1 Introduction:

Molten salt reactors (MSRs), which are generation IV reactors, are the only type of liquid fuel reactor based on the thorium-uranium fuel cycle. Compared with other breeder reactors and solid fuel reactors, an MSR exhibits the properties of higher inherent safety, higher thermoelectric conversion efficiency, less nuclear waste, and non-proliferation attributes [1-2]. A thorium-based molten salt reactor (TMSR) is designed to take full advantage of abundant thorium resources. In a TMSR, thorium-uranium fuel is dissolved in the carrier molten salt (LiF-BeF₂) in the form of fluorides [3]. The majority of nuclear fission products produced from the thorium-uranium fuel cycle is also present in the form of fluorides. Thus, long-term storage of these products may lead to the formation of harmful substances, such as F₂ and HF [4], which significantly complicates final waste treatment. As a result, it is necessary to convert fluorides into oxides for further treatment.

Pyrohydrolysis is an acceptable procedure that is routinely used for the separation and determination of fluorides and other halides [5-8]. It is a fast, reliable, and highly convenient method for the decomposition of solid samples. The pyrohydrolysis reaction is conducted by treating a sample holder in the form of a boat containing the sample in a reaction tube with superheated steam at high temperature, and then the distillate is either cooled and collected in the receiver containing the absorbing solution [9-11] or collected at temperatures near 0°C without using an absorbing solution [12]. Previous results from this laboratory showed that a series of single fluorides, such as UF₄, ThF₄, and ZrF₄ [13] could be converted into their corresponding oxides by using a homemade apparatus for pyrohydrolysis experiments. Prior work proved the feasibility of the pyrohydrolysis method for the treatment of fluorides when reprocessing spent TMSR fuel. However, alkaline, alkaline-earth, and rare-earth fluorides are difficult to hydrolyze and require higher temperatures [5, 14]. Hence, it is necessary to develop a fast pyrohydrolysis method at low temperature for the extraction of F^– from fluoride compounds. The oxides U₃O₈, WO₃, Cr₂O₃, α-Al₂O₃, and V₂O₅, and the salts Na₂W₂O₇ and Li₂W₂O₇ have all been recommended as accelerators [5, 7, 15-18].

The behavior of fluorides from spent TMSR fuel has been investigated in the past few years. Results obtained with single fluorides showed that our homemade pyrohydrolysis device could be used to transform some single fluorides into their corresponding oxides. With respect to spent TMSR fuel, which comprises hybrid molten salts containing multiple components, the feasibility of hydrolyzing single fluorides has been validated. Therefore, further study on the pyrohydrolysis behavior of mixed molten salts needs to be carried out. This requires us to first understand the pyrohydrolysis properties of the carrier salt LiF-BeF₂.

In the carrier salt LiF-BeF₂, the fluoride BeF₂ is a toxic carcinogen and intense irritant, and may release highly toxic smoke upon exposure to high temperatures or acids. In addition, experiments using BeF₂ require a series of safety assessments and approval, making it impossible to carry out LiF-BeF₂-related experiments at the moment. We therefore considered replacing Be with Al, because these two elements are positioned on a diagonal line (upper left, bottom right) in the periodic table, and would be expected to have similar properties according to the diagonal rules. Apart from this, AlF₃ and BeF₂ form similar complexes with alkali metal fluorides. For example, AlF₃ and LiF can form Li₃AlF₆ [19-22], and BeF₂ can form Li₂BeF₄ with LiF [23]. Therefore, AlF₃ was used instead of BeF₂ and LiF-BeF₂ was represented by Li₃AlF₆ in the experiment. Because of the need to avoid impurities in further treatment, no additional elements were introduced. Thus, α-Al₂O₃ was chosen as a suitable accelerator in this study. All these modifications were made with the aim of providing the necessary data and obtaining useful guidance for the subsequent pyrohydrolysis experiments of LiF-BeF₂ and other multi-component hybrid molten salts.

