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Cleaner production of vanadium oxides by cation-exchange membrane-assisted electrolysis of sodium vanadate solution
Accepted Manuscript Cleaner production of vanadium oxides by cation-exchange membrane-assisted electrolysis of sodium vanadate solution
Bo Pan, Wei Jin, Biao Liu, Shili Zheng, Shaona Wang, Hao Du, Yi Zhang PII: DOI: Reference:
S0304-386X(16)30657-0 doi: 10.1016/j.hydromet.2017.03.010 HYDROM 4543
To appear in:
Hydrometallurgy
Received date: Revised date: Accepted date:
18 September 2016 10 February 2017 13 March 2017
Please cite this article as: Bo Pan, Wei Jin, Biao Liu, Shili Zheng, Shaona Wang, Hao Du, Yi Zhang , Cleaner production of vanadium oxides by cation-exchange membraneassisted electrolysis of sodium vanadate solution. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi: 10.1016/j.hydromet.2017.03.010
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ACCEPTED MANUSCRIPT
Cleaner Production of Vanadium Oxides by Cation-Exchange Membrane-Assisted Electrolysis of Sodium Vanadate Solution
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key
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School of Chemical Engineering, Tianjin University, Tianjin 300072, P. R. China
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1
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Bo Pan,1,2,3 Wei Jin,2 Biao Liu,2 Shili Zheng,2 Shaona Wang,2 Hao Du,2,4,* Yi Zhang1,2
Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of
National Engineering Research Center of Distillation Technology, Tianjin 300072, P. R. China 4
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3
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Sciences, Beijing 100190, P. R. China*
University of Chinese Academy of Sciences, Beijing 100190, P. R. China
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Abstract: We herein report the development of a membrane-assisted electrochemical method for separating sodium and vanadium from a sodium orthovanadate solution.
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During the employed electrolysis process, Na+ ions in the anode chamber pass through the cation-exchange membrane and combine with OH− ions in the cathode
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chamber to produce a concentrated NaOH solution, resulting in lowering of the pH in the anode chamber from 13.7 to 1.81. This reaction results in the precipitation of
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92.60%-pure vanadium oxide. The effects of the electrolysis time, current density, solution temperature, and initial NaOH concentration in the cathode chamber on the process were investigated. It was found that increases in the current density and solution temperature decreased the initial NaOH concentration in the cathode chamber and enhanced sodium and vanadium separation. Using a current density of 600 A/m2 and a solution temperature of 338 K, 3641 kW·h of energy was consumed to produce 1 t of NaOH and 0.75 t of V2O5 over an electrolysis time of 7 h. Keywords: cation-exchange membrane electrolysis; sodium vanadium separation; vanadium oxide; NaOH recovery
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1.Introduction Vanadium is an important transition metal characterised by its high melting point, high plasticity, good malleability, and high fatigue resistance. It is extensively used as
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an alloying metal for the production of specialty alloy steels such as high-speed tool steels. Vanadium compounds are also used in diverse applications, such as
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pharmaceutical catalysis and energy conservation [1,2]. However, vanadium only
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accounts for about 0.02% of the earth’s crust by weight and usually co-exists with titanium, chromium, tungsten, molybdenum, lead, copper, carbon, and phosphorus in
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ores such as vanadium-titanium magnetite, vanadinite, carnotite, and black shale [3,4]. The vanadium slag generated by the vanadium-titanium magnetite smelting process is
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the most important feed for vanadium production. Due to the importance of this metal, development of clean and efficient methods for its extraction has received significant interest [5,6].
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The most widely used method for the extraction of vanadium from vanadium
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slag is sodium salt roasting–water leaching–ammonium precipitation. In this process, the vanadium slag is roasted in a high-temperature oxidising atmosphere using
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Na2CO3, NaCl, or Na2SO4 as the additive. The vanadium, which exists in the spinel mineral, is oxidised and converted into a water-soluble pentavalent vanadium salt.
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This is followed by a precipitation process in which an ammonium salt is used to obtain ammonium metavanadate or ammonium polyorthovanadate, which is further calcined to produce V2O5 [7–9]. The sodium roasting process is particularly advantageous due to its facile implementation and stable product quality. However, it has a number of disadvantages, including low vanadium recovery, requirement of high temperatures, and the production of toxic gases. In addition, during the precipitation of the ammonium compound, large volumes of high-salinity ammonium water (containing 20–50 tons/ton V2O5) are generated. This constitutes a significant burden because of the requirement for subsequent wastewater treatment.
ACCEPTED MANUSCRIPT To overcome the challenges of the sodium salt roasting process, Wang et al. [10] developed the alkaline leaching process, which employed an 80% NaOH solution as the reaction medium. The process not only requires a relatively low temperature, but also significantly enhances the vanadium recovery to ≥90%, with the by-production of Na3VO4. The entire process is efficient and clean, without the production of toxic
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gases [11] and has thus attracted significant interest. However, considering that V2O5 is a commonly used industrial material, there is a requirement to develop a clean and
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environmentally friendly method for converting Na3VO4 into V2O5.
For the conversion of Na3VO4 to V2O5 in the vanadium industry, direct
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ammonium salt precipitation is generally employed. This method involves the addition of ammonium salt to the Na3VO4 solution to precipitate ammonium vanadate,
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which is further calcined to obtain V2O5. However, the added ammonium salt mixes with the sodium salt to form high-salinity ammonia wastewater [12]. Shen et al [13]
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therefore developed a process in which Ca2+ was used to replace Na+ in the mixture, thus producing a NaOH solution, which was recycled upstream. The process also
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precipitates calcium vanadate, which can be further reacted with the NH4HCO3
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solution to precipitate NH4VO3. The process therefore involves the recycling of ammonium and avoids the generation of high-salinity ammonia wastewater. However, it generates solid CaCO3 and therefore requires an additional waste management step.
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Furthermore, Ping et al. [14] developed another method for obtaining low-valence vanadium oxide by the direct reduction of Na3VO4 using CO or H2 at
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high temperatures and the recycling of NaOH. Although the method is clean, the high temperatures employed cause the formation of a NaOH melt, which envelopes the Na3VO4 particles, hampering direct contact of the reducing gases with the latter and preventing complete reduction. Liu et al [15] also developed an electrolysis method for converting Na3VO4 into low-valence vanadium oxides. Using a NaOH concentration of 0.4 mol/L, a Na3VO4 concentration of 0.2 mol/L, and a current density of 267 A/m2, Na3VO4 was directly reduced to VOx, and the NaOH solution was recycled. This method affords efficient separation of sodium and vanadium; however, due to the migration of VO43− and the high reduction resistance in the
ACCEPTED MANUSCRIPT cathode region, the electrolysis efficiency was only 27%. Consequently, a low-cost and environmentally friendly method for separating sodium and vanadium for the clean production of vanadium products is urgently required [16,17]. The successful development of such a process would not only be beneficial to the alkaline leaching process, but would also afford a means of avoiding
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the generation of high-salinity ammonia wastewater by the current salt roasting process.
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We herein report the development of a novel membrane-assisted electrolysis separation method, which utilises a highly selective ion-exchange membrane for the
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separation of anions and cations [18,19], as has been widely applied to chlor-alkali production, desalination, drug refining, and electroplating waste recycling [20–24].
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The proposed two-compartment electrolysis system is expected to impart low resistance and low power consumption to the procedure [25,26] and will be employed
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to achieve effective separation of vanadium and sodium from a sodium orthovanadate solution.
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A flow diagram representing the extraction process of vanadium from vanadium
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slag by alkaline leaching is shown in Figure 1. The objective of the proposed membrane-assisted electrolysis separation process is to obtain a solution containing high vanadium and low sodium concentrations in the anode compartment. The major
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operation parameters, namely the temperature, current density, electrolysis time, and initial NaOH concentration in the cathode chamber, will be systematically examined
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to determine the optimal conditions.
ACCEPTED MANUSCRIPT Vanadium Slag
Alkaline Leaching
NaOH Solution
Separation
Sodium Vanadate Solution
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Vanadium Oxides
NaOH Solution
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Separation
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Electrolysis
Fig.1. Flow diagram representing the process of vanadium extraction from vanadium slag via alkaline leaching.
