Claims:We Claim:
1. A method for improving thermal shock properties of iron ore pellets comprising steps of:
? mixing of iron ore, flux, carbon source, binder and hot water to obtain a mixture;
? pelletizing the mixture with spray of hot water to obtain hot green pellets; and
? charging the hot green pellets in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.
2. The method as claimed in claim 1, wherein the iron ore is a hematite-based, magnetite based or goethite-based iron ore.
3. The method as claimed in claim 2, wherein the iron ore is selected from a group comprising ores comprising high ultra-fines, ores comprising high alumina, ores comprising high silica, ores comprising high combined moisture (loss on Ignition, LOI), ores comprising iron ore slimes, a low-grade iron ore, high grade iron ore and combination thereof.
4. The method as claimed in claim 3, wherein the low-grade iron ore comprise high alumina of about 2-6%, LOI of about 3-7% and minus 45-micron size particles of about 75-85%.
5. The method as claimed in claim 1, wherein the flux is selected from a group comprising CaO based flux, MgO based flux and combinations thereof.
6. The method as claimed in claim 5, wherein the CaO based flux is selected from a group comprising limestone, slaked lime, calcined limestone and combinations thereof.
7. The method as claimed in claim 5, wherein the MgO based flux is selected from a group comprising olivine, pyroxenite, dolomite, dunite and combinations thereof.
8. The method as claimed in claim 1, wherein the carbon source is selected from a group comprising anthracite coal, coke fines, coke breeze, carbon bearing wastes, industrial by-products and combinations thereof.
9. The method as claimed in claim 8, wherein the industrial by-products are selected from a group comprising blast furnace dust, LD sludge, Coke dust and combinations thereof.
10. The method as claimed in claim 1, wherein the binder is selected from a group comprising bentonite, organic binder, inorganic binder and combinations thereof.
11. The method as claimed in claim 10, wherein the binder is bentonite.
12. The method as claimed in claim 1, wherein the hot water has a temperature of about 65-85 °C.
13. The method as claimed in claim 1, wherein the hot green pellets formed are about 6-16 mm in size.
14. The method as claimed in claim 1, wherein the hot green pellet temperature is in the range of about 55-60 °C.
15. The method as claimed in claim 1, wherein the mixing comprises dry mixing of iron ore fines, flux, carbon source and binder followed by wet mixing with addition of hot water.
16. The method as claimed in claim 1, wherein the pelletization is carried out in a disc or drums pelletizer to obtain green pellets followed by induration in a furnace to obtain the iron ore pellets.
17. Iron ore pellets obtained by the method of any of the claims 1-16.
, Description:TECHNICAL FIELD
The present disclosure relates to the field of material sciences and metallurgy. In particular, the present disclosure relates to a method for improving thermal shock properties of iron ore pellets. The improvement in thermal shock resistance is in terms of cracking and bursting of pellets during drying operation in the pellet making. The present disclosure also relates to iron ore pellets obtained by the method.
BACKGROUND AND PRIOR ART
Iron ore pelletizing is a process in which powdered iron ore fines, fluxes, binders, and carbon-bearing solid fuels, with a preferable size of 62-67% passing 45 microns, are mixed in the presence of ~8% water to prepare a wet mixture. The wet mixture is then rolled in drum/disc pelletizer to form wet balls of size 6-16 mm (with 10-12.5 mm balls being ~65-70%) which are later indurated in an oxidizing atmosphere in a rotary grate-kiln or travelling grate furnace at a temperature of 1250-1300°C. Fluxes used here can be pyroxenite, olivine, dolomite, limestone, etc. Binder can be any organic binder or bentonite and carbon-bearing solid fuels can be coke, anthracite coal, nut coke, carbon-bearing industrial by-products such as gas cleaning plant sludges of the blast furnace, etc.
Induration furnaces consist of various zones namely, drying zone, preheating zone, firing zone, after-firing zone, and cooling zone. The drying zone (250-400°C) involves two stages, i.e. up-draught (UDD) and down-draught (DDD), to remove the surface moisture. In the drying zone, up-draught drying is done by pushing the hot air upward. If down draught drying is done instead of up-draught, then there are chances that once the temperature of the drying air/gas is below dew point, then process air will lose moisture and this moisture from the upper layers of the pellet bed will deposit on the bottom layers of pellet bed. This will increase the moisture of the bottom layer and hence pellets will become more plastic, causing deformation of the bottom layer pellet under a load of the upper layer. Therefore, there is a UDD zone first in the pellet induration furnace followed by the DDD zone.
Although the scheme of having UDD zone before DDD zone in induration furnace helps to avoid any collapse of bed voids on account of increased moisture in bottom layer pellets, still during drying of pellets in the UDD zone, the hot air which is flowing upwards reaches its dew point and loses moisture to upper layer pellets. The moisture in the upper layer pellets increases and when the pellet bed moves forward towards a higher temperature zone, green pellet in upper layer experiences higher thermal shock due to higher moisture and temperature. This results in either bursting of pellets or crack formation in pellets. Moreover, the pellets which initially entered the furnace also experience thermal shock due to exposure to high-temperature gases irrespective of the location in bed i.e. upper layer, the middle layer, or bottom layer. However, such thermal shock is maximum when the moisture in pellets is maximum which is the case for the pellets in the relatively upper layer.
Fines generation due to thermal shock results is poor bed permeability and hence poor fired pellet quality and plant productivity. Moreover, it also results in higher fines generation at pellet plants and during conveyance till end-users such as blast furnace, Corex, Midrex, etc. Cracks generated due to thermal shock also results in lower cold compressive strength (CCS) of pellets and higher Reduction Degradation index (RDI) of pellets.
Moisture Removal from the pellets in the drying stage occurs in three steps:
1. Removal of moisture from the outer regions of pellets (or shell region).
2. Movement of moisture from the Inner region of pellets (mantle and core) to the outer region of pellets (shell region).
3. Removal of moisture from the inner region of pellets that is pushed back by the outer regions of pellets.
