IN-SITU OBSERVATION OF RARE EARTH CONTAINING PRECIPITATED PHASE CRYSTALLIZATION AND SOLIDIFICATION
OF CaO-SiO2-Nd2O3 AND CaO-SiO2-Nd2O3-P2O5 MELTS
Thu Hoai Le1, Mayu Aketagawa1,2, Annelies Malfliet1, Bart Blanpain1 and Muxing Guo1
1Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 bus 2450, B-3001 Leuven, Belgium
2Department of Materials Science, Tohoku University, Aoba-yama 02, Sendai, 980-8579, Japan
Keywords: in-situ observation, phase precipitation, rare earths
Abstract
In order to optimize the recycling process, fundamental understanding of the rare earths distribution in the slag and the precipitation behavior of the REE containing compounds during slag solidification are of significant importance. In this work, “in-situ” observations of rare earth containing phase precipitation, and solidification behavior of the CaO-SiO2-Nd2O3 and CaO- SiO2-Nd2O3-P2O5 melts were performed using a confocal scanning laser microscope (CSLM) combined with an infrared imaging furnace heating (IIF). The compositions of the precipitates formed during cooling of those slags were examined using EPMA method. The addition of P2O5
was found to influence the precipitation behavior and to decrease the liquidus as well as the solidus temperatures of the slags.
Introduction
The rare earths or rare-earth elements (REEs) are key resources in the transition towards an environment-friendly, low-carbon sustainable economy. The unique magnetic, luminescent and electrochemical properties of these metallic elements allow them to play an often irreplaceable role in a wide range of applications. As the consumption of rare earth materials is rapidly increasing, their recycling has also attracted immense attention [1-6]. The present research focuses on the recycling of rare earths through the combination of pyrometallurgical and hydrometallurgical routes. In order to optimize the recycling process, fundamental understanding of the rare earths distribution in the slag and the precipitation behavior of the REE containing compounds during slag solidification are of significant importance. In our previous study, the phase diagram of the CaO-SiO2-Nd2O3 system was constructed and two possible REE recycling schemes were proposed [7]. This work continues to study the precipitation and solidification behavior of this ternary slag through in-situ CLSM observation. Since rare earths have a high affinity to phosphate structures [8], the CSLM experiment was also carried out on the ternary system with an addition of P2O5.
Experimental
Based on the obtained phase diagram data and two possible REE recycling schemes proposed in our previous work [7], two slag compositions were chosen for the present study as shown in Table 1. Samples were prepared from high purity fine powders: CaO (obtained by calcination of
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
CaCO3, 99.99%, Chempur, CAS no.: 471-34-1), SiO2 (99.9%, Merck Millipore, CAS no.:
14808-60-7), Nd2O3 (99.99%, Chempur, CAS no.: 1313-97-9), and CaHPO4 (98%, RPL, CAS no. 7757-93-9). For the quaternary CaO-SiO2-Nd2O3-P2O5 system, 5 wt% P2O5 was added to the ternary sample. The CSLM-IIF [8] was used to observe in-situ the precipitation and solidification behavior of the selected slags, which were prepared according to the isothermal-quenching procedure at 1600 oC. The slag sample was put in a Pt-20%Rh crucible placed on a sample holder inside the CSLM heating chamber. The temperature calibration was performed using pure copper, nickel and palladium, separately, as references and the sample’s temperature was observed to be 85 ± 5 oC lower than the measured temperature using the thermocouple. Before heating the sample, the CSLM chamber was evacuated and flushed with argon three times and then a constant Ar stream was maintained as 17 dm3/hour during the experiments. After being held at the desired temperature for 5 minutes, the slag was cooled to solidify temperature at a predefined cooling rate then quench to room temperature for later microscope analysis. The laser scanned images of this cooling process were captured and used for further investigation. The schematic of this experimental setup was shown in figure 1.
Figure 1: The experimental setup schematic Table 1: Compositions of samples.
Sample CaO, wt% SiO2, wt% Nd2O3, wt% P2O5, wt%
CNS CNSP
25.0 23.8
50.0 47.5
25.0 23.8
- 5
For microstructure and phase composition analysis, the samples were mounted in resin, grinded, polished and coated with carbon. The samples are then analyzed by an electron probe micro- analyzer (EPMA, JEOL JXA-8530F) equipped with standardized wavelength dispersive spectroscopy (WDS) and operated at 15 kV-15 nA. A 16.9 wt% CaO - 12.1 wt% Al2O3-54.3 wt% SiO2-16.7 wt% Nd2O3 glass was used as standard. The average accuracy of EPMA measurements on the main elements is ±1 wt%.
Results and discussion
From the previous study [7], a possible REE recycling scheme could be based on either enriching the amorphous slag in REE by the precipitation of the SiO2 and CaO.SiO2 non-REE-containing solid phases or by precipitating the REE-rich solid phase from the liquid region. By collecting either of these precipitates from the slag, a REE-enriched product can be obtained, which could serve as a REE-rich input stream in a next step of the REE recycling process. In order to examine the possibilities, the solidification behavior of a 25%CaO-50%SiO2-25%Nd2O3 slag is studied in this research. In order to investigate the influence of phosphorous, 5 wt% P2O5 was added to the ternary system by mixing CaHPO4 in the slag.
