Phosphogypsum pyrolysis resource processing technology and system under co2 atmosphere

Abstract

The application discloses a kind of CO2 Atmosphere under phosphogypsum pyrolysis resource processing technology and system, comprising: phosphogypsum is pyrolyzed under the acid synergy atmosphere formed by 600 DEG C-800 DEG C and CO2 and water vapor, realize the deep removal of eutectic phosphorus and other impurities, and output high-quality gypsum;Pyrolysis tail gas is recovered waste heat by heat exchanger, and acid waste water is generated by condensation;Waste water is recovered phosphate, fluoride and organic matter in turn by grading reaction under CO2 pressurized condition, and pollutants are converted into by-products;Purified waste water after treatment is preheated and reused in pyrolysis furnace, to realize waste water zero discharge;Part of CO2 in purified flue gas is recycled, which reduces carbon emission.The process realizes efficient purification of phosphogypsum, cascade utilization of waste heat and full recovery of valuable components at low temperature by building a closed-loop system, which significantly reduces energy consumption compared with traditional calcination process, and has no secondary pollution, with outstanding environmental and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization technology, specifically, it relates to a process and system for the resource utilization of phosphogypsum pyrolysis under CO2 atmosphere. Background Technology

[0002] Phosphogypsum is a major industrial solid waste generated during the wet-process phosphoric acid production. Specifically, for every ton of phosphoric acid produced, 4-5 tons of phosphogypsum can be produced as a byproduct. Its main component is calcium sulfate dihydrate (CaSO4·2H2O), and it also contains various impurities such as soluble phosphorus, fluorine, and organic matter. Currently, the resource utilization of phosphogypsum is mainly concentrated in the building materials sector, such as its use as a cement retarder, in the production of gypsum board, and for mine backfilling. However, these methods generally suffer from low added value, limited economic benefits, and significant susceptibility to market fluctuations, resulting in unstable disposal capacity and a long-term difficulty in improving the overall comprehensive utilization rate.

[0003] To address the difficulty in removing impurities from phosphogypsum and the resulting limitations on its high-value utilization, existing technologies have proposed various treatment pathways, primarily including water washing, lime neutralization, flotation, and high-temperature calcination. However, each method faces significant techno-economic challenges: water washing achieves good impurity removal but consumes large amounts of water, resulting in high wastewater treatment costs; lime neutralization is simple to operate and has low costs, but its ability to remove organic impurities is limited; high-temperature calcination (typically requiring temperatures above 1000℃) can effectively remove stubborn impurities such as eutectic phosphorus, but it consumes extremely high energy and is economically unfeasible. In recent years, although some studies have explored the simultaneous recovery of sulfur and calcium resources from phosphogypsum through thermochemical decomposition, these processes generally suffer from complex procedures and demanding reaction conditions, and remain far from large-scale industrial application.

[0004] Therefore, there is an urgent need to develop a new phosphogypsum treatment process. This process should not only have the ability to efficiently remove impurities and achieve resource recovery, but also be economically feasible and meet the requirements of low energy consumption and low pollution, thereby providing a practical technical path for the green and low-carbon transformation of the phosphate chemical industry. Summary of the Invention

[0005] The purpose of this invention is to address the problems of incomplete impurity removal, high energy consumption, and easy secondary pollution in the existing technology of phosphogypsum resource utilization treatment, and to provide a process and system for pyrolysis resource utilization treatment of phosphogypsum under CO2 atmosphere.

[0006] To achieve the above objectives, the present invention provides a process for the resource recovery of phosphogypsum through pyrolysis under a CO2 atmosphere, comprising the following steps: S1: The phosphogypsum material is fed into a pyrolysis furnace and pyrolyzed at a temperature of 600℃~800℃. During the pyrolysis process, CO2 gas and water vapor are introduced into the furnace to form an acidic synergistic atmosphere of CO2, which promotes the dehydration of calcium sulfate dihydrate, the release of eutectic phosphorus, the decomposition of organic impurities and the volatilization of fluorides in the phosphogypsum. S2: The high-temperature solid product (high-quality anhydrous gypsum) after pyrolysis is discharged from the pyrolysis furnace and cooled before being used as a building material raw material; the high-temperature exhaust gas generated by pyrolysis is introduced into a heat exchanger to exchange heat with the cooling medium in order to recover the waste heat of the flue gas. S3: Collect the acidic wastewater generated after condensation by the heat exchanger and send it to the wastewater treatment device; under pressure, introduce CO2 into the wastewater, and through staged control of pH value and reaction conditions, separate and recover phosphate, fluoride and organic matter in the wastewater in sequence to obtain phosphate, fluoride and organic by-products. S4: The separated purified wastewater is preheated by a heat exchanger and then reintroduced into the pyrolysis furnace as supplementary steam to participate in the reaction of step S1, thereby achieving zero discharge of process wastewater.

