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The Microbial hydrometallurgy and microbial mineral processing of metal sulphides is currently a well established technology. Over past years there has been a huge amount of developments with regards to the understanding of its both engineering perspective as well as fundamental approach with regards to the microorganisms. The huge diversity of the microorganisms, which has come into picture over the years of research and development have made the engineers to go beyond several limitations of working temperature to salt tolerance of the microorganisms in harsh conditions to deliver better technologies for the future operative plants. Today scientists have been able to deliver the various mechanisms involved in bioleaching but still there are facets to be really understood and more importantly on the front how lab scale research can be turned out into full scale operation by scaling up the research and optimizing the engineering aspects of the research. Most of the bioleaching operation has shown their productivity in commercial application of refractory gold concentrates using mesophilic microorganisms followed by the cyanide leaching to recover optimum amount of gold with an environment friendly method compared to the conventional method of roasting. Research in the area of chalcopyrite bioleaching is still continuing o solve the mysteries of jarosite precipitation and formation of passivation layer, which inhibits the copper recovery in a heap leaching of chalcopyrite by biological methods. Use of extreme thermophiles in chalcopyrite bioleaching is making a revolutionary movement to solve the mystery behind the scaling up the process, which could be possible to be solved in future. Bioleaching with other sulphide minerals together with Acid Mine Drainage (AMD) mitigation, which is a serious concern today, is taking is taking shape today in order to cater the needs of the mankind. However the biohydrometallurgy research seems to contribute to a greater extent in framing environmental friendly process with regards to hydrometallurgical operations in future and establish a developed technology to benefit human beings needs by its upcoming research and development. Keywords: Biomining, refractory gold, copper, chalcopyrite, bioleaching, nickel sulphide, biooxidation, acid mine drainage
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Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502
Vol. 1(10), 85-99, October (2012) Res.J.Recent Sci.
International Science Congress Association 85
Review Paper
Biohydrometallurgy and Biomineral Processing Technology:
A Review on its Past, Present and Future
Chandra Sekhar Gahan
1,2
, Haragobinda Srichandan
1
, Dong-Jin Kim
1
and Ata Akcil
3
1
Mineral Resource Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Yuseong-gu, Daejeon, 305-350, REPUBLIC OF KOREA
2
SRM Research Institute, SRM University, Kattankulathur - 603 203, Kancheepuram district, Chennai, Tamil Nadu, INDIA
3
Department of Mining Engineering, Mineral Processing Division, Suleyman Demirel University, TR32260, Isparta, TURKEY
Available online at: www.isca.in
Received 6
th
September 2012, revised 2012, accepted 2012
Abstract
The Microbial hydrometallurgy and microbial mineral processing of metal sulphides is currently a well established
technology. Over past years there has been a huge amount of developments with regards to the understanding of its both
engineering perspective as well as fundamental approach with regards to the microorganisms. The huge diversity of the
microorganisms, which has come into picture over the years of research and development have made the engineers to go
beyond several limitations of working temperature to salt tolerance of the microorganisms in harsh conditions to deliver
better technologies for the future operative plants. Today scientists have been able to deliver the various mechanisms
involved in bioleaching but still there are facets to be really understood and more importantly on the front how lab scale
research can be turned out into full scale operation by scaling up the research and optimizing the engineering aspects of the
research. Most of the bioleaching operation has shown their productivity in commercial application of refractory gold
concentrates using mesophilic microorganisms followed by the cyanide leaching to recover optimum amount of gold with an
environment friendly method compared to the conventional method of roasting. Research in the area of chalcopyrite
bioleaching is still continuing o solve the mysteries of jarosite precipitation and formation of passivation layer, which
inhibits the copper recovery in a heap leaching of chalcopyrite by biological methods. Use of extreme thermophiles in
chalcopyrite bioleaching is making a revolutionary movement to solve the mystery behind the scaling up the process, which
could be possible to be solved in future. Bioleaching with other sulphide minerals together with Acid Mine Drainage (AMD)
mitigation, which is a serious concern today, is taking is taking shape today in order to cater the needs of the mankind.
However the biohydrometallurgy research seems to contribute to a greater extent in framing environmental friendly process
with regards to hydrometallurgical operations in future and establish a developed technology to benefit human beings needs
by its upcoming research and development.
Keywords: Biomining, refractory gold, copper, chalcopyrite, bioleaching, nickel sulphide, biooxidation, acid mine drainage.
Introduction
The future development with regards to metal demand and
supply has motivated the current research and development to
emphasize more on the secondary metal resourses like
secondary ores and industrial byproducts or waste material
generated from various resources. The new technologies for
metal production should be novel and economically viable for
both the mining companies and entrepreneurs involved in the
process. Metal recovery by biological process has emerged as an
alternative technology today especially in metals like gold and
copper. The last decade has particularly observed the dramatic
increase in the metal prices to unprecedented high levels
followed by an unparalleled event of declining base metal
prices. Rapid increase in production costs, and finally a
sustained period of uncertainty in metal prices has developed
with time. This change in the metal prices has seriously
influenced the global mining companies. The mining industries
has always been observing the flip-flop occurring in the metal
prices exhibiting cycles of high and low metal prices, which
have forced the companies for the contraction of the industries.
The decline in the metal prices increases the cost of production
and often it is observed that the production is curtailed at times
and the mining plans are changed targeting higher metal grades
followed by cut off of the research and development
expenditures and sometimes lay-offs of research and
engineering personnel too. Biohydrometallurgical processes for
mine production and metals remediation have lower capital and
operating costs than competitive technologies and are therefore
economical to implement during mining down-cycles compared
to other processes. A comparative analysis was carried out to
observe how gold and copper biohydrometallurgical operations
took the role with the increase and decline of metal prices over
time.
Figures 1 and 2 illustrates the prices of gold and copper,
respectively ever since the inception of biomining. The reason
for selection of gold and copper is due to the full scale
operations of these two metals existing today in various
Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502
Vol. 1(10), 85-99, October (2012) Res. J. Recent Sci.
International Science Congress Association 86
industries today. The graphic representations show the
significant developments in biohydrometallurgy, where it is
peculiar to see how these technologies have been under
development for a decade or more before actually being put into
commercial practice.
Figure-1
Cumulative average gold price (US$/oz) in actual dollars
from 1968 through 2009 with significant
biohydrometallurgical applications (reprinted from
Brierley, 2010)
Figure-2
Spot average copper price (US$/lb) from 1950 through 2008
with significant biohydrometallurgical applications
(reprinted from Brierley, 2010)
Figures 1 and 2 also suggests that most biohydrometallurgical
innovations have been commercially implemented during times
of low metal prices, which could be interpreted to mean the
mining industry is more inclined to apply biohydrometallurgical
processes during leaner times
1,2
. The BioCOP™ technology
was, in fact, demonstrated as the copper price was rapidly
increasing. While increasing copper price was unlikely to be the
only reason why the technology was not fully commercialized
3
,
it may have been a contributing factor. Metal prices, of course,
are not the only motivating factor for employing
biohydrometallurgical processes
4
. Other drivers include, the
Cost of production as energy costs escalate, mining,
processing, and environmental costs substantially increase.
Some biohydrometallurgical methods for processing and
remediation are less energy intensive than alternative
technologies and can potentially reduce costs for the industry.
Also, biohydrometallurgical processing methods eliminate net
Smelter Royalties associated with smelting and refining and
potential penalty charges associated with smelting feeds with
impurities
5
. Sulfuric acid costs can be volatile depending on
demand and location
6
, while the bioleaching can be used in
some situations for on-site acid production to eliminate or
reduce acid purchases. Exploitation of ore deposits, which are
not amenable to conventional processes or otherwise difficult to
exploit for example the development of secondary copper
sulfide ore deposits, which may be too small or too remote to be
economically amenable to making a flotation concentrate either
for shipping or onsite smelting. Another example is processing
of sulfidic-refractory gold ore properties located in regions
where biooxidation technologies may be more suitable for cost
and work force reasons. A third example is ores deposits with
complex mineralogy, which could be difficult to treat by
conventional methods, where biohydrometallurgy could turn out
to be a viable alternative technology to go ahead. It can also be
reasoned here that this biohydrometallurgical techniques could
be a source of maximizing the use of existing infrastructure like
the installed solvent extraction-electrowinning plants in copper
extraction as bioleaching of copper sulfides allows to use these
existing facilities to maximize the use of the existing
investments by the industries
5
. The permissions for operating
mines by environmental authorities for the effluents generated
form the mining could be at times take very long time and
difficult too, but use of biohydrometallurgical technology could
ease the process for environmental issue over conventional
technologies as this process of biological approach is found to
be a green technology.
Historical Overview
Starting from the inception of the living world on this earth
about 3.8 billion years ago, microorganisms are known to have
appeared as the first living organisms when there was no free
oxygen with a reducing atmosphere composed of methane,
carbon dioxide, ammonia, and hydrogen. The microbial world at
that time utilized the available resources like methane or
hydrogen for their survival instead of oxygen for their
metabolism, which is currently referred as anaerobic
metabolism which is 30-50 times less effective compared to
oxygenic or aerobic metabolism. Since than throughout the
evolution microorganisms have proved to be amazingly
adaptable and enable themselves for their existence on this earth
starting from warm climatic conditions on the equatorial regions
up to the freezing cold climates in the poles. They posses best
qualities to adapt in all extreme conditions for which few of
them have been classified under the class of Extremophiles
comprising of acidophiles, methanogens and thermophillic. Few
of them also are very comfortable in pshychrophillic conditions
too. Microbes are well known to posses a much potential role in
the application of water purification and clearing up after oil
disasters and we have always been benefited from their ability
to dissolve minerals too.
Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502
Vol. 1(10), 85-99, October (2012) Res. J. Recent Sci.
