Effect of Cyanide and Oxygen Concentration on Gold Dissolution Rate

When gold and silver are dissolved, the required concentration of cyanide and oxygen is proportional. According to the reaction formula (1), 1 mol (molecular) oxygen requires 4 mol (molecular) of CN ^- , and the average ratio of the diffusion coefficients of the two is 1.5. The cyanide solution which is known to be saturated with air contains [O ₂ ] = 8.2 mg of ∕L, or 0.27 × 10 ^-3 mol (molecule). Then [CN ^- ] = 4 × 1.5 × 0.27 × 10 ^-3 = 6 × 0.27 × 10 ^-3 mol (ion), or 0.01%. In actual production, an aqueous solution containing 0.02% to 0.06% NaCN is usually used.

4Au+8NaCN+O ₂ +2H ₂ O 4NaAu(CN) ₂ +4NaOH (1)

The adjustment of the cyanide concentration in the solution is achieved by controlling the amount of cyanide input. The oxygen concentration is achieved by inflating the solution by means of an aeration machine. Under normal conditions, the aeration of the aerated machine can make the solubility of oxygen in the solution reach 7.5 ~ 8mg , L, only a certain constant value can be reached in the thin cyanide solution. Practices in most plants have shown that under atmospheric aeration conditions, the maximum dissolution rate of gold is in the range of 0.05% to 0.1% cyanide concentration; in individual cases, it is in the range of 0.02% to 0.03%. A higher cyanide concentration is used only when diafiltration cyanidation is carried out, or if ore containing more cyanide-depleted impurities is consumed, and de-gold-depleted liquid containing cyanate is recycled for recycling.

Tests have shown that when the cyanide concentration is less than 0.05%, the solubility of oxygen in the solution is large, and the diffusion rate of oxygen and cyanide in the dilute solution is faster, and the dissolution rate of gold increases with the concentration of cyanide. The line rises to the maximum. Later, as the cyanide concentration increases, the dissolution rate of gold rises slowly. When the cyanide concentration exceeds 0.15%, although the cyanide concentration is increased, the dissolution rate of gold does not increase, but decreases slightly (Fig. 1). This may be due to oxygen and CN ^- ratio imbalance. And the pH of the solution is increased to cause hydrolysis of cyanide ions:

CN ^- +H ₂ O HCN+OH ^-

Fig.1 Effect of different cyanide concentrations on the dissolution rate of gold and silver

In a low concentration cyanide solution, the dissolution rate depends on the concentration of cyanide; however, when the cyanide concentration is increased, the dissolution rate is independent of the cyanide concentration and increases with the oxygen supply pressure (Fig. 2). . To this end, the process of gold dissolution can be enhanced with an oxygen osmotic solution or high pressure aeration. If cyanidation is carried out under aeration conditions of 709.275 kPa (7 atm), the dissolution rate of gold in different characteristic ores can be increased by 10 times, 20 times, or even 30 times, and the recovery rate of gold can be increased by about 15%.

Fig. 2 Effect of different pressures and different NaCN concentrations on the dissolution rate of silver at 24 °C

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