Sludge dewatering method for activating persulfate by hydrodynamic cavitation and electrochemical oxidation

Abstract

The application discloses a sludge dewatering method for activating persulfate by means of hydrodynamic cavitation and electrochemical oxidation, wherein persulfate is added into sludge to be treated to obtain sludge containing persulfate, the sludge containing persulfate is subjected to hydrodynamic cavitation treatment to obtain hydrodynamic cavitation treated sludge, the hydrodynamic cavitation treated sludge is subjected to electrochemical oxidation treatment to obtain electrochemical oxidation treated sludge, and the electrochemical oxidation treated sludge is subjected to oxidation treatment to obtain the sludge. The treatment method provided by the application adds persulfate into sludge first, and then sequentially performs hydrodynamic cavitation treatment and electrochemical oxidation treatment, thereby improving the dewatering performance of the sludge, activating the persulfate, and generating sulfate radicals with strong oxidizing property to decompose extracellular polymeric substances in the sludge, so as to further promote cell lysis and dewatering.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sludge treatment, in particular to a sludge dewatering method for activating persulfate by hydrodynamic cavitation and electrochemical oxidation. BACKGROUND

[0002] Sludge has high water content (up to 99.9%) and large volume, and contains a large amount of bacterial cells, heavy metals and organic matter and other substances harmful to the environment, which can cause great harm to the environment if not properly treated.

[0003] The current traditional sludge dewatering methods include natural drying, thermal drying and mechanical dewatering, however, these methods have the disadvantages of low dewatering efficiency and high energy consumption to varying degrees, and the water content after treatment still cannot meet the requirements of sludge disposal, and further dewatering is required.

[0004] The current emerging sludge dewatering methods include ultrasonic, hydrodynamic cavitation and electrochemical methods. Ultrasonic is to produce a sound wave to break the cell of sludge; hydrodynamic cavitation technology is to produce a high-temperature and high-pressure environment by bubble collapse and burst to destroy the cell wall of microorganisms, improve the dewatering performance and biodegradability of sludge, reduce the amount of sludge, and realize the resource utilization of sludge; electrochemical oxidation dewatering is a relatively green sludge dewatering technology, which produces substances with strong oxidizing property to destroy the extracellular polymeric substance in sludge and promote cell lysis to promote dewatering. However, the hydrodynamic cavitation device has the disadvantages of insignificant cavitation effect and low cavitation efficiency; the core of the electrochemical method for treating sludge is the electrode, but active oxygen is produced near the anode plate during the electrochemical oxidation process, so the degree of extracellular polymeric substance and cell lysis is not high enough, and the dewatering effect is general. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application aims to provide a sludge dewatering method for activating persulfate by hydrodynamic cavitation and electrochemical oxidation. The method provided by the present application has the advantages of simple process, high reaction efficiency, small amount of reagent, low energy consumption, good dewatering performance and easy engineering application.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme

[0007] The present application provides a sludge dewatering method for activating persulfate by hydrodynamic cavitation and electrochemical oxidation. Persulfate is added to the sludge to be treated to obtain sludge containing persulfate, the sludge containing persulfate is subjected to hydrodynamic cavitation treatment to obtain hydrodynamic cavitation treated sludge, the hydrodynamic cavitation treated sludge is subjected to electrochemical oxidation treatment to obtain electrochemical oxidation treated sludge, and the electrochemical oxidation treated sludge is subjected to oxidation treatment to obtain the sludge.

[0008] The treatment method provided by the present application first adds persulfate to sludge, and then performs hydraulic cavitation treatment on the sludge. The hydraulic cavitation technology is to open the chemical bonds between molecules by using a high-temperature and high-pressure environment, so as to destroy the cell walls of microorganisms. The hydraulic cavitation technology can not only degrade macromolecules in the sludge, but also generate a large amount of hydroxyl radicals to oxidize chemical substances in the sludge, thereby improving the dewatering performance of the sludge. Subsequently, electrochemical oxidation treatment is performed. The electrochemical oxidation generates hydroxyl radicals with strong oxidizing properties, which can destroy the extracellular polymeric substance in the sludge, promote cell lysis, and improve the dewatering performance of the sludge. At the same time, the hydraulic cavitation and the electrochemical oxidation process activate the persulfate. The activated persulfate can generate sulfate radicals with strong oxidizing properties to decompose the extracellular polymeric substance in the sludge, thereby further promoting cell lysis and dewatering.

