The Blackleg Miner - Part 1

Ok - this has nothing to do with the NACODS during the NUM strike or 19th century folk songs - no, this is about a different kind of mining.

Father and son, we’re both miners. My Father mined coal until the mid-1980s but me? I’m a different kind of miner - in fact, I’m TWO different kinds of miner, thinking about it. Firstly, I’m a biominer - I worked for a while mining copper using Bacteria. You might think that a bit far-fetched but, would you believe that gold, copper and uranium are often mined in this way? Amazing though it may seem, over 25% of all the copper mined in the world is mined by Bacteria (well, Archaea too but but let’s keep it simple!). It’s not even a new process either - the Romans discovered it around 2,000 years ago!

Ok, so biomining has been around since Roman times - but what is it? Metal ores are rocks that contain a particular mineral, for example, the copper mineral chalcopyrite (pron. “cal-co-pie-rite”) is mined from the Earth in the form of an ore - varying amounts of chalcopyrite mixed in with rock. Now, for the ores that contain a high percentage of chalcopyrite and not a lot of rock - high grade ores - can be smelted to release the copper. To understand smelting, there’s the need for a bit of chemistry, sorry! First of all, chalcopyrite is a copper iron sulfide mineral, that is to say, it is a mixed sulfide of both copper and iron at once - CuFeS₂ (Cu, from the Latin cuprum, is copper; Fe, from the Latin ferrum, is iron and S is, oddly enough, sulfur - the subscript “2” shows that there are two sulfur atoms present). The ore is first crushed before being heated in the presence of oxygen, sand and coke, which (over a series of several separate steps) results in the oxidation of the mineral from CuFeS₂ into elemental copper (Cu) and slag (FeSiO₃ - the Si coming from the sand, which is mostly silicon dioxide - SiO₂). Smelting works perfectly well and is the source of 75% of the world’s copper supply, but, since the mineral has to be heated to 1200 degrees Centigrade, it’s not a cheap process, so it’s only suitable for use with these high grade ores in which there’s very little waste material. In low grade ores, there is a very small amount of chalcopyrite and a lot of rock - this would require a lot of time and energy to smelt and the resultant yield of copper is so low that smelting just isn’t financially viable. This is where the Bacteria come in handy!

Biomining is based on the fact that some Bacteria are very fond of sulfides as an energy source. Unlike humans, these organisms are lithoautotrophic, which literally means “feeds itself using rocks”. Whereas we ultimately obtain all of our carbon and energy from organic compounds such as sugars and fats, these Bacteria obtain all of their carbon from carbon dioxide gas from the atmosphere, so that they can grow without needing to supply them with a source of carbon - so it looks like they’re feeding themselves. The autotrophs that most people are familiar with are plants - plants use energy from light to fuel the process of taking up carbon dioxide - this is called photoautotrophy. Swap light for inorganic chemicals and you have lithoautotrophy. Whilst litho- comes from the Greek λίθος or lithos, meaning “rock”, it can things other than rocks, but in biomining, the lithoautotrophs really are eating the rocks!

Whilst Bacteria need a small amount of iron and copper to survive, just as we do, it’s the sulfide of chalcopyrite that they’re after. Bacteria can oxidise the sulfide present in the mineral to sulfuric acid, which then chemically dissolves the rock, exposing more of the mineral and, at the same time, dissolving the copper and iron. If the iron is in the iron (II), or ferrous, oxidation state (as it is in chalcopyrite), the bugs can use this too, instead of or as well as sulfur, forming iron (III), or ferric iron. Because of all of the acid produced, the organisms used have to be able to grow well at low pH and they also need to be able to cope with large amounts of copper - which is a very toxic metal - not to mention the high temperatures involved - you see, this process gives off a lot of heat. The way it is done is very simple - low-grade ore is simply stacked up in a big pile known as a leach heap and dilute acid is trickled through it to kick-start the Bacteria, since they don’t just tolerate low pH (acid) environments - they actually prefer them! The acid is then collected and recirculated through the heaps. Once the bugs get going and start to dissolve the rocks, the leach liquor (as the dilute acid is termed) becomes blue with copper (II) sulfate that is formed when the copper in the ore dissolves. Gradually, as this becomes darker and darker blue it is referred to as being pregnant with metals. The liquor is drained off and electrolysis used to recover high-purity elemental copper. Over a period of a few months, up to 70% of the copper in the ore can be recovered - this is the most economically viable way to handle these ores and this is where 25% of our copper comes from - mainly from operations in Chile.

Two of the best studied biomining bugs are Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans. You can guess from the names that they like acid environments and oxidise sulfur (thio) and iron (ferro) but they are capable of some neat tricks. For example, I mentioned that these organisms can use ferrous iron as an energy source, yielding ferric iron. In the dark depths of a leaching heap, the amount of oxygen in the tiny air spaces between the rocks can be very low and so, to stay alive, these bugs perform the neat trick of switching from growing aerobically (using oxygen) to anaerobically (using anything but oxygen). It just so happens that they can easily switch from using oxygen in their respiratory chain to using ferric iron, forming ferrous iron - so the situation can occur in which ferrous iron from the rock is oxidised to ferric iron by the Bacteria for energy but is then reduced back to ferrous iron for respiration. Neat huh? 

So where do I fit in? I spent a period of time looking at how the organisms used in these copper leach heaps in Chile actually work - how do they grow? Just how easy is the switch from using oxygen to using ferric iron? How do they cope with other metals? What’s their copper tolerance like? Can we make it any higher? The bugs I worked with aren’t the ones mentioned above but I can’t tell you which they were because I’m still bound by a Confidential Disclosure Agreement with the mining company involved and I’m very limited as to what I can talk about! The bugs I worked with grew at pH 1 and below - that’s low enough pH to dissolve a human body and not leave any trace - and liked to be above 80 degrees Centigrade if they could help it. Normally, when we’ve grown a culture of Bacteria in the lab and want to put it to one side for a week or so, we keep it in the fridge, slowing the metabolic rate of the bugs so that they stay as they are. With these acidothermophilic bugs, you can just keep them on the bench at room temperature - as far as they’re concerned, that’s “cold”! 

Just sort of makes me think sometimes - humans aren’t that amazing. Ok, we can build rockets and go into space and invent amazing things like the iPhone and cure diseases like leprosy…but we can’t cope with extremes of pH, temperature, salt or tolerate toxic metals like copper and uranium. We can only use oxygen (or occasionally pyruvate) for respiration - nothing interesting like ferric iron or sulfide and we can only get our carbon from food that we eat - not from the air that we breath. I think the Bacteria well and truly out-do us in all of these respects!