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State-owned farms were the first to apply chemical fertilizers on a large scale. In the 1980s, the state-owned farm system in Heilongjiang Province had a plan for the production of inorganic fertilizers, in which the production and use of calcium magnesium phosphate fertilizer were given top priority; it was decided to utilize phosphate ore resources from Yunnan to develop calcium magnesium phosphate fertilizer using the blast furnace method. However, a practical problem arises, as calcium magnesium phosphate fertilizer is an amorphous glassy substance with a fineness of 80 mesh (0.175 mm), it has no viscosity and cannot be granulated. Powdered phosphate fertilizer cannot meet the needs of mechanized fertilization on farms, and efforts by the General Administration to organize experts to develop granulated forms proved unsuccessful. Faced with **an investment of nearly 10 million yuan and a huge backlog of calcium magnesium phosphate fertilizer products, the author took on the research task of granulating calcium magnesium phosphate fertilizer.** In 1984, the author’s five-year experiment on the granulation of calcium-magnesium-phosphorus fertilizer was successful, leading to its industrial production. Taking advantage of the characteristic that \"two anhydrous mixed salts have low melting points\", and referring to the relevant data obtained from pilot tests, a molten slurry of urea-potassium salt-water was prepared in a reaction tank. Using a pressure-driven discharge method, the slurry was sent to a granulation disk through conveyance pipes and atomized by spiral nozzles, thereby enabling spray granulation on the granulation disk; as a result, multi-element granular compound fertilizers using calcium-magnesium-phosphorus fertilizer as the base material were produced (hereinafter referred to as \"compound fertilizers\"). 1 Development of a process for producing compound fertilizers from molten urine: Successful implementation of the spray granulation process, with relevant data and process parameters obtained (see Table 1). Over a period of 3 years, the author developed a method in which molten urine is introduced into a reactor and mixed in proportion with potassium salts to directly produce a ternary high-temperature molten slurry of N-K2O-H2O. This slurry is then pumped to a disk granulator where spray disk granulation is carried out to produce UNK granular fertilizers with high nitrogen content (45-0-4) or high potassium content (4-0-55), which are used as raw materials for BB fertilizers ； UN-P-K ternary granular urea-based compound fertilizer can be produced by adjusting the basic raw materials ； BB fertilizer is produced by directly mixing binary high-nitrogen, high-potassium granular fertilizer with DAP (diammonium phosphate) or processed granular MAP (ammonium monophosphate). In 1998, the co-production process was arranged by the relevant industry authorities to be put into industrial use at a urea production facility in Hebei. The first domestic production facility for producing urine-based compound fertilizers from molten urine was put into operation at the end of 1999, after more than a year of research, design, and construction. It began to produce 37-0-11 fertilizer specifically designed for corn, as well as high-concentration urine-based compound fertilizers with formulas of 15-15-15 and 15-24-6. http://www.nmtech.com.cn/jishuwang/upload/0705311511052663.jpg 2 Characteristics of the process for producing compound fertilizer from molten urine. The process for producing compound fertilizer from molten urine is also referred to as the co-production process (see Figure 1). Its main equipment includes gravity-fed slurry reactors, specialized slurry pumps, mixing feeders, disk granulators, rotary coolers, as well as systems for product grading and screening, metering and packaging, primary washing and dust removal of exhaust gases, and an automatic control system. The initial investment in equipment is only 1/2 of that required for the urea melting and spray granulation process, which provides a solid foundation for fertilizer manufacturers to achieve optimal economic benefits. http://www.nmtech.com.cn/jishuwang/upload/0705311511453689.jpg In the combined production process, it is first necessary to determine the optimal concentration of molten urea in the urea production facility. Here, the mass fraction of urea in the molten urine is set at 99.9%, and the temperature is 145°C; this corresponds to a three-stage concentrated urea solution. The materials are fed into a gravity-fed slurry reactor in a ratio of 85:7:8 by mass of molten urine, potassium chloride, and recycled liquid, respectively. As stirring takes place, a chemical reaction occurs between these materials, resulting in a low-melting-point complex salt composed of urea, potassium salts, and water – namely, the molten slurry. Upon investigation, the temperatures of the ternary melt slurries with different compositions are shown in Table 2; the melting point of the double salt is 99°C, which fully ensures the feasibility of spray granulation. When the level of the molten slurry in the reactor exceeds the horizontal height of the discharge outlet, the slurry overflows into the storage tank. It is then pumped, using a slurry pump with thermal insulation, to the adjustable spray-drying disc granulator, where it is atomized through spiral nozzles to carry out spray granulation. The powder particles are bonded and agglomerated to form qualified granules, which are sent to a rotary cooler to be cooled to room temperature. After sieving, metering, and packaging, 45-0-5 high-nitrogen or 4-0-55 high-potassium UNK granules are obtained. By adjusting the ratio of the powders and adding heavy calcium carbonate or monoammonium phosphate, high-concentration UNPK urea-based compound fertilizers such as 15-15-15 can be produced. http://www.nmtech.com.cn/jishuwang/upload/0705311512189355.jpg 3 Technical and economic advantages of the integrated production process: The melt-spraying disk granulation method used in this urine-based integrated production process features no need for drying, no backflow of material, and no pollution; it also offers advantages