It is widely known that the direct contact between solid carbon and iron oxide shows quite a fast reduction rate in comparison with CO gas reduction. However, it is difficult to sustain the physical contact between solid carbon and iron oxide. Furthermore, the contact mode will be different from the production method, which will affect the reaction kinetics strongly. In this study, different kinds of contacts between solid carbon and hematite were made to clarify the reaction mechanism. One contact mode is the deposition of the carbon layer on the hematite surface using a vacuum evaporation method. Another method is the implantation of carbon ions into hematite. Using XPS (X-ray photoelectron spectroscopy), the binding energy of Fe–O–C was measured. Heating under an argon atmosphere was carried out. Both samples showed quite different reaction behavior, though the temperature at the beginning of the reduction reaction was almost the same. From these results, it was determined that the position of carbon will change the reaction behavior drastically, because it is important how the CO gas escapes from the reaction site.
Effects of oxides (SiO2 and Al2O3) and Carbonate (CaCO3) on the reaction of hematite and carbon obtained by the mechanical milling were studied. The content of the additives were 3 mass%. The hematite contained the oxide or carbonate was mixed with graphite, and the ratio of hematite to carbon was 80 to 20. Ball milling (alumina ball and vessel) was carried out for 24 h under an argon atmosphere. The reaction rates were measured by TG–DTA and examined the effects of oxides and carbonate on the reaction rate. The main peak of reaction rate, which corresponds to the successive reaction of Fe3O4–FeO–Fe reduction, was located at 850°C for no-addition sample. When 3 mass% Al2O3 was added to the mixture of hematite and carbon, the temperature of main peak did not change. It was considered that since the vessel and balls were also made of alumina, the effect of alumina already existed in the no-addition sample. When 3 mass% SiO2 (reagent grade) was added to the mixture, the temperature of main peak was 1000°C, which meant the retardation effect on the hematite and carbon reaction. On the other hand, 3 mass% CaCO3 addition promoted the reaction and the main peak was 790°C, which was about 60°C lower than that of no-addition. Using the results of quartz and CaO addition, the reaction mechanisms for the retardation effect of SiO2 and the acceleration effect of CaCO3 were considered.
Reduction of CO2 emissions and effective use of low-grade iron resources are very important tasks for the steel industry. One of the methods to simultaneously achieve both is to lower the thermal reserve zone temperature in the blast furnace using a high-reactivity agglomerate such as an iron ore–carbon composite or iron coke in which the iron source is an ore with high combined water content. In this study, using high water content ores, the effects of the ore particle size and coal–ore mixture ratio in a composite on the reduction behavior were investigated. The impact of the ore particle size in iron coke on the gasification behavior was also investigated. A decrease in the ore particle size in the composite and an increase in the coal–ore mixture ratio lowered the temperature at which metallic iron starts to form. However, compared with hematite ore, weaker particle size and mixture ratio dependencies were obtained for composites made from ore having high combined water content. The utilization of an ore with high combined water content as the iron source is an effective method for lowering the temperature at which the gasification of iron coke begins. This temperature is about 20 K and 50 K lower than that when using coke with hematite ore and using coke only, respectively.
Decreasing the thermal reserve zone temperature is believed to increase the energy efficiency of blast furnace iron-making. To do so, either the direct reduction reaction rate or the coke gasification reaction rate must be significantly increased below 1000°C in CO-lean atmosphere. An iron ore/carbon composite (IOC) has been proposed as a new raw material that can enhance the reduction rate of iron ore. In this paper the possibility of the reduction rate enhancement was examined by preparing a kind of ideal IOC sample consisting of nano-sized Fe2O3 particles (10–70 nm in diameter) and a carbon material prepared from a thermoplastic resin. The nano-sized Fe2O3 particles in the IOC sample were surprisingly reduced to Fe in a few minutes at as low as 650°C in an inert atmosphere. This was realized by increasing the interfacial area between the Fe2O3 particles and carbon and by establishing intimate contact between the two. The carbon in the IOC sample was also completely gasified in less than 10 minutes at as low as 750°C in a CO2 atmosphere. The dramatic enhancement of the gasification rate was found to be realized by the enhancement of one of the direct reduction reactions, the reaction between Fe3O4 particles and carbon, by the aid of in-situ XRD measurement. This suggested that the enhancement of both the reduction reaction rate and the gasification reaction rate is realized by the same mechanism. These new findings will give a clue for designing IOC samples applicable to the blast furnace iron-making.
