Integrating Diffusion Dialysis with Membrane Electrolysis for

Research Article pubs.acs.org/journal/ascecg

Integrating Diffusion Dialysis with Membrane Electrolysis for Recovering Sodium Hydroxide from Alkaline Sodium Metavanadate Solution Haiyang Yan,†,∥ Chunyan Xu,‡ Yonghui Wu,§ Abhishek N. Mondal,† Yaoming Wang,*,†,∥ and Tongwen Xu*,† †

CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Materials Science, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China ‡ School of Civil & Environmental Engineering, Georgia Institute of Technology, 225 North Avenue, Atlanta, Georgia 30332, United States § Department of Chemistry, Yancheng Teachers University, No. 50, Kaifang Road, Yancheng, Jiangsu 224002, P.R. China ∥ Hefei ChemJoy Polymer Materials, Co., Ltd., B-2, Liheng Industrial Square, Fanhua Road, Hefei, Anhui230601, P.R. China ABSTRACT: Here, an energy-saving and efficient method is proposed for the clean and sustainable production of typical metal oxide (vanadium pentoxide as a case) by integrating diffusion dialysis with membrane electrolysis via cation-exchange membranes. This integrated process can achieve alkali recycle with less energy consumption. In addition, the gasesH2 and O2produced in the membrane electrolysis process can be used as byproducts. Results indicate that the total alkali recovery ratio and the vanadium rejection ratio can be as high as ∼100% and ∼92.56%, respectively. Moreover, preliminary economic evaluation indicates that the running cost can be significantly decreased from $24.52/(m3 feed) (traditional method) to $−24.18/(m3 feed) (proposed method). KEYWORDS: Vanadium, Membrane electrolysis, Diffusion dialysis, Metavanadate solution, Cation-exchange membrane

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INTRODUCTION

Vanadium pentoxide (V2O5), the most common form of vanadium,10 has been used widely as an additive during the manufacture of various steel grades,1,10 as a catalyst for organic reactions or in the oxidation of sulfur oxide, and as a ceramic coloring material for inhibiting ultraviolet transmission in glass, in photographic developers, in dyeing textiles, etc.11 V2O5 can also be used as an electrode material for supercapacitors and lithium ion batteries,12,13 and V2O5 as a raw material can synthesize VO2+ and V3+ as positive and negative electrolyte, respectively, for vanadium redox flow batteries and allvanadium photoelectrochemical storage cells.14,15 The vanadium redox flow battery, one kind of technique for energy storage, is well-suited for large-scale utility applications,14 and the photoelectrochemical storage cell has the possibility to be implemented in photoelectrochemical solar energy conversion and storage as an alternative to fossil fuels.15 Accordingly, an increasing amount of V2O5 would be needed to synthesize VO2+ and V3+ as electrolyte for vanadium redox flow battery and photoelectrochemical storage cell.

The alkaline leaching method has been introduced widely to extract metals, such as vanadium, aluminum, and tungsten, from minerals for producing the corresponding metal oxide(s).1−7 In the leaching process, the NaOH solution needs to be mixed and digested with the metal-containing materials at a high temperature to efficiently extract the metals in the form of ions such as VO3−, Al(OH)4−, and WO42−.7−9 The residual NaOH in the leaching solution should be subsequently neutralized with acid to decrease the basicity of the solution and to accelerate precipitation of the metal complex in different forms. In such cases, the residual NaOH cannot be recovered and recycled in the production circuit. Moreover, large amounts of acids are consumed in the above process, which increases the cost of the production. The salts produced from acid−base neutralization need to be treated after the precipitation process, which further increases the production cost. Hence, a clean, energy-saving, and efficient separation method is indeed essential for separating and recovering NaOH from the alkaline leaching solution, to overcome the aforementioned problems and to achieve a sustainable production. In the present study, we will take the production of vanadium pentoxide as a reference case to propose a blueprint. © 2017 American Chemical Society

Received: March 5, 2017 Revised: April 22, 2017 Published: April 26, 2017 5382

DOI: 10.1021/acssuschemeng.7b00688 ACS Sustainable Chem. Eng. 2017, 5, 5382−5393

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Simplified flowcharts of alkaline leaching of vanadium without (a) and with (b) the ion-exchange membrane (IEM) processes.

Conventionally, vanadium values can be extracted from minerals or vanadium slag by acid or alkaline leaching.2,8,16−19 The acid leaching process will produce some unwanted chemical compounds, which need to be cleaned-up by the use of relatively expensive and complicated apparatus in order to avoid polluting the environment.2 In contrast, the method of alkaline leaching has been used more commonly for the extraction of vanadium values. This method mainly comprises the steps of leaching, solid/liquid (S/L) separation, precipitation, S/L separation, and calcination, as illustrated in Figure 1a. Specifically, a caustic such as NaOH and/or Na2CO3 is mixed and digested with vanadium-containing materials to extract vanadium oxide via the leaching process. The vanadiumcontaining alkaline solution obtained from S/L separation is mainly composed of NaOH and NaVO3. The solution is strong alkaline (pH > 14) before precipitation, and the pH value should be reduced to around 6 to precipitate vanadium values. Traditionally, the pH is lowered by adding acid (e.g., HCl and H2SO4) to neutralize residual NaOH in the vanadiumcontaining alkaline solution.8 In the following precipitation process, ammonia-based materials such as NH3, NH4Cl, and (NH4)2SO4 are added and reacted with NaVO3 (reactions 1−3) to form NH4VO3 precipitate.11,16 After S/L separation, the NH4VO3 precipitation is calcined to form the product V2O5 (reaction 4). NH3 + H 2O + NaVO3 → NH4VO3 ↓ + NaOH

NH4Cl + NaVO3 → NH4VO3 ↓ + NaCl

(2)

(NH4)2 SO4 + 2NaVO3 → 2NH4VO3 ↓ + Na 2SO4

(3)

calcination

2NH4VO3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NH3 ↑ + H 2O + V2O5

(4)

The annual vanadium consumption world-wide had increased to ∼8.0 × 104 t in 2012.10 If all the vanadium is produced by the alkaline leaching method, then at least ∼3 × 105 t NaOH needs to be neutralized with ∼7.3 × 105 t HCl solution (37%), resulting in serious waste of resources and huge economic loss. Hence, it is urgent to explore a clean, energysaving, and efficient method for separating and recovering NaOH from the alkaline leaching solution to achieve sustainable production. A membrane separation process offers an alternative. Considering the particularity of NaOH recovery from the alkaline leaching solution, a pressure-driven membrane process may be unsuitable, because selective separation of different ions such as OH− and VO3− cannot be achieved via a process such as nanofiltration because the ions have the same valence states. On the contrary, ion-exchange membrane (IEM) processes (non-pressure-driven membrane processes), with a driving force of concentration gradient or electric potential difference, have a significant potential for the selective separation of different ions. Especially the diffusion dialysis process, with concentration gradient as the driving force, has received much attention in separating and recovering acid or alkali from acidic

