Flocculation of Sulfides and the Role of a Complexing Agent in it

Transcription

1 International Journal of Mineral Processing, 27 (1989) Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands m Flocculation of Sulfides and the Role of a Complexing Agent in it S. ACAR and P. SOMASUNDARAN Henry Krumb School of Mines, Columbia University, New York, N (U.S.A. (Received February 24, 1987; accepted after revision January 17, 1989) ABgfRACT Acar. S. and Somasundaran, P., Flocculation of sulfides and the role of a complexing agent in it. Int. J. Miner. Process., 27: Selective flocculation using polymers is one of the promising methods for the separation of mineral fmes. In this paper. possibilities for selective flocculation of chalcopyrite and pentlandite using polyacrylamide and polyethylene oxide are examined under different experimental conditions. Selective flocculation tests with synthetic mineral mixtures showed the selectivity to depend on 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 in non-selective flocculation. The interference by dissolved species can be eliminated by using a complexing agent such as diphenylguanidine that interacts specifically with copper minerals. Interactions between many components are possible in these systems and these are examined in order to identify the governing mechanisms. INTRODUCTION Selective flocculation using polymers is a promising method for beneficiating mineral fines [1,2J. However, the results obtained for the selective flocculation of natural ores or the synthetic mixtures do not often correlate with those from single mineral tests [3 J. This can be due to the interactions of dissolved mineral species with the component minerals of the system resulting in alterations of their surface properties [4-8 J. Such interference can be controlled by the addition of appropriate complexing reagents [9-13 J. Selection of a complexing reagent depends on its specificity towards a particular metal ion in the interfacial region [14J. It has been reported that selective flocculation of hematite from a mixture of it with quartz is possible using polyacrylic acid in the ph range from 3 to 9 in the absence of dissolved ferric ions [5 J. Addition of sufficient amounts o-f EDTA or KF was found to prevent quartz activation by iron species and thus maintain selective flocculation of hematite. Heerama et al. reported that the 1989 Elsevier Science Publishers B. V

2 112 presence ofca2+ and Mg2+ ions in iron ore pulps results in nonselective flocculation 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+ ions present in the pulp. It has also been reported that specifically adsorbing ions such as Cu2+, can reverse the zeta potential of quartz, hence enhancing its flocculation [7,8]. Such interactions can be expected to reduce the selectivity in flocculation of minerals. In this paper, flocculation characteristics of natural chalcopyrite and pentlandite in salt solutions and in mineral supernatants with and without the addition of polymers and a complexing reagent are discussed. EXPERIMENTAL Materials Natural chalcopyrite (Ward's Natural Science Establishment Inc.) and natural pentlandite (International Nickel Company) particles of less than 2.urn were used in the present study. Surface areas of chalcopyrite and pentlandite as measured by the H.E. T. nitrogen gas adsorption method were 1.22 and 1.43 m2/g, respectively. Mineral samples were stored in teflon bottles and kept under nitrogen atmosphere. Polymers Nonionic polyacrylamide was synthesized using a radiation-induced heterogeneous polymerization technique. Molecular weight of the sample was found to be 2.1 [6]. The polyethylene oxide sample was obtained from Union Carbide Corporation and had a molecular weight of 5.1. Complexing reagent Diphenylguanidine (DPG) purchased from Morton Thiocol Inc., was used as received. Inorganic reagents. Fisher certified HCl and NaOH were used to adjust the ph of the suspensions. Amend Drug and Chemical Company reagent grade NaCl (99.96%) was used to prepare kmolfm3 salt solutions used in all the experiments. Water. Triple distilled water with a specific conductivity of about 1-6 Jlmhosfcm prepared in a quartz still was used for preparing all the solutions., \

