Environment-friendly low-energy consumption tricalcium phosphate calcining process and system

Abstract

The application discloses an environment-friendly low-energy-consumption tricalcium phosphate calcining process and system, which comprises the following steps: kiln outlet flue gas denitration, in which kiln outlet flue gas is subjected to denitration in a denitration unit to remove NOx in the flue gas; first-stage kiln outlet flue gas heat recovery, in which the flue gas out of the denitration unit is heated to 600-900 DEG C in a combustion air heat exchange unit and then introduced into a kiln head of a rotary kiln to heat tertiary air into the kiln to 600-900 DEG C, and the kiln outlet flue gas is lowered to 600-800 DEG C; second-stage kiln outlet flue gas heat recovery, in which the flue gas out of the combustion air heat exchange unit is introduced into a raw material preheating unit to heat raw material from normal temperature to 500-700 DEG C, and the flue gas is lowered to 300-500 DEG C; raw material calcination, in which the preheated powdered raw material is calcined in the rotary kiln to prepare defluorinated tricalcium phosphate clinker; and clinker cooling, in which the kiln outlet clinker is cooled by a kiln head cooler to prepare a finished product. The application realizes standard flue gas emission, flue gas heat recovery, reduces system heat consumption, reduces the heat load of the rotary kiln and improves the output of the kiln system.

Description

Technical Field

[0001] This invention relates to the field of tricalcium phosphate production technology, and in particular to an environmentally friendly, low-energy-consumption tricalcium phosphate calcination process and system. Background Technology

[0002] Defluorinated calcium phosphate is a high-performance inorganic feed additive that significantly increases the yield and weight of poultry and livestock. my country's feed phosphate production started relatively late, with research beginning in the 1960s, but actual production and use only commenced in the late 1980s. Rapid development in the 1990s saw capacity increase from tens of thousands of tons to over a million tons. Currently, domestic production capacity exceeds one million tons, making China the world's second-largest feed phosphate producer after the United States. Although China's feed phosphate industry has developed rapidly due to its resource and energy advantages, problems remain, including rudimentary products, outdated technology, fragmented production, and limited application. The production of defluorinated calcium phosphate using the phosphoric acid process is a highly complex multiphase reaction. The degree of defluorination during the reaction is crucial to product quality. There are two methods for producing defluorinated calcium phosphate: sintering defluorination and melt defluorination. Both methods rely on high-temperature steam defluorination. However, the melt method, using low-grade phosphate rock as raw material, results in high impurities, high energy consumption, and poor product quality, often failing to meet feed-grade requirements and is gradually being phased out. The sintering defluorination method involves sintering phosphate rock powder and additives at high temperatures, followed by contact with water vapor to react and remove fluoride. This method is currently widely used worldwide.

[0003] The production process of defluorinated tricalcium phosphate generally employs a high-temperature method. Among these, the sodium phosphate rock sintering process is technologically advanced and produces reliable products on a large scale in developed countries such as Europe, America, and Japan. The production enterprises and technologies for this process are mainly concentrated in a few large companies, such as PCS and IMC in the United States, and Onoda Chemical Co., Ltd. in Japan, and it represents the mainstream international production process. Compared with foreign countries, my country still lags significantly in the production of defluorinated tricalcium phosphate, manifested in low output, small-scale equipment, and the lack of a large-scale industry.

[0004] The roasting temperature of defluorinated tricalcium phosphate is generally as high as 1400℃ or above, and the existing process generally uses dry rotary kiln calcination. Natural ore mined from the mine is beneficiated to increase the phosphorus pentoxide content, and a small amount of industrial-grade sulfuric acid is added. After being metered and mixed into pellets, it is sent to a rotary kiln and heated with coal tar to undergo a high-temperature reaction, removing fluorine from the phosphate rock and producing a compound calcium-phosphate mineral feed additive composed of tricalcium phosphate, sodium calcium phosphate, and other phosphates. In this process, the raw materials entering the kiln need to be granulated into pellets, resulting in a slow transfer of heat generated by fuel combustion to the materials. The calcination and defluorination time of the pellets is long, limiting the rotary kiln's output. Furthermore, the flue gas temperature exiting the rotary kiln often reaches over 700℃, with a high enthalpy carried out of the system, leading to heat loss accounting for approximately 40% of the total calcination heat consumption. Simultaneously, since the materials entering the kiln are not preheated, they require prolonged preheating and drying after entering the kiln, resulting in a relatively high heat load within the rotary kiln. This limits the kiln's capacity expansion, leading to a relatively small-scale production line for defluorinated tricalcium phosphate produced by sintering, characterized by high energy consumption, significant pollution, and high production costs. Additionally, the kiln exit temperature of around 700℃ does not reach the temperature window for SNCR denitrification, making it difficult to remove NOx using the low-investment, low-operating-cost SNCR method. This results in difficulties in NOx removal from the flue gas. Methods such as low-temperature tail gas catalytic denitrification or ozone oxidation followed by alkaline absorption are complex, requiring large investments and incurring high operating costs.

