Preparation method of sludge-based biochar for directionally enriching iron phosphate through synergistic modification of ferrous sulfate and sodium persulfate

Abstract

The invention provides a preparation method of sludge-based biochar for directionally enriching iron phosphate through synergistic modification of ferrous sulfate and sodium persulfate, and belongs to the technical field of harmless treatment and resource utilization of municipal sludge and environmental functional materials. According to the method disclosed by the invention, active phosphorus in the sludge is converted into a stable iron phosphorus compound in advance through oxidation-complexation dual effects of the ferrous sulfate heptahydrate and the sodium persulfate; the phosphorus-rich sludge is converted into more stable phosphorus-rich biochar through two-step heat treatment, the whole process of sludge pyrolysis is cooperatively regulated and controlled through Fe < 2 + > / S2O8 < 2->, integration of phosphorus release, phosphorus fixation and heavy metal passivation is achieved, the problem that in the prior art, the phosphorus retention rate is low is solved, phosphorus resources in the phosphorus-rich sludge are extremely retained and stabilized, and the phosphorus-rich sludge is obtained. The fixed rate of phosphorus in the modified sludge-based biochar with directionally enriched iron phosphate prepared by the method is greater than 99%.

Description

Technical Field

[0001] This invention belongs to the technical field of harmless treatment, resource utilization and environmental functional materials of municipal sludge, and specifically relates to a method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate by ferrous sulfate-sodium persulfate. Background Technology

[0002] In recent years, the booming development of municipal wastewater treatment and the chemical industry has led to a continuous increase in the annual production of sludge. Municipal sludge, as a byproduct of wastewater treatment, contains abundant nutrients such as nitrogen, phosphorus, and potassium, as well as rich carbon resources. Phosphorus resources, due to their non-renewable nature, are considered a "second phosphate rock." However, the coexistence of heavy metals (such as Cr and Pb) and organic pollutants in sludge, coupled with phosphorus loss and secondary pollution risks caused by traditional disposal methods such as incineration and landfill, severely restricts its resource utilization. Therefore, achieving the coordinated treatment of sludge through "harmlessness, reduction, and resource recovery" has become a key issue to be addressed in the field of environmental engineering.

[0003] Currently, sludge-based biochar pyrolysis technology, with its sludge volume reduction rate exceeding 60% and phosphorus fixation capacity, has become the mainstream resource utilization pathway for sludge. However, during conventional pyrolysis, organic phosphorus is easily converted into PH3 and PO4. x After gaseous phosphides escape, the total phosphorus retention rate remains only 60%-70%. Existing modification technologies have many limitations. Although iron salt modification can improve phosphorus fixation, it is difficult to completely suppress phosphorus volatilization during high-temperature pyrolysis. Sodium persulfate, as an advanced oxidant, can effectively degrade organic matter and release phosphorus, but it lacks a directional phosphorus fixation mechanism. In addition, although the two-step heat treatment method (pyrolysis + calcination) can optimize the pore structure, it fails to achieve a synergistic effect of phosphorus retention and functionalization. For example, patent CN111646674A recovers phosphorus through strong acid leaching, which is a cumbersome process and does not utilize the organic matter in the sludge. Patent CN110255845B requires multiple leaching and activation steps, and the phosphorus recovery rate is only 80%.

[0004] Therefore, based on the two-step thermal treatment, there is an urgent need to develop a high-efficiency, low-consumption modification technology to achieve efficient conversion of organophosphorus compounds and simultaneous stabilization of heavy metals in sludge from the source, providing an innovative solution for sludge resource utilization that combines high phosphorus retention rate and environmental safety. This will promote the industrial application of sludge resource utilization technology. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes an innovative synergistic modification technology. Through the dual oxidative-complexation action of ferrous sulfate heptahydrate and sodium persulfate, active phosphorus in the sludge is pre-converted into stable iron-phosphorus compounds; then, a two-step heat treatment process further transforms it into more stable phosphorus-rich biochar. This is achieved through the application of Fe... 2+ / S2O8 2-By synergistically regulating the entire sludge pyrolysis process, the integrated "phosphorus release-phosphorus fixation-heavy metal passivation" is achieved, solving the problem of low phosphorus retention rate in existing technologies and realizing the ultimate retention and stabilization of phosphorus resources in phosphorus-rich sludge.

