From The Economist:
The process begins when a truck arrives and dumps its load of recyclables at one end of the building. The materials are then piled on to large conveyer belts that transport them to a manual sorting station. There, workers sift through everything, taking out plastic bags, large pieces of cardboard and other items that could damage or obstruct the sorting machines. Plastic bags are especially troublesome as they tend to get caught in the spinning-disk screens that send weightier materials, such as bottles and cans, down in one direction and the paper up in another.
Corrugated cardboard is separated from mixed paper, both of which are then baled and sold. Plastic bottles and cartons are plucked out by hand. The most common types, PET (type 1) and HDPE (type 2), are collected separately; the rest go into a mixed-plastics bin.
Next, a magnet pulls out any ferrous metals, typically tin-plated or steel cans, while the non-ferrous metals, mostly aluminium cans, are ejected by eddy current. Eddy-current separators, in use since the early 1990s, consist of a rapidly revolving magnetic rotor inside a long, cylindrical drum that rotates at a slower speed. As the aluminium cans are carried over this drum by a conveyer belt, the magnetic field from the rotor induces circulating electric currents, called eddy currents, within them. This creates a secondary magnetic field around the cans that is repelled by the magnetic field of the rotor, literally ejecting the aluminium cans from the other waste materials.
Finally, the glass is separated by hand into clear, brown, amber and green glass. For each load, the entire sorting process from start to finish takes about an hour, says Bob Besso, Norcal’s recycling-programme manager for San Francisco.
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