LONGWALL MINING FOR UNDERGROUND COAL

Overview

In the method of secondary extraction known as longwall mining a relatively long mining face (typically in the range 100 to 300m but may be longer) is created by driving a roadway at right angles between two roadways that form the sides of the longwall block, with one rib of this new roadway forming the longwall face. Once the longwall face equipment has been installed, coal can be extracted along the full length of the face in slices of a given width (referred to as a "web" of coal). The modern longwall face is supported by hydraulically powered supports and these supports are progressively moved across to support the newly extracted face as slices are taken, allowing the section where the coal had previously been excavated and supported to collapse (becoming a goaf). This process is repeated continuously, web by web, thus completely removing a rectangular block of coal, the length of the block depending on a number of factors (see later notes)

Basic longwall mining principle simplified (retreating longwall in this case)

A coal haulage system is installed across the face, on modern faces an "armoured face conveyor or AFC". The roadways which form the sides of the block are referred to as "gate roads". The roadway in which the main panel conveyor is installed is referred to as the "main gate" (or "maingate"), with the roadway at the opposite end being referred to as the "tail gate" (or "tailgate") roadway.
The benefits of longwall mining compared to other methods of pillar extraction are:

• Permanent supports are only needed in the first workings portion and during installation and recovery operations. Other roof supports (longwall chocks or shields on modern longwalls) are moved and relocated with the face equipment.
  
• Resource recovery is very high - in theory 100% of the block of coal being extracted, though in practice there is always some coal spillage or leakage off the face haulage system lost into the goaf, especially if there is a lot of water on the face
  
• Longwall mining systems are capable of producing significant outputs from a single longwall face – 8 million tones per annum or more.
  
• When operating correctly the coal is mined in a systematic, relatively continuous and repetitive process which is ideal for strata control and for associated mining operations
  
• Labour costs/tonne produced are relatively low
  

Disadvantages are:

• There is a high capital cost for equipment, though probably not as high as first appears when compared to the number of continuous miner units which would be required to produce the same output.
  
• Operations are very concentrated ("all eggs in one basket")
  
• Longwalls are not very flexible and are "unforgiving" - they do not handle seam discontinuities well; gate roads have to be driven to high standards or problems will arise; good face conditions often depend on production being more or less continuous, so problems which cause delays can compound into major events.
  
• Because of the unforgiving nature of longwalls, experienced labour is essential for successful operations.
  

A major decision to be made is the size of longwall blocks. Because modern longwalls involve a large number of pieces of equipment (numbers of a magnitude of several hundred items, with many components weighing up to 30 tonnes or more), the process of recovering the equipment from a completed block, transporting it to a new block and then installing it in the new block (often with much of it being taken out of the mine for overhaul on the way) is a very major operation. Apart from the direct cost involved, production and hence income is zero during this period. Bigger longwall blocks will enable the number of relocations to be minimized, however there are limiting factors to the size of longwall blocks:

• The longer the face the more power is required on the face coal haulage system (see later notes on AFC's). The greater the power, the larger the physical size of the drive units (usually there is a drive unit at both ends of the face). The drive units have to fit into the excavation and allow room for access past them, for ventilation across the face and for some degree of roof to floor closure. Also the greater the power, the larger (and therefore heavier) the chain on the face conveyor – these chains have to be manhandled on the face at times and there are practical limitations as to the size of the chain.
  
• In some longwall installations, the heat created by the high power haulage drives may become a factor.
• Both face width and length may be governed by limitations created by lease boundaries, seam discontinuities or variations, already existing mine development and/or ventilation capacity.
  
• The ability of the mine to develop new longwall blocks so that longwall production continuity is not adversely impacted.
  
• Condition of equipment – changing out some items for overhaul or replacement during the life of a longwall block can be problematic, and is best done during a relocation.