303International Journal of Mineral Proc~..in,.. 6 (1980) 303-320e ElJevier Scientific Publiahin, Company. Amsterdam - Printed in The Netheriands

SELEcrIVE FLOCCULATION OF SYNTHETIC MINERAL MIXTURESUSING MODIFIED POLYMERS

G.C. SRESTY. and P. SOMASUNDARAN

Henry Krumb School of Mine.. ColumbiG Uniuersity, New Yor~. N.Y. 10021 (U.S.A.

(ReceiTed February 6, 1979; revised yersion accepted September 11, 1979)

ABSTRACT

Sreaty, G.C. and Somuundaran, P., 1980. Selective nocculation or synthetic mineralmixtures using modified polymeR. Int. J. Miner. Procea., 6: 303-320.

The role or the presence or active groups in polymers and operating variables 8uch asconditioning time in producing nocculation in 8ingle and mixed mineraillimes madeor hematite, quartz and chalcopyrite is examined at diUerent conditions or pH. Selec-tive nocculation was achieved on the basis or results obtained ror single mineral systemsas, for example, in the case of hematit.--quartz mixtures using 8ulfonated polymer.Flocculation was found to go through a maximum as the mineral was conditioned withthe polymer solution. lnterestin(ly, the times of maximum nocculation for variousmineraJa were sufficiently diUerent rrom each other 10 that it could be considered asa potential factor for achieviR( selectivity. Also, cleanin( of the selectively nocculatedproduct by simple redispersion in water improved the separation. Electrokinetic studiesconducted to study the mechanisms involved provided indication ror the shift of shearplane.

INTRODUCTION

In the case of number of currently available ores, mineral values are morefinely dispersed than in the past. Conventional beneficiation techniques arein general rather inefficient in the sub-sieve size range and hence, it has be-come necessary to develop new or modified processes. Among the techniquesthat are being considered at present for ("me particle beneficiation, selectivefiocculation appears to be one of most promising. Ideally this should involveaggregation of either the desired mineral species or the gangue particles intoflocs leaving the others in suspension. Separation of the unflocculated ma-terial by processes such as notation or elutriation results in the desired bene..ficiation.

Flocculation induced by dissolved polymer molecules can be attributedto charge neutralization and/or interparticle bridging. Selectivity in noccu.lation requires selective adsorption of the polymer molecules on the desired

.Present addre8: Iff Research Institute, 10 West 35th Street, Chicago, 01. 60616 (U.S.A

304

mineral particles. A majority of the commercially available polymers are,however, bulk flocculants with insufficient selectivity. In such cases, selec-tivity can be achieved either by altering the interfacial potential of themineral to create the desired electrical interactions between the polymermolecules and the mineral surface or by incorporating suitable functionalgroups into the polymer which, by formation of complexes, can makeflocculation selective. li1corporation of xanthate groups into cellulose has,for example, enabled Attia and Kitchener (1975) to obtain selective floccu-lation of copper minerals from a copper ore. Similarly modification ofstarches for selective flocculation of hematite from low-gl'ade iron ore anduse of commercial polymers containing various ionic groups for selectiveflocculation of mineral mixtures have been successfully attempted in thepast (Frommer, 1968; Usoni et aI., 1968; Read, 1972).

This paper will deal with selective flocculation of two systems: hematite-quartz and chalcopyrite-1}uartz. Effects of time of conditioning withpolymer solution of flocculation behavior of minerals, and the role of clean-ing the selectively flocculated product are described. Mechanisms involvedare discussed with the help of the above data and the results obtained forelectrokinetic properties of a selected mineral/polymer system.

EXPERIMENTAL

Natural minerals used in this study (quartz, hematite and chalcopyrite)were obtained from Wards Natural Sciences Establishment. Synthetic silica(Biosil-A) used was of reagent grade powder. Quartz and chalcopyrite werewet ground at 50% solids in a porcelain ball mill and the hematite wasground in steel ball mill. The minus 20 micron fractions were then separatedby wet screening and stored in distilled water.

Commercial polymers used are listed in Table I. The hydroxypropylcellu-lose xanthate sample was made in the laboratory by reacting one mole ofKlucel HF with three moles of potassium hydroxide and three moles ofcarbondisulfide. All the flocculants, except for starch, had an approximateaverage molecular weight of one million.

