Initial stages of coal slag interaction with high chromia sesquioxide refractories

Slagging coal gasifiers operate at temperatures as high as 1650°C in a reducing environment, requiring combustion chambers to be lined with refractories. The liner materials of choice are semi-porous high chromia refractories. Recently, a new series of high-chromia aluminia sesquioxide refractories have been developed. Both long term and short term tests are being conducted to evaluate the performance of these materials. In this study, the initial stage of slag-refractory interactions was analyzed. Samples of gasifier slag were compacted and placed upon the surface of these new chromia refractories and the temperature was raised consistent with start-up operating conditions of commercial gasifiers. The slag was completely molten by the time the furnace achieved a temperature consistent with gasifier operation conditions: 1350°C. Measurement of the slag contact angle, slag spread along the slag-refractory interface, and the loss of slag due to slag infusion into the refractory were monitored by camera. Analysis suggests a single phenomenon with an activation energy of approximately 54 kcal may be the controlling factor. Cross-section analysis of the sample and analysis of slag chemistry indicate that slag infusion preceded the slag-refractory interface front movement and that the iron component of the slag was becoming concentrated at the slag-refractory interface leading to the formation of a chromium-iron spinel phase. Results of these short term tests are critical in characterizing and understanding the results long term slag-refractory interactions.     

Effect of BaSO4 on the interfacial phenomena of high-alumina refractories with Al-alloy

The performance of high-alumina refractories used for aluminium casting significantly impacts the efficiency of metal production. The interfacial reactions with Al-alloys cause corundum and magnesium spinel deposition on the refractory surface, leading to refractory degradation. An experimental study was conducted to investigate the influence of varying barium sulphate (BaSO₄) concentrations in a high-alumina refractory on its interfacial reactions with molten Al-alloy in a horizontal tube furnace at 1523 K (1250 °C) under inert conditions. This study showed that the Al-alloy reactions with pure BaSO₄ would form barium aluminates at the interface. However, in the Al-alloy/refractory system, the interfacial behaviour was strongly influenced by the relative amount of BaSO₄, such that up to 5 wt%, the extent of alloy penetration into the refractory increased with increasing BaSO₄ contents. Electron-probe micro-analyser and X-ray diffraction studies indicated that the composition of the interface for these refractories was augmented with barium silicates and diminishing anorthite phases. In the presence of 10 wt% BaSO₄, the extent of metal penetration into the refractory decreased, whilst for 20% BaSO₄, the penetration was higher; these results were attributed to the interfacial presence of celsian (BaAl₂Si₂O₈) and unreacted barium sulphates, respectively. This study suggests that maximising the celsian formation at the interface is critical for optimising the BaSO₄ concentration for improving the refractory’s performance for Al-casting.     

Phase equilibrium,microstructure and properties of some magnesite-chromite refractories

The effect of phase equilibrium and microstructure of magnesite-chromite batches containing from 0 to 100% Egyptian chrome ore, with intervals of 10%, on their physical properties was studied. The phase equilibrium data were calculated using the phase relationships within the system M-M₂S-CMS-MR (M=MgO, S=SiO₂, C=CaO, R=R₂O₃). A computerized electron-probe microanalyser was applied to study the microstructure as well as microchemistry of the fired magnesite-chromite co-clinkers. Some physical and technological properties of the co-clinkered briquettes were also investigated by determining densification parameters, spalling resistance and load-bearing capacity.It is concluded that dense, spalling resistant and refractory magnesite, magnesite-chrome and chrome-magnesite refractories could be produced by co-clinkering of magnesite-chromite batches of 1000, 7030 and 3070 weight ratios, respectively, at 1600 °C. The prepared co-clinkers were subsequently graded, moulded and refired up to 1700 °C in order to obtain direct-bonded bricks. Meanwhile, dense chromite refractories with lower refractory quality could be processed by firing the Egyptian chrome ore up to 1600 °C.     