[0005] In summary, the problems with existing technologies are:

[0006] (1) The existing process uses a rotary kiln to calcine defluorinated tricalcium phosphate. The flue gas from the kiln carries out a large amount of heat, which is not effectively recovered and utilized, resulting in high heat consumption of the calcination system and large fuel consumption.

[0007] (2) In the existing process, the raw materials entering the kiln need to be granulated into balls. The heat generated by the combustion of fuel in the kiln is transferred to the materials slowly, and the calcination and defluorination time of the balls is long, which limits the output of the rotary kiln.

[0008] (3) In the existing process, the raw materials entering the kiln are not preheated. After entering the kiln, the materials need to be preheated and dried for a long time. The heat load inside the rotary kiln is relatively large, which limits the capacity improvement of the rotary kiln.

[0009] (4) In the existing process, the temperature of the flue gas exiting the kiln does not reach the temperature window of SNCR denitrification, making it difficult to remove NOx through the SNCR method with low investment and low operating cost.

[0010] Therefore, for the existing rotary kiln calcination process for defluorinated tricalcium phosphate, it is necessary to develop a tricalcium phosphate calcination process and system that can achieve compliant flue gas emissions, flue gas enthalpy recovery, reduced system heat consumption, reduced rotary kiln heat load, and increased kiln system output. Summary of the Invention

[0011] One of the objectives of this invention is to provide an environmentally friendly, low-energy-consumption tricalcium phosphate calcination process. This process involves adding an SNCR denitrification unit at the kiln tail and a multi-phase combined heat exchanger to recover the enthalpy of the flue gas. After the high-temperature flue gas exiting the kiln is treated by SNCR denitrification, gas-to-gas heat exchange, and gas-to-material heat exchange, the heat of the flue gas is transferred to the combustion air and raw materials entering the kiln. This achieves compliant flue gas emissions, recovers the enthalpy of the flue gas, reduces system heat consumption, reduces the heat load of the rotary kiln, and increases the output of the kiln system.

[0012] Another objective of this invention is to provide an environmentally friendly, low-energy tricalcium phosphate calcination system.

[0013] This invention is achieved through an environmentally friendly, low-energy-consumption tricalcium phosphate calcination process, comprising the following steps:

[0014] Denitrification of kiln exhaust gas: The kiln exhaust gas first passes through the denitrification unit to remove NOx from the flue gas. A reducing agent is sprayed into the bottom of the denitrification pipe of the denitrification unit so that the reducing agent reacts with the NOx in the flue gas to achieve the removal of NOx from the flue gas.

[0015] First-stage heat recovery of flue gas exiting the kiln: The flue gas exiting the denitrification unit enters the combustion air heat exchange unit through the flue gas inlet, and the tertiary air entering the kiln enters the combustion air heat exchange unit through the combustion air inlet. The heat of the flue gas exiting the kiln heats the tertiary air entering the kiln to 600-900℃ before introducing it into the kiln head of the rotary kiln. The temperature of the flue gas exiting the kiln drops from 800-1000℃ to 600-800℃, realizing the first-stage heat recovery of the flue gas.

[0016] Second-stage heat recovery of flue gas from the kiln: The flue gas exiting the combustion air heat exchange unit enters the raw material preheating unit to heat the raw material, raising it from room temperature to 500-700℃, while the flue gas temperature drops from 600-800℃ to 300-500℃, thus achieving the second-stage recovery of flue gas heat.

[0017] Raw meal calcination: Preheated powdered raw meal is calcined in a rotary kiln to prepare defluorinated tricalcium phosphate clinker;

[0018] Clinker cooling: The defluorinated tricalcium phosphate clinker exiting the kiln is cooled by the kiln head cooler to produce the finished product.

[0019] Preferably, it also includes dust removal of the kiln exhaust gas: before entering the combustion air heat exchange unit, the exhaust gas exiting the denitrification unit is first treated by the kiln tail cyclone dust collector, the collected dust is returned to the rotary kiln, and the dust-removed exhaust gas then enters the combustion air heat exchange unit.

[0020] Preferably, a portion of the preheated powdered raw material can be fed into the denitrification pipeline.

[0021] Preferably, the tertiary air entering the kiln is either air or residual air from the kiln head exiting the cooler. The residual air from the kiln head exiting the cooler is first treated by a kiln head cyclone dust collector for dust collection. The collected dust is returned to the cooler, and the dust-free residual air then enters the combustion air heat exchange unit.

[0022] Preferably, the raw material entering the raw material preheating unit is in powder form with a fineness of less than 50% residue on an 80µm sieve.

