Many recycling programs in the world today have what is known as single-stream recycling (or fully co-mingled recycling). This is a system where all recyclables (e.g. paper, plastics and metals) are mixed and collected together from the households, instead of requiring the households to sort them into different materials. Do you know how such mixed recyclables are sorted? Is sorting done by hand or by machines? 

In such systems, the “single streams” of recyclables collected are then sorted at a Material Recovery Facility (often also known as MRF – pronounced as “murf”, rhymes with “smurf”) for re-use or recycling. This of course adds to the cost of recycling. The designs of different MRFs differ to some extent, but essentially they sort out each material based on certain properties of the material. The whole stream of recyclables is fed into the processing system and travels along a conveyor belt. Humans are stationed at the start to remove cardboard, plastic bags, styrofoam, large metal objects and other material that cannot be handled by the system (and can damage or obstruct the system) or that cannot be recycled (e.g. food waste).

The common materials are then automatically sorted as follows:

A) Paper. The stream of recyclables then goes through spinning-disk screens which sort heavier materials (e.g. bottles and cans) from lighter ones (e.g. paper). These rotating disks allow the heavier materials (usually glass, metal and plastic containers) to roll down onto a conveyor belt, while the lighter materials (usually paper) travel up to a different conveyor belt. The paper is often further sorted by humans, or sometimes through automated sorting into different types of paper. 

B) Ferrous Metals (e.g. iron and steel). Strong magnets (which are usually placed above the conveyor belt) are used to separate out ferrous metals, such as iron, steel and tin cans (which are actually tin-plated steel cans).

C) Aluminum. Aluminum cans are mechanically separated by the use of eddy currents (i.e. electric currents induced in conductors when they are exposed to a changing magnetic field). An eddy current separator essentially consists of a conveyor belt with special magnetic components in the head (or rotor). The head contains a rapidly rotating system of permanent magnets – this generates high-frequency changing magnetic fields. These fields in turn induce strong eddy currents and a secondary magnetic field (which opposes the external field) in the non-ferrous metals. The opposing magnetic fields cause repulsion and the non-ferrous metals are ejected out of the material flow. (The rest of the materials fall freely into another container.)

D) Plastics. Often done by hand, separation of plastics has been automated in some Material Recovery Facilities. Plastics could be put through the automated sorter together, or in separate streams, depending on the design of the sorter. A spectroscopic (or optical) sorter is used to differentiate between different types of plastics. When illuminated by a special lamp, each type of material reflects a unique combination of wavelengths in the visible and infrared spectrums of light. These wavelengths can help to identify the material uniquely, akin to a spectral fingerprint. Based on this, the sorter then diverts each material into the proper conveyor belt or collection bin by blowing a jet of air at the material. Many types of plastics or sometimes paper or plastic / paper combinations can sorted using this process. They can be sorted to very high accuracy (more than 90%). [Development of these systems first began in the 1990s.]

E) Glass. Glass is often separated by hand into clear, brown, amber and green glass. Some MRFs automate this sorting process, for example using coloured filters to detect the colour of the glass. Glass that is broken is too small to be colour sorted and is mixed together as glass fines.

The separated items are collected and compressed into bales, which are then sent to plants (e.g. paper mills, metal smelters) for further processing and re-use by industry. Typically, there would still be residues from the sorting process – these are either fed into the system again for another round or sorting, or disposed of at the landfills or incineration plants.

Although there is still a need for human intervention at the start and along the way, there has been increasing investments in sorting technologies described above. As you can see from above, the sorting technologies depend on certain properties of the materials, for example weight, electrical conductivity, magnetic properties or light reflection / absorption.

Watch this nicely animated video (2:51 min) from the North Carolina Department of Environment and Natural Resources to find out how a Material Recovery Facility (MRF) works!

Watch this video (15:16 min) from the Boulder Country Recycling Center to find out how a Material Recovery Facility (MRF) works and see a live one in action, with commentary from a talking aluminium can!

Sources:

Wikipedia

Economist

American Recycler