Na_2CO_3-CO_2-H_2O体系处理铬渣的热力学分析

Thermodynamic analysis of chromite ore processing residue treatment in Na_2CO_3-CO_2-H_2O system

采用Na2CO3-CO2-H2O体系处理铬渣,分析该体系的反应热力学,并通过实验研究不同浸出剂对铬渣中Cr(Ⅵ)浸出的影响。研究结果表明:在75℃,当6.6<pH<10.8且c(CO32-)>1.55×10-6 mol/L时,体系处于碳酸钙的稳定区,铬酸钙、水榴石、水铝钙石及钙铁石可被分解而释放出其中的Cr(Ⅵ),而水滑石难以被分解;在温度为80℃,碳酸钠质量浓度为120 g/L,液固比为15,CO2体积分数为5%,时间为24 h的条件下处理铬渣,得到的渣经湿磨后再用此体系二次浸出,最终铬渣中Cr(Ⅵ)的质量分数降至0.13%,Cr(Ⅵ)的浸出率达到85%;毒性实验浸出液中Cr(Ⅵ)和总Cr质量浓度分别为1.21 mg/L和1.51 mg/L,均远低于HJ/T 301—2007中规定的限值,符合一般工业固体废物填埋的标准;含Cr(Ⅵ)的主要物相钙铁石、水铝钙石的质量分数明显降低且没有铬酸钙生成,这与热力学分析结果基本一致。

A new system of Na2CO3-CO2-H20 was employed to leach Cr（VI） in the chromite ore processing residue （COPR）. The thermodynamic analysis was carried out and the effect of various leaching reagents on Cr（VI） leaching was studied. The results show that the system is in the stability region of CaCO3 at 75℃ with pH in 6.6-10.8 and the CO3^2- concentration above 1.55×10^-6 mol/L, and thus CaCrO4, hydrogarnet, hydrocalumite and brownmillerite may be decomposed, resulting in the release of Cr（VI）, while hydrotalcite can hardly be decomposed. The content of Cr（VI） in COPR treated by Na2CO3-CO2-H20 leaching followed by wet-grinding and then leaching, and the leaching efficiency of Cr（VI） in COPR can reach 0.13% and 85% respectively under the optimal conditions of 80℃, liquid-solid ratio of 15, CO2 concentration in the mixture of CO2 and air of 5%, Na2CO3 mass concentration of 120 g/L and leaching time of 24 h. The Cr（VI） and total Cr mass concentrations in the leachate of toxicity characteristic leaching procedure of the COPR treated under the above optimal conditions are 1.21 mg/L and 1.51 mg/L respectively, far below the regulatory limit and meeting the landfill standard for common industrial solid waste in HJ/T 301--2007. The contents of brownmillerite and hydrocalumite regarded as the main Cr（VI）-bearing phases reduce obviously and there are no apparent diffraction peaks of CaCrO4, approximately in agreement with the thermodynamic calculation.