Waste photovoltaic panels for ultrapure silicon and hydrogen through

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Waste photovoltaic panels for ultrapure silicon and hydrogen through the low temperature magnesium silicide Pavel Dytrych, Jakub Bumba, Frantisek Kastanek, Radek Fajgar, Martin Kostejn, and Olga Šolcova Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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Industrial & Engineering Chemistry Research

Waste photovoltaic panels for ultrapure silicon and hydrogen through the low temperature magnesium silicide Pavel Dytrych, Jakub Bumba, Frantisek Kastanek, Radek Fajgar, Martin Kostejn, Olga Solcova* *Institute of Chemical Process Fundamentals of the CAS, v. v. i., Rozvojova 135/1, Prague 6, Czech Republic

ABSTRACT: Circulation technology of waste photovoltaic panels for production of ultrapure silicon and energy in the form of hydrogen storage was designed and verified. Preparation of magnesium silicide from waste photovoltaic panel’s silicon and partially oxidized magnesium was thoroughly studied. Work was focused on process optimization, thus, three groups of reactors were tested, namely the continuously evacuated open reactor, pre‐evacuated batch reactor and semi‐opened reactors. The influence of reaction temperature was evaluated in the range of 330 – 630 °C for various reaction atmospheres; argon and/or air at pressures of 5 kPa, 33 kPa and 100 kPa; and vacuum in the range of 5 ‐ 30 Pa. The effect of nitrogen and oxygen presence in the atmosphere on the resulted reaction and reaction rate was also thoroughly studied. The minimum reaction time guaranteeing the total conversion of silicon for two purities of used magnesium was also determined. The produced materials were analyzed by dispersive Raman spectroscopy, Scanning Electron Microscopy with Energy Disper‐ sive X‐Ray Spectroscopy and X‐Ray Diffraction. Finally, the reactor filling, which significantly influenced the formation of magnesium silicide, was tested and established minimally at 30% of reactor´s volume. Hydrolysis of obtained magnesium silicide by diluted acid for silicon hydrides’ (silanes) production and their subsequent thermal decomposition into the ul‐ trapure silicon and hydrogen were also successfully verified.

1.

argon as working gas at temperatures above 450 °C, in the case of magnesium silicide. For pure alkali metals, e. g. so‐ dium or potassium, the reaction starts at temperatures above 700 °C.6‐8 The reaction time for many alkali/alkali earth metals is more than one day, and this reaction also leads to an increase of volume of the reacting species. Tra‐ ditional Hohmann method involves the synthesis of sili‐ cides in a corundum beaker with argon as working gas. The reaction occurs in a metallic reactor, whereas the external source of heat, the gas burner, is used. Magnesium and other alkali metals are added in 200‐300 % excess.9 Classi‐ cal Moissan's process utilizes the high exothermic reaction of alkali metals with silicon.10, 11 The reaction mixture is heated above the melting point of alkali/alkali earth met‐ als, but the alkali/alkali earth metal reacts also with the re‐ actor´s walls and gases leading to a low quality product and low yield. The starting materials are mainly pressed into pellets and then thermally treated with temperatures be‐ tween 1100 ‐ 1500 °C and hydraulic pressure of about 100 MPa.12, 13 A powder form of magnesium silicide can also be prepared at the laboratory scale by utilizing tantalum reac‐ tors (beakers) during the solid‐solid phase reaction, whereas mono‐crystals are prepared by Bridgman method.14, 15 Other methods have also been employed to prepare high purity magnesium silicide, e. g. mechanical alloying16, spark plasma treatment17, fluidized bed rector18, repeated plastic working19, milling in a planetary mill20, ball

INTRODUCTION

A new issue concerning old used photovoltaic panels has currently become a serious challenge. The lifetime of the first generations of photovoltaic panels based on polycrys‐ talline or monocrystalline silicon has come to an end.1 Gen‐ erally, the recovery process involves dismantling of the panel with orientation towards separation of plastic, glass and metallic parts.2 The next steps are predominantly blending and thermal treatments of the material.2, 3 Chem‐ ical treatment of crystalline silicon solar cells is another possible recovery method of pure silicon from photovoltaic modules.4 The remaining silicon has to be treated by high temperature processes; however, concerning energy and material these processes are generally highly demanding.5 Moreover, only silicon with relatively high value of impu‐ rities and dopants (e.g. oxides, N2, B, P, Al) can be obtained by the above mentioned recovery processes. Therefore, similar processes are unsuitable for both reutilization and economic circulation. In fact, no process which results in silicon with high purity has been mentioned yet. A metallurgical grade silicon is used as a source for produc‐ tion of ultrapure silicon, nowadays. Silicides seem to be an‐ other promising materials except of metallurgical grade sil‐ icon. The traditional approach alkali/alkali earth metal sil‐ icides is based on the reaction of small grained alkali/alkali earth metals with silicon powder under vacuum, or with

