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Membranes 2021, 11, 575 25 of 29 the salt leakage problem can be addressed by using asymmetric bipolar membranes [44]. However, this improvement in salt leakage reduction leads to higher electrical resistance and, therefore, an increase in electrical power consumption. Based on these investigations, it could be expected that new membranes will emerge in the future, being more suitable for application in the production of concentrated bases. Thus, the final LiOH concentration will be increased with monopolar membranes by higher resistance to OH− leakage, and with bipolar membranes by reduced salt leakage and higher water dissociation efficiency. Currently, these difficulties could be addressed by specific operating conditions. For instance, it has been shown in this work that at 14 wt% LiCl concentration, lower spe- cific electricity consumption and higher electric current efficiency are obtained for LiOH production, so if a constant LiCl concentration could be established during LiOH produc- tion, this would increase energy efficiency, and would also help to slow down reduction in concentration difference between LiCl and LiOH solutions separated by the cationic membrane which, in turn, would contribute to reducing the rate of lithium transport decrease through this membrane [47]. On the other hand, with respect to Cl− leakage in the bipolar membrane, an increase in Cl− leakage rate related to simultaneous concen- tration of LiOH and HCl was observed. Given the molar mass difference between these salts, the mass concentration rate (wt%) of HCl solution is always higher compared to LiOH. Therefore, for production processes, a low HCl concentration can be set while LiOH concentration increases. In this way, Cl− leakage into the LiOH compartment through the bipolar membrane would be reduced. On the other hand, the application of a high current density would reduce the mem- brane area requirement for a given production, reducing the overall process cost, as membranes are expensive. Moreover, according to linear sweep voltammetry analysis and long-running tests, working at high current densities favors water dissociation over salt leakage [69]. However, current efficiency decreases and, therefore, specific energy consumption increases, in addition to the fact that catalytic interlayer may deteriorate, which would imply a decrease in membrane lifetime and an increase in voltage drop after a long period of operation [73]. In industry, obtaining a high concentration in the final LiOH solution (greater than 3.0 wt%) would bring benefits to the current crystallization process of LiOH·H2O pro- duction, since it would reduce the current difference between LiOH concentration and saturation concentration at which crystallization of lithium hydroxide monohydrate begins. In addition, this would encourage the use of green technologies, given the potential use of this technology driven by renewable energy sources. The Atacama Desert in Chile is charac- terized by high levels of solar radiation—between 6.7 and 10.55 kWh·m−2 per day [74,75]. The application of membrane technologies for LiOH production—such as membrane elec- trodialysis [29] and bipolar electrodialysis membranes—is a potential alternative to the LiCl to Li2CO3 and subsequent Li2CO3 to LiOH conversion steps. Obtaining Li2CO3 from lithium brine reports an approximate SEC of 1.31 kWh·kg−1 of Li2CO3 [67], which implies an equivalent SEC of 6.83 kWh·kg−1 of LiOH (considering obtaining 0.38 kg of LiOH per kg of Li2CO3 for a 59% reaction conversion [68]). Therefore, the authors estimate that specific electricity consumptions determined by BMED and other membrane technologies offset those obtained by conventional methods, with the advantage of being potentially driven by clean technologies given the conditions of the Atacama Desert. Future studies should be conducted to quantify the real impact of integrating solar energy into these processes. The design choice of a high-purity lithium hydroxide production process would be conditioned by investment costs and operating costs. Pilot tests and process simulations would allow the obtaining of key information according to different scenarios and process configurations. The latter constitutes work in progress that the authors would like to present in a follow-up to the present results.PDF Image | Bipolar Membrane Electrodialysis for LiOH Production
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