FIELD OF THE INVENTION
This invention in general relates to blast furnace operation. In particular, the
invention relates to a method of optimizing size ratio of a burden to improve
burden porosity and gas distribution and increase blast furnace productivity.
BACKGROUND OF THE INVENTION
Gas permeability plays an important role in blast furnace operation for
producing pig iron. A decrease in gas permeability essentially increases
pressure drop and therefore, non-uniformity in gas flow occurs [1]. The
pressure drop inside the blast furnace has a direct relationship with
productivity [2]. A decrease in pressure drop clearly indicates the increase in
productivity.
Therefore, it’s extremely important to maintain a good permeability condition
inside a blast furnace to operate the furnace with high productivity [3, 4].
Blast furnace surface coke size is generally between 38mm-45mm and the
center coke size is about 45mm-80 mm. Along with the high sizes, the coke
itself has high porosity, requiring a higher permeability. But the porosity of a
pellet and sinter, besides having lower sizes (Pellet size range is 5mm-15mm
and Sinter size range is 5mm-50mm), is not that high. Hence, a packed bed
consisting of sinter and pellet is having very low gas permeability involving an
increase in their permeability.
Accordingly, the present invention proposes a technology for overcoming the
above mentioned disadvantages of lower gas permeability. In this technology,
the large size range of pellet and sinter are analysed and segregated into
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several small size ranged blocks to optimize the overall gas permeability. The
size ratio (always >1) is the ratio of highest size to the lowest size present in
a sample and an increase in size ratio implies decrease in porosity and
increase in non-uniformity of the permeability. Present invention attempts to
optimize the size ratio of the pellet and sinters by segregating them into
several blocks having higher porosity.
In general practice, a centre coke having high porosity is put at the centre.
No metallic is put at the centre as it reduces the permeability at the centre.
On the other hand, surface coke and metallic are distributed at the outer
region of the blast furnace where the permeability is comparatively low. This
type of burden distribution can’t have effective gas utilization at the centre
due to non-availability of metallic burden at the centre and thereby decreases
the productivity.
Accordingly, the present invention effectively addresses these issues to
provide an improvement in the burden distribution vis-à-vis an improvement
in the burden permeability and enhancement in the gas distribution. It is also
possible to put some metallic in the centre without much affecting that
region’s permeability which essentially ensures better gas utilization. This
invention has been made to work with the same available burden.
SUMMARY OF THE INVENTION:
Accordingly, a method is provided for improving blast furnace granular zone
porosity to enhance its productivity. As per the method, the metallic burden
consists primarily of sinter and pellet with size range of sinter is between 5 to
50 mm, and wherein the size range of the pallet is between 5 to 15 mm. The
larger range of sinter is split into 7 small size ranged fractions (5-8, 8-10, 10-
15, 15-20, 20-30, 30-40 and 40-50mm) to allow decrease of size ratio (ratio
of size of largest particle to the size of smallest particle in the fraction) of
each fraction. As the porosity increases with decreasing size ratio,
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the overall porosity of the burden increases. The increasing porosity of the
burden improves the gas permeability of the blast furnace and therefore, it
allows higher gas flow through the blast furnace for the same overall
pressure drop. As the productivity increases proportionately with the gas
flow, the productivity of the blast furnace increases.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates the prior art practice.
Figure 2 shows the method of the present invention.
Figure 3 shows decrease in pressure drop for different burden mixed
according to the invention.
Figure 4 shows increase in gas velocity for different burden mixed according
to the invention.
DETAIL DESCRIPTION OF THE INVENTION
The invention effectively investigates the effect of size ratio of metallic
burden and their impact on burden porosity. It is derived from the analysis
that if the size ratio decreases down close to 1, the non-uniformity in the
burden decreases and therefore, the porosity increases. The other factors
which affect the porosity are the sphericity and the size distribution of the
particle.
Figure 1: Prior art where metallic burden consists of all size fractions together
at a time without segregation.
Figure 2: Proposed charging practice where metallic burden are charged in
different fractions with each fraction consists of segregated size fraction at a
time. Fraction 1, 2 and 3 are different segregated size fractions of sinter or
pellet.
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Figure 3: % decrease in overall pressure drop for different metallic burden.
The pressure drop is obtained from Ergun’s equation assuming total bed
height=10m, (metallic burden bed height=1m), air viscosity=3X10-5 kg.m-
1s-1, air density=0.55kg.m-3 and total pressure drop across the bed=1.54
bar.