If the moisture coming from the inner regions of the pellets does not find suitable pathways, then the vapor pressure inside pellets increases which leads to cracking or bursting of pellets. Hence, there should be enough pathways for moisture to travel. These pathways are created by open and connected pores\network of pores inside the pellets. These pores enable the moisture to move out. Still, there are chances that the pellets may not have sufficient pathways for moisture to escape as vapors which results in pellet cracking or bursting.
Low-grade iron ores consisting of higher iron ore slime content contains a higher percentage of goethite, LOI and clay mineral. The goethite contains a higher amount of chemically bonded water and during the induration process at a temperature of around 400-700°C, sudden release of chemically bonded water develops cracks in pellets and some of them even bursts into a finer powder. Moreover, Low-grade ores are naturally finer or due to their softer nature (such as for goethite, clayey, and high LOI ores), they generally get grounded to a finer size. The use of such ores containing higher super fines (particles less than 2 microns, 10 microns, etc.) in pellet making results in the formation of more compact/dense pellets. Due to increased compaction or dense pellet formation, the pathways or network of open pores are blocked which necessitates that the pellets are slowly dried to release all moisture without bursting or spalling. However, slow drying decreases productivity. On the other hand, faster movement of the machine causes faster drying and hence the bursting of pellets. High pellet compactness increases the resistance to water evaporation. Thus, low-grade ore due to finer particle size require steps which can improve its shock temperature or thermal shock behaviour during pellet making. It is also to be noted that, high goethite containing ores tend to absorb more moisture as against high hematite or magnetite ore. This further deteriorates the thermal shock behaviour of pellets.
Relevant research and prior arts to improve the thermal shock behaviour of iron ore pellets are described below:
Ward et al., in their Patent No. US3425823 reported the use of alkali humate salt in iron ore pelletizing to increase the shock temperature of pellets and decrease the disintegration of pellet under higher temperature. Disadvantage connected with this is that the raw material is costly. Secondly, higher alkali aggravates problems such as scaffold formation, burden hanging and corrodes the furnace refractory lining and increases coke consumption in the blast furnaces.
Yukio et al, in their Patent No. JPS5554530A discussed the improvement in the thermal shock property and pellet strength by introducing needle like or flaky particles in the blend. Flaky particles were obtained by crushing a cleavable lamellar mineral such as iron ore in Brazil, serpentine, serpentine asbestos, chrysotile or hornblende asbestos. The major drawback of using chrysotile and asbestos is associated with health risks. Also, chrysotile and asbestos are not good for fired pellet property due to their volume shrinkage behavior which makes the microstructure wherein chrysotile, asbestos, etc. are not completely sintered or connected with the matrix.
Pimenta et al., in their Patent No. US9175364B2 used raw material namely sodium silicate, manioc starch, and micro-silica to prepare an additive for iron ore pelletization. Pellet additive prepared showed a decrease in the generation of fines due to thermal shock inside the indurating furnace. The drawbacks associated with this additive are the high cost of starch. Moreover, addition of sodium silicate increases the sodium content of the pellets causing scaffold formation, burden hanging, etc. It also corrodes the furnace refractory lining and increases coke consumption in the blast furnaces.
Haas et al., in their paper “Effectiveness of organic binder for iron ore pelletization”, showed that by using gelled starch as partial replacement of bentonite, no breakage or spalling occurred during thermal shock test. The disadvantage of using the organic binder is that the wet balls obtained will have a rough surface. Also, organic binders are costly and give low crushing strength because of the formation of high porosity which results in dust generation in induration furnace.
Liu et al., in their paper “Effect of bentonite on the pelletizing properties of iron ore concentrate”, has shown the effect of three different types of bentonite on drop strength, crushing strength, and shock temperature of the green pellets. They concluded that the bentonite having higher montmorillonite content shows better green properties and increases the shock temperature resistance of the green pellets. The drawback of the prior art is the availability of bentonite with higher montmorillonite content.
In addition to the drawbacks mentioned in the abovementioned prior arts, one major drawback is that all the prior arts are silent on thermal shock behavior of pellets which are made from iron ore fines containing higher super-fines and low-grade raw materials containing higher clay and goethite ore.
Thus, there exists a need for improving thermal shock properties of iron ore pellets and the present disclosure achieves the same.
SUMMARY OF THE DISCLOSURE
The present disclosure relates to a method for improving thermal shock properties of iron ore pellets comprising steps of:
? mixing of iron ore, flux, carbon source, binder and hot water to obtain a mixture;
? pelletizing the mixture with spray of hot water to obtain hot green pellets; and
? charging the hot green pellets in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.
In an embodiment of the present disclosure, the method improves thermal shock resistance of iron ore pellets without addition of any material in the pellet mixture which can affect the subsequent process of iron and steel making.
In an embodiment of the present disclosure, the iron ore is a low-grade iron ore comprising high alumina of about 2-6%, LOI of about 3-7% and minus 45-micron size particles of about 75-85%.
In an embodiment of the present disclosure, the hot water has a temperature of about 65-85°C.
In an embodiment of the present disclosure, the hot green pellets formed are about 6-16 mm in size.
In an embodiment of the present disclosure, the hot green pellet temperature is in the range of about 55-60 °C.
The present disclosure also relates to iron ore pellets obtained by the method.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
In order that the disclosure may be readily understood and put into practical effect,
reference will now be made to exemplary embodiments as illustrated with reference to
the accompanying figures. The figures together with detailed description below, are
incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the
present disclosure where:

