Process for treating high phosphorus-containing wastewater

Abstract

This invention discloses a treatment process for high-phosphorus wastewater. The method involves mixing waste hydrochloric acid with ammonium-containing, high-phosphorus wastewater, and then adjusting the pH to 4.0-5.5 in the first stage to separate Fe. 3+ The generated ferric hydroxide flocs are then subjected to a second-stage pH adjustment to 8.0-9.5, utilizing Fe... 2+ In-situ ferrous hydroxide particles are generated as seed crystals, and magnesium salts are added for fluidized bed crystallization to recover magnesium ammonium phosphate. After crystallization and separation, the remaining ferrous hydroxide particles are dissolved in acid to release Fe. 2+ This invention is used for Fenton oxidation deep treatment, and finally neutralization and precipitation yield iron sludge; it realizes Fe through the stepwise utilization of iron ion valence states. 3+ with Fe 2+ The separation and synergy of these technologies not only yielded high-purity magnesium ammonium phosphate products but also provided a self-supplied iron source for the Fenton stage, solving the problems of low phosphorus recovery purity and the need for external ferrous salts in existing technologies. This approach demonstrates significant resource utilization benefits and cost advantages.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a process and method for treating high-phosphorus wastewater. Background Technology

[0002] Existing treatment methods for high-phosphorus wastewater, besides chemical precipitation, biological methods, and adsorption, also include using waste hydrochloric acid (pickling waste liquid) from the steel industry as an iron or acid source for co-treatment of phosphorus-containing wastewater. Some existing processes mix waste hydrochloric acid with phosphorus-containing wastewater and adjust the conditions to alkaline in one step, allowing iron ions to precipitate completely and removing some phosphates through flocculation. Other processes oxidize the iron ions in waste hydrochloric acid to produce coagulants such as polyferric sulfate, which are then added to the phosphorus-containing wastewater for flocculation and phosphorus removal.

[0003] Existing methods for treating high-phosphorus wastewater using waste hydrochloric acid have several drawbacks. First, the waste hydrochloric acid also contains a high concentration of Fe. 2+ and Fe 3+ While there is some overlap in the pH range of hydrolysis precipitation, the two are not entirely identical. When a one-time alkali-adjusting precipitation is used, both valence states of iron ions are simultaneously converted into hydroxide precipitates. These precipitates mix with magnesium ammonium phosphate or calcium phosphate precipitates in the wastewater, resulting in a complex composition of the sludge after solid-liquid separation. This makes it impossible to obtain high-purity phosphorus recovery products, and also makes it difficult to utilize the iron sludge separately. Secondly, when Fenton oxidation is used for further treatment in the subsequent process, the iron ions brought in by the waste hydrochloric acid are almost completely removed in the initial precipitation step. The concentration of residual dissolved iron ions in the wastewater entering the Fenton reactor is extremely low, requiring the addition of catalysts such as ferrous sulfate. This not only weakens the resource utilization value of the waste hydrochloric acid but also increases reagent costs. If attempts are made to retain the iron-containing precipitate to the Fenton stage, it will affect the purity of the phosphorus recovery products, creating a dilemma.

[0004] Therefore, in response to the problems mentioned above, this invention proposes a treatment process for high-phosphorus wastewater. Summary of the Invention

[0005] To overcome the problems in existing technologies, such as mixed precipitation due to the lack of differentiation of iron ion valence states, low purity of phosphorus recovery products, and the lack of a stable iron source in the Fenton oxidation stage requiring the addition of ferrous salts, this invention proposes a treatment process for high-phosphorus wastewater. This method is based on the fact that Fe in waste hydrochloric acid... 3+ with Fe 2+ The difference in pH during hydrolysis was addressed by adjusting the pH in stages and combining this with intermediate solid-liquid separation, first removing Fe... 3+ Removed separately in the form of ferric hydroxide, and then the Fe was adjusted. 2+Fine ferrous hydroxide particles are generated in situ under alkaline conditions to serve as seed crystals for magnesium ammonium phosphate crystallization. High-purity magnesium ammonium phosphate is then recovered via fluidized bed crystallization. The residual ferrous hydroxide particles in the wastewater are acid-dissolved to release Fe before Fenton oxidation. 2+ This achieves a synergistic unity of stepwise utilization of iron ion valence states, purification of phosphorus products, and self-supply of Fenton catalyst.

[0006] The technical solution of this invention is a treatment process for high-phosphorus wastewater, comprising the following steps: S1, waste hydrochloric acid and ammonium- and high-phosphorus wastewater are mixed in a mixing tank at a volume ratio of 1:5-1:10. The free hydrochloric acid in the waste hydrochloric acid acidically decomposes the phosphorus-containing substances in the ammonium- and high-phosphorus wastewater into orthophosphate, while the Fe carried in the waste hydrochloric acid... 3+ with Fe 2+ The ions exist stably in their ionic state under strongly acidic conditions and form soluble coordination pre-complexes with phosphate ions in the wastewater, resulting in an acidic mixed solution. The molar ratio of ammonium ion concentration to total phosphorus concentration in the ammonium-high phosphorus wastewater is ≥0.8:1, the total phosphorus concentration is 500-5000 mg / L, and the pH is 1.0-4.0. The waste hydrochloric acid is steel pickling wastewater, with a free hydrochloric acid mass fraction of 3%-8% and a total iron ion concentration of 50-120 g / L, of which Fe... 3+ It accounts for 10%-40% of the total mass of iron ions; S2, the acidic mixture is introduced into the first pH adjustment device, and alkali solution is added to adjust the pH to 4.0-5.5. The reaction is allowed to proceed for 10-30 minutes, allowing Fe to... 3+ Complete hydrolysis produces ferric hydroxide Fe(OH)3 flocs, while Fe... 2+ Under these pH conditions, the Fe(OH)3 flocs do not hydrolyze and remain in the solution. After being separated and removed by a solid-liquid separation device, a clear liquid is obtained, which mainly contains PO42-. 3- NH4 + Fe 2+ With a small amount of Ca 2+ Mg 2+ Residual ions; S3, input the clarified liquid into the second pH adjustment device, continue to add alkali solution to adjust the pH to 8.0-9.5, and let the reaction stand for 10-20 minutes to allow the Fe in the clarified liquid to rise. 2+ Partial hydrolysis generates tiny precipitate particles of ferrous hydroxide Fe(OH)2 with a particle size of 0.1-10 μm, which serve as heterogeneous seed crystals for subsequent crystallization of magnesium ammonium phosphate, resulting in an alkaline mixture containing Fe(OH)2 seed crystals. S4. The alkaline mixed solution is fed into the fluidized bed crystallization reactor. Water-soluble magnesium salts are added according to the residual total phosphorus concentration in the clarified solution at a magnesium-to-phosphorus molar ratio of 1.0:1-1.5:1. The pH in the fluidized bed crystallization reactor is controlled to be stable at 8.5-9.5, with a hydraulic retention time of 30-120 min and an upflow velocity of 5-30 m / h, to reduce the NH4+ in the wastewater. + PO4 3- With Mg 2+ Heterogeneous nucleation occurs on the surface of Fe(OH)2 seed crystals, which grow into magnesium ammonium phosphate (MgNH4PO4·6H2O, MAP) crystal particles with an average particle size of 0.3-2.0 mm, resulting in a crystallization slurry containing MAP crystals and residual Fe(OH)2 particles. S5, the crystallization slurry is fed into the first solid-liquid separation unit to separate MAP crystallized solid product and separation wastewater. The separation wastewater contains residual Fe(OH)2 particles that did not participate in crystallization and a small amount of dissolved Fe. 2+ ; S6. The separated wastewater is fed into the oxidation reaction unit. First, acid is added to the unit to adjust the pH of the wastewater to 3.0-4.0, so that the residual Fe(OH)2 particles dissolve and are converted into free Fe. 2+ Then add hydrogen peroxide solution to it, so that H2O2 reacts with Fe. 2+ It forms Fenton's reagent, which undergoes Fenton oxidation under pH 3.0-4.0 conditions with a residence time of 60-120 min, degrading recalcitrant organic matter in wastewater to obtain oxidized wastewater; S7, the oxidized wastewater is fed into the second solid-liquid separation unit, and alkali solution is added to adjust the pH to 7.0-8.5, so that the residual Fe... 3+ It precipitates out as ferric hydroxide, and after solid-liquid separation, iron sludge and deep-treated effluent are obtained.

