Advanced treatment method and system for circulating ash water in coal chemical industry

Abstract

The invention belongs to the technical field of coal chemical industry grey water treatment, and particularly relates to an advanced treatment method and system for coal chemical industry grey water circulating water. The method integrates pretreatment, complexing cracking, fractional crystallization and deep deamination, and is characterized in that a chelating structure of an organic phosphorus dispersing agent and calcium and magnesium ions is thoroughly destroyed through an advanced reduction-advanced oxidation combined process, target ions are released, and the core problems that hardness removal is hindered and phosphorus pollution is caused are solved; then, calcium and magnesium are respectively converted into calcium carbonate and magnesium ammonium phosphate particles for resource recovery by utilizing a secondary crystallization technology, so that the bottlenecks of high alkali liquor consumption and salt accumulation of a traditional high-pH magnesium ion precipitation method are broken through; and finally, the residual ammonia nitrogen is deeply removed by adopting a pollution-resistant PTFE hydrophobic membrane, so that the residual ammonia nitrogen is efficiently and stably removed, and the problem that a traditional polypropylene material membrane system is easy to pollute and block, easy to oxidize and difficult to apply to the field is solved.

Description

Technical Field

[0001] This invention belongs to the field of coal chemical ash water treatment technology, specifically relating to a method and system for deep treatment of coal chemical ash water circulating water. Background Technology

[0002] In coal chemical production, the syngas scrubbing and purification process generates a large amount of ash water with high hardness, high ammonia nitrogen, and dispersant content. Current mainstream processes use organophosphorus scale inhibitors and dispersants (such as HEDP and ATMP) to "inhibit" scale formation, which is a method of "transferring" rather than "removing" contaminants. This leads to the continuous accumulation of hardness ions and dispersants in the circulating water, causing three major problems: first, the system is forced to periodically discharge large amounts of water for replacement, wasting water resources; second, the dispersants, as recalcitrant organophosphorus pollutants, cause eutrophication of the external wastewater; and third, the high hardness and organic matter severely pollute subsequent advanced treatment units such as ammonia removal membranes, rendering them inefficient or even ineffective.

[0003] Currently, while individual technologies such as advanced oxidation, chemical precipitation, and membrane separation exist for treating similar wastewater, they all have limitations. Advanced oxidation struggles to efficiently break down stable organophosphorus-metal complexes; simple chemical precipitation produces large amounts of sludge and cannot achieve resource recovery of ammonia, phosphorus, and magnesium; conventional polymeric ammonia removal membranes are easily damaged by oxidants and microorganisms. Therefore, there is an urgent need for an integrated and innovative process that can systematically break down complexes, synergistically remove multiple pollutants, and achieve resource recovery to overcome these industry challenges.

[0004] For ash wastewater generated from syngas scrubbing in industries such as coal chemical engineering, existing technologies rely on adding organophosphorus dispersants (such as HEDP) to inhibit scaling. This leads to the continuous accumulation of calcium and magnesium hardness ions in the circulating water, increasing the risk of scaling and severely interfering with the efficiency of subsequent advanced treatment units (such as ammonia removal membranes). Simultaneously, the recalcitrant dispersants themselves cause excessive total phosphorus levels in the effluent, resulting in secondary pollution. Furthermore, conventional ammonia removal technologies face the contradiction between microbial growth and the erosion of membrane materials by chemical oxidants.

[0005] Therefore, the key technical challenges facing ash water treatment in the coal chemical industry are: 1. Solving the problem of dispersant complexation: It efficiently breaks down the stable soluble complexes formed by dispersants and calcium and magnesium ions, releasing hardness ions and creating conditions for deep hardening removal, without secondary pollution of sludge or waste gas.

[0006] 2. Achieve low-cost hardening removal and carbon dioxide fixation: Utilize calcium ions in grey water to simultaneously achieve deep hardening removal and carbon dioxide mineralization fixation through induced crystallization granulation technology, generating valuable granular calcium carbonate products.

[0007] 3. Synergistic removal and resource recovery of ammonia, phosphorus, and magnesium: Under mild conditions (pH≈9), ammonia nitrogen, phosphate (produced by the oxidation of dispersant) and magnesium ions are synergistically removed and recovered in the form of magnesium ammonium phosphate (struvite) through a two-stage cooling crystallization technology, breaking through the bottleneck of large alkali consumption and salt accumulation in traditional high pH precipitation methods.

