25% solution of sulphuric acid, and then washed, dried and crushed to 200 mesh.

By activation, some of the alumina and combined water are removed, thus reducing the colloidal condition of the bentonite (alumina combined with the water being itself colloidal, tends to make the bentonite more colloidal).

CRITERIA OF USE

Pure bentonite is a creamy yellowish to pale buff or grayish coloured mineral with specific gravity 2.0-2.2. Its refractive index is 1.447-1.550. The important criteria determining its industrial uses are as follows:

1. Chemical composition: The approximate chemical composition of bentonite is: 45- 65% SiO2, 14-25% Al2O3, 3-9% FeO+Fe2O3, 2.0-3.5% MgO, 1-5% CaO, 0.40- 2.51% Na2O, 0.5-1.5% K2O, 0.8-2.0% TiO2, and Na/Ca 4.55-2.50 (Na-bentonite) or 0.16-0.0001 (Calcium bentonite).

2. pH value: Presence of sodium/calcium ensures that bentonite is alkaline, i.e., its pH value is above 7.

3. Plasticity: Bentonite is highly plastic, and consequently it has high viscosity.

4. Swelling/adsorptive power: Bentonite has marked swelling and adsorptive properties. The high adsorption is due to the 3-layered structure of montmorillonite. The sodium bentonite is more swelling than the calcium bentonite. The former may absorb up to 5 times its weight of water, and increase in volume by up to 15 times its dry bulk (or may even be higher after processing). The swelling/adsorptive power of calcium bentonite can also be raised to this level by activating it with acid. When mixed with water, it forms into a viscous and highly plastic gel. This property is measured and expressed in terms of four parameters, namely: (i) swelling capacity, (ii) swelling index of gel value, (iii) gelling time, and (iv) gel formation index. Specific testing procedures for bentonite have been standardized. For testing swelling capacity, 2 gm of bentonite is slowly poured into 100 ml of distilled water and allowed to remain for 24 hours after which the volume of the gel formed in millilitres is measured. To test swelling index, the minimum weight of bentonite that will form gel in 10 ml of distilled water in 24 hours is determined by trial, and the swelling index is the number obtained after dividing 10 by the weight in gramme. Gelling time is tested by the time in minutes taken by 2.5 gm of dried bentonite to form gel in 25 ml of distilled water. For testing gel formation index, 1.4 gm of dried bentonite, 0.2 gm of MgO and 2.6 gm of alumina are first thoroughly mixed, then 100 ml of distilled water is added and again thoroughly shaken for 1 hour to ensure complete suspension

Clay – Bentonite 91

of the particles. After allowing the suspended particles to settle for 24 hours, the

volume of gel formed is measured which is the gel formation index.

5. Permeability: Because of its surface properties of water-binding, it has very low permeability. This is so because it adsorbs water and does not allow the water to

penetrate into it.

6. Dispersion: When dispersed in water, bentonite rapidly breaks down into miniscule

particles, even up to 0.1 micron size. The particles of sodium bentonite are smaller and they remain in suspension practically indefinitely, while those of calcium bentonite are a little coarser and they settle down after some time. The mechanism of this breaking down is not clearly known, but it is believed to have some connection with the silicon, aluminium and magnesium contents of montmorillonite. The tetravalent Si++++ ion is replaced by the trivalent Al+++ ion which, in its turn, is replaced by divalent Mg++ ion resulting in weakening of the charge and consequently the bond.

7. Base/cation exchange capacity: It means the quantity of positively charged ions (cations) that a clay mineral can accommodate on its negatively charged surface, and it is expressed as milli-equivalents per 100 gm (equivalent weight is the molecular weight of an element divided by its valency). Bentonite in general, and sodium bentonite in particular, possesses excellent base exchanging property. Sodium or potassium is exchanged readily for calcium or magnesium. The base exchanging power is further increased because bentonite breaks down readily into small particles in a liquid, thus making available a very large surface area for adsorption by virtue of which the exchange of ions takes place. The methylene blue (MB) test measures the active clay present by determining the cation exchange capacity of a sample of bentonite. The number of exchangeable ions present are determined by replacing these ions with methylene blue dye.

8. Viscosity: Viscosity is that property of a liquid which is a measure of its internal resistance to deform under shear stress, and it is measured by the stress in dynes/cm2 or Pascal (Pa) required to be applied to overcome that resistance and maintain a velocity of flow of one centimetre per second. This unit of measurement of viscosity is Poise which is 1 gm.cm.sec or 1 Pascal second (Pa.sec). It is often expressed in centipoise (cP). It is sometimes specified for bentonite meant for use as a suspension in water.

9. Toxicity: Bentonite is non-poisonous and harmless.

10. Fusibility: The fusion temperature of bentonite ranges between 13300C and 14300C.

USES AND SPECIFICATIONS

The important industrial uses of bentonite are:

1. Oil well drilling

2. Foundry

3. Refining of oils and fats

4. Construction

Kaulir Kisor Chatterjee

5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Ceramics

Refractories and abrasive wheels Pelletizing of iron ore

Cosmetics and pharmaceuticals Detergents and soaps

Insecticide and fungicide

Paints and polishes

Paper, oil cloth and linoleum Sealant

Animal and poultry feed

These uses are discussed as follows:

1. Oil well drilling: Sodium bentonite is used as an additive to the drilling mud to the extent of 20-30% depending on the drilling conditions and the quality of bentonite. Drilling mud serves to lubricate and cool the rotary cutting bits and also to bring the chips and powders up from the bore hole. The purpose of adding bentonite is threefold:

(i) It should increase viscosity of the drilling mud.

