Microbial leaching

Microbial ore leaching (microbial mining/bioleaching/biohydrometallurgy) is the process of extracting metals from ores using microorganisms. Metals are extracted from sulfide/sulfur containing low grade ores are using the activity of sulfur oxidising bacteria such as Thiobacillus ferroxidans, Sulfolobus etc. This process is commercially used for low grade copper and uranium ores. It can also be used for recovering nickel zinc, cobalt, tin, cadmium, molybdenum, lead, antimony, arsenic, selenium etc from low grade sulfide ores.

Metals exist in the insoluble sulphide form in these ores. Bacteria oxidises these metal sulphides directly or indirectly (through ferric o\iron), and converts the sulphide form of metal into soluble sulphate form, which is then leachable. Metals are finally extracted out from the leachate by different processes like, solvent partitioning, reaction with scrap iron,  electro winning etc

As high grade ores which contain higher metal content are getting depleted, ores with low metal content are tried for extracting metals. But low grade ores are not suitable for direct smelting (extracting metal from its ore by a process involving heating and melting), unlike high grade ores. Hence microorganisms which can act on low grade ores and extract the metal are employed in bioleaching/bio mining.

Microorganisms are used because they can:

·         lower the production costs.

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

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

 

Miroorganisms used for Leaching

Bioleaching reactions industrially are performed by many bacterial species that can oxidize ferrous iron and sulfur. Sulfur oxidising bacteria such as Thiobacillus ferroxidans, Sulfolobus etc, some fungi (Aspergillus niger and Penicillium simplicissimum) have also been shown to have the ability to dissolute heavy metals.

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.

T. thiooxidans and T. ferrooxidans have always been found to be present in mixture on leaching dumps. Metal sulphides are insoluble in water or acid solutions unless they are first oxidized. Thiobacillus ferrooxidans catalyzes the oxidation of iron whereas and Thiobacillus thiooxidans  catalyzes the oxidation of sulfur.

Thiobacillus is a Gram-negative bacillus which derives energy from oxidation of Fe²⁺ or insoluble sulphur (sulphides). Thiobacillus ferrooxidans or Thiobacillus thiooxidans are chemoautotrophic acidophiles and obtain energy from inorganic sources while growing in acidic medium.

For biomining, they should have some properties like:

• Need to survive the sulfuric acid produced in the process

• Resist the high metal concentrations

• Oxidize ferrous iron and sulfur compounds

• Need to form a biofilm on the mineral surface

 

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 or water with dilute sulphuric acid containing inoculum of Thiobacillus is continuously sprinkled over the pile. Water is collected at bottom. It is used to extract metals.

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

(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.

 

a)                                                      a) Slope  leaching    b) Heap leaching    c) In-situ leaching

In general, bioleaching is cleaner and safer for the environment than chemical processing. However environmental pollution with toxic products, like sulfuric acid from the pyrite leaching, and heavy metals is still possible. Another drawback of microbial leaching is the slow rate at which microbes work

 In bioleaching there are two approaches:

 1) Direct Bacterial Leaching- Direct oxidation of metal sulphide

2)   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	à	2FeSO4 + 2H2SO4

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.  Bacteria convert ferrous iron (Fe) to ferric form, which is then used in  leaching. Here, ferric iron (Fe³⁺) oxidises metal disulfide to thiosulfate  and itself gets reduced to give ferrous iron (Fe²⁺). The cycle continues.

    Thus bacteria oxidises ferrous iron to ferric iron, which in turn oxidises metal sulphide to sulphate, thus making the metal soluble and leachable.

 Bioleaching of Copper

Biological copper leaching is practiced in many countries including Australia, Canada, Chile, Mexico, Peru, Russia, United States of America etc. Copper recovery from bioleaching accounts for about 20% of the world copper production.

Thiobacillus ferrooxidans was isolated from coalmine water and studies revealed its presence in copper-leaching operations. 

Thiobacillus bioleaching is significant in commercially important recovery of copper from iron and sulphur containing ores such as chalcopyrite- CuFeS₂, chalcocite- Cu₂S and Covellite-CuS.

The majority of copper minerals in the ore are sulfides, with chalcopyrite (CuFeS₂) being the most abundant and thus economically the most important.

There are two mechanisms involved in bioleaching. The first is the direct microbial action on the sulfide mineral in the ore, and enhanced rate of oxidation of the mineral directly. This is known as the direct mechanism. Indirect process involves the microbial oxidation of ferrous to ferric ions followed by the chemical oxidation of the sulfide mineral by the ferric ion. This is known as the indirect mechanism.

 

1)      Direct oxidation of sulfide ores such as CuS (covellite)

CuS (covellite) + 20₂ è CuS0₄  

2)      Indirect oxidation via ferric-ferrous cycle

Microorganisms catalyze the oxidation of iron sulfides to create ferric sulfate. Ferric sulfate, which is a powerful oxidizing agent, then oxidizes the copper sulfide minerals to liberate copper sulphate and elemental sulphur.

