Crucible furnaces
These
furnaces melt the metal without direct contact with a burning fuel
mixture. For this reason, they are sometimes called indirect fuel-fired
furnaces.
Crucible furnaces are mostly used for
melting non ferrous metals and alloys. The metal is melted in a crucible
which is a refractory vessel made of silicon carbide, Graphite or some
other refractory material.
Three types of crucible furnace are:
1. lift out type
2. Stationary
3. Tilting
They
all utilize a container (the crucible) made out of a suitable
refractory material (for example, a clay-graphite mixture) or
high-temperature steel alloy to hold the charge. In the lift-out
crucible furnace, the crucible is placed in a furnace and heated
sufficiently to melt the metal charge.
Typical Fuels are:
· Oil,
· Gas and
· Powered coal.
When
the metal is melted, the crucible is lifted out of the furnace and used
as a pouring ladle. The other two types, sometimes referred to as pot
furnaces, have the heating furnace and containers as one integral unit.
In
the stationary pot furnace, the furnace is stationary and the molten
metal is ladled out of the container. In the tilting pot furnace, the
entire assembly can be tilted for pouring. Crucible furnaces are used
for nonferrous metals such as bronze, brass and alloys of zinc and
aluminium. Furnace capabilities are generally limited to several hundred
pounds.
The
metal in the crucible is ordinarily exposed to the furnace atmospheres.
It must be mentioned that most non ferrous metals and alloys absorb
gases, oxidize and form dross readily when melted. Steps should be taken
to reduce the contamination by fluxing, degassing and reducing the time
in melting. The thermal efficiency of crucible furnaces rarely exceeds
10 percent.
In the stationary pot furnace, the furnace is stationary and the molten metal is ladled out
of the container. In the tilting-pot furnace, the entire assembly can
be tilted for pouring; Crucible furnaces are used for nonferrous metals
such as bronze, brass and alloys of zinc and aluminium. Furnace
capacities are generally limited to several hundred pounds.
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