[0007] Preferably, the solid-liquid separation device in step S2 is one of a sedimentation tank, an inclined plate sedimentation tank, or an air flotation device. The separated Fe(OH)3 flocs are dewatered and transported as iron-containing sludge or used to prepare polyferric sulfate coagulant; in step S3, Fe 2+The particle size of the Fe(OH)2 micro-precipitates generated by hydrolysis is controlled by adjusting the stirring speed and pH increase rate in the second pH adjustment device. The stirring speed is controlled at 30-100 r / min, and the pH increase rate is controlled at 0.1-0.5 pH units / min, so that the generated Fe(OH)2 particles are distributed in the range of 0.5-5 μm. The fluidized bed crystallization reactor includes, from bottom to top, an inlet water distribution zone, a crystallization zone, a sedimentation zone, and an outlet water zone. The crystallization zone is pre-filled or not filled with external seed crystals. When external seed crystals are used, the seed crystals are quartz sand or magnesium ammonium phosphate crystals with a particle size of 0.1-0.5 mm, and the filling amount is 5%-20% of the effective volume of the crystallization zone. When no external seed crystals are filled, the Fe(OH)2 particles generated in situ in step S3 are used as the initial seed crystals, and the self-granulation of MAP particles is gradually achieved through particle collision and growth during the crystallization process.

[0008] Preferably, the water-soluble magnesium salt in step S4 is one or more of magnesium chloride, magnesium sulfate, or magnesium oxide. The addition method is continuous or intermittent. The addition amount is controlled by feedback from the effluent orthophosphate concentration detected by the online orthophosphate sensor. When the effluent orthophosphate concentration is higher than 20 mg / L, the magnesium salt addition amount is increased; when the effluent orthophosphate concentration is lower than 5 mg / L, the magnesium salt addition amount is decreased. In step S5, the first solid-liquid separation device is one or a combination of hydrocyclone, vibrating screen, or centrifuge. First, the hydrocyclone is used to separate MAP particles with a particle size greater than 0.2 mm from the fine particulate wastewater containing Fe(OH)2 particles in the crystallization slurry. Then, the vibrating screen is used to dewater the underflow of the hydrocyclone to obtain MAP crystalline solid product. The wastewater under the screen and the overflow of the hydrocyclone are combined and enter step S6.

[0009] Preferably, the acid solution in step S6 is one of sulfuric acid, hydrochloric acid, or waste acid, and the amount of acid added is such that the pH of the wastewater is controlled at 3.0-4.0, wherein the Fe released from the dissolved Fe(OH)2 particles... 2+ The concentration is 20-100 mg / L; the amount of hydrogen peroxide added is based on the ratio of H2O2 to Fe. 2+ The molar ratio was determined to be 2:1-10:1, and the ORP value of the Fenton oxidation reaction was controlled within the range of 400-600mV, which was controlled by the online ORP meter in conjunction with the hydrogen peroxide dosing pump.

[0010] Preferably, the ammonium- and phosphorus-containing wastewater is one or more of the following: livestock and poultry breeding wastewater, landfill leachate, anaerobic digestion sludge supernatant, or ammonia- and phosphorus-containing wastewater from fertilizer plants; the total iron ion concentration in the waste hydrochloric acid is 50-120 g / L and Fe 3+ The proportion of iron ions in the total mass is controlled by the storage and oxidation time of waste hydrochloric acid, which is 0-7 days, to control the Fe content. 3+The proportion is 10%-40%.

[0011] The beneficial effects of this invention are: 1. This invention uses first-stage pH adjustment to adjust Fe 3+ It is separated and removed separately in the form of ferric hydroxide, thus avoiding the presence of Fe. 3+ The precipitation of magnesium ammonium phosphate under subsequent alkaline conditions led to a decrease in purity. Simultaneously, a second-stage pH adjustment was used to remove Fe from the wastewater. 2+ In-situ generation of ferrous hydroxide particles serves as heterogeneous seed crystals for magnesium ammonium phosphate crystallization, with Fe from the same source. 3+ with Fe 2+ The process performs two functions: impurity removal and separation, and crystal seeding. This ensures that the P2O5 content in the magnesium ammonium phosphate product is stable at over 28% and the iron impurity content is controlled below 15%, solving the problem of low purity of phosphorus recovery products caused by iron ion contamination in existing technologies.

[0012] 2. This invention releases Fe from ferrous hydroxide particles remaining in wastewater after crystallization through acid dissolution. 2+ It can be used directly as a catalyst in the Fenton oxidation stage without the need for the addition of commercial iron salts such as ferrous sulfate. This not only overcomes the problem of insufficient iron source in the Fenton stage in existing technologies, which requires the addition of additional reagents, but also realizes the secondary utilization of iron in waste hydrochloric acid, saving the cost of ferrous salts and reducing the amount of solid waste generated.

[0013] 3. This invention utilizes Fe(OH)2 particles generated in situ from waste hydrochloric acid as heterogeneous seed crystals for magnesium ammonium phosphate crystallization. Compared with existing technologies that require the addition of quartz sand, garnet, or MAP crushed crystals, there is no need to purchase or prepare additional seed crystals. Furthermore, Fe(OH)2 has a high lattice matching degree with MAP, which can shorten the crystallization induction period from 45 minutes to 6 minutes and reduce the magnesium salt addition ratio from 1.8:1 to 1.2:1, significantly improving crystallization efficiency and reducing reagent consumption.

[0014] 4. This invention directly mixes waste hydrochloric acid with ammonium- and high-phosphorus wastewater for acidolysis-based synergistic pretreatment, utilizing free hydrochloric acid to replace commercial acid, while simultaneously reducing Fe... 3+ / Fe 2+ Preloaded around phosphate in a coordinated manner, it is beneficial for subsequent stepwise precipitation of valence states, thus realizing waste treatment from waste. Attached Figure Description

[0015] Figure 1 The diagram shown is a schematic representation of the process layout of this invention. Figure 2 The diagram shown is a schematic representation of the process flow of this invention. Figure 3 The diagram shown is a schematic representation of the method flow of the present invention. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Please see Figures 1-3 The present invention provides Embodiment 1: The high-ammonia and high-phosphorus wastewater used in this embodiment was taken from the anaerobic digestion slurry of a large-scale pig farm. After sedimentation to remove large suspended solids, the main water quality indicators were measured as follows: total phosphorus concentration 1260 mg / L (as P), ammonium ion concentration 1580 mg / L (as NH4+). + The ammonium-phosphorus molar ratio was approximately 1.25:1, the pH was 2.8, and the COD concentration was 3420 mg / L. The waste hydrochloric acid was taken from the pickling production line of a steel cold rolling mill. After being left to stand for 2 days, it was used. The measured free hydrochloric acid mass fraction was 5.2%, and the total iron ion concentration was 86.5 g / L, of which Fe... 3+ The total iron content was 18.7% by mass (controlled by storage oxidation time), Fe 2+ It accounts for 81.3%.

