Influence of α-Al₂O₃ and AlF₃ on pyrohydrolysis of Li₃AlF₆

Influence of α-Al 2 O 3 and AlF 3 on pyrohydrolysis of Li 3 AlF 6 1 2 3 first-author 1 3 1 3 corresp Corresponding author E-mail address: liuyuxia@sinap.ac.cn; Corresponding author E-mail address: liuyuxia@sinap.ac.cn; liuyuxia@sinap.ac.cn 1 3 corresp zhanglan@sinap.ac.cn zhanglan@sinap.ac.cn zhanglan@sinap.ac.cn Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China University of Chinese Academy of Sciences, Beijing 100049, China University of Chinese Academy of Sciences, Beijing 100049, China CAS Innovative Academy in TMSR Energy System, Shanghai 201800, China CAS Innovative Academy in TMSR Energy System, Shanghai 201800, China In this study, Li 3 AlF 6 was employed to simulate the molten salt LiF-BeF 2 to explore its pyrohydrolysis behavior and that of its components, i.e., LiF and AlF 3 , respectively. The influence of the accelerators α-Al 2 O 3 and AlF 3 on the pyrohydrolysis of LiF and Li 3 AlF 6 was investigated. Finally, the solid pyrohydrolytic products were characterized by means of X-ray diffraction (XRD), and the corresponding reaction mechanisms were proposed. These experimental results indicated that AlF 3 was completely hydrolyzed to the corresponding oxide α-Al 2 O 3 at 650°C in 1 h, whereas the complete hydrolysis of LiF and Li 3 AlF 6 required the assistance of either α-Al 2 O 3 or AlF 3 under the same conditions. The influence of the accelerator α-Al 2 O 3 and AlF 3 on the pyrohydrolytic behavior of Li 3 AlF 6 provides references for future research studies on the pyrohydrolysis of LiF-BeF 2 and multi-component molten salts. Pyrohydrolysis Accelerator Li 3 AlF 6 molten salt Reaction mechanism 1 Introduction: 1 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 1 - 2 2 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-BeF 2 ) in the form of fluorides [ 3 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 F 2 and HF [ 4 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 5 - 8 8 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 9 - 11 11 9-11 ] or collected at temperatures near 0°C without using an absorbing solution [ 12 12 ]. Previous results from this laboratory showed that a series of single fluorides, such as UF 4 , ThF 4 , and ZrF 4 [ 13 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 5 , 14 14 ]. Hence, it is necessary to develop a fast pyrohydrolysis method at low temperature for the extraction of F – from fluoride compounds. The oxides U 3 O 8 , WO 3 , Cr 2 O 3 , α-Al 2 O 3 , and V 2 O 5 , and the salts Na 2 W 2 O 7 and Li 2 W 2 O 7 have all been recommended as accelerators [ 5 5 , 7 7 , 15 15 - 18 18 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-BeF 2 . In the carrier salt LiF-BeF 2 , the fluoride BeF 2 is a toxic carcinogen and intense irritant, and may release highly toxic smoke upon exposure to high temperatures or acids. In addition, experiments using BeF 2 require a series of safety assessments and approval, making it impossible to carry out LiF-BeF 2 -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, AlF 3 and BeF 2 form similar complexes with alkali metal fluorides. For example, AlF 3 and LiF can form Li 3 AlF 6 [ 19 19 - 22 22 19-22 ], and BeF 2 can form Li 2 BeF 4 with LiF [ 23 23 ]. Therefore, AlF 3 was used instead of BeF 2 and LiF-BeF 2 was represented by Li 3 AlF 6 in the experiment. Because of the need to avoid impurities in further treatment, no additional elements were introduced. Thus, α-Al 2 O 3 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-BeF 2 and other multi-component hybrid molten salts. 