Extraction of coal in open cut mining takes place in large surface mining operations utilising heavy excavating/digging machinery. Surface mining is practical where coal deposits lie relatively near the surface. On flat land the mining operation forms a pit but in mountainous country mining may involve levelling of a mountain or the creation of a shelf or “bench” in the side of a mountain (contour mining).
The mine/land itself will generally be leased. State governments determine which land is open to exploration and mining, issue exploration and mining leases and collect royalties from producers. Planning of the mine site, including environmental impact assessments, takes place before operations can commence.
Extraction of coal involves removal of the topsoil and overburden by bulldozers and scrapers and the use of heavy front-end loaders, power shovels or hydraulic excavators to mine the underlying deposits. The topsoil is set to one side for later use in reclamation. The layer of rock and low-grade coal (the overburden) also must be removed and this may also be saved for reclamation purposes. Once removed, it is known as “spoil”.
Mining requires the use of drilling and blasting techniques. Explosives are utilised to blast rock surfaces and release coal deposits. Overburden, drilling and blasting may be contracted out, often to local companies.
Coal is loaded at the blasting face into trucks or moved by bucket-wheel excavators and is then either transported direct to the end user using trucks, railways cars, or conveyor systems, or may be subject to further processes at the open cut mine or a nearby facility. In the case of brown coal, the coal processing will often take place at the receiving facility.
Travelling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles. Coal is removed from the stockpile by tunnel conveyor systems, front end loaders, continuous slot hoppers or bunkers.
The coal delivered from the mine is called run-of-mine, or ROM, coal. This consists of coal, rocks, middlings, minerals and contamination. Contamination is usually introduced by the mining process and may include machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large variability of moisture and maximum particle size.
At the coal processing plant:
- Samples will be taken of the coal and tested. Sampling can utilise several devices. Cross cut samplers mount directly on top of the conveyor belt whilst falling stream samplers are set at the head of the belt. Generally, samples take place on ROM coal, refuse and processed coal prior to shipment.
- Crushing reduces the overall topsize of the ROM coal so that it can be more easily handled and processed. Primary crushing is performed by jaw crushers, gyratory crushers or impact crushers.
- Screening separates coal sizes by passing the coal over various vibrating screens. Screen decks can be made from different materials such as high tensile steel, stainless steel, or polyethylene.
- Coal washing takes place to remove soluble impurities and as part of gravity separation.
- The waste product is known as tailings. Tailings can be pumped to a tailings dam, combined with larger sized rejects for disposal (co-disposal), or dewatered before disposal.
Brown coal mining operations may also have a coal drying or briquette manufacturing plant. This aspect of the operation may involve the application of heat, pressure, or both to reduce the moisture content of the brown coal and extrude the resulting product into pellets or briquettes which are then allowed to harden. These are typically used in boilers to produce steam or hot water for industrial applications or, less commonly, to generate power. The briquette manufacturing process itself results in the co-generation of electricity.
Following processing, the coal or briquettes are transported to the purchaser or to a terminal facility for export.
Brown coal deteriorates rapidly and may ignite spontaneously when stockpiled, thus producers and consumers avoid accumulating large stockpiles of brown coal. However, black coal is exported and this necessitates coal storage and handling facilities. Mine owners often make substantial investments in railway networks and rolling stock or other transport infrastructure.
According to IBISWorld:
- Between 2006 and 2007, about $700 million was spent on expansions at the Gladstone Port in Queensland, together with about $320 million of public investment in various rail line upgrades and expansions in Queensland;
- The expansion in capacity at the Dalrymple Bay coal loading port (from 54.5 to 85 million tonnes per year) became operational in mid-2009 at a cost of over $400 and a further upgrade of $640 million has reportedly been committed.
- About $200 million in expansions are underway at the Abbot Point and Hay Point Coal Terminals in Queensland.
Materials and Equipment
Establishing a new mine or expanding an existing one requires large amounts of capital for mine site development and specialised equipment. The following machinery is utilised in open cut coal mining operations but is by no means exclusive or exhaustive:
- Briquette manufacturing plant
- Bucket elevators
- Bucket-wheel excavators (dredgers)
- Coal crushers
- Coal washing machinery
- Conveyors/conveyor belts
- Cranes, hoists and cables
- Cutters, loaders and other specialised mining equipment
- Dredges, barges and support craft
- Drop hammers
- Dump trucks and other road transport equipment
- Electrical equipment: charging stations for batteries, substations, switch gear, trolley lines, electric lines.
- Explosives, blast caps and explosive handling and storage facilities
- Feeder breaker
- Fuel storage tanks
- Hand tools: shovels; picks; various electric, pneumatic tools; safety equipment.
- Laboratory equipment
- Laboratory equipment
- Materials handling devices
- Mechanical Thermal Expression (MTE) plant
- Milling and crushing equipment
- Mobile machinery including excavators, bulldozers, earth moving equipment, diggers, graders, scrapers, cranes, front end loaders, heavy duty forklifts
- Office equipment and furniture
- Office machinery and equipment, including computers, fax and photocopy machines, scanners, printers, modems, file servers etc.
- PA, security and fire suppression systems
- Personal protective equipment
- Portable lights
- Portable buildings
- Port terminal handling machinery and infrastructure
- Process control equipment
- Rolling stock, railway carriages
- Satellite-guided bulldozers
- Screening plants
- Storage hoppers and bins
- Skids and pallets
- Ventilation and air-conditioning systems
- Workshop/maintenance equipment including power tools, welding equipment, pressure testers, hand tools etc.