NEW results reveal mine-to-mill optimisation can work where productivity improvements aren’t just a matter of upping blast energies.
To date, the optimisation approach has typically been used in the relatively straightforward context of autogenous and semi-autogenous grinding mills.
Recent work at the US-based Pittsboro aggregates quarry — where increasing the production of fines was anathema — has shown a fixed feed-size profile can be maintained even when the ore’s characteristics are all over the shop.
But events reveal that the learning process wasn’t all one-sided.
The research consultants from the Julius Kruttschnitt Mineral Research Centre (JKMRC) found their earlier attempt at the Bealeton quarry, also in the US and run by the same aggregate producer, didn’t turn out at well as they’d hoped.
“We were confident about the JK mine-to-mill technology and past history,” JKMRC’s Toni Kojovic told Australian Mining.
He said the Centre had a string of successful case studies behind its belt at the time. It had a bevy of figures that demonstrated the operational improvements possible.
“What was an eye-opener for us was that when we first tried to apply this [to aggregates], at Bealeton, I think we fell short.”
Kojovic said they hadn’t recognised the implications of the constraints the operation was under, such as the variability in the rock being harvested and the proximity of in-pit equipment.
“It wasn’t really a simple case of just saying here are the new recommendations, go and do them. That’s what we essentially did at Bealeton — and it backfired.”
Instrumental in that was their decision to use a side-by-side blast comparison on a single bench.
The JKMRC consultants went with site staff’s belief that the ore in the bench was uniform. They went ahead with a blast using the aggregate producer’s usual practice, followed by an adjacent blast modified according to their recommendations.
As it turned out, the rockmass change in that bench worked against the consultants. Even though the modified blast produced more of the finer fractions, a prohibitive amount of oversize was still present.
Kojovic told delegates at a recent conference in Perth that “we didn’t expect that in the same bench the ore and rockmass changed to the point of the ridiculous”. It meant the two blast designs couldn’t be compared meaningfully.
Chastened, Kojovic and his JKMRC colleague Darren Thornton, moved onto the 1.6 million tonnes per annum Pittsboro operation.
“We went into Pittsboro with all the new knowledge that we had acquired at Bealeton, and approached that job from a totally different perspective,” Kojovic said.
He said the pair were determined to understand the quarry and how the aggregates producer, Luck Stone, went about its business there — before they opened their mouths. Their new approach also involved watching and measuring day-to-day quarrying and plant operations at Pittsboro for three months before simulating alternative practices and producing guideline changes.
“If you boil it down, we wanted to reduce the amount of coarse rock, the stuff above 200 millimetres, without increasing the proportion of crusher run.” Crusher run for the quarry was the fraction measuring less than 38 millimetres.
At the same time, the JKMRC consultants were aiming to make fragmentation more consistent, regardless of where in the quarry pit the rock was being sourced from. The goal there was to allow Luck Stone to maintain relatively stable plant settings and so minimise changes needed from month to month.
The process required numerous measurements, surveys, and modelling run-throughs, according to Kojovic.
“We interrupted the plant dozens of times to collect samples. We were nosing around to learn how they were doing their blasting and crushing.”
Out of the three-month observation period came a single page of guidelines. They suggested ways of modifying Pittsboro blasts given circumstances such as the rock type being blasted, the bench height, the amount of water in the blast holes, and the extent of rockmass fracturing.
The guidelines were posted to the wall of the operation’s drill and blast office. In contrast to the approach at Bealeton, the Pittsboro staff’s knowledge and experience — gleaned via consultations — was carefully considered and married into the recommendations.
After monitoring Pittsboro’s modified blasting regime for an additional three months, the JKMRC consultants were in a position to compare operational results before and after changes to blasting were adopted.
One of the main results was that throughput at the primary crushing stage increased. The ramp-up was 9.5% (to 1035 tonnes per hour) for material Luck Stone was producing for itself from dacite tuffs and basalt, and 14.6% (to 965 tph) for andesite material the company was crushing for the quarry owner, 3M.
These figures came from comparing daily operating data for the two months featuring the same ore hardness, as indicated by the plant’s per-tonne power consumption.
Another noticeable result was that the proportion of the time the secondary crusher was in use dropped 25 percentage points, going from 75% during the three months of unmodified blasting to 50% for the modified-blasting months. In other words, the rate at which oversize was reaching this crusher had fallen.
Meantime, the proportion of crusher run going into the Pittsboro plant increased by only one or two percentage points, according to Kojovic.
But perhaps the most telling result was a comment said to have come from Luck Stone plant manager Steve Demeyer: that without the changes to blasting practice at Pittsboro, an ensuing large and unanticipated increase in ore hardness would have been “fatal” for the quarry’s productivity.
Despite encountering much harder ground, Luck Stone was able to smartly blast its way to operational improvements using mine-to-mill guidelines and insights.
* This article was based on a presentation delivered at the Marcus Evans’ conference: Optimising Productivity in Drilling and Blasting — harnessing advanced technology and techniques to optimise mineral recovery, increase performance and reduce costs. For more information about Marcus Evans’ conferences, visit www.marcusevans.com.
Julius Kruttschnitt Mineral Research Centre
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