Development of Research on Mass Transfer Mechanism Model of Nanofiltration Membrane and Its Application in Separation of Magnesium and Lithium from Salt Lake Brine

Lithium is called white oil" because of its excellent physical and chemical properties and a wide range of applications，which is of strategic significance to our development. Among the sulfuric acid method of lithium ore has problems such as low lithium recovery rate and difficult separation of lithium and sodium，which is not conducive to sustainable development. The "salt lake type" lithium resources account for 87% of the basic reserves and are the main source of lithium extraction. China's salt lake lithium resources are mainly distributed in Qinghai and Tibet with rich brine content at present. Salt lakes are mostly chlorinated and magnesium sulfate types with high content of magnesium and low content of lithium. With the development of lithium extraction technology，membrane separation has become a research hotspot for lithium extraction from salt lakes as an emerging magnesium-lithium separation technology. The nanofiltration separation technology has the advantages of no chemical reaction，no phase transition，green environmental protection and others. The nanofiltration technology as a separation process driven by pressure has good separation efficiency of magnesium and lithium，which is beneficial to the separation of magnesium and lithium from salt lake brine in the Qinghai-Tibet Plateau. According to the current situation of high magnesium-to-lithium ratio of salt lake brine in China and the characteristics of difficulty in mining and low production，the application of nanofiltration separation technology to the extraction of lithium resources in salt lakes had far-reaching significance. According to the advantages and characteristics of each separation technology，nanofiltration technology and other separation methods which were reasonably combined to realize efficient coupling technology of membrane process in practical application，the method enhanced synergy，improved lithium yield and reduced energy consumption. In addition，the nanofiltration technology also depended on the design and preparation of high-performance membrane materials when realizing high-efficiency magnesium-lithium separation process. With the development of nanofiltration materials，some researchers had carried out modification experiments on the water flux，membrane structure stability and physicochemical properties and the performance of nanofiltration membranes had been enhanced to varying degrees. Therefore，compared the negatively charged nanofiltration membrane with the intrinsically positively charged nanofiltration membrane had been greatly improved in terms of structural stability and Mg2+/Li+ separation performance. Among the membrane materials，inorganic membrane materials and polymer membrane materials which had their own advantages and disadvantages were widely used. Since then，some researchers had developed mixed matrix membranes with stable membrane pore structure and membrane skeleton. The membrane fully combined the advantages of both and the membrane had good separation performance. The nanofiltration membranes from the initial commercial negatively charged nanofiltration membranes to the nanofiltration of positively charged polymer membrane materials to the use of nanomaterials to make membranes and the current high-flux，structurally stable metal organic frameworks（MOFs）and covalent organic frameworks（COFs）membranes had shown the more stable the pore structure，the more significant the separation effect of magnesium and lithium. It was believed that the preparation of nanofiltration membranes was more inclined to adopt hybrid technology and research on metal framework materials in the future. Furthermore，the nanofiltration separation process was a membrane separation process with synergistic effects of membrane pore sieving，dielectric and Donnan effect. Based on separation reactions，the nanofiltration membrane could separate ions of different valences without introducing other solvents. The theoretical research of nanofiltration technology mainly focused on the enhancement of the mass transfer process and it was difficult to support the establishment of the mass transfer model with less theoretical basis at present. Therefore，the mass transfer model based on the theoretical study of lithium separator needed to be further explored. As the research continues，computational fluid dynamics（CFD）numerical simulation calculation technology was used to analyze the mass transfer process of nanofiltration to further understand the mass transfer enhancement in membrane separation system，and this technology achieved the purpose of reducing time，cost and risk. Finally，promoting the comprehensive utilization of salt lake resources while developing lithium resources. Accelerating the construction of a world-class salt lake industrial base and building a national clean energy industry highland were the ultimate goals of realizing the rapid，efficient and green development of lithium extraction technology from salt lakes. The advantages and disadvantages of lithium extraction technology and the institutions engaged in nanofiltration membrane related research at home and abroad were summarized. The nanofiltration membrane material was described and the future development of membrane materials was looking forward. Intensive and detailed researches on the surface mass transfer model of nanofiltration membranes played a significant role in demonstrating the mechanism of separating magnesium from lithium，as well as accelerating the extensive application of nanofiltration membranes. The characteristics of the nanofiltration membrane technology were analyzed，and the research progress of mass transfer model for nanofiltration membrane was also summarized in detail，together with the application in the separation of magnesium and lithium from salt lake brine. On this basis，the development and prospect of nanofiltration membrane technology were also proposed. © 2024 Editorial Office of Chinese Journal of Rare Metals. All rights reserved."