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Rock-Eating Bacteria "Mine" Valuable Metals

Tim Hornyak in Tokyo
for National Geographic News
November 5, 2008
 
Bacteria can leach small amounts of valuable metals from otherwise useless ore, and scientists are now applying the latest biotechnology to make the bugs better miners.

These mineral-crunching microorganisms are a type of bacteria that use minerals as their source of energy. When the life-forms break down the matter through metabolism, they squeeze out metal ores or concentrates combined with sulfur in a process called bioleaching.

The method is emerging as an increasingly important way to extract valuable minerals when conventional methods such as smelting can't do the job cheaply enough, experts say.

Development is also being spurred by the electronics industry's brisk global demand for copper.

"Certain microbes react to metal ions and help copper be leached out of low-quality ore," said Masaru Tomita of the Institute for Advanced Biosciences at Japan's Keio University.

"The ultimate goal is to establish biotechnologies to leach copper from this low-quality ore."

Bioleaching already currently accounts for an estimated 20 percent of the world's mined copper, and is in use at about 20 mines around the world.

Bioleaching

People have seen the results of microbial leaching in mine waters and rust-colored rivers for thousands of years, but it took until 1947 for bacteria to be identified as the cause.

(Related: "Rust-Breathing Bacteria: Miracle Microbes?" [April 12, 2004].)

That's when bacteria at a mine in Utah in the western United States were found to be responsible for bluish copper-bearing solutions leaking from piles of waste rock.

Since that discovery, dozens of other microorganisms useful for bioleaching have been found around the world, including in an uranium mine, a volcano, and a hot spring.

A fairly simple process called heap bioleaching, in which copper sulfide ore is crushed and put on an impermeable pad, has also increased the bacteria's usefulness.

An acidic solution containing bacteria is applied and allowed to percolate through the heap, draining into a collecting pool.

In a few months, the bioleaching is complete, with 80 to 90 percent of copper extracted from the ore. The process can be used on ores that contain only 0.5 to one percent of copper, too little for conventional refining.

Miniscule Miners

Meanwhile, recent advances in molecular technology have allowed scientists to try to optimize the growth and function of the metal-loving microbes.

Earlier this year, lead researcher Tomita began collaborating with scientists from the Chilean venture firm BioSigma.


BioSigma was set up in part by Chile's state-owned Codelco, the world's largest copper producer.

Their goal is to make the miniscule bacterial miners speedier and more efficient by identifying key genes, proteins, and metabolites.

Tomita and colleagues, for instance, now have a better understanding of the bugs' digestive systems.

The effort has so far sequenced the genomes of three organisms that rely exclusively on either iron or sulfur—or both—as an energy source. Scientists can then grow and identify the bugs.

"We are now able to know what microorganisms are present in a sample and how many of them are alive and doing their job," says Ricardo Badilla, general manager of BioSigma. "These tools enable us to control and modify a bioleaching operation at the industrial level."

The first full industrial plant using microorganisms will begin operations by the end of 2009, and Codelco hopes to be producing over 100,000 tons of copper annually from bioleaching in the next ten years.

Environmental Boon

Bioleaching may also soften the environmental blow of the mining industry, experts say.

"Bioleaching and biooxidation processes are alternatives to conventional smelting processes, which discharge large amounts of carbon dioxide, sulfur dioxide, and several kinds of toxic materials [such as] arsenic," said Chihiro Inoue, an environmental studies professor at Tohoku University in Japan.

The methods also consume large amounts of energy resources, he added.

BioSigma's Badilla agreed bioleaching reduces the "adverse impact of mining into the environment."

"There is a tenfold decrease in gaseous emissions, a twofold reduction in energy consumption, and a fivefold reduction in water consumption in comparison with conventional … technologies," Badilla said.

Bioleaching is also cheaper: A typical operation costs only half as much as conventional smelting and refining.

But a lot more research is needed, Badilla added.

"It may take more than 15 years before bioleaching starts to gradually replace conventional technology as the process is improved."
 

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