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

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

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  Nickel laterites account for almost 70% of the world’s land based nickel reserves, but less than 50% production Ni atoms

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 High Pressure Acid Leaching – complex controls, equipment, highly skilled labour, etc  Caron process – front end is energy intensive Caron process – front end is energy intensive

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

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

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

 Evaluate the efficiency of elemental sulphur or pyrite as substrates for the chemolithotrophic bacteria dissolution of nickel laterites 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 Study objectives…

Chemical composition of various oxides in the ore... Oxide% Mass SiO Fe 2 O Cr 2 O Al 2 O MgO7.5 NiO1.9 CoO0.3 CaO0.7 MnO1.3 S<0.1 P0.1 Total  100 Mineralogical Make up SiO 2 : predominant (Fe,Ni)O(OH), Fe 2 O 3.H 2 O, (Mg,Ni) 3 Si 2 O 5 (OH) 4, Ca 2 Mg 5 Si 8 O 22 (OH) 2

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

Effect of substrate type on bacterial population... pyrite sulphur Optical density at 550 nm Observations/Interpretations…  More bacteria produced with sulphur as a substrate than with FeS 2 substrate More bacteria produced with sulphur as a substrate than with FeS 2 substrate  Thermodynamically more energy created from oxidation of S 0 than Fe 2+ ions Thermodynamically more energy created from oxidation of S 0 than Fe 2+ ions

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

Effect of substrate type on recovery… pyrite Within Two WeeksOne month sulphur pyrite Observations/Interpretations…  Significant differences in leaching only observed after two weeks Significant differences in leaching only observed after two weeks sulphur

Effect of media composition on pH/redox evolution... pH E(mV vs. SHE) S 0 with bacteria FeS 2 with bacteria distilled water 9K media Observations/Interpretations…  No pH drop or increase in redox potential for sterile media 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 Decrease in pH (increase in redox) for inoculated media attributed to oxidation of substrates by bacteria distilled water 9K media FeS 2 with bacteria S 0 with bacteria

Summary of effects of substrate type… Analysis…  Overall trend show that initially abiotic oxidation of pyrite higher than biotic oxidation of sulphur Overall trend show that initially abiotic oxidation of pyrite higher than biotic oxidation of sulphur  Bacterial leaching effective after two weeks Bacterial leaching effective after two weeks  S 0 better than FeS 2 S 0 better than FeS 2 sulphur pyrite sulphur Optical density % Ni extraction (ΔpH) % Ni extraction (ΔpH)

 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 Conclusions…  Nickel laterite can be bioleached with the help of externally added sulphur containing substrates via sulphuric acid

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

Acknowledgements...

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