Swelling Behavior of Iron Ore Pellets during Reduction in H2 and N2/H2 Atmospheres at Different Temperatures

The need to develop green steelmaking techniques has led to the replacement of reducing agents such as CO with H₂. H₂ and N₂/H₂ mixtures can be used for the carbothermal reduction of iron ore. Herein, the reduction swelling index (RSI) of iron ore pellets in a forming gas (N₂/H₂) atmosphere at temperatures of 700–1000 °C is investigated and it is compared with that in pure H₂. It is showed in the experimental results that the RSI increases with increasing temperature for both the H₂ and N₂/H₂ atmospheres. The maximum swelling is reached approximately 5 min into the H₂ reduction process, while in the N₂/H₂ atmosphere, it is reached after 25–45 min of reduction, depending on the temperature. When the reduction temperature exceeds 900 °C, the RSI is greater than 20%. Scanning electron microscopy/energy-dispersive X-ray spectroscopy is performed to detect the changes in the microstructure and chemical composition of the samples. The nonreduced areas in the reduced pellets during the N₂/H₂ reduction process are analyzed using light optical microscopy.     

Rare earth extraction from bastnaesite concentrate by stepwise carbochlorination-chemical vapor transport-oxidation

A stepwise carbochlorination-chemical vapor transport-oxidation process is developed for the green rare earth extraction from a bastnaesite concentrate using carbon as reductant, chlorine gas as chlorination agent, SiCl₄ gas as defluorination agent, AlCl₃ as vapor complex former, and (O₂+H₂O) mixed gas as oxidant. Between 500 °C and 800 °C, the apparent activation energy of the carbochlorination within 2 hours changed from 17 to 10 kJ/mole roughly for the initial 20 minutes and final 1.5 hours, respectively, in the absence of SiCl₄, but these values reduced to 15 and 5.9 kJ/mole under 10 kPa of SiCl₄ gas, while the rare earth chloride conversion for 2 hours was 43 to 81 mol pct in the absence of SiCl₄ and 55 to 99 mol pct under 10 kPa of SiCl₄ gas. After carbochlorination at 550 °C for 2 hours in the (Cl₂+SiCl₄) atmosphere for efficient rare earth extraction and thorium-free volatile by-product release, throium was removed by chemical vapor transport at 800 °C for 0.5 hours in the (Cl₂+SiCl₄+AlCl₃) atmosphere and alkaline earths were separated from rare earths by oxidation at 700 °C to 1000 °C in the (O₂+H₂O) atmosphere for 0.5 hours, followed by water leaching at room temperature. Their combination allows a clean and efficient rare earth extraction from the concentrate.     

Reduction mechanisms of vanadium–titanomagnetite–non‐coking coal mixed pellet

Abstract

Experiments were carried out by testing the specimens of separate layers of iron and coal and single pellets thermogravimetrically in a nitrogen atmosphere to study the non‐isothermal reduction mechanisms of vanadium–titanomagnetite–non‐coking coal mixed pellets. The degree of reduction was measured by the weight loss. The E values of the pellet reduction were calculated based on the mass action law. It was found that with increasing temperature the reduction processes may be divided into four stages: reduction via CO and H₂ from volatiles at 400–650°C, reduction via H₂ and C generated by cracking of hydrocarbon at 650–850°C, direct reduction of carbon via gaseous intermediates at 850–1050°C and direct reduction of carbon above 1050°C.     

Behaviour of iron ore–fuel oil composite pellets in isothermal and non-isothermal reduction conditions

Abstract

Self-fluxing iron ore pellets as an alternative to the agglomeration process led to the use of low price fuel oil as a binder and reducing material. Composite pellets containing 5–15% fuel oil were isothermally and non-isothermally reduced at 750–1000°C in a flow of H₂ or N₂ gases. The total weight loss resulting from O₂ removal from the reduction of Fe₂ O₃ and from the thermal decomposition of fuel oil was continuously recorded as a function of time at different reduction conditions. The actual reduction extent at a given time was calculated from the chemical analysis of partially reduced samples at a given time and temperature. Microscopic examination and X-ray phase analysis were applied to characterise the reduction products. The isothermal reduction of composite pellets indicated that the reduction rate increased with the increase in fuel oil content at the early stages. At the later stages, the reduction rate increased in the order 12>10>5> 15% fuel oil containing pellets. The non-isothermal reduction of composite pellets in N₂ atmosphere showed the presence of an incubation period at initial reduction stages. The low intensity magnetic separation technique was applied with the aim of increasing the iron content at the expense of associated impurities. The magnetic and non-magnetic fractions were analysed and the overall recovery was determined.     