[0006] This invention provides a method for preparing sludge-based biochar with synergistic modification and directional enrichment of ferric phosphate by ferrous sulfate-sodium persulfate, comprising the following steps:

[0007] (1) Heat and dry municipal sludge; grind, crush and sieve to obtain homogeneous sludge particles;

[0008] (2) The homogeneous sludge particles are added to deionized water to prepare a sludge slurry;

[0009] (3) After the sludge slurry is evenly dispersed by ultrasonic treatment, it is placed in a magnetic stirrer, the pH is adjusted, and ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium persulfate (Na2S2O8) are added for synergistic modification to obtain modified sludge;

[0010] (4) The modified sludge slurry is placed in a centrifuge for centrifugal dehydration, and then washed and dried;

[0011] (5) The dried modified sludge was pyrolyzed using a microwave pyrolysis device, and the sludge semi-coke was obtained after cooling.

[0012] (6) The sludge semi-coke is heated and calcined under a nitrogen atmosphere, and after cooling, sludge-based biochar with modified directional enrichment of iron phosphate is obtained.

[0013] Furthermore, in step (1), the municipal sludge is heated and dried in an oven at 65℃-105℃, and after being ground and crushed, it is passed through a 100-200 mesh sieve.

[0014] Furthermore, in step (2), the solid-liquid ratio of sludge particles to deionized water is 1:10, that is, 100 mL of deionized water is added for every 10 g of sludge particles.

[0015] Further, the frequency of the ultrasonic treatment in step (3) is 20kHz-40kHz, and the time is 20min-30min.

[0016] Furthermore, in step (3), the pH is 3-4, and the synergistic modification reaction time is 90-120 min; the Fe added to the slurry 2+ and S2O8 2- The molar ratio is 2:1, and the amount of Fe added is adjusted. 2+ The ratio of sodium persulfate to phosphorus in the sludge is (1:1)-(2:1), and the corresponding mass of sodium persulfate is weighed according to the 5% solution concentration ratio to ensure that the organic phosphorus release rate is ≥95% and Fe 2+Complete oxidation is achieved at a stirring temperature of 25℃-30℃.

[0017] Furthermore, in step (4), the centrifuge speed is 9000r / min-9500r / min, the time is 5min-10min, the drying temperature is 65℃-85℃, and the time is 12h-24h, so that the modified sludge is dried to constant weight.

[0018] Furthermore, in step (5), before pyrolysis, a high-purity inert gas is introduced to replace the air atmosphere in the microwave pyrolysis device, with a gas flow rate greater than 100 mL / min; the pyrolysis temperature is 500℃, the heating rate is 10℃ / min, and the pyrolysis time is 60 min, finally obtaining sludge semi-coke synergistically modified by ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium persulfate (Na2S2O8).

[0019] Furthermore, in step (6), a tube furnace is used for calcination at a temperature of 800℃-850℃, a heating rate of 10℃ / min, a calcination time of 60min-90min, and a gas flow rate greater than 100mL / min. After natural cooling to room temperature, the material is pulverized and passed through a 200-mesh sieve.

[0020] Furthermore, the phosphorus fixation rate in the modified sludge-based biochar with directional enrichment of ferric phosphate prepared by this invention is >99%.

[0021] This invention also provides an application of sludge-based biochar that is synergistically modified with ferrous sulfate and sodium persulfate to directionally enrich ferric phosphate, specifically, the application of sludge-based biochar that is synergistically modified with ferrous sulfate and sodium persulfate to directionally enrich ferric phosphate in the lithium iron phosphate energy battery industry.

[0022] Compared with the prior art, the advantages and effects of the present invention are as follows:

[0023] 1. This invention utilizes the synergistic effect of sodium persulfate and ferrous sulfate. Sodium persulfate not only breaks down sludge cells and releases organic phosphorus, but also... 2+ Oxidized to Fe 3+ Ferrous sulfate provides a complexing iron source, simultaneously completing the three-step reaction of "phosphorus release-oxidation-complexation", with a phosphorus retention rate of ≥99%, solving the problem of phosphorus loss in traditional technology. XRD characterization shows that the synergistic modification not only ensures the phosphorus fixation rate, but also generates highly crystalline iron phosphate, making the product a precursor for the synthesis of lithium iron phosphate battery material.

