FIELD OF THE INVENTION
The present invention relates to an improved trough cover for a main trough
of Blast furnace.
BACKGROUND OF THE INVENTION
Trough cover plays a vital role in Iron making - main function of trough cover
is to cover the main trough, in order to ensure safety of cast house. The hot
metal/slag coming out from the taphole (a cylindrical hole) of blast furnace
experiences pressure difference of about 0.5-1 bar resulting in
spillage/spitting of high velocity streamed hot metal/slag. Stream of hot
metal/slag coming out of the tap hole falls over the main trough, which carry
the hot metal and slag to the ladle cars. The function of the trough cover is
to cover the main trough just in front of the taphole so that it restricts the
spitting /spillage of hot metal/slag around the cast house of the blast furnace.
The life of a trough cover depends on the refractory design and quality,
chemistry of hot metal and slag, casting duration etc. Refractory lining of the
trough cover system is normally done with either high alumina or Aluminasilicon
carbide-carbon (ASC) based castable. Small sized metallic anchor is
used as a holding arrangement for the refractory trough cover shell.
The major reason for affecting operation of the blast furnace is unexpected
failure of the trough cover leading to non availability of an operating trough
cover due to slag jam formation or corrosion of working lining refractory
castable arising out of reduction in lining thickness and deterioration of
working lining of the refractory castable.
Frequent failure of the prior art trough covers due to refractory design calls
for a need to innovate more reliable and robust refractory design and a
holding device.
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OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose an improved
trough cover for a main trough of Blast furnace, which eliminates the
disadvantages of the existing system.
Another object of the present invention is to propose an improved trough
cover for a main trough of Blast furnace, which provides a robust anchoring
system for holding the refractory castable.
A further object of the present invention is to propose an improved trough
cover for a main trough of Blast furnace, the composition of the trough cover
having zero cement to reduce the slag jam formation in the trough cover
hole with trough cover castable.
A still further object of the present invention is to propose an improved
trough cover for a main trough of Blast furnace, which enhances trough cover
life of more than one lakh throughput.
A still further object of the present invention is to propose an improved
trough cover for a main trough of Blast furnace to cover the full campaign of
the trough.
SUMMARY OF THE INVENTION
The basic objective of the invented lining design of trough cover is to create
more reliable and safe trough cover design system. In modified (invented)
design the drawbacks of the existing system is overcome by the use of small
sized ceramic anchor which enables robust anchoring/holding system for
refractory castable with trough cover shell and use of zero cement based
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refractory castable which helps in reducing the slag jam formation with
trough cover castable .
Existing practice of trough cover lining involve welding of small metallic
anchor with cover shell and then casting with high alumina/ASC castable
(100mm thickness) part by part all over the shell (shown in Figure-1).
However, during the operations, part of slag spitting out from blast furnace
taphole strikes and reacts with refractory castable on trough cover shell and
leads to jam formation. Slag jam in the castable lining, mainly in the top flat
portion, results in gradual weight increase and may break down along with
lining castable and thus making the shell naked and exposed to direct
exposure of heat.
In order to Strengthening the castable holding system with trough cover,
shell metallic anchor is replaced with newly design ceramic anchor (fig-2)
system, which can hold the refractory castable along with slag jam for longer
period of operation. Further, high alumina/ASC based conventional refractory
castable is replaced with newly developed zero cement based alumina
castable.
As lime present in refractory castable reacts with silica from slag and form a
low melting phase, hence, lime is eliminated from castable composition. In
refractory castable composition main source of lime is cement and hence,
application of zero cement based castable results in better performance.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure-1: Shows schematic drawing of a prior art trough covering refractory
lining system.
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Figure-2: shows drawing of a ceramic anchor according to the current
invention.
Figure-3: shows a detail drawing of refractory lined trough cover of blast
furnace with ceramic anchor and zero cement based high alumina castable.
DETAILED DESCRIPTION OF THE INVENTION
In order to achieve zero failure and maximum availability of the trough cover
(to cover campaign life of a trough more than 1 Lakh THM throughput) a
robust trough cover refractory system has been provided by eliminating the
drawback of the existing trough cover refractory lining design through use of
a metallic anchor for holding the refractory castable with the trough cover
shell.
In order to combat the prior art trough cover failure problem, the following
solution have been provided :-
1) Strengthening the castable holding system with trough cover shell:
The existing system of holding the refractory castable with shell (by
use of metallic anchor) found to be inadequate and an improved
anchoring system has been proposed which can hold the refractory
castable along with slag jam for longer period of operation.
2) Reducing slag jam formation with refractory castable: Lab analysis of
after service castable sample (recovered from failed trough cover)
carried out and it was found that lime present in the refractory
castable reacts with silica present in the slag and form a low melting
phase, which leads to slag jam formation, hence lime has been
eliminated from castable composition. Hence, an optimum quality of
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refractory castable having minimum amount of lime, has been
developed.
In order to fulfil the requirement of more robust anchoring system,
detail study has been carried out on different design of anchor system
and their application environment. The detail of ceramic anchor design
is shown in figure-2.
The advantages of using the newly designed ceramic anchor over
conventional metallic anchor for trough cover application are:
1) The difference of thermal expansion coefficient between the ceramic
anchor and refractory castable is very minimal compared to metallic
anchor and refractory castable, as a result chances of cracking of
refractory castable due to mismatch of thermal expansion coefficient
becomes less.
2) The anchor strength is more in case of ceramic anchor with
refractory castable as the area of contact between ceramic anchors
and refractory castable is more compared to the metallic anchor and
refractory castable.
In refractory castable composition main source of lime is cement, which is
being used as binder for castable system. Hence application of zero cement
based castable decided for trough cover lining application. From plant
observation, it was noticed that, trough cover refractory castable failure
problem is mainly concentrated around the flat portion of the cover shell.
Based on above observation, it was decided to put the ceramic anchor only
on flat & slant portion of the trough cover. The detail refractory design of
Blast furnace trough cover with ceramic anchor and no cement based high
alumina castable is shown in figure-3.
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Purpose & Application:
Plant trial of newly designed trough cover shell is carried out which achieved
more than double life compared to old design (performance data shown in
table-1).
Table-1: Plant performance data of newly designed refractory for trough
cover shell vs old design:
Items Old design: Metallic
anchor with conventional
castable refractory
New design: Ceramic
anchor with zero cement
refractory castable
Plant performance in
terms of tons of hot metal
throughput at a first
trough cover of a BF.
63680 THM throughput 132,680 THM throughputs
Plant performance in
terms of tons of hot metal
throughput at second
trough cover of the BF.
67780 THM throughput 137,370 THM throughputs
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WE CLAIM :
1. An improved trough cover for a main trough of Blast furnace to reduce
occurrence of slag jamn phenomenon and enhance operational life of
the trough cover, the improvement is characterized in that :
- the castable holding system comprises a robust ceramic anchor
enabled to hold the refractory castable for a longer period including
the spilled slag deposited on the trough cover shell; and
- the trough cover is provided with zero cement based alumina lining;
Wherein the difference of thermal expansion co-efficient between the
ceramic anchor and the refractory castable is minimized to reduce
cracking of the castables, and wherein the area of contact between the
ceramic anchor and the refractory castable is increased to allow longer
period of holding of the main trough with refractory castable.