Keywords

Introduction

The rare earth elements (REEs), including dysprosium, are increasingly valuable because of their limited known exploitable deposits in the earth’s crust [1], and because of their significant role in permanent magnets, lamp phosphors, rechargeable NiMH batteries, catalysts and other applications [2,3,4]. The recycling of Dy doped permanent magnets in electric vehicles and wind turbines is considered as part of the solution for a more green and sustainable future [5, 6]. As the largest rare-earth volume is used in permanent magnets, it is urgent to recover dysprosium, neodymium and other valuable metals from the end of life magnets. This work focuses on the REEs recovery from the slags by pyrometallurgical route, which is employed for REEs recovery as an alternative or combination with hydrometallurgical processing. However, research on the presence of rare-earth metals in slags as part of a recycling scheme has been fairly limited up to now [7, 8]. In this respect, this work aims to investigate precipitation behavior of the Dy2O3–SiO2–CaO ternary system during cooling and to optimize the cooling process for high REEs recovery .

Experimental Method

Slags were prepared by mixing reagent-grade oxides CaO (obtained by calcination of CaCO3, 99.99%, Chempur), SiO2 (99.9%, Merck Millipore) and Dy2O3 (99.99%, Chempur) with the composition 45 wt% SiO2, 49 wt% CaO and 6 wt% Dy2O3. The mixtures were respectively melted at 1673, 1773 and 1873 K for 24 h in an argon atmosphere, and then quenched in water. The schematic set-up of the quenching experiments is shown in Fig. 1.

Fig. 1
figure 1

Schematic representation of the equilibration furnace

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The microstructure and phase composition analysis of the quenched samples were investigated using an Electron Probe Micro Analyzer coupled with Wavelength Dispersive Spectroscopy (EPMA/WDS, JEOL JXA-8530F). A quantitative analysis using standards at an acceleration voltage of 15 kV and a probe current of 15 nA was performed. The slag samples were mounted in resin, grinded, well polished and Pt coated before the EPMA/WDS analysis. The CSLM-IIF was used to observe the precipitation behavior of the slag in situ . The CSLM sample was selected from the 1873 K quenched samples. The slag sample was charged into a Pt-20%Rh crucible placed on the sample holder in the heating chamber. The CSLM chamber was evacuated and flushed with argon three times before the slag sample was heated to the desired temperature while a constant Ar stream was maintained continuously during the experiments. Then the slag sample was cooled to solidify temperature at a predefined cooling rate then quench to room temperature for later microscope analysis. The temperature profile during the CSLM test is shown in Fig. 2. The laser scanned images of this cooling process were captured and used for further investigation.

Fig. 2
figure 2

Temperature profile during CSLM tests

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Results and Discussion

In consideration of slags containing lower rare earths content, a suitable cooling trajectory can enrich the rare earths in the liquid phase. The slag sample quenched from 1873 K was selected for the CLSM observation. It was remelted in the heating chamber at 1550 °C and then cooled under different cooling rate at 5, 20 and 100 °C/min, respectively. The images were recorded to show the precipitation and solidification process during cooling from 1550 °C through CSLM observation of which some snapshots are listed in Fig. 3. The composition and microstructure of the precipitates are further confirmed by EPMA/WDS with the back scattering electron (BSE) images of the equilibrated samples from 1873, 1773 and 1673 K as are shown in Fig. 4. The presence of Ca2SiO4 and Dy2O3 is determined at 1773 and 1673 K by quantification analysis through EPMA/WDS. The eutectic reaction proceeds as L → (Ca,Dy)2SiO4+Dy2O3 during cooling from above 1773 K. In the (Ca,Dy)2SiO4 solid solution, the solubility of Dy2O3 is given as 10.52 wt% at 1773 K and 5.61 wt% at 1673 K from the quantification results. It shows the C2S phase at higher temperature shows higher Dy2O3 solubility. The substitution of Dy3+ at Ca2+ site in C2S phase leads to the formation of (Ca,Dy)2SiO4 solid solution, resulting cation vacancies or oxygen interstitials to maintain charge balance in this structure. Below 1673 K, the solubility of Dy2O3 in C2S may be lower or even almost negligible.

Fig. 3
figure 3

In situ observation of precipitation behavior of the 45 wt% SiO2—49 wt% CaO—6 wt% Dy2O3 slag sample at different cooling rate with 5, 20, 100 °C/min: a first precipitate (C2S) appears b second type of precipitate appears (Dy2O3) c complete solidification

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Fig. 4
figure 4

BSE images of the quenched slag samples from equilibrium at 1873, 1773 and 1673 K

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It is observed that the first precipitates shown in Fig. 3a were found to be (Ca,Dy)2SiO4 phase and another precipitation with bright color were analyzed to be Dy2O3 as shown in Fig. 3b. As the cooling proceeds, more Dy2O3 precipitated from C2S phase. It is also found that the grain size of the precipitates (Dy2O3) increases with lower cooling rate. The precipitation and complete solidification temperatures were shown in Fig. 5 as a potential reference for the construction of a continuous cooling transformation (CCT) diagram.

Fig. 5
figure 5

The precipitation and complete solidification temperatures during cooling with different cooling rate. a: 100 °C/min b: 20 °C/min c: 5 °C/min

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Conclusion

The phase relations and precipitation behavior of Dy2O3–CaO–SiO2 system during isothermal and continuous cooling experiments with different cooling rate were studied. As the eutectic reaction proceeds as L → (Ca,Dy)2SiO4+Dy2O3 during cooling, it is possible to precipitate Dy2O3 from the Dy2O3-containing slag system. A suitable cooling trajectory can enrich the rare earths with different grain size to satisfy the practical demands.