Electric melters

For various reasons, including financial ones, the United States was slower than Japan and Europe to install continuous slab casters for the production of sheet. Electric melters cast billets continuously from about 1975 onward. Casting was done cmdely at first but the sophistication increased rapidly, culminating in the operation of a thin-slab caster at one of Nucor s plants. This opened up another avenue of attack on sheet markets, once the province of integrated mills. 

The refractory tends to degrade much faster in these furnaces, resulting in very short furnace campaigns, typically less than 2 years. Most of these furnaces are less than 40 ton (361) of glass per day however, furnaces as large as 200 ton (1801) per day have been built.6 A typical electric melter is shown in Figure 7.2. 

In all electric melters, the application of electrodes is varied dependent on the furnace construction and type of glass. Bottom, side and even over the top electrodes are in use as combinations. 

It should be noted that the electrical resistivity of apUe of nonmelted batch, or even a glass melt at temperatures well below 1000°C, is too great to allow for efficient heat generation. Consequently, aU-electric melters are started in a more or less traditional way by fossil-fuel burners and a hot crown. Once the molten glass has reached sufficient temperature, the burners and sometimes the crown are removed. 

Plasma vitrification systems can heat the waste in one of two ways either as a nontransferred arc or as a transferred arc. A nontransferred arc uses two internal electrodes. A small column of injected gas is heated by the electric arc, creating a plasma flow that extends beyond the tip of the torch. Nontransferred arcs heat by conduction and produce a dispersed heat that heats both the waste and the gas around the waste. Nontransferred arc melters can operate as in situ or ex situ processes (D11008N, pp. 3-7, 3-11). 

Manufacture of Sodium Azide was conducted at the Kankakee Ordnance Works, Joliet, Illinois, (Ref 144) by the "liquid phase process as follows For this five 12-lb bricks of sodium were melted in an electrically heated melter and the molten Na at 350°F (176.7°) dropped to a highrpressure autoclave contg 375 lbs liq ammonia and 1 lb ferric nitrate catalyst. The Na reacted to form Na amide and hydrogen ... 

Crystal glasses were traditionally melted in pot furnaces, but nowadays small continuous tank furnaces are used. Lead glasses are conveniently melted in Unit-Melter furnaces, lead-free glasses in all-electric furnaces with a daily output of several tons (cf. Fig. 102). Machine forming is being gradually introduced even for these types of glass. 

A special type of furnace is represented by the so-called mixed-melter in which roughly one half of the energy is supplied by fuel combustion and one half as electric power. 

Virtually all segments of the glass industry have now implemented 100% oxy/fuel furnace technology. Table 7.1 summarizes the completed conversions in North America. The table includes mixed melters, so-called because they get a large portion of their total melting energy from electricity. 

Joule heating is practically a requirement when a ceramic melter is employed. This means dissipating electrical energy in the molten glass between immersed electrodes. Joule heating has been shown to be feasible with sufficiently refractory electrode material such as molybdenum or even tin oxide. An auxiliary heating system has to be provided for initial start-up and for restarting. 

Pioro, L. and Pioro, 1., 2004. High Efficiency Combined Aggregate - Submerged Combustion Melter-Electric Furnace for Vitrification of High-Level Radioactive Wastes, Proc. 12 Int. Conf on Nuclear Eng. (lCONE-12), Washington, D.C., USA, April 25-29, Paper 49298,4 pages. 

Specially designed polyamides with high melt points, but very low modulus, serve to encapsulate electrical connectors. This type of polyamide is also being foamed with a Nordson Foam Melter for a gasket in microwave ovens. 

---