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Influence of pressure on the kinetics of synthetic llmenite reduction in hydrogen
Abstract
In-situ thermogravimetric measurements were used in the hydrogen reduction of poly-granular synthetic ilmenite discs at temperatures
in the range 823 to 1173 K and at pressures in the range 1.2 to 13 atm. A symmetrical beam microbalance was used, coupled
with twin reactors and twin furnaces, to minimize buoyancy and drag effects. Stable operation was achieved at high gas flow
rates where gas film transport effects were negligible. Polishing the ilmenite discs prior to reduction eliminated the formation
of dense surface metallic iron films that can impede gas diffusion into the discs. Macroscopically, the reduction reaction
proceeded topochemically and a shrinking core reaction model was found to be appropriate to predict conversion-time relationships.
It was necessary to allow for water vapor adsorption onto the reacting interface in order to model the effect of pressure
on the reduction kinetics. The observed reduction rate increased sharply with pressure up to approximately 3 atm and then
approached a plateau with further pressure increase. The porosity in the reduced ilmenite samples was very fine, with pore
diameters of typically 0.05 to 0.3 µm. Intragrain gas pressure buildup in the fine pores due to the influence of Knudsen diffusion
was incorporated into the modeling of the kinetic data.
... Under the isothermal condition, the main products are iron, TiO 2 , and MgTiO 3 , while under the nonisothermal condition, TiO 2 and MgTiO 3 can be reduced by hydrogen at a temperature of 1220 °C, and the final products are metallic iron and Mg x Ti 3-x O 5 (x in the range of 0.45-1). Vries et al. [25] reduced synthetic ilmenite disks at temperatures of 550-900 °C and pressures of 1.2-13 atm and studied the influence of pressure on the kinetics of the reduction process. The reaction rate increased sharply when the pressure reached approximately 3 atm and leveled off thereafter owing to the adsorption of water vapor. ...
The hydrogen-rich reduction of ilmenite concentrates has a significant effect on the reduction of the carbon footprint of the titanium slag production process. This study investigates the hydrogen-rich reduction of Panzhihua ilmenite concentrate pellets. The pellets were roasted at 500, 700, 900, and 1100 °C and used as the raw material for the reduction process. The effects of the pellet roasting temperature, magnetite concentrate content of the pellets, reduction temperature, and reducing gas composition on the reduction process were experimentally studied. The results showed that the average reduction rate is affected by the phase and dense structure of the pellet, reduction temperature, and hydrogen content. Further, the reduction process can be accelerated by increasing the H2 content and reduction temperature. The high compressive strength of the pellet, owing to its dense inner structure, is not conducive for the reduction process. Moreover, studies show that the H2/CO ratio has negligible effects on the reduction mechanism. The reaction mechanism of the pellet reduction process is three-dimensional diffusion. Furthermore, the effect of the pellet phase on the hydrogen-rich reduction process has been discussed.
... The temporal profiles of conversion had a sigmoid shape and presented three different stages, i.e., original induction stage, medium acceleration stage, and final deceleration stage. VRIES et al [18,19] employed a pressurized thermogravimetric microbalance to investigate the reduction of synthetic ilmenite pellets in the temperature range of 550−900 °C and pressure range of (1.2−10)×10 5 Pa. However, these isothermal experiments of reduction of Bama ilmenite using H 2 −Ar gas mixtures were carried out by WANG et al [20]. ...
The hydrogen reduction of Panzhihua ilmenite concentrate in the temperature range of 900–1050 °C was systematically investigated by thermogravimetric analysis (TG), X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. It was shown that the products of the Panzhihua ilmenite reduced at 900 °C were metallic iron and rutile. Above 1000 °C, ferrous pseudobrookite solid solution was generated. During the reduction process, element Mg gradually concentrated to form Mg-rich zone which can influence the metallization process. The reduction reaction proceeded topochemically and its related reduction kinetics were also discussed. The kinetics of the reduction indicated that the rate-controlling step was the diffusion process. The apparent activation energy of the hydrogen reduction of Panzhihua ilmenite was calculated to be 117.56 kJ/mol, which was larger than that of synthetic ilmenite under the same reduction condition.
... Numerous authors have investigated the reaction kinetics of ilmenite, and suggested various mechanisms to determine kinetic parameters. For instance, de Vries and Grey [29] found that the interfacial chemical reaction is the major mechanism contributing to the overall reduction rate of synthetic ilmenite at 1023 K in pressurized conditions, whereas the contributions from other mechanisms such as pore diffusion and gas film mass transfer were negligible. Although they noted that macroscopically the reduction reaction appeared to proceed according to a shrinking core model, the local reaction in individual grains was thought to proceed differently. ...
