Study On Conversion Of Potassium Chromate Into Sodium Dichromate In The Cleaner Process Of Chromium Compounds

As a crossing industry of the inorganic chemical industry, metallurgical industry, and materials industry, chromate manufacturing industry is also one of the most significant industries in China’s national economy. However, due to the currently used high-temperature gas-solid roasting technology in most chromate manufacturing plants in China, the poorly low conversion efficiency of chromite resources, the remarkably high energy consumption, and the huge amount of the chromium-containing residue produced in the process has resulted in serious environmental pollution and social problems. The green technical upgrading becomes more and more urgent in China’s chromate manufacturing industry. The Sub-molten Salt (SMS) process for clean manufacturing chromium compounds, which was developed by the Institute of Process Engineering, Chinese Academy of Science, has greatly improved the resource conversion efficiency, remarkably decreased the reaction temperature as well as the energy consumption, and sharply reduced the amount of chromium-containing residue from the source so as to achieve the comprehensive utilization and near zero emission of the hazardous residue. A demonstration plant for potassium-series chromates with the SMS process has been built and in operation. To develop the sodium-series chromate products, thereby satisfying the international and domestic market demands, is an important topic for the technical research on the SMS process. In this paper, a preparation process of sodium dichromate with barium salt as the conversion media and potassium chromate as the raw material was introduced and a perfect linking with the main technical units of the SMS process was realized.The following innovative progresses were achieved in this work:(1) The macro kinetics for the preparation of sodium dichromate from barium chromate and sodium hydrogen sulfate was studied. The effects of stirring rate, H+concentration and reaction temperature on barium chromate conversion rate were discussed. The control step of the reaction was determined as the chemical reaction process with a first reaction order. The parameters of kinetics and the kinetics equation were obtained.(2) The preparation process of sodium dichromate with barium salt conversion method using sodium hydrogen sulfate or sodium sulfate and sulphuric acid as sodium source and acidification medium, respectively, was investigated. The Response Surface Methodology was employed to optimize the preparation process of sodium dichromate with barium salt as the conversion intermediate. The optimized process parameters were determined as the reaction temperature of80℃, the reaction time of400min, and the ratio of liquid to solid of9:1in mL/g. Under this optimized condition, the experimental conversion rate was95%, very close to the predicted value. (3)The cooling crystallization kinetics of sodium dichromate was investigated. The effect of suspension density, stirring rate and supersaturation on this process was discussed. The nucleation and growth rate equation of sodium dichromate have been determined by the least square method, providing a good foundation for further technical research and industrial scale up.(4) The cooling crystallization and evaporation crystallization processes of sodium dichromate were studied. In the cooling crystallization process, the effects of cooling temperature, cooling rate,stirring rate on the crystal habit, main size distribution and CSD were systematically investigated. In the evaporation crystallization process, the effects of stirring rate and evaporating rate on the crystal habit, main size distribution and CSD were systematically investigated. And the experimental optimum operational conditions were finally obtained.