Final Reduction Degree Research Articles

Hydrogen reduction of ilmenite: effects of oxidation

ABSTRACT The effects of oxidation on H 2 reduction were studied on two different ilmenite ores, Ilmenite A from Senegal and Ilmenite B from South Africa. Both ores were oxidised at 700, 800, and 1000 ∘ C for 1 hour. Both ores were reduced at 800 ∘ C, while Ilmenite A was also reduced at 900, and 1000 ∘ C. Varying reduction times, 5, 15, 35, and 60 minutes, were chosen to examine the reduction path. It was found that the oxidation extent and rate were dependant on temperature. The rate of oxidation increased with increasing temperature. At 1000 ∘ C, complete oxidation was found. The samples mainly consisted of pseudobrookite. Ilmenite B underwent greater changes during oxidation due to a lower degree of weathering prior. The different degrees of weathering was one of the reasons why as-received Ilmenite B did not reach complete reduction at 800 ∘ C, while Ilmenite A did. Oxidation was seen to increase the reduction rates. The final reduction degree decreased for the more oxidised samples, at reduction temperatures of 1000 ∘ C. The extent of reduction was measured by determining the O:Fe ratio. The zero point of the ratio is set to complete metellisation of the iron, giving O:Fe=0. If this ratio is 1, then O bonded to Ti was also removed. As-received Ilmenite A reduced at 1000 ∘ C had a O:Fe ratio of −0.25, whereas 1000 ∘ C oxidised Ilmenite A had a ratio of −0.16. This suggest that, in both cases, some higher Ti-oxides were reduces to lower oxides.

Performance improvement of the H2 shaft furnace: A numerical study on hot-charge operation

This work proposes a retrofitted gas feed system for increasing thermal energy supply when the hydrogen-operated direct reduction shaft furnace adopts hot-charge operation. The potential of the new feed system is assessed further using a two-dimensional computational fluid dynamics model. The results show that the ascending gas is heated up quickly by the descending hot solid within a narrow uppermost zone when employing the conventional gas feed system, so the opportunities to obtain better furnace performance are limited since the thermal energy of the hot solid leaves the furnace in the form of high-temperature off-gas. Under a scenario adopting the hot-charge operation in conjunction with the new feed system, a high temperature favoring the endothermic reactions is maintained within almost the entire upper part of the furnace. Using an appropriate upper-row gas feed rate, the overall furnace performance can be substantially improved particularly with respect to final solid reduction degree.

Effects of Room-Temperature Center Gas Distributor Injection on the H2 Shaft Furnace Process: A Numerical Study

In the current work, a computational fluid dynamics-based model was utilized to investigate the performance of the H2 shaft furnace under a scenario where room-temperature H2 is injected through a center gas distributor (CGD) installed at the unit bottom. Modelling was conducted to simulate scenarios where the CGD operation is applied with different feed gas rates (ranging from 0 to 250 Nm3/t-pellet). The results showed that a high temperature level and thus a better internal thermochemical state can be maintained with a proper CGD gas feed rate. However, an overly high CGD feed rate (being 150 Nm3/t-pellet or a higher value) induces a detrimental scenario where the thermal energy recycled by the room-temperature CGD gas is insufficient to compensate for the decrease of sensible heat of the preheated feed gas from the bustle-pipe. This eventually results in a noteworthy chemical reserve zone of high H2 content and little solid reduction in the furnace center. A large quantity of H2 consequently remains unutilized and leaves the furnace from the top. Under the investigated conditions, the final solid reduction degree rises to maximal value when the CGD gas feed rate is 100 Nm3/t-pellet. The findings of this work revealed that the room-temperature CGD gas injection operation holds significant promise for practical applications.

Effect of reduction behavior from Fe2O3 to FeO on the formation of metallic Fe in multi-stage reduction

The current study investigated the reduction behavior of iron ore using hydrogen in four-stage reduction processes. Reduction reactions from Fe2O3 to Fe3O4 and from Fe3O4 to FeO proceeded at the first and second stages, respectively. The third and fourth stages subsequently reduced FeO to metallic iron. Iron ore mixture of hematite and goethite achieved the final reduction degree to be 87% by multi-stage reduction processes. As the duration of reduction at the first stage was decreased, the final reduction degree was improved. The first reduction stage produced the most stable iron oxides as magnetite, while the second reduction stage corresponded to the reduction condition where wustite is the most stable. Decreasing the duration time of the first reduction stage resulted in the decrease of stable magnetite formation, consequently increasing the surface area of reduced iron ore. The omission of the first reduction stage resulted in the highest degree of final reduction of 95%. Kinetic analyses for each reduction reaction were performed to derive the optimal reduction conditions. The reduction of hematite to magnetite at the initial stage of iron oxide was controlled by diffusion, and the rate-controlling step of reduction was gradually shifted to chemical reaction in the reduction from FeO to metallic iron.