The reduction rate of iron ore can be enhanced dramatically when iron ore/carbon composites (IOC) are prepared so as to maximize the interfacial area and to realize intimate contact between iron ore particles and carbon. An idea realizing this concept in practical blast furnace operation was proposed in this paper. Low grade iron ore that contains a large amount of goethite, FeOOH, was chosen as an iron ore, and brown coal derivative was chosen as a thermoplastic carbonaceous material for preparing IOC sample. Goethite, FeOOH, has layered structures combined by hydrogen bonds associated with OH groups. When FeOOH is heated up to 250 to 300°C, the OH groups are removed as H2O, leaving flat pore spaces of 0.8 nm wide between 20 nm thick Fe2O3 layers. The pore spaces are, however, closed over 300°C by the sintering of the Fe2O3 layers. The idea proposed is to insert the thermoplastic carbonaceous material into the pore space of 0.8 nm wide while the pore spaces are opened and to carbonize it to form carbon in the pore space below 500°C. This IOC is expected to realize intimate contact between the Fe2O3 layers and the carbon, and to enhance the reduction rate of the Fe2O3 dramatically. The validity of this idea was verified by preparing IOC samples using reagent grade FeOOH and a thermoplastic resin. Then it was shown that the IOC sample prepared from a low grade iron ore, SF. Robe River, and a thermoplastic extract deriving from a brown coal can realize the idea practically.
The coal composite iron ore hot briquette made by utilizing thermal plasticity of coal is recently developed as agglomerates without binder, which has several advantages to retain high density and strength during reaction at high temperatures. In this work, several hot briquettes mixed with coal and ore fines were prepared to elucidate influence of more coal amounts than molar ratio C/O=3/3 on their reaction behavior in a laboratory scale blast furnace simulator. Reaction and softening-melting tests of the briquettes were carried out in N2 stream under load with heating from room temperature to 1400°C. It was found that final gasification and reduction degrees decreased and increased with increasing coal contents mixed in the briquettes, respectively. The reduction finalized around 1000–1100°C. Larger proportion of coal provided completion of reduction at lower temperatures and more remained char, leading to smaller shrinkage of the briquettes even in higher temperatures. The crushing strength of briquettes after partial reaction decreased with reaction time as a whole, whereas it was kept above a value permitted in blast furnace operation. From gasification test of partial reacted briquettes, it was found that the rate constants were proportional to Fe/C molar ratio in the briquettes, likely due to a catalytic effect of metallic iron. The results will be overall discussed with the viewpoint of a feasible utilization of these briquettes to a blast furnace.
For iron making processes which need decreasing CO2 exhaust, coal composite iron ore hot briquette (CCB) is promising. This agglomerate contains coal with iron ore by utilizing the thermal plasticity of coal, where, carbonaceous material and iron ore are closely adjoined at the micro level, and which enables reactions to proceed from lower temperatures and finish earlier. In this study, we assumed mixed charging of CCB in ore bed aiming at improvements of permeability and gas utilization and performed high temperature reaction tests under load simulating blast furnace conditions using mixed samples consisted of iron ore mini pellet and briquette. As a result, gasification and reduction in mixed bed were accelerated around 910°C to 1000°C. This reaction acceleration was presumed to derive from mutual acceleration between gasification in briquette and indirect reduction in pellet. However, if the residual carbon in briquette is consumed completely by gasification, reduction is possibly delayed through the deterioration of permeability by sintering of pellet and briquette. It was also found that increase of reduction gas flow rate prevented the residual carbon in briquette from being consumed and accelerated reduction by sustaining the role of briquette as a spacer. And it was found that lowering thermal reserve zone temperature aggravated reducibility of pellet and delayed reduction of mixed bed. Moreover, it was found that shrinkage behavior of the bed had strong relation to amount of residual carbon in briquette and reduction behavior of pellet.
