Pressure-driven hardness removal system and method

Abstract

This invention discloses a pressure-driven hardness removal system and method. The system includes an inlet tank, an electrochemical system, a sedimentation tank, and a regulating tank connected in sequence. The electrochemical system includes an anode plate and cathode plates disposed on both sides of the anode plate. A diaphragm is provided between each cathode plate and the anode plate, forming a central circulation zone. The outer side of the diaphragm is an external reaction zone. The diaphragm is a permeable membrane, allowing the solution in the central circulation zone to pass through the diaphragm into the external reaction zone under pressure. The external reaction zone is connected to the sedimentation tank. The method of this invention is implemented using this system. The system and method of this invention can fundamentally prevent membrane scaling and maintain the hardness removal efficiency of the membrane electrochemical process.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical treatment technology and relates to a pressure-driven hardness removal system and method. Background Technology

[0002] Electrochemical technology for removing hardness from water has attracted much attention. This technology mainly utilizes electric current to induce water electrolysis on the cathode surface, generating a large amount of OH-. - With hardness ions in solution (such as Ca) 2+ Mg 2+ alkalinity ions, CO3 2- HCO3 - The reaction forms CaCO3 and Mg(OH)2 precipitates, which are deposited on the cathode surface, thereby achieving the purpose of removing hardness ions.

[0003] Currently, based on the structural form of the equipment, electrochemical hardness removal technology can be divided into two categories: 1) diaphragm-less electrochemical hardness removal technology; 2) diaphragm electrochemical hardness removal technology. Both types of technology have the following characteristics:

[0004] 1) Membrane-free electrolytic hardness removal technology works by generating an alkaline region at the cathode interface that reacts with calcium and magnesium ions and alkalinity ions in the solution, thereby producing calcium carbonate and magnesium hydroxide precipitates on the cathode surface. The scale on the cathode surface is then removed using physical or chemical methods. However, this technology is limited by the OH groups generated at the cathode. - H generated at the anode + The continuous neutralization reaction outside the interface results in low current utilization, making it difficult to meet the needs of industrial applications.

[0005] 2) The corresponding membrane electrochemical hardness removal technology involves adding a membrane between the cathode and anode to limit the OH groups generated at the cathode. - H generated at the anode + The neutralization reaction improves the current utilization rate. However, during the application process, it is inevitable that the cathode side of the diaphragm will be contaminated by calcium carbonate and magnesium hydroxide precipitation. This is the most critical bottleneck hindering the application of diaphragm electrochemical hardness removal technology.

[0006] Currently, the main technologies used in journal articles, patent documents, and applications regarding diaphragm electrochemical hardness removal technology are as follows:

[0007] 1) Regular acid washing of the diaphragm requires a large amount of acid, which leads to a lot of acid treatment problems;

[0008] 2) Periodic reversal between the cathode and anode places high demands on the materials used in both materials, and frequent reversal can shorten the electrode lifespan, affecting its industrial applications;

[0009] 3) Patent document CN113754150A discloses a stable high-hardness water electrochemical hardening system and treatment process. This method involves recirculating deeply treated alkaline softened water back to the diaphragm electrochemical system, utilizing the alkaline solution generated by the electrochemical system to treat the hardness-containing water, thereby reducing the risk of scaling and clogging inside the device and extending the cleaning cycle. However, this method does not fundamentally solve the membrane fouling problem associated with using diaphragm electrochemistry for hardening removal.

[0010] Based on the above, how to solve the problem of membrane fouling and maintain the treatment efficiency of membrane electrochemistry in the process of treating water with hardness has become an urgent problem to be solved in electrochemical hardness removal technology. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art, especially the problem of frequent diaphragm contamination during the hardening process of diaphragm electrochemical technology, which makes the diaphragm electrochemical equipment unable to operate stably. The present invention provides a pressure-driven hardening system and method that avoids diaphragm contamination by eliminating the need for reversing or acid washing.

