Thermodynamic Measurement of Di-calcium Phosphate

Abstract

The thermodynamic properties of the CaO-P2O5 system are important to develop an effective refining process in the iron and steel industry. In this study, the thermodynamic properties of (CaO)2P2O5 were investigated because the properties are necessary to develop a new dephosphorization process. The vapor of gaseous phosphorus and phosphorus oxide in equilibrium with a mixture of (CaO)2P2O5 and (CaO)3P2O5 at 1373 K to 1498 K (1100 °C to 1225 °C) were detected directly as an ion current by double Knudsen cell mass spectrometry. Comparing the ion currents with those from a mixture of Al2O3P2O5 and Al2O3, which is used as a reference mixture in this study, the Gibbs energy change of the following reaction was calculated: $$ 2{\text{CaO}}\left( {\text{s}} \right) \, + {\text{ P}}_{2} \left( {\text{g}} \right) \, + \frac{5}{2}{\text{O}}_{2} \, \left( {\text{g}} \right) = \left( {\text{CaO}} \right)_{2} {\text{P}}_{2} {\text{O}}_{5} \left( {\text{s}} \right) $$ $$ \Updelta G^{ \circ } = - 2180000 + 590T\left( { \pm 28000} \right){\text{J}}. $$