(ii) It should increase water suspension of mud.

(iii) It should seal the wall of the hole to prevent fluid loss. (iv) It should condition the wall rocks to prevent caving.

Approximately 15 tons of bentonite is required for drilling 1000 m. of well.

Sodium bentonite satisfies all the requirements. Its swelling is high and, consequently, it forms a highly viscous mass with water. When in the form of fine particles, it remains in suspension almost indefinitely. Further, being highly impermeable, it is an effective sealing agent.

For indigenous use in India, the Oil and Natural Gas Corporation (ONGC) has its own specifications and testing procedures, whereas for exports, those standardized by the American Petroleum Institute (API) are followed. The two standards prescribed (i.e. ONGC and API) are mostly similar. Fine particle size, high viscosity (minimum 15 centipoises) and low yield point (i.e., the point where a stressed material no longer deforms elastically, but begins to inelastically deform or, in other words, plasticity) are the key parameters in these specifications. Once the bentonite is confirmed to be of sodium type, these specifications are by and large taken care of.

2. Foundry: The whole system of mould-making, melting of metals, casting of the melt in mould, solidifying the cast metal to produce an object in the form of the mould, and final dressing and finishing of the object is called foundry. Moulds may be of two types:

(i) Those made of some metal (e.g., zinc) in which case the mould is called “die” and it is permanent; the casting operation is called “die casting”.

(ii) Those made of sand which are called “sand mould” or simply “mould”, and they are generally of the “use-and-throw” kind. The casting operation is called “sand casting”.

Clay – Bentonite 93

A sand mould (hereinafter referred to as mould) for solid cast is made with moist well- bonded sand rammed into the desired pattern by hand or machine, and is suited to casting of metals or alloys. Bentonite is used as a binding agent in casting of both ferrous and nonferrous metals, but more particularly iron. Plasticity when wet, and fusion are the key criteria. High plasticity ensures good binding power, while high fusion point is necessary to ensure that the molten metal/alloy cast does not melt it. Both sodium and calcium types of bentonite can be used.

Depending on the quality of casting, the Indian foundry industries use bentonite with varying physical characteristics depending on the final product, by and large following the specifications of the American Foundrymen’s Association. Nevertheless, sodium bentonite is preferred to calcium bentonite. The parameters specified by them are: swelling capacity 12-45 ml; gel time instant to 10 minutes; gel index 20-72; base-exchange capacity 60-100 milli- equivalents per 100 gm; green compressive strength 7-12 psi.

3. Refining of oils and fats: In this application, bentonite is used for decolourizing purpose. Both swelling and non-swelling varieties are used for this. Non-swelling bentonite is required to be activated. By virtue of its base-exchanging property and ability to swell, it absorbs colouring matter. So, its base exchanging power as well as swelling index should be high. It should be fine-sized, because greater the fineness, larger is the available surface area, and consequently, more will be the efficiency of decolourization. But at the same time, it should not be highly colloidal and should settle down after doing its job. For this matter, content of alumina should be within some optimum limits and that of other water-soluble salts which may be present in the combined water, should be as low as possible.

The industry generally specifies 200 mesh (approximately 50 micron) size, SiO2/Al2O3 ratio between 3.5 and 4.5, base-exchanging power 70 milli-equivalents per 100 gm, and swelling index above 8.

4. Construction: The purpose is to render a porous surface impermeable with a view to both preventing fluid loss through percolation and preventing caving of wall rock during excavation and drilling. Examples are sealing of water tanks, dams, canals, water wells etc. So, sodium bentonite, being highly impermeable, is preferred. The Indian industries specify some parameters, the important ones of which are: swelling capacity 16-20 ml; gel time instant to 5-7 minutes; gel index 35-45; base-exchange capacity 65-70 milli-equivalents per 100 gm; green compressive strength 8-9.5 psi.

However, bentonite is not suitable where the water level is fluctuating. As long as it is submerged under water, it absorbs water, swells and plugs the pores. But as soon as it is out of water, it begins to dry up, shrink and crack.

5. Ceramics: Bentonite is highly objectionable in the raw material mix for white ware because due to its high water absorption and high swelling power, it increases the filter pressing time for the raw material mix during mould preparation, and only a very small quantity (less than 1%) is sometimes added to the raw material mix for white ware, mainly with a view to improving plasticity. But, sodium bentonite is used in the glazing mixture. The semi-finished ceramic bodies are dipped in the glaze and fired at 100-13000C. The glaze should be deposited on the surface of the body uniformly, and for this purpose it is necessary that the ingredients of the mixture do not settle down but remain in suspension for a long time. The role of bentonite is to help in this by virtue of its high dispersion.

94 Kaulir Kisor Chatterjee

In 1988, the Bureau of Indian Standards (BIS) has prescribed a set of specifications the critical parameters of which are: size: (-) 45 micron; Fe2O3: 4% (max); TiO2: 3%(max); Fe2O3+ TiO2: 6% (max); CaO: 3% (max); MgO: 3% (max); CaO+MgO: 5% (max).