[0007] Further, in step S1, the CO2 acidic synergistic atmosphere is achieved by controlling the CO2 volume fraction in the furnace to 10%–15% and introducing steam; the pyrolysis furnace provides a heat source by burning bituminous coal, and the grate area heat load is 0.4 MW / m². 2 ~0.9 MW / m 2 .

[0008] Furthermore, in step S2, the waste heat recovery device is a shell-and-tube heat exchanger, the cooling medium is water, and the recovered waste heat is used to preheat combustion air, generate process steam, or provide heating.

[0009] Furthermore, in step S3, by adding a calcium-based precipitant (such as lime milk) to the wastewater, under CO2 pressurization conditions, phosphate ions are precipitated out stepwise in the form of calcium phosphate and fluoride ions in the form of calcium fluoride.

[0010] Further, in step S3, the method for graded control of pH and reaction conditions under CO2 pressurization is as follows: CO2 is introduced into the wastewater and a calcium-based precipitant is added. By controlling the amount of CO2 introduced and the amount of calcium-based precipitant added, the pH of the wastewater is graded to different ranges: first, the pH is adjusted to 3.5-4.5 so that phosphate ions precipitate out in the form of calcium phosphate; after solid-liquid separation, the pH is adjusted to 6.0-7.0 and a calcium source is added so that fluoride ions precipitate out in the form of calcium fluoride; finally, activated carbon adsorption is used to remove residual organic matter.

[0011] Furthermore, in step S5, the exhaust gas after waste heat recovery and wastewater condensation in step S2 is purified. The purification process includes dry deacidification, bag filter dust removal, and activated carbon adsorption. After the exhaust gas is purified to meet the standards, it is discharged. At the same time, a portion of the CO2 gas is separated and recycled back to the pyrolysis furnace in step S1.

[0012] The present invention also provides a system for implementing the above-described process, comprising: A pyrolysis furnace for pyrolysis of phosphogypsum, equipped with a CO2 and water vapor injection device inside; A waste heat recovery device (heat exchanger) is connected to the tail gas outlet of the pyrolysis furnace to recover the waste heat of the tail gas and condense the generated acidic wastewater. A wastewater treatment device, connected to the condensate outlet of the heat exchanger, is used to separate and recover phosphoric acid, fluoride and organic matter in wastewater under CO2 pressure conditions. A waste gas treatment device is connected to the gas outlet of the waste heat recovery device to purify the treated flue gas and separate and recycle a portion of the CO2 in it to the pyrolysis furnace. And a wastewater reuse pipeline, connecting the clean water outlet of the wastewater treatment device to the pyrolysis furnace, for reusing the treated clean water as a source of steam.

[0013] Furthermore, the temperature inside the pyrolysis furnace is controlled at 600℃~800℃.

[0014] Furthermore, the wastewater treatment device includes: at least one reaction tower for precipitating phosphates, equipped with a stirrer and a calcium-based precipitant dosing device; at least one reaction tower for precipitating fluoride salts, equipped with a stirrer and a calcium source replenishment device; and at least one adsorption tower internally filled with activated carbon.

[0015] Furthermore, the exhaust gas purification and CO2 circulation device includes a dry deacidification tower, a bag filter and an activated carbon adsorption tower connected in sequence, as well as a CO2 circulation pipeline leading from the purified flue back to the pyrolysis furnace.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention utilizes a synergistic atmosphere of CO2 and water vapor for pyrolysis. By leveraging the acidic effect of CO2, the crystal structure of phosphogypsum is efficiently destroyed at a relatively low temperature of 600℃ to 800℃, releasing eutectic phosphorus that is difficult to remove using traditional methods. Simultaneously, it promotes the volatilization of fluorine and the decomposition of organic matter. This significantly improves the efficiency of impurity removal, resulting in high-purity gypsum that can be directly used as a raw material for high-value-added building materials.

