The formation of gold in woody biomass combustion ashes

This paper investigates the enrichment of gold through combustion and ash-leaching techniques utilizing woody biomass as a fuel source. It delves into the formation of gold in ashes derived from the fixed grate combustion of pelletized woody biomass containing noble metals, conducted at a pilot-scale boiler. The biomass sample was gathered from a brownfield land at an abandoned mining area, avoiding induced phytoextraction. The fuel contained <0.05 mg/kg gold, while the bottom ash, after heat exchanger ash, deposited ash, and fly ash contained 1.52 mg/kg, 1.99 mg/kg, 2.64 mg/kg, and 3.52 mg/kg of gold, respectively. Although the amount of fly ash is lower compared to bottom ash, the concentration of gold is the highest in fly ash, which follows the after heat exchanger ash and bottom ash. The concentration of gold was enriched by a three-stage procedure of water leaching, acid leaching (10 % HCl), and alkaline leaching (5 % NaOH), after which 12.1 mg/kg and 12.6 mg/kg gold was found in the residues obtained from leached bottom ash and deposited ash, respectively. SEM was utilized to depict the morphology of gold, which appears in bottom ash as individual neat particles with a purity higher than 98 %. Pure gold particles in the size of 1-2 μm are presented in the after heat exchanger ash; meanwhile, gold in fly ash is primarily associated with potassium, sodium, sulfur, and oxygen. The findings in this study pave the way for reclaiming gold from bio-ores as well as assist in better understanding the formation of this precious metal in these secondary resources.