Transcript BACTERIAL LEACHING OF LOW GRADE NICKEL LATERITES …

The Effects of Elemental Sulphur and Pyrite
on the Leaching of Nickel Laterites Using
Chemolithotrophic Bacteria
Geoffrey S. Simate, Sehliselo Ndlovu
School of Chemical and Metallurgical Engineering
University of the Witwatersrand, Johannesburg, South Africa
Mariekie Gericke
Mintek, Randburg, South Africa
Tuesday, February 24th, 2009
SAIMM Hydrometallurgy Conference , Misty Hills, South Africa
Presentation outline…
 General Introduction
 Nickel laterites
 Current nickel recovery processes
 Alternative hydrometallurgical studies

Possible bioleaching alternative
 Experimentation
 Results and Interpretation
 Conclusions
Nickel laterite ores…

Nickel laterite is a complex ore containing several kinds of
metal elements. Nickel present as a minor constituents of
other minerals, therefore, it is not amenable to
concentration
Ni atoms
Ni atoms

Nickel laterites account for almost 70% of the world’s
land based nickel reserves, but less than 50%
production
Current recovery approach…
Pyrometallurgical methods
 Drying of ore

Calcination / reduction in kilns

Smelting

Energy intensive
Hydrometallurgical methods

High Pressure Acid Leaching – complex controls,
equipment, highly skilled labour, etc

Caron process – front end is energy intensive
Studies towards alternative hydro methods…

Conventional leaching of nickel laterites is extremely
complex and is capital intensive

Almost twice as much laterite is amenable to hydro
processing than pyro processing making hydro
processing the alternative recovery route for the future

Chemical leaching using inorganic acid media

Bacterial leaching using heterotrophic microorganisms
Possible bioleaching alternative…

Bacterial leaching using chemolithotrophic
microorganisms

With the addition of appropriate substrate such
as elemental sulphur or pyrite, bacteria can
indirectly dissolve nickel from the complex ore

Bacterial leaching using chemolithotrophic
microorganisms is a well established phenomenon
Mixed culture of chemolithotrophic bacteria…

Chemolithotrophic bacteria oxidise elemental sulphur
and pyrite to produce sulphuric acid

Sulphuric acid generated is used to chemically leach the
nickel laterites

Mixed cultures more effective because of the cooperation of the mechanisms involved

Expected competition of the iron oxidising bacteria for
oxygen could lead to reductive dissolution of ferric iron in
nickel laterites
Study objectives…

Evaluate the efficiency of elemental sulphur or
pyrite as substrates for the chemolithotrophic
bacteria dissolution of nickel laterites
Criteria used to assess effectiveness…

Microbial populations

Generation of acidity

Nickel recovery
Chemical composition of various oxides in the ore...
Oxide
SiO2
Fe2O3
% Mass
52.8
Mineralogical Make up
Cr2O3
Al2O3
MgO
NiO
CoO
21.9
1.0
SiO2: predominant
(Fe,Ni)O(OH), Fe2O3.H2O,
(Mg,Ni)3Si2O5(OH)4,
Ca2Mg5Si8O22(OH)2
2.5
7.5
1.9
0.3
CaO
0.7
MnO
1.3
S
<0.1
P
0.1
Total
 100
Experimentation and Analysis...

Leaching carried out in a platform shaker
Experimental Conditions


Same amount of sulphur by mass in each substrate
Analysis of nickelTemperature:
in solution 30°C
Agitation speed: 190 rpm
Atomic Absorption
Spectrometer
Pulp density:
5g/100mls
Apparent bacterial population determination
UV-Vis spectrophotometer
Effect of substrate type on bacterial population...
0.40
Optical density at 550 nm
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
4
8
12
16
20
24
28
32
Time (days)
Observations/Interpretations…

More bacteria produced with sulphur as a substrate than with FeS2 substrate

Thermodynamically more energy created from oxidation of S0 than Fe2+ions
Effect of substrate type on rate of pH drops…
0.1
1.0
1.5
0.2
2.0
2.5
0.1
-0.3
pH variance (ΔpH)
pH variance (ΔpH)
-0.1
Initial pH
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
0.0
-0.1
Initial pH
1.0
1.5
2.0
2.5
-0.2
-0.3
-0.4
-0.5
-1.7
Within Two Weeks
One month
Observations/Interpretations…

Initially abiotic oxidation of pyrite higher than bacterial oxidation of
sulphur

Higher rates of pH drops at higher initial pH
Effect of substrate type on recovery…
70
% Nickel extraction
% Nickel extraction
70
60
50
pyrite
40
30
20
sulphur
10
sulphur
60
50
40
30
pyrite
20
10
0
0
1.0
1.5
2.0
pH
Within Two Weeks
2.5
1.0
1.5
pH
2.0
One month
Observations/Interpretations…

Significant differences in leaching only observed after two weeks
2.5
Effect of media composition on pH/redox evolution...
2.3
E(mV vs. SHE)
2.1
1.9
pH
1.7
1.5
1.3
1.1
0.9
0.7
0.5
0
3
6
9
12 15 18 21 24 27 32
Leaching time (days)
620
610
600
590
580
570
560
550
540
530
520
0
3
6
9 12 15 18 21 24 27 32
Leaching time (days)
Observations/Interpretations…

No pH drop or increase in redox potential for sterile media

Decrease in pH (increase in redox) for inoculated media attributed to
oxidation of substrates by bacteria
Summary of effects of substrate type…
Optical density
0.40
Analysis…
0.30

0.20
Overall trend
show that initially
0.10
0.00
0
4
8
12
16
20
24
28
abiotic oxidation
32
of pyrite higher
-0.2
1.5
2.0
-0.1
(ΔpH)
-0.7
than biotic
0.1
2.5
(ΔpH)
1.0
1.0
1.5
2.0
-0.3
-1.2
sulphur
-0.5

% Ni extraction
-1.7
% Ni extraction
oxidation of
2.5
60
40
20
0
1.0
1.5
pH 2.0
2.5
Bacterial leaching
60
effective after two
40
weeks
20

0
1.0
1.5 pH 2.0
2.5
S0 better than
FeS2
Conclusions…

Nickel laterite can be bioleached with the help of externally
added sulphur containing substrates via sulphuric acid

Study shown relationships between bacterial activities
depicted by acidification, and type of substrate

Study can be extended to other difficult-to-process low
grade ores, e.g., silicate ores, oxidic converter furnace
slags, and refractory oxides
Research required…

Carrying out testworks to simulate heap leaching

Need to determine the actual amount of acid produced,
and or consumed

Need to detect, differentiate, enumerate the
microorganisms during the leaching process so as to
understand their interactive effects
Acknowledgements...
Optimisation problem…
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