Mineralogical Characterization of Anode Slimes: Part 8—“Silica” in Copper Anodes and Anode Slimes

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Copper anodes from the CCR, Inco and Kidd Creek refineries contain ≍ 120 ppm SiO2. The silica is present as a variety of complex silicate inclusions preferentially located within 500 pm of the upper anode surface and as sub-micron silicate inclusions in the bulk of the anode. Less than 10 ppm Si occurs in solid solution in the copper metal matrix. Various silicates are present in the barite/fireclay mold washes used for anode casting, and these silicates partly react with the barite and/or Cu2O during casting to form a number of barium-rich silicate phases. An unexpected Cu–Pb silicate is the product of the reaction of the silicone oil belt dressing used on Hazelett anode casting machines with Cu2O and PbO in the molten copper. During electro refining, some of the Cu-rich silicate inclusions dissolve with the formation of silica gel which seems to be the major silica carrier in the anode slimes. The silica gel commonly impregnates both the copper sulphate and the oxidate matrix phase; its stabilizing effect on copper sulphate likely contributes to anode passivation. Most of the silicate inclusions in the copper anodes, however, are not significantly leached and simply accumulate in the anode slimes. The siliceous filter aids used to collect the anode slimes also seem to be inert in the acidic electrolyte, and some filter aid particles are invariably recovered with the anode slimes. The presence of silicate species complicates the subsequent processing of the anode slimes.