Trace element impurities in anode copper from Olympic Dam, South Australia

Abstract

Anode copper is the intermediate product between copper smelting and electrorefining. Anodes are typically 98.5 to 99.8 % pure and require electrorefining to high-purity copper cathode that can be sold to customers. Impurity elements, including Ag, Au, As, Sb, Bi, Se, Te, Sn, Pb, Co, and Ni (alongside minor Fe, Al, and Si), are either dissolved within anode copper, or contained within micron-scale inclusions. To better understand the distribution of these impurities, a study of anode copper was undertaken on representative samples from the Olympic Dam anode plant, South Australia. Bulk assay for 61 elements of 269 sub-samples collected on a grid across a single anode, including the ‘ears’, shows homogeneity irrespective of location. The five most abundant impurity elements are As (mean 1279 ± 41 ppm), Se (218 ± 11 ppm), Sb (211 ± 8 ppm), Bi (202 ± 9 ppm), and Ni (195 ± 6 ppm). The assay data demonstrate that reliable, representative compositional data on the anode can be obtained from sampling and assay of only a limited part of the whole, including the ‘ears’. Trace element analysis of copper metal by laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) allows quantification of dissolved minor components. Some metals, notably Ni, occur almost completely dissolved in copper. Other elements occur only partially in dissolved form, but the proportions measured in copper metal relative to bulk assay vary significantly from only ~ 9 % for Pb, 13 % for Bi and Te, 25 % for Se, 29 % for Sn, 34 % for Sb, 55 % for As, and 60–70 % for Au, Ag, and Co. These results are consistent with the observation of a diverse suite of 1–10 µm-sized, rounded impurity inclusions containing Pb, Bi, As, Sb, Te, Se etc. that are associated with cuprite, Cu2O, and largely follow copper grain boundaries. The inclusions, each containing one or more crystalline (e.g., Cu-selenides, some Cu-Bi-arsenates, Sn-oxides and cuprite) and/or glassy phases (e.g., Cu-Pb-As-Sb-Bi-oxides), represents a trapped melt droplet, leading to the observed broad range of compositions.