A technological process for extracting struvite from seawater desalination

Abstract

The present invention relates to a technological process for extracting struvite during the seawater desalination process, belonging to the technical field of seawater desalination. It includes the following steps: synthesizing polyacrylic acid-ethylene glycol phosphate-furfural alcohol interpenetrating network; preparing the coating; painting the steel plate; building the reaction device; and producing struvite. The beneficial effects of the present invention are as follows: Using the coating technology modified by special functional groups to assist in treating concentrated seawater effectively replaces the use of EWT reagents, simplifies the technological process and reduces the use of drugs; protects the metal inner wall of the reaction kettle from electrochemical corrosion, and eliminates the use of electromagnetic liquid complexing agents to avoid environmental pollution; significantly reduces the pressure before the membrane and is easy to promote.

Description

Technical Field

[0001] The present invention belongs to the technical field of seawater desalination, and particularly relates to a technological process for extracting struvite while performing seawater desalination. Background Art

[0002] In recent years, with the increasing shortage of global fresh water resources and the continuous progress of seawater desalination technology, obtaining fresh water from the sea to alleviate the increasingly serious global water crisis has not only become a consensus in the global scientific and technological community, but also the government's proposition and countermeasure for developing new water sources in coastal countries. As an alternative and incremental technology for fresh water resources, seawater desalination has received increasing attention and support, and has become a feasible solution and important approach to solve the global water crisis in the future.

[0003] Although China has achieved great results in seawater desalination technology in the past period, there are still problems such as the dependence on imports of some key components and materials, and the urgent need to break through core technologies. In addition, under the current water price system, the cost of seawater desalination is relatively high and lacks competitiveness compared with the price of tap water. Therefore, on the premise of ensuring the water quality of the produced water, by optimizing the system design, minimizing the investment and operating energy consumption costs, and comprehensively utilizing the effective components in the concentrated seawater of seawater desalination to share the water production cost, so as to comprehensively improve the efficiency of seawater desalination projects, is an important direction for promoting the development of seawater desalination in the future.

[0004] In the past, the way to deal with the concentrated seawater obtained from seawater desalination was usually direct discharge, but direct discharge would have a potential impact on the environment. To comprehensively utilize the concentrated seawater of seawater desalination, one way is to use the effective components in the concentrated seawater to provide source water for specific industrial products. The concentrated seawater contains a large amount of Mg 2+ , as well as some trace elements, which are one of the raw materials required for the production of high-quality struvite. Ammonium magnesium phosphate, commonly known as struvite, has the molecular formula MgNH4PO4·6H2O and belongs to amorphous precipitation, which is only slightly soluble in water. Its nutrient release rate is slower than that of other soluble fertilizers, and it can be used as a slow-release fertilizer. The slow-release fertilizer of ammonium magnesium phosphate produced by using the concentrated seawater of seawater desalination and ammonium phosphate solution as the main raw materials, adding an inducer, a high-energy magnetization solution, etc., has a high market prospect and economic benefit. The production requirements of this process are that the salt water used as the raw material should contain as many divalent ions, especially Mg 2+ , and at the same time retain the necessary trace elements in seawater. Therefore, applying the concentrated seawater obtained from seawater desalination to the production of struvite can share the cost of seawater desalination.

[0005] In the prior art, three intervention schemes for the extraction of ammonium magnesium sulfate are provided, including: the front-end intervention seawater desalination process flow for the extraction of ammonium magnesium phosphate; the middle intervention seawater desalination process flow for the extraction of ammonium magnesium phosphate; the end intervention seawater desalination process flow for the extraction of ammonium magnesium phosphate. Among them, the biggest problem faced in the front-end intervention process flow is that the concentrated seawater pumped into the reaction kettle is extremely likely to damage the reaction kettle. At the same time, in the traditional electromagnetic ionic liquid, the added complexing agent has the disadvantages of being used once, not being recyclable, and requiring sealed storage to avoid direct sunlight.

[0006] To meet the requirements of process water for the production of struvite and soda ash industries, etc., share the cost of seawater desalination, and at the same time reduce the operating energy consumption and drug cost of the seawater desalination system, it is urgent to optimize the seawater desalination process at present. Summary of the Invention

[0007] The purpose of the present invention is to provide an optimized process flow for the front-end intervention of the extraction of ammonium magnesium phosphate in the comprehensive utilization of concentrated seawater for the production of struvite industry.