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milling21, etc. Commonly used particle sizes for silicon and magnesium were in 10‐300 µm range for all above men‐ tioned syntheses. These processes require: Mg, Si of ultra‐ high purity, mechanical treatment by pressurizing or mill‐ ing, advanced devices, long time of above 3 hours to a few days; therefore, they are usually exceedingly expensive. Concerning magnesium silicide optimization, procedure of reaction conditions was reported by Ioannou at al. 22, who focused on solid‐state synthesis of Mg2Si via short‐dura‐ tion ball‐milling and low‐temperature annealing from ul‐ trapure substances. In this work, magnesium powder was milled under argon atmosphere with grain sizes in the range of 149‐841 µm for 30 min or 60 min and pressed into pellets with diameters of ca. 5‐10 mm at 500 MPa. The pel‐ lets were heated at temperatures above 400 °C with maxi‐ mal time of 300 h. Ioannou at al. mentioned that minimal time for initiation of Mg2Si forming had been 1 hour. It can be concluded that existing methods are significantly en‐ ergy‐intensive; therefore, there is no realistic hope they could be implemented in eco‐recycling of solar panels.

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below 10 %, (Solartec). Silicon scraps were milled into a powder form in a mortar. The particle size of obtained sil‐ icon powder was smaller than 50 µm. After milling, the starting materials were mixed in molar ratios Mg : Si = (2 – 2.5) : 1. The weight of the starting materials was 0.6g – 10.2 g for magnesium and 0.3 g – 5.9 g for silicon. The excess of magnesium was used to compensate evaporation of mag‐ nesium and the higher content of silicon oxide, which can react with magnesium and produce magnesium oxide. The mixture of starting materials was homogenized, filled into reactor and heated under various conditions.

2.2. Types of reactors and its arrangement Figure 1 shows four reactors which were tested for prepa‐ ration of magnesium silicide; a) ceramic boat, b) batch re‐ actor, c) semi‐opened reactor with valve, and d) semi‐ opened tube reactor. The experiments in a ceramic boat (Figure 1a) were realized under argon atmosphere at approx. 33 kPa and 500 °C. Magnesium (1.1 g; 97 %) and silicon (0.6 g) were given in molar ratio 2.1:1 into the boat and and inserted into a quartz tube. After evacuating the pressure lower than 30 Pa, argon was introduced. The reaction time was 5 minutes.

Nevertheless, magnesium silicide can be really utilized as a raw material for silane production. There are existed method in with magnesium silicide is slowly dropped into a glass tube with dilute hydrochloric acid. Resulting gase‐ ous product spontaneously burns in contact with air, which indicates the formation of silanes.23. Silane is com‐ monly used for a chemical vapor deposition of thin silicon layers24, 25. A pure silicon nano‐powder from silane can be obtained by the radiofrequency thermal plasma decompo‐ sition26 or by the pyrolysis of silane27, 28. Based on above mentioned information, we designed a new three step pro‐ cess which includes a low temperature preparation of mag‐ nesium silicide, its hydrolysis by a phosphoric acid and a subsequent thermal decomposition of prepared silanes. The process enables recycling of waste photovoltaic panels (after removal of plastic’s parts) to product an ultrapure silicon in combination with hydrogen and magnesium salt fertilizer as byproduct after hydrolysis of magnesium sili‐ cide. The article is mainly focused on optimization of mag‐ nesium silicide preparation such as a reaction temperature, time, atmosphere and reactor types. An acidic hydrolysis and thermal decomposition of resulting silane to produce ultrapure silicon and hydrogen was also tested and veri‐ fied. Thus, design of a low temperature process in which a relatively small amount of heat and materials are needed for reutilization of waste silicon by using scrap magnesium is reported.

