Phosphogypsum fluidized semi-on-line low-temperature off-kiln decomposition furnace

Abstract

The application discloses a phosphogypsum fluidization semi-online low-temperature out-kiln decomposition furnace, which comprises a reduction decomposition furnace, the reduction decomposition furnace comprises a bottom fluidized bed, a middle suspension furnace and an oxidation reburning furnace arranged in sequence in the vertical direction, and the oxidation reburning furnace is arranged downstream of the middle suspension furnace; the bottom fluidized bed is provided with a fluidization air nozzle, a wind distribution plate, a raw material feeding port and a pulverized coal inlet, is used for forming a dense phase zone boiling layer and making the pulverized coal incompletely burn to generate a reducing atmosphere, and is used for decomposing CaSO4 into CaS at low temperature; the middle suspension furnace is a circular tube type furnace body, and is internally provided with a necking portion and is used for prolonging the suspension residence time of materials; and the oxidation reburning furnace is used for burning off residual CO and oxidizing part of CaS into CaO. The application realizes the low-temperature decomposition of the phosphogypsum outside the kiln, moves the reduction decomposition to outside the kiln, and realizes the oxidation calcination in the kiln, so that the decomposition temperature is low, the preheater is not blocked by skinning, the working mode can be flexibly switched, and the application is suitable for the production of the cement clinker and the acid co-production by the calcination and decomposition of the phosphogypsum.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization technology, specifically relating to a fluidized bed semi-online low-temperature external kiln decomposition furnace for phosphogypsum, which is suitable for calcining and decomposing phosphogypsum to produce acid and cement clinker, supporting the engineering realization of comprehensive utilization of industrial by-product gypsum resources. Background Technology

[0002] Phosphogypsum is a typical industrial solid waste. According to incomplete statistics, China's phosphogypsum emissions reached 90 million tons in 2025, yet the utilization rate is less than 60%, and the international comprehensive utilization rate is less than 30%. Currently, phosphogypsum is mainly treated through stockpiling using harmless disposal technologies. This method not only occupies a large amount of land but also poses significant environmental risks and safety hazards. The main component of phosphogypsum is calcium sulfate (CaSO4). In addition to calcium and sulfur, it also contains trace amounts of silicon, iron, aluminum, phosphorus, and fluorine. Except for sulfur, phosphorus, and fluorine, the other elements are commonly used in cement raw materials.

[0003] Unlike neutral and relatively pure natural gypsum and desulfurized gypsum, phosphogypsum has extremely unique raw material characteristics that decisively influence its decomposition process. First, phosphogypsum is typically acidic, with a free water content as high as 18%–25%, often requiring complex pretreatment such as drying and calcination before decomposition. Second, its coarse crystal structure, dense texture, and residual impurities such as phosphorus, fluorine, and silicon contribute to a higher decomposition temperature and more stringent decomposition conditions. Without the addition of reducing agents, the natural decomposition temperature of phosphogypsum is very high, but the decomposition rate is very low; a catalyst or highly active reducing agent, along with a longer residence time, is necessary to effectively lower the reaction temperature.

[0004] Currently, in the practice of phosphogypsum-based sulfuric acid and cement co-production projects, the preheater kiln process is commonly used. Both calcium sulfate decomposition and clinker calcination occur within the rotary kiln. The simultaneous presence of both reducing decomposition and oxidizing calcination atmospheres within the 100-meter-long kiln makes it difficult to control the kiln atmosphere, affecting stable operation. Furthermore, the cement clinker product qualification rate is low, and insufficient C3S mineral formation leads to low early strength. High energy consumption further reduces the SO2 gas concentration at the preheater outlet (5-7.5%). External preheating technology moves gypsum decomposition outside the rotary kiln to an external preheating furnace, which is beneficial for controlling the kiln atmosphere and for stable cement calcination under an oxidizing atmosphere. However, existing external preheating technologies, as seen in experimental articles and patent reports, such as the circulating fluidized bed where coal is first oxidized to generate CO gas as an intermediate phase, and then CO reacts with gypsum in a gas-solid reaction to generate calcium oxide, will produce CaS, residual CO, and sublimed sulfur from side reactions under a reducing atmosphere. This is the main technical bottleneck restricting industrialization.

[0005] In summary, the above process still has the following technical defects: 1. The calcination reaction mechanism of phosphogypsum is complex and the process window is narrow. The suitable decomposition temperature range (1000-1200℃) and reducing atmosphere conditions for phosphogypsum are very limited, close to the temperature at which the liquid phase appears during the calcination of cement clinker under an oxidizing atmosphere (1250-1280℃). The sintering range is narrow; phosphogypsum is difficult to decompose at low temperatures, while calcium sulfate easily burns at high temperatures. Even slight fluctuations in the thermal regime can lead to a decrease in decomposition rate, unstable product purity, or equipment crusting and ring formation. Furthermore, the external decomposition system is sensitive to fluctuations in the moisture and impurity content of the raw materials, requiring high pretreatment standards, making it difficult to control scale-up.

[0006] 2. The core bottleneck of phosphogypsum external decomposition technology lies primarily in the problem of material adhesion and blockage at high temperatures. Due to the presence of impurities in phosphogypsum, the material is prone to softening, agglomeration, and clumping above 1100℃. Calcium sulfate, before it has decomposed into CaO, easily adheres to the furnace walls and pipes, preventing the unit from operating continuously and stably for extended periods, resulting in a low SO2 extraction rate from phosphogypsum. This engineering challenge is considered a direct obstacle to the industrial application of this technology.

[0007] 3. Insufficient economic viability and product value. Although pre-calciner technology can increase the SO2 concentration in kiln gas in the laboratory, it still lacks a cost advantage compared to modern sulfuric acid production processes, and the quality of the co-produced cement clinker is unstable due to impurities. High equipment investment, operating energy consumption, and maintenance costs make it difficult to achieve large-scale application of this technology under current economic conditions. Summary of the Invention

[0008] To address the problems existing in the prior art, this invention provides a fluidized semi-online low-temperature external decomposition furnace for phosphogypsum, which features a compact structure, low decomposition temperature, controllable atmosphere, and strong adaptability. It realizes the engineering application of external decomposition technology for the decomposition of phosphogypsum to produce sulfuric acid and co-produce cement, achieving the transformation of phosphogypsum solid waste from harmless to highly efficient resource utilization.