A Numerical Study on the Process of the H2 Shaft Furnace Equipped with a Center Gas Distributor

In order to explore technically feasible options for improving the performance of the H2 shaft furnace (HSF), a previously built and validated computational fluid dynamics (CFD) model was employed in the current work to assess the potential of the operation based on a center gas distributor (CGD). A set of simulations was performed to mimic scenarios where different amounts of feed gas (0–30% of 1400 Nm3/t-pellet) are injected via the CGD located at the bottom of the HSF. The results showed that a relatively large stagnant zone (approximately 8.0-m in height and 0.3-m in diameter) exists in the furnace center where the gas flows are weak owing to an overly shortened penetration depth of the H2 stream solely injected from the circumferentially installed bustle-pipe. When adopting the CGD operation, however, the center gas flows can be effectively enhanced, consequently squeezing the stagnant zone and thus leading to a better overall performance of the HSF. In particular, the uniformity of the final reduction degree (mean values ranging from 0.8846 to 0.8896) of the solid phase (i.e., pellets) is well improved under the investigated condition where the total gas feed rate is fixed at 1400 Nm3/t-pellet. As for the final mean reduction degree of solid and top gas utilization degree, the two performance indicators rise to maximal values when the CGD feed ratio is increased to 20% and then slightly drop with a further increase in the ratio.

Thermodynamic studies on gas-based reduction of vanadium titano-magnetite pellets

Numerous studies have focused on the reduction thermodynamics of ordinary iron ore; by contrast, the literature contains few thermodynamic studies on the gas-based reduction of vanadium titano-magnetite (VTM) in mixed atmospheres of H2, CO, H2O, CO2, and N2. In this paper, thermodynamic studies on the reduction of oxidized VTM pellets were systematically conducted in an atmosphere of a C–H–O system as a reducing agent. The results indicate that VTM of an equivalent valence state is more difficult to reduce than ordinary iron ore. A reduction equilibrium diagram using the C–H–O system as a reducing agent was obtained; it clearly describes the reduction process. Experiments were performed to investigate the effects of the reduction temperature, the gas composition, and two types of iron ores on the reduction of oxidized VTM pellets. The results show that the final reduction degree increases with increasing reduction temperature, increasing molar ratio of H2/(H2 + CO), and decreasing H2O, CO2, and N2 contents. In addition, the reduction processes under various conditions are discussed. All of the results of the reduction experiments are consistent with those of theoretical thermodynamic analysis. This study is expected to provide valuable thermodynamic theory on the industrial applications of VTM.

Numerical analysis of the effects of magnetite properties and injection types on flash reduction behaviour

ABSTRACT In this work, a three-dimensional Eulerian–Lagrangian CFD model is developed to describe the flash reduction behaviour of magnetite in the drop tube reactor and investigate the effects of magnetite properties and injection types on particle dispersion and reduction. The results confirm that increasing final reduction degree can be achieved for concentrate particles with smaller particle size, lower particle shape factor and less particle moisture. The particles in cone injection may obtain higher final reduction degree than in surface injection. The particle trajectories show that the spreading extent of particles in the reactor enhances with the decrease of particle size and shape factor and the increase of cone angle. In addition, the evolution of particle characteristics along the reactor length are also analysed, in terms of particle temperature, particle velocity and reduction degree, aiming to understand the underlying influence mechanisms of these parameters on reduction process.

Carbothermic reduction characteristics of ludwigite and boron–iron magnetic separation

Ludwigite is a kind of complex iron ore containing boron, iron, and magnesium, and it is the most promising boron resource in China. Selective reduction of iron oxide is the key step for the comprehensive utilization of ludwigite. In the present work, the reduction mechanism of ludwigite was investigated. The thermogravimetry and differential scanning calorimetry analysis and isothermal reduction of ludwigite/coal composite pellet were performed. Ludwigite yielded a lower reduction starting temperature and a higher final reduction degree compared with the traditional iron concentrates. Higher specific surface area and more fine cracks might be the main reasons for the better reducibility of ludwigite. Reducing temperature highly affected the reaction fraction and microstructure of the reduced pellets, which are closely related to the separation degree of boron and iron. Increasing reducing temperature benefited the boron and iron magnetic separation. Optimum magnetic separation results could be obtained when the pellet was reduced at 1300°C. The separated boron-rich non-magnetic concentrate presented poor crystalline structure, and its extraction efficiency for boron reached 64.3%. The obtained experimental results can provide reference for the determination of the comprehensive utilization flow sheet of ludwigite.