[0009] In a preferred embodiment, the amount of the added persulfate is 1-2 mmol / g VSS. When the amount of the added persulfate is controlled in the range, the dewatering effect is optimal.

[0010] In a preferred embodiment, the pressure during the hydraulic cavitation treatment is 1-5 bar, and the reaction time is 5-10 min.

[0011] In actual operation, the sludge containing the persulfate is added to the hydraulic cavitation system, and a booster pump is started to perform treatment.

[0012] In a preferred embodiment, the process of the electrochemical oxidation is to insert an electrode into the activated sludge, set the distance between the electrode plates to be 1-5 cm, start the power supply, and electrolyze for 30-60 min under a current density of 100-1000 A / m 2

[0013] Further preferably, the cathode of the electrode is selected from one of a titanium electrode, a copper electrode, and an iron electrode, and is preferably a titanium electrode, and the anode is a boron-doped diamond electrode.

[0014] Further preferably, the substrate of the boron-doped diamond electrode is selected from one of polysilicon, monocrystalline silicon, and a composite substrate, and is preferably a composite substrate. The composite substrate is composed of a silicon carbide / titanium composite layer in the middle and boron-doped silicon carbide layers on both sides.

[0015] ​When the boron-doped diamond electrode adopts a composite substrate, the electrochemical oxidation effect is optimal, so that the dewatering rate of the final sludge is the highest, wherein the silicon carbide / titanium composite layer has a low thermal expansion coefficient and excellent electrical conductivity, the boron-doped silicon carbide layers on both sides have a lower thermal expansion coefficient than the silicon carbide / titanium composite layer and have a certain electrical conductivity, which is matched with the thermal expansion coefficient of diamond; at the same time, it has excellent corrosion resistance, and the electrical conductivity of the substrate is improved as a whole through the cooperation of the two, and the thermal expansion coefficient is reduced, which avoids the peeling of the diamond coating from the substrate due to the large difference in thermal expansion coefficient during high-temperature growth; and the silicon carbide and diamond can form a covalent bond (Si-C bond), which has strong bonding force and makes the composite substrate and the diamond coating have high adhesion, and at the same time has excellent electrical conductivity and strong corrosion resistance.

[0016] Further preferably, in the composite substrate, the thickness of the silicon carbide / titanium composite layer is 0.5-5mm, and the thickness of the boron-doped silicon carbide layer is 0.1-1mm.

[0017] Further preferably, in the silicon carbide / titanium composite layer, the volume fraction of silicon carbide is 70-98%, preferably 80-90%, and the volume fraction of titanium is 2%-30%, preferably 10-20%.

[0018] The volume fraction of silicon carbide and titanium is controlled within the above range, and the performance is optimal, if the volume fraction of titanium is too high, the overall thermal expansion coefficient of the substrate will be large, which is not conducive to the combination of the substrate and the diamond coating, and if the volume fraction of metal is too low, the overall electrical conductivity of the substrate will decrease, which will increase the overall material resistivity and energy consumption.

[0019] Further preferably, in the boron-doped silicon carbide layer, the concentration of boron in the boron-doped silicon carbide layer is 1×10 18 -1×10 20 atoms / cm³.

[0020] In the present application, the concentration of the doping element in the boron-doped silicon carbide layer is controlled within the above range, and the performance of the final material is optimal, and too high or too low doping concentration will reduce the electrical conductivity of the doped non-metallic inorganic material.

[0021] Further preferably, when the substrate in the boron-doped diamond electrode is a composite substrate, the preparation process is as follows: mixing silicon carbide powder and titanium powder to obtain a composite powder, laying boron-doped silicon carbide powder, composite powder and boron-doped silicon carbide powder in the mold in turn, and then sintering to obtain a composite substrate, then performing surface pretreatment on the composite substrate, and then growing a doped diamond coating on the surface of the composite substrate by chemical vapor deposition.

[0022] Further preferably, the sintering is SPS sintering, the temperature of the SPS sintering is 1000-1900 DEG C, the holding time is 10-300 min, the heating rate is 80-200 DEG C / min, the pressure is 10-60 Mpa, and the pulse duty cycle is 50%-90%. The rapid densification technology can be realized by SPS discharge plasma sintering, the grain growth is effectively controlled, and the mechanical properties of the material are improved.

