Preparation of polyphosphate-containing ammonium phosphate from ammonium sulfate and process and system therefor

Abstract

The application discloses a kind of ammonium phosphate containing polyphosphate ammonium prepared by ammonium slag as raw material and its preparation method and system, belong to ammonium technical field.Method includes: S1 preparation of pulp: ammonium slag is prepared into slurry with water, and the solid content of slurry is 55-75%;S2 acidolysis: concentrated sulfuric acid is added to the slurry obtained in step S1, and the reaction temperature is 60-100 ℃, and the amount of concentrated sulfuric acid is 20-35% of the dry basis mass of ammonium slag;S3 neutralization: urea is added to the acidolysis slurry obtained in step S2, and the reaction temperature is 80-95 ℃, and the amount of urea is 5-12% of the dry basis mass of ammonium slag;S4 spray drying: the neutralization slurry obtained in step S3 is spray dried, and the inlet air temperature is 120-250 ℃, and the outlet air temperature is 80-150 ℃;S5 high temperature polymerization: the powder obtained in step S4 is high temperature polymerized at 150-260 ℃ to obtain ammonium phosphate containing polyphosphate ammonium.

Description

Technical Field

[0001] This invention belongs to the field of industrial ammonium technology, and specifically relates to a polyphosphate-containing ammonium phosphate prepared from industrial ammonium slag as raw material, as well as its preparation method and system. Background Technology

[0002] The neutralization residue of industrial monoammonium phosphate (MAP) comes from the filter cake produced during the wet-process production of MAP after the wet-process phosphoric acid is neutralized by ammonia and then filtered. This filter cake is washed with water and dried to obtain industrial MAP residue, hereinafter referred to as MAP residue.

[0003] The composition of industrial ammonium slag is relatively complex, as shown in Table 1. The dry basis P2O5 content in the neutralized slag is 20-32%, mainly composed of a small amount of water-soluble (NH4)2HPO4 and NH4H2PO4 and a large amount of water-insoluble MgHPO4, FeMg(NH4)2(HPO4)2F3, AlMg(NH4)2(HPO4)2F3, CaSO4, SiO2, etc.

[0004] In existing technologies, industrial ammonium slag can be reused through acid hydrolysis, polymerization, and other methods.

[0005] For example, patent application number CN201910598673.4 discloses a method for preparing ammonium polyphosphate compound fertilizer using industrial monoammonium phosphate residue, the method including the following steps:

[0006] (1) Pulping: The raw material ammonium slag is mixed with water to form a uniform slurry, wherein the solid content of the uniform slurry is 40-85%.

[0007] (2) Acid hydrolysis: Slowly add acid to the uniform slurry obtained in step (1) to react and obtain acid hydrolyzed slurry. The reaction temperature is 60-100℃ and the amount of acid used is 40-60% of the raw material mass.

[0008] (3) Neutralization: Add urea to the acid hydrolysate obtained in step (2) and mix evenly. Concentrate to obtain molten raw material containing urea phosphate. The amount of urea used is 20-30% of the raw material mass.

[0009] (4) Condensation: A condensing agent is added to the molten raw material obtained in step (3), and the mixture is heated to obtain a binary fertilizer containing ammonium polyphosphate through foaming, curing and pulverization. The amount of the condensing agent is 10-30% of the raw material mass, and the reaction temperature of foaming and curing is 150-260℃.

[0010] (5) Granulation: The binary fertilizer obtained in step (4) is granulated with other compound fertilizer raw materials to obtain a compound fertilizer containing ammonium polyphosphate.

[0011] The patent requires foaming, curing and pulverizing processes, which are complex; in addition, a condensing agent needs to be added during condensation.

[0012] For example, patent application number CN201910598620.2 discloses a method for recovering nutrients from industrial monoammonium phosphate residue, the method comprising the following steps: (1) Pulping: The raw material ammonium slag is mixed with water to form a uniform slurry, wherein the solid content of the uniform slurry is 40-85%.

