Transcript Mahlangu

The SAIMM Hydrometallurgy
Conference 2009
24 – 26 February 2009
Misty Hills, Muldersdrift, Gauteng
Leaching of the arsenopyrite/pyrite
flotation concentrates using metallic
iron in a hydrochloric acid medium.
Mahlangu T, Gudyanga, F.P., and Simbi, D.J.,
2
Overview of presentation
Background
Experimental
Results and discussion
Conclusions
Acknowledgements
3
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
4
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)
5
Background: Alternatives
Exploration of reductive leaching
process as a novelty
Release sulphur as H2S – oxidise to
sulphate by strong oxidants such as
H 2O 2
6
M m S n  2nH   2ne   m M  nH 2 S ( g )
M m S n  2n(1 
(1)
p 
p
p
m
) H  2n(1  )e   M p S np 2  n(1  ) H 2 S ( g )
m
m
p
m
2
(2)
Background: Alternatives
m
M m S n  2nH   2ne  m M  nH2 S ( g )
p 
p  m
p
M m S n  2n(1  ) H  2n(1  )e  M p S np 2  n(1  ) H 2 S ( g )
m
m
p
m
m2

2
2CuFeS2  Fe  6H  Cu2 S  3Fe  3H 2 S
7
Background: Envisaged
benefits
Break down the matrix to liberate the
precious metals
Avoid the mineral surface passivation
common in the oxidative leach systems
8
Background: Pertinent
problems
Neutralisation of the leach residues
prior to cyanidation
Negative effects of residual sulphur,
even at ppm levels
9
Background: Reactions
systems
10
Background: Reaction
systems
Thermodynamic feasibility of
Reductive reaction
Hydrogen evolution side reaction
11
Background: Reaction
systems
Kinetics
Hydrogen evolution side reaction
kinetically faster than the reductive
leach reactions
12
Background: Focus areas
Effect of pH
Effect of iron/concentrate ratio
Effect of desulphurisation on gold
cyanidation
13
Experimental: Flotation
concentrate
Mineralogical composition
FeS2 FeAsS PbS CuFeS2 ZnS Sb2S3 Other
(%)
(%)
(%)
(%)
(%) (%)
(%)
58.7
27.2
0.1
0.3
1.8
0.8
11.1
14
Experimental: Flotation
concentrate
Chemical composition
Fe
(%)
As Pb Cu Zn Sb
S
(%) (%) (%) (%) (%) (%)
Au Ag Other
(g/t) (g/t) (%)
36.7 12.5 0.1 0.1 1.2 0.6 28.7 64.7 43.1
20.2
15
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
16
Experimental procedure
Reductive
leaching
500ml solution
Temperature – 105oC
45 – 60mins N2 pre-sparging
pH adjusted with HCl
Total leaching time – 300min
17
Results: Effect of pH
%S leached from
FeAsS/FeS2
15min
30min
16
14
12
10
8
6
4
2
0
0.05
60min
120min
180min
240min
300min
Direct acid leaching
Iron to concentrate
ratio = 0
0.15
0.25
0.35
pH
0.45
0.55
0.65
Inverse relationship
with pH
FeAsS  2H   Fe2  Aso  H 2 S No pyrite acid leach
G o  62.45kJ / mol
18
Results: Effect of pH
16
14
12
10
8
6
4
2
0
(a)
0.04
25
15min
30min
60min
20
15min
30min
15
60min
120min
120min
180min
10
180min
240min
240min
300min
300min
5
0
0.24
0.44
0.04
0.64
0.24
0.64
pH
pH
70
45
40
35
30
25
20
15
10
5
0
0.04
0.44
60
15min
15min
50
30min
30min
60min
40
60min
120min
120min
180min
240min
300min
30
180min
240min
20
300min
10
0
0.24
0.44
pH
0.64
0.04
0.24
0.44
0.64
pH
19
%S leachd from FeAsS/FeS 2
Results: Effect of pH & iron-toconcentrate ratio
70
60
50
0.16
40
0.32
0.64
30
0.96
20
10
0
0.1
0.15
0.25
0.34
0.44
0.54
0.62
pH
20
Results: Effect of pH & iron-toconcentrate 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
e-
H2S
H+
Fe
FeS2/FeAsS
Product layer
Fe2+
or FeCln
2-n
Fe = Fe2+ + 2e- or
Fe + nCl- = FeCln2-n + 2e-
22
Results: Galvanic interactions

2
Fe  2H  Fe  H 2
23
Results: Effect of pH & ironto-concentrate ratio
%S leachd from FeAsS/FeS2
70
60
60-70
50
50-60
40-50
40
30-40
30
20-30
20
10-20
0-10
10
0
0.96
0.1 0.15
0.25 0.34
0.44 0.54
0.62
pH
0.16
24
Results: Effect of pH & iron-toconcentrate ratio
%S leached from FeAsS/FeS2
70
60
50
15min
30min
60min
120min
180min
240min
300min
40
30
20
10
0
0
0.5
1
1.5
Iron/concentrate ratio
25
Effect of desulphurisation on Au &
Ag cyanidation
- Wet screening/sieving
Size-by-size
analysis
Cyanidation
-Au & Ag size – by – size
analysis
0.23%NaCN
pH 11
Time 48hrs
Aeration
26
Effect desulphurisation on Gold
and silver recovery
100
90
% Metal distribution
80
70
Ag
Au
60
50
40
30
20
10
0
-212+150µm
-150+106µm
-106+75µm
-75+53µm
-53µm
27
Effect desulphurisation on Gold
dissolution
16
.
12
%Au extraction
14
10
8
6
4
2
0
0
10
20
30
40
50
60
70
%S leached from arsenopyrite/pyrite
28
Effect desulphurisation on silver
dissolution
18
16
%Ag extraction
14
12
10
8
6
4
2
0
0
10
20
30
40
50
60
70
%S leached from arseonpyrite/pyrite
29
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 ironto-concentrate ratio
Galvanic interactions promote the
hydrogen evolution reaction in preference
to the reductive leach reactions
31
Conclusions
Relatively low desulphurisation
levels
Low levels of gold and silver
dissolution
Process is not effective as a pretreatment process for refractory gold
concentrates
32
Acknowledgements
Department of Metallurgical
engineering – University of
Zimbabwe
Rio Tinto Zimbabwe
Department of Materials Science
and Metallurgical Engineering –
University of Pretoria
33
Thank you
?
34