PROCEEDINGS OFTHE 7TH INTERNATIONAL MINERAL PROCESSING SYMPOSIUMiSTANBUL/TURKEY /15-17 SEPTEMBER 1998

Edited by

Suna Atak, Giiven anal & Mehmet Sabri

Innovations in MineralandCoa/Processing, Atak. ()nal& ge/ic(eds)~ 1998 BaJmIma. ftJtt8Idam.ISBN9058090132

Aocculation and selective flocculation - An overview

P. Somasundaran & Kalyan K. DasNSF 11Uc.'R c.'enter for Advanced Studies in Novel Surfactant\'. Henry Krumb School of Mine.\'. Columbia

Uni\'ersity. N. r. U.SA

ABSTRACT Flocculation/stabilization has recently become an important step in processing offine particles inmineral beneficiation. In today's world of high technology, the demand for processing of fine particulates isever- increasing with more stringent quality requirements on the finished products Flocculation/stabilization ofcolloidal systems find widespread applications in addition to minerals processing, in as diverse areas asmaterials processing, food and beverage industry, health care and personal care industries A number of factorscontrol and govern the polymer induced flocculation/stabili7.8tion processes This paper discusses some of theissues involved in flocculation and presents some of the recent advances made in the field of mineralsprocessing

1 INTRODUCTION

Treatment of the fine particulates for the recovery ofmineral values as well as for the colloidal processingof ceramic suspensions are of importance in fineparticle technology. This is due to the dwindlingmineral resources worldwide as well as ever-increasing need for manufacturing of specialitymaterials for high technology applications Asignificant proportion of the world's mineral valuesare lost as slimes in the mineral processing industries(Sresty and Somasundaran, 1978; Gebhardt andFuerstenau, 1986). To optimally recover finevaluable particles, flocculation and selectiveflocculation induced by natural and syntheticpolymers have received increased attention in the lastseveral years Considerable progress in bothfundamental understanding and practical applicationshas been made due to the development of newgroups of tailor-made polymers, more comprehensiveunderstanding of the mechanisms involved, and bettercontrol of the operating parameters (Somasundaranet al 1996) For several decades, inorganicelectrolytes, natural polymers and synthetic highmolecular weight polymers have been used ascoagulants or flocculants for clarification of effluents,acid mine drainage and treatment of paper, textile,

sugar and other industrial wastewater processingapplications Research has shown that interactionsbetween the functional groups on particulatesurface(s) and those on polymer are responsible forthe adsorption and subsequent flocculation and canbe appropriately controlled by tailoring polymerswith specific functional groups suitable for theparticular application The effectiveness of a polymerfor a given flocculation system depends on severalparameters e.g. the polymer dosage, amountadsorbed on the solid surface(s), molecular weight ofthe polymer, particle size, shape, porosity, ionicstrength of the system, the electrostatic interactions,electrokinetic properties and more importantly, theconformation of the polymer at the solid-liquidinterface. Polymeric reagents influence significantlythe flocculation/ stabilization behavior of particulatesuspensions The following recent examples illustratethe diverse areas of research involving study ofparticle-polymer interactions

The stabilizing action of sodium polyacrytate onthe colloidal dispersion of calcite has been studiedusing viscosity, ion concentration and electrophoreticmobility studies The stabilizing action was evidentfrom the sharp fall in viscosity observed at low levelsof polymer addition and the invariance of this lowviscosity throughout the remainder of the

