Claims:We claim:
1. A synthetic magnesio ferrite (SMF) comprising iron oxide in an amount of about 49- 53% by weight, calcium oxide in an amount of about 18-22% by weight, magnesium oxide in an amount of about 10-14% by weight, alumina in an amount of about 2.5-3.5 % by weight, silica in an amount of about 4.5-5.5% by weight, titanium oxide 0.18-0.22% by weight, phosphorus in an amount of about 0.08-0.12% by weight and remaining traceable elements.
2. The SMF as claimed in claim 1, wherein mineralogical phases in the SMF comprise about 28-32% of magnesio ferrite, about 14-16% of brownmillerite, about 14-18% hematite, about 8-10% magnetite, and about 14-20% wustite.
3. The SMF as claimed in claim 1 or 2, wherein the SMF has a melting temperature of about 1280-1315?.
4. A method for preparing the SMF as claimed in any one of claims 1-3, comprising:
a. mixing iron ore fines, limestone, dolomite, pyroxenite, coke, and calcined lime fines to obtain a sinter feed;
b. granulating the sinter feed by adding moisture to obtain a granulated sinter feed;
c. firing the granulated sinter feed to a temperature of about 1050?-1150? in a sinter machine for about 1 to 3 minutes followed by sintering the granulated sinter feed to obtain the SMF.
5. The method as claimed in claim 4, wherein said sintering takes about 20 to 24 minutes.
6. The method as claimed in claim 4 or 5, wherein the sinter feed comprises iron ore fines in an amount of about 38-45 wt%; limestone in an amount of about 18-22 wt%; dolomite in an amount of about 22-24 wt%; pyroxenite in an amount of about 12-14 wt%; coke in an amount of about 6-8wt%; and calcined lime fines in an amount of about 1-2 wt%.
7. The method as claimed in any one of claims 4-7, wherein the size of the iron ore fines is about -6 mm.
8. The method as claimed in any one of claims 4-7, wherein said moisture is added in an amount of about 7-7.5% by weight.
9. A method of making an iron ore sinter, comprising:
a. mixing the synthetic magnesio ferrite (SMF) as claimed in any one of claims 1-3 with iron ore fines, limestone, dolomite, pyroxenite, coke, return fines, and calcined lime fines to obtain an iron ore sinter mix;
b. granulating the iron ore sinter mix by adding moisture to obtain mini agglomerates of the iron ore sinter mix; and
c. firing the mini agglomerates of the iron ore sinter mix to a temperature of about 1000?-1100? in a sinter machine followed by sintering the mini agglomerates to obtain iron ore sinter.
10. The method as claimed in claim 9, wherein the SMF is added in an amount of about 2-10% by weight of the iron ore sinter mix.
11. The method as claimed in claim 9 or 10, wherein the SMF has a size of about minus 5 mm.
12. The method as claimed in any one of claims 9-11, wherein the iron ore sinter mix comprises iron ore fines in an amount of about 58-60 wt%; limestone in an amount of about 13-16 wt%; dolomite in an amount of about 2.5-3 wt%; pyroxenite in an amount of about 1.3-1.6 wt%; coke in an amount of about 6-8 wt%; calcined lime fines in an amount of about 1-2 wt% and return fines in an amount of about 14-18%.
13. The method as claimed in any one of claims 9-12, wherein the iron ore fines comprise about 2-3% by weight of alumina.
14. The method as claimed in any one of claims 9-13, wherein total magnesium oxide content of the iron ore sinter mix is about 1.3-1.9% by weight.
15. The method as claimed in any one of claims 9-14, wherein said moisture is added in an amount of about 6-6.5% by weight of the iron ore sinter mix.
16. The method as claimed in any one of claims 9-15, wherein said sintering takes about 20-24 minutes.
17. The method as claimed in any one of claims 9-16, wherein presence of the SMF decreases the contact angle of iron ore sinter melt.
18. The method as claimed in any one of claims 9-17, wherein the SMF promotes the formation of an iron rich magnesium phase and silico ferrites of calcium and alumina - I (SFCA-1) phase.
19. The method as claimed in any one of claims 9-18, wherein a Reduction Degradation Index (RDI) of sinter is decreased by 15-20% points and Reducibility Index (RI) is maintained at about 79.23 to about 79.37 at lower content of MgO.
20. The method as claimed in any one of claims 9-19, wherein sintering time is decreased by about 10 %.
21. The method as claimed in any one of claims 9-20, wherein generation of return sinter is decreased by about 5-8%. , Description:FIELD OF THE INVENTION
The present disclosure relates to the field of metallurgy, more particularly to iron ore sinter production. The present disclosure provides a composition and method for producing iron ore sinter with improved reduction degradation index (RDI) and reducibility index (RI) of iron ore sinter.
BACKGROUND OF THE INVENTION
The reduction process in high-temperature areas of metallurgical furnaces takes place in a real aggregate according two different schemes – direct and non-direct reduction. Their differentiation is essential for the economic operation of the aggregate. The task of non-direct area determination involves specification of temperature of direct reduction initiation which is dependent on metallurgical properties of assessed ore and reactivity of performed reducing agent, mostly coke. The quality of the product obtained from a blast furnace is significantly influenced by properties such as reducibility and disintegration of feedstock material - sinter, pellets or lump iron ore.
The reducibility refers to oxygen removal from iron oxides in gaseous atmosphere. The amount of removed oxygen is measured as mass loss of sample or through material balance of oxidation and reduction gas.
The disintegration is characterized by the rate of ore material destruction as a result of collision, abrasion, pressure and volume changes at high temperatures in the reduction atmosphere. The disintegration of iron ores mainly occurs during the reduction process from hematite to magnetite in the temperature zone of 400 °C – 700 °C, and many researchers indicate in their studies that the main reason for the disintegration is the stress concentration caused by the volume expansion of magnetite. The disintegration of ores/sinter basically depends on their reducibility. The relation between these two properties is studied in the atmosphere of hydrogen. Murakami et al. studied reduction and reducibility behavior under H2 in mixture with H2O, CO, CO2 and N2 at 500 °C and found that the reduction degree of the sinter reduced under CO-H2 gas increased with time. Results from their previous experiments showed a remarkable increase in the reduction degree (dR/dt) and disintegration index (RDI) [Murakami 2012]. Mu et al. described the effect of hydrogen addition on sinter disintegration. The increasing content of H2 with proportional decrease of CO, CO2 and N2 resulted in a higher disintegration index.
It is a well-established practice to incorporate as much flux as possible in the sinter to avoid the endothermic calcination of the flux in the blast furnace and to achieve partial slag formation in the sinter itself. MgO has different roles on sinter reducibility at different chemical composition. The effects of SiO2 and basicity on reducibility are affected by MgO, i.e., enhancement of basicity is inclined to sinter reducibility, and the enhancing SiO2 content has adverse effect on sinter reducibility, but both of which are weakened with the addition of MgO. Certain research findings which improve sinter reducibility are as follows:
(i) High basicity and lower coke consumption are helpful in improving sinter reducibility by way of favoured mineralogy.
(ii) The strong influence of types of phases present in sinter, their composition, amount/volume fraction as well as morphologies on the sinter properties is well established through several research findings. Pores in sinter can influence the reducibility (RI) and reduction degradation indices (RDI) indices depending on whether they are open or close and on the phases that are present around the pores. Besides, the level of hematite phase in sinter and its distribution is important for improved sinter reducibility.
(iii) High grade ore largely forms silico ferrities of calcium and alumina – I (SFCA-I) and low-grade ore largely forms SFCA. SFCA-I phase is the most desirable bonding phase in iron ore sinter since microstructure composed of SFCAI show higher physical strength higher reducibility than microstructure composed predominantly of SFCA.
(iv) It has been established that finely granular hematite, relict hematite, acicular ferrite and rhombic hematite matrix assemblages are reducible.
(v) High FeO sinter blended with high coke breeze addition to high MgO content have low reduction degree while low SiO2, low CaO, and high MgO sinter has high reducibility.
(vi) The addition of MgO accelerates the thermal decomposition of hematite (Fe2O3) and favours the formation of less reducible magnetite (FeO.Fe2O3)-Mg phase. MgO also restricts melt formation, this is reported to lead the increase in porosity of the sinter, which in turn provides more surface area exposed to the reducing gases, resulting in higher reducibility and reduction degradation index.
(vii) Lower silica, while keeping the basicity constant, would cause an increase in reducibility because the amount of silicate slag, which has low reducibility compared to haematite, would decrease.
(viii) the addition of MgO causes disappearance of calcium ferrite and increase in the MgO bearing phases like olivine.

