AUXILIARY AGENTS IN Gold CYANIDATION.

The mechanisms of oxidation and cyanidation of gold together with the consumption of oxygen and cyanide by gangue minerals is a serious problem. Comm only air or oxygen can be used as an oxidant in the leaching of auriferous ores because it’s readily available, cheaper and much less aggressive than most of the chemical oxidants (e.g. H₂O₂, Na₂O₂, O₃, CaO₂), but , the rate of gold cyanidation with molecular oxygen is slow compared with the chemical oxidant.

Conventional mill practice involves the sparging of oxygen or air through the pH adjusted gold leach pulp preferably prior to the addition of cyanide. This preareation oxidizes soluble sulphide into thiosulphate and finally sulphate.

If cyanide is present in the pulp, it would be oxidized to thiocyanate. These reactions obviously indicate that soluble sulphide is a major oxygen consumer. This oxidation process actually prevents the formation of passive film on the gold surface, thus gold cyanidation rate remains unaffected.

The preareation operation not only meets oxygen demand for the oxygen consumers but also produces a leach slurry saturated with dissolved oxygen which is being utilized in oxidation of gold in the cyanidation circuit. If the concentration of oxygen consumers are high (more than 1000 ppm) in the leach feed, the preareation alone would not to be enough to oxidize all of them. Then, oxygen demand would remain even high even in the cyanidation circuit. Consequently, the gold dissolution rate will drop if no additional supply of oxygen is maintained in the cyanidation circuit. Some gold mill operations maintain air or oxygen sparging in the cyanidation circuit.

The maximum utilization of oxygen in the gold mill circuit depends on its solubility in the leach pulp. The solubility of oxygen in water is temperature and pressure dependent. The solubility of oxygen increases with the decrease of temperature or increasing the oxygen pressure. The concentration of oxygen in an aqueous medium at a ambient pressure could be increased by injecting oxygen through a diffuser. The injection mechanism disperses oxygen through the aqueous medium in the form of fine bubbles.

The presence of reactive sulphides such as marcasite, pyrrhotite, realgar or chalcocite in the cyanidation feed often inhibits gold dissolution by forming a protective film on the gold surface. Nevertheless, this effect could be eliminated or minimized by intensive preareation or adding a promoter in the cyanidation pulp.

Salts of lead (e.g. lead nitrate) or mercury (e.g. mercury acetate) serve as promoters and remove soluble sulphide (S^-2) from the cyanidation solution as sparingly soluble PbS or HgS, thus keeping the gold surface clean. It is also known that these promoters accelerate the rate of gold cyanidation possibly through the development of local galvanic cells between gold and lead or mercury. A small quantity of promoter (e.g. 0.3 to 0.6 g/t) is often sufficient to counteract the harmful effect of sulphides. The addition of a lead salt to cyanidation pulp is practiced in some operations and it is also reported that an excess may cause retardation in the rate of cyanidation.