[0008] 4. Achieve stable deep ammonia removal under harsh water conditions: The use of a hydrophobic membrane made of PTFE material resistant to chemical and biological fouling can efficiently and stably remove residual ammonia nitrogen after breaking down complexes and removing most of the hardness and impurities. This solves the pain points of traditional polypropylene membrane systems, such as easy fouling and oxidation, which make them difficult to apply in this field. Summary of the Invention

[0009] This invention provides a four-stage deep treatment system and method integrating "pretreatment-complexation cracking-graded crystallization-deammonia removal". Its core lies in first thoroughly destroying the chelate structure between the organophosphorus dispersant and calcium and magnesium ions through a combined "advanced reduction-advanced oxidation" process, releasing the target ions; then, using "secondary crystallization" technology, calcium and magnesium are converted into calcium carbonate and magnesium ammonium phosphate particles for resource recovery, or finally, a fouling-resistant PTFE hydrophobic membrane is used for deep removal of residual ammonia nitrogen.

[0010] To achieve the above objectives, the present invention provides a method for deep treatment of circulating water in coal chemical ash processes, the specific steps of which are as follows: (1) Pretreatment and complexation breakdown: Ash water passes through a bag filter, a UV photoreduction reactor, and a high-concentration ozone microbubble oxidation reactor in sequence; a reducing agent is added in the UV photoreduction reactor, and strong reducing free radicals (such as eaq-) are generated under 254nm ultraviolet light excitation, which attack key sites such as CP bonds in the dispersant molecules and initially destroy the complex; under the action of high-concentration ozone microbubbles generated by a specially designed plate ozone generator, deep oxidation is carried out to completely mineralize organic phosphorus into inorganic phosphate and completely release calcium and magnesium ions.

[0011] (2) Primary calcium crystallization and CO2 fixation: The effluent after dewatering enters the primary fluidized bed crystallization granulation reactor to remove and recover calcium ions; (3) The effluent after decalcification in step (2) enters the secondary MAP crystallization reactor to remove magnesium, ammonia and phosphorus, and carry out secondary ammonium magnesium phosphate crystallization; Alternatively, a membrane method can be used for efficient and deep ammonia removal.

[0012] Preferably, in step (1), the UV light reduction treatment includes adding sodium sulfite or sodium thiosulfate as a reducing agent and controlling the oxidation-reduction potential (ORP) between -50 mV and -200 mV.

[0013] Preferably, the high-concentration ozone concentration in step (1) is 180-250 mg / L, and the ozone injection rate is 20-30 g / m³. 3 Grey water is circulated and introduced into the oxidation tower from the bottom using a 50-200 micron microbubble aerator, with a residence time (HRT) of 20-30 minutes.

[0014] Preferably, in step (2), sodium carbonate is added or a gas containing CO2 is introduced to control the pH at 9-10, the temperature at 30-80℃, and the upward flow rate at 20-70 m / h, so that calcium ions grow into particles with controllable particle size on the surface of the seed crystal in the form of calcium carbonate, thereby achieving hardening and carbon fixation; the seed crystal is fine sand or garnet powder.

[0015] Preferably, in step (3), during the secondary crystallization of magnesium ammonium phosphate, the pH is controlled between 8.8 and 9.2 by online pH, magnesium ion, and phosphate sensors, and the water temperature is lowered to below 30°C by air cooling to promote the precipitation of Mg in the solution. 2+ NH4 + and PO4 3- This process forms magnesium ammonium phosphate crystals, achieving simultaneous resource-based removal of magnesium, ammonia, and phosphorus.

[0016] Preferably, step (3) employs membrane deammoniation. The effluent from step (2) enters a membrane deammoniation module with a polytetrafluoroethylene (PTFE) hollow fiber hydrophobic membrane as its core. The pH is adjusted to above 11, and ammonia nitrogen vaporizes in the form of NH3, permeates the membrane pores, and is absorbed by the acid on the other side, achieving deep removal and recovery of ammonia nitrogen. When the acid is sulfuric acid, ammonium sulfate is obtained. The excellent chemical inertness of the PTFE membrane can withstand oxidants that may remain at the front end and prevent biofouling.

