Organic Acid-Assisted Hydrothermal Leaching of Silver from End-of-Life Photovoltaic Panels

Abstract

The aim of this study was the hydrothermal leaching of silver from waste monocrystalline silicon (m-Si) and polycrystalline silicon (p-Si) photovoltaic panel (PV) cells using organic acids, namely oxalic acid (OA) and citric acid (CA). Before leaching, two different pretreatment procedures were applied. First, the fluoropolymer backsheet was manually removed from the panel pieces and, then, the samples were subjected to high-temperature heating for the thermal degradation of the ethylene vinyl acetate (EVA) polymer. When removal by hand was not feasible, the second pretreatment procedure was followed by toluene immersion to remove the EVA and backsheet and separate the cells, glass, and films. After pretreatment, 4 M HCl leaching was applied to remove the aluminum layer from the cells. The remaining cells were subjected to hydrothermal leaching with organic acids to extract the silver. Several hydrothermal parameters were investigated, such as acid concentration (1-1.5-2 M), processing time (60-105-150 min), and temperature (150-180-210 °C), while the liquid-to-solid (L/S) ratio was fixed at 30 mL: 1 g, based on preliminary tests. Response surface methodology (RSM) was applied to optimize the hydrothermal leaching parameters. The optimized parameters were 210 °C, 95 min, 2 M CA or 210 °C, 60 min, 1 M OA. OA was more effective in Ag leaching than CA. The results were compared to HNO₃ leaching. The green leaching of silver from end-of-life PV panels with organic acids is an environmentally beneficial route.

1. Introduction

The rapid global expansion of PV installations is raising concerns about the upcoming generation of PV waste. According to recent estimates, the amount of PV waste that will be generated by 2050 in the EU-27 alone is expected to reach 21–35 million tons (MT), 47–90% more than previously predicted, due to the recently increased EU targets for PV deployment [1,2]. In recent decades, crystalline silicon (c-Si) modules have maintained 80–90% of the market share and, as such, dominate the PV waste stream. Polycrystalline silicon (p-Si) and monocrystalline silicon (m-Si) panels account for 51% and 41% of the market share of c-Si photovoltaic panels, respectively [3,4]. By 2030, 86% of waste panels discarded in the EU-27 will be of this technology [1,2]. Therefore, c-Si panels are the most valuable PV modules due to their large volume compared to other technologies.

The main components of a c-Si module are glass, polymers, silicon cells, and the aluminum frame [5,6]. Some elements such as silver, lead, tin, and other metals are also present in smaller quantities and must not be overlooked. Although Ag is only found in trace amounts in PV panels, its high economic value means that it can contribute significantly to the revenue from the recovery of panel materials [3].

Chemical leaching is the most effective and profitable method of metal recovery. Nitric acid (HNO₃) is often used in silver leaching because of its strong reactivity and its affordable price [7]. Organic acids are considered mild acids that cause little or no pollution to the environment, unlike inorganic acids. Due to environmental regulations for waste disposal, it is appealing to create effective green leaching methods for metal extraction using organic acids [8,9].

The leaching of Ag with organic acids is scarcely reported in the literature compared to inorganic acid leaching. Yang et al. employed methanesulfonic acid (MSA) and hydrogen peroxide (H₂O₂) for 1–12 h at room temperature to leach silver from discarded Si panels [10]. An iodide solution at room temperature was used to treat waste Si panels [11]. Zheng et al. used citric acid with H₂O₂ at 60 °C to leach 97% of silver [12]. Apart from these studies on silicon panels, Fransson used citric, maleic, and oxalic acid to leach silver at room temperature from CIGS solar cells; however, the results were not encouraging [13].

Conventional leaching methods use low temperatures (less than 90 °C) to extract metals from a variety of matrices. To achieve complete leaching, the low leaching temperature requires a long reaction time and a high acid concentration; this, however, raises costs and environmental impact [5,14]. The hydrothermal leaching approach is a green method that employs lower acid concentrations, shorter reaction times and does not require extra oxidants or reductants, unlike conventional hydrometallurgy [15]. Hot pressurized water is used as the green reaction medium. In this technology, the use of high temperatures (≥90 °C) accelerates the process, but increases energy consumption [16]. The method has been used to leach metals from lithium-ion batteries with organic acids [15], but to date, no hydrothermal leaching utilizing organic acids for PV cells has been reported. Therefore, this is the first attempt to leach silver from PV cells under these conditions. Our previous work reported hydrothermal silver leaching using mild inorganic acid; however, this is the first attempt to apply organic acid-assisted hydrothermal leaching for PV cells [2].

