Method and apparatus for extracting calcium sulfate for hardness removal from nanofiltration concentrate

Abstract

The application belongs to the field of sewage treatment, and discloses a method and device for extracting calcium sulfate for hardness removal from nanofiltration concentrated liquid. The device comprises an electro-catalytic reactor, an electro-softening reactor, a crystallizer, a pipeline mixer, a purifier, a dehydration separator and a seed tank. The application can remove the chroma, COD and most of the sulfate hardness in water, reduce the TDS in water, obtain high-purity calcium sulfate, and realize the resource utilization of wastewater.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment, and more specifically, relates to a method and apparatus for extracting calcium sulfate for hardening removal from nanofiltration concentrate. Background Technology

[0002] In today's environmental protection market, membrane separation technology, as a mature wastewater treatment technology, has been widely applied in various industries. Membrane separation technology can be broadly categorized into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis based on its separation performance. Among these, nanofiltration, with its 1-2 nm micropore size and the Donan effect on the membrane surface, exhibits excellent separation and interception capabilities for divalent salts and substances with molecular weights exceeding 1000 Daltons. For industries requiring zero-discharge and resource recovery of wastewater, nanofiltration's unique ability to separate monovalent and divalent salts has become an indispensable treatment unit in zero-discharge resource recovery processes. However, the concentrate produced by nanofiltration is characterized by high COD and high hardness, necessitating a solution for its subsequent treatment.

[0003] Traditional nanofiltration concentrate treatment methods include direct discharge, recirculation, and evaporation. All of these methods have drawbacks. Direct discharge is typically found in coastal areas, where the concentrate is directly discharged into the ocean. Recirculation involves mixing the concentrate with the membrane system's feed water for further concentration, but this method doesn't truly solve the concentrate disposal problem, and the membrane system's processing costs and membrane lifespan are affected over time. Evaporation involves heating and re-concentrating the concentrate in an evaporator, ultimately disposing of it as solidified salts in landfills. Furthermore, in the pharmaceutical, petrochemical, chemical fiber, textile, mining, and industrial park wastewater sectors, the water hardness (sulfate) is relatively high. After nanofiltration, the sulfate concentration in the concentrate can often reach tens of thousands or even higher. This high hardness can cause scaling and pipe blockage during evaporation, making subsequent evaporation treatment difficult. Pre-treatment of the concentrate is often necessary before evaporation, especially for high-hardness concentrates. Technologies such as dual-alkali processes, electrochemical hardening, and advanced oxidation are needed to remove calcium and magnesium hardness and various organic pollutants. The dual-alkali method for hardening introduces more ions during the pretreatment process, resulting in precipitates, which is detrimental to the overall cost control of evaporation.

[0004] In addition, in certain high-hardness water conditions, nanofiltration feed water often needs to be acidified and scale inhibitors added to prevent scaling on the nanofiltration membrane surface, thus avoiding the adverse effects of scaling on membrane performance and lifespan. The low alkalinity of the nanofiltration concentrate and the high phosphate groups after the introduction of scale inhibitors can both affect conventional electrochemical hardness removal methods that use calcium carbonate for calcium removal.

[0005] Therefore, there is an urgent need to propose a new method and apparatus for extracting calcium sulfate from nanofiltration concentrate for hardening removal. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method and apparatus for extracting calcium sulfate from nanofiltration concentrate for hardness removal. This invention can remove color, COD, and most of the hardness sulfate ions from water, reduce TDS, and simultaneously obtain high-purity calcium sulfate, achieving the resource utilization of wastewater.

[0007] To achieve the above objectives, the present invention provides an apparatus for extracting calcium sulfate for hardening removal from nanofiltration concentrate, the apparatus comprising an electrocatalytic reactor, an electrosoftening reactor, a crystallizer, a pipeline mixer, a purifier, a dehydration separator, and a seed crystal tank;

[0008] The middle inlet of the electrocatalytic reactor is connected to the nanofiltration concentrate discharge pipeline, and the upper outlet of the electrocatalytic reactor is connected to the middle inlet of the electrosoftening reactor.

[0009] The pipe connected to the upper outlet of the electric softening reactor and the pipe connected to the outlet of the pipeline mixer are combined and connected to the lower inlet of the crystallizer; the crystallizer is equipped with a constant temperature heating device and a stirring device.

