Granulation experiment was carried out by using four kinds of iron ores, and the compressive strength of granules was measured. Moreover, inducement mechanism of compressive strength of iron ore granule was investigated by adding clay or reagent hematite. The results obtained are follows:
(1) From the results of strength measurement of granule made from iron ores which have different kinds of gangue mineral and have different amount of gangue, in the case of Ore A which has little gangue quantity, the wet strength exceeded the dry strength. On the other hand, the dry strength of Ore B, C and D including the gangue minerals exceeded the wet strength.
(2) The wet strength of granules was not changed when clay or reagent hematite was added, even if the amount of added clay and reagent hematite increased. It is thought that the effect of gangue mineral and particle size on the wet strength are small, and the adhesion force between particles by liquid bridge is dominant.
(3) The dry strength of granule was improved only when clay was added. It is considered that clay mineral and iron ore adhered with electric action when clay mineral and iron ore closed with van der Waals forces range in the process of drying the water because clay mineral was charged negative and iron ore was charged positive.
Effect of ultrafine powder of Hematite and Magnetite on granulation characteristics of iron oxide was investigated by mixing and exterior coating granulation. The results obtained are follows:
(1) In the case of Hematite Ore H, particle diameter of granule is increase with increasing the amount of fine powder in both mixing granulation and exterior coating granulation. On the other hand, in the case of Magnetite Ore M, particle diameter of granule is increase with increasing the amount of fine powder in mixing granulation. In the case of exterior coating granulation using Magnetite Ore M, particle diameter of granule is increase with increasing until 20 mass% of added fine powder but particle diameter of granule is decrease when the amount of fine powder added 30 mass%.
(2) The particle diameter dependence of wet strength is relatively small in both mixing and exterior coating granulation. Except for 30 mass% of added fine Magnetite ore M in exterior coating granulation, wet strength of granule is not increase even if the amount of added fine Hematite Ore H and Magnetite ore M is increase. Therefore, it is considered that wet strength of granule is determined by adhesion force of liquid bridge.
(3) In mixing granulation, the particle diameter dependence of dry strength is relatively small. In the case of exterior coating granulation, dry strength of granule is decrease with increasing the particle diameter of granule. The dry strength of granule made from mixing granulation is larger than that of exterior coating granulation.
In order to decrease slag (SiO2, Al2O3 etc.) contents in sinter ore, it is an effective option to increase ratio of ultra fine iron ores called as Concentrate (Conc.) or Pellet Feeds (PF) in sintering mixture owing to its relatively lower slag contents than typical Sinter Feeds (SF), which, however, has a drawback to decrease the permeability of the sintering bed and sinter productivity.
This report has investigated the optimal ratio of PF/SF in a separate granulation process in terms of the productivity by means of pot sintering test with granules observation, and plant trial.
The pot test (300 mm diameter), where approximately 20% of raw materials, which include the PF, were mixed by high speed intensive mixer and then granulated by pan pelletizer, showed that the optimal PF/SF ratio was 3/1. The observation of granules revealed that P-type granules began to increase from the optimal ratio on up.
The plant trial of 8 hours at Wakayama No.5 sinter plant confirmed that the productivity came to maximum at the PF/SF ratio of 3/1 under using 20 mass% of PF in the sinter mix.
The influence of agitating conditions on agglomeration and collapse of wet iron ore mixture was investigated in the view of kinetics and matrix model analysis. At the initial stage of mixing behavior, it was found that average particle size was dependent on the mixing rate constant defined as the deviation degree of particle size and water distribution from initial state. Mixing rate constants of powder and water were almost consistent with each other and expressed by power function of Froude number of impeller. It was presumed that the water and fine particle moved together as wet aggregates during mixing at a given water level. According to the analysis of entire mixing behavior based on matrix model, it was found that the collapse indexes defined by matrix parameters increased as particle size and impact force increased. Minimum particle size at initial mixing state decreased as collapse index increased and the size of long term mixing state was expected by intrinsic increasing rates defined by maximum eigenvalue of matrix parameters.