The batch reactor (Figure 1b) was made of temperature re‐ sistant glass with an approximate volume of 50 cm3 with glass thickness of ca. 1 mm. Magnesium (0.9 g – 1.0 g; 97 %) and silicon (0.5 g) in molar ratio in the range of (2.1‐ 2.3):1 was inserted into the reactor and the air was pumped out by a rotary pump to decrease the pressure below 30 Pa and the ampule was sealed with a burner. For first experiments with reaction time of 15 or 45 minutes, 0.9 g of magnesium and 0.5 g of silicon were applied; and for 120‐minute exper‐ iment, 1 g of magnesium and 0.5 g of silicon. The reaction was performed at 500 °C in a furnace (Figure 1c). The semi‐opened reactor with a valve, which enables controlled communication with atmosphere, possessed an approximate volume of 50 cm3. For these experiments, weights of starting materials were 3.5 g of 97 % magnesium and 1.8 g of silicon in molar ratio 2.2:1; resp. 6.3 g and 3.7 g in molar ratio 2:1, and in the third experiment 10.2 g and 5.9 g in molar ratio 2:1. The mixture of starting materials was filled into the reactor, evacuated and then put into the furnace with controlled atmosphere. Within the first ar‐ rangement, atmosphere consisted of ca. 78 % nitrogen and 21 % oxygen at 5 kPa.

2. EXPERIMENTAL 2.1. Materials and experimental conditions Partially oxidized magnesium and silicon from end of life photovoltaic panels were used as starting materials for preparation of magnesium silicide (Mg2Si). Magnesium possessed the particle size of less than 200 µm, two purities of 90 and 97 % with Mg‐O content 10 % and 3 % respec‐ tively (Sigma Aldrich, Fichema s.r.o.) and the silicon scraps particle size varied between a few millimeters to several

Figure 1. a) Ceramic boat b) Batch reactor c) Semi‐opened reactor with valve d) Semi‐opened tube reactor

B


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The second arrangement was obtained by the addition of argon up to 100 kPa. The reactions of both arrangements were performed at 570 – 630 °C.

XPS spectra were charge corrected by C 1s spectral compo‐ nent binding energy set to 284.8 eV for Si, SiOx, Mg, MgOx, Mg2Si samples. The spectra were fitted using a Gaussian– Lorentzian line shape, Shirley background subtraction and a damped non‐linear least square procedure. Spectra were taken over Si 2p, Mg 2p, O 1s, C 1s, and valence band re‐ gions. Samples were repeatedly sputtered with Ar+ ions at 1 kV with current of 10 µA for 30 s to remove superficial layers. The SEM with mounted EDS was used for composi‐ tion evaluation of products based on intensity ratios of Mg Kα, Si Kα. Tungsten substrates were used as a support of the layers measured by EDS at acceleration voltage 10 ‐ 15 kV. The tungsten plates with a thin layer of studied material on the surface were attached using adhesive material to the non‐diffracting sample holders. To provide the exact dif‐ fracting plane, we used the XYZ stage coupled with correc‐ tion lasers. The measurements were carried out with a Bruker D8 Discover diffractometer equipped with a sili‐ con‐strip linear LynxEye detector and a germanium pri‐ mary monochromator providing CuKα1 radiation (λ = 1.54056 Å). Data were collected in two steps: 1. 2ϴ range of 5 ‐ 99° with a step size of 0.016 and a counting time of 1 second at each step; 2. 2ϴ range of 5 ‐ 42° with a step size of 0.016 and a counting time of 5 seconds at each step.29 The FTIR spectra of gaseous hydrolysis products before cooling and silane decomposition were measured on Ni‐ colet Impact spectrometer with spectral resolution of 1 cm‐ 1 with 100 repetitions The gas chromatography–mass spec‐ troscopy (Shimadzu QP 5050 mass spectrometer), using a 25 m long PoraBOND capillary column, programmed to 30–220 °C with temperature increase of 10 °C/min, with in‐ jector temperature of 130 °C and sampling by a gas‐tight syringe after He (atmospheric pressure) was used for de‐ tection of hydrolyzed products.

