Mining & Processing

Mining

At the Myra Falls Mine, current ore mining is underground longhole open stoping. Ore development sources typically make up 25 % of production. Stope areas of less than 20 meters in height are generally taken with only an extraction drift and inverse raises for slots. Past methods included room and pillar, Alimak, cut and fill (C&F) or some variation of C&F. The extraction from stopes is by remote controlled Scoops. The size of the stopes can reach dimensions of up to 15-25 m high by 10-15 m wide. The size of stopes is varies due to geotechnical considerations and orebody geometry. The ramp and drift development is supported with rebar bolts and screen (and sprayed with shotcrete) to ensure greater safety through stabilization of the rock mass. In some areas cable bolting is used where increased support is required. Stopes are backfilled with cemented paste-fill which is produced from processing plant tailings material. This allows higher mining recoveries as well as increased stability in the stoping area. Prior to the completion of the paste-fill plant upgrade in 2018, cemented hydraulic backfill had been used. Material is hauled by trucks through the surface decline to the surface stockpile and is then hauled to the processing crusher.

Processing

The original Myra Falls concentrator, constructed in 1966, was replaced in 1985 by the current one that has since been progressively modernized, with major flowsheet modifications made in the early 1990s and again in the mid-2000s. In its current configuration, the Myra Falls concentrator has a processing capacity of up to 3,600tpd of ore.

Material is commuted directly to the Mill via trucks. The crushing is handled at the Lynx Crusher, located on surface adjacent to the Mill. Ore from the coarse ore bin is then fed to a conventional 2-stage crushing circuit that consists of a 5X12 double deck screen and a Symons 5.5ft standard head cone crusher in open circuit followed by a 8X16 single deck and a Symons 5.5ft short head cone crusher that are configured in a reverse-closed circuit arrangement.

The crushing circuit product is then fed to two 3,600t fine ore bins, each of which feeds one of two parallel grinding lines. Each consist of an open circuit rod milling (8X12) first stage followed by a reverse closed circuit ball milling (11.5X15) second stage with 30in Knelson concentrators for gravity gold recovery and Krebs D20B hydro cyclones for classification. The Gold concentrate recovered from the Knelson concentrators is manually cleaned on a Deister concentrating table before being sent off-site for further processing.

The grinding circuit product is then fed to the flotation circuit, where Copper, Lead and Zinc bearing minerals are sequentially separated and concentrated into Copper, Lead and Zinc concentrates using conventional mechanical flotation cells. In each case, a roughing stage is followed by two cleaning stages, with an additional regrinding stage being inserted between the roughing and cleaning stages in the case of the Copper and Zinc flotation sub-circuits.

The Copper, Lead and Zinc concentrates from the flotation circuit are dewatered in dedicated conventional thickeners and filtered in Larox pressure filters. The filtered Copper and Zinc concentrates are then transported in bulk 38t tractor-trailer units 90 kilometres to Campbell River for storage and subsequent loading onto bulk carrier ships. The filtered Lead concentrate is bagged on site and then transported to a storage facility located in Vancouver.

Tailings from the flotation circuit are pumped to the paste plant where the material is dewatered in a high-rate thickener followed by a vacuum disc filter. The resulting filter cake is deposited on a reversible conveyor which can feed the dried material into either a stockpile for dry stack tailings storage or into a paste mixer. The final paste product is pumped underground as paste backfill.