Biomining is defined as extracting mineral ores or enhancing the mineral recovery from mines using microorganisms instead of traditional mining methods. Copper was the first metal extracted using microorganisms in the ancient past in the Mediterranean region. Biomining is becoming popular because it is cheap, reliable, efficient, safe, and environmentally friendly, unlike traditional mining methods. The efficiency of biomining can be increased either by finding suitable strains of microorganisms or by genetically modifying existing microorganisms, made possible due to rapid advances in the field of biotechnology and microbiology. Biomining is an application of biotechnology and is also known as microbial leaching or alternately, bio-oxidation.
MICROORGANISMS IN BIOMINING
There are different types of bacteria present in nature that oxidize metal sulfides and solubilize minerals, thus, helping in their extraction from the ores. It is very important to select suitable microorganisms to ensure the success of biomining, a process which requires knowledge of properties of microorganisms, both physiological and biochemical. Bacteria are found to be the most suitable microorganisms that can be used in biomining.
Characteristics of the bacteria used in biomining.
1. Mineral extraction involves the production of high temperatures so the bacteria should be able to survive the heat, hence, they should be thermophilic.
2. Biomining involves using strong acids and alkalis, hence, bacteria should be chemophilic.
3. Bacteria should produce energy from inorganic compounds, hence, should also be autotrophic.
4. The bacteria should be able to adhere to the solid surfaces or have the ability to form biofilms.
Identification of Bacteria Useful for Biomining Operations
There are wide varieties of bacteria with varying capabilities existing on earth; therefore, it is essential to identify precisely the types that can perform biooxidation/bioleaching effectively. Thiobacillus ferrooxidans is a chemophilic, moderately thermophilic bacteria which can produce energy from oxidation of inorganic compounds like sulfur and iron. It is the most commonly used bacteria in biomining. Several other bacteria such as T.thioxidans, Thermothrix thiopara, Sulfolobus acidocaldarius and S. brierleyi are also widely used to extract various minerals. Thermothrix thiopara is an extremely thermophilic bacteria that can survive very high temperatures between 60-75C and is used in extraction of sulfur.
Techniques like genetic engineering and conjugation are used to produce bacteria with desired characteristics to increase the rate of biooxidation thus increasing the mineral yield through biomining. It is also important to identify biomining bacteria present in colonies of other bacteria. Techniques developed for this purpose include: immune fluorescence, dot immunoassay, and dot-blot hybridization. Immuno fluorescence, this technique is generally used to identify specific antibodies or antigens present in biological fluids. Fluorescent antibodies are used to identify biomining bacteria. Dot Immunoassay This technique is used to identify ore-adhering bacteria like T.ferrooxidans and T.thiooxidans. The bacteria are applied in the form of dots on a nitrocellulose film. Antigen-antibody reaction is carried out on the film and then treated with a secondary antibody to make the reaction visible by producing a color. The sample can be approximated by comparison of the test sample with that of a known sample. Dot-blot Hybridization, This is a DNA based technique to identify biomining bacteria such as T.ferrooxidans. The bacteria are isolated from samples of ores and soil treated with sodium dodecyl sulfate (SDS). The cells are disrupted to extract DNA and the extracted DNA is then purified. The DNA obtained from ore sample is fixed on nitrocellulose membrane using southern blotting technique. Genetic probes are used to identify and distinguish various biomining bacteria used in this procedure. The DNA fragments on the membrane are treated with standard probes.
Minerals are recovered from ores by the microorganisms mainly by two mechanisms: oxidation and reduction.
The microorganisms like T.ferroxidans and T.thioxidans are used to release iron and sulfur respectively. T.ferroxidans oxidize ferrous ion to ferric ion.
4Fe++ + O2 + 4H+ --> Fe+++ + 2H2O
The bacteria attach to the surface of the ore and oxidize by a direct and indirect method.
In this method the ore is oxidized by the microorganisms due to the direct contact with the compound. 2FeS2 + 7O2 + 2H2O --> 2FeSO4 + 2H2SO4
In this method the mineral is indirectly oxidized by an agent that is produced by direct oxidation. For example, the ferric ion produced by the above reaction is a powerful oxidizing agent and can release sulfur from the metal sulphides. Thus production of ferric ion indirectly causes oxidation of metal sulfide resulting in the breaking of the crystal lattice of the heavy metal sulfide and separating the heavy metal and sulfur.
CuS + Fe+++ --> Cu+ + S + Fe++
Bacteria like Desulfovibro desulfuricans play an active role in reduction of sulfates which results in the formation of hydrogen sulphides.
4H2 + H2SO4 --> H2S + 4H2O
TYPES OF BIOMINING
Stirred Tank Biomining
This method is used for leaching from substrates with high mineral concentration. Since the method is expensive and time consuming, substrates with lower concentration are not used for leaching. Copper and refractory gold ores are well suited for this type of method. Special types of stirred tank bioreactors lined with rubber or corrosion resistant steel and insulated with cooling pipes or cooling jackets are used for this purpose. Thiobacillus is the commonly used bacteria. Since it is aerobic the bioreactor is provided with an abundant supply of oxygen throughout the process provided by aerators, pumps and blowers. This is a multi-step process consisting of large numbers of bioreactors connected to each other. The substrate moves from one reactor to another and in the final stage it is washed with water and treated with a variety of chemicals to recover the mineral.