[0009] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A fluidized bed semi-online low-temperature kiln external decomposition furnace for phosphogypsum includes a reduction decomposition furnace, which comprises a bottom fluidized bed and a middle suspension furnace arranged sequentially along a vertical direction; and an oxidation and reburning furnace disposed downstream of the middle suspension furnace. The bottom fluidized bed is equipped with a high-pressure fluidizing air inlet, fluidizing air nozzles, an air distribution plate, a raw material feed inlet, and a pulverized coal inlet. The fluidizing air nozzles are embedded in the air distribution plate to ensure that the high-pressure fluidizing air entering from the high-pressure fluidizing air inlet is evenly distributed within the bottom fluidized bed. The raw material feed inlet and the pulverized coal inlet are located on the side of the bottom fluidized bed. The raw material feed inlet of the bottom fluidized bed is connected to the feed pipe of the second-to-last stage cyclone separator of the preheater. The bottom fluidized bed is used to form a dense phase boiling layer and to generate reducing gas CO and residual carbon particles through incomplete combustion of pulverized coal, thus forming a reducing atmosphere that decomposes CaSO4 in the phosphogypsum raw material at low temperature into decomposition products containing CaS. The central suspension furnace is a cylindrical furnace body, with its bottom connected to the bottom fluidized bed. The side of the central suspension furnace is provided with a tertiary air inlet. The distance between the tertiary air inlet and the bottom fluidized bed controls the gas residence time in the dense phase zone to be greater than 0.5s and the solid residence time to be greater than 30s. The interior of the central suspension furnace is provided with at least one constriction. The central suspension furnace is used to extend the suspension residence time of solid materials. The oxidation and recombustion furnace is equipped with a tertiary air inlet, and the outlet of the central suspension furnace is connected to the inlet of the oxidation and recombustion furnace, so that the material gas discharged from the central suspension furnace, the flue gas from the rotary kiln, and the tertiary air enter the oxidation and recombustion furnace together, burn off the residual CO and pulverized coal, and oxidize CaS to CaO.

[0010] In the above technical solution, preferably, the oxidation and recombustion furnace includes an oxidation main furnace and an oxidation auxiliary furnace. Both the oxidation main furnace and the oxidation auxiliary furnace are provided with a tertiary air inlet and a pulverized coal inlet at the bottom, and a spray constriction in the middle. The furnace body wind speed is 6~10m / s, and the wind speed at the middle constriction is 15~20m / s. Furthermore, if the CO concentration in the flue gas is required to be controlled as low as 300~500ppm, the height of the oxidation auxiliary furnace is controlled to be greater than 30m to ensure that the CO is fully combusted. The outlet of the central suspension furnace is connected to the inlet of the main oxidation furnace, so that the gas discharged from the central suspension furnace, the flue gas from the rotary kiln, and the tertiary air enter the main oxidation furnace together. By adjusting the temperature and atmosphere, CO is eliminated by combustion and CaS is oxidized to CaO, and the resynthesis of CaSO4 is inhibited. The outlet of the main oxidation furnace is connected to the inlet of the last stage cyclone of the preheater. The auxiliary oxidation furnace is set on the flue gas pipeline between the gas outlet of the last stage cyclone of the preheater and the gas inlet of the second-to-last stage cyclone of the preheater. It is used to further eliminate residual CO and combustibles in the gas flow, so that the flue gas entering the downstream cyclone of the preheater along the flue gas flow direction is an oxidizing atmosphere.

[0011] In the above technical solution, preferably, the bottom fluidized bed has a conical structure, the air distribution plate is made of refractory material, and the air velocity of the fluidizing air nozzle is 30-60 m / s; Raw materials and pulverized coal are introduced into the bottom fluidized bed. A feeding box is provided at the raw material feeding port, and the pulverized coal inlet is located on the back of the feeding box and at its lower part. The pulverized coal inlet is located below the tertiary air inlet of the middle suspension furnace to form an oxygen-deficient zone in the bottom fluidized bed. The residence time of solid materials in the dense phase boiling layer is more than 30 seconds.

[0012] In the above technical solution, preferably, the tertiary air inlet of the central suspension furnace is obliquely downward, and the lower edge of the tertiary air inlet is higher than the pulverized coal inlet of the bottom fluidized bed; the length-to-diameter ratio of the central suspension furnace is 10-60, and the interior of the central suspension furnace is provided with 1-3 constrictions; the residence time of solid materials in the central suspension furnace is more than 30s; the material flow in the reduction decomposition furnace forms a dense phase zone and a dilute phase zone sequentially from the bottom upward with the tertiary air entering from the side as the boundary, the wind speed in the dense phase zone is 4-6m / s, and the wind speed in the dilute phase zone is 8-16m / s.

[0013] In the above technical solution, preferably, the bottom fluidized bed and the middle suspension furnace are arranged in a semi-online manner; A bypass pipe is provided between the bottom fluidized bed and the feed pipe of the second-to-last stage cyclone of the preheater. The bypass pipe is used to bypass the phosphogypsum raw material to the oxidation and reburning furnace above the rotary kiln smoke chamber, so as to realize the switching between the preheater kiln working mode and the pre-decomposition kiln working mode.

[0014] In the above technical solution, it is further preferred that the bypass pipeline is configured to directly bypass the material from the second-to-last stage cyclone of the preheater to the oxidation and reburning furnace through the bypass pipeline when the preheater is in the initial stage of ignition or when the bottom fluidized bed is in an abnormal operating condition, so as to operate in the preheater kiln mode. After the air from the cooling unit exchanges heat with the high-temperature clinker at the rotary kiln outlet to generate tertiary air, the material is switched to the bottom fluidized bed and operated in pre-decomposition kiln mode.

[0015] In the above technical solution, preferably, the side of the central suspension furnace is also provided with a solid waste inlet, which is used to add one or more of steel slag, sludge, sulfur slag, and biomass to the central suspension furnace to reduce coal consumption.

[0016] In the above technical solution, it is further preferred that the amount of coal used in the reduction decomposition furnace is 30-70% of the total amount of coal in the calcination system; the total tertiary air volume accounts for 30-70% of the equivalent theoretical combustion air volume of the calcination system; and it also includes a tertiary air distribution pipeline, which is used to divide the tertiary air from the cooler into three paths and send them to the central suspension furnace, the main oxidation furnace of the oxidation reburning furnace, and the auxiliary oxidation furnace, respectively.

[0017] In the above technical solution, a further preferred embodiment is that the dust collection efficiency of the final stage cyclone in the preheater is greater than 90%, and a return channel is provided between the feed pipe of the final stage cyclone and the raw material feed port of the bottom fluidized bed. The return channel is used to return some of the separated coarse particles to the bottom fluidized bed or the middle suspension furnace for recycling and decomposition. The feed pipe of the final stage cyclone is also connected to the external discharge silo to discharge the decomposition products containing CaO produced to the external discharge silo as calcium powder or calcium-based materials and cementing materials.

[0018] In the above technical solution, it is further preferred that the inlet of the last stage cyclone of the preheater and / or the feed pipe of the last stage cyclone of the preheater are provided with an auxiliary reducing agent inlet, which is used to supplement the auxiliary reducing agent for reduction adjustment or switching to the working mode of the preheater kiln.

[0019] The advantages and positive effects of this invention are: 1. This invention employs a fluidized bed design in the dense phase zone, creating dense conditions for the reducing agent at the bottom of the reduction decomposition furnace, extending the decomposition time of phosphogypsum, and thus creating conditions for low-temperature design. Under conditions of high reducibility and long residence time, the decomposition temperature of calcium sulfate in phosphogypsum is reduced, steadily shifting conventional reduction decomposition outside the rotary kiln. This provides a prerequisite for stable operating conditions of oxidation calcination inside the rotary kiln, preventing incomplete decomposition of CaSO4 leading to high-temperature fluidization (melting temperature ~1220℃) within the kiln, and ensuring the formation temperature of C3S in the clinker minerals inside the kiln. Therefore, this invention provides a low-temperature external decomposition furnace that solves the problem of industrial scale-up.

