Ball Mill

A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramicballs, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder.

Refer http://en.wikipedia.org/wiki/Ball_mill for more details.