Purification method for recovering struvite from sewage

Abstract

The invention relates to a purification method for recovering struvite from sewage, which comprises the following steps: completely transferring solid-phase struvite and impurities wrapped by the struvite into a liquid phase through acid dissolution, and breaking the binding state of the impurities and the struvite to obtain a dissolved solution; active carbon is used for adsorbing free organic impurities in the dissolved solution, separation of the impurities and target ions is achieved, and a clean solution is obtained; and by adjusting the pH value, the target ions are recrystallized into high-purity struvite in the clean liquid. According to the method, a physical and chemical purification strategy of three-step coupling of dissolution, adsorption and recrystallization is adopted, an adsorbent regeneration cycle process is coupled, and the method has the functions of deep purification and product remodeling.

Description

Technical Field

[0001] This invention belongs to the field of wastewater resource recovery technology, specifically relating to a purification method for recovering struvite from wastewater. Background Technology

[0002] Phosphorus is a major pollutant causing eutrophication in water bodies. Traditional wastewater treatment technologies remove phosphorus as a pollutant, ultimately transferring it to sludge, resulting in a huge waste of phosphorus resources. Struvite crystallization is a mature phosphorus recovery technology that can remove nitrogen and phosphorus from wastewater while obtaining potentially valuable magnesium ammonium phosphate crystals.

[0003] However, the high operating costs of the struvite crystallization method hinder its large-scale application. Improving the economic value of the recovered products is a key breakthrough in enhancing the economic feasibility of this technology. Currently, the main resource utilization pathway for struvite is as a slow-release fertilizer, but its value is limited and market acceptance is low. Therefore, exploring new ways to utilize struvite for high-value purposes is of great significance.

[0004] Recent studies have found that struvite, rich in phosphorus and nitrogen, can decompose at high temperatures to produce substances with free radical scavenging capabilities, showing potential as a flame retardant or fire extinguishing agent. Some studies have preliminarily confirmed that properly treated struvite can achieve fire extinguishing performance comparable to commercial ammonium dihydrogen phosphate dry powder fire extinguishing agents. However, struvite recovered directly from sewage often contains various organic impurities, the presence of which significantly affects the stability and effectiveness of its fire extinguishing performance. Experiments show that the fire extinguishing effect of struvite from different sources (such as aquaculture wastewater and landfill leachate) varies significantly, mainly due to differences in impurity content. In particular, samples with higher total organic carbon content show a significant decrease in fire extinguishing efficiency.

[0005] Therefore, developing an efficient and economical method to remove organic impurities from wastewater-recycled struvite is a key prerequisite for realizing its high-value utilization (such as the preparation of high-performance dry powder fire extinguishing agents). Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a purification method for recovering struvite from wastewater. The method employs a three-step coupled physicochemical purification strategy of "dissolution-adsorption-recrystallization" and a coupled adsorbent regeneration cycle process. This method combines the functions of deep purification and product remodeling.

[0007] The purification method for recovering struvite from wastewater includes:

[0008] S1: By dissolving in acid, the solid struvite and its encapsulated impurities are completely transferred into the liquid phase, breaking the bond between the impurities and the struvite, and a solution is obtained.

[0009] S2: Activated carbon is used to adsorb free organic impurities in the solution, thereby separating the impurities from the target ions and obtaining a clean solution.

[0010] S3: By adjusting the pH value, the target ions are recrystallized into high-purity struvite in the clean solution.

[0011] Optionally, step S1 specifically involves: mixing the original guano from the wastewater with a dilute nitric acid solution in a dissolving tank equipped with a mechanical stirring device, stirring until the solid completely disappears, and obtaining a homogeneous clear or slightly turbid solution.

[0012] The role of dilute nitric acid is to provide hydrogen ions to completely dissolve struvite (MgNH4PO4·6H2O), releasing Mg. 2+ NH4 + and PO4 3- Ions, while simultaneously releasing the encapsulated or adsorbed organic impurities into the solution.

