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Consultant’s Comment - Another punch in the wall-2

by wallacep created Aug 31, 2009 05:48 PM

Based on feedback from readers to our story on coal mining extraction techniques in the previous edition of The AJM, we continue our overview and provide more detail on certain methods. We also cover the equipment employed in strip mining operations.

  
Consultant’s Comment - Another punch in the wall-2


By Dmitry Przhedetsky

Draglines – one of the most popular types of mining equipment used predominantly in coal mining for overburden removal and backfill for strip mining requiring a large amount of bulk excavation, usually due to high strip ratio. Dragline is a typically electric powered rope excavator, often used in combination with other equipment for pre-strip mining (hydraulic excavators; scrapers; bucket wheel excavators or for opening the coal (hydraulic excavators or dozers and front end loaders).
Pros: ability to move large amount of rock mass; high availability; long service live; low operating cost under suitable deposit geometry.
Cons: high upfront capital cost; many variables affecting performance throughout mine life or different deposits, imposing various cut-off factors, such as depth; necessity to rehandle spoil; necessity to use in combination with other equipment. Solutions: monitoring and optimisation of dragline performance and mine design; optimised use with other equipment and technique, e.g. scrapers; cast blasting, properly called CTB (Controlled Trajectory Blasting).

Bucket wheel excavators – can be used for overburden removal in soft rock, mainly for pre-strip operations. These have been proven as the most efficient mining method for lignite mining.
Pros: low operating cost; continuous mining and conveying of waste (or coal).
Cons: only justifiable for mines with long life and constant geology.
Solutions: consideration for some new mines in Australia; combination with some other methods and equipment.

Truck and shovel – while often used as an independent method of overburden removal and coal mining itself, could be used efficiently for pre-strip operations ahead of draglines, as well as for interburden removal in multi-seam deposits.
Pros: manoeuvrability; low upfront capital cost; flexibility of mine layout in complex geological conditions.
Cons: higher operating cost; lower productivity.

Overburden Slusher (also known as BOSMIN™ Slusher) – an innovative alternative to draglines for large mass overburden removal especially beneficial for deep strip mining. In essence the system engages an excavating hoe (scraper bucket) operated by two winches placed opposite each other on the high wall and low wall sides of the strip.
Pros: allows for deep strip mining; simpler layout of operations; conversion of existing draglines.
Cons: has never been tried before.
Solutions: operational trial(s) of the system.

Underground coal gasification (UCG) - is a process of in-situ conversion of coal into gas and extraction of this gas as the end product. An oxidant pumped through injection wells reacts with coal seam and the chemical reaction generates gas, which is then pumped out through extraction wells. The received gas can be used for chemical processes and power generation.
Pros: can be applied to complex and uneconomical for other mining methods coal deposits; free from many components of environmental impact of conventional coal mining.
Cons: suitable for less than 10 per cent of known coal deposits; only applicable to deep coal deposits with specific properties of the surrounding rock, that is non-porous, etc.; is seen as a source of potential contamination of local aquifer.
Solutions: requires more industrial experience in Australia and advanced hydrogeological modelling.

The most common definition of Shortwall mining is the use of a standard continuous miner and shuttle cars across a coal face usually limited to 100 metres. Also, modern shortwall mines use mechanised roof support. When a shearer is used across a short face it is more common to refer to this method as Miniwall mining, whereas a shearer and a conveyor could be used across a face, as narrow as 24 metres (hence a confusion between a “shortened” longwall and an “extended” shortwall.) The shearers for Miniwall mining could be designed single ended, so they could be moved into the gateroad when moving the face. Also, there was a Midwall system developed (essentially a short longwall system) marketed for use in coal resources with frequent dyke and fault structures.

Whilst we cannot cover all mining methods and popular mining terms here, this knowledge is available from many specialised consultancy firms. It’s not only the “traditional” mining methods which could be improved, but many new methods are being developed for particular deposits or/and by particular suppliers of mining equipment capable of implementing an innovative idea into common practice. These new ideas are often emerging from relative (and sometimes distant) industries allowing the mining science and industry to overcome technical, economical and environmental challenges.
Special thanks to Frank Roxborough; Greg Mattila; Cliff Mallett; Bob Beatty; John Pala.
* Dmitry Przhedetsky (M Eng (Mining), FAusIMM) is a director of Rock Cognition Pty Ltd. Contact him at: dmitry@rockcognition.com.au

 





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