Method for producing acid and base by bipolar membrane electrodialysis of concentrated seawater

Abstract

The application discloses a kind of bipolar membrane electrodialysis method for preparing acid and alkali from concentrated seawater, and belongs to the technical field of seawater treatment, the method comprises the following steps: removing calcium ions and magnesium ions, impurity removal treatment, bipolar membrane electrodialyzer treatment;The impurity removal treatment is as follows: at room temperature, the pH value of the concentrated seawater after removing calcium ions and magnesium ions is adjusted to 8.5-9 by adding sodium hydroxide solution, then sodium dioxide is added to treat silicon, stirred, sodium chloride is added to increase the concentration of sodium ions by 25-28%, stirred, placed, centrifuged, and the supernatant is taken, the pH value of the supernatant is adjusted to 6.5-7 by adding hydrochloric acid solution, and the concentrated seawater after impurity removal treatment is obtained;The method can effectively remove calcium ions and magnesium ions, and the removal effect is stable.

Description

Technical Field

[0001] This invention relates to the field of seawater treatment technology, specifically to a bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater. Background Technology

[0002] Seawater desalination, as a new source of freshwater, has become an effective way to address the global freshwater shortage. With the increase in seawater desalination capacity, the corresponding volume of concentrated seawater to be treated is also increasing. The salinity of concentrated seawater can be up to twice that of the original seawater, while the average salinity of ocean water is approximately 35,000 mg / L. Currently, the most common treatment method for concentrated seawater is to remove calcium, magnesium, and sulfate ions before concentration and crystallization to obtain sodium and potassium salts. However, this method requires evaporating over 90% of the solvent, resulting in high energy consumption. Therefore, achieving energy-efficient and comprehensive utilization of concentrated seawater is an urgent problem to be solved in current seawater desalination work.

[0003] Bipolar membranes are a novel type of composite ion exchange membrane, typically composed of an anion exchange layer, a cation exchange layer, and an intermediate hydrophilic interfacial layer. Under the influence of an applied DC electric field, water molecules in the hydrophilic interfacial layer dissociate under the influence of a high potential gradient, producing hydrogen ions and hydroxide ions. Combining bipolar membranes with anion and cation exchange membranes in a specific manner constitutes the bipolar membrane electrodialysis method, enabling the preparation of acids and bases from salts without introducing new components. Compared with traditional diaphragm-free and diaphragm-based methods for electrolyzing brine, the bipolar membrane electrodialysis method for preparing acids and bases offers advantages such as greater energy efficiency, no large-scale gas generation, and a safer and more environmentally friendly production process. Applying bipolar membrane electrodialysis to the comprehensive utilization of concentrated seawater not only eliminates the high energy consumption associated with evaporation and crystallization processes but also allows the prepared acids and bases to be reused in seawater desalination (e.g., seawater acidification, seawater softening, and seawater flue gas desulfurization), significantly improving the economic benefits of concentrated seawater utilization.

[0004] However, the following problems exist when using bipolar membrane electrodialysis to comprehensively utilize concentrated seawater:

[0005] First, as mentioned in the research on the preparation of acids and bases from concentrated seawater using bipolar membrane electrodialysis (Yang Yang, Master's Thesis, Ocean University of China, May 2014), concentrated seawater has a complex composition, containing not only sodium chloride but also a large number of other anions and cations, especially high levels of calcium and magnesium ions. If concentrated seawater is used directly for acid and base preparation without any pretreatment, it will cause a large amount of precipitate to form in the alkali chamber, clogging the membrane stack and hindering the acid and base preparation process.

[0006] Secondly, as described in the research on the preparation of acids and bases from concentrated seawater using bipolar membrane electrodialysis (Yang Yang, Master's Thesis, Ocean University of China, May 2014), concentrated RO seawater, after pretreatment, was pumped into a bipolar membrane electrodialysis device. After the device operated continuously for 18 hours, elemental analysis of the cation and anion membrane surfaces revealed the presence of small amounts of silicon. Furthermore, the silicon content adsorbed on the anion membrane was higher than that adsorbed on the cation membrane, indicating that the anion membrane surface was in direct contact with the acid chamber. Silicon was likely deposited on the anion membrane surface in the form of silicic acid, leading to silicon contamination. Additionally, as described in the research progress on the modification and antifouling performance of anion exchange membranes (Cao Renqiang, Feng Zhanli, Li Yujiao, Zhao Zhijuan, Shi Shaoyuan, Journal of Process Engineering, January 2019), compared to cation exchange membranes, anion exchange membranes are more prone to organic contamination, and the contamination is more severe. Therefore, the presence of organic matter in concentrated seawater also easily causes anion membrane contamination.