[0017] 2. Compared to traditional calcination methods, this process reduces the pyrolysis temperature by 200℃ to 400℃ and energy consumption by at least 30%, resulting in significant cost reduction and efficiency improvement. Simultaneously, the waste heat generated is recovered for material preheating and steam production, and the CO2 in the exhaust gas is recycled. A portion of the purified exhaust gas is directly returned to the pyrolysis furnace via a circulating fan, utilizing its high CO2 content to maintain an acidic atmosphere within the furnace, eliminating the need for a separate CO2 separation and purification unit. This not only reduces carbon emissions but also further reduces system complexity and operating costs.

[0018] 3. This process constructs a complete closed-loop system, with a particular emphasis on the resource-based treatment of condensed acidic wastewater. Unlike traditional methods that treat acidic wastewater as a disposal burden, this invention positions it as a potential resource. By introducing CO2 pressurized staged separation technology, valuable elements such as phosphorus and fluorine in the wastewater are gradually separated and enriched, transforming them into marketable byproducts. The treated purified water is returned to the system, heated, and converted into steam for recycling, thus achieving zero wastewater discharge throughout the entire process. This design not only eliminates the risk of secondary pollution but also provides a feasible path for the green, closed-loop operation of the phosphogypsum treatment process. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a system structure diagram of the present invention; Figure 2 This is a process flow diagram of an embodiment of the present invention. Detailed Implementation

[0021] Embodiments of the present invention are described in detail below. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the embodiments of the present invention, and should not be construed as limiting the present invention.

[0022] like Figure 2 As shown, in a first aspect, the present invention provides a process for the pyrolysis and resource recovery of phosphogypsum under a CO2 atmosphere. The implementation process of this process will be described below with reference to different embodiments: Example 1 This embodiment provides a process for the pyrolysis and resource recovery of phosphogypsum under a CO2 atmosphere, using a phosphogypsum processing scale of approximately 300,000 tons per year (based on 6,000 hours of operation per year, i.e., 50 tons / hour) as an example for illustration.

[0023] The specific process flow of this embodiment is as follows: S1: Co-pyrolysis under CO2 atmosphere Phosphogypsum raw material (dry basis processing capacity 50 t / h, moisture content approximately 15%) from a phosphate chemical company is evenly fed into a pyrolysis grate furnace with a thermal power of 30MW via a conveying device. The furnace temperature is controlled at 600℃~800℃.

[0024] The system introduces approximately 60,000 Nm³ of air into the furnace through the bottom air distribution device. 3 A continuous flow of CO2 gas (partially derived from the recirculated gas after waste gas treatment within this system) maintains the CO2 volume fraction within the furnace at 10%–15%. Simultaneously, 2–3 t / h of water vapor is injected into the center of the furnace via nozzles. Under the synergistic effect of the acidic atmosphere created by CO2 and the water vapor, the calcium sulfate dihydrate in the phosphogypsum rapidly undergoes a dehydration reaction, effectively disrupting the crystal lattice structure and releasing the eutectic phosphorus previously contained within the lattice. At the same time, soluble phosphorus, fluorides, and residual organic extractants are also efficiently decomposed or volatilized under these conditions and discharged from the furnace with the flue gas.

[0025] After the pyrolysis reaction is completed, the resulting solid product is high-quality anhydrous gypsum, which is discharged from the end of the grate, cooled by a water-sealed scraper slag remover, and then sent to the finished product warehouse. It can be sold in the market as a high-quality building material raw material.

[0026] S2: High-temperature exhaust gas waste heat recovery The high-temperature flue gas discharged from the pyrolysis furnace outlet, with a temperature of approximately 900℃ to 1000℃, first enters a heat exchanger with a heat exchange area of ​​approximately 240 to 270 m². 2 A shell-and-tube heat exchanger is used. In this embodiment, high-temperature flue gas flows through the tubes, while cooling water (25°C inlet, 45°C outlet) flows through the shell. Through heat exchange, the flue gas temperature drops to approximately 130°C, while the cooling water is heated. This recovered waste heat can be used for preheating boiler feedwater, heating, or as a heat source for other stages of the process. During the heat exchange process, water vapor and some volatile substances in the flue gas condense, forming acidic condensate wastewater with a temperature of approximately 40°C–60°C and a pH value between 2 and 4. The wastewater volume is approximately 5 m³. 3 / h.

[0027] S3: Wastewater CO2 pressurized staged separation and resource recovery The acidic wastewater generated in S2 is collected in a 15 m³ well. 3The wastewater is placed in a buffer tank and then pressurized to 1.0 MPa using a corrosion-resistant pump before being sent to the wastewater treatment unit. This unit consists of three stages of reaction and separation devices connected in series.

