Apparatus and method for reducing biuret in urea and nitrogen-potassium fertilizers

Abstract

This invention relates to an apparatus and method for reducing biuret content in urea and nitrogen-potassium fertilizers, belonging to the field of urea and fertilizer production technology. The apparatus includes a urea evaporation and granulation system and a nitrogen-potassium fertilizer system. The urea tank is divided into main and auxiliary compartments in a 3:1 ratio by a partition. During normal production, only the auxiliary compartment is used to reduce residence time. In the evaporation system, the top of the heater is level with the lower liquid inlet of the separator, and symmetrically distributed double lower liquid inlets are configured to effectively eliminate backmixing dead zones. The upper tower pipeline adopts a main and auxiliary dual-pipe design with a cross-sectional area ratio of 2:1, which can be flexibly switched according to flow rate to maintain high flow velocity and is equipped with steam purging. The nitrogen-potassium fertilizer system uses a Venturi jet mixer instead of a traditional mixing tank, utilizing molten urea as the driving fluid to achieve instantaneous mixing. This invention significantly reduces the biuret content in the product through precise control of the material residence time throughout the entire process, and the apparatus has a simple structure and is easy to industrialize.

Description

Technical Field

[0001] This invention relates to the field of urea and fertilizer production technology, specifically to an apparatus and method for reducing biuret in urea and nitrogen-potassium fertilizers. Background Technology

[0002] Urea is an important nitrogen fertilizer. Biuret is a byproduct of the condensation reaction of urea at high temperatures, with the chemical formula NH₂CONHCONH₂. Biuret is toxic to crop roots and leaves, and national standards strictly limit the biuret content in agricultural urea, generally requiring it not to exceed 0.9%. Therefore, controlling and reducing the biuret content in products is one of the important technical indicators in the production of urea and urea-containing compound fertilizers.

[0003] Currently, the typical process flow of a urea plant using traditional carbon dioxide stripping technology is as follows: urea synthesis reactants are processed in a stripping tower and a distillation tower, then flash-evaporated to obtain urea at approximately 90°C and a concentration of approximately 73%, which is then sequentially fed into a primary evaporation system and a secondary evaporation system. The operating temperature of the two evaporation systems is generally between 125 and 139°C. This temperature range coincides with the conditions for rapid biuret growth and is one of the main sources of high biuret content in the product. After two stages of evaporation, molten urea with a concentration of 99.5% to 99.7% is obtained and pumped by a molten pump to a rotary granulation nozzle at the top of the granulation tower for granulation. The entire process from the discharge of the secondary evaporator to the delivery of the molten urea to the granulation nozzle is the stage with the highest system temperature; the longer the material resides in this stage, the greater the amount of biuret generated.

[0004] The existing technology mainly has the following problems: First, the assembly of the evaporator heater and separator is inadequate. In both the first and second stages of evaporation, the heater is not inserted deep enough into the separator, causing back mixing of the liquid in the space between the upper part of the heater and the separator. This increases the residence time of the material in the high-temperature zone, which is detrimental to biuret control.

[0005] Second, the design of the urine tank is flawed. The existing urine tank is divided into two compartments, a large and a small one, by a middle partition. The partition has connecting holes to balance the overall liquid level, which causes high-concentration and low-concentration urine to mix and flow back into each other in the main compartment. This results in molten urea staying in the main compartment for too long, leading to the continuous accumulation of biuret.

[0006] Third, the design of the upper tower pipeline diameter is too simple. When the urea plant produces large granulated urea or nitrogen and potassium fertilizer at the same time, the flow rate of molten urea sent to the granulation tower is reduced. If the original larger pipeline is still used, the flow velocity in the pipe will decrease and the material residence time will be prolonged, resulting in an increase in biuret.

