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1 Device Characteristics Zhongyuan Dahua Group Co., Ltd. used French KT urea slurry process technology to build a large-scale industrial (300,000 t/a) urea-based compound fertilizer device, and passed the performance assessment and completion acceptance, realizing the large-scale drum granulation process of urea-based nitrogen, phosphorus and potassium compound fertilizer. The process flow is shown in Figure 1. Various raw materials are sent from the raw material warehouse to their respective silos through belts and bucket elevators. According to different formulas, they are accurately measured and fully mixed by the electronic batching system. They are then sent to the granulator through the return belt and injected steam and urine for granulation. The granulated material enters the dryer for spheronization, drying, and drying. The material is further dried and cooled in the cooler. After the particles reach a certain strength, they are sent to the screening machine through the bucket elevator, and the fine powder is directly returned to the granulator through the belt. Large particle urea products are sent to the three-roller crusher via a belt, crushed into qualified particles, and then returned to the granulator via a belt. The finished particles enter the air boiling fluidized bed for cooling. The further cooled particles are screened by a fine screen and then coated and then sent to the finished product packaging silo. This device has realized the transformation from small and medium-sized production of urine-based compound fertilizer to large-scale production, and the overall technology ranks at the internationally advanced level. Compared with other slurry method Zhengyi, it has distinctive technical features and advantages. (1) The raw material route is flexible and products of various specifications can be produced according to soil and crop conditions. (2) The system configuration is reasonable, energy is comprehensively utilized, and production energy consumption is low. (3) Clean production, high raw material utilization, no waste liquid discharge, and exhaust gas emissions comply with environmental standards. (4) The batching system adopts static weighing to ensure stable product quality. (5) An advanced DCS control system is adopted, and key parts of the device are equipped with industrial TV monitors to enable operators to monitor and adjust the production process in a timely manner. (6) In view of the strong hygroscopicity of the product, an anti-caking agent is used, and the surface of the product particles is wrapped with oil to ensure the storage performance of the product. The comparison between this device and other urea slurry processes is shown in Table 1. 2 Problems This large-scale urine-based compound fertilizer device has the following problems in actual production. (1) When producing based on urea-heavy calcium-potassium chloride raw materials, there are problems such as difficulty in granulation, difficulty in drying, and poor compatibility between urea and heavy calcium. (2) The granulation regulator is difficult to control and causes many side reactions, resulting in low melting point eutectic, poor granulation and poor fluidity of the material. The original design added sulfuric acid and gaseous ammonia into the granulator. After the reaction, sulfate anhydride, which is beneficial to granulation, was produced, and the reaction heat was used to increase the granulation temperature. In actual production, due to the addition of separate tubes of sulfuric acid and gas ammonia, the material is mixed unevenly after spraying and the reaction is incomplete, causing local acidification of the material to produce light yellow foam, which is not only detrimental to granulation but also causes greater ammonia loss. (3) The dry and cooled dusty exhaust gas contains a large amount of woven bag wire, solid particles and other debris. Through the process of recycling waste water after washing, it blocks the inlet and outlet pipelines of the washing pump, causing serious wear and tear on the pump casing, and even blisters. (4) There is a bottleneck problem in the feeding system, which affects the production load. 3. Technical transformation As the first large-scale urea-based compound fertilizer device in China, there is little experience to learn from in production operations. After several years of production practice, Zhongyuan Dahua Group has carried out many aspects of technical transformation of the device. 3.1 Adjust the amount of heavy calcium used. Because the degree of heavy calcium ammoniation is difficult to control, the granulation conditions are unstable. After a large number of experiments, engineering and technical personnel adopted the method of using less or even no heavy calcium to solve technical problems such as difficulty in granulation, difficulty in drying, poor compatibility between urea and heavy calcium, and poor disintegration of compound fertilizer products during the production of urea-based compound fertilizer. 