Method for treating high-concentration sodium sulfate through stepwise concentration control

Abstract

The present invention relates to a method for treating high-concentration sodium sulfate through stepwise concentration control. In the present invention, bipolar membrane electrodialysis (BMED), electrodialysis (ED), nanofiltration (NF), and reverse osmosis (RO) processes can be sequentially performed when treating sodium sulfate to systematically control the concentration in each step, and the problems of increased conductivity and scaling can be minimized to very efficiently treat high-concentration sodium sulfate. In addition, since it is possible to recycle a concentrate produced during the integrated BMED-ED-NF-RO process, resources can be saved and process continuity is excellent.

Description

High-concentration sodium sulfate treatment method through stepwise concentration control

[0001] The present invention relates to a method for treating high concentrations of sodium sulfate through stepwise concentration control, and more specifically, to a method for efficiently treating high concentrations of sodium sulfate by sequentially controlling the concentration of sodium sulfate through bipolar membrane electrodialysis (BMED), electrodialysis (ED), nanofiltration (NF), and reverse osmosis (RO) processes.

[0002] This application claims priority to Korean Patent Application No. 10-2024-0191862, filed on December 19, 2024, the entire contents of which are incorporated herein by reference.

[0003] Sodium sulfate (Na2SO4) is a widely used chemical in industry for decomposing or neutralizing compounds. This sodium sulfate is generated as a byproduct during the secondary battery manufacturing process. Traditionally, this byproduct sodium sulfate was discharged into seawater or purified and reused as wastewater; however, due to the expansion of the secondary battery industry, the volume of sodium sulfate generated has surged, leading to restrictions on discharge methods. Since the discharge of large quantities of high-concentration sodium sulfate waste liquid can cause environmental pollution and have harmful effects on the ecosystem, pretreatment to reduce the concentration of discharged sodium sulfate is required to prevent these problems.

[0004] Generally, sodium sulfate pretreatment methods include physical methods such as coagulation and precipitation, chemical methods using neutralization, and biological methods using microorganisms. However, physical methods suffer from low process efficiency when treating high concentrations of sodium sulfate, chemical methods face issues such as potential waste generation and limitations in concentration reduction, and the use of microorganisms is restricted due to toxicity when sodium sulfate concentrations are high.

[0005] As a technology to improve the treatment efficiency of sodium sulfate, Korean Published Patent Application No. 10-2023-0113005 describes a method for treating wastewater containing high concentrations of sodium sulfate by precipitating and depositing salts through evaporative concentration and cooling, as a system for discharging and reducing the concentration of such wastewater. However, despite the development of various technologies, it has been difficult to effectively lower the concentration of sodium sulfate while also ensuring the economic viability and continuity of the process.

[0006] Accordingly, there is a need to develop technology capable of efficiently treating high concentrations of sodium sulfate while minimizing byproducts and enabling process recirculation.

[0007] One objective of the present invention is to provide a method for efficiently treating high concentrations of sodium sulfate through a multi-stage process.

[0008] To achieve the above objective, the present invention provides a method for treating high-concentration sodium sulfate, comprising: (i) a step of treating a high-concentration sodium sulfate solution by a bipolar membrane electrodialysis (BMED) process to produce a primary treated sodium sulfate solution; (ii) a step of treating the primary treated sodium sulfate solution by an electrodialysis (ED) process to separate it into a secondary treated sodium sulfate solution and a sodium sulfate concentrate; (iii) a step of treating the secondary treated sodium sulfate solution by a nanofiltration (NF) process to obtain a tertiary treated sodium sulfate solution; and (iv) a step of treating the tertiary treated sodium sulfate solution by a reverse osmosis (RO) process to separate it into water and a sodium sulfate concentrate.

[0009] In the present invention, the concentration of the high-concentration sodium sulfate solution may be 8 to 20%.

[0010] In the present invention, the concentration of the high-concentration sodium sulfate solution may be 12 to 15%.

[0011] In the present invention, the concentration of the primary treated sodium sulfate solution may be 4 to 6%.

[0012] In the present invention, the concentration of the secondary treated sodium sulfate solution may be 1 to 3%.

[0013] In the present invention, the sodium sulfate concentrate generated in step (ii) may be further introduced into the bipolar membrane electrodialysis process of step (i).

