Rotary kiln tail gas treatment system in iron phosphate synthesis process and use method thereof

Abstract

The application discloses a rotary kiln tail gas treatment system in a ferrophosphorus synthesis process and a use method, belongs to the ferrophosphorus synthesis equipment field, and comprises a tail gas pipeline used for being connected with a rotary kiln, wherein, according to the airflow direction, an acid liquid spraying tower, a lye spraying tower and an induced draft fan are sequentially connected in tail gas pipeline; the acid liquid spraying tower is connected with an acid liquid circulating tank used for cyclically providing the acid liquid required for spraying; according to the flow direction of the solid-liquid mixed slurry in the acid liquid circulating tank, an acid dissolving tank and a filter press are sequentially connected behind the acid liquid circulating tank; part of the filtrate of the filter press is communicated back to the acid liquid circulating tank, and the other part is communicated back to a ferrophosphorus liquid raw material position at the front end of the ferrophosphorus synthesis. The application does not directly use lye spraying to remove acid gas, but instead uses acid liquid spraying to utilize the acid gas in the tail gas, so that the tail gas emission can reach the standard, and the ferrophosphorus material can be better recovered.

Description

Technical Field

[0001] This invention relates to a rotary kiln tail gas treatment system and its usage method in the process of ferric phosphate synthesis, belonging to the field of ferric phosphate synthesis equipment. Background Technology

[0002] In the synthesis of ferric phosphate, the main components of the rotary kiln tail gas are anhydrous ferric phosphate, some unsuccessfully converted ferric phosphate dihydrate, water vapor, and SO2. Conventional rotary kiln tail gas treatment schemes typically involve first collecting ferric phosphate dust from the tail gas using a baghouse dust collector, and then treating the remaining acidic waste gas through a two-stage spray tower with added alkali. However, this scheme has significant drawbacks: the collected dust material, due to its high sulfur content, unacceptable BET surface area, small particle size, or agglomeration, cannot be properly returned to production, resulting in a decrease in raw material yield; simultaneously, the waste liquid generated by the spray tower also leads to high water treatment costs. Existing technologies also include ferric phosphate tail gas treatment schemes, such as the one disclosed in CN223760635U, which uses a cyclone separator for preliminary gas-liquid separation of the acidic tail gas, attempting to reduce the processing burden on the spray tower and lower alkali consumption. However, this solution can only reduce the amount of waste liquid generated. The acidic water mist discharged from the cyclone separator still contains a large amount of iron phosphate material. This solution does not recycle this material, and it will still be discarded as waste, which cannot solve the problem of raw material yield loss. Summary of the Invention

[0003] In order to overcome the shortcomings of the prior art, the present invention provides a rotary kiln tail gas treatment system and its usage method in the process of iron phosphate synthesis, which simultaneously achieves tail gas treatment to meet standards and efficient recovery of iron phosphate materials.

[0004] The technical solution adopted by this invention to solve its technical problem is: In a first aspect, this application provides a rotary kiln tail gas treatment system for the synthesis of ferric phosphate, comprising a tail gas pipeline for connection to the rotary kiln, and an acid spray tower, an alkali spray tower, and an induced draft fan connected in series after the tail gas pipeline in the direction of airflow; the acid spray tower is connected to an acid circulation tank for circulating the acid required for spraying; according to the flow direction of the solid-liquid mixed slurry in the acid circulation tank, an acid dissolving tank and a filter press are connected in series after the acid circulation tank; part of the filtrate from the filter press is connected back to the acid circulation tank, and another part is connected back to the ferric phosphate raw material at the front end of the ferric phosphate synthesis.

[0005] The rotary kiln tail gas treatment system provided in this application for the iron phosphate synthesis process does not directly remove acidic gases by alkaline spraying, but instead utilizes acidic gases in the tail gas by acid spraying, which not only ensures that the tail gas emissions meet the standards, but also better recovers iron phosphate materials.

[0006] Furthermore, the exhaust gas pipe is connected to the tail of the rotary kiln.

[0007] Furthermore, a heat exchanger is provided at the beginning of the exhaust gas pipe, a waste heat heating pipe is provided at the end, and an insulation layer is provided in the middle section. The heat exchanger transfers heat to the waste heat heating pipe.

