Claims:We Claim:
1. Flux coated iron ore pellets comprising of :
Fe(T) in amounts of 61.0 - 63.5 % by wt.;
SiO2 in amounts of 2.5 - 5.5 % by wt;
Al2O3 in amounts of 2.3 - 4.0 % by wt;
MgO in amounts of 0.45 - 0.80 % by wt;
LOI in amounts of 2.0 - 4.5 % by wt;
Carbon in amount of 0.80-1.60% by wt;
and having a flux coating in the range of 0.60 - 0.66 % by wt,
wherein the Tumbler index (TI) of flux coated fired pellets is in the range of 91 to 96% (+6.3 mm size) and the surface hardness in the range of 260 to 370 Hv.

2. Flux coated iron ore pellets as claimed in claim 1 wherein said flux coating is provided selectively to increase Tumbler index (TI) from 91 to 96% (+6.33 mm size) and surface hardness increased form average 260 Hv to average 370 Hv with respect to uncoated pellets.

3. Flux coated iron ore pellets as claimed in anyone of claims 1 or 2 wherein the fired pellet have basicity (B2) of 0.30 - 0.45 preferably 0.35 and (B4) in range of 0.20 - 0.30.

4. Flux coated iron ore pellets as claimed in anyone of claims 1 to 3 wherein said flux coating comprise selectively of dolomite, limestone, pyroxenite as flux coating material and slag coating material are LHF1 slag ( silicon killed), and LHF3 slag (aluminium killed).

5. Flux coated iron ore pellets as claimed in anyone of claims 1 to 4 wherein said flux coated material comprise of fineness such that 85-90% pass through less than 200 mesh which is blaine number : 2800-3200 cm2/g.
6. Flux coated iron ore pellets as claimed in anyone of claims 1 to 5 having said flux coating material in raw form preferably dolomite in the form of CaCO3.MgCO3 and limestone as CaCO3 including slag used preferably ladle furnace slag of secondary steel making process.

7. Flux coated iron ore pellets as claimed in anyone of claims 1 to 6 wherein the plate water absorption of coated material are in the range of 45-60.

8. Flux coated iron ore pellets as claimed in anyone of claims 1 to 7 having green condition thickness of 10 to 30 microns and increased thickness of transition layer of complex slag phases (20-60 microns) as fired pellets.

9. Flux coated iron ore pellets as claimed in anyone of claims 1 to 8 wherein the coat material diffuses inside the pellet and wherein selectively the dolomite coated pellets have MgO in range of 3-4% at transition layer which is richer in iron oxide, the Limestone coated pellet have CaO around 28-32% and iron oxide in the range of 50-54%, the LHF1 coated pellets have silicate slag richer in CaO and Iron oxide and LHF3 slag generated contains alumina 7-10 % which makes slag viscous owing to higher alumina to silica ratio of the slag.

10. Flux coated iron ore pellets as claimed in anyone of claims 1 to 9 wherein the fired pellets have a basicity (B2 = CaO/SiO2) of 0.30 - 0.45 and basicity [B4 = (CaO+MgO) / (SiO2+Al2O3)] of 0.20 - 0.30.

11. Flux coated iron ore pellets as claimed in anyone of claims 1 to 10 wherein said coated pellets have tumbler index value (+6.3 mm) around 96-97% for dolomite, 94-96% for limestone, 95-96% for LHF1 slag, 90-91% for LHF3 slag, and 95.5-96.5% in pyroxenite coated pellets.

12. Flux coated iron ore pellets as claimed in anyone of claims 1 to 11 having moisture content in green condition in the range of 9 to 10%.

13. A process for manufacture of flux coated iron ore pellets comprising:
(i) providing green pellets;
(ii) applying flux coating onto the surface of said green pellets ;
(iii) subjecting the green coated pellets thus obtained to induration such that during induration, higher surface temperature allows the diffusion of the flux particles towards the core of the pellet and in the process producing hard surface and surface confines pores.

14. A process as claimed in claim 13 wherein the pellet composition selectively used comprises of;
Fe(T) in amounts of 61.0 - 63.5 % by wt.;
SiO2 in amounts of 2.5 - 5.5 % by wt;
Al2O3 in amounts of 2.3 - 4.0 % by wt;
MgO in amounts of 0.45 - 0.80 % by wt;
LOI in amounts of 2.0 - 4.5 % by wt;
Carbon in amount of 0.80-1.60% by wt;
and said flux coating is carried out in the range of 0.60 - 0.66 % by wt,
such as to obtain the Tumbler index (TI) of flux coated fired pellets is in the range of 91 to 96% (+6.3 mm size) and the surface hardness in the range of 260 to 370 Hv.

