The geological characteristics of surface coal mining which distinguish it from other surface mining are the nature of formation and its relatively low value, which often require surface coal mines to move large volumes of overburden over a large area (i.e., it has a high stripping ratio). As a result, surface coal mines have developed specialized equipment and mining techniques. Examples include a dragline strip mine which mines in strips of 30 to 60 m wide, sidecasting material in pits up to 50 km long. Rehabilitation is an integral part of the mining cycle due to the significant disturbance of the involved areas.
Surface coal mines vary from being small (i.e., producing less than 1 million tonnes per annum) to large (above 10 million tonnes per annum). The workforce required depends on the size and type of the mine, the size and amount of equipment and the amount of coal and overburden. There are some typical measurements which indicate the productivity and size of the workforce. These are:
1. Output per miner expressed as tonnes per miner per year; this would range from 5,000 tonnes per miner per year to 40,000 tonnes per miner per year.
2. Total material moved expressed in tonnes per miner per year. This productivity indicator combines the coal and the overburden; productivity of 100,000 tonnes per miner per year would be low with 400,000 tonnes per miner per year being the very productive end of the scale.
Due to the large capital investment involved, many coal mines operate on a seven day continuous shift roster. This involves four crews: three work three shifts of eight hours each with the fourth crew covering rostered time off.
Mine Planning
Mine planning for surface coal mines is a repetitive process which can be summarized in a checklist. The cycle begins with geology and marketing and finishes with an economic evaluation. The level of detail (and cost) of the planning increases as the project goes through different stages of approval and development. Feasibility studies cover the work prior to development. The same checklist is used after production commences to develop annual and five-year plans as well as plans for closing down the mine and rehabilitating the area when all the coal has been extracted.
Significantly, the need for planning is ongoing and the plans need frequent updating to reflect changes in the market, technology, legislation and knowledge of the deposit learned as the mining progresses.
Geological Influences
Geological features have a major influence in the selection of the mining method and equipment used in a particular surface coal mine.
Seam attitude, commonly known as dip, represents the angle between the seam being mined and the horizontal plane. The steeper the dip the more difficult it is to mine. The dip also affects the stability of the mine; the limiting dip for dragline operations is around 7°.
The strength of coal and waste rock determines what equipment can be used and whether or not the material has to be blasted. Continuous mining equipment, such as bucketwheel excavators commonly used in eastern Europe and Germany, is limited to material of very low strength that does not require blasting. Typically, however, the overburden is too hard to be dug without some blasting to fragment the rock into smaller sized pieces which can then be excavated by shovels and mechanical equipment.
As the depth of coal seams increase, the cost of transporting the waste and coal to the surface or to the dump becomes higher. At some point, it would become more economical to mine by underground methods than by open-cut methods.
Seams as thin as 50 mm can be mined but the recovery of coal becomes more difficult and expensive as seam thickness decreases.
Hydrology refers to the amount of water in the coal and overburden. Significant quantities of water affect stability and the pumping requirements add to the cost.
The magnitude of the coal reserves and the scale of operation influences what equipment can be used. Small mines require smaller and relatively more expensive equipment, whereas large mines enjoy the economies of scale and lower costs per unit of production.
Environmental characteristics refers to the behaviour of the overburden after it has been mined. Some overburden is termed “acid producing” which means that when exposed to air and water it will produce acid which is detrimental to the environment and requires special treatment.
The combination of the above factors plus others determines which mining method and equipment is appropriate for a particular surface coal mine.
The Mining Cycle
Surface coal mining methodology can be broken into a series of steps.
Removing topsoil and either storing it or replacing it on areas being rehabilitated is an important part of the cycle as the objective is to return the land use to at least as good a condition as it was before mining began. Topsoil is an important component as it contains plant nutrients.
Ground preparation may involve using explosives to fragment the large rocks. In some instances, this is done by bulldozers with rippers which use mechanical force to break the rock into smaller pieces. Some mines where the strength of the rock is low require no ground preparation as the excavator can dig directly from the bank.
Waste removal is the process of mining the rock overlying the coal seam and transporting it to the dump. In a strip mine where the dump is in an adjacent strip, it is a sidecast operation. In some mines, however, the dump may be several kilometres away due to the structure of the seam and available dump space and transport to the dump by trucks or conveyors is necessary.
Coal mining is the process of removing the coal from the exposed face in the mine and transporting it out of the pit. What happens next depends on the location of the coal market and its end use. If fed to an onsite power station, it is pulverised and goes directly to the boiler. If the coal is low grade it may be upgraded by “washing” the coal in a preparation plant. This separates the coal and overburden to yield a higher grade product. Before it is sent to market, this coal usually needs some crushing to get it to a uniform size, and blending to control variations in quality. It may be transported by road, conveyor, train, barge or ship.
Rehabilitation involves shaping the dump to restore the terrain and meet drainage criteria, replacing topsoil and planting vegetation to return it to its original state. Other environmental management considerations include:
- water management: diversion of existing water courses and control of mine water by sediment dams and recycling so that contaminated water is not discharged
- visual planning : ensuring that the visual impact is minimized
- flora and fauna: to restoring trees and vegetation and replace indigenous wild life
- archaeology: preservation and/or restoration of culturally significant sites
- final void: what to do with the hole after mining has stopped (e.g., it may be filled in or turned into a lake)
- air blast and vibration, due to blasting, which need to be managed by specific techniques if buildings are nearby
- noise and dust, which need to be managed to avoid creating a nuisance for nearby dwellings and communities.
The impact of surface coal mining on the overall environment can be significant but with appropriate planning and control throughout all phases of the enterprise, it can be managed to meet all requirements.
Mining Methods and Equipment
Three main mining methods are used for surface coal mining: truck and shovel; draglines; and conveyor-based systems, such as bucketwheel excavators and in-pit crushers. Many mines use combinations of these, and there are also specializd techniques such as auger mining and continuous highwall miners. These constitute only a small proportion of total surface coal mining production. The dragline and bucketwheel systems were developed specifically for surface coal mining whereas truck and shovel mining systems are used throughout the mining industry.
The truck and shovel mining method involves an excavator, such as an electric rope shovel, a hydraulic excavator or a front-end loader, to load overburden into trucks. The size of the trucks can vary from 35 tonnes up to 220 tonnes. The truck transports the overburden from the mining face to the dumping area where a bulldozer will push and pile the rock to shape the dump for rehabilitation. The truck and shovel method is noted for its flexibility; examples are found in most countries of the world.
Draglines are one of the cheapest methods to mine the overburden, but are limited in their operation by the length of the boom,which is generally 100 m long. The dragline swings on its centre point and can therefore dump the material approximately 100 m from where it is sitting. This geometry requires that the mine be laid out in long narrow strips.
The main limitation of the dragline is that it can only dig to a depth of approximately 60 m; beyond this, another form of supplementary overburden removal such as the truck and shovel fleet is required.
Conveyor-based mining systems use conveyors to transport the overburden instead of trucks. Where the overburden is low strength it can be mined directly from the face by a bucketwheel excavator. It is often called a “continuous” mining method because it feeds the overburden and coal without interruption. Draglines and shovels are cyclical with each bucket load taking 30 to 60 seconds. Harder overburden requires a combination of blasting or an in-pit crusher and shovel loading to feed it onto the conveyor. Conveyor-based surface coal mining systems are most suitable where the overburden has to be transported significant distances or up significant heights.
Conclusion
Surface coal mining involves specialized equipment and mining techniques which allow the removal of large volumes of waste and coal from large areas. Rehabilitation is an integral and important part of the process.