Silver, lead and zinc
The Cannington silver mine is located in north-west Queensland, 200km south east of Mount Isa, near the township of McKinlay. The deposit was discovered by BHP Minerals in 1990 and the mine was commissioned in 1997 at a cost of some AUS$450m. Full production was achieved in early 1999, since when capacity has been expanded from 1.5Mt/y of ore to over 3Mt/y.
Cannington is the world’s largest single silver producer, representing about 6% of the world’s primary silver production, while its lead production represents about 7% of the world’s primary lead output. The lead concentrate contains 70% lead and over 3,000g/t silver with low levels of impurities. Long-term contracts for concentrate sales have been agreed with Pasminco (now Zinifex) in Australia, Metaleurop in France, Berzelius in Germany, and various smelters in Japan and Korea Zinc.
Cannington lies in the south-east corner of the proterozoic Mount Isa Block, within the metamorphics of the lower middle proterozoic eastern succession and overlain by 60m of younger sediments. It is divided by faulting into a shallow, low-grade Northern Zone and a deeper, higher grade Southern Zone. Initial mining will be undertaken in the Southern Zone. Cannington’s major economic sulphides are galena and sphalerite. The silver occurs mainly as freibergite but is also present in solid solution within galena.
At the end of 2005, the orebody contained proved sulphide ore reserves of 18.0Mt grading 477g/t silver, 10.7% lead and 3.9% zinc. Measured resources totalled 2.3Mt at 536g/t Ag, 11.94% Pb and 4.49% Zn. The metallurgical recovery rates for zinc, lead and silver were 66%, 88% and 84% respectively.
Cannington is an underground mine accessed via a 5,250m-long, 5.2m-high by 5.5m-wide decline. The main, thicker hanging-wall orebodies of the deposit are mined by transverse, longhole open sloping.
During the first year of production, a vertical hoisting shaft with a finished internal diameter of 5.6m was constructed from the surface to 650m for ore haulage. The excavation method involves stripping out a 1.8m-diameter raise-drilled pilot hole and lining with concrete from the surface. The shaft is equipped with a tower-mounted friction winder and two 9t skips in counterbalance running on rope guides. The skips are hoisted from a loading station on the 610m level and reach a final hoisting speed of 12m/s. On the surface, tipping scrolls in the shaft headframe tip the skips into a surface bin for transfer to the mill’s stockpile.
Processing steps used to produce the lead and zinc concentrates include comminution, flotation, leaching and dewatering. Waste water is pumped to a tailings dam with the flotation overflow. The overflow from the lead concentrate thickener is recycled to the lead flotation circuit. The operation of the concentrator is automated with progammable logic controllers (PLCs), an online sample analyser being used to provide continuous assays on a number of the concentrator streams for silver, lead and zinc. The silver is leached from the lead and zinc concentrates before smelting.
During the year ending June 2006, Cannington mined 3.1Mt of ore grading 461g/t silver, 10.3% lead, and 3.7% zinc. Payable metal production was 38.45Moz of silver, 266,321t of lead and 68,779t of zinc.
BHP Billiton has been implementing its Cannington Growth project since 2003, with the aim of improving recoveries, bringing the Northern Zone orebody into production and sustaining ore production at a rate of 2.4Mt/y.
After dewatering, the lead and zinc concentrates are stored in a 10,000t-capacity storage shed before being loaded into 118t-capacity side-tipping, covered trailer roadtrains by a front-end loader for transport to Cannington’s storage and rail loading facilities at Yurbi on the Matilda Highway. Supplied by Roaduser Research, the three 53.5m-long Icon roadtrains carry eight covered bins mounted on six trailers, loading and unloading taking about 20 minutes at each end of the journey.
The concentrates are stored separately in an 8,000t-capacity shed at Yurbi prior to loading into rail wagons. Queensland Rail has supplied 50 new 63t-capacity wagons for transporting the concentrates to the port of Townsville for export to world markets.
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