Claims:We Claim:

1. Iron oxide pellets comprising iron oxide green pellets having iron ore concentrate having size distribution of iron ore particles with less than 10 micron size ultrafine particles of 30 to 40%, bentonite 0.5 to 1.0 %, limestone 2.20 to 2.40 %, coke breeze 0.5 to 1.0 % and moisture level of 7.0 to 11.0 % to having GCS of 1.35-1.45 kg/pellet, Drop No. of 11-15, and green pellet porosity of <34%.

2. Iron oxide pellets as claimed in claim 1 wherein said green pellets having iron ore concentrate from 62 to 64 % such as to generate fired pellet properties selectively CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5% and porosity < 28%.

3. Iron oxide pellets as claimed in anyone of claims 1 or 2 wherein said green pellets comprising bentonite 0.5 to 1.0% as sole binder and pellet moisture 7 to 11% with beneficiated iron concentrate having iron ore size of <10 micron (30 to 40%) and >10 micron size (60 to 70%) .

4. Iron oxide pellets as claimed in anyone of claims 1 to 3 wherein said green pellets comprising:

Fe in beneficiated iron concentrate 62 to 64%
Beneficiated concentrate fineness <10 micron: 30 to 40%, and
>10 micron 60 to 70%.
Green pellet moisture 7 to 11%
Bentonite (as sole binder) 0.5 to 1.0 %
Green pellet size 6 to 16mm
coke breeze 0.5 to 1.0 %
Pellet basicity (CaO/SiO2) 0.35 to 0.40

5. Iron oxide pellets as claimed in anyone of claims 1 to 4 which is fired iron oxide pellet obtained of said green pellets wherein said fired pellet properties include selectively CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5% and porosity < 28%.

6. A process for the manufacture of iron oxide pellets as claimed in anyone of claims 1 to 4 comprising the step of providing iron oxide green pellets :
involving low grade iron ore fines through beneficiation process and obtaining said green pellets by providing iron ore concentrate having size distribution of iron ore particles with less than 10 micron size ultrafine particles of 30 to 40%, bentonite 0.5 to 1.0%, limestone 2.20 to 2.40 %, coke breeze 0.5 to 1.0 % and moisture level of 7.0 to 11.0 %

7. A process as claimed in claim 6 wherein said step of providing iron oxide green pellets involve beneficiated iron concentrate having

Chemical analysis Beneficiated Iron Concentrate
Fe(t) 62 to 64.0
SiO2 3.0 to 4.58
Al2O3 1.50 to 2.45
CaO 0.05 to 0.09
MgO 0.01 to 0.03
LOI 1.97 – 2.40

and ground beneficiated iron concentrate as said having size distribution of iron ore particles with less than 10 micron size ultrafine particles of 30 to 40%.

8. A process as claimed in anyone of claims 6 or 7 wherein for said green pellets the raw materials used comprised:

Raw material %
Iron ore fines 94.90 - 97.20
Limestone 2.20 - 2.30
Bentonite 0.40 - 1.20
Coke breeze 0.30 - 1.30

and operating conditions as hereunder:

Fe in beneficiated iron concentrate 62 to 64%
Beneficiated concentrate fineness <10 micron: 30 to 40%, and
>10 micron 60 to 70%.
Green pellet moisture 7 to 11%
Bentonite (as sole binder) 0.5 to 1.0 %
Green pellet size 6 to 16mm
coke breeze 0.5 to 1.0 %
Pellet basicity (CaO/SiO2) 0.35 to 0.40

9. A process as claimed in anyone of claims 6 to 8 wherein said green pellets were subjected to firing in pellet plant induration furnace to obtain fired pellet properties include selectively CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5% and porosity < 28%.

