A lance supporting device has a lance turntable rotatable within a plane and carrying a plurality of lance operation units each supporting a lance for inserting a gas and/or a powdered treating agent into a molten metal. Each lance operation unit has means for lifting/lowering, rotating and tilting the lance. The lance turntable carrying the plurality of lance operation units is disposed in the central hollow or cavity of a doughnut-shaped turntable which carries a plurality of vacuum degassing vessels.
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