recycle old iMac

You should prepare the Mac for recycling according to this Apple document: What to do before you sell, give away, or trade in your Mac - Apple Support  except for the reinstalling of the system. Just erase the drive.

Take a look here:

National Services - Apple

There may also be other recycling services in your community; for instance, some towns have days when they'll accept electronics for recycling. Or if it's working and you've just moved to a newer Mac, you can ask around your community to see if someone, such as a senior's home or homeless shelter, might want it.

Regards.