Clayandclaymaterials inSouthAfrica* - SAIMM · 2009. 8. 27. · plastograph,andcyclictorsiontests....

J. S. At,. Inst. Min. Metal/., vol. 91, no. 10.Oct.1991. pp. 343-363.

Clay and clay materials in South Africa*by R. O. HECKROODTt

SYNOPSISAfter a brief review of the characteristics of clay minerals and their origin, a summary is given of the industrial

uses of different types of clay materials. The mining and beneficiation of clays are discussed, with special refer-ence to the upgracing of certain types of materials.

The occurrence and resources of the more important varieties of clays are dealt with in some detail, viz residu-al kaolin derived from granite and sediments, brickmaking clays, refractory clays, bentonite, and palygorskite.

The Republic of South Africa is well endowed with a wide spectrum of clay materials, with one importantexception. Kaolin suitable for paper coating is not known to occur in the Republic, and efforts to beneficiate andtreat local materials to achieve the desired properties have so far been unsuccessful.

SAMEVATTING

'n Kort oorsig van die kenmerkende eienskappe van kleiminerale en hul onstaan word gevolg deur 'n opsom-ming van die industriele gebruike van verskillende soorte kleie. Die ontginning en veredeling van kleie wordbespreek, met spesiale verwysing na die opgradeering van sekere tipes klei.

Die voorkoms en omvang van afsettings van die meer belangrike soorte kleie word bespreek, naarnlik residu-ale kaolien afkomstig van graniet en sedimente, baksteenkleie, vuurvaste kleie, bentoniet en palygorskiet.

Die Republiek van Suid Afrika is mildelik bedeeld met 'n wye spektrum van kleimateriale, met een belangrikeuitsondering. Kaolien wat gebruik kan work as strykstof vir papier word, sover bekend, nie in die Republiekaangetref nie. Pogings om plaaslike materiale te benefisieer en te behandel, ten einde die verlangde eienskappete ontwikkel, was tot nou toe nog onsuksesvol.

INTRODUCTIONIt is difficult to define the term clay, because its

attributes differ for various disciplines. As a geologicalterm, it is used for a variety of materials, some of whichmay not possess all these attributes. In the context of thisreview, clay is defined in a general way as an earthy sub-stance containing a mixture of hydrous aluminium silicates(the clay minerals), with residual fragments of other min-erals and colloidal matter, it possesses the property of plas-ticity when wet, but becomes permanently hard when firedto a high temperature.

Furthermore, the term clay is used for materials thathave been formed by weathering processes, or are theresult of hydrothermal action, or have been deposited as asediment. It also denotes the finest fraction of a sedimentor soil in the classification of clay, silt and sand. The parti-cles of most clay minerals are very small, and it is onlyrarely that their 'equivalent' size exceeds 2 ~m. On theother hand, the particles of non-clay minerals are usuallynot smaller than about 1 to 2 ~, and most disciplines thusplace the upper limit of the clay fraction at 2 ~ since thisparticle size seems to be the natural division between theclay and the non-clay minerals.

Clay materials are used in the manufacture of a largevariety of products such as ceramics, paper, polymers,paint, and many others. In most of these applications, claymaterials play a vital and irreplaceable role. They are

. This paper is a contribution to the Metal and Mineral Series.

t Department of Civil Engineering, University of Cape Town,

Rondebosch. 7700 Cape Province.@ The South African Institute of Mining and Metallurgy, 1991. SA

ISSN oo38-223X13.00+0.oo. Paper received 7th August, 1990.

sometimes used as active ingredients, where they con-tribute to the chemical and physical requirements of theproduct, such as in ceramic bodies and glass. They arealso used as inactive ingredients or fillers in plastics andpaints, where their main function is to dilute or extend theproduct. Clays can operate as agents in many processessuch as water purification, and they are raw materials fromwhich aluminium can be extracted.

This review is presented in two parts: Part I deals withthe clay minerals, the origin of clays, the classification ofclay materials, the industrial uses of clays and the upgrad-ing of beneficiated materials; Part 11details the occur-rences and resources of clay materials in South Africa.These two parts are followed by a list of references and abibliography that are relevant to both parts.

The subject is very wide-ranging and, in the firstinstance, the reader is referred to the two general workslisted in the bibliography. More detailed published infor-mation on specific topics is referred to in the text whereappropriate.

PART I

CLA VS AND THEIR INDUSTRIAL USES

THE CLAY MINERALS

ClassificationThe clay minerals comprise a number of species of very

diverse properties. The most useful classification of clayminerals for the purpose of this review is based on theircrystal structure. This aspect has been exhaustively

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY OCTOBER 1991 343

Layer Group Subgroup Speciestype (X

=Charge per 10I'I1II18 unit) (Not aH exa~l.. are given)

Kaolinite-serpentine Kaolinite Kaolinite, dickite, nacrite, halloysite1:1 X..O Serpentine Chrysotile, antigorite, lizardite, nepouite, arnesite, berthierine, brindleyite,

baumite, greenalite, caryopilite, cronstedtite, pecoraite, kellyite

Pyrophyllite-talc Pyrophyllite Pyrophyllite, lerrip'jrophyllite

X-O Talc Talc, willemsite, ~innesotaite, kerolite, pimelite

Smectite Dioctahedral smectite Montmorillonite, beidellite, nontronite, volkhonskiteSwinelordite (intermediate between di- and trioctahedral)

X", 0,2 -0,6 Trioctahedral smectite Saponite, hectorite, sauconite, stevensite

Vermiculite Dioctahedral vermiculite Dioctahedral vermiculiteX-0,6-0,9 Trioctahedral vermiculite Trioctahedral vermiculite

2:1 Mica Dioctahedral mica Muscovite, paragonite, roscoelite, chernykhite, iIIite, phengite, alurgite,mariposite, glauconite, ce!adonite

X..I Trioctahedral mica Phlogopite, biotite, annite, siderophyllite, lepidolite, zinnwaldite, laeniolite,ephesite, hendricksite, masutomilite

Britde mica Dioctahedral brittle mica MargariteX..2 Trioctahedral brittle mica Clintonite, bityite, anandite, kinoshitalite

Chlorite Dioctahedral chlorite DonbassiteX variable Di-trioctahedral chlorite Cookeite, sudoite

Trioctahedral chlorite Clinochlore, charnosite, nimite, pennantite

2:1Inverted Palygorskite-sepiolite Palygorskite Palygorskite, yolomeriteribbons X variable Sepiolite Sepiolite, loughlinite, falcondoite

reviewed by Baileyl, and thus only a summary of the morepertinent aspects needs to be presented here.

The clay minerals are hydrous layer-silicates and arepart of the larger family of phyllosilicates. They consistessentially of continuous two-dimensional tetrahedralsheets of general composition TzOs (with T a tetrahedrallycoordinated cation, normally Si4+,AI3+,or Fe3+),which arecombined, through the sharing of common anions, withimmediately adjacent octahedral sheets (formed normallyby the cations Mgz+, AI3+,Fez+, and Fe3+ in octahedralcoordination with oxygen or hydroxyl groups). The assem-blages formed may consist of either one tetrahedral sheetlinked with one octahedral sheet, resulting in a 1:1 layer,or two tetrahedral sheets linked with one octahedral sheet,giving a 2:1 layer.

These fundamental layers mayor may not be electrostat-ically neutral-if not, the excess layer charge is neutral-ized by various interlayer units, which include cations,cation groups, or integral sheets. In the trioctahedral min-erals, all the cation positions in the octahedral sheet arefilled while, in the dioctahedral minerals, only two-thirdsof the cation positions are filled.

A convenient classification of the layer silicates intoeight groups is based on the layer type (i.e. 1:1 or 2: 1), thelayer charge and the type of interlayer unit. Further subdi-visions can be made on the basis of the type of octahedralsheet, the chemical composition and the geometry of the

stacking sequence of the layers. Such a classification sys-tem is given in Table I.

Characteristics of the Clay MineralsMost physical properties of clay minerals can be related

either to the very small size of the individual particles or tothe significant surface charge associated with these parti-cles. The effect of heat on clay minerals and theirbehaviour when in suspension with water are two othercharacteristics that are technologically very important.

Particle Size and ShapeThe ultimate crystals of the minerals in clays are

extremely small. These crystallites are generally aggregat-ed and agglomerated, sometimes as mono-mineralic units.The degree of cohesion in the aggregates varies consider-ably, and some units are broken down easily by gentle agi-tation while others are extremely difficult to separate intotheir individual crystallites.

Certain properties of powder compacts, such as texture,bulk density and porosity, depend largely on the characterof the aggregates making up the powder, while the size ofthe crystallites is the important parameter determining thecolloidal behaviour of suspensions.

The grain-size distribution of a powder can be evaluatedby a range of methods, based on geometrical symmetry,hydrodynamic symmetry, or on measurements of surfacearea. However, the crystallites of clay minerals are far

TABLE!CLASSlF1CA TION OF PHYLLOSlL1CA TES AND RELATED CLAY MINERALS

344 OCTOBER 1991 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

from spherical. The ideal shape of the crystallites is ahexagonal plate, with a very large ratio of diameter tothickness, and thus the size of such particles cannot bedescribed completely by a single parameter. This problemis partially solved by the use of the concept of 'equivalentspherical diameter' or e.s.d., which refers the size of aparticle to the radius of a sphere that will have the samebehaviour in a fluid, for example the same settlingvelocity.

The sizes of the crystallites in various clay minerals dif-fer considerably. The size range in kaolins is generallyfrom 2 to 0,3 J.I.ffi,while nearly all the particles of smec-tites are below 0,5 J.I.ffi.

Surface Charge on ParticlesClay minerals in suspension have the ability to adsorb

cations and anions, and to retain them in an exchangeablestate. This ion-exchange capacity is the result of electro-static charges on the surface of the particles, and it is gen-erally expressed in terms of milli-equivalents (me) per 100grams of material. Because both cations and anions areadsorbed at the same time, there must be both positive andnegative charges on the surface of the particles. Thecation-exchange capacities vary considerably, the valuesfor kaolinite ranging from 3 to 15 me/lOO g, for illite from10 to 40 me/lOO g, and for smectite from 80 to 150me/lOO g. There is less variation in the anion-exchangecapacities, with values of 15 to 20 me/lOO g for kaoliniteand 20 to 30 me/lOO g for smectite.

There are four mechanisms that could give rise to thesurface charge on the surface of clay particles in suspen-sion.

(a) Isomorphous lattice substitution of Al3+ for Si4+ inthe tetrahedral sheet results in unsatisfied oxide link-ages that will leave a net negative charge.

(b) The broken bonds at the edges of the layers give riseto two different types of charges. Above a pH valueof about 2,5, the exposed hydroxyls of the brokensilica tetrahedral sheet are ionized to form SiO-units, resulting in a negative charge at that point onthe broken edge, while the exposed aluminium octa-hedral groups can accept protons and thus will havea positive charge at low pH and a negative charge athigh pH (above about 9).

(c) Positive or negative charges can be generated by theionization of surface groups of the layers. The iso-electric point of silicate surfaces is at very low pHvalues, and thus SiO- units will tend to predominate.As far as AlOH- units are concerned, it depends onthe pH level whether a positive or negative charge isgenerated.

(d) The surface of the particles may be modified by thespecific absorption of ions or polyions.

The nature and magnitude of the surface charges deter-mine the behaviour of the particles when they are in adilute suspension. A clay particle in suspension will attractcations to balance the negative surface charge. Some of

these cations will be strongly adsorbed as a mono-molecu-lar layer (termed the Stem layer), while the rest of the bal-ancing cations will form a diffuse layer in which the con-centration of cations drops off exponentially with distancefrom the particle surface. This system of a charged particlewith a balancing cation layer can be regarded as a spheri-cal condenser with an associated potential, termed the zetapotential.

The magnitude of the zeta potential is determined by thetype of associated cation. Small and polyvalent cations are

adsorbed strongly, mostly in the Stem layer, with theresult that the zeta potential is small. Conversely, largemonovalent cations result in a large zeta potential.

In reality, the charges on the surface of the particles arenot evenly distributed. The edges of the crystallites canhave both positive and negative sites as a result of the bro-ken bonds, while the faces of the particles have only nega-tive sites. Depending on the pH and the electrolyte present,these sites are neutralized to different extents.