International Science Congress Association 87
Figure
-
3
View of Acid Mine Drainage pollution caused in RioTinto
thriving with acidophiles
With regards to mineral dissolution of the minerals Rio Tinto
located in the south west of Spain stands to be one of the
remarkable sites with very hostile environment for the mineral
dissolving microorganisms. Several researches carried out at
Rio Tinto enabled the researchers to compare RioTinto with
another planet Mars in our Universe. Rio Tinto located in the
heart of the Iberian pyrite belt rich in sulphur has observed
natural mining caused by the invisible miners via pyrite
biooxidation for thousands of years generating huge amount of
acidic water overloaded with several heavy metals in it. The
acidic water stream flowing in Rio Tinto can be seen as deep red
color (figure 3), which gives an idea regarding its high iron
concentration pervading the entire landscape. Due to this highly
acidic stream and heavy metal laden water, it has been very
difficult to find any vegetation or animals in this region.
Microbial studies conducted on the red stream showed the
prevalence of millions and millions of microorganism’s
dominating the ecological niche. But the question that remains
unsolved is how do they survive, which is very simple to answer
i.e. these microbes thriving in the ecological niche oxidize the
pyritic minerals generating the required energy for their survival
and replication process. This metabolic phenomena occurring
with the microorganism in this environment stands to be a very
basic model which can be related to the type of microbes that
initiated the progress of life on the earth.
Biomining microorganisms are mostly autotrophic
microorganisms, which use iron or sulphur or both as their
major energy source and CO
2
as their carbon source unlike to
the heterotrophic microorganisms using organic carbon as their
major energy source. These biomining microbes are known to
be more comfortable in the acidic environment, infact they need
acid, live in acid and they even produce acids oxidizing the
pyrite and sulphur for which they are called acidophilic or acid
loving. The other thing which is most important of these
microorganisms is that they are the Archaea responsible for
breaking down or dissolution of the mineral by iron and sulphur
oxidation. In other words they are also described as lithotrophs,
which means a rock eater as they can actually eat the rocks and
this can be easily observed in any mining complex with
sulphidic minerals mostly pyritic. Energy rich ore oxidize
without bacteria but microorganisms make the process several
thousand times faster. Using oxygen form air the bacteria
extract the metal from the ore, this releases energy which they
use to grow and to multiply. The metals become soluble in
water and Rio Tinto was among the first places in the world to
exploit this phenomenon which benefited mankind in the later
part of the time. The oldest metal object found here takes back
to 5000 years. The question still remains unanswered regarding
who created them. Historians probably don’t agree with these
characteristics but they were all Iberians. The next cultures that
were established were the Tartessians, because they have
several references in Bible and it seems that king Solomon was
very fond of their metallurgical potential and later after the
Phoenicians, there was lot of wars to dominate this Mining
complex, and finally the Romans were there from first century
after Christ’s death and ruled there for about 400 years with big
mining operations. During that period slave Laborers were used
to provide the Roman Empire with iron silver, gold and copper.
The working conditions inside the mining complex was
completely inhuman and harsh, the slaves lived and worked in
total darkness chained to rock faces and survived at best for 4
years. After the Romans came the Visigoths than the Spanish.
But it wasn’t until the end of 19th century, that full scale mining
was resumed in response to the huge demand from metals
created by the industrialization of Europe. Thereafter the Mines
at Rio Tinto were taken over by English, who abandoned
Bioleaching in favor of roasting as they could successfully
extract same amount of copper in six months compared to long
period of 3 years via bioleaching. All extraction methods
became more brutal as blasting was used in Open pit mines
(figure 4).
Figure-4
View of Open Pit Mines in RioTinto
Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502
Vol. 1(10), 85-99, October (2012) Res. J. Recent Sci.
International Science Congress Association 88
The open Pit mining is well known to be less environmental
friendly because of its high pollution level and destruction of the
landscape, which can’t be restored by any another means. The
metal of interest present in the mineral complex inside the rocks
in the mountains is recovered by destroying the big mountain
into small pieces instead of finding alternative ways to recover
the metal by restoring the nature by preserving the mountains.
By removing a mountain we may end up with huge amount of
acid mine drainage, which will influence the water table as huge
amount of undesirable metals which gets dissolved in the acidic
solutions and passes into the water resources like lakes and
rivers found nearby the mining complex. This can lead to series
of diseases caused by hazardous metal pollution in the water
resources. The toxic sulphur dioxide fumes released from the
roasting process led to the severe sickness of the miners and
also started damage the vegetation surrounding the industrial
complex within a radius of 15-20 kms. In the year 1888 both the
miners and local farmers had enough to tolerate furthermore and
came out for a demonstration against the problems and issues
considering their lives at stake. Perhaps this agitation grew so
big that it was considered to be one of the world’s first pro-
environmental demonstrations so far, but unfortunately this
demonstrated was forcibly suppressed by the military forces at
that time leading to a death toll of about 200 men, women and
children. As a thought bioleaching was reintroduced the few
years later but the golden era Rio Tinto had it in and the mining
operation was soon closed down. Today Rio Tinto and its
bacteria is once again attracting interest of an unusual kind.
The bacteria posses a vital force strong enough to survive in
outer space became known in 1969. The crew of Apollo 12 had
retrieved a camera left behind on the moon by a non man space
probe two years earlier. The camera contained perfectly signed
individuals of Streptococcus myetis bacteria commonly found in
the nose and mouth of human beings. The space scientist were
intrigued by the discovery because if such bacteria can survive
on the moon, there could be a possibility of existence of a
similar forms of life on Mars, a planet quite similar to earth and
as they could find a similar condition in Rio Tinto suggest how
life emerged on earth. Rio Tinto is considered an interesting and
good geological analogue of Mars and the reason is that they
start to describe the mineralogy of Mars and in very specific
part of Mars called meridianpi has similar mineralogy as found
in Rio Tinto produced by biological activity. It has opened
window, that there is a place on earth there is this type of
biology that produces type of mineralogy obviously gives some
connection with Mars and RioTinto. Future space scientists can
expose their instrumental conditions in Rio Tinto similar to
those in space and these bacteria might just help the biologist
solve the mystery of life on Mars, but here on earth the bacteria
are already engaged in advancing development with regards to
Biomining. Nowadays the metals are so universally employed
we take them for granted and the global demand is rapidly
increasing. In the west consumption is 10 kg per person a year.
The corresponding figure for china is only 2 kg whoever
accounts for nearly all of the increased demand for copper.
Russia, Brazil, India and Indonesia are also growing rapidly and
in great needs of metals. Countries all over the world are
literally being hovered for ore deposits. The question is to know
how to find new resources and specially to be probably less
dependent on the importation of these metals. So there is a new
interest in producing metals in Europe because of that due to the
pressure that has been now imposed by the environment of the
emerging countries and their needs in metals. Europe one of the
first centers of the Industrialization and consequently has a very
historically well developed mineral processing metal
fabrication, metal forming business and is one of the world’s
largest consumers of metals. Europe unfortunately was not as
blessed with mineral resources as set in other places in the
world like South America, Africa, Australia and most of the
easily accessible lower cost higher grade minerals have been
depleted today. In Europe there are number of small and
medium size ore deposits which still haven’t been touched. So
far they haven’t been considered worth exploiting using
ordinary smelting process. Europe also has large quantities of
mining waste with the metal content that frequently exceeds that
of recently located ore deposits.
Biomining Technology
Biomining is the utilisation of biohydrometallurgy to process
metal ores. Biohydrometallurgy is essentially the application of
biotechnology to processing minerals. Technically, the process
is a branch of hydrometallurgy, but uniquely it involves the use
of microorganisms to generate chemical oxidants, such as ferric
iron and proton acidity. Biomining can be subdivided into
bioleaching and biooxidation operations. Bioleaching involves
the solubilisation of an insoluble metal sulfide to a soluble
metal, which can be recovered from the leachate. This is most
commonly used for the recovery of copper or uranium from
low-grade ores. Biooxidation utilises mineral oxidation, but in
this case the target metal remains in an insoluble phase.
Biooxidation is often used in the pre-treatment of gold
concentrates, prior to conventional cyanide-extraction.
However, the modern application of biomining was only
initiated in the 1960's with the construction and irrigation of
heaps for the recovery of copper at the Kennecott Bingham
Canyon Copper mine, Utah, U.S.A.
7
. Since the 1980's, there has
been a large expansion in the number of heap leaching
operations for copper recovery from low-grade ores, with many
operations initiated in Chile. Between 1980 and 1998, the
amount of the world's copper produced from biomining
operations increased from 10% to 25%
8
. Biomining was
originally seen a means of extracting metals from low-grade
ores, tailings and other mine wastes. Initially used for the
recovery of copper, since the 1960's it has also been used in the
recovery of uranium, and in the mid-1980 was developed for
use in the pre-treatment of gold-bearing ores
9
. The most
important of these operations are located in developing
countries, such as Chile, Indonesia, Mexico and Peru and
Zambia. Many developing countries have significant mineral
reserves and mining is often one of their main sources of
Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502
Vol. 1(10), 85-99, October (2012) Res. J. Recent Sci.
International Science Congress Association 89
income. Biomining, with its relatively low capital and running
costs, is ideally suited to such countries
10
.
Since the 1992 Earth Summit in Rio, the concept of sustainable
technologies and development has become very popular
8
.
Biohydrometallurgical extraction procedures find favour in this
respect as they are "almost without exception more
environmentally friendly" than physicochemical processes
11
.
While the environmental costs associated with mineral
extraction and primary processing, such as ore crushing and to
some extent mineral concentration, are comparable, the process
does not require the huge amounts of energy expended during
roasting or smelting and does not produce harmful gas
emissions. Care must be taken with the resulting leach solution,
which contains highly elevated concentrations of soluble metals
and acidity, as its release into the environment could have
serious consequences. However, in the long term, the waste left
over from biological processing may be less chemically active.
The longer that the leaching process is continued, the lower the
concentration of reactive sulfide minerals left in the resulting
waste. This means that the potential for chronic pollution
generation through subsequent microbial weathering is reduced.