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ACS Sustainable Chemistry & Engineering or alkaline liquor due to its unique advantages such as low energy consumption and environmental benignity.20 The diffusion dialysis process can be realized with batch and dynamic modes, where batch diffusion dialysis is mostly used to evaluate diffusion dialysis membrane performance such as dialysis coefficient (Ui) and separation factor (S), and dynamic diffusion dialysis can be used to separate and recover acid or alkali continuously for practical application. To date, dynamic diffusion dialysis has achieved much success in treating acid solutions.21−23 In contrast, the dynamic diffusion dialysis process for alkali recovery lags far behind. The main reason is that the concentration of the recovered alkali and the alkali recovery ratio are always insufficient, since the radius of OH− ions is larger and their mobility is lower compared with H+ ions.9 If the UOH− and S values are high enough, the dynamic diffusion dialysis process could be applied potentially in separating/recovering NaOH from alkaline systems such as NaOH/Na2WO4, NaOH/NaAl(OH)4, and NaOH/NaVO3. The separation principle (NaOH/NaVO3 as an example) is illustrated in Figure 2b. The Na+ ions are permitted to be

ions and thus the alkali recovery ratio. Hence, other membrane processes may be explored to be coupled or integrated with dynamic diffusion dialysis for treating vanadium-containing alkaline feeds so as to achieve high recovery concentration, high recovery ratio, and high vanadium rejection ratio. Membrane electrolysis is an electro-membrane process, which combines ion transport across the IEM with electrochemical reaction at the electrodes.24 The main industrial application of this technology is found in chlor-alkali production, organic/inorganic acids, and alkali production from the corresponding salts.3,24−27 When it comes to alkali recovery/production, especially for the treatment of the vanadium-containing alkaline solution, the electrochemical reaction at the electrodes will occur (Figure 2b). Anode reaction: Cathode reaction:

4OH− → 2H 2O + O2 ↑ +4e− 4H 2O + 4e− → 4OH + 2H 2↑

(5) (6)

In this process, Na+ ions in the anode compartment can be transported through the CEM to the cathode compartment by a driving force of direct current and combined with OH− ions to produce NaOH, while VO3− ions remain at the anode compartment (Figure 2b). Hence, theoretically, zero vanadium leakage ratio can be achieved in the membrane electrolysis process for recovering NaOH from the vanadium-containing alkaline solution. Moreover, the produced H2 and O2 can be used as clean energy in the subsequent calcination process to lower the production cost. Based on the characteristics of dynamic diffusion dialysis and membrane electrolysis processes, their integration may bring complementary advantages in separating and recovering NaOH from the vanadium-containing alkaline solution. The dynamic diffusion dialysis process, as an eco-friendly process with low energy consumption, can be preliminarily used to separate the vanadium-containing alkaline solution to obtain dialysate and diffusate solutions. Afterward, the membrane electrolysis process, due to its zero vanadium leakage ratio, can be applied to reduce the alkali concentration of dialysate and to increase the alkali concentration of diffusate. Thereafter, high recovered alkali concentration, high alkali recovery ratio, and high vanadium rejection ratio can be achieved. The recovered alkali can be directly recycled in the circuit of V2O5 production. Meanwhile, clean energy sources H2 and O2 produced in the membrane electrolysis process can be recycled as byproducts. Vanadium-containing alkaline solution contains a large number of impurities such as components containing carbon, calcium, aluminum, silicon, phosphorus, and organics, which may complicate the membrane separation process. Hence, the simulated chemosynthesis alkaline sodium metavanadate solution containing NaOH and NaVO3 was utilized as the feed solution in the present study. In the work described in this Article, a preliminary study for integrating dynamic diffusion dialysis with membrane electrolysis was carried out. The effects of feed flow rate and flow rate ratio of feed to water on dynamic diffusion dialysis performance as well as the effect of current density on membrane electrolysis process were evaluated to obtain the optimized parameters. Specifically, the alkali recovery ratio, vanadium rejection ratio, recovered alkali concentration, energy consumption, current efficiency, and preliminary economic evaluation are discussed in detail.

Figure 2. Diagram of the integration of diffusion dialysis with membrane electrolysis. (a) Experimental apparatus: (1) diffusion dialysis membrane stack, (2) membrane electrolysis membrane stack, (3) direct current power supply, (4) feed tank, (5) water tank, (6) dialysate tank, (7) diffusate tank, (8−11) peristaltic pumps. (b) Membrane stacks and separation principles of diffusion dialysis (left) and membrane electrolysis (right).

transported from the feed (dialysate side) to the water (diffusate side) due to the concentration gradient, while the VO3− ions are less likely to pass through the cation-exchange membrane (CEM). The OH− ions, due to their smaller hydrated ion radius, have higher competition than VO3− ions and thus can diffuse along with Na+ ions to meet the requirement of electrical neutrality.20 However, the concentration gradient of NaOH between dialysate and diffusate sides would decrease drastically when large amounts of NaOH are separated from the feed, reducing the transport rate of OH− 5384


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Table 1. Properties of the Cation-Exchange Membranes (CEMs) Used in Diffusion Dialysis and Membrane Electrolysis CEM a

PVA-I PVA-IIa CMVb a

thickness (μm)

IEC (mequiv/g)

water uptake (%)

membrane resistance (Ω cm2)

transport number (%)

150−160 160−180 130−150

0.5−0.7 0.3−0.4 2.0

36−37 25−27 16−17

5−6 15−20 2.5−3.0

93−97 81−84 >96

The membrane properties were measured using the reference method.28 bData are obtained from refs 3 and 29.

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the feed solution (2.239 mol/L NaOH + 0.239 mol/L NaVO3) at room temperature for 1512 h. The mass loss ratios were calculated to evaluate the membrane alkali resistance, the equation can be expressed as follows:

