Optimized Process for Separating NaOH from Sodium Aluminate

Most bauxite-produced alumina is obtained by the Bayer process, but the production efficiency is limited by the slow gibbsite crystal growth due to th...
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Optimized Process for Separating NaOH from Sodium Aluminate Solution: Coupling of Electrodialysis and Electro-Electrodialysis Haiyang Yan,† Cuiming Wu,*,† and Yonghui Wu*,‡ †

Anhui Key Lab of Controllable Chemical Reaction & Material Chemical Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, P.R. China ‡ Department of Chemistry, Yancheng Teachers University, Yancheng 224002, P.R. China S Supporting Information *

ABSTRACT: Most bauxite-produced alumina is obtained by the Bayer process, but the production efficiency is limited by the slow gibbsite crystal growth due to the high concentration of NaOH in sodium aluminate solution. Here electrodialysis (ED) and electro-electrodialysis (EED) are coupled to separate NaOH from the sodium aluminate solution so as to enhance the gibbsite crystal growth rate and achieve high alumina production efficiency. The ED or EED process is also investigated before the coupling process to find the optimal operating conditions. The ED process indicates that the optimized current density is in the range of 45−60 mA cm−2 and the optimal membranes are CMV/AMV. The current density of 60 mA cm−2 can achieve a high recovery ratio (ηOH− 92.6%), low energy consumption (2.38 kW h kg−1), but a relatively high Al(OH)4− leakage ratio (ηAl(OH)4− 15.1%). The EED process indicates that with the optimized current density of 30 mA cm−2 and membrane CMV, the ηAl(OH)4− can be “zero” and the energy consumption can be as low as 2.07 kW h kg−1, but the treatment capability is low since OH− ions cannot be recovered directly and a single cation exchange membrane is used. The coupling process can combine the advantages of ED and EED, so that the ηOH− can keep a high value of 90.9%, the ηAl(OH)4− decreases to a low value of ∼5%, and the energy consumption remains low at 2.25 kW h kg−1. Overall, the coupling process of ED and EED is an excellent method to separate NaOH from the sodium aluminate solution.

■

INTRODUCTION Alumina (Al2O3) is used widely in the manufacture of aluminum metal, semiconductors, ceramics, and catalyst base materials.1,2 Most bauxite-produced alumina is obtained by the Bayer process, which was developed by Josef Karl Bayer in 18873,4 and mainly consists of bauxite digestion, liquor clarification, gibbsite (Al(OH)3) precipitation, and calcination5 as illustrated in Figure 1a. During the digestion process, some organic compounds in the ore are broken down to form sodium salts of succinic, acetic, and oxalic acids, etc. Predominant among these salts is sodium oxalate. Besides, there are some insoluble components composed primarily of iron oxides, sodium aluminosilicates, calcium carbonate/aluminate, etc., in the liquors.6 The majority of these impurities can be disposed after clarification, and the remaining liquors are extremely alkaline (pH > 14), containing NaOH, sodium aluminate, and some remaining impurities (the impurities are not listed in Figure 1). Gibbsite precipitation from the Bayer liquors is time consuming due to the slow gibbsite crystal growth behavior.2 The precipitation of acceptable yields needs a long time (usually 2−3 days), which grievously limits the production efficiency in the Bayer process. The gibbsite crystal growth rate is correlated with the Al supersaturation, the excess NaOH concentration (cNaOH(excess)), and the presence of impurities.2,6 For instance, sodium oxalate can have deleterious effects on the gibbsite precipitation process mainly through raising the solubility of gibbsite and affecting gibbsite particle size classification, agglomeration efficiency, etc.6 More significantly, the presence of excess NaOH can decrease the gibbsite crystal © 2015 American Chemical Society

growth rate to a large extent by the following mechanisms. (1) NaOH concentration can influence the compositions of Alrelated components. At low NaOH concentration (1−6 mol/L, cNaOH/cAl = 1.22), Al(OH)4− will dominate the liqior speciation, which can give rise to the shortest induction time and the most rapid particle growth rate in the precipitation process.7 As the NaOH concentration increases, the Alcontaining species change from Al(OH)4− to complex ions such as Al(OH)63−, Al2O(OH)62−, and (OH)3Al−(OH)− Al(OH)3−, which slows the particle growth rate and increases the induction time.7 (2) The presence of excess NaOH can directly decrease the gibbsite growth rate. According to the research of King, the gibbsite growth rate, G (nm/min), may be expressed as eq 18 G=

K (Δc)2 (c NaOH(excess))2

, Δc = c − c* (1)

where K is a growth rate constant (nm/min), cNaOH(excess) is the excess NaOH concentration (mol), Δc is the absolute supersaturation of Al, c is the instantaneous Al concentration, and c* is the equilibrium solubility of Al. The cNaOH(excess) in the sodium aluminate solution generally is very high and can reach ∼6 mol/L, not only decreasing gibbsite crystal growth rate but also causing a low yield of Al(OH)3. As a result, the purity of Received: Revised: Accepted: Published: 1876

October 26, 2014 December 28, 2014 January 1, 2015 January 1, 2015 DOI: 10.1021/ie504223e Ind. Eng. Chem. Res. 2015, 54, 1876−1886

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic Bayer process without and with the ion exchange membrane (IEM) process.