[0020] 2. In this invention, the pulverized coal used as a reducing agent adopts a "two-step" burnout mode. Under oxygen-deficient conditions in the bottom fluidized bed, the pulverized coal is first cracked and gasified to generate CO and carbon particles, which then come into full contact with phosphogypsum to form a dense phase zone, subsequently reducing and decomposing the phosphogypsum. CO is prepared from the pulverized coal through the bottom fluidized bed, serving as a gaseous reducing agent, replacing the conventional solid reducing agent of coke. Furthermore, the reduction process occurs within the reduction decomposition furnace, where the solid material is in a suspended state, resulting in a large contact area and better reduction effect, achieving the reaction conditions required for the low-temperature decomposition of CaSO4 into CaS. The flue gas exiting the reduction decomposition furnace enters the acid production system, resulting in a high sulfur dioxide content (10%-20%) and lower acid production costs.

[0021] 3. This invention is equipped with an oxidation-reburning furnace. The CaS at the outlet of the reduction decomposition furnace can contact oxygen in the tertiary air to generate CaO. Furthermore, residual CO mixes with the rotary kiln flue gas and tertiary air in the oxidation-reburning furnace, and fully contacts the oxygen introduced by the tertiary air, thus burning completely. This avoids a reducing atmosphere in the preheater, thereby preventing the formation of scale and blockage by harmful components. Moreover, the gradient zoning of the tertiary air supply dissipates excess carbon monoxide in the flue gas, solving the problems of CO emissions, scale buildup and blockage under a reducing atmosphere, and the generation of sublimated sulfur in the preheater.

[0022] 4. The decomposition furnace of this invention adopts a semi-offline design. The combination of high-pressure fluidizing air and tertiary air creates a longer residence time for materials, ensuring the possibility of preheater kiln operation during the initial ignition of the external decomposition furnace. In special working conditions such as fluctuations in phosphogypsum feed, the system can switch between preheater kiln and pre-decomposition kiln modes, thus enhancing the system's adaptability.

[0023] 5. This invention improves the decomposition ratio of phosphogypsum by circulating the material flow into the reduction decomposition furnace through the final-stage cyclone separator, thus avoiding the problem of undecomposed calcium sulfate being encapsulated in the clinker by the liquid phase, resulting in low clinker strength. Compared with traditional gypsum-based cement production processes, the rotary kiln uses clinker calcination as the dominant reaction, allowing for a 10-50% reduction in equipment size; the decomposition process occurs within the reduction decomposition furnace and is a suspension calcination process, resulting in lower system energy consumption and a corresponding reduction in flue gas volume, which further increases the SO2 concentration in the flue gas.

[0024] 6. The external decomposition furnace designed in this invention moves the decomposition of phosphogypsum from the conventional rotary kiln to a semi-online low-temperature external decomposition furnace set outside the rotary kiln. Powder and a small amount of tertiary air are injected into the bottom of the reduction decomposition furnace to create a strong reducing atmosphere. The fluidized bed design prolongs the material residence time and the uniform mixing and mass transfer with the reducing agent, thereby reducing the decomposition temperature of phosphogypsum. The flue gas from the outlet of the reduction decomposition furnace and the flue gas from the rotary kiln enter the oxidation and reburning furnace together, forming a dual-furnace external decomposition system, which prolongs the decomposition time of phosphogypsum and solves the problem of reducing atmosphere in the preheater.

[0025] This invention is applied to the process of producing acid and cement from phosphogypsum, which solves the problems of cement clinker quality and strength, increases SO2 gas concentration, and improves the production capacity of rotary kilns. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the fluidized bed semi-online low-temperature kiln external decomposition furnace for phosphogypsum provided in an embodiment of the present invention; Figure 2 A side view of the reduction decomposition furnace provided in an embodiment of the present invention; Figure 3 for Figure 1 A schematic diagram of the local structure at point D1; Figure 4 for Figure 2 A schematic diagram of the local structure at point D2; Figure 5 This is a schematic diagram of the structure of the fluidizing air nozzle and the air distribution plate provided in an embodiment of the present invention; Figure 6 A process flow diagram for the co-production of acid from phosphogypsum and cement clinker provided in an embodiment of the present invention; Figure 7 A flow chart illustrating the switchable process for co-producing cement clinker and calcium-based materials from phosphogypsum in an embodiment of the present invention.

[0027] In the diagram: 1. Preheater; 1-1. First-stage cyclone separator; 1-2. Second-stage cyclone separator; 1-3. Third-stage cyclone separator; 1-4. Penultimate-stage cyclone separator; 1-5. Final-stage cyclone separator; 2. Reduction decomposition furnace; 2-1. High-pressure fluidizing air inlet; 2-2. Fluidizing air nozzle; 2-3. Air distribution plate; 2-4. Feeding box; 2-5. Pulverized coal inlet; 2-6. Solid waste inlet; 2-7. Narrowing; 2A. Main oxidation furnace; 2B. Secondary oxidation furnace; 3. Rotary kiln; 4. Cooler; 5-1. First branch; 5-2. Second branch; 5-3. Third branch; 6. Bypass pipe; 7. Return material channel; 8. External discharge bin. Detailed Implementation

[0028] The technical solution of the present invention will now be clearly and completely described with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0030] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0031] The transformation of industrial by-product phosphogypsum from harmless disposal to resource utilization is a crucial issue in my country's comprehensive solid waste utilization. Phosphogypsum calcination for acid production and cement co-production is an economically feasible large-scale treatment method. Phosphogypsum external decomposition is essentially a complex chemical process involving element recovery. Its goal is to convert phosphogypsum into calcium sulfide (CaS) or calcium oxide (CaO), while simultaneously recovering high-concentration sulfur dioxide gas for acid production, thus forming a resource cycle of "sulfur returning to acid production and calcium returning to building materials." In current calcination processes, the clinker product of phosphogypsum as a calcium-based raw material lags significantly in quality and strength compared to conventional cement production lines. This invention addresses key technical pain points in the phosphogypsum-to-cement process, resulting in significant improvements in system energy consumption and product quality.

[0032] Example 1 like Figures 1-5 As shown, the phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace provided in this embodiment of the invention includes a reduction decomposition furnace 2, which includes a bottom fluidized bed and a middle suspension furnace arranged sequentially in the vertical direction; and an oxidation and reburning furnace arranged downstream of the middle suspension furnace.

