Conversion of Potassium Chloride into Potassium Sulfate by Four

Nov 17, 2015 - All the chemicals, such as KCl, K2SO4, (NH4)2SO4, NH4Cl etc., used in this work were of analytical grade. Distilled water was used thro...
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Conversion of Potassium Chloride into Potassium Sulfate by FourCompartment Electrodialysis: Batch Operation Process Xu Zhang,† Xiaoyao Wang,† Xianchao Liu,† Xiaozhao Han,*,† Chenxiao Jiang,‡ Qiuhua Li,‡ and Tongwen Xu*,‡ †

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China Laboratory of Functional Membranes, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei Anhui 230026, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: K2SO4 is a kind of potassic fertilizer, which is mainly prepared through converting KCl with dissolved sulfate or H2SO4. In this study, four-compartment electrodialysis was used to convert KCl with (NH4)2SO4 into K2SO4 to overcome the shortcomings of traditional methods. The phase of product crystals was identified by XRD and the crystal composition was determined by the ion content. Economic evaluation was conducted, and the effects of operation variables on conversion performance were investigated. Results showed that as current density increases from 10 to 25 mA/cm2, operation time decreases from 135 to 55 min while energy consumption increases from 0.26 to 0.50 kw·h/kg K2SO4. A slightly higher than theoretical molar ratio of (NH4)2SO4 to KCl can ensure the complete conversion of KCl into K2SO4. In addition, final K2SO4 concentrations could be increased by reducing the initial K2SO4 tank volume. Finally, higher temperature is beneficial for conversion performance and subsequent evaporation crystallization.

KCl + KHSO4 = K 2SO4 + HCl

1. INTRODUCTION Potassium is one of the three essential elements for all plants. It can develop the root, enhance the ability of drought resistance, promote photosynthesis and so on.1 Generally, one can supply potassium to plants through fertilizing the potassic fertilizers to the soil. Potassic fertilizers are usually divided into two kinds: one is potassic fertilizers containing chlorine (mainly potassium chloride); the other is chlorine-free potassic fertilizers. Potassium chloride is the basic potassic fertilizer, which is readily recoverable from naturally occurring potash raw materials, accounts for 90% of the amount of potassic fertilizers.2 But the chlorine element could be harmful or toxic for chloride-sensitive crops (such as potatoes, tomatoes, citruses and so on); moreover, it would increase soil salinity and acidulation.1 For chlorine-free potassic fertilizers, there are potassium sulfate, potassium carbonate and potassium nitrate. Among them, potassium carbonate and potassium nitrate are too expensive to be used in agriculture. However, potassium sulfate has a lower price and also excellent physical properties, and thus is more appropriate for the chloride-sensitive crops.3−5 Therefore, the preparation of potassium sulfate seems urgent and important. Generally, about 75% of potassium sulfate is generated by converting potassium chloride with dissolved sulfate or sulfuric acid. The traditional preparation methods include the Mannheim method, association−displacement method, and double decomposition method.6 The Mannheim method is mature and produces more than 50% of potassium sulfate. As seen from the reaction mechanism, this method includes two steps of reaction: KCl + H 2SO4 = KHSO4 + HCl © 2015 American Chemical Society

(R2)

The first step reaction is an exothermic reaction, whereas the second step reaction is an endothermic reaction, which needs to be conducted in a specialized heating furnace at 600−700 °C. The Mannheim method is simple and the product yield and quality is relatively stable, but it still has some shortcomings: high investment, strong corrosiveness and abrasiveness of raw materials, high reaction temperature and energy consumption, and so on. The association-displacement method is developed to decrease the reaction temperature and energy consumption of Mannheim method, but the process is more complicated; also, association and dissociation agents are expensive and thus increase product cost.7 In the double decomposition method, dissolved sulfate reacts with potassium chloride to form the potassium sulfate, and the key is to make full use of the phase diagram data to choose the optimum transformation and crystallization points. The reaction temperature is about 100− 200 °C, and the process can be easily controlled; however, a lot of mother liquors are generated that need to be disposed during the process. At the same time, it is highly corrosive and prone to damage the equiment.4,8 As a consequence, more and more attention should be paid to explore a simple method with low energy consumption and low corrosiveness. Electrodialysis (ED) is an electromembrane separation process to recover ionic species from aqueous solution without chemical consumption or waste generation.9−11 Under the application of an electric field, cations migrate toward the Received: Revised: Accepted: Published:

