FIELD OF THE INVENTION

[001] The present invention generally relates to iron ore sintering, more particularly to an unfired agglomerate for use in straight grate sintering machines to increase iron ore sintering production and decreasing fuel consumption. The invention also refers to a method of increasing iron ore sintering production and decreasing fuel consumption in straight grate sintering machines.
BACKGROUND OF THE INVENTION
[002] In iron ore sintering, a mixture of various raw material like iron ore fines, limestone fines, dolomite fines, pyroxenite fines, burnt lime and carbonaceous fuel like anthracite coal or coke breeze etc. are agglomerated into larger and porous solid lump. These agglomerates are then sized as per the requirement of the blast furnace. Feed size suitable for this agglomeration technology is 150 microns to 8 mm.
[003] For producing these agglomerates at industrial scale, most common and widely used technology in commercial application is Dwight Lloyd straight grate sintering machine. In plants using this technology, different raw material constituents are mixed and granulated in a balling drum or a high intensity mixer. The wet granules, thus produced, are charged onto endless chain of pallet cars, of straight grate sintering machine. A bed of wet/moist granules is formed and top layer of same is ignited in an ignition furnace present at the beginning of endless pallet car chain. Presence of carbon based fuel like coke breeze etc. results in the formation of a flame front because of their ignition. Suction is applied at the bottom of the bed due to which hot air moves downward and ignite the coke/coal present in the layer beneath the flame front. This ignition of coke layers results in formation of new flame front and simultaneously flame front at upper region becomes cold. This phenomenon is called movement of flame front downwards. As the flame-front moves downward, different constituents of the mixture get agglomerated/joined to each other due to partial melting and fusion. During sintering, the temperature in the sintering zone reaches as high as 1350-1400 degree centigrade which results in fusion of different constituents of sinter mix and subsequent production of large sintered porous mass. However, when this flame front reaches to bottom layer of the bed, this significantly high temperature can damage the sinter machine.
[004] Therefore, in iron ore sinter making, a bedding of hearth layer of sized iron ore sinter (generally 10-20 mm) is layered at the bottom of sinter machine. The availability of this sinter material is made by continuously screening product sinter and layering the same as hearth layer in a recycling manner. In general, about 10% volume of the sinter machine is filled with hearth layer sinter. This layering of already fired sinter as hearth in sinter machine is done for many other operational purposes. These are protection of the grate bars from exposure to high temperature, protection of center casting of sinter machine from exposure to high temperature, maintaining the permeability of the sinter bed, prevention of adjacent grate bars gap in sinter machine from choking etc.
[005] However, to achieve the operational purposes mentioned above, a significant volume of sinter machine i.e. around 10% is wasted. This hearth layer volume of sinter machine can be considered as inert volume of the sinter machine. The volume of machine which is filled by fresh raw material mixture for producing sinter can be considered as working volume of the sinter machine. Since sensible heat of the bottom most sinter and waste process gas is not utilized in sinter making therefore, significant amount of sensible heat is also lost in re-heating already sintered material layered as hearth layer in the machine.

[006] In sinter making, utilization of this inert hearth layer volume and sensible heat of bottom most sinter for sinter production is a great challenge. If some special lumps are charged as hearth layer material which can serve the same operational purpose as sinter being the hearth layer and which can get sintered at lower temperature observed by hearth layer than inert hearth layer volume of sinter machine will be converted to working volume, sensible heat of bottommost sinter and waste gas will be utilized and sinter production will be increased significantly. Additionally, the net carbon rate will be decreased because of increased net sinter production at same carbon consumption.
[007] In sinter making, a temperature of around 1300 degree centigrade or higher is required for sinter making or fusion to start. The temperature of the hearth layer does not reach this high therefore to utilize heat energy, which otherwise gets wasted in heating hearth layer sinter, a new agglomerate is required which can be layered as hearth bedding material, has adequate handling strength for transportation, which can withstand load of the sinter bed and does not get crumble, which can withstand thermal shock and finally, gets sintered at lower temperature generally observed by hearth layer in iron ore sintering.