MATERIALS AND METHODS

Experimental Setup, Membranes, and Feed Solution. Cationexchange membrane performance, i.e., ion permeability and membrane selectivity, was evaluated using a batch lab-scale diffusion dialysis setup, which contains both feed and water compartments separated by a CEM with an effective area of 6 cm2. The feed compartment was filled with 0.1 L of feed solution, and the water compartment was filled with 0.1 L of deionized water during the test. Both compartments were stirred at identical rates by motor-driven stirrers to minimize the concentration polarization effects. Diffusion dialysis was running for 1, 2, 3, or 4 h to investigate the effect of time on membrane performances, and then the solution was removed from both sides of the compartment. The dynamic diffusion dialysis setup we made is illustrated in the left of Figure 2a. The setup contained diffusion dialysis membrane stack, feed tank, dialysate tank, water tank, diffusate tank, and peristaltic pumps (Baoding Longer Precision Pump Co., Ltd., China) to precisely control the feed and water input flow rates. The separation principle of the dynamic diffusion dialysis process was elaborated in detail as shown in Figure 2b. The stack for dynamic diffusion dialysis was comprised of 13 CEMs and 14 Plexiglas spacers (thickness 0.08 cm), in which seven dialysate compartments and seven diffusate compartments were separated by CEMs and Plexiglas spacers alternatingly. The effective area of each membrane was 69 cm2, and the total effective membrane area was 897 cm2. The stack was rinsed with feed and water for a certain time to eliminate the visible gas bubbles before the experiment, and then the feed and water were kept at the conditions of the counter current to enhance the mass transfer. After a group of flow rates of feed and water was set, the samples cannot be taken until the outlet concentrations of dialysate and diffusate are stable. The setup for membrane electrolysis was done in-house as illustrated on the right of Figure 2. The experimental system contained a membrane electrolysis membrane stack, direct current power supply, anode solution tank (or dialysate tank, cathode solution tank, or diffusate tank), and peristaltic pumps (Baoding Lead Fluid Technology Co. Ltd., China) to regulate the flow rate at 0.3 L/min. The principle of the membrane electrolysis separation process is elaborated in Figure 2b. The membrane electrolysis membrane stack contained only one CEM with an effective area of 189 cm2. The electrodes of both anode and cathode were made of titanium coated with ruthenium. The electrodes and CEM were separated by two spacers (thickness 0.08 cm), and thus two different compartments were formed. The compartment between the anode and the CEM was the anode compartment (or dialysate compartment), and the compartment between the cathode and the CEM was the cathode compartment (or diffusate compartment). All the experiments were conducted at ambient temperature around 25 °C. The CEMs used in diffusion dialysis process were PVA-I and PVAII membranes provided by Shandong Tianwei Membrane Technology Co., Ltd. and used for alkali recovery. Both membranes have the anionic functional groups of −SO3−, but with different graft density; thus, the membrane properties and separation performance of PVA-I and PVA-II membranes are different. The CEM used in membrane electrolysis process was a CMV membrane provided by Asahi Glass Co. Ltd. These membranes properties are listed in Table 1. The synthetic feed solution was prepared with AR-grade chemicals, which includes 2.239 mol/L NaOH and 0.239 mol/L NaVO3 solutions. Stabilities of Diffusion Dialysis Membranes. The stabilities of both PVA-I and PVA-II membranes were investigated due to the strong basicity of the feed solution. The membranes were immersed in

ML =

m 0 − mt m0

(7)

where m0 and mt are the dry mass of the original membrane and the eroded membrane, respectively. For obtaining the mt value, the eroded membrane was washed fully by water and then dried at 102 °C until constant mass. Analytical Methods. In batch/dynamic diffusion dialysis experiments, the concentrations of Na and V were determined by the ICPOES analysis (Atomscan Advantage, Thermo Jarrell Ash Corp., USA). During the diffusion dialysis experiments, Na and V in the solutions (i.e., feed, dialysate, and diffusate) mainly exist as Na+ and VO3−, respectively. Based on the electric neutrality of solution, the OH− concentration can be calculated by the following equation:

COH− = C Na+ − C VO3 −

(8)

In the membrane electrolysis experiment, the NaOH concentration of cathode solution (diffusate solution) was determined by the acid− base titration method using the standard 0.05 mol/L HCl and methyl orange as an indicator. The pH of the anode solution (dialysate solution) was monitored by a pH meter (pH-4F, Shanghai INESA Scientific Instrument Co., Ltd.). All the experimental data were collected through three independent measurements, and the error was within ±5%. Data Analysis. In batch diffusion dialysis process, dialysis coefficients (Ui) and separation factor (S) were calculated to evaluate the membrane selectivity and permeability. The S with respect to one species over another is given as the ratio of U of the two species present in the solution. Ui and S can be calculated by the following formula:30,31

Ui =

Mi , At ΔC

S=

UOH− UVO3−

i = OH− or VO3−

(9)

(10)

Mi in eq 9 is the amount of the component i transported in moles; A is the effective area in square meters; t is the time in hours. ΔC is expressed as following:30,31 ΔC =

Cf0 − (Cft − Cdt ) ln[Cf0/(Cft − Cdt)]

C0f

(11)

Ctf

and are the feed concentrations at time 0 and t where respectively, and Ctd is the dialysate concentration at time t. To evaluate the separation performance of the dynamic diffusion dialysis process, the alkali recovery ratio (ηOH−) and vanadium rejection ratio (ηVO−3 ) are calculated as follows:

ηOH− =

ηVO − = 3

5385

Cdiffusate,OH− Q diffusate (Cdialysate,OH− Q dialysate + Cdiffusate,OH− Q diffusate)

(12)

Cdialysate,VO3− Q dialysate (Cdialysate,VO3− Q dialysate + Cdiffusate,VO3− Q diffusate)

(13)


Research Article

ACS Sustainable Chemistry & Engineering where Cdialysate,OH− and Cdiffusate,i are the concentrations of component i in the dialysate and diffusate solution, respectively; Qdialysate and Qdiffusate are the flow rates of the dialysate and diffusate solution, respectively. In experiments, the outlet volumes of dialysate and diffusate were measured with a measuring cylinder after a certain period, and then the Qdialysate and Qdiffusate can be calculated by the ratio of volume/time. In the membrane electrolysis process, energy consumption (EC, kWh/m3 or MJ/m3) is calculated as in eq 14: t

EC =

∫0 UI dt Vfeed

(14)

where U is the voltage drop across the membrane stack; I is the current applied; t is the running time; and Vfeed is the volume of the treated feed solution. Current efficiency (CE, %) is calculated as in eq 15:3

CE =

(CtVt − C0V0)F × 100% NIt

(15) −

where Ct and C0 are the concentrations of OH in cathode compartment at time t and 0, respectively; Vt and V0 are the circulated volumes in the recovery compartment at times t and 0, respectively; F is the Faraday constant (96 485 C/mol); and N is the repeating unit (N = 1).

