Research Of Intensified Oxidative Roasting Process Of Chromite Ore

Calcium-free oxidative roasting process is the technological direction of the production of chromate salts extracted from the chromite ore home and abroad. However, more than 95% of industrial chromates in China was produced by the traditional soda-lime oxidative roasting process, and minor factories employing the calcium-free oxidative roasting process also encountered many problems in the actual production process, such as slow oxidation rate of Cr(Ⅲ) (-2h), low oxidation ratio (<76%) and prone to ring-forming phenomena. To intensify the oxidative roasting process, the oxidation mechanism was investigated in this paper, and a kind of additive was selected to promote the roasting process based on the oxidation mechanism. In addition, the optimal reaction conditions were determined by series of experiments.The separate influence of the impurities of ferrous oxide and aluminum oxide on the oxidation rate of trivalent chromium and its mechanism were systematically studied using the spinal compounds synthesized in the laboratory as the starting materials in the calcium-free oxidative roasting process of chromite ore in this paper. The results show that the impurity of ferrous oxide has almost no effect on the oxidation rate of the trivalent chromium. Ferrous oxide is first oxidized to form ferric oxide, and NaFeO2 formed by the preferential reaction of ferric oxide and sodium carbonate, can further act as alkali and thus maintain a relatively rapid oxidation rate of trivalent chromium during the whole roasting process of chromite ore. The results show that the impurity of aluminum oxide obviously hinders the oxidation of trivalent chromium. This is mainly attributed to the formation of the complicated insoluble compound of MgO·(Cr203)0.5·(Al203)0.5. In addition, aluminum oxide readily reacts with sodium carbonate to form sodium aluminate which is difficult to further react with MgO·(Cr203)0.5·(Al2O3)0.5 to form Na2CrO4, resulting in the decrease in the oxidaton rate of trivalent chromium. And the results also show that the impurity of silicon oxide obviously hinders the oxidation of trivalent chromium. The main reason may be that the oxidation reaction of Na2SiO3 with Cr2O3 with the temperature higher than 1173K is reversible and thus Na2CrO4 generated would be decomposed, lowing the oxidation rate of Cr(III).Systematic oxidative roasting experiments were carried out based on the oxidation mechanism of calcium-free oxidative roasting process. The results show that, the optimized conditions are as following:the amount of alkali is 110%～120% of the theoretical value of alkali, the roasting temperature is around 1323K and the roasting time varies from 30～60min. Adding substance A to the furnace charge can greatly increase the chromium oxidation rate, the main reason is that additive A can obviously reduce the quantity of the liquid phase formed in the oxidative roasting process of chromite ore, promoting the mass-transfer process of oxygen. In the temperature range of 1223K to 1323K, with the increasing of temperature, the liquid quantity is almost unchanged facilitating the oxygen diffusion in the roasting system, and additive A has obvious positive effect on oxidation reaction of Cr(Ⅲ) with the oxidation rate of Ci(Ⅲ) of more than 98%.