[0010] The pipeline connected to the upper outlet of the crystallizer is divided into two paths, one of which is connected to the pipeline connected to the bottom outlet of the seed tank and then to the dosing port of the pipeline mixer.

[0011] The bottom discharge port of the crystallizer is connected to the lower feed port of the purifier; the pipeline connected to the upper outlet of the seed crystal tank is connected to the top purification and cleaning port of the purifier; a stirring device is installed inside the purifier;

[0012] The bottom discharge port of the purifier is connected to the inlet of the dehydration separator, and the discharge port of the dehydration separator is the collection port for the recovery of refined calcium sulfate.

[0013] According to the present invention, preferably, the bottom slag discharge port of the electrocatalytic reactor and the bottom slag discharge port of the electrosoftening reactor are both connected to the slag discharge pipeline.

[0014] According to the present invention, preferably, the electrocatalytic reactor is a tumbling electrolytic cell.

[0015] According to the present invention, preferably, the anode plate of the electrocatalytic reactor is a boron-doped diamond electrode (BDD) and / or a coated titanium electrode (DSA), and the cathode plate of the electrocatalytic reactor is a titanium plate and / or a graphite plate.

[0016] According to the present invention, preferably, the electrode spacing of the electrocatalytic reactor is controlled at 20-30 mm.

[0017] According to the present invention, preferably, the electrosoftening reactor is a diaphragm electrolyzer; the diaphragm of the electrosoftening reactor is a cation exchange membrane.

[0018] According to the present invention, preferably, the anode plate of the electro-softening reactor is a coated titanium electrode (DSA); the cathode plate of the electro-softening reactor is a stainless steel plate; and an automatic slag scraper is provided at the cathode plate of the electro-softening reactor. In the present invention, the slag scraping interval of the automatic slag scraper is adjustable.

[0019] According to the present invention, preferably, the electrode spacing of the electrosoftening reactor is controlled at 30-50 mm.

[0020] According to the present invention, preferably, the dehydration separator is a centrifugal dehydrator.

[0021] According to the present invention, preferably, the system further includes a filter; the other end of the pipe connected to the upper outlet of the crystallizer, the pipe connected to the upper outlet of the purifier, and the pipe connected to the outlet of the dehydration separator are all connected to the inlet of the filter, and the outlet of the filter is connected to the inlet pipe of the nanofiltration device.

[0022] According to the present invention, preferably, the lower middle part of the crystallizer is further provided with inclined plate packing.

[0023] Another aspect of the present invention provides a method for extracting calcium sulfate for hardening removal from nanofiltration concentrate, the method employing the above-mentioned apparatus and comprising the following steps:

[0024] S1: The nanofiltration concentrate is sent to an electrocatalytic reactor for electrocatalytic treatment to obtain electrocatalytic reactor effluent and electrocatalytic precipitate sludge; the electrocatalytic reactor effluent is sent to an electrosoftening reactor for electrosoftening treatment to obtain electrosoftening reactor effluent and electrosoftening precipitate sludge.

[0025] S2: The effluent from the electric softening reactor and the reflux water from the crystallizer mixed with seed crystals (calcium sulfate) are sent together to the crystallizer for mixing and stirring to obtain crystallizer effluent and calcium sulfate crystals;

[0026] S3: Mix a portion of the crystallizer effluent with the seed crystals from the bottom of the seed tank in a pipe mixer to obtain the crystallizer reflux water mixed with the seed crystals; stir and mix the calcium sulfate crystals with the saturated calcium sulfate aqueous solution from the top of the seed tank in a purifier to obtain purified waste liquid and purified calcium sulfate crystals;

[0027] S4: The purified calcium sulfate crystals are sent to a dehydration separator for dehydration treatment to obtain dehydrated water and calcium sulfate products.

[0028] According to the present invention, preferably, the method further includes discharging the electrocatalytic precipitation slag and the electrosoftening precipitation slag externally.

[0029] According to the present invention, preferably, the current density of the electrocatalytic treatment is 20-30 mA / cm². 2 .