[0013] (2) Acid hydrolysis: Slowly add acid to the uniform slurry obtained in step (1) to react and obtain acid hydrolyzed slurry. The reaction temperature is 60-100℃ and the amount of acid used is 40-60% of the raw material mass.

[0014] (3) Neutralization: Add urea to the acid hydrolysate obtained in step (2) and mix evenly. Concentrate and dry to obtain compound fertilizer raw material containing acid urea. The amount of urea used is 15-35% of the raw material mass.

[0015] The above patents were filed by our company a few years ago. The applicant discovered the following problems during actual production: (1) The amount of sulfuric acid used is large, and the price of sulfuric acid has increased significantly in recent years, with an increase of more than 50%, making the processing cost too high.

[0016] (2) With the upgrading of wet phosphoric acid process, especially the adoption of demagnesification process, the main effects are: after process optimization, the utilization rate of phosphorus in phosphate rock and phosphoric acid is improved, the total phosphorus content in the by-product of industrial ammonium slag is significantly reduced, the content of metal impurities such as iron, magnesium and aluminum is significantly reduced (especially magnesium content), the content of iron, magnesium and aluminum citrate-soluble complex phosphate generated by neutralization of ammonium phosphate is also reduced, and the proportion of water-soluble phosphorus is greatly increased; the previous method is no longer suitable.

[0017] (3) The large amount of sulfuric acid used results in a high concentration of sulfuric acid in the reactor, which leads to excessive viscosity of the slurry, which in turn causes serious wall adhesion, poor finished product properties, large particles, and poor uniformity.

[0018] (4) The amount of urea used is large, which not only increases the cost, but also solidifies fluorine, resulting in a high fluorine content in the product.

[0019] (5) The neutralization reaction needs to be carried out in the molten state, which makes it impossible to increase the yield, process a large amount of industrial ammonium slag, and achieve large-scale industrial production.

[0020] (6) A large amount of sulfuric acid will result in low nutrient (nitrogen and phosphorus) content in the product, which will limit its use as a raw material for fertilizer or compound fertilizer. Summary of the Invention

[0021] The purpose of this invention is to address the above-mentioned problems by upgrading and optimizing existing technologies, thereby increasing nutrient content and reducing the amount of sulfuric acid and urea used, making it more suitable for industrial production.

[0022] The main reaction process of this patent includes the following reaction formula: 2FeMg(NH4)2(HPO4)2F3+5H2SO4→Fe2(SO4)3+2MgSO4+4NH4H2PO4+6HF; 2AlMg(NH4)2(HPO4)2F3+5H2SO4→Al2(SO4)3+2MgSO4+4NH4H2PO4+6HF; H3PO4+ CH4N2O→H3PO4·CH4N2O; MgHPO4 + H2SO4 → MgSO4 + H3PO4; 5H3PO4·CH4N2O+2CH4N2O→(NH4PO3)5+5CO2+9NH3; HF + NH3 → NH4F.

[0023] The industrial ammonium slag mainly consists of a small amount of water-soluble (NH4)2HPO4 and NH4H2PO4, and a large amount of water-insoluble MgHPO4, FeMg(NH4)2(HPO4)2F3, AlMg(NH4)2(HPO4)2F3, CaSO4, and SiO2. Sulfuric acid is a strong acid, and during the reaction with the filter residue, it will displace the moderately strong acid phosphoric acid, generating substances such as MgSO4, (Fe,Al)2(SO4)3, (NH4)2SO4, CaSO4, and HF. Except for calcium sulfate, all of these substances are soluble.

