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Fluidised-bed anodic dissolution of chalcocite
HydrometaUurgy, 1 (1976) 241--257 @ Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
FLUIDISED-BED ANODIC DISSOLUTION OF CHALCOCITE
D.J. MACKINNON
Metallurgical Chemistry Section, Mineral Sciences Laboratories, Canada Centre for Mineral and Energy Technology, Department of Energy, Mines and Resources, Ottawa (Canada) (Received May 1st, 1975)
ABSTRACT MacKinnon, D.J.,1976. Fluidised-bedanodic dissolutionof chalcocite.Hydrometallurgy, 1: 241--257. The anodic dissolution of chalcocite (Cu= S) has been investigated using a fluidised-bed anode technique. Results obtained for a variety of electrolytes and experimental conditions indicate that the fluidised-bed anodic dissolution of chalcocite occurs via the formation of an intermediate copper sulphide, viz., "blue-remaining" covellite, Cu~. 1S. In stflphuric acid electrolyte the dissolution of chalcocite is inhibited after about 50% copper extraction by the vigorous evolution of oxygen gas at the platinum feeder anode. In both sulphuric acid--sodium chloride and sulphuric acid--potassium bromide electrolytes, the dissolution of chalcocite occurs to 95% copper extraction in two stages. The first stage involves the formation of Cu~ .~ S, as is the case for the sulphuric acid electrolyte1 while the second stage is attributed to the reaction between chloride (or bromide) and Cul .~ S.
INTRODUCTION Aqueous electrolysisusing sulphide anodes has given positive results (Renzoni et al.,1958) in the case of nickel sulphide matte (essentiallypure Ni3 $2 ) and as a result these anodes are n o w electrolyticallyrefined on an industrial scale by the International Nickel Co., of Canada Ltd. (Spence and Cook, 1964). In recent years there has been an increased interest in developing an industrial process for the direct electrorefining of copper matte (Cu2 S) anodes (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; K u x m a n n and Biallass, 1969; Chizhikov, 1969). These investigations have been chiefly concerned with the effect of variables such as temperature, electrolyte concentration and current density on the current efficiency during the electrodissolution of Cu2S and white metal cast anodes. In all cases an abrupt increase in the cell voltage was observed during the course of the electrolysis of Cu2S. This increase was associated with the formation of granular and porous anode slimes which adhered to the surface of the anode. Most authors (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; K u x m a n n and Biallass, 1969) found that the anodic current efficiency and the time at which the sharp
242 rise in the cell voltage occurred increased with increasing temperature and decreasing current density. Habashi and Torres-Acuna (1968) found an optimum in the anodic current efficiency for an electrolyte containing 100 kg m -3 H2SO4. However, Venkatachaiem and Mallikarjunan (1968), and Kuxmann and Biallass (1969) observed that the H2SO4 concentration had only a slight effect on the anodic current efficiency. Loshkarev and Vozisov (1953), and Habashi and Torres-Acuna (1968} reported that the sharp increase in cell voltage occurred more rapidly in concentrated acid solutions. The anode slimes, formed during the electrolysis of Cu2 S, were found to consist of elemental sulphur and covellite (CuS) (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; Chizhikov, 1969; Kuxmann and Biallass, 1969)*. Habashi and TorresAcuna (1968), and later Venkatachalem and Mallikarjunan (1971), observed the formation of digenite (Cul.sS) as an intermediate in the early stages of Cu2S electrolysis. The reported amount of elemental sulphur oxidized to sulphate varied widely from author to author. Thus Habashi and Torres-Acuna (1968) indicated that practically no oxidation of sulphur occurred, but Loshkarev and Vozisov (1953) and Venkatachalem and Mallikarjunan (1968) described measurements which resulted in 55% sulphur oxidation. Kuxmann and Biallass (19691 determined that the amount of sulphur oxidized to sulphate varied between 4 and 10%. The addition of chloride ion (Chizhikov, 1969; Venkatachalem and Mallikarjunan, 1971) to a sulphate electrolyte increased the amount of elemental sulphur in the anode slimes. In addition, the anodic current efficiency increased and the cell voltage decreased in the presence of chloride ion. It was suggested that the passivity which developed at the sulphide electrode with the progress of electrolysis in a sulphate electrolyte was hindered in the presence of chloride ion, thus permitting the continued dissolution of Cu2 S at high current efficiency and low cell voltage. The preceding discussion has summarized the results for the electrolysis of Cu2 S which were obtained using conventional planar anodes (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; Chizhikov, 1969; Kuxmann and Biallass, 1969}. These anodes were prepared by casting either molten reagent grade Cu2S or white metal. The direct dissolution of finely-divided Cu2 S flotation concentrates to form metallic copper (or copper compound) and elemental sulphur in a single step with no intermediate smelting or casting stages is also worthy of investigation. There are considerable problems involved in such a one step process, not the least of which is devising methods of conducting a sustained electrical current to the individual sulphide particles. There are essentially three systems which may be used: (i) A fluidised-bed e l e c t r o d e - a bed of conductive particles is fluidised by the * In t h e a b o v e m e n t i o n e d papers, n o a t t e m p t w a s reported t o distinguish b e t w e e n "blueremaining" c o v e l l i t e (Cu I .i S) and true c o v e l l i t e (CuS).
243 upward flow of electrolyte; a feeder electrode provides electrical contact to the bed. (ii) A c o m p a c t (pellet) electrode -- the powdered sulphide is pressed into a solid conducting cylinder; electrical contact is provided by an external wire fixed to the pellet i n such a way that the wire is insulated from the electrolyte. (iii) Electrolytic generation o f leaching agents -- the sulphide particles are suspended in an electrolyte and the oxidant, e.g. chlorine or ferric ion, is generated at an inert anode. The fluidised-bed and c o m p a c t anode systems have been studied in some detail as part of a program to obtain an increased understanding of the chemistry of sulphide electrolysis and to assess the feasibility of using sulphide concentrates in electrowinning processes. Only the results of the fluidised-bed studies will be considered here. Fluidised-bed electrodes (FBE) have a n u m b e r of advantages over conventional planar electrodes, the most important being that they contain a very large surface area per unit volume. In this respect they are particularly useful for processes that can only be carried o u t economically at low current densities. This report describes the results obtained for the electrodissolution of chalcocite (Cu2 S) using a fluidised-bed electrode technique. MATERIALS AND APPARATUS
Both synthetic chalcocite and a chalcocite flotation concentrate were used in the test work. The synthetic chalcocite was prepared according to the method of Dutrizac and MacDonald (1973). X-ray diffraction analysis of the synthetic chalcocite indicated only the orthorhombic chalcocite form of Cu2 S. Chemical analysis gave 79.9% Cu and 19.3% S. X-ray diffraction analysis of the flotation concentrate indicated chalcocite, quartz and hematite plus a minor a m o u n t of amphibole and a trace a m o u n t of magnetite. After up-grading the concentrate, chemical analysis gave 75.7% Cu, 19.2% S and 1.3% Fe. The electrolyte used in the initial test work was H2 SO4 o f varying concentrations (50--200 kg m-3). In later experiments various amounts of NaC1 or KBr were added to the H2 SO4 electrolyte. The fluidised-bed electrode cell is shown schematically in Figure 1. The cell consisted of a bed of conducting particles (Cu2S), +325 mesh, supported on a glass wool plug. Platinum sheet, in the shape of a cone, was located in the bed and served as the anodic current feeder. The counter electrode (cathode in this case) was a platinum wire located in a side arm which was separated from the fluidised-bed anode c o m p a r t m e n t by a glass frit. A capillary probe, located just above the bed, was connected externally to a saturated calomel electrode (SCE) which served as a reference electrode. The anode potential relative to the SCE and the cell voltage were measured and recorded b y means of a high input impedance Hewlett-Packard dual channel strip chart recorder (Model 7100 B). The current to the cell was supplied b y a Hewlett-Packard Regulated D.C. Power Supply, model 6 2 1 4 A (0--10 V d.c., 0--1 A). Fluidisation of the particles was achieved b y continuously circulating the anolyte b y means of a Manostat Varis-
244
I t
/ 1-
"EL%~LYTE
CAPILLARYPROBE
COUNTER ELECTRODE GLASS FRIT
DRAPNPOINT
Fig. 1. Schematic diagram of fluidised-bed electrode cell. talticP u m p (advanced model). The anolyte entered the tapered bottom of the cell via a horizontal inlet tube and left via a horizontal outlet tube located at the top of the cell.A flow rate of about 400 cc/min provided a stable fluidisedbed anode. EXPERIMENTAL The required a m o u n t o f Cu2 S (10--20 g) was placed in the cell and the p u m p was switched on to circulate the electrolyte. The circulating electrolyte was heated to the required temperature b y means of an immersion heater located in the anode c o m p a r t m e n t of the cell controlled b y a Matheson Lab-Stat. When equilibrium conditions were achieved the p o w e r supply was switched on and set to the desired current. Both the anode potential and cell voltage were recorded using the Hewlett-Packard strip chart recorder. Samples were taken periodically and analysed for copper using a Varian Techtron AA 5 atomic absorption spectrophotometer. The atomic absorption data were then entered into a c o m p u t e r program which calculated the a m o u n t of copper dissolved and also provided copper dissolution vs. time plots, from which the rate data were calculated. At the end o f the test the solution was drained from the cell and the residue was collected, washed and dried. A portion of the dried residue was analysed b y X-ray diffraction analysis and the remainder by chemical analysis.