Mill Scale has higher iron contents and produces heat by the oxidation reaction in the sintering process. For this reason, it is expected that use of Mill Scale in the sinter operation will reduce the amount of coke breeze consumption. For the purpose of examining the influence of Mill Scale on melt penetrability and sinterability, we carried out melt penetration tests and sinter pot tests.
Melt penetrability in the sintering process is an important factor that affects the quality of iron ore sinter. Our melt penetration tests lead following conclusions.
Melt penetrability of Mill Scale is so high that increased blending ratio of Mill Scale causes lower permeability, which eventually leads to lower productivity. We, however, anticipated that we may be able to control the melt penetrability if we blend Mill Scale closely with iron ore which has the nature of lower melt penetrability. We arranged that a higher ratio of Mill Scale can be blended with such iron ore in proximity in the preparatory granulation method. We used the granular in the pot tests.
In the result, oxidation reaction of Mill Scale was inhibited because Mill Scale contained in the pseudo-particle had reduced contact area with air. We confirmed that both permeability and productivity were improved by the method to control the melt penetrability mentioned above, even at higher blending ratio of Mill Scale.
For increase of sinter productivity, it is important to design sinter mixture granulation.
Moisture is indispensable for granulation as a binder between raw material particles. Once granulation is completed, moisture is dispensable during sintering because moisture vaporization is endothermic reaction.
Based on the above-mentioned view, a process of drying the granules after granulation with high moisture examined for sintering productivity by use of sinter pot test.
The main results obtained are described as follows:
(1) Drying in conjunction with high moisture granulation is effective to increase flame front speed with maintaining sintering yield;
(2) Increasing flame front speed is due to shorten the time to evaporate moisture in the wet zone of sintering packed bed in addition to increasing permeability of sintering packed bed. This effect is also evaluated and proved based on calculation of moisture transition in and out of sintering packed bed;
(3) Maintaining sintering yield is due to higher heat generation in sintering packed bed caused by higher coke combustion efficiency in addition to lower moisture concentration of sinter mixture.
(4) Collapse of granules in case of drying after granulation is avoided till the critical moisture, that is defined as the one left in the mix after higher moisture granulation makes granules to keep shape easy due to higher moisture quantity on the surface of granules.
The influence of agitating in pelletizer at HPS process on granulation and sintering properties was investigated with using typical iron ores and pellet feeds. Pellet feeds added in the sintering mixture easily formed the large aggregates bearing large amount of water and other portion of pellet feeds remained in the small size fraction in quasi-particle. The large quasi-particles generated at HPS process were mainly observed at the specific area in pelletizer due to size segregation effect. These large quasi-particles could be destructed selectively by agitating motion of impeller, resulting in achieving not only uniform quasi-particle size distribution after pelletizing but also improvement of granularity after following coke breeze coating. HPS process with agitating method showed higher permeability due to the decrease in charging density, leading to improvement of productivity. In addition, product yield also improved in spite of higher flow rate compared to conventional HPS process. Based on material and heat balance analysis, it was found that combustion efficiency improved by agitating treatment. Heat transfer from combustion heat to sensible heat of sintering cake was enhanced by agitating. In terms of rotation conditions, it was found that particle flow pattern was changed greatly and destruction efficiency decreased at the higher impeller speed according to DEM simulation. The optimum rotation speed observed in the experiment could be explained by the impact force to quasi-particle and macroscopic flow dynamics.
In order to improve particles of cokes in the sintered layer, we investigated the combustion behavior of coated cokes with fine Fe3O4 and CaO at 1173 K, 1273 K and 1473 K of experimental temperature. The reaction rates of the coated cokes were calculated by weight loss measurements using the thermo-balance electric combustion furnace. It was found that the reaction rates of the coated cokes with fine iron oxide (Fe3O4 or Fe2O3) and CaO were higher than rates of any other coated samples at almost conditions. Temperature dependence and generated CO2 gas volume of samples during combustion were measured at 1173 K and 1273 K. The temperature of coated cokes with fine iron oxide (Fe3O4 or Fe2O3) and CaO were found to be rising over 1600 K in a few minutes at 1273 K of experimental temperature. Moreover, the generated CO2 gas volume of samples were also drastically rising by the catalytic influence of CaO. The cross-sectional SEM images of coated cokes with fine iron oxide (Fe3O4 or Fe2O3) and CaO were observed. It was found that the coated layer was partially melted and surface area of coke was appeared.
