Method for preparing struvite by extracting phosphorus element through kaolinite-assisted sludge gasification slag acid leaching

Abstract

The invention relates to the technical field of sludge treatment, in particular to a method for preparing struvite by extracting phosphorus element through kaolinite-assisted sludge gasification slag acid leaching. The method comprises the following steps: S1, calcining a mixture of kaolinite and sludge in a certain proportion at 800-1000 DEG C for a period of time; s2, stirring and dissolving the calcined product with an acid solution, carrying out solid-liquid separation, and taking a phosphorus-containing supernatant; and S3, adding a certain amount of a magnesium source and an ammonium source into the phosphorus-containing supernatant, then adjusting the pH value to 9.4-10, stirring for a period of time, then carrying out solid-liquid separation, washing the filtered solid with deionized water, and drying to obtain struvite crystals. The kaolinite is introduced before the sludge is calcined, the content of phosphorus in the calcined product is increased, the utilization rate of sludge gasification slag is increased, heavy metal in the sludge can be solidified, a large amount of heavy metal can be prevented from entering phosphorus-containing supernate in the acid leaching process, and the prepared MAP struvite material is stable in crystal form, safe and reliable.

Description

Technical Field

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

[0002] Sludge is the main byproduct of sewage treatment plants. It contains a large amount of organic and inorganic substances, as well as a large number of pathogens, heavy metals and other harmful substances. Sludge treatment and disposal is an important environmental issue worldwide. Sludge contains a large amount of phosphorus, which is an essential nutrient in agricultural production. It is a non-renewable resource with decreasing reserves. The shortage of phosphorus resources has gradually attracted people's attention. As global phosphate rock resources are gradually depleted, it is of great significance to recover phosphorus from waste.

[0003] Sludge gasification slag is the solid waste remaining after the sewage treatment plant treats the sludge through high-temperature gasification. The gasification treatment technology is an efficient sludge treatment method that can reduce the volume and weight of waste and recover energy at the same time. However, during the high-temperature gasification of sludge, part of the phosphorus element will be lost, resulting in a reduction in the phosphorus element that can be extracted from the gasification slag. In the prior art, the acid leaching method is often used to extract the phosphorus element from the sludge gasification slag. For example, Chinese patent CN119100352A discloses a method for recycling phosphorus elements in sludge gasification slag, extracting phosphorus elements from the sludge gasification slag by acid leaching, adding magnesium source and ammonium source, and obtaining struvite crystals. However, since the sludge gasification slag also contains a large amount of heavy metals, acid leaching will also leach other heavy metal elements, resulting in a higher content of some heavy metal elements in the prepared struvite crystals and a slightly lower whiteness. Summary of the invention

[0004] The purpose of the present invention is to address the above-mentioned deficiencies in the prior art and to provide a method for preparing struvite by acid leaching of sludge gasification residue with the aid of kaolinite to extract phosphorus.

[0005] The method for preparing struvite by extracting phosphorus from sludge gasification residue by acid leaching assisted by kaolinite comprises the following steps:

[0006] S1. calcining a mixture of a certain proportion of kaolinite and sludge at 800-1000°C for a period of time;

[0007] S2, stirring and dissolving the calcined product with an acid solution, separating the solid and the liquid, and taking 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 to separate the solid and liquid, wash the filtered solid with deionized water, and dry to obtain struvite crystals.

[0009] Furthermore, in step S1, the mass of kaolinite in the mixture is greater than 2% and less than 10%.

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

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

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

[0013] Furthermore, in step S3, NaOH solution and H2SO4 solution are used to adjust the pH.

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

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

[0016] Furthermore, in step S3, the solid-liquid separation method is centrifugal washing with a rotation speed of 4000 rpm; the drying temperature is 60° C. and the time is 18-24 hours.

