JOHANNESBURG (miningweekly.com) – Confidence is growing in a new method of underground gold mining that has the potential to reverse the decline in South Africa’s gold output.
A raise boring machine put to the test in the stopes has passed with flying colours.
“There’s nothing, technically, that stopped us from cutting the rock, which was quite significant for us,” AngloGold Ashanti executive VP Mike MacFarlane tells Mining Weekly Online.
A local drilling contractor used conventional raise-bore equipment to drill 26 m from one sublevel to another at an angle of 26 degrees in what is one of a total of six critical projects that, if successful, could take South Africa back to being the world’s top gold producer.
“We’ve also been working in earnest on three of the other critical projects and we really haven’t had any major setback.
“Everything has pretty much gone the way we thought it was going to go, although there’s a lot more work to be done,” says MacFarlane.
The huge benefit of the proposed new method of mining is that it is the gold and only the gold that is extracted at considerably greater volume.
Companies are being engaged to come up with an ultrahigh-strength backfill that will dispense with the need to leave large pillar areas unmined in order to assure safety.
South Africa still has one of the biggest gold resources in the world, but that gold is at depth, which results in significant amounts of the precious metal having to be locked up in shaft pillars and stability pillars, because blasting snarls up the ground conditions and creates seismicity.
The proposed new way of mining dispenses with blasting, keeps people away from the danger areas; facilitates mining round-the-clock every day of the year and refocuses on revenue.
Current wasteful mining practise is considered sustainable, not only because it sterilises too much gold in the ground, but also because South Africa is not competitive with its peers in term of days worked and time at the face.
Global competitors mine 365 days a year at only 5% to 10% dilution to the 274 working days of South Africa’s deep miners, 200% dilution and only 75% of the shifts.
Greater depth also adds to travel time, which means that mineworkers in many deep South African gold mines end up spend fewer hours at the face.
Miners also have to leave the mine prior to blasting and only return after the dust has settled.
The new thinking is to mine all the gold, only the gold, all the time, and avoid leaving 40% of the gold behind in pillars.
“What we’ve proved to ourselves technically is that there’s existing equipment that's on the market right now, that’s proven technology, that can drill from one sublevel to another, over a 26 m distance; but we could probably drill further than that,” MacFarlane tells Mining Weekly Online.
The beauty of reef boring is that nobody has to work very hard physically and the machine does the work.
As there is no need to vacate the mine, productive operation can go on continuously for as long as the shift schedules allow.
New mine design is pretty much done and harder-than-concrete backfill of up to 100 MPa is on the way.
“We’ll work hard at that for the next six months,” MacFarlane comments to Mining Weekly Online.
The fourth project is tunnel boring, which is required to get to the reef to do the reef boring.
The AngloGold Ashanti team is busy with a group of people on redesigning the tunnel borer.
Prototype testing has already been done on the fifth project, reverse-circulation drilling, which involves putting a camera up the hole to locate the reef and take a picture of it in such a way that the footwall, the hanging wall and the reef are sufficiently visible to create a three-dimensional mine model.
“We’ve already done that and put the camera up the hole and the resolution was beyond our expectations the first time we tried it. It worked right away, so we were quite encouraged by that,” he says.
The sixth project involving delivering the reef to surface is something still to be worked on, but creating particular concern.
“So those are the six critical technology projects that underpin this and we’re busy over the next 18 months just working through those prototypes and ironing out the bugs so that when we try to put it all together, it will be close to ready to go the first time we try it, that’s the thinking behind prototyping and working out the problems individually and then putting it all together,” MacFarlane adds.
The years 2012 and 2013 have been set aside for prototyping and 2014, 2015 and 2016 for the end-to-end solution and proving up commercial viability.
In parallel, the company’s operations are growing increasingly receptive and talking of selectively implementing reef boring and backfilling by the end of 2014.
A couple of moths ago, a report in the archives from 1974 showed the a virtually identical reef-boring project was found to be technically viable 40 years ago but then abandoned on backfill snags.
The decision not to go ahead coincides with the beginning of the decline of the South African gold industry.
The two mirror each other, reflecting poor leadership and a decision to slow investment.
But what is heartening for today’s researchers is that the archival data reinforces that they are on the right track and that what they are proposing is technically feasible and that its commercial viability hinges on the costs and selling price of gold.