[0030] In this invention, the nanofiltration concentrate is treated by an electrocatalytic reactor:

[0031] The main principle is to utilize the high potential window and high oxygen evolution potential of the anode plate and the hydroxyl radicals generated by the cathode plate to remove organic pollutants from the nanofiltration concentrate. Simultaneously, electrolysis destroys the phosphate groups, the main component of the scale inhibitor, causing calcium to separate from the phosphate groups and preventing the scale inhibitor from affecting calcium sulfate crystallization. Specifically: an oxidation reaction occurs at the anode plate, through direct oxidation (direct electron transfer) and indirect oxidation (the generation of other active intermediate products on the electrode surface), chromophores such as carbon-carbon double bonds in the water are oxidized to achieve decolorization and COD removal. A reduction reaction occurs at the cathode plate, where a two-electron reduction reaction generates H₂O₂, which also removes some organic pollutants.

[0032] In addition, hydrolysis side reactions also occur at the cathode plate of the electrocatalytic reactor. Calcium ions, magnesium ions, and bicarbonate ions react with hydroxide ions produced by the hydrolysis at the cathode plate. However, due to the high current density, the calcium carbonate and magnesium hydroxide precipitates are washed away by the hydrogen gas produced at the cathode plate, causing the calcium and magnesium precipitates to fall off. The precipitates in the water also adsorb some organic pollutants and are eventually discharged as precipitates. Specifically: oxygen is generated from oxygen evolution at the anode plate, producing hydrogen ions, while hydrogen evolution at the cathode plate produces hydroxide ions. The anode and cathode plates form a strong acid and strong alkaline environment locally. Bicarbonate ions in the water react with the produced hydrogen and hydroxide ions. At the anode plate, some carbon dioxide is produced, and at the cathode plate, some carbonate ions are generated. Calcium and magnesium ions form calcium carbonate and magnesium hydroxide precipitates on the cathode plate surface. Due to the high current density of electrocatalysis, the hydrogen evolution reaction at the plates is intense. The generated hydrogen bubbles wash away the plate surface, causing the calcium carbonate and magnesium hydroxide scale to fall off. The calcium carbonate and magnesium hydroxide precipitates also adsorb some pollutants and settle to the bottom of the reactor, where they are discharged through the slag outlet.

[0033] Nanofiltration concentrate can remove most organic pollutants and a small amount of calcium and magnesium hardness after electrocatalytic treatment.

[0034] According to the present invention, preferably, the current density of the electrical softening treatment is 5-10 mA / cm². 2 .

[0035] In this invention, after electrocatalytic treatment, the alkalinity of the water is further reduced, primarily through the deep removal of magnesium ions in the electro-softening reactor. Because the electro-softening reactor contains a cation exchange membrane, hydrogen and oxygen evolution reactions occur at the anode and cathode plates after energization. A large number of calcium and magnesium ions in the water move towards the cathode plate after passing through the exchange membrane. Furthermore, hydroxide ions generated from water electrolysis at the cathode plate cannot pass through the exchange membrane, resulting in a strongly alkaline region at the cathode plate. Due to the different solubilities of magnesium hydroxide and calcium hydroxide (magnesium hydroxide is insoluble, while calcium hydroxide is slightly soluble), in this strongly alkaline region, magnesium ions preferentially react with hydroxide ions to form magnesium hydroxide precipitate, which adheres to the cathode plate surface. A scraper periodically removes the magnesium hydroxide precipitate from the plate surface, and the removed precipitate is discharged through the slag outlet. At the cathode plate, the calcium ion concentration also increases locally. When the local calcium ion concentration exceeds the solubility of calcium sulfate, a local supersaturation state of calcium sulfate occurs. Calcium ions combine with sulfate ions to form tiny calcium sulfate seed crystals, providing seed crystals for subsequent crystallization in the crystallizer. Electrolysis is an exothermic reaction. Due to the influence of electrical energy utilization, some electrical energy will be converted into heat energy. During the electrolysis process, the water temperature will rise, which reduces the energy consumption for heating in the crystallizer and lowers some costs.

[0036] According to the present invention, preferably, the temperature inside the crystallizer is 80-90°C; the stirring speed inside the crystallizer is 80-120 r / min. This slow stirring increases the mixing and mass transfer efficiency on the one hand, and avoids the crystals from being broken up on the other.

