membranes
Article Recovery of Acid and Base from Sodium Sulfate Containing Lithium Carbonate Using Bipolar Membrane Electrodialysis
Wenjie Gao 1, Qinxiang Fang 1, Haiyang Yan 2 , Xinlai Wei 1,2,* and Ke Wu 1,*
1 Collaborative Innovation Center for Environmental Pollution Precaution and Ecological Rehabilitation of Anhui, School of Biology, Food and Environment Engineering, Hefei University, Hefei 230601, China; [email protected] (W.G.); [email protected] (Q.F.) 2 CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, China; [email protected] * Correspondence: [email protected] (X.W.); [email protected] (K.W.)
Abstract: Lithium carbonate is an important chemical raw material that is widely used in many contexts. The preparation of lithium carbonate by acid roasting is limited due to the large amounts of low-value sodium sulfate waste salts that result. In this research, bipolar membrane electrodialysis (BMED) technology was developed to treat waste sodium sulfate containing lithium carbonate for conversion of low-value sodium sulfate into high-value sulfuric acid and sodium hydroxide. Both can be used as raw materials in upstream processes. In order to verify the feasibility of the method, the effects of the feed salt concentration, current density, flow rate, and volume ratio on the desalination performance were determined. The conversion rate of sodium sulfate was close to 100%. The energy consumption obtained under the best experimental conditions was 1.4 kWh·kg−1. The purity of the obtained sulfuric acid and sodium hydroxide products reached 98.32% and 98.23%, respectively. Citation: Gao, W.; Fang, Q.; Yan, H.; Calculated under the best process conditions, the total process cost of BMED was estimated to be USD Wei, X.; Wu, K. Recovery of Acid and −1 Base from Sodium Sulfate Containing 0.705 kg Na2SO4, which is considered low and provides an indication of the potential economic Lithium Carbonate Using Bipolar and environmental benefits of using applying this technology. Membrane Electrodialysis. Membranes 2021, 11, 152. https:// Keywords: lithium carbonate; BMED; clean production; current efficiency; recycling doi.org/10.3390/membranes11020152
Academic Editor: Natalia Pismenskaya 1. Introduction Lithium carbonate, an inorganic compound, exists as colorless monoclinal crystals or Received: 14 January 2021 a white powder. Lithium carbonate is the basic material required for the production of Accepted: 19 February 2021 secondary lithium salt and lithium metal products, so it has become the most-consumed Published: 22 February 2021 lithium product in the lithium industry. Other lithium products are essentially downstream products of lithium carbonate [1,2]. In particular, the development of new energy vehicles, Publisher’s Note: MDPI stays neutral most of which are powered by lithium batteries, has greatly promoted the use of lithium. with regard to jurisdictional claims in Global lithium consumption has increased rapidly from 48,100 tons in 2018 to 57,700 tons published maps and institutional affil- in 2019 [3]. The price of battery-grade lithium carbonate has risen from USD 6500 per ton iations. in 2015 to USD 13,000 per ton in 2019 [3]. The increasing demand and high profits have led to a surge in lithium production. As is well known, the traditional extraction processes of lithium resources from ores mainly comprise chlorination roasting, alkali roasting, and sulfuric roasting methods; heir Copyright: © 2021 by the authors. synthetic routes are shown in Scheme1[ 4–8]. In the first route, the chlorination roasting Licensee MDPI, Basel, Switzerland. method, the obtained chlorinated products are difficult to collect, and the furnace gas This article is an open access article is highly corrosive. Economic costs are high because of the complexity of the involved distributed under the terms and processes [4,7]. In the second route, the alkaline roasting method, difficulties in operating conditions of the Creative Commons Attribution (CC BY) license (https:// and maintaining equipment develop due to the high energy consumption for evaporation, creativecommons.org/licenses/by/ the high requirement for temperature control, the low recovery rate of lithium, and strong 4.0/). flocculability of the sludge after water leaching [6]. The last route, the sulfuric roasting
Membranes 2021, 11, 152. https://doi.org/10.3390/membranes11020152 https://www.mdpi.com/journal/membranes Membranes 2021, 11, x FOR PEER REVIEW 2 of 15
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flocculability of the sludge after water leaching [6]. The last route, the sulfuric roasting method, is currently the industry standard for the preparation of lithium carbonate, being method, is currently the industry standard for the preparation of lithium carbonate, being relatively mature and commonly used. β-Spodumene and sulfuric acid are calcined under relatively mature and commonly used. β-Spodumene and sulfuric acid are calcined under conditions of 250–300 °C via the displacement reaction of lithium sulfate from soluble and conditions of 250–300 ◦C via the displacement reaction of lithium sulfate from soluble and insoluble gangue. Sodium hydroxide is added to the calcined product to adjust the pH to insoluble gangue. Sodium hydroxide is added to the calcined product to adjust the pH to neutralize excess sulfuric acid and remove impurities [5,8]. Then, pure lithium sulfate is neutralize excess sulfuric acid and remove impurities [5,8]. Then, pure lithium sulfate is obtainedobtained by further removing calcium and ma magnesiumgnesium with soda ash. Sodium carbonate isis then then added added to to obtain obtain lithium lithium carbonate. carbonate. Sulfuric Sulfuric roasting roasting has hasstrong strong adaptability adaptability to var- to iousvarious rawraw materials materials and andcan handle can handle many many kinds kinds of lithium-bearing of lithium-bearing ores. The ores. process The process route isroute simple is simple and easy and easyto operate, to operate, and andthe therecovery recovery yield yield of oflithium lithium is is very very high, high, reaching reaching moremore than 90%. However, However, the route is hamper hampereded by subsequent problems. It can be seen thatthat in in addition addition to to the the massive massive use use of of raw raw materials, materials, the the production production of ofsodium sodium sulfate sulfate as aas byproduct a byproduct is also is also a major a major problem. problem. A large A large amount amount of sulfuric of sulfuric acid acidand NaOH and NaOH is con- is vertedconverted into intolow-value low-value sodium sodium sulfate. sulfate. Because Because of the oflow the purity low purityof the ofproduced the produced waste sodiumwaste sodium sulfate, sulfate, subsequent subsequent processing processing has become has become a burden a burden that thatwill willundoubtedly undoubtedly in- creaseincrease the the energy energy consumption consumption of of the the process process and and further further increase increase the the running running cost. cost. Hence,Hence, bipolar bipolar membrane electrodialysis electrodialysis (BME (BMED)D) may may be be a feasible choice to overcome thesethese challenges challenges [9]. [9].
