Analysis of zirconium containing materials using multiple digestion and spectrometric techniques

[摘要] English: The preparation of pure zirconium metal for nuclear applications is difficult due to the non-reactivity of zirconium minerals, such as zircon. The ability to accurately analyse zirconium-containing materials across the whole beneficiation chain is of crucial importance to the zirconium industry as a whole. The development of such an analytical technique is problematic, however, as the very properties which make these materials desirable also make quantification of their components extremely difficult.Certified reference materials for the fluoride-containing Necsa zirconium process products were not available. Therefore in-house reference materials were created by crystallisation of several (cation)xZrF4+x compounds. Potassium catena di-μ-fluorido-tetrafluoridozirconate(IV), cesium hexafluoridozirconate(IV) and tetraethyl ammonium catena di-μ-fluorido-bis-(trifluoridozirconate(IV)) monohydrate were prepared and characterised by X-ray crystallography and qualitative XRD. Coordination numbers for the zirconium atoms in each of these crystals were found to be 8, 6 and 7 respectively. Bridging fluorine bond lengths were determined to be approximately 2.06 and 1.97 Å for the potassium and tetraethyl ammonium complexes while terminal bond lengths were found to be 2.17 (potassium), 2.007 (cesium) and 2.15 (tetraethyl ammonium) Å. ICP-OES lower limits of detection for zirconium in the 3.25% nitric acid matrix were found to be 1.6 ppb with lower limits of quantification being ten times this value. ICP-OES zirconium recoveries for these crystals were 101(1) and 100(2)% for the potassium and cesium crystals respectively.Dissolution of various commercial and Necsa process samples was problematic and thus several digestion methods were investigated. Sulphuric acid, ammonium bifluoride and hydrofluoric acid were all investigated along with microwave assistance. A microwave-assisted acid digestion method was developed capable of complete dissolution of all zirconium compounds with ICP-OES analytical recoveries of 102.0(9), 100(2) and 101(3)% for 99.98% zirconium metal foil, ZrC and ZrH2 respectively.In order to circumvent the dissolution step a solid state GD-OES method was developed wherein sample powders were pressed into disks with a binder material, either copper or graphite. Initially instrument response across different samples was inconsistent but after optimisation of several instrument parameters, such as applied voltage and pre-burn time, a calibration curve with a R2 value of 0.9805 was achieved using multiple sample materials. This was achieved using the radio frequency glow discharge source operating at 900 V applied voltage and 14 W applied power with a 5-minute pre-burn period. Results for Necsa process products were largely in line with those achieved by the ICP-OES method.

[发布机构] University of the Free State