[0037] In this invention, the hardness in the water after electro-softening treatment mainly exists in the form of calcium hardness, and the water contains a very high concentration of sulfate ions. At room temperature, the solubility of calcium sulfate in water is 0.209 mg / ml. When the temperature rises to 80℃, the solubility of calcium sulfate decreases to 0.18 mg / ml, making it easier for calcium sulfate to reach a supersaturated state and precipitate. At 80℃, the collision between sulfate ions and calcium ions intensifies, greatly shortening (or even eliminating) the crystallization induction period of calcium sulfate. A large number of calcium sulfate seed crystals are formed in the electro-softened solution. Simultaneously, a small amount of exogenously introduced calcium sulfate seed crystals (the seed crystals) improve the crystallization efficiency of calcium sulfate. Calcium sulfate adheres to the seed crystals and rapidly deposits and expands to form large-particle-size calcium sulfate crystals. At this point, the deposition rate of calcium ions in the water can reach 10.62*10⁻⁶. -4 At a concentration of mol / (L*min), most of the calcium ions in the water will precipitate out as calcium sulfate crystals in a short time.

[0038] According to the present invention, preferably, the amount of seed crystals introduced into the crystallizer is 0.8-1.2% of the water inlet volume of the crystallizer.

[0039] In this invention, the seed tank contains a supersaturated calcium sulfate aqueous solution prepared with pure water. Therefore, solid calcium sulfate (seed crystals) is obtained at the bottom of the seed tank, while the upper part of the seed tank contains a saturated calcium sulfate aqueous solution.

[0040] According to the present invention, preferably, the method further includes using inclined plate packing to re-aggregate the calcium sulfate crystals during the precipitation process in the crystallizer, thereby improving the precipitation efficiency.

[0041] In this invention, the lower part of the crystallizer is equipped with inclined plate packing. Due to the shallow pool effect, the settling velocity of the large calcium sulfate seed crystals is reduced. The inclined tubes slow down their descent, prolonging the seed crystal adsorption and crystallization time, thus enhancing the induced crystallization rate and forming larger calcium sulfate crystals. At this point, the calcium sulfate crystal particle size can reach 30 micrometers. The softened water is weakly acidic, with the pH maintained at 5-6. Under these weakly acidic conditions, calcium is prevented from precipitating as calcium hydroxide, ensuring the purity of the calcium sulfate crystals. A portion of the crystallizer effluent is returned to the crystallizer inlet pipe, forming an internal circulation to further ensure the calcium sulfate crystallization efficiency.

[0042] According to the present invention, preferably, the method further includes sending the dehydrated water, purified waste liquid and the remaining crystallizer effluent to a filter for filtration, obtaining filtrate, which is then sent to the nanofiltration device for treatment as nanofiltration feed water, thereby reducing the nanofiltration feed water load.

[0043] In this invention, there is no need to add other equipment to treat the dehydrated water, purified waste liquid and the remaining crystallizer effluent. They are simply filtered and then re-mixed into the nanofiltration unit's inlet water, which reduces the operating load of the nanofiltration unit and saves energy.

[0044] According to the present invention, preferably, the ratio of crystallizer effluent fed into the pipeline mixer to crystallizer effluent fed to the filter is (2.5-3.5):1.

[0045] According to the present invention, preferably, the stirring speed in the purifier is 180-220 r / min.

[0046] According to the present invention, preferably, the volume ratio of saturated calcium sulfate aqueous solution to calcium sulfate crystals in the purifier is (2.5-3.5):1.

[0047] In this invention, the purifier uses a saturated calcium sulfate aqueous solution to clean and purify the calcium sulfate crystals by stirring. This mainly removes sodium ions, chloride ions, and other ions adsorbed on the surface of the calcium sulfate crystals. This not only removes impurities from the surface of the calcium sulfate crystals but also ensures that the calcium sulfate crystals do not dissolve.

[0048] According to the present invention, preferably, the dehydration process is carried out by centrifugation.

[0049] In this invention, the dehydration separator uses a centrifugal dehydrator to dehydrate the purified calcium sulfate, removing excess water to obtain solid calcium sulfate with a water content of 30%-50%. After testing, the purity of the solid calcium sulfate can reach more than 90%.

