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Selective separation and recovery of rare metals by vulcanization-vacuum distillation of cadmium telluride waste
Separation and Purification Technology 230 (2020) 115864
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Selective separation and recovery of rare metals by vulcanization-vacuum distillation of cadmium telluride waste
T
Xiaofeng Zhanga,b,c, Daxin Huanga,b,c, Wenlong Jianga,b,c, Guozheng Zhaa,b,c, Juhai Denga,b,c, ⁎ Pan Denga,b,c, Xiangfeng Konga,b,c, Dachun Liua,b,c, a
State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization, Kunming University of Science and Technology, Kunming 650093, China National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China c Yunnan Provincial Key Laboratory for Nonferrous Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hazardous waste Recycling CdTe Vulcanization Vacuum distillation
Cadmium telluride waste contains a portion of rare metals. In this paper, a new, clean and efficient vulcanization-vacuum distillation process is used to separate the rare metal Te from cadmium telluride. Taking CdTe (99.99%) and S (99.99%) as examples, the feasibility of this process was verified on a laboratory scale. Recovery of cadmium telluride waste was simplified to two-step vulcanization and vacuum distillation process. The effects of temperature and time on vulcanization-vacuum distillation were investigated. The experimental results show that the purity of Te obtained by vacuum distillation was 99.92%, and the purity of cadmium sulfide was up to 99.5%. This method provides theoretical guidance for the recycling of cadmium telluride photovoltaic modules. Compared with the traditional methods, this process has a short refining process, does no generate waste water or waste gas, and is environmentally friendly. The Te and CdS obtained from vulcanization-vacuum distillation can be utilized as raw materials for semiconductor production after subsequent processing.
1. Introduction CdTe is a direct bandgap semiconductor with a forbidden band width of 1.45 eV, which is very close to the ideal band gap width of photovoltaic materials. It has a high light absorption coefficient. For example, the light absorption coefficient is 105 cm in the visible region, allowing absorption of 90% of the light with a thickness of a few microns [1,2]. Accordingly, the production scale and application of CdTe thin-film solar cells have increased rapidly since the beginning of the 21st century [3,4]. The conversion efficiency of small-area CdTe solar cells has reached 19.6%, the module efficiency has reached 16.1%, and the average efficiency is 13%. However, there is still a big gap between the theoretical conversion efficiency of 30%, leaving plenty of room for improvement [5]. According to the European Photovoltaic Industry Association (EPIA), the contribution of photovoltaic energy to the global electricity supply may be as high as 12.6% by 2040 [6–9]. The proportion of cadmium telluride thin-film photovoltaic (CdTe-PV) in emerging technologies continues to increase. The production cost of cadmium telluride film photovoltaic is exceptionally low, and the current component cost is around $0.64/W [10]. There are two main problems with cadmium telluride photovoltaic ⁎
technology: the impact of cadmium pollution and the limited supply of metal tellurium (Te). The estimated life of cadmium telluride thin-film modules is approximately 25–35 years. When they reach the end of the intended lifetime but are not properly recycled and handled, the bracket used on the panel may harm the environment. In addition, the heavy metals contained in the materials are toxic and may cause cancer or teratogenicity. Therefore, the recycling methods must take into account these facts [11–14]. Cadmium telluride is sealed between two pieces of glass, and there is no release of Cd at normal temperature. Even at high temperatures up to 1100 °C, more than 99.96% of CdTe is sealed by the two pieces of molten glass without leakage. Various studies have shown that the recycling of cadmium telluride photovoltaic modules is feasible [15]. The European PV Accept Project reports that the energy recovery time of CdTe solar cells is only 0.9 years, and the practice of First Solar has confirmed that the recovery time is 10 months. The researchers tested four methods for treating the waste of CdTe PV module: (1) ferric chloride/hydrochloric acid solution leaching; (2) sulfuric acid oxygen pressure leaching; (3) nitric acid leaching; and (4) sulfuric acid/hydrogen peroxide leaching. The first method does not separate Te from iron (Te is not leached into the solution). Oxygen pressure leaching provides more effective separation, but the residual
Corresponding author. E-mail address: [email protected] (D. Liu).
https://doi.org/10.1016/j.seppur.2019.115864 Received 24 May 2019; Received in revised form 16 July 2019; Accepted 26 July 2019 Available online 27 July 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 230 (2020) 115864
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amount of Cd, Te and S accounts for 4−7% of the total residual amount. In addition, this process is capital intensive due to the high pressure and temperature required, and is therefore not attractive for low cost recycling. Nitric acid leaching recovers 96% of the Te, leaving Cd in solution. However, the subsequent electrolytic recovery of Cd is based on an artificial high Cd concentration (such as 10,000 ppm instead of 1000 ppm obtained from leaching). In addition, leaching with high concentrations of nitric acid requires a high temperature of 60 to 80°Celsius, inevitably producing nitric acid fume [16]. The power generation and full-scale production of First Solar represents an advanced example of cadmium telluride thin-film module recycling. The recycling process includes the following steps [17,18]:
• A shredder was used to break the glass into large pieces, and a hammer mill was used to break the broken glass into 4 mm pieces. • The semiconductor film is removed using sulfuric acid and hydrogen peroxide in a slowly rotating leaching tank. • Solid-liquid separation is performed by pouring the contents of the leaching drum into a classifier to separate the glass from the liquid. • The separation of the glass laminate is achieved by separating the • •
glass from the larger ethylene vinyl acetate (EVA) sheet through a vibrating screen. Glass cleaning removes any semiconductor material that remains on the glass. The metal component separated via sodium hydroxide precipitation is concentrated in a thickened tank, and the resulting filter cake was packaged by a third party (5NPlus/5NPV) for metal recovery.
