A diaphragm method electrochemical softening system and method based on nickel-chromium-iron-based electrodes

Abstract

This invention discloses a membrane electrochemical softening system and method based on a nickel-chromium-iron-based electrode. The softening system includes an electrochemically enhanced crystallization unit, a nickel-chromium-iron-based electrode membrane electrolysis reactor, and a solid-liquid separation unit. The softening process includes the following steps: high-hardness water is mixed with the cathode effluent of the membrane electrolysis reactor, and after controlling the pH to be >9.8, it enters the electrochemically enhanced crystallization unit for accelerated crystallization; the resulting scale-containing softened water is treated by a primary solid-liquid separation module to obtain alkaline softened water; then it is refluxed back into the membrane electrolysis reactor for electrolysis, controlling the pH of the cathode effluent to be 10.5-13 and the pH of the anode effluent to be >5.5; the cathode effluent is returned and mixed with high-hardness water to enter the electrochemically enhanced crystallization unit, while the anode effluent enters the electrochemical de-yellowing module, and after electrolysis, it enters the secondary solid-liquid separation module for solid-liquid separation to complete the softening process. This invention can reduce energy consumption and operating costs, and improve controllability and operational stability.

Description

Technical Field

[0001] This invention relates to the field of electrochemical hardening technology, and in particular to a membrane-based electrochemical softening system and method based on nickel-chromium-iron electrodes. Background Technology

[0002] The use of electrochemical methods to soften water containing hardness ions was first reported in literature in 1982. Subsequently, researchers in Germany, Israel, and other countries began to apply this technology to the softening treatment of industrial water containing hardness. Since 2002, this technology has been introduced to China from Israel, and related literature and patent reports have increased year by year. Currently, this technology has been applied to some extent in engineering practice. Compared with the traditional lime and soda ash softening method, electrochemical water softening technology has advantages such as eliminating the need for pre-adjusting alkali and adding acid to adjust the pH value, minimizing or eliminating the introduction of new ions into the water, producing less waste residue, requiring less space, being flexible in operation, and facilitating automated control, resulting in high economic and environmental benefits. In addition, compared with traditional chemical dosing methods, electromagnetic technology, and ultrasonic scale prevention technology, the advantage of electrochemical hardness removal technology is that it can remove scale-forming ions from the water, thereby increasing the concentration ratio and achieving water conservation.

[0003] Currently, electrochemical hardening technologies mainly include two types: direct electrochemical hardening and diaphragm electrochemical hardening. The former primarily utilizes the cathodic reaction at the cathode / water interface to induce calcium and magnesium ions to deposit as carbonates or hydroxides on the cathode wall. The scale layer is then periodically removed through physical scraping, chemical acid washing, or electrochemical reversal, eliminating the voltage drop caused by the scale layer and restoring electrode activity, thereby maintaining a constant hardening capacity and energy consumption. This type of technology is currently widely used in some small and medium-sized water systems. However, its current efficiency is low, generally between 5% and 25%, which significantly limits its application in large-scale water systems. Unlike direct electrochemical hardening, diaphragm electrochemical hardening improves current efficiency by adding an ion-conducting diaphragm between the anode and cathode, suppressing the neutralization of hydroxide and hydrogen ions generated at the anode and cathode. Relevant literature and published patents (Desalination, 2010, 263(1-3):285-289; CN111170424A, CN107235564A, CN113754150A, CN117466489A) indicate that the current efficiency of the diaphragm electrochemical hardening removal technology can reach over 70%, significantly improving the hardening removal effect. Furthermore, the electrolytic current density used in this technology is more than five times higher than that of the previous technology. These two advantages can significantly improve electrode utilization and equipment integration, while reducing the equipment footprint, thus meeting the hardening removal needs of large-scale water systems.

