mInternational Journal of Mineral Processing, 27 (1989) 111-123Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands

Flocculation of Sulfides and the Role of aComplexing Agent in it

S. ACAR and P. SOMASUNDARAN

Henry Krumb School of Mines, Columbia University, New York, N. ¥. 10027 (U.S.A.

(Received February 24, 1987; accepted after revision January 17, 1989)

ABgfRACT

Acar. S. and Somasundaran, P., 1989. Flocculation of sulfides and the role of a complexing agentin it. Int. J. Miner. Process., 27: 111-123.

Selective flocculation using polymers is one of the promising methods for the separation ofmineral fmes. In this paper. possibilities for selective flocculation of chalcopyrite and pentlanditeusing polyacrylamide and polyethylene oxide are examined under different experimental condi-tions. Selective flocculation tests with synthetic mineral mixtures showed the selectivity to dependon polymer type and concentration, and the presence of dissolved species from either mineral.The dissolved species are found to interact with minerals to be treated and reagents, resulting innon-selective flocculation. The interference by dissolved species can be eliminated by using acomplexing agent such as diphenylguanidine that interacts specifically with copper minerals. In-teractions between many components are possible in these systems and these are examined inorder to identify the governing mechanisms.

INTRODUCTION

Selective flocculation using polymers is a promising method for beneficiat-ing mineral fines [1,2J. However, the results obtained for the selective floc-culation of natural ores or the synthetic mixtures do not often correlate withthose from single mineral tests [3 J. This can be due to the interactions ofdissolved mineral species with the component minerals of the system resultingin alterations of their surface properties [4-8 J. Such interference can be con-trolled by the addition of appropriate complexing reagents [9-13 J. Selectionof a complexing reagent depends on its specificity towards a particular metalion in the interfacial region [14J.

It has been reported that selective flocculation of hematite from a mixtureof it with quartz is possible using polyacrylic acid in the pH range from 3 to 9in the absence of dissolved ferric ions [5 J. Addition of sufficient amounts o-fEDTA or KF was found to prevent quartz activation by iron species and thusmaintain selective flocculation of hematite. Heerama et al. reported that the

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presence ofCa2+ and Mg2+ ions in iron ore pulps results in nonselective floc-culation of iron oxide from silicious gangue [6]. Ethylenediaminetetracetate(EDTA), sodium tripolyphosphate (STPP) and sodium hexametaphosphate(DHMP) complexing reagents were tested to control Ca2+ and Mg2+ ionspresent in the pulp. It has also been reported that specifically adsorbing ionssuch as Cu2+, can reverse the zeta potential of quartz, hence enhancing itsflocculation [7,8]. Such interactions can be expected to reduce the selectivityin flocculation of minerals.

In this paper, flocculation characteristics of natural chalcopyrite and pen-tlandite in salt solutions and in mineral supernatants with and without theaddition of polymers and a complexing reagent are discussed.

EXPERIMENTAL

Materials

Natural chalcopyrite (Ward's Natural Science Establishment Inc.) and nat-ural pentlandite (International Nickel Company) particles of less than 20 .urnwere used in the present study. Surface areas of chalcopyrite and pentlanditeas measured by the H.E. T. nitrogen gas adsorption method were 1.22 and 1.43m2/g, respectively. Mineral samples were stored in teflon bottles and kept un-der nitrogen atmosphere.

Polymers

Nonionic polyacrylamide was synthesized using a radiation-induced heter-ogeneous polymerization technique. Molecular weight of the sample was foundto be 2.10 [6]. The polyethylene oxide sample was obtained from Union Car-bide Corporation and had a molecular weight of 5.10.

Complexing reagent

Diphenylguanidine (DPG) purchased from Morton Thiocol Inc., was usedas received.

Inorganic reagents. Fisher certified HCl and NaOH were used to adjust thepH of the suspensions. Amend Drug and Chemical Company reagent gradeNaCl (99.96%) was used to prepare 3-10-2 kmolfm3 salt solutions used in allthe experiments.

Water. Triple distilled water with a specific conductivity of about 10-6Jlmhosfcm prepared in a quartz still was used for preparing all the solutions. ,

\

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METHODS

Flocculation. For flocculation tests, 6 g of 20 .um mineral samples were equi-librated for 3 h in 200 ml of3'10-2 kmol/m3 NaCI solutions at the desired pHin teflon bottles on a wrist action shaker. In the case of tests involving com-plexing agents, the desired amount was added to the pulp before it was equili'-brated for 3 h. Flocculations tests were conducted in 250 cm3 beakers (withbaffles) at 3 weight percent. In the case of tests involving polymers, the desiredamount was added to the pulp and further conditioned for 3 min by stirringwith a I-inch diameter 3-blade propeller at 1200 rpm. The mineral was thenallowed to settle under gravity for 15 s and the top 50% of the suspension waswithdrawn using a specially designed suction device. The pH of the pulp wasmeasured at the end of each test. The two portions were then filtered, driedand weighed The percentage excess solids settling in 15 s was taken as a mea-sure of the flocculation of the minerals. Chemical analysis of dried residueswas conducted in the case of flocculated mineral mixtures.