Modeling and Experimental Study of Ore-Carbon Briquette Reduction under CO–CO2 Atmosphere

Iron ore-carbon briquette is often used as the feed material in the production of sponge iron via coal-based direct reduction processes. In this article, an experimental and simulation study on the reduction behavior of a briquette that is made by hematite and devolatilized biochar fines under CO–CO2 atmosphere was carried out. The reaction model was validated against the corresponding experimental measurements and observations. Modeling predictions and experimental results indicated that the CO–CO2 atmosphere significantly influences the final reduction degree of the briquette. Increasing the reduction temperature did not increase the final reduction degree but was shown to increase the carbon that was consumed by the oxidative atmosphere. The influence of the CO–CO2 atmosphere on the briquette reduction behavior was found to be insignificant in the early stage but became considerable in the later stage; near the time of the briquette reaching its maximum reduction degree, both iron oxide reduction and metallic iron re-oxidation were able to occur.

Non-isothermal reduction characteristics of roasted high alumina iron ore pellets

ABSTRACTNon-isothermal reduction of roasted Guangxi high alumina iron ore pellets with CO and H2 was conducted with high temperature synchronisation thermal analyser (NETZSCH STA 409C/CD) to understand the reduction characteristic of the ore. Chemical analysis, X-ray diffraction examination and scanning electron microscope analysis were adopted to analyse the roasted and reduced pellets. The results show that there are three main phases of Fe2O3, Al2O3 and Al3Fe5O12 in the roasted pellets. During reduction process, the iron bonded in hercynite and fayalite is difficult to be reduced by CO so that the final reduction degree is only 35.62% at the setting temperature of 1573 K. But with H2 atmosphere, the reduction degree can reach 100% at about 1520 K. It suggests that the ferric ion can completely be reduced to metallic iron by H2.

Reduction behavior and mechanism of Hongge vanadium titanomagnetite pellets by gas mixture of H2 and CO

Hongge vanadium titanomagnetite (HVTM) pellets were reduced by H2-CO gas mixture for simulating the reduction processes of Midrex and HYL-III shaft furnaces. The influences of reduction temperature, ratio of φ(H2) to φ(CO), and pellet size on the reduction of HVTM pellets were evaluated in detail and the reduction reaction kinetics was investigated. The results show that both the reduction degree and reduction rate can be improved with increasing the reduction temperature and the H2 content as well as decreasing the pellet size. The rational reduction parameters are reduction temperature of 1050 °C, ratio of φ(H2) to φ(CO) of 2.5, and pellet diameter in the range of 8 — 11 mm. Under these conditions (pellet diameter of 11 mm), final reduction degree of 95.51% is achieved. The X-ray diffraction (XRD) pattern shows that the main phases of final reduced pellets under these conditions (pellet diameter of 11 mm) are reduced iron and rutile. The peak intensity of reduced iron increases obviously with the increase in the reduction temperature. Besides, relatively high reduction temperature promotes the migration and coarsening of metallic iron particles and improves the distribution of vanadium and chromium in the reduced iron, which is conducive to subsequent melting separation. At the early stage, the reduction process is controlled by interfacial chemical reaction and the apparent activation energy is 60.78 kJ/mol. The reduction process is controlled by both interfacial chemical reaction and internal diffusion at the final stage and the apparent activation energy is 30.54 kJ/mol.

Microwave-absorbing characteristics of carbothermic reduction products of ilmenite concentrate catalyzed by sodium borate

Carbothermic reduction kinetics of ilmenite concentrate catalyzed by sodium borate and microwave-absorbing characteristics of reduction products were investigated. Kinetics results showed that the reaction was controlled by the nuclei formation model, the apparent activation energy was 100.74 kJ/mol, which was lower about 34.40 kJ/mol and 11.29 kJ/mol compared with the final stage activation energy using sodium silicate and sodium chloride as catalysts, respectively. The final reduction degree was 54.95%, 64.14% at temperatures of 1423 K and 1473 K, respectively, higher about 32.21% compared with those results by sodium silicate under the same conditions. The microwave-absorbing characteristics of reduction products were measured by the method of microwave cavity perturbation. The results demonstrated that the microwave absorbing characteristics of reduction products obtained at temperatures of 900 °C, 1100 °C and 1200 °C changed greatly. The formation of Fe occurred at the temperature of 900 °C. The decreasing of FeTiO3 content resulted in the decrease of the microwave absorbing characteristics, while, the increasing of Fe content resulted in the increase of microwave absorbing characteristics. The decrease of FeTiO3 content and the increase of FeTi2 O5 (MgTi2 O5 ) content made the microwave absorbing characteristics decrease, the increase of Fe content made the microwave absorbing characteristics increase. The decreasing trend was larger than the increasing trend, which was the main reason for the great changes of microwave absorbing characteristics of reduction products obtained at the temperature range of 850 °C–900 °C and at the temperature range of 1100 °C–1200 °C.