Figure 1: provides the general layout of the plant for iron ore pelletizing by conventional method.
Figure 2: is the schematic of up-draught drying operation in the conventional method.
Figure 3: is the schematic of down-draught drying operation in the conventional method.
Figure 4: shows the iron ore pelletizing process as per the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent product/methods do not depart from the scope of the disclosure.

Definitions:
Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

As used herein, the singular forms ‘a’, ‘an’ and ‘the’ include both singular and plural referents unless the context clearly dictates otherwise.

The term ‘comprising’, ‘comprises’ or ‘comprised of’ as used herein are synonymous with ‘including’, ‘includes’, ‘containing’ or ‘contains’ and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. Likewise, certain terms may be interchangeably used throughout the specification and thus have the same meaning even when they are referred interchangeably. For example, green pellets may be interchangeably referred as green balls, Cold Crushing strength as CCS, up-draught drying operation as UDD, down draught drying operation as DDD, etc.

The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

The term ‘about’ as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of ±10% or less, preferably ±5% or less, more preferably ±1% or less and still more preferably ±0.1% or less of and from the specified value, insofar such variations are appropriate to perform the present disclosure. It is to be understood that the value to which the modifier ‘about’ refers is itself also specifically, and preferably disclosed.

The present disclosure relates to a method for improving thermal shock properties of iron ore. The method improves thermal shock resistance of iron ore pellets without addition of any material in the pellet mixture which can affect the subsequent process of iron and steel making. The method involves processing of low-grade iron ores with high alumina, silica, the water of hydration as well as high ultrafine content. The method is cost-effective and does not disturb the working of down-stream processes such as blast furnace, Corex, Midrex, etc.

In an embodiment of the present disclosure, the method to improve thermal shock resistance of pellets can be scaled up easily to industrial scale.

