Conditions Optimization and Kinetics for the Cleaning of Ceramic

Aug 23, 2011 - The cleaning rate increased with the concentration of DTPA ... A dissolution kinetics model associated with both the concentration fact...
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Conditions Optimization and Kinetics for the Cleaning of Ceramic Membranes Fouled by BaSO4 Crystals in Brine Purification Using a DTPA Complex Solution Junjie Gu, Huiqin Zhang, Zhaoxiang Zhong, and Weihong Xing* State Key Laboratory of Materials-Oriented Chemical Engineering, Membrane Science and Technology Research Center, Nanjing University of Technology, Nanjing 210009, P. R. China ABSTRACT: We investigated the cleaning efficiency and kinetics of multichannel ceramic membranes fouled during brine purification. The foulants were first characterized by means of SEM, EDX, and XRD analyses, and we found that mainly BaSO4 crystals were deposited on the membrane surface. A cleaning solution composed of diethylenetrinitrilopentaacetic acid (DTPA), oxalic acid, and NaOH was developed to regenerate the fouled ceramic membranes. The membranes could be completely recovered with the cleaning solution. The cleaning rate increased with the concentration of DTPA (CDTPA) and temperature (T) but was not sensitive to the crossflow velocity (CFV) or transmembrane pressure (TMP). The optimized cleaning condition was CDTPA = 1.0  103 mol/L, T = 50 °C, CFV = 3.0 m/s, and TMP = 0.10 MPa. A dissolution kinetics model associated with both the concentration factor and temperature factor was developed, which fitted well the experimental results. This model was used to determine the reaction rate constants during the cleaning process at different temperatures. Based on this model, we found that the activation energy of BaSO4 dissolution using the cleaning solution consisted of DTPA, oxalic acid, and NaOH was lower than that of using pure DTPA solution. The results support the conclusion that the compound solution provided a better cleaning performance than pure DTPA solution.

1. INTRODUCTION The chlor-alkali industry is one of the largest electrochemical operations in the world. The main products are chlorine and sodium hydroxide generated simultaneously by the electrolysis of sodium chloride solutions. The current efficiency of the electrolyzers depends on the technology employed, the operation of the electrolyzers, and the purity of the brine used in the process.1 The first step of all of the currently used electrolytic processes (mercury, diaphragm, and ion-exchange membrane cells) is the preparation of purified brine.2 Saturated brine in chlor-alkali plants contains some impurities, including Mg2+, Ca2+, and SO42, that affect the performance of the processes.3 Chemical precipitation is widely used to remove these impurities in the brine purification process. The precipitates are separated from the purified brine by coagulation, flocculation, sedimentation, sand filtration, or centrifugation.2,49 However, it is difficult to produce brine with higher purity because some precipitates are so small that they cannot be removed by the above-mentioned separation processes. Recently, microfiltration (MF)/ultrafiltration (UF) membranes have been employed in combination with other treatment methods to decrease the impurities to a desirable level.1012 Fouling in membrane separation reduces the permeate flux, increases the operating pressure, decreases the product quality, and ultimately shortens the membrane life.13 In some membrane bioreactor processes, inorganic scaling occurring on MF membrane surfaces can be removed by acid cleaning.14,15 In membrane processes for brine purification developed by W. L. Gore & Associates, Inc., and Hyflux Ltd., polytetrafluoroethylene (PTFE) membranes fouled by inorganic salt sludge can be recovered by acid cleaning following a back-pulse.16,17 In our research, a ceramic membrane system was industrialized in the r 2011 American Chemical Society