2 Experimental

3 Results and discussion

3.1 Pyrohydrolysis of AlF₃

The pyrohydrolysis yield of AlF₃ was studied at different temperatures and for various reaction times, as shown in Fig. 1 (a, b). It is clearly shown in Fig. 1a that the pyrohydrolysis yield significantly increased with the reaction temperature during the initial stage up to 650°C for 1 h. Therefore, the pyrohydrolysis was conducted at the temperature of 650°C to ensure complete hydrolysis of the sample. Fig. 1b shows the relationship between the pyrohydrolysis yield and reaction time. At 650°C, AlF₃ was completely hydrolyzed in 1 h. Therefore, the optimal hydrolysis time was set to 1 h to ensure the complete hydrolysis of AlF₃. The solid products were characterized by means of XRD. As shown in Fig. 2, AlF₃ was entirely converted into α-Al₂O₃ at 650°C in 1 h. As these results were satisfactory, it was unnecessary to use an accelerator.

Fig. 1Full-screen View

Pyrohydrolysis of AlF₃ as a function of the reaction temperature and reaction time: (a)--Pyrohydrolysis of AlF₃ as a function of the reaction temperature, (b)--Pyrohydrolysis of AlF₃ as a function of the reaction time

Fig. 2Full-screen View

(Color online) XRD patterns of the sample and the pyrohydrolysis product of AlF₃

The reaction mechanism was assumed to be as follows:

$2Al{F}_3+3H_{2}O\to \text{α-}A{l}_2{O}_3+6HF$ (1)

3.2 Pyrohydrolysis of LiF

Previous studies [5] revealed that AlF₃, a readily hydrolyzable fluoride, requires low hydrolysis temperature. In contrast, the fluoride LiF cannot be easily hydrolyzed and requires high reaction temperature. In the present study, the hydrolysis behavior of LiF at 650°C was examined. The result showed that LiF was not hydrolyzable in 1h, even when the pyrohydrolysis time was prolonged to 3 h. The XRD patterns of the samples before and after treatment are shown in Fig. 3. The high hydrolysis yield of fluorides within a short time confirmed that the reaction temperature and the absence of accelerators were the main factors limiting the pyrohydrolysis of LiF under the present experimental conditions. This necessitated the use of accelerators for the pyrohydrolysis of LiF.

Fig. 3Full-screen View

(Color online) XRD patterns of the sample and the pyrohydrolysis product of LiF under different conditions

3.3 Influence of the accelerator α-Al₂O₃ on the pyrohydrolysis of LiF

3.3.1 Effects of accelerator and reaction time on pyrohydrolysis

The effects of the amount of α-Al₂O₃ and the reaction time on the pyrohydrolysis of LiF were investigated. The solid products were characterized by XRD, as shown in Fig. 4 (a, b). The results showed that LiF was not completely hydrolyzed in 1 h, even when the mass ratio of accelerator to LiF was 6:1. In contrast, the use of a 5 to 1 weight ratio of accelerator to LiF with a reaction time of 3 h enabled LiF to be transformed completely. Hence, LiF could be converted by increasing the reaction time and the amount of accelerator. The XRD patterns showed that α-Al₂O₃ participated in the reaction of LiF in the form of LiAlO₂ and LiAl₅O₈ at different mass ratios. The final research results are summarized in Table 1.

Table 1Full-screen View

Influence of α-Al₂O₃ on Pyrohydrolysis of LiF

Mass ratio (α-Al2O3 to LiF)	3:1	4:1	5:1	6:1
1h	+	+	+	+
3h	+	+	++	++

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(+ incompletely hydrolyzed, ++ completely hydrolyzed)

Fig. 4Full-screen View

(Color online) XRD patterns of the pyrohydrolysis products after (a)--1h, (b)--3h. In Fig. 4(a): Mass ratio of α-Al₂O₃ to LiF is A--4:1, B--5:1, and C--6:1 In Fig. 4(b): Mass ratio of α-Al₂O₃ to LiF is A--3:1, B--4:1, C--5:1, and D--6:1