[0033] The bottom fluidized bed has a conical structure and is equipped with a high-pressure fluidizing air inlet 2-1, a fluidizing air nozzle 2-2, an air distribution plate 2-3, a raw material feed inlet, and a pulverized coal inlet 2-5. The fluidizing air nozzle 2-2 is embedded in the air distribution plate 2-3, which is made of refractory material, to ensure that the high-pressure fluidizing air entering from the high-pressure fluidizing air inlet 2-1 is evenly distributed within the bottom fluidized bed. The air velocity of the fluidizing air nozzle 2-2 is 30-60 m / s. The raw material feed inlet and the pulverized coal inlet 2-5 are located on the side of the bottom fluidized bed. A feed box 2-4 is installed at the raw material feed inlet, which is connected to the discharge pipe of the second-to-last stage cyclone separator 1-4 of the preheater. The pulverized coal inlet 2-5 is used to supply pulverized coal to the bottom fluidized bed. It is located behind and below the feed box 2-4, and below the tertiary air inlet of the central suspension furnace, to create an oxygen-deficient zone in the bottom fluidized bed. A pulverized coal injection pipe is installed at the pulverized coal inlet 2-5. The feed box 2-4 is located 1-3 meters from the top of the fluidizing air nozzle 2-2. The tertiary air inlet is above the fluidized bed, ensuring that the residence time of the gas in the dense phase zone before the tertiary air enters the decomposition furnace is greater than 0.5 seconds. This allows the pulverized coal to fully gasify and produce carbon particles and CO, creating a high-concentration reducing atmosphere before the material enters. The bottom fluidized bed forms a dense phase boiling layer and allows the incomplete combustion and gasification of pulverized coal to produce reducing gas CO and residual carbon particles, creating a reducing atmosphere. This decomposes CaSO4 in the phosphogypsum raw material at low temperature into decomposition products containing CaS. The residence time of solid materials in the dense phase boiling layer reaches more than 30 seconds.

[0034] The bottom fluidized bed adopts a conical structure and refractory air distribution plate 2-3. High-pressure fluidizing air, combined with high-velocity fluidizing air nozzles 2-2 (30-60 m / s), forms a stable dense-phase boiling layer at the bottom upon contact with the material flow. The pulverized coal inlet 2-5 is located behind and below the feed box 2-4, and below the tertiary air inlet, ensuring an oxygen-deficient state within the bottom fluidized bed. This allows for incomplete combustion of pulverized coal, producing a large amount of CO as a reducing agent, forcibly creating a reducing atmosphere in the oxygen-deficient zone, thereby achieving the low-temperature decomposition of CaSO4 in phosphogypsum into CaS. The residence time of solid materials in the dense-phase boiling layer is over 30 seconds, providing ample reaction time for the low-temperature decomposition of phosphogypsum. The fluidized bed design keeps the material in a boiling state, resulting in extremely high heat transfer efficiency and a large heat capacity, creating conditions for lowering the decomposition temperature.

[0035] By designing a reduction decomposition furnace 2 with a bottom fluidized bed, the material residence time in the dense phase zone and the dilute phase zone (in the middle suspension furnace) is extended and controlled, reducing the reaction temperature of phosphogypsum decomposition and creating conditions for external decomposition. CO prepared by injecting coal powder under strong reducing conditions is used as a reducing agent to replace the conventional solid reducing agent. In the bottom fluidized bed, the reducing agent has a large contact area with phosphogypsum and the reduction effect is better.

[0036] The central suspension furnace has a cylindrical tubular furnace body, with its bottom connected to the bottom fluidized bed via a variable diameter connection, resulting in a length-to-diameter ratio of 10-60. A tertiary air inlet is located on the side of the central suspension furnace, angled downwards to allow tertiary air to enter at an angle, with its lower edge 2-5 mm higher than the pulverized coal inlet of the bottom fluidized bed. The interior of the central suspension furnace has 1-3 constrictions 2-7 mm to extend the suspension residence time of solid materials, which is at least 30 seconds. The tertiary air volume introduced through the central suspension furnace's tertiary air inlet is 30%-70% of the theoretical combustion air equivalent to pulverized coal in the reduction decomposition furnace 2.

[0037] The 10-60 aspect ratio of the central suspension furnace and the 1-3 internal constrictions (2-7) work together to keep solid materials in suspension for more than 30 seconds. The jetting effect of the constrictions (2-7) enhances the suspension calcination effect, ensuring full contact between the material and the reducing gas. The tertiary air inlet is obliquely downward and located above the pulverized coal inlet (2-5). The tertiary air volume is controlled at 30%-70% of the theoretical combustion air equivalent of pulverized coal in the reduction decomposition furnace 2, ensuring the maintenance of the reducing atmosphere in the central suspension furnace and providing a transition for subsequent oxidation and reburning.

[0038] The oxidation and recombustion furnace is equipped with a tertiary air inlet. The outlet of the central suspension furnace is connected to the inlet of the oxidation and recombustion furnace, so that the material gas discharged from the central suspension furnace, the flue gas from the rotary kiln 3, and the tertiary air enter the oxidation and recombustion furnace together, burn off the residual CO and pulverized coal, and oxidize CaS to CaO.

[0039] The oxidation-reburning furnace achieves a "two-step" burnout mode for pulverized coal as a reducing agent. In the bottom fluidized bed, the pulverized coal first undergoes cracking and gasification (oxygen-deficient) to generate a large amount of CO and carbon particles. The CO, acting as a reducing agent, fully contacts the phosphogypsum, subsequently reducing and decomposing it. The CO remaining in the flue gas from the middle suspension furnace outlet mixes with the rotary kiln flue gas and tertiary air in the oxidation-reburning furnace, fully contacting the oxygen introduced by the tertiary air for complete combustion. The temperature is uniform (temperature difference ±10℃), ensuring that the CO remaining at the middle suspension furnace outlet is burned off before entering the preheater, avoiding a reducing atmosphere in the preheater and thus preventing the formation of harmful components and sublimated sulfur blockage caused by CO entering the preheater. Simultaneously, some CaS is oxidized to CaO in the oxidation-reburning furnace, providing a suitable material composition for the subsequent clinker calcination in rotary kiln 3.

[0040] This invention utilizes pulverized coal to generate a reducing atmosphere containing a high concentration of CO within the reduction decomposition furnace 2, serving as a reducing agent for the decomposition of calcium sulfate into CaS. This avoids the problem of uncontrollable reducing agent dosage and reduction effect. In the oxidizing and reburning furnace, a tertiary air inlet is further installed to completely burn off the CO in the reduction decomposition furnace 2, further reducing heat consumption and increasing the temperature of the secondary air. A high-efficiency cyclone separator is installed after the oxidizing and reburning furnace, allowing newly generated CaS (containing a small amount of CaO) to quickly enter the rotary kiln, avoiding the blockage problem caused by the oxidation of CaS to CaSO4, which produces a liquid phase and enters the preheater. This maintains a predominantly oxidizing atmosphere for calcination within the kiln, consistent with conventional cement clinker calcination procedures. The calcination temperature does not decrease due to CaSO4 melting, effectively ensuring C3S generation and clinker quality. Because the calcination pressure of the rotary kiln is shared, the equipment specifications are reduced for the same output, replacing the common four-stage support rotary kiln with a three-stage support, reducing equipment investment by 10-50%. The SO2 concentration in the preheater outlet flue gas increases by 10-50%, increasing the output of the acid production system and reducing operating costs.

