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Effect of silver (Ag) catalyst in bioleaching of enargite (Cu_3AsS_4) concentrate was studied using mixed cultures of moderately thermophilic acidophilic microorganisms at 45ºC. Addition of Ag_2S enabled selective Cu dissolution from enargite while suppressing pyrite oxidation: At the highest Ag_2S concentration of 0.04%, Cu recovery reached 96% while Fe dissolution was suppressed to reach only 29% by day 72. Overall results from thermodynamic calculation, liquid/solid analyses and kinetic study suggested that Ag-catalyzed bioleaching of enargite concentrate proceeds via formation of at least two types of secondary products (chalcocite, Cu_2S; trisilver arsenic sulfide, Ag_3AsS_4): Addition of Ag_2S as Ag catalyst thermodynamically and microbiologically contributed to lowering solution redox potentials during bioleaching, consequently satisfying Eox (Cu_2S) < E_h < E_c (Ag+) to enhance enargite dissolution via formation of chalcocite intermediate. Formation of trisilver arsenic sulfide and its intermediate layer (Cu,Ag)_3AsS_4 indicated that Cu ion in the enargite lattice is gradually substituted with Ag. Such secondary products did not impose a rate-limiting step, since the Ag-catalyzed bioleaching was shown to be controlled by a chemical surface reaction, rather than diffusion through product film which was the case in the absence of Ag_2S.

This paper summarizes the results of a voltammetric study of the anodic characteristics of chalcopyrite in the potential region relevant to heap leaching in concentrated chloride solutions. Distinct peaks in the potential region of 0.7 to 0.85 V have been observed in the voltammograms, the magnitude of which depend on the chloride concentration and, particularly, the pH in the range 1 to 3. Three peaks are observed at low chloride concentrations that merge into one peak at higher concentrations. The anodic reactivity increases with increasing pH but shows a complex dependence on the chloride concentration while in the presence of added copper(II) ions, the mixed potential shifts to more positive potentials and only one peak is observed. Hysteresis between the forward and backward-going sweeps has confirmed the transient nature of the processes except at potentials below about 0.75 V in which region the system approaches steady-state behaviour. The voltammetric characteristics of chalcopyrite in this system appear to be very similar to those of covellite. Measurements of the initial rates of dissolution (in the range 1–5 × 10− 10 mol cm− 2 s− 1 depending on the conditions) have been made that confirm the voltammetric trends and compare well with previously published rates. Linear free energy relationships have been used to estimate the thermodynamic properties (Gibbs free energy − 100.7 kJ/mol) of the simplest polysulfide, namely CuS2 and possible mechanisms for the various steps in the anodic dissolution and passivation of chalcopyrite have been suggested that are consistent with the thermodynamic predictions.

Catalytic effects of silver on the dissolution of arsenic-containing copper sulfides such as enargite in acidic media were investigated. Leaching experiments showed that silver increased the enargite dissolution rate, and that the dissolution behavior was potential-dependent. The addition of silver ions and silver sulfide greatly enhanced enargite dissolution. Such behavior can be explained based on the following reaction model: enargite dissolution is enhanced by silver as the silver ions remove the hydrogen sulfide produced during enargite reduction, giving the more amenable copper sulfide. Thermodynamic calculations suggested that the addition of silver ions and controlling the potential would maintain a high enargite dissolution rate. Leaching experiments were therefore performed with the addition of silver sulfide and potential control using an automatic titration apparatus. Experimental results showed that a combination of these two factors gave high enargite dissolution; i.e., 75% copper was dissolved in 24 h at 0.750–0.850 V vs. the standard hydrogen electrode at 303 K.

