System and method for producing ammonium sulfate by using phosphogypsum

Abstract

The invention relates to a system and a method for producing ammonium sulfate by using phosphogypsum, belonging to the field of chemical industry. The system comprises a reaction tank, a filter, a filtrate tank, a filtrate pump, a circulating pump, a heater, a flash evaporation crystallizer, a crystallized slurry conveying pump, a thickener, an ammonium sulfate crystal separator, an ammonium sulfate crystal dryer and a target product conveyor. By adopting the system and the method, ammonium sulfate is produced by utilizing the byproduct phosphogypsum of the wet-process phosphoric acid device and the byproduct ammonium sulfate-ammonium carbonate solution of the calcium fluoride device, the byproduct calcium carbonate and fluosilicic acid are used for producing calcium fluoride, and the byproduct ammonium sulfate solution is used for producing an ammonium sulfate-ammonium carbonate solution in the calcium fluoride device. The invention aims to solve the problem of phosphogypsum stockpiling and the problem of recycling ammonium carbonate and calcium carbonate, waste is turned into wealth, and the environment is protected.

Description

Technical Field

[0001] This patent relates to the field of chemical production technology, and specifically to a system and method for producing ammonium sulfate using phosphogypsum and ammonium sulfate-ammonium carbonate solution. Background Art

[0002] Phosphogypsum is the main by-product in the production of wet phosphoric acid. The main component of phosphogypsum is CaSO42H2O. Every ton of phosphoric acid (P2O5) produced can produce about 4 to 5 tons of phosphogypsum. The annual domestic discharge of phosphogypsum is close to 68 million tons, while the utilization rate of phosphogypsum is only about 5%. The long-term accumulation of phosphogypsum will cause pollution of surface water and groundwater, and the establishment of a storage yard occupies a large area, is expensive, and requires a large investment. At present, phosphogypsum has become one of the largest solid wastes discharged in the chemical industry.

[0003] The comprehensive utilization of phosphogypsum mainly includes the following ways: (1) making sulfuric acid and cement; (2) making gypsum board and cement slow-release agent; (3) making alkaline soil conditioner, etc. However, these methods are subject to technical or economic constraints, and there is no final conclusion on which method is the best solution. Therefore, it is urgent to explore new ways to consume phosphogypsum.

[0004] The role of sulfur on crops is as follows: sulfur is an important component of cystine, cysteine ​​and methionine. About 90% of sulfur in plants exists in sulfur-containing amino acids; sulfur participates in the formation of chlorophyll; sulfur plays an important role in the formation and activation of certain enzymes in plants; sulfur participates in the synthesis of vitamins B and H; forms glycoside oils of cruciferous plants; sulfur is related to the protein structure that affects the cold and drought resistance of plants, and sulfur can increase the cold and drought resistance of certain crops; sulfur is related to the nitrogen fixation of rhizobia and spontaneous nitrogen-fixing bacteria. Sulfur deficiency in crops will affect their normal growth, resulting in a decrease in yield and quality. Under the development trend of high-concentration fertilizers, some areas in China are facing a crisis of soil sulfur deficiency. In some provinces and regions, 60% of the soil has already experienced sulfur deficiency to varying degrees; the application of ammonium sulfate can meet the requirements, and each ton of ammonium sulfate applied can bring in 240 kilograms of sulfur. Summary of the invention

[0005] The purpose of the present invention is to provide a system and method for producing ammonium sulfate from phosphogypsum, which utilizes the phosphogypsum produced as a byproduct of a wet phosphoric acid device and the ammonium sulfate-ammonium carbonate solution produced as a byproduct of a calcium fluoride device to produce ammonium sulfate, and the byproduct calcium carbonate is removed from the calcium fluoride device to produce calcium fluoride, and the byproduct ammonium sulfate solution is removed from the calcium fluoride device to produce ammonium sulfate-ammonium carbonate solution. This patent aims to solve the problem of phosphogypsum storage and the recycling problem of ammonium carbonate and calcium carbonate, turn waste into treasure, and protect the environment.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] .A system for producing ammonium sulfate using phosphogypsum, the system includes a reaction tank and a flash crystallizer, the output pipeline of the phosphogypsum and the output pipeline of the ammonium sulfate-ammonium carbonate solution are connected to the reaction tank, the output end of the reaction tank is connected to the flash crystallizer through a filter and a filtrate tank in sequence, and the bottom output end of the flash crystallizer is the target product.

