Applied Fundamental Research on Crystallization and Separation of Na2CrO4in the Cleaner Production of Chromium Compounds Via Aqueous Oxidation Method
|School||Hebei University of Science and Technology|
|Course||Chemical Engineering and Technology|
|Keywords||Sodium chromate Sodium aluminate Phase equilibrium Thermodynamics Kinetics Evaporative crystallization|
The chromate salts and chromate compounds are important basic chemical rawmaterials. In order to resolve the existing problem including low resource utilization andserious environmental pollution in the traditional soda ash roasting process, the Institute ofProcess Engineering, Chinese Academy of Sciences has developed a cleaner productionprocess based on aqueous oxidation in sub-molten salt medium with independentintellectual property rights in China. The crystallization and phase separation of Na2CrO4is an important part of this process as the purity and morphology of Na2CrO4intermediatewill directly affect the quality and cost of the downstream products. In this thesis, somebasic research including phase equilibrium, metastable zone, kinetics and processoptimization of Na2CrO4crystallization separation process in theNa2CrO4-NaOH-NaAlO2-H2O system were studied systematically. The followinginnovative achievements and progresses were exhibited:1) The phase equilibriums for the system Na2CrO4-NaOH-H2O andNa2CrO4-NaOH-NaAlO2-H2O at (90,120and130)℃were determined. The phasediagrams for Na2CrO4-NaOH-H2O system at90℃and Na2CrO4-NaOH-NaAlO2-H2Osystem at (120and130)℃were plotted. The study will provide some basic data for thecrystallization separation;2) The metastable zone and the effects of impurity content on the metastable zonewidth were studied. Moreover, the relationship between the boiling point and vaporpressure in the vacuum evaporation process with different NaOH concentrations was alsodiscussed, under the same concentration system, the greater the vacuum degree, the lowerthe boiling point, with the concentration increasing, the boiling point will rise, the vacuumdegreeis required to increase;3) The intermittent dynamic method was adopted to study the crystallization kineticsof sodium chromate. The nucleation and growth rate equation of sodium chromate havebeen determined by the least square method and the effect of suspension density, stirringrate, supersaturation and impurity on this process were discussed. As the suspensiondensity, stirring rate, supersaturation and impurity increases, nucleation occurs moreeasily;4) The vacuum evaporation crystallization process was optimized experimentally.The effects of some parameters, such as agitation speed, evaporation rate, crystallization temperature, impurity content, seeds added on main size distribution including CSD,purity and crystal habit were analyzed and discussed systematically. And the betteroperation conditions of crystallization were determined as the stirring rate of300r/min,the evaporation rate of about150~200mL/h, the crystallization temperature of90℃, theimpurity content of less than3g/L, adding seeds. Under the above operating conditions,sodium chromate crystal product with uniform particle size distribution and purity as highas97.5%was prepared.