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si₃N₄) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H₂-25 pct N₂, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si₃N₄-β, Si₃N₄-γ, Fe₄N, and Fe₃N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

     

Re-oxidation Kinetics of Flash-Reduced Iron Particles in H2-H2O(g) Atmosphere Relevant to a Novel Flash Ironmaking Process

A novel flash ironmaking process based on hydrogen-containing reduction gases is under development at the University of Utah. The goal of this work was to study the possibility of the re-oxidation of iron particles in a H₂-H₂O gas mixture in the lower part of the flash reactor from the kinetic point of view. The last stage of hydrogen reduction of iron oxide, i.e., the reduction of wustite, is limited by equilibrium. As the reaction mixture cools down, the re-oxidation of iron could take place because of the decreasing equilibrium constant and the high reactivity of the freshly reduced fine iron particles. The effects of temperature and H₂O partial pressure on the re-oxidation rate were examined in the temperature range of 823 K to 973 K (550 °C to 700 °C) and H₂O contents of 40 to 100 pct. The nucleation and growth kinetics model was shown to best describe the re-oxidation kinetics. The partial pressure dependence with respect to water vapor was determined to be of first order, and the activation energy of re-oxidation reaction was 146 kJ/mol. A complete rate equation that adequately represents the experimental data was developed.     

Kinetics of chlorination and oxychlorination of chromium (III) oxide

Thermogravimetric analysis (TGA) is used to study the kinetics of chlorination of Cr₂O₃ with Cl₂+N₂ and Cl₂+O₂ gas mixtures in the temperature range of 550 °C to 1000 °C. The reactivity of Cr₂O₃ toward the chlorine-oxygen gas mixture is higher than that toward the chlorine-nitrogen one. Chlorination of Cr₂O₃ proceeds with an apparent activation energy of about 86 kJ/mol between 550 °C and 1000 °C. The apparent reaction order with respect to chlorine is about 1.23 at 800 °C. At temperatures lower than 650 °C, the shrinking sphere model is the most appropriate for describing the reaction kinetics. Oxychlorination of Cr₂O₃ is characterized by an apparent activation energy of about 87 and 46 kJ/mol for temperatures lower than 650 °C and higher than 700 °C, respectively. At 800 °C and using a Cl₂+O₂ gas mixture, the maximum reaction rate is obtained when the Cl₂/O₂ molar ratio is equal to 4, confirming the formation of chromium oxychloride. At this temperature, the reaction orders with respect to chlorine, oxygen, and Cl₂+O₂ are about 1.08, 0.23, and 1.29, respectively. Mathematical fitting of the experimental data is discussed.     

Adverse effect of high purity atmosphere on sintering of manganese steels

Abstract

The Fe–(2, 3)Mn and Fe–(2, 3)Mn–0·4C specimens were sintered in the dilatometer tests for 30 min at 1150°C in an H₂–H₂ O atmosphere with a dew point of –30 and –70°C. The Mn addition at sintering of the specimens with a dew point of –30°C resulted in higher swelling with a maximum at 1150°C and processing at a dew point of –70°C resulted in minimal differences in swelling of the specimens in dependence on the Mn addition. Processing in the high purity atmosphere (–70°C) causes a loss of Mn in the form of Mn vapour transported away by the flowing atmosphere needed for alloying of the Fe matrix. On the contrary, at sintering in a low purity atmosphere (–30°C) the reaction of Mn vapour with oxygen of high partial pressure occurs near the surface of the specimens. Sintering of Mn steels requires the low purity atmosphere in contrast to the thermodynamic requirements. The sintering of Mn steels occurred under the effect of Mn vapour formed by sublimation regardless the thermodynamics for Mn–O system.     

