|
Eq. n°15
S + 0.5 O
2
+ H
2
O → H
2
SO
4
Eq. n°16
Oxidation of sulfur or sulfide to sulfuric acid increases the acid concentration, hence the metal
leaching efficiency raises. In the indirect reaction, microbes regenerate ferric iron by
oxidising ferrous iron. The process is expressed by the following chemical reactions:
MS + 2 Fe
3+
→ M
2+
+ 2 Fe
2+
+ S
Eq. n°17
2 Fe
2+
+ 0.5 O
2
+ 2 H
+
→ 2 Fe
3+
+ H
2
O
Eq. n°18
Both reactions contribute to metal bioleaching. However, in the case when acid insoluble
minerals are submitted to bioleaching, the indirect impact of microorganisms is considered as
the major role in the leaching process (Suzuki, 2001; Watling, 2006). The redox potential
determined by the oxidation of Fe
2+
to Fe
3+
is a factor affecting leaching. The leaching rate
increases with increasing redox potential and achieves a maximum efficiency at optimum
redox potential. Regeneration of the oxidant (Fe
3+
) increases the value of the redox potential
in the bioleaching solution. Precipitation of Fe
3+
with sulfate may also happen when alkali
cations are present:
3 Fe
3+
+ 2 SO
4
2-
+ 6 H
2
O + A
+
→ AFe
3
(SO
4
)
2
(OH)
6
+ 6 H
+
Eq. n°19
where: A is cation such as K
+
, Na
+
or NH
4
+
(Watling, 2006).
CHAPTER 2: COPPER METALLURGICLA SLAGS- CURRENT KNOWLEDGE AND FATE:
A REVIEW
60
Therefore, controlling the pH during bioleaching and maintaining the value of 1-2 is
important for the regeneration of ferric iron and acid by microbial organisms (Watling, 2006).
Commonly used bacteria for bioleaching are Acidithiobacillus ferroxidans and
Acidithiobacillus thiooxidans (Suzuki, 2001; Lee and Pandey, 2012). They are commonly
known as acidophilic bacteria that belong to the lithoautotrophic group. Both oxidize the
reduced forms of sulfur, while Acidithiobacillus ferrooxidans also oxidizes Fe
2+
. As the result
of the process, solubilization of metals and acid generation are perceived (Ehrlich, 1998;
Suzuki, 2001; Watling, 2006; Gadd, 2010). Additionally, acidophilic bacteria are extremely
tolerant to highly acidic conditions (Erüst et al., 2013). They are able to survive in the
environment reaching even a pH of 0.7 (Watling, 2006), which makes them especially
interesting for bioleaching. Because of the growth in strong acidity, high temperature and at
high Fe
3+
/Fe
2+
ratio, it is possible to use these microbes as accelerators of bioleaching
(Suzuki, 2001).
The efficiency of the leaching process can be improved by modification of the experimental
conditions. For example, mixing of different microbial consortIa including both autotrophic
and heterotrophic bacteria is suggested to accelerate the process of sulfide minerals leaching.
Another example is the application of indigenous bacteria. Due to the fact that these bacteria
are more adapted to high concentrations of metals, their leaching ability is higher (Watling,
2006). Additionally, bacterial attachment to the mineral surface is improved by the production
of extracellular polymeric substances where the thiol group and Fe
3+
cause the corrosion of
the surface. It was concluded that bacterial cell contact with the solid surface as energy source
is an additional factor facilitating bioleaching (Watling, 2006).
Furthermore, bacteria that belong to thermophile and hyperthermophile groups improve the
bioleaching efficiency because of their resistance to higher temperature (Bosecker, 1997; Lee
& Pandey, 2012; Erüst et al., 2013). Then, increasing the temperature in the bioleaching
process allows achieving faster dissolution rates.
Biochemical interactions between bacteria and different kind of solid materials were broadly
studied. However, very few publications considering bacterially assisted leaching of copper
slags have been reported in the literature (Table 2.4). Experiments revealed that the
application of microorganisms for metal extraction leads to satisfactory results reaching even
up to 95% of copper. Additionally, other metals such as cobalt, nickel and zinc may be
simultaneously extracted.
CHAPTER 2: COPPER METALLURGICLA SLAGS- CURRENT KNOWLEDGE AND FATE:
A REVIEW
61
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