(R1) 11937

September 1, 2015 November 6, 2015 November 17, 2015 November 17, 2015 DOI: 10.1021/acs.iecr.5b03245 Ind. Eng. Chem. Res. 2015, 54, 11937−11943

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Industrial & Engineering Chemistry Research

Figure 1. Flowchart of experimental apparatus. (1) ED module; (2) DC power supply; (3) submersible pump; (4) KCl tank; (5) K2SO4 tank; (6) (NH4)2SO4 tank; (7) NH4Cl tank; (8) anode solution tank; (9) cathode solution tank.

branes were manufactured from the polymerization of vinyl benzyl chloride (VBC) in PVDF/DMF solution, and then quaternized by trimethylamine (TMA). Their main characteristics are listed in Table S1. All the chemicals, such as KCl, K2SO4, (NH4)2SO4, NH4Cl etc., used in this work were of analytical grade. Distilled water was used throughout. 2.2. Experimental Apparatus. Figure 1 illustrates the experimental apparatus for the preparation process of potassium sulfate by ED. The employed ED module (1) was provided by Hefei Chemjoy polymer materials Co., Ltd., China. The effective area of each membrane was approximately 200 cm2, and the entire module included 24 pieces of anion exchange membranes and 24 pieces of cation exchange membranes, constituting 2 electrode compartments, 12 KCl compartments, 12 K2SO4 compartments, 12 (NH4)2SO4 compartments and 11 NH4Cl compartments. Compartments were separated by anion/cation exchange membranes and plastic partition nets (thickness ≈ 0.8 mm). The electrodes, which were made of titanium coated with ruthenium, were connected with a direct current power supply (2) (QJ3005, Ningbo Jiuyuan electronics Co., Ltd., China). 250 mL of saturated KCl, 0.08 mol/L K2SO4, 250 mL of (NH4)2SO4 and 250 mL of 0.1 mol/L NH4Cl were placed in tanks 5−7, respectively. Tanks 8 and 9 were anode and cathode solution tanks, and a K2SO4 solution (0.3 mol/L, 300 mL) was used as a rinse for them. Six submersible pumps (AP1000, Guangdong Zhenghua Electrical Appliance Co., Ltd., China, with the maximal speed of 400 L/h) were placed in tanks 4−9 to circulate the relative solutions for the ED module. Constant current and batch operation mode was adopted in this work, and once the voltage drop increased rapidly suddenly, the operation was stopped. Before the current was applied, the solution in each tank was circulated to eliminate all the visible gas bubbles inside the ED module. At different running times, voltage drop across the ED module was recorded, and samples in tanks 4−7 were taken to analyze the NH 4 + or K + concentrations, respectively. All the operations were conducted at room temperature (25 ± 3 °C), except for the case where the

cathode while anions migrate toward the anode through cation−anion exchange membranes.12 Now, ED has been proven to be a simple, efficient and environmental friendly technology.13 It has been used to demineralize and desalinize seawater or brackish water, recover organic acids, concentrate the industrial feeds and so on.14−19 On the basis of the features of ED and the preparation processes of potassium sulfate, ED could be introduced into the process of converting potassium chloride into potassium sulfate. Under the applied electric field and the separation effect of the ion exchange membranes, K+ migrates from KCl, combines with SO42− migrates from the dissolved sulfate to form the K2SO4, while Cl− migrates from the KCl, combines with the cation migrates from dissolved sulfate to generate the chlorinated salt. The entire process is similar to the double decomposition reaction, but it cannot be realized in a conventional reaction oven because neither precipitates nor gases will be produced. Here we show how a four-compartment ED works on such a double decomposition reaction. Ammonium sulfate, the byproduct of petrochemical industries and coking enterprises, was selected as the resource of sulfate radicals. Potassium chloride “reacts” with ammonium sulfate to form the potassium sulfate and ammonium chloride, which is a kind of nitrogen fertilizer. Here the effect of current density, the molar ratio of (NH4)2SO4 to KCl, initial volume and solution temperature in K2SO4 tank on conversion performance are investigated. In addition, the analysis work on the generated product crystals and preliminary economic evaluation is conducted.

