A composite catalyst for the catalytic oxidation of centrifuged mother liquor, its preparation method and application

Abstract

This invention discloses a composite catalyst for the catalytic oxidation of centrifuged mother liquor, its preparation method, and its application. The composite catalyst includes a support and an active component loaded on the support. The support is a titanium-based composite material modified with rare earth elements, and the mass ratio of the support to the active component is 100:(5-20). The rare earth-modified support is prepared by co-precipitation, followed by ultrasonic-assisted impregnation, drying, and calcination to obtain the catalyst. This invention significantly improves catalytic activity and structural stability and reduces the loss of active components through the synergistic effect of polymetallic oxides and the rare earth-modified support. It can efficiently catalyze the oxidation of concentrated centrifuged mother liquor from paraquat wastewater under mild conditions, achieving simultaneous deep degradation of COD, paraquat cations, and cyanide. It exhibits high treatment efficiency, long service life, and good regeneration performance. The catalyst preparation process of this invention is simple and the conditions are mild, making it suitable for the pretreatment of high-salt, high-COD, and recalcitrant pesticide wastewater.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment technology, specifically to a composite catalyst for the catalytic oxidation of centrifuged mother liquor, its preparation method, and its application. Background Technology

[0002] Paraquat is a highly effective contact herbicide, and its production process generates large amounts of high-concentration saline organic wastewater. Currently, the industry typically uses a stripping-evaporation concentration process for treatment: first, ammonia and low-boiling-point organic matter are recovered through stripping; then, the wastewater is evaporated and concentrated to crystallize inorganic salts, and centrifuged to obtain mixed salts and centrifuged mother liquor. However, the treatment of the centrifuged mother liquor has always been a challenge in this process. The mother liquor is enriched with high-boiling-point organic pollutants, uncrystallized ammonium salts, and small amounts of paraquat cations, with a COD as high as tens to hundreds of thousands of mg / L. It also contains pyridine heterocyclic compounds and cyanides that are difficult to biodegrade, making direct biochemical treatment impossible. Catalytic oxidation technology is an effective means of treating such high-concentration, recalcitrant organic wastewater, with the catalyst being the core component. Several catalytic oxidation methods for paraquat wastewater have been reported in existing technologies, such as using oxide catalysts supported on activated carbon at 5-7 MPa and 220-290℃ for oxidation treatment. However, such catalysts are mainly designed for raw production wastewater and are not sufficiently effective for centrifuged mother liquor after stripping and concentration. They also suffer from problems such as easy loss of active components, poor resistance to poisoning, and short service life. In addition, there are reports of using photocatalytic oxidation or electrocatalytic oxidation to treat paraquat wastewater, but photocatalysis is greatly affected by wastewater color, and electrocatalysis has high energy consumption, making it difficult to adapt to high-salt, high-COD centrifuged mother liquor systems. Therefore, developing a catalyst with high activity and high stability specifically for the catalytic oxidation pretreatment of paraquat centrifuged mother liquor is of significant practical importance.

[0003] Chinese patent CN104230081B discloses a process for treating paraquat pesticide wastewater. The process uses commercially available coconut shell activated carbon (3-5mm, specific surface area 950m² / g) as a carrier. 72.5g of copper nitrate and 40.4g of ferric nitrate are weighed and dissolved in 500mL of deionized water at a copper oxide:ferric oxide mass ratio of 3:1. Then, 500g of activated carbon is added to the impregnation solution and impregnated at room temperature for 12 hours. After filtration, the carbon is dried at 105℃ for 10 hours and calcined at 500℃ for 4 hours to obtain a catalyst for treating paraquat wastewater. However, this process requires harsh reaction conditions: 5-7MPa and 220-290℃. It is a typical wet oxidation process, demanding high-quality equipment (resistant to high temperature and high pressure corrosion), resulting in high investment costs and high energy consumption. Chinese patent application CN1576242A discloses a method for treating paraquat-containing cyanide wastewater. It proposes using activated carbon as a carrier, loading a certain amount of metal composite elements onto its surface, to prepare a supported catalyst for treating paraquat-containing cyanide wastewater. The reaction process involves reacting the paraquat-containing cyanide wastewater and the catalyst in a high-pressure reactor, stirring and heating to 100-200°C, introducing a gaseous oxidant, controlling the pressure at 0.5-1.8 MPa, reacting for 12-24 hours, and then cooling and filtering the liquid before sending it for biochemical treatment. However, the stability issue of this catalyst remains unresolved. Even with activated carbon as the carrier, under oxygen-containing conditions at 100-200°C, the activated carbon may be oxidized and depleted. Furthermore, the issue of metal component loss is not addressed. Summary of the Invention

