An investigation of the thermal behavior of magnesium ammonium phosphate hexahydrate

Struvite (MgNH₄PO₄·6H₂O) was heated to temperatures from 55 to 300 °C. X-ray diffraction analysis revealed struvite was stable at 55 °C, partially decomposed to dittmarite (MgNH₄PO₄·H₂O) at 100-200 °C, and formed an amorphous phase at 250-300 °C. Thermogravimetric analysis confirmed sample mass loss consistent with dittmarite formation at 100-200 °C and evolution of all volatiles at 250-300 °C. Fourier transform infrared (FTIR) spectroscopy detected the ν₄ NH ₄ ⁺ band in 55-200 °C solids, as expected for struvite and dittmarite. This band decreased in intensity at 250 °C, and was not evident at 300 °C, confirming loss of NH ₄ ⁺ (s) at these temperatures. FTIR spectra also showed changes in the vibrations of the ν₃ PO ₄ ^3- band. At 55 °C, splitting in the band indicated destabilization of the PO ₄ ^3- (s) group despite no change in mineralogy. Vibrations at 100-200 °C were associated with dittmarite and MgHPO₄, and at 250-300 °C, MgHPO₄ and Mg₂P₂O₇. Analysis of acid-digested solids indicated the presence of P other than P-PO₄ at 200-250 °C, confirming Mg₂P₂O₇ formation. Overall, heat treatment of struvite produces several decomposition products, complete identification of which requires the use of multiple approaches. Temperature-induced phase transformations along with emission of NH₃(g) have implications for use of struvite in multiple applications.