Powder Characterisation for Process Designers and Engineers

For process designers specifying new plant, the goal is to engineer equipment that will process and handle powders consistently and efficiently, as specified by the design brief. In contrast, engineers working as part of the manufacturing team rarely have the option of changing out equipment but strive to achieve acceptable operation using the existing plant. While these goals are somewhat different, the two groups share a need for detailed process understanding, knowledge of the interplay between powder properties and process equipment and how, in combination, they deliver product with the intended properties and quality. For the particulate handling industries this can be a significant challenge. To successfully design and operate powder processes, engineers need to determine the conditions to which a specific unit operation will subject the powder, and then measure the powder’s response to each of these environmental conditions.

Identifying process problems

Consider the example of a blend flowing from a hopper into the feed shoe of a die filling process. Each time the powder level falls below a certain point, the hopper is refilled with a new batch of feed material. However, for certain blends it is noticeable that for a short time before and after this refill, discharge flow becomes erratic, triggering process problems. When the hopper level is too low, or after recharging of the hopper, the stress in the powder at the outlet of the hopper varies. This translates through to inconsistent pressure in the shoe and variation in die filling performance with some, but not all blends.

Analysis of these relatively simple process steps reveals the variable conditions imposed on the powder. As the hopper fill level decreases, the normal stress acting on the powder near the outlet reduces and a stable arch forms, causing an interruption in flow. A stable arch may also form during recharging of the hopper, as high levels of consolidation are induced by the relatively high normal stress imposed by the additional powder. Should it be necessary to stop the process for any reason, consolidation by vibration from surrounding machinery may also become an issue. Here then, the response of the powder to consolidation, by direct compression or vibration, and its cohesive strength is highly relevant. If consolidation brings about a major change in flow properties, then problems are more likely to arise.

Rationalising powder performance

Universal powder testers incorporate bulk, shear and dynamic measurement[1] in a single instrument, and for engineers offer an intuitively sensible approach. They allow measurement of the powder in motion and permit the analysis of samples in a consolidated, moderately stressed, aerated, or fluidised state. Comparing the flow energies of conditioned samples with those of samples consolidated by compression or tapping gives a consolidation index (CI). Quantifying the response of the powder to consolidation in this way provides the insight necessary to rationalise the processing behaviour outlined above.

Comparative studies of the die filling performance of two different powders, A and B, provide an illustration of this point. Sample A (Aluminium powder) has a CI (tapped) of 1.43 while that of sample B is 2.32 (Tungsten powder). This indicates that B, a cohesive material with very fine (4 microns) angular particles, is significantly more affected by vibration than A. Die filling trials confirm that the performance of sample B deteriorates significantly if it is consolidated, as would be expected. For example, at a shoe speed of 50 mm/s, Filling Ratio falls from over 90% to less than 50% as a result of vibrational consolidation (20 taps), where Filling Ratio is the mass of powder in the die after filling relative to the mass associated with a completely full die. In contrast, Sample A demonstrates much more robust behaviour whereby filling performance is approximately the same before and after consolidation, at an equivalent shoe speed. 

In this case, a designer with access to the information provided by the powder tester has options - specify a more accommodating hopper, with more steeply angled walls or a larger outlet; pursue a policy for reducing equipment vibration; and/or install additional mechanical aides for rectifying blocked hoppers. This same information leads the manufacturing team to better operational practice with respect to hopper filling and an improved response in the event of blockage. Refilling the hopper more frequently with smaller quantities of feed is likely to be one of the best ways of reducing process upsets. For both groups it is detailed and relevant powder testing that provides the information needed to effectively manipulate either design parameters or operating practice to achieve manufacturing goals.

Reference:

[1] Jamie Clayton. Powder characterization for effective powder processing. Processing Magazine [Online] 2020. Available from: https://www.processingmagazine.com/material-handling-dry-wet/powder-bulk-solids/article/21129657/powder-characterization-for-effective-powder-processing  [Accessed 24th June 2020].