Mining
The Meadowbank operation consists of surface mining from a series of three pits all within 7 kilometres of the processing plant. Water retention dykes are being built to allow for mining beneath shallow lakes, using a unique in-water dyke construction method. The East Dyke in Second Portage Lake and the small West Channel Dyke closed off the North Portage pit area, so 16 million litres of water could be pumped out before the stripping and mining could begin.
The Portage pit went into production in late 2009 and is expected to produce ore until 2014. Following the 2013 completion of the Bay-Goose Dyke, the Goose Island pit is expected to produce ore from 2013 to 2015. Dewatering of Vault Lake will allow the Vault pit to be mined from 2015 to the end of mine life in 2019.
The mine works year-round, using conventional drilling, blasting, truck and shovel methods. The blastholes are loaded with emulsion explosives produced at an on-site plant. Each blast breaks up about 100,000 tonnes of ore or waste, three or four times per week. Hydraulic excavators scoop the broken rock into 100- or 150-tonne haul trucks. The trucks dump the ore into a gyratory crusher next to the processing plant.
The waste rock is hauled to locations where it is needed for construction, or dumped in waste storage sites or previously mined-out areas. To minimize acid generation, the sulphide-bearing waste rock is encapsulated in permafrost and capped with an insulating layer of neutralizing rock.
Processing
The 8,500-tonne/day gold processing plant at Meadowbank is designed to operate year-round using conventional technology adjusted to the Arctic climate.
To prevent freezing, the crushed ore is stored in a dome and brought into the plant on a covered conveyor. The ore is reduced in size by a secondary crusher, and then ground in a SAG mill followed by a ball mill to ensure that 80% of the ground ore is less than 60 to 90 microns diameter.
About 25% to 30% of the gold is “free gold” (not combined with other minerals), and is removed from the ground ore by a gravity circuit. The rest of the gold is leached from the ground ore using cyanide, with the gold captured using carbon-in-pulp technology and electrowinning cells. The gold-plated cathodes and gravity concentrate are smelted in an induction furnace and poured as doré bars containing about 81% gold and 19% silver. Meadowbank pours about eight to 10 doré bars per week.
The plant includes both a cyanide recycling thickener and an air-sulphur dioxide cyanide destruction circuit to ensure that no cyanide escapes to the environment. After leaching, the ground ore is essentially barren of gold, so it is pumped to the nearby tailings pond for disposal. Site water that is contaminated is prevented from contacting fresh water by diversion ditches, and is collected in the tailings pond. All water from the tailings pond is pumped back to the plant for reuse, making this a zero-discharge system and reducing the need for fresh water.