Kinetic study of niobium and tantalum hexameric forms and their substituted ions by capillary electrophoresis in alkaline medium

Abstract

In this work a capillary electrophoretic (CE) method is used for the kinetic study of the intermetallic substitutions in hexameric ions of two strategic metals, tantalum and niobium in an alkaline medium. Recently proposed processes for the production and analytical separation of tantalum and niobium that are faster, more economical and environmental friendly are based on the use of highly alkaline media. It was previously established that in these media, tantalum and niobium exist as hexameric species, HTaO (Ta) and HNb0 (Nb), which can be analysed with a CE method using an alkaline electrolyte and UV detection. However, when using the above method on an industrial sample a minor species that should correspond to the substituted TaNb form was observed. The purpose of the present study is to probe, by means of CE, the kinetic of the formation of substituted niobate-tantalate ions, TaNb (1 ≤ x ≤ 5), starting from mixtures of pure hexaniobate and hexatantalate ions. This study required the development of a new CE method allowing the separation of all the five substituted ions and their two non-substituted hexameric parent ions in less than seven minutes. In details, a previously developed separation method was transferred to a Beckman instrument and the separation improved by adjusting the total length, the applied voltage, the injection volume, the rinsing steps and the internal standard. The kinetic study shows that samples initially containing non-substituted hexameric forms of tantalum and niobium in a 1:1M ratio naturally form the five possible substituted species TaNb (1 ≤ x ≤ 5) after only a few hours which may represent an issue for future Nb-Ta separation processes operated in alkaline media. The developed method was also transferred to an Agilent instrument and the kinetic study repeated. Results obtained with the Agilent instrument corroborate those obtained with the Beckman instrument. The proposed electrophoretic separation method lays the ground for new analytical techniques that could help assessing the presence of substituted species that can be deleterious for Nb-Ta purification processes.

A fluoride-free liquid-liquid extraction process for the recovery and separation of niobium and tantalum from alkaline leach solutions

“…Surprisingly, a total loss of selectivity is observed when using the industrial Nb-Ta solution and both elements are extracted with identical extraction yields (within the experimental uncertainty). We previously showed by UV-vis spectrophotometry that the speciation of Nb in industrial solutions mainly consists of hexaniobate ions [45,48] x-8 (1 ≤ y ≤ 5), after a several hours or upon heating [55][56][57]. Taking these observations into account, given the high ratio Nb/Ta in the studied industrial samples, and due to the process by which they have been produced [45,51] Figure 2).…”

Interinstrumental transfer of a fast short‐end injection capillary electrophoresis method: Application to the separation of niobium, tantalum, and their substituted ions

“…Subsequently, the SEI method was transferred in De Cock et al. to a Beckman P/ACE CE system, due to the unavailability of the Agilent system, and in a next step adapted in order to obtain a quantitative, instead of the earlier informative, separation of not only Nb 6 and Ta 6 anions, but also all their substituted Nb 6‐x Ta x with 0 < x < 6 ions. This paper deals with the interinstrumental transfer of the new SEI method that was developed with a Beckman system to an Agilent G1600 instrument in the same laboratory.…”

“…In first investigations performed by Deblonde et al [10,11], the separation of the two Nb 6 and Ta 6 anions in a basic medium, using a 45 mM LiOH/LiCH 3 COOH BGE, an application of 10 kV, a temperature of 25.0°C, and an effective capillary length of 8.5 cm was achieved on an Agilent 7100 CE system. Subsequently, the SEI method was transferred in De Cock et al [12] to a Beckman P/ACE CE system, due to the unavailability of the Agilent system, and in a next step adapted in order to obtain a quantitative, instead of the earlier informative, separation of not only Nb 6 and Ta 6 anions, but also all their substituted Nb 6-x Ta x with 0 Ͻ x Ͻ 6 ions. This paper deals with the interinstrumental transfer of the new SEI method that was developed with a Beckman system to an Agilent G1600 instrument in the same laboratory.…”

“…The SEI method was further adapted to perform a kinetic study of Nb 6 , Ta 6 plus their substituted ions (Nb 6‐x Ta x with 0 ≤ x ≤ 6) with a Beckman P/ACE CE instrument . This paper deals with the transfer of this method optimized with a Beckman CE system to an Agilent setup.…”