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

Abstract

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.

Keywords: Aluminofluoride complexes; co-precipitate; feed calcium phosphate salt; purification; wet-process phosphoric acid.