Diaphragm method electrochemical softening system and method based on nickel-chromium-iron-based electrode

Abstract

The invention discloses a diaphragm method electrochemical softening system and method based on a nickel-chromium-iron-based electrode. The softening system comprises an electrochemical enhanced crystallization unit, an iron-chromium-nickel-based electrode diaphragm electrolysis reactor and a solid-liquid separation unit. The softening process method comprises the following steps: mixing high-hardness water with cathode effluent of the diaphragm electrolysis reactor, controlling the pH value to be greater than 9.8, and then entering the electrochemical enhanced crystallization unit for accelerated crystallization; treating the obtained softened water containing the scale crystals through a first-stage solid-liquid separation module to obtain alkaline softened clear water; refluxing the wastewater into a diaphragm electrolysis reactor for electrolytic treatment, and controlling the pH value of effluent in a cathode chamber to be 10.5-13 and the pH value of effluent in an anode chamber to be greater than 5.5; and the effluent of the cathode chamber returns to be mixed with high-hardness water and enters an electrochemical enhanced crystallization unit, and the effluent of the anode enters an electrochemical yellow removal module, is subjected to electrolytic treatment and then enters a secondary solid-liquid separation module for solid-liquid separation, so that softening treatment is completed. According to the invention, the energy consumption and the operation cost can be reduced, and the controllability and the operation stability are improved.

Description

Technical Field

[0001] The invention relates to the technical field of electrochemical hardness removal, and in particular to a diaphragm electrochemical softening system and method based on nickel-chromium-iron based electrodes. Background Art

[0002] As early as 1982, there were reports of relevant literature on the use of electrochemical means to soften water containing hardness ions. Subsequently, researchers in Germany, Israel and other countries began to use this technology for the softening treatment of hard industrial water. Since 2002, this technology has been introduced from Israel to China, and related literature and patent reports have increased year by year. At present, this technology has been applied to a certain extent in engineering practice. Compared with the traditional lime and soda softening method, the electrochemical water softening technology has the advantages of eliminating the need for pre-adjusting alkali and adding acid to adjust the pH value, no need or as little introduction of new ions into the water, less waste residue, small footprint, flexible operation, and easy automation control, and has high economic and environmental benefits. In addition, compared with the traditional chemical dosing method, electromagnetic technology, and ultrasonic anti-scaling technology, the advantage of electrochemical hardness removal technology is that it can remove the scale-forming ions in the water, thereby increasing the concentration multiple and achieving the purpose of water saving.

[0003] At present, electrochemical hardness removal technology mainly includes two types: direct electrolytic hardness removal technology and diaphragm electrochemical hardness removal technology. The former mainly promotes the deposition of calcium and magnesium ions in the form of carbonate or hydroxide on the cathode wall through the cathode reaction at the cathode / water interface, and then regularly cleans the cathode wall scale by physical scraping, chemical pickling or electrochemical reversal, eliminates the voltage drop caused by the scale layer, and restores the electrode activity, so as to maintain a constant hardness removal capacity and processing energy consumption. At present, this type of technology has been widely used in some small and medium-sized water systems. Despite this, the current efficiency of this technology is low, generally between 5% and 25%, which greatly limits the application of this type of technical equipment in large water systems. Unlike direct electrolytic hardness removal technology, diaphragm electrochemical hardness removal technology improves current efficiency by adding an ion conductive diaphragm between the cathode and the anode to inhibit the neutralization of hydroxide ions and hydrogen ions produced by the cathode and the anode. Relevant literature and public patents (Desalination, 2010, 263 (1-3): 285-289; CN111170424A, CN107235564A, CN113754150A, CN117466489A) show that the current efficiency of the membrane electrochemical hardness removal technology can reach more than 70%, and the hardness removal effect is significantly improved. In addition, the electrolysis current density used in this technology can be increased by more than 5 times compared with the previous technology. These two advantages can significantly improve the electrode utilization rate and the integration of the equipment, and reduce the equipment footprint to meet the hardness removal needs of large water systems.

[0004] At present, the main problems hindering the large-scale application of diaphragm electrochemical hardness removal technology include poor process stability, high operating energy consumption and high equipment investment cost. The problem of poor process stability is mainly caused by scaling and blockage inside the reactor and subsequent pipeline equipment, resulting in equipment shutdown. The Chinese invention patent with publication number CN106277369A discloses a scale inhibition method, in which non-scaling conductive water is used in the cathode chamber to avoid the problem of cathode scaling and blockage; the acid generated by the anode reaction is used to reduce the alkalinity in the water body to be treated, thereby reducing the downstream scaling tendency. In addition, the Chinese invention patent with publication number CN113754150A discloses a stable high-hardness water electrochemical hardness removal system and treatment process method. The crystallization process in the diaphragm electrochemical hardness removal device is transferred to the outside of the device through the process design of cathode effluent reflux, which effectively reduces the risk of scaling and blockage inside the device, and realizes the efficient and stable operation of the diaphragm electrochemical hardness removal device. The single stable operation time of the diaphragm reactor reaches more than 13 days.