245
RESULTS AND DISCUSSION
Preliminary investigations The initial test work was done using the chalcocite flotation concentrate (20 g charge) and H2 SO4 as the electrolyte. The effects of applied current, electrolyte flow rate (degree of bed fluidisation), Hs SO4 concentration, temperatuxe (25--60°C), initial Cu s+ concentration and CI- on the initial rate of Cuss dissolution were investigated. The dissolution rate of Cuss was found to be linear with time and directly proportional to the applied current (0.25 to 1.0 A). The other variables mentioned above had only a slight effect on the dissolution rate which was 0.6--0.7 g Cu/h at 0.5 A. During these preliminary tests it was observed that the anode potential increased abruptly when 40 to 50% of the copper contained in the Cu2 S had dissolved. This abrupt increase in anode potential was accompanied by O~ gassing at the Pt feeder electrode. When O2 gassing occurred the copper dissolution rate decreased, and the test was usually stopped at this point. Similar effects were observed when synthetic Cu2 S was substituted for the Cu~ S flotation concentrate. In the next section the anodic dissolution of synthetic Cu2S in H2 SO4 electrolyte is described in detail.
The Cus S/Hs S04 system The dissolution behaviour of synthetic Cu~ S at constant current (0.5 A) in 100 kg m -3 H2 SO4 at 40°C is shown in Figure 2 as a plot of copper dissolution vs. the time in hours. The rate of copper dissolution (0.64 g/h) is linear for about 10 hours. At this point the anode potential increases abruptly from 0.35 to 1.8 V vs. SCE, 02 evolution occurs, and the rate of copper dissolution slows 75
2 0 g S Y N T H E T I C Cu2S 40 °C 0.5 A
~. 5.0
a- ~
2.5
0
,
2.5
5.0
7.5
..
I0.0
T I M E (HOURS}
Fig. 2. Plot of dissolved Cu vs. time for H~SO4 electrolyte.
246 considerably. X-ray diffraction analysis indicated that the residue was a CuSlike phase i.e. "blue-remaining" covellite, Cu1.1 S. Elemental sulphur was n o t detected. The residue contained 67.8% Cu and 29.6% S. A second test was done under the same experimental conditions except that the anode potential was controlled at 0.68 volt vs. SCE b y means o f a potentiostar in order to prevent oxygen evolution at the Pt feeder electrode. The results are shown in Figure 3 as a plot of dissolved copper vs. time. The initial rate, 0.77 g Cu/h, was linear up to 5 hours, after which time it began to decrease. During the potentiostatic dissolution of Cu2 S the anodic current gradually decreased from an initial value of 0.8 to 0.05 A after 14 hours so that the reaction became extremely slow. Again, X-ray diffraction analysis of the residue indicate only the presence of "blue-remaining" covellite and chemical analysis indicated a composition of 67.9% Cu and 31.5% S. Microprobe analysis of the anode residue indicated that elemental sulphur was n o t present. I0,0
0 ?.5
00-
o 5.0 o -
2.5
20 g S Y N T H E T I C CutS I 0 0 g / t HIIS04;40=C ANODE P O T E N T I A L = 0.68 VOLT v=,
5.0
I0.0
SCE
15.0
TIME (HOURSI
Fig. 3. Plot of dissolved Cu vs. time for H2804 electrolyte. The results of the constant current (galvanostatic) and the controlled potential (potentiostatic) anodic dissolution of Cu2 S in sulphuric acid electrolyte together with X-ray diffraction, microprobe, and chemical analysis of the anodi residues suggest that the anodic dissolution of Cu2S in sulphuric acid electrolyte occurs according to the reaction: Cu2 S ÷ Cul.1S + 0.9 Cu 2÷ + 1.8 e-
(1)
The anodic dissolution of Cu2S under these conditions occurs at relatively low anodic potential until the Cu2 S particles b e c o m e converted to Cu1.1 S. At this point the anode potential increases abruptly under galvanostatic conditions to
247
value at which O2 evolves from the Pt feeder electrode. For the potentiostatic case, at anode potentials less than that required for 02 evolution, the current falls to a very low value when the Cu2 S particles b e c o m e converted to Cu1.1 S. This behaviour suggests that an additional overvoltage, possibly associated with the nucleation of elemental sulphur, is required for the continued dissolution of Cu1.1 S. In any case oxygen evolution occurs at the Pt feeder anode H20 -~ 1/~O2 + 2H ÷ + 2e:
(2)
in preference to Cu1.1 S dissolution, which indicates that an anode potential > 1 . 8 V vs. SCE* is necessary for the dissolution o f Cul .1 S under these experimental conditions. This behavior will be discussed in more detail in a later paper dealing with the fluidised-bed anodic dissolution of covellite and chalcopyrite (MacKinnon, to be published}. This result is in general agreement with that obtained for solid, planar anodes of cast Cu2S (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968; Kuxmann and Bialass, 1969}. However, the residue (deep blue in colour} contained no elemental sulphur according to X-ray diffraction and microprobe analyses. Elemental sulphur was detected in the anode slimes formed on the cast Cu2 S anodes (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968; Kuxmann and Biallass, 1969) b u t only with prolonged electrolysis after the cell voltage had increased. It should be noted that Kato and Oki (1972) in their study of the anodic dissolution of CuS observed that the anode potential increased abruptly to approximately 2 V vs. SCE after only a few minutes of polarization at a current density of 1 mA/cm 2 .
The Cu2 S/H2 S04/NaCl system A series of tests was done using 100 kg m - 3 H2 SO4 plus various amounts o f NaC1 (10, 25, 50 and 100 kg m -3 ) as the electrolyte, The effect of adding NaCl to the H2 SO4 electrolyte on the dissolution o f synthetic Cu2 S is shown in Figures 4 and 5 in plots of copper dissolved as a function o f time. The experimental conditions are presented with the Figures. At 40°C, with either 25 or 50 kg NaC1 m -3 added to the H2SO4 electrolyte, there appear to be t w o distinct stages in the dissolution o f Cu2 S. The second stage o f dissolution occurs when 40 to 50% of the copper has been removed from the Cu2 S. This is precisely the point where the reaction virtually stops when the electrolyte does not contain NaC1. Increasing the temperature to 90°C (see Fig. 5) has only a slight effect on the first stage of dissolution. However, the rate of the second stage of dissolution is substantially increased. These results are summarized in Table 1 and show that increasing the NaC1 content o f the electrolyte from 25 to 50 kg m- 3 increases the rate o f b o t h stages of Cu2 S dissolution b y a similar amount. * T h e a n o d e p o t e n t i a l e s t a b l i s h e d a t t h e P t f e e d e r e l e c t r o d e in 1 0 0 kg m -3 H 2 S O 4 a t 0.5 A
in the absence o f C u 2 S is 1.8 V vs. SCE. V i g o r o u s O~ e v o l u t i o n o c c u r s a t t h i s p o t e n t i a l .
248
7.S 1
I
i
I
I IO.O
I 15.0
lOg SYNTHETIC Cu:tS IO0 9 / t H|SO,, + 2 5 g / t N a C )
-~
5.0
o
-~ a 2,5
0
I 5.0
20.0
T I M E (HOURS)
Fig. 4. Plot of dissolved Cu vs. time for H= SO 4 - N a C I electrolyte.
15.0
I
i
i
20g SYNTHETIC Cu=S i O 0 g / t H R S O , ÷ S O g / t NoCL 0.5A SO*C
"; io,o
> o
(n a--
5,0
I 5.0
I I0.0
I 15.0
20.0
TIME (HOURS)
Fig. 5. Plot o f dissolved Cu vs. time for H~ SO 4 --NaCI electrolyte.
Increasing the NaCI content from 50 to 1 0 0 kg m -3 had no additional effect on the dissolution o f Cu2 S. The dependence o f the anode potential on time for the dissolution of Cu2 S in H2 SO4--NaCI electrolyte is presented in Figure 6. This plot shows a series of potential--time profiles obtained for the conditions indicated on the diagram. Again, as was the case for the H2 SO4 electrolyte, the anode potential increases abruptly when 40 to 50% of the copper has been dissolved o u t o f the Cu2 S. However, in this case, the potential levels o f f at about 1.1 V vs. SCE, a value considerably less than that required for 02 evolution but which corresponds to the value of the redox potential for the half-cell reaction*: * This w a s c o n f i r m e d b y measuring the potential established at the anode feeder when a current o f 0.5 A was passed through the H S SO4--NaC1 electrolyte in the absence o f Cu 2 S. The anode potential immediately rose to 1.1 V vs. SCE and remained constant. Gaseous CI 2 was observed at the Pt feeder electrode.
249 TABLE 1 Fluid-bed anodic dissolution of synthetic Cu 2 S in H2 SO4 --NaCl mixtures at 0.5 A
Cu2S
Electrolyte
(g)
(kg m - s )
10 20 20
Temp. (+ I°C)
100 H2 SO4 + 25 NaC1 100 H2 SO4 + 50 NaCl 100 H2 S04 + 50 NaCl
40 40 90
Rate 1st stage (g Cu/h)
stage
0.60 0.70 0.77
0.24 0.34 0.54
CI- ~ lhC12 + e-
Rate 2nd
(g Cu/h)
(3)
Increasing temperature allows the potential to remain at a low value for an extended time period in agreement with results obtained by previous workers (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; K u x m a n n and Biallass, 1969; Chizhikov, 1969). It~ is interesting to note that when the a m o u n t of Cu2 S is doubled, the time it takes for the anode potential to increase is also doubled (compare the t w o 40°C profiles in Fig. 6). It also appears that the second stage of Cu~ S dissolution is associated with the increase in the anode potential. This behaviour suggests that the redox reaction, equation (3), provides an electron transfer mechanism between the sulphide particles and the Pt feeder anode. Because the anode potential rises to, and remains at, the value for the discharge o f CI-, it suggests that the dissolution of Cu1.1 S is caused b y the action o f C12 p r o d u c e d at the Pt feeder anode, according to the reaction Cu1.1S + 1/2 C12 * 1.1Cu 2÷ + S O + Cl- + 1.2 er.5
I IO09/L
I H~SO4+5Og/L 0.SA
I NOCL
>o W
1.0
40eC
90eC
'~ 0 , 5 o., 0
I 5.0
I I0.0
I 15.0
20.0
T IM I: [ H O U R S ~
Fig. 6. Plot of anode potential vs. time for Hj S04--NaCl electrolyte.