Reaction behavior of a packed bed consisting of reagent pellets A or iron ore mini pellets B and coal composite iron ore hot briquettes was examined under the blast furnace simulated conditions until 1200°C. As a result, it was found that reducibility of the mixed bed is governed by reducibility of pellets, whereas gasification of briquettes in the mixed bed is hardly governed by it. However, promotion effects for reduction and gasification of the mixed beds were recognized for both pellets around 1050°C, where abnormal swelling of the easier reduced pellet A occurred remarkably with fine fibrous irons, showing a noticeable pressure drop through the bed. It was suggested that this swelling would be caused by gaseous sulfur derived from coal char in the briquettes. The mixed beds provided larger final shrinkage of the beds than the individual beds probably due to increment of the coupling reactions between reduction and gasification. It was concluded that reducibility of the pellet could influence not only the reducibility of the mixed bed but also the overall reaction behavior such as gas permeability and softening.
Increased reactivity of the reducing agent is desired for the blast furnace operation with a low reducing agent. In the present study, the catalytic effects of iron, calcium oxide, and a multi component oxide melt on the gasification reaction of reducing agent were investigated. The addition of Fe, and CaO enhanced the gasification reaction. The catalytic effects of Fe and CaO were only observed in the atmospheres of around Fe/FeO equilibration and stable CaCO3 conditions, respectively. Moreover, the conditions of contact between the reducing agent and the added material affect the reactivity of the reducing agent. Molten FeOX–SiO2–Al2O3 oxide also enhances the gasification reaction of the reducing agent. The reactivity of the carbon iron ore composite supported with Fe and multi component oxides was measured. It was found that the contact between coke and iron ore in the carbon iron ore composite enhances the reactivity.
Suppression of CO2 discharged from iron- and steel- making companies is an example of the biggest issues for the protection of global environment and sustainable growth of steelmaking industry. One of the efforts made to decrease the emission of CO2 in ironmaking process is blowing of hydrogen gas into blast furnace. Hydrogen gas can reduce iron oxide and form harmless H2O. Cementite (Fe3C) may be formed by introduction of hydrogen into blast furnace and play an important role on carburization and smelting behavior of reduced iron. The conditions that Fe3C was formed were experimentally determined by changing CO–CO2–H2 gas composition in the present work. As the result, it was found that there is a possibility of Fe3C formation by hydrogen gas introduction into blast furnace. Therefore, the effect of Fe3C on smelting behavior of reduced iron was also observed, and it was confirmed that the presence of Fe3C will have positive effect on enhancing carburization and smelting of reduced iron.
The carbide capacity of CaO–SiO2–Al2O3 slags and the rate of iron carburization through successive phases of carbon, slag, and metal are measured at 1723 K. The carbide capacity of high-basicity slag is higher than that of simulated blast-furnace (BF) slag. The iron carburization rate through CaO–SiO2–Al2O3 slags is much smaller than the rate of direct carburization or that through slags including iron oxide. It is found that the rate of carburization of iron through slags increases with increasing slag basicity. The apparent rate constant of the carburization reaction is derived to be k = 2.71 × 10–5 mol/(m2 s) for the simulated BF slag and k = 8.00 × 10–5 mol/(m2 s) for the high-basicity slag. The rate increases with increasing slag basicity, increasing carbide capacity, and decreasing oxygen partial pressure.
