Compatibility of raw materials in the production of drum-type compound fertilizers, existing problems, and countermeasures

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To read more articles on compound fertilizer technology, follow the WeChat official account: fhfjslm. Or directly add the author’s WeChat ID: xjk830103. Physical granulation technology is a method that uses physical means to process non-granular or powdered materials into fertilizers with the desired particle size and sufficient mechanical stability. 1. Compatibility of raw materials
The so-called compatibility between different materials mainly refers to the chemical reactions that occur among nutrients in various forms during mixing and stacking processes. Some of these reactions have beneficial effects; they are often utilized to improve the undesirable physical and chemical properties of materials. For instance, when calcium magnesium phosphate fertilizer is mixed with superphosphate at an appropriate ratio, the slightly alkaline nature of the calcium magnesium phosphate fertilizer can neutralize the free acids present in the superphosphate and absorb any excess moisture generated from the mixing process. The resulting product exhibits favorable physical properties. Some reactions, on the other hand, can lead to a reduction in the fertility of the fertilizer or even to a degradation of its nutrients; when certain raw materials are mixed together, they may absorb moisture, form clumps, or even lose all their fertilizing effectiveness. For example, when urea is mixed with calcium superphosphate, the following reaction occurs, resulting in the formation of calcium dihydrogen phosphate! Urea complexes and urea phosphates are produced, along with the release of crystalline water: file:///C:/Users/fxffxjk/AppData/Local/Temp/msohtmlclip1/01/clip_image001.png. As a result of this reaction, the liquid phase in the mixture increases, forming a saturated solution; the raw materials become paste-like and cannot be granulated, which sometimes leads to production interruptions. Through long-term practice and theoretical analysis, mixing tables for various raw material combinations have been developed, which can serve as one of the bases for selecting raw material pairs. file:///C:/Users/fxffxjk/AppData/Local/Temp/msohtmlclip1/01/clip_image002.png Additionally, although no chemical reaction occurs between some materials, the critical humidity of their mixtures changes. The critical humidity refers to the atmospheric humidity at which the fertilizer begins to absorb water; below this humidity level, the fertilizer does not absorb water. Generally, after mixing various fertilizers, their critical humidity is lower than that of any individual component; the values in the table represent the critical relative humidity of pure fertilizers as well as those of fertilizers made from two components. II. Main problems associated with typical compound fertilizer systems and corresponding solutions: 1. When urea is used as the nitrogen-based fertilizer, its high hygroscopicity and low softening point, along with hydrolysis that occurs during processing, lead to the formation of highly hygroscopic compounds and/or urea dimers. This limits the temperature levels allowed during processing and drying, and various technical difficulties arise during ingredient preparation. The following discusses these issues in relation to specific phosphate fertilizers. The critical relative humidity percentages for certain pure fertilizer salts and their binary mixtures are as follows: %file:///C:/Users/fxffxjk/AppData/Local/Temp/msohtmlclip1/01/clip_image004.jpg When urea is mixed with superphosphate or calcium carbonate, the main problem is that the free acids present in those compounds react with urea to form urea phosphates, urea phosphates, or urea fluorosilicates. These compounds have very high solubility in water; at 40 degrees Celsius, their solubility is approximately 5.8 times that of urea. There are three solutions: the first is to pre-treat superphosphate or calcium carbonate, and the method has been discussed earlier ； Secondly, it is necessary to control the amount of urea added. In practical operations, it has been found that as long as the urea content in the mixture does not exceed 12%, the aforementioned problems do not occur ； Third, add a certain proportion of sulfuric acid an or ammonium chloride. Sulfuric acid AN reacts with ordinary calcium carbonate, heavy calcium carbonate, and potassium salts as follows: file:///C:/Users/fxffxjk/AppData/Local/Temp/msohtmlclip1/01/clip_image005.png The ammonium gypsum, potassium gypsum, and gypsum produced in these reactions all absorb water; therefore, adding sulfuric acid AN to ordinary or heavy calcium carbonate results in hardening of the material. In the presence of urea, the following reaction occurs: file:///C:/Users/fxffxjk/AppData/Local/Temp/msohtmlclip1/01/clip_image006.png This improves the physical properties of the material. Therefore, in the presence of ammonium ions, even the use of a higher proportion of ordinary calcium carbonate and urea does not cause any problems; generally, the addition ratio is 10% of the nitrogen content. Urea is mixed with diammonium phosphate or ammonium phosphate; since ammonium phosphates contain ammonium ions, this **reduces the chances of urea reacting with phosphoric acid to form urea phosphates, making it a technically ideal combination. Furthermore, the total concentration of N and P2O5 in the saturated solution of the urea and ammonium phosphate mixed salt is lowest at a pH value of 4.5 on the isotherm; taking advantage of this characteristic, drying can be carried out with a low amount of liquid phase. 2. For compound fertilizers that use ammonium chloride as the main nitrogen source, when potassium chloride is used as the potassium source, attention should be paid to the accumulation of chlorine in the soil due to dichloride; in addition, calcium magnesium phosphate fertilizer should be added to prevent soil acidification. 3. When AN sulfate is used as the nitrogen source, AN sulfate reacts with the free acids in superphosphate and similar substances to form ammonium gypsum, which causes the fertilizer to become powdery and clumpy. This can be prevented in production through two methods: by reducing the drying temperature ； Increase the residence time inside the cooler. -------------------------------------------------------------------------------------------------