reprinted frcxn: future trends in polymer science f and

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Future Trends in Polymer SciencePolymers: Commodities Qr

(ITPR, CNR, Napels, 1984)

Reprinted frcxn:and Technology.Specialities?

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APPLICATIOIS OF POLYMERS II MIIERAL PIOCESSI8G

P. Somasundaran, Y.H.C. Wang and S. AcarSchool of Engineering and Applied Science

Columbia UniversityNew York, N.Y. 10027, U.S.A.

Polymers are being recently used in min.ralprocessing for flocculation of fines, selectiveflocculation-flotation for beneficiation of finelydispersed ores and thickening. There is alsodevelopmental work for their use as grinding aids.While in majority of cases bulk flocculation issufficient, polymers which interact selectively withdesired minerals are required in floc- flotation.Interaction of polymers with minerals is a complexfunction of system variables such as pH, ionicstrength and mode of addition. In addition inheterogeneous mineral systems it is also influenced bythe presence of dissolved mineral species. In thispaper the principles governing these interactions willbe discussed along with recent applications in theprocessing of various minerals.

INTRODUCTION

Polymers are being increasingly used in recent timesin v.rious mineral processing oper.tions includingflocculation and dewatering, effluent treatmen~ andselective flo~culation and flotation. They have evenbeen shown to have significant effect in the grindingof miner.ls. In many of these operations, theirperform.nce is governed to a large measure, by their.dsorption on mineral surfaces. Adsorption ofpolymers on minerals itself is governed by a multitudeof system properties such as pH, ionic strength,

APPLICATIONS OF POLYMERS IN MINERAL P~OCESSING 209

temperature, zeta potential and surface hydration ofthe solid, and molecular weight and nature and chargeof the functional groups of the polymer. In naturalmineral systems, in addition, the interaction ofpolymers with mineral surface is also influenced bythe presence of dissolved mineral species. The effectof a few of the above parameters has been studied fora limited number of systems (1-21) but there is notenough information on the mechanisms by which all therelevant variables determine either their adsorptionon minerals or the flocculation that results from it.

In this paper, some of the major applications ofpolymer adsorption in mineral processing will bediscussed along with problems involved reviewingmaterial presented elsewhere (22-29).

Need for Polymer Use

In mineral processing, more and more fine andultrafine mineral matter is being currently mined andbeneficiated. Considerable amount of fine waste isalso produced duriQg the processing of such mineralmatter. These wastes in the form of slimes andsludges are not easily disposed and contain a largeamount of valuable minerals in it. For example,Sbillion tons of phosphate values have been estimatedto be lost in phosphatic clay. wastes till now andevery year about 40 million tons of slimes isdiscarded. In addition to the values lost in theslimes, a serious environmental problem results fromthis' since the slime cannot be dewatered to anaoceptable solid content. Many methods have beentried to dewater these wastes, but -techniquesinvolving use of polymers appear to hold mostpromise.

Similar problems also occur in the processing ofcopper, tin, tungsten, coal and iron both in terms ofvalues last and the slimes and sludges produced.

Traditional techniques do not work efficiently forrecovering values from ultra fine material and it hasbecome necessary to develop new tech~iques. Atechnique that has been successfully used for at leasttwo mineral systems is selective flocculation usingpolymers followed by flotation. However satisfactorypolymer and processing schemes have not been found foruse in other systems where the fine particle problem

210 P. SOMASUNDARAN. Y. H. C. WANG, ETC.

exists. Successful application of selectiveflocculation in these cases will depend to a largeextent on the development of polymeric flocculantswith functional groups that will lead to selectiveadsorption of the polymers on one or more of the manyminerals in the system. Development of such reagentswill in turn depend on .a full understanding of themechanisms involved in polymer adsorption andresultant flocculation or dispersion.

Polymeric Flocculation

Physico-chemical interactions that are important indetermining flocculation are composed of the Van derWaals attractive forces, electrical double layerforces that can be attractive or repulsive in nature,bridging forces between adsorbed polymer species andparticle surfaces, and steric forces that arise fromthe overlap of the adsorbed layers which also can beattractive or repulsive in nature (26).

