The effects of thiosulfate ions on the deposition of cobalt and nickel from sulfate solutions

A detailed electrochemical study of the effects of thiosulfate ions on the reduction of nickel(II) and cobalt(II) ions in sulfate solutions in the pH range 3 to 4 has been undertaken. Even in the presence of small amounts of thiosulfate, metal sulfides are formed in addition to the metals in a reaction involving reduction of thiosulfate in the presence of these metal ions. In addition, chemical reduction of thiosulfate by metallic cobalt and nickel to form metal sulfides has been established. Reduction of cobalt ions by thiosulfate catalyzed by the metal and/or metal sulfide surface accounts for the excess anodic charge observed during open circuit contact of cobalt metal and/or cobalt sulfide with thiosulfate solutions. It has been shown that thiosulfate is responsible for the formation of black cathodic films of the metal sulfides periodically observed during electrowinning of these metals. A simple method for the control of the peroxide addition to the electrolyte in order to oxidize residual thiosulfate has been devised based on the results of this study

The electrochemistry of pyrite in chloride solutions

A detailed study of the anodic and cathodic behaviour of natural pyrite in acidic chloride solutions containing various oxidants has been conducted as part of an overall program on the fundamental aspects of the heap leaching of copper sulphide minerals. The stoichiometry of the anodic dissolution reaction depends on the potential in that it varies from less than 4F/mol Fe dissolved at potentials below 0.8 V(SHE) to 15F/mol Fe at potentials above about 1.0 V(SHE). The mixed potentials of pyrite in chloride solutions containing iron(III) are greater than those in the presence of copper(II) and both increase with agitation as a result of enhanced transport of iron(II) and copper(I) from the surface of the dissolving mineral. The mixed potentials are unaffected by the presence of dissolved oxygen confirming the low reactivity for the cathodic reduction of oxygen. The rate of anodic dissolution of pyrite in chloride solutions is independent of the acid and chloride concentration except at high chloride concentrations when the rate decreases slightly. The potential dependence of the anodic reaction roughly follows Tafel behaviour up to about 1.0 V but mechanistic conclusions are excluded due to the variable stoichiometry. Cathodic reduction of oxygen is some 30 times slower than that of 1 g/L iron(III). The reduction of copper(II) is also less significant in the leaching of pyrite due to the lower formal potential of the copper(II)/copper(I) couple than that of the iron(III)/iron(II) couple in concentrated chloride solutions. Comparative measurements have shown that pyrite will dissolve more slowly than chalcopyrite in chloride solutions and, this coupled to the low sulfate yield at low potentials, suggests that oxidation of pyrite as a source of heat in abiotic heap leaching is unlikely

Cathodic reduction of iron(III) and copper(II) on various sulfide minerals in chloride solutions

A comparative study of the electrochemical reduction of iron(III) and copper(II) ions on selected sulfide minerals in concentrated chloride solutions has been carried out as part of a broader study of the kinetics of the leaching of chalcopyrite, covellite, enargite and pyrite under chloride heap leaching conditions. Mixed potential, cyclic voltammetric and potentiostatic measurements have been made using rotating disk electrodes of massive mineral samples. For comparative purposes, arsenopyrite, platinum and gold electrodes have also been used under the same conditions. The mixed potentials of the various minerals in solutions containing 4.2 mol/L sodium chloride, 0.1 mol/L hydrochloric acid and 0.054 mol/L iron(III) and/or 0.047 mol/L copper(II) ions at 25 °C vary with time depending on the mineral reactivity. The difference between the mixed potentials and the solution potentials provide qualitative indications of mineral reactivity to dissolution with iron(III) or copper(II) as oxidants. Cyclic voltammetry conducted at potentials negative to the mixed potentials at slow sweep rates after the mixed potential measurements has shown variable reactivity of the minerals for reduction of iron(III) and copper(II) ions. The data has been analysed in terms of electrochemical kinetics using a modified Butler-Volmer approach that takes into account mass transport of the oxidized and reduced species and anodic oxidation of the minerals. The electrochemical rate constant derived from a fit of the data to the rate equation shows that all the minerals have greater reactivity for the reduction of copper(II) than iron(III) ions. The rate constant varies by about an order of magnitude within the mineral group for both iron(III) and copper(II) reduction and the rate of reduction on platinum and gold electrodes are higher for both couples than for the mineral electrodes. The ratio of the rate of copper(II) reduction to iron(III) reduction is significantly greater for the minerals containing copper than for those without copper. The observed influence of mass transport on the cathodic currents close to the mixed potentials for the reduction of copper(II) on platinum, pyrite and enargite has been quantitatively explained in terms of the effect of mass transport on the surface concentration of copper(I). An attempt has been made to correlate the kinetic data with published data on the semi-conducting properties of the metal sulfides. With the exception of covellite (which is not considered a semiconductor), the formal potentials of the copper(II)/copper(I) and iron(III)/iron(II) couples fall within the bandgaps of the metal sulfides and there does not appear to be any correlation between the energy levels of the couples in solution relative to the conduction bands of the sulfides and the reactivity for electron transfer to the metal ion couples. The effect of illumination with light of wavelength 405 nm on the cathodic currents has been demonstrated to be due to thermal and not photocurrent effects

Effect of halides in the electrowinning of zinc. II. Corrosion of lead-silver anodes