3.2 Adjust the granulation regulator by changing the amount of granulation regulator. Use less or no granulation regulator to reduce the occurrence of side reactions, reduce raw material consumption, and improve the working environment. Annual consumption of 3,940t of sulfuric acid and 2,580t of liquid ammonia can be reduced by 92.5%. This improvement has the following effects. (1) The granulation curve was corrected, the granulation temperature was greatly reduced, the operating range was recalibrated, the granulation temperature was lowered by about 10°C, the granulation range was expanded, production was stabilized, and the granulation rate was greatly improved. (2) Successfully developed high-nitrogen formulas and mass-produced 11 different formula products with a nitrogen content of ≥20%, breaking the traditional concept that the K-T process can only produce products with a nitrogen content of ≥20% by adding solid urea. (3) The return material ratio is greatly reduced from 1:4 to 1:1.5~2, which reduces the system energy consumption. The comprehensive power consumption is reduced from the design value of 45kW·h/t to 36 kW·h/t. 3.3 Wastewater Recycling (1) Remove the rear elbow and short section of the F4201 bottom drain valve and make a buffer tank with a built-in filter. During normal production, the washing liquid enters the clear liquid tank through this to reduce woven bag silk and other debris entering the clear liquid tank. The filter pore size is: 1 mm×1 mm, the buffer groove size is: Length × width × height = 500mm × 800mm × 600mm. (2) Open a hole and connect a pipe 50 mm below the upper edge of the F4201 settling tank to create a buffer tank with a built-in filter. The washing liquid overflows from this hole, which can not only maintain the liquid level in the tank, prevent gas flushing from splashing, but also effectively filter and intercept light floating debris. The filter pore size is: 1 mm×1 mm, the buffer groove size is: Length × width × height = 500 mm × 800 mm × 600 mm. (3) Install an electromagnetic flowmeter in the 16m flat pipe section from the process water to the dust removal tail gas scrubber defoaming network pipeline, and the signal is led to the main control room to facilitate control of the process water volume. The electromagnetic flowmeter can be used as an electromagnetic flowmeter that is discarded from the original urine pipeline. (4) Change the baffle weir on the south side of the underground tank to an overflow weir. The upper part of the overflow weir is flush with the bottom of the trench. A movable filter is installed on the upper part of the overflow weir. The upper part of the filter is flush with the upper part of the underground tank. The filter aperture is 1 mm × 1 mm. The original underground tank pump is put back into use. A movable filter screen is installed at the entrance of the underground tank, with a pore size of 5 mm × 5 mm. The drainage trench should be closed with a removable cover to prevent debris from entering the underground trench and blocking the pump inlet. (5) Connect a DN25 mm pipeline to the urine tank after the diversion and discharge valve at the outlet of the washing liquid pump, and install an electromagnetic flowmeter on the pipeline. The electromagnetic flowmeter can be used on the original outlet pipeline. (6) Transformation effect: By adding a filter tank and a filter in the trench, the frequency of cleaning the inlet filter of the washing pump is effectively reduced, ensuring the continuous operation of the equipment, extending the service life of the pump, and reducing the labor intensity of operators. 3.4 Transformation of the raw material conveying system Since the raw material conveying system has problems such as slow loading speed, rigidity of materials in the silo, unsmooth unloading, unstable granulation conditions, and frequent material blockage and shutdown, the following transformation was carried out. 3.4.1 Modification content (1) The grating hole of the raw material pit has been doubled from the original one, from 50mm×50mm to 100mm×100mm, thus increasing the feeding speed. (2) Make two poke holes in the lower cone part of the raw material bin, and change the dredging method from the original vibration knocking of the bin to the use of special tools to reduce the workload and extend the maintenance cycle of the bin. (3) Cut off part of the V-shaped baffle plate inside the cone of the raw material bin to expand the discharge opening and increase the discharge speed. (4) Cut off part of the discharge baffle at the bottom of the mixing hopper to expand the discharge opening and reduce the frequency of material blocking. 3.4.2 Improvement effect Before the improvement, not only was the production load low, but the silo was frequently clogged, and the silo was stopped and cleaned frequently. After the improvement, the bottleneck problems of small holes in the raw material warehouse and slow feeding of materials have been completely eliminated, the frequency of silo blockage has been greatly reduced, and the production and operation of the device have been stable. After the improvement, the output increased by 300t/d on average, reducing costs and increasing economic benefits. 