Mine excursion
A view of the construction from base camp, on top of the Bisie hill.
Leon Louw the mine. Sub-level caving often results in subsidence of the ground surface directly above the mining operations. Alphamin has developed a subsidence model, ensuring that its infrastructure is constructed outside the zone of subsidence and to ensure that the subsidence does not cause negative and long lasting environmental problems.
But, the benefits of sub-level caving might just outweigh the risks. Because all the mining activities are executed in or from relatively small openings, it is one of the safest underground mining methods around. It is also extremely cost effective and is often referred to as mining’ s dependable low-cost‘ ore factory’. In addition, it allows a large amount of flexibility with production rates.
“ We will rely on the hanging wall caving in a controlled manner, which means we won’ t have to bring in steel to support the roof. Another major factor is that we won’ t leave any pillars behind, which in a high-grade mine like Bisie, is acutely important to maximise the revenue line. There are also no long term seismic problems in the mine, as you are relieving the stress in the rock above by allowing it to collapse, so you are destressing the rock all the time,” says Faber.
Sub-level caving is a repetitive mining method that has been very successful in underground iron ore and copper mines in Sweden, where they have perfected the art of drilling long sub-level holes of almost 45m with minimum dilution.
Managing dilution
However, Faber says that Bisie will drill holes no longer than 15m to reduce the risk of excessive dilution.
“ We will have good grade control procedures in place and are in the process of compiling protocols for how to manage the cave. That, in the end, is what it is all about – managing the cave – which will be very challenging,” says Faber. To mitigate against excessive dilution feeding into the process plant, the design team decided to install inline pressure jigs from Gekko ahead of the gravity concentration plant.
“ There is a significant difference in specific gravity between the ore and the waste country rock. Test work has shown that the Gekko jigs are effective at eliminating the dilution at the very first stage in the processing plant,” says Faber. So, the philosophy is to mine as cost effectively as possible underground, and have the jigs remove the dilution on surface, rather than opt for an expensive selective mining method to reduce the dilution underground. The crushing section of the plant will be a little bigger than one would expect of a mine the size of Bisie, with a primary, secondary and tertiary section able to
[ 16 ] MINING MIRROR JANUARY 2018