Catastrophic Swelling Research Articles

Swelling and softening behavior of iron ore-spent mushroom substrate composite pellets during carbothermal reduction

In the present day, we have reached close to an optimum condition of CO2 emission when using coal or coke for iron making process. Carbon neutrality is achieved during carbothermal reduction by employing biomass as a reducing agent in composite pellets. The swelling behavior of composite pellets during reduction has detrimental effects, and catastrophic swelling leads to subsequent disintegration of pellets, which causes low gas permeability through packed beds in the industry. In this study, we used coal, spent mushroom substrate, torrefied mushroom substrate (300 °C), and de-ashed torrefied mushroom substrate as reducing agents in our iron-ore composite pellets. Volume change of the composite was investigated with heating profile (15 °C/min until 1350 °C), reduction time is 70min, the maximum swelling observed in torrefied mushroom pellets is +158%, the pellets experienced swelling in between (700°C–1000 °C) and started to soften from 825 °C. However, no swelling is observed in the coal and de-ashed torrefied pellet. On the other hand, de-ashed torrefied pellets with the addition of alkali metal oxides and soda lime glass had significant swelling and decrease in softening temperature. With the presence of Na2O and K2O in pellets ash, during reduction in some localized areas, they form low melting slags like Soda-lime (∼1000 °C) and Potash-lime (∼1200 °C) compositions. These slags start to soften at a low temperature ∼700 °C for soda-lime composition. As the temperature increases, viscosity decreases, which causes pellets to soften more, and parallelly gas evolving from the pellet during reduction (mainly CO product of the Boudouard reaction) causes these slags to expand, like a glass blowing mechanism.

Model for the approximate assessment of nitrogen content in swollen reduced iron ore from single measurements

A method of calculation has been derived to assess the nitrogen estimated content in iron reduced samples. The method is based on the review of observations and laboratory measurements of relationships between the rate of reduction and the corresponding metallic iron formation during the reduction process. The metallic iron formation has been calculated from relationships that apply to a wide variety of types of ores undergoing reduction under a nitrogen-containing gas mixture in proportions above 50% by volume. The empirical correlations found between the rates of metallization, the sample swelling index, and the equilibrium nitrogen solubility in iron can be used for determined the approximate final content of nitrogen in the reduced samples from the estimated and measured final volume of the reduced specimens. It is necessary to have an accurate analysis of the starting sample as well as the reducibility information.
 Keywords: Iron ore, nitriding, catastrophic swelling, rate of metallization, reduction degree.
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 [2]I. Mikko, M. Olli, A. Tuomas, V. Ville-Valtteri, K. Jari, P. Timo & F. Timo. “Dynamic and Isothermal Reduction Swelling Behaviour of Olivine and Acid Iron Ore Pellets under Simulated Blast Furnace Shaft Conditions”. ISIJ International. 52. 1257-1265. 10.2355/isijinternational.52.1257. 2012.
 [3]M. Kumar. “Study of reduction kinectics of iron ore pellets by noncoking coal”. Thesis of Master. National Institute of technology, Rourkela. 2009.
 [4]O. Dam. “The Influence of Nitrogen on the Swelling Mechanism of Iron Oxides During Reduction”. PhD Thesis .Univ. of London. 1983.
 [5]O. Dam and J. Jeffes. “Model for the Assessment of Chemical Composition of reduced iron ores from single measurements”. Ironmaking and Steelmaking Journal. Vol. 14, N`5. 1987.
 [6]O. Dam. “Efecto de la descomposición de gas de amoniaco (NH3) sobre el hinchamiento de óxidos de hierro durante reducción”. UCT Journal. Vol 100, 24. May 2020.
 [7]R. Agarwal and S. Hembram. “To Study the Reduction and Swelling Behavior Iron Ore Pellets”. BSc. Department of Metallurgical and Materials Engineering National Institute Of Technology, Rourkela. May 2013
 [8]Z. Chen , C. Zeilstra , J. Van der Stel , J. Sietsma & Y. Yang. “Review and data evaluation for high-temperature reduction of iron oxide particles in suspension”. Ironmaking & Steelmaking. Vol. 47. N°7. pp. 741-747. 2019.