[0017] The system used in the deep treatment method for circulating water of coal chemical ash includes: a complexation breaking module and a staged crystallization module connected sequentially along the water flow direction; the complexation breaking module includes at least a bag filter, a UV photoreduction reactor, and an ozone microbubble oxidation reactor connected sequentially; the staged crystallization module includes at least a primary fluidized bed crystallization reactor, and also includes a secondary magnesium ammonium phosphate crystallization reactor or a PTFE hydrophobic membrane deammoniation component.

[0018] Preferably, the ozone microbubble oxidation reactor is connected to a plate ozone generator.

[0019] Preferably, the secondary magnesium ammonium phosphate crystallization reactor is connected to an air-cooled cooling device.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. Eliminate dispersant pollution: By using a combination of reduction and oxidation, the complexation effect of organophosphorus dispersants is fundamentally broken, solving the core problems of hindering hardening and causing phosphorus pollution.

[0021] 2. High resource utilization efficiency and low cost: Through staged crystallization, high-purity calcium carbonate and magnesium ammonium phosphate products are recovered separately, realizing "turning waste into treasure". MAP crystallization is carried out under near-neutral conditions, which significantly reduces alkali consumption and subsequent acid adjustment costs compared to the traditional high-pH precipitation method, and avoids the accumulation of salt in the system.

[0022] 3. Thorough water purification: Hardness (Ca) 2+ Mg 2+ The removal rate is >90%, the total ammonia nitrogen removal rate is >95%, the dispersant degradation rate is >95%, and the effluent meets the high standard of reuse requirements.

[0023] 4. Stable system operation: The terminal adopts a PTFE deammoniation membrane, whose excellent oxidation resistance and fouling resistance ensure the long-term stable operation of the deep treatment unit under complex influent conditions, solving the short lifespan problem of traditional deammoniation processes. Attached Figure Description

[0024] Figure 1 This is a schematic diagram comparing the advanced reduction and advanced oxidation mechanisms in this invention.

[0025] Figure 2 This is a flow chart of the greywater recycling process for hardness removal and reuse according to the present invention.

[0026] Figure 3 This is a schematic diagram of a fluidized bed crystallization granulation reactor.

[0027] Figure 4 This diagram shows the process and equipment for secondary magnesium ammonium phosphate crystallization granulation.

[0028] Figure 5 This is a schematic diagram illustrating the deamination principle of PTFE hydrophobic membranes. Detailed Implementation

[0029] The present invention will be further described below with reference to specific embodiments. Obviously, the described embodiments are only a part of the embodiments, not all of the embodiments. Modifications or substitutions to the details and form of the technical solutions of the present invention made by those skilled in the art without inventive effort should fall within the protection scope of the present invention.

[0030] Example 1 A method for deep treatment and reuse of coal chemical ash water circulating water involving secondary hardening, phosphorus removal, and ammonia removal, which employs an advanced reduction-advanced oxidation mechanism, such as... Figure 1 As shown in the figure. For 1 m³ / h of grey water, according to Figure 2 The process in question is being carried out in a pilot-scale trial, with the following technical parameters: Raw water quality: calcium ions 350 mg / L, magnesium ions 150 mg / L, ammonia nitrogen 300 mg / L, dispersant (calculated as HEDP) 160 mg / L, TOC 800 mg / L, SS 80 mg / L.

[0031] Core process parameters: (1) Ozone oxidation: plate generator, ozone generator outlet concentration is 210-230 mg / L, pressure is 0.35 MPa, 50-200 micron microbubble aeration oxidation time is 30 min.

[0032] UV light reduction: 254nm ultraviolet light, light intensity 300-500W / m 3 For water treatment, the sodium sulfite dosage is 100-200 mg / L, the ORP is controlled at -80 mV to -120 mV, and the residence time is 10-60 seconds.

[0033] (2) Primary calcium crystallization: The temperature of the circulating water is 50-90℃. The upward flow rate is controlled by the circulating water flow rate at 10-50 m / h. The crystallizer is a three-bladed paddle mixer. The stirring speed is controlled at 20-50 rpm. The amount of 80-100 mesh silica sand or garnet seed crystals added is 10-30% of the reactor volume. The carbon dioxide flow rate is 15-20 L / m³. 3 The inlet pressure is greater than 0.2 MPa. Aeration is introduced from the bottom of the crystallization reactor via jet aeration along with the inlet water. The pH inside the crystallizer is automatically controlled at 8.5-9.5 by a liquid alkali metering pump. The reactor used is as follows: Figure 2 As shown.

