Method and device for extracting calcium sulfate from nanofiltration concentrated solution and removing hardness

Abstract

The invention belongs to the field of sewage treatment, and discloses a method and a device for extracting calcium sulfate and removing hardness from a nanofiltration concentrated solution. The device comprises an electro-catalytic reactor, an electric softening reactor, a crystallizer, a pipeline mixer, a purifier, a dehydration separator and a seed crystal tank. According to the method, chroma, COD and most hardness sulfate radicals in water can be removed, TDS in water is reduced, meanwhile, calcium sulfate with high purity can be obtained, and resource utilization of wastewater is achieved.

Description

Technical Field

[0001] The invention belongs to the field of sewage treatment, and more specifically, relates to a method and device for extracting calcium sulfate and removing hardness from nanofiltration concentrated liquid. Background Art

[0002] In today's environmental protection market, membrane separation technology, as a mature sewage treatment technology, has been widely used in the sewage treatment field of various industries. Membrane separation technology can be roughly divided into microfiltration, ultrafiltration, nanofiltration, reverse osmosis, etc. according to its separation performance. Among them, nanofiltration has a good interception and separation effect on divalent salts and substances with a molecular weight of more than 1000 Daltons due to its tiny membrane pore size of 1-2nm and the Donnan effect on the membrane surface. In industries with zero-discharge resource requirements for wastewater, nanofiltration's unique ability to separate monovalent and divalent salts has become an indispensable treatment unit in the zero-discharge resource process of wastewater. However, the concentrate produced by nanofiltration has the characteristics of high COD and high hardness of water quality, and the subsequent treatment of its concentrate needs to be solved urgently.

[0003] The treatment of traditional nanofiltration concentrate includes direct discharge, reflux reprocessing, evaporation, etc. All of the above treatment methods have disadvantages. Direct discharge treatment generally exists in offshore areas abroad, and the concentrate is directly discharged into the ocean; reflux reprocessing is to mix the concentrate into the membrane system inlet water for further concentration, but this treatment method cannot really solve the disposal problem of the concentrate, and as time goes by, the treatment cost and membrane life of the membrane system will be affected; evaporation treatment is to evaporate the concentrate by heating and reconcentrating it through an evaporator, and finally landfill it in the form of solidified salts. In addition, in the fields of pharmaceutical petrochemicals, chemical fiber textiles, mining and industrial park wastewater, the hardness of sulfate in the water is relatively high. After nanofiltration treatment, the hardness of sulfate in the nanofiltration concentrate can often reach tens of thousands or even higher concentrations. High hardness will cause the nanofiltration concentrate to scale and block the pipeline easily during the evaporation process, so it is difficult to carry out subsequent evaporation treatment. Before evaporation, the concentrate often needs to be pretreated, especially for concentrates with higher hardness. It is necessary to remove calcium and magnesium hardness and various organic pollutants in the concentrate through double alkali method, electrochemical hardness removal and advanced oxidation and other technologies. The double alkali method for hardness removal will introduce more ions and produce precipitation during the pretreatment process, which is not conducive to the overall cost control of evaporation.

[0004] In addition, in some high-hardness water, the nanofiltration inlet water often needs to be acidified and antiscalants added to prevent scaling on the nanofiltration membrane surface to avoid adverse effects on membrane performance and life. The alkalinity in the nanofiltration concentrate is low, and there are many phosphate groups after the introduction of antiscalants, which will affect the conventional electrochemical hardness removal in the form of calcium carbonate.

[0005] Therefore, it is urgent to propose a new method and device for extracting calcium sulfate and removing hardness from nanofiltration concentrated liquid. Summary of the invention

[0006] The purpose of the present invention is to address the deficiencies of the prior art and to propose a method and device for extracting calcium sulfate from nanofiltration concentrate to remove hardness. The present invention can remove chromaticity, COD, and most hardness sulfate radicals in water, reduce TDS in water, and at the same time obtain calcium sulfate with higher purity, thereby realizing resource utilization of wastewater.

