Transformation of sphalerite particles into copper sulfide particles by hydrothermal treatment with Cu(II) ions

Hydrometallurgy 75 (2004) 177 – 187 www.elsevier.com/locate/hydromet

Transformation of sphalerite particles into copper sulfide particles by hydrothermal treatment with Cu(II) ions J. Vin˜alsa,*, G. Fuentesb, M.C. Herna´ndezb, O. Herrerosc a

Department of Chemical Engineering and Metallurgy, University of Barcelona, Martı´ i Franque´s 1, E-08028 Barcelona, Spain b Department of Metallurgical Engineering, Universidad Cato´lica del Norte, Angamos 0610, Antofagasta, Chile c Faculty of Engineering, Universidad de Antofagasta, Angamos 601, Antofagasta, Chile Received 31 May 2004; accepted 31 July 2004

Abstract The nature of the hydrothermal reaction between sphalerite and copper solutions was investigated in the range 160–225 8C. Digenite (Cu1.8S) was the main reaction product at 160–212 8C, and chalcocite (Cu2S) at 225 8C. The reaction was characterized by the formation of a compact layer of copper sulfide around the sphalerite nuclei. Final particles retained the size and shape of the original ZnS. Reaction rate followed a parabolic kinetic law. No significant effect of aqueous copper concentration was observed in the range 1–10 g/L. An activation energy of 147 kJ/mol was obtained, indicating kinetic control by solid-state counter diffusion of Cu+ and Zn2+ ions through the copper sulfide layer. A possible electrochemical mechanism is discussed. The removal of zinc from digenite or chalcocite bearing copper concentrates is effective at ~225 8C, in which a high sphalerite conversion can be achieved in times allowing autoclave processing (~1 h). D 2004 Elsevier B.V. All rights reserved. Keywords: Copper concentrates; Zinc removal; Sphalerite

1. Introduction Improving bcleanQ technology in copper pyrometallurgy is expected to solve a number of environmental problems, particularly those associated with the processing of wastes containing hazardous heavy metals. Copper concentrates are usually produced by selective flotation from complex sulfide minerals. * Corresponding author. Tel./fax: +34 934021291. E-mail addresses: [email protected] (J. Vin˜als)8 [email protected] (G. Fuentes)8 [email protected] (O. Herreros). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.07.005

Sphalerite is one of the more common impurities in these concentrates. Mielczarski et al. (1999) have studied and reviewed the causes of non-selective flotation: surface activation by ions of the solution; activation by migration of ions in solid solution to the surface; and galvanic effects from contact with other sulfides. Although measures can be taken to improve selective flotation, it is currently difficult to keep zinc levels below 2% with many complex copper ores. The behavior of zinc during copper smelting has been studied by Yazawa (1974) and more recently by Degterov et al. (2000). Under the normal oxidant

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conditions, sphalerite is oxidized to ZnO, which distributes mainly between the slags and dusts of the smelting and converting processes. The prior removal of sphalerite is beneficial not only as it reduces the quantity of zinc in slags and dusts, but also because sphalerite is one of the most common carriers of extremely toxic metals (Cd, Tl, etc.). Its removal can therefore reduce the production of hazardous wastes in copper refineries. Naboichenko et al. (1979) and Naboichenko and Khudyakov (1981) studied the hydrothermal interaction of Cu2+ solutions with chalcopyrite, pyrite, galena and sphalerite in the range 130–200 8C. The reported reactions for sphalerite were, ZnS þ CuSO4ðaqÞ YCuS þ ZnSO4ðaqÞ