2. EXPERIMENTAL SECTION 2.1. Materials. The used membranes were LabA (AEM, Hefei Chemjoy Polymer Materials Co., Ltd.) and LabC (CEM, Hefei Chemjoy Polymer Materials Co., Ltd.). They were homogeneous membranes with the sulfonic acid and quaternary amine groups, respectively. LabC membranes were produced from the polymerization of sodium p-styrenesulfonate (SSS) in a polyvinylidene difluoride (PVDF)/N,Ndimethylformamide (DMF) solution, whereas LabA mem11938

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the four voltage drop−time curves appear first decreased and then increased, especially in the case of i = 25 mA/cm2. At the start of the operation, the solution concentrations in K2SO4 and NH4Cl tanks were at low levels to result in a high ED module resistance; as time progressed, the solution concentrations in the above two tanks increased, whereas those in KCl and (NH 4 ) 2SO 4 tanks decreased. Eventually, the KCl and (NH4)2SO4 solutions were very diluted, which increased the voltage drop suddenly. At that moment, the operation was stopped. Moreover, as the current density increased from 10 to 25 mA/cm2, the voltage drop presented an upward tendency, and the operation time could be shortened from 135 to 55 min because of the higher driving force.21,22 In the right part of Figure 2, NH4Cl concentration increased with the increasing of current density. Also, the higher the current density, the faster the NH4Cl concentration grows. Table S2 illustrates the effect of current density on the KCl conversion ratio, final K2SO4 concentration in K2SO4 tank and energy consumption. Except for the case of i = 25 mA/cm2, KCl conversion ratios of other three cases were more than 95%. As stated above, the initial molar ratio of (NH4)2SO4 to KCl was 1:2, which was the theoretical reaction ratio. Ideally speaking, when the operation was stopped, there was no solute in both KCl and (NH4)2SO4 tanks simultaneously. However, as a matter of fact, under the application of an electric field, different ions have different migration rates. Especially at higher electric field intensity, the phenomenon was more obvious.23 Hence, the KCl conversion ratio in the case of i = 25 mA/cm2 was so low because there still remained some KCl solute in the KCl tank but almost no (NH4)2SO4 solute in the (NH4)2SO4 tank at the end of the operation. Also, these can be confirmed by determining the final solution concentrations (0.79 mol/L for KCl and 0.06 mol/L for (NH4)2SO4). The final K2SO4 concentrations were between 83 and 92 g/L, which was less than the saturated concentration at room temperature; therefore, this would increase the energy consumption of subsequent evaporation crystallization. Next, the initial K2SO4 tank volume was investigated to increase the final K2SO4 concentration. In addition, when the current density increased from 10 to 25 mA/cm2, the energy consumption increased from 0.26 to 0.50 kw·h/kg K2SO4 accordingly. Thus, it was determined from the viewpoints of operation time and energy consumption that 15 and 20 mA/cm2 seemed appropriate for the operation. 3.2. Effect of the Molar Ratio of (NH4)2SO4 to KCl. As stated above, the entire conversion process by ED is similar to the double decomposition reaction, i.e., KCl “reacts” with (NH4)2SO4 could follow the reaction equation:

effect of solution temperature of K2SO4 tank on conversion performance was investigated. 2.3. Analyses and Data Calculations. The NH 4 + concentration was determined through oxidizing by formaldehyde first, then titrating with a calibrated NaOH solution with phenolphthalein as an indicator. The K+ concentration was analyzed by atomic absorption spectrophotometer (WFX-130B, Beijing Beifen-Ruili analytical instrument (Group) Co., Ltd., China). The generated K2SO4 crystals were analyzed through X-ray diffraction (X’Pert PRO MPD, PANalytical Co., The Netherlands) for the phase of the crystals, and the content of SO42− and Cl− were determined through ion chromatography (Dionex ICS-3000, Thermo Fisher Scientific Co., USA). The KCl conversion ratio can be calculated from the following formula: ηKCl =

C0V0 − C tVt C0V0

(1)

where C0 and Ct are the KCl concentrations in KCl tank at the start and the end of the operation, respectively. V0 and Vt are the solution volume in KCl tank at the start and the end of the operation, respectively. The energy consumption E (kw·h/kg K2SO4) was calculated as follows:20

E=

UI t dt t ,K Vt ,K M

∫C

(2)

where Ut is the voltage drop across the ED module at time t, I is the current, Ct,K and Vt,K are the K2SO4 concentration and volume in K2SO4 tank at time t. M is the K2SO4 molar molecular weight.

