Eliminating Segregation Effects From Hoppers And Bins

segregation (seg'n-ga'shen) n. In powder handling, the unintended act of separating a free-flowing particulate mixture's dif-ferent size, shape or weight components through one or a com-bination of innate or pro-cessing equipment mechanisms that may result in out-of-spec pro-ducts, and dosage or weight variations in con-sumer packages.

Segregation in bins is most often associated with free-flowing particulate solids with a significant particle size range, particle weight range or particle morphology (shape, roughness, angularity) range.

Although several segregation mechanisms contribute to batch and product inconsistencies, there are two basic and common segregation patterns that occur in bins and hoppers. The first, and most common pattern, occurs when a falling material spreads outward from the deposition point and hits the edges of the bin. This center-to-side segregation pattern allows fines to concentrate in the center of the bin and coarser particles to con-centrate on the outside of the bin. This is typical of a free flowing solid with a particle size range of three times or greater, or a solid with particles of differing roughness.

This type of segregation pattern may also take on the opposite form where coarser particles concen-trate in the bin's center and fines move to the out-side walls. This usually occurs with materials that contain less than 10 per-cent fine powder. The fine powder becomes airborne and is carried to the bin walls where it accumulates.

The second common segregation pattern occurs from top-to-bottom. The finer component fraction settles in a layer on top and the heavier, larger component penetrates this layer of fluidized fines and settles on the bottom. Sometimes these com-ponents settle in layers, as is frequently the case with material that is conveyed into the bin in batches us-ing a pneumatic system or a batch blender. This top--to-bottom segregation pat-tern is usually associated with mixtures that by vol-ume contain 20 percent or more powder.

It is not uncommon for both center-to-side and top-to-bottom segregation patterns to occur at the same time.

Solutions to segregation patterns

If center-to-side segregation is prevalent, a Diamondback Technology offset cone mixer placed above a DBT anti-segregation baffle will first blend the incoming material stream, then disperse it across the material's top surface and prevent any significant angles of repose. This configura-tion works well when the bin is nearly full or within one bin diameter below the baffle. Unfortunately, a baffle will not work for top-to-bottom segregation and will, in fact exacer-bate it.

J.R. Johanson's Diamondback Hopper eliminates unwanted product segregation

It is also im-portant to recognize that center-to--side segrega-tion is dif-ficult to pre-vent. However, selecting a hopper that provides a first-in/first--out flow will keep the particle size mix-ture existing in the bin the same as when it first entered the bin. A one-dimensional convergence Arch-breaking Diamond-back Hopper with no moving parts, con-nected to a vertical cylin-drical bin, will achieve this. If steep enough, this hopper can provide a true first-in/first-out flow from the first bit of material entering the hopper to the last bit exiting the hopper, no matter what the solids level is in the bin or hop-per. This phenomenon cannot be duplicated with a conical hopper.

First-in/first-out flow will exacerbate top-to-bottom segregation. Consequently, a Diamondback Technology anti-segregation let-down chute is essential to pre-vent top-to-bottom segre-gation. This chute keeps the material in contact with the material surface dur-ing its descent into the bin, and consequently, prevents the aeration associ-ated with mat-erial free-falling into a bin. With-out entrained air to fluidize the fines, coarse particles remain within the mass of particles and do not penetrate the top layer of fines. Unfor-tunately, the chute does not eliminate center--to-side segregation.

The segregation pattern with a typical material containing a wide range of particle sizes usually in-volves both center-to-side and top-to-bottom segregation. Consequently, the only way to eliminate a severe segregation problem is to use a combination of an anti-segregation let-down chute and an anti-segregation hopper (e.g. the Arch-breaking Diamondback Hopper) with its uniform first-in/first-out, flow pattern.

The Johanson Hopper Index, derived from the Hopper Indicizer material characterization tester, can help select the right anti-segregation Diamondback Hoppers for most applica-tions.

Using the Johanson Hopper Index

The Johanson Hopper Index (HI), which is calculated by the Johanson Hopper Indicizer, is the recom-mended mass-flow conical hopper angle measured from the vertical. In addition to predicting chute segregation, the Index can help select the right Arching-breaking Diamondback Hopper or assist is designing a minimum height hopper. (Note: The hop-per model ABD-20 or ABD-35 number refers to our catalog selections.)

For example, if 5 < HI < 15, then an ABD-20 series hopper must be used. If HI > 15, the ABD-35 series Diamondback Hopper will also work and will use less headroom.

The Hopper Index is also helpful in designing an anti-segregation minimum height hopper with the height H calculated as follows:

Where:

d = Hopper outlet diameter
D = Bin diameter
HI = Hopper Index for the hopper wall material
A = 20, B = 30 when HI > 10 deg.
A = 15, B = 25 when HI < 10 deg.
(HI + A) < (HI + B) < 45

For more information: Jerry Johanson, president, J.R. Johanson, Inc., 1237 Archer St., San Luis Obispo, CA 93401. Tel: 805-544-3775. Fax: 805-549-8282.