[0024] After the acid-hydrolyzed slurry is neutralized with urea, some hydrogen fluoride may escape during the heating reaction, while the rest will react with ammonia produced from the partial decomposition of urea to form ammonium fluoride. Actual measurements show that the fluoride content in the filter cake is 6-7%, and the fluoride content in the finished powder is 2-5%. Furthermore, the higher the amount of urea added, the higher the fluoride content in the finished product. In this patent, urea not only acts as a neutralizing agent and ammoniating agent for the acid-hydrolyzed liquid, adjusting the pH of the slurry and generating urea phosphate, but also as a small-molecule condensing agent. In addition, it can absorb the moisture generated during the reaction and react with phosphoric acid in the system to form ammonium polyphosphate.

[0025] On one hand, the present invention discloses a polyphosphate-containing ammonium phosphate prepared from industrial ammonium slag as raw material. The ammonium phosphate is obtained by acid hydrolysis, neutralization, spray drying and high-temperature polymerization of industrial ammonium slag as raw material. The total nitrogen content of the polyphosphate-containing ammonium phosphate is 7.0-15.0%, the available phosphorus content is 23.0-38.0%, the water-soluble phosphorus content is 17.0-26.0%, the phosphorus polymerization rate is 3.0-50.0%, the water-soluble MgO content is 1.7-3.2%, and the MgO activation rate (water-soluble MgO / total MgO) is ≥50%. The product is a white to light gray fine powder.

[0026] The composition of the industrial ammonium slag, on a dry basis, is as follows: N content 3.0-5.5%, total P2O5 content 20-32%, MgO content 2-5%, Fe2O3 content 0.5-2.0%, and Al2O3 content 1-4%. Specifically, the composition of the industrial ammonium slag is shown in Table 1. Table 1. Main Components of Industrial Ammonium Slag (Dry Basis)

[0027] Note: The samples in this table were taken from Hubei Xiangyun (Group) Chemical Co., Ltd. between September 2025 and March 2026.

[0028] The source of industrial ammonium slag is as follows: wet phosphoric acid is prepared by reacting magnesium-demagnesium phosphate slurry with concentrated sulfuric acid; the wet phosphoric acid is neutralized with ammonia, solid and liquid are separated, and the filter cake is washed with water and dried to obtain industrial ammonium slag.

[0029] On the other hand, see Figure 1 This invention provides a method for preparing ammonium polyphosphate, the method comprising: S1 Pulping: The ammonium slag is mixed with water to form a slurry, the solid content of which is 55-75%.

[0030] S2 Acid hydrolysis: Add concentrated sulfuric acid to the slurry obtained in step S1 to react and obtain acid hydrolyzed slurry. The reaction temperature is 60-100℃, the amount of concentrated sulfuric acid is 20-35% of the dry basis mass of ammonium slag, and the reaction time is 1-3h.

[0031] S3 Neutralization: Add urea to the acid hydrolysis slurry obtained in step S2 and react to obtain a neutralized slurry. The reaction temperature is 80-95℃, the reaction time is 0.5-1.0h, the pH at the reaction endpoint is 0.8-1.4, and the amount of urea used is 5-12% of the dry basis mass of the ammonium slag.

[0032] S4 Spray Drying: The neutralized slurry obtained in step S3 is spray dried at an inlet air temperature of 120-250℃ and an outlet air temperature of 80-150℃. In this patent, spray drying serves several purposes: first, to dry moisture; second, to facilitate polymerization; third, to directly prepare the product into powder of suitable size (usually without the need for sieving and pulverizing); and fourth, to facilitate subsequent polymerization reactions (small, fluffy particles with a large surface area, virtually no moisture, good flowability, and a high temperature, which is beneficial for subsequent polymerization reactions).

[0033] S5 High-Temperature Polymerization: The powder obtained in step S4 is polymerized at 150-260℃ to obtain ammonium phosphate containing polyphosphate, with a polymerization time of 0.1-2.0 h. This process is used to further increase the degree of polymerization.

[0034] The parameters of each raw material are shown in Table 2: Table 2 .

[0035] Preferably, the mass concentration of concentrated phosphoric acid is ≥98%.

[0036] The reaction conditions are shown in Table 3. Table 3

[0037] Preferably, in step S2, the reaction temperature is 75-85℃, the amount of concentrated sulfuric acid used is 25-35% of the dry basis mass of the ammonium slag, and the reaction time is 1.0-2.0h.