10. A process as claimed in anyone of claims 6 to 9 wherein said step of firing was carried out under operating conditions including:

Feed rate, tph 550 - 600
Bed height, mm 500 - 560
Hearth layer, mm 30 – 80
Machine speed, m/min 2.30 - 2.45
Firing temperature, oC 1250- 1330
Burn-through temperature, oC 290 - 330
Direct Recuperation (DR) temperature, oC 800 - 950

and firing cycle time as hereunder:

Updraft drying, min 8.2 - 8.7
Down draft drying, min 6.5- 7.0
Preheating, min 2.4 - 2.6
Firing and after-firing, min 15.5 - 16.5
Cooling, min 14.7 - 15.7
Total time, min 47.4 - 50.4
Machine speed, m/min 2.45 to 2.30

Dated this the 25th day of April, 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 producing iron oxide pellets from green mix comprising ultrafine ore particles and good quality pellets obtained thereof. More particularly, the present invention is directed to the production of iron ore green pellets with feed mix comprising of iron ore concentrates having ultrafine particles (less than 10 micron size of 30 to 40%), bentonite (0.5 to 1%), limestone as flux to maintain basicity (0.35 to 0.40), green pellet moisture of 7 to 11%, coke breeze (0.5 to 1.0%) leading to green pellets having GCS of 1.35-1.45 kg/pellet, Drop No. of 11-15, green pellet porosity of <34% and subsequent firing through straight grate induration furnace with firing temperature of 1250 to 1330 oC. The optimized operating parameters such as pellet size distribution, bed permeability, moisture, bentonite dosage, coke breeze addition, pellet basicity and firing temperature ensuring production of good quality pellets with desired fired pellet properties like CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5%, suitable for iron making units.

BACKGROUND OF THE INVENTION

The heat hardening of green pellets by oxidation is a process commonly used in iron ore pelletization process. The green pellets are fired in straight grate indurating machine using Corex gas at JSW Steel Ltd. The sufficient green strength of pellets is essential to avoid the distortion and breakage while transport to the induration machine and withstand the static compressive load in the pellet bed. In indurating machine, spalling of green pellets occurs in the drying stage leading to the crumbling of pellets and loss of voidage which in turn adversely impact the bed permeability.

The quality of iron ore fines required for pellet making is one of the most important parameter to get the desired properties of the pellets. Pelletization of hematite iron ore fines requires optimum level of fineness (i.e., size distribution), binder, moisture, flux, carbon, etc. to maintain green pellet moisture, basicity, and firing temperature. Lesser finer fraction (-10 microns) corresponding to increased coarser fraction in iron ore fines, does not yield sufficient strength either to green pellets or indurated pellets.

Several researchers have reported that, for good quality of green and fired pellets the -45 micron size particles in iron ore fines should be in the range of 60 to 70% and Fe content should be more than 64%. But during the course of our study, it is observed that other than -45 micron size particles, the presence of optimum range of -10 micron size particles in iron ore fines, bentonite dosage, moisture, limestone, coke breeze and firing temperature significantly influence the pellet properties.

Patent No. US 6,334,883 B1, patent dated January 1, 2002 outlines about the pellets incorporated with a carbonaceous material and iron ore mainly composed of iron oxide. The maximum fluidity of the carbonaceous material in softening and melting, and the ratio of iron oxide particles of 10 micron or smaller in the iron ore are within the range above a line which connects in the turn points A, B and C shown in below Figure 1, including the line. This permits the production of pellets incorporated with a carbonaceous material having excellent thermal conductivity and high strength. Reduction of the pellets incorporated with a carbonaceous material produces reduced iron having high strength after reduction and a low fines ratio with improved productivity.

US Patent 2004/0020326 A1 dated Feb 5, 2004 mentions a method and facility for preventing crumbling and powderization of green pellets when producing high strength green pellets using the pellets in a rotary hearth reducing furnace and for efficiently reducing the same. In general, powders of two or more types of feed stock are used. This is for adjusting the ratio between the metal oxide and carbon. Metal oxide bearing green pellets for a reducing furnace obtained by shaping a feed powder containing an iron oxide bearing powder and 5 to 30 wt % of carbon bearing powder, said feed powder containing 20 to 80 wt% of particles of not more than 10 micron size. Production of pellets using pan type pelletizer and green pellets are screened by a pellet screen and then dried by a pellet dryer and reduced by firing in a rotary hearth furnace. In this work, no flux material like limestone was used. It used higher quantity of carbon (5 to 30%) and starch as binder along with bentonite. Drying of the pellets were done outside the rotary hearth furnace.