(4)
250 All the C12 produced does n o t react with Cul.1 S as some of the gas escapes to the atmosphere, which accounts for the reduced current efficiency observed for the second stage o f Cu2S dissolution (see later). The residues obtained from the dissolution of Cu2 S in the H2 SO4 --NaC1 mixtures were subjected to both X-ray diffraction and chemical analysis. The X-ray diffraction analysis indicated t h a t the residue consisted of Cul.1 S plus sulphur. Thus the second stage o f the anodic dissolution of Cu2 S observed for the experimental conditions described above is probably represented by reaction (4) above. The results obtained from chemical analysis o f several residues are presented in Table 2. The results in Table 2 indicate t h a t a reasonable material balance was o b t a i n e d Discrepancies can occur by (a) slight oxidation of sulphur to sulphate and (b) sulphide losses during cleaning of the cell and recovery of the reaction product. The test in which 100 kg m -s HC1 was used as the electrolyte indicates that some degree of oxidation of sulphur to sulphate does occur. The results presented in Table 2 also show t h a t the copper c o n t e n t of the residue decreases with increased temperature and chloride concentration. This result is in agreem e n t with those obtained by Chizhikov (1969) and Venkatachalem and Mallikarjunan (1971) who observed an increase in the sulphur c o n t e n t o f the anodes slimes in the presence of Cl-. As shown in Table 2, the residues contained a substantial a m o u n t of copper (>20%). The fact t h a t it was n o t possible to produce residues containing less copper (<20%) is most p r o b a b l y due to the physical limitation of the anode rather than to decreased chemical reactivity of the Cu2S. Results obtained using compacted Cu2S anodes have yielded residues containing < 2% copper (MacKinnon, unpublished results). Because of its complexity, the fluidised-bed anode is difficult to describe analytically. Unlike a solid electrode, in which the reactive and boundary layer areas are well defined, the fluidised-bed anode is constantly changing as the particles make and break contact with the Pt feeder electrode. The quality of the contact between the particle and the feeder electrode may
TABLE 2 Composition of various Cu~ S residues a Electrolyte
(kgm -3)
Temp. Cu (+I°C) (%)
STOT (%)
SO (%)
SO, 2 (%)
C1(%)
Total (%)
100 100 100 100
40 70 40 70 40
45.4 77.4 53.2 72.0 59.7
14.5 63.5 -57.4 37.5
1.1 .15 .60 1.1 3.0
.02 .69 .21 .29 .20
99.7 97.7 98.4 98.4 101.6
H2SO" + 25 NaC1 H2SO4 + 25 NaC1 H2SO4 + 50 NaCl H2SO4 + 50 NaCl 100 HCI
53.2 19.5 44.4 25.0 37.0
a Starting material: synthetic Cu 2 S; 79.7% Cu; 20.2% S
251
vary widely, being affected by the nature of the solution surrounding the particle and by the length of time that the particle is in contact with the electrode. It is quite possible that sulphur on the surface of the sulphide particles could act as an insulator. Also, during the later stages of the reaction, more and more sulphur is formed, thus reducing the frequency of contact between unreacted sulphide particles and the anode feeder, and also reducing the effectiveness of the 1~C12/C1- charge transfer process.
The Cu2 S/H2 S04/KBr system Because the dissolution of Cu2S continued beyond 50% Cu extraction in the presence of C1- several tests were done in which KBr was added to the H2SO4 electrolyte in order to determine the effect of Br- on the dissolution of Cu2 S. The results obtained using 100 g/1 H2 SO4 + 50 gfl KBr as the electrolyte at a current of 0.5 A for various temperatures is shown in Figure 7 as a plot of dissolved copper vs. time. At 40°C, the Cu2 S dissolution is similar to that observed when NaC1 was added to the electrolyte i.e., there appear to be two stages to the dissolution process with the second stage proceeding at about one half the rate of the first stage. However, at higher temperatures, (70 ° and 90°C), there is initially a very slow stage (~4 h) followed by a much faster linear dissolution stage (11 h) after which the rate decreases. The results summarized in Table 3 indicate that increasing temperature causes a moderate increase in the linear rate of Cu2 S dissolution in the mixed H2 SO4--KBr electrolyte. The behaviour of the anode potential as a function of time is shown in Figure 8. For the H2 SO4--KBr electrolyte, the potential gradually increased with time until it reached a limiting value of ~0.80 volt vs. SCE. This value of the anode potential corresponds to that for the redox couple Br2/2Br- vs. the saturated calomel electrode. Thus the i n c ~ e d dissolution of Cu2S in the H2SO4/KBr 15.0
I
t
I¸
2Og SYNTHETIC CuIS I O O g / [ H z S O 4 + 5 O g / L KBr 0.5 A
I0.0
o
,o-c
X
7 0 *C
_~x~
x
a uJ 0 s.o
I S.O
i fO.O
I 15.O
20.O
TIME (HOURS)
Fig. 7. Plot of dissolved Cu vs. t i m e for H 2 SO 4 - K B r electrolyte.
252
TABLE 3 Fluid-bed anodic dissolution of synthetic Cu2 S in H 2 SO 4 --KBr mixtures at 0.5 A Cu 2 S
Electrolyte
Temp.
Linear rate
(g)
(kg m -s )
(_.1°C)
(g/h)
20 20 20 10
100 100 100 100
40 70 90 40
0.57 0.86 0.94 0.66
H= SO 4 H2 SO4 H2 SO4 H2 SO4
+ 50 + 50 + 50 + 50
KBr KBr KBr KBr
electrolyte may be attributed to the action of the Br2/2Br- redox couple servin as an electron transfer mechanism between the sulphide particles and the Pt feeder electrode. X-ray diffraction analysis of the residues obtained from the fluid-bed anodic dissolution of synthetic Cu2 S in H2 SO4--KBr mixtures indicated the presence of Cul.l S and orthorhombic sulphur as the major constituents, plus a minor amount of w-sulphur. The results from chemical analysis of the various residue: are presented in Table 4. As indicated by Table 4, the residues contained a substantial amount of copper, and as mentioned previously this is probably due to physical limitations of the fluidised-bed anode.
Current efficiency The current efficiency calculated for the anodic dissolution of Cu2 S was based on the assumption that Cu 2÷ and not Cu÷ was the dissolving species*. Th~ current efficiencies for the anodic dissolution of Cu2 S obtained for a variety of experimental conditions are summarized in Table 5. The values are based on the linear rate only, i.e., that initial straight portion of the rate plots. For the H2 SO4 and H2 SO4--NaC1 electrolytes these values refer to approximately 50% removal of the Cu from Cu2 S, i.e., to the time when O2 evolution occurs in the H2 SO4 electrolyte or to the time when the second stage of dissolution begins in the case of the H2 SO4 --NaC1 electrolyte. For the H2 SO4 --KBr electrolyte at 70 and 90°C, the current efficiency values do not correspond to the initial dissolution but rather to the linear dissolution rate that occurs about 4 hours after the start of the tests (see Fig. 7). As indicated by the data in Table 5, the current efficiency for the initial dissolution of synthetic Cu2 S is generally in excess of 100%. This implies that * Habashi and Torres-Acuna (1968) postulate that Cu + is the dissolving species in the anodic dissolution o f Cu= S b u t present no evidence to justify this claim. Even if copper was removec from t h e lattice as Cu ÷ it would be oxidized on the surface o f the anode because t~e standard potential o f the reaction Cu ÷ + Cu =÷ + e - (E °= 0.15 V) is much lower than the oxidation potential of Cu~ S (~0~5 V) and Cu ÷ is not stable under these conditions.
253
1.5
I
I
i
"~
2 0 9 S Y N T H E T I C CuzS g / I H = S O 4 + 5 O g / t KBr
I00
0.5A
0
tu
1.0
O
O.5
0 m
70
°C
90 =C
I 5.0
i010 TIME
1
15.0
20.q
(HOURS)
Fig. 8. Plot o f anode potential vs. time for H2 S04 --KBr electrolyte.
