FRAGMENTATION
Orica has successfully introduced the Vistis Bulk System in Australia at Mt Rawdon
Orica has worked with Mt Rawdon since the site’ s production start in 1998. In recent years, both parties have collaborated on a number of blasting-related, business improvement-type initiatives, including the introduction of electronic blasting systems, blast characterisation and optimisation projects, and training of site engineers and blast crews.
Following on from this, the mine expressed interest in exploring the potential for increased mill throughput via high-energy blasting. Orica agreed to provide an engineer for a two-month period to investigate this opportunity as well as cover a leave of absence of a site engineer.
The original investigation revealed that Mt Rawdon did not have a‘ burning bridge’ as such – the mill was operating near capacity as a result of years of mill optimisation projects. However, it was recognised that there was some variability in the mill feed, and if that could be controlled then average throughput rates could be increased.
The decision was made to take a mill productivity approach: improve fragmentation as a whole as well as open up more options to allow the mine to treat the material in a way that stabilised and maximised throughput.
“ The first stage of the project was to assess the effect of blast energy on fragmentation and mill throughput. Initially this was done via a tighter drill pattern, specifically with double the powder factor used in conventional blasts. While there was some increased fines production from this technique, the damage to the blast perimeter was much greater, resulting in an increased number of large rocks being sucked into the muckpile. Overall, it was hard to see a benefit in both fragmentation and mill throughput, and given that the site was already drill constrained, it was decided this solution was unsuitable for Mt Rawdon.”
Staying aligned with the original objective of mapping blast energy versus blast outcome, stage two of the project involved introducing a high-energy bulk explosive system, specifically Vistis 250.
With technical support and design from Orica, three trial blasts were undertaken in ore using the Vistis system, in conjunction with precise timing from uni tronic™ electronic blasting. These results were compared with the existing baseline of Fortis™ Advantage and the stage one high powder factor blast.
Visually the results were obvious – the higher energy of Vistis 250 appeared to deliver much finer, more consistent fragmentation than the conventional Fortis Advantage blasts.
“ Although the material was finer, the method used to assess the impact of Vistis was fundamentally flawed. In an attempt to isolate as many variables as possible, each flitch of the baseline and Vistis blasts were mined and milled immediately after each other to give a direct comparison. Given these batches of material were relatively small, and the two blast results were significantly different, the mill was never able to stabilise and the throughput was chaotic. There were some very promising signs, however, overall the results were blurred and it was very difficult to see a clear improvement.”
After considering what was learnt in stage two, it was obvious that the solution needed to be more than just blasting – it needed to be truly from the mine site to the mill. Instead of focusing on individual inputs and‘ shot-by-shot’ comparisons, stage three would assess over a much longer period of three months. In addition, the run-of-mine was redesigned to give room to blend material types and make reactive changes to the feed, and operators were trained on optimising the plant. Most importantly, the mill played a much larger role in activities traditionally left to mining( such as planning, load and haul, and drill and blast).”
Over the three-month trial period, the mill throughput rate has increased by nearly 6 % compared to the baseline. Moreover, the corresponding specific energy has been reduced by over 7 % and SAG recirculation has decreased by over 22 % The overall variation in mill throughput has also decreased due to the more consistent feed.
After seeing how significant the change was in the original Vistis 250 blasts, there was an investigation as to whether Vistis 225 could be used to expand patterns in waste and ultimately reduce the total cost of drill and blast. Depending on the geological domain, patterns were expanded between 45 % and 60 %, with no adverse impact on dig rates from this expanded pattern.“ Naturally this resulted in a lower net drill and blast cost, but perhaps more significantly the mine was able to release its drill fleet and increase the productivity of the mine.” The overall financial benefit from the mill productivity is significant. Even though the drill and blast costs in the ore have increased, the overall net benefit to Mt Rawdon( including mining and processing costs) is expected to be over A $ 5 million per annum.
Orica says that while the Vistis system is a key input to the mine to mill program, the sustained benefits achieved by Mt Rawdon are just as attributable to the operational changes they have made. Their willingness to collaborate, trial, assess and modify has allowed the mine to capitalise on the improved fragmentation provided by Vistis and deliver an improved financial result.
Dyno Nobel finer fragmentation
An underground gold mine in Canada employs long-hole blast patterns in a narrow seam underground operation. Fragmentation results from the blasting are always a concern as they have a major impact on the downstream processes. In addition, the fragmentation plays a role in the recovery and dilution of the ore. The results of fragmentation modelling were expected to determine the fragmentation distribution from 4 ft x 4 ft square and 4 ft x 3 ft staggered patterns in basalt. The stated intent of the modelling was to determine the potential fragmentation distribution that will result from the specific blast designs when loaded with either ANFO, Dyno SL, or TITAN ® 7000 RU. A fragmentation model developed in-house by Dyno Nobel was employed to model the fragmentation that would result from changes in the various blast design parameters. The fragmentation model employs four different algorithms depending on the exact circumstances to be modelled.
For the parameters examined, the delay time had the greatest impact on the fragmentation followed by the blasthole pattern. Delay times of 25 ms and 100 ms were considered in this analysis. The 25 ms delay time provided finer fragmentation in all the scenarios when compared to a delay time of 100 ms. The 4 ft x 3 ft staggered blast hole pattern produced finer and more uniform fragmentation than the 4 ft x 4 ft square blast hole pattern. There was minimal difference in the fragmentation distribution for the three explosives examined within a given blasthole pattern. However, the finest fragmentation was achieved with Dyno SL. It was
14 International Mining | MARCH 2017