The motions of particles and gas in the nearly full scale sintering beds were simulated to elucidate the effect of the magnetite ore blending on the particle agglomeration and the large scale crack formation by the simultaneous calculation of the Navier-Stokes equations and the Lagrangian DEM equations based on the sintering model in which the phase change of particles and the cohesion forces and the resistance forces due to the liquid films among particles were considered. In this study containing ratios of magnetite ores are varied from 0 mass% to 19.3 mass%. Calculated results show that the large agglomerates and cracks are not formed and the homogeneous configurations of particles are produced when the containing ratio of magnetite ores is 19.3 mass%. In the sintering bed with high magnetite containing ratio, for example 19.3 mass%, two or more magnetite ore particles contact to one hematite ore particle on average. This increases the liquid region in which the large viscous interaction forces act among close particles and those forces reduce the particle relative velocities which cause the particle collisions and agglomerations. Magnetite ore particles go into voids by their relatively high mobility in the melting zone and produce the uniform sintering beds. In this condition the final bed shrink ratio which is defined as the ratio of the final bed height and the initial bed height is 85% and it is considered that the sintering cake has the sufficient porosity for air flows.
Iso-contours of particle vertical velocities in sintering beds: (a) i calculated results without magnetite ore particle, (b) i calculated results with 5 mass% magnetite ore particles, (c) i calculated results with 10 mass% magnetite ore particles and (d) i calculated results with 19.3 mass% magnetite ore particles, where i=1, 2, 3 and 4 denote T=8 s, 16 s, 20 s and 28 s, respectively.
The motions of particles and gas in the nearly full scale sintering beds were simulated to elucidate the effect of melting volume fraction of magnetite ores on the particle agglomeration and the large scale crack formation by the simultaneous calculation of the Navier-Stokes equations and the Lagrangian DEM equations based on the sintering model in which the phase change of particles and the cohesion forces and the resistance forces due to the liquid films among particles were considered. In this study melting volume fractions of magnetite ores are varied from 60% to 88%. Calculated results show that the large agglomerates and the large scale cracks are not formed when melting volume fractions of magnetite ores become large because the large melting volume fraction of magnetite ores increases the liquid region in which the large viscous interaction forces act among close particles. If the total volume of liquid phase in the melting zone is the same, it is effective to prevent the large scale crack when the liquid phase increases uniformly rather than locally. We also examined the effect of friction forces in the melting zone on large scale cracks under the condition without magnetite ore. Calculated results show that the small friction forces prevent the formations of large scale agglomerates and cracks because the small friction forces become easy to compress the particle layers due to the weight of the particle bed and the drag force caused by air flows passing through the particle beds.
Iso-contours of contact forces between particles in sintering beds: (a) i calculated results without magnetite ore particle, (b) i calculated results with 19.3 mass% magnetite ore particles whose melting volume fractions are 60%, (c) i calculated results with 19.3 mass% magnetite ore particles whose melting volume fractions are 70% and (d) i calculated results with 19.3 mass% magnetite ore particles whose melting volume fractions are 88%, where i=1, 2, 3 and 4 denote T=8 s, 16 s, 20 s and 28 s, respectively.
Recently, the quality of sinter feed ore used in sintering process has deteriorated. In particular, T.Fe has decreased and gangue component has increased in the sinter feed ore. Increase of gangue is not only the factor to influence sinter qualities, but also the factor to increase coke ratio in the blast furnace operation as the increase of slag ratio. Therefore, to cope with the deterioration of iron ore qualities, studies on alternative iron ore resources and development of its utilization technology have been required.
In that kind of new iron ore resources, authors focus on high grade magnetite fine. In the past, there are some studies about the effect of mixing ratio and size of magnetite fine on productivity and quality, but there are few studies about magnetite fine segregation in charging. In addition, magnetite fine decreases sinter productivity by the decrease of permeability of sintering bed. A new study for using large amount of magnetite fine is required.