Dispersant-assisted hydrothermal synthesis of MnZn ferrites from raw oxides

MnZn-ferrite powders were prepared using hydrothermal syntheses of a homogenous mixture of the raw oxides, i.e., Fe₂O₃, ZnO and Mn₃O₄, at 280°C in air. The hydrothermal synthesis was performed in the presence of various amounts of an anionic dispersant. The final results of the hydrothermal reaction between the raw oxides were fine powders with a heterogeneous phase composition mostly composed of iron oxide and spinel products. The composition of the spinel products depended to a great extent on the amount of dispersant in the hydrothermally treated suspension. Without the dispersant addition, Zn ferrite and Zn manganate spinel products were formed, while in the presence of the dispersant, the ferrimagnetic MnZn-ferrite spinel product was obtained. A larger amount of the dispersant in the reaction mixture increased the conversion rate of the raw oxides into the Mn,Zn ferrite spinel product. Additionally, polyvinyl alcohol (PVA) was used during the hydrothermal synthesis in order to bind the chlorine impurities, introduced into the hydrothermally prepared powder with the raw Fe₂O₃. With the PVA burnout, the level of chlorine impurities was decreased by approximately 50%.     

Processing,microstructure and properties of ultrasonically processed in situ MgO–Al2O3–MgAl2O4 dispersed magnesium alloy composites

AZ91 alloy matrix composites are synthesized by in situ reactive formation of hard MgO and Al₂O₃ particles from the addition of magnesium nitrate to the molten alloy. The evolved oxygen from decomposition of magnesium nitrate reacts with molten magnesium to form magnesium oxide and with aluminium to form aluminium oxide. Additionally, these newly formed oxides react with each other to form MgAl₂O₄ spinel. Application of ultrasonic vibrations to the melt increased the uniformity of particle distribution, avoided agglomeration, and decreased porosity in the castings. Ultrasound induced physical phenomena such as cavitation and melt streaming promoted the in situ chemical reactions. Well dispersed, reactively formed hard oxides increased the hardness, ultimate strength, and strain-hardening exponent of the composites. Presence of well-dispersed hard oxide particles and stronger interface resulting from cavitation-enhanced wetting of reactively formed particles in the AZ91 alloy matrix improved the sliding wear resistance of the composites.     

Phase relations in the oxygen rich region of the CrWO ternary system

The phases occurring in the ternary CrWO system at 1370 K have been determined using x-ray diffraction and electron and optical microscopy. It was found that at this temperature no Cr enters WO₃ or the other tungsten oxides and no ternary CS phases appear to form. Instead, the Cr reacts to form one or two ternary rutile phases and equilibrium lies between them and the appropriate binary tungsten oxide. The compositions of these ternary oxides are Cr₂WO₆ and a previously unreported phase CrWO₄. No extended homogeneity ranges have been detected for these oxides. A phase diagram summarises the results which are also considered in the light of the formation of crystallographic shear phases in tungsten oxides.     

Nanoscale Oxide Formation at α-Al2O3–Nb Interfaces

Parts for metallurgical applications made from refractory metal–ceramic composites offer improved thermal shock resistance due to their capability for resistive heating compared to ones made solely from ceramics such as Al₂O₃. The combination of Al₂O₃ and Nb is intriguing as both show similar thermal expansion behavior over a wide temperature range. The high affinity of Nb for O to form nonprotective oxides, however, hampers its use in oxidative environments. Formation of such phases at the ceramic–metal interface can have detrimental effects on the cohesion of the composites. For this work, nanocrystalline Nb films are deposited on sapphire substrates by magnetron sputtering to study diffusion of O and high-temperature phase formation at a refractory metal–ceramic interface during heat treatment under Ar at 1600 °C. A combined approach of atom probe tomography and transmission electron microscopy for compositional and crystallographic analyses reveals that at triple junctions of the sapphire–Nb interface with Nb grain boundaries, heterogeneous nucleation of nanoscale NbO₂ occurs, which further reacts with Al₂O₃ to form AlNbO₄, while the Nb film itself remains metallic. Fast O transport through grain boundaries leads to internal oxidation at the interface, whereas regions further away from Nb grain boundaries remain unchanged.     