[0038] Preferably, in step S3, the reaction temperature is 80-85℃, the amount of urea used is 6-8% of the dry basis mass of the ammonium slag, the reaction time is 0.5-1.0h, and the final pH of the reaction is 0.8-1.2.

[0039] Preferably, in step S4, a centrifugal spray dryer is used for spray drying; the inlet air temperature is 150-220℃, the outlet air temperature is 80-120℃, the atomizer frequency is 300-400Hz, and the slurry temperature is 75-90℃.

[0040] Preferably, in step S5, a rotary kiln is used for high-temperature polymerization; the polymerization temperature is 150-180℃ and the polymerization time is 0.1-1.5h.

[0041] Preferably, embodiments of the present invention provide a method for preparing ammonium phosphate containing polyphosphate, the method comprising: S1 Pulping: The ammonium slag is mixed with water to form a slurry, the solid content of which is 55-75%.

[0042] S2 Pulping: Slowly add concentrated sulfuric acid to the pulp obtained in step S1 to react and obtain acid hydrolyzed pulp. The reaction temperature is 75-85℃, the amount of concentrated sulfuric acid is 25-35% of the dry weight of the ammonium slag, and the reaction time is 1.0-2.0h.

[0043] S3 Neutralization: Urea is added to the acid hydrolysis slurry obtained in step S2, and a neutralized slurry is obtained by reaction. The reaction temperature is 80-85℃, the amount of urea is 6-8% of the dry basis mass of the ammonium slag, the reaction time is 0.5-1.0h, and the final pH of the reaction is 0.8-1.2.

[0044] S4 Spray Drying: The neutralized slurry obtained in step S3 is spray dried using a centrifugal spray dryer; the inlet air temperature is 150-220℃, the outlet air temperature is 80-120℃, the atomizer frequency is 300-400Hz, and the slurry temperature is 75-90℃.

[0045] S5 High-temperature polymerization: The powder obtained in step S4 is sent to a rotary kiln for high-temperature polymerization to obtain ammonium phosphate containing polyphosphate. The polymerization temperature is 150-180℃ and the polymerization time is 0.1-1.5h.

[0046] Table 4 shows a comparison between this patent and existing technologies: Table 4 .

[0047] As can be seen from Table 4, this patent differs substantially from existing technologies in four aspects: the triggering method of polymerization reaction, the drying process path, the core process parameters (sulfuric acid / urea dosage), and the product form and function. Moreover, these differences are not simple process substitutions or parameter optimizations, but rather bring about good technical effects.

[0048] On the other hand, see Figure 2 This invention also provides a preparation system for ammonium polyphosphate, comprising: Slurry mixing tank: used to mix ammonium slag with water to prepare slurry.

[0049] Acidification reactor: Used to react slurry from the mixing tank with concentrated sulfuric acid to obtain acid-hydrolyzed slurry.

[0050] Neutralization reactor: Used to react the acid hydrolysis slurry from the acidification reactor with urea to obtain a neutralized slurry.

[0051] Insulated feeding tank: Used to temporarily store and keep warm the neutralization slurry from the neutralization reactor.

[0052] Centrifugal spray dryer: used for spray drying of slurry from an insulated feed tank.

[0053] Rotary kiln: Used for high-temperature polymerization of powders from centrifugal spray dryers.

[0054] Cooling device: Used to cool the powder material from the rotary kiln.

[0055] Screening structures can also be installed as needed to screen the cooled powder. The various structures are connected by pipelines, conveyor belts, or chutes.