[0008] Furthermore, the acid dissolving tank is provided with a waste heat exchange jacket, and the heat exchanger transfers heat to the waste heat exchange jacket.

[0009] Furthermore, the acid dissolution tank is equipped with a low-pressure ultrasonic cavitation component and a stirrer.

[0010] Furthermore, the acid circulation tank is connected to an acid addition pipe, an acid level gauge is installed in the acid circulation tank, an acid addition valve is installed on the acid addition pipe, and the acid addition valve is signal-connected to the acid level gauge.

[0011] Furthermore, the acid circulation tank and the acid dissolving tank are connected by a slurry discharge pipe, the slurry discharge pipe is equipped with a slurry discharge valve, the acid circulation tank is equipped with a microwave solids content meter, and the slurry discharge valve is connected to the acid level gauge and the microwave solids content meter for signal transmission.

[0012] Furthermore, the alkaline spray tower is connected to an alkaline circulation tank for circulating the alkaline solution required for spraying. The alkaline circulation tank is connected to an alkaline addition pipe. An alkaline level gauge is installed in the alkaline circulation tank. An alkaline addition valve is installed on the alkaline addition pipe, and the alkaline addition valve is signal-connected to the alkaline level gauge. The alkaline circulation tank is also equipped with a wastewater pipe, a discharge valve, and a pH meter. The discharge valve is signal-connected to both the alkaline level gauge and the pH meter.

[0013] Secondly, this application provides a method for using the rotary kiln tail gas treatment system in the iron phosphate synthesis process as described in the first aspect. The steps include: under the condition that the acid spray tower and the alkali spray tower are continuously spraying, turning on the induced draft fan to generate negative pressure to drive the tail gas of the rotary kiln through the tail gas pipeline, the acid spray tower, and the alkali spray tower in sequence, and then venting it; when the solid content of the slurry in the acid circulation tank exceeds the preset solid content, the slurry is transported to the acid dissolution tank to redissolve part of the solids, and then the slurry is continued to be transported to the filter press for filtration. Part of the filtrate is reused for spraying in the acid spray tower, and the other part is reused as the iron phosphate liquid raw material at the front end of iron phosphate synthesis.

[0014] Optionally, the induced draft fan controls the wind speed to be between 1 m / s and 3 m / s, the pH of the alkaline solution sprayed by the alkaline solution spraying tower is above 8, and the pH of the acid solution sprayed by the acid solution spraying tower is between 0.5 and 2.

[0015] The beneficial effects of this invention are as follows: Compared with existing exhaust gas treatment systems, this invention eliminates the bag filter dust collection bin and adds an acid dissolving tank and a filter press, forming a new rotary kiln exhaust gas treatment system. At the same time, the new process of this invention develops a new route to directly treat the dust collection material and the first-stage spray (acid spray tower) waste liquid on the production line, converting them into raw material ferric phosphate liquid and intermediate product ferric phosphate dihydrate, reducing the generation of waste material and waste liquid, reducing losses and treatment costs in the production process, and better solving the defects of conventional technologies.

[0016] Other features and advantages of this application will be set forth in the following description and will be apparent in part from the description or may be learned by practicing the application. The objectives and other advantages of this application may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of a rotary kiln tail gas treatment system in the synthesis process of iron phosphate provided in this application embodiment.

[0018] Figure 2 This is a schematic diagram of the principle of a rotary kiln tail gas treatment system in the synthesis process of iron phosphate provided in the embodiments of this application.

[0019] Attached reference numerals: 1. Rotary kiln; 2. Tail gas duct; 21. Heat exchanger; 22. Insulation layer; 23. Waste heat heating pipe; 3. Acid spray tower; 31. Acid circulation tank; 311. Acid level gauge; 312. Microwave solids meter; 32. Air outlet pipe; 33. Slurry discharge pipe; 331. Slurry discharge valve; 34. Acid addition pipe; 341. Acid addition valve; 4. Alkali spray tower; 41. Alkali circulation tank; 411. Alkali level gauge; 412. pH meter; 42. Exhaust duct; 43. Wastewater pipe; 431. Discharge valve; 44. Alkali addition pipe; 441. Alkali addition valve; 5. Exhaust fan; 6. Acid dissolving tank; 61. Agitator; 62. Acid level gauge; 63. Transfer pump; 64. Waste heat exchange jacket; 7. Filter press; 321. Online anemometer. Detailed Implementation

[0020] Embodiments of the present invention are described in detail below, examples of which are shown 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 are only used to explain the present invention, and should not be construed as limiting the present invention.

