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Mining

There are four main types of mining: dredging, surface mining, underground mining and insitu mining.

1. Dredging
This is a high-volume mining technique for low-value products near a plentiful source of water. Scoops/buckets are used to extract material from shallow water (often man-made lagoons). A high-tech variation of this is undersea mining, where material is sucked from the seafloor (although the only successful application of this to-date has been for gem diamonds in shallow waters).

The mining process is usually combined with the processing (typically drying and concentration) on a floating barge, which is anchored in the middle of the lagoon.

2. Surface Mining
Called 'Open-cast' if soft-rock mining (eg coal or limestone) and 'Open-pit' if hard- rock mining (eg copper and diamonds). The mining process is fundamentally different between these soft- and hard-rock operations. The former operations are usually rectangular in general shape (and advance along the seam, with waste infill behind as they advance) while the latter are oval.

Surface mines normally only extend to a depth of about 200 m, below which it is usually cheaper to extract the metal from underground. The cut-off point will depend on the economies of the two methods, with surface costs being dominated by the ore:waste (stripping) ratio, which, in turn, will depend on the shape of the orebody, the amount of overburden to be removed and the safe steepness of the wall (ie bench height v width). This latter item will depend on the type of rock and the number of fractures etc.

Hard-rock surface mining is dominated by drilling/blasting and then lifting of the broken ore either into trucks or onto conveyors for transportation to the processing plant. This lifting is usually by excavator (electric or hydraulic; with shovel or backhoe configuration) or front-end loader. The softer rocks can be recovered directly by using very powerful excavators (including the huge bucket-wheel machines).

3. Underground Mining
Access is via vertical shafts or inclined roadways (adits). There are usually two access routes (one for men and materials, and one for the ore) for safety and for ease of ventilation (fresh air comes in one and is then exhausted out of the other).

Once at the correct depth, horizontal tunnels are driven to reach the ore deposit. These are permanent structures so require strong roof supports (often including 'bolts' into the rock to tie the layers together for strength). In contrast, tunnels into the ore deposit itself are often temporary, and so the support is less substantial. Transport for men and materials can be by train, truck or man-riding conveyor belts.

There are a multitude of different extraction techniques but the main ones are:

Room and pillar - Matrix of excavated rooms with pillars left between them to hold up the roof. This method is popular for shallow mines where the mineral is thick but of relatively low value (it is a relatively wasteful method as subsequent removal of the pillars is dangerous). This method lends itself to the use of mechanised extraction.

Longwall systems (Stoping in hard-rock mining) - The mineral (which is usually in a relatively narrow seam) is extracted as a face (advancing or retreating) between two parallel roadways. This system is very popular in coal mining; using shearers (rotating drums with teeth) or plows (a fixed, chisel-like, machine that cuts slices).

Block Caving - Tunnels are driven under the ore deposit and the rock above is fragmented (by drilling/blasting and then the rock collapsing under its own weight) and the material is drawn down through 'ore passes' (see below).

Cut and Fill - Suited to irregular ore deposits of high-value minerals, this method involves mining upwards in horizontal slices, with each slice being backfilled (usually with a concrete mix to provide a suitable floor) once the fragmented ore from above has been collected.

Depending on the mine configuration, the target mineral can be collected directly from ore passes (effectively vertical tunnels used to store rock) or lifted from the floor using load-haul-dump (LHD) vehicles. Transportation to the shaft or incline can be by train, truck or conveyor.

Ore: This is separated from the waste rock at the earliest opportunity to avoid dilution (which would involve extra cost). 'Run-of-mine' is used to describe the ore as it emerges from the mine, ie before treatment.

Waste: Although some rock can be stored underground (and is ideally used to provide roof support by grinding it and pumping it back into the excavated areas) most has to be taken to the surface.

4. Insitu Mining
There are two main types of insitu mining; solution and thermal.

Solution - Involves the injection of water down drill holes into soluble deposits (most commonly salt). The mineral-rich solution is then pumped back to the surface.

Thermal - Although only still at the research stage, it is theoretically possible to burn coal insitu (by creating cracks, then injecting oxygen and a heat source) and recovering the resultant heat (in effect, an underground power station without going to the trouble of extracting the coal). This has happened spontaneously in numerous areas (particularly in India) but the difficulty has always come in controlling the burning process.

For further information in this Mining Knowledge series click the links below.
Mining Companies
Mining Equipment

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