3.3.2 Mechanism inference

When cryolite was pyrohydrolyzed in the presence of α-Al₂O₃, Silverman [24] presumed the reaction to resemble the following,

${\text{Na}}_{\text{3}}{\text{AlF}}_{\text{6}}+{\text{3H}}_{\text{2}}\text{O}+{\text{11Al}}_{\text{2}}{\text{O}}_{\text{3}}\to 6{\text{HF+Na}}_{\text{2}}\text{O}\cdot 11{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}+{\text{NaAlO}}_{\text{2}}.$ (2)

Thus, substances similar to Na₂O^.nAl₂O₃ and NaAlO₂ are produced when α-Al₂O₃ is used as an accelerator in the pyrohydrolysis of LiF. However, XRD showed that the main pyrohydrolysis product of LiF was LiAlO₂ when a small amount of α-Al₂O₃ was used, whereas large amounts of the accelerator produced LiAlO₂ and LiAl₅O₈ because the excess α-Al₂O₃ might react with LiAlO₂ to form LiAl₅O₈.

The following probable reaction processes may be considered:

When a small amount of α-Al₂O₃ was used:

${\text{2LiF+α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAlO}}_{\text{2}}\text{+2HF}.$ (3)

When a large amount of α-Al₂O₃ was used:

${\text{2LiF+5α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{+2HF}.$ (4) $\text{Which is: }2{\text{LiF+α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAlO}}_{\text{2}}\text{+2HF},$ (5) ${\text{LiAlO}}_{\text{2}}{\text{+2α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\to {\text{LiAl}}_{\text{5}}{\text{O}}_{\text{8}},$ (6) $\left(5\right)+\left(6\right)=\left(7\right){\text{ 2LiF+5α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{+2HF}.$ (7)

3.4 Influence of AlF₃ on pyrohydrolysis of LiF

3.4.1 Effect of AlF₃ and reaction time on pyrohydrolysis

The above-mentioned results showed that α-Al₂O₃, which could act as an accelerator, was the hydrolyzed product of AlF₃. Thus, we investigated the effects of AlF₃ on the dosage and reaction time of the pyrohydrolysis of LiF. The solid hydrolyzates were characterized by XRD analysis as shown in Fig. 5 (a, b). It can be seen that LiF was not completely hydrolyzed in 1 h even when the mass ratio of AlF₃ to LiF was 10:1. However, increasing the mass ratio from 4:1-6:1 to 10:1 made it possible to completely hydrolyze LiF in 3 h. This result indicated that adding an appropriate amount of AlF₃ and prolonging the reaction time could facilitate the completion of the hydrolysis of LiF. Based on the XRD patterns, LiAlO₂ and LiAl₅O₈ would form in the reaction of LiF in the presence of different weight ratios of α-Al₂O₃. The final results are summarized in Table 2.

Table 2Full-screen View

Influence of AlF₃ on Pyrohydrolysis of LiF

Mass ratio (AlF3 to LiF)	4:1	5:1	6:1	10:1
1h	+	+	+	+
3h	+	+	+	++

Show more

(+ incompletely hydrolyzed, ++ completely hydrolyzed)

Fig. 5Full-screen View

(Color online) XRD patterns of the pyrohydrolysis products for (a)--1h, (b)--3h. In Fig. 5(a): Mass ratio of AlF₃ to LiF is A--4:1, B--5:1, C--6:1, D--10:1 In Fig. 5(b): Mass ratio of AlF₃ to LiF is A--4:1, B--5:1, C--6:1, D--10:1

3.4.2 Mechanism inference

Limited literature is available regarding the use of AlF₃ as an accelerator and its reaction mechanism in pyrohydrolysis. In particular, adding appropriate amounts of AlF₃ could contribute to the completion of the pyrohydrolysis. According to the aforementioned results obtained by using α-Al₂O₃ as an accelerator, we could speculate reasonably that AlF₃ could promote the pyrohydrolysis of LiF because α-Al₂O₃ is produced by the pyrohydrolysis. Furthermore, the final solid product, when adding AlF₃ to LiF, was the same as adding α-Al₂O₃. Therefore, our speculation was verified. We could summarize the reaction mechanism as follows:

When a small amount of AlF₃ was used:

${\text{2AlF}}_{\text{3}}{\text{+3H}}_{\text{2}}\text{O}\to {\text{α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\text{+6HF,}$ (8) ${\text{2LiF+α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}{\text{O→2LiAlO}}_{\text{2}}\text{+2HF,}$ (9) $\left(8\right)+\left(9\right)=\left(10\right){\text{ LiF+AlF}}_{\text{3}}{\text{+2H}}_{\text{2}}\text{O}\to {\text{LiAlO}}_{\text{2}}\text{+4HF}.$ (10)

When a large amount of AlF₃ was used:

${\text{2AlF}}_{\text{3}}{\text{+3H}}_{\text{2}}\text{O}\to {\text{α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\text{+6HF,}$ (11) ${\text{2LiF+α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAlO}}_{\text{2}}\text{+2HF,}$ (12) ${\text{LiAlO}}_{\text{2}}{\text{+2α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\to {\text{LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{,}$ (13) $\left(11\right)+\left(12\right)+\left(13\right)=\left(14\right){\text{ LiF+5AlF}}_{\text{3}}{\text{+8H}}_{\text{2}}\text{O}\to {\text{LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{+16HF}.$ (14)

3.5 Pyrohydrolysis of Li₃AlF₆ molten salt

3.5.1 Pyrohydrolysis of Li₃AlF₆ molten salt in different components

LiF and AlF₃ may produce stable cryolite compounds [19-22]. In our previous work, molten salt with a certain proportion of Li₃AlF₆ was prepared using a mole ratio corresponding to the lowest melting point at which the eutectic salt was formed. Different proportions of the prepared molten salts and the procured cryolithionites were analyzed with XRD and the results can be seen in Fig. 6. Based on the XRD patterns, the first batch of the molten salt in the experiment was composed of Li₃AlF₆ and excess LiF (Sample A), the second contained Li₃AlF₆ and excess AlF₃ (Sample B). The purity of the procured cryolithionite was 99%, and its main constituent was Li₃AlF₆ (Sample C).

Fig. 6Full-screen View

(Color online) XRD patterns for different molar ratio of LiF-AlF₃ molten salt A--Sample A (Li₃AlF₆ and excess LiF), B--Sample B (Li₃AlF₆ and excess AlF₃), C--Sample C (Li₃AlF₆, guaranteed to be 99% pure)

The three molten salt samples were hydrolyzed at 650°C for 1 h and the hydrolysates characterized by XRD analysis. These results are detailed in Table 3 and Fig. 7 (a, b, c). The final hydrolysates of Sample A and C were Al₂O₃ and LiF (Fig. 7a, 7c), indicating the incomplete hydrolysis of these samples. The results were consistent even when the reaction time was prolonged to 3 h. Meanwhile, Sample B was completely hydrolyzed in 1 h, and the final hydrolysate was LiAlO₂ (Fig. 7b). Further analysis showed that the final hydrolysate of Sample A and C was γ-Al₂O₃, whereas that of the sample containing excess AlF₃ was α-Al₂O₃. This indicates that Sample B may be completely hydrolyzed with the acceleration effect of α-Al₂O₃.

Table 3Full-screen View

Different molar ratio of LiF-AlF₃ molten salt

	sample A (LiF-AlF3)	sample B (LiF-AlF3)	sample C (purchased)
Molar ratio	85:15	66:34	75:25
Mass ratio	7:4	3:5	13:14
Reaction time (h)	12	12	—
Maximum reaction temperature (°C)	900	900	—
Molten salt components (characterized by XRD)	Li3AlF6, LiF	Li3AlF6, AlF3	Li3AlF6 (pure 99%)
Pyrohydrolysis condition	650°C, 1 h	650°C, 1 h	650°C, 1 h
Solid products after pyrohydrolysis	Al2O3, LiF	LiAlO2	Al2O3, LiF

