Research on the process of producing urea-based compound fertilizer by high-tower granulation

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In recent years, a new force has been added to the compound fertilizer production process in our country, which is the use of spray granulation in granulation towers to produce high-concentration nitrogen, phosphorus, and potassium compound fertilizers. Once this production method was proposed, it has been favored by manufacturers and users due to its incomparable advantages. 1 Application and advantages Compared with other traditional compound fertilizer production processes, the advantages of the high-tower granulation production process of N, P, and K compound fertilizers are concentrated in the following aspects:: ①Product nutrition is uniform ； ②On the basis of high nitrogen content, the content of phosphorus and potassium can be adjusted arbitrarily ； ③The product has a smooth spherical appearance, is not easy to clump, has high strength and is conducive to fertilization. ； ④Using high tower spray granulation, it integrates granulation, drying and cooling, simplifying the production process ； ⑤The product has good water solubility and high fertilizer efficiency ； ⑥The small hole on the product particles is a natural anti-counterfeiting mark. ； ⑦There is almost no material return during the production process and the production cost is low ； ⑧The production investment is small, the scale is large, and the product output is high. In view of the above advantages, this production process has become a popular choice for many nitrogen fertilizer production companies (urea, * * ) is the first strategic choice to adjust product structure, and some companies that do not produce nitrogen fertilizers also use urea or * * High-tower granulation production is carried out by re-melting method. According to incomplete statistics, there are more than a dozen companies in our country that have adopted this new technology and have reached a certain scale of production capacity, and there are also about a dozen units under construction or to be built. It can be said that the emergence of the high-tower granulation process for producing high-concentration N, P, and K compound fertilizers is a revolution in the compound fertilizer industry. It will eliminate some traditional production processes with high cost, low output, poor quality, and large investment, and quickly form a new compound fertilizer industry pattern. 2. Process Flow The process flow diagram of high-tower granulation to produce high-concentration urea-based N, P, and K compound fertilizers is shown in Figure 1. The nitrogen in high-concentration compound fertilizers comes from urea and * Ao acid an, of which urea is the main component, is commonly known as urea-based compound fertilizer. ； The source of phosphorus is monoammonium phosphate (monoammonium phosphate) ； The source of potassium is potassium chloride (chlorine-based) or potassium sulfate (sulfur-based). During production, the powdered monoammonium phosphate, potassium chloride and additives (auxiliary materials) are heated and then added to the molten urea. After thorough mixing, they enter the granulation tower for granulation. The mist droplets crystallize, solidify and cool in the air to form finished particles. 3 Control of key points in the production process The process flow shown in Figure 1 is the basic process flow of high-tower granulation. Each enterprise and research institution has different specific processing methods for relevant links according to their different situations. As far as the overall production process is concerned, there are still quite a few problems from the perspective of production continuity and industrial scale. There are also many details that cannot be handled to a satisfactory level. In particular, the understanding of some key issues needs to be explained and analyzed from a deeper theoretical level. If these problems are not solved perfectly, they will inevitably affect the production prospects of the entire process. The author put forward some of his own views and opinions on the production process of N, P and K compound fertilizers produced by high-tower granulation, and achieved good results after production verification. http://www.nmtech.com.cn/jishuwang/upload/070412857135321.jpg3.1 The relevant information on the process principle introduces the process principle of high-tower granulation to produce high-concentration urea-based N, P, and K compound fertilizers, which are all based on the characteristics that molten urea, monoammonium phosphate, potassium chloride (or potassium sulfate) can form eutectic point compounds, but in fact, these materials cannot form so-called "eutectic point compounds" when mixed. Monoammonium phosphate and potassium chloride are added to urine (the urine temperature is usually 135°C). At this temperature, potassium chloride is very stable and does not react with urea or monoammonium phosphate. In fact, only part of the monoammonium phosphate and part of urea participate in the chemical reaction, and the reaction of free acid is negligible. First of all, due to the relationship between the temperature of monoammonium phosphate and urea itself, it will produce its own decomposition reaction, which is inevitable. Secondly, long-term contact between monoammonium phosphate and urea will cause urea to decompose and form polyphosphate. The reaction formula is:: (NH2)2CO+2NH4H2PO4→(NH4)2H2P2O7+CO2+2NH3 (1) It can be seen from formula (1) that the decomposition of urea releases ammonia gas, which will lead to the loss of nitrogen nutrients ； At the same time, CO2 is released, which will form a large number of bubbles on the liquid surface, causing the mixer to overflow or pressurize. The small pores on the particles of compound fertilizer products are also formed because the reaction continues during the