Cobalt-bearing copper slags from Luanshya (Zambian Copperbelt): Mineralogy, geochemistry, and potential recovery of critical metals

Abstract

Waste materials such as metallurgical slags can be considered as potential resources of valuable elements, including technologically critical metals. Copper slags from Luanshya, the oldest mining and smelting site in the Zambian Copperbelt, produced by smelting local Cu ores in reverberatory furnaces between 1932 and 1999, were deposited on two large dumps in the vicinity of the former ore processing area. The slags exhibit high concentrations of Co (247-5990 ppm, median: 2370 ppm) and Cu (1320-95,300 ppm, median: 8550 ppm). This multi method mineralogical investigation indicates that the slags are predominantly composed of olivine, clinopyroxene, silicate glass, and spinel-family oxides. Copper-(Fe) sulfides, cobaltpentlandite [(Co,Fe)(9)S(8)], Fe sulfides, and metallic Cu prills embedded in the silicate matrix are the major hosts of Cu and Co. The EU regulatory leaching test (EN 12457-2) indicated that the release of contaminants is relatively low and only Cu slightly exceeded the EU limit values for landfilling of inert waste. The secondary phases (Cu hydroxosulfates, Fe (oxyhydr)oxides) observed on the slag surfaces confirm that the slags undergo a weathering process on the dumps. Kinetic abiotic extraction tests were carried out in 0.5 mol/l sulfuric acid at 25 °C and with a pulp density of 1% to determine the release of metals from the milled Luanshya slags under conditions simulating hydro metallurgical recovery via agitation leaching. The Cu and Co extractability correlated with the bulk concentration of these elements, and the highest extraction yields after 24 h of leaching corresponded to 46% of the total Cu and 67% of the total Co. Despite the dramatic increase in Co prices on the global market, the Co recovery from the Luanshya slags appears to be non-economical due to the high costs of the necessary mechanical and chemical processing of the slag materials and the obtained extracts.