Behaviour of manganese oxides during magnetising reduction of Baharia iron ore by CO–CO2 gas mixture

Abstract

High manganese containing iron ore samples were isothermally reduced with a CO–CO₂ gas mixture at 600–1000°C. The course of reduction was followed by a weight loss technique. The influence of reducing gas composition and temperature on the reduction kinetics was investigated. The different phases formed during reduction were identified by X-ray phase analysis, while their structures were microscopically examined. The reduced samples were magnetically tested by means of a Davis tube tester. The effect of grain size, drum speed, and cleaning conditions on the efficiency of magnetic separation was studied using a Box-Mag wet low intensity magnetic separator. The separation efficiency was determined by analysing total iron, manganese, and acid insoluble contents in both magnetic and non-magnetic fractions. Best testing results were obtained on separation of the sample reduced with 80CO–20CO₂ (vol.-%) at 800°C. The optimum grain size for magnetic separation is below 0·15 mm while that of the drum speed is 100 rev min^-1 . The cleaning of the magnetic fraction increases the iron content and decreases the manganese and acid insoluble contents.     

Gaseous reduction of iron oxides: Part II. Pore characteristics of iron reduced from hematite in hydrogen

The pore structure of iron reduced from hematite ore by hydrogen is charactertized from measurements of pore volume, pore area and effective gas diffusivity. The measured connected pore volume within the range 0.22 to 0.34 cu cm per g is about the same as the total pore volume; that is, most of the pores in the reduced iron are interconnected. The pore area measured by the BET technique increases with decreasing reduction temperature,e.g. from 0.1 sq m per g at 1200°C to 39 sq m per g at 200°C. The effective H₂−H₂O diffusivity, measured directly at 600°C after reduction of the ore in hydrogen at the same temperature, is in excellent agreement with that derived from the reduction data. The pore-diffusivity measurements were also made at room temperature using iron samples reduced in hydrogen at 800° and 1000°C. On the basis of the pore properties measured, it appears that the reduced iron has a regular pore structure which becomes finer with decreasing reduction temperature. The effective diffusivities computed on the basis of a simple pore structure are found to be in accord with those derived previously from the reduction data.     

Effect of heat treatment on the hardness-microstructure inter-relation in a 7.5Mn-5Cr-1.5Cu alloy white iron: A modeling approach

An experimental study has been made on the effect of heat-treating temperature (800 °C 850 °C 900 °C, 950 °C, 1000 °C, and 1050 °C) and time (2, 4, 6, 8, and 10 hours) on the transformation behavior of a 7.5Mn-5Cr-1.5Cu white cast iron developed to resist aqueous corrosion in different environments. Structural changes on heat treating were monitored using hardness measurements. It was observed that on heat treating from 800 °C, hardness increased marginally with soaking period. Hardness was independent of soaking period on heat treating at 850 °C and 900 °C. On heat treating from 950 °C and higher, hardness decreased with time, the effect being pronounced at 1000 °C and 1050 °C. These changes are consistent with the resultant microstructural changes. The hardness(H) vs time(t) plots at any temperature are linear and can be represented byH = C₁ +C ₂t (T °C) The hardnessvs temperature plots as influenced by time, which, in effect, represented how effectively the alloy sustained hardness, are most appropriately represented by a third-order polynomial:H = C₁ + C₂T+ C₃T² + C₄T³ (t s) leading to a horizontal “S” shape. Based on fundamental considerations, the final model interrelating hardness with temperature and time isH = 61.8 e^2442.5/T + (0.0188 -1.6 × 10⁻⁵·T)t whereT = temperature in K;t = time in seconds; andH = Vickers hardness number, 30 kgf (VHN₃₀). The overall validity and usefulness of the model have been discussed.     

Reduction Behaviour of Wüstite Doped with MgO

Compacts made from pure wüstite and compacts doped with 2% MgO were annealed at 1000°C for 3 hrs in 50%CO‐CO₂ gas mixtures. The annealed samples were isothermally reduced at 800‐1100°C in H₂ gas. Selected samples were isothermally reduced at 1000°C with pure CO and 50%H₂‐CO gas mixture to investigate the effect of gas composition on the reduction processes. The oxygen weight loss resulting from the reduction of the samples was recorded as a function of time. X‐ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy and porosity measurements were used to characterize the annealed and reduced samples. Magnesio‐wüstite (MgO·FeO) phase was formed during the annealing of MgO doped wüstite. The MgO·FeO in turn decreased the porosity of the annealed doped samples compared to pure wüstite compacts. The influence of temperature, gas composition and MgO content on the reduction behaviour and the morphology of the annealed samples was investigated. The values of the apparent activation energy were calculated from Arrhenius plots and correlated with the reduction mechanism. The reduction rate increased with reaction temperature. In doped compacts, the MgO·FeO phase was not completely reduced both at lower reduction temperature (800°C) and during reduction with pure CO. From the activation energy values, the initial reaction stage was controlled by the combined effect of chemical reaction and gas diffusion while solid state diffusion controlled the final stage of reduction. Morphologically, metallic iron was formed in different shape structures under the effect of MgO addition and reduction conditions.     