Preparation of mineral supernatants. The mineral sample was conditionedfor three hours in 3-10-2 kmol/m3 NaCI solution at room temperature anddesired pH in 250 cm3 teflon bottles on a wrist action shaker. The suspensionwas then filtered through pretreated - with the same supernatant - # 44 What-man filter paper and the supernatant was stored in teflon bottles and used inless than a week.

RESULTS AND DISCUSSION

Flocculation

The flocculation response of natural chalcopyrite and pentlandite in thepresence of nonionic polyacrylamide and polyethylene oxide polymers was firststudied in the unmixed mode under different conditions of pH and polymerconcentration. The results given in Fig. 1 show chalcopyrite and pentlanditeto flocculate in polyacrylamide solutions at pH 7.0 with a slight decrease inflocculation at higher polymer dosage. The effect of pH on the flocculation ofchalcopyrite and pentlandite is shown in Figs. 2 and 3, respectively. In theabsence of PAM, the flocculation (i.e. coagulation) of chalcopyrite was foundto sharply decrease with increase in pH. This is attributed to the increase inthe negative potential of chalcopyrite with pH [20]. On the other hand, excel-lent flocculation of chalcopyrite was obtained in 20 ppm PAM solutions espe-ciallyat higher pH values (Fig. 2). Addition of the same amount of PAM topentlandite suspension increased flocculation from 30% to about 85% undernatural pH conditions. However, with increase in pH flocculation decreasedgradually, probably due to the formation of nickelous hydroxide on the surface

114

0 10 20 30 40

POLYMER CONC.. ppm

Fig. 1. Chalcopyrite and pentlandite flocculations as a function of polyacrylamide concentration.

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Fig. 2. Chalcopyrite flocculation as a function of pH with and without PAM or PEO.

Fig. 3. Pentlandite flocculation as a function of pH with and without PAM or PEO.

of pentlandite. In the absence of PAM, zeta potential of pentlandite is about F20 m V in the complete pH range providing some coagulation of it (Fig. 3).

It is seen from the single mineral flocculation tests that both minerals, chal-copyrite and pentlandite, are well flocculated by polyacrylamide with somemeasurable difference in the alkaline pH range. Since the flocculation selec-tivity cannot be expected to be high in this case, tests were conducted nextwith polyethylene oxide (PEO) which is partly hydrophobic. It has been re-ported in the literature that PEa preferentially flocculates the hydrophobicminerals [15-17]. PEa adsorbs by hydrogen bonding of the ether groups inaddition to hydrophobic in~raction of the -CH2.CH2- units with the hydro-

115

0 CHAU:OPYRITE I PEOto PENTLAHDITE I PEO

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401

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Fig. 4. Chalcopyrite and pentlandite flocculations as a function of polyethylene oxide concentration.

10 30

phobic sites on the mineral. Therefore it is expected that partly hydrophobicchalcopyrite could be flocculated by PEG.

Flocculation results obtained using PEG at pH 7.0 are shown in Fig. 4. Max-imum chalcopyrite flocculation was obtained at pH 7 with 5 ppm PEG addi-tion. However, pentlandite did not respond to the addition of PEG even forpolymer concentrations as high as 40ppm at the same pH level (Fig. 4). Theeffect of pH was tested at the 5 ppm PEG level. The results shown in Figs. 2and 3 indicate that chalcopyrite is flocculated by PEG in the complete pHrange, whereas pentlandite remained unflocculated under the same conditions.It is thus clear that only the naturally hydrophobic chalcopyrite, and not thepentlandite, is flocculated by PEG.

Since pentlandite did not flocculate with PEG, possibilities for selective floc-culation were explored using 50: 50 mixture of chalcopyrite and pentlandite.While studying mixed mineral systems, it is necessary to take into accountmineral entrapment in the flocs as well as heterocoagulation resulting fromelectrostatic attraction between oppositely charged particles and the effect ofsoluble species produced by either mineral. The results obtained for mixedmineral flocculation are given in Figs. 5 and 6.

