Last Month's Most Accessed Feature: Metals for the modern world: Extracting tantalum and niobium from minerals

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  • Published: Dec 1, 2017
  • Categories: Ion Chromatography
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Metals for the modern world: Extracting tantalum and niobium from mineralsTablet computers and steel

They may not be household names, but the metals tantalum and niobium are essential components of our modern world. Tantalum produces highly efficient electronic capacitors for use in tablet computers, while niobium is used to strengthen steel and other alloys and to produce the superconductors found in magnetic resonance imaging (MRI) scanners.

In their natural state, they are also generally found together, as oxides within the minerals tantalite and columbite, which are essentially the same mineral expect tantalite contains more tantalum and columbite contains more niobium. Actually extracting tantalum and niobium from these minerals is not easy, though, which helps to explain why they’re both fairly expensive (with tantalum costing $500-$2000 a kilogram). The method involves dissolving the minerals in hydrofluoric acid to convert the tantalum and niobium oxides into fluorides, which unlike the oxides can then be separated from each other using techniques such as fractional crystallization or solvent extraction.

The use of large amounts of hydrofluoric acid and organic solvents makes this method highly environmentally unfriendly and even potentially dangerous, as it tends to release toxic hydrofluoric gas. So a team of South African chemists, led by Motlalepula Nete at the University of the Free State, set about developing an alternative method for separating tantalum and niobium that didn’t involve hydrofluoric acid or organic solvents. Instead, they decided to turn to ion-exchange chromatography, even though this had already been tried with limited success.

Transformed into anions

For this approach to work, the chemists needed a way to transform the uncharged tantalum and niobium oxides into ions. Based on findings from previous studies, they decided to try dissolving the minerals in a phosphate-containing solution at high temperatures, in order to convert the oxides into metal orthophosphate anions (Ta(PO4)2 and Nb(PO4)2).

They initially tested this phosphate process on a specially-prepared mixture of tantalum and niobium oxides, while then attempting to separate the freed tantalum and niobium on various strong and weak anion exchange resins while using a phosphoric acid solution as the mobile phase. This confirmed that the phosphate process could convert the tantalum and niobium oxides into anions, which were then captured on the anion exchange resins.

The strong anion exchange resins didn’t bind the tantalum and niobium anions very firmly, though, meaning they tended to elute at similar times, preventing a clear separation. The anions bound much more firmly to the weak anion exchange resins, and could then be separated from each other by simply increasing the concentration of the phosphoric acid solution. Nete and his colleagues found that 99% of the niobium would elute at a concentration of 8M and 98% of the tantalum would then elute at a concentration of 10M, producing an almost complete separation.

Mulitple metals from minerals

Next, they tried this novel method on a sample of tantalite, which as well as tantalum and niobium also contains major concentrations of iron and manganese, lower concentrations of titanium and uranium, and trace concentrations of metals such as tungsten and aluminum. All of which could potentially interfere with the separation.

They found that the phosphate-containing solution readily dissolved the mineral, and had the added bonus of converting many of the other metals into anions. So by gradually increasing the concentration of the phosphoric acid solution, Nete and his colleagues were not only able to separate the tantalum and niobium, but also the iron, manganese and uranium.

There was, however, some interference, because a small amount of the niobium did co-elute with the tantalum fraction and the main niobium fraction was contaminated with titanium. Still, the advantages this method has over the existing fluoride-based method in terms of safety and impact on the environment mean that it has real industrial potential, say the chemists.

Hydrometallurgy, 2017, 173, 192–198: "Non-fluoride dissolution of tantalum and niobium oxides and their separation using ion exchange"


Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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