A mounting enclosure and an improved mounting arrangement for apparatus used in metal melting, refining and processing, particularly those apparatus adapted for steel making in an electric arc furnace, such as burners, lances and the like with supersonic oxygen lancing capability and injectors or the like for the introduction of particulate matter. The mounting enclosure is fluid cooled to survive the hostile environment of the electric arc furnace and is designed to occupy the step between the side wall and hearth of the furnace without any substantial change to the structure of the furnace. The mounting enclosure comprises a plurality of fluid cooling conduits surrounding an apparatus aperture and an injector aperture which are formed through the enclosure and adapted to mount an apparatus and an injector. The mounting arrangement includes utilizing the mounting enclosure to mount an apparatus with supersonic oxidizing gas lancing capability and an injector for particulate carbon in an electric arc furnace. Because the mounting enclosure is approximately the width of the step, the discharge openings of the apparatus and the injector are moved closer to the surface of the melt and toward the center of the furnace thereby providing increased efficiency. The discharge openings will now be extended to near the edge of the step so that the oxidizing gas flow pattern of the apparatus does not degrade the hearth material or other furnace equipment mounted nearby and the carbon flow pattern of the injector sufficiently agitates the slag to produce foaming.