System and method for producing superphosphoric acid

Abstract

The invention discloses a system and a method for producing superphosphoric acid, and belongs to the field of chemical engineering. The system comprises an evaporation flash crystallizer, a condensate water collecting tank, a circulating phosphoric acid pump, a superphosphoric acid delivery pump, a gas condenser, a vacuum pump and a water return tank. By adopting the system and the method, medium-pressure saturated steam is used for indirectly heating phosphoric acid to produce superphosphoric acid. The device is simple in technological process, convenient to install and small in occupied area.

Description

Technical Field

[0001] The invention relates to the technical field of chemical production, and in particular to a system and method for producing superphosphoric acid. Background Art

[0002] Wet-process phosphoric acid contains a large amount of impurities. During transportation and storage, these impurities will precipitate, causing a lot of trouble and causing the loss of P2O5. If the wet-process phosphoric acid is further concentrated to make superphosphoric acid, the polyphosphoric acid in it can chelate these impurities so that they will no longer precipitate, and the product is more suitable for transportation and storage. The mass concentration of P2O5 in the phosphoric acid solution is ≥68%, which is called superphosphoric acid. Superphosphoric acid is less corrosive than orthophosphoric acid, and low-carbon steel also has good corrosion resistance to superphosphoric acid at room temperature. When using superphosphoric acid to produce liquid fertilizer, since the polyphosphoric acid in it can chelate impurities in the acid, it can make them exist in the solution, and the solubility of polyphosphate is relatively high, so superphosphoric acid can be used to produce liquid fertilizer with higher concentration. Adding a small amount of superphosphoric acid when producing granular compound fertilizer can reduce the return ratio, reduce the amount of dust in the system, and make the particles harder and more wear-resistant. High-concentration superphosphoric acid can be used as a catalyst and dehydrating agent in the organic chemical industry.

[0003] There are usually two methods for industrial production of superphosphoric acid:

[0004] Method 1: Phosphorus ore is melted in an electric furnace, and a reduction reaction is carried out with carbon to release a mixed gas of phosphorus (P4) and carbon monoxide (CO). The phosphorus vapor is condensed into yellow phosphorus, and the phosphorus pentoxide (P2O5) produced by the combustion of yellow phosphorus is absorbed by water to obtain superphosphoric acid.

[0005] Method 2: Wet-process phosphoric acid with a phosphorus pentoxide (P2O5) mass concentration of ≥40% is subjected to multi-stage concentration and dehydration to finally obtain superphosphoric acid with a phosphorus pentoxide (P2O5) mass concentration of ≥68%.

[0006] Method 1 has high energy consumption and serious pollution, while method 2 has a long process, many equipments and the heat exchanger is prone to scaling.

[0007] Therefore, it is necessary to improve the existing superphosphoric acid production system, and it is urgent to provide a system and method with low energy consumption, low pollution, short process, less equipment, and heat exchanger that is not easy to scale. Summary of the invention

[0008] The purpose of the present invention is to provide a system and method for producing superphosphoric acid, wherein phosphoric acid from a wet-process phosphoric acid production system is evaporated, flashed, and crystallized in an evaporation flash crystallizer. Unlike the past, the evaporation flash crystallizer is an integrated device. With this system and method, superphosphoric acid can be produced with only one-stage concentration and dehydration. It is a system and method with low energy consumption, low pollution, short process, few equipment, and heat exchanger that is not easy to scale. This patent aims to save energy and reduce consumption, reduce investment in production equipment, reduce carbon emissions, and protect the environment.

[0009] The purpose of this patent can be achieved through the following technical solutions:

[0010] A system for producing superphosphoric acid comprises an evaporation flash crystallizer, a raw phosphoric acid output pipeline is connected to the top of the evaporation flash crystallizer, a gas output end in the middle of the evaporation flash crystallizer is connected to a gas condenser, and the output at the bottom of the evaporation flash crystallizer is a target product.

[0011] In the above system: the bottom of the evaporation flash crystallizer is connected to the top of the evaporation flash crystallizer through a circulating phosphoric acid pump.

[0012] In the above system: the upper part of the evaporative flash crystallizer is the evaporator, the middle part is the flash chamber, and the lower part is the crystallizer.

