Copper concentrate leaching

COPPER CONCENTRATE LEACHING IN CHLORIDE-SULFATE MEDIUM

J.P. Ibáñez, J. Ipinza, F. Guerrero, J.I. González and J. Vásquez

Departamento de Ingeniería Metalúrgica y de Materiales

Universidad Técnica Federico Santa María

Avda. España 1680, Valparaíso, Chile.

juan.ibanez@usm.cl

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 sulfuric acid, to generate “in situ” hydrochloric 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].

                        (1)[pic 1]

                                (2)[pic 2]

                                        (3)[pic 3]

The use of chloride ions in the leaching solution involved the action of the second redox couple Cu2+/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 still under discussion. It is suggested that cuprous ions are stable in the form [CuCln]1-n, and cupric ions in the form [CuCln] 2-n [16]. The cuprous complexes are more stable than the cupric group, being the reason for reaction (4) to occur [12].

                                (4)[pic 4]

                                        (5)[pic 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].

                                (6)[pic 6]

                                        (7)[pic 7]

                                (8)[pic 8]

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 potentials 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].

                                (9)[pic 9]

                                                (10)[pic 10]

The non-oxidative process was extended to include H2S removal, in the presence of oxidant agents such as Fe+3 or Cu+2 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].

                                        (11)[pic 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.

[pic 12]

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.

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