Flocculation experiments were done in 2 cm diameter and 15 cm longtest tubes at a solid content of approximately 5%. The pH of the suspensionswas adjusted when required by addition of standard solutions of HNOJ orKOH, and ionic stre~ by addition of KNO] solution. The flocculation re-sponse of the minerals is expressed as the percentage of suspended materialssettling into the bottom one-third volume fraction in the test tube during asettling time of 45 sec. One-third of the mineral that will be naturally dis-tributed in the bottom layer is excluded so that what is measured will corre-spond to that settling into the layer. The flocculation of mineral/polymersYstems was studied as a function of flocculant concentration and time ofconditioning of the mineral with flocculant.

Selective flocculation experiments were conducted using synthetic mix-tures of hematite-quartz and chalcopyrite-quartz prepared by addition

305

TABLE I

APPN)XIMATE .->rzCUtAA

WEIGIrr

~ ~P'rt~

10.~LYTB-610

..lco ~ca1 ~.

10'SEP~-AP 30

Dow Ot881cal ~

POLYST'f- SOI.r<*Aft(Sod1- Salt)

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CA"I~IC P«.Y~~

taz-r--:-tMlo.JC P«.YAca11.Ala~

f g,.2~..,+EaZ1:;:3:tM1~IC

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COM STA-=aKat.ional Starct1 and

O\~cal ~.

of equal amounts of single minerals and equilibrating the mixed fmes forone hour. These tests were conducted in 4 cm diameter and 15 cm longtest tubes at 5% solids. The pulp was fIrSt conditioned with the floccculantfor 30 sec and then, after allowing it to settle for 45 sec, the supernatant(top 67% of the total volume) was siphoned out using vacuum. For the pur-pose of cleaning, the floc portion was rediluted to the original volume withdistilled water, conditioned for 30 sec (without further addition of floccu-lant) and then after 45 sec, the supernatant was again siphoned off. Thesupernatant product (wash), the material initially siphoned (tailing) andthe final floc product (concentrate) were all analyzed for the iridividualminerals.

Electrokinetic measurements were made using a Zetameter. Towardsthis purpose, a fraction of the flocculated slurry was brought to a 0.1%solids concentration by the addition of electrolyte that resulted from cen-trifugation of the slurry from the ~e test.

306

RESULTS AND DISCUSSIONS

Flocculation of single mineral suspensions

Flocculation refers to the process of aggregation involving interparticlebridging (La Mer, 1964; Kane and La Mer, 1964). Ability of polymers tobrdige particles together arises partly from the conformational propertiesof the adsorbed polymer species. Polymer molecules are considered to ad.sorb on mineral particles by attachment at a few sites with tangling loopsand loose segments projected into the suspending medium (Silberberg,1972; Eirich, 1977). Interparticle bridging follows such adsorption ofpolymers either due to adsorption of these projected polymer segmentson uncovered surfaces of other particles or due to intertwining of theprojected polymer segments from different particles. As a result, individualparticles grow into three-dimensional networks called flocs. Such floccula-tion can be expected to depend on both the structure of adsorbed polymerspecies and the availability of uncovered particle surface for bridging.

Time of reagentization of the mineral with polymer solution can affectbotJi of the above mentioned parameters governing flocculation (Bothamand Thies, 1969). Increased reagentization can result in an increase in theadsorption density and therefore in an increase in the fraction of surfacecovered by polymer molecules. Also, a redistribution of the adsorbedpolymer segments among the" suspending medium and particle surface canoccur at the same time and this may lead to further increase in surfacecoverage even at constant adsorption density.

At a certain initial polymer concentration, surface coverage of particlesat any given time is determined by the kinetics of adsorption of the polymer(Lipatov and Sergeeva, 1972). Role of these factors in governing floccula-tion was examined for the present system 1rith the help of results obtainedfor flocculation of hematite as a function of conditioning time with poly-mer solution. Such data for hematite/Separan AP-30 (hydrolyzed polyacryl-amide) system is given in Fig. 1 where the percentage solids settled in 45 secis plotted as a function of conditioning time. Results given in this figure in-dicate that maximum flocculation is reached within short periods of con-ditioning and prolonged conditioning causes some redispersion of the floc-culated material. Conditioning times corresponding to maximum floccula-tion for the system in Fig. 1 are about 60 sec at pH 7.8 and 120 sec at pH 4.These results suggest slower kinetics of adsorption of polymer and floccula.tion at pH 4 than at pH 7.8. The active group of the polymer used in thisstudy was carboxylate with a pH value of 4.7 (Aplan and Fuerstenau, 1962).These groups will be in fully associated form at pH 4 whereas, at pH 7.8,they will be fully dissociated. Absence of sufficient number of anionicgroups on the polymer at pH 4 can indeed cause retardation of its adsorp-tion on the hematite particles that are positively charged.

