The Effects of Elemental Sulphur and Pyrite on the Leaching of Nickel Laterites Using Chemolithotrophic Bacteria

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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
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Presentation outline

• General Introduction
• Nickel laterites
• Current nickel recovery processes
• Alternative hydrometallurgical studies
• Possible bioleaching alternative
• Experimentation
• Results and Interpretation
• Conclusions

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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

• Nickel laterites account for almost 70 of the
  worlds land based nickel reserves, but less than
  50 production

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Current recovery approach

• Pyrometallurgical methods
• Hydrometallurgical methods

• Drying of ore
• Calcination / reduction in kilns
• Smelting
• Energy intensive

• High Pressure Acid Leaching complex controls,
  equipment, highly skilled labour, etc
• Caron process front end is energy intensive

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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

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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

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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
  co-operation of the mechanisms involved
• Expected competition of the iron oxidising
  bacteria for oxygen could lead to reductive
  dissolution of ferric iron in nickel laterites

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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

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Mineralogical Make
up SiO2 predominant (Fe,Ni)O(OH),
Fe2O3.H2O, (Mg,Ni)3Si2O5(OH)4, Ca2Mg5Si8O22(OH)2

Chemical composition of various oxides in the
ore...
Oxide Mass
SiO2 52.8
Fe2O3 21.9
Cr2O3 1.0
Al2O3 2.5
MgO 7.5
NiO 1.9
CoO 0.3
CaO 0.7
MnO 1.3
S lt0.1
P 0.1
Total ?? 100
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Experimental
Conditions Same amount of sulphur by mass in
each substrate Temperature 30C Agitation speed
190 rpm Pulp density 5g/100mls
Experimentation and Analysis...

• Leaching carried out in a platform shaker

• Analysis of nickel in solution
• Atomic Absorption Spectrometer

• Apparent bacterial population determination
• UV-Vis spectrophotometer

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Effect of substrate type on bacterial
population...
sulphur
Optical density at 550 nm
pyrite

• Observations/Interpretations
• More bacteria produced with sulphur as a
  substrate than with FeS2 substrate
• Thermodynamically more energy created from
  oxidation of S0 than Fe2ions

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Effect of substrate type on rate of pH drops
sulphur
pH variance (?pH)
pH variance (?pH)
pyrite
pyrite
sulphur
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

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Effect of substrate type on recovery
sulphur
pyrite
pyrite
sulphur
Within Two Weeks
One month

• Observations/Interpretations
• Significant differences in leaching only observed
  after two weeks

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Effect of media composition on pH/redox
evolution...
distilled water
S0 with bacteria
9K media
FeS2 with bacteria
E(mV vs. SHE)
pH
FeS2 with bacteria
9K media
S0 with bacteria
distilled water

• 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

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Summary of effects of substrate type

• Analysis
• Overall trend show that initially abiotic
  oxidation of pyrite higher than biotic oxidation
  of sulphur
• Bacterial leaching effective after two weeks
• S0 better than FeS2

sulphur
Optical density
pyrite
pyrite
(?pH)
pyrite
sulphur
(?pH)
sulphur
sulphur
Ni extraction
pyrite
Ni extraction
pyrite
sulphur
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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-proces
  s low grade ores, e.g., silicate ores, oxidic
  converter furnace slags, and refractory oxides

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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

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Acknowledgements...


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