2 Experimental 1 2.1 Chemicals and reagents 2 α-Al 2 O 3 , guaranteed to be 99.99% pure, was purchased from Alfa Aesar (China) Chemical Co. Ltd. LiF and AlF 3 , guaranteed to be 99.9% pure, were commercially obtained from Sigma-Aldrich (Shanghai) Trading Co., Ltd. Li 3 AlF 6 (200 mesh), guaranteed to be 99% pure, was purchased from Zibo Senow Chemical Co. Ltd. NaOH was commercially obtained from Sinopharm Chemical Reagent Co., Ltd. High-purity water (18.2 MΩ·cm) from a Milli-Q water purification system (Millipore, USA) was used for all solutions through the chemical process. All the pyrohydrolysis samples are powders and guaranteed to be 200-300 mesh. The sample mass in the experiments is approximately 80-130 mg. 2.2 Apparatus 2 The device for the pyrohydrolysis experiments is the same as that used by Dong [ 13 13 ]. The main parts of the device are: the gas supply system, steam generator, high-temperature furnace, gas control system, off-gas collection equipment, and water-cooling device. All the valves, the reaction tube, and gas pipeline are manufactured of Hastelloy-C276 alloy. The reaction boat, which is manufactured of platinum, is up to 50 mm in length, 40 mm in width, and 10 mm in height, and was shaped from 0.5-mm thick platinum foil. The reaction tube is approximately 660 mm in length and 60 mm in outside diameter with a wall thickness of 1.5 mm. In the reaction tube, the heating zone in which a constant temperature is maintained is approximately 200 mm long. The carrier gas is Ar, the temperature of the steam generator is 120°C, and the upper limit of the working temperature of the furnace is 1000°C. The HF gas produced by the reaction was absorbed with 0.1 mol/L NaOH solution. An X'PERT POWDER (PANalytical, 40 kV, 40 mA, Cu Ka) X-ray diffractometer was used for XRD measurements. 2.3 Experimental procedures 2 Accurately weighed samples and an accelerator were evenly ground and placed in a platinum reaction boat. Our previous work revealed that the thickness of the sample layer had no effect on the pyrohydrolysis yield when the sample layer was less than 1 mm thick. Hence, the thickness of the sample layer in the reaction boat was approximately 0.3-0.8 mm. The boat was placed in the central section of the reaction chamber, and the temperature of the reaction tube was adjusted to 650°C. The air tightness of the reaction tube was checked by using a pumped vacuum system. Superheated water with a flow rate of 2.5 mL/min was used in our experiment. When the reaction was complete, the reaction furnace was cooled to room temperature. The platinum boat was removed from the reaction tube and placed in an oven for 30 min. The reaction efficiency was calculated by using a weighing method. The solid products were structurally characterized by XRD. 3 Results and discussion 1 3.1 Pyrohydrolysis of AlF 3 2 The pyrohydrolysis yield of AlF 3 was studied at different temperatures and for various reaction times, as shown in Fig. 1 (a, b) Fig. 1 (a, b) . It is clearly shown in Fig. 1a 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 Fig. 1b shows the relationship between the pyrohydrolysis yield and reaction time. At 650°C, AlF 3 was completely hydrolyzed in 1 h. Therefore, the optimal hydrolysis time was set to 1 h to ensure the complete hydrolysis of AlF 3 . The solid products were characterized by means of XRD. As shown in Fig. 2 Fig. 2 , AlF 3 was entirely converted into α-Al 2 O 3 at 650°C in 1 h. As these results were satisfactory, it was unnecessary to use an accelerator. The reaction mechanism was assumed to be as follows: <math display="block" id="M1"><mrow><mn>2</mn><mi>A</mi><mi>l</mi><msub><mi>F</mi><mn>3</mn></msub><mo>+</mo><mn>3</mn><msub><mi>H</mi><mn>2</mn></msub><mi>O</mi><mo>→</mo><mtext>α-</mtext><mi>A</mi><msub><mi>l</mi><mn>2</mn></msub><msub><mi>O</mi><mn>3</mn></msub><mo>+</mo><mn>6</mn><mi>H</mi><mi>F</mi></mrow></math> 3.2 Pyrohydrolysis of LiF 2 Previous studies [ 5 5 ] revealed that AlF 3 , 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 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. 