Characterization and pH-dependent leaching behaviour of historical and modern copper slags

• Historical and modern Cu-slags display different chemistry and mineralogy.
• Cu-slags demonstrate various behaviour under exposure to pH-dependent leaching.
• Acidic conditions (pH = 2) mobilize significant quantities of metals.
• Natural slags pH and slightly alkaline (10.5) are suitable for slags disposal.
• Leaching behaviour depends on slag phase composition.

A geochemical approach combining chemical and mineralogical studies together with pH-dependent leaching experiments was applied to carry out an environmental risk assessment analysis of copper metallurgical slags. Two groups of Cu-metallurgical slags were investigated: historical (HS) and modern slags including shaft furnace slag (SFS), granulated slag (GS) and lead slag (LS) representing various smelting technologies. The metallic elements composition (Cu, Zn and Pb) of the slags, especially their distribution in mineral phases as well as their leachability at different pH conditions were determined. The slags display different chemical (e.g., Cu; HS: 5657 mg/kg; SFS: 2687 mg/kg; GS: 11,425 mg/kg, LS: 10,915 mg/kg) and phase composition, with various impurities of metallic elements in different quantities. Consequently, these materials show different leachability of metallic compounds when exposed to various pH conditions (2-13). HS and SFS slags are mainly composed of synthetic analogues of silicates such as fayalite (HS) and diopside (SFS) as well as minor sulphides. GS is an amorphous material composed of glass containing copper droplets, whereas LS is an exceptional slag type corresponding to a matte and characterized by a volumetrically major sulphides and a minor fayalite content. pH-dependent leaching experiments revealed that high concentrations of Cu, Zn and Pb may be released during leaching. Aggressive acidic conditions (pH 2) were found to be the least stable for the analysed materials, e.g., Cu: up to 5873 mg/kg for GS; Zn: up to 8919 mg/kg for LS, Pb: up to 17,186 mg/kg for LS. Alkaline pH (12-13), being generally less aggressive for slags than acidic conditions, also showed that significant amounts of metals may be mobilized (e.g., Cu: up to 114 mg/kg for HS; Zn: up to 243 mg/kg for LS; Pb: up to 5539 mg/kg for LS). pH conditions close to neutral (7-8) and those with slightly alkaline pH (10.5) appear to be the most stable for the analysed materials.