, Description:
PRIORITY INFORMATION

[001] The patent application does not claim priority from any patent application.

FIELD OF THE INVENTION

[002] The present subject matter described herein generally relates to a method of preparing wet and fired iron ore pellets and more specifically to a method for improving wet ball plasticity, crushing and drop strengths, dried pellet crushing strength as well as fired pellets compressive strength, abrasive index and Tumbler Index.

BACKGROUND

[003] Conventionally, iron ore pelletizing is a process by which the fine powdered raw materials like iron ores, fluxes, binders and carbonaceous fuel, which have been thoroughly mixed and moistened, are formed into spherical balls by rolling them on inclined rotating surfaces, e.g., in a granulating drum or on a pelletizing disc. The resulting spherical balls are then subsequently heat hardened on a travelling grate or rotary kiln furnace to attain the required crushing strength to withstand the handling and transportation as well as reduction under load conditions in blast furnaces and direct reduced iron (DRI) making furnaces.

[004] Due to increase in demand of steel, production / demand of iron ore has also increased significantly. With increased demand, quality of iron ore has continuously declined because of which more of fines in iron ore are generated. It is necessary to agglomerate these fine iron ores in form of sinter, pellets, briquette or nodules etc. Among these, pellets are preferable for agglomerating fine dust (which is preferably minus 150 micron). Additionally, due to the uniform shape and size of pellets, iron ore pellets provide a better gas permeability in blast furnace and decrease the consumption rate of coke.

[005] It is required to the iron ore pellets are of required composition to be used as a feed for blast furnace. The physical properties that influence pellet production are mainly green or wet ball quality like drop, plasticity, compressive strength as well as dried pellet compressive strength. Further, the physical properties of fired pellets for handling and transportation and bearing burden load are influenced by fired pellet tumbler index, abrasion index and compressive strength.

[006] Prior Arts
1. US6409964 describes preparation of cold bonded pellet by using iron particulate material along with water and sufficient particulate high-alumina cement binder. It shows required strength at elevated temperatures for use in conventional blast furnaces. Since Indian iron ores have higher alumina, it is not recommended to use further high alumina cement binder because alumina has high melting temperature which require more amount of heat, which in turn produce viscous slag in blast furnace. In addition, cold bonded pellets can be used in Blast furnace to certain limit only as cold bonded pellets do not have adequate strength after being subjected to reduction process and they became vulnerable to load and get broken.

2. US5000783 described prepared binder for pelletizing particulate mineral material. The binder comprises of modified native starch and water dispersible polymer material and mixture thereof. This binder is used to replace bentonite. Starch along with water dispersible polymer material shows better strength, improved abrasion strength and better balling. The major drawback of using organic binder is it shows lower crushing strength because of formation of higher porosity and less bonding phases as well as generates more dust inside induration furnace.

3. CN102337395A described use of raw materials namely cellulose ether, iron powder, starch ether and sodium bentonite to prepare pellet additive. Pellet additive prepared by using these materials shows improvement in physical and metallurgical properties, quality and yield of finished pellet. The disadvantages connected with this method are that the raw materials are costly (cellulose ether, iron powder, starch ether, etc.) and are not available in natural form. Iron powders are not naturally occurring compound and needs to be produced in certain furnaces with some specific purity and size. This becomes difficult to control. Starch Ether and cellulose ether compounds also needs to be produced in a control environment to achieve a guaranteed purity product. Further processing of these compounds in a certain proportion needs to be done carefully. All this makes this combination complex.

4. CN101215632A describes the use of an iron ore pellet binder, the composition of which in percentage by weight is sodium bentonite 20-70%, boron-containing mineral tailings 30-79%. With this binder, they showed increase in cold compressive strength and high temperature metallurgical properties. The disadvantage of the patent is higher sodium and Boron input. High sodium input in blast furnaces results in scaffold formation, hanging of burden, damages the furnace refractory lining and increase coke consumption in blast furnace. Boron containing material result in high boron in finished steel which is not desirable for many grades of finished steel.

5. US3765869 describes the method to use organic binder (low temperature binders) by reducing slag forming binders. According to this invention, the raw material used is mainly iron ore, starch/dextrin and iron powder (produced after grinding or milling of sponge iron product). Due to the chemical reaction between iron oxide of the ore, iron powder and the carbon produced and due to pyrolytic decomposition of organic binder, pellet developed greater strength after firing. The method is disadvantageous as the wet balls made using this binder has rough surfaces. Due to high surface moisture content; wet ball growth is high. Second disadvantage using this type of organic binder is that it increases the dust generation in induration.