[0007] In order to achieve the above-mentioned object, the present invention provides a device for extracting calcium sulfate and removing hardness from nanofiltration concentrate, the device comprising an electrocatalytic reactor, an electrosoftening reactor, a crystallizer, a pipeline mixer, a purifier, a dehydration separator and a seed tank;

[0008] The middle water inlet of the electrocatalytic reactor is connected to the nanofiltration concentrate discharge pipeline, and the upper water outlet of the electrocatalytic reactor is connected to the middle water inlet of the electric softening reactor;

[0009] The pipeline connected to the upper water outlet of the electric softening reactor merges with the pipeline connected to the outlet of the pipeline mixer and is connected to the lower water inlet of the crystallizer; the crystallizer is provided with a constant temperature heating device and a stirring device;

[0010] The pipeline connected to the upper water outlet of the crystallizer is divided into two paths, one of which is connected to the bottom outlet of the seed tank and connected 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 tank is connected to the top purification and cleaning port of the purifier; a stirring device is provided in the purifier;

[0012] The bottom discharge port of the purifier is connected to the feed port of the dehydration separator, and the discharge port of the dehydration separator is a collection port for recovering 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 electrolyzer.

[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 distance between the electrode plates of the electrocatalytic reactor is controlled to be 20-30 mm.

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

[0018] According to the present invention, preferably, the anode plate of the electrosoftening reactor is a coated titanium electrode (DSA); the cathode plate of the electrosoftening reactor is a stainless steel plate; and an automatic scraping plate is provided at the cathode plate of the electrosoftening reactor. In the present invention, the scraping time interval of the automatic scraping plate can be adjusted.

[0019] According to the present invention, preferably, the distance between the plates of the electric softening reactor is controlled to be 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 also includes a filter; another line of the pipeline connected to the upper water outlet of the crystallizer, the pipeline connected to the upper water outlet of the purifier and the pipeline connected to the water outlet of the dehydration separator are all connected to the inlet of the filter, and the outlet of the filter is connected to the water inlet pipeline of the nanofiltration device.

[0022] According to the present invention, preferably, an inclined plate filler is further provided in the middle and lower part of the crystallizer.

[0023] Another aspect of the present invention provides a method for extracting calcium sulfate from a nanofiltration concentrate to remove hardness, the method using the above-mentioned device, comprising the following steps:

[0024] S1: sending the nanofiltration concentrated liquid to an electrocatalytic reactor for electrocatalytic treatment to obtain electrocatalytic reactor effluent and electrocatalytic precipitation slag; sending the electrocatalytic reactor effluent to an electrosoftening reactor for electrosoftening treatment to obtain electrosoftening reactor effluent and electrosoftening precipitation slag;

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

[0026] S3: mixing part of the crystallizer effluent with the seed crystals from the bottom of the seed crystal tank in a pipeline mixer to obtain the crystallizer reflux water mixed with the seed crystals; stirring and mixing the calcium sulfate crystals with the saturated calcium sulfate aqueous solution from the upper part of the seed crystal tank in a purifier to obtain purified waste liquid and purified calcium sulfate crystals;

[0027] S4: sending the purified calcium sulfate crystals 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 comprises discharging the electrocatalytic precipitation slag and the electrosoftening precipitation slag.

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

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

[0031] It mainly utilizes the high potential window of the anode plate, the high oxygen evolution potential, and the hydroxyl radicals generated by the cathode plate to remove organic pollutants in the nanofiltration concentrate. At the same time, the phosphate group, the main component of the scale inhibitor, is destroyed by electrolysis, so that calcium and the phosphate group are separated, avoiding the influence of the scale inhibitor on the crystallization of calcium sulfate. Specifically: an oxidation reaction occurs at the anode plate, and through direct oxidation (direct electron transfer) and indirect oxidation (other active intermediates are produced on the electrode surface), the carbon-carbon double bond and other chromophores in the water can be oxidized to achieve the effect of decolorization and COD removal. A reduction reaction occurs at the cathode plate, and a two-electron reduction reaction occurs at the cathode plate to generate H2O2, which can also remove some organic pollutants.

