Claims:1) A method for improving reduction kinetics of iron ore pellets comprising coating of heat hardened iron ore pellets/fired pellets with CaO rich compound.

2) The method as claimed in claim 1, wherein the method comprises steps of:
 preparing a slurry of CaO rich compound with water;
 spraying the slurry onto the fired pellets followed by/simultaneously tumbling the pellets for homogenous coating; and
 drying the coated pellets followed by charging them in furnace for reduction;

3) The method as claimed in claim 2, wherein the slurry is sprayed onto the fired pellets at a rate of about 4 - 8 wt% of the pellets.

4) The method as claimed in claim 2, wherein the coated pellets are dried until moisture in the pellets is 2% or less.

5) The method as claimed in claim 1, wherein the pellets are coated with CaO rich compound at about 0.1 – 0.3 wt% of the pellets.

6) The method as claimed in claim 5, wherein the pellets are coated with CaO rich compound at about 0.2 wt% of the pellets.

7) The method as claimed in claim 1, wherein the CaO rich compound contains at least 75% CaO.

8) The method as claimed in claim 7, wherein the CaO rich compound is selected from a group comprising calcined lime, burnt lime, quick lime, slaked lime and combinations thereof.

9) The method as claimed in claim 8, wherein the CaO rich compound is calcined lime.

10) The method as claimed in claim 9, wherein size of the calcined lime is 90% passing 45 microns.

11) The method as claimed in claim 1, wherein the method provides for increase in coating thickness and formation of calcium ferrite rich phases on pellet surface during reduction.

12) The method as claimed in claim 11, wherein the thickness of the coating is about 90-160 microns.

13) The method as claimed in claim 12, wherein the thickness of the coating is 150 microns.