[0008] In the above system: the bottom of the flash crystallizer is connected in sequence through a thickener rake, an ammonium sulfate crystal separator and an ammonium sulfate crystal dryer.

[0009] In the above system: agitators are arranged in the reaction tank and the filtrate tank.

[0010] In the above system: the filtrate tank is connected to the flash chamber at the top of the flash crystallizer through a heater.

[0011] In the above system: the crystallizer at the lower part of the flash crystallizer is connected to the thickener rake.

[0012] In the above system: there is also an output end at the upper part of the crystallizer, and the output end is connected to the flash chamber at the upper part of the flash crystallizer through a heater.

[0013] A method for producing ammonium sulfate from phosphogypsum using the above system, the method comprising the following steps:

[0014] (1) Ammonium sulfate preparation and calcium carbonate separation process: The phosphogypsum from the boundary area and the ammonium sulfate-ammonium carbonate solution from the calcium fluoride device enter the reaction tank for reaction. After the reaction is completed, the calcium carbonate obtained after filtration is output as the target product, and the filtrate obtained is used for standby use;

[0015] (2) ammonium sulfate concentration and crystallization stage: the filtrate obtained in step (1) is heated by a heater and then enters the flash chamber of a flash crystallizer. After part of the water is evaporated, the filtrate enters the crystallizer of the flash crystallizer for crystallization;

[0016] (3) The crystals obtained in step (2) and the ammonium sulfate circulating liquid at the bottom of the crystallizer are transported to a thickener for further cooling, crystallization and thickening. When the solid content reaches ≥10%, the crystal slurry is sent to an ammonium sulfate crystal separator. The separated ammonium sulfate solution is sent to a calcium fluoride device to prepare an ammonium sulfate-ammonium carbonate solution; the separated ammonium sulfate crystals are dried and used as the target product.

[0017] In the above method: in step (1), the mass concentration of free P2O5 in the phosphogypsum is ≤0.5%, and the mass concentration of CaSO42H2O is ≥80%; the mass concentration of (NH4)2SO4 in the ammonium sulfate-ammonium carbonate solution from the calcium fluoride device is ≥10%, and the mass concentration of (NH4)2CO3 is ≥5%; the molar ratio of CaSO42H2O to (NH4)2CO3 in the reaction tank is ≥9:10;

[0018] The reaction temperature of the reaction tank is ≤80°C, and the solution residence time is ≤4 hours;

[0019] The mass concentration of (NH4)2SO4 in the filtered filtrate is ≤50%.

[0020] In the above method: the temperature of the heater in step (2) is ≥70°C; the atmospheric pressure is ≥the gas phase pressure of the flash chamber is ≥5kPa; the mass concentration of (NH4)2SO4 in the filtrate after evaporating part of the water is ≥45%.

[0021] In the above method: the mass concentration of (NH4)2SO4 in the ammonium sulfate-ammonium carbonate solution separated in step (3) is ≤60%; the mass concentration of (NH4)2SO4 in the separated ammonium sulfate crystals is ≥85%, and the mass concentration of the target product ammonium sulfate (NH4)2SO4 after drying is ≥90%.

[0022] In some specific technical solutions, the technical solutions of the present invention are as follows:

[0023] A system for producing ammonium sulfate from phosphogypsum, the system comprising a reaction tank, a reaction tank agitator, a filter, a filtrate tank, a filtrate tank agitator, a filtrate pump, a circulation pump, a heater, a flash crystallizer, a crystal slurry delivery pump, a thickener, a thickener rake, an ammonium sulfate crystal separator, an ammonium sulfate crystal dryer, and a target product conveyor.