Many metals can be recovered using biomining microbes
including, for example, copper from chalcocite (Cu
2
S), nickel
from pentlandite ((FeNi)
9
S
8
), zinc from sphalerite (ZnS), lead
from galena (PbS) and gold via the dissolution of gold-bearing
ores such as arsenopyrite (FeAsS), although not all are
commercially processed at this time. Biomining processes can
be broadly divided into two main types: irrigation-types and
stirred tank-types. Irrigation-type processes involve the
irrigation of crushed rock with a leaching solution, followed by
the collection and processing of the leachate or pregnant liquor
solution (PLS) to recover the target metals, commonly by a
solvent extraction/electro-winning (SX/EW) process. Stirred
tank-type processes use continuously operating, highly aerated
stirred tank bioreactors.
Figure-5
View of a pilot scale high temperature heap leaching operation in Mexico
Figure-6
A and B: Continuous stirred tank reactor (CSTR); C: Top view of a CSTR; D: Inside view of a CSTR
A
B
C
D
Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502
Vol. 1(10), 85-99, October (2012) Res. J. Recent Sci.
International Science Congress Association 90
The extraction of metal values from sulphidic ores and mineral
concentrates using microorganisms was termed as Biomining.
Biomining is the utilization of biohydrometallurgy to process
metal ores. Biohydrometallurgy is essentially the application of
biotechnology to processing minerals. Technically, the process
is a branch of hydrometallurgy, but uniquely it involves the use
of microorganisms to generate chemical oxidants, such as ferric
iron and proton acidity. Biomining can be subdivided into
bioleaching and biooxidation operations. Bioleaching involves
the solubilisation of an insoluble metal sulfide to a soluble
metal, which can be recovered from the leachate. This is most
commonly used for the recovery of copper or uranium from
low-grade ores. Biooxidation utilizes mineral oxidation, but in
this case the target metal remains in an insoluble phase.
Biooxidation is often used in the pretreatment of gold
concentrates, prior to conventional cyanide-extraction. In recent
years remarkable achievements have been made in developing
biomining to cater the interest of the mineral industry to match
the global demand for metals in the 21
st
century. Depletion of
high grade mineral deposits makes the traditional pyro-
metallurgical process uneconomical for metal recovery. The
search for alternative metal recovery processes to achieve
economical advantage over conventional methods motivated the
use of the biohydrometallurgical process, which in turn have
accelerated the willingness of the metal industries to use low
grade minerals
12
. Biomining is mostly carried out either by
continuous stirred tank reactors (figure 5) or heap reactors
(figure 6). Continuous stirred tank reactors are used for both
bioleaching and bio-oxidation processes collectively termed as
biomining.
Reasons for the preference of Stirred tank Biooxidation over
Heap leaching: Stirred tank biooxidation processes are mostly
applied on high grade concentrates for recovery of precious
metals like gold and silver, whereas the stirred tank bioleaching
process is used for the recovery of base metals like cobalt, zinc,
copper, and nickel from their respective sulphides, and uranium
from its oxides. Continuous stirred tank reactors are
advantageous and widely used due to the following reasons
13
.
The continuous flow mode of operation facilitates continual
selection of those microorganisms that can grow more
efficiently in the tanks, where the more efficient
microorganisms will be subjected to less wash out leading to a
dominating microbial population in the tank reactor. Rapid
dissolution of the minerals due to the dominance of most
efficient mineral degrading microorganisms utilizing the iron
and sulphur present in the mineral as the energy source.
Therefore there will be continuous selection of microorganisms
which will either catalyze the mineral dissolution or create the
conditions favorable for rapid dissolution of the minerals.
Process sterility is not required, as the objective of this process
is to degrade the minerals stating less importance on type of
microorganisms involved in it. Therefore, more importance lies
on an efficient dissolution process and the microorganisms that
carry out the dissolution process efficiently are typically the
most desirable ones. Continuous stirred tank biooxidation of
refractory gold concentrates and in one case on a cobaltic pyrite
concentrate is currently used in more than ten full-scale
operations using two different technologies with three more
plants coming up in the near future
12,14-17
. Several gold
biooxidation plants were commissioned over the last 20 years
with few new plants commissioned and is progressing fast with
rapid industrialization (table 1). Canadian-based BacTech
mining company’s bacox process is used for the treatment of
refractory gold concentrates
12
. Three plants using the bacox
process are in operation, with the most recent plant at Liazhou,
in the Shandong province of China, owned by Tarzan Gold Co.
Ltd
18,19
. Minbac Bactech bioleaching technology has been
developed jointly by bateman and mintek in Australia and
Uganda. Recently the BacTech Company has signed an
agreement on June 2008, to acquire Yamana Gold in two
refractory gold deposits in Papua New Guinea. BacTech Mining
Corporation have achieved significantly improved metal
recoveries from the test work carried out on the tailing materials
from the Castle Mine tailings deposit located in Gowganda near
Cobalt, Ontario. This metallurgical work is a precursor to
BachTech’s plan to build a bioleaching plant near Cobalt,
Ontario, to neutralize the arsenic-laden tailings prevalent in this
area, and at same time also to recover significant quantities of
Co, Ni and Ag present in the tailings. BHP Billiton Ltd operates
pilot and demonstration scale processes for the recovery of base
metals from metal sulphides of nickel, copper and zinc by
stirred tank bioleaching
20
. Bioleaching of zinc sulphides has
been widely investigated on laboratory scale by various
researchers
21-28
. The possibilities to process low grade complex
zinc sulphide ores through bioleaching have received much
attention and have been tested in pilot scale
25,29
. MIM Holdings
Pty, Ltd. holds a patent for a fully integrated process that
combines bioleaching of zinc sulphides with solvent extraction
and electrowinning of zinc metal
30
. New developments in stirred
tank processes have come with high temperature mineral
oxidation, which has been set up in collaboration between BHP
Billiton and Codelco in Chile
12
.
Stirred tank-type operations involve the processing of mineral
concentrates in large bioreactors, and offer much more control
than irrigation-based operations and therefore allow superior
leaching efficiencies in terms of rates and metal recovery.
Aeration, temperature and pH can be continually adjusted to
optimise microbial activity in a stirred-tank reactor. These are
usually arranged in series, with a continuous flow of material
into the first, which overflows to the next, and so on. Retention
time in the whole system is set to allow for sufficiently
complete microbial-mediated oxidation of the target minerals.
The feed usually consists of mineral concentrate, mixed with
water to a set pulp density, with a microbial inoculum and
additional nutrients. As with other similar forms of
biotechnological application (such as bioremediation), stirred-
tank reactors usually offer the most effective (though not
necessarily the most economic) level of processing. Most stirred
tank operations are employed for the biooxidation of refractory
Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502
Vol. 1(10), 85-99, October (2012) Res. J. Recent Sci.
International Science Congress Association 91
gold ores; the value of the gold produced displacing the higher
capital and running costs required for the implementation of
these processes. Conventionally, gold is solubilised from ores
and concentrates using cyanide. However, gold ores may be
refractory due to the presence sulphide minerals, such as with
gold-bearing arsenopyrite ores, which may occlude gold
particles from the cyanide solution. In such ores, less than 50%
of the gold may be recovered without pre-treatment.
Biooxidation is used to disrupt the sulphide mineral matrix,
making the gold accessible to the lixiviant. Total gold recovery
can be increased to over 95% through the use of such a
biooxidation process. Table 1 lists some of the commercial
stirred tank biooxidation plants pre-treating refractory gold
concentrates. Biooxidation of refractory gold concentrates in
continuous stirred tank reactors and bioleaching of copper and
nickel via heap reactors are some of the established and
commercialised technologies in present day use. Bioprocessing
of ores and concentrates provides economical, environmental
and technical advantages over conventionally used roasting and
pressure oxidation
12,17,31,32
.
Increasing demand for gold motivates the mineral exploration
from economical deposits and cheaper processing for their
efficient extraction. Different chemical and physical extraction
methods have been established for the recovery of gold from
different types and grades of ores and concentrates. Generally,
high-grade oxidic ores are pulverised and processed via
leaching, while refractory ores containing carbon are roasted at
500ºC to form oxidic ores by the removal of carbon due to
combustion and sulphur as sulphur dioxide gas. However, the
sulphidic refractory gold ores without carbon are oxidised by
autoclaving to liberate the gold from sulphide minerals and then
sent to the leaching circuit, where gold is leached out using
cyanide
33
.
In many cases pyrometallurgical processes for the pre-treatment
of refractory gold concentrates via roasting have been replaced
with continuous stirred tank reactors as a pre-treatment for
successful removal of iron and arsenic through biooxidation in
the global scenario today. The first biooxidation plant was
commissioned in 1986 by Gencor, at the Fairview mine in South
Africa. The BIOX
®
process developed by gencor, operates at
40-45°C and is used by most stirred tank operations
12
. In
contrast, plants utilising BacTech technology operate at
moderately thermophilic temperatures between 45 and 55°C.
Several more plants have been built, including a biooxidation
plant at Sansu, Ghana. Commissioned in 1994, and expanded
since, the plant processes 1000 tonnes of gold concentrate per
day, and earns nearly half of the country's foreign exchange
11
. A
commercial stirred tank operation at the Kasese Cobalt Kilembe
Mine in Uganda is used to recover cobalt from a 900 Kt dump
of cobaltiferous pyritic tailings stockpiled on the site during the
mine's operation between 1956 and 1982 (figure 7). The process
was developed by the Bureau de Recherches Géologiques et
Minières (BRGM), France.
