Some ores occur in nature as mixtures of discrete mineral particles.
Most ores, however, are made up of hard, tough rock masses that must be crushed before the valuable minerals can be released. In order to produce a crushed material suitable for use as mill feed (100 percent of the pieces must be less than 10 to 14 millimetres, or 0.4 to 0.6 inch, in diameter), crushing is done in stages.
In the primary stage, the devices used are mostly jaw crushers with openings as wide as two metres. These crush the ore to less than 150 millimetres, which is a suitable size to serve as feed for the secondary crushing stage.
In this stage, the ore is crushed in cone crushers to less than 10 to 14 millimetres. This material is the feed for the grinding mill. Gyratory, jaw and cone crushers are known types of crushing device.