A rotary device and methods for treating molten metal, a tubular sleeve for said rotary device and the use of said rotary device in the treatment of molten metal. The rotary device comprises: a tubular sleeve comprising a rotor head at one end, the rotor head comprising a gas outlet for dispersing gas into molten metal; and a hollow shaft extending inside the tubular sleeve such that at least a portion of the hollow shaft is enclosed by the tubular sleeve, wherein the hollow shaft is fluidly connected to the gas outlet of the rotor head, the tubular sleeve is formed from a refractory material that is resistant to corrosion and thermal shock, and the hollow shaft is formed from a material comprising graphite. A first method comprises: applying a layer of synthetic slag material onto an exposed surface of the molten metal; and stirring the molten metal using a rotary device comprising a rotor head, such that the molten metal flows past the layer of synthetic slag material. A second method comprises: applying a metal treatment agent to molten metal; stirring the molten metal using a rotary device comprising a rotor head; and discharging gas into the molten metal through the rotor head.