Primary Crushing

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

The primary crusher is most of the time a jaw crusher. The ore falls into the opening between a couple of metal jaws at the top and is crushed by the short, quick, motion of the one jaw which is movable — a process which is very similar to that of an animal using its jaws to chew its food.

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

Secondary Crushing
A secondary crusher is very often needed when the product from the primary crusher is too large for appropriate grinding. The main type which is used North America is the cone crusher. It is a close relative of the gyratory crusher, even though the speed of the cone crusher is faster and it is designed to handle smaller pieces of rock.

Vibrating screens are used to control the size of the final product from the crusher building. Undersize is the name of ore that falls through the openings in a screen. It will find its way into the mill on a conveyor belt. Ore which is too big to fall through the openings is called oversize and it goes back into the crusher along with a separate conveyor belt. This ore is called the circulating load.

Crusher buildings are very commonly equipped with ventilation and dust control systems designed to elimínate the buildup of dust which can cause harm.

Most mills operate every day of a week, from Monday to Sunday, 24 hours a day, compared with the mine, which generally operates only five days a week. Due to this difference in work schedules, ore that is crushed must be stored on surface in enough quantities to keep the mill going around the clock. The customary place to store the ore is in a fine ore bin next the mill. These bins generally contain enough ore to keep the mill in operation for at least 48 hours.