Lost iPhone
I have removed my device from iCloud, but I want to lock/delete my old iPhone and I can not find it. I do have the serial number though
iPhone 5s
You should have erased your device before removing it from your iCloud. If it doesn't appear in the Find My app, then you would only have the option of reporting the phone as stolen to the police using the serial number.
If your iPhone, iPad or iPod touch is lost or stolen – Apple Support