Mechanistic insights into the co-extraction of lithium and gallium from high-alumina fly ash via Na2CO3 roasting and HCl leaching: A thermodynamic perspective

Abstract

A novel thermodynamic perspective on the mechanisms underlying the co-extraction of Li and Ga from high-alumina coal fly ash (HACFA) through Na₂CO₃ roasting and HCl leaching is presented in this study. HACFA primarily comprises crystalline phases (79.9 wt%), including mullite, corundum, and quartz, alongside a minor glassy phase (20.1 wt%). Li and Ga are mainly hosted in aluminosilicate phases, particularly mullite and the glassy matrix, with Li being more concentrated in the glassy phase, likely as LiAlO₂ within alumina-rich glass (Al₂O₃). Optimal recovery is achieved by roasting at 900°C with a HACFA to Na₂CO₃ mass ratio of 1: 1.5 for 60 min, followed by leaching with 4 mol/L HCl at 90°C for 120 min, resulting in extraction efficiencies of 97.1 % for Li and 98.7 % for Ga. The Na₂CO₃ roasting process transforms inert minerals (e.g., mullite, corundum, quartz, and glassy phases) into soluble sodium silicate, sodium aluminate, and sodium aluminosilicate, thereby enhancing leaching efficiency. However, excessive roasting temperatures or acid concentrations may lead to silicic acid formation, hindering leaching efficiency. Thermodynamic simulations reveal the transformation mechanisms of Li- and Ga- bearing phases during roasting. Li leaching efficiency is governed by both the activation of original host minerals (e.g., mullite and glassy phase) and the stability of newly formed Li-bearing phases (e.g., LiAlSiO₄ (E), LiAlSiO₄, LiAlO₂, LiFeO₂, Li₂CO₃ and Li₂SiO₃), while Ga extraction primarily depends on the activation of its original host minerals. This study provides insights into the efficient recovery of critical elements from CFA.