ð1Þ

5ZnS þ 8CuSO4ðaqÞ þ 4H2 OY4Cu2 S þ 5ZnSO4ðaqÞ þ 4H2 SO4ðaqÞ

ð2Þ

with reaction (2) becoming dominant at higher temperatures. Kinetic data showed first-order dependency of the [Cu2+] for CuSO4/ZnSb1, and reaction order b1 for CuSO4/ZnSN1 ratios. The reported activation energy was 58 kJ/mol at initial rates and 96 kJ/mol at high solid conversion. On the other hand, Serova et al. (1992) have studied the hydrothermal reaction of Cu2+ with a complex Cu/Zn concentrate in the range 160–190 8C. The best reported conditions were at 190 8C, in which a concentrate containing 20.3% Zn and 8.5% Cu can be converted into a final concentrate with 6.6% Zn and 42.4% Cu. The hydrothermal reactions of chalcopyrite have been studied more extensively than those of sphalerite and several patents were already published during the 1950s (McGauley and Roberts, 1951; Roberts et al., 1953; McGauley et al., 1956). Barlett (1992) and Peterson and Wadsworth (1994) showed that the 2+ chalcopyrite reaction with Cu(aq) occurs as follows: 3CuFeS2 þ 6CuSO4ðaqÞ þ 4H2 OY5Cu1:8 S þ 3FeSO4ðaqÞ þ 4H2 SO4ðaqÞ

ð3Þ

The kinetics were reported as mixed diffusion/ chemical reaction control (Peterson and Wadsworth, 1994) with an activation energy of 90–100 kJ/mol in the range 125–200 8C. An electrochemical mechanism has been proposed in which Cu+ and Cu2+ ions diffuse in the lattices of the copper sulfide layers.

The enrichment of chalcopyrite concentrates has also been studied with the controlled injection of O2 and without Cu2+ addition, in the range 170–200 8C (Barlett, 1992; Jang and Wadsworth, 1993; Jang and Wadsworth, 1994): 1:8CuFeS2 þ 4:8O2 þ 0:8H2 OYCu1:8 S þ 1:8FeSO4ðaqÞ þ 0:8H2 SO4ðaqÞ

ð4Þ

In this case, kinetics appeared to be electrochemically controlled as the sulfide layers are highly porous. The objective of the present study is to examine the nature of the reaction of sphalerite with copper ions, as well as the major kinetics dependencies of the process. A significant departure from previous research is the extension of the temperature range to 225 8C. Reaction rates increase dramatically under these conditions, which permits to obtain extensive conversion of sphalerite particles—as large as 50 Am—within a time frame permitted by autoclave processing. Preliminary research on Chilean copper concentrates indicated the possibility of the removal of about 80% Zn by treatment at 225 8C, 1 h, 8 g/L Cu2+ (Vin˜als et al., 2004a). Under these conditions, the simultaneous removal of about 80% Cd, Tl and Bi is also obtained (Vin˜als et al., 2004b).

2. Experimental 2.1. Materials The sphalerite used in this research was taken from a single 4-cm crystal from La Unio´n (Murcia, Spain). Characterization by X-ray diffraction (XRD), and Scanning Electron Microscopy coupled by Energy Dispersive Spectrometry (SEM/EDS) confirmed a single phase product. The composition determined by Electron Microprobe Analysis (EMPA) was 61.14% Zn, 4.46% Fe and 33.74% S. The samples for autoclave experiments were prepared from the ground crystal by wet sieving. Particle sizes of b25, 25–40, 80–106 and 140–180 Am were used. 2.2. Autoclave experiments Experiments were performed in a Teflon-coated stainless steel PARR-4563 stirring reactor with a