[0024] The phosphogypsum conveying equipment from the boundary area is connected to the phosphogypsum inlet A at the top of the reaction tank, the ammonium sulfate-ammonium carbonate solution pipeline from the calcium fluoride device is connected to the ammonium sulfate-ammonium carbonate solution inlet B at the top of the reaction tank, the calcium carbonate-ammonium sulfate slurry outlet C on the side of the reaction tank is connected to the feed end of the filter, the solid discharge end of the filter is connected to the calcium carbonate conveying equipment of the defluoridation calcium device, the liquid discharge end of the filter is connected to the top inlet of the filtrate tank, and the filtrate pump outlet is connected to the pipeline connected to the circulation pump outlet and the bottom pipe inlet of the heater;

[0025] The outlet of the circulation pump is connected to the circulating liquid inlet B of the flash crystallizer through a heater, the circulating liquid outlet D of the flash crystallizer is connected to the inlet of the circulation pump, and the crystal slurry outlet F of the flash crystallizer is connected to the feed end of the thickener through a crystal slurry delivery pump;

[0026] The discharge end of the thickener is connected to the feed end of the ammonium sulfate crystal separator, the liquid discharge end of the ammonium sulfate crystal separator is connected to the ammonium sulfate solution conveying pipeline of the decalcification device, the solid discharge end of the ammonium sulfate crystal separator is connected to the feed end of the ammonium sulfate crystal dryer, and the discharge end of the ammonium sulfate crystal dryer is connected to the feed end of the target product conveyor.

[0027] In the above system: the shell inlet of the heater is connected to the steam pipeline from the boundary zone, and the shell outlet of the heater is connected to the condensate pipeline of the de-boundary zone; the dry gas inlet of the ammonium sulfate crystallizer is connected to the dry gas pipeline from the boundary zone, and the dry tail gas outlet of the ammonium sulfate crystallizer is connected to the dry tail gas pipeline of the de-boundary zone.

[0028] In the above system: the reaction tank is provided with a reaction tank agitator; the filtrate tank is provided with a filtrate tank agitator; and the thickener is provided with a thickener rake.

[0029] A method for producing ammonium sulfate from phosphogypsum using the above system, characterized in that the method comprises the following steps:

[0030] (1) Ammonium sulfate preparation and calcium carbonate separation process: phosphogypsum from the boundary area (containing free P2O5 mass concentration ≤ 0.5%, containing CaSO42H2O mass concentration ≥ 80%) enters the reaction tank from the phosphogypsum inlet A at the top of the reaction tank, and ammonium sulfate-ammonium carbonate solution from the calcium fluoride device [containing (NH4)2SO4 mass concentration ≥ 10%, containing (NH4)2CO3 mass concentration ≥ 5%] enters the reaction tank from the ammonium sulfate-ammonium carbonate solution inlet B at the top of the reaction tank, and the molar ratio of CaSO42H2O to (NH4)2CO3 is ≥ 9:10. Calcium sulfate and ammonium carbonate react in the reaction tank to generate calcium carbonate and ammonium sulfate. The reaction temperature of the reaction tank is ≤ 80°C, and the solution residence time is ≤ 4 hours. The calcium carbonate-ammonium sulfate slurry after the reaction is completed is sent to the filter feed end from the calcium carbonate-ammonium sulfate slurry outlet C on the side of the reaction tank. The calcium carbonate solid filtered out of the filter is sent to the calcium fluoride device from the solid discharge end of the filter. The filtrate filtered out of the filter [containing (NH4)2SO4 mass concentration ≤ 50%] enters the filtrate tank from the liquid discharge end of the filter through the top inlet of the filtrate tank. The filtrate in the filtrate tank is sent to the ammonium sulfate concentration and crystallization section through the filtrate pump.

