Powdered materials have characteristics such as fine particles, large specific surface area, easy moisture absorption, and easy compaction. During storage in steel cylinder silos, they are more prone to problems like bridging, mouse holes, agglomeration, and blockage compared to granular materials. Whether it is cement, fly ash, mineral powder, lime powder, starch, flour, gypsum powder, or other powdered materials, once they get clogged, it not only affects the continuity of production but may also increase the cost of manual cleaning, and even cause equipment damage and safety accidents.
Which powdered materials require storage in silos
Most fine powdered and granular materials in industrial production are suitable for the mode of closed storage in silos. These materials are fine in texture, light in weight, and are prone to dust loss when stored outdoors. Bagged storage also has problems such as cumbersome handling, moisture absorption and deterioration, and large space occupation. Silos can precisely avoid these drawbacks.
The building materials industry is the most widely used field for powdered material silo storage. Materials such as cement, fly ash, mineral powder, limestone powder, and gypsum powder are basically all stored in vertical silos. In the chemical industry, talc powder, calcium carbonate, starch, resin powder, etc., and in the food and feed industry, flour, corn flour, feed premix, etc., also commonly use silos for storage.
In addition, the metal powder from metallurgy and mining industries, dust from dust removal, and the graphite powder and lithium battery powder raw materials from the new energy industry all belong to the suitable storage materials for silos. These materials have distinct commonalities, such as fine particles and easy moisture absorption, and have high requirements for the sealing performance of the storage environment and the stability of the discharge.
Characteristics of storage for powdered materials
Powdered materials have large particle gaps and a fluffy texture. When naturally piled up, they have a high porosity and overall fluidity is extremely unstable. Under external influences such as compression, vibration, and moisture exposure, the volume of the materials will rapidly shrink, changing from a loose state to a dense and hard one, with significant reduction in fluidity.
Most powdered materials have hygroscopic properties. Water vapor in the air is absorbed by the material particles. Especially in humid environments in the south and during rainy seasons, the materials are prone to absorb water and stick together. The originally loose powder will form lumps of different sizes, directly affecting the smoothness of discharging.
At the same time, powdered materials have fine particles and are lightweight, making them highly prone to generating static electricity. During the falling and friction process of the materials in the silo, static electricity causes the particles to adhere to each other, and they will also stick to the silo wall and the inner wall of the hopper. If accumulated for a long time, it will form a buildup of materials, gradually causing blockages. Additionally, the accumulation of powdered materials has obvious dead corners retention characteristics. The longer the static storage time, the higher the probability of solidification, agglomeration, and material blockage due to accumulation.
Why powder materials often get stuck in silos
Bridge formation leading to stopped flow of materials
Bridge formation is the most common silo clogging problem in powder material silos. It occurs in almost all storage conditions for fine powder materials. Bridge formation refers to the situation where the materials above the silo discharge port stick together and support each other, forming a stable arch-like structure.
This arch-like structure can bear the weight of the materials above and suspend the discharge port below, preventing the materials from falling. At this time, the materials above the silo remain stationary, only the discharge port is empty, and the production line cannot take materials normally, directly causing a shutdown.
The formation of bridge is directly related to the fluidity of the materials. The poorer the fluidity and the finer the particles, the stronger the adhesion and friction between the particles, and the more likely they will bite each other to form an arch structure. Frequent start-stop of discharging and outflow, unstable outflow speed, and fluctuating material storage volume in the silo will all aggravate the bridge problem.
Moisture-induced caking and blockage of discharge ports
Powder materials are generally hygroscopic. Even materials that are not prone to absorbing moisture on their own will absorb moisture through ventilation gaps, feed-in ports, and maintenance ports in the silo during long-term storage. After moisture enters the material layer, it will make the powder particle surfaces wet, significantly increasing the adhesion force between the particles.
Slight moisture absorption causes the materials to cluster and have reduced fluidity. Severe moisture absorption leads to the solidification of scattered powder into hard blocks. These blocks vary in size, small ones will get stuck at the discharge port position, hindering the material descent; large ones will directly seal the discharge port, causing a complete silo clogging.
This problem is particularly prominent in rainy seasons, humid workshops, and open silo scenarios. Many enterprises’ silos have aged seals and failed ventilation valves, allowing external moisture to continuously enter the silo. Over time, large areas of material caking and blockage occur.
Silo wall friction affecting material sliding
For powder materials to slide smoothly in the silo, the core is that their own gravity is greater than the friction force of the silo wall and the adhesion force between the particles. The smoothness and material characteristics of the silo inner wall directly determine the friction force, which in turn affects the discharge effect.
