Gold extraction equilibrium in the system Cyanex 921 HCl-Au(III)

The partition of gold from hydrochloric acid solutions by the phosphine oxide Cyanex 921 has been studied. The extraction system is exothermic (Delta H degrees = -44.6 kJ/mol), the extraction being influenced by several variables. Maximum gold extraction is obtained in the HCl concentration range 5-7 M. Experimental data were analyzed numerically using the program LETAGROP-DISTR. The results showed that the extraction can be explained assuming the formation of the species HAuCl4L (log K degrees = 1.99) and HAuCl4L2 (log K degrees = 6.68) in the organic phase, where L represents the organic extractant. From the experimental data the interaction coefficients for (AuCl4-, H+) were estimated, Gold stripping is achieved effectively with water and the reaction is endothermic (Delta H degrees = 16.9 kJ/mol). Results on the extraction of HCl by Cyanex 921 are also presented.