Restoring the Reactivity of Organic Acid Solution Used for Silver Recovery from Solar Cells by Fractional Distillation

Abstract

Methanesulfonic acid (MSA) is used to recover silver (Ag) from solar cells by adding an oxidizing agent. It is possible to regenerate by substituting of H⁺ for Ag⁺, and thus it can be reused for additional reactions. However, MSA is highly hygroscopic, and as an oxidizing agent can easily decompose in the acidic environment during Ag extraction, leading to dilution due to the formation of H₂O. This H₂O in the MSA solution hinders the Ag extraction. In this study, we present a fractional distillation process for restoring the reactivity of reused MSA solutions by reducing the H₂O content. Our results showed that the reactivity of the separated MSA was restored and Ag could be recovered from the solar cell.

1. Introduction

The disposal of wastes associated with end-of-life (EoL) photovoltaic (PV) modules is problematic [1]. In general, the lifespan of a crystalline silicon (c-Si) PV module is 25–30 years, and it has been installed since the 1990s [2]. This means that the disposal of EoL PV modules will increase rapidly. In this regard, the IEA-PVPS (International Energy Agency-Photovoltaic Power Systems Programme) and IRENA (International Renewable Energy Agency) reported that the global waste generated from EoL PV modules is expected to reach 1.7–8 million tonnes by 2030 and 60–78 million tonnes by 2050 [3]. Therefore, a method to properly process PV modules is required. The European Union published the 2012/19/EU Directive, which includes guidelines on the handling of EoL PV modules. EoL PV modules are designated as waste electrical and electronic equipment (WEEE), so programs must be in place for their collection, recovery, and recycling [4]. For this reason, many recycling processes have been investigated for recovering valuable materials from PV modules with different levels of technologies [5,6,7,8,9]. In general, c-Si PV modules mostly comprise a piece of glass with aluminum frames, c-Si solar cells, an encapsulant, a back sheet, and a junction box. As Ag has a very low total weight in a PV module, it has not been considered worth recovering [10]. However, Ag is a very valuable material and has a relative value of almost half the cost of making PV modules [3]. For this reason, some studies have reported on the recovery of Ag from PV module components [11,12,13,14,15,16,17,18], including extraction with inorganic acid [17,18]. However, this method is not environmentally friendly, as it results in the generation of waste solutions. Meanwhile, Yang et al. [19] reported the recovery of Ag from c-Si solar cells by using eco-friendly methanesulfonic acid (MSA) solutions. MSA has a lot of advantages; for example, it is easy to handle, and has high conductivity, high metal salt solubility, and low toxicity [20]. However, since MSA is more expensive than conventional inorganic acids, a process that can reuse MSA must be developed for MSA-based Ag recovery to be competitive. However, it is expected that the process of recovering Ag using MSA has a side reaction to generate H₂O and the reactivity of MSA will decrease. In addition, H₂O is added during the process because MSA is highly hygroscopic. In this study, we explored possibilities of MSA solution reuse after recovering Ag using an MSA solution. Reusing the MSA can have significant economic improvements. Previous studies have already reported that reusing an acid solution is of great economic value [21,22]. Moreover, the economic value of Ag is large because, as previously mentioned, it is one of the most valuable elements on Earth. For this reason, the recovery of Ag through MSA reuse is very meaningful from an economic point of view. Meanwhile, previous studies have confirmed the side reactions in which H₂O is generated during the Ag recovery process using MSA. Because the generated H₂O can lower the acid concentration and decrease the reactivity, even if the metal solubility of the MSA is high, it can have a negative effect on the reuse of the MSA solution. Therefore, the separation behavior of MSA and H₂O was investigated by fractional distillation. As a result, it was confirmed that MSA can be reused using the optimum fractional distillation conditions. Finally, 2N grade Ag was recovered from 6″ commercial solar cells by MSA solution with restored reaction by fractional distillation.

