Sieving (classification method)

Sieving is a mechanical separation process for the size separation ( classification ) of bulk materials .

The material to be separated is placed on a sieve that is set in rotation or shaken. Technically, this is done, for example, in a mill with a plansifter , a tumbler screening machine or a vibration screening machine . Industrial screens can have a diameter of over two meters and are often arranged in several layers of screens on top of one another. The separation of solids and liquids , e.g. B. in papermaking , can be done by sieving, but mostly the related method of filtration is used. Sieving processes are used in chemistry and pharmacy , in the food industry , e.g. B. in spices , for feed and fertilizers , stones and earth , in the recycling of wood or plastic chips and in the sandpit . If a cloth (bag cloth or cheesecloth) is used instead of a sieve, one speaks of bags (from Middle High German biuteln ).

Principle: The driving force for sieving is usually the weight . In order to bring particles into contact with the sieve as often as possible, the material to be separated is moved by oscillating, vibrating and tumbling. The separation quality when sieving is determined by the probability of passage of a grain with a given mesh size.

Purpose of use:

• Classifying: To obtain one or more fractions with a defined grain size distribution from a mixture
• Protection of plants: to sort out oversized grain (too large parts) to avoid clogging, damage
• Dust removal: Adhering fine grain is separated
• Sorting: if the grain size also has other properties (e.g. flour and bran)
• for sieve analysis . With a sieve and a balance , the proportion of particles that are larger or smaller than the sieve mesh size is obtained.

Cleaning the sieve is particularly important when using machine sieving processes. It can be done with ultrasound , compressed air , brushes , or with rubber balls that bounce against the sieve from below.

literature

• Claus Bernhardt: Granulometry. Classification and sedimentation methods. German publishing house for basic industry, Leipzig 1990, ISBN 3-342-00415-0 .
• Matthias Stieß: Mechanical Process Engineering - Particle Technology. Volume 1. 3., completely revised edition. Springer, Berlin 2009, ISBN 978-3-540-32551-2 .

Individual evidence

1. ↑ Jürgen Martin: The 'Ulmer Wundarznei'. Introduction - Text - Glossary on a monument to German specialist prose from the 15th century. Königshausen & Neumann, Würzburg 1991 (= Würzburg medical-historical research. Volume 52), ISBN 3-88479-801-4 (also medical dissertation Würzburg 1990), p. 119.