[0034] (3) A membrane deammoniation module with a polytetrafluoroethylene hollow fiber hydrophobic membrane as the core is used, with the pH adjusted to above 10. The schematic diagram of the PTFE hydrophobic membrane deammoniation principle is shown below. Figure 5 As shown.

[0035] The processing results are shown in Table 1: Table 1. Treatment effects of grey water before and after treatment in Example 1

[0036] Example 2 Demonstration project for 10 m³ / h grey water treatment Raw water quality: The raw water used in Example 1 was adopted, and the treatment scale was increased by 10 times.

[0037] Process adjustments: The pretreatment uses multi-stage bag filters in parallel; the ozone system uses a modular combination of multiple plate generators; the crystallization reactor is scaled up proportionally and the internal circulation flow field is optimized.

[0038] The difference from Example 1 is that step (3) uses two-stage MAP crystallization (magnesium ammonium phosphate crystallization): pH=9.0, air-cooled to below 30°C, and 5% (by weight) of magnesium ammonium phosphate seed crystals are added. The process and equipment used are as follows: Figure 3 As shown.

[0039] Treatment results: The effluent water quality indicators were comparable to those of Example 1, with total hardness consistently below 70 mg / L and ammonia nitrogen consistently below 20 mg / L, confirming the reliability and scalability of the process. The system produces approximately 12 tons of calcium carbonate granules and 4 tons of magnesium ammonium phosphate granules annually, achieving resource recovery benefits.

[0040] The processing results are shown in Table 2: Table 2. Treatment effects of grey water before and after treatment in Example 2

[0041] This invention addresses the field of hardness removal in coal chemical ash water circulating water and has the following advantages: 1. The complexation and decomposition process of "UV light reduction-high concentration ozone microbubble oxidation" synergistic, especially the technical parameters and combination methods of using a plate ozone generator to generate high concentration ozone of 180-250 mg / L.

[0042] 2. "Graded crystallization" resource recovery technology: including specific parameters for primary high-temperature, high-pH fluidized bed crystallization to recover calcium carbonate (pH 9-10, temperature 50-80℃, upward flow rate 20-70 m / h), and specific parameters for secondary low-temperature induced crystallization to recover magnesium ammonium phosphate (pH 8.8-9.2, temperature <20℃).

[0043] 3. Application of the deep ammonia removal unit based on PTFE hydrophobic membrane in grey water after the aforementioned pretreatment and crystallization processes, especially its role as a final guarantee process, is an innovative system combination that solves the problem of ammonia removal under harsh water quality conditions.

[0044] The complete process system is formed by the specific sequential connection and synergistic effect of the above core units (complexation cracking, hierarchical crystallization, and PTFE membrane deammoniation).

[0045] The combined process solves two core problems in one fell swoop: the difficulty in treating stable complexes in high-hardness ash water and the difficulty in removing hardness at a deep depth. The effluent meets the requirements for reuse.

[0046] In this invention, the secondary MAP crystallization granulation unit can be replaced by a "chemical precipitation-tubular membrane separation" unit. That is, after the primary calcium ion crystallization granulation process, alkali is added to adjust the pH to above 11.5 to generate magnesium hydroxide precipitate, which is then separated into solid and liquid phases using a tubular membrane. The separated supernatant then enters a PTFE membrane deammoniation unit for further deammoniation. This scheme also achieves the removal of magnesium and ammonia, but the resource-based product is magnesium hydroxide, and the alkali consumption is relatively high.

[0047] High-hardness coal chemical ash wastewater is treated using crystallization granulation technology, while simultaneously recovering two high-value products: Calcium carbonate (CaCO3) granules: high-purity industrial raw material; magnesium ammonium phosphate (MAP) granules: high-quality slow-release fertilizer. This achieves a fundamental shift from "waste treatment" to "resource creation."

[0048] The characteristics of the calcium carbonate granules obtained in the embodiments of the present invention are shown in Table 3.

[0049] Table 3. Characteristics of calcium carbonate granules obtained in the embodiments of the present invention

[0050] The characteristics of the magnesium ammonium phosphate granules obtained by this invention are shown in Table 4: Table 4. Characteristics of the magnesium ammonium phosphate granules obtained in this invention

[0051] As can be seen from the above, this invention solves the problem of calcium ion interference with MAP purity by simultaneously producing two high-value products through a single process; it actively controls the purity, morphology and particle size of the products to ensure their market application value; and it transforms pollutants into slow-release fertilizers and industrial raw materials, thus realizing a circular economy.