[0024] 2. Sulfate radicals can break the bonds between heavy metals and organic matter (such as cadmium-humic acid complexes), releasing the heavy metals, which are then adsorbed or co-precipitated by iron oxides. During pyrolysis-calcination, the remaining heavy metals are further solidified in the biochar matrix, forming stable oxides, reducing the leaching concentration of heavy metals by 60%-90%.

[0025] 3. This process has significant advantages for industrial application. The process is simple and efficient, the modifiers used are inexpensive, and it does not require special equipment such as high-temperature and high-pressure equipment, thus possessing good adaptability for large-scale production. The prepared phosphorus-rich biochar has a well-developed pore structure with a specific surface area of ​​40 m². 2 / g-130m 2 / g, of which mesoporous components account for over 70%. SEM-EDS characterization shows that iron is uniformly distributed on the carbon framework, forming a high-density active site. This technology not only achieves the harmless and resource-based treatment of phosphorus-rich sludge, but also prepares precursors for new energy materials at low cost, demonstrating significant economic and environmental benefits.

[0026] It should be noted that the above general description and the following detailed description are merely exemplary and explanatory, and do not limit the disclosure of this invention. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of a method for preparing sludge-based biochar with ferrous sulfate-sodium persulfate synergistic modification and directional enrichment of ferric phosphate, according to an embodiment of the present invention.

[0028] Figure 2 The image shows the elemental mapping of the phosphorus-rich biochar prepared in Example 2 using scanning transmission electron microscopy combined with energy dispersive spectroscopy.

[0029] Figure 3 The X-ray diffraction (XRD) patterns of the phosphorus-rich biochar prepared in Examples 1-4 of this invention are shown. Detailed Implementation

[0030] The present invention will now be described in detail with reference to embodiments. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] To address the problems of large municipal sewage sludge production, inefficient recycling methods, and low value of phosphorus resource utilization products, this invention proposes a method for preparing ferric phosphate-based phosphorus-rich biochar based on synergistic modification with ferrous sulfate and sodium persulfate and a two-step heat treatment. Figure 1 The diagram shown is a process flow chart of the preparation method according to an embodiment of the present invention.

[0032] This invention provides a method for preparing sludge-based biochar with synergistic modification and directional enrichment of ferric phosphate by ferrous sulfate-sodium persulfate, comprising the following steps:

[0033] (1) The municipal sludge is heated and dried in an oven at 65℃-105℃, then ground and crushed, and passed through a 100-200 mesh sieve; grinding, crushing and sieving are carried out to obtain homogeneous sludge particles;

[0034] (2) The homogeneous sludge particles are added to deionized water to prepare sludge slurry. The solid-liquid ratio of sludge particles to deionized water is 1:10, that is, 100 mL of deionized water is added for every 10 g of sludge particles.

[0035] (3) After the sludge slurry is evenly dispersed by ultrasonic treatment, it is placed in a magnetic stirrer. The ultrasonic treatment frequency is 20kHz-40kHz, the time is 20min-30min, the pH is adjusted to 3-4, and ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium persulfate (Na2S2O8) are added for synergistic modification to obtain modified sludge; the synergistic modification reaction time is 90min-120min; the Fe added to the slurry 2+ and S2O8 2- The molar ratio is 2:1, and the amount of Fe added is adjusted. 2+ The ratio of sodium persulfate to phosphorus in the sludge is (1:1)-(2:1), and the corresponding mass of sodium persulfate is weighed according to the 5% solution concentration ratio to ensure that the organic phosphorus release rate is ≥95% and Fe 2+ Complete oxidation, with a stirring temperature of 25℃-30℃;

[0036] (4) The modified sludge slurry is placed in a centrifuge for centrifugation and dehydration, and then washed and dried. The centrifuge speed is 9000r / min-9500r / min, the time is 5min-10min, the drying temperature is 65℃-85℃, and the time is 12h-24h, so that the modified sludge is dried to constant weight.

