Continuously-operated melting

Fig. 4.12. Schematic representation of the structure of a continuously operated melting unit...

The term channel induction furnace is appHed to those in which the energy for the process is produced in a channel of molten metal that forms the secondary circuit of an iron core transformer. The primary circuit consists of a copper cod which also encircles the core. This arrangement is quite similar to that used in a utdity transformer. Metal is heated within the loop by the passage of electric current and circulates to the hearth above to overcome the thermal losses of the furnace and provide power to melt additional metal as it is added. Figure 9 illustrates the simplest configuration of a single-channel induction melting furnace. Multiple inductors are also used for appHcations where additional power is required or increased rehabdity is necessary for continuous operation (11). 

Continuous-zone-melting apparatus has been described by Pfann (op. cit., p. 171). This technique offers the advantage of a close approach to the ultimate distribution, which is usually impractical for batch operation. 

Bonded silver linings are fabricated for mild steel or copper vessels. They are soldered in situ to the walls of the vessel by means of a special tin-silver solder. The melting point of this solder is approximately 280°C, and 200°C is recommended as the maximum continuous operating temperature for linings bonded with it. Since the whole of the silver is firmly adherent to the vessel, bonded linings are suitable for operation under vacuum conditions, and provide excellent heat-transfer characteristics. 

The horizontal continuous Brodie melt crystallizer is basically an indirectly cooled crystallizer with an internal ribbon conveyor to transport crystals countercurrent to the liquid and a vertical purifier for final refining. Figure 20-8 describes the operation of a single tube unit and Fig. 20-13 depicts a multitube unit. The multitube design has been successfully commercialized for a number of organic chemicals. The Brodie purifier configuration requires careful control of process and equipment temperature differences to eliminate internal encrustations and is limited by the inherent equipment geometry to capacities of less than 15,000 tons per year per module. 

Intermittent accumulator EBM machines use a normal conventional axially fixed continuously operating extruder to prepare the melt. The accumulator is a heated reservoir where the melt is temporarily stored in the intervals between parison extrusion (similar action of a two-stage injection molding machine (IMM) as reviewed in Chapter 4. Also used is a conventional reciprocating IMM that delivers melt through a die into a blow mold. 

Carbothermal reaction requires temperatures in excess of 2000°C. After ca. 1 to 2 h the continuously operating furnace is tapped with an auxiliary electrode. The liquid silicon (m.p. 1413°C) is collected in pans or ingot molds. If necessary, further metallurgical processes such as slag extraction or air blasting with reactive gases can be carried out to rid the silicon melt of included metallic and non-metallic impurities, before the silicon solidifies into brittle blocks. 

Fig. 5.1-2. Oil-heated, continuously operated Siemens-melter. a) coaling b) burner with atomizer c) melting zone d) burner block e) outlet f) refining zone g) regenerators h) lining with refractory materials i) gas reversal mechanism j) chimney valve k) air inlet flue gas chimney...

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