Spinel Solutions Research Articles

Toward a tunable fabrication of multifunctional iron-aluminum spinels via solution combustion synthesis: The effects of fuel, heating mode, and Fe:Al precursor ratio

The solid solutions of iron and aluminum spinels (hercynite, maghemite, magnetite, and γ-alumina) are promising materials for emerging technologies such as solar thermochemical fuel production and clean dehydrogenation of fossil fuels. Solution combustion synthesis (SCS) is an attractive technique for the fabrication of these materials as it has been used for synthesis of many nanoscale oxides. However, the design space of SCS is large with many synthesis parameters affecting the properties of the combustion products. To optimize the SCS of these materials, it is important to determine which parameters yield the best properties. In the present work, FeAlOx nanocomposites were obtained by SCS using iron nitrate and aluminum nitrate as precursors and oxidizers. Two fuels (citric acid and glycine), three Fe:Al molar ratios (1:2, 1:1, and 2:1) in the precursors, and three heating modes (hotplate, muffle furnace, and microwave oven) were compared. The products were characterized by X-ray diffraction analysis, scanning electron microscopy, Brunauer-Emmett-Teller surface area analysis, and laser diffraction particle size analysis. The fuel used had the largest impact on the combustion behavior and hence the material properties. The high combustion temperature of glycine allowed for the formation of an FeAl2O4/Fe3O4 solid solution, while the lower-temperature combustion of citric acid yielded Al-substituted γ-Fe2O3 with high amorphous content. With citric acid, the specific surface area as high as 200 m2/g was achieved. The effect of Fe:Al precursor ratio was clearly seen in variations of the lattice parameter, which demonstrated the ability to tune the phase composition. The heating mode had minor effects on material properties, but some differences were observed. All products obtained by SCS in a microwave oven had low specific surface areas compared to those obtained with a hotplate and a muffle furnace.

Copper Stabilization in Glass-Ceramics: Effect of CaO and SiO2 on CuAlxFe2-xO4 Formation

Industrial activities discharge heavy metal-laden wastes; and copper is one of these metals that can be stabilized after ceramic sintering by Al- or Fe-containing precursors. Besides traditional precursors, Al- and Fe-containing solid wastes (e.g., sewage sludge incineration ash, coal fly ash) were found equally capable of copper stabilization when they were adopted in the reaction matrices. However, the effect of CaO and SiO2, the predominant components in most of the inorganic solid wastes, on metal transformation and stabilization has not been addressed. Therefore, a CaO-SiO2-CuO-Al2O3-Fe2O3 reaction matrix was proposed in this study, focusing on the effect of CaO and SiO2 on copper transformation and immobilization. Results showed that a single-phase CuAlxFe2-xO4 spinel solution was developed in the system without CaO and SiO2, whereas the addition of CaO led to the generation of two separate spinel phases: CuAl2O4 and CuFe2O4. Compared with CaO, the existence of SiO2 showed less impact on the phase transformation. The effect of sintering temperature and additive dosage was also illustrated in this study and the results provided useful guidance for the optimization of reaction systems to achieve beneficial utilization of solid wastes for heavy metal stabilization.

Thermodynamic assessment of the Fe–V–O system

The Fe–V–O system over the whole composition range was optimized according to the reliable phase equilibria and thermodynamic data. The modified quasichemical model was used to describe the liquid phase from metal alloy to oxide melt. Based upon the Compound Energy Formalism, the FeV2O4–Fe3O4 spinel solution was described by a sublattice model considering the cation distribution between tetrahedral and octahedral sites. Wüstite, corundum and (VO2)s.s. were described using a simple Bragg-Williams model. A set of self-consistent model parameters was obtained and the available phase diagram and thermodynamic data were reproduced well within experimental error limits. The complex phase relations in the Fe–V–O system at various temperatures and oxygen partial pressures were elucidated.