there is some chemical dissolution occurring (particularly in the presence of NaCl and KBr at the high temperatures) as well as the electro-chemical dissolution. The current efficiencies for the anodic dissolution o f the Cu2 S flotation concentrate are consistently less than 100%. This is because part o f the applied current is being consumed by the oxidation of the Fe impurity contained in the concentrate. When approximately 50% of the Cu has been removed from the Cu2S, the current efficiency decreases. This is particularly true for the H2 SO4 electrolyte where the decrease in the current efficiency is due to vigorous 02 evolution at the anode feeder electrode. For the H~ SO4--NaCI electrolyte, the current efficiency associated with the second stage of Cu2 S dissolution (see Figs. 4 and 5) is substantially <100% except for the test done at 90°C. The results are summarized in Table 6. The second stage of Cu~ S dissolution in the H2 S O 4 NaCl electrolyte is associated with the reaction between C12 and Cu~.l S and occurs when the anode potential increases to the value for the discharge of Cl-
TABLE 4 Composition o f various Cu= S residues a Electrolyte (kgm -s) 100 100 100 100
H2SO 4 H2SO4 H= SO4 H 2 SO 4
+ + + +
50 50 50 50
KBr KBr KBr KBrb
Temp. (+I°C)
Cu
STOT
SO
SO 42 -
Br-
Total
(%)
(%)
(%)
(%)
(%)
(%)
40 70 90 40
37.1 41.0 28.1 36.4
59.7 52.5 67.3 49.5
31.2 32.0 48.0 31.3
2.4 0.16 0.11 0.54
0.4 1.06 0.6 0.1
99.6 94.7 96.0 86.5
a Starting material: synthetic Cu 2 S; 79.7% Cu; 20.2% S b Flotation concentrate: 75.2% Cu; 19.1% S
254 TABLE 5 Current efficiencies for Cu 2 S dissolution Cu 2 S Wpe
Electrolyte (kg m - s )
Temp. (±1°C)
Linear rate (g/h)
Current efficiency (%)
Flot. conc. Synthetic Synthetic Synthetic Hot. conc. Flot. conc. Hot. conc. Flot. conc. Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Flot. conc. Flot. conc. Flot. conc. Synthetic Synthetic Synthetic
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
40 40 40 40 40 40 40 40 40 40 70 40 40 70 90 40 40 40 40 70 90
0.63 0.64 0.59 0.65 0.57 0.56 0.56 0.58 0.55 0.60 0.55 0.54 0.66 0.64 0.77 0.53 0.57 0.53 0.66 0.86 0.94
106 108 99.5 110 96.1 94.4 94.4 97.8 92.7 101 92.7 91.1 111 108 130 89.4 96.1 91.1 111 145 159
H 2 SO4 H 2 SO4 H 2SO 4 HCI H 2 SO, H 2 SO4 H 2 SO4 H~ SO4 H 2 SO, H 2SO 4 H~ SO4 H 2 SO4 H 2SO 4 H 2 SO 4 H 2SO 4 H= SO 4 H 2 SO4 H 2 SO 4 H~SO 4 H 2SO 4 H 2SO 4
+ 5 + 10 + 25 + 25 + 25 + 25 + 25 + 50 + 50 + 50 + 50 + 50 + 50 + 50 + 50 + 50 + 50
NaCI NaCI NaCI NaCI NaC1 NaCI NaCI NaCI NaCI NaCI NaC1 NaC1 NaCI KBr KBr KBr KBr
a t t h e a n o d e f e e d e r e l e c t r o d e . T h e d e c r e a s e in c u r r e n t e f f i c i e n c y f o r t h i s s t a g e o f t h e r e a c t i o n is a t t r i b u t e d t o t h e loss o f C12 t o t h e a t m o s p h e r e . T h e c u r r e n t e f f i c i e n c i e s o b t a i n e d a t 70 a n d 90°C f o r Cu2 S d i s s o l u t i o n in t h e H2 S O 4 - - K B r e l e c t r o l y t e w e r e e x t r e m e l y h i g h f o r t h e l i n e a r p o r t i o n o f t h e diss o l u t i o n c u r v e (see Fig. 7) i n d i c a t i n g a c o n s i d e r a b l e a m o u n t o f c h e m i c a l d i s s o l u t i o n . H o w e v e r , t h e i n i t i a l 3 t o 4 h o u r s Cu2 S d i s s o l u t i o n a t 70 a n d 90°C in t h i s e l e c t r o l y t e was v e r y s l o w a n d t h e c u r r e n t e f f i c i e n c i e s w e r e o n l y 3 3 . 7 a n d 4 6 . 4 % , r e s p e c t i v e l y . T h e r e a s o n f o r t h i s b e h a v i o u r is n o t k n o w n . CONCLUSION A g e n e r a l s u m m a r y o f t h e r e s u l t s o b t a i n e d f o r t h e a n o d i c d i s s o l u t i o n o f Cu2 S f o r a v a r i e t y o f e x p e r i m e n t a l c o n d i t i o n s u s i n g a f l u i d i s e d - b e d e l e c t r o d e is present e d in T a b l e 7, a n d i n d i c a t e s t h a t t h e r e a c t i o n in H2 SO4 e l e c t r o l y t e v i r t u a l l y c e a s e s a f t e r 50% Cu e x t r a c t i o n . T h i s is d u e t o a c h a n g e in t h e c o m p o s i t i o n o f t h e s o l i d s , w h i c h r e s u l t s in an a b r u p t i n c r e a s e in t h e a n o d e p o t e n t i a l w h i c h in turn causes vigorous 02 evolution at the feeder electrode and a subsequent d e c r e a s e in t h e c u r r e n t e f f i c i e n c y . T h e r e a c t i o n p r o d u c t was i d e n t i f i e d as " b l u e -
255 TABLE 6 Current efficiency for the second stage dissolution of Cu2S in H2SO4--NaCI electrolyte ]~ectrolyte (kg m -3 )
Temp. (+ 1°C)
Rate 2nd stage (g Cu/h)
Current efficiency (%)
100 H 2 SO 4 + 25 NaCI 100 H2 SO4 + 50 NaCl 100 H 2 SO 4 + 50 NaCI
40 40 90
0.24 0.34 0.54
40.5 57.3 91.1
TABLE 7 Fluidised-bed anodic dissolution of Cu~S - - General summary
]~ectrolyte
DissolvedCu
Residue
(%) H 2 SO 4
~ 50
CuS-like
Current efficiencya (%) 1
2
3
100--110
~0
--
phase
H 2 SO4--NaCI
90--95
H 2 SO4--KBr
85--90
(Cu,.1S) CuS-like 100--110 40--91 -phase + sulphur CuS-like 110--150 -40 phase + orthorhombic sulphur + e-sulphur
a (1) Linear dissolution stage; (2) dissolution stage after 50% Cu extraction; (3) initial dissolution in H~ SO 4 - K B r electrolyte at 70 and 90°C.
remaining" covellite (Cu~.z S) by X-ray diffraction analysis. These results suggest that the dissolution of Cu~.~S requires an extra overvoltage which is above that necessary for 02 evolution in the sulphate electrolyte. The anodic dissolution of Cu2 S in H~ SO4 --NaCl and H2 SO4--KBr electrolyres resulted in approximately 90--95% Cu extraction under certainvxperimental conditions. This prolonged dissolution in these mixed electrolytesis probably due to the CI- and Br- acting as charge transfer agents for the transfer of electrons from the surface of the sulphide particlesto the feeder e!ectrode~.The anodic dissolution of Cu2 S in H2 SO4--NaCI occurred in two stages. The second stage began after approximately 5 0 % of the copper was dissolved from the Cu2 S and its rate was lessthan that of the firststage of dissolution. The s e ~ n d dissolution stage was accompanied by an abrupt increase in the anode potential to the value for the discharge of Cl- and remained constant at this value for the duration of the reaction.
256
The anodic dissolution of Cu2 S in H2 SO4--KBr electrolyte at 40°C was similar to the H2 SO4--NaC1 electrolyte except that the anode potential increased gradually to a maximum value. At 70 and 90°C the Cu2Sdissolution was chara( terized by a slow initial stage (current efficiency < 50%) which lasted for 3--4 hours. The rate then increased abruptly and became linear with time (current efficiency >100%) and then decreased slightly towards the end of the reaction. Although reasonably good copper extractions (~95%) were achieved when the mixed electrolytes were used, the residues contained > 20% Cu. This result was attributed to physical limitations of the fluidised-bed electrode as discussec previously. X-ray diffraction and chemical analyses of the anodic reaction products formed in the three types of electrolytes used in this work strongly suggest that the first stage in the dissolution of Cu2 S is: C u 2 S = CUl. 1 S
+ 0.9 Cu2÷ + 1.8 e-
(1)
and that the second stage, observed for the H2 SO4--NaC1 and H2 SO4--KBr electrolytes, involves the decomposition of "blue-remaining" covellite into S and Cu 2÷: Cu,., S = 1.1 Cu 2+ + S + 2.2 e-
(4)
ACKNOWLEDGEMENTS
The author thanks J.M. Brannen for aspects of the design of the experimental apparatus and for doing the experimental work. Dr. J.E. Dutrizac prepared the synthetic Cu2 S, Mr. R.F. Pilgrim developed the computer program used for the calculations and P.E. Belanger did the X-ray diffraction analysis.
REFERENCES 1. Renzoni, L.S., McQuire, R.C. and Barker, M.V., 1958. Direct electrorefining o f nickel matte. J. Metals, 10: 414--418. 2. Spence, W.W. and Cook, W.R., 1964. The Thompson refinery. Trans. Can. Inst. Min. Metall., 67: 257--267. 3. Loshkarev, A.G. and Vozisov, A.F., 1953. Anodic solution o f copper sulphide. Zh. Prikl. Khim., 2 6 : 5 5 - - 6 2 . 4. Habashi, F. and Torres-Acuna, N., 1968. The anodic dissolution of copper(I) sulphide a n d the direct recovery o f copper and elemental sulphur from white metal. Trans. Metall. Soc., AIME, 242: 780--787. 5. Venkatachalem, S. and Mallikarjunan, R., 1968. Direct electrorefining of cuprous sulphid and copper matte. Trans. Inst. Min, Metall., Sect. C, 77: 45--52. 6. Kuxmann, U. and Biallass, H., 1969. Untersuchung zur Kupferstein Elektrolyse. Z. Erzbergbau Metallhuttenwes., 22: 53--64. 7. Chizhikov, D.M., 1969. Effect of the concentration o f chloride ions in a sulphuric acid electrolyte On the anodic dissolution o f a copper--nickel converter matte. In: D.M. Chizhikov (Editor), Issled. Protsessov Met. Isvet. Redk. Met., 166--171. 8. Venkatachalem, S. and Mallikarjunan, R., 1971. Laboratory scale studies on a new proce. dure for the recovery of electrolytic copper. Trans. Ind. Inst. Metals, June: 29--38.
257 9. Dutrizac, J.E. and MacDonald, R.J.C., 1973. The synthesis of some copper sulphides and copper sulphosalts in 500--700 gram quantities. Mater. Res. Bull., 8: 961--971. 10. MacKinnon, D.J., to he published. 11. Kato, T. and Oki, T., 1972. Anodic reaction of CuS in sulphuric acid solution. Denki Kagaku, 40: 670--674.
