Kinetics of the reaction of gold cyanidation in the presence of a thallium(I) salt

The heterogeneous kinetics of gold cyanidation in the presence of a thallium(I) salt, which is known to act as accelerator of the reaction, are described in this paper. The rate of gold dissolution in the conventional gold cyanidation decreases dramatically at pH values higher than 11.5. The presence of thallium increases this maximum value of pH to over 13.5. The kinetic orders found for each of the reagents taking part in the reaction are: 0.92 for cyanide; 0.95 for oxygen and 0.1 for thallium(I). These results, as well as the calculated activation energy (9.2 kJ mol(-1)) and the effect of pH and stirring speed on the reaction rate, have disclosed a possible mechanism by which the electrochemical reaction may take place on the gold surface. According to this mechanism, the gold surface may be divided into two areas, one anodic and another cathodic, where the gold oxidation and the thallium(I) reduction take place, respectively. The cathodic area can also be subdivided into anodic and cathodic subareas, where the oxidation of the thallium deposited on the gold surface and the oxygen reduction take place. In contrast to conventional gold cyanidation, when cyanidation takes place in the presence of a thallium(I) salt, the oxygen reduction yields hydroxide ions as a reduction product, and the rate of gold dissolution is four times faster. The equation of the reaction rate that describes the kinetics of the overall process is also presented.