FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
1 TITLE OF THE INVENTION :
A PELLET INDURATION SYSTEM AND A PROCESS FOR IMPROVING THE INDURATION OF PELLETS INVOLVING THE SAME.
2 APPLICANT (S)
Name : JSW STEEL LIMITED.
Nationality : An Indian Company.
Address : Jindal Mansion, 5-A, Dr. G. Deshmukh Marg, Mumbai - 400 026,
State of Maharastra, India.
3 PREAMBLE TO THE DESCRIPTION
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to a pellet induration system and, more particularly, pellet induration system adapted to favour avoiding distortion of quality of pellets and also enable making the production of pellets with improved productivity and cost effectiveness. More particularly, the present invention is directed to providing a pellet induration system and method wherein required size for hearth layer screen could be selectively maintained by making necessary advancement in hearth layer screen to improve the induration machine performance, pellet quality, and higher pellet plant production with desired size of pellets. Importantly, the pellet induration system involving advancement in the hearth layer system would make the production of pellets energy efficient by way of desired permeability avoiding over firing of top layers pellets which in turn favours avoiding cluster formation .Importantly, the invention by way of advancement in bed permeability would favour avoiding deterioration of pellet quality and improving productivity and efficiency of the pellet production process in totality.
BACKGROUND ART
It is known in the art that pellet is the iron bearing feed to Corex and Blast furnace (BF) in iron and steel producing industries. Pellet is an agglomerate of iron ore, formed on heating to temperatures in the range of 1300 to 1350°C. Production of iron oxide pellets from iron ore fines involves different steps from drying to induration. The ore fines are initially dried to a moisture content of less than 1% before being sent to a ball mill for grinding to the required fineness. The ground ore fines are now mixed with other additives like bentonite, limestone, carbon bearing sludge and iron ore slurry and then sent to pelletizing/balling discs to prepare green pellets/balls. These green pellets are fired in the indurating machine to get the required physical, mechanical and metallurgical properties, making them suitable feed material to iron making units.
Existing Pellet plant at of the applicants produces pellets of 8 to 16 mm size. The applicant's used to operate two pellet plants (one 4.2 Mtpa and another 3 Mtpa capacity) to feed four BFs and two Corex units. It was also experienced in the related art that the production and quality of the pellets mainly depends on the induration

machine bed permeability. Induration machine bed permeability again depends on the grate aperture of the pallet which is covered with hearth layer pellets. The hearth layer pellets protects the grate from exposure to the high temperatures. Pellet plant produces 25 to 30% of -10 mm size pellets which includes -6 mm size pellet fines also. As these fines get recycled with hearth layer material, it causes resistance to process gas flow. Hence more energy is required to push the process gas through the bed. Insufficient permeability can cause over-firing of top layer pellets leading to cluster formation. This inefficient bed permeability results in deterioration of pellet quality, increase in process fan load, reduction in productivity, increase in un-fired pellet proportion, frequent wear and tear of impeller liners.
Any abnormality in the pellet process can cause distortion of quality, decrease in production and increase in production cost. Chocking of the induration machine grate aperture by hearth layer pellets affects the pellet bed permeability. Insufficient bed permeability directly affects the induration process.
There has been therefore a need in the art to providing a pellet induration system to ensure the required size for hearth layer screen in a straight grate machine by suitably modifying the hearth layer screen to improve the induration machine performance, pellet quality, and pellet plant production to get desired size pellets, avoid chocking of grate opening, protect the grate bars of induration machine from high gas temperature and also to ensure desired bed permeability in a pellet induration process.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide a pellet induration system for improving the induration machine performance by ensuring desired bed permeability for induration of pellets, and a pellet induration process adapted to ensure desired size and quality of pellets involving desired bed permeability of induration machine.
Another object of the present invention is directed to providing a pellet induration system for improving the induration machine performance involving advancement in hearth layer screen mat formation for selectively segregating only -20 +10 mm

