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Making money from mine waste: the promise of bioleaching

Dr Marijke Fagan-Endres with one of her heap bioleaching experiments, known as a ‘box reactor’. Image provided by C. Mazzolini.

For the last few hundred years, South African mines have reaped the rewards of some of the highest-value ore on the planet. But all that is changing now, as ores deplete and social and economic difficulties mount.

Mines are thus looking for new ways to generate profit, and Dr Marijke Fagan-Endres, of the Centre for Bioprocess Engineering Research (CeBER), says
that bioleaching could be the answer.

What is bioleaching?

Bioleaching is the practice of using microorganisms such as bacteria, archaea and fungi to extract low concentrations of metals (for example copper) from waste material.

What is an extremophile?

An extremophile is a microorganism (usually bacteria and archaea) that thrives in an environment that would kill most living things. Examples would include very high or very low temperatures, high concentrations of salts, or very low pH (high acidity). They are useful for biotechnology because they can often live on unusual substances and tolerate harsh conditions.

The process is gaining traction as a way to get valuable minerals from mine tails and heaps (mine waste). To extract minerals, a heap is irrigated and then inoculated with various microorganisms. These organisms, usually extremophilic bacteria (bacteria that have adapted to thrive under extreme conditions) convert metals in the rock into metals in solution, from which they can be harvested.

Fagan-Endres’ work on heap bioleaching earned her the Most Promising Young Woman - Excellence in Research category of the Women in Engineering and the Built Environment (WiEBE) Awards in September 2014.

“I think my CV is slightly different to what they normally encounter,” she says with a smile, “so they created a category for young or emerging researchers, which I was lucky enough to be the first recipient of.”

She and her co-workers are developing sophisticated scientific methods, making use of techniques traditionally reserved for studying the human body, like magnetic resonance imaging (MRI) and X-ray computed tomography, to understand how liquid moves through a heap bioleaching system.

“One of the big challenges of heap bioleaching is that these are really huge systems, which can be kilometres in length and breadth,” she says. Add to that particle sizes ranging from grains of sand to rocks larger than your head, as well as an uneven distribution of metals in the heap, and a broad spectrum of temperatures, and you have an extremely complex process. To make it as efficient as possible, scientists need to understand exactly how it works, and this is what Fagan-Endres and her colleagues aim to do.

“Using MRI is slightly complicated in the mineral environment because you’re putting metals into a magnet, which causes all sorts of issues,” she explains. Overcoming these practical challenges was difficult, but their approach now effectively predicts liquid flow and leaching efficiency in different parts of a heap bioleaching system.

CeBER has been involved in heap bioleaching research for decades, largely through a collaboration with BHP-Billiton. Fagan-Endres’ group uses several indigenous microorganisms for heap bioleaching research, including Leptospirillum and Acidithiobacillus species, which occur naturally in mine heaps.

Other CeBER researchers are studying how these bacteria attach to ore, or how they dissolve metals. One project focusses on how bacterial ecosystems establish themselves in the heap.

Fagan-Endres says that CeBER is also involved in research into acid mine drainage (AMD) and bioflotation – another hydrometallurgical process that mines use to split ore into mineral-rich and mineral-lean fractions.

“AMD happens by the exact same process as heap bioleaching,” she says. “The only difference is, we’re trying to harness bioleaching for economic benefit, and we’re trying desperately to control AMD.”

Other researchers at CeBER are developing diagnostic techniques that can predict whether specific kinds of rock will be acid-generating or not. This will allow mines to classify waste, and make a responsible decision about how to treat it so that the environment is not harmed.

Heap bioleaching is a common practice in countries such as Chile and Peru, where they use it to reclaim copper, and the USA, where it is used to remove sulphide minerals from gold ore. Can we then expect heap bioleaching to become common practice here?

Apparently, it remains a fringe technique in South Africa. “Heap bioleaching in South Africa has been a little lean because we’ve been lucky enough to have plentiful supplies of high grade ore up to now,” says Fagan-Endres.

But as ore quality declines, Dr Fagan-Endres’ work will certainly become more and more relevant for South African mines.

Dr Marijke Fagan-Endres is a researcher and lecturer at UCT's Department of Chemical Engineering, in the CeBER research group. She holds a PhD from the University of Cambridge, and is the 2014 winner of the WiEBE Most Promising Young Woman - Excellence in Research Award.

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