Claims:WE CLAIM:
1. A process for iron oxide green pellets handling/firing in induration machine comprising:
providing green pellets from pellet plant;

screening of thus obtained green pellets based on different size,;

feeding of green pellets selectively based on the size of the green pellets involving different size pellets layer wise for desired bed permeability for uniform firing and cooling of the pellets throughout the bed for desired uniformity in pellet characteristics.

2. A process as claimed in claim 1 wherein said green pellets for said selective feeding are divided into three categories Coarse size (A): -16+12.5mm, Medium size (B): =12.5+10mm and fine size (C): -10+6mm and different size pellets are fed in layers to achieve desired permeability and produce pellets with desired mineralogy, physical and metallurgical properties.
3. A process as claimed in anyone of claims 1 or 2 wherein said pellets are fed to induration furnace selectively as three layers, two layers and single layer pellets as different size.
4. 4.A process as claimed in anyone of claims 1 to 3 wherein said different size pellets layer wise for desired bed permeability is carried out such that bed permeability of different size pellet and layer wise distribution is improved to as above as 106 JPU via a vis the bed permeability of bench mark pellets gives 92 JPU.
5. A process as claimed in anyone of claims 1 to 4 wherein said feeding of green pellets selectively based on the size is carried out such that the porosity of different size and layer wise pellet is varied from 28 to 35.27%.
6. A process as claimed in anyone of claims 1 to 5 wherein the said feeding of green pellets selectively based on the size is carried out maintaining the mineralogical phases present in the pellets to ensure selectively one or more of:
the hematite phase is varied from 46.78 to 60.47%.; the magnetite phase is varied from 2.90 to 5.49%; and the FeO content is varied from 0.47 to 0.87%. the slag phase is varied from 8.63 to 12.47%.

7. A process as claimed in any one of claim 1 to 6, carried out maintaining desired bed permeability and porosity for uniform heating of green pellets of varied sized and achieve the physical and metallurgical properties of the pellets as follows
• The T.I (+6.3mm) is varied from 88.47 to 93.60%.
• The CCS is varied from 222 to 268kg/t.
• The RDI is varied from 8.20 to 13.80%.
• The unfired pellets is varied from 1.6 to 3.5%.

8. A process as claimed in anyone of claims 1 to 7 comprising selectively involving size based layers for desired permeability selected from pellet size combinations
• B-A-C
• CA-B
• AB-C and
• B-B-B
for selectively achieving better mineralogy, higher physical (T.I and CCS) and metallurgical (RDI) properties.

9. A process as claimed in anyone of claims 1 to 8 comprising selectively varying the carbon content, firing temperature, machine speed and gas flow rate to further improve the pellet mineralogy, physical and metallurgical properties.

10. A process as claimed in anyone of claims 1 to 9 comprising carrying out the induration following:
Feed rate, tph 600 to 620
Bed height, mm 500 to 520
Hearth layer, mm 50 to 60
Machine speed, m/min 3.0 to 3.10
Firing temperature, OC 1280 to 1300oC
Burnthrough temperature, OC 270 to 300

11. A process as claimed in anyone of claims 1 to 10 comprising:
feeding of green pellets to induration furnace selectively as follows for desired pellet characteristics as defined:
Sl. No Size of the pellets, mm Layer wise pellet as different size (three and two layer) Properties
T.I(+6.3mm),% CCS,Kg/t RDI (-6.3mm),%
1 A: -16+12.5 mm (Coarse)
B: -12.5+10 mm (Medium)
C: -10 mm (Smaller) B-A-C 93.22 263 8.50
2 CA-B 92.80 256 8.92
3 AB-C 93.0 259 8.70
4 B-B-B 93.60 268 8.20

Dated this the 5th day of August, 2019
Anjan Sen
Of Anjan sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION
The present invention relates to a process for iron oxide green pellets handling/firing/heat-hardening in induration machine. More particularly, the present invention is directed to a process for the production of high strength iron ore pellets for Corex and Blast Furnace iron making units by firing the green pellets as different size and layer wise in a straight grate induration furnace. The invention favour production of better quality iron ore fired pellets for iron making units with pellet plant feed mix comprising of iron ore concentrate, bentonite (0.63), dolomite as flux to maintain basicity (3.66), green pellet moisture of 10.24%, coke breeze (0.89), sizing of the pellets as different size like coarse(A): -16+12.5mm, medium(B): -12.5+10 mm and smaller(C): -10mm, feeding of pellets in inconel basket as different size and layer wise (three, two and single layer) and subsequent firing of the pellets through straight grate induration furnace with firing temperature of 1320oC. The firing of optimized green pellet bed with different size and layer wise pellets will result in the production of better quality pellets for iron making units with desired mineralogy.

