Method for preparing phosphorus-rich biochar through sludge pyrolysis-calcination two-step heat treatment

Abstract

The invention provides a method for preparing phosphorus-rich biochar through sludge pyrolysis-calcination two-step heat treatment, and belongs to the technical field of harmless treatment and resource utilization of municipal sludge. The preparation method comprises the following steps: (1) heating and drying municipal sludge, grinding, crushing and sieving to obtain sludge particles; (2) pyrolyzing the sludge particles in the step (1) in a microwave pyrolysis device, and cooling to obtain sludge semicoke; and heating and calcining the sludge semicoke in the step (2) in a nitrogen or carbon dioxide atmosphere, and cooling to obtain the phosphorus-rich biochar. Compared with commercial activated carbon, the phosphorus-rich biochar prepared by the method disclosed by the invention is rich in a large amount of phosphorus element, the source is more environment-friendly, and the harm caused by mass production and accumulation of municipal sludge is solved; the phosphorus-rich biochar disclosed by the invention is more practical, can be applied to grassland gardens instead of phosphate fertilizer, or can be mined as a phosphate ore source instead of phosphate ore.

Description

Technical Field

[0001] The invention belongs to the technical field of harmless treatment and resource utilization of municipal sludge, and in particular relates to a method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination. Background Art

[0002] Optimize the existing sludge disposal process technology and develop and apply advanced process routes. Focusing on the technical bottlenecks of sludge harmless treatment and resource utilization, it is emphasized that it is necessary to focus on breakthroughs in sludge stabilization and harmless treatment, resource utilization, coordinated disposal, and reduction in sewage treatment plants. Technical equipment, carry out innovative sludge treatment and resource utilization technology applications. Sludge has complex components and a wide range of sources, and contains a large amount of undecomposed organic matter. How to harmlessly and resourcefully treat and dispose of municipal sludge has become one of the important conditions for promoting the peaceful and stable development of my country's ecological environment.

[0003] Studies have pointed out that a large amount of phosphorus in sewage is enriched in sewage sludge, but at the same time heavy metals such as cadmium, lead, copper, zinc and nickel will also be deposited in the sludge. In the study, most thermal treatment technologies use high-temperature incineration to remove heavy metals, but this will cause a large amount of phosphorus to volatilize and be lost at high temperatures. The "two-step method" process is a process route that can not only retain a high phosphorus fixation rate, but also remove and stabilize heavy metals in sludge to a large extent; the phosphorus in the product has high bioavailability and industrial utilization, and the risks of pollutants such as heavy metals are lower than the national application standards.

[0004] At present, there have been some relevant feasibility studies on the two-step pyrolysis process for treating municipal sludge, but there are few studies on the effects of calcination conditions on the products. In addition, in today's advocacy of promoting resource utilization, there are few reports on technical processes for the safe and efficient use of products.

[0005] In summary, developing a municipal sludge thermal treatment technology that can retain a large amount of phosphorus while promoting the removal or safe and stable storage of heavy metals is an important issue that needs to be solved urgently. Summary of the invention

[0006] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a phosphorus-rich biochar and a preparation method thereof using a two-step thermal treatment of municipal sludge. The method can not only retain a high phosphorus fixation rate, but also promote the removal or safe and stable storage of heavy metals. The prepared phosphorus-rich biochar has high bioavailability and industrial applicability.

[0007] The objective of the present invention is achieved through the following technical solutions:

[0008] The present invention provides a method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0009] (1) heating and drying municipal sludge, grinding, crushing, and sieving to obtain sludge particles;

[0010] (2) placing the sludge particles in step (1) in a microwave pyrolysis device for pyrolysis, and obtaining sludge semi-coke after cooling;

[0011] (3) The sludge semi-coke obtained in step (2) is heated and calcined in a nitrogen or carbon dioxide atmosphere, and then cooled to obtain phosphorus-rich biochar.

[0012] Furthermore, in the method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, the municipal sludge heated and dried in step (1) is ground and crushed, and then passed through a 100-200 mesh sieve.

[0013] Furthermore, in the method for preparing phosphorus-rich biochar by two-step pyrolysis-calcination heat treatment of sludge, in step (2), before pyrolysis, high-purity inert gas is introduced to replace the air atmosphere in the microwave pyrolysis device, and the gas flow rate is greater than 100 mL / min;

[0014] The pyrolysis temperature is 300°C-700°C, the heating rate is 5°C / min-15°C / min, and the pyrolysis time is 30min-90min.

