‘Why Not Put It Back?’ Solution To Tailings Just Not That Simple

It seems like an obvious solution to a mountainous problem.

“Why not put it back in the hole?”

Debbie Boots asked that question as she sat in her Montana log home, down the road from where the proposed Rock Creek mine would create a 300-foot-high, 340-acre tailings pile.

Like many other Inland Northwest residents, Boots is upset at the prospect of a potentially polluting eyesore growing over the next 30 years in the green Clark Fork River valley.

Residents look at these piles of industrial waste - already littering the region - and want to know why the rock can’t just be stored back underground once the silver and copper is removed from mines.

At some mines, in fact, the sandy leftovers known as tailings are put back in the hole. North Idaho’s Lucky Friday mine is a notable example.

But mining experts say “backfilling” is complicated, expensive and sometimes impractical. That’s especially true for projects like the one Asarco Inc., wants to build at Rock Creek.

“You can’t just say, ‘Shove it back in.’ I wish you could,” said Lani Boldt, a civil engineer with the U.S. Bureau of Mines in Spokane. “It depends on the ore body, the operating life of the mine, the grinding and milling process … “

There’s a lot of interest in backfilling, though. Putting tailings underground can stabilize a mine. That protects workers, and can prevent the earth above the mine from sinking.

Also, tailings piled on the surface are unsightly until the landscape is “reclaimed” with grasses and trees.

That can take decades, plus dollars and diligence that aren’t always available.

The biggest worry about tailings piles is their potential to pollute surface or ground water.

Modern mine operations are efficient at extracting metals from rock. What’s left is mostly silica sand.

But there’s always something mixed in with the sand, including trace metals and whatever acids or soaps are used to extract the precious metals.

In the case of gold mines, such as the proposed Crown Jewel project in Washington’s Okanogan County, a dilute cyanide solution is used to extract the metal.

That’s why environmental officials sometimes require that a clay or polyurethane lining go under the tailings.

Pumps are installed to capture seeping water that might carry toxins.

Another threat is erosion or even collapse of tailings impoundments.

The Rock Creek impoundment would be just 400 feet from the Clark Fork River.

The Lucky Friday silver mine near Mullan puts half of its tailings into an impoundment near the South Fork of the Coeur d’Alene River.

The rest of the sand that comes out of its processing mill is pumped back underground.

For nearly a decade, the sand has been mixed with concrete and used to stabilize the mine, which is prone to mini-earthquakes called rockbursts.

“Sandfill’ is not quite a slurry, not quite a paste,” explained mine manager Tom Fudge during a tour of the mine.

At the end of a dark and humid tunnel a mile underground, he showed how the process worked.

The tunnel led to a “room” where the walls had been reinforced with chain link bolted to the rock.

When the miners finished drilling the vein of silver there, they drove metal bars into the floor and built a rock dam at the entrance.

A pipe led to the top of the dam. The brown sandfill would ooze out of the pipe and fill the room, leaving just enough space for expansion.

When the sandfill hardens, it becomes a ceiling under which the miners work as they chase the silver deeper and deeper.

Silver Valley mines are good candidates for backfilling for several reasons.

For one thing, much of the ore is high grade, meaning it’s more profitable and the owners can afford the added expense.

For another, the veins are narrow and steeply pitched.

In contrast, the silver and copper deposits in the Cabinet Mountains are big and horizontal.

Mines like the one proposed at Rock Creek consist of huge rooms instead of narrow tunnels.

“They’d have to build all kinds of dams and reinforcements to hold the tailings in place,” said Art Bookstrom of the U.S. Geological Survey.

Those bulkheads would require the use of cement, which is another expense.

The cost of cemented backfill at the Rock Creek mine has been estimated at $3 to $12 per ton, compared to surface disposal costs of between 80 cents and $2.50 per ton.

Cement comes with an environmental price tag, Boldt noted, because it also must be mined.

Under no circumstances can all of a mine’s tailings be put back underground.

They just won’t fit.

“If you take out a cubic foot of rock, you’ll probably only be able to put half of it back before air spaces between individual particles stop you,” Boldt said.

That’s one reason state and federal officials haven’t required backfilling for the Rock Creek mine.

Even if half of the tailings were put underground, they wrote in draft environmental impact statement, “a surface impoundment could not be eliminated or significantly reduced in size.”

Judy Hutchins wants them to take another look at backfilling.

The Montana woman is among environmentalists campaigning against the Rock Creek mine.

“Fewer tailings could make the impoundment shallower, more stable - maybe let them move it back from the river,” Hutchins said.

“A lot of the reaction here is that Asarco wants to make a lot of money from the mine, then leave a ‘gut pile’ behind.”

, DataTimes ILLUSTRATION: Color Photo