Figure 4: % increase in gas velocity for different metallic burden. The gas
velocity is obtained from Ergun’s equation assuming total bed height=10m,
(metallic burden bed height=1m), air viscosity=3X10-5 kg.m-1s-1, air
density=0.55kg.m-3 and total pressure drop across the bed=1.54 bar.
In present method the sinter burden of size range 5-50mm is segregated into
7 parts having size range 5-8, 8-10, 10-15, 15-20, 20-30, 30-40 and 40-
50mm. The average porosity of the whole range 5-50mm is 0.558. The size
ratio of the whole range is 10. Due to this large size range, the randomness
in the packing is too large and therefore, the porosity distribution is not
uniform and the overall porosity is low. But when these whole size range is
segregated into 7 small size ranged parts, the size ratio for each fraction got
reduced and therefore each fraction got size ratio below 2 (i.e. 2 > size ratio
> 1) and hence randomness associated with each fraction is very less and
hence the porosity of each fraction is high. Present invention proposed a
method for charging these 7 segregated parts once at a time over others in
place of the whole range.
Similarly pellet also can be segregated into several parts. But since, in pellet
~90% of the pellet lies in the size range 10-15 mm, the improvement in
porosity is negligibly small upon segregated charging. Therefore, in present
technology only sinter is segregated and pellet is not segregated. But the
pellet and sinter must be charged separately.
In this method the whole burden is segregated into small size ranged blocks
for which the porosity is more than the whole burden without segregation
due to low size ratio. If the whole burden is divided into n blocks, the
EXAMPLE:
Plant data analysis reveals that usually the pressure drop in the granular zone
of the blast furnace contributes to 26% of the overall pressure drop and the
major pressure drop contribution in granular zone comes from the metallic
burden (sinter + pellet) as coke has very high permeability than metallic.
To evaluate the effectiveness of the present method experiments have been
performed to calculate the porosity of sinter and pellet burden. As a first set
of experiments approximately 110 kg of sinter and pellet from actual blast
furnace operation is taken and screened to obtain the size distribution
present in the burden. Next, these materials are segregated based on their
size ranges and for each fraction the porosity test was done. The experiments
for measuring the porosity are performed in a cylindrical pot of diameter =
202.5 mm and height = 385 mm. The sinter and pellets of different size
ranges are put into the pot via random packing. The total weight of the
material is noted each time. These experiments have been done for each size
ranged as well as for the whole range of sinter and pellet. The true density of
pellet and sinter has been measured in gas pycnometer. From the value of
true density and the material weight, one can easily obtain the volume of
material required for random packing. At this point, porosity of each fraction
is obtained by calculating the pot volume and the solid volume using the
following formula: Porosity= 1-solid volume/total volume of the pot. The
porosity values are shown in Table 1.
Table 2 shows the change in overall porosity in granular zone for different
metallic burden. The overall improvement of porosity in granular region is
~7% for 30% pellet charging (present case). The increase in granular zone
porosity surely decreases the overall pressure drop. The reduction of pressure
drop across the granular region can be calculated using Ergun’s equation (see
figure 3). The Reynolds number of the flow is usually ~ 200 and hence both
the laminar and the turbulent parts of the equation need to be accounted.
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Using Ergun’s equation it is observed that the pressure drop in granular
region is reduced by 27.8%. Therefore, as the pressure drop in the granular
zone of the blast furnace contributes to 26% of the overall pressure drop,
overall reduction of pressure drop in the blast furnace is 7.23%.
Now, if overall pressure drop in the blast furnace is kept constant, then due
to increasing bed permeability, the overall gas velocity (U) will increase and
can be obtained from Ergun equation.

WE CLAIM :
1. A method of optimizing size ratio of a burden to improve burden porosity
and gas distribution and increase blast furnace productivity, the method
comprising :-
- segregating the size range of metallic burden into a plurality of small
size ranges such than the size ratio of whole range metallic is less than
about 2; and
- charging of one small size of metallic burden at a time.
2. The method as claimed in claim 1, wherein the burden charging
increases the gas velocity.
3. The method as claimed in claim 1 or claim 2, wherein the increased gas
velocity increases the rate of iron ore reduction.
4. The method as claimed in any of claims 1 to 3, wherein the increase in
gas velocity increases the rate of iron ore reduction enhancing blast
furnace productivity.