[0014] In the present invention, the concentration of the sodium sulfate concentrate produced in step (ii) may be 8 to 20%.

[0015] In the present invention, the sodium sulfate concentrate produced in step (iv) may be further introduced into the electrodialysis process of step (ii).

[0016] In the present invention, the concentration of the sodium sulfate concentrate produced in step (iv) may be 4 to 6%.

[0017] In the present invention, the reverse osmosis process in step (iv) can be performed under pressure conditions of 20 to 30 bar.

[0018] According to the present invention, by sequentially performing bipolar membrane electrodialysis (BMED), electrodialysis (ED), nanofiltration (NF), and reverse osmosis (RO) processes during sodium sulfate treatment, the concentration can be systematically controlled at each stage, and high concentrations of sodium sulfate can be treated very efficiently by minimizing conductivity increase and scaling problems. Furthermore, since the concentrate generated during the BMED-ED-NF-RO integrated process can be recirculated, resources can be saved and the continuity of the process is excellent.

[0019] Figure 1 shows the change in conductivity and concentration at the BMED stage in a process circulation process according to one embodiment of the present invention.

[0020] The terms used herein are for describing embodiments of the invention and are not intended to limit the invention. In this specification, the singular form includes the plural form unless specifically stated otherwise in the text. As used herein, "comprises" and / or "comprising" are for specifying mentioned components and / or steps and do not exclude the presence or addition of other components and / or steps.

[0021] Unless otherwise defined, all terms used herein (including technical and scientific terms) may be used in the sense generally understood by those skilled in the art to which the present invention pertains. Furthermore, commonly used dictionary terms are interpreted as having meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or excessive sense unless specifically defined otherwise.

[0022] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0023]

[0024] The present invention relates to a method for efficiently treating sodium sulfate (Na2SO4) through a multi-stage process.

[0025] Processes that directly treat high concentrations of sodium sulfate suffer from membrane scaling issues and high energy consumption. Furthermore, high sodium sulfate concentrations can impair process efficiency due to increased conductivity during the dialysis process, and relying solely on a single dialysis process limits concentration control. While reverse osmosis can be used as an alternative, it presents a problem of poor efficiency due to scaling issues under high concentration conditions.

[0026] As a measure to solve these problems and improve the efficiency of concentration reduction, a multi-stage process combining two or more processes can be applied. However, since the concentration of the sodium sulfate solution changes during the stepwise processing and the optimal conditions for treatment differ in each process, even if low-concentration sodium sulfate is produced by simply combining various processes, the efficiency of concentration reduction is low compared to the time and cost consumed in the multi-stage process, resulting in poor economic feasibility. Furthermore, when using various processes, the types and concentrations of by-products and concentrates generated in each process also differ, which may lead to the problem of complex by-product treatment.

[0027] The present invention provides a sodium sulfate treatment method that can effectively lower the concentration of high-concentration sodium sulfate using a multi-stage process while offering excellent process efficiency, economic feasibility, and continuity. The method is characterized by sequentially performing bipolar membrane electrodialysis (BMED), electrodialysis (ED), nanofiltration (NF), and reverse osmosis (RO) processes during sodium sulfate treatment.

[0028]

[0029] Specifically, a sodium sulfate treatment method according to one embodiment of the present invention may include the following steps:

[0030] (i) a step of treating a high-concentration sodium sulfate solution using a bipolar membrane electrodialysis (BMED) process to produce a primary treated sodium sulfate solution;

[0031] (ii) a step of separating the above-mentioned primary treated sodium sulfate solution into a secondary treated sodium sulfate solution and a sodium sulfate concentrate by treating the primary treated sodium sulfate solution with an electrodialysis (ED) process;

[0032] (iii) a step of treating the above secondary treated sodium sulfate solution with a nanofiltration (NF) process to obtain a tertiary treated sodium sulfate solution; and

[0033] (iv) A step of separating the above-mentioned tertiary treated sodium sulfate solution into water and sodium sulfate concentrate by treating it with a reverse osmosis (RO) process.

[0034] Hereinafter, the process and apparatus for treating sodium sulfate at each stage of the BMED-ED-NF-RO integrated process according to the present invention will be described in detail.

