Conversion Of Potassium Chromate Intermediates Into Sodium Chromate In The Cleaner Production Of Chromium Compounds With Sub-Molten Salt Medium

Chromium is an important national strategic resource and chromate series products are important basic raw materials in chemical industries, light industries and advanced alloy materials manufacturing industries. The low resource conversion efficiency and serious environmental problems in the currently-used traditional production technology of chromate salts in china has spawned wide public concerns. The Institute of Process Engineering, Chinese Academy of Sciences has recently developed a new cleaner manufacturing process for chromium compounds. Based on the sub-molten salt techniques,the cleaner process significantly decreases the reaction temperature and energy consumption,remarkably improves the utilization efficiency of the main element chromium, and successfully achieves the comprehensive utilization of chromite resources and the zero emission of chromium-containing residue.The sub-molten salt cleaner manufacturing techniques use potassium hydroxide as the reaction medium. In relatively low temperatures, the intermediate produced is potassium chromate which is then used for manufacturing downstream chromate series products, such as potassium dichromate and chrome oxide. To achieve the conversion of chromate products from potassium salts to sodium salts, the clean manufacturing process of sodium chromate was systematically investigated with potassium chromate as the raw materials, sodium nitride as the conversion medium, and conventional crystallization as the conversion means. The topic of this thesis has strong application background and certain scientific importance.The following innovative progresses were mainly achieved:(1) Through investigating the solubility change trend of potassium nitrate and Na2CrO4 with the temperature in mixture respectively, cooling crystallization method is chosen to separate KNO3 from the mixture solution and evaporation crystallization method is chosen to separate Na2CrO4 from the mixture solution.(2) The influence of operation parameters on the separation efficiency of potassium nitrate in the cooling crystallization process was investigated. The optimal operating conditions of cooling crystallization process were obtained. Based on the analysis of the sodium chromate crystal composition in the evaporation crystallization process, the changing tendency of the purity of the Na₂CrO₄ crystals was found, and the separation and purification method of the sodium chromate crude crystal was proposed.(3) The industrial production cycle of Na₂CrO₄ was simulated. It was found that, after several cycles, the yield of potassium nitrate is about 66%, and the yield of Na₂CrO₄ is about 10%, providing a basis for the further optimization of the conversion process.(4) The refining methods of the Na₂CrO₄ crude crystal were proposed. The experimental results demonstrated that, the purity of sodium chromate crystal can be improved from 78.3% to 94% after washing with fresh saturated solution of sodium chromate, and the purity of sodium chromate crystal can be improved from 81.4% to 92% after the dissolution and recrystallization process. The refined sodium chromate products will meet the requirements in the industrial process.(5) Through studying the method of ion exchange to separate potassium ion in the solution of Na₂CrO₄, the suitable ion exchange resin was selected and the dynamic ion exchange experiments were carried out.