2. Introduction
Bioleaching is the simple and effective technology for metal
extraction from low grade ores and mineral concentrate by
the use of micro organisms.
Commonly used microorganisms are:
Mesophiles
Moderately thermophilic bacteria
extremophiles
3. History
Copper recovery from mine waters in the Mediterranean area
3000 years ago.
The role of bacteria in bioleaching was shown in 1947.
In 1950´s copper dump leaching.
In 1960´s the first industrial copper heap leaching operation.
First industrial gold bioleaching plant in 1980´s
Nowadays about 40 plants in industrial use for copper, gold,
zinc, cobalt, uranium.
4. Mesophiles -(Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans
and species of Ferroplasma)
Moderately thermophilic bacteria-(Sulfobacillus and Acidithiobacillus
cladus)
Extremophiles- (Sulfolobus metallicus and Metallosphaera sedula.)
Fungi – Fungal strains - Aspergillus niger and Penicillium simplicissimum
Organisms involved can be
conveniently classified into:
6. Process of Bioleaching
• Biomining encompases two processes
• 1. Bioleaching : Solubilization of one or more
components of complex solid by contact with
liquid phase. Solubilization is mediated by
bacteria. Hence, Bacterial leaching or
Bioleaching.
• Metal of interest is extracted from respective
ores by bacterial action.
• 2. Biooxidation: Bacterial oxidation of reduced
Sulphur accompanying the metal.
7. Procedure
Bacteria perform the key reaction of regenerating the major ore
oxidizer, mostly ferric ion. This reaction takes place in the cell
membrane of bacteria.
In the first step, disulfide is spontaneously oxidized to
thiosulfate by ferric iron (Fe3+), which in turn is reduced to
give ferrous iron (Fe2+):
• FeS2+6 Fe3++3 H2O⟶7 Fe2++S2O3
2-+6 H+ spontaneous
In second step Microorganisms catalyze the oxidation of
ferrous iron and sulphur, to produce ferric iron and sulphuric
acid:
• Fe2+ + 1/4O2 + H+ ---> Fe3+ + 1/2 H2O
• S + 3/2O2 + H2O ---> H2SO4
8. Thiosulfate is also oxidized by bacteria to give
sulfate:
• S2O3
2- +2O2+H2O⟶2 SO4
2-+2 H+ (sulfur oxidizers)
The ferric iron produced in reaction (2) oxidized
more sulfide as in reaction (1), closing the cycle
and given the net reaction
• 2 FeS2+7O2+2 H2O⟶2 Fe2++4SO4
2+4H+
The net products of the reaction are soluble
ferrous sulfate and sulfuric acid.
9. Mechanism involves in bioleaching
Two processes are used in bioleaching:
Direct bioleaching
Indirect bioleaching
10. In direct bioleaching
In indirect method of bioleaching of minerals
bacteria produce strong oxidizing agent which
reacts with metals and extract them from the ores.
11. In direct bioleaching minerals which are
susceptible to oxidation undergoes direct
enzymatic attack by the microorganisms.
Direct bioleaching
12. REACTIONS INVOLVED
Generation of ferric ions in indirect bioleaching
4FeSO + O + 2H SO 2Fe (SO4) + 2H O.
Cu S + 2Fe (SO4) 2CuSO +2FeSO +S
Direct bioleaching invoves:
CuS +2O T. ferroxidans CuSO
14. Commercial process of bioleaching
Naturally occur bioleaching process is very slow.
For commercial extraction of metal by
bioleaching the process is optimized by
controlling the pH, temperature, humidity, o2 and
co2 concentrations.
These processes are:
Slope leaching
In-situ leaching
Heap leaching
15. Slope leaching
In slope leaching the ore is finely ground and kept in large pile in a slope
which is subjected to continuous sprinkling of aqueous solution of
microorganisms. The leach liquor collected at the bottom of the ore is
processed further for metal recovery.
