bioleaching

Extraction of metals from electronic waste by Bacterial leaching

Dr.M.Meyyappan,Professor of Physics,URCW.Karaikudi-630003

Introduction
Microbiological processes (bacterial leaching)  are successfully used in mining  and biohydrometallurigal recovery of metals  from low-percentage ores(copper bioleaching from chalcopyrites),mines with low yield after long use. It is now extended in the process of recovery of metals particularly copper from electronic scarp and gold from jewellery waste. For example printed circuit boards (PCBs) which are part of almost all electronic devices among which copper is the dominant component.The uses of electronics devices in industries, research and academic institutions,  government and private offices, transport and communication systems are increasing day-by-day enormously  which  ultimately results with dumping of electronic waste every where.It is estimated that about 50 million tones of e-waste is produced annually world over.of which only 10 % is recycled and the remaining are dumped as waste, which slowly pollute our environment. Recycling of such e-waste will help for our economical and industrial growth. Recycling of these wastes with economically viable process at all places is now possible with bacterial leaching.Microbiological leaching uses the natural ability of microorganisms to transform metals present in the wastein a solid form to a dissolved form.The bacterial leaching can be formed either in alkaline or acidic medium depending upon the specific nature of the bacteria employed and metals to be extracted from the waste. Among major groups of bacteria, the most commonly used are acidophilus (acidithiobacillus ferrooxidans and acidithiobacillus thiooxidans) and sulfolobus sp. Fungi such as Penicillium sp  and Aspergillus niger are also used in bioleaching technique.The bioleaching process is cheaper and simpler to perform at all places in comparison to conventional techniques.,since microorganisms easily adapt to changing and even extreme living conditions. It requires less energy, less space and well suitable even   for smaller resource of metals to be extracted. As it is eco-friendly in comparison to chemical methods, it is now becoming as green technology.
Bioleaching of Electronic waste
The metal content in the elements of mobile phones is about 28% including 10-20 % of copper1-5 % of lead and 1-3 % nickel with very small amount of some precious metals The traditional methods used for processing electronic scrap are pyro and hydrometallurgical methods Bacterical leaching for the extraction of copper ,zinc, nickel, tin, aluminum from electronic waste has been discovered and used successfully. Bioleaching from electronic waste is a complex process determined  by many factors-types of microporganisms,pH value of the medium,concentration of ferrous ion (Fe²⁺) in the system ,toxicity of ingredients,fineness of the material particles(smaller the size of the particles of the crushed samples of waste greater will be the efficiency) and temperature. Bioleaching is analogous to metal sulfide leaching..Material heterogeneity of waste, in particular of alkaline particulate components of the solid phase may hinder the bioleaching process and slow down its dynamics.High aluminum concentrations and alkaline character of the non-metallic components in the invironment inhibits the growth of bacteria like acidithiobacillus ferrooxidans and acidithiobacillus thiooxidans.The addition of acidifying agent improves the efficiency of the process.In such conditions nickel,aluminum,zinc and copper are converted into soluble components in solution with effectiveness close to 90 %.The concentration Fe³⁺ ions,pH and the number of used microorganisms play a crucial role in the leaching process of metals from solid state to solution.Adding higher dose of A.ferrooxidans,iron ion Fe²⁺ to the system, one can achieve the pH value of the solution for the most convenient environment required for bacterial leaching.For example pH in the range 1.5 to 2.0 adjusted with H₂SO₄  improves the efficiency of copper extraction from the waste.
Suggestions
 After analyzing the bioleaching techniques used for extraction of copper and other metals, we found some bio-physical parameters to improve this method
1.Bacteria can grow under very diverse conditions, which is why they are found nearly everywhere on earth. Although bacteria are good at adapting to their environments, certain conditions promote bacterial growth . These conditions include temperature,moisture,oxygen and pH.
The bacterial growth can be improved by improving the adaptability of the bacteria in any changing environment. This can be performed by changing the temperature of the system periodically within certain convenient range particularly from 25-40^oC(The optimum temperature for Acidithiobacillus is 28 -35^oC)  and the pH value of the solution  may also be changed periodically around 7, the neutral pH within tolerable range, before the process of extraction is performed. However the process of extracting the metals from the waste must be done at the optimum temperature and optimum pH.
2.A mixture of A.ferrooxidans and A.thiooxidans may be used. For their survival one will try to dominate the other by growing fast, utilizing the environment to the maximum extent.
3.Water content in food materials provides an excellent environment for bacteria to grow. The essence of waste vegetables will provide the best environment to the growth of bacteria.
4.Although some bacteria (anaerobe) can survive without oxygen ,in general the presence of oxygen greatly affect the growth of bacteria (aerobes) b.Almost all bacteria require CO₂ for their growth. The optimum abundance of CO₂ varies from bacteria to bacteria.
5.The bacteria used for bioleaching thrive on the sulphuric compounds of some metals.Since in nature copper is usually combined with sulphur, these bacteria like copper ore. By oxidizing copper sulphides insoluble in water, the microbes turn them into readily soluble compounds and the process is very fast.To activate this process without any lagging  vegetable waste of onion and garlic may be added in the system, since both onion and garlic are rich with sulphur.A small amount of copper sulphate may be added to the system before leaching which will improve the efficiency of the process considerably.
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