Tailings storage facility breach at Mount Polley copper and gold mine

6 August 2014

The tailings storage facility at Imperial Metals Corporation’s Mount Polley mine was breached, early Monday morning, releasing an undetermined amount of water and tailings in the early morning of 4 August. The cause of the breach is unknown at this time. Mount Polley is an open pit copper/gold mine with a developing underground project, located in south-central British Columbia, Canada.

The Mount Polley mine site. Tailings ponds is to the right. Click to enlarge. Click to enlarge.

On Tuesday, Imperial reported that “The tailings dam breach that caused a water and tailings discharge … has stabilized.” Imperial said that the exact quantities of water and tailings discharged have yet to be determined. The tailings are alkaline with an average ph of 8.5 and are not acid generating.

Mount Polley mine has been placed on care and maintenance status. Imperial does not know at this time how long it will take to restore operations.

The Cariboo Regional District (a regional government that functions independently of the provincial government) reported that waterways affected by this event include Quesnel Lake, Polley Lake, Hazeltine Creek and Cariboo Creek. Additionally the Horsefly Likely Forest Service Road (Ditch Road) has been washed out at Hazeltine Creek. A water-use ban remains in place and includes recreational water activities, bathing, and drinking etc. Everyone in this area is advised to use bottled water until further notice.

In the Mount Polley mill, run-of-mine ore from the open pits is hauled to the crusher. The crusher has three stages of crushing involving five crushers, twenty conveyors and four sets of screens. The ore is dumped into the feed pocket of the primary gyratory crusher and the product is discharged to the grinding circuit at finer than approximately 20 mm particle diameter.

The grinding circuit consists of two parallel rod mill/ball mill circuits and a pebble mill circuit. Crusher product is first split between to two rod mills where it has water added to form slurries. The slurries are pumped to cyclones that classify the ore particles by size.

The larger particles flow to feed the ball mills while the fine particles are discharged to the second stage of grinding: the pebble mill circuit. The ball mills are in “closed circuit”, meaning that the discharge is pumped to the classifying units (cyclones) and the particles will not pass to the next grinding stage until they are fine enough to be classified as such.

The second stage grinding circuit (the pebble mill circuit) also consists of mills, pumps and cyclones. Three pebble mills receive the coarse product from the cyclones, fed by pumps. The pebble mills are so named as they use pebbles (rocks obtained from the crusher) for grinding. Particles finer than 200 microns are then pumped as slurry to the flotation circuit.

The flotation circuit separates the valuable minerals from the waste minerals. The valuable minerals, mostly in the form of sulfides, are separated from waste minerals (gangue minerals) by floating and being collected and upgraded or cleaned to produce a concentrate. Initial separation is done in a rougher/scavenger circuit, where the waste minerals are discarded as tailings (which flow by gravity to the tailings impoundment area).

Imperial Metals president Brian Kynoch told CBC News:

“It’s very close to drinking water quality, the water in our tailings,” he said. “There’s almost everything in it but at low levels.... No mercury, very low arsenic and very low other metals.”

Imperial is an exploration, mine development and operating company based in Vancouver, British Columbia. The Company operates the Mount Polley copper/gold mine in British Columbia and the Sterling gold mine in Nevada. Imperial has 50% interest in the Huckleberry copper mine and has 50% interest in the Ruddock Creek lead/zinc property, both in British Columbia. Imperial is in development of its wholly owned Red Chris copper/gold property in British Columbia.