Figure-7
Bioleaching plant at the Kasese mine, Uganda, for extraction
of cobalt from pyrite tailings
The plant processes some 245 tonnes of tailings per day,
recovering approximately 92% of the cobalt
15,34
. The BioNIC
®
process has been commercialised by BHP-Billiton for the
extraction of nickel from low-grade ores, and is based on the
BIOX process
7
. Pilot-scale plants in South Africa and Australia
have demonstrated the viability of the process, and Queensland
Nickel have decided to proceed with a plant aimed at processing
approximately 5,000 tonnes of nickel per year
35
. Biomining
using highly aerated, carefully controlled stirred tank
bioreactors is highly effective, with mineral decomposition
occurring within days rather than weeks or months as with
irrigation-type systems. However, due to the level of
engineering and process control involved, these are considerably
more expensive operations than irrigation-type processes.
Efficient aeration is difficult to achieve, and constitutes the
largest individual running cost. Another major constraint of
these systems is that only approximately 20% pulp densities can
be maintained
12
. At densities greater than this, efficient aeration
becomes very difficult, and shear forces due to the motion of the
impellers physically damages the mineral-leaching
microorganisms, affecting leaching efficiency.
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Table-1
Plants using continuous stirred tank biooxidation for pretreatment of refractory gold concentrates (Brierley, 2010)
Plants using continuous stirred tank biooxidation for pretreatment of refractory gold concentrates
Industrial plant and location & owner
Concentrate
treatment capacity
(tons)
Operating
years
Current status/Performance/Reasons for
closure
Fairview, Barberton, South Africa/Pan
African Resources
62 1986 - present Gold field’s BIOX
®
Sao Bento, Brazil/AngloGold Ashanti 150 1991-2008
Gold field’s BIOX
®
. A single-stage reactor was
used to pretreat concentrate for pressure
oxidation (under care and maintenance)
Harbour Lights, Western Australia 40 1991 - 1994
Gold field’s BIOX
. Ore deposit depleted
(decommissioned)
Wiluna, Western Australia/Apex Minerals 158 1993-present Gold field’s BIOX
®
Ashanti, Obuasi, Ghana/AngloGold Ashanti 960 1994-present Gold field’s BIOX
®
Youanmi, Western Australia 120 1994-1998 BacTech Bacox
Tamboraque, san Mateo, Peru/Gold Hawk
Resources
60 1998-2003 Gold field’s BIOX
®
Beaconsfield, Tasmania,
Australia/Beaconsfield Gold
~70 2000-present BacTech Bacox
Laizhou, Shandong Province,
China/Eldorado Gold
~100 2001-present BacTech Bacox
Suzdal, Kazakhstan/Centroserve 196 2005-present Gold field’s BIOX
®
Fosterville, Victoria, Australia/North gate
Minerals
211 2005-present Gold field’s BIOX
®
Bogoso, Ghana/Golden Star Resources 750 2006-present BacTech Bacox
Jinfeng, China/Eldorado Gold 790 2006-present Gold field’s BIOX
®
Kokpatas, Uzbekistan/Navoi Mining and
Metallurgy
1,069 2008-present Gold field’sBIOX
®
Coricancha, Peru 60 1998-20008
Gold field’sBIOX
®
Temporarily stopped
(under care and maintenance)
Heap bioleaching is a rapidly emerging technology for the
extraction of base metals from sulfide minerals. Significant
attention has been focussed on the development of bioheap
leaching in recent years
7
. Heap bioleaching is mostly practiced
on low grade copper ores with 1-3% copper and mainly on
secondary copper sulphide minerals such as covelite (CuS) and
chalcocite (Cu
2
S) (figure 8). In heap leaching, the crushed
secondary sulphidic ores are agglomerated with sulphuric acid
followed by stacking onto leach pads which are aerated from the
base of the heap. Then the ore is allowed to cure for 1-6 weeks
and further leached with acidic leach liquor for 400-600 days. A
copper recovery of 75-95% is obtained within this period of
time.
As the construction of heap reactors are cheap and easy to
operate it is the preferred treatment of low grade ores
36
.
Commercial application of bioheap leaching designed to exploit
microbial activity, was pioneered in 1980 for copper leaching
(figure 8). The Lo Aguirre mine in Chile processed about
16,000 tones of ore/day between 1980 and 1996 using
bioleaching
37
. Numerous copper heap bioleaching operations
have been commissioned
7
, since then Chile produces about
400,000 tones of cathode copper by bioleaching process,
representing 5% of the total copper production
38
. The
Talvivaara Mining Company Plc. (figure 9, 10 and 11) started
an on-site pilot heap in June 2005 and the bioheap leaching
commenced in August 2005. Talvivaara have started full
production since 2010. Production of nickel is approximately
33,000 tones and has the potential to provide 2.3% of the
world's current annual production of primary nickel. The first
shipment of commercial grade nickel sulphide started in
February 2009
39
.
Figure-8
A: Heap Bioleaching of Copper
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Figure-9 Figure-10
Heap leaching operations at Talvivaara Production process of heap leaching at Talvivaara, Finland
Figure-11
Flow sheet of the down stream processing at Talvivaara, for metal recovery
Heap leaching can be traced as far back as the 1600’s in Spain,
Germany, Sweden and China
40
. One of the most notable earlier
applications of bioleaching was at the Rio Tinto mines in south-
western Spain. At the beginning of the 1890s, heaps of low
grade copper ore were constructed and left for 1 to 3 years for
natural decomposition. Although, this practice was maintained
for several decades, the contribution of mineral solubilising
bacteria was not confirmed until much later. Reference to iron
(II) oxidation by Acidithiobacillus ferrooxidans was first made
in 1951
41
. In addition, in 1961, the leachate at the Rio Tinto
mines was found to contain Acidithiobacillus ferrooxidans
42
.
There was a slow progression to commercial application of
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biohydometallurgy (including bioleaching) where activity of
micro-organisms would be facilitated. In 1970, Lacey and
Lawson reported that the iron (II) oxidation by Acidithiobacillus
ferrooxidans was half a million to a million times faster than
abiotic chemical oxidation by dissolved oxygen
43
. As a result of
this and similar work, copper leaching from heaps was initiated
in 1980 and several copper heap bioleach operations have been
set up since then
42
. Irrigation-type processes can involve in situ,
dump, or heap bioleaching. In situ bioleaching describes the
process where metals are solubilised and recovered directly
from the ore body itself. This process was employed to recover
uranium from low-grade ore at the denison mine, ontario,
Canada. In this operation, blasted ore in an underground stope
was flooded intermittently with AMD, and aerated. The leach
liquor was removed after periods of about three weeks and the
uranium recovered. During 1988, this process recovered nearly
350 tons of uranium, at a then value of US$ 25M
9,44
. The
suitability of such operations is entirely dependent on the local
hydrogeology, which must facilitate good leachate recovery
without significant loss into the surrounding environment. Loss
of leachate to underlying soil, porous rock and groundwater
would have a severe environmental impact due to the chemical
compositions of these liquors. Dump leaching involves the
recovery of metals from dumps of very low-grade mine ores and
mine wastes. The Bingham Canyon biomining project in the
1950's is an example of this process; the largest of the dump
leaching operations on this site comprised four billion tonnes of
low-grade copper waste
11
. The Bala Ley plant, owned by
Codelco in Chile runs a dump leaching operation, where huge
quantities of low-grade copper ore are subjected to cycles of
preconditioning, irrigation, rest, conditioning and washing. With
each step taking up to a year to complete, a single cycle may run
for many years.
Heap leaching (figure 12) is similar to dump leaching, but
involves the construction of carefully designed heaps of ore,
usually of low-grade, in specially prepared areas. The ore is first
crushed and then agglomerated, usually with sulfuric acid
before being stacked in heaps up to 10 m high on pads lined
with an impermeable barrier, such as a high density
polyethylene liner. The design of a heap operation may include
aeration pipes, added during construction, to allow forced
aeration of the heap. A leaching solution, often the raffinate left
following metal extraction from the PLS, is used to irrigate the
heap from the top. This may or may not be supplemented with
inorganic nutrients and a microbial inoculum. The PLS may be
recycled to the top of the heap, as an “intermediate leach
solution”. The heaps are designed with the optimisation of
microbial activity in mind, and leaching efficiency is therefore
superior to dump leaching operations. Adjustments can be made
to the aeration rate, if forced aeration is employed, which may
help to control temperature as well as the availability of oxygen
and carbon dioxide. Irrigation can be controlled in terms of flow
rate and composition, in an attempt to ensure that sufficient
nutrients are supplied to the microbial population, without
saturating the heap
7
. Table 2 lists some copper heap leaching
operations. While heap leaching operations are mainly
employed for the bioleaching of copper, a heap leaching
operation was constructed for the biooxidation of refractory
gold ore by the newmont mining corporation at the gold quarry
mine in Nevada, USA. The process utilises a mixture of
mesophilic, moderately thermophilic and thermophilic
microorganisms, and allows low-grade ore containing as little as
1 g gold t
-1
to be processed economically
11,16
. Heap leaching
operations are almost exclusively used to treat graded but
unprocessed ores. However, GeoBiotics LLC have developed
the GEOCOAT™ process, which involves coating inert, support
rock with a thin layer of ore concentrate. This process offers
much shorter leaching times than standard heap leaching, while
avoiding the capital and running cost associated with stirred
tank operations. This process is in use at the Agnes gold mine in
South Africa
45
.
Irrigation-type processes allow only minimal control over
reaction conditions within the rock pile. These processes rely on
microbial activities to produce the ferric iron lixiviant ultimately
responsible for the extraction of the target metal from the ore.
This requires an adequate supply of oxygen and carbon dioxide,
which is difficult to achieve in a large heap. Internal
temperature is difficult to measure and control, and depends on
several factors, including heap height, local climate and
irrigation and aeration rates. It is also intrinsically linked to the
sulfide mineral content of the rock. The higher the sulfide
content, the higher the temperature is likely to become. Internal
temperatures between 65-80°C are not uncommon
16,46
.