3 113 METHODS Flocculation. For flocculation tests, 6 g of 2.um mineral samples were equilibrated for 3 h in 2 ml of3'1-2 kmol/m3 NaCI solutions at the desired ph in teflon bottles on a wrist action shaker. In the case of tests involving complexing agents, the desired amount was added to the pulp before it was equili'- brated for 3 h. Flocculations tests were conducted in 25 cm3 beakers (with baffles) at 3 weight percent. In the case of tests involving polymers, the desired amount was added to the pulp and further conditioned for 3 min by stirring with a I-inch diameter 3-blade propeller at 12 rpm. The mineral was then allowed to settle under gravity for 15 s and the top 5% of the suspension was withdrawn using a specially designed suction device. The ph of the pulp was measured at the end of each test. The two portions were then filtered, dried and weighed The percentage excess solids settling in 15 s was taken as a measure of the flocculation of the minerals. Chemical analysis of dried residues was conducted in the case of flocculated mineral mixtures. Preparation of mineral supernatants. The mineral sample was conditioned for three hours in kmol/m3 NaCI solution at room temperature and desired ph in 25 cm3 teflon bottles on a wrist action shaker. The suspension was then filtered through pretreated - with the same supernatant - # 44 Whatman filter paper and the supernatant was stored in teflon bottles and used in less than a week. RESULTS AND DISCUSSION Flocculation The flocculation response of natural chalcopyrite and pentlandite in the presence of nonionic polyacrylamide and polyethylene oxide polymers was first studied in the unmixed mode under different conditions of ph and polymer concentration. The results given in Fig. 1 show chalcopyrite and pentlandite to flocculate in polyacrylamide solutions at ph 7. with a slight decrease in flocculation at higher polymer dosage. The effect of ph on the flocculation of chalcopyrite and pentlandite is shown in Figs. 2 and 3, respectively. In the absence of PAM, the flocculation (i.e. coagulation) of chalcopyrite was found to sharply decrease with increase in ph. This is attributed to the increase in the negative potential of chalcopyrite with ph [2]. On the other hand, excellent flocculation of chalcopyrite was obtained in 2 ppm PAM solutions especiallyat higher ph values (Fig. 2). Addition of the same amount of PAM to pentlandite suspension increased flocculation from 3% to about 85% under natural ph conditions. However, with increase in ph flocculation decreased gradually, probably due to the formation of nickelous hydroxide on the surface

4 POLYMER CONC.. ppm Fig. 1. Chalcopyrite and pentlandite flocculations as a function of polyacrylamide concentration. 1.:::::= I a ẉ..i l- I- w a I If -.[ m- 4 2 ppm Polymer 6. 2ppm PAM 5ppm PEO o- lt 3xl-2kmol/m' ph ph 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. 8 -L NoCI of pentlandite. In the absence of PAM, zeta potential of pentlandite is about F 2 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, chalcopyrite and pentlandite, are well flocculated by polyacrylamide with some measurable difference in the alkaline ph range. Since the flocculation selectivity cannot be expected to be high in this case, tests were conducted next with polyethylene oxide (PEO) which is partly hydrophobic. It has been reported in the literature that PEa preferentially flocculates the hydrophobic minerals [15-17]. PEa adsorbs by hydrogen bonding of the ether groups in addition to hydrophobic inraction of the -CH2.CH2- units with the hydro-