[0012] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0013] A pressure-driven hardness removal system includes an inlet tank, an electrochemical system, a sedimentation tank, and an equalization tank connected in sequence. The electrochemical system includes an anode plate and cathode plates disposed on both sides of the anode plate. A diaphragm is provided between the two cathode plates and the anode plate, forming a central circulation zone between the two diaphragms. The outer side of the diaphragm is an external reaction zone. The diaphragm is a water-permeable diaphragm, allowing the solution in the central circulation zone to pass through the diaphragm into the external reaction zone under pressure. The external reaction zone is connected to the sedimentation tank.

[0014] In the aforementioned pressure-driven hardness removal system, preferably, the central circulation zone is provided with a liquid outlet, which is connected to the water inlet tank and the regulating tank, respectively.

[0015] In the aforementioned pressure-driven hardness removal system, preferably, the central circulation zone is provided with a liquid inlet, and the water inlet tank is connected to the liquid inlet of the central circulation zone via a pump.

[0016] In the aforementioned pressure-driven hardness removal system, preferably, the electrochemical system is equipped with a power source, the anode plate is connected to the positive terminal of the power source, and the cathode plate is connected to the negative terminal of the power source.

[0017] In the aforementioned pressure-driven hardness removal system, preferably, the permeable membrane is one or more of the following: microfiltration membrane, ultrafiltration membrane, nanofiltration membrane, glass fiber membrane, and asbestos membrane.

[0018] As a general technical concept, the present invention also provides a pressure-driven hardening removal method, implemented using the above-mentioned pressure-driven hardening removal system, including the following steps:

[0019] A portion of the hardness-containing wastewater is sent to the central circulation zone of the electrochemical system through the inlet tank, while another portion is sent to the external reaction zone of the electrochemical system. The operating pressure of the central circulation zone is set to P1, and the operating pressure of the external reaction zone is set to P2, where P1 > P2. Power is supplied to initiate an electrochemical reaction, removing hardness from the wastewater. The solution from the high-pressure central circulation zone permeates through the diaphragm into the low-pressure external reaction zone to prevent scaling of the diaphragm. A precipitant is provided in the external reaction zone. The effluent from the external reaction zone is sent to the sedimentation tank for solid-liquid separation. The resulting effluent from the sedimentation tank is sent to the regulating tank to adjust the pH to neutral, yielding hardness-removed effluent.

[0020] In the aforementioned pressure-driven hardening removal method, preferably, the operating pressure P1 of the central circulation zone is 0.02MPa to 0.3MPa.

[0021] In the aforementioned pressure-driven hardness removal method, preferably, during the electrochemical reaction process, the amount of water permeating the membrane in the central circulation zone is maintained at 0.1% to 5% of the circulating water volume in the central circulation zone, and every 12h to 48h, the pressure in the central circulation zone is increased to 1.2P1 to 2P1, and after running for 1min to 60min, the operating pressure P1 of the central circulation zone is restored.

[0022] In the aforementioned pressure-driven hardness removal method, preferably, the hardness-containing wastewater sent to the central circulation zone accounts for 5% to 30% of the hardness-containing wastewater sent to the external reaction zone;

[0023] During the electrochemical reaction, the pH value of the solution in the central circulation zone is controlled at 0-5, and the pH value of the solution in the external reaction zone is controlled at 9-12.

[0024] The precipitant is one or more of CO2, sodium carbonate, and sodium bicarbonate.

[0025] In the aforementioned pressure-driven hardness removal method, preferably, a portion of the effluent from the central circulation zone is discharged into the regulating tank to adjust the pH value to neutral, while another portion of the effluent from the central circulation zone is returned to the inlet tank.

[0026] Compared with the prior art, the advantages of the present invention are as follows:

[0027] (1) This invention utilizes different water inlet methods for the cathode and anode to generate H by electrolyzing water at the anode. +This process ensures that the anolyte solution is acidic and maintains a certain pressure for circulation, while the cathode is a static reaction zone. A water-permeable diaphragm is used to maintain a certain permeability of the acidic solution from the anolyte to the cathode. Furthermore, by adjusting the anolyte pressure, the permeability from the anolyte to the cathode can be increased, thereby fundamentally preventing diaphragm scaling. This eliminates the need for acid washing or electrode reversal cleaning operations and maintains the diaphragm's electrochemical hardness removal efficiency.