[0037] (5) The dried modified sludge was pyrolyzed using a microwave pyrolysis device. Before pyrolysis, high-purity inert gas was introduced to replace the air atmosphere in the microwave pyrolysis device, and the gas flow rate was greater than 100 mL / min. The pyrolysis temperature was 500℃, the heating rate was 10℃ / min, and the pyrolysis time was 60 min. Finally, sludge semi-coke was obtained by synergistic modification of ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium persulfate (Na2S2O8).

[0038] (6) The sludge semi-coke is heated and calcined under a nitrogen atmosphere using a tubular furnace at a temperature of 800℃-850℃, a heating rate of 10℃ / min, a calcination time of 60min-90min, and a gas flow rate greater than 100mL / min. After natural cooling to room temperature, it is pulverized through a 200-mesh sieve and cooled to obtain modified sludge-based biochar with directional enrichment of iron phosphate.

[0039] The modified sludge-based biochar with directional enrichment of ferric phosphate prepared by this invention has a phosphorus fixation rate of >99%.

[0040] Table 1 shows the composition of the municipal sludge used in the embodiments of the present invention:

[0041]

[0042] a GB / T 17664-1999.FC(%)=100%-Volatile matter(%)-Ash(%)-Moisture(%).

[0043] b Measured using the EA 3000 element analyzer.

[0044] c Calculated by difference(O(%)=100%-C(%)-H(%)-S(%)-N(%)-Ash(%)).

[0045] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosed invention. This application is intended to cover any modifications, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein.

[0046] To better illustrate the technical means and product effects of the present invention, the preferred embodiments of the present invention will be described below.

[0047] Example 1

[0048] A method for preparing high-energy-efficiency sludge-based phosphorus-rich biochar materials through synergistic modification with ferrous sulfate and sodium persulfate includes the following steps:

[0049] (1) Heating and drying the residual sludge from the municipal sewage treatment plant, heating and drying the sludge in an oven at 105℃, then crushing and grinding it, and passing it through a 200-mesh sieve to obtain sludge particles;

[0050] (2) Add 30g of sludge particles to 300mL of deionized water at a solid-liquid ratio of 1:10 to prepare sludge slurry; and stir for 30min under ultrasonic conditions until uniform. The ultrasonic treatment frequency is 25kHz.

[0051] (3) After adjusting the pH of the solution obtained in (2) to between 3 and 4, add 18.5 mg / L MFeSO4·7H2O (corresponding to Fe) at an iron-to-phosphorus ratio of 1:1.2+ Synergistic modification was achieved by reacting 1.034 g of Na2S2O8 solution with 5% Na2S2O8 solution for 90 min to obtain modified sludge.

[0052] (4) The modified sludge is placed in a centrifuge and centrifuged at 9000 r / min for 10 min to dehydrate. After dehydration, it is washed, dried at 65℃ for 24 h, and then passed through a 200-mesh sieve.

[0053] (5) The dried modified sludge is put into a microwave pyrolysis furnace, the pyrolysis conditions are set, and high-purity nitrogen is introduced to replace the air atmosphere in the furnace. After the replacement is completed, the microwave oven is started to pyrolyze the sludge. The pyrolysis temperature is 500℃, the heating rate is 10℃ / min, and the pyrolysis time is 60min. Nitrogen is continuously introduced during the pyrolysis process, and the gas flow rate is greater than 100mL / min. After the pyrolysis is completed, the sludge is cooled to obtain sludge semi-coke.

[0054] (6) The sludge semi-coke is placed in a tubular furnace for calcination. Before calcination, nitrogen is introduced to replace the air in the furnace. Calcination is carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 800℃, a heating rate of 10℃ / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar is obtained.

[0055] Example 2

[0056] The difference between this embodiment and embodiment 1 is that: in step (3), the iron-phosphorus ratio is set to 1.2:1, and 5% Na2S2O8 solution (54 mL) is added. Other preparation methods are the same as in embodiment 1.