81

concentration region (Rogan et al 1994) Besidesapplication in materials processing industries,polymeric flocculation also finds widespreadapplication in paper and pulp making industry Inone interesting study involving flocculation of woodpolymers (unbleached thermo-mechanical pulpsuspensions) with linear cationic polyacrylamides ofdifferent charge densities (but same molecularweight), formation of the polyelectrolyte complexeswas observed and its role in the destabilization of thecolloidal particles was delineated. The properties ofthe polyelectrolyte complexes was found to dependon the charge density of the cationic polyelectrolyte( Nylund et aI, 1995 ). In another interestingapplication involving DNA, the flocculation ofpositively charged latex micro spheres and negativelycharged single-stranded DNA of varying chainlength from 3-mers up to 1400 mers wasinvestigated The coagulation kinetics at the optimumpolymer dose was found to be diffilsion limited Theauthors have proposed a mechanism which dependsupon the relative spacing of charged groups on thepolymer backbone and particle surface, the size andinherent flexibility of the polymer in question and thenature of the initial polymer-particle interactions(Walker and Grant, 1996)

Attempts have also been made in the recent pastto develop predictive mathematical models forflocculation and selective flocculation processes(Moudgil and Behl, 1992; Behl et at, 1993; Hogg,1994) . Many mechanisms involving particle-particleand particle-polymer interactions have beenproposed. In addition to chemical factors,hydrodynamical conditions in flocculation/dispersionare found to have significant effect, which isespecially important for practical applications, wheremechanical agitation is used to ensure proper mixingof added reagents, to aid dispersion prior toselective flocculation and then to promote flocformation and growth ( Hogg, 1992).

adsorption of polymer, or by using activators thatinduce adsorption of polymers on desired minerals

(Somasundaran, 1978)Adsorption of polymers on solid surfaces is

dependent on polymer properties such as molecularweight and configuration, distribution of functionalgroups, solid properties such as surface charge,ox:idation state and solution properties such as ionicstrength, temperature and the solvent power for thepolymer It is this dependence of adsorption that caneffectively be used for obtaining selective adsorption

(Somasundaran, 1979) Aggregation betweenparticles can take place if the interparticle repulsiveforces do not exceed the corresponding attractiveforces (Figure I). Therefore, aggregation can beachieved if the solution properties are adjusted suchthat the electrostatic repulsive force is minimized.Dispersion can be affected by maximizing theelectrostatic repulsive forces

The various forces responsible for the adsorptionof the polymers on the mineral surfaces resultprimarily from three types of bonding namely,electrostatic, hydrogen and covalent bonding (Srestyet al. 1978). The solution chemistry conditions, therheology of the solid suspension and the polymerproperties dictate the nature of interactions Oftenseveral mechanisms may be operative at the sametime in controlling the adsorption/flocculationbehavior of the system.

Adsorption of the polymers on the particle Surfaceis a complex phenomenon which can be considered

+AEP\A.SlOH

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ATTRACTIONetERGY2 THEORETICAL ASPECTS

2 I Adsorption of polymers a,1d .'ilIrface coverage

Polymer molecules can be made to selectively adsorbon particles by (i) adjusting the chemical compositionof the suspending medium, (ii) introducing intopolymer. active functional groups that will formcomplexes or salts with the metal atoms on thesurfaces of the desired minerals and (iii) usingdepressants such as sodium silicate that would adsorbon the undesired mineral surface thereby preventing

Figure I Sum of repulsive and attractive interactionsbetween particles as a function of distance ofseparation between them under various conditions

82

polymer but remains well dispersed with the anionicpolymer Polymers can also flocculate similarlycharged particles provided the magnitude of theinterfacial potential is not very high Under suchconditions, the electrostatic repulsive forces are notstrong enough to prevent the particles from comingwithin the polymer bridging range of each other.

to be an irreversible process particularly in dilutesuspensions that are often encountered in flocculationsystems Generally, the residual concentration ofpolymers in the system is only a fraction of the initialconcentration thereby suggesting a strong adsorptionaffinity of the polymers on the particle surfaces(Linke and Booth, 1960) Some of the issues relatedto the bonding characteristics in the flocculationsystem are briefly addressed here H):dro~en bonding

Surface oxygen species on the particles may formhydrogen bonds with the nonionic polymers therebyassisting the flocculation process Such adsorptionmechanism is possible when the electrical doublelayer repulsions are not very strong.