[0015] Furthermore, in the method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, step (3) uses a tubular furnace for calcination;

[0016] Furthermore, in step (3), the calcination temperature is 700°C-900°C, the heating rate is 5°C / min-15°C / min, the calcination time is 30min-90min, and the gas flow rate is greater than 100mL / min.

[0017] Furthermore, in the method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, the phosphorus fixation rate of the prepared phosphorus-rich biochar is greater than 83%.

[0018] Furthermore, in the method for preparing phosphorus-rich biochar by two-step pyrolysis-calcination heat treatment of sludge, heating and calcination are carried out in a carbon dioxide atmosphere in step (3), and the biologically available ratio of the prepared phosphorus-rich biochar is greater than 20%.

[0019] Furthermore, in the method for preparing phosphorus-rich biochar by two-step sludge pyrolysis-calcination heat treatment, the pyrolysis temperature in step (2) is 500°C, the inert atmosphere in step (3) is a carbon dioxide atmosphere, and the calcination temperature is 900°C.

[0020] Furthermore, the phosphorus fixation rate in the prepared phosphorus-rich biochar reached 95.46%, and the bioavailability ratio reached 52.60%.

[0021] The present invention also provides phosphorus-rich biochar obtained by two-step thermal treatment of sludge pyrolysis-calcination, which is prepared by using the method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination.

[0022] The present invention also provides an application of phosphorus-rich biochar obtained by two-step thermal treatment of sludge pyrolysis-calcination in phosphate rock or phosphate fertilizer.

[0023] Advantages and effects of the present invention:

[0024] 1. The present invention provides a method for preparing phosphorus-rich biochar by a two-step heat treatment of sludge pyrolysis-calcination. Compared with commercial activated carbon, the prepared phosphorus-rich biochar is enriched with a large amount of phosphorus and has a more environmentally friendly source, thus solving the hazards caused by the large-scale production and accumulation of municipal sludge.

[0025] 2. The phosphorus-rich biochar of the present invention is more practical and can be applied to grasslands and gardens instead of phosphate fertilizers, or can replace phosphate mining as a source of phosphate rock, which overall reduces the environmental damage and resource waste caused by phosphate mining and phosphate fertilizer production, and solves the problems of excessive municipal sludge production, inefficient treatment methods and low utilization rate.

[0026] It should be noted that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a process flow chart of a method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination in an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The present invention is described in detail below in conjunction with embodiments. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. In the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the scope of protection of the present invention.

[0029] In view of the problems of large municipal sludge production and low recycling means, the present invention proposes a phosphorus-rich biochar using a two-step thermal treatment of municipal sludge and a preparation method thereof, such as Figure 1 Shown is a process flow chart of the preparation method of an embodiment of the present invention.

[0030] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0031] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 100-200 mesh sieve;

[0032] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting pyrolysis conditions, pyrolysis target temperature of 300°C-700°C, heating rate of 5°C / min-15°C / min, pyrolysis time of 30min-90min, introducing high-purity inert gas to dilute the furnace air atmosphere for replacement, the gas flow rate is greater than 100mL / min, after the replacement is completed, start the microwave oven to pyrolyze the sludge, and cool it after the pyrolysis is completed to obtain sludge semi-coke;

[0033] (3) placing the sludge semi-coke obtained in step (2) in a tube furnace, setting calcination conditions, calcination target temperature of 700°C-900°C, heating rate of 5°C / min-15°C / min, calcination time of 30min-90min, introducing carbon dioxide gas or nitrogen into the furnace to replace air before calcination, with a gas flow rate greater than 100mL / min, maintaining a carbon dioxide gas or nitrogen atmosphere, starting the calcination furnace, and calcining the sludge semi-coke; after cooling, obtaining phosphorus-rich biochar.

[0034] In order to better illustrate the technical means and product effects of the present invention, the preferred embodiments of the present invention are described below.