The technical paper titled “Influence of beneficiation plant pellet grade iron ore fines on pellet quality”, T Umadevi et.al., Steel Grips, 2013 mentions that the concentrate from beneficiation plant at JSW Steel Ltd. (i.e., pellet grade fines) is coarser (-45 micron in the range of 40-45%). Optimum particle size of the concentrate is required to get the desired properties of the pellets. Two ball mills were set up to grind the concentrate to obtain the optimum particle size for pellet making. Pelletization studies were carried out in laboratory by varying the ball mill discharge of 52 to 68% of -45 micron size to optimize the pellet grade size to achieve desired physical and metallurgical properties of the pellets. The desired physical and metallurgical properties were obtained with the iron ore fineness of -45 micron particles of 64%. The paper does not mention about the ultrafine particles i.e. <10 µm size and other parameters like bentonite, moisture, firing temp, carbon content etc.

The technical paper titled “Influence of pellet basicity (CaO/SiO2) on iron ore pellet properties and microstructure”, T Umadevi et.al., ISIJ international, Vol.51, No. 1, (2011), pp 14-20, mentions that the quality of the pellets depends on the type of raw materials, gangue content, flux proportion and their subsequent treatment to produce pellets. The limestone addition, i.e., basicity (CaO/SiO2 of 0.40 to 0.50.) of pellet decides the mode, temperature and the amount of melt formed. The test result revealed that the pellet properties were influenced by the bonding phases present in the pellet. The tumbler index increased from 93.15 to 95.38% and cold crushing strength increased from 176 to 264 kg/p with increase in pellet basicity from 0.08 to 1.15. RDI of the pellet decreased initially from 16.3 to 10.9% with pellet basicity of 0.08 to 0.33 and again increased from 10.9 to 13.6 with increase in pellet basicity from 0.33 to 1.15. This effect is due to the change in structural properties of the pellet during reduction. The paper does not mention about the optimum ultra-fines size i.e. <10 micron size, green pellet moisture, bentonite, carbon percentage and firing temperature.

The gamut of literature on pellet making refers that, the size distribution of iron ore fines for green ball formation with sufficient green strength should have -45 micron size particles in the range of 60 to 70% and -150 microns to be 100%. However, there is no mention of the size distribution of finer fractions such as -10 micron particles and -25 micron particles. During the course of present study, inventors observed that the presence of ultrafine particles (-10 micron size particles) play crucial role in balling of the fines. The beneficiated concentrate contains 62-64% Fe, 3-4.5% SiO2, 2.5-3.5% Al2O3 to produce desired quality of pellets for iron making units.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to a process for production of good quality iron oxide pellets making use of ultrafine ore particles(-10 micron size particles) in selective proportion in green pellet feed mix and the pellet product having improved properties obtained thereof suitable for use in iron making units.

A further object of the present invention is directed to a process for preparation of good quality iron oxide green pellets (GCS of 1.35-1.45 kg/pellet, Drop No. of 11-15) using feed mix comprising of iron ore concentrate (optimum size distribution of iron ore particles with less than 10 micron size ultrafine particles of 30 to 40%) and optimum additives like bentonite, moisture, limestone and coke breeze.

A still further object of the present invention is directed to a process for production of good quality iron oxide/ore pellets involving optimized use of additives in feed mix such as bentonite dosage (0.5 to 1%), flux for maintaining basicity (0.35 to 0.40), green pellet moisture (7 to 11%), and coke breeze (0.5 to 1.0%) to ensure good quality of green pellets.

A still further object of the present invention is directed to a process for production of good quality iron oxide/ore fired pellets involving heat hardening through straight grate induration furnace with optimization of furnace operating parameters(firing temperature of 1250 to 1330 oC) to produce good quality fired pellets having CCS of 242-258 kg/pellet, TI of 94-96%, RDI of 9.5-11.2% by subjecting to firing through straight grate induration furnace suitable for economized use in iron making units.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to iron oxide pellets comprising iron oxide green pellets having iron ore concentrate having size distribution of of iron ore particles with less than 10 micron size ultrafine particles of 30 to 40%, bentonite 0.5 to 1.0 %, limestone 2.20 to 2.40 %, coke breeze 0.5 to 1.0 % and moisture level of to having GCS of 1.35 to 1.45 kg/pellet, Drop No. of 11 to 15, green pellet porosity of <34%.

