Bioleaching/Biocorrosion
Metals/Biomining
Presented to: Dr. Michael Broaders
1. Biocorrosion
By Lisa Smith
Physicochemical interactions between
a metallic material and its environment can lead to corrosion.
Corrosion is a “naturally occurring
process by which materials fabricated of pure metals and/or other mixtures
undergo chemical oxidation from ground state to an ionized species” (Beech,
2003). The process proceeds through a series of oxidation (anodic) and
reduction (cathodic) reactions of chemical species in direct contact with, or
in close proximity to, the metallic surface.
In natural habitats and man-made
systems, surface-associated microbial growth, i.e. biofilms, influence the
physico-chemical interactions between metals and the environment, frequently
leading to deterioration of the metal. For example in a marine environment the
presence of a biofilm can accelerate corrosion rates of carbon steel by several
orders of magnitude. However, in contrast, certain types of biofilms produce a
protective barrier effect resulting in a significant decrease in corrosion rates
of metals.
Deterioration of metal under a
biological influence is termed biocorrosion or microbiologically influenced
corrosion (MIC).
“Biocorrosion is a result of
interactions between metal surfaces and bacterial cells and their metabolites”
(Beech, Sunner, 2004).
The main types of bacteria
associated with metals in terrestrial (and aquatic) environments are
sulfate-reducing bacteria (SRB), sulfur-oxidising bacteria,
iron-oxidising/reducing bacteria, manganese –oxidising bacteria and bacteria
secreting organic acids and slime. These organisms coexist in naturally
occurring biofilms.
SRB are the main group of
microorganisms and are generally anaerobic, however some genra tolerate oxygen
and even grow in its presence. They are distributed within two domains: Archaea
and Bacteria.
There is increasing recognition that microbes such as
bacteria play an even larger role in all forms of corrosion than previously
thought. It is now reported that up to 70% of all corrosion in water systems is
caused or accelerated by microbes.
2. Biooxidation
By Deborah Mc Auliffe
Many
biotechnology-derived processes use microorganisms to help ease the usage of
harmful chemicals in various industrial processes. The mining industry uses
microorganisms and their natural ability to digest, absorb, and change the
quality of different chemicals and metals, to refine ores.
Biooxidation
also uses microorganisms, not to extract metals, but to make the metals ready
for extraction. Oxidation is the chemical reaction in which an element is
changed by the addition of oxygen. Rust is an example of the oxidization of
iron.
Biooxidation is
mainly used in gold mining. Gold is often found in ores with gold particles
scattered throughout, called refractory ores, and the small particles of gold
are covered by insoluble minerals. These minerals make the extraction
difficult. Therefore, microorganisms that can "eat away" at the
mineral coating are used to pre-treat the gold ores before they can be
extracted.
Bioleaching of
copper, and biooxidation of refractory gold ores are the only well-established
large scale processes that are commercially carried out today.
Currently, 25
percent of all copper worldwide is produced through biomining. The process is
used on a variety of other metals such as gold and uranium. Biomining is not
yet a proven or profitable technology to apply to other metals such as zinc,
nickel and cobalt.
3. Bioleaching
By Marian Cummins
3.1 History of Bioleaching
Although
mining is one of the oldest technologies known it has succeeded in escaping the
major
technological advances seen in that of agriculture and medicine. Many minerals
and metals are mined today in exactly the same manner, as they were hundreds of
years previous. The crude ores are dug from the earth, crushed and the mineral
is extracted by either by extreme heat or due to the addition of toxic
chemicals. But due to the environmentally unfriendly aspect of these mining
techniques new methods, which are kinder and more environmentally friendly, are
being used which uses microorganisms, which leach out the metals- that of
Bioleaching
One of the earliest
recordings of bioleaching comes from Cyprus Spain
Prior to electrolysis, the recovery
of the copper from copper sulphate was by cementation (precipitation). It is thought
that this process was known in Pliny time but no written records of this have
survived. Its is known that the Romans used to place scrap iron into the river
and over a period of a few months the copper precipated around the iron. The
pure copper was then recovered by smelting, but what the Romans didn’t realize
was microorganisms played a major biological contribution to this process by
generating the copper in the water. The Chinese were also aware of the process
(cementation) as documented by King Lui- An (177-122 BC). The Chinese
implemented the commercial production of copper from copper sulphate when the
Chiangshan cementation plant started operation in 1096 with an annual
production of 190ton Cu/annum. Bioleaching and cementation were also described
by Paracelsus the Great (1493-1541). He noted the copper deposition onto iron
at a spring in the Zifferbrunnen in Hungary
Even though these earlier
bioleaching operations were difficult to document, it is known that copper
leaching was well established at the Rio Tinto mine in Spain
In the 1940’s in America Ohio
River , this discharge was attributed to the weathering of
subbitumous coal. Naturally enough this pollution incident was unacceptable and
it led to widespread investigation by universities and several US government
institutions, such as the US Bureau of Mines as to the source of the pollution.