[0023] An environmentally friendly, low-energy tricalcium phosphate calcination system includes a rotary kiln, a cooler, a kiln tail flue, a denitrification unit, a combustion air heat exchange unit, and a raw material preheating unit. The kiln tail flue, rotary kiln, and cooler are connected in sequence. The flue gas outlet of the kiln tail flue is connected to the flue gas inlet of the denitrification unit. The flue gas outlet of the denitrification unit is connected to the flue gas inlet of the combustion air heat exchange unit. The flue gas outlet of the combustion air heat exchange unit is connected to the flue gas inlet at the bottom of the raw material preheating unit. The combustion air outlet of the combustion air heat exchange unit is connected to the kiln head hood of the rotary kiln. The discharge port at the bottom of the raw material preheating unit is connected to the kiln tail flue. A kiln head burner is installed at the kiln head of the rotary kiln.

[0024] Preferably, the denitrification unit includes a denitrification pipe, a denitrification system, and a spray gun. The denitrification pipe is located above the kiln tail flue chamber, and the flue gas outlet at the top of the denitrification pipe is connected to the flue gas inlet of the combustion air heat exchange unit. The denitrification system is connected to the spray gun, and the spray gun is located at the bottom of the denitrification pipe.

[0025] More preferably, the discharge port at the bottom of the raw material preheating unit is connected to the denitrification pipeline.

[0026] Preferably, a kiln tail cyclone dust collector is provided between the flue gas inlet of the combustion air heat exchange unit and the flue gas outlet of the denitrification unit. The flue gas inlet of the kiln tail cyclone dust collector is connected to the flue gas outlet of the denitrification unit, the flue gas outlet of the kiln tail cyclone dust collector is connected to the flue gas inlet of the combustion air heat exchange unit, and the discharge port of the kiln tail cyclone dust collector is connected to the kiln tail smoke chamber.

[0027] Preferably, the combustion air inlet of the combustion air heat exchange unit is connected to the low-temperature waste air outlet of the cooler.

[0028] In a further preferred embodiment, a kiln head cyclone dust collector is provided between the combustion air inlet of the combustion air heat exchange unit and the cooler. The waste air inlet of the kiln head cyclone dust collector is connected to the low-temperature waste air outlet of the cooler, the discharge port of the kiln head cyclone dust collector is connected to the cooler, and the waste air outlet of the kiln head cyclone dust collector is connected to the combustion air inlet of the combustion air heat exchange unit.

[0029] Preferably, the combustion air heat exchange unit includes a gas-to-gas heat exchanger, the flue gas inlet of the gas-to-gas heat exchanger is connected to the flue gas outlet of the denitrification unit, the flue gas outlet of the gas-to-gas heat exchanger is connected to the flue gas inlet of the raw material preheating unit, and the combustion air outlet of the gas-to-gas heat exchanger is connected to the kiln head hood of the rotary kiln.

[0030] More preferably, the gas-to-gas heat exchanger has an air inlet valve and a fan installed on the combustion air inlet pipe, and an air outlet valve installed on the combustion air outlet pipe.

[0031] Preferably, the raw material preheating unit includes at least one cyclone preheater, and the cyclone preheater includes a cyclone cylinder and an air duct and a material pipe connected to the cyclone cylinder.

[0032] The present invention has the following advantages and beneficial effects:

[0033] 1. This invention increases the temperature of the flue gas exiting the kiln by preheating the raw materials before they enter the kiln, thus meeting the temperature window for SNCR denitrification. A denitrification pipeline is installed at the kiln tail, and an SNCR denitrification system is arranged to ensure that the NOx emissions of the flue gas meet environmental protection standards. Moreover, it requires less investment and has low operating costs.

[0034] 2. This invention fully recovers the enthalpy of the flue gas exiting the kiln by setting up a multi-phase combined heat exchanger at the kiln tail. The first stage of heat exchange is a gas-to-gas phase heat exchange, transferring the heat from the high-grade flue gas to the tertiary air entering the kiln, increasing the temperature of the combustion air inside the kiln. The second stage is a gas-to-solid phase heat exchange, transferring the heat from the cooled flue gas to the raw materials entering the kiln, increasing the temperature of the raw materials. Through this two-stage heat recovery, the heat load inside the rotary kiln is reduced, and the heat loss carried away by the flue gas is minimized, thereby reducing the heat consumption of the kiln system and saving fuel.

[0035] 3. This invention improves the calcination capacity of the rotary kiln by setting a raw material preheating unit at the kiln tail, so that the raw material entering the kiln is powder and can be directly calcined in the kiln. This avoids the problem of slow heat transfer speed of the pellets in the kiln and long defluorination time of the pellets after granulation.

[0036] 4. By setting a raw material preheating unit at the kiln tail, the raw materials entering the kiln are preheated to 500-700°C, saving the drying and heating time required after the materials enter the kiln, reducing the heat load inside the rotary kiln, and improving the calcination capacity of the rotary kiln. At the same time, it replaces the existing dry long kiln process, which can reduce the size of the kiln for the same output, which is conducive to the large-scale production of single lines.