[0004] The purpose of this invention is to provide a composite catalyst for the catalytic oxidation of centrifugal mother liquor, its preparation method, and its application. The catalyst of this invention has high catalytic activity, strong stability, and minimal loss of active components. It can efficiently degrade COD, paraquat, and cyanide in high-concentration, recalcitrant centrifugal mother liquor. It is applicable to milder working conditions and has a longer service life, thereby solving the technical problems mentioned in the background art, such as easy loss of active components, poor resistance to poisoning, and short service life of existing catalysts in the treatment of mother liquor after paraquat concentration.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A composite catalyst for the catalytic oxidation of centrifuged mother liquor includes a support and an active component loaded on the support. The support is a titanium-based composite material modified with rare earth elements, and the mass ratio of the support to the active component is 100:(5-20).

[0006] Furthermore, the active component is at least three of the oxides of copper, iron, cerium, and manganese.

[0007] Furthermore, the support is a lanthanum-modified titanium dioxide-alumina composite support, wherein the mass ratio of titanium dioxide to alumina is (2-4):1, and the loading of lanthanum is 1-5% of the total mass of the support.

[0008] Furthermore, the molar ratio of copper oxide: ferric oxide: cerium dioxide: manganese oxide in the active component is (2-4): (1-2): (1-1.5): (0-0.5).

[0009] Furthermore, the composite catalyst is a spherical particle with a particle size of 2-6 mm, a specific surface area of ​​150-250 m² / g, and a pore volume of 0.3-0.6 cm³ / g.

[0010] A method for preparing a composite catalyst for the catalytic oxidation of centrifuged mother liquor includes the following steps: Step 1: Dissolve titanium source, aluminum source and lanthanum source in deionized water in proportion, add precipitant to adjust pH to 8-10 to form coprecipitate, age, filter, wash and dry the precipitate, calcine at 500-700℃ for 4-8 hours, crush and sieve to obtain lanthanum modified titanium-aluminum composite carrier. Step 2: Dissolve copper salt, iron salt and cerium salt in deionized water according to the proportion of active components to prepare an impregnation solution with a total metal ion concentration of 0.5-2.0 mol / L; Step 3: Add the lanthanum-modified titanium-aluminum composite carrier obtained in Step 1 to the impregnation solution in Step 2, and impregnate at 40-60℃ for 6-12 hours, during which ultrasonic-assisted dispersion is performed for 20-40 minutes. Step 4: Dry the impregnated solid at 80-120℃ for 8-12 hours, and then calcine it at 400-600℃ for 3-6 hours to obtain the composite catalyst.

[0011] Furthermore, the titanium source mentioned in step 1 is titanium oxysulfate or titanium tetrachloride, the aluminum source is aluminum nitrate or aluminum sulfate, and the lanthanum source is lanthanum nitrate.

[0012] Furthermore, the precipitant is ammonia or ammonium carbonate.

[0013] Furthermore, in step 3, the ultrasonic power is 200-400W and the ultrasonic frequency is 40-60kHz.

[0014] The application of the composite catalyst for the catalytic oxidation of centrifuged mother liquor in the pretreatment of paraquat wastewater after concentration is as follows: the centrifuged mother liquor after concentration of paraquat wastewater and the composite catalyst are added to a high-pressure reactor, the reaction pressure is controlled at 2.5-4.5 MPa, the reaction temperature is 180-240℃, air or oxygen is introduced as an oxidant, and the reaction residence time is 60-120 minutes. The amount of catalyst used is 1-8% of the mass of the mother liquor after centrifugation of the concentrated paraquat wastewater.

[0015] The treatment mechanism of the mother liquor from the centrifugation of concentrated paraquat wastewater using the composite catalyst of the present invention is as follows: COD removal mechanism: (1) Free radical oxidation: CuO catalyzes the generation of ·OH free radicals, which non-selectively attack organic matter, decompose macromolecules into small organic acids, and finally mineralize them into CO2 and H2O.

[0016] (2) Lattice oxygen participation: CeO2 through Ce 3+ / Ce 4+ The system continuously provides lattice oxygen to oxidize organic matter in the oxygen-deficient region, ensuring the depth of the reaction.

[0017] (3) Synergistic effect: Free radicals and lattice oxygen work together to achieve efficient COD removal (>91%).

