Recycling an old (2012) iMac

If you decide to recycle, sell, or give away the computer locally, see:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If you have no other way to dispose of your iMac Apple has a recycle scheme.

Go to Apple Trade In and follow the prompts. Have the serial number for the Mac on hand because part of the process is to enter this so that Apple's systems know that it is old and of little to no monetary value. Nevertheless, you can use this method to get it to Apple for disposal.

You might also drop it off for recycling at your local Apple Store, Staples or Office Max.

They all recycle.

Make sure you wipe the drive per the guidance that @Jan Hedlund posted above.