Evaluation of the Merrill–Crowe process for the simultaneous removal of platinum, palladium and gold from cyanide leach solutions

Abstract

Studies were conducted to evaluate the Merrill–Crowe cementation method for the simultaneous extraction of platinum, palladium and gold associated with copper and nickel from cyanide solutions, particularly high temperature eluates from carbon adsorption and desorption process. The paper reviews the Merrill–Crowe method and its potential for platinum and palladium recovery in the presence of gold, copper and nickel. In a mechanically stirred tank reactor and in the presence of pure nitrogen gas, statistically designed experiments were performed at two levels of the five operating parameters which were the free cyanide concentration, temperature, cementation time, base and precious metal concentrations and their studied ranges were 50–150ppm, 60–90°C, 15–90min, 175–1350ppm and 220–880ppm, respectively.The optimum experimental conditions resulted in a solution with free cyanide concentration of 150ppm at 60°C and a reaction time of 90min, base metal concentration of 175ppm and precious metal concentration of 880ppm. 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 (0.15g 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′1. It was also found that the ratio k′1/k′2 was smallest for Pt, in other words the deviation from 1st order linear kinetics was greatest.