[0037] 5. This invention prevents the combustion air heat exchange unit from becoming clogged due to dust accumulation during long-term use by treating the flue gas and combustion air entering the combustion air heat exchange unit through dust collection. Attached Figure Description

[0038] Figure 1This is a schematic diagram of the calcination system provided in Embodiment 1 of the present invention;

[0039] Figure 2 This is a schematic diagram of the calcination system provided in Embodiment 2 of the present invention.

[0040] In the picture:

[0041] a-Air; g2-Secondary air; g3-Tertiary air entering the kiln; g4-Residual air at the kiln head; g5-Low-temperature flue gas; R-Raw material; F-Pulverized coal; K-Defluorinated tricalcium phosphate clinker;

[0042] 1-Raw material preheating unit; 101-First stage cyclone separator; 102-Second stage cyclone separator; 103-Distribution valve;

[0043] 2-Combustion air heat exchange unit; 201-Gas-to-gas heat exchanger; 202-Fan; 203-Inlet valve; 204-Outlet valve; 205-Kiln tail cyclone dust collector; 206-Kiln head cyclone dust collector;

[0044] 3-Denitrification unit; 301-Denitrification pipeline; 302-Spray gun; 303-Denitrification system;

[0045] 4-Kiln tail smoke chamber; 5-Rotary kiln; 6-Cooler; 7-Kiln head burner; 8-Kiln head hood.

[0046] The dashed line with an arrow indicates the airflow direction; the solid line with an arrow indicates the flow direction of raw material or pulverized coal. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" 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.

[0049] This invention provides an environmentally friendly, low-energy-consumption tricalcium phosphate calcination process, comprising the following steps:

[0050] Denitrification of kiln exhaust gas: Before entering the kiln tail cyclone dust collector 205, the kiln exhaust gas first passes through the denitrification unit 3 to remove NOx from the flue gas. The NOx concentration in the flue gas entering the denitrification pipe is generally 800-1500 ppm, and the flue gas temperature is 800-1000℃. A reducing agent (ammonia water) is sprayed into the bottom of the denitrification pipe 301 of the denitrification unit 3. The reducing agent in the denitrification pipe 301 reacts with the NOx in the flue gas to achieve the removal of NOx from the flue gas.

[0051] Dust removal from kiln exhaust: The high-temperature flue gas exiting the kiln contains dust. Before entering the combustion air heat exchange unit 2, the flue gas treated by the denitrification unit 3 is first treated by the kiln tail cyclone dust collector 205. The collected dust is returned to the rotary kiln 5, and the dust-removed flue gas then enters the combustion air heat exchange unit 2.

[0052] First-stage heat recovery of kiln exhaust gas: After being treated by the kiln tail cyclone dust collector 205, the exhaust gas enters one channel of the combustion air heat exchange unit 2 through the exhaust gas inlet. The tertiary air g3 enters another channel of the combustion air heat exchange unit 2 through the combustion air inlet. The high-temperature exhaust gas exiting the kiln is the heating source, and the tertiary air entering the kiln is the heat absorption source. The exhaust gas exiting the kiln heats the tertiary air entering the kiln. The heat of the exhaust gas exiting the kiln is used to heat the tertiary air entering the kiln to 600-900℃ before it is introduced into the kiln head of the rotary kiln 5. The exhaust gas exiting the kiln drops from 800-1000℃ to 600-800℃, thus realizing the first-stage heat recovery of the exhaust gas.

[0053] Second-stage heat recovery of flue gas from the kiln: The flue gas from the combustion air heat exchange unit 2 enters the raw material preheating unit 1 to heat the raw material, raising it from room temperature to 500-700℃, while the flue gas temperature drops from 600-800℃ to 300-500℃, thus achieving the second-stage recovery of flue gas heat; the raw material entering the raw material preheating unit 1 is in powder form with a fineness of less than 50% residue on an 80μm sieve;

[0054] Raw material calcination: The preheated powdered raw material is fed into rotary kiln 5 for calcination to prepare defluorinated tricalcium phosphate clinker K; a portion of the preheated powdered raw material can also be fed into denitrification pipe 301. Since the flue gas temperature in denitrification pipe 301 is 800-1000℃, the temperature of the material entering the kiln can be increased, and the heat load of rotary kiln 5 can be reduced.

[0055] Clinker cooling: The defluorinated tricalcium phosphate clinker exiting the kiln is cooled by the kiln head cooler 6 to produce the finished product.

[0056] The tertiary air g3 entering the kiln is either air or residual air g4 exiting the cooler 6. The residual air exiting the cooler 6 is first treated by the kiln head cyclone dust collector 206 for dust collection. The collected dust is returned to the cooler 6, and the residual air after dust removal enters the combustion air heat exchange unit 2.