[0018] Mechanism of paraquat cation removal: (1) Adsorption and enrichment: The Lewis acid sites of Fe2O3 selectively adsorb paraquat cations (containing pyridine rings), thereby increasing the local concentration.

[0019] (2) Ring-opening degradation: ·OH radicals attack the pyridine ring, and Fe2O3 polarizes the π electron cloud to promote the breaking of CN / CC bonds, thus achieving ring opening.

[0020] (3) Deep mineralization: the ring-opening products are further oxidized into CO2, H2O and inorganic nitrogen, with a removal rate of >99%.

[0021] Cyanide removal mechanism: (1) Adsorption and capture: Fe2O3 and CeO2 surface metal sites adsorb CN⁻ through coordination.

[0022] (2) Oxidative transformation: ·OH free radicals or CeO2 lattice oxygen will convert CN - It is oxidized to cyanate (OCN⁻).

[0023] (3) Hydrolysis and mineralization: OCN⁻ is hydrolyzed to generate CO2 and NH3, or directly oxidized to CO2 and N2, with a removal rate of >98%.

[0024] Compared with the prior art, the beneficial effects of the present invention are: This invention significantly improves the catalytic activity and structural stability of the catalyst in high-salt, high-COD paraquat centrifuged mother liquor by using a lanthanum-modified titanium-aluminum composite carrier in conjunction with multi-metal oxide active components. It effectively reduces the leaching and loss of active components and can achieve simultaneous and efficient degradation of organic matter, paraquat cations, and cyanide under mild reaction conditions. It has a long service life and excellent regeneration performance. Detailed Implementation

[0025] The embodiments of the present invention will be described in detail below; however, the embodiments of the present invention are not limited thereto. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Example

[0026] (1) Preparation of the carrier: Weigh 200g of titanium oxysulfate (calculated as TiO2) and 100g of aluminum nitrate (calculated as Al2O3) and dissolve them in 2L of deionized water. Add 15g of lanthanum nitrate (calculated as La2O3) and stir until completely dissolved. Under vigorous stirring, slowly add ammonia water to adjust the pH to 9.0 to form a coprecipitate. After the precipitation is complete, continue stirring and aging for 4 hours. Filter and wash with deionized water until no sulfate ions are detected (tested with barium chloride solution). Dry the filter cake at 105℃ for 12 hours, then place it in a muffle furnace and heat it to 600℃ at a heating rate of 5℃ / min. Calcinate at this temperature for 6 hours. After natural cooling, crush and sieve the particles with a particle size of 3-5mm to obtain the lanthanum-modified titanium-aluminum composite carrier (TiO2:Al2O3 mass ratio = 2:1, La2O3 loading 3.2%). The specific surface area of ​​the carrier is 210m² / g and the pore volume is 0.42cm³ / g.

[0027] (2) Preparation of impregnation solution: Weigh 72.5g of copper nitrate, 60.6g of iron nitrate and 43.4g of cerium nitrate according to the molar ratio of copper oxide: ferric oxide: cerium dioxide = 3:1.5:1, dissolve them in 500mL of deionized water, and prepare an impregnation solution with a total metal ion concentration of 1.2mol / L.

[0028] (3) Loading: Add 500g of the carrier obtained in step (1) to the impregnation solution in step (2) and impregnate in a 50°C water bath for 10 hours. During this period, use an ultrasonic cleaner (power 300W, frequency 50kHz) to assist in dispersion for 30 minutes.

[0029] (4) Drying and calcination: The impregnated solid was filtered out, dried at 105℃ for 10 hours, and then placed in a muffle furnace and heated to 500℃ at a heating rate of 3℃ / min. It was then calcined at a constant temperature for 4 hours and naturally cooled to obtain composite catalyst C1. The total loading of active components in composite catalyst C1 was determined to be 8.5% (mass percentage), the specific surface area was 185 m² / g, and the pore volume was 0.36 cm³ / g. Example

[0030] (1) Preparation of the carrier: 300g of titanium tetrachloride (calculated as TiO2) and 75g of aluminum sulfate (calculated as Al2O3) were weighed and dissolved in 2.5L of deionized water. 25g of lanthanum nitrate (calculated as La2O3) was added and stirred until completely dissolved. Under vigorous stirring, ammonium carbonate was slowly added dropwise to adjust the pH to 9.5 to form a coprecipitate. After the precipitation was complete, stirring and aging continued for 5 hours. The mixture was filtered and washed with deionized water until no chloride ions were detected (tested with silver nitrate solution). The filter cake was dried at 110℃ for 10 hours and then placed in a muffle furnace and heated to 650℃ at a heating rate of 5℃ / min. It was calcined at a constant temperature for 5 hours. After natural cooling, the mixture was crushed and sieved to obtain particles with a particle size of 4-6mm, thus obtaining the lanthanum-modified titanium-aluminum composite carrier (TiO2:Al2O3 mass ratio = 4:1, La2O3 loading 4.8%). The specific surface area of ​​the carrier was determined to be 180m² / g and the pore volume was 0.38cm³ / g.