[0021] The following disclosure provides many different embodiments or examples for implementing different structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed.

[0022] To address the shortcomings of existing technologies, the inventors of this application, after in-depth research, discovered that the fundamental reason why baghouse dust cannot be reused is that, under high temperature and trace amounts of SO2 atmosphere, the iron phosphate dust in the rotary kiln exhaust gas adsorbs acidic gases and undergoes weak chemical bonding on its surface, leading to an increase in the surface energy of the dust particles. Under the mechanical force of bag filtration, it is extremely prone to secondary agglomeration, and the specific surface area (BET) decreases significantly, resulting in excessive sulfur content. If a "two-stage alkaline spray" or "water washing + alkaline spray" scheme is directly adopted, although acidic gases can be removed, the iron phosphate dust will quickly transform into agglomerates that are difficult to redisperse after encountering water or alkaline solutions. As a result, the iron and phosphorus elements in it cannot be reused by the production system and can only be treated as solid waste or low-grade by-products. Based on this, this application proposes a tiered treatment approach of first acid dissolution and capture, followed by alkaline neutralization. The acidic spray solution is used to dissolve the iron phosphate dust in the exhaust gas into a soluble iron phosphate salt solution, thereby avoiding the physical agglomeration and surface passivation of the dust at the source and laying a chemical foundation for the full recovery of subsequent materials.

[0023] like Figure 1 As shown in the embodiment of this application, a rotary kiln tail gas treatment system is provided in the process of iron phosphate synthesis. The system includes a rotary kiln 1, which is connected to a tail gas pipe 2. The tail gas pipe 2 is connected to an acid spray tower 3 (first-stage spray tower). The acid spray tower 3 has an air outlet pipe 32 and a slurry discharge pipe 33. The air outlet pipe 32 is connected to an alkaline spray tower 4 (second-stage spray tower), and the slurry discharge pipe 33 is connected to an acid dissolving tank 6. The alkaline spray tower 4 is connected to an induced draft fan 5 through an induced draft pipe 42, and the acid dissolving tank 6 is connected to a filter press 7 through a slurry pipe.

[0024] This application does not use dust collector bags; its design principle lies in the differentiated resource utilization of different components in the exhaust gas. The primary acid spray tower utilizes the solubility of ferric phosphate (FePO4) and its precursor, ferric phosphate dihydrate (FePO4·2H2O), in low pH (pH=0.5~2) phosphoric acid solutions to efficiently dissolve and collect solid ferric phosphate material entrained in the exhaust gas into the liquid phase. This design avoids the drawbacks of solid-gas separation in traditional baghouse dust collectors, transforming dust collection into liquid-phase absorption. This ensures that the collected material exists in an ionic state in the acid solution, solving the problems of excessive sulfur content and BET non-compliance in the collected dust.

[0025] In actual production, the exhaust gas temperature at the outlet of the exhaust gas pipeline is between 350℃ and 450℃. With the exhaust gas pipeline fully insulated, the exhaust gas temperature will drop sharply at the half-meter position where the exhaust gas pipeline contacts the primary spray tower. The main reason is that the exhaust gas directly enters the spray tower and comes into contact with the alkaline solution (existing technology) in the circulating spray water tank. Condensation and pipe blockage often occur at this position, resulting in poor exhaust gas discharge.

[0026] like Figure 1 As shown, the exhaust gas pipe 2 is located at the kiln tail of the rotary kiln (in rotary kiln terminology, the feed section is called the kiln tail, and the discharge section is called the kiln head), and the exhaust gas flow direction is opposite to the material flow direction inside the rotary kiln 1. A heat exchanger 21 (e.g., a plate heat exchanger) is installed at the outlet of the exhaust gas pipe, along with an insulation layer 22 for the exhaust gas pipe, to construct a two-stage waste heat utilization system.