The CSLM experiments were performed for these slags with a cooling rate of 20 °C/min. Since the slag was transparent before the precipitation of the first phase, the microstructure (i.e. grain boundaries) of the crucible bottom could be observed. By maintaining the focus of the microscope on the bottom of the crucible, the onsets of the precipitates were detected by the appearance, movement and growth of a new structure in the liquid. The CLSM images were continuously recorded during the experiments and the representative images are shown in Figures 2 and 3 for the CNS and CNSP slags, respectively. BSE images of the solidified slags are presented in figure 4. In figures 2 and 3, the first image represents the droplet sample in a complete liquid state at the indicated temperature. Circles, arrows are used to mark most visible precipitates, which appear as a brighter or darker color, depending on their positions. The figure 2 shows a top view of the precipitates growing in the ternary slag. At 1469 oC, two precipitates marked by circles A and B, could be clearly seen. These started to grow from heterogeneous nucleation sites at the bottom of crucible up to above 120 µm in size (Figure 2b, c). They were observed to have dendritic structure with very long needle-like branches as the white phase shown in figure 4a. Figure 2d shows another type of precipitate that started to form at 1429 oC, which was transparent and its formation could only be confirmed by its movement within the liquid phase. As the temperature decreased, the newly formed precipitate grew, as indicated by the yellow arrow in figure 2e and f. For this slag, the compositional analysis shows the existence of two precipitated phases: Nd-rich needles and round-shaped SiO2 particles (Figure 4a), which may obtained by the heat treatment progress [10]. The results indicate that the Nd-rich phase is the phase observed in figure 2b while SiO2 is the transparent phase that started to form at lower temperature, and show good agreement with our previous study [7].
When P2O5 was added to the slag, different precipitate morphologies were observed (Figure 3).
In particular, as shown in figure 3b and c, the new irregular-shape precipitates prefer to form at the grain boundaries of the crucible. A BSE image of the samples indicates that the precipitate has a dendritic structure (Figure 4b). As the temperature decreased to 1223oC, the precipitate particle number increased but there was no significant increase in its size (Figure 3 d -3f). In addition, no other type of precipitate was further formed during the cooling of the slag, which is in good agreement with the compositional analysis (Figure 4b).
Figure 2: The CNS slag a cooling rate of 20 °C/min (a) transparent liquid slag b) first precipitates appearing (c) first precipitates growing and moving (d) second type of precipitate forming in a dashed circle (e) second type of precipitate appearing (f) both types of precipitates growing and moving.
Figure 3: The CNSP slag at a cooling rate of 20°C/min a) transparent liquid slag b) precipitate appearing c) precipitate growing (d, e, f) precipitates increasing and growing.
Figure 4: Back scattering electron (BSE) images of (a) 25%CaO-50%SiO2-25%Nd2O3 and (b) 23.8%CaO-47.5%SiO2-23.8%Nd2O3-5%P2O5 slags after the CSLM experiments. In Figure a, the needle-like white structure is a Nd-rich phase.
Figure 5 shows the on-set precipitation temperature (liquidus) and solidification complete temperature (solidus) as a function of time for the different samples. With the addition of 5%
P2O5 and the cooling rate of 20 oC/min, the liquidus temperature and the solidus temperature decrease from 1484 to 1399oC and from 1284 to 1040oC, respectively.
Figure 5: Precipitation and complete solidification temperatures with cooling rate at 20°C/min of 25%CaO-50%SiO2-25%Nd2O3 and 23.8%CaO-47.5%SiO2-23.8%Nd2O3-5%P2O5 slags.
As a follow-up of this preliminary study to evaluate the potential of recycling REE from the waste through either the precipitation of the none-REE-containing solid phase from the Liquid + SiO2 region or the precipitation of the REE-rich ternary compound from the Liquid + Ca2+xNd8- x(SiO4)6O2-0.5x region, CSLM experiments and EPMA analysis will be done at different cooling rates and different compositions. These data are necessary to construct the CCT diagram. which is a practical tool for the REE recyclers to select appropriate slag compositions and cooling rates and to optimize the process.
Conclusion
Precipitation of the rare earth rich ternary compound were in-situ observed in 25%CaO- 50%SiO2-25%Nd2O3 system with a cooling rate of 20 oC/min. The addition of P2O5 influences the precipitation behavior and decreases the liquidus as well as the solidus temperatures of the slags. The results provide information on the selection of suitable flux materials for rare earth containing wastes recycling processes where slags are considered as the starting secondary raw materials, as an alternative to rare earth ores. The efficiency of REEs recovery and slag recycling can thus be enhanced by a properly engineered hot stage slag treatment process.
Acknowledgement
The authors acknowledge the support from the Hercules Foundation (project no. ZW09-09) in the use of the FEG-EPMA and from the GOA/13/008 project at the KU Leuven.
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