[0013] Optionally, the mass concentration of the dilute nitric acid solution is 8%-12%. If the concentration is too low, the dissolution efficiency will be low and the liquid consumption will be high; if the concentration is too high, it may cause nitrogen oxides to escape or aggravate the corrosion of the equipment.

[0014] Alternatively, the mass ratio of the original struvite to the dilute nitric acid solution is 1:(8-12). This ratio ensures complete dissolution, prevents the system from becoming too viscous, and facilitates subsequent stirring and mass transfer.

[0015] Alternatively, the stirring speed can be 200-400 rpm.

[0016] Optionally, step S2 specifically involves: transferring the above-mentioned solution to an adsorption tank, adding granular activated carbon, and performing contact adsorption under stirring; after adsorption is completed, stopping stirring, using the natural density difference between the activated carbon and the solution for sedimentation separation, and achieving separation through a precision sieve.

[0017] Activated carbon, with its large specific surface area and abundant surface functional groups, effectively captures various organic molecules in solution through physical and chemical adsorption. The target ion is Mg. 2+ NH4 + PO4 3- .

[0018] Alternatively, the dosage of activated carbon is 10-30 mg / L of solution, which is the cost-effective dosage based on the organic load (typically 100-500 mg / L in TOC) of typical wastewater struvite solutions.

[0019] Further optionally, the adsorption time is 0.5-2 hours to ensure adsorption equilibrium is reached; the stirring conditions are 200-400 rpm to ensure that the activated carbon particles are uniformly suspended and fully contacted with the solution; and the mesh size of the precision sieve is 300-400 mesh.

[0020] Alternatively, segmented or fluidized bed adsorption modes can be used to improve adsorption efficiency.

[0021] Optionally, the purified solution after separation needs to be tested for total organic carbon (TOC). This invention requires the TOC content of the purified solution to be below 0.5%. This threshold is a key indicator to ensure the stable and compliant fire extinguishing performance of recrystallized struvite. If the TOC exceeds the standard, the liquid can be returned to the adsorption tank for secondary adsorption until a clean solution is obtained.

[0022] Optionally, step S3 specifically involves: transferring the cleaned liquid to a recrystallization tank, adding an alkaline regulator under stirring, and recrystallizing; after crystallization, allowing it to settle, separating the solid and liquid, washing the solid with deionized water, and drying it at 60-100℃ to obtain the purified struvite product.

[0023] Alternatively, the pH of the solution can be adjusted to 9.0-10.0, preferably 9.5, using NaOH solution as the alkaline adjuster. This pH range is the thermodynamically advantageous range for struvite crystallization, ensuring the preservation of Mg... 2+ NH4 + PO4 3- It efficiently binds crystals while inhibiting the formation of other phosphate impurities (such as magnesium phosphate).

[0024] Alternatively, the crystallization process can be carried out with gentle stirring at 100-400 rpm for 30-120 minutes. Stirring helps to promote uniform nucleation and growth and prevents crystal agglomeration.

[0025] Optionally, step S3 may include the following activated carbon regeneration and recycling steps: The saturated activated carbon is removed from the adsorption tank and regenerated in a regeneration furnace at 800-900℃ for 0.5-2 hours under inert gas (such as nitrogen) protection to restore its adsorption capacity and achieve recycling. The high temperature carbonizes and vaporizes the adsorbed organic matter (mainly CO2 and H2O), thereby clearing the pores of the activated carbon. The adsorption capacity of the regenerated activated carbon can be restored to more than 90% of its initial value.

[0026] The present invention has the following beneficial effects:

[0027] (1) The purification effect is significant and the versatility is strong: the first "dissolution-adsorption" impurity removal route for sewage struvite was created, which can deeply remove complex and different types of organic impurities, so that the TOC of the final product is stably controlled below 0.5%, which solves the problem of uncontrollable performance caused by impurity differences in struvite from different sources, and realizes the standardization of raw materials.

