What are the working principles of 4 types of compound fertilizer agglomeration/granulation process.

What is compound fertilizer granulation? 4 types of granulation method principle

Compound fertilizer granulation method its principle is the use of chemical (or) physical methods to process non-granular or powdery materials into granular fertilizers that meet the required size range and have sufficient mechanical stability. There are four main types commonly used in fertilizer industry production: agglomeration granulation, slurry coating granulation, melt condensation granulation and extrusion granulation. Their granulation mechanisms are different, which are described below.

Theoretical analysis of compound fertilizer agglomeration and granulation method principle: physical and chemical levels

• Under certain liquid phase conditions, mechanical agitation causes particles to collide with each other and come into close contact under the squeeze of mechanical action. Capillary forces caused by the surface tension of the liquid phase hold them together. Capillary particles are related to the contact area between particles. Relatively speaking, small particles are much larger than large particles. When the bonded particles reach a certain limit size (determined by the relationship between the particle’s surface area and its mass), the capillary force cannot withstand the vibration of their collision with other particles and will no longer bond and grow.
• The liquid phase can be introduced as water or saturated steam, or as a solution of one or more fertilizer components. Once the liquid phase comes into contact with the solid particles, it will dissolve the material from the particle surface, and chemical reactions may or may not occur depending on the properties of the material. If materials dissolved in the liquid phase recrystallize, the liquid phase acts as a binder to hold them together. The bonding quality obviously affects the strength of the finished particles, and the bonding or agglomeration process is controlled by some factors, such as molecular and electrostatic forces caused by the close proximity of ions, the surface porosity of the original solid particles and the crystal structure of the void material, etc. . Reunion and granulation are mainly bridging and bonding of materials.
• As mentioned above, the existence of liquid phase is the prerequisite for agglomeration and granulation. Controlling the amount of liquid phase is also the most important way to control the agglomeration and granulation operation. For example, too much liquid phase will lead to excessive granulation, and too little liquid phase will result in insufficient granulation due to too little bridging, resulting in poor particle strength. The basic factors that affect the amount of liquid phase are moisture, temperature and return material, and they are related to each other.
• The moisture content is not only affected by the water or steam added during the granulation process, or the chemical reaction in the granulator, but also by the moisture content of the feed, the amount of returned material, and the moisture content of the returned material.
• The granulation temperature affects the solubility of the material. Generally, increasing the temperature will increase the solubility of the material and increase the amount of liquid phase. The temperature inside the granulator is basically determined by the temperature of the raw materials and return materials, the amount of water and/or steam added, and the heat generated by the chemical reaction.
• Theoretically, for each mixture there is an optimal amount of liquid phase for granulation efficiency. The liquid phase consists of the water contained in the material and the salts dissolved in the water. Since the solubility of fertilizer salts increases with temperature, the warmer the temperature, the less water is needed. So, for any given mix, there is an optimal moisture content at each temperature.
• Although the return ratio is determined by the selection of the granulator and its granulation efficiency, the return ratio, the size of the returned particles, the return temperature, etc., all have a significant impact on the granulation operation.
• For example, if the return ratio is increased under certain other conditions, insufficient granulation will occur due to insufficient liquid phase; conversely, if the return ratio is reduced, excessive granulation will occur due to too much liquid phase. Phenomenon. The size of the returned particles has exactly the same impact on the granulation operation. For the same mass of returned material, the surface area of small particles is larger than that of large particles, so a larger amount of liquid phase is required to ensure good granulation. Now because other conditions are constant, that is, the amount of liquid phase is also constant, if the returned material particles are too small, insufficient granulation will occur, and vice versa.

