Basic principle of leaching gold by thiosulfate method (2)

3) Kinetics of immersion gold Thiosulfate is used as a leaching agent for gold and silver . When the pH is too high, S ₂ 0 ₃ ^2- undergoes disproportionation reaction to produce S ^2- , resulting in vulcanization of heavy metals, especially silver. Precipitate. However, the disproportionation reaction product sulfite can react with any sulfide in the solution, which is beneficial to the stable existence of S ₂ 0 ₃ ^2- and inhibit the precipitation of metal sulfide. In essence, the disproportionation reaction of S ₂ 0 ₃ ^2- is reversible and is in a state of dynamic equilibrium in solution. In order to maintain a moderate pH of the medium, it is most suitable to use an ammonia solution as a medium in which thiosulfate is stably present. The pH of the ammonia thiosulfate solution can be around 10, the potential is stable at about 200 mV, and the pH of the solution is independent of the thiosulfate concentration. Ammonia has a great influence on the anodic oxidation of thiosulfate, which can significantly reduce the oxidation rate of S ₂ 0 ₃ ^2- . The higher the ammonia concentration, the faster the oxidation rate of S ₂ 0 ₃ ^2- decreases. When the ammonia concentration is 1.0 mol/L, the oxidation rate is only a quarter of that in the absence of ammonia. The leaching rate of gold also increases with increasing ammonia concentration and decreasing S ₂ 0 ₃ ^2- oxidation rate. However, excessive ammonia will cause an increase in hydroxide ions, which is detrimental to gold leaching.
When the gold ore is leached with the ammonia thiosulfate solution, the ammonia concentration and the thiosulfate concentration have an effect on the complex of gold, silver and copper . Under leaching conditions (pH=10, E=200 mV) (shaded in Figures 1 to 5), use S ₂ 0 ₃ ^2- and 1-3 mol/L NH with a concentration of 1.0 mol/L. The solution of ₃ / NH ₄ ⁺ leaches gold and silver, and the stable gold and silver complex ions entering the solution are Au ( NH ₃ ) ₂ ⁺ and Ag (S ₂ 0 ₃ ) ₂ ^{3- respectively} , as shown in Fig. 1 and Fig. 2 Show. When Cu ²⁺ is present in the solution, the concentration of ammonia ions in the solution decreases due to the formation of copper-ammonium complex ions, and gold is stably present from Au(NH ₃ ) ₂ ⁺ to Au(S ₂ 0 ₃ ) ₂ ^3- . The stable complex ion form of copper varies with S ₂ 0 ₃ ^2- concentration (0.1~1.0 mol/L), ammonia concentration (1.0~3.0 mol/L) and Cu ²⁺ concentration (0.0063~0.05 mol/L). And change. In low concentration S ₂ 0 ₃ ^2- (0.1 mol/L) and low concentration NH ₃ /NH ₄ ⁺ (1.0 mol/L) solution, copper is Cu(NH ₃ ) regardless of the high or low concentration of Cu ²⁺ . ₄ ^{2+ is} stable, as shown in Figure 3. In high concentration S ₂ O ₃ ^2- (1.0mol/L) and low concentration NH ₃ /NH ₄ ⁺ (1.0 mol/L) solution, high concentration of Cu ²⁺ (0.05 mol/L) is Cu (S ₂ 0) ₃ ) ₃ ^{4- is} stable, while low concentration Cu ²⁺ (0.0063 mol/L) is stable in Cu(NH ₃ ) ₄ ²⁺ , as shown in Figure 4. High concentration of Cu ²⁺ (0.05 mol/L) is Cu(NH ₃ ) in high concentration S ₂ 0 ₃ ^2- (1.0 mol/L) and high concentration NH ₃ /NH ₄ ⁺ (3.0 mol/L) solution. ₄ ^{2+ is} stable, as shown in Figure 5. It can be seen that under leaching conditions, the stability of the tetraammine copper complex is worse than that of cuprous thiosulfate.

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Figure 4

Figure 5

   

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