[0028] In the first-stage reaction tower, CO2 is introduced into the wastewater along with a metered amount of lime slurry (Ca(OH)2). Under pressure and the carbonation system formed by CO2, the pH value is precisely controlled, causing most of the phosphate ions in the wastewater to precipitate as calcium phosphate (Ca3(PO4)2). The precipitate is then dewatered by a plate and frame filter press and recovered as phosphorus resources.

[0029] The filtrate enters the second-stage reaction tower, where the pH is further adjusted or a calcium source is added to cause the fluoride ions in the wastewater to precipitate out as calcium fluoride (CaF2), which is then recovered by pressure filtration.

[0030] The filtrate from the second stage finally enters the third-stage adsorption tower containing activated carbon, where trace amounts of organic extractants in the wastewater are adsorbed and removed by the activated carbon.

[0031] S4: Water recycling and waste gas purification After the above three-stage treatment, the valuable resources contained in the wastewater have been fully recovered, and the remaining water has been purified. The volume of this purified wastewater is approximately 5 m³. 3 The wastewater is pumped at a rate of / h to the heat exchanger of the waste heat recovery system for preheating, and then injected into the pyrolysis furnace in the form of steam as a supplement to the reaction medium in the aforementioned pyrolysis reaction step (S1). Thus, the entire process achieves the goal of closed-loop wastewater recycling and zero discharge.

[0032] After heat exchange and wastewater condensation and recovery, the flue gas temperature is significantly reduced, and it then enters the waste gas treatment system consisting of a dry acid removal tower, a bag filter, and an activated carbon adsorption tower. In the dry acid removal tower, hydrated lime powder is injected to deeply remove residual acidic gases such as HF and SO2 from the flue gas; the bag filter ensures that the outlet particulate matter concentration does not exceed 10 mg / Nm³. 3 The terminal activated carbon adsorption tower serves as a backup treatment unit, used to adsorb any trace organic pollutants that may be present. The purified flue gas is ultimately discharged into the atmosphere through a 30-meter-high chimney. To maintain the required CO2 atmosphere within the pyrolysis furnace, a bypass is installed after the induced draft fan, extracting approximately 25,000 Nm³ of CO2. 3 The exhaust gas produced per hour is pressurized by a circulating fan and then sent back to the pyrolysis furnace, thus achieving partial recycling of CO2.

[0033] The process and system described in this embodiment not only achieves efficient and low-carbon resource utilization of phosphogypsum at the technical level, successfully producing high-quality building material raw materials (such as gypsum board and cement retarder), but also effectively recovers valuable by-products such as phosphorus and fluorine from wastewater. The entire process involves zero wastewater discharge, achieving the goal of zero wastewater discharge. Overall, this process demonstrates significant environmental and economic benefits, providing a green and sustainable technical path for the resource utilization of phosphogypsum.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0035] Example 2 This embodiment provides a process and system for the pyrolysis and resource utilization of phosphogypsum under a CO2 atmosphere, using a phosphogypsum processing scale of approximately 150,000 tons per year (based on 6,000 hours of operation per year, i.e., 25 tons / hour) as an example for illustration.

[0036] The specific process flow of this embodiment is as follows: S1: Co-pyrolysis under CO2 atmosphere Phosphogypsum raw material (dry basis processing capacity 25 t / h, moisture content approximately 15%) from a phosphate chemical company is uniformly fed into a 15MW pyrolysis furnace using a conveying device. The furnace temperature is controlled between 650℃ and 750℃.

[0037] The system introduces approximately 30,000 Nm³ of air into the furnace through the bottom air distribution device. 3 A flow rate of CO2 gas (50% of which comes from the recirculated gas after waste gas treatment in this system) is used to maintain the CO2 volume fraction in the furnace at 12%–15%. Simultaneously, 1.2–1.5 t / h of water vapor (derived from the preheated and evaporated purified wastewater) is injected into the center of the furnace through nozzles. Under the synergistic effect of the acidic atmosphere created by CO2 and the water vapor, the calcium sulfate dihydrate in phosphogypsum undergoes a rapid dehydration reaction, effectively disrupting the crystal lattice structure and releasing the eutectic phosphorus previously contained within the lattice. At the same time, soluble phosphorus, fluorides, and residual organic extractants are also efficiently decomposed or volatilized under these conditions and discharged from the furnace with the flue gas.

[0038] After the pyrolysis reaction is completed, the resulting solid product is high-quality anhydrous gypsum, which is discharged from the end of the grate, cooled by a water-sealed scraper slag remover, and then sent to the finished product warehouse. It can be sold in the market as a high-quality building material raw material.