[0007] Regarding the first problem mentioned above, as described in the "Research on the Comprehensive Utilization of Concentrated Seawater by Electrodialysis and Bipolar Membrane Electrodialysis" by Zhang Wei, Master's Thesis, Zhejiang University of Technology, December 2017, the following method can be used: First, the concentrated seawater is pretreated using chemical precipitation to remove some calcium and magnesium ions. Then, a monovalent selective electrodialysis membrane stack is used to further remove calcium and magnesium ions, obtaining a salt solution rich in sodium chloride. This reduces the calcium and magnesium ion content in the solution to below 10 ppm, thus avoiding the influence of calcium and magnesium ions on the acid-base preparation process.

[0008] Regarding the second problem mentioned above, as described in the research on the preparation of acids and bases from concentrated seawater using bipolar membrane electrodialysis (Yang Yang, Master's Thesis, Ocean University of China, May 2014), it can be solved by adjusting the pH value of the softened concentrated seawater to 2.0 to prevent organic and silicon contamination of the anion exchange membrane.

[0009] However, the above solutions still have the following problems:

[0010] First, as described in the research on the application of monovalent selective electrodialysis in the treatment of acidic heavy metal wastewater (Li Fuqin, Zhang Yingong, Zhu Min, Wang Shaozhou, Guo Yanfu, Water Treatment Technology, March 2022), commercial monovalent selective cation exchange membranes suffer from unstable performance. Similarly, as described in the article on the removal of calcium and magnesium ions from seawater using CO2 (Zhang Jijun, Yuan Junsheng, Li Xia, Chemical Industry and Engineering Progress, March 2012), when precipitating calcium and magnesium ions from seawater, the precipitation rate of magnesium ions is low, resulting in a high magnesium ion content entering the commercial monovalent selective cation exchange membrane. However, due to the unstable performance of the commercial monovalent selective cation exchange membrane, the magnesium ion content after treatment fluctuates significantly.

[0011] Secondly, as described in "Contamination and Treatment of Anion Exchange Resins" by Dong Taiyi in "Fertilizer Design" (December 1983), contamination of anion exchange resins is mainly caused by colloidal silica and high molecular weight organic acids. Leakage caused by colloidal silica contamination is more severe at higher water temperatures or pH values. Similarly, as described in "Research Progress on Contamination and Prevention of Strong Base Anion Exchange Resins" by Xiao Zunhong and Xu Hede in "Journal of Shaoyang Teachers College" (October 1999), colloidal silica cannot be removed by ion exchange after entering the resin. This can lead to silica leakage during operation due to hydrolysis of colloidal silica. If the amount of alkali used for each regeneration is insufficient, the temperature of the regenerated solution is too low, or the flow rate of the regenerated solution is too slow, silica will remain at the bottom of the resin and leak due to hydrolysis during operation. This indicates that silica contamination is also affected by temperature and flow rate. Lowering the pH value can only alleviate silica contamination; the problem will still exist. Furthermore, lowering the pH value can lead to higher acid consumption and increased corrosion of the equipment. Summary of the Invention

[0012] To address the shortcomings of existing technologies, this invention provides a bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater. This method can effectively remove calcium and magnesium ions with stable removal results, and can also avoid contamination of the anion exchange membrane by organic matter and silicon, thereby reducing acid consumption and equipment corrosion.

[0013] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0014] A bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater includes: removal of calcium and magnesium ions, impurity removal treatment, and bipolar membrane electrodialysis treatment.

[0015] To remove calcium and magnesium ions, concentrated seawater is taken and stirred at room temperature at a stirring speed of 20-60 rpm. Sodium hydroxide aqueous solution is added to the concentrated seawater until the pH value is 11.5-12. The mixture is filtered, and the filtrate is taken and stirred at room temperature at a stirring speed of 20-60 rpm. Carbon dioxide is continuously introduced into the filtrate, while sodium hydroxide aqueous solution is added to maintain the pH value at 11.5-12. The mixture is stirred for 40-50 minutes, then calcium carbonate is added and stirred for 40-50 minutes. The mixture is filtered, and hydrochloric acid aqueous solution is added to adjust the pH value to 7. The mixture is then passed through an electrodialysis machine to remove divalent and higher valence metal ions, resulting in concentrated seawater after the removal of calcium and magnesium ions.

[0016] In the process of removing calcium and magnesium ions, the ratio of concentrated seawater to treated calcium carbonate is 10L:100-110g.

[0017] The molar concentration of the sodium hydroxide aqueous solution is 1 mol / L.

[0018] The carbon dioxide is introduced at a rate of 0.8-0.9 L / min;

[0019] The molar concentration of the hydrochloric acid aqueous solution is 1 mol / L;

[0020] The electrodialysis unit consists of 20 cation exchange membranes and 20 anion exchange membranes. The cation exchange membranes are cation exchange membranes with sulfonic acid groups, which can selectively allow monovalent cations to pass through while blocking divalent cations. The anion exchange membranes are general-purpose anion exchange membranes.