15. A process as claimed in anyone of claims 13 or 14 wherein the basicity of fired pellet is maintained to have basicity (B2) of 0.30 - 0.45 preferably 0.35 and (B4) in range of 0.20 - 0.30.

16. A process as claimed in anyone of claims 13 to 15 wherein said flux coating used comprise selectively of dolomite, limestone, pyroxenite as flux coating material and slag coating material are LHF1 slag ( silicon killed), and LHF3 slag (aluminium killed).

17. A process as claimed in anyone of claims 13 to 16 wherein said flux coated green pellets prior to firing comprise of selective fineness such that 85-90% pass through less than 200 mesh which is blaine number : 2800-3200 cm2/g.

18. A process as claimed in anyone of claims 13 to 17 wherein said flux coating material used in raw form selected is preferably dolomite in the form of CaCO3.MgCO3 and limestone as CaCO3 including slag used preferably ladle furnace slag of secondary steel making process.

19. A process as claimed in anyone of claims 13 to 18 wherein the plate water absorption of coated material is maintained in the range of 45-60.

20. A process as claimed in anyone of claims 13 to 19 wherein the flux coating thickness varies and is controlled such that said pellets in green condition have a coating thickness of 10 to 30 microns which subsequently increases to thickness of transition layer of complex slag phases (20-60 microns) as fired pellets.

21. A process as claimed in anyone of claims 13 to 20 wherein the basicity of the fired pellets is maintained such as to attain a basicity (B2 = CaO/SiO2) of 0.30 - 0.45 and basicity [B4 = (CaO+MgO) / (SiO2+Al2O3)] of 0.20 - 0.30.

22. A process as claimed in anyone of claims 13 to 21 wherein moisture content in green condition is maintained in the range of 9 to 10%.

23. A process as claimed in anyone of claims 13 to 22 wherein said green pellets are produced involving a disc pelletizer having inclination of 41 to 450 preferably about 440 with disc rotation of 16 to 22 rpm, preferably 18 rpm for 20 minutes and the coating of pellets carried out for additional 4-6 minutes preferably 5 minutes..

24. A process as claimed in anyone of claims 13 to 23 comprising reducing gases involved including selectively CO, hydrogen and nitrogen preferably in combination of 58-63 % CO, 30-35 % hydrogen gas, and remaining 5-7 % nitrogen for reduction of coated pellets. The reducibility of the coated pellets is found to be 96-97 %.

25. A process as claimed in any of claims 13 to 24 wherein green coated pellets are provided to have lower moisture content in the range of 9 to 10% compared to the uncoated pellets in the range of 10.2 to 10.3% to facilitate the faster drying of green pellets in drying zone of induration furnace.

Dated this the 27th day of December, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION

The present invention relates to flux coated iron ore pellets with improved surface hardness & tumbler index and a process for manufacturing such pellets. More particularly, the present invention is directed towards a process for improving the tumbler index of iron ore pellets by applying the flux rich coating of dolomite, limestone, pyroxenite or LHF slag on green iron pellets. The coating of such materials helps in formation of thin hard layer of complex slag enriched with iron oxide on the surface of pellets. The coating material gets adhered to the green pellet surface during pelletization in laboratory disc pelletizer or plant disc pelletizer before entering the pellets in induration furnace. The higher temperature between 1000 to 1330 0C during induration helps the coated layer to get diffuse inside the pellet surface up to certain depth. The pellets after firing consist of the fluxed rich layer at the surface or shell of the pellets followed by transition layer of flux and iron oxide rich slag and lastly the core consist of original pellet composition. The coated material in case of limestone or dolomite get calcined during induration and diffuses slowly inside the pellets surface and helps in maintaining the overall porosity and reducibility as compared to non-coated pellets. The process comprises of making green pellets in disc pelletizer then spraying the coated flux once the pellet reach the mean particle size (MPS) of 10.50 - 12.50 mm in disc pelletizer. In order to control the coating thickness it is more appropriate that the coating material be sprayed after screening of pellets and before entering the pellets in induration furnace.