[0007] Fourth, the residence time in the nitrogen and potassium fertilizer mixing process is long. Nitrogen and potassium fertilizer production equipment is usually equipped with two mixing tanks, using low-pressure steam heated by coils to keep the materials in a molten state. The residence time in the mixing process is relatively long, resulting in a high biuret content in the nitrogen and potassium fertilizer, which is difficult to meet customer requirements. Summary of the Invention

[0008] The purpose of this invention is to overcome the above-mentioned shortcomings of the prior art and provide an apparatus and method for reducing biuret in urea and nitrogen-potassium fertilizers. By improving the structural assembly of the evaporator separator and heater, optimizing the management of the urea tank compartment, setting up a dual-diameter upper tower pipeline switching system, and using a jet mixer to replace the traditional melting tank, the residence time of materials in the high-temperature zone is reduced from multiple aspects, effectively reducing the biuret content in the product.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: On the one hand, the present invention provides an apparatus for reducing biuret in urea and nitrogen-potassium fertilizers, including a urea evaporation granulation system and a nitrogen-potassium fertilizer system; The urea evaporation and granulation system includes a urine tank, a urine pump, a first-stage evaporation heater, a first-stage evaporation separator, a second-stage evaporation heater, a second-stage evaporation separator, a melt pump, a granulation tower, and a pipeline system. The urine tank is divided into a main compartment and a secondary compartment by a partition, with a volume ratio of 3:1 between the main compartment and the secondary compartment. The partition is made of 10mm thick S30403 stainless steel plate. The bottoms of the main compartment and the secondary compartment 23 are respectively connected to pipes on both sides of a connecting valve, which is a shut-off valve. The return pipeline is led to the bottom of the main compartment. The top of the tubes of the first-stage evaporator heater is level with the liquid outlet of the first-stage evaporator separator to eliminate the back mixing space at the bottom of the separator; the bottom of the first-stage evaporator separator is provided with two liquid outlets symmetrically distributed at 180° to each other. The liquid outlets are located on the bottom plane of the separator and close to the heater. The diameter of the liquid outlets is 3% to 4% of the diameter of the separator; the two liquids are directly merged through a pipe in the lower part of the first-stage evaporator heater and form a first-stage evaporator liquid outlet pipe; The top of the tubes of the second-stage evaporator heater is level with the liquid outlet of the second-stage evaporator separator; the bottom of the second-stage evaporator separator is also provided with two liquid outlets symmetrically distributed at 180° to each other, with the same structural parameters as the first-stage evaporator separator, and the two liquids merge into the second-stage evaporator liquid outlet pipe; a sight glass is installed on the second-stage evaporator liquid outlet pipe to observe whether the molten urine in the pipe and the liquid discharge at the bottom of the separator are normal. The pipelines for the urea granulation tower are divided into an upper main pipeline and an upper secondary pipeline, with a cross-sectional area ratio of 2:1 between the upper main pipeline and the upper secondary pipeline. A top three-way reflux valve is installed on the upper main pipeline. A lower three-way reflux valve is installed between the upper main pipeline and the upper secondary pipeline, serving as a shared reflux valve for both pipelines. An upper pipeline three-way purge valve is installed before the upper shut-off valve on the upper pipeline to purge the pipeline before the shut-off valve. A granulation reflux pipeline three-way purge valve is installed after the top three-way reflux valve to purge the pipeline after the valve. A dust collector is installed at the top of the granulation tower, and the reflux pipeline is led from the top three-way reflux valve to the bottom of the main compartment of the urea tank. The nitrogen and potassium fertilizer system includes a vibrating screen, a bucket elevator, a weighing feeder, a jet mixer, and a rotary drum granulator. The jet mixer has a venturi structure and is equipped with a jacket, through which steam is introduced for heating and insulation. The outlet pipeline of the melt pump is connected to the rotary drum granulator, the nitrogen and potassium fertilizer jet mixer, and the urea granulation tower. The small urea particles obtained at the bottom of the granulation tower can be transported to the rotary drum granulator for use as seed crystals.

