More stoping methods

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

With the development of drills capable of creating large-diameter (15-cm) holes up to 60 metres long, conventional blast-hole stoping has been replaced in part by a more efficient stoping method.

Vertical crater retreat (VCR), also known as vertical retreat mining (VRM), reduces the cost of mining wide, steeply dipping orebodies. The stope has a similar shape, but instead of small-diameter longholes drilled in fans, large-diameter blastholes are drilled vertically from a top sill to break through into a bottom sill on the sublevel below. This allows the ore to be broken into the bottom sublevel in successive horizontal slices using the same blasthole for each successive deck or blast. No slot raise is required.

Only the bottom of each hole is loaded for each successive blast. This breaks off a slice of ore from the bottom of the ore block, which falls into the drawpoint level below, where it can be mucked out. Mucking provides room in the stope for the next blast. Dilution is controlled by removing just enough ore to create a sufficient void for the following blast.

This mining method is particularly safe, because the miner does not enter the area where the ore is blasted. Drilling and loading are done from the top sill. The method also eliminates the need to support the ground in the stope after each blast. Mucking can be done using remote-controlled load-haul-dump machines.

— Room-and-pillar — In cases where the orebody is narrow and flat-lying, as is the case with many coal, potash, salt and Mississippi Valley-type lead-zinc deposits, room-and-pillar mining is often used. As the name suggests, ore is mined from large voids or rooms, and pillars of ore are left between the rooms to support the overlying strata. The ore pillars remain upon completion of mining and are not recovered.

— Shrinkage stoping — Shrinkage stoping is a flexible mining method for narrow orebodies that need no backfill during stoping. Successive horizontal slices of ore, usually about 3 metres high, are taken along the length of a stope in a manner similar to cut-and-fill. The ore is removed from the stope through drawpoints at the bottom horizon spaced about every 7.5 metres along strike. Just enough ore is left in the place to provide a floor from which to work when taking the next cut. This requires considerable planning and co-ordination.

When the ore is blasted, it fills a space about 1.5 times the size of the space it filled as a solid mass. This is called swell and is an important factor in determining how much ore to draw from the bottom of the stope in order to maintain adequate working room. The broken ore is drawn down from chutes below, thus “shrinking” the volume of broken ore in the stope.

The process is continued upward until the stope either reaches the next level or is stopped at some predetermined elevation. Horizontal crown pillars are left behind at the top of the stope.

Shrinkage stoping depends on gravity to keep the broken ore moving to the draw points, so it works only in steeply-dipping orebodies. There is no provision for support, so the wall rocks must be strong and competent. The orebody must also be wide enough to allow a working width all the way up the stope, generally no fewer than two metres.

— Sublevel caving — In sublevel caving, ore is developed from a series of sublevels spaced at regular intervals throughout the orebody. Mining begins at the top of the orebody. A series of ring patterns is drilled and blasted from each sublevel. Broken ore is mucked out after each blast and the overlying waste rock caves on top of the broken ore.

This technique is inexpensive, highly mechanized and yields a large amount of muck. It is normally used in massive, steeply dipping orebodies with considerable strike length. Since dilution and low recoveries are unavoidable, sublevel caving is used to mine low-grade, low-value orebodies.

— Stope and mine control — The mine has an engineering department that keeps the plans, designs and schedules up to date, and makes sure the ore is extracted safely and efficiently. Surveyors keep a running check on the progress of mine workings and on the volume of ore removed. Safety supervisors ensure that the workings remain stable and that the staff follow safe working practices.

The mine geology department keeps up-to-date plans and sections of all levels, including information on ore grades and ground conditions. It also usually directs ongoing underground exploration for more ore. Development headings and stopes have to be sampled. In a large mine, this amounts to hundreds of samples, all of which must be assayed or evaluated on the same day. The results are used as guides and checks on production and serve as the mine’s quality control program.