Abstract:

Sherritt developed the Dilute Pressure Oxidation (Dilute POX) process for the recovery of base and/or precious metals from high impurity or refractory sulphide concentrates. In this process, the autoclave is operated at a lower slurry solids content than what is required for autothermal operation to control the acidity of the pressure leach solution, with heat recovery used to maintain the autoclave temperature.

This study presents the results from treating high silver refractory gold feeds. Operating under Dilute POX conditions suppressed the formation of argentojarosite, which increased the silver extractions in direct cyanidation of the POX residue to over 90%, from 0 to 30% under autothermal conditions, while maintaining high gold extractions in cyanidation (>98%) and high sulphide oxidation (>97%) in POX. The relationships between free acid in solution and silver extraction, and between magnesium in solution and effective free acid, were consistent with previous Dilute POX studies with arsenical copper concentrates.

With lower sulphate in the solids under these conditions, formation of basic ferric sulphate was reduced or eliminated, iron and arsenic in solution were reduced and the majority of the sulphide in the feed was converted to free acid. Based on these results, operating under Dilute POX conditions may eliminate the need for hot curing in POX flowsheets for refractory sulphide feeds, and/or allow for treating feeds with higher carbonate contents.

Keywords: Refractory, sulphides, gold, silver, pressure oxidation, cyanide amenability tests, magnesium sulphate, basic ferric sulphate, hot curing, Dilute POX

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Scott Pavelich

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