3.3 Influence of the accelerator α-Al 2 O 3 on the pyrohydrolysis of LiF 2 3.3.1 Effects of accelerator and reaction time on pyrohydrolysis 3 The effects of the amount of α-Al 2 O 3 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) 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-Al 2 O 3 participated in the reaction of LiF in the form of LiAlO 2 and LiAl 5 O 8 at different mass ratios. The final research results are summarized in Table 1 Table 1 . (+ incompletely hydrolyzed, ++ completely hydrolyzed) 1 1 Mass ratio (α-Al 2 O 3  to LiF) 1 1 3:1 1 1 4:1 1 1 5:1 1 1 6:1 1 1 1h 1 1 + 1 1 + 1 1 + 1 1 + 1 1 3h 1 1 + 1 1 + 1 1 ++ 1 1 ++ 3.3.2 Mechanism inference 3 When cryolite was pyrohydrolyzed in the presence of α-Al 2 O 3 , Silverman [ 24 24 ] presumed the reaction to resemble the following, <math display="block" id="M2"><mrow><msub><mrow><mtext>Na</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><mo>+</mo><msub><mrow><mtext>3H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>+</mo><msub><mrow><mtext>11Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mo>→</mo><mn>6</mn><msub><mrow><mtext>HF+Na</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>⋅</mo><mn>11</mn><msub><mrow><mtext>Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mo>+</mo><msub><mrow><mtext>NaAlO</mtext></mrow><mtext>2</mtext></msub><mo>.</mo></mrow></math> Thus, substances similar to Na 2 O . nAl 2 O 3 and NaAlO 2 are produced when α-Al 2 O 3 is used as an accelerator in the pyrohydrolysis of LiF. However, XRD showed that the main pyrohydrolysis product of LiF was LiAlO 2 when a small amount of α-Al 2 O 3 was used, whereas large amounts of the accelerator produced LiAlO 2 and LiAl 5 O 8 because the excess α-Al 2 O 3 might react with LiAlO 2 to form LiAl 5 O 8 . The following probable reaction processes may be considered: When a small amount of α-Al 2 O 3 was used: <math display="block" id="M3"><mrow><msub><mrow><mtext>2LiF+α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2HF</mtext><mo>.</mo></mrow></math> When a large amount of α-Al 2 O 3 was used: <math display="block" id="M4"><mrow><msub><mrow><mtext>2LiF+5α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>+2HF</mtext><mo>.</mo></mrow></math> <math display="block" id="M5"><mrow><mtext>Which is:      </mtext><mn>2</mn><msub><mrow><mtext>LiF+α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2HF</mtext><mo>,</mo></mrow></math> <math display="block" id="M6"><mrow><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+2α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mo>→</mo><msub><mrow><mtext>LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mo>,</mo></mrow></math> <math display="block" id="M7"><mrow><mrow><mo>(</mo><mn>5</mn><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mn>6</mn><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mrow><mo>(</mo><mn>7</mn><mo>)</mo></mrow><msub><mrow><mtext>      2LiF+5α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>+2HF</mtext><mo>.</mo></mrow></math> 3.4 Influence of AlF 3 on pyrohydrolysis of LiF 2 3.4.1 Effect of AlF 3 and reaction time on pyrohydrolysis 3 The above-mentioned results showed that α-Al 2 O 3 , which could act as an accelerator, was the hydrolyzed product of AlF 3 . Thus, we investigated the effects of AlF 3 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) Fig. 5 (a, b) . It can be seen that LiF was not completely hydrolyzed in 1 h even when the mass ratio of AlF 3 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 AlF 3 and prolonging the reaction time could facilitate the completion of the hydrolysis of LiF. Based on the XRD patterns, LiAlO 2 and LiAl 5 O 8 would form in the reaction of LiF in the presence of different weight ratios of α-Al 2 O 3 . The final results are summarized in Table 2 Table 2 . (+ incompletely hydrolyzed, ++ completely hydrolyzed) 1 1 Mass ratio (AlF 3 to LiF) 1 1 4:1 1 1 5:1 1 1 6:1 1 1 10:1 1 1 1h 1 1 + 1 1 + 1 1 + 1 1 + 1 1 3h 1 1 + 1 1 + 1 1 + 1 1 ++ 3.4.2 Mechanism inference 3 Limited literature is available regarding the use of AlF 3 as an accelerator and its reaction mechanism in pyrohydrolysis. In particular, adding appropriate amounts of AlF 3 could contribute to the completion of the pyrohydrolysis. According to the aforementioned results obtained by using α-Al 2 O 3 as an accelerator, we could speculate reasonably that AlF 3 could promote the pyrohydrolysis of LiF because α-Al 2 O 3 is produced by the pyrohydrolysis. Furthermore, the final solid product, when adding AlF 3 to LiF, was the same as adding α-Al 2 O 3 . Therefore, our speculation was verified. We could