SchemeScheme 1. 1. ReactionReaction scheme scheme for for the the synthesis synthesis of oflithium lithium carbonate carbonate by bychlorination chlorination roasting, roasting, acid acid roasting,roasting, and and alkali alkali roasting. A bipolar membrane is composed of an anion membrane and a cation membrane [10–12]. A bipolar membrane is composed of an anion membrane and a cation membrane [10– The anion and cation membranes are bonded by an intermediate catalytic layer. When 12]. The anion and cation membranes are bonded by an intermediate catalytic layer. When a current is applied to the bipolar membrane, water was split into hydrogen ions and a current is applied to the bipolar membrane, water was split into hydrogen ions and hy- hydroxide ions in the interfacial layer of the bipolar membrane [13–15]. Electrodialysis (ED) droxide ions in the interfacial layer of the bipolar membrane [13–15]. Electrodialysis (ED) technology is an electromembrane separation process in which anions and cations migrate technology is an electromembrane separation process in which anions and cations migrate to opposite electrodes under the action of a direct current electric field, thereby achieving to opposite electrodes under the action of a direct current electric field, thereby achieving ion separation, concentration, and purification [16–18]. BMED technology is based on ion separation, concentration, and purification [16–18].+ BMED technology− is based on tra- traditional electrodialysis, in which H2O splits into H and OH in the interfacial layer of ditional electrodialysis, in which H2O splits into H+ and OH− in the interfacial layer of the the bipolar membrane [19,20]. BMED can simultaneously achieve high-salinity wastewater bipolar membrane [19,20]. BMED can simultaneously achieve high-salinity wastewater desalination and acid and alkali production without introducing other components [21,22]. desalinationThe produced and acid acid and alkaliand alkali can be producti directlyon or indirectlywithout reusedintroducing in industrial other components applications, enhancing the value of high-salinity wastewater. BMED has been researched in waste salt
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Membranes 2021, 11, 152 3 of 14 [21,22]. The produced acid and alkali can be directly or indirectly reused in industrial ap- plications, enhancing the value of high-salinity wastewater. BMED has been researched intreatment waste salt and treatment acid–base and reuse acid–base in the productionreuse in the processes production of neopentylprocesses of glycol, neopentyl ore mining, gly- col,and ore phenyl mining, glycine. and phenyl [21,23, 24glycine.]. [21,23,24]. InIn order order to to solve solve the the problem problem of of the the sodium sodium sulfate sulfate waste waste salt salt produced produced in in the the lithium lithium carbonatecarbonate production production process, process, BMED BMED techno technologylogy was was introduced introduced into into the the above above process process toto recycle recycle the the waste waste salts, salts, as as shown shown in in Figure Figure 1;1; the the waste waste salt salt (sodium (sodium sulfate) sulfate) can can be be convertedconverted into into sodium sodium hydroxide hydroxide and and sulfuric sulfuric acid, acid, and and both both products products can can be be used used as as raw raw materialsmaterials in in the the upstream upstream process. process. Therefore, Therefore, a aclosed-loop closed-loop lithium lithium carbonate carbonate production production processprocess can can be be achieved achieved by by introducing introducing BMED BMED processing. processing. The The main main purpose purpose of of this this work work isis to to verify verify the the feasibility feasibility of of BMED BMED for for the the treatment treatment of of sodium sodium sulfate sulfate waste waste containing containing lithiumlithium carbonate. carbonate. The The influence influence of of feed feed salt salt concentration, concentration, current current density, density, volume volume ratio, ratio, andand flow flow rate rate on on BMED BMED performance performance was was investigated. investigated.
. Figure 1. A flow chart of the preparation of lithium carbonate by acid roasting and bipolar membrane Figureelectrodialysis 1. A flow (BMED)chart of technology.the preparation of lithium carbonate by acid roasting and bipolar mem- brane electrodialysis (BMED) technology. 2. Experiment 2.2.1. Experiment Materials 2.1. MaterialsWe used self-made simulated sodium sulfate waste salt (0.3 mol·L−1) containing lithiumWe used carbonate; self-made its components simulated sodium are shown sulfate in Tablewaste1 [salt5]. (0.3 H2SO mol·L4, NaOH,−1) containing NaCO3 lith-, and iumother carbonate; chemicals its used components in this study are shown were all in analytically Table 1 [5]. pure H2SO (Meifeng4, NaOH, Chemical NaCO3, Instrumentand other chemicalsCo. Ltd., used Hefei, in China).this study The were anion all analytically exchange, cation pure (Meifeng exchange, Chemical and bipolar Instrument membranes Co. Ltd.,were Hefei, purchased China). from The Tokuyama anion exchange, Co., Tokyo, cation Japan. exchange, Table2 andlists bipolar the main membranes properties ofwere the purchasedthree membranes from Tokuyama used in theCo., experiment. Tokyo, Japan. Table 2 lists the main properties of the three membranes used in the experiment. Table 1. Chemical composition of simulated feed. Table 1. Chemical composition of simulated feed. Components (mg·L−1) Li+ Ca+ Mg+ ComponentsSimulated (mg·L feed−1) Li+ 4500Ca+ 3.28Mg 0.20+ Simulated feed 4500 3.28 0.20
Table 2. Properties of the membranes applied to the BMED experiments a.
Membrane Characteristics Neosepta AMX Neosepta CMX Neosepta BP-1 IEC (meq·g−1) 1.4–1.7 1.5–1.8 - Thickness (µm) 120–180 220–260 200–350 Area resistance (Ω·cm2) 2.0–3.5 2.0–3.5 - Voltage drop (V) - - 1.2–2.2 Current efficiency (%) - - >98 Transport number (%) 91 98 >98 a Data from the manufacturer’s instruction manual.
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Table 2. Properties of the membranes applied to the BMED experiments a.
Membrane Neosepta AMX Neosepta CMX Neosepta BP-1 Characteristics IEC (meq·g−1) 1.4–1.7 1.5–1.8 - Thickness (μm) 120–180 220–260 200–350 Area resistance 2.0–3.5 2.0–3.5 - (Ω·cm2) Voltage drop (V) - - 1.2–2.2 Current efficiency (%) - - >98 Transport number 91 98 >98 (%) Membranes 2021, 11, 152 4 of 14 a Data from the manufacturer’s instruction manual.
