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

[摘要] English: Zirconium occurs in nature as a component of the lithosphere in various molecular fractionswithin a number of mineral ores. Since its discovery in 1789, many chemical processes havebeen developed to have zirconium in its pure and malleable form for different uses in variousindustries. 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 thealigning of nuclear arcs, its chemical and radiation resistance, metallurgical properties as wellas its low thermal neutron capture cross section. For this purpose the metal has to beextremely pure (>99.9 %) and devoid of the elements which can render it unusable as fuelrod 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-elementquantification of zirconium and its associated impurities in ultra-pure metal (foil:>99.98 % and rod: >99 %) and zirconium(IV) tetrafluoride samples at threshold andone-tenth of threshold by using commercially available equipment such as ICP-OES,iii) identify and compare the different analytical techniques andiv) determine the LOO/LOO of zirconium and its associated impurities and performmethod validation on these analytical methods.Various digestion techniques, including individual mineral acids and their combinations, aswell as microwave-assisted digestion were investigated with varying degrees of success.These included bench-top and microwave digestions with sulphuric acid (98 %), phosphoricacid (80 %) and aqua regia (nitric acid (55 %):hydrochloric acid (32 %), 3:1). The bench-topdigestions of the zirconium rod samples by mineral acids gave average zirconium recoveriesof 100.6 % for the sulphuric acid, 57.6 % and 89.6 % for phosphoric acid and aqua regiarespectively, while the average recoveries for the bench-top digestions of the zirconium foilwere 101.9 % for the sulphuric acid, 100.8 % and 85.1 % for the phosphoric acid and aquaregia, respectively. Microwave-assisted digestions of the metal samples with these mineralacids gave an average of 88.2 % for the phosphoric acid digestion, 100.2 % and 100.3 % forthe sulphuric acid and aqua regia respectively for the zirconium rod digestion. The zirconiumrecoveries for the metal rod gave average recoveries of 32.7 %, 5.6 % and 97.4 % forphosphoric acid, aqua regia and sulphuric acid, respectively. Excellent recoveries for thezirconium(IV) tetrafluoride dissolutions were obtained at 99.5 % at the optical emissionwavelengths of 343.823 nm and 101.7 % at 339.198 nm. Trace elements, which includedaluminium, chromium and silicon, were quantified in this sample at 1.9 ppm, 0.1 ppm and0.5 ppm, respectively. Potassium hexafluorozirconate was obtained by reacting KF and ZrF4and gave zirconium recoveries of 100.9 % at 343.823 nm and 100.5 % at 339.198 nm. Theproduct was also characterized using IR and the quantification of K using AA. The LOO andLOQ 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 thethreshold of their permissible concentrations in the nuclear grade zirconium. The resultsobtained ranged from 98 % to 103 %. The elements were then batched into 3 groups whichwere quantified respectively, followed by their combinations and ultimately all the elementswere quantified in a single batch at one-tenth of the threshold and at the threshold. Theresults 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, WandU) at threshold recovery. Recoveries between 98 % and 103 % for Group 1, 99 % and101 % for Group 2 and 99 and 102 % for Group 3 elements were obtained at one-tenth ofthe threshold. The quantification results obtained for the element combinations of Groups 1and 2 at the threshold concentrations ranged from 99 % to 102 %, which were similar also forGroups 1 and 3 combinations while 98 % to 103.5 % were obtained for the Groups 2 and 3combinations. At one-tenth of the threshold the recoveries were obtained between 98 % and102 % 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 elementscombined, recoveries between 98.8 % and 102.3 % were obtained at threshold recoverywhile 97.8 % and 102 % were obtained at one-tenth of the threshold concentrations. Poorrecoveries at one-tenth of the threshold for boron, cadmium and uranium were obtained inthe quantifications of the element mixtures - this was due to these elements beingquantitatively analyzed close to their LOQ's.The experimental results obtained for the quantitative analyses of zirconium and its specifiedimpurities for nuclear purposes were validated using the hypothesis test of the t-statisticvalue (tcrit of ±2.31 for the pooled results in the quantification of zirconium metal samples andtcrit of ±4.30 for the quantitative analyses of zirconium and its impurities) at 95 % confidenceinterval to determine the acceptability of the results as recommended by IS017025. Otherstatistical parameters, such as the accuracy, precision and specificity, were investigated andthe results were shown to be reproducible for all the experimental measurements.

[发布机构] University of the Free State