Reduction Behavior and Structural Evolution of Iron Ores in Fluidized Bed Technologies—Part 2: Characterization and Evaluation of Worldwide Traded Fine Iron Ore Brands

The reduction of iron ore fines by means of fluidized bed technology has become an alternative route of ironmaking. To investigate the reduction behavior of fine ores and to improve fluidized bed processes, a lab scale fluidized bed facility is installed. The reactor is charged with different kinds of fine ores. The reactors get fluidized by a reducing gas mixture containing H2, H2O, CO, CO2, CH4, and N2 in variable range. Additionally, a sampling system is installed, which enables the sampling of bed material during reduction. All samples are analyzed regarding their grain size distribution, chemistry, and morphology. The comparison of the raw input ore with the reduced ore—bed samples during the tests and final material—leads to a new way of fine ore characterization. After the determination of a standardized process, globally traded iron ore fines are tested under the same process conditions. The results enable the validation of the morphological evolution of different iron ore types and brands (hematitic, magnetitic, and limonitic) during the reduction with CO‐rich reducing gas in fluidized bed processes. Further investigation on the structure and porosity of the different phases is done to evaluate the relation to reduction rate and final reduction degree.

Iron ore reduction in a small fluidized bed

To optimize the process and improve the degree of reduction, a two-stage experiment was designed. It was carried out in a micro-fluidized bed. It is operated as a differential reactor to ensure every particle in the micro-fluidized bed is at equal temperature, concentration and residence time. The experiment used iron ore powder from Yangdi. The reducing gas and temperature in the pre-reduction stage were, respectively, pure CO and 625 °C. In the final reduction stage, the temperature and reducing gas were, respectively, 800 °C and pure CO. The residence time was between 10 min and 60 min. In addition to the experiment, a microscopic technique was also carried out. The test shows a significant influence of the temperature and residence time of the two-step experiment on the final reduction degree.

Reduction Behavior of Hematite Composite Containing Polyethylene and Graphite with Different Structures with Increasing Temperature

Plastic wastes can play the significant role of reductants for iron oxides by supplying reducing gases, e.g., H2 and CO, through pyrolysis. However, there is a problem that the thermal degradation temperature of plastics is significantly lower than the reduction temperature of iron oxide. If a large temperature difference can be formed within the composite granule, the reducing gases thus released can be used as reductants. Therefore, in this paper, the reduction behavior of the composites prepared by using hematite, graphite, and polyethylene regents with different compositions and structures was examined in order to understand the reduction behavior and to attain a high utilized ratio of polyethylene to reduction. Reduction experiments of the composites were carried out under an inert gas flow, and gases formed during reduction were continuously analyzed. The reduction degree of hematite was calculated using the concentration of these gases. Further, micro- and macrostructures of the reduced composite were observed. The addition of polyethylene to a hematite composite containing polyethylene and graphite led to a decrease in the final reduction degree, under the same ratio of carbon to oxygen in the composite. This indicates that the contribution of polyethylene to the reduction reaction was limited. This is because the crack formation enhanced the direct outward flow of the composite without contributing to the reduction reaction. In contrast, the double-layer composite, in which the polyethylene content in the inner layer is larger than that in the outer, shows an effective utilization of polyethylene as compared to the homogeneous composite.

Reduction of Iron Ore Fines with CO-rich Gases under Pressurized Fluidized Bed Conditions

Pressurized fluidized bed processes for the reduction of iron ore fines using CO-rich reducing gas formed by coal gasification are an upcoming ironmaking technology. They operate in a continuous multi-staged countercurrent mode at an absolute pressure of 4 bar.For process optimization, a laboratory-scale pressurized fluidized bed reactor was built to perform experiments similar to industrial conditions. The reactor is operated as a differential reactor to ensure a constant gas concentration and temperature within the reactor volume.Three-stage experiments with three different iron ores were carried out at an absolute pressure of 4 bar in batch-wise operation. The reducing gas consisted of a mixture of H2, H2O, CH4, CO2 and CO. The CO-concentration was varied in a range between 30% and 50%, the residence time in pre-reduction between 30 and 60 min and the operating temperature from 400°C to 850°C. In correspondence to the tests, microscopic techniques were applied to determine the importance of morphological properties of the ores on their reduction behavior.A higher temperature in the pre-reduction stage causes an increase of porosity as well as an increase of the final reduction degree. Comparing different ore types, a significant correlation between the morphology of the employed ores and the final reduction degree was found.