Electrostatic bonding

This is a particularly probable mechanism when thecharge density of the polymer is relatively high andopposite in sign to the mineral particle The surfacepotential of the solid particulates gets neutralized bythe adsorption of the oppositely charged polymerspecies even at a very low concentration of thereagent and this can even lead to charge reversalEffective flocculation by interparticle bridging canoccur even when the zeta potential is non-zero(either positive or negative) and the electrostaticrepulsive forces are not very high The role ofelectrostatic forces was illustrated in the case offlocculation of negatively charged synthetic silica(Biosil-A) with cationic, Nalcolite-610 andstabilization with anionic polymer Separan AP-30,Figure 2 (Sresty et ai, 1978 ) It may be noticedfrom this figure that the silica particles undergoexcellent flocculation in the presence of the cationic

Covalent bonding

The most widespread mechanism of flocculation maybe the result of strong covalent bonding between themetal ions present on the particulate surface(s) andthe active groups of the interacting polymer (s)Flocculation of kaolin by polyacrylamides wasreported as a result of the formation of salt-typecompounds by reaction between the polymer and theCa 2- ions present in kaolin (Michaels and Morelos.

1955)Sufficient coverage of the particulate surface

facilitates polymer bridging leading to a progressivebuild-up of the fine particles into three-dimensionalnetworks which are reported to be stabilized uponcomplete coverage (LaMer and Smelie, 1962) Theseauthors predicted existence of an optimum polymeradsorption layer and reported a value of 05 surfacecoverage by the polymer molecules for maximumflocculation. Subsequently, the concept of stericstabilization due to complete coverage of theparticles was suggested [t was reasoned that thestability could be disturbed if the Gibbs free energydue to interpenetration of the polymer chain isnegative (Napper. 1970) The interactions leading tothe change in entropy and enthalpy of the free energycomponent due to the interpenetration of the polymerchains depend on several factors. namely, theinteractions among the polymer, solvent and theinorganic electrolyte species (Napper. 1970; Napperand Netschey. 1971) In such cases flocculation canproceed if the increase in entropy due to the releaseof S(uvent molecules outweighs the loss of entropydue to interpenetration of the polymeric chains TheGibbs free energy change is considered in such casesto be determined essentially by the change in entropydue to the release of the solvent molecules and due tothe decrease in randomness of the polymer chain and

Figure 2 Percentage of synthetic silica settled as afunction of concentration of Nalcolyte -610 andSeparan AP- 30, reagentizing time 30 seconds;settling time: 45 seconds (Sresty, Raja andSomasundaran, 1978 )

83

by the enthalpy of dissolvation of the polymer chain.Therefore, for flocculation to occur , the increase inentropy due to the release of solvent moleculesshould outweigh the loss of entropy due to theinterpenetration of polymer chains and increase inenthalpy of dissolvation

three interactions, VT determines if a collision willbe aborted or be successful collision leading toadhesion. DL VO ( Derjaguin and Landau, 1941;Verwey and Overbeek, 1948) and HHF (Hogg,Healy and Fuerst~ 1966) theories considerthe London van der Waals attractive energy andelectrical double layer energy and yield anexpression for the total energy, VT, which showsthat it will be attractive at large and smalldistances In the case of polymeric flocculation,the interaction between polymer layers adsorbedbetween two particles and also between such alayer and a bare particle playa predominant role.Of particular interest in the case of polymericflocculation is the nature and magnitude of V s,which can be positive or negative depending onthe solvent power of the medium for the exposedportion of the adsorbed layer Interpenetration ofthe branches of the polymer adsorbed on differentparticles will be favorable if the solvent power isnegligible, thus assisting flocculation

i\~ = i\Ht - T i\ s.