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2.1.Chemicals
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2.Materials and methods
The NaOH (>96%) and Na3VO4.12H2O (>99%) employed herein were both of
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analytical reagent grade, and were purchased from Sinopharm Chemical Reagent Co.
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Ltd. All aqueous solutions were prepared using high-purity Milli-Q water (resistivity = 18 MΩ·cm). The anode was a platinum (Pt) plate (99.999%, area = 7.5 cm2), while the cathode was a Ni plate (99.999%, area = 7.5 cm2). A DuPont N-117 Nafion
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membrane with an area of 12 cm2 was employed for the cation exchange membrane.
2.2．Electrolysis of the Na3VO4 solution The electrolysis cell employed herein was a two-chamber cell composed of Plexiglas, which exhibits excellent chemical stability in both acidic and alkaline environments. Each of the two chambers measured 2 cm × 8 cm × 8 cm. The electric power was supplied by a Maynuo DC Source Meter (M8852 30 V/20 A), and a water bath (DF-101S) was used to control the system temperature. Prior to each experiment, the Pt and Ni electrodes were repeatedly rinsed and cleaned with ethanol and water in an ultrasonic cleaner to obtain contamination-free
ACCEPTED MANUSCRIPT surfaces. The membrane was then equilibrated overnight using a hydrochloric acid solution and then rinsed with demineralised water prior to each experiment. The two electrodes were then placed in the respective chambers of the cell and were maintained at 8 mm from one another，where 8 mm should be replaced with the appropriate distance. with the DuPont N-117 Nafion cation membrane separating the
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anode and cathode chambers. The prepared electrolytic cell was placed in the water bath to control its temperature（298 K-338 K）, the anode chamber was filled with a
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Na3VO4 solution, and the cathode chamber was filled with a NaOH solution. All electrolysis experiments were performed under constant electric current.
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The sodium and vanadium concentrations were then analysed using inductively coupled plasma (ICP) (Optima 7300V，P.E, America). A diffractometer coupled to a
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copper anode tube was used to obtain the X-ray diffraction (XRD) (Empyrean, PANalytical B.V, Holland ) patterns and to identify the crystalline compounds. The
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3.1. Electrolysis mechanism
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3.Results and discussion
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pH of the solution was measured using a Mettler Toledo FE20 pH meter.
Figure 2 shows a schematic representation of the cation-membrane electrolysis
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system. The objective of the electrolysis process, as mentioned earlier, was to obtain a solution with high vanadium and low sodium concentrations in the anode chamber.
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The reactions taking place at the anode and cathode were oxygen evolution (Eq. 1) and hydrogen evolution (Eq. 2), respectively: 4OH− → 2H2O + O2 + 4e−
(1)
4H2O + 4e− → 4OH− + 2H2
(2)
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Fig. 2. A schematic representation of the two-chamber membrane-assisted electrochemical approach.
As the electrochemical process progressed, the cathode chamber was filled with
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OH− ions, which induced cation migration to the cathode chamber to balance the
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charge. Because of the high cation selectivity of the cation-exchange membrane, Na+ ions passed through it to the cathode chamber to form a NaOH solution, thereby
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completing the recycling of NaOH, as shown in Figure 2. The negatively charged vanadate ions remained in the anode chamber, where a high-vanadium low-sodium
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solution was consequently formed. Furthermore, the pH in the anode chamber progressively decreased caused by the oxygen evolution reaction, resulting in the polymerisation and precipitation of the vanadate ions. As indicated, for a vanadium concentration of 0.5 mol/L and a solution pH >7, the aggregations comprise V2O74− and V4O124−, while at pH 2–7, they comprise V10O286−, V10O27(OH)5-, and V10O26(OH)24-, and at pH <2, V2O5 is precipitated[27]. The membrane-assisted electrolysis process thus enables the simultaneous recycling of NaOH and the formation of various vanadium oxide species. For optimization of this process, the effect of the electrolysis time on the
ACCEPTED MANUSCRIPT proposed method was initially investigated. Figure 3(a) shows the variation in the sodium concentration in the anode and cathode chambers with varying electrolysis times. To enhance separation, the initial sodium concentration in the anode chamber should be as high as possible, while that in the cathode chamber should be low. This minimises the migration resistance of the Na+ ions through the membrane.
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Considering the solubility of Na3VO4 at 298 K， the concentration of the Na3VO4 solution in the anode chamber in the present study was ~0.5 mol/L, and that of the
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NaOH solution in the cathode chamber was also ~0.5 mol/L. This was achieved through the utilisation of an appropriate conductivity. After 9 h of continuous
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electrolysis, the NaOH concentration in the anode chamber had decreased from 52.6 to 6.2 g/L, while that in the cathode chamber had increased from 17.5 to 64.9 g/L,
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indicating that the membrane electrolysis process effectively reduced the sodium concentration in the original solution to achieve separation of sodium from vanadium.
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Figure 3(b) shows the recorded chronopotentiogram for a current density of 600 A/cm2. As shown, the voltage increases with increasing reaction time, due to the
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gradual decrease of the sodium ion concentration in the anode chamber by the
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electromigration effect. This causes the solution resistance to also increase. However, because of the significant increase in the difference between the Na+ concentrations in the cathode and anode chambers, the migration of Na+ ions to the cathode chamber is
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substantially hampered, and this contributes to the increase in the solution resistance. As the electrolysis process progressed, no vanadium was detected in the cathode
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chamber, indicating that the Nafion cation membrane effectively prevented the migration of vanadium anions to the cathode chamber. Concentrated NaOH was thus formed in the cathode chamber, while high vanadium and low sodium concentrations were achieved in the anode chamber solution. The separation of sodium and vanadium was consequently accomplished as designed.
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(b)
60
5.6 5.2
50 40
Potential (V)
NaOH concentration (g·L-1)
(a)
Cathode Anode
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4.4 4.0
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8
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Time (h)
Fig. 3. Variation in (a) the NaOH concentration, and (b) the electrolysis voltage over time. The solution in the
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anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode
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chamber was 90 mL 17.5 g/L NaOH. The current density was 600 A/m2, the temperature was 298 K, and the electrolysis time was 9 h.
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3.2. Effect of current density
Current density is an important factor for determining the efficiency of the
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proposed electrolysis process for separating vanadium and sodium. Its specific effect was investigated using values of 600, 800, and 1000 A/m2. Figure 4 shows the
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resulting variations of the NaOH concentration and solution pH in the anode chamber.
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As shown, the separation efficiency is generally enhanced with increasing current density. More specifically, when the current density is increased, greater quantities of OH− are generated in the cathode chamber, and additional Na+ is driven through the
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cation-exchange membrane. As electrolysis proceeds, the transfer rate of Na+ through the membrane decreases, due to the following two reasons. Firstly, the continuous
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migration of Na+ to the cathode chamber results in a significant increase in the difference between the Na+ concentrations of the two chambers, which is unfavourable for further Na+ migration. Secondly, the H+ ions generated in the anode chamber compete with the Na+ ions for passage through the cation-exchange membrane, and the H+ ions that pass through the membrane neutralise the OH− ions in the cathode chamber, thereby altering the pH of the cathodic solution [28,29].
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(a) 60
(b)
40
600 A/m2 800 A/m2 1000 A/m2
12 10 8
30
pH
Concentration (g·L-1)
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600 A/m2 800 A/m2 1000 A/m2
50
6
20 4 10
2 0
2
4 6 Time (h)
8
0
10
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8
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Time (h)
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Fig. 4. Effect of current density on (a) the NaOH concentration, and (b) the pH in the anode chamber. The solution
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in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode
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chamber was 90 mL 17.5 g/L NaOH. The temperature was 298 K.
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3.3.Effect of solution temperature
The effect of solution temperature on the NaOH concentration and the pH of the
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anode chamber are shown in Figure 5. As indicated, an increase in temperature is favourable for the separation of vanadium and sodium. This is mainly because of an
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increase in the Na+ diffusion coefficient with increasing temperature, resulting in
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accelerated migration of the Na+ ions. The porosity of the membrane is also increased upon increasing the temperature, thus further accelerating the migration of Na+ ions.
