As with most of the issues confronting the open cut mining industry, forward planning can make all the difference.
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Whether it’s final voids, rehabilitation or managing spontaneous combustion issues, getting the mine plan right before the first sod is turned is vital.
And by spontaneous combustion we are not just talking about smouldering.
The landforms created post-mining can also be susceptible to spontaneous combustion.
These ‘created’ landforms given the right conditions and depending on the reactive nature of the rock and the quality of the rehabilitation work could pose spontaneous combustion risks.
In the future, or sooner depending on the price and demand for our coal, the first large scale open cut mines in the Upper Hunter will close and spontaneous combustion in these closed mines may become a problem if it is not managed appropriately.
Mining spoils or overburden that contain reactive pyrite need to be buried to prevent exposure to the atmosphere after mining. It is also important that this work is done in a reasonable timeframe that reduces the amount of time the spoil is exposed.
According to spontaneous combustion expert Dr Basil Beamish (an Adjunct Associate Professor at the School of Mining Engineering, University of New South Wales) the coal industry has been developing new strategies to effectively manage the issue of spontaneous combustion and this includes risk-based assessment of the hazard for mine closure.
This topic was recently covered at the Australasian Institute of Mining and Metallurgy Life-of-Mine Conference in Brisbane.
‘The trick is to stop it happening by burying the overburden in a systematic manner that keeps the reactive rocks away from air and water,” he said.
“Capping thickness and using the right materials, for example clay or other inert rock is also essential to prevent water and air getting to the reactive material and causing spontaneous combustion post mining.”
He said gases from spontaneous combustion that has reached high temperatures can be hazardous in confined spaces.
Locally the mines with the highest outbreaks of spontaneous combustion according to a thesis undertaken by Kiera Shiels (‘Fugitive Emissions from Open Cut Mines in NSW”, University of Sydney, 2011, ) in consultation with staff from the NSW Office of Environment and Heritage are Anglo American ‘s Drayton and it neighbour BHP Billiton’s Mt Arthur.
This is put down to the fact they mined Greta Coal Measures.
In Anglo American’ Mining Operation Plan for Drayton signed off last year and covering the mine’s closure there are actions that the company is planning to do to minimise spontaneous combustion including capping potential areas with between 2-5m of weathered sandstones and heavy clays.
There will be aerial and terrestrial laser surveys and regular review of rehabilitated area to confirm any instances of spontaneous combustion and implementation of required mitigation responses.
For at least five-years post closure the company will continue to complete aerial surveys to monitor the landform topography in order to identify any areas of the site where the capping of inert material has not been achieved or where additional maintenance work may be required.
In Mt Arthur’s Annual Environmental Management Report (FY15) it states there was a 13 per cent decrease in the overall amount of area affected by spontaneous combustion.
This reduction was put down to excavation and loading out of affected material and capping works. But the mine received complaints relating to odour and smoke. The two mines organised a joint thermal imagery scan flight over affected areas.