14) Iron ore pellets obtained by the method of any of the claims 1-13.
, Description:TECHNICAL FIELD
The present disclosure relates to the field of metallurgy. Particularly, the present disclosure relates to a method for improving reduction kinetics of iron ore pellets without affecting the fines generation behaviour of pellets, requiring any change in chemical composition, porosity, etc. of fired pellets as well as in operating parameters of the pelletizing furnace. The method comprises coating of heat hardened iron ore pellets/fired pellets with CaO rich compound. Also, disclosed are the reduced iron ore pellets obtained by the method.
BACKGROUND AND PRIOR ART
In India, during mining and beneficiation, more than 60% of the total iron ore is generated as fines. Agglomeration of these fines is required in order to utilize the valuable raw materials, decrease the load on the mines and to reduce environmental pollution. Quality of Iron ore agglomerates, such as sinter and pellets, play a vital role in decreasing the reducing agent consumption and increasing the productivity of a blast furnace. To maximize the production and minimize the fuel rate of the blast furnace, reducibility of iron-bearing burden plays a significant role.
The purpose of the blast furnace is to chemically reduce and physically convert solid iron oxides into liquid pig iron. The blast furnace is a continuously operating shaft furnace that is based on the counterflow principle. Iron bearing burden (such as pellets, sinter, and lump ore) and metallurgical coke are charged in separate layers from the top of the furnace. In the lower part of the furnace, the hot blast (air at ~1000–1200 °C) is blown above the liquid level through tuyeres. Additionally, auxiliary reducing agents are injected via tuyeres. In front of each tuyeres, the hot blast reacts with carbon fuel and produces a reducing gas (CO and H2). The reducing gas ascends in the furnace, heats the descending burden and reduces iron oxides. The reducing gas which is generated in front of the tuyeres ascends to the top in 5 to 20 seconds after going through numerous chemical reactions and leaves the furnace with a temperature of ~100–200 °C. The efficiency of this process is largely determined by the ability of the reducing gas to remove oxygen from iron oxides. Since reducing gases moves towards the top very quickly, kinetics of reduction of iron-bearing burden becomes very important for the efficient operation of a blast furnace in terms of fuel rate and productivity.
Since mechanical mining generates huge amount of fines (which can only be utilized through the pellet making route) as well as depleting iron ore quality necessitates beneficiation, which further necessitates its agglomeration via pelletizing, the proportion of pellets are continuously being increased in iron making units such as blast furnace. To improve the efficiency of the blast furnace in terms of fuel rate and productivity, the reduction kinetics of iron ore pellets should be higher.
In order to improve the reduction behaviour of iron ore pellets, various methods have been reported.
Mohanty et al in a paper titled “Effect of Basicity on the Reduction Behaviour of Iron Ore Pellets” established the effect of basicity on the reducibility and suggested 1.6 basicity (i.e. CaO/SiO2) for higher reducibility of Iron Ore Pellets. The disadvantage of this process is higher dust generation associated with high basicity pellets as well as the process compels the pellet makers to increase the basicity (or CaO content) of the pellets. In actual plant operation, the chemistry of the pellet is governed by Blast Furnace burden composition i.e. Sinter, Pellet, and Lump ore ratio, and hence chemistry of one has a direct effect on the chemistry of another. Therefore, chemistry of the pellet cannot be changed just for the sake of reducibility.
Botelho et al in Patent No US 20140096650 A1 disclosed an invention related to the improvement of reducibility of iron ore pellets from a catalytic effect, generated by the addition of metallic Fe and Ni. But the disadvantage associated with the above art is that the addition of metallic Fe alone does not provide a significant effect on the degree of metallization. Also, this process is commercially not encouraged due to the high cost of Metallic Ni and its availability.
Sugiyama et al in Patent No US4372779A reported the Iron ore pellets containing coarse ore particles, having a particle size distribution consisting of 25-40 wt. % of coarse ore with particle size greater than 0.1 mm, less than 21 wt. % of medium size ores with the particle sizes of 0.1-0.04 mm, and more than 39 wt. % of fine ore having a particle size smaller than 0.04 mm. These pellets showed excellent reducibility at high temperatures and physical strength. The disadvantage associated with this art is the raw material preparation or grinding step wherein particle sizes need to be controlled to produce the required particle size distribution. Additionally, pellets consisting of excessive content of coarse particles are unable to meet other requirements of green ball qualities such as drop strength and green ball compressive strength.
Isao Fujita et al in Patent No CA 1099519 A described an invention related to the fired iron-ore pellets having at least two precipitated slag phases of different chemical compositions that appear among iron-oxide grains. The pellets having two different slag phases contain not less than 4% by weight of CaO + SiO2, so that large cracks may be produced in the course of reduction. The cracks thus formed cause each pellet to be split into several pieces. These pellets thus exhibit excellent and improved reducibility. However, major disadvantage associated with this prior art is the effect of crack generation on the pellet crushing strength which decreases by the presence of cracks in pellets. Moreover, this results in higher fines generation in the blast furnace and hence decreases furnace permeability.
Y Kazuharu et al in Patent No JPS55107738 (A) described a process to increase the reducibility of green pellets and enhance the productivity thereof by specifying the fine ore blending rate of the pellets and carrying out pelletizing with water of a value higher than the proper value to flatten the pellets. Fine iron ore is blended at a rate of 20-50% in a blending process, and the water content of the blend before pelletizing is adjusted to a value of 0.5 - 2.0 % higher than the maximum value of proper water content (9-10%). The blend is then pelletized with a pelletizer. The blending rate and the water content enable the resulting pellets to be flattened. By this flattening, reducibility of the pellets is enhanced. It is to be noted that though flattening an agglomerate increases its surface area from which reducing gases can penetrate and hence increase reducibility, flattened agglomerate cannot be fired in an induration furnace because of improper gas flow and thus main disadvantage associated with it is poor control over the firing of agglomerate and hence, poor agglomerate in terms of strength, which is not suitable for transportation and metallurgical processing.
Porous iron ore pellets and a process for manufacturing same was described by Kazumasa Taguchi et al in Patent No US4350523. Pellets having a pore size distribution consisting of more than 30% of pores having a diameter greater than 10 microns and a balance of pores having a diameter smaller than 10 microns, a total porosity greater than 30%, and FeO content less than 1% by weight was described. Improvement in pore size, as well as porosity, can result in high reducibility or better reduction kinetics of pellets, but this kind of higher porosity has an adverse effect on pellet strength, which in turn affects the efficiency of the pellet making process by generating higher fines in the form of chips and dust, after induration.
Ravi Bollina et al in his work “Effect of Lime coating of iron ore pellets on iron ore production” reported improvement in reducibility of pellets. The disadvantage associated with this process is that lime was coated onto the pellet after green balling and before firing. This resulted in an increase in the reducibility. But lime-coated pellets when fired, generate porous phases at the surface which generates huge fines during transportation. As a result, a very negligible or no coating is available at the iron-making end-use, and therefore, the benefits of coating are not realized. Also, the fines generation behaviour results in poor reduction degradation.
Sterneland et al in his work “The Use of Coated Pellets in Optimizing the Blast Furnace Operation” presented the Use of Coated Pellets in Optimizing the Blast Furnace Operation. Olivine, dolomite, and quartzite respectively were applied as coating agents onto the regular olivine pellets and it was concluded that: i) Dust generation was significantly decreased when using coated pellets in the blast furnace; ii) Sticking was prevented by the coating material; and iii) Gas utilization was higher for all coated pellets, with a lower variation, indicating a smoother blast furnace operation. The disadvantage associated with this process is the addition of the binder in the form of bentonite in the coating material. A two-material system, wherein one is prime coating material and the other is a binder, requires capital investment such as storage unit, drying and grinding unit (for mentioned fluxes as well as for bentonite), proportioning unit, mixing unit, etc. Additionally, bentonite addition results in increasing silica, alkali, and alumina load of the pellets and hence affects slag rate and operation of the blast furnace. Thus, the process is complex, costly and undesirable.
US7442229B2 described a method of coating lumps such as pellets, briquettes, granulates, lump ore etc. The coating material is selected from a group of “cluster abating materials” or “slag modifying material” such as lime or magnesia bearing material like lime, limestone, magnesite, olivine, dolomite, serpentine, ilmenite. A binder is chosen for applying coating onto lumps such as bentonite, alkali metal salt of carboxymethyl cellulose (CMC), sodium chloride and sodium glycolate, and other polysaccharides or synthetic water-soluble polymers. Moreover, a dispersion stabilizer was also chosen to maintain stability of the slurry solution, such as organic dispersants including polyacrylates, polyacrylate derivatives and the like; and inorganic dispersants including caustic soda, ash, phosphates and the like. Preferred stabilizers include both organic and inorganic stabilizers including xanthan gums or derivatives thereof, cellulose derivatives such as hydroxyethyl cellulose carboxymethylcellulose and synthetic viscosity modifiers such as polyacrylamides and the like. Disadvantage associated with this is the need of binders and dispersions for slurry preparation which makes the whole process costly as well as complex from the point of operating easily. Moreover, use of such binders and dispersions on the feed of blast furnace causes them to enter blast furnace wherein the volatile material present in them gets vaporized due to increase in temperature and reports in off-gas and once the temperature of off-gas decreases, they settle in the pipes of blast furnace such as down-comers, gas cleaning plants etc and blocks the pipelines over a period of time.
KR100782750B1 described a method of coating lime and sugar containing viscous solution over the sintered ores used in blast furnace. Such coatings are claimed to form calcium carbonate when enter the blast furnace due to availability of CO2 and catalytic effects of sugar. The coating is claimed to decrease the reduction degradation which may be due to blocking pores on the surface and hence lower reduction at lower temperature in blast furnace. However, the patent is silent over reduction kinetics of the ores which may get decreased by presence of calcium carbonate over the surface. Moreover, use of such binders (sugar etc.) on the feed of blast furnace causes them to enter blast furnace wherein the volatile material present in them gets vaporized due to increase in temperature and reports in off-gas and once the temperature of off-gas decreases, they settle in the pipes of blast furnace such as down-comers, gas cleaning plants etc, and blocks the pipelines over a period of time. Additionally, usage of sugar makes the whole process costly and technically challenging due to requirement of handling high viscous solution comprising lime and sugar.
US20200224285A1 described a process for coating raw materials used for direct reduction process. Disadvantage associated with this is the requirement of complex drying after coating which requires drying in CO2 free or H2O free environment and a temperature which is at least 50 degree more than the decomposition temperature of each coating material. The said drying also claimed to increase the temperature of coated pellets and same pellets (with increased temperature) are charged in direct reduction furnace. The art also necessitates coating porosity for the success of the invention. Moreover, the patent is silent over reduction kinetics of pellets in blast furnace. The art is also silent on blast furnace process as a whole and is more focussed on clustering phenomenon in shaft furnace used in direct reduction processes such as Midrex etc.
WO 2017/006200 Al described a two-layer coating method for decreasing the clustering phenomenon in direct reduction furnaces such as Midrex. The process is disadvantageous (1) since it proposes two coatings which makes the process of coating costly (2) One coating is lime based but second coating is cement based which increases the alumina content of the pellets and hence higher slag rate in blast furnace. (3) First coating needs to be dried first and then second coating needs to be applied which makes the process complex and techno-economically unattractive. Further, the art is silent over reduction kinetics of pellets in blast furnace and blast furnace process in its entirety. The art in only focussed on direct reduction processes such as Midrex.
Ali et al in their work “Coating of iron oxide pellets for direct reduction” described a method of coating where, bauxite and serpentine alone and limestone in combination with bentonite found to be effective for countering stickiness. This procedure is not advantageous for downstream process of steel making in electric arc furnace due to higher silica input from bauxite, serpentine or bentonite. Moreover, the art is silent on reduction kinetics of agglomerates in blast furnace process. The art is focussed on direct reduction process such as HyL, Midrex etc and is silent on blast furnace iron making route.
Thus, there exists a need for reduced pellets and a method/process for improving reduction kinetics of iron ore pellets without affecting the reduction fines generation behaviour of pellets, changing the chemistry or operating regimes of the pelletizing plant, etc and the present disclosure achieves the same.

