International PLACER DOME’S BIG BELL

As with most new gold mines in Australia, the property already has a production history (underground operations ceased in 1955). Mining today, however, is considerably less selective and is geared towards higher tonnages but lower grades. With a mill throughput of 7,500 tonnes per day, the operatio n must mine about 100,000 tons (91,000 tonnes) of ore and waste each day to feed the mill. This works out to a 12:1 strip ratio, which is unusual for a gold mine in Australia. Sixteen million tons (14.5 million tonnes) of pre-production stripping was completed to expose the orebody.

Adopting what one Placer official described as “conservative Canadian standards” for open-pit mining would have boosted that strip ratio significantly and made the project uneconomic, Peter Neilans, mine superintendent, said. To maintain steeper pit slopes, artificial support systems are used, including tensioned and untensioned cable bolts up to 100 ft (30 m) long. This minimizes waste removed and enables the company to maintain wall slopes between haulage ramps that are 59 degrees on the footwall and 55 degrees on the hangingwall. “We invest more than $1 million per year in bolting, which allows us to take a lower strip,” Neilans said.

The present milling rate is possible because the company is mining oxide ore; with sulphide material, daily throughput will drop to 4,800 tonnes, said George Paspalas, senior metallurgist. Sufficient oxide reserves are on hand for one to 1 1/2 years of production, he added.

The company has stuck with proven technology, most of which is utilized at other mines it operates in Australia and North America. The mill includes a cip (carbon-in-pulp) circuit and a 22-ft-diameter sag (semi-autogenous grinding) unit driven by a 2,000 kw, variable-speed motor. All power is diesel-generated on-site. Six power generators (3 mw each) supply the power. One unit is usually down for maintenance.

Leach tanks have open-impeller agitators and the residence time is 20 hours for oxide ore and about 28 hours for primary ore. Loaded carbon is acid- washed before elution and the gold is recovered by a standard electro- refining process. About 0.053 oz of gold is recovered from each ton of ore (1.8 g per tonne), the company said.

Paspalas said the oxide material “has been close to feasibility” but he added that it has been much more abrasive than anticipated. Grinding media consumption is also double and lime consumption is about 22% higher than feasibility, but this will drop when fresh ore is processed.

“Carbon management has been very good and we have had excellent recoveries from Day One,” he said. Recoveries have been averaging about 92% for oxide material grading 0.058 oz (1.9 g) gold; fresh ore is expected to grade about 0.088 oz (3 g). Mine reserves consist of 22 million tons (20 million tonnes) averaging 0.096 oz (3.3 g) gold. Of this total, 3.3 million tons grading 0.058 oz (3 million tonnes grading 1.9 g) are classified as oxide and transition ore, 10 million tons at 0.099 oz (9.1 million tonnes at 3.4 g) as primary ore, and 8.7 million tons grading 0.1 oz (7.9 million tonnes grading 3.4 g) as underground ore.

Underground development is expected to begin in the fourth year of production, when a decline will be started from the pit bottom. The ore zone appears to be quite homogenous at depth and has been drilled to about a mile below surface. David Duval is the former west coast editor of The Northern Miner newspaper. Flowsheet

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