[0057] An environmentally friendly, low-energy tricalcium phosphate calcination system includes a rotary kiln 5, a cooler 6, a kiln tail flue 4, a denitrification unit 3, a kiln tail cyclone dust collector 205, a combustion air heat exchange unit 2, and a raw material preheating unit 1. The kiln tail flue 4, the rotary kiln 5, and the cooler 6 are connected in sequence, as are the kiln tail flue 4, the denitrification unit 3, the kiln tail cyclone dust collector 205, the combustion air heat exchange unit 2, and the raw material preheating unit 1.

[0058] Specifically, the denitrification unit 3 is used to remove NOx from the high-temperature flue gas exiting the kiln. The flue gas inlet at the bottom of the denitrification unit 3 is connected to the flue gas outlet of the kiln tail smoke chamber 4, and the flue gas outlet at the top of the denitrification unit 3 is connected to the flue gas inlet of the kiln tail cyclone dust collector 205. The discharge port of the kiln tail cyclone dust collector 205 is connected to the kiln tail smoke chamber 4, and the flue gas outlet of the kiln tail cyclone dust collector 205 is connected to the flue gas inlet of the combustion air heat exchange unit 2. The flue gas outlet of the combustion air heat exchange unit 2 is connected to the flue gas inlet at the bottom of the raw material preheating unit 1. The combustion air outlet of the combustion air heat exchange unit 2 is connected to the kiln head hood 8 of the rotary kiln 5. The flue gas outlet at the top of the raw material preheating unit 1 discharges low-temperature flue gas. The feed inlet at the top of the raw material preheating unit 1 is used for raw material feeding. The discharge outlet at the bottom of the raw material preheating unit 1 is connected to the kiln tail smoke chamber 4. A kiln head burner 7 is provided at the kiln head of the rotary kiln 5.

[0059] Combustion air heat exchange unit 2 is located after denitrification unit 3. The high-temperature flue gas exiting the kiln heats the tertiary air entering the kiln in combustion air heat exchange unit 2. Raw material preheating unit 1 is located after combustion air heat exchange unit 2. The flue gas exiting combustion air heat exchange unit 2 enters raw material preheating unit 1 to preheat the raw material. The raw material from raw material preheating unit 1 enters rotary kiln 5 through a feed pipe. The head of kiln head burner 7 is located at the center of the kiln head of rotary kiln 5, and fuel is injected from the kiln head. A kiln tail cyclone dust collector 205 is installed between denitrification unit 3 and combustion air heat exchange unit 2 to collect dust from the flue gas exiting the kiln and return it to rotary kiln 5. The dust-removed flue gas then re-enters combustion air heat exchange unit 2, preventing blockage due to dust accumulation during long-term use.

[0060] The combustion air heat exchange unit 2 includes a gas-to-gas heat exchanger 201. The flue gas inlet of the gas-to-gas heat exchanger 201 is connected to the flue gas outlet of the kiln tail cyclone dust collector 205, and the flue gas outlet of the gas-to-gas heat exchanger 201 is connected to the flue gas inlet of the raw material preheating unit 1. The combustion air outlet of the gas-to-gas heat exchanger 201 is connected to the kiln head hood 8 of the rotary kiln 5. An inlet valve 203 and a fan 202 are installed on the pipe of the combustion air inlet of the gas-to-gas heat exchanger 201, and an outlet valve 204 is installed on the pipe of the combustion air outlet of the gas-to-gas heat exchanger 201.

[0061] The combustion air inlet of the gas-to-gas heat exchanger 201 is connected to the low-temperature waste air outlet of the cooler 6. A kiln head cyclone dust collector 206 is installed between the combustion air inlet of the gas-to-gas heat exchanger 201 and the cooler 6. The waste air inlet of the kiln head cyclone dust collector 206 is connected to the low-temperature waste air outlet of the cooler 6, the discharge port of the kiln head cyclone dust collector 206 is connected to the cooler 6, and the waste air outlet of the kiln head cyclone dust collector 206 is connected to the combustion air inlet of the combustion air heat exchange unit 2.

[0062] The combustion-supporting tertiary air consists of air or residual air from the kiln head of the cooler 6. The combustion-supporting tertiary air is blown into the air-to-air heat exchanger 201 by the blower 202. An air inlet valve 203 and an air outlet valve 204 are respectively installed before and after the air-to-air heat exchanger 201, so that the air volume of the combustion-supporting tertiary air is adjustable. The heated combustion-supporting tertiary air is connected to the kiln head hood 8 of the rotary kiln 5 and enters the rotary kiln 5.

[0063] The denitrification unit 3 includes a denitrification pipe 301, a denitrification system 303, and a spray gun 302. The denitrification pipe 301 is located above the kiln tail flue chamber 4. The flue gas outlet at the top of the denitrification pipe 301 is connected to the flue gas inlet of the combustion air heat exchange unit 2. The denitrification system 303 is connected to the spray gun 302. The spray gun 302 is located at the bottom of the denitrification pipe 301. The reducing agent (ammonia water) is sprayed into the denitrification pipe 301 through the denitrification system 303 and the spray gun 302 to reduce NOx in the kiln tail flue gas to N2.