[0028] (2) Qualitative improvement in product performance: The thermal decomposition behavior of purified struvite is closer to that of pure struvite, and the release of flame-retardant active ingredients (such as NH3 and PO· free radicals) is more concentrated and efficient; After it is prepared into dry powder fire extinguishing agent, the key indicators such as fire extinguishing time and anti-reignition ability can be comparable to or even better than commercial ammonium dihydrogen phosphate, and the batch stability is excellent.

[0029] (3) High process integration, with purification and morphology reshaping functions: This method not only removes impurities, but also obtains new struvite crystals with more uniform particle size and morphology through recrystallization. This "reconstruction" process is more conducive to subsequent crushing, hydrophobic modification and dry powder compounding, simplifying the downstream fire extinguishing agent processing technology. Detailed Implementation

[0030] Example 1

[0031] This embodiment provides a purification method for recovering struvite from sewage, the specific steps of which are as follows:

[0032] (1) In the dissolving tank, add 100 kg of nitric acid solution with a mass fraction of 10%, then add 10 kg of unpurified guano powder recovered from aquaculture wastewater, turn on the top mechanical stirring device, and stir at a speed of 300 rpm until the solid is completely dissolved to obtain a homogeneous solution.

[0033] (2) The solution is pumped to the adsorption tank, and granular activated carbon is added to the adsorption tank. The amount added is calculated based on the solution volume as 20 mg / L. The mechanical stirring of the adsorption tank is turned on and stirred at 300 rpm for 1 hour to ensure that the activated carbon is in full contact with the solution and adsorbs organic matter.

[0034] (3) Turn off the stirring and let the activated carbon settle naturally. The solution is filtered and separated through the screen set at the bottom of the adsorption tank to obtain the purified solution. The total organic carbon content of the purified solution is 0.3%, which meets the requirements.

[0035] (4) The purified solution is pumped to the recrystallization tank, mechanical stirring is turned on, and sodium hydroxide solution is slowly added through the dosing port at a speed of 300 rpm to adjust the pH value of the solution to 9.5. Stirring is continued for 30 minutes, and struvite crystals can be seen to gradually precipitate out.

[0036] (5) Turn off the stirring, let it stand and settle, collect the stalagmite solids at the bottom from the outlet, dry them at 80°C, and obtain about 9.5 kg of purified stalagmite product;

[0037] (6) Collect the saturated activated carbon separated in the adsorption tank and send it to the activated carbon regeneration device. Regenerate it at 850°C for 2 hours under nitrogen protection, and then cool it down for later use.

[0038] Tests showed that the dry powder fire extinguishing agent prepared from the purified struvite product obtained in this embodiment, at a concentration of 1m³, exhibited good performance. 3 In the standard fire extinguishing test chamber, its fire extinguishing time is comparable to that of commercial ammonium dihydrogen phosphate fire extinguishing agent, and its performance is stable.

[0039] Example 2

[0040] This embodiment provides a purification method for recovering struvite from sewage, which is the same as in Embodiment 1, except that:

[0041] In step (1), the struvite is derived from the landfill leachate treatment process and has a high initial TOC content.

[0042] In step (2), the dosage of granular activated carbon is increased to 30 mg / L and the adsorption time is extended to 1.5 hours.

[0043] In step (4), adjust the pH to 10.0.

[0044] In step (6), the thermal regeneration temperature is 900℃.

[0045] After purification, the TOC content of the product decreased from the original 2.1% to 0.4%, and the fire extinguishing efficiency of the dry powder fire extinguishing agent prepared from it was significantly improved compared with that before purification, reaching the usable standard.