The temperature, time, and concentration of organic acids are optimized to improve the efficiency of the hydrothermal leaching of Ag. The response surface methodology (RSM) with a Box–Behnken design (BBD) is a statistical technique used to design the experiments [14]. Instead of varying one parameter at a time, this strategy modifies all the tested parameters simultaneously to examine how they affect silver leaching. This facilitates the identification of possible interactions between the parameters. The entire experimental design, which requires more time and resource requirements, is avoided without compromising the insight into how the parameters affect the process. Each factor is defined in this design at three different levels: low, medium, and high [17]. The tested parameter levels were selected based on literature studies using organic acids for leaching [12,15,18].

The aim of this work was to utilize organic acids (CA, OA) to evaluate the hydrothermal leaching of silver from monocrystalline silicon (m-Si) and polycrystalline silicon (p-Si) photovoltaic (PV) panel waste and examine through RSM experimental design how the parameters (acid concentration, time, and temperature) affect the process. The use of mild organic acids eliminates the generation of harmful wastewater.

2. Materials and Methods

2.1. Materials

Discarded monocrystalline (m-Si) and polycrystalline silicon (p-Si) photovoltaic panels were collected from domestic companies in Chania and Heraklion, Greece, and are depicted in Figure 1. The panel specifications are reported by Savvilotidou and Gidarakos [19]. Due to external damage caused by severe weather conditions, the panels were discarded, but their layered structure was intact.

Oxalic acid (C₂H₂O₄, Merck, Darmstadt, Germany) and citric acid (C₆H₈O₇, Merck) were used as the leaching media. Oxalic acid is a dicarboxylic with two carboxylic groups and citric acid is tricarboxylic acid (Supplementary Materials Section S1).

2.2. Pretreatment Procedures

Initially, the PV panels were dismantled, removing the cables, junction boxes, and aluminum frames [19]. Then, the panels were cut to 4 × 4 or 5 × 5 cm². To reach the desired materials in the waste panels, the sandwiched structure of panels must be destroyed and the EVA layer encapsulating the cells must be removed [20]. Once the EVA layer was eliminated by heat, in order to avoid harmful emissions of fluorinated compounds, the backsheet must be removed first. The backsheet consists of layers of fluorinated and phthalate polymers (polyvinyl fluoride or Tedlar, polyethylene terephthalate or PET) [2]. In this work, the backsheet was detached by hand whenever possible or by immersion in toluene for a full day. Therefore, two pretreatment procedures were applied: (i) the backsheet was manually peeled off and then the panel was heated to eliminate the EVA, or (ii) the panels were immersed in toluene to destroy the polymer structures, thus separating the glass, cells, and ribbons, as shown in Figure 2. The experimental procedure is illustrated in Figure 3, and the steps are described in detail in the following subsections.

2.2.1. Thermal Treatment of EoL Si PV Panels

For the panel pieces whose backsheet was manually removed, the EVA polymers were burned in an oxidizing atmosphere in a Nabertherm lab furnace. The samples were heated from 20 °C to 600 °C at a rate of 19 °C per minute and maintained at 600 °C for 30 min. After the heat treatment, the samples were sieved to separate the glass, cells, and metallic ribbons. In the subsequent phases, only the cells were used for hydrothermal treatment.

2.2.2. Toluene Treatment of EoL Si PV Panels

For the panel pieces whose backsheet could not be detached by hand, the pieces where fully immersed in toluene (99%) in a large vial with a lid and left there for at least 24 h, at room temperature, without stirring. Toluene as an organic solvent causes the swelling and separation of EVA [20]. Toluene is a nonpolar, non-hazardous solvent with a capacity to dissolve within the EVA matrix and cause a noticeable swelling of the polymer and depolymerize the PV backsheet [21,22,23]. After the PV immersion in toluene, the glass, cells, and ribbons were mostly separated from the swollen resins. Some glass pieces where still attached to the swollen EVA, but only the cells were used in the subsequent experiments. The cells were heated at 120 °C to remove the excess toluene.

2.3. Characterization of EoL Si PV Panels

To determine the chemical composition, the cells were first ground in a mill (19 FRITCH pulverisette, Pittsboro, NC, USA) and then examined by X-ray fluorescence analysis (XRF, Rigaku Primus IV, Tokyo, Japan).