Utilization of self reducing pellet in blast furnace is one of the effective technologies to mitigate the CO2 emissions in the steel industry. However, there are not sufficient researches on how the pellet behaves around cohesive zone. Therefore, purpose of this study is to clarify the effect of slag melting behavior on metal-slag separation behavior. In order to simulate the behavior around cohesive zone, electrolytic iron powder, carbon powder, and synthetic slag were prepared as reduced iron, residual carbon and slag components, respectively. They were well mixed as given mass ratios decided from the composition of the self reducing pellet. Different kinds of slag compositions were adopted to change properties, such as melting temperature and viscosity. The mixtures were pressed into tablets and used as experimental samples. “In-situ” observations of metal-slag separation behavior in the mixtures during constant rate heating were done by a laser microscope combined with infra-red furnace and metal-slag separation temperatures were decided. Following results were obtained. The metal-slag separation behavior was dominated largely by agglomeration behavior of liquid phase of iron. The separation surely occurred when both phases of iron and slag change to liquid phases. At the same time, the separation could also occur even if a small amount of solid phases still remained in the mixture. In this experimental condition, the effect of slag melting temperature on the separation was larger than the effect of slag's viscosity.
In the present work, we measured the density and the surface tension of CaO–SiO2–Al2O3–R2O (CaO/SiO2=0.67, Al2O3=20 mass%, R2O=10.8 mol%, R=Li, Na, K) quaternary melts at elevated temperature using the double-bob Archimedean method and the ring method, respectively. The density of the CaO–SiO2–Al2O3–R2O melts decreased with temperature due to the thermal expansion of the melts. In addition, the density of the CaO–SiO2–Al2O3 ternary melt decreased with the addition of the alkali oxides. We converted the density into the molar volume to have a consideration on the microstructure of the melts. The molar volumes of the CaO–SiO2–Al2O3–R2O melts are proportional to cube of the cationic radius for the alkali ions, which indicates the structural roles of the alkali oxides are mainly charge compensator to AlO45– anions. The surface tension of the CaO–SiO2–Al2O3–R2O melts changed only slightly with temperature. The surface tension of the CaO–SiO2–Al2O3 ternary melt increased with the addition of Li2O. In contrast, the surface tension decreased with the addition of Na2O or K2O. As for quaternary system, the surface tension increased with the ionic potential of the additive alkali ions; it is the same tendency with the binary alkali silicate melts. However, the mechanism of the surface tension change with the addition of the alkali oxides has not been clarified in the present study.
The influence that temperature has on bed pressure drop, grain size and morphology of iron fines in a hot-state visualizable fluidized bed was investigated in this work. It was found that there are both a start sticking temperature (Ts, 673 K–773 K) and a critical defluidization temperature (Tc, 923 K–973 K) of the iron particles in the fluidized bed. Not only the bed pressure drop but the agglomeration degree of the iron particles shows a strong dependence on temperature. It is also confirmed experimentally that the crystallized iron fines without whisker show a certain viscosity even at a temperature far from the melting point.
The technology of self-reducing pellets for ferro-alloys production is becoming an emerging process due to the lower electric energy consumption and the improvement of metal recovery in comparison with the traditional process. This paper presents the effects of reduction temperature, addition of ferro-silicon and addition of slag forming agents for the production of high carbon ferro-chromium by utilization of self-reducing pellets. These pellets were composed of Brazilian chromium ore (chromite) concentrate, petroleum coke, Portland cement, ferro-silicon and slag forming components (silica and hydrated lime). The pellets were processed at 1773 K, 1823 K and 1873 K using an induction furnace. The products obtained, containing slag and metallic phases, were analyzed by scanning electron microscopy and chemical analyses (XEDS). A large effect on the reduction time was observed by increasing the temperature from 1773 K to 1823 K for pellets without Fe–Si addition: around 4 times faster at 1823 K than at 1773 K for reaction fraction close to one. However, when the temperature was further increased from 1823 K to 1873 K the kinetics improved by double. At 1773 K, the addition of 2% of ferro-silicon in the pellet resulted in an increasing reaction rate of around 6 times, in comparison with agglomerate without it. The addition of fluxing agents (silica and lime), which form initial slag before the reduction is completed, impaired the full reduction. These pellets became less porous after the reduction process.