[0032] In addition, hydrolysis side reactions will also occur at the cathode plate of the electrocatalytic reactor. Calcium ions, magnesium ions, and bicarbonate ions will react with the hydroxide produced by the hydrolysis of the cathode plate. However, due to the high current density, the calcium carbonate and magnesium hydroxide precipitates will be washed away by the hydrogen produced at the cathode plate, causing the calcium and magnesium precipitates to fall off. The precipitates in the water will also adsorb some organic pollutants and eventually be discharged in the form of precipitates. Specifically: the anode plate generates oxygen to produce hydrogen ions, and the cathode plate generates hydrogen to produce hydroxide ions. The positive and negative plates form a strong acid and strong base environment locally. The bicarbonate in the water will react with the produced hydrogen ions and hydroxide ions. A part of the anode plate produces carbon dioxide, and a part of the cathode plate generates carbonate. Calcium and magnesium ions will generate calcium carbonate and magnesium hydroxide precipitates on the surface of the cathode plate. The current density of electrocatalysis is high, so the hydrogen evolution reaction at the plate is violent. The generated hydrogen bubbles wash the surface of the plate, causing the calcium carbonate and magnesium hydroxide to fall off. The calcium carbonate and magnesium hydroxide precipitates will also adsorb some pollutants and sink to the bottom of the reactor and be discharged through the slag discharge port.

[0033] The nanofiltration concentrate can remove most of the 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 electric softening treatment is 5-10ma / cm 2 .

[0035] In the present invention, after electrocatalytic treatment, the alkalinity in the water is further reduced, and the depth removal of magnesium ions is mainly carried out in the electric softening reactor. Since there is a cation exchange membrane in the electric softening reactor, after power is turned on, hydrogen and oxygen evolution reactions occur at the anode and cathode plates, and a large amount of calcium and magnesium ions in the water move to the cathode plate after passing through the exchange membrane, and the hydroxide produced by electrolysis of water at the cathode plate cannot pass through the exchange membrane, so a strong base area appears at the cathode plate. Due to the different solubility of magnesium hydroxide and calcium hydroxide, magnesium hydroxide is insoluble and calcium hydroxide is slightly soluble. In the strong base area, magnesium ions will preferentially react with hydroxide to generate magnesium hydroxide precipitation attached to the surface of the cathode plate, and the scraper will regularly scrape off the magnesium hydroxide on the surface of the plate, and the scraped magnesium hydroxide precipitation is discharged through the slag discharge port. At the cathode plate, the calcium ion concentration will also increase in the local area. When the local calcium ion concentration is higher than the solubility of calcium sulfate, a local calcium sulfate supersaturation state will occur, and the calcium ions will combine with sulfate to form tiny calcium sulfate seeds, which provide seeds for subsequent induction of crystallization in the crystallizer. Electrolysis is an exothermic reaction. Affected by the utilization rate of electric energy, part of the electric energy will be converted into heat energy. The water temperature will increase during the electrolysis process, reducing the energy consumption of heating in the crystallizer and reducing some costs.

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

[0037] In the present invention, the hardness in the water after the electric softening treatment mainly exists in the form of calcium hardness, and there is a very high concentration of sulfate in the water. The solubility of calcium sulfate in water is 0.209 mg / ml at room temperature. When the temperature rises to 80°C, the solubility of calcium sulfate will drop to 0.18 mg / ml, and calcium sulfate in the water is more likely to reach a supersaturated state and precipitate. At 80°C, the collision between sulfate and calcium ions intensifies, and the crystallization induction period of calcium sulfate will be greatly shortened (or even disappear). A large number of calcium sulfate seeds are formed in the solution after the electric softening, and a small amount of calcium sulfate seeds (the seeds) are introduced from the outside, so that the crystallization efficiency of calcium sulfate is improved, and calcium sulfate is attached to the seeds and rapidly deposited and expanded to form large-diameter calcium sulfate crystals. At this time, the deposition rate of calcium ions in the water can reach 10.62*10 -4 mol / (L*min), most of the calcium ions in the water will precipitate in the form of 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 amount of the crystallizer.

