Mineralogical Framework And Chemical Treatment Methods Of Chromite Ore Processing Residue

Chromate is widely applied in steel, textile, leather and other manufacturing industries. In chromate making process, chromite ore processing residue (COPR) would be made as by-product. Hexavalent chromium (Cr(VI)) in COPR could cause disease, so much as cancer in human organs. As the rapid development of industry in our country, nearly one million tons of COPR are produced every year. Considering the COPR produced over the years, natural environment and human health at various locations in China are seriously threatened. Therefore, it is very important in theory research field and engineering practice to systematically study the characteristics and appropriate treatment methods of the COPR produced in our country.Three aspects of the research contents of this thesis are:1) applying advanced techniques such as X-ray fluorescence (XRF), X-ray diffraction technology (XRD) and environmental scanning electron microscopy (ESEM) to systematically analyse COPR, and consequently developing the mineralogical framework for China-based COPR; 2) studying the effects of acid solutions (HNO3 or H2SO4) on the leach of Cr(VI) from COPR by batch tests respect to reaction time and COPR particle size, and laying the foundation of the subsequent research for chemical treatment methods; 3) using chemical reduction methods to study the effectiveness of treatment methods with ferrous sulfate (FeSO4) or a cationic polysulfide reagent (CaSx) as reducing agents.The main achievements of this thesis are as follows:(1) The mineralogical framework for China-based COPR was developed. Elemental composition, mineral composition and surface feature of COPR were identified, respectively. The result shows that the dominant elements of COPR are Ca, Fe, Mg, Al, Si and Cr by descending order of the content, which are evenly distributed throughout of COPR particles. The weight percentage of chromium is around 6%. COPR particle consists of brownmillerite, periclase, calcite, dolomite, hydroandradite, chromite and quartz. Hydrocalumite and larnite are possibly contained in COPR. Cr(â…¥) possibly exists in hydroandradite which presents inside COPR particle. Cr(â…¢)-bearing brownmillerite occurs in the rim of COPR particles. As a whole, a low content of Cr evenly distributes in COPR particles, while slightly high concentration of Cr occurs inside particles.(2) The leaching behavior of Cr(â…¥) from COPR was investigated by batch leaching tests. According to the test results, aqueous Cr(â…¥) accounted for most of the content of total Cr in leachate. It was difficult for coarse-sized particles in this study completely reacting with acid. Comparing with acid type used, the particle size may have a more significant impact on the release of Cr(â…¥) from COPR. It was easier to lower the pH value in leachate and release Cr(â…¥) from COPR with H2SO4 relative to HNO3. In addition, the leach of Cr(â…¥) from COPR was persistent. The acid neutralization capacity of COPR was relatively large, which suggests that the application of acid treatment method for COPR disposal may be costly.(3) Chemical reduction treatment methods of COPR were studied systematically. When COPR was permeated with FeSO4-H2SO4 solution, Cr(â…¥) concentration in leachate could be controlled to a low value in a relatively short time. However, the subsequently blocking remanent COPR prevented further reaction with FeSO4. In the whole column experiment process, Cr(â…¥) concentrations could not be detected in leachates from COPR treated with CaSx and permeated with DI water. Complemented with the results of XRD and ESEM, the treatment with CaSx and permeated with DI water is more effective compared with the treatment using FeSO4-H2SO4 solution.This thesis systematically developed a mineralogical framework for China-based COPR, investigated leaching characteristics of Cr(â…¥) and evaluated the effectiveness of chemical reduction treatments of COPR. They lay the foundation for future large-scale treatment of COPR.