Factors affecting caking behaviour of powdersa

21st February 2013

Caking compromises the value of both raw materials and products in many processes. Tim Freeman shows how using powder rheometers can help to overcome the problem.

Caking, the formation of agglomerated material from discrete particles or granules, is a widespread problem in powder processing. Many feed materials, as well as the products used or manufactured by the food, chemical and pharmaceutical industries, are sold as free-flowing, easily processable powders. Caking compromises their value by adversely affecting either in-process or end-use performance.

Agglomeration can occur over time via a number of mechanical or chemical routes: one especially common mechanism is the migration or absorption of water. For achemical and process engineers, the key to its control lies in managing environmental conditions such that the feed or product remains in an optimal state. Described here, and illustrated by experimental studies, is the way in which powder rheometers can assist by measuring the changes in powder properties during the caking process - as a function of humidity and consolidation for example.

Essential to solids processing is the ability to control manufacturing variables in such a way as to produce powders with the required properties for specific applications. Packed into bags, kegs, bulk containers or tankers, these powders leave the manufacturing site in a relatively well-defined state. However, powder condition immediately before use may be markedly changed, by transportation and as the result of storage.

Caking is one of the primary mechanisms by which powders deteriorate when stored, and humidity, temperature and consolidation are all influencing factors. Water can cause limited dissolution of the material and the subsequent formation of crystal bridges, which bind primary particles together into larger agglomerates. Prolonged consolidation on the other hand may promote mechanical aggregation by physically forcing the particles closer together. The sensitivity of different powders to these effects varies considerably, so storage conditions that suit one material may be unsuitable for another.

Powder rheometry

Powder rheometers measure the dynamic properties of a sample, characterising flowability in a very direct way. Basic flow energy (BFE) is a key baseline measure and is defined as 'the energy required to rotate a helical blade down through a sample at a controlled rotational velocity. It is derived from precise measurements of both the axial and rotational forces acting on the blade as it passes through the powder (Fig. 1). Precision engineered instruments that use well-defined, automated test methodologies can measure BFE reproducibly, making it a highly differentiating parameter for powders across the entire cohesivity spectrum.

Agglomerate formation during caking changes the BFE of a sample, a number of mechanisms conspiring to increase BFE. Firstly, caking increases the strength of the inter-particulate bonds which have to be overcome before flow can occur. Secondly, the caked bulk is stiffer, the packing of the particles presenting greater resistance to movement. Fine powders tend to be relatively cohesive with the ability to trap air in inter-particulate pockets and as a result are often highly compressible, with an almost 'spongy' quality. With these materials, movement of the rheometer blade impacts only a small portion of the sample; the compressible powder easily absorbs the induced displacement and BFE is therefore low. In contrast, larger agglomerates pack more closely leaving little space for entrained air, so any displacement induced by the blade is transmitted very effectively through the bed, giving a much larger flow or transmission zone, and a corresponding increase in BFE.

Finally, if caking is associated with a change in moisture content it can increase BFE by changing the bulk density of the sample, as higher bulk densities often go hand in hand with a higher BFE. Together these changes make monitoring BFE a sensitive and productive way of detecting caking and assessing how it is influenced by environmental conditions, as the following study shows.

Consolidation on caking behaviour

Comparing BFE profiles as a function of time for samples held under different conditions helps engineers to identify the optimum storage conditions for a given powder. The results of one such investigation are displayed in Fig. 2. Here two sets of data are shown: one for a sample held under no applied consolidating load, the other consolidated by an applied pressure of 9kPa.

For this powder blend, a slight increase in BFE can be seen during the initial four day period for both the unconsolidated and consolidated powders. However, in the following days the BFE rises rapidly, in a more pronounced way for the consolidated powder. At five and a half, the consolidated powder is twice as resistant to flow as it was when loaded into the storage vessel, whereas the unconsolidated powder takes until day eight to achieve a similar state. In both cases, BFE continues to rise rapidly with no sign of a plateau.

In a bin and hopper the powder sits under the consolidating pressure of its own weight. The properties of this powder suggest that minimising this pressure will be beneficial in terms of caking, so how can this be achieved?

One option is to run the bin at a relatively low fill level, topping up at a regular frequency with lower volumes. This potentially confers advantage in two ways. Firstly it reduces the head of powder acting on the material in the hopper, where the material is under greatest consolidating pressure. Secondly it reduces the residence time of powder in the bin, thereby limiting the extent of caking. However this second point deserves closer scrutiny because in fact, residence time will only be uniformly reduced if the bin is running in a mass flow, rather than funnel flow regime.

With mass flow, all the powder in the bin is in motion: as material is withdrawn from the hopper, powder transits through the unit on a first in, first out basis. A hopper with walls steeper than a limiting value defined on the basis of the shear properties of the powder will deliver this performance. Where these criteria are not met, funnel flow can develop. The hopper angle is insufficiently steep to ensure that powder flows smoothly down the walls so material builds up (Fig. 3) and the residence time of powder within the hopper becomes non-uniform. Some powder remains in the vessel only briefly, entering the centre of the 'funnel' and exiting almost immediately. More importantly from the point of view of caking, other portions of the powder population remain towards the base of the vessel for considerable amounts of time, under the consolidating pressure of the material above. These conditions are ideal for caking and are to be avoided.

While the above example illustrates the importance of considering caking within the dynamic environment that applies in a feed bin, many associate the phenomenon more closely with the 'static' condition of powder stored for a certain period of time in, for example, a closed keg. Here, studies such as the one detailed earlier quantify the likely extent of a caking problem and inform decisions about how often the material needs to be tumbled or agitated to keep it in a fit state for subsequent processing.

Tim Freeman is Director of Operations with Freeman Technology, Welland, UK.


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