Method and system for purifying melamine production off-gas

Abstract

The present application belongs to the technical field of melamine tail gas treatment, and particularly relates to a purification method and system for melamine production tail gas. The purification method for melamine production tail gas provided by the present application is for tail gas generated in the production of melamine by the gas phase method. The tail gas is sequentially subjected to molten urea washing, ammonia water washing, and then urea hydrolysis treatment, to obtain pure tail gas containing only ammonia and carbon dioxide. The present application has the beneficial effect that, after the tail gas is washed by molten urea and ammonia water and further treated by urea hydrolysis, the melamine, melamine acid, biuret, urea, isocyanic acid, cyanamide and other components originally contained in the tail gas only contain pure CO2 and NH3 after purification, the purification degree of the tail gas is very high, and the obtained tail gas can be directly applied to the production of fine chemical products.

Description

Technical Field

[0001] This invention belongs to the field of melamine tail gas treatment technology, specifically relating to a method and system for purifying melamine production tail gas. Background Technology

[0002] At present, the main production process of melamine is the urea decomposition method, which is to produce gaseous melamine by condensation reaction of molten urea under certain pressure and atmosphere and with the action of a catalyst.

[0003] However, for every ton of melamine produced by this reaction, 1120 Nm³ of byproduct is also generated. 3 The NH3 around and 560Nm 3 The reaction tail gas, composed of CO2 on the left and right sides, together with the system circulating gas, constitutes the reaction gas discharged from the reactor.

[0004] Furthermore, it is worth noting that the composition of the circulating gas from the urea scrubbing tower is extremely complex. This is mainly because, after being washed and purified in the urea scrubbing tower by a circulating urea solution containing a small amount of water and trace solid particles (mainly melamine, cyanuric acid, and biuret) as well as a small amount of isocyanic acid (HNCO) and cyanamide (NH2CN), the excess scrubbing and purification gas is discharged from the system as production tail gas. It still contains trace amounts of isocyanic acid and cyanamide, which are entrained by mist caused by gas-liquid contact in the urea scrubbing tower, resulting in the addition of trace amounts of liquid urea, water, and trace amounts of solid impurities in the tail gas.

[0005] Currently, in order to make fuller use of resources, based on the resources and co-production conditions of each manufacturer, the tail gas from melamine by-products has been used to produce chemical products containing ammonia or reacting with ammonia, such as ammonia water, ammonium bicarbonate, ammonium nitrate, urea, sodium carbonate, ammonium chloride, and liquid ammonia. The applications of these products generally do not have high requirements for trace amounts of impurities such as melamine, cyanuric acid, isocyanic acid, cyanamide, and biuret. In fact, most of the impurities can be converted into tail gas products during the utilization process.

[0006] However, if the exhaust gas is used to produce high-purity carbon dioxide and industrial-grade ammonium bicarbonate, ammonium carbonate, and other products, the trace amounts of harmful impurities such as melamine, isocyanate, and cyanamide carried in the gas cannot meet the purity requirements of the raw materials.

[0007] There is no systematic and clear introduction or report in the industry regarding the deep purification and upgrading technology of melamine production tail gas. Only patent CN1538961A reports an improved method for producing high-purity melamine with high yield. However, the purpose of this patent is to prepare high-purity melamine, and its process belongs to the "liquid phase method" for producing melamine.

[0008] The production processes and conditions for the "liquid phase method" and the "gas phase method" differ significantly. The "liquid phase method" operates at a pressure of 7 MPa, and the urea production unit is integrated into the melamine production process. The isocyanate and cyanamide in the tail gas do not require separate treatment; they are discharged along with ammonia and carbon dioxide into the urea process. In contrast, the "gas phase method" operates at a pressure of 0.5 MPa. The urea unit is separate from the melamine production system, and the melamine tail gas cannot be used by the urea unit; it can only be used to process other tail gas products.

