AMC Consultants has been engaged by Silver Mines Limited to complete aspects of a Value Engineering Study (VES) for the Bowdens Silver Mine in New South Wales, Australia. The package of work is intended to update and improve the understanding of the geological, metallurgical, geotechnical and mining engineering components of the previous project studies. The work will lead into a Definitive Feasibility Study and, ultimately, into development of the project. As part of the VES, AMC proposed to implement a geometallurgical programme to improve the characterization of the processing behaviour of the ore.
Part of this programme was a trial of the Equotip rebound hardness tester to measure the hardness of the drill core, and for comparison with the results of conventional comminution testing. The Equotip is a small, portable device that has been demonstrated to be an effective tool for orebody hardness measurement. It consists of a pen-like impact device containing a small, spring-mounted, tungsten carbide–tipped spear. The spear is released and passes through the pen, strikes the sample and rebounds. The impelled velocity of the spear is measured when the permanent magnet, built into the spear, passes through a wire coil near the base of the pen. The device also measures the velocity of the spear as it rebounds from the test object. The hardness of the material (Leeb hardness) is calculated as the ratio of the rebound velocity and the impelled velocity, multiplied by 1,000. The method is non-destructive and individual measurements only take a few seconds.
Figure 1 The Equotip tool (left) and data capture device (right)
After completing orientation tests, Equotip measurements were conducted on the core remaining from six intervals on which standard comminution tests had been carried out in 2012. The objective of the tests was to determine whether a useful correlation could be made between Equotip hardness measurements and the results of the earlier SMC and Bond work index tests. All measurements were carried out on quarter core-in-core trays positioned on a heavy wooden workbench in the core shed. The hardness of the core was measured with the Equotip at approximately 2 cm intervals over the 10 m – 15 m intervals corresponding to the 2012 comminution test samples. The hardness measurements were averaged over each metre and were relatively consistent within each intercept. Scattered very low values appeared related to small areas of alteration or imperfect contact between the instrument and the core.
The results were plotted against the comminution test results from 2012. The Leeb hardness measurements show remarkably strong correlations with Bond ball work index (BBWi) and the A x b parameter derived from SMC Tests®. The BBWi provides an estimate of specific energy consumption in a ball mill and the A x b parameter, which has no formal units, is a rock breakage parameter that is valuable for semi-autogenous grinding (SAG) mill design. An exponential model provided an excellent fit with R-squared values close to 1.
Figure 2 Scatterplot of Leeb hardness versus Bond ball work index
The tests demonstrated that the Equotip can be used to map hardness variations in drill core at a scale that can be correlated against comminution test data and geological logs. At a cost in the order of $5/m, the Equotip is much less expensive than conventional comminution testing and the tests are non-destructive. Logging of all the drill core from the deposit is therefore a cost-effective option. The tool has the potential to provide a much-improved estimate of power consumption, mill throughput, and operating costs for the Bowdens project.
As well as having application in feasibility studies, hardness mapping using the Equotip, could potentially be applied to many operating mines where the milling circuit struggles to handle fluctuations in ore hardness. Variability in the quality of mill feed may reduce throughput, increase recirculating load, and increase power consumption. Equotip testing could be used to map ore hardness in sufficient detail that Bond work index or A x b could be estimated on a block-by-block basis in the mineral resource and ore reserves models. Comminution characteristics could then be included in block value estimation, and mine scheduling, resulting in reduced variability in the hardness characteristics of the run of mine ore, and consequently higher mill throughput and/or lower energy consumption. For more information on hardness estimation or geometallurgy, please contact Ian Lipton.
Corporate Consulting Manager / Principal Geologist