Chemical process in which thiourea dissolves gold and silver

Many studies have shown that gold can be dissolved in the acidic solution of thiourea in the presence of oxidant, and the effect of thiourea in sulfuric acid solution is better, and the gold-forming complex cation Au(SCN ₂ H ₄ ) _{2 is} also proved. The form of ⁺ is present in the acidic solution of thiourea. Thiourea must dissolve gold and silver from a zero valence state to a +1 oxidized state, and thiourea can also be oxidized in an acidic solution. In the presence of oxidants such as hydrogen peroxide, high- iron ions, etc., thiourea can be gradually oxidized into a variety of products, the first is the formation of dithiocarbazone [HN(NH ₂ )CSSC(NH ₂ )NH, simply written as RSSR, Wherein R is C(NH ₂ )NH], which acts as a selective oxidant for gold and silver. The reaction of thiourea to RSSR is reversible, but if the potential of the solution is too high, the RSSR will be oxidized to the next step, such as cyanamide, hydrogen sulfide and elemental sulfur, so the potential of the thiourea leaching solution is strictly controlled. It is important to minimize the loss of thiourea.
The basic reaction of thiourea leaching gold and silver can be described as follows:
Gold oxidation:
Au=====Au ⁺ +e ^- E ^Ó¨ =1.692V (1)
Gold is dissolved in thiourea:
Au+2(TU)→Au(TU) ₂ ⁺ +e ^- E ^Ó¨ =0.38V (2)
Formation of dithiocarbamidine:
2(TU)=====RSSR+2H++2e- E ^Ó¨ =0.42V (3)
This reaction is very fast, the generated RSSR is a reactive oxidant, and is necessary in the dissolution of gold. The reaction of equations (2) and (3) gives the total reaction:
Au+RSSR+2H ⁺ =====Au (TU) ₂ ⁺ (4)
A similar reaction occurs in silver:
Ag+RSSR+2H+=====Ag(TU) ₂ ⁺
The reaction (4) had E ^Ө = 0.04 V and ΔG ^Ө = -7750 J/mol.
In a Fe ³⁺ containing solution, Fe ³⁺ acts as an oxidizing agent:
Fe ³⁺ +e ^- =====Fe ²⁺ E ^Ó¨ =0.77V (5)
Adding reactions (2) and (5) gives:
Au+Fe ³⁺ +2(TU)=====Au(TU) ₂ ⁺ +Fe ²⁺ E ^Ó¨ =0.39V (6)
There is a similar reaction to silver:
Ag+Fe ³⁺ +2(TU)=====Ag(TU) ₂ ⁺ +Fe ²⁺
The ^{ΔG Ө of the} reaction (6) = -37 595 J/mol, which has a larger tendency to dissolve than the reaction (4). Therefore, the addition of oxidant and the formation of dithiocarbamidine are also beneficial for gold dissolution. [next]
From the data listed in Table 1, it is known that the preferred oxidizing agents are high-valent iron salts, dithiocarbamidine, manganese dioxide, and dissolved oxygen. Playa Corzine tests showed: When bleaching powder, potassium permanganate, potassium weight chromium as an oxidant to dissolve a small amount of gold precipitate elemental sulfur solution occurs very quickly, indicating that at this time thiourea quickly decomposed and oxidized Invalid.

Oxygen is more oxidizing in acid. Since the thiourea acid solution must contain some iron and other impurities into the immersion liquid when processing the gold-containing mineral raw material, it is only necessary to continuously fill the slurry with air when the gold is dissolved, and the high-priced iron salt can be regenerated, and the solution is in the solution. Dissolved oxygen itself is sufficient to oxidize the gold and transfer it into the immersion liquid.
Experiments have shown that the impregnation of oxygen into the slurry during immersion in gold provides a more stable oxidizing atmosphere, while the more active oxidant such as hydrogen peroxide consumes too much thiourea because the RSSR is irreversibly oxidized to the next step. product. The RSSR generation response is:

When the concentration of thiourea and the pressure of dissolved oxygen are constant, the driving force of the gold dissolution reaction increases with the decrease of the pH of the medium and the concentration of the thiourea complex. When the pH of the medium is constant, the driving force of the dissolution reaction will follow the dissolved oxygen. The pressure and the concentration of thiourea increase and increase.
The figure below shows the potential-pH diagram of the Au(Ag)-SCN ₂ H ₄ -H ₂ O system at 25 °C. As can be seen from the figure, the line 1 in which gold is dissolved to form Au(SCN ₂ H ₄ ) ₂ ⁺ is at a potential of 0.3739 V, and the line ₂ in which silver is dissolved to form Ag (SCN ₂ H ₄ ) ₃ ⁺ is 0.1142 V. Therefore, from the thermodynamics, the Ag line potential is lower than the Au line, and thiourea dissolves silver more easily than gold. And both the 1 line and the 2 line intersect the 4 line, the pH corresponding to the intersection of the gold dissolution line 1 is 1.78, and the pH corresponding to the intersection of the silver dissolution line 2 is 6.17. That is to say: the pH of gold dissolution is not more than 1.78, and the pH of silver dissolution is not more than 6.17. Only when the pH of the thiourea solution is lower than the pH corresponding to the intersection point, the oxidant can oxidize gold and silver and complex with thiourea to form a complex ion into the solution. If the pH of the solution is greater than the pH corresponding to the intersection, the oxidation reaction of thiourea will be enhanced, and the resulting dithiocarbamidine will be decomposed into S ^2- , H ₂ S, CNNH _2, etc., and the dissolved Au (SCN _{2 ) will be} dissolved. H ₄ ) ₂ ⁺ and Ag (SCN ₂ H ₄ ) ₃ ⁺ complex ions are reduced to metal precipitates. The higher the pH of the solution, the stronger the tendency of thiourea oxidation, and the more the reduction of dissolved gold and silver complex ions. The dithiocarbamium formed at this time does not oxidize gold or silver. [next]

The organic complexing agent is a thiourea, seen from the data in Table 2, while thiourea has a higher selectivity acidic solution gold dissolution also found that when the ingredients contain mercury, copper, bismuth, decreases thiourea The efficiency of gold dissolution increases the consumption of thiourea.