If unsatisfied positive sites on the edges remain, the par-ticles will tend to attract each other in an edge-to-facemanner, resulting in a 'card house' structure. If the posi-tive sites are largely satisfied, the particles will tend topack in a face-to-face manner as a 'card pack' structure.

Behaviour in Clay-Water SuspensionsThe behaviour of clay-water suspensions is very sensi-

tive to strain rates. Only very dilute suspensions behave asNewtonian liquids, in which the rate of shear is directlyproportional to the shear stress. Suspensions with higherconcentrations of charged particles are usually shear-thin-ning or thixotropic. At even higher concentrations ofsolids, the system starts to show plastic behaviour with ayield stress that depends on the solids content.

Rheology of Clay Suspensions. Suspensions of clay materi-als with relatively high solids contents are used in the slip-casting process for the manufacture of ceramics. The flowproperties and stability of such suspensions depend criti-cally on the characteristics of the colloidal fraction, partic-ularly on the zeta potential of the particles.

If the potential is low (of the order of 10 mV), the parti-cles can approach each other closely enough for the short-range forces of attraction to operate, with the result thataggregates of particles, or flocs, form. In this flocculatedstate, dilute suspensions are unstable and will sedimentout, forming a distinct layer of flocs and leaving a clearfluid. However, if the potential is high (of the order of 50mV), the particles will repel one another so strongly thatthey cannot come close enough to aggregate. The suspen-sion is now diSpersed or deflocculated and it will not sedi-ment out. Therefore, the replacement of alkali ions (Na+,K+, and Li+) with alkali earth ions (Ca2+, Mg2+) willchange a stable suspension to one that will sediment out.

In general, the natural exchangeable cation in clay mate-rials is predominantly calcium, with the result that the zetapotential will be low and the suspension will be flocculat-ed. If the zeta potential is steadily increased by the pro-


gressive replacement of the calcium with a deflocculatingcation (generally sodium), the degree of the flocculationwill decrease, as shown in Fig. 1. The result of this changeis that the viscosity of a suspension will decrease until thepoint is reached at which the attractive forces are neutral-ized. Further additions of sodium will result in over-dis-persion, causing the viscosity to increase again as the sys-tem starts to coagulate. The particles unite to form largeraggregations that will not necessarily sediment out.

Apparent visoosity (poise)

25

20

15

10

5

00 o~ OA O~

Deflocculant addition (mass "to)

Fig. 1-Deflocculatlon curve of a typical kaolin

A partially deflocculated suspension is usuallythixotropic, meaning that, if it is left undisturbed, its vis-cosity will increase as a result of a build-up of structurewithin it. This thixotropic condition is readily destroyed byagitation.

Deflocculation thus makes it possible to form suspen-sions of high solids content and low viscosity. This is tech-nologically important since less water needs to be removedduring the drying of slip-cast articles. The ability of a claymaterial to be dispersed is evaluated by its critical concen-tration, defined as the solids concentration of a fully dis-persed suspension having a viscosity of 5 P (poise).

Plasticity of Clay Suspensions. Plasticity can be definedrather formally as the property that allows a material to bedeformed continuously and without rupture under anapplied force that exceeds a certain minimum or yieldvalue, and to retain the new shape when the applied forceis removed. The definition describes the relationshipbetween the applied stress and the resultant strain, and isthus purely qualitative, with no units of plasticity defined.

The underlying problem in a consideration of the plas-ticity of clay bodies is that this attribute is a composite,and not a fundamental, property. Because ceramists aremuch concerned with the shaping of a clay-water mass


0,8

using different methods and materials, different aspects ofthis behaviour become important as cases vary. The plas-ticity of a clay material is thus frequently referred to in ageneral way as 'workability', while specific aspects aredescribed by such imprecise terms as 'short' or 'fat'depending on the feel of the material, or 'strong' and'lean' if the moulding behaviour is considered.

Plasticity is exhibited by clay materials within a compar-atively narrow range of water addition~. If not enoughwater is present, the mass cannot be moulded without rup-ture while, if too much water has been added, the massbecomes a pourable slip. The range between these twoextremes is sometimes termed the workability range. Itvaries with different clays; some materials will show awide range and others a short range.

It is generally accepted that plasticity is associated withparticles of colloidal size, i.e. particles so small that theirbehaviour is dominated by their surface character and notby their bulk properties. The larger the proportion of col-loidal particles, the more plastic the material will be andthe longer its workability range, but at the same time theamount of water needed to develop optimum plasticity isgreater. The size distribution of all the particles also has animportant influence on plasticity. Because of its betterpacking density, a material consisting only of colloidalparticles is less plastic than a material composed of a rangeof particle sizes.

The type and amount of electrolyte (certain kinds of sol-uble salts and acids) that is present in the water also influ-ence the plastic behaviour of a clay material considerably,because the balance between the attractive and repulsiveforces between the clay particles depends on the type ofexchangeable cations and anions absorbed on the surfaceof the particles.

Because of the complex and composite nature of plastic-ity, its precise definition by the measurement of a singleattribute is not possible. It can thus be evaluated only bycomparison of relevant behaviours: by subjective evalua-tion, by comparison of stress-strain relationships, or bycorrelation with physical and mechanical properties.

In practice, the proportion of water required to develop abody with a specific, well-recognized consistency is usedas a measure of plasticity in a number of techniques, suchas those devised by Atterberg and Pfefferkorn. The alter-native approach involving the degree of deformation atstresses higher than the yield stress is used in the Linseistest. the BCRA compression plastometer, the Brabenderplastograph, and cyclic torsion tests.

Effect of Heat on KaolinBecause of the particular importance of clays to ceram-

ics, the effect of heat on this material has been studiedextensively, and there is a great deal of literature coveringall aspects of its behaviour as a ceramic raw material.During ruing, the kaolin takes part in a series of reactionsthat lead to the formation of a glass-rich phase. On cool-ing, this phase forms the vitreous bond in the ceramic

body. One of these reactions is the fonnation of the miner-al mullite through a series of exothennic and endothennictransfonnations.

To be of use as a refractory material, clay must have,fIrst of all, a high fusion temperature. Because there is nodefinite melting point as such, but rather progressivefusion, the refractoriness of a clay material is evaluated incomparison with the behaviour of standardized materials.The most commonly used procedure is to compare thefusion of a conical test piece made from the clay materialwith that of cones made from a series of ceramic bodies ofincreasingly higher refractoriness. The fusion point used inthis comparison is the point at which the cone has squattedto a specifIed degree. The refractoriness of the materialthen refers to the code of the standardized body; for exam-ple, Seger Cone 34 has an end-point of 1750°C.

ORIGIN OF CLAYS

Most clay minerals are formed by the transfonnation ofprimary silicates or volcanic glass as the result ofhydrothennal or nonnal weathering processes. The verylarge amount of work done on the synthesis of clay miner-als, which has been fully reviewed by Grim (seeBibliography), allows some general conclusions to bedrawn regarding the environmental conditions favouringthe fonnation of various clay minerals.

Hydrothermal AlterationAlthough the alteration products of rocks subjected to

hydrothennal action are not generally argillaceous, all theclay minerals have been found in hydrothennal bodies.The interrelationships between the different factors thatinfluence the hypogene development of clay minerals arecomplex and frequently obscured by subsequent supergeneevents.

At low temperatures and pressures, the fonnation ofkaolinite minerals is apparently enhanced by acidic condi-tions, while alkaline conditions favour the formation ofeither smectite or mica, depending on the concentration ofmagnesium or potassium. If the temperature is somewhatabove 350°C and the pressure moderate, pyrophyllitefonns instead of kaolinite, while boehmite fonns if there isan excess of alumina. At more elevated temperatures andpressures, the relationships become complicated. Forexample, mica can fonn under acidic conditions, whilekaolinite or pyrophyllite can form in the presence ofexcess potassium, depending on the particular conditions.

WeatheringThe types and nature of the soils formed by normal

weathering processes depend on a complex interactionbetween a number of factors, such as the parent rock(composition, texture, and structure), climatic conditions,time, topography, and the presence or absence of vegeta-tion. It is thus possible that soils containing kaolin andsoils containing smectite can both develop from the sameparent rock, or that soils with the same clay-mineral com-position can result from different parent rocks, depending

on the weathering conditions.Kaolinite is formed from acid igneous rocks if the

weathering conditions are such that potash and magnesiaare rapidly removed after the breakdown of the parentminerals. Under conditions of poor drainage and low rain-fall, which allow the potash and magnesia to remain in theweathering environment, smectite and illite are fonned.Similarly, kaolinite is fonned from basic igneous rocks ifthe magnesia is removed as soon as it is released. If theleaching rate is less rapid, smectite may be fonned fastand kaolinite at a later stage. The renewed weathering ofsediments follows the same pattern as for acid igneousrocks. It seems that the other alkalis and alkaline earthsplay a relatively minor role in detennining the type of claymineral fonned, although it appears that the presence ofcalcium inhibits the development of kaolinite whileenhancing the fonnation of smectite.

Halloysite has only rarely been found in weatheringproducts, and special conditions are required for its fonna-tion. It is thought that the hydrous form of this mineral canresult from the weathering of plagioclase if the conditionsare neutral or slightly acidic.

CLAY MATERIALSClay materials can be classifIed on the basis of their

composition, properties, origin, locality, or many otherparameters. A useful classification when concerned withthe use of the clay is whether the deposit was fonned insitu. or whether it consists of material transported awayfrom the place of fonnation. Residual or primary depositsare generally not contaminated with other material, whilesedimentary or secondary deposits are generally classifIedas far as grain size is concerned.

A certain nomenclature has been developed over theyears, such as kaolin, ball clay, bentonite, and fullersearths to describe groups of clay materials of economicimportance. Such descriptive tenns are not always used(;onsistently, and some confusion exists as to the meaningof the tenns.

KaolinKaolin, also sometimeS called China clay, is a white,

fine-grained, naturally occurring earthy material. Thename kaolin is derived from the Chinese 'Kao-ling' orKauling', meaning 'high ridge'. This is the name of arange of hills near Yaochao Fu in the province of Kiangsi,where the material has been exploited for centuries.

The tenn kaolinite is used to denote the specifIc mineralspecies, as well as the mineral group comprising kaolinite,dickite, and nacrite. The tenn kaolin is less well-defInedand is frequently used synonymously with 'kaolinitic claymaterial'. In the case of residual clays derived fromcoarsely grained feldspathic rocks, kaolin is used here onlyfor the material that is extracted from the weathered rockand that consists essentially of kaolinite, while in the caseof residual clays derived from fIne-grained sediments, ortransported clays of similar composition, the tenn is usedfor the materials as such, even though they may contain


substantial amounts of minerals other than kaolinite.

Ball ClayHistorically, the term ball clay has been used to define

the fine-grained, highly plastic kaolinitic sedimentaryclays found specifically in the Dorset and Devonshireareas in the south of England. The better grades of thismaterial fire white, and are sought after as the plastic com-ponent in ceramic whiteware bodies. The term ball claydoes not describe any particular property of the clay mate-rial, nor does it have any geological significance, but it isbelieved that it is derived from the original method ofextraction, which involved cutting out of the clay in openpits into 25 cm cubes. During handling between pit, railwagon, or barge to factory, the cubes became rounded-hence the term ball clay. An alternative explanation is thatthe term is derived from the digging implement used for-merly, which was known as a tubal.

Certain local clays are sometimes called ball clays main-ly because of their highly plastic nature and the similaritybetween their superficial appearance and that of Englishball clays.

BentoniteOriginally, the term bentonite was used to describe a

highly plastic clay from Wyoming, but it was later broad-ened to include all clays that are produced by the alterationof volcanic ash in situ and that are largely composed ofsmectite. The rheological properties of bentonite dependlargely on the structural characteristics and composition ofthe smectite, as well as on the type of exchangeable cationpresent. Consequently, there is a tendency in commercialusage to restrict the term bentonite to material with highcolloidal and thixotropic properties, similar to theWyoming material.

Fullers EarthsSince early times, oil and dirt particles have been

removed from wool by a water slurry of absorbent earth,which ranged from fine-grained silts to clays. The processis called 'fulling'. At present, the term fullers earths isapplied to any clay that has an adequate decolorizing andpurifying capacity without having to be chemically acti-vated. Fullers earths are not only used to clean wool, butalso for decolorizing and purifying oils. Clays composedof palygorskite and certain kinds of smectite have superioradsorptive activity and are used extensively in this role.