[0027] First-stage waste heat (such as...) Figure 2 ①) At the contact point between the exhaust gas pipe 2 and the acid spray tower 3, the temperature inside the pipe is maintained above 120℃ via a waste heat heating pipe 23 to prevent ferric phosphate dust from condensing and clogging the pipe upon cooling. This design absorbs heat at the beginning of the exhaust gas pipe and releases it at the end, providing supplemental heating to the contact point between the exhaust gas pipe and the primary spray tower. Simultaneously, the beginning of the exhaust gas pipe is directly connected to the kiln tail, allowing a continuous flow of high-heat exhaust gas into the pipe. The energy dissipated during heat exchange at this point is negligible. This prevents the high-temperature exhaust gas discharged from the rotary kiln 1 from rapidly decreasing in temperature during transport, which could cause ferric phosphate dust in the exhaust gas to condense and adhere to the inner wall of the pipe, leading to blockage. This ensures continuous and stable exhaust gas transport while fully recovering waste heat from the exhaust gas, reducing external heating energy consumption, and extending the system's trouble-free operating cycle.

[0028] like Figure 1 As shown, the acid spray tower 3 is equipped with an acid circulation tank 31, an air outlet pipe 32, a slurry discharge pipe 33, and an acid addition pipe 34. The acid circulation tank 31 of the acid spray tower 3 treats the exhaust gas from the rotary kiln 1 using an acid circulation spray method. The acid circulation tank 31 is equipped with an acid level gauge 311 (online level gauge) and a microwave solids content meter 312 (online microwave solids content meter). In this embodiment, the acid circulation spray uses an acid solution with a pH of 0.5~2 as the spray liquid, which can efficiently dissolve and capture anhydrous ferric phosphate and dihydrate ferric phosphate dust in the exhaust gas, preventing dust from penetrating the filter and entering the subsequent alkaline spray system, while also providing a basis for subsequent material recovery.

[0029] like Figure 1 As shown, the air outlet pipe 32 of the acid spray tower is connected to the alkali spray tower 4, and the air outlet pipe 32 is equipped with an online anemometer 321.

[0030] like Figure 1As shown, the acid spray tower slurry discharge pipe 33 is located at the bottom of the acid circulation tank 31. The slurry discharge pipe 33 is connected to the acid dissolution tank 6, and the slurry discharge pipe 33 is equipped with a slurry discharge valve 331 (electromagnetic pneumatic ball valve). The slurry discharge valve 331 is interlocked (signal connection) with the microwave solids content meter 312. When the microwave solids content meter 312 detects that the slurry solids content reaches 35%, it automatically opens the ball valve to discharge the slurry, and automatically closes it after discharge. The function of this interlock setting is: when the microwave solids content meter detects that the slurry solids content in the acid circulation tank is greater than 35%, it automatically opens the electromagnetic pneumatic ball valve of the slurry discharge pipe to discharge the high solids content slurry to the acid dissolution tank for processing. This setting not only ensures that the slurry concentration in the acid circulation tank is always within a reasonable range, avoiding excessive concentration that would reduce spraying efficiency and cause pipe blockage, but also enables timely discharge and recycling of the collected iron phosphate material, achieving continuous material collection and recovery.

[0031] like Figure 1 As shown, the acid addition pipe 34 is located at the top of the acid circulation tank 31. The acid addition pipe 34 is equipped with an acid addition valve 341 (electromagnetic pneumatic ball valve), which is interlocked with the acid level gauge 311. When the acid level in the circulation tank drops to 20%, the discharge valve is automatically closed, and the addition valve is opened to replenish the acid solution until the level recovers to 70%, at which point the addition valve is closed again. The function of this interlock is to automatically maintain the stability of the acid level in the circulation tank when the level is below 20% of the tank height, opening the acid addition valve to replenish the acid solution until the level recovers to 70%, and then automatically closing the addition valve again. This setting can automatically maintain the stability of the acid level in the circulation tank, ensuring the continuous and stable operation of the spraying system without the need for frequent manual adjustments.