2. Materials and Methods

2.1. Recovery of Ag Using Reused Organic Solvent

A 6″ mono c-Si commercial solar cell was applied to confirm the reuse of the MSA solution. The solar cell was broken into many pieces, which were immersed in a reused MSA solution with H₂O₂ (30 wt.%) for 1–12 h. The ratio of reused MSA and H₂O₂ was used 90:10. The stirring speed was 80 RPM and the experiment proceeded at 25 °C.

2.2. Fractional Distillation of Organic Compounds

A 1:1 mixture of MSA and H₂O was used to investigate the optimum conditions for the separation behavior of H₂O. A rotary evaporator (Hahnshin S&T HS-2005S-N, Gimpo-si, Korea) was used for the separation experiment. First, the mixture of MSA and H₂O was placed in a round flask and distilled at 80 RPM for 3 h. The heating medium was silicone oil and the temperature of the coolant was 4 °C. As shown in

Table 1. Experimental conditions of the fractional distillation process. MSA: methanesulfonic acid.

Contents	(a)	(b)	(c)
Distillation temperature (°C)	100	150	200
Mixing ratio (MSA:H2O)	1:1
Heating medium	Silicone oil
Time (hr)	3
Flask rotation speed (RPM)	80
Coolant temperature (°C)	4

, the heating temperatures of silicone oil for fractional distillation were 100, 150, and 200 °C and are indicated in (a), (b), and (c), respectively. The detailed experimental conditions are shown in Table 1.

2.3. Recovery of Ag Using a Separated MSA Solution

After confirming the optimal conditions for fractional distillation, the Ag recovery process from the solar cells was carried out until four times as described in Section 2.1. To recover Ag leached from the solar cell, HCl (35 wt.%) was put into the MSA-based leach solution with stirring at 200 RPM for 1 h. The precipitated AgCl was separated by centrifugation device. Recovered AgCl was converted to Ag metal powder by chemical wet process with stirring at 200 RPM for 1 h.

The concentration of Ag in the solution was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Shimadzu ICPS-1000IV, Kyoto, Japan). Fourier-transform infrared (FT-IR, Bruker ALPHA-P, Billerica, MA, USA) spectroscopy was used to investigate the qualitative analysis of fractionally separated MSA and distilled H₂O. The phases of the recovered AgCl and converted Ag powder were also analyzed by X-ray diffractometer (XRD, Rigaku DMAX-2500, Tokyo, Japan). Glow discharge mass spectrometer (GDMS, Thermo VG 9000, Waltham, MA, USA) was used to analyze the concentrations of the various impurities in the Ag metal powder.

3. Results and Discussion

3.1. Extraction Behavior of Ag Using Reused Organic Solution

In the previous experiment, MSA and H₂O₂ were mixed and the optimum process was determined [19]. As shown in Figure 1, however, the reactivity of MSA for Ag extraction from solar cells was significantly reduced. The solubility of the Ag in the reused solution was reduced by about 15% even though the experiment was conducted for up to 12 h. This is because H₂O was formed as a side reaction in the Ag extraction process using MSA and H₂O₂. The mechanism of the chemical reaction used to extract Ag with MSA and H₂O₂ is shown as follows [23].

2Ag + H₂O₂ → Ag₂O + H₂O

(1)

Ag₂O + 2CH₃SO₃H → 2CH₃SO₃Ag + H₂O

(2)

2CH₃SO₃H + 2Ag + H₂O₂ → 2CH₃SO₃Ag + 2H₂O

(3)

The H₂O generated by the side reaction lowers the concentration of MSA, which reduces the extraction reactivity of Ag from the solar cell. In an acidic environment, in addition, H₂O₂ further decomposes into H₂O and O₂ due in an acid-catalyzed reaction [24]. In an acidic solution, it is possible for H₂O₂ to be decomposed and become diluted solution due to the formation of H₂O [25].

H₂O₂ + H → H₂O + 1/2O₂ + H

(4)

It is important to develop methods to control the H₂O in solution because it has very strong effects on the physicochemical properties of acid [26]. Reduced reactivity with Ag in MSA solution is attributed to an increased amount of H₂O, which results in a lower Ag dissolution rate from the solar cell. Also, MSA is hygroscopic and the process of extracting Ag requires a relatively long process time. Therefore, more H₂O can be added to the MSA solution during the process. As a result, the reactivity of reused MSA is significantly lowered in the Ag extraction process.