INDUSTRIAL USE OF CLA VS

For some applications, the clay materials must complywith very stringent specifications while, for other uses, avariety of clay materials is acceptable. In any assessment.of the usefulness of a clay material, it should be borne inmind that the requirements to be met by a clay are well-defined for some applications, but that there is no suchinformation available for the majority of uses.Frequently, the reasons why a certain clay is used arenot disclosed by the user, or the advantages that areallegedly gained through the use of a particular clayhave no real scientific basis. For many applications,

cost is a major, or even overriding, factor in the use of aparticular clay.

The materials are always considered in their final bene-ficiated condition. Some kaolins require extensive benefi-ciation as, for example, the primary kaolins, while othermaterials are only mined selectively and marketed in thecrude form, or are merely milled without any attempt toremove impurities.

Clay Materials for CeramicsIn the South African context, the ceramics industry is

the major user of clay materials. Clays are used in themaking of ceramic products because they facilitate theshaping of the body and the handling of the unfired arti-cles. The clay fraction also plays a major part in the reac-tions taking place during vitrification, producing strength,permanency and other desirable properties in the article.

A large variety of clay materials are used as ceramic rawmaterials. Most of these materials are kaolins orkaolinitic-micaceous clays, and the major minerals presentin these clay materials are kaolinite, illite, and quartz, withlesser or trace amounts of smectite, organic matter,feldspar, pyrophyllite, iron, and titanium-bearing minerals,etc. Other clay materials that are of interest in the produc-tion of ceramics are bentonite, attapulgite, pyrophylIite,and talc, but they are used to a much lesser extent.

A primary distinction within this group of clay materialsis based on colour. As far as ceramics are concerned, thefired colour, and not the natural or unfired colour, isimportant. Two main groups of ceramic clays can thus bedistinguished: dark-firing materials that are generally usedonly in the manufacture of structural clay products, andthose materials that generally fire to a light colour and areused in whitewares. Another primary distinction is basedon the plasticity or workability of the materials, which aredefined broadly as plastic clays, semi-plastic clays, orclays of low plasticity.-

Whitewares 2,3

Practically all whitewares are produced from fabricatedbodies. Each kind of whiteware dictates certain specificproperties that must be met by the clay materials if theyare to be acceptable as raw materials for a particular appli-cation. For example, porcelain bodies must fire to a verywhite colour, stoneware bodies must have good plasticityand firing behaviour, and sanitary-ware bodies must haveexcellent casting properties.

Beneficiated primary kaolin is sought after because ofits purity, but many residual kaolinitic clays also fIre toacceptable whiteness. As such clay materials are generallynot plastic, high-quality plastic clays that fire to a lightcolour are incorporated in fabricated whiteware bodies.

Structural Clay Products'The properties of the clay materials used in the making

of structural clay products (face bricks, common bricks,hollow bricks and tiles, sewer pipes, floor tiles, roofingtiles, etc.) are poorly defined. Their fired colour rangesfrom yellow or dark ivory to deep red, and they have a rea-


sonably long firing range. They are generally not veryplastic; otherwise, problems are encountered during thedrying stage, particularly with products such as hollowblocks and roofing tiles. Some clays have natural bloatingproperties, and they are used for the production oflightweight aggregates.

A large variety of kaolinitic clay materials is used for themaking of structural clay products. Frequently only a sin-gle naturally occurring material is used; sometimes two ormore materials are blended to give the required properties,and only occasionally are other materials, such as fluxesand fillers, added to adjust the properties of a clay materialor blend of clay materials.

RefractoriesSCertain naturally occurring kaolinitic clays have low

flux contents (such as iron oxides, alkalis, and earth alka-lis), and thus have the ability to be fired to high tempera-tures (in excess of Seger cone 27 or 1605°C) without melt-ing or deforming. Both plastic 'fire clay' and non-plastic'flint clay' types of refractory clay are found naturally.Beneficiated primary kaolins are sometimes pressed intobriquettes and sintered to a high density for use as a grogor chamotte in refractories.

Refractory clays are used for the manufacture of a greatvariety of refractories for the metallurgical industry, inpower generation, and for the construction of kilns andfurnaces.

Fillers, Extenders, and CarriersClay materials have long been used as inert fillers or

extenders, the only aim being to reduce the cost of theproducts. However, improved methods of refining theclays, as well as a better understanding of their properties,have made them valuable, and even essential, componentsof many materials.

PaperThe paper industry is a major consumer of kaolin, which

is used both as a filler and as an ingredient of the surfacecoating. The kaolins must pass a series of very stringenttests, such as for abrasiveness, brightness, rheologicalbehaviour, and particle-size distribution.

Filler kaolins must have a high brightness (reflectancevalues ranging from 80 to 84 per cent are acceptable) anda low 'grit' content (the residue on a 325-mesh screenshould be less than 0,15 per cent). The presence of grit(usually fine quartz) is also evaluated by the ValleyAbrasion Test, and the abrasion value should not exceed13 to 16 mg. The particle-size distribution is not critical,but the bulk of the particles should be between 1 and 10J.UIl.The clay must disperse easily.

Coating kaolins are generally chemically bleached toachieve very high brightness, and values in the range 85 to88 per cent are usually required for these materials. Theclays are fractionated to have practically no particles largerthan 2 J.UIl,and the bulk of the material should be smallerthan 1 J.UIl.A most important property is the rheologicalbehaviour of the kaolin in suspension. Suspensions with a

solids content higher than 71 per cent, but with sufficientfluidity to pass freely through fine screens or to spreadevenly at high coating-machine speeds, are generallyrequired. Other important properties are abrasiveness(which must be low), degree of oil adsorption, and opacity(covering power).

PaintKaolin, bentonite, and attapulgite are widely used in

paint formulations. The kaolins usually consist mainly ofkaolinite and are water-washed, chemically bleached, andfractionated to particular particle-size ranges (generally 0,5to 5 J.UIl).Sometimes the kaolins are surface-modified bycladding of the particles with a variety of organic materialsto make them hydrophobic, or they are calcined and thenfinely ground.

The surface characteristics of paint (eg. sheen) can becontrolled by the addition of a clay material, which per-mits high pigment loading and adds to the hiding power ofthe paint. The amount of clay in the paint formula varieswith the type of paint, and ranges from as low as 2 to 5 percent in some enamels to more than 50 per cent in someinterior-grade paints.

The specifications for paint-grade clays are diverse, butproperties such as oil absorption, particle-size distribution,colour, and grit content (abrasiveness) are important.

RubberBoth kaolin and bentonite are used in compounding rub-

ber. The 'hard' kaolins produce rubbers, which have ahigh modulus of rupture and tensile strength, and goodresistance to abrasion, and stiff uncured compounds, whilethe 'soft' kaolins produce weaker rubbers, with lowerresistance to abrasion, and soft uncured compounds.

In 'hard' kaolin, 90 per cent of the particles are less than2 J.UIl,while the 'soft' kaolins are coarser, with only about60 per cent of their particles finer than 2 J.UIl.The grit con-tent of the materials must be low (in high-grade kaolins itis less than 0,3 per cent), but brightness is not very impor-tant.

PolymersKaolin is used extensively in polymers because it is easi-

ly dispersed in the resin and does not separate out The lowrelative density and light colour of kaolin are also advan-tages. Various types of kaolin are used, including organic-clad materials.

Printing InksThe kaolins used in printing inks must have good bright-

ness, be free from grit, and must be very fine (with sub-stantially all the particles smaller than 2 J.UIl).Organic-dadkaolins are used extensively.

Clays for Foundry-moulding SandsThe moulds used for the shaping of metal by the casting

process are composed essentially of sand and clay.Naturally occurring sands were used originally, but syn-thetic or fabricated sands are now preferred since they canbe prepared to meet exacting specifications with a high


degree of control of their properties. The granular particlesare generally quartz sand, but calcined clay (chamotte) orzircon sand is used for special applications. A large pro-portion of used moulding sand is reclaimed and reconsti-tuted by the addition of fresh clay to replace the clay mate-rial destroyed by the high operating temperatures.

The function of the clay fraction is to bind the sand par-ticles together, and the properties of moulding sand influ-enced by the type and amount of clay used are the greenand dry compressive strength, hot strength, and flowabili-ty.

The most commonly used clay material for mouldingsands is bentonite. The exchangeable cation carried by themontmorillonite in the bentonite determines to a consider-able degree the binding properties of the clay material. Theprincipal clay for moulding sands in the United States ofAmerica, produced from the Wyoming region, carriessodium as the exchangeable cation, and it is common prac-tice to chemically treat calcium- and magnesium-carryingbentonites with soda ash to change their properties in thedirection of those possessed by Wyoming bentonite.However, for certain casting conditions, the properties ofnon-sodium montmorillonite are equal, or even superior,to those of sodium montmorillonite.

Highly plastic kaolinitic clays, such as ftreclays and ballclays, are also widely used as binders for moulding sands.These clays generally have lower bonding power than ben-tonites, but they possess certain other desirable propertiesand are frequently mixed with bentonite in this applica-tion. Certain very fine-grained mite clays have a bondingpower approaching that of montmorillonite and are alsomixed with bentonite.

Clays for Drilling MudsDuring rotary drilling, such as in the search and recov-

ery of petroleum, a fluid is maintained in the hole at alltimes. The fluid is pumped continuously to the bottom ofthe hole through the hollow drill stem and, as it rises t() thesurface in the annular space between the stem and the wallof the hole, it removes the cuttings of the drilling process.

The rheological properties of drilling muds are of primeimportance. The viscosity of the fluid must be higher thanthat of water to ensure the efficient removal of cuttings,but the fluid must still be pumpable and the optimum vis-cosity for general drilling conditions is about 15 cP (cen-tipoise). The fluid should also be strongly thixotropic toprevent the settling of cuttings in the hole during tempo-rary interruptions in pumping. It is very important that therheological behaviour of the drilling mud should be alteredrelatively little by large variations in the concentrations ofelectrolytes caused, for example, when the hole suddenlyencounters masses of salt or gypsum. Another function ofdrilling fluid is to confine formation fluids from ingressinto the hole. As these formation fluids are generally underconsiderable pressure, drilling muds of high density areprepared by the addition of weighting materials such asfinely ground barytes. The penetration of water from thedrilling mud into the formations must also be restricted


through the building up of an impervious coating on thewall of the hole.

Drilling fluids usually have the following composition(by volume): 65 to 98 per cent water, 2 to 30 per cent clay,and 0 to 35 per cent weighting material. The clay usedmost extensively under severe drilling conditions is sodi-um-carrying bentonite from Wyoming. This materialyields large volumes of mud per ton, develops very highthixotropy, and builds up a thin but highly imperviouslayer on the wall of the hole. If highly saline conditions areencountered, clays composed of palygorskite are used,since the rheological properties of these muds vary littlewith large changes in electrolyte content.

Miscellaneous UsesCertain bentonites, halloysites, and kaolins are used in

the manufacture of catalysts for petroleum cracking andother organic reactions. Clay materials are also used asfillers and carriers in a multitude of products, such asasphalt and linoleum, medicines, pharmaceuticals and cos-metics, adhesives, pesticides, and soaps, or in the prepara-tion of goods such as leather and fabrics. Specificationsare seldom available and a large variety of clays seem tobe satisfactory. Low cost appears to be an overriding con-sideration in many applications.

For more information on the less frequent, and some-times unusual, uses of clays, the reader is referred to theextensive review by Grim that is listed in theBibliography.

MINING AND BENEFIClATION

The choice of the mining and benefication techniques tobe used at a particular clay deposit depends on a widerange of economic and technical factors. The economicfactors are usually obvious, and include items such as thedistance from the market or the cost of capital equipment,power, and labour, but their relative importance is far fromconstant, and they are critically influenced by many exter-nal factors. Some of the more important technical factorsthat need to be considered include the nature and size ofthe deposit, the quantity and quality of product required,and environmental aspects. The interrelationships of thesefactors are unique for each deposit.

