Induction furnaces have become the most widely used furnaces for melting iron and, increasingly, for non-ferrous alloys. These furnaces have excellent metallurgical control and are relatively pollution free (in comparison to cupola furnaces). The two most common induction furnaces are the coreless furnace and the channel furnace.

The basic principle of induction melting furnaces is that a high voltage in the primary coil induces a low-voltage, high current across the metal charge which acts as a secondary coil. Because of electrical resistance in the metal this electrical energy is converted into heat which melts the charge (Metal Asia, 1999c). Once the metal is in its molten state the magnetic field produces a stirring motion. The power and frequency applied determine the stirring rate. This is controlled to ensure complete melting of the charge and adequate mixing of alloy and fluxing materials, and to minimise temperature gradients in the charge. Excessive stirring, on the other hand, can increase lining damage, increase oxidation of the alloys, generate excess slag and increase inclusions and gas pick-up.

In a coreless furnace, the refractory-lined crucible is completely surrounded by a water-cooled copper coil. This prevents the primary coil from overheating. In channel furnaces, the coil surrounds an inductor. Induction furnaces are available in capacities from a few kilograms to 75 tonnes. Coreless induction furnaces are more typically in the range of 5 tonnes to 10 tonnes. Some large channel units have a capacity of over 200 tonnes. Channel induction furnaces are also commonly used as holding furnaces.

Induction furnaces are very efficient and are available in different sizes. They are able to melt a wide range of metals but little refining of the metal is possible. Induction furnaces require much cleaner scrap than cupola furnaces and somewhat cleaner scrap than electric arc furnaces. The capital costs are higher than those of electric arc furnaces but the operating costs are lower due to reduced refractory wear. Other advantages of induction furnaces are that they are relatively simple, very small quantities of any metal composition can be melted and the melting time is relatively short - around 1 hour - allowing metal to be delivered at small, regular intervals.

Approximately 60% of the energy supplied to the furnace is transferred to the charge. Around 30% of the energy is lost to the cooling water, an additional 7% lost from radiation and convection losses, and the remainder is lost in the furnace's electrical system.

Energy consumption can be as low as 550 kW.h/tonne but these figures are achieved only with high utilisation factors and for higher-frequency furnaces. Figures of around 650-750 kW.h/tonne are more typical (Jain, 1986). In comparing the overall efficiency of these systems with that of fuelbased furnaces, it should be remembered that the electricity has to be generated and even modern power stations do not reach a 40% efficiency, which means the overall fuel consumption is well over 2000 kW.h/ tonne.