TYPES AND MECHANISM OF MICROBIAL LEACHING

1. DR. N. BANU
ASSOCIATE PROFESSOR
VELS UNIVERSITY

2. Bacterial leaching – winning of metals with
the aid of bacteria. Thiobacillus.
Definition: convertion of sparingly soluble
metal compounds by biochemical reaction
mechanisms into water-soluble metal
sulfates.
Microbial leaching – used for recovering
metal from copper and uranium ores.

3. Thiobacilli:
 Gram-negative; non-spore forming rods.
 0.5 µm thick and 1.3 µm long
 Chemolithoautotrophic, aerobic.
 Energy from elementary sulfur, reduced sulfur
compounds and CO2.
 End product of oxidation: sulfate.
 Acid pH (2.0 – 3.0)
 Species : Tiobacillus thioxidans & T.
ferrooxidans

4. Isolated by Waksman and Joffe in 1922.
Rods, 1-2 µm length, occurs individualy,
pairs or short chains.
Oxidizes elementary sulfur, thiosulfate to
sulfuric acid and sulfate.
pH – 1.5 – 1.
Production of acid leads to the
decomposition of the rock.

5.  Isolated in 1947 by Colmer and Hinkle from
coal pit waters containing sulfuric acid.
 Identical to T. thiooxidans but slow oxidation
of elementary sulfur.
 In addition to sulfur and reduced sulfur, it also
oxidizes bivalent iron.
 Ferrobacillus ferrooxidans and F. sulfooxidans
– identical, but differentiated by using
inorganic sulfur compounds.

6. Chemoautotrophs
Oxidize bivalent iron, ele.sulfur, sulfides
and leach molybdenite and chalcopyrite.
Temp. 45 – 70°C; pH – 2; Belongs to
Sulfolobus (S.Brierleyi)
Facultative thermophiles: oxidizes pyrite,
pentlandite, and chalcopyrite.

7. Direct Bacterial Leaching
Indirect Bacterial Leaching
Direct Bacterial Leaching:
A physical contact exists between bacteria
and sulfide mineral. Pyrite is oxidized to
iron iii sulfate.
2FeS2 +7O2 2FeSO4+2H2SO4
4FeSO4 + O2 +2H2SO4 2Fe2(SO4)3
+2H2O

8. Oxidation of nonmetallic compounds by T.
thiooxidans:
Covellite (CuS)Molybdenite(MoS2)
Chalcocite (Cu2S) Stibnite (Sb2S3)
Sphalerite (ZnS) Cobaltsulfide(CoS)
Galena (PbS) Millerite (NiS)
MeS +2O2 MeSO4

9. Sulfur minerals can be oxizied by Fe3+ ions
and leached by an exclusively
geochemical mechanism without the
interaction of bacteria. Leaching agent is
merely produced or regenerated by
microorganisms.
MeS +Fe2(SO4)3 MeSO4 +2FeSO4
+S
2S +3O2 + 2H2O T.thio 2H2SO4

10. UO2 +Fe(SO4)3 UO2SO4 +2FeSO4
2FeS2 +H2O +71/2O2 T. ferrooxidans
Fe(SO4)3
+H2SO4

11.  Nutrients
 pH
 Temperature
 Chemistry and Mineralogy of the leaching
material
 Grain size and concentration of the substrate
 Surface-active agents and organic
extractants.
 Heavy metals.
 Light.

12. Lab:
Percolator leaching
Suspension leaching in shake flasks
Column leaching
Technical leaching process:
Dump leaching
Heap leaching
In situ leaching
Stirred tank Process.

13.  Kennecott copper corp. at Bingham Canyon,
USA. – dump leaching.
 Chino Mine in New Mexico, USA. Dump
leching.
 Miami Mine in Arizona, USA – in situ leaching.
 Uranium ore:
 Stanrock Mine in the Elliot Lake region of
ontario, Canada. – in situ.
 Milliken Mine of the Rio Algom Mine Ltd.
Canada – in situ.