Power to the pits

Extracting valuable commodities in British Columbia is dictated by geography and geology so mines often wind up in far-flung places where ore has been deposited in concentrations that may be worth mining. BC has abundant mineral wealth but it is largely low-grade ore so the mines are typically large and therefore the challenges of access and delivering water and electricity also become large. Other hurdles include First Nations and community buy-ins, environmental assessments and commodity prices. There are 24 mines under review and it’s unlikely they will all be developed in the short to medium term.

One of the roles of the Mining Association of BC is to lobby for transmission lines (T/L’s), and as Byng Giraud, Vice-president of Policy and Communications of the MABC explains, “Infrastructure raises all kinds of issues. We need roads, power lines and other infrastructure. In fact, what we really need is community support to build a mine, for a mine to be developed there has to be a greater public good. Throughout much of our history, infrastructure has come first (railroads, for example) because nobody can establish a large operation in an area that has nothing to offer. It is often infrastructure that helps to develop the economics of a community. World-class deposits can lay idle for decades as in the Yukon because there is no way to get to them or to get the product out economically due to inadequate infrastructure.”

Other hot issues around mining are commodity pricing and energy; the viability of a mine is often contingent on whether they can feed off the grid. Giraud continues, “the carbon tax can hit a mine really hard, there’s much incentive for ‘power switching’ from diesel to electric where possible to reduce fuel consumption which can put the climate action goals in potential conflict with BC Hydro’s (BCH’s) energy conservation program.”

When you consider that less than 0.03% of BC’s land base is used for mining, the fact that about seven mines in BC consume 14% to 17% of all power generated by BCH is impressive. There are a few important underground and coal mines sprinkled around BC, but their power requirements are small by comparison to the large energy-hungry open pits that need up to 930 GWh (million watt hours)/year and there are many smaller mines, some with no more than 10 employees operating outside of the BCH service territory running off diesel generators. Even if they were near a T/L, it might not be worth using it.

The biggest energy users in BC are the hard rock mines; there is only one major underground gold mine in BC, QR mine near Quesnel, which is fed off of a 25KV distribution feeder. The rest of the large hard rock mines are open pit operations that feed off the BC Hydro grid from 69kV (kilovolts) to 230kV via either privately built or BCH operated and maintained T/L’s.

Kemess operates a copper and gold mine about 400 kms north of Prince George. They own and maintain a 360 km 130kV T/L that is fed from the Kennedy Substation near Mackenzie. They process up to 52,000 tonnes/day.

Moving south, Huckleberry, a copper and molybdenum mine, also operates a private 120 km 138 kV T/L fed out of BCH’s Houston substation. They process 19,000 tonnes/day. Endako, about 200 kms west of Prince George, is rare in that molybdenum is their primary product. They produce about 16,200 kg of MoS₂ daily, and are fed off the grid from of BCH’s Glennanan substation via a short (~20km) 69kV BCH operated T/L.

Gibraltar, a copper and molybdenum mine near Williams Lake, is fed off of the 69kV system along a BCH-owned tap from Soda Creek currently processes 42,000 tons/day and is ramping up to 55,000 tons/day following the completion of a major expansion in 2009. Mount Polley, a gold and copper mine also near Williams Lake is also fed off the Soda Creek tap via a 50km BCH owned 69kV line– the last 14 km were built privately. The mine processes about 19,000 tonnes/day.

The largest mine in BC, in fact the largest open pit copper mine in Canada, is the Highland Valley Copper (HVC) mine near Logan Lake. HVC owns 2 -138kV substations and has an internal distribution system which includes ring loops with feeder lines to mobile substations (13.8kV) that feed their electric shovels and drills in the Lornex and Valley open pits. The mine produces 238,000 tonnes/day and mills 130,000 tonnes of it daily. The T/L’s to the HVC substations are owned and maintained by BCH.

HVC has taken energy conservation seriously. In 1986 they installed in-pit crushers and overland conveyors to feed the Highland Mill and in spite of the increased electrical consumption they reduced their electricity consumption per tonne milled by a third of what it had been at startup in 1972. They have the distinction of being BCH’s single largest connection at over 120MW; their energy consumption in 2008 was a whopping 930 GWh, equivalent to energizing some 90,000 BC homes. To put that into perspective, a distribution feeder for a community might be about 12 MW including residential, commercial and light manufacturing customers.

