A rotary device for dispersing gas in molten metal comprises a hollow shaft and a hollow rotor attached to the shaft, the rotor having a plurality of vanes extending from the shaft towards the periphery of the rotor and dividing the rotor into a plurality of compartments, each compartment having an inlet adjacent the shaft and an outlet adjacent the periphery of the rotor, and the rotor having means for passing gas from the discharge end of the shaft into the compartments, wherein the discharge end of the shaft opens into a manifold in the rotor and the inlets for the compartments are present in the wall of the manifold. When the device is rotated in molten metal contained in a vessel and gas is passed down the shaft, metal is drawn into the manifold and breaks up the gas stream emerging from the shaft into very small bubbles. The gas/metal dispersion flows into the compartments through the inlets and out through the peripheral outlets and the gas is dispersed through the whole body of molten metal.