[0035] Example 1

[0036] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0037] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 100-mesh sieve;

[0038] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting pyrolysis conditions, pyrolysis target temperature of 300° C., heating rate of 10° C. / min, pyrolysis time of 60 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, gas flow rate greater than 120 mL / min, after replacement, starting the microwave oven to pyrolyze the sludge, cooling after pyrolysis, and obtaining sludge semi-coke;

[0039] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 700°C, the heating rate to 10°C / min, the calcination time to 60min, introducing nitrogen gas into the furnace to replace the air before calcination, the gas flow rate to be greater than 100mL / min, maintaining the nitrogen gas atmosphere and starting the calcination furnace to calcine the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0040] Example 2

[0041] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0042] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 150-mesh sieve;

[0043] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting the pyrolysis target temperature to 500° C., the heating rate to 10° C. / min, the pyrolysis time to 90 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, the gas flow rate to be greater than 120 mL / min, after the replacement is completed, starting the microwave oven to pyrolyze the sludge, and cooling after the pyrolysis is completed to obtain sludge semi-coke;

[0044] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 800°C, the heating rate to 5°C / min, the calcination time to 60min, introducing nitrogen into the furnace to replace the air before calcination, the gas flow rate to be greater than 100mL / min, maintaining the nitrogen gas atmosphere and starting the calcination furnace to calcine the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0045] Example 3

[0046] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0047] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 150-mesh sieve;

[0048] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting the pyrolysis target temperature to 500° C., the heating rate to 15° C. / min, the pyrolysis time to 30 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, the gas flow rate to be greater than 120 mL / min, after the replacement is completed, starting the microwave oven to pyrolyze the sludge, and cooling after the pyrolysis is completed to obtain sludge semi-coke;

[0049] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 900°C, the heating rate to 10°C / min, the calcination time to 30min, introducing nitrogen into the furnace to replace the air before calcination, the gas flow rate to be greater than 100mL / min, maintaining the nitrogen gas atmosphere and starting the calcination furnace to calcine the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0050] Example 4

[0051] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0052] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 200-mesh sieve;

[0053] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting the pyrolysis target temperature to 700° C., the heating rate to 10° C. / min, the pyrolysis time to 60 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, the gas flow rate to be greater than 100 mL / min, after the replacement is completed, starting the microwave oven to pyrolyze the sludge, and cooling after the pyrolysis is completed to obtain sludge semi-coke;

[0054] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 900°C, the heating rate to 10°C / min, the calcination time to 60min, introducing nitrogen gas into the furnace to replace the air before calcination, the gas flow rate to be greater than 100mL / min, maintaining the nitrogen gas atmosphere and starting the calcination furnace to calcine the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0055] Example 5

[0056] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0057] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 100-mesh sieve;

[0058] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting the pyrolysis target temperature to 300° C., the heating rate to 5° C. / min, the pyrolysis time to 90 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, the gas flow rate to be greater than 120 mL / min, after the replacement is completed, starting the microwave oven to pyrolyze the sludge, and cooling after the pyrolysis is completed to obtain sludge semi-coke;

[0059] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 900°C, the heating rate to 5°C / min, the calcination time to 90min, introducing carbon dioxide gas into the furnace to replace the air before calcination, with a gas flow rate greater than 150mL / min, maintaining the carbon dioxide gas atmosphere and starting the calcination furnace to calcine the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0060] Example 6

[0061] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0062] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 200-mesh sieve;

[0063] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting the pyrolysis target temperature to 500° C., the heating rate to 10° C. / min, the pyrolysis time to 60 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, the gas flow rate to be greater than 120 mL / min, after the replacement is completed, starting the microwave oven to pyrolyze the sludge, and cooling after the pyrolysis is completed to obtain sludge semi-coke;

[0064] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 800°C, the heating rate to 10°C / min, the calcination time to 60min, introducing carbon dioxide gas into the furnace to replace the air before calcination, with a gas flow rate greater than 100mL / min, maintaining the carbon dioxide gas atmosphere and starting the calcination furnace to calcine the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0065] Example 7

[0066] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0067] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 200-mesh sieve;

[0068] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting the pyrolysis target temperature to 500° C., the heating rate to 10° C. / min, the pyrolysis time to 60 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, the gas flow rate to be greater than 120 mL / min, after the replacement is completed, starting the microwave oven to pyrolyze the sludge, and cooling after the pyrolysis is completed to obtain sludge semi-coke;