While Van der Waals forces arise from Londondispersion forces, Keesom interactions betweenpermanent dipoles and Debye interactions betweenpermanent and induced dipoles and the electricalforces arise from the charging of the particles insolution, bridging forces result from the possibilitythat a long chain adsorbate with several active groupson it can induce aggregation by attaching itself totwo or more particles. It has been considered thatthis type of aggregation can take place only whenparticles are partially covered with polymers. Thishypothesis has not however been experimentallyproven. In fact, bridging should be possible evenwhen the particles are fully covered if somedetachment and reattachment of the adsorbed polymer ispossible. Even though polymers have been consideredin the past to adsorb irreversibly, desorption hasbeen achieved recently under special conditions (29).Steric forces arise from the overlap of the adsorbedlayers and can be repulsive or attractive in naturedepending on whether these layers prefer to be incontact with the solvent or not. If the solvent powerof the surrounding fluid for the adsorbed layer isminimal, then the layers on different particles willshow a tendency to interpenetrate into each other andthereby promote aggregation of the particles. On theother hand, if the solvent power is high, then theadsorbed layers will prefer to be in contact with the

APPLICATIONS OF POLYMERS IN MINERAL PROCESSING 211

medium and there will neither be interpenetration ofthe adsorbed layers nor any aggregation. Theinterpenetration of the layers will take place if thefree energy of interpenetration of polymeric chainsinto each other is nelative. For this condition to bemet, the increase in entropy due to the release ofsolvent molecules upon any desolvat1on or the polymersshould predominate over the loss of entropy of thepolymeric chains themselves and the increase inenthalpy due to the desolvat1on. Since this processis an entrop1c phen08enon, it is clear that thetemperature will have a major role to play. The ionicstrength level will also have a great influence. Ionsdue to their hydration will make fewer water moleculesavailable for solvation for the polymers and this willessentially impart a tendency to the polymer toprecipitate and to interpenetrate. On the basis ofconsiderations of the above forces, one can deriveconditions for flocculation and means for controllingthe extent of it.

The most common method used for flocculationinvloves use of polymers. Such large 801ecules whenadsorbed on the particles can, essentially shift thesurface of contact between them so that the electricalnature of the interfaces, which might not have be'enoriginally conducive to aggregation, assumes asecondary role. It can be seen from Figure 1 that thezeta potential of both the positively and negativelycharged hematite fines is reduced to a .small valuethat is characteristic of the polymer. Under thiscondition, the coated particles will not repell eachother and hence ca~ be expected to flocculate.Indeed, the efficiency of polymeric flocculation canbe expected to depend on, in addition to the extent ofthe adsorption of polymers, also the configuration ofthe adsorbed species.

Polymer adsorption on minerals itself is a verycomplex process since it is influenced by a nu.ber ofpolymer properties, mineral properties and solutionproperties. Forces responsible for polyer adsorptionarise mainly from electrostatic, hydrogen and covalentbonding.

Polymer Charge

of a polymerelectrostatic

In thecharged

largecan

number otbe a major

caseunits,

with abonding

212 P. SOMASUNDARAN, Y. H. C. WANG, ETC

I . 311o-ZWOI/m3 HoC!pH

C 2.7~0.1tI. 1t.2~ 0.1

60> 40e

.; 20c~ 0z

~ -202 -40c... -60wN

.A---'::==i:--- - - ~ -- - - -~.=k--

,J I I I I 'I'" I I

0.1 1 10 100 1000

TOTAL POLYMER CO~ENTRATION, "'t/kt

Figure 1. Zeta potential of hematite as a function ofpolycarylamide concentration.

factor. The role of electrostatic forces indetermining adsorption and flocculation is clearlyseen in Figures 2 and 3 (30). It can be seen that boththe adsorption and flocculation as determined bysettling rate decrease with increasing anionicity ofthe polymer. The effect of the anionicity onadsorption is as~expected for this since the negativecharge on the copolymers should produce a reduction inthis adsorption of the negative kaolinite particles.Flocculation under constant dosage conditions usedhere also decreased with increase in anionicity but itshould be noted that the optimum dosage and theflocculation under such conditions can be differentfor each polymer.

Ionic Strength.

The explanation based on the electrostaticinteractions between the similarly charged polymer andthe mineral particles is supported by the ionicstrength effects seen in Figure 4. When the ionicstrength of the solution is increased, the adsorptionof the anionic polymer increases and becomes equal to


Figure 2. Adsorption isotherms ofpolyacrylamide-polyacrylic acid copolymers ofdifferent charge densities on kaolinite at pH4.5. (30). .

that of the nonionic polymer at 1 kmol/m3 HaCl. Theadsorption of the nonionic polymer itself is notaffected to any significant extent by the ionicstrength increase. Evidently under the high ionicstrength condition, the electrostatic repulsion iseliminated by the compression of the double layer andthe adsorption of the nonionic and the anionicpolymers becomes identical to ea:h other.

.eJ!