[0021] The method for preparing the treated calcium carbonate is as follows: at 30-40℃, heavy calcium carbonate is mixed with water and stirred at a stirring speed of 20-60 rpm for 40-60 min. Sodium dodecyl sulfate aqueous solution is added and stirred for 40-60 min. Hexadecyltrimethylammonium chloride aqueous solution is added and stirred for 40-60 min. The mixture is then centrifuged at a centrifugation speed of 3000-4000 rpm for 15-20 min. The precipitate is collected, dried, and the treated calcium carbonate is obtained.

[0022] In the preparation of the treated calcium carbonate, the ratio of heavy calcium carbonate, water, sodium dodecyl sulfate aqueous solution, and hexadecyltrimethylammonium chloride aqueous solution is 110-120g:1000-1200mL:480-500mL:460-480mL.

[0023] The average particle size of the heavy calcium carbonate is 15 μm;

[0024] The concentration of the sodium dodecyl sulfate aqueous solution is 15 g / L;

[0025] The concentration of the hexadecyltrimethylammonium chloride aqueous solution is 15 g / L;

[0026] The impurity removal process involves adding sodium hydroxide aqueous solution to the concentrated seawater after removing calcium and magnesium ions at room temperature to adjust the pH value to 8.5-9, then adding sodium to treat silica, stirring at 20-60 rpm for 1.5-2 hours, adding sodium chloride until the sodium ion concentration increases by 25-28%, stirring for 4-5 hours, letting it stand for 20-25 hours, centrifuging at 12000-13000 rpm for 20-30 minutes, taking the supernatant, adding hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 6.5-7, and obtaining the concentrated seawater after impurity removal.

[0027] The ratio of concentrated seawater used to remove calcium and magnesium ions to sodium-treated silica used in the impurity removal process is 10L:80-90g.

[0028] In the impurity removal process, the molar concentration of the sodium hydroxide aqueous solution is 1 mol / L;

[0029] The molar concentration of the hydrochloric acid aqueous solution is 1 mol / L;

[0030] The method for preparing sodium-treated silica is as follows: at 20-35°C, silica is mixed with an aqueous sodium chloride solution, stirred at a stirring speed of 20-60 rpm for 40-60 min, centrifuged at a centrifugation speed of 12000-13000 rpm for 10-15 min, the precipitate is collected, dried, and sodium-treated silica is obtained.

[0031] In the preparation of sodium-treated silica, the ratio of silica to sodium chloride aqueous solution is 100g:1000-1100mL.

[0032] The sodium chloride aqueous solution has a mass concentration of 10%.

[0033] The average particle size of the silica is 40 nm;

[0034] The bipolar membrane electrodialysis process involves using concentrated seawater after impurity removal as feed liquid, which is then fed into the bipolar membrane electrodialysis unit for treatment. The cathode solution, alkaline solution, feed liquid, acid solution, and anolyte are all circulated until the conversion rate of the feed liquid reaches 70%, at which point the treatment is completed. Then, the alkali and acid are collected from the tanks containing the alkaline solution and the tanks containing the acid solution, respectively.

[0035] In the bipolar membrane electrodialysis process, constant current is used, and the current is controlled at 1.5A;

[0036] The flow rates of catholyte, alkaline solution, feed solution, acid solution, and anolyte are all 20 L / h;

[0037] Each bipolar membrane electrodialysis unit consists of a membrane stack, a flow meter, a centrifugal pump, and five water tanks.

[0038] The membrane stack consists of four bipolar membranes, three anion exchange membranes, and three cation exchange membranes. The anion exchange membranes have an ion exchange capacity of 1.1 mmol / g and a membrane resistance (25℃, 0.1 mol / L NaCl aqueous solution) of 2.5 Ω•cm. 2 The ion exchange capacity of the cation exchange membrane is 1 mmol / g, and the membrane resistance (25℃, 0.1 mol / L NaCl aqueous solution) is 3 Ω•cm. 2 Water dissociation voltage of bipolar membrane (100 mA / cm) 2 The value of the water dissociation efficiency (at 25℃, 0.5 mol / L Na2SO4 aqueous solution) was 1.2 V, and the water dissociation efficiency (at 25℃, 0.5 mol / L Na2SO4 aqueous solution) was 98%.

