Patent GB-1413508-A (GB)
Title: Secondary refining process
IPC: B22D23/10
Filed: 1972-08-25
Abstract: 1413508 Electroslag melting BRITISH STEEL CORP 23 Aug 1973 [25 Aug 1972] 39829/72 Heading H5M [Also in Division B3] In an electroslag refining process, metal to be refined is introduced into the slag bath in the mould, and a non-consumable current conductor passes current to the slag to establish a potential difference between the non-consumable conductor and both the metal pool beneath the slag bath and the metal to be refined in the slag bath, the metal pool and the metal to be refined both being maintained permanently of negative polarity with respect to the non-consumable conductor. In Fig. 1, the non-consumable current conductor comprises a non-consumable electrode 4 immersed in the slag bath 10 and made of a material having a melting point above 2000‹ C. and capable of withstanding chemical attack from compounds, e.g. fluorides in the slag. The electrode 4 may be made of iridium, zirconium boride, vitreous carbon, refractory oxide, a cermet, or a core of graphite coated with one of the above materials. Preferably, however, the non-consumable electrode is a water cooled copper electrode. The metal to be refined is in the form of a consumable electrode 3 (of iron or steel) introduced into the slag bath 10. The electrode 3 and mould base plate 2 are connected to the negative pole of a D.C. source whilst the non-consumable electrode 4 is connected to the positive pole. In operation the slag bath 10 is heated by current through it between electrodes 4,3 and between non-consumable electrode 4 and refined metal in pool 11. The applied voltage across the interface of the slag bath 10 and molten pool 11 is sufficient to ensure cathodic protection of both the electrode 3 and metal pool 11, thus eliminating electrochemical oxidation and minimizing inclusions of oxygen, sulphur, hydrogen and carbon in the pool 11 and growing ingot 12. These inclusions migrate to the non-consumable electrode 4 where they accumulate or leave the slag bath in gaseous form. The consumable electrode 3 is lowered and the non-consumable electrode 4 is raised as melting progresses. The distance between the tip of the consumable electrode 3 and molten pool 11 is maintained constant by maintaining a potential difference between electrode 3 and pool 11 and measuring the voltage across the slag bath, by determining the position of the electrode 3 relative to the pool 11 by a radioactive source, or by load cells which weigh the ingot being produced and initiate a consequent movement of the electrode 3. More than one non-consumable electrode may be provided. The non-consumable rod electrode 4 may be replaced by an annular electrode or a ring of electrodes (Fig. 2, not shown), or one or more non-consumable electrodes may be set in and insulated from the wall of the mould at the level of the slag bath (Fig. 3, not shown). In Fig. 4 (not shown). The wall of the mould acts as the non-consumable current conductor, short circuiting being prevented by a frozen slag layer formed between the mould and the growing ingot. In Fig. 5 (not shown) the non- consumable current conductor is formed by a plasma torch directed on to the slug surface, whereas in Fig. 8 (not shown) it is formed by the upper part of the mould, the lower part being separated from it by electrical insulation. In Figs. 6 and 7 a metal coating is applied to an arbor which is mounted centrally in the mould and connected electrically to the negative pole of the D.C. source. A ring of consumable electrodes interdigitated with the non-consumable electrodes is also connected to the negative pole. In any of the above embodiments, the metal to be refined may be supplied in the molten state through an aperture in the side wall of the mould level with the slag bath, instead of by means of consumable electrodes.