Abstract
Alkalinity generation and toxic trace metal (such as vanadium) leaching from basic oxygen furnace (BOF) steel slag particles must be properly understood and managed by pre-conditioning if beneficial reuse of slag is to be maximised. Water leaching under aerated conditions was investigated using fresh BOF slag at three different particle sizes (0.5–1.0, 2–5 and 10 × 10 × 20 mm blocks) and a 6-month pre-weathered block. There were several distinct leaching stages observed over time associated with different phases controlling the solution chemistry: (1) free-lime (CaO) dissolution (days 0–2); (2) dicalcium silicate (Ca2SiO4) dissolution (days 2–14) and (3) Ca–Si–H and CaCO3 formation and subsequent dissolution (days 14–73). Experiments with the smallest size fraction resulted in the highest Ca, Si and V concentrations, highlighting the role of surface area in controlling initial leaching. After ~2 weeks, the solution Ca/Si ratio (0.7–0.9) evolved to equal those found within a Ca–Si–H phase that replaced dicalcium silicate and free-lime phases in a 30- to 150-μm altered surface region. V release was a two-stage process; initially, V was released by dicalcium silicate dissolution, but V also isomorphically substituted for Si into the neo-formed Ca–Si–H in the alteration zone. Therefore, on longer timescales, the release of V to solution was primarily controlled by considerably slower Ca–Si–H dissolution rates, which decreased the rate of V release by an order of magnitude. Overall, the results indicate that the BOF slag leaching mechanism evolves from a situation initially dominated by rapid hydration and dissolution of primary dicalcium silicate/free-lime phases, to a slow diffusion limited process controlled by the solubility of secondary Ca–Si–H and CaCO3 phases that replace and cover more reactive primary slag phases at particle surfaces.
Highlights
Steelmaking slag is an important industrial by-product, with an annual global production of 170–250 million tonnes
X-ray diffractometer (XRD) analysis (SI Fig. S2) showed it contains phases structurally matched to larnite, brownmillerite (dicalcium aluminoferrite; Ca2(Al, Fe)2O5), free-lime (CaO) and a Wüstite-like phase (FeO)
After 20 min of reaction, the pH of all tests was ≥ 9, the maximum pH value was recorded after ~1 day and the pH value gradually decreased with time
Summary
Steelmaking slag is an important industrial by-product, with an annual global production of 170–250 million tonnes (van Oss 2016). It is produced when CaO (or limestone/dolomite) is added to the steel furnace as a flux that reacts with process impurities (primarily silica) and separates them from the molten steel (Bobicki et al 2012; Eloneva et al 2010). EAF slag has a similar chemical composition to BOF slag, its composition varies slightly with the type of scrap steel used, and it contains similar mineral phases (Tossavainen et al 2007; Yildirim and Prezzi 2011)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have