BIOLEACHING

BIOLEACHING

• Bioleaching or Metal Bioleaching or Biomining is a process in Mining and Biohydrometallurgy (natural processes of interactions between microbes and minerals) that extracts valuable metals from a low-grade ore with the help of microorganisms such as Bacteria or Archaea.
• Bioleaching is an alternative to more traditional physical and chemical methods of mineral processing.
• The application of Biomining processes predates by centuries the understanding of the role of microorganisms in Metal extraction. However, the modern era of biomining began with the discovery of the bacterium Thiobacillus ferrooxidans.
• Bioleaching techniques are often more effective than traditional mining applications and can even be used to clean Mine Tailings Sites.

METALS EXTRACTED FROM BIOLEACHING

• Bioleaching is one of several applications within Biohydrometallurgy and several methods are used to recover

a) Antimony

b) Arsenic

c) Cobalt

d) Copper

e) Gold

f) Lead

g) Molybdenum

h) Nickel

i) Silver

j)  Uranium

k) Zinc

MICROORGANISMS USED IN BIOLEACHING

a) Thiobacillus sp.

• The most commonly used microorganisms in Bioleaching are Thiobacillus thiooxidans and Thiobacillus ferrooxidans.
• Two new species of acidophilic Thiobacilli, Thiobacillus prosperus (Halotolerant metal-mobilizing bacteria) and Thiobacillus cuprinus (Facultatively chemolithoautotrophic bacterium) which oxidizes metal sulfides but does not oxidize ferrous iron.
• Thiobacillus thiooxidans and Thiobacillus ferrooxidans are Mesophilic bacteria which grow best at temperatures of 25 – 35 °C.

b) Leptospirillum sp.

• Leptospirillum ferrooxidans is another acidophilic Obligately Chemolithotrophic, Ferrous iron oxidizing bacterium.
• Leptospirillum ferrooxidans tolerates lower pH values and higher concentrations of Uranium, Molybdenum and Silver than Thiobacillus ferrooxidans, but it is more sensitive to Copper and unable to oxidize Sulfur or sulfur compounds. Therefore, by itself, Leptospirillum ferrooxidans cannot attack mineral sulfides. This can only be done together with Thiobacillus ferrooxidans or Thiobacillus thiooxidans.
• Leptospirillum ferrooxidans are Mesophilic bacteria which grow best at temperatures of 25 – 35 °C.

c) Thermophilic bacteria

• Acidianus brierleyi, formerly associated with the genus Sulfolobus is a Chemolithoautotrophic, Facultatively aerobic, Extremely acidophilic Archaeon growing on ferrous iron, elemental sulfur and metal sulfides. Under anaerobic conditions elemental sulfur is used as an electron acceptor and is reduced to H₂S.
• Members of the genus Sulfolobus are Aerobic, Chemolithotrophic bacteria oxidizing ferrous iron, elemental sulfur and sulfide minerals. The same compounds are used as energy source by Sulfobacillus thermosulfidooxidans, a spore-forming facultatively autotrophic bacterium.

d) Heterotrophic microorganisms

• Heterotrophic bacteria and fungi which require organic supplements for growth and energy supply may contribute to Metal leaching.
• The Heterotrophic microorganisms do not have any benefit from the metal leaching. Among the bacteria, members of the genus Bacillus are most effective in metal solubilization, with regard to the fungi, Saccharomyces cerevisiae, Aspergillus and Penicillium simplicissium are the most important ones.

FEATURES OF MICROORGANISMS USED IN BIOLEACHING

• Single celled organisms.
• Chemosynthetic metabolism.
• Should derive Carbon-di-oxide and Oxygen from Atmosphere.
• Requires Acidic pH.

