In industrial process engineering, mixing is a unit operation that involves manipulating a heterogeneous physical system, with the intent to make it more homogeneous. Familiar examples include pumping of the water in a swimming pool to homogenize the water temperature, and the stirring of pancake batter to eliminate lumps.
At an industrial scale, efficient mixing can be difficult to achieve. A great deal of engineering effort goes into designing and improving mixing processes. Mixing at industrial scale is done in batches (dynamic mixing) or with help of static mixers.
Typical example of a mixing process in the industry is concrete mixing, where cement, sand, small stones or gravel and water are commingled to a homogeneous self-hardening mass used in the construction industry. Another classical mixing process is mulling foundry molding sand, where sand, bentonite clay, fine coal dust and water are mixed to a plastic, moldable and reusable mass, applied for molding and pouring molten metal to obtain sand castings that are metallic parts for automobile, machine building, construction or other industries.