Impact of Cooling Rate on the Mineralogy and Leaching Behaviour of Copper Slag

Avoiding and minimizing the leaching of heavy elements from non-ferrous metal slags is a precondition for their effective valorisation, where minimum requirements are regulated by standards and legislation of different local regions. Whilst most laboratory-scale studies on slag leaching have focused on factors such as slag bulk composition and solution pH, limited research has been dedicated to systematically investigating the influence of slag mineralogy and the underlying mechanisms driving leaching behaviour. This study examines the impact of cooling rate on the mineralogical characteristics of copper slag that has been carefully sampled from an industrial slag cleaning process and explores how these characteristics affect the leachability of various elements. Bulk leaching experiments were conducted on slag samples cooled at different rates, followed by detailed analysis of their microstructure, phase composition and elemental distribution. The deportment of key elements, including Cu, Pb, Zn, Mo, As and Sb, was quantitatively determined using advanced microanalytical techniques such as electron microprobe X-ray microanalysis and laser ablation ICP-MS. Additionally, an in situ micro-leaching experiment was developed to assess the stability of individual phases within the slag samples. The results reveal that slow-cooled slag exhibits higher leachability for several elements, including Na, K, Ca, Mo and Sb, when compared to quenched slag. This increased leachability is attributed to the formation of new phases in the slow-cooled slag, which are relatively unstable and more prone to leaching under the experimental conditions.