[0057] A method for preparing sludge-based biochar with ferrous sulfate-sodium persulfate synergistic modification and directional enrichment of ferric phosphate includes the following steps:

[0058] (1) First, the residual sludge from the municipal sewage treatment plant is heated and dried. After the sludge is heated and dried in an oven at 105°C, it is crushed and ground and passed through a 200-mesh sieve to obtain sludge particles.

[0059] (2) Add 30g of sludge particles to 300mL of deionized water at a solid-liquid ratio of 1:10, and stir evenly under ultrasonic conditions. The ultrasonic treatment frequency is 25kHz and the time is 30min to prepare sludge slurry.

[0060] (3) After adjusting the pH of the solution obtained in (2) to between 3 and 4, add 23.7 mg / L MFeSO4·7H2O (corresponding to Fe) at an iron-to-phosphorus ratio of 1.2:1. 2+ 1.324 g) and 5% Na2S2O8 solution were reacted for 90 min to carry out synergistic modification and obtain modified sludge;

[0061] (4) The modified sludge is placed in a centrifuge and centrifuged at 9000 r / min for 10 min to dehydrate. After dehydration, it is washed, dried at 65℃ for 24 h, and then passed through a 200 mesh sieve.

[0062] (5) The dried modified sludge is put into a microwave pyrolysis furnace, the pyrolysis conditions are set, and high-purity nitrogen is introduced to replace the air atmosphere in the furnace. After the replacement is completed, the microwave oven is started to pyrolyze the sludge. The pyrolysis temperature is 500℃, the heating rate is 10℃ / min, and the pyrolysis time is 60min. Nitrogen is continuously introduced during the pyrolysis process, and the gas flow rate is greater than 100mL / min. After the pyrolysis is completed, the sludge is cooled to obtain sludge semi-coke.

[0063] (6) The sludge semi-coke is placed in a tubular furnace for calcination. Before calcination, nitrogen is introduced to replace the air in the furnace. Calcination is carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 800℃, a heating rate of 10℃ / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar is obtained. Figure 2 The image shows the elemental mapping of the phosphorus-rich biochar prepared in this embodiment using scanning transmission electron microscopy combined with energy dispersive spectroscopy.

[0064] Example 3

[0065] The difference between this embodiment and embodiment 1 is that: in step (3), the iron-phosphorus ratio is set to 1.5:1, and 71 mL of 5% Na2S2O8 solution is added. Other preparation methods are the same as in embodiment 1.

[0066] A method for preparing sludge-based biochar with ferrous sulfate-sodium persulfate synergistic modification and directional enrichment of ferric phosphate includes the following steps:

[0067] (1) First, the residual sludge from the municipal sewage treatment plant is heated and dried. After the sludge is heated and dried in an oven at 105°C, it is crushed and ground and passed through a 200-mesh sieve to obtain sludge particles.

[0068] (2) Add 30g of sludge particles to 300mL of deionized water at a solid-liquid ratio of 1:10, and stir evenly under ultrasonic conditions. The ultrasonic treatment frequency is 25kHz and the time is 30min to prepare sludge slurry.

[0069] (3) After adjusting the pH of the solution obtained in (2) to between 3 and 4, add 31.5 mg / L MFeSO4·7H2O (corresponding to Fe) at an iron-to-phosphorus ratio of 1.5:1. 2+ Synergistic modification was achieved by reacting 1.759 g of sludge with 71 mL of 5% Na2S2O8 solution for 90 min to obtain modified sludge.

[0070] (4) The modified sludge is placed in a centrifuge and centrifuged at 9000 r / min for 10 min to dehydrate. After dehydration, it is washed, dried at 65℃ for 24 h, and then passed through a 200 mesh sieve.

[0071] (5) The dried modified sludge is put into a microwave pyrolysis furnace, the pyrolysis conditions are set, and high-purity nitrogen is introduced to replace the air atmosphere in the furnace. After the replacement is completed, the microwave oven is started to pyrolyze the sludge. The pyrolysis temperature is 500℃, the heating rate is 10℃ / min, and the pyrolysis time is 60min. Nitrogen is continuously introduced during the pyrolysis process, and the gas flow rate is greater than 100mL / min. After the pyrolysis is completed, the sludge is cooled to obtain sludge semi-coke.