L\H( arises from polymer dissolvation and L\S(increases from released solvent molecules anddecreases from interpenetrating polymersFlocculation under complete surface coverage ispossible only if the free energy of interpenetration ofadsorbed layers, ~ is negative. It has been foundthat partial coverage allows the free loops of thepolymers adsorbed on one particle to adsorb on thebare region of another particle and thus produceinterparticle bridging and hence cause flocculation

2.2 Physico-chemica/ collditilJl/S

3 IMPORTANT FACTORS IN FLOCCULATIONThe physico- chemical conditions responsible forselective aggregation in solution are (a) collisionsand forces governing collision rate (b) adhesionduring collisions; repulsive and attractive forcescontrolling the probability of adhesion (c) effect ofdissolved mineral ions and other ions in water on thesurface properties of individual minerals

3. I PreJreatmenl

Collisions:

Collisions generally result from thermal motions thatare determined by particle size, temperature,viscosity etc. Controlled agitation by stirringenhances both the rates of collision and adhesionFlocculation assisted by such external forces areknown as orthokinetic flocculation Collision ratesbetween particles of different sizes can be calculated(Levich, 1962)

Adhesion

Flocculation can be enhanced or retarded by externaladdition of ions which subsequent to adsorption onthe mineral surfaces either activate or depressflocculation Selective flocculation of hematite -quartz using anionic polyacrylamide has beenreported to be effective after the addition ofCalgonR and sodium fluoride (Read, 1972) Treatmentprior to the flocculation may also consist ofcomminution and liberation of constituentcomponents, mechanical and chemical dispersion,and any other process to remove possible coatingmatters. In the flocculation system, dispersants playasignificant role in obtaining full liberation particularlyfor removing interparticle adhesion However, thecomplexity of polymer-polymer interactions can havea detrimental effect on the successive flocculation

Many investigators have found that methods usedsuccessfully for the separation of synthetic mixturesof pure mineral components often fail when real oresamples are used The primary reason for the loss inselectivity has been attributed to the coating andcementing matters on one or more solid surfaces. Ithas been shown that removal of surface coating andorganic cementing matters can significantly enhanceselective separation of francolite from phosphateslimes Pretreatment used to remove the coatinginclude solubilization using acid or cornplexing agentand oxidation by chemicals like hydrogen peroxide

The important interactions that decide whether or notparticles will collide and adhere are thefollowingi) London Van der Waals attractive forces, V"

ii)Double layer forces that can be attractive orrepulsive in nature, V.1 and

iii)Steric forces that arise from the overlap ofadsorbed layers, V s These forces can also beattractive or repulsive in nature depending onwhether the adsorbed layers are in contact withthe solvent or not. The combination of the above

84

and ion exchange with excess salt (Anderson andSomasundaran, 1993) One way to prevent 'slimecoating' and 'hetero-coagulation' is by the use ofdispersants to prevent particle-particle adhesion Theobjective criterion in the selection of dispersants isthe minimum interference by these reagents on thesubsequent adsorption of the tlocculant Bothinorganic (e.g sodium hexa-metaphosphate) andorganic ( eg low molecular weight polymers likeP AA, PEO and PVP) reagents are known to beeffective dispersants for mineral systems

32 Conditioning time

The conditioning time of the mineraI with thepolymer determines the extent of surface coveragesignificantly Prolonged conditioning may result in re-dispersion possibly due to the saturation coverage ofthe particle surface by the polymer molecules(Michaels, 1954; Kitchener, 1972) The role ofpolymer conformation is important in inter-particlebridging mechanism. Extended chains of thepolymers have been found to be more effective inbridging The coiling of polymer molecules ismaximum at their respective iso-electric points andany excess charge resulting from the solutionconditions may lead to an extended conformationthus facilitating inter-particle bridging

in some others ( Sato and Yoshida, 1967). Themobility of the polymer molecules and solvent powerof the suspending medium depend on the temperatureand affect significantly the adsorption behavior of thepolymers on the solid surfaces It is generallybelieved that when the suspending medium is a poorsolvent for the polymer, the polymer adsorption rateis greater The conformation of the polymer isaffected by the nature of the solvent and, hence, to agreat extent determines the chances of collision ofthe polymer molecules with the particulate materialsParticle morphology and surface chemical propertiesof the solid also contribute to the kinetics and extentof adsorption (Kraus and Dugone,1955; Frisch et aI.1959; Linke and Booth, 1960; Stromberg and Kline,1961)

