Solvent extraction reagents for gold recovery from alkaline cyanide solutions

Three different solvent extraction systems for the recovery of gold from alkaline cyanide solutions are reviewed. These include the modified amine extractants in which the basicity of simple alkyl amines, with respect to the aurocyanide anion, is controlled by the addition of alkyl phosphorous esters. The selectivity of such modified amines for gold extraction from alkaline cyanide solutions is excellent Next, solvating extractants, undiluted alkyl phosphorous esters and phosphine oxides, are discussed. The significance of ion pair extraction by a solvation mechanism is evident from data reported in the literature for these extractants. The industrial application of these solvating extractants is demonstrated for the treatment of waste electroplating solutions and for the selective extraction of gold from mercury in a carbon eluate strip solution. Finally, research on the use of substituted guanidines for gold extraction from alkaline cyanide solutions is reviewed. These guanidines have a greater basicity than amines, and extraction can be accomplished at higher pH values without the use of modifiers. In addition to the di- through pentasubstituted guanidines discussed by Henkel researchers, recent research at the University of Utah shows that monosubstituted guanidines are quite effective extractants as well. The substituted guanidines is the most recent gold solvent extraction system studied, and the experimental results indicate that excellent extractions can be achieved at pH 11 and stripping above pH 13 provided that an appropriate alcohol is used in the diluent (carrier). The suitability of the substituted guanidine solvent extraction system for treating dilute leach solutions has been demonstrated in pilot-plant testing.