Fig. 5 shows the effect of PEG addition on the amount of chalcopyrite-pen-tlandite mixtures settled at three different pH values. Flocculation obtainedin the absence of PEG can be considered to result from simple coagulationinduced by the dissolved species. Addition of PEG led to increased flocculationof the chalcopyrite-pentlandite mixture. Although chalcopyrite recovery isfound to increase with increasing pH the assay of it in the presence and in theabsence of PEG remained the same indicating no enrichment of chalcopyritedue to PEG addition (Fig. 6a). As seen from Fig. 6b pentlandite recovery alsoincreased in the presence of PEG throughout the pH range studied and toe

116

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If

4 . 8 10 12

pH

Fig. 5. Flocculation of chalcopyrite and pentlandite mixtures as a function of pH with and withoutPEO.

assay remained practically the same again suggesting no enrichment due toPEG addition.

A possible reason for the lack of selectivity can be heterocoagulation of chal-copyrite with pentlandite, however, this is not the case in the acid and naturalpH regions [20]. Besides heterocoagulation, a major reason for the lack ofselectivity can be the activation of pentlandite or deactivation of chalcopyrite

117

by dissolved species such as CU2+, Ni2+, Fe2+ and 82- and their hydroxy spe-cies from the minerals present in the system. It has been reported that specif-ically adsorbing ions can change and/or even reverse the zeta potential of amineral and thereby cause heterocoagulation as well as enhanced polymer floc-culation [7,8,21]. In order to investigate the effect of dissolved mineral cationson the flocculation of chalcopyrite and pentlandite, tests were next conductedin mineral supernatants in the absence of any polymer.

The effect of the pentlandite supernatant on the flocculation characteristicsof chalcopyrite is illustrated in Fig. 7. It can be seen that chalcopyrite is dis-persed by the supernatant in the acid pH range. Above about pH 6.0, however,increased flocculation of chalcopyrite is obtained in the pentlandite superna-tant. On the other hand, pentlandite flocculation is enhanced in the presenceof chalcopyrite supernatant throughout the pH range studied (Fig. 8).

Fig. 7. Flocculation of cbalcopyri~ in the presence and in the absence of pentlandi~ supernatant.

2 4 6 8 10 12 14

pH

Fig. 8. Flocculation ofpentlandite in the presence and in the absence of chalcopyrite supernatant.

118

Dissolution and precipitation/ readsorption of mineral species in mixed min-eral systems is considered to be a major reason for the observed effects. Thiswas further confirmed by electrophoretic mobility and by electron spectros-copy for chemical analysis (ESCA) results [14,27,28). In the presence of su-pernatants zeta potentials of chalcopyrite and pentlandite are found to be re-duced considerably due to the adsorption or surface precipitation of dissolvedions and their hydroxy species on the mineral surfaces. ESCA spectrum ofchalcopyrite conditioned in pentlandite supernatant and washed free of thesupernatant showed that nickel ions can be found on the surface of chalcopyr-ite. Similarly, the spectrum of pentlandite conditioned in chalcopyrite super-natant and washed also showed presence of copper on the pentlandite surface.

The extent of dissolution of chalcopyrite and pentlandite was determined byanalyzing supernatants at different pH values using Direct Coupling PlasmaSpectrometry (DCP). The results are given in Tables I and II.

The above results clearly show that the amount of dissolved species is sig-nificant enough under certain pH conditions to have an effect on the floccu-lation. In order to minimize the effect of the dissolved species on selectivity inflocculation, an attempt was made next to complex them usingdiphenylguanidine.

TABLE I

Chemical analysis of chalcopyrite supernatant

34.3 (natural)79

1112.4

TABLE II

Chemical analysis of pentlandiu supernatant

34.1 (natural)79

1112.5

119

Effect of diphenyiguanidine on flocculation

Diphenylguanidine (DPG) has been extensively used in the flotation sepa-ration of INCO matte, which consists of CU2S and Ni3S2' due to its unusualselectivity for copper minerals (22-25]. Based on this information, the effectof DPG on the flocculation of chalcopyrite and pentlandite was tested.

The effect of addition of complexing agent diphenylguanidine to the naturalchalcopyrite and pentlandite suspensions in the presence and in the absenceof PEG is shown in Figs. 9 and 10. In these figures flocculation power*l (26 ]of the reagent is plotted as a function of pH at different dosages.

Addition ofDPG to the chalcopyrite suspension during conditioning causeddispersion of the system at pH < 5 at the concentration levels of 10-3 kmol/m3, 10-4 kmol/m3 and 10-6 kmol/m3 (Fig. 9). Above pH 5, addition of 10-4kmol/m3 DPG increased the chalcopyrite flocculation significantly. In thepresence of PEG the dispersion effect of 10-4 kmol/m3 DPG is absent at acidpH region. Furthermore, flocculation power of PEG is also absent in the nat-ural and alkaline pH ranges. These results clearly suggest the DPG and PEGhave a significant combined effect on the chalcopyrite flocculation. In contrastto the above, pentlandite suspension did not respond to the addition of the

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Fig. 9. Flocculation of chalcopyrite in the presence of: (a) DPG; and (b) DPG and PEG.