Polymer adsorption and flocculation involvesseveral steps eg transport of polymer molecules tothe suspended particles followed by initial attachmentof the polymer chain, which then rearranges to givean equilibrium configuration (Akers, 1972). Collisionof the particles with the adsorbed polymers result information of flocs which may subsequently break-upAdsorption and flocculation steps have beenvisualized (Gregory, 1988) as either due to diffiIsion(rapid adsorption) or induced by the motion of thefluid in the system (slow adsorption). The effect ofcharge density and molecular mass on theflocculation kinetics of silica suspensions has beeninvestigated and it was found that the rate of polymer(copolymers of acrylarnide and dimethylarninoethylacrylate) adsorption could be slow compared to theparticle collision rate at low charge densities and lowpolymer molecular weights (Lee and Gregory, 1991)

Differences in the rate of adsorption or in the rateof other rate controlling steps could result fromchanges in the process conditions eg thereagentizing times in hematite-quartz system usingSeparan AP -30 Maximum flocculation for hematitewas observed with about 60 seconds reagentizing atneutral pH whereas for quartz the correspondingtime was found to be - 600 seconds (Sresty etal,1978 ) Using a simple technique which involvedimmersion of a glass plate in cationic polymersolutions for short periods followed by deposition ofsilica spheres on the adsorbed polymers to decoratethem for monitoring, , it was shown that adsorptioncould be complete in less than one minute In the firstfew seconds, there was very little adsorptionfollowed by a .. catastrophic" sudden increase inadsorption, FigureJ (Somasundaran and Sivakumar1988; Onoda and Somasundaran, 1987)

33 Kilteti(,'s

Rate of uptake of polymer by the solid substrate isinfluenced by molecular weight, structure andconfiguration of the polymer molecule, polymerconcentration, agitation, temperature, ionic strengthand pH Rate of adsorption increases with increase inagitation and polymer concentration ( Stromberg andKline, 1961) The kinetics of the adsorption of thepolymer molecules on the particulate surfaces isgoverned by the rate of diffiJsion of these moleculesto the surfaces It is also important to note that thekinetics and the extent of adsorption are dependenton the porous nature of the particles With porousparticles, the extent of adsorption may also varydepending on the solution properties of the systemand hence profoundly affect the adsorption behaviorThe solution chemistry conditions as well as thepolymer properties including the molecular weightand conformation of the polymer, its dosage and thetemperature are some of the important parameterswhich govern the diffiJsion and adsorption of thepolymer on the surface However, an increase inmolecular weight of the polymer can result in anincrease in adsorotion in ceratin cases and a decrease

85

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, f 101Ol'.u ~.cf fll(. N:

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Figure J Adsorption of polyacrylamide ( PAM) onglass as a function of time for different PAMconcentrations (Somasundaran and Sivakurnar, 1988)

quartz. In a similar fashion, selective flocculationbetween galena and calcite failed in the absence ofother additives since the zeta potentials of theseminerals are not sufficiently high at natural pH toproduce required dispersion of fines prior to selectiveflocculation (Yarar and Kitchener, 1970)

The selectivity of starch in its interaction with ironoxide depends to a large extent on the functionalgroups present in the starch molecule (Iwasaki et al.I %9) Selective flocculation of a hematite-quartzmixture using causticized com starch was carried outin our laboratory. It was found that the recovery ofiron oxide increased sharply with flocculantconcentration The perfonnance of the selectiveflocculation of hernatite-quartz mixture is shown inFigure 4 where separation index is plotted as afunction of the starch concentration.Separation index takes into account both therecovery and grade of the mineral of interest and isdefined as " [ % of valuable mineral recovered in the

concentrate + % of gangue rejected in tailings 100 ]II ()()" A value of unity denotes complete separationwhile a value of zero implies total failure of selectiveseparation. The results show that a satisfactoryseparation index of 0 7 could be obtained after a one-