Table 1. Chemical composition of monocrystalline (m-Si) and polycrystalline silicon (p-Si) cells (RSD 0.5–1.5%).

Element (% w/w)	p-Si Cell	m-Si Cell
Si	64.46	65.18
Al	6.16	9.87
Cu	0.25	0.93
Ag	1.13	1.37
Sn	0.10	0.28
Pb	0.13	0.35

displays the results. As shown, the composition of the m-Si and p-Si panels does not differ significantly, especially in terms of the Ag content. Indeed, the main difference between m-Si and p-Si panels is the purity of the Si crystals [24]. In our previous study [2], the mild hydrothermal leaching of Ag with HNO₃ was used on both m-Si and p-Si panels, and the results showed a comparable behavior and performance for both panel types. The present study involves the leaching of m-Si cells with CA and p-Si cells with OA. However, it is anticipated that the acids will have similar efficiency for both types of cells. In most literature studies, only one panel type is considered (i.e., m-Si) and is denoted as c-Si, implying that the results will be similar for both types of c-Si panels [25,26].

2.4. Hydrothermal Leaching Experiments

In the experiments, 1 g cell was leached with 30 mL organic acid in a hydrothermal reactor. First, each sample was ground into fine powder in a mortar. The S/L ratio was kept constant for all tests. Grinding facilitates breaking the chemical and mechanical bonds between Ag and Al electrodes and Si layer and makes the surface area for reaction larger. Prior to Ag leaching, each sample was subjected to Al removal by immersion in 4 N HCl for 1 h, followed by centrifugation to obtain the cell for the hydrothermal test. This step was applied as studies have reported that citric acid could leach Al [27]. The hydrothermal leaching process was conducted in 100 mL steel reactors (Techinstro, Nagpur, India) with an internal PTFE layer at 150–210 °C for 1–2.5 h without agitation with 1–2 M organic acid. After cooling, the resulting sample was filtered through a 0.45 μm syringe filter for analysis in Atomic Absorption Spectroscopy (AAS, Shimadzu, Kyoto, Japan) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS, Agilent, Santa Clara, CA, USA).

2.5. Experimental Design: Response Surface Methodology

Three different parameters in the hydrothermal leaching process were investigated: (i) temperature, (ii) time, and (iii) concentration of organic acid (CA or OA).

The experiments were designed using the Box–Behnken design (BBD) response surface methodology. Instead of altering one parameter at a time, this design simultaneously modifies all the investigated factors to examine their effects on leaching, detecting potential interactions among the factors. The BBD saves time and resources by avoiding the full experimental design, without loss of information. Each factor has three distinct levels: low, middle, and high [14,17]. The experimental design was performed by the Minitab software (version 18.1), as displayed in

Table 2. Response surface methodology with a Box–Behnken experimental design.

Run Order	Concentration (M)	Time (min)	Temperature (°C)
1	1.5	105	180
2	1	60	180
3	1.5	150	210
4	1	150	180
5	2	105	150
6	1.5	60	210
7	1	105	210
8	1.5	105	180
9	2	105	210
10	1.5	150	150
11	1	105	150
12	1.5	60	150
13	2	60	180
14	2	150	180
15	1.5	105	180

. In the experiments, m-Si panels were leached with CA and p-Si panels with OA.

The concentration of organic acid (CA or OA) ranged between 1 M (low level) and 2 M (high level), based on literature studies addressing organic acid leaching [12,18]. Zheng et al. [12] studied Ag citric acid leaching by varying the concentration in range of 0.5–2.5 M; however, the best results were achieved when the organic acid concentration was in range of 1–2 M. Thus, this was the focus of the present study. To achieve hydrothermal conditions, temperatures higher than 90 °C are employed. The temperature was varied between 150 °C (low level) and 210 °C (high level), based on the hydrothermal leaching conditions with organic acids (citric and oxalic acid) of Li et al. [15]. Also, the time ranged from 60 min (low level) to 150 min (high level), because, as the (cold) hydrothermal reactor was placed in the hot furnace, there was a delay phase until the desired temperature was reached; so, shorter times were not chosen. However, it is obvious that the reaction time of the hydrothermal reactor at the desired temperature was shorter than the times reported.