Mining MethodsBecause of the relatively low unit price of mined clay

materials, their winning is restricted to low-cost opera-tions. Short-range variation in the properties and quality ofthe clay materials, as well as localized contamination,affect the mining costs adversely. Other important factorsthat need to be taken into account are the subsequent meth-ods of beneficiation, production level, size and locality oforebody, infrastructure, and ecological aspects.

A large variety of 'dry' mining methods for the loosen-ing, loading, and hauling of clay materials are available,and the choice of mining system will depend on many fac-tors. These methods offer the ability to mine selectively. In'wet' mining, the faces of the clay pit are washed downwith high-pressure water jets or 'monitors'. The pit wash

is collected in a sump at the bottom of the pit and, fromthere, it is pumped to a treatment plant This method doesnot allow short-range selective mining.

BeneficiationBecause sedimentary clays are seldom beneficiated to

any significant extent, this section deals exclusively withthe beneficiation of residual kaolins, particularly for use inthe ceramic and paper industries. In this context, beneficia-ti~n means the separation of the kaolin from the quartz,micaceous minerals, and other unwanted materials thatmake up the rest of the crude ore.

The method and degree of beneficiation are determinedby the properties of the crude clay material and therequirements of the user, such as the degree of consolida-tion of the crude material, the particle-size distributions inthe different phases, the character of the contaminantphases, and the required brightness, grit content, plasticity,particle-size distribution, etc.

Ore-dressing techniques are based on differences in oneor more of the properties of the gangue and the mineral tobe benefici~ted; for example, differences in particle size,shape, density, magnetic susceptibility, colour, or surfacechemistry. Unfortunately, the properties of the major min-erals in the crude ore of kaolin are very similar. Forinstance, the densities of kaolinite, mica, quartz, andfeldspar are all very close to 2,6 gjcm2 and density separa-tion is not practical. The magnetic susceptibility, colour,and surface chemistry of these major components of thecrude material are also very similar, and beneficiation isgenerally feasible only if there is a significant difference inthe particle size and shape of the various minerals presentin the crude kaolin.

The two basic routes that can be followed in the benefi-~iation ~f kaoli~ from the crude material are dry process-109,or alr ~ota~on, and wet processing or classification bysedimentation m a water suspension.

Dry ProcessingThis p~ess. involves the drying and pulverizing of the

raw matenal and the removal of the fine material by aircu~nts. For example, the crude material is passed througha pnmary crusher and fed continuously into a rotary dryer,where the moisture is reduced from typically between 20and 25 per cent to between 1 and 2 per cent. The driedmaterial then passes through a pulverizer or attritor fromwhich particles of the required fineness are lifted by aircurrents. A series of separators and cyclones is used toachieve a good separation of particle sizes.

The dry process is relatively simple, but it is not used inthe preparation of high-grade materials, especially whenvery low grit levels are required. The reason is that thecrushing and dry attrition processes result in some break-down of the large quartz grains into micrometre and sub-micrometre particles, which are then captured and retainedin the clay fractions.

capital requirement and energy consumption are also high-er than those for a dry process. However, materials thatwill meet specifications with small tolerance limits can beproduced more easily by a wet process. A modem wetbeneficiation process comprises the steps of disintegration,refining or fractionation, and dewatering.

Most raw kaolins will disintegrate or disperse in waterwith only gentle agitation. Comminution is thus unneces-sary, and indeed undesirable, since it would increase thelevel of very fme quartz by crushing the coarse grains ofquartz.

The finest screen or sieve that can be used in practice, a4OO-meshsieve, has openings of about 37 J.U11.However,such a screen is not fine enough to remove all the mica andquartz, and kaolin produced by screening will thus stillcontain an appreciable amount of these materials.Furthermore, with such very fme materials, other methodsof separation, such as sedimentation in refining tanks,hydrocycloning and centrifuging are usually more eco-nomical.

The dewatering of the slurry, containing in general morethan 70 per cent water, usually takes place in two stages:thickening, followed by drying. Raked tanks or centrifugesare mostly used for the initial thickening of the slurries.The thickened slurries are dewatered either by spray-dry-ing or f1ltration, followed by final drying in rotary kilns,band dryers, or tray dryers. For effective f1ltration of thefine-particle slurry, pressure drops across the septum ofconsiderably more than one atmosphere are needed.Vacuum filters are thus not used for dewatering kaolinslurries, but rather discontinuous filter presses capable ofworking at high pressures (up to 1 MPa).

Spray-dried material has a low bulk density, and it posesproblems in shipping. Filter-pressed material, extruded as'noodles' and dried, is preferred for bulk transportation.Material to be bagged is usually in the form of dried andpulverized f1ltercakes.

UPGRADING OF BENEFICIATED MATERIALS

Efforts to upgrade the quality of clay materials arealmost exclusively restricted to beneficiated kaolins, andsuch efforts are mainly concerned with improving eitherthe whiteness or the rheological properties. Chemical ormechanical treatments, magnetic separation, flotation, ordifferential sedimentation can be used to improve thebrightness of the kaolin, while fractionation, disintegrationand surface treatments may be effective in improving itsrheological properties.

Chemical TreatmentHydrosulphites of sodium, zinc, and calcium (also called

dithio~ites, the salts of the organic radical (S204}2-),added m amounts of up to 0,5 per cent by mass to the rawclay in the presence of sulphuric acid are often very eff'ec-tive in improving the brightness of natural kaolins. Ferricsalts are converted to the ferrous form by this treatment,

Wet Processing resulting in a reduction of the brown colour. This treat-This method is a much more complex procedure, while the ment affects only the free oxides of iron, and has little

JOURNALOF THESOUTHAFRICANINSTITUTEOF MININGAND METALLURGY OCTOBER1991 351

influence on pyrites and iron-bearing aluminosilicates.The brightness of some kaolins is adversely affected by

the presence of small amounts of carbonaceous matter,which imparts a grey colour. The carbon can be removed,without destruction of the kaolinite, by an ozone treatment.

In fired products, lime is by far the most effective agentand, when it is added in the form of a very fine powder,clays containing up to 0,5 per cent iron oxide can be firedto a whiteness acceptable for most purposes.

Magnetic SeparationSpecific contaminants can be removed by sophisticated

selective or 'piggy-back' flotation procedures, but similarbenefits can be obtained by high-intensity magnetic sepa-ration operated at field strengths of over 20 kG. This tech-nique is effective in removing very fine magnetic andpara-magnetic particles, such as hematite, pyrite, biotitemicas, and iron-stained quartz or anatase, from the kaolinslurry, improving the brightness of the kaolin correspond-ingly. The capital cost and total operating costs are high,making this method suitable only for the production ofhigh-quality products. Nevertheless, it has become one ofthe main methods used to improve the brightness of paper-grade kaolins.

Mechanical TreatmentKaolins frequently consist of aggregates of small parti-

cles of kaolinite strongly bound together. If the surfaces ofthese aggregates are iron-stained, the brightness of thematerial can be improved if the aggregates are tom apartor delaminated so that uncontaminated surfaces can beexposed. Delamination of the aggregates may also result inan improvement of the rheological properties of the kaolin.

Normal dispersion actions will not break down theaggregates, but delarnination can be achieved with somekaolins by high-energy pugging and extrusion of a plasticmass with a water content of as low as 18 to 20 per cent.In such a very stiff condition, the particles of kaolinitecannot slide easily past one another and the high shearstresses tear the aggregates apart. The aggregates of somekaolins can also be delaminated by wet grinding with spe-cial very coarse sand or glass beads. The mills are rubber-lined to avoid comminution of the sand or glass beads.

The rheological properties of a kaolin can also beimproved in certain instances if the kaolinite particles canbe broken by very high-energy mechanical treatment. Theeffectiveness of such disintegration to lower the viscosityof the kaolin suspension would depend on the initial sizeand shape of the particles, and whether they can be brokendown in such a way that they can move past one anothermore easily in a suspension.

FractionationThe rheological properties of kaolin are adversely affect-

ed by the presence of smectites in amounts as small as I or2 per cent. The smectites are naturally extremely fine-grained and can be eliminated by elutriation, using equip-ment such as decanter centrifuges to remove the very finefraction from a kaolin suspension.


PARTn

OCCURRENCE AND RESOURCES OF CLAYMATERIALS

The Republic of South Africa is well endowed with awide range of clay materials. The occurrence andresources of the more important clay materials are summa-rized under the categories of residual kaolin derived fromgranites and from sediments, plastic clay, brickmakingclay, refractory clay, bentonite, and palygorskite.

GRANITE-DERIVED RESIDUAL KAOUN6,7

Residual kaolin derived from granitic rocks occurs in alarge number of localities in the south-western and south-ern regions of the Cape Province, as well as inNamaqualand. Smaller and isolated deposits are also foundin Natal and the Transvaal.

Cape Kaolin8The generic term Cape kaolin is used for that group of

kaolins formed by in situ weathering of the coarsely por-phyritic granite of the Cape Granite Suite, possibly withsome hydrothermal alteration. The kaolinite is generallyfairly coarse and therefore rather non-plastic. Because theweathered material has not been transported and is thusnot contaminated, the refined kaolin exhibits a high degreeof brightness.

The residual material, or growan, consists mainly of amixture of kaolinite, quartz, and mica. Small amounts ofhalloysite are sometimes observed, and the presence of asmectite-illite mixed-layer mineral has also been reported.The unbeneficiated weathered granite has a pronouncedbimodal particle-size distribution, as can be seen from Fig.2. The fraction above about 40 JlIn consists essentially ofquartz and mica, while the fraction below that size is prac-tically pure kaolin. Because of the sharpness of thebimodal distribution, most of the kaolinite in the weath-ered material can be removed by simple fractionation pro-cesses. If the processes are efficient in removing most ofthe particles above about 20 Jlm, the beneficiated kaolinwill be virtually free of quartz and mica.

There can be no doubt regarding the primary nature ofthe kaolin deposits of the Western Cape. Proof of the insitu decomposition of the Cape Granite is clearly given bythe retention of the granitic texture in the undisturbedweathered material. Furthermore, transition zones fromfully weathered to partially weathered to unweathered rockhave frequently been found in boreholes, where the par-tially weathered material is characterized by the presenceof biotite and grains of unaltered white feldspar.

One of the factors thought to have contributed to thevery deep surface weathering of the coarse porphyriticgranite is the strain in the big feldspar crystals. This strainis the result of the micropoikilitic intergrowth of feldsparand quartz, and enhances the disintegration of the rock.Another factor is the presence of prominent faults andjoints, which allow good circulation of ground water. Astructural control seems likely in many of the occurrences.

Percentage In Interval15

10

5

00,2 2 20

Particle size (~)200 2000

Fig. 2- Typical bimodal particle-size distribution of w88thered Cape Granite

The shape of the deposits is generally trough- and funnel-shaped with steep sides, and they parallel the local faultpattern. The extensive leaching that is required for the for-mation of kaolin is usually associated with a hot, humidtropical climate. Such a climate existed during the earlyand middle Miocene period.

The presence of stanniferous mineralization in theStellenbosch region, as well as the widespread occurrenceof spherical nodules and thin veins of tourmaline, aretaken as evidence of hydrothermal activity. However, cas-siterite has not been found elsewhere, and tourmaline isalso characteristic of the Cape Granite. The remarkabledepth of weathering is taken as another argument in favourof at least some low-temperature hydrothermal activity.

The consensus of opinion is thus that the kaolin depositshave been formed mainly by surface weathering, but thequestion of whether hydrothermal activity exerted anyinfluence has not yet been settled.

Properties of Cape Kaolin9,10The Cape kaolins are remarkable mainly for three rea-

sons:(a) the high yield of the deposits--commonly 50 per

cent or more;(b) the virtual absence of grit, mica, and other impurities

in the minus 20 JlD1fraction, which makes beneficia-tion simple;

(c) the high degree of brightness of the beneficiatedkaolin.

The beneficiated kaolin consists mainly of a rather poor-ly crystallized kaolinite, with small amounts of quartz andmica. The crystal habit of the kaolinite is poorly defined,and scanning electron microscopy has revealed the pres-ence of numerous stacks or booklets of kaolinite plates.

Typical chemical analyses of Cape kaolins are given inTable 11.As far as the major elements are concerned, theCape kaolins are similar to the kaolins of England andGermany. The Fe:zO3contents of some of the Cape kaolinsmay be as low as 0,15 per cent, but their TiO2 content isdistinctly higher than that of the English kaolins. Thechemical analyses of the American and Brazilian kaolins,on the other hand, show low K2O contents, reflecting thevirtual absence of mica. but the TiO2 contents are high.

