The present invention relates to an apparatus for injecting gas into a vessel. The apparatus may include a gas flow duct and a central body within a forward end region of the duct. The central body and the gas flow duct form an annular nozzle for the discharge of gas from the duct. A plurality of flow directing vanes are disposed about the central body to impart swirl to a gas flow through the nozzle. The flow directing vanes have substantially straight leading end portions radiating outwardly from the central body and extending along the duct. The vanes also have substantially helical trailing end portions extending helically about the central body toward the front end of the duct and transition portions joining the leading end portions to the trailing end portions. The transition portions are shaped so as to merge smoothly with both the leading end portions and the trailing end portions and to smoothly and progressively change shape between them.