If the silo inner wall is not polished, has rough surfaces with burrs, or has a layer of solidified accumulated material adhered to it after long-term use, it will significantly increase the resistance to material sliding. Powder materials close to the silo wall will be held back by friction and cannot slide synchronously with the central materials.
Over time, the powder materials at the silo wall will continuously accumulate, compact, and solidify, gradually narrowing the discharge channel. As the channel becomes narrower, the material flow becomes more disordered, eventually resulting in intermittent discharging or even complete blockage.
Discharge port size affects discharge capacity
The discharge port is the only channel for material discharge in the silo. The rationality of its size design directly determines the stability of powder material discharge. Many small silos and custom silos, to adapt to small conveying equipment, deliberately reduce the size of the discharge port.
For uniformly granular and well-flowing materials, a small-sized discharge port will not cause significant problems, but for powder materials with large fluctuations in fluidity, they are prone to clustering. If the discharge port is too small, the materials will be severely restrained by the squeezing force during descent, and the fine powder will be compressed and compacted, forming a blockage layer.
At the same time, insufficient discharge port size will create a typical “central flow” state in the silo. Only a small amount of material in the center can slide down, while the surrounding materials remain stationary, and the stationary materials gradually solidify and accumulate, eventually completely blocking the discharge port.
The cone angle of the hopper affects the overall flow pattern
The cone angle of the lower hopper of the silo is a key parameter that determines the overall flow pattern of the materials, and it is also an easily overlooked factor that can cause silo blockage. The size of the cone angle directly matches the resting angle of the materials. Once the parameters do not match, the flow pattern will completely become chaotic.
If the cone angle of the hopper is too small, the inner wall of the hopper is too smooth, and the gravitational component of the material’s descent is insufficient, unable to overcome the frictional force of the particles. The material will stick to the inner wall of the hopper and accumulate statically, unable to slide down smoothly, gradually forming accumulated materials and solidifying the silo blockage.
If the cone angle is too large, although the resistance of the material’s descent is small, it will cause the discharge speed to be too fast and the flow rate to be uneven. When the material descends rapidly, a negative pressure will be formed, drawing in external air, further intensifying the dusting, stratification, and clustering of the materials, indirectly triggering bridge formation and silo blockage problems.
Long-term storage leads to material compaction
Many enterprises’ silos do not operate continuously 24 hours a day, and there may be situations where materials are stored without being discharged. Powdered materials that are stored for a long time in the silo will be under the continuous pressure from the upper layer materials.
The originally loose powdered materials will gradually expel the air from the particle gaps under the continuous pressure, shrink in volume, and increase in density, gradually being compacted and solidified. The longer the storage time, the higher the degree of compaction of the materials and the worse the fluidity.
When the silo is restarted for discharge after several days or even longer intervals, the compacted materials have lost their loose fluidity and cannot naturally fall, either getting stuck in the silo or breaking down to form large chunks that block the discharge port. This is the core reason for frequent silo blockages after resuming production.
Dead corners inside the warehouse cause material retention
Inadequate design of the silo structure, internal protrusions, residual welding seams, and non-flowing transition sections of the hopper will all form multiple dead corners in the silo. The materials at these dead corner positions will never be able to participate in normal discharge due to their own gravity.
The stagnant materials will remain in the dead corner area for a long time， constantly being compacted, getting damp, and solidifying, gradually thickening. When the accumulated stagnant materials reach a certain level， they will occupy the normal discharge channels, interfering with the overall material flow pattern.
At the same time， the solidified accumulated materials in the dead corners will periodically fall off, large chunks of solidified materials falling into the discharge port, directly causing sudden silo blockage. This type of material blockage problem is extremely repetitive and cannot be completely resolved without optimizing the structure and cleaning the dead corners.
How to reduce powder material blockage through silo design
When designing, the first step is to adjust the cone angle of the hopper according to the characteristics of the material. Based on the repose angle and fluidity of different powder materials, set a reasonable cone shape to ensure that the material can form a continuous flow and avoid local blockage. Secondly, reasonably design the size of the discharge port, match the diameter with the fineness of the material and the production requirements, and prevent small openings from limiting flow and causing material blockage.
The inner wall of the silo needs to be treated precisely, grinding all weld seams and protrusions smooth, and combining with wear-resistant and smooth liners to reduce the friction of the silo wall and prevent material adhesion and accumulation. At the same time, optimize the internal structure of the silo, eliminating unnecessary protrusions and steps, making the transition section of the hopper smooth and fluent, and eliminating storage as much as possible.