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programmable heating controller. The stirring speed was 800 min1 in all experiments. The system was raised to the working temperature at a rate of ~5 8C/ min , then maintained at this temperature for a fixed time (nominal time), until it was cooled to the ambient temperature, again at a rate of ~5 8C/min. After this treatment, the pH and the solution volume were measured. Copper, zinc and iron in the final solution were determined by A.A. Residual solids were characterized by XRD, reflected-light microscopy and SEM/EDS and selected samples were analyzed by EMPA. In order to detect the possible formation of elemental sulfur, samples of residual solids were treated by CS2. No significant amounts of elemental sulfur were detected in the range studied. The nature of the reaction of sphalerite with copper ions was investigated in the range 160–225 8C. 0.1 g of 25–40 Am sphalerite particles was treated with 100 cm3 of slightly acidified ( H2SO4, pH 1.3) copper sulfate solutions of known concentrations. Under these conditions, there is sufficient Cu2+ excess to assume practically constant copper concentration, pH and solution volume. However, other experiments were performed with b25 Am sphalerite and a much lower excess of Cu2+ (0.8 g sphalerite in 80 cm3 of solution), in order to correlate the Cu2+ consumed with the Zn2+ and Fe2+ in solution as well as with the SO42 increment. The kinetics of the process was also studied in the range. Experiments were performed in the same way as previously described for large excess Cu2+/sphalerite, studying the effects of particle size, copper concentration and temperature. The fraction of sphalerite reacted was computed as the ratio Zn2+ in solution/initial Zn in the solid. Selected samples of intermediate and final solid products were studied in SEM to measure the thickness of the layer of copper sulfides.

3. Results and discussion 3.1. Nature of the reaction 3.1.1. Solid phase composition and textures Fig. 1 shows the complete transformation of sphalerite into copper sulfide—in this case Cu2S—at

179

Fig. 1. Transformation of ZnS to Cu2S (225 8C, Cu2+ 5 g/L, pH 1.1). (A) Initial sphalerite. (B) 60 min. (C) 120 min (SEM–BSE).

225 8C. The reaction occurs across a compact copper sulfide layer and the final copper sulfide particles retain practically the same size and shape of the original sphalerite. The results obtained by XRD on solid phase composition are shown in Fig. 2 and Table 1. Digenite (theoretically Cu1.8S) was the dominant, almost exclusive solid product in the range 180–212 8C. Amongst the numerous phases of composition approaching bCu2SQ, the preferential formation of digenite is probably due to its thermodynamic stability under the conditions of pH, EH and temperature studied. Whilst digenite (at ~200 8C) and sphalerite

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Fig. 2. XRD spectra of reacted solids at different temperatures (conditions as in Table 1). (1) Sphalerite. (2) Digenite. (3) Covellite. (4) Chalcocite-Q. (5) Chalcocite-M.

are both isometric, digenite’s structure (space group Fm3m) and cell parameter (5.589 A) (Will et al., 2002) are quite different from those of sphalerite (space group F-43m, 5.409 A) (Skinner, 1961). Fig. 3 shows the digenite layers observed in this temperature interval. No intermediate covellite (CuS) layer was detected between ZnS nuclei and digenite. However, on increasing the reaction temperature to 225 8C, the stability of digenite decreases under the conditions studied. It is initially formed during the heating process but evolves to chalcocite by 225 8C;

first tetragonal chalcocite (Chalcocite-Q) (Fig. 4) then monoclinic chalcocite (Chalcocite-M) (Fig. 1C). The true phase during longer reaction times is probably hexagonal chalcocite (Chalcocite-H), but this phase reverts to chalcocite-M below 103 8C (Gaines et al., 1997). The formation of small amounts of covellite during the hydrothermal reaction of sphalerite was also detected in the interval 160–200 8C (Table 1). Covellite was found as relatively large laminar crystals over an initially formed digenite layer in

J. Vin˜als et al. / Hydrometallurgy 75 (2004) 177–187 Table 1 Fraction reacted and phase composition of final solids (25–40 Am sphalerite) Temp. [Cu2+] (8C) (g/L)

Time pHf (min)

Sphalerite Reaction products reacted (%) (XRD) (I i /I j : ratio of main peak intensity)