FLUIDISED-BED ANODIC DISSOLUTION OF CHALCOCITE
D.J. MACKINNON
Metallurgical Chemistry Section, Mineral Sciences Laboratories, Canada Centre for Mineral and Energy Technology, Department of Energy, Mines and Resources, Ottawa (Canada) (Received May 1st, 1975)
ABSTRACT MacKinnon, D.J.,1976. Fluidised-bedanodic dissolutionof chalcocite.Hydrometallurgy, 1: 241--257. The anodic dissolution of chalcocite (Cu= S) has been investigated using a fluidised-bed anode technique. Results obtained for a variety of electrolytes and experimental conditions indicate that the fluidised-bed anodic dissolution of chalcocite occurs via the formation of an intermediate copper sulphide, viz., "blue-remaining" covellite, Cu~. 1S. In stflphuric acid electrolyte the dissolution of chalcocite is inhibited after about 50% copper extraction by the vigorous evolution of oxygen gas at the platinum feeder anode. In both sulphuric acid--sodium chloride and sulphuric acid--potassium bromide electrolytes, the dissolution of chalcocite occurs to 95% copper extraction in two stages. The first stage involves the formation of Cu~ .~ S, as is the case for the sulphuric acid electrolyte1 while the second stage is attributed to the reaction between chloride (or bromide) and Cul .~ S.
INTRODUCTION Aqueous electrolysisusing sulphide anodes has given positive results (Renzoni et al.,1958) in the case of nickel sulphide matte (essentiallypure Ni3 $2 ) and as a result these anodes are n o w electrolyticallyrefined on an industrial scale by the International Nickel Co., of Canada Ltd. (Spence and Cook, 1964). In recent years there has been an increased interest in developing an industrial process for the direct electrorefining of copper matte (Cu2 S) anodes (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; K u x m a n n and Biallass, 1969; Chizhikov, 1969). These investigations have been chiefly concerned with the effect of variables such as temperature, electrolyte concentration and current density on the current efficiency during the electrodissolution of Cu2S and white metal cast anodes. In all cases an abrupt increase in the cell voltage was observed during the course of the electrolysis of Cu2S. This increase was associated with the formation of granular and porous anode slimes which adhered to the surface of the anode. Most authors (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; K u x m a n n and Biallass, 1969) found that the anodic current efficiency and the time at which the sharp
242 rise in the cell voltage occurred increased with increasing temperature and decreasing current density. Habashi and Torres-Acuna (1968) found an optimum in the anodic current efficiency for an electrolyte containing 100 kg m -3 H2SO4. However, Venkatachaiem and Mallikarjunan (1968), and Kuxmann and Biallass (1969) observed that the H2SO4 concentration had only a slight effect on the anodic current efficiency. Loshkarev and Vozisov (1953), and Habashi and Torres-Acuna (1968} reported that the sharp increase in cell voltage occurred more rapidly in concentrated acid solutions. The anode slimes, formed during the electrolysis of Cu2 S, were found to consist of elemental sulphur and covellite (CuS) (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; Chizhikov, 1969; Kuxmann and Biallass, 1969)*. Habashi and TorresAcuna (1968), and later Venkatachalem and Mallikarjunan (1971), observed the formation of digenite (Cul.sS) as an intermediate in the early stages of Cu2S electrolysis. The reported amount of elemental sulphur oxidized to sulphate varied widely from author to author. Thus Habashi and Torres-Acuna (1968) indicated that practically no oxidation of sulphur occurred, but Loshkarev and Vozisov (1953) and Venkatachalem and Mallikarjunan (1968) described measurements which resulted in 55% sulphur oxidation. Kuxmann and Biallass (19691 determined that the amount of sulphur oxidized to sulphate varied between 4 and 10%. The addition of chloride ion (Chizhikov, 1969; Venkatachalem and Mallikarjunan, 1971) to a sulphate electrolyte increased the amount of elemental sulphur in the anode slimes. In addition, the anodic current efficiency increased and the cell voltage decreased in the presence of chloride ion. It was suggested that the passivity which developed at the sulphide electrode with the progress of electrolysis in a sulphate electrolyte was hindered in the presence of chloride ion, thus permitting the continued dissolution of Cu2 S at high current efficiency and low cell voltage. The preceding discussion has summarized the results for the electrolysis of Cu2 S which were obtained using conventional planar anodes (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; Chizhikov, 1969; Kuxmann and Biallass, 1969}. These anodes were prepared by casting either molten reagent grade Cu2S or white metal. The direct dissolution of finely-divided Cu2 S flotation concentrates to form metallic copper (or copper compound) and elemental sulphur in a single step with no intermediate smelting or casting stages is also worthy of investigation. There are considerable problems involved in such a one step process, not the least of which is devising methods of conducting a sustained electrical current to the individual sulphide particles. There are essentially three systems which may be used: (i) A fluidised-bed e l e c t r o d e - a bed of conductive particles is fluidised by the * In t h e a b o v e m e n t i o n e d papers, n o a t t e m p t w a s reported t o distinguish b e t w e e n "blueremaining" c o v e l l i t e (Cu I .i S) and true c o v e l l i t e (CuS).
243 upward flow of electrolyte; a feeder electrode provides electrical contact to the bed. (ii) A c o m p a c t (pellet) electrode -- the powdered sulphide is pressed into a solid conducting cylinder; electrical contact is provided by an external wire fixed to the pellet i n such a way that the wire is insulated from the electrolyte. (iii) Electrolytic generation o f leaching agents -- the sulphide particles are suspended in an electrolyte and the oxidant, e.g. chlorine or ferric ion, is generated at an inert anode. The fluidised-bed and c o m p a c t anode systems have been studied in some detail as part of a program to obtain an increased understanding of the chemistry of sulphide electrolysis and to assess the feasibility of using sulphide concentrates in electrowinning processes. Only the results of the fluidised-bed studies will be considered here. Fluidised-bed electrodes (FBE) have a n u m b e r of advantages over conventional planar electrodes, the most important being that they contain a very large surface area per unit volume. In this respect they are particularly useful for processes that can only be carried o u t economically at low current densities. This report describes the results obtained for the electrodissolution of chalcocite (Cu2 S) using a fluidised-bed electrode technique. MATERIALS AND APPARATUS
Both synthetic chalcocite and a chalcocite flotation concentrate were used in the test work. The synthetic chalcocite was prepared according to the method of Dutrizac and MacDonald (1973). X-ray diffraction analysis of the synthetic chalcocite indicated only the orthorhombic chalcocite form of Cu2 S. Chemical analysis gave 79.9% Cu and 19.3% S. X-ray diffraction analysis of the flotation concentrate indicated chalcocite, quartz and hematite plus a minor a m o u n t of amphibole and a trace a m o u n t of magnetite. After up-grading the concentrate, chemical analysis gave 75.7% Cu, 19.2% S and 1.3% Fe. The electrolyte used in the initial test work was H2 SO4 o f varying concentrations (50--200 kg m-3). In later experiments various amounts of NaC1 or KBr were added to the H2 SO4 electrolyte. The fluidised-bed electrode cell is shown schematically in Figure 1. The cell consisted of a bed of conducting particles (Cu2S), +325 mesh, supported on a glass wool plug. Platinum sheet, in the shape of a cone, was located in the bed and served as the anodic current feeder. The counter electrode (cathode in this case) was a platinum wire located in a side arm which was separated from the fluidised-bed anode c o m p a r t m e n t by a glass frit. A capillary probe, located just above the bed, was connected externally to a saturated calomel electrode (SCE) which served as a reference electrode. The anode potential relative to the SCE and the cell voltage were measured and recorded b y means of a high input impedance Hewlett-Packard dual channel strip chart recorder (Model 7100 B). The current to the cell was supplied b y a Hewlett-Packard Regulated D.C. Power Supply, model 6 2 1 4 A (0--10 V d.c., 0--1 A). Fluidisation of the particles was achieved b y continuously circulating the anolyte b y means of a Manostat Varis-
244
I t
/ 1-
"EL%~LYTE
CAPILLARYPROBE
COUNTER ELECTRODE GLASS FRIT
DRAPNPOINT
Fig. 1. Schematic diagram of fluidised-bed electrode cell. talticP u m p (advanced model). The anolyte entered the tapered bottom of the cell via a horizontal inlet tube and left via a horizontal outlet tube located at the top of the cell.A flow rate of about 400 cc/min provided a stable fluidisedbed anode. EXPERIMENTAL The required a m o u n t o f Cu2 S (10--20 g) was placed in the cell and the p u m p was switched on to circulate the electrolyte. The circulating electrolyte was heated to the required temperature b y means of an immersion heater located in the anode c o m p a r t m e n t of the cell controlled b y a Matheson Lab-Stat. When equilibrium conditions were achieved the p o w e r supply was switched on and set to the desired current. Both the anode potential and cell voltage were recorded using the Hewlett-Packard strip chart recorder. Samples were taken periodically and analysed for copper using a Varian Techtron AA 5 atomic absorption spectrophotometer. The atomic absorption data were then entered into a c o m p u t e r program which calculated the a m o u n t of copper dissolved and also provided copper dissolution vs. time plots, from which the rate data were calculated. At the end o f the test the solution was drained from the cell and the residue was collected, washed and dried. A portion of the dried residue was analysed b y X-ray diffraction analysis and the remainder by chemical analysis.