On the basis of the electrostatic consideration pHalso can be expected to have a larger effect on theadsorption of the anionic polymer. This is indeedfound to be the case of the adsorption of thepolyac~ylamide and its sulfonated copolymer. (SeeFigure 5). Under alkaline and high temperatureconditions, adsorption of the nonionic polyacrylamidewas found to be reduced and as suggested by theobserved increase in viscosity of the polymersolutions, this effect is considered t~ be due to thehydrolysis of the polymer.

P. SOMASUNDARAN, Y. H. C. WANG, ETC.214

Settling rate of kaolinite as a function of thecharge density of the polyacrylamide-polyacrylicacid copolymers at pH 4.5 (30).

Figure 3.

In addition to the chemical variables, hydrodynamicconditions can also be expected to affectflocculation. The type of mixing used can be criticalowing to the effect of the resultant agitation on thedistributio~ of the polymer uniformly in the pulp. Inthis regard, the point of addition of the polymer, theconcentration of the polymer solution and the mannerof addition C2n be expected to have an influence onthe polymer .j:- take, flocculation and even subsequentbreak-up of t~e flocs.

As 1nd1cat',"L1 ~ar11er, the nature of the flocculationof f1nes obto .I,ed 1n polymer solut1ons 1s dependentupon a number of factors 1ncluding polymercharacteristics, solid and solvent propert1es as wellas the hydrodynamic conditions used dur1ng theflocculation 3nd preflocculat1on stages. It 1s to be


Figure 4. Equilibrium adsorption of polyacrylamide (PAM)and sulfonated polYlcrylamide (PAHS> onkaolinite as 8 function of 10nic strength (31).

0"0E

>-~

Figure 5. Comparison of the pH effect on the adsorption ofpolyacrylamide (PAM) and sulfonatedpolyacrylamide (PAHS) on kaolinite (22).

P. SOMASUNDARAN, Y. H. C. WANG. ETC.218

16

,4

2

~O

Figure 6. Various flocculation responses together withadsorption obtained with Ha-kaolinite at pH 4.5as a function of polyacrylamide (PAM 0.4-0)

dosage (23).

noted here that different properties of theflocculated systems can be the determining criterionin different processes. For example, whereassupernatant clarity will be important when water is tobe recycled, it can be the sediment volume or thesettling rate that will be important in effluenttreatment. For filtr8tion, on the other hand cakestrength and cake moisture content can be theimportant parameters. Different factors (see Table 1)can be manipulated in the system depending on whichproperty 1s to be optimized.

z2.-~~0U)0c

~

~e

>-~

cnZ1&1Q


For example, in a recent study (23) of theflocculation of kaolinite clay using polyacrylamidesit was clearly seen that optimum polymer concentrationand anionicity depended on the flocculation propertystudied (see Fisure 6-8). Both the settlins rat'e andthe supernatant clarity with the nonionicpolyacrylamide flocculent shoved at pH -.5 a markedincrease to a maximum at about 25 mS/kg, whereas withthe anionic polymer settlinl rate and supernatantclarity shoved maxima at 10-25 milks, but the systemwas totally dispersed at hiSh concentrations.Sediment volume did not show any measurable chanseover the entire concentration ranse of nonionicpolymer. In contrast, with the anionic polyacrylamide

Figure 7. Various flocculation responses together withadsorption obtained with "a-kaolinite at pH 4.5as a function of 33J hydrolyzed polyacrylamide(HPAH 0.4-33> dosage. (23>

P. SOMASUNOAAAN, Y. H. C. WANG, ETC.218

Factors 1n FlocculationTable 1.

Polymer Type

- Polymer Dosage

Polymer Concentration

Temperature

- pH

Ionic Strensth

Use of Two or More Polymers andOrder ot Addition

- Method and Speed of Mixing ofPolymer with Suspension

~LYACRYLAMIOE (MW-0.48Ioel/Mo-KAOLINITEClo to..", IOS810"1_-V...NoCIpMo4.510.2 CONO.SPEED 01200,...SILo 0.025 COHO. TIME 0 10-,ftTo 241~

1 ..."10.- a aa

~1-1.0c8

.!Q.8.Jl-I-~

-a,a

0I

~Set.

~.t. 0

::~:::=~J~~" VIs.

. , ~.;~~\~ ~ T~

l-I-

~!~, .

. . . . . ]40'"0 10 20 JO 40 ~

~\.YMP CMA~.£ DENSITY. ~..

Figure 8. Diagram illustrating the eftect of polymercharge density on various tlocculation responsesof H8-kaolinite at pH 4.5 (23).