brine purification process. Ceramic membranes were fouled by inorganic salt sludge during filtration and recovered by backflushing and acid cleaning. Beyond the experimental results obtained in the laboratory, the permeability of the ceramic membranes decreased noticeably after 2 years of operation, and acid cleaning had no effect on the recovery of the membrane flux. The phenomenon of the inefficacy of acid cleaning in membrane recovery during long-term operation has scarcely been reported. No relevant cleaning method was applied in the industrial operation. Therefore, a feasible cleaning strategy is necessary for the ceramic membrane process for brine purification. Metal chelating agents with carboxylate functional groups are used to remove divalent cations from the organic complexes, which could improve the cleaning performances of fouled membranes.18,19 Diethylenetrinitrilopentaacetic acid (DTPA) is a commonly used chelating agent to keep metal ions such as Ba2+, Ca2+, and Sr2+ in solution and, hence, prevent scale formation in industrial processes. DTPA is also employed to enhance the dissolution of low-solubility salts, and it is widely used to control barite scaling in drilling machinery and oil wells.2025 However, there do not yet appear to be any reported work about the cleaning of fouled membranes used in brine purification by metal chelating reagents. In this work, we determined the chemical composition of the foulants by carrying out microanalysis on the surface of membranes fouled in the brine purification process. A cleaning Received: January 27, 2011 Accepted: August 23, 2011 Revised: August 16, 2011 Published: August 23, 2011 11245

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Table 1. Ranges of All of the Parameters Investigated in the Cleaning Experiments parameter 3

CDTPA (10

Figure 1. Flow sheet of the ceramic membrane system for brine purification (CMSBP).

Figure 2. Filtration pressure curve of the CMSBP within 1 month of running after 2 years of operation. Points ac are the pressures after HCl treatments. The arrow represents the increasing trend of the regenerated filtration pressure.

solution was developed to regenerate the fouled membranes. Regeneration experiments were designed to investigate the factors influencing the cleaning efficiency, and the cleaning kinetics was also investigated. Both the cleaning solution and optimized cleaning conditions were applied in the industrial operation of brine purification.

2. MATERIALS AND METHODS 2.1. Ceramic Membrane System for Brine Purification. The membranes were fouled in a ceramic membrane system for brine purification (CMSBP).12 The flow sheet of the CMSBP running in a chlor-alkali plant is presented in Figure 1. Crude salt containing impurities (Ca2+, Mg2+, SO42, organic matter, solid suspension, etc.) was dissolved in the salt dissolving tank to form saturated brine. Na2CO3, NaOH, and BaCl2 were introduced to precipitate Ca2+, Mg2+, and SO42, respectively, in the reactor. At the end of the precipitation reaction, the saturated brine in the reactor was pumped to the ceramic membrane filter to remove the suspended solids. The primary brine was produced for the ion-exchange membrane electrolysis. The retentate was pumped to the plate and frame filter to obtain cakes. To maintain a constant flux, the filtration pressure of the ceramic membrane filter needed to increase with time as a result of fouling and could be regenerated by back-flushing. The ceramic membranes were regenerated with hydrochloric acid (HCl) solution after several days of running because of the decreasing efficiency of the back-flush. Although the HCl treatment could effectively eliminate the membrane fouling, the regenerated filtration pressure still increased noticeably after long-term operation for 2 years (Figure 2), suggesting an accumulation of acid-insoluble foulants. 2.2. Membranes. The membranes, supplied by Jiangsu Jiuwu High-Tech Co. Ltd., Nanjing City, P. R. China, were

mol/L)

range 0.510.0

T (°C) CFV (m/s)

1866 1.06.0

TMP (MPa)