In general, at least 80 per cent of the beneficiated kaolinconsists of particles smaller than 10 JlD1,and 50 per centsmaller than 2 JlD1.In Fig. 3, the range in particle-size dis-tribution of the Cape kaolins is compared with the distri-

Particle size (J&m)50

20

10

5

2

0,520 50 80 95 99

Percentage smaller99,9

- Cape kaolin

Filler grade

- - - Caramlc grade

'-'-'-, Coating grade

Fig. 3-Comparlson between the particle-size distribution ofCape kaolin and those of typical English kaollns

J\.;vRNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY OCTOBER 1991 353

Material SiOZ A1Z03 FezO3 TiOz MgO Cao KzO NazO Loss onignition

Cape kaolinsOarS 47,1 37,3 0,51 0,67 0,23 0,08 1,13 0,11 12,89OayK 48,6 35,2 1,22 0,76 0,43 0,13 1,84 0,10 11,76

English kaolins:Paper grade Coating 47,8 37,0 0,58 0,03 0,16 0,04 1,10 0,10 13,1

Filler 48,7 36,0 0,82 0,05 0,25 0,06 2,12 0,10 11,9

Ceramic grade Oay 1 47,8 36,6 0,7 0,1 0,3 0,3 2,1 0,1 12,3Oay2 47,8 36,0 0,8 0,1 0,3 0,3 2,3 0,1 12,3

Gennan kaolins:Paper grade Filler 48,8 35,0 0,8 0,5 - - 2,5 0,1 11,5Ceramic grade 48,0 36,0 0,6 0,4 - - 1,8 0,2 12,2

American kaolinsPaper grade Coating 45,3 38,4 0,30 1.4 0,25 0,05 0,05 0,25 13,95

Brazilian kaolinsPaper grade Coating 45,1 37,75 1,90 1,10 - - 0,10 0,10 13,85

TABLEllTYPICALCHEMICALANALYSESOFBENEFICIATEDKAOLINS'

bution of typical English ceramic-grade, coating-grade,and filler-grade kaolins, The Cape kaolins seem to beslightly fmer than those English clays normally used forceramics, but similar to the filler grades. Materials of thecorrect particle-size distribution for a coating-grade kaolincan obviously be prepared by a suitable fractionation pro-cess.

It is possible to beneficiate kaolins with very highbrightness values without resorting to bleaching or asophisticated mineral-separation technique. Brightnessvalues ranging from 85 to 94 are general for raw kaolin,while the fired material has values of 93 to 95. The bright-ness values of the English filler grades for the paper indus-try range from 80 to 83.

Regarding ceramic applications, kaolin is non-plastic,deflocculates well and has good casting properties. Its drystrength is understandably low, while the PCE. is betweenSeger cones 35 and 36.

The critical concentration of beneficiated Cape kaolin,when dispersed with sodium acrylate, is unfortunately low(59 to 61 per cent) when compared with that of coating-grade materials (70 to 72 per cent). The low critical con-centration of Cape kaolin is probably due to its particleshape and size distribution and to the presence of smallamounts of smectitic material.

Beneficiated Cape kaolin is thus a high-grade ceramicraw material, and it is also suitable as a filler or extenderpigment in the paper, paint, and polymer industries; as acarrier for pesticides; and as one of the components in themanufacture of medicines, cosmetics, and pharmaceuti-cals. Unfortunately, its poor rheological properties whenbeneficiated makes it less suitable as a component inpaper-coating formulations.

Occurrences of Cape KaolinThrough the years, a number of surveys have been made

of various kaolin-bearing regions by private interests,while a concerted exploration programme was undertakenby the Geological Survey during the period 1975 to 1979.Most of the known occurrences can conveniently begrouped into four areas: the Cape Peninsula, Vredenburg-Saldanha, Brackenfell-Kuilsrivier, Stellenbosch-SomersetWest A number of small and economically unimportantoccurrences are also scattered throughout the WesternCape wherever Cape Granite is found.

None of the known important kaolin deposits of theCape Peninsula occurs outside the Fish Hoek-Noordhoek-Kommetjie valley. Although granite-derived kaolin isknown to occur widely in the Cape Peninsula, and newlocalities continue to be exposed through road works andbuilding operations, such deposits are usually sterilized byurban development. The exploitable deposits are at FishHoek (Brakkloof), Kommetjie (Imhoff's Gift), andNoordhoek (Chaplin's Estate, Good Hope, Dassenberg).

At present beneficiated kaolin is produced only from theBralddoof deposit, but the exploitation of the Noordhoekdeposits must be imminent, provided that an acceptablesolution to the ecological restraints can be found. Theamount of beneficiated kaolin that could be recoveredfrom the Noordhoek occurrences is estimated to be of theorder of 10 Mt.

Kaolin has been found in many places in the Vredenburgpeninsula, but the results of extensive prospecting during1979 were rather disappointing in that no large depositswere found. However, a promising, though rather small,deposit was subsequently uncovered on the farmLangeklip 47. The occurrence of kaolin in this area seemsto be much more restricted and less well-developed than inthe Cape Peninsula, even though there are strong similari-ties between the two areas, such as the presence of coarse-grained granite and fracture zones and, presumably, thesame climatic conditions during the geological past More


active erosion conditions in the Vredenburg-Saldanha areamay account for the removal of much of the weatheredmaterial, but indications of hydrothermal alteration arealso totally lacking in this area. The experience gainedwith the Langeklip occurrence indicates that there couldwell be a fair number of medium to small kaolin occur-rences in this area capable of yielding good-quality kaolin.Based on present knowledge, however, it seems doubtfulwhether more than about 1 Mt of beneficiated kaolin ofgood quality could be produced from this area.

Five noteworthy occurrences were delineated in theBrakenfell-Kuilsrivier area during the 1978 explorationprogramme of the Geological Survey. They occur atHazendal, Riverside, Chateau, Montford, and Teen-die-bult.

Although the size of these deposits appears to be rathersmall-it is estimated that, at most, 2 Mt of beneficiatedkaolin of good quality could be produced from them-theadvantages of this area are the well-developed infrastruc-ture, the relatively low value of the ground, and the small-er impact of ecological restraints.

Of the various occurrences investigated by theGeological Survey in the vicinity of Stellenbosch andSomerset West, only those on the property of theStellenbosch Airstrip and on Verzicht are of interest. TheAirstrip deposit, in particular, is large and could yieldabout 5 Mt of good-quality beneficiated kaolin.

Namaqualand"ll,12Kaolin deposits are present in a very large area in the

north-western part of the Republic, covering parts of theVredendal, Vanrhynsdorp, and south-westernNamaqualand Districts, the main occurrences being cen-tred approximately on Bitterfontein. Deposits of good-quality primary kaolin were formed in this region by surfi-cial weathering of gneissic and metamorphic rocks. Theresidual material has many similarities to that derived fromthe granites in the Western Cape and consists mainly ofkaolinite and quartz, while halloysite has been reportedfrom some occurrences. The beneficiated Bitterfonteinkaolins are similar in most respects to beneficiated Capekaolin.

It is convenient to divide the occurrences geographicallyinto an eastern group (near Nuwerus), a western group(near Landplaas), and a northern group (in the vicinity ofGaries). The more important deposits are as follows:

in the Nuwerus area: Nieuwhoudts Naauwte, Erd VarkGat, De Toekomst, Kersbosvlei,

in the Landplaas area: Moedgewin, Klipkraal,Hendriksvlei, Katdoringvlei,

in the Garies area: Rondabel.Kaolin is not produced from any of these deposits at pre-

sent, although a number of deposits were exploited in thepast on a very limited scale. It is estimated that the follow-ing amounts of good-quality kaolin of high brightnesscould be beneficiated from these deposits:

Nuwems deposits 0,5 to 0,6 Mt

Landplaas deposits 2,2 to 4,0 MtGaries deposits 1,5 Mt.

The formation of the deposits around Nuwerus isascribed to normal surface-weathering processes on felds-pathic rocks poor in ferromagnesian minerals. The envi-ronmental conditions at the time the deposits were formedwere such that the potash was leached rapidly, with theresult that the feldspar was altered directly to kaolinite,and not through an intermediate stage of sericite. Chemicalanalyses of fresh rock, partially weathered rock, and fullyweathered material show the removal of SiO2 and K2Oand an increase in H2O. There is no change in the ironoxide content, and the conclusion is thus that the whitekaolin was formed from rocks low in iron minerals.Analyses for trace elements indicated that there was noincrease in the elements that are associated with hydrother-mal action. The absence of the minerals fluorite, topaz,and tourmaline also points to a non-hydrothermal genesisfor these clay deposits.

Other OccurrencesApart from the major deposits of granite-derived kaolin

in the Western Cape and the Namaqualand regions, only afew other deposits of good-quality kaolin that could be ofeconomic importance are known, but it is always possiblethat more occurrences may be found.

NatalKaolin derived from granite occurs at a number of local-

ities in the Inanda-Ndwedwe area of Natal. Although theextent of the deposits is limited and none of them arebeing exploited at present, good-quality material is foundnear Coqweni, Nozandla, and Appelsbosch13.

The deposits are clearly residual and their formation isascribed to the downward percolation of surface water,which leached the feldspar-rich granite of the BasementComplex. The kaolinization took place particularly in azone between the upper and the lower levels of the fluctu-ating water table, immediately below the contact betweenthe granite and the overlying sediments of the NatalGroup, as well as along fault zones.

Cape ProvinceA very large deposit of good-quality kaolin occurs as

narrow lenses in a pegmatitic granite on the farmRondeheuwel, north of Mossel Bay. The kaolin is derivedfrom the weathering of vertical dolerite dykes in theGeorge granite. The kaolinization of the dolerite and adja-cent granite extends to about 30 m below the remnants of ahorizontal capping of silcrete, which covers a wide area inthis district.

The kaolin is used in a milled form by the ceramicsindustry for the production of whiteware. Apart fromselective mining and hand cobbing to remove iron-stainedfracture and joint portions, it is not otherwise beneficiated.

RESIDUAL KAOLIN DERIVED FROM SEDIMENTS

Residual kaolin deposits derived from sedimentary rocksare particularly well developed around Grahamstown in


Type Deposit

Coastal Plain Avenue ParkMelrose

Grahamstown Strowan

)Peneplane BlakesCoronation

Palmer}

Elandskloof

MayfieldBeaconsfieldWebberWallaceUpper Gletwyn

CrousGlenhoek

the Eastern Cape. These white-firing clay materials areextensively used in the ceramics industry and as fillermaterial where low abrasiveness is not a critical require-ment. Other kaolin occurrences derived from sedimentaryrocks are found in Natal in the Ndwedwe-AppelsboschDistrict, in the Southern Cape near Albertinia, and in theTransvaal at Zebediela. These occurrences have not beeninvestigated nearly to the same extent as those in theGrahamstown region, and it is likely that there are manyoccurrences that are not recorded.

Ceramic Clays from Grahamstown14,15The clays from the Grahamstown region are probably

the best known and most widely used ceramic raw materi-als in South Africa today. This area has a long history inceramic manufacture, and extensive brick productionoccurred as early as 1875. Prospecting over the years hasbeen intensive but probably by no means exhaustive, andnew deposits that could be economically viable may in alllikelihood still be discovered.

The Grahamstown clays are characterized by theirdiverse character. The large variations (even within a sin-gle deposit) in mineralogical composition, particle-sizedistribution, plasticity, colour, and vitrification behaviourare the result of their origin.

There is no doubt that the Grahamstown clay depositsare residual, i.e. they were formed in situ by the weather-ing of Cape and Karoo sediments. The extensive andunusually deep weathering of the shales is ascribed to theextraordinarily complete peneplanation that existed in theGrahamstown area at some period during Miocene toTertiary times. The drainage under those conditions wasvery ineffective, with the consequence that water soakeddeeply down and leached out the soluble constituents ofthe sediments. During the dry seasons, the water wasdrawn to the surface by capillary action, where the dis-solved iron precipitated as oxides along the joint and bed-ding surfaces. Likewise, the dissolved silica precipitated asquartz in fracture zones and as the extensive silcrete cap-pings now present over the leached areas. The depositsassociated with a specific peneplane are all of the sameage.

