A mixer system for aggressive (toxic, hazardous, corrosive, etc.) materials which are mixed (agitated, suspended, circulated, etc.) in a tank with an impeller connected to a drive shaft has the impeller and its shaft mounted in an assembly which extends through an opening into the tank and closes that opening. The assembly has a cylindrical hub with a passageway through which the shaft extends out of an open end of the hub. Bearings which are made of materials which are resistant to the aggressive material in the tank are mounted in the hub and journal and support the shaft. There are no dynamic seals which close the confinement region so that the aggressive materials can be present therein. In the event that it is desirable that the bearings be replaceable or the confinement region be sterilized, or purged, an expandable elastomeric collar may be disposed either at the end of the hub below the bearings, which may then be mounted in a cartridge in a hub which is removable therefrom, or in a plate spaced from the hub. Purge or pressure release passages may be provided which are opened and through which purging or sterilizing gas is introduced into the confinement region.