Study On Evaporation Crystallization Process Of Sodium Molybdate

Sodium molybdate is an important metal salt compound.Molybdenum is a kind of trace element necessary for human body,animal and plant,and low toxicity and low pollution of sodium molybdate,which makes sodium molybdate more widely used in corrosion inhibition,agents,agriculture,catalysts,dyes,enamel and other fields.In view of the backward production mode for sodium molybdate in the domestic,the low purity,the flake-shaped crystal,the small particles and the wide size distribution,the crystallization process of sodium molybdate was systematically studied and optimized in this paper.The specific research contents are as follows:The law of crystallization thermodynamics determines the choice of the crystallization process and the yield crystallization process.In this paper,the solubility data of sodium molybdate dihydrate in pure water was firstly determined at the temperature range of 20-80℃ by static equilibrium method.And the density and boiling point of saturated sodium molybdate solution were also measured.The phase diagram of the ternary water-salt system of sodium molybdate and sodium carbonate was experimentally determined and given in the paper.The production of sodium molybdate products was characterized using XRD and Raman analysis.The results show that the obtained product is sodium molybdate crystal.The solubility of sodium molybdate increases with the temperature but the change is negligible.The experiment proves that the dissolution equilibrium time of sodium molybdate solution can be determined by density method.The sodium carbonate has a significant effect on the salting out of sodium molybdate.And the purity and crystallization rate of the product can be controlled by adjusting the pH of the solution.In crystallization kinetics,the nucleation rate and growth rate has an important influence on the size and distribution of the product.In this paper,the MSMPR model was used for calculation.The particle size distribution and residence time of continuous decompression evaporation crystallization of sodium molybdate under different operating conditions were determined by steady state equilibrium method.The nucleation and growth rate were calculated with the first square method.Finally,the data of sodium molybdate evaporation and crystallization kinetics were fitted by software,and the growth and nucleation rate equation models were obtained.The equation shows that in order to obtain a larger particle size sodium molybdate crystal,the supersaturation should be appropriately reduced while keeping the crystal density low.Increase crystal retention time.Based on the research of thermodynamics and kinetics,the detailed decompression and crystallization process of sodium molybdate was studied in detail and optimized.The effects of operating parameters(such as solution pH,supersaturation,suspension density and residence time)on the particle size distribution and crystal shape of sodium molybdate during continuous vacuum evaporation were studied.The optimum crystallization process conditions within the scope of the investigation were determined.It provides a basis for industrial crystal production and crystallizer design and amplification of sodium dihydrate molybdate.The crystal shape of sodium molybdate was changed into needle shape by adding sodium hexametaphosphate.The effects of pH value,supersaturation and addition amount of crystal form controlling agent on the particle size of needle crystal were investigated,and a large and uniform crystal grain size was obtained.Finally,the fluidity,dissolution rate and bulk density of the needle crystals and the plate crystals were investigated and compared.Experiments have shown that the dissolution rate of acicular sodium molybdate is higher.