Fig. 1. Structure chart of the vacuum distillation furnace: 1, stainless steel shell; 2, valve 3, thermocouple; 4, insulation materials; 5, graphite resistance; 6, crucible; 7, condenser; 8, vacuum gauge.
stoichiometric ratio of 1:1.1, and a cylindrical shape (1.5 cm in height and 2 cm in diameter) was produced in a steel mold and further densified under a pressure of 8 MPa. The vacuum heating furnace was filled with argon, and an appropriate amount of small cylinder (30 g) was placed in the bottom of the graphite crucible and heated. The temperature was 200–500 °C, the pressure (Ar) was 1 atm, the heating rate was 10 °C/min, and the temperature was maintained for 10–30 min. The 400 °C and 30 min vulcanization product was crushed, the particles (diameter 0.3–0.8 cm) were screened using a sieve, and 20 g of the particles (diameter 0.3–0.8 cm) were distilled at 550–800 °C. The pressure was 15 Pa, the heating rate was 10 °C/min, and the temperature was maintained for 15–90 min. The crystalline phase of vulcanization product was identified by Xray diffraction (XRD) using Cu Kα (λ = 1.5418 Å) radiation with a scanning rate of 2°/min on an instrument manufactured by Rigaku Corporation of Japan. Volatilized and non-volatilized residues were collected at random at six locations. Samples were then mixed to form are presentative concentration. High-content elements were determined by chemical titration. The contents of Cd and S in the condensate were determined by Agilent 7700x Inductively coupled plasma atomic emission spectrometry (ICP-AES). An electron probe micro analyzer (EPMA, JEOL Ltd., Japan) was used to scan the elemental distribution of the samples. The recovery rate of Te is derived by the following formula: Recovery\;rate = (m 0*w0)/(m1*w1) where m0 is the mass (g) of the raw material; w0 is the Te content (%) in the raw material; m1 is the mass (g) of the condensate; w1 is the Te content (%) in the condensate.
Vacuum metallurgy is a metallurgical process carried out in a defined system below atmospheric pressure. This process has been used in alloy separation and secondary resource recovery [19–26]. Therefore, this paper uses vulcanization-vacuum distillation to separate the rare metal Te from CdTe, providing theoretical guidance for the recycling of cadmium telluride photovoltaic modules. Compared with the traditional methods, the process has significant advantages: no waste gas generated, highly effective resource utilization, short refining process, good working conditions, and compliance with the requirements of energy saving and clean production. The CdS and Te obtained by vulcanization-vacuum distillation can be directly used as raw materials for producing semiconductors after subsequent processing. 2. Materials and methods 2.1. Materials and device The experimental raw materials are cadmium telluride powder produced by a company in Sichuan and sulfur powder produced by a company in Yunnan. The chemical constituents of cadmium telluride and sulfur were quantitatively determined by atomic absorption spectrometry (WFX-320, Beijin Beifen-Ruili Analytical Instrument (Group) Co., Ltd.), chemical titration and gravimetric method. The purity of cadmium telluride powder was 99.99%, and the purity of sulfur was 99.99%. The experimental device is a small vertical vacuum distillation furnace, and the photograph and structural drawing are shown in Fig. 1. The heating principle involves heating of the graphite resistance by the current and continuous generation of radiation. The heat generated by the radiation is transmitted to the volatile components on the surface of the alloy for evaporation, and the heat is transferred from the surface to the alloy by heat conduction. The system pressure can be maintained at 15 Pa. After heating and evaporating under vacuum, the alloy volatilizes into a gas phase and condenses on the condenser.
3. Theoretical analysis 3.1. Gibbs free energy calculation and dynamics discussion The probability of a displacement reaction is determined by the change in Gibbs free energy (ΔG). When ΔG is negative, the reaction is spontaneous at constant pressure and constant temperature. Conversely, when ΔG is positive, the reaction is not spontaneous. The ΔG can be expressed by formula (1):
ΔG = ΔG0 + RTlnQ
2.2. Methods
(1)
ΔG is the standard Gibbs free energy, and its changes are accompanied by the conversion of the standard state reactant into a standard state
Cadmium telluride and sulfur were uniformly mixed in a 2
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Fig. 2. Relationship between △G° and temperature.
Fig. 4. Relationship curves between the vapor pressure and temperature of the sulfurized product. S2 (118–600 C°), Te2 (449–980 C°), CdS (530–980 C°).
product. R is the general gas constant, T is the temperature, and Q is the reaction quotient. According to the calculations by HSC CHEMISTRY 6.0, the relationship between the ΔG and temperature of reaction can be plotted in Fig. 2, which indicates that the ΔG of reaction is negative at 200–500 °C and gradually decreases as the temperature increases. This finding indicates that reaction occurs spontaneously and proceeds more easily under standard conditions at higher temperature. Therefore, Te is expected to be replaced by sulfur from cadmium telluride via a displacement reaction in a temperature range of 200–500 °C. Vulcanization process is shown in Fig. 3.
to reduce the vapor pressure of Te, thereby allowing condensation and continuous evaporation of the Te in high temperature regions. The elements, which evaporate into the vapor phase, are condensed to achieve separation (Fig. 5). 4. Results and discussion 4.1. Sulfurization reaction 4.1.1. Differential thermal analysis Fig. 6 shows the TG and SDTA curves of CdTe and sulfur in an argon atmosphere with Al2O3 as a reference. The initial mass reduction (115–240 °C, Δm = 14.5%) is attributable to the volatilization of sulfur. Within this temperature range, a small amount of sulfur reacts with cadmium telluride to form Te and CdS. The endothermic peak (118 °C) is the sulfur melting endotherm, and the endothermic peak (210 °C) is the sulfur evaporation endotherm. When the temperature is 400–500 °C, melting and volatilization of the generated Te cause a slight decrease in mass (Δm = 0.5%). As the temperature increases, Te evaporates and absorbs heat. The heating rate of Fig. 6(b, c) is too fast, the image dispersion is large, and the temperature shows hysteresis effect.
