Introduction Chapter 1 Page 1 Bioleaching is the extraction of metals from ores using the principal components of water, air and microorganisms, all of which are found readily within the environment. The development of new effective techniques, using these sources, to process mineral material is one of the decisive factors of scientific and technological progress. Lover metal content and a more complicated mineral composition of ores to be processed coupled with growing environmental awareness have made mining and recovery of values more expensive (Ehrlich 1988). Our natural mineral wealth has been exploited considerably to a greater extent during the past 50 years. With increase in industrialization along with population growth, the demand of metals has increased and is likely to go up further in years to come. This has lead to irreversible impacts like depleting high-grade ores with simultaneous generation of solid wastes and effluents containing metals. It is thus important to tackle the problem for control of pollution and recovery of metal values in a cost-effective method. Biotechnology holds greater importance in mineral engineering for the development of economically viable processes for utilization of wastes and low grade ores through biochemical leaching methods and up gradation of ores through bio beneficiation. In these processes the natural ability of microorganisms belonging to various groups has been effectively utilized (Mishra et al 2005, Mohapatra et al 2009). Worldwide reserves of high-grade ores are diminishing at an alarming rate due to the rapid increase in the demand for metals. However there exist large stockpiles of low and lean grade ores yet to be mined. But the recovery of metals from them using conventional techniques is very expensive due to high energy and capital inputs required. Another major
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Introduction
Chapter 1 Page 1
Bioleaching is the extraction of metals from ores using the principal
components of water, air and microorganisms, all of which are found
readily within the environment. The development of new effective
techniques, using these sources, to process mineral material is one of the
decisive factors of scientific and technological progress. Lover metal
content and a more complicated mineral composition of ores to be
processed coupled with growing environmental awareness have made
mining and recovery of values more expensive (Ehrlich 1988).
Our natural mineral wealth has been exploited considerably to a greater
extent during the past 50 years. With increase in industrialization along
with population growth, the demand of metals has increased and is likely
to go up further in years to come.
This has lead to irreversible impacts like depleting high-grade ores with
simultaneous generation of solid wastes and effluents containing metals. It
is thus important to tackle the problem for control of pollution and
recovery of metal values in a cost-effective method.
Biotechnology holds greater importance in mineral engineering for the
development of economically viable processes for utilization of wastes and
low grade ores through biochemical leaching methods and up gradation of
ores through bio beneficiation. In these processes the natural ability of
microorganisms belonging to various groups has been effectively utilized
(Mishra et al 2005, Mohapatra et al 2009).
Worldwide reserves of high-grade ores are diminishing at an alarming rate
due to the rapid increase in the demand for metals. However there exist
large stockpiles of low and lean grade ores yet to be mined. But the
recovery of metals from them using conventional techniques is very
expensive due to high energy and capital inputs required. Another major
Introduction
Chapter 1 Page 2
problem is environmental costs due to high level of pollution from these
technologies. Environmental standards continue to stiffen, particularly
regarding toxic wastes, so costs for ensuring environmental protection will
continue to rise.
Biohydrotechnology is regarded as one of the most promising and the most
revolutionary solution to these problems, compared to pyrometallurgy or
chemical metallurgy. It dramatically reduces the capital costs with
opportunity to reduce environmental pollution. Biological processes are
carried under mild conditions, usually without adding toxic chemicals. The
products of biological processes wind up in aqueous solution, easy to
containment and treatment than gaseous waste e.g. sulfur dioxide. (Preston
D et al. 2004, Ehrlich 1988)
Nowadays bioleaching occupies an increasingly important place among
the available mining technologies. Today bioleaching is no longer a
promising technology but an actual economical alternative for treating
specific mineral ores. An important number of the current large-scale
bioleaching operations are located in developing countries. This situation is
determined by the fact that several developing countries have significant
mineral reserves and by the characteristics of bioleaching that makes this
technique especially suitable for developing countries because of its
simplicity and low capital cost requirement. (Acevedo 2002)
1.1 History of biomining
The origin of mining industry goes back with beginning of civilization. It
has been carried out since at least Neolithic time. The earliest use of
microbial processes for mining occurred long before it was clear that
microbes were responsible for the effects observed. At the Rio Tinto (Rd
River) mine in Seville, Spain, copper mine workings were rediscovered in
Introduction
Chapter 1 Page 3
1556. Evidence suggests that the mine used water from the Rio Tinto water