Natural carbonaceous matter aided the bioleaching of Cu from chalcopyrite concentrates. The oxidative dissolution of chalcopyrite was enhanced more significantly by anthracite than carbonaceous matter in double refractory gold ore (DRGO). This was achieved through Galvanic interactions between the chalcopyrite and natural carbonaceous matter. Measurement of impedance verified that the electro-resistance is smaller in anthracite, which has a greater graphitic degree than carbonaceous matter in DRGO. The electron shuttle between chalcopyrite and the Fe3+ /Fe2+ redox couple was facilitated not only by the amounts of carbonaceous matter but also the degree of graphitization of carbonaceous matter. A higher graphitization degree increased the electron conductivity of the carbonaceous matter to help mediate Cu bioleaching while avoiding direct contact of thermophile cells with refractory copper sulfides.

Part I of this series described the results of a study of the electrochemistry of pyrite under conditions similar to those encountered in acidic bioleaching of the mineral. In this paper, the electrochemical data obtained in the previous paper is used to derive estimates of the rate of leaching of pyrite based on a mixed potential model for the kinetics as a function of the solution composition and temperature. The results of the calculated rates of dissolution are compared with those obtained in a limited study of the dissolution of a milled pyrite sample at controlled potentials. It is demonstrated that the rate of dissolution calculated from the anodic current density in a solution without iron(III) when the potential is held at the mixed potential measured in the presence of iron(III) compared well with the rate predicted by the linear polarization method and that obtained from the change in the solution potential with time in a batch dissolution of a pyrite electrode in solutions containing iron(III). The dissolution rate calculated from the steady-state currents at the mixed potential in the absence of iron(III) decreased with increasing sulphate ion concentration, increased with increasing temperature and also with increasing rotation speed of the pyrite electrode. The results of leaching experiments with milled pyrite under conditions of controlled solution potential has shown that the rate of dissolution is significantly higher at a potential of 0.85 V than at 0.80 V. Approximately 90% of the sulphide associated with pyrite is oxidised to elemental sulphur. The results of the leach experiments quantitatively agree well with those derived from the electrochemical measurements. The detrimental effect of high sulphate concentrations on the rate of dissolution has been confirmed by the leach tests.

A detailed study of the electrochemical behavior of pyrite under typical bio-leaching conditions has been carried out with the focus on the effect of the concentration of sulphate ions on the mixed potential, cyclic voltammetric and potentiostatic measurements. The mixed potential decreases with increasing sulphate ion concentration and the addition of iron(II) and increases as expected with increasing iron(III) concentration and agitation at constant pH. Temperature, pH and the concentration of dissolved oxygen have minimal effects on the mixed potential in the presence of iron(III). Cyclic voltammetric and potentiostatic data confirm that the rate of anodic oxidation of pyrite decreases with increasing sulphate ion concentration and increases with increasing temperature. The pH and dissolved oxygen concentration have little effect while addition of chloride ions inhibits the anodic oxidation of pyrite. Studies of the cathodic reduction of iron(III) and dissolved oxygen have shown that the rate of the former is several orders of magnitude more reversible(rapid) than that of dissolved oxygen which only exhibits measurable reactivity at potentials well below the mixed potentials in the presence of iron(III). The reduction of iron(III) is inhibited by sulphate ions due to the formation of electrochemically less reactive sulphate complexes.

The use of chloride in the heap leaching of sulfide minerals requires the regeneration of the oxidants copper(II) and iron(III). While this is possible in a sulfate system by the use of bacterially catalysed oxidation of iron(II), this can only economically be achieved by chemical reaction with dissolved oxygen in chloride systems due to the sensitivity of bacteria to high concentrations of chloride ions. The kinetics of the reduction of dissolved oxygen by copper(I) in acidic chloride solutions has been studied under possible heap leach conditions. The results confirm some published data that the rate is second-order in copper(I). The rate increases with increasing acid concentrations but decreases with increasing chloride and copper(II) concentrations. Peroxide has been shown to be a detectable intermediate and a modified mechanism has been proposed which is consistent with the kinetic data. The use of the derived rate equation in a modified rate expression for the copper-catalysed auto-oxidation of iron(II) has been derived.

