Method for preparing struvite by acid leaching of phosphorus element from sludge gasification slag assisted by kaolinite

Abstract

This invention relates to the field of sludge treatment technology, and more particularly to a method for preparing struvite by acid leaching of phosphorus from sludge gasification residue with kaolinite assistance. The method includes the following steps: S1, calcining a mixture of kaolinite and sludge in a certain proportion at 800-1000℃ for a period of time; S2, dissolving the calcined product with an acid solution, separating the solid and liquid, and collecting the phosphorus-containing supernatant; S3, adding a certain amount of magnesium and ammonium sources to the phosphorus-containing supernatant, adjusting the pH to 9.4-10, stirring for a period of time, separating the solid and liquid, washing the filtered solid with deionized water, and drying to obtain struvite crystals. This invention introduces kaolinite before sludge calcination, increasing the phosphorus content in the calcined product, improving the utilization rate of sludge gasification residue, and solidifying heavy metals in the sludge. It avoids a large amount of heavy metals entering the phosphorus-containing supernatant during acid leaching, resulting in a stable and reliable MAP struvite material.

Description

Technical Field

[0001] This invention relates to the field of sludge treatment technology, and in particular to a method for preparing struvite by acid leaching of phosphorus from sludge gasification residue assisted by kaolinite. Background Technology

[0002] Sludge is a major byproduct of wastewater treatment plants, containing large amounts of organic and inorganic matter, as well as pathogens, heavy metals, and other harmful substances. Globally, sludge treatment and disposal is a significant environmental issue. Sludge contains a large amount of phosphorus, an essential nutrient for agricultural production. As a non-renewable resource with dwindling reserves, the shortage of phosphorus resources is increasingly attracting attention. With the gradual depletion of global phosphate rock resources, recovering phosphorus from waste is of great importance.

[0003] Sludge gasification residue is the solid waste remaining after sludge treatment at wastewater treatment plants undergoes high-temperature gasification. Gasification is a highly efficient sludge treatment method that can reduce waste volume and weight while recovering energy. However, during high-temperature gasification, some phosphorus is lost, resulting in a reduction in the amount of phosphorus that can be extracted from the gasification residue. Existing technologies often use acid leaching to extract phosphorus from sludge gasification residue. For example, Chinese patent CN119100352A discloses a method for recovering phosphorus from sludge gasification residue by acid leaching. This method involves adding magnesium and ammonium sources to extract struvite crystals. However, since sludge gasification residue also contains a large amount of heavy metals, acid leaching will also leach out other heavy metals, resulting in a higher content of some heavy metals and slightly lower whiteness in the prepared struvite crystals. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned shortcomings of the prior art by proposing a method for preparing struvite through acid leaching of phosphorus from sludge gasification residue assisted by kaolinite.

[0005] The present invention discloses a method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue, comprising the following steps:

[0006] S1. Calcine a mixture of kaolinite and sludge in a certain proportion at 800-1000℃ for a period of time;

[0007] S2. After dissolving the calcined product with an acid solution by stirring, separate the solid and liquid components and take the phosphorus-containing supernatant.

[0008] S3. Add a certain amount of magnesium source and ammonium source to the phosphorus-containing supernatant, then adjust the pH to 9.4-10, stir for a period of time, and then separate the solid and liquid. Wash the filtered solid with deionized water and dry it to obtain struvite crystals.

[0009] Furthermore, in step S1, the mass of kaolinite in the mixture is greater than 2% and not greater than 10%.

[0010] Furthermore, in step S1, the calcination time is 3-4 hours.

[0011] Furthermore, in step S2, acid leaching is performed at room temperature with 0.2 mol / L H2SO4 solution and stirring for 8 hours, with a liquid-to-solid ratio of 10 mL / g.

[0012] Furthermore, in step S2, the solid-liquid separation method is centrifugal washing at a speed of 4000 rpm.

[0013] Furthermore, in step S3, the pH is adjusted using NaOH solution and H2SO4 solution.

[0014] Furthermore, the concentration of NaOH solution was 1 mol / L, and the concentration of H2SO4 solution was 0.5 mol / L; the reaction was stirred for 18-24 hours.