[0035] In describing the present invention, a multi-stage process that sequentially performs the bipolar membrane electrodialysis (BMED), electrodialysis (ED), nanofiltration (NF), and reverse osmosis (RO) processes may be referred to as the “BMED-ED-NF-RO integrated process.”

[0036] In describing the present invention, the term “sodium sulfate solution” means a solution containing sodium sulfate, and does not exclude the presence of organic and / or inorganic substances other than sodium sulfate in said solution.

[0037] In describing the present invention, unless otherwise noted, the concentration unit “%” refers to a mass percentage concentration (wt%) representing the mass of a solute dissolved in a unit mass of solution as a percentage. Additionally, in the present invention, the concentration of a solution refers to the concentration of sodium sulfate in the solution, unless otherwise noted.

[0038] In describing the present invention, a high-concentration sodium sulfate solution may refer to a solution in which the concentration of sodium sulfate is 8% or more, specifically 8 to 20%, preferably 8 to 15%, specifically 9 to 15%, more specifically 10 to 15%, and even more specifically 10% (±0.5%).

[0039] In the present invention, a bipolar membrane electrodialysis (BMED) step is performed as a primary process for high-concentration sodium sulfate treatment.

[0040] Bipolar membrane electrodialysis is a technique for treating a solution using an ion exchange membrane, and can be performed using a bipolar membrane electrodialysis device in which multiple cells comprising a cation exchange membrane (CEM), a bipolar membrane, and an anion exchange membrane (AEM) are stacked between an anode and a cathode.

[0041] In the above device, the electrodes (anode and cathode) serve to supply current to induce an electrolytic reaction, the cation exchange membrane selectively moves cations to produce acid, and the anion exchange membrane selectively moves anions to produce base. In the above device, the bipolar membrane has a double-layer structure, with one side blocking cations and the other side blocking anions, and electrolysis is performed between the layers to produce hydrogen ions and hydroxide ions, each of which reacts with anions and cations to form acids and bases.

[0042] When a sodium sulfate solution is introduced into the desalination chamber formed by the above-mentioned cation exchange membrane and anion exchange membrane, cations (Na + ) moves through the cation exchange membrane, and anions (SO4 2- ) moves to the opposite side through the anion exchange membrane. As a result, ions are removed in the desalination chamber, and the concentration of the sodium sulfate solution decreases.

[0043] In the present invention, process efficiency and economic feasibility can be secured by performing the bipolar membrane electrodialysis first among the multi-stage treatment processes of sodium sulfate. Since the efficiency of electrodialysis increases with higher ion concentrations, it is desirable to perform bipolar membrane electrodialysis at the initial stage when the ion concentration is highest. Furthermore, using bipolar membrane electrodialysis enables the constant maintenance of acid, base, and salt conductivity.

[0044] In the present invention, the bipolar membrane electrodialysis can be performed at a temperature of 25 to 40°C. If the temperature is too low, the ion mobility may be low, which may result in reduced efficiency, and if the temperature is too high, the durability of the membrane may be compromised.

[0045] In the present invention, the bipolar membrane electrodialysis may be performed using a sodium sulfate solution having a sodium sulfate concentration of 8% or more, preferably 10% or more, specifically 12% or more, and the concentration of the solution may be 20% or less, preferably 18% or less, specifically 15% or less. If the concentration of sodium sulfate is too low, a problem may occur in which the efficiency decreases due to a low ion concentration, and if the concentration is too high, a problem may occur in which sodium sulfate precipitates and interferes with current transmission.

[0046] In the present invention, the concentration of high-concentration sodium sulfate can be reduced to 5% by using the bipolar membrane electrodialysis as a primary treatment process for sodium sulfate.

[0047] Specifically, in one embodiment of the present invention, the concentration of the sodium sulfate solution treated by the bipolar membrane electrodialysis may be 4 to 6%, preferably 4.5 to 5.5%, specifically 5% (±0.1%).

[0048] After the above bipolar membrane electrodialysis step, electrodialysis (ED) is performed as a secondary process. In the present invention, by introducing a solution whose concentration has been adjusted to a level of 5% through primary treatment into the electrodialysis process, the efficiency of concentration reduction by electrodialysis can be maximized.

