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MICROBIAL LEACHING OF METALS

Pavan.Ma, Chaitanya.A.S

b

a II B.Tech., Dept. of Chemical Engg., Gayatri Vidya Parishad,Vizag, E-mail: pawan_cool87@yahoo.co.in

b II B.Tech., Dept. of Chemical Engg., Gayatri Vidya Parishad,Vizag, E-mail: ayyagarisri@sifymail.com

Keywords: Biohydrometallurgy, thiobacillus, ferroxidans, glycogalius, redoxolysis.

ABSTRACT:

With the help of biotechnology, the race is on to discover the magic bug that will

revolutionize the mining industry. The aim of this abstract is to present an eco- friendly

technique of extracting the metals, keeping in view the environmental and economical

factors. The issue under lime light-BIOLEACHING is the outstanding solution for

mining technology. Environmental standards continue to stiffen, particularly regarding

toxic wastes, so costs for ensuring environmental protection will continue to rise. The

products of biological processes are more amenable to containment and treatment than

gaseous waste.

The most important player in the bioleaching process is Acidithiobacillus ferrooxidans. It

is a chemoautotrophic acidophile, whose unique ability to oxidise metals, leading to

leaching. A consortium of microorganisms namely Acidithiobacillus thiooxidans,

Leptospirillum ferrooxidans and thermophilic bacteria are involved in bioleaching. In

any event, biomining is now at the top of mining technology, and future development of

the technology appears promising.

1. INTRODUCTION

Future sustainable development requires measures to reduce the dependence on

nonrenewable raw materials and the demand for primary resources. Improvement of

already existing mining techniques can result in metal recovery from sources that have

not been of economical interest until today. The current methods will surely prevail for

many years to come, but biological processes are generally less energy-intensive and less

polluting than most nonbiological ones. Metal-winning processes based on the activity of

microorganisms offer a possibility to obtain metals from mineral resources not accessible

by conventional mining. Microbes such as bacteria and fungi convert metal compounds

into their water-soluble forms and are biocatalysts of these leaching processes.

In general, bioleaching is a process described as being “the dissolution of metals from

their mineral source by certain naturally occurring microorganisms to transform elements

so that the elements can be extracted from a material when water is filtered trough it”. In

the case of copper, copper sulfide is microbially oxidized to copper sulfate, in the

aqueous phase. “Biomining” concerns mostly applications of microbial metal

mobilization processes in large-scale operations of mining industries for an economical

metal recovery.

In an ecofriendly way, Microbial technology offers an economic alternative for the

mining industry, at a time when high-grade mineral resources are being depleted.

Bacterial extraction of metals is not a new technology as it has been in application for ore

leaching for centuries, though the mechanism remained unknown.

2. HISTORICAL BACKGROUND

It is an age-old idea, which the present scientific community is capitalizing upon.

Evidence suggests that the mine used water from the Rio Tinto mines in Spain contained

a very high concentration of ferric iron owing to microbial activity in the area. The Rio-

Tinto mines in southwestern Spain are usually considered as the cradle of

biohydrometallurgy. These mines have been exploited since pre-Roman times for their

copper, gold, and silver values. As a consequence, to the ban of open-air ore roasting and

its resulting atmospheric sulfur emissions in 1878 in Portugal, hydrometallurgical metal

extraction has been taken into consideration in other countries more intensely.

Efforts to establish bioleaching at the Rio-Tinto mines had been undertaken in the

beginning of the 1890s. Heaps (10 m in height) of low-grade ore (containing 0.75% Cu)

were built and left for one to three years for “natural” decomposition. 20 to 25% of the

copper left in the heaps were recovered annually. Although industrial leaching operations

were conducted at the Rio-Tinto mines for several decades, the contribution of bacteria to

metal solubilization was confirmed only in 1961, when Thiobacillus ferrooxidans was

identified in the leachates. In 1947, Thiobacillus ferrooxidans was identified as part of the

microbial community found in acid mine drainage.

3. MICROBES AT WORK

For many years, the only microorganism thought to be important in the leaching of

metals from ores was the rod-shaped bacterium Thiobacillus ferrooxidans. This

microorganism was discovered in the acidic water draining coalmines in 1957. T.

ferrooxidans is acidophilic or acid loving; it tends to live in hot springs, volcanic fissures

and sulfide ore deposits that have a high concentration of sulfuric acid. It is also

moderately thermophilic, thriving in the temperature range between 20 and 35 degrees C.

The bacterium gets energy for growth from the oxidation of either iron or sulfur. The iron

must be in the ferrous, or bivalent, form (Fe++), and it is converted by the action of the

bacterium into the ferric, or trivalent, form (Fe++).

