COPPER CONCENTRATE LEACHING IN CHLORIDE-SULFATE MEDIUM
J.P. Ibáñez, J. Ipinza, F. Guerrero, J.I. González and J. Vásquez Departamento de Ingeniería Ingeniería Metalúrgica Metalúrgica y de Materiales Materiales Universidad Técnica Federico Santa María Avda. España España 1680, Valparaíso, Valparaíso, Chile. Chile.
[email protected] [email protected]
ABSTRACT
The mixed leaching of a Chilean copper concentrate (predominantly chalcopyrite) was studied by using sulfate-chloride aqueous solution under normal pressure at different temperatures at constant pH. The use of sea water to increase the chloride concentration was studied as well. The leaching rate in sulfate-chloride solution was faster than in sulfate solution by a factor of ten in 7 days. The use of sea water led to a dissolution of 28% of the total copper in the same period of time at 60 °C, when the chloride concentration was around 60 g/L the dissolution was near to 36% at 60 °C. The iron content in the concentrate was around 30%, and remains in the solid after 7 days of leaching. The brown, reddish and yellow color solutions resulting after experiments indicate the presence of different copper chloride complexes ions. Additional experimental work is carrying out to improve the copper recovery by analyzing addition of sodium chloride on the copper concentrate cured with sulf uric uric acid, to generate “in situ” hydrochloric h ydrochloric acid.
INTRODUCTION
Chalcopyrite (CuFeS2) is the most important copper mineral [1, 2] with the highest concentrate production, which is traditionally treated by smelting technology [3]. This copper sulfide has a high stability in aqueous systems and it is refractory to normal hydrometallurgical processes [4]. The leaching of chalcopyrite has been widely investigated, seeking for parameters affecting the kinetics of the dissolution [5, 2, 6] and supporting the best theoretical mechanism to explain the dissolution [7], for both chemical and bacterial leaching [8, 9]. The dissolution rate of chalcopyrite concentrate in sulfate media is slower to the leaching of secondary copper sulfide [10]; being the passivation of the mineral surface at high solutions potentials, like a dense layer of elemental sulfur or a polysulfide CuSn, one of the widely accepted reasons although the nature of the passivation film is still controversial [7, 11]. The dissolution of the chalcopyrite with oxygen in acidic solutions can be represented by reaction (1). When the leaching proceeds, a ferric oxidation of chalcopyrite can be carried out by reaction (2), being dissolved by ferric ions, producing ferrous ions which are re oxidized by oxygen in acidic solutions according to reaction (3). This last step is slow at ambient pressure [12]. CuFeS O 4H +
CuFeS 4Fe +
4Fe
+
= Cu
= Cu
O 4H
+
+
+
+
Fe
+
5Fe
= 4Fe
+
0
2S 2H O
2S
0
(1) (2) (3)
2H O
The use of chloride ions in the leaching solution involved the action of the second redox 2+ + couple Cu /Cu . It is advantageous due to the aggressive nature of the leaching and to the stability of cuprous ions by the formation of chloro-complexes, being a more effective process than a regular leaching in sulfate solutions with ferric ions as the oxidant agent. Some possible reasons of these phenomena are the higher rates of electron transfer in chloride solutions, as the reduction in chalcopyrite passivation [13, 14]. General reactions of dissolution of chalcopyrite under a sulfate chloride media are shown by equation (4) and (5) [15]. The prevailing redox couple of copper chloride complexes are 1-n still under discussion. It is suggested that cuprous ions are stable in the form [CuCln] , and 2-n cupric ions in the form [CuCln] [16]. The cuprous complexes are more stable than the cupric group, being the reason for reaction (4) to occur [12]. +
CuFeS 3Cu
+
= 4Cu
+
Fe
2S
0
(4)
+
4Cu
O 4H
+
= 4Cu
+
2H O
(5)
The oxidative model presented above is not enough to explain a rate promotion by the presence of ferrous and cupric ions. A reductive/oxidative dissolution model was proposed to interpret this enhancement of chalcopyrite leaching, presented by reactions (6) (7) and (8). The first step is the reduction of chalcopyrite by ferrous ions in the presence of cupric ions, represented by reaction (6) and the final step is an oxidation of intermediate Cu2S by ferric ions [17]. +
CuFeS 3Cu Cu S 4Fe Cu S 4 H
+
+
+
3Fe +
= 2Cu
= 2Cu S 4Fe
+
(6)
+
(7)
S 2H O
(8)
S4Fe
2O = 2Cu
+
The intermediate Cu2S in reaction (6) is formed only at potentials below the critical potential that is function of the ferrous and cupric ions concentrations. If that potential is higher, Cu2S is not formed and reaction (1) or (2) occurs. Also, Cu2S is leached faster than CuFeS2, increasing copper extractions at low pote ntials in the presence of these ions [18]. A non-oxidative/oxidative process has been proposed for the dissolution of chalcopyrite without any oxidizing reagent by reaction (9), with the formation of soluble cupric ions and hydrogen sulfide which are metastable products, being covellite precipitated by reaction (10). The rate of reaction (9) is governed by a rapid dissolution to assure the equilibrium and a diffusion of the soluble species away from the mineral surface [19]. CuFeS 4H +
Cu
+
= Cu
+
+
Fe
2H S = CuS 2H
2H S
+
(9) (10)
The non-oxidative process was extended to include H2S removal, in the presence of oxidant +3 +2 agents such as Fe or Cu and sustain the reaction (9). This is represented by reaction (11) [19]. Assuming that the rate of reaction (11) is rapid compared to the rate of diffusion of H2S from the surface, the equilibrium at the surface will be perturbed by the removal of H2S by oxidation [18]. H S 2Fe
+
= S 2Fe
+
2H
+
(11)
As we saw on this review, there is no consensus about the nature of the rate determining step and the mechanism involved in the dissolution. This work studies the effect of parameters such as initial ions concentration in the leaching solution and temperature in the dissolution of a chalcopyrite concentrate.
