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Thermodynamic and kinetic limitations of gold leaching in ferric chloride media Ljubetic, Kresimir Eduardo

Abstract

The global gold industry is under increasing pressure to search for alternative technologies to replace cyanidation due to the inefficiency of cyanide in treating low grade refractory gold ores and increased public scrutiny on the use of cyanide in gold mining. Chloride has been identified as a promising candidate to replace cyanide. The objective of this research was to gain fundamental knowledge on the thermodynamic and kinetic limitations of gold dissolution in ferric chloride media at moderate temperatures. First, thermodynamic calculations were done to determine gold and iron speciation in chloride solution at varying initial ferric concentration, total chloride concentration, solution potential, and type of chloride salts. Then the effects of these variables on the kinetics of gold dissolution in ferric chloride media were studied by batch leaching and electrochemical tests using pure gold as the model mineral. Finally, the efficacy of ferric chloride media for gold leaching from real ores was tested by batch leaching of an oxide ore sample. The speciation calculation showed that gold dissolves as Au(I) species in ferric chloride solution and that the predominant ferric species is FeCl₂+, which was considered to be the main oxidizer. The batch leaching tests showed that the gold extraction increased with the initial ferric concentration up to 0.3 M; increasing total chloride concentration and the solution potential also had positive effects on gold dissolution; the kinetics of gold dissolution was significantly slower in the presence of the divalent salts. Gold extraction was thermodynamically controlled at low free chloride concentrations and low solution potentials. When these two variables were sufficiently high, the dissolution process was under kinetic control. The potentiodynamic tests revealed that the dissolution kinetics was cathodically controlled at ferric concentrations up to 0.3 M with sufficient chloride; while at sufficiently high ferric concentrations, the anodic process controlled the dissolution process, the kinetics of which increased with the total chloride concentrations. Gold was extracted from the oxide ore sample, but the drop in the solution potential led to a low overall extraction. The dissolution kinetics was enhanced by acid curing as pretreatment and increasing leaching temperature.

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