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

1. 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 • Background • Experimental • Results and discussion • Conclusions • Acknowledgements

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 routes • Roasting – 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

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. 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

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

14. Experimental: Flotation concentrate Mineralogical composition

15. Experimental: Flotation concentrate Chemical composition

16. 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

17. Experimental procedure

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

19. Results: Effect of pH

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

21. Results: 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

22. H2S H+ e- Fe FeS2/FeAsS Product layer Fe2+ or FeCln2-n Fe = Fe2+ + 2e- or Fe + nCl- = FeCln2-n + 2e- Results: Galvanic interactions

23. Results: Galvanic interactions

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

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

26. Effect of desulphurisation on Au & Ag cyanidation Cyanidation

27. Effect desulphurisation on Gold and silver recovery

28. Effect desulphurisation on Gold dissolution

29. Effect desulphurisation on silver dissolution

30. 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

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

34. Thank you ?