[0028] (2) This invention separates the cathode and anode and utilizes electrochemistry to generate a large amount of OH- at the cathode. - This eliminates the need to add alkaline agents such as sodium hydroxide and calcium hydroxide during the hardness removal process, reducing the introduction of ions into the water. After hardness removal, the acidic water from the anode is used to adjust the pH of the alkaline solution at the cathode, eliminating the need to add acidic agents such as sulfuric acid and hydrochloric acid. Attached Figure Description

[0029] Figure 1 This is a process flow diagram of the pressure-driven hardening removal method according to Embodiment 1 of the present invention.

[0030] Legend:

[0031] 1. Inlet tank; 2. Pump; 3. Anode plate; 4. Diaphragm; 5. Cathode plate; 6. Sedimentation tank; 7. Equalization tank; a. Wastewater containing hardness; b. Calcium and magnesium scale residue; c. Effluent from sedimentation tank; d. Effluent after hardness removal. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention. All materials and instruments used in the following embodiments are commercially available.

[0033] Example 1

[0034] A pressure-driven hardness removal system of the present invention includes an inlet tank 1, an electrochemical system, a sedimentation tank 6, and an adjustment tank 7 connected in sequence. The electrochemical system includes an anode plate 3 and cathode plates 5 disposed on both sides of the anode plate 3. A diaphragm 4 is provided between the two cathode plates 5 and the anode plate 3, and the two diaphragms 4 form a central circulation zone. The outer side of the diaphragm 4 is an external reaction zone. The diaphragm 4 is a water-permeable diaphragm, which allows the solution in the central circulation zone to enter the external reaction zone through the diaphragm 4 under pressure. The external reaction zone is connected to the sedimentation tank 6.

[0035] In this embodiment, the central circulation zone is provided with a liquid outlet, which is connected to the water inlet tank 1 and the regulating tank 7 respectively.

[0036] In this embodiment, the central circulation zone is provided with a liquid inlet, and the water inlet tank 1 is connected to the liquid inlet of the central circulation zone through the pump 2.

[0037] In this embodiment, the electrochemical system is equipped with a power source, with the anode plate 3 connected to the positive terminal of the power source and the cathode plate 5 connected to the negative terminal of the power source.

[0038] In this embodiment, the water-permeable membrane is a microfiltration membrane, which is used to separate the cathode and anode.

[0039] A certain RO membrane pretreatment system traditionally uses sodium hydroxide and sodium carbonate to remove hardness, followed by adding acid to adjust the pH. The influent calcium ion concentration is 400-500 mg / L, and the pH is around 8. Sodium hydroxide is added to adjust the pH to 9-10, and sodium carbonate is added to reduce the calcium ion concentration to below 100 mg / L. The effluent is then acidified to bring the pH back to 7. This water quality is used to conduct experiments for the present invention.

[0040] A pressure-driven hardening removal method according to the present invention, such as Figure 1 As shown, the pressure-driven hardening system of this embodiment is implemented, including the following steps:

[0041] (1) A portion of the hardness-containing wastewater a is sent to the central circulation zone of the electrochemical system through the inlet tank 1. The central circulation zone then sends a portion of the effluent into the inlet tank 1 through the liquid outlet to form a circulation, maintaining the wastewater volume of the inlet tank 1 at 1m³. 3 Another portion of the hardness-containing wastewater (a) is sent to the external reaction zone of the electrochemical system, with the flow rate in the external reaction zone controlled at 5 m³ / s. 3 / h, the operating pressure P1 of the central circulation zone is 0.2 MPa (the pressure is controlled by the circulation flow rate), and the operating pressure P2 of the external reaction zone is 0.

