June/July 2014

Energy

Wait until you’re full – part-time processing can equal big energy savings

By Michelle Levesque

Energy costs are increasingly a drain on an operating mine’s budget. However, miners can make low-cost changes like running mills at full capacity through careful scheduling to save millions.

In 2008, a consultant conducted an energy use study, which began with an energy audit at a base metal milling facility that assessed electricity use to identify potential savings. After gathering and analyzing a year’s worth of hourly electricity data for each of the plant’s 292 pieces of electrical equipment, 10 energy conservation measures were identified that could reduce this facility’s energy use by about five per cent. In 2012, MIRARCO (Mining Innovation Rehabilitation and Applied Research Corporation) conducted a subsequent analysis to determine the influence of plant use relative to design capacity on unit electricity consumption (kilowatt per hour/tonne of ore milled). An electricity part-load model for the plant was derived to illustrate how much electricity per tonne of ore processed was consumed at different throughputs. Not surprisingly, the most efficient use of electricity occurred when the plant operated at design capacity. If the plant studied had been operated at 100 per cent of design capacity instead of 74 per cent, the electricity savings would have amounted to 14 per cent. In energy management terms, this was the “elephant in the room.”

A mill can operate below design capacity for many reasons such as unplanned maintenance, changes in ore characteristics, fluctuating market conditions or a shortage of ore supply. But if the reason is due to reduced ore production, a mill can implement an intermittent schedule to ensure the facility operates at design capacity. Management must consider the costs of ore storage, additional material handling, and recovery losses and labour, as well as operational issues in the mill and downstream processes when making such a decision. But if the savings outweigh the costs, this presents an opportunity for milling facilities to improve their bottom line.

Following the initial audit, MIRARCO surveyed 13 mills operating in Ontario in 2012 to examine how these facilities were run. The three base metal mills that we looked at were operating between 47 per cent and 74 per cent of design capacity, whereas the 10 gold milling facilities were run between 59 per cent and 120 per cent of design capacity. It should be noted that nine out of these 10 gold mills were operating above 75 per cent.

It became clear that these companies could save electricity and, ultimately, money if they modified their operating schedules. This would entail shutting down the facility periodically while stockpiling ore, then processing the ore, and repeating the cycle. For example, a facility operating at 50 per cent of design capacity could adopt a two-week on/two-week off schedule; a mill running at 75 per cent of design capacity could have a one-week shutdown period for every three weeks of operation.

One mill in Ontario has already started to realize the efficiencies that can be gained by modifying its schedule and optimizing milling operations. North American Palladium has been operating its Lac-des-Iles mill intermittently since 2010. Still, in 2012, when the facility operated on a two-week on/two-week off schedule, it ran at 74 per cent capacity, demonstrating that there is room for improvement and it could benefit from additional savings.

After completing our follow-up survey, we developed part-load models for the three base metal facilities we looked at by using the part-load model derived from the original audit. We also adopted cost models to reflect economies of scale. Intermittent operation of the three base metal facilities in 2012 would potentially have reduced electricity consumption by between 16 per cent and 36 per cent.

Understanding billing structures is also important to maximize financial savings. In Ontario, facilities with an average demand greater than five megawatts are considered “Class A” customers and their demand charges can be significant. By shutting down mills during peak demand hours in Ontario, thereby decreasing energy consumption when power is most expensive, miners can reduce costs even further. Shutting down a mill can potentially reduce its total electricity load by 90 per cent, with only equipment like lights, fans and agitators remaining in operation. In 2012, every megawatt saved by a “Class A” facility during the five peak demand hours corresponded to savings of roughly $275,000 per year. For a mill with an average demand of five megawatts, this translates to more than $1 million in annual savings; the savings would be proportionally greater for larger operations.

 Michelle Levesque Michelle Levesque is a researcher
at MIRARCO. She is pursuing a PhD
in natural resources engineering from
Laurentian University and the focus of
her research concerns energy
management for the mining sector.

It is possible to significantly reduce energy expenditures at milling facilities, especially those with low utilization, considering the current billing structure in Ontario. Energy conservation measures can provide savings, but it is equally important that whenever energy is used at a facility, it is done as efficiently as possible. Confronting the elephant in the room may be intimidating, but the rewards are worthwhile and sustainable in terms of both efficiency and overall savings.

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