Study Of Remediation Of Chromite Ore Processing Residue And Clean Preparation Technology Of Chromium Oxide

Chromates, as one of the main products of inorganic chemicals, are widely used in many aspects such as electroplating, tannage, printing and dyeing, medicine, paint, catalyst, oxidant and metal inhibitor, playing a very important role in the national economy. However, the chromate industry produces a large amount of solid hazardous wastes mainly including chromite ore processing residue (COPR) and sulphates containing hexavalent chromium (Cr(Ⅵ)), which are the first and second hazard in the process, respectively.Chromates production is beginned with the roasting process of chromite ore. There are two main processes for extracting chromate from chromite ore. One is the high lime roasting process characterised by large amount of COPR produced, and the other is the non-lime roasting process with less amount of COPR. The trends of chromates production is the non-lime roasting process in the future in China, but there is plenty of COPR to be treated in the past several decades.There are also two processes for producing chromium oxide (Cr2O3): one is the pyrolysis of sodium dichromate and ammonium sulphate, and the other is pyrolysis of chromic anhydride (CrO3). Both processes have the problem of high consumption of sulphuric acid and produce hazardous material of suphates containing Cr(VI).So this dissertation mainly focused on the theory and technology of COPR remediation with starch by hydrothermal process, with tannage sludge by pyrolysis process and with wastewater containing hydrofluoric acid. The theory and technology of clean production of Cr2O3was also investigated with starch by hydrothermal process and dry reduction process. The main argumentations and original conclusions were made as follows:The formation of acicular ettringite bearing Cr(Ⅵ) was the main reason of incomplete remediation of COPR with sulphur-containing reductants in aqueous solution at the ambient temperature. The thermodynamic calculation showed that increasing temperature could effectively suppress ettringite formation, which was further verified by experimental results, subsequently. The COPR treated with starch by hydrothermal process could meet the standard of entering the landfill yard of household garbage under the optimized conditions.Carbon monoxide and hydrocarbon organic compounds released from the decomposition of grease and protein of tannage sludge could reduce Cr(Ⅵ) in COPR to trivalent chromium (Cr(Ⅲ)) for the remediation of COPR at temperature of over400℃. And COPR treated with tannage sludge by pyrolysis process also met the standard of entering the landfill yard of household garbage.Thermodynamic calculation and kinetic study of the leaching of COPR with hydrofluoric acid as well as the remediation technology of COPR with wastewater containing hydrofluoric acid were investigated in this work. Results showed that hydrofluoric acid and wastewater containing hydrofluoric acid could also destroy the structures of minerals containing Cr(Ⅵ), leading to release Cr(Ⅵ) in COPR. And the leaching kinetics of acidic Cr(Ⅵ) from COPR followed the shrinking unreacted core model and was controlled by inner diffusion. Results also showed that the remediation effect of column leaching was very close to that of the pond leaching. The COPR treated with wastewater containing hydrofluoric acid for30days met the standard of entering the landfill yard of general industrial wastes. And the COPR treated with wastewater containing hydrofluoric acid for60days by column leaching, further column leached by ferrous sulphate solution for1day, could meet the standard of entering the landfill yard of household garbage. In addition, the Cr(Ⅵ) content of the leachate treated with ferrous sulphate solution and calcium hydroxide reached the national sewage discharge standard.The kinetics and technology of preparation of Cr2O3by hydrothermal process of Cr(Ⅵ) with starch as the reductant were also conducted. Results showed that the kinetics could be described by the kinetic model of isothermal irreversible reaction with constant volume, and the hydrothermal reduction was controlled by the chemical reaction. Homogeneously distributed Cr2O3powder with particle size of660-880nm was obtained with Cr(Ⅵ) reduction rate of more than98%by hydrothermal process. Furthermore, ultra-fine near-spherical or spherical Cr2O3powder with uniform size of110-130nm could be prepared by the application of surfactant in the hydrothermal process.The thermodynamics and technology of preparation of Cr2O3by dry-process of Cr(Ⅵ) with starch were carried out. Results indicated that the reduction product varied with the kind of chromates and the reduction temperature. The reduction product of sodium chromate was sodium chromate(Ⅲ)(NaCrO2), while the reduction product of sodium dichromate depended on the temperature. Cr2O3was obtained with the reduction temperature of lower than550℃, and Cr2O3gradually tranformed to NaCrO2with increasing the reduction temperature. Irregular Cr2O3powder with the particle size of418-438nm was produced with the Cr(Ⅵ) reduction rate of above98%under the optimal conditions.