BACKGROUND OF THE INVENTION
During firing of pellets in straight grate induration furnace, the green pellets pass through different thermal treatments, namely drying, preheating, heating and cooling. The pellet bed is fired with downdraft firing using corex gas and cooled with updraft cooling using ambient air. Coke breeze, which is used as fuel in the pellet mix, supplies the necessary energy for uniform heating of the pellet. The physicochemical conditions, e.g. the temperature and oxygen partial pressure mainly depends on the amount of fuel incorporated in the pellet mix. As a result the percentage and the distribution of various phases in the pellets vary, leading to deviation in quality. Based on the above conditions in the induration machine, the fired pellets often have non-uniform phases and structures of the core to the shell of the pellet depending upon their size and position of the pellets within the pellet bed from top to the bottom of the induration furnace. The time difference between the reduction and oxidation of the pellet depends on the size of the pellet as well as the position of the pellet in the bed. Usually mixed pellets of different sizes varying from 16 to 6 mm pellets are firing in induration furnace at Pellet plant.
The physical, metallurgical and microstructural properties have a major influence on iron ore pellet reduction behavior during processing in a blast furnace. Most of the iron ore pellet plants produce the iron ore pellets in the size range of 16 to 6 mm. The sintering intensity, thermo-chemical conditions and formation of different phases vary across the cross section of the pellet. The time required for various reactions within the pellet is directly proportional to the pellet size. The firing sequence of pellets in the straight grate induration furnace at JSW Steel is downdraft firing using the corex gas and cooling is the updraft cooling using ambient air. The firing time, temperature, oxidation time and oxygen partial pressure are decisive for the structural changes within the pellets; the nature of the changes affects, in turn, the physical and metallurgical properties of the pellets. Based on the above conditions in the induration machine, the fired pellets often have non-uniform phases and structures of the core to the shell of the pellet depending upon their size and position of the pellets within the pellet bed from top to the bottom of the induration furnace. The pellets at the top layer exposed to higher temperature compared to middle and bottom layers. The sintering intensity and the formation of hematite, magnetite and slag phases differ across the bed based on fuel rate and pellet size and this in turn affects the pellet properties. The time difference between the reduction and oxidation of the pellet depends on the size of the pellet as well as the position of the pellet in the bed. The pellet size as well as position of the pellet in the furnace has a marked effect on the formation of different phases and microstructure. The microstructural phases influences on pellet physical and metallurgical properties. Usually, mixed pellets of different sizes varying from 16 to 6 mm pellets are fired in induration furnace.

The technical paper titled “Effect of Iron Ore Pellet Size on its Properties and Microstructure”, T Umadevi et.al., Steel Research int., 2009 mentions that the properties of the pellets mainly depends on the size of the pellets. In this work, detailed studies were carried out on pellets of different size produced from a 4.2 Mtpa pellet plant for their physical, metallurgical and micro-structural properties. It was observed that the pellets in the size range of +8 to -12 mm showed good strength and lower RDI. In this paper there is no mention of the effect of size wise pellets as layer wise.

The technical paper titled “Investigation of factors affecting pellet strength in straight grate induration machine”, T Umadevi et.al., Ironmaking and Steelmaking, Vol.35, No.5, (2018), pp 321-326, mentions that the quality of the pellets across the pellet bed with mixed size of the pellets i.e 16 to 6mm. This study mainly focuses on the distribution of phases and their impact on cold crushing strength at different carbon levels (1.20 and 1.35%), pellets from different layers of theinduration bed in an industrial straight grate were characterized. In this paper there is no mention of the effect of size wise pellets as layer wise distribution.
Worldwide all pellet plants follow the firing of the green pellets as mixed pellets of different sizes varying from 16 to 6 mm in straight grate induration furnace/ rotary kiln. Most of the pellet plants produce the pellets for iron making units by firing the mixed size pellets (-16+6mm). In the present investigation, it was found that the size of the pellets and the position of the pellets in the pellet bed influences on mineralogy, physical and metallurgical properties of the pellets. For a detailed study the pellet plant green pellets of size -16 to 6mm were collected.
The aim of this study is to optimize the pellet bed with different size pellets as layers to make uniform firing and cooling of the pellet throughout the bed to achieve the desired mineralogy, physical and metallurgical properties. In this study the distribution of phases in different layers as different size pellets and their impact on pellet physical, metallurgical and microstructural phases of the pellets are examined.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide a process for iron oxide green pellets handling/firing/heat-hardening in induration machine wherein green pellets are fired in induration furnace as different size pellets fed in selective layer wise disposition.

