Method for treating sludge through subcritical oxidation method

Abstract

The invention discloses a method for treating sludge by a subcritical oxidation method. The method comprises the following steps: S1, mixing a high-entropy oxide catalyst and sludge, and injecting the mixture into a subcritical oxidation reactor; and S2, filling oxygen into the subcritical oxidation reactor, and carrying out subcritical oxidation treatment on the sludge. According to the method, the high-entropy oxide is adopted as the catalyst, and the high-entropy oxide is good in thermal stability and stable in catalytic performance, so that oxygen is quickly decomposed into free radicals with strong oxidizing capacity under the action of the catalyst, the reaction speed is increased, and the reaction activation energy is reduced. Compared with a subcritical oxidation treatment process without using a catalyst, the method has the advantages that the removal rate of the chemical oxygen demand of the sludge can be remarkably increased under the condition of the same reaction temperature and pressure, and the reaction temperature can be greatly reduced under the condition of the same removal rate of the chemical oxygen demand, so that the loss of the catalyst is reduced, the energy consumption is reduced, and the treatment efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of sludge treatment, in particular to a method for treating sludge using a subcritical oxidation process. Background Art

[0002] In sewage treatment plants, nearly 70% of pollutants (including new pollutants such as antibiotics, microplastics, etc.) will be transformed or transferred into sludge. Sludge is rich in nutrients required by plants such as nitrogen (N), phosphorus (P), potassium (K), etc., and also contains a large amount of heavy metals and pathogenic microorganisms. If it is not handled properly, it will not only cause environmental pollution, but also cause waste of resources. In addition, sludge treatment is an energy and cost-intensive process, accounting for about 50% of the total operating cost of wastewater treatment. Sludge is treated by subcritical oxidation method, which can achieve stabilization, reduction and resource utilization.

[0003] Subcritical oxidation is a process for treating wastewater by thermochemical oxidation of organic matter in water phase. It is very effective in treating high-concentration biodegradable organic wastewater at high temperature (180-300℃) and certain pressure (10-150bar oxygen). Since the material structure of residual sludge is very similar to that of high-concentration organic wastewater, subcritical oxidation can also be used to treat sludge. Compared with traditional technologies, it has the characteristics of small footprint, high treatment efficiency per unit time, low investment cost per ton of sludge and high sludge resource potential.

[0004] However, the existing sludge subcritical oxidation treatment method has the disadvantages of high energy consumption, rapid catalyst loss and low treatment efficiency. Summary of the invention

[0005] The purpose of the present invention is to solve the technical problems of high energy consumption, rapid catalyst loss and low treatment efficiency of subcritical oxidation method, and to provide a method for treating sludge by subcritical oxidation method.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for treating sludge by subcritical oxidation, comprising the following steps:

[0008] S1, mixing the high entropy oxide catalyst and the sludge and injecting them into a subcritical oxidation reactor;

[0009] S2. Filling oxygen into the subcritical oxidation reactor to perform subcritical oxidation treatment on the sludge.

[0010] In some embodiments, in step S1, the amount of the high entropy oxide catalyst added is 0.1-0.5 wt % of the sludge mass.

[0011] In some embodiments, the high entropy oxide catalyst is added in an amount of 0.2 to 0.3 wt % of the sludge mass.

[0012] In some embodiments, the high entropy oxide catalyst is obtained by co-precipitation and sintering.

[0013] In some embodiments, the co-precipitation method comprises:

[0014] A1. dissolving at least five different metal salts in deionized water to obtain a solution having a total cation concentration of 0.8 to 1.2 mol / L; dissolving sodium carbonate in deionized water to form a solution having a total cation concentration of 1.4 to 1.7 mol / L;

[0015] A2. Add a solution having a concentration of 0.8 to 1.2 mol / L to a solution having a concentration of 1.4 to 1.7 mol / L, and keep the mixture under continuous stirring for 15 to 60 minutes. Then filter the gel-like coprecipitate, wash it repeatedly with deionized water, and dry it at a temperature of 50 to 70° C. for 5 to 24 hours to obtain a precursor powder.

[0016] In some embodiments, the sintering method comprises:

[0017] B1, calcining the precursor powder at a temperature of 400-500° C. for 0.5-1.5 h at a heating rate of 5-15° C. / min, so that the water in the material is completely gasified;

[0018] B2. Sintering in a muffle furnace at a temperature of 900-1200°C at a heating rate of 5-10°C / min in a static air atmosphere for 1-2 hours;

[0019] B3. After rapid air quenching, a formed high entropy oxide catalyst is obtained.