[0042] (2) Power is turned on to carry out an electrochemical reaction to remove hardness from the wastewater containing hardness a. The solution in the high-pressure central circulation zone passes through the diaphragm 4 into the low-pressure external reaction zone to prevent scaling of the diaphragm 4. During operation, the pH value of the external reaction zone is 10-11, and the pH value of the central circulation zone is 2-3. CO2 gas is passed through the external reaction zone to precipitate calcium ions. The effluent enters the sedimentation tank 6. The sediment (calcium and magnesium scale b) is discharged from the bottom of the sedimentation tank 6. The effluent c from the sedimentation tank enters the regulating tank 7 to adjust the pH to neutral, thus obtaining the hardness-removed effluent d.

[0043] In this embodiment, the pH will continue to decrease during the circulation process in the central circulation zone, so some solution is discharged into the regulating tank 7 to neutralize the effluent.

[0044] In this embodiment, the anode to cathode permeation rate is <20L / h, that is, the water permeation rate through the diaphragm 4 in the central circulation zone is kept at 2% of the circulating water volume in the central circulation zone. Every 24 hours, the pressure in the central circulation zone is increased to 0.4MPa. After running for 10 minutes, the pressure is restored to 0.2MPa to keep the external reaction zone from removing hardness.

[0045] In this embodiment, the hardness-containing wastewater a sent to the central circulation zone accounts for 20% of the hardness-containing wastewater a sent to the external reaction zone.

[0046] After treatment by the diaphragm electrochemical hardness removal system of this embodiment, the calcium ion concentration of the hardness-removed effluent d can be stably reduced to below 100 mg / L, and the pH value of the effluent is 6-8, without introducing any other ions during the process.

[0047] After running continuously for more than 7 days, no contamination was observed in the diaphragm, except that the hardness and efficiency remained unchanged.

[0048] Comparative Example 1

[0049] A diaphragm electrochemical method for removing hardness is basically the same as the method in Example 1, except that the pressure in the control center circulation zone and the external reaction zone is the same, both being 0.2 MPa.

[0050] At the start of operation, the calcium ion concentration in the effluent with hardness d can stably decrease to below 100 mg / L. However, after 12 hours, with the same influent flow rate in both the central circulation zone and the external reaction zone, the pH value in the central circulation zone gradually increases, while the pH value in the external reaction zone gradually decreases. This is because at the beginning of operation, the diaphragm is free of fouling, and its electrochemical efficiency is at its highest, resulting in the generation of H+ in the central circulation zone. + It is acidic, and OH- is generated in the external reaction zone. - It is alkaline; however, with membrane fouling, electrochemical efficiency decreases, and H₂ is generated in the central circulation zone. + As the pH value decreases, the external reaction zone generates OH-. - As the pH value gradually decreases, the hardness removal efficiency is affected, leading to a continuous increase in the calcium ion concentration in the effluent while the efficiency continues to decline. After 24 hours of operation, the calcium ion concentration in the effluent reaches 400 mg / L, making further operation impossible. A large amount of calcium carbonate precipitate appears on the cathode side of the diaphragm, indicating severe contamination.

[0051] Comparative Example 2

[0052] A membrane electrochemical method for hardness removal is basically the same as the method in Example 1, except that: a non-permeable cation membrane is used to separate the anode and cathode, the operating pressure of the central circulation zone is controlled at 0.2 MPa, and the operating pressure of the external reaction zone is 0. Since the cation membrane is non-permeable, the water permeation is 0.

