Cleaner production of basic chromium sulfate – with a review of sustainable green production options

Add time:07/19/2019    Source:sciencedirect.com

Ferrochromium and chromium based compounds are produced worldwide from chromite ore. Chrome chemicals are from chromium based compounds. Basic-chromium-sulfate (BCS) is a chrome chemical mostly required as main tanning agent in chrome tanning process in leather industry and also to synthesize other chromic compounds. During the preparation of basic-chromium- sulfate, sodium dichromate containing chromium (VI) is reduced to chromium (III) by sulfur-di-oxide in a packed tower at a chemical process industry (B&M Chemicals, India) where the BCS is produced at suboptimal efficiency due to inefficient (suboptimal) absorption of sulfur-di-oxide (as the gas enters the tower at a higher temperature) followed by reaction. Improper reduction leads to the presence of unconverted chromium (VI) that imparts toxicity, not only through product basic-chromium- sulfate but also through leather and articles produced using this chemical. Design and operating conditions of the packed bed absorber were found and effects of pH, temperature and concentration of SO2 on the performance of absorption were studied. The reaction is favored at pH 4. The overall efficiency of conversion in the tower is increased from 70% to 80% by operating the column at optimal operating conditions obtained using response surface method. Dispersion is achieved by random packing that provides complex countercurrent flow path resulting in greater probability of entraining gas inside the tower. Profiles of basic chromium sulfate are obtained along the height of the tower by solving mathematical models of the reactive absorption process. Chromium (VI) is 99% reduced to chromium (III) in the product. Leached residues and liquid wastes are treated with either ferrous sulfate or sodium sulfide to convert chromium (VI) to chromium (III) to provide sustainable technology. Treated solid waste is stored before safe disposal in hazardous landfill facility. As chromium (VI) is highly toxic, several sustainable methods are suggested to eliminate it from effluent to provide cleaner and sustainable production environment in the industry.

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