Flocculation response of quartz as a function of conditioning time withSeparan AP-30 is shown in Fig. 2. Good flocculation of quartz was observedusing the anionic polymer at pH 7 where quartz is negatively changed.

307

aw-'....w'"

'"a

-'0'"

#

0 600 1200 1800 2400

CONOITIONING TIME. SECONOS

Fig- 1. Percentage or hematite fines settled as a runction or time or reagentizing withSeparan AP-30; settling time, 45 sec.

0~....w'"

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0 400 800

COt«>ITIONING TIME, SECONDS

1200

Fig. 2. Percentage or quartz fines settled as a runction or time or reagentizing withSeparan AP.30; settling time, 45 sec.

308

Flocculation of negatively charged quartz by this anionic polymer is prob-ably due to hydrogen bonding or contamination of quartz during its grind-ing in poreelain mill. However, maximum in flocculation was not as sharpin thjs cue as in the case of hematite and conditioning time correspondingto the maximum is at about 600 sec. The relatively slow flocculation isprobably due to the weak forees of adsorption between the anionic polymerand similarly charged quartz particles. The luge difference observed herebetween the optimum reagentization times for hematite and quartz suggeststhe possibility of using times of conditioning of mineral with polymer solu-tion for achieving a limited selectivity in flocculation.

Adsorption of polymer molecules on mineral particles is governed main-ly by three types of bonding, namely, electrostatic, hydrogen and covalentbonding. The predominance of any of the above three bonding mechanismsover other depends on the particular mineral/polymer system and proper.ties of the suspending medium. A combination of the above mechanismscan also be operative under favourable conditions. The role of electrostaticforees in controlling adsorption of polymers and flocculation is furtherdiscussed in this section.

Electrostatic bonding is the predominant mechanism by which ionicpolymers can adsorb on mineral particles that are usually charged in solu-tions. Polymer molecules, by adsorption on oppositely charged particles,can neutralize the charge on particle surface and decre~ the interparticlerepulsive forees that prevent aggregation of particles (Ries and Meyers,1968). Adsorption of ionic polymers on similarly charged particles is not

100

.. NAl-CO\. YTE - 610

. SEPARAN - AP30

pH' 35 TO 36

aw-'....w- 7-.0::i0..

iI

H" . .0 eo 120 ~

~"CONC., ~M lORY SQ..IOS IASIS)

Fi,. 3. Percentage of synthetic silica settled as a function of concentration of Nalcolyte-610 and Separan AP-30; reagentizing time, 30 sec; settling time, 45 sec.

309

pOIIible if the interfacial potential is sufficiently high to introduce electro-static repulsion. For example, results shown in Fig. 3 indicate that syntheticsilica (BiosU-A) can be flocculated with the cationic polyacrylamide,Nalcolyte-610, but not with the anionic polyacrylamide, Separan AP-30.Addition of Separan AP-30 did not result in any increase in the fraction ofsilica seWed in 45 sec. Since the silica particles are highly negatively chargedin the pH range tested, electrostatic forces can evidently be considered asresponsible for controlling their flocculation. It is to be noted, however,from Fig. 4, that good flocculation of ilematite was obtained with both ofthese polymers eventhough anionic Separan was slightly more effective thancationic Nalcolyte. Figure 5 shows the flocculation response of hematite atvarious concentrations of sodium polystyrenesulfonate as a function of pH.Anionic polystyrenesulfonate flocculates hematite better at lower pH valueswhere the mineral is positively charged than at higher pH values where it isnegatively charged.

These results indicate the complex nature of mechanisms involved inflocculation of mineral suspensions. Though electrostatic forces are observedto govern flocculation of some of the above systems, observed nocculationcannot be explained on the basis of electrostatic bonding alone for all cases.For the present systems, it is suggested that flocculation is dependent on anumber of mechanisms, the predominance of anyone mechanism beingdependent on the particular mineral/polymer combination and physico-chemical properties of the suspending medium.