Among the other microorganisms, taking part is T. thioxidans, a rod-shaped bacterium

not unlike T. ferrooxidans that grows on elemental sulfur and some soluble sulfur

compounds. T. ferrooxidans and T. thiooxidans combined, for example, are more

effective in leaching certain ores than either organism is alone. Similarly, the

combination of Leptospirillium ferrooxidans and T. organoparus can degrade pyrite

(FeS2) and chalcopyrite (CuFeS2), a feat neither species can accomplish alone.

Sulfolobus acidocaldarius and S. brierleyi oxidize sulfur and iron for energy, relying on

either carbon dioxide or simple organic compounds for carbon. Ordinarily oxygen is

required by Sulfolobus, - as in other aerobic organisms, the oxygen serves as the ultimate

acceptor of the electrons removed in the process of chemical oxidation. The potential of

Sulfolobus species to leach metals is only now being recognized: because of the

extraordinary ability of these organisms to attack resistant mineral structures.

IMAGES OF T.FERROOXIDANS

4. PRINCIPLE OF MICROBIAL LEACHING

Mineralytic effects of bacteria and fungi on minerals are based mainly on three

principles, namely acidolysis, complexolysis, and redoxolysis.

Microorganisms are able to mobilize metals by (1) the formation of organic or inorganic

acids (protons); (2) oxidation and reduction reactions; and (3) the excretion of

complexing agents. Sulfuric acid is the main inorganic acid found in leaching

environments. Sulfur-oxidizing microorganisms such as thiobacilli form it.

All facts have been combined and a mechanism has been developed which the following

features characterize: (1) cells have to be attached to the minerals and in physical contact

with the surface; (2) cells form and excrete exopolymers; (3) these exopolymeric cell

envelopes contain ferric iron compounds that are complexed to glucuronic acid residues.

These are part of the primary attack mechanism; (4) thiosulfate is formed as intermediate

during the oxidation of sulfur compounds; (5) sulfur or polythionate granules are formed

in the periplasmatic space or in the cell envelope.

4.1 MECHANISM:

A generalized reaction can be used to express the biological oxidation of a

mineralsulphide involved in leaching:

MS + 2O2 MSO4 .......... (1)

Where M is a bivalent metal.

There are two major mechanisms of bacterial leaching. One involves the ferric-ferrous

cycle (indirect mechanism), whereas the other involves physical contact of the organism

with the insoluble sulphide (direct mechanism) and is independent of the indirect

mechanism. An important reaction mediated by Acidithiobacillus ferrooxidans is:

4FeSO4 + O2 +2H2SO4 2Fe2(SO4)3 ……….(2)

Ferric sulphate is a strong oxidising agent capable of dissolving a wide range of metal

sulphide minerals. Leaching brought about by ferric sulphate is termed indirect leaching

because it proceeds in the absence of both oxygen and viable bacteria. This mode is

responsible for leaching several minerals:

CuFeS2 (chalcopyrite) + 2Fe2(SO4)3 CuSO4 + 5FeSO4 + 2S0 ……(3)

FeS2(pyrite) + Fe2(SO4)3 FeSO4 + 2S0 ………(4)

UO2 + Fe2(SO4)3 + 2H2SO4 UO2(SO4)4-3 + FeSO4 + 4H+…….(5)

Elemental sulphur generated by indirect leaching can be converted to sulphuric acid by

Acidithiobacillus ferrooxidans:

2S0 + 3O2 + 2H2O 2H2SO4 ………(6)

This sulphuric acid maintains the pH at levels favorable to the growth of bacteria and

helps in the effective leaching of oxide minerals:

CuO(Tenorite) + 2H2SO4 CuSO4 + H2O ………(7)

UO3 + 3H2SO4 UO2(SO4)4-3 + H2O + 4H+……..(8)

In the direct mechanism of leaching by bacteria, intimate contact and adhesion to the

mineral takes place prior to enzymatic attack by the organism. The direct mechanism is

inferred from scanning electron micrographs, which demonstrate bacterial adhesion on

mineral surfaces. The direct mechanism is further confirmed by the leaching of synthetic

sulphides free of iron, where only the direct attack of the bacteria can lead to leaching:

CuS(covellite) + 2O2 CuSO4 ……….(9)

ZnS(sphalerite) + 2O2 ZnSO4 ……….(10)

4.2 Bacterial Attachment on Mineral Surfaces:

It is known that the formation of extra cellular polymeric substances plays an important

role in the attachment of thiobacilli to mineral surfaces such as sulfur, pyrite, or covellite.