The cause of the sulphuric acid was due to the oxidation of pyrite, which is
present in the subbitumuous coal, but it was also noted that this oxidation
occurred much more rapidly than could be contributed to by that of inorganic
chemistry. Also an important observation was that of the presence of sulphur
oxidizing bacteria. And in 1950 a couple of years after the incident a new
species was identified that of Thiobacillus ferrooxidans. This organism is able
to oxidize elemental sulphur and ferrous ions at a much higher rate than that
achieved by inorganic chemistry. It is this catalysis of the oxidation of
ferrous ions that makes Thiobacillus ferrooxidans and other iron and sulphur
oxidizing microorganisms such important catalysts in the bioleaching process.
3.2
Why has Bioleaching become such an attractive alternative?
Bioleaching is a very
attractive alternative to to the conventional mining techniques and it is very
desirable in today’s world due to the continued depletion of high grade
reserves and so it allows the more economically extraction of minerals by from
low grade ores, it also arise from the resulting tendency for mining to be
extended deeper underground and also it is a much more environmental friendly
alternative to that of the conventional mining methods to which there is a
growing awareness of the environmental issues associated with the smelting of
sulphide minerals and the burning of sulphur rich fossil fuels and of course
there is the enormous energy costs that is associated with the conventional
methods. Bimining also improves recovery rates, reduces capital and operating
costs.
There has being a very
widespread and rapid interest in the exploitation of biomining especially in
the copper industry, due to the fact that the copper in the low grade ore is
bound up in a sulfide matrix, it can be recovered by traditional smelting only
at great cost. In addition the world is running out of smelting capacity
because of the depletion of the high-grade ores means that more ore has to be
smelted to produce the same amount of copper. Oxidising bacteria can reduce the
need for these expensive smelters. Whereas a new smelter can cost 1 billion
dollars the technology required for biomining I pretty uncomplicated.
In order to understand the
process of microbial mining or biomiining a number of considerations must be
understood and answered, such as what microorganisms are involved in the
extraction of the metals from the rocks and where in nature do they occur? What
biochemical functions do these microorganisms perform and what do they require
in the need of nutrient and environmental conditions in order to maintain their
activity? What are the constraints of the commercial exploitation of such
biological techniques? And what impact will the new tools of genetic
engineering have on the future of biomining?
3.3 General Properties of the
Microorganisms
The bacteria involved in
biomining are among the most remarkable life forms known. They are described as
chemolithotrophic, which basically means rock eating, that is they obtain their
energy from the oxidation of inorganic substances. Many of them are also
autotrophic that is they utilize carbon dioxide in the atmosphere as the carbon
source. These microorganisms live in very inhospitable environments, which
other microbes would find it impossible to survive or tolerate; for example the
sulphuric acid and soluble metals concentrations are often very high. Some
thermophilic microorganisms require temperatures above 50 degree Celsius (122
degree Fahrenheit), and a few strains have been found at temperatures close to
that of the boiling point of water.
4.4 Specific Microorganisms
For many years the general
impression was that Thiobacillus ferrooxidans was the only microorganism
responsible for the leaching proceeds. As previously stated this microorganism
wasn’t discovered until 1957 in the acidic water draining coal mines, where it
was then determined the relationship between the existence of this
microorganism and the dissolution of metals in copper- leaching operations.
Since its discovery in the Rio Tinto Mine in Spain a wealth of information has
be collected regarding its characteristics and also more importantly on the
role it plays in bioleaching of the metals.