OBJECTIVE OF THE INVENTION

[007] An object of the present subject matter is to provide a method of preparing wet and fired iron ore pellets.
[008] Another object of the present subject matter is to provide a method to increase the crushing strength of fired iron ore pellets.
[009] Another object of the present subject matter is to increase the tumbler index of the fired pellets.
[0010] Another object of the present subject matter is to decrease the abrasion index of fired pellets.
[0011] Another object of the present subject matter is to decrease the fine generation during pellet production and transportation.
[0012] Another object of the present subject matter is to provide a method to improve wet ball properties namely, Wet Compressive Strength (GCS).
[0013] Another object of the present subject matter is to develop a method to improve wet ball properties namely, Drop strength.
[0014] Another object of the present subject matter is to optimize the plasticity of the wet unfired iron ore pellets.
[0015] Another object of the present subject matter is to increase the Dry Compressive Strength (DCS) of dried iron ore pellets.
[0016] Another object of the present subject matter is to increase production of the pellet by sacrificing extra crushing strength obtained by present method.
[0017] Yet another object of the present subject matter is to improve the wet, dry and fired pellet strength of iron ore pellets prepared from difficult to pelletize category iron ores.
[0018] Yet another object of the present subject matter is to provide a method to increase silicate bond as well as make uniform silicate bond across the iron ore pellet volume.
[0019] Still another object of the present subject matter is to improve the natural ballability of iron ore fines.

SUMMARY OF INVENTION

[0020] Before the present system is described, it is to be understood that this application is not limited to the particular method or process, as there can be multiple possible steps/embodiments that are not expressly illustrated in the present disclosures. It is also to be understood that the terminology used in the description is for the purpose of describing the particular process only and is not intended to limit the scope of the present application. This summary is provided to introduce aspects related to a method of preparing wet and fired iron ore pellets and the aspects are further elaborated below in the detailed description. This summary is not intended to identify essential features of the proposed subject matter nor is it intended for use in determining or limiting the scope of the proposed subject matter.

[0021] The present subject matter describes a method of preparing wet and fired iron ore pellets. The method comprises steps to prepare iron ore pellet with an improved wet unfired pellets qualities, dried unfired pellet qualities and fired pellet qualities. The steps include mixing an iron ore fine, flux, solid fuel, binder with an amorphous siliceous material and water. Preparing a wet mix of mixed iron ore fine, flux, solid fuel, binder and amorphous siliceous material in a mixer. Preparing iron ore pellet from the mixer, wherein the iron ore pellet are prepared in a pelletizer, wherein the iron pellets are of 8-16 mm diameter. Further, firing the pellet in an induration furnace to obtain fired iron ore pellet.

[0022] In one embodiment, the iron ore fine is one of hematite, magnetite, goethite, high quartz or high alumina based ore.

[0023] In one embodiment, the flux is one of limestone, olivine, dolomite or slaked lime.
[0024] In one embodiment, the solid fuel is one of anthracite coal, carbon-bearing sludge, nut coke, coke breeze.

[0025] In one embodiment, the amorphous siliceous material is one of pyrogenic micro-silica or silica fumes powder. The dosage of the amorphous siliceous material in the mixture is 0.2 – 1 wt.% of base mixture of the iron ore, the flux, the solid fuel and the binder.

[0026] In one embodiment, the size of the micro-silica is 100% passing of 35-45 µm having an amorphous siliceous content in range of 95-99% , density of 0.25 to 0.40 g/cc, SiO2 content of 90-94%, crystalline mineral impurity of 0.5- 1% and specific surface area 15000-225000 cm2/g.

[0027] In one embodiment, a natural ballability index range (K value range) of the iron ore pellet is in range of 0.54-0.72.

[0028] In one embodiment, a drop strength of the unfired iron ore pellet increase by 2.3 points of a base pellet, wherein the base pellet is iron ore pellet without addition of amorphous siliceous material.

[0029] In one embodiment, a wet compressive strength of the unfired iron ore pellet increase by 0.51 points of the base pellet and a compressive strength of the dried iron ore increase of 0.31 points of the base pellet. The base pellet is iron ore pellet without addition of amorphous siliceous material.

[0030] In one embodiment, a plasticity of the unfired iron ore pellet decrease by 16.7% of the base pellet.