[0031] (2) Ammonium sulfate concentration and crystallization section: The filtrate from the filtrate pump [containing (NH4)2SO4 mass concentration ≤ 50%] is mixed with the circulating liquid from the circulating pump and enters the heater (8) for heating. The heated solution temperature is ≥ 70°C. It exits the heater and enters the flash chamber at the top of the flash crystallizer through the circulating liquid inlet B of the flash crystallizer. The gas phase pressure (≥ 5kPa) in the flash chamber is lower than the atmospheric pressure. The circulating liquid removes part of the water [containing (NH4)2SO4 mass concentration ≥ 45%] through flash evaporation. At the same time, the temperature is reduced and flows into the crystallizer below the flash chamber from the outlet C at the bottom of the flash chamber; the flash steam coming out of the gas phase outlet A at the top of the flash chamber goes to the boundary area.

[0032] The circulating liquid from the flash chamber enters the crystallizer from the bottom outlet E, where ammonium sulfate crystallizes and gradually grows. The grown crystals sink to the bottom of the crystallizer and go to the thickener with a part of the ammonium sulfate circulating liquid from the crystal slurry outlet F through the crystal slurry transfer pump; the rest of the circulating liquid goes to the circulating pump from the circulating liquid outlet D.

[0033] (3) Ammonium sulfate crystallization, filtration and drying process: The crystallization slurry from the crystallization slurry delivery pump continues to cool, crystallize and thicken in the thickener. When the solid content reaches ≥10%, the crystallization slurry is sent to the ammonium sulfate crystallization separator. The separated ammonium sulfate solution [containing (NH4)2SO4 mass concentration ≤60%] is sent to the calcium fluoride device to prepare ammonium sulfate-ammonium carbonate solution; the separated ammonium sulfate crystals [containing (NH4)2SO4 mass concentration ≥85%] are dried in the ammonium sulfate crystallization dryer, and the dried ammonium sulfate crystals are taken as the target product-ammonium sulfate [containing (NH4)2SO4 mass concentration ≥90%] and go to the boundary area.

[0034] In some of the most specific technical solutions, the more detailed steps are as follows:

[0035] (1) Ammonium sulfate preparation and calcium carbonate separation process: phosphogypsum from the boundary area (containing free P2O5 at a mass concentration of 0.5% to 0.1% and CaSO42H2O at a mass concentration of 80% to 98%) enters the reaction tank from the phosphogypsum inlet A at the top of the reaction tank, and ammonium sulfate-ammonium carbonate solution from the calcium fluoride device [containing (NH4)2SO4 at a mass concentration of 10% to 40% and (NH4)2CO3 at a mass concentration of 5% to 30%] enters the reaction tank from the ammonium sulfate-ammonium carbonate solution inlet B at the top of the reaction tank, and the molar ratio of CaSO42H2O to (NH4)2CO3 is 9:10 to 10:9. Calcium sulfate reacts with ammonium carbonate in the reaction tank to generate calcium carbonate and ammonium sulfate. The reaction temperature of the reaction tank is 60°C to 80°C, and the solution retention time is 2 to 4 hours. The calcium carbonate-ammonium sulfate slurry after the reaction is completed is sent to the filter feed end from the calcium carbonate-ammonium sulfate slurry outlet C on the side of the reaction tank. The calcium carbonate solid filtered out of the filter is sent to the calcium fluoride device from the solid discharge end of the filter. The filtrate filtered out of the filter [containing (NH4)2SO4 mass concentration of 30% to 50%] enters the filtrate tank from the liquid discharge end of the filter through the top inlet of the filtrate tank. The filtrate in the filtrate tank is sent to the ammonium sulfate concentration and crystallization section through the filtrate pump.

[0036] (2) Ammonium sulfate concentration and crystallization section: The filtrate from the filtrate pump [containing (NH4)2SO4 mass concentration of 30% to 50%] is mixed with the circulating liquid from the circulating pump and then enters the heater for heating. The heated solution temperature is 70°C to 90°C. It exits the heater and enters the flash chamber at the top of the flash crystallizer through the circulating liquid inlet B of the flash crystallizer. The gas phase pressure (5kPa to 20kPa) in the flash chamber is lower than the atmospheric pressure. The circulating liquid removes part of the water [containing (NH4)2SO4 mass concentration of 45% to 65%] through flash evaporation. At the same time, the temperature is reduced and flows into the crystallizer below the flash chamber from the bottom outlet C of the flash chamber; the flash steam coming out of the gas phase outlet A at the top of the flash chamber goes to the boundary area.

