Ceramic
hearth blast furnaces faced severe
hearth wear & hot spots, leading
to lowering hot metal production.
Faster ramp-up after blow-in,
lower coke rate, lower hot metal
silicon and high operating temperature
strained the hearth refractory
system. At the stage, it was a
must to prevent any conditions
that severely impede efficient
heat transfer. It was also imperative,
as in the thick hearth lining,
the hot face attains a high temperature
immediately after the start-up.
High temperature across large
cross-section on the hot face
leads to high thermal expansion
inducing thermal stress, causing
severe cracks along horizontal
planes.
Carbon hearth emerged as obvious
alternative as carbon refractories
are known to be superior because
of their high thermal conductivity,
superior thermal shock resistance,
volume stability over a wide temperature
range, non-wettability with hot
metal and resistance to alkali
& slag attack.
In the carbon hearth, above the
bottom cooling system, a graphite
layer having a very high thermal
conductivity is laid. The graphite
cooling course distributes heat
to the entire cooling system.
In an uncooled underhearth, the
graphite course carries heat to
the sidewall cooling system. In
both the cases, it results in
lowering the penetration and increased
life of hearth. Above the graphite
layer and below the ceramic cup,
micro-pore carbon blocks are used.
Carbon blocks are used between
the micro-pore carbon blocks.
The performance of the carbon
hearth largely depends on the
rate of dissolution of carbon
in unsaturated liquid metal. This
continues to take place until
hot metal begins to freeze at
around 11500C. Thus the
extent of wear depends on the
rate of cooling i.e. efficiency
of the cooling system and the
heat extraction by carbon &
graphite blocks. The positioning
of 11500C isotherm within
the ceramic cup is the key to
minimize the wear of the hearth.
The ceramic cup is a relative
insulator in comparison with carbon
& graphite blocks and therefore,
the freeze line of hot metal can
be maintained in the ceramic cup
with proper selection of the materials
and the cooling system.
The surroundings of carbon &
graphite blocks are rammed with
carbon or graphite ramming masses,
respectively, in order to provide
the hearth system a higher degree
of stability.
