The Investigation Of Mg²⁺/Li⁺Separation Process By Nanofiltration

The salt lakes located in the west of China boost abundant lithium resource, therefore, the majority of them have the structural characteristics of high Mg2+/Li+ratio, which has led to the problem that traditional method of precipitation is unsuitable for the lithium recovery from this kind of salt lake brine. Nanofiltration (NF) membranes are new kinds of membranes which have properties between those of ultrafiltration (UF) and reverse osmosis (RO).and they have the characteristic of monovalent and bivalent ion separation. So, in this work, we use nanofiltration to separate Mg2+and Li+in the brine to reduce the ratio of them and enrich the lithium concentration. After that, the efficiency of adsorption by the ion sieve will be improved subsequently. In the meantime, it can also apply to refining the desorptive solution.Firstly, operation conditions, including operation time, operation pressure, solution temperature, pH value and Mg2+/Li+ratio were investigated to study the separation of magnesium and lithium by4different type of nanofiltration membranes (DK-2540, DL-2540, NF270, HL-2540). Compared with the other membranes, DL-2540and NF-270were found as the suitable ones to process the brine with high Mg+and Li+separation factor and water flux.Secondly, the effect of other ions on the Mg2+and Li+separation was also investigated. And it turned out that, monovalent ions such as Na+and K+, have the negative impact on the separation due to the fierce competition with Li+, and divalent ion such as Ca2+has the far less impact on the separation. The rank of ion rejections (R(Mg2+)> R(Ca2+)> R(Na+)> R(Li+)> R(K+)) were shown in the experiment of mixed solution. On this basis, the competition model of different cation with Li+was set up, it can be used to predict the separation factor of the two-component solution.DL and NF membranes which were already selected were used to process East Taijinar, West Taijinar, Qarham old brine, as well as ion sieve desorptive solution. The results show that, in processing East Taijinar, West Taijinar and desorptive solution, nanofiltration has the good separation factor and water flux and the Mg2+/Li+ratio has reduced by2-4times. As for Qarham old brine, it was less feasible to process by nanofiltration. In this work, the impact of membrane fouling on the water flux was also discussed finally. It was found that after chemical cleaning, membrane flux can be fully restored.