2.2. Setup 2.2.A Setup self-assembled BMED stack was used for the experiment. The stack consisted of six bipolarA self-assembled membranes, five BMED anion stack exchange was used membranes, for the experiment. and five Thecation stack exchange consisted mem- of six branesbipolar comprising membranes, five five repeating anion exchange BM–A–C–BM membranes, units, one and of five which cation is exchangeshown in membranes Figure 2. Thecomprising effective membrane five repeating area BM–A–C–BM of each membrane units, was one 189 of which cm2. The is shown adjacent in Figuremembranes2. The wereeffective separated membrane by a spacer area of with each a membrane thickness of was 0.75 189 mm. cm2 .A The ruthenium-coated adjacent membranes titanium were electrodeseparated plate by were a spacer connected with a thickness on either ofside 0.75 of mm.the membrane A ruthenium-coated stack, respectively, titanium and electrode the twoplate plates were were connected connected on either with sidea DC of powe the membraner supply (WYL1703, stack, respectively, Hangzhou and Siling the two Electri- plates calwere Instrument connected Co. with Ltd., a DCHangzhou, power supply China). (WYL1703, Four circulating Hangzhou loops Siling ensured Electrical streams Instrument recir- culatedCo. Ltd., into Hangzhou,the stack and China). were formed Four circulating between two loops neighboring ensured streams membranes recirculated with four into peristalticthe stack pumps and were (Baoding formed Le betweenad Fluid twoTechnology neighboring Co. membranesLtd., Baoding, with China), four peristalticthe salt compartment,pumps (Baoding acid Leadcompartment, Fluid Technology base comp Co.artment, Ltd., Baoding, and electrode China), thecompartment. salt compartment, Ac- cordingacid compartment, to different experimental base compartment, schemes, and aqueous electrode sodium compartment. sulfate solutions According of different to differ- concentrationsent experimental and volumes schemes, (0.1–0.5 aqueous mol·L sodium−1) were sulfate fed solutionsinto the salt of chamber different (the concentrations compart- − mentand between volumes the (0.1–0.5 anion molexchange·L 1) were membrane fed into an thed cation salt chamber exchange (the membrane) compartment at different between flowthe rates, anion and exchange 500 mL membrane deionized and water cation (containing exchange a membrane)few drops of at the different corresponding flow rates, electrolyteand 500 mLsolution) deionized was fed water into (containingthe acid chamber a few and drops the alkali of the chamber corresponding at different electrolyte flow ratessolution) (200–600 was mL·min fed into−1) [23]. the acidFor circulation, chamber and 600 the mL alkali of 0.3 chambermol·L−1 aqueous at different Na2SO flow4 solu- rates −1 −1 tion(200–600 was added mL·min to the)[ anode23]. For and circulation, cathode chambers 600 mL of which 0.3 mol are·L connectedaqueous Natogether2SO4 solution inter- nally,was and added the toflow the rate anode was and 200–600 cathode mL·min chambers−1. The which BMED are recovery connected experiment together was internally, car- −1 riedand out the in flow electrostatic rate was suppression 200–600 mL· minmode,. with The BMEDthe current recovery density experiment ranging wasfrom carried 10 to 50 out −2 mA·cmin electrostatic−2, and the suppression voltage could mode, be read with from the current the DC density power rangingsupply board. from 10 The to50 salt mA cham-·cm , berand conductivity the voltage was could measured be read using from thea portable DC power conductivity supply board. meter The (DDBJ-350, salt chamber INESA con- Scientificductivity Instrument was measured Co. Ltd., using Shanghai, a portable China), conductivity and the meter experiment (DDBJ-350, was INESAstopped Scientific when theInstrument salt chamber Co. conductivity Ltd., Shanghai, was China),lower than and 500 the uS·cm experiment−1. was stopped when the salt chamber conductivity was lower than 500 uS·cm−1.
Figure 2. Schematic diagram of the BMED membrane reactor used in the experiment. Figure 2. Schematic diagram of the BMED membrane reactor used in the experiment.
2.3. Analysis and Calculations
The concentrations of H2SO4 and NaOH were determined using a digital titrator (Continuous RS, VITLAB, Muhltal, Germany), with phenolphthalein used as an indicator. The acid and alkali concentrations of the product were calculated using Equation (1) [25]:
Cc × Vc Ct = (1) Vr
−1 where Ct is the molar concentration of H2SO4/NaOH (mol·L ) at time t, Cc is the concen- tration of the NaOH/H2SO4 standard solution, and Vr and Vc are the volume of the acid compartment at time t and the volume of the titration solution, respectively. The current efficiency (η, %) of the BMED process was calculated using Equation (2) [26]:
Z(C V − C V )F η = t t 0 0 × 100% (2) NIt Membranes 2021, 11, 152 5 of 14
−1 where Ct and C0 are the molar concentrations of sulfuric acid (mol·L ) at times t and 0, re- spectively; Vt and V0 are the volumes of produced acid at time t and time 0, respectively; Z is the absolute ion valence (Z = 2 for sulfuric acid); F is the Faraday constant (96,485 C·mol−1); I (A) is the applied current; N is the repeating unit (N = 5) of the apparatus; and t (min) is the test time. The energy consumption E (kW·h·kg−1) was calculated using Equation (3) [26]:
Z t UIdt E = (3) 0 CtVt M
where E (kW·h·kg−1) is the energy consumption; U (V) is the voltage drop across the BMED −1 apparatus; I (A) is the applied current; Ct (mol·L ) and Vt (L) are the concentration and volume of feed concentration, respectively, at time t; and M is the molar mass of sodium sulfate (142.04 g·mol−1).
