Chromium Oxidation Mechanism Of Complex Phase During The Late Stages Of The Chromite Roasting Process

Soda ash roasting of the chromite ore is a clean method for the production of chromium compounds, which was widely being used in the chromate production of our country. However, the problems of slow chromium oxidation rate, low recovery rate, easier kiln-ringing, and high percentage content of chromium in residues, were still not solved in the roasting process. So the further studies on the mechanism of chromite roasting, the reason for chromium difficult oxidation in late stages, and the influences of the complex phase components in the roasting process are crucial to realize clean production processof the chromium compounds.Physi-chemical properties of the intermediates which were produced in the late stages of chromite roasting process and the factors of chromite oxidation were investigated in detail. The difficult oxidation mechanism of chromite in late stages was proposed based on the transformation of chromite spinel structure in the roasting process. Additionally, the reasons of kiln-ringing in the actual production were explored and the ways to solve the problems were put forward in the roasting process. The main results were listed as follows:The roasting intermediates were obtained in the late stages of chromite roasting with soda ash (the conversion yield of chromium about 70%), which main ingredients had the water soluble phase Na2CrO4 and NaA102 and the water insoluble MgCr0.4Fe1.6O4 spinel phase and complex sodium-silicate-aluminate phase such as Mg1.5Na9Si12Al12O48 and NaSiAlO4 phases.In the chromite oxidation process, the accumulation of vice phases such as the sodium-silicate-aluminate phase and the effective concentration of Na+ decreased, lead to the chromium difficult oxidation in late stages, the chromium oxidation rate was only 50%-60% when re-roasted the intermediates; In industrial production, for improving the chromium oxide rate can be added appropriate amount of alkali, increased the appropriate temperature and blast volume in the late stages of roasting process.Utilizing the Na in silicate-aluminate phase indicated that, the CaCO3 was added into the roasting process, and the sodium-silicate-aluminate phase reacted with CaCO3 to generate calcium-silicate-aluminate phase with high melting point, such as Ca2Fe12Mg0.4Sio.4O5. The replacement mechanism between Na and Ca reduced the redundant reaction of Na with Al, Fe and Si to improve the concentration of Na+, but also diluted the amount of liquid phase in the roasting reaction system, to promote the diffusion of oxygen and chromite oxidation. However, the high toxicity CaCrO4 (Cr6+) would be produced with the addition of CaCO3, leading to serious pollution to environment.Na+could promote the transformation of spinel structure in high temperature, and the chromite spinel was deeper decomposed with the higher concentration of Na+. The Cr of chromite reacted with Na to form Na2CrO4, the Cr content was decreased gradually, and the MgCr0.4Fe1.6O4 spinel transformed to MgFe2O4 spinel at last.The micro-analysis indicated that, the lattice fringes in the spinel phase of chromite before and after reaction were regular and clear, and the dislocations were not obviously detected. Therefore, the decomposition mechanism of spinel was speculated by point defects which were generated by atomics thermal vibration in the lattice at high temperature, the formation of defects and the diffusion of the metal ions in reaction process needed greater activated energy to limit the chromium conversion rate of the roasting reaction in the late stages.The phase composition and morphology of "ring" materials generated in the actual production were studied. The main phases of "ring" materials contains more sodium-chromate, sodium-aluminate and aluminum-silicate phase, these phases led to the increase of reactants viscosity and the worse flowability in the rotary kiln. The spinel phase was embedded by the part of aluminum silicate phases, resulting in harder separation of main phases and minor phases. The separated main phases with high melting point which was re-added into the furnace with further deteriorate the roasting circumstance, increasing the risk of kiln-ring. The further treatment of main phases was proposed to resolve the above problems. The results showed the main phase was exposed in the air with aging to benefit aluminum silicate phase decomposition, and resulting in reduction of the kiln-ringing phenomenon.