3.2. Saturated vapor pressure The relationship between the saturated vapor pressure and temperature of a pure component can be expressed by Eq. (2):
lgp* = AT−1 + BlgT + CT + D
(2)
where p* is the saturated vapor pressure of the pure component, A, B, C and D are the evaporation constants of each element in the alloy. The relationship between the vapor pressure and temperature of each component in the vulcanization product is shown in Fig. 4. The calculation results show that the equilibrium vapor pressure increases with increasing temperature. As shown in Fig. 4, the saturated vapor pressure of the material decreases in the following order: S ＞ Te ＞ CdS. It is noteworthy that the pressure corresponding to the melting point of Te (450 °C) is 24 Pa. Under the actual processing conditions, the temperature in the high temperature region of the system is generally above 450 °C. Therefore, when the system pressure is below 24 Pa, the Te vapor does not reach saturation in the high temperature region, thereby facilitating the volatilization of Te. At this pressure, when the steam enters the condensation zone below 450 °C, the Te vapor is supersaturated when it reaches the condensation condition. Therefore, the system creates suitable conditions under vacuum
4.1.2. XRD diffraction and EPMA Sulfide was prepared at a temperature of 200 °C to 500 °C, a pressure (Ar) of 1 atm and a heating rate of 10 °C/min. The temperature was maintained for 30 min. The phase evolution of the experimental samples characterized by XRD spectra is shown in Fig. 7. The observed phases were CdTe, Te and CdS. When the temperature rises to 200 °C, the morphology is characterized by three-phase crystal forms (CdTe, Te and CdS), and the main peak is CdTe (23.8). When the temperature is at
Fig. 3. Schematic diagram of the reaction mechanism of cadmium telluride and sulfur. 3
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Fig. 5. Schematic of the vacuum distillation process.
displacement reaction, the formed Te is separated from the reaction interface of cadmium telluride and sulfur and enters the surface of the melt, and the formed cadmium sulfide forms a solid product layer. Therefore, the reaction of cadmium telluride with sulfur melt belongs to a shrinkage core model with a solid product layer. According to the theory of heterogeneous reaction kinetics, the reaction comprises the following steps: (a) external diffusion of sulfur in the melt through the liquid boundary layer; (b) internal diffusion of sulfur and elemental Te in the melt through the solid product layer; (c) interfacial chemical reaction. Therefore, the rate-determining step involves two resistive steps, namely internal diffusion through the liquid metal and interfacial chemical reaction. According to the Arrhenius equation, the chemical reaction rate generally increases with increasing temperature. It can be concluded that the process is controlled by an interfacial chemical
400 °C and 500 °C, the morphology is characterized by a two-phase crystal form with a major peak at Te (27.6). As the reaction temperature increased, the content of CdS and Te gradually increased, and the cadmium telluride peak disappeared. Sulfide was prepared at a temperature of 300 °C and 400 °C, a pressure (Ar) of 1 atm and a heating rate of 10 °C/min. The temperature was maintained of10- 30 min. The phase evolution of the experimental samples characterized by XRD spectra is shown in Fig. 8. At 400 °C, holding for 30 min, the sulfurized product is a two-phase crystalline form, and the rest have cadmium telluride peaks. Therefore, 400 °C, 30 min is the best curing conditions. Photographs and electron probe results of the sulfide are shown in Fig. 9. We can clearly see the distribution of each element in the sulfide. In the reaction system, cadmium telluride and sulfur undergo a
Fig. 6. Thermal analysis of CdTe and S (TG and SDTA). 4
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Fig. 7. X-ray diffraction patterns of sulfurized products at different temperature.
content in the condensate increases at higher temperature. When the temperature is between 600 °C and 650 °C, the sulfur content in the condensate fluctuates significantly, 0.039% increases to 0.391%; The recovery rate does not increase significantly after 600 °C. Therefore, the distillation temperature of 600 °C may be regarded as the best extraction Te. Within this range, the purity of Te remains above 99.93%. Under the conditions of distillation temperature of 600 °C and pressure of 15 Pa, the selected holding time was 15 min, 30 min, 45 min, 60 min, 75 min and 90 min. The composition analysis of the condensate obtained with different holding time is shown in Fig. 11(b). As illustrated in Fig. 11(b), the shorter the distillation time, the higher the purity of the condensate Te and the lower the impurity content in Te. When the distillation time was 15 min, the temperature was 600 °C, and the residual pressure was 15 Pa, the purity of Te reached 99.929%, and the contents of CdS and S in Te were 0.034% and 0.037%, respectively. As the time of vacuum distillation was extended, the amount of Cd and S in the condensate slowly increased. The Cd element mainly existed in the form of CdS. The S element existed in the two main forms of CdS and elemental sulfur. As the distillation time prolongs, cadmium sulfide begins to slowly evaporate, resulting in an increase in the levels of sulfur and cadmium in the condensate. As the distillation time increased from 15 to 90 min, the Te recovery rate
reaction at a lower temperature, and if the reaction rate exceeds the diffusion rate, the rate-determining step of the process changes from the interfacial chemical reaction to diffusion. Therefore, the kinetics of the process vary with the reaction conditions.