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RESULTS AND DISCUSSION Dialysis Coefficient (Ui) and Separation Factor (S) in Batch Diffusion Dialysis Process. Dialysis coefficient and separation factor are the two significant parameters for diffusion dialysis membranes, in which the dialysis coefficient represents the ion permeability, and the separation factor represents the membrane selectivity. The batch diffusion dialysis performances are evaluated by two different kinds of commercial CEMs, PVA-I and PVA-II, giving guidance for dynamic diffusion dialysis process. Figure 3a depicts the concentrations of OH− and VO3− in the water (diffusate side) as a function of time. The concentrations of OH− and VO3− increased more rapidly for PVA-I membrane as compared with PVA-II membrane, which indicated that PVA-I membrane has high OH− permeability but low ion selectivity. For instance, after 4 h of batch diffusion dialysis, the OH− concentration of PVA-I membrane increases to 0.2171 mol/L, which is 2.6 times than that of PVA-II membrane, while the VO3− concentration of PVA-I membrane increases to 0.0014 mol/L, 9.7 times higher than that of PVA-II membrane. Figure 3b shows the dialysis coefficient (UOH−) and separation factor (S) for both membranes. For PVA-I membrane, UOH− = 4.3 × 10−3 m/h and S = 35.3 after 1 h, while for PVA-II membrane, UOH− = 1.9 × 10−3 m/h and S = 68.8 after 1 h. The values are compared with previous values of different membranes, as shown in Table 2. The UOH− is determined mainly by the nature of membrane materials.9 FSB membrane, due to its hydrophobic fluorinated polymer matrix and no alkali transport promoter, has relatively low alkali permeability (3.1 × 10−3 m/h).9 PVA-based CEM is of higher alkali permeability than the SPPO-based membrane due to the highly hydrophilic nature of PVA chains.32−34 Besides, the −OH groups of PVA-based membrane can further enhance the transport of hydrated OH− through hydrogen-bonding. Both commercial PVA-I and PVA-II membranes have supports, which should reduce their permeability when compared with the other membranes without supports.34 Furthermore, the ion-exchange capacity (IEC) of membrane also affects the

Figure 3. Batch diffusion dialysis performances of PVA-I and PVA-II membranes: (a) OH− and VO3− concentrations in the diffusate with respect to time during the batch diffusion dialysis process, and (b) OH− dialysis coefficient (UOH−) and separation factor (S) with respect to time during the batch diffusion dialysis process.

membrane permeability.35 For the membranes prepared from PVA and multisilicon copolymer, the UOH− decreases with decreasing IEC, as can be verified from Table 2. This is because the ion-exchange groups can promote the ion transportation of Na+ ions. However, there is a trade-off effect between membrane permeability and selectivity. For instance, PVA-II membrane has the lowest value of UOH−, but a relatively high separation factor of 68.8. Figure 3b also shows that the PVA-I membrane has higher UOH− values and lower separation factors compared to PVA-II membranes during 4 h operation. Meanwhile, the measured UVO3− values of PVA-I membrane increased from 1.23 × 10−4 to 2.47 × 10−4 m/h gradually as a function of time, and those of PVA-II membrane are stable within the range of 2.18 × 10−5− 2.72 × 10−5 m/h. Therefore, the separation factors of both membranes decreases as the time prolongs. Interestingly, the increase of UVO3− in PVA-I membrane may be caused by the membrane corrosion, which will be discussed later in the investigation of diffusion dialysis membrane stabilities. Effect of Feed Flow Rate (Qfeed) and Flow Rate Ratio (Qfeed/Qwater) on Dynamic Diffusion Dialysis Performance. The operating parameters (i.e., Qfeed and Qfeed/Qwater) can significantly influence the concentration of the recovered alkali, alkali recovery ratio (ηOH−), and vanadium rejection ratio (ηVO3−). Both PVA-I and PVA-II membranes were investigated in dynamic diffusion dialysis process to select an optimal 5386


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Table 2. Dialysis Coefficients (UOH−) and Separation Factors (S) of PVA-I and PVA-II Membranes and Previously Reported Membranes diffusion dialysis performancea

a

cation exchange capacity (IEC) (mmol/g)

UOH−(m/h)

S

NaOH/Na2WO4

(1.4−2.2) × 10−3

63−301

0.33−0.53

NaOH/Na2WO4 NaOH/NaAl(OH)4

(1.4−1.9) × 10−3 (0.1−2.0) × 10−2

8.6−22.4 5.0−30.1

yes

1.8

NaOH/NaAl(OH)4

3.1 × 10−3

8.5

PVA-multisilicon copolymer

no

0.7−1.1

NaOH/Na2WO4

(1.0−1.1) × 10−2

16−19

PVA-I


yes

0.5−0.7

NaOH/NaVO3

4.3 × 10−3

35.3

PVA-II


yes

0.3−0.4

NaOH/NaVO3

1.9 × 10−3

68.8

membrane

composition

support

ref32

SPPO or SPPO-multisilicon copolymer

no

1.4−2.3

ref33

PVA−PECs

no

ref9

fluorinated polymer (FSB)

ref34

feed solution

The run temperature is ∼25 °C, and the run time is 1 h.

membrane for practical application. For PVA-I membrane, due to its high permeability, the Qfeed/Qwater (mL/min:mL/min) ratios were set at 0.75:0.75, 0.75:1.5, 0.75:3, 1.5:1.5, 1.5:3, 1.5:6, and 3:3, while the Qfeed/Qwater (mL/min:mL/min) ratios for the PVA-II membrane were set at 0.375:0.375, 0.375:0.75, 0.375:1.5, 0.75:0.75, 0.75:1.5, 0.75:3, 1.5:1.5, 1.5:3, 1.5:6, and 3:3. Dynamic Diffusion Dialysis Performances of PVA-I Membrane. Figure 4 illustrates NaOH and NaVO3 concentrations in the diffusate and dialysate, ηOH− and ηVO3−, in different experimental conditions. When Qfeed = 0.75 or 1.5 mL/min, as the Qfeed/Qwater changed from 1:1 to 1:4, all the concentrations decreased, as can be seen in Figure 4a, and the ηOH− increased but the ηVO3− decreased undesirably, as shown in Figure 4b. The reduced ηVO3− means that more VO3− ions are leaked from the feed to the diffusate in diffusion dialysis separation process, resulting in a low extraction efficiency of vanadium, which is disadvantageous for the production of V2O5. Hence, the optimal Qfeed/Qwater is selected as 1:1 for PVA-I membrane. As the flow rate increases under the optimized flow rate ratio, NaOH and NaVO3 concentrations of diffusate decreases from 1.695 to 1.045 mol/L and from 0.102 to 0.051 mol/L, respectively. NaOH and NaVO3 concentrations of dialysate increased from 0.646 to 1.111 mol/L and from 0.117 to 0.175 mol/L, respectively, while the ηOH− decreased from 65.12% to 44.28%, and the ηVO3− increased from 61.88% to 80.11%. Those values means that low flow rate can result in a high recovered alkali concentration and high ηOH− but low ηVO3−. Another factor, water osmosis from the water side to the feed side, is also considered to evaluate the diffusion dialysis performance. The flow rates of feed and water were precisely controlled during the experiments, and the outlet flows (Qdialysate and Qdiffusate) can be calculated as volume/time. Hence, the transported water quantity (ΔQ) can be calculated from the equation ΔQ = Qdialysate − Qfeed. Table 3 shows that ΔQ increases as Qwater increases, while Qfeed remains fixed. As is well known, the osmotic pressure of dilute solution can be calculated by the Van’t Hoff equation,36