[0009] Therefore, it is necessary to provide a purification technology with low operating costs and easy industrial application, which can systematically and comprehensively separate, purify and enhance various harmful impurities in the by-product tail gas of melamine production by the "gas phase method", so that the tail gas can become a raw material for producing high-purity, high-value-added food-grade and industrial-grade fine chemical products. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a method and system for purifying melamine production tail gas.

[0011] In this invention, the tail gas produced by the gas-phase method (i.e., the one-step urea process) for producing melamine is classified and treated according to the nature and characteristics of the impurities that may be present in it.

[0012] The present invention provides a method for purifying melamine production tail gas, specifically: the tail gas generated from the gas-phase method of melamine production is washed sequentially with molten urea and ammonia water, and then treated with urea hydrolysis to finally obtain pure tail gas containing only ammonia and carbon dioxide.

[0013] In the above method, preferably, the temperature of the molten urea washing is 140-150°C, and the temperature of the ammonia washing is 100-150°C.

[0014] Preferably, the temperature for the urea hydrolysis treatment is 150–160°C.

[0015] In this invention, since the melting temperature of urea is 132.7℃, biuret precipitate will be generated when the temperature is higher than 150℃, which will increase the viscosity of the solution and affect the normal operation of the equipment. Therefore, the temperature for washing the exhaust gas with molten urea must be controlled within the range of 140-150℃.

[0016] In addition, regarding the temperature of ammonia water washing, the temperature of the ammonia water solution can be between 100℃ and 150℃. However, if the temperature is too high, the amount of water vaporization during the washing process will be large, requiring more fresh water to be added, which will increase the cost.

[0017] The present invention provides a method for purifying melamine production tail gas, comprising the following steps:

[0018] The tail gas generated from the gas-phase preparation of melamine in S1 first enters the molten urea scrubbing tower. The tail gas is then fully washed by the countercurrent contact between the liquid molten urea and the tail gas. The resulting scrubbing liquid is returned to the molten urea scrubbing tower for use as tail gas scrubbing liquid recycling, or pumped into the urea scrubbing tower. In addition, the tail gas obtained from the washing process, which does not contain solid impurities, enters the ammonia scrubbing tower.

[0019] In the ammonia scrubbing tower, S2 uses an ammonia solution to scrub the tail gas after it has been scrubbed with molten urea in S1 again. The scrubbing liquid is pressurized by a pump, heat exchanged by a heat exchanger, and then introduced into the urea hydrolysis tower for hydrolysis reaction. The purified gas obtained is directly introduced into the tail gas collection device.

[0020] S3 When the washing liquid in S2 enters the urea hydrolysis tower and completes hydrolysis in the urea hydrolysis tower, the obtained ammonia solution is cooled by a heat exchanger and returned to the ammonia scrubbing tower as the washing liquid for recycling; the hydrolysis gas generated in the urea hydrolysis tower is discharged from the urea hydrolysis tower and incorporated into the purified tail gas, and then enters the tail gas collection device.

[0021] In this invention, the liquid molten urea obtained by heating urea at 133-140℃ is relatively pure. Furthermore, calculations show that an average of 3000 kg of molten urea is required per ton of tail gas for washing. Besides containing approximately 0.5% water, it contains almost no other impurities. Therefore, during tail gas treatment, by adding a washing process using liquid molten urea, the gas and liquid materials flow counter-currently within the tower for sufficient contact. Solid impurities such as melamine, cyanuric acid, and biuret in the reaction gas are transferred to solids during the contact washing process. In the urea solution, solid impurities in the exhaust gas are removed, and the washing process can also partially remove gaseous harmful impurities such as isocyanate and cyanamide. As a result, some impurities will be added to the washing liquid, such as melamine, cyanuric acid, biuret, isocyanate, and cyanamide. These components are all finished or intermediate products in the preparation of melamine. This part of the washing liquid can be transferred with the urea solution to the urea washing tower and reactor in the existing process. The melamine contained therein can be re-vaporized, and the remaining impurities can be converted into melamine in the reactor.