Synthesis of hexagonal plate-like Al4Si2C5 and the effect of Al4Si2C5 addition to Al2O3–C refractory

Hexagonal plate-like Al₄Si₂C₅ particles were synthesized for the first time via a carbothermal reduction process with controlled heating temperature and raw materials ratio, and their oxidation behavior was investigated. Al₄O₄C, Al₂OC, SiC and Al₄SiC₄ formed as intermediate products when the batch mixture was heated in argon atmosphere, and Al₄Si₂C₅ then formed at above 1800 °C. Possible reaction mechanisms responsible for the formation of this ternary carbide were discussed based on the reactions at the initial and later stages of the carbothermal reduction process. Al₄Si₂C₅ added to the Al₂O₃–C refractory initially reacts with CO to form Al₂O₃, SiO₂ and C. After the reaction, Al₂O₃ react with SiO₂ to form mullite on the surfaces of the refractories, which inhibit the oxidation of the refractories.     

Morphology and formation mechanism of the pyrochlore phase in ZnO varistor materials

The secondary phases in ZnO varistor materials, having Bi₂O₃, Sb₂O₃, MnCO₃ and CoO as additives, were investigated using transmission electron microscopy with an energy dispersive X-ray spectrometer and X-ray diffractometer scans. The information about the morphology of the pyrochlore phase and its formation reaction were obtained. The pyrochlore phase is formed from the reaction between the spinel and Bi₂O₃ phase. Below 1200° C, the spinel phase reacts with the liquid Bi₂O₃ phase present at the multiple grain junction and forms the pyrochlore phase. The pyrochlore phase nucleates at the corners of multiple grain junction adjacent to the spinel grain and grows towards the centre of the grain junction.     

New Approach for the Application of Functional Ceramic Material in Carbon Bonded Doloma Refractories to Reduce Emissions

Doloma carbon bricks with graphite contents of approximately 2 wt% are widely used in the production of stainless steels in argon oxygen decarburisation (AOD) or in vacuum oxygen decarburisation (VOD) vessels as lining material. The application of doloma refractories is connected with metallurgical benefits such as high oxidic stability of its oxides, and the ability to bond sulphur from the hot metal. The production and application of carbon bonded refractories is linked with environmental harmful emissions in the broadest sense. Amongst the aspect of environmental friendly refractory systems this work has observed and shown the interaction of functional ceramic material TiO₂ with the organic binder system. In the centre of this work is the aspect of increased residual carbon content of the binder resin due to TiO₂ addition. The increased residual carbon content of the binder resin connected with improved mechanical, physical and thermomechanical properties due to sub‐micro scaled TiO₂ addition offers the feasibility to reduce the total carbon content without downgrading the brick properties. This aspect has not been observed yet and is of high interest with respect to reduced emissions and environmental friendly refractories. Previous works have investigated the influence of TiO₂ on other carbon bonded refractory systems such as alumina carbon and magnesia carbon. As illustrated in this work and previous work, TiO₂ is working completely different in the Doloma Carbon system from other systems.     

Mechanistic investigation of the field-activated combustion synthesis (FACS) of tungsten carbide with or without nickel additive

The activation of self-propagating combustion reactions in the W-C system and its composite with Ni as additive was achieved by using an electric field. The reaction mechanisms of Field-Activated Combustion Synthesis (FACS) of tungsten carbide and its composite have been investigated by using sample-quenched method. Through turning off electric field during FACS process, a series of combustion products with different phase compositions have been obtained. Layer to layer X-ray and microscopic analyses of these combustion products across quenched combustion front suggested that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. W₂C is the intermediate phase between WC and reactants (W and C). Metal additive produces liquid phase and accelerates the diffusion between solid reactants (W and C), which facilitates the formation of W₂C and the transformation from W₂C to WC phase. Moreover, melted Ni reacts with W and W₂C to form mixed compounds of type W _x C _y M _z .     