EXPERIMENTAL Materials
Copper concentrate was obtained by flotation from operations in Chile, being classified into narrow size fraction using a cyclo-sizer obtaining a 12.3 µm average size. The chemical analysis reported in the operation showed 26.8% Cu, 26.7% Fe, 31.3% S, 8.1% SiO2, 2.7% Al2O3, 0.2% As and others elements. The chemical analysis was made by atomic absorption spectroscopy (AAS) and the results were 24% Cu and 27.8% Fe. The salt supplied by Sociedad Punta de Lobos (SPL) was 97.8 % NaCl and 1.4 % sulfate with a particle size of 100% -1/2".
Leaching experiments
Agitated leaching experiments were carried out in a 500 mL thermostatic jacketed glass reactor with an effective volume of 200 mL of solution. The pulp was mechanically stirred at 800 rpm. The lid contained ports for continuous measuring of temperature, pH and redox potential using an Ag/AgCl reference electrode. The experimental setup is schematically showed in Figure 1.
Figure 1 - Schematic diagram of the experimental setup. (1) vertical mixer; (2) temperature controller; (3) glass reactor; (4) ORP & pH meter; (5) thermostatic bath; (6) plate heater.
One type of experiments consisted in to mix copper concentrate with 200 mL of acid solution (0.2 M H2SO4). The percentage of solids in the concentrate pulp is considered constant in 25 %. Another type of experiments consisted in to mix copper concentrate with concentrated sulfuric acid and sodium chloride, with chemical curing times of 0 and 20 h.
The temperature was studied in the range of 40 to 70 °C. The concentration of chloride ion varied between 0 and 90 g/L using NaCl (provided by SPL). The samples were withdrawn every 2 h, adding an equal volume of the leaching solution to replace that removed. The filtered solutions were analyzed for copper and iron by AAS. Measurement of the redox potential and pH, were made in the solution, immediately after sampling of the mineral pulp.
RESULTS AND DISCUSSIONS
Effect of temperature -
Figure 2 shows the results of leaching with 30 g/L Cl at different temperatures of the concentrate pulp. The copper extraction increases significantly by increasing the temperature of the pulp. The difference of copper extraction between 50 and 70 °C, is highly significant in the control region by diffusion in the layer of product (sulfur) and less important in the region of chemical control.
-
Figure 2 – Effect of the temperature on the kinetic of copper extraction with 30 g/L Cl .
Effect of sodium chloride concentration
The effect of different concentrations of chloride ion added directly in the mineral pulp to 70 °C was studied. Figure 3 shows that the addition in the range of 40 to 80 g/L Cl-, does
not significantly change the extraction of copper, however the high temperature of the pulp. This result shows that the concentration of chloride in the pulp does not affect the kinetic behavior of the leaching of copper concentrate and only influences the thermodynamic equilibrium and the distribution of the complex chloride-copper species.
Figure 3 – Effect of chloride concentration on copper extraction from a copper concentrate in NaCl-H2SO4-H2O solution.
Effect of chemical curing and conditioning time
The chemical curing consists in mixing copper concentrate with sodium chloride and a fraction of the acid consumption, added as concentrated sulfuric acid. This mixture is conditioned for a certain period of time to promote the sulfating of the sulfide species content in the concentrate. In this experiment, 30 kg NaCl/ton of concentrate is mixed with 20% of the stoichiometric consumption of sulfuric acid, without and with 20 days of conditioning at room temperature. Figure 4 shows that the conditioning produces a 20% increase in the copper extraction, probably due to the "in situ" formation of HCl, whose pKa= -9 is significantly smaller than the first dissociation of sulfuric acid (pKa= -6.6). The reaction for formation of hydrochloric acid with a temperature under 50 °C is given by: NaCl H SO = NaHSO HCl
(12)
With temperatures over 50 °C, the next reaction occurs: 2NaCl H SO = Na SO 2HCl
(13)
The concentrate sulfating will be more efficient with reaction (13), which depends on the heat generated by the exothermic reactions that occurs in the mixture, at expense of the dissolution of the copper and iron sulfides.
