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The influence of roasting temperature upon gold recovery from a refractory gold ore Carter, Ralph

Abstract

The results of this investigation indicate that the gold in the cyanide tailings of refractory gold calcines is occluded in fused or recrystallized iron oxide particles formed during roasting. These high temperature particles cannot be prevented by controlling the roasting atmosphere or by maintaining a low furnace temperature. The above conclusions have been reached following a microscopic examination of calcines produced at various furnace temperatures with different atmospheres, and a comparison with calcines produced by decomposing samples of completely sulphated concentrate under such conditions that the maximum particle temperature varied between 650 C and 1100 C. This comparison showed that the particle temperature during a normal roast is several hundred degrees higher than the furnace temperature. An approximate relationship between the two is established for roasting the "refractory" arsenical gold concentrate examined during this investigation. The unavoidable high particle temperatures occurring in furnace roasts are not upheld by theory since calculated radiation and convection losses exceed the heat generated. Rate of reaction and rate of oxygen diffusion calculations, however, do explain the failure of controlled atmospheres to give low particle temperature roasts at moderate furnace temperatures where the reaction is not diffusion controlled. Because high particle temperatures could not be prevented during any furnace roast, and because low particle temperatures are essential for complete gold extraction by cyanidation, aqueous medium oxidation under high oxygen pressures was studied. Gold recoveries from calcines produced by this means was over 98% compared with 80% from furnace calcines. It was shown that a medium with a high hydroxyl concentration gives adequate and rapid oxidation of the sulphides by increasing the chemical driving force by the oxygen-hydroxyl half cell potential.

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