In situ high-temperature X-ray and neutron diffraction of Cu–Mn oxide phases

Copper–manganese oxides were analyzed by in situ high-temperature powder neutron and X-ray diffraction to investigate their crystal structure. Cu–Mn spinel was found to form a continuous solid solution with cubic symmetry between Mn₃O₄ and Cu₂MnO₄. A high-temperature phase with approximate composition Cu₅Mn₄O₉ was shown to have hexagonal symmetry. The cation distribution and lattice parameters of Cu–Mn spinel were resolved through Rietveld refinement of in situ neutron diffraction data. The results demonstrated that the Cu ion has a lower octahedral site preference than manganese ions, and quenching is not a reliable method to determine the equilibrium structure in the system.     

铁磷模拟HLW熔体的腐蚀性能

在铁磷模拟ＨＬＷ熔体和硼硅酸盐熔体ＤＷＰＦ内测量了六种耐火材料的动态腐蚀速度,测量在 １０００～１３００℃之间进行,在铁磷熔体中,致密氧化铝和氧化铬耐火材料有最低的熔线腐蚀速度,二氧化硅、锆英石和ＡＺＳ耐火材料的腐蚀速度比较高．同时,氧化铝和氧化铬耐火材料在铁磷熔体中的腐蚀速度小于它们在硼硅酸盐熔体ＤＷＰＦ中的腐蚀速度．对氧化铬耐火材料来说,其在三种含有模拟ＨＬＷ废料的铁磷熔体中的熔线腐蚀速度＜０．１ｍｍ／ｄａｙ．可以认为商品制造的致密氧化铝和氧化铬耐火材料是可以用来熔化很多铁磷ＨＬＷ废料的,甚至可以熔化含有１６ｗｔ％氧化钠的ＨＬＷ废料．     

Enlarged Co?O Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt‐containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen‐deficient perovskites. Here, a systematic study of spinel ZnFe_xCo_2?xO₄ oxides (x = 0–2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal–oxygen covalency and an enlarged Co? O covalency by 10–30 at% Fe substitution is responsible for the activity enhancement. While the pH‐dependent OER activity of ZnFe_0.4Co_1.6O₄ (the optimal one) indicates decoupled proton–electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p‐band center relative to Fermi level governed by the spinel's cation deficient nature.     

Enlarged CoO Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction

Cobalt‐containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen‐deficient perovskites. Here, a systematic study of spinel ZnFe_xCo_2?xO₄ oxides (x = 0–2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO₂ is developed. The distinctive OER activity is found to be dominated by the metal–oxygen covalency and an enlarged Co?O covalency by 10–30 at% Fe substitution is responsible for the activity enhancement. While the pH‐dependent OER activity of ZnFe_0.4Co_1.6O₄ (the optimal one) indicates decoupled proton–electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p‐band center relative to Fermi level governed by the spinel's cation deficient nature.     

Preparation,electrical and photoelectrochemical properties of magnesium doped iron oxide sintered discs

Sintered discs of magnesium substituted iron oxides have been prepared by gel routes or classical solid state reaction techniques and their crystallographic, electrical, and photoelectrochemical properties examined. The discs, sintered between 1200 and 1400°C for 20 hours, contain both α-Fe₂O₃ and a spinel phase. The electrical properties of the discs have been correlated with spinel concentration, the electrical resistivity decreasing with increasing spinel content. The materials were found to be consistently n-type by Seebeck voltage measurements. Photoelectrochemical measurements in aqueous solution showed however that the sintered disc electrodes exhibit photocathodic currents that are characteristic of p-type semiconductors. Evidence is presented which suggests that inhomogeneities in the near surface region of the discs are responsible for the observed photoelectrochemical properties.     

