F. Habashi (1966, 1967) pointed out that the kinetics of leaching gold from cyanide solution is essentially an electrochemical dissolution process, generally following the following reaction formula:
2Au+4NaCN+O 2 +2H 2 O→2NaAu(CN) 2 +2NaOH+H 2 O 2
A similar reaction formula can be written for the dissolution of silver . This conclusion is based on the following facts:
1 each time 2mol of metal is dissolved, it consumes 1mol (molecular) oxygen;
2 2 mol (molecular) cyanide is consumed per 1 mol of metal dissolved;
3 hydrogen peroxide is formed when gold or silver is dissolved, and 1 mol (molecular) H 2 O 2 is produced per 2 mol of metal dissolved;
4 Experiments have shown that the dissolution of gold and silver in NaCN + H 2 O 2 during anaerobic is a slow process (Table below). Because 2Au+4NaCN+H 2 O 2 →2NaAu(CN) 2 +2NaOH
The reaction is rare. In fact, if a large amount of hydrogen peroxide is present in the solution, the cyanide ion will be oxidized to the oxycyanate ion which does not dissolve the gold due to the following reaction:
CN - +H 2 O 2 →CNO - +H 2 O
It inhibits the dissolution of gold and silver.
Gold and silver dissolution rate | |||
Dissolution mass / mg | Time /min | Remarks | |
Cyanide + O 2 | Cyanide + H 2 O 2 | ||
Gold 10 | 5~10 | 30~90 | 1943 |
Silver 5 | 15 | 180 | 1951 |
Despite the differences in the theory that gold is dissolved in cyanide solution, RWZurilla (1969) et al., by collecting and measuring H 2 O 2 experiments that diffused from the gold surface (no longer participating in the reaction), showed that 85% of the gold was In Bodland's hydrogen peroxide theory, only 15% of the gold is dissolved by Elsner's oxygen theory, ie the reduction of O 2 does not directly produce OH - but always involves the intermediate H 2 O 2 The generation. The generated H 2 O 2 can promote the formation of Au(CN) 2 : [next]
O 2 +2H 2 O+2e - →H 2 O 2 +2OH -
2Au+4CN - +H 2 O 2 →2Au(CN) 2 - +2OH -
This reaction is achieved by O 2 dissolved by the introduction of air into the solution. This conclusion can increase the cyanide dissolution rate of gold by adding a small amount of H 2 O 2 to the solution. If a large amount of H 2 O 2 is added, the surface of the gold particles is passivated and the dissolution rate is lowered to obtain a strong proof.
The dissolution mechanism of gold in cyanide solution is essentially an electrochemical corrosion process. According to the electrochemical corrosion point of view, two adjacent surfaces of the corroded metal, one is the cathode and the other is the anode (the anode is gold; the cathode is another mineral or another region of gold), as shown in Fig. 1. In the figure, A 1 represents the area of ​​the gold particles as the cathode region; A 2 represents the area of ​​the anode region.
Electrochemical corrosion of the electrode reaction is as follows:
Cathodic reaction O 2 +2H 2 O+2e - =====H 2 O 2 +2OH -
Anode reaction 2Au(CN) 2 - +2e - =====2Au+4CN -
When the two equations are subtracted, the total response is: [next]
2Au+4CN - +O 2 +2H 2 O=====2Au(CN) 2 - +H 2 O 2 +2OH -
The interaction between gold and cyanide solution is a typical gas, solid and liquid heterogeneous reaction process. Therefore, its reaction rate should be subject to the general multiphase reaction kinetics. The reaction consists of four steps: the O 2 and CN- dissolved in the solution diffuse through the boundary layer (δ) to the gold surface, the gold surface adsorbs O 2 and CN-, and the gold surface dissolves the electrochemical reaction of gold, and the reaction product Au(CN) 2 - diffuses into the interior of the solution through the boundary layer.
Since the electromotive force of the electrochemical reaction of gold dissolution is large, the reaction speed is fast. Therefore, like most heterogeneous reactions, the dissolution rate of gold is generally controlled by diffusion. Therefore, enhanced diffusion, enhanced agitation and aeration are the main ways to enhance leaching.
Studies have shown that the gold dissolution rate increases with increasing cyanide concentration in the low cyanide concentration range (see Figure 2). When the cyanide concentration increases to a certain limit, the gold dissolution rate is no longer increased. The effect of oxygen concentration in the solution has another characteristic: at low cyanide concentration, the dissolution rate is independent of the oxygen pressure in the upper part of the solution (two lines coincide); at high cyanide concentration, the dissolution rate increases with the increase of oxygen partial pressure. . In other words, the reaction rate at high oxygen concentration depends on the diffusion of cyanide ions through the diffusion layer to the anode region; at high cyanide concentrations, it depends on the diffusion of oxygen through the diffusion layer into the cathode region. At a fixed oxygen pressure, the reaction rate increases as the cyanide concentration increases, and finally approaches a plateau value, which is the ultimate speed at that oxygen pressure. This plateau value is proportional to oxygen.
[next]
In the electrochemical corrosion system of cyanide-dissolved gold, the cyanidation process is a typical diffusion control process. The factor that affects the maximum polarization of the cathode and anode is concentration polarization, which is determined by Fick's law. In the anolyte, the CN-to-gold particle surface diffusion rate is:
Where D(CN-)——the diffusion coefficient of CN-, cm 2 /s;
A1 - surface area where the anode reacts, cm 2 ;
c(CN-)——concentration of CN- outside the diffusion layer, mol/L;
c(CNi - ) - concentration of CN- in the diffusion layer, mol / L;
Δ—the thickness of the diffusion layer, cm.
Since the chemical reaction rate is very fast, c(CNi - ) tends to zero, then:
Where D(O 2 )——02 has a diffusion coefficient of cm2/s;
c(O 2 )——the concentration of O 2 outside the diffusion layer, mol/L;
c(O 2 i)——the concentration of O 2 in the diffusion layer, mol/L;
A2 - surface area at which the cathode reacts, cm2.
Since the chemical reaction rate is very fast, [O 2 ]i tends to zero, then:
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