SUMMARY OF THE DISCLOSURE
The present disclosure relates to a method for improving reduction kinetics of iron ore pellets without affecting the fines generation behaviour of pellets, changing the chemistry or operating regimes of the pelletizing plant, etc. The method comprises coating of heat hardened iron ore pellets/fired pellets with CaO rich compound.
In an embodiment of the present disclosure, the method comprises steps of:
 preparing a slurry of CaO rich compound with water;
 spraying the slurry onto the fired pellets followed by/simultaneously tumbling the pellets for homogenous coating; and
 drying the coated pellets followed by charging them in furnace for reduction;
In an embodiment of the present disclosure, the pellets are coated with CaO rich compound at about 0.1 – 0.3 wt% of the pellets and the CaO rich compound is selected from a group comprising calcined lime, burnt lime, quick lime, slaked lime and combinations thereof.
In an embodiment of the present disclosure, the method provides for reduction in HOSIM time, decrease in fines generation during reduction and decrease in dense phases on the surface of the pellets.
In an embodiment of the present disclosure, the method provides for increase in coating thickness and formation of calcium ferrite rich phases on pellet surface during reduction.
The present disclosure also relates to iron ore pellets obtained by said 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 comparison of the mechanisms of reduction between lime-coated and non-coated iron ore pellets. Stage 1- Fired pellet without reduction, (a) Non-coated Fired Pellets, (b) Lime-coated Fired Pellets; Stage 2 - Fired pellet initial reduction, (c) Reduced portion of pellet showing presence of dense phases at surface, (d) Reduced portion of pellet showing decrease in the presence of dense phases at surface, (e) Unreduced portion of pellet; Stage 3 - Fired pellet further reduction after a fixed time duration, (f) reduced portion, (g) unreduced portion.

