Magnesium removal from phosphoric acid by precipitation: Optimization by experimental design

High content of magnesium in phosphate and phosphoric acid affects negatively the performance and operating conditions in phosphate industry. A content of more than 0.3% in phosphate increases the P2O5 losses during phosphate digestion and filtration, and also increases steam consumption and solid settling kinetics during concentration. In this work, the removal of magnesium from phosphoric acid by precipitation in one of the compounds, MgAlF5 or MgAl2F8, was studied. Magnesium precipitation is achieved by the simultaneous addition of aluminum and fluorine. The experimental design methodology was used to carry out this work. Tests were conducted according to the NEMRODW software using industrial quality phosphoric acid. The screening study of parameters affecting the removal efficiency of magnesium from industrial wet phosphoric acid showed that from the following parameters: temperature, F/Mg and Al/Mg ratios, aluminum form and fluorine form, only temperature and F/Mg ratio have an effective influence on magnesium removal. The optimization of magnesium removal from phosphoric acid was performed according to the response surface methodology using a composite matrix. By applying this methodology, the optimum parameters corresponding to a maximum magnesium removal efficiency in phosphoric acid were determined. The values of the optimum parameters obtained by this method are T = 80 °C, ratios: Al/Mg = 1 and F/Mg = 16