Uranium Acid Leaching
Leaching is the chemical attack in order to produce a uranium solution. This is the main stage and has direct influence in costs. The type of ore and gangue determine the leaching process to be used. Thus, the process can be acid or alkaline.
Acid leaching is used more frequently because crushing and grinding promote good conditions for leaching, and also allows a better solid-liquid separation. This is not a selective process because there are impurities in the solution. The equipments employed must resist the acid attack.
The leaching agents to be employed are usually sulphuric, nitric, and phosphoric acid.
Alkaline leaching is more selective for uranium and very useful in the treatment of uranium ores where limestone is the main impurity. This type of gangue is very reactive and consumes high dosages of acid such as 100 kg/t.
The alkaline process has high cost and requires fine grinding and high temperature with residence time very long during leaching. The alkaline process is justified when the ore has a high grade of uranium.
4.2.1 Acid Leaching.
In this leaching process uranium is in solution in the form of uranyl ion UO22+. For hexavalent uranium, the reaction is:
UO3 + 2H+ = UO22+ + H2O
For tetravalent uranium is necessary uranium oxidation. The reaction can be represented as fallow:
UO2 + 2H+ +1/2 O2 = UO22+ + H2O
- Uranium oxidation is fast in acid conditions and is favored by ferric ions.
UO2 + 2Fe3+ = UO22+ + 2Fe2+
Ferric ion acts like a catalyzer transferring an electron. Other oxidants are no effective under normal conditions of pressure and temperature because oxidation-reduction potential of iron is closer to oxidation potential of uranium than other oxidants.
UO2 = UO22+ + 2e- Eo = -442 mV
Fe3+ + e- = Fe2+ Eo = -770 mV
MnO2 + 4H+ + 2e- = 2H2O + Mn2+ Eo = -1208 mV
O2 + 4H+ + 4e- = 2H2O Eo = -1229 mV
ClO3- + 4H+ +6e- = Cl- +2H2O +1/2O2 Eo = -1450 mV
Other oxidants need the presence of ferric ion so that the oxidant can react with iron and this with uranium.
- MnO2 + 2Fe2+ + 4H+ = 2Fe3+ + Mn2+ + 2H2O
UO2 + 2Fe3+ = UO22+ + 2Fe2+
Tetravalent uranium can be leached under appropriate conditions of oxidation and acidity. Pitchblende and Uraninite are leached at pH 1.5 to 2.0. Dadivite is leached with solution 7N sulphuric acid. Acid leaching preferably employs sulphuric acid because is economic and easy acquisition. Reactions between uranyl ion and sulphate ion are represented in the following reactions;
UO22+ + SO42- = UO2 SO4 K = 50
UO22+ + 2SO42- = [UO2 (SO4)2]2- K = 350
UO22+ + 3SO42- = [UO2 (SO4)3]4- K = 2500
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