Gold leaching in non-cyanide lixiviant systems: critical issues on fundamentals and applications

Thermodynamics and speciation of the Au(0/I/III) system are reviewed on the basis of published data on disproportionation of Au(I) and solubility of gold metal and Au(I/III) salts. Kinetics of gold oxidation from rotating gold discs, gold colloid, gold-silver alloy and gold ores reported in the literature are reviewed to compare and contrast different lixiviant systems. Linear correlations of stability constants of Au(I) complexes with respect to relevant complexes of Ag(l) and Cu(I) show the stability order of Au(I) complexes with different ligands: CN- > HS- > S2O32- > SC(NH2)(2) > OH- > I- > SCN- > SO2- > NH3 > Br- > Cl- > CH3CN. In the case of Au(III) the> stability order of complexes is CN- > OH- > SCN- > Br- Cl-. The Eh-pH and Eh-log[Cl-] diagrams of Au(0/I/III)III)-OH--NH3 and Fe(II/III)-Cu(0/I/II)-Au(0/I/III)-Cl- systems are revised to incorporate complex species such as AuOH0, Au(OH)(2)(-) and AuCl2-. Solubility of gold salts follows the order NaAuCl4 > KAuCl4 > AuCl3 > Au(NH3)(4)(NO3)(2) > Au2S/H-2 S > Au(OH)(3)/NaOH > Au(OH)(3). Solubility of gold metal in the presence of oxidants decreases in the order MgS/H2S much greater than Fe(III)/Cl- > Cu(II)/Cl- > Cu(II)/NH3 > NaOH/H2O at a given temperature, but increases with increasing temperature. Results from rotating disc studies in different lixiviant systems show that the rate of gold oxidation depends on the temperature and concentration of oxidant and ligand. Silver dissolves faster than gold and thus Ag(l) catalyses the gold dissolution by redox-displacement. The surface chemical reaction mechanism for gold dissolution is rationalized on the basis of electrochemical and adsorption theory. Rate of gold leaching in different lixiviant systems is represented by a shrinking sphere/core model with an apparent rate constant of 10(-5) s(-1). (C) 2004 Elsevier Ltd. All rights reserved.