Research On The Preparation Of Potassium Dichromate By Potassium Chromate Purification-Electrolysis

The chemical industry gradually realized that cleaner production is necessary for the future development of enterprises, with the promotion of cleaner production concept and the government’s increasing emphasis on environmental issues. In recent years,electrochemical synthesis has made great progress in the industrial production as a green production technology. The purification of potassium chromate solution, electrochemical synthesis of potassium dichromate and crystallization process of potassium dichromate electrolyte were investigated in this paper.Removal of Ca2+, Mg2+, Al3+ and Fe3+ ions from potassium chromate solution were studied by fractional precipitation. The effects of K2CO3 dosage, pH, reaction temperature,reaction time, aging time and amount of coagulant were systematically studied on the process of potassium chromate purification. Results showed that the optimum conditions of potassium chromate electrolyte purification were the dose of K2CO3 10g/L, pH=11,amount of diatomite adding 0.5g/L, 90 ℃ reaction 4 hours, aging 4 hours under room temperature in the first step, and adjusting the pH=8, diatomite dosage 0.2g/L, 90 ℃reaction 2 hours, aging 2 hours under room temperature in the second step. Under the optimum conditions the concentration of Ca2+, Mg2+ and Fe3+ was less than 4mg/L, and the concentration of Al3+ was less than 5mg/L.Meanwhile, the preparation of potassium dichromate was investigated by electrolysis with asbestos diaphragm and cation-exchange membrane. According to the experimental results, although the asbestos diaphragm was low requirement for the concentration of impurity ions in the electrolyte, but the conversion rate and current efficiency were quite low. The asbestos diaphragm was not suitable for the preparation of potassium dichromate by electrolysis, due to the permeability of asbestos diaphragm. The optimum conditions with D-5 cation-exchange membrane were current density 0.2A/cm2, electrolysis temperature 80 ℃, KOH concentration 50g/L and K2CrO4 concentration 400g/L. The conversion rate, current efficiency and DC power consumption were 96.5%, 95.2% and728.5kW?h?t-1 under the optimum conditions. Furthermore, the effect of impurity ions concentrations on the voltage and current efficiency were studied by continuouselectrolysis.Finally, the crystallization of potassium dichromate solution was researched, including the effect of conversion on the crystalline purity in the electrolysis process and the impact of crystallization conditions on the crystal size distribution and morphology. The conversion rate of potassium dichromate was greater than 90% and the purity of potassium dichromate crystal could satisfy the superior product requirement of GB28657-2012 under the optimum conditions of initial concentration of potassium dichromate 500g/L, stirring speed 300r/min and cooling rate 0.5℃/min.