DR CHRIS BRYAN
Mining is perhaps the second oldest profession (well, certainly the third oldest) but as with most things it seems, nature has been doing it for far longer. As life coalesced from the primordial soup, there was no oxygen in the atmosphere and photosynthesis was not even a glimmer in evolution’s eye. For millions of years, all life on Earth was underpinned by energy from mineral sources rather than sunlight - early life was lithotrophic, literally ‘rock-eating’.
Exposed sulfide minerals such as pyrite were abundant and microorganisms got their energy through the oxidation of the iron and sulphur they contained. These activities produce a corrosive mix of ferric iron and sulphuric acid which attacks nearby minerals resulting in the dissolution of many metals contained therein. Today we do not tend to find naturally occurring outcrops of sulphide minerals, but where such minerals are exposed to moisture and air through extractive (mining) activities they are rapidly colonised by the descendents of these ancient tiny miners. In the absence of any preventative measures this microbial weathering can produce metal-rich acid mine drainage (AMD) causing severe environmental damage.
Cornwall hosts a wealth of mine wastes and the actions of the naturally occurring microorganisms shape and colour the landscape we see today. As a result Cornwall produces huge amounts of AMD and, as elsewhere, this must be remediated at significant cost, in perpetuity. While some legal departments may argue over the definition of ‘perpetuity’, this is somewhat Clintonesque - we will be remediating mine drainage for as long as water drains from them. Following from the Wheal Jane disaster in 1992, when the failure of an adit plug led to the sudden release of 50 million litres of AMD into the Carnon River, a treatment plant was installed at the former Wheal Jane mine site. Due to the vast interconnected nature of Cornwall’s underground mines, this plant effectively pumps out water from a huge area of West Cornwall – some 150 million litres per day.
Nevertheless, humans have been exploiting these biological processes to actively recover metals since antiquity, albeit unknowingly: in 166 AD a Greek naturalist and physician described in situ leaching of permeable copper ores in Cyprus while the observation by Diego Delgardo in the 1500’s of scrap iron being transformed into copper (simple displacement chemistry) in the Rio Tinto surely fuelled alchemists' beliefs that base metals could be converted to gold.
Now known as biomining or biohydrometallurgy, this process has been used commercially since the early 1960’s with the construction and irrigation of heaps for the recovery of copper at the Kennecott Bingham Canyon Copper mine. Between 1980 and 1998, the amount of the world's copper produced from biomining operations increased from 10% to 25%. The biooxidation of gold-bearing arsenopyrite ores is the basis of the BIOX® process developed by Gencor in the 80’s. In these processes microbial activity renders occluded gold accessible to cyanide, increasing recovery rates to 95-98% and up to 3% of the world’s gold is produced this way.
Text commissioned for the publication Natural Alchemy, 2014. The project was a collaboration between Dr. Chris Bryan of the University of Exeter’s Environment and Sustainability Institute (ESI) and artist Oliver Raymond-Barker. It is part of the Creative Exchange Program supported by the ESI and Falmouth University’s Research in Art, Nature and the Environment (RANE).
All text copyright the author.
All text copyright the author.