The molten metal IS the lubrication for the impeller and base-volute. Running the pump dry will result in damage. The only question is how much damage. This impeller is an example of a very costly mistake.
A broken impeller is probably the best possible outcome of running a pump dry. Most likely, the damage would be more expensive. In the worst case event, the only salvageable components are the motor mount, motor, and shaft coupling.
RUNNING A PUMP DRY VOIDS THE WARRANTY.
When troubleshooting motor problems, remove the shaft and impeller to avoid damage. Once the motor is operating properly according to your pump manual and you have verified the motor is turning in the correct direction; then reinstall the shaft and impeller.
Verify the shaft and impeller turn freely by hand BEFORE THE PUMP IS PREHEATED OR PLACED IN THE MOLTEN METAL.
Once fully preheated and introduced into the molten metal following the procedure found in the manual, again verify the shaft and impeller turn freely by manual during the shaft. ALWAY USE PROPER PROTECTIVE SAFETY GEAR FOR THIS PROCEDURE. Once you have determined shaft and impeller turn freely, the pump is ready to be energized.
NEVER TURN THE SHAFT COUNTER CLOCKWISE. THIS MAY LOOSEN OR UNTHREAD THE IMPELLER FROM THE SHAFT. WITHOUT THE IMPELLER, THE SHAFT IS UNBALANCED AND WILL BREAK. A BROKEN SHAFT MAY ALSO DAMAGE THE POSTS AND BASE.