Anomalous Enrichment Mechanisms of Selenium from Coal to Fly Ash: The Key Roles of Zinc-/Fe-Bearing Minerals and Implications for Potential Recovery

Abstract

Unraveling the anomalous enrichment mechanisms of selenium (Se) in fly ash during coal combustion is critical for Se emission control and potential recovery. Our results indicated that combustion process can greatly enrich Se in fly ash, particularly concentrate in ash produced from co-combustion of tire-derived fuels and coal. SeO2 (g) mainly captured by iron oxides at 300-700 ℃ while only CaO can retain Se above 900 ℃. Nevertheless, amorphous phases dominated the mineralogy of iron and calcium in coal fly ash, especially for the high-Ca ash sample. The enrichment of Se in fly ash by CaO and Fe-silicates at high temperature (> 1000 ℃) was excluded due to few CaO existing in ash and Fe-silicates showing no correlation with Se. Grain-scale Se distribution displayed a decreasing order of zinc-bearing large iron oxides >> Ca aluminate > Ca aluminosilicate or silicate. Extremely high Se contents (0.1-0.36 wt.%) was notable in large irregular tire-derive zinc-containing domains.Three selenium association including SeO2 (s) (4.7-21.9%), Ca-bound selenium (15.6-38.4%) and Fe-bound Se (43.5-75.3%) constituted the total Se in ash samples, suggesting surface chemical reactions of SeO2 (g) with Fe minerals were the primary selenium enrichment pathway during coal combustion.

Keywords: high-Se fly ash, Se speciation, enrichment mechanisms, metal recovery, coal power plants