Chemical potential of oxygen for iron-rutile-ilmenite and iron-ilmenite-ulvospinel equilibria

Abstract

The chemical potential of oxygen corresponding to the iron-rutile-ilmenite (IRI) and iron-ilmenite-ulvospinel (IIU) equilibria has been measured employing solid-state galvanic cells, Pt, Fe + TiO₂ + FeTiO₃//Fe+FeO, Pt and Pt, Fe + FeTiO₃ + Fe₂TiO₄//(Y₂O₃) ZrO₂//Fe+FeO, Pt in the temperature range of 875 to 1275 K and 900 to 1373 K, respectively. The cells are written such that the right-hand electrodes are positive. The electromotive force (emf) of both the cells was found to be reversible and to vary linearly with temperature over the entire range of measurement. The chemical potential of oxygen for IRI equilibrium is represented by Δμ_O2(IRI) = -550,724 - 29.445T + 20.374T InT(±210) J mol^-1 (875 ≤T≤ 1184 K) = -620,260 + 369.593T - 27.716T lnT(±210) J mol^-1 (1184≤T≤ 1275 K) and that for IIU equilibrium by Δµ o₂(IIU) = -501,800 - 49.035T + 20.374T lnT(±210) J mol^-1 (900 ≤T≤ 1184 K) = -571,336 + 350.003T- 27.716T lnT(=-210) J mol^-1 (1184 ≤T≤ 1373 K) The standard Gibbs energy changes for IRI and IIU equilibria have been deduced from the measured oxygen potentials. Since ilmenite contains small amounts of Ti³⁺ ions, a correction for the activity of FeTiO₃ has been incorporated by assuming ideal mixing on each cation sublattice in the FeTiO₃-Ti₂O₃ system. Similarly, the ulvospinel contains some Fe³⁺ ions and a correction for the activity of Fe₂TiO₄ has been included by modeling the Fe₂TiO₄-Fe₃O₄ system. The third-law analysis of the results obtained for IRI equilibrium gives ΔH ₂₉₈ ⁰ = -575 (±1.0) kJ mol^-1 and for IIU equilibrium yields ΔH ₂₉₈ ⁰ = -523.7 (±0.7) kJ mol^-1}. The present results suggest that Fe²⁺ and Ti⁴⁺ cations mix almost ideally on the octahedral site of spinel lattice in Fe²TiO⁴, giving rise to a configurational contribution of 2R In 2 (11.5256 J mol^-1 K^-1) to the entropy of Fe₂TiO₄.