Spent-medium leaching of germanium, vanadium and lithium from coal fly ash with biogenic carboxylic acids and comparison with chemical leaching

Coal fly ash (CFA), produced in coal-fired power plants, is categorized as hazardous waste and has caused serious environmental impacts. The CFA contains significant amounts of critical metals and can be considered as a secondary resource for these metals. Therefore, through an appropriate recycling process, reducing CFA stocks helps to reduce its severe environmental impacts and to provide a potential metal resource. In the current research, a spent-medium bioleaching process with Pseudomonas putida and Pseudomonas koreensis was introduced for recovering germanium, vanadium, and lithium from CFA. For this purpose, organic acids were produced with the mentioned microorganisms and used for leaching experiments after salt roasting CFA with Na2CO3. The effect of different parameters has been investigated for CFA leaching in the presence of biogenic acids. The results showed that the highest recoveries of metals were obtained for the leaching test with organic acids produced by Ps. putida, at 500 rpm agitation speed, 3% pulp density and 75 °C. The Ge, V, and Li recoveries at optimum conditions were 83%, 98% and 97%, respectively. Kinetic modeling was employed to determine the effect of different parameters on the leaching efficiency. The results showed that reagent diffusion to the surface of the particles was a rate-limiting step in the leaching process. The activation energies for Ge, V and Li leaching, were 37.1 kJ/mol, 28.6 kJ/mol and 15.7 kJ/mol, respectively.