In an embodiment of the present disclosure, the method for improving thermal shock properties of iron ore pellets comprises steps of:
? mixing of iron ore, flux, carbon source, binder and hot water to obtain a mixture;
? pelletizing the mixture with spray of hot water to obtain hot green pellets; and
? charging the hot green pellets in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.
In an embodiment of the present disclosure, the method improves thermal shock resistance in terms of cracking and bursting at a specific temperature of interest.
In an embodiment of the present disclosure, the iron ore is a hematite-based, magnetite based or goethite-based iron ore.
In an embodiment of the present disclosure, the iron ore is selected from a group comprising ores comprising high ultra-fines, ores comprising high alumina, ores comprising high silica, ores comprising high combined moisture (loss on Ignition, LOI), ores comprising iron ore slimes, a low-grade iron ore, high grade iron ore and combination thereof.
In another embodiment of the present disclosure, the iron ore is an ore comprising high ultra-fines.
In another embodiment of the present disclosure, the iron ore is an ore comprising high alumina.
In another embodiment of the present disclosure, the iron ore is an ore comprising high silica.
In another embodiment of the present disclosure, the iron ore is an ore comprising high combined moisture (loss on Ignition, LOI).
In another embodiment of the present disclosure, the iron ore is an ore comprising iron ore slime.
In another embodiment of the present disclosure, the iron ore is low-grade iron ore.
In an embodiment of the present disclosure, the low-grade iron ore comprise high alumina of about 2-6%, LOI of about 3-7% and minus 45-micron size particles of about 75-85%.
In an embodiment of the present disclosure, the flux is selected from a group comprising CaO based flux, MgO based flux and combinations thereof.
In an embodiment of the present disclosure, the CaO based flux is selected from a group comprising limestone, slaked lime, calcined limestone and combinations thereof.
In an embodiment of the present disclosure, the MgO based flux is selected from a group comprising olivine, pyroxenite, dolomite, dunite and combinations thereof.
In an embodiment of the present disclosure, the carbon source is selected from a group comprising anthracite coal, coke fines, coke breeze, industrial by-products and combinations thereof.
In an embodiment of the present disclosure, the industrial by-products are selected from a group comprising blast furnace dust, LD sludge, Coke dust and combinations thereof.
In an embodiment of the present disclosure, the binder is selected from a group comprising bentonite, organic binder, inorganic binder and combinations thereof.
In another embodiment of the present disclosure, the binder is bentonite.
uIn an embodiment of the present disclosure, the hot water has a temperature of about 65-85 °C.
In another embodiment of the present disclosure, the hot water has a temperature of about 65°C, about 66°C, about 67°C, about 68°C, about 69°C, about 70°C, about 71°C, about 72°C, about 73°C, about 74°C, about 75°C, about 76°C, about 77°C, about 78°C, about 79°C, about 80°C, about 81°C, about 82°C, about 83°C, about 84°C or about 85°C.
In an embodiment of the present disclosure, the hot green pellets formed are about 6-16 mm in size.
In another embodiment of the present disclosure, the hot green pellets formed are about 6 mm in size, about 7 mm in size, about 8 mm in size, about 9 mm in size, about 10 mm in size, about 11 mm in size, about 12 mm in size, about 13 mm in size, about 14 mm in size, about 15 mm in size or about 16mm in size.
In an embodiment of the present disclosure, the hot green pellet temperature is in the range of about 55-60 °C.
In another embodiment of the present disclosure, the hot green pellet temperature is in the range of about 55°C, about 56°C, about 57°C, about 58°C, about 59°C or about 60°C.
In an embodiment of the present disclosure, hot water is used in both mixer and pelletizing equipment during iron ore pelletization.
In an embodiment of the present disclosure, the hot water used in pelletizing is heated by means of but not limiting to electrical heating, solar power heating, steam injection and waste hot gas/air injection in the water tanks as a means to use sensible heat of waste gas/air.
In an embodiment of the present disclosure, the mixing comprises dry mixing of iron ore fines, flux, carbon source and binder followed by wet mixing with addition of hot water.
In an embodiment of the present disclosure, hot water is added at any stage of formation of mixture. For ex: In one embodiment, hot water is added to a mixture of iron ore, flux, carbon source and binder is added to the mixture later. In another embodiment, hot water and binder is added to a mixture of iron ore, flux and carbon source. In yet another embodiment, mixture of iron ore and flux or mixture of flux and carbon source are obtained followed by adding hot water to the mixture along with other ingredients. Thus, different permutations and combinations which are well within the knowledge of the person skilled in the same field.
In an embodiment of the present disclosure, the pelletization is carried out in a disc or drums pelletizer to obtain green pellets followed by induration in a furnace to obtain the iron ore pellets.
In an embodiment, the present disclosure relates to a method for improving thermal shock properties of iron ore pellets comprising steps of:
? mixing of iron ore, flux, carbon source, binder and hot water having a temperature of about 65-85 °C to obtain a mixture;
? pelletizing the mixture with spray of hot water having a temperature of about 65-85 °C to obtain hot green pellets; and
? charging the hot green pellets having a temperature in the range of about 55-60 °C in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.
In an embodiment, the present disclosure relates to a method for improving thermal shock properties of iron ore pellets comprising steps of:
? mixing of low-grade iron ore, olivine flux, limestone, carbon bearing wastes, bentonite and hot water having a temperature of about 65-85 °C to obtain a mixture;
? pelletizing the mixture with spray of hot water having a temperature of about 65-85 °C to obtain hot green pellets; and
? charging the hot green pellets in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.
In an embodiment, the present disclosure relates to a method for improving thermal shock properties of iron ore pellets comprising steps of:
? mixing of low-grade iron ore, olivine flux, limestone, coke breeze, bentonite and hot water having a temperature of about 65-85 °C to obtain a mixture;
? pelletizing the mixture with spray of hot water having a temperature of about 65-85 °C to obtain hot green pellets; and
? charging the hot green pellets in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.
In an embodiment, the present disclosure relates to a method for improving thermal shock properties of iron ore pellets comprising steps of:
? mixing of low-grade iron ore, olivine flux, limestone, carbon bearing wastes, coke breeze, bentonite and hot water having a temperature of about 65-85 °C to obtain a mixture;
? pelletizing the mixture with spray of hot water having a temperature of about 65-85 °C to obtain hot green pellets; and
? charging the hot green pellets in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.
In an embodiment, the present disclosure relates to a method for improving thermal shock properties of iron ore pellets comprising steps of:
? mixing of low-grade iron ore, iron ore slime, olivine flux, limestone, carbon bearing wastes, coke breeze, bentonite and hot water having a temperature of about 65-85 °C to obtain a mixture;
? pelletizing the mixture with spray of hot water having a temperature of about 65-85 °C to obtain hot green pellets; and
? charging the hot green pellets in induration furnace followed by firing to obtain the iron ore pellets with improved thermal shock properties.