4 SELECTIVITY

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~c:=c

A serious problem often faced by the practicingengineers in devising efficient selective flocculationschemes is the lack of selectivity in the adsorptionof polymers on the desired mineral surfaces due tothe non- specificity of the commercially availablepolymeric reagents This has necessitated research inthe realm of selective flocculation by incorporatinginto the polymer chain specific functional groupsthat can complex or chelate with the solid surfacesand thereby cause selective adsorption/flocculation.

The success of separation by selective flocculationdepends on the liberation of the particulate mattersfrom each other. preferential adsorption of theflocculants on the particles of interest. bridging ofthe flocs by the adsorbed polymer molecules andeffective separation of the flocculated mass from thesuspension without re-dispersion of the flocs Thepreferential adsorption of a polymeric reagent in acomplex system often depends on the interfacialpotential on the particle surface, fractional coverageof the particle surface by the flocculant, the chemicalnature of the flocculant and the solids present in thesystem

In a study involving separation of galena fromother minerals, it was found that galena flocculationwith anionic polyacrylamide could be enhanced bythe addition of lead nitrate and retarded by sodiumsulfide It was also found that galena dried in contactwith air did not undergo selective flocculation fromquartz and this was attributed to zeta potential ofgalena being zero in a medium containing oxidizedlead salts and hence undergoing coagulation with

Figure 4 Separation index achieved from theselective flocculation of hematite-quartz mixture as afunction of concentration of starch, reagentizing time30 sec, settling time; 45 sec

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3 . ~---,.,..O. 0 O_~EO&65- P«O+ttr4_/.JOPG

202 ~ ~ ~ ~ I~ j4'ZOpH

Figure 5 Recovery and assay of: (a) chalcopyrite and (b) pentlandite

stage cleaning of the flocculated product (Sresty andSoma.'IlJndaran, 1980)

Complexing agents have also been used toenhance the selectivity of flocculation processDiphenylguanidine (DPG), a complexing agent, hasbeen extensively used in the flotation separation ofINCa matte, which consists of CuzS and Ni3Sz dueto its unusual selectivity for copper minerals (Tipmanet al., 1976) The effect of addition of DPG in thepresence and absence of polyethylene oxide (PEO)on chalcopyrite and pentlandite flocculation indicatedthe possibility of selectivity The results of selectiveflocculation tests conducted with synthetic mixturesof chalcopyrite and pentlandite is shown in Figures5a and 5bThe recovery and assay of chalcopyrite is found toincrease from about 700/0 to 900/0 and from 500/0 toabout 600/0 respectively, in the presence of DPG andPEG together throughout the pH range studiedHowever, the recovery and assay of pentlandite wasfound to reduce compared to the results obtainedwhen only PEG was present (Acar andSomasundaran, 1989).

Selective flocculation of hematite from itsmixtures with quartz, calcite etc has been achievedby many investigators ( Usoni et al, 1968; Yarar andKitchener, 1970 Read, 1971) Subsequent to thesuccessful use of many chelating type collectors inthe flotation-separation. applications have beenattempted by the incorporation of some of the activegroups into the polymeric chain (Iwasaki et al,1969; Attia and Kitchener, 1975; Clauss et aI 1976)

