This discusses the leaching of coppe anode slimes resulting from eletrowinnin of copper

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University of Pretoria
NSC 421: Project presentation
Recovery of precious metals from anode slime
Kennedy Nyalungu
Supervisor: Dr D. Groot

2. 2
Presentation Outlined
• Introduction
• Project Background
• Experimental Procedure
• Results and Discussions
• Conclusion
• Questions and suggestions

3. What are anode slime?
• Insoluble residue that collects at the bottom of solution or cell during the
electrorefining of copper.
• PGM’s, gold, silver, copper and other metals.
• Colour: Blackish
• Produced as by-products at Palabora
• R146 053 per tonne
(www.indiamart.com, 22/10/13)
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4. Industrial background
• Electrorefining processing of copper
(Davenport et al 1999)
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5. Scientific background
• Elements found in a typical copper anode
• Anode purity : 99.5% Cu
• BaSO4 used as a wash
• Area : 1 m by 1 m
(Kennecott Utah copper manual)
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6. Scientific background
• Which elements form slime?
(Davenport et al 1999)
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Element %into 'slime' %into electrolyte
Cu <0.2 >98.8
Au 100 0
Ag >99 <1
Se 98 2
Te 98 2
Pb 98 2
Bi 60% with 0.1% Pb in anode 40
Sb 60% with 0.1% As, Bi Pb and Sb 40
(each) in anode
As 25% with 0.1% As in anode 75
S 1 99
Ni 1 99
Co 1 99
Fe 0 100

7. Scientific background
• How do slime form?
(Kennecott Utah copper manual)
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8. Scientific background
• How do slime form?
(Kennecott Utah copper manual)
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9. Scientific background
• How do slime form?
(Kennecott Utah copper manual)
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10. Literature study
• Metals in focus
– Copper
– Nickel
– Silver
– Gold
– Selenium
– Tellurium
(www.statesymbolsusa.org, 31/05/13)
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11. Literature study
• Processing: Pyrometallurgical or Hydrometallurgical methods
• Leaching process
– Selective dissolution of metal values from ores
• Why leaching?
– Low capital costs
– Minimum air pollution (SO2 elimination)
– Equipment can be installed in modules
– low energy consumption
(www.denvermineral.com, 23/10/13)
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12. Literature study
• Leaching agent
– Thiourea
• Why Thiourea and not cyanide?
– Low toxicity
– High silver complexing ability
– Higher rates than Cyanide rates
– No passivation in acidic solutions
(Kennecott Utah copper manual)
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13. Hypothesis and Research Outcomes
• Hypothesis
– After the anode slimes have been decopperised, gold and silver can
be leached selectively
• Objective
– The aim of the project is to recover gold and silver from copper
anode slime.
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14. Experimental procedure
• Proposed methodology
• Variable parameters during each stage
– Residence time
– Temperature
– Reagent concentration
– Solid-liquid ratio
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15. Experimental design
• Decopperisation
– Sulphuric acid (400 g/l)
– Leaching time, 2 hours
– Hydrogen peroxide (50 ml)
– Temperature 30 0C
– Solid-liquid ratio of 1:1
– 100 g of sample
• Thiourea leach
– 10 g/l Thiourea solution
– Temperature of 30 0C
– Reaction time of 2 hours
– Hydrogen peroxide (50 ml)
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16. Experimental design
• Equipment
– Hot plate
– Stirrer
– Glass reactor
– Filter paper
• Safety
– Risk assessment form
– Harmful chemicals in use
• Cause genetic defects
• Skin irritation
• Breathing difficulties if inhaled
– Precautions
• Read labels before use
• Wear protective clothes
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17. Results and Discussion
• Material characterization (XRD)
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18. Results and Discussion
• Material characterization (XRF)
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Element Composition
Ba (%) 39.23
Cu (%) 22.6
S (%) 19.31
Ag (%) 9.11
Ni (%) 2.65
Si (%) 2.05
Se (%) 1.17
Al (%) 0.71
Te (%) 0.63
Pb (%) 0.47
Fe (%) 0.33
Au (%) 0.24
As (%) 0.12

19. Results and Discussion
• Scanning Electron Microscopy (SEM)
(SEI)
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20. Results and Discussion
• Decopperisation
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21. Results and Discussion
• Decopperisation
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Run 1
(S90) Run 2 Run 3 Run 4
Time 2 hours 3 hours 3 hours 4 hours
S:L ratio 1 to 1 1 to 1 1 to 2 1 to 2
Temperature (0C) 90 40 40 60
Acid Conc. (g/l) 400 400 400 600
Oxidant No Yes Yes Yes

22. Results and Discussion
• Decopperisation
– Run 4
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23. Results and Discussion
• Thiourea leach
• XRF results
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Element Composition
Ba (%) 64.75
S (%) 22.47
Si (%) 4.21
Ni (%) 2.71
Se (%) 1.52
Fe (%) 1.2
Pb (%) 0.74
Al (%) 0.6
Mn (%) 0.26
Cu (%) 0.25
Bi (%) 0.03

24. Results and Discussion
• Thiourea leach
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Au Ag Cu
R6 0 874.1 27972 25.8
R6W1 1 570.3 20480 10.7
R6W2 2 239.9 8390 2.1
R6W3 3 8 531 1.8
R6W4 4 0 222.3 0.2

25. Results and Discussion
• Gold recovery during washing stages
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26. Conclusion
• Decopperisation
– Temperature, combination oxidizing agent and reagent concentration have the
greatest effect on the copper recovery.
– The highest copper recovery of 42 023 ppm is obtained at 4 hours with 600 g/l
of sulphuric acid.
– Washing of residue is necessary as most of the copper is recovered during
washing stages.
• Thiourea leach
– The highest gold recovery is 874.1 ppm and silver is 27972.3 ppm
– Gold and silver failed to leach selectively under the experiment conditions
tested.
– Gold and silver recoveries increase with in reagent concentration and time.
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27. Recommendations
• Washing of the residue is necessary as it greatly affect the results.
• An oxidant need to be added just after the process has started.
• Temperature should not be raised to a higher value to avoid evaporation
loses when using an open reactor.
• More residence time is required to achieve higher gold and silver yields.
• All the copper has to be removed completely for selectively recovering
gold and silver.
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28. Questions and Suggestions ?
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