A further object of the present invention is directed to a process for pellet induration for achieving improved pellet properties such as T.I >93.0% and CCS >240 kg/p .

A still further object of the present invention is directed to a process for pellet induration to favour reduction in unfired pellets <2.50%.

A still further object of the present invention is directed to a process of pellet induration with pellet characteristics including Mineralogy: FeO= 0.7%, Hematite phase:>55.0%, and uniform distribution of slag phase around the hematite grains from core to shell.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a process for iron oxide green pellets handling/firing in induration machine comprising:
providing green pellets from pellet plant;

screening of thus obtained green pellets based on different size,;

feeding of green pellets selectively based on the size of the green pellets involving different size pellets layer wise for desired bed permeability for uniform firing and cooling of the pellets throughout the bed for desired uniformity in pellet characteristics.

A further aspect of the present invention is directed to a process wherein said green pellets for said selective feeding are divided into three categories Coarse size (A): -16+12.5mm, Medium size (B): =12.5+10mm and fine size (C): -10+6mm and different size pellets are fed in layers to achieve desired permeability and produce pellets with desired mineralogy, physical and metallurgical properties.

A still further aspect of the present invention is directed to a process wherein said pellets are fed to induration furnace selectively as three layers, two layers and single layer pellets as different size.

A still further aspect of the present invention is directed to a process wherein said different size pellets layer wise for desired bed permeability is carried out such that bed permeability of different size pellet and layer wise distribution is improved to as high as 106 JPU vis a vis the bed permeability of bench mark pellets gives 92 JPU.

A still further aspect of the present invention is directed to a process wherein said feeding of green pellets selectively based on the size is carried out such that the porosity of different size and layer wise pellet is varied from 28 to 35.27%.

Another aspect of the present invention is directed to aprocess wherein the said feeding of green pellets selectively based on the size is carried out maintaining the mineralogical phases present in the pellets to ensure selectively one or more of :
the hematite phase is varied from 46.78 to 60.47%.;the magnetite phase is varied from 2.90 to 5.49%; and the FeO content is varied from 0.47 to 0.87%. the slag phase is varied from 8.63 to 12.47%.

Yet another aspect of the present invention is directed to a process carried out maintaining desired bed permeability and porosity for uniform heating of green pellets of varied sized and achieve the physical and metalurgical properties of the pellets as follows
• The T.I (+6.3mm) is varied from 88.47 to 93.60%.
• The CCS is varied from 222 to 268kg/t.
• The RDI is varied from 8.20 to 13.80%.
• The unfired pellets is varied from 1.6 to 3.5%.

A still further aspect of the present invention is directed to aprocess comprising selectively involving size based layers for desired permeability selected from pellet size combinations
• B-A-C
• CA-B
• AB-C and
• B-B-B
for selectively achieving better mineralogy, higher physical (T.I and CCS) and metallurgical (RDI) properties.

A still further aspect of the present invention is directed to aprocess comprising selectively varying the carbon content, firing temperature, machine speed and gas flow rate to further improve the pellet mineralogy, physical and metallurgical properties.

Another aspect of the present invention is directed to aprocess comprising carrying out the induration following:
Feed rate, tph 600 to 620
Bed height, mm 500 to 520
Hearth layer, mm 50 to 60
Machine speed, m/min 3.0 to 3.10
Firing temperature, OC 1280 to 1300oC
Burnthrough temperature, OC 270 to 300

Yet another aspect of the present invention is directed to aprocess comprising :
feeding of green pellets to induration furnace selectively as follows for desired pellet characteristics as defined:
Sl. No Size of the pellets, mm Layer wise pellet as different size (three and two layer) Properties
T.I(+6.3mm),% CCS,Kg/t RDI (-6.3mm),%
1 A: -16+12.5 mm (Coarse)
B: -12.5+10 mm (Medium)
C: -10 mm (Smaller) B-A-C 93.22 263 8.50
2 CA-B 92.80 256 8.92
3 AB-C 93.0 259 8.70
4 B-B-B 93.60 268 8.20