In addition, a complete sealing structure needs to be equipped, optimizing the sealing effect of the feed port, maintenance port, and ventilation valve, reducing the entry of external water vapor and moisture into the silo, and preventing material moisture and caking from the source.
Which flow-aiding devices are suitable for powder silos
Structural design alone cannot completely eliminate silo blockage. Combining with appropriate flow-aiding devices can continuously ensure stable material flow and is a necessary configuration for powder silos.
The air flow-aiding air cushion is the most commonly used device. It is installed on the inner wall of the hopper and uses a small amount of airflow to disturb the material, avoiding powder adhesion and compaction, and is suitable for most dry powder materials such as cement, fly ash, and mineral powder. The pneumatic breaker is suitable for solving the problem of bridge formation. It uses instantaneous high-pressure airflow to impact the arched material layer， breaking the material support structure and restoring material flow.
For ultra-fine powders that are prone to compaction and caking, a vibrating discharger can be combined. Through small and high-frequency vibration, it continuously discharges the material to prevent accumulation and solidification. It should be noted that flow-aiding devices do not need to be blindly selected in large quantities. Based on the fineness, humidity, and storage conditions of the material, a single or combined configuration can be matched. Overloading equipment installation will instead cause resource waste.
How to reduce the risk of powder material blockage
Firstly, avoid long-term static storage of materials. Try to implement first-in-first-out, reducing the accumulation and solidification time of materials in the silo. During intermittent shutdowns of the production line, regular short-term discharging can be carried out to allow the materials in the silo to flow slightly, preventing compaction and caking.
Secondly, control the moisture content of the feed. Before the raw materials perform moisture detection. Moist and water-excess materials are prohibited from entering the silo to reduce the probability of moisture-induced blockage. At the same time, keep the ventilation and air-exchange system of the silo running normally to promptly discharge the humid air in the silo.
In addition, try to avoid frequent start-stop of discharging and overloading of storage. The material storage in the silo should not be too low or accumulate for a long time. Maintaining a reasonable material level can maintain a stable material flow pattern and reduce bridge formation and material stagnation problems.
What to do when powder materials block the silo
When a silo blockage occurs, avoid hasty operations. Incorrect methods of clearing can easily damage the equipment and even lead to safety accidents.
For minor blockages or intermittent material discharge, start the auxiliary flow equipment and use air flow and vibration to guide the materials. Most minor bridging and wall sticking problems can be directly resolved. After the clearing is completed, continue to discharge for a period of time to empty the unstable retained materials in the silo.
When there is a severe bridging or complete silo blockage, first stop the machine and cut off the power supply. Observe the blockage position through the silo inspection port and use special tools to gently clear it. Do not directly strike the silo body or hopper. Violent striking can cause the silo to deform, weld seams to crack, and create new storage dead corners.
If there is a large area of lumped blockage, gradually remove the lumped materials, thoroughly clear the discharge channel, and check if there is water accumulation or dampness inside the silo. Investigate the root cause of the silo blockage to avoid secondary blockage.
How to do the daily maintenance of powder material silos
Regularly check the sealing parts, ventilation valves, and dust removal equipment of the silo. Replace aging sealing components in time to ensure the silo is dry, without dust, and free from moisture intrusion. Regularly clean the accumulated materials on the silo wall to prevent the accumulation of materials from thickening and solidifying, changing the silo structure and forming dead corners.
At the same time, regularly check the operation status of the auxiliary flow equipment and discharge equipment. Repair faulty equipment in time to ensure the normal operation of the auxiliary flow and guidance functions. At regular intervals, conduct a comprehensive inspection of the silo interior, clear the retained materials in dead corners, and check for wear and deformation of the cone angle and discharge port.
In addition, keep records of the working conditions and summarize the blockage patterns of powder materials in different seasons and with different raw materials. Take preventive measures in advance to shift from passive clearing to active prevention.
Conclusion
The blockage of powdery materials in silos is not caused by a single factor. Instead, it is the result of the interaction of material properties, silo design, discharge structure, agitator equipment, and operation management. Compared to dealing with the blockage after it occurs, it is more important to fully consider the flow characteristics of powdery materials during the project design stage. Select appropriate silo structures, cone angles, discharge port sizes, and agitator schemes based on different materials, and ensure proper moisture control and equipment maintenance during daily operation. Integrating preventive measures throughout the design, construction, and operation processes can significantly reduce the risk of silo blockage and ensure the long-term, safe, and stable operation of the silo.