I’ve sometimes wondered how recycling plants deal with unwanted materials (like staples in magazines, plastic windows in envelopes, etc) mixed up with the recyclables. Apparently I wasn’t the only one to wonder this; Slate magazine has an article explaining the rather ingenious processes involved in pulping paper for recyling.

When bales of sorted paper arrive at a mill, they’re fed into a huge, blenderlike contraption along with water and chemicals. The resulting pulp then goes through a number of purification steps. First, a long chain called a ragger is lowered into the swirling mixture; things like twine and wire wrap around the chain and get pulled out. A metal screen at the bottom of the pulper picks out more contaminants—this should be when your plastic window fragments are removed. Next, the slurry is spun around in a cone-shaped hydrocyclone—which separates out higher-density items like stones and bits of metal (like staples)—and then it’s screened again through a finer mesh. Finally, if the pulp is being made into high-quality product like white office paper, air bubbles and detergents are pumped in to wash away unwanted ink particles.

The answers to more “recycling stumpers” at Slate.