A new review paper on alkaline wastes is now online, after been accepted by the Journal of Cleaner Production. This paper focuses on the environmental impacts associated with alkaline residues, such as coal fly ash, bauxite residues or red mud, steelworks slags, concrete crusher fines, flue gas desulphurisation waste, Air Pollution Control (APC) residues, Solvay process waste, and chromite ore processing residue (COPR).
While there are significant differences in bulk mineralogical composition among these residues, all are characterised by the presence of Na, Ca or Mg oxides that rapidly hydrate to produce soluble hydroxides. A large range of Ca and Na silicate, aluminate and aluminosilicate phases are also present, hydrating and dissolving to generate alkalinity. Finally, any Ca and Mg carbonates present provide a stable (low solubility) alkaline phase. Initially, dissolution of soluble Na or Ca hydroxides tends to dominate (pH > 12). The leachates of these residues have pH above typical regulatory thresholds for discharge to water bodies (usually 9), and a major influence on the mobility of contaminants potentially soluble at high pH, because they form oxyanions (e.g. As, Cr, Mo, Se and V).
The paper discusses the environmental impacts of the residues, highlighting the acute episodes, such as the dam failure in Ajka, Hungary, as well as the widespread chronic impact of the leachate on receiving waters, especially in old and abandoned disposal sites. Pollutant pressure and biological impacts are also reviewed.
The management options for the residues and their leachates are also discussed, distinguishing active and passive treatment options. Potential reuses of these materials, in construction materials, as agricultural amendments, and in environmental applications are identified. The mechanisms of carbon sequestration by alkaline residues are assessed, and the potential for enhancing its rate as a climate change off-setting measure for the industry is evaluated. The potential for recovery of metals critical to e-technologies, such as vanadium, cobalt, lithium and rare earths, from alkaline residues is considered. Finally, research needs are identified, including the need to better understand the biogeochemistry of highly alkaline systems in order to develop predictable passive remediation and metal recovery technologies.
Reference: Gomes, H.I., Mayes, W.M., Rogerson, M., Stewart, D.I., Burke, I.T., (2015) Alkaline residues and the environment: A review of impacts, management practices and opportunities. Journal of Cleaner Production. (In Press)