[0022] Of course, if the washing liquid flowing out of the molten urine scrubbing tower contains few impurities and still has a certain washing capacity, the flowing washing liquid can be directly reused for washing the tail gas in the molten urine scrubbing tower after being pumped.

[0023] In addition, the exhaust gas after being washed with molten urea still contains trace amounts of isocyanic acid, cyanamide, and liquid urea carried by the mist.

[0024] Regarding isocyanate, Professor Zhu Xiaolei's team at Nanjing University of Technology has studied the reaction mechanism of TiO2 catalytic hydrolysis of isocyanate: HNCO + H2O → CO2 + NH3. Their research results show that although isocyanate can be hydrolyzed into ammonia and carbon dioxide, the process requires catalyst conditions and high reaction temperature, which increases the cost. Furthermore, it does not systematically and comprehensively remove various gaseous, liquid, and solid impurities in the exhaust gas. Therefore, it has not been widely used in the exhaust gas treatment industry.

[0025] In this invention, in order to better remove trace amounts of isocyanate contained in the exhaust gas, ammonia water is used to wash the exhaust gas at high temperature, taking advantage of the hydrolysis reaction of isocyanate and its unique acidity. On the one hand, a very small amount of isocyanate may undergo a hydrolysis reaction to generate ammonia gas. On the other hand, since ammonia water is alkaline, hydrogen cyanide can react with it as follows to generate cyanamide (monocyanamide or aminocyanide): HOCN + NH3·H2O → NH2CN + 2H2O.

[0026] Although the hydrolysis reaction mentioned above in this invention has a small reaction degree at room temperature, the reaction degree can meet the production requirements when the temperature is above 100°C, and can completely remove most of the isocyanate and cyanamide. Specifically, in the ammonia water scrubbing tower, the newly generated and existing cyanamide in the ammonia water solution undergo a hydrolysis reaction under the conditions of temperature >100°C and pH >10 to generate urea: NH2CN + H2O = CO(NH)2. The tail gas emission temperature is 140°C, and the temperature after washing with ammonia water solution is about 116°C. Since some ammonia gas is added as backflushing gas during the melamine production process, the actual ammonia content in the tail gas is greater than 66%. Under the conditions of temperature above 100°C, there is a large amount of free ammonia in the ammonia water solution, which makes the pH of the ammonia water solution >10.

[0027] Of course, if the temperature and pH are low, such as between 60-100℃ and pH < 10, although the reaction of isocyanate to cyanamide in the solution proceeds normally: HOCN + NH3·H2O → NH2CN + 2H2O, the NH2CN in the solution will polymerize to form dicyandiamide: 2NH2CN → C2H4N4 (dicyandiamide). The generated C2H4N4 will ultimately pollute the purified exhaust gas. In addition, if the temperature of the ammonia solution is < 60℃, the solution is prone to crystallization, clogging equipment and pipelines, making the washing process impossible. Therefore, the operating temperature in the ammonia scrubbing tower must be > 100℃ and pH > 10 to truly achieve a good effect in purifying the exhaust gas.

[0028] Based on the above description, in this invention, ammonia water is used to wash the tail gas containing isocyanate, cyanamide, and urea. After treatment, trace amounts of isocyanate, cyanamide gas, and liquid urea impurities in the tail gas are washed, absorbed, and converted into urea solution by the ammonia solution. At the same time, after washing and refining, some of the water in the washing liquid vaporizes to form purified gas containing only ammonia, carbon dioxide, and water vapor, which can be directly collected and used as high-quality raw material gas for the dry production of ammonium bicarbonate and ammonium carbonate.

[0029] Furthermore, the washing solution containing trace amounts of urea, obtained after washing with ammonia water, is pressurized by a pump and sent to a urea hydrolysis reactor. There, it is heated to approximately 160°C with steam to hydrolyze the dissolved urea, converting it into ammonia and carbon dioxide.