containing a very high concentration of ferric ion owing to microbial
activity in the area. When the water from this river was irrigated into
copper containing deposits, the copper dissolved and later precipitated as
smaller deposits. (Acevedo 2002, Dave et al. 2012, Ehrlich1988)
The roots of hydrometallurgy may be traced back to the period of
alchemists.Although the people at that time likely believed this process to
be magic; we now know that it was the first recorded use of
biomineralization. Also probably such methods may have been used since
prehistoric times and probably the Greeks and Romans extracted copper
from mine water more than 2000 years ago (Bosecker, 1997). It is reported
by Rawlings (2004), pre-Romans recovered silver, and the Romans
recovered copper from a mineral deposit located in the Seville province of
southern Spain. This deposit was later on became the site of the Rio Tinto
mine. Leaching of copper was practiced in Norway in the 15th century, in
Germany in 16th century and in England in the 18th century. In early 19th
century, heap and dump leaching was practiced. In 1947, actual evidence of
microbial leaching was obtained through the pioneer workers Colmer and
Hinkel. They isolated pure culture of Acidithiobacillus ferooxidans from mine
water. This gram negative chemolithotroph could oxidize the sulfide part
of minerals to sulphuric acid and ferrous ion to ferric at a very low pH. The
industrial scale bioleaching of copper in heaps has had a chequered 400
year career. Johnson and Hallberg (2009) reported that during the 18th and
19th centuries, the practice of allowing underground shafts and adits at
these mines to flood, and then periodically releasing and capturing the
metal-rich waters to recover copper from the leached subterranean rocks
by adding scrap iron (cementation) was adapted. This was, essentially, an
ignorant application of ‘‘in situ’’ leaching, which later deliberately applied
Introduction
Chapter 1 Page 4
for uranium biomining from worked-out mines in Canada in 1970s. Over
the past 20 years this technology has blossomed with annualized world
copper production from the process increasing from 0.2% to approximately
8-10%. Bioleaching of copper in heaps was first recorded at the Rio Tinto
mine in 17th century. The first modern industrial scale copper heap
bioleach, producing 14,000tpa, commenced in 1980 at Lo Aguirre in Chile.
The first stand-alone mine using copper bioleaching – solvent extraction –
electrowinning was the Girilambone Copper Operation (managed by
Straits Resources and commissioned 1993) in central NSW, Australia
(Acevedo 2002, Ehrlich 1988, Rossi 1990).
1.2 Mineral bioprocessing mechanisms.
Biomining is a combination of chemical and biological reactions. During
metal sulphide bioleaching, metal sulphides are oxidized to metal ions and
sulphate by aerobic, acidophilic Fe2+ iron and/or sulphur compound
oxidizing bacteria or Archaea (Schippers, 2007).
Bioleaching is a heterogeneous reaction that takes place at the interface
between a solid and liquid phase and some-times a gaseous phase. At the
boundary between the two phases, a diffusion layer is formed. The
dissolution of mineral ore takes place through the following stages: (1)
diffusion of reactant through the diffusion layer, (2) adsorption of the
reactant on the solid, (3) chemical reaction between the reactant and the
solid, (4) desorption of the product from the solid and (5) diffusion of the
product through the diffusion layer. Any of these stages (1) - (5) may be the
rate controlling step depending on its relative speed to the others (Baba et
al., 2012)
A generalized reaction can be used to express the biological oxidation of a
mineral sulphide involved in leaching:
Introduction
Chapter 1 Page 5
MS + 2O2 MSO4, Where M is a bivalent metal (1)
There are two dominant mechanisms, which are considered to be involved
in bioleaching.
In the first mechanism, bacterial membrane directly interacts with the
sulphide surface using enzymatic mechanisms. This mechanism is referred
to as the direct mechanism. Cell attachment to suspended mineral particles
takes place within minutes or hours with cell preferentially occupying
irregularities of the surface structure (Boon2001).The second mechanism
involve oxidation of reduced metal mediated through ferric (Fe+3)
generated from the microbial oxidation of ferrous ions (Fe2+) compounds
present in the mineral. Ferric ion is an oxidizing agent and is chemically
reduced to ferrous ions. Ferrous ions can be microbially oxidized to ferric
ions again. In this case, iron has a role as electron carrier. It has been
proposed that no direct physical contact is needed for the oxidation of iron
.The following equations describe the direct and indirect mechanism for
the oxidation of metal sulphides. (Thore et al.2007, Ana et al. 2003, Ehrlich
1988, Schippers et al. 1999)
Metal sulphides can be directly oxidized by A. ferrooxidans to soluble
metals sulphates according to equation,
MS + 2O2 M+2 +SO4-2 (2.1)
Because the metal sulphides exist in an insoluble form and the metal
sulphate (MSO4) is usually water soluble, this reaction is able to transform
a solid phase to a liquid one to which further treatment can be provided to
recover the metal. Theoretically, the mechanism can be continued until all
the substrate (MS) is converted to product (MSO4). Examples of the direct
stirred tank reactors. One feature of both types of processes is that, unlike
most other commercial fermentation processes, neither is sterile, nor any
attempt is made to maintain the sterility of the inoculum (Rawlings, 2002).