[0049] Electrodialysis can be performed using an electrodialysis device that includes multiple cells containing a cation exchange membrane (CEM) and an anion exchange membrane (AEM) between an anode and a cathode.

[0050] When power is supplied to the above electrodialysis device, cations (Na₂S) + ) moves through the cation exchange membrane and anions (SO4 2- ) moves through an anion exchange membrane, and ion separation occurs. In the concentration chamber, a high concentration of sodium sulfate (sodium sulfate concentrate) is formed, and in the dilution chamber, a low concentration of sodium sulfate solution is formed.

[0051] In the present invention, the electrodialysis can be performed at a temperature of 20 to 35°C. If the temperature is too low, the ion mobility may be low, which may result in reduced efficiency, and if the temperature is too high, the durability of the membrane may be compromised.

[0052] In the present invention, the electrodialysis can be performed using a sodium sulfate solution with a concentration of 4 to 6% via the bipolar membrane electrodialysis. If the concentration is too low, a problem may occur in which ions cannot move, and if the concentration is too high, precipitation of sodium sulfate may become a problem, which may cause problems with continuous operation.

[0053] In the present invention, a sodium sulfate solution that has been treated by primary treatment is subjected to electrodialysis treatment to obtain a sodium sulfate solution with a low concentration of 2%.

[0054] Specifically, in one embodiment of the present invention, the concentration of the sodium sulfate solution secondarily treated by electrodialysis may be 1 to 3%, preferably 1.5 to 2.5%, specifically 2% (±0.1%).

[0055] In the present invention, the sodium sulfate concentration of the concentrate produced together with the secondary treated sodium sulfate solution may be about 8 to 20%, for example, 10 to 15%.

[0056] In one embodiment of the present invention, the sodium sulfate concentrate generated in the electrodialysis step may be introduced into the bipolar membrane electrodialysis step. In the above embodiment, the process sequence of concentrate recirculation is not limited and may be performed by collecting and introducing the concentrate separately during the BMED-ED-NF-RO integrated process to continuously proceed with the process, or by initiating a separate process using the collected concentrate after the entire process is completed.

[0057] Specifically, in the above electrodialysis process, a sodium sulfate concentrate with a concentration of 10% is produced together. Since a high-concentration sodium sulfate solution with a concentration of 10% is used in the preceding bipolar membrane electrodialysis step, it is possible to recycle the sodium sulfate concentrate formed by electrodialysis back into bipolar membrane electrodialysis.

[0058] After the above electrodialysis step, a nanofiltration (NF) process is performed as a third process.

[0059] Nanofiltration is a technology that separates substances through a fine filter. It can block particles (ions or molecules) larger than a certain size through the fine pores of the filter, and can selectively separate charges in charged nanofiltration membranes. For the nanofiltration described above, a nanofiltration device comprising a nanofiltration membrane, a pressure pump, a tank where the solution to be treated is located, and a water tank where the filtered water and concentrate are collected may be used.

[0060] In the process of nanofiltration of a sodium sulfate solution, the concentration of sodium ions in the filtrate is relatively higher and the concentration of sulfate ions can be lower because cations are more likely to pass through depending on the charge on the surface of the nanofiltration membrane.

[0061] In the present invention, a sodium sulfate solution with a concentration of 2% generated in the electrodialysis step can be treated by a nanofiltration process to selectively remove multivalent ions. Through this, membrane fouling (scaling) problems that may occur in the subsequent reverse osmosis process can be prevented, thereby improving the efficiency of the process.

[0062] Furthermore, introducing nanofiltration prevents the problem of salt pH decreasing and impurities increasing during the bipolar membrane electrodialysis process in the process circulation. Since this enables an increase in salt pH and a reduction in impurity levels compared to the process excluding nanofiltration, process efficiency can be improved.

[0063] In the present invention, the nanofiltration can be performed at a temperature of 20 to 40°C. If the temperature is too low, problems with ion movement may occur, and if the temperature is too high, problems with the durability of the membrane may occur.

[0064] In the present invention, the nanofiltration can be performed under conditions of 35 bar or less. If the pressure is too high, the electricity cost may be high, which may reduce economic feasibility.