π = CRT

Figure 4. Dynamic diffusion dialysis performances of PVA-I membrane: (a) NaOH and NaVO3 concentrations in the diffusate and dialysate, and (b) alkali recovery ratio (ηOH−) and vanadium rejection ratio (ηVO3−). Qfeed/Qwater = 1:1, 1:2, or 1:4. The X-axis refers to the flow rate of feed (Qfeed), while the flow rate of water can be obtained according to the corresponding Qfeed/Qwater.

where π is the osmotic pressure of the solution, C is the total concentration of all species in the solution, R is gas constant, and T is Kelvin temperature of the solution. Here, this equation is hypothesized to be suitable. In the present study, the equation can be expressed as the following:

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Table 3. Effect of the Flow Rate of Feed and Water on the Flow Rate of Dialysate and Diffusate, the Water Transport (ΔQ), and the Logarithmic Mean Value of ΔCtotal for PVA-I Membrane Qfeed/Qwater

Qfeed (mL/min)

Qwater (mL/min)

Qdialysate (mL/min)

Qdiffusate (mL/min)

ΔQ (mL/min)

ΔC total (mol/L)

1:1 1:2 1:4 1:1 1:2 1:4 1:1

0.750 0.750 0.750 1.500 1.500 1.500 3.000

0.750 1.500 3.000 1.500 3.000 6.000 3.000

0.877 0.964 1.065 1.748 1.786 1.840 3.252

0.623 1.286 2.685 1.252 2.714 5.660 2.748

0.127 0.214 0.315 0.248 0.286 0.340 0.252

1.643 1.846 2.119 2.138 2.136 2.071 2.859

π = C totalRT = 2(C NaOH + C NaVO3−)RT

reach as high as 1.045−1.306 mol/L, the ηOH− can be achieved at 44.28−55.95%, and the ηVO3− can be kept at the range of 70.62−80.11%. The recovered alkali with high concentration can be directly reused in the leaching procedure in production of V2O5. Dynamic Diffusion Dialysis Performances of PVA-II Membrane. Figure 5 illustrates NaOH and NaVO3 concentrations in the diffusate and dialysate, ηOH− and ηVO3− in different operation conditions when PVA-II membrane is used. When the Qfeed/Qwater = 1:1, as the flow rate increases, the NaOH and NaVO3 concentrations of diffusate decreases from

(17)

High difference of the osmotic pressure can result in high water transport from the dialysate to the diffusate side, which is the same as forward osmosis.37 The difference of the osmotic pressure between dialysate and diffusate can be calculated using the following equation: Δπ = ΔC totalRT

(18)

Figure 2b indicats that the ΔCtotal between the inlet of feed and the outlet of diffusate is the total concentration difference of all species in the feed and diffusate, and ΔCtotal between the outlet of dialysate and the inlet of water is the total concentration of all species in the dialysate. To simplify the ΔCtotal, a logarithmic mean value (ΔC total ) of ΔCtotal from the inlet of feed and to the outlet of dialysate is calculated as the overall ΔCtotal during the dynamic diffusion dialysis process, and the equation can be expressed as ΔC total =

(C total,feed − C total,diffusate − C total,dialysate) ln[(C total,feed − C total,diffusate)/C total,dialysate]

(19)

Hence, eq 17 can be substituted as Δπ = ΔC totalRT =

(C total,feed − C total,diffusate − C total,diffusate) ln[(C total,feed − C total,diffusate)/C total,dialysate]

RT (20)

It is interesting to find that the ΔC total increases with the increase in Qwater when the Qfeed is fixed, as can be seen in Table 3. The Δπ increased according to eq 20, resulted in a high water transport, and thus the increased value of ΔQ. As Qwater increases, the ηOH− increases, but the duration of mass transfer is shortened, so as to lower the concentration of recovered alkali. In addition, ηVO3− decreased with an increase in the value of Qwater. Hence, theoretically speaking, the low Qwater is better. The optimal Qfeed/Qwater is 1:1 for PVA-I membrane. When Qfeed/Qwater = 1:1, with increasing flow rate, the ΔC total also increases, as shown in Table 3, so as to increase Δπ and ΔQ. But the increase of ΔC total has no obvious influence on the improvement of dynamic diffusion dialysis performance due to the fact that the ηOH− does not increase, as can be seen from Figure 4b. It means that the high ΔC total may not result in a better dynamic diffusion dialysis performance. The flow rate of feed and water may play a more important role in the dynamic diffusion dialysis separation performance than ηOH− and ηVO3−. Overall, the Qfeed/Qwater is optimized as 1:1 for PVA-I membrane, the flow rate is optimized as 1.5−3.0 mL/min to obtain a high ηVO3−. The concentration of recovered alkali can

Figure 5. Dynamic diffusion dialysis performances of PVA-II membrane: (a) NaOH and NaVO3 concentrations in the diffusate and dialysate, and (b) alkali recovery ratio (ηOH−) and vanadium rejection ratio (ηVO3−). Qfeed/Qwater = 1:1, 1:2, or 1:4. The X-axis refers to the flow rate of feed (Qfeed), while the flow rate of water can be obtained according to the corresponding Qfeed/Qwater. 5388


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Table 4. Effect of the Flow Rate of Feed and Water on the Flow Rate of Dialysate and Diffusate, the Water Transport (ΔQ), and the Logarithmic Mean Value of ΔCtotal for PVA-II Membrane Qfeed/Qwater

Qfeed (mL/min)

Qwater (mL/min)

Qdialysate (mL/min)

Qdiffusate (mL/min)

ΔQ (mL/min)

ΔC total (mol/L)