245
RESULTS AND DISCUSSION
Preliminary investigations The initial test work was done using the chalcocite flotation concentrate (20 g charge) and H2 SO4 as the electrolyte. The effects of applied current, electrolyte flow rate (degree of bed fluidisation), Hs SO4 concentration, temperatuxe (25--60°C), initial Cu s+ concentration and CI- on the initial rate of Cuss dissolution were investigated. The dissolution rate of Cuss was found to be linear with time and directly proportional to the applied current (0.25 to 1.0 A). The other variables mentioned above had only a slight effect on the dissolution rate which was 0.6--0.7 g Cu/h at 0.5 A. During these preliminary tests it was observed that the anode potential increased abruptly when 40 to 50% of the copper contained in the Cu2 S had dissolved. This abrupt increase in anode potential was accompanied by O~ gassing at the Pt feeder electrode. When O2 gassing occurred the copper dissolution rate decreased, and the test was usually stopped at this point. Similar effects were observed when synthetic Cu2 S was substituted for the Cu~ S flotation concentrate. In the next section the anodic dissolution of synthetic Cu2S in H2 SO4 electrolyte is described in detail.
The Cus S/Hs S04 system The dissolution behaviour of synthetic Cu~ S at constant current (0.5 A) in 100 kg m -3 H2 SO4 at 40°C is shown in Figure 2 as a plot of copper dissolution vs. the time in hours. The rate of copper dissolution (0.64 g/h) is linear for about 10 hours. At this point the anode potential increases abruptly from 0.35 to 1.8 V vs. SCE, 02 evolution occurs, and the rate of copper dissolution slows 75
2 0 g S Y N T H E T I C Cu2S 40 °C 0.5 A
~. 5.0
a- ~
2.5
0
,
2.5
5.0
7.5
..
I0.0
T I M E (HOURS}
Fig. 2. Plot of dissolved Cu vs. time for H~SO4 electrolyte.
246 considerably. X-ray diffraction analysis indicated that the residue was a CuSlike phase i.e. "blue-remaining" covellite, Cu1.1 S. Elemental sulphur was n o t detected. The residue contained 67.8% Cu and 29.6% S. A second test was done under the same experimental conditions except that the anode potential was controlled at 0.68 volt vs. SCE b y means o f a potentiostar in order to prevent oxygen evolution at the Pt feeder electrode. The results are shown in Figure 3 as a plot of dissolved copper vs. time. The initial rate, 0.77 g Cu/h, was linear up to 5 hours, after which time it began to decrease. During the potentiostatic dissolution of Cu2 S the anodic current gradually decreased from an initial value of 0.8 to 0.05 A after 14 hours so that the reaction became extremely slow. Again, X-ray diffraction analysis of the residue indicate only the presence of "blue-remaining" covellite and chemical analysis indicated a composition of 67.9% Cu and 31.5% S. Microprobe analysis of the anode residue indicated that elemental sulphur was n o t present. I0,0
0 ?.5
00-
o 5.0 o -
2.5
20 g S Y N T H E T I C CutS I 0 0 g / t HIIS04;40=C ANODE P O T E N T I A L = 0.68 VOLT v=,
5.0
I0.0
SCE
15.0
TIME (HOURSI
Fig. 3. Plot of dissolved Cu vs. time for H2804 electrolyte. The results of the constant current (galvanostatic) and the controlled potential (potentiostatic) anodic dissolution of Cu2 S in sulphuric acid electrolyte together with X-ray diffraction, microprobe, and chemical analysis of the anodi residues suggest that the anodic dissolution of Cu2S in sulphuric acid electrolyte occurs according to the reaction: Cu2 S ÷ Cul.1S + 0.9 Cu 2÷ + 1.8 e-
(1)
The anodic dissolution of Cu2S under these conditions occurs at relatively low anodic potential until the Cu2 S particles b e c o m e converted to Cu1.1 S. At this point the anode potential increases abruptly under galvanostatic conditions to
247
value at which O2 evolves from the Pt feeder electrode. For the potentiostatic case, at anode potentials less than that required for 02 evolution, the current falls to a very low value when the Cu2 S particles b e c o m e converted to Cu1.1 S. This behaviour suggests that an additional overvoltage, possibly associated with the nucleation of elemental sulphur, is required for the continued dissolution of Cu1.1 S. In any case oxygen evolution occurs at the Pt feeder anode H20 -~ 1/~O2 + 2H ÷ + 2e:
(2)
in preference to Cu1.1 S dissolution, which indicates that an anode potential > 1 . 8 V vs. SCE* is necessary for the dissolution o f Cul .1 S under these experimental conditions. This behavior will be discussed in more detail in a later paper dealing with the fluidised-bed anodic dissolution of covellite and chalcopyrite (MacKinnon, to be published}. This result is in general agreement with that obtained for solid, planar anodes of cast Cu2S (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968; Kuxmann and Bialass, 1969}. However, the residue (deep blue in colour} contained no elemental sulphur according to X-ray diffraction and microprobe analyses. Elemental sulphur was detected in the anode slimes formed on the cast Cu2 S anodes (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968; Kuxmann and Biallass, 1969) b u t only with prolonged electrolysis after the cell voltage had increased. It should be noted that Kato and Oki (1972) in their study of the anodic dissolution of CuS observed that the anode potential increased abruptly to approximately 2 V vs. SCE after only a few minutes of polarization at a current density of 1 mA/cm 2 .
The Cu2 S/H2 S04/NaCl system A series of tests was done using 100 kg m - 3 H2 SO4 plus various amounts o f NaC1 (10, 25, 50 and 100 kg m -3 ) as the electrolyte, The effect of adding NaCl to the H2 SO4 electrolyte on the dissolution o f synthetic Cu2 S is shown in Figures 4 and 5 in plots of copper dissolved as a function o f time. The experimental conditions are presented with the Figures. At 40°C, with either 25 or 50 kg NaC1 m -3 added to the H2SO4 electrolyte, there appear to be t w o distinct stages in the dissolution o f Cu2 S. The second stage o f dissolution occurs when 40 to 50% of the copper has been removed from the Cu2 S. This is precisely the point where the reaction virtually stops when the electrolyte does not contain NaC1. Increasing the temperature to 90°C (see Fig. 5) has only a slight effect on the first stage of dissolution. However, the rate of the second stage of dissolution is substantially increased. These results are summarized in Table 1 and show that increasing the NaC1 content o f the electrolyte from 25 to 50 kg m- 3 increases the rate o f b o t h stages of Cu2 S dissolution b y a similar amount. * T h e a n o d e p o t e n t i a l e s t a b l i s h e d a t t h e P t f e e d e r e l e c t r o d e in 1 0 0 kg m -3 H 2 S O 4 a t 0.5 A
in the absence o f C u 2 S is 1.8 V vs. SCE. V i g o r o u s O~ e v o l u t i o n o c c u r s a t t h i s p o t e n t i a l .
248
7.S 1
I
i
I
I IO.O
I 15.0
lOg SYNTHETIC Cu:tS IO0 9 / t H|SO,, + 2 5 g / t N a C )
-~
5.0
o
-~ a 2,5
0
I 5.0
20.0
T I M E (HOURS)
Fig. 4. Plot of dissolved Cu vs. time for H= SO 4 - N a C I electrolyte.
15.0
I
i
i
20g SYNTHETIC Cu=S i O 0 g / t H R S O , ÷ S O g / t NoCL 0.5A SO*C
"; io,o
> o
(n a--
5,0
I 5.0
I I0.0
I 15.0
20.0
TIME (HOURS)
Fig. 5. Plot o f dissolved Cu vs. time for H~ SO 4 --NaCI electrolyte.
Increasing the NaCI content from 50 to 1 0 0 kg m -3 had no additional effect on the dissolution o f Cu2 S. The dependence o f the anode potential on time for the dissolution of Cu2 S in H2 SO4--NaCI electrolyte is presented in Figure 6. This plot shows a series of potential--time profiles obtained for the conditions indicated on the diagram. Again, as was the case for the H2 SO4 electrolyte, the anode potential increases abruptly when 40 to 50% of the copper has been dissolved o u t o f the Cu2 S. However, in this case, the potential levels o f f at about 1.1 V vs. SCE, a value considerably less than that required for 02 evolution but which corresponds to the value of the redox potential for the half-cell reaction*: * This w a s c o n f i r m e d b y measuring the potential established at the anode feeder when a current o f 0.5 A was passed through the H S SO4--NaC1 electrolyte in the absence o f Cu 2 S. The anode potential immediately rose to 1.1 V vs. SCE and remained constant. Gaseous CI 2 was observed at the Pt feeder electrode.
249 TABLE 1 Fluid-bed anodic dissolution of synthetic Cu 2 S in H2 SO4 --NaCl mixtures at 0.5 A
Cu2S
Electrolyte
(g)
(kg m - s )
10 20 20
Temp. (+ I°C)
100 H2 SO4 + 25 NaC1 100 H2 SO4 + 50 NaCl 100 H2 S04 + 50 NaCl
40 40 90
Rate 1st stage (g Cu/h)
stage
0.60 0.70 0.77
0.24 0.34 0.54
CI- ~ lhC12 + e-
Rate 2nd
(g Cu/h)
(3)
Increasing temperature allows the potential to remain at a low value for an extended time period in agreement with results obtained by previous workers (Loshkarev and Vozisov, 1953; Habashi and Torres-Acuna, 1968; Venkatachalem and Mallikarjunan, 1968 and 1971; K u x m a n n and Biallass, 1969; Chizhikov, 1969). It~ is interesting to note that when the a m o u n t of Cu2 S is doubled, the time it takes for the anode potential to increase is also doubled (compare the t w o 40°C profiles in Fig. 6). It also appears that the second stage of Cu~ S dissolution is associated with the increase in the anode potential. This behaviour suggests that the redox reaction, equation (3), provides an electron transfer mechanism between the sulphide particles and the Pt feeder anode. Because the anode potential rises to, and remains at, the value for the discharge o f CI-, it suggests that the dissolution of Cu1.1 S is caused b y the action o f C12 p r o d u c e d at the Pt feeder anode, according to the reaction Cu1.1S + 1/2 C12 * 1.1Cu 2÷ + S O + Cl- + 1.2 er.5
I IO09/L
I H~SO4+5Og/L 0.SA
I NOCL
>o W
1.0
40eC
90eC
'~ 0 , 5 o., 0
I 5.0
I I0.0
I 15.0
20.0
T IM I: [ H O U R S ~
Fig. 6. Plot of anode potential vs. time for Hj S04--NaCl electrolyte.