[008] To increase the processing capacity of sinter plants and utilizing hearth layer inert volume, few researchers have reported different ways and methods. Some of them are reported here.
[009] Method for increasing yield of sintering ore by utilizing siderite was described by Zhang Yixian et al in patent no CN103834799A wherein sized siderite ore is used as hearth layer and roasted siderite was added to sinter production for calculating overall sinter yield.
[010] A method of providing insulation to grate bars is described by Sathaye, Jayant Moreshwar in Patent Application No. 1784/CHE/2012 wherein a mixture of steel making slag (12-20 mm size) and iron ore sinter (10-20 mm size) is charged as hearth layer in sinter making.
[011] A supporting plate clean chute segregation distributor and segregation distributing device for segregating wet granulated raw material and increasing productivity is reported by Kejian Liu et. al., Patent No. CN102032790 (A).
[012] An apparatus for improving air permeability of sintering layer to improve quality and productivity of sinter produced by mounting a puff pipe on the lower part of a backward inclined plate of a charging unit is reported by Byun Sang Geun et al., Patent No. KR20020088119 (A).
[013] A Method of increasing sinter rate was described by Bradwell Cyril in patent no US2767074A in which he proposed a method of increasing sintering rate by preheating the mix with hot water and steam.
[014] Above cited inventions either mentioned new design incorporation in sinter plant for production increase or using some different material as hearth layer in sinter making which is completely different in composition and quality as compared to sinter. The retrofitting of any new design in running sinter plant is very difficult as well as cost involve is also many time very high. Use of material totally different in chemical composition also does not support for production increase. Additionally, no invention has suggested to use sensible heat of bottom sinter and waste gas for sintering. To address the drawbacks in the work available till date, a two-step method is proposed wherein at first step a carbon free special unfired agglomerate is developed which can be heat hardened or sintered at relatively low temperature as compared to iron ore sintering. In the second stage, these special agglomerates are layered as hearth bedding material and subsequently heat hardened using sensible heat of both bottom-most sinter of sinter plant and waste gas in straight grate iron ore sintering.
OBJECTIVES OF THE INVENTION
[015] The primary object of this invention is to provide an unfired agglomerate to be used as hearth layer in a straight grate sintering machine to increase iron ore sintering production and decreasing fuel consumption.
[016] Another object of the invention is to provide an unfired agglomerate with low sintering temperature.
[018] Yet another object of the invention is to provide an unfired agglomerate with sufficient handling and transportation strength to be used as hearth layer in a straight grate sintering machine.
[019] Yet another object of the invention is to provide an iron ore agglomerate with sufficient compressive strength and thermal shock resistance to be used as hearth layer in a straight grate sintering machine.
[020] Yet another object of the invention is to provide a method for preparing an unfired agglomerate.
[021] A further object of the invention is to provide a method to increase iron ore sintering production and decreasing fuel consumption in a straight grate sintering machine.
[022] A still further object of the invention is to provide a method to form sintered mass of un-fired agglomerate of hearth layer using sensible heat of bottom sinter and waste gas.
SUMMARY OF THE INVENTION
[023] This invention relates to an unfired agglomerate, a method for preparing the unfired agglomerates and a method by which working volume of an iron ore sintering machine is increased, fuel requirement is reduced and productivity of sinter plant is increased.
[024] In accordance with an embodiment of an invention there is provided an unfired agglomerate for use in straight grate sintering machine as hearth layer. The unfired agglomerate comprising iron ore fines, lime fines, MgO based flux, B2O3 bearing material and hydraulic binder.
[025] In accordance with said embodiment, the unfired agglomerates are in form of briquettes, bricks, pellets etc. of iron ore and are free from any fuel. These un-fired agglomerates are developed with sufficient mechanical strength required for handling and transportation, have sufficient thermal shock resistance required for sustaining thermal profile observed by hearth layer region in sinter making, have lower sintering temperature for getting sintered at lower temperature observed in hearth layer is sinter making.