The semi‐opened tube reactor had a screw‐thread with a little hole of 1.2 mm. (Figure 1d). The reactor volume was 36 cm3. The hole permitted the gaseous and adsorbed spe‐ cies to be pumped out of the reactor and its small diameter prevented evaporation of magnesium during heating. The reactor was filled with the reaction mixture, inserted into a quartz tube connected to a vacuum pump and heated to 330 °C ‐ 630 °C for 15 ‐ 120 minutes under pressure below 30 Pa. The weights of starting materials were for magne‐ sium (2.7 g; 97 %) and silicon (1.5 g) in molar ratio 2.1:1, resp. 6.5 g and 3.5 g in molar ratio 2.1:1.

2.3. Magnesium silicide hydrolysis Magnesium silicide prepared as described in 2.2., was used as a starting material for hydrolysis by solution of phos‐ phoric acid. The purity of magnesium silicide was 97.8 % and concentration of phosphoric acid 25 %. The laboratory apparatus consisted of a two‐neck round‐bottom flask equipped with the dropping funnel for an acid and with the connector to a vacuum pump and a cuvette, which enabled vacuum inside the flask and also a silane’s collection. For the experiment 0.1 g of magnesium silicide was filled into the flask. The dropping funnel was filled with phosphoric acid and the apparatus was evacuated by the vacuum pump through the connector until constant pressure 6 Pa. In the next step the connector to vacuum pump was closed and phosphoric acid was dropped inside the flask. Generated silicon hydrides were collected into the connected cuvette. The gaseous mixture was then cooled down in acetone (‐ 20 °C) to condensate water and higher silanes and to purify silane for other use.

3. Results and Discussion

The reaction on solid‐solid interface of magnesium and sil‐ icon particles predominantly depends on reaction temper‐ ature and time as well as on the contact between magne‐ sium and silicon particles. Moreover, it can also be influ‐ enced by reaction atmosphere, mainly the presence of ox‐ ygen or nitrogen, evaporation of magnesium and another various factors. (To observe the influence of reaction at‐ mosphere’s composition (Ar, O2, N2, vacuum) and its time of contact with raw mixture on magnesium silicide synthe‐ sis, various constructions and arrangements of reactors were chosen.) For that reason, this study is focused on op‐ timization of these mentioned factors. The standard for‐ mation enthalpy at temperature 25 °C is approximately ‐ 77.8 to ‐79.1 kJ/mol in solid‐solid reaction according (1) and ‐373.5 kJ/mol according reaction 2:30, 31

2.4. Silane decomposition Pure silane (>99.9 %) separated from obtained gaseous product of hydrolysis was inserted into an evacuated cu‐ vette (50 cm3) equipped with a thin platinum wire (d = 0.1 mm, l = 10 cm). The starting pressure of silane was 1000 Pa at ambient temperature. The voltage of 5.2 V with the cur‐ rent of 1 A was applied for 130 minutes for heating of the platinum wire.

2.5. Analysis procedure Products were observed visually, by dispersive Raman Ni‐ colet Almega XR Spectrometer and SEM with EDS (Scan‐ ning Electron Microscope with Energy Dispersive X‐Ray Spectroscopy probe, Tescan Indusem). All Raman disper‐ sive spectra were collected in 256 exposures with excitation wavelength of 473 nm (power of 8‐10 mW) and resolution of 2 cm‐1. The purity of starting materials was determined by XPS (X‐Ray Photoelectron Spectrometer, Kratos ESCA 3400) and SEM with EDS. The base pressure inside the XPS chamber was lower than 5.0 10‐7 Pa and as a source of X‐ Ray, the polychromatic Mg Kα at 1253.4 eV was used. The

2 Mg (s) + Si (s) ‐> Mg2Si (s)

(1)

2 Mg (g) + Si(s) ‐> Mg2Si (s)

(2).

In all experiments the excess of magnesium was applied to compensate evaporation of magnesium and occurrence of silicon oxide, which together with the relatively high evap‐ oration of magnesium limits the reaction rate and energy

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Table 2. Semi‐opened reactor with a valve – influence of added nitrogen to the reaction atmosphere

required for a successful progress of reaction. The oxygen content in the reaction atmosphere also limits the for‐ mation of Mg2Si similarly as the presence of nitrogen, which can lead to magnesium nitrides production. Thus, argon atmosphere, which suppresses intrusion of water, ni‐ trogen and oxygen from outside to the reactor, can be ap‐ plied; nevertheless, the price of argon is not favorable for scale up and a real application. Vacuum usage seems to be the best choice as it allows desorption of adsorbed gases and water from the surface of starting materials. Vacuum also affects phase transition of magnesium, which rapidly increases the reaction rate caused by the gas‐solid reaction rather than the solid‐solid one. Thus, both vacuum and ar‐ gon atmospheres were tested.