Some problems of hydrogen in reactor structural materials: A review

The review concerns some problems of hydrogen in the main reactor structural materials used in the core of nuclear reactors. Zirconium alloys, steels, and vanadium alloys, as well as the hydrogen and helium synergetic effect exerted on the radiation resistance, are considered. The main sources resulting in the accumulation of hydrogen isotopes in reactor materials are discussed. The causes and consequences of hydride embrittlement of zirconium alloys at relatively low temperatures are analyzed. It is shown that hydrogen can induce the embrittlement of vessel steels through the weakening interatomic bond forces and a stabilization of radiation-induced defects. It is demonstrated that hydrogen in the presence of helium behaves like a gas that enhances the irradiation effect on the microstructure and properties of materials in many cases. The irradiation with simultaneous introduction of hydrogen and helium causes, in particular, a catastrophic swelling of chromium steels and vanadium alloys, whereas in the case of austenitic steel the effect is less pronounced.

Numerical simulation of iron whisker growth with changing oxygen content in iron oxide using phase-field method

Phase-field method is used to simulate the growth of iron whiskers on a gas–solid interface by changing the oxygen content in iron oxide. Results show that oxygen content significantly influences not only the crystal lattice but also the diffusion direction of iron ion vacancies and iron ions, wherein iron ions diffuse to low-oxygen-content areas and iron ion vacancies diffuse to high-oxygen-content areas. The catastrophic swelling of the micro-area volume and the growth of iron whiskers are caused by the enrichment of iron ions in low-oxygen-content areas. The simulation revealed the effects of the reduction rate and the oxygen potential of gas. Iron whiskers are longer and stronger when the oxygen potential of reducing gas decreases from −133.96kJ·mol−1 to −160.01kJ·mol−1 or when the reduction rate increases from 6.3×10−3mol·s−1 to 7.3×10−3mol·s−1. Iron whiskers can appear earlier with a low oxygen potential or a high reduction rate. Both oxygen potential and reduction rate promote the nucleation and growth of iron whiskers.

Effects of Reducing Gas on Swelling and Iron Whisker Formation during the Reduction of Iron Oxide Compact

AbstractThe effects of different types of reducing gas on swelling and iron whisker formation during the reduction of iron oxide compacts were investigated. The compacts sintered in air at 1273 K were reduced at 1173 K in different reducing atmospheres. The results indicated that catastrophic swelling can happen in CO but not when H2 is present in the reducing gas mixture. Scanning electron microscope (SEM) micrographs showed that catastrophic swelling was caused by a large amount of long iron whiskers formed during the reduction. The presence of N2 and CO2 in CO changed the amount of long iron whiskers and its distribution, which determined the extent of swelling.

Effects of Firing and Reduction Conditions on Swelling and Iron Whisker Formation during the Reduction of Iron Oxide Compact

The effects of firing and reduction conditions on swelling and iron whisker growth were investigated. The iron oxide compacts sintered in air were reduced in the tubular high temperature reactor by carbon monoxide or hydrogen. The experimental results showed that catastrophic swelling was suppresed by increasing the firing temperature or the firing time at moderate firing temperatures. The severest swelling happened in the reduction temperature of 900–1000°C with carbon monoxide. No catastrophic swelling was observed when hydrogen was used as the reducing gas. The SEM micrographs showed that catastrophic swelling was caused by a large amount of long whiskers formed during the reduction. The number of nucleation sites of whisker growth was limited when the firing temperature was increased above 1000°C, which also suppressed catastrophic swelling. On the basis of exprimental data and SEM results, a conceptual diagram was formulated to describe the effects of firing and reduction conditions on iron whisker growth and swelling.

Reduction swelling behaviour of haematite/magnetite agglomerates with addition of MgO and CaO

The influence of three kinds of CaO and MgO additives (dolomite, burnt lime and serpentine) on the reduction swelling behaviour of haematite–magnetite (H–M) concentrates pellets was studied. Burnt lime and dolomite increased the reduction swelling index of H–M oxidised pellets, while the reduction swelling index was able to be reduced when serpentine was added. CaO accelerated the formation and growth of metallic iron whiskers and led to abnormal swelling of the magnetite briquettes, while MgO was able to be dissolved in wüstite and reduced the migration rate of Fe2+; therefore, there was no catastrophic swelling in either the haematite or magnetite briquettes. As far as H–M concentrate pellets were concerned, because the solubility of CaO in magnetite was greater than that in the primary haematite and the secondary haematite generated from magnetite during the oxidation was easy to be reduced to wüstite, there was abnormal swelling in the reduced H–M pellets with CaO addition.