[0039] Both the cathode and anode of the bipolar membrane electrodialysis unit are made of titanium coated with ruthenium. The partition of the bipolar membrane electrodialysis unit has a size of 9cm*12cm, and the effective area of ​​the partition is 88cm².2 ;

[0040] Five water tanks are used to hold cathodic solution, alkaline solution, feed solution, acid solution and anolyte respectively, with an initial volume of 3L for each of the following:

[0041] Both the catholyte and the anolyte are aqueous solutions of sodium sulfate with a molar concentration of 0.5 mol / L;

[0042] The alkaline solution is an aqueous solution of sodium hydroxide with a molar concentration of 0.01 mol / L;

[0043] The acid solution is an aqueous solution of sulfuric acid with a molar concentration of 0.01 mol / L.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] (1) In the bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater of the present invention, in order to avoid the problem of low removal rate of magnesium ions by chemical precipitation, calcium carbonate is added when removing calcium and magnesium ions by chemical precipitation. As mentioned in the paper "Bionic Preparation and Assembly of Inorganic Nanoparticles in Dodecyl Sulfate (Ca,Mg) System", anionic surfactants and cationic surfactants have a certain inducing effect on the formation of magnesium hydroxide. Heavy calcium carbonate can also adsorb calcium and magnesium ions and induce the formation of calcium carbonate and magnesium carbonate, thereby reducing the content of calcium and magnesium ions, improving the removal rate of calcium and magnesium ions in chemical precipitation, ensuring low calcium and magnesium ion concentrations after entering the electrodialyzer, thereby reducing the large difference in calcium and magnesium ion concentrations between different batches due to the unstable effect of the monovalent selective electrodialysis membrane in the electrodialyzer, and further reducing the impact of high calcium and magnesium ion content.

[0046] (2) In the bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater of the present invention, in order to avoid organic pollution and silicon pollution, sodium-treated silica is used. Sodium-treated silica is silica with sodium ions adsorbed on its surface. As described in the simulation of the interaction process between sodium ions and nano silica surface based on molecular dynamics. Zhang Fengjuan, Zhang Lei, Lü Zhenhu, Mairyemuguli Anwaier, Yu Weichu, Wang Muqun, Dong Jingfeng, Zhuang Weijie. Oilfield Chemistry. February 2024, there is an interaction between sodium ions and silica, indicating that silica has a certain adsorption and fixation effect on sodium ions; as described in the influence of humic acid on the aggregation kinetic mechanism of nano silica in water. Xin Xin, Jiao Ruyuan, Sun Hongyan, Huo Lijuan, Zhang Weijun, Wang Dongsheng. China Water & Wastewater. June 2025, under alkaline conditions, the structure of humic acid becomes an extended chain structure. Therefore, it is inferred that it combines with sodium ions and plays a bridging role, thereby promoting particle aggregation. Therefore, this invention first adds sodium to treat silica. A certain amount of sodium ions are adsorbed on the surface of silica. Sodium ions achieve adsorption equilibrium in concentrated seawater. In adsorption equilibrium, sodium ions can promote the formation of silica from silicon and can also adsorb humic organic matter in concentrated seawater onto the silica surface through bridging, thereby removing organic matter and silicon and avoiding organic and silicon pollution. After treatment for a period of time, sodium chloride is added. As the sodium ion concentration increases, it can further promote the adsorption of organic matter and silicon to reach a new adsorption equilibrium, and can also improve the acid-base current efficiency in subsequent acid and alkali production.

[0047] (3) The bipolar membrane electrodialysis method for acid and alkali production from concentrated seawater of the present invention can effectively remove calcium and magnesium ions and the removal effect is stable, thereby avoiding precipitation in the alkali chamber. It can also reduce the organic matter and silicon content in concentrated seawater without using a large amount of acid to adjust the pH value, thus avoiding the pollution of the anion membrane by organic matter and silicon. Attached Figure Description

[0048] Figure 1 This is a flowchart of the bipolar membrane electrodialysis method of the present invention. Detailed Implementation

[0049] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention are now described.

[0050] The concentrated seawater used in the examples and comparative examples all came from the same batch. The concentrated seawater contained sodium ions at a concentration of 16210 mg / L, potassium ions at 587 mg / L, calcium ions at 829 mg / L, magnesium ions at 2265 mg / L, chloride ions at 29800 mg / L, sulfate ions at 3902 mg / L, carbonate ions at 356 mg / L, bromine at 114 mg / L, and silicon at 3.5 mg / L.

[0051] The electrodialysis unit used in the examples and comparative examples is the same electrodialysis unit. The membrane stack of the electrodialysis unit consists of 20 cation exchange membranes and 20 anion exchange membranes. The cation exchange membranes are cation exchange membranes with sulfonic acid groups, which can selectively permeate monovalent cations and block divalent cations. The anion exchange membranes are general-purpose anion exchange membranes.

[0052] The bipolar membrane electrodialysis units used in the examples and comparative examples each consist of a membrane stack, a flow meter, a centrifugal pump, and five water tanks.

