The new
generation blast furnaces with
large volumes operate at high
top pressure, increased blast
temperature, injection at tuyeres
e.g. coal and oxygen. This has
resulted in an increase in tapping
rate, tapping temperature and
duration of tapping. Such changes
have prompted radical changes
in the materials and applications
of refractories in the BF cast
house areas, particularly tap
hole clay.
The tap hole is prone to early
erosion and therefore for obtaining
a trouble free cast, a good and
strong tap hole of requisite strength
is required, for which a good
quality tap hole clay is essential.
Earlier in most of the blast furnaces
hydrous mass was used, which doesn’t
provide strong tap hole. Particularly
this hydrous mass, water bonded
was found to be responsible for
oxidizing the carbon lining around
the tap hole, leading to breakouts.
To achieve a trouble free tapping
a constant tap hole length need
to be ascertained. The expansion
of the tap hole need to be controlled
with hot metal and slag volume.
The delivery speed of the hot
metal stream should have minimum
possible variation. Any turbulence
in the stream at the tap hole
exit should be avoided. The operation
and closing of the tap hole need
to be smooth, to the extent possible.
High temperature and Si content
of the hot metal affects the performance
of the tap hole clay adversely.
Material design for tap hole clay
has undergone a radical change.
Superior grade Al2O3-SiC-C based
anhydrous tap hole clays are put
in regular use in medium to large
size blast furnaces. Both silicious
and aluminous aggregates are used
along with various additives and
binders. Phenolic resins, tar
and various oils are used as binders
along with Carbides (SiC) or nitrides
(Si3N4) as additives. Carbides
and Nitrides impart corrosion
and abrasion resistance properties
where it also improves sinter
ability of the mix.
These anhydrous tap hole masses
maintain good tap hole length,
offer smooth & controlled
drainage of metal & slag and
protect the hearth in tap hole
region, the zet impulse of the
tap hole is lowered, which reduces
waering of trough lining. Slight
expansion has been recognised
as a desirable property as negative
PLC (permanent linear change)
leads to gap formation.
The hearth should be empty as
long as possible; otherwise the
permeability of the blast furnace
will be influenced negatively.
This is influenced by 'Gun-up'
to ‘Knock-out' (GUTKO) time
practiced in any individual plant.
To achieve the best results the
GUTKO time should optimum for
any plant depending on the charging
rate of the furnace, number of
tap holes available and tap hole
diameter.
Installation
Techniques
In the classification, based on
appearance, the tap hole clay
is always in 'mud state'; the
reason why, it is also called
tap hole mud.
It is applied by a electro-mechanically/hydraulically
operated mud gun, the reason why,
it is also called mud
gun mass. The capacity
of the mud gun largely depends
on the pressure inside the blast
furnace.
The mud gun mass is extruded from
mud gun into the tap hole through
electro-mechanic/hydraulic pressure.
Pressure gauge installed on the
mud gun monitors the pressure
being applied for the extrusion.
A higher pressure reading reflects
the excessive hardness of the
material, where as, a lower pressure
reading reflects the softness
of the material. A good flowability
help in formation of desired mushroom
inside the blast furnace. Mud
gun is removed after 5-10 minutes
of the extrusion, once the tap
hole clay is sintered on the hot
face and seals the hole.
For opening the tap hole for metal
cast, drilling machine is used.
An optimum drilling time is the
reflection of a good quality tap
hole clay. At the manufacturing
end, this property is ascertained
by evaluating the cold crushing
strength of the coked material.
Depending on the mud gun capacity,
pressure being applied on mud
gun, capacity of the drilling
machine and GUTKO
time, the tap hole clay is tailor
made to cater the need of an individual
plant.
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