Table 1. Properties of the Cation Exchange Membrane (CEM) and Anion Exchange Membrane (AEM)a manufacturer Asahi Glass Co. Ltd., Japan Beijing Tingrun Membrane Technology Development Co. Ltd., China Heifei Chemjoy Polymer Materials Co. Ltd., China

a

water uptake (%)

burst strength (MPa)

area resistance (Ω cm2)

transport number (%)

2.0b 2.2b 1.8−2.2

33−40

0.16 0.16 >0.25

2.5−3.0 1.5−3.0 2−5

>96 >96 95−99

160−230 ∼145

1.8−2.0 0.8−1.0

24−28 35−40

>0.25 >0.35

5−9 0.5−1.5

90−95 >98

∼200

0.8−1.0

35−40

>0.35

0.5−1.5

>98

membrane name

membrane type

thickness (μm)

IEC (meq/g)

CMV AMV JCM-II-07

CEM AEM CEM

130−150 110−150 160−230

JAM-II-07 CJEDC145 CJEDA200

AEM CEM AEM

Data are supplied by the manufacturers except where noted. bData are obtained from ref 22.

decrease the energy consumption, and other processes are required to overcome the problem of high ηAl(OH)4−. Electro-electrodialysis (EED) is a combination of electrolysis and electrodialysis with a single CEM or AEM.16 EED has been used in the production of acids and regeneration of NaOH from spent caustic.17−19 Patents US 5,141,610 and US 5,198,085 reported that NaOH etchants of aluminum could be continuously converted to solid Al(OH)3 and aqueous NaOH by EED.20,21 A high Na+ recovery ratio (∼100%) could be achieved, and ηAl(OH)4− was “zero” due to the peculiar ion transport mechanisms, namely, the Na+ ions in the etchants are electromigrated through CEM to the cathode compartment and then combined with the OH− ions which are produced by electrolysis of H2O; the Al(OH)4− ions undergo electrolysis to produce Al(OH)3 in the anode compartment or reaction compartment. However, here EED only uses a single CEM. Besides, a large amount of OH− ions in the etchants cannot be recovered directly. On the contrary, electrolysis of H2O is required in the cathode compartment to produce OH− ions. Hence, the treatment capability is lower than that of ED. On the basis of the characteristics of ED and EED processes, their coupling may bring complementary advantages in separating the sodium aluminate solution. In the preliminary stage, ED can be utilized to separate the solution rapidly and recover the OH− ions directly without need of the electrolysis of H2O. Afterward, EED can be used for further separation to achieve high ηOH− and low ηAl(OH)4−. Hence, the gibbsite crystal growth rate and the yield of Al(OH)3 can be enhanced, which elevates the production efficiency in the Bayer process.

NaOH in the mother liquor is also low, which is disadvantageous to the circuit use of the mother liquor. Hence, it is urgent to separate NaOH from the sodium aluminate solution before the procedures of seeding and precipitation. The addition of the separation process such as the ion exchange membrane (IEM) process during the Bayer process is shown in Figure 1b. The cNaOH(excess) can be decreased while the Δc can be increased due to the decrease of c*. Hence, the gibbsite crystal growth rate and yield as well as particle sizes7 of Al(OH)3 can be enhanced. In addition, the separated NaOH solution can be directly used as the mother liquor in the Bayer circuit. Electrodialysis (ED) is an electromembrane separation process with cation and anion exchange membranes (CEMs and AEMs) arranged in an alternating pattern to separate several cells.9 Under a driving force of direct current, anions and cations can migrate toward the anode and cathode, respectively.10 Nowadays, ED has been used widely for separation and concentration of inorganic or organic ions.11−14 Our previous report revealed that NaOH could be effectively separated from the simulated chemosynthesis sodium aluminate solution by ED.15 However, the energy consumption was higher than 12 kW h kg−1 due to the use of a high current density of 350 mA cm−2 and low effective area of 5.73 cm2 for each membrane. Besides, the alkali recovery ratio (ηOH−) was lower than 50% and the Al(OH)4− leakage ratio (ηAl(OH)4−) was higher than 12% since the Al(OH)4− ions are inevitably migrated from the feed solution to the recovery solution through AEM under the direct current. Hence, the parameters of the experiments need to be optimized to 1877