3. Results and Discussion 3.1. Effect of Feed Concentration on BMED The feed concentration is the key factor that can affect BMED performance [27]. For this reason, the influence of the feed concentration was investigated in the range of 0.1 to 0.5 mol·L−1. The current density was 30 mA·cm−2, and the flow rate of all solutions was 500 mL·min−1. Figure3 shows that the C H+ in the acid compartment and COH− in the base com- partment increased gradually as a function of time. It can be seen that the concentrations of acid and base increased as a function of time, indicating that H+ and OH− were con- tinuously produced by BPM and transported from the cation exchange layer (CEL) and anion exchange layer (AEL) to the acid and base compartments, respectively. Meanwhile, 2− the SO4 ions were transported from the salt compartment to the acid compartment and combined with H+ ions to produce sulfuric acid, while the Na+ ions were transported from the salt compartment to the base compartment and combined with OH− ions to produce sodium hydroxide. The greater the feed concentration, the higher the acid–base concentration obtained, which is in line with the theoretical situation. But we can see that as the salt concentration increases, the acid and base concentrations start to differ over time. This may be due to the following reasons. A fixed current density means a steady water splitting, resulting in a constant rate of proton formation. As the concentration of the salt chamber increases, high salt concentration leads to high osmotic pressure, which in this case increases the driving force for the diffusion of water molecules from the salt chamber to the alkali chamber, thus resulting in the enlargement of the volume of the alkali chamber. Figure3b shows the change in salt chamber conductivity as a function of time. It can be seen that with the progress of the experiment, the conductivity of the salt chamber gradually decreased, and it could be reduced to 500 uS·cm−1 within the range of 0.1 to 0.5 mol·L−1, indicating that BMED has good conversion efficiency for sodium sulfate within the indicated concentration range. Figure3c shows the voltage drop under different feed concentrations. It can be seen that the voltage drop decreased due to water splitting on the bipolar membrane at the beginning of the experiment. It then reached a plateau, and as the feed concentration increased, the voltage drop became smaller. In the later stage of the experiment, the concentration of the salt chamber dropped sharply, resulting in an increase in the resistance of the membrane stack and an increase in voltage drop. Figure3d shows the current efficiency and energy consumption. It can be seen that the current efficiency decreased as feed concentration increased. According to Donnan equilibrium theory [28,29], the selectivity of an ion exchange membrane decreases with an increase in electrolyte concentration. As a result, more ions can be transported through the ion exchange membrane, resulting in a decrease in current efficiency at high salt concentrations. In addition, with the gradual increase in salt concentration, the osmotic pressure around the bipolar membrane also increases, and it becomes difficult for water molecules to migrate to the interfacial layer of the bipolar membrane, thus reducing the water supply required for Membranes 2021, 11, x FOR PEER REVIEW 6 of 15
mol·L−1, indicating that BMED has good conversion efficiency for sodium sulfate within the indicated concentration range. Figure 3c shows the voltage drop under different feed concentrations. It can be seen that the voltage drop decreased due to water splitting on the bipolar membrane at the beginning of the experiment. It then reached a plateau, and as the feed concentration increased, the voltage drop became smaller. In the later stage of the experiment, the concentration of the salt chamber dropped sharply, resulting in an increase in the resistance of the membrane stack and an increase in voltage drop. Figure 3d shows the current efficiency and energy consumption. It can be seen that the current efficiency decreased as feed concentration increased. According to Donnan equilibrium theory [28,29], the selectivity of an ion exchange membrane decreases with an increase in electrolyte concentration. As a result, more ions can be transported through the ion ex- change membrane, resulting in a decrease in current efficiency at high salt concentrations. In addition, with the gradual increase in salt concentration, the osmotic pressure around Membranes 2021, 11the, 152 bipolar membrane also increases, and it becomes difficult for water molecules to mi- 6 of 14 grate to the interfacial layer of the bipolar membrane, thus reducing the water supply required for water splitting. Theoretically, with increasing feed salt concentration, the re- sistance of the membrane stack decreases, and energy consumption decreases. However, water splitting. Theoretically, with increasing feed salt concentration, the resistance of the when the feed salt concentration is greater than 0.3 mol·L−1, energy consumption starts to membrane stack decreases, and energy consumption decreases. However, when the feed increase, which may be due to the large concentration gradient of sulfate ions in the acid salt concentration is greater than 0.3 mol·L−1, energy consumption starts to increase, which chamber and saltmay chamber be due toat thethe largelater stage concentration of the experiment, gradient of leading sulfate ionsto the in diffusion the acid chamberof and salt sulfate ions fromchamber the acid at chamber the later to stage the salt of the chamber experiment, [30]. At leading the same to time, the diffusion the increased of sulfate ions from sulfate in the feedthe leads acid chamber to an increase to the in salt osmotic chamber pressure [30]. Ataround the same the bipolar time, the membrane, increased sulfate in the which inhibits feedwater leads splitting to an in increase the bipolar in osmotic interface pressure of the around bipolar the membrane. bipolar membrane, Reduced which inhibits water injectionwater in a bipolar splitting membrane in the bipolar interface interface increases of the energy bipolar consumption. membrane. Reduced Based on water injection in the comprehensivea bipolar acid membraneand base yield, interface it can increases be seen energythat when consumption. the salt concentration Based on the of comprehensive −1 the feed was aboutacid and0.2–0.3 base mol·L yield,, ithigh can current be seen efficiency that when and the saltlow concentrationenergy consumption of the feed was about could be achieved0.2–0.3 with mol high·L− acid1, high and current base yield. efficiency and low energy consumption could be achieved with high acid and base yield.
2.0 70 (a) Acid concentration Alkali concentration (b) 0.1 mol/L 1.8 60 0.2 mol/L 1.6 0.3 mol/L (mol/L) 0.4 mol/L 50 1.4 0.5 mol/L
1.2 40
1.0 30 0.8 0.1 mol/L 0.2 mol/L 0.6 0.3 mol/L 20
0.4 mol/L Conductivity(ms/cm) 0.4 0.5 mol/L 10 0.2
0.0 0 120 100 80 60 40 20 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Time (min) Time(min) 32 2.0 90 (c) Energy consumption Current efficiency 30 0.1 mol/L (d) 0.2 mol/L ) 80
28 Current efficiency 0.3 mol/L 1.6 70 26 0.4 mol/L 0.5 mol/L 24 kw·h/kg 60
22 ( 1.2 50 20 40 18 0.8 ( 16 30 % Voltage drop(V)
14 ) 20 0.4 12 10 10 Energy consumption Energy 8 0.0 0 0 20 40 60 80 100 120 0.1 0.2 0.3 0.4 0.5 Time(min) Concentration(mol/L)
Figure 3. EffectFigure of feed3. Effect concentration of feed concentration on BMED performance: on BMED performance: (a) acid concentration (a) acid concentration and alkali concentration, and alkali (b) electrical conductivity,concentration, (c) voltage drop (b) electrical across the conductivity, apparatus, and (c) (voltaged) energy drop consumption across the apparatus, and current and efficiency. (d) energy consumption and current efficiency. 3.2. Effect of Current Density on BMED 3.2. Effect of Current Density on BMED In the operation of BMED, current density is one of the most important factors that determine current efficiency, energy consumption, capital cost, and other operation perfor- mance indicators [21]. Current density in the range of 10 to 50 mA·cm−2, commonly used
