2.5 Surface mining
In surface mining (sometimes called 'opencast' mining in the UK), the coal seam is accessed by removing the rock overburden; a process that benefits from economies of scale by using some of the world's largest machines (Figure 15).
Figure 16 shows schematically how surface mines are organized. Topsoil is removed and stored or used immediately for land restoration elsewhere. Shallow or soft overburden is removed by draglines, hydraulic shovels and dump trucks, but at deeper levels with harder layers of rock explosives may have to be used. When the first coal is reached, seams are usually worked by bench mining methods (Figure 16). The top surface of coal exposed on each bench is carefully cleaned to remove any adhering waste rock. This careful exclusion of non-coal material enables high-quality coal to be produced consistently by surface mining. The clean coal is then broken up by diggers or with the aid of explosives, and loaded into trucks using mechanical shovels. Bench methods allow every seam to be worked, not just the thickest. Seams as thin as 0.1 m can be worked where they form part of the overburden to a lower, profitable seam.
The precise sequence of overburden and coal removal in a surface mine depends on the area covered by the mine, and the thickness and vertical spacing of the coal seams. In some cases, a single large pit is opened and the overburden is then moved around within it to gain access to different parts of the coal seam (Figure 17). In mountainous areas, benches may be cut into the slopes and the removal of coal and overburden may result in the mountain being levelled.
The critical factor is the angle at which the seams dip beneath the surface. In the case of horizontal strata (Figure 13) and an almost flat land surface, wherever the coal seam is present beneath the surface, the overburden is roughly the same. Consequently, the potential size of a surface operation to mine a horizontal coal seam is limited only by environmental legislation, ownership of the land and any areas of rising ground that would increase the overburden ratio. Finding seams that remain horizontal over vast areas is uncommon, however. Where the strata dip beneath the surface, overburden increases as the seams dip to deeper levels, thereby posing a limit on mine size.
Figure 18 shows a cross-section through coal-bearing rocks that dip at 45° to the horizontal. By counting the squares in the overburden, and by assuming a maximum stripping ratio of 20:1, determine whether it would be economic to strip mine the coal seam down to depth A and additionally down to depth B. You can assume that the coal seam covers 125 mm2 squares down to depth A and 150 squares to depth B.
Down to depth A
The coal seam covers 125 mm2 squares, whereas the overburden covers 25 cm2 squares, i.e. 25 × 100= 2500 mm2 squares.
So, the ratio of overburden: coal is 2500:125, or 20:1. This stripping ratio is equal to the 20:1 limit, so this coal seam could be strip mined to this depth.
Down to depth B
The coal seam covers 150 mm2 squares, whereas the overburden covers 33 cm2 squares, i.e. 33 × 100= 3300 mm2 squares.
So, the ratio of overburden: coal is 3300:150, or 22:1. This stripping ratio is higher than the 20:1 limit, so surface mining of the coal seam to this depth would not be economic.
Clearly, at greater depths, proportionately more overburden has to be removed per volume of coal, and this steadily increases the stripping ratio.
Your answer to Question 6 shows how the extent of surface mining is limited laterally in the direction of dip, the limits of a mine becoming narrower as the dip increases, because the stripping ratio increases more with depth of excavation. A dip of 45° is at or above that where unconfined rock becomes unstable and begins to slip: surface mining of coal with steeper dips is too dangerous to be undertaken, and, if at all, the seams must be won by underground operations. At lesser dips, surface mining takes the form of operations that are along and parallel to the outcrop of coal seams: this is termed strip mining. Such operations are widespread in the western USA (Box 1) and are particularly appropriate for single, horizontal coal seams at shallow depths beneath flat topography, as in some Australian coalfields and the low-grade brown coals of Germany. Only a narrow strip is 'sterilized' from agricultural use for a year or so as the strip mine progresses across country. Bench mining, as shown in Figure 16, is favoured where there are a number of seams within 100 m of the surface.
In the UK, surface mines are now located in areas of shallow coal where underground mining has ceased, and include districts where the old mining methods had extracted only a proportion of the coal. Typically, surface bench mines are favoured in the UK where access to large areas is highly restricted. They usually operate for between three to five years, cover about 2 km2, and contain about two million tonnes of coal in a dozen or so seams. In 2004, just under half of UK coal production came from 42 surface mines (Figure 19). This is half the number of operating surface and underground mines just eight years before.