[0039] S2: High-temperature exhaust gas waste heat recovery The high-temperature flue gas discharged from the pyrolysis furnace outlet, with a temperature of approximately 900℃~1000℃, first enters a heat exchanger with a heat exchange area of ​​approximately 130 m². 2 A shell-and-tube heat exchanger is used. In this embodiment, high-temperature flue gas flows through the tubes, while cooling water (25°C inlet, 45°C outlet) flows through the shell. Through heat exchange, the flue gas temperature drops to approximately 130°C, while the cooling water is heated. This recovered waste heat can be used for preheating boiler feedwater, heating, or as a heat source for other stages of the process. During the heat exchange process, water vapor and some volatile substances in the flue gas condense, forming acidic condensate wastewater with a temperature of approximately 40°C–60°C and a pH value between 2 and 4, with a wastewater volume of approximately 2.5 m³. 3 / h.

[0040] S3: Wastewater CO2 pressurized staged separation and resource recovery The acidic wastewater generated in S2 is collected in an 8 m... 3 The wastewater is placed in a buffer tank and then pressurized to 1.0 MPa using a corrosion-resistant pump before being sent to the wastewater treatment unit. This unit consists of three stages of reaction and separation devices connected in series.

[0041] In the first-stage reaction tower, CO2 is introduced into the wastewater along with a metered amount of lime slurry (Ca(OH)2), approximately 60 kg / h. Under pressure and the carbonation system formed by CO2, the pH value is precisely controlled to 4.0–4.5, causing most of the phosphate ions in the wastewater to precipitate as calcium phosphate (Ca3(PO4)2). The precipitate is then dewatered by a plate and frame filter press and used for phosphorus resource recovery (yield approximately 65 kg / h).

[0042] The filtrate enters the second-stage reaction tower, where the pH is further adjusted to 6.5–7.0 or a calcium source (CaCl2 solution) is added to precipitate fluoride ions in the wastewater as calcium fluoride (CaF2) (yield of approximately 14 kg / h), which is then recovered by pressure filtration.

[0043] The filtrate from the second stage finally enters the third-stage adsorption tower containing activated carbon, where trace amounts of organic extractants in the wastewater are adsorbed and removed by the activated carbon.

[0044] S4: Water recycling and waste gas purification After the above three-stage treatment, the valuable resources contained in the wastewater have been fully recovered, and the remaining water has been purified. The volume of the purified wastewater generated is approximately 2.5 m³. 3 The wastewater is pumped at a rate of / h to the heat exchanger of the waste heat recovery system for preheating, and then injected into the pyrolysis furnace in the form of steam as a supplement to the reaction medium in the aforementioned pyrolysis reaction step (S1). Thus, the entire process achieves the goal of closed-loop wastewater recycling and zero discharge.

[0045] After heat exchange and wastewater condensation and recovery, the flue gas temperature is significantly reduced, and it then enters the waste gas treatment system consisting of a dry acid removal tower, a bag filter, and an activated carbon adsorption tower. In the dry acid removal tower, hydrated lime powder is injected to deeply remove residual acidic gases such as HF and SO2 from the flue gas; the bag filter ensures that the outlet particulate matter concentration does not exceed 10 mg / Nm³. 3 The terminal activated carbon adsorption tower serves as a backup treatment unit, used to adsorb any trace organic pollutants that may be present. The purified flue gas is ultimately discharged into the atmosphere through a 30-meter-high chimney. To maintain the required CO2 atmosphere within the pyrolysis furnace, a bypass is installed after the induced draft fan, extracting approximately 12,000 Nm³ of CO2. 3 The exhaust gas produced per hour is pressurized by a circulating fan and then sent back to the pyrolysis furnace, thus achieving partial recycling of CO2.

[0046] The process and system described in this embodiment not only achieves efficient and low-carbon resource utilization of phosphogypsum at the technical level, successfully producing high-quality building material raw materials (such as gypsum board and cement retarder), but also effectively recovers valuable by-products such as phosphorus and fluorine from wastewater. The entire process involves zero wastewater discharge, achieving the goal of zero wastewater discharge. Overall, this process demonstrates significant environmental and economic benefits, providing a green and sustainable technical path for the resource utilization of phosphogypsum.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0048] Example 3 This embodiment provides a process and system for the pyrolysis and resource utilization of phosphogypsum under a CO2 atmosphere, which is particularly suitable for phosphogypsum raw materials with high organic matter content (organic matter ≥ 0.3%). The example is a phosphogypsum processing scale with an annual processing capacity of about 300,000 tons (based on 6,000 hours of operation per year, i.e., 50 tons / hour).