DOI: 10.1021/ie504223e Ind. Eng. Chem. Res. 2015, 54, 1876−1886

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Figure 2. Schematic separation setups: (a) ED setup, (b) EED setup, and (c) coupling setup. Setups contain the units of (1) ED membrane stack with three cell pairs, (2) EED membrane stack, (3, 4) direct current power supplies, (5) electrode tank, (6) feed tank, (7) recovery tank, (8−10) peristaltic pumps with a flow rate of 0.38 L min−1, and (11, 12) peristaltic pumps with a flow rate of 0.24 L min−1.

calcium carbonate, which may complicate the membrane process. Hence, the simulated chemosynthesis sodium aluminate solution containing NaOH and NaAl(OH)4 is utilized as the feed in the present study. In addition, a

Meanwhile, the separated NaOH solution can be used in the Bayer circuit. Sodium aluminate solution in the Bayer circuit is complex and contains a range of impurities such as sodium oxlate and 1878

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respectively. Feed tank (6) and recovery tank (7) were used as common tanks. Other conditions were the same as those in the single ED or EED setup. The solution in each tank was circulated to eliminate all visible gas bubbles before the operating current was applied.23 All experiments were performed at room temperature and a constant current density. The samples in the feed or recovery compartments were taken at intervals to analyze the concentrations of OH− and Al(OH)4− ions. 2.3. Ions Titration and Data Calculations. 2.3.1. Ions Titration. The Al(OH)4− concentration was determined by addition of excess disodium ethylenediamine tetraacetic acid (EDTA−Na) and then back-titration with CuSO4 using 1-(2pyridylazo)-2-naphthol (PAN) as an indicator.24 The OH− concentration was determined by addition of excess HCl and then back-titration with Na2CO3 using methyl orange as an indicator.13,25 2.3.2. Alkali Recovery Ratio (ηOH−), Al(OH)4− Leakage Ratio (ηAl(OH)4−), and Separation Efficiency (S). ηOH− and ηAl(OH)4− can be calculated from eq 215

preliminary investigation for the feasibility of ED, EED, and their coupling for separating NaOH from the simulated solution is carried out with low membrane areas as well as limited operating time. The effect of operation conditions on the separation efficiency is investigated, including the current density, operating time, and membrane type. In particular, the ηOH−, ηAl(OH)4−, energy consumption, and electro-osmosis transport are discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. The membrane categories and properties are listed in Table 1. All chemicals were of analytical grade. Deionized water was used. Three kinds of solutions were used with a volume of 250 mL. Sodium aluminate solution containing NaOH and NaAl(OH)4 was prepared by a chemosynthesis method as described previously.15 The concentrations of NaOH and NaAl(OH)4 were ∼1.5 and ∼0.8 mol L−1, respectively. The initial recovery solution was 0.2 mol L−1 NaOH, and the electrode rinse solution used in ED was 0.3 mol L−1 Na2SO4. 2.2. Experimental Setup. 2.2.1. Setup for ED. The selfmade ED setup and membrane stack are illustrated in Figure 2a. The setup contained the following sections: (a) ED membrane stack (1), (b) direct current power supply (3) (GX1761SL5A, Hangzhou Yuhang Siling Electrical Instrument Ltd., China), (c) tanks for electrode rinse, feed, and recovery solutions (5−7), and (d) peristaltic pumps (8−10) (BT100L with a pump head of 2 × YZ15, Baoding LeadFluid Technology Co. Ltd., China) to regulate the flow rate at 0.38 L min−1 for each compartment. The membrane stack, which had three cell pairs, was comprised of two electrode compartments, three feed compartments, and three recovery compartments as shown in Figure 2a. The compartments were formed by alternately arranging cation or anion exchange membranes (CEM or AEM) and plexiglas spacers. The spacer, with a thickness of 10 mm, contained an oval hole with an area of 20 cm2 in the middle and used silicon rubber as the seals. The digital photo is shown in Figure S1, Supporting Information. The electrodes were made of titanium coated with ruthenium. Each membrane had an effective area of 20 cm2. 2.2.2. Setup for EED. Figure 2b shows the EED setup and membrane stack. The setup contained the following sections: (a) EED membrane stack (2) in which the electrodes were made of titanium coated with ruthenium, (b) direct current power supply (4), (c) tanks for feed and recovery solutions (6, 7), and (d) peristaltic pumps (11, 12). The EED membrane stack was comprised of a feed compartment and a recovery compartment, which were separated by one CEM and two spacers. Two different membrane areas were used, and the thickness of the spacers as well as the flow rate of the solutions was changed accordingly. When the membrane effective area was 20 cm2, the apparatus was similar to the ED apparatus, with a spacer thickness of 10 mm and solution flow rate of 0.38 L min−1. When the membrane area was 90 cm2, the spacer had a thickness of 0.8 mm and contained a rectangular hole with an area of 9 × 10 cm2 in the middle. The solution flow rate was 0.24 L min−1. 2.2.3. Coupling Setup. ED and EED setups were coupled as shown in Figure 2c. ED was used for the preliminary separation of the feed solution, after which EED was carried out for further separation to achieve a high alkali recovery ratio. The effective areas of each membrane in ED and EED were 20 and 90 cm2,

