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Membranes 2020, 10, 371 2 of 14 lithium extraction from high Mg/Li brine. Solvent extraction is an efficient separation technology; both of the separation factors (SFLi–Na, SFLi–Mg) can reach hundreds or even more than one thousand [24,25]. However, the extraction reagent has a slight solubility in aqueous solution [26], which is not suitable for treating brine directly. The ion-sieve absorption method is considered to be an effective approach to extract lithium from the high Mg/Li ratio brines thanks to its low cost, high selectivity, and nontoxicity [27]. However, the ion-sieve absorption method faces the following problems: (1) it is difficult to prepare the high absorption capacity absorbent; and (2) there is a significant loss of capacity in the desorption process when acids or oxidants are used as desorption agents. The above problems seriously restrict its large-scale industrial application [20]. Nanofiltration (NF), as an important membrane separation technology, has been successfully applied for separating lithium and magnesium from a high Mg/Li brine because of its selective rejection of divalent ions and monovalent ions based on Donnan exclusion [28,29]. However, it also suffers from the following problems: (1) This technology can only treat brine with very low sodium and potassium content, and it usually takes 1–2 years to obtain this kind of brine [30–32]. (2) The salinity in the type of brine after potassium removal is too high to meet the operation condition for this purpose, which needs to be diluted with water (the amount of water used for dilution is usually several times than the brine). This process not only needs to consume a large amount of fresh water, but also increases the amount of water to be treated. In our previous work, we have proved that the EID method shows an excellent lithium extraction properties from the high mass ratio of brine [20,21,33]; the mass ratio of brine can be decreased from the initial 58.5 in the brine to 0.93 in the obtained anolyte. Although the mass ratio of Mg/Li in the anolyte is much lower than the original brine, the lithium concentration in the anolyte is only 1–2 g·L−1, which is far from the lithium concentration required to precipitate lithium carbonate. For this reason, we need to concentrate the anolyte and remove the residual impurities (e.g., Mg2+, Ca2+, and SO42−) in it. Theoretically, all kinds of concentration methods (like reverse osmosis, electrodialysis, evaporation, and so on) [34,35] and impurity removal methods (like nanofiltration, solvent extraction, and so on) [24,28,29] can be used to treat the obtained anolyte. Notably, the total salt concentration of the obtained anolyte is between 20 and 30 g·L−1, which is an ideal range for NF and reverse osmosis (RO) treatment. Therefore, we proposed an integrated lithium carbonate preparation process combining EID, NF, RO, evaporation, and precipitation processes to prepare Li2CO3 from a high Mg/Li brine. The aim of the main processes are as follows: (1) the EID method is used to maximize the separation of magnesium and lithium from the brine to obtain a low Mg/Li anolyte; (2) removing the multivalent ions (e.g., Mg2+, Ca2+, and SO42−) from the obtained anolyte via the NF method; (3) concentrating the permeate flow produced by NF with the RO method; (4) further increasing the lithium concentration by evaporation; and (5) precipitating Li2CO3 by adding Na2CO3. 2. Materials and Methods 2.1. Membranes The membrane used in the EID method is a heterogeneous anionic membrane (MA-3475), which was purchased from Beijing Anke Membrane Separation Technology & Engineering Co., LTD. (Beijing, China agent). The heterogeneous anionic membrane selectively allows the anions to pass through and reject the cations. The NF (NF2) and RO (RO5) membranes used for the experiment are disc tube membranes, which were made by RisingSun Membrane Technology Co., Ltd., (Beijing, China). Specifically, the membrane areas of the NF membrane and RO membrane are both 2.2 m2, and the operation pH are in the range of 3–11. The permeate flux and desalination rate of NF were 42 L·m−2·h−1 and 98%, respectively, which were obtained at 25 ◦C, operating pressure of 0.7 MPa, and test salt concentration of MgSO4 of 2 g·L−1. Further, the permeate flux and desalination rate of RO were 42 L·m−2·h−1 and 99.5%, respectively, which were obtained at 25 ◦C, operating pressure of 1.55 MPa, and test salt concentration of NaCl of 2 g·L−1.PDF Image | Membrane Process for Preparing Lithium Carbonate
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