In Parts 1 and 2 of this series, which describe the results of a study of the dissolution of chalcopyrite under conditions that could be expected in a heap leaching process for primary copper minerals, it was shown that enhanced leaching of chalcopyrite from several copper concentrates in dilute acidic chloride solutions can be achieved by controlling the potential in a "window" of 560-600 mV (SHE) in the presence of dissolved oxygen. It was also found that the rate is linear and essentially independent of the initial concentration of chloride and cupric ions under these conditions. Furthermore, the rate appears to be largely independent of the source of the mineral and is strongly dependent on the temperature (activation energy = 72 kJ mol- 1). In this part, additional kinetic data on the effects of fine pyrite on the rate complemented by detailed mineralogical analysis of the residues will be used to demonstrate that sulfur forms a soluble intermediate such as H2S in the dissolution reaction. A summary of the results of a detailed study of the kinetics of the copper ion catalysed oxidation of H 2S by dissolved oxygen is presented which provides further support for a mechanism for the dissolution of chalcopyrite under heap leach conditions which involves an initial step involving non-oxidative dissolution to form H2S and either cupric ions or a covellite-like surface as the initial products.

The leaching rate of chalcopyrite (CuFeS2) in H2SO4 solutions depends on the redox potential determined by the concentration ratio of Fe3+ to Fe2+, and the rate is higher at redox potentials below a critical value (critical potential). In actual leaching systems, different metal ions are released from coexisting minerals to the aqueous phase. The present study investigated the effects of coexisting metal ions on the critical potential of chalcopyrite leaching. Shaking-flask leaching experiments were carried out with 0.1 g of ground chalcopyrite and 10 cm3 of 0.1 kmol m− 3 H2SO4 containing 0.1 kmol m− 3 Fe2+ and 0.001 kmol m− 3 of the metal ions at 298 K in air. The initial redox potential was adjusted by adding Fe3+, and the amount of Cu extracted after 24 h was investigated as a function of the potential. The results indicate that the critical potential increases by the addition of Ag+ or Bi3+ but is not affected by Pd2+, Hg2+, Cd2+, Zn2+, Ni2+, Co2+, or Mn2+. The results were interpreted by a reaction model assuming the formation of intermediate Cu2S due to the reduction of chalcopyrite and subsequent oxidation of the Cu2S at potentials below the critical potential. Catalytic effects of metal ions on chalcopyrite leaching are also discussed based on the experimental results and the proposed model.

Copper extraction during the oxidative leaching of chalcopyrite (CuFeS2) by ferric ions or by dissolved oxygen in sulfuric acid solutions is known to be faster at low redox potentials but slower at potentials above a critical value. The present study shows that this phenomenon occurs only when cupric and ferrous ions coexist, based on electrochemical measurements. Using a chalcopyrite electrode prepared from an ore lump sample, effects of the addition of 0.1 kmol m−3 cupric and ferrous ions on anodic polarization curves and AC impedance spectra were investigated in 0.1 kmol m−3 sulfuric acid with stirring at 298 K in nitrogen. Without cupric and/or ferrous ions, the anodic current increased monotonically with increasing applied potential. When cupric and ferrous ions coexisted, active-passive behavior was observed, i.e., the current increased with increasing potential to reach a maximum and it suddenly decreased at a certain potential, whereby the current became less dependent on the potential. In the active region, i.e., at low potentials, the anodic current with coexisting cupric and ferrous ions was larger than that without these ions. These results agree well with the results of leaching experiments reported previously, and indicate that coexisting cupric and ferrous ions promote the anodic dissolution of chalcopyrite at low potentials. The analysis of the AC impedance spectra indicates that a high-resistance passive layer grows on the chalcopyrite surface without cupric and/or ferrous ions, and that coexistence of these ions causes a formation of another product layer and inhibits the passive layer growth in the active region. To interpret the active-passive behavior of chalcopyrite, a reaction model, assuming the formation of intermediate Cu2S in the active region, is discussed based on the experimental results.