0w...0-0-W..0

~I'

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F"". 4. Percentace or hematite fines settled as a runction or concentration or Nalcolyte-610 and Separan AP.30; reagentizing time, 30 sec; settling time, 45 sec.

310

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Selective flocculation of aynthetic mineral mix tures

Preferential flocculation of a single or a group of mineral particulatesfrom a suspension containing more than one mineral is referred to as selec-tive flocculation. Success of the selective flocculation technique in benefici-ating heterogeneous natural ores will depend on a number of factors, someof which include: (a) good dispersion of the fine particles in suspension;(b) preferential adsorption of the dissolved polymer molecules on particlesand subsequent fiocculation; and (c) effective separation of the fiocs fromthe suspension. Results of the tests on selective flocculation of hematiteand chalcopyrite from their mixtures with quartz; and effect of the abovementioned parameters on the separation are discussed in this section.

Hematite-quartz mixture using sodium polystyrenesulfonatePreferential adsorption of long-chain alkyl sulfonates on iron oxide mine-

rals is well known in flotation. Chemical aspects of notation collectors andpolymeric flocculants are similar in several ways. Experiments were con-ducted to determine whether such selectivity can be obtained with sulfonate

811

aw..J0-

S..a~a#

FiC. 6. Percentage of hematite fines and quartz fines settled u a function of concentra-tion of sodium poIysty~nesulfonate; reacentizinc time, 30 see; settling time, 45 see.

polymers in the flocculation of hematite. The flocculation responses forsingle mineral suspensions of hematite and quartz are shown in Fig. 6 as afunction of concentration of sodium polystyTenesulfonate. The percentageof hematite settling in 45 sec is higher than. that for quartz and the resultssuggest that separation of hematite from quartz by $elective flocculationshould be possible unless there are strong interactions between these twominerals.

The results obtained for selective flocculation tests are shown in Fig. 7.The recovery and grade in terms of % Fel0) in the settled portion areshown in this figure as a function of flocculant concentration. Grade of thesettled portion is observed to increase marginally with flocculant concen-tration, attain a maximum and then decrease in the higher concentrationrange. However, the recovery of iron oxide is found to increase sharplywith increase in flocculant concentration and reach a constant value ofabout 75%. The decrease in the grade of the settled.portion at higher floc-culant conce.ltration indicates commencement of flocculation on entrap-ment of quartz particles in this concentration range. The settled portionwas cleaned by redispersing in water to remove the weakly flocculatedand mechanically entrained quartz particles. The grade and recovery obtainedafter one cleaning operation are also given in Fig. 7. Cleaning of the settledportion has improved the grade with a simultaneous decrease in reco~ery ofiron oxide.

312

.,..g ~

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;r /.f- .,?, ~104

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pH' 17 TO 78

/~

I

40~ I I 1_'250 60 120 180 2»-

POLYMER COfC.. PfIM IORYsa..m BASIS!

Fig. 7. Recovery and grade of' the concentrate f'rom selective Oocculation of' hematite-quartz mixture as a f'unction or concentration of'sodium polystyrenesulronate; reagentiz-ing time, 30 sec; settling time, 45 sec.

In order to evaluate the perfonnance of flocculation, and in particular,cleaning, both grade and recovery will have to be considered. Separation in-dex incorporates both of these variables and is defmed as "(percentage ofvalue mineral recovered in the concentrate + percentage of gangue rejectedin the tailing - 100)/100". A value of either plus or minus unity for separa-tion index refers to complete separation of the minerals and a value closeto zero refers to failure of the process. Calculated values of separation indexfor the results shown in Fig. 7 are given in Fig. 8. These results indicate thatcleaning of the flocculated product has improved the separation significantly.The importance of cleaning the flocculated slurry has been discussed earlierby Friend et al. (1973) and Clauss et al. (1976).