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40 30 20
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Concentration (g·L-1)
50
(b)
298 K 318 K 338 K
0
2
298 K 318 K 338 K
10 8 6 4
10
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14 12
pH
(a) 60
2 4 6 Time (h)
8
10
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0
2
4
6
8
10
Time (h)
Fig.5. Effect of temperature on (a) the NaOH concentration, and (b) the pH in the anode chamber. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was 90 mL 17.5 g/L NaOH. The current density was 600 A/m2.
ACCEPTED MANUSCRIPT 3.4. Effect of initial NaOH concentration in the cathode chamber The effect of the initial Na+ concentration in the cathode chamber was investigated by varying the NaOH concentration between 17.5 and 72.4 g/L at 298 K and at a current density of 600 A/m2 (see Figure 6). As the initial NaOH concentration in the cathode chamber was increased, it became more difficult for the Na+ ions
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present in the anode chamber to pass through the membrane into the cathode chamber, resulting in suppressed separation of the sodium and vanadium. Furthermore, upon
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increasing the NaOH concentration, the viscosity of the NaOH solution increased,
(a) 60
(b)
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8
pH
30 20
17.5 g/L 35.2 g/L 72.4 g/L
10 0
14 12
40
0
2
4
6
8
10
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17.5 g/L 35.2 g/L 72.4 g/L
4 2 0
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4 Time (h)
6
8
10
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Time (h)
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Concentration (g·L-1)
50
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thus inhibiting Na+ migration.
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Fig. 6. Effect influence of the initial cathode NaOH concentration on (a) the NaOH concentration, and (b) the pH in the anode chamber. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH
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52.6 g/L), the current density was 600 A/m2, and the temperature was 298 K.
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4.Power consumption 4.1.Effect of current density Figure 7(a) shows the effect of current density on the electrolysis voltage with time. As shown, the electrolysis voltage increases as the electrolysis process progresses. This is primarily due to the electrochemical reactions that occur at the anode, as expressed by Eqs. 1 and 3. When the OH− concentration in the anode chamber is high, Eq. 1 dominates [30]. However, as the electrolysis process progresses, the anode chamber becomes acidic, due to the continuous evolution of H+,
ACCEPTED MANUSCRIPT as in shown in Eq. 3: 2H2O → 4H+ + O2 + 4e−
(3)
In addition, Figure 7(b) shows the variation in the electrolysis voltage upon reducing the acidity of the anode solution. In this case, the electrolysis voltage increases with increasing current density for a given solution acidity, and increases
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with decreasing pH (increasing acidity) of the anode chamber solution. This indicates that the voltage increase may be due to the polarisation effect on the electrode due to
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the increase in current density and the decrease in OH− concentration. This condition can be expressed by the Nernst equation (Eq. 4), where E is the actual reaction
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potential, Eɵ is the standard reaction potential, [OH−] is the OH− concentration of the solution, n is the transfer electron number of the reaction, F is Faraday’s constant, R is
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Avogadro’s constant, and T is the reaction temperature. As the OH− concentration decreases, the potential of the anodic oxygen evolution reaction (Eq. 1) increases. In
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addition, with a decreasing OH− concentration in the anode chamber, the dominant reaction at the anode changes from Eq. 1 to Eq. 3, with the standard potential
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increasing from 0.40 to 1.23 V [31], thus accounting for the observed voltage increase.
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Furthermore, with decreasing pH, both the ion concentration and the conductivity in the anode chamber decrease, thus contributing to the observed increase in electrolysis voltage.
6.0
5.0 4.5
600 A/m2 800 A/m2 1000 A/m2
4.0 3.5
0
2
(4)
6.0 5.5
Voltage (V)
Potential (V)
5.5
(b)
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(a)
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E = Eɵ + (RT)ln([OH−]4)/(nF)
4
6
8
5.0 4.5
600 A/m2 800 A/m2 1000 A/m2
4.0
10
3.5
0
2
4
6
Time (h)
8
10
12
14
pH
Fig. 7. Effect of current density on the electrolysis voltage over (a) time, and (b) pH. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was 90 mL 17.5 g/L NaOH. The temperature was 298 K.
ACCEPTED MANUSCRIPT The relationship between the current efficiency and density in the cathode chamber was also investigated. The current efficiency was calculated based on the NaOH generated in the cathode chamber, as follows: η = (cE−cB)VNAe/MIt
(5)
where η is the current efficiency, cB and cE are the NaOH molar concentrations at the beginning and end of the reaction, respectively, V is the volume of the cathode
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chamber, NA is Avogadro’s number, e is the electron charge, M is the molecular
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weight of NaOH, I is the electrolytic current, and t is the electrolysis time. Figure 8 shows the relationships between the current density and both the electric
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power and the current efficiency. As shown, the current efficiency decreases with increasing current density. This is due to the neutralisation of the generated H+ ions by
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the OH− ions generated in cathode chamber, resulting in a waste of energy and a decrease in the current efficiency. As the electric power can be given by W = UIt, a
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decrease in the current density is beneficial to achieving reduced electric power consumption. Thus, although increasing the current density enhances the separation of
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consumption of the system.
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sodium and vanadium, it decreases the current efficiency and increases the power
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0.52
0.51
6400
Electric power Current efficiency
0.50
Current efficiency
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Electric power (kW·h)
6800
6000 0.49 600
800
1000
Current density (A/m2)
Fig .8. Effect of current density on both current efficiency and power consumption. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was 90 mL 17.5 g/L NaOH. The power consumption was calculated for the production of 1 t NaOH and 0.75 t V2O5.
4.2.Effect of solution temperature Figure 9 shows the relationships between temperature and both the electrical
ACCEPTED MANUSCRIPT power consumption and the current efficiency. As shown, the electrical power consumption significantly decreased upon increasing the temperature, as a higher temperature decreases the power consumption by enhancing the migration of ions. Moreover, higher temperatures decrease the polarisation of the electrolyte concentration, and thus facilitate the diffusion of sodium ions, which also decreases
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power consumption. Furthermore, the porosity of the ion-exchange membrane increases with increasing temperature, resulting in decreased resistance to the
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migration of Na+ ions, and hence contributing to the reduction in power consumption. A higher temperature is also beneficial to the electrolysis reactions at both electrodes,
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thereby enhancing the current efficiency. As such, it was clear that a higher temperature was favourable, as it significantly decreases the electrolysis voltage, and
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therefore decreases the electrical energy consumption of the electrolysis process.
5000
0.64 0.60
Electric power Current efficiency
4000
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3000
300
0.56
Current efficiency
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0.68
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Electric power (kW·h)
6000
0.52 320
340
Temperature (K)
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Fig. 9. Effect of temperature on both current efficiency and power consumption. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was
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90 mL 17.5 g/L NaOH. The current density was 600 A/m2. The power consumption was calculated for the production of 1 t NaOH and 0.75 t V2O5.
5. Analysis of the membrane electrolysis products As previously discussed, vanadium oxides can be produced in the anode chamber at a certain pH. After prolonged electrolysis, we observed that an acidic solution with a pH of 1.81 was obtained. This was accompanied by the formation of a red precipitate upon cooling of the anode solution at room temperature. This precipitate
ACCEPTED MANUSCRIPT was then filtered and analysed by XRD. As shown in Figure 10(a), the precipitate was determined to be NaOH·3V2O5. An obtained scanning electron microscopy (SEM) image (Figure 10(b)) also revealed that the precipitate was of a uniform shape and size. ICP was then employed to determine the composition of the precipitates obtained by membrane electrolysis and acid precipitation, and the results are presented in Table
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1. As shown in the table, the precipitate obtained following membrane electrolysis was composed of 92.60% V2O5 and 7.40% Na, indicating that almost all the sodium been
successfully
separated
using
the
proposed
cation-exchange
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had
membrane-assisted electrolysis process. The composition of the corresponding
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product obtained via the acid precipitation method is also provided in the table for comparison. As can be seen, the proposed method produced significantly enhanced
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results. 2000
1200 800
HNaV6O16·4H2O
400
10
20
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0
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Indensity
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(a) 1600
30
40
50
60
2Theta/ °
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Fig. 10. (a) XRD pattern, (b) the SEM image of the obtained product. Table 1 Composition of the products obtained from the proposed electrolysis method and from direct acidification
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Method
V2O5
Na
Membrane electrolysis
92.60%
7.40%
Acid precipitation
86.32%
13.68%
Conclusion 1) A novel cation-exchange membrane-assisted electrolysis method for the effective separation of a sodium vanadate solution to obtain sodium and vanadium was developed.