Fe2O3 has a colouring effect. TiO2 (over a period of time) makes the product coloured and so they are objectionable. Besides, TiO2 has a high melting point and it will unnecessarily increase the firing temperature.

Lime (CaO) is highly hygroscopic. So, if it is present in clay, the product will absorb water over the course of time on exposure, and ultimately, crumble. Also, at 11000 C (i.e. below the firing temperature), CaO reacts with alumina and silica and forms new compounds, mostly silicates. Some of these silicates lower the fusion point of clay. Also, if lime is present in the form of CaCO3 or CaSO4, then CO2 or SO3 is expelled on heating, and the ware is left more porous. Finally, lime makes the melt more fluid and it reduces the range between softening and flowing temperature. Sometimes this range may be as short as 400C only. The result is that it becomes difficult to control the temperature of the furnace to remain within this range. For these reasons, lime is very objectionable in bentonite. MgO is also highly refractory in nature and so is undesirable in white ware, because white wares, by definition, are non-refractory in nature. Besides, it is hygroscopic, absorbing 120% of its volume of water slowly over a period of time.

6. Refractories and abrasive wheels: In alumina refractories which are otherwise not plastic, bentonite is used as a binder for imparting green strength. For the same reason, it is also used in crucibles and abrasive wheels. Fe2O3 and TiO2 are objectionable because, when exposed to high operational temperatures to which the refractories are exposed, they may combine to form low-melting iron-titanate glass causing blisters on the refractory body and consequent increase in porosity. High green strength of bentonite is a necessary criterion for use in both refractories and abrasive wheels.

7. Pelletizing of iron ore: Loose fines of very small particle size (les than 325 mesh) of iron ore that cannot be sintered are formed into pellets. To the iron ore fines, 0.5-3.0% bentonite is added as a binder. Coke breeze and some flux (limestone) may also be added. The mixture is placed in cones, drums or discs, out of which discs are relatively more flexible with regard to types of ore and they can be controlled better. A disc pelletizer is a rotating inclined flat plate table. The particles gradually coalesce first, into very small pellets, which go on taking more and more particles and keep enlarging in size till they attain specified sizes. Water is sprayed as required. These are called green pellets, and the operation is called balling. The control is effected by changing the angle (20-800) and the rpm. of the disc (+8) of the disc. The green pellets are then heat treated at a temperature of above 12000 C – generally around 13150C – in grate kilns, followed by air cooling with a view to obtaining necessary strength.

The role of bentonite is as a binder to impart enough bonding strength to the pellets for withstanding the stress of rotation and heat treatment. The parameters of significance include swelling capacity, gelling index, gel time, base exchange capacity and green compressive strength. The values for these parameters as preferred by industries are: gelling Index 20-25; gel time 7-10 minutes; base exchange capacity 60-65 milli-equivalents per 100 gm; green compressive strength 7-8 psi. Sodium bentonite which satisfy these specifications, is preferred.

8. Cosmetics and pharmaceuticals: In both applications, the purpose is to make a paste. Bentonite increases viscosity and also, it is non-toxic and harmless. It is especially suitable

Clay – Bentonite 95

for use in the therapeutic intestinal absorbent preparation. Liquid bentonite (1-2% concentration) containing its minerals absorbs toxins and bacteria responsible for various types of intestinal infections and, being inert, it passes through the body undigested after delivering mineral nutrients. In naturopathy it is used in mud packs. Due to its non-toxicity and high absorption, it is can be applied for a smoothening effect in skin diseases. The therapeutic use of a type of clay believed to be bentonite was in vogue amongst the aboriginal people in many countries since centuries.

In the field of pediatric and geriatric healthcare (where patients have difficulty in swallowing tablets), liquid bentonite (0.5-5.0% concentration) finds use as a gelling agent in the manufacture of suspensions. The criteria are low solubility and good rheological property. But the disadvantage of using bentonite is that the drug is bonded tightly and is trapped, causing its rate of release to slow down.

9. Detergents, soaps and purification of sewerage water: Sodium bentonite is the constituent of many detergents used in UK, USA and Germany for scouring textiles. The role of bentonite is to adsorb the dirt particles through exchange of cations. Besides the type of bentonite, swelling capacity, fineness of size, pH and content of grit particles are important criteria. The industries specify 10 ml swelling capacity, (-) 240 mesh size, neutral to slightly alkaline solution (pH 8-9) and freedom from grit. The sodium type of bentonite and the swelling capacity ensure good cation-exchanging and adsorptive capacities. Fine size means longer duration of suspension and larger availability of surface area for adsorption. Acidic or strongly alkaline solution may corrode the textile and grit will tend to damage the threads while scouring.

Bentonite is used in certain soaps to the extent of 25 percent.

Bentonite is effective in purification of sewage and turbid water. Here again, the criteria are its base-exchanging capacity, fine particle size with large available surface area and ability to remain in suspension for a long time.