A further aspect of the present invention is directed to iron oxide pellets wherein said green pellets having iron ore concentrate from 62 to 64 % such as to generate fired pellet properties selectively CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5% and porosity < 28%.

A still further aspect of the present invention is directed to iron oxide pellets wherein said green pellets comprising bentonite 0.5 to 1.0% as sole binder and pellet moisture 7 to 11% with beneficiated iron concentrate having iron ore size of <10 micron (30 to 40%) and >10 micron size(60 to 70%) .

A still further aspect of the present invention is directed to iron oxide pellets wherein said green pellets comprising:

Fe in beneficiated iron concentrate 62 to 64%
Beneficiated concentrate fineness <10 micron: 30 to 40%, and
>10 micron 60 to 70%.
Green pellet moisture 7 to 11%
Bentonite (as sole binder) 0.5 to 1.0 %
Green pellet size 6 to 16mm
coke breeze 0.5 to 1.0 %
Pellet basicity (CaO/SiO2) 0.35 to 0.40

A still further aspect of the present invention is directed to iron oxide pellets which is fired iron oxide pellet obtained of said green pellets wherein said fired pellet properties include selectively CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5% and porosity < 28%.

Another aspect of the present invention is directed to a process for the manufacture of iron oxide pellets as described above comprising the step of providing iron oxide green pellets :
involving low grade iron ore fines through beneficiation process and obtaining said green pellets by providing iron ore concentrate having size distribution of iron ore particles with less than 10 micron size ultrafine particles of 30 to 40%, bentonite 0.5 to 1.0 % ,limestone 2.20 to 2.40 %, coke breeze 0.5 to 1.0 % and moisture level of 7 to 11 %.

Yet another aspect of the present invention is directed to said process wherein said step of providing iron oxide green pellets involve beneficiated iron concentrate having

Chemical analysis Beneficiated Iron Concentrate
Fe(t) 62 to 64.0
SiO2 3.0 to 4.58
Al2O3 1.50 to 2.45
CaO 0.05 to 0.09
MgO 0.01 to 0.03
LOI 1.97 – 2.40

and ground beneficiated iron concentrate as said having size distribution of iron ore particles with
less than 10 micron size ultrafine particles of 30 to 40%.

A further aspect of the present invention is directed to said process wherein for said green pellets the raw materials used comprised:
Raw material %
Iron ore fines 94.90 - 97.20
Limestone 2.20 - 2.30
Bentonite 0.40 - 1.20
Coke breeze 0.30 - 1.30

and operating conditions as hereunder:
Fe in beneficiated iron concentrate 62 to 64%
Beneficiated concentrate fineness <10 micron: 30 to 40%, and
>10 micron 60 to 70%.
Green pellet moisture 7 to 11%
Bentonite (as sole binder) 0.5 to 1.0 %
Green pellet size 6 to 16mm
coke breeze 0.5 to 1.0 %
Pellet basicity (CaO/SiO2) 0.35 to 0.40

A still further aspect of the present invention is directed to said process as claimed in anyone of claims 6 to 8 wherein said green pellets were subjected to firing in pellet plant induration furnace to obtain fired pellet properties include selectively CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5% and porosity < 28%.

A still further aspect of the present invention is directed to a process wherein said step of firing was carried out under operating conditions including:
Feed rate, tph 550 - 600
Bed height, mm 500 - 560
Hearth layer, mm 30 – 80
Machine speed, m/min 2.30 - 2.45
Firing temperature, oC 1250- 1330
Burn-through temperature, oC 290 - 330
DR temperature, oC 800 - 950

and firing cycle time as hereunder:

Updraft drying, min 8.2 - 8.7
Down draft drying, min 6.5- 7.0
Preheating, min 2.4 - 2.6
Firing and after-firing, min 15.5 - 16.5
Cooling, min 14.7 - 15.7
Total time, min 47.4 - 50.4
Machine speed, m/min 2.45 to 2.30