ηi =

cri(t ) ·Vr(t ) − cri(0) ·Vr(0) , i = OH−or Al(OH)4 − c fi(0) ·Vf (0) (2)

where cri(t) and cri(0) are the concentration of component i in the recovery compartment at time t and 0, respectively, cfi(0) is the concentration of component i in the feed compartment at time 0, Vr(t) and Vr(0) are the volume of recovery solution at time t and 0, respectively, and Vf(0) is the volume of feed solution at time 0. Van der Bruggen et al. calculated the separation efficiency (S) between components A and B as eq 326 S(t ) =

(cA(t )/cA(0)) − (c B(t )/c B(0)) (1 − cA(t )/cA(0)) + (1 − c B(t )/c B(0))

(3)

where cA(t) and cB(t) are the concentrations of A and B correspondingly in the dilute phase (feed solution) at time t and cA(0) and cB(0) are the concentrations of A and B in the feed solution at time 0, respectively. It should be noted that the volume change of the feed solution has been ignored for eq 3. If the volume change cannot be ignored, eq 3 should be replaced by the following formula S(t ) =

cA(t )·V (t ) cA(0)·V (0)

(1 −

cA(t )·V (t ) cA(0)·V (0)

−

c B(t )·V (t ) c B(0)·V (0)

) + (1 −

c B(t )·V (t ) c B(0)·V (0)

)

(4)

where V(t) and V(0) are the volumes of feed solution at time t and 0, respectively. In the present study, there is a relatively obvious volume change for feed or recovery solution as the operating time prolongs, indicating the presence of the electro-osmosis transport. Therefore, eq 4 should be used for calculating the S values. The amount of substance of A or B in the feed solution, shown as cA(t)V(t) and cB(t)V(t) in eq 4, can be calculated as eq 5 according to the material balance principle cA(t ) ·V (t ) = cA(0) ·V (0) − (cAr(t )·Vr(t ) − cAr(0)·Vr(0)) c B(t )V (t ) = c B(0) ·V (0) − (c Br(t )·Vr(t ) − c Br(0)·Vr(0)) (5)

where cAr(t) and cAr(0) are the concentration of A in the recovery solution at time t and 0, respectively, Vr(t) and Vr(0) 1879

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Figure 3. Effect of the current density on ED performance with JCM/JAM membranes including (a) the OH− and Al(OH)4− concentrations in the recovery compartment, (b) the OH− recovery ratio(ηOH−), Al(OH)4− leakage ratio (ηAl(OH)4−), and separation efficiency (S) after the experiments, (c) the voltage drop across the membrane stack, and (d) the current efficiency and energy consumption.

where F is the Faraday constant (96 485 C mol−1) and N is the repeating unit of the stack (N = 3 for ED and 1 for EED). E can be calculated as eq 9 in the coupling process

are the volume of recovery solution at time t and 0, respectively, and cBr(t) and cBr(0) are the concentration of B in the recovery solution at time t and 0, respectively. Hence, eq 4 can be replaced by eq 6 S(t ) =

c Br(t )·Vr(t ) − c Br(0)·Vr(0) c B(0)·V (0) c Br(t )·Vr(t ) − c Br(0)·Vr(0) c B(0)·V (0)

−

cAr(t )·Vr(t ) − cAr(0)·Vr(0) cA(0)·V (0)

+

cAr(t )·Vr(t ) − cAr(0)·Vr(0) cA(0)·V (0)

E=

∫0

t

UI dt (cr(t ) ·Vr(t ) − cr(0)·Vr(0))M

(cr(t ) ·Vr(t ) − cr(0)·Vr(0))F NIt

(UEDIED + UEEDIEED)dt (cr(t )Vr(t ) − cr(0)Vr(0))M

(9)

3. RESULTS AND DISCUSSION 3.1. ED Process. 3.1.1. Effect of Current Density on ED Performance. Figure 3a−d demonstrates the ED performance under different current densities. The cation and anion exchange membranes (CEM and AEM) were JCM and JAM, respectively. The operating time was 180 min for each experiment. Figure 3a shows that the concentrations of both OH− and Al(OH)4− in the recovery solution increase with the current density, since a high current density can lead to a high driving force.28 The OH− concentration increases slowly at the end of operating, while the Al(OH)4− concentration increases more rapidly. Therefore, Al(OH)4− leakage from the feed solution becomes more serious as a function of time, which should be due to the change of the feed compositions. As the OH− concentration and pH in the feed solution decrease with time, some complex Al-related ions such as OAl(OH)32− and

(7)

where U is the voltage drop across the membrane stack, I is the current applied in the stack, cr(t) and cr(0) are the concentration of NaOH in the recovery compartment at time t and 0, respectively, Vr(t) and Vr(0) are the circulated volume in the recovery compartment at time t and 0, respectively, M is the molar mass of NaOH, and t is the operating time. η (%) can be calculated as eq 827 η=

t

where UED and UEED are the voltage drops across ED and EED membrane stacks, respectively, and IED and IEED are the currents in ED and EED, respectively.