ACCEPTED MANUSCRIPT 2) During the proposed electrolysis process, the Na+ ions in the anode chamber pass through the cation-exchange membrane to combine with OH− ions, and generate a concentrated NaOH solution in the cathode chamber, thus resulting in a decrease in the chamber pH from 13.7 to 1.81. This induces the precipitation of high-purity vanadium oxide in the anode chamber.
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3) Variation in the electrolysis conditions significantly affected the sodium and vanadium separation efficiency. Increasing the current density was beneficial to the
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separation, although an increase in power consumption was also observed. In contrast, higher temperatures increased the diffusion coefficient of the ions, thus enhancing the
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conductivity of the electrolyte and the reactions, and thereby decreasing the power consumption. The initial concentration of the NaOH in the cathode chamber was also
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critical to the process, with a higher concentration increasing the diffusion resistance of Na+ and decreasing the sodium and vanadium separation efficiency.
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4) For a current density of 600 A/m2 and temperature of 338 K, 3641 kW·h of energy was required to produce 1 t of NaOH and 0.75 t of V2O5. The precipitation product of
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the process was determined to be NaOH·3V2O5, with analysis revealing that the
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separation quality was higher than that achieved by the direct acidification of a sodium vanadate solution. We expect that the electrolysis process proposed herein could be considered a low-cost and environmentally friendly method for separating
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sodium and vanadium for the clean production of vanadium products, and so would avoid the generation of high-salinity ammonia wastewater resulting from current salt
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roasting processes.
Acknowledgements We acknowledge financial support from the National Basic Research Program of China (973 Program) under Grant No.2013CB632605, the Program of National Natural Science Foundation under Grant，No. 51404227，51604254，51604253.
ACCEPTED MANUSCRIPT Reference: 1. Hope B K. An assessment of the global impact of anthropogenic vanadium[J]. Biogeochemistry, 1997, 37(37):1-13. 2.Peterson J R, Smathers D B. An overview of industrial capacity and other factors [J]. Journal of Nuclear Materials.1986, 141–143 (3), 1113–1116.
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3.Yao Zhong. Review of Vanadium Processing in China [J].Engineering Sciences.2005, (3), 58-62.
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4.Hu Z, Fei H, Liu L. Comprehensive Utilization of Vanadium-Titanium Magnetite Deposits in China Has Come to a New Level [J].Acta Geologica Sinica.2013, 87 (1), 286-287.
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5. Howard R L, Richards S R, Welch B J, et al. Vanadium distribution in melts intermediate to ferroalloy production from vanadiferous slag[J]. Metallurgical & Materials Transactions B, 1994, 25:1(25):27-32.
NU
6. Liu B, Du H, Wang S N, et al. A novel method to extract vanadium and chromium from vanadium slag using molten NaOH - NaNO 3, binary system[J]. AIChE Journal, 2013, 59(2):541-552.
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7. Zhang Y, Su Z, Liu B, You Z, Yang G, Li G. Jiang T. Sodium stannate preparation from stannic oxide by a novel soda roasting–leaching process [J]. Hydrometallurgy.2014, 146 (3), 82-88.
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Vanadium Titanium.2012,33(1),1-7.
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8.Sun Chao-hui. Analysis on New Vanadium Technologies and Prospects of Vanadium Industry [J].Iron Steel
9.Tao C, Yu Y, Liu Z, Du J, Fan X. Vanadium Slag Calcium Roasting Process Enhanced by Borocalcite [J].Iron Steel Vanadium Titanium.2014,35(4),7-13.
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10. Wang Z H, Zheng S L, Wang S N, et al. Research and prospect on extraction of vanadium from vanadium slag by liquid oxidation technologies[J]. Transactions of Nonferrous Metals Society of China, 2014, 24(5):1273–1288.
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11. Liu Hui-bin, Du Hao, Liu Biao, Zheng Shi-li, Zhang Yi.Dissolution behavior of vanadium slag in KOH submolten salt[J]. The Chinese Journal of Nonferrous Metals.2013,23(4), 1130-1139. 12.Zhu Shou-chuan. Engineering application of deoxidation-neutralization+evaporation concentration process to the treatment of wastewater with sedimentated vanadium[J].Industry water treatment.2009, 29 (9), 84-87. 13.Shen Xiao-qing, Li Zhong-jun, Process of Recovering Vanadium from Vanadium Leaching Acid[J].Henan Industry. 1999(1), 16-18. 14. Li P, Xu H B, Zhang Y, et al. A cleaning preparation method of vanadium oxide: China[P]. 2009 15. Liu B, Zheng S, Wang S, et al. The redox behavior of vanadium in alkaline solutions by cyclic voltammetry method [J]. Electrochimica Acta, 2012, 76(8): 262-9.
ACCEPTED MANUSCRIPT 16.Fang L C. Study on Treating Wastewater Containing V 2O5 Produced from Vanadium-bearing Waste Slag [J].Guangzhou Chemical Industry.2011,39(18),112-115. 17.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 [J].Environment International 2015, 80, 79-88. 18. Nagarale R K, Gohil G S, Shahi V K. Recent developments on ion-exchange membranes and
PT
electro-membrane processes[J]. Advances in Colloid & Interface Science, 2006, 119(2-3):97-130. 19. Xu T, Xu T. Ion exchange membranes: State of their development and perspective[J]. Journal of Membrane
RI
Science, 2005, 263(1-2):1-29.
20.Tzanetakis N, Taama W M, Scott K. Salt splitting in a three-compartment membrane electrolysis cell
SC
[J].Filtration & Separation 2002, 39 (3), 30-38.
21.Garriga C.Sandra,Valorization of brines in the chlor-alkali industry. Integration of precipitation and membrane
NU
processes [J]..Fusion Engineering & Design 2011, 86 (4-5), 330–340.
22.Yue W, Zhang L, Liu,X, Liu,G, Liu,Y. Influence of current density on transfer characteristics in electrolysis
MA
cell of chlor-alkali industry [J].Ciesc Journal,2015,No,3.21.
23.Wang Ming-yong, Xing Hai-qing, Wang Zhi,Guo Zhan-cheng.Investigation of Chlor-Alkali Electrolysis
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Intensified by Super Gravity [J].Acta Phys. -Chim. Sin.2008, 24 (3), 520-526.
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24.Shen Pei-kang, Wang Sheng-long, Hu Zhi-yi, Li Yong-liang, Zeng Rong, Huang Yue-qiang. Hydrogen Production by Alcohol Electrolysis [J].Acta Phys. -Chim. Sin. 2007, 23 (1), 107-110. 25.Faverjon F, Rakib M, Durand G. Electrochemical study of a hydrogen diffusion anode-membrane assembly for
CE
membrane electrolysis [J].Electrochimica Acta 2005, 51 (3), 386-394. 26. Fischer R, Seidel H, Morgenstern P, Förster H J, Thiele W, Krebs P. Treatment of Process Water Containing
AC
Heavy Metals with a Two-Stage Electrolysis Procedure in a Membrane Electrolysis Cell [J]. Engineering in Life Sciences 2005, 5 (2), 163–168. 27. Robert E. The Hydrolysis of Cations[M].The hydrolysis of cations, Wiley, 1976:2385-2387. 28.Nakagawa T, Kishimoto N, Asano M, Mizutani H. Removal of Persistent Organic Compounds and Total Nitrogen in Wastewater by Ozonation Combined with Electrolysis Using Two-Compartment Electrolytic Flow Cell with Solid Electrolyte [J]. Journal of Japan Society on Water Environment 2008, 31 (31), 359-365. 29.Jin W, Tolba R, Wen J, Li K, Chen A. Efficient extraction of lignin from black liquor via a novel membrane-assisted electrochemical approach [J]. Electrochimica Acta 2013, 107 (107), 611-618. 30.Jin W, Du H, Zheng S, Xu H, Zhang Y. Comparison of the oxygen reduction reaction between NaOH and KOH
ACCEPTED MANUSCRIPT solutions on a Pt electrode: the electrolyte-dependent effect [J].Journal of Physical Chemistry B, 2010, 114 (19), 6542-6548.