0.010.30

multichannel ceramic membranes. The membranes were made of a ZrO2 selective layer sintered on an α-Al2O3 support. They were 1.0 m long with 19 circular channels having an inner diameter of 4.0 mm. The filtration area of these multichannel membranes was 0.24 m2. The total area of the membranes used in the CMSBP was over 90 m2, which could guarantee 30 m3/h output of primary brine. The membranes in this study had been used in the CMSBP for 2 years and could not be regenerated by HCl treatment. They were replaced from the industrial plant and cut into short membrane segments with a length of 0.1 m. These short membrane segments were investigated in experiments. 2.3. Cleaning Reagents. Considering the remarkable performance of DTPA in dissolving barite scale in oil wells,2025 DTPA was selected as the main cleaning agent. Because oxalic acid is a typical dicarboxylic acid chelating promoter,26 it was selected as the cleaning promoter. DTPA has a strong chelating ability when the pH is greater than 12;23,27,28 as a result, NaOH was used in the cleaning solution to adjust the pH. Therefore, we mixed DTPA (Shanghai Nanxiang Reagent Co. Ltd., Shanghai, P. R. China) and oxalic acid (Shanghai Lingfeng Chemical Reagent Co. Ltd., Shanghai, P. R. China) in deionized water to prepare the cleaning solution. The cleaning solution could be prepared in different concentrations, but the molar concentration of oxalic acid was always equal to the molar concentration of DTPA. The pH of the solution was adjusted to 13 using NaOH (Xilong Chemical Reagent Co. Ltd., Shantou, P. R. China). 2.4. Experimental Procedure. The cleaning solution was used to regenerate the fouled membranes with a crossflow filtration setup that was described in detail previously.29 Keeping the permeate line open, the cleaning solution was maintained at a constant volume by recycling permeate back into the feed tank. Temperature was controlled with hot water recycling in the jacket. To evaluate the efficiency, the pure water flux (PWF) was measured after the regeneration experiments. The effects of the concentration of DTPA (CDTPA), temperature (T), crossflow velocity (CFV), and transmembrane pressure (TMP) were investigated in regeneration experiments. Flux curves were monitored during the course of experiments to determine the effective cleaning time. The ranges of all of the parameters investigated in the cleaning experiments are presented in Table 1. 2.5. Characterization. After the samples had been sputtered with a thin layer of gold, the microstructure of the membranes was observed under a field-emission scanning electron microscope (S-4800, Hitachi, Tokyo, Japan) at an acceleration voltage of 5.0 kV. Elemental analysis was conducted with an energydispersive X-ray (EDX) spectrometer attached to the microscope. The crystalline structure of the foulants was determined by the X-ray diffraction (XRD, D8 Advance, Bruker, Karlsruhe, Germany) with Cu Kα radiation at 40 kV and 30 mA. XRD patterns were recorded in the range of 5° e 2θ e 60° with a scanning step of 0.05°. The concentration of metallic elements in the cleaning solution after regeneration was analyzed by 11246

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Industrial & Engineering Chemistry Research inductively coupled plasma (ICP) optical emission spectroscopy (Optima 2000 DV, Perkin-Elmer, Wellesley, MA). The PWF of the membrane was measured with a crossflow filtration setup29 (T = 25 °C, CFV = 3.0 m/s, and TMP = 0.10 MPa).

3. RESULTS AND DISCUSSION 3.1. Membrane Regeneration. 3.1.1. Characterization of the Fouled Membranes. The fouled membranes were rinsed with 1.5 wt %