Furthermore, several studies have indicated that the mineral texture composed of skeletal hematite, columnar calcium ferrite, and glassy silicate was degraded especially. Therefore, the effect of coexisting minerals on RDI should be considered. Thereupon fracture strength of columnar calcium ferrite was evaluated 3using indentation microfracture method to study aggravation of RDI. In any case, decrease in an amount of skeletal hematite leads certainly to improvement of RDI. Several mechanisms have been proposed regarding the formation of the skeletal hematite: (a) dissolving of magnetite, which coexists with silicate melt at a high temperature, and crystallizing out abruptly at a temperature range of hematite crystallization in phase diagrams (Inazumi et al.), (b) incongruent melting of the calcium ferrite in the heating process (Inoue and Ikeda, (c) dissolving of SiO2 into the CaO-Fe2O3 melt (Matsuno et al.), and (d) assimilation of the CaO-Fe2O3 melt and silicate melt (Haruna et al.).

The literature in the classification of iron ore sinter phase mineralogy through QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy) stated about the conceptual iron ore sinter mineralogy as shown below. The sintered material cools and crystallizes into several mineral phases of different chemical compositions and morphologies. These are mainly haematite, magnetite, SFCAs, calcium silicates, and silicate glass (as summarized in Table 1). Minor phases include wustite, anorthite, Ca-Mg-Fe silicates, and residual minerals such as quartz, free lime, olivine, and periclase. Magnetite, hematite and SFCA occur in various different morphologies exhibiting unique physico-chemical behaviors.
Table 1

Previous research show that the secondary hematite is a main factor to result in low temperature reduction degradation of sinter, and its formation relates with MgO content in raw materials of sinter, but the mechanism has been not very clear until now. The mass changes of samples with different proportions of MgO and Fe2O3 during heating-up, constant temperature sintering and cooling process were measured to understand quantitatively the effect of MgO content on transformation between Fe2O3 and Fe3O4 by a thermogravimetric method, simultaneously the X-ray diffraction (XRD) and the optical microscopy (OM) were used to follow the changes of mineral phase and microstructures in samples for investigation of the mechanism of MgO depressing the secondary hematite formation in sintering process of iron ores. The results show that after addition of MgO which reacts with Fe2O3 to form magnesium ferrite and magnesioferrite, the former promotes the formation of the magnesioferrite during the heating-up process and the latter is more stable than the magnetite in low temperature to depress the oxidation of magnetite in the cooling process, so resulting in decreasing of the secondary hematite in sinter. Therefore, it makes clear the reason to improve the low temperature reduction degradation of sinter after addition of MgO in the raw materials.