... As part of any proposed process, the fundamentals of the reaction need to be investigated to aid in the optimisation of the process [1,45]. The present work has unambiguously shown the influence of shaking intensity on the process of contaminant removal in Fe 0 /H 2 O systems on the example of methylene blue discoloration by metallic iron (Fe 0 ). ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe0). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min-1 was directly compared. Investigated materials were scrap iron (Fe0), granular activated carbon (GAC), and deep sea manganese nodules (MnO2). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0 to 9.0 g/L of each reactive material (systems I, II and III), or (iii) 5 g/L Fe0 and 0 to 9.0 g/L GAC or MnO2 (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8 to 5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min-1, a plateau between 50 and 150 min-1, and a sharp increase of MB discoloration at shaking intensities ≥ 200 min-1. At 300 min-1, increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe0/MnO2 and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe0 surface should not exceed 50 min-1.
... Thus, an intensive work has been done in the past about the reactivity and kinetic determination of the reduction of ilmenite for metallurgical applications using H 2 (Bardi et al., 1987;Grey et al., 2007;Sun et al., 1992bSun et al., , 1993Vries and Grey, 2006;Zhao and Shadman, 1991), CO (Park and Ostrovski, 2003;Zhao and Shadman, 1990), CH 4 (Zhang and Ostrovski, 2001) or carbon (Wang and Yuan, 2006) as reducing agents. ...
The objective of this study was to establish the kinetic of both reduction and oxidation reactions taking place in the chemical-looping combustion (CLC) process using ilmenite as an oxygen carrier. Because of the benefits of using of pre-oxidized ilmenite and the activation of the ilmenite during the redox cycles, the reactivity of both the pre-oxidized and activated ilmenite was analyzed. The experimental tests were carried out in a thermogravimetric analyzer (TGA), using H2, CO or CH4 as reducing gases, and O2 for the oxidation step. Thus, the reactivity with the main reacting gases was analyzed when natural gas, syngas or coal are used as fuels in a CLC system. The changing grain size model (CGSM) was used to predict the evolution with time of the solid conversion and to determine the kinetic parameters. In most cases, the reaction was controlled by chemical reaction in the grain boundary. In addition, to predict the behaviour of the oxidation during the first redox cycle of pre-oxidized ilmenite, a mixed resistance between chemical reaction and diffusion in the solid product was needed. The kinetic parameters of both reduction and oxidation reactions of the pre-oxidized and activated ilmenite were established. The reaction order for the main part of the reduction reactions of pre-oxidized and activated ilmenite with H2, CO, CH4 and O2 was n=1, being different (n=0.8) for the reaction of activated ilmenite with CO. Activation energies from 109 to 165 kJ mol⁻¹ for pre-oxidized ilmenite and from 65 to 135 kJ mol⁻¹ for activated ilmenite were found for the different reactions with H2, CO and CH4. For the oxidation reaction activation energies found were lower, 11 kJ mol⁻¹ for pre-oxidized and 25 kJ mol⁻¹ for activated ilmenite.
... As part of any proposed process, the fundamentals of the reaction need to be investigated to aid in the optimisation of the process [1,45]. The present work has unambiguously shown the influence of shaking intensity on the process of contaminant removal in Fe 0 /H 2 O systems on the example of methylene blue discoloration by metallic iron (Fe 0 ). ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe(0)). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min(-1) was directly compared. Investigated materials were scrap iron (Fe(0)), granular activated carbon (GAC), and deep sea manganese nodules (MnO(2)). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0-9.0 g/L of each reactive material (systems I, II and III), or (iii) 5g/L Fe(0) and 0 to 9.0g/L GAC or MnO(2) (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8-5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min(-1), a plateau between 50 and 150 min(-1), and a sharp increase of MB discoloration at shaking intensities >or=200 min(-1). At 300 min(-1), increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe(0)/MnO(2) and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe(0) surface should not exceed 50 min(-1).
Reduction of chromium-bearing vanadium–titanium sinter (CVTS) was studied under simulated conditions of a blast furnace, and thermodynamics and kinetics were theoretically analyzed. Reduction kinetics of CVTS at different temperatures was evaluated using a shrinking unreacted core model. The microstructure, mineral phase, and variation of the sinter during reduction were observed by X-ray diffraction, scanning electron microscopy, and metallographic microscopy. Results indicate that porosity of CVTS increased with temperature. Meanwhile, the reduction degree of the sinter improved with the reduction rate. Reduction of the sinter was controlled by a chemical reaction at the initial stage and inner diffusion at the final stage. Activation energies measured 29.22–99.69 kJ/mol. Phase transformations in CVTS reduction are as follows: Fe2O3→Fe3O4→FeO→Fe; Fe2TiO5→Fe2TiO4→FeTiO3; FeO·V2O3→V2O3; FeO·Cr2O3→Cr2O3.
Both response and selectivity are key issues for a gas sensing material. Here, we presented a porous Ag/Ag2O-SnO2 composite with excellent sensing performance towards hydrogen. The optimal weight ratio of Ag to SnO2 was 3 wt% in the Ag/Ag2O-SnO2 composite, which gave the highest response of 40 to 200 ppm hydrogen at 170 °C. And what's more, it was nearly insensitive to carbon monoxide and methane at this temperature. The porous microspheres composite exhibited high response and rapid response (less than 5 s) to trace amount of hydrogen. The high response and excellent selectivity of this composite could be attributed to the high catalytic activity of Ag/Ag2O nanoparticles built on the surface of SnO2 microspheres, the Schottky barrier and low working temperature.