[0031] (2) Preparation of impregnation solution: Weigh 96.6g of copper nitrate, 80.8g of iron nitrate and 65.1g of cerium nitrate according to the molar ratio of copper oxide: ferric oxide: cerium dioxide = 4:2:1.5, dissolve them in 600mL of deionized water, and prepare an impregnation solution with a total metal ion concentration of 1.5mol / L.

[0032] (3) Loading: Add 500g of the carrier obtained in step (1) to the impregnation solution in step (2) and impregnate in a 60°C water bath for 8 hours. During this period, use an ultrasonic cleaner (power 400W, frequency 60kHz) to assist in dispersion for 20 minutes.

[0033] (4) Drying and calcination: The impregnated solid was filtered out, dried at 120℃ for 8 hours, and then placed in a muffle furnace and heated to 550℃ at a heating rate of 3℃ / min. It was then calcined at a constant temperature for 5 hours and naturally cooled to obtain composite catalyst C2. The total loading of active components in composite catalyst C2 was determined to be 11.2% (mass percentage), the specific surface area was 162 m² / g, and the pore volume was 0.33 cm³ / g. Example

[0034] (1) Preparation of the carrier: 250g of titanium oxysulfate (calculated as TiO2) and 100g of aluminum nitrate (calculated as Al2O3) were weighed and dissolved in 2.2L of deionized water. 10g of lanthanum nitrate (calculated as La2O3) was added and stirred until completely dissolved. Under vigorous stirring, ammonia water was slowly added dropwise to adjust the pH to 8.5 to form a coprecipitate. After the precipitation was complete, stirring and aging continued for 3 hours. The mixture was filtered and washed with deionized water until no sulfate ions were detected. The filter cake was dried at 100℃ for 12 hours and then placed in a muffle furnace and heated to 550℃ at a heating rate of 5℃ / min. It was then calcined at a constant temperature for 7 hours. After natural cooling, the mixture was crushed and sieved to obtain particles with a particle size of 2-4mm, thus obtaining the lanthanum-modified titanium-aluminum composite carrier (TiO2:Al2O3 mass ratio = 2.5:1, La2O3 loading 2.1%). The specific surface area of ​​the carrier was determined to be 230m² / g and the pore volume was 0.45cm³ / g.

[0035] (2) Preparation of impregnation solution: Weigh 72.5g of copper nitrate, 40.4g of iron nitrate, 43.4g of cerium nitrate and 12.5g of manganese nitrate according to the molar ratio of copper oxide: ferric oxide: cerium dioxide: manganese oxide = 3:1:1:0.5, dissolve them in 550mL of deionized water, and prepare an impregnation solution with a total metal ion concentration of 1.0mol / L.

[0036] (3) Loading: Add 500g of the carrier obtained in step (1) to the impregnation solution in step (2) and impregnate in a 45°C water bath for 12 hours. During this period, use an ultrasonic cleaner (power 250W, frequency 45kHz) to assist in dispersion for 40 minutes.

[0037] (4) Drying and calcination: The impregnated solid was filtered out, dried at 90℃ for 12 hours, and then placed in a muffle furnace and heated to 450℃ at a heating rate of 3℃ / min. It was then calcined at a constant temperature for 6 hours and naturally cooled to obtain composite catalyst C3. The total loading of active components in composite catalyst C3 was determined to be 7.8% (mass percentage), the specific surface area was 198 m² / g, and the pore volume was 0.39 cm³ / g.

[0038] Comparative Example 1: Preparation of activated carbon-supported catalyst according to existing technology: Commercially available coconut shell activated carbon (3-5 mm, specific surface area 950 m² / g) was used as the carrier. 72.5 g of copper nitrate and 40.4 g of ferric nitrate were weighed and dissolved in 500 mL of deionized water at a copper oxide:ferric oxide mass ratio of 3:1. 500 g of activated carbon was added to the impregnation solution and impregnated at room temperature for 12 hours. After filtration, the carbon was dried at 105℃ for 10 hours and calcined at 500℃ for 4 hours to obtain activated carbon-supported catalyst D1.