[0010] On the other hand, the present invention also provides a method for reducing biuret in urea and nitrogen-potassium fertilizers using the above-mentioned device, comprising the following steps: Step 1, Urine Tank Operation Management: During normal production, close the connecting valve, and the urine only flows in the secondary compartment, where it is pumped into a first-stage evaporation heater to reduce residence time. When high-concentration urine needs to be discharged to the urine tank through the return pipeline before the urea system starts circulating, open the connecting valve in advance to mix the high-concentration urine with the low-concentration urine in the main compartment to prevent crystallization. After the main compartment stops feeding, lower the overall liquid level of the main and secondary compartments to the minimum limit of 5%, close the connecting valve, and then gradually restore the liquid level of the secondary compartment to the normal operating level of 15%. Step 2, Evaporation and Concentration Operation: After being heated by the first-stage evaporation heater, the urine enters the first-stage evaporation separator. The liquid phase is quickly discharged from two symmetrical 180° liquid outlets at the bottom of the separator. After the two liquids merge, they are sent to the second-stage evaporation heater for further concentration through the first-stage evaporation liquid outlet pipe. Molten urea is discharged from two symmetrical liquid outlets at the bottom of the second-stage evaporation separator. After merging, it flows out through the second-stage evaporation liquid outlet pipe. The flow status is monitored in real time through the sight glass. Step 3, Upper Tower Pipeline Switching Operation: When the flow rate to the granulation tower is large, operate the upper tower main pipeline alone; when the flow rate to the granulation tower is less than 50% of the normal flow rate, first open the upper tower auxiliary pipeline, then shut down the upper tower main pipeline; after shutting down the upper tower main pipeline, close its upper and lower shut-off valves, open the lower three-way return valve, and purge the pipeline with 0.7MPa saturated steam for 3-5 minutes through the upper tower pipeline three-way purging valve to prevent pipeline crystallization and blockage; simultaneously reduce the number of dust collector fans in the granulation tower to 1 / 3 of the normal number to prevent excessive airflow from affecting the granulation quality; Step four, nitrogen and potassium fertilizer production operation: After being screened by a vibrating screen, lifted by a bucket elevator, and metered by a weighing feeder, the potassium fertilizer is sent to a jet mixer. Molten urea at a temperature of 138℃ and a pressure of 2.5MPa is used as the driving fluid to draw the potassium fertilizer into the mixer for rapid mixing through the Venturi effect. The temperature of the material at the mixer outlet is controlled to be 120-125℃, and the amount of steam in the jacket is adjusted to maintain the molten state of the material. After mixing, the material is directly sent to a rotary drum granulator for granulation to produce nitrogen and potassium fertilizer.

[0011] Compared with the prior art, the present invention has the following advantages: 1. By aligning the top of the heater tubes with the bottom liquid outlet of the separator, the backmixing space at the bottom of the separator is eliminated; the two 180° symmetrical bottom liquid outlets allow the liquid phase to be discharged quickly, effectively reducing the residence time of the material in the high-temperature zone of the evaporation system and lowering the biuret content in urea.

[0012] 2. The urine tank adopts a main and auxiliary compartment design with a volume ratio of 3:1. During normal production, the connecting valve is closed, and the urine only flows in the auxiliary compartment, completely eliminating cross-contamination and back mixing between the main and auxiliary compartments, and effectively shortening the material residence time.

[0013] 3. The upper tower pipeline adopts a dual pipeline design with a cross-sectional area ratio of 2:1, consisting of a main pipeline and a secondary pipeline. By switching the pipeline, the appropriate flow rate of molten urea in the pipeline can be maintained under different operating conditions, reducing the residence time of the material in the high-temperature pipeline and effectively controlling the growth of biuret.

[0014] 4. The Venturi structure jet mixer is used to replace the traditional melting and mixing tank. The power of molten urea is used to directly draw in potassium fertilizer and quickly mix it before it enters the rotary drum granulation. This greatly shortens the residence time, significantly reduces the biuret content in nitrogen and potassium fertilizers, and improves product quality.

[0015] 5. This invention is rationally designed, simple in structure, safe and reliable, and easy to operate. It requires minimal modification to existing devices and has good potential for widespread application. The following embodiments further illustrate the technical solution of this invention in detail. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the device process of the present invention.