[0039] In the present invention, the seed tank contains a supersaturated calcium sulfate aqueous solution prepared with pure water, so that solid calcium sulfate (seed crystals) is obtained at the bottom of the seed tank, and a saturated calcium sulfate aqueous solution is formed at the top of the seed tank.

[0040] According to the present invention, preferably, the method further comprises using inclined plate fillers to re-agglomerate the calcium sulfate crystals during the precipitation process in the crystallizer to improve the precipitation efficiency.

[0041] In the present invention, the lower part of the crystallizer is provided with an inclined plate filler, which is affected by the shallow pool effect, and the sedimentation velocity of the large-particle calcium sulfate seed crystals formed is reduced. The inclined tube delays its falling stroke, prolongs the crystal seed adsorption crystallization time, strengthens the induced crystallization rate, and forms larger calcium sulfate crystals. At this time, the calcium sulfate crystal particle size can reach 30 microns. The water after electrical softening presents weak acidity, and the pH is maintained at 5-6. Under weak acid conditions, calcium is avoided from being precipitated in the form of calcium hydroxide precipitation, and the purity of the calcium sulfate crystals is guaranteed. A part of the crystallizer effluent will flow back to the crystallizer water inlet pipeline, forming an internal circulation to further ensure the calcium sulfate crystallization efficiency.

[0042] According to the present invention, preferably, the method also includes sending the dehydrated water, purified waste liquid and the remaining part of the crystallizer effluent to a filter for filtration, obtaining the filtrate and sending it to a nanofiltration device for treatment as nanofiltration inlet water, which can reduce the nanofiltration inlet load.

[0043] In the present invention, there is no need to add other equipment to specifically treat the dehydrated water, purified waste liquid and the remaining crystallizer effluent, and they are only re-mixed into the nanofiltration device inlet water after filtration, thereby reducing the operating load of the nanofiltration device and saving energy consumption.

[0044] According to the present invention, preferably, the ratio of the crystallizer effluent fed into the pipeline mixer to the 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, in the purifier, the volume ratio of the saturated calcium sulfate aqueous solution to the calcium sulfate crystals is (2.5-3.5):1.

[0047] In the present invention, the purifier uses a saturated calcium sulfate aqueous solution to clean and purify the calcium sulfate crystals by stirring, mainly removing other ions such as sodium ions and chloride ions adsorbed on the surface of the calcium sulfate, which can not only remove impurities on the surface of the calcium sulfate crystals, but also ensure that the calcium sulfate crystals will not dissolve.

[0048] According to the present invention, preferably, the dehydration treatment is performed by centrifugation.

[0049] In the present invention, the dehydration separator uses a centrifugal dehydrator to dehydrate the purified calcium sulfate, remove excess water and 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] According to the characteristics of different water qualities, the process flow provided by the present invention can efficiently remove large molecular organic pollutants in the nanofiltration concentrate, remove most of the hardness sulfate ions in the water without adding external agents, reduce the TDS in the water, and at the same time obtain high-purity calcium sulfate, thereby realizing the resource utilization of wastewater.

[0052] The present invention fully considers water quality conditions, removes calcium and magnesium ions in the form of calcium sulfate and magnesium hydroxide precipitation, and obtains calcium sulfate crystals with higher purity through crystallization.

[0053] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0055] Figure 1 A schematic diagram of a device for extracting calcium sulfate and removing hardness from a nanofiltration concentrated liquid provided in Example 1 of the present invention is shown.

