This article discusses two major mechanisms of segregation: sifting and fluidization segregation.
What is Sifting Segregation?
Sifting segregation occurs when small, fine particles move through large, coarser particles. This is the most common means for particles to separate. In order for sifting to occur, they material must be:
* easy flowing
* have a range of particle sizes
* have fairly large (> 100 mesh) particles
* have some means of interparticle motion (such as forming a pile).
The particles segregate in a horizontal or side-to-side segregation pattern as shown in this photo of a stockpile of ore. (see Sifting segregation image) Notice how the fines concentrate in the center while the coarse particles move to the perimeter.
What is Fluidization Segregation?
Fluidization segregation affects fine particles, as they tend to remain airborne longer because they are less permeable to air than coarser or heavier particles. As a tank is being filled, the fine, light material tends to settle on top, while the coarse, heavier particles fall rapidly, creating a vertical or top-to-bottom segregation pattern.
The type of flow pattern that develops in your tank - funnel flow or mass flow - effects segregation.
What are Funnel Flow and Mass Flow?
There are two types of flow patterns that can develop in a silo or tank: funnel flow and mass flow. Funnel flow occurs when some of the material in your silo moves while the rest remains stationary. The walls of the hopper section of your silo are not steep or smooth enough to overcome the friction that develops between them and your bulk solid. Funnel flow can result in decreased live or usable capacity, stagnant material that can cake or spoil, and segregation problems, and/or, in fact, can cause structural failure. The first material that enters your silo is usually the last material to discharge. (see funnel flow image)
Mass flow occurs when all the material in your tank is in motion whenever you discharge product. The hopper walls are steep and smooth enough to overcome the friction that develops between the wall surface and bulk solid, ensuring flow along them. Remember, because all the material is moving, stable ratholes cannot form, making mass flow suitable for cohesive solids, powders, materials that degrade or spoil, and especially solids that segregate. (see mass flow image)
Solids flow property tests identify how a solid will flow and allow you to design a reliable silo or hopper to store and discharge your product. Always keep in mind that the feeder design is as critical as the tank design. They must work in unison!
How Do Sifting and Fluidization Segregation Affect My Process Operation?
Each of these segregation mechanisms can affect your process in different ways. For example, in all likelihood, your process requires a uniformly mixed product to ensure quality. As you load your product into its silo, perhaps it segregates by the sifting mechanism. If the silo or tank exhibits a funnel flow pattern (some material moves while most remains stagnant, likely at the walls) at some point in your process, you will discharge all fine particles or all coarse particles, which will affect your process negatively. The reason this is due to the preferential flow channel that forms, usually above the outlet, causing the center (fine particles) to empty first. As such, funnel flow exhibits a first-in-last-out flow sequence.
As another example, a typical approach to filling silos is to use a pneumatic conveying system to transfer product from trucks, railcars, etc. Typically, a pneumatic conveying line enters the silo in the top, center. This means of filling silos causes the coarser, heavier particles to be driven to the bottom, while the finer, lighter particles remain airborne and settle on top. Perhaps you have designed your tank for mass flow (all the material moves whenever any is withdrawn — flow at the walls). If your product segregates top-to-bottom, and you are discharging from your tank to bulk bags or boxes, most of the containers will be filled with coarse particles, while the last few containers will contain all fines. This will obviously affect your product quality and costs. Although in many handling situations, a mass flow pattern is preferred, in this case, mass flow enhances segregation. Mass flow exhibits a first-in-first-out flow sequence, allowing the product at the bottom (coarser or heavier particles) to discharge first.
A Mass Flow Approach to Solving Your Sifting Segregation Issues
Using a mass flow pattern will not only prevent the flow problems typically experienced by funnel flow (ratholing, flushing, etc.) but it will minimize sifting segregation effects.
Because of the first-in-first-out type flow sequence, the side-to-side segregated particles will be remixed as they discharge from the outlet. One thing to keep in mind about mass flow is that as you empty your silo, and the head of material reaches the hopper section, a velocity gradient is created, causing the center to empty faster than the material at the walls. The flow pattern is still considered mass flow. However, your product may segregate slightly due to this velocity difference.
A Mass Flow Approach to Solving Your Fluidization Segregation Issues — How Can This Be?
As stated above if you experience top-to-bottom (fluidization) segregation, a mass flow pattern will be a detriment. However, there is a way to prevent this problem. Simply change the way the product enters the silo from top, center to entering tangentially somewhere near the top. As the material enters from the side, it swirls around the tank walls and segregates side-to-side. A side-to-side segregation problem is corrected by using mass flow.
Another approach would be to use a specially designed letdown chute as shown. Although used mostly to minimize particle attrition, this device can be used to minimize fluidization segregation by forcing the material to remain in contact along the chute. As the particles flow along the chute, their velocities are slow enough to prevent separation of fines and coarse particles by fluidization. (see letdown chute image).
There are other methods to resolving segregation issues not listed here. Keep in mind that in order to address a segregation problem, you must know how your material flows. Laboratory tests are available to identify your material’s flow properties such that using this information wisely will result in segregation free flow.
Joseph Marinelli is a consulting engineer and president of Solids Handling Technologies. He has been providing testing and consulting services since 1972. As a former consultant with Jenike & Johanson Inc., he has years of experience testing powders and designing bins and feeders for reliable flow. He lectures frequently on the topic of powder handling, and has published several papers, including and article in a chemical encyclopedia and two in a food powder book. For more information, contact Joe at 803-802-5527 or [email protected]
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