[0004] The main problems hindering the large-scale application of diaphragm electrochemical hardening technology include poor process stability, high operating energy consumption, and high equipment investment costs. Regarding the problem of poor process stability, the main issue is scale buildup and blockage inside the reactor and downstream piping, leading to equipment shutdowns. Chinese invention patent CN106277369A discloses a scale inhibition method that uses non-scaling conductive water in the cathode chamber to avoid cathode scale buildup and blockage; it also utilizes the acid generated by the anolyte reaction to reduce the alkalinity of the water to be treated, thereby reducing the tendency for downstream scale buildup. Furthermore, Chinese invention patent CN113754150A discloses a stable high-hardness water electrochemical hardening system and treatment process. Through a cathode effluent recirculation process design, the crystallization process within the diaphragm electrochemical hardening device is transferred outside the device, effectively reducing the risk of scale buildup and blockage inside the device. This achieves efficient and stable operation of the diaphragm electrochemical hardening device, with a single stable operation time of over 13 days for the diaphragm reactor.

[0005] To address the high energy consumption issue of diaphragm electrochemical hardening technology, Chinese invention patent CN113666547A discloses a low-energy-consumption dual-electrode induction diaphragm electrolysis circulating water descaling and scale inhibition device. This device combines direct electrode electrolysis and induction electrode electrolysis by embedding an induction electrode between the anode and cathode. Simultaneously, a PTFE membrane isolates the anode and cathode from the induction electrode, constructing a total of four chambers: an anode chamber, a cathode chamber, an induction anode chamber, and an induction cathode chamber. This significantly reduces the energy consumption for acid and alkali production during electrolysis. A major reason for the high energy consumption of diaphragm electrochemical hardening reactors is the introduction of the diaphragm. Besides the increased inter-electrode resistance caused by the diaphragm itself, its introduction also increases the distance between the anode and cathode electrodes, leading to increased liquid junction resistance, ultimately resulting in higher tank pressure and increased energy consumption. Based on this, Chinese invention patent CN118026426A discloses a membrane-free, high-efficiency electrochemical coupling process for descaling. This invention uses a conductive cathode porous filter element as the electrochemical descaling cathode, and connects it to a scale separation system via a water pump through the top of the cathode porous filter element. The water pump draws in high-concentration OH groups near the cathode porous filter element. - The water-based method eliminates the need for membrane materials, thus achieving high-efficiency H2O. + and OH - The separation of these components greatly facilitates the removal of hardness ions from water. This invention not only improves the stability of the process but also reduces energy consumption and material investment costs to some extent, thus enhancing economic efficiency.

[0006] It is evident that existing diaphragm electrochemical softening technology has made significant progress in terms of stability and economy. However, compared to traditional chemical softening technologies, it still lacks a clear advantage in terms of operating energy consumption and equipment investment. A key factor contributing to this problem is the electrode material, especially the anode material containing precious metals. Its high price not only limits the overall investment cost of the equipment, but the limited electrode quantity also forces designers to increase the electrode current density to improve the processing capacity of a single unit, thus shortening the electrode lifespan and increasing equipment energy consumption. Furthermore, in some circulating water treatment scenarios, the presence of corrosive ions such as fluoride ions in the treated water often significantly reduces the lifespan of precious metal electrodes, indirectly causing unplanned equipment downtime and increasing material replacement and maintenance costs. Therefore, improving existing technologies from the perspective of electrode materials to enhance the economics of this type of technology remains crucial. Summary of the Invention

[0007] To address the issues of high equipment investment and unstable processes in complex water systems containing corrosive ions in diaphragm electrochemical softening technology, this invention, based on a stable high-hardness water electrochemical hardening removal system and treatment process disclosed in patent CN113754150A, further improves the electrochemical softening system from the perspective of electrode materials, providing a diaphragm electrochemical softening system and method based on nickel-chromium-iron-based electrodes.

[0008] In a first aspect, the present invention provides a membrane-based electrochemical softening system based on a nickel-chromium-iron electrode, which is achieved by the following technical solution.

[0009] An electrochemical softening system based on nickel-chromium-iron-based electrodes using a diaphragm method includes an electrochemically enhanced crystallization unit, a solid-liquid separation unit, and a nickel-chromium-iron-based electrode diaphragm electrolysis reactor.

[0010] The electrochemically enhanced crystallization unit includes an electrochemically enhanced crystallization anode and an electrochemically enhanced crystallization cathode; a high-hardness water inlet is connected to the electrochemically enhanced crystallization unit, and the softened water outlet of the electrochemically enhanced crystallization unit is connected to the primary solid-liquid separation module of the solid-liquid separation unit.

