Electrical resistance furnace

There are large-scale operations using direct-heat resistance furnaces. These are mainly in melting bulk materials where the Hquid material serves as a uniform resistor. The material is contained in a cmcible of fixed dimensions which, coupled with a given resistivity of the material, fixes the total resistance within reasonable limits. The most common appHcation for this type of direct-heat electric resistance furnace is the melting of glass (qv) and arc furnaces for the melting of steel (qv). 

Vitreous silica is used for gas-heated or electrically heated devices ia various shapes, eg, as a tube or muffle because of its electrical resistivity, impermeabihty, and low expansion. In its simplest form, an electric-resistance furnace consists of a vitreous siUca tube or pipe on which the resistance element is wound (see Furnaces, ELECTRIC). Because of its iadifference to temperature gradients, a tubular furnace of vitreous siUca maybe made to operate at different temperatures at various portions of the tube, either by arrangement of the heating elements or by cooling sections of the tube with water. Vitreous siUca pipes may be employed ia vacuum-iaduction and gas-fired furnaces (see Vacuum technology) (221). 

Ca.rhothermic Reduction. Sihcon carbide is commercially produced by the electrochemical reaction of high grade siUca sand (quartz) and carbon in an electric resistance furnace. The carbon is in the form of petroleum coke or anthracite coal. The overall reaction is... 

Apart from use in metallurgical research and measurements, solid electrolytes have also been put to use as heating elements in electrical resistance furnaces. In order to prevent electrolysis from occurring, alternating currents must be used. In contrast to metallic heating elements, they may be used in air at around 2000 °C. In view of the fact that their conductivity must be very low at room temperature, there is a need for them either to be kept continuously warm, or to be preheated with an auxiliary heating element. 

Remembering that the temperature of the caustic soda solution used in the silicol process is above 100" C., frequently rising to 120° C., it was thought that a higher temperature might perhaps produce the suspected reaction ferro-silicon was accordingly heated in an atmosphere of steam in an electric resistance furnace to a temperature of 300° C., but still no hydrogen was produced. Consequently it was concluded that the explanation of the smaller consumption of caustic soda than would be anticipated from theoretical considerations must be explained on some basis other than the reaction of silicon with water. 

Geotech Development Corporation offers a proprietary Cold Top ex situ vitrification process for the treatment of contaminated soil. The system melts the soil using an electric resistance furnace that can operate at temperatures of up to 5200°F. The vendor claims that wastes are transformed into an essentially monolithic, vitrified mass. The process is termed cold top vitrification because soil is added to the top of the melt to act as an insulator and to minimize the loss of volatile metals into the off-gas treatment system. The technology has been evaluated in a pilot-scale facility and is commercially available. 

A heliarc-welded, all-nickel can of 850-mL volume was filled with an intimate mixture of NiF, (290 g, 3 mol) and anhyd KF (52 g, 9 mol). The can was valved to a tank of F2 gas and a vacuum pump and was heated by an electric-resistance furnace. The can was heated slowly to 500 C under 10 atm of F2 and then cooled to 250 C, while still under several atm of F2. Several such cycles were carried out before using the device for the regeneration of F2. For the regeneration of F2, the salt was fluorinated at 250 C until no more F2 was taken up. The can was then cooled to 225 C and evacuated to remove the excess F2 and any volatile impurities. The temperature was then raised until the desired F, pressure (at 400 C, 25 atm) was achieved. 

FURNACES,ELECTRIC - RESISTANCE FURNACES] (Vol 12) -nylon m [POLYAMIDES - FIBERS] (Vol 19)... 

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