Research On Desalination Of High Salinity Alkali Solution By Nanofiltration

With the continual development of international economic, the need of soda ash is increasing and has a well prospect. At present, the main methods of producing soda ash in China, include Solvay process, combined method and trona processing. In order to ensure the process of evaporation and crystallization smoothly and improve the quality of product when obtain soda ash from trona, we should discharge proper mother liquor during trona processing, which contains large amounts of NaCl and high concentration of Na₂CO₃ and NaHCO₃ also. If NaCl is separated from discharged mother liquor, while Na₂CO₃ and NaHCO₃ components are retained to return to the system, we can not only reduce emissions, but also increase the economic efficiency. In this paper, we made a detailed research on desalination effect of high salinity alkali solution by nanofiltration to provide the basis for practical application.The experiment use GE-DK1812C roll nanofiltration membrane to make research on the changes of rejections about nanofiltration membrane to CO32-, HCO3- and Cl-, separation coefficient, membrane flux and recovery rate at different feed flow, operating pressure and feed concentration; in experiment, the raw material solution is composed of NaCl:187.9g/L, Na₂CO₃:30.5g/L and NaHCO₃:7.4g/L. The experiment results indicate that feed concentration and operating pressure have greater impact to the performance of nanofiltration membrane. Ion separation factor, membrane flux and recovery rate are all increased with increase of operating pressure and decrease of feed concentration. Feed flow affects experiment results mainly by changing the flow conditions near the surface of membrane and the concentration polarization, the effect of which is less obvious than other factors; ion rejection and membrane flux are slightly increasing with the increase of feed flow. Under optimum conditions, separation factors of CO32-/Cl- and HCO3-/Cl- are 25.7 and 1.8.When the feed solution is two element system of sodium carbonate - sodium chloride, the raw material has a higher utilization rate compared to ternary system of sodium carbonate - sodium hydrochloride - sodium chloride. Under the conditions that feed solution is composed of Cl-:0.622mol/L and CO32-:0.0646mol/L and operating pressure is 12bar, the rejection of carbonate and chloride ions are 93.5% and 4.4%; meanwhile, the separation coefficient of carbonate and chloride ions is 14.9, which is larger than total separation coefficient 5.5 of carbonate, bicarbonate and chloride ions before feed solution changed. So changing HCO3- in mother liquor to CO32- and then making treatment by nanofiltration can improve utilization rate of raw material.