[0053] At the start of operation, the calcium ion concentration in the effluent with hardness d could be stably reduced to below 100 mg / L. However, after 3 hours, with the same influent flow rate to the central circulation zone and the external reaction zone, the pH value in the central circulation zone gradually increased, while the pH value in the external reaction zone gradually decreased. The calcium ion concentration in the effluent reached 220 mg / L, and the efficiency continued to decrease. After 6 hours of operation, the calcium ion concentration in the effluent reached 400 mg / L, making it impossible to continue operation. A large amount of calcium carbonate precipitate appeared on the cathode side of the diaphragm, indicating severe pollution.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A pressure-driven based hardness removal method, characterized in that, The hardness removal method based on pressure driving is implemented by a hardness removal system based on pressure driving, which comprises a water inlet tank (1), an electrochemical system, a sedimentation tank (6) and a regulating tank (7) connected in sequence, the electrochemical system comprises anode plates (3) and cathode plates (5) arranged on both sides of the anode plates (3), diaphragms (4) are arranged between the two cathode plates (5) and the anode plates (3) respectively, a central circulation zone is formed between the two diaphragms (4), and the outer side of the diaphragm (4) is an external reaction zone; the diaphragm (4) is a water-permeable diaphragm, so that the solution in the central circulation zone is driven by pressure to pass through the diaphragm (4) and enter the external reaction zone, and the external reaction zone is communicated with the sedimentation tank (6). The hardness removal method based on pressure driving comprises the following steps: Part of the wastewater (a) containing hardness is sent to the central circulation zone of the electrochemical system through the water inlet tank (1), and the other part of the wastewater (a) containing hardness is sent to the external reaction zone of the electrochemical system; the operating pressure of the central circulation zone is set as P1, the operating pressure of the external reaction zone is set as P2, P1>P2, a power supply is connected, an electrochemical reaction is carried out, the hardness of the wastewater (a) containing hardness is removed, the solution in the central circulation zone with high pressure penetrates through the diaphragm (4) and enters the external reaction zone with low pressure, so as to prevent the diaphragm (4) from being scaled, a precipitating agent is arranged in the external reaction zone, the effluent of the external reaction zone is sent to the sedimentation tank (6) to carry out solid-liquid separation, the effluent (c) of the sedimentation tank is sent to the regulating tank (7), the pH value is adjusted to neutral, and the effluent (d) of hardness removal is obtained.

2. The pressure-driven hardness removal method according to claim 1, wherein, The operating pressure P1 of the central circulation zone is 0.02-0.3 MPa.

3. The pressure-driven based hardness removal method of claim 1, wherein, During the electrochemical reaction, the water amount of the central circulation zone penetrating through the diaphragm (4) is 0.1%-5% of the circulating water amount of the central circulation zone, and every 12-48 hours, the pressure of the central circulation zone is increased to 1.2P1-2P1, and after 1-60 minutes of operation, the operating pressure P1 of the central circulation zone is restored.

4. The pressure-driven hardness removal method according to any one of claims 1 to 3, characterized in that, The wastewater (a) containing hardness sent to the central circulation zone accounts for 5%-30% of the wastewater (a) containing hardness sent to the external reaction zone. During the electrochemical reaction, the pH value of the solution in the central circulation zone is controlled to be 0-5, and the pH value of the solution in the external reaction zone is controlled to be 9-12. The precipitating agent is one or more of CO2, sodium carbonate and sodium bicarbonate.

5. The pressure-driven hardness removal method according to any one of claims 1 to 3, characterized in that, Part of the effluent of the central circulation zone is discharged into the regulating tank (7) to adjust the pH value to neutral, and the other part of the effluent of the central circulation zone is sent back to the water inlet tank (1).

6. The pressure-driven hardness removal method according to any one of claims 1 to 3, characterized in that, The central circulation zone is provided with a liquid outlet, and the liquid outlet is communicated with the water inlet tank (1) and the regulating tank (7) respectively.

7. The pressure-driven hardness removal method according to any one of claims 1 to 3, characterized by, The central circulation zone is provided with a liquid inlet, and the water inlet tank (1) is communicated with the liquid inlet of the central circulation zone through a pump (2).

8. The pressure-driven hardness removal method according to any one of claims 1 to 3, characterized by, The electrochemical system is provided with a power supply, the anode plates (3) are connected with the positive pole of the power supply, and the cathode plates (5) are connected with the negative pole of the power supply.

9. The pressure-driven hardness removal method according to any one of claims 1 to 3, characterized by, The water permeable membrane is one or more of a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, a glass fiber membrane, and an asbestos membrane.

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