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Development of recycling techniques in 1st and 2nd generation waste photovoltaic panels

Savvilotidou Vasiliki

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Summary

Waste electrical and electronic equipment (WEEE) or e-waste is globally considered as one of the fastest growing and complex waste streams. The European Directive on WEEE (2012/19/EU) aims at sustainable production and consumption and sets targets for collection, reuse, recycling and recovery. From 15th August 2018 and onwards, WEEE is classified within the six new categories, as stipulated in Annex III of the recast Directive instead of the existing ten categories. End-of-Life (EoL) photovoltaic (P/V) panels consist one of the newest WEEE under category 4 and, thus, a current and future challenge, since their management is yet to be compiled. Because of the P/V market growth and its continuous expansion, the International Renewable Energy Agency (IRENA) has predicted that waste P/V panels will amount to 1.7-8.0 million tonnes by 2030 and to 60-78 million tonnes by 2050. P/Vs are considered as “clean” energy technologies with positive impacts on energy security and climate change, however, the proper management of EoL P/Vs is an indispensable issue that should be particularly addressed and evaluated from a life-cycle viewpoint.The purpose of this thesis is to develop recycling techniques for P/V panels in order to recover valuable components, taking into consideration that they represent one of the newest and most promising sources of secondary raw materials. P/V panels based on different technology, namely polycrystalline silicon (p-Si) and monocrystalline silicon (m-Si) panels classified in the 1st generation of photovoltaics, as well as copper indium selenide (CIS) and amorphous silicon classified in the 2nd generation of photovoltaics, were studied. Aiming at sustainable management of waste panels, various investigations were carried out including four different approaches, (a) the delamination of P/V panels, (b) the recovery of valuable metals (semiconductors), rather than simple recovery of bulk materials, (c) the reuse of glass or plastic in cement mortar production, and (d) the valorization of glass in the production of glass-ceramics for applications in the construction sector.One of the main problems in the management of P/V panels is their complex and multilayer structure which differs depending on the cell technology. Initially, investigations were conducted on the delamination of P/V panels by comparing thermal, mechanical and chemical treatment techniques. The comparison and optimal approach were determined based not only on the efficiency of delamination, but also on the mass flow of precious (silver) and critical (indium) metals during these processes. These high-tech metals have been included in the European (EU) list of critical raw materials (CRMs) and their recycling is a priority in order to contribute to a circular economy and reduce the risks pertinent to expensive and scarce resources. The content of silver and indium in the treated mass was determined. Also, their pre-concentration yield and losses in each treatment technique were calculated. Finally, selective recovery of these metals was achieved using a hydrometallurgical process, including leaching and precipitation. Apart from high-value recyclable materials, bulk materials including glass and plastic from P/Vs were recycled and reused as partial replacement of fine aggregates or cement in cement mortar production. Various parameters, among which, the type of waste (glass or plastic), the amount (%) and particle size of waste, as well as the resource replaced (fine aggregates or cement) were studied. Physical, mechanical and thermal properties of cement mortars were determined and compared to reference mortars. Also, the resistance of mortars to corrosive environments, as well as their potential toxicity were examined. The last part of this study was the valorization of specific wastes generated from the energy sector, i.e. P/V glass and lignite fly ash, and the production of glass-ceramics. Various parameters, such as mixing ratio, melting and sintering temperatures, and others were investigated to determine the optimal conditions. The physical and mechanical properties of the produced glass-ceramics were examined. Also, the chemical composition, mineralogy and microstructure, as well as the chemical stability were determined.Delamination and separation of the major components contained in P/V panels were achieved through a combination of processes, namely a thermal process and a gravimetric separation, leading to intact and reusable components. Under this combination of processes, 91-94% of silver was pre-concentrated from the p-Si and m-Si panels and around 96% of indium was pre-concentrated from the CIS panel as well. Through selective recovery, i.e. leaching and precipitation, In2O3 and AgCl were recovered achieving 74.8 and 98.7-99.2% recovery. In addition, cement mortars containing up to 20% glass as replacement of sand or cement exhibited high strength and resistance to corrosion comparable with those of reference mortars, whereas plastic addition resulted in enhanced thermal properties by reducing the thermal conductivity of cement mortars (from 0.77 to 0.45 W/m·k). Finally, melting of glass and lignite fly ash mixture at 1200 oC and sintering of the produced glass at 700-800 oC resulted in dense and homogeneous glass-ceramics. Specifically, the results showed that the produced glass-ceramics can be used in the construction sector as brick pavers, since their compressive strength and water absorption were 113-148 MPa and 0.02-0.07% respectively, thus complying with the standard specification, ASTM C 1272.In conclusion, metal, glass and plastic parts consisting more than 90% of P/V panels can be reused, recovered or recycled towards an integrated sustainable management of waste P/V panels, indicating potential future applications.

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