4.2. Vacuum distillation First, the effect of distillation temperature on the volatile release of Te and CdS was investigated with a distillation temperature of 550800 °C, a distillation time of 30 min, a residual pressure of 15 Pa and 20 g vulcanization sample. Fig. 10 shows an XRD pattern of volatile component and residue under optimized conditions (600 °C, 15 Pa, 30 min). Te is enriched in the condensate and CdS is enriched in the residue, indicating that the vulcanization-vacuum distillation is effective for separating Te and CdS. The composition analysis of the condensate obtained with different holding temperature is shown in Fig. 11(a). As illustrated in Fig. 11(a), when the temperature was increased from 550 °C to 800 °C, the recovery rate of Te increased from 71.75% to 99.16%, and the decrease in the mass of the residue sample indicated that the average recovery rate of Te was controlled by temperature. When the temperature is in the range of 550 °C and 800 °C, the Cd
Fig. 8. (a) X-ray diffraction patterns of sulfurized products at different time in 300 °C; (b) X-ray diffraction patterns of sulfurized products at different time in 400°C. 5
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Fig. 9. Product photographs and electron probe results of sulfurized product (400 °C, 30 min).
efficiency of Te and CdS. When the sulfurized product is heated under vacuum, molecules of the metals generated leave the surface of the material and they may encounter other gas molecules. If no collision occurs until a molecule reaches the condenser, Langmuir postulates that the maximum evaporation rate of the metal will be achieved. It can be described as ωmax = 2.624 × 10 - 2α pi * M/ T , where ωmax is the
increased from 71.25% to 98.56%. Photographs and electron probe results of the residue and condensate are shown in Fig. 12. We can clearly see the distribution of each element. The tellurium in the residue is mainly distributed at the edge of the granule. Sulfur is very dispersed in the condensate. Evaporation rate affects volatilization time and separation
Fig. 10. XRD patterns of the (a) volatile component and (b) residue obtained under the optimized conditions (600 °C, 15 Pa, 30 min). 6
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Fig. 11. (a) Effect of vacuum distillation temperature; (b) Effect of vacuum distillation time.
maximum evaporation rate; α is the accommodation coefficient, the probability of a distilled atom leaving the surface of the melting material, which is taken as close to unity; p i *is the saturated pressure of the pure element; M is the molecular weight of the distilled element; T
is the melting surface temperature. The results of evaporation rates of Te and CdS in material at 600 °C are 2.728 g·cm−2 min−1 and 3.079 × 10−4 g cm−2 min−1, respectively. Therefore, the evaporation rate of Te is much higher than that of CdS, and Te will evaporate
Fig. 12. Product photographs and electron probe results: (a) residue; (b) condensate. (600 °C 30 min 15 Pa). 7
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doi.org/10.1016/j.seppur.2019.115864.
preferentially in a relatively short time, which is consistent with the calculation results of saturated vapor pressure. Based on the kinetic calculation of evaporation rate, we can conclude that with an increase in distillation time, Te with a higher evaporation rate, will gradually completely volatilize, while CdS will slowly volatilize, resulting in the increase in impurity content of the volatile matter. As shown in Fig. 11(b), Te recovery rate tends to reach a maximum when distillation time exceeds 30 min. Continuing to increase distillation time will only lead to CdS volatilization to form impurities and rapidly reduce the purity of Te. Taking into account the extension of distillation time will increase financial and human resources, so in a higher purity and higher recovery rate, cost savings, improve resource utilization, choose 30 min for the best distillation time. Cadmium sulfide is enriched in the residue, up to 99.5%, and further processing and purification can be used as raw material for the production of cadmium telluride photovoltaic modules. Our previous work proved that vulcanization-vacuum distillation can successfully separate Te from cadmium telluride and obtain different products. Studying the enrichment of the semiconductor layer and the effect of impurity elements in the semiconductor layer on the separation effect is our next step. The problems remaining to be solved for the industrialization of the vacuum distillation process include how to optimize the experimental conditions and further explore continuous and automated vacuum distillation processes.
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5. Conclusion Vulcanization-vacuum distillation technology was proposed for the first time to separating and recovering tellurium from cadmium telluride waste. The feasibility of this process was verified on the laboratory scale and semi-technical scale, thus providing theoretical guidance for the recycling of cadmium telluride photovoltaic modules. Smelting of cadmium telluride and sulfur can produce Te and CdS. With a temperature of 550–800 °C, a vacuum distillation time of 30 min and a residual gas pressure of 15 Pa, Te was enriched to 99.93%, and the recovery rate was 94.14%. The contents of Cd and S in the Te were 0.034% and 0.037%, respectively. The purity of cadmium sulfide in the vacuum distillation crucible can reach 99.5%. Meanwhile, the CdS collected by vulcanization-vacuum distillation can be used as a raw material for semiconductor production after purification. Compared with the existing methods, the vulcanization-vacuum distillation approach features no waste gas or wastewater, high resource utilization rate, short refining process, good working conditions, and meets the requirements of energy saving and clean production. Acknowledgements This work has been founded by the Fund of National Natural Science Foundation of China [grant numbers U1502271], the National Key Research and Development Program of China [grant numbers 2016YFC0400404], the Cultivating Plan Program for the Leader in Science and Technology of Yunnan Province [grant numbers 2014HA003], the Program for Nonferrous Metals Vacuum Metallurgy Innovation Team of Ministry of Science and Technology[grant numbers 2014RA4018], Postgraduate project of scientific research foundation of Yunnan Provincial Department of Education[grant numbers 2017YJS066], Personal Training Project of Kunming University of Science and Technology [grant numbers KKZ3201552016], Personal Training Project of State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization [grant numbers CNMRCUTS1605], Science and technology planning project of Yunnan Province [grant numbers 2017FB082]. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// 8
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Selective separation and recovery of rare metals by vulcanization-vacuum distillation of cadmium telluride waste
T
Xiaofeng Zhanga,b,c, Daxin Huanga,b,c, Wenlong Jianga,b,c, Guozheng Zhaa,b,c, Juhai Denga,b,c, ⁎ Pan Denga,b,c, Xiangfeng Konga,b,c, Dachun Liua,b,c, a
State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization, Kunming University of Science and Technology, Kunming 650093, China National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China c Yunnan Provincial Key Laboratory for Nonferrous Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hazardous waste Recycling CdTe Vulcanization Vacuum distillation
Cadmium telluride waste contains a portion of rare metals. In this paper, a new, clean and efficient vulcanization-vacuum distillation process is used to separate the rare metal Te from cadmium telluride. Taking CdTe (99.99%) and S (99.99%) as examples, the feasibility of this process was verified on a laboratory scale. Recovery of cadmium telluride waste was simplified to two-step vulcanization and vacuum distillation process. The effects of temperature and time on vulcanization-vacuum distillation were investigated. The experimental results show that the purity of Te obtained by vacuum distillation was 99.92%, and the purity of cadmium sulfide was up to 99.5%. This method provides theoretical guidance for the recycling of cadmium telluride photovoltaic modules. Compared with the traditional methods, this process has a short refining process, does no generate waste water or waste gas, and is environmentally friendly. The Te and CdS obtained from vulcanization-vacuum distillation can be utilized as raw materials for semiconductor production after subsequent processing.