in BMED processes, was selected in this experiment. The concentration of sodium sulfate in the salt chamber was 0.3 mol·L−1. The flow rate of all circulating loops was 500 mL·min−1. Figure4 shows the effect of current density on BMED operation. When the current density was 10 mA·cm−2, the experimental time was more than 240 min, and its desalting effect on sodium sulfate was very poor. Therefore, the experimental inflection point (after which desalting is considered unable to be carried out) was selected here for drawing processing. As shown in Figure4a, the concentration of C H+ in the acid compartment and COH− in the base compartment increased with time, which indicates that sodium sulfate waste salt was successfully converted into sulfuric acid and sodium hydroxide, and the higher the current efficiency, the higher the concentration of generated acid and base. It can be seen from the diagram that when the current efficiency was 20–40 mA·cm−2, BMED was well able to convert sodium sulfate to acid and alkali. However, when the current density was 10 mA·cm−2, it can be seen from Figure4b that the conductivity of the salt chamber Membranes 2021, 11, 152 7 of 14
decreased slowly. It took more than 240 minutes to reduce the conductivity of the salt chamber below 500 uS·cm−1, indicating that sodium sulfate decomposed when the current density was 10 mA·cm−2. The conversion efficiency was very low, and it was difficult to convert salt into acid and base. Figure4c shows the variation curve of the voltage drop over time. Its basic trend is the same as that described above. Due to the low ion concentration in the initial acid and base chambers, the resistance of the membrane stack was larger. With the decomposition of the contents of the salt chamber by BMED, the concentration in the acid–base chamber increased gradually, and the resistance decreased and tended to a steady state [31]. In the late stage of the experiment, the salt chamber concentration decreased sharply, and the membrane stack resistance rose once again. In addition, the voltage drop increased with increasing current density, which is consistent with the theory [32]. Figure 4d shows the energy consumption and current efficiency with an increase in current density and a gradual increase in energy consumption; this fits most of the research results [33–35]. However, with increasing current density and current efficiency, high current density will, in theory, increase the water splitting, which increases with the amount of ion transport across the membrane; the current efficiency should then drop but, here, the opposite is observed. After ruling out a current density of 10 mA·cm−2 due to its desalination taking a long time with low current efficiency, the observations in the remaining four groups could be explained in three possible ways. Firstly, the concentration of the salt chamber is low, so the four groups of current density can effectively complete transformation; as a result, the influence of current density on the conversion rate is eliminated, and the difference in reaction time is small. The increase in the number of protons passing through the anion exchange membrane brought by high current density is not enough to reduce the current efficiency. In addition, due to the increase in current density, more H+ ions are generated near the bipolar membrane, which accelerates the reaction process and improves Membranes 2021, 11, x FOR PEER REVIEWthe current efficiency [36]. Lastly, in the case of consuming the same amount of charge,8 of 15 the acid chamber H+ ions increase with increasing current density, resulting in an increase in current efficiency [37]. Thus, there is a growing trend. In general, a current efficiency of 30 mA·cm−2 provided better processing performance.
1.2 (a) Acid concentration Alkali concentration (b) 10 mA/cm2 2 1.0 40 20 mA/cm (mol/L) 2
) 30 mA/cm 40 mA/cm2 0.8 32 50 mA/cm2 ms/cm (
0.6 24 10 mA/cm2 20 mA/cm2 0.4 16 30 mA/cm2 40 mA/cm2
2 Conductivity 0.2 50 mA/cm 8
0.0 0 240 200 160 120 80 40 0 40 80 120 160 200 240 0 40 80 120 160 200 240 Time (min) Time(min) 20 2.0 90 (c) Energy consumption Current efficiency 2 (d)
10 mA/cm ) 80 18 2 20 mA/cm Current efficiency 1.6 30 mA/cm2 70
) 16 40 mA/cm2 kw·h/kg
V 60 2
50 mA/cm (
( 1.2 14 50
12 40 0.8 ( 30 10 % Voltage drop Voltage 20 ) 0.4 8 10 Energy consumption Energy 6 0.0 0 0 40 80 120 160 200 240 10 20 30 40 50 2 Time(min) Current density(mA/cm )
FigureFigure 4. Effect 4. Effect of current of current density density on BMED on BMED performance: performance: (a) acid (a) acid concentration concentration and and alkali alkali concen- concen- tration,tration, (b) (electricalb) electrical conductivity, conductivity, (c) (voltagec) voltage drop drop across across the the apparatus, apparatus, and and (d ()d energy) energy consump- consumption tionand and current current efficiency. efficiency.
3.3. Effect of Flow Rate on BMED The influence of flow rate in the range of 200 to 600 mL·min−1 was studied. A current density of 30 mA·cm−2 was selected, and the sodium sulfate concentration in the salt cham- ber was 0.3 mol·L−1. Figure 5 shows the effect of flow rate on performance. The energy consumption for pump could be neglected compared with the energy applied for BEMD stack, so it was not considered in the experimental scale study.
1.2 50 (a)Acid concentration Alkali concentration (b) 45 200 ml/min 1.0 300 ml/min
(mol/L) 40 400 ml/min 35 0.8 500 ml/min 30 600 ml/min
0.6 25
200 ml/min 20 0.4 300 ml/min 15 400 ml/min Conductivity(ms/cm) 500 ml/min 10 0.2 600 ml/min 5
0.0 0 70 60 50 40 30 20 10 0 10203040506070 0 102030405060 Time (min) Time(min) 2.0 80 12.4 (c) 200 ml/min (d) Energy consumption Current efficiency
12.2 ) 1.8 300 ml/min 70 12.0 efficiency Current 400 ml/min 1.6 11.8 500 ml/min 60
) 11.6 1.4
600 ml/min kw·h/kg V 11.4 50 (
( 1.2 11.2 11.0 1.0 40 10.8 0.8
10.6 30 (
10.4 0.6 % Voltage drop 10.2 20 ) 0.4 10.0 10 9.8 0.2 Energy consumption Energy 9.6 0.0 0 0 102030405060 200 300 400 500 600 Time(min) Flow rate of system(mL/min)
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1.2 (a) Acid concentration Alkali concentration (b) 10 mA/cm2 2 1.0 40 20 mA/cm (mol/L) 2
) 30 mA/cm 40 mA/cm2 0.8 32 50 mA/cm2 ms/cm (
0.6 24 10 mA/cm2 20 mA/cm2 0.4 16 30 mA/cm2 40 mA/cm2
2 Conductivity 0.2 50 mA/cm 8
0.0 0 240 200 160 120 80 40 0 40 80 120 160 200 240 0 40 80 120 160 200 240 Time (min) Time(min) 20 2.0 90 (c) Energy consumption Current efficiency 2 (d)
10 mA/cm ) 80 18 2 20 mA/cm Current efficiency 1.6 30 mA/cm2 70
) 16 40 mA/cm2 kw·h/kg
V 60 2
50 mA/cm (
( 1.2 14 50
12 40 0.8 ( 30 10 % Voltage drop Voltage 20 ) 0.4 8 10 Energy consumption Energy 6 0.0 0 0 40 80 120 160 200 240 10 20 30 40 50 2 Time(min) Current density(mA/cm )
Figure 4. Effect of current density on BMED performance: (a) acid concentration and alkali concen- tration, (b) electrical conductivity, (c) voltage drop across the apparatus, and (d) energy consump- Membranes 2021, 11, 152 tion and current efficiency. 8 of 14
3.3. Effect of Flow Rate on BMED 3.3.The Effect influence of Flow of Rate flow on rate BMED in the range of 200 to 600 mL·min−1 was studied. A current −2 densityThe of 30 influence mA·cm ofwas flow selected, rate in theand rangethe sodium of 200 sulfat to 600e mLconcentration·min−1 was in studied. the salt A cham- current −1 berdensity was 0.3 of mol·L 30 mA. ·Figurecm−2 was 5 shows selected, the effect andthe of flow sodium rate sulfate on performance. concentration Thein energy the salt consumptionchamber was for 0.3pump mol could·L−1. be Figure neglected5 shows compared the effect with of flowthe energy rate on applied performance. for BEMD The stack,energy so it consumption was not considered for pump in the could experimental be neglected scale compared study. with the energy applied for BEMD stack, so it was not considered in the experimental scale study.