(4)
250 All the C12 produced does n o t react with Cul.1 S as some of the gas escapes to the atmosphere, which accounts for the reduced current efficiency observed for the second stage o f Cu2S dissolution (see later). The residues obtained from the dissolution of Cu2 S in the H2 SO4 --NaC1 mixtures were subjected to both X-ray diffraction and chemical analysis. The X-ray diffraction analysis indicated t h a t the residue consisted of Cul.1 S plus sulphur. Thus the second stage o f the anodic dissolution of Cu2 S observed for the experimental conditions described above is probably represented by reaction (4) above. The results obtained from chemical analysis o f several residues are presented in Table 2. The results in Table 2 indicate t h a t a reasonable material balance was o b t a i n e d Discrepancies can occur by (a) slight oxidation of sulphur to sulphate and (b) sulphide losses during cleaning of the cell and recovery of the reaction product. The test in which 100 kg m -s HC1 was used as the electrolyte indicates that some degree of oxidation of sulphur to sulphate does occur. The results presented in Table 2 also show t h a t the copper c o n t e n t of the residue decreases with increased temperature and chloride concentration. This result is in agreem e n t with those obtained by Chizhikov (1969) and Venkatachalem and Mallikarjunan (1971) who observed an increase in the sulphur c o n t e n t o f the anodes slimes in the presence of Cl-. As shown in Table 2, the residues contained a substantial a m o u n t of copper (>20%). The fact t h a t it was n o t possible to produce residues containing less copper (<20%) is most p r o b a b l y due to the physical limitation of the anode rather than to decreased chemical reactivity of the Cu2S. Results obtained using compacted Cu2S anodes have yielded residues containing < 2% copper (MacKinnon, unpublished results). Because of its complexity, the fluidised-bed anode is difficult to describe analytically. Unlike a solid electrode, in which the reactive and boundary layer areas are well defined, the fluidised-bed anode is constantly changing as the particles make and break contact with the Pt feeder electrode. The quality of the contact between the particle and the feeder electrode may
TABLE 2 Composition of various Cu~ S residues a Electrolyte
(kgm -3)
Temp. Cu (+I°C) (%)
STOT (%)
SO (%)
SO, 2 (%)
C1(%)
Total (%)
100 100 100 100
40 70 40 70 40
45.4 77.4 53.2 72.0 59.7
14.5 63.5 -57.4 37.5
1.1 .15 .60 1.1 3.0
.02 .69 .21 .29 .20
99.7 97.7 98.4 98.4 101.6
H2SO" + 25 NaC1 H2SO4 + 25 NaC1 H2SO4 + 50 NaCl H2SO4 + 50 NaCl 100 HCI
53.2 19.5 44.4 25.0 37.0
a Starting material: synthetic Cu 2 S; 79.7% Cu; 20.2% S
251
vary widely, being affected by the nature of the solution surrounding the particle and by the length of time that the particle is in contact with the electrode. It is quite possible that sulphur on the surface of the sulphide particles could act as an insulator. Also, during the later stages of the reaction, more and more sulphur is formed, thus reducing the frequency of contact between unreacted sulphide particles and the anode feeder, and also reducing the effectiveness of the 1~C12/C1- charge transfer process.
The Cu2 S/H2 S04/KBr system Because the dissolution of Cu2S continued beyond 50% Cu extraction in the presence of C1- several tests were done in which KBr was added to the H2SO4 electrolyte in order to determine the effect of Br- on the dissolution of Cu2 S. The results obtained using 100 g/1 H2 SO4 + 50 gfl KBr as the electrolyte at a current of 0.5 A for various temperatures is shown in Figure 7 as a plot of dissolved copper vs. time. At 40°C, the Cu2 S dissolution is similar to that observed when NaC1 was added to the electrolyte i.e., there appear to be two stages to the dissolution process with the second stage proceeding at about one half the rate of the first stage. However, at higher temperatures, (70 ° and 90°C), there is initially a very slow stage (~4 h) followed by a much faster linear dissolution stage (11 h) after which the rate decreases. The results summarized in Table 3 indicate that increasing temperature causes a moderate increase in the linear rate of Cu2 S dissolution in the mixed H2 SO4--KBr electrolyte. The behaviour of the anode potential as a function of time is shown in Figure 8. For the H2 SO4--KBr electrolyte, the potential gradually increased with time until it reached a limiting value of ~0.80 volt vs. SCE. This value of the anode potential corresponds to that for the redox couple Br2/2Br- vs. the saturated calomel electrode. Thus the i n c ~ e d dissolution of Cu2S in the H2SO4/KBr 15.0
I
t
I¸
2Og SYNTHETIC CuIS I O O g / [ H z S O 4 + 5 O g / L KBr 0.5 A
I0.0
o
,o-c
X
7 0 *C
_~x~
x
a uJ 0 s.o
I S.O
i fO.O
I 15.O
20.O
TIME (HOURS)
Fig. 7. Plot of dissolved Cu vs. t i m e for H 2 SO 4 - K B r electrolyte.
252
TABLE 3 Fluid-bed anodic dissolution of synthetic Cu2 S in H 2 SO 4 --KBr mixtures at 0.5 A Cu 2 S
Electrolyte
Temp.
Linear rate
(g)
(kg m -s )
(_.1°C)
(g/h)
20 20 20 10
100 100 100 100
40 70 90 40
0.57 0.86 0.94 0.66
H= SO 4 H2 SO4 H2 SO4 H2 SO4
+ 50 + 50 + 50 + 50
KBr KBr KBr KBr
electrolyte may be attributed to the action of the Br2/2Br- redox couple servin as an electron transfer mechanism between the sulphide particles and the Pt feeder electrode. X-ray diffraction analysis of the residues obtained from the fluid-bed anodic dissolution of synthetic Cu2 S in H2 SO4--KBr mixtures indicated the presence of Cul.l S and orthorhombic sulphur as the major constituents, plus a minor amount of w-sulphur. The results from chemical analysis of the various residue: are presented in Table 4. As indicated by Table 4, the residues contained a substantial amount of copper, and as mentioned previously this is probably due to physical limitations of the fluidised-bed anode.
Current efficiency The current efficiency calculated for the anodic dissolution of Cu2 S was based on the assumption that Cu 2÷ and not Cu÷ was the dissolving species*. Th~ current efficiencies for the anodic dissolution of Cu2 S obtained for a variety of experimental conditions are summarized in Table 5. The values are based on the linear rate only, i.e., that initial straight portion of the rate plots. For the H2 SO4 and H2 SO4--NaC1 electrolytes these values refer to approximately 50% removal of the Cu from Cu2 S, i.e., to the time when O2 evolution occurs in the H2 SO4 electrolyte or to the time when the second stage of dissolution begins in the case of the H2 SO4 --NaC1 electrolyte. For the H2 SO4 --KBr electrolyte at 70 and 90°C, the current efficiency values do not correspond to the initial dissolution but rather to the linear dissolution rate that occurs about 4 hours after the start of the tests (see Fig. 7). As indicated by the data in Table 5, the current efficiency for the initial dissolution of synthetic Cu2 S is generally in excess of 100%. This implies that * Habashi and Torres-Acuna (1968) postulate that Cu + is the dissolving species in the anodic dissolution o f Cu= S b u t present no evidence to justify this claim. Even if copper was removec from t h e lattice as Cu ÷ it would be oxidized on the surface o f the anode because t~e standard potential o f the reaction Cu ÷ + Cu =÷ + e - (E °= 0.15 V) is much lower than the oxidation potential of Cu~ S (~0~5 V) and Cu ÷ is not stable under these conditions.
253
1.5
I
I
i
"~
2 0 9 S Y N T H E T I C CuzS g / I H = S O 4 + 5 O g / t KBr
I00
0.5A
0
tu
1.0
O
O.5
0 m
70
°C
90 =C
I 5.0
i010 TIME
1
15.0
20.q
(HOURS)
Fig. 8. Plot o f anode potential vs. time for H2 S04 --KBr electrolyte.