[0050] The beneficial effects of the technical solution of the present invention are as follows:

[0051] The process provided by this invention can efficiently remove macromolecular organic pollutants from nanofiltration concentrates, targeting different water quality characteristics. It can remove most of the hardness sulfate in the water without adding external reagents, reduce the TDS in the water, and obtain high-purity calcium sulfate, thus realizing the resource utilization of wastewater.

[0052] This invention takes into full account water quality conditions, removes calcium and magnesium ions by precipitation of calcium sulfate and magnesium hydroxide, and obtains high-purity calcium sulfate crystals through crystallization.

[0053] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0054] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0055] Figure 1 A schematic diagram of an apparatus for extracting calcium sulfate for hardening from nanofiltration concentrate provided in Embodiment 1 of the present invention is shown.

[0056] The annotations in the attached figures are explained as follows:

[0057] 1. Electrocatalytic reactor, 2. Electrosoftening reactor, 3. Crystallizer, 4. Pipeline mixer, 5. Purifier.

[0058] 6. Dehydration separator, 7. Seed tank, 8. Nanofiltration concentrate discharge pipeline, 9. Slag discharge pipeline, 10. Pipeline connecting the upper outlet of the electric softening reactor, 11. Pipeline connecting the outlet of the pipeline mixer, 12. Pipeline connecting the bottom outlet of the seed tank, 13. Pipeline connecting the upper outlet of the seed tank, 14. Filter, 15. Nanofiltration unit, 16. Inlet water of the nanofiltration unit, 17. Product water of the nanofiltration unit. Detailed Implementation

[0059] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0060] Example 1

[0061] This embodiment provides a device for extracting calcium sulfate for hardening removal from nanofiltration concentrate, such as... Figure 1 As shown, the device includes an electrocatalytic reactor 1, an electrosoftening reactor 2, a crystallizer 3, a pipeline mixer 4, a purifier 5, a dehydration separator 6, and a seed crystal tank 7.

[0062] The middle inlet of the electrocatalytic reactor 1 is connected to the nanofiltration concentrate discharge pipeline 8, and the upper outlet of the electrocatalytic reactor 1 is connected to the middle inlet of the electrosoftening reactor 2; the bottom slag discharge ports of the electrocatalytic reactor 1 and the bottom slag discharge ports of the electrosoftening reactor 2 are both connected to the slag discharge pipeline.

[0063] The electrocatalytic reactor 1 is a tumbling electrolytic cell; the anode plate of the electrocatalytic reactor 1 is a boron-doped diamond electrode (BDD), and the cathode plate of the electrocatalytic reactor 1 is a titanium plate; the electrode spacing of the electrocatalytic reactor 1 is controlled at 20 mm; the electrosoftening reactor 2 is a diaphragm electrolytic cell; the diaphragm of the electrosoftening reactor 2 is a cation exchange membrane; the anode plate of the electrosoftening reactor 2 is a coated titanium electrode (DSA); the cathode plate of the electrosoftening reactor 2 is a stainless steel plate; an automatic slag scraper (not shown) is provided at the cathode plate of the electrosoftening reactor 2; the electrode spacing of the electrosoftening reactor 2 is controlled at 50 mm;

[0064] The pipe 10 connected to the upper outlet of the electric softening reactor merges with the pipe 11 connected to the outlet of the pipe mixer and connects to the lower inlet of the crystallizer 3; the crystallizer 3 is equipped with a constant temperature heating device and a stirring device.

[0065] The pipeline connected to the upper outlet of the crystallizer 3 is divided into two paths. One path, together with the pipeline 12 connected to the bottom outlet of the seed tank, is connected to the inlet of the pipeline mixer 4.

[0066] The bottom discharge port of the crystallizer 3 is connected to the lower feed port of the purifier 5; the upper outlet of the seed tank is connected to the pipe 13, which is connected to the top purification and cleaning port of the purifier 5; the purifier 5 is equipped with a stirring device.

[0067] The bottom discharge port of the purifier 5 is connected to the inlet of the dehydration separator 6, and the discharge port of the dehydration separator 6 is a collection port for the recovered refined calcium sulfate. The dehydration separator 6 is a centrifugal dehydrator.

[0068] The system also includes a filter 14; the other end of the pipe connected to the upper outlet of the crystallizer 3, the pipe connected to the upper outlet of the purifier 5, and the pipe connected to the outlet of the dehydration separator 6 are all connected to the inlet of the filter 14, and the outlet of the filter 14 is connected to the inlet pipe of the nanofiltration device.