160

5

60

1.2

4

180

5

60

1.2

14

190*

10*

200

1

60

1.1

29

200

2

60

1.2

29

200

5

60

1.1

25

200

10

60

1.2

34

212 225

5 5

60 15

1.1 1.1

53 32

225

5

60

1.1

60

120* 1.2* 81*

181

Table 2 shows the results of a stoichiometric experiment at 190 8C to correlate the mole of sphalerite reacted with the mole of Cu2+ consumed, as well as the mole SO42 generated. Sphalerite reacts stoichiometrically giving a molar ratio (Zn/Fe) in the

Digenite, covellite I dig/I cov ~ 1 Digenite, covellite I dig/I cov ~ 4 Digenite, possible anilite (Cu7S4) in trace* Digenite, covellite I dig/I cov ~ 10 Digenite, covellite I dig/I cov ~ 13 Digenite, covellite I dig/I cov ~ 30 Digenite, covellite I dig/I cov ~ 30 Digenite Digenite, chalcocite-Q Chalcocite-M

* Experiment performed with sphalerite particles of b25 Am and pH0=1.5.

some anomalous reacted grains (Fig. 5). These abnormal grains appear when the initial digenite layer is physically separated from the sphalerite core due to a rupture, probably of mechanical origin (Fig. 5). It seems covellite develops when digenite is not in galvanic contact with sphalerite. However, the relative amount of covellite decreases with increasing temperature in the interval 160–200 8C. Higher copper concentration also reduces the formation of covellite. For [Cu2+] 10 g/L, covellite is practically absent even at 190 8C. 3.1.2. Stoichiometry In accordance with the solid phase composition, the expected general stoichiometry for the sphalerite reaction would be: ð2  xÞCu2þ ðaqÞ þ ð5  xÞ=4ZnS þ ð1  xÞH2 O ! Cu2 xS þ ð5  xÞ=4Zn2þ ðaqÞ þ þ ð1  xÞ=4SO2 4ðaqÞ þ 2ð1  xÞH

ð5Þ

where theoretically, x=0 for chalcocite and x=0.2 for digenite.

Fig. 3. Digenite layers at different temperatures. (Cu 5 g/L, 60 min) (A) 160 8C, (B) 180 8C, (C) 200 8C, (D) 212 8C (SEM–BSE).

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J. Vin˜als et al. / Hydrometallurgy 75 (2004) 177–187 Table 2 Results of a stoichiometric experiment (0.850 g sphalerite, b25 Am, 190 8C, 2 h) Solutions

Volume (cm3)

Cu2+ (g/L)

S (g/L)

Zn2+ (g/L)

Fe2+ (g/L)

pH

Initial Final

80 76

9.96 1.82

6.56 7.43

0.00 5.56

0.00 0.41

1.5 1.2

Stoichiometric ratios (molar)

Fig. 4. Optical micrograph of a layer showing lamellae transformation of digenite (medium gray) to chalcocite (white) at 225 8C. (Cu2+ 5 g/L, pH 1.1, 15 min).

aqueous solution practically identical to that found in the sphalerite sample. The stoichiometric coefficients for (DCu/Zn+Fe) and (DSO4/Zn+Fe) in the aqueous solution, were 1.52 and 0.18, respectively. Taking x=0.2, these values are consistent with the general reaction (5), as digenite was the phase obtained at 190 8C. Considering the reaction stoichiometry and the molar volume of the pure phases (23.2, 25.7 and 27.4 cm3/mol for ZnS, Cu1.8S and Cu2S, respectively), the theoretical Pilling–Bedworth ratios are 0.93 for digenite and 0.95 for chalcocite. These values suggest that the layers could have a low but significant microporosity, despite the fact that SEM observations on layers formed at z200 8C did not detect pores.

