Development and Scale-up of Chemical Process for Conversion of Impure Sodium Diuranate to Uranium Peroxide

Uranium mills following the sulfuric acid leaching route produce Uranium Peroxide (UP) as uranium ore concentrate (yellow cake), which is then refined to produce nuclear grade uranium products. Whereas the mills processing ores by alkaline leaching route generate Sodium Diuranate (SDU) as yellow cake, which commonly contains impurities viz. Zr, SiO2, Mo, Organic Carbon (OC). These impurities form muck, interfacial crud and acid insolubles during digestion and solvent extraction (SX) stages in refining process. Hence, SDU needs to be pre-treated before processing in the uranium refinery. India has substantial U reserves (>50%) hosted in carbonate minerals matrix which obviously need alkaline processing for extraction of U. In the present study, the origin of impurities in a SDU produced in an Indian Uranium mill was traced back to the mineralogical composition of the ore, chemical reactivity of minerals to the reagents, and build-up of impurities in process streams due to existence of multiple recycle streams in the milling process flowsheet. A process scheme has been developed for converting SDU into highly pure Heat Treated Uranium Peroxide (HTUP). The scheme involves digestion of SDU in sulfuric acid medium followed by separation of insolubles from the liquor which is later subjected to pH controlled U-peroxide precipitation process. The U-peroxide formed was heat treated to drive-off active oxygen and water of crystallization. A HTUP product assaying about 91% U3O8 was produced from an impure SDU assaying 74% U3O8. Process has been demonstrated at large scale (about 100 kg/batch) and engineering data and scale up parameters have been generated.