[0008] Due to the existence of concentrated salt water electrolyte, electrochemical corrosion is extremely likely to occur in the reaction kettle cavity and pipeline. The reaction equation is as follows:

[0009] Anode: 2Fe–4e - ==2Fe 2+

[0010] Cathode: O2 + 2H2O + 4e- == 4OH -

[0011] The present invention provides a polymer material that can reversibly complex metal ions, which protects the reaction kettle and intervenes in the reaction regulation to replace the EWT reaction solvent:

[0012] I. Synthesis of polyacrylic acid-acryloyl cystamine-polyfurfuryl alcohol interpenetrating network

[0013] A. Synthesis of furfuryl alcohol resin (PFA): Add 100 g of furfuryl alcohol to a round-bottom flask, add 20 ml of deionized water, heat and stir under a nitrogen atmosphere, control the temperature at 60 °C for 30 minutes. Then, dilute 5 ml of concentrated sulfuric acid to 30 ml and gradually add it dropwise to the flask, controlling the temperature at 70-80 °C. Cool to room temperature, add ammonia water to adjust the pH to neutral, centrifuge the product, and vacuum dry overnight.

[0014] B. Synthesis of polyacrylic acid - acryloylcystamine (PAA - PACA): Add 1 g of acrylic acid into a round - bottom flask, then add 100 ml of deionized water. Heat and stir under a nitrogen atmosphere, control the temperature at 75 °C for 30 minutes. Drop 20 g of acrylic acid and an equivalent amount of ammonium persulfate into the reaction system respectively over 1 hour. After dropping, stir at a constant temperature for 2 hours. After the reaction is completed, dry overnight under vacuum. Grind the product, disperse it in toluene solution, add 20 - 30% of cystamine relative to the equivalent amount of acrylic acid added into the reaction system, protect it with nitrogen, heat and reflux. After 6 hours, filter the solid powder, wash it three times with dichloromethane, and then dry it;

[0015] C. Weigh PFA and PAA - PACA with a mass ratio of 1.5:1 and dissolve them in anhydrous ethanol. Stir and reflux at 80 °C, add 10% of the mass equivalent of N - hydroxymethylacrylamide, and react at a constant temperature for 2 hours to obtain polyacrylic acid - ethylene glycol phosphate - polyfurfuryl alcohol interpenetrating network structure (PAA - PACA - PFA).

[0016] II. Coating ratio:

[0017]

[0018]

[0019] Add deionized water and the raw materials in the above - mentioned ratio in the dispersion tank. After dispersing evenly, grind it on the machine until the particle size is less than 30 μm.

[0020] III. The struvite reaction device includes: a PAA - PACA - PFA filling column, a reactor, the inner wall of the pipeline, filter sand; the net and the stirring paddle are both sprayed with PAA - PACA - PFA;

[0021] The struvite reactor adopts a stirred - type reactor, and the reactor mainly includes 20 - 30 m 3 Two concentration tanks, which use a solar photovoltaic battery pack to assist in distillation and concentration. The purified distilled water is used as industrial distilled water, 3 m 3 Three reaction kettles, three feeding and discharging pumps, four metering pumps, one centrifuge, and all the water - passing equipment are sprayed and coated with PAA - PACA - PFA. The PAA - PACA - PFA filling column is a filling column connected before the reactor, with the same diameter as the pipeline and a length of 5 cm, filled with PAA - PACA - PFA mixed with SiO2 particles.

[0022] IV. Steps for producing struvite: Pump a certain amount of concentrated seawater into the reaction kettle, turn on the metering pump, and after stirring for 5 minutes, start pumping the inducer (NaH2PO4) and diluent (NH4HCO3) into the kettle with the metering pump. It takes about 1.5 - 2 hours to finish pumping. Adjust the pH value to about 8 with ammonia water, continue stirring for 1 hour, then use the discharge pump to pump the liquid into the centrifuge, and centrifuge for about 1 hour to discharge the material. The ratio of the structure-forming ions is n(Mg 2+ ):n(NH4 + ):n(PO4 3- ) = 1:1:1.03.

