Primary Crushing

The milling process begins with the primary crusher, which is most often located below the mine workings so that broken ore can be dropped down an ore pass to be crushed and then hauled to the surface in a skip. This is so because loading skips with small, 15-cm-wide pieces of ore is more efficient than loading it with larger chunks.

The primary crusher is usually a jaw crusher. Ore falls into the opening between a pair of metal jaws at the top and is crushed by the short, rapid, motion of the one movable jaw — a process not unlike an animal using its jaws to chew food.

A few large-tonnage mills use a gyratory crusher as the primary crusher. It consists of a heavy, gyrating head which works inside a crushing bowl fixed to the main frame. Rock falling into the bowl is caught and broken up by the gyrating head.