[0013] A method for producing superphosphoric acid using the above system is provided, wherein the phosphoric acid from the boundary zone and the circulating phosphoric acid are transported to an evaporation flash crystallizer, and evaporation, flash evaporation and crystallization are performed in the evaporation flash crystallizer in sequence to obtain the target product.

[0014] In the above method, saturated steam from the boundary zone exchanges heat with phosphoric acid in the tube side, and the phosphoric acid after heat exchange enters the flash chamber for flash evaporation. The material after flash evaporation contains superphosphoric acid, and the material after flash evaporation is transported to the crystallizer. A part of the superphosphoric acid in the crystallizer enters the evaporator as circulating phosphoric acid, and the other part of the superphosphoric acid is used as the target product.

[0015] In the above method: the mass concentration of P2O5 in the circulating phosphoric acid is ≤80%, and the temperature is ≤210°C; the mass concentration of P2O5 in the phosphoric acid in the critical zone is ≤62%, and the temperature is 20-40°C.

[0016] In the above method: the concentration of hot phosphoric acid in the flash chamber is 68% to 80% by mass of P2O5.

[0017] In the above method: the mass concentration of P2O5 in the target product superphosphoric acid is 68% to 80%, and the temperature is 190°C to 210°C.

[0018] In the above method: the mass ratio of circulating phosphoric acid to target product superphosphoric acid is ≥50:1.

[0019] In some more specific technical solutions, the system and method described in the present invention are as follows:

[0020] A system for producing superphosphoric acid comprises an evaporation flash crystallizer, an evaporator, a flash chamber, a crystallizer, a condensed water collecting tank, a circulating phosphoric acid pump, a superphosphoric acid delivery pump, a gas condenser, a vacuum pump, and a water return tank. The phosphoric acid pipeline, which is a raw material from the boundary zone, is connected to the phosphoric acid inlet A at the top of the evaporator at the upper part of the evaporation flash crystallizer, and the superphosphoric acid outlet J at the lower part of the crystallizer at the lower part of the evaporation flash crystallizer is connected to the superphosphoric acid pipeline, which is a target product of the deboundary zone, through the superphosphoric acid delivery pump.

[0021] In the above system: the circulating phosphoric acid outlet M in the middle of the lower crystallizer of the evaporation flash crystallizer is connected to the raw material from the boundary area - phosphoric acid pipeline through a circulating phosphoric acid pump.

[0022] In the above system: the medium-pressure steam from the boundary zone is connected to the upper steam inlet C of the upper evaporator of the evaporation flash crystallizer, and the lower condensate outlet D of the upper evaporator of the evaporation flash crystallizer is connected to the condensate pipeline of the de-boundary zone through a condensate collecting tank.

[0023] In the above system: the upper gas phase outlet F of the flash chamber in the middle of the evaporative flash crystallizer is connected to the lower gas phase inlet C of the gas condenser, the non-condensable gas outlet A at the top of the gas condenser is connected to the non-condensable gas pipeline of the de-boundary zone through a vacuum pump, the upper circulating cooling water inlet B of the gas condenser is connected to the circulating cooling water inlet pipeline from the de-boundary zone, and the bottom circulating cooling return water outlet D of the gas condenser is connected to the circulating cooling return water pipeline of the de-boundary zone through a return water tank.

[0024] A method for producing superphosphoric acid using the system of claim 1, the method comprising the following steps:

[0025] (1) Phosphoric acid feeding and evaporation: Saturated steam (temperature ≤ 240°C, pressure ≤ 3.41 MPa) from the boundary zone enters the evaporator shell side from the upper steam inlet C of the evaporator at the top of the evaporator flash crystallizer, and exchanges heat with the circulating phosphoric acid in the tube side; the saturated steam is cooled into condensed water, goes to the condensed water collection tank from the lower condensed water outlet D of the evaporator, and returns to the boundary zone from the bottom of the condensed water collection tank.