1:1 1:2 1:4 1:1 1:2 1:4 1:1 1:2 1:4 1:1

0.375 0.375 0.375 0.750 0.750 0.750 1.500 1.500 1.500 3.000

0.375 0.750 1.500 0.750 1.500 3.000 1.500 3.000 6.000 3.000

0.486 0.508 0.536 0.879 0.920 0.938 1.667 1.678 1.716 3.188

0.264 0.617 1.339 0.621 1.330 2.813 1.333 2.822 5.784 2.813

0.111 0.133 0.161 0.129 0.170 0.188 0.167 0.178 0.216 0.188

1.893 1.861 1.990 2.353 2.495 2.642 2.798 2.932 2.926 3.868

a significant role in the dynamic diffusion dialysis performance, which can, however, be adjusted by controlling the feed and water flow rates. Overall, for PVA-II membrane, the optimized feed flow rate and Qfeed/Qwater are ∼0.75 mL/min and ∼1:2, respectively, in consideration of high ηVO3− of ∼92.56%, low concentration of VO3− of 0.0102 mol/L in diffusate, high ηOH− of ∼66.59%, and acceptable concentration of recovered alkali ∼0.933 mol/L. Comparison of Dynamic Diffusion Dialysis Performances between PVA-I and PVA-II Membranes. For PVA-I membrane, high concentration of recovered alkali (1.045−1.306 mol/L) and relatively high ηOH− (44.28−55.95%) can be achieved when the flow rate is 1.5−3.0 mL/min (Qfeed/Qwater = 1), while for PVA-II membrane, high ηOH− of ∼66.59% and acceptable concentration of recovered alkali (∼0.933 mol/L) can be achieved when the feed flow rate and Qfeed/Qwater are ∼0.75 mL/min and ∼1:2, respectively. The probable reason is that PVA-I membrane has higher ion permeability (UOH− = 0.0043 m/h) than PVA-II membrane (0.0019 m/h) according to the batch diffusion dialysis experiments. However, when using PVA-I membrane for dynamic diffusion dialysis process, the highest value of ηVO3− is only 80.11%, which can be easily attained by using PVA-II membrane (the value of ηVO3− is higher than 91.48% when the feed flow rate is higher than 0.75 mL/min). The reason is that PVA-II membrane is more stable in alkaline solution and has higher separation factor (68.8). Hence, a stable membrane should be selected for the dynamic diffusion dialysis process. Stabilities of Diffusion Dialysis Membranes. Figure 6 shows that the mass loss ratio of PVA-I membrane increased rapidly for the first 40 h and then increased slowly from ∼28% to ∼35% at the time between 40 and 200 h, and the mass loss ratio is stable at ∼35% after 200 h. It is to be noted that the mass loss ratio of PVA-II membrane is relatively quite stable and is as low as 2−6%. The values are much lower than those of PVA-I membrane. The low mass loss ratio indicates that PVA-II membrane is more stable in alkaline solution, and thus is more feasible toward the recovery of alkali from the alkaline feed. Membranes PVA-I and PVA-II are composed of PVA and multisilicon copolymer.34 The copolymer contains a large amount of −Si(OCH3)3 groups, which can undergo sol−gel reaction with PVA −OH groups to form a cross-linked silica network. The silica network can significantly enhance the amorphous region within the hybrid membranes.38 Proper dosage of silica can prevent the aggregation of silica particles and obtain a more homogeneous distribution of silica particles within the PVA matrix,39 thus elevating membrane stability and

1.679 to 0.936 mol/L and from 0.049 to 0.006 mol/L, respectively, NaOH and NaVO3 concentrations of dialysate increases from 0.786 to 1.935 mol/L and from 0.156 to 0.209 mol/L, respectively, the ηOH− decreases from 53.71% to 29.92%, and the ηVO3− increases from 85.32% to 97.58%. Although high flow rate can achieve acceptable recovered alkali (0.936 mol/L) and high ηVO3− (97.58%), the ηOH− is only 29.92%, and the NaOH concentration of dialysate is still quite high (1.935 mol/ L). Hence, high flow rate is harmful for alkali recovery due to the short time of mass transfer between feed and water. As mentioned above, increasing water flow rate can improve the ηOH− at the fixed feed flow rate. When the feed low rate is 0.375 mL/min, as the Qfeed/Qwater changes from 1:1 to 1:4, the ηOH− increases from 53.71% to 85.37% rapidly, but the concentration of recovered alkali decreases from 1.679 to 0.597 mol/L undesirably and the ηVO3− decreases from 85.32% to 77.64%. Comprehensively, when the feed flow rate is 0.375 mL/min, the optimal Qfeed/Qwater = 1:2 in consideration of high recovered alkali concentration (1.201 mol/L), high ηOH− (77.97%), and relatively high ηVO3− (80.74%). When the feed flow rate is 0.75 mL/min, with the change of Qfeed/Qwater (from 1:1 to 1:4), all the ηVO3− values are higher than 91%, the ηOH− increases from 51.78% to 72.34% rapidly, but the concentration of recovered alkali decreases from 1.431 to 0.471 mol/L undesirably. Hence, the optimal Qfeed/Qwater = 1:1−1:2 to obtain a high concentration of recovered alkali (0.931−1.431 mol/L). When the feed flow rate is 1.5 mL/min, the PVA-II membrane exhibited an outstanding vanadium rejection performance (>94.5%), but the ηOH− was only 45.40% and the NaOH concentration of dialysate was very high (1.025 mol/L) when Qfeed/Qwater = 1:1. When the Qfeed/Qwater changed to 1:2 or 1:4, the concentration of recovered alkali decreased to a low range of 0.411−0.723 mol/L, though the ηOH− can reach to 58.57−66.96%. The water osmosis is also investigated. Table 4 indicates that, when the Qfeed is fixed, the ΔQ increases with the Qwater, which is in accordance with the results of PVA-I membrane. But the overall value of water transport is lower than that with PVA-I membrane, which should be partly attributed to the different membrane erosion in alkaline solution. PVA-II membrane is more stable, as can be reflected by the lower mass loss ratio, which will be discussed in next section. The stable membrane is more effective to hinder water osmosis and maintain membrane selectivity. When the Qfeed/Qwater is fixed, the water transport increases with the increase in the flow rate due to the increase of ΔC total . As with PVA-I membrane, the ΔC total does not play 5389


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Figure 6. Membrane mass loss ratio with respect to the corrosion time.

selectivity. However, excess dosage of the copolymer would result in more serious phase separation between organic and inorganic phases,40 and thus a reduced membrane stability and selectivity in alkaline solution can be observed. Integration of Dynamic Diffusion Dialysis with Membrane Electrolysis. In the dynamic diffusion dialysis process, the operation conditions were optimized as follows: PVA-II membrane with the feed flow rate of 0.75 mL/min, and the water flow rate of 1.5 mL/min (Qfeed/Qwater = 1:2). Results showed that the flow rate ratio of dialysate to diffusate is 0.92:1.33 (Table 4). The ηOH− can reach up to ∼66.59%, and the concentration of recovered alkali is ∼0.933 mol/L. However, the residual concentration of NaOH in dialysate is still high (0.676 mol/L), which needs to be reduced further to attain the pH of ∼6 in the vanadium precipitation process as mentioned before. Hence, in the present study, membrane electrolysis was utilized to treat the dialysate and diffusate from the dynamic diffusion dialysis process, as shown in Figure 2a. Meanwhile, the gases of H2 and O2 can be produced in the cathode and anode compartment in membrane electrolysis process (Figure 2b), which can be recycled as clean energy in the subsequent calcination process in the production of V2O5. The membrane electrolysis is a batch treatment process, in which the volume ratio of anode solution (360 mL of dialysate from diffusion dialysis) to cathode solution (520 mL of diffusate from diffusion dialysis) is the same as the flow rate ratio of dialysate to diffusate in diffusion dialysis process (0.92:1.33). In membrane electrolysis process, effect of current density was investigated in details. The operating time of each batch experiment was determined by the occasion when the pH of anode solution decreases to ∼6. Figure 7a showed that the operating time decreases as the current density increases from 10 to 50 mA/cm2. A high current density can enhance the electrolytic reactions in the anode and cathode as well as ion migration can happen from the anode compartment to the cathode solution.24 When the pH of dialysate decreases to ∼6, the concentration of NaOH in diffusate can be increased from ∼0.933 mol/L to as high as ∼1.455 mol/L. Meanwhile, the total ηOH− can reach almost 100% theoretically. Besides, the leakage ratio of VO3− from the dialysate to the diffusate can almost reach zero due to the special separation mechanism of membrane electrolysis, as shown in Table 5, which is advantageous for the extraction of

