Targeted Recovery of Germanium from Coal Fly Ash Using Quaternary Ammonium Salt Extractant

Coal fly ash serves as a vital source of germanium production. The efficient and highly selective recovery of germanium poses considerable challenges in the context of sustainable development and environmental protection. This study designed and synthesized choline-like quaternary ammonium salts to enable efficient and selective germanium extraction. The investigation delved into the influence of leaching conditions, germanium concentration, extractant structures, extractant concentration, pH levels, isooctanol concentration, tributyl phosphate concentration, and extraction time on the efficiency of germanium recovery. Under optimized conditions (0.068 mol/L extractant, 10% isooctanol, 50% tributyl phosphate, 40% solvent oil, pH 0.74, O/A = 1:4, room temperature, 1 min extraction), a 94.72% germanium extraction efficiency was attained through two successive extractions. The germanium extraction capacity of an organic phase with 0.068 mol/L extractant (BHMA-16) was 3.51 g/L. Impurity ions, such as iron, aluminum, nickel, copper, zinc, and arsenic, were nearly not extracted. Single-step stripping with 0.3 M NaOH yielded a 97.81% germanium recovery. Furthermore, analysis using the slope method, FT-IR, and NMR confirmed the extracted complex structure as [R4N][Ge(OH)2(HL)]. This research revealed that electrostatic interactions and hydrogen bonding mediated the specific binding between the extractant and germanium–tartrate complex through a “lock-and-key” model, which constituted the primary mechanism for targeted germanium recovery from coal fly ash leachate.