(6)

Ions A and B represent Al(OH)4− and OH− correspondingly in this study. The S value is between 0 and 1 since Al(OH)4− ion is transported more slowly than OH− ion. 2.3.3. Energy Consumption (E) and Current Efficiency (η). E (kW h kg−1) can be calculated as formula 7 in the ED or EED process27 E=

∫0

(8) 1880

DOI: 10.1021/ie504223e Ind. Eng. Chem. Res. 2015, 54, 1876−1886

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Industrial & Engineering Chemistry Research

Figure 4. Effect of membrane type on ED performance with a current density of 60 mA cm−2 including (a) the OH− and Al(OH)4− concentrations in the recovery compartment, (b) the separation efficiency, and (c) the voltage drop across the membrane stack.

(OH)3Al−O−(OH)3Al2− can be transformed to simple ions such as Al(OH)4− and the transport resistance decreases.29 Figure 3b shows that the ηOH− increases from 39.6% to 81.5% as the current density increases from 30 to 75 mA cm−2 and then becomes stable until 90 mA cm−2. ηAl(OH)4− increases from 5.2% to 42.6% as the current density increases from 30 to 90 mA cm−2, and the S values decrease from 0.76 to 0.32 gradually. OH− ion transport can influence the S and Al(OH)4− transport strongly. At high ηOH− value, OH− ions are mostly transferred to the recovery compartment and the OH− concentration in the feed is low. Hence, migration of Al(OH)4− ions from the feed to the recovery solution is accelerated, resulting in an increase of ηAl(OH)4− and a decrease of S values. In particular for the highest current density of 75−90 mA cm−2, ηOH− has achieved the highest value and hence the ηAl(OH)4− increases persistently and the S value decreases rapidly. Overall, proper current density is needed to obtain suitable ηOH−, ηAl(OH)4−, and S values. Figure 3c shows that the voltage drop decreases with time when the current density is lower than 60 mA cm−2, which can be attributed to the enrichment of ions in the recovery compartment and thus the reduced membrane stack resistance. However, the voltage drop increases at a later period of operating when the current density increases to 75 or 90 mA cm−2. This should be attributed to the reduction of ions in the feed solution and thus the enhanced membrane stack resistance. Figure 3d indicates that the energy consumption dramatically increases from 1.63 to 5.11 kW h kg−1 with the current density,

and the current efficiency decreases from 73.4% to 50.3%. More energy is consumed at higher current density to overcome the membrane stack resistance. Besides, the S value decreases and the cr,OH− value increases slowly at a later period of operating when the current density is high (Figure 3a and 3b). Accordingly, the (cr(t)Vr(t)−cr(0)Vr(0)) values in eqs 7 and 8 are relatively low, leading to higher energy consumption and lower current efficiency. In summary, the current density can influence the ED performance significantly and should be selected as 45−60 mA cm−2 to achieve a balanceable ηOH− of 55.6−67.1%, ηAl(OH)4− of 9.2−16.8%, separation efficiency of 0.60−0.72, energy consumption of 2.19−3.02 kW h kg−1, and current efficiency of 62.2−68.7%. 3.1.2. Effect of Membrane Type on ED Performance. Three kinds of commercial membranes were used to separate NaOH from sodium aluminate solution. The current density was fixed at 60 mA cm−2. Figure 4a shows that CJ-EDC/EDA membranes can lead to both higher OH− and higher Al(OH)4− concentrations as compared with JCM/JAM membranes, indicating the higher permeability of CJ-EDC/EDA membranes. However, Figure 4b shows that the separation efficiency is lower than JCM/JAM membranes. The CMV/AMV membranes have the highest OH− concentration and generally the lowest Al(OH)4− concentration in the recovery compartment, indicating both excellent alkali permeability and selectivity of CMV/AMV membranes. The high permeability and selectivity lead to the highest separation efficiency, as shown in Figure 4b. For 1881