[0046] Example 3

[0047] This embodiment provides a purification method for recovering struvite from wastewater, similar to Embodiment 1, except that this embodiment purifies struvite powder recovered from landfill leachate via a stirred crystallization method. This raw material has a high impurity content (TOC approximately 2.5%) and poor fire extinguishing performance. The specific steps are as follows:

[0048] Dissolution: In a dissolution tank, mix 5 kg of the guano powder with 50 kg of 9% nitric acid solution and stir at 350 rpm for 40 minutes until completely dissolved.

[0049] Adsorption purification: The solution was pumped into the adsorption tank, and granular activated carbon was added at a relatively high ratio of 25 mg / L. Adsorption was carried out by stirring at 320 rpm, and the adsorption time was extended to 1.5 hours to ensure sufficient capture of complex organic matter.

[0050] Separation and monitoring: After settling, the purified solution was separated and the TOC was measured to be 0.4%.

[0051] Recrystallization: Transfer to a recrystallization tank, adjust the pH to 9.8 with NaOH solution while stirring at 300 rpm for 45 minutes. After crystallization, precipitate, wash, and dry at 80℃.

[0052] Results: The TOC of the purified product decreased to 0.4%. After ball milling it to about 20 μm and mixing it with hydrophobic silica, the extinguishing time in a 1 m³ fire extinguishing experiment was improved from ineffective extinguishing (>60 seconds) before purification to 8.2 seconds.

[0053] Example 4

[0054] This embodiment provides a purification method for recovering struvite from wastewater, which is the same as in Embodiment 1, except that: this embodiment processes granular struvite (particle size 1-2 mm) obtained from food wastewater through fluidized bed crystallization. This raw material itself has certain fire extinguishing properties, but its performance can be further improved. The specific steps are as follows:

[0055] Dissolving: Mix 10 kg of granular struvite with 100 kg of 11% nitric acid solution and stir to dissolve. Due to the dense nature of the particles, the dissolving time is slightly longer, approximately 60 minutes.

[0056] Adsorption purification: Given the low initial TOC of the raw material (approximately 0.8%), the activated carbon dosage was reduced to 15 mg / L, and adsorption was carried out by stirring for 1 hour.

[0057] Separation and monitoring: The TOC of the purified solution after separation was 0.2%.

[0058] Recrystallization: Adjust the pH to 9.5 and stir for 30 minutes to recrystallize. This step not only purifies the product but also transforms it from particulate matter into a uniform powder that is easier to pulverize later.

[0059] Results and Comparison: The TOC of the purified powder product was 0.2%. Comparative experiments showed that the extinguishing agent prepared from the purified product had a significantly shorter extinguishing time (4.1 seconds) than the unpurified product (5.8 seconds), and its anti-reignition performance was enhanced.

[0060] Example 5

[0061] This embodiment provides a purification method for recovering struvite from wastewater, which is the same as in Embodiment 1, except that: this example focuses on demonstrating the effect of activated carbon regeneration cycle and uses two-stage series adsorption to optimize purification efficiency. The specific steps are as follows:

[0062] Dissolution: 8 kg of struvite from mixed sources (aquaculture and food wastewater) was dissolved in 12% nitric acid at a mass ratio of 1:10.

[0063] Series adsorption: The solution first enters the first adsorption tank (add 20 mg / L of fresh activated carbon, adsorb for 1 hour), and the supernatant then enters the second adsorption tank (add 10 mg / L of regenerated activated carbon, adsorb for 0.5 hours).

[0064] Separation and monitoring: After two-stage adsorption, the TOC of the purified solution is extremely low, reaching 0.15%.

[0065] Recrystallization: Adjust the pH to 9.5 and crystallize to obtain a high-purity product.

[0066] Regeneration effect: The activated carbon saturated in the first adsorption tank was regenerated at 850℃ under N2 protection. Tests showed that the activated carbon after three regenerations still maintained more than 88% of the adsorption capacity of the same solution as the new carbon. This example demonstrates the feasibility and economy of activated carbon recycling.