Fig. 1.2 Major process options used in biomining (Johnson, 2010).
Introduction
Chapter 1 Page 35
1.9 Bioleaching technology
Bioleach heap leaching is a sub-category of an irrigation based biomining
processes. Irrigation based processes can be categorised based on the type
of resources to be processed dump leaching, heap leaching and in situ
leaching. Heap leaching deals with the newly mined materials
(intermediate grade oxides and secondary sulphides deposited in the form
of a heap on an impervious natural surface or a synthetically prepared pad
leached with circulation, percolation, and irrigation of the leaching
medium (Pradhan et al., 2008). Primary sulphides like chalcopyrite are also
suitable for this type of leaching.
Bioleach heap technology is emerged as the predominant technology route
for the recovery of metals from low-grade ores. In terms of revenue
generated, it is the most significant industrial application of biomining
(Rawlings, 2002). The technology has been in use since the 1960’s for the
acid leaching of copper oxide minerals, and since the 1970’s for the cyanide
leaching of gold and silver.
The static bioleaching techniques are based on the principle of circulating
water and air through heaps of ore coarsely fragmented to activate the
growth of microorganisms that amplify the oxidation of the sulphidic
minerals (Morin et al., 2006). This process involves stacking crushed ore
into piles constructed on an impermeable layer fitted with a solution
drainage system, or arranged on a slope to facilitate drainage. In many
cases the ore is agglomerated through tumbling with acid and/or irrigation
solution prior to stacking.
1.9.1 Basic Heap Design and the Importance of Heat Generation
Heap bioleaching technology relies on very similar operational heap
configurations, and civil and geotechnical engineering as for conventional
Introduction
Chapter 1 Page 36
acid heap leaching. The basic configuration comprises an impermeable
leaching pad upon which the material to be leached is stacked. The PLS is
collected via a system of drainage pipes contained in a 1–2-m inert
overburden material at the bottom of the heap. An aeration distribution
pipe system is used to supply air into the heap.The PLS typically reports to
intermediate ponds and eventually to conventional solvent extraction and
electrowinning, while the raffinate is added to the top of the heap via a
network of irrigation pipelines. The leach material can either be run-of-
mine or crushed and agglomerated ore. The microbial reactions described
previously occur at the ore–liquid interface and in the solution and result
in the release of copper from the mineral to the solution phase. The key
distinguishing process feature of marginal metal sulfide heaps is the
achievement of elevated heap temperature (above 55ºC) in order to
facilitate effective recovery. The attainable temperature is the net result of
heat-generation and heat-loss/heat-retention factors. Heat generation is
mainly the result of microbial oxidation of sulfur to sulfate (Petersen and
Dixon 2002). The two main factors that govern the overall capacity for heat
generation are therefore the available sulfur content of the ore and
microbial activity.
Introduction
Chapter 1 Page 37
Fig. 1.3 Whole-ore biooxidation heap flow diagram (Thomas et al. 2007) 1.9.2 Physical, chemical and biological factors affecting bioheapleaching
Although heap leaching appears to be a very simple process in concept, the
sub processes taking place within the ore bed are rather complex and their
interactions not yet fully understood.
To unravel the processes underlying heap bioleaching it is useful to
distinguish between phenomena taking place at different scales within the
heap (Peterson and Dixon, 2007a). Beginning at the heap scale, we can
distinguish a number of transport effects. More specifically these include:
(1) Solution flow: In unsaturated, coarsely granular packed beds solution
generally flows along tortuous pathways but remains stagnant in pores
and crevices between particles. This strongly influences heap
performance in terms of reagent delivery and product removal from the
reaction sites within the ore particles.
Introduction
Chapter 1 Page 38
(2) Heat flow: Heat of reaction is significant in sulphide leaching. It is
transported through the heap downward as sensible heat with the
flowing solution and upward as latent heat with the flow of humid air.
Depending on air and solution flow rates, heaps can assume certain
temperature profiles and judicious manipulation of these variables
allow a certain degree of control.
(3) Gas flow: Although gas flow is usually well distributed in aerated
heaps, ensuring ample supply of oxygen throughout, the supply of CO2
may be limited under certain conditions. In non-aerated heaps O2
availability may also be limited, and gas distribution patterns are
complex.
