Evidence of an iron-gold corrosion cell during the Zadra pressure elution of loaded carbon

CIM Bulletin, Vol. 1, No. 1091, 2006
G.A. Vickell, C. Taschereau, and A.R. Laplante
The file is a zipped PDF document.Extremely fine free gold was originally discovered on the surface of carbon fines within the carbon fines settling hopper at Omai Gold Mine Ltd. (OGML). This discovery prompted an intensive metallurgical investigation to determine the genesis of this gold and how to control it so as to prevent fine gold losses to carbon fines and tailings. The initial scope of work involved an extensive carbon handling circuit sampling campaign for the purpose of defining the extent of the fine gold occurrence within the carbon handling circuit. This campaign revealed that very fine free gold was present in the various carbon elution, screening, and reactivation products.Scanning electron microscopy studies demonstrated that the fine gold had a dendritic morphology similar to Merrill-Crowe gold precipitates. This observation gave rise to the thought that the fine gold was most likely the by-product of a cementation process as opposed to some native occurrence. Fugitive fine gold discharge from the electrowinning circuit was tested, but eliminated as a possible source of the fine gold. The initial sampling campaign also demonstrated that the fine gold was present in the eluted carbon before and after the carbon reactivation kiln, thereby ruling out the possibility that the fine gold genesis was the result of a high-temperature reduction process. It also revealed that the fine gold was not entering the carbon handling circuit via loaded carbon transfers from the carbon-in-pulp circuit to the carbon elution vessel.As implausible as it seemed at the time, attention then shifted to the carbon elution stage as a possible source of the fine free gold.At this juncture of the investigation, it was realized that, notwithstanding operating temperature, the Merrill-Crowe and carbon elution processes occur under similar physical and chemical conditions. The authors therefore hypothesized that higher temperatures such as those present during the carbon elution process, combined with the presence of a metal or substance of adequate electrochemical ‘nobility,’ might theoretically precipitate or cement gold from high-grade gold cyanide eluate during the loaded carbon elution cycle. Pursuing this possibility, the investigation subsequently identified significant quantities of grinding circuit fine iron and magnetic mineral grit entrained in the loaded carbon stream ahead of the carbon elution vessel.Laboratory and plant tests were designed and executed to explore the hypothesis that metallics and/or mineral magnetics were mediating the fine gold cementation phenomenon. The tests demonstrated that fine steel retrieved from grinding circuit pulps was capable of cementing fine gold from loaded carbon eluate under identical carbon elution conditions.Several remedial options were subsequently investigated to minimize or eliminate the phenomenon. These options included testing a magnetic separator to remove entrained steel and magnetic mineral grit from the loaded carbon prior to elution, optimizing loaded carbon screen washing effectiveness, barren solution oxidation, and carbon-in-pulp viscosity control. Each of the options were successful to varying degrees but none one of them was capable of halting the phenomenon completely.It has since been established that the gold cementation phenomenon described herein occurs at many other carbon-in-leach and carbon-in-pulp gold plants worldwide. Furthermore, it has since been proven that the phenomenon is also mediated by certain mineral sulphides such as those referred to during the coarse of this investigation as magnetic grit.Personal communication with those plants where the phenomenon is known to occur has established that fine gold production is variable in extent from site to site.In conclusion, the genesis of fine free gold within the carbon handling circuit at OGML was proven to result from a cementation process that is mediated by an iron-gold corrosion couple during the Zadra pressure elution of loaded carbon. The fundamental science and plant operating conditions of the phenomenon were investigated as were potential corrective methods.The authors recommend that those operations with a Zadra pressure elution circuit undertake audits to determine the extent of fine gold in their carbon handling circuit and quantify the extent of fine gold losses to carbon fines and tailings. It is further recommended that applied research into this phenomenon be undertaken for the purpose of characterizing its precise physico-chemical nature and, potentially, identifying effective remediations.
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