[0053] The membrane stack consists of four bipolar membranes, three anion exchange membranes, and three cation exchange membranes. The bipolar membranes, anion exchange membranes, and cation exchange membranes used in the examples and comparative examples were all from the same production batch and were unused new membranes. The anion exchange membranes had an ion exchange capacity of 1.1 mmol / g and a membrane resistance (25°C, 0.1 mol / L NaCl aqueous solution) of 2.5 Ω•cm. 2 The ion exchange capacity of the cation exchange membrane is 1 mmol / g, and the membrane resistance (25℃, 0.1 mol / L NaCl aqueous solution) is 3 Ω•cm. 2 Water dissociation voltage of bipolar membrane (100 mA / cm) 2 The value of the water dissociation efficiency (at 25℃) was 1.2V, and the water dissociation efficiency (at 25℃, 0.5mol / L Na2SO4 aqueous solution) was 98%.

[0054] Both the cathode and anode of the bipolar membrane electrodialysis unit are made of titanium coated with ruthenium. The partition size of the bipolar membrane electrodialysis unit is 9cm*12cm, and the effective area of ​​the partition is 88cm². 2 .

[0055] Five water tanks are used to hold cathodic solution, alkaline solution, feed solution, acid solution and anolyte respectively, with an initial volume of 3L for each of them.

[0056] Both the cathode solution and the anolyte are aqueous solutions of sodium sulfate with a molar concentration of 0.5 mol / L.

[0057] The alkaline solution is an aqueous solution of sodium hydroxide with a molar concentration of 0.01 mol / L.

[0058] The acid solution is an aqueous solution of sulfuric acid with a molar concentration of 0.01 mol / L.

[0059] Example 1

[0060] according to Figure 1 The flowchart illustrates a bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater, as detailed below:

[0061] 1. Removal of calcium and magnesium ions: Take 10L of concentrated seawater and stir it at 20rpm at room temperature. Add 1mol / L sodium hydroxide aqueous solution to the concentrated seawater until the pH value is 11.5. Filter and collect the filtrate. Stir the filtrate at 20rpm at room temperature. Then continuously pass carbon dioxide into the filtrate, controlling the carbon dioxide passage rate to be 0.8L / min. At the same time, maintain the pH value at 11.5 by adding 1mol / L sodium hydroxide aqueous solution. Stir for 40min. Add 100g of treated calcium carbonate and stir for 40min. Filter and collect the filtrate. Add 1mol / L hydrochloric acid aqueous solution to adjust the pH value to 7. Pass the solution into an electrodialysis machine to remove divalent and higher valence metal ions, and obtain concentrated seawater after removing calcium and magnesium ions.

[0062] The method for preparing the treated calcium carbonate is as follows: at 30°C, 110g of heavy calcium carbonate is mixed with 1000mL of water and stirred at 20rpm for 40min. Then, 480mL of sodium dodecyl sulfate aqueous solution is added and stirred for 40min. Next, 460mL of hexadecyltrimethylammonium chloride aqueous solution is added and stirred for 40min. The mixture is then centrifuged at 3000rpm for 15min. The precipitate is collected, dried, and the treated calcium carbonate is obtained.

[0063] The average particle size of the heavy calcium carbonate is 15 μm;

[0064] The concentration of the sodium dodecyl sulfate aqueous solution is 15 g / L;

[0065] The concentration of the hexadecyltrimethylammonium chloride aqueous solution is 15 g / L;

[0066] 2. Impurity Removal Treatment: At room temperature, add a 1 mol / L sodium hydroxide aqueous solution to all the concentrated seawater obtained in step 1 after removing calcium and magnesium ions to adjust the pH value to 8.5. Then add 80g of sodium to treat silica and stir at 20 rpm for 1.5h. Add sodium chloride until the sodium ion concentration increases by 25% and stir for 4h. Let stand for 20h and centrifuge at 12000 rpm for 20min. Take the supernatant and add a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 6.5 to obtain the concentrated seawater after impurity removal treatment.

[0067] The method for preparing sodium-treated silica is as follows: at 20°C, 100g of silica is mixed with 1000mL of sodium chloride aqueous solution with a mass concentration of 10%, stirred at a stirring speed of 20rpm for 40min, centrifuged at a centrifugation speed of 12000rpm for 10min, the precipitate is taken, dried, and sodium-treated silica is obtained.

[0068] The average particle size of the silica is 40 nm;

[0069] 3. Bipolar membrane electrodialysis treatment: Take 3L of concentrated seawater obtained from step 2 after impurity removal as feed solution and pass it into the bipolar membrane electrodialysis unit for treatment. Constant current is used during treatment, and the current is controlled at 1.5A. The flow rates of catholyte, alkali solution, feed solution, acid solution and anolyte are all controlled at 20L / h. The catholyte, alkali solution, feed solution, acid solution and anolyte are all circulated until the conversion rate of the feed solution reaches 70%. The treatment is then completed. The alkali and acid are collected from the water tank containing the alkali solution and the water tank containing the acid solution, respectively.

