Nickel hydroxide recovery from stainless steel pickling liquors by selective precipitation

Stainless steel pickling liquor is obtained when a steel surface is treated with hydrofluoric and nitric acid mixtures. Industrial treatments of this dangerous and toxic effluent are focused on acid recovery, while metals are considered as a waste to be managed. Thus, a process taking into account the need to recover valuable metals is under development. This work presents the optimization of the first precipitaiton stage of this new process, with the aim of separating metals (iron and chromium) as fluorides by crystallization, keeping nickel in solution. Experimental conditions that lead to the best yield of reaction were 60-65 °C, pH 4.1-4.2, and free fluoride from 14 to 15% of alkali stream as potassium fluoride. Crystals obtained are mainly hydrated K₂FeF₅ and CrF₃, with a little fraction of other oxides and fluorides phases. The entire content of nickel remaining in solution precipitates as Ni(OH)₂ in another stage.