3. RESULTS AND DISCUSSION 3.1. Effect of Current Density. The initial molar ratio of (NH4)2SO4 to KCl was kept at 1:2, the initial (NH4)2SO4 concentration was about 1.7 mol/L, and the initial K2SO4 tank volume was 700 mL. Figure 2 shows the voltage drops across the ED module and NH4Cl concentrations in NH4Cl tank under different current densities. In the left part of Figure 2, all

2KCl + (NH4)2 SO4 = K 2SO4 + 2NH4Cl

(R3)

As seen from the equation, the theoretical molar ratio of (NH4)2SO4 to KCl was 1:2. In fact, to make the targeted reactant react completely, more additional reactants are needed. From here, the effect of the molar ratio of (NH4)2SO4 to KCl was investigated. Current density was set as 15 mA/cm2, and other operation parameters were the same as those described above. Figure 3 indicates the effect of the molar ratio of (NH4)2SO4 to KCl on voltage drops across the ED module and NH4Cl concentrations. From the left part of Figure 3, it seems that the molar ratio has hardly any effect on the voltage drops and operation time, except for the case where the molar ratio is 2:2. But for the NH4Cl concentration−time curves in the right part

Figure 2. Effect of current density on voltage drops across ED module and NH4Cl concentration in NH4Cl tank when the molar ratio of (NH4)2SO4 to KCl was 1:2. 11939

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Figure 3. Effect of molar ratio of (NH4)2SO4 to KCl on voltage drops across ED module and NH4Cl concentration in NH4Cl tank. Figure 4. Effect of the K2SO4 tank initial volume on voltage drops across ED module and NH4Cl concentration in the NH4Cl tank.

of Figure 3, as the molar ratio increased, the concentration increased slightly. The reason is mainly that a higher molar ratio could afford more NH4+, under the effect of concentration difference diffusion and electric field driven, more NH4+ transfers via the membrane into the NH4Cl tank. Table S3 shows the effect of the molar ratio of (NH4)2SO4 to KCl on KCl conversion ratio, final K2SO4 concentration and energy consumption. Except for the case where the molar ratio is 1.0:2, the KCl conversion ratios of other cases are all more than 98%. This effectively demonstrated that excessive (NH4)2SO4 can fully promote the conversion of KCl into K2SO4. With regard to final K2SO4 concentration and energy consumption, it seems that effect of molar ratio is not obvious; final K 2 SO 4 concentration changed between 89 and 94 g/L whereas energy consumption changed between 0.34 and 0.38 kw·h/kg K2SO4. To sum up, a high molar ratio of the (NH4)2SO4 to KCl can promote the complete conversion of KCl into K2SO4 without affecting other performances obviously. But excessive (NH4)2SO4 would cause unnecessary waste of resources. Hence, it is suggested that a ratio that is slightly higher than the theoretical molar ratio (1.0:2) is preferable. 3.3. Effect of the Initial Volume of K2SO4 Tank. As stated in section 3.1, to decrease the energy consumption of subsequent evaporation crystallization, final K2SO4 concentrations could be increased through reducing the K2SO4 tank initial volume. Here, the effect of K2SO4 tank initial volume on conversion performance was investigated. The molar ratio of (NH4)2SO4 to KCl was 1:1, current density was kept at 15 mA/ cm2, and the initial (NH4)2SO4 concentration was about 3.4 mol/L. Figure 4 illustrates the effect of K2SO4 tank initial volume on voltage drops across the ED module and NH4Cl concentrations in NH4Cl tank, it seems that K2SO4 tank initial volume has hardly any effect on the two items. The operation time and the final NH4Cl concentrations are about 100 min and 2.1 mol/L, respectively. Table S4 illustrates the effect of K2SO4 tank initial volume on the KCl conversion ratio, final K2SO4 concentration in K2SO4 tank and energy consumption. In allusion to KCl conversion ratio and energy consumption, K2SO4 tank initial volume has no obvious impacts. However, with the K2SO4 tank initial volume reduced from 700 to 400 mL, the final K2SO4 concentration increased from 93.69 to 123.50 g/L. Specially, the solution was saturated in the case of 400 mL; also, there were some crystals separated in the bottom of the K2SO4 tank.

