Thermodynamics of Fe_tO-P₂O₅-CaO and Fe_tO-P₂O₅-MgO Ternary Slags Saturated with Solid Iron

Abstract

As a fundamental study on the physico-chemical properties of phosphate slags, a measurement has been made on the equilibrium of hydrogen-water vapor gas mixtures with the phosphate slags of Fe_tO-P₂O₅-CaO and Fe_tO-P₂O₅-MgO ternary systems contained in solid iron crucible at 1 400°C. From the results obtained, the activity of iron oxide and the ratio of ferrous to ferric iron (Λ= Fe³⁺/Fe²⁺) have been determined as a function of slag composition. The activity of iron oxide in these slags markedly deviated to the direction of the composition of 3CaO·P₂O₅ and 3MgO·P₂O₅ compound respectively.
In view of ionic theory, the approximate validity of regular solution model for cations were examined to evaluate the thermodynamic properties of these ternary slag containing P₂O₅. It was confirmed that the regular solution model was satisfied within experimental error except for extremely FeO rich region. The values of interaction energy concerning P₂O₅ are as follows,
α₃₄ (P⁵⁺Ca²⁺) =-50900 ± 400 cal/mol
α₃₅ (P⁵⁺Mg²⁺)=-32200 ± 700 cal/mol