Iron Making in Blast Furnace
This is the blast furnace
on a longitudinal cross section, as it look like. Now, let us start with the
different section. At the top we can
find there is a charging device, placed in a top cone. The device shown is
conventional two bell charging and the most recent charging system is the bell less charging. Next to the
charging region is a cylindrical section, called the throat. Throat section is used for the final burden distribution over the stockline.
Wall of the throat is also
used to project armour for deflecting the charge material away from the wall.
After throat comes the shaft. This is a very important
region in the blast furnace where most
of the solid state reduction takes place through counter current gas-solid
interaction. It is the longest
portion of the furnace and maintenance of good permeability across section in this region is a challenging
task. Also you can find that stack is
little tappered out. It is not
perfectly cylindrical, and it is basically a cone. This tapering out is to accommodate the expansion of the solid as
it get heated up during its course of descent
and heat exchange with the
ascending hot gas.
Another thing you can find on the shaft wall- is a
cooling arrangement. In the course of heat exchange with the hot ascending gas,
refractory as well as the wall get heated up.
Unless this heat is extracted
out there could be heat accumulation and increase in refractory temperature, which is not good for the refractory lining and its life-period.
Since blast furnace is a continuous reactor it is not
economic for intermittent shutdown and
replacement of refractory. Therefore
adequate cooling arrangement on the wall is required
to drive out the excess heat. Cooling is done using some water cooled copper plates placed on the shaft wall, called
the copper staves. In copper staves, there
are arrangement for cold water going in and hot water coming out.
Now, after the shaft we have another cylindrical section, called the belly region.
Softening of solid burden (except coke) starts here. And just below the belly you have another conical region with tapering in,
is called the bosh. Again, why it is tappered in? Here, the solid become liquid and it shrink in volume. So, some
tapering is required such that
shrinking volume is accommodated inside to restrict creation of voids and
charge slips etc
And just below the bosh, you have the hearth. This is
basically the reservoir for liquid metal
and slag. Near the top of the hearth,
there is a slag notch to take out the lighter
slag floating over the liquid iron. An
iron notch is there near the bottom of the hearth to tap out the hot metal.
So, these are the different longitudinal section in blast furnace and below that it has the foundation. And now let us see different internal zones in a blast furnace. Blast furnace is a complicated reactor which separates two regions-dry and wet regions separated by an interface, called the cohesive zone. In cohesive zone softening of solid burden starts and everything is liquid (except coke) below the cohesive zone. So, in dry zone (shaft region) burden remains solid, and in the wet zone (belly, bosh region) burden becomes liquid. Gas moves up against solid coke and liquid iron and slag in wet zone and against solid in the dry zone. The cohesive zone has a shape of inverted V, following the solid isotherm inside the furnace.
Then comes the combustion zone, the heart of heat
generation in the furnace. Here preheated air blast after entering through
tuyers combusts the coke particles around it-
called the raceway What are
the reaction you can expect? First carbon is burned with oxygen
in air forming CO2 and then CO2
reacts
with carbon to form CO, forming a CO rich
gas in the raceway. Because,
thermodynamically CO2 is not
stable in presence of carbon at high temperature.
In the slide above you can find two regions in the raceway, A and A-prime. In
region A, carbon get oxidized to CO2, which
subsequently gasify carbon to CO in the
A-prime region.
In combustion zone there are several tuyers across the
periphery of the lower part of the bosh
region, with gas being sourced from a bustle pipe. You can see only two tuyeres
on the longitudinal cross section and
on a plan view you can see all the tuyers, as shown in the slide. Each ballon shaped region at the tip of tuyere is
called the raceway. Carbon burning takes place only in the raceway. Beyond raceway, oxygen does not reach. So, there exists a coke region in the central region,
which remains inactive,
called the Deadman’s coke. It is shown by the central circular region in the plan view and
in longitudinal cross section by an
inverted V shape. This is the steady
state shape of the deadman’s zone in
blast furnace. This is a very
important zone because this is the only solid
burden at the bottom of the furnace which holds the over burden. The deanman’s coke might be seating on the hearth bottom (seating deadman) or
it might be floating on the liquid iron (floating deadman).
Now, let us talk about cohesive zone, which I have
mentioned as an interface separating the
dry and wet zone in the blast furnace.
In blast furnace, solid-liquid transition does not takes place along a particular horizontal line, that is,
above that line everything is solid and blow it liquid.
In the blast furnace, the cohesive zone takes the shape
of an inverted V-shape. It basically follow the shape of the isotherm
of the solid burden. The thickness of
the cohesive zone is defined by two
lines-start of softening and end of softening defined by the solidus and liquidus isotherm of the slag. It is to be noted that coke does not melt
in the blast furnace and as a result
coke in the cohesive zone offers the path gas to pass, called the coke slits.
In the figure shown in the slide, coke slits are shown in purple color, and the fused mass of iron and slag
in red. So, alternate layers of fused
mass and coke slits may be observed. Now, if you follow the ore layer from
periphery to the center, you can find
that the ore layer when it approach near the center in cohesive zone, it has become red, means it has softened.
