AFTER tailoring non-explosive rock-breaking products to its block-caving needs, De Beers’ Finsch diamond mine in South Africa has doubled the number of draw point hang-ups it can clear in a work shift.
In the block-caving mining method, the ore body is undercut so that the bottom portion starts collapsing — or caving — due to the mass of overlying material.
The ore fragments fall under gravity, gathering at draw points for collection and later processing. Meantime, the caving process gradually propagates upward and further into the ore body.
But too-large fragments can block a draw point portal. The culprit can be a single boulder or a stringed arch of smaller boulders. Block-caving miners call such blockages “hang-ups”.
Hang-ups need to be cleared quickly, for good reasons.
Stresses created by the blockage transmit to important mine structures, such as draw point brows and extraction-level tunnels, according to De Beers. Damage can go as far as causing footwall heave and failure of tunnel sidewalls.
Company drill and blast engineer Llewellyn Dippenaar said “rat holes” were another serious side-effect of not clearing hang-ups quickly enough. The phenomenon is where functioning draw points are overused, so that corresponding ore columns are eaten through and waste rock begins reporting to the draw points too early.
In severe cases, a draw point can become uneconomical and ore reserves unrecoverable.
Conversely, Dippenaar said, good draw control maximised the ore that could be extracted via block caving.
Dippenaar said the Finsch mine had experienced difficulties in trying to clear hang-ups with the explosives conventionally used for the task.
“In the past we would drill and charge the hang-ups with normal commercial explosive and wait ’til shift change-over to blast the big rocks,” he said.
If a second blast was needed before a hang-up was fully cleared, another wait for a shift change-over ensued.
Production pressures associated with failing to blast down a hang-up the first time around meant staff were tempted to go overboard with the explosives. However, according to Dippenaar, in most cases overcharging not only pulverised the rock plugging a draw point, it also damaged the draw point brow or the stiff brow support.
“Then we started shopping around, started talking to people. We looked at conventional explosives versus propellants,” he said.
According to a South African-based non-explosives supplier, propellants don’t detonate but instead deflagrate. Deflagration reactions generally propagated at speeds of 200-1000 metres a second, while detonation reaction speeds were between 1000 and 10,000 metres a second. Detonation produces peak pressures within millionths of a second, an order of magnitude faster than for deflagration. Maximum pressures developed are also around 10 times greater.
The result is that propellants produce no leading supersonic shock wave — just a sustained pressure front created by the gaseous reaction products.
The spin-off the Finsch mine staff spotted was that propellants had peak particle velocities that were below those known to cause damage to mine structures.
“So it looked very effective and we started playing around with it,” Dippenaar said.
At first, Finsch mine’s road-testing of the propellant-based materials, or “non-explosives” as they are also known, produced an appreciable rate of misfires and blowouts. The bugbears responsible included drilling holes too shallow or too deep, having too big a toe burden, and insufficient tamping.
Undeterred, Dippenaar, who says he loves a challenge, joined a few other colleagues to experiment with the propellant-based product. They found that by replacing fuse heads with detonators they could increase the speed of the initial reaction and break ground every single time.
“We eventually ended up making holes and not even tamping it and it would still break the rock. Then we said, ‘OK, now we’ve got a breakthrough here.’ “
Now that use of propellant-based rock-breaking was in full swing at Finsch, dedicated hang-up clearing crews have opened up around 35 draw points per shift, according to Dippenaar. Previously, using conventional explosives, the mine would typically achieve only half of that, if it was lucky.
A key factor in the productivity improvement was the reduction in the amount of toxic gases produced in blasts. The quicker re-entry permitted blast staff to follow-up recalcitrant hang-ups the same day.
And as hoped, damage to brows and brow supports decreased. Fly rock was reported to be minimal as well, and, according to Dippenaar, the pre-blast removal of personnel didn’t need to be as drastic as for conventional explosives.
In response to being asked whether the eventual payoff had been worth the cost of establishing non-explosive rock-breaking at Finsch, Dippenaar said “definitely”.
“Repairing one brow support is in the region of R80,000,” he said.
“That on its own covers the cost, not even mentioning the availability of draw points for drawing ore.” He also said the value of ore lost due to tunnel or brow damage could run into the millions.
Dippenaar said De Beers was now in the process of testing the propellant-based blasting product in its open cast operations.
*This article was based on a presentation delivered at the Marcus Evans’ conference: Optimising Productivity in Drilling and Blasting — harnessing advanced technology and techniques to optimise mineral recovery, increase performance and reduce costs. For more information about Marcus Evans’ conferences, visit www.marcusevans.com.
Drill and blast specialist
De Beers Consolidated Mines