The Recycling Process, Part III: Reprocessing
Now that we’ve separated the stuff we want from what we don’t, we have to make it into something someone will buy. Most important is the clean-up: remember, this stuff is garbage. Imagine you’re recycling HDPE (#2) milk bottles. You will have issues with dirt, labels, odor, and, well, milk (or what’s left of it). Whether it’s milk, soda, laundry detergent, or any other product, you will have the same issues of dirt, labels, residual product, goo, and odor: they’ll just be a bit different for each kind of bottle.
Dr. Bill Carroll explains Reprocessing, the third step in the recycling process.
First, we chop the material into quarter- to half-inch-square flakes. Then it’s off to a wash tank where water, heat, and some kind of soap or grease-cutter are combined to remove the dirt, labels, product, and goo from the flakes. Then there’s another separation step.
Bottles are complex devices, particularly when you consider the labels and caps. Milk bottles are #2; the caps are generally the same, but they may be color-coded to designate what kind of milk it is. The highest-value recycle is called “natural,” that is, colorless. Flecks of colored cap are a problem when mixed with “natural” bottles. Soda bottles are even trickier. The bottles are PET (#1), but the caps are PP (#5). One is clear; the other is opaque. And they don’t melt together well.
Different techniques are needed for different materials. For soda bottles we use the difference in density between bottle material and cap material: bottle plastic sinks and caps float. At the end, cap material is scraped off the top of the water, and bottle material is dredged up from the bottom of the tank. Watch the process here.
For milk bottles we can physically remove the caps in an earlier step (yes, and recycle them). If a stray gets through and gets ground up, we can remove the colored bits using an optical system like what we described in the previous post to separate out the little colored bits.
Once washed, we dry the material in hot air, and we use a centrifugal separator called a cyclone to remove fine bits of dirt and labels from the clean flake much like the inside of a Dyson vacuum cleaner does.
Finally, we need to turn the dried plastic flakes into something people want to buy. Typically, that’s plastic pellets. An extruder—basically, an industrial Play-Doh Fun Factory—melts the flake and pushes the melt through a screen that catches the last bits of label and unmeltable stuff, then through a die that produces a strand similar to the way you might make spaghetti. The strand is cooled and then chopped into small pellets.
The pellets are sold to people who make other kinds of products: pipe, house wrap, bottles, and other things.