[0020] In some embodiments, in step S1, the moisture content of the sludge is 80-99 wt%.

[0021] In some embodiments, the water content of the sludge is 85-90 wt %.

[0022] In some embodiments, in step S2, the reaction temperature in the subcritical oxidation process is 200-280°C, the reaction time is 30-60 minutes, and the injection pressure of oxygen is 10-150 bar.

[0023] In some embodiments, the reaction temperature in the subcritical oxidation process is 200-260° C., the reaction time is 60 minutes, and the injection pressure of oxygen is 10 bar.

[0024] The beneficial effects of the present invention compared with the prior art include:

[0025] The subcritical oxidation method for treating sludge provided by the present invention adopts high-entropy oxide (HEO) as a catalyst. The high-entropy oxide has good thermal stability and stable catalytic performance, so that oxygen is quickly decomposed into free radicals with strong oxidizing ability under the action of the catalyst, thereby accelerating the reaction speed and reducing the reaction activation energy. Compared with the subcritical oxidation treatment process without using a catalyst, under the same reaction temperature and pressure conditions, the present invention can significantly improve the removal rate of chemical oxygen demand of sludge, and under the same chemical oxygen demand removal rate, the reaction temperature can be greatly reduced, and the dehydration performance of the sludge after the reaction is further improved, thereby reducing catalyst loss, reducing energy consumption, and improving treatment efficiency.

[0026] In some embodiments, the high entropy oxide catalyst is obtained by co-precipitation and sintering. The high entropy oxide catalyst obtained by this synthesis method can avoid the dissolution of the catalyst active components during use, and will not cause catalyst loss and activity reduction, thereby increasing its repeated service life.

[0027] Other beneficial effects of the embodiments of the present invention will be further described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a flow chart of the method for treating sludge by subcritical oxidation method in an embodiment of the present invention.

[0029] Figure 2a and 2b SEM images of the high entropy oxide (HEO) prepared in Example 1 at different magnifications.

[0030] Figure 2c These are the XRD diagrams of the sample prepared in Example 1 before reaction without adding sludge, after one reaction, and after six reactions.

[0031] Figure 2d This is the EDS image of the high entropy oxide prepared in Example 1.

[0032] Figure 3a This is a schematic diagram of the COD removal rate of the sludge in Example 1.

[0033] Figure 3b These are SEM images of the HEO catalyst in Example 1 before reaction (left picture) and after 6 reactions (right picture). DETAILED DESCRIPTION

[0034] The present invention is further described below with reference to the accompanying drawings and in combination with preferred embodiments. It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other without conflict.

[0035] like Figure 1 As shown, an embodiment of the present invention provides a method for treating sludge by subcritical oxidation based on a high entropy oxide catalyst, comprising the following steps:

[0036] S1. Mix the high entropy oxide catalyst and the sludge and inject them into a subcritical oxidation reactor (such as a reactor).

[0037] S2. Fill the subcritical oxidation reactor with oxygen and perform subcritical oxidation treatment on the sludge.

[0038] In some embodiments, in step S2, oxygen is charged into the reactor at an injection pressure of 10-150 bar, the temperature in the reactor is raised to 200-280° C., and maintained for 30-60 minutes to obtain a reaction slurry after subcritical oxidation. Preferably, the reactor temperature is maintained at 200-260° C., subcritical oxidation is performed for 60 minutes, and the oxygen injection pressure is 10 bar.

[0039] In some embodiments, in step S1, the high entropy oxide catalyst is obtained by co-precipitation and sintering.

[0040] Further, the coprecipitation method includes: dissolving a metal salt (greater than or equal to five, including magnesium, aluminum, calcium, titanium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, tungsten, preferably, selecting five metals of Cu, Mn, Fe, Ni, Co, or five metals of Cu, Mn, Fe, Mg, Co) in deionized water to obtain a solution with a total cation concentration of 0.8 to 1.2 mol / L (each cation concentration is 0.16 to 0.24 mol / L, and the metal cations are added to the deionized water in an equimolar ratio). Dissolve Na2CO3 (sodium carbonate) in deionized water to form a solution with a concentration of 1.4 to 1.7 mol / L. The coprecipitation method is carried out in a reverse addition manner, that is, slowly (preferably at a drop rate of 1-2 drops / second) adding a solution with a concentration of 0.8 to 1.2 mol / L to a solution with a concentration of 1.4 to 1.7 mol / L, so as to better generate a gel-like coprecipitate. The mixture is kept under continuous stirring for 15 to 60 minutes, and then the gel-like coprecipitate is filtered, repeatedly washed with deionized water, and dried at a temperature of 50 to 70° C. for 5 to 24 hours to obtain a precursor powder.

