The evaluation of various oxidants used in acid leaching of uranium

SAIMM Hydrometallurgy Conference, February 2009

Riaan Venter – Senior Process Engineer

Contents

• Introduction

• Mineralogy

• Simple chemistry

• Oxidants

• Conclusions

Introduction

• In nature uranium occurs in tetravalent (U4+) and hexavalent (U6+) states.

• U4+ has very low solubility in acid and alkaline solutions.

• U4+ must be oxidised to U6+ state, which has higher solubility.

• Proper oxidising conditions must be maintained to achieve high uranium extraction.

Introduction

When selecting an oxidant on commercial scale:

• Effectiveness to maintain oxidising environment

• Availability and cost– Logistics to get to site– Administering to leach

Introduction

Oxidants considered

• Manganese dioxide (MnO2) as pyrolusite

• Sodium chlorate (NaClO3)

• Hydrogen peroxide

– As H2O2

– As Caro’s acid (H2SO5)

• Oxygen in pressure leaching circuits

• Sulphur dioxide/air (oxygen) mixture

Mineralogy

• Primary uranium ores found in veins or pegmatites

• Secondary ores found in weathered zones of primary deposits and precipitated in sediments

• Multiple oxides, complex associations with rare earths

Mineralogy

• Minerals containing uranium in the tetravalent state:– Uraninite (predominantly tetravalent)

• Witwatersrand ores in SA• Rossing in Namibia

– Coffinite• Kayelekera in Malawi

• Minerals containing uranium in the hexavalent state:– Carnotite

• Langer Heinrich in Namibia• Trekkopje in Namibia

– Phosphate deposits• Bakouma in CAR• Commercial producers of phosphoric acid

Acid Leach Flowsheet

(Lunt and Holden, 2006)

Chemistry

• U4+ must be oxidised to U6+

• Ferric iron acts as principle oxidant

• Iron is present as constituent in the ore or introduced as metallic iron

• Uranium dissolution in sulphuric acid

UO2 + 2Fe3+ → UO22+ + 2Fe2+

2Fe2+ + MnO2 + 4H+ → 2Fe3+ + Mn2+ + 2H2O

Oxidants

Manganese dioxide (MnO2)

• Traditional oxidant used as pyrolusite on Witwatersrand ores and elsewhere

• Oxidation reaction:

2Fe2+ + MnO2 + 4H+ → 2Fe3+ + Mn2+ + 2H2O

• Each mole of MnO2 requires 2 moles of acid

• Commercially available pyrolusite contains 30% to 50% of MnO2. Remainder potential acid consumers.

• First choice for Witwatersrand mines in the past as it was freely available. Was used in preference to SO2/air.

• Vaal Reefs South Uranium Plant still operates with this flowsheet and has been for the past 30 years.

Oxidants

Manganese dioxide (MnO2)

• Advantages:– Added as a slurry– No special agitation required and does not dissociate if not reacted

immediately.• Disadvantages:

– Needs milling circuit to produce slurry with fine solids– Availability and environmental considerations weighs against it.

• South Africa is an exporter of manganese ores from Northern and Eastern Cape, but long distances results in increased transport costs.

• Relatively low MnO2 concentrations in commercially available pyrolusite results in high volumes needed to be transported to site.

– Environmental effects involves Mn2+ in solution. Pyrite oxidation will lower pH on slimes dams and soluble manganese might enter water sources.

OxidantsSodium Chlorate (NaClO3)

• First choice oxidant in North American uranium plants• Oxidation reaction:

6Fe2+ + NaClO3 + 6H+ → 6Fe3+ + NaCl + 3H2O

• Each mole of NaClO3 requires 3 moles of acid • In South Africa traditionally not considered as a result of cheaper

and more convenient alternatives• Advantages:

• NaClO3 added to the leach as a solution, i.e. no special agitation necessary

• Disadvantages:• During reaction in the leach chloride ions goes into solution with

adverse effects on IX and materials of construction• Sodium chlorate is relatively expensive• Auto-ignition risk when in contact with organic – needs

procedures and proper engineering for safe handling

OxidantsHydrogen Peroxide (H2O2)

• Used as oxidant in 1980’s in Australia, pyrolusite replaced with Caro’s Acid.

• Can be used as H2O2 or can be made up to Caro’s acid by reaction with H2SO4

H2O2 + H2SO4 ↔ H2SO5 + H2O

• Oxidation reaction:

Caro’s acid: 2Fe2+ + H2SO5 + 2H+ → 2Fe3+ + H2SO4 + H2O

Hydrogen peroxide: 2Fe2+ + H2O2 + 2H+ → 2Fe3+ + 2H2O

• Each mole of H2O2 or H2SO5 requires 1 mole of acid

OxidantsHydrogen Peroxide (H2O2)• Advantages:

– Compared to pyrolusite:• Similar extractions to pyrolusite reported with reduction in

acid consumption• Reduction in lime consumption• No manganese or gangue minerals• Cleaner oxidant handling• Simplified effluent handling

• Disadvantages:– H2O2 will dissociate if no rapid reaction with iron in solution– Difficulties experienced with dispersion on existing operation

when changed from pyrolusite to H2O2

– H2O2 is hazardous to handle and transport - needs procedures and proper engineering for safe handling

– Supply could become a challenge

OxidantsHydrogen Peroxide (H2O2)

• Using Caro’s acid reduces H2O2 consumption, more efficient use of H2O2

• H2O2 can also be used for precipitation of UO4 instead of ADU, which eliminates ammonia from the flowsheet

• Gold plants already using H2O2 for cyanide destruction storage, use and availability is an advantage

• Previously discounted based on cost, but reported to produce a purer product

• Very favourable from environmental perspective, produces only water

• Production of Caro’s acid was traditionally a challenge as a result of high heat generation by reaction of H2SO4 with H2O2. • Lately made in situ by feeding both into a funnel device with

heat dissipated into slurry below.