[0017] Reference numerals: 1. Vibrating screen; 2. Bucket elevator; 3. Weighing feeder; 4. Jet mixer; 5. Large particle molten urea pipeline; 6. Jacket; 7. Upper tower pipeline tee purge valve; 8. Granulation reflux pipeline tee purge valve; 9. Blower; 10. Granulation nozzle; 11. Top tee reflux valve; 12. Upper tower auxiliary pipeline; 13. Upper tower main pipeline; 14. Lower tee reflux valve; 15. First stage evaporator heater; 16. First stage evaporator liquid outlet pipe; 17. Second stage evaporator heater; 18. Sight glass; 19. Second stage evaporator liquid outlet pipe; 20. Molten pump; 21. Reflux pipeline; 22. Urine pump; 23. Secondary compartment; 24. Main compartment; 25. Connecting valve. Detailed Implementation

[0018] To enable those skilled in the art to better understand the present application, the technical solutions in specific embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by those skilled in the art.

[0019] The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0020] The following is a detailed description of an apparatus and method for reducing biuret in urea and nitrogen-potassium fertilizers according to the present invention, with reference to the accompanying drawings.

[0021] Example 1 like Figure 1 As shown, this embodiment provides a device for reducing biuret in urea and nitrogen-potassium fertilizers, which mainly consists of the following key components: First, there is the urine tank, which is divided into a main compartment 24 and a secondary compartment 23 by a 10mm thick S30403 stainless steel partition. The volume ratio of the main compartment 24 to the secondary compartment 23 is 3:1. The connecting valve 25 is a shut-off valve, with pipes on both sides connected to the bottom of the main compartment 24 and the bottom of the secondary compartment 23, respectively, for connecting the two compartments under specific operating conditions. The return line 21 is led out from the top three-way return valve 11 and to the bottom of the main compartment 24, introducing the molten urea returned from granulation into the main compartment.

[0022] Following this is an evaporation system, which mainly includes an evaporator heater 15 and an evaporator separator. The evaporator heater 15 is connected to the evaporator separator so that the top of the heater tubes is level with the separator's liquid outlet, thus minimizing the backmixing space at the bottom of the separator. The bottom of the evaporator separator has two liquid outlets symmetrically distributed at 180° to each other, located on the bottom plane of the separator, positioned as close as possible to the heater. The diameter of the liquid outlets is 3% to 4% of the separator diameter, ensuring rapid liquid discharge and reducing residence time. The liquid discharged from the two liquid outlets merges directly through a pipe in the lower part of the heater, and then flows into a single evaporator liquid outlet pipe 16.

[0023] Next is the second-stage evaporation system, which mainly includes a second-stage evaporation heater 17 and a second-stage evaporation separator. The assembly method and liquid outlet setting of the second-stage evaporation heater 17 and the second-stage evaporation separator are the same as those of the first-stage evaporation system: the top of the heater tubes is level with the liquid outlet of the separator, and the bottom of the separator has two liquid outlets symmetrically distributed at 180° to each other, which merge and flow into the liquid outlet pipe 19 of the second-stage evaporation system. A sight glass 18 is installed on the liquid outlet pipe 19 of the second-stage evaporation system, allowing operators to observe at any time whether the molten liquid in the pipe is flowing normally and whether the molten liquid at the bottom of the separator is being discharged normally, so as to promptly detect and handle any abnormalities.

[0024] Following this is the upper tower pipeline system, mainly comprising the melting pump 20, the upper tower main pipeline 13, and the upper tower auxiliary pipeline 12. After being pressurized by the melting pump 20, the molten urea is sent to the granulation tower via pipelines divided into the upper tower main pipeline 13 and the upper tower auxiliary pipeline 12, with a cross-sectional area ratio of 2:1. Each pipeline is equipped with a shut-off valve. A top three-way reflux valve 11 is installed on the upper tower main pipeline 13; a lower three-way reflux valve 14 is installed between the upper tower main pipeline 13 and the upper tower auxiliary pipeline 12, serving as a shared reflux valve for both pipelines. An upper tower pipeline three-way purge valve 7 is installed before the upper shut-off valve on the upper tower pipeline, primarily used to purge the pipeline before this shut-off valve; a granulation reflux pipeline three-way purge valve 8 is installed after the top three-way reflux valve 11, primarily used to purge the pipeline after the top three-way reflux valve 11. A dust collector is installed at the top of the granulation tower, equipped with a fan 9. The return line 21 is led from the top three-way return valve 11 to the bottom of the main compartment 24 of the urine tank.

