Modern Quarrying Q3 2021

Rooikraal was the first within the AfriSam Group to acquire a licence to blast within 500 m of structures.

AT THE QUARRY FACE

block. The improvement in energy retention leads to several benefits such as reduced flyrock, improved fragmentation, and reduced vibration and noise, among others. Improving blast performance Not only does the Varistem help reduce potential safety incidents such as flyrock and vibration, but it also improves blasting performance and decreases costs. The plugs con- tain blast energy for longer. Maximum blast containment, explains Sterley, means greater fragmentation and micro-fractures, with more tonnage processed at less cost. Following fractures created by the explosive shock waves, the Varistem improves expanding gas containment. The greater the gas containment time within the ore mass fractures, the better the fragmentation. “Getting fragmentation right is substantially cheaper than having to do secondary blasting or mechanical breakage,” he says. In some cases, Varistem has achieved up to a 25% increase in fragmentation, which in turn reduces crushing costs and increases crusher throughput. Independent studies also show that the Varistem can increase blast pattern spread by 10% or more, with the same fragmentation for major savings in drilling and explosive costs. “There are several secondary cost savings which result from increased fragmentation – easier rock removal, reduced secondary breakage costs, increased truck capacity, and less wear on crushing equipment – all of which contribute to the cost benefit,” says Sterley.

The blast analysis clearly indicated that the time to stemming movement of the Varistem side of the block was multiple times longer than on the non-Varistem side of the block.

the use of blast-improvement and containment plugs is that they could improve the effectiveness of stemming material in the blasthole. As a result, this would better contain the explosive energy within the rock mass and yield a more controlled and efficient blast. Of the various products that have come to market to improve blast performance, flyrock control and productivity, stemming plugs have probably been looked upon with some incredulity. “We were initially sceptical and eventually did a test run and the results were amazing. The intent of the trial was to demonstrate the energy retention capabilities of the Varistem blast stemming plugs through conducting a split blast,” explains Sterley. The trial run indicated a massive improvement in energy retention. A key takeaway was the increase in time to stemming movement. The blast analysis clearly indicated that the time to stemming movement of the Varistem side of the block was multiple times longer than on the non-Varistem side of the

and spacing, which improves our fragment size distribution and the total cost of operation,” he says. Stemming plugs To further reduce the likelihood of stemming ejection and flyrock, the operation became one of the first in South Africa to adopt stemming plugs. “We recently started using Varistem stemming plugs, supplied locally by ERG Industrial,” says Sterley. The stemming plugs are placed between the explosive column charge and stemming. Plugs work by creating an additional blocking effect within the drill hole when blast energy is released, directing more energy into the block and less energy upwards, out of the drill hole. Upon detonation of the explosives, the plug is forced upward into the stemming material and “locks up”. The explosive gases and energy are prevented from travelling (venting) up through the drill hole. Typically, the loss of explosive energy through stemming ejection reduces the performance of the blast. The fundamental theory promoting

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MODERN QUARRYING QUARTER 3 - 2021