However, it is unwise to increase the final K2SO4 concentration by decreasing the K2SO4 tank initial volume blindly, because lots of crystals separated would block the pipelines to affect the performance of ED module during the operation process. In brief, on the premise of the excellent performance of the ED module, the final K2SO4 concentrations could be increased by reducing the K2SO4 tank initial volume, and the smaller volume, the better. 3.4. Effect of the Solution Temperature of K2SO4 Tank. To investiage the solution temperature of K2SO4 tank, the molar ratio of (NH4)2SO4 to KCl was set as 1:1, the initial (NH4)2SO4 concentration was about 3.4 mol/L, the initial K2SO4 tank volume was 400 mL and the current density was kept at 15 mA/cm2. Figure 5 and Table S5 show the effect of the solution temperature of the K2SO4 tank on voltage drops across the ED

Figure 5. Effect of the solution temperature of K2SO4 tank on voltage drops across ED module and NH4Cl concentration in NH4Cl tank.

module, NH4Cl concentrations in the NH4Cl tank, KCl conversion ratio, final K2SO4 concentration in the K2SO4 tank and energy consumption. As shown in the left part of Figure 5, it seems that as the solution temperature increases from room temperature to 50 °C, the voltage drop decreases slightly, and the operation time decreases from 102 to 95 min; also, this could be confirmed through the data of energy consumption in Table S5: energy consumption decreased from 11940

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composition could be obtained. As seen from Table 1 that illustrates the specific crystal composition, there exists some

0.37 to 0.31 kw·h/kg K2SO4 as the temperature increased. The reason may be that higher temperature can enhance the activity of ions in solution. In the right part of Figure 4, the NH4Cl concentration−time curves of the three temperatures have little difference: the final NH4Cl concentrations are all between 2.1 and 2.3 mol/L. As illustrated in Table S5, with the increasing solution temperature, the KCl conversion ratios of the three cases are more than 99%, and the final K2SO4 concentration increases from 123.50 to 135.29 g/L. As stated in section 3.3, when the K2SO4 tank initial volume was 400 mL, there were some crystals separated in the bottom of the K2SO4 tank to block the pipelines at room temperature. However, as the temperature increased, the solubility of dissolved salts increased, so the final K2SO4 concentration increased and there were no crystals in the tank. Also, after the operation, if the K2SO4 solution was cooled, some crystals would separate out, and this is favorable for the subsequent evaporation crystallization. The effect of the solution temperature in the K2SO4 tank on conversion performance should be discussed from two aspects. On one hand, higher temperature could result in lower energy consumption and higher solution solubility. This is beneficial for ED performance and subsequent evaporation crystallization. On the other hand, a certain quantity of heat needs be consumed to increase the solution temperature, if the heat is the industrial waste heat, it is profitable; if the heat needs additional heating, it is more costly. 3.5. Analysis of the Generated Product Crystals. In general, potassium sulfate fertilizers are often used in the form of a solid. In this work, the product crystals were generated through heating and evaporating the solution in the K2SO4 tank. Some analysis work was conducted on these product crystals. First, X-ray diffraction (XRD) studies were carried out to identify the phase of the crystals. Figure 6 shows the XRD

Table 1. Specific Product Crystal Composition item

K+ (wt %)

NH4+ (wt %)

SO42− (wt %)

Cl− (wt %)