[026] In accordance with said embodiment, the unfired agglomerate comprises lime fines (2) comprising CaO content of 10-20% by weight of the unfired agglomerate, MgO based flux (3) comprising MgO content of 1-2% by weight of the unfired agglomerate, B2O3 bearing material (4) comprise B2O3 content of 0.5-10% by weight of the unfired agglomerate. The B2O3 bearing material is selected from the group consisting of calcined colemanite, calcined boric acid, orthoboric acid or meta-boric acid. Further, the hydraulic binder is selected from the group consisting of ground granulated blast furnace slag and portland cement.
[027] In accordance with said embodiment, the iron ore fines are coarser in size to facilitate pelletizing or briquetting wherein finer fraction reporting in bonding phases is below 15% and more preferably 10%. Further, the B2O3 bearing material is ground to finer size to report mainly ion bonding phases which is below 25 micron for pellet making and below 150 micron for briquetting or extrusion.
[028] In accordance with said embodiment, the agglomerate has a sintering temperature in the range of 838-1200 degree centigrade.
[029] In accordance with another embodiment of the invention a method for preparing unfired agglomerate for use as hearth layer in straight grate sintering machine is provided. The method comprises preparing a base mixture of iron ore fines, lime fines, MgO based flux, B2O3 bearing material, and hydraulic binder; preparing an intermediate mixture by adding water to the base mixture in appropriate amount; sizing the intermediate mixture to form sized agglomerates; and atmospheric curing for 7-10 days of the sized agglomerates to form unfired agglomerates to be used as hearth layer in straight grate sintering machine. The unfired agglomerates are cured or dryed using waste exit gas of the sintering machine.
[030] The curing of these agglomerate is done under atmospheric drying. After curing, primary drying of these un-fired agglomerates just before charging can be provided by waste process air heat if required. These agglomerates are used as hearth layer in sinter making, during which due to sensible heat of bottom most sinter layer and sensible heat of waste process air, two major reactions occur. First of these reaction is melting of bonding phases due to lower eutectic temperature of bonding phases formed due to special additive addition (B2O3 based material). Second reaction is bonding of different constituents of un-fired hearth layer material to form sintered mass like the iron ore sinter formed above hearth layer.
[031] In accordance with another embodiment of the present invention there is provided a method for increasing iron ore sintering production and decreasing fuel consumption in straight grate sintering machines, the method comprising the steps of charging unfired agglomerate as hearth layer on pallet car of the machine, forming a bed by layering raw material comprising iron ore fines and carbonaceous fuel over the hearth layer, igniting top layer of the bed to sinter the raw material, and applying suction from wind boxes placed below the heath layer.
[032] In accordance with said embodiment, the hearth layer is sintered by sensible heat and waste exit process gas.
[033] In accordance with said embodiment, wherein size of unfired agglomerates varies in the range of 8 to 25 mm, preferably in the range of 10 to 15 mm.
[034] In accordance with said embodiment, height of the bed varies in the range of 20-100 mm.

BREIF DESCRIPTION OF THE DRAWINGS
[035] The following drawings are illustrative of preferred embodiment for enabling the present invention, are descriptive of some methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.
Figure 1: illustrates an exemplary unfired agglomerate, according to the present invention.
Figure 2 and 3: depicts results of bonding phase temperature study in a dilatometer.
Figure 4: depicts a graph illustrating the temperatures at which 10% shrinkage occurred.
Figure 5: illustrates a conventional unfired agglomerate.
Figure 6: illustrates a conventional sinter mix.
Figure 7: illustrates a flow chart depicting the method step of preparing unfired agglomerate according the present invention.
Figure 8: illustrates conventional process of a sinter plant.
Figure 9: illustrates process of sinter plant according to the present invention.
Figure 10: illustrates an exemplary preparation of unfired agglomerates, in accordance with the invention.
Figure 11: illustrates an exemplary layering process of unfired agglomerates as hearth layer.
Figure 12: illustrates an exemplary sintering sintering process, in accordance with the invention.
