Abstract: A vacuum/reduced pressure refining vessel (1) is connected, by an upstream duct (5) to a dry dust collector (3) that uses a filter (2). The dry dust collector (3) is connected to a reduced pressure evacuating apparatus (4) by a downstream duct (6), and a block valve (7) is provided in the upstream duct (5). At the start of a vacuum/reduced pressure refining treatment, a non-oxidizing gas is injected into the upstream duct at a position that is upstream of the block valve (7) to substantially replace the oxygen in the upstream duct and to hermetically close the upstream duct. After the hermetically closed state has been established in the upstream duct, the block valve (7) is opened and operation of the dry dust collector (3) is started.

Abstract: A refining furnace used for vacuum refining has a sealing cover mounted on the refining furnace so as to cover the furnace throat of the refining furnace and to be in close contact with a sealing flange (10) formed on the refining furnace. The adhesion of metal and slag to the sealing flange is prevented without using any flange cover. A sealing flange (10) is formed on the outer surface of the refining furnace (1) at a position on a conical part (8) extending between the furnace throat (9) and a straight body part or on the straight body part. A slag-stopping dummy flange (11) is formed on the outer surface of the refining furnace (1) at a position between a furnace throat (9) and the sealing flange (10). The slag-stopping dummy flange (11) has an outer circumference lying on the inner side of the inner circumference of a sealing cover (4) and on the outer side of the inner circumference of a lower end part of a dust collecting hood for atmospheric refining.