Process for recovering gold and silver from refractory ores

A hydrometallurgical process for the recovery of precious metal from an ore or concentrate containing arsenopyrited or pyrite wherein precious metal is occluded in arsenopyrite or pyrite, which process comprises:

• (a) forming in a common volume space a gas phase comprising air and water vapor and a liquid slurry comprising the ore or concentrate as the solid phase and acid and water as the liquid phase of the slurry;
• (b) effecting in the slurry between the arsenopyrite or pyrite and an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3 an oxidation-reduction reaction having a standard potential between about 0.90 and about 1.20 volts on the hydrogen scale, thereby solubilizing in the liquid phase the arsenic, iron and sulfur in the arsenopyrite, or the iron and sulfur in the pyrite, all as the oxidation products, and producing in the liquid phase nitric oxide in which the nitrogen has a valence of plus 2, as the reduction product;
• (c) releasing nitric oxide from the liquid phase into the gas phase;
• (d) oxidizing the nitric oxide in the gas phase, in which an oxygen partial pressure above the ambient oxygen partial pressure in air is maintained by continuous addition of an oxygen containing gas, to form an oxidized nitrogen species in which the nitrogen has a valence of at least plus 3, the total amount of oxygen added being at least in an amount stoichiometrically required for solubilization in the liquid phase of the arsenic, iron and suphur in the arsenopyrite or the iron and sulfur in the pyrite;
• (e) absorbing the oxidized nitrogen species into the slurry wherein the oxidized nitrogen species become available for the oxidation-reduction reaction of step (b) whereby the nitrogen, in its oxide forms, functions as a catalyst for the transport of oxygen from the gas phase to the oxidation-reduction reactions in the slurry, thereby permitting the total of the oxidized nitrogen species and nitric oxide in the system to be less than a stoichiometric balance required for the oxidation of the arsenic, iron and sulphur;
• (f) subjecting the slurry to a solid-liquid separation to produce a solid residue and a liquid fraction;
• and
• (g) recovering precious metal from the solid residue.