[0026] The circulating phosphoric acid (containing P2O5 mass concentration ≤80%, temperature ≤210°C) from the circulating phosphoric acid pump and the raw material from the boundary zone - phosphoric acid (containing P2O5 mass concentration ≤62%, temperature at room temperature) are mixed and enter the evaporator from the phosphoric acid inlet A at the top of the evaporator at the upper part of the evaporator-flash crystallizer, and enter the evaporator tube side from the evaporator tube side inlet B. The phosphoric acid flows down along the inner wall of the tube in the form of a film and exchanges heat with the saturated steam in the shell side; the phosphoric acid in the tube side is heated (temperature ≤230°C), and the heated phosphoric acid flows into the flash chamber below the evaporator from the connecting port E between the evaporator and the flash chamber.

[0027] (2) Flash evaporation and dehydration of hot phosphoric acid: The hot phosphoric acid from the upper evaporator of the evaporative flash crystallizer is flashed in the flash chamber. When the gas phase pressure is lower than the atmospheric pressure (≤100 kPa), a part of the water is removed, the concentration of the hot phosphoric acid is increased (containing P2O5 mass concentration ≤80%), and part of the orthophosphoric acid is dehydrated to obtain superphosphoric acid. At the same time, the temperature is reduced (temperature ≤210°C) and flows into the crystallizer below the flash chamber from the bottom outlet H of the flash chamber.

[0028] The gas coming out of the gas phase outlet F at the upper part of the flash chamber enters the lower part of the gas condenser, and the circulating cooling water (temperature ≤30℃) from the boundary zone enters the upper part of the gas condenser to cool the gas to ≤40℃ and remove the moisture therein. The circulating cooling return water and the condensate in the gas go to the return water tank from the bottom outlet D of the gas condenser, and then go to the boundary zone from the return water tank. The return water temperature is ≤40℃; the clean non-condensable gas coming out of the non-condensable gas outlet A at the top of the gas condenser goes to the vacuum pump and is sent to the boundary zone by the vacuum pump.

[0029] (3) Superphosphoric acid crystallization, circulation and discharge: Superphosphoric acid from the flash chamber in the middle of the evaporative flash crystallizer enters the crystallizer from the bottom outlet K. A portion of the superphosphoric acid in the crystallizer is used as circulating phosphoric acid and is mixed with the raw material, phosphoric acid, from the circulating phosphoric acid outlet M in the middle of the crystallizer through a circulating phosphoric acid pump. The mixture is then fed into the evaporator through the phosphoric acid inlet A at the top of the evaporator in the evaporative flash crystallizer. The other portion of the superphosphoric acid is used as the target product, superphosphoric acid (containing a P2O5 mass concentration of ≤80% and a temperature of ≤210°C), and is discharged from the superphosphoric acid outlet J at the bottom of the crystallizer through a superphosphoric acid delivery pump to the boundary area. The mass ratio of the circulating phosphoric acid to the target product, superphosphoric acid, is ≥50:1.

[0030] In some more specific technical solutions, a system and method for producing superphosphoric acid includes mixed feeding of raw materials - phosphoric acid and circulating phosphoric acid, indirect heat exchange between phosphoric acid and saturated steam, evaporation of hot phosphoric acid under negative pressure, dehydration, non-condensable gas washing, and treatment of superphosphoric acid, characterized in that the production process includes: phosphoric acid feeding, evaporation operation, hot phosphoric acid flash evaporation, dehydration operation, superphosphoric acid crystallization, circulation, and discharge operation, the steps are as follows:

[0031] (1) Phosphoric acid feeding and evaporation: Saturated steam (temperature 220℃~240℃, pressure 2.37MPa~3.41MPa) from the boundary zone enters the evaporator shell side from the upper steam inlet C of the evaporator at the upper part of the evaporator flash crystallizer, and exchanges heat with the circulating phosphoric acid in the tube side; the saturated steam is cooled into condensed water, goes to the condensed water collecting tank from the lower condensed water outlet D of the evaporator, and returns to the boundary zone from the bottom of the condensed water collecting tank.

[0032] The circulating phosphoric acid (containing 68% to 80% P2O5 mass concentration and 190°C to 210°C) from the circulating phosphoric acid pump and the raw material from the boundary zone - phosphoric acid (containing 40% to 62% P2O5 mass concentration and room temperature) are mixed and enter the evaporator from the phosphoric acid inlet A at the top of the evaporator at the upper part of the evaporation flash crystallizer, and enter the evaporator tube side from the evaporator tube side inlet B. The phosphoric acid flows down along the inner wall of the tube in the form of a film and exchanges heat with the saturated steam in the shell side; the phosphoric acid in the tube side is heated (temperature 210°C to 230°C), and the heated phosphoric acid flows into the flash chamber below the evaporator from the connecting port E between the evaporator and the flash chamber.