[0049] The specific process flow of this embodiment is as follows: S1: Enhanced pyrolysis Phosphogypsum raw material (dry basis processing capacity 50 t / h, moisture content approximately 18%, organic matter content ≥0.3%) from a phosphate chemical company is uniformly fed into a pyrolysis furnace with a thermal power of 30MW via a conveying device. The furnace temperature is controlled at 750℃~800℃ (still within the range of 600~800℃).

[0050] The system introduces approximately 65,000 Nm³ of air into the furnace through the bottom air distribution device. 3A flow rate of CO2 gas (50% of which comes from the recirculated gas after waste gas treatment in this system) is used to maintain the CO2 volume fraction in the furnace at 14%–15% (not exceeding the range of 10%–15%). Simultaneously, 3.5–4.0 t / h of water vapor (derived from the preheating and evaporation of purified wastewater) is injected into the center of the furnace through nozzles to promote the water vapor reforming reaction of high-boiling-point organic matter. Under the synergistic effect of the acidic atmosphere created by CO2 and the water vapor, calcium sulfate dihydrate in phosphogypsum rapidly undergoes a dehydration reaction, effectively disrupting the crystal lattice structure and releasing the eutectic phosphorus originally contained in the lattice. At the same time, high-content organic matter is efficiently decomposed or gasified, and soluble phosphorus and fluorides are also discharged from the furnace with the flue gas.

[0051] After the pyrolysis reaction is completed, the resulting solid product is high-quality anhydrous gypsum with an organic carbon residue of <0.02% and a whiteness of over 85%. It is discharged from the end of the grate, cooled by a water-sealed scraper slag remover, and then sent to the finished product warehouse. It can be sold in the market as a high-value-added building material raw material (such as high-grade gypsum board, self-leveling mortar, etc.).

[0052] S2: High-temperature exhaust gas waste heat recovery The high-temperature flue gas discharged from the pyrolysis furnace outlet, with a temperature of approximately 900℃~1000℃, first enters a heat exchanger with a heat exchange area of ​​approximately 270 m². 2 A shell-and-tube heat exchanger is used. In this embodiment, high-temperature flue gas flows through the tubes, while cooling water (25°C inlet, 45°C outlet) flows through the shell. Through heat exchange, the flue gas temperature drops to approximately 120°C (still within a reasonable range), while the cooling water is heated. This recovered waste heat can be used for preheating boiler feedwater, heating, or as a heat source for other stages of the process. During the heat exchange process, water vapor and some volatile substances in the flue gas condense, forming acidic condensate wastewater with a temperature of approximately 40°C–60°C and a pH value between 2 and 3, with a wastewater volume of approximately 6 m³. 3 The concentration of organic matter (TOC) in the wastewater is about 50% higher than that of conventional phosphogypsum.

[0053] S3: Wastewater CO2 pressurized staged separation and resource recovery The acidic wastewater generated in S2 is collected in an 18 m³ container. 3 The wastewater is placed in a buffer tank (increasing the buffer volume to cope with water quality fluctuations), and then pressurized to 1.0 MPa using a corrosion-resistant pump before being sent to the wastewater treatment unit. This embodiment still uses a three-stage series reaction separation device, but the retention time of each stage is appropriately extended for high-organic-matter wastewater.

[0054] In the first-stage reaction tower, CO2 is introduced into the wastewater along with a metered amount of lime slurry (Ca(OH)2). Under pressure and the carbonation system formed by CO2, the pH value is precisely controlled to 4.0–4.5, causing most of the phosphate in the wastewater to precipitate as calcium phosphate (Ca3(PO4)2). Since organic matter may interfere with precipitation, the lime slurry dosage is appropriately increased (approximately 140 kg / h) to ensure a phosphate removal rate >95%. The precipitate is dewatered using a plate and frame filter press and then recovered as phosphorus resources.

[0055] The filtrate enters the second-stage reaction tower, where the pH is further adjusted to 6.5–7.0 and a calcium source (CaCl2 solution) is added to precipitate fluoride ions in the wastewater as calcium fluoride (CaF2), achieving a fluoride removal rate of >90%. The fluoride is then recovered by pressure filtration.