[0037] The circulating liquid from the flash chamber enters the crystallizer from the bottom outlet E, where ammonium sulfate crystallizes and gradually grows. The grown crystals sink to the bottom of the crystallizer and go to the thickener with a part of the ammonium sulfate circulating liquid from the crystal slurry outlet F through the crystal slurry transfer pump; the rest of the circulating liquid goes to the circulating pump from the circulating liquid outlet D.

[0038] (3) Ammonium sulfate crystallization, filtration and drying process: The crystallization slurry from the crystallization slurry delivery pump continues to cool, crystallize and thicken in the thickener. When the solid content reaches 10% to 30%, the crystallization slurry is sent to the ammonium sulfate crystallization separator. The separated ammonium sulfate solution [containing (NH4)2SO4 mass concentration of 40% to 60%] is sent to the calcium fluoride device to prepare ammonium sulfate-ammonium carbonate solution; the separated ammonium sulfate crystals [containing (NH4)2SO4 mass concentration of 85% to 95%] are dried in the ammonium sulfate crystallization dryer, and the dried ammonium sulfate crystals are used as the target product - ammonium sulfate [containing (NH4)2SO4 mass concentration of 90% to 99%] and sent to the boundary area by the target product conveyor.

[0039] Beneficial effects of the present invention:

[0040] The production of ammonium sulfate using phosphogypsum can solve the problem of phosphogypsum storage. Ammonium sulfate can be used as an industrial raw material and can also solve the problem of sulfur deficiency in cultivated land. Every ton of ammonium sulfate produced can consume about 1.3 tons of phosphogypsum produced as a byproduct of wet-process phosphoric acid, about 0.73 tons of ammonium carbonate produced as a byproduct of the calcium fluoride device, and about 0.76 tons of calcium carbonate can be produced at the same time; 0.76 tons of calcium carbonate can be removed from the calcium fluoride device to produce about 0.6 tons of calcium fluoride, consume about 2 tons of fluorosilicic acid (containing about 18% H2SiF6 mass concentration) produced as a byproduct of wet-process phosphoric acid, and about 0.73 tons of ammonium carbonate can be returned to the ammonium sulfate device to produce 1 ton of ammonium sulfate. The ammonium sulfate and calcium fluoride devices utilize each other's byproducts, while consuming phosphogypsum and fluorosilicic acid produced as byproducts of wet-process phosphoric acid, achieving a win-win situation, turning waste into treasure, and protecting the environment. The cost of producing ammonium sulfate using phosphogypsum is 65% of the natural gypsum method and 91% of the direct sulfuric acid neutralization method. The decalcification device using ammonium sulfate solution is beneficial to the condensation of water vapor and the absorption of NH3 and CO2 gases to form ammonium sulfate-ammonium carbonate solution; the production of ammonium sulfate using ammonium sulfate-ammonium carbonate solution is beneficial to increasing the initial concentration of ammonium sulfate and facilitating the crystallization of ammonium sulfate. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is the process flow diagram of this patent

[0042] Among them, there are a reaction tank 1, a reaction tank agitator 2, a filter 3, a filtrate tank 4, a filtrate tank agitator 5, a filtrate pump 6, a circulation pump 7, a heater 8, a flash crystallizer 9, a flash chamber 9-1, a crystallizer 9-2, a crystal slurry delivery pump 10, a thickener 11, a thickener rake 12, an ammonium sulfate crystal separator 13, an ammonium sulfate crystal dryer 14, and a target product conveyor 15. DETAILED DESCRIPTION

[0043] 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.

[0044] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0045] like Figure 1 A system for producing ammonium sulfate from phosphogypsum, characterized in that the system comprises a reaction tank 1, a reaction tank agitator 2, a filter 3, a filtrate tank 4, a filtrate tank agitator 5, a filtrate pump 6, a circulation pump 7, a heater 8, a flash crystallizer 9, a crystal slurry delivery pump 10, a thickener 11, a thickener rake 12, an ammonium sulfate crystal separator 13, an ammonium sulfate crystal dryer 14, and a target product conveyor 15.