crystallization process and gas is released from the inside to the outside. In addition, polyphosphate is a very viscous substance. If too much polyphosphate is produced, it will increase the viscosity of the slurry and directly affect the fluidity of the slurry, thus affecting the normal progress of nozzle granulation. The main factors affecting this reaction are temperature and moisture. The higher the temperature and the higher the moisture, the faster the reaction. Therefore, these two process indicators should be strictly controlled, and the length of mixing time also needs to be strictly controlled. Therefore, in fact, what is formed by mixing these substances together should only be a mixture, not a compound, and the term eutectic point is also inaccurate. Due to the addition of a large amount of "impurities" to the urea melt, the crystallization temperature has dropped, but the drop will not be too much. If the temperature control of compound fertilizer production is performed below the crystallization temperature of urea (132.6°C), urea will often crystallize due to precipitation and separation, thus affecting the normal production. 3.2 Pretreatment of raw materials The pretreatment of raw materials is crucial to the entire production process.: One is the moisture content in the raw materials, the second is the temperature control of the raw materials before being added to urine, and the third is the fineness of the raw materials. Some researchers believe that high moisture content can directly reduce the crystallization temperature and viscosity of urea, making the granulation process relatively simple and smooth. In fact, this is not the case. High moisture content has three major disadvantages:: First of all, material transportation is difficult. Since monoammonium phosphate and potassium chloride are highly hygroscopic, the moisture mass fraction in these two raw materials is often as high as about 5%. Adhesion will occur during the transportation process. Over time, it will form hard blocks and cause equipment failure. ； Secondly, high moisture will aggravate the negative reaction between urea and monoammonium phosphate, resulting in increased ammonia loss and polyphosphate production. ； Third, due to the high moisture content of the raw materials, it will directly lead to high moisture content of the finished product. At 30°C, the critical relative humidity of urea reaches 75%, but when combined with ammonium phosphate and potassium fertilizers, its critical relative humidity is only 40% to 50%. It is a particularly hygroscopic fertilizer, and its high moisture content will directly affect the quality of the product. Therefore, the moisture in the raw materials must be removed at the beginning of the process to control its mass fraction to about 1%. In addition, in order to prevent supercooled raw materials from being instantly chilled when added to the urea melt and causing local urea crystallization, monoammonium phosphate, potassium chloride and auxiliary materials must be preheated before entering the mixer, so that moisture can be removed while heating the raw materials. It is necessary to control the moisture content and avoid excessive temperature. The temperature is preferably 100 to 110°C, because when it exceeds 110°C, monoammonium phosphate will accelerate the decomposition rate and cause ammonia loss. In addition, the fineness of the raw materials should be controlled to 0.3~0.8mm, because too large particles will block the nozzle, and too fine particles will increase the difficulty of mixing. 3.3 Granulation Granulation is the core of the entire production process of high-tower granulation to produce urea-based N, P, and K compound fertilizers. There is a big difference between the granulation of compound fertilizer and the granulation of urea. Urea is a single material with relatively stable crystallization temperature, complete physical and chemical data, and complete design calculation theory. Due to the large changes in the nutrients of N, P, and K in compound fertilizers, it is difficult to obtain physical and chemical data such as crystallization temperature, density, viscosity, entropy, and enthalpy. Therefore, there are no strict theoretical calculations in the granulation of compound fertilizers. They are all conceptual estimates based on urea production. A series of equipment, including the granulation tower, also adopt conceptual designs. The author puts forward the following issues that need attention:: (1) Full mixing of materials is the basis for smooth granulation. This requires that the mixing efficiency of the equipment used must be high. (2) Strictly control the material temperature. As mentioned above, since there is no so-called eutectic compound formed, the temperature of the material cannot be controlled too low, which will cause local urine to precipitate and crystallize and block the nozzle. (3) The granulator and nozzle used for granulation must be modified to be suitable for the granulation production of compound fertilizer. Especially the structure of the nozzle must have a special structure inside to prevent material from layering and sticking to the wall. The form can be diverse. The nozzle hole diameter of the nozzle is larger than that of the nozzle for urea, and most diameter sizes are 2.5 to 3.0mm. (4) Since the thermal enthalpy value of compound fertilizer is higher than that of urea, its crystallization and cooling speed are slower. Compared with urea, the height of the granulation tower required is much higher than that of urea. Even if the height of the granulation tower reaches 100m, the temperature of the finished product is often still above 60°C, and it needs to continue cooling. ； In addition, since the density and particle diameter of compound fertilizer are larger than that of urea, the diameter of the prilling tower required is also larger than that of urea.