[0011] The solid-liquid separation unit includes a primary solid-liquid separation module, an electrochemical yellowing removal reactor, and a secondary solid-liquid separation module; the outlet of the primary solid-liquid separation module is connected to the anode chamber and cathode chamber of the iron-chromium-nickel-based electrode membrane electrolysis reactor, respectively.

[0012] The iron-chromium-nickel based electrode membrane electrolysis reactor is divided into an anode chamber and a cathode chamber by a microporous membrane. The anode chamber is equipped with a membrane electrolysis anode, and the cathode chamber is equipped with a membrane electrolysis cathode. The outlet of the cathode chamber is connected to the inlet of high-hardness water. The inlet of the anode chamber is also connected to the inlet of high-hardness water through a pipeline. The outlet of the anode chamber is connected to the electrochemical yellowing removal reactor, and the outlet of the electrochemical yellowing removal reactor is connected to the secondary solid-liquid separation module.

[0013] Furthermore, both the diaphragm electrolytic anode and the diaphragm electrolytic cathode are made of an iron-chromium-nickel-based alloy, comprising iron, chromium, nickel, molybdenum, and impurity elements. The mass fractions of iron, chromium, nickel, and molybdenum are 45–88%, 9–25%, 3–25%, and ≤3.5%, respectively, with the remainder being impurity elements. Preferably, the mass fractions of iron, chromium, nickel, and molybdenum are 68.5%, 23%, 5%, and 3%, respectively, with the remaining 0.5% being impurities.

[0014] Furthermore, the microporous membrane is an ion exchange membrane, a microporous ceramic membrane, a microporous plastic membrane, a wood fiber membrane, or an asbestos membrane, preferably a microporous plastic membrane.

[0015] Furthermore, the cathode and anode chambers of the chromium-nickel based electrode membrane electrolysis reactor both have a bottom-in, top-out flow direction.

[0016] Furthermore, both the electrochemically enhanced crystallization anode and the electrochemically enhanced crystallization cathode are made of carbon steel electrodes.

[0017] Furthermore, the primary solid-liquid separation module and the secondary solid-liquid separation module are selected from sedimentation tanks, filtration devices, or a combination of both, with filtration devices being preferred for the primary and secondary solid-liquid separation modules; the electrochemical yellowing removal reactor is equipped with an electrochemical yellowing removal anode and an electrochemical yellowing removal cathode, both of which are made of carbon steel.

[0018] Furthermore, both the cathode chamber and the anode chamber of the iron-chromium-nickel based electrode diaphragm electrolytic reactor are connected to the pickling tank via pipelines.

[0019] Furthermore, the bottom of the electrochemically enhanced crystallization reactor and the bottom of the secondary solid-liquid separation module are connected to the primary solid-liquid separation module via pipelines.

[0020] Furthermore, the system also includes a PLC automation control system to enable near- and remote control of process parameters.

[0021] Secondly, the present invention provides a method for softening water using a membrane electrochemical softening system based on iron-chromium-nickel-based electrodes, which is achieved by the following technical solution.

[0022] A method for softening water using a membrane electrochemical softening system based on a nickel-chromium-nickel-based electrode, comprising the following steps:

[0023] S1. High-hardness water is mixed with the cathode effluent of the iron-chromium-nickel-based electrode membrane electrolysis reactor. After the pH value of the mixed water is controlled to be above 9.8, it enters the electrochemical enhanced crystallization unit for accelerated crystallization, and the resulting alkaline softened water containing scale crystals with a total hardness of 0-2.0 mmol / L is obtained.

[0024] S2. The alkaline softened water containing scale crystals is sent to the first-stage solid-liquid separation module to obtain alkaline softened clean water, and the turbidity is controlled below 20 NTU;

[0025] S3. Alkaline softened water is refluxed into the cathode and anode chambers of the iron-chromium-nickel-based electrode membrane electrolysis reactor for electrolysis treatment. At the same time, some hard water is mixed into the anode chamber. The pH value of the cathode chamber effluent is controlled between 10.5 and 13 by adjusting the electrolysis current, the flow rate of alkaline softened water and hard water, and the pH value of the anode chamber effluent is controlled above 5.5.