1.2 50 (a)Acid concentration Alkali concentration (b) 45 200 ml/min 1.0 300 ml/min
(mol/L) 40 400 ml/min 35 0.8 500 ml/min 30 600 ml/min
0.6 25
200 ml/min 20 0.4 300 ml/min 15 400 ml/min Conductivity(ms/cm) 500 ml/min 10 0.2 600 ml/min 5
0.0 0 70 60 50 40 30 20 10 0 10203040506070 0 102030405060 Time (min) Time(min) 2.0 80 12.4 (c) 200 ml/min (d) Energy consumption Current efficiency
12.2 ) 1.8 300 ml/min 70 12.0 efficiency Current 400 ml/min 1.6 11.8 500 ml/min 60
) 11.6 1.4
600 ml/min kw·h/kg V 11.4 50 (
( 1.2 11.2 11.0 1.0 40 10.8 0.8
10.6 30 (
10.4 0.6 % Voltage drop 10.2 20 ) 0.4 10.0 10 9.8 0.2 Energy consumption Energy 9.6 0.0 0 0 102030405060 200 300 400 500 600 Time(min) Flow rate of system(mL/min)
Figure 5. Effect of flow rate on BMED performance: (a) acid concentration and alkali concentration, (b) electrical conductivity, (c) voltage drop across the apparatus, and (d) energy consumption and current efficiency.
Figure5a shows the influence of flow rate on the acid and base yield, and it can be seen that the concentrations of acid and base increased as time passed. Figure5b shows the change curve of salt chamber conductivity with time. It can be seen that with the passage of time, the conductivity values of the salt chambers with different flow rates were all reduced to 500 uS·cm−1. As the current density was the same, the salt concentration with a low flow rate needed a longer time. Figure5c shows the effect of flow rate on the voltage drop of the BMED reactor. As the experiment progressed, the voltage drop showed a tendency to fall first, then to stabilize, and eventually rise. The initial descent was due to the introduction of acids and bases through the bipolar membrane. After about 7 minutes, the BMED reactor state began to stabilize, and the voltage drop also reached a stable state. It can be seen that as the flow rate increased, the voltage drop became larger and larger, and the time required for the experiment became shorter. Figure5d shows the energy consumption and current efficiency at different flow rates. Energy consumption decreased with increasing flow rate mainly due to the short reaction time required by a high flow rate under the same current density. After reaching 500 mL·min−1, it started to rise again, which may have been because an increase in the feed liquid requires more energy for circulation. By contrast, the current efficiency was relatively stable, but the overall current efficiency still presented an upward trend. Since the number of H+ ions in the bipolar membrane was constant, the ratio of H+/Na+ at low flow rates was higher, and the current efficiency was lower [24]. Therefore, a flow rate of 500 mL·min−1 was more appropriate.
3.4. Effect of Volume Ratio on BMED Volume ratio, as one of the parameters of BMED operation, is also an important factor affecting performance [38]. Therefore, in this experiment, the volume ratio of the acid Membranes 2021, 11, x FOR PEER REVIEW 10 of 15
ment progressed, acid and alkali were gradually produced, and the electrolyte concentra- tion increased. Then, there was a plateau, and the voltage drop reached the lowest value. At this time, the average conductivity of the acid–base salt chamber was at a high level, and the BMED operation reached a steady state. In the later stage of the experiment, the voltage drop began to rise due to the greatly reduced conductivity of the salt chamber. When the volume ratio was 1:2, the voltage drop in the steady state was the lowest. The energy consumption and current efficiency are shown in Figure 6d. As the volume ratio increased, the current efficiency gradually decreased and dropped below 30% for a ratio of 1:5. This is mainly due to the increase in volume ratio—with the prolonging of reaction Membranes 2021, 11, 152 9 of 14 time, the concentration in the acid–base chamber increased, which led to an increase in ion back-diffusion and hydrolysis. The change in energy consumption was the opposite. When the volume ratio was 1:1, the energy consumption was 1.35 kWh/kg, and when the volume(alkali) ratio chamber was increased and the to salt 1:5, chamber the energy was consumption varied over 1:1,increased 1:2, 1:3, to 1:4,2.67 andkWh/kg—an 1:5. We chose a increasecurrent of densitymore than of 3097%. mA On·cm the− 2basis, and of the the sodium high yield sulfate of acid concentration and base, 1:2 inwas the selected salt chamber as wasthe optimal 0.3 mol ·volumeL−1. The ratio. flow rate of all circulation loops was 500 mL·min−1. Figure6 shows the effect of volume ratio on BMED performance.
45 (a) Acid concentration 2.7 Alkali concentration (b) 40 1:1 2.4 1:2
(mol/L) 35 2.1 1:3 1:4 30 1.8 1:5
25 1.5
20 1.2 1:1 1:2 0.9 15 1:3
1:4 Conductivity(ms/cm) 0.6 10 1:5 0.3 5
0.0 0 270 240 210 180 150 120 90 60 30 0 30 60 90 120 150 180 210 240 270 0 50 100 150 200 250 300 Time (min) Time(min) 13.0 3.0 80 (c) 2.8 (d) Energy consumption Current efficiency ) 2.6 70
12.5 Current efficiency 2.4 2.2 60
) 12.0 kw·h/kg
V 2.0 50 (
( 1.8 11.5 1.6 40 1.4 11.0 1.2
1:1 30 ( 1.0
1:2 % 10.5 0.8 Voltage drop Voltage 1:3 20 ) 1:4 0.6 10.0 1:5 0.4 10
Energy consumption Energy 0.2 9.5 0.0 0 0 50 100 150 200 250 300 1:1 1:2 1:3 1:4 1:5 Time(min) Volume ratio
FigureFigure 6. Effect 6. Effect of volume of volume ratio ratio on BMED on BMED performance: performance: (a) acid (a )concentration acid concentration and alkali and concen- alkali concentra- tration,tion, ( b)) electrical electrical conductivity, conductivity, (c ()c voltage) voltage drop drop across across the the apparatus, apparatus, and and (d) energy (d) energy consump- consumption tionand and current current efficiency. efficiency.
