Salt is water soluble. Powdered shell is not. Put It in a bucket of water and give it a stir.
Either decant carefully, pour through a cheesecloth type material tight enough to catch the shell, or dip out water with a cup.
Probably have to do a few rinses with fresh water. When satisfied, pour into shallow, wide container and let water evaporate.
Place in the oven at 300 degrees for an hour and a half to use the essential oils of oysters as an air "freshener."