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying nonlimiting illustrative drawings and examples.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: shows theprocess flow diagram for pellet making.
Figure 2: shows schematically the implementation of firing/heat hardening of pellets in induration machine indicating different zones of the indurating machine.
Figure 3: shows a setup for pellet bed permeability measuring.
Figure 4: Basket configuration used for charging and firing of the green pellets for experimental trials.
Figure 5: is graphical presentation of Green pellet bed permeability with different size pellets as layers.
Figure 6: shows graphically three, two and single layer size wise pellet phase analysis (Layer wise average).
Figure 7 (A-D):shows theMicrostructures of different size pellets subjected to layer wise induration.
Figure 8: shows graphically the effect of different size pellet as layer wise on T.I of the pellet.
Figure 9: shows graphically the effect of different size pellet as layer wise on CCS of the pellet.
Figure 10: shows graphically the effect of different size pellet as layer wise on RDI of the pellet

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention relates to a process for green pellets handling/firing/heat-hardening in induration machine wherein green pellets are fired in induration furnace as different size pellets fed in selective layer wise disposition. More specifically, present invention deals with a process for the production of high strength iron ore pellets for Corex and Blast Furnace iron making units has been developed by firing the green pellets as different size and layer wise in a straight grate induration furnace.

Production of iron oxide pellets from iron ore fines involves different steps from drying to induration. The beneficiation plant concentrate (Fe: 59 to 65%) is ground to achieve -45 micron 65 to 68%. The ground ore is now mixed with other additives like bentonite, limestone, dolomite and coke breeze and then sent to palletising/balling discs to prepare green pellets/balls of size -16+6 mm. These green pellets are fired in the induration furnace to get the required physical, mechanical and metallurgical properties, making them suitable feed material to iron making units. Accompanying Figure 1 shows the schematic diagram of the conventional iron ore pelletization process.

Accompanying Figure 2 schematically shows the induration process implemented in a straight grate induration machine. It comprises the following components/zones:
Numerals Description
1 Pallet car
2 Updraft drying zone
3 Down draft drying zone
4 Preheating zone
5 Firing and after firing zone
6 Cooling and after cooling zone
7 Hood exhaust fan
8 Updraft drying fan
9 Wind box exhaust fan
10 Wind box recuperation fan
11 Cooling air fan
12 Hood exhaust fan

The green pellets, after sizing through a roller conveyor (-16+6mm), 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 contain 8 to 10% moisture. The travelling grate carries the pellets through the indurating furnace where they are subjected to sequential zones of updraft drying, downdraft drying, and 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 1000oC by downdraft air flowing through the bed and the hot gas is recycled from thecooling zone 1. During this stage, the 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 reaction from the preheating zone continues in the firing stage. Here, the pellet charge is heated to an optimum temperature for a controlled period, and temperature is raised to 1280 to 1300oC. The 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 1000oC, and this gas is directed to the firing and preheating zones, where it is further heated by the burners with COREX gas fuel. The gases from the cooling stage 2 are of lower temperature and used for drying of pellets (<80oC).
The features of the process according to present invention are described hereunder with reference to following examples:
Example 1:
Green pellets were collected from pellet plant and the size analysis was carried out and the green pellet mix proportion and size analysis is shown in Table 1 and Table 2 respectively. The green pellets are divided into three sizes as A- coarse, B- Medium and C- Smaller size based on size of the pellets (Table 2).

Table 1 Green pellet mix proportion
Raw material %
Beneficiation plant concentrate 93.0 - 93.49
Dolomite 3.66 – 3.72
Coke breeze 0.89 – 1.00
Bentonite 0.63-0.65
CRM dust + ESP dust + SMS Sludge 1.33 – 1.45
Green Pellet moisture, % 10.24-10.30

Table 2 Size analysis of green pellets and nomenclature
Size, mm Nomenclature of green pellets based on size %
+16 & -16+12.5 Coarse A 27.43
-12.5+10 Medium B 53.6
-10 +8, -8+6 Smaller C 18.97

Example 2:

The pellet bed permeability was carried out under this example on different layer as different size pellets using bed permeability measuring equipment and is shown in Figure 3.

The formula used for calculation of bed permeability is as follows;

Whereas,
JPU – Japanese permeability unit
F – air flow rate (m3/min)
A – cross sectional area of bed (m2)
L – bed height (mm)
?P – pressure drop across bed (mmWC)

The test plan for green pellet bed permeability and plant basket trials are shown in Table 3. The plant basket trials have been carried out by filling the basket with 20 kg of pellets as different layers as different sizes (Table 3) and fired in the 116 m length pellet plant induration machine (Figure 2). The basket was kept in the center of the bed on the hearth layer and it covers the entire bed height of the induration machine. The basket used for plant trials is shown in Figure 4. The induration process parameters during the trials were given in Table 4.

