Recovery of Te for the scrap of Bi₂Te₃-based alloys by vacuum metallurgy

The initial materials were produced by Hubei Sagreon New Energy Co., Ltd, and the content of elements has been shown on table 1. In this research, we used a vacuum tube furnace developed by ourselves. As shown in figure.1, the entire vacuum device contains a thermocouple, heater, graphite crucible, condenser and glass casing. At first, 50 g of the n-type bismuth telluride-based scrap were washed with distilled water and absolute ethanol, dried in a vacuum drying oven at 60 °C and subsequently put into a high-purity graphite crucible. Then put the crucible in the quartz tube for the experiment at the conditions of 873 K–1073 K. A dynamic vacuum level of approximately 0.1 Pa in the distillation chamber was obtained by a mechanical vacuum pump. The heating rate was controlled at 5 K min⁻¹, and the temperature was controlled by an automatic temperature control system when the target distillation temperature was reached. As the temperature inside the crucible reached the target temperature, the material started evaporating. After the end of distillation, as the temperature in the chamber dropped to room temperature, the condenser was cleaned off and weighed. The vacuum pump continued running during the cooling process, ensuring that the system was always in a vacuum state, thus preventing gas leakage and the oxidization of the metal. After the first vacuum distillation, take out the condensate and put it into a new crucible again. Repeat the above steps for the second and third vacuum distillation.

The phases of the distillation products were examined by x-ray diffraction (XRD) in a Philips X'pert Pro diffractometer with Kα radiation (λ = 1.5418 Å). The fracture morphologies of distillation products were observed by field-emission scanning electron microscopy (FESEM, FEI/ Nova400 NanoSEM). The content of each element in the distillation product is analyzed by an x-ray fluorescence spectrometer (XRF/Panalytical Axios).

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The principle of vacuum distillation is to use the difference in saturated vapor pressure between elements to achieve the purpose of separating the subject metal and impurity components under a certain temperature and vacuum conditions. In a vacuum environment, elements with a higher saturated vapor pressure are preferentially volatilized into the gas phase, while other elements are not volatilized and remain in the residue, thereby achieving the separation of main elements and impurity elements. Under the same system pressure, the saturated vapor pressure of pure metal is only related to temperature. The saturated vapor pressure at different temperatures can be calculated by formula 1 [20]. The results are shown in table 2.

$$\begin{eqnarray*}&&\mathrm{lg}\,P=A{T}^{-1}+B\,\mathrm{lg}\,T+CT+D\end{eqnarray*}$$

The required parameters A, B, C and D are evaporation constants. We calculated the saturated vapor pressure of various elements in the scrap of Bi₂Te₃-based alloys in the temperature range of 473 to 973 K, as shown in figure 2.

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Figure 2 shows that the saturated vapor pressure of each component in the scrap of Bi₂Te₃-based alloys increases with temperature. At the same temperature, the saturated vapor pressure of Te is higher than that of other components (especially Bi). In the vacuum distillation process, Se and Te are easier to volatilize into the tube, while Bi tends to stay in the residue. Therefore, this experiment can separate Bi, Te and Se through vacuum distillation, and Te can evaporate into the vapor phase and candense to achieve separation [21].

In the range of 473 K–1173 K, the saturated vapor pressure of Te and Se is always higher than that of Bi. After comprehensively considering the volatilization rate of Bi, Se Te elements and the temperature selection of Hageman's work [15], we choose 873 K–1073 K as the temperature range of our experiment.

For dilute liquid alloy solutions, the 'Separation Coefficient β' at the distillation temperature is calculated by formula 2 [16]:

$$\begin{eqnarray*}&&{\beta }_{A}=\displaystyle \frac{{\Upsilon }_{A}}{{\Upsilon }_{B}}\,\cdot \,\displaystyle \frac{{p}_{A}^{\ast }}{{p}_{B}^{\ast }}\end{eqnarray*}$$

In the formula, ${p}_{A}^{\ast }$ and ${p}_{B}^{\ast }$ are the saturated vapor pressures of the pure substances of the elements A and B; ${\Upsilon }_{A}$ and ${\Upsilon }_{B}$ are the activity coefficients of A and B. If ${\beta }_{A}$ ≫ 1 or ≪ 1, the two components A-B can be separated well. We calculated the separation coefficients of Te-Se and Te-Bi at 873 K, 973 K, and 1073 K respectively, as shown in table 3.

It can be seen from table 3 that the Te-Se separation coefficient is about 20 while the separation coefficient of Te-Bi is 9 orders of magnitude lower than that of 1 at 873–1073 K. It means that Bi is remained in the liquid phase while Te and Se are concentrated into the vapor phase. Therefore, Bi can be collected in residue.

According to the Langmuir evaporation rate formula, the evaporation rate of component A in the melt is expressed as formula 3 [16]:

$$\begin{eqnarray*}&&{W}_{A}={\Upsilon }_{A}{N}_{A}\left({P}_{A}^{0}\right)\sqrt{\displaystyle \frac{{M}_{A}}{2\pi RT}}\end{eqnarray*}$$

Where ${W}_{A}$ is the evaporation rate $\left(g/\left({{\rm{cm}}}^{2}\,\cdot \,S\right)\right)$ of metal A at temperature T; ${\Upsilon }_{A}$ is the activity coefficient of the component in the melt; N_A is the concentration of component A in the melt; P_A is the saturated vapor pressure of component A at temperature T; P₁ is the system space vapor partial pressure; ${M}_{A}$ is the atomic weight of component A; R is the universal constant of gas, R = 8.314 $Pa\,\cdot \,{m}^{3}\,\cdot \,{\left(mol\,\cdot \,K\right)}^{-1}.$ We calculated the evaporation rate of elements in the scrap of Bi₂Te₃-based alloys at 873 K, 973 K, and 1073 K respectively, as shown in table 4.

