An electric induction furnace is used in several gold mining operations. In the operation of the furnace, a high frequency electric current is passed through the primary coil, inducing a secondary current in the charge, heating it by resistance to the desired temperature. The charge may consist either of single of zinc precipitates, gold cathodes or a quantity of loose pieces such as scrap. Even comparatively fine turnings may be melted successfully if a moderate amount of heavier scrap is used. While high frequency heating is effective over a wide range, about 1,000 cycles have been found to be the most practical for one tone furnace. Smaller furnaces need higher frequencies.
Basically, the heat is developed in the outer part of the charge and is quickly carried to the center by conduction, which is rapid through solid metals. After the charge starts to melt and a pool is formed in the bottom of the furnace, a stirring effect occurs. This not only carries heat to the center of the charge, but accelerates melting by washing molten metal against the unmelted solid metal. It also mixes the charge thoroughly assuring uniformity. The flow lines in the molten bath assure the absence of dead spots and that every part of the bath is moved. The vigor of the stirring can be adjusted by varying the power input.
The high frequency induction furnace is basically an air transformer in which the primary is a coil of water cooled copper tubing and the secondary is the mass of metal to be melted. The induction comprises an outer shell made of a resistant material and carries the trunnions on which the furnace pivots in tapping or pouring. The coil consisting of a helix of water cooled copper tubing is lined with a layer of refractory material that forms a protective coating against metal leaks. This layer is continued above and below the coil against the support, the firebrick top and base. The coil lining and the firebrick bottom provide a cavity into which the refractory lining of the furnace is built. This lining may take the form of a thin wall crucible or supporting shell packed into the cavity with granular refractory or it may be in the form of a sintered lining that holds the charge or bath of metal and is molded at the top on one side to form the pouring spout. Due to peculiarities in construction of the furnace in which the primary coil is fairly close to the metal bath, the selection of suitable refractories is an important consideration.
Cross section of an electric induction furnace