[0041] By designing the decomposition furnace in a semi-offline manner, the gradient entry of the tertiary air is effectively utilized to regulate the two-step reaction of pulverized coal, thereby controlling the incomplete combustion of CO. This makes the reducing atmosphere controllable and only present in the reduction decomposition furnace 2. It avoids the problem of CO entering the preheater in conventional designs, which would exacerbate the scaling of harmful components and blockage caused by sublimated sulfur.

[0042] In a preferred embodiment of the present invention, the oxidation and recombustion furnace includes an oxidation main furnace 2A and an oxidation auxiliary furnace 2B. Both the oxidation main furnace 2A and the oxidation auxiliary furnace 2B have tertiary air inlets and pulverized coal inlets at their bottoms, and a central constriction. The furnace body wind speed is 6-10 m / s, and the wind speed at the central constriction is 15-20 m / s. The outlet of the central suspension furnace is connected to the inlet of the oxidation main furnace 2A, allowing the gas discharged from the central suspension furnace to enter the oxidation main furnace 2A along with the flue gas and tertiary air from the rotary kiln 3. By adjusting the temperature and atmosphere, CO combustion is eliminated, CaS is partially oxidized to CaO, and the resynthesis of CaSO4 is inhibited. The outlet of the oxidation main furnace 2A is connected to the inlet of the preheater's final stage cyclone separator 1-5. The airflow is slowed down in the preheater's final stage cyclone separator 1-5, with some coarse particles returning to the reduction decomposition furnace 2 for circulation, and some particles entering the rotary kiln. The auxiliary oxidation furnace 2B is located on the flue gas pipeline between the gas outlet of the last-stage cyclone separator 1-5 of the preheater and the gas inlet of the penultimate-stage cyclone separator 1-4 of the preheater. It is used to further eliminate residual CO and combustibles in the gas flow, ensuring that the flue gas entering the downstream cyclone separator of the preheater along the flue gas flow direction is in an oxidizing atmosphere. If the CO concentration in the flue gas is required to be controlled to below 300-500 ppm, the height of the auxiliary oxidation furnace should be greater than 30m to ensure complete CO combustion. The gas flow after gas-solid separation in the last-stage cyclone separator 1-5 of the preheater enters the auxiliary oxidation furnace 2B and then enters the downstream cyclone separator of the preheater along the flue gas flow direction, avoiding sublimation of sulfur and residual CO.

[0043] The staged setup of the main oxidation furnace 2A and the auxiliary oxidation furnace 2B enables the step-by-step elimination of CO. The main oxidation furnace 2A undertakes the main tasks of CO combustion and CaS oxidation, while the auxiliary oxidation furnace 2B serves as the last line of defense, ensuring that the flue gas entering the preheater is completely in an oxidizing atmosphere, thus completely eliminating the generation of sublimated sulfur and the problem of scaling and blockage under a reducing atmosphere.

[0044] In a preferred embodiment of the present invention, the bottom fluidized bed and the middle suspension furnace are arranged in a semi-online manner. A bypass pipe 6 is provided between the bottom fluidized bed and the feed pipe of the second-to-last stage cyclone 1-4 of the preheater. The bypass pipe 6 is used to bypass the phosphogypsum raw material to the oxidation main furnace 2A of the oxidation and reburning furnace in the upper part of the rotary kiln smoke chamber, so as to realize the switching between the preheater kiln working mode and the pre-decomposition kiln working mode.

[0045] Specifically, the bypass pipe 6 is configured to directly bypass the feed from the second-to-last stage cyclone 1-4 of the preheater to the oxidation and reburning furnace during the initial ignition stage or when the reduction and decomposition furnace 2 experiences abnormal operating conditions, and operate in preheater kiln mode; after the air from the cooler 4 exchanges heat with the high-temperature clinker at the outlet of the rotary kiln 3 to obtain tertiary air, the feed is switched to the reduction and decomposition furnace 2 and operated in pre-decomposition kiln mode.

[0046] The decomposition furnace of this invention features a semi-online layout, forming a self-contained system. The reducing agent is pulverized coal, and the air supply primarily consists of tertiary air from the cooler and a small amount of high-pressure fluidizing air from the bottom of the fluidized bed. The incomplete combustion of pulverized coal in the dense phase zone produces a large amount of CO, which acts as a reducing agent for phosphogypsum, providing a strong reducing environment conducive to phosphogypsum decomposition. During the initial ignition phase, or under special conditions such as unstable feed or pressure on the bottom fluidized bed, the system can flexibly switch to preheater kiln mode by closing the valves of the three tertiary air paths and opening the valves of the bypass pipes. Once the high-temperature clinker at the rotary kiln outlet has exchanged heat with the cooler air to obtain tertiary air, the bottom fluidized bed is then put into operation, switching to pre-decomposition kiln mode. This design significantly enhances the system's adaptability to fluctuations in phosphogypsum feed, avoids the risk of the entire production line shutting down due to decomposition furnace failure, and improves the system's reliability and industrial operability.

[0047] As a preferred embodiment of the present invention, it also includes a tertiary air distribution pipeline for dividing the tertiary air from the cooler 4 into three paths, which are respectively sent to the central suspension furnace, the main oxidation furnace 2A of the oxidation reburning furnace, and the auxiliary oxidation furnace 2B.

[0048] The tertiary air distribution pipeline achieves gradient zoning of tertiary air supply. The first branch, 5-1, sends tertiary air into the central suspension furnace, controlling 30%-70% of the theoretical combustion air equivalent of pulverized coal in the reduction decomposition furnace 2, maintaining the reducing atmosphere in the bottom fluidized bed and the central suspension furnace. The second branch, 5-2, sends air into the main oxidation furnace 2A, burning off residual CO and oxidizing some CaS. The third branch, 5-3, sends air into the auxiliary oxidation furnace 2B, further ensuring the oxidizing properties of the flue gas. This gradient air supply mode precisely controls the two-step reaction of pulverized coal, making the reducing atmosphere controllable and present only in the bottom fluidized bed and the central suspension furnace.

[0049] In a preferred embodiment of the present invention, the inlet of the last stage cyclone 1-5 of the preheater and / or the feed pipe of the last stage cyclone 1-5 of the preheater are provided with an auxiliary reducing agent inlet, which is used to supplement the auxiliary reducing agent for reduction adjustment or switching to the working mode of the preheater kiln.

[0050] The auxiliary reducing agent inlet allows a small amount of coke to be added as an auxiliary reducing agent from the inlet of the final stage cyclone 1-5 and the feed pipe, providing a flexible reducing agent supply solution for supplementing adjustment or switching to the preheater kiln working mode.

[0051] As a preferred embodiment of the present invention, the dust collection efficiency of the final stage cyclone in the preheater is greater than 90%, and a return channel 7 is provided between the feed pipe of the final stage cyclone in the preheater 1-5 and the raw material feed port of the bottom fluidized bed. The return channel 7 is used to return some of the separated coarse particles to the bottom fluidized bed or the middle suspension furnace for recycling and decomposition.