[0023] Further preferably, the surface pretreatment process of the composite substrate comprises the following steps: surface grinding of the composite substrate, sand blasting treatment, and finally, seed implantation of the composite substrate subjected to the sand blasting treatment in the diamond nanocrystal suspension.

[0024] In the application, the oxide layer is removed by grinding the composite substrate, the roughness is reduced, the surface is flattened, then the surface micro-roughness is increased by sand blasting treatment, more nucleation sites are provided, the surface area is increased, the seed adhesion capacity is improved, the surface micro-pits are generated, and the seed mechanical engagement is facilitated. The grinding provides a good surface basis for the sand blasting, and the sand blasting creates an ideal microstructure for the seed implantation. Through the above-mentioned surface pretreatment means, the high-quality doped diamond coating is finally grown.

[0025] If only grinding treatment is performed without sand blasting, the surface is too smooth and lacks micro-roughness, the seed is difficult to adhere and fix on the substrate surface, and the nucleation sites are insufficient, which affects the growth quality of the subsequent diamond film. If only sand blasting treatment is performed without grinding, the original impurities and oxide layer on the surface cannot be completely removed, the sand blasting effect is uneven, the surface treatment is inconsistent, and the substrate surface may have large protrusions and defects, which affect the uniformity of the subsequent film growth.

[0026] Further preferably, the grinding speed is controlled to be 15-30 rpm, and the grinding time is controlled to be 5-20 min.

[0027] Further preferably, the sand blasting pressure is controlled to be 0.1-0.3 MPa, the sand blasting time is controlled to be 5-60 s, and the sand blasting distance is controlled to be 10-30 cm.

[0028] Further preferably, the diamond nanocrystal suspension is obtained by dispersing nanodiamonds in pure water, and the particle size of the nanodiamonds is 1-25 nm.

[0029] Further preferably, the seed implantation is performed under ultrasonic waves, and the ultrasonic time is 30-60 min.

[0030] Further preferably, the process for growing the boron-doped diamond coating by chemical vapor deposition comprises: placing the surface-preprocessed composite substrate into a chemical deposition furnace, introducing gases with a mass flow ratio of hydrogen:methane:boration = 100-120:2-10:0.05-3, growing at a pressure of 1-4 KPa and a temperature of 650-1000 DEG C, and growing for 2-4 times, wherein the composite substrate is replaced in a forward-reverse manner after each growth, and the growth is continued, and each growth lasts for 10-40 hours.

[0031] Preferably, the oxidation treatment is performed under stirring at a rotation speed of 100-200 rpm, and the oxidation treatment lasts for 1-2 hours.

[0032] Principle and advantages

[0033] The process provided by the application comprises adding persulfate to the sludge, and then performing hydrodynamic cavitation treatment, wherein the hydrodynamic cavitation technology is used to open chemical bonds between molecules in a high-temperature and high-pressure environment, so as to destroy the cell wall of microorganisms, degrade macromolecules in the sludge, and generate a large amount of hydroxyl radicals to oxidize chemical substances in the sludge and improve the dewatering performance of the sludge; subsequently, electrochemical oxidation treatment is performed, the hydroxyl radicals generated by the electrochemical oxidation have strong oxidizing properties and can destroy the extracellular polymeric substance in the sludge, promote cell lysis, and improve the dewatering performance of the sludge; meanwhile, the persulfate is activated during the hydrodynamic cavitation and electrochemical oxidation processes, and the activated persulfate can generate sulfate radicals with strong oxidizing properties to decompose the extracellular polymeric substance in the sludge, so as to further promote cell lysis and dewatering.

[0034] In the electrochemical oxidation process, when the boron-doped diamond electrode with a composite substrate is used as the anode, the silicon carbide / titanium composite layer in the composite substrate has a low thermal expansion coefficient and excellent electrical conductivity, the boron-doped silicon carbide layers on both sides have a lower thermal expansion coefficient than the silicon carbide / titanium composite layer and have a certain electrical conductivity, which matches the thermal expansion coefficient of diamond; meanwhile, the composite substrate has excellent corrosion resistance, and the combination of the two can improve the electrical conductivity of the substrate and reduce the thermal expansion coefficient, thereby avoiding the peeling of the diamond coating from the substrate due to the large difference in thermal expansion coefficient during high-temperature growth of the diamond coating; the silicon carbide and diamond can form a covalent bond (Si-C bond), which has strong bonding force and makes the composite substrate and the diamond coating have high adhesion, and the composite substrate has excellent electrical conductivity and strong corrosion resistance, so that the efficiency of electrochemical oxidation can be greatly improved, the activation effect of the persulfate can be improved, and the final dewatering efficiency can be improved.