The green coated pellets so formed are subjected to induration and helps in increasing the TI of pellets from 91 to 96% (+6.3 mm size) and the surface hardness of pellets from average 260 Hv to average 370 Hv.

BACKGROUND OF THE INVENTION
In iron ore pelletization process, moisture, flux and binders are added to the iron ore powder, mixed, pelletized and subsequently thermally treated to attain the required properties. Tumbler index of pellets is basically governed by Blaine number of iron ore fines, moisture, binder, additives, basicity and firing temperature. Many researchers and literature reported that the pellets are externally coated with different metal oxides during DRI making or BF iron making to reduce the dust generation and sticking of pellets. Results from these investigations indicate that the sticking behavior of pellets is a result of the growth of fibrous iron precipitates (iron whiskers) that hooked to each other and finally crystallizes during initial stage of metallization.

The prior patents, viz., Berrun-Castanon, J. et al. US5181954; Goetzman, H. US3975182; Sandoval, J. GB1514777 state that one way to prevent the pellet sticking is to coat the iron surfaces with a coating material that is inactive under the reducing conditions in the shaft furnace. However, a single coating has drawbacks that include ineffective agglomerate prevention during reduction and the loss of the coating prematurely during shipment or movement prior to the reduction. However, such coating on fired pellets does not help in increasing any physical properties.

Patent No. WO 2015068104 A1 discloses the methods and compositions for producing coated iron oxide pellets comprising an outer coating of cement exhibiting a reduced sticking index while retaining a high level of metallization following reduction. The improved coated iron oxide pellets can be used to produce direct reduced iron (DRI) with improved productivity. The coating was provided after firing of pellets.

German Patent Application No. 2,061,346 describes the method of forming a coating of ceramic powder on iron ore pellets. This ceramic powder can be limestone, dolomite, talc (basic magnesium silicate), lime or cement. This patent application however does not include or suggest any specific method or apparatus to apply said coating on the iron ore pellets. The significant problem of finding a way of handling a suspension of cement in large industrial quantities remains.

Patent No. WO 2017006200 A1 states that iron ore pellets including a core comprising iron ore, a first coating comprising lime, and a second coating comprising cement, wherein the first coating is disposed between a surface of the core and the second coating. A process for manufacturing the iron ore pellets whereby the first coating is applied to the core to form a coated core, the surface area coverage of the first coating is measured, the second coating is applied to the coated core, and the surface area coverage of the second coating is measured. A process for manufacturing reduced iron pellets is also provided whereby the iron ore pellets are reduced with a reducing gas at temperatures up to 1100 ºC.
US Patent 2013/0111809 A1 discloses a method wherein green pellets comprising of inner core containing an iron oxide, a carbonaceous material for reduction, and a slag-form wherein the coating layer is formed as a protective layer, which is disposed off so as to encapsulate the surface of the inner core layer which contains an inorganic compound (includes an alkali oxides) having a melting point 750 °C or higher but less than 1100 °C, and a combustion layer (carbonaceous material), which is disposed off so as to encapsulate the surface of the protective layer.

Thus, there has been a need to develop a formulation and process of producing shell coating on iron ore pellets in green condition which not only helps in controlling the pellets moisture and diffusion of such coating on pellet surface but also help improving tumbler index of fired pellet products for advantageous application in blast furnace, Corex, DRI making or any other iron making process.

The fired pellets made with binders were studied to investigate the effect of firing temperature and time on pellet hardness. The increase in temperature and induration time shows a pronounced effect on hardness of the pellets. However, it was observed that the hardness values are decreasing from surface to the core of the pellet in all conditions. The hardness of fired pellets made with organic binders has shown improvement over without binder but less than that of inorganic binder (bentonite). Hardness values of 150 - 332 Hv were achieved with increase in temperatures up to 1200 °C. Pellets made with higher silica and alumina containing iron ores with basicity of 0.20 to 0.40 have shown tendency of formation of fragile surface due to poor slag bonding on surface. This resulted in lower surface hardness, lower abrasion & tumbler index, and higher dust generation.

None of the above literature and patents discloses the usage of flux coating through limestone, Dolomite and ladle furnace slag on green pellets. Similarly, the gases and their composition used herein for the reduction are fully different from the above mentioned work to determine the sticking or clustering behavior of pellets. The rate of oxide diffusion inside the pellets and coating thickness of fired pellets is key factor to enhance the mechanical abrasion index and surface hardness of pellets.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide flux coated iron ore pellets by applying flux rich coating on the green iron ore pellets prior to induration to achieve improved physical properties of fired iron ore pellets and a process for producing such pellets favoring improved surface hardness of fired pellets with improved tumbler index.