3.5. PerformanceAs shown Evaluation in Figure 6a, the acid–base concentration increased with time. At the beginningThe above of experiments the experiment, demonstrate the rates that of acidthe sa andlt concentration, base increase current in each density, experiment flow were rate,not and affected volume by ratio the volumeeach have ratio. a significant However, influence it is obvious on BMED thatwith performance. increasing Through volume ratio, thethe BMED final process, obtained sodium concentration sulfate wastes was can higher. be converted As the volumeinto sulfuric ratio acid gradually and sodium increased, hydroxide,the alkali and chamber the products concentration can be wasused gradedas raw lowermaterials than in the the acid upstream chamber production concentration, process,and when thus achieving the volume closed-loop ratio reached production. 1:4, the concentration in the alkali chamber began toIn decrease. order to Thisfurther was verify mainly the effect due toof theBMED fact on that sodium when sulfate the volume waste salt of thetreatment, salt chamber a simulatedbecame larger, waste thesalt experimentsolution was took used longer, for experiments. while the The Na2 treatmentSO4 content was increased performed sharply underwith the the abovementioned increase of volumetric optimal ratio. experimental In this case, conditions. the osmotic In addition, pressure after will each also ex- increase, periment,which will we increaseused pure the water leakage circulation of water, to causing clean the the membrane alkali chamber stack tofor produce more than more 30 water; min.this As caused shown the in volumeFigure 7a, of BMED the alkali efficiently chamber converted to become the larger,simulated and salt the solution concentration into then showed a downward trend [39]. This phenomenon can be attributed to the increased difference between the salt and acid chambers as the volume ratio increased. The number of sodium hydrate ions entering the alkali chamber was higher than the number of sulfate hydrate ions entering the acid chamber, and the amount of water carried by the hydrated ions also increased [40]. Figure6b shows the change in conductivity of the salt chamber, and its change trend was basically the same as for acid and alkali concentration. Figure6c shows the voltage drop versus time curve. The volume ratio had the same influence on the voltage drop as the flow rate. The initial voltage drop was mainly due to the fact that the initial acid–base chamber was pure water with very low conductivity. As the experiment progressed, acid and alkali were gradually produced, and the electrolyte concentration increased. Then, there was a plateau, and the voltage drop reached the lowest value. At this time, the average conductivity of the acid–base salt chamber was at a high level, and the BMED operation reached a steady state. In the later stage of the experiment, the voltage drop began to rise due to the greatly reduced conductivity of the salt chamber. When the volume ratio was 1:2, the voltage drop in the steady state was the lowest. The energy consumption and current efficiency are shown in Figure6d. As the volume ratio increased, the current efficiency Membranes 2021, 11, 152 10 of 14
gradually decreased and dropped below 30% for a ratio of 1:5. This is mainly due to the increase in volume ratio—with the prolonging of reaction time, the concentration in the acid–base chamber increased, which led to an increase in ion back-diffusion and hydrolysis. The change in energy consumption was the opposite. When the volume ratio was 1:1, the energy consumption was 1.35 kWh/kg, and when the volume ratio was increased to 1:5, the energy consumption increased to 2.67 kWh/kg—an increase of more than 97%. On the basis of the high yield of acid and base, 1:2 was selected as the optimal volume ratio.
3.5. Performance Evaluation The above experiments demonstrate that the salt concentration, current density, flow rate, and volume ratio each have a significant influence on BMED performance. Through the BMED process, sodium sulfate wastes can be converted into sulfuric acid and sodium hydroxide, and the products can be used as raw materials in the upstream production process, thus achieving closed-loop production. In order to further verify the effect of BMED on sodium sulfate waste salt treatment, Membranes 2021, 11, x FOR PEER REVIEW 11 of 15 a simulated waste salt solution was used for experiments. The treatment was performed under the abovementioned optimal experimental conditions. In addition, after each experi- ment, we used pure water circulation to clean the membrane stack for more than 30 min. Asacid shown and in alkali. Figure The7a, basic BMED trend efficiently is consistent converted with the the above. simulated However, salt solution because into the acidsimu- andlated alkali. salt The solution basic contained trend is consistent other ions, with the the overall above. voltage However, drop becausewas higher the than simulated that of saltthe solution pure feed, contained and the other reaction ions, time the overallwas also voltage shortened, drop which was higher was mainly than that due of to the the purepresence feed, and of lithium the reaction ions timeand wascarbonate also shortened, ions. It can which be seen was from mainly Figure due to7b thethat presence the final of lithiumacid and ions base and concentrations, carbonate ions. current It can efficiency, be seen from and Figureenergy7 bconsumption that the final were acid higher and basethan concentrations, the pure feed. current When efficiency,the solution and contains energy lithium consumption ions, the were lithium higher ions than and the the pure sul- feed.fate When ions may the solutionpromote containseach other, lithium which ions, accelerates the lithium the reaction ions and process the sulfate and better ions trans- may promotefers from each the other, salt chamber which accelerates to the corresponding the reaction compartment. process and better At the transfers same time, from CO the32− + + 2− saltwill chamber combine to thewith corresponding Na in the form compartment. of HCO3 , which Atthe leads same to a time, relative CO 3increasewill combinein the mi- + + 2− withgration Na in amount the form and of migration HCO3 , which efficiency leads of to lithium a relative ions. increase The existence in the migration of CO3 has amount a ben- 2− andeficial migration effect efficiencyon the experimental of lithium process ions. The to existencea certain ofextent. CO3 Tablehas 3 a lists beneficial the composition effect on theof experimental the BMED products. process The to a purity certain levels extent. of Tablesulfuric3 lists acid the and composition sodium hydroxide of the BMEDreached products.98.32% and The 98.23%, purity levelsrespectively, of sulfuric as determined acid and sodiumusing an hydroxide atomic absorption reached spectrograph 98.32% and 98.23%,(240 FS respectively, AA, Agilent as Technologies, determined using Inc., Santa an atomic Clara, absorption CA, USA), spectrograph which can be (240 used FS in AA, the Agilentproduction Technologies, of lithium Inc., carbonate Santa Clara, to achieve CA, USA), closed-loop which canproduction. be used in the production of lithium carbonate to achieve closed-loop production.