Figure 2: provides schematic of iron making routes through blast furnace; (a) Conventional pellets as feed; and (b) pellets obtained in the present disclosure.

Figure 3: provides (a) Hosim time and (b) Hosim no of pellets tested in the study; P1 - uncoated pellet; P2 - pellet coated with 0.1% lime; P3 - pellet coated with 0.15% lime; P4 - pellet coated with 0.2% lime; and P5 - pellet coated with 0.25% lime.

Figure 4: illustrates optical microscopic studies on the surface of pellets (a) base case – pellet surface with no coating; (b) pellet surface with 0.1% lime coating; (c) pellet surface with 0.15% lime coating; (d) pellet surface with 0.2% lime coating; and (e) pellet surface with 0.25% lime coating.

Figure 5: illustrates lime-coating thickness over iron ore pellets estimated using an optical microscope. P1 - uncoated pellet; P2 - pellet coated with 0.1% lime; P3 - pellet coated with 0.15% lime; P4 - pellet coated with 0.2% lime; and P5 - pellet coated with 0.25% lime.

Figure 6: illustrates quantification of phases in reduced pellet surface – (a) Calcium Ferrite Phase in the surface (b) Wustite phase in the surface (c) Fayalite phase in the surface; P1 - uncoated pellet; P2 - pellet coated with 0.1% lime; P3 - pellet coated with 0.15% lime; P4 - pellet coated with 0.2% lime; and P5 - pellet coated with 0.25% lime.