During the sintering process, as temperature increases, hematite from the ore is being reduced to magnetite. With increase in MgO addition the formation of magnesiospinel phase starts to form along with magnetite. Most of the Mg ions exist in the magnetite phase. The magnesiospinel chemical formula is [(Mg, Fe)O.Fe2O3)] which is said to be more stable than magnetite due to the solid state diffusion of Mg2+ which replaces some of the Fe2+ from the magnetite lattice.

The reducibility depends on the physical and chemical characteristics of the sinter particle size, porosity, mineralogical composition and internal physical structure. As per the laboratory studies, the reducibility of the low silica sinter (73.95–80.32%) was higher than high silica sinter (63.05–72.62%). Low silica sinter reducibility was not affected that much with increase in MgO addition, but in high silica sinter (silica>60%) the sinter reducibility was decreased to below 63% at MgO addition more than 2.8%. The minimum sinter reducibility required for blast furnace operation is 65.0%. The microstructural phases of low and high silica sinters were greatly affected by MgO addition. The decrease in sinter reducibility is associated with a decrease in hematite, SFCA, and pore phase and increase in magnesiospinel and silicate/slag phase. Magnetite/magnesiospinel content of the low silica sinter and high silica sinter increased with increase in MgO addition. The reducibility of the sinter is directly related to sinter magnetite content. Reducibility of both low and high silica sinter decreased with increase in sinter magnetite/magnesiospinel content. During the reduction process the conversion of hematite into magnetite/magnesiospinel takes place. The higher the amounts of magnetite/magnesiospinel in sinter, higher the FeO content and lower the reduction rate.

The key factors affecting the structure and quality of iron ore sinter are the properties of the iron ore, their composition mainly alumina and silica content, granulation efficiency, nucleus stability and primary sinter melt volume and, flux and their granulometry. All the mentioned parameter controls the sinter minerology and phase distribution.

In the sintering process, main raw material used is iron ore fines, Quality of the iron ore fines influences the process of sintering significantly. Ore with higher alumina content is usually detrimental in forming the sinter matrix due to the low reactivity of alumina bearing minerals and the high viscosity of primary melts. A small increase in the alumina content of sinter blend can have a significant adverse impact on the strength and reduction degradation characteristics of the final sinter leading to deterioration in gas permeability in the upper part of the blast furnace. The most harmful effect of alumina is to worsen the sinter RDI, which increases as the alumina content rises. Industrial experience with the blast furnace shows that within a 10-10.5% CaO content range an increase of 0.1% in the alumina content raises the RDI by 2 points.

The strength and quality of sinter deteriorate as the alumina content rises. Alumina promotes the formation of SFCA, which is beneficial for sinter strength, but the strength of the ore components is lower, since a high alumina content in their lattice has been reported to be the main cause of the observed lower strength. Alumina increases the viscosity of the primary melt that forms during the sintering process, leading to a weaker sinter structure with more interconnected irregular pores. It could also alter the composition and properties of the primary melt formed in the sinter located near the cohesive zone of the blast furnace, which would have a negative impact on the gas and liquid permeability and reducibility of sinter in the lower part of the blast furnace.

An increase in the mean particle size of the iron ore fines promotes the productivity of the sintering machine, saves the specific fuel consumption but reduces the sinter strength. Dense low alumina iron ores give a better sinter strength and lower specific fuel consumption. Very high level of micro-fines in the ore decreases the granulation efficiency and consequently, decreases the bed permeability and affects productivity of sintering adversely.

High quality agglomerate in the form of sinter improves productivity of blast furnace and reduces coke consumption. With low availability of high-grade iron ore, now low-grade iron ores with high alumina, hydrates and other impurities are needed to be used. These impurities and gangue specially alumina, decreases the quality of iron ore sinter and productivity significantly and therefore it has become important to overcome these issues by means of innovative approach. To improve quality of iron ore sinter and increase its productivity, various researchers have reported different ways and method. Some of these are listed below.

CN100336918C relates to a chemical additive for sintering iron ore power. The chemical additive for sintering iron ore power is a powdery compounding mixture whose granularity is greater than or equal to 50 meshes. The chemical additive for sintering iron ore power is composed of 20 to 45 wt% of oxygen enriching agent, 5 to 15 wt% of free radical initiator, 20 to 45 wt% of fluxing agent, 2 to 15 wt% of intensifying agent, 2 to 15 wt% of catalyst and 5 to 20 wt% of nucleating agent. The content of compound needle type calcium ferrite (SFCA) can be enhanced by 15%, the reduction degree (RI) can be enhanced by 7.83%, the index (RDI<+315>) of the low temperature reduction pulverization can be enhanced by 8.2%, and stering solid consumption can be reduced by 8%.

CN109852745B relates to a magnesium-iron-calcium complex composite flux for iron-making sinter, which comprises the following components in percentage by mass: organic polymer 0.25 to 0.75 percent of sticky material; 11 to 13 percent of oxygen increasing sintering agent; 86.25 to 88.75 percent of flux carrier. The organic high-viscosity material is one or a mixture of two of carboxymethyl cellulose and water-soluble resin, and the mass percentage is 0.25-0.75.0%. The oxygen-increasing sintering agent is iron oxide red (Fe2O3) The granularity is less than or equal to 0.044mm, and the mass percentage is 11-13%. The flux carrier is one or a mixture of two of high-iron high-calcium dead burned magnesium powder and calcite powder, and the mass percentage of the carrier is 86.25-88.75%. The invention can bond more than 90% of sintering mixture into balls, improve the air permeability of the sintering material layer, and is beneficial to increasing the material layer height and reducing the consumption of solid fuel so as to realize low-temperature sintering. Improve the air permeability of the sinter bed, increase the sintering speed and yield, can sinter by low negative pressure air draft, save the power consumption, save the energy.

