Mineralogy and Leaching of Steel Slag (BOF/LD/Converter steel slag)

Integrated steelworks produce steel from iron-ore. Co-produced are vast amounts of slags in the Blast Furnace process (~200-250 kg slag/ton of metal) and Basic Oxygen Furnace (BOF) process (~100 kg slag/ton of steel). Whereas BF-slag is in high demand for cement use, BOF-slag addition to cement doesn’t create synergy, despite mineralogical similarity.

BOF slag mineralogy can be predicted using thermochemical modelling for the multicomponent system CMAFfSP, using FactSage, and petrological observations of BOF slag confirm phenocrysts of Magnesio-Wuestite (Liquidus-phase) followed by alpha-C2S, in a groundmass of co-crystalizing Wuestite, Srebrodolskite (C4AF), C2S and lime. Incorporation of minor constituents (Ti, V, Cr) occurs in Wuestite (Cr) and C2S (V) and C4AF (V, Ti), but cannot be predicted for lack of solid solution models. Cr and V can cause leaching concerns for slag applications.

Studying hydration reactions on finely ground BOF slag indicates that hydrogarnet, hydrotalcite, portlandite and CSH-gel tend to form as product and that all primary slag phases contribute in the reaction. The leaching of Vanadium is pH-controlled during C2S dissolution.

Application of BOF slag as secondary raw material requires adequate characterization. Grinding BOF slag to cement-fineness can produce a high-strength binder, however, requiring the aid of specific additives.

C=CaO; M=MgO; A=Al2O3; F=Fe2O3; f=FeO; S=SiO2; P=P2O5; C2S=Ca2SiO4