Field of invention
The present invention relates to a process for the manufacture of reducible sinter
having high strength. The ultra high reducible sinter of the present invention results in
improved blast furnace operation and especially helps in raising the reduction efficiency.
Background of the invention
Modern blast furnaces operate with auxiliary fuel injection through tuyers. In multi-fuel
injection system through tuyere like Coal Dust Injection, natural gas, oil, and tar etc., the
stack permeability in such furnaces is adversely affected due to increase in ore-coke ratio.
Increasing the pulverized coal/fuel injection rate increases the gas permeability resistance
in a blast furnace. Moreover, such furnaces demand high reducibility sinter with lower
Reduction Degradation index (RDI).
Burden materials are composed of iron oxides associated initially with impurities. When
these materials are subjected to mechanical stresses at normal conditions as well as at
higher temperature under reducing atmosphere, their disintegration starts at point of
weakness. The extent of disintegration depends upon reduction temperature, reducing
potential of gas, reduction time and mineralogical properties of sinter. Maximum break
down occurs between 400-600°C with CO/CO2 ratio of about 1.0 and a longer reduction
time in upper stack region of blast furnaces. Obviously the furnaces will run smoother as
the burden descends through the critical temperature zone of 400-600°C as fast as possible.
In this respect high productivity blast furnaces will be least affected, where as the blast
furnaces with sluggish burden movement shall be adversely affected.
For effective utilization of enriched tuyere gases it is necessary that the ferrous
burden is of the high reducibility index. This facilitates achievement
of lower coke rate. Under presently known processes, due to sluggish burden movement,
operator prefers low RDI sinter because certain varieties of iron ore is usually characterized
by high alumina and which has negative effect on reduction degradation index.
It is also desirable for fuel efficiency, smooth blast furnace operation and hot metal quality
that the ferrous materials are reduced as far as possible in the zone of indirect reduction
(1000°C) during their period of residence in this critical temperature regime. The
improvement of the reducibility of sinter alongwith maintaining the desire strength also
confirmed to be effective in raising the reduction efficiency of blast furnace.
In general, there is a positive correlation between RDI and Reducibility (RI). If RDI is low,
reducibility index will also be low.
High FeO content in sinter compensates the adverse effect of alumina in sinter but reduces
the reducibility index. High SiC>2 content improves the strength but reduces also the
reducibility index.
In conventional sintering process where reducibility index is high i.e. 69 to 72%, the
reduction degradation index also high and varies from 37 to 40%.
For blast furnace (BF) process intensification the ferrous burden should be highly reducible
with low CDI, low gangue and should possess superior high temperature properties for
facilitating alternative fuel injection in blast furnaces.
It has been observed that decrease in permeability is the ultimate limiting factor and hence its
control through burden distribution and improvement in quality of raw materials is of vital
importance for achieving high injection rate in blast furnaces.
The most important quality parameters of sinter are Reduction Degradation
Index (RDI) and Reductibility Index (RI). For good blast furnace operation
particular emphasis has been given on RDI of sinter for maintaining good
stock permeability. Further the improvement of the reducibility of sinter is
confirmed to be also effective in raising the reduction efficiency of blast
furnaces.
Therefore, high reducibility index (RI>75%) with low Reduction Degradation
Index (RDI < 27%) sinter is very much essential for alternative fuel injection
system and operation of high productivity blast furnaces.
Reducibility index of hitherto known sinter is low i.e. <65% in high FeO
sinter (FeO>9%) and in low FeO sinter (<7-8%). Reduction degradation index
is very high i.e. more than 30%.
Furthermore, the vertical sintering speed (VSS) in top layer of the sinter bed
in the sinter manufacture pot is high and in bottom layer, it is low.
In conventional process heat losses through side plate of the pallet is found to
be high due to the absence of heat resistance materials leading to substantial
leakage of fresh air through side wall.