5 115 CHAU:OPYRITE I PEO to PENTLAHDITE I PEO C I&! -' l- I- I&! U) uo c ::; uo 81 1 I I P--.,. 3.IO-2tmol/m3 ph: 7. HaCI Jf 2 u POLYMER CONC., ppm Fig. 4. Chalcopyrite and pentlandite flocculations as a function of polyethylene oxide concentration. phobic sites on the mineral. Therefore it is expected that partly hydrophobic chalcopyrite could be flocculated by PEG. Flocculation results obtained using PEG at ph 7. are shown in Fig. 4. Maximum chalcopyrite flocculation was obtained at ph 7 with 5 ppm PEG addition. However, pentlandite did not respond to the addition of PEG even for polymer concentrations as high as 4ppm at the same ph level (Fig. 4). The effect of ph was tested at the 5 ppm PEG level. The results shown in Figs. 2 and 3 indicate that chalcopyrite is flocculated by PEG in the complete ph range, whereas pentlandite remained unflocculated under the same conditions. It is thus clear that only the naturally hydrophobic chalcopyrite, and not the pentlandite, is flocculated by PEG. Since pentlandite did not flocculate with PEG, possibilities for selective flocculation were explored using 5: 5 mixture of chalcopyrite and pentlandite. While studying mixed mineral systems, it is necessary to take into account mineral entrapment in the flocs as well as heterocoagulation resulting from electrostatic attraction between oppositely charged particles and the effect of soluble species produced by either mineral. The results obtained for mixed mineral flocculation are given in Figs. 5 and 6. Fig. 5 shows the effect of PEG addition on the amount of chalcopyrite-pentlandite mixtures settled at three different ph values. Flocculation obtained in the absence of PEG can be considered to result from simple coagulation induced by the dissolved species. Addition of PEG led to increased flocculation of the chalcopyrite-pentlandite mixture. Although chalcopyrite recovery is found to increase with increasing ph the assay of it in the presence and in the absence of PEG remained the same indicating no enrichment of chalcopyrite due to PEG addition (Fig. 6a). As seen from Fig. 6b pentlandite recovery also increased in the presence of PEG throughout the ph range studied and toe

6 116 1&1..J 1&1 II) II) ;:j II) If ph Fig. 5. Flocculation of chalcopyrite and pentlandite mixtures as a function of ph with and without PEO. assay remained practically the same again suggesting no enrichment due to PEG addition. A possible reason for the lack of selectivity can be heterocoagulation of chalcopyrite with pentlandite, however, this is not the case in the acid and natural ph regions [2]. Besides heterocoagulation, a major reason for the lack of selectivity can be the activation of pentlandite or deactivation of chalcopyrite

7 by dissolved species such as CU2+, Ni2+, Fe2+ and 82- and their hydroxy species from the minerals present in the system. It has been reported that specifically adsorbing ions can change and/or even reverse the zeta potential of a mineral and thereby cause heterocoagulation as well as enhanced polymer flocculation [7,8,21]. In order to investigate the effect of dissolved mineral cations on the flocculation of chalcopyrite and pentlandite, tests were next conducted in mineral supernatants in the absence of any polymer. The effect of the pentlandite supernatant on the flocculation characteristics of chalcopyrite is illustrated in Fig. 7. It can be seen that chalcopyrite is dispersed by the supernatant in the acid ph range. Above about ph 6., however, increased flocculation of chalcopyrite is obtained in the pentlandite supernatant. On the other hand, pentlandite flocculation is enhanced in the presence of chalcopyrite supernatant throughout the ph range studied (Fig. 8). 117 Fig. 7. Flocculation of cbalcopyri in the presence and in the absence of pentlandi supernatant ph Fig. 8. Flocculation ofpentlandite in the presence and in the absence of chalcopyrite supernatant.

8 118 Dissolution and precipitation/ readsorption of mineral species in mixed mineral systems is considered to be a major reason for the observed effects. This was further confirmed by electrophoretic mobility and by electron spectroscopy for chemical analysis (ESCA) results [14,27,28). In the presence of supernatants zeta potentials of chalcopyrite and pentlandite are found to be reduced considerably due to the adsorption or surface precipitation of dissolved ions and their hydroxy species on the mineral surfaces. ESCA spectrum of chalcopyrite conditioned in pentlandite supernatant and washed free of the supernatant showed that nickel ions can be found on the surface of chalcopyrite. Similarly, the spectrum of pentlandite conditioned in chalcopyrite supernatant and washed also showed presence of copper on the pentlandite surface. The extent of dissolution of chalcopyrite and pentlandite was determined by analyzing supernatants at different ph values using Direct Coupling Plasma Spectrometry (DCP). The results are given in Tables I and II. The above results clearly show that the amount of dissolved species is significant enough under certain ph conditions to have an effect on the flocculation. In order to minimize the effect of the dissolved species on selectivity in flocculation, an attempt was made next to complex them using diphenylguanidine. TABLE I Chemical analysis of chalcopyrite supernatant (natural) TABLE II Chemical analysis of pentlandiu supernatant (natural)

