Microbial Leaching - Chemistry, Process

 Microbial leaching

Microbial ore leaching (bioleaching) is the process of extracting metals from ores with the use of microorganisms. This method is used to recover many different precious metals like copper, lead, zinc, gold, silver, and nickel.

Microorganisms are used because they can:

·    very efficiently extract metals when their concentration in the ore is low.

     lower the production costs.

·   cause less environmental pollution in comparison to the traditional leaching methods.

·         

Miroorganisms used for Leaching

The most commonly used microorganisms for bioleaching are Thiobacillus thiooxidans and T. ferrooxidans. The other microorganisms may also be used in bioleaching viz., Bacillus licheniformis, B. luteus, B. megaterium, B. polymyxa, Leptospirillum ferrooxidans, Pseudomonas fluorescens, Sulfolobus acidocaldarius, Thermothrix thioparus, Thiobacillus thermophilica, etc.

Chemistry of Microbial Leaching  

T. thiooxidans and T. ferrooxidans have always been found to be present in mixture on leaching dumps. Thiobacillus is the most extensively studied Gram-negative bacillus bacterium which derives energy from oxidation of Fe²⁺ or (sulphides) insoluble sulphur.

In bioleaching there are two approaches:

1)      Direct Bacterial Leaching- Direct oxidation of metal sulphide

2)      Indirect Bacterial Leaching – Oxidation of ferrous iron content of the ore to ferric iron. The ferric iron in turn, chemically oxidises the metal to be recovered. 

Direct Bacterial Leaching

In direct bacterial leaching a physical contact exists between bacteria and ores and oxidation of minerals takes place through several enzymatically catalyzed steps. Direct oxidation of metal sulphide occurs here. For example, pyrite is oxidized to ferrous sulphate as below:

	T. ferrooxidans
2FeS2 + 7O2 + 2H2O		2FeSO4 + 2H2SO4

Ores, like pyrite (FeS), are first oxidized by ferric iron (Fe³⁺) to thiosulfate (S₂O₃²⁻) in the absence of bacteria.

Indirect Bacterial Leaching
In indirect bacterial leaching microbes are not in direct contact with minerals but leaching agents are produced by microorganisms which oxidize them.

In the first step, metal disulfide is spontaneously oxidized to thiosulfate by ferric iron (Fe³⁺), which in turn is reduced to give ferrous iron (Fe²⁺):

(1)   FeS₂+6Fe³⁺+3H₂O⟶7Fe²⁺+S₂O₂⁻³+6H⁺ (spontaneous)

Bacteria act in the second step and recover Fe from ferrous iron (Fe) which is then reused in the first step of leaching:

(2) 4Fe²⁺+O²+4H⁺⟶4Fe³⁺+2H₂O (iron oxidizers)

Thiosulfate is also oxidized by bacteria to give sulfate:

(3) S₂O₂⁻³+2O₂+H₂O⟶2SO₂⁻⁴+2H⁺ (sulfur oxidizers)

The ferric iron produced in reaction (2) oxidize more sulfide as in reaction (1), giving the net reaction:

(4)   2FeS₂+7O₂+2H₂O⟶2Fe²⁺+4SO₂⁻⁴+4H⁺+2FeS₂+7O₂+2H₂O⟶2Fe²⁺+4SO₄²⁻+4H⁺

The net products of the reaction are soluble ferrous sulfate and sulfuric acid. The critical reaction is the oxidation of sulfide by ferric iron. The main role of the bacterial step is the regeneration of this reactant. 

Copper leaching has a very similar mechanism. 

Bacteria oxidises the ore and also regenerates the major ore oxidizer (ferric iron). The microbial oxidation process 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 while reducing oxygen to water.

Leaching Process
There are three commercial methods used in leaching:

(i) Slope Leaching. Ores are ground first to get fine pieces. It is dumped in large piles down a mountain side leaching dump. Water containing inoculum of Thiobacillus is continuously sprinkled over the pile. Water is collected at bottom. It is used to extract metals and generate bacteria.

(ii) Heap Leaching. The ore is dumped in large heaps called leach dump. Further steps of treatment are similar to slope leaching- Water containing inoculum of Thiobacillus is continuously sprinkled over the pile. Water collected at the bottom is used to extract metals and generate bacteria

(iii) In situ Leaching. In this process ores remain in its original position in earth. Surface blasting of rock is done just to increase permeability of water. Thereafter, water containing Thiobacillus is pumped through drilled passage to the ores. Acidic water seeps through the rock and collects at bottom. From this, mineral is extracted and water is reused after generation of bacteria.

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