Electrochemical flotation separation of chalcocite from molybdenite

Electrochemical flotation separation of chalcocite from molybdenite

International Journal of Mineral Processing, 10 ( 1 9 8 3 ) 8 9 - - 9 4 89 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Nethe...

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International Journal of Mineral Processing, 10 ( 1 9 8 3 ) 8 9 - - 9 4

89

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ELECTROCHEMICAL FLOTATION SEPARATION OF CHALCOCITE FROM MOLYBDENITE

S. CHANDER and D.W. FUERSTENAU

Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720 (U.S.A.) (Received February 20, 1982; revised and accepted July 9, 1982)

ABSTRACT Chander, S. and Fuerstenau, D.W., 1983. Electrochemical flotation separation of chalcocite from molybdenite. Int. J. Miner. Process., 10: 89- 94. The technique of electrochemical flotation has been used to separate chalcocite from molybdenite. Either chalcocite can be depressed and molybdenite floated or vice versa, through appropriate choice of the potential of flotation. Under the conditions investigated in this study molybdenite is found to be relatively inactive, electrochemically. The recovery of chalcocite is affected by the rate of oxidation, as controlled by the change in sweep rate to alter the potential of chalcocite from the conditioning potential to the potential of flotation.

INTRODUCTION

Since the development of the flotation process for beneficiation of sulfide minerals with xanthates as collectors~ many studies have been made in efforts to improve, control and understand flotation behavior (Granville et al., 1972; Chander and Fuerstenau, 1975a; Poling, 1976; and Woods, 1976). From the large number of these investigations that have been directed at mineral surface/collector ion (or molecule) interactions, it is apparent that the oxidation of both the mineral and the collector play an important role in the collection process. It is generally believed that collector ions oxidize at the mineral surface to form either metal-reagent salts or the reagent disulfide. Electrochemical techniques of cyclic- and single-sweep voltammetry have been used in recent years to delineate reaction mechanisms in flotation of sulfide minerals with thiol collectors (Woods, 1971; Chander and Fuerstenau, 1974a). An advantage of these techniques is that investigations over the entire stability range of water can be carried out in a short time. Cyclic voltammetry can be used to elucidate electrochemical reaction mechanisms whereas singlesweep voltammetry is amenable to quantitative analysis of electrochemical data. 0301-7516/83/$03.00

© 1983 Elsevier Science Publishers B.V.

90 This paper presents the results of our continuing investigations to develop an electrochemical flotation cell in which the potential of sulfide mineral particles is controlled by imposing an external potential. A Hallimond tube flotation cell modified to convert the bed of mineral particles into an electrode (Chander and Fuerstenau, 1975b) has been used to separate chalcocite from molybdenite through control of potential of the mineral particles. REVIEW OF ELECTROCHEMICALREACTIONS OF CHALCOCITEAND MOLYBDENITE The reactions of chalcocite and molybdenite in solutions of diethyldithiophosphate collector are briefly reviewed in this section to help understand the response of these minerals in an electrochemical flotation cell. Since the present investigation was carried out at pH 9.3, this review is limited to reactions in slightly alkaline solutions. Chalcocite reactions in the absence of collector Chalcocite undergoes reduction to give copper and soluble sulfur species (Chander and Fuerstenau, 1974a), according to the reaction: Cu2S + H ÷ + 2 e- -~ 2 Cu + HS-

(1)

at potentials less than - 0 . 4 V. The reverse reaction for eq. 1 occurs in the potential range - 0 . 5 to +0.2 V. Chalcocite oxidizes according to the reaction: Cu:S + H20 ~ [Cu 2÷, Cu20, CuO, Cu(OH)2] + [S, CuS, $20~-, etc.] + H* + e-

(2)

The exact nature of the products, which is a function of the oxidation conditions, is poorly understood at present. The copper-containing solid oxidation product is, however, known to be hydrophilic and retards charge transfer at the chalcocite electrode when present in excessive amounts. Very little direct evidence exists regarding the nature of the sulfur-containing oxidation products. These products are expected to have little influence on the flotation response in collector solutions. The reverse reaction for eq. 2 occurs in the potential range - 0 . 5 to +0.2 and its extent depends upon the a m o u n t of oxidation products present initially. Chalcocite reactions in the presence of diethyldithiophosphate Chalcocite reacts electrochemically with diethyldithiophosphate ion, DTP- as follows: DTP- ~ DTP ° + eCu2S + ( 2 - m)DTP ° + mDTP-~ 2Cu DTP + [S] + m e-

(3) (4)

91

Cu:S + (4 - n) DTP ° + n DTP- -* Cu(DTP)2 + [S] + n e-

(5)

[S] implies that the form of the sulfur species is'unidentified and DTP ° is the discharged dithiophosphate species. Reaction 3 occurs at a potential of about 0.0 V whereas reactions 4 and 5 occur at about 0.2 V. The reverse reaction for eq. 3 occurs at potentials of about - 0 . 3 V whereas the reverse of eqs. 4 and 5 occurs in the potential range - 0 . 6 to - 0 . 9 V.

