Characterisation and extraction of copper and cobalt from smelting slag

Tina Phiri

Smelting slag is a significant potential resource for cobalt and copper. The recovery of cobalt and copper from smelting slag could substantially augment the supply of these metals which are crucial for facilitating the transition towards green energy while also solving environmental issues associated with slag waste disposal. However, the complex mineralogical and chemical composition of smelting copper slag poses a significant processing challenge. Current approaches for processing of smelting slag involve high temperatures ranging from 1200 °C to 1500 °C. This study aimed to assess the global distribution of smelting slag waste, investigate the mineralogical and chemical characteristics, perform thermochemical simulations using the HSC Chemistry® software, and develop efficient technologies for cobalt and copper extraction from smelting slag. Different instruments and techniques were utilised for sample characterisation, including XRD, XRF, ICP-OES, FTIR, and FESEM-EDS. In the pyrometallurgical reduction experiments, the effects of charcoal, borax, temperature, and time were studied. Finally, metal dissolution was investigated using hydrometallurgical techniques involving acidic and alkaline leaching systems. The XRD analysis revealed that the primary phases found in the smelting slag feed sample were Fe2SiO4 and Fe3O4. Chemical analysis of the feed using ICP-OES and XRF indicated that the smelting slag contains significant amounts of valuable metals such as Cu, Co, Fe, Ti, Ca, Na, Mg, Al, K, and Mn. The FESEM-EDS study verified the existence of a silicate-magnetite matrix and yielded valuable data on the metal embedment in this complex matrix. A novel carbocatalytic reduction technique was developed, which utilised charcoal as a reducing agent and Na2B4O7 as a network modifier of the smelting slag at temperatures ranging from 800 to 1000 °C. The results demonstrated that the carbocatalytic reduction process has efficacy in expediting the morphological transformation of the complex smelting slag microstructure to amorphous and metallic phases at a low temperature of 850 °C. Leaching experiments of the carbocatalytic reduction product in H2SO4 resulted in high extractions of 85.57% Cu, 90.73% Co, 91.85% Fe. An alternative alkaline glycine leaching of the feed sample showed excellent selectivity for Cu, yielding 90.28% extraction at 40 °C. Therefore, the development of this novel technique will contribute to the efficient extraction of Co and Cu from smelting slag, all while significantly minimising the environmental impacts of slag waste.

Characterisation and extraction of copper and cobalt from smelting slag

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