[0056] The present invention has the following advantages: (1) Suitable for the new process of ammonium slag; (2) The amount of sulfuric acid and urea used is small, resulting in low cost; (3) Low production difficulty, low sulfuric acid consumption, which reduces the viscosity of the slurry; no melting is required. (4) The product has high nutrient content, good water solubility, and uniform particle size; (5) Enables continuous production, suitable for industrial production; (6) Spray drying yields powder with low moisture content (≤2%) and uniform particle size. The product has stable moisture content, which is conducive to long-term storage and makes subsequent polymerization reactions more uniform and controllable. (7) The degree of polymerization can be controlled in the high-temperature polymerization process, offering high flexibility. The polymerization rate is adjustable within a wide range of 5-50%, and products can be customized according to different crop needs and different fertilizer effect periods; (8) The product can be used directly as fertilizer raw material without granulation, saving energy consumption in the granulation process. Attached Figure Description

[0057] Figure 1 This is a flowchart of the preparation method of ammonium polyphosphate provided in the embodiments of the present invention; Figure 2 This is a schematic diagram of the preparation system containing ammonium polyphosphate provided in the embodiments of the present invention; Figure 3 This is a picture of a product containing ammonium polyphosphate. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0059] Example 1 See Figure 1 Example 1 provides a method for preparing ammonium polyphosphate, the method comprising: S1 Pulping: In the slurry preparation tank, ammonium slag and water are mixed to prepare slurry with a solid content of 55-75%.

[0060] S2 Pulping: In the acidification reactor, concentrated sulfuric acid is slowly added to the slurry obtained in step S1 to react and obtain acid hydrolysate. The reaction temperature is 75-85℃ and the reaction time is 1.0-2.0h. The amount of concentrated sulfuric acid used is 25-35% of the dry weight of the ammonium slag.

[0061] S3 Neutralization: In the neutralization reactor, urea is added to the acid hydrolysis slurry obtained in step S2, and the reaction is carried out to obtain a neutralized slurry. The reaction temperature is 80-85℃, the reaction time is 0.5-1.0h, the final pH of the reaction is 0.8-1.2, and the amount of urea used is 6-8% of the dry basis mass of the ammonium slag.

[0062] S4 Spray Drying: The neutralized slurry obtained in step S3 is spray dried using a centrifugal spray dryer; the inlet air temperature is 150-220℃, the outlet air temperature is 80-120℃, the atomizer frequency is 300-400Hz, and the slurry temperature is 75-90℃.

[0063] S5 High-temperature polymerization: The powder obtained in step S4 is sent to a rotary kiln for high-temperature polymerization to obtain ammonium phosphate containing polyphosphate. The polymerization temperature is 150-180℃ and the polymerization time is 0.1-1.5h.

[0064] S6 Cooling and sieving: The powder obtained in step S5 is cooled and sieved using a cooling device and a sieving device.

[0065] S7 Packaging; The powder obtained in step S6 is packaged.

[0066] The prepared ammonium phosphate containing polyphosphate is a light gray fine powder, and its parameters are shown in Table 5. Table 5 .

[0067] As shown in Table 5, the ammonium phosphate containing polyphosphate prepared by this patent has a high nutrient content, typically greater than 35%, and can be used directly as a fertilizer or as a fertilizer raw material. It exhibits a high magnesium activation rate, exceeding 50%; the product particles are uniform and free of lumps; and it has a low sulfur content.

[0068] Example 2 (35% sulfuric acid, 6% urea) Take 1000g of ammonium slag (dry basis, see Table 1, item 1 for specific parameters), add 400g of water, and stir to prepare a uniform slurry. Add 350g of 98% sulfuric acid, stir evenly, and heat and stir at 85℃ for 1 hour. Add 60g of urea, and neutralize at 85℃ for 0.5 hours until pH=0.86. Spray dry (inlet air 160℃, outlet air 88℃, atomization frequency conversion 300Hz), pumping the neutralized slurry into the feed inlet of the spray dryer through a peristaltic pump to obtain the dried finished powder. Preheat the drying chamber to 180℃, and place the obtained spray-dried powder in the drying chamber for a high-temperature polymerization reaction for 1 hour.

[0069] The slurry has low viscosity throughout the process, resulting in uniform particle size in the final product.