3.2. Fractional Distillation Behavior of an Organic Solvent and H₂O Mixture

Table 2. The amount of residual MSA and H₂O distilled by the following process conditions: (a) 100 °C, (b) 150 °C and (c) 200 °C.

Contents	(a)	(b)	(c)
Residual MSA (mL)	490	400	310
Distilled H2O (mL)	110	200	290

shows the fractional distillation behavior according to process conditions. We can use this process to separate a mixture of two miscible liquids that have boiling points that are different to each other. This process is carried out using a rotary evaporator apparatus. This method can separate a mixture of MSA (boiling point is about 290 °C) and H₂O (boiling point is 100 °C) at 1 atm, respectively. In the case of (a) and (b), the fractional distillation was not affected much. However, in the case of (c), MSA and H₂O were almost separated. In the case of (a) in which the fractional distillation temperature was 100 °C, the distilled H₂O was only 110 mL and the remaining MSA was 490 mL. In the case of (b) in which the distillation temperature was 150 °C, it was confirmed that MSA still contained a large amount of H₂O, although the distilled H₂O increased to 200 mL. In the case of (c) in which the fractional distillation temperature was 200 °C, it was found that the distilled H₂O was 290 mL and the remaining MSA was 310 mL. It was confirmed that the amount of separated MSA did not change after 3 h in all conditions. It can be seen that a temperature of about 200 °C is required for the H₂O mixed with the MSA to be sufficiently separated.

Table 3. Location of relevant indicator bands in the MSA-H₂O system.

Location Wavenumber (cm−1)	Description
3000–3400	O–H stretch
3030	CH3 asymmetric stretch
2940	CH3 asymmetric stretch
1630	O–H bend
1415	CH3 asymmetric bend
1320	SO3 asymmetric stretch
1180	SO2 asymmetric stretch
1120	SO3 symmetric stretch
1050	SO3 symmetric stretch
980	CH3 rock
880	S–OH stretch
780	C–S symmetric stretch
760	C–S symmetric stretch

shows the location wavenumber and description of the MSA-H₂O system to allow for a better understanding of the FTIR analysis results. It is based on various literature cited [27,28,29,30,31,32,33]. The FTIR analysis results, which were obtained through before and after fractional distillation, are shown in Figure 2 and Figure 3. Pure MSA, MSA + H₂O, and separated MSA were analyzed by FTIR to investigate the reactivity behavior of MSA. Figure 2a shows typical FTIR peaks of pure MSA [25,26]. There were CH₃ bands at 3030, 2940, 1415, and 980 cm⁻¹, SO₃ bands at 1320 and 1120 cm⁻¹, a S–OH stretch vibration at 880 cm⁻¹, and a C–S stretch vibration at 760 cm⁻¹, as shown in Table 3. In the case of Figure 2b, the height of the FTIR peak was lowered overall, except for the C–S stretch vibration at 780 cm⁻¹, because the MSA was mixed with H₂O. In Figure 2c, on the other hand, we can see that the result of the FTIR analysis line is located in the middle of Figure 2a,b at parts of the CH₃ and SO₃ band and the C–OH stretch vibration at 3030, 2940, 1320, 1120, and 880 cm⁻¹. It is considered that the result of the restoration of the reactivity of MSA was due to fractional distillation. Generally, the intrinsic physicochemical properties of the corresponding molecule can be confirmed by the wavenumber of each IR absorption peak [34]. The f actor that determines the peak intensity in the infrared spectra is the concentration of molecules in the sample [35]. In other words, the concentration of the sample is related to the height of the FTIR peak. The height of the peak increases as the concentration of the analytical sample increases. For this reason, the IR peak of Figure 2c, which corresponds to separated MSA, is in the middle of Figure 2a (pure MSA) and Figure 2b (MSA mixed with H₂O). This result indicates that fractional distillation increases the MSA concentration, which means that the reactivity with Ag can be restored. Meanwhile, pure H₂O and distilled H₂O by fractional distillation are shown in Figure 3a,b. Pure H₂O and distilled H₂O showed differences in FTIR analysis results. First, the height of the O–H stretch at 3320 cm⁻¹ of the distilled H₂O was found to be lower than that of pure H₂O, because the other components were mixed in the distilled H₂O. In addition, the SO₂ band was found at 1180 cm⁻¹, the SO₃ band was found at 1050 cm⁻¹ and the C-S vibration was found at 780 cm⁻¹ in the FTIR analysis of the distilled H₂O. This means that MSA is also decomposed during fractional distillation process.