Figure 7. Effect of current density on membrane electrolysis performances: (a) NaOH concentration of diffusate and pH of dialysate with respect to time, (b) the voltage drop across the membrane stack with respect to time, and (c) energy consumption and current efficiency at different current densities.

vanadium. After the treatment by membrane electrolysis process, the dialysate can be directly introduced to the vanadium precipitation process rather than neutralizing with acid; the recovered alkali (diffusate) can be directly reused in the circuit of vanadium oxide production. Figure 7b shows the change of voltage drop across the membrane stack as a function of time. As the current density increases, the overall value of voltage drop increases gradually. The reason is that the high current can result in high voltage 5390


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integration of diffusion dialysis with membrane electrolysis and single membrane electrolysis are 293.4 and 593.3 MJ/(m3 feed), respectively. After treatment with the IEM process, the NaOH in the feed can be recovered efficiently as mentioned previously. This process need not consume HCl to neutralize the waste NaOH. Therefore, the energy consumption for the IEM process can be offset by recycling the NaOH. In contrast, the running costs of the traditional method (without IEM process) refers mainly to the consumptions of acid for the neutralization process. Table 6 shows the economic analysis between the traditional method (without membrane process) and the proposed method (diffusion dialysis + membrane electrolysis). In the traditional method, the NaOH in feed solution is neutralized with the acid (e.g., HCl) to reduce the pH of the feed solution. In the process, large amounts of NaOH resource go into the waste and also large amounts of HCl need be to consumed, which increases the running cost ($24.52/(m3 feed)). When the IEM process is introduced, the cost for acid can be ignored. For membrane electrolysis, the running cost referring to the energy consumption can only be based on the membrane electrolysis process, which can be offset by the recycled NaOH, so that the total running cost can be decreased to a negative value of $−17.3/(m3 feed). A negative running cost means that the value of the recycled NaOH is higher than the cost of energy consumption for membrane eletrolysis. For the integration of diffusion dialysis with membrane electrolysis, the total running cost can be further decreased due to the low energy consumption of the dynamic diffusion dialysis process. The total running cost is as low as $−24.18/(m3 feed), much lower than that of the traditional method. Besides, in the membrane electrolysis process, a clean energy of H2 and a comburent of O2 can be produced in the cathode and anode compartments, respectively. If the produced clean energy is recycled and used in the subsequent process during the V2O5 production, the total running cost would be further reduced. Hence, the integration of dynamic diffusion dialysis with membrane electrolysis has a strong practicability and is an energy-saving and efficient method to separate and recover NaOH in the clean and sustainable production of V2O5.

Table 5. Effect of the Current Density on the Leakage of V in the Membrane Electrolysis Process concentration of V (mol/L) in diffusate

C0

Ct

leakage ratio of V in membrane electrolysis

10 mA/cm2 20 mA/cm2 30 mA/cm2 40 mA/cm2 50 mA/cm2

1.02 × 10−2

1.03 × 10−2

∼0

1.02 × 10−2

1.01 × 10−2

∼0

1.02 × 10−2

1.03 × 10−2

∼0

1.02 × 10−2

1.01 × 10−2

∼0

1.02 × 10−2

1.03 × 10−2

∼0

30 mA/cm2

0

1.23 × 10−4

∼0

conditions diffusion dialysis + membrane electrolysis

membrane electrolysis

drop when the electric resistance of the membrane stack is fixed.41 Energy consumption and current efficiency of the membrane electrolysis process are also calculated and represented in Figure 7c. As the current density increases, the energy consumption increases from 230.5 to 369.2 MJ/(m3 feed) gradually; while the current efficiency is relatively stable with a range of 89.57−93.98%. High current density can lead to a high voltage drop but a reduced experimental time. Hence the current efficiency can be stable as calculated from eq 15. Overall, the current density is optimized as ∼30 mA/cm2 in consideration of short operating time and relatively low energy consumption (293.4 MJ/(m3 feed)). Besides, single membrane electrolysis process is investigated as a comparison with the integration process. The anode solution was the feed (2.239 mol/L NaOH + 0.239 mol/L NaVO3, 300 mL), the initial cathode solution is deionized water (300 mL), and the current density was 30 mA/cm2. Table 5 shows that the leakage ratio of VO3− from the anode solution to the cathode solution can reach nearly zero, as the same as the integration process. But the energy consumption of the single membrane electrolysis process is much higher (593.3 MJ/(m3 feed), the data calculated from eq 14) and twice that of the integration process, indicating that the single-membrane electrolysis process needs more energy to separate and recover NaOH from the alkaline sodium metavanadate solution. Preliminary Economic Evaluation. A preliminary economic evaluation mainly referring to the running cost is carried out to prove the advantage and practicability of the integration of diffusion dialysis and membrane electrolysis. According to the previous section, the energy consumptions required to the

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CONCLUSION In this present study, alkaline sodium metavanadate solution containing NaOH and NaVO3, which result from the production of vanadium pentoxide, has been separated efficiently by integrating dynamic diffusion dialysis with membrane electrolysis process. The batch diffusion dialysis process showed that commercial PVA-I membrane has higher permeability (4.3 × 10−3 m/h) but lower selectivity (35.3) compared to PVA-II membrane (1.9 × 10−3 m/h, 68.8). The

Table 6. Economic Analysis of Traditional Method (without IEM Process), Single Membrane Electrolysis, and Proposed Method (Diffusion Dialysis + Membrane Electrolysis Process) without IEM process cost for energy consumption ($/m3 feed) saved cost for NaOHa ($/m3 feed) cost for acidb (such as HCl, $/m3 feed) total running cost ($/m3 feed) a

0 0 24.52 (81.72 kg/m3/30% × $0.09/ kg) 24.52

membrane electrolysis

diffusion dialysis + membrane electrolysis

13.60 (593.3MJ/m3/3.6 × $0.0825/ kWh) −30.90 (−89.56 kg/m3× $ 0.345/kg) 0

6.72 (293.4MJ/m3/3.6 × $0.0825/ kWh) −30.90 (−89.56 kg/m3× $0.345/kg) 0

−17.3

−24.18

The price of NaOH (100%) is about $0.345/kg. bThe price of HCl (30%) is about $0.09/kg. 5391