Influence of Manganese Oxide and Silica on the Morphological Structure of Hematite Compacts

AbstractDry compacts of pure Fe2O3 and Fe2O3 doped with either (2–6 mass%) MnO2, (2.5–7.5 mass%) SiO2 or with both (2–6% MnO2 + 7.5% SiO2) were indurated at 1373 K for 6 hours and physically and chemically characterized. The fired compacts were isothermally reduced with pure CO gas at 1073–1373 K. The O2‐weight loss was continuously recorded as a function of time using TGA technique. The external volume of pure and doped compacts was measured at different reduction conditions and the volume change was calculated. The structural changes accompanying the reduction process were visually and microscopically examined and the different phases were identified by X‐ray diffraction analysis. After firing, manganese ferrite (MnFe2O4) phase was identified in MnO2‐doped compacts. In pure Fe2O3 compacts, the external volume of compacts was increased with reduction temperature, showing a maximum swelling value at 1198 K. Catastrophic swelling was observed in MnO2‐doped Fe2O3 compacts, the volume change increased with MnO2 content showing catastrophic swelling in compacts containing 6%MnO2 at 1248 K. The catastrophic swelling was attributed to the formation of dense metallic iron whiskers and plates in a highly porous structure. Unlike in MnO2‐doped samples, no considerable volume changes were detected in SiO2‐doped Fe2O3 and (MnO2 + SiO2)‐doped Fe2O3 compacts where the presence of silica greatly hindered the swelling phenomenon at all reduction temperatures.

Influence of SiO<sub>2</sub> and/or MnO<sub>2</sub> on the Reduction Behaviour and Structure Changes of Fe<sub>2</sub>O<sub>3</sub> Compacts with CO Gas

Pure Fe 2 O 3 and Fe 2 O 3 doped with either 2.5-7.5mass% SiO 2 , 6.0mass% MnO 2 and (6.0%MnO 2 + 75%SiO 2 ) compacts annealed at 1 473 K for 6h were prepared. The different phases were identified by X-ray diffraction (XRD) and their structures were examined by optical and scanning electron microscopes. The magnetic properties were measured with Vibrating Sample Magnetometer (VSM). Total porosity and pore size distribution were determined and their external volume was also measured. Unlike in SiO 2 doped samples where no new phases were detected, manganese ferrite (MnFe 2 O 4 ) was identified in MnO 2 -containing samples. Annealed compacts were isothermally reduced with CO at 1 073-1 373 K and the O 2 -weight loss was continuously recorded. It was found that the reduction rate of SiO 2 -Fe 2 O 3 samples increases at the early stages with SiO 2 mass% due to the increase in their original porosity. At final stages, the rate was retarded due to the formation of hardly reducible fayalite (Fe 2 SiO 4 ). In MnO 2 -containing samples, the reduction rate was retarded at initial stages due to the presence of manganese ferrite. At later stages, the formation of fayalite-manganoan [(Fe, Mn) 2 SiO 4 ] greatly hindered the reduction process. A catastrophic swelling (ΔV= 405%) was recorded in 6%MnO 2 -Fe 2 O 3 compacts and was greatly diminished to 55.95% in presence of 7.5% SiO 2 due to the decrease in size and number of metallic iron whiskers and plates. The reduction mechanism of pure and doped samples was predicted from the correlations between reduction kinetics and the microscopic examinations of partially and completely reduced samples.

Volume Changes of Iron Oxide Compacts under Isothermal Reduction Conditions

Compacts made from chemically grade Fe2O3 were fired at 1473K for 6 hrs. The fired compacts were isothermally reduced either by hydrogen or carbon monoxide at 1073–1373K. The O2 weight‐loss resulting from the reduction process was continuously recorded as a function of time using TGA technique, whereas the volume change at different reduction conditions was measured by displacement method. Porosity measurements, microscopic examination and X‐ray diffraction analysis were used to characterize the fired and reduced products. The rate of reduction at both the initial and final stages was increased with temperature. The reduction mechanism deduced from the correlations between apparent activation energy values, structure of partially reduced compacts and application of gas‐solid reaction models revealed the reduction rate (dr/dt) at both the initial and final stages. At early stages, the reduction was controlled by a combined effect of gaseous diffusion and interfacial chemical reaction mechanism, while at the final stages the interfacial chemical reaction was the rate determining step. In H2 reduction, maximum swelling (80%) was obtained at 1373K, which was attributed to the formation of metallic iron plates. In CO reduction, catastrophic swelling (255%) was obtained at 1198K due to the formation of metallic iron plates and whiskers.