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31. Zha Quan-xing. Introduction of Electrode Process Kinetics [M].Beijing: Science Press, 2002;pp 257-259.
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Highlights We proposed an effective method for separating Na and V from a Na3VO4 solution
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Na+ in the solution was recovered in the form of NaOH solution using this method
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The electrolysis conditions influenced sodium and vanadium
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separation efficiencies
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The energy consumption for the production of the products was also calculated
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No waste water was created during the membrane electrolysis process
Bo Pan, Wei Jin, Biao Liu, Shili Zheng, Shaona Wang, Hao Du, Yi Zhang PII: DOI: Reference:
S0304-386X(16)30657-0 doi: 10.1016/j.hydromet.2017.03.010 HYDROM 4543
To appear in:
Hydrometallurgy
Received date: Revised date: Accepted date:
18 September 2016 10 February 2017 13 March 2017
Please cite this article as: Bo Pan, Wei Jin, Biao Liu, Shili Zheng, Shaona Wang, Hao Du, Yi Zhang , Cleaner production of vanadium oxides by cation-exchange membraneassisted electrolysis of sodium vanadate solution. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi: 10.1016/j.hydromet.2017.03.010
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Cleaner Production of Vanadium Oxides by Cation-Exchange Membrane-Assisted Electrolysis of Sodium Vanadate Solution
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key
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School of Chemical Engineering, Tianjin University, Tianjin 300072, P. R. China
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1
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Bo Pan,1,2,3 Wei Jin,2 Biao Liu,2 Shili Zheng,2 Shaona Wang,2 Hao Du,2,4,* Yi Zhang1,2
Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of
National Engineering Research Center of Distillation Technology, Tianjin 300072, P. R. China 4
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3
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Sciences, Beijing 100190, P. R. China*
University of Chinese Academy of Sciences, Beijing 100190, P. R. China
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Abstract: We herein report the development of a membrane-assisted electrochemical method for separating sodium and vanadium from a sodium orthovanadate solution.
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During the employed electrolysis process, Na+ ions in the anode chamber pass through the cation-exchange membrane and combine with OH− ions in the cathode
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chamber to produce a concentrated NaOH solution, resulting in lowering of the pH in the anode chamber from 13.7 to 1.81. This reaction results in the precipitation of
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92.60%-pure vanadium oxide. The effects of the electrolysis time, current density, solution temperature, and initial NaOH concentration in the cathode chamber on the process were investigated. It was found that increases in the current density and solution temperature decreased the initial NaOH concentration in the cathode chamber and enhanced sodium and vanadium separation. Using a current density of 600 A/m2 and a solution temperature of 338 K, 3641 kW·h of energy was consumed to produce 1 t of NaOH and 0.75 t of V2O5 over an electrolysis time of 7 h. Keywords: cation-exchange membrane electrolysis; sodium vanadium separation; vanadium oxide; NaOH recovery
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1.Introduction Vanadium is an important transition metal characterised by its high melting point, high plasticity, good malleability, and high fatigue resistance. It is extensively used as
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an alloying metal for the production of specialty alloy steels such as high-speed tool steels. Vanadium compounds are also used in diverse applications, such as
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pharmaceutical catalysis and energy conservation [1,2]. However, vanadium only
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accounts for about 0.02% of the earth’s crust by weight and usually co-exists with titanium, chromium, tungsten, molybdenum, lead, copper, carbon, and phosphorus in
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ores such as vanadium-titanium magnetite, vanadinite, carnotite, and black shale [3,4]. The vanadium slag generated by the vanadium-titanium magnetite smelting process is
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the most important feed for vanadium production. Due to the importance of this metal, development of clean and efficient methods for its extraction has received significant interest [5,6].
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The most widely used method for the extraction of vanadium from vanadium
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slag is sodium salt roasting–water leaching–ammonium precipitation. In this process, the vanadium slag is roasted in a high-temperature oxidising atmosphere using
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Na2CO3, NaCl, or Na2SO4 as the additive. The vanadium, which exists in the spinel mineral, is oxidised and converted into a water-soluble pentavalent vanadium salt.
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This is followed by a precipitation process in which an ammonium salt is used to obtain ammonium metavanadate or ammonium polyorthovanadate, which is further calcined to produce V2O5 [7–9]. The sodium roasting process is particularly advantageous due to its facile implementation and stable product quality. However, it has a number of disadvantages, including low vanadium recovery, requirement of high temperatures, and the production of toxic gases. In addition, during the precipitation of the ammonium compound, large volumes of high-salinity ammonium water (containing 20–50 tons/ton V2O5) are generated. This constitutes a significant burden because of the requirement for subsequent wastewater treatment.
ACCEPTED MANUSCRIPT To overcome the challenges of the sodium salt roasting process, Wang et al. [10] developed the alkaline leaching process, which employed an 80% NaOH solution as the reaction medium. The process not only requires a relatively low temperature, but also significantly enhances the vanadium recovery to ≥90%, with the by-production of Na3VO4. The entire process is efficient and clean, without the production of toxic
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gases [11] and has thus attracted significant interest. However, considering that V2O5 is a commonly used industrial material, there is a requirement to develop a clean and
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environmentally friendly method for converting Na3VO4 into V2O5.
For the conversion of Na3VO4 to V2O5 in the vanadium industry, direct
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ammonium salt precipitation is generally employed. This method involves the addition of ammonium salt to the Na3VO4 solution to precipitate ammonium vanadate,
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which is further calcined to obtain V2O5. However, the added ammonium salt mixes with the sodium salt to form high-salinity ammonia wastewater [12]. Shen et al [13]
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therefore developed a process in which Ca2+ was used to replace Na+ in the mixture, thus producing a NaOH solution, which was recycled upstream. The process also
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precipitates calcium vanadate, which can be further reacted with the NH4HCO3
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solution to precipitate NH4VO3. The process therefore involves the recycling of ammonium and avoids the generation of high-salinity ammonia wastewater. However, it generates solid CaCO3 and therefore requires an additional waste management step.
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Furthermore, Ping et al. [14] developed another method for obtaining low-valence vanadium oxide by the direct reduction of Na3VO4 using CO or H2 at
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high temperatures and the recycling of NaOH. Although the method is clean, the high temperatures employed cause the formation of a NaOH melt, which envelopes the Na3VO4 particles, hampering direct contact of the reducing gases with the latter and preventing complete reduction. Liu et al [15] also developed an electrolysis method for converting Na3VO4 into low-valence vanadium oxides. Using a NaOH concentration of 0.4 mol/L, a Na3VO4 concentration of 0.2 mol/L, and a current density of 267 A/m2, Na3VO4 was directly reduced to VOx, and the NaOH solution was recycled. This method affords efficient separation of sodium and vanadium; however, due to the migration of VO43− and the high reduction resistance in the
ACCEPTED MANUSCRIPT cathode region, the electrolysis efficiency was only 27%. Consequently, a low-cost and environmentally friendly method for separating sodium and vanadium for the clean production of vanadium products is urgently required [16,17]. The successful development of such a process would not only be beneficial to the alkaline leaching process, but would also afford a means of avoiding
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the generation of high-salinity ammonia wastewater by the current salt roasting process.
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We herein report the development of a novel membrane-assisted electrolysis separation method, which utilises a highly selective ion-exchange membrane for the
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separation of anions and cations [18,19], as has been widely applied to chlor-alkali production, desalination, drug refining, and electroplating waste recycling [20–24].
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The proposed two-compartment electrolysis system is expected to impart low resistance and low power consumption to the procedure [25,26] and will be employed
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to achieve effective separation of vanadium and sodium from a sodium orthovanadate solution.
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A flow diagram representing the extraction process of vanadium from vanadium
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slag by alkaline leaching is shown in Figure 1. The objective of the proposed membrane-assisted electrolysis separation process is to obtain a solution containing high vanadium and low sodium concentrations in the anode compartment. The major
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operation parameters, namely the temperature, current density, electrolysis time, and initial NaOH concentration in the cathode chamber, will be systematically examined
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to determine the optimal conditions.