[0046] The phosphogypsum conveying equipment from the boundary area is connected to the phosphogypsum inlet A at the top of the reaction tank 1, the ammonium sulfate-ammonium carbonate solution pipeline from the calcium fluoride device is connected to the ammonium sulfate-ammonium carbonate solution inlet B at the top of the reaction tank 1, the calcium carbonate-ammonium sulfate slurry outlet C on the side of the reaction tank 1 is connected to the feed end of the filter 3, the solid discharge end of the filter 3 is connected to the calcium carbonate conveying equipment of the de-calcium fluoride device, the liquid discharge end of the filter 3 is connected to the top inlet of the filtrate tank 4, and the outlet of the filtrate pump 6 is connected to the pipeline connected to the outlet of the circulation pump 7 and the bottom pipe inlet of the heater 8;

[0047] The outlet of the circulation pump 7 is connected to the circulating liquid inlet B of the flash crystallizer 9 through the heater 8, the circulating liquid outlet D of the flash crystallizer 9 is connected to the inlet of the circulation pump 7, and the crystal slurry outlet F of the flash crystallizer 9 is connected to the feed end of the thickener 11 through the crystal slurry delivery pump 10;

[0048] The discharge end of the thickener 11 is connected to the feed end of the ammonium sulfate crystal separator 13, the liquid discharge end of the ammonium sulfate crystal separator 13 is connected to the ammonium sulfate solution conveying pipeline of the decalcification device, the solid discharge end of the ammonium sulfate crystal separator 13 is connected to the feed end of the ammonium sulfate crystal dryer 14, and the discharge end of the ammonium sulfate crystal dryer 14 is connected to the feed end of the target product conveyor 15.

[0049] The shell side inlet of heater 8 is connected to the steam pipeline from the boundary zone, and the shell side outlet of heater 8 is connected to the condensed water pipeline of the de-boundary zone; the dry gas inlet of ammonium sulfate crystallizer dryer 14 is connected to the dry gas pipeline from the boundary zone, and the dry tail gas outlet of ammonium sulfate crystallizer dryer 14 is connected to the dry tail gas pipeline of the de-boundary zone.

[0050] The reaction tank 1 is provided with a reaction tank agitator 2; the filtrate tank 4 is provided with a filtrate tank agitator 5; and the thickener 11 is provided with a thickener rake 12.

[0051] The method for producing ammonium sulfate from phosphogypsum using the above system comprises the following steps:

[0052] (1) Ammonium sulfate preparation and calcium carbonate separation process: phosphogypsum from the boundary area (containing free P2O5 at a mass concentration of 0.18% and CaSO42H2O at a mass concentration of 92%) enters the reaction tank 1 from the phosphogypsum inlet A at the top of the reaction tank 1, and the ammonium sulfate-ammonium carbonate solution from the calcium fluoride device [containing (NH4)2SO4 at a mass concentration of 30.24% and (NH4)2CO3 at a mass concentration of 11.41%] enters the reaction tank 1 from the ammonium sulfate-ammonium carbonate solution inlet B at the top of the reaction tank 1, and the molar ratio of CaSO42H2O to (NH4)2CO3 is 1:1. Calcium sulfate and ammonium carbonate react in the reaction tank 1 to generate calcium carbonate and ammonium sulfate. The reaction temperature of the reaction tank 1 is 75°C, and the solution retention time is 3 hours. The calcium carbonate-ammonium sulfate slurry after the reaction is completed is sent to the feed end of the filter 3 from the calcium carbonate-ammonium sulfate slurry outlet C on the side of the reaction tank 1. The calcium carbonate solid filtered out of the filter 3 is sent to the calcium fluoride device from the solid discharge end of the filter 3. The filtrate filtered out of the filter 3 [containing (NH4)2SO4 mass concentration of 39.8%] enters the filtrate tank 4 from the liquid discharge end of the filter 3 through the top inlet of the filtrate tank 4. The filtrate in the filtrate tank 4 is sent to the ammonium sulfate concentration and crystallization section through the filtrate pump 6.