[0018] Among them, Fe in waste hydrochloric acid 3+ The proportion of Fe in the total iron mass is controlled by the storage oxidation time (0-7 days, controlling Fe). 3+ (Ratio 10-40%), the principle lies in the Fe in waste hydrochloric acid 2+ It undergoes slow oxidation upon contact with air (oxygen): 4Fe 2+ + O2 + 4H + → 4Fe 3+ + 2H2O. The rate of this oxidation reaction is affected by a variety of environmental factors, including storage temperature, the ratio of the open area of ​​the container to the volume of the solution, whether stirring or disturbance is involved, the initial concentration of free acid, and the types of impurity ions in the solution.

[0019] In this example, the same batch of fresh waste hydrochloric acid was used (initial total iron concentration 105 g / L, free HCl mass fraction 6.2%, Fe...). 3+ The iron (initially accounting for 4.8% of the total iron mass) was packaged in 10L open plastic drums and subjected to natural oxidation experiments under the following three storage conditions: Condition 1: Temperature 10±2℃, stand still, cover the container opening with a dustproof cloth but allow natural air circulation, the liquid surface in contact with air is approximately 0.07m². 2 (The bucket is 30cm in diameter), and the liquid level is about 15cm.

[0020] Condition 2: Temperature 25±2℃, store in still condition, otherwise the same as condition 1.

[0021] Condition 3: Temperature 25±2℃, microporous aeration head at the bottom, aeration rate of 0.5L / min, simulating a scenario of violent disturbance or long-term open-air stirring storage.

[0022] Samples were taken every 24 hours, and total iron and Fe were determined using the potassium dichromate oxidation-ferrous ammonium sulfate titration method. 2+ Concentration, Fe difference subtraction method 3+ Concentration, calculate Fe 3+ The proportion of iron to the total mass is shown in Table 1: Table 1. Fe in waste hydrochloric acid under different storage conditions 3+ Change in proportion over time (unit: %)

[0023] Table 1 shows that temperature is the most significant factor affecting the oxidation rate. Fe2+ after standing for 7 days at 25℃... 3+ The proportion can reach 28.2%, while it is only 14.3% at 10℃. Aeration can significantly accelerate oxidation; after 7 days, Fe... 3+ The proportion is as high as 67.5%, far exceeding the upper limit of 40% of the preferred range of this invention. Therefore, in practical applications, vigorous stirring or aeration should be avoided, and the storage days should be adjusted appropriately according to seasonal temperature changes.

[0024] The concentration of free hydrochloric acid directly affects Fe 2+ The oxidation rate of H. According to chemical reaction kinetics, H + The higher the concentration, the more Fe 2+ The more stable the environment, the slower the oxidation rate. Take the same batch of waste hydrochloric acid, dilute it with deionized water or concentrate it using rotary evaporation, and prepare four samples with free hydrochloric acid mass fractions of 3%, 5%, 7%, and 10% (initial Fe...). 3+ The proportions were all controlled at around 5%, and the samples were stored at 25°C under static conditions. The Fe content of each group was measured after 7 days. 3+ The proportions and results are shown in Table 2: Table 2. Fe after 7 days of storage at different free acid concentrations 3+ Proportion (25℃, let stand)

[0025] Table 2 shows that the higher the free acid concentration, the more Fe... 2+ The less easily it is oxidized, the more Fe will be after 7 days. 3+ The lower the proportion. Waste hydrochloric acid typically has a free acid mass fraction between 3-8% when it leaves the factory, corresponding to the 3-7% range in the table. Within this range, after 7 days of storage, Fe... 3+The ratio can reach 26-36%, which is within the control range of 10-40% of this invention.

[0026] Process flow: (1) The above-mentioned waste hydrochloric acid and ammonium-containing high-phosphorus wastewater were continuously fed into a mixing tank via metering pumps at a volume ratio of 1:8. The mixing tank was equipped with a paddle agitator with a stirring speed of 60 r / min. After the free hydrochloric acid (mass fraction 5.2%) in the waste hydrochloric acid was mixed with the phosphorus-containing wastewater at pH 2.8, the pH of the mixed solution dropped to 0.95. Under this strongly acidic condition, the organophosphorus compounds (mainly hydroxyethylidene diphosphonic acid and its derivatives) and polyphosphates in the ammonium-containing high-phosphorus wastewater were completely acidified into orthophosphates. At the same time, the Fe carried in the waste hydrochloric acid was also degraded. 3+ with Fe 2+ These ions exist stably in hydrated form under strongly acidic conditions and do not undergo hydrolysis or precipitation. These iron ions form soluble coordination pre-complexes (such as FeH₂PO₄) with phosphate ions released from wastewater. 2+ FeHPO4 + These pre-complexes, although not yet precipitated, have altered the chemical environment of phosphate at the molecular level, creating conditions for subsequent stepwise precipitation of iron ions. The hydraulic retention time in the mixing tank is 20 minutes, and an acidic mixture is discharged.

[0027] Specifically, this step utilizes the existing free hydrochloric acid in the waste hydrochloric acid to replace the additional industrial sulfuric acid or hydrochloric acid. This achieves efficient acidolysis of organophosphorus and polyphosphates without introducing additional anions besides sulfate or chloride ions. Furthermore, the iron ions in the waste hydrochloric acid are pre-loaded around the phosphate ions in a coordinated form, which facilitates stepwise precipitation according to valence state in subsequent steps. This step directly treats waste with waste, saving on reagent costs. Moreover, the acidolysis efficiency is not significantly affected by fluctuations in the original pH of the wastewater because the strongly acidic environment provided by the waste hydrochloric acid far exceeds the buffering capacity of the wastewater itself.

[0028] (2) The acidic mixture is fed by gravity into the first pH adjustment device, which is a reaction vessel equipped with a stirrer and an automatic pH control system. A 10% sodium hydroxide solution is slowly added as an alkali solution. A pH sensor monitors the pH changes in the vessel in real time. When the pH rises to the 4.0-5.5 range, the frequency of the alkali dosing pump is adjusted by a PID controller to stabilize the pH at 4.8 ± 0.2. Under this pH condition, the reaction residence time is 25 min. At pH 4.8, Fe... 3+ The hydrolysis solubility product Ksp(Fe(OH)3) = 2.79 × 10 -39 Calculations show that the residual Fe 3+ The concentration can be reduced to 10 -6 Below mol / L, it almost completely hydrolyzes to form brownish-red ferric hydroxide flocs. And Fe...2+ The Fe(OH)2 remains dissolved under these pH conditions because the initial precipitation pH of Fe(OH)2 is approximately 7.5, much higher than 4.8. During stirring, the generated Fe(OH)3 flocs gradually collide and grow, forming visible flocs. After the first stage of pH adjustment, the mixture is sent to a solid-liquid separation device; in this embodiment, an inclined plate sedimentation tank is used for separation. The Fe(OH)3 flocs settle rapidly on the surface of the inclined plate, and the supernatant is collected through a collection tank to obtain the separated clear liquid. The Fe(OH)3 flocs discharged from the bottom of the sedimentation tank are dewatered by a plate and frame filter press and then transported as iron-containing sludge for the preparation of polyferric sulfate coagulant. The total phosphorus concentration in the separated clear liquid was measured to be 1180 mg / L, and the Fe... 3+ When the concentration is below 0.5 mg / L, Fe 2+ The concentration is 7.2 g / L (concentration after dilution, Fe in the original waste hydrochloric acid). 2+ The theoretical value after mixing at a volume ratio of 1:8 is approximately 86.5 × 0.813 / 9 ≈ 7.82 g / L, which meets expectations after deducting a small amount of loss due to entrainment by the flocs.