[0070] Example 2

[0071] according to Figure 1 The flowchart illustrates a bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater, as detailed below:

[0072] 1. Removal of calcium and magnesium ions: Take 10L of concentrated seawater and stir it at 60rpm at room temperature. Add 1mol / L sodium hydroxide aqueous solution to the concentrated seawater until the pH value is 12. Filter and collect the filtrate. Stir the filtrate at 60rpm at room temperature. Then continuously pass carbon dioxide into the filtrate, controlling the carbon dioxide flow rate at 0.9L / min. At the same time, maintain the pH value at 12 by adding 1mol / L sodium hydroxide aqueous solution. Stir for 50min. Add 110g of treated calcium carbonate and stir for 50min. Filter and collect the filtrate. Add 1mol / L hydrochloric acid aqueous solution to adjust the pH value to 7. Pass the solution into an electrodialysis machine to remove divalent and higher valence metal ions, and obtain concentrated seawater after removing calcium and magnesium ions.

[0073] The method for preparing the treated calcium carbonate is as follows: at 40°C, 120g of heavy calcium carbonate is mixed with 1200mL of water and stirred at 60rpm for 60min. Then, 500mL of sodium dodecyl sulfate aqueous solution is added and stirred for 60min. Next, 480mL of hexadecyltrimethylammonium chloride aqueous solution is added and stirred for 60min. The mixture is then centrifuged at 4000rpm for 20min. The precipitate is collected, dried, and the treated calcium carbonate is obtained.

[0074] The average particle size of the heavy calcium carbonate is 15 μm;

[0075] The concentration of the sodium dodecyl sulfate aqueous solution is 15 g / L;

[0076] The concentration of the hexadecyltrimethylammonium chloride aqueous solution is 15 g / L;

[0077] 2. Impurity Removal Treatment: At room temperature, add a 1 mol / L sodium hydroxide aqueous solution to all the concentrated seawater obtained in step 1 after removing calcium and magnesium ions to adjust the pH value to 9. Then add 90g of sodium to treat silica and stir at 60 rpm for 2 hours. Add sodium chloride until the sodium ion concentration increases by 28% and stir for 5 hours. Let stand for 25 hours and centrifuge at 13000 rpm for 30 minutes. Take the supernatant and add a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 7 to obtain the concentrated seawater after impurity removal treatment.

[0078] The method for preparing sodium-treated silica is as follows: at 35°C, 100g of silica is mixed with 1100mL of sodium chloride aqueous solution with a mass concentration of 10%, stirred at 60rpm for 60min, centrifuged at 13000rpm for 15min, the precipitate is collected, dried, and sodium-treated silica is obtained.

[0079] The average particle size of the silica is 40 nm;

[0080] 3. Bipolar membrane electrodialysis treatment: Take 3L of concentrated seawater obtained from step 2 after impurity removal as feed solution and pass it into the bipolar membrane electrodialysis unit for treatment. Constant current is used during treatment, and the current is controlled at 1.5A. The flow rates of catholyte, alkali solution, feed solution, acid solution and anolyte are all controlled at 20L / h. The catholyte, alkali solution, feed solution, acid solution and anolyte are all circulated until the conversion rate of the feed solution reaches 70%. The treatment is then completed. The alkali and acid are collected from the water tank containing the alkali solution and the water tank containing the acid solution, respectively.

[0081] Comparative Example 1

[0082] Based on the bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater in Example 1, the addition of calcium carbonate is omitted in the first step of removing calcium and magnesium ions. That is, the first step of removing calcium and magnesium ions is changed to:

[0083] Take 10L of concentrated seawater and stir it at 20rpm at room temperature. Add a 1mol / L sodium hydroxide aqueous solution to the concentrated seawater until the pH value is 11.5. Filter and collect the filtrate. Stir the filtrate at 20rpm at room temperature. Then, continuously pass carbon dioxide into the filtrate, controlling the carbon dioxide flow rate at 0.8L / min. At the same time, maintain the pH value at 11.5 by adding a 1mol / L sodium hydroxide aqueous solution. Stir for 80min, filter, collect the filtrate, and add a 1mol / L hydrochloric acid aqueous solution to adjust the pH value to 7. Pass the solution into an electrodialysis apparatus to remove divalent and higher valence metal ions, obtaining concentrated seawater after removing calcium and magnesium ions.

[0084] The remaining operations are consistent with those in Example 1.

[0085] Comparative Example 2

[0086] Based on the bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater in Example 1, the addition of sodium-treated silica is omitted in the second impurity removal step, that is, the second impurity removal step is changed to:

[0087] At room temperature, add a 1 mol / L sodium hydroxide aqueous solution to all the concentrated seawater obtained in step 1 after removing calcium and magnesium ions to adjust the pH value to 8.5, add sodium chloride until the sodium ion concentration increases by 25%, stir for 4 hours, let stand for 20 hours, centrifuge at 12000 rpm for 20 minutes, take the supernatant, add a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 6.5, and obtain the concentrated seawater after impurity removal treatment;

[0088] The remaining operations are consistent with those in Example 1.