T. ferrooxidans is rod shaped
(usually single or in pairs), non- spore forming, gram negative, and single
pole flagellated ( HORAN, 1999;KELLY and HARRISON, 1984; LEDUC and FERRONI,
1994; MURR, 1980). T. ferrooxidans is also acidophilic; it tends to be found in
hot springs
As important and all T.
ferrooxidans is in the leaching process another important microorganism taking
part ii that of T. thioxidans, this is also a rod shaped bacteria, very similar
to T. ferrooxidans but it can’t oxidized Fe3+ it is also gram
negative Its maximum growth rate is at 35 degrees C, and it is the dominant
microbe found at low Ph environments. It has being found that mixed cultures of
bacteria are responsible for the extraction of metals from their ores such as
is the case with the combined effects of T ferrooxidans and T. thiooxidans are
more effective in leaching certain ores together than as an individual
organism. Also Leptospirillium ferrooxidans and T. organaparus can degrade
pyrite (FeS2) and chalopyrite (CuFeS2), a feat, which
neither species can do alone.
4.5 Bioleaching Processes
The process of bioleaching
falls under 2 methods that of direct leaching and indirect leaching. Direct
leaching is the process where the bacteria attack the minerals which are
susceptible to leaching by enzymes. By obtaining the energy from the inorganic
material the bacteria aid in the transferring of electrons from iron or sulphur
to oxygen. The more oxidized product is
generally the more soluble the product. The inorganic material never enter the
bacterial cell, the electrons released by the oxidation reaction are
transported through the cell membrane (and in aerobic organisms) to oxygen
atoms forming water. ATP (adenosine triphosphate) is produced when the
transferred electrons give up their energy.
Indirect leaching, in cons tract does not
occur by the bacteria attacking the minerals. The bacteria produce ferric iron
(Fe3+) by oxidizing soluble ferrous iron (Fe2+) which is
a powerful oxidizing agent that reacts with the other metals, and transforms
them into a soluble oxidisable form in a sulphuric acid solution. In this way
the ferrous iron is produced again and is rapidly oxidized by the bacteria thus
it is a continuous cycle. This indirect leaching is generally known as
bacterial assisted leaching. T. ferrooxidans can speed up the oxidation of iron
by a factor of more than a million than without the bacteria being present in
the solution.
3.6 The Process
In the case of the extraction
of copper from its ore the aforementioned bacteria T.ferrooxidans and T.
thiooxidans are involved in this process, which is a 2-stage process that of
direct and indirect as previously discussed.
In stage 1, the bacteria
break down the mineral arsenopyrite (FeAsS) by oxidizing the sulphur ant the
metal (arsenic ions) to a higher oxidation state whilst reducing dioxygen by H2
and Fe3+ This allows the soluble products to dissolve as such
FeAsS(s)
-> Fe2+(aq)
+ As3+(aq) + S6+(aq)
This process of direct leaching as described previously
occurs at the cell membrane of the bacteria. The electrons pass into the cells
and are used in biochemical processes to produce energy for the bacteria to
reduce oxygen molecules to water.
In stage 2, that of indirect
leaching the bacteria oxidise Fe2+ to Fe3+
(whilst reducing O2).
Fe2+ -> Fe3+
They then oxidise the metal to a
higher positive oxidation state. With the electrons gained from that, they
reduce Fe3+ to Fe2+ to continue the cycle. This stage
involves both ditect and indirect leaching.
M3+ -> M5+
The gold is now separated from the
ore and in solution.
The process for copper is very
similar. The mineral chalcopyrite (CuFeS2) follows the two stages of
being dissolved and then further oxidised, with Cu2+ ions being
left.
In the process of extracting
copper (Cu2+) from a mixture, the copper ions are removed by solvent
extraction, which leaves the other ions in solution. The copper is removed by
bonding to a ligand, which is essentially a large molecule consisting of a
number of smaller groups each processing a lone pair. The ligand is then dissolved
in kerosene (organic solvent) and shaken with the resultant reaction:
Cu2+(aq) +
2LH(organic) -> CuL2(organic)
+ 2H+(aq)
Electrons
are donated to the copper, producing a complex, copper bonded to 2 molecules of
the ligand. As this complex has no charge as as it is no longer attracted to
the polar water molecules it dissolves in the kerosene and is then seperated
from solution.This initail reaction is reversible as so is pH dependent. The
copper ions go back into an aqueoeus solution by adding concentrated acid.
To
increase the purity of the copper an electric current is added to the copper
ions as it passes through an electro-winning process. The copper ions which
have a 2+ charge are attracted to the
negative electrode and thus collected.
