Niobium and tantalum beneficiation using gas-phase fluorination

Abstract

The processing of minerals containing tantalum and niobium is a challenge that has most modern researchers focused on optimising the processes that have already reached scientific maturity. Ore digestion in aqueous mixtures of sulfuric and hydrofluoric acid, followed by selective liquid-liquid extraction, is the method of choice for recovery of tantalum and niobium from the parent minerals. As this method has significant environmental and practical drawbacks, there is a need for a new process to beneficiate these minerals. The Advanced Metals Initiative (AMI) programme of the Department of Science and Technology (DST) proposes that no tantalum or niobium values should leave South Africa without some degree of local beneficiation. A significant strategic advantage may be gained from developing a process which is economically viable and more environmentally friendly. This thesis proposes a technology which would circumvent many of the drawbacks of wet chemical systems. The proposed technology would use anhydrous fluorinating gases (HF(g) and F2) to convert the oxidic minerals to oxyfluorides and/or fluorides, followed by thermal separation. Since little is known about the reaction between the fluorinating agents mentioned and the ores containing Ta/Nb, a detailed study of these reactions and possible products for the current concept is realised. Oxyfluorides are the most probable intermediates during the fluorination process. As part of the research, the most likely oxyfluoride intermediates were synthesised. The details of their spectral and crystallographic properties are discussed. Their thermal properties were investigated; this showed that oxyfluorides can be used to develop a thermal separation process in either the high temperature (600-900 ºC) or low temperature region (150-200 ºC). Thermogravimetric analysis also suggests a difference in the decomposition pathways for niobium and tantalum oxyfluorides. Dioxyfluoride is the most stable of the oxyfluorides and is a necessary byproduct, regardless of which other oxyfluoride is synthesised, and may occur even during the synthesis of the pentafluorides. It was therefore considered imperative to understand the decomposition kinetics of the dioxyfluoride compounds, to calculate the decomposition activation energies, and to construct physical decomposition models for these compounds. By means of mechanistic methods, it is shown that the decomposition of the oxyfluorides occurs via Avrami-Erofeev A2 or A3 models and that for this process the activation energy for TaO2F (320 kJ.mol-1) is roughly double that for NbO2F (156 kJ.mol-1). Once the characterisation of the possible reaction intermediates had been completed, the reaction and interaction of F2 and anhydrous HF with pure metal oxides of Ta and Nb were investigated. To this end, both thermogravimetric and differential scanning calorimetry were employed. Thermodynamic calculations indicated that for both these fluorinating agents, the corresponding pentafluorides were the preferred (indeed the only) reaction products, though the experimental results showed that a whole range of oxyfluorides form. The data collected showed no evidence of a two-step mechanism, as has been observed for Nb2O5, for the fluorination of Ta2O5 with elemental fluorine. However, in both cases the rate-limiting step is governed by the contracting volume (R3) mechanistic. The activation energy for the Ta2O5 + F2 reaction is 63-67 kJ.mol-1, and leads to the formation of the pentafluoride without detectable oxyfluoride formation. A single ore containing tantalum and niobium was selected for study and characterised prior to evaluating its reaction with the chosen fluorinating gases. As the reaction products have a substantially more complex matrix, they were shown to be far less self-evident than in the studies conducted on the pure oxides. Nevertheless, it is shown that separation using this methodology is indeed feasible. Aided by techniques such as SEM and ICP-OES, it could be shown that physical and chemical changes occur in the mineral during the fluorination reaction. The concluding chapter considers the information assimilated during this study and provides likely scenarios for a process based on the selective volatilisation of tantalum and niobium fluorides and oxyfluorides. Two likely processes are postulated, the first one involving partial fluorination and sublimation, the second one complete fluorination to the pentafluoride.