HCl solution to simulate the industrial hydrochloric acid treatment. After the hydrochloric acid treatment, the PWF of the fouled membrane was 227 L 3 m2 3 h1, which was only about 28.4% of the initial flux of a new membrane (∼800 L 3 m2 3 h1). The surface of the fouled membrane was analyzed by EDX and XRD. The EDX data reported in Table 2 suggest that the surface was composed of O, S, and Ba, with the absence of a signal for elemental Zr. According to the contaminants derived from the precipitation step in the brine purification, the elemental composition detected on the membrane surface suggested that the foulant might be barium sulfate. The XRD pattern of the fouled membrane surface reveals that only barite was present on the membrane (Figure 3). Therefore, the acid-insoluble foulant was assumed to be due to BaSO4 crystals deposited on the membrane surface. SEM micrographs of fouled membrane are represented in Figure 4a. There was a cake layer with a thickness of about 4 μm on the membrane surface. The particles of the cake layer were orthorhombic or rice-shaped, which is the typical morphology of BaSO4 crystals.30,31 The sizes of the particles were between 0.05 and 0.30 μm. 3.1.2. Effect of the Cleaning Solution. A cleaning solution was used to regenerate the fouled membrane for 100 min (CDTPA = 1.0  103 mol/L, T = 50 °C, CFV = 3.0 m/s, and TMP = 0.10 MPa). The PWF of the membrane was completely recovered to a value of 855 L 3 m2 3 h1. The surface of the regenerated membrane was analyzed by EDX. The results showed that the main metallic element composing the surface was Zr. Ba could no longer be detected (Table 2). The regenerated membrane was also analyzed by field-emission scanning electron microscopy (FESEM; Figure 4b). The SEM micrograph of the cross section of the regenerated membrane reveals that the cake layer disappeared. No BaSO4 crystals were present on the membrane surface. Instead, partially sintered particles of the top layer of the membrane were exposed. The used cleaning solution was collected after the regeneration experiments. ICP spectroscopy was applied to analyze the concentrations of metallic elements in it. Only Ba was detected, and the concentration was 34.83 mg/L. Neither Zr nor Al was detected in the solution, which suggests that the cleaning solution did not exert a corrosive effect on the membrane. An unfouled membrane was treated with a cleaning solution for 180 min (CDTPA = 1.0  103 mol/L, T = 50 °C, CFV = 3.0 m/s, and TMP = 0.10 MPa). The flux decreased by no more than 5% throughout the experiment. This minimal variation suggests negligible adsorption of the solute on the membrane surface. Therefore, the cleaning solution had no adverse effect on the lifetime and permeability of the membrane. It therefore has potential in industrial operation. 3.2. Optimization of the Cleaning Conditions. 3.2.1. Effect of the Concentration of DTPA on the Regeneration Efficiency. Regeneration experiments were conducted using cleaning solutions with different concentrations of DTPA (CDTPA = 0.5, 1.0, 5.0, and 10.0  103 mol/L). The flux curves of the cleaning solutions are presented in Figure 5. When CDTPA increased, the

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Table 2. EDX Data for the Membrane Surface fouled (wt %)

regenerated (wt %)

C

0.00

31.44

O

44.35

37.03

Al

0.00

0.39

S

13.12

0.00

Zr

0.00

31.15

Ba

45.52

0.00

Figure 3. XRD pattern of the fouled membrane surface.

Figure 4. SEM micrographs of the cross sections and surfaces of the ceramic membranes at different magnifications: (a) fouled membrane (δ is the thickness of the cake) and (b) regenerated membrane.

slope of the climbing phase became larger. The time of the climbing phase represents the effective cleaning time. Obviously, the effective cleaning time decreased with increasing CDTPA. At CDTPA = 0.5  103 mol/L, the flux grew gently and leveled off after ∼120 min, whereas the flux increased sharply and leveled off within only 10 min at CDTPA = 10.0  103 mol/L. This phenomenon suggests that the cleaning rate was strongly dependent on CDTPA. The higher the value of CDTPA, the greater the cleaning rate. This CDTPA-dependent cleaning rate is attributed to the faster dissolution of BaSO4 crystals at higher concentrations of the chelating agent. Considering both the 11247

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Figure 5. Flux versus regeneration time for cleaning solutions of different concentrations (CDTPA). Conditions: T = 50 °C, CFV = 3.0 m/s, and TMP = 0.10 MPa.

Figure 7. Flux recovery versus regeneration time at different CFVs. Conditions: CDTPA = 1.0  103 mol/L, T = 50 °C, and TMP = 0.10 MPa.

Figure 6. Flux recovery versus regeneration time at different temperatures. Conditions: CDTPA = 1.0  103 mol/L, CFV = 3.0 m/s, and TMP = 0.10 MPa.