[0089] Comparative Example 3

[0090] Based on the bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater in Example 1, the addition of sodium chloride is omitted in the second processing step, that is, the second impurity removal step is changed to:

[0091] At room temperature, add a 1 mol / L sodium hydroxide aqueous solution to all the concentrated seawater obtained in step 1 after removing calcium and magnesium ions to adjust the pH value to 8.5. Then add 80g of sodium to treat silica, stir at 20 rpm for 5.5h, let stand for 20h, centrifuge at 12000 rpm for 20min, take the supernatant, add a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 6.5, and obtain the concentrated seawater after impurity removal treatment.

[0092] The method for preparing sodium-treated silica is as follows: at 20°C, 100g of silica is mixed with 1000mL of sodium chloride aqueous solution with a mass concentration of 10%, stirred at a stirring speed of 20rpm for 40min, centrifuged at a centrifugation speed of 12000rpm for 10min, the precipitate is taken, dried, and sodium-treated silica is obtained.

[0093] The average particle size of the silica is 40 nm;

[0094] The remaining operations are consistent with those in Example 1.

[0095] Comparative Example 4

[0096] Based on the bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater in Example 1, the addition of sodium to treat silica and sodium chloride is omitted in the second processing step. That is, the second impurity removal step is modified as follows:

[0097] At room temperature, add a 1 mol / L sodium hydroxide aqueous solution to all the concentrated seawater obtained in step 1 after removing calcium and magnesium ions to adjust the pH value to 8.5. Then centrifuge at 12000 rpm for 20 min, take the supernatant, add a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 6.5, and obtain the concentrated seawater after impurity removal treatment.

[0098] The remaining operations are consistent with those in Example 1.

[0099] Comparative Example 5

[0100] Based on the bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater in Example 1, in the second step of the impurity removal process, the addition of sodium to treat silica and sodium chloride is omitted, and the phrase "adding a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH to 6.5" is changed to "adding a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH to 2". In other words, the second step of the impurity removal process is modified as follows:

[0101] At room temperature, add a 1 mol / L sodium hydroxide aqueous solution to all the concentrated seawater obtained in step 1 after removing calcium and magnesium ions to adjust the pH value to 8.5. Then centrifuge at 12000 rpm for 20 min, take the supernatant, add a 1 mol / L hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 2, and obtain the concentrated seawater after impurity removal treatment.

[0102] The remaining operations are consistent with those in Example 1.

[0103] Test case

[0104] Following the methods of Examples 1-2 and Comparative Examples 1-5, 50 batches of feed solutions were continuously treated (the catholyte, alkali solution, feed solution, acid solution, and anolyte were replaced in each treatment) to ensure that the feed solutions came from the same batch. The calcium and magnesium ion concentrations in the concentrated seawater after calcium and magnesium ion removal in Step 1 of Examples 1-2 and Comparative Example 1 were tested, and the differences between the maximum and minimum calcium and magnesium ion concentrations were recorded. The results are as follows:

[0105]

[0106] Then, after completion, the bipolar membrane electrodialysis unit was disassembled to observe whether there was any blockage in the dialysis unit's partitions, and whether there was any contamination on the surfaces of the bipolar membrane, anion exchange membrane, and cation exchange membrane (specifically, whether there was any precipitation or suspended solids). The observation results are as follows:

[0107]

[0108] The average acid and alkali production current efficiency of the 50 batches of treated liquid was taken, and the results are as follows:

[0109]

[0110] The results of this test show that, compared with Example 1, Comparative Example 1 has a large difference in the removal rates of calcium and magnesium ions between different batches, and is prone to clogging or membrane fouling during long-term acid and alkali production. Compared with Example 1, Comparative Examples 2-5 have problems such as clogging or membrane fouling during long-term acid and alkali production and low acid and alkali current efficiency.

