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