Improved hydrogen production with 100% fuel conversion through the redox cycle of ZnFeAlOx oxygen carrier in chemical looping scheme

Chemical looping is a promising way to produce hydrogen due to the inherent purification feature and CO2 capture. Operating oxygen carrier at high reduction degree is desirable because it can improve hydrogen production yield and reduce solid flux rate in a fluidized bed chemical looping reactor. However, the high reduction degree generally has a negative effect on cyclic stability, necessitating the trade-off between the hydrogen production reactivity and stability. In this article, we prepared a new Zn-Fe-Al mixed spinel as the oxygen carrier showing both the high reactivity and stability. It has the nearly 100% fuel conversion and stable hydrogen yield of ~2.23 mmol/g, which is about two times higher than that of Fe2O3 material. The mechanism study indicates that the high cyclic performance is due to the embedded interface structure induced by the reversible resolution of Fe active component in the spinel solution. This work provides a new design scheme of oxygen carrier for other chemical looping applications.

Thermodynamic modeling of isotherms of oxygen solubility in liquid metal of Fe – Mg – Al – O system

Studying the interaction between oxygen and magnesium and aluminum dissolved in liquid iron is an important task in order to choose optimal parameters for refining and casting of steels. Relevance of this research is caused by determining the possibility and conditions for formation of unfavorable refractory particles of magnesium oxideand magnesian spinel in a metal melt. In the course of this research, thermodynamic modeling of phase equilibria implemented in liquid metal of such systems as Fe – Mg – O, Fe – Al – O and Fe – Mg – Al – O within the temperature range of 1550 – 1650 °С was carried out. Calculation was made using the technique of constructing the solubility surfaces for the metal components which connects quantitative changes in composition of a liquid metal with qualitative changes in composition of products obtained as a result of interaction of a metallic melt’s components. The modeling method was based not only on usin­g equilibrium constants of reactions occurring between components of the systems under research in the selected temperature range, but also on taking into account the values of interaction parameters of the first order (according to Wagner) of elements in liquid iron. In order to simulate activities of the oxide melt conjugated with the metallic one, approximation of the theory of subregular ionic solutions was used. To model activities of oxides solid solution, approximation of the theory of regular ionic solutions was used. And the theory of ideal ionic solutions was used for the solid solution of spinels. In the course of the work, isotherms of oxygen’s solubility in liquid metal of systems Fe – Mg – O, Fe – Al – O and Fe – Mg – Al – O have been constructed, and regions of thermodynamic stability of oxide phases conjugated with the metallic melt have been determined. In particular, compositions area of a liquid metal which is going to be in equilibrium with the solid solution of spinels | FeAl 2 O 4 , MgAl 2 O 4 | solid solution has been determined for Fe – Mg – Al – O system. Results obtained in the course of thermodynamic modeling have been compared to experimental data.

An Experimental Study on the Reduction Behavior of Dust Generated from Electric Arc Furnace

To improve the utilization value of electric arc furnace dust (EAFD) containing zinc, the reduction behavior of non-agglomerate dust was investigated with carbon and ferrosilicon in an induction furnace. The experimental results show that when the temperature increases, the zinc evaporation rate increases. When the reducing agent is carbon, zinc evaporation mainly occurs in the range of 900–1100 °C. When the reducing agent is ferrosilicon, zinc begins to evaporate at 800 °C, but the zinc evaporation rate is 90.47% at 1200 °C and lower than 99.80% with carbon used as a reducing agent at 1200 °C. For the carbon reduction, the iron metallization rate increases with a rise in the temperature. When the reducing agent is ferrosilicon, with an increase in temperature, the metallization rate first increases, then decreases, and finally, increases, which is mainly due to the reaction between the metallic iron and ZnO. In addition, the residual zinc in the EAFD is mainly dispersed in the form of a spinel solution near the metallic phase.

Thermodynamic Modeling of Isotherms of Oxygen Solubility in Liquid Metal of the Fe–Mg–Al–O System