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

[0018] The present invention introduces kaolinite before calcining the sludge. Due to the unique layered structure of kaolinite, more reaction sites and more surface areas are provided, the contact area with phosphorus in the sludge is increased, the adsorption of phosphorus element and the interaction and chemical reaction between phosphorus and other mineral substances are promoted, the fixation of phosphorus in the sludge is enhanced during the high-temperature calcination process, the volatilization of phosphorus element is avoided, and the phosphorus content in the calcined product is increased. Moreover, under the condition of high-temperature calcination, an appropriate amount of kaolinite can promote the conversion of organic phosphorus to inorganic phosphorus, and improve the biological effectiveness and recycling rate of phosphorus. Moreover, because kaolinite soil can change the volatilization characteristics of heavy metals, the viscosity of slag and the melting point of fly ash can be reduced to a certain extent, and the fluidity of slag in a molten state can be improved, thereby further promoting the heavy metals to form a stable slag crystal structure, improving the fixation rate of heavy metals, solidifying the heavy metals in the sludge, and avoiding a large amount of heavy metals from entering the phosphorus-containing supernatant during the acid leaching process. The prepared MAP struvite material has stable crystal form, safety and reliability.

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

[0020] The phosphorus extraction technology provided by the present invention has controllable reaction conditions, and can selectively dissolve phosphates by adjusting the type and concentration of the acid solution while avoiding dissolving other impurities, thereby improving the extraction purity.

[0021] The MAP product preparation process provided by the present invention has low cost, high yield, high efficiency and high product purity. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

[0027] Figure 4 The pharmaceutical-grade powdered kaolinite in Example 3 and the alternative embodiment 3 is the XRD diffraction pattern of MAP prepared from the high-quality kaolin mine in Maoming, Guangdong;

[0028] Figure 5 The statistical diagram of the acid leaching P recovery rate and total P recovery rate after calcination of sludge and kaolin at different mass ratios;

[0029] Figure 6 is the total phosphorus content in the sludge kaolinite calcined slag in Comparative Examples 1-2 and Examples 3 and 4;

[0030] Figure 7 The distribution of various forms of phosphorus in Comparative Examples 1-2 and Examples 3 and 4 in the sludge kaolinite pyrolysis calcined slag. DETAILED DESCRIPTION

[0031] The following are specific embodiments of the present invention and the accompanying drawings to further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

[0032] Comparative Example 1: Kaolin - 0%

[0033] Step S1001: 10 g of powdered activated sludge passed through a 60-mesh sieve was placed in a crucible, and calcined in a muffle furnace at a heating rate of 10° C. / min, and calcined at 900° C. for 4 h to obtain 4.473 g of sludge pyrolysis calcined slag, with a coke yield of 44.73%.

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

[0035] Step S1003: After stirring, high-speed centrifugation is performed to obtain 10 mL of phosphorus-containing supernatant.

[0036] Step S1004: Place 10 mL of the phosphorus-containing supernatant in a 50 mL beaker, and add 1.75 mL of a 1 mol / L MgCl2 solution and 1.75 mL of a 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 mixture at room temperature on a magnetic stirrer for 24 hours.

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

[0039] Comparative Example 2: Kaolin-2%

[0040] Step S2001: 9.8 g of powdered activated sludge passed through a 60-mesh sieve and 0.2 g of pharmaceutical-grade powdered kaolinite were fully mixed and placed in a crucible, and calcined in a muffle furnace at a heating rate of 10°C / min. Calcinated at 900°C for 4 hours to obtain 4.533 g of sludge kaolinite pyrolysis calcined slag, with a coke yield of 45.33%.

[0041] Step S2002: Take 2 g of sludge kaolinite pyrolysis calcined slag and place it in a 50 ml beaker, add 20 ml of 0.2 mol / L H2SO4 solution, and then stir and acid-leach for 8 hours at room temperature on a magnetic stirrer.

[0042] Step S2003: After stirring, high-speed centrifugation is performed to obtain 10 mL of phosphorus-containing supernatant.

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

[0044] Step S2005: The pH value was adjusted to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring, and then stirred at room temperature on a magnetic stirrer for 24 hours.