[0005] In response to the high energy consumption of diaphragm electrochemical hardness removal technology, a Chinese invention patent with publication number CN113666547A discloses a low-energy dual-electrode induction diaphragm electrolysis circulating water descaling and anti-scaling device, which realizes the combination of direct electrode electrolysis and induction electrode electrolysis by embedding an induction electrode between the positive and negative electrodes. At the same time, a PTFE membrane is used to isolate the positive and negative electrodes from the induction electrodes, and a total of four chambers, namely, an anode chamber, a cathode chamber, an induction anode chamber and an induction cathode chamber, are constructed, which significantly reduces the energy consumption of electrolysis acid and alkali production. One of the main reasons for the energy consumption of the diaphragm electrochemical hardness removal reactor is the introduction of the diaphragm. In addition to the increase in interelectrode resistance caused by the diaphragm itself, its introduction also increases the distance between the positive and negative electrodes, resulting in an increase in liquid junction resistance, which ultimately increases the equipment tank pressure and energy consumption. Based on this, the Chinese invention patent with publication number CN118026426A discloses a membrane-free high-efficiency electrochemical coupling process descaling system and method. The invention uses a conductive cathode porous filter element as an electrochemical descaling cathode, and connects it to the scale separation system through a water pump through the top of the cathode porous filter element. The water pump is used to pump the high-concentration OH near the cathode porous filter element. - The water method eliminates the need for diaphragm materials and can achieve high efficiency H + and OH - The invention not only improves the stability of the process, but also reduces energy consumption and material investment costs to a certain extent, thus improving economic efficiency.

[0006] It can be seen that the existing diaphragm electrochemical softening technology has made great progress in stability and economy. Despite this, compared with the traditional chemical softening treatment technology, this technology still does not have obvious advantages in terms of operating energy consumption and equipment investment. Among them, a key factor causing this problem is the electrode material, especially the anode material containing precious metals. Its expensive price not only limits the overall investment cost of the equipment, but the limited electrode input also forces designers to increase the electrode current density to increase the processing capacity of a single device, which in turn shortens the life of the electrode and increases the energy consumption of the equipment. In addition, in some circulating water treatment scenarios, since the treated water often contains some corrosive ions such as fluoride ions, the service life of precious metal electrodes is often greatly reduced, indirectly causing unplanned shutdown of equipment and increasing the cost of material replacement and maintenance. Therefore, how to improve the existing technology from the perspective of electrode materials and improve the economy of such technology is still very critical. Summary of the invention

[0007] In order to solve the problems of large equipment investment in diaphragm electrochemical softening technology and unstable process in complex water quality systems containing corrosive ions, the present invention further improves the electrochemical softening system from the perspective of electrode materials on the basis of a stably operating high-hardness water electrochemical dehardening system and treatment process method disclosed in patent CN113754150A, and provides a diaphragm electrochemical softening system and method based on nickel-chromium-iron based electrodes.

[0008] In a first aspect, the present invention provides a diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes, which is realized by adopting the following technical scheme.

[0009] A diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes, comprising an electrochemical strengthening crystallization unit, a solid-liquid separation unit, and an iron-chromium-nickel based electrode diaphragm electrolysis reactor;

[0010] The electrochemically enhanced crystallization unit comprises an electrochemically enhanced crystallization anode and an electrochemically enhanced crystallization cathode; the electrochemically enhanced crystallization unit is connected to a high hardness water inlet, and the softened water outlet of the electrochemically enhanced crystallization unit is connected to a 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 water outlet of the primary solid-liquid separation module is respectively connected to the anode chamber and cathode chamber of the iron-chromium-nickel based electrode diaphragm electrolysis reactor;

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

[0013] Furthermore, the diaphragm electrolysis anode and the diaphragm electrolysis cathode are made of iron-chromium-nickel based alloy, and the constituent elements include iron, chromium, nickel, molybdenum and impurity elements, wherein the mass fractions of iron, chromium, nickel and molybdenum are 45-88%, 9-25%, 3-25% and ≤3.5% respectively, and the remainder is impurity elements. Preferably, the mass fractions of iron, chromium, nickel and molybdenum are 68.5%, 23%, 5% and 3% respectively, and the other 0.5% is 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 flow directions of the cathode chamber and the anode chamber of the chromium-nickel-based electrode diaphragm electrolysis reactor are both bottom-in and top-out.