Although it is certain that extensive leaching played themajor role in the genesis of the deposits, the presence ofsmall amounts of the minerals alunite, natro-alunite,wardite, and potassium feldspar in all the deposits (irre-spective of the nature and age of the parent rock orwhether the occurrences are associated with theGrahamstown Peneplane or the Coastal Plain) is strongevidence that there must have been a certain amount oflow-level hydrothermal action at some time during thegenesis of these deposits.

The Grahamstown Clay DepositsThere are numerous occurrences of clay in the

Grahamstown region, all associated with a peneplane-either the Coastal Plain or the Grahamstown Peneplane-and covering an area stretching over a distance of more

than 35 km from east to west. Because most of the occur-rences are of limited extent or their material is of poorquality, only a few have been prospected in any detail orare in actual production. At present, only two of thesedeposits are known to be associated with the Coastal Plain,the rest of the deposits all being associated with theGrahamstown Peneplane.

The deposits occur as relict conical hills or on the edgesof the Peneplane, and are generally partly covered by awell-developed capping of silcrete or a thick overburdenof rubble. The thickness and hardness of the silcrete cap-ping are decisive factors in the economic exploitation ofthe deposits. The original structural features of the parentrock, such as folds, joints, and bedding planes, are clearlypreserved, while vein quartz is frequently found in associ-ation with shear zones. In many deposits there is heavystaining by iron oxides along the parting planes, but thestaining is not penetrative and much of the iron oxide canbe removed by a process of hand cobbing.

Because the quality of the kaolin depends on the litholo-gy of the parent rock, good-quality kaolin is found onlywith the right combination of peneplane formation andgeology. The physiographic and stratigraphic relationshipsof the important deposits in the Grahamstown area aresummarized in Table Ill.

TABLEIIIGEOLOGICAL RELA TIONSIllPS OF THE GRAHAMSTOWN CLAY

DEPOsrrs

Geological setting

Witteberg Group

Prince Albert Form

Dwyka Formation

Witteberg Group

There are numerous other occurrences associated withthe Grahamstown Peneplane. They are invariably foundclose to the 650 m contour and just below theGrahamstown Silcrete Formation. The presence of deeplyweathered material below the silcrete layer, away from thepresent edges of the Grahamstown Peneplane, is wellknown, but the thick silcrete cover makes occurrences ofthis kind economically unattractive.

Properties of the Grahamstown ClaysAlthough the mineralogy of the clay materials shows


Content, %, in clays derived from

Minerals Witteberg shale Owyka tillite Ecca shale

Kaolinite 20 to 70 Up to 70 20 to 35

Mica - 2M1 10 to 25 Low 10

Quartz 30 to 60 20 to 60 55 to 70

Pyrophyllite Up to 35 0 0

Feldspar 5 0 0

large variations-not only from deposit to deposit but alsowithin the deposits-some very clear compositional differ-ences are apparent between the clay materials derivedfrom different parent rocks, as can be seen from Table IV.These differences are of great importance to ceramists. Forexample, clays derived from Witteberg shales containappreciable but varying amounts of pyrophyllite, as wellas small amounts of feldspar, while clays derived fromDwyka tillite or Ecca shales are devoid of pyrophyllite andalso have less micaceous minerals than the Wittebergclays. These differences in mineralogical composition arereflected in the differences in ceramic properties.

TABLEIVMINERALOGY OF TIlE GRAHAMSTOWN CIA Y DEPOSITS

It would appear that the type of parent rock has a dis-tinct influence on the particle-size distribution of theweathered material. For example, the clays derived fromthe Dwyka Formation have a narrower particle-size distri-bution than those derived from the Bokkeveld andWitteberg shales. There is, unfortunately, no markedantiparallelism in the particle-size distributions of the indi-vidual minerals, with the result that there is a considerableamount of quartz with a grain size smaller than 20 j.l.minall the clays. Refining processes based on particle-size dif-ferentiation would thus be ineffective in producing a mate-rial consisting essentially of kaolinite.

It must be stressed that in most cases there is a consider-able variation within the occurrences, and only generalcomparisons can be made. The type of parent rock has, tosome extent, an influence on the ceramic properties of theclay materials, and some broad trends can be recognized.

The plasticity of the clays derived from the Dwyka tilliteand shale of the Prince Albert Formation can be describedas fair, while the Witteberg clays generally have goodplasticity. The dry strengths of the clays derived from theWitteberg shale tend to be significantly higher than thoseof the Beca and Dwyka clays.

The clays derived from Dwyka tillite and Ecca shalehave a good white colour in their natural state, and thebrightness of these materials compares fairly favourablywith that of the beneficiated kaolins from the WesternCape. They are, indeed, frequently used as fIllers where alow abrasiveness is not required. The fired colour of all theGrahamstown clay materials is also very good, and thematerials are acceptable in this respect as materials for theproduction of whitewareslO.16.

The Grahamstown clay materials are, with only fewexceptions, utilized for ceramics without any real benefici-ation, apart from limited selective mining, rough handsorting, and sometimes cobbing to remove excessive ironstaining. In general, the materials are supplied and used inthe crude form, and any beneficiation is really only inci-dental during the manufacturing process of the user, forexample the screening out of coarse quartz during the wetpreparation of a ceramic body.

The greatest problem faced by the clay producers in theGrahamstown region is the large variation in the propertiesof the weathered material within and between most of thedeposits. These variations are the result of the rapid short-range changes in the original lithology of the parent rock,but the contorted structural nature of the parent rock fre-quently makes meaningful selective mining unrealistic.The quality of the clay material supplied is thus generallynot consistent, and some of the properties can change con-siderably over a short time. The consumer should thus bewell aware not only that there are real differences betweenthe different Grahamstown clays, but also that materialfrom a particular deposit can vary greatly with time.

Other OccurrencesResidual kaolin derived from sedimentary rocks is also

found in various other parts of the Republic of SouthAfrica. These occurrences have not been investigatednearly to the same extent as those in the Grahamstownregion, and it is likely that there are many occurrences thatare not recorded. Deposits that could be of economicimportance are found in Natal, the Transvaal and the CapeProvince.

NatalSediment-derived residual kaolin occurs along the

Tongaat-Durban road at Kingscliff and Afgrond. Theweathering of the shales of the Natal Group was accompa-nied by the leaching of calcium, magnesium, and phospho-rus, as well as by the oxidation of iron, resulting in thealteration of feldspar and mica to kaolinite and illite. Thelenses of kaolinized shale are small in extent, and the shalehas long been used by the local population for decoratingtheir homes and for medicinal purposes.

In the unflfed state, the clay materials are yellowish tocream-coloured, and there is no significant improvementafter beneficiation to minus 10 j.I.ffi,nor after a chemicalbleaching process. The materials have low plasticity and arelatively short firing range, with sharp densificationbetween about 1150 and 12()()OC.They fife to a yellowish-cream colour13.

The poor colour of both the unflfed and flfed materialsmake them unsuitable as fillers in the paper industry or asraw materials for ceramic whitewares. Even if colour werenot important, the presence of quartz in the fme fractionswould preclude their use in applications where low abra-siveness is required. .

The clays could be used as one of the raw materials inthe production of coloured ceramics. However, the ton-


nage of material available is small and the total reservesmust be increased considerably before these clays can beregarded as the raw material for a ceramics industry.

Cape ProvinceThere are a number of occurrences of clay materials near

Albertinia, in the Riversdale District, with apparently simi-lar features to those of the Grahamstown region. The claysare derived from steeply dipping Bokkeveld shales andargillaceous sandstones, and the deposits are found near orjust below silcrete and ferricrete capstones. These clays areused extensively as raw materials in the whiteware ceram-ics industrylo.

TransvaalA large deposit of white kaolin occurs just north of

Zebediela, probably derived from a toff or lava band of theBlack Reef Formation. The clay material consists mainlyof kaolinite, with minor quantities of quartz and sericite,and its colour ranges from white and cream to ochre andred. The white material is generally found towards the topof the band and is thought to be the result of strong leach-ing. The material is used in the ceramics industrylo.

THE WESTERN CAPE PLASTIC CLAYSThere are two well-known occurrences of high quality

plastic clays in the Western Cape: in the Kraaifonteinregion (on the farm Eersterus), and in the Brackenfellregion (on the farm Groenland). Because of their generalappearance and high plasticity, they are known locally asball clays.

The country rock on which the clay deposits and associ-ated sediments rest belongs to the Malmesbury Formation.This formation consists of argillaceous shales, phyllites,and arenaceous rocks, which are folded and have under-gone regional metamorphism. The Malmesbury Formationis completely covered by surface formations in the vicinityof the clay deposits, and there are thus no outcrops of theserocks. A few kilometres west of Brackenfell, post-Malmesbury Cape Granite outcrops. These rocks alsooccur as a large batholithic mass to the south of thedeposits, forming the Bottelary hills.

The occurrences are overlain by up to 6 m of Cape Flatssand of Tertiary to Quarternary age. The sand is unconsoli-dated. contains no carbonate minerals, and varies in colourfrom off-white to rusty-brown. The bottom 3 m of the sandis admixed with a grey clay. The clay measure is about 3mthick and grades downwards into impure peat or lignite,which varies in thickness from 0,3 to I m. The clay mea-sure can be divided into an Upper Suite and a Lower Suite.Recognizable plant remains are evident in the peat, whichalso contains much clay material either admixed with theplant remains or as individual thin layers. The clay mea-sure is underlain by a thick layer of sand of variable claycontent 17.

Properties of the Plastic Clays!O,!6The Westem Cape plastic clays consist largely of kaoli-

nite, with varying amounts of quartz and sometimes smallamounts of smectite. Micaceous minerals are completely


absent, and these clays thus differ noticeably from theEnglish ball clays in this respect As far as their particle-size distribution is concerned, a direct comparison with theEnglish ball clays is not possible, but it seems that the clayfraction of the Cape clays is finer-grained than the Englishball clays. The particle-size distribution of the Cape claysis distinctly bimodal, with a large clay fraction (where theparticles are all extremely small) and a much coarser frac-tion (consisting mainly of quartz grains). The ratio of clayfraction to coarse fraction varies considerably throughoutthe deposits.

The plastic behaviour and dispersion properties of thesematerials are complex, but depend largely on the amountof clay fraction and organic matter present in them. Thesematerials are important components of fabricated white-ware bodieslo, 16.

Origin of the Plastic ClaysA major and still unsolved problem is the origin of the

clay measures. The kaolinite of this clay is disordered,while that derived from the granites is fairly well ordered.On the other hand, the Malmesbury shales contain appre-ciable amounts of quartz and micaceous minerals, whilethe plastic clay is practically monomineralic. Furthermore,most of the plastic clays from the Western Cape, if not all,are associated with units of peat or lignite. The presence ofmacchia pollen and palm spores indicates contrasting cli-mates of cool, wet winters and dry, windy summers to sub-tropical conditions. The clay unit is probably of lateCainozoic age.

One possibility is that the clay measures were depositedeither in a lacustrine or fluvio-lacustrine environment. Thepresence of the organic matter would then indicate a slop-ing or hilly country from which plant matter was derived.Under such conditions, the sandy units and the peat unitswould indicate deposition in shallow water, whereas thevery fme-grained pure-clay units point to deep-water con-ditions.

However, it is confusing to find thin clay units, only afew millimetres thick, within peat or lignite units. If theessentially monomineralic nature of the clay units is alsotaken into consideration, an alternative possibility shouldbe considered: namely, that the clay units were formed bythe in situ alteration of volcanic ash deposited periodicallyin shallow, highly acidic waters IS.

BRICKMAKINGCLAYSA large variety of clay materials is used locally for the

making of structural clay products, but such clays invari-ably contain large proportions of kaolinite and quartz,while micaceous minerals are seldom absent These claysgenerally contain appreciable amounts of iron compounds,with the result that they fire to a dark colour. It should bestressed that these materials are place-bound, and only invery exceptional circumstances would these materials betransported to other production centres.

