Bioleaching rare earth elements from coal fly ash by Aspergillus niger

Rare earth elements (REEs) are abundant in certain mining coal, and further enriched in coal fly ash (CFA) after coal combustion. If the appropriate leaching technology is chosen, the CFA can be used as an unconventional source of REEs. In this paper, culture conditions and bioleaching process of Aspergillus niger (A. niger) were optimized, the mechanism of acid production and factors of the bioleaching process of A. niger were discussed, and the bioleaching mechanism of REEs from CFA by A. niger was expounded. The results showed that organic nitrogen sources affected the morphology and acid production capacity of A. niger, and the A. niger used in this study mainly secreted oxalic acid (4.05 mg/L). Secondly, the initial pH value, the most influential factor, further affected the bioleaching process by affecting the acid production of A. niger. Therefore, by optimizing the initial pH value to enhance the acid production capacity of A. niger and developing the best bioleaching system, the CFA can reach a leaching rate of 30.91 % for total REEs. Finally, based on the test results of organic acids and pH, combined with previous studies, we speculated the main mechanism of REES bioleaching from CFA by A. niger as proton exchange and organic ligand complexation. Microbial leaching (e.g., A. niger) integrates economic, environmental, and resource use values.