Show more

Fig. 7Full-screen View

(Color online) XRD patterns of the pyrohydrolysis products of Li₃AlF₆ samples (a)--Sample A (Li₃AlF₆ and excess LiF), (b)--Sample B (Li₃AlF₆ and excess AlF₃), (c)--Sample C (Li₃AlF₆, guaranteed to be 99% pure)

3.5.2 Mechanism inference

Silverman reported that the solid hydrolysates of Na₃AlF₆ were NaAlO₂ and NaF [24]. The general pyrohydrolysis reaction of Na₃AlF₆ could be expressed as follows:

$N{a}_{\text{3}}Al{F}_{\text{6}}+\text{2}{H}_{\text{2}}O=NaAl{O}_{\text{2}}+\text{2}NaF+\text{4}HF.$ (15)

In comparison, in our experiment, the XRD results showed that the main hydrolysates of sample A (Li₃AlF₆ and excess LiF) and C (Li₃AlF₆, guaranteed to be 99% pure) were Al₂O₃ and LiF. Sample B (Li₃AlF₆ and excess AlF₃) yielded LiAlO₂ only because of an excess of AlF₃. Thus, the reaction of Li₃AlF₆ is assumed to be as follows,

when the Li₃AlF₆ is pure or with an excess of LiF (sample A and C),

${\text{2Li}}_{\text{3}}{\text{AlF}}_{\text{6}}{\text{+3H}}_{\text{2}}\text{O}\to {\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}\text{+6LiF+6HF}\text{.}$ (16)

when Li₃AlF₆ is used with an excess of AlF₃ (sample B),

${\text{Li}}_{\text{3}}{\text{AlF}}_{\text{6}}{\text{+2H}}_{\text{2}}\text{O}\to {\text{LiAlO}}_{\text{2}}\text{+2LiF+4HF,}$ (17) ${\text{2AlF}}_{\text{3}}{\text{+3H}}_{\text{2}}\text{O}\to {\text{α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\text{+6HF,}$ (18) ${\text{2LiF+α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAlO}}_{\text{2}}\text{+2HF,}$ (19) $\left(17\right)+\left(18\right)+\left(19\right)=\left(20\right):{\text{ Li}}_{\text{3}}{\text{AlF}}_{\text{6}}{\text{+2AlF}}_{\text{3}}{\text{+6H}}_{\text{2}}\text{O}\to {\text{3LiAlO}}_{\text{2}}\text{+12HF}.$ (20)

3.6 Influence of AlF₃ and acceleratorα-Al₂O₃ on Pyrohydrolysis of Li₃AlF₆

Complete hydrolysis was difficult to achieve because the direct hydrolysate of pure Li₃AlF₆ might produce LiF. Therefore, the effects of AlF₃ and the accelerator α-Al₂O₃ on the pyrohydrolysis of Li₃AlF₆ were explored. Fig. 8 shows the XRD patterns of the solid hydrolysates. The results showed that adding appropriate amounts of α-Al₂O₃ (the mass ratio of the accelerator to the sample was 3:1) and AlF₃ (the mass ratio of AlF₃ to the sample was 6:1) might trigger the reaction of Li₃AlF₆ until the hydrolysis was completed. Based on the XRD patterns, adding α-Al₂O₃ produced LiAlO₂ and LiAl₅O₈, whereas adding AlF₃ yielded LiAl₅O₈ and Al₂O₃.

Fig. 8Full-screen View

(Color online) XRD patterns of pyrohydrolysis products of Li₃AlF₆ sample with α-Al₂O₃ and AlF₃ A--Li₃AlF₆ sample, B--Li₃AlF₆ sample mix with α-Al₂O₃, C--Li₃AlF₆ sample mix with AlF₃

3.6.1 Mechanism inference

Based on the results of the effects of AlF₃ and α-Al₂O₃ on the pyrohydrolysis of LiF and the reaction mechanism on cryolite proposed by Silverman [24]_, the reaction mechanisms of α-Al₂O₃ and AlF₃ for the pyrohydrolysis of Li₃AlF₆ were deduced as follows.