summarize the reaction mechanism as follows: When a small amount of AlF 3 was used: <math display="block" id="M8"><mrow><msub><mrow><mtext>2AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+3H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mtext>+6HF,</mtext></mrow></math> <math display="block" id="M9"><mrow><msub><mrow><mtext>2LiF+α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>O→2LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2HF,</mtext></mrow></math> <math display="block" id="M10"><mrow><mrow><mo>(</mo><mn>8</mn><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mn>9</mn><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mrow><mo>(</mo><mrow><mn>10</mn></mrow><mo>)</mo></mrow><msub><mrow><mtext> LiF+AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+2H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+4HF</mtext><mo>.</mo></mrow></math> When a large amount of AlF 3 was used: <math display="block" id="M11"><mrow><msub><mrow><mtext>2AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+3H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mtext>+6HF,</mtext></mrow></math> <math display="block" id="M12"><mrow><msub><mrow><mtext>2LiF+α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2HF,</mtext></mrow></math> <math display="block" id="M13"><mrow><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+2α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mo>→</mo><msub><mrow><mtext>LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>,</mtext></mrow></math> <math display="block" id="M14"><mrow><mrow><mo>(</mo><mrow><mn>11</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>12</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>13</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>14</mn></mrow><mo>)</mo></mrow><msub><mrow><mtext>  LiF+5AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+8H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>+16HF</mtext><mo>.</mo></mrow></math> 3.5 Pyrohydrolysis of Li 3 AlF 6 molten salt 2 3.5.1 Pyrohydrolysis of Li 3 AlF 6 molten salt in different components 3 LiF and AlF 3 may produce stable cryolite compounds [ 19 19 - 22 22 19-22 ]. In our previous work, molten salt with a certain proportion of Li 3 AlF 6 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 Fig. 6 . Based on the XRD patterns, the first batch of the molten salt in the experiment was composed of Li 3 AlF 6 and excess LiF (Sample A), the second contained Li 3 AlF 6 and excess AlF 3 (Sample B). The purity of the procured cryolithionite was 99%, and its main constituent was Li 3 AlF 6 (Sample C). 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 Table 3 and Fig. 7 (a, b, c) Fig. 7 (a, b, c) . The final hydrolysates of Sample A and C were Al 2 O 3 and LiF ( Fig. 7a, 7c 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 LiAlO 2 ( Fig. 7b Fig. 7b ). Further analysis showed that the final hydrolysate of Sample A and C was γ-Al 2 O 3 , whereas that of the sample containing excess AlF 3 was α-Al 2 O 3 . This indicates that Sample B may be completely hydrolyzed with the acceleration effect of α-Al 2 O 3 . 1 1   1 1 sample A (LiF-AlF 3 ) 1 1 sample B (LiF-AlF 3 ) 1 1 sample C (purchased) 1 1 Molar ratio 1 1 85:15 1 1 66:34 1 1 75:25 1 1 Mass ratio 1 1 7:4 1 1 3:5 1 1 13:14 1 1 Reaction time (h) 1 1 12 1 1 12 1 1 — 1 1 Maximum reaction temperature (°C) 1 1 900 1 1 900 1 1 — 1 1 Molten salt components (characterized by XRD) 1 1 Li 3 AlF 6 , LiF 1 1 Li 3 AlF 6 , AlF 3 1 1 Li 3 AlF 6 (pure 99%) 1 1 Pyrohydrolysis condition 1 1 650°C, 1 h 1 1 650°C, 1 h 1 1 650°C, 1 h 1 1 Solid products after pyrohydrolysis 1 1 Al 2 O 3 , LiF 1 1 LiAlO 2 1 1 Al 2 O 3 , LiF 3.5.2 Mechanism inference 3 Silverman reported that the solid hydrolysates of Na 3 AlF 6 were NaAlO 2 and NaF [ 24 24 ]. The general pyrohydrolysis reaction of Na 3 AlF 6 could be expressed as follows: <math display="block" id="M15"><mrow><mi>N</mi><msub><mi>a</mi><mtext>3</mtext></msub><mi>A</mi><mi>l</mi><msub><mi>F</mi><mtext>6</mtext></msub><mo>+</mo><mtext>2</mtext><msub><mi>H</mi><mtext>2</mtext></msub><mi>O</mi><mo>=</mo><mi>N</mi><mi>a</mi><mi>A</mi><mi>l</mi><msub><mi>O</mi><mtext>2</mtext></msub><mo>+</mo><mtext>2</mtext><mi>N</mi><mi>a</mi><mi>F</mi><mo>+</mo><mtext>4</mtext><mi>H</mi><mi>F</mi><mo>.