Another object of the present invention is directed to said process to improve tumbler index of ore pellets by forming a hard slag layer wherein addition of surface coating materials help in formation of low melting slag basically magnesio-ferrite, silico-ferrite, calcium-silicate and calcium-aluminate which are rich in iron. The slag adhere to pellet surface very tightly which can’t be removed even after tumbling of pellets.

A still further object of the present invention is directed to the said process to control the moisture in green pellets by spraying of dry ground flux/slag on the pellet surface which reduce the overall pellet moisture and hence quick drying of pellets in downdraft and updraft zones of induration furnace.

A further object of the present invention is directed to the said process to restrict the dust generation in pellet making as well in blast furnace, Corex, DRI making or any other iron making process.

A further object of the present invention is directed to the said process to maintain reducibility of the flux coated iron ore pellets wherein the slag covered iron grains revealed large number of micro-pores. The surface of coated pellets having increased micro-pores facilitates easy access for the reducing gases to reach to the core of the pellets.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to flux coated iron ore pellets comprising of;
Fe(T) in amounts of 61.0 - 63.5 % by wt.;
SiO2 in amounts of 2.5 - 5.5 % by wt;
Al2O3 in amounts of 2.3 - 4.0 % by wt;
MgO in amounts of 0.45 - 0.80 % by wt;
LOI in amounts of 2.0 - 4.5 % by wt;
Carbon in amount of 0.80-1.60% by wt;
and having a flux coating in the range of 0.60 - 0.66 % by wt,
wherein the Tumbler index (TI) of flux coated fired pellets is in the range of 91 to 96 % (+6.33 mm size) and surface hardness in the range of 260 to 370 Hv.

A further aspect of the present invention is directed to said flux coated iron ore pellets wherein said flux coating is provided selectively to increase Tumbler index (TI) from 91 to 96% (+6.33 mm size) and surface hardness increased form average 260 Hv to average 370 Hv with respect to uncoated pellets.

A still further aspect of the present invention is directed to said flux coated iron ore pellets wherein the fired pellet have basicity (B2) of 0.30 - 0.45 preferably 0.35 and (B4) in range of 0.20 - 0.30.

A still further aspect of the present invention is directed to said flux coated iron ore pellets wherein said flux coating comprise selectively of dolomite, limestone, pyroxenite as flux coating material and slag coating material are LHF1 slag ( silicon killed), and LHF3 slag (aluminium killed).

Another aspect of the present invention is directed to said flux coated iron ore pellets wherein said flux coated material comprise of fineness such that 85-90% pass through less than 200 mesh which is blaine number : 2800-3200 cm2/g.
Yet another aspect of the present invention is directed to said flux coated iron ore pellets as claimed in anyone of claims 1 to 5 having said flux coating material in raw form preferably dolomite in the form of CaCO3.MgCO3 and limestone as CaCO3 including slag used preferably ladle furnace slag of secondary steel making process.

A further aspect of the present invention is directed to said flux coated iron ore pellets wherein the plate water absorption of coated material are in the range of 45-60.

A still further aspect of the present invention is directed to said flux coated iron ore pellets having green condition thickness of 10 to 30 microns and increased thickness of transition layer of complex slag phases (20-60 microns) as fired pellets.

A still further aspect of the present invention is directed to said flux coated iron ore pellets wherein the coat material diffuses inside the pellet and wherein selectively the dolomite coated pellets have MgO in range of 3-4% at transition layer which is richer in iron oxide, the Limestone coated pellet have CaO around 28-32% and iron oxide in the range of 50-54%, the LHF1 coated pellets have silicate slag richer in CaO and Iron oxide and LHF3 slag generated contains alumina 7-10 % which makes slag viscous owing to higher alumina to silica ratio of the slag.

A still further aspect of the present invention is directed to said flux coated iron ore pellets wherein the fired pellets have a basicity (B2 = CaO/SiO2) of 0.30 - 0.45 and basicity [B4 = (CaO+MgO) / (SiO2+Al2O3)] of 0.20 - 0.30.

A still further aspect of the present invention is directed to said flux coated iron ore pellets wherein said coated pellets have tumbler index value (+6.3 mm) around 96-97% for dolomite, 94-96% for limestone, 95-96% for LHF1 slag, 90-91% for LHF3 slag, and 95.5-96.5% in pyroxenite coated pellets.