Bioheaps are large amounts of low grade ore and effluents from extraction processes that contain trace amounts of minerals. Such effluents are usually stacked in large open space heaps and treated with microorganisms to extract the minerals. Bioheaps are also called biopiles, biomounds and biocells. They are also used for biodegradation of petroleum and chemical wastes. The low grade ores like refractory sulfide gold ore and chalocite ore (copper) are crushed first to reduce the size then treated with acid to promote growth and multiplication of chemophilic bacteria. The crushed and acid-treated ore is then agglomerated so that the finer particles get attached to the coarser ones, and then treated with water or other effluent liquid. This is done to optimize moisture content in the ore bacteria that is inoculated along with the liquid. The ore is then stacked in large heaps of 2-10 feet high with aerating tubes to provide air supply to the bacteria thus promoting biooxidation.
Advantages of using bioheaps are that they are:
· Cost effective
· of simple design and easy to implement
· and very effective in extracting from low concentration ores
Disadvantages of using bioheaps are that they:
· Are time consuming (takes about 6-24 months),
· have a very low yield of mineral,
· require a large open area for treatment,
· Have no process control,
· are at high risk of contamination,
· Have inconsistent yields because bacteria may not grow uniformly in the heap.
In this method the mineral is extracted directly from the mine instead of collecting the ore and transferring to an extracting facility away from the site of the mine. In-situ biomining is usually done to extract trace amounts of minerals present in the ores after a conventional extraction process is completed. The mine is blasted to reduce the ore size and to increase permeability and is then treated with water and acid solution with bacterial inoculum. Air supply is provided using pipes or shafts. Biooxidation takes place in-situ due to growing bacteria and results in the extraction of mineral from the ore.
Factors Effecting Biomining
Biomining success depends on various factors some of which are discussed below.
Choice of Bacteria
This is the most important factor that determines the success of bioleaching. Suitable bacteria that can survive at high temperatures, acid concentrations, high concentrations of heavy metals, remaining active under such circumstances, are to be selected to ensure successful bioleaching.
Crystal Lattice Energy
This determines the mechanical stability and degree of solubility of the sulfides. The sulfide ores with lower crystal lattice energy have higher solubility, hence, are easily extracted into solution by the action of bacteria.
Rate of oxidation by the bacteria depends on the particle size of the ore. The rate increases with reduction in size of the ore and vice-versa.
Composition of ore such as concentration of sulfides, amount of mineral present, and the extent of contamination has direct effect on the rate of bio-oxidation.
Biooxidation requires a pH of 2.5-3 for maximum results. The rate of biooxidation decreases significantly if the pH is not in this range since the activity of acidophilic bacteria is reduced.
The bacteria used in biomining are either mesophilic or thermophilic. Optimum temperature is required for biooxidation to proceed at a fast rate. Optimum temperature range for a given bacteria is between 25-35° C depending on the type of ore being selected. The rate of biooxidation is reduced significantly if the temperature is above or below the optimum temperature.
The bacteria used in biomining are aerobic thus require an abundant supply of oxygen for survival and growth. Oxygen can be provided by aerators and pipes. Mechanical agitation is also an effective method to provide continuous air supply uniformly and also to mix the contents.
The ratio of ore/sulfide to the leach solution (water + acid solution + bacteria inoculum) should be maintained at optimum level to ensure that biooxidation proceeds at maximum speed. The leach solution containing leached minerals should be removed periodically and replaced with new solution.
Adding small amounts of surfactants like Tween 20 to the leaching process increases the rate of biooxidation of minerals from sulfide ores. The surfactants decrease the surface tension of the leach solution, thus, wetting the ore and resulting in increased bacterial contact which ultimately increases the rate of biooxidation.
BIOMINING OF COPPER
Copper was the first metal extracted by bioleaching. It is the metal most commonly extracted from oxide ores by this method. In the United States, alone, about 11% of copper is produced from low grade ores by bioleaching technique every year. Copper is available in mines across the world in more than 350 types of ores, but it is mainly present along with sulfur. Copper from low-grade ores like copper sulfide minerals is most commonly extracted by biooxidation since it is not economically viable to use conventional metallurgical techniques.
Low grade copper ore is brought to the dump leaching site. The dump surface is wetted uniformly with water and sulfuric acid using sprayers to maintain acidity which helps the growth of acidophilic bacteria and bacterial inoculum. Air is supplied to the dump through channels constructed for this purpose while building the dump. Biooxidation takes place over the course of time and copper is leached into the solution which is collected at the bottom of the heap. The leach solution rich in copper is treated chemically using electrowinning and solvent extraction techniques to extract pure copper.
In this technique the leach solution containing copper leached from the dump is circulated through an electrowining cell and electricity is passed. Pure copper is obtained from the cell in the form of electro-won cathode. An Electrowining cell is basically a simple electro-voltaic cell with a lead or graphite anode and aluminum cathode with the leach solution being the electrolyte. When the electricity is passed through the cell, the copper ions present in the electrolyte are reduced to metallic copper and become deposited over the cathode.
Copper solvent extraction systems consist of three loops. In the first loop the leach solution containing copper obtained from dump leaching is passed through the extraction chamber. Here the leach solution comes in contact with organic extractant which extracts copper from it. Leach solution and organic extractant are passed through the leaching chamber for further leaching. The copper-rich organic extractant then enters the second loop and passes through stripping chamber. The stripping chamber consists of highly acidic electrolyte which strips copper from the organic extractant. The organic extractant is directed back to the extraction chamber in the first loop. The copper-rich acidic electrolyte enters the third loop and is subjected to electrowinning to extract pure copper. The spent electrolyte is directed back to the stripping chamber in the second loop.