1 Introduction

Rare earth (RE) elements were becoming increasingly important in the transition to a green, low-carbon economy [1–4]. Waste phosphor attracts considerable attention as a material for recycling REEs due to a large amount of REEs [5]. REEs, especially terbium, are the most critical raw materials, with the highest supply risk in Refs. [6, 7]. From now on, many researches start on REEs recycling from waste phosphor; however, less than 1 % of the REEs were recycled in 2011 [8]. Technically, Ce and Tb in the CeMgAl11O19: Tb3+ (CTMA) are difficult to recycle by leaching or mechanochemical treatment due to their stable aluminate crystal structure [9–13].

As is known to all, CTMA has a distorted magnetoplumbite structure. Figure 1 shows the unit cell of magnetoplumbite [14]. The structure consists of spinel-like blocks separated by mirror planes. The spinel blocks are composed of Al3+ and O2−, while the mirror planes contain O2− and large ions that are often trivalent rare earth cations. In the unit cell of CTMA, the rare earth ions such as Tb3+ incorporated as activators in the phosphors are built into the mirror planes at the site of the large cations such as Ce3+. The Mg2+ is located at Al3+ sites in the spinel-like blocks. The Al3+ sites in the aluminates are of three different types which are in octahedral, tetrahedral, and fivefold surrounding of oxygen ions, respectively [15].

Fig. 1
figure 1

Projection of unit cells of magnetoplumbite

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The alkaline fusion of waste phosphor as a pretreatment process makes it possible to increase the leaching rate [16, 17]. However, there are few literatures on the CTMA alkali fusion mechanism from a viewpoint on structure. Therefore, the mechanism of CTMA via fusion with sodium hydroxide was discussed in this study.

2 Experimental

In this study, CTMA powders with the average particle size of 2–4 μm were obtained from the Dalian Luminglight, China. CTMA powders were mixed with sodium hydroxide, according to the 1:1 NaOH/CTMA mass ratio by ball milling. Differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis were carried out using the NETZSCH STA 409 C/CD thermal analyzer. The reference material was α-Al2O3 powder, and the parent glass powder samples (<74 μm) were heated from 23 to 700 °C at the heating rate of 10 °C·min−1. The mixtures were then placed into 200 ml iron crucibles. Fusion was performed in the furnace at 150, 200, 250, 275, 300, 325 and 350 °C for 2 h separately, and quenching processing was needed to keep the crystal structure in the condition. The main chemical reaction during the alkali fusion process is:

$${\text{CTMA}} + {\text{NaOH}} + {\text{O}}_{ 2} \to {\text{NaAlO}}_{ 2} + {\text{MgO}} + {\text{CeO}}_{ 2} + {\text{Tb}}_{ 2} {\text{O}}_{ 3} + {\text{ H}}_{ 2} {\text{O}}$$
(1)

The fusion product was cleaned several times by stirring at 200 r·min−1 for 20 min with deionized water at 60 °C to remove NaOH and NaAlO2. All of the intermediate and final products obtained were collected and dry roasted at 120 °C, and then ground to particle size of smaller than 52 μm. X-ray diffraction (XRD) analysis was performed using Philips APD-10 X-ray diffractometer with Cu Kα radiation, 40 kV voltage, and 150 mA current at 10 min−1 scanning rate, with 2θ range of 10°–100°. The morphology and the mean particle size were observed by scanning electron microscope (SEM, JSM-6510A, Japan).

3 Results and discussion

3.1 Simultaneous thermal analysis

Figure 2 shows the typical DSC–TG curves of the mixture. From the TG curve, the total weight loss between room temperature (RT) and 274.04 °C is measured to be about 24.44 %, and it is the evaporation of water due to high moisture-absorption of sodium hydroxide. And there is an endothermic peak at 150 °C and a turning point at 200 °C in both the TG and DSC curves, at which the reaction may begin. However, the DSC curve shows an exothermic peak located at 274.04 °C and the maximum endothermic peak located at 294.06 °C, which means that the alkaline fusion process takes place at 274.04 °C and finishes at 294.06 °C. According to the TG curve, the weight loss between 274.04 and 400.00 °C is measured to be about 4.22 %, and the DSC curve shows an endothermic peak located at 310.9 °C. The temperature range of the endothermic peak in the DSC curve fits well with that of weight loss in the TG curve, which means that the reaction starts at 150 °C and finishes at about 400 °C.

Fig. 2
figure 2

DSC–TG curves of mixture (CTMA) under flowing air

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3.2 Phase and microstructural analyses

Figure 3 shows XRD patterns of alkaline fusion CTMA products at different temperatures. The product were treated by washing. When the temperature is lower than 250 °C, the main crystal phase hardly changes due to the low reaction rate between solid phases at low temperature. When the alkali temperature increases to 300 °C, diffraction peak intensities of CTMA significantly reduce, and the main crystal phase is CTMA phase, but the Ce0.6Tb0.4O2−x phase appears. It shows that CTMA phase starts to decompose, but not completely. When the alkali temperature increases to 325 °C, even 350 °C, diffraction peak of CTMA disappears, and the main crystal phases are Ce0.6Tb0.4O2−x (JCPDS 52-1303), Tb2O3 (JCPDS 23-1418) and MgO (JCPDS 45-0946).

