Challenges in solid aggregate handlingSeth Price | September 25, 2022
In many industrial processes, powders, agricultural products, crushed raw materials or other ground solids must be fed through hoppers, feeding tubes, grates, silos and other processing equipment. Everything from cement to metal powders to grain are considered part of the solids handling world.
Hoppers can feed solids to other process equipment using gravity or through pressure, by adding a pressurized gas to create a fluidized bed. The hoppers may have augers or mixing paddles to break up clumps, homogenize components or to aid in feeding.
Solids handling techniques speed up production by delivering raw materials to processing equipment, but it is not without its safety hazards and technical challenges.
Practical issues with solids handling
Besides safety hazards, there are several common solids handling problems that can cause erratic flow, overflow conditions or generally cause troubleshooting headaches and delays to production. The inconsistent feed makes predicting throughput difficult, and can wreak havoc in a control loop that uses a level control or the feed rate to decide when to add more material to the hopper.
Perhaps the most common problem is clogging of orifices and feed ports. While there are many causes of clogs, there are several failure modes worth discussing separately. General clogs lead to inconsistency in feed rate and false level control signals.
Clogs are particularly common with particles of varying sizes and shapes, such as in crushed materials. Spherical materials of consistent particle size have significantly fewer clogging issues, but many materials cannot be spheroidized. Often, they can be screened to ensure a relatively consistent size distribution, so this should be used whenever possible.
Ratholing is when the center of a hopper empties, yet material still clings to the sides. It is sometimes called pipelining or channeling. This will cause erratic flow, as sometimes material will slide into the “rathole” but other times it won’t. It is often difficult to detect as well, because the material stuck to the sides of the hopper can fool level sensors. It can even stick to sight glasses, so technicians think the hopper is fuller than reality.
This phenomenon is most common with irregularly shaped particles that may interlock, sticking together rather than flowing freely. It is also common in particles that are hydroscopic, where water is absorbed from the air and helps clump particles together. Ratholing is an effect of stagnant motion in the hopper.
Spherical particles are less likely to rathole. If the nature of the process does not allow for rounded particles, ratholing can be limited by keeping powders dry. Hoppers can be tapped mechanically to cause material to fall from the hopper walls. Inert gas can be injected at varying flow rates or locations to mix things up a bit.
Closely related to ratholing is arching or bridging, where material falls from the center, but a bridge forms over the top of the hole. It has similar effects on production, varying flow rates. It also has similar remedies, as changing feed locations and speed, agitation, tapping and other such processes. These actions will cause the arches to fall.
Denser and larger particles tend to settle out of a fluidized bed first. With the wrong combination of feeding flow rates and particle size distribution, it is possible to have the material segregate, where less dense material travels farther, faster and remains suspended in air (or a process gas) for a longer time. Segregation’s biggest effects will depend on the specific process, but it may result in only certain particle sizes passing through and participating in the process, while denser particles build up, eventually clogging the upstream parts of the process.
Tighter particle size distributions are less likely to segregate noticeably. Screening particles to narrow the particle size distribution and limiting moisture to reduce aggregation will minimize the effects of segregation.
Besides feed problems, all solids handling equipment is also subject to erosion. Anyone who has ever used a sandblaster or bead blaster knows that high pressure gas propelling particles lead to wear on the surface — that’s the point of these devices. Wearing away paint, grease, rust and so on is great in a sandblaster, but not so great along a pipeline transporting sand into a furnace at a glass manufacturer.
Erosion is a fact of life, but it can be limited by reducing the angle of turns in pipelines and lowering particle velocities. Pipes at right angles will erode much more quickly than pipes with shallow angles. Also, rounded particles and soft particles will lead to much slower erosion than jagged or hard particles.
Any flammable substance that is ground to a fine enough powder is still flammable, and if contained, explosive. Explosive mixtures of powder and air in hoppers can be a major safety hazard, particularly in closed hoppers.
Dispersed powders can quickly form an explosive mixture. Virtually anything, when ground up to a fine powder, can explode. Dust explosions require five elements: a combustible dust, dispersion of the dust, containment, an oxidant and an ignition source. Any dust not ignited in the initial explosion is dispersed by the shockwave, leading to secondary and tertiary explosions. The Dust Explosion Pentagon shows these ingredients, and is commonly used as a reminder of the hazards.
Unfortunately, many solids handling hoppers and lines provide all of these elements. Powders are flammable, moving around (and getting dispersed) and often contained in a hopper, tube or pipe. Atmospheric oxygen is often enough of an oxidant for dust explosions. As for the ignition source, any open flame, hot surface, or static discharge can set off the explosion.
A number of techniques are used to remove some of these elements from the dust explosion equation. Some hoppers are backfilled with nitrogen or argon instead of air to remove the oxygen from the environment. Grounding of all devices, including level sensors, mixers and other pieces limits one ignition source. Explosively actuated vents reduce the pressure spike and direct it in a safer direction, should the dust ignite.
Another major hazard is present when there is a lot of material, such as in a grain silo. In certain instances, workers may be tempted to walk onto a pile of grain or a pile of aggregate material for one reason or another. This is incredibly dangerous.
The material may only pack at 40% leaving large voids in the pile of aggregate. As the worker walks across the surface, the void space could collapse, pulling the worker in and burying them in the material. This has killed farmers, firefighters, coal power plant workers, and many others. Workers should not walk on loose material, and firefighters (on site or in the community) should be trained in safe fire-fighting techniques in grain silos, storage bins and aggregate piles.
Solids handling has always been a part of human history, and will continue to play an important role in industrial processes. By thorough process design and proper training, powders can be handled safely and effectively.