The dissolution of chalcopyrite in chloride solutions: Part 2: Effect of various parameters on the rate

In Part 1 of this series, which describes 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 range of 560-600 mV (SHE) in the presence of dissolved oxygen. Based on the results of these experiments, this paper reports on an extensive study of the kinetics of the dissolution of several chalcopyrite concentrates in chloride solutions under various conditions in especially designed reactors. It will be demonstrated that the rates of dissolution at constant potential in the range of 580-600 mV by control of the oxygen supplied to the reactor are approximately constant for up to 80% dissolution for sized fractions of the concentrates. The rate of dissolution of chalcopyrite under these conditions is largely independent of the pulp density, iron and copper ion concentrations which could be expected in a heap leach operation, the acidity and the chloride ion concentration. Variation of the temperature in the range 25 to 75 °C under otherwise constant conditions resulted in an activation energy for dissolution of two different concentrates of 72 kJ mol-

The kinetics of dissolution of synthetic covellite, chalcocite and digenite in dilute chloride solutions at ambient temperatures

An experimental study of the comparative kinetics of dissolution of various synthetic copper sulphides (covellite, chalcocite, and digenite) in dilute chloride solutions has been carried out at ambient temperatures such as could be encountered in the heap leaching of secondary copper sulphide minerals. The dissolution of sized synthetic covellite particles was carried out in dilute HCl solutions containing known concentrations of copper(II) and iron(III) at controlled potentials. The results show that the rate of dissolution is similar at potentials of 600 and 650 mV, but is predictably less at a potential of 550 mV. The rate of dissolution is remarkably similar to that of chalcopyrite under similar conditions and is largely independent of Cl- and HCl concentration in the range 0.2 to 2.5 M and 0.1 to 1 M respectively. The effect of temperature is significant and an activation energy of 71.5 kJ mol - 1 was derived which confirms a chemical or electrochemical rate-determining reaction on the mineral surface. A mineralogical study of the residue after leaching shows that most of the sulphur is associated with unreacted covellite and occurs as isolated globules on the surface with over 90% of the unreacted covellite surface free of sulphur. Dissolution of synthetic chalcocite and digenite is rapid compared with that of covellite under the same conditions. At a potential of 500 mV, the relatively rapid initial dissolution of chalcocite and digenite does not proceed beyond about 50% and 45% copper dissolution, respectively, as predicted from the thermodynamics. These results confirm the formation of a covellite-like phase as an intermediate which cannot be leached at a potential of 500 mV. An increase in the potential results in rapid dissolution of this "secondary covellite," relative to primary covellite. These results provide useful information for the conditions that should be used for the heap leaching of ores containing secondary copper sulphide minerals

The dissolution of chalcopyrite in chloride solutions: Part 1. The effect of solution potential

A study of the dissolution of several copper concentrates under controlled potential conditions in chloride solutions has demonstrated that the rate of dissolution of chalcopyrite is strongly dependent on the potential of the solution. Linear rates were obtained for the dissolution of chalcopyrite in solutions containing 0.2 M HCl and 0.5 g L- 1 Cu(II) at 35 °C. The rate is enhanced within a range of potentials of 550 to 620 mV (versus SHE) and the presence of dissolved oxygen is essential for enhanced rates within this potential window. Even though dissolved oxygen is important in order to achieve acceptable rates of dissolution, excessive oxidation can increase the potential into a region in which passivation is possible. Reduction of the potential into the optimum region results in restoration of enhanced rates of dissolution. On the other hand, dissolution at low potentials (< 540 mV) results in reduced rates of copper dissolution which increase significantly when the potential is subsequently increased to above 580 mV. Mineralogical studies have shown that chalcopyrite remains un-leached and small amounts of covellite appear to be formed on the surface of the chalcopyrite at low potentials. All concentrates appear to dissolve at roughly the same rate under the same conditions, despite differences in the composition and mineralogy of the samples

The dissolution of chalcopyrite in chloride solutions; Part 3. Mechanisms

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

Enhanced leaching of chalcopyrite at low potentials in chloride solutions 2. Mechanisms

In a previous paper in this series, it was shown that enhanced leaching of chalcopyrite from a number of concentrates in acidic chloride solutions can be achieved by controlling the potential in a range of 560-600 mV (SHE) in the presence of dissolved oxygen. The addition of fine pyrite enhanced the rate of dissolution under these conditions. Based on the results of these leaching experiments and detailed mineralogical analyses of the residues, a mechanism involving non-oxidative dissolution of the mineral coupled to oxidation of the product hydrogen sulfide is proposed. The latter reaction is shown to occur predominately by a copper ion- catalyzed reaction with dissolved oxygen. The results of an independent study of the kinetics of this reaction will be presented which will demonstrate that the rates are consistent with those obtained for the dissolution of the mineral. The possible involvement of a covellite-like surface layer on the chalcopyrite under some conditions will also discussed as it relates to the mechanism. It will also be shown that fine pyrite particles can also act as a catalyst surface for the oxidation of hydrogen sulfide. This mechanism is consistent with the mineralogy which confirmed the formation of secondary sulfur which is not directly associated with chalcopyrite except at high pH (about 2) values but is found as discrete globules and associated with fine pyrite if present. A comparison of this mechanism with those proposed in other more limited studies of the dissolution of chalcopyrite under similar conditions in sulfate solutions has been made

Enhanced leaching of chalcopyrite at low potentials in chloride solutions 1. Concentrates

It is well known that the leaching of chalcopyrite under ambient conditions is extremely slow even under highly oxidizing conditions. This has been attributed to so-called passivation of the mineral that occurs at potentials above about 0.65 V (SHE). However, previous reports have suggested that the mineral can be more effectively dissolved at lower potentials in sulfate solutions and mechanisisms for this reaction have been proposed. This paper will report on an extensive study of the kinetics of the dissolution of a number of chalcopyrite concentrates in chloride solutions under various conditions in specially designed reactors with the objective of developing a heap leach process for primary copper sulfide ores. It will be demonstrated that enhanced rates of dissolution can be achieved at ambient temperatures By the application of controlled potentials in the range 560-600 mV, depending on the concentration of chloride ions. However, control of the potential by the use of electrochemical or chemical oxidation of iron(II) or copper (I) ions is ineffective unless carried out in the presence of dissolved oxygen. The rates of dissolution are approximately constant for up to 80% dissolution for sized functions of the concentrates with an activation of energy of about 80 kJ/mole. Chalcopyrite from different sources appears to dissolve at approximately the same rate which is largely independent of the iron and copper ion concentrations, the acidity and chloride ion concentration but depends in some cases on the presence of additives such as fine pyrite