[0033] (2) Flash evaporation and dehydration of hot phosphoric acid: The hot phosphoric acid from the upper evaporator of the evaporative flash crystallizer is flashed in the flash chamber. When the gas phase pressure is lower than the atmospheric pressure (100 kPa to 90 kPa), part of the water is removed, the concentration of the hot phosphoric acid is increased (containing P2O5 mass concentration of 68% to 80%), and part of the orthophosphoric acid is dehydrated to obtain superphosphoric acid. At the same time, the temperature is reduced (temperature 190°C to 210°C) and flows into the crystallizer below the flash chamber from the outlet H at the bottom of the flash chamber.

[0034] The gas coming out of the gas phase outlet F at the upper part of the flash chamber enters the lower part of the gas condenser, and the circulating cooling water (temperature 0℃~30℃) from the boundary zone enters the upper part of the gas condenser to cool the gas to 10℃~40℃ and remove the moisture therein. The circulating cooling return water and the condensate in the gas go to the return water tank from the bottom outlet D of the gas condenser, and then go to the boundary zone from the return water tank. The return water temperature is 10℃~40℃; the clean non-condensable gas coming out of the non-condensable gas outlet A at the top of the gas condenser goes to the vacuum pump and is sent to the boundary zone by the vacuum pump.

[0035] (3) Superphosphoric acid crystallization, circulation, and discharge: Superphosphoric acid from the flash chamber in the middle of the evaporative flash crystallizer enters the crystallizer from the bottom outlet K, and a portion of the superphosphoric acid in the crystallizer is mixed with the raw material, phosphoric acid, from the circulating phosphoric acid outlet M in the middle of the crystallizer through the circulating phosphoric acid pump and then enters the evaporator through the phosphoric acid inlet A at the top of the evaporator in the evaporative flash crystallizer. The other portion of the superphosphoric acid is used as the target product, superphosphoric acid (containing a P2O5 mass concentration of 68% to 80% and a temperature of 190°C to 210°C), and is discharged from the superphosphoric acid outlet J at the bottom of the crystallizer through the superphosphoric acid delivery pump to the boundary area. The mass ratio of the circulating phosphoric acid to the target product, superphosphoric acid, is 50 to 100:1. Beneficial effects of the present invention:

[0036] The energy consumption of producing superphosphoric acid with wet phosphoric acid is about 20% lower than that of producing superphosphoric acid with thermal phosphoric acid, and the production cost is only 70% to 80% of that of thermal phosphoric acid. When the medium-pressure steam produced as a byproduct of the sulfuric acid unit is available, the first-stage concentration and dehydration are adopted, and the evaporation, flash evaporation and crystallization of phosphoric acid are completed in the integrated equipment of the evaporation flash crystallizer. This system can be used to concentrate dilute phosphoric acid and produce superphosphoric acid. It is a system and method with a short process, small footprint, few equipment, wide application, and heat exchanger that is not easy to scale. This patent aims to save energy and reduce consumption, reduce investment in production equipment, reduce carbon emissions, and protect the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is the process flow diagram of this patent

[0038] Among them, there are evaporation flash crystallizer 1, evaporator 1-1, flash chamber 1-2, crystallizer 1-3, condensate collecting tank 2, circulating phosphoric acid pump 3, superphosphoric acid delivery pump 4, gas condenser 5, vacuum pump 6, and return water tank 7. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present patent to clearly and completely describe the technical solutions in the embodiments of the present patent. Obviously, the described embodiments are part of the embodiments of the present patent, not all of the embodiments. The components of the embodiments of the present patent described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present patent provided in the drawings is not intended to limit the scope of the patent claimed for protection, but only represents the selected embodiments of the present patent. Based on the embodiments in this patent, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this patent. The embodiments of the present patent are described in detail below, and examples of the embodiments are shown in the drawings, where the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below by reference to the drawings are exemplary and are intended to be used to explain the present patent, and cannot be understood as limitations on the present patent.