The
copper can also be concentarted and recovered by using scrap iron which
replaces the copper in the reaction as thus:
Cu2+(aq) +
Fe(s) -> Cu(s)
+ Fe2+(aq)
As
described biomining has being extremely successful in the case of copper.But
gold can also be obtained in a similar manner. Up until recently the gold
mining industry depended on high grade ores near the surface og the earth.But
by the 1980 and the depletion of these ores forced miners to rely on the lower
grade ores which were located deeper in the mines. These low grade ores were
more difficult to process in comparsion to the high grade ores at the surface
as they were naturally oxidized by bacteria, sunlight and water. But the low
grade ores are generally encased in sulphide minerals a and so processing of
these ores requires roasting or pressure oxidation and then treatment with
cyanide.Biomining means that the costly procedures of roasting and pressure
oxidation can be surpasssed by usinf T. ferrooxidans for the pretreatment of the
gold ores. The first mine to take advantage of this was Fairview
mine in South Africa
Although gold and copper are
probably the most important and valuable metals and undoubtedly this is what
has pushed the huge interest there now is in biomining. But biomining has also
played a big part in the phosphates industry. Phosphates are definitely not as
valuable as the metals but their extraction is definitely plays a part in big
time mining. Phosphates for fertilisers is the world’s second largest
agricultural chemical (after nitrogen); about 5 .5 million tons are produced
every year in the US
The traditional method of extracting
phosphates from ores was by burning at high temperatures with the resultant of
solid phosphorus, or else by treatment with sulphuric acid with phosphoric acid
and huge amounts of useless low-grade gypsum being the result. But with the
process of biomining a much milder technique was available. This new technique
used two bacteria that of Pseudomonas cepacia E-37 and Erwinia herbicola, these
bacteria were chosen from hundreds of bacteria as they have the unusual
ability: a direct oxidative pathway of converting glucose into gluconic and 2
ketogluconic acids, which means that sulfuric acid doesn’t have to be used in
the process and also this milder technique it performed at room temperature and
so it is a much more environmentally friendly process.
3.7 Methods to increase biomining
efficiencies and the impacts of Genetic Engineering on Biomining
As biomining is now at an all
time high it the next challenge is to increase its efficiency. At the present
time it is only indigenous microorganisms that naturally occur in dumps or mine
run off that are used in the bioleaching process. So now the focus is on
finding microbial strains that are better suited to large scale industrial
processing. One draw back is that the
bioleaching process releases large amounts of heat and
Can raise the temperature so
much that the bacteria that are being in use are killed or slowed down. To
combat this work has being and is still currently being done on using
Archaebacteria for use in biomining. These primitive thermophiles, or heat
loving bacteria are so far poorly studied and they are found in deep-sea vents
and in hot springs Yellowstone National Park ,
Iceland New Zealand Western
Australia
Another challenge is to find
or engineer strains that can stand up to the presence of heavy metals such as
mercury, arsenic, cadmium, these metals poison the microbes currently being
used in biomining and thus slow down biomining. Some steps have being taken
toward finding resistant strains to these poisons by showing that some microbes
have enzymes that can work in 2 ways that of protecting their basic activities
from heavy metals or by pumping the metals out of the bacteria. Also some work
but not a lot has being done on identifying genes that help the microbes deal
with the heavy metals and these genes may be used to genetically engineer
resistant strains. The genetically engineering of bacteria to resist heavy
metal poisoning is not an easy achievement.
As much less
is known about the Thiobacillus species and the other microbes used in
biomining than is known for E. coli, which is of course a lab favourite? But
hopefully this genetically engineering of these microorganisms will take place
at a much quicker pace than the two millennia is took the Roman miners at Rio
Tinto to become a major improvement in biomining, but it is fairly safe to say
that these developments will take place sooner rather than later as biomining
has become a worldwide accepted process
3.8 Metal extraction operations
Insitu leaching is a
promising alternative for the recovery of metals from low-grade ores, which are
in inaccessible places. Also this has the advantages as this technology has
minimal impact on the environment and it is currently used to extract residual
minerals from abandoned mines. The way this is performed is the leaching
solution is applied directly to the walls and the roof of the intact stope (an
underground excavation from which the ore has being removed) or else to the
rubble of the fractured workings. Insitu leaching has been successful to the
recovery of copper and uranium,
Dump leaching is also a
method employed for the extraction of metals but as in the case for copper it
is not a very fast or efficient process. The dumps often contain boulders and
large rocks which have a very low surface- to – volume ratio for the action of
bacteria. Also the interior of a large dump is low in oxygen, which is a
requirement for the oxidation of iron and sulphur compounds, and the
temperature can also rise to over 50 degree C. because the oxidation process
are exothermic. Also there is a significant channeling of the acidified water
as it percolates through the rocks, so the copper solubiislation is restricted
to only a minor portion of the dump. Despite these disadvantages dump leaching
is a low cost and a low-tech method or recovery.
