Low Temperature Phase Transition of Ilmenite during Oxidation by Chlorine

Abstract

The phase transition of natural and synthetic ilmenite during oxidation by Cl₂ at 390–560°C was characterized. The ilmenite phase was converted to crystallographic shear (CS) phases during the oxidation. One third of the Fe²⁺ in the ilmenite phase was converted to FeCl₃. The remaining Fe²⁺ was oxidized to Fe³⁺ in the CS phases. The CS phase obtained from synthetic ilmenite was the Fe₂Ti₃O₉ (M₅O₉) phase. A small amount of Mg²⁺ impurity in natural ilmenite caused the resulting structure to be the M₁₃O₂₃ structure when it was oxidized to the CS phase at temperatures up to 490°C, with the M₅O₉ structure also formed at temperatures above 540°C. The presence of Mg²⁺ was crucial in determining the particular structure of the CS phase and its thermal stability. These observations provide a useful way to get an oxidation product with a particular structure of the CS phase. They showed that there can be significant effects on the phase transition and thermal stability of the products from the presence of an impurity element.