*lFlocculation power= (Po-P,)/Po. where: Po=the mass (g) of solid in the top 50% of the su-pernatant; P,= the mass (g) of solid in the top 50% of the supernatant when a flocculant is used;+ 100% corresponds to the complete flocculation and -100% corresponds to the complete dis-persion of the suspension.

120

PENTLANDITE

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same concentrations ofDPG (Fig. 10). There is also no effect of the presenceof PEa along with D PG on the flocculation of pentlandite.

The above results suggested possible selective action by DPG in chalcopyr-ite-pentlandite mixtures. This was tested by conducting selective flocculationtests with synthetic mixtures of chalcopyrite and pentlandite. It can be seenfrom Fig. 11 that flocculation of the mixture is enhanced by the presence ofDPG and PEa together, particularly in the acid pH region. The effect of thedissolved species is suppressed in this pH range by the presence of DPG. Therecovery and assay of chalcopyrite is found to increase from about 70% to 90%and from 50% to about 60%, respectively, in the presence of DPG and PEatogether throughout the pH range studied (Fig. 12a). On the other hand, therecovery and assay of pentlandite are reduced compared to the results obtainedwhen only PEa is present (see Figs. 6b and 12b). From the comparison of theresults in Figs. 6a and 6b and with those in 12a and 12b, it can be concluded

80

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Fig. 12. Recovery and...y of: (a) chalcopyrite; and (b) pentlandire.

that enhanced enrichment of chalcopyrite is obtained when 10-4 kmol/m3 DPGand 5 ppm PEa are employed together.

Lack of selectivity in the flocculation of mixed sulfides is found to be due tointeractions of dissolved mineral species with the particle surface. This is sup-ported by the results of flocculation tests in mineral supernatants which showedthat while chalcopyrite is dispersed by the pentlandite supernatant, pentlan-dite is flocculated by the chalcopyrite supernatant. Evidence for the interac-tion of dissolved mineral species with particle surfaces was obtained from theelectrokinetic and ESCA studies [14,27,28]. Electrokinetic studies with chal-

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122

copyrite and pentlandite showed that zeta potentials of these minerals are re-duced significantly by the presence of the other's supernatant [27]. Surfacechemical analysis of the chalcopyrite sample treated with pentlandite super-natant and washed showed a nickel peak along with copper peaks [28]. Simi-larly, ESCA spectrum of pentlandite showed a copper peak when the formermineral was treated with chalcopyrite supernatant and washed.

SUMMARY

( 1) Single mineral flocculation tests showed that both natural chalcopyriteand pentlandite are flocculated with polyacrylamide polymer. Natural chal-copyrite is also flocculated with polyethylene oxide, but not pentlandite.

(2) Polyethylene oxide polymer may be used to flocculate chalcopyrite fromits mixture with pentlandite. Selectivity of the process may be enhanced bymodifying the polymer adsorption using complexing reagents and by changingthe medium pH and temperature.

(3) Dissolved mineral species can affect the flocculation process. The re-sults of flocculation tests with binary mixtures showed nonselectivity of chal-copyrite que to possible activation of pentlandite by dissolved species. Theevidence of this is seen from the results of the flocculation tests in the presenceof supernatants. It is to be noted that in these cases enhanced selectivity ofchalcopyrite is obtained when complexing reagent diphenylguanidine is em-ployed along with the polymer.

ACKNOWLEDGEMENTS

The authors acknowledge the National Science Foundation (MSM-86-17183,CBT -86-15524), the Inco Inc. and the Union Carbide Corporation for supportof this work.

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7 Critchley, J .K. and Jewitt, S.R., 1979. The effect of Cu + + ions on zeta potential of quartz.

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23 Cynamid's Nitrogen Chemicals Digest, IV, 1950. The Chemistry of Guanidine. AmericanCynamid Company, Stamford, Conn.

24 Boldt, J .R., Jr. and Queneau, P ., The Winning of Nickel Longman S., Canada Ltd.25 Sproule, W.K., Harwurt, G.A. and Rose, E., 1945. U.S. Patent 2, 432, 456.26 UsOni, L., Rinelli, G. and Marabini, A.M., 1968. Selective properties of flocculants and pos-

sibilities of their use in flotation of fine minerals. In: 8th Int. Miner. Process. Congr., Len-ingrad (Leningrad: Institut Mekhanobr 1969), Vol 1, pp. 514-533 (in Russian); paper D13,14 pp. (in English).

27 Acar, S. and Somasundaran, P., 1989. Effect of Dissolved Mineral Species on the Electroki-netic Behavior of Sulfides. To be published.

28 Acar, S. and Somasundaran, P., in prep. ESCA Characterization of Sulfide Mineral Surfacesin Flocculation Systems.