Quantification of tantalum in series of tantalum-containing compounds

Abstract

The main objective of this research was firstly the successful digestion of a series of tantalum containing compounds, with specific exclusion of HF, and secondly, the accurate quantification of tantalum with the help of inductively-coupled plasma optical-emission spectrometry (ICP-OES). The series of tantalum containing compounds include TaF5, TaCl5, Ta2O5, Ta-metal, Ta-CRM (certified reference material), as well as two types of tantalite-ore. The goal of the study of the pentahalide compounds was to verify the use of ICP-OES as analytical method in terms of tantalum recovery. The aqueous dissolution and subsequent quantification of both these compounds was very successful and yielded recovery results of 96 % and 100 % for the fluoride and chloride salts respectively. Validation of the dissolution and quantification of the pentafluoride was within an acceptable range, while the dissolution and quantification methods for the pentachloride was considered exceptionally successful with a recovery of 100.05 % and a t-value = 0.95. Digestion results showed that tantalum pentoxide was considerably more difficult to dissolve than the pentahalides, but good results were eventually obtained after experimentation with various digestion methods. The most successful digestion method was obtained with the help of a high temperature (1100 °C) lithium tetraborate flux, after which the pentoxide melt was dissolved with a mixture of phosphoric acid and methanol. Tantalum recoveries of ca. 97 % were obtained with this method. This study also showed that the CRM and the tantalum metal did not dissolve easily with any of the developed digestion methods. Despite the fact that tantalum is present in the CRM in the form of tantalum pentoxide (0.236 %), the method developed for tantalum pentoxide dissolution did not succeed in completely dissolving the CRM. A qualitative study showed that there are a variety of elements present in the CRM, including Al, Si and Fe, of which especially Al has the potential of interfering with tantalum quantifications at certain chosen wavelengths. Tantalum recoveries of 89.75 % were obtained for the CRM. Tantalum metal is known to be chemically inert in some of the most extreme chemical environments. Therefore it was no surprise that none of the digestion methods investigated during this study could achieve complete dissolution of the metal, and a maximum recovery of only 90 % was obtained for the most successful method. Of the two types of ore analysed, Tan-A yielded the best result with a calculated tantalum pentoxide content of 27.69 % compared to the accepted value of 27.17 %. The Sample 1 ore yielded an average tantalum pentoxide content of 29.98 % and although this result was not statistically the same as the results obtained by previous studies, the digestion and subsequent recovery was still considered successful. The natural magnetic characteristics of the Sample 1 ore were used to separate some of the impurities from the original ore before digestion. This magnetic separation was seen as a huge success and indicated possibilities for future studies. The two main components that were almost completely separated from the original ore were the iron- and titanium-associated ores. The metal oxide content after separation was 1.41 % and 0.83 % for ironand titanium oxides respectively, where it was 18.48 % and 10.68 % before separation. Lastly, complete method validation was performed on all the developed digestion and quantification methods for all the tantalum containing compounds to determine efficiency, accuracy and precision in order to statistically judge the effectiveness of each method. This validation has also identified a few gaps in this study, which indicated excellent opportunities for interesting and insightful future studies.