[0053] (2) Ammonium sulfate concentration and crystallization section: The filtrate from the filtrate pump 6 [containing (NH4)2SO4 mass concentration of 39.8%] is mixed with the circulating liquid from the circulating pump 7 and enters the heater 8 for heating. The heated solution temperature is 85°C. It exits the heater 8 and enters the flash chamber 9-1 at the top of the flash crystallizer 9 from the circulating liquid inlet B of the flash crystallizer 9. The gas phase pressure (10 kPa) of the flash chamber 9-1 is lower than the atmospheric pressure. The circulating liquid removes part of the water [containing (NH4)2SO4 mass concentration of 55.24%] through flash evaporation. At the same time, the temperature is reduced and flows from the bottom outlet C of the flash chamber 9-1 into the crystallizer 9-2 below the flash chamber 9-1; the flash steam coming out of the gas phase outlet A at the top of the flash chamber 9-1 goes to the boundary area.

[0054] The circulating liquid from the flash chamber 9-1 enters the crystallizer 9-2 from the bottom outlet E, and ammonium sulfate crystallizes and gradually grows in the crystallizer 9-2. The grown crystals sink to the bottom of the crystallizer 9-2 and go to the thickener 11 through the crystal slurry outlet F with a part of the ammonium sulfate circulating liquid through the crystal slurry transfer pump 10; the rest of the circulating liquid goes to the circulating pump 7 from the circulating liquid outlet D.

[0055] (3) Ammonium sulfate crystallization, filtration and drying process: The crystallization slurry from the crystallization slurry delivery pump 10 continues to cool, crystallize and thicken in the thickener 11. When the solid content reaches 18.39%, the crystallization slurry is sent to the ammonium sulfate crystallization separator 13. The separated ammonium sulfate solution [containing (NH4)2SO4 mass concentration of 45.21%] is sent to the calcium fluoride device to prepare ammonium sulfate-ammonium carbonate solution; the separated ammonium sulfate crystals [containing (NH4)2SO4 mass concentration of 90%] are dried in the ammonium sulfate crystallization dryer 14, and the dried ammonium sulfate crystals are sent to the boundary area as the target product-ammonium sulfate [containing (NH4)2SO4 mass concentration of 96%] by the target product conveyor 15.

[0056] The results of the patent operation are shown in Table 1.

[0057] Table 1 Performance evaluation results of 300 tons / day ammonium sulfate production unit using phosphogypsum

[0058]

[0059]

Claims

1. A system for producing ammonium sulfate from phosphogypsum, characterized in that: The system comprises a reaction tank (1) and a flash crystallizer (9); an output pipeline for phosphogypsum and an output pipeline for ammonium sulfate-ammonium carbonate solution are connected to the reaction tank (1); the output end of the reaction tank (1) is connected to the flash crystallizer (9) via a filter (3) and a filtrate tank (4) in sequence; the output end at the bottom of the flash crystallizer (9) is the target product.

2. The system for producing ammonium sulfate from phosphogypsum according to claim 1, characterized in that: The bottom of the flash crystallizer (9) is connected in sequence through a thickener rake (12), an ammonium sulfate crystal separator (13) and an ammonium sulfate crystal dryer (14).

3. The system for producing ammonium sulfate from phosphogypsum according to claim 1, characterized in that: Agitators are provided in the reaction tank (1) and the filtrate tank (4).

4. The system for producing ammonium sulfate from phosphogypsum according to claim 1, characterized in that: The filtrate tank (4) is connected to the flash chamber (9-1) at the top of the flash crystallizer (9) through a heater (8).

5. The system for producing ammonium sulfate from phosphogypsum according to claim 2, characterized in that: The crystallizer (9-2) at the lower part of the flash crystallizer (9) is connected to the thickener rake (12).

6. The system for producing ammonium sulfate from phosphogypsum according to claim 1, characterized in that: The upper part of the crystallizer (9-2) is also provided with an output end, which is connected to the flash chamber (9-1) at the upper part of the flash crystallizer (9) through a heater (8).