[0064] The raw material preheating unit 1 includes at least one stage of cyclone preheater, which includes a cyclone tube and a duct and a feed pipe connecting the cyclone tube. The discharge port at the bottom of the raw material preheating unit 1 is also connected to the denitrification pipe 301. In this embodiment, two stages of cyclone preheaters are provided, namely a first stage cyclone tube 101 and a second stage cyclone tube 102. The flue gas inlet of the second stage cyclone tube 102 is connected to the flue gas outlet of the gas-to-gas heat exchanger 201. The discharge port of the second stage cyclone tube 102 is connected to the kiln tail flue chamber 4 and the denitrification pipe 301 through a distribution valve 103. The flue gas outlet of the second stage cyclone tube 102 is connected to the flue gas inlet of the first stage cyclone tube 101. The discharge port of the first stage cyclone tube 101 is connected to the connecting pipe between the second stage cyclone tube 102 and the gas-to-gas heat exchanger 201.

[0065] The combustion air inlet and outlet pipes of the cyclone preheater, rotary kiln 5, cooler 6, denitrification pipe 301, and gas-to-gas heat exchanger 201 are all lined with refractory materials.

[0066] The present invention will now be described in further detail.

[0067] Example 1

[0068] Please see Figure 1This embodiment provides an environmentally friendly, low-energy tricalcium phosphate calcination system. It mainly consists of a rotary kiln 5, a cooler 6, a kiln tail smoke chamber 4, a denitrification pipeline 301, a kiln tail cyclone dust collector 205, a gas-to-gas heat exchanger 201, and a cyclone preheater.

[0069] A kiln head hood 8 is installed at the front end of the rotary kiln 5, and a kiln head burner 7 is connected in front of the kiln head hood 8. Fuel is injected and burned through the kiln head burner 7 to provide heat for the calcination of the material inside the rotary kiln 5. The kiln tail of the rotary kiln 5 is connected to the kiln tail flue chamber 4. The flue gas generated by combustion inside the rotary kiln 5 is discharged through the kiln tail flue chamber 4. The NOx concentration in the combustion flue gas is generally 800-1500 ppm. The calcination temperature of the material inside the kiln is about 1400℃. The clinker exiting the kiln enters the cooler 6. Air a is introduced into the cooler 6 to cool the clinker to below 150℃. The hot air at the front end of the cooler 6 is secondary air g2, with a temperature of 800-1200℃, which provides combustion air for calcination inside the kiln.

[0070] The denitrification pipe 301 is located above the kiln tail flue chamber 4. A spray gun 302 is installed at the bottom of the denitrification pipe 301. Ammonia water in the denitrification system 303 is sprayed into the denitrification pipe 301 through the spray gun 302. The residence time of flue gas in the denitrification pipe 301 is greater than 0.5s, which reduces NOx in the flue gas to N2.

[0071] The gas-to-gas heat exchanger 201 is located after the denitrification pipeline 301. A kiln tail cyclone dust collector 205 is installed between the denitrification pipeline 301 and the gas-to-gas heat exchanger 201 to collect dust from the flue gas and return it to the rotary kiln 5. The dust-removed flue gas then enters the gas-to-gas heat exchanger 201, preventing blockage due to dust accumulation during long-term use. The high-temperature flue gas in the gas-to-gas heat exchanger 201 heats the tertiary air g3 entering the kiln. The high-temperature flue gas exiting the kiln tail cyclone dust collector 205 serves as the heating source, while the tertiary air g3 entering the kiln acts as the heat absorption source. The heat from the high-temperature flue gas exiting the kiln heats the tertiary air entering the kiln to 600–900°C before introducing it into the rotary kiln 5. The temperature of the flue gas exiting the kiln drops from 800–1000°C to 600–800°C, thus achieving the first-stage recovery of flue gas heat. The preheated tertiary air g3 is connected to the kiln head hood 8 through a pipe, and after being mixed with the secondary air g2, it enters the kiln to provide combustion air for fuel combustion.

[0072] The cyclone preheater has two stages. After gas-solid heat exchange and cyclone separation, the raw material temperature increases, while the flue gas temperature decreases to 300–500℃. The raw material is preheated in the two-stage cyclone preheater. The raw material R is in powder form with a fineness of less than 50% residue on an 80µm sieve and is at room temperature. The raw material is fed into the outlet duct of the second-stage cyclone 102 through a pipe for gas-solid heat exchange. Driven by the airflow, it enters the first-stage cyclone 101. After gas-solid separation in the first-stage cyclone 101, the material is fed from the feed pipe of the first-stage cyclone 101 into the outlet duct of the gas-to-gas heat exchanger 201. Driven by the airflow, it enters the second-stage cyclone 102. After gas-solid separation in the second-stage cyclone 102, the raw material is fed into the kiln tail flue chamber 4.

