Metal recovery from spent stainless steel pickling solutions

In stainless steel production a defined surface finish of the products is gained by pickling with aggressive acids. However, pickling lines generate significant amounts of waste products, such as metal oxide particles, metal enriched acid solutions and metal containing neutralization sludge from waste water treatment. Thus, valuable metals, such as chromium and nickel, are irrecoverably lost: Only in Europe, more than 2500 t/a of nickel with a current value of 40 million €/a are deposited, although nickel is regarded as a strategic metal.Additional treatment steps for metal recovery can contribute to a sustainable production. Thus, in the current study the application of membrane electrolysis for nickel recovery from spent pickling solutions was investigated. To proof the industrial application, the investigations were carried out with both, synthetic and industrial solutions. Parameter adjustment included the variation of feed metal concentration, current density, and concentrate conductivity.It could be shown that elemental nickel can be recovered, when iron is removed from the solution. Otherwise, the deposited product consists of metal oxides and hydroxides. Both, current efficiency and specific product deposition rate increased with increasing current density. Deposition rate was not influenced by the concentrate conductivity.The valuable metals recovered by membrane electrolysis can be fed back to internal or external recycling. Thus, the formation of highly hazardous waste is avoided and natural resources can be preserved.

► Spent pickling solutions from the steel industry cause a loss of valuable metals. ► Fe, Cr and Ni can be recovered by membrane electrolysis. ► The electrodeposited product consists of metal oxides and hydroxides. ► Metal deposition of 500 g/m2 h was achieved at a current density of 120 mA/cm2. ► The product can be recycled to the steel production.