10. Insecticide and fungicide: In this application, the role of bentonite is as a carrier. There are two types of carriers — liquid for spraying and granules for sprinkling. Due to non- toxicity, bentonite does not increase the poisonous effects of the insecticide and fungicide. In the liquid type, its role is to emulsify the powders for spraying (an emulsion is a dispersion of liquid in another immiscible liquid). Since it is to be sprayed, specific gravity (2.3-2.6) and bulk density (27-31 lb/cu.ft.) are very important.

So as far as granules are concerned, the neutral to slightly acidic non-swelling calcium bentonite with high water-holding capacity (25%) and pH 6-7 is preferred, and the bentonite is used in calcined form (bentonite clay black granules). For making this product, crushed bentonite is subjected to heat in an oil-fired rotary kiln, cooled and again crushed. The specifications of calcined bentonite stipulated by industries for this use are: size (+) 52 to (-) 22 mesh; bulk density 0.8 gm/ml (min); pH in 10% aqueous solution 5-7; attrition 0.25%; adsorptive capacity 18% (min); practically free from dust and other foreign matter.

11. Paints and polishes: In the case of emulsion paints, the role of bentonite is as an emulsifying agent (as above) to prevent settling of pigment. In water paints and polishes, it is used to increase viscosity and to make pastes.

12. Paper, oil cloth and linoleum: Where off-colour is acceptable or where colour is not important, calcium bentonite can be used in the manufacture of paper, oil cloth (smooth paper thickly coated with linseed oil) and linoleum (floor-covering canvas thickly coated with linseed oil). Addition of 10% bentonite, by virtue of its bonding power, helps to improve the

96 Kaulir Kisor Chatterjee

retention of china clay (used as a coating material in high quality paper to make the surface glazed) from 45% to 84%. In addition, bentonite increases the smooth feel of the surface.

13. Sealants: Sealant is an organic substance soft enough to pour or extrude and capable of subsequent hardening to form a permanent bond with the substance. One of the components of a sealant is what is called pigment, which performs various functions like acting as a filler, colouring, shielding ultraviolet light, etc. Bentonite acts as an excellent thickener for water emulsion type sealants. Montmorillonite particles exist in the form agglomerates up to several millimetres in size, but in 5-6% concentration in water, they break down to less than 0.1 micrometer. These nano-sized particles enable even the heaviest of the pigments to remain in suspension.

14. Animal and poultry feed: Bentonite is used as a pellet-binder. Besides, it provides additional mineral nutrients. Its use increases egg size and shell hormones.

15. Others:

(i) Rubber (as a reinforcing and stiffening agent). (ii) Welding rod (as a binder for the coating).

CLAY – ATTAPULGITE (FULLER’S EARTH)

In some earlier literature, the non-swelling type calcium bentonite made up predominantly of montmorillonite and containing overwhelmingly more of alumina (14-25%) than magnesia ( 2.0-3.5%) was referred to as attapulgite or fuller’s earth because of some commonality in their properties and usage — particularly after the former is acid-activated. This is still so in the UK. But now, in many countries including India, attapulgite is recognized as a distinct type of clay made up of a different group of clay minerals namely, palygorskite (of which the important member minerals, are sepiolite and attapulgite) and in which magnesia (11-18%) instead of alumina is predominant. Its other name, fuller’s earth, owes its origin to the popular practice of the old-time washer men who used this material for removing oily dirt from woolen garments by first kneading the latter with this material and then shaking and fluffing it. The process was called “fulling” and the washer men, “fullers”. It is believed that this practice was in vogue in Cyprus as early as in 5000 BC, and the clay was then known by the name “cymolean earth”. According to the definition proposed by Ladoo R.B. and Myers W.M in 1953, fuller’s earth is an inexact term applied to certain natural clays that have an ability to absorb colouring materials from oils of animal, vegetable and mineral origin. This definition is followed in certain countries like Brazil. In some parts of India, particularly Rajasthan, it is locally known by the name “Multani Mitti” because traditionally, it used to be sent to and marketed in Multan, Pakistan. Geologically, attapulgite occurs in association with dolomitic limestone.

As per the records, the production of attapulgite in India (then combined with Pakistan) was about 3,000 tons in 1924 which rose to about 12,000 tons in 1946. In 1949, the productions in India and Pakistan were about 4,640 and 5,050 tons respectively. In India it has been notified as a minor mineral and the Union Government does not publish its production statistics.

CRITERIA OF USE

Attapulgite is soft and dirty white to buff in colour. It is not used for making any fired product and its firing characteristics are not relevant. But the most important properties are:

1. Chemical composition: Theoretically, attapulgite is a compound consisting of magnesia, alumina, silica and 5-7% water. But the commercially important

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2. 3. 4.

5. 6.

7. 8.

attapulgite as found in nature contains varying amounts of calcium, alkalis and iron. It is the alkalis (sodium and potassium) that is important from the point of view of industrial usage.

pH value: It is slightly acidic having pH of 6.5-7.

Plasticity: It is nonplastic with low water retention.

Structure: Attapulgite has a unique acicular (needlelike) structure comprising three- dimensional chains made of hollow fibers. This prevents it from swelling like montmorillonite (bentonite) which is made up of two-dimensional platelets. But when attapulgite is treated with a liquid, the needles tend to interlock and this gives it strengthening and thickening or thixotropic property. This is not plasticity, but pseudoplasticity. Unlike other thickeners, the strength makes attapulgite a stable thickener.