(33S hydrolyzed) both the sediment volume and theslurry viscosity showed increases with polymerconcentration in the 0 to 50 and 25 to 50 mllkg rangesrespectively. Viscosity and sediment volume, however,remained at the highest levels up to the highesttested concentrations of anionic polymer. Thedifferent effect3 of polymers on the above re3pon3eswere di3cus3ed in terms of variou3 characteristic3 offloC3 and floc-agaregatea. While settling rate andpercent 30lid settled can be expected to be governede33entially by the coar3er region of the particle sizedi3tribution, by the den3ity .and compre3sive strengthof the floC3, and by the structure of any threedimen3ional network3 that can trap the fluid, the3upernatant clarity will depend on any 3hift in thefine region of the size distribution and on theability of the polymers and the network to capture theultrarine3 and colloid3. Sediment volume and 31urryVi3c03ity on the other hand can depend on, in additionto the floc size distribution and the compre33ive3trength of the floc3, a130 the thickneS3 of thead30rbed layer3 of the polymer on the particle3.

All the .easured responses were sens1t1ve to the.n10n1c1ty of the polymer used as the flocculent.Sett11nl rate showed a m8x1mum w1th 01 hydrolys1s .ndthe supern.t.nt cl.r1ty exh1b1ted 8 m.x1mum 1n the 0-201 hydrolys1s ranle. Wh11e the .n10n1c1ty. d1d nothave a s1gn1f1cant effect on the v1scos1ty in the 0 -201 ranle, further 1ncre.ses in an10nic1ty produced ameasurable decrease in v1scos1ty. These effects were.ccounted for in terms of electrost8t1c 1nteractionsbetween the polymer l.yers on the particles 8ndalterations in the conf1guration of the adsorbedpolymers due to the presence of the charged groups.In th1s study, flocculat10n was correlated .lso withboth adsorption density 8nd est1mated surf.ce coveralefor the non10n1c and 331 hydrolyzed poly.crylam1des.Max1mum sett11ng r.te was obta1ned with the nonion1cflocculent at 0.1 .nd w1th the hydrolyzed sample at0.2 surface coverale. Supern.tant clar1ty showed amax1mum at a surface coverage of N.-k.olinite by thehydrolyzed polyacrylam1de of 0.1. At h1gher surfacecoverages (such as 0.5> cons1dered in the p.st to beoptimum for flocculation, complete d1spersion wasobtained with both the nonion1c and the an10nicpolymer.

220 P. SOMASUNDARAN, Y. H. C. WANG, ETC.

Clearly, the polymer properties have a major effecton the type of flocculation obtained. Additional workis indeed needed to establish the effect of many othertypes of cationic, non ionic and anionic co-polymers aswell as block-polymers.

Selective Flocculation or Disoersion

The extent of selective flocculation or dispersionthat can be achieved with minerals depends on theselective adsorption of flocculants on one or more,but not all, mineral types followed by their8ggregation (2'). Separation of the flocculatedparticles from the others can then be achieved byusing conventional techniques such as flotation orsedimentation. In the case of flotation, it isnecessary to take into account the interactions offlocculants with flotation agents.

Flocculant reacents that are used for inducingselectivity include polymers, dispersants andactivators. Adsorption of polymers is usually not asselective as that of surface active or inorganicreagents. Partly this is due to the hydrogen bondingnature of the polymer adsorption. Selectiveadsorption or polymer molecules can be achieved byadjustinl the chemical composition of the suspendingmedium and thereby the surrace potential on the.ineral or by introducinl into the polymer activefunctional groups that will form complexes or saltswith the metal species on the surface of the desiredmineral. In the former case the dependence ofadsorption of ionic polymers on surface charle isexploited. The dependence of polymer flocculation onelectrostatic interaction is illustrated in Filure 9which gives data for settling of flocculated silica inthe presence of an anionic and a cationic polymer. Itcan be seen that only the cationic nalcolyte is ableto produce any significant flocculation of thenegatively charged quartz particles. Based on such apremise, selective separation of quartz from itsmixtures with hematite, calcite, etc. has beenachieved by several investigators (32-34) using, forexample, anionic polyacrylamide.

As mentioned earlier, selective flocculation C8nalso be obtained by incorporating groups that C8n formcovalent bonding with the mineral surface species. Itis well known that adsorption of collectors onminerals depends on the presence of functional groups


Figure 9. Diagrem illustrating settlinl or syntheticsilica in the presence of cationic Nalcolyte-610and anionic Separan AP-30 (41).

such as carboxylate and mercaptan in them. It shouldbe possible to obtain selectivity during flocculationby incorporating such groups tha~ are already known toadsorb selectively. Such an approach has beensuccessfully tested in the past by a number of workers<35-40). For example, tests withhydroxypropylcellulose xanthate containing mercaptanas the active group has been observed to produce goodflocculation of chalcopyrite with pratically no effecton quartz < see Figure 10). Resu.lts or tests withsynthetic 8ixtures or these minerals are shown inFigure 11 IS a function of concentration of thexanthlte <41). An excellent separation index of 0.75was obtained after a one-stage cleaning operation inwhich quartz particles that have been trapped in thebulk chllcopyrite flocs, particullrly at high polymerconcentrations, were wIshed away.

