MINERAL PROCESSING APPLIED TOADVANCED MATERIALS TECHNOLOGY

P. Somasundaran and C. MaJtesh,

Henry Kromb School of Mines,

Columbia University,

New York, New York 10027.

ABSTRACf of electronic components. optical fibers and evenbiomaterials. Production of such materials, maintaining thequality in terms of such properties as size, surface area,shape and purity consistently requires new and modifiedprocessing techniques often with use combined use of manymechanical, chemical, electrochemical andpyrometallurgicalsubprocesses.

The ever increasing need for treating fme complexmineral bodies has led to the evolution and rapiddevelopment of advanced processing techniques for fmegrinding, classification and mineral separation andpurification. With the introduction of these newtechnologies and understandin~ of underlying fundamentalprinciples, mineral processing IS now being al?plied tomaterIals ranging from ceramics and composites tosuperconductors. In this review, current and potentialapplications of mineral en~ineering techniques to advancedmaterials processing are dIScussed. Particufarly, materialprocessing involvin~ ultrafine grinding. dispersion andflocculation and thm film formation is examined. Noveltechniques of solid/liquid separation involvingmanipulation of interfaces and use of high intensity highgradient superconductor magnetic equipment forsolid/solid separation are discussed.

Research in the field of mineral processing has longsince been aimed at solving these problems. Judicious useof interfacial phenomena has led to the evolution anddevelopment of processes aimed at treatin~ low grade andfine 'ilained ores. Grinding being a key urnt operation hasreceived much attention in the field of mineral processing,particularly the mechanisms of ,grinding and the kineticsmvolved. In this paper the applications of mineralprocessing techrnques to advance materials technology arediscussed.

GRINDINGINTRODUcnON

The recent discovery of high temperaturesuperconducting ceramics and high performance structuralceramics has drawn the attention of researchers worldwidetowards the emerging field of advanced ceramics and hasincreased the awareness towards the need for improvin~current ceramic processing methods. While the productionof advanced ceramics involves sophisticated chemicalprocessing and is closely dependent on the quality andpurity of the compounds used, the basic raw materials - theoxides and silicates - are among the most abundant in theearth's crust. The only drawback faced is that the oresmined are complex and fmely disseminated. This would notonly require fmer grinding for liberation, but alsodevelopment of novel processing techniques for upgradingsuch ores. The future of ceramics lies in efficient andeconomic extraction of the raw materials and thedevelopment of new processing methods designed toeliminate the flaws inherent to traditional ceramicprocessing methods. Most of these defects are generatedduring powder processing prior to firing. hence advances inthe field of ceramic processing win depend upon a bettercontrol of the powders processed and of the subsequentsuspension stability since most ceramics are fabricated, atleast in part, using solid/liquid mixtures (Bleier, 1984).Ultrapure minerals are also being used for the fabrication

Grinding of coarse-grain material to produce finepowders is of concern in many areas of ceramics, healthsciences, agriculture, etc. The science of fine grinding inceramics has lagged considerably behind othertechnological advances. Fines and ultrafines presentunusual difficulties during their I?rocessing. It has beensuggested in the past that reductIons in mass and increasein surface energy and surface area are mainly responsiblefor it (Somasundaran, 1976). However, morphology andsurface mineralogical and chemical heterogeneity also playa major role in determining the interfacial behavior of fineswhich is very critical in downstream processes and theseproperties can be controlled to a large extent by usingappropriate grinding methods (Somasundaran andRoussev, 1985).

Major differences occur between the morphologiesof particles prepared using different mechanisms. Forexample, a sample of - 400 mesh quartz particles preparedby impact show surface characteristics unlike thoseprepared by abrasion using a mortar and pestle. The onesprepared by abrasion undergo significant erosion incomparison to those from the impact tester. Groovesresultin& from such erosion can have a profound effect onthe wettIng characteristics of the particles (Somasundaran,1984). Prolonged grinding often used in the preparation of

3

ADVANCED MATERIALS4

0 OJ O.Z 0.3 0.4 05 Ob 0.7 08 0.9

PARTICLE SIZE. mm

Comparison of density of quartz of different sizeproduced by grinding in water and air andammonia.

1.0TIME. M'nII'.'

Conversion of massicot to litharge and sulfur togalena during dry grinding in a ball mill.Fig.2

Fig.!