[0040] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0041] like Figure 1A system for producing superphosphoric acid, characterized in that the system comprises an evaporation flash crystallizer 1, an evaporator 1-1, a flash chamber 1-2, a crystallizer 1-3, a condensed water collecting tank 2, a circulating phosphoric acid pump 3, a superphosphoric acid delivery pump 4, a gas condenser 5, a vacuum pump 6, and a return water tank 7. The phosphoric acid pipeline, which is a raw material from the boundary zone, is connected to the phosphoric acid inlet A at the top of the evaporator 1-1 at the upper part of the evaporation flash crystallizer 1, and the superphosphoric acid outlet J at the lower part of the crystallizer 1-3 at the lower part of the evaporation flash crystallizer 1 is connected to the superphosphoric acid pipeline, which is a target product of the deboundary zone, through the superphosphoric acid delivery pump 4.

[0042] The circulating phosphoric acid outlet M in the middle of the lower crystallizer 1-3 of the evaporation flash crystallizer 1 is connected to the raw material from the boundary area - the phosphoric acid pipeline through the circulating phosphoric acid pump 3.

[0043] The medium-pressure steam from the boundary zone is connected to the upper steam inlet C of the upper evaporator 1-1 of the evaporation flash crystallizer 1, and the lower condensate outlet D of the upper evaporator 1-1 of the evaporation flash crystallizer 1 is connected to the condensate pipeline of the boundary zone through the condensate collecting tank 2.

[0044] The upper gas phase outlet F of the flash chamber 1-2 in the middle of the evaporative flash crystallizer 1 is connected to the lower gas phase inlet C of the gas condenser 5, the non-condensable gas outlet A at the top of the gas condenser 5 is connected to the non-condensable gas pipeline of the de-boundary zone through a vacuum pump 6, the upper circulating cooling water inlet B of the gas condenser 5 is connected to the circulating cooling water pipeline from the de-boundary zone, and the circulating cooling return water outlet D at the bottom of the gas condenser 5 is connected to the circulating cooling return water pipeline of the de-boundary zone through a return water tank 7.

[0045] The method for producing superphosphoric acid using the above system comprises the following steps:

[0046] (1) Phosphoric acid feeding and evaporation: Saturated steam (temperature 232°C, pressure 2.96 MPa) from the boundary zone enters the shell side of evaporator 1-1 from the upper steam inlet C of evaporator 1-1 at the upper part of evaporation flash crystallizer 1, and exchanges heat with the circulating phosphoric acid in the tube side; the saturated steam is cooled into condensed water, goes to condensed water collecting tank 2 from the lower condensed water outlet D of evaporator 1-1, and returns to the boundary zone from the bottom of condensed water collecting tank 2.

[0047] The circulating phosphoric acid (containing P2O5 mass concentration of 76% and temperature of 199°C) from the circulating phosphoric acid pump 3 and the raw material from the boundary zone, namely phosphoric acid (containing P2O5 mass concentration of 54% and temperature of room temperature), are mixed and enter the evaporator 1-1 from the phosphoric acid inlet A at the top of the evaporator 1-1 at the upper part of the evaporator-flash crystallizer 1, and enter the tube side of the evaporator 1-1 from the tube side inlet B of the evaporator 1-1. The phosphoric acid flows down along the inner wall of the tube in the form of a film and exchanges heat with the saturated steam in the shell side. The phosphoric acid in the tube side is heated (temperature of 216°C), and the heated phosphoric acid flows into the flash chamber 1-2 below the evaporator 1-1 from the connecting port E between the evaporator 1-1 and the flash chamber 1-2.

[0048] (2) Flash evaporation and dehydration of hot phosphoric acid: The hot phosphoric acid from the upper evaporator 1-1 of the evaporative flash crystallizer 1 is flashed in the flash chamber 1-2. When the gas phase pressure is lower than the atmospheric pressure (93.5 kPa), a part of the water is removed, the concentration of the hot phosphoric acid is increased (containing 76% by mass concentration of P2O5), and a part of the orthophosphoric acid is dehydrated to obtain superphosphoric acid. At the same time, the temperature is reduced (temperature 199°C) and flows into the crystallizer 1-3 below the flash chamber 1-2 from the bottom outlet H of the flash chamber 1-2.

