The determination of titanium, zirconium and hafnium in molybdenum, niobium and tantalum alloys by ion exchange Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r781wj620

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  • New alloys containing refractory metals have been developed by the aero-space industry to meet the demand for thermally resistant materials. The chemical analysis of these alloys has been handled successfully by the use of ion exchange. Hydrofluoric acid, either alone or combined with hydrochloric acid, has here-to-fore been used to dissolve these alloys and complex their constituents, thus forming metal-fluoride anionic species which are preferentially adsorbed by the resin. Large volumes of eluents, made up of rather concentrated hydrofluoric-hydrochloric acid solutions, are necessary to separate the adsorbed species. The anion exchange behavior of titanium, zirconium, hafnium, molybdenum, tantalum and niobium, in the aqueous and mixed solvent sulfuric-oxalic acid system, was studied for three reasons: 1. The lack of methods designed for the separation and determination of small amounts of titanium, zirconium or hafnium in molybdenum, tantalum, and niobium base alloys. 2. The inconveniences associated with the use of hydrofluoric acid solutions. 3. The recent findings in ion exchange chromatography that the introduction of an organic solvent into the eluent enhances the chance of finding the proper conditions of separation. The distribution coefficient concept and the plate theory were utilized to find the proper conditions of separation. The effect of oxalic acid concentration, sulfuric acid percentage and methanol percentage on the elution character of the six metal ions was investigated. It was found that greater adsorption is favored at low percentage of sulfuric acid and high concentration of oxalic acid. In the presence of both acids, methanol decreases the adsorption of molybdenum, titanium, tantalum and niobium, and increases the adsorption of zirconium and hafnium. As a result of the previous studies, 17 separation procedures, involving different combinations of the six metals investigated, were developed using the concept of the minimum height column. These procedures were tested on synthetic metal mixtures and found adequate. The range in which some of these procedures could be used is wide. As low as 0.05% titanium and 0.05% zirconium could be separated from a molybdenum base alloy and determined accurately. Large amounts of constituents can be separated and determined as well. In addition to designing these separation procedures, some conclusions were drawn regarding the complex formation involved: 1. Titanium does not form anionic complexes in the presence of sulfuric acid and needs high level of sulfate ion to form such complexes. 2. Zirconium sulfate complexes are more stable than the hafnium ones. 3. Molybdenum forms sulfato complexes which seem to be in slow equilibrium with some unadsorbable species. 4. All six metal oxalato complexes are stable at low hydrogen ion concentration. At high acidities these complexes break down due to the repression of the oxalic acid dissociation. Methanol tends to increase the adsorption of metal ions from strong acid solutions. In the presence of oxalic acid, however, the adsorption decreases. This is, probably, due to the suppression of the dissociation of oxalic acid.
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