[0015] Furthermore, in step S3, the magnesium source is a 1 mol / L magnesium chloride solution, and the ammonium source is a 1 mol / L ammonium chloride solution. The Mg in the supernatant is then... 2+ NH 4+ :PO4 3- The molar ratio is controlled at 1:1:1.

[0016] Furthermore, in step S3, the solid-liquid separation method is centrifugal washing at a speed of 4000 rpm; the drying temperature is 60℃ and the time is 18-24h.

[0017] Furthermore, the kaolinite is pharmaceutical grade kaolinite, and the sludge is dried, pulverized, and passed through a 60-mesh sieve as powdered activated sludge.

[0018] This invention introduces kaolinite before sludge calcination. Due to its unique layered structure, kaolinite provides more reaction sites and a larger surface area, increasing the contact area with phosphorus in the sludge. This promotes the adsorption of phosphorus and the interaction and chemical reaction between phosphorus and other minerals, which helps fix phosphorus. During high-temperature calcination, it enhances the fixation of phosphorus in the sludge, prevents phosphorus volatilization, and increases the phosphorus content in the calcination product. Moreover, under high-temperature calcination conditions, an appropriate amount of kaolinite can promote the conversion of organic phosphorus to inorganic phosphorus, improving the bioavailability and reuse rate of phosphorus. Furthermore, because kaolinite can alter the volatility characteristics of heavy metals, it can reduce the viscosity of slag and the melting point of fly ash to a certain extent, improving the fluidity of slag in the molten state. This further promotes the formation of a stable slag crystal structure for heavy metals, increases the fixation rate of heavy metals, and solidifies the heavy metals in the sludge. This prevents a large amount of heavy metals from entering the phosphorus-containing supernatant during acid leaching. The resulting MAP struvite material has a stable crystal structure and is safe and reliable.

[0019] With the increase of kaolinite content, a small portion of apatite-form inorganic phosphorus (AP) in the sludge is transformed into apatite-form inorganic phosphorus (NAIP). NAIP is unstable and is more easily decomposed under acid leaching conditions. Kaolinite can regulate the phosphorus element in the sludge under high-temperature calcination conditions, further promoting the release of phosphorus resources during acid leaching. At the same time, an appropriate amount of kaolinite can promote the conversion of organic phosphorus to inorganic phosphorus in the sludge, increase the inorganic phosphorus content, and increase the acid leaching rate of phosphorus.

[0020] The phosphorus extraction technology provided by this invention allows for controllable reaction conditions and enables selective dissolution of phosphates by adjusting the type and concentration of acid, while avoiding the dissolution of other impurities, thereby improving extraction purity.

[0021] The MAP product preparation process provided by this invention has low cost, high yield, high efficiency, and high product purity. Attached Figure Description

[0022] Figures 1a-1f TG-DTG analysis of sludge kaolinite at different mass ratios in Comparative Examples 1 and 2 and Examples 1-4, respectively;

[0023] Figure 2a XRD patterns of MAPs prepared at different acid leaching concentrations with a liquid-to-solid ratio of 10 ml / g;

[0024] Figure 2b XRD patterns of MAPs prepared at different acid leaching concentrations with a liquid-to-solid ratio of 20 ml / g;

[0025] Figure 2c XRD patterns of MAPs prepared at different acid leaching concentrations with a liquid-to-solid ratio of 50 ml / g;

[0026] Figure 3 The XRD diffraction patterns of Comparative Examples 1 and 2 and Examples 1-4 are shown.

[0027] Figure 4 XRD diffraction patterns of MAP prepared from high-quality kaolinite from Maoming, Guangdong Province, in Example 3 and Alternative Example 3;

[0028] Figure 5 Statistical graphs showing the acid leaching P recovery rate and total P recovery rate after calcination of sludge and kaolin with different mass ratios;

[0029] Figure 6 The total phosphorus content in the kaolinite calcination residue of sludge in Comparative Examples 1-2 and Examples 3 and 4;

[0030] Figure 7 The distribution of various forms of phosphorus in sludge kaolinite pyrolysis calcination residue in Comparative Examples 1-2 and Examples 3 and 4. Detailed Implementation

[0031] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0032] Comparative Example 1: Kaolin-0%

[0033] Step S1001: Place 10g of powdered activated sludge that has passed through a 60-mesh sieve into a crucible and calcine it in a muffle furnace at a heating rate of 10℃ / min. Calcine it at 900℃ for 4h to obtain 4.473g of sludge pyrolysis calcination residue with a coke yield of 44.73%.

