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The leaching of chalcopyrite Jones, David Llewellyn

Abstract

The anodic dissolution of chalcopyrite has been examined in chloride and sulphate solutions from 20°C to 175°C. In chloride the yield of elemental sulphur is nearly 100%, whereas in sulphate solutions it is 75% or less, the remainder being oxidized. It is postulated that in sulphate solutions the copper dissolves as a thiosulphate complex, which can decompose either to a soluble form, or to cupric sulphide and another sulphur species, probably dithionate. The anodic polarization of chalcopyrite displays two important regions: first a diffusion region in which the current is highly time-dependent, but potential independent and second a higher current region, attributed to the build up of a space-charge in the mineral. This space-charge current is largely time independent but is linearly dependent on potential. At low temperatures chloride solutions and sulphate solutions give similar polarization curves but at 90°C and above, the space-charge region in chloride solutions starts at much lower potentials (500 mV) whereas in sulphate solutions it does not. Sulphuric acid solutions passivate the mineral at high temperatures and potentials. The electrochemical results correlate well with leaching experiments using ferric sulphate and ferric chloride, and there is every reason to believe that for chalcopyrite an electrochemical mechanism is operative during leaching. Ferric chloride is an equal or more effective oxidizing agent than ferric sulphate; its (ferric chloride) effectiveness is increased by fine particle size, high [Fe⁺⁺⁺]. Ferric sulphate leaching is relatively unaffected by these factors, but decreased by [Fe⁺⁺]. Linear kinetics are normally observed with both reagents, but under optimum conditions with ferric chloride a rapid initial dissolution precedes the linear stage. Ferric sulphate appears to selectively attack the mineral along grain boundaries, whereas ferric chloride does not. Mixed potential measurements indicate that in ferric sulphate the reaction is under mixed control, and that both anodic and cathodic reactions are quite irreversible. In cupric chloride the leaching reaction is under anodic control, and the cathodic reaction is highly reversible. Ferric chloride leaching appears to be cupric chloride leaching in reality; the ferric ions serve the purpose of depressing the [Cu⁺], thus raising the potential of the Cu⁺⁺/Cu⁺ couple. Thus, ferric chloride leaching is also under anodic control.

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