Selective recovery of lithium from high-aluminum fly ash by using alkali-dissolution-assisted HBTA–TOPO synergistic extraction

The process of recycling lithium (Li) from common coal fly ash using strong acids can release substantial quantities of hazardous low-value residues, and the complex ionic composition of the acidic leaching solution hinders the selective extraction of Li. Accordingly, a green and economical approach was developed for the selective recovery of Li and extraction of valuable Al-rich residues from high-alumina fly ash (HAFA). In HAFA, the glass phase (the main Li-bearing phase) can be selectively dissolved in the mild NaOH solution at 95 °C for 2 h, and valuable mullite phase (the main Al-bearing phase) was retained in the leaching residue (Al2O3 content 49.00 wt%). The leaching efficiencies of Li, Si, and Al were 91.60%, 43.19%, and 3.56%, respectively. Among the primary metallic elements present in HAFA, only Li, Na, K, and Al in the glass phase were readily dissolved in the alkaline solution. Meanwhile, the synergistic extraction system of HBTA and TOPO has good complexing ability with Li + compared to Al3+, K+, and Na+. Therefore, Li+ can be selectively extracted from alkaline leaching solutions using HBTA and TOPO. The Li extraction efficiency reached 99.68% after three consecutive extractions under the optimal conditions. Finally, the used alkali leaching solution and synergistic extractants can be regenerated via Ca(OH)2 precipitation and HCl reverse extraction. These findings can provide new strategies for the green and overall utilization of HAFA.