3. Blowing
In
The process of starting a newly lined furnace is called
blowing in. This operation involves four main steps, viz.
drying, filling, lighting and operating until routine
practice is achieved.
There is no standard practice of blowing-in.
Each plant follows its own method. The
main steps are briefly discussed below.
Step 1 - Drying
The new lining of a furnace contains a significant
proportion of moisture . This moisture must be slowly
and completely removed before the temperature of the
furnace is raised. This operation is known as drying.
During drying the furnace is slowlyheated.
Different techniques for drying :
- Blowing of hot blast in to BF .
4. Step 2 - Filling
At the end of drying, the furnace is cleared off all the remains
of drying
and then filling of furnace is taken up. .
Filling of the furnace starts from hearth . The common
method is --
Filling up the hearth with light kindling wood and shavings
saturate with oil up to the tuyere level, over this , timber &
above that coke is charged up to the bosh level.
Limestone sufficient to flux the ash in the charged coke, is
also charged along with the coke after the initial coke blanks ,
Over the coke blanks, light burden consisting of ore, lime
stone and coke ( i.e. the ratio of iron ore to coke is low, about
0·5-0·6 of the normal ratio ) are charged up to stock line.
5. Steps 3 & 4 -- Lighting and operating until routine practice is
reached.
After filling , the furnace is lighted by inserting red-hot bars
through
the tuyeres or slag hole and iron notch.
Burning is allowed with natural draught for the first 24 - 36
hours ; And after that a light blast is applied. As soon as good
amount of gas emerges from the furnace top , blast is blown
with gradual increase until it reaches normal level , in 6 - 7
days time.
Tap holes are kept open for hot gases to escape out during
the early period. Tap holes are closed , when the gases
coming out through tap holes almost stops .
6. When sufficient slag accumulate, it is drained out.
As the material descends , furnace is kept full by charging
fresh burden.
The ratio of iron ore is gradually increased in the burden
.After the hearth is full , slag & hot metal are drained out.
After the first cast is over charging and tapping schedules
are established and are strictly adhered to until routine
production is established.
It generally takes nearly a week to establish normal routine
practice (i.e. metal and slag of desired composition are
produced at the desired intervals) .
7. Bankin
g
BF after blowing-in, is expected to run continuously for 5-7
years.
Some times , furnace has to be shut down for unforeseen
reasons like
– labour troubles, shortages of raw materials, serious
breakdowns , etc.
‘ Banking ’ is resorted to as an emergency measure when
such unforeseen events require a furnace shutdown.
Since the furnace can not be put 'on' or 'off ’ readily, the
furnace needs to be shutdown in such a way that when
production is to be resumed , it can be done with minimum of
efforts and wastages.
The temporary shutdown of blast furnace is known as
8. During banking , reduction in combustion rate is achieved not
by extinguishing the fire but by taking blast of, covering the
fire with excess coke, closing all air inlets and covering the
stock with fine material. The fire is thereby maintained inside
for future use.
During banking operation, the blast is taken off, the blow
pipes are dropped and the tuyere openings are plugged with
special refractory mass , to prevent air from drafting through.
Thus, hearth heat is preserved and the furnace could be
returned to operation with a minimum effort.
9. The stock level is inspected every day when the furnace is
banked.
A slow downward movement of the stock indicates slow
consumption of coke by air that is infiltrating in. A rapid drop
in stock level can lead to deficiency of coke at the time of
restarting the furnace.
The furnace is restarted by replacing the tuyeres and notches
and, charging sufficient coke to fill up space created by the
sinking of the stock. The rest of the practice normally takes
course similar to blowing-in practice.
It may take a week to bring back the furnace operation to
normalcy.
10. Blowing-
out
Blowing – out is , the process of stopping the furnace operation
-
- when business conditions deteriorate and production is no
longer
required,.
Common method of blowing – out is given below.
Charging is stopped and the stock is allowed to descend until
minimum
of it remains inside. The last cast is carried out as completely as is
feasible. Towards the end , the remaining stock is quenched with
water and the furnace becomes cold . The furnace is cleared off
the quenched material , by raking operation .
-Starting the furnace after blowing-out , takes less time and less
effort than starting a ‘bankedfurnace’.
- However, the costs in connection with blowing out, raking out and
11. Back
drafting:
To perform various maintenance functions such as the
replacement of tuyeres or repairing cables , it is necessary to
take the furnace off blast for short periods , often less than
two hours,. In such instance, the furnace is not banked but is
back drafted.
The blast is put off and the bustle pipe is put under negative
pressure to force furnace gases to flow in reverse direction
into the stove where it is burned. Air may be drawn in
through the bleeders downwards to achieve this.
12. Fannin
g
Some times , full production capacity of the furnace is not
required for reasons like – less demand for iron ,
raw materials shortage etc.
In such cases , production is decreased
by reducing the input blast.
If the blast is reduced to a value less than 25% of the normal
volume then this technique is known is 'fanning‘.
During fanning a positive pressure is
maintained in hot blast system.
Fanningis far better than total shutting off the blast , because
by adopting fanning ,
13. Blowing
down :
Generally ‘ blowing down’ is carried out at the end of the
campaign ,
i.e. refractory lining has worn out , and hence there is need for
relining the furnace.
Blowing a furnace down means running the furnace without
charging until the burden level in the furnace is reduced to
approximately the tuyere level.
Reasons for preferring this technique
to banking or blowing-out a furnace are – (1)
the procedures of banking and blowing-out blast
furnaces are expensive. (2) Often banking results in
difficult start ups
because of water problems, or any of unforeseen reasons. (3)
when a furnace is blown-down it can be thoroughly inspected
for leaking coolers or staves etc..