such as energy savings, reduced material loss, and lower production costs. We utilize the visibility, adjustability of disk granulation, and the cyclic mechanism of automatic backfeeding to achieve spray disk granulation in a co-production process without backfeeding. Theory and practice show that the larger the diameter of the disk granulator, the greater the drop in height experienced by the material, and the longer the path followed by the material as it rolls. To ensure continuous production, the amount of powder fed onto the disk and the amount of slurry sprayed should be synchronized with the granulation output. Therefore, the diameter, side height, and rotation speed of the granulation disk become the main parameters for determining the production scale of the compound fertilizer manufacturing plant as well as the choice of associated equipment (see Table 3). Once the diameter, edge height, and rotation speed of the granulation disk are determined, the angle between the disk and the ground level is adjusted so that the mixture within the disk moves automatically into the designated area based on parameters such as its overall density and angle of repose. When the well-structured, smooth and qualified particles rise on the surface of the storage area above the horizontal line at the bottom edge of the granulation disk, they automatically overflow and proceed to the cooling process, enabling one-time granulation via disk spraying without any backflow of material. According to experts’ calculations, reducing 1 ton of return material in the production of compound fertilizers saves 25.37 yuan in costs; therefore, for a compound fertilizer production facility with an annual output of 100,000 tons, adopting a melt-spraying granulation process without return material will result in an increase in profits of 1,011,400 yuan. Of course, the workload and working conditions for operators should be improved. http://www.nmtech.com.cn/jishuwang/upload/0705311512543436.jpg If a spray granulation machine equipped with functions of internal grading, internal feedback, and internal crushing – a \"three-interior\" type machine – is used in the combined production process, it is also possible to achieve the same effect as disk spray granulation without any feedback, but at a cost 40 times higher than that of a disk granulator. The molten slurry produced by the co-production process, also known as a special \"binder\" for granulation, is a complex salt with a low melting point and stable chemical properties. It is atomized using a specially designed spiral nozzle and sprayed onto the material bed on the granulation disk, thereby causing the powder particles to \"bond and agglomerate.\" Under the action of various external forces such as rolling, collision, and compression, granulation is achieved, avoiding the increase in moisture content in the material that occurs when steam and recycled water are used in the so-called \"urea molten spray granulation\" process. In particular, slurries with stable chemical properties prevent the increase in the liquid phase volume of the material and the problems of product moisture absorption and scaling caused by the chemical reactions between urea and potassium salts, ammonium phosphates, and free acids. Therefore, in the co-production process, from the slurries and powders used to the processing methods employed, strict controls are applied to ensure that the moisture content does not exceed specified limits. As a result, there is no need to dry the product, which not only saves on the substantial costs associated with drying equipment but also reduces energy and fuel consumption. For a plant with an annual production capacity of 100,000 tons, this leads to economic savings of over one million yuan for the enterprise. The production of UNK high-potassium 5-0-57 granular fertilizer through urine co-production spray granulation also takes advantage of the heat source provided by urea solution and the high temperature of melted urine, thereby overcoming the technical challenges associated with potassium fertilizer granulation. Both crystalline forms of potassium chloride and potassium sulfate are non-sticky, do not easily form granules, and are even less suitable for being turned into potassium fertilizer granules with a particle size matching that of nitrogen and ammonium phosphate fertilizers, for use as raw materials in BB fertilizers. By using the molten slurry produced as a by-product of urine as a \"binding agent\" and powdered potassium chloride or potassium sulfate as the base material, and employing a spray granulation process without drying, UNK granular potassium fertilizer can be produced in one step. This approach not only solves the problem of finding suitable raw materials for BB fertilizer but also addresses issues related to the chemical compatibility between potassium fertilizer and nitrogen and phosphorus fertilizers, representing a technological breakthrough. 4 Comparison of the two spraying granulation processes: The molten slurry produced by the urine co-production process is a complex salt with a low melting point and stable chemical properties; it is sprayed onto a disk granulator to cause the powder particles to stick together and form granules. This granulation process does not require the use of steam or water, and no increase in the liquid phase occurs as a result of chemical reactions during granulation; therefore, the product does not need to be dried, and there is no backflow of material. Molten urea is used for spray granulation; no potassium salts are added to the molten urea solution. Granulation is generally carried out by introducing steam in a drum granulator, and the material needs to be dried. As a result, the process is complex, and the investment required is higher than that of the urine co-production process (see Table 4). http://www.nmtech.com.cn/jishuwang/upload/0705311513364357.jpg5 Conclusion: The author has developed a process technology for producing urine-based compound fertilizers from molten urine, one that incorporates independent intellectual property rights of China. The author is willing to collaborate with enterprises to work together toward achieving soil-based fertilization in agriculture!