In this research, the control method of magnetite fine segregation by magnetic force at charging and the improvement of sinter productivity by this method was studied. The effect of upper segregation of magnetite fine was studied through the analysis of melting behavior and interfacial reaction of calcium ferrite melts into hematite substrate and magnetite substrate.
A situation of sintering raw materials becomes more severe in this century. In the sintering process, magnetite iron ore is expected as high grade and condensation raw materials. However, there are not sufficient researches on the solid-solid reactions between magnetite and CaO in the sintering process. Therefore, purpose of this study is to clarify the initial liquid formation behavior by investigating the solid-solid reactions between magnetite and CaO under Ar or air atmosphere.
In order to simply simulate the sintering raw materials, mixture samples of iron oxide and CaO were prepared. Reagent magnetite and reagent hematite were prepared as iron oxide. The mixtures, which have the weight ratio of iron oxide/CaO=80/20, 85/15 and 90/10, were pressed into tablet shape and used as experimental samples. “In-situ” observations of the initial liquid formation behavior in the samples were carried out by a laser microscope combined with infra-red furnace. The samples were heated at 50°C/min under Ar or air atmosphere. The initial liquid formation temperatures were decided from this observation.
In the case under Ar atmosphere, the initial liquid formation temperature of magnetite-CaO mixture samples became higher than that of hematite-CaO mixture samples. This difference was caused from CaO·FeO·Fe2O3 formation before liquid formation between magnetite and CaO. In the case under air atmosphere, the initial liquid formation temperature of magnetite-CaO mixture samples became lower than that of hematite-CaO mixture samples, and the temperatures were decreased with the increasing of magnetite. This reason was because 4CaO·FeO·8Fe2O3 was formed by the solid-solid reactions between magnetite and CaO. It was found that the sintering atmosphere and magnetite blending ratio to raw materials have the influence on the initial liquid formation temperature when the magnetite iron ore is used in the sintering process.
This study has been performed to understand the reaction behavior of wüstite particles and the effect of existing state of CaO component in the iron ore sintering bed for their effective utilization as an agglomeration agent. Changes in the structure and pressure drops of a sintering bed were measured by using a laboratory-scale sintering simulator.
When wüstite particles were mixed with model pellet of raw materials, pressure drop of the sintering bed did not show significant change independent of average CaO content between 5 and 10 mass% CaO. This is because most of wüstite particles and model pellets kept their initial shapes and therefore the structural change of the sintering bed did not occur. On the contrary, pressure drop of the bed changed drastically when wüstite particles were mixed with CaO particles with 1.0 - 2.0 mm in particles size. In these cases, wüstite and CaO particles were melted and agglomerated each other. Addition of CaO component at the vicinity of the wüstite particles seems to decrease in liquidus temperature of local composition and promote the melt formation. In order to effectively utilize wüstite containing materials as an agglomeration agent, it is essential to arrange sufficient amount of CaO component close to such particles for melt formation at lower temperature.
This study was carried out to examine the way to suppress the CO2 emissions from the iron ore sintering process through effectively utilizing the oxidation heat of iron containing material, namely iron-bearing agglomeration agent. Increase in the oxidation ratio of iron-bearing agglomeration agent in the process will lead to decrease in the amount of CO2 emissions. In order to increase the oxidation ratio, the effect of melt formation in the iron oxide layer on the oxidation reaction of metallic iron particle covered by adhering layer of CaO, SiO2 and magnetite ore was discussed. Changes in the O2 concentration in outlet gas and sample bed temperature were measured by using a laboratory scale sintering simulator. Oxidation ratios of samples were calculated by the concentration of outlet gas.
Oxidation ratio of metallic iron particles with adhering layer was higher than that of simple metallic iron particles. Melt formation of the iron oxide layer at surface of metallic iron particles would promote the oxidation compared to thatat solid state. When CaO concentration of the adhering layer was increased, oxidation ratio of metallic iron particle was increased until 50 mass%-CaO. In the case of CaO concentration more than 50 mass%, however, oxidation ratio was kept approximately 0.9. It suggests that an excess amount of melt formed on the surface of metallic iron particle seems to prevent the diffusion of oxygen. Adjusting the composition and amount of melt formed on the surface of iron-bearing agglomeration agent would lead higher reaction ratio.
