| Chromite ore processing residue(COPR) is one of the most dangerous wastes and it has caused serious environmental pollution and damage for its higher toxicity. Many studies on the innocuous treatment and resource utilization of COPR have been made. It is important to choose an economical way to leach the Cr(VI) in insoluble compounds of COPR effectively in the research field of wet process of COPR. We analysed the thermodynamics of treating COPR in Na2CO3-CO2-H2O system from the view of alkaline process detoxification of COPR and studied the practical experimental laws based on the theoretical analysis. Finally we evaluated the detoxification effect of COPR using toxicity characteristic leaching procedure(TCLP).The thermodynamic analysis of treating COPR in Na2CO3-CO2-H2O system showed that Cr(VI)-bearing minerals in COPR such as calcium chromate, hydrogarnet, hydrocalumite and brownmillerite are unstable and may transform into stable calcium carbonate when pH value is in the range from 6.6 to 10.8 and the concentration of CO32- is above 1.55×10-6 mol/L at 75℃. Moreover, lowering temperature favors to decompose these minerals from the point of view of thermodynamics. However, hydrotalcite present in COPR can not be decomposed by CO32-. When pH value is higher than 12 at 75℃in the system, hydrotalcite can be decomposed into Mg(OH)2.The experimental study on alkaline process detoxification of COPR was carried on based on the thermodynamic analysis. It can be divided into two parts:treat COPR with Na2CO3-CO2-H2O system directly and treat oxidizing roasted COPR with Na2CO3-CO2-H2O system.Through the experimental study on treating COPR with Na2CO3-CO2-H2O system directly, the effects of kinds of leaching reagents, reaction temperature, concentration of sodium carbonate and reaction time on the Cr(VI) concentrations of treated residues and the Cr(VI) leaching rates of COPR is clear. Besides, the Cr(VI) concentration of treated residue reduced to 0.13% and the Cr(Ⅵ) leaching rate of COPR reached 85% under the optimum experimental conditions.Experiments of oxidizing roasting COPR showed that conditions such as dosage of sodium carbonate, roasting temperature, time and patterns have important influence on the Cr(Ⅵ) oxidation rates of clinkers and the oxidation rate can reach up to 98%. It had two ways to treat clinkers obtained by COPR roasting with Na2CO3-CO2-H2O system. One was to treat clinkers with Na2CO3-H2O and the effects of concentration of sodium carbonate, liquid-solid ratio, temperature, time and particle size on the leaching of clinkers had been studied. Thus, the Cr(Ⅵ) concentration of treated residue reduced to 0.20% and the Cr(Ⅵ) leaching rate of clinker reached 95.8% under the optimum conditions. The other was to treat clinkers with CO2-H2O system and the effects of temperature, aeration rate, time, liquid-solid ratio, stirring rate and dosage of sulfuric acid on the leaching of clinkers had been studied. Thus, the Cr(Ⅵ) concentration of treated residue reduced to 0.20% and the Cr(Ⅵ) leaching rate of clinker reached 95.6% under the optimum conditions.The TCLP results of COPR treated with Na2CO3-CO2-H2O system directly were not perfect. But if the treated COPR was leached secondly with the same system after wet grinding process or if the treated COPR was heap leached with oxalic acid or sodium oxalate, the treated COPR could meet the standards about landfilling as general industrial solid wastes in HJ/T301-2007. |
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