[0073] After the raw material exiting the cyclone preheater enters the kiln tail smoke chamber 4, it is calcined in the rotary kiln 5, where it is sintered into clinker. The clinker exiting the kiln is cooled by the cooler 6 to produce clinker products. Part of the hot air exiting the cooler 6 enters the kiln as high-temperature secondary air g2; the other part is discharged from the kiln head as kiln head exhaust air g4. The temperature of the kiln head exhaust air is 200-400℃, which provides a drying heat source for the raw material and fuel grinding process.

[0074] Example 2

[0075] Please see Figure 2 Unlike Example 1, the tertiary air g3 entering the kiln is sourced from hot air exiting the cooler 6, with a temperature of 200–400°C. Its enthalpy is higher than that of ambient air, which helps to increase the temperature of the tertiary air entering the kiln and reduce heat consumption. This example is suitable for situations where the raw materials and fuels have low moisture content and the residual air g4 at the kiln head has abundant heat. After being drawn from the kiln head cooler 6, the tertiary air g3 enters the kiln head cyclone dust collector 206 for dust collection before entering the gas-to-gas heat exchanger 201, preventing blockage due to dust accumulation during long-term use.

[0076] The feed pipe of the second-stage cyclone separator 102 is equipped with a feed distribution valve 103. The feed pipe is divided into two branches: one branch connects to the kiln tail flue chamber 4, and the other branch connects to the denitrification pipe 301. The opening degree of the feed distribution valve 103 can be adjusted between 0 and 100% to realize the distribution of raw materials. When the raw materials are distributed into the denitrification pipe 301, since the flue gas temperature in the denitrification pipe 301 is 800-1000℃, the temperature of the material entering the kiln can be increased, and the heat load of the rotary kiln 5 can be reduced.

[0077] In summary, this invention preheats the raw materials before they enter the kiln, increasing the temperature of the flue gas exiting the kiln and meeting the temperature window for SNCR denitrification, thus ensuring that NOx emissions meet environmental standards. Furthermore, it requires less investment and has lower operating costs. By incorporating a multi-phase combined heat exchanger at the kiln tail, the enthalpy of the flue gas exiting the kiln is fully recovered. The first stage is a gas-to-gas heat exchange, transferring heat from the high-grade flue gas to the tertiary air entering the kiln, increasing the temperature of the combustion air inside the kiln. The second stage is a gas-to-solid heat exchange, transferring heat from the cooled flue gas to the raw materials entering the kiln, increasing the temperature of the raw materials. Through this two-stage heat recovery, the heat load inside the rotary kiln is reduced, minimizing heat loss carried away by the flue gas, thereby reducing the kiln system's heat consumption and saving fuel. By setting up a raw material preheating unit at the kiln tail, the raw materials entering the kiln are preheated to 500-700℃, saving the drying and heating time required after the materials enter the kiln, reducing the heat load inside the rotary kiln, and improving the calcination capacity of the rotary kiln. At the same time, it replaces the existing dry long kiln process, which can reduce the size of the kiln for the same output, which is conducive to the large-scale production of single lines.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An environmentally friendly, low-energy-consumption tricalcium phosphate calcination process, characterized in that: Includes the following steps: Denitrification of kiln exhaust gas: The exhaust gas first passes through the denitrification unit to remove NOx from the exhaust gas. A reducing agent is sprayed into the bottom of the denitrification pipe of the denitrification unit, so that the reducing agent reacts with the NOx in the exhaust gas to achieve the removal of NOx from the exhaust gas. The exhaust gas outlet of the kiln tail flue is connected to the exhaust gas inlet of the denitrification unit, the exhaust gas outlet of the denitrification unit is connected to the exhaust gas inlet of the combustion air heat exchange unit, the exhaust gas outlet of the combustion air heat exchange unit is connected to the exhaust gas inlet at the bottom of the raw material preheating unit, and the combustion air outlet of the combustion air heat exchange unit is connected to the kiln head hood of the rotary kiln. First-stage heat recovery of flue gas exiting the kiln: The flue gas exiting the denitrification unit enters the combustion air heat exchange unit through the flue gas inlet, and the tertiary air entering the kiln enters the combustion air heat exchange unit through the combustion air inlet. The heat of the flue gas exiting the kiln heats the tertiary air entering the kiln to 600~900℃ before introducing it into the kiln head of the rotary kiln. The temperature of the flue gas exiting the kiln drops from 800~1000℃ to 600~800℃, realizing the first-stage heat recovery of the flue gas. Second-stage heat recovery of flue gas from the kiln: The flue gas exiting the combustion air heat exchange unit enters the raw material preheating unit to heat the raw material, raising it from room temperature to 500~700℃, while the flue gas temperature drops from 600~800℃ to 300~500℃, thus achieving the second-stage recovery of flue gas heat. Raw meal calcination: Preheated powdered raw meal is calcined in a rotary kiln to prepare defluorinated tricalcium phosphate clinker; Clinker cooling: The defluorinated tricalcium phosphate clinker exiting the kiln is cooled by the kiln head cooler to produce the finished product.

