The optimum experimental conditions resulted in a solution with free cyanide concentration of 150 ppm at 60 掳C and a reaction time of 90 min, base metal concentration of 175 ppm and precious metal concentration of 880 ppm. Under these conditions, cementation yield obtained was 91; 54; 19% for Au(I), Pd(II) and Pt(II) respectively. Zn addition was 1.5 times the stoichiometric amount for complete cementation for the low level case, while it was 0.68 times less the stoichiometric for the high level, in order to monitor competitive aspects of cementation from a mixed solution on a time scale and at conditions that might be relevant to industrial operations.
In contrast to Au cementation reaction, Pd and Pt cementation did not go to completion even though excess zinc powder (0.15 g Zn i.e. 1.5 to 3.4 stoichiometric ratio) was used with their mono-ionic solutions, which gave 48 and 36% of Pd(II) and Pt(II) respectively. In preliminary kinetic studies, deviations from the first-order rate law were observed. The rate of cementation of Au(I) was about 5 times higher than for Pd(II) and 15 times higher than for Pt(II), based on the apparent rate constant, k鈥?/sup>1. It was also found that the ratio k鈥?/sup>1/k鈥?/sup>2 was smallest for Pt, in other words the deviation from 1st order linear kinetics was greatest.