[0025] The outlet pipeline of the molten pump 20 is simultaneously connected to the large-particle molten urea pipeline 5 of the rotary drum granulator, the nitrogen-potassium fertilizer injection mixer 4, and the urea granulation tower in three directions. The small-particle urea obtained at the bottom of the granulation tower can be transported to the rotary drum granulator through pipelines to participate in the granulation process of large-particle urea as seed crystals.

[0026] Finally, there is the nitrogen and potassium fertilizer system, which mainly consists of a vibrating screen 1, a bucket elevator 2, a weighing feeder 3, a jet mixer 4, a jacket 6, and a rotary drum granulator. The jet mixer 4 adopts a venturi structure, and a jacket 6 is set on its outer side to introduce steam to heat and keep the material in a molten state to prevent crystallization.

[0027] Example 2 This embodiment provides a method for reducing biuret in urea and nitrogen-potassium fertilizers using the apparatus of Embodiment 1, specifically including the following parts: Firstly, regarding the normal operation of the urine tank: During normal production, the connecting valve 25 between the main compartment 24 and the secondary compartment 23 of the urine tank remains closed, and the urine only flows within the secondary compartment 23. The urine from the flash evaporation tank, with a temperature of approximately 90°C and a concentration of approximately 73%, enters the secondary compartment 23. After being pressurized by the urine pump 22, it is sent to a section of the evaporation heater 15 to reduce the residence time of the urine in the tank.

[0028] Secondly, regarding the operation before the urea system begins circulation, if high-concentration urine needs to be discharged into the urine tank through the return line 21 before circulation, the connecting valve 25 should be opened in advance to allow the high-concentration urine to enter the main compartment 24 and mix thoroughly with the low-concentration urine, preventing crystallization of the high-concentration urine in the pipes or tank. After the main compartment 24 stops feeding, the overall liquid level in both compartments of the urine tank should be lowered to the minimum limit of 5%, then the connecting valve 25 should be closed, and the liquid level in the auxiliary compartment 23 should be gradually restored to the normal operating level of 15%.

[0029] Next comes the evaporation and concentration operation. Urine pump 22 sends urine from secondary compartment 23 into the first-stage evaporator heater 15. The heated urine exits from the top of the heater and enters the first-stage evaporator separator. The gas phase is led out from the top of the first-stage evaporator separator to the first-stage evaporator condenser; the liquid phase is rapidly discharged from two symmetrical 180° outlets at the bottom of the separator, reducing liquid accumulation and backmixing at the bottom. The two liquid streams merge and form the first-stage evaporator drain pipe 16, sending the concentrated urine from the first stage into the second-stage evaporator heater 17. The second-stage evaporator heater 17 further heats and concentrates the urine. The gas phase is led out from the top of the second-stage evaporator separator to the second-stage evaporator condenser, while molten urea is discharged from the two symmetrical outlets at the bottom of the separator, merging and entering the second-stage evaporator drain pipe 19. Operators can monitor the flow status of the molten urine and whether the discharge is normal in real time through the sight glass 18.