This paper summarizes experimental results obtained from a series of laboratory scale electrowinning tests conducted over 5 months to quantify the effects of halides (chloride, fluoride and bromide) on the performance and corrosion of lead-silver anodes under conditions similar to those used during the electrowinning of zinc. The parameters investigated include operating anode potential, corrosion rate and anode scale/cell mud generation rates. Information was also obtained on the consumption of halides and manganese ions and the composition of the anode scale and cell mud. The results have confirmed plant observations of excessive anode corrosion and chlorine emissions at a chloride concentration of 400 mg/L but not at a concentration of 200 mg/L. It has also confirmed the importance of maintaining a suitable manganese(II) concentration in the electrolyte. Bromide and fluoride ions, albeit at lower concentrations, do not have measurable effects on anode corrosion. Although a definitive explanation for enhanced local corrosion at high chloride concentrations has not been advanced, the nature of the accelerated corrosion suggests that a crevice-like corrosion process is responsible for localized massive corrosion. This has been attributed to the presence of high acidity and permanganate ions between the manganese oxide layer and the alloy surface

The effects of halides in the electrowinning of zinc. I. Oxidation of chloride on lead-silver anodes

A study of the kinetics of the anodic oxidation of chloride ions on the surface of lead-silver anodes used for the electrowinning of zinc has shown some interesting and practically important results. Oxidation of chloride does not occur on a PbSO4 surface but occurs readily on a PbO2 surface. Oxidation of chloride occurs at the mass-transport controlled rate at the operating potentials of anodes during the electrowinning of zinc in the absence of manganese in the electrolyte. However, the rate of oxidation of chloride ions is significantly lower in the presence of manganese ions in the concentration range 1 to 5 g/L. These results have been compared with plant observations that confirm the lower rate of chlorine generation in the presence of manganese. The use of periodic current reversal does not increase the rate of oxidation of chloride ions in the presence of manganese ions. The rate of anodic oxidation of chloride in the presence of manganese ions is too low to be of practical value in controlling the chloride levels during the electrowinning of zinc

The oxidation of manganese ions on lead alloys during the electrowinning of copper

Manganese ions are often encountered as impurities in the electrolyte used in the electrowinning of copper. The effects of manganese ions in the electrolyte on the anodic behaviour of lead-calcium-tin anodes used in the electrowinning of copper have not previously been definitively established. Potentiostatic oxidation of rotating disc electrodes of Pb-Ca-Sn anodes has been used to investigate the anodic behaviour of these anodes and the oxidation reactions of manganese ions in sulphuric acid solutions. The electrochemical measurements were complemented by chemical analyses of the oxidized manganese species formed during the anodisation process. The oxidation of manganese on these anodes was found to occur at potentials above about 1.7 V and is also accompanied by oxidation of the lead and evolution of oxygen at potentials above about 2.0 V. The amount of manganese oxidized increases with increasing oxidation potential. The presence of manganese in the electrolyte decreases the rate of formation of lead dioxide and reduces the fraction of the charge associated with oxygen evolution. Higher concentrations of manganese(m and lower oxidation potentials favour the production of manganese (III) which can form a MnOOH intermediate layer, while permanganate is produced at low concentrations of manganese(II). Mn02 powder produced by chemical precipitation reactions involving manganese (III) and permanganate(VII) ions, was also formed in the bulk electrolyte and on the walls of the cell. Mechanisms for the various reactions are presented together with recommendations for minimization of permanganate formation in copper tankhouse electrolytes

The effects of dithionate and thiosulfate ions on the deposition of cobalt and nickel from sulfate solutions

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

Electrochemical oxidation of iron (II) ions on lead alloy anodes

The anodic oxidation of iron(II) ions on Pb-Ca-Sn anodes in sulphate solutions has been studied using cyclic voltammetry and potentiostatic methods. The oxidation of iron(II) on these anodes does not occur on a lead sulphate surface but occurs readily on a lead dioxide surface. The rate of the oxidation of iron(II) at the anodes is controlled by mass transport to the anode surface. Chemical reaction between iron(II) and PbO2 on the surface of the anodes is rapid. In the presence of manganese ions, oxidation of iron(II) also occurs on the surface of manganese oxides which can also rapidly oxidise iron(II). The presence of iron(II) in the electrolyte will increase the rate of "sulfation" of anodes during power disruptions in copper tankhouses. Limited experiments with a Pb-Ag anode showed very similar results

Assessment of options for economic processing of preg-robbing gold ores

One of the challenges facing the gold industry in the twenty-first century is the continuing need to identify new reserves of economically treatable ores. Discoveries of large, metallurgically simple orebodies are becoming increasingly rarer, forcing companies to investigate options for the processing of ores which may present recovery, economic and/or environmental issues, requiring innovative approaches to their treatment. Ores which contain naturally occurring carbonaceous material ('pregrobbing ores') is an example of one such ore type. Newmont's Jundee operation contains zones of carbonaceous ore within the oxidised ore body. A program consisting of detailed laboratory work and extensive plant trials has been undertaken to assess the options of the processing of this material. Laboratory tests demonstrated the advantage of carbon-in-leach (CIL) over direct cyanide leaching and carbon-in-pulp (CIP) for treatment of these ores. The laboratory program identified several factors which would enhance overall gold recoveries when preg-robbing ore was being processed, including: keeping preg-robbing ore separate from non-graphitic ore, maximising gravity recovery and ensuring the plant operates in 'pure' CIL mode i.e. no leaching before first adsorption tank. Plant trials were undertaken to assess the economic sustainability of treating moderate preg-robbing ores through a modified gravity/leach/adsorption circuit. Results indicated that where good operational control of the circuit was maintained to ensure high gravity recovery, minimal leaching prior to carbon contact and maintenance of good, active carbon inventory, then acceptable and sustainable overall gold recoveries could be obtained. However, plant trials did not produce gold recovery as high as laboratory tests. Using these relatively simple plant modifications potentially allows a plant to treat ores with preg-robbing index (PRI) values up to 1. When PRI values rise to 1·3-1·6 leach recoveries can drop from.85 to <40%, indicating a more intensive approach may be required, including kerosene addition and higher carbon inventories and activities