</mo></mrow></math> In comparison, in our experiment, the XRD results showed that the main hydrolysates of sample A (Li 3 AlF 6 and excess LiF) and C (Li 3 AlF 6 , guaranteed to be 99% pure) were Al 2 O 3 and LiF. Sample B (Li 3 AlF 6 and excess AlF 3 ) yielded LiAlO 2 only because of an excess of AlF 3 . Thus, the reaction of Li 3 AlF 6 is assumed to be as follows, when the Li 3 AlF 6 is pure or with an excess of LiF (sample A and C), <math display="block" id="M16"><mrow><msub><mrow><mtext>2Li</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><msub><mrow><mtext>+3H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mtext>+6LiF+6HF</mtext><mtext>.</mtext></mrow></math> when Li 3 AlF 6 is used with an excess of AlF 3 (sample B), <math display="block" id="M17"><mrow><msub><mrow><mtext>Li</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><msub><mrow><mtext>+2H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2LiF+4HF,</mtext></mrow></math> <math display="block" id="M18"><mrow><msub><mrow><mtext>2AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+3H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mtext>+6HF,</mtext></mrow></math> <math display="block" id="M19"><mrow><msub><mrow><mtext>2LiF+α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2HF,</mtext></mrow></math> <math display="block" id="M20"><mrow><mrow><mo>(</mo><mrow><mn>17</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>18</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>19</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mrow><mo>(</mo><mrow><mn>20</mn></mrow><mo>)</mo></mrow><mo>:</mo><msub><mrow><mtext>  Li</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><msub><mrow><mtext>+2AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+6H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>3LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+12HF</mtext><mo>.</mo></mrow></math> 3.6 Influence of AlF 3 and acceleratorα-Al 2 O 3 on Pyrohydrolysis of Li 3 AlF 6 2 Complete hydrolysis was difficult to achieve because the direct hydrolysate of pure Li 3 AlF 6 might produce LiF. Therefore, the effects of AlF 3 and the accelerator α-Al 2 O 3 on the pyrohydrolysis of Li 3 AlF 6 were explored. Fig. 8 Fig. 8 shows the XRD patterns of the solid hydrolysates. The results showed that adding appropriate amounts of α-Al 2 O 3 (the mass ratio of the accelerator to the sample was 3:1) and AlF 3 (the mass ratio of AlF 3 to the sample was 6:1) might trigger the reaction of Li 3 AlF 6 until the hydrolysis was completed. Based on the XRD patterns, adding α-Al 2 O 3 produced LiAlO 2 and LiAl 5 O 8 , whereas adding AlF 3 yielded LiAl 5 O 8 and Al 2 O 3 . 3.6.1 Mechanism inference 3 Based on the results of the effects of AlF 3 and α-Al 2 O 3 on the pyrohydrolysis of LiF and the reaction mechanism on cryolite proposed by Silverman [ 24 24 ] , the reaction mechanisms of α-Al 2 O 3 and AlF 3 for the pyrohydrolysis of Li 3 AlF 6 were deduced as follows. The mechanism of the reaction of Li 3 AlF 6 with α-Al 2 O 3 : <math display="block" id="M21"><mrow><msub><mrow><mtext>Li</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><msub><mrow><mtext>+2H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2LiF+4HF,</mtext></mrow></math> <math display="block" id="M22"><mrow><msub><mrow><mtext>2LiF+α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2HF,</mtext></mrow></math> <math display="block" id="M23"><mrow><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+2α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mo>→</mo><msub><mrow><mtext>LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>,</mtext></mrow></math> <math display="block" id="M24"><mrow><mrow><mo>(</mo><mrow><mn>21</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>22</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>23</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mrow><mo>(</mo><mrow><mn>24</mn></mrow><mo>)</mo></mrow><msub><mrow><mtext> Li</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><msub><mrow><mtext>+3α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+3H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAlO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>+6HF</mtext><mo>.