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPNAYING DRAWINGS

Figure 1: Process flow diagram for pellet making.
Figure 2: Firing of pellets in induration machine.
Figure 3: show graphical plot of influence of green pellet moisture on (a) GCS and (b) drop number.
Figure 4: show graphical plot of influence of bentonite addition on (a) GCS and (b) drop number.
Figure 5: shows graphically the influence of coke breeze addition on (a) CCS and (b) T.I of the pellet.
Figure 6: shows the graphically the influence of firing temperature on (a) CCS and (b) T.I of the pellet.

The present invention is directed to a process for production of good quality iron oxide pellets making use of ultrafine ore particles(-10 micron size particles) in selective proportion in green pellet feed mix and the pellet product having improved properties obtained thereof suitable for use in iron making units. The invention optimize the use of additives in feed mix such as bentonite dosage (0.5 to 1%), flux for maintaining basicity (0.35 to 0.40), green pellet moisture (7 to 11%), and coke breeze (0.5 to 1.0%) to ensure good quality of green pellets. Thereafter, green pellets are heat hardened through straight grate induration furnace with optimization of furnace operating parameters (firing temperature of 1250 to 1330 oC) to produce good quality fired pellets.

Process description for pellet making:

The production of iron oxide pellets from iron ore fines involves different steps from drying to induration. The low grade iron ore fines having 55-60% Fe was enriched to 62-64% Fe through beneficiation process followed by wet grinding & filtration. The ground ore fines are mixed in mixer with bentonite and limestone to maintain basicity of 0.35-0.40 and coke breeze to achieve the uniform firing of the pellets. The green mix was fed to pelletizing/balling disc to prepare green pellets/balls. In balling disc, moisture was added to compensate for the required green pellet moisture. The produced green pellets were screened to get the green pellets size in the range of 6 to 16 mm as per the plant requirement. These green pellets are fired in straight grate indurating furnace for pellet hardening to achieve the required physico-chemical, mechanical and metallurgical properties. Accompanying Figure 1 shows the schematic diagram of the steps involved iniron ore pelletization process according to present invention.

Accompanying Figure 2 shows the schematic diagram of firing of green pellets in a straight grate induration machine. The items/zones and fans at various locations in induration system indicated in Figure 2 are as follows:

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

The green pellets, after sizing through a roller conveyor, are discharged onto the pallet car(1)/travelling grate of 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 temperature. The travelling grate carries the pellets through the indurating furnace where they are subjected to the sequential zones of updraft drying(2), downdraft drying(3), preheating(4), firing & after firing(5) and first & second cooling(6). 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(2) the hot gas is recycled from the cooling zone(6). The drying is continued in a subsequent downdraft stage(3) to remove free water of the top of the bed by relatively hot gases coming from the firing zone(5) 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 generate cracks due to high internal stresses. In the preheating zone(4), the pellets are heated to about 500 to 1000 oC by downdraft air flowing through the bed and the hot gas is recycled from the cooling zone (6). 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(4) continue in the firing zone(5). Here pellet charge is heated to an optimum temperature for a controlled period. Strength of the pellets increases at this stage because of re-crystallization and formation of slag phase. The off-gas from the firing zone(5) is recuperated to the drying zone. The circulation of air/hot gases in different zones is activated by hood exhaust fan(7), updraft drying fan(8), wind box exhaust fan(9), wind box recuperation fan(10) & cooling air fan(11).

Detailed description of the invention:

The quality of iron ore fines required for pellet making is one of the most important parameter to get the desired properties of the pellets which are required for blast furnace/corex iron making units. The produced pellets for iron making units should have optimum physical and physicochemical characteristics to withstand the number of physical and chemical transitions during its processing. Pelletization of hematite iron ore fines requires high level of fineness of iron ore. The lower finer fraction corresponding to coarser size of fines, does not provide sufficient strength both to the green pellets and indurated pellets. Due to depletion of high grade iron ore fines and increasing demand and production of iron and steel it is essential to utilize low grade iron ore fines through beneficiation process.