Mitzmager, 1968). However the increased efficiency of wetgrinding can also be due to physical reasons. Cushioningeffects due to the presence of a bed of fines will be lessduring wet grinding than in dry grinding. since the fineparticles tend to remain suspended in the water in theformer case. It has also been reported that grinding inorganic liquids is more efficient than in water(Somasundaran, 1978).

fine and ultrafine particles produces major differences in.in addition to morpholo~, surface crystal structure andeven chemical composition (Un and Somasundaran, 1972).This has been confirmed during prolonged grinding ofquartz, calcite and massicot-sulrur mixture. Gravimetric,thermogravimetric and X-ray diffraction analyses of thef,found particles showed measurable changes in propertiesm all cases. Thus, the density of quartz particles was foundto decrease as a function of grinding time appaJ:ently due tothe creation of deep amorphous layers on the particles(Fig. 1). In the case of massicot and calcite, polymorphictransitions were found to alter their structures to lithargeand ar~onite respectively. On the other hand, in the caseof grindIng of masicot or litharge with sulfur, solid statereactions took place causing total chemical conversion ofthe samples to the sulfides (Fig2).

Metals and metal sulfides have been reported toundergo rapid surface oxidation on grinding. Such surfacescan acquire higher hydrophilic properties and can adsorblarge quantities of water vapor. Increased reactivity ofsamples due to ~nding is well-known. Increase in cationexchange capaCIty of such minerals as alumina, silica andmica during grinding (Gregg, 1%8) has also been observedin the past

Surfactants have been widely reported as effectivegrinding aids (Somasundaran and Un, 19n). The effect ofadding a flotation agent called Flotigam P on wet ball-milling of quartz and limestone is shown in Fig.3 as anexample. In some cases. however, the presence ofsurfactants has.led to a decrease in the grinding efficiency.Hydrophobization of the particles by the adsorbedsurfactants can result in the attachment of air bubbles tothem and consequent levitation. The grindin$ efficiency canbe expected to be lower if the particles remain levitated.Grinding is in general found to be more efficient in thepresence of inorganic electrolytes. In the ceramic industry,~ding of metallic and refractory-type materials is foundto be more efficient when multivalent electrolytes are usedas additives (EI-Shall and Somasundaran, 1982). Effect ofAlCI) and Cao on wet grinding of quartz is shown inFigure 4. The effect of the reagents on grinding has beenshown to be their cumulative effect on subprocessesinvolved such as crack initiation and propagation,flocculation of ground product, pulp flow, adhesion ofground product to mill walls and grinding medium.

Future technological developments in the electronicindustry are aimed at desiening devices of the order ofmicrometers. For this minIaturization of electronic devices,fme fabrication of other surrounding devices is necessary.Attainable dimensions of the devices depend on the size ofthe ceramic particles: the smaller the particles, the moreminiature the solid device can become (Ichinose, 1987).Therefore, manufacturing particles with sizes that suit thedesired ceramics is a prerequisite. Grindin~ is an usefultool for achieving such results. Work done In mineralprocessing to elucidate the mechanism.~ and means toImprove the efficiency of this operation stands to provebeneficial in the manufacture of high performanceelectronic devices.

Thus it is clear that particles can show differences inmorphology depending on how they are produced. Suchdifferences can have a major effect on the wettability ofparticles and hence, on their aggregation. For example, inthe study of contact angle of teflon and nitrocellulosesubjected to various mechanical treatments it was foundthat the receding contact angle decreased and theadvancing angle increased as the surface was made rougher(Oliver, et ai, 1980).

A major problem in the r roduction of fines is thepoor efficient;'! of the process 0 grinding. It has been statedthat the grinding efficiency is of the order of 1 %. In thisregard an understanding of the effects of changes in thephysico-chemical environment on comminution is essentialsince such changes have been clearly shown to producebeneficial or detrimental effects. Use of grindin~ aids is areported practice in the cement industry. Grinding in wateris usually more effective than dry grinding. This effect ofwater has been ascribed to a reversible reaction betweenunsatisfied surface bonds and water molecules {Un and

MINERAL PROCESSING APPLIED TO ADVANCED MATERIALS 5

[A) Effect of addition of AlCl3 on wet ball millingof quartz in acidic solutions.

Fig.4

[B) Change in different pro~rties of quartzsuspensions due to lO-S Kmol/m3 CaCl2 addition.

ELECfROSTAllC STABILlZAll0N: theexistence of a charge on the particle surface (or its creationby adsorption of a charge control agent on the colloid)induces a net charge of opposite sign in the dispersionmedium by principle of electroneutrality. These so-calledcounter ions, which neutralize the particle charge, surroundthe particle, givin~ rise to the electrical double layer. It isthe mutual repulsIon of these double layers that preventsaggregation of particles when colloidal stabilization iselectrostatical1y control1ed.

