The Recycling Process, Part II: Separation
Now that we have collected a big pile of stuff, we need to separate what we want from what we don’t want. Separation is important because while in principle you could melt all this plastic together, doing so wouldn’t maximize its value or make something very usable. Here’s a comparison. Did you ever melt your crayons together when you were a kid? Did it come out a pretty color? Same thing happens with mixed plastics; so sorting is key.
Recall the scavengers I mentioned in the last posting, working at a landfill. They sort manually, picking out only the desired materials. This was how most sorting happened even in the United States until the mid-1990s, although it wasn’t done at a landfill; rather it was done from a conveyor belt in a processing plant. In some cases, it’s still done this way today.
Sorting garbage is a crummy job. But in principle, if your eye can see it and your hand can pick it, you should be able to design a machine to do the same thing. And in the past few years systems have been developed to do that task.
First, you need to have each piece separate from all the others on a conveyor. This part of the process is called singulation. Then you have to be able to tell one plastic from another because different plastics are used for different things. In the next step you need a machine that can remove the selected article from the conveyor. To do that, the grabber needs to locate each article once it’s been identified. Here is how it’s done.
Dr. Bill Carroll explains the second step of the recycling process--separation and sorting.
Singulation. Plastics are shipped to the facility as bales to minimize volume and shipping cost. They are coarsely separated by a bale breaker, a machine that pulls them apart, and then finely separated into individual pieces in a tumbler—a long rotating drum like a clothes dryer that bumps them apart.
Identification. Articles are distinguished by various wavelengths of light in a process called spectroscopy. Visible light distinguishes color, which your eye can see. The chemical identification of different plastics, however, requires infrared light, which is outside the visible spectrum. The process is a bit more complicated than just distinguishing colors, but it works pretty well. Finally, there are other machines that use X-rays to identify items by the atoms they contain, especially atoms heavier than oxygen, like chlorine or metals.
Tracking and Removal. Once an article passes under the light source and is identified by its reflected light, a computer tracks it until it gets to the end of the line. Then a blast of air moves it in the correct direction. This has probably been the biggest boon to automated sorting: cheap computer power that follows articles and orchestrates removal.
You can see a process that uses these principles here and here.
Bill Carroll is vice president of industry issues for Occidental Chemical Corporation in Dallas, an adjunct professor of chemistry at Indiana University, and a former president of the American Chemical Society. He operated a plastics recycling business for OxyChem between 1989 and 1994.