If all that is needed is minimal suspension of slowly settling particles, the following mixer should be sufficient for a 5.0 m diameter vessel with a 7.5 m height and four vertical baffles to prevent uncontrolled swirling.  A 1.25 m diameter pitched-blade impeller with 45 degree angled blades and rotating at 45 rpm should be sufficient.  The impeller power will be about 1.8 kW, so a 3.75 kW motor is recommended.
 
If the shaft does not have a bottom support, it needs to be 89 mm in diameter.  If it has a bottom support, the shaft diameter can be reduced to 51 mm.