[0052] The return channel 7 returns some of the coarse particles separated by the final stage cyclone separators 1-5 to the fluidized bed decomposition furnace for circulation, while some particles enter the rotary kiln. This increases the decomposition ratio of phosphogypsum and avoids the problem of undecomposed calcium sulfate being encapsulated in the clinker by the liquid phase, resulting in low clinker strength.

[0053] As a preferred embodiment of the present invention, the side of the central suspension furnace is also provided with a solid waste inlet 2-6, which is used to add one or more of steel slag, sludge, sulfur slag and biomass to the central suspension furnace to reduce coal consumption.

[0054] The installation of solid waste inlets 2-6 allows solid wastes such as sulfur slag, steel slag, sludge, and biomass to be used as alternative raw materials and fuels, achieving the synergistic treatment of multiple solid wastes while reducing coal consumption.

[0055] The core working principle of this invention is as follows: Pulverized coal undergoes incomplete combustion under oxygen-deficient conditions in the dense phase zone of the bottom fluidized bed via a pulverized coal injection pipe, producing a large amount of CO, which acts as a reducing agent for phosphogypsum. The phosphogypsum raw material, after being preheated stage by stage in preheater 1, is fed into the bottom fluidized bed through feed boxes 2-4. Under fluidized conditions, it comes into full contact with CO, and CaSO4 is reduced and decomposed into CaS, while simultaneously releasing flue gas containing a high concentration of SO2. The residence time of solid materials in the bottom fluidized bed is over 30 seconds, and the residence time of solid materials in the central suspension furnace is also over 30 seconds. This dual long residence time design ensures the complete progress of the decomposition reaction.

[0056] After the CaS-containing gas is discharged from the central suspension furnace, it enters the oxidation-reburning furnace together with the kiln tail gas and tertiary air from the rotary kiln 3. In the oxidation-reburning furnace, the residual CO comes into contact with the oxygen in the tertiary air and burns out, and some of the CaS is oxidized to CaO. The gas-solid mixture after being treated in the oxidation-reburning furnace enters the final stage cyclone separator 1-5 of the preheater for gas-solid separation. The separated solid material is sent to the rotary kiln 3 to be calcined into clinker under an oxidizing atmosphere. The separated SO2-containing gas is discharged through the preheater 1 and enters the acid production system.

[0057] The tertiary air comes from the cooler 4 and is divided into three gradient air supply paths: the first branch 5-1 is sent to the central suspension furnace (the air volume is 30%-70% of the theoretical combustion air volume of pulverized coal) to maintain the reducing atmosphere in the reduction decomposition furnace 2; the second branch 5-2 is sent to the main oxidation furnace 2A to burn off CO and oxidize CaS; the third branch 5-3 is sent to the auxiliary oxidation furnace 2B to further ensure that the flue gas entering the preheater is an oxidizing atmosphere.

[0058] This invention achieves the separation of calcium sulfate decomposition and clinker calcination processes, solving the problem of difficult-to-control kiln atmosphere in traditional processes.

[0059] The process flow for co-producing cement clinker using the fluidized bed phosphogypsum semi-online low-temperature kiln external decomposition furnace of this invention, coupled with external pre-decomposition of gypsum in the kiln, is as follows: like Figure 6 As shown, phosphogypsum, after being filtered, dried, and homogenized by a gradient drying and homogenizing feeding device, enters preheater 1 for staged preheating. It is then fed into reduction decomposition furnace 2 through the feed pipe of the second-to-last stage cyclone separator 1-4. High-pressure fluidizing air, pulverized coal, and preheated raw materials are introduced to the bottom of reduction decomposition furnace 2. The preheated raw materials undergo reduction decomposition in a reducing atmosphere. The outlet material contains CaS, CaSO4, and a small amount of CaO. The exhaust gas from reduction decomposition furnace 2 and the flue gas from the rotary kiln enter the main oxidation furnace 2A, where tertiary air is introduced for oxidation treatment, causing CO combustion and oxidizing some CaS to CaO. The oxidized material is then fed into the last stage cyclone separator 1-5 of the preheater. The solids after gas-solid separation in the last stage cyclone separator quickly enter the kiln tail flue chamber 3-1 and are then calcined in rotary kiln 3 to produce high-temperature clinker. After cooling in cooler 4, qualified cement clinker is obtained. After the cooling air undergoes heat exchange, the secondary air enters the kiln, and the tertiary air is divided into three paths, entering the reduction decomposition furnace 2, the main oxidation furnace 2A, and the auxiliary oxidation furnace 2B respectively. The flue gas separated by the final stage cyclone separator enters the auxiliary oxidation furnace 2B, where the added tertiary air burns off the reducing gases and fuel particles in the flue gas. The flue gas exiting the auxiliary oxidation furnace enters the preheater, undergoes multi-stage heat exchange, and is then discharged for acid production.

[0060] Example 1 A process for co-producing acid from phosphogypsum and cement clinker using a fluidized semi-online cryogenic external precalciner is implemented as follows: After being filtered and dehydrated, phosphogypsum is batched with other auxiliary raw materials, including phosphogypsum (93.51% by mass, dihydrate basis), boiler slag (0.79%), bauxite (0.86%), and sulfur slag (0.5%). After homogenization, the mixture enters preheater 1. After multi-stage preheating, the raw material is fed into reduction decomposition furnace 2 through the feed pipes of the second-to-last cyclone separator 1-4 for decomposition.

[0061] After decomposition, the material is carried by the airflow into the main oxidation furnace 2A of the oxidation and recombustion furnace. There, it combines with the high-temperature flue gas containing SO2 drawn from the kiln tail flue and tertiary air to burn off the incompletely burned CO in the reduction decomposition furnace 2, while simultaneously promoting the oxidation of some CaS to CaO. The oxidized material is then carried by the airflow into the final stage cyclone separator 1-5, where gas-solid separation occurs. The solid material enters the kiln tail flue 3, and then enters the rotary kiln 3 for calcination into clinker. Finally, it passes through the kiln hood into the cooler 4, where it is cooled to obtain cement clinker.

[0062] After the air exchanges heat with the clinker, the secondary air enters the kiln, and the tertiary air is divided into three paths through valves: into reduction decomposition furnace 2, oxidation main furnace 2A, and oxidation auxiliary furnace 2B.

[0063] Powdered coal is added to the bottom of both the reduction decomposition furnace 2 and the oxidation reburning furnace. The reducing agent (CO ≥ 2%, 800-1000℃) is generated from the powdered coal at the bottom of the reduction decomposition furnace 2. The amount of coal used in the reduction decomposition furnace is 30-70% of the total coal consumption of the combustion system. Powdered coal is added to the bottom of the oxidation reburning furnace as an auxiliary means to increase the temperature and prevent the temperature from being too low, resulting in incomplete CO combustion. In addition, coke is added as an auxiliary reducing agent from the inlet of the final stage cyclone separators 1-5 and their feed pipes as a supplement.

