Harmless treatment and resource utilization of stainless steel pickling sludge via direct reduction and magnetic separation

The process of neutralization of stainless steel pickling wastewater will produce a lot of pickling sludge, and improper treatment of this sludge poses a serious threat to the environment. This paper presents a new process for disposing stainless steel pickling sludge in an environmentally friendly manner while effectively reusing the elements contained in the sludge. This process involved direct reduction with magnetic separation. First, the harmful components of sludge were turned into harmless components by converting Fe3+, Cr3+, and Ni2+ to Fe, Cr, and Ni metals by direct reduction. The temperature of reduction was 1473 K, which was lower than the existing process temperature (1673 K). Then, the reduction product was subjected to magnetic separation, whose efficiency is high, to separate metals and nonmetals. Consequently, the metal alloy powder (Ni + Cr + Fe) and nonmetallic phase, which were harmless, were collected. The nonmetallic phase was mainly composed of CaF2. The recovery rates of Fe, Cr, and Ni were 95.3%, 88.7%, and 97.53%, respectively, and their grades were 69.31%, 7.60%, and 16.71%, respectively. Alloy powder can be used as raw material for stainless steel smelting. (C) 2019 Published by Elsevier Ltd.