Conversely, if the sulfide content is too low, the temperature
may not be high enough to allow for sufficiently rapid mineral
dissolution, rendering the heap uneconomical. The heterogenic
nature of heaps, with steep pH, nutrient and temperature
gradients creating different macro- and micro environmental
conditions adds the unpredictable performance of the heap as a
whole
47
. The BioNIC
®
process has been commercialised by
BHP-Billiton for the extraction of nickel from low-grade ores,
and is based on the BIOX process
7
. Pilot-scale plants in South
Africa and Australia have demonstrated the viability of the
process, and Queensland Nickel has decided to proceed with a
plant aimed at processing approximately 5,000 tonnes of nickel
per year
35
. Biomining using highly aerated, carefully controlled
stirred tank bioreactors is highly effective, with mineral
decomposition occurring within days rather than weeks or
months as with irrigation-type systems. However, due to the
level of engineering and process control involved, these are
considerably more expensive operations than irrigation-type
processes. Efficient aeration is difficult to achieve, and
constitutes the largest individual running cost. Another major
constraint of these systems is that only approximately 20% pulp
densities can be maintained
12
. At densities greater than this,
efficient aeration becomes very difficult, and shear forces due to
the motion of the impellers physically damages the mineral-
leaching microorganisms, affecting leaching efficiency.
Research Journal of Recent Sciences ______
_
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International Science Congress Association
Model of heap leaching process (Reprinted from BiomineWiki)
Industrial Copper heap bioleaching operations throughout the world (adapted from Brierley, 2010)
Industrial Copper heap bioleaching operations throughout the world*
Industrial plant
and location/owner
Lo Aguirre, Chile/Sociedad Minera Pudahuel
Mount Gordon (Formerly Gunpowder), Australia/Aditya Birla
Mt Leyshon, Australia (Formerly Normandy Poseidon)
Cerro Colorado, Chile/BHP Billiton
Girilambone, Australia/Straits Resources & Nordic Pacific
Ivan-Zar, Chile/ Compañía Minera Milpro
Punta del Cobre, Chile/Sociedad Punta del Cobre
Quebrada Blanca, Chile/Teck Resources
Andacollo Cobre, Chile/ Teck Resources
Dos Amigos, Chile/CEMIN
Zaldivar, Chile/Barrick Gold
Lomas Bayas, Chile/Xstrata
Cerro Verde, Peru/Freeport McMoran
Lince II, Chile/Antofagasta Plc
Monywa, Myanmar/Myanmar No.1 Mining Enterprise
Nifty Copper, Australia/Aditya Birla
Morenci, Arizona/Freeport McMoran
Lisbon Valley, Utah/Constellation Copper
Jinchuan Copper, China/Zijin Mining Group
Spence, Chile/BHP Billiton
Whim Creek and Mons Cupri, Australia strait Resources
Skouriotissa Copper, Cyprus/Hellenic Copper
Source Brierley consultancy LIC.
*Copper dump bioleach operations are not included in this table. About 7% of the world’s 17Mt of
copper is produced by heap
bioleaching. Another 8
_
________________________________
______________
International Science Congress Association
Figure-12
Model of heap leaching process (Reprinted from BiomineWiki)
Table-2
Industrial Copper heap bioleaching operations throughout the world (adapted from Brierley, 2010)
Industrial Copper heap bioleaching operations throughout the world*
and location/owner
Cathode copper production
(tons/year)
15,000
1980
Mount Gordon (Formerly Gunpowder), Australia/Aditya Birla
33,000 1991-
2008 (On care and maintenance)
Mt Leyshon, Australia (Formerly Normandy Poseidon)
750
1992
115,000
Girilambone, Australia/Straits Resources & Nordic Pacific
14,000
1993
10,000 - 12,000
1994
Punta del Cobre, Chile/Sociedad Punta del Cobre
7,000-8,000
75,000
21,000
10,000
150,000
60,000
54,200
27,000
Closed 2009 (high mining costs)
Monywa, Myanmar/Myanmar No.1 Mining Enterprise
40,000
16,000
1998
380,000
27,000 projected
10,000
200,000
Whim Creek and Mons Cupri, Australia strait Resources
17,000
8000
*Copper dump bioleach operations are not included in this table. About 7% of the world’s 17Mt of
bioleaching. Another 8
-
13% of the world’s copper is produced by dump bioleaching
______________
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Res. J. Recent Sci.
95
Industrial Copper heap bioleaching operations throughout the world (adapted from Brierley, 2010)
Operational status
1980
-1996 (Ore depletion)
2008 (On care and maintenance)
1992
-95 (stockpile depleted)
1993-present
1993
-2003 (Ore depletion)
1994
-present (currently leaching
primary ore)
1994 - present
1994 - present
1996 - present
1996 - present
1998 - present
1998 - present
1997 - present
Closed 2009 (high mining costs)
1998 - present
1998
– present (oxide/sulphide)
2001 - present
2006 - present
2006 - 2009
2007 - present
2006 - present
1996 - present
*Copper dump bioleach operations are not included in this table. About 7% of the world’s 17Mt of
13% of the world’s copper is produced by dump bioleaching
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Future Challenges in Biohydrometallurgy
Commercial challenges: Biomining technologies are mostly
developed by mining companies, biotechnology companies,
government laboratories, university research scientists and
engineers, and mining consultants. The journey from the
laboratory research to commercial application of biomining
technologies developed by these various organizations faces
much more bottlenecks or limitations together with very
difficult challenges. The total amount of time required to
research, develop, pilot and commercialize the technology,
which normally takes at least ten years or more as two decades
thereby leading to long-term commitment by Research and
development unit with proper cooperation from financial terms
and management issues. The biomining site are normally site
specific for which every technology developed needs an onsite
pilot plant followed by demonstration scale and finally leading
to a Full scale operation depending upon the conditions and the
results obtained which can be very costly affair. The biomining
technology should also be such a feasible technology, which can
either outcompete or at par to compete with the existing
technology with respect to every parameters taken into
consideration by the mining companies. To be specific the
bioleaching technology should be able to compete in making
required concentrate fulfilling industries requirement at par to
pressure oxidation, roasting/smelting, and emerging chemical
leaching processes. Similarly metals bioremediation should also
be competitive to the existing technology of alkaline
precipitation, ion exchange and reverse osmosis, which are used
by the industry under special conditions. The rate of success is
also another risk factor which needs to be incorporated during
the study. The risks involved in commercializing new
technologies are detailed by others
48,49
. Failure in technology is
a common thing for any research organization or mining
company but it indirectly hampers the name and fame of the
institutions involved in final design and commissioning. The
new technologies always requires and urges for a huge capital
investment since the process units may need to establish the
front-end and the back-end of the actual biomining process.
Intellectual property Rights (IPR) is another very important
factor as Mining companies often balk at paying licensing fees
or royalties for technology due to various reasons like changes
in ore type and newer processes negating the value of the
technology; licensing fees may stifle business deals, mining
companies may be reluctant to open production logs which is
necessary to assess licensing fees. Licensing fees or royalties
for biomining processes is impossible to negotiate particularly
for metal production as it could be many factors which can
affect the production, which are not in the hands of the owner.
Process guarantees for mine production and environmental
biotechnologies is the most difficult part as discussed above.
Finally the availability of skilled engineers and scientists is
always been a problem lying with mining companies
50
.
Technical challenges: The technical challenges and
opportunities faced by research and development units are the
bioheap leaching of primary sulfide minerals despite of few
steps have been taken to understand and progress with respect to
chalcopyrite
5,51
. However much more attention has to be given
to chalcopyrite biochip leaching to understands the problems
faced during the leaching process like reasons for aestivation
occurring in heaps due to jar site precipitation and iron
hydroxides and which re the jar site formers and the diffusion
barriers for the leachant to progress the leaching and more
importantly the leaching kinetics and finally how to get
optimized conditions suitable for luxuriant bacterial growth and
good recovery of copper minerals and economic feasibility.
Emphasis should also be given in understanding the
fundamentals of the slightly reducing conditions that can occur
within the heap when thermophiles are used for dissolution of
chalcopyrite and other primary copper sulfide minerals
50
.
Presence of silicate minerals bound to the complex polymetallic
sulphides has been a big issue in the design of heap
bioleaching
51,52
. Bioheap leaching model development,
integration, and validation developed via heap leaching aspects
taking hydrology and heat balance into account need to be given
a through thought to understand mathematical modeling aspects
to use the process in robust conditions. Other direction which
could be looked into lies in better understanding of secondary
copper sulfide heap leaching even though the crushed ore heap
leaching of secondary copper sulfides has been widely used
over a decade lacking some information in production issues.
Further addition to all the aspects discussed above a lot of
questions lies unanswered like the time taken by the
microorganisms prevalent in the bioheap conditions where the
source of inoculum is the raffinate together with natural growth
of microbes in the stacked ore. Benefits of inoculation of
microorganisms in the heap together with extent of aeration
requirement in the heap to get better recovery and amount of
aeration required. Is the temperature issue important in the heap
bioleaching of secondary sulphides and the reasons behind it as
slow rate of oxidation of sulfides in the absence of pyrite, which
needs to be solved? Finally the microbe-mineral interface
interactions followed by mechanisms involving in the heap with
respect to galvanic interactions, oxidation-reduction potential,
and pH and dissolved metals ion concentration leading to toxic
effects on the microbes together with down steam processing
issues. In-situ leaching is one of the growing concerns in each
and every nick and corner around the world as urbanization
have forced human habitation nearby the mining operation
which have made it very important to decrease mining foot
prints. In-situ mining would drastically decline the impacts of
mining on human habitation but this issue needs to be retrospect
and efficiently accomplished and is of course a big challenge for
tomorrows mining organizations and intellectuals.
Technological advancement is required to development
sustainable technologies to treat decommissioned cyanide-
leached heaps by rotating biological contactors by developing
methods to treat cyanide-, thiocyanate-, and metal-contaminated
waters resulting from gold treatment
53
. However, the technology
has not yet been successful and developed and needs to be
evaluated considering various environmental impacts, but
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Biotechnological approach can lead to a huge cost savings in
return. Technology for stabilizing sulfide-bearing wastes is also
another aspect to be looked into, which tends to be huge
challenge to future biomining professionals.