In an embodiment, the present disclosure provides a method of processing raw materials of pelletizing for preparing green balls/pellets (a common name for moist or wet balls). In the conventional method, powdered iron ore, fluxes, and carbon-bearing solid fuels such as coke/coal, etc. are mixed in mixer equipment in the presence of a binder such as bentonite. During mixing, a pre-decided amount of water is added to form a wet mixture which is used in the preparation of green balls of size (6-16 mm). In a pellet plant wherein, dry grinding is under operation, and ground raw materials in the form of powder are used for pelletizing, addition of water results in the formation of green balls which has a very low temperature close to ambient temperature in the range of 35°C-40°C. When these green balls are charged in an induration furnace, they are subjected to drying by an air of temp 250-400°C first wherein the pellets suffer cracking and bursting due to thermal shock. These cracks and bursting result in fines generation in pelletizing as well as lower crushing strength of final product pellet.
In an embodiment, the present disclosure provides a method of raw material processing which results in improved resistance of these green balls to thermal shock inside induration furnace. The proposed method comprises increasing the green ball temperature in the range of 55 – 60 °C. In this method, first, the temperature of the processed water is raised to 65-85°C. This hot water is then used in the mixer equipment wherein powdered iron ore, fluxes, solid fuel, and binder are mixed to form a wet mixture. This results in the formation of a hot wet mixture. This hot mix is then transferred to balling equipment such as disc or drum wherein green balls are formed. Water used during balling is also raised to 65-70°C prior to its use in pelletizing. Once the hot green balls are formed, these are transferred to the induration furnace for pellet making. These green balls, by virtue of their higher temperature, suffer a significantly lower thermal shock when dried.
In an embodiment, the present disclosure provides a method of preparing green/wet balls to improve the thermal shock resistance of the pellets. The method comprises addition of hot water in the range of 65-85°C in the ground raw materials for pellet making. Based on the temperature of the ground ore itself, the temperature of the water can be optimized to achieve a green ball temperature in the range of 55-60 °C before entering the induration furnace.
In the conventional process of iron ore pelletizing, ground and powdered form of iron ore, fluxes, solid carbon-bearing fuel, and bentonite are mixed in the presence of water. The temperature of the water is not a control parameter in conventional iron ore pelletizing. The wet mixture of iron ore, fluxes, solid carbonaceous fuel such as coke or coal, and binder is produced in the mixer equipment which is then sent to balling equipment such as discs or drums. In these discs or drums, green balls of required size, i.e. 6-16 mm (with 10 -12.5 mm fraction being 65-70%) are produced. Due to the usage of water which is at a lower temperature, the temp of green balls is also lower. When these green balls are charged in the induration furnace, they are exposed to an air of temperature around 250-400 °C. Due to this high-temperature exposure, several green balls experience thermal shock which results in cracking and bursting of pellets. Further, as the green balls are dried in the up-draught drying zone, the moisture from the lower layers in the pellet bed reports in the air. This hot air upon cooling down to below its dew point releases excess moisture. During drying of pellets in the middle and upper-layer in the pellet bed, the temperature of the drying air decreases and hence it loses moisture which is absorbed by upper layer pellets. This mechanism causes an increase in the moisture in the upper layer pellets to more than what is present when the pellet enters the induration furnace. As the machine moves forward to a relatively high-temperature zone, these high moisture pellets are exposed to air with high temperatures. Due to high-temperature exposure and high moisture content in pellets, there is a rapid release of moisture, and due to the rapid release of moisture, these pellets suffer even more thermal shock which results in fines generation, pellet bursting, cracking, etc. The pellet bursting phenomenon results in generating fines in the bed. This results in poor permeability of pellet bed and hence poor firing and poor productivity of the pelletizing process. Moreover, crack generated in pellets causes a decrease in the CCS of pellets and hence more fines generation in transportation as well as poor performance in iron making furnaces.
In an embodiment, the present disclosure provides a method to increase the green ball temperature before entering the induration furnace. In this method, water used in the pellet making is heated to increase its temperature in the range of 65-85°C. This hot water is then used in the pellet making so that the temperature of green balls formed is increased enough that during its transportation from balling equipment to induration furnace, after any drop in its temperature, it is still adequate and in the range of 55-60°C. With an increase in the temperature of green balls, from 35°C - 40°C range to 55°C – 60°C range, the thermal shock which these pellets suffer decreases significantly. This results in a decrease in bursting and cracking of pellets. Lower bursting results in lower fines generation in the pellet making, as well as the proper firing of pellets. The fines generated otherwise (in the absence of hot green balls) would enter the void of bed and causes air channelling and hence poor firing of pellets. Also, lower crack generated in the pellets also ensures higher CCS of pellets. The increase in the temperature of water can be done using electrical heating, solar heating or injection of steam in the water tank which is used for pellet making.
In an embodiment, the present disclosure provides a method of increasing green ball temperature in iron ore pelletizing. In the conventional route of iron ore pelletizing, which has a dry grinding system for producing powdered material with particle size suitable for the pellet making, iron ore fines, fluxes, carbon-bearing fuels such as anthracite coal, coke breeze, etc., are charged in a dryer. The dryer drives out the moisture of these materials and the dried material is then charged in the grinding mill. These grinding mills grind the materials to produce powdered fines with sizes suitable for pelletizing. The powdered material is charged in a mixer in which bentonite and water are added to produce the wet mixture. This mixture is then conveyed to balling equipment such as discs or drums. Wet balls (or green balls) of suitable size (generally 6-16 mm) are formed in discs or drums and the same is transferred to the induration furnace for firing.