[0030] (NH)2CO+H2O→2NH3+CO2; Since the components of the hydrolyzed gas are the same as those of the tail gas, the hydrolyzed gas and the tail gas can be combined and introduced into the reaction tail gas, and collected at the same time, as a high-quality raw material gas for the dry production of ammonium bicarbonate and ammonium carbonate.

[0031] In the above method, preferably, in S1, the flow rate of the molten urea is 10-15 t / h, and the flow rate of the exhaust gas is 2000-3000 m³ / h. 3 / h.

[0032] Preferably, in S2, the flow rate of ammonia water is 10-20 t / h, and the flow rate of tail gas is 2000-3000 m³ / h. 3 The washing liquid is pressurized by a pump to a pressure of 0.5-0.8 MPa per hour, and then heated to 140-150°C by a heat exchanger.

[0033] Preferably, the ammonia solution in S3 is cooled to 90-100°C via a heat exchanger.

[0034] In addition, the present invention also provides a purification system used in the above method, specifically including: a molten urea scrubbing tower, an ammonia scrubbing tower, and a urea hydrolysis tower;

[0035] The molten urea scrubbing tower is equipped with a molten urea scrubbing tower tail gas inlet, a molten urea inlet, a molten urea scrubbing tower tail gas outlet, a second molten urea inlet, and a molten urea scrubbing tower washing liquid outlet;

[0036] The ammonia scrubbing tower is equipped with an ammonia scrubbing tower tail gas inlet, an ammonia scrubbing tower tail gas outlet, a first ammonia inlet, a second ammonia inlet, and an ammonia scrubbing tower scrubbing liquid outlet.

[0037] The urea hydrolysis tower is equipped with a urea hydrolysis tower washing liquid inlet, a purified gas outlet, and an ammonia water outlet;

[0038] The tail gas outlet of the molten urea scrubbing tower is connected to the tail gas inlet of the ammonia scrubbing tower. The washing liquid outlet of the molten urea scrubbing tower is split into two paths after passing through the first pump device, and is respectively connected to the second molten urea inlet of the molten urea scrubbing tower and the urea scrubbing tower.

[0039] The washing liquid flowing out of the urea scrubbing tower contains a large amount of urea. Therefore, after passing through the first pump, it is divided into two streams. The main component of the reflux liquid is liquid urea, which still has the ability to remove impurities from the gas. Therefore, it can be used again to scrub the tail gas. Of course, if the impurity content is high, the washing liquid will directly enter the urea scrubbing tower in the existing process.

[0040] The tail gas outlet of the ammonia scrubbing tower is connected to a tail gas collection device. The washing liquid outlet of the ammonia scrubbing tower is connected to the washing liquid inlet of the urea hydrolysis tower via a heat exchanger after passing through a second pump device. The purified gas outlet of the urea hydrolysis tower is connected to a tail gas collection device. The ammonia outlet of the urea hydrolysis tower is connected to the second ammonia inlet of the ammonia scrubbing tower via a heat exchanger.

[0041] The beneficial effects of this invention are as follows:

[0042] First, a method for purifying melamine production tail gas is provided. Specifically, molten urea and ammonia are used to wash the impurity-containing waste gas. After further hydrolysis with urea, pure tail gas containing only NH3 and CO2 is obtained. After purification, the tail gas can be directly used as a high-quality raw material for the dry production of ammonium bicarbonate and ammonium carbonate.

[0043] Secondly, the ammonia washing gas is completed under isobaric conditions, with a small washing liquid volume and low power consumption. The amount of urea that can be converted into urea in the tail gas of each ton of melamine is only 0.1 kg. Therefore, the steam consumed by the hydrolysis reaction is less than the kilogram level. Furthermore, due to the small amount of material transferred and the small amount of impurities processed in the purification process, the equipment specifications required for the treatment process are small, and the investment is relatively small.