Crystallization of glass in fireclay refractories: part II: Detailed study on the mullite crystal content of the ‘synthetic’ glass

A glass of composition similar to that found in fireclay refractories was synthesized and subsequently nucleated with Cr₂O₃, V₂O₅ and TiO₂. These glasses were heat-treated for crystallization of the mullite phase. The mullite content, the crystallization of mullite and the effect of temperature on the rate were investigated. The concentration of the nucleating oxides and the size and charge of their cations influence both the mullite content obtained and the crystallization rate.     

Sintering behaviour of spinel-alumina composites

Study of alumina-magnesia binary phase diagram reveals that around 40–50 wt% alumina dissolves in spinel (MgAl₂O₄) at 1600°C. Solid solubility of alumina in spinel decreases rapidly with decreasing temperature, which causes exsolution of alumina from spinel phase. Previous work of one of the authors revealed that the exsolution of alumina makes some interlocking structures in between alumina and spinel phases. In the present investigation, refractory grade calcined alumina and spinel powder were used to make different batch compositions. Green pellets, formed at a pressure of 1550 kg cm⁻² were fired at different temperatures of 1500°, 1550° and 1600°C for 2 h soaking time. Bulk density, percent apparent porosity, firing shrinkage etc were measured at each temperature. Sintering results were analysed to understand the mechanism of spinel-alumina interactions. SEM study of present samples does not reveal the distinct precipitation of needle shaped α-alumina from spinel, but has some effect on densification process of spinel-alumina composites. Microstructural differences between present and previous work suggest an ample scope of further work in spinel-alumina composites.     

Defects‐Induced In‐Plane Heterophase in Cobalt Oxide Nanosheets for Oxygen Evolution Reaction

Cobalt oxides as efficient oxygen evolution reaction (OER) electrocatalysts have received much attention because of their rich reserves and cheap cost. There are two common cobalt oxides, Co₃O₄ (spinel phase, stable but poor intrinsic activity) and CoO (rocksalt phase, active but easily be oxidatized). Constructing Co₃O₄/CoO heterophase can inherit both characteristic features of each component and form a heterophase interface facilitating charge transfer, which is believed to be an effective strategy in designing excellent electrocatalysts. Herein, an atomic arrangement engineering strategy is applied to improve electrocatalytic activity of Co₃O₄ for the OER. With the presence of oxygen vacancies, cobalt atoms at tetrahedral sites in Co₃O₄ can more easily diffuse into interstitial octahedral sites to form CoO phase structure as revealed by periodic density functional theory computations. The Co₃O₄/CoO spinel/rocksalt heterophase can be in situ fabricated at the atomic scale in plane. The overpotential to reach 10 mA cm^?2 of Co₃O₄/CoO is 1.532 V, which is 92 mV smaller than that of Co₃O₄. Theoretical calculations confirm that the excellent electrochemical activity is corresponding to a decline in average p‐state energy of adsorbed‐O on the Co₃O₄/CoO heterophase interface. The reaction Gibbs energy barrier has been significantly decreased with the construction of the heterophase interface.     

Synthesis of nanocrystalline magnesium aluminate (MgAl2O4) spinel powder by the urea-formaldehyde polymer gel combustion route

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by the urea-formaldehyde (UF) polymer gel combustion route. A transparent gel formed from magnesium nitrate, aluminium nitrate and UF after drying underwet self-sustained combustion when initiated with a burning splinter. The combustion product on calcinations at 850 °C formed MgAl₂O₄ spinel. Calcination of the combustion product resulted in particle coarsening. The powder obtained by planetary ball milling of the spinel had a median particle size of 1.58 μm. The spinel particles are agglomerates of nanocrystallites of size in the range 10-30 nm. The compacts prepared by uni-axial pressing of the spinel powder sintered to >99% TD at 1600 °C.