Study On Alkali Production By Electrodialysis Metathesis

Caustic soda,baking soda and soda ash are all important basic inorganic chemical products,which are widely used in all walks of life and in daily life.The traditional alkali industry requires saturated sodium chloride as a raw material,and there are problems such as high energy consumption,large pollution,difficult separation of products,and easy corrosion of equipment.Electrodialysis metathesis（EDM）is a new type of mass transfer separation process,which can realize the transformation and concentration of substances in solution at the same time.It has the advantages of low process energy consumption,no product separation,and easy replacement of membrane materials.Therefore,based on EDM technology,this paper studies a new process for preparing caustic soda concentrated solution and a new process for preparing baking soda/soda ash concentrated solution by carbon absorption,both using Na Cl and NH₃·H₂O as raw materials.It aims to provide a green,low-carbon and high-efficiency road for the alkali-making industry that is suitable for low-concentration systems.First,the preparation process of caustic soda concentrated solution by electrodialysis metathesis method was explored.Taking the product concentration,purity,energy consumption and yield as indicators,under the three conditions of operating voltage,NH₃·H₂O flowing rate,and feeding rate,the optimal process conditions were explored,and the membrane types were investigated in order to increase the concentration of caustic soda,but the results showed that the product concentration was not high under the optimal process conditions,and the energy consumption was high compared with other caustic soda preparation methods.By analyzing the change of ion concentration in each product chamber,especially the migration law and path of OH^-in the system,it is found that the difficulty of dissociation of ammonia water molecules makes the ion concentration of the raw material chamber where the ammonia water is located low and the mass transfer resistance is large,which leads to the application of electrodialysis metathesis method to the main reasons for the problems of high energy consumption and low product purity in the preparation of caustic soda are.Compared with the dissociation rate of ammonia water,the reaction rate of ammonia water and carbon dioxide is faster,and ammonium bicarbonate or ammonium carbonate,as a strong electrolyte,exists in the form of ions in the solution,which can break through the limitation of low ion concentration in the raw material chamber.Therefore,on the basis of the previous preliminary investigation,it is proposed to introduce CO₂into the ammonia water to ionize the ammonia water and convert it into ammonium bicarbonate or ammonium carbonate for migration,so as to prepare sodium bicarbonate or sodium carbonate concentrated solution by electrodialysis metathesis method and exploring a new pathway for carbon neutrality at the same time.The process of preparing baking soda concentrated solution by electrodialysis metathesis was further carried out.As the control factor of electrodialysis,the current density was first investigated,after the current density was determined,the gas-liquid ratio was adjusted so that Na HCO₃did not contain CO₃^2-,and the molar ratio between NH₃·H₂O and CO₂was maintained,the influence of the feeding rate was investigated,and it was found that a too low feeding rate would increase the mass transfer resistance and increase the energy consumption,finally,the water replenishment rate in Na HCO₃was adjusted to make the Na HCO₃concentration close to saturation,and the water replenishment rate in NH₄Cl was increased to reduce the same ion migration phenomenon,thereby improving the product purity.On the basis of the process of preparing concentrated solution of baking soda,the ventilation rate of carbon dioxide in ammonia water was further reduced,and the ammonia water was converted into ammonium carbonate for migration,and the process of preparing concentrated solution of soda ash by electrodialysis metathesis was studied.The research results show that by adjusting the gas-liquid ratio,the Na₂CO₃product can be basically free of HCO₃^-and OH^-,but the purity of Na₂CO₃is low,and there is a large amount of NH₄⁺,it is inferred that HCO₃^-and CO₃^2-may be"ammonia hydrate ions"in ammonia water to migrate.