CuFeS₂ (chalcopyrite) + 2Fe₂(S0₄)₃ (Ferric sulphate) è CuS0₄ + 5FeS0₄ (Ferrous sulphate) + 2S

Elemental sulphur generated by indirect leaching can be converted to sulphuric acid by Thiobacillus ferrooxidans:

2S + 30₂ + 2H₂Oè2H₂S0₄

Copper is then leached by the sulfuric acid formed. The sulphuric acid maintains the pH at levels favourable to the growth of bacteria. 

It can also be of environmental concern as acid mine drainage.

Direct oxidation can also occur for Chalcopyrite, as shown with Covellite.

Bioleaching operations worldwide for copper follow the same pattern.

Ø copper ore mined from open pits is segregated

Ø higher-grade material is concentrated for smelting, while the lower-grade ore is subjected to leaching

Ø The ore is piled on an impermeable surface as a dump 

Ø The top is levelled

Ø Leach solution (water or water with dilute sulphuric acid) is flooded or sprayed onto the dump

Ø Leach solutions enriched with copper exit at the base of the dump and are recovered

Ø Copper recovery from leachate/leach solutions is either by solvent partitioning or reaction with scrap iron.

 

(Electrowinning- electroextraction, is the electrodeposition of metals from their ores)

     Practically, 50-70% copper can be recovered from a low grade ore by bio mining. Costs are about ½ or 1/3^rd of direct smelting. However, it is a slow process and may need many years for reasonable recovery of the metal.

 

 Bioleaching of Uranium

Uranium leaching or Microbial recovery of uranium from low grade ores proceeds is an indirect process. Thiobacillus ferrooxidans is involved in uranium extraction and it does not directly attack on ore but on the iron oxidants.

Low grade uranium ores are of two types- carbonate rich and pyrite containing. Insoluble tetravalent uranium oxide (UO₂) is present in low grade ores.

Pyrite containing uranium deposits are leached by the action of Thiobacillus ferrooxidans. Ferric sulphate and sulphuric acid is obtained by the action of Thiobacillus ferrooxidans from the pyrite within uranium ore.

2FeS₂+H₂O+7 ½[O₂] è Fe₂[SO₄]₃+ H₂SO₄

In the subsequent leaching process, H₂SO₄/FeSO₄ solution oxidizes the insoluble tetravalent uranium in the ore to soluble hexavalent uranium sulphate

UO₂+Fe₂(SO₄)₃ è UO₂SO₄+2FeSO₄

pH required for the reaction is 1.5-3.5.  Temperature: around 35⁰ C

The dissolved Uranium is extracted from the leachate/leach liquor using organic solvents such as tributyl phosphate/ion exchange resins and subsequently precipitated out.

Thiobacillus ferrooxidans does not directly interact with uranium minerals. The role of Thiobacillus ferrooxidans in uranium leaching is the best example of the indirect mechanism. Bacterial activity is limited to oxidation of pyrite and ferrous iron.

Bioliberation of Gold

Gold is recovered from its ores by sodium cyanide leaching process which converts gold to a soluble cyanide complex.

4Au+8NaCN+O₂+2H₂O è 4NaAu(CN)₂+4NaOH

Low grade sulfidic gold ores cannot be subjected to sodium cyanide extraction. Sulphidic gold ores are mainly two types: pyrite and arsenopyrite.

Pretreatments like roasting or pressure oxidation is required to free gold from enclosing sulphides prior to cyanide leaching. Such pretreatments can be costly and are substituted by the action of iron and sulphur oxidising acidophilic bacteria. These bacteria can oxidise sulphidic ores, this breaks the sulphide matrix and results in improved accessibility of gold for leaching with cyanide. Thus, bio-oxidation using Thiobacillus ferrooxidans is done in low grade sulphide ores followed by cyanide extraction which improves gold recovery from such low grade ores. This is a less costly, less polluting alternative to other oxidative pretreatments such as roasting and pressure oxidation.

Recently, bio-oxidation of gold ores has been implemented as a commercial process, and is under study worldwide for further application to refractory gold ores. Heap leaching technology is used for commercial processes involving gold ores.

 Bacterial leaching is a revolutionary technique used to extract various metals from their ores. Traditional methods of extraction such as roasting and smelting are very energy intensive and require high concentration of elements in ores. Bacterial leaching is possible with low concentrations and requires little energy inputs. The process is environment friendly even while giving extraction yields of over 90%.

 Bioleaching offers several advantages, such as:

1.      The ability to economically process low grade sulfide ores

2.      The ability to process ores that may not be feasible to be smelted for environmental reasons

     However, Bioleaching and mine wastes cause pollution – AMD or acid mine drainage, which is the acid rich effluent produced as a result which can result in pollution of environment.