[0072] (6) The modified sludge semi-coke is placed in a tubular furnace for calcination. Before calcination, nitrogen is introduced to replace the air in the furnace. Calcination is carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 800℃, a heating rate of 10℃ / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar is obtained.

[0073] Example 4

[0074] The difference between this embodiment and embodiment 1 is that: in step (3), the iron-phosphorus ratio is set to 2:1, and 105 mL of 5% Na2S2O8 solution is added. Other preparation methods are the same as in embodiment 1.

[0075] A method for preparing sludge-based biochar with ferrous sulfate-sodium persulfate synergistic modification and directional enrichment of ferric phosphate includes the following steps:

[0076] (1) First, the residual sludge from the municipal sewage treatment plant is heated and dried. After the sludge is heated and dried in an oven at 105°C, it is crushed and ground and passed through a 200-mesh sieve to obtain sludge particles.

[0077] (2) Add 30g of sludge particles to 300mL of deionized water at a solid-liquid ratio of 1:10, and stir evenly under ultrasonic conditions. The ultrasonic treatment frequency is 25kHz and the time is 30min to prepare sludge slurry.

[0078] (3) After adjusting the pH of the solution obtained in (2) to between 3 and 4, add 44.5 mg / L of FeSO4·7H2O (corresponding to Fe) at an iron-to-phosphorus ratio of 2:1. 2+ 2.495g) and 5% Na2S2O8 solution were reacted for 90 min to carry out synergistic modification and obtain modified sludge;

[0079] (4) The modified sludge is placed in a centrifuge and centrifuged at 9000 r / min for 10 min to dehydrate. After dehydration, it is washed, dried at 65℃ for 24 h, and then passed through a 200 mesh sieve.

[0080] (5) The dried modified sludge is put into a microwave pyrolysis furnace, the pyrolysis conditions are set, and high-purity nitrogen is introduced to replace the air atmosphere in the furnace. After the replacement is completed, the microwave oven is started to pyrolyze the sludge. The pyrolysis temperature is 500℃, the heating rate is 10℃ / min, and the pyrolysis time is 60min. Nitrogen is continuously introduced during the pyrolysis process, and the gas flow rate is greater than 100mL / min. After the pyrolysis is completed, the sludge is cooled to obtain sludge semi-coke.

[0081] (6) The sludge semi-coke is placed in a tubular furnace for calcination. Before calcination, nitrogen is introduced to replace the air in the furnace. Calcination is carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 800℃, a heating rate of 10℃ / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar is obtained. Figure 3 The X-ray diffraction (XRD) patterns of the phosphorus-rich biochar prepared in Examples 1-4 of this invention are shown.

[0082] Comparative Example 1

[0083] The difference between this comparative example and Example 1 is that it does not include the ferrous sulfate-sodium persulfate synergistic modification treatment in step (3).

[0084] A method for preparing sludge-based biochar with ferrous sulfate-sodium persulfate synergistic modification and directional enrichment of ferric phosphate includes the following steps:

[0085] (1) First, the residual sludge from the municipal sewage treatment plant is heated and dried. After the sludge is heated and dried in an oven at 105°C, it is crushed and ground and passed through a 200-mesh sieve to obtain sludge particles.

[0086] (2) The sludge particles are put into a microwave pyrolysis furnace, the pyrolysis conditions are set, and high-purity nitrogen is introduced to replace the air atmosphere in the furnace. After the replacement is completed, the microwave oven is started to pyrolyze the sludge. The pyrolysis temperature is 500℃, the heating rate is 10℃ / min, and the pyrolysis time is 90min. Nitrogen is continuously introduced during the pyrolysis process, and the gas flow rate is greater than 100mL / min. After the pyrolysis is completed, the sludge semi-coke is obtained.

[0087] (3) The sludge semi-coke is placed in a tubular furnace for calcination. Before calcination, nitrogen is introduced to replace the air in the furnace. Calcination is carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 800℃, a heating rate of 10℃ / min, and a calcination time of 60 min. After cooling, unmodified biochar is obtained.

