| Summary: | The INCO Ltd. Copper Refinery Electrowinning Department (CRED) in Copper Cliff, Ontario,
processes a Cu, Ni, Co, Fe, As, S and precious metals containing residue produced by the INCO
Pressure Carbonyl (IPC) plant. This residue is treated batchwise in two successive leaching
stages. The non-oxidative First-Stage Leach (FSL) process extracts the vast majority of the Ni,
Co, Fe and As values. The Second-Stage Leach (SSL) process comprises the aqueous pressure
oxidation of a predominantly Cu₂S feed under acid-deficient conditions. The basic copper
sulfate (BCS) precipitated in this stage is subsequently redissolved with spent electrolyte to
separate the copper values from the final residue rich in precious and platinum group metals.
The present study is part of a joint INCO-UBC research effort aiming at the optimization of both
leaching stages. The purpose of this project is to provide a better understanding of the first-stage
chemistry in general and of the behavior of arsenic in particular, since elevated concentrations of
this element are detrimental to the efficiency of the second-stage system.
The test work shows that the FSL process is insensitive to physicomechanical parameters such as
agitation rate and pulp density, as long as the "solids off-bottom condition" is satisfied. In
contrast, the effect of temperature and reactor residence time on impurity extraction is
pronounced, especially in the case of arsenic. On the basis of thermodynamics and experimental
observations made in the course of the investigations, a tentative reaction model is proposed to
explain reprecipitation of this element (and nickel) under copper-depleted first-stage conditions.
The presence of organic electrowinning additives in solution does not influence the Ni, Co, Fe
and As recoveries achieved in the first stage. The iron and, to a lesser extent, nickel extraction
depend on the availability of acid in solution, whereas cobalt is leached principally by metathesis
reactions. Two scenarios for the leaching of arsenic are postulated, yet neither one can be
confirmed; the concentration of arsenic is either below the detection limit of the equipment
(XRD) or is obscured by the presence of other elements (SEM).
Arsenic redissolution from a copper-depleted FSL batch cannot be accomplished by the
introduction of pregnant copper electrolyte to the first-stage flash tank under the conditions of the
laboratory simulation. In order to mitigate arsenic related second-stage leaching difficulties it is
recommended that a potential probe be installed between the pressure let-down vessel and the
filter feed tank. The on-line potential readings would allow timely changes in the make-up to
avoid copper depletion of subsequent batches. Simplified calculations have shown that the use of compressed air instead of "push steam" for
discharging of first-stage reactors leads to an unacceptable increase in the copper level in
solution.
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