[0065] After the above nanofiltration step, a reverse osmosis (RO) step is performed as a fourth process. In the present invention, by introducing a sodium sulfate solution with a concentration of 2% from which polyvalent ions have been removed into the reverse osmosis process, optimal efficiency can be secured in the reverse osmosis process and high-purity water can be obtained.

[0066] Reverse osmosis is a technique that removes solutes through a semipermeable membrane by applying external pressure higher than the osmotic pressure to move water from an area of ​​high concentration to an area of ​​low concentration, thereby removing impurities and ions.

[0067] For the above reverse osmosis, a device comprising a reverse osmosis membrane (semipermeable membrane), a pressure pump, a tank for collecting reverse osmosis-treated water, and piping for collecting raw water / waste may be used. Since water molecules in the sodium sulfate solution pass through the membrane and are separated by external pressure, a concentrated solution remains on one side after treatment, while water with a lower concentration is collected on the opposite side.

[0068] In the reverse osmosis process, if there are many polyvalent ions in the treatment solution, membrane fouling may occur because they are strongly adsorbed to the surface of the membrane by electrical attraction; however, in the present invention, by introducing a nanofiltration-treated solution into the reverse osmosis process, the problem of membrane fouling can be prevented and the efficiency of the process can be improved.

[0069] In the present invention, the reverse osmosis can be performed at room temperature. If the temperature is too low, a problem with the precipitation of sodium sulfate may occur, and if the temperature is too high, a problem with the durability of the membrane may occur.

[0070] In the present invention, the reverse osmosis can be performed under conditions of 35 bar or less, preferably 20 to 30 bar, and more preferably 20 to 25 bar. If the pressure is too high, a problem may arise where the power consumption increases.

[0071] If nanofiltration is not performed in the preceding process, the reverse osmosis operating pressure increases due to the osmotic pressure of sulfate ions, but by performing a nanofiltration process before reverse osmosis, it is possible to perform the reverse osmosis process under pressure conditions of 20 to 25 bar.

[0072] In the present invention, a sodium sulfate solution with a concentration of 2% that has been treated by nanofiltration is introduced into a reverse osmosis process to separate it into high-purity water and a sodium sulfate concentrate. In this way, the present invention can reduce the concentration of sodium sulfate from a level of 10% or more to 0.5% or less, specifically to a level of 0.01 to 0.5%, through a stepwise process.

[0073] In the present invention, the sodium sulfate concentration of the concentrate produced together with the 4th treated sodium sulfate solution is at a level of about 5%, specifically 4 to 6%, preferably 4.5 to 5.5%, more specifically 5% (±0.1%).

[0074] In one embodiment of the present invention, the sodium sulfate concentrate generated in the reverse osmosis step may be introduced into the electrodialysis step. In the above embodiment, the process sequence of concentrate recirculation is not limited and may be carried out by separately collecting and introducing the concentrate during the BMED-ED-NF-RO integrated process to continuously proceed with the process, or by using the collected concentrate for the next process after the entire process is completed.

[0075] Specifically, in the reverse osmosis process, a sodium sulfate concentrate with a concentration of 5% is produced along with water. Since a sodium sulfate solution with a concentration of 5% is used in the preceding electrodialysis step, it is possible to recirculate the sodium sulfate concentrate formed by reverse osmosis by feeding it into the electrodialysis process. Therefore, it is possible to conserve resources, minimize by-products, and maintain the continuity of the process.

[0076] As described above, the present invention allows for obtaining high-purity water from a high-concentration sodium sulfate solution through an integrated BMED-ED-NF-RO process. By utilizing the stepwise process of the present invention, high-concentration sodium sulfate can be efficiently treated by systematically lowering the concentration of sodium sulfate while minimizing issues of increased conductivity and scaling. Furthermore, since the concentrate generated during the multi-stage process can be recirculated, resources can be saved and process continuity maintained.

[0077]

[0078] Examples

[0079] The present invention will be explained in more detail below through examples. However, these examples are merely for illustrating the present invention and are not limited thereto.

