The hunt for rare earths: Removing metal oxides to improve rare earth detection

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  • Published: Apr 9, 2012
  • Author: Jon Evans
  • Channels: Ion Chromatography
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Technology critical

The hunt for rare earths: Removing metal oxides to improve rare earth detection 

From toiling away pretty much unnoticed, rare earth metals have lately found themselves making headlines. Comprising the 15 lanthanide elements, from lanthanum to lutetium, together with scandium and yttrium, which are often found in the same ore deposits as the lanthanides, the rare earths are crucial to many of our modern technologies. The specific configuration of electrons around their nuclei provide the rare earths with a variety of highly useful optical, electronic and magnetic properties. As such, they are essential to the workings of everything from mobile phones to hard disk drives to wind turbines.

Their recent notoriety stems from concerns about whether the supply of rare earths can keep up with the growing worldwide demand for these modern technologies. Despite their name, most of the rare earth metals are not actually that rare in the Earth's crust, but unlike metals such as iron and copper they don't form rich deposits. Most deposits contain less than 5% rare earth metals, making them tricky and expensive to extract.

Furthermore, one country, China, currently monopolises the supply of rare earths, accounting for around 95% of rare earth production. To make matters worse, in order to spur its own high-tech industries, China has begun to restrict its rare earth exports, prompting the US, the EU and Japan to lodge a complaint with the World Trade Organization.

 

Interfering oxides

Rare earths also have other uses; for example, their concentrations in rocks provide a handy marker for many geological processes. So for both modern technologies and geology, it's useful to be able to detect and accurately measure the concentration of specific rare earths in rocks and ore.

Although there are several techniques for doing this, most of which involve inductively coupled plasma-mass spectrometry (ICP-MS), they struggle to detect concentrations much below the microgram per gram level. In many rocks, however, rare earths are only present at nanogram per gram levels.

Part of the reason for the lack of sensitivity is that other compounds in the rock tend to interfere with the analysis, especially metal oxides such as barium oxide. This led a team of French scientists to wonder whether the sensitivity could be improved by separating the rare earth metals from the metal oxides by ion-exchange chromatography (IEC) prior to analysis by ICP-MS.

'By removing all other trace elements, we [can] increase the accuracy and the reproducibility of the rare earth measurements,' team member Marc Ulrich at the University of New Caledonia told separationsNOW.

 

Correcting for drift

Ulrich and his colleagues came up with a method that involves dissolving the rock samples in an acid mixture before loading it onto an IEC column. They then employ a mixture of hydrochloric acid and nitric acid to elute the metal oxides from the column and nitric acid on its own to elute the rare earths.

Using this method, the scientists found that they could indeed separate the interfering metal oxides, including oxides of nickel, chromium, strontium and barium, from the rare earths, allowing the rare earths to be detected by ICP-MS at nanogram per gram levels. Testing the method on eight geological reference materials, including three samples with very low concentrations of rare earths, the scientists measured concentrations for individual rare earths that were very close to previously published values (mainly within 5%), even though these values were obtained using much more complicated analytical procedures.

The scientists were able to improve this accuracy still further, as well as improve the reproducibility of the method, by spiking the samples with the rare earth element thulium. This allowed them to correct for the unavoidable drift that occurs over multiple analyses, reducing the difference in the measured concentrations between different analyses to less than 1%.

So now we have a method that can easily detect rare earths at very low concentrations. Whether we can actually utilize rare earths at these low concentrations is another matter altogether.

Related Links

Geostandards and Geoanalytical Research, 2012, 36, 7 - 20: "Accurate measurement of rare earth elements by ICP-MS after ion-exchange separation: Application to ultra-depleted samples"


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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