45 (a) Pure feed 45 60 (b) Pure feed 40 Simulated feed Pure feed 40 Simulated feed 35 50 Simulated feed 35 30 30 40 25 25 30 20 20 1.5 15 15 Voltage drop(V) 1.0 10 10
0.5 Electrical conductivity(ms/cm) Electrical 5 5
0 0 0.0 0 102030405060 Acid(mol/L) Base(mol/L) EC(kw·h/kg) CE(%) Time(min)
Figure 7. Comparison of BMED experiments using simulated feed and pure feed: (a) the experimental process and (b) the Figure 7. Comparison of BMED experiments using simulated feed and pure feed: (a) the experimental process and (b) the experimental results. experimental results.
Table 3. Component analysis of BMED products.
Component (mg·g−1) Product SO42− Na+ Li+ Ca+ Mg+ H2SO4 (98 - a 12.99 wt. %) Feed NaOH (dry 11.55 - weight) H2SO4 (98 - 17.06 0.07 -- b -- Simulated wt. %) feed NaOH (dry 16.43 - 1.64 -- -- weight) a High content, did not detect the item; b low content, was not detected.
3.6. Economic Analysis
Membranes 2021, 11, 152 11 of 14
Table 3. Component analysis of BMED products.
Component (mg·g−1) Product 2− + + + + SO4 Na Li Ca Mg a H2SO4 (98 wt. %) - 12.99 Feed NaOH (dry weight) 11.55 - b H2SO4 (98 wt. %) - 17.06 0.07 – – Simulated feed NaOH (dry weight) 16.43 - 1.64 – – a High content, did not detect the item; b low content, was not detected.
3.6. Economic Analysis We calculated the economic performance of the BMED process for treating sodium sulfate waste containing lithium carbonate, as shown in Table4. The relevant data in Table4 are based on previous calculation methods [ 41,42]. The processing capacity was calculated according to 8640 h per year with membrane stack cost being 1.5 times the membrane cost, and peripheral equipment cost being 1.5 times the membrane stack cost. The membrane was considered to have a service life of 3 years with an annual interest rate of 8% and maintenance cost of 10% of the total investment cost. Therefore, the cost of treating sodium sulfate with BMED was calculated as follows:
Table 4. Process cost analysis of bipolar membrane electrodialysis.
Parameters BMED Process Remarks Feed volume (L) 1.0 Feed salt concentration (mol·L−1) 0.3 Current density (mA·m−2) 30 Batch experiment time (min) 52 Effective each membrane area (cm2) 189 −1 Energy consumption (kWh·kg Na2SO4) 1.469 −1 Treatment capacity (kg Na2SO4·year ) 424 1 year, 8640 h Price of bipolar membrane ($·m−2) 800 Price of mono membrane ($·m−2) 200 Membrane lifetime and amortization of the 3 Based on the literature a and actual conditions peripheral equipment (year) Electricity charge ($·kWh−1) 0.0825 Based on China’s electricity Membrane cost ($) 128.52 Apparatus cost ($) 192.78 Peripheral equipment cost ($) 289.17 Total investment cost ($) 481.95 Amortization ($·year−1) 160.65 Interest ($·year−1) 38.556 Interest rate, 8% Maintenance ($·year−1) 48.195 10% of total investment cost Total fixed cost ($·year−1) 247.401 −1 Total fixed cost ($·kg Na2SO4) 0.5835 −1 Energy cost ($·kg Na2SO4) 0.1212 −1 Total process cost ($·kg Na2SO4) 0.705 a H. Strathmann, and G.H. Koops, Process economics of electrodialytic water dissociation for the production of acid and base. in: A.J.B. Kemperman, (Ed.), Handbook on bipolar membrane technology, Enschede: Twente University Press, 2000, pp. 191–220. Membranes 2021, 11, 152 12 of 14
−1 −1 total fixed cost ($·kg Na2SO4) = total fixed cost ($·year )/process capacity −1 (kg Na2SO4·year ) −1 −1 total process cost ($·kg Na2SO4) = total fixed cost ($·kg Na2SO4)/total energy −1 cost ($·kg Na2SO4) It can be seen that under the premise of meeting processing standards, the cost of the −1 process is USD 0.705 kg Na2SO4. Taking into account the cost of originally disposing of sodium sulfate waste containing heavy metals and the benefits associated with the production of sulfuric acid and sodium hydroxide, the use of BMED for the treatment of sodium sulfate waste salt containing lithium carbonate is very beneficial.
4. Conclusions Continuous and stable treatment of sodium sulfate wastes containing lithium carbon- ate can be achieved by using a process combining bipolar membranes and electrodialysis. BMED can convert sodium sulfate wastes into sulfuric acid and sodium hydroxide, both of which can be used as raw materials for upstream processes, realizing closed-loop produc- tion and presenting economic benefits through reduced costs. The effects of the feed salt concentration, current density, flow rate, and volume ratio on the performance of BMED were investigated. When the feed salt concentration was increased to 0.5 mol, the perfor- mance of BMED began to decline, which may have been due to the increase in osmotic pressure caused by the high concentration and the intensification of ion diffusion. To ensure acid–base yield, 0.2–0.3 mol·L−1 was selected as the best salt concentration for the feed. Regarding current density, high current density leads to high energy consumption. In order to maximize the economic benefits, we chose 30 mA·cm−2 as the optimal current density. The influence of the flow rate and volume ratio on BMED is also not to be underestimated. The best flow rate and volume ratio were 500 mL·min−1 and 1:2, respectively. Under the optimal technological conditions, the simulated wastewater treatment achieved the ex- pected effect. The existence of lithium carbonate had a beneficial effect on the experimental process to a certain extent. The obtained sulfuric acid and sodium hydroxide products were determined by atomic absorption spectrometry, and the impurity content was low, with purity rates reaching 98.32% and 98.23%, respectively. The economic cost of the −1 BMED process was calculated as USD 0.705 kg Na2SO4. Therefore, BMED treatment of sodium sulfate waste salts containing lithium carbonate promotes the recycling production of lithium carbonate and has good economic and environmental benefits.
Author Contributions: Conceptualization, X.W. and K.W.; investigation, W.G.; formal analysis, H.Y.; data curation, W.G. and Q.F.; writing—original draft preparation, W.G.; writing—review and editing, X.W., supervision, K.W. All authors have read and agreed to the published version of the manuscript. Funding: This project was supported by the Natural Science Research Project of Anhui Province (No. KJ2019A0825); China National Critical Project for Science and Technology on Water Pollution Prevention and Control (2017ZX07603-003). Conflicts of Interest: The authors declare no conflict of interest.
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