T [°C] 570 630 570 630

Table 1: Reaction parameters for Boat and Batch type of reactors Si

Mg (97%)

Pressure [kPa] 100 100 5 5

Mg2Si Yes + nitride Yes + nitride Yes + nitride Yes + nitride

The reaction was retarded by impurities; adsorbed water evaporating during the heating period; oxygen, nitrogen and those, which cannot be eliminated by continuous evacuation (SiOx, MgOx, etc.). Obtained results confirmed that neither an open reactor (ceramic boat), nor a batch reactor was a suitable arrangement for the preparation of magnesium silicide. To overcome the oxygen influence on the reaction, which limits the formation of Mg2Si, higher reaction temperatures of 570°C and 630°C, at reaction time of 2 hours were applied. Two arrangements, with and with‐ out Ar, were used for testing (see Experimental part). Ob‐ tained results are summarized in Table 2. Magnesium sili‐ cide with small fraction of Mg3N2 was obtained and Raman spectra of prepared samples were identical for all cases. Figure 2 shows Raman spectra of prepared samples and pure magnesium silicide, which illustrates a reference spectrum. Characteristic phonon modes demonstrate the presence of both magnesium silicide and nitride in pre‐ pared samples. These modes are summarized together with reference phonon modes obtained from literature32, 33, 34 data in Table 3. These results confirmed the fact that even a small amount of nitrogen negatively influenced the pu‐ rity of reaction products, where not only desired magne‐ sium silicide was formed but also the magnesium nitride. Nevertheless, the abundance of magnesium nitride was be‐ low 5 mass %. It was also found that the argon atmosphere inhibited evaporation of adsorbed species on magnesium and silicon surface, which suppressed formation of magne‐ sium silicide. Therefore, the final optimization of reaction conditions was performed in the semi‐opened tube reactor (Figure 1d) under continuous vacuum. Determination of the lowest reaction temperature and time belongs to the

Mg2Si

Reac‐ tor

Pres‐ sure

type

[Pa]

Boat

30

5

0.3

0.6

Partially

30

15

0.3

0.6

Partially

30

30

0.3

0.6

Yes

[min.] [g]

Atmosphere Ar + air Ar + air air air

To obtain information on above mentioned reaction con‐ ditions, four reactors were suggested, designed and applied (see Figure 1). In fact, they could be divided into three groups based on various abilities to affect reaction condi‐ tions; continuously evacuated open reactor (ceramic boat), batch pre‐evacuated reactor and semi‐opened reactors. First, a ceramic boat (evacuated open reactor) (see Figure 1a) and a batch reactor (see Figure 1b) were applied. Reac‐ tion conditions of all experiments are summarized in Table 1 including information on magnesium silicide formation. As can be seen, the reaction in the combustion boat was relatively fast; nevertheless, conversion on magnesium sil‐ icide was low. It is supposed that a prompt start of reaction could be caused by the magnesium’s ability to evaporate quickly. Thus, even a small deviation in temperature or ho‐ mogeneity of magnesium and silicide mixture could cause a local massive evaporation of magnesium resulting in stopping the reaction. Therefore, continuously evacuated semi‐opened reactors in two arrangements were designed: a semi‐opened reactor with a valve, see Figure 1c, and a semi‐opened tube reactor, see Figure 1d. The construction of semi‐opened reactor with a valve enabled a controlled addition of air; thus, the influence of nitrogen and oxygen on the reaction rate and purity of products could be tested.

Time

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[g]

Batch

5

15

0.5

0.9

No

5

45

0.5

0.9

Partially

5

70

0.5

0.9

Partially

5

120

0.5

1.0

Yes

Figure 2. Raman spectra of Mg2Si and Mg3N2

D


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Table 3. Phonon modes comparison of prepared sample Mg3N2 + Mg2Si from Figure 2 and magnesium nitride from literature Subst.

Mg2Si Mg2Si Mg3N2

Mg3N2

Mg3N2

Exp. [cm‐1] 272

357

227

326, 357

395

Lit [cm‐1]

340

213

338

379

259

Figure 4. SEM/EDS of Mg2Si prepared in semi‐open tube reactor at 400 °C for 25 min

most important reaction parameters. Therefore, the wide temperature’s range between 330 °C ‐ 630 °C and time be‐ tween 15 – 50 minutes at p = 5 – 30 Pa were tested.