[0034] Step S1002: Take 2g of sludge pyrolysis calcination residue and place it in a 50ml beaker. Add 20ml of 0.2mol / L H2SO4 solution and then stir and acid-leach at room temperature on a magnetic stirrer for 8h.

[0035] Step S1003: After stirring, centrifuge at high speed and take 10 mL of phosphorus-containing supernatant.

[0036] Step S1004: Place 10 mL of phosphorus-containing supernatant into a 50 mL beaker, add 1.75 mL of 1 mol / L MgCl2 solution and 1.75 mL of 1 mol / L NH4Cl solution.

[0037] Step S1005: Adjust the pH to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring; then stir the reaction at room temperature on a magnetic stirrer for 24 h.

[0038] Step S1006: After the reaction is completed, the solid is centrifuged and washed multiple times. The filtered solid is dried at 60℃ for 24 hours to obtain struvite crystals.

[0039] Comparative Example 2: Kaolin - 2%

[0040] Step S2001: 9.8g of powdered activated sludge that has passed through a 60-mesh sieve and 0.2g of pharmaceutical-grade powdered kaolinite are thoroughly mixed and placed in a crucible. The mixture is then calcined in a muffle furnace at a heating rate of 10℃ / min for 4 hours at 900℃ to obtain 4.533g of sludge-kaolinite pyrolysis calcination residue with a coke yield of 45.33%.

[0041] Step S2002: Take 2g of sludge kaolinite pyrolysis calcination residue and place it in a 50ml beaker. Add 20ml of 0.2mol / L H2SO4 solution and then stir and acid leaching at room temperature on a magnetic stirrer for 8h.

[0042] Step S2003: After stirring, centrifuge at high speed and take 10 mL of phosphorus-containing supernatant.

[0043] Step S2004: Place 10 mL of phosphorus-containing supernatant into a 50 mL beaker, add 1.81 mL of 1 mol / L MgCl2 solution and 1.81 mL of 1 mol / L NH4Cl solution.

[0044] Step S2005: Adjust the pH to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring. Then stir the reaction on a magnetic stirrer at room temperature for 24 h.

[0045] Step S2006: After the reaction is completed, the solid is centrifuged and washed multiple times. The filtered solid is dried at 60℃ for 24 hours to obtain struvite crystals.

[0046] Example 1: Kaolin-4%

[0047] Step S3001: 9.6g of powdered activated sludge that has passed through a 60-mesh sieve and 0.4g of pharmaceutical-grade powdered kaolinite are thoroughly mixed and placed in a crucible. The mixture is then calcined in a muffle furnace at a heating rate of 10℃ / min for 4 hours at 900℃ to obtain 4.627g of sludge-kaolinite pyrolysis calcination residue with a coke yield of 46.27%.

[0048] Step S3002: Take 2g of sludge kaolinite pyrolysis calcination residue and place it in a 50ml beaker. Add 20ml of 0.2mol / L H2SO4 solution and then stir and acid leaching at room temperature on a magnetic stirrer for 8h.

[0049] Step S3003: After stirring, centrifuge at high speed and take 10 mL of phosphorus-containing supernatant.

[0050] Step S3004: Place 10 mL of phosphorus-containing supernatant into a 50 mL beaker, add 1.75 mL of 1 mol / L MgCl2 solution and 1.75 mL of 1 mol / L NH4Cl solution.

[0051] Step S3005: Adjust the pH to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring. Then stir the reaction on a magnetic stirrer at room temperature for 24 h.

[0052] Step S3006: After the reaction is completed, the solid is centrifuged and washed multiple times. The filtered solid is dried at 60℃ for 24 hours to obtain struvite crystals.

[0053] Example 2: Kaolin-6%

[0054] Step S4001: After thoroughly mixing 9.4 g of powdered activated sludge that has passed through a 60-mesh sieve and 0.6 g of pharmaceutical-grade powdered kaolinite, place the mixture in a crucible and calcine it in a muffle furnace at a heating rate of 10 °C / min for 4 h at 900 °C to obtain 4.698 g of sludge-kaolinite pyrolysis calcination residue with a coke yield of 46.98%.

