Understanding powder caking

By Tim Freeman, Freeman Technology

Caking, or unwanted agglomeration, presents major problems in powder processing, capable of making even those powders that are normally free-flowing and easily processable, challenging. Its control depends on understanding the influence of, and closely managing, environmental conditions. Modern powder testers help make the link between different variables and the tendency towards caking, providing useful information for the determination of an optimal environment.

A significant number of materials in the food, chemical and pharmaceutical industries, from raw feeds, additives and intermediates through to manufactured products, are sold as free-flowing, easily processable powders, however caking can adversely affect their performance. Caking (agglomeration) occurs via a number of mechanisms, both mechanical and chemical, with the migration and absorption of water often the most influential. Control can be achieved by managing environmental conditions in a way that keeps feed or product in an optimal state. Powder rheometers can assist by measuring powder properties that indicate the progression of caking – as a function of humidity and consolidation for example enabling a greater understanding of which factors are most influential at different stages of processing.

Why powders cake

Powdered materials are generally susceptible to caking. Humidity, temperature and consolidation are all influencing factors. Changes in temperature can result in condensation. Humid conditions and condensed water can cause partial dissolution of powder particles, forming crystalline bridges between them to create agglomerates. Prolonged consolidation, during which external pressures lead to particles being quite literally forced together, can lead to mechanical aggregation.

To control the influence of chemical and mechanical agglomeration, it is necessary to manage environmental conditions during processing and storage. In addition, to ensure that powders remain in an optimal condition until they are used, the choice of packing containers and type of transportation should also be assessed and managed to ensure the product maintains those properties required for its final application.

Through testing and understanding the behaviour of each material, it becomes possible to assess and minimize the risk of caking at different points in the process, to maximize and maintain product quality. Powder testing results can inform, for example, decisions about how often the material needs to be tumbled or agitated to keep it in a fit state for subsequent processing and whether it can maintain its quality if packed in bags, kegs, bulk containers or tankers.

Modern powder testing techniques deliver an array of powder properties data. Using a powder rheometer it is a relatively quick and easy task to gather data that describe, for example, the impact of consolidation on the rate and severity of caking under conditions analogous to those found in a plant or storage facility. Through appropriate testing it is therefore possible to determine which conditions promote caking in specific materials. This enables the establishment of the manufacturing and storage protocols most appropriate for the materials being processed.

Powder testing

Powder rheometers measure the dynamic properties of a sample, characterizing flowability in a very direct way. Basic flow energy (BFE), defined as ‘the energy required to rotate a blade down through a sample at a controlled rotational and vertical velocity’, is a key baseline measure. BFE is derived from precise measurements of both the axial and rotational forces acting on the blade as it passes through the powder (see figure 1).

Figure 1: Measuring BFE with a powder rheometer

Precision engineered instruments using well-defined, automated test methodologies 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 for a number of reasons:

  • Caking increases the strength of the inter-particulate bonds that must be overcome before the material will flow
  • The caked bulk is stiffer, the close packing of the larger particles presenting greater resistance to movement
  • If caking is associated with a change in moisture content it can increase BFE by altering the bulk density of the sample; higher bulk densities often go hand in hand with a higher BFE

Testing in practice…

The impact of consolidation on caking of a powder blend

Several samples of a blend were taken and stored for different periods of time, ranging from zero to ten days. For every sample stored without consolidation, another sample was stored whilst being subjected to compaction with a consolidating stress of 9kPa. Tests were carried out on each sample and the resulting BFEs are shown in Figure 2 (as a function of time).

 

Figure 2: Investigating the impact of consolidation on caking by tracking changes in BFE as a function of time

For the powder blend under test, there is a slight increase in BFE during the initial four day period, in both the unconsolidated and consolidated samples. However, in the following days the BFE rises rapidly, in a more pronounced way for the consolidated powder. At five and a half days, the consolidated powder is twice as resistant to flow as it was when first loaded into the storage vessel, whereas the unconsolidated powder does not reach a similar state until day eight.  In both cases, the BFE continues to rise rapidly with no sign of a plateau. These observations indicate that there will be advantages in storing this blend under conditions of low applied stress and that storage periods of more than four days should be avoided.

What are the practical consequences?

Consider the results of this study in the context of selecting a storage bin for this blend. When resident in a bin and hopper, the powder sits under the consolidating pressure of its own weight. The properties of this powder blend suggest that minimising this pressure will be beneficial in avoiding caking, so how can this be achieved?

Running the bin at a relatively low fill level, while topping up more frequently with lower volumes, is one option. Not only does this reduce the head of powder acting on the material in the hopper (where the material is under greatest consolidating pressure), it also shortens the powder’s residence time in the bin, limiting the opportunity for caking. However this second point deserves closer scrutiny because residence time will only be reduced uniformly if the bin is running in a mass flow, rather than funnel flow, regime.

Mass flow means that all the powder in the bin is in motion: 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 (subject to the outlet diameter being sufficiently large also). Where these criteria are not met, funnel flow can develop. The hopper angle is not steep enough to ensure the smooth flow of powder down the walls. Material builds up (see figure 3) and the residence time of powder in the hopper becomes non-uniform.

 

Figure 3: Comparing powder transit through a hopper under A) mass flow and

B) funnel flow conditions

Some powder remains in the vessel only briefly, entering the centre of the ‘funnel’ and exiting almost immediately. More importantly where caking is concerned, some powder remains towards the base of the vessel for a considerable time, under the consolidating pressure of the material above, creating ideal conditions for caking.

The example described demonstrates the importance of considering caking in the dynamic environment of a feed bin. However, caking is also closely associated with the ‘static’ condition of powder stored in, for example, a closed keg. Here, similar studies can quantify the likely extent of a caking problem and provide data that inform decisions about how often the material should be tumbled or agitated to maintain its good condition for subsequent processing.

Continuing management

Since caking is a problem that can affect the in-process or end-use performance of a material, it has the potential to significantly diminish a product’s value. BFE, a dynamic powder property measured using a powder rheometer, can be used to investigate the impact of consolidation on the rate and severity of caking. An analogous approach enables quantification of the effect of other environmental variables such as humidity, and can also be used in determining the effectiveness of anti-caking agents. It therefore supports chemical and process engineers in their efforts to limit caking and develop optimal storage conditions.