Suspending

In process engineering, suspending refers to the dispersion of insoluble powders in a liquid.

Producing a homogeneous suspension can be difficult:

1. if the powder particles are very small
2. if the powder particles are poorly wettable (hydrophobic)
3. if the liquid has a high viscosity
4. if the density of the liquid differs from the density of the powder particles
5. if the suspension process is carried out on a large (industrial) scale

Regarding 1)

A powder consisting of dust-like particles has an extremely large surface area. Such powders tend to stick together and form agglomerates. The surrounding air is located between the dust particles. This property is referred to as the porosity of the powder. If you try to mix the dust in water, it can only be done if high shear forces and turbulence are generated. In practice, this is done with the help of mixing tools that resemble a ship's propeller.

Regarding 2)

Hydrophobic powders resist wetting with water. On a hydrophobic surface, water would bead up or form large drops. In this case, it may help to reduce the surface tension of the water, for example by adding a surfactant. A homogeneous suspension can only be created when shear and turbulence occur.

Regarding 3)

If the liquid has a high viscosity, it is more difficult to mix it completely. A propeller stirrer would not be sufficient. A spiral stirrer would be suitable. It may be necessary to provide an additional heavy-duty stirrer at a high speed.

Regarding 4)

Large differences in density make a suspension inhomogeneous. Depending on the situation, the powders either float to the surface of the liquid or settle to the bottom of the mixing container. Both effects are used in water clarification / water treatment.

• So-called flocculation agents are added to the water. These combine with the dirt particles and float up. They can be separated on the surface.
• In the other case, the chemicals introduced combine with the dirt particles and settle to the bottom of the mixing tank. The water above is clean.

Regarding 5)

Large agitators can block if the density of the powder is higher than the density of the liquid. This can happen when the agitator is switched off. The powder then sinks slowly to the bottom. There it can build up to a very high packing density. The sludge at the bottom condenses into a solid mass. After a long time, the agitator is blocked and can no longer be operated.

The situation is comparable to the solidification of powder at rest in a silo.

amixon^® mixers can be used to process challenging and difficult suspensions. In practice, this is done, for example, during fermentations. For large batches, two systems from amixon^® can be used: the conical mixer and the Gyraton^® mixer. Both systems can be used in a variety of ways, especially when large volumes are involved.