NV Sidgwick published his calculation of R=2.5×10 29 in 1952 and calculated that the oxidation-reduction potential of Au + is E=-0.045V, which is 0.526V higher than the -0.571V in the figure.
Chu Jianhua et al. (1984) conducted a potential detection using Wangshuirong gold preparation solution, and found that the measured value was 0.258~0.570V higher than the theoretical value in the figure, which is very close to the above Siqiwick measurement. In order to carry out the verification, the gold cyanide gold and gold powder direct cyanide gold preparation liquid were respectively used for the determination, and the potential error measured by the different preparation liquids under the same conditions was only 0.01V. This result indicates that the above-mentioned measured value is believable, and accordingly, Au(CN) 2 - +e - =====Au+2CN - is proposed, and the correction curve equation of the reaction formula is:
E=1.0638+0.1182 lg(1+l0 pH-9.4 )-0.1182pH
The correction curve is shifted upward by 0.5238V, which is -0.0472V, parallel to the traditional theoretical curve in the figure.
In view of the fact that the reduction potential of Au(CN) 2 - /Au is so high, the electromotive force of the primary battery composed of Au and C lines is small (E max =0.136V), and its driving force for gold cyanidation is not large. In today's cyanide production practice, the increase of [CN - ] total concentration and the use of carbon slurry method (reducing Au + concentration in the immersion liquid) are used to reduce the reduction potential of Au(CN) 2 - and increase the gold cyanide battery. The electromotive force is necessary to increase the cyanide driving force.
When the Ag(CN) 2 - /Ag potential was measured using a solution prepared by dissolving AgNO 3 in NaCN solution, the measured value was close to the theoretical curve in the graph at pH > 9.4, and the measured value was lower than the theoretical curve at pH < 8.4. The latter should be due to the acid effect of Ag(CN) 2 - . It shows that the theoretical potential-pH curve of Ag(CN) 2 - in the figure is correct. Since the Ag (CN) 2 - line reduction potential is low in the figure, the cyanidation driving force of silver is large and it is easy to cyanide.
Regarding the kinetics of gold and silver dissolution in cyanide solution, since the electromotive force of gold cyanide battery measured in the past is very large (E max =0.862V), early researchers have inferred the cyanide kinetics of gold. The process is controlled by diffusion. In 1963, the gold cyanide activation energy measured by KJ Cath-ro et al. was 12.97 (weak stirring intensity, low oxygen concentration) ~59.999 kJ/mol. Obviously the former is controlled by diffusion, while the latter is mainly controlled by the chemical reaction of the interface layer. Therefore, how to improve the chemical reaction rate of the gold cyanide interface layer is an important measure to improve the cyanidation efficiency and strengthen the cyanide operation process.
The gold cyanidation process can be written as the following two reactions:
2Me+4CN - +O 2 +2H 2 O=====2Me(CN) 2 - +4OH - (1)
2Me+H 2 O 2 +4CN-=====2Me(CN) 2 - +2OH - (2)
The above reaction is carried out according to the formula (2) to a lesser extent (15%), and is mainly carried out according to the formula (1) (85%), that is, [next]
2Au+4CN - +O 2 +2H 2 O====2Au(CN) 2 - +4OH -
   None of the above reactions are sufficient to oxidize gold to Au + ions into the solution. However, according to the Nernst equation, the potential of a metal in its solution is related to the ion activity a of this metal:
E=E Ó¨ +(RT/nF)lna(Me n+ )
The potential equation of gold at 25 °C is E=1.73+0.059lga(Au + )
Therefore, the potential of gold decreases as the Au + activity in the solution decreases, which is the basis for the dissolution of gold in the cyanide solution. In summary, the activity of Au + is drastically reduced in the presence of CN - ions, and therefore, Au + ions and CN - ions can form a very strong complex ion Au(CN) 2 - .
The thermodynamic characteristics of the cyanide leaching process can be summarized as:
1 The cyanide solution dissolves gold to form a reduction electrode potential of the complex ion, which is much lower than the reduction electrode potential of the free gold ion, so the cyanide solution is a good solvent and complexing agent for gold.
2 The reaction in which gold is dissolved in a cyanide solution to form a complex ion indicates that the gold complex ion Au(CN) 2 - is stable in an aqueous solution.
The reduction potential of 3 gold free ions is higher than that of silver ions, but the reduction potential of gold complex ions is lower than that of silver complex ions. This shows that the cyanide solution dissolves gold more easily than silver. [next]
4 In the range of pH 9-10, the electrode potential of the gold complex ion decreases with increasing pH. It is indicated that in this range, increasing the pH is advantageous for dissolving gold; but above this range, their electrode potentials are almost constant, and pH has no effect on the dissolved gold. If the pH is less than 9, CN - is easily converted to HCN, which not only causes loss of cyanide, but also pollutes the environment.
5 At the same pH, the electrode potential of the complex ion of gold decreases as the activity of the complex ion decreases. Silver also has the same rules.
â‘¥ cyanidation process, such as a strong oxidizing agent used, will cause the CN - oxidation of CNO -, which will lead to increased consumption of cyanide.
It should be noted that although the dissolution of gold is thermodynamically feasible, there are still some kinetic difficulties.
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