Hydration of dicalcium silicate and diffusion through neo-formed calcium-silicate-hydrates at weathered surfaces control the long-term leaching behaviour of basic oxygen furnace (BOF) steelmaking slag

Stewart, Douglas I.; Bray, Andrew W.; Udoma, Gideon; Hobson, Andrew J.; Mayes, William M.; Rogerson, Mike; Burke, Ian T.

Abstract

© 2018 The Author(s) 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 (Ca 2 SiO 4 ) dissolution (days 2–14) and (3) Ca–Si–H and CaCO 3 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 CaCO 3 phases that replace and cover more reactive primary slag phases at particle surfaces.

Citation

Stewart, D. I., Bray, A. W., Udoma, G., Hobson, A. J., Mayes, W. M., Rogerson, M., & Burke, I. T. (2018). Hydration of dicalcium silicate and diffusion through neo-formed calcium-silicate-hydrates at weathered surfaces control the long-term leaching behaviour of basic oxygen furnace (BOF) steelmaking slag. Environmental science and pollution research, 25(10), 9861-9872. https://doi.org/10.1007/s11356-018-1260-7

• Journal Article Type: Article
• Acceptance Date: Jan 11, 2018
• Online Publication Date: Jan 25, 2018
• Publication Date: 2018-04
• Deposit Date: Feb 6, 2018
• Publicly Available Date: Jul 5, 2018
• Journal: Environmental Science and Pollution Research
• Print ISSN: 0944-1344
• Publisher: Springer Verlag
• Peer Reviewed: Peer Reviewed
• Volume: 25
• Issue: 10
• Pages: 9861-9872
• DOI: https://doi.org/10.1007/s11356-018-1260-7
• Keywords: Steel slag; Vanadium; Alkaline waste; Leaching; Particle size; Reuse
• Public URL: https://hull-repository.worktribe.com/output/583445
• Publisher URL: https://link.springer.com/article/10.1007%2Fs11356-018-1260-7
• Contract Date: Jul 5, 2018