[0035] Compared with the prior art, the application has at least the following advantages:

[0036] 1. The sludge is broken and dewatered by using the technologies of hydraulic cavitation, electrochemical oxidation and persulfate oxidation, and the dewatering efficiency is high;

[0037] 2. The hydraulic cavitation can destroy the cell wall of the microorganism in the sludge, the electrochemical oxidation can destroy the extracellular polymer in the sludge, and the two can promote cell lysis and dewatering and increase the reaction efficiency;

[0038] 3. The hydraulic cavitation and electrochemical oxidation activate the persulfate, the activated persulfate generates sulfate radicals to further decompose the sludge treated by the hydraulic cavitation and electrochemical oxidation, and the three synergistically promote cell lysis and dewatering;

[0039] 4. The process is simple, the reaction efficiency is high, the amount of reagent used is small, the energy consumption is low, the dewatering performance is good, and the process is easy for engineering application. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 A sludge dewatering process flow chart of hydraulic cavitation synergized with electrochemical oxidation to activate persulfate. DETAILED DESCRIPTION

[0041] Example 1

[0042] 1L of municipal sludge with a water content of 98% was taken, 1.5mmol / g VSS of sodium persulfate was added, and after stirring and dissolving, it was put into a hydraulic cavitation device. The booster pump was turned on, the pressure was adjusted to 3bar, and the reaction was carried out for 5min. Then the electrode plate was inserted into the sludge, the cathode was titanium electrode, the anode was boron-doped diamond electrode (the substrate was polysilicon), the electrode plate spacing was 2cm, the current density was set to 550A / m 2 , the power was turned on, the reaction was carried out for 40min, then the power was turned off, and the stirring was carried out at 100rpm for 1h. After the reaction was completed, the water content of the sludge was measured to be 48.3%.

[0043] Comparative Example 1

[0044] 1L of municipal sludge with a water content of 98% was taken, 1.5mmol / g VSS of sodium persulfate was added, and after stirring and dissolving, it was put into a hydraulic cavitation device. The booster pump was turned on, the pressure was adjusted to 3bar, and the reaction was carried out for 5min. Then the electrode plate was inserted into the sludge, the cathode was titanium electrode, the anode was boron-doped diamond electrode (the substrate was polysilicon), the electrode plate spacing was 2cm, the current density was set to 550A / m 2 , the power was turned on, the reaction was carried out for 40min, then the power was turned off, and the stirring was carried out at 100rpm for 1h. After the reaction was completed, the water content of the sludge was measured to be 48.3%.

[0045] Example 2

[0046] Take a certain municipal sludge 1 L of moisture content of 99%, add sodium persulfate 2 mmol / g VSS, stirring and dissolving, then put into the hydrodynamic cavitation device, open the booster pump, adjust the pressure to 5 bar, react for 10 min, then insert the electrode plate in the sludge, the cathode is titanium electrode, the anode is boron-doped diamond electrode (the substrate is polysilicon), the electrode plate spacing is 2 cm, the current density is set to 800 A / m 2 , turn on the power, react for 60 min, then turn off the power, stir at 100 rpm for 1 h, after the reaction is completed, the moisture content of the sludge is measured to be 40.5%.

[0047] Comparative Example 2

[0048] Take a certain municipal sludge 1 L of moisture content of 99%, add sodium persulfate 2 mmol / g VSS, stirring and dissolving, then insert the electrode plate in the sludge, the cathode is titanium electrode, the anode is boron-doped diamond electrode (the substrate is polysilicon), the electrode plate spacing is 2 cm, the current density is set to 800 A / m 2 , turn on the power, react for 60 min, then turn off the power, stir at 100 rpm for 1 h, after the reaction is completed, the moisture content of the sludge is measured to be 72.6%.

