Zirconium and hafnium separation in different inorganic and natural compounds
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Zirconium (Zr) and hafnium (Hf) co-exist in the mineral zircon ore and its treated modified form, namely Plasma-Dissociated Zircon (PDZ), which always contain a small amount of Hf ranging between 1-3 %. The physical and chemical properties of Zr and Hf are almost identical and their separation is notoriously difficult, tedious and involves expensive processes. The purpose of this study was to initially investigate the possible separation of (Zr/Hf)O2 in inorganic salts and PDZ ((Zr/Hf)O2.SiO2) and apply these optimum separation conditions to separate Zr and Hf from PDZ. The dissolution of the inorganic salts were done using the flux fusion technique during which a mixture of 90.9 % ZrO2 and 9.09 % HfO2 (try to replicate the natural abundance in minerals) were fused with NH4F.HF as flux. The successful dissolution of the metal oxides was confirmed by total and accurate recovery of 100.6(2) % for ZrO2 while unexpectedly high HfO2 recovery was (121.2(9) %) was obtained. Possible solution matrix effects such as high F- concentration, were suspected as reasons for the high Hf recovery. H2SO4 was added to flux mixture and excess fluoride was removed by the evaporation of HF. This variation led to excellent Hf recoveries and quantitative results indicated the recovery of 100.1(2) % for Zr and 100(2) % for Hf. This method was subsequently used for the dissolution of PDZ and the analytical results indicated the presence of 66.0(4) % for Zr and 1.43(1) % for Hf. These fluoride solutions were subsequently investigated for the possible separation of Zr and Hf using, ion exchange, solvent extraction and microwave assisted dissolution. The separation of Zr and Hf in the fluoride matrix was investigated with an ion exchange process. Three different anion resins, namely Dowex Marathon wba, Dowex 21k and Amberlite IRA-900 were investigated for elemental separation. The strong anion exchanger resin, Amberlite IRA-900 was selected and different experimental parameters such as flow rate, eluent and eluent concentrations were investigated. Quantitative results indicated the preferential elution of Zr over Hf. At 0.05 M HCl only Zr was eluted while Hf was completely retained in the column and the recovery of Zr was 86.44 % from the inorganic Zr/Hf mixture. The optimum conditions, Amberlite IRA-900 resin, 0.05 M HCl and 20 cm column length, which are developed for the inorganic Zr/Hf mixture, were applied on the PDZ material and the recovery of Zr was 24(6) %. The isolated of ZrO2 from this reaction mixture, was re-dissolved using NH4F.HF as flux. The concentration of Zr was quantitatively determined using ICP-OES. The obtained average metal recoveries were 77.8(7) and 0.11(0) % for Zr and Hf respectively which are extremely promising, pointing to the separation of the two elements and the removal of the Hf from the Zr. The drawback to this method is, the low Zr recovery (compared to the amounts initially used in the separation process). Solvent extraction was the next technique to be investigated for separation of Zr and Hf as an alternative to ion exchange due to low Zr recoveries obtained in ion exchange separation method. The results obtained using MIBK as an extractant from H2SO4 solutions indicated a slight preferential extraction of Zr into the organic layer, leaving Hf in the aqueous layer with recoveries of 65(1) % Zr and 4(1) % Hf. Microwave assisted digestion in H2SO4 of both (Zr/Hf)O2 and PDZ were inconclusive. Validation of the analytical results using ICP-OES was also performed. Most of the results obtained for the Zr and Hf quantification in the inorganic salt ((Zr/Hf)O2), were accepted at the 95% confidence interval. However, other results indicated poor precision and accuracy of Hf hence the null hypothesis was rejected.