Bioleaching: Introduction, Methods, Application, Copper, Microorganisms, and Processes

Bioleaching: Introduction, Methods, Application, Copper, Microorganisms, and Processes!

Introduction to Bioleaching:

Leaching process was first observed in pumps and pipelines installed in mine pits containing acid water. This process was later on employed for recovering metals from ores containing low quantity of the metal. Presently certain metals from sulfide ores and other ores are extracted by employing only leaching method.

Extraction of metals from low-grade ores by employing microorganism is called as bio­leaching. Large quantities of low-grade ores are produced during the separation of higher-grade ores and are generally discarded in waste heaps. Metals from such ores cannot economically be processed with chemical methods. There are large quantities of such low-grade ores especially copper ores, which can be processed profitably by bio-leaching.

Copper and Uranium are presently produced commercially by employing bioleaching process. However, problems may creep in when the large scale bioleaching process of a waste dump is improperly managed because seepage of leach fluids containing low pH and metals into natural water supplies and ground water causing metal pollution.

Mechanism of Bioleaching:

The process of bioleaching is accomplished by two ways:

(i) Direct bioleaching

(ii) Indirect bioleaching

(i) Direct Bioleaching:

Thiobacillus ferrooxidans is oftenly used in microbial leaching. It is an autotrophic, aerobic, gram (-) negative rod shaped bacterium. It synthesizes its carbon substances by CO₂ fixation. It derives the required energy for CO₂ fixation either from the oxidation of Fe²⁺ to Fe³⁺or from the oxidation of elemental sulphur or reduced sulphur compounds to sulfates.

Thiobacillus thiooxidans oxidizes insoluble sulphur to sulphuric acid, which takes place in the periplasmic space. It is possible to dissolve iron through direct bacterial leaching as shown in the above reactions.

(ii) Indirect Bioleaching:

This leaching process takes place without direct involvement of microorganisms but they indirectly support the leaching by producing agents responsible for oxidation of minerals. It can be explained by the process of oxidation of pyrite. Pyrite is a common rock mineral that is found in association with many ores. The pyrite is initially oxidized to elemental sulphur equation 4, which is subjected to further oxidation by Thiobacillus ferrooxidans due to which sulphuric acid is formed which is shown in equation 2.

Thiobacillus thiooxidans and Thiobacillus ferrooxidans are generally seen associated with leaching dumps. In pilot plant reactors of 50 liter capacity, leaching can be performed continuously in a cascade series with recycling of cells and leachates.

In the laboratory better yields of bioleaching products can be obtained under optimal conditions, like control of temperature, O₂ and CO₂ adjustments, maintenance of pH between 2 and 3, and eh around – 300mv with very finely ground ores in a tower (percolator). However, conditions and yield cannot be achieved in a commercial scale because it is expensive.

Thiobacillus thiooxidans and Thiobacillus ferrooxidans are generally used in bioleaching methods. However, a number of other microorganisms such as Thiobacillus concretivorus, Pseudomonas florescens, P.putida, Achromobacter, Bacillus licheniformis, B. cereus, B luteus, B. polymyxa, B. megaterium and several thermophilic bacteria like Thiobacillus thermophilica, Thermothrix thioparus, Thiobacillus TH1 and Sulfolobus acidocaldarius. Because of more rapid growth rate the thermophilic bacteria may significantly accelerate the bioleaching process.

Processes of Bioleaching:

i. Commercial Processes:

Methods described below are generally employed in large scale bioleaching processes:

(a) Slope Leaching:

In this method finely powdered ore, approximately 10,000 tons are made into large piles along the slopes of a mountain, and water containing Thiobacillus is continuously sprinkled. Metals are extracted from the water that collects at the bottom of the mountain. The water is recycled again after metal extraction and regeneration of the bacteria in an oxidation pool (Fig. 12.1a).

(b) Heap Leaching:

The ore is arranged in a big heap, which is treated with water as in slope leaching. The recovery of metals and other processes are conducted just like in slope leaching (Fig. 12.1b).

