Enhancing the quantification of critical elements in WTE and coal ash via alkaline fusion: Superiority of lithium metaborate (LiBO2)

Authors: KUMAR, VIKRAM - University Of Illinois; SCOTT, JOHN - University Of Illinois; ZHAO, LINDUO - University Of Illinois; BARGON, MARGARITA FYODO - University Of Illinois; BARAL, ANIRUDDHA - University Of Illinois; ROESLER, JEFFREY - University Of Illinois; Sharma, Brajendra; GARG, NISHANT - University Of Illinois

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2025
Publication Date: 8/7/2025
Citation: Kumar, V., Scott, J.W., Zhao, L., Bargon, M.R., Baral, A., Roesler, J., Sharma, B.K., Garg, N. 2025. Enhancing the quantification of critical elements in WTE and coal ash via alkaline fusion: Superiority of lithium metaborate (LiBO2). Energy and Fuels. https://doi.org/10.1021/acs.energyfuels.5c02683.
DOI: https://doi.org/10.1021/acs.energyfuels.5c02683

Interpretive Summary: The transition to low-carbon energy technologies has increased the demand for rare earth elements (REEs). Because of the heterogeneously distributed REEs around the globe and to minimize environmental impact, there is an increased focus on recovering these from industrial waste by-products, such as coal fly ash, sediments, mine tailings, and sewage sludge. A potential new feedstock, waste-to-energy bottom, and fly ash (WTE ash) generated from the incineration of municipal solid waste can address the increasing demand for REEs. To improve and accelerate the analysis of REEs in WTE ash, alkaline fusion using lithium borate was conducted for WTE ash digestion followed by analysis of elements including 16 REEs. It was found that this approach successfully decomposed refractory phases in WTE ashes and resulted in good digestion efficiencies for most elements. The results from this study will help utilize a waste product, recover needed REEs, divert it from landfills, and reduce its environmental impact. The work also has the potential to be translated to biomass gasification ash, which can help improve the economic value, utilization, and demand for farm products.

Technical Abstract: Waste-to-energy ash (WTE ash) is a potential feedstock that can address the increasing demand for critical materials, such as rare earth elements (REEs), and increase supply chain resilience. Critical materials concentration in WTE ash can have spatiotemporal variation due to changes in waste composition and incineration conditions, suggesting the need to identify ashes suitable for critical materials recovery. To identify ashes suitable for critical materials recovery, powdered ash specimens must first be converted into a liquid form by digestion with inorganic acids and then assessed for critical materials concentration via ICP-MS and ICP-OES. Acid digestion with inorganic acids alone can often fail to dissolve critical materials residing in the refractory minerals completely and require significant time (~hours). These limitations can be overcome with alkaline fusion using an appropriate flux. In this article, we evaluate the feasibility of using LiBO2, Li2B4O7, and their combination as fluxes for WTE ash digestion by measuring the elemental concentrations of 39 elements, including 16 REEs, on two standard reference materials (BCR 176R and SRM 1633c). By comparing the measured elemental concentrations with published data, we conclude that alkaline fusion with LiBO2, Li2B4O7, and their combination results in good digestion efficiencies (>85 percent) for most elements. Additionally, our findings indicate that digestion efficiencies for certain elements are sensitive to the flux composition – the increased presence of Li2B4O7 in the flux reduces the digestion efficiency of Fe, Ti, As, Na, Pb, and Cd. These findings suggest that alkali fusion with LiBO2 can successfully decompose refractory phases in WTE ashes, enabling ash identification for critical materials recovery.