The induration furnace consists of several zones for treating the green balls. The first one is up-draught drying in which hot air of around 250°C is pushed from the bottom of the bed to dry the bed. The second zone is down-draught drying in which hot air at ~350-400°C air is pushed from the top to bottom. The third zone is the pre-heating zone in which temperature is gradually raised to firing temp which is 1250-1300°C. The fourth zone is the firing zone in which pellets are fired at peak firing temp for a pre-decided time. The fifth zone is the after-firing zone which provides a gap between the firing and cooling zone. Thereafter comes cooling zone #1 in which ambient air is pushed from the bottom to cool hot pellets in an oxidizing environment. Finally, cooling zone 2 in which pellets are further cooled to achieve temperature suitable for further handling. Induration machine consists of an endless pallet car in which pellets are charged. As this endless pallet car moves forwards, pellets in the pallet car pass through different zones resulting in the production of fired pellets.
When the green pellets are charged in the induration furnace, they enter the up-draught drying zone. The temperature of the pellets entering the up-draught drying zone is 35-40°C based on the temperature of ground ore from the ball mills and water used in mixer equipment. When these pellets enter the up-draught drying zone, a hot air of temperature ~250 °C is pushed from the bottom. Sudden exposure of wet pellets to high-temperature air results in rapid drying of pellets which also results in cracking and bursting of some pellets. During drying of bottom layer pellets, the moisture is driven away by circulating air, however, this air when reaches the upper layer gets cooled below and hence loses excess moisture to upper layer pellets. This way, moisture in the upper layer pellets increases. Once the bed reaches down-draught drying, an air of temperature 350-400°C is pushed downward. Due to an even higher temperature and moisture content of pellets, these pellets suffer even more thermal shock which results in the bursting and cracking of pellets. Fines generated by the bursting of pellets enter the void space in the pellet bed. This results in choking of the bed and hence, when such bed enters preheating or firing zone, circulating hot air finds it difficult to penetrate such densely packed bed. Due to this, there is a channeling of air and hence pellets get poorly fired which results in lower crushing strength of pellets. Moreover, the pellets which contain cracks are lower in crushing strength even if they get fired in the induration furnace. The bursting of pellets results in fines generation and lower CCS pellets results in further fine generation during transportation. Lower CCS pellets also affect the performance of iron-making furnaces such as Midrex, Corex, blast furnace, etc.
The phenomenon of the pellets bursting and cracking due to thermal shock is even more sensitive for low-grade ores containing high alumina, goethite, LOI, and clay minerals. Moreover, low-grade ores are either naturally finer, or due to their softer nature (such as for goethite, clay, and high LOI ores), they generally get grounded to a finer size in ball mills. The use of such ores containing higher super fines (particles less than two microns, 10 microns, etc.) in the pellet making results in the formation of more compact/dense pellets. Due to increased compaction or dense pellet formation, the pathways or network of open pores are blocked which necessitates that the pellets are slowly dried to release all moisture without bursting or spalling. However, slow drying decreases productivity. Faster movement of the machine causes faster drying and hence the bursting of pellets. High pellet compactness increases the resistance to water evaporation. Thus, low-grade ore due to finer particle size requires steps that can improve its shock temperature or thermal shock behavior during pellets making. It is also to be noted that, high goethite containing ores tend to absorb more moisture as against high hematite or magnetite ore. This further deteriorates the thermal shock behavior of pellets.
In an embodiment, the present disclosure provides a method to improve thermal shock resistance of such green balls i.e. produced from low-grade iron ores containing high alumina, ultra-fines, silica, clay minerals, combined moisture (LOI) by means of increasing the temperature of green balls. The enabler to increase the temperature of the green balls is ground ore temperature and water temperature. Thus, thermal shock resistance of pellets is improved without the addition of any foreign material in the pellet mixture which can affect the functioning of downstream processes such as the iron and steel making processes. Moreover, the present disclosure can be scaled up to the industrial level relatively easily and in a much-controlled manner.
It is to be understood that the foregoing description is illustrative not a limitation. While considerable emphasis has been placed herein on particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.
Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure, certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments may be practiced and to further enable those of skill in the art to practice the embodiments. Accordingly, following examples should not be construed as limiting the scope of the embodiments herein.
EXAMPLES:
Example 1: Comparison of conventional method versus method of the present disclosure
Figure 1 provides the general layout of the plant for iron ore pelletizing. Figure 1 shows the conventional method of preparing iron ore pellet in which ground materials in dry form are utilized for the pellet making after receiving them from the dry grinding system. Part no 1.1a is iron ore bin containing ground iron ore, part no 1.1b is flux bin containing ground fluxes such as limestone, dolomite, olivine, pyroxenite, etc., part no 1.1c is carbon-bearing solid fuel bin containing ground anthracite coal, coke fines, coke breeze, etc. part no 1.1d is bentonite bin containing ground bentonite powder, part no 1.2 is a mixer, 1.3 pelletizer, 1.3a- wet pellets produced in pelletizer, 1.4- conveyor belt, and 1.5 is a traveling grate induration furnace. Part no A is a water addition system in the mixer and Part B is a water spraying system in a disc pelletizer.