A still further aspect of the present invention is directed to said flux coated iron ore pellets having moisture content in green condition in the range of 9 to 10%.

A further aspect of the present invention is directed to a process for manufacture of flux coated iron ore pellets comprising:
(i) providing green pellets;
(ii) applying flux coating onto the surface of said green pellets ;
(iii) subjecting the green coated pellets thus obtained to induration such that during induration, higher surface temperature allows the diffusion of the flux particles towards the core of the pellet and in the process producing hard surface and surface confines pores.

A still further aspect of the present invention is directed to said process wherein the pellet composition selectively used comprises of;
Fe(T) in amounts of 61.0 - 63.5 % by wt.;
SiO2 in amounts of 2.5 - 5.5 % by wt;
Al2O3 in amounts of 2.3 - 4.0 % by wt;
MgO in amounts of 0.45 - 0.80 % by wt;
LOI in amounts of 2.0 - 4.5 % by wt;
Carbon in amount of 0.80-1.60% by wt;
and said flux coating is carried out in the range of 0.60 - 0.66 % by wt,
such as to obtain the Tumbler index (TI) of flux coated fired pellets is in the range of 91 to 96 % (+6.33 mm size) and surface hardness in the range of 260 to 370 Hv.

A still further aspect of the present invention is directed to a process wherein the basicity of fired pellet is maintained to have basicity (B2) of 0.30 - 0.45 preferably 0.35 and (B4) in range of 0.20 - 0.30.

Another aspect of the present invention is directed to a process wherein said flux coating used comprise selectively of dolomite, limestone, pyroxenite as flux coating material and slag coating material are LHF1 slag ( silicon killed), and LHF3 slag (aluminium killed).

Yet another aspect of the present invention is directed to said process wherein said flux coated green pellets prior to firing comprise of selective fineness such that 85-90% pass through less than 200 mesh which is blaine number : 2800-3200 cm2/g.
A still further aspect of the present invention is directed to a process wherein said flux coating material used in raw form selected is preferably dolomite in the form of CaCO3.MgCO3 and limestone as CaCO3 including slag used preferably ladle furnace slag of secondary steel making process.

A still further aspect of the present invention is directed to said process wherein the plate water absorption of coated material is maintained in the range of 45-60.

A still further aspect of the present invention is directed to said process wherein the flux coating thickness varies and is controlled such that said pellets in green condition have a coating thickness of 10 to 30 microns which subsequently increases to thickness of transition layer of complex slag phases (20-60 microns) as fired pellets.

A still further aspect of the present invention is directed to a process wherein the basicity of the fired pellets is maintained such as to attain a basicity (B2 = CaO/SiO2) of 0.30 - 0.45 and basicity [B4 = (CaO+MgO) / (SiO2+Al2O3)] of 0.20 - 0.30.

A still further aspect of the present invention is directed to a process wherein moisture content in non coated green pellet is maintained in the range of 10.2 to 10.3%.

A still further aspect of the present invention is directed to process wherein said green pellets are produced involving a disc pelletizer having inclination of 41 to 450 preferably about 440 with disc rotation of 16 to 22 rpm, preferably 18 rpm for 20 minutes and the coating of pellets carried out for additional 4 - 6 minutes preferably 5 minutes..

A still further aspect of the present invention is directed to a process comprising reducing gases involved including selectively CO, hydrogen and nitrogen preferably in combination of 58-63 % CO, 30-35 % hydrogen gas, and remaining 5-7 % nitrogen for reduction of coated pellets wherein the reducibility of the coated pellets are found to be 96-97 %.

A still further aspect of the present invention is directed to a process wherein the green coated pellets are provided to have lower moisture content in the range of 9 to 10% compared to the uncoated pellets in the range of 10.2 to 10.3% to facilitate faster drying of green pellets in drying zone of induration furnace.