[0031] In one embodiment, a compressive strength of the fired dried iron ore pellet is increase by 71.5 points of base pellet with addition of 0.5% micro-silica.

[0032] In one embodiment, a tumbler index increase by 0.09 to 0.13 % of base pellet and an abrasion index decrease by 0.06 to 0.26 % points of the base pellet.

[0033] In another embodiment, return pellet fine decrease by 0.2% of the base pellet.
BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The foregoing summary, as well as the following detailed description of embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the disclosure, there is shown in the present document example constructions of the disclosure, however, the disclosure is not limited to the specific methods and device disclosed in the document and the drawings. The detailed description is described with reference to the following accompanying figures.

[0035] Figure 1 illustrates Schematic of Micro-Silica addition in pellet making operation, in accordance with an embodiment of the present subject matter.

[0036] Figure 2 illustrates a pellet cross-section showing cracked quartz particle, silicate bonds and hematite, in accordance with an embodiment of the present subject matter.

[0037] Figure 3 illustrates a variation of natural ballability with variation in ore composition and micro-silica addition, in accordance with an embodiment of the present subject matter.

[0038] Figure 4 illustrates a comparison of wet ball compressive strength, GCS unit, in accordance with an embodiment of the present subject matter.

[0039] Figure 5 illustrates comparison of drop number of pellets prepared with and without micro-silica addition, in accordance with an embodiment of the present subject matter.

[0040] Figure 6 illustrates wet ball deflection before crushing (plastic behaviour), in accordance with an embodiment of the present subject matter.

[0041] Figure 7(A), Figure 7(B), Figure 7(C) illustrates Comparison of CCS of pellets fired at different temperature (with and without addition of micro-silica), in accordance with an embodiment of the present subject matter.

[0042] Figure 8 illustrates CCS of pellets prepared with and without addition of micro-silica and fired in basket test, in accordance with an embodiment of the present subject matter.

[0043] Figure 9 illustrates CCS of commercially produced pellets with and without addition of micro-silica, in accordance with an embodiment of the present subject matter.

[0044] The figure depicts various embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION

[0045] Some embodiments of this disclosure, illustrating all its features and process, will now be discussed in detail. The words "comprising", “having”, and "including," and other forms thereof, are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. It must also be noted that as used herein and in the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Although any process and methods similar or equivalent to those described herein can be used in the practice, the exemplary method for preparing magnesium salts and micro silica is now described. The disclosed process is merely example of the disclosure, which may be embodied in various forms.

[0046] Various modifications to the embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. However, one of ordinary skill in the art will readily recognize that the present disclosure is not intended to be limited to the embodiments illustrated but is to be accorded the widest scope consistent with the principles and features described herein.

[0047] The present subject matter describes the method by which wet ball properties namely drop strength, green compressive strength, green ball plasticity as well as fired pellet compressive strength and fired pellet tumbler/abrasion maybe improved. The present subject matter involves the method of preparing fired iron ore pellets by application of additives for method of iron ore pelletizing along with iron ore fines (hematite, magnetite, goethite etc.), fluxes (limestone, olivine, dolomite, slaked lime etc.), binder (bentonite) and solid fuel (anthracite coal, carbon bearing sludge, nut coke, coke breeze etc.). Additive used in the present subject matter maybe siliceous material which is amorphous in nature. In one embodiment, iron ore pellets maybe obtained by adding siliceous material like silica fume or micro-silica etc. in the pellet raw material mixture.
[0048] In accordance with present subject matter, amorphous siliceous material mainly micro-silica is being added along with iron ore fines (hematite, magnetite, goethite etc.), fluxes (limestone, olivine, dolomite, slaked lime etc.), binder (bentonite) and solid fuel (anthracite coal, Carbon bearing sludges, nut coke, coke breeze etc.). The mixed material is then carried to disc pelletizer to obtain wet balls. The obtained wet balls are noted to have better crushing strength, optimal plasticity, better drop strength. The wet balls are unloaded on roller screen from where undersized balls were screened out and required sized balls were transferred to travelling grate furnace for induration. After induration or firing, an increase in tumbler strength and compressive strength were found and return fines generation decreased.