So, ore was solid away from cohesive zone and
gets softened in cohesive zone. You may also note that no such
transition has taken place for coke
as indicated by no change in color.
Let us explain temperature profile and shape of cohesive zone. Usually blast furnace is dominated by central flow and therefore more amount of gas passes through the central region maintaining temperature of the central region higher than the region near wall.
Now consider the stock profile. It is usually V shapped because burden moves through the central region at faster speed than
that near the wall due to resistance offered by the wall. So, alternate
softened region and coke slit appears in the cohesive zone. Gas passes
through these coke slits, which distribute the gas both in the peripheral
(through coke slits placed near wall
region) and central region (through coke slits near central region).
Now, come to the temperature profile and reaction
in the blast furnace across the longitudinal cross section above
tuyere location. Gas temperature is shown by dotted line; while solid temperature is
represented by the solid line. We can
see that there exists an isothermal
zone in the middle part of the blast furnace, where temperature of the gas and solid are same (may be taken as 1200K)
and no heat exchange takes place. It
also indicates that heat balance in
the lower and upper part of the furnace is maintained separately. In lower
part of the furnace gas temperature decreases from 2200K to 1200K (a decrease by 1000K), while the solid
is heated from 1200K to 1600K (a rise by 400K). It obviously indicates much higher heat capacity of solid
compared to that of the gas and large
heat capacity of the solid may be attributed to several endothermic reactions
in the lower part of the furnace. In
contrast, in the upper part of the furnace solid is heated from 300K to 1200K (an increase by 900K) and the gas is cooled from 1200K to say
500K (a decrease by 700K),
indicating sufficient heat capacity of gas
to preheat the solid.
At the upper part of the thermal reserve zone there
exists a zone called the chemical reserve zone, where iron-wustite equilibrium is achieved.
Now, let us see the major reactions that takes place
across the longitudinal section of the furnace. In the upper part of the furnace the
reduction of higher order oxides takes place,
namely hematite to magnetite and magnetite to wustite by indirect
reduction. It is called indirect reduction because the CO
generated by carbon gasification at the lower part of the furnace is used for these reduction. Carbon does not take part in the reduction through
its gasification at comparatively lower
temperature.
3Fe2O3+CO=2Fe3O4+CO2 Fe3O4+CO=3FeO+CO2
Wustite reduction takes place at
comparatively lower part of the
furnace. Wustite undergoes both the direct and indirect
reduction; indirect reduction at comparatively
upper zone and direct reduction at comparatively lower zone, where
carbon directly participate in the
reaction through in-situ carbon gasification takes place.
FeO+CO=Fe+CO2 - indirect reduction FeO+C=Fe+CO - direct reduction
The direct reduction
is a misnomer-direct reduction takes place through
gaseous intermediates. Carbon takes part in the reaction through
carbon gasification reaction 9endothermic
reaction), which takes place only if the temperature is high. Therefore
direct reduction takes place only in the lower part of the furnace where temperature above 1200K.
The direct reduction of wustite can pictorially be represented as:
FeO + C = Fe + CO
FeO + CO = Fe + CO2
CO2 + C = 2CO
All other direct reductions (direct reduction of P2O5, MnO, SiO2) also take place in the lower part of the
furnace, as indicated in the slide.
Final sulphur reaction takes place at the slag metal interface
through the reaction: CaO + S +
C = CaS + CO
Here, Sulphur get reduced to CaS. Therefore it is favoured under blast furnace reducing atmosphere.
In this lecture, what we have discussed different sections of blast furnace across the longitudinal direction, namely upper cone with charging device, cylindrical throat, truncated conical shaft with tapered out, cylindrical belly, truncated conical bosh with tapering in, and cylindrical hearth from top to bottom. Blast furnace is tapered out at shaft and tapered in at bosh to accommodate the material expansion and contraction. And there exists a long isothermal zone in the middle of the longitudinal section and a Chemical Inactive Zone exists in the upper part of the isothermal zone. The dry and wet zone in a blast furnace is interfaced by a cohesive zone, which has a shape of inverted shape and here the solid softening takes place. The coke provides the path for ascending gas in the cohesive zone. Combustion of carbon takes place in the balloon shapped raceway originating from tuyere-tip. Oxygen cannot penetrate beyond raceway and in the central region the coke remains inactive-called the deadman’s coke. The deadman zone extends from hearth upto cohesive zone and it can either seat at the bottom of the hearth or float over the liquid in the hearth.
Higher order oxides are reduced by indirect reduction in
the upper part of the furnace. Indirect
reduction of wustite attains equilibrium in the chemical reserve zone. All
direct reductions takes place at the lower part of the furnace
including the direct reduction of
wustite. In
direct reduction carbon directly participate in the reaction through in-situ gasification reaction, which is only
possible at higher temperature at the lower part of the furnace.
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