DOI: 10.1021/ie504223e Ind. Eng. Chem. Res. 2015, 54, 1876−1886

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with the highest Al(OH)4− rejection also show the lowest EO transport. As CMV/AMV membranes can have a low energy consumption of 2.38 kW h kg−1, high separation efficiency of 0.94−0.73, high ηOH− of 92.6%, and high current efficiency of 85.8%, they will be used in the ED/EED coupling process. 3.2. EED Process. The sodium aluminate solution was separated by membrane CMV, CJ-EDC, or JCM. The current density was set at a low value of 30 or 50 mA cm−2 to reduce energy consumption. The feed or recovery solution is recirculated during the experiments. The concentration of feed decreases continuously with time, and the limiting current density can be reached,32 at which point the voltage reaches the limiting voltage, resulting in a dramatic increase of the curves in Figure 5a. Hence, each experiment should be stopped before the limiting voltage, and a suitable operating time is determined accordingly. Figure 5a shows that the operating time is controlled by the membrane area, current density, and membrane category. First, the operating time of 20 cm2 CMV membrane (575 min) is 4.4 times longer than that of 90 cm2 (130 min) under 50 mA cm−2 current density, indicating a lager membrane area can reduce the operating time. Second, the operating time under 50 mA cm−2 is shorter than that under 30 mA cm−2, for a high current density can enhance the electrolytic reactions in the anode and cathode as well as ion migration from the feed to the recovery solution. Finally, CMV membrane has the shortest operating time due to its high permeability, while JCM membrane needs the longest operating time. Figure 5b shows that the OH− concentration in the recovery solution (cr,OH−) is in the range of 1.40−1.65 mol L−1 at the end of operating. ηOH− as calculated is higher than 90%. cr,OH− of CMV membrane increases more rapidly due to high alkali permeability. Figure 5c shows that CMV membrane can achieve zero Al(OH)4− leakage, while CJ-EDC shows the highest Al(OH)4− leakage. The leakage may be attributed to back diffusion of Al(OH)4− from the feed to the recovery solution by a driving force of concentration difference.33 The CMV membrane, due to its low water transport and probably dense structure, can better restrain the migration of large-size ions such as Al(OH)4− and hence shows a lower leakage ratio than the other membranes. The effect of the current density on the energy consumption, current efficiency, and EO transport is also investigated. A membrane area of 90 cm2 is taken as an example since a larger membrane can separate the feed solution more effectively. Figure 5d shows that the energy consumption increases with the current density, since high current density can lead to a high voltage drop across the membrane stack. However, higher current density has no obvious effect on the current efficiencies of CMV and JCM membranes or only slightly reduces the current efficiency of CJ-EDC membrane. High current density can enhance the electrolysis of H2O and the concentration of NaOH in the recovery compartment (reaction 1). Accordingly, the operating time (t in eq 8) can be reduced, and the current efficiency remains relatively stable.

instance, the separation efficiencies of CJ-EDC/EDA and JCM/ JAM membranes are 0.85 and 0.83 after operating 30 min and then decrease to 0.55 and 0.60 correspondingly. Meanwhile, the separation efficiency of CMV/AMV membranes is as high as 0.94 initially and then decreases to 0.73. Figure 4c shows that the voltage drops of CJ-EDC/EDA and JCM/JAM membranes gradually decrease with the time. The data for these two membranes are similar, since OH− concentrations in the recovery solutions are similar. The voltage drop of CMV/AMV membranes also decreases in the early stage of operating but then increases in the latter stage of operating. The increase in the latter stage is attributed to the highest alkali permeability of CMV/AMV membranes, which leads to the lowest ion concentration in the feed compartment and thus the enhanced resistance and voltage drop. ED performance after 180 min operating including ηOH−, ηAl(OH)4−, energy consumption, and current efficiency are calculated and shown in Table 2. Membranes CJ-EDC/EDA Table 2. Effect of Membrane Type on ED Performance Including Alkali Recovery Ratio (ηOH−), Al(OH)4− Leakage Ratio (ηAl(OH)4−), Energy Consumption, Current Efficiency, and EO Transport after Operating membrane type

ηOH− (%)

ηAl(OH)4− (%)

energy consumption (kW h kg−1)

current efficiency (%)

EO transport (mL)

CMV/ AMV CJ-EDC/ EDA JCM/JAM

92.6

15.1

2.38

85.8

28

72.6

21.9

2.77

67.3

63

67.1

16.8

3.02

62.2

54

have a higher ηAl(OH)4− value (21.9%) than the other membranes (15.1−16.8%), which is due to the low membrane selectivity. Membranes CMV/AMV have higher ηOH− (92.6%) than CJ-EDC/EDA (72.6%) or JCM/JAM membranes (67.1%) because of the high OH− concentration in the recovery compartment (cr(t)). High cr(t) can result in a high value of (cr(t)Vr(t) − cr(0)Vr(0)) in eq 7 or 8. Hence, the energy consumption of CMV/AMV is the lowest (2.38 kW h kg−1) and the current efficiency is the highest (85.8%). Water transport, a common phenomenon in ED, has also been investigated in this study. Both osmosis and electroosmosis (EO) can cause water transport.30 Nevertheless, water transport by osmosis is at least eight times smaller than that by EO when the concentration difference across the membrane is below 1 mol L−1.31 As here the initial concentration difference of NaOH (∼1.3 mol L−1) is close to 1 mol L−1, the EO transport can generally represent water transport and is calculated from the following formula EO transport = Vr(t ) − Vr(0)