[0088] Performance Analysis

[0089] 1. Heavy metal content analysis of phosphorus-rich biochar

[0090] Inductively coupled plasma atomic emission spectrometry (ICP) was used to detect the content of heavy metal ions in the biochar samples prepared in each example and comparative example. The specific method is as follows: 0.1 g of sample was weighed and placed in the polytetrafluoroethylene digestion vessel of a microwave digester. 8 mL of 65% HNO3, 1 mL of 30% H2O2, and 2 mL of 30% HF were added to the sample. The microwave digester was then used, and the temperature was controlled according to the method shown in Table 2.

[0091] Table 2 is a parameter table for microwave digestion temperature setting method;

[0092]

[0093] After digestion and cooling, the samples were removed and filtered using a 0.45 μm filter. After volume adjustment, the heavy metal concentration in the samples was determined by ICP. Table 3 shows the heavy metal content (mg / kg) of unmodified sludge biochar and ferrous sulfate-sodium persulfate synergistic modified phosphorus-rich sludge biochar.

[0094] Table 3 shows the heavy metal content (mg / kg) in the biochar materials prepared in Examples 1-4 and the comparative examples.

[0095]

[0096] Table 3 shows that the heavy metal content of the sludge biochar modified by ferrous sulfate-sodium persulfate was reduced compared with that of the unmodified sludge biochar. Compared with the comparative example, Example 2 showed the best removal effect on heavy metals, with a significant reduction in the total amount of heavy metals, indicating that synergistic modification is effective in reducing heavy metals.

[0097] 2. Analysis of phosphorus fixation rate in phosphorus-rich biochar

[0098] The phosphorus fixation law in the biochar samples prepared in each example and comparative example was detected using a standardized phosphorus speciation determination method (SMT) developed under the framework of the European Committee for Standardization and Testing. The specific method is as follows: 20 mL of 3.5 M hydrochloric acid was added to 0.2 g of sample, and the sample was extracted by constant temperature shaking for 16 h. The supernatant was then extracted using a 0.45 μm filter and the phosphorus content in the sample was analyzed by molybdenum blue assay. The results are shown in Table 4.

[0099] Phosphorus yield R in the sample yield The calculation formula is as follows:

[0100] R yield =M2 / M1×100%(1)

[0101] In the formula, M1 and M2 are the masses of biochar before and after heat treatment, respectively.

[0102] The formula for calculating the phosphorus fixation rate ΨP in the sample is as follows:

[0103] ΨP=C2 / C1×R yield ×100% (2)

[0104] In the formula, C1 and C2 are the phosphorus concentrations in the samples before and after heat treatment, respectively.

[0105] Table 4 shows the phosphorus yield and fixation rate in the biochar materials prepared in each example and comparative example.

[0106]

[0107] According to Table 4, compared with the comparative examples, it can be seen that the phosphorus concentration and fixation rate of phosphorus-rich biochar prepared by the two-step pyrolysis-calcination method after synergistic modification with different iron-phosphorus ratios in Examples 1-4 are improved, with the fixation rate ranging from 94.26% to 99.99%. As can be seen from the table, the optimal iron-phosphorus ratio is 1.2:1, at which point the phosphorus fixation rate reaches 99.99%, almost achieving complete fixation of phosphorus.

[0108] In summary, the phosphorus-rich biochar prepared by the two-step thermal treatment of ferrous sulfate and sodium persulfate synergistic modification and pyrolysis calcination in this invention achieves better results with lower precipitation cost. It can not only reduce the risk of heavy metal pollution when utilizing phosphorus-rich biochar in sludge, but also achieve a phosphorus retention rate of more than 99% in sludge. This shows that this invention can achieve the preset goal of harmless and resource-based treatment of municipal sludge.

[0109] 3. Specific surface area analysis of phosphorus-rich biochar

[0110] The specific surface area of ​​sludge-based phosphorus-rich biochar was determined using a fully automated specific surface area and porosity analyzer (BET), and the results are shown in Table 5.

[0111] Table 5 shows the specific surface area (m²) of the biochar materials prepared in each embodiment and comparative example. 2 / g).

[0112]

[0113] As shown in Table 5, the synergistic modification of ferrous sulfate and sodium persulfate significantly increases the specific surface area of ​​sludge-based phosphorus-rich biochar, providing more binding sites for phosphorus fixation. This is beneficial for subsequent acid leaching separation and electrode material loading. The optimal effect is achieved at an iron-to-phosphorus ratio of 1.2:1.