Fig. 3
figure 3

XRD patterns of alkaline fusion production at different temperatures

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However, from the details of the XRD patterns at different temperatures in Fig. 4, when the temperature increases, the CTMA diffraction peak splits and shifts to large angle in Fig. 4a. The results show the decease of CTMA crystalline size and significant phase transformation. And the diffraction peak intensities of CTMA significantly reduce. From Fig. 4b, the diffraction peak of CeO2 (JCPDS 34-0394) first appears at 275 °C, and those of Tb2O3 (JCPDS 23-1418), Ce0.6Tb0.4O2−x and MgO (JCPDS 45-0946) first appear at 300, 325, and 325  °C, respectively, which fit well with DSC–TG curves in Fig. 2.

Fig. 4
figure 4

Details of XRD patterns at different temperatures: a CTMA phase transition process and b product phases

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The phase analysis from Fig. 4 is listed in Table 1. According to the results of XRD analysis, the transformation of the crystalline structure in the alkaline fusion process is shown in Fig. 5. It shows that, firstly, Tb and Mg in CTMA [P63/mmc (194)] move from the crystal lattice, and the main phase changes into CeAl11O18 [JCPDS 48-0055, P63/mmc (194)]. And then they are decomposed into NaAlO2 and CeO2 with the reaction. Increasing temperature benefits the formation of Ce0.6Tb0.4O2−x from Tb2O3 and CeO2. The details of phases’ crystalline structures are obtained from Table 2. It shows that the CTMA is decomposed step by step in the alkaline fusion process.

Table 1 Transformation of crystalline structure in alkaline fusion process
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Fig. 5
figure 5

Phases transition in alkaline fusion process

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Table 2 Lattice parameters of phases in alkaline fusion process
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Figure 6 shows SEM images of CTMA and alkaline fusion product at different temperatures. The raw CTMA are irregular particles with smooth surface and size of 2–4 μm as depicted in Fig. 6a. However, the smooth surface becomes rough after alkaline fusion at 275 °C as shown in Fig. 6b, but the particles are complete. When the alkaline fusion occurs at 300 or 325 °C as shown in Fig. 6c, d, the particles are disintegrated gradually and finally break into smaller grains with the size of 300–400 nm, but are severely agglomerated.

Fig. 6
figure 6

SEM images of CTMA and alkaline fusion product at different temperatures: a CTMA before alkaline fusion, b 275 °C, c 300 °C, and d 325 °C

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3.3 Acid soluble analyses

The products were dissolved completely in 2 mol·L−1 HCl after alkaline fusion at 500 °C for 2 h with the solid-to-liquid ratio of 1:5, temperature of 60 °C, and time of 2 h. The rare earth oxide (REO) would be recycled by oxalic acid precipitation and calcination. It means that the alkaline fusion is an effective method for recycling of Ce and Tb from CTMA.

3.4 Model and mechanism analyses

From the SEM results obtained at the “macro” level, it is reasonable to conclude that the alkaline fusion process can be described by the shrinking core model [18]. Figure 7 illustrates the model of the mechanism of the alkaline fusion reaction. It is hypothesized that: (1) NaOH turns into molten salt at about 300 °C; (2) the reaction occurs on the surface of the particle, and the product diffuses into the surrounding liquid phase; (3) then the surface becomes rougher with small pores and cracks, particularly for the particle with a high conversion rate; (4) the reaction proceeds until the decomposition of particle is completed, in the end, the smaller grains aggregate in the liquid phase.

Fig. 7
figure 7

Schematic representation of probable mechanism

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At the “micro” level, Fig. 8 illustrates the basic premises of the mechanism of unit cell. When the NaOH melts to ion liquid, Na+ may substitute for RE ion sites in mirror plane initially, due to that Na+ radius of 102 pm is close to that of RE ions (Ce3+ 103.4 pm, Tb3+ 92.3 pm). Then the RE ions combine with free OH− into REO and H2O. However, it results in much more defects around the sodium ion sites due to their different valence states, such as interstitial sodium ions and oxygen vacancies. The lattice constant of the mirror plane increases, and oxygen vacancies, sharing in the sites of vertex (V′′O1) and edges (V′′O2) in alumina octahedral, would be produced, which will break the stucture of spinel block. The defect generally provides an extensive perturbation of the surrounding lattice; it would increase the diffusion amount of Na+. The neighboring oxygen ions, the aluminum ions, and Na+ regroup to NaAlO2 in the spinel blocks, and Mg2+ would combine with free OH− into MgO and H2O.

Fig. 8
figure 8

Probable mechanism of CTMA unit cell

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4 Conclusion

The mechanism of CeMgAl11O19: Tb3+ alkaline fusion with sodium hydroxide was examined by various techniques to elucidate their roles in the expected BMA transformations. XRD analysis indicates that the transformation sequence involves CeAl11O18 as the intermediate phases between CTMA and the final product. Because sodium ions substitute for the cerium and terbium ions in the mirror plane during the process, more oxygen vacancies and interstitial sodium ions produce and cause the unit cell [P63/mmc (194)] to break from the mirror plane. Then during the process sodium ions substitute for the magnesium ions in the spinel block successively. Finally, the CTMA is decomposed into NaAlO2, CeO2, Tb2O3, and MgO. At the same time, some of CeO2 and Tb2O3 continue to form Ce0.6Tb0.4O2−x .