| Chromium chemicals extracted from chromite ore are indispensable raw materials in the national economy and defence. However, the manufacturing process of these chromium chemicals causes serious pollution which restricts the sustainable development of chromium salt industry. So it is of great importance to develop clean prodction technology to solve the pollution in the chromate production process. The lime-free roasting process is gradually becoming the most popular production method for chromium chemicals in china, but the problems of slow chromium oxidation and low recovery rate, as well as the difficult of comprehensive utilization of chromium-containing residues, are still not solved. So the further studies on the influencing law among the components in oxidative roasting process and the oxidative reaction mechanism are crucial to enhance the oxidation roasting reaction and achieve clean production process.The reaction behaviors of chromium, iron and aluminum components in soda-ash roasting process were systematically investigated based on the thermodynamic calculation and kinetic analyses. Additionally, the influence of these reaction rules on the chromium oxidation was assessed in this paper. The chromium oxidation reaction mechanism was proposed based on the results of chromic spinel oxidative kinetic and the structure changing in roasting process. Then the pellet roasting technology without recycled residue was investigated to improve the chromate production efficiency. The iron enrichment and separation rules in chromite ore processing residue were also studied. Finally, an economical and effective method for comprehensive utilization of the residue was proposed. The main results and progresses were listed as follows:1) Thermodynamic calculation of reactions involved in the chromite ore oxidation roasting process shows that sodium carbonate not only reacts with chromium component in chromite ore to form sodium chromate but also reacts with other components to form various compounds containing sodium. The binary compounds of Na2O·Fe2O3, Na2O·Al2O3and Na2O·SiO2, as well as the multiple compounds of Na3MgAlSi2O8and Na2O·Al2O3·2SiO2, could be formed under the conventional roasting conditions. These binary components containing sodium are not stable and can further react with chromium components to form sodium chromate. In contrast, the multiple compounds mentioned above are more stable than sodium chromate and thus they couldn’t be transformed into sodium chromate. The magnesium component in chromite ore can transform into MgO·Fe2O3, MgO·Al2O3, MgO·SiO2and2MgO·Si02, but these compounds containing magnesium can further be decomposed by sodium carbonate to form magnesium oxide and sodium salts of ferrate, aluminate and silicate.2) The reaction kinetics of components containing iron in chromite oxidation roasting system was studied. The results demonstrated that the components containing iron such as MgO·Fe2O3and Fe2O3could react rapidly with Na2CO3to from Na2O·Fe2O3and all the reaction kinetic equations corresponded with diffusion controlling model for solid state reactions. Na2O·Fe2O3could react with pure chromium-bearing spinels which made the Na2O·Fe2O3transform into Na2Cr04completely, but the chromium oxidation rate in this process is more slowly than that between Na2CO3and chromium spinels under the same conditions. The iron atom could diffuse into chromium-bearing spinels and formed an iron-magnesium-chromium special MgFexCr(2-x)O4in oxidation roasting process, significantly effecting the chromium oxidation kinetics. The kinetic rate limiting step of chromium oxidizing reaction of Na2O·Fe2O3and MgFexCr(2-x)O4was the diffusion of Cr3+in chromium-bearing spinel. This reaction process has a high activation energy and low chromium oxidation rate.3) The magnesia-alumina spinel could react rapidly with Na2CO3to from Na2O Al2O3and the reaction kinetic equations corresponded with diffusion controlling model for solid state reactions. Na2O·Al2O3could react with part of chromium-bearing spinels under oxidizing atmosphere, but the Na2O·Al2O3could not transform into Na2Cr04completely. The composition and structure of chromium-bearing spinel impacted the reaction of Na2O·Al2O3and these spinels significantly. When Na2O·Al2O3reacted with MgO·Cr2O3in oxidizing atmosphere, Na2CrO4could be formed, while some aluminum atom diffused into spinel structure and formed an aluminum-magnesium-chromium spinel Mg(CrAl)O4. The thermodynamic calculation showed that Mg(CrAl)O4could react with Na2CO3, Na2O·Fe2O3and Na2O Al2O3, by which the chromium in spinel of Mg(CrAl)O4could be oxidized and transformed into Na2Cr04. However, the results of kinetic study showed that the reaction rate of Al2O3and Mg(CrAl)O4was extremely slow, which made the oxidizing reaction of chromium in Mg(CrAl)O4hardly proceed under the oxidizing roasting conditions. The components of MgO, MgO Fe2O3and Fe2O3in roasting system could accelerate the reaction rate of Na2O·Al2O3and MgO·Cr2O3, while they had no effect on the reaction of Na2O·Al2O3and Mg(CrAl)O4.4) The results of chromite ore oxidizing kinetics indicated that it could be divided into two stages in the chromium oxidizing process. The reaction rate of chromium oxidation in the initial stage is rapidly, while the rate in the later stage is slowly. Further analyses illustrated that the chromite ore was decomposed by Na2CO3and most of chromium was oxidized and then transformed into Na2Cr04, while a part of Na2CO3reacted with iron and aluminum containing components forming Na2O·Fe2O3and Na2O Al2O3respectively in the initial stage. When Na2CO3was completely consumed, the reaction of chromium-bearing spinel in chromite ore and Na2O Al2O3or Na2O·Fe2O3formed in the initial stage became the dominating reactions for chromium oxidation and the later stage began. The kinetics rules indicated that the formation of Na2O·Al2O3and the Mg(CrAl)O4spinel structure present in chromite ore was the main reason of low chromium oxidation rate. The formation of Na2O·Fe2O3had no effect on the final oxidation ratio of chromium but it was the main cause of low chromium oxidation rate in the later stage.5) The method of chromite ore oxidation roasting with pellet burden material was investigated. The conclusion obtained by the experimental results could be made as follows:the disadvantage effects of the liquid phase in the roasting process could be avoided and the soda ash roasting of chromite ore without recycled chromium-containing residue could be realized by using the pellet burden material. The pellet formed by pulverized chromite ore and soda ash, could be successfully roasted in the laboratory rotary kiln in the roasting temperature range of1000℃to1100℃and the chromium oxidation rate reached above98%.6) The properties of chromite ore processing residue produced from lime-free roasting process were investigated, and then the efficient and economical method for iron separation and recovery was studied. The results showed that the iron-bearing minerals were dispersed among the residue as a manner of superfine grains or scattered into the spinels structure in isomorphism, resulting in the difficulty of the direct magnetic separation. By employing the process of carbon reduction sintering followed by magnetic separation, the iron could be separated. The results showed that the reduction temperature and chromium content in the residue had a great influence on the iron separation and recovery. The optimized parameters for the iron reduction and separation were as following:the reduction temperature in the range of1100℃to1300℃, reduction time was90-120min, the amount of carbon added was about1.2times of stoichiometric requirement. The metallic iron particles generated under the above reduction conditions were coarse and could be readily magnetic seperated from the residue. Total iron grade of magnetic concentrate and iron recovery could reach up to80%and95%, respectively. |
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