South Korean researchers have developed a method to upcycle silicon recovered from end-of-life PV modules into high-value silicon nitride (Si₃N₄). The process achieved recycled silicon purity of 99.95% through optimized milling, acid etching, and sedimentation before nitridation.
A South Korean research team has developed a method to upcycle silicon recovered from end-of-life (EoL) PV modules into silicon nitride (Si₃N₄), a high-value ceramic used in the automotive, aerospace, electronics, medical, energy, and manufacturing industries due to its high strength, thermal stability, wear resistance, and electrical insulation properties.
“To the best of our knowledge, this is the first demonstration of converting silicon recovered from actual EoL PV modules into Si₃N₄,” corresponding author Jin-Seok Lee told pv magazine. “Rather than simply recovering silicon as a secondary raw material, we aimed to demonstrate a practical pathway for giving recycled silicon a new, higher-value application.”
Lee said the team’s next step is to move from proof-of-concept research toward a practical, scalable recycling and upcycling process.
“We are currently working with Wonkwang S&T, a Korean PV recycling company, to develop mobile PV recycling technology that can process end-of-life PV modules closer to where they are generated,” he said. “Through this approach, we aim to reduce transportation costs by approximately 30% and carbon emissions by more than 10% compared with conventional centralized recycling.”
The researchers used a single end-of-life Suntech STP200-18/Ub module containing 54 polycrystalline silicon cells based on an aluminum back-surface field (Al-BSF) architecture.
After removing the junction box and aluminum frame, they separated the glass from the ethylene-vinyl acetate (EVA)/cell/EVA/backsheet laminate using a hot knife. They then cut the laminate into cell-sized pieces and milled the material at 400 rpm, 600 rpm, or 800 rpm to assess the effects of milling speed on particle agglomeration and impurity removal.
The researchers removed large backsheet fragments and residual EVA using 3 mm and 0.5 mm sieves, respectively. They then combusted the remaining organic materials in air at 600 C for one hour.
Particle-size analysis and scanning electron microscopy (SEM) imaging were then used to assess agglomeration. The recovered silicon underwent a two-stage purification process, comprising 20 minutes in 36 wt% hydrochloric acid (HCl) to remove aluminum, copper, tin, and lead, followed by 30 minutes in 36 wt% nitric acid (HNO₃) to dissolve silver. Both treatments were conducted under stirring and ultrasonication.
To remove acid-resistant titanium dioxide (TiO₂) originating from the backsheet, the researchers dispersed 10 g of powder in 1 liter of water for 20 minutes, allowed it to settle for between 5 minutes and 20 minutes, and then removed 800 mL of supernatant. They identified 5 minutes as the optimal settling time.
Following characterization, the highest-purity powder, produced at 400 rpm, was selected for nitridation. Purified and unpurified powders were ball-milled in ethanol for 20 hours to an average particle size of approximately 1 µm. They were then nitrided under a flow of 95% nitrogen and 5% hydrogen, first at 1,350 C for one hour and then at 1,450 C for 10 minutes.
The researchers subsequently used X-ray diffraction (XRD) to calculate silicon conversion and determine the proportions of the alpha and beta crystalline phases of silicon nitride (α-Si₃N₄ and β-Si₃N₄). They used SEM to compare particle morphology and inductively coupled plasma optical emission spectrometry (ICP-OES) to measure the purity of the final ceramic.
“We found that particle agglomeration during milling strongly affects the subsequent removal of metallic impurities,” said Lee. “By controlling the milling conditions and combining stepwise acid etching with a simple sedimentation process, we were able to effectively control both metallic and ceramic impurities in the recovered silicon.”
The researchers found that severe particle agglomeration at 800 rpm resulted in a residual aluminum concentration of 4,290 ppm after HCl etching, compared with just 189 ppm at 400 rpm.
“This clearly showed that optimization of a recycling process cannot be based simply on more intensive milling,” Lee said.
“We were impressed by the effectiveness of the relatively simple sedimentation process,” he added. “In only 5 minutes, 71.4% of the TiO₂ impurity could be removed while maintaining a silicon recovery rate of 92.3%. Most importantly, after controlling these impurities, the recycled silicon reached 99.95% purity, and the resulting Si₃N₄ contained 93.1% α-Si₃N₄. In comparison, Si₃N₄ synthesized from recycled Si without the additional purification process contained only 54.7% α-Si₃N₄. This demonstrated that impurities originating from waste PV modules can directly affect the properties of the final upcycled product.”
The findings are described in the study “Upcycling silicon recovered from photovoltaic waste into silicon nitride via the field-applicable control of metal and ceramic impurities,” published in Materials Today Sustainability. Researchers from the Korea Institute of Energy Research and Chungnam National University participated in the study.
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