[0064] High-pressure fluidizing air, pulverized coal, and preheated raw materials are introduced to the bottom of the reduction decomposition furnace 2. The residence time in both the dense phase and dilute phase zones is >30s. The material containing CaS and CaSO4 at the decomposition furnace outlet, together with the rotary kiln flue gas and tertiary air, enters the main oxidation furnace 2A and then enters the final stage cyclone separators 1-5. After gas-solid separation, the flue gas containing SO2 enters the auxiliary oxidation furnace 2B, and then enters the preheater step by step to the acid production process.

[0065] In the decomposition furnace outlet material, part (30-50%) of CaS in the main oxidation furnace 2A reacts to CaO along with the airflow. The material containing CaS (10-30%), CaO (5-40%) and CaSO4 (30-70%) enters the rotary kiln 3 to complete the oxidation and calcination reaction and produce qualified clinker.

[0066] Comparative Example 1 (Traditional preheater kiln process) This comparative example uses a traditional preheater kiln process, where both calcium sulfate decomposition and clinker calcination occur within the rotary kiln. The simultaneous presence of both reduction decomposition and oxidation calcination atmospheres within the 100-meter-long kiln makes it difficult to control the kiln atmosphere and affects stable operation. This results in low cement clinker product qualification rates, low early strength, and high energy consumption, further reducing the SO2 gas concentration at the preheater outlet (to only 5-7.5%).

[0067] Comparative Example 2 (Existing external kiln decomposition technology) The process employs a circulating fluidized bed where coal is first oxidized to produce CO gas as an intermediate phase, which then reacts with gypsum in a gas-solid reaction to generate calcium oxide. However, under a reducing atmosphere, CaS, residual CO, and sublimed sulfur produced by side reactions are generated, leading to severe scaling and blockage of the preheater, making long-term continuous and stable operation of the unit impossible.

[0068] Compared to Comparative Example 1, this invention moves the calcium sulfate decomposition process from inside the rotary kiln 3 to an external decomposition furnace. The rotary kiln 3 only undertakes the clinker calcination task, maintaining a predominantly oxidizing atmosphere for calcination, consistent with conventional cement clinker calcination procedures, effectively ensuring clinker quality. Because the calcination pressure of the rotary kiln is shared, the equipment specifications required for the same output are reduced; the common four-stage support rotary kiln is replaced with a three-stage support rotary kiln, reducing equipment investment by 10-50%. The SO2 concentration in the preheater outlet flue gas increases by 10-50%, increasing the output of the acid production system and reducing operating costs.

[0069] Compared to Comparative Example 2, this invention extends the material residence time through fluidized bed design and creates highly reducing conditions by using a tertiary air gradient zoned air supply and a two-step pulverized coal combustion mode, thereby lowering the decomposition temperature of calcium sulfate. The temperature reduction and CO re-combustion ensure that the preheater does not form a crust, avoiding the formation of sublimed sulfur and overcoming the core technical bottleneck of external decomposition. The semi-online layout and bypass piping design allow the system to flexibly switch between preheater kiln and pre-decomposition kiln modes, greatly enhancing the system's industrial adaptability.

[0070] Example 2 (Bypass production of calcium-based materials) like Figure 7 As shown, the final stage cyclone discharge pipe is also connected to the external discharge silo, which discharges the decomposition products containing CaO produced to the external discharge silo as calcium powder or calcium-based admixtures and cementing materials.

[0071] Increase the height of the tertiary air inlet in the reduction decomposition furnace to 5-10 m above the bottom of the fluidized bed. Increase the proportion of high-pressure fluidizing air from 8-15% of the equivalent theoretical combustion air to 20-30%. Increase the fluidizing air nozzle velocity to over 60 m / s. Ensure the gas residence time in the dense phase zone before the tertiary air enters exceeds 1 second, and the solid residence time exceeds 1 minute. Increase the operating temperature of the reduction decomposition furnace to 950-1000℃ to enhance the decomposition of CaSO4 and the conversion of CaS to CaO. Close the inlet feed pipe through the feed valve of the final stage cyclone separator and connect to the external discharge bin 8 to bypass the decomposed calcium powder or calcium-based materials with CaO as the main component. For cooling the calcium powder or calcium-based materials, a new cooling system can be added, or a rotary kiln can be used as the cooling channel for the calcium powder or calcium-based materials.

[0072] Furthermore, the reducing atmosphere in the reduction decomposition furnace is increased, and the CO concentration in the flue gas at the preheater outlet is controlled to be below 300-500 ppm. The height of the auxiliary furnace is controlled to be greater than 30m, and the tertiary air in the oxidation and reburning furnace reacts fully with CO to ensure complete combustion of CO.

[0073] Example 3 (Production of cementitious materials) Based on Example 1, compared to desulfurized gypsum, phosphogypsum generally contains 2-10% SiO2. For phosphogypsum materials containing silicon, only phosphogypsum is used in the raw material batching, without adding other auxiliary materials such as aluminum and iron. The calcined and decomposed phosphogypsum produced by the system of Example 1 is used to remove SO2. The clinker from the kiln is a calcium-based material containing calcium silicate, dicalcium silicate, CaO and a small amount of tricalcium silicate. This material is a cementitious material with certain cementing properties and can be used as an admixture for aggregates, blocks or concrete additives, etc.

[0074] Table 1. Composition data of cementitious materials using only phosphogypsum as a raw material and after calcination.

[0075] This invention employs a fluidized bed reductive decomposition furnace 2, coupled with an oxidation and reburning furnace, to separate the calcium sulfate reduction decomposition from the clinker oxidation and calcination process. The fluidized bed design extends the material residence time, and the three-stage air supply and two-step coal combustion create highly reducing conditions, thereby lowering the calcium sulfate decomposition temperature, achieving low-temperature decomposition of phosphogypsum, increasing the calcium sulfate decomposition rate, and preventing undecomposed calcium sulfate from being encapsulated in the clinker by the liquid phase, resulting in low clinker strength. The lower temperature and CO reburning ensure that the preheater does not form a crust, avoiding the formation of sublimed sulfur and overcoming the core technical bottleneck of external kiln decomposition. The decomposition process occurs within the reductive decomposition furnace 2 and is a suspension calcination, resulting in lower system energy consumption. Oxidative atmosphere calcination can be achieved within the rotary kiln, with virtually no calcium sulfate decomposition occurring inside the kiln. The increased kiln temperature improves the clinker calcination quality and correspondingly increases the clinker strength. No residual calcium sulfate or calcium sulfide remains in the clinker, ensuring clinker stability. Compared with the traditional gypsum cement production process, the rotary kiln only undertakes the clinker calcination task, and the equipment size can be reduced by 10-50%. Due to the increased production capacity, the system heat consumption is reduced, the amount of combustion flue gas is reduced, and the SO2 concentration is increased.

