The Chemical behaviour of silver in the pressure oxidation of complex sulphide ores and concentrates

As supplies of high-grade base metal concentrates diminish, the treatment of complex sulphide materials, which often contain a significant fraction of silver must be considered. Acid pressure oxidation is a promising process for these materials and silver recovery will play a major role in the economy of the process. However, in acid pressure oxidation processing, silver is incorporated into the jarosite specie and the cyanidation of the resulting residue yields very low recovery. Efforts to recover silver from complex sulphide concentrates therefore require an understanding of the chemical behaviour of silver in the pressure oxidation process. In the present work, pressure oxidation of various complex sulphides was investigated in the temperature range 110-150°C to shed new light on the deportment of silver from the source minerals to the residues with subsequent recovery by cyanidation. Silver sulphide reacts in the autoclave under the conditions used for the oxidation of complex sulphide and silver ion is released into the solution. Once the silver is released, the sulphide ion (S²⁻) at the surface of the mineral is oxidised to elemental sulphur by ferric ions. The ratecontrolling step is the oxidation of the sulphide ions at the surface of the mineral and in the absence of ferric, the overall reaction is slow. The rate of reaction increases with increasing temperature and the concentration of ferric ions. However, the rate of reaction decreases with increasing silver sulfate concentration. The kinetics is controlled by chemical reaction with activation energy of 72.6 kJ/mol. The aqueous silver released by the trace silver-bearing minerals can co-precipitate with iron and lead sulphate as a complex jarosite species. In this work, the precipitation kinetics of aqueous silver and ferric in the presence of a lead sulphate suspension were conducted at a temperature range of 110-150°C and under conditions that were relevant to zinc pressure oxidation. The major collector of silver on the oxidation residue was confirmed to be the plumbojarosite specie. The kinetics of the precipitation increases with increasing temperature and concentration of initial ferric sulphate and lead sulphate suspension. The initial acid level had a pronounced negative effect on the rate of formation of jarosite by reducing the rate of hydrolysis of ferric ion. The effect of temperature and seeding on the kinetics indicate that the reaction rate is controlled by a chemical reaction step, which is thought to be occurring at the surface of the jarosite crystals with activation energy of 114.3 kJ/mol. Silver precipitation is fast and finished before iron precipitation. The precipitation of iron continues. This usually leads to the zoning of the silver-rich jarosite in the core of the jarosite structures. During cyanidation of such a residue, only the silver ion in the outer crystal planes of the jarosite structures is thought to be accessible to the free cyanide ions in the pulp leading to low extraction of silver. In this study, it was found that silver could also react with sulphur to form silver sulphide in an acid media containing ferrous and ferric sulphate. Under deoxygenated condition, the reaction apparently involves the reduction of aqueous silver to silvery white flakes (metallic silver) and the subsequent reaction of metallic silver with sulphur. The reduction of aqueous silver is very rapid under highly reducing conditions. The reaction of metallic silver with sulphur is believed to be the rate-determining step because of the positive effect of sulphur on the rate of reaction. Under the pressure oxidation condition, the presence of oxygen minimises the silver sulphide formation by oxidising ferrous to ferric. Also, any silver sulphide formed is oxidised to aqueous silver. Silver precipitation as silver sulphide, therefore does not occur under the pressure oxidation condition. The addition of soluble iodide ion to low to medium temperature autoclave oxidation experiments confirmed that silver could be precipitated as silver iodide in the autoclave discharge residue. The reaction of aqueous silver ion and iodide to form silver iodide (Agl) is therefore an effective way of deporting silver into a recoverable form. Although silver iodide has a low solubility product, silver extraction by cyanidation from the autoclave precipitate which is formed at high temperature is not complete, which suggests that some silver ions are still captured by the jarosite phase during iron precipitation. The deportment of some silver ions to the jarosite phase is promoted by the decrease in stability of silver iodide at high temperature and the increase in the stability of jarosite specie at high temperature. Therefore, in the presence of iodide and under the optimal conditions that favour jarosite precipitation, some aqueous silver may react preferentially with hydrolysed ferric and sulphate to form jarosite. The resulting jarosite specie will impact the ability to recover silver from oxidation residues. Three different complex sulphides were oxidised in the temperature range 110-150°C, and potassium iodide was added to the initial solution to promote Agl in the autoclave residue. High zinc extraction into solution was achieved. The resulting residues were subjected to cyanidation for silver recovery. High recovery of silver (up to 98%) was achieved because its deportment into the lead-hydronium jarosite during the pressure oxidation was minimised, while its deportment into the cyanide-soluble silver iodide was promoted. Silver recovery decreased slightly at high temperatures (140 and 150°C) because of the enhanced stability and kinetics of lead-hydronium jarosite formation, but this could be overcome by increasing acidity and iodide concentration of the autoclave slurry. It is believed that it is now possible to control the chemical behaviour of silver in the pressure oxidation of complex sulphide ores and concentrates at 110-150°C. Addition of small amounts of soluble iodide combined with careful control of temperature, oxidation kinetics and solution composition can ensure that most of the silver in the feed material forms silver iodide in the autoclave residue. Silver iodide in the residue can be quickly and completely leached in weak cyanide solution or perhaps in alternate silver lixiviant.

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