China operates more than 100 top coal caving faces, and produces over 200 Mtpa. With this experience, the Chinese can teach the Australian mining industry a thing or two about longwall top coal caving.
The caving method is being used to produce up to 7 Mtpa per mine, and the technique can be used to mine steeply dipping and extremely thick seams.
“Mines with thick seams can become much more productive in their cost per tonne and the amount of coal that they are producing,” CSIRO senior mining engineer Patrick Humphries told Australian Mining.
“Like all people in the coal industry, mine managers want more tonnes at lower cost,” he said.
Top coal caving enables the mine to extract more tonnes as you retreat, driving down costs.
“There are a large percentage of thick coal seams here in Australia that we can not fully extract with our current longwall technology,” Humphries said.
“Mines are unable to extract coal from longwall faces over 4.5 m to 4.8 m high, so operators are sterilising the coal, and leaving a lot of it behind,” he said.
This is not only a waste, but a safety risk, as the risk of spontaneous combustion is increased when leaving fuel sources in mined out areas.
High-reach longwall faces are cutting higher and higher in Australia, with the highest of those cutting up to 4.8 m high.
“This can cause stability issues, and the risk of coal falling from the face into the workspace is increased,” Humphries said.
With top coal caving, the initial face cut is much lower, down to around 3 m high.
“Top coal caving can remove potentially hazardous coal, and it has a positive effect on cost and face stability, as the initial cut is lower than traditional longwalls,” Humphries said.
Humphries has recently released a report into top coal caving through the Australian Coal Association Research Program (ACARP).
The research looked at geotechnical issues and caving assessment, subsidence, operating issues with ventilation, gas and dust, and the different ways of operating longwalls to suit Australian conditions.
“We have developed engineering design tools to characterise coal seams, so that miners can evaluate the potential of top coal caving recovery and production in an operation,” Humphries said.
The top coal caving method is very similar to conventional longwall mining at the front end.
“However, the caving technologies at the back are completely new, and the system is a fundamental shift in thinking,” Humphries said.
“Operators need to know the ground that they are mining to estimate caving recovery and how they will operate the face.”
DBT has built Australia’s first top coal caving longwall in the New South Wales’ Hunter Valley.
“They will go through a learning curve with the Chinese,” Humphries said.
“We have a wealth of knowledge in conventional longwall mining and there will be a learning period to bring top coal caving in Australia up to the same standard.”
There is about 6 billion tonnes of thick seams coal in Australia, a significant proportion of which is perfect for top coal caving, according to Humphries. Potential top coal caving reserves are spread 75% to 25% between Queensland and New South Wales respectively.
“A very large portion of mines in Queensland would benefit from the technology,” Humphries said.