[0044] Third, after treating the melamine tail gas using the method of the present invention, the initial melamine content in the tail gas is reduced from 0.6 mg / Nm³. 3 0.2 mg / Nm³ of cyanuric acid 3 Biuret 5mg / Nm 3 Urea 50mg / Nm 3 Isocyanate 12mg / Nm 3 6 mg / Nm of cyanamide 3 After purification, it contains only 0.001 mg / Nm³ of urea. 3 The exhaust gas is highly purified. Attached Figure Description

[0045] Figure 1 A schematic diagram of the system used in the melamine production tail gas purification method provided by the present invention;

[0046] In the diagram: 1-Melted urea scrubbing tower, 11-Melted urea scrubbing tower tail gas inlet, 12-First molten urea inlet, 13-Melted urea scrubbing tower tail gas outlet, 14-Second molten urea inlet, 15-Melted urea scrubbing tower washing liquid outlet, 2-Ammonia scrubbing tower, 21-Ammonia scrubbing tower tail gas inlet, 22-Ammonia scrubbing tower tail gas outlet, 23-First ammonia inlet, 24-Second ammonia inlet, 25-Ammonia scrubbing tower washing liquid outlet, 3-Urea hydrolysis tower, 31-Purified gas outlet, 32-Urea hydrolysis tower washing liquid inlet, 33-Ammonia outlet, 4-First pump unit, 5-Second pump unit, 6-Heat exchanger. Detailed Implementation

[0047] To enable those skilled in the art to better understand the present invention, the present invention will now be further described in conjunction with specific embodiments.

[0048] Example 1

[0049] Taking the exhaust gas from a 30,000-ton / year melamine plant as an example, using the attached... Figure 1 The purification system shown purifies the exhaust gas.

[0050] S1, the melamine reaction tail gas from the production system at 0.3 MPa and 140°C, is washed in a 1000×6000 mm diameter molten urea scrubbing tower with 12 t / h of fresh liquid molten urea at 140°C. The tail gas flow rate is 2541 m³ / h. 3 / h, the washing liquid after washing is introduced into the urine washing tower of the existing process; the tail gas obtained by washing without solid matter at 0.3Mpa and 140℃ enters the ammonia water scrubbing tower with a diameter of Φ1000×6000mm.

[0051] S2 uses a 10t / h, 100℃ ammonia solution to wash the tail gas entering from S1. The tail gas flow rate is 2541m³. 3 During the washing process, isocyanate, cyanamide, and liquid urea in the gas are completely dissolved in ammonia water. At the same time, isocyanate and cyanamide undergo hydrolysis to obtain a 30% ammonia water solution containing a small amount of urea at 95°C. The pressure of this solution is increased to 0.6 MPa by a pump, and after being heated to 148°C by a heat exchanger, it is introduced into a 1000×3000 mm urea hydrolysis tower for hydrolysis. Meanwhile, the purified gas obtained directly enters the tail gas collection device.

[0052] After being washed with ammonia, the exhaust gas underwent air stripping and humidification, which increased the water vapor content by 330 kg and the volume by 410 m³. 3 / h, due to humidification and water vaporization, the exhaust gas temperature drops to 90-95℃;

[0053] After the washing liquid in S2 is introduced into the urea hydrolysis tower, it is heated to 153°C by steam for hydrolysis. A small amount of hydrolysis gas is discharged from the hydrolyzer and incorporated into the purified tail gas.

[0054] After hydrolysis, the ammonia solution is cooled to 100°C by a heat exchanger and can be returned to the ammonia scrubbing tower as the scrubbing liquid.

[0055] The comparison of the content of each component in the melamine production tail gas before and after two washings is shown in Table 1 below.

[0056] Table 1. Content of each component in the exhaust gas before and after washing.

[0057] impurities melamine cyanuric acid biuret Urea isocyanate Cyanide mg / Nm 3 )]]> 0.6 0.2 5 50 12 6 mg / Nm 3 )]]> 0 0 0 0.001 0 0

[0058] As can be seen from the table above, after two washing processes and deep purification treatment in the urea hydrolysis tower, almost all the impurities such as melamine, cyanuric acid, urea, biuret, and cyanamide originally contained in the melamine tail gas can be removed. The resulting tail gas contains only CO2 and NH3, and has a high purity, which can be directly used to produce fine chemical raw materials such as ammonium bicarbonate and ammonium carbonate.

