The invention relates to a rotary device for dispersing a gas in a molten metal. The device comprises a hollow shaft at one end of which is attached a rotor. The rotor has a roof and a base which are spaced apart and connected by a plurality of vanes. A compartment is defined between each adjacent pair of vanes and the roof and the base, and each compartment has an inlet and first and second outlets. A flow path is defined through the shaft into the inlets of the compartments and out of the first and second outlets. Each first outlet is disposed radially outwardly of the respective inlet and arranged to disperse gas laterally of the rotor in use, and each second outlet is disposed in the roof of the rotor and arranged to disperse gas upwardly from the rotor in use.