there is some chemical dissolution occurring (particularly in the presence of NaCl and KBr at the high temperatures) as well as the electro-chemical dissolution. The current efficiencies for the anodic dissolution o f the Cu2 S flotation concentrate are consistently less than 100%. This is because part o f the applied current is being consumed by the oxidation of the Fe impurity contained in the concentrate. When approximately 50% of the Cu has been removed from the Cu2S, the current efficiency decreases. This is particularly true for the H2 SO4 electrolyte where the decrease in the current efficiency is due to vigorous 02 evolution at the anode feeder electrode. For the H~ SO4--NaCI electrolyte, the current efficiency associated with the second stage of Cu2 S dissolution (see Figs. 4 and 5) is substantially <100% except for the test done at 90°C. The results are summarized in Table 6. The second stage of Cu~ S dissolution in the H2 S O 4 NaCl electrolyte is associated with the reaction between C12 and Cu~.l S and occurs when the anode potential increases to the value for the discharge of Cl-
TABLE 4 Composition o f various Cu= S residues a Electrolyte (kgm -s) 100 100 100 100
H2SO 4 H2SO4 H= SO4 H 2 SO 4
+ + + +
50 50 50 50
KBr KBr KBr KBrb
Temp. (+I°C)
Cu
STOT
SO
SO 42 -
Br-
Total
(%)
(%)
(%)
(%)
(%)
(%)
40 70 90 40
37.1 41.0 28.1 36.4
59.7 52.5 67.3 49.5
31.2 32.0 48.0 31.3
2.4 0.16 0.11 0.54
0.4 1.06 0.6 0.1
99.6 94.7 96.0 86.5
a Starting material: synthetic Cu 2 S; 79.7% Cu; 20.2% S b Flotation concentrate: 75.2% Cu; 19.1% S
254 TABLE 5 Current efficiencies for Cu 2 S dissolution Cu 2 S Wpe
Electrolyte (kg m - s )
Temp. (±1°C)
Linear rate (g/h)
Current efficiency (%)
Flot. conc. Synthetic Synthetic Synthetic Hot. conc. Flot. conc. Hot. conc. Flot. conc. Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Flot. conc. Flot. conc. Flot. conc. Synthetic Synthetic Synthetic
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
40 40 40 40 40 40 40 40 40 40 70 40 40 70 90 40 40 40 40 70 90
0.63 0.64 0.59 0.65 0.57 0.56 0.56 0.58 0.55 0.60 0.55 0.54 0.66 0.64 0.77 0.53 0.57 0.53 0.66 0.86 0.94
106 108 99.5 110 96.1 94.4 94.4 97.8 92.7 101 92.7 91.1 111 108 130 89.4 96.1 91.1 111 145 159
H 2 SO4 H 2 SO4 H 2SO 4 HCI H 2 SO, H 2 SO4 H 2 SO4 H~ SO4 H 2 SO, H 2SO 4 H~ SO4 H 2 SO4 H 2SO 4 H 2 SO 4 H 2SO 4 H= SO 4 H 2 SO4 H 2 SO 4 H~SO 4 H 2SO 4 H 2SO 4
+ 5 + 10 + 25 + 25 + 25 + 25 + 25 + 50 + 50 + 50 + 50 + 50 + 50 + 50 + 50 + 50 + 50
NaCI NaCI NaCI NaCI NaC1 NaCI NaCI NaCI NaCI NaCI NaC1 NaC1 NaCI KBr KBr KBr KBr
a t t h e a n o d e f e e d e r e l e c t r o d e . T h e d e c r e a s e in c u r r e n t e f f i c i e n c y f o r t h i s s t a g e o f t h e r e a c t i o n is a t t r i b u t e d t o t h e loss o f C12 t o t h e a t m o s p h e r e . T h e c u r r e n t e f f i c i e n c i e s o b t a i n e d a t 70 a n d 90°C f o r Cu2 S d i s s o l u t i o n in t h e H2 S O 4 - - K B r e l e c t r o l y t e w e r e e x t r e m e l y h i g h f o r t h e l i n e a r p o r t i o n o f t h e diss o l u t i o n c u r v e (see Fig. 7) i n d i c a t i n g a c o n s i d e r a b l e a m o u n t o f c h e m i c a l d i s s o l u t i o n . H o w e v e r , t h e i n i t i a l 3 t o 4 h o u r s Cu2 S d i s s o l u t i o n a t 70 a n d 90°C in t h i s e l e c t r o l y t e was v e r y s l o w a n d t h e c u r r e n t e f f i c i e n c i e s w e r e o n l y 3 3 . 7 a n d 4 6 . 4 % , r e s p e c t i v e l y . T h e r e a s o n f o r t h i s b e h a v i o u r is n o t k n o w n . CONCLUSION A g e n e r a l s u m m a r y o f t h e r e s u l t s o b t a i n e d f o r t h e a n o d i c d i s s o l u t i o n o f Cu2 S f o r a v a r i e t y o f e x p e r i m e n t a l c o n d i t i o n s u s i n g a f l u i d i s e d - b e d e l e c t r o d e is present e d in T a b l e 7, a n d i n d i c a t e s t h a t t h e r e a c t i o n in H2 SO4 e l e c t r o l y t e v i r t u a l l y c e a s e s a f t e r 50% Cu e x t r a c t i o n . T h i s is d u e t o a c h a n g e in t h e c o m p o s i t i o n o f t h e s o l i d s , w h i c h r e s u l t s in an a b r u p t i n c r e a s e in t h e a n o d e p o t e n t i a l w h i c h in turn causes vigorous 02 evolution at the feeder electrode and a subsequent d e c r e a s e in t h e c u r r e n t e f f i c i e n c y . T h e r e a c t i o n p r o d u c t was i d e n t i f i e d as " b l u e -
255 TABLE 6 Current efficiency for the second stage dissolution of Cu2S in H2SO4--NaCI electrolyte ]~ectrolyte (kg m -3 )
Temp. (+ 1°C)
Rate 2nd stage (g Cu/h)
Current efficiency (%)
100 H 2 SO 4 + 25 NaCI 100 H2 SO4 + 50 NaCl 100 H 2 SO 4 + 50 NaCI
40 40 90
0.24 0.34 0.54
40.5 57.3 91.1
TABLE 7 Fluidised-bed anodic dissolution of Cu~S - - General summary
]~ectrolyte
DissolvedCu
Residue
(%) H 2 SO 4
~ 50
CuS-like
Current efficiencya (%) 1
2
3
100--110
~0
--
phase
H 2 SO4--NaCI
90--95
H 2 SO4--KBr
85--90
(Cu,.1S) CuS-like 100--110 40--91 -phase + sulphur CuS-like 110--150 -40 phase + orthorhombic sulphur + e-sulphur
a (1) Linear dissolution stage; (2) dissolution stage after 50% Cu extraction; (3) initial dissolution in H~ SO 4 - K B r electrolyte at 70 and 90°C.
remaining" covellite (Cu~.z S) by X-ray diffraction analysis. These results suggest that the dissolution of Cu~.~S requires an extra overvoltage which is above that necessary for 02 evolution in the sulphate electrolyte. The anodic dissolution of Cu2 S in H~ SO4 --NaCl and H2 SO4--KBr electrolyres resulted in approximately 90--95% Cu extraction under certainvxperimental conditions. This prolonged dissolution in these mixed electrolytesis probably due to the CI- and Br- acting as charge transfer agents for the transfer of electrons from the surface of the sulphide particlesto the feeder e!ectrode~.The anodic dissolution of Cu2 S in H2 SO4--NaCI occurred in two stages. The second stage began after approximately 5 0 % of the copper was dissolved from the Cu2 S and its rate was lessthan that of the firststage of dissolution. The s e ~ n d dissolution stage was accompanied by an abrupt increase in the anode potential to the value for the discharge of Cl- and remained constant at this value for the duration of the reaction.
256
The anodic dissolution of Cu2 S in H2 SO4--KBr electrolyte at 40°C was similar to the H2 SO4--NaC1 electrolyte except that the anode potential increased gradually to a maximum value. At 70 and 90°C the Cu2Sdissolution was chara( terized by a slow initial stage (current efficiency < 50%) which lasted for 3--4 hours. The rate then increased abruptly and became linear with time (current efficiency >100%) and then decreased slightly towards the end of the reaction. Although reasonably good copper extractions (~95%) were achieved when the mixed electrolytes were used, the residues contained > 20% Cu. This result was attributed to physical limitations of the fluidised-bed electrode as discussec previously. X-ray diffraction and chemical analyses of the anodic reaction products formed in the three types of electrolytes used in this work strongly suggest that the first stage in the dissolution of Cu2 S is: C u 2 S = CUl. 1 S
+ 0.9 Cu2÷ + 1.8 e-
(1)
and that the second stage, observed for the H2 SO4--NaC1 and H2 SO4--KBr electrolytes, involves the decomposition of "blue-remaining" covellite into S and Cu 2÷: Cu,., S = 1.1 Cu 2+ + S + 2.2 e-
(4)
ACKNOWLEDGEMENTS
The author thanks J.M. Brannen for aspects of the design of the experimental apparatus and for doing the experimental work. Dr. J.E. Dutrizac prepared the synthetic Cu2 S, Mr. R.F. Pilgrim developed the computer program used for the calculations and P.E. Belanger did the X-ray diffraction analysis.
REFERENCES 1. Renzoni, L.S., McQuire, R.C. and Barker, M.V., 1958. Direct electrorefining o f nickel matte. J. Metals, 10: 414--418. 2. Spence, W.W. and Cook, W.R., 1964. The Thompson refinery. Trans. Can. Inst. Min. Metall., 67: 257--267. 3. Loshkarev, A.G. and Vozisov, A.F., 1953. Anodic solution o f copper sulphide. Zh. Prikl. Khim., 2 6 : 5 5 - - 6 2 . 4. Habashi, F. and Torres-Acuna, N., 1968. The anodic dissolution of copper(I) sulphide a n d the direct recovery o f copper and elemental sulphur from white metal. Trans. Metall. Soc., AIME, 242: 780--787. 5. Venkatachalem, S. and Mallikarjunan, R., 1968. Direct electrorefining of cuprous sulphid and copper matte. Trans. Inst. Min, Metall., Sect. C, 77: 45--52. 6. Kuxmann, U. and Biallass, H., 1969. Untersuchung zur Kupferstein Elektrolyse. Z. Erzbergbau Metallhuttenwes., 22: 53--64. 7. Chizhikov, D.M., 1969. Effect of the concentration o f chloride ions in a sulphuric acid electrolyte On the anodic dissolution o f a copper--nickel converter matte. In: D.M. Chizhikov (Editor), Issled. Protsessov Met. Isvet. Redk. Met., 166--171. 8. Venkatachalem, S. and Mallikarjunan, R., 1971. Laboratory scale studies on a new proce. dure for the recovery of electrolytic copper. Trans. Ind. Inst. Metals, June: 29--38.
257 9. Dutrizac, J.E. and MacDonald, R.J.C., 1973. The synthesis of some copper sulphides and copper sulphosalts in 500--700 gram quantities. Mater. Res. Bull., 8: 961--971. 10. MacKinnon, D.J., to he published. 11. Kato, T. and Oki, T., 1972. Anodic reaction of CuS in sulphuric acid solution. Denki Kagaku, 40: 670--674.