The presence of gold in ore occurs in most cases in the form of natural gold. The dissolution and leaching of natural gold is often achieved by the complexation of gold and cyanide ions. It is very important to use zinc reduction method to recover gold from the leachate. Therefore, it is very important to understand the thermodynamic relationship between gold cyanide and zinc replacement gold. The relevant chemical reactions and thermodynamic relationships are as follows:
Au + +e 
Au
E=1.73+0.0591 1
Au 3 + +3e Au
E=1.498+0.0197 2
Au(CN) 2 - Au + +2CN -
pCN=19+0.5lg 3
Au(CN) 2 - +e Au+2CN -
E=-0.68+0.0591lg +0.118pCN 4
H + +CN - HCN
Let A=a +a HCN , then
pH=9.4-lgA+lg[1+10 pH -9.4 ]-pCN 5
2 H + +2e H 2
E=-0.0591pH-0.0295lgP 6
O 2 +4H + +4e 2H 2 O
E=1.23+0.0148lgP -0.0591pH 7
H 2 O 2 +2H + +2e 2H 2 O
E=1.77+0.0295lga -0.0591pH 8
O 2 +2H + +2e H 2 O 2
E=0.68+0.0295lg -0.0591pH 9
Zn 2 + +2e Zn
E=-0.76+0.0295lga Zn 2 + 10
Zn(CN) 4 2- Zn 2 + +4CN -
pCN=4.2+0.250lg
Zn(CN) 4 2- +2e Zn+4CN
E=0.118pCN-1.26+0.0295lga Zn (CN)
Figure 1 Au, Zn-H 2 O system potential-pH diagram
Figure 2 U-H 2 O system potential-pH diagram
The difference between Fig. 1 and Fig. 2 is that the abscissa in Fig. 1 represents both pH and pCN. The relationship between pCN and pH can be calculated according to formula 5, combined with the concentration of cyanide during heap leaching, A in formula 5 is 1 × 10 - 2 mol / L, the value of pH corresponding to pCN can be calculated by the following table. Show:
table
pH | 0 | 2 | 4 | 6 | 8 | 9.4 |
pCN | 11.4 | 9.4 | 7.4 | 5.4 | 3.4 | 2.3 |
From Figure 4, the following conclusions can be drawn:
First, the line of Figure 11 shows that when the gold ore is cyanide heap leaching, the gold leaching potential is related to the pH value. At first, the potential decreases as the pH increases; when the pH reaches 9.4, the potential is lowest; thereafter, the pH is increased and the potential remains substantially unchanged. It can be seen from Equation 5 that the pH value of the leaching solution is related to the amount of sodium cyanide added, and increases with the increase of the cyanide concentration. Therefore, the potential first decreases with the increase of the cyanide concentration, but after a certain amount, the cyanide is increased. The concentration of the compound remains substantially unchanged. It should be noted that cyanide is easily decomposed at low pH. In order to avoid the unnecessary consumption of cyanide, in the production practice, the protective base is generally used, and after the pH of the leaching solution reaches 9, the cyanide is added.
Second, the 7 line in Figure 1 is above the 1 line, indicating that oxygen is a good oxidant for cyanide leaching, and its presence or absence is one of the key factors for cyanide leaching. Modern research has shown that dissolved oxygen in water generates H 2 O 2 during the cyanidation gold leaching process, as shown by the 9-line in the figure. The line is also above the 1 line, indicating that H 2 O 2 also contributes to gold leaching. However, in practice the production of H 2 O 2 is rarely used as an oxidizing agent, because the H 2 O 2 is a strong oxidant (as shown in line ⑧ shown), may result CN - is oxidized into CNO -, thereby increasing the consumption of cyanide, In addition, the use of air (inflated) as an oxidant is inexpensive.
Third, 10 in Figure 1, with The line, as well as the dotted lines to the right of the line parallel to the (11) line, indicate that zinc can reduce Au(CN) 2 - into elemental gold in any pH range, and is itself oxidized to various conditions of various ions. The oxidation product may be a soluble or insoluble matter such as Zn 2 + , Zn(CN) 4 2 - , Zn(OH) 2 , HZnO 2 - and ZnO 2 2 - depending on the concentration of cyanide in the solution and the pH. Some heap leaching projects using zinc powder replacement process must pay great attention to the fact that once insoluble Zn(OH) 2 is formed , it will seriously hinder the smooth progress of production, reduce the recovery rate of gold, and increase the consumption of zinc. When the solution is insufficiently basic, the soluble ZnO 2 2 - is hydrolyzed to form insoluble Zn(OH) 2 , and the reaction is
ZnO 2 2 - +2H 2 O ZN(OH) 2 ↓+2OH -
Therefore, it is necessary to maintain sufficient cyanide and alkali amount in the solution during the replacement process.
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