Development of a capillary electrophoresis method for the analysis in alkaline media as polyoxoanions of two strategic metals: Niobium and tantalum

Abstract

Tantalum (Ta) and niobium (Nb) are two strategic metals essential to several key sectors, like the aerospace, gas and oil, nuclear and electronic industries, but their separation is really difficult due to their almost identical chemical properties. Whereas they are currently produced by hydrometallurgical processes using fluoride-based solutions, efforts are being made to develop cleaner processes by replacing the fluoride media by alkaline ones. However, methods to analyze Nb and Ta simultaneously in alkaline samples are lacking. In this work, we developed a capillary zone electrophoresis (CE) method able to separate and quantify Nb and Ta directly in alkaline media. This method takes advantage of the hexaniobate and hexatantalate ions which are naturally formed at pH>9 and absorb in the UV domain. First, the detection conditions, the background electrolyte (BGE) pH, the nature of the BGE co-ion and the internal standard (IS) were optimized by a systematic approach. As the BGE counter-ion nature modified the speciation of both ions, sodium- and lithium-based BGE were tested. For each alkaline cation, the BGE ionic strength and separation temperature were optimized using experimental designs. Since changes in the migration order of IS, Nb and Ta were observed within the experimental domain, the resolution was not a monotonic function of ionic strength and separation temperature. This forced us to develop an original data treatment for the prediction of the optimum separation conditions. Depending on the consideration of either peak widths or peak symmetries, with or without additional robustness constraints, four optima were predicted for each tested alkaline cation. The eight predicted optima were tested experimentally and the best experimental optimum was selected considering analysis time, resolution and robustness. The best separation was obtained at 31.0°C and in a BGE containing 10mM LiOH and 35mM LiCH3COO.The separation voltage was finally optimized, resulting in the separation of Nb, Ta, and IS in less than 2.5min, which is three times faster than any CE method ever reported for the separation of Nb and Ta (acidic media included). Some figures of merit of the method were determined such as linearity ranges and limits of quantitation. Finally, the method was successfully applied to the analysis of a real industrial sample.

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

“…In first investigations performed by Deblonde et al. , 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.…”

“…These hexameric forms are formed at a pH above 9.0, absorbed in the UV spectrum, and possess different electrophoretic mobilities. The method was first developed in and further optimized in on an Agilent G7100 instrument.…”

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

“…This similar radius also complicated the analytical separation of both compounds in previously tested techniques [1,2,4]. As stated above, the physicochemistry of tantalum and niobium was studied earlier and it was proven that, in addition to their UV absorption, the hexaniobate and hexatantalate ions, which are naturally formed at pH above 9, have readily different electrophoretic mobilities and can thus be separated by means of CE methods [9,15]. A difference in electrophoretic mobility is determined by a difference in the charge on the hydrated radius ratio of an ion.…”

“…Furthermore because of the reduced analysis time, lower voltages can be applied when facing a joule heating effect. Prerequisite for a short-end injection is a highly efficient method to compensate the loss of resolution due to the reduced migration length [15,[18][19][20].…”

“…7In a first part of the actual study the earlier developed method for the separation of Ta 6 and Nb 6 [15] was transferred to a Beckman instrument in the same laboratory and evaluated for the separation of all substituted ions. During this inter-instrumental transfer the earlier defined guidelines by De Cock et al [21,22] were applied.…”