Western Cape19Clays suitable for the manufacture of structural clay

products (face and common bricks, floor tiles, roofingtiles, sewer pipes, etc.) are found throughout the WesternCape. The [ued colour of these materials ranges from yel-low or dark ivory to deep red, and they are generally notvery plastic. There is a fair variation in the properties ofthese clays, making it possible to produce a fairly widerange of products. A grouping found useful for thesebrickmaking clays takes into account the geological for-mation to which they belong, as well as their mineralogicalcomposition and [ICingbehaviour.

The geological relationships of the materials are usuallyeasily established. In the majority of cases, it is quite clearwhether the material is a Malmesbury shale (e.g. by itscharacteristically steeply dipping layered structure) or amore recent sediment (e.g. by its horizontal bedding, orbecause it overlies the Malmesbury sediments).

The firing range of these materials is a distinctiveparameter. Firing range refers to the increase in tempera-ture required to increase the degree of vitrification or den-sification by a certain amount In the evaluation of theseclays, the firing range is specified as the difference in thetemperatures resulting in water absorptions of 15 and 10percent.

Based on the above criteria, the brickmaking clays of theWestern Cape can be classified into three groups.

Group AThe clay materials allocated to this group are all

Malmesbury shales and weathered phyllites, and they con-sist mainly of kaolinite, hydrous mica, and quartz, thekaolinite and hydrous mica being present in about equalamounts. They are generally red-burning, are not veryplastic, and are characterized by their short [ICingrange-as low as 25°C but not more than 70°C. Their occurrenceis widespread: from Cape Town to Paarl, from GordonsBay to Kalbaskraal and, in all likelihood, even furtherafield. They are really suitable only for the making ofcommon bricks, but face bricks and heavy tiles can be pro-duced with careful production control.

Group BThe materials in this group are also Malmesbury shales

and weathered phyllites. These clays consist of kaolinite,hydrous mica, and quartz, with generally more kaolinitethan hydrous mica. Their firing colours are similar to thoseof the Group A materials, but their [ICingranges are longerthan those of Group A, making it easier to produce a con-sistent product. Their plasticity is usually still too poor forthese materials to be used for anything other than bricksand heavy tiles. The Group B materials have been foundonly in the central area-at Bellville, Brackenfell,Stellenbosch, Durbanville and Kalbaskraal-but they mayhave a wider distribution.

Group CThe materials belonging to this group are all recent sedi-

ments. They consist mainly of kaolinite and quartz, butvery small amounts of hydrous mica are also present Theyhave long firing ranges (generally over 120°C), fire to a

light buff to golden-brown colour, and are generally moreplastic than Group A and Group B materials. Materialswith these characteristics are known as yet only atKoelenhof, Dassenberg (near Kalbaskraal), andPhesantekraal. They are very important for the productionof sewer pipes and tiles, while the addition of these claysto Group A and B materials considerably facilitates theproduction of a better-class product such as face bricks.

ResourcesFor all practical purposes, raw materials with properties

adequate for the production of common bricks arewidespread and limitless in the Western Cape. Materialsfor the making of attractive face bricks are also readilyavailable, particularly in the central area of the region. Onthe other hand, the distribution of the more plastic claymaterials with Group C characteristics appears to be muchmore limited. The relative abundance of the various mate-rials is directly related to geological facts: the wholeregion consists largely of the Malmesbury formation,while the more recent sediments are understandably morelimited and sporadically developed.

Transvaal20The clay-based ceramics industry of the Transvaal is

well served by a wide range of raw materials suitable forthe manufacture of face and common bricks, roof tiles, andfloor tiles. These materials range from hard shales of lowplasticity to soft clays of very high plasticity, and theydevelop [ued colours ranging from light beige and yellowto deep red. Two broad classes of brickmaking materialscan be defined based on age: namely, those of Proterozoicage and those of the Vryheid Formation of the EccaGroup.

Proterozoic ClaysThe Proterozoic lithologies that can be used for brick-

making belong to the Witwatersrand, Transvaal andVentersdorp Supergroups. Despite the vast age span andextensive surface exposures, these weathered products ofshales and igneous rocks account for only about 20 percent of the clay-brick produ~tion of the Transvaal.

The Proterozoic materials exhibit remarkable similaritiesin their brickmaking properties, and only two types needbe identified. The shales, composed mainly of anhydrousmicas and quartz in their unweathered state, weather toillite and hydromicas. The hard, impervious nature of theindurated shales and poorly developed weathering proftlesof the Transvaal Highveld ensure that degradation of theprimary mineralogy is incomplete; thus, these shales arerefractory and of low plasticity. The vitrification range isvery long, and the water absorption does not fall below 15per cent at practicable temperatures for brick kilns.

Lavas and minor intrusives of ultrabasic to andesiticcomposition constitute the second group of Proterozoicbrickmaking materials. The anhydrous high-temperatureprimary minerals, mainly pyroxene and plagioclase, areunstable and break down readily to a clay-rich assemblageof calcic smectite and interlayered chlorite-illite. Kaolin


and quartz are minor accessories. The iron content is usu-ally between 10 and 20 per cent F~O3' yielding very dark-red to blue-black vitrified fired colours. In contrast to theshales, these clays have a very high plasticity, very shortvitrification range, and a strong tendency to crack duringdrying, all of which would lead to very poor yields of goodquality products even in the most sophisticated of brick-works. The two Proterozoic types of raw materialsdescribed above are complementary and can be blended toproduce bricks of reasonable quality, although most suchproduction is for the non-face market.

Karoo ClaysThe sedimentary rocks of the Karoo Sequence used by

the Transvaal brickmaking industry represent a part of thecoal-measures facies of the Vryheid Formation.

The same lithological sequence is widely observed andcan be generalized, from the base upwards, as follows,

(a) Low-grade coal or a very plastic black clay of highcarbon content is present.

(b) A plastic non-refractory kaolinite of low quartz con-tent occurs, and grades from black at the base to lightpurple-grey at the top with decreasing carbon con-tent.

(c) A hard refractory flint clay occurs within the plastickaolinite at many localities and, with increasing fre-quency, to the east and north of Pretoria until theflint clay is the predominant lithology.

(d) A white to pale yellow or brown kaolinite of moder-ate refractoriness and plasticity, and of higher quartzcontent than the plastic kaolin, is present. The quartzcontent tends to increase upwards, and a shaley tex-ture is generally present. This unit tends to be pre-dominant on the West Rand, and absent or poorlydeveloped to the east of Johannesburg.

(e) A sandstone horizon frequently occurs as the nextmember of the sequence. This unit is generally medi-um- to coarse-grained but varies considerably inthickness, induration, and lateral persistence.

(f) Above the sandstone on the East Rand, there is asandy, iron-stained kaolinite. This unit invariably hasa significant content of smectite-group minerals,which impart a short fuing range and low vitrifica-tion temperature.

The origin of t~e clay sequence described above is con-troversial, and models implying both diagenetic and pri-mary origins for the clay mineralogy have been suggested.

Brickmaking PropertiesIt can be deduced from the descriptions given above that

there is a correlation between plasticity and refractoriness,and that, in general, the highly plastic clays vitrify morerapidly and at lower temperatures than do the less plasticvarieties. This applies to both the Proterozoic and theKaroo clays.

The highly plastic, smectitic clays of both Proterozoicand Ecca age have a fuing range of approximately 50°C,and reach a water absorption of 10 per cent by 950°C. TheProterozoic shales of low plasticity have a fuing range of

about 300°C, and do not achieve a water absorption of 10per cent below the practical temperature limit for brick-making kilns of 1200°C. The Ecca shales and clays ofmoderate plasticity have a good balance of forming, dry-ing, and firing properties that render them ideal for brick-making purposes. Typical fuing ranges for the kaoliniticEcca shales are between 100 and 150°C, and a waterabsorption of 10 per cent is generally achieved at between1025 and 1070°C.

Natal and Orange Free StateZ°There is not much information available on the brick-

making clays of this region. However, it appears that allthe suitable clays are red-burning and thus no light-coloured bricks can be produced.

The friable carbonaceous shales of the Pietermaritzburgand Vryheid Formations of the Ecca Group are used exten-sively for brick manufacture in the Tugela Basin region.These shales, which are often intruded by minor dolerite,are generally overlain by Quartemary Berea Red Sand sed-iments. The hard material weathers rapidly on exposure,and sufficient plasticity develops within a few days.

The siltstones and shales of the Volksrust Formation ofthe Ecca Group are exploited in the Odendaalsrus area,while Beaufort shales are used near Bloemfontein. In theKimberley area, the Prince Albert shales of the EccaGroup are mined for brickmaking.

REFRACTORY CLA YS21-24

A classification generally used in South Africa is basedon two important quality-determining parameters: bulkdensity (Table V), and alumina content on a calcined basis(Table VI). Bulk density is inversely related to porosityand to some extent reflects the plasticity of the clay, aswell as the resistance of the fired product to slag attack,while the alumina content gives an indication of the refrac-toriness of the material23.

TABLEVBULK DENSITY OF REFRACTORY CLAYS

Type of clay Bulk density

Flint claySemi-flint fire-claySemi-plastic fire-clayPlastic fire-clay

Higher than 2,322,00 to 2,321,90 to 2,00

Lower than 1,90

TABLEVIALUMlNA CONTENf OF REFRACfORY CLAYS

Grade of clay Al2O3 content after calcining%

I11IIIIV

Higher than 44,044 to 4040 to 38

Lower than 38


Occurrences of Refractory ClaysRefractory clays are known to occur only in the

Transvaal, and a full inventory of all the deposits of suchmaterials was compiled in 1987 by Brede1l22,who notedwhether their exploitation was active, abandoned, or dor-mant. The refractory clays in the Witwatersrand area rangefrom plastic to semi-flint types, and are confined to theVryheid Fonnation of the Karoo Sequence, which wasdeposited on the uneven floor of older rocks-mainly theChuniespoort Group of the Transvaal Sequence. The totalproduction of high-quality flint clay and a large proportionof the production of semi-flint clay come from thePretoria-Belfast area. These deposits are generally locatednear the edges of the numerous outliers of Ecca sedimentsfringing the continuous sheet of Karoo rocks extendingsouthwards.

The main productive deposits in the Witwatersrand regionare at Boksburg, Brakpan, Springs, Modder East, Marievale,Olifantsfontein, Lawley and Vereeniging21.24.25.Severaloccurrences in both the East Rand and the West Rand wererecently investigated in detail by Brede1l22.The type andgrade of the refractory clays found in this region vary con-siderably, and materials ranging from plastic to semi-flintclays are sometimes found in one deposit.

Flint and semi-flint types of refractory clay occur in thevicinity of Bon Accord, Hammanskraal Boekenhouts-kloofdrift, Nooitgedacht, Witbank, Middelburg,Bronkhorstspruit, Belfast and Soutpansberg21-23. Thedeposits of high-quality material tend to be thinner andsmaller than the more plastic refractory clays of theWitwatersrand area.

The better-quality refractory clays are used for the man-ufacture of fireclay refractory bricks, while the clays oflow refractoriness (the so-called chocolate-brown plasticclays) are used for the making of building bricks, tiles andpipes. Some of the clays fire to a good whiteness and arethus suitable for the production of whitewareslO.

Flint clay is sold either in the raw state or as a calcinedmaterial (chamotte). It is used in the manufacture of high-quality super-duty fireclay bricks and is much in demandby the steel industry.

Resources of Refractory ClaysAs pointed out by Brede1l22,the lack of intensive explor-

ation, even in producing areas, as well as the confidentialnature of company data, renders the calculation of theresources of refractory clays highly conjectural.Comparisons between estimations by different investiga-tors are also made difficult because of the inexact termi-nology used.

Witwatersrand RegionBrede1l22summarized his calculations of resources of

refractory clays on the East Rand as follows:(1) indicated resources of semi-flint and plastic clay

with good refractoriness (pcE 34 to 35) within theboundaries of active mining areas: 10,5 Mt

(2) indicated resources of semi-flint, semi-plastic and

plastic clay with good refractoriness (PCE 34 to 35)in areas where mining is potentially feasible: 11,3 Mt

(3) indicated resources of semi-flint, semi-plastic andplastic clay with low refractoriness (pcE 26 to 33) inareas where mining is potentially feasible: 22,4 Mt

(4) refractory clay sterilized by urban development (as at1987):...26,6 Mt.

