Immobilization of Chromium in Stainless Steel Slag Using Low Zinc Electric Arc Furnace Dusts

A new synergistic treatment of stainless steel slag and low zinc content electric arc furnace (EAF) dusts is proposed to immobilize harmful chromium in stainless steel slag. The effects of ZnFe2O4 addition on the mineralogic phase and chromium leachability of CaO-SiO2-MgO-Al2O3-Cr2O3 synthetic slag were investigated to explore the feasibility of this method. The mineralogic phases in stainless steel slag were investigated by scanning microscopy equipped with energy-dispersive spectroscopy and X-ray diffraction. The leaching concentration values of chromium and zinc were evaluated according to an alkaline digestion for the hexavalent chromium (US-EPA-3060A) and toxicity characteristic leaching procedure (TCLP, US-EPA-1311) method, respectively. It was found that all synthetic slags mainly contain alpha-Ca2SiO4, merwinite and spinel phase, in line with the calculation results by FactSage. The crystallization of spinel and merwinite phases was enhanced by the addition of ZnFe2O4 but suppressed the precipitation of alpha-Ca2SiO4. It was revealed that the leaching concentration of chromium was depressed by adding ZnFe2O4 and was far below the chemical limits defined in the French proposal for a criterion and evaluation methods for waste ecotoxicity (0.1 mg L-1). In addition, the zinc leaching concentration meets the sanitary landfill standard. The proposed synergistic treatment method was further validated on industrial stainless steel slags and EAF dusts. Experimental results indicated that the synergistic treatment method can immobilize chromium effectively and the final slags can be disposed of by sanitary landfilling or recycling as constructional materials. A schematic technologic route diagram of the synergistic treatment was also proposed.