Mining gold turns to scientific world

Alison Calkins

Saturday, Sept. 28, 1996 | 11:59 a.m.

ELKO -- If you're lookin' for gold in the Carlin Trend, you won't be needin' your pickax and pan.

Instead, you'd best pack up a chemistry set, a giant pressure cooker and a few zillion microscopic bugs.

Oh, and don't forget your electron microscope, because these days you can't even see the gold left in them thar hills.

It's the new age of mining microscopic gold from refractory ore -- the ore out of which it's difficult to get gold because of the presence of sulfides and active carbon.

It's the gold that doesn't respond to traditional leaching chemicals such as cyanide because of those pesky sulfides and carbon. It's the gold left in low-grade ore typically dumped in a waste pile.

And it's all going on at Barrick Gold Co. and Newmont Gold Co., neighbors in the mineral-rich territory about 30 miles northwest of Elko.

Barrick operates the largest autoclave facility in the world at its Goldstrike property, processing ore from the open-pit Betze-Post mine and the newly opened underground Meikle mine.

An autoclave is a giant pressure cooker that oxidizes sulfides in the ore and exposes the micron-sized gold particles within.

At Goldstrike, Barrick operates six autoclaves, which are the heart of the process that produces 2 million ounces of gold a year.

From the bottom, an 82-foot-by-15-foot-diameter autoclave is reminiscent of a submarine.

Inside the lead-lined tank, crushed and ground ore is agitated and subjected to temperatures and pressure that reach 430 degrees Fahrenheit and 430 pounds per square inch.

"We can accomplish in 40 to 50 minutes what would take Mother Nature a couple of million years to do," said Process Manager Andy Bolland.

After the autoclave, the process continues with the addition of lime and cyanide. By then, the ore is a liquid "slurry" to which active carbon is added and to which the gold is strongly attracted.

An electric current later separates the two and the gold particles drop out onto steel wool. Then it's smelted and poured.

"We get 200 ounces of gold from one ton of carbon," Bolland said.

Newmont has bugs, ammonium thiosulfate and a roaster to claim the gold from its stubborn refractory ore.

The bugs are actually naturally occurring microscopic bacteria that love to eat iron but dislike gold.

After the bugs are done munching the iron from the ore, the gold can be extracted by conventional cyanide leaching methods.

Using a partially patented process, the company is in the midst of a $12.5 million, three-year demonstration project that's expected to produce 100,000 ounces of gold from 3 million tons of low-grade ore at the Newmont Gold quarry.

It's called bioleaching and it doesn't look like much.

Small black hoses, called dripemitters, crisscross piles of crushed ore, dripping what appears to be water over the rock.

But that liquid is loaded with the Thiobacillus ferrooxidan and Leptospirillum ferrooxidan bacteria that are raised in low pH solutions inside nearby tanks.

A ton of ore may contain 1.5 trillion bacteria. Initially, the bacteria are sprayed on the ore as it's piled.

When the bacteria have done their duty, they become dormant or die and their 0.0004- to 0.001-inch bodies decompose.

Like the autoclave process at Goldstrike, it's a way to speed past Mother Nature.

"The oxidation of the ore, which normally occurs over millions of years, is accelerated into a matter of months," said Richard Perry, Newmont's manager of leaching.

Half the ore used in the demonstration project contains that organic carbon to which gold is strongly attracted -- so much that cyanide leaching can't dissolve it.

That's where ammonium thiosulfate comes in.

In December 1994, Newmont received a patent to use the chemical to leach gold from carbon.

Like the bioleaching heap, the chemical is first sprayed on the crushed ore as it's piled, and then applied via the dripemitters.

So far, the demonstration project has produced 16,000 ounces of gold, with 9,500 attributed to the bio-thiosulfate process and 6,500 to the bio-cyanide process.

Newmont, the first company to use bioleaching pretreatment of low-grade refractory gold ores on a large scale, began developing the process in 1988.

Satisfied that it's working, Newmont is already preparing a section of its 682 square miles of Carlin Trend property for a commercial-scale refractory leaching process.

"Eight to 10 million tons a year is our goal," Perry said. "That's what will be coming out of the pit."

Newmont also boasts the largest refractory ore treatment plant -- or roaster -- in the world.

The $340 million plant uses heat and oxygen to free the gold from higher grade ore so it can be cyanide leached. It dry-grinds, crushes and treats up to 10,000 tons of refractory ore per day.

"Bioleaching or biooxidizing does the same thing essentially that the roaster is doing," Perry said. "It's just that the different processes are more economical on the different grades of ores."

Barrick and Newmont are leading the way into the new age of gold mining.

"It's an exciting time to be in the minerals industry," Perry said. "For so many years, the industry was stagnant. But there have been amazing technological advances in the past 10 to 20 years. There are two ways to go into the future -- as an innovator or a fast copier. We prefer to be an innovator."