A method of RDI improvement composition of sinter ore, preparing method for the same, and method of surface treatment of sinter ore using the same is described Patent Publication No. KR20050009771A wherein a combination of boron compound, chlorides and lithium compound were used for the same. The method stated about the usage of calcium chloride and lithium compound improves the RDI of sintered ore which is very low in alumina content ~1.8%. However, addition of such element like chlorides and lithium will cause maintenance problem because of corrosion caused by these elements and their compounds.

KR101167381B1 stated that the present invention relates to a method for improving the low temperature reduction differentiation index of sintered ore. Preparing a sintered ore and charging the sintered ore coated with the differentiation inhibitor into a blast furnace.
According to the present invention, since the differentiation inhibitor prepared by mixing the magnesium chloride aqueous solution with the aqueous calcium chloride solution is coated on the surface of the sintered ore, the sintered ore is charged into the blast furnace so that the coating is uniformly applied to the sintered ore without precipitation or solidification of the calcium chloride aqueous solution even at a low temperature. Therefore, there is an advantage that the quality can be stabilized by improving the low temperature reduction differentiation index of the sintered ore.

CN85100645A directed to a double-sphere sintering technology is a kind of energy consumption that reduces iron ore concentrate sintered process. Improve the double-sphere sintering technology method of sinter quality and output, be the bead of iron ore concentrate being made different basicity respectively, with addition of an amount of fuel, behind the mixing, it is good to obtain metallurgical performance at lower sintering temperature, is the self-fluxed sinter of main bonding phase with the calcium ferrite. Its reducing property significantly improves, and the solid fuel consumption of low temperature reduction degradation index and sintering process obviously reduces, and productivity improves greatly.

A method of increasing sinter rate is described in U.S. Patent No. 2,767,074 in which the inventor has proposed a method of increasing sintering rate by preheating the mix with hot water and steam. Preheating the mix by hot water and steam does not work to the expectation as todays raw material with high combined moisture results in cold process air generation to decrease mix temp as well as provide extra moisture for moisture front formation.

In US4657584, a process is provided for sintering iron-bearing particulate materials in which the final sinter contains 3/5.5% MgO yet has substantially the same strength as a low MgO limestone sinter. At least 20% of the MgO in the final sinter product must come from free MgO-bearing material other than dolomite having a particle size smaller than 3/8 inch, but not more than 20% of it can have a particle size smaller than 1/64 inch. Also, not more than 25% of the MgO in the final product may come from dolomite. The invention includes the relatively higher strength MgO-containing sinter produced.

A method for improving strength and size composition of sinter is described by in CN103343218B in which special brucite fibers are used for improving sinter quality. Materials like brucite fibers are not so easily available for commercial usage and will be costly replacement of any flux.

The above-cited inventions have a major drawback that they do not cover how the sinter quality and productivity will be improved or at least not be decreased if the raw material quality deteriorates. Additional drawbacks are that they either suggest a new design incorporation in the sinter plant for production increase or suggest use of some different materials as a hearth layer in sinter making whose chemical composition is not the same as the sinter.

Thus, there is a need to develop an efficient way to improve the quality of the iron ore sinter even when produced using low-grade iron ores such as high alumina-containing iron ores and the like. The present disclosure attempts to address this need.
SUMMARY OF THE INVENTION
The present disclosure provides a synthetic magnesio ferrite (SMF) comprising iron oxide in an amount of about 49- 53% by weight, calcium oxide in an amount of about 18-22% by weight, magnesium oxide in an amount of about 10-14% by weight, alumina in an amount of about 2.5-3.5 % by weight, silica in an amount of about 4.5-5.5% by weight, titanium oxide 0.18-0.22% by weight, phosphorus in an amount of about 0.08-0.12% by weight and remaining traceable elements.

The present disclosure also provides a method for preparing the SMF, comprising: (a) mixing iron ore fines, limestone, dolomite, pyroxenite, coke, and calcined lime fines to obtain a sinter feed; (b) granulating the sinter feed by adding moisture to obtain a granulated sinter feed; and (c) firing the granulated sinter feed to a temperature of about 1050?-1150? in a sinter machine for about 1 to 3 minutes followed by sintering the granulated sinter feed to obtain the SMF.