7. A method for producing ammonium sulfate from phosphogypsum using the system of claim 1, characterized in that: The method comprises the following steps: (1) Ammonium sulfate preparation and calcium carbonate separation process: The phosphogypsum from the boundary area and the ammonium sulfate-ammonium carbonate solution from the calcium fluoride device enter the reaction tank for reaction. After the reaction is completed, the calcium carbonate obtained after filtration is output as the target product, and the filtrate obtained is used for standby use; (2) ammonium sulfate concentration and crystallization stage: the filtrate obtained in step (1) is heated by a heater and then enters the flash chamber of a flash crystallizer. After part of the water is evaporated, the filtrate enters the crystallizer of the flash crystallizer for crystallization; (3) The crystals obtained in step (2) and the ammonium sulfate circulating liquid at the bottom of the crystallizer are transported to a thickener for further cooling, crystallization and thickening. When the solid content reaches ≥10%, the crystal slurry is sent to an ammonium sulfate crystal separator. The separated ammonium sulfate solution is sent to a calcium fluoride device to prepare an ammonium sulfate-ammonium carbonate solution; the separated ammonium sulfate crystals are dried and used as the target product.

8. The method according to claim 7, characterized in that: In step (1), the mass concentration of free P2O5 in the phosphogypsum is ≤0.5%, and the mass concentration of CaSO42H2O is ≥80%; the mass concentration of (NH4)2SO4 in the ammonium sulfate-ammonium carbonate solution from the calcium fluoride device is ≥10%, and the mass concentration of (NH4)2CO3 is ≥5%; the molar ratio of CaSO42H2O to (NH4)2CO3 in the reaction tank is ≥9:10; The reaction temperature of the reaction tank is ≤80°C, and the solution residence time is ≤4 hours; The mass concentration of (NH4)2SO4 in the filtered filtrate is ≤50%. The preferred embodiment of step (1) is as follows: In step (1), the mass concentration of free P2O5 in the phosphogypsum is 0.5% to 0.1%, and the mass concentration of CaSO42H2O is 80% to 98%; the mass concentration of (NH4)2SO4 in the ammonium sulfate-ammonium carbonate solution from the calcium fluoride device is 10% to 40%, and the mass concentration of (NH4)2CO3 is 5% to 30%; the molar ratio of CaSO42H2O to (NH4)2CO3 in the reaction tank is 9:10 to 10:9; The reaction temperature of the reaction tank is 60°C to 80°C, and the solution residence time is 2 to 4 hours; The mass concentration of (NH4)2SO4 in the filtered filtrate is 30% to 50%.

9. The method according to claim 7, characterized in that: The temperature of the heater in step (2) is ≥70°C; the atmospheric pressure is ≥the gas phase pressure of the flash chamber is ≥5kPa; the mass concentration of (NH4)2SO4 in the filtrate after evaporating part of the water is ≥45%; The preferred embodiment of step (2) is as follows: In step (2), the temperature of the heater is 70°C to 90°C; the gas phase pressure in the flash chamber is 5kPa to 20kPa; and the mass concentration of (NH4)2SO4 in the filtrate after evaporating part of the water is 45% to 65%.

10. The method according to claim 7, characterized in that: The mass concentration of (NH4)2SO4 in the ammonium sulfate-ammonium carbonate solution separated in step (3) is ≤60%; the mass concentration of (NH4)2SO4 in the separated ammonium sulfate crystals is ≥85%, and the mass concentration of the target product ammonium sulfate (NH4)2SO4 after drying is ≥90%; The preferred embodiment of step (3) is as follows: The solid content in step (3) reaches 10% to 30%; the mass concentration of (NH4)2SO4 in the separated ammonium sulfate-ammonium carbonate solution is 40% to 60%; the mass concentration of (NH4)2SO4 in the separated ammonium sulfate crystals is 85% to 95%, and the mass concentration of the target product ammonium sulfate (NH4)2SO4 after drying is 90% to 99%.

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