The%20SAIMM%20Hydrometallurgy%20Conference%202009

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The SAIMM Hydrometallurgy Conference 2009 24
26 February 2009 Misty Hills, Muldersdrift,
Gauteng
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• Leaching of the arsenopyrite/pyrite flotation
  concentrates using metallic iron in a
  hydrochloric acid medium.
• Mahlangu T, Gudyanga, F.P., and Simbi, D.J.,

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Overview of presentation

• Background
• Experimental
• Results and discussion
• Conclusions
• Acknowledgements

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Background ores

• Au Ag bearing arsenopyrite/pyrite concentrate
• Au occurring in sub-microscopic form and/or in
  solid solution
• Ores prevalent in the Central and Southern parts
  of Zimbabwe

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Background Processing routes

• Roasting custom roasting plant in Kwekwe (now
  not operational)
• Alternatives bioleaching of concentrates (pilot
  plant operated for a while in the 1990s)

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Background Alternatives

• Exploration of reductive leaching process as a
  novelty
• Release sulphur as H2S oxidise to sulphate by
  strong oxidants such as H2O2

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Background Alternatives
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Background Envisaged benefits

• Break down the matrix to liberate the precious
  metals
• Avoid the mineral surface passivation common in
  the oxidative leach systems

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Background Pertinent problems

• Neutralisation of the leach residues prior to
  cyanidation
• Negative effects of residual sulphur, even at ppm
  levels

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Background Reactions systems
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Background Reaction systems

• Thermodynamic feasibility of
• Reductive reaction
• Hydrogen evolution side reaction

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Background Reaction systems

• Kinetics
• Hydrogen evolution side reaction kinetically
  faster than the reductive leach reactions

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Background Focus areas

• Effect of pH
• Effect of iron/concentrate ratio
• Effect of desulphurisation on gold cyanidation

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Experimental Flotation concentrate

• Mineralogical composition

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Experimental Flotation concentrate

• Chemical composition

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Experimental Reagents

• Iron shavings screened washed and stored under
  deoxygenated conditions
• AR grade reagents of
• HCl H2SO4 ferric sulphate
• potassium dichromate
• High purity nitrogen
• Sodium cyanide
• Sodium hydroxide

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Experimental procedure
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Results Effect of pH

• Direct acid leaching
• Iron to concentrate ratio 0
• Inverse relationship with pH
• No pyrite acid leach

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Results Effect of pH
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Results Effect of pH iron-to-concentrate ratio
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Results Effect of pH iron-to-concentrate ratio

• FeS2 Fe 4H 2Fe2 2H2S
• ?Go -58.14 kJ/mol
• FeAsS 2H Fe2 Aso H2S
• ?Go -62.45 kJ/mol
• FeAsS Fe 2H Feo Aso Fe2 H2S
• ?Go -62.43 kJ/mol

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Results Galvanic interactions
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Results Galvanic interactions
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Results Effect of pH iron-to-concentrate ratio
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Results Effect of pH iron-to-concentrate ratio
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Effect of desulphurisation on Au Ag cyanidation
Cyanidation
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Effect desulphurisation on Gold and silver
recovery
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Effect desulphurisation on Gold dissolution
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Effect desulphurisation on silver dissolution
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Conclusions

• Reductive leach of the arsenopyrite/pyrite
  concentrate thermodynamically feasible
• FeAsS both chemical and reductive leach
  reactions operational
• FeS2 postulated to leach through a reductive
  leach reaction

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Conclusions

• Process strongly influenced by both pH and
  iron-to-concentrate ratio
• Strong interaction between pH and
  iron-to-concentrate ratio
• Galvanic interactions promote the hydrogen
  evolution reaction in preference to the reductive
  leach reactions

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Conclusions

• Relatively low desulphurisation levels
• Low levels of gold and silver dissolution
• Process is not effective as a pre-treatment
  process for refractory gold concentrates

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Acknowledgements

• Department of Metallurgical engineering
  University of Zimbabwe
• Rio Tinto Zimbabwe
• Department of Materials Science and Metallurgical
  Engineering University of Pretoria

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• Thank you
• ?