Reduction Kinetics and Catastrophic Swelling of MnO2-doped Fe2O3 Compacts with CO at 1073-1373 K

Pure Fe2O3 and Fe2O3 doped with either 2, 4, or 6 mass% MnO2 annealed at 1473 K for 6 h were isothermally reduced with carbon monoxide at 1073–1373 K. The oxygen weight loss resulted from the reduction at a given temperature was continuously recorded as a function of time. Reflected and scanning electron microscopes were used to characterize the annealed and reduced samples whereas the different phases developed were identified by X-ray phase analysis technique. The external volume of partially and completely reduced samples was measured by displacement method and the volume change (ΔV%) was calculated. At a given temperature, the influence of MnO2 mass% on the reduction behaviour and volume change of Fe2O3 compacts was investigated. The doping of MnO2 showed different effects during the reduction of Fe2O3 which is temperature dependant. At <1198 K, the rate of reduction decreased at early stages with the increase in MnO2 mass% due to the presence of hardly reducible manganese ferrite phase (MnFe2O4). At final reduction stages the retardation effect was attributed to the formation of dense iron manganese oxide (FeO0.899, MnO0.101). At ≥1198 K, the presence of MnO2 promoted the reduction of Fe2O3 and the catastrophic swelling resulted from the formation of both metallic iron plates and whiskers was observed. Maximum swelling (ΔV%) was measured at 1198 and at 1248 K for pure Fe2O3 and MnO2-doped compacts respectively and it increased with the increase in MnO2 mass% resulting about 405% for 6 mass% MnO2-doped samples. The reduction mechanism was predicted from the correlation between the apparent activation energy values, testing of different mathematical formulations derived from gas-solid reaction model and the microscopic examination of partially reduced compacts.

Kinetics and catastrophic swelling during reduction of iron ore in carbon bearing pellets

The rate of reaction between iron oxides and charcoal, agglomerated together as pellets, has been correlated with the swelling of the pellets and with the morphology of the iron formed. The experimental techniques employed were thermogravimetric analysis, X-ray diffraction analysis, and microscopic examination of partially reduced pellets. The experiments were performed in the temperature range 1223–1423 K. The conditions that favour the growing of iron in the form of whiskers, and hence lead to catastrophic swelling, have been determined.

Structural changes occurring during reduction of hematite and magnetite pellets containing coal char.

The structural changes that occurred during the reduction of hematite and magnetite pellets containing coal char, in the temperature range between 900°C and 1200°C, were studied.The behavior of the pellets was complex. Significant temperature gradients within the pellet were attained during the process, and the reduction was accompanied by catastrophic swelling of the pellets between 900°C and 1000°C, while shrinkage was observed at 1100 and 1200°C. Observed microstructural features such as intragranular porosity and cracking of oxide grains resulted from severe lattice disturbances occurring upon transformation of hematite to magnetite.The catastrophic swelling observed for hematite and magnetite pellets 900 and 1000°C was due to the filamentary or whisker growth of iron outward from the wustite surfaces. The whisker formation appears to be due to a changing reducing potential of the gas phase within the pellet coupled with the presence of calcium oxide on the wustite surface. The shrinkage reported at 1100 and 1200°C is accounted for in terms of sintering of iron filaments.The pellets strength was found to decrease between 900°C and 1000°C, and increased at higher temperatures. The weakness at low temperatures was due to the absence of bonding between iron filaments or whiskers. The high strength obtained at higher temperatures (1100 and 1200°C) was produced by sintering of iron filaments.

The Softening Properties of Alkali-Containing Iron Oxide Briquetts Under Reducing Conditions

Abstract The behaviour of alkali-containing sintered hematite briquettes was studied under simulated blast furnace conditions. Alkalis lower the strength of the briquette because of the formation of oxide phases which have low softening temperature ranges. Softening is more pronounced with sodium than with potassium. The shrinkage of the briquette during reduction increases with both alkali concentration and temperature. The detrimental effects of alkalis are neutralized when the silica content is high. Magnesia additions increase the strength of the silica-containing briquettes. However, if lime or alumina is added, their strength decreases. Catastrophic swelling occurs only at low loads. Resume Le comportement de briquettes d'hematite frittees contenant des alcalins a ete etudie en stimulant les conditions d'un haut-fourneau. Les alcalins abaissent la resistance de la briquette a cause de la formation de phases d'oxydes qui ramollissent dans les domaines de temperatures peu elevees. Ce ramollissement es...