Microbes and mining

Noise is anathema to the modern mining industry, as it generally means wasted energy. In looking for better, less-loud ways of winning metals, the industry is increasingly turning to techniques that involve biological processes. Its most powerful ally is the smallest and most silent of all lifeforms: bacteria.

In using bacteria to extract metals from rocks, miners are taking a leaf out of nature’s book. Bacteria have been doing it since the dawn of life — producing enzymes that speed up the oxidation of mineral sulphides and release their heavy metals into solution.

Prospectors have always recognized the significance of red rivers stained by metals that have been fixed by algae. They use them to find what they call gossans, the rusty outcrops of decomposing sulphide deposits. Inside the gossans, helping the decomposition along, are invisible hungry bacteria with tongue-twisting names like Thiobacillus thiooxidans and Leptospirillum ferrooxidans.

It was not until the early 1950s that the importance of bacteria as natural sulphide destroyers became clear. Since then, many different types with the talent have been identified, with varying tolerances and biochemical roles. Individuals of the most voracious type are a few thousandths of a millimetre in size, rod-shaped, and able to move around. They flourish in the acid conditions they help to create, and attack almost every kind of sulphide. Other types are more choosy, shunning common sulphides like pyrite and arsenopyrite. Some more than others can tolerate higher temperatures, stronger acids and higher concentrations of dissolved metals.

Bacteria are great survivors. Even under quite extreme conditions there are types that are capable of breaking down sulphides. Selective breeding and genetic engineering have added to their abilities, for instance by producing strains that are more tolerant of dissolved toxins such as mercury.

The types that decompose sulphides work in complicated ways. They attack the minerals directly and assist chemical reactions that have the same effect. Even with simple sulphides there is normally a chain of reactions. For instance, they commonly contain impurities — sometimes trace elements that are toxic to bacteria. Also, there may be subtle complexities at the atomic level that test the versatility of bacteria.

Dissolving copper out of rock (that is, leaching) is an old production technique. Pliny the Elder (79-23 AD) wrote that it was widely used in his time, particularly in Spain. However, “bio-leaching,” the deliberate use of bacteria to speed the chemical release of metals from ores, is a relatively recent development. It was pioneered in the U.S. in the late 1950s and so far has been used mainly on ores of copper and uranium. It can be used on raw ore or on mill products such as mineral concentrates. A related technique knowns as “bio-oxidation” is used by the gold mining industry to decompose sulphide concentrates and expose tiny grains of gold for chemical recovery.

Miners were using bacteria to leach copper from sulphide ore long before they realized it. At the Rio Tinto mine in Spain, it was once the practice to expose heaps of broken sulphide ore to the atmosphere and precipitate copper from water that had percolated through the heaps. The effectiveness of this simple technique puzzling: was the local climate responsible, or was there something strange about the ore itself? The real reason, undoubtedly, was that the heaps were an ideal breeding ground for oxidizing bacteria.

The potential for modern, large-scale commercial use of bacteria to leach copper is demonstrated by a project on the drawing boards for the giant Escondida copper mine in northern Chile.

Its purpose would be to recover copper from sulphide mineralization that would otherwise be left on waste dumps. Escondida has an enormous inventory of that sort of mineralization, amounting to 2.3 billion tonnes at the latest reckoning. The average copper content is 0.55% — not particularly low by the standards of some copper mines but below Escondida’s economic grade for recovering the copper into sulphide concentrates.

Although the project is at an advanced stage of study and design, a decision to proceed is at least a year away and will depend on environmental approvals and an improved outlook for the copper market. If the go-ahead is given, the scale and sophistication of the project would make Escondida a world leader in the bio-leaching of copper.

Right now, it can just be described in terms of general intentions and best estimates. Starting with rock from existing dumps, the intention is to engineer two large heaps of material from which copper will be leached by bacteria-charged solutions of dilute sulphuric acid and ferric sulphate.

“‘Engineer’ is the right word,” says Chris Willows, chief consultant of Rio Tinto Technical Services. “Great care has to be taken in constructing the leach pads and making them impervious. The heaps must be built so that air will reach the bacteria and permeability is maintained, and there must be allowance for constant monitoring of what’s going on inside. A lot of pipework and monitoring equipment are involved.”

The material going on to the heaps will not be crushed beforehand: blasting and handling will have reduced most of it to fist size and smaller. It will not be uniform in terms of sulphide content, copper mineralogy, type or size distribution — all factors that will have some bearing on how it responds to bio-leaching.

Over 30 years or so, the heaps will be built in stages to a final height of about 130 metres. Rock will be trucked to two leach pads, each of which will cover about 8 sq. km. Each heap will eventually contain about a billion cubic metres of treated material.

Common strains of bacteria will be used, ones that specialize in decomposing sulphides. They will be encouraged to multiply by blowing air through the heaps, which will provide the carbon dioxide they use to generate bio-mass. If they like their surroundings, they will multiply to the tune of billions per litre of solution. Trace elements needed for growth, such as potassium, will be readily available in the rock heaps.

Temperatures within the heaps will be continuously monitored — oxidizing sulphides give off heat that could build up locally and make the bacteria uncomfortable. The surface of the heaps will be insulated against the freezing winds that blow through the Escondida area from the Andes. The leach pads will be about 3,000 metres above sea level, yet it’s unlikely energy will be required to heat the leach solutions.

Copper will be leached from the heaps as dissolved sulphate, in weak solutions that will be pumped to a solvent extraction plant. Here, the concentration of dissolved copper will be increased using an organic reagent. In the final stage, eletrowinning, high purity metal will be recovered as sheets called cathodes by passing electricity through tanks of the enriched solution. Solvent extraction-electrowinning is already being used on a large scale to recover copper from oxide ores at Escondida. That type of ore is expected to run short in a few years.

How much copper does Escondida stand to gain from its use of micro-organisms? It’s a question that can only be answered with estimates, but they are based on a great deal of testwork, including the experimental treatment of 200,000 tonnes of typical material in a trial heap. And bio-leaching is a process that can continue for a long time after it has reached its peak, so its total yield is not entirely predictable. Empirical results over time are the best measure of its effectiveness.

Escondida expects the project to yield 84,000 tonnes of high-purity (cathode) copper a year initially, building up to 234,000 tonnes a year as material is added to the heaps at a faster rate and more electrowinning capacity becomes available. By comparison, last year Escondida produced 136,700 tonnes of cathode copper by conventional leaching of oxide ores, or 18% of the mine’s total copper production.

Typical material is expected to have released about a third of its copper after it has been treated for 250 days. It would go on yielding copper for as long as reactive fluids and bacteria can reach copper mineral and find an outlet. Willows explains: “About half the copper going into the heaps would start out in sulphides that respond extremely well to bacterial leaching. The other half would be in a sulphide called chalcopyrite, which leaches relatively slowly however you do it.

“We can’t expect to recover all the copper, but I’d be surprised if we didn’t eventually collect at least fifty per cent of what was there to begin with. Although this implies that a considerable amount is left behind, bear in mind that without bacteria, the likelihood is that none of this copper could be extracted economically.”

Setting the project up is expected to cost US$500 million and require less water per tonne of ore than the normal method of treating the same type of mineralization.

— The preceding is from Review, a quarterly publication of London-based Rio Tinto. The author was formerly chief geologist for the company.