[0029] Specifically, this step achieves Fe 3+ with Fe 2+ The present invention achieves valence state separation by precisely controlling pH in Fe... 3+ Complete precipitation and Fe 2+ The narrow, almost non-settling temperature range (4.0-5.5) allows for acid-induced complex breakdown before precipitation, enabling Fe... 3+ The Fe(OH)3 flocs generated by hydrolysis not only remove iron itself, but also remove residual suspended solids, colloids, and some organic pollutants from the wastewater through adsorption and co-precipitation, thus purifying the water. Furthermore, they also remove Fe... 3+ Pre-separation avoids Fe 3+ Entering the subsequent magnesium ammonium phosphate crystallization stage, because Fe 3+ Once precipitated under alkaline conditions, it will mix into the MAP product, making it difficult to separate by physical methods and severely affecting the product purity.

[0030] (3) The clarified liquid is introduced into a second pH adjustment device, which is also equipped with a stirrer and an automatic pH control system. A 10% sodium hydroxide solution is continuously added to gradually increase the pH from 4.8 to 8.5-9.5. In this embodiment, the final pH is controlled at 9.0 ± 0.2. During the pH increase, the pH increase rate is controlled at 0.3 pH units / minute, while the stirring speed is set to 50 r / min. When the pH exceeds 7.5, the Fe in the clarified liquid... 2+ Hydrolysis begins to form Fe(OH)₂ precipitate. Since the solubility product of Fe(OH)₂, Ksp = 4.87 × 10⁻⁶, this process is necessary. -17 At pH 9.0, Fe was equilibrated.2+ The concentration is approximately 0.05 mg / L, with the vast majority of Fe... 2+ The Fe(OH)₂ crystals precipitate. However, under rapid stirring conditions, the generated Fe(OH)₂ does not have time to grow into large particles, but instead remains suspended in water as tiny particles with a diameter of 0.5-5 μm. These particles are light green in color and have a high specific surface area and abundant surface hydroxyl active sites. The reaction residence time in this step is 15 min, resulting in an alkaline mixture containing Fe(OH)₂ seed crystals. The concentration of Fe(OH)₂ particles (calculated as Fe) in this mixture is approximately 6.8 g / L, with a uniform particle size distribution and no obvious agglomeration.

[0031] Specifically, Fe(OH)₂ and MAP (magnesium ammonium phosphate) in this step have similar crystal structures and surface charge characteristics, and Fe(OH)₂ and magnesium ammonium phosphate have a good matching relationship in crystallography. According to the Joint Committee on Standards for Powder Diffraction cards: Fe(OH)₂ belongs to the hexagonal crystal system with cell parameters a=0.3258nm and c=0.4605nm; MAP belongs to the orthorhombic crystal system with cell parameters a=0.692nm, b=0.619nm and c=1.114nm, but the crystal structure of MAP often exhibits pseudo-hexagonal symmetry, and its (001) and (101) crystal planes are the most important growth crystal planes. The interplanar spacing of Fe(OH)₂ (001) is d(001) = c = 0.4605 nm, while that of MAP (001) is d(001) = c = 1.114 nm; they are not directly matched. However, the interplanar spacing of Fe(OH)₂ (100) is d(100) = a × √3 / 2 = 0.282 nm (the formula for calculating the interplanar spacing of hexagonal (100) crystals is d(100) = a·√3 / 2), while the interplanar spacing of MAP can be calculated using the formula 1 / d 2 =h 2 / a 2 +k 2 / b 2 +l 2 / c 2 Calculation, substituting h=1, k=0, l=1, yields: 1 / d 2 =1 / 0.692 2 +0 / 0.619 2 +1 / 1.114 2=2.088+0+0.806=2.894, d(101)=1 / √2.894=0.588nm. The interplanar spacing of Fe(OH)2 (100) (0.282 nm) is twice that of 0.564 nm. The mismatch δ between Fe(OH)2 (100) and MAP (101) (0.588 nm) is calculated by the following formula: δ=(d_MAP - 2d_Fe(OH)2) / [(d_MAP+2d_Fe(OH)2) / 2]×100%=(0.588-0.564) / [(0.588+0.564) / 2]×100%=0.024 / 0.576×100%≈4.2%. Therefore, the Fe(OH)2 surface can effectively reduce the energy barrier for heterogeneous nucleation of MAP and promote the rapid precipitation of MAP on its surface. Compared to externally added seed crystals, in-situ generated Fe(OH)2 particles are uniformly dispersed, abundant, and require no additional purchase or preparation. Furthermore, the Fe(OH)2 particles themselves are stable under alkaline conditions and do not release Fe. 2 + Interference with MAP crystallization (because Fe in the solution at this time) 2+ The concentration has been controlled at an extremely low level by the precipitation reaction. At the same time, the stirring speed and pH increase rate are controlled to regulate the seed crystal size. Too fast stirring will produce excessively fine particles (<0.1μm), which can easily penetrate the subsequent solid-liquid separation equipment; too slow stirring will result in excessively large seed crystal size (>10μm), which will reduce the specific surface area and the number of nucleation sites.

[0032] (4) The alkaline mixture containing Fe(OH)2 seed crystals is pumped into a fluidized bed crystallization reactor, which consists of an inlet water distribution zone, a crystallization zone, a sedimentation zone and an outlet water zone from bottom to top.

[0033] Optionally, the inlet water distribution zone is located at the bottom of the reactor, with a height of 0.4 m and a diameter of 1.6 m. An inlet is located at the center of the bottom, through which the wastewater and magnesium salt mixture enters. A porous water distribution plate, 8 mm thick, with an opening ratio of 12% and 8 mm diameter holes arranged in an equilateral triangle with a spacing of 15 mm, is installed 0.15 m above the inlet. The water distribution plate's function is to evenly distribute the inlet water upwards, preventing jet short-circuiting, and supporting the particles in the upper crystallization zone. A 0.25 m buffer zone is maintained above the water distribution plate to ensure a more uniform water velocity distribution before the water enters the crystallization zone.

[0034] Optionally, the crystallization zone is located above the inlet water distribution zone, and is a cylindrical straight pipe section with a height of 4.5m, an inner diameter of 1.6m, and a height-to-diameter ratio of approximately 2.8. The crystallization zone is the main part of the reactor and has no internal components to ensure uniform fluidization. During operation, wastewater flows through the crystallization zone at an upward flow velocity of 5-30 m / h, driving the Fe(OH)2 seed crystals and growing MAP particles to form a stable fluidized bed. According to Stokes' law, the particles are fluidized when the drag force and gravity of the upward water flow are balanced. In this embodiment, seed crystals with a diameter of 0.1-0.5 mm are in a fully fluidized state at an upward flow rate of 12 m / h, while MAP particles with a diameter greater than 1.0 mm tend to settle in the lower part of the crystallization zone, forming a natural gradation along the height direction: the bottom particles have the largest diameter (1.0-2.0 mm), the middle particles have a moderate diameter (0.5-1.0 mm), and the upper particles have the smallest diameter (0.1-0.5 mm) or are only seed crystals. Three sampling ports are set along the height direction on the sidewall of the crystallization zone (located at 1.0 m, 2.5 m, and 4.0 m from the water distribution plate, respectively) to monitor particle size distribution and water quality changes.

