If the iron has a switch that shuts it off (tip switch) if it tips over then it is likely a mercury switch especially if it is a bit older. If it doesn’t shut off if it tips over then it does not contain mercury.  If it has mercury in it, then it has to be managed as a mercury containing device. If it doesn’t, it is not a banned item and could potentially be managed in scrap metal depending on if it is actually metal.  And if it has a mercury switch – you can identify if the switch is mercury easily – glass (you’ll see it);  metal – rock it back and forth and you’ll feel the weight.