Removal of Al3+ and Mg2+ ions in wet-process phosphoric acid via the formation of aluminofluoride complexes

With the decrease in the phosphate rock grade, the minor element ratios (MER) [(Fe2O3 wt% + Al2O3 wt% + MgO wt%)/P2O5 wt%] of wet-process phosphoric acid (WPA) exhibits a linear upward trend. This can lead to a huge challenge for the high-quality production of feed calcium phosphate salt (FCPS). In the present study, we proposed a novel and economical strategy to precipitate Al3+ and Mg2+ via the formation of aluminofluoride complexes (NaMgAlF6·H2O) with the anhydrous sodium sulfate (Na2SO4) and hydrofluoric acid (HF) as precipitation agents. Because of the low solubility of the complexes in WPA, the removal efficiencies of Al3+ and Mg2+ ions could reach 99.5% and 64.8%, respectively. The maximum mass loss of P2O5 was less than 0.5%. The precipitates could be separated and converted into the HF and Na2SO4 for reuse, thus further decreasing the cost of WPA purification.