| Bromine,known as“marine element”,is widely used in industrial and agricultural production and is closely related to people's lives.As a large salt-producing country,the bitter bittern produced by salt production in China is rich in bromine resources every year.The traditional bromine extraction methods are air blowing and steam distillation,which have the disadvantages of low yield,huge equipment,and are not conducive to the extraction of low-concentration bromine resources.Therefore,it is necessary to develop new processes to improve the utilization of bromine resources.In addition to bromine resources,bittern also contains more sodium,potassium and chlorine,so the overall development and utilization of Na+,K+//Cl-,Br-quaternary system will help to improve the comprehensive utilization level of bittern in China.For the separaion of Na(Cl,Br)solid solution,the phase equilibrium of Na Cl-Na Br-H2O,Na Cl-Na Br-HCl-H2O and Na Cl-Na Br-CH3OH system was studied.The solubility was calculated by Pitzer model and its extended model,and the process of producing high purity Na Cl and Na Br was established.The phase equilibrium data of ternary system Na Cl-Na Br-H2O and quaternary system Na Cl-Na Br-HCl-H2O under different raw material ratios at 298K were determined by isothermal dissolution equilibrium method,and the equilibrium solid phase was characterized by XRD.The results showed that the equilibrium solid phase was Na(Cl,Br)or Na(Cl,Br)·2H2O solid solution,and the content of Na Cl was much higher than that of Na Br,which proved that Na Br produced salting-out effect on Na Cl in aqueous solution.Due to the co-ion effect of Cl-,the content of Na Cl in the equilibrium solid phase of the quaternary system was further increased.Based on this,a process for separating the coexistence system of Na Cl-Na Br was designed.The mother liquor was fractionated,the high purity HCl and HBr were collected by stages,and the solid phase precipitated from the crystallization was returned to the starting point for recycling,forming a complete process.In view of the fact that the crystallization of chlorine and bromine coexistence system is easy to form solid solution,the stable phase equilibrium of alcohol-salt system formed by Na Cl and Na Br dissolved in anhydrous methanol at 273K,298K and 323K was studied,and the corresponding phase diagrams were drawn.The results showed that the Na Cl-Na Br-CH3OH phase diagram had one invariant point and two univariant curves.Compared with the traditional water-salt system,this system had not only the crystallization region of Na(Cl,Br)solid solution,but also the crystallization region of Na Cl single salt and the co-crystallization region of Na Cl and solid solution.With the increase of Na Br content,Na Cl content decreased significantly,which proved that Na Br had a stronger salting-out effect on Na Cl than in aqueous solution,and the solubility of the two solutes in methanol decreased with the increase of temperature.According to the Na Cl-Na Br-CH3OH phase diagram obtained in this paper and the Na Cl-Na Br-H2O phase diagram at 298K,the process for separating Na Cl and Na Br components from Na(Cl,Br)solid solution was designed,and Na Cl and high purity Na Br were obtained.Based on the theory of Pitzer electrolyte solution,the theoretical calculation of the solubility of the Na Cl-Na Br-H2O system at 298K was carried out.The results showed that the experimental values were basically consistent with the calculated values,proving the feasibility of the Pitzer model in calculating the solubility of stable system containing solid solution.For the Na Cl-Na Br-CH3OH system,the Pitzer model was extended and the Pitzer parameters were modified to make it suitable for the electrolyte-alcohol system.The solubility of the Na Cl-Na Br-CH3OH system at 298K was calculated.The results showed that the experimental values were basically consistent with the calculated values,which proved that the extended Pitzer model in this paper was feasible in calculating the solubility of electrolyte-alcohol system. |
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