In this paper, we studied the interaction of magnesium and aluminum dissolved in liquid iron with oxygen, which is an important problem for choosing the optimal parameters of steel refining and casting. The relevance of the study is determined by the possibility and conditions for the formation of adverse refractory particles of magnesium oxide and magnesian spinel in the melt. We performed the thermodynamic modeling of phase equilibria realized in the liquid metal of the Fe–Mg–O, Fe–Al–O, and Fe–Mg–Al–O systems in the temperature range of 1550–1650°C. The calculation was carried out using a technique for constructing the surface of the component solubility in a metal, which associates quantitative changes in the liquid metal composition with changes in the composition of the interaction products of molten metal components. The modeling method was based both on the use of equilibrium constants of reactions occurring between the components of the studied systems in the selected temperature range and on the account of the values ​​of the first-order interaction parameters (according to Wagner) of elements in liquid iron. To simulate the activities of an oxide melt conjugated with a metal melt, the approximation of the theory of subregular ionic solutions was used. The approximation of the theory of regular ionic solutions was used to model the activity of a solid solution of oxides while the theory of perfect ionic solutions was used to model the activity of a solid solution of spinels. The isotherms of oxygen solubility in the liquid metal of the Fe–Mg–O, Fe–Al–O, and Fe–Mg–Al–O systems were plotted, and the regions of thermodynamic stability of oxide phases conjugated with the metal melt were determined. In particular, for the Fe–Mg–Al–O system, the region of liquid metal compositions, in equilibrium with which there will be a solid solution of spinels |FeAl2O4, MgAl2O4|ss, was determined. The obtained results of thermodynamic modeling are compared with experimental data.

Kinetics of the topochemical reaction of the solid solutions of magnesia spinels: Mg(Cr0.5Fe0.5)2O4, Mg(Al0.5Cr0.5)2O4, Mg(Al0.5Fe0.5)2O4 with sulphur oxides

Spinel-containing materials belong to an important group of refractories used as high-temperature unit linings. A crucial element of the conditions in which they are used is gaseous corrosion caused by sulphur oxides: SO2 and SO3. In previous investigations into reactions of magnesia spinels with sulphur oxides it was found that spinels’ reactivity could be considerably influenced by a phase transition (order – disorder) in the cation sublattice, resulting in a change of their reactivity in relation to SO2/SO3.The aim of the study was to investigate the kinetics of topochemical reactions between equimolar solid solutions of magnesia spinels and sulphur oxides before and after the order – disorder phase transformation in the structure begins. Research into the reaction of equimolar solid solutions Mg(Cr0.5Fe0.5)2O4, Mg(Al0.5Cr0.5)2O4, Mg(Al0.5Fe0.5)2O4 with SO3 was undertaken due to the fact that spinels form solid solutions in basic refractories. To conduct kinetic measurements, a semi-flow reactor was designed and constructed, in which investigations were carried out at the temperatures of 773 and 973 K and time range: 0–7 h. A mixture of air and SO2 (13%) was used.The obtained results have been compared with the kinetic results from the previous work obtained on two-cation spinels: MgAl2O4, MgFe2O4, MgCr2O4. The influence of the degree of inversion in spinel structure on the kinetics of the process was discussed.

Mass Spectrometry Study of the Thermodynamic Properties of Spinel Solid Solutions

The partial pressure values of vapor species over spinel solid solutions and the boundary of the solid spinel solutions in the MgAl2O4 (solid solution) + Al2O3 region are determined on the basis of the data obtained by Knudsen effusion mass spectrometric method in the temperature range 2000–2200 K. The obtained thermodynamic information is compared to the information available in the literature. The values of oxide activities, Gibbs energies, enthalpies and entropies of formation of solid solutions show a slightly negative deviations from ideality.

Enhanced hydrogen production performance through controllable redox exsolution within CoFeAlOx spinel oxygen carrier materials

Chemical looping performance can be improved through the controllable exsolution of the redox phase in a spinel solution.

Corrosion of high chromia refractory materials by basic coal slag under simulated coal gasification atmosphere

In this paper, dynamic corrosion experiment of a high chromia refractory interaction with basic coal slag under slagging gasifier conditions was conducted by using rotary drum corrosion test with the FactSage thermodynamic analysis. The microstructures and chemical compositions of the corroded samples were analyzed by scanning electron microscopy (BEI and EDS), and the corrosion mechanism was investigated by combining thermodynamic simulation and SEM analysis. The results show that the simulation results were consistent with the results of corrosion test. Reaction layer and penetration layer are formed from the surface to the interior of the sample after corrosion. The (Mg, Fe) (Al, Cr)2O4 spinel solution was formed in the reaction layer, which make the matrix structure become dense and change the overall structure of the particles’ uniformity. Corrosion of Cr2O3 aggregate is relatively weak by slag. The Cr2O3 dissolves into the slag through the formed spinel solution layer on the surface of aggregates. While, Cr2O3 and Al2O3 dissolve into molten slag through the spinel solution layer formed in the matrix. ZrO2 in the matrix directly dissolved into molten slag and penetrates inner the matrix with the penetration of the slag to form a ZrO2-free region. The liquid sintering of the matrix has happened in the melt, causes the structure of the penetration layer become dense, which is different from that of the original sample.

