The modern process of recycling, particularly under a single-stream system, relies on highly automated facilities called Materials Recovery Facilities (MRFs). These large-scale industrial plants efficiently separate a mixed stream of paper, plastics, metals, and glass into distinct, high-purity commodity grades. This transformation involves a series of mechanical, magnetic, and optical sorting steps, each tailored to isolate a specific material based on its unique properties.
Initial Material Handling and Sizing
The sorting process begins on the tipping floor, where collection trucks unload their mixed cargo of recyclables. Front-end loaders push the material onto a primary conveyor belt. Manual pre-sort workers are positioned along the conveyor to remove large, non-recyclable items such as hoses, bulky plastics, or film plastic. This prevents contamination and protects the automated machinery from damage.
The material then travels to a series of mechanical screens designed to separate items based on size and shape. A common device is the trommel screen, a large, rotating, cylindrical drum with numerous holes. As the material tumbles inside, smaller, heavier objects like broken glass fragments and dirt fall through the holes, while larger items continue along the system.
Other facilities use disc screens, which utilize a series of rotating discs set at specific angles. These screens separate materials into two primary streams. The flat, two-dimensional fraction consists mainly of paper and cardboard. The heavier, three-dimensional container fraction includes bottles and cans, setting the stage for material-specific sorting technologies.
Mechanical Separation of Metals and Fiber
Once the material is separated by size, specialized mechanical equipment targets metals and fiber products. Ferrous metals, which contain iron, are extracted using large, powerful overhead magnets, often electromagnets. As the mixed stream passes underneath, the magnetic field lifts steel and tin cans clear of the conveyor belt and drops them into a collection chute.
Non-ferrous metals, primarily aluminum beverage cans, require a different approach because they are not magnetic. These items are separated using an eddy current separator, which operates on the principle of electromagnetic induction. A rapidly spinning rotor containing a series of alternating-pole magnets creates a fluctuating magnetic field at the end of the conveyor belt.
The induced current creates a magnetic field that opposes the original magnetic field. This interaction generates a powerful repulsive force that effectively “kicks” the non-ferrous metal away from the main stream and over a divider. Simultaneously, the fiber fraction (paper and cardboard) is routed over specialized oscillating screens. These screens allow rigid containers to fall through, while the flat fiber material is carried across the top.
High-Tech Sorting for Plastics and Glass
The separation of different types of plastic is technologically advanced, as polymers often look similar but have distinct chemical compositions. This process is primarily handled by Near-Infrared (NIR) optical sorters, which use spectroscopy to identify different plastic resin types. As plastic containers travel rapidly, the NIR system scans each item by directing infrared light onto its surface.
Each plastic polymer (such as PET, HDPE, or PP) absorbs and reflects specific wavelengths of infrared light, creating a unique spectral “fingerprint.” The optical sorter’s computer analyzes this signature in milliseconds to identify the exact resin type. Once identified, a precisely timed burst of compressed air ejects the targeted plastic item onto its correct conveyor belt, separating it from the rest of the stream.
Glass is often broken during collection and is typically separated early using screens that filter out small fragments. While some facilities attempt to sort the broken glass (cullet) by color using optical scanners, the primary focus is removing contaminants. The small, heavy glass fragments that fall through the sizing screens are then cleaned through steps like density separation, where light materials float and heavy impurities sink.
Final Quality Check and Baling
After the automated and mechanical systems finish, the separated streams undergo a final quality check to ensure material purity meets manufacturer requirements. This is often accomplished at secondary manual picking stations, where workers remove any remaining non-target materials or contaminants. This step is necessary because the presence of even small amounts of foreign material can significantly reduce the market value and usability of the recycled commodity.
Once a material stream is verified as clean, it is sent to a baler, a powerful hydraulic press that compresses the loose items into dense, uniform blocks. Materials like corrugated cardboard, aluminum cans, and segregated plastic types are compacted into these bales, which are tied with wire for stability. This process dramatically reduces the volume, making storage and transportation much more efficient. The resulting bales are the final commodity products sold to manufacturers.