What is microbial Leaching?
- The process by which metals are dissolved from ore-bearing rocks using micro-organisms is termed as microbial leaching.
- In present time, a number of ores cannot be economically processed with chemical methods because of their low metal content.
- Moreover, in course of separation of higher-grade ores, large quantities of low-grade ores are produced, which are generally discarded in waste heaps.
- There are vast quantities of such low-grade copper ores throughout the world, which cannot be profitably purified by means of conventional chemical methods, but could be processed by microbial leaching.
- There are also significant quantities of nickel, lead, and zinc ores which could be leached.
- Leaching was first discovered as a process occurring in pumps and pipelines installed in mine pits containing acidic water.
- It was subsequently developed for the recovery of metal from low-grade ores.
- For many metals, there are now leaching methods which permit extraction from metal sulfides or other ores.
- The metals are converted to water-soluble metal sulfates with the aid of biochemical oxidation processes.
- In commercial applications, copper and uranium have been widely produced through the micro-organisms.
- However, there have been difficulties in extrapolating the results from laboratory and pilot-plant studies into practical field conditions.
- In addition, problems may arise when the large-scale leaching process of a waste dump is improperly managed.
- Leach fluids containing large quantities of metals and having extrememly low pH values (pH 3) can seep from such dumps into nearby natural water supplies and ground waters, causing enormous and lasting damage.
Microorganisms used for leaching:
- The two most commonly used organisms in microbial leaching are Thiobacillus thiooxidans and Thiobacillus ferrooxidans.
- A number of others may also be used including-
- Thiobacillus concretivorus, Pseudomonas fluorescens, P. putida, Achromobacter, Bacillus licheniformis, B. cereus, B. luteus, B. polymyxa, B. megaterium, and several thermophilic bacteria including Thiobacillus thermophilica, Thermothrix thioparus, Thiobacillus TH1, and Sulfobus acidocaldarius.
- The heterotrophic organisms listed have not as yet actually been used, but it seems likely that processes will be developed by which metals are extracted from ores with microbially produced organic acids via chelate and salt formation.
- Because of their more rapid growth rate, the thermophilic bacteria may significantly accelerate the leaching process.
Process of microbial ore leaching:
- Thiobacillus ferrooxidans is the organism that has been extensively studied.
- Itis a gram-negative rod-shaped bacterium which is 0.5-0.8 μm X 1.0-2.0 μm in size.
- An autotrophic aerobe, it can obtain carbon for biosynthesis solely from CO2 fixation, and obtains its energy from the oxidation of Fe2+ to Fe3+ or from the oxidation of elemental sulphur and reduced sulphur compounds to sulphate.
- 4FeSO4 + 2H2SO4 + O2 ——–> 2 Fe2(SO4)3 + 2H2O ————–(1)
- 2 So+ 3 O2 + 2H2O —————> 2 H2SO4 —————-(2)
- 2FeS2 + 7 O2 + 2H2O ———> 2 FeSO4 + 2H2SO4 ————–(3)
- The oxidation of insoluble sulfur to sulfuric acid, which is also performed by Thiobacillus thiooxidans, occurs in the periplasmic space.
- According to equation 3, iron is dissolved through direct bacterial leaching.
- In addition to this leaching process performed only by micro-organisms, there is another process, ‘indirect, bacterially supported leaching’, which takes place slowly in the absence of microbes.
- The oxidation of pyrite can be used as an example.
- Pyrite is a common rock mineral that is found in association with many ores.
- The following equation describes the initial oxidation of pyrite by ferric ions:
- FeS2 + Fe2(SO4)3 ——-> 3 FeSO4 + 2 So ———–(4)
- The sulfur which is formed via this process is re-oxidized as shown in equation 2.
- Examination of leaching dumps always shows the presence of mixtures of T. thiooxidans and T. ferrooxidans.
- In pilot-plant reactors (50 liter), leaching can be performed continuously in a cascade series with recycling of the cells and leachate.
- Yields such as those in other areas of micro-biology can be attained in the laboratory under optimal condition (temperature control, O2 and CO2 adjustment, maintenance of pH around 2-3 and Eh around -300 mV) with very finely ground ores in a tower (percolator), or better yet in fermenters under optimal conditions and yields cannot be realized due to the high cost.
