FIELD OF THE INVENTION
The present invention relates to a process for improvement in quality of iron ore agglomerates generated in steel plant for metallurgical application. In particular, it relates to a process to provide adequate strength to iron ore agglomerates to withstand the pressure and temperature conditions of the blast furnace.
BACKGROUND OF THE INVENTION
A large percentage of iron ore fines generated in the steel industry are predominantly recycled via the agglomeration making route. Sintering is a generic term that is used to describe a high temperature process in which a raw materials mixture is converted into a particular form of agglomerate known as sinter. Iron ore sinters are suitable porous materials obtained by agglomeration processes of iron ore fine particles. These agglomerates are widely employed in blast furnaces to produce pig iron. Sufficient cold handling strength of sinter is required to minimise fines generation and different processes are used for this purpose.
Sinter is produced at relatively low temperatures (~1050ºC) without complete melting of the iron ore particles. Thus, the microstructure of sinter consists largely of unreacted ore particles, bonded by fairly large amount of complex ferrite phase, as well as, partially reduced hematite or magnetite phases. The type and amount of flux and fuel can thus affect the sinter quality.
Sinters are a product of re-crystallisation and partial melting of iron ore fines and slag formation. Thus the quality of sinter is very much dependent on its microstructural characteristics.

Reduction Degradation Index (RDI) of sinter is high temperature property which affects the productivity of a blast furnace. As the burden moves down the furnace there is change in phases due to presence of reducing gases in the furnace. Secondary hematite/ Hematite are converted to magnetite, resulting in volume change and generation of cracks. Thus RDI of sinter can be largely attributed to the presence of secondary hematite that is formed due to the re-oxidation of magnetite formed during sinter. Formation of magnetite is dependent on the amount of reducing gases available which is supplied by the solid fuel. Thus to maintain adequate RDI proportionately solid fuel needs to be increased. The challenge, therefore, is to produce sinter with sufficient high temperature strength without increasing the solid fuel.
JP55119138 discloses use for improving the sinter quality. The patent discloses that after sintering of iron ore it is crushed by means of a crusher, powder is removed by means of a vibrating screen and the sintered ore is charged into a bunker for cooling. With the sintered ore at that time, its temperature is 600-700ºC, its composition mainly consists of magnetite, and a reducing powdering index in a blast furnace is excellent. An inert gas such as Ar, N2, etc. is blasted from a nozzle on which a cone is placed, in the bunker and the sintered ore 100 is cooled at 100-150 ºC. C and extracted and the reducing powdering index does not deteriorate. However, it does not report on quantitative effect of the use of inert gases. It is also doubtful that at the bunker the temperature of sinter will be 600-700 ºC.
OBJECTS OF THE INVENTION
In view of the foregoing limitations inherent in the prior-art, object of the invention is to provide sufficient strength to the above mentioned iron ore sinter

to withstand the disintegration that can happen during handling, transportation and blast furnace process.
Another object of the invention is to improve percentage of magnetite phase of the sinter and thus improve the reduction disintegration index (RDI) and reduce overall consumption of solid fuel in sinter making process.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a process to produce agglomerates of sinter, the process comprising steps of preparing a green mix, then charging the green mix into sinter machine for assimilation of cake, the cake comprising magnetite as one of the major constituents, then spraying inert or nitrogen gas (N2) or combination over assimilated cake after the burn through point (BTP) is achieved to provide the reduction in oxygen in the atmosphere to restrict the formation of secondary haematite from magnetite and dislodging the sintered cake into the cooler.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates various steps of a sintering process to produce agglomerates of sinter in accordance with an embodiment of the invention.
FIG. 2 shows a block diagram describing various stages through which the sintering process is executed in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a process to produce
agglomerates of sinter, the process comprising steps of preparing a green mix,
charging the green mix into sinter machine for assimilation of cake, the cake
comprising magnetite as one of the major constituents, spraying inert or nitrogen
gas (N2) or combination over assimilated cake after the burn through point (BTP)
is achieved to provide the reduction in oxygen in the atmosphere to restrict the
formation of secondary haematite from magnetite; and dislodging the sintered
cake into the cooler.
Shown in FIG. 1 is a process (100) defining various steps to produce
agglomerates of sinter.
At step (104) a green mix is prepared.
In sintering, ore fines below 10mm in size are mixed at bedding blending yard with
various silica and lime bearing fluxes (pyroxenite, lime, limestone etc) and coke as
fuel. A pile of layered raw materials is thus created; these mixed materials are
further sent to sinter plant with the help of conveyor belt.
The preferred material used in granulation of the green mix has been shown in Table 1
and Table 2:



The mixed material (base mix) is further mixed in a mixing drum (204) (shown in FIG. 2) at sinter plant and lime fines and moisture as per the requirement is added for granulation (green mix).
At step (108), the green mix is charged into a sinter machine (208) for assimilation of cake and filled upto a height of 600 cm. The cake comprises magnetite as one of the major constituents. The top of the green mix is ignited using a movable burner hood (212) fuelled by coke oven gas while suction is applied across the bed. Ignition temperature is maintained at 1500 ̊C while suction during ignition is maintained at 600 mmWC. The speed of the belt is such that ignition is continued for 2 minutes and after the ignition process the suction is increased to 1500 mmWC.
As already discussed, Reduction Degradation Index (RDI) of sinter is high temperature property which affects the productivity of a blast furnace. As the burden moves down the furnace there is change in phases due to presence of reducing gases in the furnace. Secondary hematite/ Hematite are converted to magnetite, resulting in volume change and generation of cracks.
At step (112), inert gas or nitrogen gas (N2) or combination is sprayed over assimilated cake after burn through point (BTP) is achieved at a gas injection point (216). This is to provide the reduction in oxygen in the atmosphere to restrict the formation of secondary haematite from magnetite.

The temperature of the assimilated cake over which inert or N2 gas or combination is sprayed is maintained at 500 ̊C or more.
One of the methods to spray the gas can be done using nozzles.
The inert or nitrogen gas (N2) or combination flow rate is maintained at 2400-2600 Nm3 per tonne of gross sinter for 1.5 to 3.0 min.
At step (116), the sintered cake is dislodged into a cooler (220).
The process (100) as mentioned above reduces the reduction in degradation index (RDI) achieved by 3.0 – 4.0 compared to the same process followed without using Nitrogen or inert gas or combination. Similarly, it also reduces the solid fuel consumption upto 5-7 kg per tonne with respect to the same process (100) without using nitrogen or inert gas or combination.
Experimental Analysis
The above mentioned process for producing agglomerates of sinter and its benefit can be validated by the following examples. The following examples should not be construed to limit the scope of invention.
Raw materials were weighed in the proportions shown in Table 3 to get the desired sinter chemistry shown in Table 4.

The raw materials were charged into the granulation drum which was then inclined at 45̊and set into rotation at a constant speed of 25 rpm. The materials were allowed to ‘dry mix’ for 15 minutes, following which half the total amount of water was added and mixed intensely for 2 minutes. Rest of the water was then added and mixed. The granules were allowed to grow without mixing during the last minute of granulation. The total amount of moisture used was 6% of base mix and the wet mixing time was 5 minutes.
Green mix from the granulation drum was then transferred to the sinter machine as mentioned above in the description. Ignition temperature was 1500 ̊ C while suction during ignition was maintained at 600 mmWC. The sinter machine speed was maintained such that the Ignition was continued for 2 minutes after which the suction was increased. The increased suction was maintained throughout the experiment. The experiment was considered to be complete when the windbox thermocouple recorded the maximum temperature (BTP-Burn through point, Completion of sintering process).
Stabilized sinter was screened into various size fractions and tested for its chemical constituents, Tumbler Index (TI), Abrasion Index (AI), Reduction Degradation Index (RDI) and Reducibility Index (RI). Chemical analysis was done using ICP-AES. Sinter in the 10-40 mm size range was taken and the Tumbler test was conducted with all parameters as per the standard. Tumbler Index values, RDI and RI tests were performed. Quantitative and Qualitative phase analysis of sinter was also carried out.
Three experiments were conducted at each level to check for reproducibility. In nitrogen injection tests, when the temperature of waste gas reached its maximum, a hood with nozzles connected to the nitrogen line was placed on the

sinter strand, just after the BTP was achieved, and nitrogen gas flow was started. The nitrogen gas flow was 2500Nm3 and was monitored with the help of a rotameter. The result of experiments is shown in Table 5.

Advantages
Sufficient strength is obtained to iron ore sinter to withstand the disintegration that can happen during handling, transportation and blast furnace process.
Improved percentage of magnetite phase of the sinter and thus improved reduction disintegration index (RDI) and reduced overall consumption of solid fuel in sinter making process is obtained.
This is also beneficial for sinter making process to produce sinter for charging in blast furnace. It also helps in higher productivity, and lesser overall solid fuel consumption in the sinter-making.

WE CLAIM:
1. A process to produce agglomerates of sinter, the process comprising steps
of:
preparing a green mix;
charging the green mix into sinter machine for assimilation of cake, the cake comprising magnetite as one of the major constituents;
spraying inert or nitrogen gas (N2) or combination over assimilated cake after the burn through point (BTP) is achieved to provide the reduction in oxygen in the atmosphere to restrict the formation of secondary haematite from magnetite; and
dislodging the sintered cake into the cooler.
2. The process as claimed in claim 1, wherein the temperature of the assimilated cake over which inert or N2 or combination gas is sprayed is 500oC or more.
3. The process as claimed in claim 1, wherein the inert or N2 gas or combination flow rate is maintained at 2400-2600 Nm3 per tonne of gross sinter for 1.5 to 3.0 min.