Furthermore, the heat pattern in sinter manufacturing units is not uniform
throughout the layer. Temperature is low in top layer and near the side plate
and high in bottom later. Main reason for this is the regenerated heat which
is not utilized uniformly throughout the sinter, resulting in low reducibility
and non-uniform strength of sinter. This is the major draw back in
conventional sintering process.
Detailed description
To meet the abovementioned objects, the process of manufacturing sinter
comprises suitable selection of process parameters, construction of
manufacturing pot and chemistry of raw materials.
The sinter so produced possesses high reducibility index (RI > 76%) with
lower degradation (RDI < 27%) which is achieved by higher percentage of
primary hematitie (20-30%) with large size of hematite crystal (40-50 µm)
and calcium ferrite [5 CaO2SiO2, 9 (Fe AD2O3] phase in finished sinter.
The above sinter mineralogy is achieved by selecting appropriate heat
pattern i.e. average bed temperature 1300-1400°C, preferably 1350-1360°C
and thereby obtain optimum sinter chemistry i.e. Fe>57% and CaO/SiOa 2.1
to 2.37 with low ratio of nucleai/powder ratio i.e. 1.35 to 1.64 in final sinter
mix.
For eliminating heat loss from side walls of the unit, the sinter pot is
modified and side wall of the pot was fully covered by heat resistance
(insulation) materials for prevention of heat losses during sintering. In
conventional process there is no heat insulation materials are used in side
plates for prevention of heat losses.
The preferred heat insulation material is ceramic fiber blanket which is
capable of being used in the temperature range of 1260 to 1400°C with a
thermal conductivity of around 0.126 w/mk at 600°C. The thickness of the
ceramic fibre is preferably 10mm.
Layer wise control in vertical sintering speed for efficient utilization of
regenerated heat of preheated air through the bed and reduction of heat
losses through side wall decreased the degradation and increased the
reducibility index even at low sinter bed temperature <1300°C.
The process comprising firstly, the selection of ore fines such that the Fe
content is more than 63.5% and Al2O3/SiO2 ratio is between 0.8 to 1.1. This
ore is purely hematite and porous type which is good for higher reducibility
index. The size of the ore fines is selection such that 70% fines has size -3mm.
The raw material, other than ore fines, may also comprise minor ingredients
such as mixed flux, coke, LD slag, flue dust, mill scale, lime etc.
The fines are then subjected to mixing and balling to achieve a degree of
granulation of 50-68%, nucleai/power ratio of 1.3 to 1.6 and bulk density of
2.0.
During the process of manufacture the vertical sintering speed (VSS) is
selected such that VSS is low in top layer and high in bottom layer for
utilization of regenerated heat more in top layer than bottom layer. Similarly
sinter bed temperature in middle layers were kept higher than bottom layer.
Preferably, the VSS for 1st layer from top having height of approximately
140mm is selected between 11.93 to 13.25 mm/min, for second layer 12.56 to
13.65 mm/min and for 3rd layer 13.71 to 15 mm/min.
The average sinter bed temperature is selected between 1350 to 1390°C. At
even more than 1300°C bed temperature the reducible phase like calcium
ferrite is not dissolved. Preferably, the temperature of 1st layer was
maintained between 1288 to 1355°C, 2nd layer between 1378 to 1420°C and
3rd layer between 1360 to 1415°C.
The sinter so produced is found to comprise 55.5 to 58% Fe, 9.37 to 11% FeO
and CaO/SiOa ratio of 2.14 to 2.37. RDI was found to be between 24.2 to 27%
and RI between 71.6 to 76%.
The invention will now be described with reference to non-limiting examples
and accompanying drawings in which'-
Description of accompanying drawings
Figure 1 shows a conventional sinter pot.
Figure 2 shows the modified sinter pot of the present invention with side wall
insulation.
Figure 3 shows the flowchart of the process for manufacturing sinter in
accordance with the present invention.
Figure 1 shows a conventional sinter pot (l) which essentially has a grate bar
(2) and a wind box (3). This sinter pot is modified in the present invention by
providing side wall insulation as shown in figure 2. The sinter pot (l) is
provided with a 10 mm thick side wall insulation (4) of ceramic fibre blanket.