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: Fired pellets may be interchangeably referred as heat hardened pellets or heat hardened iron ore pellets, reduced pellets as iron ore pellets in certain paragraphs, 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 improving reduction kinetics of iron ore pellets without affecting the fines generation behaviour of pellets during reduction, changing the chemistry or operating regimes of the pelletizing plant, etc. Accordingly, the present disclosure provides a method to improve reduction kinetics of iron ore pellets comprising coating of heat hardened iron ore pellets/fired pellets with CaO rich compound.
In an embodiment, the present disclosure relates to increasing the reduction kinetics of iron ore pellets without affecting the fines generation behaviour of pellets during reduction, without changing the chemistry or operating regimes of the pelletizing plant, etc. The method to increase reduction kinetics of iron ore pellets comprises coating of heat hardened iron ore pellets/fired pellets with CaO rich compound.
In an embodiment of the present disclosure, the method comprises steps of:
 preparing a slurry of CaO rich compound with water;
 spraying the slurry onto the fired pellets followed by/simultaneously tumbling the pellets for homogenous coating; and
 drying the coated pellets followed by charging them in furnace for reduction;
In another embodiment of the present disclosure, the method comprises steps of:
 preparing a slurry of CaO rich compound with water;
 spraying the slurry onto the fired pellets followed by tumbling the pellets for homogenous coating; and
 drying the coated pellets followed by charging them in furnace for reduction;
In another embodiment of the present disclosure, the method comprises steps of:
 preparing a slurry of CaO rich compound with water;
 spraying the slurry onto the fired pellets and simultaneously tumbling the pellets for homogenous coating; and
 drying the coated pellets followed by charging them in furnace for reduction;
In another embodiment of the present disclosure, the slurry is sprayed onto the fired pellets at a rate of about 4 - 8 wt% of the pellets.
In another embodiment of the present disclosure, the coated pellets are dried until moisture in the pellets is 2% or less.
In another embodiment of the present disclosure, the coated pellets are dried until at least 98% of moisture is removed.
In another embodiment of the present disclosure, the pellets are coated with CaO rich compound at about 0.1 – 0.3 wt% of the pellets.
In another embodiment of the present disclosure, the pellets are coated with CaO rich compound at about 0.2 wt% of the pellets.
In another embodiment of the present disclosure, the CaO rich compound contains at least 75% CaO.
In another embodiment of the present disclosure, the CaO rich compound is selected from a group comprising calcined lime, burnt lime, quick lime, slaked lime and combinations thereof.
In another embodiment of the present disclosure, the CaO rich compound is calcined lime.
In another embodiment of the present disclosure, size of the calcined lime is 90% passing 45 microns.
In another embodiment of the present disclosure, the method provides reduction in HOSIM time by about 30-40 minutes, decrease in fines generation during reduction by about 0.5-0.7% and decrease in dense phases on the surface of the pellets by about 40-65%.
In another embodiment of the present disclosure, the dense phases are fayalite and wustite.
In another embodiment of the present disclosure, the method provides for increase in coating thickness and formation of calcium ferrite rich phases on pellet surface during reduction.
In another embodiment of the present disclosure, the thickness of the coating is about 90-160 microns.
In another embodiment of the present disclosure, the thickness of the coating is 150 microns.
The present disclosure also relates to iron ore pellets obtained by said method. In other words, the iron ore pellets are reduced iron ore pellets obtained by the method.
In another embodiment of the present disclosure, the cold crushing strength of iron ore pellets remains unaffected while reducibility increases.
In an embodiment of the present disclosure, the present disclosure relates to a method for improving the reduction kinetics of pellets without requiring any change in chemical composition, porosity, etc. of fired pellets at pellet plants, without changing the operating parameters of pelletizing plants, etc. The method described here is very simple, efficient, and cost-effective.
In an embodiment of the present disclosure, the method enables better productivity and fuel efficiency in blast furnace iron making.
In an embodiment, the present disclosure relates to a coating material that is easily available, simple to produce, available cheaply as by-products in the steel industry, do not necessarily require grinding, etc., and beneficial to the blast furnace process.
In an embodiment of the present disclosure, the coating material itself acts as a binder so that it adheres to the Iron Oxide pellet easily.
In another embodiment, the present disclosure provides a coating material on iron ore pellets and a method which can be done as near as possible to blast furnace so that coating remains intact and its effects are visible on Blast Furnace operation.
In another embodiment, the present disclosure provides a method for improving the reduction kinetics of iron ore pellets, in which the gas utilization of coated pellets is higher to ensure a decrease in fuel rate and increase productivity.
In another embodiment of the present disclosure, the coating material is CaO rich compound selected from a group comprising calcined lime, burnt lime, quick lime, slaked lime and combinations thereof, preferably calcined lime.
In an embodiment of the present disclosure, the coating results in the formation of suitable phases at the pellet surface during reduction. This ensures that the surface pores are intact during the reduction of these pellets inside the blast furnace. The coating also ensures to minimize phases which can result in the filling/blocking of surface pores with melts and hence improved reduction kinetics. Moreover, it results in decrease in dense phases at the surface which retards the gas penetration inside pellets.
Since the coating is applied on fired pellets, the present disclosure ensures that no process change is required in the pellet plant for improving the reduction kinetics of pellets. Moreover, since the coating is being applied on fired pellets, this is done as near as possible to the blast furnace so that the pellets undergo minimal abrasion. Also, this ensures that the coating serves its purpose of improving the reduction kinetics of pellets instead of being lost in the dust and fines, etc.
In iron ore pelletizing, raw materials such as iron ore fines, a combination of fluxes such as limestone, dolomite, olivine, etc., and carbonaceous fuel such as coke breeze or anthracite coal are mixed in pre-decided proportion and are subjected to drying. These dried fines are grounded in ball mills and the ground powder is obtained. The ground powder is mixed with a suitable binder such as bentonite and water. The wet mixture thus produced is then rolled into spherical bodies of size 6-16 mm in balling discs or drums. These spherical bodies are called as green pellets. These green pellets are heat hardened in an induration furnace. An induration furnace consists of different heat treatment zones in which pellets are dried, preheated, fired and cooled. Pellet heat hardened or fired in induration furnace are transported to iron making units such as blast furnace for pig iron production. When these pellets are charged in iron making units such as blast furnaces, these are reduced and melted to form hot metal (pig iron) and slag.
In the iron ore pelletizing unit, there are multiple subunits. Improving the reduction kinetics of pellets by changing something at the pellet plant is very difficult as all subunits are connected, and one has a direct relation with others. For example, coarse grinding of raw material for increasing porosity will affect green ball quality and hence increase green ball reject. This also results in poor crushing strength of fired pellets. Poor green ball quality affects furnace permeability, etc.