[0056] The following are the descriptions of the reference numerals:

[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 connected to the upper water outlet of the electric softening reactor, 11 pipeline connected to the outlet of the pipeline mixer, 12 pipeline connected to the bottom outlet of the seed tank, 13 pipeline connected to the upper outlet of the seed tank, 14 filter, 15 nanofiltration device, 16 water inlet of the nanofiltration device, 17 water production of the nanofiltration device. DETAILED DESCRIPTION

[0059] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present invention more thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0060] Example 1

[0061] This embodiment provides a device for extracting calcium sulfate and removing hardness from a 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 tank 7;

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

[0063] The electrocatalytic reactor 1 is a tumbling electrolyzer; 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 distance between the pole plates of the electrocatalytic reactor 1 is controlled at 20 mm; the electrosoftening reactor 2 is a diaphragm electrolyzer; 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 scraper plate (not shown) is provided at the cathode plate of the electrosoftening reactor 2; the distance between the pole plates of the electrosoftening reactor 2 is controlled at 50 mm;

[0064] The pipeline 10 connected to the upper water outlet of the electric softening reactor and the pipeline 11 connected to the outlet of the pipeline mixer merge and are connected to the lower water inlet of the crystallizer 3; the crystallizer 3 is provided with a constant temperature heating device and a stirring device;

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

[0066] The bottom discharge port of the crystallizer 3 is connected to the lower feed port of the purifier 5; the pipeline 13 connected to the upper outlet of the seed tank is connected to the top purification and cleaning port of the purifier 5; a stirring device is provided in the purifier 5;

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

[0068] The system further includes a filter 14; another pipeline connected to the upper water outlet of the crystallizer 3, the pipeline connected to the upper water outlet of the purifier 5, and the pipeline connected to the water 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 water inlet pipeline of the nanofiltration device;

[0069] An inclined plate filler (not shown) is also provided in the lower middle portion of the crystallizer.

[0070] This embodiment also provides a method for extracting calcium sulfate from a nanofiltration concentrate to remove hardness, the method using the above-mentioned device, comprising the following steps:

[0071] S1: sending the nanofiltration concentrated liquid to the electrocatalytic reactor 1 for electrocatalytic treatment to obtain electrocatalytic reactor effluent and electrocatalytic precipitation slag; sending the electrocatalytic reactor effluent to the electrosoftening reactor 2 for electrosoftening treatment to obtain electrosoftening reactor effluent and electrosoftening precipitation slag; discharging the electrocatalytic precipitation slag and the electrosoftening precipitation slag;

[0072] The current density of the electrocatalytic treatment is 20 mA / cm 2 , electrolysis time is 1h;

[0073] The current density of the electric softening treatment is 7.5 mA / cm 2 , electrolysis time is 1h;

[0074] S2: The effluent from the electric softening reactor and the reflux water from the crystallizer mixed with the seed crystals are sent to the 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 fillers to improve the sedimentation efficiency of calcium sulfate crystals in the crystallizer 3, and obtaining crystallizer effluent and calcium sulfate crystals (particle size 30 microns);

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

[0076] S3: mixing part of the crystallizer effluent with the crystal seeds from the bottom of the seed tank 7 in the pipeline mixer 4 to obtain the crystallizer reflux water mixed with the crystal seeds (forming an internal circulation); stirring and mixing the calcium sulfate crystals with the saturated calcium sulfate aqueous solution from the upper part 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] In the purifier 5, the volume ratio of the saturated calcium sulfate aqueous solution to the calcium sulfate crystals is 3:1.

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

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

[0080] The nanofiltration concentrate treated in this embodiment is the nanofiltration concentrate wastewater from a certain 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 the process of this embodiment, the water quality is reduced to COD: 25 mg / L, calcium: 481 mg / L, magnesium: 45 mg / L, 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 embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A device for extracting calcium sulfate from nanofiltration concentrate to remove hardness, characterized in that: The device comprises an electrocatalytic reactor, an electrosoftening reactor, a crystallizer, a pipeline mixer, a purifier, a dehydration separator and a seed tank; The middle water inlet of the electrocatalytic reactor is connected to the nanofiltration concentrate discharge pipeline, and the upper water outlet of the electrocatalytic reactor is connected to the middle water inlet of the electric softening reactor; The pipeline connected to the upper water outlet of the electric softening reactor merges with the pipeline connected to the outlet of the pipeline mixer and is connected to the lower water inlet of the crystallizer; the crystallizer is provided with a constant temperature heating device and a stirring device; The pipeline connected to the upper water outlet of the crystallizer is divided into two paths, one of which is connected to the bottom outlet of the seed tank and connected 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 tank is connected to the top purification and cleaning port of the purifier; a stirring device is provided in the purifier; The bottom discharge port of the purifier is connected to the feed port of the dehydration separator, and the discharge port of the dehydration separator is a collection port for recovering refined calcium sulfate.