Steel production requires iron ore and coal, and accompanies greenhouse gas emissions. The annual production of the world crude steel increased from 0.9 to 1.3 billion tonnes since 2002, and is anticipated to increase due to the continuous economic growth in developing countries especially in Asia. About two thirds of the crude steel is produced by BF–BOF route whose ratio in the total crude steel production has gradually been increasing. The BF–BOF process is an excellent process in terms of productivity and energy efficiency, the process, however requires high grade iron ore and coal, and the large steel production by the BF–BOF route will accelerate the exhaustion of the high grade raw materials. The CO2 emissions are also anticipated to increase as does the steel production, since approximately 2 tonnes of CO2 is generated per tonne of steel by the BF–BOF process. On the other hand, world in-use steel stock has been increasing year by year due to the continuous large steel production, and will be recycled as a large amount of scrap to the market in the future. Steelmaking by feeding Iron Nuggets to EAF together with the scrap can significantly decrease the CO2 emissions while satisfying the steel quality by diluting the impurities containing in the scrap, as well as conserving the natural resources of high grade iron ore and coal. Steel production by using the direct reduction processes together with the scrap is a suitable and reasonable solution for a sustainable iron and steel making.
Iron-coke having various amount of M.Fe were produced in laboratory scale and the influence of M.Fe content in Iron-coke on reaction behavior under the condition simulating blast furnace has been investigated. Cold strength of Iron-coke products was decreased with an increase of mixing ratio of iron ore mostly due to a prevention of dilatation of coal particles by iron ore, resulting in weak bonding of coal particles. Nevertheless formed Iron-coke with iron ore in the fraction up to 30% would have enough strength for use in blast furnace as nut coke. Both CRI and JIS-reactivity were enhanced by increasing ratio of mixed iron ore, confirming the catalysis effect of M.Fe. The temperature at which carbon consumption started was lowered with an increase of T.Fe in coke. Formed Iron-coke containing 43% of T.Fe started reaction consuming its carbon at lower temperature than conventional coke by 150°C. Furthermore, consumed carbon ratio was improved by M.Fe installation to coke due to increasing gasification. Process evaluation with using Iron-coke in blast furnace was performed by BIS test. It was revealed that using formed Iron-coke having 43% of T.Fe for blast furnace resulted in an increase of shaft efficiency by 6.8%. It was found that to lower the reducing agent rate in blast furnace by decreasing the temperature of thermal reserve zone, lowering the beginning temperature of coke reaction was effective. Usage of Iron-coke having M.Fe catalyst within coke matrix is one of the methods.
To evaluate influences on the softening and melting behavior of blast furnace raw materials, the examinations of high temperature properties in various types of raw materials for blast furnaces were executed. Shrinking behavior before reaching approximately 1300°C is estimated by using softening viscosity taking into consideration pores of particles, and void ratio as well as liquid phase ratio of the layer. The softening viscosity of the solid phase of approximately 1010 Pa·s was obtained and the influence of the mineral species of the solid phase or temperature is small. Melting reduction sharply progresses through expanding the liquid phase deriving from FeO and gangue. And this behavior is affected with position of the reduction curve and the liquidus curve. Sinter forms a solid phase from FeO and gangue due to a rise of reduction degree according to melting reduction. This solid obstructs contact of the liquid phase including FeO with coke, which restricts the progress of melting reduction. On the other hand, with lumpy ore and pellet, melting reduction rapidly progresses without interruption, since no solid phase is formed even when FeO decreases due to melting reduction.