[0070] Dry finished product specifications: N: 7.57%; available P2O5: 30.07%; water-soluble P2O5: 24.75%; water-soluble phosphorus content: 82.3%; phosphorus polymerization rate: 3.36%; total MgO: 3.96%; water-soluble MgO: 2.23%.

[0071] Polymer product specifications: N: 8.23%; available P2O5: 32.63%; water-soluble P2O5: 23.81%; water-soluble phosphorus content: 72.97%; phosphorus polymerization rate: 46.40%.

[0072] When the product of Example 1 is dissolved in water, it contains a small amount of insoluble matter, forming a milky white turbid liquid. However, its solubility is better than that of the ammonium slag raw material and the prior art (CN201910598673.4).

[0073] Example 3 (25% sulfuric acid, 6% urea) Take 1000g of ammonium slag (dry basis, see Table 1, item 1 for specific parameters), add 600g of water, and stir to prepare a uniform slurry. Add 270g of 98% sulfuric acid, stir evenly, and heat and stir at 75℃ for 1 hour. Add 60g of urea, and neutralize at 80℃ for 0.5 hours until pH=1.0. Spray dry (inlet air 200℃, outlet air 117℃, atomization frequency conversion 320Hz). Pump the neutralized slurry into the feed inlet of the spray dryer using a peristaltic pump for spray drying to obtain the dried finished powder. Preheat the rotary kiln to raise the kiln temperature to 150-180℃. The measured front end temperature was 126℃, and the middle end temperature was 156℃. Pour the obtained spray-dried powder into the rotary kiln for high-temperature polymerization reaction, maintaining rotation and heating. After about 10 minutes, the polymerized powder is discharged.

[0074] The slurry has low viscosity throughout the process, resulting in uniform particle size in the final product.

[0075] Dry finished product specifications: N: 7.6%; available P2O5: 36.84%; water-soluble P2O5: 26.89%; water-soluble phosphorus content: 73%; phosphorus polymerization rate: 34%; total MgO: 5.8%; water-soluble MgO: 3.70%.

[0076] Polymer product specifications: N: 8.39%; available P2O5: 36.94%; water-soluble P2O5: 25.90%; water-soluble phosphorus content: 70.1%; phosphorus polymerization rate: 46%; total MgO: 5.86%; water-soluble MgO: 3.13%.

[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A polyphosphate-containing ammonium phosphate prepared from industrial ammonium slag, characterized in that, It is obtained by acid hydrolysis, neutralization, spray drying and high-temperature polymerization using industrial ammonium phosphate slag as raw material; the total nitrogen content of the polyphosphate ammonium phosphate is 7.0-15.0%, the available phosphorus content is 23.0-38.0%, the water-soluble phosphorus content is 17.0-26.0%, the phosphorus polymerization rate is 3.0-50.0%, the water-soluble MgO content is 1.7-3.2%, and the MgO activation rate is ≥50%.

2. The ammonium phosphate containing polyphosphate prepared from industrial ammonium slag according to claim 1, characterized in that, The composition of the ammonium slag, on a dry basis, is as follows: N content is 3.0-5.5%, total P2O5 content is 20-32%, MgO content is 2-5%, Fe2O3 content is 0.5-2.0%, and Al2O3 content is 1-4%.

3. The ammonium phosphate containing polyphosphate prepared from industrial ammonium slag according to claim 1, characterized in that, The source of the industrial ammonium slag is as follows: wet phosphoric acid is prepared by reacting magnesium-removed phosphate slurry with concentrated sulfuric acid; the wet phosphoric acid is neutralized with ammonia, and the solid and liquid are separated. The filter cake is washed with water and dried to obtain industrial ammonium slag.

