The hand mortar and pestle has for centuries been the standard equipment for achieving size reduction and is still very widely used for small scale purposes. Size reduction by compression can also be carried out on a small scale using end-runner mill and edge-runner mill which are mechanized forms of mortar and pestle-type compression comminution.
The end-runner mill consists of a weighted pestle mounted eccentrically in a ceramic, granite or metal mortar, which is rotated by a motor. The pestle rotates by friction and is free to rise and fall in the mortar so that its grinding action involves both impact and shear, the material being crushed and rubbed between it and the rotating mortar.
Spring-loaded scrapers ensure that material is constantly returned to the grinding area and at the end of the operation the pestle can be swung clear of the mortar to facilitate emptying and cleaning.
End run er mill provides moderately fine powder and operates successfully with fibrous materials, bark, woods fruits, leaves, etc.
Wet grinding with very viscous material such as ointments and paste is also possible.
Edge runner mill, also known as Chilean mill or Roller stone mill consists of one or two heavy steel or granite rollers mounted on a horizontal shaft and turned round a central vertical shaft on a bed of steel or granite. The stones may vary from 0.5 to 2.5 m in diameter, the larger size weighing up to about 6 tonnes.
The material to be ground is kept in the path of the runner by scrapers. The reduction is partly due to crushing: by the weight of the stones, but more to friction between the surfaces of contact between the runners and the bed stone.
Although the edge runner mills are gradually being replaced by more sophisticated machines they are still used, particularly for reducing extremely tough and fibrous materials – roots and barks to the form of powder.
1. It produces fine particles
2. Requires less attention during the milling operation
1. It is not suitable for milling sticky materials
2. Machine noise leading to noise pollution
This article titled “Edge-Runner Mills and End-Runner Mills” discusses Edge-Runner and End-Runner Mills,