[0026] S4. The effluent from the cathode chamber of the iron-chromium-nickel-based electrode membrane electrolysis reactor is returned and mixed with high-hardness water before entering the electrochemical enhanced crystallization unit; the effluent from the anode enters the electrochemical yellowing removal reactor, and after yellowing removal, it enters the secondary solid-liquid separation module for further solid-liquid separation to complete the softening treatment.

[0027] Furthermore, the cathode and anode of the iron-chromium-nickel based electrode membrane electrolysis reactor periodically switch polarities, while simultaneously switching their respective effluent flow direction and flow rate.

[0028] Furthermore, the cathode and anode of the electrochemical enhanced crystallization unit periodically switch polarities; the minimum electrolysis current of the electrochemical yellowing removal reactor is set to ensure complete yellowing removal from the softened water, and the cathode and anode of the electrochemical yellowing removal reactor periodically switch polarities.

[0029] Furthermore, when the molar ratio of inorganic carbon to calcium hardness in hard water is less than 0.8, inorganic carbon is added to the hard water to adjust the molar ratio of inorganic carbon to calcium hardness to the range of 0.8 to 1.3; the inorganic carbon is selected from carbon dioxide, water-soluble carbonates or bicarbonates.

[0030] Thirdly, the present invention provides an application of a membrane-based electrochemical softening system based on a nickel-chromium-iron electrode, which is achieved by the following technical solution.

[0031] The above-mentioned membrane electrochemical softening system based on iron-chromium-nickel electrodes is applied in the softening treatment of circulating cooling water, nanofiltration concentrate, reverse osmosis concentrate, and easily scale-forming water rich in calcium and magnesium hardness ions.

[0032] Furthermore, it is suitable for softening corrosive, scaling-prone water containing fluoride and chloride ions.

[0033] This application has the following beneficial effects.

[0034] This invention uses iron-chromium-nickel-based electrodes in the electrodes of a membrane electrochemical softening reactor, which not only reduces the overall investment and operating costs of this technology from a material perspective, but also reduces the risk of abnormally shortened lifespan or failure of precious metal anodes caused by unstable water quality, thus helping to further improve the controllability and operational stability of the technology. In addition, the low cost and controllability of this type of electrode material allows for flexible adjustment of the electrode input, thereby effectively reducing the operating tank pressure and consequently reducing energy consumption and operating costs. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of the present invention.

[0036] The system comprises: 1. Electrochemically enhanced crystallization unit; 2. Solid-liquid separation unit; 3. Iron-chromium-nickel based electrode diaphragm electrolysis reactor; 101. Electrochemically enhanced crystallization anode; 102. Electrochemically enhanced crystallization cathode; 103. Electrochemically enhanced crystallization power source; 104. High-hardness water inlet; 201. Primary solid-liquid separation module; 202. Secondary solid-liquid separation module; 203. Electrochemical yellowing removal power source; 204. Electrochemical yellowing removal reactor; 205. Electrochemical yellowing removal anode; 206. Electrochemical yellowing removal cathode; 207. Softened product water outlet; 301. Diaphragm electrolysis anode; 302. Anode chamber; 303. Microporous diaphragm; 304. Diaphragm electrolysis cathode; 305. Cathode chamber; 306. Diaphragm electrolysis power source; 307. Acid washing tank. Detailed Implementation

[0037] The present patent application will be further described below with reference to the embodiments.

[0038] like Figure 1 As shown, a membrane electrochemical softening system based on iron-chromium-nickel-based electrodes includes an electrochemically enhanced crystallization unit 1, a solid-liquid separation unit 2, and an iron-chromium-nickel-based electrode membrane electrolysis reactor 3.

[0039] Among them, the high-hardness water inlet 104 of the electrochemical enhanced crystallization unit 1 is the water inlet. The softened water effluent from the electrochemical enhanced crystallization unit 1 enters the first-stage solid-liquid separation module 201. The filtered alkaline softened water flows back into the iron-chromium-nickel-based electrode membrane electrolysis reactor 3. The effluent from the cathode chamber 305 of the iron-chromium-nickel-based electrode membrane electrolysis reactor 3 flows back into the electrochemical enhanced crystallization unit 1 after converging with the high-hardness water inlet. The effluent from the anode chamber 302 of the iron-chromium-nickel-based electrode membrane electrolysis reactor 3 is electrolyzed by the electrochemical yellowing removal reactor 204 and then enters the second-stage solid-liquid separation module 202 for filtration. After removing the hydroxides of iron, chromium, and nickel, the softening treatment is completed. The softened clean water discharged from the softened water outlet 207 is used downstream.

