GB1386611A - Protection of graphite electrodes - Google Patents

Abstract

1386611 Coated electrodes FOSECO INTERNATIONAL Ltd 27 Nov 1972 [26 Nov 1971 19 July 1972] 55128/71 and 33839/72 Heading B2E [Also in Divisions C1 C7 and H5] A graphite arc furnace electrode which is supplied with electric current for heating has a protective coating over its entire surface except that which is intended to make electrical contact with the electrode clamp. The coating comprises 15-90% by weight of a matrix material having a melting point below 1000‹C and which is a graphite melting material, and 10-85% by weight of a refractory filler. In an arm furnace in which an electrode is supplied with current through an electrode clamp and fed into the furnace, the portion of the electrode in contact with the clamp is uncoated. The coating may be applied as the electrode is being lowered into the furnace or alternatively it may be coated, except for the surface to be engaged by the clamp, away from the furnace. The coating may be applied automatically on raising or lowering the electrode through a spray ring or a system of individual jets between the electrode clamp and the furnace chamber. The coating may be applied as a powder onto a hot electrode or a suspension on to a hot or cold electrode by spraying, brushing, dipping, rolling, or smearing. The coating application may be assisted electrostatically. The electrode may be heated in the furnace or may be heated before or after application of the coating by, flame, induction, microwave or infra red heating. The coating comprises the matrix having a melting point below 1000‹C and a refractory filler and which may be suspended and dissolved in a liquid carrier e.g. water. After heating the matrix fuses so that in use it forms with the filler a liquid or plastic impermeable layer. Matrices may be boric oxide, metaboric acid, boric acid, salts of these acids e.g. sodium borate, or vanadium pentoxide. Alternatively the matrices may be combinations of glaze forming materials e.g. phosphates, fluorides, silicates e.g. alkali metal phosphates, aluminium ortho phosphate, alkali metal silicates, glass, calcium fluoride, sodium aluminium fluoride, sodium borofluoride. Combinations of matrix materials may be used. Chrome ore may be added to certain of these materials to improve wetting of the graphite. Fillers may be refractory oxides, carbides, nitrides or borides such as chromic oxide, zirconium oxide, titanium oxide, silicon carbide, zirconium carbide, tungsten carbide, boron nitride, titanium boride or zirconium boride. 80% of the filler particles are less than 0.5mm preferably less than 0.2mm and more preferably 0.06mm. Small percentages e.g. less than 10% of refractory fibrous material e.g. asbestos lagwool, glass fibre or aluminosilicate fibre may be incorporated in the coating. More than one layer of coating material may be applied and a referred furnace electrode may be removed at intervals to have a further coating applied.