November 27, 2009

YK TIMES

The Robertson Shaft headframe, the tallest structure in the Northwest Territories, may be converted to a geothermal plant which could heat half of Yellowknife.

FEATURE | Demolition & environmental engineering

1,400 ounces of gold discovered during remediation of Yellowknife mine

PETER KENTER

correspondent

The Con Mine, located just south of Yellowknife, produced more than 160,000 kilograms of gold between 1938 and 2003. But the contractor now demolishing it says there are millions of dollars in gold left in the old girl yet.

And once the site is remediated, it could heat half the city.

Among the buildings being spared by request of Yellowknife City Council is the Robertson Shaft headframe, the tallest structure in the Northwest Territories.

Although plans for the structure could involve anything from an indoor garden to a condominium development, the building might also be used as the operations centre for a geo-thermal heat plant.

Preliminary studies indicate that geo-thermal energy raises the temperature in many of the shafts to in excess of 35 degrees Celsius. As groundwater slowly fills the shafts, the old mine may provide the community with enough heat energy to heat half the city.

The 340-hectare mining and processing site is held under lease from the Northwest Territories. Part of the contract with mine operator, Newmont Mining Corporation, involves remediation of the site.

It includes demolition of dozens of unwanted buildings, capping of tailing ponds, and the clean-up of soil containing unwanted by-products of gold refining, such as arsenic.

The company supports recycling and is carefully segregating any building materials — such as steel I-beams — that can be recovered.

But among the material still lodged in some of the buildings: gold-rich ore, and dust containing the precious metal in various states of refinement.

“We started demolition in the summer, but work is on hold for the winter, as of the first week of November,” says Bill Lyle, Director, Reclamation and Closure with Newmont. “To date, we’ve probably recovered about 1,400 ounces of gold.”

As part of the refining process, gold ore is crushed and heated in a smelter, with slag rising to the top of the smelting bucket and heavier gold and silver sinking to the bottom. The molten material is poured into conical molds and cooled, with the worthless slag broken off the base of the cone.

The remaining gold cones, referred to as “buttons,” are further refined into bars that are weighed and assayed for purity.

The gold find at Con is no surprise to Newmont.

Gold is a heavy metal that tends to settle in cracks, crevices and to any low points in the gold mining and processing facility as gravity conducts its own sorting enterprise.

“When you take a look at the buildings and the processes involved as the ore is crushed, milled, processed and refined through flotation, it becomes fairly obvious where any of the low-lying areas in that operation exist,” says Lyle.

“When the facility is operating, there’s no economic incentive to remove it.

“After the processor shuts down for the last time, we look carefully around the tanks, the sumps and the pump areas and investigate all of the ventilation systems and the ductwork. There’s also an autoclave in the facility where we expect to find some gold remnants.

“And wherever you find gold, you can usually expect to find some silver as well.”

The company is using employees of the mine who are familiar with the gold processing operation to assist in locating likely gold deposits.

The logistics of recovering gold from a demolished building is as much a matter of economics and market forces as it is engineering.

“Any time you demo a building or a processor, you look at the economic conditions associated with it,” says Lyle. “If you think the price of gold or whatever material you expect to find is high enough, you can make a greater effort to segregate and reclaim that material.

“Gold is just another material we deal with, based on market prices for demolished and reclaimed material.”

At Canadian spot prices approaching $1,200 per ounce, Lyle says its helping defray the cost of demolition and reclamation. Even at a base $1,000 an ounce it’s $1.4 million so far.The demolition of the remaining buildings may take up to two years to complete, weather permitting.

In the meantime, some buildings deemed to have historical importance or a potential for re-use have been given a reprieve.

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