[0040] The diaphragm electrolytic anode 301 and the diaphragm electrolytic cathode 304 are both made of iron-chromium-nickel-based alloy, and the constituent elements include iron, chromium, nickel, molybdenum and other impurity elements. The mass fractions of iron, chromium, nickel and molybdenum are 68.5%, 23%, 5% and 3% respectively, and the other 0.5% is impurity.

[0041] The cathode chamber 305 and anode chamber 302 of the iron-chromium-nickel based electrode membrane electrolysis reactor 3 both have a bottom-in, top-out flow direction.

[0042] The microporous diaphragm 303 of the iron-chromium-nickel based electrode membrane electrolysis reactor 3 is a microporous PTFE plastic diaphragm with an average pore size controlled at 0.5 μm.

[0043] Both the electrochemically enhanced crystallization anode 101 and the electrochemically enhanced crystallization cathode 102 of the electrochemically enhanced crystallization unit 1 are made of carbon steel electrodes.

[0044] The solid-liquid separation unit includes a primary solid-liquid separation module 201, an electrochemical yellowing removal reactor 204, and a secondary solid-liquid separation module 202. The primary solid-liquid separation module 201 uses filtration separation with a filtration accuracy of 1 μm; both the electrochemical yellowing removal cathode 206 and the electrochemical yellowing removal anode 205 are made of carbon steel; the secondary solid-liquid separation module 202 uses filtration separation with a filtration accuracy of 0.5 μm.

[0045] The process method for softening high-hardness industrial circulating cooling water using a membrane electrochemical softening system based on iron-chromium-nickel electrodes includes the following steps:

[0046] 1) High-hardness industrial circulating cooling water and the effluent from the cathode chamber 305 of the iron-chromium-nickel-based electrode membrane electrolysis reactor 3 are mixed, and the pH value of the mixed water is controlled above 9.8. The mixture enters the electrochemical enhanced crystallization unit 1 for accelerated crystallization treatment to obtain alkaline softened water containing scale crystals with a total hardness of 0-2.0 mmol / L.

[0047] 2) The alkaline softened water containing scale crystals is treated by the first-stage solid-liquid separation module 201 to obtain alkaline softened clean water with turbidity controlled below 20 NTU;

[0048] 3) Alkaline softened water is refluxed into the cathode chamber 305 and anode chamber 302 of the iron-chromium-nickel based electrode membrane electrolysis reactor 3 for electrolysis treatment. At the same time, some hard water and the inlet water of the anode chamber 302 are mixed and enter the anode chamber 302. By adjusting the electrolysis current, the flow rate of alkaline softened water and hard water, the pH value of the effluent from the cathode chamber 305 is controlled between 10.5 and 13, and the pH value of the effluent from the anode chamber 302 is controlled above 5.5.

[0049] 4) The effluent from the cathode chamber 305 of the iron-chromium-nickel based electrode membrane electrolysis reactor 3 is returned and mixed with high-hardness water to enter the electrochemical enhanced crystallization unit 1. The effluent from the anode chamber 302 enters the electrochemical yellowing removal reactor 204. After treatment, it enters the secondary solid-liquid separation module 202 for further solid-liquid separation to complete the softening treatment.

[0050] The diaphragm electrolysis cathode 304 and diaphragm electrolysis anode 301 periodically switch polarity via the diaphragm electrolysis power supply 306, while simultaneously switching their respective effluent flow direction and flow rate, with a switching cycle of 24 hours.

[0051] The electrochemically enhanced crystallization cathode 102 and electrochemically enhanced crystallization anode 101 are periodically switched in polarity by the electrochemically enhanced crystallization power supply 103, with a switching cycle of 1 hour.