Figure 2 is the schematic of the drying zone of the induration furnace (1.5) in fig 1. It shows green pellets layered on a pallet car inside the induration furnace. A layer of fired pellets at the bottom of the car is layered first which is used as refractory protection and called as hearth layer. Above that, green pellets are layered. When hot air from the bottom enters the furnace; bottom layer wet pellets experience a thermal shock due to which some pellets get cracked or burst. When the hot air reaches the upper layer its temperature decreases, and as a result, its moisture-holding capacity decreases. This results in condensation of some of the moisture of hot air into the upper layer wet pellets of the pellet bed. This causes an increase in the moisture of upper layer pellets. When the pallet cars containing green pellets move forward into down draught drying zone, these high moistened pellets at the top (and some at the middle layer also) comes under direct contact with high-temperature air of ~350-400 °C.
Figure 3 shows the schematic of down draught drying showing hot air of 350-400°C entering from top. It also shows top layer wet pellets with high moisture content as well as bottom layer dried pellets and hearth layer at the bottom. When hot air enters the bed from the top, it is in direct contact with top layer pellets. Due to high temperature air and high moisture in pellets, these pellets experience thermal shock due to the rapid release of moisture from pellets. This again causes cracks and bursting of some pellets in the bed.
When these pellets are burst, it generates fines which go into the void space in the bed. During firing, hot air does not flow properly through these choked voids, and hence, pellets near to them gets poorly fired and hence lower crushing strength. Moreover, cracks present in the pellets also lowers the crushing strength of pellets. Broken pellets in the final output due to lower crushing strength or pellet bursting increases the fines generation in pellets and hence lower productivity. Low CCS pellets also perform poorly in the iron making furnaces. Additionally, low grade ores with high LOI, high Ultrafine/superfine content, high clay, alumina or silica, perform even worse during drying operation and hence in the present disclosure, such detrimental effects of low-grade ore, iron slimes, mineralogical composition, super-fines/ultra-fines content have been negated or decreased by means of producing the green balls themselves at higher temperature.
Figure 4 shows the iron ore pelletizing process as per the present disclosure. It differs from the conventional method in a way that instead of water at ambient temperature, hot water is used in both mixer and pelletizing equipment. The water used in pelletizing can be heated by means of electrical heating of water, through solar power heating or through steam injection or through waste hot gas/air injection in the water tanks as a means to use sensible heat of waste gas/air.
Once the water temperature is increased in the range of 65-85°C, it is used in the mixer and pelletizing equipment. Water temperature can be optimized to produce green balls of temperature 55-60°C. Once the green balls with higher temperature are produced, the bottom layer pellets (mentioned in figure 2) experience lower thermal shock. Moreover, since the temperature of the green balls in the bed is high therefore, the decrease in the temperature of hot air is lower while it moves from the bottom to top. Hence, the moisture carrying capacity of hot air at the top of the bed is more than what it can hold in conventional method. Therefore, it releases lesser moisture to top layer. This results in lesser increase in the moisture of the top layer (mentioned in figure 3). Moreover, the thickness of the top layer bed which experience increase in moisture also decreases. Such bed when moves forward and comes under down draught drying, experience lower thermal shock as compared to that in conventional method. This result in lesser bursting and cracking. When the temperature of the pellet is 55-60°C, the bursting is completely avoided. If the temperature of the green ball is significantly increased such as 65°C and beyond, its drying rate increase drastically due to moisture from core of the pellet also going out simultaneously with that in the mantle and shell. This results in excess crack in pellets (although bursting still avoided) and hence pellet temperature should be optimized between 55-60°C for best results.
Example 2: Effect of green ball temperature on thermal shock properties
Five sets of pellets were made for establishing the effect of green ball temperature on thermal shock properties. These pellets were tested for green ball qualities such as Green compressive strength (GCS), Dry Compressive strength (DCS), Drop No, Moisture and Thermal shock resistance. The thermal shock test was designed with heating patterns similar to Tata Steel 6MTPA Pellet plant. The peak shock temperature given to pellet was 500°C wherein pellets were heated to reach 500 °C at a heating rate of ~67 °C/min. The number of burst pellets, crack containing pellets and un-cracked pellets were counted and reported.
Iron ore fines (Fe Source), olivine flux (MgO and SiO2 source), Limestone (CaO source), Carbon bearing wastes, and Coke breeze, etc. are grounded together in the commercial pellet plant of Tata Steel Ltd., situated at Jamshedpur, India. This co-grinding of raw material produces a ground ore concentrate (GOC) which contains the required amount of total iron, CaO, MgO, and carbon fuel. This ground ore concentrate was used in the present example along with bentonite and iron ore slime. Ground ore concentrate, bentonite and iron ore slime are mixed with hot water to obtain a mixture which is then pelletized with spray of hot water to obtain hot green pellets. The hot water temperature is varied from 65-85°C.
Table 1 shows the chemical analysis of raw materials. Table 2 shows the particle size in terms of minus 45 microns sizes. Table 1 and 2 shows that iron ore slime used in the present disclosure is higher in alumina and LOI as well as contains higher minus 45-micron size particles.
Table 1: Raw Material Chemical Analysis

