
133Melting Furnace Fundamentals
The combination of these requirements eliminates most of the available refrac-
tory compositions. Of the remaining choices, those which have by far become
the most popular for melting and holding furnaces are refractories based on the
alumina–silica system. Figure 7.8 illustrates the system, which features the ter-
nary compound mullite (Al
6
Si
2
O
13
) in addition to pure corundum (Al
2
O
3
) and silica
(SiO
2
). The refractories used in contact with molten metal and dross in aluminum
melting and holding furnaces have a high alumina content (60%–85% Al
2
O
3
) and
typically consist mostly of a mixture of corundum and mullite