3.5 Technical transformation of gas ammonia pipeline 3.5.1 Technical transformation measures The technical transformation measure is to add a gas ammonia pipeline at the inlet of the granulator. Its advantages are as follows. (1) Increase the pH value. The pH value of the raw materials mixed in the urea-based compound fertilizer granulator should be adjusted to neutral or close to neutral, that is, pH value 6 to 6.5. Under this condition, the MAP in the material has the best viscosity and is most conducive to granulation. If the pH value is too low, MAP has poor viscosity and is not easy to granulate, making granulation difficult. ； If the pH value is too high, the water-soluble phosphorus in MAP will degrade into insoluble phosphorus and affect crop absorption. Ammonia is an alkaline substance, and introducing gaseous ammonia can increase the pH value and improve the viscosity of MAP. (2) Improve the granulation conditions. The optimal granulation temperature of urea-based compound fertilizer is 60-65 ℃. The temperature inside the granulator of this device is 50~60℃, and occasionally exceeds 60℃ in summer. The reaction between gaseous ammonia and monoammonium phosphate is an exothermic reaction, which can increase the temperature of the material and facilitate the granulation of the material. At the same time, as the temperature of the material increases, the evaporation rate of water accelerates, the moisture content of the material at the outlet of the granulator decreases, and the water content in the product will also decrease, which can greatly reduce the agglomeration of the product. The national standard requirement for water content is <2%, and most of our products are <2%. Experiments show that when the water content is <1%, the agglomeration phenomenon is significantly reduced. 3.5.2 Transformation effect (1) Before transformation ① The pH value of the material at the outlet of the granulator was about 5.5, and the granulation properties of the material at the outlet of the granulator were poor. ； ②The particle strength is low, and the material easily enters the dryer and is easily powdered. ； ③As the circulation volume increases, the process screen is prone to clogging. (2) After transformation, ① gaseous ammonia is introduced, and the pH value of the material can be controlled at 6 to 6.5, achieving optimal granulation conditions. ； ②Through the reaction between gaseous ammonia and monoammonium phosphate, the pH value of the granulating material is increased. At the same time, part of the heat is released by the reaction, which improves the balling rate of the material. ； ③The moisture absorption point of the material is increased, the moisture absorption is reduced, and the sticky material inside the equipment is reduced, which is beneficial to the drying of the material. ； ④The circulation volume is reduced and the power consumption is reduced. 4 Benefit Analysis 4.1 Economic Benefit Data from the financial statements of Zhongyuan Dahua Group Co., Ltd. show: Since the device was put into operation in April 2002, it has been * * In accordance with quality standards, a total of 22 formulas of compound fertilizer products with total nutrients ≥40%, ≥42%, and ≥45% have been produced. In 2002, we produced 60,000 t/a of products and achieved sales revenue of 5.275 million yuan, with a profit and loss balance. ； In 2003, 120,000 t/a of compound fertilizer was produced, with sales revenue of 152.5 million yuan and total profit of 8.16 million yuan. ； In 2004, it produced 168,000 t/a of compound fertilizer, achieved sales revenue of 270.63 million yuan, and total profit of 36.51 million yuan. 4.2 Social benefits The product of this device has reasonable formula, stable quality, high utilization rate and significant fertilizer effect. Based on an average increase of 6.5 kg of grain per kilogram of pure nutrients, the annual grain production can increase by 877.5 million kg. Since the Zhongyuan Dahua 300,000 t/a industrial test device was put into operation, it has obviously promoted related industries in surrounding areas, such as railways, road transportation, agriculture, etc. After the product was put on the market, users generally reported that it has good fertilizer efficiency and has obvious effects on improving crop yield and quality. 5 Conclusion (1) Zhongyuan Dahua’s 300,000 t/a urea-based compound fertilizer plant did not fully consider the characteristics of the actual materials in the design, resulting in the plant being unable to operate safely, stably, and for a long time. After technical transformation, the problems caused by the design flaws were solved and the optimal operating state of the system was achieved, so that the plant can operate safely and stably, and achieve the production target of 1,000 tons per day. (2) Urine-based compound fertilizer has been * * It is included in the recent development of chemical fertilizer varieties and has great room for development in the country. The technical transformation experience of Zhongyuan Dahua Group's 300,000 t/a urea-based compound fertilizer device has been applied and promoted in the new urea-based compound fertilizer devices of four companies in Hubei, Heilongjiang, Ningxia, Xinjiang and other places.