Detailed characterisation of precious metals and critical elements in anode slimes from the Olympic Dam copper refinery, South Australia

Abstract

Together with silver (Ag) and gold (Au), which are almost always recovered, the impurity elements, arsenic (As), antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb), selenium (Se) and tellurium (Te) are major components of anode slimes, a by-product of the electrorefining of impure copper. The Olympic Dam electrorefinery, South Australia, generates raw slimes (~30 % Cu, 5–10 % Ag, 0.5–1 % Au, 3–4 % Bi, ~2–3 % Sb, and 5–6 % As), which subsequently undergo decopperisation and then pH-neutralisation. Decopperisation involves treatment with steam and acid at 90 ◦C, giving a slime that contains ~ 1 % Cu, 8–17 % Ag, ~2% Au, ~7% Bi, ~4–5 % Sb, and ~ 3 % As. pH-neutralisation is achieved by addition of NaOH and is the final step prior to cyanidation and precious metal recovery. Bulk chemical compositions, gross morphology and particle size distributions are complemented by highmagnification backscatter imaging and energy-dispersive X-ray analysis of individual particles in raw, decopperised, and pH-neutralised anode slimes. In a first micron-to-nanoscale approach, we use electron backscatter diffraction mapping to assess the crystallinity of individual component phases and a scanning transmission electron microscopy study on micron-sized slices extracted in-situ to confirm the crystal structure of a conspicuous BiAsO4 phase as rooseveltite. The results establish an in-depth understanding of element deportments in slimes and their evolution during sequential stages of treatment. Characterisation studies for Te, Se, Sb, Bi, and As are essential prerequisites for any design of metallurgical circuits for by-product recovery from anode slimes and/or electrolyte. This study aims to demonstrate that complementary cutting-edge microanalytical techniques widely used in mineralogy provide a level of detail essential when considering refinery slimes as a new source of critical commodities.