[0049] In accordance with Figure 1 illustrates, the Figure 1 illustrates a schematic of Micro-Silica addition in pellet making operation is disclosed. During the process of making the pellet making iron ore (1), fluxes (2), solid fuels like anthracite coal, coke fines/breezes or carbon bearing sludge (3) are grounded in ball mill are conveyed into mixer (6). In the same mixer, Bentonite (4) and Micro-silica (5) are added along with required amount of pre-determined water. This wet mixture is further transported to balling disc or drum (7). Wet ball formed in this equipment (7) are further conveyed to induration machine for firing.

[0050] In one exemplary embodiment, the method of preparing wet and fired iron ore pellets is described. The method comprises steps to prepare iron ore pellet with an improved wet unfired pellets qualities, dried unfired pellet qualities and fired pellet qualities. The steps include mixing an iron ore fine, flux, solid fuel, binder with an amorphous siliceous material and water. Preparing a wet mix of mixed iron ore fine, flux, solid fuel, binder and amorphous siliceous material in a mixer. Preparing iron ore pellet from the mixer, wherein the iron ore pellet are prepared in a pelletizer, wherein the iron pellets are of 8-16 mm diameter. Further, firing the pellet in an induration furnace to obtain fired iron ore pellet.

[0051] Further, the present subject matter discloses use of siliceous material that may lead to increase of silicate melt and uniform distribution of silicate bond in pellet volume. Due to this uniformly distributed bonding phase, strength of pellets is increased without affecting the other qualities of pellet. The use of additive leads to improvement in drop strength, wet ball crushing strength, dry ball crushing strength, fired pellet tumbler/abrasion and cold crushing strength and pellet return fines generation are noted to be decreased.

[0052] Silicate melt or silicate bonds are important bonding phases in iron ore pellets other than the recrystallized hematite bonds itself. However, obtaining high alumina iron ore silicate bonds becomes more important as recrystallized hematite bond formation is less (as the temperature required for forming recrystallized bond is higher with increase in alumina). Additionally, recrystallized hematite bonds of high alumina ore may result in high reduction degradation of pellets as alumina dissolution maybe higher in recrystallized hematite as Al3+ and Fe3+ may dissolve in each other more during recrystallization reaction.

[0053] Though silicate melts are important bonds for iron ore pellets but all siliceous material, which are added during iron ore pelletizing, do not result in silicate melt or silicate bond formation. Especially crystalline siliceous materials do not participate much during induration. Moreover, quartzite, which is more than 94% silica, not only remain inert during induration but also forms cracks in quartz particle itself as well as in pellets due to alpha to beta polymorphic transformation at ~550 °C and results in less CCS.

[0054] As illustrated by Figure 2, the figure illustrates quartzite particle with cracks inside it as well as silicate bonds in pellet. These silicate bonds are not uniformly distributed across the pellet structure. As these siliceous materials are present with iron ore and are not uniformly distributed in ore itself. Therefore at few points the silicate bonds will be more and at other locations silicate bonds will be absent. Addition of finer micro-silica leads to uniform distribution of silicate bonds as well as amorphous silica leads to formation of silicate bonds.

[0055] Further, wet ball qualities namely wet ball drop strength, crushing strength and plasticity etc. depends on iron ore mineralogy and its textural features which determine the natural ballability. Any method that may results in improvement of natural ballability or balling behaviour would result in improving the wet ball qualities.

[0056] In one embodiment, the iron ore pellets are required to possess the below physical properties:

(i) Wet Ball Property: Wet ball property is determined by two parameters that are drop strength and wet compressive strength.

(a) Drop Strength: The wet pellets during preparation are conveyed on disc or drum pelletizer to induration unit via several conveyors. During the process the pellets are subjected to sudden drop from certain height from disc/drum to conveyor and from one conveyor to another conveyor and finally from conveyor to induration machine. During this mode of transportation, wet unfired pellets experience sudden impact force for fall. The drop number of wet pellets is required to be high enough to endure the transportation from balling circuits to furnace without breaking or generating cracks. Drop strength is mainly govern by moisture, binder type and mineralogy of ore and its surface characteristics.

Further, a minimum amount of plasticity is required for better drop number or drop strength. On other side, excess plasticity is not intended as excess plastic balls may lead to deformed balls inside the furnace on the pellet bed leading to lower the bed permeability and poor firing and lower productivity.

(b) Wet Compressive Strength: Wet pellets are required to have the strength to sustain static load, plastic deformation and breakage when the wet pellets are placed as a bed on travelling grate furnace. Excess Plastic deformation leads to lowering of permeability of pellet bed and lead to poor or inadequate firing. Breakage of wet pellets results in lowering of production, high fines generation, filling of bed void-age by fines and thus lower permeability of bed etc.