The present disclosure also provides a method for preparing an iron ore sinter, comprising: (a) mixing the SMF with iron ore fines, limestone, dolomite, pyroxenite, coke, return fines, and calcined lime fines to obtain an iron ore sinter mix; (b) granulating the iron ore sinter mix by adding moisture to obtain mini agglomerates of the iron ore sinter mix; and (c) firing the mini agglomerates of the iron ore sinter mix to a temperature of about 1000?-1100? in a sinter machine followed by sintering the mini agglomerates to obtain iron ore sinter.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a flowchart of a process for sintering/preparing SMF and a process for sintering/preparing iron ore sinter according to the present disclosure.
Figure 2 shows the effect of sinter feed alumina and the addition of SMF on hemisphere form temperature.
Figure 3 shows the effect of sinter feed alumina and the addition of SMF on the contact angle of the sinter melt.
Figure 4 shows a mechanism of sintering reaction according to the conventional sintering process versus the sintering reaction according to the present disclosure.
Figure 5 shows the X-ray diffraction analysis of two SMFs prepared according to the present disclosure.
Figure 6 shows the dilatometer data showing hemisphere and flow temperature of SMF.
Figure 7 shows the effect of sinter alumina and the addition of SMF on the tumbler index of the iron ore sinter.
Figure 8 shows the effect of sinter alumina and the addition of SMF on the reduction degradation index (RDI) of the iron ore sinter.
Figure 9 shows the effect of sinter alumina and the addition of SMF on the reducibility index (RI) of the iron ore sinter.
Figure 10 shows the effect of sinter alumina and the addition of SMF on the amount of return fines generated during iron ore sintering.
Figure 11 shows the effect of sinter alumina and the addition of SMF on the sintering time of iron ore.
Figures 12 and 13 show SEM-EDX analysis of iron ore sinter obtained according to the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having”, or “including but not limited to” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Reference throughout this specification to “one embodiment”, “an embodiment”, or “some embodiments” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in one embodiment”, “in an embodiment”, or “in some embodiments” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.
The term “about” as used herein encompasses variations of +/-10% and more preferably +/-5%, as such variations are appropriate for practicing the present invention.
Iron ore sinter is the primary feed material for making iron in a blast furnace. The iron ore sinter is produced by a sintering process that converts a mixture (iron ore sinter mix/iron ore sinter feed) of fine-sized raw materials, including iron ore, coke, limestone, return fines, etc. into an agglomerated product called iron ore sinter for charging into the blast furnace. The production of high-quality iron ore sinter is important for production of high-quality iron, furnace productivity, and lower consumption of reducing agents.
One of the important factors affecting the structure and quality of iron ore sinter is the temperature of primary melt formation and the primary melt volume formed from slag and other components of the iron ore sinter mix. During sintering, the temperature in the sintering zone reaches as high as 1350? which results in an incipient melt formation and fusion of different constituents of sinter mix and subsequent production of large sintered porous mass. A melt is formed due to reaction of fluxing components with the constituents of iron ore at high temperature. This melt promotes the diffusion of different components of the slag forming compounds across the grains through the melt and subsequent solidification of this melt binds the grains to each other. Therefore, an early formation of this melt and amount of the melt formation plays a significant role in determining the quality of sintering.
The quality of iron ore sinter is reflected by properties such as reducibility and disintegration of the iron ore sinter which in turn depend on several factors including the quality of iron ore. As discussed above, with low availability of high-grade iron ore, low-grade iron ores with high alumina, hydrates, silica content, and other impurities are required to be used. Even a small increase in the alumina content of the iron ore sinter mix has a significant adverse impact on the yield, strength and the reduction degradation characteristics of the iron ore sinter obtained from the mix. This is because alumina in the iron ore sinter mix is believed to promote and stabilize SFCA (silico-ferrite of calcium and aluminum) phase which is less desirable than SFCA-I phase as the microstructure composed of the SFCA-I phase shows higher physical strength and higher reducibility than the microstructure composed predominantly of SFCA. Sinter with SFCA morphology shows a lower shatter index, lower tumbler index and lower RDI.
An increase in the alumina content of the iron ore also demands a high sintering temperature and a longer sintering time to promote melt formation and to produce a sinter with reasonable quality. As a result, the fuel rate increases, and the sintering productivity decreases.
It is a well-established practice to add compounds containing MgO as a flux in the iron ore sinter mix. However, increasing the amount of MgO to counter the effects of high alumina is also not very helpful. This is because an increased amount of MgO reduces the binding phase strength and the sinter strength.
The present disclosure attempts to address these problems by providing a novel fluxing agent called synthetic magnesioferrite (SMF) for doping the iron ore sinter mix to improve the quality of the iron ore sinter. The present disclosure also provides a method of producing SMF and a method of producing iron ore sinter by adding SMF to the iron ore sinter mix.
The addition of SMF provides several advantages. The iron ore sinter produced by addition of SMF shows improved reduction degradation index and increased reducibility of sinter. The addition of SMF to the iron ore sinter mix decreases the sintering time by increasing sintering reaction kinetics. Further, the addition of SMF enables production of a high-quality iron ore sinter from low-grade ores such as a high alumina-containing iron ore or high crystallized water iron ore with increased productivity and at the same time, does not increase the net carbon consumption.
The SMF has a lower melting point because of which it starts melting at lower temperature and facilitates melting and assimilation of other raw materials (primary melt formation) in the iron ore sinter mix. This is particularly beneficial for high alumina-containing iron ores since the presence of high alumina increases the melting point and lowers the viscosity of the sinter mix. The addition of SMF counteracts these effects of high alumina and provides a better-quality iron ore sinter.
Both the temperature and the viscosity of primary melt formation are significantly important in iron ore sintering. With the presence of SMF in the sinter mix along with higher alumina, the temperature of primary melt formation is lowered down, thus bonding phases form at a lower temperature and remain at that temperature or higher temperature for longer duration as the temperature of that zone is still increasing.
On the other hand, in the absence of SMF, the temperature of primary melt formation is high because of which the melt will form at a much later stage in sintering and will get cooled immediately, resulting in a thermal shock and brittle phases formation. Additionally, since the viscosity of the primary melt is high in the absence of SMF, it does not flow effectively to nearby pores and will not bind the grains effectively.
The SMF of the present disclosure comprises iron oxide in an amount of about 49- 53% by weight, calcium oxide in an amount of about 18-22% by weight, magnesium oxide in an amount of about 10-14% by weight, alumina in an amount of about 2.5-3.5 % by weight, silica in an amount of about 4.5-5.5% by weight, titanium oxide 0.18-0.22% by weight, phosphorus in an amount of about 0.08-0.12% by weight, including values and ranges therebetween for each of the above-mentioned component, and remaining traceable elements (Table 2).
Table 2: Chemical composition of SMF
Fe2O3 CaO SiO2 MgO Al2O3 TiO2 P
49-53% 18-22% 4.5-5.5% 10-14% 2.5-3.5% 0.18-0.22% 0.08-0.12%