2. The device for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 1, wherein: 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; The electrocatalytic reactor is a tumbling electrolyzer; 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 distance between the plates of the electrocatalytic reactor is controlled at 20-30 mm; The electric softening reactor is a diaphragm electrolyzer; the diaphragm of the electric softening reactor is a cation exchange membrane; The anode plate of the electrosoftening reactor is a coated titanium electrode; the cathode plate of the electrosoftening reactor is a stainless steel plate; an automatic scraping plate is provided at the cathode plate of the electrosoftening reactor; The distance between the plates of the electric softening reactor is controlled at 30-50 mm; The dehydration separator is a centrifugal dehydrator.

3. The device for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 1, wherein: The system further comprises a filter; another pipeline connected to the upper water outlet of the crystallizer, the pipeline connected to the upper water outlet of the purifier and the pipeline connected to the water outlet of the dehydration separator are all connected to the inlet of the filter, and the outlet of the filter is connected to the water inlet pipeline of the nanofiltration device; An inclined plate filler is also arranged in the middle and lower part of the crystallizer.

4. A method for extracting calcium sulfate from a nanofiltration concentrate to remove hardness, characterized in that: The method adopts the device described in any one of claims 1 to 3, and comprises the following steps: S1: sending the nanofiltration concentrated liquid to an electrocatalytic reactor for electrocatalytic treatment to obtain electrocatalytic reactor effluent and electrocatalytic precipitation slag; sending the electrocatalytic reactor effluent to an electrosoftening reactor for electrosoftening treatment to obtain electrosoftening reactor effluent and electrosoftening precipitation slag; S2: sending the effluent from the electric softening reactor and the crystallizer reflux water mixed with seed crystals into the crystallizer for mixing and stirring to obtain crystallizer effluent and calcium sulfate crystals; S3: mixing part of the crystallizer effluent with the seed crystals from the bottom of the seed crystal tank in a pipeline mixer to obtain the crystallizer reflux water mixed with the seed crystals; stirring and mixing the calcium sulfate crystals with the saturated calcium sulfate aqueous solution from the upper part of the seed crystal tank in a purifier to obtain purified waste liquid and purified calcium sulfate crystals; S4: sending the purified calcium sulfate crystals to a dehydration separator for dehydration treatment to obtain dehydrated water and calcium sulfate products.

5. The method for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 4, wherein: The method further comprises discharging the electrocatalytic precipitation slag and the electrosoftening precipitation slag; The current density of the electrocatalytic treatment is 20-30 mA / cm 2 ; The current density of the electric softening treatment is 5-10ma / cm 2 .

6. The method for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 4, wherein: The temperature in the crystallizer is 80-90°C; the stirring speed in the crystallizer is 80-120r / min; In the crystallizer, the amount of seed crystal introduced is 0.8-1.2% of the water inlet amount of the crystallizer.

7. The method for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 4, wherein: The method also includes using inclined plate fillers to re-agglomerate the calcium sulfate crystals during the precipitation process in the crystallizer to improve the precipitation efficiency.

8. The method for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 4, wherein: The method further comprises sending the dehydrated water, purified waste liquid and the remaining part of the crystallizer effluent to a filter for filtration, obtaining a filtrate and sending it to a nanofiltration device for treatment as nanofiltration influent; Preferably, the ratio of the crystallizer effluent fed into the pipeline mixer to the crystallizer effluent fed to the filter is (2.5-3.5):

1.

9. The method for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 4, wherein: The stirring speed in the purifier is 180-220r / min; In the purifier, the volume ratio of the saturated calcium sulfate aqueous solution to the calcium sulfate crystals is (2.5-3.5):

1.

10. The method for extracting calcium sulfate and removing hardness from nanofiltration concentrate according to claim 4, wherein: The dehydration treatment is carried out by centrifugation.

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