(ii) Dry Ball Properties: In the drying zone, surface moisture of pellets is removed. Due to the removal of surface moisture, dry pellet loses the plasticity and thus any resistance to mechanical shock is also decreased (mechanical shock occurs due to small vibrations from machine movement). Therefore, dry pellets are required to maintain enough strength to withstand the load of layers above, pressure of gases flowing across bed as well as small mechanical vibrations arising due to machine movement.

(iii) Fired Pellet Properties: Post induration, fired pellets are required to have enough strength to withstand their handling and transportation as well as stresses occurring inside it during metallurgical treatment in Blast furnaces or DRI making unit etc. These physical strengths are measured in terms of Tumbler/Abrasion index and Cold compressive strength.

(a) Tumbler/Abrasion: Tumbler and Abrasion Index is measured in terms of +6.3 mm fraction, 0.5 to 6.3 mm fraction and minus 0.5 mm fraction of pellets after tumbling in a rotary drum of specific make for specific time. Percentage of +6.3 mm (Tumbler index) particles after tumbling should be high enough to sustain handling and transportation. Percentage of Minus 0.5 mm (Abrasion index) should be as minimum as possible because increase in abrasion results in leads to high dust generation during conveyance.

(b) Cold Crushing strength: If the crushing strength is not adequate then pellet is likely to be prone to breakage during its metallurgical operation (because of the load of the burden over it) and will worsen the permeability and productivity of iron making furnaces.

[0057] In one embodiment, natural ballability of ores plays a very significant role. Natural ballability plays a vital role in determining how the ore will behave during balling operation and what will be the quality of green ball in terms of drop number, plasticity, green compressive strength etc. Natural ballability or Static ballability refers to the assembly or coherence character of a fine material under the action of water droplets and it is measured as
K= Wm/(Wc-Wm)
Where, K= Natural Ballability Index of material,
Wm is maximum molecular water content of the material (wt%)
And Wc is Maximum Capillary water content of the material.

[0058] The natural ballability value is classified into different groups based on magnitude of K and these groups are defined in Table 1

TABLE 1
S. No Condition Inference
1 K < 0.2 No Ballability
2 K= 0.2-0.35 Weak Ballability
3 K= 0.35-0.60 Moderate Ballability
4 K= 0.60-0.80 Fair Ballability
5 K > 0.80 Excellent Ballability

[0059] Maximum molecular water content: Molecular water is the water adsorbed on particle surface by molecular interactions. Molecular water content of a powder material is hence an evaluation of the specific surface area and hydrophilic character of the material. It is measured by using compression method or centrifugal method. By removing all the excess moisture after applying certain compression force, the residual moisture is measured and is known as molecular moisture.

[0060] Maximum capillary water content: Capillary water is the water existing in capillary pores between particles, and is dependent on particle size, shape and porosity of the material. It is measured by first allowing a bed of ore powder to absorb maximum amount of water by means of capillary action then measure the amount of moisture in the ore by heating.

[0061] In one embodiment, addition of special siliceous material, K value or natural ballability of ore is found to improve and the details of the same is provided in exemplary case study.

[0062] In one exemplary case study, the special siliceous material added is micro-silica, which is amorphous in nature and is 100% less than 38 microns on wet sieve basis. Micro-amorphous silica includes silica sols, gels, powders, and porous glasses. These consist of ultimate particles of the inorganic polymer (SiO2)n, where a silicon atom is covalently bonded in a tetrahedral arrangement to four oxygen atoms. Each of the four oxygen atoms is covalently bonded to at least one silicon atom to form either a siloxane, –Si–O–Si–, or a silanol, –Si–O–H–, functionality. The bond distances and bond angles in amorphous silica are similar to those of cristobalite. Si–O bond distances are approximately 0.16 nm, and Si–O–Si bond angles are about 148?.
Amorphous silica dissolves or depolymerizes in water according to equation 1:
SiO2 (s) +2H2O (l) ? H4SiO4 (aq) ……. (1)
H4SiO4 (aq), which is either a mono-silicic or an ortho-silicic acid, is also expressed as Si(OH)4 (aq) or H2SiO3 (aq).