[0035] Optionally, the sedimentation zone is located above the crystallization zone and consists of a combination of a truncated cone section and a cylindrical section. The truncated cone section is 0.8 m high, with a lower inner diameter of 1.6 m and an upper inner diameter of 2.8 m, resulting in an expansion ratio of 1.75. The function of the truncated cone section is to reduce the upward flow velocity, causing larger particles to detach from the fluidized state and settle back into the crystallization zone. Above the truncated cone section is the cylindrical sedimentation zone, 1.2 m high and with an inner diameter of 2.8 m. Inside the cylindrical sedimentation zone, two layers of inclined plate assemblies are installed. The lower end of the first layer of inclined plates is 0.3 m from the bottom of the sedimentation zone, and the second layer of inclined plates is 0.5 m apart from the first layer. Each layer of inclined plates is made of 1.5 mm thick polypropylene plates, with an inclination angle of 60°, a plate spacing of 50 mm, and a plate length of 1.0 m. The function of the inclined plate assembly is to increase the sedimentation area and shorten the particle settling distance, allowing fine Fe(OH)2 particles and unevolved MAP microcrystals to be intercepted or settled as they pass through the inclined plates. The bottom of the sedimentation zone is conical with a cone angle of 60°. The bottom of the cone is connected to the top of the crystallization zone, and the settled particles return to the crystallization zone by gravity to continue participating in the crystallization reaction. A crystallization slurry overflow outlet is provided on the side wall of the sedimentation zone 0.2m from the upper edge of the frustum section for continuous discharge of wastewater containing residual Fe(OH)2 particles.

[0036] Optionally, the effluent zone is located at the top of the sedimentation zone, with a height of 0.5m and an inner diameter of 2.8m. A ring-shaped triangular overflow weir is installed at the top, with a weir plate height of 0.15m, a tooth depth of 30mm, and a tooth spacing of 50mm to ensure uniform effluent flow. A water collection trough, 0.2m wide, is located outside the overflow weir, with an outlet at the bottom through which the supernatant is discharged from the reactor. A defoaming plate (a circular plate with a diameter of 2.4m, 0.1m above the liquid surface) is installed in the center of the effluent zone to prevent air bubbles from carrying particles into the effluent.

[0037] Under an upward flow velocity of 12 m / h, a fluidization gradient forms along the height direction within the reactor. The bottom of the crystallization zone (0-1.0 m height) exhibits the highest flow velocity and strongest turbulence, primarily containing large MAP particles (0.8-2.0 mm in diameter). These particles, due to their settling velocity exceeding the upward flow velocity, are in a state of weak fluidization or a moving bed, with mutual friction promoting surface smoothing and densification. In the middle of the crystallization zone (1.0-3.0 m height), the flow velocity gradually decreases, mainly containing medium-sized particles (0.3-0.8 mm in diameter), which are in a fully fluidized state, uniformly suspended and slowly rising. In the upper part of the crystallization zone (3.0-4.5 m height), the flow velocity further decreases, mainly containing fine particles (0.1-0.3 mm in diameter) and Fe(OH)₂ seed crystals. Some of these fine particles are carried into the sedimentation zone by the water flow. Upon entering the sedimentation zone, due to the increased diameter, the upward flow velocity drops sharply from 12 m / h to approximately 3.9 m / h, lower than the settling velocity of Fe(OH)₂ seed crystals (particle size <10 μm), but higher than the settling velocity of MAP microcrystals (>0.1 mm). Therefore, most MAP microcrystals settle back into the crystallization zone, while Fe(OH)₂ particles are carried out of the reactor by the water flow into the effluent zone, thus achieving natural separation of MAP crystals and Fe(OH)₂ particles. The median particle size of the particles periodically discharged from the bottom of the crystallization zone can reach 1.1 mm, while the median particle size of the Fe(OH)₂ particles carried out by the overflow is approximately 1.2 μm, a difference of nearly three orders of magnitude.

[0038] A static mixer is installed before the inlet at the bottom of the reactor to mix the magnesium salt solution. The total phosphorus concentration remaining in the clarified liquid (1180 mg / L, calculated as P, converted to PO4) is used. 3- (Concentration approximately 3620 mg / L), calculated based on a magnesium-to-phosphorus molar ratio of 1.2:1, the required magnesium ion concentration is approximately 1.2 × 1180 / 31 ≈ 45.7 mmol / L, or about 1.1 g Mg. 2+ / L. In this embodiment, magnesium chloride hexahydrate (MgCl2·6H2O) was prepared into a 15% (w / w) solution and continuously injected into the inlet pipeline via a metering pump. In the fluidized bed crystallizer, the wastewater was uniformly raised through a bottom distribution plate, while a small amount of alkaline solution (5% (w / w) sodium hydroxide) was added to the reactor via a pH control system to stabilize the pH in the crystallization zone at 9.0 ± 0.2. The upward flow rate was controlled at 12 m / h, and the hydraulic retention time was 60 min. Under these conditions, the NH4+ in the solution... + (The concentration of 1580 mg / L is approximately 176 mg / L after dilution, and the calculated dilution factor is 9 times. The actual NH4+ concentration is...) + Approximately 176 mg / L, or 9.8 mmol / L), PO4 3- (Approximately 1180 / 31 = 38.1 mmol / L) and supplemented Mg 2+Heterogeneous nucleation (45.7 mmol / L) occurred on the surface of Fe(OH)2 seed crystals. Fe(OH)2 seed crystals provided a large number of nucleation sites, and MAP crystals grew outward with Fe(OH)2 particles as the core, forming core-shell structured crystalline particles. As the crystals circulated, collided, and grew in the fluidized bed, the particles gradually increased in size. The sedimentation zone of the reactor was equipped with inclined plates. Larger MAP particles remained in the crystallization zone and continued to grow because their settling velocity was greater than their rising velocity, while smaller particles rose with the water flow. After the operation stabilized, the crystal particles were periodically discharged from the bottom of the crystallization zone. The average particle size of the MAP particles was measured to be 1.1 mm, and the particles were white or light gray (slightly gray due to the small amount of Fe(OH)2 in the core). The residual total phosphorus concentration in the supernatant of the crystallization zone was detected to be 32 mg / L, and the total phosphorus removal rate (relative to the supernatant of step (2)) reached 97.3%. The crystallization slurry was continuously discharged from the overflow port of the sedimentation zone and entered the subsequent separation section.

[0039] Specifically, this step utilizes in-situ generated Fe(OH)2 seeds, significantly improving the nucleation and growth rates of MAP crystallization. In traditional MAP crystallization processes without seeds, homogeneous nucleation requires high supersaturation (typically a saturation index SI>2.0) and an induction period lasting tens of minutes; however, with the presence of Fe(OH)2 seeds in this invention, the critical supersaturation for heterogeneous nucleation is reduced to SI≈1.2, and the induction period is shortened to 2-5 minutes. This allows for higher crystallization efficiency at a lower magnesium salt dosage ratio (Mg:P=1.2:1 instead of the traditional 1.5-2:1) and a shorter hydraulic residence time, while the generated MAP particles are larger and more uniformly distributed, which is beneficial for subsequent separation. Furthermore, since step (2) has already... 3+ Pre-separation is performed; only Fe remains in this step. 2+ The Fe(OH)2 seed crystals are generated, and the amount of seed crystals is controlled at a low level (approximately 6.8 g / L based on Fe, accounting for approximately 5%-8% of the total mass of the crystal particles). Therefore, the iron impurity content in the MAP product is controllable and will not affect its quality as a slow-release phosphate fertilizer.