The basket was marked as different layers (Figure 4) and pellets were collected as per test trial plan with typical (Table 3) three layers (Top, Middle and bottom), two layers and single layer. Representative samples from all these layers were tested in the laboratory for chemical analysis, cold crushing strength (CCS) and mineralogical studies. After mixing the different layers as a single mix, these were subjected to a tumbler index (TI) and reduction degradation index (RDI). Tumbler Index was measured as per ISO 3271 whereas Cold crushing strength (CCS) was measured as per ISO 4700. For microstructural studies, 4 pellets from each layer were collected representatively.

Table 3 Test plan for bed permeability and plant basket trials
Three layer pellets
Expt 1 Expt 2 Expt 3 Expt 4 Expt 5 Expt 6
Top A A B B C C
Middle B C A C B A
Bottom C B C A A B
Two layer pellets
Expt 7 Expt 8 Expt 9 Expt 10 Expt 11 Expt 12
Top- Middle- Bottom A BC B CA C AB
BC A CA B AB C
Single layer pellet Conventional pellet
Expt 13 Expt 14 Expt 15 Expt 16
Top A B C Bench Mark
Middle A B C
Bottom A B C

Table 4 Induration machine parameters
Feed rate, tph 600 to 620
Bed height, mm 500 to 520
Hearth layer, mm 50 to 60
Machine speed, m/min 3.0 to 3.10
Firing temperature, OC 1280 to 1300oC
Burnthrough temperature, OC 270 to 300

Example 3:
Properties for green pellets used for different trials and fired pellets obtained are evaluated under this example.
Green pellet properties
The green crushing strength and drop number of the pellets is 1.10 to 1.20 kg/p and 13 to 20 No (number) respectively, which meets the desired green pellet properties.
Pellet bed permeability
The bed permeability test results are shown in Figure 5. Bed permeability of thepellet plant pellets is 91.7 JPU. Except single layer pellet bed with smaller size pellets ( C-C-C), pellet bed with three layers, two layers and single layer of different size pellets showed bed permeability similar or greater than the bed permeability of pellet plant pellets.The highest bed permeability was achieved with a single layer pellet of coarser size (A-A-A). Because of bigger size, the pellet bed voidage is more and the bed permeability was also more.
Chemical analysis of the fired pellets
Table 5 shows the chemical analysis of the fired pellets as layer wise as different size pellets.
Table 5 Chemical analysis of different size pellets as layer wise
Fe,% SiO2,% Al2O3,% CaO,% MgO,% FeO,% B2
62.0 to 62.5 4.40 to 4.65 3.22 to 3.42 1.46 to 1.62 0.73 to 0.82 0.30 to 0.90 0.30 to 0.40

Mineralogical studies
The formation of mineralogical phases in iron ore pellets depends on the size of the pellets as well as the position of the pellets within the pellet bed in induration furnace from the top to the bottom. In the induration machine the pellets are fired as downdraft firing and cooling is an updraft cooling. The pelletization kinetics depends on size the as well as the position of the pellet in the pellet bed.

The phase analysis of layer wise (three, two and single layer) with different size pellets are shown in the Table 6 and average value (phase analysis) of each test is shown in Figure 6. In the chosen experimental condition, the following layer wise distribution showed best mineralogical phase distribution: B-A-C, CA-B, AB-C and B-B-B. Figure 7 shows the micrographs of layer wise as different size pellets. To further improve the mineralogy, the carbon content, firing temperature, machine speed and gas flow rate vary to further improve the results in other combination as well.
Table 6 Three, two and single layer size wise pellet phase analysis (Layerwise)