[0041] The sintering method includes: preliminarily calcining the synthesized precursor powder at a temperature of 400-500°C for 0.5-1.5h, with a heating rate of 5-15°C / min, so that the water in the material is completely vaporized. Then, sintering is performed in a muffle furnace at a temperature of 900-1200°C at a heating rate of 5-10°C / min in a static air atmosphere for 1-2h. After sintering, the sample is taken out of the furnace by rapid air quenching to obtain a formed high entropy oxide catalyst, which is stored at room temperature.

[0042] In some embodiments, in step S1, the water content of the sludge is 80-99 wt %. Preferably, the water content of the sludge is 85-90 wt %.

[0043] In some embodiments, in step S1, the amount of high entropy oxide catalyst added is 0.1-0.5 wt% of the sludge mass. Preferably, the amount of high entropy oxide catalyst added is 0.2-0.3 wt% of the sludge mass.

[0044] The method for treating sludge by subcritical oxidation provided by the embodiment of the present invention adopts high entropy oxide as a catalyst. The high entropy oxide has good thermal stability and stable catalytic performance, so that oxygen is quickly decomposed into free radicals with strong oxidizing ability under the action of the catalyst, thereby accelerating the reaction speed and reducing the reaction activation energy. Under the same reaction conditions, compared with the subcritical oxidation treatment process without using a catalyst, under the same reaction temperature and pressure conditions, the embodiment of the present invention can increase the removal rate of sludge COD (chemical oxygen demand) by 10%, and under the same COD removal rate, the reaction temperature is reduced by 15%, and the dehydration performance of the sludge after the reaction is further improved, achieving the goals of reducing catalyst loss, reducing energy consumption, and higher treatment efficiency.

[0045] In addition, high entropy oxide is an oxide solid solution with a single crystalline phase formed by five or more different metal cations in equal or approximately equal molar ratios. The number of phases in the equimolar multi-component metal cations is significantly reduced compared to the maximum value allowed by the Gibbs phase law, and even a single phase may be formed. The formation of a single-phase solid solution may be related to the larger configuration entropy (Sconfig) of the mixture. It can be seen that high entropy oxide is a class of highly active catalyst materials that affects the phase stability of the solid solution by accurately controlling the configuration entropy of the material. Multiphase catalysts are prone to serious catalyst active component dissolution during use due to the limitations of the synthesis method (commonly used coprecipitation and impregnation method), which will cause catalyst loss and activity decline, reducing its repeated service life. The high entropy oxide catalyst in the embodiment of the present invention is obtained by coprecipitation and sintering. The high entropy oxide catalyst obtained by the synthesis method can avoid the occurrence of catalyst active component dissolution during use, and will not cause catalyst loss and activity decline, thereby increasing its repeated service life. The high entropy oxide in the preferred embodiment of the present invention is preferably a (Cu, Mn, Fe, Mg, Co)O type high entropy oxide or a (Cu, Mn, Fe, Ni, Co)O type high entropy oxide having a stable phase structure (an entropy-stable single-phase Fm-3m solid solution) and excellent degradation performance. The preferred high entropy oxide is a magnetic catalyst material mainly formed on the basis of Fe (iron) and can be better recovered, thereby reducing the cost of sludge treatment.

[0046] The following is a detailed description of the method for treating sludge by subcritical oxidation in conjunction with specific examples. The chemical substances used in the following examples, unless otherwise specified, are commercially available and are of analytical grade.