This study has been performed to understand the characteristic of KR slag, which is a by-product of the desulfurization process of steel, aiming at its effective utilization in the iron ore sintering process. KR slag contains CaO, metallic iron and FeO component and has been partly utilized in the sintering process as CaO source. However, its oxidation behavior has not been evaluated. Effective utilization of KR slag will lead to reduction of CO2 emission by decreasing the amount of coke breeze and increase the ratio of formed melt during sintering.
Oxidation and melting behaviors of KR slag in the sintering bed were examined by using a laboratory-scale sintering simulator.
Higher oxidation ratio was obtained for KR slag than that for metallic iron particle. Iron oxide layer formed on the surface of iron particles in KR slag were easily melted and it would keep the oxidation rate of metallic iron at a certain level. The effect of mixing ratio of KR slag on the structure and permeability of the sintering bed was examined. The amount of formed melt increased with increase in the mixing ratio of KR slag.
On the other hand, when the melt ratio became to be above 15%, bed permeability was significantly decreased. This would be caused by blocking the gas flow path in the sintering bed with the excess amount of melt. Therefore, mixing ratio of KR slag is necessary to control the melt ratio below 15%.
This study has been performed to clarify the oxidation characteristics of iron-bearing agglomeration agents charged with coke in the iron ore sintering bed as an approach toward the reduction of CO2 emissions from the sintering process. A series of oxidation experiments were carried out for metallic iron particles and magnetite concentrate pellets using laboratory-scale sintering simulator, which gave the changes in their O2 consumption rate and reaction ratios during sintering.
When metallic iron particles were used with coke, oxidation reaction of metallic iron was suppressed at an initial few seconds in the case of 25% metallic iron addition. However, it was not suppressed in the case of the higher metallic iron addition. The final oxidation ratio of metallic irons was approximately 0.8 in any case of the mixing ratio.
When magnetite pellet and that containing 20 mass%CaO were used as agglomeration agent for sintering, reaction ratio of the former was always higher than that of the latter. It suggests that melt-formation led to a decrease in the specific surface area for the oxidation reaction. When magnetite pellets were charged with coke, oxidation of magnetite was suppressed by a decrease in the oxygen partial pressure around the burning coke particles.
The performance of iron ore sinter in a blast furnace such as strength and reducibility is strongly affected by the amount and the chemical composition of liquid phase in sinter during the sintering process. Thus, it is necessary to control the production of liquid phase during the sintering process. Iron ore sintering process consists of rapid heating of pulverized raw materials, partial reduction reaction of iron ores as well as liquid phase production, which are caused by combustion of cokes and suction of flaming gas. The heating time of raw materials is too short that the chemical reactions within the sinter never reach the thermodynamic equilibriums. Nevertheless, controlling the melt production could be attempted based on the thermodynamic data such as the phase diagrams. In this study, liquidus lines coexisting with iron oxides and/or 2CaO·SiO2 in the FeOx-CaO-SiO2 system have been measured at 1573 K in the range of oxygen partial pressures between 10–6 atm and 10–2 atm. In addition, the effect of Al2O3 content on the liquidus has also been investigated as Al2O3 is a major gangue component affecting the quality of sinter. It has been found that the homogenous liquid region extends over a wide range of CaO/SiO2 (C/S) ratio between 0.6 and 2 or even more for all the measurement oxygen partial pressures. It has also been found that the FeOx content in liquid phase at C/S=1.0 in equilibrium with solid FeOx is almost constant to be 40-45 mass% irrespective of oxygen partial pressures. However, the FeOx content decreases with an increase in Al2O3 content, indicating that the homogenous liquid region in the FeOx-rich side becomes narrower. As a consequence, it is considered that the amount of slag melt produced in sinter during the sintering process becomes smaller with increasing Al2O3 content.