[0063] When amorphous micro-silica is added in the mixture for iron ore pelletizing, with the addition of water, it partially undergoes the reaction as mentioned in equation (1). Thus, formation of H4SiO4 results in more and more (SiO4)4- ions in the mixture resulting in better electrostatic interaction in the mixture and thus better natural ballability. This in turn results in improvement of green balling operation and thus wet ball qualities like drop number, green compressive strength, plasticity and finally dry compressive strength.

[0064] In another exemplary embodiment, the effect of addition of micro-silica on natural ballability variation is noted. Iron-ore from different mines is collected and grounded with different proportion of dry and wet ores wherein dry ore is soft and wet ore is harder in regard to Bond Work Index values. Table 2 provides ratio of dry circuit and wet circuit ores for grinding and ballability measurement.

TABLE 2
Mix Ratios Dry Circuit Ore % Wet circuit Ore %
Mix Ratio 1 100 0
Mix Ratio 2 75 25
Mix Ratio 3 50 50
Mix Ratio 4 25 75
Mix Ratio 5 0 100

[0065] In one implementation, the mix ratio maybe grounded for 15 min in lab scale ball mill. The ball mill product is screened at 0.5 mm and the products obtained were used for natural ballability measurement. The Figure 2 illustrates the natural ballability variation of ground ores with and without addition of 0.5% micro-silica. The Set 1 in the Figure 3 represents the ground product of mix ratio 1-5 without addition of Micro-silica and the Set 2 represents the ground product of Mix ratio 1-5 with addition of Micro-silica.

[0066] It maybe noted from the Figure 3 that the natural ballability is significantly improved with the addition of amorphous micro-silica in the mixture and many K values (Natural ballability Index) were above 0.6 above which mixture ballability is fair as per K value interpretation. It is therefore noted that the addition of micro-silica has a significant effect with regard to improving ballability of iron ore for pellet making. The main reason for improving the natural ballability of ores is extra SiO44- ions which forms with the addition of water to micro-silica.

[0067] In another exemplary case study, the effect of micro-silica on wet ball qualities is noted. Ground iron ore, limestone, Olivine, Anthracite Coal, bentonite and Micro-silica are used as raw material. For base case, base iron ore pellets without micro-silica addition are prepared and tested for wet ball properties. These properties are compared with the wet ball properties of pellets prepared with additional dozing of micro-silica in the mixture. The Table 3 defines the material balance for making wet balls.
TABLE 3
Raw Materials Base Case Trial 1
Iron ore, wt.% 92.62 92.62
Limestone, wt.% 2.27 2.27
Olivine, wt.% 3.16 3.16
Anthracite Coal, wt.% 1.43 1.43
Bentonite, wt.% 0.52 0.52
Micro-Silica, wt.% 0% of above base mixture 0.5% of above base mixture

[0066] Effect of Micro-silica on wet/green compressive strength of pellets:
The Figure 4 illustrates the compressive strength of wet /green pellets. It is evident that the compressive strength of green pellets increases significantly when micro-silica is used for pellet making. This further contributes to increase in balling disc productivity, decrease in green ball rejects and increase in CCS of pellets. Balling disc productivity increases the residence time of green ball in balling disc can be decreased for getting the required GCS. Green ball reject will decrease because of lower undersize due to lower green ball collapse/breakage during conveying. CCS increases as better GCS leads to better voidage in induration machine as high GCS pellet would not deform much and therefore will not decrease the pellet bed voidage. Better voidage leads to better circulation of processing air and hence better thermal exposure which leads to better CCS.

[0067] Effect of Micro-Silica on Drop Strength of wet unfired pellets:
The Figure 5 illustrates the drop strength of green pellets. Drop strength of green pellets is increased significantly when micro-silica is used for pellet making. This will contribute to increase in balling disc productivity, decrease in green ball reject. Balling disc productivity will increase because residence time of green ball in balling disc can be decreased for getting the required drop number. Green ball reject will decrease because of lower undersize due to lower green ball collapse/breakage during conveying. This also provides the opportunity to decrease the binder/bentonite consumption.

[0068] Effect of Micro-Silica on plasticity of wet unfired pellets:
During the testing of the compressive strength of green/wet pellets, pellets initially get slightly deformed before getting crushed. The extent to which they are getting deform depends on their plasticity. Lower the deflection lesser the plasticity. The Figure 6 illustrates the average green ball deformation of pellets prepared in this work. It is to be noted that though plasticity is required by wet pellets, excess plasticity may lead to poor bed voidage as excess plastic pellets will deform under load and decrease the permeability of bed. With the addition of micro-silica at 0.5%, plasticity decreased by 16.7% (since average deflection decreased to 0.5 from 0.6).