Fabrication, characterization, and electrical conductivity properties of Pr6O11 nanoparticles

Pr6O11 nanoparticles were obtained by subsequent thermal decomposition of the as-prepared precipitate formed under ambient temperature and pressure using NaOH as precipitant.The calcination process was affected,for 1 h in static air atmosphere,at 400-700 °C temperature range.The different samples were characterized using X-ray diffraction(XRD),transmission electron microscopy(TEM),field emission scanning electron microscopy(FE-SEM),thermogravimetric analysis(TGA),in situ electrical conductivity,and N 2 adsorption/desorption.The obtained results demonstrated that nano-crystalline Pr6O11,with crystallites size of 6-12 nm,started to form at 500 °C.Such value increased to 20-33 nm for the sample calcined at 700 °C.The as-synthesized Pr6O11 nanoparticles presented high electrical conductivity due to electron hopping between Pr(III)-Pr(IV) pairs.     

FeCe nanocomposite with high iron content as efficient catalyst for generation of CO_x-free hydrogen via ammonia decomposition

FeCe nanocomposite catalysts with different iron contents were synthesized by a facile co-precipitation method.The as-prepared materials were characterized by various techniques including powder X-ray diffraction(XRD),N_2 adsorption/desorption and high-resolution transmission electron microscopy(HRTEM).Catalyst with the highest iron content(90 FeCe) shows the best activity for the hydrogen generation via ammonia decomposition.83% NH_3 conversion is achieved at 550℃ and nearly full conversion of NH_3 is realized at 600℃ with a GHSV of 24000 cm~3/(g_(cat)·h).The large content and small size crystal particles of iron species are responsible for the good catalytic performance.Temperatureprogrammed reduction by hydrogen(H_2-TPR) was performed to investigate the interaction between cerium and iron species.It is found that slight cerium can exert strong interaction with iron compound thus effectively prevent the self-aggregation of active iron species,so as to improve the catalytic activity for ammonia decomposition.     

Rate of reduction of pyrrhotite in hydrogen

The hydrogen-reduction of spheroidal pyrrhotite particles, varying in diameter from about 0.4 to 1.6 cm, was investigated at 600, 800, and 900°C. Microscopic examination of partially reduced samples showed a well-defined interface between unreacted pyrrhotite and a porous iron shell. The rate of reduction is controlled primarily by counter-current diffusion of H₂ and H₂S in the gas-film boundary layer and in the porous iron layer. The magnitude of the ratio of effective diffusivity to the molecular interdiffusivity for H₂-H₂S, derrived from the reduction data, suggests that the structure of the porous iron formed is similar to that of an idealized porous material.     

Deterioration of electromotive force of chromel-alumel thermocouples in reducing atmospheres at high temperatures

The deterioration of electromotive force (emf) of Chromel-Alumel (CA) thermocouples in 80 pct H₂ + 15 pct CO + 5 pct CO₂ has been analyzed in terms of the corrosion behavior of Chromel. Emf of the CA thermocouple deteriorated drastically in 80 pct H₂ + 15 pct CO + 5 pct CO₂. After exposure for about 1000 hours at 900 °C, the decrease of emf was about 16 mV. The deterioration process could be separated into three terms. The first term, which has the smallest time constant of about 20 hours, was attributed to carbon deposition on the Chromel surface in the temperature range of 600 to 700 °C. The second term, which has a time constant of about 100 hours, was attributed to the severe internal oxidation of chromium in the temperature range of 500 to 800 °C. The third term, having the largest time constant of several thousand hours, might be attributed to the moderate and gradual preferential oxidation of chromium in Chromel in the range 800 to 900 °C. Boron nitride (BN) coating on CA thermocouples could reduce this deterioration of emf; the decrease of emf was improved to about 3 °C during 700 hours test at 900 °C.     

