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The reaction of silver with cyanide solutions under oxygen pressure Deitz, George Albert

Abstract

An investigation of the kinetics of dissolution of silver in NaCN solutions in the presence of gaseous oxygen under pressure has been carried out with a view to establishing the reactions involved and the factors influencing the rate. The reaction studies were made using a pressure vessel and the reaction was followed by periodic sampling and analysis of the solution. The variables investigated included cyanide concentration, oxygen pressure, temperature and the presence of metal hydroxides in the solution. Linear, zero order reaction rate curves were generally obtained. The rate of dissolution of silver was found to increase linearly with increasing cyanide concentration and level off at a value which was proportional to the oxygen pressure. At constant cyanide concentration, a similar dependence of the rate on oxygen pressure was observed. The apparent activation energy of the reaction is 2400 cal./gm-mole. The results indicate that the rate of reaction is determined principally by the diffusion of the reactants in solution to the silver surface. At low cyanide concentrations, when oxygen is present in excess, the rate is controlled by the diffusion of cyanide ions. Conversely, when cyanide is in excess, the diffusion of oxygen is rate controlling. The principle reaction appears to be: 2Ag + 4NaCN + O₂ + 2H₂O → 2NaAg(CN)₂ + 2NaOH + H₂O₂ which is in agreement with the findings of previous investigators. The reaction rate was found to decrease with increasing pH on addition of metal hydroxide salts to the solution. In the case of KOH, the rate decrease is believed to be due to the lowered solubility of oxygen and a lowered potential for its reduction. The effect of Ca(OH)₂ and Ba(OH)₂ in lowering the rate is more marked. This appears to be due to the formation of an insoluble peroxide coating on the silver surface, which was identified by x-ray diffraction measurements. The thermodynamic properties of silver in dilute aqueous solutions, the suggested possible dissolution reactions and the effect of the cyanide ion concentration on the thermodynamic properties of these reactions are presented and discussed in the form of potential-pH diagrams, as developed by Pourbaix.

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