(10)

where Vr(t) and Vr(0) are the volume of recovery solution at time t and 0, respectively. Here t is the duration of the whole operation. Table 2 shows that the EO transport is lowest for CMV/AMV membranes (28 mL) and highest for CJ-EDC/ EDA membranes (63 mL), which is in accordance with the trend of ηAl(OH)4− in this study. The similar trend between EO transport and ηAl(OH)4− may be attributed to the membrane structure. Membranes with a dense polymer network and high steric hindrance can not only reduce the migration of Al(OH)4− ion but also minimize the water molecules transported per ion.30,31 Therefore, CMV/AMV membranes

2H 2O → H 2 ↑ +2OH−

(reaction 1)

Membranes CMV, CJ-EDC, and JCM under 30 mA cm−2 have energy consumptions of 2.07, 2.21, and 2.53 kW h kg−1 and current efficiencies of 91.4%, 85.6%, and 77.5%, respectively. The results indicate that the membrane with high permeability 1882

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Figure 5. EED performance including (a) voltage drop across the membrane stack, (b) OH− concentration in the recovery compartment, (c) Al(OH)4− concentration in the recovery compartment, (d) current efficiency and energy consumption, and (e) EO transport after operating.

increase to 20, 34, and 43 mL, respectively. In particular, the CMV membrane is of the lowest EO transport. Therefore, dense membrane structure can effectively minimize the water transport30 and also restrain the back diffusion of Al(OH)4− ions. Zero Al(OH)4− leakage in EED could then be achieved. Overall, EED can be optimized with a CMV membrane area of 90 cm2 and current density of 30 mA cm−2. The Al(OH)4− leakage can be neglected, and the energy consumption is only

can achieve high ion migration and thus low energy consumption and high current efficiency. Figure 5e shows that the EO transport increases with the current density, for more ions in the hydrated ions form34 can be transported under an intensive current density. The EO transports of CMV, JCM, and CJ-EDC membranes are 13, 21, and 28 mL correspondingly when the current density is 30 mA cm−2. As the current density increases, the EO transports 1883

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Figure 6. Coupling performance including (a) voltage drop across the membrane stack and (b) the OH− and Al(OH)4− concentrations in the recovery compartment.

2.07 kW h kg−1. Nevertheless, the treatment capability is lower as compared with ED. For example, the operating time of EED is 575 min under 50 mA cm−2, which is three times longer than that of ED (180 min under 60 mA cm−2) when the effective area of each membrane is 20 cm2. 3.3. Coupling Process. Both ED and EED can show a high ηOH− value (>90%) under the optimized conditions, which means that most of the NaOH component is removed after separation. The separated NaOH solution can be directly used as the mother liquor, and removal of NaOH can enhance the gibbsite crystal growth rate and yield of Al(OH)3. However, the ED process has a high ηAl(OH)4− of ∼15%, which would reduce the yield of Al(OH)3 and the purity of NaOH in the recovery solution. The EED process has a low treatment capability. Hence, ED is coupled with the EED process to overcome the above defects. CMV/AMV membranes were used, and the EED current density was 30 mA cm−2. The effects of ED current density and ED operating time on the coupling performance are investigated through ηOH−, ηAl(OH)4−, energy consumption, and total operating time. Figure 6a shows that the voltage drop decreases with decreasing ED current density from 60 to 45 mA cm−2, since a low current density can result in a low voltage drop with a relatively constant resistance. The operating time of EED, which is determined by the limiting voltage, decreases with increasing ED operating time or ED current density. The total operating time as listed in Table 3 decreases from 190 to 180 min as ED operating time increases from 90 to 150 min under

