Lining is also the designation for the melting crucible or wear crucible without the concrete rings or fixed ceramic components. Nowadays dry tamping compounds are generally used. Damp compounds are sometimes still used in aluminium and zinc furnaces. There are 3 different types of material: acidic, basic and neutral compounds. Mixtures of the 3 above types of material are also used for certain applications.
Acidic compounds contain approx. 98.5% SiO2
Neutral compounds contain approx. 84.5% Al2O3 and up to 13% MgO
Basic magnetic compounds contain approx. 88.0% MgO, up to 10% Al2O3 and approx. 2% SiO2. The application temperatures of acidic compounds are normally in the area of 1,600 °C and can reach up to a maximum of 1,700 °C for short periods.
For neutral compounds, the normal application temperatures are in the area of 1,650 °C and can reach up to a maximum of 1,750 °C for short periods.
Basic magnetic compounds have a normal application temperature of 1,650 °C and a maximum of 1,800 °C for short periods.
In a 1-t crucible furnace with 1,000 kW for example, the maximum temperature can be reached within 2 minutes, after which the furnace must be emptied immediately and re-charged with scrap. The furnace is now brought up to temperature for approx. 3 minutes at high output, so that the crucible is not cooled down too much from the extremely high temperature required. With this procedure, charge numbers of from 40 to 70 can be achieved, depending on the method of operation, compound and charge materials.
Dry compounds are usually supplied in 25 kg sacks or disposable containers weighing up to 1,600 kg. The compounds are supplied ready for use with sintering agents, and do not need to be mixed. Acidic compounds containing boric anhydride contain no crystalline water, so that these can be used with temperature increases of up to 150 K/h. High-alumina and magnesitic compounds are specially mixed and supplied by manufacturers for individual applications.