[0032] like Figure 1 As shown, the alkaline spray tower 4 is equipped with an alkaline circulation tank 41, an exhaust pipe 42, a wastewater pipe 43, and an alkaline addition pipe 44. The alkaline circulation tank 41 of the alkaline spray tower 4 treats the tail gas from the acid spray tower 3 using an alkaline circulation spray method. The alkaline circulation tank 41 is equipped with an alkaline level gauge 411 (online level gauge) and a pH meter 412 (online pH meter). In this embodiment, a 32% sodium hydroxide solution is used as the alkaline solution, which can completely neutralize acidic gases such as SO2 in the tail gas, and the SO2 concentration in the emitted tail gas can be controlled at 30 mg / m³. 3 The following meet national environmental emission standards. Furthermore, such as... Figure 1 As shown, the exhaust duct 42 of the alkali spray tower is directly connected to the exhaust fan 5 for venting. The exhaust fan 5 is a variable frequency fan, and it is interlocked with the online anemometer 321 for control. The online anemometer stabilizes the air velocity in the exhaust duct at 2 m / s and adjusts the exhaust fan frequency in real time to ensure that the negative pressure in the rotary kiln is stable at -50±5 Pa, thus avoiding negative pressure fluctuations from affecting the calcination process.

[0033] like Figure 1As shown, the wastewater pipe 43 of the alkali spray tower is located at the bottom of the alkali circulation tank 41. The wastewater pipe 43 discharges into the water treatment system and is equipped with a discharge valve 431 (electromagnetic pneumatic ball valve). The discharge valve 431 is interlocked with the pH meter 412. When the pH of the alkali solution drops below 8, the valve automatically opens to discharge the depleted alkali solution, and then new solution is added. The function of this interlock is: when the pH meter detects that the pH of the alkali circulation tank is less than 8, it indicates that the alkali solution has been completely consumed. The wastewater pipe valve automatically opens to discharge the depleted alkali solution, and then new alkali solution is automatically added to ensure the neutralization efficiency of the secondary spray.

[0034] like Figure 1 As shown, the alkali addition pipe 44 is located at the top of the alkali circulation tank 41. The alkali addition pipe 44 is equipped with an alkali addition valve 441 (electromagnetic pneumatic ball valve). The alkali addition valve 441 is interlocked with the alkali level gauge 411 to automatically maintain a stable liquid level in the alkali tank. This setting is similar to the level interlock of the first-stage spray system, which can automatically maintain a stable liquid level in the alkali circulation tank and ensure continuous operation of the alkali spray.

[0035] like Figure 1 As shown, the acid dissolution tank 6 is equipped with a stirrer 61, an acid level gauge 62 (online level gauge), a transfer pump 63, a low-pressure ultrasonic cavitation assembly (not shown), and a waste heat exchange jacket 64. The low-pressure ultrasonic cavitation assembly has a frequency of 20kHz~40kHz, which can utilize the cavitation effect to break down the agglomeration structure of iron phosphate particles and accelerate the dissolution process. Furthermore, the secondary waste heat from the tail gas of the rotary kiln 1 is introduced into the heat exchange jacket of the acid dissolution tank to maintain the temperature of the acid dissolution system at 60~70℃, making full use of the waste heat of the tail gas to accelerate the acid dissolution process.

[0036] This application involves feeding a high-solids slurry rich in iron and phosphorus ions into an acid dissolution tank for deep redissolution, and utilizing the residual heat of the tail gas to maintain an acid dissolution environment of 60-70°C, accelerating the complete dissolution of agglomerates. After separation by a filter press, the resulting clarified filtrate has extremely high reuse value: one stream can be directly used as supplementary acid for the acid spray tower, realizing the recycling of the spray medium; the other stream can be directly reused as raw material for ferrophosphate solution at the synthesis front end, without additional treatment. The small amount of filter residue produced by the filter press is actually incompletely dissolved high-purity dihydrate ferrophosphate intermediate, which can be returned to the production line as a seed crystal or additive. Thus, this system achieves atomic-level complete recovery of ferrophosphate material from the tail gas, eliminating solid waste generation and significantly reducing the cost of purchasing ferrophosphate raw materials.

[0037] like Figure 1 As shown, filter press 7 is a frame filter press. The filter cloth of the frame filter press is acid-resistant filter cloth, which can withstand the corrosion of acidic slurry and ensure the stable operation of the filter press.