Iron Ore Reduction in a Laboratory-scale Fluidized Bed Reactor-Effect of Pre-reduction on Final Reduction Degree

Fluidized bed processes for iron ore reduction (e.g. FIOR®, FINMET®, FINEX®) operate in a continuous multi-staged countercurrent mode. To optimize iron ore reduction, different operating conditions occur in each stage.Significant influence of the first reduction stage on the final reduction degree was observed in industrial plant. The iron ore particles seem to “memorize” the precursor autoclave conditions.To optimize iron ore reduction, as well as to develop new reduction processes, it is necessary to understand this phenomenon. Industrial plants operate on pressures up to 12 bar and temperatures up to 900°C. So a laboratory-scale pressurized fluidized bed reactor was built to perform experiments similar to industrial conditions.Two-stage experiments with Mt. Newman hematite ore from Western Australia at variable operating conditions show significant influences of temperature and residence time of the pre-reduction stage on the final reduction degree. Microscopical analysis showed the influence of mineralogy and texture on the reduction behavior of original respectively partly reduced iron ore. Formed magnetite in the pre-reduction stage causes a degradation of final reduction degree. Additionally the amount of newly formed magnetite depends on pre-reduction temperature, composition of the reducing gas and residence time.

A new approach to kinetic study of wet atmosphere activation of fused iron catalyst

A new experimental method has been proposed enabling the determination of the effect of reduction degree ( R) and concentration of water vapour ( c H 2O ) on the activation rate ( r= dR dt ) of the iron catalyst for ammonia synthesis. A wetless mixture of H 2 and N 2 (3:1) was introduced into the reaction chamber of an automated flow thermobalance. The applied reactor can be considered as a reactor with ideal mixing. A monolayer of catalyst grains was reduced in wet atmosphere produced by reduction itself. The reduction degree was calculated from the loss of mass recorded during the reduction. The measured reduction rate and a known fixed value of the flow rate of the gas mixture enabled us to calculate the concentration of water vapour. The experiments differ from each other by the initial mass of the catalyst and by the flow rate. The variability of these factors resulted in the variation of c H 2O within the industrially important range of 1,000–10,000 ppm. The final reduction degree varied from 78% to 95%. The results of the series of experiments performed at ca. 550°C were presented in the form of a 3-D diagram: r= r( R, c H 2O ). The diagram cut by planes at c H 2O =const yields curves r= r( R). These curves were described by a Gauss-like function. The diagram cut by plane R=10% yields the initial rate curve r= r( c H 2O ). The curve was described by an exponential function. The model function was proposed in order to describe the 3-D diagram, too. The acquisition of data by the proposed method is relatively easy, especially because there is no need, similarly as in the industrial installations, to saturate gases with water vapour.

Evolution of Porosity Profiles of Magnetite Phase during High Temperature Reduction of Hematite.

The reduction of initially nonporous hematite to porous magnetite by CO+CO2 (3:97) mixture was monitored thermogravimetrically at 850°C. For the series of six kinetic runs the grains of diameter ca. 1.5 mm were used. The final reduction degree varied from 13 to 100%. After each kinetic run the microscopic observations of the central cross-section of grains were done in quantitative way. The observations yielded the values of local porosity. The empirical equations were found describing the continuous exponential decrease of local porosity with the distance from the external surface of the grain. The value of total porosity obtained by mercury porosimetry agrees in a reasonable way with microscopic data. The classical shrinking core model (SCM) was fitted to kinetic data. The model took into account the gas-solid reaction occurring at sharp defined interface as well as the pore diffusion phenomena occurring inside the magnetite layer. The model was also modified. The local value of porosity was introduced to the definition of effective diffusivity (Deff). In this way Deff was allowed to vary with the distance from the external surface of the grain. The corrected three parameter SCM yielded slightly worse results. It implies that apart from the spatial variation of Deff the temporal one should be also considered. Indeed, it was found that the local porosity of the already reduced layer varied also with time. However, the data are not accurate enough to permit the temporal variation of Deff to be included in the model calculation.