In order to elucidate the mechanisms by which such polymers act, zeta-potential measurements of hematite particles were made as a function of con-centration of the flocculant. Measurements were done under variable andcons'c:ant ionic strength conditions at two pH values, i.e., 7.8 where the hema-tite particles are negatively charged and 4 where the hematite particles arepositively charged *. The results obtained are given in Fig. 9a, b. At pH 4, ad-sorption of the anionic polymer on the positively charged hematite particlesis found to produce a decrease in the zeta potential and even make the sur-face negatively charged. Tests could not be continued at pH 4 and ionic

.Point or zero ch&fJe ror the massive red hematite (95% pure) used in this study is atpH 5 (Sresty, 1977)

313

strength of 10-1 M above a polymer concentration of 100 ppm as there wasexcessive flocculation making the measurements difficult. Excellent floccu-lation was observed even after the charge reversal of hematite particles.Similar results were earlier obtained by Somasundaran et al. (1966) foraggregation of alumina using sodium dodecylsulfonate.

Such charge reversal normally suggests specific adsorption of the polymermolecules due to forces that are non-electrical in nature. In this case, how-ever, it is most interesting to note that polymer adsorption has also causeda decrease in zeta potential of similarly charged particles at pH 7.8. Underconstant ionic strength conditions, adsorption of negatively charged mole-cules cannot normally be expected to make the mineral particles lessnegative. The observed results clearly suggest a significant shift in the shearplane due to adsorption of the massive polymer molecules. In fact, underthese conditions, the zeta potential obtained can be considered to be a goodmeasure of the adsorbed polymer layer rather than that of the originalmineral particles themselves.

314

>2

...~I-ZIIIl-

f

~IIIN

0.

~50 100 150

POLYSTYREN( SA.FONATE CONC.. PPM (MY SOLIOS BASISI

Fig. 9a. Zeta potential or hematite fines as a function or concentration of sodium poly-stynnesulronate, natural ionic strenlth. b. Zeta potential or hematite fines as a functionor concentration or sodium polystyrenesulfonate; ionic strenlth 10-1 M KNO3.

Hematite-quartz mixture using causticized corn starchSelectivity of com starch in depressing iron oxide during anionic flotation

of silica and in selectively flocculating iron oxides are well known (Cookeet aI., 1952; Iwasaki et aI., 1969). The selectivity of starch in depressing ironoxides during flotation of silica from iron ores depends to a large extent onthe functional groups present in the starch molecule (Iwasaki et al., 1969).Chang (1952) has reported oxidized starch to produce most preferentialadsorption on hematite particles than other types.

315

Flocculation responses of hematite and quartz fines in the presence ofcausticized com starch are shown in Fig. 10. Increase in settling of hema-tite due to flocculation is larger than that of quartz. Also. the differencebetween the flocculation response of these two minerals in the presenceof causticized com starch is greater than that in the presence of sodiumpolYltyrenesulfonate suggesting better selectivity with starch than withthe latter. Results of the selective flocculation experiments using hematite-quartz mixtures are given in Fig. 11. In this case. a satisfactory separationindex of 0.7 was in fact obtained after a one-stage cleaning of the flocculatedproduct.

A single stage cleaning of the flocculated product has produced significantimprovement in the separation index for both of tJie above mentioned sys-tems. The effect of multiple-stage cleaning of the flocculated product onboth grade and recovery was determined at two concentrations of starch.Multiple-stage cleaning of the flocculated slurry produced significant im.provement in tJie grade owing to further removal of entrained quartz. Asexpected. recovery. however. decreased during each stage of cleaning. Theeffect of multiple-stage cleaning on the separation index obtained for selec-tive flocculations are shown in Figs. 12 and 13. Due to tJie existence of anapparent upper limit of grade that can be obtained in this case. improve-ment in separation index occurred only during the f"llSt few stages of thecleaning. Under these conditions. scaverging of the tailings might be neces-sary to obtain economically acceptable separation levels. These results also

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.. HEHATITE pH -78

to

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FiC. 10. P~tace or hematite rUles and quartz fines Mttled - . runction or concentra-tioa or stareh; reagentizing time, 30 sec; settling time, 45 sec.

eo

40

316

x~z..

z2:..cc

:..,\II

100 200 300

STARCH C~. PPM (DAY SOU OS BASIS!

Fig. 11. Separation index achieved from selective flocculation of hematite-quartz mix-ture as a function of concentration of starch; reagentizing time, 30 sec; settling time, 45 sec.