2. The environmentally friendly, low-energy tricalcium phosphate calcination process as described in claim 1, characterized in that, It also includes dust removal of kiln exhaust gas: before entering the combustion air heat exchange unit, the exhaust gas exiting the denitrification unit is first treated by the kiln tail cyclone dust collector, and the collected dust is returned to the rotary kiln. The dust-removed exhaust gas then enters the combustion air heat exchange unit.

3. The environmentally friendly, low-energy tricalcium phosphate calcination process as described in claim 1, characterized in that, A portion of the preheated powdered raw material can be fed into the denitrification pipeline.

4. The environmentally friendly, low-energy-consumption tricalcium phosphate calcination process as described in claim 1, characterized in that, The three-stage air entering the kiln is either air or residual air from the kiln head exiting the cooler. The residual air from the kiln head exiting the cooler is first treated by a kiln head cyclone dust collector for dust collection. The collected dust is returned to the cooler, and the dust-free residual air then enters the combustion air heat exchange unit.

5. The environmentally friendly, low-energy-consumption tricalcium phosphate calcination process as described in claim 1, characterized in that, The raw material entering the raw material preheating unit is in powder form with a fineness of less than 50% residue on an 80µm sieve.

6. An environmentally friendly, low-energy tricalcium phosphate calcination system, characterized in that, The system includes a rotary kiln, a cooler, a kiln tail flue, a denitrification unit, a combustion air heat exchange unit, and a raw material preheating unit. The kiln tail flue, rotary kiln, and cooler are connected in sequence. The flue gas outlet of the kiln tail flue is connected to the flue gas inlet of the denitrification unit. The flue gas outlet of the denitrification unit is connected to the flue gas inlet of the combustion air heat exchange unit. The flue gas outlet of the combustion air heat exchange unit is connected to the flue gas inlet at the bottom of the raw material preheating unit. The combustion air outlet of the combustion air heat exchange unit is connected to the kiln head hood of the rotary kiln. The discharge port at the bottom of the raw material preheating unit is connected to the kiln tail flue. A kiln head burner is installed at the kiln head of the rotary kiln.

7. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 6, characterized in that, The denitrification unit includes a denitrification pipe, a denitrification system, and a spray gun. The denitrification pipe is located above the kiln tail flue chamber. The flue gas outlet at the top of the denitrification pipe is connected to the flue gas inlet of the combustion air heat exchange unit. The denitrification system is connected to the spray gun, which is located at the bottom of the denitrification pipe.

8. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 7, characterized in that, The discharge port at the bottom of the raw material preheating unit is connected to the denitrification pipeline.

9. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 6, characterized in that, A kiln tail cyclone dust collector is installed between the flue gas inlet of the combustion air heat exchange unit and the flue gas outlet of the denitrification unit. The flue gas inlet of the kiln tail cyclone dust collector is connected to the flue gas outlet of the denitrification unit, the flue gas outlet of the kiln tail cyclone dust collector is connected to the flue gas inlet of the combustion air heat exchange unit, and the discharge port of the kiln tail cyclone dust collector is connected to the kiln tail smoke chamber.

10. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 6, characterized in that, The combustion air inlet of the combustion air heat exchange unit is connected to the low-temperature waste air outlet of the cooler.

11. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 10, characterized in that, A kiln head cyclone dust collector is installed between the combustion air inlet of the combustion air heat exchange unit and the cooler. The waste air inlet of the kiln head cyclone dust collector is connected to the low-temperature waste air outlet of the cooler. The discharge port of the kiln head cyclone dust collector is connected to the cooler. The waste air outlet of the kiln head cyclone dust collector is connected to the combustion air inlet of the combustion air heat exchange unit.

12. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 6, characterized in that, The combustion air heat exchange unit includes a gas-to-gas heat exchanger. The flue gas inlet of the gas-to-gas heat exchanger is connected to the flue gas outlet of the denitrification unit. The flue gas outlet of the gas-to-gas heat exchanger is connected to the flue gas inlet of the raw material preheating unit. The combustion air outlet of the gas-to-gas heat exchanger is connected to the kiln head hood of the rotary kiln.

13. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 12, characterized in that, The gas-to-gas heat exchanger has an inlet valve and a fan installed on the combustion air inlet pipe, and an outlet valve installed on the combustion air outlet pipe.

14. The environmentally friendly, low-energy tricalcium phosphate calcination system as described in claim 6, characterized in that, The raw material preheating unit includes at least one stage of cyclone preheater, and the cyclone preheater includes a cyclone cylinder and air ducts and material pipes connected to the cyclone cylinder.

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