From waste to sortable material - processing and characterization of recycled glass

Abstract

Recycling glass is a key step toward sustainable material use, reducing the need for virgin resources, minimising environmental impacts, and supporting circular economy goals. This thesis focuses on the preparation and characterization of clear and green recycled glass as a foundation for future research in automated optical colour-sorting technologies. The work demonstrates that simple, yet effective procedures are sufficient to prepare high-quality cullet at laboratory scale. Post-consumer glass was collected, cleaned with resource-efficient methods, and processed through manual and mechanical crushing. Sieving techniques were employed to achieve standardised particle size fractions, particularly in the range of 4–10 mm, which is considered optimal for optical sorting. By comparing different crushing techniques and analysing particle size distribution, the study highlights how fragmentation behaviour differs between clear and green glass and how these differences can affect sorting performance. In addition to particle size, the thesis examines essential physical properties such as particle density, bulk and tapped density, porosity, and flow behaviour. These characteristics determine how cullets behave during handling, storage, transport, and separation, and therefore play a crucial role in recycling efficiency. A combination of manual and instrument-based methods was applied to ensure accuracy and practical relevance. The findings underline that the crushing method and particle properties directly influence downstream processes. Excessive fines hinder sorting efficiency, while controlled fragmentation produces cullets better suited for automated systems. Overall, the study provides a comprehensive characterization of recycled glass and establishes a reproducible preparation method. In doing so, it creates a robust foundation for future research and contributes to the development of more precise, efficient, and scalable glass recycling systems.