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NUM8ER OF Q.EANINGS

Fig. 12. Recovery and grade or the concentrate, and separation index achieved rromselective flocculation or hematite-Quartz mixtures using starch as a runction or numberor cleaning stages; reagentizing time, 30 sec: settling time, 45 sec.

317

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Fit- 13. Reco..ry and ITade of the concentrate, and aeparation index achieved fromaelective nocculation of hematite-quutz mixture u8inc starch as a function of numberof cle.niDI stages; reacentizinc time, 30 see; settling time, 45 sec.

show that optimum amount of cleaning depends on the concentration ofpolymer added. In the present case, use of higher ~olymer concentrationsare found to require a larger number of cleaning stages.

Chalcopyrite-quartz mixture using xanthateSelective adsorption of xanthates on heavy minerals such as chalcopyrite,

galena and sphalerite in preference to quartz and calcite has resulted in itsextensive use as collecton in the flotation of sulfide minerals. An ideallyselective flocculant can be made for separating these minerals from gangueby incorporating the sulfi1Ydryl group into long chain polymen. Hydroxy-propylcellulose supplied by Hercules, Inc., under the name "Klucel HF"is a high-molecular weight surface-active polymer with foaming tendency.This polymer also possesses a long flexible chain which can be helpful forflocculation. Hydroxypropylcellulose xanthate was prepared by reactingthis polymer with three moles of potassium hydroxide and three molesof carbondisulfide.

Flocculation responses of both chalcopyrite and quartz fines as a functionof concentration of hydroxypropylcellulose xanthate at the correspondingnatural pH values for these systems is illustrated in Fig. 14. It is seen thatthis polymer can be effective in flocculating chalcopyrite with practicallyno effect on quartz. The results obtained for the selective flocculation ofchalcopyrite from chalcopyri~uartz mixture are given in Fig. 15. Anexcellent separation index of 0.75 was obtained after a one-stage cleanin~operation.

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CONCLUSIONS

Flocculation of mineral suspensions is sensitive to many operating vari-ables and the effect of some of these variables such as the type of activegroup present in the polymer molecules is very significant. Results obtainedin this study have indicated the existence of an optimum reagentizing timeof the mineral with the polymer solution, which was dependent on theparticular mineral/polymer system and the electrolytic properties of the sus-pending medium. ConPitions that permit favourable electrostatic forcesbetween mineral particles and polymer molecules do induce flocculation,but all of the results obtained could not be explained on the basis of anelectrostatic mechanism alone.

Selective flocculation experiments conducted with mixed mineral systemsunder conditions selected on the basis of the results obtained for singlemineral suspensions further confirms selective flocculation to be a feasibleprocess. Selectivity of the polymers towards flocculation can be improvedsignificantly by the incorporation of suitable functional groups into thepolymeric chains. Cleaning of the nocculated product is observed to beessential for obtaining satisfactory separation.

Electrokinetic tests conducted for hematite/polystyrenesulfonate sys-tem at different pH values gave indications for a shift of the shear plane.In the case of a polymer-coated particle, the results suggest that the valueof the zeta potential obtained to be characteristic of the adsorbed polymerlayer itself rather than that of the original particle.

ACKNOWLEDGEMENT

The support of the Particulate and Multiphase Processes Program of theNational Science Foundation and the International Nickel Company isgratefully acknowledged.

REFERENCES

Aplan, F.F. and Fuerstenau, D.W., 1962. Principles oC non-metallic mineral flotation.In: D.W. Fuerstenau (Editor), Froth Flotation. 50th Anniversary yolume, AIME,p.170.

Atti.. Y.A.L and Kitchener, J.A., 1975. Dev.lopm~nt oC compJe~ing polymers Cor theselective flocculation oC copper mineral.. In: M. Carta (Editor), Proc. XIth Int.Concr. Miner. Procesa. UniYersata di Caciiari. p. 1233.

Botbam. R. and Thies. C., 1969. The efCect oC stereorecularity upon the adsorptionbehavior of hiCh molecular weicht poly (iaopropylacrylate). J. Colloid Interface Sci.,31: 1.

Chant. C.C.. 1952. Subatituted starches in amine flotation of iron ores. Trans. AlME.199: 922.

Clau-. C.R.A, Appleton, E.A. and Vink, J.J., 1976. Flocculation or caaiterite in mi~.tuns with quartz using a modified polyacrylamide nocculant. Int. J. Miner. Process.3: 27.

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