The UK has approximately 43 × 106 t of coal reserves in operational and nonoperational surface mines. (75% of this coal is in Scotland, the rest being shared equally by England and Wales.) It is becoming increasingly difficult to estimate how long this coal will supply the UK demand, as cheaper imports of coal from around the world have forced the European coal industry, both in surface and underground mines, into rapid decline since the mid-1990s. As a direct consequence, 53% of the coal that is consumed in the UK is now imported and this figure is likely to increase in the future.
The decline in the production of surface mined coal in the UK is not reflected elsewhere in the world where in general, production from surface mines has increased in recent years. Despite the fact that most coal reserves are only exploitable through underground mining, globally some 40% of coal is now surface mined. The USA produces 50% of its coal from such mines (see Box 1); Australia, 70%; and Venezuela and Columbia close to 100%. However, China — by far the largest producer of coal — is dependent on deep coal seams and only mines 7% of its coal at the surface.
Box 1 Surface coal mining at the Black Thunder Coal Mine, Powder River, Wyoming
In the USA, three states (Montana, Illinois and Wyoming) possess over 56% of the country's coal reserves (Figure 20a). Until recently, Wyoming laid claim to the largest single coal mine in the USA; the Black Thunder surface mine. Black Thunder, located in the Powder River Basin (Figure 20b), opened in 1977. In 2003, the mine produced 51.5 × 106 t of coal.
To place this in perspective, the whole of the UK produced just over half this amount of coal in the same year.
The coal at Black Thunder is of Early Tertiary age (56.5-65 Ma), so it is significantly younger than the Carboniferous coal of the UK. (The ages of the various coal deposits are discussed further in Section 4.) The principal Wyodak seam is an impressive 22 m thick (Figure 21a). This single coal seam covers some 35 000 km2 of the Powder River Basin, making it the most important in the USA. Its great thickness and the fact that it is composed of the remains of woody material suggest that the coal formed in a stagnant tropical mire where the rate of subsidence was sufficient to allow peat to accumulate over thousands of years. This extensive mire was associated with a delta that moved westward during the Palaeocene.
At Black Thunder, the seam is gently dipping, and locally splits into two beds separated by up to 18 m of waste. The coal is low-sulphur and sub-bituminous and has an ash content of around 5%.
Strip mining techniques are used at Black Section 2 Thunder. They start with the removal of topsoil by mechanized scrapers ahead of the pit and this soil is trucked and placed on areas undergoing restoration. Then, the overburden is stripped away to a depth of 15-75 m to reach the Wyodak seam in a two-stage process.
Around 20 to 30% of the overburden is removed by blasting it directly into areas where the seam has already been mined. Moving such large volumes of rock in this way requires an enormous amount of explosive. The remaining overburden is removed by four large draglines. The largest, called 'Ursa Major', has a 110 m-long boom and carries a 122 m3 bucket, which can clear 150 t of overburden in one scoop (Figure 21b).
Once the coal seam is exposed, it too is blasted to ease its removal. Electric mining shovels then load the coal onto trucks, which transport it to a nearby crushing plant. A 3.5 km-long conveyor belt takes the coal to a further crusher and then into silos from which it is loaded onto trains.
With a low-sulphur and ash content, Wyoming's coal is well suited to fuel power stations without any preparation except crushing. Indeed, 96.7% of coal produced in Wyoming is used to generate electricity in 22 US states. With reserves of over 900 × 106 t, the Black Thunder mine could continue to produce coal at its current rate until 2022.
The quantity of coal extracted per worker-day in surface mining can be many times that in underground mines — a factor that is reflected in the generally lower cost of surface mined coal (helped also by lower capital and operating costs). Other advantages of surface mining are that geological problems are more easily resolved and the working environment is safer for mining personnel.
One disadvantage for mine operators is that some major coal-producing countries, including the UK and US, have legislation requiring rehabilitation of the mined area, or laws prohibiting surface mining on land where rehabilitation would not be possible. In such cases, and where the coal is deeper than the economic stripping ratio, underground mining is the only option.