[0049] The gas coming out from the gas phase outlet F at the upper part of the flash chamber 1-2 enters the lower part of the gas condenser 5, and the circulating cooling water (temperature 25°C) from the boundary zone enters the upper part of the gas condenser 5 to cool the gas to 40°C and remove the moisture therein, and the circulating cooling return water and the condensate in the gas go to the return water tank 7 from the bottom outlet D of the gas condenser 5, and go to the boundary zone from the return water tank 7, and the return water temperature is 35°C; the clean non-condensable gas coming out from the non-condensable gas outlet A at the top of the gas condenser 5 goes to the vacuum pump 6, and is sent to the boundary zone by the vacuum pump 6.

[0050] (3) Superphosphoric acid crystallization, circulation and discharge: Superphosphoric acid from the flash chamber 1-2 in the middle of the evaporative flash crystallizer 1 enters the crystallizer 1-3 from the bottom outlet K. A portion of the superphosphoric acid in the crystallizer 1-3 is used as circulating phosphoric acid and is mixed with the raw material, phosphoric acid, from the circulating phosphoric acid outlet M in the middle of the crystallizer 1-3 through the circulating phosphoric acid pump 3. The mixture is then mixed with the raw material, phosphoric acid, and enters the evaporator 1-1 through the phosphoric acid inlet A at the top of the evaporator 1-1 in the evaporative flash crystallizer 1. Another portion of the superphosphoric acid is used as the target product, superphosphoric acid (containing a P2O5 mass concentration of 76% and a temperature of 199°C), and is discharged from the superphosphoric acid outlet J at the bottom of the crystallizer 1-3 through the superphosphoric acid delivery pump 4 to the boundary zone. The mass ratio of the circulating phosphoric acid to the target product, superphosphoric acid, is 80:1.

[0051] The results of the patent operation are shown in Table 1.

[0052] Table 1 Performance evaluation results of 100 tons / day superphosphoric acid unit

[0053]

[0054]

Claims

1. A system for producing superphosphoric acid, characterized in that: The system comprises an evaporative flash crystallizer (1), a raw material phosphoric acid output pipeline is connected to the top of the evaporative flash crystallizer (1), a gas output end in the middle of the evaporative flash crystallizer (1) is connected to a gas condenser (5), and the output at the bottom of the evaporative flash crystallizer (1) is a target product.

2. The system for producing superphosphoric acid according to claim 1, characterized in that: The bottom of the evaporation flash crystallizer (1) is connected to the top of the evaporation flash crystallizer (1) via a circulating phosphoric acid pump (3).

3. The system for producing superphosphoric acid according to claim 1, characterized in that: The upper part of the evaporation flash crystallizer (1) is an evaporator (1-1), the middle part is a flash chamber (1-2), and the lower part is a crystallizer (1-3).

4. A method for producing superphosphoric acid using the system of claim 1, characterized in that: The method comprises the following steps: transporting phosphoric acid from the boundary zone and circulating phosphoric acid to an evaporation flash crystallizer (1), and performing evaporation, flash evaporation and crystallization in the evaporation flash crystallizer in sequence to obtain a target product.

5. The method according to claim 4, characterized in that: The saturated steam from the boundary zone exchanges heat with the phosphoric acid in the tube. The phosphoric acid after heat exchange enters the flash chamber for flash evaporation. The material after flash evaporation contains superphosphoric acid. The material after flash evaporation is transported to the crystallizer. A part of the superphosphoric acid in the crystallizer enters the evaporator as circulating phosphoric acid, and the other part of the superphosphoric acid is used as the target product.

6. The method according to claim 5, characterized in that: The mass concentration of P2O5 in the circulating phosphoric acid is ≤80%, and the temperature is ≤210°C; the mass concentration of P2O5 in the phosphoric acid in the boundary zone is ≤62%, and the temperature is 20-40°C.

7. The method according to claim 5, characterized in that: The concentration of hot phosphoric acid in the flash chamber is 68% to 80% by mass of P2O5.

8. The method according to claim 5, characterized in that: The target product superphosphoric acid has a P2O5 mass concentration of 68% to 80% and a temperature of 190°C to 210°C.

9. The method according to claim 5, characterized in that: The mass ratio of the circulating phosphoric acid to the target product superphosphoric acid is ≥50:1.

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