Figure 8. Flux recovery versus regeneration time at different TMPs. Conditions: CDTPA = 1.0  103 mol/L, T = 50 °C, and CFV = 3.0 m/s.

efficiency and cost, a concentration of CDTPA = 1.0  103 mol/L is proposed to be used for the cleaning solution. The cleaning rate of regeneration was related to CDTPA, but the flux reached equivalent steady values for all concentrations. The concentrations of Ba in the used cleaning solutions of different CDTPA were all around 34.5 mg/L. The PWFs of the membranes were all completely recovered after regeneration. Therefore, cleaning solutions with any CDTPA could completely eliminate the foulant as long as the effective cleaning time guaranteed. It was noticed that all flux curves decreased slightly with time after reaching peaks (Figure 5), which might be due to the refouling of the DTPA molecules or a DTPABa complex on the membrane surface or in the membrane pores. Some researchers have also reported decreased cleaning efficiency in several membrane cleaning processes and considered refouling of the solute to be the cause.32,33 As a result, an excessively long cleaning time is unfavorable, and the regeneration was considered to be finished within the effective cleaning time. 3.2.2. Effect of the Temperature on the Regeneration Efficiency. Setting the peak flux of the cleaning solution as 100% regeneration, the flux curve can be transformed into a flux recovery curve. Figure 6 shows the flux recovery curves of the regeneration experiments at different temperatures. It is obvious that flux recovered faster at higher temperatures. Desorption of the BaDTPA complex from the barite surface is believed to be the ratecontrolling step in the dissolution process.27,28 A relatively high temperature is favorable to the activation and desorption of the BaDTPA complex.27,28 Therefore, raising the cleaning temperature could accelerate the cleaning process and provide a short effective cleaning time. Because it is easy to obtain a hot water supply with a temperature of 50 °C in industrial operation, T = 50 °C is appropriate for industrial cleaning.

3.2.3. Effect of the Crossflow Velocity on the Regeneration Efficiency. The flux recovery curves of regeneration at different CFVs are presented in Figure 7. The curves seem similar to each other. The rate-controlling step in a flowing dissolution process of barite scale is the surface reaction.27,28 Therefore, a high CFV, which provides a turbulent flow to eliminate the resistance of diffusion, is assumed to have little influence on the dissolution efficiency. Actually, the profiles of flux recovery at different CFVs almost overlapped. Therefore, the dissolution rate of the fouling was independent of the CFV. As a result, it is convenient to apply the same value of 3.0 m/s to the working CFV in industrial cleaning. 3.2.4. Effect of the Transmembrane Pressure on the Regeneration Efficiency. The flux recovery curves of regeneration at different TMPs are presented in Figure 8. The variation of TMP in these experiments changed only the flow velocity of the cleaning solution through the cake pores and membrane pores. The flow velocity parallel to the membrane surface did not alter. The cleaning rates at TMP = 0.10 and 0.30 MPa were almost the same, but that at TMP = 0.01 MPa was much smaller. This phenomenon was considered to be due to the decrease of the dissolution rate of the BaSO4 crystals at low TMP. Because the size of the membrane pores was about 50 nm and the Reynolds number of the flow in the membrane pores was even smaller than 1  104, the flow was laminar. The diffusion efficiency in laminar flow depends strongly on the flow velocity. Even though the surface reaction step controls the dissolution of barite under flowing conditions,27,28 the mass diffusion between the BaSO4 crystals and the cleaning solution becomes the rate-controlling step when the flow velocity is too small to eliminate the diffusion resistance. The flow velocity in pores at TMP = 0.01 MPa was not enough to eliminate the diffusion resistance, so the cleaning rate 11248