ACCEPTED MANUSCRIPT Vanadium Slag
Alkaline Leaching
NaOH Solution
Separation
Sodium Vanadate Solution
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Vanadium Oxides
NaOH Solution
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Separation
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Electrolysis
Fig.1. Flow diagram representing the process of vanadium extraction from vanadium slag via alkaline leaching.
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2.1.Chemicals
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2.Materials and methods
The NaOH (>96%) and Na3VO4.12H2O (>99%) employed herein were both of
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analytical reagent grade, and were purchased from Sinopharm Chemical Reagent Co.
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Ltd. All aqueous solutions were prepared using high-purity Milli-Q water (resistivity = 18 MΩ·cm). The anode was a platinum (Pt) plate (99.999%, area = 7.5 cm2), while the cathode was a Ni plate (99.999%, area = 7.5 cm2). A DuPont N-117 Nafion
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membrane with an area of 12 cm2 was employed for the cation exchange membrane.
2.2．Electrolysis of the Na3VO4 solution The electrolysis cell employed herein was a two-chamber cell composed of Plexiglas, which exhibits excellent chemical stability in both acidic and alkaline environments. Each of the two chambers measured 2 cm × 8 cm × 8 cm. The electric power was supplied by a Maynuo DC Source Meter (M8852 30 V/20 A), and a water bath (DF-101S) was used to control the system temperature. Prior to each experiment, the Pt and Ni electrodes were repeatedly rinsed and cleaned with ethanol and water in an ultrasonic cleaner to obtain contamination-free
ACCEPTED MANUSCRIPT surfaces. The membrane was then equilibrated overnight using a hydrochloric acid solution and then rinsed with demineralised water prior to each experiment. The two electrodes were then placed in the respective chambers of the cell and were maintained at 8 mm from one another，where 8 mm should be replaced with the appropriate distance. with the DuPont N-117 Nafion cation membrane separating the
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anode and cathode chambers. The prepared electrolytic cell was placed in the water bath to control its temperature（298 K-338 K）, the anode chamber was filled with a
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Na3VO4 solution, and the cathode chamber was filled with a NaOH solution. All electrolysis experiments were performed under constant electric current.
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The sodium and vanadium concentrations were then analysed using inductively coupled plasma (ICP) (Optima 7300V，P.E, America). A diffractometer coupled to a
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copper anode tube was used to obtain the X-ray diffraction (XRD) (Empyrean, PANalytical B.V, Holland ) patterns and to identify the crystalline compounds. The
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3.1. Electrolysis mechanism
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3.Results and discussion
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pH of the solution was measured using a Mettler Toledo FE20 pH meter.
Figure 2 shows a schematic representation of the cation-membrane electrolysis
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system. The objective of the electrolysis process, as mentioned earlier, was to obtain a solution with high vanadium and low sodium concentrations in the anode chamber.
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The reactions taking place at the anode and cathode were oxygen evolution (Eq. 1) and hydrogen evolution (Eq. 2), respectively: 4OH− → 2H2O + O2 + 4e−
(1)
4H2O + 4e− → 4OH− + 2H2
(2)
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Fig. 2. A schematic representation of the two-chamber membrane-assisted electrochemical approach.
As the electrochemical process progressed, the cathode chamber was filled with
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OH− ions, which induced cation migration to the cathode chamber to balance the
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charge. Because of the high cation selectivity of the cation-exchange membrane, Na+ ions passed through it to the cathode chamber to form a NaOH solution, thereby
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completing the recycling of NaOH, as shown in Figure 2. The negatively charged vanadate ions remained in the anode chamber, where a high-vanadium low-sodium
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solution was consequently formed. Furthermore, the pH in the anode chamber progressively decreased caused by the oxygen evolution reaction, resulting in the polymerisation and precipitation of the vanadate ions. As indicated, for a vanadium concentration of 0.5 mol/L and a solution pH >7, the aggregations comprise V2O74− and V4O124−, while at pH 2–7, they comprise V10O286−, V10O27(OH)5-, and V10O26(OH)24-, and at pH <2, V2O5 is precipitated[27]. The membrane-assisted electrolysis process thus enables the simultaneous recycling of NaOH and the formation of various vanadium oxide species. For optimization of this process, the effect of the electrolysis time on the
ACCEPTED MANUSCRIPT proposed method was initially investigated. Figure 3(a) shows the variation in the sodium concentration in the anode and cathode chambers with varying electrolysis times. To enhance separation, the initial sodium concentration in the anode chamber should be as high as possible, while that in the cathode chamber should be low. This minimises the migration resistance of the Na+ ions through the membrane.
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Considering the solubility of Na3VO4 at 298 K， the concentration of the Na3VO4 solution in the anode chamber in the present study was ~0.5 mol/L, and that of the
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NaOH solution in the cathode chamber was also ~0.5 mol/L. This was achieved through the utilisation of an appropriate conductivity. After 9 h of continuous
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electrolysis, the NaOH concentration in the anode chamber had decreased from 52.6 to 6.2 g/L, while that in the cathode chamber had increased from 17.5 to 64.9 g/L,
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indicating that the membrane electrolysis process effectively reduced the sodium concentration in the original solution to achieve separation of sodium from vanadium.
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Figure 3(b) shows the recorded chronopotentiogram for a current density of 600 A/cm2. As shown, the voltage increases with increasing reaction time, due to the
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gradual decrease of the sodium ion concentration in the anode chamber by the
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electromigration effect. This causes the solution resistance to also increase. However, because of the significant increase in the difference between the Na+ concentrations in the cathode and anode chambers, the migration of Na+ ions to the cathode chamber is
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substantially hampered, and this contributes to the increase in the solution resistance. As the electrolysis process progressed, no vanadium was detected in the cathode
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chamber, indicating that the Nafion cation membrane effectively prevented the migration of vanadium anions to the cathode chamber. Concentrated NaOH was thus formed in the cathode chamber, while high vanadium and low sodium concentrations were achieved in the anode chamber solution. The separation of sodium and vanadium was consequently accomplished as designed.
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(b)
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Potential (V)
NaOH concentration (g·L-1)
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Fig. 3. Variation in (a) the NaOH concentration, and (b) the electrolysis voltage over time. The solution in the
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anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode
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chamber was 90 mL 17.5 g/L NaOH. The current density was 600 A/m2, the temperature was 298 K, and the electrolysis time was 9 h.
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3.2. Effect of current density
Current density is an important factor for determining the efficiency of the
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proposed electrolysis process for separating vanadium and sodium. Its specific effect was investigated using values of 600, 800, and 1000 A/m2. Figure 4 shows the
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resulting variations of the NaOH concentration and solution pH in the anode chamber.
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As shown, the separation efficiency is generally enhanced with increasing current density. More specifically, when the current density is increased, greater quantities of OH− are generated in the cathode chamber, and additional Na+ is driven through the
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cation-exchange membrane. As electrolysis proceeds, the transfer rate of Na+ through the membrane decreases, due to the following two reasons. Firstly, the continuous
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migration of Na+ to the cathode chamber results in a significant increase in the difference between the Na+ concentrations of the two chambers, which is unfavourable for further Na+ migration. Secondly, the H+ ions generated in the anode chamber compete with the Na+ ions for passage through the cation-exchange membrane, and the H+ ions that pass through the membrane neutralise the OH− ions in the cathode chamber, thereby altering the pH of the cathodic solution [28,29].
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(a) 60
(b)
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600 A/m2 800 A/m2 1000 A/m2
12 10 8
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Concentration (g·L-1)
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Fig. 4. Effect of current density on (a) the NaOH concentration, and (b) the pH in the anode chamber. The solution
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in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode
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chamber was 90 mL 17.5 g/L NaOH. The temperature was 298 K.
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3.3.Effect of solution temperature
The effect of solution temperature on the NaOH concentration and the pH of the
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anode chamber are shown in Figure 5. As indicated, an increase in temperature is favourable for the separation of vanadium and sodium. This is mainly because of an
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increase in the Na+ diffusion coefficient with increasing temperature, resulting in
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accelerated migration of the Na+ ions. The porosity of the membrane is also increased upon increasing the temperature, thus further accelerating the migration of Na+ ions.