Starch and related polysaccharides have beenwidely applied in the mineral processing industry as

depressants, flocculants and selective flocculants(Weissonbom et aI. 1994) Also modified starchesare regularly used in other industries ego in papermanufacturing where it is used as an additive in theparticulate suspension When the molecular weight ofthe starch is lowered, converted starch is obtained.Modification can also be carried out by substitutingdifferent functional groups into the polymer chainNumerous mechanisms for the adsorption of starchand related poly-saccharides onto particles have beenproposed. Hydrogen bonding and hydrophobicinteractions between polysaccharides and paniclesurfaces are mainly responsible for the adsorptionHowever, more recent evidences have shown thatchemical interactions may playa significant role(Laskowski et aI., 1991 ). Using spectroscopictechniques, it has been shown that many derivativesof starch such as dextrin, fructose, glucose andsucrose have complexing abilities with metal ions

Polymers and surfactants have also been used incombination with each other for flocculation!stabilization. The stability of suspensions in thepresence of polymers and surfactants are governedby many factors besides the electrostatic interactionsIn the case of stahilizatiOfI, considerations have beengiven to the role of the adsorbed polymer andsurfactant layer on particle surfaces which canproduce repulsive steric interactions (Sato and Ruch,1980 ) For j1occulatiOfI of suspensions, bridging byadsorbed surfactants and polymers is a widelyaccepted mechanism in addition to chargeneutralization In the case of flocculation, severalfactors affect the performance eg flocculant dosage,

87

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molecular weight, charge density, as well as mode ofaddition

5 RECENT ADVANCES

In a study on the selective flocculation of ultra-fineiron ore Weissenborn et aL, 1995 carried out an in -depth investigation of the mechanism of theadsorption of starch and its components namelyamylopectin and amylose onto hematite surfaceUsing OiffiJse Reflectance Infrared FourierTransform (DRIFT) Spectroscopy, these authorsdetermined the nature of the adsorbed species on thehematite surface and also the extent of adsorption Itwas found that wheat starch and amylopectin werestrongly adsorbed on the surface ostensibly by theformation of a surface complex rather thanadsorption by hydrogen bonding. Using the dataobtained from adsorption and thermogravimetricresults together with supportive evidence frominfrared and zeta potential results, the authorsproposed preferential adsorption of starch ontohematite due to the higher concentration of metaladsorption sites They suggested that flocculationoccurs by classical bridging mechanism

Jarnstrom et al., 1995 have studied flocculation ofkaolin suspensions induced by modified starchesThey measured the fluctuations in the turbidity inflowing kaolin suspensions using a fibre-opticalsensor It was found that a cationically modifiedstarch was an excellent tlocculant. The effect of ionicstrength was also studied and it was observed thatthe flocculation response was satisfactory if the ionicstrength was maintained between I and 100 roM.However, it was interesting to note that theflocculation response was poor at I mM NaCIconcentration when the kaolin suspension was pre-treated with Na-polyacrylate. But the flocculationwas satisfactory in presence of Na-polyacrylate athigh temperatures and when the salt and starchconcentrations were high. The authors attributed theflocculation of kaolin by the cationically modified andconverted starch to a bridging mechanism.

One interesting study on the adsorption andflocculation of negatively charged polystyrene latexparticles using cationic modified polyacrylarnides wasconducted by Eriksson et al,1993. In this work.conditions were chosen such that the adsorbedpolymers approached equilibrium conditions beforethe occurrence of appreciable flocculation Polymerswith very low charge density was found to formloops and tails at the latex surface and theyflocculated the latex particles by bridging But

polymers with higher charge density adsorbed in aflat configuration and flocculation occurred bycharge neutralization These authors suggested thatflocculation could also result from the long rangeattraction between the positive and negative chargecenters on the particles They found that at maximumadsorption of highly charged polymers the latexparticles acquire a high positive charge. This has beenargued as the mean distance between positivecharges on the polymer being smaller than thedistance between the charges on the latex, such thation binding of the polymer to the ionized latexsurface groups can not take place without thesimultaneous attachment of an excessive positivecharge to the latex surface.