(c) In-Situ Leaching:

This process is carried with an ore which remains in its original location in the earth. The permeability of ore is increased by sub-surface blasting. Several passages, as shown in Fig. 12.1c are drilled through the ore. A well like pit is also made out at the bottom of the ore. Now acidic water containing Thiobacillus is pumped through drilled passages of the ore. The acidic water percolates through the ore and collects in the pit at the bottom of the ore. The water is pumped out from the pit and the minerals are extracted. The water after extraction of minerals, is reused after regeneration of bacteria.

ii. Copper Leaching:

To have an idea of bioleaching process copper leaching by bacteria is described as an example. Covellite, chalcocite and chalcopyrite are generally used as copper ores for bioleaching processes. Apart from containing copper, the ores also contain other elements like iron, zinc and sulphur. For example – Chalcopyrite contains 26% copper, 25.9% iron, 20.5% zinc and 33% sulphur.

Mechanism of Copper Leaching:

During the oxidation of Chalcopyrite the following reaction occurs:

2CuFeS₂ + 8 ½ O₂ + H₂SO₄ → 2 CuSO₄ + Fe₂ (SO₄)₃ + H₂O

Similarly covellite is oxidized to copper sulphate

CuS + 2O₂ → CuSO₄

Generally heap leaching process is employed in copper leaching process but sometimes a combination of heap leaching and in situ leaching processes are used. The solution (Sulphate/Fe³ solution) is sprinkled over the heap which percolates through the ore and collects at the bottom pit. The solution collecting in the bottom pit will include copper metal, which is removed by precipitation. The remaining water with Fe³⁺ is used again in the leaching process after adjusting the pH to 2.0 with the help of H₂SO₄. An outline of microbial leaching of copper is illustrated in Fig. 12.2.

Bioleaching of copper has been used in the United States, Australia, Canada, Mexico, South Africa, Portugal, Spain and Japan. About 5% of the world copper production is obtained through bioleaching.

iii. Uranium Leaching:

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 + 2FeSO.

Uranium bioleaching process is more significant economically. In situ microbial leaching is greater acceptance since it eliminates the expense of moving vast amounts of material. For instance thousand tons of uranium ore must be handled in other than bioleaching processes, to obtain one ton of uranium. This is an indirect leaching process since the microbial attack is not on uranium ore directly but on the Iron oxidant. Ferric sulphate and sulfuric acid can be produced by T.ferrooxidans from the pyrite within the uranium ore.

2FeS₂ + H₂O + 7.5 O₂ → Fe₂ (SO₄)₃ + H₂SO₄

For the initial production of the Fe³⁺ leach solution the pyrite reaction is used. For carrying this reaction pilot plants with surface reactors are used which are similar to trickling filters used in sewage operations.

For getting optimum uranium leaching the incoming air should passes a pH of 1.5-3.5, temperature of 35°C and CO₂ 0.2%. However, certain thermophilic strains require a temperature optimum of 45-50°C.

The dissolved uranium is extracted from the leach liquor, in commercial processes with organic solvents like tributyl phosphate and the uranium is subsequently precipitated from the organic phase. The organic solvents which remain in the water system after extraction of uranium may be toxic and hence cause problems when the microbiological system is reused.

Microbial leaching of refractory precious metal ores to enhance recovery of gold and silver is one of the most promising applications. Gold is obtained through bioleaching of arsenopyrite/pyrite ore and its cyanidation process. Silver is more readily solubilized than gold during microbial leaching of iron sulphide.

Similarly silica is leached from ores like magnesite, bauxite, dolomite and basalt by Bacillus licheniformis. The silica is accumulated by B.licheniformis by process of adsorption which is readily separated. This technology of obtaining silica from magnesite is being adapted by Salam works of Burn, Standard Co. Ltd, Tamil Nadu in collaboration with the department of Biotechnology, Govt. of India.

Ore leaching by microbes has potential for use in the extraction of other metals such as zinc, cobalt and nickel. New reactor systems are likely to be developed to increase the efficiency of bioleaching in terms of cost and kinetics. These innovations are expected to extend the scope of bleaching applications.