Preparation of chromium oxide by hydrothermal reduction of sodium chromate aqueous solution with hydrogen and application to chromite ore processing

Abstract

Chromium oxide (Cr2O3) has wide application in various industries. Its traditional production process, uses sodium carbonate (Na2CO3), which is converted into cheaper byproducts, including sodium sulfate (Na2SO4) and sodium bisulfate (NaHSO4). These byproducts contain hexavalent chromium (Cr(VI)), which can lead to serious environmental pollution. In this work, a cleaner production process for Cr2O3 through the hydrothermal reduction of a sodium chromate (Na2CrO4) aqueous solution with hydrogen (H2) was studied. Nearly single-phase chromium oxide hydroxide (CrOOH) formed as a solid product after the hydrothermal reduction. Then, Cr2O3 was obtained by the thermal decomposition of CrOOH. The advantages of the new process are remarkable. First, the raw material, the Na2CrO4 aqueous solution, was the direct intermediate product in the pressure oxidative leaching of chromite ore with sodium hydroxide (NaOH). Second, the reduction efficiency of Cr(VI) increased up to 97.6%, and the total yield of chromium from Na2CrO4 to Cr2O3 was as high as 94.2%. Finally, 96.6% of sodium could be converted into NaOH, so the aqueous solution in the hydrothermal hydrogen reduction process could be recycled as the leaching agent for the pressure oxidative leaching process of chromite ore. Consequently, a complete cleaner and shorter preparation route from chromite ore to Cr2O3 was achieved by integrating three processes: the pressure oxidative leaching of chromite ore, the hydrothermal reduction of the Na2CrO4 aqueous solution, and the thermal decomposition of CrOOH. The new method achieves the high-efficiency use of chromium resources, cyclic utilization of material, and extremely low environmental pollution of Cr(VI), and well exemplifies the principle of the 3Rs (Reduce, Recycle, Reuse).