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40 30 20
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Concentration (g·L-1)
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298 K 318 K 338 K
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Fig.5. Effect of temperature on (a) the NaOH concentration, and (b) the pH in the anode chamber. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was 90 mL 17.5 g/L NaOH. The current density was 600 A/m2.
ACCEPTED MANUSCRIPT 3.4. Effect of initial NaOH concentration in the cathode chamber The effect of the initial Na+ concentration in the cathode chamber was investigated by varying the NaOH concentration between 17.5 and 72.4 g/L at 298 K and at a current density of 600 A/m2 (see Figure 6). As the initial NaOH concentration in the cathode chamber was increased, it became more difficult for the Na+ ions
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present in the anode chamber to pass through the membrane into the cathode chamber, resulting in suppressed separation of the sodium and vanadium. Furthermore, upon
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increasing the NaOH concentration, the viscosity of the NaOH solution increased,
(a) 60
(b)
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pH
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17.5 g/L 35.2 g/L 72.4 g/L
10 0
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Concentration (g·L-1)
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thus inhibiting Na+ migration.
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Fig. 6. Effect influence of the initial cathode NaOH concentration on (a) the NaOH concentration, and (b) the pH in the anode chamber. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH
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52.6 g/L), the current density was 600 A/m2, and the temperature was 298 K.
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4.Power consumption 4.1.Effect of current density Figure 7(a) shows the effect of current density on the electrolysis voltage with time. As shown, the electrolysis voltage increases as the electrolysis process progresses. This is primarily due to the electrochemical reactions that occur at the anode, as expressed by Eqs. 1 and 3. When the OH− concentration in the anode chamber is high, Eq. 1 dominates [30]. However, as the electrolysis process progresses, the anode chamber becomes acidic, due to the continuous evolution of H+,
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(3)
In addition, Figure 7(b) shows the variation in the electrolysis voltage upon reducing the acidity of the anode solution. In this case, the electrolysis voltage increases with increasing current density for a given solution acidity, and increases
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with decreasing pH (increasing acidity) of the anode chamber solution. This indicates that the voltage increase may be due to the polarisation effect on the electrode due to
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the increase in current density and the decrease in OH− concentration. This condition can be expressed by the Nernst equation (Eq. 4), where E is the actual reaction
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potential, Eɵ is the standard reaction potential, [OH−] is the OH− concentration of the solution, n is the transfer electron number of the reaction, F is Faraday’s constant, R is
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Avogadro’s constant, and T is the reaction temperature. As the OH− concentration decreases, the potential of the anodic oxygen evolution reaction (Eq. 1) increases. In
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addition, with a decreasing OH− concentration in the anode chamber, the dominant reaction at the anode changes from Eq. 1 to Eq. 3, with the standard potential
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increasing from 0.40 to 1.23 V [31], thus accounting for the observed voltage increase.
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Furthermore, with decreasing pH, both the ion concentration and the conductivity in the anode chamber decrease, thus contributing to the observed increase in electrolysis voltage.
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600 A/m2 800 A/m2 1000 A/m2
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Fig. 7. Effect of current density on the electrolysis voltage over (a) time, and (b) pH. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was 90 mL 17.5 g/L NaOH. The temperature was 298 K.
ACCEPTED MANUSCRIPT The relationship between the current efficiency and density in the cathode chamber was also investigated. The current efficiency was calculated based on the NaOH generated in the cathode chamber, as follows: η = (cE−cB)VNAe/MIt
(5)
where η is the current efficiency, cB and cE are the NaOH molar concentrations at the beginning and end of the reaction, respectively, V is the volume of the cathode
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chamber, NA is Avogadro’s number, e is the electron charge, M is the molecular
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weight of NaOH, I is the electrolytic current, and t is the electrolysis time. Figure 8 shows the relationships between the current density and both the electric
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power and the current efficiency. As shown, the current efficiency decreases with increasing current density. This is due to the neutralisation of the generated H+ ions by
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the OH− ions generated in cathode chamber, resulting in a waste of energy and a decrease in the current efficiency. As the electric power can be given by W = UIt, a
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decrease in the current density is beneficial to achieving reduced electric power consumption. Thus, although increasing the current density enhances the separation of
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consumption of the system.
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sodium and vanadium, it decreases the current efficiency and increases the power
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0.52
0.51
6400
Electric power Current efficiency
0.50
Current efficiency
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Electric power (kW·h)
6800
6000 0.49 600
800
1000
Current density (A/m2)
Fig .8. Effect of current density on both current efficiency and power consumption. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was 90 mL 17.5 g/L NaOH. The power consumption was calculated for the production of 1 t NaOH and 0.75 t V2O5.
4.2.Effect of solution temperature Figure 9 shows the relationships between temperature and both the electrical
ACCEPTED MANUSCRIPT power consumption and the current efficiency. As shown, the electrical power consumption significantly decreased upon increasing the temperature, as a higher temperature decreases the power consumption by enhancing the migration of ions. Moreover, higher temperatures decrease the polarisation of the electrolyte concentration, and thus facilitate the diffusion of sodium ions, which also decreases
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power consumption. Furthermore, the porosity of the ion-exchange membrane increases with increasing temperature, resulting in decreased resistance to the
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migration of Na+ ions, and hence contributing to the reduction in power consumption. A higher temperature is also beneficial to the electrolysis reactions at both electrodes,
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thereby enhancing the current efficiency. As such, it was clear that a higher temperature was favourable, as it significantly decreases the electrolysis voltage, and
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therefore decreases the electrical energy consumption of the electrolysis process.
5000
0.64 0.60
Electric power Current efficiency
4000
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3000
300
0.56
Current efficiency
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0.68
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Electric power (kW·h)
6000
0.52 320
340
Temperature (K)
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Fig. 9. Effect of temperature on both current efficiency and power consumption. The solution in the anode chamber was composed of 90 mL 80.7 g/L Na3VO4 (NaOH 52.6 g/L), and the solution in the cathode chamber was
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90 mL 17.5 g/L NaOH. The current density was 600 A/m2. The power consumption was calculated for the production of 1 t NaOH and 0.75 t V2O5.
5. Analysis of the membrane electrolysis products As previously discussed, vanadium oxides can be produced in the anode chamber at a certain pH. After prolonged electrolysis, we observed that an acidic solution with a pH of 1.81 was obtained. This was accompanied by the formation of a red precipitate upon cooling of the anode solution at room temperature. This precipitate
ACCEPTED MANUSCRIPT was then filtered and analysed by XRD. As shown in Figure 10(a), the precipitate was determined to be NaOH·3V2O5. An obtained scanning electron microscopy (SEM) image (Figure 10(b)) also revealed that the precipitate was of a uniform shape and size. ICP was then employed to determine the composition of the precipitates obtained by membrane electrolysis and acid precipitation, and the results are presented in Table
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1. As shown in the table, the precipitate obtained following membrane electrolysis was composed of 92.60% V2O5 and 7.40% Na, indicating that almost all the sodium been
successfully
separated
using
the
proposed
cation-exchange
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had
membrane-assisted electrolysis process. The composition of the corresponding
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product obtained via the acid precipitation method is also provided in the table for comparison. As can be seen, the proposed method produced significantly enhanced
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results. 2000
1200 800
HNaV6O16·4H2O
400
10
20
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0
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Indensity
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(a) 1600
30
40
50
60
2Theta/ °
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Fig. 10. (a) XRD pattern, (b) the SEM image of the obtained product. Table 1 Composition of the products obtained from the proposed electrolysis method and from direct acidification
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Method
V2O5
Na
Membrane electrolysis
92.60%
7.40%
Acid precipitation
86.32%
13.68%
Conclusion 1) A novel cation-exchange membrane-assisted electrolysis method for the effective separation of a sodium vanadate solution to obtain sodium and vanadium was developed.
ACCEPTED MANUSCRIPT 2) During the proposed electrolysis process, the Na+ ions in the anode chamber pass through the cation-exchange membrane to combine with OH− ions, and generate a concentrated NaOH solution in the cathode chamber, thus resulting in a decrease in the chamber pH from 13.7 to 1.81. This induces the precipitation of high-purity vanadium oxide in the anode chamber.
