When casting aluminium, furnaces are needed for melting the charge (to a casting temperature of 680 to 750 °C) and making it up, for holding the melt at temperature and for the casting operation itself. Making up a charge means preparing the type and quantity of the raw materials to be charged to the furnace, for example solid or molten primary and secondary aluminium alloys. Alloying elements, such as copper or manganese, are added in pure form or as master alloys.
As even small quantities of water in the charge materials can cause violent explosions if they come into contact with molten aluminium (the aluminium combines with the oxygen of the water with release of large quantities of heat), all additions to the melt have to be dry in the interests of industrial safety and the prevention of accidents.
Special types of melting furnace
Different types of furnace are used depending on the aim and the method of heating. A common feature is that they are all refractory lined.
- Crucible furnaces with a tall crucible, narrow at the top, made from either a clay"graphite mixture, silicon carbide or cast iron (an inner coating prevents dissolution of the iron by the aluminium) are used for melting, holding at temperature or casting. They are often tiltable and fitted with a cover, contain up to 800 kilograms of the melt (molten metal) and are heated all the way round by gas or oil flames or via an electrical resistance coil.
- Hearth furnaces are flat tanks (hence the name "tank furnaces" as well), often tiltable, for example for continuous casting. The large surface area of the molten bath is heated by a flame from above, allows a large throughput, but also causes a lot of oxidation.
- In induction furnaces, the metal to be melted is heated by eddy currents, which are induced in it by an alternating current on the outside. The eddy currents also result in good mixing. The crucible (with a capacity of up to 12 tonnes for melting, and more than 20 tonnes for holding at temperature) ensures oxidation is low (thanks to a small melt surface area in relation to the melt volume). The melting rate is high.
- Cylindrical rotary drum furnaces rotate slowly about their horizontal axis, have a capacity of up to 20 tonnes and are used above all for remelting scrap because it is possible to use fluxes in them to minimise oxidation and absorb impurities as well as to purify the melt.
Melting a charge takes some four to eight hours depending on the furnace output. A layer of aluminium oxide always forms on the surface of the molten metal as a result of the reaction of the melt with the oxygen in the air, a melting loss that can result in a loss of between one and several per cent of the metal. Together with the impurities from the melt purification, so-called "dross" is formed and this is sent for reprocessing.
Lower energy requirement
In order to heat up a kilogram of aluminium from room temperature to a casting temperature of 750 °C one requires theoretically 1.1 megajoules of thermal energy (which is equivalent to 0.3 kilowatt-hours). Thanks to heat recovery, modern hearth furnaces are the most economical but at best only use 45 per cent of the energy in the oil or the gas so that the actual energy requirement for melting is about 2.8 megajoules (0.7 kilowatt-hours) per kilogram. Transporting the melt to the foundry by road also contributes to energy saving.