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

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Title: The%20SAIMM%20Hydrometallurgy%20Conference%202009


1
The SAIMM Hydrometallurgy Conference 2009 24
26 February 2009 Misty Hills, Muldersdrift,
Gauteng
2
  • Leaching of the arsenopyrite/pyrite flotation
    concentrates using metallic iron in a
    hydrochloric acid medium.
  • Mahlangu T, Gudyanga, F.P., and Simbi, D.J.,

3
Overview of presentation
  • Background
  • Experimental
  • Results and discussion
  • Conclusions
  • Acknowledgements

4
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

5
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)

6
Background Alternatives
  • Exploration of reductive leaching process as a
    novelty
  • Release sulphur as H2S oxidise to sulphate by
    strong oxidants such as H2O2

7
Background Alternatives
8
Background Envisaged benefits
  • Break down the matrix to liberate the precious
    metals
  • Avoid the mineral surface passivation common in
    the oxidative leach systems

9
Background Pertinent problems
  • Neutralisation of the leach residues prior to
    cyanidation
  • Negative effects of residual sulphur, even at ppm
    levels

10
Background Reactions systems
11
Background Reaction systems
  • Thermodynamic feasibility of
  • Reductive reaction
  • Hydrogen evolution side reaction

12
Background Reaction systems
  • Kinetics
  • Hydrogen evolution side reaction kinetically
    faster than the reductive leach reactions

13
Background Focus areas
  • Effect of pH
  • Effect of iron/concentrate ratio
  • Effect of desulphurisation on gold cyanidation

14
Experimental Flotation concentrate
  • Mineralogical composition

15
Experimental Flotation concentrate
  • Chemical composition

16
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

17
Experimental procedure
18
Results Effect of pH
  • Direct acid leaching
  • Iron to concentrate ratio 0
  • Inverse relationship with pH
  • No pyrite acid leach

19
Results Effect of pH
20
Results Effect of pH iron-to-concentrate ratio
21
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

22
Results Galvanic interactions
23
Results Galvanic interactions
24
Results Effect of pH iron-to-concentrate ratio
25
Results Effect of pH iron-to-concentrate ratio
26
Effect of desulphurisation on Au Ag cyanidation
Cyanidation
27
Effect desulphurisation on Gold and silver
recovery
28
Effect desulphurisation on Gold dissolution
29
Effect desulphurisation on silver dissolution
30
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

31
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

32
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

33
Acknowledgements
  • Department of Metallurgical engineering
    University of Zimbabwe
  • Rio Tinto Zimbabwe
  • Department of Materials Science and Metallurgical
    Engineering University of Pretoria

34
  • Thank you
  • ?
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