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

[0017] Furthermore, the primary solid-liquid separation module and the secondary solid-liquid separation module select a sedimentation tank, a filtration device or a combination of the two, and the primary solid-liquid separation module and the secondary solid-liquid separation module preferably use a filtration device; the electrochemical yellowing removal reactor is provided with an electrochemical yellowing removal anode and an electrochemical yellowing removal cathode, and the electrochemical yellowing removal anode and the electrochemical yellowing removal cathode are both made of carbon steel.

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

[0020] Furthermore, the system also includes a PLC automatic control system to achieve local and remote control of process parameters.

[0021] In a second aspect, the present invention provides a method for softening water using a diaphragm electrochemical softening system based on an iron-chromium-nickel based electrode, which is achieved by adopting the following technical solution.

[0022] A method for softening water using a diaphragm electrochemical softening system based on an iron-chromium-nickel-based electrode, using the diaphragm electrochemical softening system based on a nickel-chromium-iron-based electrode according to any one of claims 1 to 6, the method for softening water comprising the following steps:

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

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

[0025] S3. The alkaline softened water is refluxed into the cathode chamber and anode chamber of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor for electrolysis treatment, and part of the high hardness water is mixed into the anode chamber, and the pH value of the cathode chamber effluent is controlled between 10.5-13 by adjusting the electrolysis current, alkaline softened water and high hardness water flow, and the pH value of the anode chamber effluent is controlled to be above 5.5;

[0026] S4. The effluent from the cathode chamber of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor is returned and mixed with high-hardness water to enter the electrochemical enhanced crystallization unit; the effluent from the anode enters the electrochemical yellowing removal reactor, and after yellowing, 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 diaphragm electrolysis reactor periodically switch polarity and simultaneously switch the respective effluent flow directions and flow rates.

[0028] Furthermore, the cathode and anode of the electrochemical enhanced crystallization unit periodically switch polarity; the minimum electrolysis current of the electrochemical yellowing removal reactor is set based on the standard of softening the produced water to completely remove the yellowing, and the cathode and anode of the electrochemical yellowing removal reactor periodically switch polarity.

[0029] Furthermore, when the molar ratio of inorganic carbon to calcium hardness in high-hardness water is less than 0.8, inorganic carbon is added to the high-hardness 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 carbonate or bicarbonate.

[0030] In a third aspect, the present invention provides a use of a diaphragm electrochemical softening system based on a nickel-chromium-iron based electrode, which is achieved by adopting the following technical solution.

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

[0032] Furthermore, it is suitable for softening the scale-forming water containing fluoride ions and chloride ions which is corrosive.

[0033] This application has the following beneficial effects.

[0034] The present invention uses iron-chromium-nickel based electrodes for the electrodes of the diaphragm electrochemical softening reactor, which not only reduces the overall investment cost and operating cost of this type of technology from the material perspective; it can also reduce the risk of abnormal shortening or failure of the precious metal anode due to unstable water quality, which helps to further improve the controllability and operating stability of the technology; in addition, the low-cost and controllable characteristics of this type of electrode material are conducive to flexible regulation of the electrode input amount, thereby effectively reducing the operating cell pressure, and further reducing energy consumption and operating costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a structural schematic diagram of the present invention.

[0036] Among them, 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 supply; 104. high hardness water inlet; 201. primary solid-liquid separation module; 202. secondary solid-liquid separation module; 203. electrochemical yellowing removal power supply; 204. electrochemical yellowing removal reactor; 205. electrochemical yellowing removal anode; 206. electrochemical yellowing removal cathode; 207. softened water outlet; 301. diaphragm electrolysis anode; 302. anode chamber; 303. microporous diaphragm; 304. diaphragm electrolysis cathode; 305. cathode chamber; 306. diaphragm electrolysis power supply; 307. pickling tank. DETAILED DESCRIPTION

[0037] The present patent application is further described below in conjunction with embodiments.

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

[0039] Among them, the high hard water inlet 104 of the electrochemically enhanced crystallization unit 1 is the water inlet, the softened water outlet of the electrochemically enhanced crystallization unit 1 enters the primary solid-liquid separation module 201, the filtered alkaline softened water flows back into the iron-chromium-nickel-based electrode diaphragm electrolysis reactor 3, the effluent reflux of the cathode chamber 305 of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor 3 and the high hard water inlet meet and enter the electrochemically enhanced crystallization unit 1, the effluent of the anode chamber 302 of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor 3 is electrolytically treated in the electrochemical yellowing removal reactor 204 and then enters the secondary solid-liquid separation module 202 for filtration, and the hydroxides of iron, chromium and nickel are intercepted and removed to complete the softening treatment, and the softened clean water discharged from the softened water outlet 207 is supplied to downstream use.