Extraction or loss of these exopolymers prevents cell attachment resulting in decreased

metal leaching efficiencies. It was concluded that a direct contact between bacterial cells

and solid surfaces is needed for an effective metal mobilization.

Interactions between microorganisms and the mineral surface occur on two levels. The

first level is a physical sorption because of electrostatic forces. Due to the low pH levels,

the microbial cell envelopes get positively charged leading to electrostatic interactions

with the mineral phase. The second level is characterized by chemical sorption where

chemical bonds between cells and minerals might be established (e.g., disulfide bridges).

In addition, extra cellular metabolites are formed and excreted during this phase in the

near vicinity of the attachment site. In the presence of elemental sulfur, sulfur-oxidizing

microorganisms from sewage sludge form a filamentous matrix similar to a bacterial

glycocalyx suggesting the relative importance of these extracellular substances in the

colonization of solid particles.

5. BACTERIAL LEACHING TECHNIQUES:

The two major techniques used in leaching are percolation and agitation leaching.

Percolation leaching involves the percolation of a lixiviant through a static bed, whereas

agitation leaching involves finer particle sizes agitated in a lixiviant. Due to the large-

scale operations involved in bacterial leaching, percolation leaching is preferred

commercially. The principal commercial methods are in situ, dump, heap and vat

leaching. In situ leaching involves pumping of solution and air under pressure into a mine

or into ore bodies made permeable by explosive charging.

The resulting metal-enriched solutions are recovered through wells drilled below the ore

body. Three types of ore bodies are generally considered for In situ leaching: surface

deposits above the water table, surface deposits below the water table and deep deposits

below the water table. Dump leaching involves uncrushed waste rock that is piled up.

These dumps generally contain about 0.1-0.5% copper, too low to recover profitably by

conventional procedures. Some of these dumps are huge, containing in excess of 10

million tons of waste rock. Heap leaching requires the preparation of the ore, primarily

size reduction, so as to maximize mineral-lixiviant interaction and the laying of an

impermeable base to prevent lixiviant loss and pollution of water bodies. Essentially,

both dump and heap leaching involve the application of the lixiviant to the top of the

dump or heap surface and the recovery of metal laden solution that seeps to the bottom by

gravity flow. The dilute sulphuric acid sprinkled on top percolates down through the

dump, lowering the pH and promoting the growth of acidophilic microorganisms. The

acid run-off is collected at the bottom of the dump, from where it is pumped to a recovery

station. Copper is extracted from the acid run-off by cementation or solvent extraction or

electrowining. All the above processes are essentially uncontrolled from a biological and

engineering standpoint. Besides these processes are slow in nature and require long

periods to recover a portion of the metal.

Vat leaching as currently applied to oxide ores involves the dissolution of crushed

materials in a confined tank. More controls can be brought in for enhanced recovery by

the use of bioreactors, though necessarily these involve higher costs. However, for ore

concentrates and precious metals they are being considered actively.

5.1 Bioleaching of Copper:

Biological copper leaching is practiced in many countries including Australia, Canada,

Chile, Mexico, Peru, Russia and the United States of America. Copper recovery from

bioleaching accounts for about 25% of the world copper production. Following the initial

isolation of Acidithiobacillus ferrooxidans from coalmine water in 1947, studies quickly

disclosed its presence in copper-leaching operations. Acidithiobacillus ferrooxidans is

also found in the Malanjkhand Copper Mines. The physical configurations of bioleaching

operations worldwide for copper are mostly uniform. Typically, copper ore mined from

open pits is segregated, higher-grade material is concentrated to produce feed for

smelting, while the lower-grade ore is subjected to leaching. The ore is piled on an

impermeable surface until a dump of suitable dimension forms. After the top is leveled,

leach solution is flooded or sprayed onto the dump. A copper dump represents a complex

and heterogenous microbiological habitat. It contains solids ranging in size from boulders

to fine sand and includes material of complex mineralogy. Bacterial colonization occurs

mainly in the top one meter or so. The temperature may reach 900C in the interior of the

dump and supports a range of thermophillic microorganism, which are often anaerobic, or

microaerophilic. In these regions, indirect leaching by ferric sulphate also prevails. The

exterior of the dump is at ambient temperature and undergoes changes in temperature

reflecting seasonal and diurnal fluctuations. Many different microorganisms have been

isolated from copper dumps, some of which have been studied in the laboratory. These

include a variety of mesophilic, aerobic iron and sulphur oxidizing microorganisms;

thermophilic iron and sulphur oxidising microorganisms; and anaerobic sulphate

reducing bacteria. Some are heterotrophic bacteria, which indirectly affect metal

solubilisation by affecting the growth and activity of metal solubilising bacteria. Others

are protozoa, which interact with and prey on different types of bacteria. Leach solutions

enriched with copper exit at the base of the dump and are conveyed to a central recovery

facility. In most large-scale operations the leach solution, containing 0.5-2.0 g copper per

liter is pumped into large cementation units containing iron scrapings for cementation and

then electrolysis. A typical large dump may have an operating life of over ten years.