DISPERSION TECHNOWGY

Most particles in liquids are electrically charged, thenature and magnitude of which are dependent on themechanisms responsible for the charge generation on theirsurfaces and the properties of the solids and solutioncomprising the system. The point of zero charge of a soliddefines the solution condition at which the particles possesszero net charge and is an important experimentallyaccessible property of the system since its location withrespect to solution conditions will essentially determine therepulsion or lack of it between particles and consequentlythe stability of a suspension of these particles. The chargeat the solid-li~uid interface can be mOdified by controlledadsorption of Ionized chemical species or polymers. This ispossible since it is not the surface potential that is directlycontrolling the interactions between the particles, but aninterfaciaf potential at some distance away from the surfacethat the particle will have to work against as they approacheach other. Control of this potential, which can be assumedfor convenience to be the potential at the shear plane,known as the zeta potential, is P:Ossible usually by adjustingthe ionic composition and pH of the medium. Figure 5shows the zeta potential of alumina as a function of pH andits effect on the settling rate of the suspensions(Somasundaran, 1975). Stability is minimum near zero zetapotential conditions, as indicated by the maximum inpercent solid settled. The effect of added polymer(polyacrylic acid) is also indicated in the figure. Thepresence of polymer leads to hi$her settling rates and theconformation of the polY1!ler is Instrumental in determiningthe solids settled in addition to the zeta potential (Huang,et ai, 1988). It is estimated that aggregation can usually beobtained in the absence of steric forces if the zeta ~tentialis less than 15 to 20 m V or if the particles are oppositelycharged (Somasundaran, 1980).

Most ceramic greenware is fabricated usingsolid/liquid suspensions. The elements of ceramicprocessing that rely, to various degree, on suspensionstability are powder formation, powder dispersion andpowder packing. The traditional medium for the processingof ceramic powders has been water but in recentdevelopments, organic media have sometimes beenpreferred (Reed, 1988). For instance, some ceramics ofcurrent interest such as nitrides or carbides, tend to developoxide surfaces when processed in water and hence non-aqueous media are preferred when working with thesematerials. But whatever be the dispersin.8 media, forsuccessful processing it is of paramount Importance to havea stable dispersion of the solid particles in the liquid.Unagglomerated powders with a narrow size distributionhave enormous ~tential in advancing the science ofceramics processing.

Phenomenologically, suspension stabili.tr may becharacterized by the disJ>C?rsion state of the individualparticles: without stabilizing additives, naked, unchargedparticles usually undergo very rapid aggregation. Colloidalparticles have indeed a natural tendency to collide witheach other, being subject to Brownian motion. These.collisions happen to be aggregative due to the action ofLondon-Van der Waals attractive forces between thecolloids. The resulting aggregates are morphologicallycompact and ti~tly bound. Potentially, these aggregatesrepresent a major source of defects in structural ceramicsand in magnetic media. Colloid stabilization is aimed atpreventing this type of aggregation.

. In order to sta~ilize a dispersion, it is necessary toproYIde stron~ repulsive forces between the particles. Twobasic mechanisms are ~enerally accepted as be~ able toprovide such stabilization (Sato and Ruch, 1980): 1.Electrostatic stabilization and, 2. Sterle stabilization.

ADVANCED MATERIALS6

STERIC ST ABILIZ~ TIO~: When two particleswith a layer of macromolecules adsorbed on their surface

approach one another, the overlap of the adsorbed layercreates an osmotic pressure which keeps the particles apart.This mechanism known as steric stabilization can also beachieved by attaching short chains to particle surfacesthrough reactive end groups, giving rise to "hairy" particleswhich cannot approach one another closer than thecombined thickness of the two adsorbed layers.

>E

-J<t~Z\&It-oA.

<t .t-\&I .N Recently there has been an increasing awareness of

the value in applying dispersion science to ceramicsprocessing. A particularfy important example is colloidalstabilization using macromolecular (polymeric) additives.In the presence of p<?lymers, a colloidal dispersion canundergo charge stabilization, steric stabilization orflocculation. These f henomena are always attributed to theextent and nature 0 the ~olymer adsorption on thesubstrate, type and magrntude of the surface charge on theparticles, and the conformation of the adsorbed polymer atthe solid-liquid interface.