1. Introduction CdTe is a direct bandgap semiconductor with a forbidden band width of 1.45 eV, which is very close to the ideal band gap width of photovoltaic materials. It has a high light absorption coefficient. For example, the light absorption coefficient is 105 cm in the visible region, allowing absorption of 90% of the light with a thickness of a few microns [1,2]. Accordingly, the production scale and application of CdTe thin-film solar cells have increased rapidly since the beginning of the 21st century [3,4]. The conversion efficiency of small-area CdTe solar cells has reached 19.6%, the module efficiency has reached 16.1%, and the average efficiency is 13%. However, there is still a big gap between the theoretical conversion efficiency of 30%, leaving plenty of room for improvement [5]. According to the European Photovoltaic Industry Association (EPIA), the contribution of photovoltaic energy to the global electricity supply may be as high as 12.6% by 2040 [6–9]. The proportion of cadmium telluride thin-film photovoltaic (CdTe-PV) in emerging technologies continues to increase. The production cost of cadmium telluride film photovoltaic is exceptionally low, and the current component cost is around $0.64/W [10]. There are two main problems with cadmium telluride photovoltaic ⁎
technology: the impact of cadmium pollution and the limited supply of metal tellurium (Te). The estimated life of cadmium telluride thin-film modules is approximately 25–35 years. When they reach the end of the intended lifetime but are not properly recycled and handled, the bracket used on the panel may harm the environment. In addition, the heavy metals contained in the materials are toxic and may cause cancer or teratogenicity. Therefore, the recycling methods must take into account these facts [11–14]. Cadmium telluride is sealed between two pieces of glass, and there is no release of Cd at normal temperature. Even at high temperatures up to 1100 °C, more than 99.96% of CdTe is sealed by the two pieces of molten glass without leakage. Various studies have shown that the recycling of cadmium telluride photovoltaic modules is feasible [15]. The European PV Accept Project reports that the energy recovery time of CdTe solar cells is only 0.9 years, and the practice of First Solar has confirmed that the recovery time is 10 months. The researchers tested four methods for treating the waste of CdTe PV module: (1) ferric chloride/hydrochloric acid solution leaching; (2) sulfuric acid oxygen pressure leaching; (3) nitric acid leaching; and (4) sulfuric acid/hydrogen peroxide leaching. The first method does not separate Te from iron (Te is not leached into the solution). Oxygen pressure leaching provides more effective separation, but the residual
Corresponding author. E-mail address: [email protected] (D. Liu).
https://doi.org/10.1016/j.seppur.2019.115864 Received 24 May 2019; Received in revised form 16 July 2019; Accepted 26 July 2019 Available online 27 July 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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amount of Cd, Te and S accounts for 4−7% of the total residual amount. In addition, this process is capital intensive due to the high pressure and temperature required, and is therefore not attractive for low cost recycling. Nitric acid leaching recovers 96% of the Te, leaving Cd in solution. However, the subsequent electrolytic recovery of Cd is based on an artificial high Cd concentration (such as 10,000 ppm instead of 1000 ppm obtained from leaching). In addition, leaching with high concentrations of nitric acid requires a high temperature of 60 to 80°Celsius, inevitably producing nitric acid fume [16]. The power generation and full-scale production of First Solar represents an advanced example of cadmium telluride thin-film module recycling. The recycling process includes the following steps [17,18]:
• A shredder was used to break the glass into large pieces, and a hammer mill was used to break the broken glass into 4 mm pieces. • The semiconductor film is removed using sulfuric acid and hydrogen peroxide in a slowly rotating leaching tank. • Solid-liquid separation is performed by pouring the contents of the leaching drum into a classifier to separate the glass from the liquid. • The separation of the glass laminate is achieved by separating the • •
glass from the larger ethylene vinyl acetate (EVA) sheet through a vibrating screen. Glass cleaning removes any semiconductor material that remains on the glass. The metal component separated via sodium hydroxide precipitation is concentrated in a thickened tank, and the resulting filter cake was packaged by a third party (5NPlus/5NPV) for metal recovery.