Raw Materials T. Fe CaO MgO SiO2 Al2O3 LOI
Ground Ore Concentrate 60.14 1.16 1.47 3.11 2.69 4.82
Bentonite 10.17 1.33 1.97 39.51 15.61 22.39
Iron Ore Slime 59.45 0.04 0.06 2.90 5.49 4.97
Table 2: Raw Material Size

Particle size Ground Ore Concentrate Iron Ore Slime Bentonite
Minus 45 µm, vol.% 64.86 77.97 78.01
Table 3 shows the Material balance for making pellets in the present disclosure. Bentonite was used over and above the total mix used in the pellet making. For all the sets of pellets, the green ball temperature was varied from 35°C to 70 °C. And the hot green balls were tested for thermal shock resistance. Table 4 shows the green ball quality data of pellets. It can be seen that the moisture content of pellets increased with increasing in slime content of pellets as iron ore slimes containing high alumina, goethite, etc. absorb more moisture during pelletizing. Because of more ultra-fines contributed by iron ore slime, GCS, DCS and drop no have also increased significantly.
Table 3: Material Balance for making pellets

Raw Material Set 1 Set 2 Set 3 Set 4 Set 5
Ground Ore Concentrate, % 100 95 90 80 60
Iron Ore Slime, % 0 5 10 20 40
Bentonite, wt.% of total mix 0.5 0.5 0.5 0.5 0.5

Table 4: Green ball physical properties

Properties Tested Set 1 Set 2 Set 3 Set 4 Set 5
Moisture, wt. % 8.33 8.39 8.45 8.51 8.57
GCS, kg/p 1.04 1.10 1.15 1.41 2.09
Drop No, avg. count 7.5 7.7 8.7 9.4 12.6
DCS, kg/p 4.03 4.51 4.97 5.81 7.03
Table 5: Relative variation in burst and cracked pellets with change in green ball temperature and slime content

Set Id Green Ball Temperature
35 °C 40 °C 45 °C 50 °C 55 °C 60 °C 65 °C 70 °C
Set 1 100.0 88.8 55.5 55.5 38.8 27.8 83.3 116.7
Set 2 100.0 94.5 77.8 55.5 38.8 33.3 61.2 94.5
Set 3 111.2 94.5 83.3 61.2 38.8 33.3 55.5 72.2
Set 4 116.7 100.0 88.8 66.7 44.5 33.3 55.5 66.7
Set 5 122.2 100.0 88.8 66.7 50.0 38.8 50.0 66.7
Table 5 shows the data for thermal shock resistance in terms of cracked and burst pellets for a constant peak shock temperature of 500 °C. Table 5 shows the relative change in cracked and burst pellets. It is evident from the table that on increasing the iron ore slime content in the pellet, a total of cracked and burst pellet increased. With increasing in the green ball temperature, bursting and cracking decreased. After 65°C, the cracked pellet again started increasing because high rates of drying which occurs simultaneously at core, mantle and shell region instead of shell first, then mantle and finally core. The relative variation in table 5 is calculated considering set 1 pellet at 35 °C as base case.
The above data clearly shows that producing green balls at higher temperature such as in the range of 55-60°C improves the thermal shock properties. 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.