[0052] This invention provides comparative data on the long-term operating flux decline of PTFE deammoniation membrane and conventional PP membrane under the same simulated feed water conditions, or experimental data on oxidant immersion resistance, to demonstrate its performance advantages.

[0053] This invention uses a PTFE deammoniation membrane for deammoniation, and its differences from conventional PP membranes are shown in Table 5: Table 5. Comparative Analysis of Core Performance of PTFE Deammoniation Membrane and Conventional PP Membrane

[0054] As shown in Table 5, under the same simulated influent (especially wastewater containing organic matter, grease, and other easily contaminants) during long-term operation, the flux decline rate of PTFE membranes is expected to be significantly slower than that of PP membranes. Its stronger antifouling ability results in a longer stable operating cycle and a lower cleaning frequency. In immersion experiments with oxidants of the same concentration and duration (such as 1000-5000 ppm NaOCl solution), the tensile strength of PP membranes will decrease significantly, and surface cracking may occur; while the physical and chemical properties of PTFE membranes are expected to remain almost unchanged.

Claims

1. A method for advanced treatment of coal chemical grey water circulating water, characterized in that, The method comprises the following steps: (1) pretreatment and complex breaking: the grey water passes through a bag filter, a UV light reduction reactor, and a high-concentration ozone micro-bubble oxidation reactor in sequence; a reducing agent is added in the UV light reduction reactor, and strong reducing free radicals are generated under excitation of 254 nm ultraviolet light to mineralize organic phosphorus into inorganic phosphate and release calcium and magnesium ions; (2) primary calcium crystallization and CO2 fixation: the water after breaking is introduced into a primary fluidized bed crystallization granulation reactor to remove and recover calcium ions; (3) the water after decalcification in step (2) is introduced into a secondary MAP crystallization reactor to remove magnesium, ammonia, and phosphorus and perform secondary magnesium ammonium phosphate crystallization. Or a membrane method is used for efficient deep ammonia removal treatment.

2. The method of claim 1, wherein, In step (1), the UV light reduction treatment comprises adding sodium sulfite or sodium thiosulfate as a reducing agent and controlling the oxidation-reduction potential ORP to be between-50 mV and-200 mV.

3. The method of claim 1, wherein, In step (1), the high-concentration ozone micro-bubble oxidation reactor is a plate-type ozone generator, the ozone concentration is 180-250 mg / L, and the ozone input is 20-30 g / m 3 Grey water is circulated in the water, and a 50-200 micron micro-bubble aerator is used to introduce the oxidation tower at the bottom, and the residence time HRT is 20-30 min.

4. The method of claim 1, wherein, In step (2), sodium carbonate is added or CO2-containing gas is introduced, the pH is controlled to be 9-10, the temperature is controlled to be 30-80℃, and the upflow velocity is controlled to be 20-70 m / h; the seed crystal is fine sand or garnet powder.

5. The method of claim 1, wherein, In step (3), when the secondary magnesium ammonium phosphate crystallization is performed, the pH is controlled to be 8.8-9.2 through online pH, online magnesium ion, and phosphate sensor control, and the water temperature is reduced to below 30℃ through air cooling to realize synchronous resourceization removal of magnesium, ammonia, and phosphorus.

6. The method of claim 1, wherein, In step (3), the membrane method is used for ammonia removal, and the water in step (2) is introduced into a membrane ammonia removal assembly with polytetrafluoroethylene hollow fiber hydrophobic membrane as the core, and the pH is adjusted to be higher than 11.

7. A coal chemical grey water recycling water advanced treatment and reuse system for implementing the method of any one of claims 1-6, characterized in that, It comprises: a complex breaking module and a staged crystallization module connected in sequence along the water flow direction; the complex breaking module at least comprises a bag filter, a UV light reduction reactor, and an ozone micro-bubble oxidation reactor connected in sequence; the staged crystallization module at least comprises a primary fluidized bed crystallization reactor and further comprises a secondary magnesium ammonium phosphate crystallization reactor or a PTFE hydrophobic membrane ammonia removal assembly.

8. The system of claim 7, wherein, The ozone micro-bubble oxidation reactor is connected with a plate-type ozone generator.

9. The system of claim 7, wherein, The secondary magnesium ammonium phosphate crystallization reactor is connected with an air cooling device.

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