However, at 380°C magnesium silicide was partially de‐ tected in the mixture of pure magnesium and silicon; and finally, temperature above 400°C for 2 h guaranteed pure magnesium silicide formation calculated against silicon.

Moreover, magnesium of two purities (90 and 97 %) was used to gain information concerning the influence of mag‐ nesium oxide on reaction speed. Obtained results for both purities of magnesium are depicted in Figure 3, where axis x stands for reaction temperature and axis y for yield of magnesium silicide. The obtained data for yields were av‐ eraged from four experiments. It is evident that a critical temperature boundary lies in a narrow interval between 360°C – 400°C for both purities of Mg. There was almost no evidence of magnesium silicide at 360°C.

The obtained results were repeatedly corroborated by other experiments, and in all cases at least three experi‐ ments were performed. It is obvious from Figures 4, 5 and Tables 4, 5 that the reaction led to a complete conversion of silicon to Mg2Si, where only a small fraction of oxygen could be seen on the surface of silicon. The following experiments were focused on determination of the lowest reaction time and magnesium purity inevita‐ ble for total conversion to magnesium silicide at the lowest possible temperature of 400 °C (see Figure 3).

Table 4: SEM/EDX element measurement of Mg2Si pre‐ pared in semi‐open tube reactor at 400 °C for 25 min Element

Norm. C [wt.%]

Atom. C [at. %]

Error (1±σ) [wt. %]

O

3.16

4.94

0.67

Mg

64.68

66.47

2.23

Si

32.16

28.60

0.94

Total

100.00

100.00

Table 5: Composition of Mg2Si prepared in semi‐open tube reactor at 400 °C for 25 min Substance

System

Magnesium Cubic silicide

Space group* Comp. [%] Fm3m

97.8

Magnesium Hexagonal P63/mmc

2.1

Silicon

Mg3(PO4)2 (aq) + SinH(2n+2) (3) As can be seen from Figure 7 for 31 m/z fractions, the pre‐ dominant gaseous product of hydrolysis was silane (ca 90%), followed by disilane (ca 8%) and trisilane (ca 1.5%), which can be easily separated. Elimination of water and higher silanes was verified by lowering temperature below their boiling points. Decomposition of silane over hot platinum wire, which is the third step of the new process, was verified. Time de‐ pendence of decomposition is described by FTIR spectrum (Figure 8). Rapid reaction rate at the beginning of the re‐ action can be explain by free surface of hot platinum wire. Therefore, a pure crystalline silicon was quickly deposited on the wire surface. The obtained results were repeatedly corroborated by three other experiments.

Table 6: Time and Mg purity dependence on formation of Mg2Si at 400 °C, 5‐10 Pa Time

Mg2Si /Mg

Mg2Si /Mg

[min]

(97 %)

(90 %)

15

Partially

No

25

Yes

No

30

Yes

No

40

Yes

Partially

50

Yes

Yes

Table 7: Semi‐opened tube reactor, T= 400 °C, cont. vac‐ uum, p = 30 Pa, t = 30 min, reactor vol. 36 cm3 Si [g]

Mg (97 %) [g]

Mg2Si

1.48

2.7

No

3.54

6.5

Yes

F


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at 1000 Pa in 50 cm3 cuvette. The reaction rate of silane de‐ composition was retarded by deposition of ultrapure sili‐ con on the surface of platinum wire. The design of the whole process brings a new way of reuti‐ lization of waste photovoltaic panels to produce an ul‐ trapure silicon and energy or enables hydrogen storage in the form of silanes using a pure waste‐less approach. The formation of magnesium silicide proceeds at 200 °C lower temperature in comparison to existing processes15‐17.. More‐ over, the traditional processes utilize high toxic and envi‐ ronmental unfriendly substances as hydrogen chloride, or chlorine, which are highly corrosive and, thus, require the expensive technological devices. It can be concluded that the new designed tree‐steps technology significantly ex‐ ceeded traditional approaches in lower price and environ‐ mental impact as well in energy saving.

Figure 8. FTIR of thermal decomposition of silane in time over hot platinum wire.