[0052] The electrochemical yellowing removal cathode 206 and electrochemical yellowing removal anode 205 are periodically switched in polarity by the electrochemical yellowing removal power supply 203, with a switching cycle of 24 hours.

[0053] The cathode chamber 305 and anode chamber 302 of the iron-chromium-nickel based electrode diaphragm electrolysis reactor 3 are periodically cleaned using a hydrochloric acid aqueous solution containing corrosion inhibitors in pickling tank 307. The cleaning cycle is 7 days, and each cleaning session lasts 3-6 hours. The pH of the hydrochloric acid aqueous solution containing corrosion inhibitors is controlled below 2. The corrosion inhibitor used is an aniline-hexamethylenetetramine corrosion inhibitor system, with an aniline concentration of 2 g / L and a hexamethylenetetramine concentration of 3 g / L.

[0054] During operation, the alkaline softened mud water at the bottom of the electrochemical enhanced crystallization unit 1 and the non-alkaline softened mud water at the bottom of the secondary solid-liquid separation module 202 are periodically sent to the primary solid-liquid separation module 201 for solid-liquid separation.

[0055] Example 1

[0056] The physicochemical parameters of the circulating cooling water in a chemical plant are shown in Table 1. The softening treatment was carried out using the method of this invention, and a long-term performance stability study was conducted. The molar ratio of inorganic carbon to calcium hardness calculated from Table 1 is 1.05, close to 1, therefore no sodium carbonate supplementation is required. The control parameters of the electrochemical hardening process are shown in Table 2. The long-term operation results are shown in Table 3. It can be seen that, under the premise of ensuring stable operation of the diaphragm electrochemical softening, the investment cost of the equipment is nearly half that of the diaphragm electrochemical softening equipment based on precious metal electrodes, demonstrating significant technical and economic benefits.

[0057] Table 1 Physicochemical parameters of circulating cooling water in a chemical plant

[0058]

[0059] Table 2 Control parameters of electrochemical hardening process for circulating cooling water in a chemical enterprise

[0060]

[0061]

[0062] Table 3. Comparison of Electrochemical Hardening Effect and Economic Efficiency of Circulating Cooling Water in a Chemical Enterprise

[0063]

[0064] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A membrane-based electrochemical softening system based on a nickel-chromium-iron electrode, characterized in that: It includes an electrochemically enhanced crystallization unit (1), a solid-liquid separation unit (2), and an iron-chromium-nickel based electrode membrane electrolysis reactor (3). The electrochemically enhanced crystallization unit (1) includes an electrochemically enhanced crystallization anode (101) and an electrochemically enhanced crystallization cathode (102); the electrochemically enhanced crystallization unit (1) is connected to a high-hardness water inlet (104), and the softened water outlet of the electrochemically enhanced crystallization unit (1) is connected to the first-stage solid-liquid separation module (201) of the solid-liquid separation unit (2); The solid-liquid separation unit (2) includes a primary solid-liquid separation module (201), an electrochemical yellowing removal reactor (204), and a secondary solid-liquid separation module (202); the outlet of the primary solid-liquid separation module (201) is connected to the anode chamber (302) and the cathode chamber (305) of the iron-chromium-nickel based electrode membrane electrolysis reactor (3), respectively. The iron-chromium-nickel based electrode membrane electrolysis reactor (3) is divided into an anode chamber (302) and a cathode chamber (305) by a microporous membrane (303). The anode chamber (302) is equipped with a membrane electrolysis anode (301), and the cathode chamber (305) is equipped with a membrane electrolysis cathode (304). The outlet of the cathode chamber (305) is connected to the high hardness water inlet (104). The inlet of the anode chamber (302) is also connected to the high hardness water inlet (104) through a pipeline. The outlet of the anode chamber (302) is connected to the electrochemical yellowing removal reactor (204), and the outlet of the electrochemical yellowing removal reactor (204) is connected to the secondary solid-liquid separation module (202). The diaphragm electrolytic anode (301) and diaphragm electrolytic cathode (304) are both made of iron-chromium-nickel-based alloy, and the constituent elements include iron, chromium, nickel, molybdenum and impurity elements. The mass fractions of iron, chromium, nickel and molybdenum are 45-88%, 9-25%, 3-25% and ≤3.5% respectively, and the balance is impurity elements.