sample 1 sample 2 sample 3

38.76 41.14 42.86

13.63 15.45 12.25

34.92 32.01 33.28

12.69 11.40 11.61

impurity ions (Cl− and NH4+) in the final product crystals, and this would influence the property and quality of potassium sulfate fertilizers. Also, from the data of Table 1, the average purity of K2SO4 could be calculated to be 60.54%. As per the Chinese Government standard “Potassium Sulfate for Agricultural Use, GB20406-2006”, for the moment, K2SO4 generated in this study could not be used as potassic fertilizers for chloride-sensitive crops. The main reason could be that the ion selectivity of membrane cannot achieve 100%, so the ion leakage phenomenon might happen during the operation. In future research, some measures, such as using the monovalent selective membranes, modifying the operating conditions, altering the operation mode and so on, should be taken to decrease the leakage ratio of impurity ions. 3.6. Preliminary Economic Evaluation. The above experimental results confirm the feasibility of conversion of potassium chloride into potassium sulfate by ED. Nevertheless, the economic viability should also be considered. The economic cost of product K2SO4 generated by the ED process may consist of three parts: cost of raw materials, cost of ED process, and cost of subsequent evaporation crystallization. First, raw materials include KCl and (NH4)2SO4. As stated in section 1, (NH4)2SO4 is the byproduct of petrochemical industries and coking enterprises. Here, its cost would be neglected. Referring to the Web site: www.Agronet.com.cn, the average price of KCl is about 350 $/t KCl; converting the cost into the form of K2SO4, the cost of raw materials is 253 $/t K2SO4. Second, the cost of the ED process mainly includes energy cost and investment cost; taking the case of 20 mA/cm2 in section 3.1 for example, the ED process cost can be calculated by the procedure as reported in the literature,24−26 and the results are illustrated in Table 2. As seen from the table, the total cost could be 200 $/t K2SO4. Third, to our best knowledge, the cost of evaporating water is less than 160 $/t H2O; on this basis, the subsequent evaporation crystallization cost of K2SO4 can be calculated to be 17.39 $/t K2SO4. Adding the three parts together, the economic cost of product K2SO4 is about 470.39 $/t K2SO4. Also, according to the Web site: www. Agronet.com.cn, the average price of K2SO4 is about 514.40 $/t K2SO4. Comparing the two prices, it can be concluded that the ED process of preparing K2SO4 is a low-cost, low-energy, and competitive process.

Figure 6. XRD pattern of the product crystal.

4. CONCLUSIONS To overcome the shortcomings of traditional preparation methods, the ED process was investigated to prepare potassium sulfate, which is an important kind of potassic fertilizer. Potassium chloride “reacts” with ammonium sulfate, which is a byproduct of petrochemical industries and coking enterprises, to form the potassium sulfate and ammonium chloride. First, various operation parameters, such as current density, molar ratio of (NH4)2SO4 to KCl, initial volume and solution temperature of the K2SO4 tank were investigated. Results

patterns. The peaks at 2θ angles of 30.02, 30.74, and 30.92 could be attributed to the structure of arcanite (K2SO4) according to JCPDS 05-0613. Also, the peaks at 2θ angles of 21.36, 40.6, and 65.22 can be attributed to the structure of KCl according to JCPDS 41-1476. Second, the content of anions SO42− and Cl− of product crystals was determined by ion chromatography. And by combining the content of K+ and NH4+ through the atomic absorption spectrophotometer and titration method, the crystal 11941

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(NH4)2SO4 to KCl on KCl conversion ratio, final K2SO4 concentration and energy consumption (Table S3), effect of the K2SO4 tank initial volume on KCl conversion ratio, final K2SO4 concentration and energy consumption (Table S4), and effect of the solution temperature of K2SO4 tank on KCl conversion ratio, final K2SO4 concentration and energy consumption (Table S5) (PDF).

Table 2. Estimation of Process Cost in ED ED process repeating units current density (mA/cm2) effective membrane area (m2) current efficiency (%) energy consumption (kWh/kg) process capacity (kg/year) electricity change ($/kg) energy cost for K2SO4 ($/kg) energy cost for the peripheral equipment ($/kg) total energy cost ($/kg) membrane life and amortization of the peripheral equipment (year) anion exchange membrane price ($/m2) cation exchange membrane price ($/m2) membrane cost ($) stack cost ($)

11 20 0.02 36.5 0.43 521.93 0.1 0.043 0.002

peripheral equipment cost ($) total investment cost ($) amortization ($/year) Interest ($/year) maintenance ($/year)

129.6 216.0 43.2 17.28 21.6

total fixed cost ($/year) total fixed cost ($/kg) total process cost ($/kg)

82.08 0.16 0.20

0.045 5 60 60 57.6 86.4

remarks

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*X. Han. Tel.: +86-551-6290-5769. E-mail: [email protected]. *T. Xu. Tel.: +86-551-6360-1587. E-mail: [email protected]. Notes

annual operation days = 300

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the Fundamental Research Funds for the Central Universities (Nos. JZ2015HGBZ0097, JZ2015HGQC0203).

