Ionic Liquid Technology for the Separation of Rare Earths

Ionic liquids possess some interesting properties for solvent extraction experiments such as a negligible volatility, a low flammability and high structure tuneability. Moreover, their ionic structure and metal complex solvation power is totally different from apolar aliphatic or aromatic solvents. Even though ionic liquids are considered as safer and more environmentally friendly alternatives for traditional organic diluents, they have one main disadvantage which is their slower extraction kinetics due to the higher viscosity and mass transport of this kind of solvents.

In the last years, the supply of rare earths and NdFeB magnets has been under a constant pressure due to a cheaper production process of China resulting in a quasi-monopoly and its strong export. Therefore, the recovery of rare-earths from end-of-life materials such as NdFeB magnets becomes strategically very interesting as it reduces the rare-earth supply dependency on China.

In the first results part of this PhD dissertation, the basic extractant trihexyl(tetradecyl)phosphonium in combination with chloride and nitrate anions is used to separate some main transition metals from the rare earths present in NdFeB or SmCo magnets. The process is based on a salting-out procedure by using high concentrations of salt or acid in the aqueous phase. The most promising process was tested on a real NdFeB magnet, which was first roasted and leached selectivily to remove the iron. Than, the remaining transition metals were removed by solvent extraction with trihexyl(tetradecyl)phosphonium chloride in the presence of 3.5 M of NH4Cl in the aqueous phase. Afterwards, the rare earths were precipitated by the addition of oxalic acid and calcinated. In this way, a highly pure mixture of the rare-earth oxides was produced which can be used directly as starting material for the production of NdFeB magnets. The processes are operated in that way that they minimize the amount of waste streams and the amount of chemicals consumption. Moreover, the ionic liquid or even aqueous phases are reused to obtain a closed and environmentally friendly process.

The second part of this PhD dissertation focuses on the use of the ionic liquid betainium bis(trifluoromethylsulfonyl)imide for the extraction of metals. An innovative process, increasing the reaction and extraction rate by reducing the ionic liquid phase viscosity during the extraction process, is worked out. In this method, called homogeneous liquid-liquid extraction, the aqueous/ionic liquid mixture is heated above its critical temperature, at which one homogeneous phase is formed. Afterwards, the mixture is cooled and two phases are reformed. In this way, mixing and reaction between the metal and the extractants occurs at molecular scale in the homogeneous state, whereas phase and metal separation can be achieved by cooling down and obtaining two phases.

The ionic liquid betainium bis(trifluoromethylsulfonyl)imide, in combination with trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide and water was used as well for a triphasic extraction system. In this triphasic system, three different metals (Sc(III), Y(III) and Sn(II)) can be separated in one single step, a separation that cannot be achieved when working with the conventional two phases.