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By Sarah Hurst
For Mining News 

Pogo bounds up steep learning curve

Teck-Pogo management confident challenges can be met as Alaska's newest mine heads for full production; more miners needed

 

Last updated 3/26/2006 at Noon



In the early days of exploration at a property near the small town of Delta Junction in Interior Alaska, a bunch of geologists who'd had a few too many beers began jumping up and down. They may have been jumping for joy, imagining a rich gold vein beneath their feet, or more likely they were jumping to keep warm, as this region is one of the coldest on the planet in winter. Someone nicknamed the strange dance the Pogo dance, and the name stuck. Today in the same spot, Pogo mine is producing its first gold bars, a joint venture between Canadian mining major Teck Cominco and Japan's Sumitomo.

An Alaska Miners Association tour of Pogo in mid-March coincided perfectly with the beginning of operations. As a result, the 30 places on the tour were booked up months in advance and there was a long waiting list of people hoping somebody would drop out. A total of 15 people could go underground at one time, loaded into a windowless metal box on wheels. Once the vehicle arrived at its destination (in one case after backing several hundred feet out of the portal to avoid an oncoming truck) the visitors had plenty of freedom to walk around in the mud and chip off pieces of the valuable rock.

Having waited a long time to see Pogo mine and endured a three-hour early morning bus ride from Fairbanks, the geologists in the group were keen to spend as much time underground as possible. About another three hours, as it turned out. "To a geologist it's like a cathedral down there," one said on his reluctant return to the surface, "a cathedral of quartz." That is certainly true: even to the completely un-geological eye there is a clear line separating the white-and-rust quartz from the gray wall rock.

On the other hand, the sparkling dots in the quartz could be gold or could be pyrite; the miner who drove us down there wasn't too sure. The largest visible gold here is the size of a match head, according to Jack DiMarchi, Teck-Pogo's chief geologist. Apparently the company isn't worried about the possibility of giving too much gold away, as they provided 100 pounds of quartz chunks to the group as souvenirs. To extract an estimated 3.7 million ounces of gold from 7.7 million tons of ore over a 10-year mine life, a lengthy journey through the mill's processes is required.

Drift and fill mining

The drift and fill method of mining is being used here. Drifts are horizontal tunnels 15 feet square that lead off the main decline. At the end of each of the two shifts per day, blasting takes place. To blast one block of about 250 tons of ore, 60 holes have to be drilled. This is considerably less ore than in other methods of mining such as long-hole stoping, which progress more quickly. The temperature underground is a comfortable of 44-45 degrees.

At the time of the tour, the mill was shut down. One employee said it has been working about 12 hours out of every 24. Pogo's first gold bar was produced on Feb. 12 and about five more were produced in the subsequent month. Unsurprisingly, the company would like to get up to full speed as soon as possible and start making money to off-set the $347 million capital cost of the mine. "We've got start-up issues, maintenance, that's what we're here for - it's a learning curve," said Karl Hanneman, the mine's manager of public and environmental affairs.

An 11-foot diameter conveyor tube that will bring ore from the mine to the mill is not yet in operation, so ore is being trucked to the mill instead.

As in most metals mines, ore is first crushed and ground to a 50-micron average size in a huge SAG mill and ball mill.

It then goes through a gravity circuit and flotation cells that produce a 10 percent concentrate, meaning that the concentrate is 10 percent of the weight of the ore.

The concentrate is reground and run through a cyanide leach circuit, with a carbon in pulp process to pull the gold out of the solution.

The source of the carbon is burnt coconut shells, which have the right physical properties to be resistant and handle the process without breaking down.

Gold made into sludge

All that remains on the production side is for the gold to be made into a sludge that is melted in the furnace and then poured to form a gold bar. But that still leaves the much larger volume of tailings to be dealt with. And in Pogo's case the tailings pose specific challenges. Two giant Larox filters that emit periodic high-pitched roars like wounded dinosaurs squeeze the water out of the tailings to produce a filter cake containing only 15 percent moisture.

The filtered tailings go to the dry stack tailings facility on a windy slope by the side of a mountain. A dozer shovels the tailings to make a fairly even surface, and additional material is folded in "like raisins in a pudding," as Hanneman puts it. "The properties of the material make it very impermeable," he added. Dry stack tailings allow Pogo to avoid having to maintain and monitor a tailings pond for years or decades after closure. A dam creates a speed bump to catch water that comes down from the valley, and a diversion ditch above the tailings facility enables the mine to divert as much water around the property as possible, in accordance with EPA requirements.

The mine's permits call for zero discharge of water. The water that is removed by the filters contains a high level of arsenic. It goes to the water treatment plant, where the arsenic is converted to iron arsenate, which is bright red, but very benign. Cement is then added and the muddy mixture is pumped back underground to fill the mine's workings with paste. The tour group was able to climb up onto the flat, slippery surface of the paste underground and see the deep footprints left by miners who tramped around in it when it was even wetter.

Stench would signal evacuation

Tour participants were given a safety briefing before going underground and kitted out with hard hats and lamps, safety goggles, steel-toed boots, fluorescent vests and belts with self-rescuers to convert carbon monoxide to carbon dioxide in case of emergency. There are phones at various places inside the mine and if an evacuation becomes necessary, a stench will be activated - a really strong garlic and rotten egg smell. To ensure miners recognize the smell, it's activated as a test every six months, though fortunately not on the day of the tour.

Pogo still requires more underground miners, and although it's relatively easy to send new recruits to the nearby Delta Mine Training Center, it's harder to find experienced miners.

Teck-Pogo makes no secret of the fact that it has been poaching employees from Alaska's other mines and those mines have on occasion poached some back.

One obvious example is geologist Paul Jensen, formerly of Fort Knox gold mine in Fairbanks, whose name is now on an office door at Pogo.

Local company Taiga Ventures provides the buses that transport workers from Fairbanks or Delta Junction to Pogo, up the 50-mile winding mountain road built for the mine by Alaska Interstate Construction.

It's a good road and it will need to be.

The Pogo story has only just begun.

 

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