Bioleaching:

Microbial leaching is the process by which metals are dissolved from the ore bearing rocks using microorganisms. It is also called as biomining or microbial leaching.

Microbial technology renders helps in case of recovery of ores which cannot be economically processed with chemical methods because they contain low-grade metals. The low-grade ores contain a significant amount of Ni, Pb and Zinc ores which can be processed by microbial leaching. Bioleaching of Cu and Uranium are most commonly used.

1. Microorganism used for leaching:

Thiobacillus thiooxidans, Thiobacillus ferroxidans and Thiobacillus thermophilica
Bacillus licheniformis, B. luteus, B. megaterium, B. polymyxa, Leptospirillum ferrooxidans, Pseudomonas fluorescens, Sulfolobus acidocaldarius, Thermothrix thioparus 
a. Chemistry of microbial leaching

T. thiooxidans and T. ferroxidans have always been found to be present in a mixture of leaching on dumps. Thiobacillus is the most extensively studied Gram-negative bacillus bacterium which derives energy from the oxidation of Fe2+ or insoluble sulfur. In bioleaching there are two following reaction mechanisms:-
1. Direct bacterial leaching (Direct the action of bacteria on the ore to extract metal)
Here a physical contact exists between bacteria and ores and oxidation of minerals takes place through several enzymatically catalyzed steps. For instance, certain bacteria (e.g., T. ferrooxidans) can transfer electrons (coupled with ATP production) from iron or sulfur to oxygen. That is these organisms can obtain energy from the oxidation of Fe2+ to Fe3+or from the oxidation of sulfur and reduced sulfur compounds to sulfate as illustrated below. For example, pyrite is oxidized to ferric sulfate as below

                                    T.ferroxidans
→⟶⟶⟶⟶⟶⟶⟶⟶⟶FeS2+7O2+2H2O                                               2FeSO4+2H2SO4
                                                                                            

2. Indirect bacterial leaching:
In this indirect method, the bacteria produce strong oxidizing agents such as ferric iron and sulfuric acid on oxidation of soluble iron or soluble sulfur respectively. Ferric iron or sulfuric acid, being powerful oxidizing agents react with metals and extract them. For indirect bioleaching, an acidic environment is absolutely essential in order to keep ferric iron and other metal. 

3. Commercial Process of Bioleaching:
The naturally occurring mineral leaching is very slow. The microbial bioleaching process can be optimized by creating ideal conditions— temperature, pH, and nutrient, O2, and CO2 supply, etc. A diagrammatic representation of the general bioleaching process is depicted in Fig. 32.1. The desired microorganisms with nutrients, acid, etc., are pumped into the ore bed. The microorganisms grow and produce more acid. The extracted leach liquor is processed for metal recovery. The leach liquor can be recycled again and again for further metal extraction.


4. Types of bioleaching process:

a. Slope leaching:
The ore is finely ground and dumped in large piles down a mountainside (Fig. 32.2A). This ore is then subjected to a continuous sprinkling of water containing the desired microorganism (T. ferrooxidans). The water collected at the bottom is used for metal extraction. The water can be recycled for the regeneration of bacteria.

b. Heap leaching:
In this case, the ore is arranged in large heaps (Fig. 32.2B) and subjected to treatments as in slope leaching.
c. In situ leaching:
The ore, in its original natural place, is subjected to leaching (Fig. 32.2C). Water containing the microorganisms is pumped through drilled passages. In most cases, the permeability of the rock is increased by subsurface blasting of the rock. As the acidic water seeps through the rock, it collects at the bottom which is used for metal extraction. This water can be recycled and reused.
5. Advantages of Bioleaching:
When compared to conventional mining techniques, bioleaching offers several advantages. Some of them are listed below.
1. Bioleaching can recover metals from low-grade ores in a cost-effective manner.
2. It can be successfully employed for concentrating metals from wastes or dilute mixtures.
3. Bioleaching is environmentally friendly since it does not cause any pollution (which is the case with conventional mining techniques).
4. It can be used to produce refined and expensive metals which otherwise may not be possible.
5. Bioleaching is a simple process with low cost technology.
6. It is ideally suited for the developing countries.
The major limitation or disadvantage of bioleaching is the slowness of the biological process. This problem can, however, be solved by undertaking an in depth research to make the process faster, besides increasing the efficiency. 

6. Selected examples of microbial bioleaching are briefly described below:
a. Bioleaching in desulfurization of coal:
The process of removal of sulfur containing pyrite (FeS2) from high sulfur coal by microorganisms is referred to as bio desulfurization. High sulfur coal, when used in thermal power stations, emits sulfur dioxide (SO2) that causes environmental pollution. By using the microorganisms Thiobacillus ferrooxidans and T. thiooxidans, the pyrite which contains most of the sulfur (80-90%) can be removed. Thus, by employing bioleaching, high sulfur coal can be fruitfully utilized in an environment friendly manner. In addition, this approach is quite economical also.