4. The method for preparing ammonium polyphosphate as described in any one of claims 1-3, characterized in that, The method includes: S1 Pulping: The ammonium slag is mixed with water to form a slurry, the solid content of which is 55-75%; S2 Acidolysis: Concentrated sulfuric acid is added to the slurry obtained in step S1, and the reaction is carried out to obtain acid-hydrolyzed slurry. The reaction temperature is 60-100℃, and the amount of concentrated sulfuric acid used is 20-35% of the dry weight of the ammonium slag. S3 Neutralization: Urea is added to the acid hydrolysis slurry obtained in step S2, and a neutralized slurry is obtained by reaction. The reaction temperature is 80-95℃, and the amount of urea used is 5-12% of the dry weight of the ammonium slag. S4 Spray drying: The neutralized slurry obtained in step S3 is spray dried with an inlet air temperature of 120-250℃ and an outlet air temperature of 80-150℃. S5 High-temperature polymerization: The powder obtained in step S4 is polymerized at a high temperature of 150-260℃ to obtain ammonium phosphate containing polyphosphate.

5. The method according to claim 4, characterized in that, In step S2, the reaction temperature is 75-85℃, the amount of concentrated sulfuric acid used is 25-35% of the dry basis mass of the ammonium slag, and the reaction time is 1.0-2.0h.

6. The method according to claim 4, characterized in that, In step S3, the reaction temperature is 80-85℃, the amount of urea used is 6-8% of the dry basis mass of the ammonium slag, the reaction time is 0.5-1.0h, and the final pH of the reaction is 0.8-1.

2.

7. The method according to claim 4, characterized in that, In step S4, a centrifugal spray dryer is used for spray drying; the inlet air temperature is 150-220℃, the outlet air temperature is 80-120℃, the atomizer frequency is 300-400Hz, and the slurry temperature is 75-90℃.

8. The method according to claim 4, characterized in that, In step S5, a rotary kiln is used for high-temperature polymerization; the polymerization temperature is 150-180℃ and the polymerization time is 0.1-1.5h.

9. The method according to claim 4, characterized in that, The method includes: S1 Pulping: The ammonium slag is mixed with water to form a slurry, the solid content of which is 55-75%; S2 Pulping: Slowly add concentrated sulfuric acid to the pulp obtained in step S1 to react and obtain acid hydrolysis pulp. The reaction temperature is 75-85℃, the amount of concentrated sulfuric acid is 25-35% of the dry weight of ammonium slag, and the reaction time is 1.0-2.0h. S3 Neutralization: Urea is added to the acid hydrolysis slurry obtained in step S2, and a neutralized slurry is obtained by reaction. The reaction temperature is 80-85℃, the amount of urea is 6-8% of the dry basis mass of the ammonium slag, the reaction time is 0.5-1.0h, and the final pH of the reaction is 0.8-1.

2. S4 Spray Drying: The neutralized slurry obtained in step S3 is spray dried using a centrifugal spray dryer; the inlet air temperature is 150-220℃, the outlet air temperature is 80-120℃, the atomizer frequency is 300-400Hz, and the slurry temperature is 75-90℃. S5 High-temperature polymerization: The powder obtained in step S4 is sent to a rotary kiln for high-temperature polymerization to obtain ammonium phosphate containing polyphosphate. The polymerization temperature is 150-180℃ and the polymerization time is 0.1-1.5h.

10. The preparation system for ammonium polyphosphate as described in any one of claims 1-3, characterized in that, The system includes: Slurry mixing tank: used to mix ammonium slag with water to prepare slurry; Acidification reactor: used to react slurry from the slurry preparation tank with concentrated sulfuric acid to obtain acid-hydrolyzed slurry; Neutralization reactor: Used to react the acid hydrolysis slurry from the acidification reactor with urea to obtain a neutralized slurry; Insulated feeding tank: used to temporarily store and keep warm the neutralization slurry from the neutralization reactor; Centrifugal spray dryer: used for spray drying of slurry from an insulated feed tank; Rotary kiln: used for high-temperature polymerization of powders from centrifugal spray dryers; Cooling device: Used to cool the powder material from the rotary kiln.

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