Example:
Although many research works have been conducted, further investigation on the effect of iron ore fines size on green and fired quality of pellet is carried out by way of present invention for further improvement. Detailed investigations have been carried out on different particle size iron ore fines (by controlled grinding in lab) to optimize the iron ore fines’ fineness to achieve the desired green and fired properties of the pellets.

(i) To get the good green pellet properties optimum iron ore fines size is required and this is achieved through grinding. To achieve the optimum pellet feed size of beneficiation plant product needs more grinding. Table 1 and Table 2 shows the chemical analysis and size analysis of material collected for pelletization studies respectively. To study the effect of 10 micron size on green and fired pellet properties along with other parameters detailed pelletization studies have been carried out using beneficiation plant concentrate (after controlled grinding in lab to achieve different percentage of 10 micron size) The mix proportion of the green mix is shown in Table 3.

Table 1: Chemical analysis of the raw material

Chemical analysis, % Proximate analysis, %
Beneficiated Iron Concentrate Limestone Bentonite Coke breeze
Fe(t) 62 to 64.0 0.17 8.15 Ash 17.25
SiO2 3.0 to 4.58 0.34 46.62 VM 2.49
Al2O3 1.50 to 2.45 0.17 19.66 FC 80.27
CaO 0.05 to 0.09 55.59 2.37
MgO 0.01 to 0.03 0.88 2.87
LOI 1.97 – 2.40 --- ---

Table 2: Size analysis of the raw material

Size in microns Beneficiated Iron Concentrate (before grinding), % Beneficiated Iron Concentrate (After grinding), %
+150 6.0 to 17.0 1.0 to 2.0
-150+75 32 to 58.0 98.0 to 99.0
-75+45 25 to 45.5 82.6 to 94.1
-45+25 19.4 to 33.80 62.9 to 78.0
-25+10 12.5 to 25.0 44.2 to 60.3
-10 6.8 to 15.0 26.0 to 42.0

Table 3: Raw material mix proportion

Raw material %
Iron ore fines 94.90 - 97.20
Limestone 2.20 - 2.30
Bentonite 0.40 - 1.20
Coke breeze 0.30 - 1.30

(ii) This study aims at studying the effect of hematite iron ore fineness i.e., <10 micron and >10 micron size fraction on pellet properties by considering other parameters like moisture addition, bentonite dosage, coke breeze addition and firing temperature at pellet basicity of 0.35 to 0.40).

(iii) Pelletizing experiments have been carried out by grinding the beneficiation plant concentrate to different fineness (to achieve different 10 micron size fraction from 26 to 42%) in pellet feed. The feed mix for each experiment consist of iron ore fines, limestone (to maintain pellet basicity 0.35), bentonite (varied from 0.3 to 1.2%), coke breeze (varied from 0.3 to 1.1%) and green pellet moisture (varied from 7 to 11%).

Table 4 shows size analysis of ground material of beneficiation plant concentrate for pelletization studies. The -10 micron size in iron ore fines was varied from 26 to 42%. The pellet mix was fed to balling disc to prepare the green pellets. The balling disc parameters are maintained as follows:
• Disc diameter : 450 mm
• Disc inclination : 45o
• Disc speed : 38 rpm

Total nine number of experiments were carried out using different combination of dry & beneficiation concentrate 1, beneficiation plant concentrate 2.

Table 4: Ground Iron ore fines size of beneficiation plant concentrate (Feed to pellet making)

(iv) Green pellet properties like drop number and green crushing strength were determined for each experiment by standard test method.

(v) For firing, each set of experiment comprises 20 kg of green pellets. The green pellets were kept in rectangular stainless steel baskets (350 mm long) and fired in pellet plant induration furnace. The firing temperature was varied from 1250 to 1330 oC based on coke breeze content. The basket was kept at the centre of the pellet bed on the top of the hearth layer. The corresponding operating parameters of the furnace are shown in Table 5. Fired pellets were subjected to evaluations of chemical, physical and metallurgical properties. The firing cycle time is shown in Table 6. The optimization of green pellet moisture, bentonite was carried out to achieve the desired green pellet properties by maintaining -10 micron size from 26 to 42%. Later optimization of coke breeze, and firing temperature was carried out by maintaining the pellet basicity 0.35.

