A case study that explores the potential for increased mill throughout using high-energy explosives
The Vistis™ Bulk System is a range of high-energy bulk explosives for use in hard rock metal mines.
Vistis produces over three-and-a-half times the detonation pressure and triple the relative bulk strength of ANFO. The energy levels generated, and the speed with which they are released, delivers optimal fragmentation that reduces overall operating costs either through pattern expansion, without impacting load and haul rates, or by providing broken stock that increases milling efficiency.
The range consists of two primary bulk explosives: Vistis and Vistan™. The Vistis range is designed specifically for wet blasthole applications while the Vistan range is designed for use in dry or dewatered blastholes.
Mt Rawdon overview
Mt Rawdon is an open pit gold mine located 75 km south-west of Bundaberg in Queensland. It is owned by the Evolution group, Queensland’s largest gold producer with its four operating sites. Mt Rawdon is a low-grade mine (0.71-1.0 g/) yielding approximately 100 000 oz of gold per annum.
Orica has worked with Mt Rawdon since the site’s commencement 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 the 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).
Ore blasting result
Over the three-month trial period, the mill throughput rate has increased by nearly six per cent compared to the baseline. Moreover, the corresponding specific energy has been reduced by over seven per cent and SAG recirculation has decreased by over 22 per cent. The overall variation in mill throughput has also decreased due to the more consistent feed.
Waste blasting – solutions and the result
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 per cent, 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 $5 m per annum.
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.
Mark Boon, General Manager Mt Rawdon Operation, said that Orica worked closely with Mt Rawdon to understand what was important to the mine before proposing changes. The close working relationship, measurement of all the critical metrics around mining and milling and changing the way the milling processing was operated all contributed to the results. The step change achieved from the Vistis system has been a key enabler of the mine to mill success and achieving significant value for Mt Rawdon.
A video case study can be viewed online at www.orica.com/vistis