The tapping
rate, temperature of pig iron
and duration of tapping have increased
considerably in a modern blast
furnace resulting in stringent
service conditions for troughs/runners.
Thus the conventional hydrous
mass or Al2O3-C mass is no longer
adequate to survive the stringent
operating conditions. In trough
the wear process starts with corrosion
but erosion and cracking also
come into play. While corrosion
could be tackled by chemical properties
of material, the process of erosion
and cracking could be eliminated
by proper engineering of the physical
properties of the material.
Al2O3-SiC-C material, which has
excellent corrosion resistant,
has been accepted as a standard
base material for wear. As regards
to physical characteristics, some
design and engineering need to
done in the material, as under.
The wear lining must show a slight
positive expansion over the entire
range of their working temperature.
Also, during cooling there must
be an overall 'leftover' positive
expansion in the trough. Under
this condition the wear lining
material will not show any crack
and slow penetration of hot metal
at localised places can be stopped.
To accommodate the expansion,
avoiding any bulging out, expansion
allowances need to be provided
by way of inserting compressible
high temperature material in the
wear lining near the blast furnace
shell. The safety lining surface
should be smooth enabling main
lining to slip over it, during
expansion.
On many occasions, due to operational
constraints a trough is cooled
down after drainage and hence
working lining temperature falls
down to ambient temperature. In
such a situation, a negative PLC
(permanent linear change) value
of material will allow for cracks
opening in the lining which again
will not seal up during heating.
These cracks would be vulnerable
areas of attack during subsequent
running of trough. Hence the PLC
value of material at operating
temperature should be slightly
positive or material should be
volume stable.
Both tensile and compressive hot
strength of the material should
be high. While the high tensile
strength takes care of any crack
initiation tendency, the compressive
strength determines rate of erosion
of wear lining by flowing iron
& slag streams at high temperature.
Addition of fine powders of Si,
Al and Mg to the extent of 1-
5% helps in improving these hot
strengths of the material.
Thermal conductivity is very important
in a 'pooling type' trough where
due to long stretch of iron holding
continuously, refractory lining
reach almost steady state of thermal
equilibrium. High value of thermal
conductivity would convey more
amount of heat to safety lining
and supportive structure. In the
absence of cooling of trough (i.e.
trough embedded) in cast house
with no external face exposed
to any cooling, the increased
amount of heat evacuation to supporting
structure would cause the back
up lining temperatures to go up
thereby damaging supporting structure
and also the movement behind wear
lining would cause cracks in wear
lining. However, if the trough
is a cooled one, the higher thermal
conductivity of both wear and
back-up lining is desirable, so
that the temperature gradient
at hot face could be kept steep
containing the wear factors.