[0069] Effect of Micro-Silica on Dried Pellet Crushing strength:
Dried pellet Compressive strength increased from 3.49 kg/p to 3.8 kg/p. This will contribute to increase in CCS of pellets as better DCS will lead to better resistant to load during induration.

[0070] In another exemplary case study, the effect of micro-silica on Fired Pellet CCS at Lab Scale is studied. The iron ore pellets are fired at different temperatures in lab scale rapid heating muffle furnace. After firing, CCS of fired pellets are measured and are illustrated in Figure 7(A), Figure 7(B) and Figure 7(C). CCS of fired pellets increased from 222 kg/p in base case to 271 kg/p in micro-silica doped pellets fired at 1300 °C firing temp. and at 1290 °C firing temp CCS increased from 222 kg/p in base case to 250 kg/p and at 1270 °C firing temp CCS increased from 183 kg/p to 232 kg/p. The temperature noted by pellets varies across the height and width of bed. Even though the target firing temp is 1300 °C for hematite ore based pellets the actual temp seen by pellets inside bed varies from 1300 degree Celsius towards lower values. To study that effect, the average CCS of base case and micro-silica doped pellet is calculated by average of CCS at 1270, 1290 and 1300 and comparing the same. Average base case CCS in this case is 209 kg/p while with doping of 0.5% micro-silica CCS increased to 251 kg/p which is 42 points more as compared to base value.

[0071] In yet another exemplary case study, the effect of addition of micro-silica in pellet mix on fired pellet CCS at Pilot Level Basket Test is studied. Green/wet pellets prepared at lab are fired in the commercial pellets plant. The wet unfired pellets are kept in Inconel basket/cage and these baskets are placed inside the commercial pellet plant induration furnace. After the baskets reaches the discharge end, the baskets with fired pellets inside them are carefully retrieved. This procedure of firing pellets inside pellet plant gives close simulation of firing profile to actual firing in plant than in muffle furnace.

[0072] For carrying the pilot level basket test, raw material of iron ore, limestone, olivine, anthracite coal, bentonite and micro-silica are used. In the base case, base iron ore pellets without addition of micro-silica were prepared and for comparison, pellets with 0.5% micro-silica addition are prepared. Separate inconel baskets were filled with wet pellets. The baskets were carefully placed inside induration furnace and retrieved after firing at the discharge end with fired pellets inside them. The fired pellets were then carefully taken out and tested for compressive crushing strength (CCS). The CCS values obtained with and without addition of micro-silica and fired in basket test is as illustrated in Figure 8. A significant improvement in CCS by 71.5 points is observed by addition of 0.5% micro-silica in the pellet base mixture.

[0073] In another exemplary case study, the case study highlights the effect of Micro-Silica addition in pellet mix on wet ball and Fired pellet quality as well as on pellet production in a trial at commercial pellet plant. Micro-silica is tried at commercial pellet plant, which has a grate area of 4 m (w) X 192 m (L) = 768 m2. The micro-silica in the raw material mixture added in a very controlled manner, out of two numbers of bentonite bins, one bentonite bin is emptied completely and filled with micro-silica. The bin filled with micro-silica is used for controlled dozing of micro-silica in the mixer while other bentonite bin is used for adding regular bentonite in the whole mix.

[0074] With the addition of micro-silica at 0.3 wt.% there is a significant increase in the CCS. Upon realizing the increase in CCS, machine speed and machine feed is increased to realize impact of micro-silica addition on the increase in production with same CCS prior to trial i.e. without micro-silica addition. Comparative Data of the trial with the base values are as illustrated in the Figure 9. The Figure 9 shows that the CCS of pellets produced with addition of 0.3% micro-silica were 29 points higher in crushing strength.

[0075] As the improvement in the crushing strength is noted, induration machine feed and speed is increased to determine the impact of micro-silica addition on increase in production. With increase in machine feed rate and machine speed, CCS is again noted to be down at the level of 214 kg/p. Other quality data are shown in the Table 4.

[0076] From Table 4, it is noted that, with same CCS of 214 kg/p, production increase from 6360 tonnes in one shift to 6680 tonnes, which is 380 tonnes more or 5.03% increase with respect to base period. In terms of product pellet quality, even at higher level of production tumbler index improved from 95.11% in base period to 95.2% when micro-silica is used. Abrasion index also improved from 4.33% to 4.27%.

