Thermal desorption of molybdenum(VI)-oxide from loaded metal oxide ion exchangers

A new procedure for the final purification of 99Mo, generated for medical use by the reaction 235U (nth,f)99Mo, has been developed. It consists in the sorption of molybdate species on metal oxide ion exchangers from nitric acid medium followed by a thermal desorption of the volatile molybdenum (VI)-oxide from the adsorber matrix by applying temperatures up to 12500C. This method could effciently substitute the existing final purification of 99Mo, which is currently performed by sublimation of MoO3 from the residue of an evaporated 99Mo-concentrate - a labour intensive and time consuming operation. The investigations presented in this paper describe the thermal desorption of molybdenum (VI)-oxide species from Mo-loaded Al2O3 and SnO2 ion exchangers. The influence of temperature, sublimation time, Mo-loading, carrier gas atmosphere and grain size of the oxide particles on the sublimation yields has been determined. The assumption that water vapour present in the carrier gas applied during the volatilization would lead to a drastic increase in the sublimation yields was confirmed for both oxides. The release of molybdenum (VI)-oxide from the porous Al2O3 exchanger was found to be in good agreement with the bulk diffusion mechanism based on FICK's law. SnO2 exchanger demonstrated a distinct superiority for practical applications. Heated up to 12500C in presence of an O2/H2O-carrier gas, this less porous oxide permits quantitative Mo-yields within 30 minutes. Sublimation experiments with other fission products retained on the SnO2 exchanger resulted in the following decontamination factors: 1.5 for Te, 40 for Ru and >> 100 for Sn and Zr. (orig.)