OxidantsOxygen

• Uranium ores containing sulphidic minerals can be leached by adding oxygen at elevated temperatures and pressures

• Addition of sulphides to ores that do not contain sulphidic minerals has also been proposed

• Sulphuric acid and ferric sulphate are generated in-situ by reaction of oxygen with sulphides

OxidantsOxygen

• With pyrite and uraninite present the following reactions are expected:

Pyrite oxidised to produce soluble iron and acid:

2FeS2 + 7O2 + H2O → 2Fe2SO4 + 2H2SO4

Ferrous iron oxidised to ferric iron by oxygen:

2FeSO4 + H2SO4 + ½O2 → Fe2(SO4)3 + H2O

Iron hydrolysis removes iron at T > 170 ºC

Fe2(SO4)3 + 6H2O → 2Fe(OH)3 + 3H2SO4

Fe2(SO4)3 + 3H2O → Fe2O3 + 3H2SO4

Jarosite also forms during hydrolysis

3Fe3+ + 2SO42- + xM+ + (7-x)H2O → Mx(H3O)1-x[Fe3(SO4)2(OH)6] + (5+x)H+

OxidantsOxygen• Dissolution of uranium:

U4+ oxidised by Fe3+

UO2 + Fe2(SO4)3 → UO2SO4 + 2FeSO4

Dissolution of U4+ by oxygen

UO2 + H2SO4 + ½O2 → UO2SO4 + H2O

U6+ dissolves in presence of H2SO4

UO3 + H2SO4 → UO2SO4 + H2O

Acid consuming gangue leached by acid

CaCO3 + H2SO4 → CaSO4 + CO2↑ + H2O

• Reactions proceed more rapidly at high pressure and temperature

OxidantsOxygen

• Advantages:– Improved extraction– Decreased operating costs– Decreased impurities and free acid in leach solutions– Improved slurry filtration– Increased recovery of gold and uranium from pyrite

• Disadvantages– Increased corrosion and maintenance

– Production of soluble SiO2 which contaminates resin and forms crud

OxidantsSulphur dioxide/air (SO2/air)

• Used in iron and manganese removal in cobalt circuits

• First suggested by workers in USA, operating conditions and detailed chemistry determined by GML in South Africa

• Oxygen in the air, together with SO2 is responsible for oxidation

2Fe2+ + SO2 + O2 → 2Fe3+ + SO42-

• SO2 and O2 required in solution.

OxidantsSulphur dioxide/air (SO2/air)

• Mass transfer limits maximum reaction rate as a result of lower solubility of O2 compared to SO2

• Oxidation rate is controlled by the SO2/O2 ratio and rate of oxygen mass transfer, independent of Fe2+ concentration

• Higher oxidation rates obtained in solutions than in slurries

• Once SO2 flow rate increase above O2 mass transfer rate oxidation rate decreases as a result of reducing conditions caused by the SO2

• Main contributor to reduced SO2 efficiency is side reaction that produces H2SO4

SO2 + 1/2O2 + H2O → H2SO4

OxidantsSulphur dioxide/air (SO2/air)

• Large scale implementation has a number of challenges:

– For slurries dispersion agitation is necessary with more installed and utilised power

– SO2 source:

When taken from a sulphur burner or acid plant care must be taken to ensure that the SO2/air ratio is satisfactory, might have higher nitrogen content

– Any SO2 escaping from the leach tanks need to be passed through a scrubbing system

– Relatively low SO2 concentrations in the gas stream gives rise to large gas flow rates

– SO2/air mixture might need to be introduced against large slurry heads which will influence the partial pressure of the gas mixture.

– Oxidising Fe in a solution stream is easier but need to ensure that sufficient Fe is oxidised to oxidise the uranium

Conclusion

• Traditionally pyrolusite was used as oxidant in acid leach of uranium in Southern Africa

– Availability and logistics as well as environmental issues seem to be changing this tendency

– Increased acid consumption as a result of gangue minerals

• SO2/air:

– Works satisfactory but has engineering challenges.

– Very attractive option if acid plant on site and with low sulphur prices

– Lower acid consumption, as SO2 converted to acid

• Hydrogen peroxide:

– An attractive alternative with no environmental effects

– Reduced acid and oxidant consumption compared to pyrolusite.

– Handling and transport might be a challenge and availability an issue

– Might be costly compared to other oxidants

Conclusion

• Sodium chlorate:– Not really considered as a result of introduction of chlorides into

the system– Also has an explosion risk

• Oxygen: – Works well at high temperatures and pressures. – Sulphuric acid and ferric sulphate generated in-situ

Conclusion

• All the oxidants described work adequately in acid leaching of uranium.

• Possible to engineer solutions to introduce the oxidant into the leach slurry on plant scale.

• Factors that play a significant role in selecting a suitable oxidant:– Availability and supply of the oxidant – Cost of the oxidant– Environmental impact of the oxidant. – All three of these issues will be impacted on by the location of

the plant and possible sources of oxidant close to the operation.

Acknowledgements

The authors would like to thank the management of GRD Minproc (Pty)Ltd for permission to present this paper and to acknowledge the input of their colleagues in undertaking uranium projects and uranium feasibility studies.