Occurrence of critical elements in biomass ashes and their potential industrial and ecological significance: A preliminary critical overview

A preliminary evaluation on the occurrence of 40 critical elements (CE) in 8 biomass ashes (BAs) and the potential industrial and ecological significance of these CE was conducted based on our own studies and reference data. The elemental and phase-mineral composition of these BAs is variable and depends mostly on the type of combusted biomass. The presence of minerals and phases in BAs includes species among carbonates, silicates, sulphates, chlorides, oxyhydroxides, and phosphates, as well as glass and char. It was found that only 12 CE such as B, Ba, Be, Cu, Ga, Ge, Mg, Mn, Ni, P, Si, and Sr in specific BAs are the most prospective for recovery. Some strong associations of CE in BAs were identified. Many CE such as rare earth elements (REE), As, Be, Bi, Ge, Nb, Sb, Sc, Ta, and W associate with Si-, Ti- and Al-bearing minerals and glass; some CE (B, Ba, Ga, Mn, Ni, Sr, and V) associate with salt minerals; whereas certain CE (Co, Ni, and Sr) associate with Fe and Mg oxides. However, the modes of trace CE occurrence in BAs as discrete minerals were almost unidentified, excluding the presence of As, B, Ba, Cu, Mn, V and some REE in specific minerals. Most trace CE probably occur dominantly as impurities in common minerals and phases of BA. BAs are promising sources for extraction of some CE that could yield fractions meeting the cut-off grade for industrial recovery after some pre-treatment, but more detailed investigations are required for that purpose.