In situ leaching
In in situ leaching ore is subjected to bioleaching in its natural occurrence,
aqueous solution of microorganisms is pumped through drilled passages
with in the ore. The leach liquid collected at the bottom of the ore used for
metal extraction.
Heap leaching
In heap leaching ore is arranged in heap and goes through the same
procedure as in slope leaching. The aqueous solution containing
microorganism works on the heap of ore and produces the leach liquor. The
leach liquor is used for metal recovery.
19. FACTORS EFFECTING BIOMINING:
Success of biomining and efficiency in recovery of minerals depends on various
factors some of which are discussed below.
(a) Choice of Bacteria - This is the most important factor that determines the
success of bioleaching. Suitable bacteria that can survive at high temperatures,
acid concentrations, high concentrations of heavy metals, remaining active
under such circumstances, are to be selected to ensure successful bioleaching.
(b) Crystal Lattice Energy - This determines the mechanical stability and
degree of solubility of the sulfides. The sulfide ores with lower crystal lattice
energy have higher solubility, hence, are easily extracted into solution by the
action of bacteria.
(c) Surface Area - Rate of oxidation by the bacteria depends on the particle
size of the ore. The rate increases with reduction in size of the ore and vice-
versa.
20. Cont…
(d)Ore Composition – Composition of ore such as concentration of
sulfides, amount of mineral present, and the extent of contamination, has
direct effect on the rate of bio-oxidation being selected. The rate of
biooxidation is reduced significantly if the temperature is above or below
the optimum temperature.
(e) Acidity - Biooxidation requires a pH of 2.5-3 for maximum results.
The rate of biooxidation decreases significantly if the pH is not in this
range since the activity of acidophilic bacteria is reduced.
(f) Temperature - The bacteria used in biomining are either mesophilic
or thermophilic. Optimum temperature is required for biooxidation to
proceed at a fast rate. Optimum temperature range for a given bacteria is
between 25-35° C depending on the type of ore
21. (g) Aeration - The bacteria used in biomining are aerobic thus require an
abundant supply of oxygen for survival and growth. Oxygen can be provided by
aerators and pipes. Mechanical agitation is also an effective method to provide
continuous air supply uniformly and also to mix the contents.
(h) Solid-liquid Ratio - The ratio of ore/sulfide to the leach solution (water +
acid solution + bacteria inoculum) should be maintained at optimum level to
ensure that biooxidation proceeds at maximum speed. The leach solution
containing leached minerals should be removed periodically and replaced with
new solution.
(i) Surfactants - Adding small amounts of surfactants like Tween 20 to the
leaching process increases the rate of biooxidation of minerals from sulfide
ores. The surfactants decrease the surface tension of the leach solution, thus,
wetting the ore and resulting in increased bacterial contact which ultimately
increases the rate of biooxidation.
22. Benefits of bioleaching
Simple
Inexpensive
Employed for collecting metals from waste and drainages
Use to extract refines and expensive metals which is not
possible by other chemical processes
no poisonous sulfur dioxide emissions as in smelters
no need for hi pressure or temperature
ideal for low-grade sulfide ores
Environment friendly process
23. Disadvantages
Time consuming
(takes about 6-24 months or longer)
Have a very low yield of mineral
Requires a large open area for treatment
May have no process control
High risk of contamination
Inconsistent yield because bacteria cannot
grow uniformly
24. The future of Bioleaching
Isolate new bacterial strains from extreme environments, such as mine-drainage
sites, hot springs, and waste sites, and use these to seed bioleaching processes.
Improve isolates by conventional mutation and selection or by genetic
engineering. One possibility would be to introduce arsenic resistance into some
bioleaching organisms, which could then be used in gold bioleaching.
Heterotrophic leaching is a solution for wastes and ores of high pH (5.5) where
many of the acidophiles would not grow. Fungi like Trichoderma horzianum
have been shown to solubilize MnO2, Fe O3, Zn, and calcium phosphate
minerals.
The population dynamics within the bioleaching dumps and the relative
importance of various organisms and mechanisms needs to be understood
