A CSIRO breakthrough in block caving is helping companies cost-effectively mine large, low-grade deposits with greater control. TIM TREADGOLD reports
Article from resourceful: Issue 10
If you're working in an underground mine, there is no power harder to harness than gravity, for the simple reason that the ore being mined has to be hauled to the surface. But, there is a way of using gravity deep beneath the Earth's surface: it's called block caving.
Technically difficult and painstaking in its preparation, block caving is pretty much what it sounds like – dig underneath a block of ore and let in cave in. Once 'caved' the broken ore is collected and hauled to the surface for processing.
If the preparation is not done well or the ore does not fracture into pieces small enough to be easily handled by underground equipment, then an expensive problem can be created.
One of the best ways to avoid a 'blocked' cave, and to reduce ore to a manageable size, is to apply the rock-fracturing techniques a team from CSIRO has helped develop and which has become a worldwide standard in block cave mining.
"Hydraulic fracturing of the ore to be caved has proved to be extremely successful in a number of mines in Australia and overseas," says CSIRO senior research technician, Kevin Quinlan.
The process involves drilling a series of holes into the orebody, inserting inflatable packers that are filled with water, sometimes with a gel added to prevent leakage, and then apply pressure which expands natural cracks in the rock to 'pre-condition' the ore.
In cold climates, nature has a similar way of cracking rocks because water that has entered a natural fracture expands as it freezes, exerting pressure which extends the fracture.
In a man-made situation, once the hydraulic fracturing has done its work, the ore is blasted in a conventional way and gravity takes over, with the ore dropping to be collected and lifted to the surface in manageable pieces.
Principal users of the technique are companies working with large, low-grade orebodies such as copper miners in Chile and gold miners in Australia.
The Cadia and Ridgeway goldmines of Newcrest Mining in western New South Wales are examples of successful block caving.
Done well, block caving is a cost-effective way of extracting large volumes of ore in an underground setting.
But, like many advances in science and technology a better understanding of block caving started in a trial and error fashion. Problems with a cave at an Australian copper mine in the 1990s provided an example of what can go wrong if the ore is not fully fractured when blasted.
The formation of an 'air-gap' meant that a large portion of the ore high up in the area being caved became stuck, presenting a complex problem which required considerable time and money to be fixed.
"Pre-conditioning with hydraulic fracturing can play an important role in ensuring that events such as that do not occur," Mr Quinlan says.
There are a number of aims of preconditioning, which generally starts with drilling a number of holes spaced about 50 metres apart into the orebody, ensuring the best alignment with natural fractures in the rock before inserting the inflatable packing units.
"Hydraulic pre-conditioning can help induce caving and provides greater control of the cave," Mr Quinlan says.
"It also helps reduce the size of rock fragments and helps produce closely spaced fractures to prevent blockages making the ore easier to handle as it drops into the collection zone."
"Pre-conditioning is also useful in destressing the ore, especially as you get deeper underground, reducing the risk of rock outbursts, which can be both a safety and cost factor," he says.
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