Man Seung Lee

BACKGROUND Sludge resulting from petroleum refining contains metal oxides such as nickel, copper, aluminum, iron, zinc, calcium, and magnesium. Since the demand for nickel is increasing, a process was proposed to recover pure nickel metal powder from this sludge. In this study, leaching, cementation, precipitation, solvent extraction, and chemical reduction operations were carried out and employed for the recovery of nickel powder from sludge. RESULTS Nickel oxide was first completely dissolved using 0.3 mol L −1 H 2 SO 4 solution at a pulp density of 50 g L −1 . The leaching solution contained Al(III), Ca(II), Cu(II), Fe(III), Mg(II), Ni(II), and Zn(II). Most of the copper, magnesium, nickel, and zinc were completely dissolved at this step. Copper(II) was removed by cementation with iron powder. After oxidizing Fe(II) to Fe(III) using H 2 O 2 , Fe(III) and Al(III) were simultaneously separated by precipitation of their hydroxides at a solution pH of 4. Five stages of cross‐current extraction of Zn(II) with saponified Cyanex 272 led to a raffinate containing only Ni(II), Ca(II), and Mg(II). Nickel metal powders were recovered from this raffinate by chemical reduction with hydrazine. CONCLUSION The mass balance of the continuous experiments verified that 99% of the nickel was recovered from the sludge as metal powders with extra high purity. In these separation steps, optimum conditions were obtained and a simple and efficient process was proposed to recover pure nickel metal powders from the sludge. © 2023 Society of Chemical Industry.