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: shows the process flow for coating of green pellet at bench scale followed by firing of pellets.
Figure 2: shows the Binary slag system of MgO-FeO showing the effect of dolomite on surface of pellets and the shifting of melting temperature in binary system from surface to the core of pellets.
Figure 3a: shows the Binary slag system for calcium silicate and effect of coating on lowering the temperature of slag generated in transition zone of pellets.
Figure 3b: shows the Binary slag system for calcium Aluminate and effect of coating for increasing the temperature of slag generated at transition zone in pellets.
Figure 4: illustrates the optical micrographs of dolomite and limestone coated pellets at 200X and 500X using inverted optical microscope (Model : Axiovert 40 MAT, Carl Zeiss).
Figure 5a: shows schematically the process flow for green pellet making followed by coating in lower deck or lower screen and subsequent firing of pellets
Figure 5b: shows the photograph of actual coating of green iron ore pellets.

Figure 6: shows the graphical representation of the coated pellet properties against various coated materials.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS
The present invention is directed to provide flux or shell coated iron ore pellets and a process for increasing the tumbler index and surface hardness of iron ore pellets which in turn reduces the sticking of pellets in DRI making or in blast furnace and also reduce fines generation in pellet making and in iron making unit (Corex, Blast furnace and DRI units). The working of the invention is illustrated with the help of following example:

Examples:
According to an embodiment of the present invention iron ore pellets are coated in green condition with various fluxes or slag as coating materials. The raw material in pellet making consists of iron ore, limestone, dolomite, coke breeze, and bentonite. The pellet feed consists of 92-94% iron ore fines, 1-1.5% limestone, 1-1.5% dolomite, 0.8-1.4% coke breeze, 0.6-1.2% bentonite and 0.5-0.6 % of coating material. The said coating materials are Limestone, Dolomite Pyroxenite as flux coating and LHF1 and LHF3 as source of slag. Secondary steel making slag silicon killed denoted as LHF 1 and Secondary steel making slag aluminium killed denoted as LHF 3.

The green pellets having the MPS of 10.50 -12.50 mm are fired at the temperature of 1280-1340 OC, which is based on type of coating material present in pellets. The composition of reducing gases used in invention is of (i) 30-35% CO and 60-65% H2 gas and remaining 3 - 6% nitrogen and (ii) 58-63% CO and 30-35% hydrogen gas and remaining is 5 - 7% nitrogen. The fired pellets are reduced at temperature of 950 OC and the degree of reduction is found to be 94 - 96% index.

Accompanying Figure 1 shows the process of pellet coating in bench scale disc pelletizer, where the feed material as mentioned above is mixed and pelletized in desired MPS of 10.5-12.5 mm. The sized pellets are then coated with single coat material of flux. The pellets were then subjected for firing in raising hearth furnace. The green pellets are coated with coating material having thickness of 10-30 micron while the thickness is linearly increased with firing temperature and time. Usage of LHF3 slag contains maximum micro-porosity and lowest coating thickness of 15 microns.

Accompanying Figure 2, 3a and 3b show the effect of coating on melting temperature of slag generated during firing of pellets in transition zone. Figure 2 reveals that the presence of excessive MgO,CaO and SiO2 on pellet surface decreases the melting point of slag in transition layer. MgO having strong affinity towards FeO forms isomorphous magnesio-ferrite, which generates strain at the pellet surface due to deeper diffusion inside the pellet surface and hence increases the hardness on the pellet surface.

Similarly, Figure 3a reveals the binary slag system of CaO-SiO2 that while coating with limestone, formation of solid solution of calcium-silicate takes place such slag covers the pellet periphery and firmly adheres to pellet surface thereby increasing the surface hardness and tumbler index. Similar slag phases are formed when LHF1 slag coating is done on green pellets. Figure 3b represents the binary slag system of CaO-Al2O3, The pellets coated with LHF3 slag gives negative effect owing to higher alumina content in the range of 26-27% resulting in higher slag melting temperature of more than 1400 oC. At firing temperature of 1280-1360 oC, the slag is more viscous and difficult to diffuse into the pellet leading to poor slag bonding and therefore, the tumbler index as well as the surface hardness decrease.

Accompanying Figure 4 represents the optical micrograph of coated pellets in fired condition. The dolomite coated pellets show the phases like MF and MF1. MF1 is rich in MgO content whereas MF is iron rich magnesio-ferrite having thickness in the range of 30 - 40 microns. Higher coating thickness (35 - 50 microns) was observed in limestone coated pellets as compared to the dolomite coated pellets (30 - 40 microns) which is attributed to the more affinity of iron with lime.

