BIOLEACHING

Bioleaching is the extraction of specific metals from their ores through the use of bacteria. This is much cleaner than the tradtitional heap leaching using cyanide.[1] Bioleaching is one of several applications within biohydrometallurgy and several methods are used to recover copper, zinc, lead, arsenic, antimony, nickel, molybdenum, gold, and cobalt.

The Process

The extraction of gold from its ore can involve numerous ferrous and sulfur oxidizing bacteria, including Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans (formerly known as Thiobacillus). For example, bacteria catalyse the breakdown of the mineral arsenopyrite (FeAsS) by oxidising the sulfur and metal (in this case arsenic ions) to higher oxidation states whilst reducing dioxygen by H2 and Fe3+. This allows the soluble products to dissolve.

FeAsS(s) → Fe2+(aq) + As3+(aq) + S6+(aq)

This process actually occurs at the cell membrane of the bacteria. The electrons pass into the cells and are used in biochemical processes to produce energy for the bacteria to reduce oxygen molecules to water.

In stage 2, bacteria oxidise Fe2+ to Fe3+ (whilst reducing O2).

Fe2+ → Fe3+

They then oxidise the metal to a higher positive oxidation state. With the electrons gained, they reduce Fe3+ to Fe2+ to continue the cycle.

M3+ → M5+

The gold is now separated from the ore and in solution.

The process for copper is very similar. The mineral chalcopyrite (CuFeS2) follows the two stages of being dissolved and then further oxidised, with Cu2+ ions being left.