9 119 Effect of diphenyiguanidine on flocculation Diphenylguanidine (DPG) has been extensively used in the flotation separation of INCO matte, which consists of CU2S and Ni3S2' due to its unusual selectivity for copper minerals (22-25]. Based on this information, the effect of DPG on the flocculation of chalcopyrite and pentlandite was tested. The effect of addition of complexing agent diphenylguanidine to the natural chalcopyrite and pentlandite suspensions in the presence and in the absence of PEG is shown in Figs. 9 and 1. 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 caused dispersion of the system at ph < 5 at the concentration levels of 1-3 kmol/ m3, 1-4 kmol/m3 and 1-6 kmol/m3 (Fig. 9). Above ph 5, addition of 1-4 kmol/m3 DPG increased the chalcopyrite flocculation significantly. In the presence of PEG the dispersion effect of 1-4 kmol/m3 DPG is absent at acid ph region. Furthermore, flocculation power of PEG is also absent in the natural and alkaline ph ranges. These results clearly suggest the DPG and PEG have a significant combined effect on the chalcopyrite flocculation. In contrast to the above, pentlandite suspension did not respond to the addition of the 18, , """", 4 f 2 z u I C '..I-2! u ' 8_.,,:..I """; ' eo: 6 6i : '. 'b-o CHALCOPYRITE 1O-mol/m3 DPG 6 1-4kmoll m3 DPG 1-6kmol/m3 DPG a: 4 III 2 z!2 I- C..J -2 u u -4..J -6 -en -8,..6 CHALCOPYRITE A 1mot/m3 DP6. ppm PEO + lo-4kmol/lf!' DP6. ppm PEO -1"" 1"" ph ph Q 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 5% of the supernatant; P,= the mass (g) of solid in the top 5% of the supernatant when a flocculant is used; + 1% corresponds to the complete flocculation and -1% corresponds to the complete dispersion of the suspension. b

10 12 E Go Z t: c..i U U..I. PENTLANDITE 6 lo-4kmol/m3 DPG a lo-8kmol/m3 bpg ::::::::;:;ẕ / -- -1'.. L c.".i.i ph 8: W 2. Go Z 2- c oj C.) C.) oj -2 1,.. u PENTLANOITE mol/m3 OPG.. pprl PEO PEO+l-4.MOV..3 OPG I. 5pp.. :::::1;..., ", too',.. b Fit. 1. Flocculation of pentlandite in the p of: (a) DPG; and (b) DPG and PEO. same concentrations ofdpg (Fig. 1). There is also no effect of the presence of PEa along with D PG on the flocculation of pentlandite. The above results suggested possible selective action by DPG in chalcopyrite-pentlandite mixtures. This was tested by conducting selective flocculation tests with synthetic mixtures of chalcopyrite and pentlandite. It can be seen from Fig. 11 that flocculation of the mixture is enhanced by the presence of DPG and PEa together, particularly in the acid ph region. The effect of the dissolved species is suppressed in this ph range by the presence of DPG. The recovery and assay of chalcopyrite is found to increase from about 7% to 9% and from 5% to about 6%, respectively, in the presence of DPG and PEa together throughout the ph range studied (Fig. 12a). On the other hand, the recovery and assay of pentlandite are reduced compared to the results obtained when only PEa is present (see Figs. 6b and 12b). From the comparison of the results in Figs. 6a and 6b and with those in 12a and 12b, it can be concluded