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was lower than that at the TMP = 0.10 and 0.30 MPa. As a result, an appropriate TMP is necessary, and the favorable value was determined to be 0.10 MPa. 3.3. Cleaning Kinetics. The flux recovery curve represents the dissolution degree of the BaSO4 cake. The experimental results in Figures 5 and 6 demonstrate that the dissolution rate of BaSO4 crystals was strongly concentration- and temperature-dependent. The concentration factor was not taken into account in the kinetics model developed by Dunn and Yen,27 and no existing model fitted the experimental results. Therefore, in this section, we investigate the dissolution kinetics of BaSO4 with the consideration of the concentration factor. 3.3.1. Kinetics Model. The stoichiometric equation for the irreversible dissolution of BaSO4 in the cleaning solution to form a complex can be described by the general reaction BaSO4 ðsÞ þ DTPA 5 w BaDTPA 3 þ SO4 2

ð1Þ

Based on the experimental results, we made following assumptions: (a) the decreased rate of the cake resistance is equal to the dissolution rate of BaSO4 and (b) the dissolution rate, expressed as the increase rate of dissolved Ba2+ concentration in the solution, is first-order in the concentrations of both BaSO4(s) remaining on membrane and DTPA. The concentration of solid BaSO4 is equal to C∞  Ct, where C∞ is the final concentration of Ba2+ and Ct is the concentration of Ba2+ in the solution at any time t. The concentration of DTPA at time t is C0  Ct, where C0 is the initial concentration of DTPA. Therefore, the reaction rate of can be expressed as rBa ¼ kCs CD ¼ kðC∞  Ct ÞðC0  Ct Þ

ð2Þ

where rBa is the reaction rate, namely, the dissolution rate of BaSO4, at steady state (mol 3 L1 3 min1); k is the reaction rate constant (L 3 mol1 3 min1); Cs is the concentration of solid BaSO4 remaining on the membrane (mol 3 L1); CD is the concentration of DTPA remaining in solution (mol 3 L1); C∞ is the final concentration of Ba2+ in solution (mol 3 L1); Ct is the concentration of Ba2+ in solution at time t (mol 3 L1); and C0 is the initial concentration of DTPA in solution (mol 3 L1). Because rBa ¼ dCt =dt integration yields Z Ct Z t dCt ¼ k dt 0 0 ðC0  Ct ÞðC∞  Ct Þ   1 C 0  Ct C0 ln  ln ¼ kt C0  C∞ C∞  Ct C∞

Figure 9. Linear fitting of the rate constants associated with temperature of 18, 50, and 66 °C. The individual symbols are the calculated experimental data, and the solid lines are the fitting curves.

ð3Þ

ð4Þ ð5Þ

In this equation, C∞ could be calculated as 0.25  103 mol 3 L1 according to the ICP data. Ct during the regeneration process could be calculated from the flux recovery values on Figure 6. Defining the left part of the eq 5 as S, values of S corresponding to various values of Ct were obtained. Therefore, rate constants associated with different temperatures could be calculated by linear fitting with S and t. The linear fitting curves are shown in Figure 9. k(18 °C), k(50 °C), and k(66 °C) were determined to be 46.40, 108.67, and 174.96 L 3 mol1 3 min1, respectively. Experimental data in Figure 5 were calculated and transformed into concentration curves of dissolved Ba2+ in cleaning solution. The kinetics model and the k(50 °C) value were applied to

Figure 10. Dissolved Ba2+ in cleaning solutions of different concentrations (CDTPA). Conditions: T = 50 °C, CFV = 3.0 m/s, and TMP = 0.10 MPa. The individual symbols are the experimental data, and the solid lines are the curves simulated from the kinetics model, where k(50 °C) = 108.67 L 3 mol1 3 min1.

