The disposal of toxic mine tailings is one of the most-discussed environmental issues in the mining industry. The leftover material often contains toxic heavy metals or sulphide ores, which can develop into a significant environmental hazard. A foolproof and safe way to get rid of these inconvenient tailings has not yet been found: dams can break or leak, water from pond storages can leak into nearby soil, submarine tailings are critical to control.
The need for a solution is growing. According to the US Bureau of Land Management (BLM), there are nearly 31,000 abandoned, publicly-owned, mine lands registered in the US alone. Around 75% of these sites need further investigation. Other figures from the American Metal Market suggest that there are around 560,000 public and private abandoned sites throughout the US – many of which pose a risk to the environment.
Canadian clean tech company BacTech has presented an exciting new solution called bioleaching, based on rock-eating bacteria, such as the Acidithiobacillus and Leptospirillum bacterium, to clear abandoned mine sites. Powered by the sulphides in the tailings, the little helpers munch their way through the toxic material and, as a nice side-effect, accelerate the breakdown of minerals.
BacTech president and CEO Ross Orr is convinced that this cheap and clean bioleaching technology can provide a sustainable solution. Mining-technology.com spoke to Orr to find out more about his vision of how rock-munching bacteria could eat their way through abandoned mining sites all over the world.
Elisabeth Fischer: Why is disposing of mine tailings so important?
Ross Orr: The number one problem in North America and obviously South Africa and South America is acid mine drainage (AMD). Sulphides are sitting in the tailings, exposed to wind, rain and sun. There they start to oxidise, creating a sulphuric acid, which then makes its way out of the tailings, kicking up and liquefying heavy metals.
The metals then get dumped into the local river, which changes the pH of the water in a critical way. In the US, the state of Pennsylvania is the largest generator of acid because of coal tailings that have been left behind a hundred years ago, laying there for ages, poisoning the water and the environment. It's just not good.
EF: How can bioleaching help?
RO: One of the benefits or positive side-effects of bioleaching is the ability to neutralise or stabilise arsenic into a ferric arsenate. The bacteria we use for bioleaching to oxidise the sulphide materials left behind, occur naturally and are harmless to humans, animals and the environment. The process can stabilise these toxins from minerals and prevent further harmful AMD. For the bacteria this is a 'garden of Eden' environment, where they can thrive and multiply and achieve in six days what would normally take 20 years to occur naturally.
EF: What kind of projects are you hoping to use bioleaching on?
RO: At the beginning, we literally went looking for arsenic. Where acid mine drainage is coming out of tailings, they bring heavy metals with it, including arsenic. That led us to Snow Lake in Manitoba, Canada, where we found a large stockpile of concentrate, not tailings but actual concentrate, which has over 20% arsenic in it. At the same time it also contains gold.
We went to the state government, knowing that most governments today are broke and need to do more for the environment. We suggested them to clean up the problem without charging them anything but that we would keep the gold that we'd recover. Governments are a lot more willing to talk to you when you work for free. But the recovered metals belong to us and we sell them like a common mining company.
There are two types of customers for us: the governments and the privately owned tailings. Each group has a much different endpoint in their game. The governments just want to see the tailings cleaned up and they are not necessarily in there trying to split the process-outcomes with you, whereas the private mining companies of course realise that they have a value. But the only way to unlock the value is to do it with our technology and they are forced to bring us in as a partner. One has the green factor and one doesn't. We wouldn't do it unless we were compensated with metal.
EF: What types of bacteria do you use?
RO: There are many different strains but we have probably 60 different that we keep in our refrigerator. When they are not given all the things they like, they technically hibernate and go to sleep. So when we lower the temperature and deprive them of sunlight then they go into a dormant state.
When we do a project, we usually put more than one strain of bacteria into the process and what then happens is some sort of Darwinian process where one dominant strain will take over. That allows us to spend 30 days doing the project ramp-up and determining which one of the bacteria is going to be our workforce.
Their benefit is, as we like to say, they are non-unionised, they work 24 hours a day and they don't whine.
EF: How do the bacteria work?
RO: Traditional mining approaches such as smelting or roasting need intense heat to recover the metals. But there are strict limits on the amount of arsenic that can be burnt and the general industry rule is that smelters cannot treat base or precious metal concentrates that contain more than 1.5% arsenic. Bioleaching doesn't need any heat and we're going after the material that's been left behind because it couldn't get recovered with traditional methods.
In case of tailings, we go to the site and refloat the tailings to make another concentrate. And the bacteria don't care - as long as they have sulphides to get their energy from they are going to continue to attack for sulphides and liberate the metals for recovery.
We predominantly use tanks for our projects, where the bacteria oxidise the sulphides within optimal operating and living conditions. At the beginning we use five or six but by the time we get to the end we have two settling tanks. One tank has a pregnant solution of base metals such as zinc, copper, nickel. The bacteria create a sulphuric acid in the tank by oxidising the sulphides. When that sulphuric acid touches the base metal they liquefy and then you recover them using conventional technology such as electro-winning.
The other tank will have the precious metals, which are basically residues and the stuff that has not been devoured. We take that residue and just put it back into a conventional gold circuit to recover the metal.
The nice thing is that we don't have to separate the concentrate into copper, zinc and gold because we just put everything into the same tank and remove the metal in various stages along the way.
EF: Can the bacteria survive and work in different environments?
RO: Our biggest concern with the bacteria is cooling them down as they have an exothermic reaction, a chemical reaction accompanied by the release of heat. That allows us to work in sites such as in Snow Lake, Manitoba, in the wintertime. We have sites in Mexico too and our biggest concern there is to keep the temperature below 75°C so that the bacteria don't blow up.
EF: How much interest has there been in this method?
RO: It's definitely coming on. The difference between what's going on today versus five or six years ago is huge. A lot of that what's happening is still academia because there are only two companies, Gold Fields and REBgold, who do it on a commercial basis.
I think it's the beginning of a wave of environmental technologies in mining all over but with the understanding that it takes a long time to have the industry to craft something new. The dollars are big but the mistakes are big too. We take comfort in the fact that our technology is commercial. Will it be the technology of choice in ten years? I doubt it - there will probably be something else that has come along.
But for us, as long as we have an inventory of projects to work on, than we're going to be busy for the next 25 years. That's really what we are trying to do right now. Getting our hands on assets where we know our technology can work and we'll get to them sooner or later. Maybe, just not tomorrow.