The fate of noble metals and rare earth elements during pelletized biomass combustion

The extraction of rare earth elements (REEs) and noble metals (NMs) from unconventional resources is playing a crucial role under the context of industrialization and reserve depletions. Plants used for phytoextraction are promising materials for the recovery of metals, but the biomass needs to be reduced to a manageable amount and volume prior to the extraction process. This paper investigates the combustion process of biomass focusing on NMs and REEs flow. The plants harvested from a brownfield land were pelletized and incinerated in a fixed-grate pilot-scale boiler, meanwhile, solid remains from various points in the combustion and flue gas system were captured and analyzed. The results show that levels of NMs in deposited ash and fly ash are greater than in bottom ash. Meanwhile, the higher REE concentration in bottom ash compared to that in other solid residuals demonstrates the less ability of these compounds to escape from the combustion chamber. Generally, the concentrations of REEs and NMs in the solid residues are significantly higher compared to biomass. SEM-EDS analyses of the contaminated solid remains indicate that gold forms individual particles with purity higher than 95 wt% in the bottom ashes, and this finding adds novel insights into gold phytomining.