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

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

[0042] The microporous membrane 303 of the iron-chromium-nickel-based electrode membrane electrolysis reactor 3 is a microporous PTFE plastic membrane, and the average pore size of the micropores is controlled at 0.5 um.

[0043] The electrochemically strengthened crystallization anode 101 and the electrochemically strengthened crystallization cathode 102 of the electrochemically strengthened crystallization unit 1 are both 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 adopts a filtering separation method with a filtering accuracy of 1 μm; the electrochemical yellowing removal cathode 206 and the electrochemical yellowing removal anode 205 are both made of carbon steel; the secondary solid-liquid separation module 202 adopts a filtering separation method with a filtering accuracy of 0.5 μm.

[0045] The process for softening high-hardness industrial circulating cooling water by a diaphragm electrochemical softening system based on an iron-chromium-nickel based electrode comprises the following steps:

[0046] 1) High-hardness industrial circulating cooling water and the effluent from the cathode chamber 305 of the Fe-Cr-Ni-based electrode diaphragm electrolysis reactor 3 are mixed, the pH value of the mixed water is controlled to be above 9.8, and the mixed water enters the electrochemical strengthening 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 primary solid-liquid separation module 201 to obtain alkaline softened clean water, and the turbidity is controlled below 20 NTU;

[0048] 3) The alkaline softened clean water flows back into the cathode chamber 305 and the anode chamber 302 of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor 3 for electrolysis treatment, and at the same time, part of the high-hardness water and the inlet water of the anode chamber 302 are mixed and enter the anode chamber 302, and the pH value of the water outlet from the cathode chamber 305 is controlled to be between 10.5 and 13 by adjusting the electrolysis current, the alkaline softened clean water and the high-hardness water flow, and the pH value of the water outlet from the anode chamber 302 is controlled to be above 5.5;

[0049] 4) The effluent from the cathode chamber 305 of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor 3 is returned and mixed with high-hardness water to enter the electrochemical strengthening crystallization unit 1, and the effluent from the anode chamber 302 enters the electrochemical yellowing removal reactor 204, and after treatment, 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 the diaphragm electrolysis anode 301 periodically switch polarity through the diaphragm electrolysis power supply 306, and simultaneously switch the respective water outlet flow direction and flow rate, and the switching cycle is 24 hours.

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

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

[0053] The cathode chamber 305 and the anode chamber 302 of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor 3 are regularly cleaned by using the hydrochloric acid aqueous solution containing the corrosion inhibitor in the pickling tank 307. The cleaning cycle is 7 days, and each cleaning time is 3 to 6 hours. The pH of the hydrochloric acid aqueous solution containing the corrosion inhibitor is controlled below 2. The corrosion inhibitor is an aniline-hexamethylenetetramine corrosion inhibition 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 electrochemically 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 circulating cooling water of a chemical enterprise are shown in Table 1. The method of the present invention was used for softening treatment, and the long-term performance stability was investigated. The molar ratio of inorganic carbon and calcium hardness calculated from Table 1 is 1.05, which is close to 1, so there is no need to add sodium carbonate. The control parameters of the electrochemical hardness removal process are shown in Table 2. The long-term operation effect is shown in Table 3. It can be seen that, under the premise of ensuring the stable operation of the diaphragm electrochemical softening, the investment cost of the equipment is nearly half of that of the diaphragm electrochemical softening equipment based on precious metal electrodes, and the technical and economic benefits are obvious.

[0057] Table 1 Physical and chemical parameters of circulating cooling water quality in a chemical enterprise

[0058]

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

[0060]

[0061]

[0062] Table 3 Comparison of electrochemical hardness removal effect and economic efficiency of circulating cooling water in a chemical enterprise

[0063]

