An investigation of the thermal behavior of magnesium ammonium phosphate hexahydrate

Abstract

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.