Typically a sixth of a mine’s operating costs is in power consumption. To understand the power requirements it helps to review the open pit process. The initial operation; drilling, blasting, and using large electric scoop shovels to move the muck into trucks as well as pumping water -as the pit gets deeper more groundwater is encountered -accounts for 10% -12% of a mine’s electricity bill. While the trucking is powered by diesel, some mines operate conveyors which increases the amount of electricity used to get ore to the mill but reduces diesel consumption. As carbon tax and caps come into effect, it may be that more conveyor lines replace trucking. This can mean reduced fuel consumption but increased electricity consumption because it can take over 20,000 hp to move the tens of thousands of tonnes of material a day to the mill. The overall energy consumption goes down but the capital expenditures can only be justified for large quantities of ore.

In the crushing process, large electric motors (~700 hp) are used to crush the extracted rock. Some mines have three crushers and up to three stages of crushing while others may have in-pit crushing and conveying. Crushing and conveying rock accounts for another 5% -10% of the power consumed.

The largest proportion of energy is consumed in the grinding circuit of the size reduction process; 50% to 60% goes into grinding the extracted rock into a fine powder that allows the target mineral to be concentrated. There are different ways to pulverize the rock into fine powder that is mixed with water into a slurry and then concentrated. The rod mill and ball mill combination uses motors that ranges from 2400 hp to 5000 hp. The semi-autogenous grinding mills use motors up to 15,000 hp followed by ball mills driven by 4000 hp to 5000 hp motors. The new style sag mill process being proposed for new mines may use up to 28,000 hp using wrap around motors to drive the mills.

The flotation process where the slurry is stirred into a froth that the target minerals will float and are later collected and dewatered takes another 10% -12% of the electricity consumption. Availability of water is often a limiting factor in the volume of material that can be processed; most of the water that is used is captured, reused and not released into the environment. Pumping the tailings to the ponds and pumping reclaimed water back to the mill, depending on the topography, accounts for another 10% -12% of electricity consumption. The remainder is used for offices, workshops, and camps.

To optimize their operation, most mines operate 24/7; the availability target for the mills is in the 92% -96% range. Most mines run at 85% -95% of full load so the consumption curve is quite flat. Most mines have stockpiles or parallel systems so while a crusher or mill line may stop for maintenance the processing never stops. Unplanned interruptions of the grinding or flotation process can have a devastating effect
on a mine. So what is important to BCH mining customers?

Mike Brandson, Senior Key Account Manager for BCH explains, “Continuous, reliable electricity delivered at a stable voltage is the most important thing for our customers. Even a short interruption, say a millisecond outage through switching or some error or other event can have a big impact on a mining operation, it can take 40 -60 minutes to get everything back on line and a lot longer to reach stable operations. A longer unplanned outage, even a couple of hours from a storm or equipment failure will have a major financial impact because it can take 2 -3 days to get everything back up and running with stability. So reliability is extremely important.”

Occasionally BCH will need outages to carry out maintenance or repairs. As much as possible BCH tries to align these needs with their customer’s planned maintenance shut downs. Some of the T/L’s are in remote areas that are subject to river erosion, landslides, rock and snow avalanche, trees, debris flows, and some of these mines are at the end of very long T/L’s.

Brandson continues, “We do not guarantee service but we do everything we can to minimize outages and we give them as much notice as possible when we do have to interrupt. That way our customer can prepare by going through a controlled shut down, however, they still lose production and once it is lost it can never be recovered.”

The other issue that is extremely important for the BCH customers is voltage stability. Brandson concludes, “Our power is clean so power quality is not a concern, but what we call a 69kV line is a nominal value, that line may operate anywhere from 64kV to 66kV and any new customer needs to design their electrical equipment with this in mind. We do not have the ability to guarantee a voltage so if a customer happens to be at the end of a very long line, their equipment has to match what we can provide.”

Given the multiple hits of low copper, molybdenum and coal prices, the credit crisis, higher electricity prices and carbon taxes, it might be a challenge to bring the mines under review in BC into production in the short term. The provision of T/L’s to new mines and the cost of electricity that they use will be key factors to their evolution.

Tonia Jurbin, P. Eng. is a freelance writer and geotechnical engineer based in the Greater Vancouver area. www.toniajurbin.com.