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 EngineeringUniversity 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

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

Criteria used to assess effectiveness…

Microbial populations

Generation of acidity

Nickel recovery

Study objectives…

Chemical composition of various oxides in the ore...

Oxide % Mass

SiO252.8

Fe2O321.9

Cr2O31.0

Al2O32.5

MgO 7.5

NiO 1.9

CoO 0.3

CaO 0.7

MnO 1.3

S <0.1

P 0.1

Total 100

Mineralogical Make up

SiO2: predominant(Fe,Ni)O(OH), Fe2O3.H2O,

(Mg,Ni)3Si2O5(OH)4,

Ca2Mg5Si8O22(OH)2

Experimentation and Analysis...

Leaching carried out in a platform shaker

Apparent bacterial population determination

UV-Vis spectrophotometer

Analysis of nickel in solution

Atomic Absorption Spectrometer

Experimental Conditions

Same amount of sulphur by mass in each substrateTemperature: 30°C

Agitation speed: 190 rpmPulp density: 5g/100mls

Effect of substrate type on bacterial population...

pyrite

sulphur

Opt

ical

den

sity

at

550

nm

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

Effect of substrate type on rate of pH drops…

pyrite

pyrite

sulphur sulphur

Observations/Interpretations…

Initially abiotic oxidation of pyrite higher than bacterial oxidation of

sulphur

Higher rates of pH drops at higher initial pH

Within Two Weeks One month

pH v

aria

nce

(ΔpH

)

pH v

aria

nce

(ΔpH

)

Effect of substrate type on recovery…

pyrite

Within Two Weeks One month

sulphur

pyrite

Observations/Interpretations…

Significant differences in leaching only observed after two weeks

sulphur

Effect of media composition on pH/redox evolution...

pH

E(m

V v

s. S

HE

)

S 0 with bacteria

FeS2 with bacteria

distilled water

9K media

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

distilled water

9K media

FeS 2 with bacteria

S0 with

bacteria

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

pyrite

pyrite

pyrite pyrite

pyrite

sulphur

sulphur

sulphur

sulphur

Opt

ical

den

sity

% N

i ext

ract

ion

(ΔpH

)%

Ni e

xtra

ctio

n

(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...

My job responsibility: Min distance {Q – A } Subject to availability of Question

and Answer Constraints

Optimisation problem…

THANK YOU ALL…