[0056] The filtrate from the second stage finally enters the third-stage adsorption tower containing activated carbon. To cope with the high organic load, the height of the activated carbon layer was increased from 1.5 m to 2.5 m, and a two-stage series adsorption (i.e., two adsorption towers operating in series) was adopted to ensure the organic matter removal rate. Trace organic extractants in the wastewater are fully adsorbed by the activated carbon. After the above treatment, the TOC of the purified wastewater is <15 mg / L.

[0057] S4: Water recycling and waste gas purification After the above three-stage treatment, the valuable resources contained in the wastewater have been fully recovered, and the remaining water has been purified. The volume of this purified wastewater is approximately 6 m³. 3 The wastewater is pumped at a rate of / h to the heat exchanger of the waste heat recovery system for preheating, and then injected into the pyrolysis furnace in the form of steam as a supplement to the reaction medium in the aforementioned pyrolysis reaction step (S1). Thus, the entire process achieves the goal of closed-loop wastewater recycling and zero discharge.

[0058] After heat exchange and wastewater condensation and recovery, the flue gas temperature is significantly reduced, and it then enters the waste gas treatment system consisting of a dry acid removal tower, a bag filter, and an activated carbon adsorption tower. In the dry acid removal tower, hydrated lime powder is injected to deeply remove residual acidic gases such as HF and SO2 from the flue gas; the bag filter ensures that the outlet particulate matter concentration does not exceed 10 mg / Nm³. 3 The terminal activated carbon adsorption tower serves as a backup treatment unit, used to adsorb any trace organic pollutants that may be present. Due to the high concentration of organic matter at the inlet, the activated carbon replacement cycle has been shortened to two months (but it remains existing equipment; no new unit has been added). The purified flue gas is ultimately discharged into the atmosphere through a 30-meter-high chimney. To maintain the required CO2 atmosphere inside the pyrolysis furnace, a bypass is installed after the induced draft fan, extracting approximately 28,000 Nm³ of CO2. 3The exhaust gas per hour (still within a reasonable range) is pressurized by a circulating fan and sent back to the pyrolysis furnace, thereby achieving partial recycling of CO2.

[0059] The process and system described in this embodiment were used to verify the adaptability of the process to high-organic-matter phosphogypsum. The total organic matter removal rate reached over 96%, successfully producing high-quality building material raw materials. Simultaneously, the purity of calcium phosphate and calcium fluoride recovered from wastewater reached 90% and 85%, respectively (achieved through optimized sedimentation conditions). The entire process resulted in zero wastewater discharge, achieving the goal of zero wastewater discharge. Overall, this process demonstrates significant environmental and economic benefits, providing a green and sustainable technological path for the resource utilization of high-organic-matter phosphogypsum.

[0060] like Figure 1 As shown, a second aspect of the present invention also provides a pyrolysis resource recovery system for phosphogypsum under a CO2 atmosphere, the system comprising: A pyrolysis furnace for pyrolysis of phosphogypsum, equipped with a CO2 and water vapor injection device inside; A waste heat recovery device (heat exchanger) is connected to the tail gas outlet of the pyrolysis furnace to recover the waste heat of the tail gas and condense the generated acidic wastewater. A wastewater treatment device, connected to the condensate outlet of the heat exchanger, is used to separate and recover phosphoric acid, fluoride and organic matter in wastewater under CO2 pressure conditions. A waste gas treatment device is connected to the gas outlet of the waste heat recovery device to purify the treated flue gas and separate and recycle a portion of the CO2 in it to the pyrolysis furnace. And a wastewater reuse pipeline, connecting the clean water outlet of the wastewater treatment device to the pyrolysis furnace, for reusing the treated clean water as a source of steam.

[0061] The temperature inside the pyrolysis furnace is controlled between 600℃ and 800℃.

[0062] The wastewater treatment device includes: at least one reaction tower for precipitating phosphates, equipped with a stirrer and a calcium-based precipitant dosing device; at least one reaction tower for precipitating fluoride salts, equipped with a stirrer and a calcium source replenishment device; and at least one adsorption tower filled with activated carbon.