[0059] Examples 2-4

[0060] Unlike Example 1, the flow rate of the ammonia solution in the ammonia scrubbing tower was adjusted, and the flow rate of the ammonia solution was set to 8t / h, 5t / h, and 2t / h respectively; the rest of the operation was the same as in Example 1.

[0061] The content of each component in the exhaust gas before and after washing is shown in Table 2 below.

[0062] Table 2. Impurity content of tail gas before and after washing under different flow rates of ammonia solution.

[0063]

[0064] Obviously, as can be seen from the table above, the flow rate of the ammonia solution in the ammonia scrubbing tower directly affects the scrubbing effect of the tail gas. The results of Example 1 show that the scrubbing effect of the tail gas is best when the flow rate of the ammonia solution is 10t / h. Of course, although a larger flow rate can achieve a better scrubbing effect, it will undoubtedly increase the cost of the scrubbing liquid raw material and increase energy consumption. Therefore, the optimal flow rate of the ammonia solution is controlled at 10t / h. In addition, Examples 2 to 4 show that when the flow rate of the ammonia solution decreases, the content of impurities carried by the tail gas after scrubbing increases, indicating that the ammonia solution does not thoroughly scrub the components of the tail gas.

[0065] Comparative Example 1

[0066] Unlike Example 1, only a molten urine scrubbing tower was installed, and the exhaust gas output from the molten urine scrubbing tower was directly detected. The detection results are shown in Table 3 below.

[0067] Table 3. Content of various components in the exhaust gas output from the molten urea scrubber.

[0068] impurities melamine cyanuric acid biuret Urea isocyanate Cyanide <![CDATA[Content (mg / Nm 3 )]]> 0 0 3 50 12 6

[0069] The data in the table above shows that the original impurities in the exhaust gas, such as melamine and cyanuric acid, can be completely removed during the molten urea washing process. Furthermore, after washing in the molten urea scrubbing tower, the content of biuret in the exhaust gas is also reduced, while the content of other impurities does not change significantly.

[0070] Comparative Example 2

[0071] Based on Comparative Example 1, an ammonia scrubbing tower was added, but the urea hydrolysis tower was not installed. The components of the exhaust gas output from the ammonia scrubbing tower were detected. The contents of each component in the exhaust gas output from Comparative Example 1 after being scrubbed with 10t / h of ammonia solution at 140℃ are shown in Table 4 below.

[0072] Table 4. Content of various components in the exhaust gas output from the ammonia scrubbing tower.

[0073] impurities melamine cyanuric acid biuret Urea isocyanate Cyanide <![CDATA[Content (mg / Nm 3 )]]> 0 0 1.1 10.5 0 0

[0074] The data in the table above shows that after being washed by the ammonia water scrubbing tower, components such as isocyanate and cyanamide in the exhaust gas can be completely removed. In addition, the content of biuret and urea is also reduced. Only some urea and biuret remain in the exhaust gas. If a urea hydrolysis tower is not added, the purity of the obtained exhaust gas is still not high.

[0075] The above analysis clearly shows that by using the method and system of this invention to purify the tail gas generated during the "gas-phase method" for melamine production, all kinds of impurities in the tail gas can be completely removed, and the tail gas ultimately contains only CO2 and NH3 components, which can be directly collected and used as high-quality raw material gas for the dry production of ammonium bicarbonate and ammonium carbonate.

Claims

1. A method for purifying melamine production tail gas, characterized in that, The tail gas produced by the gas-phase method for manufacturing melamine is washed with molten urea and ammonia water in sequence, and then treated with urea hydrolysis to obtain pure tail gas containing only ammonia and carbon dioxide. The temperature for washing with molten urea is 140-150°C, the temperature for washing with ammonia is 100-150°C, and the pH is greater than 10.