Figure 13: illustrates a sintered unfired agglomerate used as hearth layer, in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION
[036] The preferred embodiments of the disclosed subject matter are described in more detail hereinafter with reference to the accompanying drawings. The disclosed subject matter may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, the exemplary embodiments are provided so that this disclosure is thorough and complete, and will convey the scope of the disclosed subject matter to those skilled in the art. Like reference numerals in the drawings denote like elements.
[036] It is to be noted that, as used in the specification and the appended claims, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or”, unless the content clearly dictates otherwise.
[037] Hereinafter, an unfired agglomerate, method of preparing said unfired agglomerate and a method for of increasing iron ore sintering production and decreasing fuel consumption in straight grate sintering machines will be explained in more detail.
[038] In Iron ore sintering, working volume of the sinter machine is always less than that of actual volume because around 10% of the volume is used as hearth bedding layer for protection of machinery and other operation related purposes like bed permeability etc. Also, it results in significant heat losses as sensible heat of bottom most sinter and waste gas is not used. This heat is absorbed by hearth layer sinter which in turn protects the metallic machine parts below from exposure to high temperature. The hearth volume of the sinter machine is thus a kind of inert volume where apart from heat consumption, no other reaction related to sintering happens. Converting the hearth volume into working volume partially or completely as well as using sensible heat of the bottom sinter and waste gas partially or completely will result in significant fuel saving and production increase.
[039] To utilize hearth layer volume of sinter machine for sinter production, unfired agglomerate of required size (10-25 mm or more preferably 10-15 mm) is disclosed according to an embodiment of an invention with following property:
1) Handling and transportation strength- so that it can be conveyed from the point of production to hearth layer bin of sinter plant and finally as hearth layer in sinter making.
2) Compressive strength- so that it can withstand the compressive loading resulting from the bed load above it.
3) Thermal shock resistance- since when the flame front of the bed reaches to bottom layer, the hearth layer agglomerate will undergo a sudden increase in the temperature which can result in crumbling/fracture of the hearth layer agglomerate. Crumbling of hearth layer agglomerate is not desirable as it will result in poor bed permeability and fines generation. To avoid the same, hearth layer pellet should have sufficient thermal shock resistance.
4) Most importantly, the special material to be used as hearth layer should have lower sintering temperature so that this hearth layer material gets sintered at lower temperature. Since hearth layer agglomerates cannot be formed with carbon fuel addition as any fuel addition in hearth layer agglomerate will generate heat which in turn can damage the sinter machine grate bars, center casting and other accessories. Therefore, Carbon free un-fired agglomerate with lower sintering temperature are used to be layered as hearth bedding material to get sintered at lower temperature.
[040] Referring to figure 1, an exemplary unfired agglomerate according to the present invention is depicted. The agglomerate is produced which has sufficient handling strength, compressive strength, thermal shock resistance and lower sintering temperature. The agglomerate comprises iron ore fines (1), lime fines (2), low LOI MgO flux (3) and a binder (5) which can provide it enough handling strength, compressive strength and thermal shock resistance and a special additive was added to decrease its sintering temperature so that these agglomerates can be sintered at lower temperature when charged as hearth layer in sinter making. Special additive added in these agglomerates is B2O3 based compound (4), colemanite, which decreases the melting point of bonding phases and results in sintering at lower temperature. These agglomerates formed were cured for 7-10 days before being charged as hearth layer in sinter making. In 7-10 days curing, these agglomerates achieve sufficient strength for handling purposes.