4. Conclusion Circulation technology of waste photovoltaic panels for production of ultrapure silicon and energy in the form of hydrogen storage was designed and verified. The influence of reaction temperature, time, atmosphere, reactor’s type and magnesium purity on formation of magnesium silicide from waste silicon photovoltaic panels was thoroughly studied. Three groups of reactors were applied: continu‐ ously evacuated open reactor, batch pre‐evacuated reactor and semi‐opened reactors. It was found, that neither the opened nor the batch pre‐evacuated reactor was appropri‐ ate for successful treatment, because of strong magnesium evaporation in the first case and retarding reaction by ad‐ sorbed impurities in the second one. These problems were solved by newly designed semi‐opened tube reactors, which enabled controlled communication between the re‐ actor and furnace atmosphere. Thus, magnesium evapora‐ tion was suppressed and free leaving of adsorbed impuri‐ ties was enabled. Magnesium silicide was successfully pre‐ pared both in vacuum and argon atmospheres; neverthe‐ less, it was also found that even a small addition of air re‐ sulted in production of magnesium nitride.

AUTHOR INFORMATION Corresponding Author *[email protected], tel. 00420296780179

ORCID Olga Solcova: orcid.org/0000‐0001‐8327‐0257 ACKNOWLEDGMENT The support of Grant Agency of the Czech Republic (grant No. 15‐14228S) is gratefully acknowledged.

ABBREVIATIONS REFERENCES 1

Kang, S.; Yoo, S.; Lee, J.; Boo, B.; Ryu, H. Experimental in‐ vestigations for recycling of silicon and glass from waste photovoltaic modules, Renewable Energy 2012 47, 152. 2 Granata, G.; Pagnanelli, F.; Moscardini, E.; Havlik, T.; Toro L. Recycling of photovoltaic panels by physical operations, Solar Energy Materials and Solar Cells 2014, 123, 239. 3 Radziemska, E; Ostrowski, P.; Cenian, A.; Sawczak, M. Chemical, thermal and laser processes in recycling of pho‐ tovoltaic silicon solar cells and modules. Ecological Chemis‐ try and Engineering. S 2010, 17.3, 385. 4 Klugmann‐Radziemska, E.; P. Ostrowski, P. Chemical treat‐ ment of crystalline silicon solar cells as a method of recov‐ ering pure silicon from photovoltaic modules. Renewable Energy 2010, 35, 1751. 5 M. V. Fthenakis, M. V., End‐of‐life management and recy‐ cling of PV modules. Energy Policy 2000, 28, 1051. 6 Gire, G. Décomposition thermique des siliciures mag‐ nésium. Comptes Rendus Hebd. Seances Acad. Sci. 1933, 196, 1405. 7 Lebeau, P; Bossuet, P. Sur le siliciure de magnesium. Comptes Rendus Hebd. Seances Acad. Sci. 1908, 146, 284. 8 Wöhler, L.; Schuff, W. Über die Silizide der Erdalkalien. Z. Anorg. Allg. Chem. 1932, 201, 33. 9 E. Hohmann, Silizide und Germanide der Alkalimetalle. Z. anorg. Chem. 1948, 257, 113. 10 Kieffer, R; Benesovsky, F. Hartstoffe, Springer, Vienna, 1963, pp. 455‐548.

The minimal reaction temperature which guarantee the to‐ tal conversion of silicon from photovoltaic panels to mag‐ nesium silicide was established at 400°C. It was also deter‐ mined that the minimal reaction time significantly de‐ pends on magnesium purity. For 97 % magnesium, the to‐ tal conversion was achieved after 25 minutes; however, for 90 % magnesium, the required reaction time was twice as long, particularly 50 minutes. Finally, the reactor filling, which substantially influenced the formation of magne‐ sium silicide, was evaluated and established minimally at 30% of reactor´s volume. In all experiments, the conver‐ sion to magnesium silicide related to silicon was at least 99%, for both 90% and 97% purity of Mg. Hydrolysis of prepared magnesium silicide was success‐ fully verified by 25 %w/w H3PO4. It was confirmed by GC‐ MS, that major gaseous product is silane (>90 %), which was purified by elimination of water and higher silanes by lowering temperature below their boiling points. Subse‐ quently, pure silane was totally decomposed into ultrapure silicon and hydrogen on hot platinum wire during 130 min

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Waste photovoltaic panels for ultrapure silicon and hydrogen through

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