[0038] This application also utilizes the aforementioned rotary kiln tail gas treatment system to develop a new rotary kiln tail gas treatment process during ferric phosphate synthesis: Reference Figure 2 Through the action of the induced draft fan 5, the tail gas from the rotary kiln 1 during the ferric phosphate synthesis process (containing anhydrous ferric phosphate, ferric phosphate dihydrate, water vapor, sulfur dioxide, etc.) passes through the acid spray tower 3. Under the acid circulation spray of the acid spray tower 3, the anhydrous ferric phosphate and some unsuccessfully converted ferric phosphate dihydrate in the tail gas are dissolved, adsorbed, and settled into the acid circulation tank 31 of the acid spray tower 3 to form a slurry, so as to recover iron and phosphorus elements. (When the valve is open) the slurry flows by gravity through the slurry discharge pipe 33 to the acid dissolution tank 6. The slurry is discharged into the acid dissolution tank 6 for a certain period of acid dissolution treatment, and then enters the filter press 7. After solid-liquid separation by the filter press 7, the filtrate is split into two paths for reuse: 50% of the filtrate is used as supplementary acid for the acid spray tower 3 to realize the closed-loop circulation of the spray liquid and reduce the consumption of fresh phosphoric acid; the remaining 50% of the filtrate is returned to the front end as raw material iron-phosphorus solution. The filter residue can be directly returned to the production line as intermediate ferric phosphate dihydrate or made into additives.

[0039] The acid solution in acid spray tower 3 is a phosphoric acid solution with a pH between 0.5 and 2. The acid solution in acid spray tower 3 can be a mixture of 50%–70% filter press filtrate and 30%–50% phosphoric acid solution (preferably 30% phosphoric acid solution + 70% filter press filtrate). When the microwave solids analyzer 312 detects that the solids content of the slurry in the acid circulation tank 31 of acid spray tower 3 is greater than 35%, the slurry discharge valve 331 of the slurry discharge pipe opens, and the slurry flows by gravity from the slurry discharge pipe 33 to the acid dissolving tank 6.

[0040] When the liquid level in the acid circulation tank 31 is less than 20% of the height of the acid circulation tank 31, close the slurry discharge valve 331 of the slurry discharge pipe 33 and open the acid addition valve 341 of the acid addition pipe 34. When the liquid level in the acid circulation tank 31 reaches 70% of the height of the acid circulation tank, close the acid addition valve 341 of the acid addition pipe 34.

[0041] After the slurry flows by gravity through the slurry discharge pipe 33 to the acid dissolution tank 6, when the liquid level in the acid dissolution tank 6 reaches 70%, the slurry discharge valve 331 of the slurry discharge pipe 33 is closed (the acid level gauge is interlocked with the slurry discharge valve). The slurry is stirred and acid-dissolved in the acid dissolution tank 6 for 4 to 6 hours. Preferably, the slurry undergoes ultrasonic-assisted acid dissolution in the acid dissolution tank, in conjunction with a waste heat exchange system (…). Figure 2 (②) By strictly maintaining the system at 60~70℃, the process only takes 2 hours to complete the full dissolution, which is twice as efficient as the original process.

[0042] The two-stage waste heat utilization system constructed in this application is not merely a simple energy recovery, but also crucial for ensuring the long-term stable operation of the entire exhaust gas treatment system. The first stage of waste heat utilization (corresponding to...) Figure 2①) The heat from the high-temperature tail gas at the kiln tail is absorbed by the heat exchanger at the first end and transferred to the waste heat heating pipe connected to the acid spray tower at the end, ensuring that the inner wall temperature of the pipe is maintained above 120℃. This effectively prevents the humid and hot acidic tail gas from condensing due to temperature drop before entering the spray tower, thus avoiding the accumulation of moisture and scale on the inner wall of the pipe end, which would block the air duct and significantly extend the continuous operation cycle of the system. Second-stage waste heat utilization (corresponding to...) Figure 2 (②) The waste heat from the exhaust gas is introduced into the heat exchange jacket of the acid dissolution tank, precisely controlling the temperature of the acid dissolution system at 60~70℃. At this temperature, the chemical dissolution kinetics of ferric phosphate agglomerates is significantly improved, greatly enhancing material turnover efficiency.

[0043] like Figure 2 As shown, the mixed acidic gas composed of water vapor and SO2 in the exhaust gas is adsorbed and neutralized by the alkaline solution circulating spray in the alkaline solution spray tower 4, and the gas exiting the alkaline solution spray tower 4 is directly discharged into the air.

