Method validation for the quantification of impurities in Zirconium metal and other relevant Zr compounds.

English: Zirconium occurs in nature as a component of the lithosphere in various molecular fractions within a number of mineral ores. Since its discovery in 1789, many chemical processes have been developed to have zirconium in its pure and malleable form for different uses in various industries. These industries include the nuclear, jewellery, medicine and cosmetic industries. It is considered extremely important in the nuclear industry and is used, for example, in the aligning of nuclear arcs, its chemical and radiation resistance, metallurgical properties as well as its low thermal neutron capture cross section. For this purpose the metal has to be extremely pure (>99.9 %) and devoid of the elements which can render it unusable as fuel rod cladding material in the nuclear reactor. The objectives of this study were to: i) develop an alternative digestion method for zirconium to hydrofluoric acid, ii) develop an effective and efficient analytical method for the multi-element quantification of zirconium and its associated impurities in ultra-pure metal (foil: >99.98 % and rod: >99 %) and zirconium(IV) tetrafluoride samples at threshold and one-tenth of threshold by using commercially available equipment such as ICP-OES, iii) identify and compare the different analytical techniques and iv) determine the LOO/LOO of zirconium and its associated impurities and perform method validation on these analytical methods. Various digestion techniques, including individual mineral acids and their combinations, as well as microwave-assisted digestion were investigated with varying degrees of success. These included bench-top and microwave digestions with sulphuric acid (98 %), phosphoric acid (80 %) and aqua regia (nitric acid (55 %):hydrochloric acid (32 %), 3:1). The bench-top digestions of the zirconium rod samples by mineral acids gave average zirconium recoveries of 100.6 % for the sulphuric acid, 57.6 % and 89.6 % for phosphoric acid and aqua regia respectively, while the average recoveries for the bench-top digestions of the zirconium foil were 101.9 % for the sulphuric acid, 100.8 % and 85.1 % for the phosphoric acid and aqua regia, respectively. Microwave-assisted digestions of the metal samples with these mineral acids gave an average of 88.2 % for the phosphoric acid digestion, 100.2 % and 100.3 % for the sulphuric acid and aqua regia respectively for the zirconium rod digestion. The zirconium recoveries for the metal rod gave average recoveries of 32.7 %, 5.6 % and 97.4 % for phosphoric acid, aqua regia and sulphuric acid, respectively. Excellent recoveries for the zirconium(IV) tetrafluoride dissolutions were obtained at 99.5 % at the optical emission wavelengths of 343.823 nm and 101.7 % at 339.198 nm. Trace elements, which included aluminium, chromium and silicon, were quantified in this sample at 1.9 ppm, 0.1 ppm and 0.5 ppm, respectively. Potassium hexafluorozirconate was obtained by reacting KF and ZrF4 and gave zirconium recoveries of 100.9 % at 343.823 nm and 100.5 % at 339.198 nm. The product was also characterized using IR and the quantification of K using AA. The LOO and LOQ for zirconium were determined to be about 4 ppb at the two most sensitive wavelengths (343.823 nm and 339.198 nm) for the zirconium quantification. The elements were first quantified individually at one-tenth of the threshold and at the threshold of their permissible concentrations in the nuclear grade zirconium. The results obtained ranged from 98 % to 103 %. The elements were then batched into 3 groups which were quantified respectively, followed by their combinations and ultimately all the elements were quantified in a single batch at one-tenth of the threshold and at the threshold. The results obtained ranged from 99 % to 102 % for Group 1 (AI, Cr, Hf and Fe), 98 % to 102 % for Group 2 (B, Cd, Co, Cu and Mn) and 100 % to 102 % for Group 3 (Mo, Ni, Si, Ti, Wand U) at threshold recovery. Recoveries between 98 % and 103 % for Group 1, 99 % and 101 % for Group 2 and 99 and 102 % for Group 3 elements were obtained at one-tenth of the threshold. The quantification results obtained for the element combinations of Groups 1 and 2 at the threshold concentrations ranged from 99 % to 102 %, which were similar also for Groups 1 and 3 combinations while 98 % to 103.5 % were obtained for the Groups 2 and 3 combinations. At one-tenth of the threshold the recoveries were obtained between 98 % and 102 % for Groups 1 and 2, 70 % and 103.5 % for Groups 1 and 3 while 4 % and 102 % were achieved for Groups 2 and 3. In the quantitative analyses of all the elements combined, recoveries between 98.8 % and 102.3 % were obtained at threshold recovery while 97.8 % and 102 % were obtained at one-tenth of the threshold concentrations. Poor recoveries at one-tenth of the threshold for boron, cadmium and uranium were obtained in the quantifications of the element mixtures - this was due to these elements being quantitatively analyzed close to their LOQ's. The experimental results obtained for the quantitative analyses of zirconium and its specified impurities for nuclear purposes were validated using the hypothesis test of the t-statistic value (tcrit of ±2.31 for the pooled results in the quantification of zirconium metal samples and tcrit of ±4.30 for the quantitative analyses of zirconium and its impurities) at 95 % confidence interval to determine the acceptability of the results as recommended by IS017025. Other statistical parameters, such as the accuracy, precision and specificity, were investigated and the results were shown to be reproducible for all the experimental measurements.