Digging for victory

Coal mining has come a long way since the dark days of canaries in cages, Davy lamps and grime covered miners with pick axes. In a modern mine you are more likely to come across a highly-qualified automation engineer tasked with keeping huge machines grinding away at the coal face.

The choice of mining method is largely determined by the geology of the coal deposit. Underground mining currently accounts for about 60 per cent of world coal production, although in several important coal producing countries surface mining is more common. Surface mining accounts for around 80 per cent of production in Australia, while in the US it is used for about 67 per cent.

Above or below

There are two main methods of underground mining: room-and-pillar and longwall mining. In room-and-pillar mining, coal deposits are mined by cutting a network of 'rooms' into the coal seam and leaving behind 'pillars' of coal to support the roof of the mine. These pillars can be up to 40 per cent of the total coal in the seam - although this coal can sometimes be recovered at a later stage. This can be achieved in what is known as 'retreat mining', where coal is mined from the pillars as workers retreat. The roof is then allowed to collapse and the mine is abandoned.

Longwall mining involves the full extraction of coal from a section of the seam or 'face' using mechanical shearers. A longwall face requires careful planning to ensure favourable geology exists throughout the section before development work begins. The coal 'face' can vary in length from 100-350m.

Self-advancing, hydraulically-powered supports temporarily hold up the roof while coal is extracted. When coal has been extracted from the area, the roof is allowed to collapse. Over 75 per cent of the coal in the deposit can be extracted from panels of coal that can extend 3km through the coal seam.

The main advantage of room-and-pillar mining over longwall mining is that it allows coal production to start much more quickly, using mobile machinery that costs under $5m (longwall mining machinery can cost $50m).

The choice of mining technique is site specific but always based on economic considerations; differences even within a single mine can lead to both methods being used.

When it comes to longwall systems Randy Cooper, global product line manager at Joy Mining Machinery explains that there are two technologies available - plow and shears. " As the world's largest producer of coal, China realises that, for full recovery of its coal reserves, they must mine their lower seams which usually overlay the thicker seams," he says.

"Historically, the lower seams were mined first, causing subsidence to travel upward, fracturing the thinner seams and rendering them unrecoverable. Now, these previously sacrificed overlying thinner seams must be taken first, and that is where the question is being asked: shearer or plow for low-seam longwalling?

"Most modern plow systems generally can produce 1.8 million tonnes of clean coal in a year. Under the same circumstances, a high horsepower shearer - even taking some additional rock to maintain minimum working height - will mine more than twice that, resulting in higher clean tonnes per-man-year and much lower costs per tonne."

Surface mining - also known as opencast or opencut mining - is only economic when the coal seam is near the surface. This method recovers a higher proportion of the coal deposit than underground mining as all coal seams are exploited - 90 per cent or more of the coal can be recovered. Large opencast mines can cover an area of many square kilometres and use very large pieces of equipment, including: draglines, which remove the overburden; power shovels; large trucks, which transport overburden and coal; bucket wheel excavators; and conveyors.

The overburden of soil and rock is first broken up by explosives; it is then removed by draglines or by shovel and truck. Once the coal seam is exposed, it is drilled, fractured and systematically mined in strips. The coal is then loaded on to large trucks or conveyors for transport to either the coal preparation plant or direct to where it will be used.

Preparation and transport

Coal straight from the ground, known as run-of-mine (ROM) coal, often contains unwanted impurities such as rock and dirt and comes in a mixture of different-sized fragments.

However, coal users need coal of a consistent quality. Coal preparation - also known as coal beneficiation or coal washing - refers to the treatment of ROM coal to ensure a consistent quality and to enhance its suitability for particular end-uses.

The treatment depends on the properties of the coal and its intended use. It may require only simple crushing or it may need to go through a complex treatment process to reduce impurities.

To remove impurities, the raw ROM coal is crushed and then separated into various size fractions. Larger material is usually treated using 'dense medium separation'. In this process, the coal is separated from other impurities by being floated in a tank containing a liquid of specific gravity, usually a suspension of finely ground magnetite. As the coal is lighter, it floats and can be separated off, while heavier rock and other impurities sink and are removed as waste.

The smaller size fractions are treated in a number of ways, usually based on differences in mass, such as in centrifuges. A centrifuge is a machine which turns a container around very quickly, causing solids and liquids inside it to separate. Alternative methods use the different surface properties of coal and waste.

In 'froth flotation', coal particles are removed in a froth produced by blowing air into a water bath containing chemical reagents. The bubbles attract the coal but not the waste and are skimmed off to recover the coal fines. Recent technological developments have helped increase the recovery of ultra fine coal material.

The way that coal is transported to where it will be used depends on the distance to be covered. Coal is generally transported by conveyor or truck over short distances. Trains and barges are used for longer distances within domestic markets, or alternatively coal can be mixed with water to form a coal slurry and transported through a pipeline.

Ships are commonly used for international transportation, in sizes ranging from Handymax (40-60,000 deadweight tonnes, or DWT), Panamax (60-80,000 DWT) to large Capesize vessels (80,000+ DWT). Around 700 million tonnes of coal was traded internationally in 2003 and around 90 per cent was seaborne trade. Coal transportation can be very expensive - in some instances it accounts for up to 70 per cent of the delivered cost of coal.

Measures are taken at every stage of coal transportation and storage to minimise environmental risks.

Safety at coal mines

The coal industry takes the issue of safety very seriously. Coal mining deep underground involves a higher safety risk than coal mined in opencast pits. However, modern coal mines have rigorous safety procedures, health and safety standards and worker education and training, which have led to significant improvements in safety levels in both underground and opencast mining.

There are still problems within the industry. The majority of coal mine accidents and fatalities occur in China. Most accidents are in small scale town and village mines, often illegally operated, where mining techniques are labour intensive and use very basic equipment.

The Chinese government is taking steps to improve safety levels, including the forced closure of small-scale mines and those that fail to meet safety standards.