3.3. Recovery of Ag from Organic Solvent with Restored Reactivity

We confirmed that concentration of MSA can be restored by fractional distillation as a result of FTIR. Figure 4 shows the Ag extraction behavior by repeated use of MSA by ICP-AES analysis as a function of the reaction time. The yellow dotted line in Figure 4 indicates that Ag is entirely dissolved in the solution. From this, we can see that in all the cases where MSA was reused, the yellow line is reached within 4 h. It is indicated that the reactivity of MSA and Ag was improved by all fractional distillation experiments. In other words, the MSA concentration was restored through fractional distillation, and it was confirmed that the Ag could be dissolved in the reused MSA.

Figure 5 shows the XRD spectrums of AgCl recovered and Ag converted from AgCl by a wet chemical process. The converted Ag powder only shows the phase of Ag. As a result, pure Ag powder can be obtained using this simple chemical process.

Table 4. The purity levels of the as-recovered Ag powders.

Elements	Concentration (ppmw)	Elements	Concentration (ppmw)
Be	<0.001	Cr	0.76
B	0.89	Mn	0.42
Na	3.4	Fe	16
Mg	0.71	Co	0.01
Al	4.8	Ni	0.89
Si	65	Cu	2
P	38	Zn	0.36
S	0.5	Ga	<0.1
Cl	4000	Ge	<0.1
K	0.71	Ag	Matrix
Ca	1.1	Sn	2.8
Ti	0.47	Pb	480

provides a summary of the GDMS results for the purity levels of the as-recovered Ag powders. It can be confirmed that the Ag metal converted from AgCl had a purity of ~99.5% (2N), with an especially large amount of Cl and Pb present in the recovered Ag as impurities. It is expected that Pb dissolved due to the reaction of MSA solution and Pb. This is because Pb exists as a glass frit together with the Ag present in the solar cell [36]. Not only Ag but Pb also dissolves well in the MSA and H₂O₂ solution [37]. The dissolved Pb is considered to be precipitated together with PbCl₂ when AgCl is leached by HCl [38]. For this reason, Pb and Cl were detected as 480 ppm and 4000 ppm in the GDMS analysis, respectively. In addition, it is expected that a large amount of Cl was detected in the Ag powder because not all of the AgCl was converted to Ag. As a result, 2N-grade Ag could be recovered from solar cells and it would be possible to recover higher purity Ag through the optimization condition of Ag conversion from AgCl and additional processes such as the electrochemical method.

4. Conclusions

MSA, an environmentally friendly organic acid, was used to recover Ag from solar cells. MSA is highly soluble, environmentally friendly, and easy to handle. However, when Ag is recovered using MSA and H₂O₂, a side reaction that generates H₂O occurs and H₂O₂ decomposes in H₂O under acidic conditions. Furthermore, because MSA is hygroscopic, the concentration of MSA decreased after the Ag extraction reaction, and the reactivity with Ag decreased. In order to improve the reactivity with Ag and to be able to reuse the MSA solution, fractional distillation was applied as a method to separate MSA and H₂O. MSA and H₂O were separated at a fractional distillation temperature of 200 °C, and MSA could be reused for recovering Ag. The Ag extracted from the solar cells was recovered in the form of AgCl, and the Ag was converted through the chemical wet process. The purity of converted Ag was 99.5%, and it was confirmed that Pb and Cl existed as impurities in the converted Ag. However, it may be possible to recover higher purity Ag from solar cells through the optimization conversion condition of Ag and additional process such as the electrorefining process.