Selective elimination of natural radionuclides during the processing of high grade monazite concentrates by caustic conversion method

This work is directed for removal of the nondesired species (²²⁸Ra, ²²⁶Ra, ²²³Ra, ²¹⁰Pb, Th(IV) and Fe(III)) in the rare earth chloride (RECl₃) liquor before separation of Ln(III). The different factors affecting elimination of radium-isotopes, lead (²¹⁰Pb), Th(IV) and Fe(III) from the RECl₃ liquor, have been investigated and optimized. The results indicated that the activity concentration of radionuclides in RECl₃ liquor was above the safe limits required during the separation process of Ln(III). Adjustment of pH 3±0.1 leads to eliminate 14±1% of radionuclides and Th(IV), and 40±3% of Fe(III), while 12±1% of Ln(III) was lost. The developed method shows that more than 95% of the nondesired species was selectively removed when the liquor was eliminated by potassium sulfate or sulfuric acid solutions in presence of Ba/Pb-carrier (1: 1). About 20-83% of Ln(III) was lost when the non-desired species removed by sodium or ammonium sulfate or potassium chromate solutions. Fe(III) interfered with Ln(III) when radionuclides and Th(IV) were eliminated by 2.6M H₂SO₄ in presence of Ba/Pb-carrier. Finally, use of 0.23M K₂SO₄ or 2.6M H₂SO₄ was efficient to reduce level of ²²⁸Ra, ²²⁶Ra, ²²³Ra and ²¹⁰Pb to the safe limits in viewpoint of radiation protection. In addition, the interfered Th(IV) and Fe(III) were also eliminated efficiently from RECl₃ liquor before the chemical processing of Ln(III).