[0069] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 900°C, the heating rate to 10°C / min, the calcination time to 60min, introducing carbon dioxide gas into the furnace to replace the air before calcination, the gas flow rate to be greater than 100mL / min, maintaining the carbon dioxide gas atmosphere, starting the calcination furnace, and calcining the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0070] Example 8

[0071] A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, comprising the following steps:

[0072] (1) Heat and dry the excess sludge from the municipal sewage treatment plant, crush and grind the dried sludge, and pass it through a 200-mesh sieve;

[0073] (2) putting the municipal sludge powder of step (1) into a microwave pyrolysis furnace, setting the pyrolysis target temperature to 700° C., the heating rate to 10° C. / min, the pyrolysis time to 60 min, introducing high-purity nitrogen to dilute the furnace air atmosphere for replacement, the gas flow rate to be greater than 100 mL / min, after the replacement is completed, starting the microwave oven to pyrolyze the sludge, and cooling after the pyrolysis is completed to obtain sludge semi-coke;

[0074] (3) placing the sludge semi-coke obtained in step (2) in a tubular furnace, setting the calcination target temperature to 900°C, the heating rate to 10°C / min, the calcination time to 60min, introducing carbon dioxide gas into the furnace to replace the air before calcination, the gas flow rate to be greater than 100mL / min, maintaining the carbon dioxide gas atmosphere, starting the calcination furnace, and calcining the sludge semi-coke; and obtaining phosphorus-rich biochar after cooling.

[0075] Performance Analysis

[0076] 1. Analysis of phosphorus yield and fixation rate in phosphorus-rich biochar

[0077] (1) Investigate the effect of calcination under nitrogen atmosphere on the yield and fixation rate of phosphorus

[0078] The yield and fixation rate of phosphorus in the phosphorus-rich biochar prepared in Examples 1-4 were detected by the SMT method. 20 ml of 3.5 M hydrochloric acid was added to 0.2 g of the sample, and the sample was shaken and extracted for 16 h. The supernatant was extracted using a 0.45 μm filter head and the phosphorus content in the sample was analyzed by molybdenum blue test. The sample yield calculation formula is as follows:

[0079] R yield =M2 / M1×100% (1)

[0080] Where M1 and M2 are the masses of biochar before and after heat treatment, respectively.

[0081] The phosphorus fixation rate of the sample was calculated as follows:

[0082] ψ P =C2 / C1×R yield ×100% (2)

[0083] Where C1 and C2 are the phosphorus concentrations in the sample before and after heat treatment, respectively.

[0084] Table 1 The yield and fixation rate of phosphorus in phosphorus-rich biochar samples of Examples 1-4

[0085]

[0086]

[0087] According to Table 1, when calcined in a nitrogen atmosphere, the phosphorus element in the phosphorus-rich biochar can obtain a higher fixation rate, reaching more than 83%.

[0088] (2) Investigate the effect of calcination under carbon dioxide atmosphere on the yield and fixation rate of phosphorus

[0089] The SMT method was used to detect the yield and fixation rate of phosphorus in the phosphorus-rich biochar prepared in Examples 5-8.

[0090] Table 2 Yield and fixation rate of phosphorus in phosphorus-rich biochar samples of Examples 5-8

[0091]

[0092] According to Table 2, when calcined in a carbon dioxide atmosphere, the phosphorus element in the phosphorus-rich biochar can obtain a higher fixation rate, reaching more than 83%.

[0093] 2. Analysis of phosphorus bioavailability of phosphorus-rich biochar

[0094] The bioavailable content and bioavailable ratio of phosphorus in the phosphorus-rich biochar prepared in Examples 1-8 were analyzed by 2% formic acid extraction.

[0095] The samples were extracted with 2% formic acid at a solid-liquid ratio of 1:100 and allowed to stand for 16 hours. The supernatant was collected and the phosphorus content of the samples was determined using the molybdenum blue colorimetric method. The bioavailable ratio was calculated using the following formula:

[0096] θ P =C2 / C1×100%

[0097] Where C1 and C2 are the total phosphorus concentration of the sample and the concentration of phosphorus in the corresponding extract, respectively.

