Recovery of Neodymium and Dysprosium from NdFeB Magnets using Ionic Liquid Technology ,,

The first part of this PhD thesis shows how ionic liquids can be employed as a green alternative to replace the conventional organic phase in solvent extraction processes of rare-earth elements. In a first approach, the selective leaching of rare earths from NdFeB magnets and their solvent extraction from a nitrate media is presented. More specifically, neodymium and dysprosium were separated from cobalt by extracting them to the ionic liquid trihexyl(tetradecyl)phosphonium nitrate (Cyphos IL 101 nitrate). Afterwards neodymium and dysprosium were separated using ethylenediaminetetraacetic acid (EDTA) as a selective stripping agent. The purified metals were recovered as oxalates and then transformed into the corresponding oxides by calcination. In a second approach, the recovery and separation of rare earths from chloride media is presented. Neodymium and dysprosium were separated by using Cyphos IL 101 in its thiocyanate form combined with the molecular extractant CyanexÒ 923. The addition of a molecular extractant provided a beneficial lower viscosity of the organic phase and a higher loading capacity in comparison with the pure ionic liquid. Stripping of the metals from the loaded organic phase was carried out with water and the rare earths were also recovered as oxalates. In the second part of this PhD dissertation, the deep-eutectic solvent choline chloride:lactic acid (molar ratio 1:2) was employed to dissolve the magnets. The solvent extraction process was carried out by contacting the deep-eutectic solvent (more polar phase) with ionic liquids and conventional extractants diluted in toluene (less polar phase). Iron, boron and cobalt were separated from neodymium and dysprosium using the ionic liquid tricaprylmethylammonium thiocyanate (Aliquat 336 SCN) diluted in toluene. The separation of neodymium and dysprosium was assessed by using two different types of extractants, a mixture of trialkylphosphine oxides (Cyanex 923) and bis(2-ethylhexyl)phosphoric acid (D2EHPA). Based on the distribution ratios, separation factors and the easiness of the subsequent stripping, Cyanex 923 was chosen as the most adequate extractant. This new methodology offers higher selectivities and efficiencies than the corresponding aqueous system. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to elucidate the mechanisms for extraction of cobalt and iron from the deep-eutectic solvent. Furthermore, the feasibility of scaling up this separation process was tested in a mixer settler setup. Finally, considering that the correct quantification of metal ions in aqueous solutions is essential for the evaluation of a solvent extraction system and also for the follow up of continuous systems, practical and easy guidelines for the correct preparation of aqueous samples and posterior quantification by total-reflection X-ray fluorescence (TXRF) were developed. The most important parameters that play a role in the calibration of a TXRF apparatus such as the choice of the standard element and the concentration ratio between the analyte and the standard were discussed.Acknoledgements Abstract Abbreviations and symbols Table of contents Outline Chapter 1 Introduction 1.1 The rare-earth elements 1.1.1 The rare-earth crisis: the wakeup call 1.1.2 How to solve the problem? 1.1.3 Recycling of NdFeB magnets 1.2 Hydrometallurgy as a tool for the recovery of rare earth elements 1.21 Leaching 1.22 Solvent extraction 1.3 Upscaling 1.3.1 Mixer-settlers 1.4 Total-reflection X-ray fluorescence (TXRF) 1.5 References Chapter 2 Objectives Chapter 3 Practical guidelines for best practice on total reflection X-ray fluorescence spectroscopy: analysis of aqueous solutions 3.1 Introduction 3.2 Experimental 3.2.1 Materials and methods 3.2.2 Formulas 3.3 Results and discussion 3.3.1 Parameter studies and optimization 3.3.2 Calibration curves 3.3.3 Carrier, blank problems and detector contamination 34 Conclusions 35 References Chapter 4 Extraction and separation of neodymium and dysprosium from used NdFeB magnets: an application of ionic liquids in solvent extraction towards the recycling of magnets 71 4.1 Introduction 4.2 Experimental 4.2.1 Materials and methods 4.2.2 Synthesis of trihexyl(tetradecyl)phosphonium nitrate 4.2.3 Selective leaching and direct magnet dissolution 4.2.4 Solvent extraction 4.2.5 Separation of Co(II) from Nd(III) and Dy(III) 4.3 Results and discussion 4.3.1 Leaching 4.3.2 Separation of Co(II) from Nd(III) and Dy(III) 4.3.3 Separation of Nd(III) and Dy(III) 4.4 Conclusions 4.5 References Chapter 5 Separation of neodymium and dysprosium using a phosphonium thiocyanate ionic liquid combined with neutral extractants: a process relevant for the recycling of end-of-life NdFeB magnets 5.1 Introduction 5.2 Experimental 5.2.1 Materials and methods 5.2.2 Equipment and characterization 5.2.3 Solvent extraction 5.2.4 Theory 5.3 Results and discussion 5.3.1 Extraction parameters 5.3.2 Scrubbing and Stripping 5.3.3 Mechanism of extraction 5.3.3 Recycling of the organic phase 5.4 Conclusions 5.5 References Chapter 6 Separation of rare earths and other valuable metals from deep-eutectic solvents: a new alternative for the recycling of used NdFeB magnets 129 6.1 Introduction 6.2 Experimental 6.2.1 Materials and methods 6.2.2 Equipment and characterization 6.2.3 Synthesis of DES 6.2.4 Leaching 6.2.5 Solvent extraction 6.3 Results and discussion 6.3.1 Dissolution 6.3.2 Solvent extraction 6.3.3 Separation by mixer-settlers 6.3.4 Recycling of the less polar phase 6.3.5 Comparison with conventional systems 6.3.6 EXAFS 6.4 Conclusions 6.5 References Chapter 7 Conclusions and outlook Safety aspects List of publications Attended conferences and trainingsnrpages: 208status: publishe