[0040] (5) The crystallization slurry is fed into a hydrocyclone for classification and separation. The feed pressure of the hydrocyclone is 0.2 MPa, the underflow diameter is 8 mm, and the overflow diameter is 20 mm. Under the action of centrifugal force, MAP particles with a diameter greater than 0.2 mm enter the underflow, while particles containing residual Fe(OH)2 (mostly <10 μm in diameter) and a small amount of dissolved Fe are separated. 2+Fine particulate wastewater is discharged from the overflow port. The underflow (approximately 15% of the feed volume) enters a vibrating screen for dewatering. The screen mesh size is 0.15 mm. The MAP crystals retained on the screen are rinsed with a small amount of clean water and then dried at 105°C for 2 hours to obtain MAP crystalline solid product. The wastewater under the screen is combined with the overflow from the hydrocyclone and proceeds to subsequent steps. After weighing and testing, the recovered MAP solid product in this embodiment has a mass of 8.2 kg / ton of wastewater (equivalent to treating 1 ton of original wastewater), a moisture content of 8.5%, and a Fe(OH)2 impurity mass fraction of 11.2% (calculated as iron oxide). The total phosphorus content (based on P2O5) is 29.6%, and the heavy metal content is: arsenic 0.8 mg / kg, cadmium 0.2 mg / kg, lead 1.5 mg / kg, chromium 2.1 mg / kg, and mercury <0.05 mg / kg, all meeting the limits specified in GB / T 23349-2009 "Ecological Indicators for Arsenic, Cadmium, Lead, Chromium, and Mercury in Fertilizers". This product can be used directly as a slow-release phosphate fertilizer in farmland or sold as a raw material for industrial phosphate production.

[0041] (6) The separated wastewater from step (5) is fed into an oxidation reaction device, which is a three-stage series reaction tank with stirring and automatic pH control. First, a 30% sulfuric acid solution is added to the first-stage reaction tank to lower the pH of the wastewater from approximately 9.0 to 3.5 ± 0.2. Under this acidic condition, the Fe(OH)2 particles carried in the wastewater dissolve rapidly. Fe(OH)2 + 2H + → Fe 2+ + 2H2O; The dissolution process is completed within 10 minutes, releasing Fe. 2+ The concentration was measured to be 65 mg / L. Then, a 27.5% hydrogen peroxide solution was added to the reaction vessel, and the reaction proceeded according to the formula: H₂O₂ and Fe. 2+ The dosage was calculated based on a molar ratio of 6:1, meaning the hydrogen peroxide concentration was approximately 65×34 / 56×6 / 1000≈0.237g / L (as H2O2). In actual operation, the redox potential of the reaction system was monitored using an online ORP meter, controlling the ORP value within the range of 450-550mV. When the ORP was below 450mV, the dosage of H2O2 was automatically increased; when it was above 550mV, the dosage was reduced. The Fenton reaction was carried out within a hydraulic retention time of 90 minutes, during which the pH was maintained between 3.0 and 4.0 using an online pH meter and acid / base dosing pump. The hydroxyl radicals (·OH) generated by the Fenton reaction have extremely strong oxidizing power (oxidation potential 2.80V), capable of oxidizing recalcitrant organic matter in wastewater into carbon dioxide, water, and simple small molecule acids. After the reaction, the COD of the oxidized wastewater decreased from 850mg / L before entering the Fenton reaction to 58mg / L, achieving a removal rate of 93.2%. Simultaneously, some Fe... 2+Oxidized to Fe 3+ Furthermore, ·OH can directly oxidize some of the remaining phosphite to orthophosphate, but after the preceding MAP crystallization, the total phosphorus in the wastewater is already low, and the Fenton stage does not contribute much to the further removal of total phosphorus.

[0042] Specifically, this step uses the Fe(OH)2 particles separated in step (5) as the iron source for the Fenton reaction. Existing Fenton oxidation processes typically require the addition of ferrous sulfate, which not only increases reagent costs but also introduces sulfate ions, leading to an increase in sulfate levels in the effluent. In this invention, the Fe(OH)2 particles are the portion that was not consumed by MAP crystallization after seed crystal formation in step (3), and if not utilized, would become solid waste; they are converted into Fe through acid dissolution. 2+ This perfectly satisfies the requirements of the Fenton reaction for Fe. 2+ The need for a catalyst enables the secondary utilization of iron. Simultaneously, the acid dissolution process requires lowering the pH to 3-4, which happens to be the optimal pH range for the Fenton reaction, eliminating the need for additional acid adjustment.

[0043] (7) The oxidized wastewater is sent to the second solid-liquid separation device (a vertical flow sedimentation tank is used in this embodiment). A 10% sodium hydroxide solution is added to the mixing reactor before the sedimentation tank to adjust the pH of the wastewater to 7.5 ± 0.2. Under this pH condition, the Fe remaining after the Fenton reaction... 3+ (Concentration approximately 50 mg / L, with some Fe) 2+ During the reaction, the oxidized material was completely hydrolyzed to form Fe(OH)3 flocs. The surface loading of the sedimentation tank was 1.2 m³. 3 / (m 2 The hydraulic retention time is 2.5 h. The Fe(OH)3 flocs settle to the bottom and are pumped to the sludge thickening tank. The supernatant is discharged as deep-treated effluent.

[0044] The tested effluent from the advanced treatment process showed the following parameters: total phosphorus 0.3 mg / L, COD 46 mg / L, pH 7.4, suspended solids <10 mg / L, and total iron concentration 0.8 mg / L, meeting the Class I discharge standard of the Integrated Wastewater Discharge Standard GB 8978-1996. The iron sludge at the bottom of the sedimentation tank, after plate and frame filtration, yielded iron sludge cake with a solids content of approximately 35%, mainly composed of Fe(OH)3, with a dry basis iron content (calculated as Fe2O3) of approximately 58%. This cake can be returned to the iron smelting process as an iron-containing raw material or used to prepare polyferric sulfate coagulant.

[0045] This invention is illustrated in Example 2: The ammonium- and phosphorus-containing wastewater used in this embodiment was taken from the leachate in the equalization pond of a municipal solid waste landfill. The leachate had been pretreated to remove most of the suspended solids.

[0046] The water quality indicators are: total phosphorus concentration 780 mg / L (as P), ammonium ion concentration 1150 mg / L (as NH4+). + The ammonium-phosphorus molar ratio was approximately 1.47:1, the pH was 3.2, and the COD concentration was 2850 mg / L. The waste hydrochloric acid was taken from another steel cold rolling mill. This mill's pickling wastewater was freshly taken without prolonged storage. The measured free hydrochloric acid mass fraction was 6.8%, and the total iron ion concentration was 112 g / L, of which Fe... 3+ Fe accounts for only 6.5% of the total iron mass fraction. 2+ It accounts for 93.5%. Compared with Example 1, the Fe in the waste hydrochloric acid of this example is... 3+ The proportion is low, so the amount of Fe(OH)3 flocs produced in step (2) is small, but the Fe in step (3) can be used to generate Fe(OH)2 seed crystals. 2+ More abundant.