[0023] Specifically, the benefits of the present invention are as follows:

[0024] 1. Utilize the polymer interpenetrating network structure, introduce a coating technology modified with insoluble and reversibly utilizable special functional groups to assist in treating concentrated seawater, effectively replace the use of EWT reagents, simplify the process flow and reduce the use of drugs;

[0025] 2. Bond polymers containing functional groups such as hydroxyl (-OH), carboxyl (-COOH), and mercapto (-SH) to the inner walls of the pipelines and reaction kettles to protect the metal inner walls of the reaction kettles from electrochemical corrosion, and avoid using electromagnetic liquid complexing agents to prevent environmental pollution;

[0026] 3. Significantly reduce the pressure before the membrane, making this solution easy to promote. Description of the Drawings

[0027] Figure 1 Schematic diagram of the front-end intervention seawater desalination process for ammonium magnesium phosphate extraction;

[0028] Figure 2 Schematic diagram of the reversible conversion process of the coating during the production of ammonium magnesium phosphate;

[0029] Figure 3 Schematic diagram of the packed column. Detailed Embodiments

[0030] The present invention will be further described below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention and cannot be used to limit the protection scope of the present invention.

[0031] Example 1

[0032] Synthesis of a reversible metal ion complexing polymer material, polyacrylic acid - ethylene glycol phosphate - polyfurfuryl alcohol interpenetrating network:

[0033] 1) Synthesis of furfuryl alcohol resin (PFA): Add 100 g of furfuryl alcohol and 20 ml of deionized water into a round-bottom flask, heat and stir under a nitrogen atmosphere, control the temperature at 60 °C for 30 minutes. Then, dilute 5 ml of concentrated sulfuric acid to 30 ml and gradually add it dropwise into the flask, controlling the temperature at 70 - 80 °C. Cool down to room temperature, add ammonia water to adjust the pH to neutral. After centrifuging the product, dry it overnight under vacuum;

[0034] 2) Synthesis of polyacrylic acid - acryloyl cystamine (PAA - PACA): Add 1 g of acrylic acid and 100 ml of deionized water into a round-bottom flask, heat and stir under a nitrogen atmosphere, control the temperature at 75 °C for 30 minutes. Drop 20 g of acrylic acid and an equivalent amount of ammonium persulfate into the reaction system respectively, continuously for 1 hour. After dropping, stir at a constant temperature for 2 hours. After the reaction ends, dry it overnight under vacuum. Grind the product, disperse it into toluene solution, add 20 - 30% of cystamine relative to the equivalent amount of acrylic acid added into the reaction system, under nitrogen protection, heat and reflux. After 6 hours, filter the solid powder, wash it three times with dichloromethane, and then dry it;

[0035] 3) Weigh PFA and PAA - PACA with a mass ratio of 1.5:1 respectively, dissolve them in absolute ethanol, stir and dissolve under reflux at 80 °C, add 10% of the mass equivalent of N - hydroxymethyl acrylamide, and react at a constant temperature for 2 hours to obtain polyacrylic acid - ethylene glycol phosphate - polyfurfuryl alcohol interpenetrating network structure (PAA - PACA - PFA).

[0036] Example Two

[0037] Other parts are the same as those in Example One. The preparation of the polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network coating formula and spraying samples in Example One includes the following content:

[0038] The above coating components are: water: 12, PAA - PACA - PFA: 40, defoamer: 1 - 3, dispersant 1 - 3, titanium dioxide: 10, thickener: 0.2, film-forming aid: 3, barium sulfate: 10, waterborne epoxy resin: 30. Add deionized water and the raw materials in the table in the above proportions into a dispersion tank. After dispersing evenly, grind it on a machine until the particle size is lower than 30 μm; After grinding and degreasing the cold-rolled steel plate, spray the coating onto the sample plate at a speed of 0.3 ml / s, with a wet film thickness of ~75 μm, and dry it in an oven at 50 °C.

[0039] Example Three

[0040] The struvite reaction device includes: a PAA - PACA - PFA filling column, a reactor. The inner wall of the pipeline, the filter screen, and the stirring paddle are all sprayed with PAA - PACA - PFA;

[0041] The struvite reactor adopts a stirred reactor, and the reactor mainly includes 20 - 30 m3 There are 2 concentration tanks, which use a solar photovoltaic battery pack to assist in distillation and concentration. The purified distilled water is used for industrial distilled water, 3m 3 There are 3 reaction kettles, 3 discharging and feeding pumps, 4 metering pumps, 1 centrifuge, and the water passing equipment is coated with PAA - PACA - PFA.