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

[0046] Example 1: Kaolin-4%

[0047] Step S3001: 9.6 g of powdered activated sludge passed through a 60-mesh sieve and 0.4 g of pharmaceutical-grade powdered kaolinite were fully mixed and placed in a crucible, and calcined in a muffle furnace at a heating rate of 10°C / min. Calcinated at 900°C for 4 hours to obtain 4.627 g of sludge kaolinite pyrolysis calcined slag with a coke yield of 46.27%.

[0048] Step S3002: Take 2 g of sludge kaolinite pyrolysis calcined slag and place it in a 50 ml beaker, add 20 ml of 0.2 mol / L H2SO4 solution, and then stir and acid-leach for 8 hours at room temperature on a magnetic stirrer.

[0049] Step S3003: After stirring, high-speed centrifugation is performed to obtain 10 mL of phosphorus-containing supernatant.

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

[0051] Step S3005: The pH value was adjusted to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring, and then stirred at room temperature on a magnetic stirrer for 24 hours.

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

[0053] Example 2: Kaolin-6%

[0054] Step S4001: 9.4 g of powdered activated sludge passed through a 60-mesh sieve and 0.6 g of pharmaceutical-grade powdered kaolinite were fully mixed and placed in a crucible, and calcined in a muffle furnace at a heating rate of 10°C / min. The mixture was calcined at 900°C for 4 hours to obtain 4.698 g of sludge kaolinite pyrolysis calcined slag with a coke yield of 46.98%.

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

[0056] Step S4003: After stirring, high-speed centrifugation is performed to obtain 10 mL of phosphorus-containing supernatant.

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

[0058] Step S4005: The pH value was adjusted to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring, and then stirred at room temperature on a magnetic stirrer for 24 hours.

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

[0060] Example 3: Kaolin-8%

[0061] Step S5001: 9.2 g of powdered activated sludge passed through a 60-mesh sieve and 0.8 g of pharmaceutical-grade powdered kaolinite were fully mixed and placed in a crucible, and calcined in a muffle furnace at a heating rate of 10°C / min. The mixture was calcined at 900°C for 4 hours to obtain 4.779 g of sludge kaolinite pyrolysis calcined slag with a coke yield of 47.79%.

[0062] Step S5002: Take 2 g of sludge kaolinite pyrolysis calcined slag and place it in a 50 ml beaker, add 20 ml of 0.2 mol / L H2SO4 solution, and then stir and acid-leach on a magnetic stirrer at room temperature for 8 hours.

[0063] Step S5003: After stirring, high-speed centrifugation is performed to obtain 10 mL of phosphorus-containing supernatant.

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

[0065] Step S5005: The pH value was adjusted to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring, and then stirred at room temperature on a magnetic stirrer for 24 hours.

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

[0067] Example 4: Kaolin-10%

[0068] Step S6001: 9.0 g of powdered activated sludge passed through a 60-mesh sieve and 1.0 g of pharmaceutical-grade powdered kaolinite were fully mixed and placed in a crucible, and calcined in a muffle furnace at a heating rate of 10°C / min. The mixture was calcined at 900°C for 4 hours to obtain 4.864 g of sludge kaolinite pyrolysis calcined slag with a coke yield of 48.64%.

[0069] Step S6002: Take 2 g of sludge kaolinite pyrolysis calcined slag and place it in a 50 ml beaker, add 20 ml of 0.2 mol / L H2SO4 solution, and then stir and acid-leach on a magnetic stirrer at room temperature for 8 hours.

[0070] Step S6003: After stirring, high-speed centrifugation is performed to obtain 10 mL of phosphorus-containing supernatant.

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

[0072] Step S6005: The pH value was adjusted to 9.4-9.5 by adding 1 mol / L NaOH solution and 0.5 mol / L H2SO4 solution while stirring, and then stirred on a magnetic stirrer for 24 hours.

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

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

[0075] Table 2 is a table showing the contents of heavy metals in the acid leaching solutions of Comparative Examples 1 and 2 and Examples 1-4. It can be seen from Table 2 that the addition of an appropriate amount of kaolinite significantly reduces the contents of heavy metals in the acid leaching solutions, and the reduction in the contents of elements such as Fe and Mn is more obvious.