60 mA cm−2 and increases from 180 to 227 min as ED current density decreases from 60 to 45 mA cm−2. Figure 6b shows that the cr,Al(OH)4− value increases with the ED operating time or ED current density. For instance, the value with ED operating 120 min under 60 mA cm−2 is similar to that with ED operating 150 min under 45 mA cm−2. The cr,OH− can reach up to 1.46−1.56 mol L−1 after operating. Table 3 shows that ηOH− can reach up to 89.5−91.7%, and ηAl(OH)4− increases from 2.9 to 8.2% with increasing ED operating time but decreases from 8.2% to 4.8% with decreasing ED current density. Hence, ED operating time should be optimized as 120 min when ED current density is 60 mA cm−2 to achieve a low ηAl(OH)4− (∼5%). Table 3 also shows that the energy consumptions are in the range of 2.22−2.35 kW h kg−1 when ED current density is 60 mA cm−2 and decrease to 1.95 kW h kg−1 with 45 mA cm−2. Hence, low ED current density can reduce energy consumption but enhance operating time. Overall, the coupling process can combine the ED advantage of high treatment capability with the EED advantage of zero Al(OH)4− leakage. For example, when the ED current density and ED operating time are 60 mA cm−2 and 120 min correspondingly, ηOH− can reach up to 90.9%, much higher than the previous value (12%), and the energy consumption is as low as 2.25 kW h kg−1, lower than onequarter of the previous value (>12 kW h kg−1). Hence, the ED/ EED coupling process may be more economically competitive. As this separation process is a simply preliminary study, the scalability and energy consumption are still limited for industrialization. Typical refineries produce in excess of 1 × 107 kg Al2O3 per day. The whole energy consumption of the Bayer process is ∼3.35 kW h per kg Al2O3, among which ∼0.064 kW h is consumed in the precipitation step.35 In the present work, production of 1 kg of Al2O3 approximately needs to separate 1 kg of NaOH. Therefore, ∼0.081 kg of Al2O3 can be produced per day with 6 batches separation (4 h for each batch). The membrane area may be increased from 20 cm2 to 2 m2 after scaling up, and the number of ED repeating units may be increased from 3 to 600. Accordingly, ∼615 ED setups are needed to satisfy the practical production of refineries. For the EED process, only one membrane can be used for each setup, which may be the bottleneck for the scale-up application. The energy consumption can be reduced after the scale-up application. Taking the ED process for organic acids production

Table 3. Coupling Performance Including Total Operating Time, ηOH−, ηAl(OH)4−, and Energy Consumption total operating time (min)

ηOH− (%)

ηAl(OH)4− (%)

energy consumption (kW h kg−1)

− 90

190

91.4

2.9

2.22

− 120

185

90.9

4.5

2.25

− 150

180

91.7

8.2

2.35

− 150

227

89.5

4.8

1.95

experimental condition ED60 mA cm−2 min + EED30 mA cm−2 ED60 mA cm−2 min + EED30 mA cm−2 ED60 mA cm−2 min + EED30 mA cm−2 ED45 mA cm−2 min + EED30 mA cm−2

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Industrial & Engineering Chemistry Research Notes

as an example, the energy consumption decreases from 2.58 to 1.03 kW h kg−1 when the effective membrane area increases from 7.07 cm2 to 2.4 m2 and the number of repeating units increases from 1 to 10.36,37 Similarly, energy consumption of the ED/EED coupling process may be decreased from 2.25 to lower than 1.0 kW h kg−1. Though the value is still much higher than that of the precipitation step in the Bayer process (0.064 kW h kg−1), the higher energy consumption should be at least partly offset by the increased production efficiency. The aluminate decomposition efficiency is only 40−50% for the traditional Bayer process35 and can be elevated largely in the short term after NaOH separation. The separated NaOH can be reused in the Bayer circuit, which can also bring economic benefits. Overall, ED and EED processes can be potentially coupled to separate sodium aluminate solution, though more work needs to be done for scale-up application.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Science Foundation of China (Nos. 21176053 and 21376204).

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4. CONCLUSION Sodium aluminate solution containing NaOH and NaAl(OH)4 is separated by electrodialysis (ED), electro-electrodialysis (EED), and their coupling process. The ED process has high treatment capability. Under an optimal current density of 60 mA cm−2, the CMV/AMV membranes exhibit high performance including a ηOH− of 92.6%, separation efficiency of 0.94− 0.73, energy consumption of 2.38 kW h kg−1, and current efficiency of 85.8%. The EED process can achieve a high concentration (1.40−1.65 mol L−1) and purity of NaOH in the recovery solution due to zero Al(OH)4− leakage. The energy consumption is only 2.07 kW h kg−1 with a CMV membrane area of 90 cm2 and current density of 30 mA cm−2. The coupling of ED and EED can achieve both high treatment capability and low Al(OH)4− leakage, together with low energy consumption. Energy consumption is 2.25 kW h kg−1, ηOH− can reach up to 90.9%, and ηAl(OH)4− can be reduced to ∼5% with a ED current density of 60 mA cm−2 and ED operating time of 120 min. Accordingly, the gibbsite crystal growth rate and yield of Al(OH)3 can be enhanced substantially, which is advantageous to the enhancement of the production efficiency in the Bayer process. Besides, the separated NaOH solution can be directly used as the mother liquor for circuit usage, and the production cost can be saved. Hence, the coupling process is of high significance in alumina industry. Further studies are needed for the scale-up application of ion exchange membrane (IEM) process, including scale formation on the membrane, stability of the membrane process, reduction of the energy consumption, and enhancement of the treatment capability especially for the EED process. Besides, the membrane fouling of the complex organic and/or inorganic components in the Bayer process liquors should be overcome to improve the IEM process performance.

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ASSOCIATED CONTENT

S Supporting Information *

Digital photo of ED setup with geometric characteristics in detail (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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