[0080]

[0081] Experimental Example: Measurement of reverse osmosis operating pressure difference and salt pH and concentration according to process configuration

[0082]

[0083] According to the present invention, a sodium sulfate treatment process was performed in the order of BMED-ED-NF-RO, and the sodium sulfate treatment process was performed under the same conditions, excluding the NF step. The pressure difference during reverse osmosis operation, which is the final step in each process, and the salt pH and concentration of BMED during process circulation were measured and are shown in Table 1 below.

[0084] Classification Process Configuration RO Operating Pressure (bar) BMED Salt pH BMED Salt Na (g / L) BMED Salt S (g / L) Comparative Example BMED-ED-RO 3 3.5 1.8 5 1 1.2 Example BMED-ED-NF-RO 2 2.5 3.5 4 6 0.3

[0085] Experimental results showed that when nanofiltration was excluded from the BMED-ED-NF-RO process, the operating pressure in the final reverse osmosis stage was 33.5 bar, requiring high pressure conditions. On the other hand, when the nanofiltration stage was introduced before reverse osmosis, it was confirmed that the operating pressure was significantly lowered to 22.5 bar.

[0086] From the above results, it was found that by using the BMED-ED-NF-RO sequential process of the present invention during sodium sulfate treatment, the operating pressure of reverse osmosis can be reduced and process efficiency can be secured.

[0087] In addition, it was confirmed that when the process is circulated without nanofiltration in the BMED-ED-NF-RO process, the desalination solution containing sodium sulfate and sulfuric acid is continuously discharged, causing the pH of the salt to drop and the process efficiency to decrease due to hydrogen ions and impurities, whereas when sulfate ions are removed using nanofiltration and the solution is used for electrodialysis, the salt pH of the BMED process is relatively high and the impurity content is low, thus increasing the process efficiency.

[0088] Figure 1 shows the changes in conductivity and concentration in the BMED process during process circulation in each process, and it was confirmed that even if nanofiltration is added according to the present invention, the process conditions are maintained as in the process excluding nanofiltration.

[0089]

[0090] The present invention is not limited to the embodiments described above but can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without altering the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1. (i) a step of producing a primary treated sodium sulfate solution by treating a high-concentration sodium sulfate solution using a bipolar membrane electrodialysis (BMED) process; (ii) a step of separating the above-mentioned primary treated sodium sulfate solution into a secondary treated sodium sulfate solution and a sodium sulfate concentrate by treating the primary treated sodium sulfate solution with an electrodialysis (ED) process; (iii) a step of treating the above secondary treated sodium sulfate solution with a nanofiltration (NF) process to obtain a tertiary treated sodium sulfate solution; and (iv) A step of separating the above tertiary treated sodium sulfate solution into water and sodium sulfate concentrate by treating it with a reverse osmosis (RO) process. A high-concentration sodium sulfate treatment method comprising 2. In Paragraph 1, A high-concentration sodium sulfate treatment method in which the concentration of the high-concentration sodium sulfate solution is 8 to 20%.

3. In Paragraph 1, A high-concentration sodium sulfate treatment method in which the concentration of the high-concentration sodium sulfate solution is 12 to 15%.

4. In Paragraph 1, A high-concentration sodium sulfate treatment method in which the concentration of the first-hand treated sodium sulfate solution is 4 to 6%.

5. In Paragraph 1, A high-concentration sodium sulfate treatment method in which the concentration of the secondary treated sodium sulfate solution is 1 to 3%.

6. In Paragraph 1, A method for treating high concentration sodium sulfate, further comprising the step of introducing the sodium sulfate concentrate produced in step (ii) above into the bipolar membrane electrodialysis process of step (i) above.

7. In Paragraph 1, A high-concentration sodium sulfate treatment method in which the concentration of the sodium sulfate concentrate produced in step (ii) above is 8 to 20%.

8. In Paragraph 1, A method for treating high concentration sodium sulfate, further comprising the step of introducing the sodium sulfate concentrate produced in step (iv) above into the electrodialysis process of step (ii) above.

9. In Paragraph 1, A high-concentration sodium sulfate treatment method in which the concentration of the sodium sulfate concentrate produced in step (iv) above is 4 to 6%.

10. In Paragraph 1, A method for treating high concentration sodium sulfate, wherein the reverse osmosis process in step (iv) above is performed under pressure conditions of 20 to 30 bar.

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