[0076] Figures 1a-1fTG-DTG analysis of sludge kaolinite of different mass ratios in Comparative Examples 1, 2 and Examples 1-4 respectively. Through TG-DTG analysis of kaolinite sludge of different ratios, the loss on ignition decreases with the increase of kaolinite content at the same temperature. Thermogravimetry reflects the heat absorption and release characteristics of the sample. The addition of kaolinite affects the heat absorption and release of the sludge. Organic phosphorus will decompose during the calcination process, and inorganic phosphorus may undergo reactions such as crystal transformation. Within a certain temperature range, phosphorus-related compounds will undergo dehydration, decomposition and other processes, resulting in mass changes. New peaks will appear on the DTG curve, or the shape, position, and intensity of the original peaks will change. From the DTG curve, kaolinite will affect the reaction rate of compounds in the sludge, and may affect the mineral phase of phosphorus and form new phases, thereby fixing more phosphorus and heavy metals, increasing the leaching amount of phosphorus and reducing the leaching amount of heavy metals.

[0077] By changing the acid leaching concentration and liquid-to-solid ratio in Comparative Example 1, the process conditions for successfully synthesizing MAP were determined. Figure 2a XRD of MAP prepared with liquid-to-solid ratio of 10 ml / g and different acid leaching concentrations. Figure 2b XRD of MAP prepared with different acid leaching concentrations at a liquid-to-solid ratio of 20 ml / g. Figure 2c XRD of MAP prepared with different acid leaching concentrations at a liquid-to-solid ratio of 50 ml / g; MAP can be well synthesized under the condition of an acid leaching concentration of 0.2 mol / L, and XRD has obvious diffraction peaks. If the sulfuric acid concentration is too high, the synthesis of MAP will be limited, and impurity ions such as heavy metals will also increase to form amorphous precipitates. If the liquid-to-solid ratio is too high, the concentration of P leached out by acid is low, and it is difficult to synthesize MAP.

[0078] Figure 3 The XRD diffraction patterns of Comparative Examples 1 and 2 and Examples 1-4 are analyzed by analyzing the XRD of the product prepared from the H2SO4 solution with an acid leaching concentration of 0.2 mol / L. It can be seen that there are obvious diffraction peaks at 2θ=15.81°, 20.85°, 21.45°, 30.60° and 33.28°, which coincide with the PDF card of MAP struvite (MgNH4PO4·6H2O), and it can be verified that the struvite product was successfully synthesized and prepared.

[0079] If the acid leaching concentration is high, the recovered precipitation product will not show obvious diffraction peaks. The precipitation product is mainly amorphous. This is because the high acid leaching concentration can form too many impurity ions that precipitate, generate a large number of crystal nuclei, and thus form a lot of fine crystals, making the precipitation amorphous. The sludge calcined product with kaolinite added has a higher leaching rate of phosphorus, higher MAP purity, higher whiteness, and better crystallinity.

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

[0081] Table 3 shows the P recovery rate of the acid leaching solution, the amount of MAP synthesis and the total P recovery rate 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 leaching solution 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. Among them, the P content in the sludge used in this example is 8.33wt.%, Figure 5 The statistical diagram of the acid leaching P recovery rate and total P recovery rate after calcination of sludge and kaolin at different mass ratios is shown in Table 3 and Figure 5 It can be seen that since the P element only exists in activated sludge, keeping the total mass unchanged and replacing part of the mass of activated sludge with powdered kaolinite, at a certain proportion, can promote the fixation of phosphorus in the sludge during high-temperature calcination and promote the release of phosphorus in the sludge during acid leaching. When kaolinite is 8% of the total mass, the P recovery rate of 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 kaolin can enhance the release of phosphorus in activated sludge, thereby further strategically recovering phosphorus resources, increasing the added value of materials, and having certain economic and environmental benefits.