2. The membrane-based electrochemical softening system based on a nickel-chromium-iron electrode according to claim 1, characterized in that: The microporous membrane (303) is an ion exchange membrane, a microporous ceramic membrane, a microporous plastic membrane, a wood fiber membrane, or an asbestos membrane.

3. The membrane-based electrochemical softening system based on a nickel-chromium-iron electrode according to claim 1, characterized in that: The cathode chamber (305) and anode chamber (302) of the chromium-nickel based electrode membrane electrolysis reactor (3) both flow in a bottom-in, top-out direction.

4. The membrane-based electrochemical softening system based on a nickel-chromium-iron electrode according to claim 1, characterized in that: Both the electrochemically enhanced crystallizing anode (101) and the electrochemically enhanced crystallizing cathode (102) are made of carbon steel electrodes.

5. The membrane-based electrochemical softening system based on a nickel-chromium-iron electrode according to claim 1, characterized in that: The primary solid-liquid separation module (201) and the secondary solid-liquid separation module (202) are selected from sedimentation tanks, filtration devices or a combination of the two; the electrochemical yellowing removal reactor (204) is equipped with an electrochemical yellowing removal anode (205) and an electrochemical yellowing removal cathode (206), both of which are made of carbon steel.

6. A method for softening water using a membrane electrochemical softening system based on iron-chromium-nickel-based electrodes, characterized in that: Using the membrane electrochemical softening system based on nickel-chromium-iron electrodes as described in any one of claims 1-5, the water softening method includes the following steps: S1. The high-hardness water is mixed with the cathode effluent of the iron-chromium-nickel-based electrode membrane electrolysis reactor (3), and the pH value of the mixed water is controlled to be above 9.8 before entering the electrochemical enhanced crystallization unit (1) for accelerated crystallization. The resulting alkaline softened water containing scale crystals with a total hardness of 0-2.0 mmol / L is obtained. S2. The alkaline softened water containing scale crystals is sent to the first-stage solid-liquid separation module (201) to obtain alkaline softened clean water, and the turbidity is controlled below 20 NTU; S3. Alkaline softened water is refluxed into the cathode chamber (305) and anode chamber (302) of the iron-chromium-nickel-based electrode membrane electrolysis reactor (3) for electrolysis treatment. At the same time, some high-hardness water is mixed into the anode chamber (302). The pH value of the cathode chamber outlet water is controlled between 10.5 and 13 by adjusting the electrolysis current, the flow rate of alkaline softened water and high-hardness water, and the pH value of the anode chamber outlet water is controlled above 5.

5. S4. The effluent from the cathode chamber (305) of the iron-chromium-nickel based electrode membrane electrolysis reactor (3) is returned to be mixed with high-hardness water and enters the electrochemical enhanced crystallization unit (1); the effluent from the anode enters the electrochemical yellowing removal reactor (204), and after yellowing removal, it enters the secondary solid-liquid separation module (202) for further solid-liquid separation to complete the softening treatment.

7. The method for softening water using a membrane electrochemical softening system based on an iron-chromium-nickel-based electrode according to claim 6, characterized in that: The cathode and anode of the iron-chromium-nickel based electrode membrane electrolysis reactor (3) periodically switch polarities, while simultaneously switching their respective effluent flow direction and flow rate.

8. The method for softening water using a membrane electrochemical softening system based on an iron-chromium-nickel-based electrode according to claim 6, characterized in that: The cathode and anode of the electrochemical enhanced crystallization unit (1) periodically switch polarities; the minimum electrolysis current of the electrochemical yellowing removal reactor (204) is set to ensure thorough yellowing removal of softened water, and the cathode and anode of the electrochemical yellowing removal reactor (204) periodically switch polarities.

9. The method for softening water using a membrane electrochemical softening system based on an iron-chromium-nickel-based electrode according to claim 6, characterized in that: When the molar ratio of inorganic carbon to calcium hardness in hard water is less than 0.8, inorganic carbon is added to the hard water to adjust the molar ratio of inorganic carbon to calcium hardness to the range of 0.8 to 1.3; the inorganic carbon is selected from carbon dioxide, water-soluble carbonates or bicarbonates.

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