P. SOMASUNDARAN, Y. H. C. WANG, ETC.222

Figure 10. Percentage solids settled of chalcopyrite finesand quartz fines as ~ function of concentrationof hydroxypropylcellulose xanthate (41).

Selective flocculation or dispersion of natural oresis often made d1rf1cult ow1na to interference from thedissolved ions. Like in the case of flotation,additives that can complex with such dissolved ions oradsorb on mineral particles selectively to mod1ry themcan be used to enhance selectivity in such cases.Thus, separation or hemat1te.-quartz mixtures us1nianionic polyacrylamide has been reported to bepromoted by the addition of callon (principally sodiumhexametaphosphate) and sodium fluoride (42).Similarly, rlocculation of heavy minerals has beendepressed by the addition or sodium sulfide,polyphosphates and polyacrylates (43).

223APPLICATIONS OF POLYMERS IN MINERAL PROCESSING

Flaure11. Separation index achieved in selectiveflocculation of chalcopyrite-quartz mixture as .function of concentration ofhydroxypropylcellulose xanthate (41).

In this relard it is important to note thatselection of any dispersant has to be made so thatthere will be minimum interference by these reagentson subsequent 8dsorption of floccu18nts. Indeed allthese realents includinl the flocculant itself shouldnot interfere with 8ny downstre8m process much 8Sflot8tion, filtration or pelletizinl. Selectivefloccu18tion h8S been successfully used recently on acommercial sc81e for the beneficiation of low gradeiron ore. This process uses starch which floccu18tethe hematite leaving the qu8rtz and silicatesdispersed. The commercial use of this technololY is,however, currently limited to beneficiation of theTilden iron ore and to that of potash in a Co.inco

224 P. SOMASUNDARAN, Y. H. C. WANG. ETC.

plant in Saskatchewan, where clay flot8tion is8chieved by the joint use of 8 synthetic flocculantand a cationic collector. Even though the process8ppears to hold potential for other ores, variousproblems existing both at the basic and applied levelwill have to be solved before this potenti.l can befully realized. For example, 8 major proble. is thatmost of the currently available long chain polymers8re bulk flocculants and lack the desiredspecificity. As mentioned earlier, specificity c8n,however, be introduced by incorporating active groupsinto the polymers. However, past work on selectiveflocculation using such modified polymers deals mostlywith binary mineral systems in which the valuable.ineral was a metal sulfide (galena, pyrite orsphalerite), or a .etal or its oxide (he.atite,chrom1te, iron and t1t8nium) and the other componentwas a gangue minerals. Kitchener et 81. (44,45) haveclaimed selective flocculation of sulfide mineralswith mercaptan substituted polyols (xanthates).However, use of high molecular weight polymers withsubstituted mercaptan groups or other specific metllchelating groups as selective floccu18nts appears tobe limited. One such use is that of Clauss et al.(46) who reportedly obtained selective flocculation ofcassiterite from quartz-cassiterite .ixtures undercertain conditions using hydroxamic Icid substitutedpolyacrylamide. Hech8nisms involved in such processesare, however, largely unestablished.

Reports of separation by selective flocculation ofmulti-component natural ores themselves are scant.Commercial applications mentioned earlier, Carta et al(47) reported a study on the beneficiation ofultrafine fluorite from latium. For .ost systems,however, selective flocculation is not easily achievedeven under conditions when excellent selectivity isexpected. This fact becomes easily evident uponexamining, for example, the results obtained by Usoniet ale (48) during their investigation of theselective properties of anionic, cationic and nonionicpolymer.s as flocculants for several mineralsindividu8lly and then in combination with each other.They observed the prediction of selective flocculationon the basis of the results from single mineral teststo agree fairly well with the results obtained forpyrite-quartz and sphalerite-quartz mixtures usingnonlonic separan but to fail for mixtures ofgalena-quartz and sph81erite-quartz using anionic

APPLICATIONS OF POLYMERS IN MINERAL PROCESSING225

Concluding Not!

ACKNOWLEDGEMENT- - - - - -- -

The authorThermodynamicsProgram of(CPE-83-18163).

acknowledgesHultiphase andthe National

the supportParticulate

Science

of theProcessingFoundation

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reprinted frcxn: future trends in polymer science f and

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