Experimental studies on solid‐state reduction of pickling sludge generated in the stainless steel production

Abstract

Neutralized pickling sludge generated during stainless steel production contains large amounts of calcium fluoride, water and a relatively low metal content, which makes it difficult to directly recycle back to the steelmaking process. It would also pose a potential environmental problem if deposited outdoors over a long term. However, metals such as iron, nickel and chromium, in the form of oxides and/or hydroxides, as well as calcium fluoride in the sludge, are of economic concern to stainless steel makers. Therefore, to develop an economically and technically suitable recycling process, a comparison study on reducing both pickling sludge and mixtures of metal oxides, using petroleum coke (P‐coke) and graphite as reductant, respectively, was carried out at temperatures up to 1400°C using thermogravimetric (TG) analysis. Meanwhile, laboratory‐scale tests on reducing pickling sludge, using P‐coke as reductant, were also conducted in a Tammann furnace at 800–1200°C. Results of the comparison study by TG demonstrate that the reduction behaviour of pickling sludge may be approached by the reduction behaviour of the metal oxide mixture having the same composition as those in the pickling sludge. Results of the Tammann furnace tests show that temperature has a significant influence on reduction. The reduced metals in the reduction products exist mostly in the form of alloys such as ‘(Fe,Ni)’ and ‘Cr–Fe–Ni’.