[0077] With regard to quality of wet ball, even though there is slight decrease in wet ball compressive strength (GCS) from 1.31 kg/p to 1.24 kg/p but this is due to lower residence time in balling disk for increased production rate. Due to improved interaction of micro-silica with the pellet constituents, dried ball compressive strength increased from 4.63 kg/p in base period to 4.87 kg/p during micro-silica application. With increased production rate, drop number also increased from 12.77 to 15.1. Table 4 also shows that during the base period and during addition of micro-silica in the mixture the firing the average firing temp of firing zone were almost similar. However, the machine speed of travelling grate is higher during micro-silica addition, which implies that unfired pellets were getting less residence time at the same temp as base period but the fired pellet qualities are still better during micro-silica application period. From Table 4, it is also evident that pellet fines generated during micro-silica application period were 0.2% lower than base period and further leads to production increase as well as lower dust generation and subsequent environmental problem.
TABLE 4

[0078] Exemplary embodiments discussed above may provide certain advantages. Though not required to practice aspects of the disclosure, these advantages may include those provided by the following features.

[0079] Although embodiments of method of preparing wet and fired iron ore pellets are explained, it is to be understood that the appended claims are not necessarily limited to the specific process described. Further, the specific process are disclosed as examples of embodiments of process method of preparing wet and fired iron ore pellets.

Claims:WE CLAIM:

1. A method to prepare fired iron ore pellet, wherein the method comprises of:
mixing an iron ore fine, flux, solid fuel, binder with an amorphous siliceous material and water;
preparing a wet mix of mixed iron ore fine, flux, solid fuel, binder and amorphous siliceous material in a mixer;
preparing iron ore pellet from the mixer, wherein the iron ore pellet are prepared in a pelletizer, wherein the iron pellets are of 8-16 mm diameter;
firing the pellet in an induration furnace to obtain fired iron ore pellet.

2. The method of claim 1, wherein the iron ore fine is one of hematite, magnetite, goethite, high quartz or high alumina based ore.

3. The method of claim 1, wherein the flux is one of limestone, olivine, dolomite or slaked lime.

4. The method of claim 1, wherein the solid fuel is one of anthracite coal, carbon-bearing sludge, nut coke, coke breeze.

5. The method of claim 1, wherein the amorphous siliceous material is one of pyrogenic micro-silica or silica fumes powder, wherein the dosage of the amorphous siliceous material in the mixture is 0.2 – 1 wt.% of base mixture of the iron ore, the flux, the solid fuel and the binder.

6. The method of claim 5, wherein the size of the micro-silica is 100% passing of 35-45 µm having an amorphous siliceous content in range of 95-99% , density of 0.25 to 0.40 g/cc, SiO2 content of 90-94%, crystalline mineral impurity of 0.5- 1% and specific surface area 15000-225000 cm2/g.

7. The method of claim 1, wherein a natural ballability index range (K value range) of the iron ore pellet is in range of 0.54-0.72.

8. The method of claim 1, wherein a drop strength of the unfired iron ore pellet increase by 2.3 points of a base pellet, wherein the base pellet is iron ore pellet without addition of amorphous siliceous material.

9. The method of claim 1, wherein a wet compressive strength of the unfired iron ore pellet increase by 0.51 points of the base pellet and a compressive strength of the dried iron ore increase of 0.31 points of the base pellet, wherein the base pellet is iron ore pellet without addition of amorphous siliceous material.

10. The method of claim 1, wherein a plasticity of the unfired iron ore pellet decrease by 16.7% of the base pellet, wherein the base pellet is iron ore pellet without addition of amorphous siliceous material.

11. The method of claim 1, wherein a compressive strength of the fired dried iron ore pellet is increase by 71.5 points of base pellet with addition of 0.5% micro-silica, wherein the base pellet is iron ore pellet without addition of amorphous siliceous material.

12. The method of claim 1, wherein a tumbler index increase by 0.09 to 0.13 % of base pellet and an abrasion index decrease by 0.06 to 0.26 % points of the base pellet, wherein the base pellet is iron ore pellet without addition of amorphous siliceous material.

13. The method of claim 1, wherein return pellet fine decrease by 0.2% of the base pellet, wherein the base pellet is iron ore pellet without addition of amorphous siliceous material.