Fig. 1. Structure chart of the vacuum distillation furnace: 1, stainless steel shell; 2, valve 3, thermocouple; 4, insulation materials; 5, graphite resistance; 6, crucible; 7, condenser; 8, vacuum gauge.
stoichiometric ratio of 1:1.1, and a cylindrical shape (1.5 cm in height and 2 cm in diameter) was produced in a steel mold and further densified under a pressure of 8 MPa. The vacuum heating furnace was filled with argon, and an appropriate amount of small cylinder (30 g) was placed in the bottom of the graphite crucible and heated. The temperature was 200–500 °C, the pressure (Ar) was 1 atm, the heating rate was 10 °C/min, and the temperature was maintained for 10–30 min. The 400 °C and 30 min vulcanization product was crushed, the particles (diameter 0.3–0.8 cm) were screened using a sieve, and 20 g of the particles (diameter 0.3–0.8 cm) were distilled at 550–800 °C. The pressure was 15 Pa, the heating rate was 10 °C/min, and the temperature was maintained for 15–90 min. The crystalline phase of vulcanization product was identified by Xray diffraction (XRD) using Cu Kα (λ = 1.5418 Å) radiation with a scanning rate of 2°/min on an instrument manufactured by Rigaku Corporation of Japan. Volatilized and non-volatilized residues were collected at random at six locations. Samples were then mixed to form are presentative concentration. High-content elements were determined by chemical titration. The contents of Cd and S in the condensate were determined by Agilent 7700x Inductively coupled plasma atomic emission spectrometry (ICP-AES). An electron probe micro analyzer (EPMA, JEOL Ltd., Japan) was used to scan the elemental distribution of the samples. The recovery rate of Te is derived by the following formula: Recovery\;rate = (m 0*w0)/(m1*w1) where m0 is the mass (g) of the raw material; w0 is the Te content (%) in the raw material; m1 is the mass (g) of the condensate; w1 is the Te content (%) in the condensate.
Vacuum metallurgy is a metallurgical process carried out in a defined system below atmospheric pressure. This process has been used in alloy separation and secondary resource recovery [19–26]. Therefore, this paper uses vulcanization-vacuum distillation to separate the rare metal Te from CdTe, providing theoretical guidance for the recycling of cadmium telluride photovoltaic modules. Compared with the traditional methods, the process has significant advantages: no waste gas generated, highly effective resource utilization, short refining process, good working conditions, and compliance with the requirements of energy saving and clean production. The CdS and Te obtained by vulcanization-vacuum distillation can be directly used as raw materials for producing semiconductors after subsequent processing. 2. Materials and methods 2.1. Materials and device The experimental raw materials are cadmium telluride powder produced by a company in Sichuan and sulfur powder produced by a company in Yunnan. The chemical constituents of cadmium telluride and sulfur were quantitatively determined by atomic absorption spectrometry (WFX-320, Beijin Beifen-Ruili Analytical Instrument (Group) Co., Ltd.), chemical titration and gravimetric method. The purity of cadmium telluride powder was 99.99%, and the purity of sulfur was 99.99%. The experimental device is a small vertical vacuum distillation furnace, and the photograph and structural drawing are shown in Fig. 1. The heating principle involves heating of the graphite resistance by the current and continuous generation of radiation. The heat generated by the radiation is transmitted to the volatile components on the surface of the alloy for evaporation, and the heat is transferred from the surface to the alloy by heat conduction. The system pressure can be maintained at 15 Pa. After heating and evaporating under vacuum, the alloy volatilizes into a gas phase and condenses on the condenser.
3. Theoretical analysis 3.1. Gibbs free energy calculation and dynamics discussion The probability of a displacement reaction is determined by the change in Gibbs free energy (ΔG). When ΔG is negative, the reaction is spontaneous at constant pressure and constant temperature. Conversely, when ΔG is positive, the reaction is not spontaneous. The ΔG can be expressed by formula (1):
ΔG = ΔG0 + RTlnQ
2.2. Methods
(1)
ΔG is the standard Gibbs free energy, and its changes are accompanied by the conversion of the standard state reactant into a standard state
Cadmium telluride and sulfur were uniformly mixed in a 2
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Fig. 2. Relationship between △G° and temperature.
Fig. 4. Relationship curves between the vapor pressure and temperature of the sulfurized product. S2 (118–600 C°), Te2 (449–980 C°), CdS (530–980 C°).
product. R is the general gas constant, T is the temperature, and Q is the reaction quotient. According to the calculations by HSC CHEMISTRY 6.0, the relationship between the ΔG and temperature of reaction can be plotted in Fig. 2, which indicates that the ΔG of reaction is negative at 200–500 °C and gradually decreases as the temperature increases. This finding indicates that reaction occurs spontaneously and proceeds more easily under standard conditions at higher temperature. Therefore, Te is expected to be replaced by sulfur from cadmium telluride via a displacement reaction in a temperature range of 200–500 °C. Vulcanization process is shown in Fig. 3.
to reduce the vapor pressure of Te, thereby allowing condensation and continuous evaporation of the Te in high temperature regions. The elements, which evaporate into the vapor phase, are condensed to achieve separation (Fig. 5). 4. Results and discussion 4.1. Sulfurization reaction 4.1.1. Differential thermal analysis Fig. 6 shows the TG and SDTA curves of CdTe and sulfur in an argon atmosphere with Al2O3 as a reference. The initial mass reduction (115–240 °C, Δm = 14.5%) is attributable to the volatilization of sulfur. Within this temperature range, a small amount of sulfur reacts with cadmium telluride to form Te and CdS. The endothermic peak (118 °C) is the sulfur melting endotherm, and the endothermic peak (210 °C) is the sulfur evaporation endotherm. When the temperature is 400–500 °C, melting and volatilization of the generated Te cause a slight decrease in mass (Δm = 0.5%). As the temperature increases, Te evaporates and absorbs heat. The heating rate of Fig. 6(b, c) is too fast, the image dispersion is large, and the temperature shows hysteresis effect.
