Fertilizer production technology - Granulation part -1- Wet granulation - drum granulation
Date:2024/07/02
Drum granulation method (rotary drum granulator) is a method of granulating raw material particles into the required size by using water or granulating accelerator in a granulation container such as a disc granulator or drum fertilizer dryer, which belongs to wet granulation method. The production process is to continuously feed the raw material powder into the granulation container, such as a slow rotating disk or drum, spray an aqueous solution or granulation accelerator in the container, the particles roll along the rotating direction of the granulation container, collide with another particle, combine and grow larger, and finally grow into the desired size of particles.
The principle of drum granulation
Liquid cross-linking force plays a very important role in drum granulation. In the granulation process, when the amount of liquid added is very small, the surface of the particles is not completely wet, the air is in a continuous phase between the particles, the liquid presents a dispersed phase, and a small part of the particles form liquid cross-linking, but the particles are not connected to each other. In this state, the gas-liquid interfacial tension binding force of gas-liquid cross-linking between raw material particles is very weak, and the particles repeatedly combine and disintegrate, making it difficult to form large particles. When the amount of liquid is appropriate, the surface of the particles is completely wet, a large part of the air between the raw material particles is replaced by the liquid, the air becomes a dispersed phase, the liquid becomes a continuous phase, the liquid cross-linked around the air is formed, and they are connected into a network, the liquid cross-linked around the air is formed. At this time, the interfacial tension of liquid crosslinking and capillary negative pressure strongly bind the primary particles together. When the amount of liquid is too large, the space between the particles is completely filled by the liquid, all the air is driven away, the liquid cross-linking between the particles disappears, the gas-liquid interfacial tension disappears, only the surface tension of the liquid is left, the adhesion between the particles dissolves, and the whole becomes a slurry. Therefore, the amount of liquid added is a very important factor in drum granulation, that is, too much or too little liquid added will hinder the granulation process.
The main factors affecting granulation
1. Particle size and distribution of raw materials
The smaller the particle size of the raw material particles, the larger the specific surface area (surface area per unit mass or surface area per unit volume), the smaller the space fraction between the particles, and the larger the solid-liquid interfacial tension between the particles as liquid-liquid cross-linking. Therefore, the smaller the particle size of the raw material, the higher the particle strength.
2. Add granulation accelerator
Depending on the type of raw material, particle and surface characteristics, using only water may result in low pelletizing efficiency or low pelletizing strength. Adding granulation accelerator may be an effective treatment method.
3. The agglomeration state of raw material powder and particles
Secondary particles are composed of irregular primary particles, the surface is uneven, there are many internal cavities, the space is filled with liquid, forming a large number of free water structure. If the amount of rotation is insufficient, it will eventually result in particles with high water content, low volume density and low strength.
4. Add water
Water is the most important factor in the formation of liquid cross-linking between particles. If the speed and water amount are appropriate, the granulation efficiency will be improved, and factors such as particle strength and surface state after granulation will also be improved. However, this should be confirmed in preliminary experiments, as the optimal amount of addition depends on the type of raw material and granulation equipment.
5. Type and rotation speed of granulation equipment
In general, the larger the diameter of the granulation container, the higher the granulation efficiency, the higher the obtained particle strength, and the smoother the surface. The rotation speed of the granulation equipment is also important. If the rotation speed is slow, the particles will be deposited at the bottom of the granulation container, and the height of the particles will be low, and the rolling distance to the bottom will be short, resulting in insufficient rolling. If the speed is too high, the rolling speed of the particles can not keep up, and under the action of centrifugal force, disintegration may occur.
The principle of drum granulation
Liquid cross-linking force plays a very important role in drum granulation. In the granulation process, when the amount of liquid added is very small, the surface of the particles is not completely wet, the air is in a continuous phase between the particles, the liquid presents a dispersed phase, and a small part of the particles form liquid cross-linking, but the particles are not connected to each other. In this state, the gas-liquid interfacial tension binding force of gas-liquid cross-linking between raw material particles is very weak, and the particles repeatedly combine and disintegrate, making it difficult to form large particles. When the amount of liquid is appropriate, the surface of the particles is completely wet, a large part of the air between the raw material particles is replaced by the liquid, the air becomes a dispersed phase, the liquid becomes a continuous phase, the liquid cross-linked around the air is formed, and they are connected into a network, the liquid cross-linked around the air is formed. At this time, the interfacial tension of liquid crosslinking and capillary negative pressure strongly bind the primary particles together. When the amount of liquid is too large, the space between the particles is completely filled by the liquid, all the air is driven away, the liquid cross-linking between the particles disappears, the gas-liquid interfacial tension disappears, only the surface tension of the liquid is left, the adhesion between the particles dissolves, and the whole becomes a slurry. Therefore, the amount of liquid added is a very important factor in drum granulation, that is, too much or too little liquid added will hinder the granulation process.
The main factors affecting granulation
1. Particle size and distribution of raw materials
The smaller the particle size of the raw material particles, the larger the specific surface area (surface area per unit mass or surface area per unit volume), the smaller the space fraction between the particles, and the larger the solid-liquid interfacial tension between the particles as liquid-liquid cross-linking. Therefore, the smaller the particle size of the raw material, the higher the particle strength.
2. Add granulation accelerator
Depending on the type of raw material, particle and surface characteristics, using only water may result in low pelletizing efficiency or low pelletizing strength. Adding granulation accelerator may be an effective treatment method.
3. The agglomeration state of raw material powder and particles
Secondary particles are composed of irregular primary particles, the surface is uneven, there are many internal cavities, the space is filled with liquid, forming a large number of free water structure. If the amount of rotation is insufficient, it will eventually result in particles with high water content, low volume density and low strength.
4. Add water
Water is the most important factor in the formation of liquid cross-linking between particles. If the speed and water amount are appropriate, the granulation efficiency will be improved, and factors such as particle strength and surface state after granulation will also be improved. However, this should be confirmed in preliminary experiments, as the optimal amount of addition depends on the type of raw material and granulation equipment.
5. Type and rotation speed of granulation equipment
In general, the larger the diameter of the granulation container, the higher the granulation efficiency, the higher the obtained particle strength, and the smoother the surface. The rotation speed of the granulation equipment is also important. If the rotation speed is slow, the particles will be deposited at the bottom of the granulation container, and the height of the particles will be low, and the rolling distance to the bottom will be short, resulting in insufficient rolling. If the speed is too high, the rolling speed of the particles can not keep up, and under the action of centrifugal force, disintegration may occur.