Reduction Behavior of Panzhihua Titanomagnetite Concentrates with Coal

The reduction behavior of the Panzhihua titanomagnetite concentrates (PTC) briquette with coal was investigated by temperature-programmed heating under argon atmosphere in a vertical tube electric furnace. The mass loss behavior of the PTC-coal mixture was checked by thermogravimetric analysis method in argon with a heating rate of 5 K (5 °C)/ min. It was found that there are five stages during the carbothermic reduction process of the PTC. The devolatilization of coal occurred in the first stage, and reductions of iron oxides mainly occurred in the second and third stages. The reduction rate of iron oxide in the third stage was much higher than that in the second stage because of the significant rate of carbon gasification reaction. The iron in the ilmenite was reduced in the fourth stage. In the final stage, the rutile was partially reduced to lower valence oxides. The phase transformation of the briquette reduced at different temperatures was investigated by X-ray diffraction (XRD). The main phases of sample reduced at 1173 K (900 °C) are metallic iron, ilmenite (FeTiO₃), and titanomagnetite (Fe_3–x Ti _x O₄). The traces of rutile (TiO₂) were observed at 1273 K (1000 °C). The iron carbide (Fe₃C) and ferrous-pseudobrookite (FeTi₂O₅) appeared at 1473 K (1200 °C). The titanium carbide was found in the sample reduced at 1623 K (1350 °C). The shrinkages of reduced briquettes, which increased with increase in the temperature, were found to depend greatly on the temperature. With increasing the reduction temperature to 1573 K (1300 °C), the iron nuggets were observed outside of the samples reduced. The nugget formation can indicate a new process of ironmaking with titanomagnetite similar to ITmk3 (Ironmaking Technology Mark 3).     

The effect of hydrogen addition on the carbon-deposition behaviour during the reduction of pellets for the blast furnace process

With the application of large amount of pulverised coal injection into the blast furnace, the hydrogen content in the gas will increase, which accelerates the reduction of iron ore in lump zone of the blast furnace as well as carbon-deposition reaction. This study has investigated the effect of hydrogen addition on carbon-deposition reaction during the reduction of pellets through thermodynamic calculation and experiment. The results show that H₂ can promote the carbon-deposition reaction, while the increase of temperature and CO₂ can significantly inhibit it. The preference region of temperature for C formation is about 600°C. Moreover, the promotion effect of H₂ on the carbon-deposition reaction at 700°C is better than that at 600°C. The SEM observation results show that the generated carbon is mainly distributed on the surface of the pellet, and only a little carbon is located inside the pellet. The agglomerated carbon could be more easily formed due to the dramatic carbon-deposition reaction caused by the lower temperature or higher H₂ content. But, most of the carbon just exists as an individual particle at the lower carbon-deposition reaction rate. The results of SEM–EDS reveal that carbon deposited is primarily in the form of elemental carbon rather than in the form of cementite. The study also shows that with increasing reduction time, the rate of carbon-deposition increases, mainly due to the promotion effect of reduced iron during the reduction process of pellets.     

Electrochemical synthesis of tetragonal oxide tungsten bronze nanofilms on platinum

We are the first to synthesize nanofilms of tetragonal oxide tungsten bronze (OTB) on a Pt(110) substrate by the electrolysis of the K₂WO₄–Na₂WO₄–WO₃ melt at 700 and 750°C. The composition and the morphology of OTB are shown to depend on the deposition potential and the WO₃ concentration in the melt. The laws of formation of tetragonal OTB films are discussed. The synthesized OTB samples are found to have a good thermal stability in the temperature range 20–800°C.     

Sintering of Fe–C–BN steel compacts in different atmospheres studied by dilatometry and DTA/TG

Abstract

In the present work, 2%hBN was admixed with Fe–0·8C, and both dilatometric and differential thermal analysis/thermogravimetry investigations were conducted in Ar and N₂ atmospheres, followed by microstructural studies and mechanical testing. The α–γ phase transformation in both atmospheres was found to occur in the temperature range of plain iron and not, as expected, in that common for Fe–C. In the Ar atmosphere, liquid phase formation is recognised by endothermic differential thermal analysis signal and shrinkage in the dilatometer during the heating stage at ～1275°C. In contrast to the activating effect of the inert Ar for the decomposition of hBN, the deactivating effect of the N₂ atmosphere is visible from the dilatometry results: sintering in N₂ even resulted in slight expansion during the isothermal stage. Therefore, the, at first surprising, conclusion can be drawn that the chemically inert Ar is activating the sintering process while the more reactive N₂ passivates it.