The SMF comprises various mineralogical phases such as magnesio ferrite, brownmillerite, hematite, magnetite, wustite, and the like. In some embodiments, the SMF comprises about 28-32% of magnesio ferrite, about 14-16% of brownmillerite, about 14-18% hematite, about 8-10% magnetite, and about 14-20% wustite. In some embodiments, the melting temperature of the SMF ranges from about 1280? to about 1315?, including values and ranges thereof, such as about 1280, 1285, 1290, 1295, 1300, 1305, 1310, or 1315 ?.
Also provided herein is a method for preparing the SMF. To prepare SMF, iron ore fines, limestone, dolomite, pyroxenite, coke, and calcined lime fines are mixed to obtain a sinter feed. The sinter feed is granulated in a mixing equipment by adding moisture to obtain granulated sinter feed. The granulated sinter feed is added to a heating bed in a sintering machine and is fired. The top layer of the granulated sinter feed is fired to a temperature of about 1050?-1150? for about 1 to 3 minutes. After about 1-3 minutes, firing is stopped and the fired sinter feed is allowed to sinter for about 20-24 minutes to obtain SMF.
In some embodiments, the sinter feed to prepare the SMF comprises about 38-45 wt% iron ore fines, about 18-22 wt% limestone, about 22-24 wt% dolomite, about 12-14 wt% pyroxenite, about 6-8 wt% coke and about 1-2 wt% calcined lime fines, including values and ranges thereof. The size of the iron ore fines employed to prepare the SMF is -6 mm. The sinter feed is granulated in a mixing equipment by mixing the sinter feed with about 7-7.5% by weight of moisture for about 15-20 minutes. The granulated sinter feed is added to a hearth layer in a sintering machine and the top layer of the sinter feed is fired/ignited to a temperature of about 1050?-1150? for about 1 to 3 minutes. The sintering machine can be a bottom suction draught type or any other furnaces like muffle, microwave, tubular etc. can be employed for firing and sintering.
In some embodiments, the granulated sinter feed is fired/ignited to a temperature of about 1050, 1060, 1070, 1080, 1090, 1100, 1110, 1120, 1130, 1140, or 1150? for about 1-3 minutes, after which firing/ignition is stopped and the fired sinter feed is sintered for about 20-24 minutes to obtain the SMF. A flowchart of the process for preparing the SMF is shown in Figure 1.
Also provided herein is a method of preparing iron ore sinter by adding the SMF as a fluxing agent. The SMF is crushed/ground to a size of about -5 mm and crushed SMF is employed for making iron ore sinter. About 90-100% of SMF employed for iron ore sintering has a size of about -5 mm.
The method of preparing iron ore sinter comprises: a) mixing the SMF with iron ore fines, limestone, dolomite, pyroxenite, coke, return fines, and calcined lime fines to obtain an iron ore sinter mix; b) granulating the iron ore sinter mix in a mixing equipment by adding moisture to obtain mini agglomerates of the iron ore sinter mix; and c) firing the mini agglomerates of the iron ore sinter mix to a temperature of about 1000?-1100? in a sinter machine followed by sintering the mini agglomerates to obtain iron ore sinter.
In some embodiments, the iron ore sinter mix comprises SMF in an amount of about 2-10 wt%, iron ore fines in an amount of about 58-60 wt%; limestone in an amount of about 13-16 wt%; dolomite in an amount of about 2.5-3 wt%; pyroxenite in an amount of about 1.3-1.6 wt%; coke in an amount of about 6-8 wt%; calcined lime fines in an amount of about 1-2 wt% and return fines in an amount of about 14-18 wt%, including values and range thereof. In some embodiments, the SMF is added in an amount of about 2%, 4%, or 6% by weight of the iron ore sinter mix. In some embodiments, the iron ore employed in the sintering comprises about 2-3.5 % by weight of alumina. The total magnesium oxide content of the iron ore sinter mix is maintained at about 1.3-1.9% by weight.
The iron ore sinter mix is granulated in a mixing equipment by adding moisture, for example, about 6-6.5% by weight of moisture, for about 15-20 minutes to obtain mini agglomerates. The mini agglomerates of iron ore sinter mix are added to a hearth layer in a sintering machine and the top layer of the mini agglomerates is fired/ignited to a temperature of about 1000?-1100? for about 1 to 3 minutes. After this, the firing is stopped, and the fired mini agglomerates are allowed to sinter for about 20-24 minutes to obtain iron ore sinter.
In the present method of making iron ore sinter, the addition of SMF lowers the melting temperature of the iron ore sinter mix and increases the viscosity of the primary melt. The addition of SMF is particularly beneficial for sintering of iron ores containing high alumina. The presence of high alumina increases the melting point and lowers the viscosity of the sinter mix. As can be seen from Figure 2, the addition of SMF lowers the melting temperature of the sinter mix containing high alumina iron ore. Figure 3 shows that the addition of SMF decreases the contact angle of sinter melt containing high alumina iron ore. A lower melting temperature and increased viscosity due to the addition of SMF lead to earlier formation of primary melt and better assimilation of the constitutes of the iron ore sinter mix which in turn provides a better-quality iron ore sinter.
