The reason why cyanide selectively dissolves gold , silver and some of their complexes is due to the aeration of alkali metal (or alkaline earth metal) cyanide. In practice, an aerated solution of sodium cyanide (or calcium cyanide) is usually used, along with a few bases (usually lime). The addition of the base is to inhibit the hydrolysis of cyanide to avoid the loss of vanatilization.
If the dissolution reaction is considered to be an electrochemical erosion process, the dissolution of gold can also be considered as the process of dissolving gold into the solution on the anode surface (Fig. 1). When oxygen on the surface of the cathode receives electrons, the reaction between the anode and cathode sections is:
Anode area Au+2CN - 
Au(CN) 2 - +e
Cathode area O 2 +2H 2 O+2e - H 2 O 2 +2OH -
Further reaction H 2 O 2 +2e 2OH -
In electrochemical corrosion systems, the most important factor affecting the polarization of anodes and anodes is concentration polarization, which is determined by Fick's law:
A 1 [(O 2 )-(O 2 )i] (1)
A 2 〔(CN - )-(CN - )i) (2)
In the middle with - the diffusion rate of CN - and O 2 , respectively, mol (molecular) / s;
D CN and - the diffusion coefficient of cyanide and dissolved oxygen, respectively, cm 2 /s;
(CN - ) and (O 2 )- are the concentration of CN - and O 2 in the whole solution, respectively, mol (molecular) ∕ mL;
(CN - )i and (O 2 )i- are the concentration of CN - and O 2 at the interface, respectively, mol (molecular) ∕ mL;
A 1 and A 2 - are the surface area of ​​the reaction between the cathode and the anode, respectively, cm 2 ;
δ-Nernst interface layer thickness, cm.
Figure 1 Diagram of the dissolution of gold in cyanide solution
In metal interface CN - and through the stagnant layer O 2 as compared to the speed, assuming the chemical reaction at the interface quickly, then, as soon as they reach the metal surface will be consumed immediately, that is to say:
(CN - )i=0 (O 2 )i=0
Therefore, equations (1) and (2) can be simplified to:
A 1 [O 2 ]
A 2 [CN - 〕
Because the metal dissolution rate is twice the rate of oxygen consumption and is one-half the rate of cyanide consumption, it is:
Gold dissolution rate = 2 A 1 [O 2 ]
Or the dissolution rate of gold = A 2 [CN - 〕
When the above reaction formula reaches equilibrium,
2 A 1 〔O 2 〕= A 2 [CN - 〕
However, because the total surface area of ​​the metal in contact with the water phase is A = A 1 + A 2 , thus:
Gold dissolution rate = (3)
It can be seen from this equation that when the cyanide concentration is low, the first term is negligible compared to the second term of the denominator. Therefore, equation (3) can be simplified as:
Gold dissolution rate = (CN - )=k 1 (CN - )
The value calculated by this formula is in agreement with the experimental results of Fig. 2, that is, when the cyanide concentration is low, the dissolution rate depends only on the cyanide concentration.
Fig. 2 Effect of different pressures and different NaCN concentrations on the dissolution rate of silver at 24 °C
Similarly, when the cyanide concentration is high, the second term is negligible compared to the first term of the denominator, then equation (3) can be simplified as:
Gold dissolution rate = 2 [O 2 ]=k 2 [O 2 ]
The value calculated by this formula also coincides with the experimental results of Fig. 2, that is, when the cyanide concentration is high, the dissolution rate depends only on the oxygen concentration.
when [CN - 〕=4 [O 2 ]
which is
At this point, the dissolution rate reaches the limit. The average value of the diffusion coefficient found in Table 1 is:
=2.76×10 -5 cm 2 ∕s
=1.83×10 -5 cm 2 ∕s
The average ratio of the two diffusion coefficients:
≈1.5
That is, the molar average ratio of the ultimate dissolution rate of the two is:
4 =4 ≈6
Table 1 Diffusion coefficient and average
Temperature ∕ °C | KCN∕% | D CN ∕cm 2 ·s -1 | D O2 ∕cm 2 ·s -1 |
|
18 | - | 1.72×10 -5 | 2.54×10 -5 | 1.48 |
25 | 0.03 | 2.01×10 -5 | 3.54×10 -5 | 1.76 |
27 | 0.0175 | 1.75×10 -5 | 2.20×10 -5 | 1.26 |
average value | - | 1.83×10 -5 | 2.76×10 -5 | 1.50 |
At this time, the dissolution rate of gold reaches the limit dissolution rate. It is in good agreement with the experimental values ​​shown in Table 2, 4.6 to 6.8.
Table 2 Ratio of cyanide and oxygen concentration to limiting dissolution rate
metal | Temperature ∕ °C | P O2 ∕Pa | (O 2 )∕mol·L -1 | (CN - )∕mol·L -1 |
|
gold | 25 | 101325 | 1.28×10 -3 | 6.0×10 -3 | 4.69 |
25 | 21278 | 0.27×10 -3 | 1.3×10 -3 | 4.85 | |
25 | 101325 | 1.28×10 -3 | 8.8×10 -3 | 6.88 | |
silver | twenty four | 757911 | 9.55×10 -3 | 56.0×10 -3 | 5.85 |
twenty four | 344505 | 4.35×10 -3 | 25.0×10 -3 | 5.75 |
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