The Effect of Iron Oxide Concentration on Redox Equilibrium of the Melts of FeO-Fe<sub>2</sub>O<sub>3</sub>-RP<sub>2</sub>O<sub>6</sub> System (R=Mg, Ca, Ba) with Gas Phase

Abstract

To elucidate the redox equilibrium of high concentration transition metal ions in a highly viscous oxide melts with gas phase, the effect of iron concentration on the redox equilibriums in the oxide melts of FeO-Fe2O3-RP2O6 system (R=Mg, Ca, Ba) at 1200°C with CO2 atmosphere, which oxygen partial pressure is about 10-3.6 atm, was studied in this report.The [Fe2+]/[Fe3+] ratios decreased at low iron oxide concentration and then increased after reaching their minimum values, as iron oxide concentration increased. These ratios also decreased at low iron oxide level and increased at high iron oxide level, as R was exchanged by Mg, Ca, Ba. The minimum values of [Fe2+]/[Fe3+] ratios shifted to the low iron oxide concentration by these exchange.It is discussed from these experimental results that the reaction between free ferrous ion and ferric oxiacid ion such asFe2++1/4 O2+(x-1/2)O2-=FeOx(2x-3)-contributes mainly to the redox equilibrium at the low iron oxide concentration, and that the reaction between oxiacid ions of ferrous and ferric ions such asFeOy(2y-2)-+1/4 O2=FeOx(2x-3)-+{1/2-(x-y)}O2-contributes mainly at the high iron oxide concentration.The relation between [Fe2+]/[Fe3+] and [O2-], oxygen ion concentration, given by the combination of the above two redox reactions, agrees with the experimental results about the relation between iron oxide concentration and [Fe2+]/[Fe3+] ratios in various melts, if the oxygen ion concentration increases with the increase of iron oxide concentration.The increase of oxygen ion concentration was recognized from the infra-red spectrums of glasses, obtained by quenching the melts in the redox equilibrium. Comparing the I. R. spectrums of FeO-Fe2O3-RP2O6 glasses with RP2O6 glasses, it was observed that the increase of iron oxide concentration corresponded to the exchanging R by Mg, Ca, Ba in RP2O6 glasses, the increase of basicity in the melts. This increase seemed to accompany with the change of the form (for example, chain length) of the phosphorus oxiacid ions in the melts. The formation of ferrous oxiacid ion in the melt was found to be possible from the shift of absorption peak by ferrous ion in the optical absorption spectrum in the above glass.Discussion about activities of various ions was not successful in this paper.