Figure 4 explains the difference in the mechanism of conventional sintering of iron ore and sintering of iron ore according to the present disclosure where the SMF is added.
Figure 4, left side, shows a schematic of conventional sintering. Figure 4(a) shows a schematic image of sinter feed agglomerates. It shows high alumina coarser iron ore fines surrounded with finer high alumina iron ore, coke breeze and fluxes (CaO and MgO bearing materials). During sintering, coke particles in the mixture get oxidized and generate in-situ heat. This heat energy results in calcination of fluxes and de-hydroxylation of combined moisture in ores. Once the temperature is sufficiently high for the reaction to take place, these materials will react to form a complex melt in sinter. These phases are highly viscous due to the presence of high alumina along with low iron high magnesia rich phase as shown in Figure 4(b). Due to high viscous nature, these slag phases do not flow out to nearby pores efficiently. Upon solidification, these phases result in the generation of large amount columnar SFCA, porous sinter structure with large size pores, low platy SFCA along with the high amount of relict or primary hematite and secondary hematite also. These phases are schematically shown in Figure 4(c). As discussed earlier, sinter with these phases show poor physical and metallurgical properties. Owing to higher viscosity of the melts, the Mg richer slag is not able to get uniformly distributed in the sinter slag phase and hence the actual effect of MgO is not seen in sinter properties.
Figure 4, right side, shows a schematic of sintering according to the present disclosure. Figure 4(d) shows a schematic image of sinter feed agglomerates for the SMF-added sintering process. It shows coarser iron ore fines surrounded with finer iron ore, coke breeze, fluxes (CaO and MgO bearing materials) and SMF particles. During sintering, coke particles in the mixture will get oxidized and generate in-situ heat. This heat energy results in calcination of fluxes, de-hydroxylation of combined moisture in ores as well as melting of SMF. Since the melting point of SMF is lower, it will form melt at a lower temperature and start reacting with other constituents of sinter mix making at an early stage. The temperature of sinter reaction is quite lowered compared to the conventional process and hence these constituents react at a lower temperature to form the complex melt in sinter. These melts are relatively low in viscosity due to the presence of SMF for high alumina ore sintering (see Figure 4(e)). This low viscous nature of melts enables them to flow out to nearby pores efficiently. As these melt flow out to cover more granules of sinter mix, they absorb more iron and hence upon solidification, these phases result in the generation of platy SFCA, porous sinter structure with small size pores, low columnar SFCA along with the relatively low amount of relict or primary hematite. Finer size of SMF (<5mm) results in a better contact with iron and flux grains and hence results in iron richer magnesio ferrite formation and SFCA-1 phase generation.
The addition of SMF: (i) promotes the formation of an iron rich magnesium phase and silico ferrites of calcium and alumina - I (SFCA-1) phase; (ii) decreases the Reduction Degradation Index (RDI) of sinter by 15-20% points; (iii) maintains the Reducibility Index at about 79.23 to about 79.37 at a lower content of MgO; (iv) decreases the sintering time by about 10%; and (v) decreases the generation of return sinter by about 5-8%.
It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the 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 herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein
Examples
Example 1: Preparation of SMF
A synthetic magnesio ferrite (SMF) was produced using a pilot pot test facility. To produce SMF, a sinter feed was prepared by mixing iron ore fines, limestone, dolomite, pyroxenite, calcined lime fines and coke breeze followed by granulation of this mixture in the presence of about 7-7.5% by weight of moisture to obtain granulated sinter feed. The SMF was prepared in a sinter pot of height 600 mm with a square cross-section of side length 300 mm. The sintering pot contains a removable grate bar set up at the bottom. The granulated sinter feed was charged in the sinter pot. Iron ore sinter of size 10-25 mm was charged at the bottom of the pot as hearth layer. The height of the hearth layer was maintained at 50 mm over which sinter feed to prepare the SMF was charged. The mixture was ignited with the help of an LPG fired ignition furnace. Flame temperature or firing temperature was kept at 1100 °C. The firing time for the top layer of the bed was 2 minutes after which the firing was stopped and the fired sinter feed was allowed to sinter for 23 minutes to obtain the SMF. The composition of SMF is shown in Table 3.
Table 3: Chemical composition of synthetic magnesio ferrite
Fe2O3 CaO SiO2 MgO MnO Al2O3 TiO2 P
50 20.73 4.72 12.62 0.11 3.38 0.21 0.1