(Zn/Fe)solution/ (Zn/Fe)solid

DCusolution/ P

DS / P solution

0.99

1.48

0.18

Zn+Fe solution

Zn+Fe solution

However, EMPA of these layers gives a sum of components very close to 100% (Table 3), which indicates, in any case, very low microporosity. 3.2. Kinetics 3.2.1. Effect of reaction time SEM measurements of copper sulfide layers obtained at different times showed nonlinear rates. Fig. 6 shows the results obtained at 200 8C in which a parabolic rate law was obtained, strongly suggesting kinetic control by solid layer diffusion. Consequently, kinetic data were treated by the following shrinking core model: F ðaÞ ¼ 1  ð2=3aÞ  ð1  aÞ2=3 ¼ kex t

ð6Þ

kex ¼ kPL =r02

ð7Þ

where a is the fraction reacted, t the time, k ex the experimental rate constant, r 0 the initial particle radius

Table 3 EMPA data on sphalerite core and layers of reaction products

S Zn Fe Cu Total

Fig. 5. Covellite laths on digenite layer in partially or completely void grains (180 8C, 60 min, Cu2+ 5 g/L, pH 1.1) (SEM–BSE).

Sphalerite core*

Digenite (200 8C)**

Chalcocite-M (225 8C)**

32.92% (0.37)std 59.65% (2.43)std 4.35% (2.21)std 2.39% (0.68)std 99.31%

20.42% (0.73)std 0.64% (0.29)std 0.15% (0.06)std 78.27% (1.87)std 99.48% molar Cu/S=1.9

20.12% (0.43)std 0.36% (0.38)std 0.06% (0.05)std 79.16% (0.69)std 99.70% molar Cu/S=2.0

Apparatus: Cameca SX-50; Program: PAP. Conditions: 20 kV; 20 nA; counting time 10 s. Standards: Pyrite (S Ka); Sphalerite (Zn Ka); Pyrite (Fe Ka); Chalcopyrite (Cu Ka). * Average 20 points. ** Average 10 points.

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183

Fig. 6. Effect of the reaction time on the growth of copper sulfide layer.

Fig. 8. Plot of 12/3a(1a)2/3 vs. time for different particle radius.

and k PL the rate constant for product-layer diffusion control (Sohn and Wadsworth, 1979).

radius as it was expected from the theoretical model (Eqs. (6) and (7)).

3.2.2. Effect of particle size Fig. 7 shows the conversion/time data at 200 8C, [Cu2+] 5 g/L and pH 1.3–1.1, for three shortened particle sizes: r 0=16F4, 47F2 and 80F10 Am. Fig. 8 is the application of the conversion model. Rate conversion decreases very significantly with increase in particle size. Fig. 9 shows a plot of the experimental rate constants from the inverse of the square particle

3.2.3. Effect of copper concentration The copper concentration effect was studied at 200 8C, pH 1.3–1.1, r 0 16F4 Am and for copper concentrations: 1, 2, 5 and 10 g/L. Reaction rate was not very sensitive to the aqueous copper concentration as is shown in Fig. 10. In order to corroborate the data obtained from solution analysis, SEM measurements of the copper sulfide layers were also performed for different copper concentrations. Layer thickness observed at 200 8C after 1 h were almost identical (1.3–1.4 Am) in the interval Cu2+ 1– 10 g/L, confirming that if copper concentration is in excess of the stoichiometric consumption requirements, the reaction rate is practically independent of [Cu2+]. This effect is similar to that found by Peterson and Wadsworth (1994) for the hydrothermal reaction of chalcopyrite. At least in the present study, this effect is also clearly inconsistent with the control by pore diffusion of aqueous copper ions to the sphalerite interface. More likely, diffusion control involves copper ions in solid state. Consequently, the gradient of these ions from the water–copper sulfide interface

Fig. 7. Effect of particle radius on the conversion rate.

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Fig. 9. Experimental rate constants against the inverse of the square particle radius.

to the copper sulfide–sphalerite interface would be limited by the gradient of lattice vacancies and not by any gradient of the aqueous copper ions. Considering that in chalcocite and digenite (also in covellite) the Cu ions of the lattice are Cu+ (Nakai et al., 1978, Termes et al., 1987), it seems reasonable to expect that copper diffuses as Cu+ in these layers.

