Relation between Local Structural Environment of Iron Ions and FeO_1.33 Activities in the FeO_x-CaO-SiO₂ slags

Abstract

Oxidation states and coordination structures of iron ions in FeO_x-CaO-SiO₂ slags and their dependencies on the oxygen partial pressures and slag compositions have been measured by Mössbauer spectroscopy. The relationship between local structural environment of iron ions and FeO_1.33 activities has also been investigated.

Variation in the slag composition causes Fe ions to have such a local structural environment that the degree of polymerization of the whole silicate network structure is retained. The ratio of the number of Fe³⁺ in the octahedral coordination to that of Fe³⁺ in the tetrahedral coordination (x_Fe3+(Oh) / x_Fe3+(Td)) converges to the range between 0.5 to 1.0 with decreasing the degree of polymerization of matrix silicate network structure. Since the x_Fe3+(Oh) / x_Fe3+(Td) values of Fe₃O₄ and Ca₂Fe₂O₅ crystals are reported to be 0.5 and 1, respectively, it is considered that Fe ions tend to form microcrystalline clusters having the similar structures to those of Fe₃O₄ and/or Ca₂Fe₂O₅ in the slags with x_Fe3+(Oh) / x_Fe3+(Td) of 0.5 to 1.0.

By comparing the oxidation states and coordination structures of iron ions with FeO_1.33 activities measured by authors elsewhere, it has been found that FeO_1.33 activities divided by FeO_x concentraions (γ_FeO1.33) are the largest in the slags with x_Fe3+(Oh) / x_Fe3+(Td) of 0.5 to 1.0. The larger γ_FeO1.33 values correspond to the poorer affinity of iron oxides with matrix silicates. As a consequence, it is considered that the formation of microcrystalline clusters having the similar structures to those of Fe₃O₄ and/or Ca₂Fe₂O₅ results in the increase in the γ_FeO1.33 values, i.e., the FeO_1.33 activities.