Introduction
Microbial recovery of minerals, also known as bioleaching or biomining, is a process that leverages the metabolic activities of microorganisms to extract valuable metals from ores. This method is particularly useful for low-grade ores that are not economically viable for traditional mining methods. The process involves the use of bacteria and archaea, which can oxidize sulfide minerals and solubilize metals. Here's a detailed overview of the process, with examples:
Mechanisms of Bioleaching
1.Direct Bioleaching:
- Involves direct contact between microbes and the mineral.
- Microbes attach to the mineral surface and enzymatically convert the metal sulfides into soluble metal sulfates.
2. Indirect Bioleaching:
- Microbes produce biochemicals (like ferric iron and sulfuric acid) that react with the mineral to solubilize metals.
- The process occurs in two steps: the microbial oxidation of iron and sulfur, followed by chemical oxidation of the mineral by the produced ferric iron or acid.
Types of Microorganisms Involved in Bioleaching
1. Acidophilic Chemolithotrophs:
-Acidithiobacillus ferrooxidans: Oxidizes ferrous iron and sulfur, common in the leaching of copper, uranium, and gold.
- Leptospirillum ferrooxidans: Mainly oxidizes ferrous iron.
2. Sulfur-Oxidizing Bacteria:
-Thiobacillus thiooxidans: Converts elemental sulfur to sulfuric acid, aiding in mineral dissolution.
2FeS2 + 7 O2 + 2H2O ———> 2 FeSO4 + 2H2SO4
3. Fungi:
- Some fungi produce organic acids that can leach metals from minerals.
Steps in the Bioleaching Process
1. Ore Preparation:
- Crushing and grinding of the ore to increase surface area.
- Sometimes the ore is agglomerated to improve the permeability of leaching solutions.
2. Inoculation:
- Introduction of microbial cultures into the ore.
- Can be naturally occurring microbes or specific strains introduced to enhance the process.
3. Leaching:
- Heap Leaching: Stacked ore heaps are irrigated with an acidic leaching solution containing the microbes.
- Finely ground ores (up to 100,00 tons) are dumped in large piles down a mountainside and continuously sprinkled with water containing Thiobacillus.
- The water is collected at the bottom and reused after metal extraction and possible regeneration of the bacteria in an oxidation pool.
- In-situ Leaching: Solution is pumped through an ore body without removing the ore
- Water containing Thiobacillus is pumped through drilled passages to unextracted ore which remains in its original location in the earth.
- In most cases, the permeability of the rock must be first increased by subsurface blasting of the rock.
- The acidic water seeps through the rock and collects in the bottommost cavity from which it is pumped, the minerals extracted, and the water re-used after regeneration of bacteria
- Stirred Tank Leaching: Ore is mixed with the microbial solution in large tanks.
4. Recovery:
- The leachate containing dissolved metals is collected and processed to recover the metals, often through precipitation, solvent extraction, or electrowinning.
Examples of Bioleaching
1. Copper Bioleaching:
- One of the most common applications.
- Example: Escondida Mine in Chile uses bioleaching for copper extraction.
- Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans are key microbes involved.
- If chalcocite, chalcopyrite, or covellite are used for the production of copper, several metals are usually found together.
- For example, chalcopyrite contains 26% copper, 25.9% iron, 2.5% zinc, and 33% sulphur.
- Chalcopyrite is oxidized as follows: 2 CuFeS2 + 8 ½ O2 + H2SO4 ———> 2 CuSO4 + Fe2 (SO4)3 + H2O
2. Gold Bioleaching:
- Often used for refractory gold ores that are not amenable to conventional cyanidation.
- Example: Fairview Mine in South Africa.
- Bacteria like Acidithiobacillus ferrooxidans help to break down sulfide matrices, liberating the gold.
3. Uranium Bioleaching:
- Example: Denison Mine in Canada.
- Bacteria oxidize the uranium from the ore, making it soluble for extraction.
4. Nickel Bioleaching:
- Example: The Moa Bay in Cuba and Soroako in Indonesia.
- Uses Acidithiobacillus ferrooxidans and other bacteria to extract nickel from laterite ores.
Advantages of Bioleaching
1.Environmental Benefits:
- Lower emissions of sulfur dioxide compared to smelting.
- Reduced need for high temperatures and pressures, saving energy.
- Less environmental impact as it can be conducted in situ without large-scale excavation.
2.Economic Benefits:
- Cost-effective for low-grade ores and waste materials.
- Reduced operational costs due to lower energy and equipment needs.
- Possibility to recover metals from tailings and waste rock, adding value to mining operations.
Challenges and Future Directions of Bioleaching
1.Slow Process:
Bioleaching is generally slower than traditional methods, which can limit its applicability for some projects.
2.Control of Microbial Activity:
Ensuring optimal conditions for microbial growth and activity can be complex and site-specific.
3.Research and Development:
Ongoing research is aimed at improving the efficiency and speed of bioleaching processes, as well as expanding the range of metals that can be recovered.
Conclusion
In summary, microbial recovery of minerals or Bioleaching is a sustainable and increasingly important method for extracting valuable metals from ores, particularly those that are difficult or uneconomical to process using traditional methods. The continued advancement in microbial and biotechnological research is expected to enhance the efficacy and applicability of bioleaching in the mining industry.
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