Powder Consolidation - Why & How


by Robert G. McGregor

Marketing/National Sales Manager – Laboratory

Brookfield Engineering Laboratories, Inc.

Tablet making machines can suddenly jam in the middle of a high speed production run. What’s the reason? One cause is the consolidation of the powder in the feed hopper which means that the cohesive strength of the powder to stick together is stronger than the force of gravity driving it down into the feeder. The inevitable consequence is erratic flow behavior or possibly a total stoppage of flow altogether. It’s time to bring out the baseball bat and bang that hopper.

Bags of fine quality powder, perhaps cocoa or starch for example, can be produced to spec and hauled by truck to distant destinations, far away from the processing plant where originally manufactured. Upon arrival, however, the powder has hardened or consolidated into clumps in the sacks that contain the cargo. Discharging the powder into feed bins is problematical, but that’s only the beginning. The clumps may break up, but smaller clumps may still exist and cause blockages which eventually affect discharge of the powder from the feed hopper.

Piles of pulverized powder materials, such as limestone, are stockpiled outdoors until ready for use. The material on the surface for all practical purposes is free flowing. The powder in the middle of the pile may have consolidated and be hard to move. How fast does this happen and does it cause downstream problems for subsequent processing operations?

Consolidation of powder occurs naturally because the weight of the material on itself (self-weight) causes a gradual settling action. The fundamental questions to answer are two-fold:

1. What is the nature of the powder to begin with? Is it a free-flowing material or is it cohesive?

2. Over time how fast does it consolidate and change its flow behavior? Is there a tendency to become more cohesive or possibly, worst of all, non-flowing?

There is a proven scientific method for making these determinations. Shear cell technology (See Figures 1 and 2) has been around for several decades and provides the predictive means for defining whether a powder is cohesive. The ASTM has documented the procedure in D6128. The powder is placed in a trough, compressed to a defined compaction force, and sheared to measure how much strength it has. This tells how much force is required to cause the powder particles to slide against each other. If the force required is greater than the self-weight of the powder, then trouble awaits. (See Figure 3) When the test is repeated at subsequent time intervals, the upward shift in the flow function quantifies an increase in cohesiveness, which portends greater difficulty with flow behavior. (See Figure 4)

Understanding the science is the first step toward solving the problem of erratic flow behavior. Knowing that there is proven instrumentation which can help to avert the problem is the second step (See Figure 5). The recent breakthroughs with this instrumentation include ease of use, rapid data output due to shortened test methods, and less expensive models. These advancements are moving powder flow testing into QC departments where everyday measurements can help to avoid serious operational problems.

It’s time to check out what’s happening in the world of powder flow measurement and begin predicting potential flow issues before they cripple your plant.

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Fig. 1: Shear Cell Contains Powder To Be Tested

Fig. 2: Vane Lid Compresses Powder and Causes Consolidation

Fig. 3: Flow Function Shows How Much Strength a Powder Achieves When Consolidated

Fig. 4: Time Consolidation Test Shows How Quickly a Powder Gains Strength

Fig. 5: Powder Flow Tester Provides QC Test Capability To Measure Powder Flowability


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