3.2. Saturated vapor pressure The relationship between the saturated vapor pressure and temperature of a pure component can be expressed by Eq. (2):
lgp* = AT−1 + BlgT + CT + D
(2)
where p* is the saturated vapor pressure of the pure component, A, B, C and D are the evaporation constants of each element in the alloy. The relationship between the vapor pressure and temperature of each component in the vulcanization product is shown in Fig. 4. The calculation results show that the equilibrium vapor pressure increases with increasing temperature. As shown in Fig. 4, the saturated vapor pressure of the material decreases in the following order: S ＞ Te ＞ CdS. It is noteworthy that the pressure corresponding to the melting point of Te (450 °C) is 24 Pa. Under the actual processing conditions, the temperature in the high temperature region of the system is generally above 450 °C. Therefore, when the system pressure is below 24 Pa, the Te vapor does not reach saturation in the high temperature region, thereby facilitating the volatilization of Te. At this pressure, when the steam enters the condensation zone below 450 °C, the Te vapor is supersaturated when it reaches the condensation condition. Therefore, the system creates suitable conditions under vacuum
4.1.2. XRD diffraction and EPMA Sulfide was prepared at a temperature of 200 °C to 500 °C, a pressure (Ar) of 1 atm and a heating rate of 10 °C/min. The temperature was maintained for 30 min. The phase evolution of the experimental samples characterized by XRD spectra is shown in Fig. 7. The observed phases were CdTe, Te and CdS. When the temperature rises to 200 °C, the morphology is characterized by three-phase crystal forms (CdTe, Te and CdS), and the main peak is CdTe (23.8). When the temperature is at
Fig. 3. Schematic diagram of the reaction mechanism of cadmium telluride and sulfur. 3
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Fig. 5. Schematic of the vacuum distillation process.
displacement reaction, the formed Te is separated from the reaction interface of cadmium telluride and sulfur and enters the surface of the melt, and the formed cadmium sulfide forms a solid product layer. Therefore, the reaction of cadmium telluride with sulfur melt belongs to a shrinkage core model with a solid product layer. According to the theory of heterogeneous reaction kinetics, the reaction comprises the following steps: (a) external diffusion of sulfur in the melt through the liquid boundary layer; (b) internal diffusion of sulfur and elemental Te in the melt through the solid product layer; (c) interfacial chemical reaction. Therefore, the rate-determining step involves two resistive steps, namely internal diffusion through the liquid metal and interfacial chemical reaction. According to the Arrhenius equation, the chemical reaction rate generally increases with increasing temperature. It can be concluded that the process is controlled by an interfacial chemical
400 °C and 500 °C, the morphology is characterized by a two-phase crystal form with a major peak at Te (27.6). As the reaction temperature increased, the content of CdS and Te gradually increased, and the cadmium telluride peak disappeared. Sulfide was prepared at a temperature of 300 °C and 400 °C, a pressure (Ar) of 1 atm and a heating rate of 10 °C/min. The temperature was maintained of10- 30 min. The phase evolution of the experimental samples characterized by XRD spectra is shown in Fig. 8. At 400 °C, holding for 30 min, the sulfurized product is a two-phase crystalline form, and the rest have cadmium telluride peaks. Therefore, 400 °C, 30 min is the best curing conditions. Photographs and electron probe results of the sulfide are shown in Fig. 9. We can clearly see the distribution of each element in the sulfide. In the reaction system, cadmium telluride and sulfur undergo a
Fig. 6. Thermal analysis of CdTe and S (TG and SDTA). 4
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Fig. 7. X-ray diffraction patterns of sulfurized products at different temperature.
content in the condensate increases at higher temperature. When the temperature is between 600 °C and 650 °C, the sulfur content in the condensate fluctuates significantly, 0.039% increases to 0.391%; The recovery rate does not increase significantly after 600 °C. Therefore, the distillation temperature of 600 °C may be regarded as the best extraction Te. Within this range, the purity of Te remains above 99.93%. Under the conditions of distillation temperature of 600 °C and pressure of 15 Pa, the selected holding time was 15 min, 30 min, 45 min, 60 min, 75 min and 90 min. The composition analysis of the condensate obtained with different holding time is shown in Fig. 11(b). As illustrated in Fig. 11(b), the shorter the distillation time, the higher the purity of the condensate Te and the lower the impurity content in Te. When the distillation time was 15 min, the temperature was 600 °C, and the residual pressure was 15 Pa, the purity of Te reached 99.929%, and the contents of CdS and S in Te were 0.034% and 0.037%, respectively. As the time of vacuum distillation was extended, the amount of Cd and S in the condensate slowly increased. The Cd element mainly existed in the form of CdS. The S element existed in the two main forms of CdS and elemental sulfur. As the distillation time prolongs, cadmium sulfide begins to slowly evaporate, resulting in an increase in the levels of sulfur and cadmium in the condensate. As the distillation time increased from 15 to 90 min, the Te recovery rate
reaction at a lower temperature, and if the reaction rate exceeds the diffusion rate, the rate-determining step of the process changes from the interfacial chemical reaction to diffusion. Therefore, the kinetics of the process vary with the reaction conditions.