[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A fluidized bed semi-online low-temperature external decomposition furnace for phosphogypsum, characterized in that, It includes a reduction decomposition furnace, which comprises a bottom fluidized bed and a middle suspension furnace arranged sequentially along a vertical direction; and an oxidation and reburning furnace disposed downstream of the middle suspension furnace; The bottom fluidized bed is equipped with a high-pressure fluidizing air inlet, fluidizing air nozzles, an air distribution plate, a raw material feed inlet, and a pulverized coal inlet. The fluidizing air nozzles are embedded in the air distribution plate to ensure that the high-pressure fluidizing air entering from the high-pressure fluidizing air inlet is evenly distributed within the bottom fluidized bed. The raw material feed inlet and the pulverized coal inlet are located on the side of the bottom fluidized bed. The raw material feed inlet of the bottom fluidized bed is connected to the feed pipe of the second-to-last stage cyclone separator of the preheater. The bottom fluidized bed is used to form a dense phase boiling layer and to generate reducing gas CO and residual carbon particles through incomplete combustion of pulverized coal, thus forming a reducing atmosphere that decomposes CaSO4 in the phosphogypsum raw material at low temperature into decomposition products containing CaS. The central suspension furnace is a cylindrical furnace body, with its bottom connected to the bottom fluidized bed. The side of the central suspension furnace is provided with a tertiary air inlet. The distance between the tertiary air inlet and the bottom fluidized bed controls the gas residence time in the dense phase zone to be greater than 0.5s. The interior of the central suspension furnace is provided with at least one constriction. The central suspension furnace is used to extend the suspension residence time of solid materials. The oxidation and recombustion furnace is equipped with a tertiary air inlet, and the outlet of the central suspension furnace is connected to the inlet of the oxidation and recombustion furnace, so that the material gas discharged from the central suspension furnace, the flue gas from the rotary kiln, and the tertiary air enter the oxidation and recombustion furnace together, burn off the residual CO and pulverized coal, and oxidize CaS to CaO.

2. The phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace according to claim 1, characterized in that, The oxidation and reburning furnace includes a main oxidation furnace and an auxiliary oxidation furnace. Both the main oxidation furnace and the auxiliary oxidation furnace are equipped with a tertiary air inlet and a pulverized coal inlet at the bottom, and a spray constriction in the middle. The furnace body wind speed is 6~10m / s, and the wind speed at the middle constriction is 15~20m / s. The outlet of the central suspension furnace is connected to the inlet of the main oxidation furnace, so that the gas discharged from the central suspension furnace, the flue gas from the rotary kiln, and the tertiary air enter the main oxidation furnace together. By adjusting the temperature and atmosphere, CO is eliminated by combustion and CaS is oxidized to CaO, and the resynthesis of CaSO4 is inhibited. The outlet of the main oxidation furnace is connected to the inlet of the last stage cyclone of the preheater. The auxiliary oxidation furnace is set on the flue gas pipeline between the gas outlet of the last stage cyclone of the preheater and the gas inlet of the second-to-last stage cyclone of the preheater. It is used to further eliminate residual CO and combustibles in the gas flow, so that the flue gas entering the downstream cyclone of the preheater along the flue gas flow direction is an oxidizing atmosphere.

3. The phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace according to claim 1, characterized in that, The bottom fluidized bed has a conical structure, the air distribution plate is made of refractory material, and the air velocity of the fluidizing air nozzle is 30-60 m / s; Raw materials and pulverized coal are introduced into the bottom fluidized bed. A feeding box is provided at the raw material feeding port, and the pulverized coal inlet is located on the back of the feeding box and at its lower part. The pulverized coal inlet is located below the tertiary air inlet of the middle suspension furnace to form an oxygen-deficient zone in the bottom fluidized bed. The residence time of solid materials in the dense phase boiling layer is more than 30 seconds.

4. The phosphogypsum fluidized semi-online low-temperature external decomposition furnace according to claim 1, characterized in that, The tertiary air inlet of the central suspension furnace is inclined downward, and the lower edge of the tertiary air inlet is higher than the pulverized coal inlet of the bottom fluidized bed; the length-to-diameter ratio of the central suspension furnace is 10-60, and the interior of the central suspension furnace has 1-3 constrictions; the residence time of solid materials in the central suspension furnace is more than 30 seconds; the material flow in the reduction decomposition furnace forms a dense phase zone and a dilute phase zone sequentially from the bottom upward with the tertiary air entering from the side as the boundary, the wind speed in the dense phase zone is 4-6 m / s, and the wind speed in the dilute phase zone is 8-16 m / s.

5. The phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace according to claim 1, characterized in that, The bottom fluidized bed and the middle suspension furnace are arranged in a semi-online configuration. A bypass pipe is provided between the bottom fluidized bed and the feed pipe of the second-to-last stage cyclone of the preheater. The bypass pipe is used to bypass the phosphogypsum raw material to the oxidation and reburning furnace above the rotary kiln smoke chamber, so as to realize the switching between the preheater kiln working mode and the pre-decomposition kiln working mode.

6. The phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace according to claim 5, characterized in that, The bypass pipeline is configured to directly bypass the material from the second-to-last stage cyclone of the preheater to the oxidation and reburning furnace during the initial stage of ignition or when abnormal conditions occur in the bottom fluidized bed, so as to operate in the preheater kiln mode. After the air from the cooling unit exchanges heat with the high-temperature clinker at the rotary kiln outlet to generate tertiary air, the material is switched to the bottom fluidized bed and operated in pre-decomposition kiln mode.

7. The phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace according to claim 1, characterized in that, The side of the central suspension furnace is also provided with a solid waste inlet, which is used to add one or more of steel slag, sludge, sulfur slag, and biomass to the central suspension furnace in order to reduce coal consumption.

8. The phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace according to claim 2, characterized in that, The coal consumption of the reduction decomposition furnace is 30-70% of the total coal consumption of the calcination system; the total tertiary air volume accounts for 30-70% of the equivalent theoretical combustion air volume of the calcination system; it also includes a tertiary air distribution pipeline, which is used to divide the tertiary air from the cooler into three paths, which are respectively sent to the central suspension furnace, the main oxidation furnace of the oxidation reburning furnace, and the auxiliary oxidation furnace.

9. The phosphogypsum fluidized semi-online low-temperature kiln external decomposition furnace according to claim 2, characterized in that, The dust collection efficiency of the final stage cyclone separator in the preheater is greater than 90%. A return channel is provided between the feed pipe of the final stage cyclone separator and the raw material feed port of the bottom fluidized bed. The return channel is used to return some of the separated coarse particles to the bottom fluidized bed or the middle suspension furnace for recycling and decomposition. The feed pipe of the final stage cyclone separator is also connected to the external discharge silo to discharge the decomposition products containing CaO to the external discharge silo as calcium powder or calcium-based materials and cementing materials.

10. The phosphogypsum fluidized semi-online low-temperature external decomposition furnace according to claim 2, characterized in that, The inlet of the last stage cyclone separator of the preheater and / or the feed pipe of the last stage cyclone separator of the preheater are provided with an auxiliary reducing agent inlet, which is used to supplement the auxiliary reducing agent for reduction adjustment or switching to the working mode of the preheater kiln.

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