Selective separation and stepwise enrichment of Li, Ga, and REEs from high-alumina fly ash via oxalic acid leaching

Recovering rare metals from fly ash is a highly effective approach for transforming waste into sustainable resources. This study achieves selective separation and stepwise enrichment of lithium (Li), gallium (Ga) and rare earth elements (REEs) from high-alumina coal fly ash (HACFA) by via oxalic acid leaching. The results indicate that the optimal mass ratio of fly ash and sintering agent (Na2CO3 and CaCO3) is 1:0.45:1.46. During the sintering process, the generation of NaAlO2 and Ca2SiO4 greatly improves the efficiency of leaching trace elements from fly ash, which may be related to the disruption of the bridging oxygen bond structure, resulting in the dissociation of the Si-O (Al-O) polymer network structure in HACFA. The best conditions for an oxalic acid leaching experiment are a concentration of 0.1 mol/L and a period of 4 days. Under these circumstances, Li and Ga leaching rates were 95.75 % and 87.60 %, respectively, with a retention coefficient of 95.78 % for REEs in the residue. By adjusting the pH, 87.46 % of the Ga in HACFA was recovered by precipitation of Ga(OH)3 in the leaching solution at pH 6. After calcination and dissolution of the filter residue, the removal rate of Ca2+ was 98.24 % by using an NH4F dosage coefficient of 1.3. Heavy rare earth elements in the solution undergo significant precipitation at a pH of 8.5-9.5. At pH 10.5, 79.73 % of REEs in HACFA was enriched as REE(OH)3 precipitate.