[0067] Example 6

[0068] This embodiment provides a purification method for recovering struvite from wastewater, which is the same as in Embodiment 1, except that this example focuses on enhanced purification of specific impurities (oils). Struvite recovered from certain industrial wastewaters (such as slaughterhouse wastewater) may contain oily impurities. The specific steps are as follows:

[0069] Dissolving: Dissolve the grease-containing struvite with 10% nitric acid.

[0070] Adsorption purification: While adding conventional granular activated carbon (20 mg / L), an additional small amount of hydrophobically modified activated carbon (5 mg / L) was added to specifically enhance the adsorption of oil molecules. The mixture was stirred and adsorbed for 1.2 hours.

[0071] Separation and monitoring: The purified solution not only met the TOC standard (0.3%), but also showed that the characteristic peaks of the oil and fat basically disappeared through infrared spectroscopy.

[0072] Recrystallization: Recrystallization under normal conditions (pH 9.5).

[0073] Results: The final product not only met the fire extinguishing performance standards, but also improved the flowability of the raw material for the fire extinguishing agent, reducing the angle of repose from 52° before purification to 43°.

Claims

1. A purification method for recovering struvite from sewage, characterized by, include: S1: By dissolving in acid, the solid struvite and its encapsulated impurities are completely transferred into the liquid phase, breaking the bond between the impurities and the struvite, and a solution is obtained. S2: Activated carbon is used to adsorb free organic impurities in the solution, thereby separating the impurities from the target ions and obtaining a clean solution. S3: By adjusting the pH value, the target ions are recrystallized into high-purity struvite in the clean solution.

2. The purification method according to claim 1, characterized in that, Step S1 specifically involves mixing the original guano stone recovered from the sewage with a dilute nitric acid solution in a dissolving tank equipped with a mechanical stirring device, stirring until the solid completely disappears, and obtaining a homogeneous clear or slightly turbid solution.

3. The purification method of claim 2, wherein, The mass concentration of the dilute nitric acid solution is 8%-12%, the mass ratio of the original struvite to the dilute nitric acid solution is 1:(8-12), and the stirring speed is 200-400 rpm.

4. The purification method of claim 1, wherein, Step S2 specifically involves: transferring the above-mentioned solution to an adsorption tank, adding granular activated carbon, and performing contact adsorption under stirring; after adsorption is completed, stopping stirring, using the natural density difference between the activated carbon and the solution for sedimentation separation, and achieving separation through a precision sieve.

5. The purification method of claim 4, wherein, The activated carbon dosage is 10-30 mg / L of solution, the adsorption time is 0.5-2 hours to ensure adsorption equilibrium is reached; the stirring conditions are 200-400 rpm, and the mesh size of the precision sieve is 300-400 mesh.

6. The purification method of claim 4, wherein, The purified solution after separation needs to be tested for total organic carbon. The TOC content of the purified solution must be less than 0.5% to ensure that the fire extinguishing performance of recrystallized struvite is stable and meets the standards. If the TOC exceeds the standard, the liquid will be returned to the adsorption tank for secondary adsorption until a clean solution is obtained.

7. The purification method according to claim 1, characterized in that, Step S3 is as follows: the clean liquid is transferred to a recrystallization tank, an alkaline regulator is added under stirring, and recrystallization is carried out; after crystallization, the liquid is allowed to settle, the solid and liquid are separated, the solid is washed with deionized water and dried at 60-100℃ to obtain the purified struvite product.

8. The purification method according to claim 7, characterized in that, Adjust the pH of the solution to 9.0-10.0, using NaOH solution as the alkaline adjuster. Crystallize for 30-120 minutes with gentle stirring at 100-400 rpm.

9. The purification method according to claim 1, characterized in that, After step S3, the following activated carbon regeneration and recycling steps may also be included: the saturated activated carbon is taken out from the adsorption tank and heated in a regeneration furnace at 800-900℃ for 0.5-2 hours under inert gas protection to regenerate it, so as to restore its adsorption capacity and realize recycling.