[0042] The PAA - PACA - PFA packed column is connected before the reactor, has the same diameter as the pipeline, and is a packed column filled with PAA - PACA - PFA mixed with SiO2 particles with a length of 5 cm.

[0043] The concentrated seawater is evaporated and concentrated to 1 / 10 of its original volume using a rotary evaporator. After taking an appropriate amount of the concentrated liquid, elemental analysis is carried out using atomic spectroscopy to determine the content of Mg 2+ . The concentrated liquid is added to a metal container internally coated with PAA - PACA - PFA coating. After adding H3PO4 in an equimolar amount according to Mg 2+ and so on, ammonia water and 1 / 10 volume of EWT electronic water are added, and the pH is adjusted to 7.5 - 8.5 until the white precipitate struvite is produced. After grinding the produced struvite powder, X - ray powder diffraction is carried out. The ray dose is 40 keV, and the diffraction angle measurement range is 5 - 60°. According to the full width at half maximum of the diffraction peak, the crystallization degree of the produced struvite is determined. It can be seen that the crystallization degree of the produced struvite is good.

[0044] Example 4

[0045] The other parts are the same as those in Examples 1, 2, and 3. The steps of producing struvite using the device in Example 3 are as follows:

[0046] A certain amount of concentrated seawater is pumped into the reaction kettle. The metering pump is started. After stirring for 5 min, the inducer (NaH2PO4) and the diluent (NH4HCO3) are pumped in using the metering pump, which takes about 1.5 - 2 h to finish. Ammonia water is added to adjust the pH value to about 8. After continuing to stir for 1 h, the liquid material is pumped into the centrifuge using the discharging pump, and centrifuged for about 1 h to discharge. The ratio of the structure - forming ions n(Mg 2+ ):n(NH4 + ):n(PO4 3- ) = 1:1:1.03.

[0047] The following processes occur in the reaction:

[0048] A. Water pretreatment. Through the PAA - PACA - PFA interpenetrating network structure coating to assist crystallization, cations with a valence of more than two in seawater are basically removed, and ammonium magnesium sulfate precipitate is formed in this step. The calcium and magnesium removal rate is 95.85%, among which calcium is 91.06% and magnesium is 97.25%;

[0049] B. Seawater desalination. After the front-end intervention process, the recovery rate reaches 80%, and the pressure before the membrane is about 2.0 MPa.

[0050] Example Five

[0051] The present invention further analyzes the influence of the concentrated seawater quality on the removal rate of divalent cations such as calcium and magnesium; analyzes the influence of supersaturation, pH value, etc. on the crystal morphology, formation rate, yield and purity of magnesium ammonium phosphate:

[0052] A. Supersaturation: When the supersaturation of the reaction is between 1 and 5, the removal rate of phosphate and the formation of magnesium ammonium phosphate crystals can reach the best;

[0053] B. pH value: From the experimental results and theoretical analysis, a large amount of struvite is formed when the pH value is between 8.0 and 10.0, and the purity is relatively high;

[0054] C. Ratio of structure-forming ions: Mg 2+ , NH4 + , PO4 3- The stoichiometric ratio of the three to react to form struvite is 1:1:1. When the remaining mass concentration of NH4 + is between 30 and 80 mg / L, the purity is the highest. When the molar ratio of Mg 2+ PO4 3- is greater than 1, the formation of struvite is rapid, and the removal amount of phosphorus increases with the increase of the ratio between the two. However, when n(Mg 2+ ) / n(PO4 3- ) > 1.05, there is no significant effect on the removal rate of phosphorus;

[0055] D. Coprecipitation of calcium ions: The increase in the concentration of Ca 2+ will reduce the size of struvite crystals, inhibit the growth of struvite, and even replace struvite to form amorphous calcium phosphate;

[0056] E. Reaction time: The reaction is a reaction driven by kinetics. Prolonging the time will not increase the removal rate of phosphorus. However, prolonging the reaction time can increase the particle size of struvite crystals. A particle size of 2 - 4 mm is beneficial for industrial processing and better exerting the slow-release effect of struvite.

[0057] Example Six

[0058] The paint sample plates produced in Examples 1 and 2 and the comparison steel plates are placed in a beaker containing concentrated seawater with 10 times the volume of seawater for an aging test. After 30 days, the result shows that the steel plates coated with the paint have no obvious change and have better stability.