Oxidation states and coordination structures of iron ions in FeOx-CaO-SiO2 slags and their dependencies on the oxygen partial pressures and slag compositions have been measured by Mössbauer spectroscopy. The relationship between local structural environment of iron ions and FeO1.33 activities has also been investigated.
Variation in the slag composition causes Fe ions to have such a local structural environment that the degree of polymerization of the whole silicate network structure is retained. The ratio of the number of Fe3+ in the octahedral coordination to that of Fe3+ in the tetrahedral coordination (xFe3+(Oh) / xFe3+(Td)) converges to the range between 0.5 to 1.0 with decreasing the degree of polymerization of matrix silicate network structure. Since the xFe3+(Oh) / xFe3+(Td) values of Fe3O4 and Ca2Fe2O5 crystals are reported to be 0.5 and 1, respectively, it is considered that Fe ions tend to form microcrystalline clusters having the similar structures to those of Fe3O4 and/or Ca2Fe2O5 in the slags with xFe3+(Oh) / xFe3+(Td) of 0.5 to 1.0.
By comparing the oxidation states and coordination structures of iron ions with FeO1.33 activities measured by authors elsewhere, it has been found that FeO1.33 activities divided by FeOx concentraions (γFeO1.33) are the largest in the slags with xFe3+(Oh) / xFe3+(Td) of 0.5 to 1.0. The larger γFeO1.33 values correspond to the poorer affinity of iron oxides with matrix silicates. As a consequence, it is considered that the formation of microcrystalline clusters having the similar structures to those of Fe3O4 and/or Ca2Fe2O5 results in the increase in the γFeO1.33 values, i.e., the FeO1.33 activities.
In general, Fe content in iron ore is gradually decreasing. This fact affects worse performance of BF operation, for example, increase of RAR and Slag ratio. Depletion of high grade iron ore deposits is moving us to use concentrates in sintering process.
Through magnetite concentration deteriorates reducibility because of high FeO content in sinter product. Such situation makes it to promote oxidation of magnetite iron ore during sintering process for improving sinter reducibility. In addition, promoting oxidation of magnetite possibly increases sinter strength with using oxidation heat is exothermal reaction.
ISIJ sinter research group for utilization of magnetite concentration suggests that restricting melt formation is critical for promoting oxidation of magnetite concentration.
In this paper, It is confirmed that “Separate Granulation” has been examined to apply their suggestion by sinter pot test.
The main results obtained are described as follows:
(1) “Separate Granulation” in case that magnetite is pre-granulated with high Al2O3 iron ore without limestone and coke breeze resulted in decrease of FeO in sinter and improvement of both sinter reducibility and sinter strength.
(2) Sinter micro structure featured restriction of pore, low circle factor and small mineral texture, which supported that melting restriction worked during sintering.
(3) Magnetite decreased and hematite increased as sinter mineral, which corresponded with decrease of FeO content.
(4) These facts shown (1) to (3) concluded that “Separate Granulation” is effective to improve both sinter reducibility and sinter strength due to restriction of melting during sinter reaction.
Quantification techniques of mineral phases in iron ore sinter has been developed by using Rietveld analysis of X-ray diffraction patterns and applied to iron ore sinters made by the sinter pot test in a laboratory. Rietveld refinements successfully determined phase fractions and structural parameters in co-existing phases such as hematite (α-Fe2O3), magnetite (Fe3O4), multi-component calcium-ferrites (SFCA: Ca2 (Ca, Fe, Al)6 (Fe, Al, Si)6O20 and SFCA-I: Ca2(Ca, Fe)3 (Ca, Fe, Al)8 (Fe, Al)8 O20) and other minor phases.
It was found that strength of iron ore sinter was correlated with quantity of magnetite, and not with quantity of calcium-ferrites. And the lattice constants of calcium-ferrites were different among the specimens obtained from different positions within the sinter.
Rietveld analysis was shown to be a straightforward and high-throughput technique for quantification of mineral phases in the iron ore sinter and the obtained crystallographic parameters can be used as a useful indicator to investigate the sinter reaction.