Investigation of phase evolution during the formation of calcium potassium sodium orthophosphate

Abstract The formation of calcium potassium sodium orthophosphate (Ca2KNa(PO4)2) from sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), potassium dihydrogen phosphate (KH2PO4) and calcium carbonate (CaCO3) was investigated using differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and powder X-ray diffraction (XRD). DTA showed five distinct thermal events attributed to dehydration of NaH2PO4·2H2O, dehydration of NaH2PO4 and KH2PO4, the transition of amorphous calcium sodium phosphate to β-CaNaPO4, the decomposition of CaCO3 to CaO, and the crystallization of Ca2KNa(PO4)2. TGA showed 5.6% weight loss between 70 and 170 °C due to the dehydration of NaH2PO4·2H2O to NaH2PO4; the 7.3% weight loss between 170 and 280 °C is due to the dehydration of NaH2PO4 and KH2PO4; a continuous weight loss over 280–760 °C due to the reaction of melted alkali metal phosphates with CaCO3 and an approximate 8% weight loss between 670 and 840 °C due to the calcination of residual CaCO3. XRD analysis, as a function of temperature, supported the evolution of these events and phases, showed the formation of other intermediates: β-CaKPO4 and calcium potassium pyrophosphate (CaK2P2O7). The proposed mechanism of the reaction to Ca2KNa(PO4)2 involves the formation and consumption of these intermediates: acid alkali metal pyrophosphate, metaphosphate, CaK2P2O7 and β-CaNaPO4.