[0069] Inclined plate packing (not shown) is also provided in the lower middle part of the crystallizer.

[0070] This embodiment also provides a method for extracting calcium sulfate for hardening removal from nanofiltration concentrate. The method uses the above-mentioned apparatus and includes the following steps:

[0071] S1: The nanofiltration concentrate is sent to electrocatalytic reactor 1 for electrocatalytic treatment to obtain electrocatalytic reactor effluent and electrocatalytic precipitate sludge; the electrocatalytic reactor effluent is sent to electrosoftening reactor 2 for electrosoftening treatment to obtain electrosoftening reactor effluent and electrosoftening precipitate sludge; the electrocatalytic precipitate sludge and electrosoftening precipitate sludge are discharged externally;

[0072] The current density of the electrocatalytic treatment is 20 mA / cm². 2 The electrolysis time is 1 hour;

[0073] The current density of the electrical softening treatment is 7.5 mA / cm². 2 The electrolysis time is 1 hour;

[0074] S2: The effluent from the electro-softening reactor and the crystallizer return water mixed with seed crystals are sent together to crystallizer 3 for mixing and stirring. The deposition rate of calcium sulfate crystals can reach 10.62*10. -4 mol / (L*min), using inclined plate packing to improve the settling efficiency of calcium sulfate crystals in the crystallizer 3, to obtain crystallizer effluent and calcium sulfate crystals (particle size 30 micrometers);

[0075] The temperature inside the crystallizer is 80°C; the stirring speed inside the crystallizer is 100 r / min; and the amount of seed crystals introduced into the crystallizer 3 is 1% of the water volume entering the crystallizer.

[0076] S3: Mix a portion of the crystallizer effluent with the crystal seeds from the bottom of the seed tank 7 in the pipe mixer 4 to obtain the crystallizer reflux water mixed with the crystal seeds (forming an internal circulation); mix the calcium sulfate crystals with the saturated calcium sulfate aqueous solution from the top of the seed tank 7 in the purifier 5 at a stirring speed of 200 r / min to obtain purified waste liquid and purified calcium sulfate crystals.

[0077] Inside the purifier 5, the volume ratio of saturated calcium sulfate aqueous solution to calcium sulfate crystals is 3:1.

[0078] S4: The purified calcium sulfate crystals are sent to the dehydration separator 6 for centrifugal dehydration to obtain dehydrated water and calcium sulfate products.

[0079] The method further includes sending the dehydrated water, purified waste liquid and the remaining crystallizer effluent to filter 14 for filtration, obtaining filtrate, which is then sent to nanofiltration device 15 for treatment as nanofiltration feed water; and the ratio of crystallizer effluent sent to pipeline mixer 4 to crystallizer effluent sent to filter 14 is 3:1.

[0080] The nanofiltration concentrate treated in this embodiment is nanofiltration concentrate wastewater from an industrial park, and its water quality conditions include: COD: 550 mg / L, calcium: 1854 mg / L, magnesium: 457 mg / L, sulfate: 16750 mg / L, and alkalinity: 750 mg / L.

[0081] After treatment using the process described in this embodiment, the water quality was reduced to COD: 25 mg / L, calcium: 481 mg / L, magnesium: 45 mg / L, and sulfate: 8578 mg / L. 2.72 g of calcium sulfate product with a purity of 93% after drying can be recovered per liter of water.