Conformation and orientation of the adsorbedmolecules also playa major role in polymericflocculation (Tjipangangjara and Somasundaran,1991). However, the exact role of the conformationis stiD not well understood mainly due to the complexnature of the adsorption of the polyelectrolytes andthe lack of reliable techniques to monitor theconformation in situ. Fluorescence and Electron SpinResonance spectroscopy techniques are powerfultools to probe the micro structure, orientation andconformation of adsorbed surfactant and polymerlayers ( Chander and Somasundaran, 1987; Chanderet aI 1988, Somasundaran and Yu, 1994) Thedetails regarding the application of these techniquesare beyond the scope of this paper and can be foundin the above references

Enhanced flocculation obtained by maneuveringpH shift could also be obtained by using acombination of polymers, one of which can adsorbstrongly and another that can bridge A combinationof two or more polymers produces synergism in theflocculation or stabilization of suspensionsFlocculation of alumina suspensions with sequentialaddition of polystyrene sulfonate and cationicpolyacrylamide was found to be superior to thatobtained by a single polymer The cationic polymerdid not produce any flocculation by itself while theanionic polystyrene sulfonate had a minor effect.However, excellent flocculation was obtained withthe dual polymer even when the charge on aluminaparticles were significant, Figure 6 (Yu andSomasundaran. 1993)From adsorption studies. it was found that cationicpolyacrylamide when used alone, does not adsorb onthe alumina surface due to electrostatic repulsionHowever, both polystyrene sulfonate and poly-acrylamide co-adsorbed completely on alumina whenthey were used in combination with each other,Figure 7( Yu and Somasundaran, 1993).

88

R8IiduaI PSS COftC. ~

Figure7 Adsorption isotbenn of individual polymersand their combination on the alumina surface; fJ ,premixture of polystyrene sulfonate and cationicpolyacrylamide; V , 12 million mol wt. polystyrenesulfonate; 0 , 4600 mol. wt. polystyrene sulfonate; 0, cationic polyacrylamide (Yu and Somasundaran,1993)

Figure 6. Comparison of flocculation response ofalumina with double tlocculants to that with singletlocculants; A. , polystyrene sulfonate followed bycationic polyacrylamide, 0 , cationic polyacrylamidefollowed by polystyrene sulfonate; V, premixture ofpolystyrene sulfonate and cationic polyacrylamide; 0,4600 mol wt. polystyrene sulfonate alone, O,cationicpolyacrylamide alone

arising due to interactions of the various processparameters, the industrial success has been ratherlimited The benchmark Tilden Iron Ore mine'ssuccess in selective flocculation is the first industrialapplication of this technique Considering that anumber of parameters govern adsorption/flocculationof the fine particulates, it is necessary to make anintegrated effort to correlate the influence of theprocess parameters as the development of futureselective flocculation technology wiD essentially bean evolutionary process

This co-adsorption is attributed to the interactionof complexes between the two at the solid/liquidinterface. The superior flocculation in the case ofdual polymers was explained on the basis ofelectrostatic attraction of the anionic polystyrenesulfonate on positively charged alumina surface. thepre-adsorbed polymer molecules acting as anionic"anchors " for the adsorption of the long chain

cationic polymer and cause good interparticlebridging and hence excellent flocculation

ACKNOWLEDGMENTS6 CONCLUSIONS

The authors acknowledge the financial supports ofNational Science Foundation (rNT-9415417) andIUCR Center for Advanced Studies in NovelSurfactants (NSF/EEC-9804618)

Flocculation /stabilization of colloidal suspensions isof great technological importance Based on ourknowledge of colloid and surface chemistry,significant achievements have been made in obtainingdesired end results In the case of mineral processing,selective flocculation is a promising techniqueAttempts have been made to exploit this technologyto achieve selective separation both in the laboratoryscale as well as at a industrial level However,because of the enormous complexity of the system

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fi

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Frisch,HL., M. Y Hellman & J.L.Lundberg, 1959Adsorption of polymers: polystyrene on carbon,J Polym. Sci, 38 441

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