The Effects of Elemental Sulphur and Pyrite on the Leaching of Nickel Laterites Using Chemolithotrophic Bacteria

1. The Effects of Elemental Sulphur and Pyrite on the Leaching of Nickel Laterites Using Chemolithotrophic Bacteria Geoffrey S. Simate, SehliseloNdlovu School of Chemical and Metallurgical Engineering University of the Witwatersrand, Johannesburg, South Africa MariekieGericke Mintek, Randburg, South Africa Tuesday, February 24th, 2009 SAIMM Hydrometallurgy Conference , Misty Hills, South Africa

2. Presentation outline… • General Introduction • Nickel laterites • Current nickel recovery processes • Alternative hydrometallurgical studies • Possible bioleaching alternative • Experimentation • Results and Interpretation • Conclusions

3. Nickel laterite ores… • Nickel laterite is a complex ore containing several kinds of metal elements. Nickel present as a minor constituents of other minerals, therefore, it is not amenable to concentration Ni atoms Ni atoms • Nickel laterites account for almost 70% of the world’s land based nickel reserves, but less than 50% production

4. Current recovery approach… • Pyrometallurgical methods • Hydrometallurgical methods • Drying of ore • Calcination / reduction in kilns • Smelting • Energy intensive • High Pressure Acid Leaching – complex controls, equipment, highly skilled labour, etc • Caron process – front end is energy intensive

5. Studies towards alternative hydro methods… • Conventional leaching of nickel laterites is extremely complex and is capital intensive • Almost twice as much laterite is amenable to hydro processing than pyro processing making hydro processing the alternative recovery route for the future • Chemical leaching using inorganic acid media • Bacterial leaching using heterotrophic microorganisms

6. Possible bioleaching alternative… • Bacterial leaching using chemolithotrophic microorganisms • With the addition of appropriate substrate such as elemental sulphur or pyrite, bacteria can indirectly dissolve nickel from the complex ore • Bacterial leaching using chemolithotrophic microorganisms is a well established phenomenon

7. Mixed culture of chemolithotrophic bacteria… • Chemolithotrophic bacteria oxidise elemental sulphur and pyrite to produce sulphuric acid • Sulphuric acid generated is used to chemically leach the nickel laterites • Mixed cultures more effective because of the co-operation of the mechanisms involved • Expected competition of the iron oxidising bacteria for oxygen could lead to reductive dissolution of ferric iron in nickel laterites

8. Study objectives… • Evaluate the efficiency of elemental sulphur or pyrite as substrates for the chemolithotrophic bacteria dissolution of nickel laterites Criteria used to assess effectiveness… • Microbial populations • Generation of acidity • Nickel recovery

9. Mineralogical Make up SiO2: predominant (Fe,Ni)O(OH), Fe2O3.H2O, (Mg,Ni)3Si2O5(OH)4, Ca2Mg5Si8O22(OH)2 Chemical composition of various oxides in the ore...

10. Experimental Conditions Same amount of sulphur by mass in each substrate Temperature: 30°C Agitation speed: 190 rpm Pulp density: 5g/100mls Experimentation and Analysis... • Leaching carried out in a platform shaker • Analysis of nickel in solution • Atomic Absorption Spectrometer • Apparent bacterial population determination • UV-Vis spectrophotometer

11. Effect of substrate type on bacterial population... sulphur Optical density at 550 nm pyrite Observations/Interpretations… • More bacteria produced with sulphur as a substrate than with FeS2 substrate • Thermodynamically more energy created from oxidation of S0 than Fe2+ions

12. Effect of substrate type on rate of pH drops… sulphur pH variance (ΔpH) pH variance (ΔpH) pyrite pyrite sulphur Within Two Weeks One month Observations/Interpretations… • Initially abiotic oxidation of pyrite higher than bacterial oxidation of sulphur • Higher rates of pH drops at higher initial pH

13. Effect of substrate type on recovery… sulphur pyrite pyrite sulphur Within Two Weeks One month Observations/Interpretations… • Significant differences in leaching only observed after two weeks

14. Effect of media composition on pH/redox evolution... distilled water S0 with bacteria 9K media FeS2 with bacteria E(mV vs. SHE) pH FeS2 with bacteria 9K media S0 with bacteria distilled water Observations/Interpretations… • No pH drop or increase in redox potential for sterile media • Decrease in pH (increase in redox) for inoculated media attributed to oxidation of substrates by bacteria

15. Summary of effects of substrate type… Analysis… • Overall trend show that initially abiotic oxidation of pyrite higher than biotic oxidation of sulphur • Bacterial leaching effective after two weeks • S0 better than FeS2 sulphur Optical density pyrite pyrite (ΔpH) pyrite sulphur (ΔpH) sulphur sulphur % Ni extraction pyrite % Ni extraction pyrite sulphur

16. Conclusions… • Nickel laterite can be bioleached with the help of externally added sulphur containing substrates via sulphuric acid • Study shown relationships between bacterial activities depicted by acidification, and type of substrate • Study can be extended to other difficult-to-process low grade ores, e.g., silicate ores, oxidic converter furnace slags, and refractory oxides

17. Research required… • Carrying out testworks to simulate heap leaching • Need to determine the actual amount of acid produced, and or consumed • Need to detect, differentiate, enumerate the microorganisms during the leaching process so as to understand their interactive effects

18. Acknowledgements...

19. Optimisation problem… My job responsibility: Min distance {Q – A } Subject to availability of Question and Answer Constraints THANK YOU ALL…