Types of surface 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 mining process 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. Types of Coal 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.

4. Insitu Mining

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 types of mining of 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 effectFind Article, 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.