[0039] Comparative Example 2: Using the same preparation method as Example 1, except that lanthanum nitrate was not added during the preparation of the support, and all other conditions were exactly the same, an unmodified titanium-aluminum supported catalyst D2 was obtained.

[0040] Application example: Catalyst performance evaluation 1. Experimental conditions The concentrated mother liquor of paraquat wastewater was used as the treatment target, and its water quality indicators were: COD 25.51×10⁻⁶. 4 ppm, ammonia nitrogen 5.89×10 4 The concentrations of paraquat cationic residue were 7800 ppm, cyanide 65 ppm, and pH was 4-5. In a 2L high-pressure reactor, 1.5L of centrifuged mother liquor and 75g of the catalyst to be tested (5% dosage) were added. After sealing, the mixture was stirred and heated to 200℃. Compressed air was introduced to maintain the pressure inside the reactor at 3.0MPa, and the reaction residence time was 90 minutes. After the reaction, the mixture was cooled, samples were taken for testing, and then analyzed. The results of wastewater treatment using five catalysts (C1, C2, C3, D1, and D2) are shown in Table 1 below.

[0041] 2. Analytical Methods COD was determined using the potassium dichromate method; paraquat residues were determined using high performance liquid chromatography; cyanide was determined using isonicotinic acid-pyrazolone spectrophotometry; and the amount of catalyst active components dissolved was determined using inductively coupled plasma atomic emission spectrometry.

[0042] 3. Evaluation Results

[0043] The evaluation results in Table 2 above show that: The catalysts C1-C3 prepared in this invention achieved a COD removal rate of over 91% for the concentrated mother liquor of paraquat wastewater, and a removal rate of over 98% for paraquat cations and cyanides, significantly superior to the comparative catalysts. The amount of active component leaching from the catalysts of this invention was significantly lower than that of the comparative catalysts, indicating that the rare earth-modified titanium-aluminum composite support has a better anchoring effect on the active component and the catalyst has higher stability. In Example 2, catalyst C2, with a higher active component loading, exhibited the best catalytic activity. Although Comparative Example 1 (activated carbon support) had a large specific surface area, the active component was easily lost under high temperature and high pressure conditions, and the activity was low, possibly because activated carbon has poor stability under an oxidizing atmosphere. Compared with Example 1, Comparative Example 2 (unmodified support) had slightly lower activity and a significantly increased amount of active component leaching, indicating that lanthanum modification helps to improve the stability and activity of the catalyst.

[0044] Catalyst lifetime test: Ten batches of catalyst C1 were continuously run under the same reaction conditions (each batch reacted for 90 minutes, and the catalyst was filtered out after the reaction and used directly in the next batch) to investigate the catalyst stability. The results are shown in Table 3 below.

[0045] The results showed that after 10 batches of continuous operation, the COD removal rate of catalyst C1 remained above 89%, the paraquat cation removal rate remained above 97%, and the dissolution of active components remained at a low level, indicating that the catalyst of the present invention has good stability.

[0046] Example 4: Effect of different process conditions on catalytic effect The effects of reaction temperature, pressure, and time on COD were investigated using composite catalyst C1. The experimental results are shown in Table 4 below:

[0047] According to the analysis results in Table 4: (1) Effect of reaction temperature The reaction was carried out at a constant pressure of 3.0 MPa and a time of 90 minutes at 160℃, 180℃, 200℃, 220℃, and 240℃, respectively, with COD removal rates of 72.3%, 84.6%, 91.5%, 93.8%, and 94.2%, respectively. Considering both removal efficiency and energy consumption, the optimal reaction temperature is 200-220℃.

[0048] (2) Effect of reaction pressure The reaction was carried out at a fixed temperature of 200℃ for 90 minutes, and at pressures of 2.0 MPa, 2.5 MPa, 3.0 MPa, 3.5 MPa, and 4.0 MPa, respectively, with COD removal rates of 78.5%, 87.3%, 91.5%, 92.6%, and 93.1%, respectively. Considering both equipment investment and removal efficiency, a reaction pressure of 2.5-3.5 MPa was selected as the optimal range.