Brede1l22 also conservatively estimated the inferredresources of all refractory clays on the West Rand to be inexcess of 200 Mt, of which about 70 per cent would con-sist of clay with good refractoriness (pcE 34 to 35), 27 percent of clay with low refractoriness (PCE 29 to 33), and 3per cent of non-refractory clay.

Pretoria-Belfast RegionIn 1965, Bennetts23 estimated the probable ore reserves

of flint clay to be of the order of 1 to 2 Mt, but stated thatunderground mining could increase this figure to over 50Mt. At the same time, however, the industry estimated theproven reserves of high-quality flint clay at about 3,5 Mt,with additional estimated reserves of about 4 Mt, as report-ed by HeckroodtZl.

For his estimation of resources, Brede1l22grouped therefractory clays into three categories:

(i) Grade I flint clay: demonstrated resources of 15,2Mt, of which only 3,4 Mt represent measuredreserves under the prevailing conditions; inferredresources of 1,0 Mt;

(ii) Grade I semi-flint and Grade 11and III flint clay:demonstrated resources of 4,6 Mt; inferred re-sources of 1,0 Mt;

(iii) Grade I to III plastic clay: demonstrated resourcesof 1,2 Mt; inferred resources of 0,5 Mt.

GenesisBrede1l22 suggested that the purity and type of the

refractory clay deposited was determined by pH-controlleddifferential flocculation, rather than by the nature of thesource material or post-depositional alteration. In acidicenvironments, as could have been caused by the concen-tration of organic acids in swampy basins during the earlystages of Ecca deposition, almost pure kaolinite wouldhave been deposited from a mixed clay-mineral suspen-sion. In the smaller proximal basins and at low pH, kaolin-ite particles would have flocculated in an edge-to-facemanner, giving rise to small deposits of flint clay. At high-er pH, flocculation tends to be face-to-face and, therefore,deposits of plastic clay would have formed in the larger,more distal brackish basins. The gradual displacement ofthe acidic conditions by a fluvial and lacustrine environ-ment would have reduced the efficiency of differentialflocculation and eventually led to the deposition of shale.

By comparing the rubidium and strontium contents ofthe refractory clays from different regions with the con-centration of these trace elements in kaolinite derived fromin situ weathering of probable parent rocks, Bredell22con-cluded that the West Rand clays received most of theirkaolinite from Halfway House Granite, with a possible


contribution from Witwatersrand shale, while the claysfrom the East Rand were derived from both HalfwayHouse Granite and granite from the Bushveld Complex.The granite from the Bushveld also served as the mainsource of kaolinite for both the flint and the plastic clays inthe Pretoria-Belfast area, with possible contributions fromRooiberg felsite, shales of the Pretoria Group, andWaterberg arkose.

BENTONITE24

Clay materials similar to the Southern Bentonites of theUSA are found in a number of localities in the Republic.The exchangeable cations are mainly calcium and magne-sium, but the clay can readily be converted to a high-swelling sodium bentonite by treatment with sodium car-bonate. The bentonite can also be effectively activated bystrong acids such as hydrochloric or sulphuric acid togreatly increase its bleaching action.

Occurrences of BentoniteBentonites are known to occur in four localities in South

Africa.

Koppies District, OFSMajor deposits of bentonite are found on the farms

Ocean and Blaauwboschpoort, with smaller occurrenceson other farms in the vicinity. The material occurs belowan overburden of 12 m of Ecca shale, which occupiesembayments in the Swaziland schists. The occurrences areflat-lying and lenticular, with a hard footwall and hanging-wall of shale, caused by local silicification, probablythrough the release of silica from the bentonite.

Knysna DistrictThe deposit on Roode Farm (near Plettenberg Bay) is

part of a succession of clays and sands of the Upper WoodBeds of the Enon Formation. The deposit is lenticular, dip-ping eastwards, with a thickness that varies from 0,15 to2,5 m and an overburden that could be as thick as 61 m.The material is sometimes interstratified with sandstone.

Wodehouse DistrictA bentonite layer, between 1 and 2 m thick, occurs in

the Red Beds on the farms Pronks Berg and Tijger Klip.The bentonite is underlain by yellowish sandstone andoverlaiD by red mudstone, which has undergone extensivesilicification. Thin lenses of sandstone are interbeddedlocally. It was estimated that up to 1 Mt of bentoniticmaterial is available but, because of its variable quality,the occurrence may not be economically viable.

Northern NatalA deposit of impure bentonite occurs in the Upper

Lebombo Stage of the Stormberg Series, near Mkuze. Thereserves are probably large. This deposit was probablyformed by the alteration of perlite, perlitic pillow lava, andtoff.

Properties of BentoniteThe bentonites from the Koppies, Knysna and

Wodehouse Districts are very similar and consist mainly


of montmorillonite and some quartz. Cristobalite is gener-ally present, sometimes only in trace amounts. The cation-exchange capacity of good-quality material ranges from 94to 99 me per 100 g of dry clay, the exchangeable cationsbeing essentially calcium and magnesium in equal propor-tions. The endothermic loss of crystalline water peaks atabout 700°C.

The bentonite occurring in northern Natal consists ofsmectite, some kaolinite, and a high proportion of cristo-halite. This material is similar to the Ultramarina bentonitefrom M~mbique and it may have ceramic applicationssince it fires white and imparts high dry strength to thebodylO.

The presence of cristobalite and the absence of largeamounts of quartz in the bentonites from the Koppies,Knysna and Wodehouse Districts point to the in situ alter-ation of layered volcanic tuff, rather than the alteration ofSwaziland schists or Ecca shales, as the origin of thesematerials. Bedding laminations and the presence of thinlenses of sandstone in the deposits in the WodehouseDistrict are evidence that the ash was deposited in shallowwater, while it is also known that at least eight volcanicvents of Stormberg age exist within 8 km of these occur-rences.

PAL YGORSKITE24,26,27

The resources of palygorskite in the Republic are large,but their exploitation is limited. Considerable tonnages ofthis material are imported into the country, even thoughsome of the local palygorskite can be beneficiated to com-pete on quality with the foreign material.

Palygorskite is a product of neoformation during pedo-genesis in arid to semi-arid regions, together with the for-mation of smectite or carbonates. At present, palygorskiteis known to occur at Dwaalboom (Rustenburg District),Northam, Naboomspruit, Crecy, Immerpan, Lydenburgand Hotazel.

There are many similarities between the deposits. Thepalygorskite layer, between 1 and 4 m thick, usuallyoccurs below an overburden of black turf or surface lime-stone. The top part of the layer is generally dissected bycarbonate veins. The best-quality material is found in themiddle part, which grades downwards into sandy or grav-elly palygorskite.

It would seem that the parent material had no influenceon the genesis of the palygorskite, since it is found abovegranite, basalt, norite and manganese ore. Palygorskite isnever found in association with lacustrine sediments.

The extent of this resource is not known, but it is clearthat it is large. The genetic association of palygorskite withsurface limestones could assist in exploration and, becauseof the widespread distribution of calcrete in South Africa,it is probable that the distribution of palygorskite is farmore extensive than is known at present

CONCLUSIONAs this review has shown, clays, though less glamorous

than many other raw materials, are vital to technological

societies. They are required for the manufacture of build-ing materials, refractories, crockery, sanitary ware, paper,paint, pharmaceuticals and a host of other products.Certain clay materials are also importimt catalysts, or theyare used as absorbents in many applications.

The Republic of South Africa is fortunate in the widerange of clays that it possesses, although many of thedeposits are yet to be exploited. Further study and explo-ration may yet reveal additional sources.

REFERENCES1. BAIU!Y, S. W. Structures of layer silicates. Crystal structures of clay

minerals and their X-ray identification. Brindley, G. W., andBrown, G. (OOs.). London, Mineralogical Society, 1980. pp. 1-124.

2. RYAN, W. Propertks of ceramic raw materials. Oxford, PergamonPress, 1978. pp. 42-72.

3. WORRAlL, W. E. Ceramic raw materials. Oxford, Pergamon Press,1982. pp. 16-85.

4. CLEWS, F. H. Heavy clay technology. London, Academic Press,1969.481 pp.

5. CHEsTERS, J. H. Refractories for iron and steel making. London, TheMetals Society, 1974. 493 pp.

6. HECKROODT,R. O. ResidwJl kaolins of the Republic ofSouJh Africa.Pretoria, Geological Survey. (In preparation.)

7. MURRAY, L 1., and HECKROODT,R. O. South African kaolins. Devel.in Sedimentology, vol. 27. 1979. pp. 6O1-!JO8.

8. HECKROODT, R. O. The primary kaolins of the Western Cape,Republic of South Africa.lnterceram.. vol. 30.1981. pp. 99-100.

9. HocKROODT, R. O. The properties of the Cape kaolins. Ann. Geol.Surv. S. Afr., vol. 13. 1979. pp. 9-13.

10. SKAWRAN,D. W. (comp.). Ceramic raw materials of Southern Africa.CSIR Technical Notes, XlBOU-KER Series. Pretoria, CSIR, 1968.

11. HEYSTEK,H., DE JAGER, D. H., DE W AAL,P., and URLI, G. L P. Thekaolin deposits of the area between Bitterfontein and Landplaas,VanrhynsdOlp District. Pretoria, Geological Survey, Bull. 36, 1961.71 pp.

12. ERASMUS, D. J. Die voorkoms en ekonomiese potensiaal van tweekaolienafsettings naby Bitterfontein, Vanrhynsdorp Distrik.Johannesburg, Rand Afrikaans University, M.Se. thesis, 1980.

13. MAZlBUKO,G. University of Zululand, uncompleted thesis,1987.

14. SMUTS, J. The geology, mineralogy and chemistry of theGrahamstown clay deposits. Grahamstown, Rhodes University,1983.

15. HECKROODT, R. 0., and BOHMANN, D. Genesis of South Africanresidual kaolins from sedimentary rocks. Proceedings 8thInternational Clay Conference, Denver. Schultz, Van Oben, andMumpton (eds.). Bloomington, Clay Minerals Society, 1985. pp.128-134.

16. SKAWRAN,D. W., and HECKROODT,R. O. The influence of plasticclays on the ceramic properties of a whiteware body. Interceram..no. 2nO. 1970. pp. 2-6.

17. McMDLAN, M. D. Personal communication, 1988.18. BOHMANN,D. Personal communication, 1990.19. HECKROODT,R. O. The brickrnaking clays of the Western Cape. AM.

Geol. Surv. S. Afr., vol. 13. 1978. pp. 1-8.20. EBORN,A. C., and HUOHES,M. Personal communication, 1990.21. HECKROODT,R. O. Refractory raw materials: Investigation reports

on the processing of certain minerals in the Republic of SouJh Africaand SouJh West Africa. VI. Pretoria, Depamnent of Planning, 1967.pp. 18-27.

22. BREDELL, J. H. Refractory clays in the Transvaal, Pretoria,Geological Survey of South Africa, Memoir 73. 1987. 176 pp.

23. BENNE'ITS,K. P. The flint-clay deposits of the area between Pretoriaand Belfast, Transvaal. Bull. Geol. Surv. S. Afr. no. 45. 1965.79 pp.

24. SrnMIDT, E. R. Clay. Mineral resources of the Republic of SouJhAfrica. Coetzee, C. B. (00.). Pretoria, Geological Survey, Depamnentof Mines, 1976. pp. 275-287.

25. HEYSTEK,H., and CHASE, B. U. R. A study of the brown plastic fire-clays occurring in the Transvaal, South Africa. Trans. Brit. Ceram.Soc., vol. 52. 1953. pp. 482-496.

26. GERMIQUET,J-D. A mineralogical investigation of some palygorskitedeposits in South Africa with reference to beneficiation and industri-al use. Johannesburg, Rand Afrikaans University, M.Se. thesis, 1977.pp. 1-138.

27. HEYSTEK, H., and SCHMIDT, E. R. The mineralogy of the atta-pulgite-rnontmorillonite deposit in the Springbok Flats, Transvaal.Trans. Geol. Soc. S. Afr., vol. 56. 1953. pp. 99-115.

BIBLIOGRAPHYGRIM, R. E. Applied clay mineralogy. New York, McGraw-HiU, 1962.

422 pp.GRlMSHAW,R. W. The cJaemjstry and physics of clays and allied ceramic

materials. London, Ernest Benn Limited, 1971. 1024 pp.


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