pellets for the hearth layer to avoid any chocking of the induration machine grate aperture.
A further object of the present invention is directed to providing a pellet induration system for improving the induration machine performance wherein required air draft can be maintained through bed to avoid over-firing of top layer pellets leading to cluster formation.
A further object of the present invention is directed to providing a pellet induration system for improving the induration machine performance wherein un-fired pellet proportion can be reduced.
A further object of the present invention is directed to providing a pellet induration system for improving the induration machine performance wherein excess load of process fan and frequent wear and tear of impeller liners can be avoided.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a pellet induration system wherein green pellets after sizing through roller conveyor are discharged onto the travelling grate induration machine on top of the hearth layer and between the side layers of the fired pellets whereby the hearth layer pellets are provided to protect the grate bars from high gas temperature comprising induration machine bed permeability control means for uniform firing of the layers of pellets comprising :
means to selectively circulate pellets for hearth layer involving a hearth layer station with separate bunker at the feeding point of the indurations machine, said hearth layer station comprising hearth layer screen mat formation comprising means for segregating only -20 +10 mm pellets for the hearth layer there by avoiding any chocking of the induration machine grate aperture by the hearth layer pellets and favouring desired bed permeability.
A further aspect of the present invention is directed to a pellet induration system wherein said hearth layer screen mat formation comprises screen mat arrangement wherein first plurality of segments comprise of 10 mm size mat, followed by mat of 20 mm size .

A still further aspect of the present invention is directed to a pellet induration system wherein first four segments comprise of 10 mm size mat such that the total area to segregate minus 10 mm pellets is about 4800 X 2250 mm.
A still further aspect of the present invention is directed to a pellet induration system comprising means adapted for increasing or decreasing the screening area of said hearth layer station.
A still further aspect of the present invention is directed to a pellet induration system wherein said means for increasing or decreasing the screening area of said hearth layer station comprises rubber sheet at first segment.
Yet another aspect of the present invention is directed to a pellet induration system wherein the hearth layer screen mat formation is adapted such that cumulative +10 mm size is increased to a level of 69 to 87%, pellet mean size is increased from 11.23 to 12.06%, and recirculation of pellet fines (-6.13 mm) is reduced to the level of 0.78 to 0.17%.
A still further aspect of the present invention is directed to a process for improving the induration of pellets involving the pellet induration system as described above comprising:
carrying out the pellet induration by selective screening of segregating only -20 + 10 mm pellets for hearth layer there by avoiding any chocking of the induration machine grate aperture by the hearth layer pellets and favouring desired bed permeability.
A still further aspect of the present invention is directed to the said process for improving the induration of pellets wherein:
(i) total area of hearth layer screen to segregate the +10 mm size pellet is
increased from 2400 x 2250 mm to 4800 x 2250 mm. (ii) the cumulative +10 mm size is increase from 69 to 87% (pellet mean size
11.23 to 12.06%) and recirculation of pellet fines (-6.13 mm) is reduced from
0.78 to 0.17%. (iii) unfired proportion is reduced from 7.04 to 5.25 %.

(iv) CCS of the pellet increased to about 250 kg/pellet.
(v) the RDI (-6.13 mm) is decreased from 12.15 to 11.33%.
(vi) specific power consumption reduced from 67.55 to 64.94 Kwh/t of pellet; and
(vii) burnthrough temperature improved from 314 to about 330°C.
The objects and advantages of the invention are described in greater details with reference to the following accompanying non limiting illustrative drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the schematic illustration of the conventional grinding circuit and iron ore pelletization system for formation of green pellets.
Figure 2: is the schematic diagram of pellet induration system for straight grate induration process for iron ore pellets.
Figure 3: is the schematic diagram of the top view of vibro screen mat arrangement showing the conventional hearth layer mat screen configuration.
Figure 4: is the schematic diagram of the modified hearth layer screen mat specification according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURES
The present invention is directed to providing a pellet induration system wherein green pellets after sizing through roller conveyor are discharged onto the travelling grate induration machine on top of the hearth layer and between the side layers of the fired pellets whereby the hearth layer pellets are provided to protect the grate bars from high gas temperature and ensure required bed permeability for desired consistent pellet quality after induration.
Reference is first invited to the accompanying Figure 1 that illustrates schematically the stages of conventional grinding and iron ore pelletization process. The ore fines