3. Results and Discussion

The efficiency of organic acids for the hydrothermal leaching of Ag from c-Si panels was studied without the use of any additional oxidizing agents. When hot water is applied under pressure, the hydrothermal method can be used to leach metal ions using organic acids without an oxidizing agent, unlike conventional leaching with low-temperature organic acids that requires the use of agents such as H₂O₂. Citric acid has been previously studied as a promising substitution of inorganic acids for the leaching of valuable metals from lithium-ion batteries [16]. Overall, the mechanisms that govern the mobilization and leaching of metals from solid samples with organic acids are acidolysis and complexolysis [9]. During complexation, the formerly leached metal participates in ligand formations to yield soluble complexes [28]. The acid dissociation constant (pKa) is one of the factors that often determines the potency of organic acids. Oxalic acid has two carboxyl groups, while citric acid has three. The pKa values during their dissociation reactions are shown in Section S1 [29] (Supplementary Materials). In addition to their important complexing properties, organic acids pose fewer environmental risks and do not generate secondary contaminants [30].

3.1. Hydrothermal Leaching with Citric Acid

For citric acid, the experiments with the highest yield were obtained for 105 min, at temperatures of 180–210 °C and concentrations of 1.5–2 M. When the experimental time increased to 150 min, the yield decreased (other conditions 1–2 M, 150–180 °C), indicating that very long treatment times are not desirable. This could be attributed to the effect of pressure increasing with time, as the decomposition constants of weak acids rise with higher applied pressure, making the acids weaker [31]. The shorter time of 60 min did not favor Ag leaching, probably because the reactor requires some time to reach the desired temperature. Once the hydrothermal reactor was placed in the hot furnace, the time was recorded, and this time was longer than the actual time at the desired temperature, as it included the preheating period. This is clearly illustrated in Figure 4, showing the contour plot of Ag versus temperature and time. In Figure 4, the concentration of Ag increased when the time was around 105 min and the temperature was 210 °C. These conditions yielded the highest results when the concentration of citric acid was 2 M (413 ppb). The interaction diagram of the concentration–time and concentration–temperature parameters depicted in Figure S1 (Supplementary Materials) clearly confirms these conditions. Indeed, metal–organic acid complexes become more soluble at higher temperatures, which improves the solubility of metals [16]. The effects of temperature–time and concentration–time on Ag leaching are also depicted in the surface plots illustrated in Figures S2 and S3 (Supplementary Materials), confirming these observations. The effect of the main parameters on the Ag leaching yield is displayed in Figure 5. As can be seen, the optimal time was about 100 min and temperature was 210 °C, while the effect of concentration was less pronounced. The interaction plot for the parameters is displayed in Figure S3 (Supplementary Materials), revealing that no significant interactions occurred between concentration–temperature and time–temperature, while slight interactions existed only for concentration–time. When the treatment time was 60 min, the yield slightly increased with the increase in the acid concentration; however, when the time was 150 min, the yield decreased with the increase in the concentration, as prolonged times may lead to increased pressure affecting the dissociation constants. Considering the Pareto chart in Figure 6, it is shown that all parameters were not statistically significant at the α = 0.1 level. However, at the α = 0.2 level, the temperature became statistically significant, while the other factors did not. According to the RSM design optimization, the optimum conditions for citric acid leaching were 210 °C, 2 M, and a time of 95 min, which were verified experimentally. The analysis of variance (Table S1, Supplementary Materials) suggests that only the temperature parameter is significant for CA leaching (significance level α = 0.2). The test does not detect lack-of-fit because the P-value for lack-of-fit is apparently greater than α, suggesting that lack-of-fit is not statistically significant. Also, the model summary yields a R-sq of 60%, which is not sufficiently high, but this can be explained by the relatively low performance of CA in leaching Ag. Plotting the residuals versus order confirms the hypothesis that they are independent of each other because, when presented in time order, they do not show trends or patterns. As shown in Figure S5 (Supplementary Materials), the residuals are not correlated because they fall randomly around the centerline.

