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Mining facts

Mining and Mineral Processing at the Mount Polley mine

In the Mount Polley Mine, run-of-mine ore from the open pits and underground is hauled to the crusher.  The crusher has three stages of crushing involving five crushers, twenty conveyors and four sets of screens.  Ore is dumped by the surface mining fleet into the feed pocket of the primary gyratory crusher, and is then crushed in three stages to produce a product at finer than 16 mm for the grinding circuit. Periodically, the crusher also used for the production of aggregates used in tailings construction and other tasks.

The grinding circuit consists of two parallel rod mill/ball mill circuits and a pebble mill circuit. Crusher product is first split between two rod mills where water is added to form slurries.  The rod mill discharge is pumped to the primary hydrocyclones that classify the particles by size.  The larger particles flow to feed the ball mills while the fine particles report to two flash flotation cells. The ball mills are in “closed circuit”, meaning that the discharge is pumped to the classifying units (primary hydrocyclones) and the particles will not pass to the next grinding stage until they are fine enough to feed through the flash flotation cells.  The underflow from the flash flotation cells is pumped to the secondary hydrocyclones, the flash flotation product can report directly to the concentrate circuit or to regrind for further upgrading.

The coarse particles classified by the secondary hydrocyclones reports to three pebble mills for further size reduction. The pebble mills are in “closed circuit” with the secondary hydrocyclones and product that is sized at 65% finer than 200-mesh is fed to the flotation circuit. Pebbles obtained from the triple deck screen in the crusher are used as grinding media in the pebble mills.

The flotation circuit separates the valuable minerals from the rest of the crushed rocks. With the addition of reagents, the valuable minerals, mostly in the form of sulphide minerals chalcopyrite and bornite, are separated by flotation and are collected and upgraded to produce a concentrate. Initial separation is completed in a rougher/scavenger circuit, where the remaining minerals are discarded as tailings (which flow by gravity to the Tailings Storage Facility).  Rougher concentrate is reground in a regrind mill and further upgraded in a cleaner circuit to produce the final concentrate product. Cleaner tailings are recycled to the scavenger circuit.

The concentrate from the flotation circuit is dewatered in two stages: the thickener settles particles and decants water so that the settled particles form a sludge by sedimentation and have a reduced water content of roughly 25%-30%; pressure filtration further reduces water content to approximately 8%. The water removed is utilized as process water. The filtered concentrate is stored in the load-out building and loaded onto 40-tonne trucks for shipping. Tailings materials generated by mill operations are piped via gravity to the TSF.

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Mining facts

How does Mount Polley decide to process?

Gold and Copper

The broken rock in the open pit is segregated into ore and waste based on the assay of rock cuttings sourced from the blasthole production drills.

This assay analysis determines the sulphide copper percentage and gold grams per tonne for material within the vicinity of each blasthole. 

The forecasted metal prices and metallurgical recovery is used to calculate copper equivalent percentage.

A forecast of mining and treatment costs establishes the mining cutoff grade and hence the material to be trucked to the plant for processing.

The plant uses three stage crushing, and conventional rod and ball mills with a flotation and dewatering circuit to produce a copper /gold concentrate.

When the mine is up-and-running, it mine moves 80,000-90,000 tonnes of material per day, the mill has a capacity to process 17,800 to 22,000 tonnes per day (tpd) of ore depending on hardness. Mount Polley concentrates are trucked to facilities at the Port of Vancouver and then shipped to overseas smelters.