[041] Iron ore fines, Calcined lime, Calcined colemanite or other B2O3 bearing material after calcination, Calcined or low LOI MgO bearing flux like olivine, Hydraulic cement like ground BF slag or other Portland cement are mixed together in preferred proportion. Enough water is added either to the mix or to the CaO bearing materials to make slaked solution. After subjection to proper mixing, unfired agglomerates are formed in the form of either of pellets, briquettes, extruded products etc. These are subjected to atmospheric curing for gaining handling and transportation strength. The CaO content of the agglomerates is in the range of 8-14%. Total CaO content of these un-fired agglomerates is achieved by calcined lime to achieve better handling strength as well as thermal shock resistance. Calcined colemanite was added in the mix at 0.5 -7% by weight so that the colemanite content in bonding phases is 5-70%. The size of the calcined colemanite to be kept at the size which reports in bonding phases for ex: below 25 microns for pellet making and below 150 microns for briquetting and extrusion. Total material which reports in bonding phases to be kept below 15% and more preferably 10%. The size of the agglomerate to be layered as hearth needs to be kept at 10-25 mm, however, the smaller they are in the mentioned range is better for subsequent sintering in hearth layer and colemanite consumption. Due to thermal gradient, enough heat will not reach to inside of these agglomerates and hence higher colemanite may be required for these agglomerate to get sintered (as higher colemanite in bonding phases will decrease the sintering temperature to even below 900 degrees Celsius). After the completion of first step i.e. preparation of special agglomerate for hearth layer, cured agglomerates are layered as hearth layer in sintering. After layering cured agglomerates as hearth layer, wet granulated mixture is charged over it in sinter making. Because of the treatment of these hearth layer agglomerates by sensible heat of waste gas as well as bottom sinter near to hearth layer, a significant part of the agglomerates is sintered and achieve sufficient strength to be charged to Blast furnace etc.
[042] In iron ore sintering, a minimum of 10% shrinkage occurs due to incipient melt formation, solidification and fusion. Therefore, to develop the low temperature sinter-able agglomerate, its bonding phase should undergo a minimum of 10% shrinkage on heating. The bonding phase temperature was studied with addition of B2O3 bearing colemanite ore to iron ore. To know how much colemanite is required in bonding phases which can produce a minimum of 10% shrinkage, colemanite and iron ore fines were added in different proportion and were heated in a dilatometer wherein the temperature required for different degree of shrinkage can be noted. It is to be noted that when small tablet made of iron ore powder was subjected to the heating in dilatometer equipment, no deformation, shrinkage etc. were observed even till 1300°C (see fig 2) however, when tablet made from colemanite powder was subjected to heating in dilatometer, 10% shrinkage was observed at 886°C and complete flow temperature was found to be 1038 °C (see fig 3). Temperature of 10% shrinkage of sinter mixture was also measured to know what is the minimum temperature required if colemanite is not added in the special hearth layer un-fired agglomerate. Temperature at which 10% shrinkage was observed in sinter mix as well as in various formulations of special hearth layer un-fired agglomerate with colemanite percentage being varied from 0-100% is shown in fig 4. As observed from the data, with just 10 percent colemanite in the bonding phases, the temperature of 10% shrinkage decreased significantly to 1164 °C. With the addition of lime and other constituents of sintering, this can be further reduced. Based on this finding, unfired agglomerate, suitable for charging as hearth layer were developed. Bonding mechanism of conventional unfired agglomerate, sinter mix and special un-fired agglomerate as proposed in present invention is shown in Table 1.
Conventional unfired agglomerate Conventional Sinter mix Special low temp sinter-able unfired agglomerate
Figures Figure no. 5 Figure no. 6 Figure no. 1
Processing step 1 Above mixture mixed with required water and agglomerates using briquetting, pelletizing or extrusion etc. and cured with different techniques. Above mixture granulated after adding required water and then layered on Dwight Lloyd straight grate sintering machine. Above mixture is mixed with required amount of water, agglomerated in required size and cured in atmospheric curing. Particle size of calcined colemanite added should smaller enough such that it reports only in bonding phases as shown.
Processing step 2 (a) No further Processing Top layer ignited using external fuel and then burning of coke fines in the mix generates significantly high temperature >1300 °C required for incipient fusion and sintering. The product sinter is then sized, screened and sent to Blast furnace Agglomerate of preferred size layers as hearth layer in sintering machine.
Processing step 2(b) No further Processing No further Processing Heating of the hearth layer agglomerate from sensible heat of bottom sinter and waste gas. Incipient fusion at particles interface due to low eutectic temperature in the range of 838-1200 degree Celsius.
