Grinding or comminution is the process of reducing the size of particles. In a broader sense, comminution is used to describe desirable size reduction while attrition is used to describe accidental or unwanted size reduction. An adjunct to grinding is classification or particle size separation.
Methods
In general, grinding is divided into media or medialess grinding or milling. In media milling, balls, pebbles, or other media, such as sand, are used in a stirred mixture along with the sample material to be ground. The collisions of the media with the sample material break the particles into smaller pieces.
Common methods of media milling or grinding include ball milling, bead milling, attritor milling, sand milling, horizontal milling, vertical milling, and vibratory milling. In each case, media, typically larger than the material to be ground, is added to a chamber containing the material to be ground. The entire collection is then stirred, rotated or otherwise agitated. By choosing the correct media material (sand, steel, alumina, or zirconia) and the correct size (1 mm to 10 cm), sample particles can be milled down to average sizes of 0.1 micron very readily. By controlling the time, applied energy, and the size of the grinding media, almost any size particle can be obtained. While media mills may be operated dry without any added liquid, it is commonplace to mill the material in a solvent or water with additional dispersants. Wet milling produces the finest particles and provides a ready-to-use dispersion at the same time.