Activities of components in Ca2SiO4-Ca3P2O8 solid solution at 1573 K

Abstract

In steelmaking processes, it is known that P2O5 in dephosphorisation slags often exists in solid solutions between Ca2SiO4 and Ca3P2O8, <C2S-C3P>SS, and thus the solid solutions play an important role in the dephosphorisation reaction. Hence, the knowledge of thermochemical properties, ie phase relations and activities of components, in slags containing <C2S-C3P>SS are necessary for the effective removal of phosphorus from hot metal. In this study, the P2O5 activities in <C2S-C3P>SS were measured through the gas equilibrium method. In the present experiment, oxide samples were equilibrated with Cu-P liquid alloys at 1573 K under a stream of Ar + H2 + H2O or Ar + H2 + CO2 gas mixture in which oxygen potential was fixed and then equilibrium phosphorus concentrations in the Cu-P alloys, [mass%P]Cu, were analysed with ICP-OES. The underlying reaction can be expressed by: 2[P]Cu + (5/2)O2(gas) = P2O5 in oxide K = aP2O5 (pO2/atm)-5/2 [mass%P]Cu-2 Where: K denotes the equilibrium constant aP2O5 represents the P2O5 activity pO2 is the oxygen partial pressure in the gas phase Firstly, the experiments were performed on MgO + Mg3P2O8, in which the P2O5 activity has already been reported, to determine the value for the equilibrium constant, K. Subsequently, the P2O5 activities were measured in <C2S-C3P>SS + CaSiO3 and <C2S-C3P>SS + CaO with the same experimental procedure. The P2O5 activity in <C2S-C3P>SS coexisting with CaO was found to be about seven digits lower than that with CaSiO3 even at the same composition of <C2S-C3P>SS. This phenomenon was explained in terms of the activities of Ca2SiO4 and Ca1.5PO4 (= (1/2)Ca3P2O8). These activities are determined depending on temperature and the composition of <C2S-C3P>SS, while the P2O5 activity depends not only on these variables but also on the second phase coexisting with <C2S-C3P>SS. The Ca2SiO4 and Ca1.5PO4 activities exhibited large negative deviations from Raoult’s law, which implied the strong chemical affinity between the components. Experimental errors in these experiments would be cancelled out in calculating aP2O5 since they would be involved in both experimental procedures for MgO + Mg3P2O8 and <C2S-C3P>SS + CaO or <C2S-C3P>SS + CaSiO3, which makes the present results more reliable.