Thus, reduction kinetics of iron ore pellets plays an important role in improving the efficiency of the blast furnace in terms of productivity and fuel rate. It is known that higher the reducibility or faster the reduction kinetics, better is for blast furnaces. Moreover, fewer fines generation during the reduction in blast furnaces, better is for furnace permeability. Reduction kinetics is measured at the laboratory scale in terms of HOSIM time and reduction fines generation behaviour by HOSIM number. In this test, pellets with a fixed weight are placed in a furnace. These pellets are subjected to a pre-decided temperature and gas composition as per the working of a blast furnace. Once these pellets are reduced to certain pre-decided value, the time of reduction is noted. Afterwards, they are taken outside and tumbled and screened at 3.15 mm screen. The time required for reduction is taken as HOSIM time and percentage fines generation (minus 3.15 mm particles, wt. %) is noted as HOSIM Number.
In an embodiment, the present disclosure comprises of coating fired pellet with calcined lime. In this method, a lime solution is prepared by mixing calcined lime and water. The said mixture is then sprayed evenly onto the fired pellets. The fired pellets are tumbled to ensure even and homogenous coating of lime on fired pellets. The coated pellets when subjected to reduction, ensure suitable metallurgical phases at the surfaces of pellets to enable better reduction of the interior part of pellets.
In an embodiment, the process comprises of slurry preparation of lime and water with pre-decided solid content (~4 - 8 wt. % lime). The slurry is then sprayed evenly onto the surface of the fired pellets at a rate of 4 - 8 wt. % of pellets. The fired pellets are tumbled to ensure an even and homogenous coating of lime on fired pellets. Once the pellets are coated with lime, they are ready to be charged as a ferrous feed for the blast furnaces. A drying operation of coated pellets to bring the moisture at acceptable levels of 2% or lower before charging in the blast furnace provides even better results in blast furnace operation.
It is known that the surfaces get reduced first and then the inner part of pellets are reduced during reduction of pellets. Moreover, during reduction, a certain amount of low melting phase or highly dense phase formation is unavoidable. These low melting phases block the pores at the surfaces and relatively dense phase retards the gas penetration inside pellets and hence inner part of pellets does not get reduced completely or it takes a longer time for reduction. Coating of the fired pellet with lime before charging these pellets inside the furnace ensures that the surface pores remain unaffected due to formation of suitable phases at the surfaces of reduced pellets and avoidance of certain low melting or dense phases at the surfaces. In the present disclosure, lime coated pellets are tested for HOSIM time and HOSIM number for evaluation of reduction kinetics and reduction fines generation behaviour and the results were compared with non-coated fired pellets.
Thus, the present disclosure relates to improving reduction kinetics of fired or heat hardened pellets. Heat hardened pellets produced at pellet plant are transported to iron making units such as blast furnace for pig iron production. When these pellets are charged in iron making units such as blast furnaces, these are reduced and subsequently melted to form hot metal (pig iron) and slag. The reduction kinetics of these pellets plays an important role in improving the efficiency of the blast furnace in terms of productivity and fuel rate. It is known that the higher the reducibility or faster the reduction kinetics, the better is for blast furnaces. Moreover, fewer fines generation during the reduction in blast furnaces, better is for furnace permeability.
The phenomenon of reduction happening at the surface of pellets plays a major role in improving the reduction behaviour of pellets. Once a pellet is subjected to a reduction inside an iron making unit such as blast furnace, the shell region of the pellet is reduced first followed by mantle (middle) and finally core (central region). During the reduction of shell region, the surface when reduced results in the formation of phases such as Wustite and Fayalite. These phases are denser and do not provide enough access to the inner part of pellet for further reduction. This results in a decrease in reduction kinetics of pellets as soon as the surface region of the pellet is reduced. Once the pellets are coated with CaO bearing compound such as lime in the present disclosure, it results in the formation of phases which favours the reduction of the inner volume of pellets. During reduction, lime coated pellets favour the formation of calcium ferrite bearing phases on the surface. Calcium ferrite being a low-density material than hematite favours the stabilization of surface pores which enables reducing gases access to enter pellet inner volume for carrying out reduction reactions. Moreover, lime coated pellets when subjected to reduction results in decreasing the formation of wustite and Fayalite at the surface as compared to non-coated pellets. To further elaborate the difference in reduction mechanism of lime-coated fired pellets vs non-coated pellets, the mechanism of reduction is discussed through schematics of pellets subjected to reduction operation [Figure 1].
Figure 2(a) shows the conventional route of blast furnace iron making using iron ore pellets, sinter, lump ore, and coke as feed. Figure 2(b) shows the same route with the incorporation of the present invention. In the conventional route (Figure 2a), iron ore pellets, iron ore sinter, lump ore, and coke are charged in the blast furnace. In blast furnaces, these raw materials are processed, and hot metal is produced, and slag is generated. In the present disclosure, iron ore pellets are coated with lime wherein lime % is >= 0.2 wt.% of pellets. Lime is mixed with water to make a slurry and that slurry is sprayed onto pellets. The coated pellets are tumbled to ensure a homogenous coating. Once the coating process is completed, the coated pellets are charged into the blast furnace along with iron ore sinter, lump ore and coke for hot metal production.
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: COATING OF FIRED PELLETS WITH CALCINED LIME
Initially, a slurry is prepared by mixing calcined lime and water with pre-decided solid content (~4 - 8 wt. % lime). The slurry is then sprayed evenly onto the fired pellets at a rate of 4 - 8 wt. % of pellets. The fired pellets are tumbled to ensure even and homogenous coating of lime on fired pellets. The coated pellets when subjected to reduction, ensure suitable metallurgical phases at the surfaces of pellets to enable better reduction of the interior part of pellets.
EXAMPLE 2: COMPARATIVE MECHANISM OF REDUCTION FOR COATED AND NON-COATED PELLETS
Once the pellets are coated with calcined lime, it results in the formation of phases which favours the reduction of the inner volume of pellets. During reduction, lime coated pellets favour the formation of calcium ferrite bearing phases on the surface. Calcium ferrite being a low-density material than hematite favours the stabilization of surface pores which enables reducing gases access to enter pellet inner volume for carrying out reduction reactions. Moreover, lime coated pellets when subjected to reduction results in decreasing the formation of wustite and Fayalite at the surface as compared to non-coated pellets. Figure 1 and Table 1 provides the comparison of pellets under reduction wherein one set of fired pellets is coated with lime and the other is non-coated. It shows how fired pellets when get coated by the lime result in faster reduction kinetics as compared to non-coated pellets.
Table 1: Comparison of pellets under reduction wherein one set of fired pellets is coated with lime and the other is non-coated
Stage Identification Non-coated Fired Pellets Lime-coated Fired Pellets
Stage 1- Fired pellet without reduction
Non-coated Fired Pellets Lime-coated Fired Pellets
Stage 2 – Fired pellet initial reduction Partially reduced pellet with surface covered with dense phases such as Wustite, Fayalite which chokes pores and retards gas penetration for further reduction Partially reduced pellet with surface covered with decrease in amount of dense phases such as Wustite, Fayalite and higher amount of low density phases such as calcium ferrite which enables gas penetration for further reduction
Stage 3 – Fired pellet further reduction after a fixed time duration
Presence of significantly unreduced portion due to slow reduction kinetics
Higher reduction as compared to non-coated pellet due to faster reduction kinetics

