Kinetics of platinum extraction from spent reforming catalysts in aqua-regia solutions

Platinum content of two commercial spent reforming catalysts were extracted in aqua-regia solutions under atmospheric pressure and at temperatures up to 100 °C. Three factors, including presence of coke, catalyst particle size, and impeller agitation speed were first tested to study the relative importance of mass-transfer resistances during Pt extraction reaction. Catalyst particle sizes < 100 µm and agitation speeds > 700 rpm eliminated the internal and external mass-transfer resistances, respectively. The effect of other factors, including HNO3-to-HCl volume ratio, liquid-to-solid mass ratio, and the reaction temperature on the extraction rate of platinum were then examined. Pt extraction rate was significantly increased by increasing liquid-to-solid mass ratio and the reaction temperature. Kinetic modeling using power-law rate equation for Pt extraction revealed that increasing liquid-to-solid mass ratio increases the acid concentration, as a major reactant. This quantitatively accounted for the increased Pt extraction rate. The effect of temperature on Pt extraction kinetics was studied using the Arrhenius equation. The activation energy for the platinum surface dissolution reaction was calculated as 72.1 kJ/mol. This large value indicates that Pt extraction in aqua-regia solution is controlled by surface chemical reaction. The reaction order was 1.5 for Pt concentration in solid and 1.3 for the hydrogen ion molarity in solution.