Fundamental Physical Processes: Sieving, Grinding, Mixing, and Drying

Sieving: Separating Solid Particles by Size

Sieving is a physical method used for separating heterogeneous solid mixtures. It involves passing a mixture of solid particles of different sizes through a sieve or colander. The smaller particles pass through the pores, while larger particles are retained by the screen.

This is a very simple method generally used for solid heterogeneous mixtures, such as separating sand (which passes through the sieve) from stones (which are retained). The screen openings are usually of different sizes and are selected according to the size of the particles in the mixture.

Application of Screening

To apply this method, it is necessary that both phases are present in the solid state. Metal or plastic sieves are used, which retain the larger particles and allow those of smaller diameter to pass. Examples of mixtures separated by sieving include:

• Marble chips mixed with sand
• Flour and cork
• Table salt and pieces of rock
• Pebble and sand
• Glass beads and sugar
• Bits of cork and sand
• Sawdust and fine salt

Grinding and Pulverization

Grinding is a process that seeks to extract juices from various agricultural products, such as sugar cane or grapes. The term is also commonly used to refer to the pulverization and dispersion of solid material.

Materials subjected to grinding can include food products (cereal grains, grapes, olives, etc.) or non-food materials (stones or other solid matter).

Mixing: A Complex Unit Operation

Mixing is one of the unit operations of chemical engineering that is most difficult to subject to scientific scrutiny. So far, no formula or equation has been developed to accurately calculate the degree of completion achieved by the mixture or the speed with which the process is performed under specific conditions.

It is sometimes suggested that only the power consumption of a mixer provides a true measure of the degree to which a mixture is complete, based on the idea that it takes a definite amount of work to mix the material particles within the container.

However, this is rarely true in practice due to interference that is impossible to assess, such as cross-currents and eddy currents, which are set up inside the container (even when mixing plastics and solids).

Drying: Food Preservation by Water Extraction

Drying is a method of food preservation involving the extraction of water, which prevents the growth of microorganisms and subsequent putrefaction. The drying of food using the sun and wind to prevent spoilage has been known since ancient times.

Water is usually removed by evaporation (air drying, sun drying, smoking, or wind drying). However, in the case of freeze-drying, food is frozen first, and then water is removed by sublimation.

Bacteria and microorganisms require water to grow. Drying effectively prevents them from surviving in the food. Furthermore, drying often creates a hard outer layer, which helps prevent microorganisms from entering the food.