Update: It’s New Year’s Eve and the Ars employees are enjoying a winter break (inevitably filled with some fun rides and whatever black mirror the thing is). So let’s resurface a few site archive favorites that are appropriate for the occasion, like this tour of a facility that will inevitably be busy after the holiday season. Our Sims Municipal Recycling Center story originally ran on December 7, 2015, and appears to remain unchanged below.
BROOKLYN, New York — A conveyor belt causes material to fly past at speeds that require both concentration and quick eye movements if you want to track a single item. Above the constant roar of all the heavy equipment, it is just possible to distinguish the brief hiss of high-pressure air jets. Those jets are produced where the conveyor belt ends and most of the material dumps onto a second belt below. However, each hiss causes a carefully chosen item to jump off the end of the belt and float to another collection area, where yet another conveyor belt takes it in its way.
The process of carefully choosing which items to sift out is all done without human intervention. It is based on how that object reflects light that is beyond the range of human vision.
All this happens in just one of the dozen stations in a modern recycling center, each of which isolates a single class of materials based on their physical properties. In recent decades, recycling has gone from a manual process to an extremely automated one, where things like infrared sensors and tiny air jets mix with huge front loaders and huge material balers.
A lot of technological innovation has gone into figuring out how to take a chaotic mix of items and separate them into relatively pure material streams. But being able to do this is not enough – it must be made into a sustainable business. So today much of the innovation taking place in the recycling industry has to be doubly focused on the economy.
Large-scale materials science
Recycled materials are only valuable when they are pure: a collection of a single type of metal or plastic that can serve as a raw material for manufacturing or other industrial processes. In fact, the economics of recycling would be spectacular if you could get people to separate a dozen separate classes of recyclables and then take them to a recycling center.
Unfortunately, there would be almost no recycled materials, as hardly anyone would bother to do all the sorting and carting. In fact, sorting recyclables into more than one or two separate streams reduces the recycling rate. Single-stream recycling also has a major transport advantage. When trucks do not need to have space for individual recyclables, they are more likely to fill up completely before taking the material to its destination.
That shifts the problem of separating pure materials to the recycling center itself. Here too, the economy limits choices: the more people involved in carefully distinguishing each type of recyclable material, the more expensive the process. For recycling, automation is key. But how can a machine distinguish and separate different types of material?
The answer is that no machine can do that. Instead, a combination of hardware is used with different machines separating specific material classes. To get the technical details of how all these machines work, we visited each type of hardware at the Sims Municipal Recycling Center in Brooklyn and had Eadaoin Quinn explain the process to us.
Smash and grab
When barges and trucks arrive at the facility, the material (often still in the clear bags) is dumped as a single stream onto a large conveyor belt. It is quickly divided into two streams; having two parallel tracks allows the facility to continue operating even if key hardware components are unavailable for maintenance, Quinn told Ars. Typically, the site operates for two eight-hour shifts a day.
The first material to use is glass. A lot breaks down during transport; large asymmetric steel rollers break the rest into small pieces. These simply fall through the cracks and are collected below. The rest of the material is too big and goes on. Sims can separate clear glass for reuse; any colored material is usually used as construction filling.
The next station uses a huge rotating magnet to pull out all the ferrous metals, ranging from cans (which contain very little tin) to car parts. The problem here is that some of the metal could be buried under other recyclables that weigh more than the magnet can lift. To overcome this, the magnetic drum is located above a conveyor belt that stands on a vibrating platform. The vibrations give everything in the recycling stream a small boost toward the magnet, allowing it to bond to the metal.
The vibrating platform is large and heavy enough to physically shake the entire facility, and it is responsible for much of the noise.
Anything not attracted to the magnet falls off the conveyor belt and onto a second below it and continues into the facility. The rotating drum is arranged so that its magnetic force drops on the opposite side of the platform from its rotation, allowing the metals to fall into a separate conveyor belt.
It’s not 100 percent effective — Quinn says you’ll often see a city’s metal hangers running on a dry cleaning hook in a plastic bag and carrying it between the metals. But it’s a lot better than where it started.
Other metals are sorted by a similar machine. Materials like aluminum aren’t naturally attracted to magnets, so the hardware uses an electric field to create enough attraction for them to cling to the drum. Again, most other materials immediately fall off the end of the conveyor belt and fall onto a new one below; the drum lifts all remaining metals to a separate conveyor belt.