(Fig. 6): 160 8C 0.5F0.1 Am; 180 8C 0.8F0.1 Am; 200 8C 1.4F0.1 Am; 212 8C 2.5F0.1 Am; 225 8C 4.2F0.1 Am. The obtained k PL from SEM measurements were similar to those obtained from solution analysis with a maximum difference of less than 25% (Fig. 12).

3.2.4. Temperature effect Temperature effect was studied at [Cu2+] 5 g/L , pH 1.3–1.1, r 0=16F 4 Am and 160, 180, 200, 212 and 225 8C. Fig. 11 is the plot of the reaction model. The Arrhenius plot of the product-layer rate constants (k PL) from data in Fig. 11 is shown in Fig. 12. The estimated constants from SEM layer measurements were also included in Fig. 12, computed from layer thickness obtained after 1 h at various temperatures (Figs. 1 and 3), and assuming a parabolic rate law

Fig. 10. Effect of the copper concentration.

Fig. 11. Plot of 12/3a(1a)2/3 vs. time for different temperatures.

J. Vin˜als et al. / Hydrometallurgy 75 (2004) 177–187

185

Fig. 12. Arrhenius plot of the rate constants.

The obtained activation energy in the interval 180– 225 8C was 147 kJ/mol (35 kcal/mol), a typical value for diffusion in solid state, which confirms this kinetic reaction control for the hydrothermal conversion of the sphalerite. The rate at 160 8C was not considered in the determination of the activation energy, as at this temperature conversion is very slight and, in relative terms, the formation of covellite was of the same magnitude as that of digenite (Table 1). Peterson and Wadsworth (1994) reported an activation energy for the hydrothermal reaction of chalcopyrite, significantly lower (99.7 kJ/mol) than that obtained in the present study for sphalerite, in spite of the fact that the ionic diffusion must occur through the same lattice (digenite). This may be due to the reported intermediate layer of covellite, the presence of a different counter diffusing ion, Fe2+, and mixed chemical-diffusion kinetics. On the other hand, the product layer rate constants for sphalerite are much lower than for chalcopyrite at V200 8C. For instance, the reported k PL for chalcopyrite was about 3 Am2/h at 150 8C (Peterson and Wadsworth, 1994). Values of this magnitude for sphalerite were not reached until about 200 8C (Fig. 12). Consequently, whereas chalcopyrite treatment can be technically feasible at b200 8C, the analogous treatment for sphalerite requires N200 8C. However, the high activation energy for sphalerite permits to obtain high

reaction rates at 225 8C (~17 Am2/h), which ensure high sphalerite conversion in times allowing autoclave processing (~1 h). 3.3. Overall mechanism Hydrothermal conversion of the sphalerite to chalcocite or digenite involves cathodic (the reduction of Cu2+ to Cu+) and anodic processes (the oxidation of 1/5 or 1/6 of the S2 from sphalerite to SO42, for chalcocite and digenite, respectively). Fig. 13 shows a schema of the possible mechanism. In this formulation, it is assumed counter diffusion of Cu+ and Zn2+ through the lattice of the copper sulfide. Only two interfaces are considered—sphalerite/copper sulfide and copper sulfide/water—because no experimental evidence of an intermediate covellite zone was found. For comparative purpose, all the reactions are written for 1-mol ZnS. 3.3.1. Sphalerite/copper sulfide interface Cu+ ions displace the Zn2+ ions of sphalerite with the consequent growth of the copper(I) sulfide phase. At 225 8C, chalcocite would be the more stable arrangement, whereas digenite would be more stable at b225 8C. However, the formation of digenite would require an oxidation process to compensate the cation deficit. In this formulation, the release of electrons is assumed, but

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Digenite: þ 1:5Cu2þ ðaqÞ þ 1:5eY1:5CuðsÞ

ð13Þ

2 1=6Cu1:8 SðsÞ þ 4=6H2 OY0:3Cuþ ðsÞ þ 1=6SO4ðaqÞ

þ 8=6Hþ ðaqÞ þ 1:3e 2þ Zn2þ ðsÞ YZnðaqÞ

ð14Þ ð12Þ

3.3.3. Overall Chalcocite: 2þ ZnSðsÞ þ 1:6Cu2þ ðaqÞ þ 0:8H2 OY0:8Cu2 SðsÞ þ ZnðaqÞ þ þ 0:2SO2 4ðaqÞ þ 1:6HðaqÞ

ð15Þ

Digenite: Fig. 13. Schema of the possible mechanism.

