Slag from a pool of molten slag (34) floating on molten metal in a plasma arc treatment chamber (36) is removed by providing a slag container (38) located inside the chamber which has a slag intake pipe (40) extending from a bottom of the container (38) downwardly into the slag pool. A vacuum line (48) is connected to the top of the container and a vacuum source (46). When a vacuum is applied to the inside of the container, the pressure prevailing in the chamber pushes the slag from the pool through the intake pipe into the container. When the container becomes filled with molten slag, the latter enters a section (44) of the line communicating the top of the container with the vacuum source. As the slag enters the pipe section, it is cooled and solidified to prevent further slag flow into or gas/air flow through the vacuum line so that the container can be lifted out of the chamber while the slag in the container is still in its molten state without causing slag spillage.