[0047] Example 1

[0048] The preparation process of (Cu, Mn, Fe, Mg, Co)O type high entropy oxide is as follows: 0.1 mol of each of CuCl2, MnCl2, FeCl2, MgCl2, and CoCl2 is dissolved in 500 mL of deionized water to prepare a solution A with a total cation concentration of 1 mol / L (each cation concentration is 0.2 mol / L). 0.7 mol of Na2CO3·10H2O is dissolved in 500 mL of deionized water to form a solution B with a concentration of 1.4 mol / L. Solution A is slowly added to solution B (drop speed is 1-2 drops / second), ultrasonically stirred for 30 minutes, and then the gel-like coprecipitate is filtered, repeatedly washed with deionized water, and dried in an oven at 60°C for 24 hours to obtain a precursor powder. The precursor powder is preliminarily calcined at a temperature of 450°C (heating rate of 10°C / min) for 1 hour, and then sintered in a muffle furnace at a temperature of 1050°C at a heating rate of 10°C / min in a static air atmosphere for 2 hours. After sintering, the sample is taken out of the furnace by rapid air quenching, and the precipitated metal salts of the five components (Cu, Mn, Fe, Mg, Co) are completely converted into an entropy-stable single-phase Fm-3m solid solution, that is, a formed high-entropy oxide catalyst is obtained and stored at room temperature. Figure 2a and 2b The following are SEM images of high entropy oxide (HEO) prepared at different magnifications. It can be seen from the figure that the surface of the prepared high entropy oxide is rough and has many gaps. According to BET data, its specific surface area is 6.1252 m 2 / g, and its average pore size is 32.143nm, which belongs to the mesoporous range (2-50nm). Figure 2d Shown is the EDS image of the prepared high entropy oxide, from which it can be seen that the five components (Cu, Mn, Fe, Mg, Co) are evenly distributed, and no segregation or other phase formation trend is observed.

[0049] The concentrated sludge from the secondary sedimentation tank of a water purification plant was sampled. The sludge was kept at 4°C, with a moisture content of 98.2±0.2wt%, a COD of 10920mg / L, and a pH of 7.1. The sludge was treated by a method based on high entropy oxide subcritical oxidation, specifically:

[0050] The (Cu, Mn, Fe, Mg, Co)O type high entropy oxide was added as a catalyst to 100 mL of sludge according to the addition amount of 0.2 wt% of the catalyst by weight of the sludge, and then injected into the reactor. After the reactor was sealed, 10 bar of oxygen was filled into the reactor, and the temperature of the reactor was controlled at 200°C (with an error of 0.5°C) by a program temperature riser. The timing was started at the same time, the stirring speed was 200r / min, and the subcritical oxidation was 60min. After the reaction was completed and cooled to room temperature, the gas in the reactor was released, the reactor was opened, and the reaction slurry after subcritical oxidation was obtained. The reaction slurry was separated into solid and liquid by filter paper, and the reaction solution was stored at 4°C. The chemical oxygen demand (COD), organic acid pH, TP, and TN of the mixed sludge sample after the reaction were sent to the detection agency for analysis. According to the test, the total COD (chemical oxygen demand) removal rate of the sludge is 71.22%, the sludge settling performance is greatly improved, and the organic acid pH, TP, and TN in the treated sludge sample are basically the same as those in the sludge before treatment. Compared with the subcritical oxidation treatment process without using a catalyst (COD removal rate is 64.72%), the total COD removal rate can be increased by more than 10%, achieving the goal of reducing catalyst loss (catalyst loss per reaction is less than 5%), reducing energy consumption, and achieving higher treatment efficiency.

[0051] In order to verify the repeated service life of the catalyst prepared in this example, the solution after the above reaction was filtered and separated to obtain the HEO catalyst, which was washed with distilled water for several times before the next experiment. Figure 3a As shown in the figure, after 6 reactions, the COD removal rate of the sludge decreased from 71.22% to 67.21%, a decrease of only 4.01%, indicating that the HEO catalyst has good reusability. Figure 3b Shown are SEM images of the HEO catalyst before reaction (left) and after 6 reactions (right). It can be seen that the reused HEO also has good crystal phase and structural stability.

[0052] Example 2

[0053] The concentrated sludge (water content 90wt%) from the secondary sedimentation tank of a water purification plant was treated by a method based on high entropy oxide subcritical oxidation, specifically:

[0054] The catalyst used is (Cu, Mn, Fe, Ni, Co)O type high entropy oxide (which is prepared by the coprecipitation method and sintering method in the above embodiment 1, the difference is that the metal salts used are CuCl2, MnCl2, FeCl2, NiCl2, CoCl2), the high entropy oxide catalyst is added to the sludge according to the addition amount of 0.25wt% catalyst by weight of the sludge, and both are injected into the reactor, the reactor is sealed, and 10bar of oxygen is filled into the reactor, the temperature is controlled at 240℃ (error is 0.5℃) by a program temperature riser, and the timing is started at the same time, and the subcritical oxidation is 60min. After cooling to room temperature, the gas in the reactor is released, the reactor is opened, and the reaction slurry after subcritical oxidation is obtained. According to the test, the total COD removal rate of the sludge is 70%, and the sludge settling performance is greatly improved. Compared with the subcritical oxidation (283°C) treatment process without using a catalyst, the reaction temperature of the subcritical oxidation (240°C) using this catalyst is reduced by about 15%, achieving the goals of reducing catalyst loss, reducing energy consumption, and improving treatment efficiency.

