Title: The%20SAIMM%20Hydrometallurgy%20Conference%202009
1The 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.,
3Overview of presentation
- Background
- Experimental
- Results and discussion
- Conclusions
- Acknowledgements
4Background 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
5Background Processing routes
- Roasting custom roasting plant in Kwekwe (now
not operational) - Alternatives bioleaching of concentrates (pilot
plant operated for a while in the 1990s)
6Background Alternatives
- Exploration of reductive leaching process as a
novelty - Release sulphur as H2S oxidise to sulphate by
strong oxidants such as H2O2
7Background Alternatives
8Background Envisaged benefits
- Break down the matrix to liberate the precious
metals - Avoid the mineral surface passivation common in
the oxidative leach systems
9Background Pertinent problems
- Neutralisation of the leach residues prior to
cyanidation - Negative effects of residual sulphur, even at ppm
levels
10Background Reactions systems
11Background Reaction systems
- Thermodynamic feasibility of
- Reductive reaction
- Hydrogen evolution side reaction
12Background Reaction systems
- Kinetics
- Hydrogen evolution side reaction kinetically
faster than the reductive leach reactions
13Background Focus areas
- Effect of pH
- Effect of iron/concentrate ratio
- Effect of desulphurisation on gold cyanidation
14Experimental Flotation concentrate
- Mineralogical composition
15Experimental Flotation concentrate
16Experimental 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
17Experimental procedure
18Results Effect of pH
- Direct acid leaching
- Iron to concentrate ratio 0
- Inverse relationship with pH
- No pyrite acid leach
19Results Effect of pH
20Results Effect of pH iron-to-concentrate ratio
21Results 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
22Results Galvanic interactions
23Results Galvanic interactions
24Results Effect of pH iron-to-concentrate ratio
25Results Effect of pH iron-to-concentrate ratio
26Effect of desulphurisation on Au Ag cyanidation
Cyanidation
27Effect desulphurisation on Gold and silver
recovery
28Effect desulphurisation on Gold dissolution
29Effect desulphurisation on silver dissolution
30Conclusions
- 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
31Conclusions
- 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
32Conclusions
- Relatively low desulphurisation levels
- Low levels of gold and silver dissolution
- Process is not effective as a pre-treatment
process for refractory gold concentrates
33Acknowledgements
- Department of Metallurgical engineering
University of Zimbabwe - Rio Tinto Zimbabwe
- Department of Materials Science and Metallurgical
Engineering University of Pretoria
34