Hydrothermal synthesis of struvite and its phase transition: Impacts of pH, heating and subsequent cooling methods

MAP (magnesium, ammonium, and phosphate) precipitation under hydrothermal condition was investigated in the present study. The hydrothermal mediated solution of a MAP in equimolar ratios (1:1:1) was prepared for crystal growth in a reactor autoclave. In the hydrothermal experiments, the reactor was heated in a furnace at varying temperatures (60, 80 and 120 °C) for 24 h, and then followed by (i) air cooling and (ii) water quenching to room temperature. Struvite could be formed in the solution with initial pH values (7-10) when the reactor worked at 60 °C for 24 h and was subsequently air-cooled and water-quenched to room temperature. Working with the temperature of 80 °C and subsequent air cooling yielded struvite, dittmarite, and newberyite in the solution with pH values of 9 and 10. By heating reactor at temperatures of 80 and 120 °C and water quenching, however, a pure dittmarite could be grown in the solution with any pH values (7-10). Dittmarite was also the major phase precipitated in the solution with any pH condition at 120 °C for the longer reaction time (96 h). A small impurity of sylvite was unexpectedly co-precipitated in all processes. SEM micrograph showed that the crystal solids have irregular prismatic-shaped morphology. The optimal pH for MAP crystallization was predicted to be about 9. The present results provided valuable information for the understanding of phosphate recovery from effluent by the hydrothermal treatment.