</mo></mrow></math> The mechanism of the reaction of Li 3 AlF 6 with AlF 3 : <math display="block" id="M25"><mrow><msub><mrow><mtext>Li</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><msub><mrow><mtext>+2H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2LiF+4HF,</mtext></mrow></math> <math display="block" id="M26"><mrow><msub><mrow><mtext>2AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+3H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mtext>+6HF,</mtext></mrow></math> <math display="block" id="M27"><mrow><msub><mrow><mtext>2LiF+α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><msub><mrow><mtext>+H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>2LiAlO</mtext></mrow><mtext>2</mtext></msub><mtext>+2HF,</mtext></mrow></math> <math display="block" id="M28"><mrow><msub><mrow><mtext>LiAlO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mtext>+2α-Al</mtext></mrow><mtext>2</mtext></msub><msub><mtext>O</mtext><mtext>3</mtext></msub><mo>→</mo><msub><mrow><mtext>LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>,</mtext></mrow></math> <math display="block" id="M29"><mrow><mrow><mo>(</mo><mrow><mn>25</mn></mrow><mo>)</mo></mrow><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>26</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>27</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>28</mn></mrow><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>29</mn></mrow><mo>)</mo></mrow><msub><mrow><mtext>  Li</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>AlF</mtext></mrow><mtext>6</mtext></msub><msub><mrow><mtext>+14AlF</mtext></mrow><mtext>3</mtext></msub><msub><mrow><mtext>+24H</mtext></mrow><mtext>2</mtext></msub><mtext>O</mtext><mo>→</mo><msub><mrow><mtext>3LiAl</mtext></mrow><mtext>5</mtext></msub><msub><mtext>O</mtext><mtext>8</mtext></msub><mtext>+48HF</mtext><mo>.</mo></mrow></math> 4. Conclusion 1 The pyrohydrolysis behavior and reaction mechanism of LiF, AlF 3 , and Li 3 AlF 6 were investigated. The effects of AlF 3 and α-Al 2 O 3 on the pyrohydrolysis of LiF and Li 3 AlF 6 were explored. These results are expected to serve as an important guideline for future experiments on the molten salt LiF-BeF 2 and the pyrohydrolysis of multi-component molten salt mixtures. The following conclusions were drawn: (1) The pyrohydrolysis efficiency of AlF 3 increased with increasing reaction temperature and time. After pyrohydrolysis at 650°C for 1 h, AlF 3 was completely hydrolyzed and converted to α-Al 2 O 3 . (2) LiF cannot be hydrolyzed in 1 h or 3 h, at 650°C. The addition of AlF 3 and the accelerator α-Al 2 O 3 promoted the completely pyrohydrolysis of LiF. (3) The results showed that neither Sample A (Li 3 AlF 6 and excess LiF) nor C (Li 3 AlF 6 , guaranteed to be 99% pure) may be completely hydrolyzed, and their final hydrolysates were γ-Al 2 O 3 and LiF, respectively. 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Sun et al ., One-step synthesis of high-purity Li2BeF4 molten salt . J Fluorine Chem . 181 , 30 - 35 ( 2016 ). doi: 10.1002/chin.201613013 http://doi.org/10.1002/chin.201613013 . One-step synthesis of high-purity Li2BeF4 molten salt H. P. Silverman , F. J. Bowen , Pyrohydrolysis of cryolite fluoride-bearing materials . Anal Chem . 31 , 160 - 165 ( 1960 ). doi: 10.1021/ac60156a021 http://doi.org/10.1021/ac60156a021 . Pyrohydrolysis of cryolite fluoride-bearing materials 10.1007/s41365-018-0465-2 This work was supported by the "Strategic Priority Research Program" of the Chinese Academy of Sciences (No.XDA02030000). 2017-11-29 2018-02-05 2018-03-22 2018-07-25 2018-09-01 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2018 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-018-0465-2 http://dx.doi.org/10.1007/s41365-018-0465-2 . 2018 Nuclear Science and Techniques 09 29