[0055] Step S4002: Take 2g of sludge kaolinite pyrolysis calcination residue and place it in a 50ml beaker. Add 20ml of 0.2mol / L H2SO4 solution and then stir and acid-leach at room temperature on a magnetic stirrer for 8h.

[0056] Step S4003: After stirring, centrifuge at high speed and take 10 mL of phosphorus-containing supernatant.

[0057] Step S4004: Place 10 mL of phosphorus-containing supernatant into a 50 mL beaker, add 1.79 mL of 1 mol / L MgCl2 solution and 1.79 mL of 1 mol / L NH4Cl solution.

[0058] Step S4005: Adjust the pH to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring. Then stir the reaction on a magnetic stirrer at room temperature for 24 h.

[0059] Step S4006: After the reaction is completed, the solid is centrifuged and washed multiple times. The filtered solid is dried at 60°C for 24 hours to obtain struvite crystals.

[0060] Example 3: Kaolin-8%

[0061] Step S5001: 9.2g of powdered activated sludge that has passed through a 60-mesh sieve and 0.8g of pharmaceutical-grade powdered kaolinite are thoroughly mixed and placed in a crucible. The mixture is then calcined in a muffle furnace at a heating rate of 10℃ / min for 4 hours at 900℃ to obtain 4.779g of sludge-kaolinite pyrolysis calcination residue with a coke yield of 47.79%.

[0062] Step S5002: Take 2g of sludge kaolinite pyrolysis calcination residue and place it in a 50ml beaker. Add 20ml of 0.2mol / L H2SO4 solution and then stir and acid leaching at room temperature on a magnetic stirrer for 8h.

[0063] Step S5003: After stirring, centrifuge at high speed and collect 10 mL of phosphorus-containing supernatant.

[0064] Step S5004: Place 10 mL of phosphorus-containing supernatant into a 50 mL beaker, add 1.79 mL of 1 mol / L MgCl2 solution and 1.79 mL of 1 mol / L NH4Cl solution.

[0065] Step S5005: Adjust the pH to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring. Then stir the reaction on a magnetic stirrer at room temperature for 24 h.

[0066] Step S5006: After the reaction is completed, the solid is centrifuged and washed multiple times. The filtered solid is dried at 60°C for 24 hours to obtain struvite crystals.

[0067] Example 4: Kaolin-10%

[0068] Step S6001: After thoroughly mixing 9.0g of powdered activated sludge that has passed through a 60-mesh sieve and 1.0g of pharmaceutical-grade powdered kaolinite, place the mixture in a crucible and calcine it in a muffle furnace at a heating rate of 10℃ / min for 4 hours at 900℃ to obtain 4.864g of sludge-kaolinite pyrolysis calcination residue with a coke yield of 48.64%.

[0069] Step S6002: Take 2g of sludge kaolinite pyrolysis calcination residue and place it in a 50ml beaker. Add 20ml of 0.2mol / L H2SO4 solution and then stir and acid leaching at room temperature on a magnetic stirrer for 8h.

[0070] Step S6003: After stirring, centrifuge at high speed and take 10 mL of phosphorus-containing supernatant.

[0071] Step S6004: Place 10 mL of phosphorus-containing supernatant into a 50 mL beaker, add 1.69 mL of 1 mol / L MgCl2 solution and 1.69 mL of 1 mol / L NH4Cl solution.

[0072] Step S6005: Adjust the pH to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring. Then stir the reaction on a magnetic stirrer for 24 hours.

[0073] Step S4006: After the reaction is completed, the solid is centrifuged and washed multiple times. The filtered solid is dried at 60°C for 24 hours to obtain struvite crystals.

[0074] Table 1 shows the industrial analysis of the dried sludge, which has a high volatile content and its mass decreases during high-temperature calcination.

[0075] Table 2 shows the heavy metal content in the acid leaching solutions of Comparative Examples 1 and 2 and Examples 1-4. As can be seen from Table 2, the addition of an appropriate amount of kaolinite significantly reduces the heavy metal content in the acid leaching solution, with a more significant reduction in the content of elements such as Fe and Mn.