. i4 5 6 7 8 9 10 11

pH

Comparison of stability of alumina suspensionswith zeta potential of the particles at various pHvalues in the presence and absence of polymer.

I . . i; --. ,8-

Long chain ionic surfactants can also adsorb onmineral particles and cause destabilization of suspensionsto a extent ~reater than inorganic ions can. In Fig. 6.stability ratio of an alumina-sodium dodecyl sulfonatesystem is shown along with sulfonate adsorption density andthe zeta potential (Somasundaran and Fuerstenau, 1966). Itshould be noted that the ionic strength of the system ismaintained at a constant level of 2 x 10-3 M under allconditions and therefore the observed changes inelectrokinetic and stability isotherms are not due to thedouble layer compression one gets with the addition of asalt. The sudden increase in the rate of adsorption of thesurfactant at about 10-4 M has been attributed to lateralassociation between adjacent hydrocarbon chains of theadsorbed surfactant ions and this process has been termedhemimicellization since the aggregates formed areanalogous to two-dimensional micelles. Recently they havebeen called solloids (colloids on.solids) to include othertypes of adsorbed aggregates.

It is generally accepted that, in non-polar media.charge formation by ionic surfactants is due to theiradsor:-ption on the colloid surface followed by dissociationof a few of the adsorbed ions (Novotny, 1981). This weakdissociation leads to sufficient surface charge to induceelectrostatic stabilization. An im~rtant aspect to be notedis that in non-polar media. dispersions can be drasticallyaffected by mmute amounts of impurities, especially thosethat influence the surface charge. Presence of water,therefore, is very critical because of its ionizing properties.In Fi~re 7, the effect of water on a stable dispersion ofalumma in cyclohexane is illustrated (MalbreI andSomasundaran. 1989). Aerosol OT as surfactant was addedin this case to disperse the particles in concentration suchthat a monolayer of surfactant was present on the aluminaparticles. Figure 7a shows the settling rate of the alumina-supernatant interface as a function of residual waterconcentration at~o different surfactant concentrations(8.5 and 26 x 10- M). Figure 7b shows the correspondin~water adsorption isotherms. As the water concentration Inthe system is increased, the suspension exhibits a succession

Settling index, adsofP;tion density andelectro f horetic mobility of alumina - sodiumdedecy sulfonate system as a function of sulfonateconcentration.

MINERAL PROCESSING APPLIED TO ADVANCED MATERIALS 7

ADVANCED SEPARAllON TECHNIQUES

The basic raw materials necessary for the productionof advances materials are present in the earth's crust butoccur in the form of finely disseminated ores together withunwanted impurities. Conventional separation techniquesus~ such processes as flotation and flocculation fail m thecolloidal and micron ranges. In the mineral industry,ma~netic separation has been successfully employed butlimited to a few systems primarily because relatively fewminerals exhibit magnetIc responses of practical use withthe feeble magnetic forces offered by available magnets.The emergence of powerful magnets with higher gradientshas enabled wider application of magnetic separation atleast on a laboratory scale recently in the mineral industry.Superconducting m~ets can produce extremely intenseand uniform magnetic fields wIth lower powerconsumption. By incorporating these superconductingmagnets into separators it is possible to have .large workingvolumes to accommodate finer as well as coarser particlesat high capacities. Commercial application of high gradientmagnetic separation has been limited so far, to ourknowledge, to the beneficiation of kaolin. Use ofextraneous reagents to pretreat the ore to generate areas ofhigher magnetic susceptibility might possibly extend theapplication of the magnetic separation.

Combining the principles of mineral process~ andthe discovery of superconducting magnets it should bepossible to design advanced separation techniques usingsuper intensity high gradient magnetic equipment toachieve separation of colloidal and ultrafine YarticuJates.Herein possibly lies a potential application 0 mineralprocessIng which if properly harnessed could lead to theelimination of several current limitations in the separationindustry.

Fig.7 (a) Effect of water on the alumina in cyclohexanesuspension settling rate.

(b) Adsorption of water on alumina from AerosolOTjcyclohexane solution.

of flocculated and dispersed states. At low waterconcentrations, the settling occurs rapidly. The onset ofstabilization corresponds to a sh~ increase in the amountof water adsorbed on alumina, showing that the adsorptionof water indeed plays a critical role in the stabilizationphenomenon. Even though a monolayer of surfactant isadsorbed on alumina throu9;hout the entire range of waterconcentration studied, stabilization takes place only whentraces of water are added to the system. Smce thestabilization by Aerosol OT is considered to be due to apartial dissociation of the surfactant at the interface, it isreasonable to conclude that water is required for thedissociation of ions to take place in non polar media. Athigher water concentrations, a sharp increase in thesuspension settling rate at both surfactant concentrationsstudied is seen. However, as the surfaCtant concentration isincreased, the water concentration at which flocculationtakes place is also found to increase.