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3) Variation in the electrolysis conditions significantly affected the sodium and vanadium separation efficiency. Increasing the current density was beneficial to the
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separation, although an increase in power consumption was also observed. In contrast, higher temperatures increased the diffusion coefficient of the ions, thus enhancing the
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conductivity of the electrolyte and the reactions, and thereby decreasing the power consumption. The initial concentration of the NaOH in the cathode chamber was also
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critical to the process, with a higher concentration increasing the diffusion resistance of Na+ and decreasing the sodium and vanadium separation efficiency.
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4) For a current density of 600 A/m2 and temperature of 338 K, 3641 kW·h of energy was required to produce 1 t of NaOH and 0.75 t of V2O5. The precipitation product of
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the process was determined to be NaOH·3V2O5, with analysis revealing that the
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separation quality was higher than that achieved by the direct acidification of a sodium vanadate solution. We expect that the electrolysis process proposed herein could be considered a low-cost and environmentally friendly method for separating
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sodium and vanadium for the clean production of vanadium products, and so would avoid the generation of high-salinity ammonia wastewater resulting from current salt
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roasting processes.
Acknowledgements We acknowledge financial support from the National Basic Research Program of China (973 Program) under Grant No.2013CB632605, the Program of National Natural Science Foundation under Grant，No. 51404227，51604254，51604253.
ACCEPTED MANUSCRIPT Reference: 1. Hope B K. An assessment of the global impact of anthropogenic vanadium[J]. Biogeochemistry, 1997, 37(37):1-13. 2.Peterson J R, Smathers D B. An overview of industrial capacity and other factors [J]. Journal of Nuclear Materials.1986, 141–143 (3), 1113–1116.
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3.Yao Zhong. Review of Vanadium Processing in China [J].Engineering Sciences.2005, (3), 58-62.
RI
4.Hu Z, Fei H, Liu L. Comprehensive Utilization of Vanadium-Titanium Magnetite Deposits in China Has Come to a New Level [J].Acta Geologica Sinica.2013, 87 (1), 286-287.
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5. Howard R L, Richards S R, Welch B J, et al. Vanadium distribution in melts intermediate to ferroalloy production from vanadiferous slag[J]. Metallurgical & Materials Transactions B, 1994, 25:1(25):27-32.
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6. Liu B, Du H, Wang S N, et al. A novel method to extract vanadium and chromium from vanadium slag using molten NaOH - NaNO 3, binary system[J]. AIChE Journal, 2013, 59(2):541-552.
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7. Zhang Y, Su Z, Liu B, You Z, Yang G, Li G. Jiang T. Sodium stannate preparation from stannic oxide by a novel soda roasting–leaching process [J]. Hydrometallurgy.2014, 146 (3), 82-88.
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Vanadium Titanium.2012,33(1),1-7.
D
8.Sun Chao-hui. Analysis on New Vanadium Technologies and Prospects of Vanadium Industry [J].Iron Steel
9.Tao C, Yu Y, Liu Z, Du J, Fan X. Vanadium Slag Calcium Roasting Process Enhanced by Borocalcite [J].Iron Steel Vanadium Titanium.2014,35(4),7-13.
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10. Wang Z H, Zheng S L, Wang S N, et al. Research and prospect on extraction of vanadium from vanadium slag by liquid oxidation technologies[J]. Transactions of Nonferrous Metals Society of China, 2014, 24(5):1273–1288.
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11. Liu Hui-bin, Du Hao, Liu Biao, Zheng Shi-li, Zhang Yi.Dissolution behavior of vanadium slag in KOH submolten salt[J]. The Chinese Journal of Nonferrous Metals.2013,23(4), 1130-1139. 12.Zhu Shou-chuan. Engineering application of deoxidation-neutralization+evaporation concentration process to the treatment of wastewater with sedimentated vanadium[J].Industry water treatment.2009, 29 (9), 84-87. 13.Shen Xiao-qing, Li Zhong-jun, Process of Recovering Vanadium from Vanadium Leaching Acid[J].Henan Industry. 1999(1), 16-18. 14. Li P, Xu H B, Zhang Y, et al. A cleaning preparation method of vanadium oxide: China[P]. 2009 15. Liu B, Zheng S, Wang S, et al. The redox behavior of vanadium in alkaline solutions by cyclic voltammetry method [J]. Electrochimica Acta, 2012, 76(8): 262-9.
ACCEPTED MANUSCRIPT 16.Fang L C. Study on Treating Wastewater Containing V 2O5 Produced from Vanadium-bearing Waste Slag [J].Guangzhou Chemical Industry.2011,39(18),112-115. 17.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 [J].Environment International 2015, 80, 79-88. 18. Nagarale R K, Gohil G S, Shahi V K. Recent developments on ion-exchange membranes and
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electro-membrane processes[J]. Advances in Colloid & Interface Science, 2006, 119(2-3):97-130. 19. Xu T, Xu T. Ion exchange membranes: State of their development and perspective[J]. Journal of Membrane
RI
Science, 2005, 263(1-2):1-29.
20.Tzanetakis N, Taama W M, Scott K. Salt splitting in a three-compartment membrane electrolysis cell
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[J].Filtration & Separation 2002, 39 (3), 30-38.
21.Garriga C.Sandra,Valorization of brines in the chlor-alkali industry. Integration of precipitation and membrane
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processes [J]..Fusion Engineering & Design 2011, 86 (4-5), 330–340.
22.Yue W, Zhang L, Liu,X, Liu,G, Liu,Y. Influence of current density on transfer characteristics in electrolysis
MA
cell of chlor-alkali industry [J].Ciesc Journal,2015,No,3.21.
23.Wang Ming-yong, Xing Hai-qing, Wang Zhi,Guo Zhan-cheng.Investigation of Chlor-Alkali Electrolysis
D
Intensified by Super Gravity [J].Acta Phys. -Chim. Sin.2008, 24 (3), 520-526.
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24.Shen Pei-kang, Wang Sheng-long, Hu Zhi-yi, Li Yong-liang, Zeng Rong, Huang Yue-qiang. Hydrogen Production by Alcohol Electrolysis [J].Acta Phys. -Chim. Sin. 2007, 23 (1), 107-110. 25.Faverjon F, Rakib M, Durand G. Electrochemical study of a hydrogen diffusion anode-membrane assembly for
CE
membrane electrolysis [J].Electrochimica Acta 2005, 51 (3), 386-394. 26. Fischer R, Seidel H, Morgenstern P, Förster H J, Thiele W, Krebs P. Treatment of Process Water Containing
AC
Heavy Metals with a Two-Stage Electrolysis Procedure in a Membrane Electrolysis Cell [J]. Engineering in Life Sciences 2005, 5 (2), 163–168. 27. Robert E. The Hydrolysis of Cations[M].The hydrolysis of cations, Wiley, 1976:2385-2387. 28.Nakagawa T, Kishimoto N, Asano M, Mizutani H. Removal of Persistent Organic Compounds and Total Nitrogen in Wastewater by Ozonation Combined with Electrolysis Using Two-Compartment Electrolytic Flow Cell with Solid Electrolyte [J]. Journal of Japan Society on Water Environment 2008, 31 (31), 359-365. 29.Jin W, Tolba R, Wen J, Li K, Chen A. Efficient extraction of lignin from black liquor via a novel membrane-assisted electrochemical approach [J]. Electrochimica Acta 2013, 107 (107), 611-618. 30.Jin W, Du H, Zheng S, Xu H, Zhang Y. Comparison of the oxygen reduction reaction between NaOH and KOH
ACCEPTED MANUSCRIPT solutions on a Pt electrode: the electrolyte-dependent effect [J].Journal of Physical Chemistry B, 2010, 114 (19), 6542-6548.
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CE
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SC
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31. Zha Quan-xing. Introduction of Electrode Process Kinetics [M].Beijing: Science Press, 2002;pp 257-259.
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Highlights We proposed an effective method for separating Na and V from a Na3VO4 solution
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Na+ in the solution was recovered in the form of NaOH solution using this method
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The electrolysis conditions influenced sodium and vanadium
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separation efficiencies
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The energy consumption for the production of the products was also calculated
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No waste water was created during the membrane electrolysis process