Michael Nicol

The recovery of base metal sulfides has been emphasized in the study of the flotation of platinum group minerals (PGMs). However, in some ore bodies such as the Platreef of the Bushveld Complex, sulfide minerals do not make up the majority of the more significant PGM mineralization. It is therefore important to develop a fundamental understanding of how non-sulfide and sulfide PGMs respond respectively to the current reagents being used in the flotation of these minerals. The interactions between platinum and palladium sulfides and tellurides with the thiol collectors, ethyl xanthate and diethyl dithiophosphate, have been studied using the electrochemical techniques of mineral rest potential and voltammetry measurements. The differences in the interaction between the two types of minerals with these reagents were of key interest. Rest potential measurements indicated stronger electrochemical interactions of the minerals with ethyl xanthate than diethyl dithiophosphate. These measurements also indicated that there are different forms of oxidized collector present on the mineral surfaces after interaction with ethyl xanthates. Voltammetry measurements support these observations and suggest differences of adsorption on Pt and Pd sulfides than on Pt and Pd tellurides. Voltammetry also showed that there was minimal electrochemical interaction between the Pt and Pd minerals and diethyl dithiophosphate.

The goal in the electrowinning of metals is the production of high quality and purity cathodes. In a nickel/cobalt electrowinning operation, one of the ongoing issues is the occurrence of black deposits on the nickel and cobalt cathodes.An investigation has been made into the effects of dithionate and thiosulfate ions on the reduction and oxidation processes of cobalt and nickel. Dithionates are not responsible for the formation of black metal sulphides. In the presence of small amounts of thiosulfate, metal sulphides are formed in addition to the metals in a reaction involving reduction of thiosulfate in the presence of these metal ions. Chemical reduction of thiosulfate by metallic cobalt and nickel to form metal sulphides has been established. In addition, reduction of cobalt ions by thiosulfate catalysed by the metal/metal sulphide surface accounts for the excess anodic charge observed during open circuit of cobalt metal/sulphide in thiosulfate solutions.

Mass transport of copper ions to the cathode during the electrowinning of copper is important in determining the optimum current density in order to achieve deposits of acceptable physical and chemical quality. The results of pilot scale tests using full size cathodes and anodes (both lead-calcium-tin and titanium mixed metal oxide, MMO) will be described. In these tests, silver ions have been used as a tracer in order to determine local mass transfer coefficients to 16 sections of each cathode. The results have shown that mass transfer is slightly higher at the top and bottom of the cathode and that the distribution is more uniform with MMO anodes than with conventional lead alloy anodes. The results agree quantitatively with previously published data obtained using half width electrodes. Measurement of the mass of each section has also been used to establish the current distribution over the surface of the cathodes. In addition, the reduced cell voltage obtained with the MMO anodes has been quantified as have the voltage drops at the contact of the anode header bars with the busbar.

A number of sulfide minerals of copper, nickel and iron can only be effectively leached by methods that require elevated temperatures in acidic solutions. While suitable for concentrates, these processes are not applicable to tank or heap leaching of low grade ores. It is now generally accepted that many of these minerals are subject to so-called passivation under oxidising conditions. However, the mechanisms involved in this passivation are not well understood and this study will review some of these mechanisms and show that the passivation of several of these important minerals appears to follow similar trends. Electrochemical measurements using millerite, covellite and chalcopyrite electrodes were used to study the similarities of leaching and passivation processes taking place in oxidative acidic media. Thus, potentiostatic current-time transient experiments have demonstrated the resemblance with selective dissolution of metals from alloy systems. A model will be proposed which involves depletion of metal from outer layers of the minerals and the limiting step being diffusion of the metal ions through sulfur sub-lattice in these minerals. Potential step polarization experiments have enabled estimates to be made of the selfdiffusion coefficients of copper, nickel and iron which were in the range of published values extrapolated from higher temperatures. The well known parabolic leaching behaviour of these minerals is supported by solid state diffusion as the rate limiting step.

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.

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). A study has been made of the rate of leaching of copper in dilute chloride solutions from a low-grade ore which contained chalcopyrite as the only copper sulfide mineral. Leaching has been carried out in batch stirred reactors using milled ore and in 1 m columns using crushed ore in solutions of chloride at various potentials. It has been observed that enhanced rates of dissolution of copper from chalcopyrite in chloride media can be achieved at ambient temperatures by the application of controlled potentials in a "potential window" between 560 and 600 mV (SHE). These results generally confirm the results presented in Part 1 of this series of papers which detailed the results obtained on the dissolution of a concentrate from the same ore. A reduced sensitivity of the rate of leaching in columns to the potential has been observed which is interpreted in terms of a potential distribution at the surface of chalcopyrite grains. This potential distribution can vary both locally due to mineralogical effects and vertically due to potential variations down the column as observed in the differences of the potential in the column inlet and outlets. A linear rate of approximately 0.1% copper dissolution from a low grade chalcopyrite ore under typical heap leach conditions has been measured in a solution of 50 g/L chloride at a pH of 1 and a potetitia1 of about 600 mV for the feed solution.