CONCLUSIONS

Ultrastructure processing of crystalline ceramicsbegins with the controlled formation of ~wders, andcontinues through controlled particle packing todensification. In this paper, the application of mineralprocessing techniques to these features have beenconsidered. Parameters critical to fine grindin~, grindingefficiency and dispersion stability have been discussed.Finally the potential of magnetic separation whensuperconducting magnets are incorporated is also outlined.

ACKNOWLEDGMENT

Financial support from the National ScienceFoundation (MSM-86-17183) and the New York Miningand Mineral Resources Research Institute (NY MMRRI) isgratefully acknowledged.

To achieve reliability and reproducibility in ceramicsprocess~ uniformly packed green microstructures mustbe obtained. Uniform particle packing, the consequence ofa stable dispersion of uniform-size powder, not onlyinfluences sinterability, but also results in uniform,controlled shrinkages with minimum flaws and microcracks.Optimum control over the dispersion stability is aprerequisite for achieving the desired results. Factorsinfluencing dispersion stability, a.8&lomeration and settlingare outlined in the above discussIons. Advances in the fieldof interfacial technology will prove beneficial in the designof better processing techniques.

ADVANCED MATERIALS-8

REFERENCES

Bleier, A., 1984, in Ultrastructure Processin& of CemP1i~.Glasses and Comoosites, L.L.Hench and D.R.Ulricheds.. John Wiley, New York, p 391.

El-Shall, H.. and Somasundaran, p.. 1982, "Mechanisms ofGrinding Modification by Chemical Additives:Organic Reagents", SME Preprint, pp 82-167.

Gregg, SJ.. 1968, Chem.lnd., 11, pp 611-617.

Huang, Y.B.. Tjipangandjara, K.F., and Somasundaran, P ..1988, "Correlation of Alumina Flocculation withAdsorbed Polyacrylic Acid Conformation" in~Int. Sxml!- nn the r:roo~~~n and Pr~~nB ~f Fin~Particles, Montreal, AJ. Plumpton Ed., CaD. Inst. ofMining & Met., p 269.

Ichinose, N., 1987, Introduction to Fme Ceramics, JohnWiley, New York.

Lin, IJ., and Mitzmager, A, 1%8, Trans.AIME, 241, pp412-418.

Un, IJ.. and Somasundaran, P., 1972, Powder Tech., 6, pp171-180.

Malbret. C.A., and Somasundaran. P., 1989, accepted forpublication in J. CoiL Inter.Sd.

Novotny, V., 1981, Colloids and Surfaces, 2 P 373.

Oliver, J.F., Huh, C., and Mason, S.G., 1980, Colloids andSurfaces, 1, pp 78-104.

Reed, J.S., 1988, Introdu~ion to the Princi~les of CeramicProcessing, John Wiley, New York

Sato, T., and Ruch, R., 1980, Stabilization of ColloidalDis~ersions ~ PolYmer Adsor1}tion, Marcel Dekker,New York

Somasundaran, P., and Fuerstenau, D. W., 1966,J.Phys.Chem., 70, p 90.

Somasundaran, P, and Un, IJ., 1972, Ind. Eng. Chern. Proc.Des. and Dev., II,pp321-331.

Somasundaran, P.,1975,AIChESymp.Series, 71, p 1.

Somasundaran, P, 1976, in Research Needs in MineralProcessin&, P .Somasundaran and D. W.Fuerstenaueds., New York, pp 125-133.

Somasundaran, P, 1978, "Theories of Grinding", in CeramicProcessin2 Before Firin2" , G. Y .Onoda and L.L.Hench-edS;fOhDWiley, New York, pp 105-124.

Somasundaran, P., 1980, in Fine Particles Processin2 - Vol.2, P .Somasundaran ecf:;-AIME New Y ork,pp 947-976.

Somasundaran, P, Mining Eng., 36, 1984, pp 1177-1186.

Somasundaran, P and Roussev, R, 1985, in PowderMetallur2V and Related Hi2h Temoerature Material~P .Ramafiishna ed., Oxford-and IBM Publishing,pp59.-68.