[0114] In summary, the phosphorus-rich biochar prepared by the synergistic modification of ferrous sulfate and sodium persulfate through a two-step thermal treatment method of pyrolysis and calcination in this invention can not only achieve extremely high phosphorus yield and phosphorus fixation rate, but also reduce the risk of heavy metal pollution during the resource utilization of sludge-based phosphorus-rich biochar. At the same time, it realizes the harmless and resource-based treatment of sludge and the low-cost preparation of precursors for new energy materials, which has significant economic and environmental benefits.

Claims

1. A method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate using ferrous sulfate-sodium persulfate, characterized in that, Includes the following steps: (1) Heat and dry municipal sludge; grind, crush and sieve to obtain homogeneous sludge particles; (2) The homogeneous sludge particles are added to deionized water to prepare a sludge slurry; (3) After the sludge slurry is evenly dispersed by ultrasonic treatment, it is placed in a magnetic stirrer, the pH is adjusted, and ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium persulfate (Na2S2O8) are added for synergistic modification to obtain modified sludge; (4) The modified sludge slurry is placed in a centrifuge for centrifugal dehydration, and then washed and dried; (5) The dried modified sludge was pyrolyzed using a microwave pyrolysis device, and the sludge semi-coke was obtained after cooling. (6) The sludge semi-coke is heated and calcined under a nitrogen atmosphere, and after cooling, sludge-based biochar with modified directional enrichment of iron phosphate is obtained.

2. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, In step (1), the municipal sludge is heated and dried in an oven at 65℃-105℃, and after being ground and crushed, it is passed through a 100-200 mesh sieve.

3. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, In step (2), the solid-liquid ratio of sludge particles to deionized water is 1:10, that is, 100 mL of deionized water is added for every 10 g of sludge particles.

4. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, The frequency of the ultrasonic treatment in step (3) is 20kHz-40kHz, and the time is 20min-30min.

5. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, In step (3), the pH is 3-4, and the synergistic modification reaction time is 90-120 min; Fe added to the slurry 2+ and S2O8 2- The molar ratio is 2:1, and the amount of Fe added is adjusted. 2+ The ratio of sodium persulfate to phosphorus in the sludge is (1:1)-(2:1), and the corresponding mass of sodium persulfate is weighed according to the 5% solution concentration ratio to ensure that the organic phosphorus release rate is ≥95% and Fe 2+ Complete oxidation is achieved at a stirring temperature of 25℃-30℃.

6. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, In step (4), the centrifuge speed is 9000r / min-9500r / min, the time is 5min-10min, the drying temperature is 65℃-85℃, and the time is 12h-24h, so that the modified sludge is dried to constant weight.

7. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, In step (5), before pyrolysis, a high-purity inert gas is introduced to replace the air atmosphere in the microwave pyrolysis device, with a gas flow rate greater than 100 mL / min; the pyrolysis temperature is 500℃, the heating rate is 10℃ / min, and the pyrolysis time is 60 min, finally obtaining sludge semi-coke synergistically modified by ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium persulfate (Na2S2O8).

8. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, In step (6), a tube furnace is used for calcination at a temperature of 800℃-850℃, a heating rate of 10℃ / min, a calcination time of 60min-90min, and a gas flow rate of more than 100mL / min. After natural cooling to room temperature, the material is pulverized and passed through a 200-mesh sieve.

9. The method for preparing sludge-based biochar by synergistic modification and directional enrichment of ferric phosphate with ferrous sulfate-sodium persulfate according to claim 1, characterized in that, The modified sludge-based biochar with directional enrichment of ferric phosphate prepared by this invention has a phosphorus fixation rate of >99%.

10. The application of a sludge-based biochar with ferrous sulfate-sodium persulfate synergistic modification and directional enrichment of ferric phosphate as described in any one of claims 1-9, characterized in that, Specifically, this involves the application of a sludge-based biochar that uses ferrous sulfate and sodium persulfate to synergistically modify and directionally enrich ferric phosphate in the lithium iron phosphate energy battery industry.

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