Examples: Processes for manufacture of ultra high reducible sinter
Ultra high reducible sinter was manufactured following the process as
illustrated in the flowchart of figure 3. High grade ore fines with Fe content
of 63.7% and particle size of -3mm 70% was taken. The chemical analysis of
raw materials is shown in table 1.
Sinter chemistry was thus optimized w.r.t. RDI & RI. In high Fe sinter, FeO
content was also kept in high. Fe content was 55.5 to 58.1% and FeO 9.3 to
11.0% and basicity (CaO/SiO2) was 2.1 to 2.37. Sinter with high Fe (>58%)
and high FeO >11.0 Reducibility Index was not deteriorated. In conventional
sintering reducibility index was adversely affected with high FeO% (>9"10%)
in sinter.

WE CLAIM:
1. A process for the manufacture of reducible sinter comprising:
(i) providing a sinter pot having side wall insulation;
(ii) providing raw materials comprising ore fines with Fe content of at least 63.5%
and particle size of-3mm 70%;
(iii) balling said raw materials;
(iv) heating the said raw materials obtained in step (iii) in said sinter pot at average
bed temperature of 1350 to 1390°C and with vertical sintering speed selected
such that vertical sintering speed is low in top sinter layer and high at bottom
sinter layer.
2. Process as claimed in claim 1 wherein said side wall insulation is ceramic fibre
blanket.
3. Process as claimed in claim 2, wherein said ceramic fibre blanket has thickness of 10
mm.
4. Process as claimed in claim 2. wherein said ceramic fibre is so selected such that it is
capable of being used in temperature range of 1260 to 1400 C with a thermal
conductivity of around 0.126 w/mk at 600°C.
5. Process as claimed in claim 1, wherein said raw material preferably has Al2O3/SiO2
ratio between 0.8 to 1.1.
6. Process as claimed in claim 1, wherein said raw material comprises other minor
ingredients such as mixed flux, coke, Ling Donawiz slag, flue dust, mill scale, lime.
7. Process as claimed in claim 1, wherein said balling is carried out to obtain a degree of
granulation of 50 to 68%, nucleai/power ratio of 1.3 to 1.6 and bulk density of 2.0.
8. Process as claimed in claim 1, wherein VSS for first layer of sinter bed from top is
selected between 11.93 to 13.25 mm/min, for second layer 12.56 to 13.65 mm/min
and for third layer 13.71 to 15 mm/min.
9. Process as claimed in claim 8, wherein each of said first, second and third layer has
height of 140mm.
10. Process as claimed in any preceding claim, wherein high reducible sinter obtained
thereby has RDI of between 24.2 to 27% and Rl between 71.6 to 76%.
11. Process as claimed in any preceding claim, wherein high reducible sinter obtained
thereby comprises 55.5 to 58% Fe, 9.37 to 11% FeO and CaO/SiO2 ratio of 2.14 to
2.37.
12. High reducible sinter manufactured by the process as claimed in any preceding claim,
having RDI of between 24.2 to 27% and Rl between 71.6 to 76%.
13. A process for manufacture of high reducible sinter as substantially herein described in
the text, examples and accompanying figure 2.

Process for the manufacture of ultra high reducible sinter having high strength. The process yields sinter having RDI of between 24.2 to 27% and RI between 71.6 to 76% and comprising 55.5 to 58% Fe, 9.37 to 11% FeO and CaO/SiO2 ratio of 2.14 to 2.37. The process comprises providing a sinter pot having side wall insulation, providing raw materials comprising ore fines with Fe content of at least 63.5% and particle size of -3mm 70%, balling the raw materials and heating the raw materials in the sinter pot at average bed temperature of 1350 to 1390°C and with VSS selected such that VSS is low in top sinter layer and high at bottom sinter layer. The ultra high reducible sinter of the present invention results in improved blast furnace operation and especially helps in raising the reduction efficiency.