Thiosulfate is used as a leaching agent for gold and silver , and it encounters kinetic difficulties in practical applications. The effect of temperature on gold and silver leaching is more complicated, as shown in Figure 6.
Figure 6 
The amount of gold dissolved is the largest at 65 ° C and 140 ° C and the smallest at 100 ° C. Above 65 ° C, the amount of gold dissolved decreases due to a decrease in the concentration of Cu 2+ catalyst, S 2 0 3 2- oxidation, and the formation of CuS precipitates covering the surface of the gold particles. At a temperature higher than 100 ° C, copper sulfide adhering to the surface of the gold particles is dissolved in the ammoniated oxygen-containing solution. The copper sulfide film is eliminated, the gold particle surface is re-exposed, and S 2 0 3 2- and Cu(NH 3 ) 4 2+ are regenerated, so that the gold dissolution rate is rapidly increased. However, when the temperature exceeds 140 ° C and S 2 0 3 2- is rapidly oxidized, the concentration is significantly reduced, and the amount of gold dissolved is greatly reduced. [next]
Baghdad Saliang et al. studied the dissolution kinetics of gold and silver in sodium thiosulfate solution by rotating disk method. It is pointed out that the dissolution rate of gold and silver is linear with temperature in the temperature range of 45~85 °C. In order to avoid violent decomposition of thiosulfate, the leaching temperature should be controlled at 65~75 °C. Gold and silver were dissolved in the solution, and the actual activation energies were 17.55 kJ/mol and 21.4 kJ/mol, respectively. This indicates that the leaching process is diffusion-controlled, and it is considered that the Na 2 S 2 O 3 concentration exceeds 0.13 mol/L, which is unfavorable to the reaction kinetics.
Experimental studies have shown that when the surface of gold and silver discs gradually forms sulfur and sulfide precipitates, the velocity of the solvent reaching the metal surface is lowered, which hinders the dissolution of the solvent through the dissolution process of the sulfide layer. If sodium sulfite [m (Na 2 S0 3 ):m (Na 2 S 2 0 3 )=1:1] is added to the solution, the precipitation of sulfur and sulfide on the metal surface can be prevented because the hot sodium sulfite solution can dissolve the finely divided Sulfur, producing sodium thiosulfate:
Na 2 S0 3 +S ==== Na 2 S 2 0 3
In addition, sodium sulfite is cheap and non-toxic, and can also increase the alkalinity of the solution. It is also a solvent for gold itself.
Kakowski and others have also found that copper ions catalyze the dissolution of gold by thiosulfate, which can increase the dissolution rate of gold by 17 to 19 times. At the same time, however, it forms a copper sulfide barrier on the gold surface, allowing the leaching reaction to be diffusion controlled. Baghdad Saliang believes that the reaction of copper sulfate in the addition of sodium sulfite is:
2CuS0 4 +4Na 2 S 2 0 3 +Na 2 S0 3 +H 2 0 ==== 2Na 3 [Cu(S 2 0 3 ) 2 ]+2Na 2 S0 4 +H 2 SO 4
When sodium sulfite is not added, it is:
2CuS0 4 +2Na 2 S 2 0 3 +3H 2 0+3/20 2 ==== Cu 2 S+3H 2 S0 4 +2Na 2 S0 4
Visible sulfite reaction can induce a stable copper complex thiosulfate (K no = 6 × 10 -13); sodium sulfite is not generated thio sulfate hydrolyzed into Cu 2 S and H 2 S0 4. Therefore, the addition of sodium sulfite can reduce the amount of sodium thiosulfate and greatly reduce the amount of sulfuric acid and sodium sulfate produced. For this reason, it is recommended that when leaching gold with thiosulfate, the mass ratio of each component of the leaching agent should be m(Na 2 S 2 0 3 ):m (Na 2 S0 3 ):m(CuS0 4 )=1: 1 : 0.7.
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