Figure 4 – Effect of chemical curing and conditioning on copper extraction from a copper concentrate in NaCl-H 2SO4-H2O solution, 30 kg NaCl/ton of concentrate, without and with 20 days of conditioning at room temperature.
Figure 5 shows the effect on the kinetics copper extraction of sodium chloride addition during curing and concentrated conditioning. The experiment was performed with a conditioning time of 30 days and with acid leaching at 70 °C. It is noted that without f NaCl, the copper extraction is less than 5% after 50 h of leaching. However, the addition of 30 kg NaCl/ton produces a complete extraction of the copper content in the concentrate in the same period of time. By decreasing the addition to the half reduces the copper extraction to around of 70% after 50 h, which is indicating a non-linear relationship between dosage for curing and the leaching answer of the concentrate.
Figure 5 - Effect of sodium chloride addition during the curing and conditioning of the concentrated, pH=2 at 70 °C and conditioning time of 30 days.
Figure 6 shows the effect of conditioning time on the iron extraction from their sulfides contained in the concentrate, with a conditioning time of 20 days and addition 15 kg NaCl/ ton for the curing. It can be seen that this leaching medium produces a significant extraction of iron from the copper concentrate. In effect, the iron extraction increases from 5% in the sample without conditioning time until around 50% in the sample with conditioning, in a time period of 50 hours. Pyrite, principal iron species in the copper concentrate, is generally not attacked during leaching and as a consequence, the ferrous iron in solution appears by the dissolution of the chalcopyrite. This ferrous ion is oxidized to ferric ion in presence of cupric ion, contributing to the dissolution of the chalcopyrite. For this reason, some studies have focused on the catalytic effect on the leaching of the initial addition of copper sulfate in the pulp.
Effect of chloride concentration on the redox potential
Figures 7 shows comparatively the evolution of the redox potential and the copper extraction for 15 and 30 kg NaCl/ton added for curing the concentrate for 20 days. The agitated leach was carried out at pH = 2 and 70 °C.
Figure 6 – Effect of conditioning time on iron extraction from a copper concentrate in NaClH2SO4-H2O solution; 15 kg NaCl/ton concentrate at 70 °C and conditioning time of 20 days.
The results show that the addition of sodium chloride in the curing has a positive and significant effect on the kinetics of the process. In effect, with 30 kg/ton is dissolved 80% of the copper in 45 h, whereas with 15 kg/ton reach the same copper extraction with more than 100 h. In both cases, the redox potential is under 700 mV, significantly less than the value normally found in the bacterial leaching (about 750 mV). At time zero, the redox potential is 500 mV and increases as the concentration of chloride in the pulp decreases. This demonstrates that it is possible to control the redox potential of the pulp, adding sodium chloride continuously during the leaching. This condition of less oxidizing redox potential in the pulp significantly enhances the leaching of chalcopyrite. The lower value of redox potential reached in the pulp should be 2+ + 3+ 2+ probably to the domain of the redox couple Cu /Cu on the Fe /Fe .
Figure 8 shows the leaching process with sea water and distilled water, at pH 2 and 70 °C. Both samples have the same kinetic behavior, increasing their redox potential according the leaching proceeds. Other ions present in sea water, apparently, do not affect the leaching process, being feasible the use in copper concentrate leaching.
100
700
] % [ 80 n o i t c 60 a r t x e 40 r e p p 20 o C
(a)
] V 650 [ m l a i c n 600 t e o p x o 550 d e R
Copper ext. ORP
0
500 0
10
20
30
40
50
Time [h] 100
700
(b)
] % [ 80 n o i t c 60 a r t x e 40 r e p p o 20 C
Copper ext.
0
] V 650 m [ l a i c n 600 e t o p x o 550 d e R
500 0
20
40
60
80
100
120
Time [h]
Figure 7 - Evolution of redox potential and copper extraction as a function of time for a conditioning time of 20 days at pH= 2 and 70 °C. (a) 30 kg NaCl/ton; (b) 15 kg NaCl/ton.
Figure 8 - Ionic force water effect on the copper extraction and redox potential in the copper concentrate leaching.
CONCLUSIONS
The chemical curing and the conditioning time are the most relevant variables in the kinetics of the leaching process. The addition of 15 kg NaCl/ton concentrate with 20 days of conditioning time allow a copper extraction higher than 80% with 50 h of leaching. The addition of sodium chloride increases significantly the kinetics of copper extraction from the chalcopyrite concentrate, probably due to the lower redox potential of the pulp (near to 550 mV). The sea water does not affect the kinetics of leaching of copper concentrate. However, recirculation of water recovered could affect the kinetics of the process.
ACKNOWLEDGMENTS
We thank InProMet (Innovación en Procesos Metalúrgicos) form the Departamento de Ingeniería Metalúrgica y de Materiales of the Universidad Técnica Federico Santa María for supporting this work.
REFERENCES
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