The reduction degree and rate of the initial ilmenite concentrate and pre-oxidation samples by hydrogen were investigated by using thermogravimetric analysis system. H2-TPR was used to detected reduction properties of the pre-oxidation products which were oxidized at different temperatures. The phase transformations and micromorphology changes during pre-oxidation and hydrogen reduction were identified by XRD and SEM. The FeTiO3 was decomposed into TiO2 and Fe2O3 at the temperature below 900 °C. With the temperature rising, Fe 2TiO5 was the only stable phase in high temperatures. The reduction of pre-oxidation ilmenite concentrate was faster than the natural sample, which was attributed to the micropores generated on the surface of ilmenite concentrate during oxidation. The main phases of the reduction product were metallic iron, ferrous pseuodbrookite.
Introduction Experimental Results and Discussion Conclusions
The kinetics of fluidised bed hydrogen reduction of preoxidised ilmenite concentrates was studied as a function of the different operating parameters of gas velocity, pressure and temperature. The reduction kinetics is controlled by gas starvation at typical fluidised bed flowrates and pressures. Gas starvation was found to dominate at H2 flowrate to ilmenite ratios less than 2 L min?1 g?1. At higher ratios, a form of attenuated gas starvation exists due to the small equilibrium constant for ilmenite reduction. This leads to large variations in the kinetic driving force for small variations in water vapour in the bed. Within the gas starvation region, there is an apparent linear dependence of reduction rate on pressure when a constant fluidising velocity is used. However if the supply of hydrogen to the reactor is kept constant, then the reduction rate is independent of gas pressure above 1 bar. At hydrogen supplies above 2 L min?1 g?1 the reduction rate increases with pressure but the relationship is not linear. An important contributing factor to reduction mechanisms is the very fine intrapore diameters (median pore diameter
Scanning electron microscopy and transmission electron microscopy techniques were used to characterize the products from hydrogen reduction of polygranular synthetic ilmenite discs at temperatures in the range 823 to 1173 K and pressures in the range 1 to 13 atm. Reduction commences at grain boundaries and cracks and advances progressively to grain interiors. Within individual grains, the morphology of the reduction products was found to be crystallographically controlled. Near parallel bands of metallic iron (Fem
) form within each grain, aligned with the basal plane of ilmenite (il) (0001)il
. The separation between bands is of the order of 1 μm and is relatively constant with change of pressure and temperature. In the interband region, conversion of ilmenite to rutile occurs preferentially parallel to
\( \{ 11\ifmmode\expandafter\bar\else\expandafter\=\fi{2}0\} _{{il}} \) ilmenite planes, generating platelets of rutile that grow normal to the Fem
bands. The intergrain duplex morphology of the reduction products closely resembles cellular precipitation in alloys. At reduction temperatures above ∼1000 K, the interband region comprises dense, nonporous oriented intergrowths of rutile platelets and residual ilmenite, whereas below ∼900 K, the interband region contains a fine, filamentary network of pores. In the intermediate temperature regime, a change from dense to porous interband region occurs with increasing pressure. The observations have been interpreted in terms of the relative rates of interfacial chemical reaction and solid-state diffusion, with the latter having a controlling influence at lower temperatures or higher pressures.
Examination of wustite single crystals reduced in CO/CO2 and H2/H2O gas mixtures has shown that in all cases a dense iron layer is formed initially on the oxide surface and that the porous growth of iron which is obtained under certain experimental conditions occurs as a direct result of the breakdown of this initial dense iron layer. Possible mechanisms of the iron layer breakdown are examined and compared with the experimental observations. A qualitative model of the breakdown process involving the nucleation of gas bubbles and the expansion of these bubbles in the iron layer is presented.
The thermal oxidation of titanium by water vapour proceeds according to a linear-parabolic rate law resulting from a reaction–diffusion mixed regime. The growth of TiO2 takes place by rapid diffusion of substitutional hydroxide ions generated at the gas–scale interface. Kinetic calculations based upon these assumptions are in agreement with the experimental observations.
The behavior of metallic niobium in pure water vapor (without added oxygen) was studied between 800 and 1000C. Linear kinetics curves were observed (except for a short initial period) with a homographic influence of H2O pressure. Hydrogen additions in the gas phase largely modified the reaction rates. The oxide formed (NbO2) is different from that predicted by thermodynamics (Nb2O5). Taking into account all these experimental observations, a mechanism is proposed in which the limiting step is oxygen incorporation into the oxide lattice.
In situ gravimetric measurements and microscopic examinations were used to determine the mechanisms of oxygen removal from synthetic ilmenite disks between 823 and 1353 K. Under a hydrogen atmosphere, iron was observed to form a layer of low porosity on the surface of samples early in the reduction. This created diffusion limitations for hydrogen to the reaction front and for the escape of water vapor. A shrinking core reduction model, modified to include the growth of this iron film, was capable of predicting the conversion-time relationships of ilmenite samples. An activation energy of 43.2 +/- 2.6 kcal/gmole was determined to be representative of reaction control over the temperature range 823-1023 K.
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