Bacterial Leaching in the mining industry

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Bacterial Leaching in the mining industry

Introduction

What is bacterial leaching

Bacterial leaching is the process by which micro-organisms are used to extract valuable elements from compounds. The bacteria achieve this extraction by oxidising the compounds; (this is how the bacteria obtain their energy to live) separating the compound into its separate elements. 

How is it applied to the mining industry

In mining, bacteria such as the Thiobacillus thio-oxidans obtain the energy they need to live by oxidising S2- ions. The S2- ions are present in insoluble minerals of copper, zinc and lead. The oxidation of the S2- ions by bacteria releases these valuable metal ions into solution (from where the metallic elements can be extracted).

An example of where widespread leaching can be found

San Manuel in Arizona, a mine consisting of five holes drilled into an ore deposit, an acidic leaching solution containing bacteria is pumped down the central hole where the bacteria do their work. The resulting solution is pumped from the other holes and processed. The leaching solution is recycled.

Leaching Gold

Explanation

Between 15% and 30% of the world’s gold reserves occur as refractory minerals - microscopic particles of gold encapsulated in a mineral matrix. A traditional method of extracting this gold is to use froth flotation to separate the refractory minerals from any unwanted oxide ores and non-metallic minerals present. This produces a sulphide concentrate, which is then roasted to liberate the gold. The gold is extracted by treating the resulting mixture with an aerated solution of sodium cyanide, a process called cyanidation. However, roasting converts any sulphur in the refractory minerals to sulphur dioxide and any arsenic to arsenic (III) oxide, both of which have environmental pollutant qualities. Cyanidation is also difficult. The mineral matrix acts as an impervious physical barrier and shields most of the gold particles from attack by cyanide ions.

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The biological approach is to treat the refractory sulphide concentrate with the thermophilic bacterium Suipholobus acidocalderius. These bacteria catalyse the oxidation of the encapsulating sulphide minerals by dioxygen under aqueous conditions. When tested at 70°C, cyanidation of the resulting extract led to increase in gold recovery from 10% to 100%.


Chemical equations

Taking the refractory mineral arsenopyrite (idealised formula FeAsS) as the example, the overall oxidation process is represented by the following equation.

2FeAsS+ 7O2 + 4H+ + 2H2O  2Fe3+ +2H3AsO4 + 2HSO4-

Bacterial oxidation occurs in two stages:

Stage 1

Stage 2

Leaching copper

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