[0047] The specific adjustments are as follows: (1) The volume ratio of waste hydrochloric acid to wastewater was adjusted to 1:6 (because the total phosphorus concentration in the wastewater was slightly low, the proportion of waste hydrochloric acid was appropriately reduced to avoid over-acidification). After mixing, the pH was about 1.1 and the acid hydrolysis time was 40 min.

[0048] (2) First-stage pH adjustment to 5.0±0.2 (due to Fe) 3+ (A lower proportion and a slightly higher pH are beneficial for floc coagulation), and a reaction time of 20 minutes. The amount of Fe(OH)3 flocs separated was significantly less than in Example 1, and only a small amount of iron-containing sludge was produced after pressure filtration.

[0049] (3) The pH was adjusted to 9.2 ± 0.2 in the second stage, with the pH increase rate controlled at 0.2 pH units / min, the stirring speed at 40 r / min, and the reaction residence time at 20 min. The median particle size of the generated Fe(OH)2 seed crystals was 2.3 μm, and the specific surface area was 38.7 m². 2 / g.

[0050] (4) Based on the total phosphorus concentration of the clarified liquid (745 mg / L after step (2)), magnesium salt was added at a ratio of Mg:P = 1.1:1. In this embodiment, magnesium sulfate (MgSO4·7H2O) was used to prepare a 20% mass fraction solution. The fluidized bed upflow rate was adjusted to 10 m / h, and the hydraulic retention time was 80 min. The pH of the crystallization zone was stabilized at 9.0-9.2.

[0051] (5) The hydrocyclone operates at a pressure of 0.18 MPa, and the separation of underflow and overflow is good.

[0052] (6) The pH of the Fenton oxidation section was adjusted to 3.2±0.2, and Fe was measured after dissolving Fe(OH)2. 2+ The concentration is 48 mg / L, and H2O2 is added according to the formula H2O2:Fe2+ The molar ratio was 8:1, the ORP was controlled at 480-550mV, and the residence time was 100min.

[0053] (7) Neutralize the pH to 7.8±0.2, and discharge the effluent after sedimentation.

[0054] Comparative Example 1 provided by the present invention: This example is used to verify the promoting effect of Fe(OH)2 in-situ seed crystals on MAP crystallization in this invention, and a comparative example is set up. The water quality (pig biogas slurry) and the same steps (1) and (2) as in Example 1 are used, but Fe is not carried out in step (3). 2+ Instead of hydrolyzing to generate seed crystals, the clear liquid from step (2) is directly adjusted to pH 9.0 and then fed into a fluidized bed crystallization reactor without adding any seed crystals (including no Fe(OH)2 or external seed crystals such as quartz sand). The rest of the operation is the same as in Example 1.

[0055] After the operation stabilized, the following phenomena were observed in the fluidized bed crystallizer, as detailed in Table 3: Table 3 Comparison of key indicators between Example 1 and Comparative Example (without seed crystals)

[0056] First, it takes up to 45 minutes from mixing the magnesium salts to the appearance of tiny crystal particles visible to the naked eye, while in Example 1 it only takes 6 minutes.

[0057] Secondly, most of the discharged crystal particles are needle-shaped or fine powder, with a median particle size of only 0.12 mm as measured by a laser particle size analyzer, and a wide particle size distribution (0.05-0.5 mm), making it difficult to effectively separate them using a hydrocyclone.

[0058] Third, the residual total phosphorus concentration in the supernatant of the crystallization zone was 86 mg / L, and the total phosphorus removal rate was only 88.2% (97.3% in Example 1).

[0059] Fourth, in order to achieve a similar phosphorus removal rate, we tried to increase the Mg:P ratio to 1.8:1, which reduced the residual total phosphorus to 41 mg / L, but it was still higher than the 32 mg / L in Example 1, and the amount of magnesium salt used increased by 50%.

[0060] Fifth, because no seed crystals were used, the MAP crystals produced by homogeneous nucleation were small and easily encapsulated tiny suspended matter, resulting in increased impurity content in the product. The P2O5 content was only 22.4%, which did not meet the quality requirements for slow-release phosphate fertilizer.

[0061] Comparative Example 2 is provided in this invention: This example is used to verify Fe in step (2). 3+To clarify the necessity of pre-separation, a comparative example was set up. The same water quality and waste hydrochloric acid as in Example 1 were used, but Fe was not separated in step (2). 3+ The separate separation directly adjusts the pH of the acidic mixture from step (1) to 9.0 in one step, so that Fe 3+ and Fe 2+ Simultaneously, precipitation was carried out, followed by subsequent steps. The main comparison was of the purity of the MAP product, and the results are shown in Table 4. Table 4. Comparison of product purity between Example 1 and Comparative Example (separation without valence state)

[0062] In step (4) during the MAP crystallization process, both Fe(OH)3 and Fe(OH)2 particles exist as seed crystals. However, the surface properties of Fe(OH)3 are not well matched with MAP, and some Fe(OH)3 particles are not completely encapsulated by MAP. As a result, a large amount of iron hydroxide is mixed into the final MAP product.

[0063] The MAP product obtained in step (5) is dark gray. The mass fraction of Fe(OH)2+Fe(OH)3 impurities is as high as 34%, and the P2O5 content is only 21.3%, which is far below the fertilizer standard requirements.

[0064] Because the product has too high an iron content, it cannot be used directly as a phosphate fertilizer and requires additional acid washing to remove iron, which increases processing costs.

[0065] Comparative Example 3 is provided in this invention: This example is used to verify the effect of the oxidation time of waste hydrochloric acid storage on Fe. 3+ The influence of the ratio and the adaptability of this invention to actual waste hydrochloric acid fluctuations were discussed, using waste hydrochloric acid from the same source (initial Fe). 3+ Samples (approximately 5%) were stored naturally for 0, 2, 5, and 7 days. The same batch of pig biogas slurry (water quality same as in Example 1) was treated according to the procedure in Example 1, where some parameters in steps (2), (3), and (4) were based on Fe... 3+ Proportional fine-tuning (Fe) 3+ When the proportion is high, the reaction time of step (2) should be slightly extended to ensure Fe 3+ Complete precipitation; Fe 3+ When the proportion is low, the amount of alkali added in step (2) should be appropriately reduced, while the other parameters remain unchanged. The results are shown in Table 5. Table 5. Storage time of different waste hydrochloric acid (different Fe) 3+ Process effects under (proportion)

[0066] Table 5 shows that even if Fe in waste hydrochloric acid 3+The proportion fluctuates between 5% and 41%, and the process of this invention can operate stably and obtain qualified effluent (total phosphorus ≤ 0.5 mg / L) and high-purity MAP product (P2O5 content ≥ 28%). When Fe 3+ When the proportion increases, the amount of Fe(OH)3 flocs generated in step (2) increases, which is beneficial for removing more organic matter, but the Fe in step (3) can be used to generate seed crystals. 2+ The reduction leads to a smaller Fe(OH)2 seed crystal size and a smaller Fenton segment Fe 2+ The concentration decreased. However, the Fenton fraction of Fe... 2+ The concentration range of 48-72 mg / L can meet the requirements for catalytic oxidation because the H2O2 dosage can be adjusted proportionally. This indicates that the present invention has good adaptability to the source and storage conditions of waste hydrochloric acid, and in practical applications, there is no need for strict pretreatment of waste hydrochloric acid, which greatly improves the engineering applicability of the process.

