Inside the shaft

The following is an excerpt from the ninth edition of Mining Explained, published by The Northern Miner.

Once a stope has been mined out, it is often necessary to backfill it with some waste material so that the ore adjacent to the stope can be mined without affecting the structural integrity of the underground workings. Backfill can be waste rock from underground, sand brought down from surface, or mill tailings that have been processed so that all the fine fraction has been removed.

This material is often mixed with a bonding agent, such as cement, so that it stands up once the ore adjacent to it is mined out. Backfill may be placed immediately, during the mining cycle, or once the stope is completely mined out.

Ore haulage

In track mines, ore is drawn from the chutes on various working levels into rail cars known as ore cars. These are hauled, or trammed, by electric or diesel locomotives to a station where the cars are dumped. The ore falls down a short finger raise into a main ore pass. Ore cars vary in capacity from one to 20 tonnes.

The ore pass is a fairly large raise, usually 2.5-3.5 metres (8-11 ft.) in diameter, into which the broken ore is dumped. The main ore pass extends from the uppermost level all the way down to the deepest level in the mine. Short finger raises are excavated from each level into the main ore pass. The ore falls to an underground crusher station. Control chutes are usually established at various intervals in the ore pass system to provide ore storage above the crusher. To prevent over-sized chunks of ore from plugging up the ore pass, steel rails are placed in a grid-work pattern, called a grizzly, over the dumping station.

Common practice is to install a large jaw or gyratory crusher in the underground crusher station. This unit crushes the ore, usually to less than 15 cm (6 inches). The ore then falls into a large chamber or bin below the crusher. From here, it is fed into a loading pocket near the shaft bottom.

Skipping ore to surface

The most common conveyance for carrying the ore to surface is the skip. These are self-dumping buckets and are usually operated in counterbalance in two separate shaft compartments to reduce the amount of power needed for hoisting. In other words, the weight of the empty skip descending will compensate in part for the weight of the other skip that is being hoisted.

Skips are constucted of lightweight alloys and carry loads of three to greater than 20 tonnes. They are filled from the loading pockets at the base of the ore pass.

In some mining situations, hoisting by conveyor belts may be less costly, and in mines developed by a decline rather than a shaft, it is usual to tram the ore directly to surface by truck or rail car. Whatever way it gets to surface, the ore goes to the mill for crushing, grinding and beneficiation.

In some mining situations, the orebody at depth can be a long distance from the main shaft. When this happens, an internal shaft may be needed for further development at depth. Such an underground shaft is called a winze, and it has its own hoist with skips and cages.

At the bottom of any shaft or winze is a sump. It holds the groundwater that seeps into the mine so that it may be pumped either to surface or to the mine’s water circuit.

Shafts are divided into separate compartments: one for the skips; one for the cage; one for the manway (a series of ladders for emergency access); and one for service equipment, such as water discharge lines, electric power cables, compressed air, communication and “data highways” for carrying electronic information. It is also common to have one or more raises to surface for ventilation.