Investigation of the Equilibrium Germanium-Substituted Wustite with Metal and Spinel Solutions in the Fе-Gе-O System

The phase equilibrium in the Fe-Ge-O system is investigated by measuring EMF of galvanic elements with solid oxygen-conductive electrolytes. The samples for investigation were prepared by the solid phase reaction at 900 °C. The samples were analyzed to determine the total and phase composition. The gross composition of the samples was refined by chemical analysis. The iron content was determined by the standard method of bichromatometric titration, while that of germanium was determined by the photometric method, which is based on measuring the optical density of the colored colloid solution of germanium phenyl fluoronate, which is formed upon the interaction of germanium dioxide with phenylfluorone in the acidic medium. The phase composition was established using X-ray diffractometry and electron probe microanalysis using a DRON-3 diffractometer and a JEOL JSM-6460LV computer controlled scanning (raster) microscope. The experimental function of EMF galvanic elements of temperature is formulated. After processing the obtained data, the equilibrium oxygen pressures over the samples were calculated, the solubility of germanium in iron monoxide (wustite) was determined, the phase composition of the regions bordering the wustite region was established, the isothermal sections of the state diagram of the system were constructed at 1173 and 1273 K in the equilibrium region of the wustite solution with metallic and spinel phases

Thermodynamic modeling of oxide phases in the Fe–Mn–O system

A critical evaluation and thermodynamic modeling for thermodynamic properties of all oxide phases and phase diagrams in the Fe–Mn–O system are presented. Optimized Gibbs energy parameters for the thermodynamic models of the oxide phases were obtained which reproduce all available and reliable experimental data within error limits from 298K to above the liquidus temperatures at all compositions covering from known oxide phases, and oxygen partial pressure from metal saturation to 0.21bar. The optimized thermodynamic properties and phase diagrams are believed to be the best estimates presently available. Two spinel phases (cubic and tetragonal) were modeled using Compound Energy Formalism (CEF) with the use of physically meaningful parameters. The present Fe–Mn spinel solutions can be integrated into a larger spinel solution database, which has been already developed. The database of the model parameters can be used along with a software for Gibbs energy minimization in order to calculate any type of phase diagram section and thermodynamic properties.

Thermodynamic Stability of Spinel Phase at the Interface Between Alumina Refractory and CaO–CaF2–SiO2–Al2O3–MgO–MnO Slags

The interfacial reaction between alumina refractory and CaO–CaF2–SiO2–Al2O3–MgO–MnO slag was observed at 1873 K to estimate the stability of the spinel phase using computational thermodynamics under refining conditions of Mn‐containing steels. The concentration of MnO formed by the slag–steel reaction in the CaO–CaF2–SiO2–Al2O3–MgO melts generally increased by decreasing the CaO/SiO2 ratio of the initial melts. No intermediate compounds were formed at the refractory–slag interface when the initial CaO/SiO2 ratio was 0.5, whereas CaAl12O19 (CA6) and Mg(Mn)Al2O4 (spinel), identified from TEM analysis using EDS mapping and SAED patterns, were observed at the refractory–slag interface when the CaO/SiO2 ratio was 1.0 or greater. The (at.%Mg)/(at.%Mn) ratio in the spinel solution increased by increasing the CaO/SiO2 ratio, which originated from the fact that MgO activity continuously increased as the CaO/SiO2 ratio increased. From thermodynamic analysis considering the equilibrium constant (KSP) and activity quotient (QSP) of the spinel formation reaction at the slag–refractory interface and the bulk slag phase, the precipitation–dissolution behavior of the spinel phase was predicted, which exhibited good consistency with the experimental results. Hence, the dissolutive corrosion mechanism of alumina refractory into the CaO–CaF2–SiO2–Al2O3–MgO–MnO slag was proposed.