Improvement in reduction kinetics of pellets by coating fired pellets by lime ensures that physical properties of pellets are un-altered and hence reduction fines generation behaviour does not deteriorate. Cold compressive strength of the pellet is also not changed as no change in fluxing amount, flux type, porosity or firing profile, etc. at the pellet plant is made. Since the coating is done with CaO, it does not induce any harmful effect on blast furnaces as CaO is beneficial for blast furnace operation. Since lime itself acts as a binder, no external binder is required for adequate adhesiveness of coating (lime) with the substrate (pellet). This type of coating is done as near to Blast Furnaces as possible with a change in water addition for making lime slurry to avoid excess water ingress into the Blast furnace from the top through coated pellets.
EXAMPLE 3: EFFECT OF LIME COATING ON THE REDUCTION BEHAVIOUR OF IRON ORE PELLETS
To establish the effect of lime coating on the reduction behaviour of iron ore pellets, a series of tests were done wherein pellets were coated with lime at different dosages. Water and lime were mixed to produce a lime solution or slurry. This solution was sprayed onto lime coated pellets. During spraying, coated pellets were manually tumbled to ensure a homogenous coating on pellets. This series consists of five sets of pellets, and these are given in table 1. The lime coated pellets were tested in HOSIM instrument for reduction time (Hosim time) and fines generation due to reduction (Hosim Number).
Table 1 Pellet identification with different dosages of lime coating
Pellet Id Coating Material* Water for Slurry Preparation**
P1 No Coating NA
P2 0.1 % Lime 5%
P3 0.15 % Lime 5%
P4 0.20 % Lime 5%
P5 0.25% Lime 5%
*Coating material: Weight % basis pellets to be coated.
**Water for slurry or solution preparation: Weight % basis pellets to be coated.