[0082] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for extracting calcium sulfate for hardening removal from nanofiltration concentrate, characterized in that, The apparatus used in the method includes an electrocatalytic reactor, an electrosoftening reactor, a crystallizer, a pipeline mixer, a purifier, a dehydration separator, and a seed crystal tank; The middle inlet of the electrocatalytic reactor is connected to the nanofiltration concentrate discharge pipeline, and the upper outlet of the electrocatalytic reactor is connected to the middle inlet of the electrosoftening reactor. The pipe connected to the upper outlet of the electric softening reactor and the pipe connected to the outlet of the pipeline mixer are combined and connected to the lower inlet of the crystallizer; the crystallizer is equipped with a constant temperature heating device and a stirring device. The pipeline connected to the upper outlet of the crystallizer is divided into two paths, one of which is connected to the pipeline connected to the bottom outlet of the seed tank and then to the dosing port of the pipeline mixer. The bottom discharge port of the crystallizer is connected to the lower feed port of the purifier; the pipeline connected to the upper outlet of the seed crystal tank is connected to the top purification and cleaning port of the purifier; a stirring device is installed inside the purifier; The bottom discharge port of the purifier is connected to the inlet of the dehydration separator, and the discharge port of the dehydration separator is a collection port for the recovery of refined calcium sulfate; The device also includes a filter; the other end of the pipe connected to the upper outlet of the crystallizer, the pipe connected to the upper outlet of the purifier, and the pipe connected to the outlet of the dehydration separator are all connected to the inlet of the filter, and the outlet of the filter is connected to the inlet pipe of the nanofiltration device. The method includes the following steps: S1: The nanofiltration concentrate is sent to an electrocatalytic reactor for electrocatalytic treatment to obtain electrocatalytic reactor effluent and electrocatalytic precipitate sludge; the electrocatalytic reactor effluent is sent to an electrosoftening reactor for electrosoftening treatment to obtain electrosoftening reactor effluent and electrosoftening precipitate sludge. S2: The effluent from the electric softening reactor and the crystallizer return water mixed with seed crystals are sent together to the crystallizer for mixing and stirring to obtain crystallizer effluent and calcium sulfate crystals; S3: Mix a portion of the crystallizer effluent with the seed crystals from the bottom of the seed tank in a pipe mixer to obtain the crystallizer reflux water mixed with the seed crystals; stir and mix the calcium sulfate crystals with the saturated calcium sulfate aqueous solution from the top of the seed tank in a purifier to obtain purified waste liquid and purified calcium sulfate crystals; S4: The purified calcium sulfate crystals are sent to a dehydration separator for dehydration treatment to obtain dehydrated water and calcium sulfate product; The method further includes sending the dehydrated water, purified waste liquid and the remaining crystallizer effluent to a filter for filtration, obtaining filtrate, which is then sent to the nanofiltration device for treatment as nanofiltration feed water.

2. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, The bottom slag discharge ports of the electrocatalytic reactor and the electrosoftening reactor are both connected to the slag discharge pipeline. The electrocatalytic reactor is a tumbling electrolytic cell; The anode plate of the electrocatalytic reactor is a boron-doped diamond electrode and / or a coated titanium electrode, and the cathode plate of the electrocatalytic reactor is a titanium plate and / or a graphite plate. The electrode spacing of the electrocatalytic reactor is controlled at 20-30 mm; The electrosoftening reactor is a diaphragm electrolyzer; the diaphragm of the electrosoftening reactor is a cation exchange membrane. The anode plate of the electro-softening reactor is a coated titanium electrode; the cathode plate of the electro-softening reactor is a stainless steel plate; and an automatic slag scraper is provided at the cathode plate of the electro-softening reactor. The electrode spacing of the electrosoftening reactor is controlled at 30-50 mm; The dehydration separator is a centrifugal dehydrator.

3. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, Inclined packing is also provided in the lower middle part of the crystallizer.

4. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, The method further includes discharging the electrocatalytic precipitation slag and the electrosoftening precipitation slag externally; The current density of the electrocatalytic treatment is 20-30 mA / cm². 2 ; The current density of the electrical softening treatment is 5-10 mA / cm². 2 .

5. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, The temperature inside the crystallizer is 80-90℃; the stirring speed inside the crystallizer is 80-120 r / min; The amount of seed crystals introduced into the crystallizer is 0.8-1.2% of the water inlet volume of the crystallizer.

6. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, The method also includes using inclined plate packing to re-aggregate the calcium sulfate crystals during the precipitation process in the crystallizer, thereby improving precipitation efficiency.

7. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, The ratio of crystallizer effluent fed into the pipeline mixer to crystallizer effluent fed to the filter is (2.5-3.5):

1.

8. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, The stirring speed inside the purifier is 180-220 r / min; Inside the purifier, the volume ratio of saturated calcium sulfate aqueous solution to calcium sulfate crystals is (2.5-3.5):

1.

9. The method for extracting calcium sulfate for hardening removal from nanofiltration concentrate according to claim 1, wherein, The dehydration process is carried out by centrifugation.

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