A Look at Optical Sorting

Operators of recycling centers are on a neverending quest to become more efficient and effective in segregating materials that flow through their facilities. The technology for processing and sorting recyclables continues to evolve, and optical sorting systems are now commonly used in many recycling centers to sort recyclables, produce high quality commodities, reduce labor costs and increase throughput.

To children who are touring a recycling center it may appears that “magical” forces are in play as eddy currents repel aluminum, magnets pull ferrous metals and compressed air is used to blast materials over divider walls. In reality, there are basic forces of nature and laws of physics that are used to process the materials. 

During the past five years, some of the biggest advancements in sorting technology can be found with optical sorting systems that use light to identify various materials. More specifically, optical sorting examines the properties of light waves that are being reflected from the different materials. 

How it works

The actual process of optical sorting is fairly basic. Recyclable material is loaded onto an accelerator belt or high-speed conveyor in an effort to thin out the layer of material. The conveyor moves the material under a bright light. As the material passes under the light, some of that light is absorbed by the recyclables on the belt while other light waves are reflected. A series of camera lenses that are pointed at the conveyor belt are able to see and record the light waves that bounce off the recyclables.

Each material generates a unique light “signature” that is read with a spectrometer. These spectral signatures (also called light fingerprints) are used to identify different types of materials. The ability of the lens and spectrometer to identify material is impressive. Optical sorting can be used to identify and separate a wide variety of materials including plastics, glass, wood, paper, cardboard and many other items. 

Additionally, the computer can be programed to target different types of plastics, including polyethylene terephthalate (PET), colored and natural high-density polyethylene (HDPE), polyvinyl chloride (PVC), polypropylene (PP) and others. Different types of wood and colors of paper and glass can also be identified.

Once the spectrometer identifies a specific item on the conveyor belt as a “targeted” or desired item, the physical part of sorting process occurs. The computer controlling the process calculates the position of the desired recyclable on the belt and triggers a blast of compressed air in the area of the belt where the targeted material is located. The jet of air separates the item from the rest of the material on the conveyor belt. Most sorting systems include a quality control step to ensure purity of the commodity that is being sorted.

Optical sorting is faster and more effective than manual sorting and can result in a significant reduction of labor on the sort line.

Maintenance

Like all machinery, good maintenance is necessary to ensure the system is running at peak performance. Optical sorting systems should be inspected every day to ensure all of the lights are working and that the lenses are clear and free of dust and obstructions. Lights that are burned out need to be replaced. Air nozzles must also be reviewed to make sure they are not clogged and have the right amount of pressure to perform the physical separation. The computer that controls the process must be occasionally re-calibrated to ensure it is not missing any desired items on the belt.

The future of optical sorting

Advancements will continue in the sorting of recyclables at recycling centers. For optical sorting systems we will see continued improvements with lenses and cameras while computers get faster. All of the improvements lead to systems that are better and quicker at selecting and recovering materials.    

Will Flower is general manager with Winters Bros. Waste Systems in Long Island, N.Y.