Bridging and ratholing are common problems in bulk material handling, and some materials are notorious for poor flow properties. Learn about the most challenging bulk materials and see proof of why AirSweep is the best flow aid for the job.
What are bridging and ratholing?
Bridging happens when bulk materials form a stable arch above an outlet, completely stopping the flow. This usually occurs because of particle cohesion, moisture, or poor hopper design.
Ratholing creates a narrow channel through the material, leaving stagnant product clinging to the container walls. It’s common with materials that have poor flow properties or stick easily to surfaces.
Both problems cause production stoppages, material spoilage, and costly manual intervention—headaches no plant manager wants.
What materials are prone to bridging and ratholing?
Flow problems can be caused by many factors, including vessel design or plant environment (like humidity and temperature). However, these materials have inherent properties that affect how they behave when they are stored, transported, or mixed.
1. Cement and Fly Ash
Cement and fly ash are ultra-fine powders that easily absorb moisture. This leads to clumping, compacting, and cohesion that quickly cause bridging. When stored under pressure, these materials become even tougher to handle because the pressure forces particles closer together, increasing their density and cohesion. The compacted material can form stable arches or stick tightly to hopper walls, making flow nearly impossible without external intervention.
Watch: AirSweep Cement Mix Material Flow Test
2. Food Powders
Most food or beverage processing facilities or commercial bakeries use fine powders like flour, cornstarch, cocoa, and milk powder. These powders are hygroscopic, or readily absorb moisture from the air. Flour and starch compact under their own weight, while cocoa and milk powder stick to hopper walls due to moisture or static charges. These properties make bridging and ratholing and bridging frequent problems.
Food and beverage manufacturers also have to meet strict sanitation standards. Stagnant material can become spoiled, and manually clearing out material blocks may cause contamination.
Watch: AirSweep Moist Cornstarch Material Flow Test
3. Chemicals and Fertilizers
Many chemicals and fertilizers absorb moisture, leading to cohesion and clumping. Granular fertilizers often bridge because of uneven particle sizes, while fine chemical powders compact or stick to equipment surfaces.
Aside from slowing down operations, flow interruptions in fertilizers and chemicals disrupt batching accuracy and downstream processes like mixing and packaging.
4. Plastic Pellets and Resins
Plastic pellets and resins used in plastic production and packaging pose unique challenges. Smooth, spherical pellets interlock under pressure, causing bridging. Irregularly shaped compact and form stable arches in hoppers.
Recycled plastic and resins also pose more challenges. They have varying particle sizes and shapes, increasing the chances of bridging and compacting. As “green manufacturing” becomes a bigger priority because of sustainability goals, reliable flow systems ensure that recycling operations remain efficient, sustainable, and cost-effective.
Watch: AirSweep Plastic Regrind Material Flow Test
5. Coal and Aggregates
Coal and aggregates are typically used in energy production, mining, and construction. These materials often have irregular particle sizes and varying moisture content. Fine coal dust, in particular, sticks together due to cohesion and compaction. Material blocks can choke boiler feed lines and delay energy production—a costly problem for any plant.
6. Metal powders
Metal powders are extremely fine, dense, and prone to static charge buildup. These properties cause bridging, compaction, and ratholing, particularly in precision processes like additive manufacturing.
7. Sawdust and Wood Chips
Sawdust and wood chips are typically used in paper and pulp industries, wood processing, and biomass energy production. They are lightweight and irregularly shaped, which makes them prone to interlocking and bridging in hoppers and silos. Their fibrous nature also causes the material to compress and compact over time, creating blockages that restrict flow.
Watch: AirSweep Sawdust Material Flow Test
Solve bridging and ratholing for good
Do you work with a tough material that is prone to bridging and ratholing? Have you tried other flow aids but still resort to manual cleaning or hammering the vessel? AirSweep provides a proven solution to keep your materials flowing, eliminate costly disruptions, and maximize productivity. To learn how AirSweep can improve your plant’s operations, contact us for a consultation.
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