Practical Aspects of the Electric Melting of Glass
JSGT 1950 V34 T238-T253
The amount of current which passes through glass rises progressively with rise of temperature producing an effect described as self-deregulating. This effect increases with the steepness of the resistivity-temperature curve and the margin for regulation of the current has to be progressively wider the steeper the curve. The conditions for melting glass entirely by electricity are favourable in the case of commercial soda–lime–silica glasses for which the tangent to the resistivity–temperature curve is small (7° 20' at 1450° for the glass SiO2 71·6, CaO+MgO 12·7, Na2O 15·0%). For silicate of soda with a still smaller tangent, the current regulation during melting is easy; on the other hand the curves for borosilicate and lead glasses are much steeper. Experience has suggested that glasses for which the angle is at least 2·5 times that of the 15% Na2O soda–lime–silica glass should preferably be melted by a combination of fuel and electric heating. By choosing the position in the bath and the diameter of the electrodes the distribution of the temperature in the glass can be controlled and oxidation of the electrodes (particularly those of graphite) can be prevented. Flame heating and submerged electrode heating produce different temperature gradients in the glass. The conditions governing the location of electrodes in full electric melting and in combined fuel and electric melting are discussed. The calculation of the energy required to operate an electric melting furnace is described. A table of data of the thermal characteristics of various refractory and insulating materials makes it possible to determine the heat required for the furnace structure. The heat absorbed in the glass from the stage of introducing the batch to its removal as sheet (by Fourcault machines) is shown to be approximately 0·7 kWh/kg of glass, or 0·6 kWh. if 30% cullet is used. In practice, the overall consumption in each of two furnaces with tanks of Corhart blocks and two Fourcault machines per furnace to work out the glass is 1·8 to 2·0 kWh/kg of glass, the power input being approximately 2000 kW, the voltage between phases and the furnace output 25–28 tons per 24 h. Applications of electric melting to small, single-phase current furnaces, and to large furnaces in which the melting is carried out in a large chamber and the refining in one of a series of cells run at a high temperature are described.
Edouard V. Borel
