GB1473091A - - Google Patents

Abstract

1473091 Internal resistance heating ELEMELT Ltd 14 March 1974 [4 Jan 1973] 486/73 Heading H5H A furnace for melting materials which are only conducting when molten, e.g. glass, alumina, sodium silicate or magnesium chloride comprises a furnace chamber having a bottom wall 12 and a peripheral wall 17, an inner electrode arrangement at or near the centre of the chamber and an outer electrode arrangement at or near the peripheral wall 17, an A.C. supply connected to the inner and outer electrode arrangements in such a way that current flows radially between the inner and outer electrode arrangements and the establishment of peripheral voltage gradients in the peripheral wall 17 is avoided, and means for diverting current from the bottom wall of the chamber. Such an arrangement permits the peripheral wall 17 to be formed of or lined with a material, e.g. 96% chromic ocide, 4% titanium oxide, which although chemically non-reactive, has too high an electrical conductivity to be used normally. As shown the outer electrode arrangement comprises angularly spaced electrodes 19a-19i and the inner electrode arrangement comprises a concentric array of electrodes 20a-20i. To divert current from the bottom wall 12 insulating blocks 21 which shroud the lower ends of the electrodes are provided. The blocks 21 are made of zirconium orthosilicate. The blocks cause the formation of a stagnant layer of glass in contact with the bottom wall and thus allow it to be formed of or lined with chromic oxide material. Alternatively, the diversion of current to the bottom wall may be prevented by a relatively poorly conducting lining on the bottom wall. Either or both electrode arrangements may be in the form of cylindrical electrodes, the outer cylindrical electrode forming the wall of the chamber and being composed of the chromic oxide material and the inner cylindrical electrode, also being formed of chromic oxide and forming the inner wall of an annular chamber. An intermediate concentric array of electrodes may be interposed between the inner and outer electrode arrays to enable the outer portion of the chamber, which is subjected to the greater heat loss, to be heated more than the inner portion of the chamber. An outlet for glass may be provided in the centre of the bottom wall or may extend radially between the electrode arrays. When a central outlet is provided it may be formed as a duct having an adjustable crosssection for forming fibres. The duct may be heated by current between an annular electrode about the duct exit and the inner electrode array. Current regulators may control the current between the inner and outer electrode arrays. Reference has been directed by the Comptroller to Specification 1,177,070.