5.2 Bioleaching of Uranium:

Uranium leaching proceeds by the indirect mechanism as Acidithiobacillus ferrooxidans

does not directly interact with uranium minerals. The role of Acidithiobacillus

ferrooxidans in uranium leaching is the best example of the indirect mechanism. Bacterial

activity is limited to oxidation of pyrite and ferrous iron. The process involves periodic

spraying or flooding of worked-out stopes and tunnels of underground mines with

lixiviant. Another method in use for uranium extraction is vat leaching. Bioleaching has

also been used succesfully to obtain uranium from waste gold ore.

5.3 Bioliberation of Gold:

Iron- and sulphur-oxidising acidophilic bacteria are able to oxidise certain sulphidic ores

containing encapsulated particles of elemental gold, resulting in improved accessibility of

gold to complexation by leaching agents such as cyanide. Bio-oxidation of gold ores is a

less costly, less polluting alternative to other oxidative pretreatments such as roasting and

pressure oxidation.

6. Even Bacteria Have to Deal with Waste Management…

Yet another example of biotechnology in mining involves a process called metal

precipitation. This process is used to treat wastewater from mining operations, which, as

described above, contains high concentrations of dissolved SO42- and Fe

2+, as well as

excess acid. This material cannot be released directly into the environment as it is

extremely hazardous to stream and river ecology. In what is essentially, a reversal of the

procedure described that leads to acid mine drainage, sulphate-reducing bacteria like

Desulfovibrio and Desulfotomaculum oxidize organic matter or H2 by using sulphate as

an electron acceptor according to the following reactions:

2 CH2O + SO42- --------> H2S + 2 HCO

3-

5 H2 + SO42---------> H2S + 4 H2O

The sulphide present then immediately reacts with any dissolved metal, producing an

insoluble metal sulphide:

M2+ + S

2- --------> MS (precipitate)

The metal sulphide is very inert and can be disposed of, provided it is not exposed to

oxygen, lest the process begin again.

In many mining operations, this process is carried out in engineered anaerobic swamps. A

large enclosure is first lined to make it impermeable. The wastewater is then allowed to

enter the area where the pH is adjusted with limestone. As the pH increases, some iron

precipitates and the rest of the material is passed into an area containing organic waste

material. The degradation of the organic waste maintains the anoxic state of the

containment pond. Microbial action reduces the sulphate present to metal sulphides that

are then permanently precipitated within the passive reactor. The top of the reactor can be

covered with topsoil and planted to restore the aesthetic potential of the area.

7. ADVANTAGES

• This approach is an eco-friendly and economic process.

• Highly efficient process and wastage of the metal is minimized as far as possible.

• Employs comparatively less manual labour.

• Easy & flexible process.

8. DISADVANTAGES

• Microbial culture and maintenance is a complex task.

• Process is very slow.

• The waste mine water may contain toxic substances and their disposal is a big

problem.

9. CONCLUSION

The current panorama of bioleaching in developing countries is encouraging. It is

expected that in the coming years several new commercial-size bioleaching plants will be

installed. It is likely that heap leaching will continue to be the choice for low-grade ores

and tailings, while tank bioleaching technology will probably increase its application for

gold, copper and other base-metal concentrates. The use of thermophilic bacteria and

archea will be a major contribution, increasing the leaching rates and metal recoveries

and allowing for the treatment of recalcitrant ores such as chalcopyrite.

Developing countries should increase their efforts in research and development in

bioleaching technology, as they have comparative and competitive advantages in this

area. International cooperation should also be considered in the establishment of new

operations that can significantly contribute to the economic and social development of

these countries.

10. REFERENCES

1) Davis Jr., R.A., Welty, A.T., Borrego, J., Morales, J.A., Pendon, J.G. and J.G. Ryan.

2000. Rio Tinto estuary (Spain): 5000 years of pollution. Environmental Geology. 39:

1107-1116.

2) Brierley, C.L. and J.A. Brierley. 1997. Microbiology for the Metal Mining Industry. in

Manual of Environmental Microbiology. (Ed.) C.J. Hurst. ASM Press, Washington D.C.

3) Brierley, C.L. 1995. Bacterial oxidation. Engineering and Mining Journal. 196:42-44.

4) Acevedo, F. 2000. The use of reactors in biomining processes. Electronic Journal of

Biotechnology.

online.

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