Fig.1.4 A heap indicating the major heap-scale transport effects (Jochen et al. 2007)
The next level, at the meso-scale, represents a cluster of particles within a
heap bed. Here, two processes contribute to the overall rate of leaching:
(1) Diffusion transport: Diffusion is the main mode of transport of
dissolved constituents from and to the moving solution into pore spaces
between particles, and into cracks and fissures within particles. The
effect of pore diffusion on overall kinetics is determined by the length
Introduction
Chapter 1 Page 39
of the diffusion path, which can be significant for systems with poor
solution distribution (Peterson and Dixon, 2007b)
(2) Microbial population dynamics: This encompasses the complex
interactions of different types of microorganisms in the liquid phase
and on the mineral surface. It includes the growth behaviour of each
strain as a function of temperature and concentration of dissolved
constituents (acid, Fe2+ and Fe3+ iron, O2, CO2 as carbon source, etc.),
and any synergies between these and the concomitant iron and sulphur
oxidation reactions.
Fig.1.5 A cluster of particles within a heap bed (left) and the microbial colonies inhabiting the moist pore space between particles (right) (Peterson and Dixon 2007) (3) Grain topology
As the rate of leaching is usually proportional the total mineral surface
available to leaching, a further degree of complexity is added by the
distribution of grains of different size and accessibility within the ore. This
aspect is referred to as “mineral topology” Fig.1.6 illustrates, in simplified
form, the reaction network and associated mass exchanges found in heap
bioleaching. For this reaction to proceed at any particular location in the
heap, oxygen must be transferred from the gas phase into the solution
phase. Associated with this is the transport of air at the heap scale as well
as the gas–liquid mass transfer kinetics at the solution–gas interface. Acid,
Introduction
Chapter 1 Page 40
on the other hand, must be supplied with the solution fed to the heap and
then transported to reaction sites by solution flow and interparticle
diffusion. Given the reagent concentrations, microbial population
characteristics and prevailing temperature, the oxidation reactions will
proceed at a certain rate. The ferric iron generated will have to migrate by
diffusion to mineral grains, which may be located deep inside a particle far
from the location of the microbial oxidation. The mineral is now oxidized
at a rate again depending on prevailing concentrations and temperatures.
The released heat of reaction determines, in interaction with all global gas
and liquid flow phenomena, the local temperature.
Finally, the dissolved copper migrates through the particle and stagnant
solution into the flowing solution to be transported out of the heap.
(Peterson and Dixon 2007)
Fig.1.6 The mineral biooxidation reaction-transport network between gas, liquid and solid phases in heaps (Peterson and Dixon 2007)
Finally, the smallest scale at which sub-processes of heap bioleaching need
to be analyzed is that of the individual mineral grain. Here leaching is
governed by the electrochemical interactions between the mineral grains
and reagents in solution.
Introduction
Chapter 1 Page 41
1.9.3 Microbiology of bioleach heap
A wide variety of microorganisms consisting mainly of bacteria and
Archaea are found in natural leaching environments such as bioheap. The
majority of known acidophilic microorganisms have been isolated from
such natural environments. Understanding the microbiology of a bioheap
is important for advancement in commercial bioheap applications. Such
knowledge will increase the applications to various types of ores as well as
to the diversity of mineral deposits that can be processed by bioheap
technology. It will also enable the better control of conditions to improve
upon the leaching rates, metal recoveries and cost of production. A limited
comprehension is available of what actually occurs in a full-scale
microbiologically operated bioheap, despite the commercial achievement
in the copper ore bioheap leaching (Pradhan et al., 2008). Although
oxidative dissolution of simple and complex sulphide ores and
concentrates may be mediated by pure cultures of iron-oxidizing
acidophiles, as has often been described in laboratory studies, axenic
cultures are never found in actual biomining operations. Consortia of
microorganisms with synergistic (and sometimes complimentary)
metabolic physiologies have been identified in all commercial scale
systems that have been examined (Johnson, 2010).
These have tended to show, as would be expected, that microbial diversity
is far greater in heaps and dumps, which are highly heterogeneous and
uncontrolled environments, than in stirred tanks where conditions are far
more homogeneous. Both operate essentially as ‘‘inorganic’’ systems in
that, while inorganic nutrients (ammonium and phosphate) are added to
stimulate microbial activity, organic carbon is not. This, together with the
primary energy sources available being the sulphide minerals themselves,
means that the dominant prokaryotes present are invariably chemo-
Introduction
Chapter 1 Page 42
autotrophic iron and sulphur oxidizers. However, organic carbon derived
from living (as exudates) and dead (as lysates) primary producers can
accumulate in leach liquors, and can support the growth of mixotrophic
and heterotrophic acidophiles. Hence, it is possible, as noted by Johnson
and Hallberg (2009), to divide micro-organisms in biomining operations
into three: (i) ‘‘primary acidophiles,’’ iron-oxidizing prokaryotes that
generate ferric iron and are responsible for initiating mineral dissolution;
(ii) ‘‘secondary acidophiles,’’ sulphur-oxidizing acidophiles that generate
sulphuric acid from reduced sulphur produced during mineral dissolution
and help maintain pH conditions that are conducive for the biooxidation of