Biohydrometallurgy Future Prospects
Biomining is going to take a major role both in
biohydrometallurgical operations and biomineral processing
together with bioremediation as an advantageous technology to
deal with the problems and issues related to the rising
environmental concerns around the globe. Biomining itself
tends to be an economical and technically viable technology for
small deposits which are unable to support huge financial
crunch involved in the process of extracting metals values and
remediating polluted industrial sites. It also offers an
opportunity as an auxiliary process for on-site generation of acid
for adjunct operations, such as base metal oxide heap leaching,
while in some cases might be more feasible taking
environmental concerns into act. Though commercial success in
certain minerals have been seen for years but technical
challenges are huge which is unsolved till date. Emphasis has
given to biooxidation pretreatment of sulfidic-refractory gold
concentrates in stirred-tank reactors together with dump and
heap bioleaching of secondary copper sulfide ores and Nickel
sulphides while development is on its way to reach success
with regards to heap leaching of low-grade chalcopyrite ores
and Uranium oxides
5,54
. A huge amount of opportunities still
lies in the biomining technology to be fully realized such as
bioleaching of zinc sulphides and black shale
55-57
. Further more
to add up with the future is in-situ leaching using microbial
processes.
Conclusion
Biohydrometallurgy and Biomineral processing is well known
for its application in a variety of base-metal sulphides, mostly
either in full scale operation or demonstration scales in various
countries around the globe. One of the interesting things
observed in his Bioprocessing minerals and metallurgy industry
is its environmental friendly process together with economical
process. Biohydrometallurgy application especially in case of
refractory gold cconcentrate has been replacing roasting plants
in recent years. The replacement of roasting plants in countries
like China has shown the progress of biohydrometallurgy as a
promising technology for future. Utilisation of bioleaching
process for treatment of industrial and municipality waste is also
taking its pace slowly and steadily. The use of
Biohydrometallurgy techniques for the treatment of low grade
base-metal dumped at mines site, which are in fact costly to
treast by smelters have considered biopprocessing as
advantageous process. Sometimes it has been observed that
complex ploymetallic ores containing more than one metal
values makes the flotation process difficult to produce high
value concentrates, therefore bioleaching of several metal values
prior to flotation helps the selective preparation oc concentrates.
It is expected that research and industrial developments via heap
bioleaching of low-grade primary ores and concentrates together
with tank bioleaching and biooxidation makes a greater leap in
the technological advancement of Mineral and metal industry in
the days to come.
Acknowledgements
Authors are thankful for the research support provided by
leading foreign research institute recruitment program through
the national research foundation of Korea (NRF) funded by the
ministry of education, science and technology (MEST) (2011-
00123). Financial support from the research project of the Korea
institute of geoscience and mineral resources (KIGAM) and the
energy and mineral resources engineering program grant funded
by the ministry of knowledge economy, Korea (GP2011-001-2)
is gratefully acknowledged.
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... The first biooxidation unit for the pretreatment of gold ores was commercialized in 1986 by Gencor at the Fairview mine in South Africa (van Aswegen et al. 1991). Due to the minimal ecological impact on the environment and a number of other advantages, tank biooxidation is successfully used to remove iron and arsenic from gold-bearing ores, gradually replacing the indicated physicochemical and pyrometallurgical technologies (Gahan et al. 2012). ...
... Heap leaching of copper is widely used in the biomining industry, but all existing technologies are based on the extraction of copper from a secondary mineral-chalcocite (Cu 2 S) (Olson et al. 2003;Gahan et al. 2012). Chalcocite is readily leached under the action of protons to form covellite (CuS) (Eq. ...
Chapter
Biohydrometallurgy is a modern, steadily developing alternative metal production technology based on the use of microorganisms and their metabolic products, such as ferric iron, sulfuric acid, etc. for the extraction of metals from ores. Microbiological processing of ores and concentrates has economic, technical and, most importantly, environmental advantages over traditional technologies. Heap leaching is successfully used for recovery of copper from a secondary mineral—chalcocite (Cu2S). However, the main world reserves of copper are found in the form of chalcopyrite (CuFeS2). Chalcopyrite is the most refractory mineral and undergoes chemical or biological oxidation at a very low rate. One of the most common ways to enhance copper extraction from chalcopyrite is the use of thermophiles. Besides, the intensity of biooxidation of sulfide minerals depends on the pH, redox potential, Fe²⁺/Fe³⁺ ratio, metals ion concentration and the microorganisms used. It was revealed that the mixed cultures and consortia of moderate thermophilic microorganisms were more efficient and stable in the oxidation of chalcopyrite than pure cultures. From this point of view, developing and optimizing microbial associations for use in commercial copper leaching systems remain an important challenge. In this paper bioleaching of chalcopyrite by pure and mixed cultures of moderate thermophilic bacteria S. thermosulfidooxidans and thermotolerant sulfur or iron oxidizing bacteria L. ferriphilum CC, as well as the influence of physicochemical factors on this process have been investigated.
... The first biooxidation unit for the pretreatment of gold ores was commercialized in 1986 by Gencor at the Fairview mine in South Africa (van Aswegen et al. 1991). Due to the minimal ecological impact on the environment and a number of other advantages, tank biooxidation is successfully used to remove iron and arsenic from gold-bearing ores, gradually replacing the indicated physicochemical and pyrometallurgical technologies (Gahan et al. 2012). ...
... Heap leaching of copper is widely used in the biomining industry, but all existing technologies are based on the extraction of copper from a secondary mineral-chalcocite (Cu 2 S) (Olson et al. 2003;Gahan et al. 2012). Chalcocite is readily leached under the action of protons to form covellite (CuS) (Eq. ...
Chapter
Metal-rich natural and artificial habitats are extreme environments for the development and evolution of unique microbial communities, which have adapted to the toxic levels of the metals. Diverse bacterial groups have developed abilities to deal with the toxic metals by bioaccumulation of the metal ions inside the cell actively or passively, extracellular precipitation, efflux of heavy metals outside to the microbial cell surface, biotransformation of toxic metals to less toxic forms, and metal adsorption on the cell wall. Metalophilic microbes are found in all bacterial and archaeal groups studied, but mostly appear among aerobic and facultative anaerobic chemoheterotrophic and chemolithoautotrophic microorganisms of the Bacillus, Pseudomonas, Staphylococcus, Actinobacteria, Cuprividus, Acidobacterium, Acidithiobacillus, Thiobacillus, Ferroplasma, and Sulfolobus genera. The phenomenon of microbial heavy metal resistance has fundamental importance and is particularly relevant in microbial ecology, especially in connection with the roles of microbes in biogeochemical cycling of heavy metals and in the bioremediation of metal-contaminated environments. The heavy metal resistance mechanisms and different applications of metal resistant/metalophilic bacteria and archaea have been expounded deeply in this chapter.
... Nowadays, scientists have the ability to deliver various mechanisms involved in bioleaching, but still some facts remain to be really understood and more importantly, how lab scale research can be turned into full-scale operation by scaling up the research and optimising the research engineering aspects. The extreme thermophiles usage in chalcopyrite bioleaching is making a revolutionary movement for solving the mystery behind the process of scaling up (Gahan et al., 2012). ...
... Therefore, sterility is not needed in the systems, owing to the microorganisms' continuous selection that will catalyse the metals dissolution. Biooxidation of refractory gold concentrates is currently used by CSTR in more than ten full-scale plants (Gahan et al., 2012). A rotating-drum reactor, as an alternative to the stirred-tank reactor, has technical advantages in E-wastes treatment at high pulp densities and reduces global energy consumption. ...
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Full-text available
Nowadays, large amount of municipal solid waste is because of electrical scraps (i.e. waste electrical and electronic equipment) that contain large quantities of electrical conductive metals like copper and gold. Recovery of these metals decreases the environmental effects of waste electrical and electronic equipment (also called E-waste) disposal, and as a result, the extracted metals can be used for future industrial purposes. Several studies reported in this review, demonstrated that the biohydrometallurgical processes were successful in efficient extraction of metals from electrical and electronic wastes. The main advantages of biohydrometallurgy are lower operation cost, less energy input, skilled labour, and also less environmental effect in comparison with pyro-metallurgical and hydrometallurgical processes. This study concentrated on fundamentals and technical aspects of biohydrometallurgy. Some points of drawbacks and research directions to develop the process in the future are highlighted in brief.
... Бактериально-химическими методами добывается около 20% меди и значительная часть урана (США, Канада, Мексика, Перу, Испания, Австралия и др.). Функционирует около 15 про-мышленных установок бактериального выщелачивания в восьми странах (ЮАР, Австралия, Бразилия, США, Канада, Замбия, Гана, Россия) [28,29]. Микробиологами выявлено множество бактерий и архей, ускоряющих процесс окисления сульфидных минералов [30][31][32][33][34][35][36][37] (таблица). ...
Article
Full-text available
Acidophilic chemolithotrophic microorganisms are used in biohydrometallurgy for the extraction of metals from sulphide ores. Some types of microorganisms belonging to this group are capable of generating electricity under certain conditions. This circumstance determined a recent upsurge of research interest in their use in biofuel cells. Under a constant supply of the substrate to the bioelectrochemical system, acidophilic chemolithotrophic microorganisms are capable of producing electricity for a prolonged period of time. The use of extremophiles in microbial fuel cells is of particular interest, since these microorganisms can serve as bioelectrocatalysts at extreme pH, salinity and temperature, while the vast majority of microorganisms are unable to survive under these conditions. Therefore, selection of optimal conditions and approaches to controlling the work of acidophilic chemolithotrophic microorganisms in such fuel cells is of particular importance. On this basis, a technology for the simulteneous bioleaching of metals from poor ores and the generation of electricity can be developed. Biofuel cells operating at low pH values using acidophilic chemolithotrophic microorganisms are yet to be investigated. The number of studies on acidophilic electroactive microorganisms is very limited. In this regard, the purpose of this review was to consider the prospects for the use of acidophilic chemolithotrophic microorganisms as bioagents in microbial fuel cells. The reviewed publications demonstrate that chemolithotrophic microorganisms can act as both anodic (metal-reducing, sulphur-oxidizing microorganisms) and cathodic (metal-oxidizing prokaryotes, sulfate reducers) highly efficient bioagents capable of using mining wastes as substrates.
