Master Slag Production for the Recovery of Rare Earth Elements
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The Rare Earth Elements (REEs) is a term describing a group of 17 elements which are critical components in many technological applications due to their magnetic or optical properties. In contrast to the name, these elements are relatively abundant, but distributed in low concentration throughout the Earth's crust. Recovery of these elements is currently focused mainly in China, but the strategic importance of these elements within the EU motivates innovation in local resource extraction. The development of a pyrometallurgical approach for the recovery of REEs from iron mine tailings containing apatite has been the main focus of the present work. This was realized through the production of a Master Slag, i.e. a slag of uniform relative composition of Nd2O3-SiO2-CaO made from an Apatite Concentrate produced by LKAB in Kiruna, Sweden, which is a Swedish iron mining company. An experimental investigation into the Nd2O3-SiO2-B2O3 phase diagram was also performed for the intention of developing fundamental thermodynamic data for the Nd-Fe-B system - commercially known as the system used in permanent magnets. The developed thermal process of producing the Master Slag includes two main steps, i.e. (1) removal of phosphorus, which was achieved with a 99.7% reduction, and (2) melting of the remaining material at 1873 K for 60 min in an induction furnace; followed by rapid cooling. Scanning Electron Microscope - Energy Dispersive X-ray Spectrometry (SEM-EDS), X-Ray Diffraction (XRD) and Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) were used for the chemical analysis of the obtained material. The produced Master Slag proved to consist of three phase regions, these differed mainly by their neodymium concentration. The Nd-rich phase contained the compounds Ca2Nd3(SiO4)3F and variations of Ca2Nd3(SiO4)2(PO4)O with an overall Nd-concentration of 20-48% by weight. However, sampling at a lower temperature seems to indicate a higher Nd-concentration in the Nd-rich phase. The evaluation of the Nd2O3-SiO2-B2O3 phase diagram proved to offer several critical issues. A method for equilibriation of samples at high temperature (1673 K) without vaporization of B, as well as the optimization of the equilibriation time to secure reproducible results were the focus in the present work.