Theoretical analysis of the chemical and physical process of melt condensation compound fertilizer granulation method principle

• Melt condensation granulation is to heat the material or use the reaction heat between raw materials to make the material basically ejected through the nozzle in the form of anhydrous liquid or melt. The melt can be directly condensed into granules in the refrigerant, or it can be coated on the particles of the granulator to condense. This is repeated until the particle size gradually increases to the required size.
• At the same time, when the melt is coated on the particles, the particles will also cluster or stick to larger particles and condense into finished particles. Melt condensation granulation can take advantage of the physical properties of the lowest eutectic point after mixing two dry salts.
• Melt condensation granulation utilizes this physical characteristic to operate at a lower temperature, which not only saves energy, but also reduces side reactions or ammonia loss. However, because of this, melt condensation granulation is subject to certain restrictions on the selection and formulation of raw materials, and there are strict requirements on raw material measurement, mixing time, operating temperature, etc.
• Melt condensation and granulation requires less or even no material return. The production system produces less dust. The product is bead-shaped and has a smooth surface, but the particle size is not easy to adjust within a wide range. The most common melt granulation process is tower granulation. Friends who are interested can click on the link below:

The principle, process and theoretical support of slurry spray compound fertilizer granulation principle.

Slurry coating granulation principle is the uses of all or most of the materials to enter the granulation equipment in the form of slurry. In this way, the slurry consisting of solid matter suspended in liquid is coated on the returned particles, and then dried (drying can also be performed simultaneously during the coating and granulation process) and sieved.

The part with qualified particle size is used as the finished product, and the rest is together with the fine particles under the sieve. And the particles that are too large on the screen are crushed together as return materials, and sent back to the granulator to repeat slurry coating, drying, screening, and crushing. The particles are coated layer by layer, similar to onion skins, and the particle size gradually increases to meet the required size range.

Although the particle size of the returned material usually has a wide distribution range and due to the mechanical action of the granulator, there may actually be some agglomeration and granulation, but it is mainly slurry coating granulation. In order to save energy consumption, the less moisture content in the slurry, the better. However, in order to ensure the fluidity of the slurry, the slurry must contain a certain amount of water, causing the granulation process to be controlled by the water balance of the system, and the return material is usually relatively large.

To sum up, due to the larger specifications of the granulation equipment and its downstream equipment, investment and energy consumption increase accordingly. In recent years, the development of new technologies such as internal material return has made it possible to basically eliminate external material return, greatly simplifying the process and saving investment and energy consumption.

In fertilizer production, slurries are usually prepared by reacting phosphoric, sulfuric or nitric acid, or even some mixture of these acids, with ammonia. Urea is often used as a supplemental nitrogen source in slurry coating granulation formulations. Potassium salt can be added to the granulator to mix with the slurry when needed. Therefore, slurry coating and granulation is more convenient and flexible, and can be used to produce a variety of granular fertilizer products of different grades, and the particles are rounder and harder. This is the main reason for its widespread adoption.

Principle and theoretical analysis of compound fertilizer extrusion granulation method:

Extrusion granulation is a dry granulation process in which solid materials rely on external force to agglomerate. The particles of the material are closely approached under high pressure, causing molecular force, electrostatic force, and valence force to tightly combine the particles.

Extrusion granulation can be regarded as a continuous process. Under the action of external force, the air is first removed when conveying materials, and the particles are rearranged to eliminate the gaps between the materials, and the materials are densified. When continuing to pressurize and increase density, there will be different situations depending on the properties of the material. For brittle materials, part of the particles are crushed and fine powder fills the gaps between the particles. In this case, if the free chemical bonds on the newly created surface cannot be quickly saturated with atoms or molecules from the surrounding atmosphere, then the newly created surfaces will contact each other and form strong composite bonds.

When plastic materials are squeezed, the particles deform and move around each other, generating strong van der Waals forces. In fact, in most cases, these two bonding mechanisms exist at the same time, and under certain operating conditions (such as feeding, etc.), the extrusion process may tend to proceed according to one of the mechanisms. In the final stage of the extrusion process, due to the large amount of energy provided to the system in the form of pressure, hot spots may be formed at the contacts between particles, causing the material to melt. Solid bridges form when the temperature drops and the material cools.

Conclusion:

Therefore, in this article, we focus on the principles of granulation of four types of compound fertilizers, and the theoretical basis of physics and chemistry. So how to make good compound fertilizers? Only good compound fertilizer equipment can produce high-quality compound fertilizer, and only good compound fertilizer technology can produce good compound fertilizer.