Characterization of a precipitate sludge from a sulfuric acid plant

• Sulfur sludge is a waste by-product formed during copper and nickel smelting. • Multi-method characterisation of a sample from the Harjavalta Smelter is reported. • It is exceptionally enriched in Pb, Se, Hg, Cd and As. • The material is dominated by finely intergrown PbSe, SePbHg, Pb and Se phases. • Physical separation was unsuccessful; hydrometallurgy may also material processing. Materials characterization is essential for both waste management, but also as the first stage in determining the potential for waste reprocessing as part of the circular economy. This paper describes in detail the multi-method characterisation of a filter press sulfur sludge sample from Boliden’s Harjavalta Smelter in Finland. This material represents the filter press cake precipitate after it has been clarified and filtered from the sulfuric acid plant. The sample was characterized geochemically and mineralogically, as well as for Acid Mine Drainage (AMD) potential. Magnetic and gravity separation process tests were also conducted to further investigate processing options for extracting any valuable metals. The study showed the sludge is chemically highly complex and mineralogically/materially challenging, mainly because of its extreme composition. In conclusion, it is suggested that a hydrometallurgical process path to neutralize this sample is the best way forward, which will be developed in future work.