To confirm the amount of magnesio ferrite phase formation, the SMF was subjected to X ray diffraction test. The test reveals the quantitative amounts of phases present in the sinter (Figure 5).
Example 2: Melting behavior of SMF
To understand the melting behavior of SMF, the temperature of the primary melt formation as well as viscosity of the primary melt was studied using dilatometer. In a dilatometer equipment, a small tablet of powdered SMF sample was heated. With increase in temperature, the sample undergoes different degree of shrinkage, deformation and becomes hemispherical due to primary melt formation. Finally, with a further increase in the temperature, the hemispherical sample formed earlier melts completely and flows away. The temperature at which samples flow can be correlated to the viscosity of the melt. If the temperature is high, then it can be correlated to its high viscosity. If the temperature at which spherical shape is formed is high, it can be correlated to its high primary melt formation temperature. The powder tablet of the SMF was tested in a dilatometer. Figure 6 shows the results of the dilatometer test.
Figure 6 shows hemisphere formation in the range of 1302-1312? and a decrease in the contact angle upon hemisphere formation.
Example 3: Effect of SMF on hemisphere formation and contact angle in dilatometer tests
The dilatometer test described in Example 2 was conducted with high alumina and low alumina iron ore sinter as it is and the sinter with the addition of SMF in different proportion to understand the effect of SMF on the hemisphere formation and the contact angle of the melt.
Variation in temperature of hemisphere formation and contact angle is shown in Figures 2 and 3, respectively. The samples included: iron ore sinter mix with 2.3% alumina, iron sinter mix with 3.3% alumina, iron ore sinter mix with 3.3% alumina + 2% SMF, iron ore sinter mix with 3.3% alumina + 4% SMF, and iron ore sinter mix with 3.3% alumina + 6% SMF. Both the hemisphere formation temperature and the flow temperature decreased with the addition of SMF. This indicates that the addition of SMF to sinter making raw materials decreases the temperature of primary melt formation and decreases the viscosity of the primary melt which would promote early start of sintering process, i.e., at low temperature and therefore provide more time for sintering to occur.
It is evident from Figure 3 that with an increase in alumina, the contact angle of melt was higher. It is also evident that with the addition of SMF to sinter feed containing high alumina iron ore, the contact angle decreased significantly. The contact angle has been correlated with the viscosity of the material, i.e., the lower the contact angle, the lower the viscosity. Therefore, with the addition of SMF to the sinter feed containing high alumina iron ore, the viscosity of the primary melt would decrease due to which the primary melt will flow out into the pores easily through diffusion (Figure 4). Such melts, on cooling, transform to desirable mineralogical phases in the sinter. The contact angle is also one of the ways to measure the wettability of the material. The lower the contact angle, the better is the wettability of the liquid to cover the surface of the material. When the melt is in better contact with the grains of iron oxide, it will promote better diffusion (Figure 4).
Example 4: Preparation of iron ore sinter by addition of SMF
7 sets of iron ore sinter were prepared by adding crushed SMF to iron ore fines, limestone, dolomite, pyroxenite, coke, return fines, and calcined lime fines to obtain an iron ore sinter feed. The sinter feed of 100 kg was mixed in a mixer drum and moisture was added to convert the feed into micro balls having a mean particle size of 2. 5mm. The micro balls were transferred to a pot sinter. In all set of trials, the suction rate and the ignition flame temperature for firing the sinter in the sintering process was kept constant, 1300 mm of water column and at 1000?, respectively.
During the sintering process, the time to complete the sintering process was noted, i.e., after achieving the burn through the temperature of the sinter bed (maximum temperature of waste gas). The fired sinter was then stabilized by dropping the whole mass of sinter for 4 times from 2-meter height. After dropping, minus 5 mm fraction of sinter fines was removed and weighed and the remaining sinter was further screened in the size range -40 mm to +10 mm for a tumbler test. The sinter was then tested for Reduction degradation index and microstructural analysis.
In trial 1, sinter with iron ore containing 2.3% alumina and no SMF was prepared in the pot grate sintering set-up. In trial 2, sinter with iron ore containing 3.3% alumina and no SMF was prepared in the pot grate sintering set-up. The quality and productivity of the sinter produced in trial 1 and 2 were compared. In trials 3, 4, and 5, sinter with iron ore containing 3.3% alumina and SMF in increasing order were prepared to establish the effect of SMF in sinter making. In trial 3, 4 and 5, the SMF was added at 2%, 4% and 6 % by weight, respectively. The sinter was made with target chemistry, i.e., the basicity (CaO/SiO2) of 2.3 and MgO at 1.9 %. The coke rate in all set of trials was kept constant at 6.5 %. In trials 6 and 7, the SMF was added at 4% by weight; however, the total MgO content of the final sinter was maintained at about 1.5% and 1.3%, respectively. Table 4 provides chemical composition of sinter produced in 7 sets of trial.
Table 4: Chemical composition of sinter and other raw materials used in this study
Material T. Fe FeO CaO MgO SiO2 Al2O3 LOI
Sinter of Trial 1 55.16 9.88 11.32 1.86 4.82 2.51 0
Sinter of Trial 2 54.28 10.03 11.26 1.9 4.85 3.12 0
Sinter of Trial 3 54.62 10.45 11.41 1.88 4.89 3.17 0
Sinter of Trial 4 54.71 10.62 10.89 1.93 4.78 3.19 0
Sinter of Trial 5 54.67 10.88 11.26 1.94 4.91 3.22 0
Sinter of Trial 6 54.71 10.03 10.23 1.5 4.46 3.21 0
Sinter of Trial 7 54.71 10.06 10.16 1.3 4.38 3.11 0
Low Alumina Iron Ore 61.23 0.21 0.05 0.04 3.23 2.41 2.6
High Alumina Iron Ore 59.44 0.13 0.06 0.03 3.62 2.96 3.99
Limestone Fines 0.6 0 50.2 1.6 4.1 0.7 42.32
Dolomite Fines 2 0.08 27.18 19.24 8.19 0.49 40.01
Pyroxenite Fines 2.78 1.29 22.32 23.5 23.11 1.03 2.78
Lime Fines 0.32 - 66.35 0.88 1.54 0.79 21.24
Coke Fines/breeze - - - 0.24 5.6 5.1 81.26
The results of the test are provided in Figures 7, 8, 9, 10, and 11. Figure 7 shows that the Tumbler index dropped by 1.3 point when alumina increased from 2.3 to 3.3% in sinter. With the addition of SMF to 6% of the sinter mix, tumbler improved as compared to sinter made with 3.3% alumina. Figure 8 shows that the RDI increased by 5.2 points to 26.52 with increase in alumina and was brought down to 18.45 with the addition of 6% SMF. Figure 9 shows that the reducibility index was maintained or slightly increased by the addition of SMF. Sintering time and return fines were also increased when the alumina content increased which were decreased by the addition of SMF as shown in Figures 10 and 11.
From Figures 7-11, it can be seen that when the SMF was added in such a way that the final MgO content in sinter was kept at 1.3% and 1.5% MgO, the RDI and RI values were still comparable with the sinter made with alumina content of 3.3%. The similar effect was seen in the sintering parameters and higher temperature properties of the sinter.
It is evident that the addition of SMF to iron sinter making raw material resulted in improved quality of sinter at increased reducibility and decrease RDI even with higher level of alumina and higher crystallized water content. A decrease in required sintering temperature results in better sinter quality and productivity while using high alumina iron ore which is otherwise not achieved.
Example 5: SEM analysis of iron ore sinter
The composition of slag bond was studied using EDS which is shown in Figures 12 and 13. An average aluminium content was calculated to see how much aluminium and magnesium is going into the slag bonds to acess the extent to which SMF helps in bringing the alumina in the slag phase through melting. The study shows that the addition of SMF in sinter increases the alumina content in slag phase along with CaO, MgO and Fe2O3. This results in generation of more iron rich SFCA in sinter phase and also the silicon content in SFCA is less.