11 Q W -J l- I- W (II (II Q (II # CHALCOPYRITE + PENTLANOITE 3a1-z.mol/",sNaCl Opp", PEO 6 5 pplr PEO /.' OPG O' I I I I I ph Fig. 11. Flocculation of chalcopyriu and pentlandite mixture. ",*> l- E >- % 6 II 4 '- "'-_...,,A,./ ": -..- : - CHALCOPYRITE + PEHTLANOITE _1/,.3 HaO. OPtNDPEO.6 5ppm PEO+I-4"'al/m3PG 8 I : I. i t I. i. ). I.!4 z -< -' z A. It > u w 1. CHALC<FYRITE + PEHTLANDITE 3.1-2_1/..3 HoC1. Oppm PEO. A PPIII PEG + l-4kmol/m3 DPG 81 II I I soj I I 4 '\ 8-.--c:;--t "'-"'-" "6 11 I 6 4 t: z j z A... c.... c 2' I I I I I Q b Fig. 12. Recovery andy of: (a) chalcopyrite; and (b) pentlandire. 1-.,'..,1,..,'... m R 14 that enhanced enrichment of chalcopyrite is obtained when 1-4 kmol/m3 DPG and 5 ppm PEa are employed together. Lack of selectivity in the flocculation of mixed sulfides is found to be due to interactions of dissolved mineral species with the particle surface. This is supported by the results of flocculation tests in mineral supernatants which showed that while chalcopyrite is dispersed by the pentlandite supernatant, pentlandite is flocculated by the chalcopyrite supernatant. Evidence for the interaction of dissolved mineral species with particle surfaces was obtained from the electrokinetic and ESCA studies [14,27,28]. Electrokinetic studies with chal- ph

12 122 copyrite and pentlandite showed that zeta potentials of these minerals are reduced significantly by the presence of the other's supernatant [27]. Surface chemical analysis of the chalcopyrite sample treated with pentlandite supernatant and washed showed a nickel peak along with copper peaks [28]. Similarly, ESCA spectrum of pentlandite showed a copper peak when the former mineral was treated with chalcopyrite supernatant and washed. SUMMARY ( 1) Single mineral flocculation tests showed that both natural chalcopyrite and pentlandite are flocculated with polyacrylamide polymer. Natural chalcopyrite is also flocculated with polyethylene oxide, but not pentlandite. (2) Polyethylene oxide polymer may be used to flocculate chalcopyrite from its mixture with pentlandite. Selectivity of the process may be enhanced by modifying the polymer adsorption using complexing reagents and by changing the medium ph and temperature. (3) Dissolved mineral species can affect the flocculation process. The results of flocculation tests with binary mixtures showed nonselectivity of chalcopyrite que to possible activation of pentlandite by dissolved species. The evidence of this is seen from the results of the flocculation tests in the presence of supernatants. It is to be noted that in these cases enhanced selectivity of chalcopyrite is obtained when complexing reagent diphenylguanidine is employed along with the polymer. ACKNOWLEDGEMENTS The authors acknowledge the National Science Foundation (MSM , CBT ), the Inco Inc. and the Union Carbide Corporation for support of this work. REFERENC 1 Read, A.D. and Hollick,C.T., Miner. Sci. Eng.,8 (3). 2 MOUdgil, B.M., The Role of Polymer-Surfactant Interactions in the Interfacial Processes. Doctorate Research Proposal. Columbia University, New York. 3 Somasundaran, P., In: P. Somasundaran and N. Arbiter (Editors), Beneficiation of Mineral Fines, Problems and Research Needs. p Attia, Y.A.I., Development of a selective flocculation process for a complex copper ore. Int. J. Miner. Process., 4: Somasundaran, P., Selective flocculation of fines. Presented at Symposium on the Physical Chemistry of Mineral-Reagent Interactions in Sulfide Flotation, College Park, April 6-7, Heerema, R.H., Lipp, R.J. and Iwasaki, 1.,1979. Complexation of calcium iron in selective flocculation of iron ore. Presented at the Annual AIME Meeting, New Orleans, Febr. 21, 1979.

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