are initially dried to a moisture content of less than 1% before being sent to a ball mill for grinding to the required fineness. The ground ore fines are then mixed with other additives like bentonite, limestone, carbon bearing sludge and iron ore slurry and then sent to pelletizing / balling discs to prepare green pellets/balls. These green pellets are fired in the indurating machine to get the required physical, mechanical and metallurgical properties, making them suitable feed material to iron making units.
Reference is now invited to the accompanying Figure 2 that schematically shows the pellet induration system in a straight grate machine. The green pellets, after sizing through a roller conveyor, are discharged onto the travelling grate induration machine on top of the hearth layer and between the side layers of the fired pellets. The hearth layer pellets protect the grate bars from high gas temperatures. The green pellets as obtained in the previous step, contain 8 to 9% free moisture and are 8 to 16 mm in size. The traveling grate carries the pellets through the indurating furnace where they are subjected to the sequential zones of updraft drying, downdraft drying, preheating firing and after firing and first and second cooling. The first stage of drying is updraft to prevent the condensation of the water and pellet deformation in the bottom layer of the pellet bed. For updraft drying the hot gas is recycled from the cooling zone 2. The drying is continued in a subsequent downdraft stage to remove free water of the top of the bed by relatively hot gases coming from the firing zone of the furnace. In these two zones it is necessary to use drying air of sufficient temperature to cause rapid water evaporation yet not so high that the pellets are destroyed by high internal pressures. In the preheating zone, the pellets are heated to about 500 to 1000 °C by downdraft air flowing through the bed and the hot gas is recycled from the cooling zone 1. During this stage, pellets are completely dried and reactions such as removal of combined moisture, decomposition of carbonates, coke combustion and conversion of iron oxide to hematite take place.
The reactions from the preheating zone continue in the firing stage. Here pellet charge is heated to an optimum temperature for a controlled period, and temperature is raised to 1300°C. Strength of the pellets increases at this stage because of re-crystallization and formation of slag phase. Some of the off-gas from the firing zone is recuperated to the drying zone. Five interconnected process fans are provided to circulate air throughout the different zones of the indurating

machine. After firing, the fired pellets undergo cooling where ambient air is drawn upward through the bed. The off-gas leaving the first stage of cooling has a temperature of around 1000°C, and this gas is directed to the firing and preheating zones where it is further heated by the burners with Corex gas fuel. The gas from the cooling stage-II is of lower temperature and is used for drying of pellets.
Any abnormality in the pellet process can cause distortion of quality, decrease in production and increase in production cost. Chocking of the induration machine grate aperture by hearth layer pellets affects the pellet bed permeability. Insufficient bed permeability directly affects the induration process. To get the required size for hearth layer screen to improve the induration machine performance, pellet quality, and pellet plant production, changes have been made at hearth layer screen to get desired size pellets.
The green pellet feeding arrangement involves providing series of conveyors known as Hearth Layer (HL) series, to circulate 10 to 15% of pellets for hearth layer. Importantly, a Hearth layer station is provided with a separate bunker of 50 tons capacity for distribution of pellets. Provision is also made to circulate pellets either by vibro screen route or thru bypass route. Pellets are conveyed through vibro screen, HL series conveyors, and get stored in a separate bunker called "hearth layer bunker" of 100 tons capacity, provided at feeding point of the indurating machine. Quadrant gate arrangement is provided at the bin discharge of pellets throughout the width of the machine and flexibility to increase or decrease the height of the fayer. Rapping device has been provided at the return side of the machine to clear the pallet grate aperture that gets chock due to under size pellets getting struck in grate aperture.
Accompanying Figure 3 shows the top view of vibro screen mat arrangement. Total effective screening area of the vibro screen is 6000 x 2500 mm. It is schematically shown in the Figure 3 that the first two segments mats are made to take out -5mm size pellet while second and third segments are made to separate -10mm pellet. Fifth segment is made to take pellet of +10 to -20mm size pellets on HL series conveyors. This screen is designed to segregate fines, under size, required size and oversize separately. The HL screen route is providing 67.48% of required pellets. The actual required pellet size percentage is above 90% of -20 +10 mm. Fines. (-6.3mm size

material) percentage is more in HL screen route. But the actual purpose of screen was to segregate only -20 +10 mm pellets for hearth layer. The existing HL screen is thus inefficient to segregate the required size of pellets for hearth layer and required modification for performance improvement of pellet induration system with improved bed permeability.
Accompanying Figure 4 shows the hearth layer screen mat specification after modification. Before the modification mat area to segregate -10mm size pellet was 2400 x 2250 mm. To provide pellet size of -20 to +10 mm to hearth layer through the HL screen route in percentage above 90% first to top segment mat was replaced with 10mm size mat. Because of this modification, total area to segregate minus 10 mm pellet has been increased to 4800 x 2250 mm. Option is also provided to increase or decrease the screening area by providing a rubber sheet at first segment. As a result of the above said modification in HL screen mat specification, the cumulative +10 mm size is increased to a level of 69 to 87%, pellet mean size is increased from 11.23 to 12.06%, and recirculation of pellet fines (-6.13 mm) is reduced to the level of 0.78 to 0.17%.
The process of pellet induration according to the present invention is thus directed to carrying out the pellet induration by selective screening of segregating only -20 + 10 mm pellets for hearth layer there by avoiding any chocking of the induration machine grate aperture by the hearth layer pellets and favouring desired bed permeability. Importantly, the process for improving the induration of pellets involves:
(i) total area of hearth layer screen to segregate the +10 mm size pellet is
increased from 2400 x 2250 mm to 4800 x 2250 mm. (ii) the cumulative +10 mm size is increase from 69 to 87% (pellet mean size
11.23 to 12.06%) and recirculation of pellet fines (-6.13 mm) is reduced from
0.78 to 0.17%. (iii) unfired proportion is reduced from 7.04 to 5.25 %. (iv) CCS of the pellet increased to about 250 kg/pellet. (v) the RDI (-6.13 mm) is decreased from 12.15 to 11.33%. (vi) specific power consumption reduced from 67.55 to 64.94 Kwh/t of pellet; and (vii) burnthrough temperature improved from 314 to about 330°C.