[0030] Next is the switching operation of the upper tower pipeline. When the urea unit is not producing large-particle urea or nitrogen and potassium fertilizer, the flow rate of molten urea to the granulation tower is relatively large, and the upper tower main pipeline 13 is operated separately for granulation. When the urea unit simultaneously produces large-particle urea or nitrogen-potassium fertilizer, causing the molten urea flow rate to the granulation tower to be less than 50% of the normal amount, the pipeline switching should be performed according to the following steps: First, open the upper tower auxiliary pipeline 12 with a smaller cross-sectional area and confirm that the flow rate is normal; shut down the upper tower main pipeline 13 and close the upper and lower shut-off valves on the upper tower main pipeline; open the lower three-way return valve 14; through the upper tower pipeline three-way purging valve 7, introduce saturated steam at a pressure of 0.7MPa into the upper tower main pipeline for purging, and the purging time is 3-5 minutes to prevent the pipeline from being blocked by crystallization due to material solidification; as the granulation flow rate of the upper tower gradually decreases, simultaneously reduce the number of dust collector fans 9 at the top of the granulation tower to 1 / 3 of the normal number to prevent excessive air volume from affecting the forming quality of urea granules; during the adjustment process, continuously observe and analyze the product quality of urea granules to ensure that they meet the standards. By switching between the main and auxiliary pipelines in the upper tower, the flow rate of molten urea in the upper tower pipeline can be effectively maintained, reducing the residence time of the material in the high-temperature pipeline, thereby controlling the growth of biuret in small and large granular urea.

[0031] Finally, the production operation of nitrogen and potassium fertilizer is carried out. Potassium fertilizer is screened by vibrating screen 1 to remove particles that do not meet the particle size requirements. The qualified potassium fertilizer is then lifted by bucket elevator 2 and sent to the potassium fertilizer silo for storage. During production, the potassium fertilizer is accurately metered by weighing feeder 3 and then fed into the Venturi-structured jet mixer 4. The molten urea pump 20 sends molten urea from the evaporation system at a temperature of 138°C and a pressure of 2.5MPa as the driving fluid into the jet mixer 4. Utilizing the Venturi effect, a negative pressure is formed at the throat of the mixer, drawing the potassium fertilizer in from the side and rapidly and uniformly mixing it with the molten urea within the mixer.

[0032] With the addition of potassium fertilizer, the temperature of the mixture will decrease. Operators should adjust the steam supply in jacket 6 as needed, based on the target material temperature control at the outlet of jet mixer 4 (120-125℃), to supplement heat and ensure the material remains in a molten state, preventing crystallization.

[0033] After potassium fertilizer and molten urea are mixed, the mixture is directly fed into a rotary drum granulator for granulation to produce qualified nitrogen and potassium fertilizer products. Compared with the traditional melting and mixing tank process, this invention uses a jet mixer instead of a melting and mixing tank, which significantly shortens the mixing and residence time of potassium fertilizer and urea, effectively reduces the biuret content in the nitrogen and potassium fertilizer, and improves product quality.

[0034] Finally, it should be noted that the described embodiments are merely some, not all, of the embodiments of the present invention. Those skilled in the art will understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the claims and their equivalents; that is, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A device for reducing biuret in urea and nitrogen-potassium fertilizers, characterized in that, It includes a urea evaporation and granulation system and a nitrogen and potassium fertilizer system; the urea evaporation and granulation system includes a urea tank, an evaporation system, and an upper tower pipeline system; The urine tank is divided into a main compartment and a secondary compartment by a partition, and the bottoms of the main compartment and the secondary compartment are connected by a pipeline with a control valve. The evaporation system includes at least one evaporation unit containing a heater and a separator. The top of the heater is flush with the liquid outlet of the separator, and the bottom of the separator has multiple symmetrically distributed liquid outlets. The upper tower pipeline system includes parallel upper tower main pipelines and upper tower auxiliary pipelines with different cross-sectional areas. The upper tower pipeline system is equipped with valve groups for switching between the main and auxiliary pipelines and for backflow. The nitrogen and potassium fertilizer system includes a jet mixer, whose motive fluid inlet is connected to the molten urea output from the evaporation system.

2. The device for reducing biuret in urea and nitrogen-potassium fertilizers according to claim 1, characterized in that, The volume ratio of the main compartment to the secondary compartment is 3:1; the control valve on the pipeline with the control valve is a shut-off valve; the system's return pipeline is led to the bottom of the main compartment.