4.2. Vacuum distillation First, the effect of distillation temperature on the volatile release of Te and CdS was investigated with a distillation temperature of 550800 °C, a distillation time of 30 min, a residual pressure of 15 Pa and 20 g vulcanization sample. Fig. 10 shows an XRD pattern of volatile component and residue under optimized conditions (600 °C, 15 Pa, 30 min). Te is enriched in the condensate and CdS is enriched in the residue, indicating that the vulcanization-vacuum distillation is effective for separating Te and CdS. The composition analysis of the condensate obtained with different holding temperature is shown in Fig. 11(a). As illustrated in Fig. 11(a), when the temperature was increased from 550 °C to 800 °C, the recovery rate of Te increased from 71.75% to 99.16%, and the decrease in the mass of the residue sample indicated that the average recovery rate of Te was controlled by temperature. When the temperature is in the range of 550 °C and 800 °C, the Cd
Fig. 8. (a) X-ray diffraction patterns of sulfurized products at different time in 300 °C; (b) X-ray diffraction patterns of sulfurized products at different time in 400°C. 5
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Fig. 9. Product photographs and electron probe results of sulfurized product (400 °C, 30 min).
efficiency of Te and CdS. When the sulfurized product is heated under vacuum, molecules of the metals generated leave the surface of the material and they may encounter other gas molecules. If no collision occurs until a molecule reaches the condenser, Langmuir postulates that the maximum evaporation rate of the metal will be achieved. It can be described as ωmax = 2.624 × 10 - 2α pi * M/ T , where ωmax is the
increased from 71.25% to 98.56%. Photographs and electron probe results of the residue and condensate are shown in Fig. 12. We can clearly see the distribution of each element. The tellurium in the residue is mainly distributed at the edge of the granule. Sulfur is very dispersed in the condensate. Evaporation rate affects volatilization time and separation
Fig. 10. XRD patterns of the (a) volatile component and (b) residue obtained under the optimized conditions (600 °C, 15 Pa, 30 min). 6
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Fig. 11. (a) Effect of vacuum distillation temperature; (b) Effect of vacuum distillation time.
maximum evaporation rate; α is the accommodation coefficient, the probability of a distilled atom leaving the surface of the melting material, which is taken as close to unity; p i *is the saturated pressure of the pure element; M is the molecular weight of the distilled element; T
is the melting surface temperature. The results of evaporation rates of Te and CdS in material at 600 °C are 2.728 g·cm−2 min−1 and 3.079 × 10−4 g cm−2 min−1, respectively. Therefore, the evaporation rate of Te is much higher than that of CdS, and Te will evaporate
Fig. 12. Product photographs and electron probe results: (a) residue; (b) condensate. (600 °C 30 min 15 Pa). 7
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preferentially in a relatively short time, which is consistent with the calculation results of saturated vapor pressure. Based on the kinetic calculation of evaporation rate, we can conclude that with an increase in distillation time, Te with a higher evaporation rate, will gradually completely volatilize, while CdS will slowly volatilize, resulting in the increase in impurity content of the volatile matter. As shown in Fig. 11(b), Te recovery rate tends to reach a maximum when distillation time exceeds 30 min. Continuing to increase distillation time will only lead to CdS volatilization to form impurities and rapidly reduce the purity of Te. Taking into account the extension of distillation time will increase financial and human resources, so in a higher purity and higher recovery rate, cost savings, improve resource utilization, choose 30 min for the best distillation time. Cadmium sulfide is enriched in the residue, up to 99.5%, and further processing and purification can be used as raw material for the production of cadmium telluride photovoltaic modules. Our previous work proved that vulcanization-vacuum distillation can successfully separate Te from cadmium telluride and obtain different products. Studying the enrichment of the semiconductor layer and the effect of impurity elements in the semiconductor layer on the separation effect is our next step. The problems remaining to be solved for the industrialization of the vacuum distillation process include how to optimize the experimental conditions and further explore continuous and automated vacuum distillation processes.
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5. Conclusion Vulcanization-vacuum distillation technology was proposed for the first time to separating and recovering tellurium from cadmium telluride waste. The feasibility of this process was verified on the laboratory scale and semi-technical scale, thus providing theoretical guidance for the recycling of cadmium telluride photovoltaic modules. Smelting of cadmium telluride and sulfur can produce Te and CdS. With a temperature of 550–800 °C, a vacuum distillation time of 30 min and a residual gas pressure of 15 Pa, Te was enriched to 99.93%, and the recovery rate was 94.14%. The contents of Cd and S in the Te were 0.034% and 0.037%, respectively. The purity of cadmium sulfide in the vacuum distillation crucible can reach 99.5%. Meanwhile, the CdS collected by vulcanization-vacuum distillation can be used as a raw material for semiconductor production after purification. Compared with the existing methods, the vulcanization-vacuum distillation approach features no waste gas or wastewater, high resource utilization rate, short refining process, good working conditions, and meets the requirements of energy saving and clean production. Acknowledgements This work has been founded by the Fund of National Natural Science Foundation of China [grant numbers U1502271], the National Key Research and Development Program of China [grant numbers 2016YFC0400404], the Cultivating Plan Program for the Leader in Science and Technology of Yunnan Province [grant numbers 2014HA003], the Program for Nonferrous Metals Vacuum Metallurgy Innovation Team of Ministry of Science and Technology[grant numbers 2014RA4018], Postgraduate project of scientific research foundation of Yunnan Provincial Department of Education[grant numbers 2017YJS066], Personal Training Project of Kunming University of Science and Technology [grant numbers KKZ3201552016], Personal Training Project of State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization [grant numbers CNMRCUTS1605], Science and technology planning project of Yunnan Province [grant numbers 2017FB082]. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// 8