Figure 5a reveals the process of coating pellets in green condition at pellet plant where the green pellets made at disc pelletizer-301 were transferred for screening through belt conveyor-302. The pellets were properly distributed through pellet distributor on screening deck consisting of upper deck-213 and lower deck-216. The upper deck--213 screen remove oversize pellets (>16 mm in diameter) and lower deck-217 removes -6 mm size pellets. The oversize and undersize pellets are then again transferred to the disc pelletizer for pelletization through belt conveyor-215.

The Dozer system-211 is attached with vibrator-212 which is electrically driven. The dozer is placed exactly on the exit side of the lower deck screen-216 where the coating material will coat the green pellets before entering into the induration furnace-402. The coating material are ground in the ball mill-103 where the hot air is passed to ball mill for drying through hot air generator-102. The material is fed to ball mill from hoppers-101.The ground material is transferred to the dozer system through pneumatic line-104. Figure 5b shows the actual setup in pellet plant for coating purpose.

Accompanying Figure 6 reveals the effect of pellet coating on the physical properties of pellets. Pellets coated with dolomite showed better RDI properties as compared to non-coated pellets. Similarly the RDI of LHF3 coated pellets is poor as compared to LHF1 slag coated pellets. The tumbler index of dolomite pellets is found in the range of 95-96% as compared to 90-92% of non-coated pellets. However, the coating did not show any appreciable impact on CCS of pellets. So far the pore density in surface region is concerned; the coated pellets in all cases are having higher pore density than that of non-coated pellets owing to the calcination of flux and/or incipient fusion of slag.

EXPERIMENTAL RESULTS:
The process according to the present invention was established by conducting series of trials to produce pellets by coating the green pellets at different dozing rate of dolomite, limestone, Pyroxenite and slag based on their bulk density. The coated green pellets were fired and tested for metallurgical properties as shown in Table 1. It was surprisingly found that the addition of coating helps in increasing the surface hardness of pellets with higher pores density compared to non-coated pellets. Further studies (Table 1) shows the hardness taken at different layer of coated pellets mainly the surface, transition layer and core of the pellets. The micro-hardness is measured using Vicker hardness tester (Model: Zhµ, Make: Zwick Roell, Germany).

Table 1: Properties of the pellets
Coating material Microhardness (Vicker hardness number, Hv) CCS (kg/pellet)
Transition layer Surface Core Tumbler (+6.3 mm)

Non coated pellets Nil 260 242 262 90.25
Dolomite coated 374 395 263 265 96.12
Limestone coated 363 362 241 262 94.25
LHF1 Slag coated 365 375 250 264 95.33
LHF3 Slag coated 237 245 251 267 90.45
Pyroxinite coated 365 375 250 268 96.01

Table2: EPMA analysis of coated fired pellets
Location in pellet CaO MgO SiO2 Al2O3 FeO
Dolomite at surface 36.2 10.7 2.47 4.4 21.5
Dolomite at transition 21.3 3.5 9.45 7.16 44.7
Limestone at surface 44.2 1.02 26.5 2.74 12.8
Limestone at transition 30.2 1.29 4.57 1.64 52.4
LHF1 slag at surface 18.4 3.83 18.5 6.84 25.3
LHF1 slag at transition 13.3 3.45 30.2 4.51 51.3
LHF3 slag at surface 32.1 1.36 7.45 9.45 18.4
LHF3 slag at transition 21 0.98 6.45 8.45 45.2
Pyroxenite at surface 1.24 15.5 44.5 1.25 11.5
Pyroxenite at transition 4.56 4.56 45.3 4.35 21.8

The EPMA analysis of pellets revealed that the coated pellets form complex slag at the transition zone which helps to moves the ternary slag point towards low melting point region as shown in Table 2. It is also observed that the MgO has diffused into iron grains at the transition layer which are higher in iron oxide content so diffuse and adhere to pellet core. However the surface is rich in calcium magnesium silicate. The presence of such slag composition at the surface of the pellets allows more micro pores to form at the surface of the pellets. In case of limestone coated pellets the surface is rich in calcium silicate slag the calcium silicate slag the slag composition have higher melting point compared to slag composition form at the transition layer CaO to combine with iron oxide and provide good wettability of slag and diffusion in the pellet.

Pellets coated with slag basically LHF3 which is alumina rich slag shows that the Alumina silica ratio of slag is more than 1; at this point the slag is very viscous and required higher temperature for diffusion more than 1380-1420oC. Similarly pellet coated with LHF1 having similar effect to that of dolomite coated pellets but the slag formed is deficient in MgO but form calcium ferrite and calcium silicate.