Morphological Changes of Panzhihua Ilmenite During Oxidation Treatment

Morphological changes with the phase transitions were investigated in detail for the oxidation roasting of Panzhihua ilmenite in air from 873 K to 1173 K (600 A degrees C to 900 A degrees C). It was found that a thin hematite layer of 1 to 2 mu m formed rapidly at the initial stage of the oxidation process, independent of the oxidation temperature, on the surface of ilmenite particles, and the thickness of the hematite layer kept nearly constant with increasing time of oxidation. The morphology inside an ilmenite particle, however, changed with the oxidation temperature. Needlelike rutile network enwrapped by hematite grains was observed after oxidation below 1073 K (800 A degrees C), and the structure can be well preserved during the oxidation process, although grain growth of rutile and hematite did occur with the extension of the oxidation time. The morphological changes at 1073 K (800 A degrees C) and higher showed two distinct stages, where in the first stage ilmenite was oxidized to form rutile and hematite with morphology similar to that of oxidation below 1073 K (800 A degrees C). The as-formed rutile and hematite were not stable and recombined to form pseudobrookite in the second stage, which caused significant morphology change; i.e., the needlelike rutile network gradually dissolved to form the final morphology of irregular rutile grains dispersed in the pseudobrookite matrix with some isolated hematite grains. Moreover, the oxidation temperature also has a great effect on the rate of morphological changes, i.e., the increased evolution rate with increasing the oxidation temperature.