air retention capacity

Fine-dispersed, dry powders consist of particles surrounded by a wafer-thin layer of air. This enclosed air significantly influences the flow and compaction behaviour of the bulk material. The term "air retention capacity" refers to the ability of a powder to retain enclosed air in its bulk. This behaviour varies significantly depending on the particle size, shape, surface properties and density of the solid particles.

A pronounced air retention capacity results in a low bulk density and reduced packing density. The powder appears "loosened" or "fluffy". If the powder is mechanically agitated – for example, in a mixer, during pneumatic conveying or during filling – the air between the particles can cause temporary fluidisation. In this state, the solid particles behave similarly to a liquid: they can move relatively freely against each other and exhibit low internal friction.

Powders with high air retention capacity pose particular challenges in terms of handling. They tend to compact poorly and retain a large volume after filling. This makes precise dosing, packaging and transport difficult. In industrial practice, this can be countered by taking specific measures. For example, filter immersion probes are inserted into bags or containers to allow the air trapped during filling to escape in a controlled manner. 

In most cases, it is desirable to reduce the air retention capacity sustainably through compaction.

Fluidizable powders can be compacted in the amixon® mixer. This is achieved by means of vacuuming. In this state, the powders can be mixed and wetted evenly. This causes them to lose their air retention capacity. Granulation in the amixon® ring layer mixing granulator RMG is a continuous process. This process also serves to compact the material.

Another method is compression. Roller presses are used for dry granulation. In this process, fluidisable powders are compressed under high pressure to form pellets and then crushed. This produces granule-like, free-flowing powders with a defined density that do not require any binding agents or subsequent drying.

Understanding and controlling air retention capacity are therefore important aspects in the process engineering of powder mixing, granulation and bulk material processing as a whole. They influence process control, product quality, dosing accuracy and the cost-effectiveness of mixing and conveying processes.