A gamut of applications

Mixing is the bringing of dissimilar substances into contact with each other, whereby at least one of the substances is a liquid. The mixing process is achieved through movement and velocity differences in the liquid.

Stirring is the mechanical agitation of the liquid in order to achieve mixing.

Applications

Stirring and mixing is a basic operation that has a huge variety of industrial applications. We can classify mixing and stirring processes based on the phases present. However, stirring and mixing processes always have the presence of a continuous liquid phase in common. In addition, a solid phase (powder, granules) may be present, as can a second liquid phase (droplets), a gaseous phase (bubbles) or a combination thereof.

Blending and homogenizing

In the simplest application, this involves miscible liquids. This is called blending or homogenizing. An example is the mixing of different batches of oil.

Suspending

Stirring can also serve to keep solid particles in motion in a fluid. This is called suspending. Examples are suspension polymerisation (used in the manufacture of plastics), the dissolving of salts or the leaching out of metal ores.

Dispersing and emulsifying

In liquids that are immiscible, it is possible – by stirring – to distribute the one liquid into the other in the form of fine droplets. This is called a liquid dispersion. Dispersing occurs in polymerizations (styrene in water to make polystyrene), extraction processes (e.g. in copper extraction) or in the food industry (coffee creamers). A stable liquid dispersion is called an emulsion. Emulsifying can be achieved by stirring so hard that the particles become very small and/or by adding a surface-active substance.

Gas dispersion

By means of stirring, one can also achieve intensive contact between a liquid and a gas. This is called gas dispersion and examples of its use are in water purification, the manufacture of antibiotics and in the "hardening" of oils.

Stirring and mixing are always obtained by bringing about flow rate and velocity differences in the liquid. This costs energy. Therefore, one of the challenges for mixing and stirring technology is to create a mixing process that is both efficient and low-energy.