Table 5: Operating condition of indurating furnace at pellet plant.

Feed rate, tph 550 - 600
Bed height, mm 500 - 560
Hearth layer, mm 30 – 80
Machine speed, m/min 2.30 - 2.45
Firing temperature, oC 1250- 1330
Burn-through temperature, oC 290 - 330
Direct Recuperation (DR) temperature, oC 800 - 950

Table 6: Firing cycle time

Updraft drying, min 8.2 - 8.7
Down draft drying, min 6.5- 7.0
Preheating, min 2.4 - 2.6
Firing and after-firing, min 15.5 - 16.5
Cooling, min 14.7 - 15.7
Total time, min 47.4 - 50.4
Machine speed, m/min 2.45 to 2.30

(vi) The green and fired properties of the pellets with different percentage of 10 micron size are shown in Table 7.
Table 7: Green and fired properties of the pellets

Where GCS - Green pellet strength, CCS - Cold crushing strength, TI - Tumbler index (+6.3mm),% & RDI - Reduction degradation index stands respectively.

Results of experiments and conclusion:

From Table 7, it was found that -10 micron size particles in iron ore fines <30% and >40% showed poor fired pellet properties. On the basis of experimental results, 30 to 40% of -10 µm size particles in pellet mix is required to achieve the desired green and fired properties of the pellets required for iron making units. Figures 3 and 4 show the effect of moisture and bentonite addition on green pellet properties respectively wherein Figure 3 shows influence of green pellet moisture on GCS and drop number and Figure 4 shows influence of bentonite addition on GCS and drop number.

Figures 5 and 6 show the effect of coke breeze and firing temperature on CCS and T.I of pellets respectively wherein Figure 5 shows the influence of coke breeze addition on CCS and T.I of the pellet and Figure 6 shows the influence of firing temperature on CCS and T.I of the pellet.

The present and proposed practice of iron ore pelletization according to present invention are thus summarized as below.

Present Practice: Less than 10 µm size particles in the pellet feed of iron ore fines is varying from 20 to 45%. Wide fluctuations in ultra-fines size particles in the pellet feed is found influencing the quality of pellets. Besides the ultrafine particles (<10 micron) of iron ore fines, other parameters like green pellet moisture and bentonite dosage with respect to iron ore fineness, pellet basicity, coke breeze and firing temperature also influence the green and fired properties of the pellets.

Proposed pelletization practice as ascertained based on the experiments carried out under the present invention to produce good quality green as well as fired pellets are summarised as follows:

Fe in beneficiated iron concentrate 62 to 64%
Beneficiated concentrate fineness <10 micron: 30 to 40%, and
>10 micron 60 to 70%.
Green pellet moisture 7 to 11%
Bentonite (as sole binder) 0.5 to 1.0 %
Green pellet size 6 to 16mm
coke breeze 0.5 to 1.0 %
Pellet basicity (CaO/SiO2) 0.35 to 0.40
Firing temperature 1250 to 1330 oC

It is thus possible by way of the present invention to provide a process for production of iron oxide green pellets having GCS of 1.35-1.45 kg/pellet, Drop No. of 11-15, green pellet porosity of <34% using feed mix comprising of iron ore concentrate (optimum size distribution of iron ore particles with less than 10 micron size ultrafine particles of 30 to 40%) and optimum additives like bentonite, moisture, limestone and coke breeze. More importantly, with Fe content in beneficiated iron concentrate varying from 62 to 64% and optimized operating parameters including firing temperature of 1250 to 1330 oC, with selective firing cycle time and machine speed for pellet making in straight grate induration furnace, the desired fired pellet properties like CCS of >240 kg/pellet, TI (+6.3 mm) of >94%, and RDI (-6.3 mm) of <11.5% with fired pellet porosity should be less than 28%,was achieved with the pellet mix comprising of -10 micron size particles (30 to 40%) and >10 micron size(60 to 70%) in beneficiated iron ore concentrate, have been achieved making the product suitable for iron making units.