... Biomining is a more environmentally friendly biotechnology for metal recovery from sulfidic ores than traditional pyrometallurgical techniques in which products from microbial metabolism are harnessed as chemical oxidants, such as ferric iron (Fe 3+ ), to leach or solve high-value metals including gold, copper, zinc, nickel, and cobalt (Gahan et al. 2012;Brierley 2016). In addition, biooxidation refers to a type of biomining that is used as a pre-treatment of gold-containing refractory ores and concentrates. ...
Article
Full-text available
Biooxidation of gold-bearing refractory mineral ores such as arsenopyrite (FeAsS) in stirred tanks produces solutions containing highly toxic arsenic concentrations. In this study, ferrous iron and inorganic sulfur-oxidizing Acidithiobacillus strain IBUN Ppt12 most similar to Acidithiobacillus ferrianus and inorganic sulfur compound oxidizing Acidithiobacillus sp. IBUNS3 were grown in co-culture during biooxidation of refractory FeAsS. Total RNA was extracted and sequenced from the planktonic cells to reveal genes with different transcript counts involved in the response to FeAsS containing medium. The co-culture’s response to arsenic release during biooxidation included the ars operon genes that were independently regulated according to the arsenopyrite concentration. Additionally, increased mRNA transcript counts were identified for transmembrane ion transport proteins, stress response mechanisms, accumulation of inorganic polyphosphates, urea catabolic processes, and tryptophan biosynthesis. Acidithiobacillus spp. RNA transcripts also included those encoding the Rus and PetI proteins involved in ferrous iron oxidation and gene clusters annotated as encoding inorganic sulfur compound metabolism enzymes. Finally, mRNA counts of genes related to DNA methylation, management of oxidative stress, chemotaxis, and motility during biooxidation were decreased compared to cells growing without mineral. The results provide insights into the adaptation of Acidithiobacillus spp. to growth during biooxidation of arsenic-bearing sulfides.
... Several studies focused on the development of (bio-)hydrometallurgical processes to efficiently recover Co and/or Ni from mine tailings or metallurgical residues [10][11][12][13][14]. Bioleaching processes, which involved the use of microorganisms to convert insoluble metal sulfides to soluble metal sulfates, showed great potential to recover Co and/or Ni from sulfidic mine tailings due to their relative simplicity of operation, and low capital and operating costs [10,[15][16][17][18]. However, these processes required long retention time (few days to several weeks) and their efficiencies can be quite variable, especially when applied to tailings containing low sulfur contents [10,19]. ...
Chapter
Tailings from inactive gold mines, that are not yet successfully restored (generation of As- and Co-contaminated neutral mine drainage), represent a promising secondary source of strategic metals including Co and Ni. Three different mine tailings (sites A, B and C) from CobaltCobalt Mining Camp were collected and characterized. Preliminary chemical leachingLeaching tests were conducted with inorganic acids (HCl, H2SO4 and HNO3) to solubilize Co and Ni at different concentrations (0.01–0.5 N). The influence of the number of the leachingLeaching steps on the recovery of Co and Ni was also evaluated. Promising concentrations of Co (0.7%) and Ni (0.3%) were reported in tailings from site A, while lower concentrations were measured in tailings from sites B and C (0.02–0.1%), requiring pre-concentration steps (not tested in this preliminary study) before leachingLeaching to reduce operating costs. More than 85% of both Co and Ni were solubilized from tailings from site A after only 30 min using H2SO4 (0.25 N) at room temperature. Lower efficiencies (36–62%) were observed for tailings from sites B and C, which can be partially explained by the higher amounts of acid-consuming minerals present in the gangue. Additional experiments are required to better understand the mechanisms involved in Co and Ni solubilization and to optimize operating conditions in terms of Co and Ni recovery.
Article
A response surface capable of describing the extraction of copper with high statistical confidence (R² = 0.9973) was obtained using a central composite factorial design (CCD). The parameters used were the initial concentration of Fe²⁺ ions ([Fe²⁺]i) and pulp density (ρpulp). The results evidenced that chalcopyrite leaching was strongly influenced by the solution potential, which was a function of the [Fe²⁺]i:ρpulp ratio. The optimal parameters obtained for maximizing the copper extraction percentage were those that satisfied a [Fe²⁺]i:ρpulp ratio of ≈ 80 (mmol L⁻¹/%), in the range from 200 to 398 mmol L⁻¹ of Fe²⁺. The [Fe²⁺]i:ρpulp ratio of ≈ 80 allowed an optimal range of solution potential for most of the experiment duration, which provided a copper extraction of 91 ± 3% in 28 days, under moderate conditions. The leaching residues were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffractometry (XRD), and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS). The mathematical model, together with the calculated Nernst potentials of the main oxidation–reduction reactions of chalcopyrite, indicated that the copper extraction was governed by experimental conditions that favored chalcopyrite reduction coupled with the chalcocite oxidation reaction. Hypotheses to explain the reasons for certain experimental conditions that could increase or decrease chalcopyrite dissolution were formulated and are extensively discussed. These findings contribute to the development of new routes for the processing of chalcopyrite mineral.
Article
Inactive mines provide a great source of bacterial diversity for studying acidophilic communities and their biotechnological applications, but prospecting of these anthropogenic environments in Colombia has been limited. Conventional microbiological methods were used to isolate acidophilic bacterial strains from effluents emanating from the Colombian gold mine ‘El Zancudo’ (Titiribí, Antioquia). Despite the drainage waters having circumneutral pH, all of the isolated strains were phylogenetically related to the extreme acidophile Acidithiobacillus genus. However, based upon 16S rRNA gene sequences the mesophilic sulfur-oxidizing indigenous strains could not be assigned to a species. Pure cultures were selected by screening in medium with soluble inorganic arsenic (III) and their mineral-oxidative activity was evaluated at 30 °C in Erlenmeyer flasks with arsenopyrite ore under rotary shaking conditions. The indigenous strains were able to catalyze arsenopyrite oxidation in a mixed culture with a pulp density of 10%, maintaining their growth in the presence of >80 mM leached arsenic. This research provides information regarding the isolation of arsenic resistant bacterial communities from neutral effluents from El Zancudo mine and the possibility of the isolated strains to be useful in the biooxidation pretreatment of refractory gold-bearing arsenopyrite ores and concentrates.
Article
The copper, cobalt and nickel ores are still currently mined in the world. Its complex mineralogy creates extraction challenges by means of conventional metallurgical methods. Meanwhile, dealing with mesophilic strains in leaching process requires a compromise between solid loading and microbiota activity and growth. That is why, the influence of solid loading with fine or coarse particulates, the cell disturbance during the metal–microbes interactions depending upon the influence of gangue nature as well as metallic ions concentration on bacterial tolerance and the chemical and biological pathways involved in bioleaching mechanism of complex ores are summarised in detail in this paper. The current trends in mechanism research and diverse discovered set of microbiota and bacterial population coupled with bacterial adaptation methods contribute to optimise and improve the metals leaching performance and knowledge. In addition, the different existing complex mineralogical structures elaborate a main indirect mechanism with two different transitory mechanisms, before metal is converted into metal sulphate as wealthily explained in this comprehensive review. More data for cost analysis concomitant with extraction efficiency of metals using mesophilic bioleaching process are needed. However, it does not mean that other options are excluded in order to set a bio-hydrometallurgical chain. In fact, to consider also the concentration and purification of the pregnant leaching solution via phase separation and solvent extraction will be helpful. This obeys to the idea of option trees, where possible options are then systematically gaged with respect to critical criteria.
Chapter
Biotechnology relevant to gold exploration, mining, recovery, and waste disposal is illustrated with respect to microbiological aspects of gold mineralization, Biooxidation of refractory sulfide ores and concentrates, cyanide-free gold dissolution, and biodegradation of cyanide containing effluents. Current industrial status of technological innovations in the bioreactor processing and heap bioleaching of refractory sulfide ores and concentrates are discussed. Biodetoxification and degradation of cyanides in waste tailings and waters are critically analyzed with examples from industrial practice. Prospects for direct biodissolution of gold are brought out. Recovery of gold from spent leach cyanide solutions and electronics wastes is examined. Bright future prospects for Biotechnology in gold exploration, mining, extraction, and waste disposal are emphasized.
Article
Full-text available
Cueva de Villa Luz (a.k.a. Cueva de las Sardinas) in Tabasco, Mexico, is a stream cave with over a dozen H2S-rich springs rising from the floor. Oxidation of the H2S in the stream results in a abundant, suspended elemental sulfur in the stream, which is white and nearly opaque. Hydrogen sulfide concentrations in the cave atmosphere fluctuate rapidly and often exceed U.S. government tolerance levels. Pulses of elevated carbon monoxide and depleted oxygen levels also occassionally enter the cave. Active speleogenesis occurs in this cave, which is forming in a small block of Lower Cretaceous limestone adjacent to a fault. Atmospheric hydrogen sulfide combines with oxygen and water to form sulfuric acid, probably through both biotic and abiotic reactions. The sulfuric acid dissolves the limestone bedrock and forms gypsum, which is readily removed by active stream flow. In addition, carbon dioxide from the reaction as well as the spring water and cave atmosphere combines with water. The resultant carbonic acid also dissolves the limestone bedrock. A robust and diverse ecosystem thrives within the cave. Abundant, chemoautotrophic microbial colonies are ubiquitous and apparently act as the primary producers to the cave's ecosystem. Microbial veils resembling soda straw stalactites, draperies, and 'u-loops' suspended from the ceiling and walls of the cave produce drops of sulfuric acid with pH values of <0.5-3.0 ±0.1. Copious macroscopic invertebrates, particularly midges and spiders, eat the microbes or the organisms that graze on the microbes. A remarkably dense population of fish, Poecilia mexicana, fill most of the stream. The fish mostly eat bacteria and midges. Participants in an ancient, indigenous Zoque ceremony annually harvest the fish in the spring to provide food during the dry season.