[0055] Example 3

[0056] The concentrated sludge (water content 85wt%) from the secondary sedimentation tank of a water purification plant was treated by a method based on high entropy oxide subcritical oxidation, specifically:

[0057] The catalyst used is (Cu, Mn, Fe, Mg, Co)O type high entropy oxide (which is prepared by the coprecipitation method and sintering method in the above embodiment 1). The high entropy oxide catalyst is added to the sludge in an amount of 0.25wt% of the catalyst by weight of the sludge, and both are injected into the reactor. After the reactor is sealed, 10 bar of oxygen is filled into the reactor, and the temperature is controlled at 280°C (with an error of 0.5°C) by a program temperature riser. At the same time, the timing is started, and the subcritical oxidation is 60 minutes. After cooling to room temperature, the gas in the reaction is released and the reactor is opened. According to the test, the total COD removal rate of the sludge is 83%, and the sludge settling performance is greatly improved. Compared with the subcritical oxidation treatment process without using a catalyst, the total COD removal rate can be increased by more than 15%, achieving the goal of reducing catalyst loss, reducing energy consumption, and higher treatment efficiency.

[0058] It can be seen from the embodiments of the present invention that the high entropy oxide catalyst of the present invention has the advantages of small catalyst dosage, low reaction temperature, short reaction time, etc., can improve the COD removal rate, and does not affect the TN, TP and organic acid content in the sludge hydrolyzate, so that nutrients and organic acids can be more easily recovered from the sludge.

[0059] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art of the present invention, several equivalent substitutions or obvious variations can be made without departing from the concept of the present invention, and the performance or use is the same, which should be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for treating sludge by subcritical oxidation, characterized in that: The steps include: S1, mixing the high entropy oxide catalyst and the sludge and injecting them into a subcritical oxidation reactor; S2. Filling oxygen into the subcritical oxidation reactor to perform subcritical oxidation treatment on the sludge.

2. The method for treating sludge by subcritical oxidation method according to claim 1, characterized in that: In step S1, the amount of high entropy oxide catalyst added is 0.1-0.5 wt% of the sludge mass.

3. The method for treating sludge by subcritical oxidation according to claim 2, characterized in that: The addition amount of the high entropy oxide catalyst is 0.2-0.3 wt % of the sludge mass.

4. The method for treating sludge by subcritical oxidation according to any one of claims 1 to 3, characterized in that: The high entropy oxide catalyst is obtained by co-precipitation and sintering.

5. The method for treating sludge by subcritical oxidation method according to claim 4, characterized in that: The coprecipitation method comprises: A1. dissolving at least five different metal salts in deionized water to obtain a solution having a total cation concentration of 0.8 to 1.2 mol / L; dissolving sodium carbonate in deionized water to form a solution having a total cation concentration of 1.4 to 1.7 mol / L; A2. Add a solution having a concentration of 0.8 to 1.2 mol / L to a solution having a concentration of 1.4 to 1.7 mol / L, and keep the mixture under continuous stirring for 15 to 60 minutes. Then filter the gel-like coprecipitate, wash it repeatedly with deionized water, and dry it at a temperature of 50 to 70° C. for 5 to 24 hours to obtain a precursor powder.

6. The method for treating sludge by subcritical oxidation as claimed in claim 5, characterized in that: The sintering method comprises: B1, calcining the precursor powder at a temperature of 400-500° C. for 0.5-1.5 h at a heating rate of 5-15° C. / min, so that the water in the material is completely gasified; B2. Sintering in a muffle furnace at a temperature of 900-1200°C at a heating rate of 5-10°C / min in a static air atmosphere for 1-2 hours; B3. After rapid air quenching, a formed high entropy oxide catalyst is obtained.

7. The method for treating sludge by subcritical oxidation method according to claim 1, characterized in that: In step S1, the water content of the sludge is 80-99wt%.

8. The method for treating sludge by subcritical oxidation as claimed in claim 7, characterized in that: The water content of the sludge is 85-90wt%.

9. The method for treating sludge by subcritical oxidation method according to claim 1, characterized in that: In step S2, the reaction temperature in the subcritical oxidation process is 200-280°C, the reaction time is 30-60 minutes, and the injection pressure of oxygen is 10-150 bar.

10. The method for treating sludge by subcritical oxidation as claimed in claim 9, characterized in that: The reaction temperature in the subcritical oxidation process is 200-260° C., the reaction time is 60 minutes, and the injection pressure of oxygen is 10 bar.

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