[0098] Table 3 Bioavailable content and bioavailable ratio of phosphorus in phosphorus-rich biochar samples of Examples 1-8

[0099] Example <![CDATA[C1(mg / g)]]> <![CDATA[C2(mg / g)]]> Bioavailable ratio (%) Example 1 36.25 4.55 12.55 Example 2 39.99 6 15.00 Example 3 34.56 12.58 36.40 Example 4 37.75 16.78 44.45 Example 5 48.77 24.45 50.13 Example 6 47.37 9.52 20.10 Example 7 52.24 27.48 52.60 Example 8 43.95 22.85 51.99

[0100] From Table 1 and Table 2, it can be seen that the phosphorus-rich biochar prepared by the present invention can obtain a higher phosphorus yield and fixation rate regardless of calcination in a nitrogen atmosphere or a carbon dioxide atmosphere. The optimal heat treatment conditions are a microwave pyrolysis temperature of 500°C and a carbon dioxide calcination temperature of 900°C, and the phosphorus fixation rate reaches 95.46%.

[0101] According to Table 3, compared with nitrogen atmosphere, the phosphorus in the phosphorus-rich biochar prepared by calcination under carbon dioxide atmosphere has higher bioavailability, the optimal heat treatment conditions are microwave pyrolysis temperature of 500°C and carbon dioxide calcination at 900°C, and the bioavailability ratio reaches 52.60%, which fully demonstrates that the phosphorus in the phosphorus-rich biochar prepared by the present invention is efficiently utilized.

[0102] From the above analysis, it can be seen that the method for preparing phosphorus-rich biochar by two-step heat treatment of sludge pyrolysis-calcination in the present invention can obtain not only a higher phosphorus yield and fixation rate, but also a higher bioavailable content and bioavailable ratio by calcining under a carbon dioxide atmosphere, indicating that the present invention can achieve the preset goal of harmless and resource-based treatment and disposal of municipal sludge.

Claims

1. A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination, characterized in that: The following steps are involved: (1) heating and drying municipal sludge, grinding, crushing, and sieving to obtain sludge particles; (2) placing the sludge particles in step (1) in a microwave pyrolysis device for pyrolysis, and obtaining sludge semi-coke after cooling; (3) The sludge semi-coke obtained in step (2) is heated and calcined in a nitrogen or carbon dioxide atmosphere, and then cooled to obtain phosphorus-rich biochar.

2. A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination as claimed in claim 1, characterized in that: The heated and dried municipal sludge is ground and crushed, and then sieved through 100-200 mesh.

3. The method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination as claimed in claim 1, characterized in that: Step (2) Before pyrolysis, high-purity inert gas is introduced to replace the air atmosphere in the microwave pyrolysis device, and the gas flow rate is greater than 100 mL / min; the pyrolysis temperature is 300° C.-700° C., the heating rate is 5° C. / min-15° C. / min, and the pyrolysis time is 30 min-90 min.

4. The method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination as claimed in claim 1, characterized in that: Step (3) is calcined in a tubular furnace at a temperature of 700°C-900°C, a heating rate of 5°C / min-15°C / min, a calcination time of 30min-90min, and a gas flow rate greater than 100mL / min.

5. The method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination as claimed in claim 1, characterized in that: The phosphorus fixation rate of the prepared phosphorus-rich biochar is greater than 83%.

6. A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination as claimed in claim 1, characterized in that: In step (3), heating and calcination are carried out under a carbon dioxide atmosphere, and the bioavailability ratio of the prepared phosphorus-rich biochar is greater than 20%.

7. A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination as claimed in claim 1, characterized in that: The pyrolysis temperature in step (2) is 500°C, the inert atmosphere in step (3) is a carbon dioxide atmosphere, and the calcination temperature is 900°C.

8. A method for preparing phosphorus-rich biochar by two-step thermal treatment of sludge pyrolysis-calcination as claimed in claim 7, characterized in that: The phosphorus fixation rate of the prepared phosphorus-rich biochar reached 95.46%, and the bioavailability ratio reached 52.60%.

9. A phosphorus-rich biochar produced by a two-step thermal treatment of sludge pyrolysis and calcination, characterized in that: The method for preparing phosphorus-rich biochar by a two-step thermal treatment of sludge pyrolysis-calcination as described in any one of claims 1 to 8 is used for preparation.

10. Use of the phosphorus-rich biochar obtained by the two-step thermal treatment of sludge pyrolysis and calcination as claimed in claim 9 in phosphate rock or phosphate fertilizer.

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