The mechanism of the reaction of Li₃AlF₆ with α-Al₂O₃:

${\text{Li}}_{\text{3}}{\text{AlF}}_{\text{6}}{\text{+2H}}_{\text{2}}\text{O}\to {\text{LiAlO}}_{\text{2}}\text{+2LiF+4HF,}$ (21) ${\text{2LiF+α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAlO}}_{\text{2}}\text{+2HF,}$ (22) ${\text{LiAlO}}_{\text{2}}{\text{+2α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\to {\text{LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{,}$ (23) $\left(21\right)+\left(22\right)+\left(23\right)=\left(24\right){\text{ Li}}_{\text{3}}{\text{AlF}}_{\text{6}}{\text{+3α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+3H}}_{\text{2}}\text{O}\to {\text{2LiAlO}}_{\text{2}}{\text{+LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{+6HF}.$ (24)

The mechanism of the reaction of Li₃AlF₆ with AlF₃:

${\text{Li}}_{\text{3}}{\text{AlF}}_{\text{6}}{\text{+2H}}_{\text{2}}\text{O}\to {\text{LiAlO}}_{\text{2}}\text{+2LiF+4HF,}$ (25) ${\text{2AlF}}_{\text{3}}{\text{+3H}}_{\text{2}}\text{O}\to {\text{α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\text{+6HF,}$ (26) ${\text{2LiF+α-Al}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+H}}_{\text{2}}\text{O}\to {\text{2LiAlO}}_{\text{2}}\text{+2HF,}$ (27) ${\text{LiAlO}}_{\text{2}}{\text{+2α-Al}}_{\text{2}}{\text{O}}_{\text{3}}\to {\text{LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{,}$ (28) $\left(25\right)+\left(26\right)+\left(27\right)+\left(28\right)=\left(29\right){\text{ Li}}_{\text{3}}{\text{AlF}}_{\text{6}}{\text{+14AlF}}_{\text{3}}{\text{+24H}}_{\text{2}}\text{O}\to {\text{3LiAl}}_{\text{5}}{\text{O}}_{\text{8}}\text{+48HF}.$ (29)

4. Conclusion

The pyrohydrolysis behavior and reaction mechanism of LiF, AlF₃, and Li₃AlF₆ were investigated. The effects of AlF₃ and α-Al₂O₃ on the pyrohydrolysis of LiF and Li₃AlF₆ were explored. These results are expected to serve as an important guideline for future experiments on the molten salt LiF-BeF₂ and the pyrohydrolysis of multi-component molten salt mixtures. The following conclusions were drawn:

(1) The pyrohydrolysis efficiency of AlF₃ increased with increasing reaction temperature and time. After pyrohydrolysis at 650°C for 1 h, AlF₃ was completely hydrolyzed and converted to α-Al₂O₃.

(2) LiF cannot be hydrolyzed in 1 h or 3 h, at 650°C. The addition of AlF₃ and the accelerator α-Al₂O₃ promoted the completely pyrohydrolysis of LiF.

(3) The results showed that neither Sample A (Li₃AlF₆ and excess LiF) nor C (Li₃AlF₆, guaranteed to be 99% pure) may be completely hydrolyzed, and their final hydrolysates were γ-Al₂O₃ and LiF, respectively. Sample B (Li₃AlF₆ and excess AlF₃) could be completely hydrolyzed and converted into LiAlO₂.

(4) Adding appropriate amounts of AlF₃ and α-Al₂O₃ could accelerate the complete hydrolysis of Li3AlF6.

In consideration of the diagonal effect, according to which the chemical properties of BeF₂ and AlF₃ are similar, we presumed that the behavior of AlF₃ could be extended to BeF₂. We verified the feasibility of converting AlF₃ to Al₂O₃, which can be used as an accelerator in the pyrohydrolysis reaction. Our results suggest that BeF₂ can act as an accelerator with a self-accelerating effect when reprocessing spent TMSR fuel.