[0049] (3) Effect of reaction time At a constant temperature of 200℃ and pressure of 3.0MPa, the COD removal rates were 52.6%, 78.4%, 91.5%, 93.2%, and 93.8% for reactions lasting 30, 60, 90, 120, and 150 minutes, respectively. Considering the overall treatment efficiency, the optimal reaction time is 90-120 minutes.

[0050] Catalyst regeneration test After 10 batches of operation, the composite catalyst C1 was removed, washed with deionized water to remove surface deposits, and then regenerated by calcination at 500°C for 3 hours. The regenerated catalyst was then used to treat the centrifuged mother liquor under the same conditions (200°C, 3.0 MPa, 90 min), achieving a COD removal rate of 90.2%, close to the 91.5% of the fresh catalyst, indicating that the catalyst of this invention has excellent regeneration performance.

[0051] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A composite catalyst for the catalytic oxidation of centrifuged mother liquor, characterized in that, It includes a carrier and an active component loaded on the carrier. The carrier is a titanium-based composite material modified with rare earth elements, and the mass ratio of the carrier to the active component is 100:(5-20).

2. The composite catalyst for the catalytic oxidation of centrifuged mother liquor according to claim 1, characterized in that, The active component is at least three of the oxides of copper, iron, cerium, and manganese.

3. The composite catalyst for the catalytic oxidation of centrifuged mother liquor according to claim 1, characterized in that, The carrier is a lanthanum-modified titanium dioxide-alumina composite carrier, wherein the mass ratio of titanium dioxide to alumina is (2-4):1, and the lanthanum loading is 1-5% of the total mass of the carrier.

4. The composite catalyst for the catalytic oxidation of centrifuged mother liquor according to claim 1, characterized in that, The molar ratio of copper oxide: ferric oxide: cerium dioxide: manganese oxide in the active components is (2-4): (1-2): (1-1.5): (0-0.5).

5. The composite catalyst for the catalytic oxidation of centrifuged mother liquor according to claim 1, characterized in that, The composite catalyst is a spherical particle with a particle size of 2-6 mm, a specific surface area of ​​150-250 m² / g, and a pore volume of 0.3-0.6 cm³ / g.

6. A method for preparing a composite catalyst for the catalytic oxidation of centrifuged mother liquor according to any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Dissolve titanium source, aluminum source and lanthanum source in deionized water in proportion, add precipitant to adjust pH to 8-10 to form coprecipitate, age, filter, wash and dry the precipitate, calcine at 500-700℃ for 4-8 hours, crush and sieve to obtain lanthanum modified titanium-aluminum composite carrier. Step 2: Dissolve copper salt, iron salt and cerium salt in deionized water according to the proportion of active components to prepare an impregnation solution with a total metal ion concentration of 0.5-2.0 mol / L; Step 3: Add the lanthanum-modified titanium-aluminum composite carrier obtained in Step 1 to the impregnation solution in Step 2, and impregnate at 40-60℃ for 6-12 hours, during which ultrasonic-assisted dispersion is performed for 20-40 minutes. Step 4: Dry the impregnated solid at 80-120℃ for 8-12 hours, and then calcine it at 400-600℃ for 3-6 hours to obtain the composite catalyst.

7. The method for preparing a composite catalyst for the catalytic oxidation of centrifuged mother liquor according to claim 6, characterized in that, The titanium source mentioned in step 1 is titanium oxysulfate or titanium tetrachloride, the aluminum source is aluminum nitrate or aluminum sulfate, and the lanthanum source is lanthanum nitrate.

8. The method for preparing a composite catalyst for the catalytic oxidation of centrifuged mother liquor according to claim 6, characterized in that, The precipitant is ammonia or ammonium carbonate.

9. The method for preparing a composite catalyst for the catalytic oxidation of centrifuged mother liquor according to claim 6, characterized in that, In step 3, the ultrasonic power is 200-400W and the ultrasonic frequency is 40-60kHz.

10. The application of the composite catalyst for catalytic oxidation of centrifuged mother liquor according to claim 6 in the catalytic oxidation pretreatment of mother liquor after concentration of paraquat wastewater, characterized in that, The application method is as follows: after concentrating paraquat wastewater, the centrifuged mother liquor and the composite catalyst are added to a high-pressure reactor, the reaction pressure is controlled at 2.5-4.5 MPa, the reaction temperature is 180-240℃, air or oxygen is introduced as an oxidant, and the reaction residence time is 60-120 minutes. The amount of catalyst used is 1-8% of the mass of the mother liquor after centrifugation of the concentrated paraquat wastewater.

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