[0044] The alkali solution in the alkali spray tower 4 is a sodium hydroxide solution with a concentration between 28% and 32%. When the liquid level in the alkali circulation tank 41 is less than 20% of its height, the discharge valve 431 of the wastewater pipe 43 is closed, and the alkali addition valve 441 of the alkali addition pipe 44 is opened. When the liquid level in the alkali circulation tank 41 reaches 70% of its height, the alkali addition valve 441 of the alkali addition pipe 44 is closed. Furthermore, when the pH meter 412 detects that the pH of the alkali circulation tank 41 in the alkali spray tower 4 is less than 8, the discharge valve 431 of the wastewater pipe 43 is opened, and the wastewater flows by gravity from the wastewater pipe 43 to the water treatment plant. After passing through a membrane concentration + MVR evaporation crystallization system, industrial-grade sodium sulfate byproducts can be separated. The condensate generated by evaporation can be returned to the alkali preparation stage for recycling, achieving zero wastewater discharge.

[0045] like Figure 2 As shown, to ensure that the rotary kiln 1 is in a negative pressure environment, the air velocity in the outlet duct 32 is controlled at 1~3m / s (preferably 2m / s). The air velocity in the outlet duct 32 is monitored by an online anemometer 321. The variable frequency induced draft fan 5 is interlocked with the online anemometer 321 to achieve a stable negative pressure range for the rotary kiln 1, specifically -50Pa±5Pa. This completely avoids the impact of negative pressure fluctuations on the calcination of the rotary kiln and ensures the stable quality of the iron phosphate product.

[0046] This invention achieves full recovery of ferric phosphate materials, significantly improving production yield. Unlike existing technologies such as CN223760635U, this application does not merely reduce the burden on the spray tower using a cyclone separator. Instead, it employs a primary acid spray to directly capture ferric phosphate dust from the exhaust gas. Combined with an online acid dissolution-pressure filtration process, the captured ferric phosphate slurry is directly converted into reusable raw material iron-phosphate solution and intermediate product dihydrate ferric phosphate. This achieves 100% recovery of ferric phosphate materials from the exhaust gas, completely solving the problems of unusable unqualified dust and raw material yield loss in conventional solutions. Practical verification shows that this solution can increase the overall yield of ferric phosphate production by more than 3%. Control measures such as solid content-discharge valve interlock, liquid level-addition valve interlock, and wind speed-induced draft fan interlock can automatically maintain the concentration and liquid level stability of the spray system, while ensuring the stability of the negative pressure in the rotary kiln. This solves the problems of large fluctuations, easy blockage, and impact on the main production system in the existing tail gas treatment system, and realizes the unattended fully automatic operation of the entire system. The insulation and heating structure of the exhaust gas pipeline effectively prevents the iron phosphate in the exhaust gas from condensing and clogging the pipeline during transportation. Compared with conventional exhaust gas pipelines, the system's fault-free operation cycle is extended by more than 2 times, significantly reducing equipment maintenance costs.

[0047] The combination of primary acid spraying and secondary alkali spraying not only achieves the standard treatment of acidic gases such as SO2, but also further reduces alkali consumption and waste liquid generation compared with existing technologies. At the same time, with the full recycling of materials, waste is turned into treasure. While meeting environmental protection requirements, it significantly reduces the raw material cost of production and has significant economic and environmental benefits.

[0048] In the two-stage waste heat recovery design, the first stage maintains the pipeline temperature above 120℃ to prevent dust condensation, and the second stage heats the acid dissolution system to 60~70℃ to accelerate dissolution, thus realizing the effective utilization of exhaust gas waste heat.

[0049] In addition, the reuse of filtrate from the filter press reduces the consumption of fresh phosphoric acid in the acid spray tower.

