Mine On-Line Service (MOLS) carried out grade control study in a low grade nickel mine, utilising the all new Percussive Drill Sampler Analyzer (PDSA) trailer-mounted version, manufactured by IMA Engineering.
The goal of the study was to compare the current mine grade control method with the new continuous on-line sampling and analysis method, and calculate its effect to the mine economy. The test was done during normal blast hole production drilling utilizing 140mm bit size and 12 degrees inclined holes. A drilling pattern with 3.6m burden, 5m hole spacing and 1.5m subdrill was used. The 15m high 100,000t bench was located in waste-ore contact zone. The cut off grade used was 0.07% Ni.
Currently the mine analyzes the cuttings directly from the piles with hand held XRF – analyzer for ore grade control. One analysis from each blast hole drill cuttings pile on ground determines the location for ore – waste rock border.
The IMA PDSA collects all the drill cuttings from the rig’s pre-separator and analyzes its elemental compositions immediately while drilling. Multi-elemental analysis for four elements was made every 15 seconds, resulting in an average sample length of 25 cm of drill hole. X, Y, Z – coordinates were given to each analysis. Accurate small scale 2X2x2 meter block model was created using inverse distance variation method for the grades between the blast holes visualizing ore structure in the bench.
The study revealed significant difference between the current and the on-line grade control method’s results. The ore had relatively large small scale grade variations which the current grade control method could not detect. The handheld XRF, with a very low sampling frequency (one to three analysis per cuttings pile) and a small analysis area (few square millimeters per analysis), gives biased results.
Uncertainties in grade control lead to increased ore loss and waste rock dilution. In this case, MOLS was able to detect that the ore – waste limit had been placed on waste rock side, causing a 7.6% increase in waste rock dilution. PDSA also located previously undetected ore mass within the waste rock, causing a 5.5% ore loss. Most of these losses could be avoided using the on-line grade control tools.
Biased sampling results for ore-waste decision making have a significant impact on the mine’s economy. The projected added value created by the PDSA on-line drill cuttings analysis in the test bench only was from 60,000 to 160,000 USD, whether the losses are compared to cut-off grade or to mine average ore grade . Depending on annual utilization of the drill and PDSA, the investment to the On-Line Grade Control system had a projected payback time from weeks to months.