BIOLEACHING:
Bioleaching is a phenomenon in which micro-organisms are used to extract metals from their ores and minerals. In this process, the micro-organisms use the minerals and ores as a substrate and produce energy. It is also known as biomining. Bioleaching is one of the applications of hydrometallurgy and many metals could be extracted like iron, zinc, nickel, copper, cadmium, lead etc. The micro-organisms involved in bioleaching are fungi and bacteria such as thiobacillus. Other includes are:
Mesophiles
Thermophiles
Acidophilic
Psychrophilic
There are almost 11 species of bacteria which are known for bioleaching of sulfur ores, based on contact and non-contact mechanisms for extraction of metals. The micro-organisms perform leaching by either of the three principles i.e. Redoxo-lysis, complex-lysis and acidolysis and produce organic and inorganic acids to mobilize metals, the main of them all is sulfuric acid, in leaching environment.
BIOLEACHING AND ENVIRONMENT:
Environment affects the process and rate of biomining. Based on the environment of bioleaching there are two categories of bioleaching:
Natural bioleaching
Artificial bioleaching
NATURAL BIOLEACHING:
This is a type, naturally occurring in the environment, the main key players are microbes and transfer of electrons from microbes to metals in order to oxidize them, and formation of energy reserves for them as the metabolism is connected with their electron transport chain. The process is slow and it produces a large amount of waste.
MECHANISM:
In the mechanism of bioleaching, there are two ways: direct bioleaching and indirect bioleaching. Each type of mechanism involves ligand induction followed by surface protonation of metal sulfide surfaces. The final product for every mechanism is FeSO4 and H2SO4
DIRECT BIOLEACHING
In this type oxidation of metals is done directly, as the microorganisms give electrons to metals (reduced). For this mechanism the main thing is the attachment of microbes (adsorption) to the surface, otherwise, electron transfer will not occur. Examples of the microbes involved in: are Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Thiobacillus thiooxidans, giving the extraction of copper, iron, nickel, followed by chemotaxis.
This type of leaching is called contact leaching and an enzymatic relation is required b/w microbes and cell membrane.
INDIRECT LEACHING
(Non-contact leaching)
In this type of biomining, there is no need of microbial attachment on the metal surface and the electrons are not gained by the metals directly from microbes, but the reduced metals are oxidized by taking electrons from Fe3+ions (oxidizing agents). These ions are produced by the oxidation of ferrous ions. When the electrons are given to metals, the ferric ions are converts into ferrous ions, which in return are oxidized to ferric ions again by microbes, i.e. why the mechanism is called an indirect mechanism. In this model, surfaces of metal sulfides get degraded due to chemical attack by ferric ions.
(Co-operative bioleaching)
This type of mechanism is independent of direct and indirect leaching, this includes the following steps:
1. Cells get adsorbed on the surface of the metal (physical contact)
2. Exo-polymers are formed and secreted by the cells (extracellular envelopes)
3. These envelopes contain ferric ions in binding with residues of glucouronic acid
4. All the sulfur compounds such as FeS2, MoS2, PbS, CuFeS2, As2S3, As3S4, ZnS, MnS2 produce thiosulphate as an intermediate.
5. Formation of polythiosulphte or sulfur granules in the envelops which are the energy reserves for microbes.
ARTIFICIAL BIOLEACHING:
As we know that natural bioleaching is a very slow process to be used at commercial levels, artificial bioleaching i.e. bio-mining is preferred which is actually microbial biomining with a modified environment. Artificial biomining is a hydrometallurgical operation based on chemical engineering, molecular and microbiology biology. At commercial levels, biomining is attained by optimizing O2, CO2, pH, temperature, humidity etc. The micro-organisms produce more acid while they are growing on the metal surface, the metal is recovered by extraction from liquor, could be reused.
PROCESS:
There are three types of artificial bioleaching.
Slope leaching
Heap leaching
In-situ leaching
SLOPE BIOLEACHING
In this type of bioleaching, the first step is to grind the metal ores to have a fine powder. The reason is that the smaller the size is, the greater will be the surface area for microbial action. After grinding the fine pieces input into the dump from the mountainside, in which inoculum is added with water, while water is also collected from bottom in order to use it to recover the metals. The liquor is reused as it contains biomining bacteria.
HEAP BIOLEACHING
The first step is dumping of metal ores n larger heaps named as leaching heaps, then inoculum of bacteria such as thiobacillius species are showered on the ores, the bacteria grow and the water is collected from bottom to recover metal and liquor is collected in oxidation ponds (reused).
Heap bioleaching includes the use of many heap plants such as stacker, grasshopper, pad drainage system, transportation plant, copper leachates, heap irrigations etc. In heap bioleaching, temperature and aeration play a crucial role in using thermophile bacteria. Heap biomining is significant for the extraction of copper and gold ores.
IN-SITU BIOLEACHING:
As it is clear from the name that the ore is bioleached at its natural place where it is originally present. Using drilled passages, water is irrigated to the ores containing potential microbes, as the rocks are hard, to make the rocks permeable, subsurface blasts are performed. Water (containing thiobacillus sp.) seeps through it and get collected by pumping from the bottom from where it is used to recover or extract the metal. The liquor then can be recycled and reused.
In situ bioleaching has many advantages such as it is cost-effective, inaccessible could also be reached. Similarly, the problems with this method are the porosity of rocks for which blasts are performed.
Many ores are subjected to artificial biomining such as covellite, chalcocite and chalcopyrite. There are many metals are extracted commercially using these methods include gold, silver, silica, uranium etc.
CONCLUSION:
Varying the physiochemical and chemical properties of ores and microbes, the environment can be changed as the same is for metal extraction. Biotechnology offers advancements by providing genetically modified strains which have more ability to extract metals even from low-grade ores. Genome sequencing of biomining microbes has shown that these microbes have a high tolerance to heavy metals, cysteine synthesis and attachment to mineral cells.
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