Specific gravity: It is very light having specific gravity of only 0.6-0.7, and being nonplastic, it can remain in suspension in oil and water.

Base exchanging power: The base-exchanging power (also known as cation- exchanging power) means the quantity of positively charged ions (cations) that it can accommodate on its negatively charged surface, and it is expressed as milli- equivalents per 100 gm (equivalent weight is the molecular weight of an element divided by its valency). Sodium or potassium is exchanged readily for calcium or magnesium. Attapulgite possesses good base exchanging power due to the alkalis present in it and also the hollow fibers. The charged ions inside the hollow fibers can trap impurities of oils etc. This power can be enhanced by activation and activated attapulgite is sometimes loosely equated with calcium bentonite.

Catalytic power: This can be enhanced by activation.

Oil retention: It can retain oil to the extent of 55-72% depending on the type of oil; but after air and steam blowing, it comes down to only 20-25 percent.

ACTIVATION

Before use, attapulgite is generally activated for increasing the efficiency of its base- exchanging capabilities. There are three common processes:

1. In an earlier practiced process in Germany, dried bentonite was mixed with soda ash. The product was known as toxoton or tonsil. Instead of soda ash various long-chain synthetic polymers like carboxy methyl cellulose (CMC), starch (poly-phosphates), etc. can also be added.

2. Attapulgite is dried at 1100C, finely ground, then digested with 96% sulphuric acid or hydrochloric acid for several hours, and finally washed and dried.

3. Attapulgite is sun-dried, crushed to 100 mesh size, heated with a 25% solution of sulphuric acid or hydrochloric acid, and then washed, dried and crushed to 200 mesh.

Activation aims at producing modifications on the surface of the clay mineral crystals and at developing a capacity to adsorb colouring matter and other impurities in oils through increased surface area of the clay particles and increased catalytic power.

Clay – Attapulgite (Fuller’s Earth) 99

USES

After activation, attapulgite is used for the following industrial purposes.

1. Bleaching and purification: It is used for removing colouring matter and other impurities from vegetable, animal and mineral oils, wine, fruit juice, saccharine juice, vinegar, sulphur, waxes, etc. This is done by the base-exchanging ability of attapulgite — specially activated attapulgite. In addition, its lightness combined with nonplastic nature enables it to remain in suspension in the liquid for a long time facilitating efficient interaction. Slight acidity prevents it from chemically reacting with acids of fruit juice, vinegar, etc. Relatively low oil retention after air and stem blowing keeps the loss of oil within limits.

2. Cleaning: Due to its strong base-exchanging power, catalytic power, nonplastic nature and high oil retention, attapulgite (activated) can be used for removing oil spillage from factory floors, for absorbing excess oil and grease off the brake bands on the winches of cranes and for cleaning oily dirt from woolen clothes. The latter use has been the oldest one as explained in the first paragraph, and in this, the nonplastic nature enables it to be shaken off easily. Attapulgite is also used for cleaning soldiers who are contaminated with chemical weapons. There is record of its widespread use in the 16th century by women as mud-pack for facial application. Now, archeologists are treating the marble surface of Tajmahal with fuller’s earth for getting rid of the yellow patches caused by the action of suspended particulate matter (SPM) and bringing back the original shine. A pack of fuller’s earth is first applied on the surface, left for a few days and then dusted off. The oily pollutants are taken away by the fuller’s earth by virtue of its base exchanging power and hollow fibers while at the same time its nonplasticity does not allow it to stick to the surface.

3. Simulation of explosion: Fine grained attapulgite produces a much larger plume than ordinary dirt due to nonplastic nature and low specific gravity. Thus, special effects can be produced with a small and safe charge of explosive.

4. Oil well drilling: Drilling mud serves to lubricate and cool the rotary cutting bits and also to bring the chips and powders up from the bore hole. Generally, bentonite is preferred as an additive to the mud. But in some cases where the water in the rock is acidic, attapulgite is used. On account of its slightly acidic nature, it does not react with the water. Also, its lightness helps it to keep the mud particles in suspension.

5. Insecticide carrier: Lightness and nonplastic nature are the criteria.

6. Pharmaceuticals: In this application, it is mixed with bentonite to adjust the latter’s plasticity and specific gravity (see chapter on bentonite). It can remain in suspension both in water and in alcohol. Its concentration varies depending on the purpose to be

served as follows:

i. Adsorbent———————————————-10-50% concentration

ii. Viscosity modifier———————————— 2-10% concentration

iii. Bindingagent——————————————2-10%concentration

iv. Suspending agent in creams, ointments, etc.——- 1-10% concentration

v. Suspending agent in oral medicine—————– 0.5-2.5% concentration

100 Kaulir Kisor Chatterjee

vi. Emulsion stabilizer in creams, ointments, etc. —– 2-5% concentration vii. Emulsion stabilizer in oral medicine —————-1-5% concentration viii. Stabilizing agent—————————————0.5-2.5% concentration

(1) Petrochemicals: Used as a catalyst for oil cracking (in ‘cracking’, molecules are broken down under high temperature (with or without a catalyst) into smaller units, and a new type of hydrocarbon, namely, olefin is produced. By cracking, light gases, petroleum coke, fuel oil, etc. can also be produced).