It can be seen from table 4 that the evaporation rate is affected remarkably by the change of distillation temperature. The evaporation rate can increase even ten times with the temperature increment of 100 K. It is concluded that Te and Se can volatilize quickly and concentrate into vapor phase than Bi. Also it can be predicted that Se and Te cannot be separated well by vacuum distillation technology. The evaporation rate of Bi is less than 9 orders of magnitude and above of Te and Se. In the vacuum distillation process, Bi will only evaporate a small amount, which is easier to separate.

The influence of distillation temperature on Te, Se, Bi in the scrap of Bi₂Te₃-based alloys was studied. The sample was prepared at a temperature of 873 K to 1073 K, a pressure of 0.1 Pa, a heating rate of 10 °C min⁻¹ and maintained for 120 min. The XRD patterns of volatile components at different temperatures (873 K, 973 K, 1073 K) is shown in figure 3. It can be seen that the observed phases are Te and Se, while no obvious characteristic diffraction peaks of Bi are detected, presumably due to the small vapor pressure and low evaporation rate of Bi. It indicates a vacuum distillation is effective for separating Te and Bi from the scrap of Bi₂Te₃-based alloys. However, the presence of Se phase can be detected in XRD patterns at different temperatures. Since the saturated vapor pressures of Te and Se are close, In vacuum distillation, both elements volatilize and condense in vacuum distillation, in conformity with other's [22] observation.

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The SEM of condensate and resident under the conditions of a temperature of 873 K, a pressure of 0.1 Pa, and a vacuum distillation time of 1h is shown in figure 4. From figure 4(a), it can be clearly seen the 200 micron-sized polygonal tellurium crystals, and a little flocculent substance is also found on the surface of tellurium crystal, which is consistent with the SEM of tellurium deposit at high temperature in [23]. Figure 4(b) shows that the surface of the residue is relatively flat with many pores on the surface. The pores may be caused by the volatilization of tellurium and selenium during distillation [24].

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The effect of distillation temperature on the volatilization of Se and Te was investigated under pressures of 0.1 Pa at a constant distillation time of 120 min. The highest recovery rate can be obtained only by volatilizing Te and leaving other elements in the residue. The recovery efficiencies of Te can be calculated as formula 4:

$$\begin{eqnarray*}&&{\rm{Recovery}}\,\mathrm{efficiency}=\displaystyle \frac{{{\rm{m}}}_{{\rm{0}}}\times {\omega }_{0}}{{{\rm{m}}}_{{\rm{raw}}}\times {\omega }_{{\rm{raw}}}}\end{eqnarray*}$$

where m_raw is the mass of raw material(g); ω_raw is the content of Se, Te and Bi in raw material(%); m₀ is the mass of condensate(g); ω₀ is the content of Se, Te and Bi in condensate(%).

As shown in figure 5, the recovery rate of Te increases from 86.62% to 90.34% with the temperature increased from 873 K to 1073 K. It is important to note that with a single distillation of 873 K the recovery rate of Te can be easily purified from the initial scrap at 47.30% to 86.62%. When the temperature rises, the recovery rate does not increase significantly, suggesting that after the distillation at 1073 K for 2 h, the Te content has become saturated, and more distillation time has no obvious effect on increasing the Te recovery rate. However, the Se content in the condensate is always maintained at 3% and does not change too much with the increase of the distillation temperature. It may due to the very little difference of the saturated vapor pressure between Se and Te. In the vacuum distillation process, both Se and Te are easy to volatilize and condense the same time [25].

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Figure 6 displays the EDS diagram of condensate at different temperatures. As shown in table 5, three regions were selected for EDS analysis. It can be seen that after two hours of vacuum distillation, the content of Bi is always zero, proving that Bi hardly evaporates and condenses during the distillation. Another phenomenon that cannot be ignored is that the content of selenium is still around 4%–10% at different temperatures, the result is also consistent with our previous studies. The results indicate that it is difficult to remove selenium from the scrap of Bi₂Te₃-based alloys at a distillation temperature of 873 K–1073 K. The findings from Guo's work [26] seem consistent with our study. For example, after a distillation temperature of 923 K maintained for 120 min, the recovery efficiency of Se and Te was 98.09% and 97.82%, respectively, which suggests Se is easier to collect than Te during distillation.

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Figure 7 shows the contents of the scrap of different distillation times under the condition of the distillation temperature of 1073 K and the pressure of 0.1 pa. It can be seen from figure 7 that the longer the distillation time, the lower the Te purity in the condensate, and the higher the impurity content in Te. With the extension of the distillation time to 3 h, we can see that there is an obvious increase in the amount of Bi in the condensate, indicating Te and Se have been completely evaporated in the raw material, and further extension of distillation time will lead to the evaporation and condensation of Bi. It can also be seen that no matter how the distillation time is prolonged, the content of Se is always between 3% and 4% [27].

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After that, the recovery of Te from the scrap of Bi₂Te₃-based alloys is achieved through a multi-stage distillation. The method is to first distill at 1073 K for 2 h, then distill the condensate again at 873 K for 2 h, and finally distill the condensate at 723 K for 2 h.

As shown in figure 8, after the multi-stage distillation, the purity of Te increased from 41.53% of the raw material to 98.88%. And Bi was not detected in the condensate after the third distillation. This is probably due to the temperature of 723 K is relatively low and it's difficult for Bi to volatilize and condense during distillation. The volatiles are a mixture of Te and Se, while Bi is collected in the residue. The recovery efficiency is conducive to further purification of Te. This means that Te scrap of Bi₂Te₃-based alloys can be purified and separated by multistage vacuum distillation, and then applied to production again.

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