Yi Zhang, Zuohu Li, Hongbin Xu, Shili Zheng, and Tao Qi. Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, P O Box 353 Sub 26, Beijing, 100190, China
Chromic oxide is an important basic chemical and finds many applications including metallurgical materials, green pigments, construction materials, refractory materials and catalysts. In the traditional production process, the intermediate from chromite ore to chromic oxide includes sodium chromate (Na2CrO4), sodium dichromate (Na2Cr2O7), and chromic anhydride (CrO3). Long technical route directly leads to a high production cost. Furthermore, the environmental problem of the emitted yellow smoke in the calcination of CrO3 to produce Cr2O3 and the discharged chromium-containing sodium sulfate and sodium bisulfate is very serious, which has spawned worldwide public environmental concerns. With a design objective to eliminate pollution at the source, a short and clean production process of chromic oxide has been developed by the Institute of Process Engineering, Chinese Academy of Sciences. The only intermediate product is sodium chromate (Na2CrO4) or potassium chromate (K2CrO4) in the production process of chromic oxide from chromite ore. As hydrogen gas is used as the reduction agent of hexavalent chromium to trivalent chromium, the alkali metal-containing byproduct is only sodium hydroxide (NaOH) or potassium hydroxide (KOH) and the reaction process is rather clean and highly efficient. The produced sodium hydroxide or potassium hydroxide is used as the reaction media from chromite ore to sodium chromate or potassium chromate in the Sub-Molten Salt (SMS) process. Therefore, higher resource utilization efficiency and zero emission of the chromium-containing waste are achieved. By combining XRD, XPS, TG-DSC, FT-IR, ICP-AES and chemical titration, the fundamental research results demonstrate that, the chemical reaction from chromite ore includes four steps: (1) the oxidation and separation of the trivalent chromium in chromite ore to hexavalent chromium through the SMS process (2Cr2O3 + 3O2 + 8MOH = 4M2CrO4 + 4H2O, M = Na or K); (2) the reduction of hexavalent chromium to trivalent chromium (2M2CrO4 + 3H2 = 2MCrO2 + 2MOH + 2H2O); (3) the hydrolysis of alkali metal chromium oxide (MCrO2 + H2O = MOH + CrOOH); and (4) the thermal decomposition of chromic hydroxide (2CrOOH = Cr2O3 + H2O). It was also found that, the first two steps are very easy and quick and the chemical conversion ratio of trivalent and/or hexavalent chromium is up to 99% or higher, the yield of chromic oxide is subject to the conversion ratio of the third step, and the purity and of chromic oxide product is subject to the conversion ratio of the last step. Based on the clean process mentioned above, a demonstration industrial production line with an annual capability of 2000 tons of chromic oxide has been built up and run in China. In the production line, the industrial yield of chromium can be achieved as high as an average of 98.5% and the production cost of chromic oxide can be decreased by 17.1% when compared to the traditional production process. The clean process has exhibited a promising prospect in the industrial production of chromic oxide.