1. The SAIMM Hydrometallurgy Conference 2009 24 – 26 February 2009 Misty Hills, Muldersdrift, Gauteng

2. 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. 3 Overview of presentation  Background  Experimental  Results and discussion  Conclusions  Acknowledgements

4. 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. 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. 6 Background: Alternatives  Exploration of reductive leaching process as a novelty  Release sulphur as H 2 S – oxidise to sulphate by strong oxidants such as H 2 O 2

7. 7 Background: Alternatives

8. 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. 9 Background: Pertinent problems  Neutralisation of the leach residues prior to cyanidation  Negative effects of residual sulphur, even at ppm levels

10. 10 Background: Reactions systems

11. 11 Background: Reaction systems  Thermodynamic feasibility of  Reductive reaction  Hydrogen evolution side reaction

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

13. 13 Background: Focus areas  Effect of pH  Effect of iron/concentrate ratio  Effect of desulphurisation on gold cyanidation

14. 14 Experimental: Flotation concentrate Mineralogical composition

15. 15 Experimental: Flotation concentrate Chemical composition

16. 16  Iron shavings: screened washed and stored under deoxygenated conditions  AR grade reagents of  HCl; H 2 SO 4 ; ferric sulphate;  potassium dichromate;  High purity nitrogen  Sodium cyanide  Sodium hydroxide Experimental: Reagents

17. 17 Experimental procedure Reductive leaching 500ml solution Temperature – 105 o C 45 – 60mins N2 pre-sparging pH adjusted with HCl Total leaching time – 300min

18. 18 Results: Effect of pH Direct acid leaching  Iron to concentrate ratio = 0  Inverse relationship with pH  No pyrite acid leach

19. 19 Results: Effect of pH

20. 20 Results: Effect of pH & iron-to- concentrate ratio

21. 21 FeS 2 + Fe + 4H + = 2Fe 2+ + 2H 2 S  G o = -58.14 kJ/mol FeAsS + 2H + = Fe 2+ + As o + H 2 S  G o = -62.45 kJ/mol FeAsS + Fe + 2H + = Fe o + As o + Fe 2+ + H 2 S  Go = -62.43 kJ/mol Results: Effect of pH & iron-to- concentrate ratio

22. 22 Results: Galvanic interactions H2SH2S Fe 2+ or FeCl n 2-n H+H+ FeS 2 /FeAsS Product layer Fe Fe = Fe 2+ + 2e - or Fe + nCl - = FeCl n 2-n + 2e - e-e-

23. 23 Results: Galvanic interactions

24. 24 Results: Effect of pH & iron- to-concentrate ratio

25. 25 Results: Effect of pH & iron-to- concentrate ratio

26. 26 Effect of desulphurisation on Au & Ag cyanidation 0.23%NaCN pH 11 Time 48hrs Aeration Cyanidation

27. 27 Effect desulphurisation on Gold and silver recovery

28. 28 Effect desulphurisation on Gold dissolution

29. 29 Effect desulphurisation on silver dissolution

30. 30 Conclusions  Reductive leach of the arsenopyrite/pyrite concentrate thermodynamically feasible  FeAsS – both chemical and reductive leach reactions operational  FeS 2 – postulated to leach through a reductive leach reaction

31. 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. 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. 33 Acknowledgements  Department of Metallurgical engineering – University of Zimbabwe  Rio Tinto Zimbabwe  Department of Materials Science and Metallurgical Engineering – University of Pretoria

34. 34 Thank you ?