The Solubility and Speciation of Tantalum in Fluoride-Bearing Aqueous Solutions

English Abstract:

Recent investigations (in press) have demonstrated that niobium is mobile in aqueous fluids at elevated temperature, and that this mobility increases with higher fluoride activity. There are, however, no data on the solubility and speciation in aqueous solutions of its geochemical “twin”, tantalum. Such data would permit assessment of the relative mobility of niobium and tantalum in hydrothermal fluids and their fractionation in various geologic environments. As with niobium, hard acid-soft base (HSAB) considerations suggest that fluoride is the ligand most likely to form stable complexes which will facilitate the development of significant concentrations of tantalum in hydrothermal fluids. Results of previous experiments by Zaraisky et al. (2008) conducted with columbite-(Mn) in fluoride-bearing solutions at elevated temperature and pressure suggest that tantalum is an order of magnitude less soluble than niobium, but no thermodynamic data or equilibrium constants were derived from these experiments.

In this study, we determined the solubility of tantalum oxide in aqueous solutions at elevated temperature (100-250 °C), variable fluoride activity (~10^-5-10⁰ mF), and variable pH (2.0 and 2.8). Our experiments involved determining the solubility of tantalum oxide in F-bearing aqueous solutions at temperatures up to 250 °C and vapour-saturated water pressure. The experiments were performed using the autoclave solubility method and employed a technique involving teflon tubes, which was developed previously for determining the solubility of REE fluorides in F-bearing solutions.

Preliminary results indicate that the speciation and solubility of tantalum are dependent on the activity of fluoride in the aqueous solution. At low fluoride activity (~ <10^-2.5 mF), niobium solubility is low, whereas at higher fluoride activity (~ >10^-2.5 mF), the solubility of tantalum increases by multiple orders of magnitude. However, at an equivalent fluoride activity the solubility of tantalum is more than an order of magnitude less than that of its twin, niobium. The rapid increase in tantalum solubility at higher fluoride activity is similar to that observed for Nb, and suggests that tantalum may dissolve as a hydroxide or hydroxyl-fluoride species in our experiments.