Heap leaching is the most
popular metal extraction method used, in particular used for copper. In this
method the ore is heaped onto open air leach pads with a base of asphalt or
impervious plastic sheeting. The heaps are no more than a few meters high by a
few meters wide so as to allow the oxygen to diffuse to all parts of the heap
and reduce the build- up of heat from the leaching process. The heaps are
sprayed with sulphuric acid (for copper extraction) and with cyanide (for gold
extraction) which contain a fraction of the bacterial population, the rest
being attached to the mineral, in a controlled manner and the run off is
collected on the plastic shheting.When the desired metal concentration is
obtained, the rich liquor is pumped to the solvent extraction section and then
sent to the electro winning, where the fine metal is recovered or where the
purity can also be increased as described earlier in the bioleaching process.
The finely ground copper concentrate provides a large surface to volume ratio
and so promote bioleaching. Nutrients such as phosphates can be added to
promote growth if necessary. Heap leaching is a more environmentally friendly
option and is also more economic and it is especially attractive for mines in
remote areas or for small operations where only a small body of ore is to be
extracted. Although heap operation is simple and adequate to handle large
volumes of minerals, but their productivity and yields are limited due to the
severe difficult in maintaining an adequate process control.
Heap and dump leaching present a number
of advantages such as simple operation, low investment and operation costs and
acceptable yields. On the other hand the processes suffer from some serious
limitations such as the piled material is very heterogeneous and practically no
close process control can be exerted, except for intermittent pH adjustment and
the addition of some nutrients. The rates of oxygen and carbon dioxide transfer
that can be obtained are low, and extended periods of operation are required in
order to achieve sufficient conversions (Acevedo and Gentina, 1989).
Heap leaching can also be
used for the recovery of gold. Most of the world’s gold reserves contain the
metal bound up in the small particles in the rocks. After grinding up the rocks
the gold is then recovered by gravity separation or by treatment with cyanide.
But more often theses techniques are proving to be ineffective in the recovery
of the gold. For example if the gold is found associated with pyrite, usually
arsenopyrite, it can’t be recovered by gravity, while the cyanide reacts with
the pyrite before it can complex with the gold, making the process too
expensive and environmentally hazardous due to the large releases of cyanide.
The gold in this case can be recovered by oxidizing the pyrite at high pressure
in an autoclave or by roasting, followed by recovery with cyanide. Both
techniques are very expensive and also pose a serious environmental risk as the
liberated gases contain arsenic.
Sometimes the gold can also be
found as fine particles in carbonaceous sulphide ores. By grinding the gold is
liberating but it has a tendency to stick to the carbonaceous compounds making
it difficult to recover by conventional techniques.
It was discovered that
microorganisms could oxidize the gold bearing pyrite and arsenopyrite ores and
also the carbonaceous ones. Also this process of using microorganism’s means
that the cyanide quantities needed is sufficiently reduced. Commercial
bacterial oxidation of refractory gold ores (those that were difficult to
recover by conventional methods) was first used at Gencor’s Fairview South Africa
The technology used at Fairview Fairview South Africa
Bioreactors also have their own
drawbacks associated with their operation. The choice of material for their
construction is important and also the costs involved at maintaining them at
their correct temperature. The temperature inside the reactors can rise rapidly
to 50 degree C or higher, whereas the microorganisms predominantly prefer
temperatures of 20-40 degree C, so the reactors have d to be cooled to keep the
microorganisms alive. Although some plants are currently using extremely
thermophilic microorganisms which can grow at higher temperatures as is the
case with the Youanmi plant in Australia
3.9 Examples of current
Industrial Bioleaching Operations
■ Acid Mine Drainage
■ Rio Tinto ,
Spain
■ Dump Leaching
■ Bagdad , USA
■ Pinto Valley , USA
■ Sierrita , USA
■ Morenci , USA
■Heap Leaching
■ Cerro Colorado , Chile
■ Cananea ,
Mexico
■ Chuquicamata SBL, Chile
■ Collahuasi ,
Chile
■ Giilambone ,
Australia
■ Ivan Zar Chile
■ Morenci , USA
■ Punta del Cobre , Chile
■ Bioleaching of Gold Concentrates
■Ashanti, Ghana
■ Fairview , Zambia
■ Harbour Lights, Australia
■ Mount Leyshon , Australia
■ Sao
Bento , Brazil
■ Wiluna , Australia
■ Youanmi , Australia
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