Study On The Structure Design Of Highly Charged Ultra-thin Composite Nanofiltration Membrane And Its Separation Performance For Magnesium And Lithium

With the rapid development of industry and economy,the demand for lithium resources in various industries is increasing.Salt lakes in Clina are rich in lithium resources,but the mass ratio of Mg²⁺/Li⁺ is relatively high.So the key to retract lithium efficiently is to separate magnesium and lithium effectively.This paper focused on the separation of Mg²⁺and Li⁺ from salt brine with high Mg²⁺/Li⁺,and the following work has been carried out.Nanofiltration?NF?membranes with different structures and chargeability were prepared by using different amine materials.The relationship between the chargeability and structure of nanofiltration membranes and the selective separation performance of Mg²⁺ and Li⁺ was studied.The structure and performance of the charged nanofiltration membrane with the best separation performance were optimized,and a high positive charged ultra-thin composite nanofiltration membrane was prepared.Interfacial polymerization was used to prepare composite NF membranes with different structures and charge properties by using various amine polymers as the aqueous phase monomers.It can be seen from the experimental results that NF membranes prepared by using polyethyleneimine as the amine monomer exhibits positive chargeability owing to a large amount of amine groups from polyethyleneimine macromolecular,and NF membranes prepared by the piperazine or m-phenylenediamine as the aqueous phase monomer exhibited negative chargeability.During the separation process of magnesium and lithium from simulated salt lake brine,positively charged NF membrane exhibited a better for the separation of Mg²⁺ and Li⁺ than negatively charged NF membrane,and the NF membrane with higher positive chargeability is a better choice for the separation of Mg²⁺and Li⁺.In order to optimize the structure and performance of the above positive charged composite NF membrane,the influence of the interfacial polymerization conditions on the structure and properties of the composite NF membrane was explored in this paper.The experimental results showed that there was a dense high positive hydrophilic polyamide film on the surface of high positively charged NF membrane prepared in this work.Due to the similar hydration radius of Mg²⁺and Li⁺are and different number of charges they carried,the difference in the rejection performance of NF membrane is mainly affected by the electrostatic repulsion between membrane and the ions.After the filtration of simulated brine using the high positive charged ultra-thin NF membrane prepared,the mass ratio of Mg²⁺ and Li⁺decreased fiom the initial 20 to 1.3,and the separation factor SMgLi were 0.05.The rejection of membrane for Mg²⁺ and Li⁺ were 95%and 19%,respectively,and the water flux was 40.16 L/?m²h?.The high positively charged NF memb rane prepared in this paper exhibited excellent separation performance for Mg²⁺ and Li⁺,which lays a foundation for the subsequent research and application of NF membrane separation for Mg²⁺ and Li⁺ from salt lake with high mass ration of Mg²⁺ and Li⁺.