[0067] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A treatment process for high-phosphorus wastewater, characterized in that, Includes the following steps: S1, waste hydrochloric acid and ammonium- and high-phosphorus wastewater are mixed in a mixing tank at a volume ratio of 1:5-1:

10. The free hydrochloric acid in the waste hydrochloric acid acidically decomposes the phosphorus-containing substances in the ammonium- and high-phosphorus wastewater into orthophosphate, while the Fe carried in the waste hydrochloric acid... 3+ with Fe 2 + The ions exist stably in the ionic state under strongly acidic conditions and form soluble coordination pre-complexes with phosphate ions in the wastewater to obtain an acidic mixture. S2, the acidic mixture is introduced into the first pH adjustment device, and alkali solution is added to adjust the pH to 4.0-5.

5. The reaction is allowed to proceed for 10-30 minutes, allowing Fe to... 3+ Complete hydrolysis produces ferric hydroxide Fe(OH)3 flocs, while Fe... 2+ Under this pH condition, the Fe(OH)3 flocs do not hydrolyze and remain in the solution. The Fe(OH)3 flocs are separated and removed by a solid-liquid separation device to obtain the separated clear liquid. S3, input the clarified liquid into the second pH adjustment device, continue to add alkali solution to adjust the pH to 8.0-9.5, and let the reaction stand for 10-20 minutes to allow the Fe in the clarified liquid to rise. 2+ Partial hydrolysis generates tiny precipitate particles of ferrous hydroxide Fe(OH)2 with a particle size of 0.1-10 μm, which serve as heterogeneous seed crystals for subsequent crystallization of magnesium ammonium phosphate, resulting in an alkaline mixture containing Fe(OH)2 seed crystals. S4. The alkaline mixture is fed into the fluidized bed crystallization reactor. Water-soluble magnesium salts are added according to the residual total phosphorus concentration in the clarified liquid, at a magnesium-to-phosphorus molar ratio of 1.0:1-1.5:

1. The pH in the fluidized bed crystallization reactor is controlled to be stable at 8.5-9.5, with a hydraulic retention time of 30-120 min and an upflow velocity of 5-30 m / h, to reduce the NH4+ in the wastewater. + PO4 3- With Mg 2+ Heterogeneous nucleation occurs on the surface of Fe(OH)2 seed crystals, which grow into magnesium ammonium phosphate (MAP) crystal particles with an average particle size of 0.3-2.0 mm, resulting in a crystallization slurry containing MAP crystals and residual Fe(OH)2 particles. S5, the crystallization slurry is fed into the first solid-liquid separation unit to separate MAP crystallized solid product and separation wastewater. The separation wastewater contains residual Fe(OH)2 particles that did not participate in crystallization and a small amount of dissolved Fe. 2+ ; S6. The separated wastewater is fed into the oxidation reaction unit. First, acid is added to the unit to adjust the pH of the wastewater to 3.0-4.0, so that the residual Fe(OH)2 particles dissolve and are converted into free Fe. 2+ Then add hydrogen peroxide solution to it, so that H2O2 reacts with Fe. 2+ It forms Fenton's reagent, which undergoes Fenton oxidation under pH 3.0-4.0 conditions with a residence time of 60-120 min, degrading recalcitrant organic matter in wastewater to obtain oxidized wastewater; S7, the oxidized wastewater is fed into the second solid-liquid separation unit, and alkali solution is added to adjust the pH to 7.0-8.5, so that the residual Fe... 3+ Iron sludge and deep-treated effluent are obtained by solid-liquid separation after precipitation in the form of ferric hydroxide.

2. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: The ammonium- and phosphorus-containing wastewater has a molar ratio of ammonium ion concentration to total phosphorus concentration ≥ 0.8:1, a total phosphorus concentration of 500-5000 mg / L, and a pH of 1.0-4.

0. The waste hydrochloric acid is steel pickling wastewater with a free hydrochloric acid mass fraction of 3%-8% and a total iron ion concentration of 50-120 g / L, of which Fe... 3+ It accounts for 10%-40% of the total mass of iron ions.

3. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: The clarified solution in step S2 mainly contains PO4. 3- NH4 + Fe 2+ With a small amount of Ca 2+ Mg 2+ Residual ions.

4. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: In step S2, the solid-liquid separation device is one of a sedimentation tank, an inclined plate sedimentation tank, or an air flotation device. The separated Fe(OH)3 flocs are dewatered and transported as iron-containing sludge or used to prepare polyferric sulfate coagulant.

5. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: Fe in step S3 2+ The particle size of the Fe(OH)2 micro precipitates generated by hydrolysis is controlled by adjusting the stirring speed and pH increase rate in the second pH adjustment device. The stirring speed is controlled at 30-100 r / min, and the pH increase rate is controlled at 0.1-0.5 pH units / min, so that the particle size of the generated Fe(OH)2 particles is distributed in the range of 0.5-5 μm.

6. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: The fluidized bed crystallization reactor comprises, from bottom to top, an inlet water distribution zone, a crystallization zone, a sedimentation zone, and an outlet water zone. The crystallization zone may or may not be pre-filled with external seed crystals. When external seed crystals are used, the seed crystals are quartz sand or magnesium ammonium phosphate crystals with a particle size of 0.1-0.5 mm, and the filling amount is 5%-20% of the effective volume of the crystallization zone. When no external seed crystals are filled, the Fe(OH)2 particles generated in situ in step S3 are used as the initial seed crystals, and the self-granulation of MAP particles is gradually achieved through particle collision and growth during the crystallization process.

7. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: In step S4, the water-soluble magnesium salt is one or more of magnesium chloride, magnesium sulfate, or magnesium oxide. The addition method is continuous or intermittent. The addition amount is controlled by feedback from the effluent orthophosphate concentration detected by the online orthophosphate sensor. When the effluent orthophosphate concentration is higher than 20 mg / L, the magnesium salt addition amount is increased; when the effluent orthophosphate concentration is lower than 5 mg / L, the magnesium salt addition amount is decreased.

8. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: In step S5, the first solid-liquid separation device is one or a combination of a hydrocyclone, a vibrating screen, or a centrifuge. First, a hydrocyclone is used to separate MAP particles with a diameter greater than 0.2 mm from the fine particulate wastewater containing Fe(OH)2 particles in the crystallization slurry. Then, a vibrating screen is used to dewater the underflow of the hydrocyclone to obtain MAP crystalline solid product. The wastewater under the screen and the overflow of the hydrocyclone are combined and enter step S6.

9. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: In step S6, the acid solution is one of sulfuric acid, hydrochloric acid, or waste acid. The amount of acid added is based on controlling the pH of the wastewater at 3.0-4.

0. The Fe released from the dissolved Fe(OH)₂ particles... 2+ The concentration is 20-100 mg / L; the amount of hydrogen peroxide added is based on the ratio of H2O2 to Fe. 2+ The molar ratio was determined to be 2:1-10:1, and the ORP value of the Fenton oxidation reaction was controlled within the range of 400-600mV, which was controlled by the online ORP meter in conjunction with the hydrogen peroxide dosing pump.

10. The treatment process for high-phosphorus wastewater according to claim 1, characterized in that: The total iron ion concentration in the waste hydrochloric acid was 50-120 g / L and Fe 3+ The proportion of ferric and ferrous ions in the total mass is controlled by the storage and oxidation time of waste hydrochloric acid, which is 0-7 days, to control the Fe content. 3+ The proportion is 10%-40%, used to meet the requirements for Fe in step S2. 3+ The requirement is to remove organic matter and colloids through flocculation, while ensuring that sufficient Fe remains in step S3. 2+ Iron source used to generate Fe(OH)2 seed crystals and for Fenton oxidation in step S6.

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