Enrichment of noble metals and rare earth elements in solid residues coming from biomass combustion

Recovering rare earth metals (REEs) and noble metals (NMs) from nontraditional minerals is increasingly important due to rising industrial demand and declining natural reserves. This study investigates the enrichment of these metals through biomass combustion and ash-leaching techniques. The biomass contained less than 0.05 mg/kg of Au, Ag, and Nd and was combusted in a pilot-scale boiler. The resulting ashes underwent a three-stage leaching process: water leaching, acid leaching (10 % HCl), and alkaline leaching (5 % NaOH), which further concentrated the metals. After treatment, bottom ash contained 12.1, 14.2, and 54.9 mg/kg of Au, Ag, and Nd, respectively. Ash collected after the heat exchanger contained 21.5 mg/kg Ag and 20 mg/kg Nd. Deposited ash contained 12.6, 13.1, and 11.7 mg/kg of Au, Ag, and Nd, respectively. SEM-EDS analysis revealed that Au occurred as nearly pure particles (>98 %) in bottom ash, and as small pure particles (1–2 µm) in after heat exchanger ash, but as compounds in fly and deposited ash. Ag was typically integrated within solid compounds in bottom ash and formed surface coatings on fly ash particles. REEs displayed diverse formation patterns: in bottom ash, Pr, Nd, and Ce were mainly associated with compound phases, while Er appeared as discrete particles; in after heat exchanger ash, Pr, Nd, and Er were integral to solid phases. The results show that NMs and REEs can be potentially recovered from bio-ores and offer insights into how these elements form in unconventional resources.