(2) Adhesives and sealants: Adhesive is an organic or inorganic substance capable of bonding together other substances by surface attachment. On the other hand, sealant is an organic substance soft enough to pour or extrude and capable of subsequent hardening to form a permanent bond with the substance. In sealants, addition of attapulgite gives a non-sag property due to its unique structure and strengthening power. Besides, it acts as a thixotropic modifier.

In an adhesive, one of the components is what is called pigment which performs various functions like improving certain properties, colouring, etc. When fine-grained attapulgite (grain size 0.12-0.14 micrometer) is added to water- and solvent-based adhesive preparations, its functions are as follows:

i. It provides both thixotropy and suspending properties for pigments.

ii. It improves whiteness.

(9) Cement grouting: When attapulgite is used in cement grouting applied to the porous surface of a dry tile, the quantity of water within the grouting composition is maintained for a sufficient time so as to allow the cement to cure adequately.

CLAY – POZZOLANIC CLAY

True “pozzolana” is a fine, sandy volcanic ash and “pozzolan” is an acidic alumino- siliceous material. Thus, pozzolanic clay (also called ‘pozzolanic ash’) has the chemical composition essentially similar to that of clay, but if the original meaning of the term clay is considered, then it is an exception. As per the classical meaning, clays are essentially ceramic materials, and the word “ceramic” derived from Greek originally meant fired and fused common clays (see the chapter Clays—General). Pozzolanic clay is not used in any kind of ceramics.

HISTORY

It was originally discovered and dug at Pozzuoli near Visuvius, Italy Subsequently, it has been found at a number of other sites. The Romans mixed it with lime and water and used for making concrete structures, including underwater ones, in 100 BC. The Roman port Cosa was built of pozzolana that was poured underwater using a long tube to avoid its mixing with sea water.

CRITERIA OF USE

The colour of pozzolana may be white, black, grey or red. But the main criteria of its industrial use are chemical. Pozzolana reacts at room temperature with calcium hydroxide in presence of water to form calcium silico-aluminium hydrate compounds (C-S-H) which is a compact substance with low porosity and having cement-like properties at room temperature. Further, due to the acidic nature of pozzolan, the C-S-H produced from it is resistant to corrosion by sulphates and it is also resistant to water leakage and spalling due to low porosity.

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1. 2.

USES

Mortar: Finely ground and mixed with lime and water, pozzolanic clay acts like Portland cement and makes a strong mortar that can set under water. It is therefore suited for use in underwater structures like dams, pillars of bridges, etc.

Portland cement-pozzolan (pozzolanic cement): Modern pozzolans are actually a mix of pozzolana and portland cement which is called portland cement-pozzolan or pozzolanic cement. This can also be used underwater, and in addition, its high acidity makes it especially resistant to corrosion from sulphates. Fully set pozzolanic cement is stronger than portland cement alone due to it being less porous. Low porosity also makes it resistant to water leakage and spalling. These advantages make it most suited for use underwater in oil wells as well as offshore oil wells.

SUBSTITUTES

There are other substances which possess pozzolanic properties and are used for making pozzolanic cement. These are:

(1) Fly ash: Fly ash is a waste generated in thermal power plants. As per American Standard for Testing material (ASTM), there are two classes of fly ash — class-F and class-C. Class-F fly ash is generated from burning of anthracite and bituminous coal, and it has strong pozzolanic property and practically no cementing ability alone. Class-C fly ash, on the other hand, is generated from burning of lignite and sub- bituminous coal, and it has self-cementing properties with moderate pozzolanic power. Compared to class-C fly ash, which is also used for mixing with cement, class-C fly ash has slower setting rate with easier workability. Consequently, this fly ash is preferred as a pozzolanic material to make concrete admixture. The concrete based on this fly ash is called “green concrete” because it mitigates environmental problem by making use of an environmentally hazardous industrial waste. However, according to the American Concrete Institute, both classes of fly ash can be used — class-F to the extent of 15-25% and class-C, 20-35 percent.

(2) Highly reactive metakaolin (HRM) : It is a powerful pozzolan manufactured from kaolin and used for making “white concrete”. Metakaolin literally means changed kaolin, and it is the change of kaolinite crystals into a disordered amorphous state. This is effected by dehydroxylization (calcining), i.e. removal of the water of the kaolinite crystals by heating at 500-10000C over a defined period of time.

(3) Silica fume or micro-silica or fluffy silica or simply silica dust: It is the waste material generated as smoke or fume from ferrosilicon, semiconductor and other industries. It is amorphous, lightweight, nanometre-sized, fluffy and free-flowing silica powder having specific surface area of the order of 22000 m2/kg.

(4) Ricehuskash:Itisobtainedfrompaddyfields.

Class-F fly ash has the advantage of low heat of hydration facilitating easy workability, but it also suffers from the disadvantage of low early strength. On the other hand mortar

Clay – Pozzolanic Clay 103

based on micro-silica, metakaolin and rice husk ash has high early strength, reduced permeability, greater durability, less efflorescence, resistance to degradation caused by alkali- silica reaction (ASM), but it also has the disadvantage of high heat of hydration with reduced workability. This necessitates use of some super-plasticizer and chilled water for preparing the admixture. Sometimes, a mixture of this fly ash and micro-silica is used in order to combine the advantages of both.