[0063] The exhaust gas purification and CO2 circulation device includes a dry deacidification tower, a bag filter and an activated carbon adsorption tower connected in sequence, as well as a CO2 circulation pipeline leading from the purified flue back to the pyrolysis furnace.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A process for the resource utilization of phosphogypsum through pyrolysis under a CO2 atmosphere, characterized in that, Includes the following steps: S1: The phosphogypsum is fed into a pyrolysis furnace and pyrolyzed at a temperature of 600℃~800℃. During the pyrolysis process, CO2 gas and water vapor are introduced into the furnace to form a CO2 acidic synergistic atmosphere, which promotes the dehydration of calcium sulfate dihydrate, the release of eutectic phosphorus, the decomposition of organic impurities and the volatilization of fluorides in the phosphogypsum, so as to obtain high-quality anhydrous gypsum products and high-temperature tail gas. S2: The high-temperature exhaust gas generated in step S1 is introduced into the waste heat recovery device to exchange heat with the cooling medium to recover the waste heat of the flue gas, and acidic wastewater is condensed in the process. S3: Collect the acidic wastewater generated by condensation in step S2 and send it to the wastewater treatment device. Under the condition of CO2 pressurization to 1.0 MPa, the pH value and reaction conditions are controlled in stages to separate phosphate, fluoride and organic matter in the wastewater in sequence, so as to recover phosphate, fluoride and organic by-products. S4: The wastewater separated and purified in step S3 is preheated by the waste heat recovery device and then reintroduced into the pyrolysis furnace described in step S1 as supplementary steam to achieve zero wastewater discharge.

2. The process according to claim 1, characterized in that, In step S1, the CO2 acidic synergistic atmosphere is achieved by controlling the CO2 volume fraction in the furnace to be between 10% and 15% and by introducing water vapor.

3. The process according to claim 1, characterized in that, In step S1, the pyrolysis furnace provides a heat source by burning bituminous coal, and the grate area heat load is 0.4 MW / m². 2 -0.9 MW / m 2 .

4. The process according to claim 1, characterized in that, In step S2, the waste heat recovery device is a shell-and-tube heat exchanger, the cooling medium is water, and the recovered waste heat is used to preheat combustion air, generate process steam, or provide heating.

5. The process according to claim 1, characterized in that, In step S3, the method for graded control of pH and reaction conditions under CO2 pressurization is as follows: CO2 is introduced into the wastewater and a calcium-based precipitant is added. By controlling the amount of CO2 introduced and the amount of calcium-based precipitant added, the pH of the wastewater is graded to different ranges: First, the pH is adjusted to 3.5~4.5 so that phosphate ions precipitate out in the form of calcium phosphate; after solid-liquid separation, the pH is adjusted to 6.0~7.0 and a calcium source is added so that fluoride ions precipitate out in the form of calcium fluoride; finally, activated carbon adsorption is used to remove residual organic matter.

6. The process according to claim 1, characterized in that, It also includes step S5: purifying the tail gas after waste heat recovery and wastewater condensation in step S2. The purification process includes dry deacidification, bag filter dust removal and activated carbon adsorption in sequence. The tail gas is discharged after purification meets the standards, and a portion of the CO2 gas is separated and recycled back to the pyrolysis furnace in step S1.

7. A CO2 atmosphere pyrolysis resource recovery system for implementing the process described in any one of claims 1 to 6, characterized in that, include: (1) A pyrolysis furnace for pyrolysis of phosphogypsum, which is equipped with a CO2 gas injection device and a water vapor injection device. (2) Waste heat recovery and wastewater generation device, connected to the tail gas outlet of the pyrolysis furnace, used to recover waste heat from flue gas and condense it to generate acidic wastewater; (3) Wastewater treatment device, connected to the condensate outlet of the waste heat recovery and wastewater generation device, used to sequentially separate and recover phosphates, fluorides and organic matter in wastewater under CO2 pressure conditions; (4) Wastewater reuse pipeline, connecting the purified water outlet of the wastewater treatment device to the pyrolysis furnace, for reusing purified wastewater as a source of steam to the pyrolysis furnace; (5) Waste gas purification and CO2 recycling device, which is connected to the gas outlet of the waste heat recovery and wastewater generation device, is used to purify the tail gas and recycle part of the CO2 back to the pyrolysis furnace.

8. The system according to claim 7, characterized in that, The temperature inside the pyrolysis furnace is controlled at 600℃~800℃.

9. The system according to claim 7, characterized in that, The wastewater treatment device includes: at least one reaction tower for precipitating phosphates, equipped with a stirrer and a calcium-based precipitant dosing device; at least one reaction tower for precipitating fluoride salts, equipped with a stirrer and a calcium source replenishment device; and at least one adsorption tower filled with activated carbon.

10. The system according to claim 7, characterized in that, The exhaust gas purification and CO2 circulation device includes a dry deacidification tower, a bag filter and an activated carbon adsorption tower connected in sequence, as well as a CO2 circulation pipeline leading from the purified flue back to the pyrolysis furnace.

Images