[0082] Figure 6 is the total phosphorus content in the sludge kaolinite calcined slag in Comparative Examples 1-2 and Examples 3 and 4. Wherein, K0-K10 corresponds to Kaolin, 0%-Kaolin, 10%, 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 It indicates the char yield after sludge pyrolysis at the corresponding pyrolysis temperature, WR sludge It indicates the weight proportion of sludge in the pyrolysis raw material. It can be seen that the appropriate amount of kaolinite has a certain significant improvement in the retention of total phosphorus content in the sludge, which corresponds to the acid leaching rate.

[0083] Figure 7 The distribution of various forms of phosphorus in comparative examples 1-2 and examples 3 and 4 in the sludge kaolinite pyrolysis calcined slag, from Figure 7It can be seen that with the increase of kaolinite content, the content of apatite inorganic phosphorus (NAIP) has a small increase. A small part of apatite inorganic phosphorus (AP)-transforms to non-apatite inorganic phosphorus (NAIP). NAIP is unstable and is more easily decomposed under acid leaching conditions. When the kaolinite addition amount is from 2% to 8%, 2.29% of apatite inorganic phosphorus is transformed into non-apatite inorganic phosphorus. The NAIP content increases by 1.55mg, and the change of AP is 0.69mg. According to Figure 7 When the addition amount of kaolinite was 8%, the content of organic phosphorus (OP) was low, which also verified that an appropriate amount of kaolinite could promote the conversion of organic phosphorus to inorganic phosphorus.

[0084] Table 1

[0085]

[0086] Table 2

[0087]

[0088]

[0089] Table 3

[0090]

[0091] For matters not mentioned above, the prior art applies.

[0092] Although some specific embodiments of the present invention have been described in detail through examples, those skilled in the art should understand that the above examples are for illustration only and are not intended to limit the scope of the present invention. Those skilled in the art to which the present invention belongs may make various modifications or supplements to the specific embodiments described or replace them in a similar manner, but they will not deviate from the direction of the present invention or exceed the scope defined by the attached 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 in the protection scope of the present invention.

Claims

1. A method for preparing struvite by acid leaching of phosphorus from sludge gasification residue assisted by kaolinite, characterized in that: The steps include: S1. calcining a mixture of a certain proportion of kaolinite and sludge at 800-1000°C for a period of time; S2, stirring and dissolving the calcined product with an acid solution, separating the solid and the liquid, and taking 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 to separate the solid and liquid, wash the filtered solid with deionized water, and dry to obtain struvite crystals.

2. The method for preparing struvite by acid leaching of sludge gasification residue with kaolinite as claimed in claim 1, characterized in that: In step S1, the mass of kaolinite in the mixture is greater than 2% and less than 10%.

3. The method for preparing struvite by acid leaching of kaolinite-assisted sludge gasification residue to extract phosphorus as claimed in claim 1, characterized in that: In step S1, the calcination time is 3-4 hours.

4. The method for preparing struvite by acid leaching of kaolinite-assisted sludge gasification residue to extract phosphorus as claimed in claim 1, characterized in that: In step S2, 0.2 mol / L H2SO4 solution is used for acid leaching at room temperature for 8 h with stirring, and the liquid-to-solid ratio is 10 mL / g.

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

6. The method for preparing struvite by acid leaching of kaolinite-assisted sludge gasification residue to extract phosphorus as claimed in claim 1, characterized in that: In step S3, NaOH solution and H2SO4 solution are used to adjust the pH.

7. The method for preparing struvite by acid leaching of sludge gasification residue with kaolinite as an aid of phosphorus extraction according to claim 6, characterized in that: The concentration of NaOH solution is 1 mol / L, and the concentration of H2SO4 solution is 0.5 mol / L; stir the reaction for 18-24 hours.

8. The method for preparing struvite by acid leaching of sludge gasification residue with kaolinite as an aid of phosphorus extraction according to 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. 2+ :NH 4+ :PO4 3- The molar ratio was controlled to be 1:1:

1.

9. The method for preparing struvite by acid leaching of sludge gasification residue with kaolinite as claimed in claim 1, characterized in that: In step S3, the solid-liquid separation method is centrifugal washing with a rotation speed of 4000 rpm; the drying temperature is 60° C. and the time is 18-24 hours.

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

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