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

BackgroundExperimental Results and discussionConclusionsAcknowledgements

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 routesRoasting – 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

(2) )()1()1(2)1(2M

(1) )(22

2m

2

2

2 gSHm

pnSM

p

me

m

pnH

m

pnS

gSnHmMnenHSM

m

nppn

nm

)()1()1(2)1(2M

)(22

2m

2

2

2 gSHm

pnSM

p

me

m

pnH

m

pnS

gSnHmMnenHSM

m

nppn

nm

SHFeSCuHFeCuFeS 22

22 3362

8

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

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

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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

FeS2 FeAsS PbS CuFeS2 ZnS Sb2S3 Other(%) (%) (%) (%) (%) (%) (%)

58.7 27.2 0.1 0.3 1.8 0.8 11.1

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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

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Iron shavings: screened washed and stored under deoxygenated conditions

AR grade reagents ofHCl; H2SO4; ferric sulphate;potassium dichromate;High purity nitrogenSodium cyanideSodium hydroxide

Experimental: Reagents

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Experimental procedure

Reductive leaching

500ml solutionTemperature – 105oC45 – 60mins N2 pre-spargingpH adjusted with HClTotal leaching time – 300min

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Results: Effect of pH

02468

10121416

0.05 0.15 0.25 0.35 0.45 0.55 0.65

pH

%S

leac

hed

from

F

eAsS

/FeS

2

15min 30min 60min 120min 180min 240min 300min Direct acid leaching

Iron to concentrate ratio = 0

Inverse relationship with pH

No pyrite acid leach

molkJG

SHAsFeHFeAsSo

o

/45.62

2 22

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Results: Effect of pH

0

2

4

6

8

10

12

14

16

0.04 0.24 0.44 0.64

pH

15min

30min

60min

120min

180min

240min

300min

(a)

0

5

10

15

20

25

0.04 0.24 0.44 0.64

pH

15min

30min

60min

120min

180min

240min

300min

05

1015202530354045

0.04 0.24 0.44 0.64

pH

15min

30min

60min

120min

180min

240min

300min

0

10

20

30

40

50

60

70

0.04 0.24 0.44 0.64

pH

15min

30min

60min

120min

180min

240min

300min

20

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

0

10

20

30

40

50

60

70

0.1 0.15 0.25 0.34 0.44 0.54 0.62

pH

%S

leac

hd

fro

m F

eAsS

/FeS

2

0.16

0.32

0.64

0.96

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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

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

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Results: Galvanic interactions

H2S

Fe2+ or FeCln2-n

H+

FeS2/FeAsS

Product layer

Fe

Fe = Fe2+ + 2e- orFe + nCl- = FeCln

2-n + 2e-

e-

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Results: Galvanic interactions

222 HFeHFe

24

0.1 0.15 0.25 0.34 0.44 0.54 0.620.16

0.960

10

20

30

40

50

60

70

%S

lea

chd

fro

m F

eAsS

/FeS

2

pH

60-70

50-60

40-50

30-40

20-30

10-20

0-10

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

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

0

10

20

30

40

50

60

70

0 0.5 1 1.5

Iron/concentrate ratio

%S

leac

hed

from

FeA

sS/F

eS2

15min

30min

60min

120min

180min

240min

300min

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Effect of desulphurisation on Au & Ag cyanidation

Size-by-size analysis

- Wet screening/sieving-Au & Ag size – by – size analysis

0.23%NaCNpH 11Time 48hrsAeration

Cyanidation

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Effect desulphurisation on Gold and silver recovery

0

10

20

30

40

50

60

70

80

90

100

-212+150µm -150+106µm -106+75µm -75+53µm -53µm

% M

etal

dis

trib

uti

on

Ag

Au

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Effect desulphurisation on Gold dissolution

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70

%S leached from arsenopyrite/pyrite

%A

u e

xtra

ctio

n

.

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Effect desulphurisation on silver dissolution

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60 70

%S leached from arseonpyrite/pyrite

%A

g ex

trac

tion

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ConclusionsReductive 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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ConclusionsProcess 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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ConclusionsRelatively 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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AcknowledgementsDepartment 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

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