In a plant producing magnesium pellets and fines from ingots, an explosion in the fines transfer screw originating at the sieving station spread to the storage container, causing the opening of a roof vent on a box housing this container as well as on a vent protecting the adjoining room. The fire broke out on the roof and was able to spread, despite the presence of fire walls in the adjacent box, following the transfer of ignited products via the 2 open vents. External first responders, notified by neighbours, and company personnel quickly controlled the outbreak. The explosion had occurred outside of the operating period for fines production as a consequence of the accidental start-up of this Archimedes screw by an employee. The screw break that caused friction on the sidewalls and thus sparks in the transfer duct not yet saturated in magnesium triggered this explosion. No injuries were reported. Following the accident, the operator installed a speed regulator on the screw to make a soft start. The inspection authorities for classified facilities requested that the operator examine solutions to prevent a fire from spreading via the explosion vents intended to protect the various premises.