Physical and metallurgical properties of the pellets
Figure 8 Tumbler Index (T.I) of different size pellets as layer wise. The tumbler index (T.I.) is a measure of resistance to generate fines during handling and transportation. The T.I of Layer wise pellet like B-A-C, CA-B, AB-C and B-B-B showed a higher tumbler index compared to other pellets as different size as the layer. The single layer pellet with finer size (C-C-C) and coarser size (A-A-A) pellets showed a poor tumbler index because of higher porosity. Except finer and coarser size pellet as single layer other pellets of different size and layer wise pellets showed similar T.I of bench mark pellets tumbler index. To further improve the tumbler index, the carbon content, firing temperature, machine speed and gas flow rate vary to further improve the pellet mineralogy in other combination as well.
Table 7 and Table 8 shows the FeO content and strength of different size pellets as layer wise respectively. Figure 9 shows the effect of different size pellet as layer wise on pellet CCS. The size of the pellet, the position of the pellet within the pellet bed, thermo-chemical conditions like temperature and oxygen partial pressure influences the formation of the different phases within the pellets which affects the pellet strength. The layer wise pellet like B-A-C, CA-B, AB-C and B-B-B showed higher CCS compared to other pellets as different size as layer. The single layer pellet with fine (C-C-C) and coarser (A-A-A) size pellets showed poor CCS. Other pellets of different size and layer wise pellets showed similar CCS as per bench mark pellet CCS. To further improve the pellet strength, the carbon content, firing temperature, machine speed and gas flow rate is varied to further improve the pellet mineralogy in other combination as well.
Table 7 FeO content in layer wise different size pellets
Sl.No Expt No FeO,%
Layers Top Middle Bottom Average
1 Conventional method (Expt 16) Bench mark 0.75 0.53 0.6 0.63
2 Expt 1 A-B-C 1.20 0.35 0.30 0.62
3 Expt 2 A-C-B 1.18 0.42 0.4 0.67
4 Expt 3 B-A-C 0.20 0.60 0.38 0.35
5 Expt 4 B-C-A 0.22 0.43 1.00 0.55
6 Expt 5 C-B-A 0.8 0.85 0.92 0.86
7 Expt 6 C-A-B 0.82 0.87 0.68 0.76
8 Expt 7 A-BC 1.15 0.43 0.42 0.67
9 Expt 8 BC-A 0.40 0.40 0.95 0.58
10 Expt 9 B-CA 0.41 0.6 0.7 0.57
11 Expt 10 CA-B 0.55 0.45 0.42 0.47
12 Expt 11 C-AB 0.80 0.71 0.65 0.72
13 Expt 12 AB-C 0.72 0.48 0.44 0.55
14 Expt 13 A-A-A 1.02 0.79 0.90 0.90
15 Expt 14 B-B-B 0.40 0.25 0.31 0.32
16 Expt 15 CCC 0.9 0.82 0.74 0.82

Table 8 Strength of layer wise different size pellets
Sl.No Expt No CCS, kg/p Unfired pellets,%
Layers Top Middle Bottom Average
1 Conventional method (Expt 16) Bench mark 250 271 226 249 2.7
2 Expt 1 A-B-C 206 265 278 250 2.7
3 Expt 2 A-C-B 208 275 264 249 2.7
4 Expt 3 B-A-C 262 252 274 263 1.8
5 Expt 4 B-C-A 263 267 220 250 2.6
6 Expt 5 C-B-A 226 240 225 230 3.2
7 Expt 6 C-A-B 228 235 245 236 3.0
8 Expt 7 A-BC 210 272 263 248 2.7
9 Expt 8 BC-A 258 268 222 249 2.7
10 Expt 9 B-CA 265 248 224 246 2.8
11 Expt 10 CA-B 238 258 271 256 2.0
12 Expt 11 C-AB 227 238 242 236 3.1
13 Expt 12 AB-C 238 268 270 259 1.9
14 Expt 13 A-A-A 223 230 213 222 3.5
15 Expt 14 B-B-B 264 273 268 268 1.6
16 Expt 15 C-C-C 220 230 234 228 2.4

The reduction degradation (RDI) of the pellets indicates their tendency to generate the fines RDI -6.3mm) during reduction in the shaft furnace. Figure 10 shows theeffect of different size pellet as layer wise on RDI of the pellet.

In the typical conditions chosen, the layer wise pellet like B-A-C, CA-B, AB-C and B-B-B showed lower RDI compared to other pellets as different size as the layer. The single layer pellet with fine (C-C-C) and coarser (A-A-A) size pellets showed higher RDI. To further improve the pellet RDI, the carbon content, firing temperature, machine speed and gas flow rate is varied to further improve the pellet mineralogy in other combination as well.
Based on above experimental trials, new Proposed Practice of feeding of green pellets to induration furnace are as follows:
Sl. No Size of the pellets, mm Layer wise pellet as different size (three and two layer) Properties
T.I(+6.3mm),% CCS,Kg/t RDI (-6.3mm),%
1 A: -16+12.5 mm (Coarse)
B: -12.5+10 mm (Medium)
C: -10 mm (Smaller) B-A-C 93.22 263 8.50
2 CA-B 92.80 256 8.92
3 AB-C 93.0 259 8.70
4 B-B-B 93.60 268 8.20