Concepts for the Extraction of Rare Earth Elements from Spent Phosphors

The rare earth elements (REE) are nowadays mainly used for permanent magnets, catalysts, hy-drogen storage alloys in rechargeable batteries, for glasses, ceramics, and polishing agents. Other important areas of application are phosphors in different types of lamps (fluorescent tubes, energy-saving lamps, etc.). These materials consist of a host lattice (RE- or halophosphatebased) which is doped by an activator. After excitation by e.g. UV-radiation the phosphor emits light, whereas the kind of phosphor used in an illuminant strongly depends on its application.

Fluorescent lamps are often collected and treated at their end of life by disintegration and sorting. However, the obtained fine fractions do not only comprise of REE or halophosphate-containing phosphors but also include small amounts of mercury and are therefore disposed till today in special landfills. Due to the enormous price increase of the rare earths during 2010 and 2011, a lot of activities started for extracting these elements from secondary resources such as for example phosphors. All recycling concepts for this task mainly work with hydrometallurgical methods. The large number of different compounds in phosphors with sometimes very different behaviors with respect to their solubility in aqueous phases forms a major challenge to achieve good yields as well as acceptable purities of the produced rare earth concentrates by a process with a reasonable number of operation steps. Therefore, this paper gives at first a brief overview about the compounds, used as phosphors and about existing concepts or investigations. It further describes some experimental research done at the Chair of Nonferrous Metallurgy at the Montanuniversitaet Leoben. Basically the studies concentrate on three major concepts. Two of them cope with the (multi)step acidic leaching of spent phosphors in order to separate the so called halophosphate based phosphors from the rare earth containing ones and a subsequent precipitation in order to extract the REE from the hydrochloric solutions quantitatively. The third concept deals with an alkaline leaching using solutions of sodium hydroxide under elevated temperatures. The aim of this process was the dissolving of undesired elements like aluminium and silicon while the REE form an insoluble residue of RE-hydroxides.
The experiments on laboratory scale showed that a preliminary halophosphate leaching at pH-values of 0 - 1.2 leads to significant losses of REE, while a direct hot acidic leaching of the spent phosphor and a subsequent precipitation of REE with oxalic acid achieves good yields. Nevertheless the obtained solids are contaminated with calcium and barium. Therefore further experiments will aim to an optimization of the procedure by variation of the precipitation temperature, concentration and amount of the used acid and precipitation agent. Additionally a subsequent pH-adjustment should be tested in order to dissolve calcium and barium to improve the product quality.
A hot alkaline leaching seems to be inappropriate to produce a high grade RE concentrate. By calculating mass balances it could be shown that just one third of the aluminium and calcium present in the raw material was dissolved by this method.

Copyright: © Lehrstuhl für Abfallverwertungstechnik und Abfallwirtschaft der Montanuniversität Leoben
Quelle: Depotech 2012 (November 2012)
Seiten: 6
Preis: € 3,00
Autor: Dr. Alexander Poscher
Priv.-Doz. Dr. mont. Stefan Luidold
Prof.Dipl.-Ing. Dr. Helmut Antrekowitsch
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