[0076] Figures 1a-1fTG-DTG analysis was performed on sludge containing kaolinite at different mass ratios in Comparative Examples 1 and 2, and Examples 1-4. Through TG-DTG analysis of sludge containing kaolinite at different proportions, the loss on ignition rate decreased with increasing kaolinite content at the same temperature. Thermogravimetric analysis (TGA) reflects the endothermic and exothermic characteristics of the samples. The addition of kaolinite affected the endothermic and exothermic effects of the sludge. During calcination, organic phosphorus decomposes, and inorganic phosphorus may undergo crystal transformation reactions. Within a certain temperature range, phosphorus-related compounds undergo dehydration and decomposition, leading to changes in mass. New peaks appear on the DTG curve, or the shape, position, and intensity of existing peaks change. The DTG curves indicate that kaolinite affects the reaction rate of compounds in the sludge, potentially influencing the phosphorus mineral phase and forming new phases, thereby fixing more phosphorus and heavy metals, increasing phosphorus leaching, and reducing heavy metal leaching.

[0077] By changing the acid leaching concentration and liquid-solid ratio in Comparative Example 1, the process conditions for successful synthesis of MAP were determined. Figure 2a XRD patterns of MAP prepared at different acid leaching concentrations with a liquid-to-solid ratio of 10 ml / g. Figure 2b XRD patterns of MAP prepared at different acid leaching concentrations with a liquid-to-solid ratio of 20 ml / g. Figure 2c XRD patterns of MAP prepared with different acid leaching concentrations at a liquid-to-solid ratio of 50 ml / g were obtained. MAP can be synthesized well under an acid leaching concentration of 0.2 mol / L, and the XRD shows obvious diffraction peaks. If the sulfuric acid concentration is too high, the synthesis of MAP is limited, and the amount of impurity ions such as heavy metals will increase, forming amorphous precipitates. If the liquid-to-solid ratio is too high, the concentration of P leached out by acid is low, and it is not easy to synthesize MAP.

[0078] Figure 3 To compare the XRD diffraction patterns of Examples 1 and 2 and Examples 1-4, XRD analysis was performed on the products prepared by acid leaching with 0.2 mol / L H2SO4 solution. Obvious diffraction peaks were observed at 2θ = 15.81°, 20.85°, 21.45°, 30.60°, and 33.28°, which correspond to the PDF card of MAP struvite (MgNH4PO4·6H2O), confirming the successful synthesis and preparation of the struvite product.

[0079] If the acid leaching concentration is high, the recovered precipitate will not show obvious diffraction peaks, and the precipitate will mainly consist of amorphous substances. This is because a high acid leaching concentration results in an excessive number of impurity ions that can form precipitates, generating a large number of crystal nuclei, which in turn form numerous fine crystals, making the precipitate amorphous. Sludge calcination products with added kaolinite exhibit higher phosphorus leaching rates, higher MAP purity, higher whiteness, and better crystallinity.

[0080] Figure 4The XRD diffraction patterns of MAP prepared from high-quality kaolinite from Maoming, Guangdong Province, in Example 3 and Alternative Example 3 are shown. Figure 4 It can be seen that MAP was successfully synthesized from pharmaceutical-grade kaolin and high-quality kaolin ore from Maoming, Guangdong, under high-temperature calcination conditions with sludge, indicating that the kaolinite used in this invention is also applicable to other kaolinite clay minerals and has strong applicability.

[0081] Table 3 shows the P recovery rate, MAP synthesis amount, and total P recovery rate of the acid leachate after calcination of sludge and kaolin (Comparative Examples 1, 2, and Examples 1-4) with different mass ratios. The P recovery rate of the acid leachate is the ratio of the leached P content to the P content in the sludge, and the total P recovery rate is the ratio of the P content in the final MAP to the P content in the sludge. In this example, the P content in the sludge was 8.33 wt.%. Figure 5 Statistical graphs of P recovery rate and total P recovery rate after acid leaching of sludge and kaolin with different mass ratios after calcination, from Table 3 and Figure 5 It can be seen that since phosphorus (P) only exists in activated sludge, by replacing a portion of the activated sludge with powdered kaolinite while keeping the total mass constant, phosphorus fixation in the sludge can be promoted during high-temperature calcination and phosphorus release during acid leaching at a certain proportion. When kaolinite accounts for 8% of the total mass, the recovery rate of P in the acid leaching solution reaches 72.41%, and the total P recovery rate reaches 72.16%. Therefore, this example shows that an appropriate amount of kaolinite can enhance the release of phosphorus from activated sludge, thereby enabling further strategic resource recovery of phosphorus, increasing the added value of materials, and having certain economic and environmental benefits.

