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Membranes 2021, 11, 575 23 of 29 The lowest SEC value obtained was 5.97 kWh per LiOH kg in Test 5 after the first 120 min of processing, when reaching a LiOH concentration of 2.05 wt%. This can be attributed to the lower current density used (500 A·m−2) and the advantage of increasing the number of membranes in the stack to four LiOH compartments. However, current efficiency (CE) when concentrating the LiOH solution to 3.35 wt% was 0.58, and as LiOH concentration increased to 4.13 wt%, the current efficiency decreased to 0.46. Thus, part of the electric current was consumed in leakage by Cl− migration across the bipolar membrane, rather than in OH− production. This is consistent with the results obtained in the LSV tests. This behavior evidences the advantage of using operational current densities higher than the first limiting current density in bipolar membranes. According to our results, process energy efficiency is affected with increasing LiOH concentration according to OH− leakage effects and cation membrane deswelling. On the other hand, increasing HCl solution concentration promotes a higher leakage of Cl− ions into the LiOH compartment, affecting current efficiency in the bipolar membrane in the generation of OH− ions, which also affects solution purity. Table 8 presents a summary of results obtained in long-running tests. The best result was achieved in Test 2, where a Neosepta BP membrane, CMB cationic membrane and current density of 1000 A·m−2 were used. The possibility was shown of achieving LiOH solutions between 3.34 and 4.35 wt% concentration, with a range of 96.0–95.4% purity. Product purity is therefore inversely proportional to the final concentration obtained. The lowest average specific electricity consumption (SEC) was achieved in Test 1 between 6.94 and 8.71 kWh per kilogram of LiOH. On the other hand, the highest electrical current efficiency (CE) was achieved in Test 2, in the range of 0.77–0.59, with an SEC between 7.57 and 9.45 kWh per kilogram of LiOH. The CMB membrane presented a high current efficiency despite its higher electrical resistance, which can be attributed to a higher resistance to co-ion leakage. However, it is not recommended to use it with LiCl solutions higher than 14 wt%. This membrane was tested in long-running tests at 25 wt% LiCl concentrations, showing damage to its structure after two hours of processing due to the high osmotic pressure reached, causing its rupture and contamination between LiCl and LiOH solutions. Consequently, these results are not presented. 3.4.3. Specific Electricity Consumption (SEC) Comparison Specific BMED electricity consumption depends mainly on membranes’ characteristics and their interaction with concentrated solutions under a specific electric current density. The SEC results obtained in this work were determined by final LiOH concentration, and are comparable to other membrane processes used for lithium recovery and LiOH pro- duction. Specific energy consumptions between 5.43 and 6.20 kWh·kg−1 of LiOH from lithium-rich solutions produced from salt lake brine have been reported for processes such as nanofiltration, reverse osmosis, and conventional electrodialysis integrated with BMED, achieving final LiOH concentrations between 0.58 and 1.03 M (approximately 1.37–2.41 wt%), with current efficiencies between 0.36 and 0.44% [64]. Regarding direct BMED application, SEC in the order of 6.60 kWh·kg−1 was reported from 0.725–0.730 wt% LiCl solutions containing organic solvents reaching 0.3 M LiOH concentrations (approx- imately 0.71 wt%) [28], while recently, from LiCl solutions of 100 g·L−1 (approximately 9.5 wt%), SECs between 2.78 kWh·kg−1 and 7.80 kWh·kg−1 of LiOH have been reported, with current efficiencies between 0.7384 and 0.1445 at 640 A·m−2 using CMB and BP mem- branes, achieving LiOH concentrations of 1.08 M and 2.40 M (approximately 2.5 wt% and 5.4 wt% LiOH), respectively [64]. These reports are consistent with the results presented in this work, demonstrating the influence of LiCl concentration and the degree of final LiOH concentration on process performance. The same technology has been used with different starting solutions. Thus, a 0.5 M Li2SO4 feed [65] reports a specific electricity consumption of 7 kWh·kg−1, which increases when working with more concentrated solutions. Meanwhile, the use of aqueous Li2CO3PDF Image | Bipolar Membrane Electrodialysis for LiOH Production
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