[0049] Example 3

[0050] Take a certain municipal sludge 1 L of moisture content of 99%, add sodium persulfate 1 mmol / g VSS, stirring and dissolving, then put into the hydrodynamic cavitation device, open the booster pump, adjust the pressure to 2 bar, react for 8 min, then insert the electrode plate in the sludge, the cathode is titanium electrode, the anode is boron-doped diamond electrode (the substrate is polysilicon), the electrode plate spacing is 2 cm, the current density is set to 300 A / m 2 , turn on the power, react for 30 min, then turn off the power, stir at 100 rpm for 1 h, after the reaction is completed, the moisture content of the sludge is measured to be 53.8%.

[0051] Comparative Example 3

[0052] Take a certain municipal sludge 1 L of moisture content of 99%, add sodium persulfate 1 mmol / g VSS, stirring and dissolving, then put into the hydrodynamic cavitation device, open the booster pump, adjust the pressure to 2 bar, react for 8 min, then stir at 100 rpm for 1 h, after the reaction is completed, the moisture content of the sludge is measured to be 68.3%.

[0053] Example 4

[0054] 1. Preparation of boron-doped silicon carbide powder:

[0055] According to the doping amount of boron is 1 × 10 20atoms / cm³, high purity SiC powder (purity≥99.9%), B4C powder (purity≥99.9%), and sintering aids Al2O3, Y2O3 were ball-milled at a speed of 800 rpm for 24 hours to obtain a mixed powder, wherein the mass fraction of the sintering aid Al2O3 in the mixed powder was 0.5%, and the mass fraction of Y2O3 in the mixed powder was 0.5%. Then, under the protection of argon, the mixed powder was first heated to 1000°C at a heating rate of 15°C / min and kept for 1 hour, and then heated to 1800°C and kept for 2 hours.

[0056] 2. Preparation of the composite substrate:

[0057] SiC powder (80 vol%) and titanium powder (20 vol%) were mixed and ball-milled in a ball mill at a speed of 1200 rpm for 36 hours to obtain a mixed powder. The boron-doped SiC powder, the mixed powder, and the boron-doped SiC powder were sequentially laid in a mold, and the thickness of each doping layer was 0.5 mm, and the thickness of the composite layer in the middle was 3 mm. SPS sintering was performed at a temperature of 1550°C, a holding time of 120 min, a heating rate of 150°C / min, a pressure of 40 MPa, and a pulse duty cycle of 80%.

[0058] 3. Preparation of boron-doped diamond electrodes

[0059] The sintered sandwich structure composite substrate was surface treated by grinding at a speed of 20 rpm for 15 min, sand blasting (pressure 0.2 MPa, time 30 s, distance 20 cm), and then seed implantation in a 10 nm diamond suspension (4 g / L) for 45 min under ultrasonic. The boron-doped diamond was grown by hot filament chemical vapor deposition (HFCVD) at a gas flow ratio (H2:CH4:B2H6) of 110:5:2, a pressure of 2 KPa, a temperature of 800°C, and a growth time of 3 times, 30 h each time, and a positive and negative replacement after each growth. A composite substrate with a resistivity of 0.2 mΩ·cm was prepared, and the resistivity of existing single crystal silicon is 2 mΩ·cm, which is 10 times lower than that of existing single crystal silicon substrate.

[0060] 4. Sludge dewatering

[0061] 1L of municipal sludge with a water content of 98% was taken, 1.5 mmol / g VSS of sodium persulfate was added, and after stirring and dissolving, it was put into a hydrodynamic cavitation device. The booster pump was turned on, the pressure was adjusted to 3 bar, and the reaction was carried out for 5 min. Then, the electrode plates were inserted into the sludge, with titanium as the cathode and boron-doped diamond as the anode, and the distance between the electrode plates was 2 cm. The current density was set to 550 A / m 2 , the power was turned on, and the reaction was carried out for 40 min. After the power was turned off, the sludge was stirred at 100 rpm for 1 h. After the reaction was completed, the water content of the sludge was measured to be 41.1%.

