Study On Crystal Habit Control And Caking Of Sodium Chloride

Regulation and control of crystal habit and hence development of multiple crystal morphologies of sodium chloride is the main approach to improve the added value of sodium chloride products and the market competitiveness of domestic salt enterprises.However,the sodium chloride crystals obtained in industrial production only have one type of crystal habit,limiting its wide applications.Furthermore,the sodium chloride products produced will face serious caking problems,influencing the quality of products and economic benefits of salt enterprises.In order to resolve these problems,the growth,dissolution,and caking of sodium chloride crystals were systematically studied in the thesis.The preparation methods of sodium chloride with different crystal habits were established,and the formation mechanism of sodium chloride with different crystal habits was discussed.The caking behavior of sodium chloride crystals was studied and predicted.The above studies provide the research basis and guidelines for the preparation of high-quality sodium chloride crystals with strong anti-caking ability and desired morphology.Firstly,spherical crystal of sodium chloride was prepared by reduced pressure evaporation crystallization method,and the effects of stirring rate,evaporation intensity and evaporation fraction were investigated.The optimal crystallization conditions were determined and the formation mechanism of spherical sodium chloride was explored.In addition,the dissolution of sodium chloride in methanol-water mixed solvents was studied.It was found that the crystals could be dissolved into spherulites.The effects of the composition of solvent mixtures and the particle size of crystals were investigated.It was found that the spherulites were easily obtained by dissolving sodium chloride in methanol-water mixed solvent with methanol mass fraction of 0.5 and the smaller the particle size of sodium chloride was,the more easily the spherulites were formed.What's more,the flake habit of sodium chloride crystals was prepared by small-scale liquid evaporation crystallization method.The effects of amounts of liquid and evaporation temperature on crystal habit were studied.The mode of crystal growth was found to be spiral growth as observed by microscope and scanning electron microscope.Then,the surface structure of two kinds of cubic crystals of sodium chloride was analyzed by atomic force microscope,and their dissolution kinetics and mechanism in methanol-water solvent mixtures were further investigated.On the basis of young's equation,the solid-liquid surface free energy between water and sodium chloride crystals of different particle sizes were calculated from contact angle measured by capillary rise method.Taking considerations of solubility of sodium chloride and dissolution mechanism and kinetics of inorganic salt,the reasonable explanation of sodium chloride dissolving into spherulites can be obtained.Finally,the effects of relative humidity on moisture absorption of sodium chloride crystals was investigated by DVS,and the roles of several anti-caking agents and sodium chloride crystals of spherical shape on anti-caking ability were further studied.In addition,the tiling states of spherical and cubic crystals of sodium chloride were simulated by PFC3 D software.Then the caking behavior of the crystals was predicted by combining the factors of moisture absorption of the crystals and the amount of critical crystal bridges.The prevention and control strategies of caking of sodium chloride crystals were given according to the regulation and control over environmental humidity,crystal habit and particle size distribution,etc.The above research has not been reported in the literature.