Avoiding and minimizing the leachingLeaching of heavy elementsElements from non-ferrousFerrous metalMetal slagsSlag is a precondition for their effective valorisationValorisation, where minimum requirements are regulated by standards and legislation of different local regions. Whilst most laboratory-scale studies on slagSlag leachingLeaching have focused on factors such as slagSlag bulk composition and solution pH, limited research has been dedicated to systematically investigating the influence of slagSlag mineralogyMineralogy and the underlying mechanisms driving leachingLeaching behaviour. This study examines the impact of coolingCooling rate on the mineralogical characteristics of copper slagCopper slag that has been carefully sampled from an industrial slag cleaningSlag cleaning process and explores how these characteristics affect the leachability of various elementsElements. Bulk leachingLeaching experiments were conducted on slagSlag samples cooled at different rates, followed by detailed analysisAnalysis of their microstructureMicrostructure, phase composition and elemental distributionDistribution. The deportmentDeportment of key elementsElements, 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-leachingLeaching experiment was developed to assess the stability of individual phases within the slagSlag samples. The results reveal that slow-cooled slagSlag exhibits higher leachability for several elementsElements, including Na, K, Ca, Mo and Sb, when compared to quenched slagSlag. This increased leachability is attributed to the formationFormation of new phases in the slow-cooled slagSlag, which are relatively unstable and more prone to leachingLeaching under the experimental conditions.