Created by orebeneficiation on 2/15/2012 4:04:40 AM
Some factors need to be taken into account when choosing mining method for a certain ore deposit. These main factors are location, depth, ore characteristic, geometry, morphology, environment, economics, and even mining tradition etc. Depending on these different factors, there are mainly four mining types for choice: insitu mining, dredging, open pits or quarries, under ground mines.
Insitu mining
The process initially involves drilling of holes into the ore deposit. 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 is 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.
Dredging
Dredging is an excavation activity or operation usually carried out at least partly underwater, in shallow seas or fresh water areas with the purpose of gathering up bottom sediments and disposing of them at a different location. This is a high-volume mining technique for low-value products near a plentiful source of water. 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.
Open pits and quarries
There are many alternatives within open pits and quarries but the great principles are identical. Most industrial materials and shallow metallic deposits (< 300 m) are mined by this method, which is the cheapest in practice. The scale of the projects, and particularly their depth, is conditioned by an economic threshold above which it is better to continue mining through underground workings.As a rule, after the stripping operations (removal of the soil and superficial horizons), actual mining is carried out in successive steps, imparting a roughly conical shape to the mine. The mining of each step or bench produces a tonnage of extracted material corresponding to the overburden surrounding the deposit, which is sent directly to the waste dump, and a tonnage corresponding to the ore that is selectively routed either for storage or directly to the processing plant. The variation in the ratio between the tonnage of waste to be extracted and the quantity of ore recoverable (also called stripping ratio) strongly conditions the economic viability of the mine. If this ratio becomes too high, especially when the quarry is deepened, it is no longer economically profitable to continue strip mining.
Underground (quarries and) mines
When deposits are difficult to reach from the surface (depth, cliffs permitting side access), the only alternative is underground workings. A broad range of methods are available (chamber and pillar, long-wall, under-level caving, under-level stoping and filling, shrinkage) all of which are roughly adapted to the characteristics of the ore or the geometry of the deposit: dip of the layers or veins,thickness, continuity of the mineralization, grade of ore (disseminated or massive). The workings are generally opened by levels with a 60 m vertical spacing and then sublevels at 15 m intervals. Two criteria are vital for all these workings: selectivity of the ore and its percentage recovery.All the operations conducted in the ore are connected to one another and to the surface by a series of passages, all opened in the overburden surrounding the deposit: shafts, inclines, drifts, chutes, cross-cuts for personnel and machine access, for removal of ore and drainage water, as well as for ventilation. This organisation of the operations has the following consequences:
- the ore extraction capacities are generally much lower than for surface quarries;
- the quantity of waste produced per unit of ore mined is much lower than for surface quarries;
- the ground area of this type of underground mine is considerably smaller than for surface quarrying, except for subhorizontal layers;
- the mechanical risks are different (subsurface collapse, structural weakness around shafts and other inclines).
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