Claims

1. A method for dewatering sludge by hydrodynamic cavitation synergized with electrochemical oxidation of activated persulfate, characterized by: The method comprises the following steps: adding persulfate into sludge to be treated to obtain sludge containing persulfate, performing hydrodynamic cavitation treatment on the sludge containing persulfate to obtain hydrodynamic cavitation treated sludge, performing electrochemical oxidation treatment on the hydrodynamic cavitation treated sludge to obtain electrochemical oxidation treated sludge, and performing oxidation treatment on the electrochemical oxidation treated sludge to obtain the sludge. The process of the electrochemical oxidation treatment is as follows: inserting electrodes into activated sludge, setting the distance between the electrode plates to be 1-5 cm, turning on the power supply, and electrolyzing for 30-60 min under a current density of 100-1000 A / m 2 ​ The anode of the electrode is a boron-doped diamond electrode. The substrate in the boron-doped diamond electrode is a composite substrate, which is composed of a silicon carbide / titanium composite layer in the middle and boron-doped silicon carbide layers on both sides.

2. The method according to claim 1, characterized in that: The addition amount of the persulfate is 1-2 mmol / g VSS.

3. The method according to claim 1, characterized in that: During the hydrodynamic cavitation treatment, the pressure is 1-5 bar, and the reaction time is 5-10 min.

4. The method of claim 1, wherein the method is characterized by: The cathode of the electrode is selected from one of a titanium electrode, a copper electrode and an iron electrode.

5. The method of sludge dewatering by hydrodynamic cavitation synergized with electrochemical oxidation of persulfate according to claim 1, characterized by the fact that: In the composite substrate, the thickness of the silicon carbide / titanium composite layer is 0.5-5 mm, and the thickness of the boron-doped silicon carbide layer is 0.1-1 mm. In the silicon carbide / titanium composite layer, the volume fraction of silicon carbide is 70-98%, and the volume fraction of titanium is 2%-30%. The concentration of boron in the boron-doped silicon carbide layer is 1 x 1016 18 -1 x 1018 20 atoms / cm3.

6. The method according to claim 1, wherein the method comprises the following steps: adding persulfate into sludge to be treated to obtain sludge containing persulfate, performing hydrodynamic cavitation treatment on the sludge containing persulfate to obtain hydrodynamic cavitation treated sludge, performing electrochemical oxidation treatment on the hydrodynamic cavitation treated sludge to obtain electrochemical oxidation treated sludge, and performing oxidation treatment on the electrochemical oxidation treated sludge to obtain the sludge. When the substrate in the boron-doped diamond electrode is a composite substrate, the preparation process comprises the following steps: mixing silicon carbide powder and titanium powder to obtain a composite powder, laying boron-doped silicon carbide powder, the composite powder and boron-doped silicon carbide powder in a mold in sequence, then sintering to obtain a composite substrate, and then performing surface pretreatment on the composite substrate and growing a boron-doped diamond coating on the surface of the composite substrate by chemical vapor deposition. The sintering is SPS sintering, the temperature of the SPS sintering is 1000-1900 ℃, the holding time is 10-300 min, the heating rate is 80-200 ℃ / min, the pressure is 10-60 Mpa, and the pulse duty cycle is 50%-90%.

7. The method according to claim 6, characterized in that: The surface pretreatment process of the composite substrate comprises the following steps: first, surface grinding of the composite substrate, then sand blasting treatment, and finally, seed implantation of the composite substrate subjected to the sand blasting treatment in a diamond nanocrystal suspension. During the grinding, the grinding speed is controlled to be 15-30 rpm, and the time is controlled to be 5-20 min. During the sand blasting treatment, the pressure is controlled to be 0.1-0.3 MPa, the time is controlled to be 5-60 s, and the distance is controlled to be 10-30 cm. The diamond nanocrystal suspension is obtained by dispersing nanodiamond in pure water, and the particle size of the nanodiamond is 1-25 nm. The seed implantation is performed under ultrasonic waves, and the time of the ultrasonic waves is 30-60 min.

8. The method according to claim 6, characterized in that: The process of growing the boron-doped diamond coating by chemical vapor deposition comprises the following steps: placing the composite substrate subjected to the surface pretreatment in a chemical deposition furnace, inputting gases with a mass flow ratio of hydrogen:methane:boration = 100-120:2-10:0.05-3, growing under a pressure of 1-4 Kpa at a temperature of 650-1000 ℃, growing for 2-4 times, replacing the composite substrate in a positive and negative manner after each growth, and then continuing the growth, and the time of each growth is 10-40 h.

9. The method according to claim 1 or 2, characterized in that: The oxidation treatment is carried out under stirring at a rotation speed of 100-200 rpm for a time of 1-2 h. The oxidation treatment is carried out under stirring at a rotation speed of 100-200 rpm for a time of 1-2 h.

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