*MgO flux added is either low lOI or calcined so that it does not consumes heat for calcination etc
Table 1: Mechanism of bonding and subsequent strength gain for bonding phases.
[043] Referring to figure 7, shown therein are steps of a method for preparing an unfired agglomerate according to the present invention. The method comprises the steps of preparing a base mixture (20) of iron ore fines, lime fines, MgO based flux, B2O3 bearing material, and hydraulic binder. Preparing an intermediate mixture (21) by adding water to the base mixture in appropriate amount. Sizing the intermediate mixture to form sized agglomerates (22), and atmospheric curing (23) of the sized agglomerates to form unfired agglomerates to be used as hearth layer in straight grate sintering machine.
[044] In accordance with another embodiment, the present invention relates to a method for increasing iron ore sintering production and decreasing fuel consumption in straight grate sintering machines.
[045] The basic difference for operation purpose of the present invention and conventional sintering process is explained herewith. First Schematic of conventional sinter plant operation is shown in fig 8 and is described below.
[046] Separate stock piles of iron ore fines, fluxes, solid fuels, by-products etc. are stored in raw material yard. These raw materials are transported to proportioning unit wherein raw material are weighed and proportioned as per the required material balance. This proportioned material is conveyed to mixing and granulation unit where water and lime fines are added and mixing is done and wet granules are formed. The granulated mixture is then transported to feeding hopper of sinter plant straight grate furnace. Feeding hopper feeds the material on endless pallet car of sintering machine. Before feeding the mixture granules on the pallet cars, sinter of size (10-25 mm) are layered as hearth layer using hearth layer sinter bins. This bed of un-sintered mix having hearth layer at bottom and granules over it moves forward with the movement of endless car in forward direction. When the bed comes under ignition furnace, top layer gets ignited and due to suction from bottom, flame front proceeds downwards and sintering occurs. Hearth layer in the process acts as a heat barrier between hot bottom sinter and metallic grate bars and maintains the permeability of bed as finer granules do not reach the gap between adjacent grate bars.
[047] On sintering completion, sinter machine is tilted and sinter cake falls on spike crusher and crushed sinter is transported to sinter cooler. Cooled sinter, thereafter, screened into three fractions, minus 5 mm which goes to granulation unit for recycling, plus 5 mm which goes to blast furnace stock house and some amount of 10-25 mm size sinter which is recycled back in sinter plant for its use as hearth layer.
[048] The Schematic of modified sinter plant operation with special low temperature heat hard-enable agglomerate as hearth layer for sinter production is shown in fig 9. Instead of sinter screened from the produced sinter in cyclic manner, special low temperature heart hard-enable and carbon free agglomerates will be transported to hearth layer bin of sinter plant as shown in fig 9. These will get sintered and report in sinter production and will be transported to blast furnace as feed.

[049] Schematic of the method of increasing iron ore sintering production and decreasing fuel consumption in straight grate sintering machines are shown in figure 10-12.
