Abstract: The present invention concerns apparatus for treating a bath of liquid metal and in particular aluminum or alloys thereof by injecting gas, comprising a rotary agitator, the lower end of which, in the rest condition, rests on a gas injection plug disposed at the bottom of the bath and which, under the action of the pressure of the gas emitted by the plug, rises and, thus being supported by a fluid bearing, may, under the effect of an external couple, rotate freely about its axis and allow a multitude of regularly dispersed gas bubbles to escape through the space which separates the agitator from the plug, enabling intimate contact between the gas and the liquid metal and thus to improve the efficiency of the treatment. It may be used in treating metals from which hydrogen and non-metallic impurities are to be removed.