In modern mining industry several new processes are used which are easily applicable and economical and environment friendly. Application of biotechnology in mining industry to extract different metals is one of the above said new age techniques. The area of discipline in which microbes are used to extract different metals from ores, is collectively termed as bio-hydrometallurgy or microbial mining.

Bioleaching is a part of microbial mining. In microbial leaching or bioleaching metals can be extracted from large quantities of low grade ores This process has been used for centuries to recover copper form the drainage water of mines the role of microbes in the bioleaching came into notice some 40 years ago.

Mechanisms Involved in Bioleaching

Two processes are used for bioleaching, direct action and indirect action. In direct action bacteria act on the ore directly to extract metal. In indirect action the bacteria produces some chemical substance like acids or ions which lead to extraction of metal from the ore. In mining industry both the direct and indirect method is used in combination to extract metal economically.

I) Direct Bioleaching

In direct bioleaching minerals which are susceptible to oxidation undergoes direct enzymatic attack by the microorganisms. Certain bacteria such as Thiobacillus ferrooxidans oxidize ferrous ions to ferric ions and in this process it transfers the electrons form iron to oxygen. Iron is extracted from the iron ore pyrite (FeS2) in a more soluble form. Thibacillus thiooxidans extract sulphur from iron ore pyrite in the form of sulphates which is also a more soluble form.

II) Indirect Bioleaching

In the indirect method of bioleaching of minerals bacteria produces strong oxidizing agents which react with metals and extract them from the ores. Bacteria produce oxidizing agents such as ferric ions and sulphuric acid on oxidation of soluble iron and soluble sulphur respectively. Acidic environment is a necessary condition for indirect bioleaching to keep the ferric ion and other metals in solution. The continuous oxidation of iron, sulphur, metal sulphides and carbonate ions in solution help to keep the acidic environment.

Microbes Involved in Bioleaching

The bacteria which are naturally involved in growing up among rocks are most suitable and applicable ones for bioleaching. The most commonly used microorganisms for bioleaching are Thiobacillus ferrooxidans and Thiobacillus thiooxidans. Thiobacillus ferrooxidans is a gram negative, rod-shaped, motile, non-spore forming bacterium. It derives the energy of the growth from the oxidation of iron or sulphur in ore. This microorganism oxidises ferrous ions (Fe2+) to ferric ions (Fe3+) and converts sulphides, thiosulphate, sulphur to sulphate. Thiobacillus thiooxidans has similar functions and it grows mostly on sulphur ores or rocks.

Sulfolobus acidocaldarius and Sulfolobus brierlevi are two other bacteria which are involved in bioleaching of copper and molybdenum from ores like chalcopyrite and molybdenite. They are thermophilic and acidophilic bacteria which mostly grow in acidic hot springs. Two other bacteria like Leptospirillum ferrooxidans and Thiobacillus organoparpus work in a combination to extract iron and copper from pyrite and chalcopyrite.

Some bacteria like Pseudomonas aeruginosa can also be used to extract high value metal like uranium from low quality uranium ore containing around 0.02% uranium. This microorganism extracts around 100mg uranium per one lire solution in less than 10 seconds. Rhizopus arrhizus is another bacterium that can extract uranium from low grade uranium ores.

Fungi have not shown a broad area of examples in the field of bioleaching. Some of the fungi associated with bioleaching are Aspergillus niger and Aspergillus oryzae. While A.niger extracts copper and nickel, A.oryzae has been found to extract gold by bioleaching. The use of more bacteria and fungi are still in the experimental stage.

Commercial Process of Bioleaching

The naturally occurring bioleaching process is a very slow process. For commercial extraction of metals by bioleaching the process is optimized by controlling the temperature, pH, humidity, O2 & CO2 concentration etc. The appropriate microorganisms along with nutrients and acids are pumped in to the ore bed and the extracted leach liquor is collected for metal recovery. The different processes of commercial bioleaching are as follows.

I) Slope Leaching

In slope leaching the ore is finely ground and kept in large pile in a slope which is subjected to continuous sprinkling of aqueous solution of microorganisms. The leach liquor collected at the bottom of the ore is processed further for metal recovery.

II) In-situ Leaching

In in situ leaching ore is subjected to bioleaching in its natural occurrence. Aqueous solution of microorganism is pumped through drilled passages within the ore. The leach liquor collected at the bottom of the ore used for metal extraction.

III) Heap Leaching

In heap leaching the ore is arranged in heap and goes through the same treatment such as in slope leaching. The aqueous solution containing microorganisms works on the heap of ore and produces the leach liquor. The leach liquor is used for metal recovery.

In all these three processes of commercial bioleaching the aqueous solution of microorganisms can be reused.

Advantages of Bioleaching

1. Extraction of metals form low grade ores
2. Very economical process
3. Employed for collecting metals from wastes or drainages
4. Environment friendly process
5. Used to extract refined and expensive metals which is not possible by other chemical
processes

Disadvantages of Bioleaching

1. Very slow process
2. Dependency on several atmospheric conditions decreases the efficiency of the process.

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