Uranium Precipitation into Yellow cake Formation
4.4.1 Precipitation.
Uranium concentrate is precipitated from the final solution obtained from elution or re-extraction. In alkaline processes, uranium concentrate is precipitated by sodium hydroxide at pH 12 or by reducing UO2+2 ions with hydrogen.
2Na2UO2(CO3)3 + 6NaOH = NaU2O7 + 6Na2CO3 + 3H2O
In acid processes, uranium concentrate is precipitated by ammonium. Also, it is possible use magnesia, sodium hydroxide or hydrogen peroxide.
Precipitation by ammonium starts with a neutralization with lime or magnesia until pH 3.5 to 4.0. With this first reaction ammonium dosage is reduced and some impurities such as iron, aluminium or thorium are eliminated.
2[UO2 (SO4)3]4 - + 2NH3 = (NH4)2U2O7 + 4SO4 2-
The precipitated is not ammonium diuranate (ADU) else a mixture of diuranates, oxides, and sulphates. Its proportion in the concentrate depends on temperature, speed, and pH.
With low temperatures, pulp is viscous and difficult to be pulped and pumped.
With high temperatures, there is an excess of sulphate. Precipitation is good to 40 oC and pH 5.5. Immediately, pH increases its value until 7.5. The ADU formed is very crystalline.
Hydrogen peroxide precipitates uranium like uranium peroxide.
UO2 (SO4) +H2O2 + 2H2O = UO4 2H2O + H2SO4
This operation is done between 35 to 60 oC. Ferric ion is interference because reacts with hydrogen peroxide. For this reason iron must be precipitated first.
4.4.2 Filtration and drying.
Yellow cake is separated from the solution by settling and filtration. Drying is done under controlled temperature because can be produced a thermolysis.
9(NH4)2U2O7 + Q = 14NH3 + 6U3O8 + 15H2O + 2N2
Temperature changes gradually:
Humectation water is eliminated until 175 oC.
Ammonium is liberated and appear hydrated oxides from 260 to 320 oC.
Product is converted to UO3 until 400 oC.
Oxide UO2 is formed until 600 oC.
5.0 Final product.
5.1 Specifications.
The final product must have certain physical and chemical specifications which depend of its application. Generally, impurities can be accepted in certain range. Per example, Fe and C are accepted until few ppms.
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