A sustainable method for germanium, vanadium and lithium extraction from coal fly ash: Sodium salts roasting and organic acids leaching

The paper proposes an environmentally friendly process for recovering germanium, lithium, and vanadium from coal fly ash (CFA) using thermal pretreatment and hydrometallurgy. To this end, three sets of experiments were conducted: parameter screening, optimization, and kinetic modeling. The Taguchi method was employed to develop screening tests and ascertain the most influential parameters. The salt type (NaCl, NaNO3, Na2CO3, Na2SO4), the CFA/salt ratio, the organic acid type (malic, oxalic, citric, and acetic acids), the acid concentration, and the processing time were investigated. The results indicated that while the acid type has the most significant effect on the leaching process, the salt/CFA ratio has the least effect on metal recoveries. The second set of experiments used response surface methodology (RSM) to optimize the dissolution process and obtain the highest recoveries of Ge, Li, and V. Additionally, mathematical models were suggested to predict the metals recoveries. Citric acid and NaCO3 were selected as the optimal leaching and roasting agents. The optimum condition to obtain maximum recoveries was 0.5 M citric acid, a CFA/NaCO3 ratio of 1:0.5, and a leaching time of 60 min. During the final set of experiments, four tests were conducted under optimal conditions at various temperatures for kinetic modeling and activation energy calculation. At 30 °C, the highest recoveries for Ge, V, and Li were 98.15%, 75.31%, and 97.30%, respectively. The “interfacial transfer and diffusion across the product layer” model was discovered to govern leaching kinetics. The activation energies for Ge, V, and li were 24.50 kJ/mol, 34.16 kJ/mol, and 49.82 kJ/mol, respectively.