ZnSðsÞ þ 1:5Cu2þ ðaqÞ þ 4=6H2 OY5=6Cu1:8 SðsÞ

it is also possible that this process could be effected through positive electron holes coupled with cation vacancies. The Zn2+ ions diffuse through the copper sulfide lattice to the copper sulfide/water interface. Chalcocite: ZnSðsÞ þ

2Cuþ ðsÞ YCu2 SðsÞ

2 þ þ Zn2þ ðaqÞ þ 1=6SO4ðaqÞ þ 8=6HðaqÞ

4. Conclusions (1)

þ

Zn2þ ðsÞ

ð8Þ

Digenite: 2þ ZnSðsÞ þ 1:8Cuþ ðsÞ YCu1:8 SðsÞ þ ZnðsÞ þ 0:2e

ð9Þ

3.3.2. Copper sulfide/water interface Cu2+ ions from the solution are adsorbed and reduced to Cu+ ions in surface cathodic sites. For electronic balance, S2 ions of the Cu(I) sulfide are oxidized to sulfate in surface anodic sites. The Cu+ ions diffuse through the copper sulfide lattice to the sphalerite/copper sulfide interface. For ionic charge balance, Zn2+ ions desorb to the aqueous solution. Chalcocite: þ 1:6Cu2þ ðaqÞ þ 1:6eY1:6CuðsÞ

þ 1:6Hþ ðaqÞ þ 1:6e 2þ Zn2þ ðsÞ YZnðaqÞ

þ ð1  xÞH2 O ! Cu2x S þ ð5  xÞ=4Zn2þ ðaqÞ þ þ ð1  xÞ=4SO2 4ðaqÞ þ 2ð1  xÞH

(2)

(3) ð11Þ ð12Þ

The reaction of sphalerite with Cu2+ sulfate solutions at pH 1.1–1.3 produces mainly digenite (Cu1.8S) at 180–212 8C and chalcocite (Cu2S) at 225 8C. The general stoichiometry of the hydrothermal reaction can be written as: ð2  xÞCu2þ ðaqÞ þ ð5  xÞ=4ZnS

ð10Þ

2 0:2Cu2 SðsÞ þ 0:8H2 OY0:4Cuþ ðsÞ þ 0:2SO4ðaqÞ

ð16Þ

The reaction occurs through a compact layer of copper sulfide surrounding the sphalerite nuclei. Final copper sulfide particles retain the size and shape of the original sphalerite. Reaction rate follows a parabolic kinetic law, indicative of a solid layer diffusion control. The negligible effect of the aqueous copper concentration and the very high activation energy (147 kJ/mol) are consistent with a reaction control by solid state counter diffusion of Cu+ and Zn2+ through the copper sulfide layer.

J. Vin˜als et al. / Hydrometallurgy 75 (2004) 177–187

(4)

The removal of sphalerite from digenite or chalcocite bearing copper concentrates is effective at ~225 8C, in which a high conversion can be achieved in times allowing autoclave processing (~1 h).

Acknowledgements The authors wish to thank the Universidad Cato´lica del Norte (Chile) for the financial support of this research in the framework of a Doctorate Program in collaboration with the Universidad de Barcelona (Spain). The support of the bServeis Cientı´ficoTe`cnics de la Universidad de BarcelonaQ and Mrs. E. Vilalta in the characterization studies is also gratefully acknowledged.

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