Table 3. Summary of Activation Energy of BaSO4 Dissolution Ea (kcal 3 mol1)

reagent

5.32

cleaning solution

this study

9.59

DTPA

Dunn and Yen27

DTPA

Putnis et al.20

10.7

ref

simulate the concentration curves of dissolved Ba2+. The experimental and simulated results are presented in Figure 10. The model fitted the experimental results well. 3.3.2. Activation Energy. The Arrhenius equation can be applied to obtain the activation energy based on three rate constants associated with different temperatures. From the Arrhenius rate law   Ea ð6Þ k ¼ A exp RT where k is the reaction rate constant (L 3 mol1 3 min1), A is the frequency factor (L 3 mol1 3 min1), Ea is the activation energy (kJ 3 mol1), R is the ideal gas constant (8.314J 3 mol1 3 K1), and T is the temperature (K). The Arrhenius plot for the rate constants determined above gave Ea = 22.34 kJ 3 mol1 (r2 = 0.992), or 5.32 kcal 3 mol1, and A = 4.69  105 L 3 mol1 3 min1 for the cleaning solution. The Ea values reported in the literature for the dissolution of BaSO4 by DTPA are listed in Table 3. The activation energy obtained in this study is much smaller than those from the references, which suggests that the cleaning solution could depress the activation 11249

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Industrial & Engineering Chemistry Research energy of the dissolution. It has been reported that oxalate ions can catalyze the surface complexation reaction between DTPA and the BaSO4 crystal surface by forming a two-ligandsurface complex to improve the dissolution performance.26 As a result, the complex cleaning solution developed in this study provides a better cleaning performance than a simple DTPA solution.

4. CONCLUSIONS In this work, cleaning solutions composed of diethylenetrinitrilopentaacetic acid (DTPA), oxalic acid, and NaOH were developed to regenerate the fouled membranes used in the ceramic membrane system for brine purification (CMSBP). The fouling mechanism of the membrane was found to be the deposition of BaSO4 crystals on the membrane surface. BaSO4 crystals with sizes in the range of 0.050.30 μm constituted a cake layer with a thickness of about 4 μm. The cleaning solution could eliminate the cake layer to recover the pure water flux (PWF) of the membrane. The cleaning solution did not exhibit a corrosive influence on the membrane. The cleaning rate increased with the concentration of DTPA (CDTPA) and temperature (T) but was not sensitive to the crossflow velocity (CFV) or transmembrane pressure (TMP). Fouled membranes were proposed to be regenerated with cleaning solution (CDTPA = 1.0  103 mol/L) under favorable cleaning condition (T = 50 °C, CFV = 3.0 m/s, and TMP = 0.10 MPa). A dissolution kinetics model associated with both the concentration factor and the temperature factor was developed. The model was found to fit the experimental results well. The rate constants at different temperatures, the activation energy, and the frequency factor of BaSO4 dissolution by the cleaning solution were obtained. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86-25-83172288. Fax: +86-25-83172292. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (No. 2009CB623400), the National Natural Science Foundation of China (Nos. 20806038, 21076102), the Research Project of Natural Science for Universities Affiliated to Jiangsu Province (No. 10KJB530002), the Joint Innovation Fund of Jiangsu Province (No. BY2009107) of China, and the Research project for environmental protection of Jiangsu Province (No. 201018). ’ REFERENCES (1) O’Brien, T. F.; Bommaraju, T. V.; Hine, F. Handbook of ChlorAlkali Technology; Springer: New York, 2005; Vol. 1: Fundamentals. (2) O’Brien, T. F.; Bommaraju, T. V.; Hine, F. Handbook of ChlorAlkali Technology; Springer: New York, 2005; Vol. 2: Brine Treatment and Cell Operation. (3) Madaeni, S. S.; Kazemi, V. Treatment of saturated brine in chloralkali process using membranes. Sep. Purif. Technol. 2008, 61 (1), 68–74. (4) Khodorkovskaya, S. I.; Petrenko, S. A.; Zarazilov, I. S.; Voroshilov, G. N.; Samoilenko, V. I.; Aranovich, E. L.; Plekhov, N. A. Removing magnesium and calcium compounds from sodium chloride brine. USSR patent SU1263628, 1986. (5) Yagishita, A.; Hine, F. Method of purifying brine for electrolytic chemical production. U.S. Patent 4,746,441, 1988.

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