Claims

1. A bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater, characterized in that, include: Removes calcium and magnesium ions, undergoes impurity removal treatment, and is processed using a bipolar membrane electrodialysis unit; The removal of calcium and magnesium ions involves stirring concentrated seawater at room temperature, adding sodium hydroxide aqueous solution until the pH value is 11.5-12, filtering, collecting the filtrate, stirring the filtrate at room temperature, continuously passing carbon dioxide into the filtrate, and simultaneously maintaining the pH value at 11.5-12 by adding sodium hydroxide aqueous solution, stirring, adding treated calcium carbonate, stirring, filtering, collecting the filtrate, adding hydrochloric acid aqueous solution to adjust the pH value to 7, and passing it through an electrodialysis machine to remove divalent and higher-valent metal ions, thus obtaining concentrated seawater after the removal of calcium and magnesium ions; In the process of removing calcium and magnesium ions, the ratio of concentrated seawater to treated calcium carbonate is 10L:100-110g. The method for preparing the treated calcium carbonate is as follows: at 30-40℃, heavy calcium carbonate is mixed with water and stirred, sodium dodecyl sulfate aqueous solution is added and stirred, hexadecyltrimethylammonium chloride aqueous solution is added and stirred, centrifuged, the precipitate is collected and dried to obtain the treated calcium carbonate. The impurity removal process involves adding sodium hydroxide aqueous solution to the concentrated seawater after removing calcium and magnesium ions at room temperature to adjust the pH value to 8.5-9, adding sodium to treat silica, stirring, adding sodium chloride until the sodium ion concentration increases by 25-28%, stirring, letting stand, centrifuging, taking the supernatant, adding hydrochloric acid aqueous solution to the supernatant to adjust the pH value to 6.5-7, and obtaining the concentrated seawater after impurity removal. The ratio of concentrated seawater used to remove calcium and magnesium ions to sodium-treated silica used in the impurity removal process is 10L:80-90g. The method for preparing sodium-treated silica is as follows: at 20-35°C, silica is mixed with an aqueous sodium chloride solution, stirred, centrifuged, the precipitate is collected, and dried to obtain sodium-treated silica. In the preparation of sodium-treated silica, the ratio of silica to sodium chloride aqueous solution is 100g:1000-1100mL. The sodium chloride aqueous solution has a mass concentration of 10%. The average particle size of the silica is 40 nm.

2. The bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater according to claim 1, characterized in that, In the process of removing calcium and magnesium ions, the molar concentration of the sodium hydroxide aqueous solution is 1 mol / L. The carbon dioxide is introduced at a rate of 0.8-0.9 L / min; The molar concentration of the hydrochloric acid aqueous solution is 1 mol / L; The electrodialysis unit consists of 20 cation exchange membranes and 20 anion exchange membranes. The cation exchange membranes are cation exchange membranes with sulfonic acid groups, which can selectively allow monovalent cations to pass through while blocking divalent cations. The anion exchange membrane is a general-purpose anion exchange membrane.

3. The bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater according to claim 1, characterized in that, In the preparation of the treated calcium carbonate, the ratio of heavy calcium carbonate, water, sodium dodecyl sulfate aqueous solution, and hexadecyltrimethylammonium chloride aqueous solution is 110-120g:1000-1200mL:480-500mL:460-480mL. The average particle size of the heavy calcium carbonate is 15 μm; The concentration of the sodium dodecyl sulfate aqueous solution is 15 g / L; The concentration of the hexadecyltrimethylammonium chloride aqueous solution is 15 g / L.

4. The bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater according to claim 1, characterized in that, In the impurity removal process, the molar concentration of the sodium hydroxide aqueous solution is 1 mol / L; The molar concentration of the hydrochloric acid aqueous solution is 1 mol / L.

5. The bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater according to claim 1, characterized in that, The bipolar membrane electrodialysis process involves using concentrated seawater after impurity removal as feed solution, which is then fed into the bipolar membrane electrodialysis unit for treatment. The cathode solution, alkaline solution, feed solution, acid solution, and anolyte are all circulated until the conversion rate of the feed solution reaches 70%, at which point the treatment is completed. The alkaline solution and acid solution are then collected from the water tank containing the alkaline solution and the water tank containing the acid solution, respectively.

6. The bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater according to claim 1, characterized in that, In the bipolar membrane electrodialysis process, constant current is used, and the current is controlled at 1.5A; The flow rates of catholyte, alkaline solution, feed solution, acid solution, and anolyte are all 20 L / h.

7. The bipolar membrane electrodialysis method for producing acid and alkali from concentrated seawater according to claim 1, characterized in that, Each bipolar membrane electrodialysis unit consists of a membrane stack, a flow meter, a centrifugal pump, and five water tanks. The membrane stack consists of 4 bipolar membranes, 3 anion exchange membranes, and 3 cation exchange membranes; Both the cathode and anode of the bipolar membrane electrodialysis unit are made of titanium coated with ruthenium. Five water tanks are used to hold cathodic solution, alkaline solution, feed solution, acid solution and anolyte respectively, with an initial volume of 3L for each of the following: Both the catholyte and the anolyte are aqueous solutions of sodium sulfate with a molar concentration of 0.5 mol / L; The alkaline solution is an aqueous solution of sodium hydroxide with a molar concentration of 0.01 mol / L; The acid solution is an aqueous solution of sulfuric acid with a molar concentration of 0.01 mol / L.

Images