THE EFFECT OF PHASE DECOMPOSITION IN QUATERNARY CHROMITE SPINEL ON THE EXTRACTION OF CHROMIUM AS SODIUM CHROMATE DURING OXIDATIVE ALKALI ROASTING

摘要

Natural chromite mineral is a solid solution of spinel end members namely: chromite (FeCr_2O_4), hercynite (FeAl_2O_4), magnetite (Fe_3O_4), magnesio-chromite (MgCr_2O_4), (MgAl_2O_4), and magnesio-ferrite (MgFe_2O_4). A phase transformation in chromite minerals occurs in oxidizing conditions during the soda-ash roasting reaction. This phase transformation plays an important role in improving the extraction efficiency of Cr~(3+) from mineral phase. The extraction efficiency of chromium (Cr~(3+)) from chromite to a water-soluble sodium chromate (Cr~(6+)) salt depends upon the decomposition kinetics of the spinel phase. An investigation of the phase changes in chromite ores in the absence of sodium carbonate was carried out in air, argon, and in argon +5 percent hydrogen gas mixture. The thermodynamics of phase transformation reaction in chromite solid solution phase is discussed using the solution model proposed by Sack and Ghiorso~(1). The microstructural changes and the changes in elemental compositions of different phases were examined by SEM and electron probe microanalysis (EPMA) respectively. The experimental results indicate that the solid solution spinel phase in chromite mineral undergoes spinodal decomposition under the influence of oxygen partial pressure and temperatures. The structure of phases formed and their morphologies suggest that the phase decomposition may be similar to those observed by Burkert et. al.~(2), although the scale of phase separation in chromite appears to be strongly dependent on oxygen partial pressure.