A contribution to the study of the separation of zirconium-hafnium in the MIBK-HSCN-HCl system

This dissertation presents the laboratory-scale studies, related to the zirconium/hafnium separation by the solvent extraction technique, in the MIBK-HSCN-HCl (methyl isobutyl ketone-thiocyanic acid-hydrochloric acid) system. The MIBK system constitutes one of the main processes for the production of nuclear grade Zr O2 and Hf O2, based on the preferential extraction of hafnium in the organic phase as a thiocyanic complex. The hafnium and zirconium oxides were obtained within nuclear purity requirements. Considerable variations in thiocyanate concentration and free acid content in the system were also observed. The process conditions that were monitored were found to be extremely favourable for the thiocyanic acid decomposition reactions to occur. The formation of a large amount of solid organic materials, both in the organic and aqueous process solutions were observed. In view of these facts, some additional studies were carried out to evaluate the influence of thiocyanate concentration and free acid content of the aqueous phase, parameters directly related to the thiocyanic acid chemical stability. The best results of zirconium/hafnium separation efficiency were obtained for free acid values in the range of 0.5 to 1.0 N, the thiocyanate concentration of the aqueous phase of 2.5 M and the thiocyanate concentration of the organic phase of 2.0 M. The experimental procedures used in this work consisted of single-batch extraction, discontinuous countercurrent and continuous countercurrent operations. In the latter, a battery of glass mixer-settlers was used. Analytical control of the process was carried out using X-ray fluorescence spectroscopy and optical emission spectrography. Additionally, analytical procedures for the determination of free acid content and thiocyanate concentration in the process solutions were evaluated and implemented in these studies. (author). 105 refs., 45 figs., 58 tabs