[050] Iron ore fines (1), Limes fines (2), Low LOI MgO flux or calcined MgO bearing flux (3) and Hydraulic binder like ground granulate blast furnace slag or Portland cement etc. (4), Low melting B2O3 bearing compound like calcined colemanite (A) etc. and optimum water are mixed together in a mixing unit (5) which can be a plough and shear mixture/intensive mixture/muller mixture etc. This wet mixture is charged into wet agglomeration units like disc pelletizer (6) or briquetting (7) or any other cold agglomeration units like extrusion, drum pelletizer etc. Suitable size range of these agglomerates is 8-25 mm and preferable size range is 10-15 mm size. After production of these unfired wet agglomerates, they need to be subjected to atmospheric curing for 7-10 days and then conveyed to hearth layer bin (8). From hearth layer bin (8) the unfired agglomerates, having sufficient strength to sustain load of bed above, are charged on the pallet car of straight travelling grate machine (12) to form the hearth layer (14). Simultaneously, at a small distance ahead of the hearth layer bin, feed mix bin (9) discharges the fresh material onto drum feeder (10). Drum feeder discharges the feed onto segregation plate (11) which layers the feed onto the hearth layer (14). As the machine moves forward, a bed (13) comprising of un-sintered wet mixture granules over cured unfired agglomerates as hearth layer (14) is formed. As the machines moves forward, bed (13) comes under ignition hood (15). Ignition hood ignites the top layer of the bed (13) and a flame front (17) is generated. Since the suction is applied from the bottom wind boxes (16), fuel in the lower level gets ignited while the earlier ignited zone gets sintered. This process of fuel getting ignited in lower level and subsequent sintering continues and hence flame front travel downwards as shown in fig 7 (c) wherein (18) represent sintered zone, (17) flame front zone and (20) un-sintered zone. When the flame front (17) reaches bottom of the pallet car where unfired agglomerates are layered as hearth layer, due to high temperature of flame front zone (~1350-1400 °C) temperature of the hearth layer reaches ~1200 °C. Since the temperature is greater than the temperature required for formation of bonding phases due to presence of colemanite (which lowers the sintering temperature in the range of 838-1200 degree Celsius), sintering of hearth layer materials occurs. The bottom-most part of hearth layer does not reach a temperature of 1200 and therefore does not get strongly sintered but it protects the grate from overheating which is one purpose of the hearth layer usage in sinter making.
[051] The invention is now described by way of non-limiting example:
[052] Example: Iron ore, Lime fines, Olivine, Ground granulated blast furnace slag and calcined colemanite of mentioned chemistry in table 2 was used for preparing agglomerate of 10-15 mm size in the form of pellets.
Table 2: Chemistry of raw materials
MATERIAL T. Fe B2O3 CaO MgO SiO2 Al2O3 LOI
NOA IRON ORE 62.93 0.00 0.08 0.01 2.58 2.77 3.36
OLIVINE 5.26 0.00 0.41 50.12 39.43 0.65 1.57
LIME Fines 0.17 0.00 68.90 0.56 1.31 0.56 27.54
BF Slag 0.00 0.00 36.56 7.49 33.48 20.25 0.00
Calcined Colemanite 0.83 55.45 32.82 2.61 6.00 0.67 0.00
[053] Iron ore used for pellet preparation was first screened at 0.5 mm. 0.5-10 mm fraction was then crushed below 5 mm and fresh minus 0.5 mm fraction was again screened. Plus 0.5 mm fraction was then ground using ball mill to minus 0.5 mm. Different fraction screened earlier at 0.5 mm fraction and material ground using ball mill was then mixed and homogenized so that a relatively coarser mix was obtained for pellet making such that minus 25-micron fraction is not significantly higher. Calcined colemanite was grounded such that minus 25-micron fraction is maximum possible. This is done so that calcined colemanite reports in bonding phases of unfired pellets. Olivine and Blast furnace slag was grounded for pellet making using ball mill.
[054] These materials were then mixed is preferred proportion and pellets of size 10-15 mm were prepared. These pellets were cured under atmospheric curing for 10 days.
[055] Handling strength in terms of cold compressive strength achieved after 10 days was 52 kg/pellets. Two sets of pot grate sintering test were taken wherein in base case 10-20 mm sized sinter was used as hearth layer while in other set Calcined Colemanite added special low temperature sinter-able unfired pellets were used as hearth layer. Table 3 provides pot test results comparison.
Table 3: Pot test results comparison

[056] The sintering time of both the test was same. As provided in above table, production of the sintering machine increased by 16.77 percent from 65.64 to 76.65 (difference of 76.65 and 65.64 divided by 65.64 is 16.77 %) as hearth layer material need not to be screened from the freshly produced sinter in trial 1. Also since no fuel was added for additional sinter Production therefore net coke breeze saving was 14.36 percent (difference of 78.28 and 91.40 divided by 91.40 is 14.36%)
[057] OBSERVATION: Significant amount of Hearth layer unfired agglomerate got sintered to the bottom sinter due to high sensible heat of bottom sinter and process air. Sintered Hearth layer as shown in image below resulted in increased production.
[058] It is also to be understood that the invention is not limited by the specific example and embodiment described hereinabove, but includes such changes and modifications as may be apparent to one skilled in the art.