MECHANISMS INVOLVED IN BIOLEACHING

• Thiobacillus thiooxidans and Thiobacillus ferrooxidans have always been found to be present on the leaching dump.
• The species of Thiobacillus is most extensively studied Gram negative bacteria which derives energy from oxidation of Fe²⁺.
• Two mechanisms are used for Bioleaching. They are

a) Direct Bioleaching (Contact Bioleaching)

b) Indirect Bioleaching

Mechanisms of Bioleaching

a) Direct Bioleaching or Contact Bioleaching

• In Direct Bioleaching, a physical contact exists between Microorganisms and Ores, and Oxidation of mineral takes place through enzymatically catalyzed steps.
• Example - Pyrite is oxidized to Ferric sulphate.

b) Indirect Bioleaching

• In Indirect Bioleaching, microorganisms produce strong Oxidizing agent (Ferric ion and Sulfuric acid) which reacts with metals and extract them from the ores.
• For Indirect bioleaching, acidic environment is absolutely essential in order to keep Ferric iron and other metals in solution. Acidic environment maintained by oxidation of iron, sulfur, metal sulphides or by dissolution of carbonate ions.
• Example – Bioleaching of Uranium.

COMMERCIAL BIOLEACHING PROCESS

• Naturally occurring bioleaching process is very slow.
• For commercial extraction of metal by Bioleaching, the process is optimized by controlling the pH, Temperature, Humidity, Oxygen and Carbon-di-oxide concentrations.
• There are four commercial process used in Bioleaching. They are

a)     Slope Leaching or Dump Leaching

b)    Heap Leaching

c)     In situ Leaching

d)    Vat Leaching

a) Slope Leaching or Dump Leaching

• In Slope Leaching or Dump Leaching process, the ores are first ground to get fine pieces and then dumped into large Leaching dump.
• Water containing inoculum of Thiobacillus is continuously sprinkled over the ore.
• Water is collected from the bottom and used to extract metals and generate bacteria in an Oxidation pond.

b) Heap Leaching

• In the Heap leaching, the ore is dumped into large Heaps called Leach heaps.
• Water containing inoculum of Thiobacillus is continuously sprinkled over the ore.
• Water is collected from the bottom and used to extract metals and generate bacteria in an Oxidation pond.

c) In situ Leaching

• In In situ Leaching process, the ore remains in its original position in the earth.
• Surface blasting of earth is done to increase the permeability of water.
• Water containing Thiobacillus is pumped through drilled passage to the ores.
• Acidic water seeps through the rock and collects at the bottom. Again, water is pumped from bottom.
• Mineral is extracted and water is reused after generation of bacteria.

d) Vat Leaching

• Vat leach units are Rectangular containers (drums, barrels, tanks or vats), usually very big and made of wood or concrete, lined with material resistant to the leaching media equipped with agitators.
• The treated ore is usually coarse. The leaching reagents may be added to the leaching object in different ways.

Dump Leaching and In situ Leaching

Heap Leaching

FACTORS AFFECTING BIOLEACHING

1) Physico-chemical factors

• Temperature - Affects leaching rate, microbial composition and activity.
• pH - Needs to be low (Acidic) to obtain the fastest leaching rates and to keep ferric iron and metals in solution.
• Ferric oxygen reaction - Electron acceptor needed in Chemical and Biological oxidation.

2) Microbiological factors

• Microbial diversity culture - Mixed cultures tend to be more robust and efficient than Pure culture.
• Population density - High population density tends to increase the leaching rate.
• Metal Tolerance - High metal concentrations may be toxic to microbes.
• Choice of Bacteria - Suitable bacteria that can survive at High temperatures, Acid concentrations, High concentrations of heavy metals, remaining active under such circumstances.

3) Mineral factors

• Composition - Provides electron donor and trace elements.
• Particle size - Affects the available mineral/liquid contact area.
• Surface area - Rate of oxidation by the bacteria increases with reduction in size of the ore and vice versa.
• Porosity - Cracks and pores in the particles give rise to the internal area.
• Presence of other Metal sulfide - Mineral having the lowest potential is generally oxidized first.

Advantages of Bioleaching

• Simple process.
• Eco-friendly process.
• Inexpensive technique.
• Employed for collecting metals from waste and drainages.
• Use to extract refines and expensive metals which is not possible by other chemical processes.
• No poisonous sulfur dioxide emission as in smelter.
• No need of high pressure and temperature.
• Ideal for low grade Sulphide ores.

Disadvantages of Bioleaching

• Time consuming (takes 6 to 24 months or longer).
• Have a very low yield of minerals.
• Requires a large open area for treatment.
• May have no process control.
• High risk of contamination.
• Inconsistent yield because bacteria cannot grow uniformly.