INVESTIGATION OF WUSTITE AND SPINEL SOLUTION EQUIBALANCE IN THE FE–GE–O SYSTEM

The method of EMF measurement of a galvanic cell with solid electrolyte (EMF method) has been used for examination of the equilibrium conditions of wustite and spinel solutions in the Fe-Ge-O system in the temperature range of Т = 1100÷1300 K. X-ray diffraction and micro X-ray spectral methods were used for determination of phase composition. It is found that magnetite and iron germanate are restrictedly dissolved in each other. Isothermal sections of the phase diagram of the Fe–Fe3O4–Fe2GeO4 system are plotted at Т = 1273 and 1173 K.

High Chrome Refractory Characterization: Part I. Impact of Melt Reduction/Oxidation on the Corrosion Mechanism

High Cr2O3 containing Monofrax™ K‐3 is a robust refractory that is used in the fiberglass industry and used in radioactive waste glass melters worldwide. Monofrax™ K‐3 is tolerant of transition metal oxides but contains highly reduced solid solutions of spinels, that is, (Mg,Fe2+)(Al,Cr)2O3. Conversely, many of the waste feeds being processed are highly oxidizing. The K‐3 refractory corrosion was tested in sealed crucibles starting with slurried melter feed instead of prereacted glass called for by ASTM C621. Testing the refractory coupon during the feed‐to‐glass conversion exposes the refractory to the oxidizing and reducing species being released during vitrification, for example, NO3−, NO2−, CO2, CO, O2. Corrosion rates measured in highly oxidizing (high nitrate) feeds were ~1.8–2.8 times higher than those determined using prereacted glass or reduced feeds. Confirmatory corrosion rates were measured on Monofrax™ K‐3 coupons immersed in oxidizing feed in a 1/100th‐scale HLW pilot‐scale melter. Corrosion is heterogeneous or incongruent as Ni and Fe in the waste glass exchange with Mg and Al in the refractory. An insoluble NiFe2O4 spinel corrosion product is formed that can build up a protective layer along the refractory walls or spall and settle to the melter floor depending on melt pool convection/agitation.

Critical Evaluation and Thermodynamic Modeling of the Mg–Mn–O (MgO–MnO–MnO2) System

All phase diagram and thermodynamic data of the MgO–MnO and MgO–MnO–MnO2 systems were critically evaluated and thermodynamically optimized to obtain one set of thermodynamic model parameters. The MgMn2O4–Mn3O4 spinel solutions were modeled with two sublattices Compound Energy Formalism by considering the cation distribution between tetrahedral and octahedral sites. The slag phase and monoxide solid solution were also described using the Modified Quasichemical Model and Bragg–Williams random mixing model, respectively. The optimized thermodynamic model parameters can be used with a Gibbs energy minimization routine to calculate the phase diagram and thermodynamic properties of the system at oxygen partial pressures between metal saturation and 1 atm.

Critical evaluation and thermodynamic modeling of the Al–Mn–O (Al2O3–MnO–Mn2O3) system

Abstract In the present study, a complete thermodynamic description of the Mn–Al–O (MnO–Mn 2 O 3 –Al 2 O 3 ) system has been carried out. All the experimental phase diagram data from metal saturation to 1 atm oxygen partial pressure, thermodynamic data for spinel and liquid phases and structural data for spinel were critically evaluated and optimized in order to obtain Gibbs energies of all phases as functions of temperature and composition. The MnAl 2 O 4 –Mn 3 O 4 spinel solution was modeled using the Compound Energy Formalism considering the cation distribution between tetrahedral and octahedral sites. The slag phase was described using the Modified Quasichemical Model considering both, the Mn 2+ and Mn 3+ species.

Investigation into the equilibrium of the wustite and spinel solutions in the Fe-Ge-O system

Equilibrium conditions of the wustite and spinel solutions in the Fe-Ge-O system in temperature range T = 1100–1300 K are investigated by measuring the emf of the galvanic element with solid electrolyte (the emf method). X-ray diffractometry and electron probe microanalysis were used to determine the phase composition. It is established that the iron magnetite and germanate are partially miscible in one another. The isothermal sections of the phase diagram of the Fe-Fe3O4-Fe2GeO4 system at T = 1273 K and 1173 K are constructed.

ChemInform Abstract: Critical Assessment and Thermodynamic Modeling of the Cu—Fe—O System

AbstractReview: 99 refs.