We claim:

1. An unfired agglomerate for use in straight grate sintering machine as hearth layer comprising:
? iron ore fines (1),
? lime fines (2),
? Magnesium Oxide (MgO) based flux (3),
? Boron oxide (B2O3) bearing material (4), and
? hydraulic binder (5).
2. The unfired agglomerate as claimed in claim 1, wherein the lime fines (2) comprise CaO content of 10-20% by weight of the unfired agglomerate.
3. The unfired agglomerate as claimed in claim 1, wherein the MgO based flux (3) comprise MgO content of 1-2% by weight of the unfired agglomerate.
4. The unfired agglomerate as claimed in claim 1, wherein the B2O3 bearing material (4) comprise B2O3 content of 0.5-10% by weight of the unfired agglomerate.
5. The unfired agglomerate as claimed in claim 1, wherein the hydraulic binder (5) is 2-10% by weight of the unfired agglomerate.
6. The unfired agglomerate as claimed in claim 1, wherein the B2O3 bearing material is selected from the group consisting of calcined colemanite, calcined boric acid, orthoboric acid/or meta-boric acid.
7. The unfired agglomerate as claimed in claim 1, wherein the hydraulic binder (5) is selected from the group consisting of ground granulated blast furnace slag and portland cement.
8. The unfired agglomerate as claimed in claim 1, wherein iron ore fines (1) are coarser in size to facilitate pelletizing or briquetting wherein finer fraction reporting in bonding phases is below 15% and more preferably 10%.
9. The unfired agglomerate as claimed in claim 1, wherein the B2O3 bearing material is grounded to finer size to report mainly ion bonding phases which is below 25 micron for pellet making and below 150 micron for briquetting or extrusion.
10. The unfired agglomerate as claimed in claim 1, wherein the unfired agglomerate has a sintering temperature in the range of 838-1200 degree centigrade.
11. The unfired agglomerate as claimed in claim 1, wherein the unfired agglomerate is arranged as hearth layer in the form selected from the group consisting of briquettes, bricks, pellets and the like.
12. A method for preparing unfired agglomerate as claimed in claim 1 for use as hearth layer in straight grate sintering machine, wherein the method comprises the steps of:
? preparing a base mixture (20) of iron ore fines, lime fines, MgO based flux, B2O3 bearing material, and hydraulic binder,
? preparing an intermediate mixture (21) by adding water to the base mixture in appropriate amount,
? sizing the intermediate mixture to form sized agglomerates (22), and
? atmospheric curing (23) of the sized agglomerates to form unfired agglomerates to be used as hearth layer in straight grate sintering machine.
13. The method for preparing unfired agglomerate as claimed in claim 12, wherein the atmospheric curing is done for 7-10 days.
14. The method for preparing unfired agglomerate as claimed in claim 12, wherein the sized agglomerates are selected from the group consisting of briquettes, bricks, pellets and the like.
15. The method for preparing unfired agglomerate as claimed in claim 12, wherein the unfired agglomerates are cured or dryed using waste exit gas of the sintering machine.
16. A method for increasing iron ore sintering production and decreasing fuel consumption in straight grate sintering machines, the method comprising the steps of:
? charging unfired agglomerate as claimed in claim 1 as hearth layer on pallet car (12) of the machine,
? forming a bed (13) by layering raw material over the hearth layer,
? igniting top layer of the bed (13) to sinter the raw material, and
? applying suction from wind boxes (16) placed below the heath layer.
17. The method as claimed in claim 16, wherein the raw material comprises of iron ore fines and carbonaceous fuel.
18. The method as claimed in claim 16, wherein the hearth layer is sintered by sensible heat and waste exit process gas.
19. The method as claimed in claim 16, wherein size of unfired agglomerates varies in the range of 8 to 25 mm.
20. The method as claimed in claim 16, wherein size of unfired agglomerates varies in the range of 10 to 15 mm.
21. The method as claimed in claim 16, height of the bed varies in the range of 20-100 mm.