Maximum five days before you leave Belgium, you must deregister as a Belgian resident. It is very important that you do this. Failure to deregister could have important consequences for visa requirements, taxes, health insurance registration and so on if you return to Belgium in the future.
In Antwerp, you can deregister using the e-service on the website of the city of Antwerp or at your local government office. You can also ask the International Staff Office (residencepermit.staff@uantwerpen.be) to apply for the Model 8 (proof of deregistration) for you and fill in the letter of attorney.
When you have left Belgium, please send your identity card back to the local government office.