Molybdenite reactions in the absence o f collector Molybdenite is a relatively noble mineral amongst the various sulfides with large rest potentials (Allison et al., 1972). Its rest potential corresponds to a mixed potential involving oxygen reduction and oxidation of molybdenite to molybdate (Chander and Fuerstenau, 1974b). Recent studies in our laboratories show that the rest potential of molybdenite also depends upon the crystal orientation. The results will be published in the proceedings of the 3rd International Symposium on Hydrometallurgy (Krishnaswamy and Fuerstenau, 1983). Under oxidizing conditions molybdenite oxidizes to molybdate ions and the oxidation products of sulfur: MoS2 + 8 OH- ~ MoO~- + 2 IS] + 4 H20 + 6 e-

(6)

The oxidation product of metal in the sulfide is a soluble species in the case of molybdenite as compared to an insoluble, hydrophilic oxide/hydroxide in the case of chalcocite. This is an important difference which may give rise to differences in the flotation response of these two minerals, particularly in the absence of a collector.

Molybdenite reactions in the presence of diethyldithiophosphate Rest potential measurements of a molybdenite electrode in diethyldithiophosphate solutions show that the electrode measures mixed potentials (Chander and Fuerstenau, 1974b). The a m o u n t of the collector adsorbed in alkaline solutions is relatively small indicating relatively less activity of molybdenite towards electrochemical oxidation of the collector. METHODS AND MATERIALS

A conventional three-electrode system with mineral particles as the working electrode, a saturated calomel as the reference electrode, and a "cap f o r m " platinum electrode (Coleman Instruments) as the counter electrode was used. The electrode potential was controlled by a 68TS1 Wenking potentiostat in conjuction with an SMP 69 Wenking Motor potentiometer. Current-potential curves were recorded on an MFE X-Y recorder (Model 815M). The experimental cell used to investigate the effect of potential on flotation was a modified HaUimond tube which has been described elsewhere (Chander and Fuerstenau, 1975b). The electrical contact to the particles to

92 be floated was obtained b y replacing the conventional sintered-glass disk by a g o l d , o a r e d sinteredglass disk (the gold coating was produced by vacuum deposition). The electrical contact to the gold-coated sintered-glass disk was made with a tungsten wire that was also coated with gold. Purified nitrogen gas, after flowing through a copper furnace for the removal of traces of oxygen, was used for leviation in the flotation experiments. In carrying o u t a flotation test, 120 ml of 0.025 M sodium borate solution containing 2 X 10 -3 M KDTP was placed in the cell and nitrogen was bubbled through it for at least 30 minutes. A weighed amount of the mineral mixture to be floated (0.2 g chalcocite + 0.2 g molybdenite) was then added to the cell and a potential of - 1 . 2 V was applied. Nitrogen gas was bubbled through the t o p section of the cell to keep the partial pressure of oxygen low. The powder was then conditioned b y gentle agitation with a magnetic stirrer for a period of t w o minutes. The potential was changed continuously with the m o t o r potentiometer to bring the mineral particles to the potential desired for flotation. Natural chalcocite from Superior, Arizona, obtained from Ward's Natural Science Establishment, Inc., Rochester, N.Y., was crushed in a roll crusher and a 48 X 65 mesh fraction was used. No special care was taken to prevent oxidation during crushing b u t the flotation tests were made only one day after the sample was prepared. It is anticipated that unless a sample becomes excessively oxidized, the cathodic pre-treatment of the sample will remove any oxides at the chalcocite surface (Chander and Fuerstenau, 1974a). Molybdenite from Fortland, Arendal, Norway was also crushed in a roll crusher and the 48 X 65 mesh fraction was used for flotation. The flotation concentrate was analyzed b y dissolving chalcocite in acid and analyzing for the copper content. The water used in all the experiments was prepared b y distilling tap water in a Barnstead Laboratory still and then passing the distillate through a twostage Heraeus Quartz still. Except for the potassium diethyldithiophosphate, only analyzed reagent-grade chemicals were used. Technical-grade diethyldithiophosphoric acid was obtained from Aldrich Chemical Co., Inc., and its potassium salt was prepared and purified b y a m e t h o d described b y Bode and Arnswald (1962). The nitrogen which was purified to remove traces of oxygen b y passage through a column of Kieselguhr impregnated with copper heated to 200 ° C, was bubbled through distilled water prior to its entry into the electrochemical flotation cell. RESULTS AND DISCUSSION The electrochemical flotation results for chalcocite are given in Table I. Chalcocite does n o t float at potentials less than 0.0 V and floats readily at potentials greater than +0.2 V. The rate of oxidation, controlled by the rate of increase of potential from the initial negative potential of - 1 . 2 V to the potential of flotation, does effect the flotation. If the rate of oxidation is

93 TABLE I The effect of potential on the flotation behavior of chalcocite in 2 x 10 3 M DTP at a pH of 9.3 (0.025 M sodium borate). Potential of preconditioning - 1 . 2 V Potential of flotation (V) -- 1.2 -1.0 -0.5 0.0 + 0.2 + 0.5

Sweep rate ot reach the flotation potential 5 mV/s

50 mV/s

No flotation No flotation No flotation No flotation Good flotation Good flotation