Hosim Results

Figure 3(a) provides Hosim time and 3(b) Hosim no of pellets tested in this study. It is evident from figure 3(a) that Hosim time i.e. reduction time of pellets decreased significantly by 46 minutes for lime coated pellets. Moreover, figure 3(b) shows that fines generation due to reduction has not increased. In fact, fines generation has decreased with lime-coating over pellets.

Optical microscopy for coating thickness
Figure 4 shows the optical microscopy images of pellets with and without the coating of lime. Figure 5 shows the thickness of lime coating over iron ore pellets estimated using an optical microscope. It is evident that lime coating thickness increased with increase in lime dosage.
Quantification of phases at pellet surface after reduction in Hosim test
Fines generated from the surfaces of reduced pellets were collected and analyzed for phases using X-Ray diffraction. Figure 6 provides these quantifications which shows that Calcium ferrite phase increased gradually with increased lime dosage for coating. Wustite and Fayalite first increased at 0.1% lime coating and afterwards decreased with increased lime dosage for coating. Increase in Fayalite and wustite could be due to increased melt formation due to presence of calcium oxide at a lower level, however, with the increased calcium oxide, these melts contain more and more CaO and hence promotes lower Fayalite and wustite and higher calcium ferrite phase. For 0.1% lime dosage for coating, reduction time (Hosim time) did not decrease. This could be due to the counter-effect of calcium ferrite and combination of Fayalite and wustite. For a higher dosage of lime coating, Hosim time reduced significantly as Fayalite and wustite decreased while calcium ferrite increased.
The above studies confirm that lime coating of the fired pellets have a significant effect on reduction behaviour of iron ore pellets wherein lime coating improves reduction kinetics of pellets. Moreover, increased reduction kinetics did not affect the fines generation behaviour negatively.