Removal of magnesium and aluminum from wet phosphoric acid by ralstonite precipitation: Thermodynamic and performance analysis, mechanistic studies and precipitate characterization

The purification of wet-process phosphoric acid has always posed a challenge for both academic and industrial researchers. In order to address this issue, a treatment unit was developed to mitigate magnesium and aluminum impurities from industrial semi-aqueous wet process phosphoric acid (SWPPA) (39.4 wt% P 2 O 5 ) through ralstonite precipitation using Na + /F − as the precipitator (F − provided by HF). Thermodynamic analysis, DFT calculation and separation experiments results were used to investigate the impact of sodium sources properties on impurities removal rates and H 3 PO 4 yield. Additionally, the effects of reaction temperature, reaction time, precipitating agent dosage, and aging time on precipitation efficiency were also examined. The results show that the ability of sodium source to ionize Na + and react with H 3 PO 4 and HF plays a crucial role in determining the impurity removal performance of magnesium and aluminum. The precipitants NaCl and Na 2 CO 3 exhibit the highest removal rates for magnesium and aluminum, which is attributed to the easy ionization of Na + ions. In comparison to Na 2 CO 3 , the higher H 3 PO 4 yield with NaCl as precipitator can be achieved due to its lower nucleation rate and higher crystal growth rate in the phosphoric acid system. NaCl and HF as precipitants effectively reduces the concentrations of magnesium and aluminum in the untreated SWPPA solution from 23.47 g MgO/kg P 2 O 5 to 3.29 g MgO/kg P 2 O 5 , 48.98 g Al 2 O 3 /kg P 2 O 5 to 15.54 g Al 2 O 3 /kg P 2 O 5 . The removal rates for magnesium and aluminum are found to be 85.98% and 68.27%, respectively, with minimal residue of Na + /F − .