3.2. Hydrothermal Leaching with Oxalic Acid

In the Ag leaching experiments, a concentration of 1 M oxalic acid was optimal, while increasing the concentration above 1 M was inhibitory. The decrease in leaching yield with the increase in the concentration was attributed to the relatively low solubility of oxalic acid in water. Indeed, oxalic acid concentrations higher than 1 M produced saturated solutions, which were not conducive to acidolysis. The acid concentration is already at its maximum in a saturated solution and additional acid additions do not actively dissolve in the solution. This can lead to a reduction in the effective concentration of acid available for the acidolysis reaction [32]. The Pareto chart shows that the oxalic acid concentration was a significant parameter at the 95% confidence level, as shown in Figure 7. Oxalic acid has two carboxyl groups and dissociates in two steps, with the first step having pKa₁ = 1.2, indicating a relatively stronger acid than citric acid (pKa₁ = 2.79) in this step. However, at a higher temperature and pressure, the pKa values increase for both acids, but the relative difference may remain. The highest Ag concentrations were achieved at experimental conditions of 180 °C and 1 M for 60–150 min (1201–1376 ppb). The main effects plot also supports that the low concentration favored Ag leaching (Figure 8), while the other parameters were not so important. The surface plot of Ag versus the temperature and concentration indicated that the highest yields were obtained when the concentration was low and the temperature was high (Figure 9). The contour plot in Figure 10 of Ag versus the concentration and time shows that short times were enough for Ag leaching. The optimized conditions by the Minitab software for oxalic acid leaching were concentration 1 M, time 60 min, and temperature 210 °C, and were experimentally validated. The analysis of variance (Table S2, Supplementary Materials) indicates that only the concentration is a significant parameter (significance level α = 0.05), while the other parameters and their interactions are not. Also, the lack-of-fit is not statistically significant as the p-value > α, meaning that the test detects no lack-of-fit. The plot of the residuals versus order (Figure S6, Supplementary Materials) verifies the assumption that the residuals are independent from one another, as they show no trends or patterns. As shown in Figure S6 (Supplementary Materials), the residuals on the plot fall randomly around the center line and, therefore, are not correlated. Additionally, the model summary estimates R-sq 85%, which is sufficient when considering the reduced performance of organic acids to leach Ag. To further examine the effect of concentration, experiments were performed at lower oxalic acid concentrations (0.6, 0.8, 1 M), as shown in Figure 11. For these experiments, the other conditions were kept at 180 °C and 150 min. As shown in the Pareto chart (Figure 7), the effect of the other parameters was insignificant; so, they were kept constant, and only the concentration was varied to test whether lower concentrations of OA had a desirable effect on Ag leaching. It is obvious that 1 M oxalic acid yielded the best results (Figure 11). The Ag distribution in the cell grid is not uniform as there are denser and less dense lines. This uneven Ag coating is most likely the result of heat treatment during cell processing or cutting during the preparation of samples in the lab. This discrepancy in Ag content in the cell material could be the main cause of the error in the testing process, explaining the relatively large error bars in Figure 11 [33].

Comparing the obtained Ag concentrations between CA and OA leaching, it is concluded that OA was more effective than CA. The maximum Ag concentration achieved by leaching with citric acid was only 30% of that achieved with oxalic acid. As discussed previously, this may be due to the fact that OA is a stronger acid in the first dissociation step than CA; therefore, at short reaction times, it has a better leaching efficiency than CA. On the other hand, very long reaction times did not have a positive effect on leaching for both organic acids. This can be attributed to the effect of the pressure increasing with time in the hydrothermal reactor. The decomposition constants of weak acids increase with a higher applied pressure, thus making the acids weaker [31].

3.3. Comparison to HNO₃ Leaching

For comparison, leaching was also carried out with HNO₃ (65%) according to the procedure of Dias et al. [5]. The strong inorganic acid proved to be much more effective for leaching Ag from the EoL panels, revealing that the efficiency of the organic acids was less than 5% compared to 65% HNO₃. Inorganic acids are produced from minerals and do not contain carbon atoms, unlike organic acids. Because organic acids contain carbon atoms, they are less acidic than inorganic acids and, therefore, exhibit a lower leaching efficiency [34]. Future research may indicate different organic acids and different experimental conditions and procedures.

4. Conclusions

Recycling end-of-life PV panels is challenging due to the complex structure and the small amounts of various elements that coexist in their numerous components. In this study, hydrothermal leaching experiments with organic acids were conducted using a Box–Behnken design and response surface technique. The objective of hydrothermal Ag leaching was to develop a rapid and green method that does not require oxidants and can use organic acids. The organic acids employed in this study are inexpensive and safe for the environment. Oxalic acid leaching proved to be more effective than citric acid in Ag leaching. In fact, the highest yield achieved with citric acid leaching was only 30% of that achieved with oxalic acid. The optimized parameters for citric acid leaching were 210 °C, 2M, and a time of 95 min, while for oxalic acid, a concentration of 1 M, time of 60 min, and temperature of 210 °C. However, the strong inorganic acid (65% HNO₃) proved far more effective than the examined organic acids; therefore, more research is needed to establish efficient leaching routes when utilizing organic acids.