Chalcopyrite leaching in sulfuric acid solutions depends on the redox potential determined by the concentration ratio of ferric to ferrous ions, and the leaching rate is higher at redox potentials below a critical value. Previously, the authors have proposed a reaction model to interpret this phenomenon. The present study applied the model to interpret the catalytic effect of silver ions on chalcopyrite leaching. The model assumes that at lower potentials, chalcopyrite leaching proceeds in two steps: first, chalcopyrite is reduced by ferrous ions to form Cu 2 S that is more rapidly leached; next, the intermediate Cu 2 S is oxidized by ferric and/or dissolved oxygen to release cupric ions. During the chalcopyrite reduction, hydrogen sulfide is released to the liquid phase. Silver ions react with the hydrogen sulfide to form silver sulfide precipitate and decrease the concentration of hydrogen sulfide in the liquid phase, causing a rise in the critical potential of Cu 2 S formation and broadening of the potential range where rapid copper extraction takes place. To confirm the model, the redox potential dependence of chalcopyrite leaching was investigated in the presence of various concentrations of silver ions with 0.1 kmol m −3 sulfuric acid containing known concentrations of ferrous and ferric ions at 298 K in air. The critical potential increased with increasing concentrations of silver ions. This agrees with the model proposed here but cannot be explained by the conventional model proposed by Miller et al.

The effects of ferrous ions on chalcopyrite oxidation with ferric ions in 0.1 mol dm−3 sulfuric acid solutions were investigated by leaching experiments at 303 K in nitrogen. With high cupric ion concentrations, the chalcopyrite oxidation was enhanced by high concentrations of ferrous ions and copper extraction was mainly controlled by the concentration ratio of ferrous to ferric ions or the redox potential of solutions. Ferrous ions, however, suppressed the chalcopyrite oxidation when cupric ion concentrations were low. A reaction model, which involves chalcopyrite reduction to intermediate Cu2S by ferrous ions and oxidation of the Cu2S by ferric ions, was proposed to interpret the results.

Oxidative leaching of chalcopyrite with dissolved oxygen and/or with ferric ions is promoted by high concentrations of ferrous ions in sulfuric acid solutions containing cupric ions. This paper proposes a reaction model to interpret this phenomenon and the thermodynamics of the leaching is discussed. The model considers the leaching to take place in two steps: (1) reduction of chalcopyrite to Cu2S by ferrous ions in the presence of cupric ions and (2) oxidation of the Cu2S to cupric ions and elemental sulfur by dissolved oxygen and/or by ferric ions. The intermediate Cu2S is more amenable to oxidation than chalcopyrite, causing enhanced copper extraction. The model predicts that the formation of intermediate Cu2S and ferrous-promoted chalcopyrite leaching occur when the redox potential of the solution is below a critical potential that is a function of the ferrous and cupric ion concentrations. To confirm this, flask-shaking leaching experiments were carried out with 0.1 mol dm−3 sulfuric acid solutions containing known concentrations of ferrous, ferric, and cupric ions at 303 K in air. The results agreed well with the predictions, i.e. copper extraction was enhanced at solution potentials below the critical potential predicted with the model.

The oxidation of iron(II) is an essential step in the oxidative leaching of uranium(IV) minerals such as uraninite and one of the reagents used in practice for this oxidation is sodium chlorate. This paper presents the results of an investigation into the kinetics of the oxidation of iron(II) by chlorate in solutions typical of the leaching of uranium. The results have confirmed the known form of the rate equation which is first-order in each of iron(II), chlorate and acid concentration. An overall rate equation has been derived which can be used to model the rate of iron(II) oxidation as a function of the reactant concentrations and the temperature. The half-life was found to be less than 1 min under typical leach conditions at the Olympic Dam operation in South Australia and this species cannot be considered to be the direct oxidant for uranium(IV) minerals.