Alkaline Leaching of Uranium

Uranium is soluble in alkaline solutions of carbonate or ammonium and forms the complex UO₂(CO₃)₃^4- that is very stable.

The main reactions are listed below.

Hexavalent uranium.

UO₃ + 3CO₃^2- + H₂O = [UO₂(CO₃)₃]^{4 -} + 2(OH)^-

Tetravalent uranium requires oxidant conditions.

UO₂ + 3CO₃^2- + H₂O + ½ O₂ = [UO₂(CO₃)₃]^4- + 2(OH)^-

OH^- ions increase pH. Uranium precipitation can occur if pH values are higher than his precipitation pH.

2[UO₂(CO₃)₃]^{4 -} + 6(OH)^- + 2Na⁺ = U₂O₇Na₂ + 6(CO₃)^2- + 3H₂O

In order to avoid uranium precipitation must be added bicarbonate so that OH- ion can be neutralized.

UO₃ + CO₃^2- + 2HCO₃^- = [UO₂(CO₃)₃]^{4 -} + H₂O

Bicarbonate is added considering the type of uranium mineral and gangue present in the ore. Sulphide, silicates and aluminates consume carbonate, and calcium hydroxide can be employed.

2FeS₂ + 7O₂ + 8CO₃^2- + 6H₂O = 2Fe(OH)₂ + 4SO₄^2- + 8HCO₃

SiO₂ + H₂O + CO₃^2- = SiO₃^2- + 2HCO₃^2-

Al₂O₃ + 3H₂O + CO₃^2- = 2AlO₂^2- + 2HCO₃^- + 2H₂O

Ca(OH)₂ + 2HCO₃^- = CaCO₃ + CO₃^2- + 2H₂O

Normal parameters during leaching are 40-50 g/l Na₂CO₃ and 10-20 g/l NaHCO₃. Alkaline leaching is appropriate for ores with high content of limestone due to the high consumption of acid. Limestone is not attacked by carbonates.

According to reaction (b) tetravalent uranium needs oxygen in alkaline leaching. Oxygen can be obtained directly or introduced in the ore.

Oxidation is possible employing oxidant reagents or catalyzers that act like intermediaries between the oxidant agent and the ore.

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