0.

2. The method for purifying melamine production tail gas as described in claim 1, characterized in that, The steps include the following: The tail gas produced by the S1 gas phase method for melamine preparation first enters the molten urea scrubbing tower. The tail gas is then fully washed by the countercurrent contact between the liquid molten urea and the tail gas. The resulting scrubbing liquid is returned to the molten urea scrubbing tower for use as tail gas scrubbing liquid recycling, or pumped into the urea scrubbing tower. In addition, the tail gas obtained from washing, which does not contain solid impurities, enters the ammonia water scrubbing tower. In the ammonia scrubbing tower, S2 uses an ammonia solution to scrub the tail gas after it has been scrubbed with molten urea in S1 again. The scrubbing liquid is pressurized by a pump, heat exchanged by a heat exchanger, and then introduced into the urea hydrolysis tower for hydrolysis reaction. The purified gas obtained is directly introduced into the tail gas collection device. S3 When the washing liquid in S2 enters the urea hydrolysis tower and completes hydrolysis in the urea hydrolysis tower, the resulting ammonia solution is cooled by a heat exchanger and then returned to the ammonia scrubbing tower as the washing liquid of the ammonia scrubbing tower for recycling. The hydrolyzed gas produced in the urea hydrolysis tower is discharged from the urea hydrolysis tower and incorporated into the purified tail gas, which then enters the tail gas collection device.

3. The method for purifying melamine production tail gas as described in claim 2, characterized in that, In S1, the flow rate of molten urea is 10~15 t / h, and the flow rate of tail gas is 2000~3000 m³ / h. 3 / h.

4. The method for purifying melamine production tail gas as described in claim 2, characterized in that, In S2, the flow rate of ammonia water is 10~20 t / h, and the flow rate of tail gas is 2000~3000 m³ / h. 3 / h, the washing liquid is pressurized by a pump to a pressure of 0.5~0.8 MPa, and then heat-exchanged to 140~150℃ by a heat exchanger.

5. The method for purifying melamine production tail gas as described in claim 2, characterized in that, In S3, the ammonia solution is cooled to 90~100℃ via a heat exchanger.

6. The method for purifying melamine production tail gas as described in claim 2, characterized in that, The purification system is implemented using the following components: a molten urea scrubbing tower (1), an ammonia scrubbing tower (2), and a urea hydrolysis tower (3). The molten urea scrubbing tower (1) is provided with a molten urea scrubbing tower tail gas inlet (11), a first molten urea inlet (12), a molten urea scrubbing tower tail gas outlet (13), a second molten urea inlet (14), and a molten urea scrubbing tower washing liquid outlet (15). The ammonia scrubbing tower (2) is provided with an ammonia scrubbing tower tail gas inlet (21), an ammonia scrubbing tower tail gas outlet (22), a first ammonia inlet (23), a second ammonia inlet (24), and an ammonia scrubbing tower washing liquid outlet (25). The urea hydrolysis tower (3) is provided with a purified gas outlet (31), a urea hydrolysis tower washing liquid inlet (32), and an ammonia water outlet (33). The tail gas outlet (13) of the molten urea scrubbing tower is connected to the tail gas inlet (21) of the ammonia scrubbing tower. The washing liquid outlet (15) of the molten urea scrubbing tower is split into two paths after passing through the first pump device (4), and is connected to the second molten urea inlet (14) of the molten urea scrubbing tower and the urea scrubbing tower, respectively. The tail gas outlet (22) of the ammonia scrubbing tower is connected to a tail gas collection device. The washing liquid outlet (25) of the ammonia scrubbing tower is connected to the washing liquid inlet (32) of the urea hydrolysis tower after passing through the second pump device (5) and the heat exchanger (6). The purified gas outlet (31) of the urea hydrolysis tower is connected to a tail gas collection device. The ammonia outlet (33) of the urea hydrolysis tower is connected to the second ammonia inlet (24) of the ammonia scrubbing tower (2) through the heat exchanger (6).

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