Stoneware Clay

CLAY – OTHERS

Stoneware clay is similar to fireclay, with the most important difference being in vitrifiability. While fireclay is poorly vitrifiable, stoneware clay has strong vitrifiability. It starts fusing at 12500C. A minor difference lies in the mineralogy. In addition to kaolinite (fireclay), montmorillonite is also present in stoneware clay. Plasticity is high, it is a good conductor of heat and is strongly resistant to acids and alkalis. Raw and fired colours are not of any relevance. The main uses of stoneware clay are as follows:

1. Chemical stoneware: Chemical stoneware and chemical porcelain (both glazed and unglazed) are used in chemical industries for holding chemicals. High plasticity facilitates moulding into shapes, and due to strong vitrifiability, it completely fuses at 12800C to a solid, compact and impervious mass. The charge consists of stoneware clay, quartz, ball clay and feldspar.

2. Ceramic tower packing material: Cooling towers are used in plants requiring heat transfer as in the case of the spent steam in thermal power plants. They are also used in chemical industries engaged in the manufacture of organic chemicals, petrochemicals, alkalis, acids, etc. and also in room heaters. The packing materials include partition rings, saddles, honeycombs, etc. made in various shapes and sizes. The good heat conductivity is the key criterion. The other criteria and the charge are the same as for chemical stoneware.

3. Stoneware crockery: They are used in kitchens, hotels, etc. as kitchenware. The charge and the criteria are the same as for chemical stoneware. Moderate heat resistance, easy cleanability, nontoxicity are important advantages.

4. Sewerage pipes: Such pipes are made from impure stoneware clay having lower plasticity, which can be made up by blending with some plastic clay. After moulding, the pipes of required diameters are fired. On firing, the pipes vitrify into solid, compact and impervious products. Fired colour is no criterion.

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Pipe Clay

Pipe clay is an exceptionally pure, white, siliceous type of ball clay. Like ball clay, it is also highly plastic. Smoking pipes made of this clay were once very popular, and to that it owed its name.

Brick Clay and Brick Shale

The differences between brick clay (also called brick earth) and brick shale lie mainly in hardness and mineralogy. The former is soft while the latter is hard and indurated. Mineralogically, the former is composed predominantly of kaolinite and montmorillonite and the latter of illite. The hardness of the brick shale may render it somewhat less plastic than the brick clay. But both are used for making common bricks.

Manufacturing process: Brick is an artificial stone made by forming clay into rectangular blocks which are hardened either by burning in kiln or by sun-drying. For ease of handling and laying, bricks must be small and light. The effective limit on the width of a brick is set by the distance which can conveniently be spanned between the thumb and the fingers of one hand — normally about 4 inches.

History: The history of bricks dates back to at least 3000 BC, when Babylonians and Egyptians used them in construction works. During those times, raw clay or shale was crushed, mixed with water, kneaded manually, made into different shapes and sizes with hand and then dried in the sun. These sun-dried bricks served the purpose in the dry climate of the Middle East. But later on, in damp places like England, they were found to be inadequate, and the technique of burning the clay with fire was invented. Nowadays, coal-fired brick kilns are employed. Such coal-fired bricks are strong with compressive strength varying from 35-125 kg/cm2 depending on the quality of the brick and its purpose of use.

Criteria of use: The clay or shale for this purpose is generally impure — (calcareous, ferruginous, etc)-occurring near the surface and is low-cost to support the low-investment brick industry. The properties particularly looked for in the clay are plasticity, low shrinkage on cooling after firing and poor vitrifiability. Plasticity facilitates moulding into desired shapes and sizes; and low shrinkage ensures retention of the shape and size after firing without cracking. Poor vitrifiability prevents the brick from forming into a very hard solid mass that cannot be easily broken to suit the different spaces, and it also makes the brick porous and rough. Porosity helps it to trap air pockets that serves to insulate the building, and both the porosity and roughness together help it to bond with the mortar better by soaking water (up to 20%). Fired colour may be anything and usually not uniform. Refractoriness is not at all required.

Substitution:

(1) Fly ash brick: Nowadays fly ash bricks are gaining acceptance as a substitute of clay bricks. Since use of fly ash will mitigate the environmental problem of disposal created by its huge accumulation in power plants, governments are encouraging its

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use in brick-making. For making fly ash bricks, a new technology called “Fal-G” using fly ash, lime and gypsum is being popularized. In this technology, the raw materials are ground, and water is added to obtain a semi-dry mass. The mass so obtained is shaped into bricks by machine moulding, and then the pressed bricks are subjected to specific curing cycle in sun or in air and steam to gain the required strength. Now, the thrust is on the use of more and more fly ash in the mix and for this purpose technology is constantly being improved.

(2) Sand-lime brick: Another substitute is sand-lime brick (also called “calcium silicate bricks”). These are used particularly where clay bricks are scarce or where consistently high-strength bricks are required. Sand-lime bricks are compact with water absorption less than 7.15% by weight after 24 hours. Here, calcium hydroxide [Ca(OH)2], instead of cement, is mixed with sand and water to form a paste which is then pressed into solid, perforated or hollow bricks and hardened under high-pressure steam whereby calcium hydroxide and silica combine to yield calcium hydrosilicate, and strongly bonded bricks become ready. These are ordinarily off-white in colour, but pigments can be added to make coloured bricks.

