Novel method for improving iron recovery from electric arc furnace slag: on-site hot modification

The iron element in electric arc furnace (EAF) slag is extremely difficult to recycle due to the low specific magnetic susceptibility of the RO phase (a solid solution of FeO, MgO, CaO, and MnO). Landfilling EAF slag is strictly forbidden for environmental consideration because of poisonous Cr 6+ leaching. The original RO phase could be transformed to a spinel structure, whose specific magnetic susceptibility is much higher than that of other minerals, through hot modification, resulting in significantly increased iron recovery by magnetic separation. Precipitation of spinel crystals encloses chromium, such that iron and chromium could be recovered simultaneously. The chromium in obtained iron concentrates is considerably useful for stainless steel making rather than polluting the environment. As a result, recovering iron and chromium is truly beneficial for cleaner production. Hot modification of EAF slag should be conducted at 1500–1600 °C for at least 60 min to guarantee homogeneous liquid slag. The liquid slag was poured onto an iron mold to obtain modified slag (MS) through air quenching. MS was characterized by thermodynamic analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive spectroscopy to correlate the relationship between mineral structures and iron recovery. The iron recovery rate of MS first increased and then decreased with increasing modifier. It was less than 10% when the modifier addition amount was below 12 wt.%, but it increased rapidly as the modifier addition amount increased from 16 to 24 wt.%, mainly due to spinel formation. The highest iron recovery rate was 81.9% when the modifier amount reached 20 wt.%. Meanwhile, Cr 6+ was enriched in the spinel phase but was not observed in other minerals. Industrial tests were performed on-site with the modifier ranging from 12 to 18 wt.% because additional heat was not provided during the tests. Results showed that MS with 18 wt.% modifier addition exhibited an iron recovery rate of 61.0%, much higher than that (34.6%) of the original slag.