In future blast furnace operation which aims at low carbon consumption, it is indispensable to optimize the quality of carbonaceous and ferrous burdens, not evaluations of each individual but both sides of them considering interactions under the coexistence. Therefore, a simultaneous evaluation method of carbonaceous and ferrous burdens at cohensive zone of blast furnaces by softening-melting test which simulated the temperature profile in blast furnaces determined by reactivity of coke was developed. The effect of sinter ore reducibility and coke reactivity on sinter soften-melting property at cohensive zone of blast furnaces were evaluated. With increasing CRI, coke reactivity, gasification start temperature lowers as a result of increasing the reduction rate of sinter at 900–1000°C and increasing softening shrinkage resistance at the initial stage of the shrinkage near 1200°C.
Practical ways of estimating temperatures of coke flowing into raceway as well as the gas core depth validated by direct measurement were suggested for the purpose of better describing the combustion behaviors of injected coal fine in blast furnace raceway. The numerical simulation model was refurnished with the scheme, and helped clarifying an intriguing effect of coal gasification or decomposition degrees before exiting tuyere tip on attained combustibility of coal at raceway boundary. Another scheme for quantitative expression of pressure losses caused exclusively by coal fine residence in and around raceway was also developed by shutting off coal injection for minutes at practical operation. The obtained results lead to developing the advanced tuyere, CD (converged and diverged) tuyere which enabled reducing the interaction between injectants and hot blast inside tuyere and thus improving overall gas permeability even under conditions of massive coal injection over 250 kg/tHM.
Top gas recycling is considered as one of the highest potential technologies to improve reduction efficiency and correspondingly to reduce carbon consumption. As a typical nitrogen free ironmaking process, pre-reduction shaft furnace of COREX® process (COREX® shaft furnace) for short is suitable to adopt the technology aiming to cut down CO2 emission. Under the premise of constant total injection volume, three kinds of reducing gas injection methods are numerically studied by employing a two-dimensional mathematical model. The method that 20% of total reducing gas in volume fraction is blasted through normal inlet (NI) while the rest through down pipe inlet (PI) rather than deadman inlet (DI) could apparently improve gas flow in the inactive zone located near the bottom direct reduced iron (DRI) outlet, thus increasing DRI reduction degree to 61% under present calculation conditions. Meanwhile, either decreasing the vertical height of PI or increasing its diameter makes further improvement on furnace efficiency. After adopting top gas recycling to the shaft furnace by NI+PI method with optimal parameters, CO utilization ratio reaches above 46% when DRI reduction degree correspondingly increases by 12%, what's more, CO2 emission from the whole process is reduced by about 540 Nm3/tHM. The results prove that top gas recycling technology promotes reduction efficiency inside shaft furnace and greatly reduces the greenhouse gas emission, which will contribute to suppressing global warming.
Because the cupola does not require a reducing agent, CO2 generation can be greatly reduced in comparison with the blast furnace process. Moreover, because the latent heat of the cupola off-gas can be utilized effectively in steel plants by blast furnace gas recovery equipment, the cupola was introduced in JFE Steel East Japan Works. In this report, the effects of coke diameter and oxygen concentration of blast on the reaction rates of coke combustion, coke gasification and carburization in the cupola were studied. Operation was simulated from the viewpoints of these reactions and heat transfer between coke/scrap and shaft gas. In addition, the changes in the off-gas composition, coke rate, hot metal composition, were observed in an operating small cupola under several conditions of coke diameter and oxygen concentration of blast.
A controlling method of the radial mixed coke ratio distribution under high coke mixed charging, which is called FCG (Flow Control Gate) dynamic control method was studied with the aim of stable operation with high productivity and low RAR at Chiba No. 6 blast furnace. For this purpose, scale model experiments and mathematical burden distribution model calculations were performed. The effects of FCG dynamic control with ore and coke simultaneous discharging from the respective top bunkers on the mixed coke ratio distribution were examined and applied to Chiba No. 6 blast furnace. After application of FCG dynamic control, improvements of gas utilization efficiency: +0.2%, gas permeability at cohesive zone (the part of lower shaft): –14.7% and coke ratio: –4.2 kg/t (with constant RAR) were confirmed. Since June 2007, high productivity operation with low RAR has been conducted at Chiba No. 6 blast furnace.