No No No No No No

flotation flotation flotation flotation flotation flotation

TABLE II The separation of chalcocite from molybdenite in an electrochemical flotation cell from a 50 : 50 percent mixture. Potential of preconditioning --1.2 V Potential of flotation (V)

Molybdenite recovery (%)

Chalcocite recovery (%)

Chalcocite in conc. (%)

--1.2 -0.6 + 0.3 (50 mv/s)* + 0.3 ( 5 my/s)*

62 39 21 17

14 6 63 100

18 14 75 86

*Rate at which the potential is changed from the conditioning potential to the flotation potential. high, f o r e x a m p l e , with a sweep rate o f 50 m V / s , c h a l c o c i t e does n o t float; whereas if t h e rate o f o x i d a t i o n is small, f o r e x a m p l e , with a sweep rate o f 5 m V / s , c h a l c o c i t e floats. M o l y b d e n i t e was n o t depressed b y a p p l y i n g low potentials and it floats even at p o t e n t i a l s o f - 1 . 2 V. This e l e c t r o c h e m i c a l inactivity o f m o l y b d e n i t e was utilized t o separate c h a l c o c i t e / m o l y b d e n i t e m i x t u r e in t h e e l e c t r o c h e m i c a l f l o t a t i o n cell and t h e results are given in Table II. A t low potentials, c h a l c o c i t e can be depressed and m o l y b d e n i t e floated. A t high potentials, c h a l c o c i t e floats readily, leaving m o l y b d e n i t e in the cell. T h e r e c o v e r y o f c h a l c o c i t e is b e t t e r if t h e rate o f o x i d a t i o n is small, t h a t is, t h e f l o t a t i o n separation is b e t t e r if t h e electrode p o t e n t i a l is increased at a rate o f 5 m V / s t h a n at a sweep rate o f 50 m V / s . T h e results are as e x p e c t e d based o n t h e reactions s u m m a r i z e d in a previous section. CONCLUSIONS The t e c h n i q u e o f e l e c t r o c h e m i c a l f l o t a t i o n has been successfully e m p l o y ed to separate c h a l c o c i t e f r o m m i x t u r e s o f m o l y b d e n i t e and chalcocite.

94

Either molybdenite or chalcocite can be floated depending upon the potential. If molybdenite flotation is desired, the addition of a conventional molybdenite collector such as fuel oil should help improve its recovery. Depression of molybdenite at high potentials is considered to result primarily because of the competition with a more hydrophobic constituent, namely, chalcocite. ACKNOWLEDGEMENTS

Support of the National Science Foundation for this research is acknowledged. The assistance of Dr. J.M. Wie in analyzing the samples is also acknowledged.

REFERENCES Allison, S.A., Gould, L.A., Nicol, M.J. and Granville, A., 1972. A determination of the products of reaction between various sulfide minerals and aqueous xanthate solution, and a correlation of the products with electrode rest potentials. Metall. Trans., 3: 2613--2618. Bode, H. and Arnswald, W., 1962. Untersuchungen fiber substituierte, Dithiophosphate, I. Mitteilung, Die Diathyldithiophosorsaure und ihr Natriumsalz. Z. Anal. Chem., 185: 99--110. Chander, S. and Fuerstenau, D.W., 1974a. The effect of potassium diethyldithiophosphate on the electrochemical properties of platinum, copper and copper sulfide in aqueous solutions. Electroanal. Chem. Interfacial Electrochem., 56: 217--247. Chander, S. and Fuerstenau, D.W., 1974b. Effect of potassium diethyldithiophosphate on the interfacial properties of molybdenite. Trans. IMM, 83: C180--C185. Chander, S. and Fuerstenau, D.W., 1975a. Electrochemical reaction control of contact angles on copper and synthetic chalcocite in aqueous potassium diethyldithiophosphate solutions. Int. J. Miner. Process., 2: 333--352. Chander, S. and Fuerstenau, D.W., 1975b. Effect of potential on the flotation and wetting behavior of chalcocite and copper. Trans. SME]AIME, 258: 284--285. Granville, A., Finkelstain, N.P. and Allison, A.A., 1972. Review of reactions in the flotation system galena-xanthate-oxygen. Trans. IMM, 81: C1--C30. Krishnaswamy, P. and Fuerstenau, D.W., 1983. Electrochemical mechanisms for the aqueous dissolution of molybdenite. Proc. 3rd Int. Symposium on Hydrometallurgy, Atlanta, Georgia, in press. Poling, G.W., 1976. Reactions between thiol reagents and sulfide minerals. In: M.C. Fuerstenau (Editor), Flotation, Vol. 1, AIME, New York, N.Y., pp. 334--363. Woods, R., 1971. The oxidation of ethyl xanthate on platinum, gold, copper and galena electrodes, relation to the mechanism of mineral flotation. J. Phys. Chem., 75: 354--362. Woods, R., 1976. Electrochemistry of sulfide flotation. In: M.C. Fuerstenau (Editor), Flotation, Vol. 1. AIME, New York, N.Y., pp. 258--333.