Science - May 26, 2011

PhD student finds a new path to gold

Bacteria can be used to help extract gold from pyrite. This new procedure is more sustainable than the old method.

The equipment Alex Hol uses to extract gold from pyrite.
It was not unknown for gold prospectors in the Wild West to be deceived by pyrite. This mineral looks deceptively like a clump of raw gold. ‘That's why it is also known as fool's gold', says PhD student Alex Hol. 'But they were not really being that stupid as there really is gold in pyrite.' Hol developed a method for extracting this invisible gold from pyrite (and the related mineral arsenopyrite) - like a modern gold prospector or alchemist.
Pyrite (FeS2) contains a little gold, about one hundred grams per ton of the mineral, explains Hol. But it is not easy to extract that gold. The gold particles are firmly embedded in the mineral's crystal structure. 'But the current high gold prices make it worth the effort of separating out that gold. There are factories in places like Peru, Australia and South Africa that are doing just that.'
Sulphuric acid
The trick is to break down the crystal structure, which releases the gold. The usual technique is based on oxidation (chemically or by using bacteria) of the iron in the pyrite. It is possible to extract up to ninety percent of the gold using this way but it is not a sustainable method. Not only does the process require a lot of energy and oxygen, it also produces sulphuric acid as a waste product, which cannot easily be used for other purposes.
Hol is taking a totally different approach. The heart of his procedure is the use of bacteria to reduce the sulphide in pyrite. 'This releases hydrogen sulphide (H2S, the smell of rotting eggs, ed.), which you can easily extract from the solution and process to elemental sulphur. Bioreduction is a fantastic idea for breaking down pyrite.'
However, partial oxidation of pyrite is still required first. That is why the recipe Hol finally came up with is a combination of bio-oxidation and bioreduction. He calls his procedure Paroxsul: partial oxidation of sulphides.
Alex Hol. Bioreduction of sulphide minerals to recover invisible gold. PhD degree ceremony on Thursday 31 May at 16:00.