3. The device for reducing biuret in urea and nitrogen-potassium fertilizers according to claim 1, characterized in that, The evaporation system includes a first-stage evaporation heater and a separator arranged in series, as well as a second-stage evaporation heater and a separator; each separator has two liquid outlets symmetrically distributed at 180° to each other at its bottom, located on the bottom plane of the separator and arranged close to the heater; the diameter of the liquid outlet is 3% to 4% of the diameter of the separator.

4. The device for reducing biuret in urea and nitrogen-potassium fertilizers according to claim 3, characterized in that, A sight glass is installed on the lower liquid pipe of the two-stage evaporator heater.

5. The device for reducing biuret in urea and nitrogen-potassium fertilizers according to claim 1, characterized in that, The cross-sectional area ratio of the main pipeline to the auxiliary pipeline is 2:1; the valve group includes a top three-way reflux valve installed on the main pipeline and a lower three-way reflux valve installed between the main pipeline and the auxiliary pipeline as a shared reflux valve; the upper pipeline system also includes an upper pipeline three-way purge valve and a granulation reflux pipeline three-way purge valve for purging the inside of the pipeline.

6. The device for reducing biuret in urea and nitrogen-potassium fertilizers according to claim 1, characterized in that, The nitrogen and potassium fertilizer system is also equipped with a vibrating screen, a bucket elevator and a weighing feeder in sequence upstream of the jet mixer, and a rotary drum granulator connected downstream; the outside of the jet mixer is equipped with a jacket for heating and heat preservation by introducing steam.

7. A method for reducing biuret in urea and nitrogen-potassium fertilizers using the apparatus as described in any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: Keep the control valve between the main compartment and the secondary compartment closed, so that urine only flows in the secondary compartment and is sent to the evaporation system to reduce urine retention time; Step 2: After being heated by the heater, the urine enters the separator. The liquid phase is quickly discharged and merged from multiple symmetrical outlets at the bottom of the separator to prevent the liquid from accumulating and back-mixing at the bottom of the separator. Step 3, pipeline switching: Based on the molten urea flow rate to the granulation tower, select the main upper tower pipeline with a larger cross-sectional area to operate independently, or switch to the secondary upper tower pipeline with a smaller cross-sectional area when the flow rate decreases to maintain a suitable flow velocity in the pipeline, and perform steam purging on the pipeline that is not in use. Step 4, spray mixing: When producing nitrogen and potassium fertilizer, molten urea is fed into the spray mixer as the driving fluid. The metered potassium fertilizer is drawn in by the negative pressure effect for rapid mixing. After mixing, it is sent to the rotary drum granulator for granulation.

8. The method according to claim 7, characterized in that, In step one: when there is high-concentration urine that needs to be discharged to the urine tank through the return pipeline before the urea system starts circulating, the control valve is opened in advance to mix the high-concentration urine with the low-concentration urine in the secondary compartment to prevent crystallization; after the feed to the secondary compartment is stopped, the liquid level of the main and secondary compartments is reduced to the minimum limit of 5%, then the control valve is closed, and then the liquid level of the main compartment is gradually restored to the normal operating liquid level of 15%.

9. The method according to claim 7, characterized in that, In step three: when the flow rate to the granulation tower is less than 50% of the normal flow rate, first open the auxiliary pipeline to the upper tower, and then shut down the main pipeline to the upper tower; after shutting down the main pipeline to the upper tower, purge the pipeline with saturated steam at a pressure of 0.7 MPa for 3 to 5 minutes through the three-way purge valve; at the same time, reduce the number of dust collector fans in the granulation tower to 1 / 3 of the normal number.

10. The method according to claim 7, characterized in that, In step four, the temperature of the molten urea, which serves as the driving fluid, is 138°C and the pressure is 2.5 MPa. During the mixing process, the amount of steam used in the jacket is adjusted to maintain the molten state of the material, based on the target of controlling the material temperature at the outlet of the jet mixer to be between 120°C and 125°C.

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