Article
Full-text available
Bio-oxidation is a well-established technology for the pre-oxidation of refractory gold-bearing concentrates in agitated reactors, as well as for the heap leaching of crushed secondary copper sulphide ores. Smelting seems set to remain the economically-preferred route, in the short- to medium term, for the processing of clean, high grade copper concentrates. However, bioleaching offers one hydrometallurgical alternative for base metal recovery in cases where, for example (a) only a very low grade flotation concentrate can be produced at an acceptable recovery, (b) penalty elements in the concentrate eliminate the smelting option, (c) geographical location or road, rail and/or harbour logistics prevent the economic transport of the concentrate to a smelter, or (d) authorities require local value-addition. The flexibility and economics of direct agitated bioleaching could be greatly enhanced if control could be exercised over the extent to which the intermediate elemental sulfur, formed during base metal sulphide oxidation, is oxidized to sulfate. Furthermore, since bioleaching produces large quantities of gypsum residues from neutralization with lime and limestone, maximising the density of the gypsum residues deserves specific attention during the process design of an agitated bioleach process. Indirect bioleaching can provide a relatively low-energy processing option with virtually perfect and self-regulating acid and iron balances for concentrates of secondary copper sulphides, or of chalcopyrite concentrate that leaches under low redox potential at moderate temperature. That is provided a suitable configuration is chosen for the bacterial ferrous iron oxidation step so that it can tolerate iron precipitation. Whether copper heap bioleaching is possible on ore in which chalcopyrite and pyrite are the major minerals depends on the reactivity of the pyrite to reach sufficiently high temperatures for acceptable leach kinetics as well as the feasibility of simultaneously finding an economic combination of crush size for liberation, acid consumption at temperature, heap height, and permeability to irrigation liquor and air. Temperatures of the order of 80°C and above are more readily achievable in heaps with lift heights greater than the normal 4 to 8 m, and pyrrhotite provides a more favorable heat source than pyrite. More refined laboratory scale test procedures and data interpretation is needed for predicting the acid consumption during commercial scale heap leaching. This requirement is more critical where high temperature heap bioleaching needs to be considered due to the greater acid-gangue reaction at higher temperature. Operator-advisory software is now available for the administration of a large number of cells under various stages of heap leaching that require counter-current leaching and manipulation of the irrigation and aeration rates to maximize heat accumulation within the operating range of the drippers/sprinklers while also controlling the copper tenor in the liquor feed to solvent extraction. As more challenging ore-types are being explored, classification or dense medium separation steps may more frequently be incorporated into heap bioleaching circuits for either the separate treatment of different classification products or the treatment of only a concentrated portion of the ore mined. Some of the uranium ores currently being explored will almost certainly be exploited by heap leaching. The employment of microbial oxidation activity in uranium ore heaps is suggested as a means of maintaining a high redox potential for the leaching of reduced uranium minerals in such heaps.
Article
Hydrothermal simulation experiments were performed with contemporary sediments from Lake Chapala to assess the source of the lake tars. The precursor-product relationships of the organic compounds were determined for the source sediments and their hydrous pyrolysis products. The pyrolysis products contained major unresolved complex mixtures of branched and cyclic hydrocarbons, low amounts of n-alkanes, dinosterane, gammacerane, and immature and mature hopane biomarkers derived from lacustrine biomass sources. The results support the proposal that the tar manifestations in the lake are not biodegraded petroleum, but were hydrothermally generated from lacustrine organic matter at temperatures not exceeding about 250 °C over brief geological times.
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Despite the increasing sophistication of process design and improvement tools and a substantial catalogue of operating plant characteristics, the mineral resource industry still manages to be plagued by operating plants that fail because they are neither smart nor safe. While the actual number of plants that fall into this category is not that substantial, the adverse end effects are generally widely publicised and they do little to reflect the complexity and broad expanse of knowledge that is required to achieve the ultimate objective of the development of our mineral resources in an economically and environmentally sustainable manner. The general public and the finance industry are generally totally uniformed about projects that ramp-up to name plate capacity on time and on budget. There are many reasons why these "wayward" metallurgical plants come into being, ranging from scientific-engineering incompetence, project owner/developer avarice, insufficient attention to future market/product requirements, impossible conditions imposed by our generally ignorant government/political authorities, through to obvious bad luck, but more likely a combination of all such factors. This presentation attempts to provide an overview of what should be regarded as some very basic ground rules, ranging from a proper understanding of the resource itself (location, geology, mineralogy) through to completion and detailed analysis of risk mitigation strategies before it is too late. Along the way potential problems of over-design as well as under-design will be canvassed, as will be the less than adequate generation of an expanded pool of "top gun" metallurgists/process engineers. The presentation will be illustrated by a number of disasters as well as examples of what can be achieved.
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Minerals biooxidation is now accepted as a viable technology for the pretreatment of refractory sulfidic gold ores and concentrates, and for the leaching of base metals from their ores and concentrates. Tank bioleaching or biooxidation is successful in achieving high metal recoveries, but both capital and operating costs are relatively high. Heap biooxidation has lower costs, but to date has suffered from low metal extraction rates and low ultimate metal recoveries. These disadvantages may outweigh the lower capital and operating costs of heap processes. GeoBiotics has developed and patented the GEOCOAT® biooxidation and bioleaching technology, which combines the high recoveries of tank processes with the low costs of heap-based processes. The process has been commercialized for the pretreatment of a refractory sulfidic gold concentrate. GeoBiotics is also developing the GEOLEACH™ technology for bioleaching and biooxidation of gold and base metal ores in heaps (Fig. 5.1).
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A process by which ore that has usually been crushed and often agglomerated to be stacked on a prepared containment system is known as heap leaching. They use cyanide as the leachant with feeds that are characterized as oxide gold ores in the case of gold. Sulfuric acid is the only commercialized technology for copper heap leaching, but there has also been some experimentation with other lixiviants like ammonia. Copper oxide ore leaching is just about dissolution process but sometimes need a mix of dissolution and bioleaching, where the bioleaching makes the copper available to acid leaching. In the case of nickel laterite ores, heap leaching them is more complicated than copper or gold ores.
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An efficient and simple method for the vacuum impregnation of stone is described, based on the formation of an airtight jacket around the object allowing a vacuum to be applied to the surface and acting as a wick for the consolidating material. /// Une méthode simple et efficace pour l'imprégnation sous vide de la pierre est décrite, basée sur la formation d'une enveloppe étanche à l'air autour de l'objet permettant d'appliquer le vide à la surface et agissant comme une mèche pour le matériel de consolidation. /// Ein wirksames und einfaches Verfahren zur Vakuumimpregnierung von Stein wird beschrieben, das auf der Bildung eines luftdichten Mantels um den Gegenstand basiert, welcher die Aufbringung eines Vakuums auf die Oberfläche erlaubt und als Docht für das Konsolidierungs-material wirkt.
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Stromatolites are sedimentary structures produced by the sediment-trapping, binding and/or precipitation activity of microbial communities, in particular by photosynthetic cyanobacteria. They occur today in a wide range of aquatic habitats, both marine and non-marine, from shallow subtidal to supratidal and in lakes, streams and thermal springs. Although uncommon today, stromatolites were widespread in the past, and are the most conspicuous fossils in Precambrian rocks. It has been suggested that microbes played a major role in the development of the banded-iron formations that are widespread in Precambrian rocks, and that they played a crucial role in the formation of atmospheric oxygen. -from Authors
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A broth containing the sulfate reducing bacterium Desulfovibrio desulfuricans was used to treat samples of reagent calcium sulfate, gypsum-rock specimens, fragments from a marble monument with a black weathering crust rich in gypsum, and a marble monument with similar crust. Calcite was found to have formed on all treated surfaces suggesting that this microbe has the potential to clean crusted marble monuments whilst also regenerating calcite, the parent mineral of the marble. /// Un bouillon de culture contenant une bactérie réductrice des sulfates, le Desulfovibrio desulfuricans, a été utilisé pour traiter des échantillons de sulfate de calcium, des spécimens de pierre en gypse, des morceaux provenant d'un monument de marbre recouverts d'une croûte noire de vieillissement riche en gypse, et un monument de marbre avec une croûte semblable. Il s'est formé de la calcite sur toutes les surfaces traitées, suggérant que cette bactérie peut nettoyer les monuments de marbre recouverts d'une croûte, tout en régénérant le calcite qui est le principal constituant du marbre. /// Calciumsulfate, gipshaltige Gesteine sowie gipsreiche schwarze Verwitterungskrusten von zwei Denkmälern aus Marmor wurden mit einer Desulfovibrio desulfuricans Kultur in Nährbouillon behandelt. Die Bakterien reduzieren Sulfate. Die Untersuchungen ergaben, daß sich auf allen behandelten Oberflächen Calcit (Calciumcarbonat) gebildet hatte. Dies scheint die Möglichkeit zu eröffnen, mit Hilfe dieser Bakterien Objekte aus Marmor zu reinigen. Der gebildete Calcit ist wiederum der Hauptbestandteil von Marmor.
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Lipases, hydrolytic enzymes that act on glycerol-ester bonds, are often used in conservation for their ability to degrade aged oil films, as a non-toxic and often less aggressive alternative to highly polar organic solvents and/or alkaline mixtures. One such enzyme has been used to remove layers of an aged acrylic resin (Paraloid B72) in two instances, a fifteenth-century tempera painting on panel and a nineteenth-century oil painting on canvas. A plausible mechanism for the action of the enzyme is discussed.