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hydrogarnet in hydrothermal process. J. Hazard. Mater. 2011, 189, 827−835. (6) Paulino, J. F.; Afonso, J. C.; Mantovano, J. L.; Vianna, C. A.; Silva Dias da Cunha, J. W. Recovery of tungsten by liquid−liquid extraction from a wolframite concentrate after fusion with sodium hydroxide. Hydrometallurgy 2012, 127−128, 121−124. (7) Bohlouli, A.; Afshar, M. R.; Aboutalebi, M. R.; Seyedein, S. H. Optimization of tungsten leaching from low manganese wolframite concentrate using Response Surface Methodology (RSM). Int. J. Refract. Hard Met. 2016, 61, 107−114. (8) Schemel, R.; Rodriguez, D.; Salazar, R. Method for leaching and recovering vanadium from vanadium bearing by-product materials. U.S. Patent 4,539,186, Sept 3, 1985. (9) Yan, H.; Xue, S.; Wu, C.; Wu, Y.; Xu, T. Separation of NaOH and NaAl(OH)4 in alumina alkaline solution through diffusion dialysis and electrodialysis. J. Membr. Sci. 2014, 469, 436−446. (10) Imtiaz, M.; Rizwan, M. S.; Xiong, S.; Li, H.; Ashraf, M.; Shahzad, S. M.; Shahzad, M.; Rizwan, M.; Tu, S. Vanadium, recent advancements and research prospects: A review. Environ. Int. 2015, 80, 79−88. (11) Morgan, K. A., Estates, H. Recovery of vanadium values. U.S. Patent 4,061,712, Dec 6, 1977. (12) Yan, Y.; Li, B.; Guo, W.; Pang, H.; Xue, H. Vanadium based materials as electrode materials for high performance supercapacitors. J. Power Sources 2016, 329, 148−169. (13) Wu, H.; Qin, M.; Li, X.; Cao, Z.; Jia, B.; Zhang, Z.; Zhang, D.; Qu, X.; Volinsky, A. A. One step synthesis of vanadium pentoxide sheets as cathodes for lithium ion batteries. Electrochim. Acta 2016, 206, 301−306. (14) Li, X.; Zhang, H.; Mai, Z.; Zhang, H.; Vankelecom, I. Ion exchange membranes for vanadium redox flow battery (VRB) applications. Energy Environ. Sci. 2011, 4, 1147−1160. (15) Liu, D.; Zi, W.; Sajjad, S. D.; Hsu, C.; Shen, Y.; Wei, M.; Liu, F. Reversible electron storage in an all-vanadium photoelectrochemical storage cell: synergy between vanadium redox and hybrid photocatalyst. ACS Catal. 2015, 5, 2632−2639. (16) Burwell, B. Process of recovering high purity vanadium compositions, U.S. Patent 3,320,024, May 16, 1967. (17) Pitts, F.; Magalas. Process for recovering vanadium values from acidic sulfate solution, U.S. Patent 4,126,663, Nov 21, 1978. (18) Pitts, F. Process for recovering vanadium values from acidic chloride solution, U.S. Patent 4,150,092, Apr 17, 1979. (19) Aarabi-Karasgani, M.; Rashchi, F.; Mostoufi, N.; Vahidi, E. Leaching of vanadium from LD converter slag using sulfuric acid. Hydrometallurgy 2010, 102, 14−21. (20) Luo, J.; Wu, C.; Xu, T.; Wu, Y. Diffusion dialysis-concept, principle and applications. J. Membr. Sci. 2011, 366, 1−16. (21) Zhang, X.; Li, C.; Wang, H.; Xu, T. Recovery of hydrochloric acid from simulated chemosynthesis aluminum foil wastewater by spiral wound diffusion dialysis (SWDD) membrane module. J. Membr. Sci. 2011, 384, 219−225. (22) Zhang, X.; Li, C.; Wang, X.; Wang, Y.; Xu, T. Recovery of hydrochloric acid from simulated chemosynthesis aluminum foils wastewater: an integration of diffusion dialysis and conventional electrodialysis. J. Membr. Sci. 2012, 409−410, 257−263. (23) Zhuang, J.-X.; Chen, Q.; Wang, S.; Zhang, W.-M.; Song, W.-G.; Wan, L.-J.; Ma, K.-S.; Zhang, C.-N. Zero discharge process for foil industry waste acid reclamation: Coupling of diffusion dialysis and electrodialysis with bipolar membranes. J. Membr. Sci. 2013, 432, 90− 96. (24) Kumar, M.; Tripathi, B. P.; Saxena, A.; Shahi, V. K. Electrochemical membrane reactor: Synthesis of quaternary ammonium hydroxide from its halide by in situ ion substitution. Electrochim. Acta 2009, 54, 1630−1637. (25) Kumar, M.; Tripathi, B. P.; Shahi, V. K. Electro-membrane reactor for separation and in situ ion substitution of glutamic acid from its sodium salt. Electrochim. Acta 2009, 54, 4880−4887.

dynamic diffusion dialysis process showed that PVA-II membrane has high rejection ratio of vanadium (>91.48%) and low water transport along with high alkali resistance (mass loss ratio of only 2−6%). The VO3− rejection ratio appeared as high as ∼92.56% with only 0.0102 mol/L residue in diffusate. The NaOH recovery ratio can stretch up to ∼66.59% with the concentration of ∼0.933 mol/L under the optimized running conditions (Qfeed = 0.75 mL/min and Qwater = 1.5 mL/min). The residual concentration of NaOH in dialysate was 0.676 mol/L, which was further treated by the membrane electrolysis process. The membrane electrolysis process reduced the pH of the dialysate side to ∼6, and enhanced the concentration of NaOH in the diffusate side from ∼0.933 to ∼1.455 mol/L correspondingly. The leakage ratio of VO3− from the dialysate to the diffusate almost reached zero. The energy consumption is as low as 293.4 MJ/(m3 feed) under the optimized current density of 30 mA/cm2.The dialysate after the treatment of membrane electrolysis can be directly introduced into the vanadium precipitation process rather than neutralizing with acid; the recovered alkali (diffusate) can be directly reused and recycled in the circuit of V2O5 production. Besides, H2 and O2 produced in the membrane electrolysis process can be recycled as clean energy in the subsequent calcination process to save the production cost. Preliminary economic evaluation indicated that the running cost of the proposed method (integration of diffusion dialysis with membrane electrolysis, $−24.18/(m3 feed)) is much lower than that of the traditional method (without membrane process, $24.52 /(m3 feed)). Hence, integration of dynamic diffusion dialysis accompanied by membrane electrolysis process is a clean, energy-saving, efficient, and sustainable method for separation and recovery of NaOH in the production of V2O5.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: + 86 551 3601587. ORCID

Yonghui Wu: 0000-0001-9521-9344 Tongwen Xu: 0000-0001-6000-1791 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (Nos. 21476220, 21490581, and 21376204), National High Technology Research and Development Program 863 (No. 2015AA021001), and K.C. Wong Education Foundation (2016).

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Integrating Diffusion Dialysis with Membrane Electrolysis for

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