What is coal?
Coal is a combustible, sedimentary, organic rock formed from ancient vegetation, which has been consolidated between other rock strata and transformed by the combined effects of microbial action, pressure and heat over a considerable time period. This process is referred to as 'coalification'.
Layered between other sedimentary rocks, coal is found in seams ranging from less than a millimetre in thickness to many metres.
Coal is mainly composed of carbon (50 to 98 per cent), hydrogen (2 to 13 per cent) and oxygen, and smaller amounts of nitrogen, sulphur and other elements. It also contains a little water and grains of inorganic matter that remain as a residue known as ash when coal is burnt.
Initially peat, the precursor of coal, was converted into lignite or brown coal with low organic 'maturity'.
Over many millions of years, chemical and physical changes occur to change the lignite. Over time coal became harder and more mature, at which point they are re-classified as brown coal, sub-bituminous or hard coals. Under the right conditions, the progressive increase in the organic maturity continued, ultimately to form anthracite.
Australia's oldest deposits of black coal, found in New South Wales and Queensland, were formed between 225 and 180 million years ago. However, younger black coals mined in Queensland, South Australia and Tasmania are between 140 and 180 million years old. Victoria's brown coal deposits are young by comparison, formed less than 45 million years ago.
Open cut mining
Most of Rio Tinto Coal Australia's operations use the open cut method to mine coal. This type of mining is only economical when the coal seam is relatively close to the surface and recovers a higher proportion of the coal deposit than underground methods.
Our open cut mines use draglines, electric shovels, large haul trucks, excavators and high capacity conveyors to mine the coal seams.
There are two main methods of extracting coal by underground mining: room-and-pillar and longwall mining, the latter used by Rio Tinto Coal Australia's underground operation, Kestrel Mine.
Longwall mining involves the use of mechanised shearers to cut and remove the coal at the face, which can vary in length from 100-250 metres.
Self-advancing, hydraulic-powered supports temporarily hold up the roof while the coal is extracted. The roof over the area behind the face, from which the coal has been removed, is then allowed to collapse. Over 75 per cent of the coal in the deposit can be extracted using this method.
Uses of coal
Coal has a wide range of important uses - the major ones being:
- Electricity generation
- Steel production
- Cement manufacture.
Of all the electricity generated worldwide, about 40 per cent is produced from coal. It is also the principal form of reductant in steel industries, with over 66 per cent of world steel production dependent on coal.
Source: Australian Coal Association (ACA)