1.5 times membrane cost 1.5 times stack cost

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REFERENCES

(1) Making chloride-free potash fertilizers. Phosphorus Potassium 1988, 156, 8. (2) Grzmil, B.; Kic, B. Single-stage process for manufacturing of potassium sulphate from sodium sulphate. Chem. Pap. 2005, 59, 476. (3) Tomaszewska, M. Preliminary studies on conversion of potassium chloride into potassium sulfate using membrane reactor. J. Membr. Sci. 2008, 317, 14. (4) Dave, R.; Ghosh, P. Efficient recovery of potassium chloride from liquid effluent generated during preparation of schoenite from kainite mixed salt and its reuse in production of potassium sulfate. Ind. Eng. Chem. Res. 2006, 45, 1551. (5) Phinney, R. The production of potassium sulphate. European Patent EP19860103871, October 29, 1986. (6) Zhang, Y.; Cao, J.; Ren, B.; Xie, Y. A Deneral Description for Production Method of Potassium Sulfate (In Chinese). Chem. Fert. Des. 2003, 41, 4. (7) Chen, D.; Guo, Y.; Deng, T. Current research status of production technology of potassium sulfate (In Chinese). Inorg. Chem. Ind. 2010, 42, 4. (8) EI-Diwani, G.; Abouel-Fotouh, A. M.; Hawash, S. I. Kinetic study for potassium sulfate production from potassium chloride in ammoniacal solution. Afinidad 2004, 61 (511), 225. (9) Huang, C.; Xu, T.; Zhang, Y.; Xue, Y.; Chen, G. Application of electrodialysis to the production of organic acids: State-of-the-art and recent developments. J. Membr. Sci. 2007, 288, 1. (10) Readi, O.; Girones, M.; Nijmeijer, K. Separation of complex mixtures of amino acids for biorefinery applications using electrodialysis. J. Membr. Sci. 2013, 429, 338. (11) Wang, X.; Zhang, X.; Wang, Y.; Du, Y.; Feng, H.; Xu, T. Simultaneous recovery of ammonium and phosphorus via the integration of electrodialysis with struvite reactor. J. Membr. Sci. 2015, 490, 65. (12) Marder, L.; Bernardes, A. M.; Ferreira, J. Z. Cadmium electroplating wastewater treatment using a laboratory-scale electrodialysis system. Sep. Purif. Technol. 2004, 37, 247. (13) Zheng, Y.; Li, Z.; Wang, X.; Gao, X.; Gao, C. The treatment of cyanide from gold mine effluent by a novel five-compartment electrodialysis. Electrochim. Acta 2015, 169, 150. (14) Jones, R.; Massanet-Nicolau, J.; Guwy, A.; Premier, G.; Dinsdale, R.; Reilly, M. Removal and recovery of inhibitory volatile fatty acids from mixed acid fermentations by conventional electrodialysis. Bioresour. Technol. 2015, 189, 279. (15) Ghyselbrecht, K.; Huygebaert, M.; Van der Bruggen, B.; Ballet, R.; Meesschaert, B.; Pinoy, L. Desalination of an industrial saline water

interest rate, 8% 10% the investment cost

showed that 15 and 20 mA/cm2 seem appropriate for the operation. A slightly higher than theoretical molar ratio of (NH4)2SO4 to KCl can ensure the complete conversion of KCl into K2SO4. On the premise of excellent performance of the ED module, the final K2SO4 concentration increases from 93.69 to 123.50 g/L with the initial K2SO4 tank volume reduced from 700 to 400 mL. Higher temperature is beneficial for ED performance and subsequent evaporation crystallization, but the heat source needs to be carefully considered. Second, XRD studies were carried out to identify the phase of the crystals. Results showed that there mainly existed arcanite (K2SO4) and KCl in the product crystals. Crystal composition was obtained through determining the ion content, and there existed some impurity ions (Cl− and NH4+) in the product crystals. Third, economic evaluation of the ED process was conducted; results show that the process is low-cost, low energy consuming, and competitive. In brief, this is a simple, low-cost, low energy consuming, and environmentally friendly process. However, the shortfall is that some impurity ions still exist in the product crystals, so some measures need to be taken to improve the process in the future.

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AUTHOR INFORMATION

Corresponding Authors

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03245. Properties of cation exchange membrane and anion exchange membrane (Table S1), effect of current density on KCl conversion ratio, final K2SO4 concentration and energy consumption when the molar ratio of (NH4)2SO4 to KCl is 1:2 (Table S2), effect of molar ratio of 11942

DOI: 10.1021/acs.iecr.5b03245 Ind. Eng. Chem. Res. 2015, 54, 11937−11943

Article

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DOI: 10.1021/acs.iecr.5b03245 Ind. Eng. Chem. Res. 2015, 54, 11937−11943