[0082] Figure 6 The total phosphorus content in the kaolinite calcination residue of sludge from Comparative Examples 1-2 and Examples 3 and 4 is given. K0-K10 correspond to 0%-10% Kaolin, and the total phosphorus content is defined as phosphorus retention (PS, mg P / g SS): PS = PC * Y char / WR sludge PC represents the concentration of phosphorus (mg P / g char), Y char WR represents the coke yield after sludge pyrolysis at the corresponding pyrolysis temperature. sludge This indicates the weight percentage of sludge in the pyrolysis feedstock. It can be seen that an appropriate amount of kaolinite significantly increases the retention of total phosphorus in the sludge, corresponding to the acid leaching rate.

[0083] Figure 7 To compare the distribution of various forms of phosphorus in Examples 1-2 and Examples 3 and 4 in the pyrolysis calcination residue of sludge kaolinite, from... Figure 7It can be seen that with the increase of kaolinite content, the content of apatite-form inorganic phosphorus (NAIP) increases slightly. A small portion of apatite-form inorganic phosphorus (AP) transforms into non-apatite-form inorganic phosphorus (NAIP), and NAIP is unstable and more easily decomposed under acid leaching conditions. With kaolinite addition ranging from 2% to 8%, 2.29% of the apatite-form inorganic phosphorus transforms into non-apatite-form inorganic phosphorus. The NAIP content increases by 1.55 mg, while the AP content changes by 0.69 mg. Figure 7 When the kaolinite content was 8%, the organic phosphorus (OP) content was low, which also verifies that an appropriate amount of kaolinite can promote the conversion of organic phosphorus to inorganic phosphorus.

[0084] Table 1

[0085]

[0086] Table 2

[0087]

[0088]

[0089] Table 3

[0090]

[0091] For any points not covered above, existing technologies shall apply.

[0092] Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue, characterized in that: Includes the following steps: S1. Calcine a mixture of kaolinite and sludge in a certain proportion at 800-1000℃ for a period of time; S2. After dissolving the calcined product with an acid solution by stirring, separate the solid and liquid components and take the phosphorus-containing supernatant. S3. Add a certain amount of magnesium source and ammonium source to the phosphorus-containing supernatant, then adjust the pH to 10, stir for a period of time, separate the solid and liquid, and wash and dry the filtered solid with deionized water to obtain struvite crystals. In step S1, the mass of kaolinite in the mixture is greater than 2% and not greater than 10%. In step S1, the calcination time is 3-4 hours.

2. The method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue as described in claim 1, characterized in that: In step S2, acid leaching was performed at room temperature for 8 hours with stirring of a 0.2 mol / L H2SO4 solution, and the liquid-to-solid ratio was 10 mL / g.

3. The method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue as described in claim 1, characterized in that: In step S2, the solid-liquid separation method is centrifugal washing at a speed of 4000 rpm.

4. The method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue as described in claim 1, characterized in that: In step S3, the pH is adjusted using NaOH solution and H2SO4 solution.

5. The method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue as described in claim 4, characterized in that: The NaOH solution concentration was 1 mol / L, and the H2SO4 solution concentration was 0.5 mol / L; the reaction was stirred for 18-24 h.

6. The method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue as described in claim 1, characterized in that: In step S3, the magnesium source is a 1 mol / L magnesium chloride solution, and the ammonium source is a 1 mol / L ammonium chloride solution. The Mg in the supernatant is... 2+ NH 4+ :PO4 3- The molar ratio is controlled at 1:1:

1.

7. The method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue as described in claim 1, characterized in that: In step S3, the solid-liquid separation method is centrifugal washing at a speed of 4000 rpm; the drying temperature is 60℃ and the time is 18-24 h.

8. The method for preparing struvite by acid leaching of phosphorus from kaolinite-assisted sludge gasification residue as described in claim 1, characterized in that: The kaolinite is pharmaceutical grade kaolinite, and the sludge is dried, pulverized, and passed through a 60-mesh sieve as powdered activated sludge.

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