Types of microbial ore leaching:
Three methods have practical application:
1. Slope leaching:
- Finely ground ores (up to 100,00 tons) are dumped in large piles down a mountainside and continuously sprinkled with water containing Thiobacillus.
- The water is collected at the bottom and reused after metal extraction and possible regeneration of the bacteria in an oxidation pool.
2. Heap leaching:
- The ore is arranged in large heaps and treated as in slope leaching.
3. In-situ leaching:
- Water containing Thiobacillus is pumped through drilled passages to unextracted ore which remains in its original location in the earth.
- In most cases, the permeability of the rock must be first increased by subsurface blasting of the rock.
- The acidic water seeps through the rock and collects in the bottommost cavity from which it is pumped, the minerals extracted, and the water re-used after regeneration of bacteria.
Examples of microbial ore leaching
Copper leaching:
- If chalcocite, chalcopyrite, or covellite are used for the production of copper, several metals are usually found together.
- For example, chalcopyrite contains 26% copper, 25.9% iron, 2.5% zinc, and 33% sulphur.
- Chalcopyrite is oxidized as follows:
- 2 CuFeS2 + 8 ½ O2 + H2SO4 ———> 2 CuSO4 + Fe2 (SO4)3 + H2O
- Covellite is oxidized to copper sulfate:
- CuS + 2 O2 ————> CuSO4 ————–(6)
- Copper leaching plants have been in wide use throughout the world for many years, generally operated as simple heap leaching processes but sometimes as combinations of heap and in-situ leaching.
- The leaching solution (sulfate/Fe3+ solution) carries the microbial nutrients in and the dissolved copper out.
- The solution is sprinkled over the heap and percolates through the rock pile to the lower level where the copper-rich-liquid is collected.
- The copper containing solution (up to 0.6g/l) is removed, the copper is precipitated, and the water is reused after readjusting the pH to 2.
- Countries in which microbial leaching of copper has been widely used include the United States, Australia, Canada, Mexico, South Africa, Portugal, Spain, and Japan.
- About 5% of the world copper production is obtained via microbial leaching.
- A single installation in the United States has produced up to 200 tons of copper per day.
Uranium leaching:
- Although less uranium than copper is obtained by microbial leaching, the uranium process is more significant economically.
- Because a thousand tons of uranium ore must be handled to obtain one ton of uranium, in-situ microbial leaching is gaining greater acceptance, since it eliminates the expense of moving such vast amounts of material.
- In the uranium leaching process, insoluble tetravalent uranium is oxidized with a hot H2SO4/ Fe3+ solution to soluble hexavalent uranium sulfate.
- UO2 + Fe2 (SO4)3 ————-> UO2SO4 + 2 FeSO4 —————-(7)
- This is an indirect leaching process since the microbial attack is not on the uranium ore directly but on the iron oxidant.
- Ferric sulphate and sulphuric acid can be produced by T. ferrooxidans from the pyrite within the uranium ore.
- 2 FeS4 + H2O+ 7.5 O2 ———-> Fe2(SO4)3 + H2SO4 ——(8)
- The pyrite reaction is used for the initial production of the Fe3+ leach solution.
- Pilot plants operate with surface reactors similar to the trickling filters used in sewage.
- Optimal uranium leaching conditions are pH 1.5-3.5, 35oC and 0.2% CO2 in the incoming air.
- Some thermophilic strains are known which have a temperature optimum of 45-50oC.
- In commercial processes, the dissolved uranium is extracted from the leach liquor with organic solvents such as tributylphosphate and the uranium is subsequently precipitated from the organic phase.
- Adsorption of the uranyl ions with ion exchangers is another possibility.
- The organic solvents which remain in the water system after extraction may be toxic and hence cause problems when the microbiological system is reused.
- In-situ leaching has the disadvantage that the permeability of the rock may be low and the drilled passages may not always allow an adequate supply of nutrients and oxygen to enter deeply into the ore.
- In such situations the heap system is often still used commercially for leaching of uranium.
- Areas where uranium leaching has been carried out include the United States, Canada, and South Africa.