Claims

1. A technological process for extracting struvite from the seawater desalination process, characterized in that: It includes the following steps:

1. Synthesize polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network; 2. Prepare the coating and apply it to the steel plate. The components of the coating include: polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network; 3. Prepare the gel filling column; 4. Set up the reaction device; 5. Produce struvite; The synthesis of polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network adopts the following steps: A. Synthesis of furfuryl alcohol resin: Add 100 g of furfuryl alcohol to a round - bottom flask, add 20 ml of deionized water, heat and stir under a nitrogen atmosphere, control the temperature at 60 °C. After 30 minutes, dilute 5 ml of concentrated sulfuric acid to 30 ml and gradually add it dropwise to the flask, control the temperature at 70 - 80 °C, cool down to room temperature, add ammonia water to adjust the pH to neutral. After centrifuging the product, dry it overnight under vacuum; B. Synthesis of polyacrylic acid - acryloyl cystamine: Add 1 g of acrylic acid to a round - bottom flask, add 100 ml of deionized water, heat and stir under a nitrogen atmosphere, control the temperature at 75 °C for 30 minutes. Then, drop 20 g of acrylic acid and an equivalent amount of ammonium persulfate into the reaction system respectively, for 1 hour. After dropping, stir at a constant temperature for 2 hours. After the reaction is completed, dry it overnight under vacuum. Grind the product and disperse it in toluene solution. Add 20 - 30% of cystamine relative to the equivalent amount of acrylic acid added to the reaction system, under nitrogen protection, heat and reflux. After 6 hours, filter the solid powder, wash it three times with dichloromethane, and then dry it; C. Weigh furfuryl alcohol resin and polyacrylic acid - acryloyl cystamine with a mass ratio of 1.5:1 and dissolve them in anhydrous ethanol, stir and reflux at 80 °C, add 10% of the mass equivalent of N - hydroxymethylacrylamide, and react at a constant temperature for 2 hours to obtain a polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network structure; The reaction device for struvite includes: a polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network structure filling column, a reactor, the inner wall of the pipeline, a filter screen, and a stirring paddle, all of which are sprayed with polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network structure materials; The struvite reactor uses a stirred reactor, and the reactor mainly includes 20 - 30m 3 Two concentration tanks, which use a solar photovoltaic battery pack for auxiliary distillation and concentration. The purified distilled water is used as industrial distilled water, 3m 3 Three reaction kettles, three feeding and discharging pumps, four metering pumps, one centrifuge, and the water passing equipment are all sprayed and coated with a polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network structure; The polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network structure filling column is connected before the reactor, has the same diameter as the pipeline, and is a filling column with a length of 5 cm and a mixture of polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network structure and SiO2 particles; 2. The technological process for extracting struvite in the seawater desalination process according to claim 1, wherein: The steps for producing struvite: Pump a certain amount of concentrated seawater into the reaction kettle, start the metering pump, and after stirring for 5 minutes, start to pump the inducer NaH2PO4 into the kettle using the metering pump. The diluent NH4HCO3 is added within 1.5 - 2 hours. Adjust the pH value to about 8 by adding ammonia water, continue stirring for 1 hour, then use the discharge pump to pump the liquid material into the centrifuge, and centrifuge for about 1 hour to discharge the product. The ratio of the structure-forming ions is n(Mg 2+ ):n(NH4 + ):n(PO4 3- ) = 1:1:1.

03.

3. The technological process for extracting struvite from the seawater desalination process according to claim 1, wherein: The preparation of the coating includes the following content: The components of the coating are: water: 12 parts, polyacrylic acid - acryloyl cystamine - polyfurfuryl alcohol interpenetrating network structure: 40 parts, defoamer: 1 - 3 parts, dispersant: 1 - 3 parts, titanium dioxide: 10 parts, thickener: 0.2 parts, film - forming aid: 3 parts, barium sulfate: 10 parts, water - based epoxy resin: 30 parts. Add deionized water and the above - proportioned raw materials to a dispersion tank, disperse evenly, and then grind on a machine until the particle size is less than 30 μm; 4. The technological process for extracting struvite from the seawater desalination process according to claim 1, characterized in that: The application of the coating to the steel plate is to polish and degrease the cold - rolled steel plate, then spray the coating onto the sample plate at a speed of 0.3 ml / s, with a wet film thickness of ~75 μm, and dry it in an oven at 50 °C.

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