Most keys are brass or a nickel-brass mixture. Such amalgams cannot be recycled in the usual recycling system which is why specialist metal recyclers like scrap dealers are the only recycling channel.
Keys
Recyclable?
Blue Bins & Chutes
Other Recycling
Recommended
Specialised Recycling
Scrap metal dealers may take these but unless you have a large quantity, don't expect them to pay you for them. A key only weighs about 10g or about 5 cents worth of metal. If you have a kilogram or more of keys, you might be able to sell them for scrap.
Scrap Metal Dealers
Alternative
Specialised Recycling
UnPackt has a key collection service: drop your old keys off and they will be sent for recycling when the volume is large enough to be accepted by a scrap dealer.