[0064] The embodiments of this specific implementation method are all preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Therefore, any equivalent changes made based on the structure, shape, and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes, characterized in that: It comprises an electrochemically enhanced crystallization unit (1), a solid-liquid separation unit (2), and an iron-chromium-nickel-based electrode diaphragm electrolysis reactor (3); The electrochemically enhanced crystallization unit (1) comprises 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 a primary solid-liquid separation module (201) of the solid-liquid separation unit (2); The solid-liquid separation unit (2) comprises a primary solid-liquid separation module (201), an electrochemical yellowing removal reactor (204) and a secondary solid-liquid separation module (202); the water outlet of the primary solid-liquid separation module (201) is respectively connected to the anode chamber (302) and the cathode chamber (305) of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor (3); The iron-chromium-nickel-based electrode diaphragm electrolysis reactor (3) is divided into an anode chamber (302) and a cathode chamber (305) by a microporous diaphragm (303); a diaphragm electrolysis anode (301) is provided in the anode chamber (302), and a diaphragm electrolysis cathode (304) is provided in the cathode chamber (305); the water outlet of the cathode chamber (305) is connected to the high-hardness water inlet (104); the water inlet of the anode chamber (302) is also connected to the high-hardness water inlet (104) through a pipeline, the water outlet of the anode chamber (302) is connected to the electrochemical yellowing removal reactor (204), and the water outlet of the electrochemical yellowing removal reactor (204) is connected to the secondary solid-liquid separation module (202).

2. The diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes according to claim 1, characterized in that: The diaphragm electrolysis anode (301) and the diaphragm electrolysis cathode (304) are both made of an iron-chromium-nickel based alloy, and the constituent elements include iron, chromium, nickel, molybdenum and impurity elements, wherein the mass fractions of iron, chromium, nickel and molybdenum are 45-88%, 9-25%, 3-25% and ≤3.5% respectively, and the remainder is impurity elements.

3. The diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes 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.

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

5. The diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes according to claim 1, characterized in that: The electrochemically strengthened crystallized anode (101) and the electrochemically strengthened crystallized cathode (102) are both made of carbon steel electrodes.

6. The diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes according to claim 1, characterized in that: The first-level solid-liquid separation module (201) and the second-level 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 provided with an electrochemical yellowing removal anode (205) and an electrochemical yellowing removal cathode (206), and the electrochemical yellowing removal anode (205) and the electrochemical yellowing removal cathode (206) are both made of carbon steel.

7. A method for softening water using a diaphragm electrochemical softening system based on an iron-chromium-nickel based electrode, characterized in that: Using the diaphragm electrochemical softening system based on nickel-chromium-iron based electrodes as described in any one of claims 1 to 6, the method for softening water comprises the following steps: S1. High hardness water is mixed with the cathode effluent of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor (3), and the mixed water is controlled to have a pH value of 9.8 or above and then enters the electrochemically enhanced crystallization unit (1) for accelerated crystallization to obtain alkaline softened water containing scale crystals with a total hardness of 0-2.0 mmol / L; S2. The alkaline softened water containing scale crystals is sent to the primary solid-liquid separation module (201) to obtain alkaline softened water, and the turbidity is controlled below 20NTU; S3. The alkaline softened clean water is refluxed into the cathode chamber (305) and the anode chamber (302) of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor (3) for electrolysis treatment, and part of the high-hardness water is mixed into the anode chamber (302), and the pH value of the cathode chamber outlet water is controlled to be between 10.5 and 13 by adjusting the electrolysis current, the alkaline softened clean water and the high-hardness water flow rate, and the pH value of the anode chamber outlet water is controlled to be above 5.5; S4. The effluent from the cathode chamber (305) of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor (3) is returned and mixed with high-hardness water to enter the electrochemical enhanced crystallization unit (1); the effluent from the anode enters the electrochemical yellowing removal reactor (204), and after yellowing, enters the secondary solid-liquid separation module (202) for further solid-liquid separation to complete the softening treatment.

8. The method for softening water by a diaphragm electrochemical softening system based on an iron-chromium-nickel based electrode according to claim 7, characterized in that: The cathode and anode of the iron-chromium-nickel-based electrode diaphragm electrolysis reactor (3) periodically switch polarity and simultaneously switch the respective outlet water flow directions and flow rates.

9. The method for softening water by a diaphragm electrochemical softening system based on an iron-chromium-nickel based electrode according to claim 7, characterized in that: The cathode and anode of the electrochemical enhanced crystallization unit (1) periodically switch polarity; the minimum electrolysis current of the electrochemical yellowing removal reactor (204) is set based on the standard of completely removing yellow from softened produced water; the cathode and anode of the electrochemical yellowing removal reactor (204) periodically switch polarity.

10. The method for softening water by a diaphragm electrochemical softening system based on an iron-chromium-nickel based electrode according to claim 7, characterized in that: When the molar ratio of inorganic carbon to calcium hardness in high-hardness water is less than 0.8, inorganic carbon is added to the high-hardness 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 carbonate or bicarbonate.

Images