GB795790A - Improvements in or relating to the recovery of gold - Google Patents

Abstract

Gold is separated from arsenical and sulpharsenical ores, concentrates and residues and roasted materials, which may contain also cobalt and/or nickel and/or iron, by treatment with one or more strong mineral acids followed by treatment in the same medium with an oxidizing agent supplied as such or produced in situ, the redox potential of the medium being measured, and maintained during the oxidation treatment at such a value that the gold is oxidized and solubilized. Thereafter any solid residue is separated and gold recovered from the solution. Mineral acids referred to are a mixture of sulphuric and nitric acids, hydrochloric acid and sulphuric acid. Suitable oxidizing agents are chlorine (which may be produced in situ by the addition of aqueous hydrochloric acid or sodium chloride to a sulphuric acid mixture or the addition of sodium chlorate to a hydrochloric acid mixture) and sodium or calcium hypochlorites, and the salts may be added as aqueous solutions. It is preferred to employ a temperature of about 70 DEG C. and an acidity greater than normal. When using a mixture of sulphuric and nitric acids it may be necessary, e.g. when using chlorine as oxidizing agent, to remove some of the arsenious oxide before effecting solubilization of the gold: in this case the residue of metallic arsenides which remains insoluble may be separated from the liquid, arsenic removed therefrom by suitable means such as hot water washing, and the solids resuspended in the liquid for completion of the solubilization after restoration of the redox potential. If, however, hydrochloric acid or sodium chloride are used as oxidizing agents with sulphuric and nitric acids, the gold is solubilized before the arsenic is oxidized. After oxidation the liquor may be neutralized, e.g. with lime or calcium carbonate, with continuous maintenance of the redox potential above the level at which gold reprecipitates and with pH measurement. Thus with the solution from an ore containing iron and cobalt, and after obtaining an Fe/As ratio of about 1, the neutralization should continue until ferric arsenate is precipitated, then gold can be separated by adding sodium sulphide or iron or, as described in an example, by adsorption on charcoal. Any silver present may be recovered by passing the gold-free solution over copper or iron. In examples: (1) a roasted speiss containing cobalt, nickel, iron and arsenic was reacted at 90 DEG C. with hydrochloric acid and aqueous sodium chlorate added; and (2) smaltite containing cobalt, nickel and arsenic was reacted with a mixture of sulphuric and nitric acids and chlorine added as oxidizing agent. Specification 795,793 is referred to.