Table 1 shows the size analysis of hearth layer screen pellets before and after modification. Table 2 shows the pellet plant data before and after modification.
Table: 1

Size, mm Before modification After modification
+ 16 1.66 2.15
+ 12.5 24.43 33.56
+ 10 41.40 51.50
+8 29.00 11.16
+6.3 1.21 1.46
-6.3 2.30 0.17
Cum +10 67.49 87.21
Tabl e: 2
Particulars Before modification After modification
Production, t 10224 10719
Unfired pellet, % 7.04 5.25
CCS, kq/pellet 216 250
RDI (-6.13mm), % 12.15 11.33
Power, kwh/t 67.55 64.94
Burnthrough temp., °C 314 330
It is thus possible by way of the present invention to providing a system and method for pellet induration by providing selective size of green pellets on hearth layer to avoid choking of grate aperture and resultant reduction of air draft and enhance bed permeability favorable for required quality of pellets and to protect the grate bars from high gas temperature with enhanced productivity and without any interruption of plant operation.

We Claim:
1. A pellet induration system wherein green pellets after sizing through roller
conveyor are discharged onto the travelling grate induration machine on top
of the hearth layer and between the side layers of the fired pellets whereby
the hearth layer pellets are provided to protect the grate bars from high gas
temperature comprising
induration machine bed permeability control means for uniform firing of the layers of pellets comprising :
means to selectively circulate pellets for hearth layer involving a hearth layer station with separate bunker at the feeding point of the indurations machine ,said hearth layer station comprising hearth layer screen mat formation comprising means for segregating only -20 +10 mm pellets for the hearth layer there by avoiding any chocking of the induration machine grate aperture by the hearth layer pellets and favouring desired bed permeability.
2. A pellet induration system as claimed in claim 1 wherein said hearth layer screen mat formation comprises screen mat arrangement wherein first plurality of segments comprise of 10 mm size mat , followed by mat of 20 mm size .
3. A pellet induration system as claimed in claim 3 wherein first four segments comprise of 10 mm size mat such that the total area to segregate minus 10 mm pellets is about 4800 X 2250 mm.
4. A pellet induration system as claimed in anyone of claims 1 to 3 comprising means adapted for increasing or decreasing the screening area of said hearth layer station.
5. A pellet induration system as claimed in claim 4 wherein said means for increasing or decreasing the screening area of said hearth layer station comprises rubber sheet at first segment.

6. A pellet induration system as claimed in anyone of claims 1 to 5 wherein the hearth layer screen mat formation is adapted such that cumulative +10 mm size is increased to a level of 69 to 87%, pellet mean size is increased from 11.23 to 12.06%, and recirculation of pellet fines (-6.13 mm) is reduced to the level of 0.78 to 0.17%.
7. A process for improving the induration of pellets involving the pellet induration system as claimed in anyone of claims 1 to 6 comprising:
carrying out the pellet induration by selective screening of segregating only -20 + 10 mm pellets for hearth layer there by avoiding any chocking of the induration machine grate aperture by the hearth layer pellets and favouring desired bed permeability.
8. A process for improving the induration of pellets as claimed in claim 7 wherein:
(i) total area of hearth layer screen to segregate the +10 mm size pellet is
increased from 2400 x 2250 mm to 4800 x 2250 mm. (ii) the cumulative +10 mm size is increase from 69 to 87% (pellet mean size
11.23 to 12.06%) and recirculation of pellet fines (-6.13 mm) is reduced from
0.78 to 0.17%. (iii) unfired proportion is reduced from 7.04 to 5.25 %. (iv) CCS of the pellet increased to about 250 kg/pellet. (v) the RDI (-6.13 mm) is decreased from 12.15 to 11.33%. (vi) specific power consumption reduced from 67.55 to 64.94 Kwh/t of pellet; and (vii)burnthrough temperature improved from 314 to about 330°C.
9. A pellet induration system and a process for improving the induration of pellets involving the same substantially as herein described and illustrated with reference to the accompanying examples and figures.