Study On Some Key Scientifical Issues During The Preparation Process Of Potassium Sulfate By Selective Electrodialysis

Potassium sulfate is a kind of important chloride-free potash fertilizer for chloride-sensitive crops.However,there is no pure potassium sulfate in nature.Traditional preparation methods,such as Mannheim method,association-displacement method and double decomposition method,have some shortcomings:high energy consumption,strong corrosiveness and so on.Thus,it is urgent and important to find a new,simple,eco-friendly and energy-efficient process to prepare potassium sulfate.Electrodialysis,as an electric driven separation process,can remove the charged ions from the homogeneous solutions.It has series of advantages,such as high current efficiency and eco-friendly.Nowdays,electrodialysis has been used mainly to concentrate and desalinize seawater,produce organic acid and treat industrial wastewaters.Selective electrodialysis(SED)is a new type of electrodialysis process to separate ions which have the same electrical property.SED has important applications in resourceful treatment waste water.Here,SED is used to prepare potassium sulfate,the works concerned are as follows:1.An experimental study was carried out to prepare potassium sulfate from potassium chloride by SED.Effects of sulfate species,current density and molar ratio of ammonium sulfate to potassium chloride were investigated.Results showed that ammonium sulfate was regarded as the optimum sulfate source because of the lowest energy consumption and cheap raw materials.When current density was set as 20 or 30 mA/cm2,the generated products can conform to the Chinese Government standard(GB20406-2006).A higher molar ratio can promote the substitution reaction more completely but result in larger loss of K+ ions and more increment of NH4+ ions in products.Moreover,a mathematical model was established to predict the product concentration.Results showed that there is a good agreement between experimental and computational concentrations.Finally,preliminary economic evaluation of SED process was conducted,and results show that the cost of potassium sulfate prepared in this study was higher than the commercial price.The main reason is the small experimental apparatus and the high price of monovalent anion selective membrane.2.The monovalent anion selective membranes were prepared by BPPO and tertiary amines.Effects of the carbon chain length of tertiary amine,tertiary amine/bromomethyl ratio,heat treatment temperature and compositing polypyrrole were investigated.Results showed that with the increasing of carbon chain length of the grafted tertiary amines,membrane monovalent anion selectivity PSO4Cl increased.Smaller amine/bromomethyl ratios can promote PSO4Cl higher but result in higher membrane area resistance.Higher heat treatment temperature can increase the crystallinity of membranes.And the resultant membranes with the improved crystallinity are expected to exhibit the high monovalent anion selectivity PSO4Cl due to the pore-size sieving effect.Compositing polypyrrole on the membrane surface can improve the membrane surface density.Results showed that after compositing polypyrrole,membrane monovalent anion selectivity PSO4Cl was greatly improved,but the membrane surface resistance also increased rapidly.Comparing the anion selective membranes prepared by different methods reported by previous literatures,the membrane prepared in this study has relatively higher monovalent anion selectivity,and the disadvantage was that the membrane area resistance is relatively high.In conclusion,a simple,green method for potassium sulfate preparation was proposed in this thesis,qualified products were prepared in laboratory studies.In order to apply this potassium sulfate preparation method to practice,on the one hand,it is necessary to develop the new excellent ion exchange membranes,on the other hand,the pilot production study was also indispensable.