Valorization of BOF Steel Slag by Reduction and Phase Modification: Metal Recovery and Slag Valorization
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INTRODUCTION
STEEL slag is classified as basic oxygen furnace (BOF) slag, electric arc furnace (EAF) slag, and ladle metallurgy slag. The EU produced around 20 million tons steel slag annually in the recent years, of which 46 wt pct is BOF slag.[1] Recycling and reutilization of such large amounts of steel slag are of great importance, both for the sustainability of the metallurgical industry and for the environment. After modification of its chemistry and solidification mineralogy by means of hot stage engineering, steel slag can be recycled for internal use in the steelmaking process, and utilized for road construction, for cementitious substitutes, and as agricultural fertilizer.[2–8] Using the slag for construction or agricultural purposes, however, does not consider the large amount of Fe present in the slag. Typically, BOF slags contain from 14 to 29 wt pct of Fe, present in iron oxides and iron-containing minerals.[9] ‘‘Zero waste’’ of BOF slag can be achieved by both metal recovery and slag utilization. However, the difficulty of controlling the P distribution between slag and the recovered metal prevents the slag from achieving a full reutilization. Because of P, the recovered Fe has a limited application. Also, P stabilizes b dicalcium silicate (b-C2S). The complete removal of P from BOF slag leads to disintegration of the slag due to the transformation of b-C2S to
CHUNWEI LIU, SHUIGEN HUANG, PATRICK WOLLANTS, BART BLANPAIN, and MUXING GUO are with the Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, Box 2450, 3001 Heverlee, Belgium. Contact e-mail: [email protected] Manuscript submitted November 18, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
c dicalcium silicate (c-C2S) during cooling.[10] Thus, it is favorable to concentrate P in the oxides while limiting its concentration in metallic Fe. In previous studies, the carbothermic reduction of slag has been described to recover Cr and/or Fe from stainless steel slag, hot metal dephosphorization slag, and other steelmaking slags,[11–14] without focusing on improving the purity and controlling the morphology of the metal. Furthermore, the utilization of the remaining slag by modifying the chemical composition did not receive as much attention as the metal. Ye et al. studied the reduction of steel slags to recover both metals (Fe, Mn, V, and Cr) and oxide materials. The oxides could be reused as cementitious materials and/or desulfurization fluxes in secondary metallurgy, but no chemical modification of the oxides was considered.[15] To utilize the steel slag as aggregates in road construction, Yang et al. studied the chemical modification of the slag after reduction, focusing on avoiding the disintegration of slag caused by the transformation of b-C2S to c-C2S.[16] Kim et al. proposed a two-stage reduction of EAF slag followed by water quenching, with the aim to reuse the amorphous slag in the cement preparation.[17] However, the concurrent implementation of reduction to achieve high value-added metal products, and chemical improve
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