(3) Concrete hollow brick: An aggregate is made up of sand, fly ash dust, gravel and clay and the aggregate should be a mix of different sizes such as +10-12.5 mm (15%), +4.75-10mm (40%), 300 micron -4.75 mm (35%) and (-)300 micron (10%). This aggregate is thoroughly mixed with water in 1:6 ratio so as to obtain a mixture of uniform colour and consistency, and then pressed into bricks, cured for 70-80 hours and sun- or steam-dried.

(4) Kimberlite tailings: Kimberlite is the host rock for diamond. After mining kimberlite and processing it for recovering the diamonds, huge quantities of kimberlite tailings are generated. Hollow bricks experimentally made of 70% kimberlite and 30% clay have been reported to be promising.

Terracotta Clay and Terracotta Shale

Terracotta articles are fired products used mainly for decorative purpose (e.g., flower vase, toys, show pieces). The clay and shale suitable for making terracotta articles are generally impure (ferruginous) and are similar in properties (e.g., good plasticity, low shrinkage, poor vitrifiability) to brick clay and brick shale except that the fired colour should be uniform and pleasing. Mineralogy is also similar, i.e., the shale is mainly composed of illite and the clay, of kaolinite and montmorillonite.

Roofing Tile Clay

Clay roofing tiles are popular in certain regions and amongst certain sections of people. They are low-cost roofing materials, give good insulation against heat, have aesthetic appeal and the damaged portions are easily replaceable. The clay suitable for making such tiles are, like brick and terracotta clays, composed of kaolinite and montmorillonite.

For making roofing tiles the clays collected from the deposits are stored and allowed to weather for a period of about 6 months and then mixed with water. The mixture is left to age for a week. This aged clay is then crushed, extruded cut into slabs, stored overnight, pressed

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into tiles, sun-dried and then finally fired at 800-9000C under oxidizing conditions. At this temperature, only partial vitrification takes place.

The clay should be soft, plastic and easily vitrifiable and should have low shrinkage. Softness and plasticity facilitate moulding and cutting into shapes and sizes. Vitrifiability will tend to reduce porosity of the fired tile so that there should not be leakage of rain water, although complete vitrification is not allowed so as to maintain a little porosity which can trap air and provide thermal insulation. Low shrinkage helps retention of size after firing. In roofing tile clays, contents of 20% sand and 5-6% Fe2O3 are desirable. The former tends to regulate the degree of vitrification and impart the desired porosity, while the latter gives a pleasing red colour after firing because, at the firing temperature and in the oxidizing conditions, it does neither melt nor is reduced to iron (its melting temperature is 11000C).

Pottery Clay

The clay used for making ordinary pottery is also referred to in literature as ordinary clay and common clay. It is probably the oldest use of clay. Ancient men living as early as 10,000 years ago knew how to make not only fired potteries, but even painted ones. So much evidence of such painted potteries belonging to the period 8000-4000 BC have been found, that some historians have described the civilization during that whole period as “Painted Pottery Civilization”. Artifacts comprising potteries with attractive designs painted on them have been dug out from the ruins of ancient civilizations like Mohenjo Daro and Harappa (Indus Valley), Nile Valley, Hoang-Ho (China), Turkestan (Central Asia), Chaldea (Mesopotamia), Parsepolis (Iran) that flourished during the period 5000-3000 BC. This long tradition has made pottery-making develop more as a folk art than an industry.

Potters make use of the locally available clay, and the product range is as wide as the variations in the nature of clay and in the craftsmanship of people from place to place. As a result, the art of pottery has become very location-specific — some particular place having become famous for a specific type of product. But generally, a few characteristics stand out as common amongst all pottery clays. They should be coarse, soft, plastic, low-shrinkage and non-refractory. Vitrification is regulated through the duration of firing and depending on the requirements, a product can be made to be a high-porosity one or a solid impervious one. In the manufacture of pottery, iron content (Fe2O3) of 0.6 – 0.7% can be tolerated — more particularly because the firing temperature is too low to melt and reduce the oxide to iron, and also because colour is not a criterion.

Besides pottery, the ordinary clay also finds some special industrial uses after processing. These are as follows:

1. Filler in rubber: Rubber clays consist principally of flat platelets, which accounts for its elasticity and hardness. The effect varies considerably with different clays. Hard clays reinforces the stiffness of rubber in a greater degree than soft clays. Stiffened rubber is suited to the requirement abrasion-resistant products like footwear. Both hard and soft clays improve acid resistance of rubber when used in acid tank lining. Ordinary clay acts as a good reinforcing agent after regeneration. Regenerated clay is produced by decomposing ordinary clay with sulphuric acid and then treating the resulting aluminium sulphate solution containing silica with sodium silicate solution. The precipitate formed is filtered, washed, dried and ground. Regenerated clay has smaller particle size.

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2. Glazed tile: A process to manufacture low-cost glazed tiles from common clay was developed by the Central Glass and Ceramic Research Institute (CGCRI), India, way back in 1984. The process required comparatively low firing temperature of 9500C for a comparatively short duration of 4-6 hours.

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