[0050] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0051] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A rotary kiln tail gas treatment system for iron phosphate synthesis, comprising a tail gas pipe (2) for connection to a rotary kiln (1), characterized in that, Following the airflow direction, an acid spray tower (3), an alkaline spray tower (4), and an induced draft fan (5) are connected in series after the exhaust gas pipe (2); the acid spray tower (3) is connected to an acid circulation tank (31) for circulating the acid required for spraying; following the flow direction of the solid-liquid mixture in the acid circulation tank (31), an acid dissolving tank (6) and a filter press (7) are connected in series after the acid circulation tank (31); part of the filtrate from the filter press (7) is connected back to the acid circulation tank (31), and the other part is connected back to the ferric phosphate raw material at the front end of the ferric phosphate synthesis.

2. The rotary kiln tail gas treatment system for the iron phosphate synthesis process according to claim 1, characterized in that, The exhaust gas pipe (2) is connected to the tail of the rotary kiln (1).

3. The rotary kiln tail gas treatment system for the ferric phosphate synthesis process according to claim 1, characterized in that, The exhaust gas pipe (2) is provided with a heat exchanger (21) at the beginning, a waste heat heating pipe (23) at the end, and a heat insulation layer (22) in the middle section. The heat exchanger (21) transfers heat to the waste heat heating pipe (23).

4. The rotary kiln tail gas treatment system for the iron phosphate synthesis process according to claim 3, characterized in that, The acid dissolving tank (6) is provided with a waste heat exchange jacket (64), and the heat exchanger (21) transfers heat to the waste heat exchange jacket (64).

5. The rotary kiln tail gas treatment system for the iron phosphate synthesis process according to claim 1, characterized in that, The acid dissolution tank (6) is equipped with a low-pressure ultrasonic cavitation component and a stirrer (61).

6. The rotary kiln tail gas treatment system for the iron phosphate synthesis process according to claim 1, characterized in that, The acid circulation tank (31) is connected to an acid addition pipe (34), an acid level gauge (311) is installed in the acid circulation tank (31), an acid addition valve (341) is installed on the acid addition pipe (34), and the acid addition valve (341) is signal connected to the acid level gauge (311).

7. The rotary kiln tail gas treatment system for the ferric phosphate synthesis process according to claim 6, characterized in that, The acid circulation tank (31) and the acid dissolution tank (6) are connected by a slurry discharge pipe (33). A slurry discharge valve (331) is provided on the slurry discharge pipe (33). A microwave solid content meter (312) is provided in the acid circulation tank (31). The slurry discharge valve (331) is connected to the acid level gauge (311) and the microwave solid content meter (312) via signal connection.

8. The rotary kiln tail gas treatment system for the ferric phosphate synthesis process according to claim 1, characterized in that, The alkaline spray tower (4) is connected to an alkaline circulation tank (41) for circulating alkaline solution required for spraying. The alkaline circulation tank (41) is connected to an alkaline addition pipe (44). An alkaline level gauge (411) is installed in the alkaline circulation tank (41). An alkaline addition valve (441) is installed on the alkaline addition pipe (44). The alkaline addition valve (441) is signal-connected to the alkaline level gauge (411). The alkaline circulation tank (41) is equipped with a wastewater pipe (43). A discharge valve (431) is installed on the wastewater pipe (43). A pH meter (412) is installed in the alkaline circulation tank (41). The discharge valve (431) is signal-connected to the alkaline level gauge (411) and the pH meter (412).

9. The method of using the rotary kiln tail gas treatment system in the ferric phosphate synthesis process as described in any one of claims 1 to 8, characterized in that, While the acid spray tower (3) and the alkali spray tower (4) are continuously spraying, the induced draft fan (5) is turned on to generate negative pressure to drive the tail gas of the rotary kiln (1) through the tail gas pipe (2), the acid spray tower (3), and the alkali spray tower (4) in sequence, and then vent it. When the solid content of the slurry in the acid circulation tank (31) exceeds the preset solid content, the slurry is transported to the acid dissolution tank (6) to redissolve part of the solids, and then the slurry is transported to the filter press (7) for filtration. Part of the filtrate is reused for spraying in the acid spray tower (3), and the other part is reused as the raw material for ferric phosphate solution at the front end of ferric phosphate synthesis.

10. The method of use according to claim 9, characterized in that, The induced draft fan (5) controls the wind speed to be between 1 m / s and 3 m / s. The pH of the alkaline solution sprayed by the alkaline solution spraying tower (4) is above 8, and the pH of the acid solution sprayed by the acid solution spraying tower (3) is between 0.5 and 2.

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