Mechanism of in-situ formation of spinel and its effect on the mechanical properties of Al2O3–C refractories

This work looked at the in-situ formation mechanism of magnesia alumina spinel in Al₂O₃–C refractories with magnesia addition at different firing temperatures. A comprehensive study on the mechanical properties of Al₂O₃–C refractories was performed in comparison to traditional analogs. The magnesia alumina spinel was in-situ formed at the firing temperature of 1150 °C in Al₂O₃–C refractories. With the increase of the firing temperature, the Al₂O₃ phase was gradually dissolved in spinel phase to form aluminum rich spinel phase, resulting in a decrease in its lattice constant due to the defects formation. The formed spinel phase was homogenously distributed and bonded well with corundum, improving the interfacial bond, load transferring capacity and crack propagation resistance. The formation of spinel phase also enhanced the sintering of the alumina matrix owing to the solid solution of alumina in the spinel. Therefore, the mechanical properties such as cold modulus of rupture and hot modulus of rupture in Al₂O₃–C refractories achieved a substantial enhancement compared with traditional refractories.     

Preliminary investigations of the production of MgAlON bonded refractories

The aim of the work was to find an appropriate composition for the formation of MgAlON bonding phase for Al₂O₃ and MgO based refractories. The first step was the preparation of pure MgAlON. AlN and Al₂O₃ were used as starting powders and either MgO or MgAl₂O₄ was added as a source of magnesium. The results verified the possibility to produce MgAlON under the prevailing conditions. Afterwards, MgAlON bonded alumina and magnesia refractories were investigated. The obtained results confirmed the possibility of the production of MgAlON bonded alumina refractories. However, in the case of magnesia-based samples MgAlON was not formed and instead stoichiometric spinel, AlN and alumina rich spinel were detected in the bonding phase. Additionally, the joining between the MgO grains and the matrix was poor with wide gaps on the interface.     

Compositions Based on MgAl2O4, Al6Si2O13, AND Al2TiO5

Binary and ternary fused compositions based on alumomagnesian spinel, aluminum titanate, and mullite are studies using various techniques of physicochemical analysis. The minimum incipient melting temperature in the MgAl₂O₄ – Al₆Si₂O₁₃ – Al₂TiO₅ system is [1645 ± 15] °C. Fused materials with tailored phase compositions are obtained under industrial conditions. Technological properties of a fused composite corundum – alumomagnesian spinel – mullite – aluminum titanate are studied.     

Reaction of high-alumina refractory with chromium vapors in vacuum

Conclusions The main cause of the failure of high-alumina refractory of mullite composition during service in vacuum furnaces is the chemical reaction of chromium vapors with the alumina, as a result of which a readily melting vitreous silicate substance is formed together with gaseous silicon monoxide and corundum with chromium dissolved in it. Therefore, in order to increase the life of linings of vacuum furnaces it is necessary to use high-alumina refractories with a maximally high content of Al₂O₃.Translated from Ogneupory, No. 6, pp. 38–40, June, 1969.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.     

The structure and composition of high-frequency fused mullite

Conclusions The high-frequency smelting of a charge containing 69–75% Al₂O₃ and 31–25% SiO₂ gives a product with a stable phase composition of mullite and glass. Corundum occurs only in the form of discrete crystals.Mullite from a charge with alumina modulus 3.0 contains a minimum of glass and a maximum of Al₂O₃, so that it is the most interesting one from the standpoint of refractories production.Firing the fused materials promotes the formation of mullite from the vitreous phase. A similar phenomenon occurs during the firing of products manufactured from fused mullite by the ceramics technology. In this case the pre-firing stage can be dispensed with.Deceased.Translated from Ogneupory, No. 4, pp. 40–46, April, 1977.     

Synthesis and Properties of Fusion-Cast Refractories in the MgO – Al2O3 System

Results of a study of fusion-cast refractories in the MgO – Al₂O₃ high-alumina system are reported. A casting technology for spinelide refractories is proposed. The materials synthesized are analyzed for phase composition by x-ray diffractometry and petrography and are shown to consist of corundum, MgAl₂O₄ spinel and their solid solutions. The materials are tested for corrosion resistance in industrial glass melts, and a corundum spinelide material with 5% Mg is recognized as the most promising for practical use.     

Mullite refractories from bauxites of the iksinskoe deposit

Bauxites with 76% Al₂O₃ and 2.65% Fe₂O₃ were used to produce mullite refractories and mixtures for steel-teeming ladles. Tests in service have shown that the strength of these refractories is no worse than or comparable with that of refractories based on commercial alumina and Chinese bauxites.Translated from Ogneupory, No. 2, pp. 28–31, February, 1995.     

Properties of compositions based on the system Al2O3-Mg(Al1−x,Crx)2O4

Conclusions Magnesia-alumina spinel and highly chromic spinel MgAl_0.4Cr_1.6O₄ and MgCr₂O₄ retard the grain growth of corundum during firing of the product. Low-chromic spinel MgAl_1.6Cr_0.4O₄ and MgAlCrO₄ added in small quantities intensify the corundum recrystallization.Small (5–10% by weight) additions of spinel Mg(Al_1–x, Cr_x)₂O₄ increase the strength of the corundum specimens, but only the high-alumina spinel improves their sintering.The sintering of mixtures of Al₂O₃ and Mg(Al_1–x Cr_x)₂O₄ is impaired during the substitution of magnesia-alumina spinel by magnesia chromite, and with an increase in the quantity of spinels from 5 to 30%.A small addition (5–10%) of high-alumina spinel of the composition Mg(Al_1–x, Cr_x)₂O₄ where x 0.5 to the alumina precalcined at 1450°C enables us to obtain dense, strong, and thermally shock-resistant corundum products.Translated from Ogneupory, No. 3, pp. 53–56, March, 1972.     

Properties of silica sol bonded corundum‐spinel castables for steel ladles

In order to overcome the shortcoming of the calcium aluminate cement (CAC) bonded castables, we prepared corundum‐spinel castables using silica sol as binder and tabular corundum, sintered magnesia‐alumina spinel, and reactive alumina as raw materials in this study. The effect of spinel grain size and solid content of silica sol on the flow value, sintering, mechanical strength and microstructure of the specimens treated at varying temperature of 400, 1000, 1500, and 1650°C for 5 hours in an air atmosphere were studied by SEM and EDS analyses. The results indicate that silica sol is suitable as a binder for corundum‐spinel systems. And silica sol with solid content of 25% bonded samples containing ≤90 μm spinel perform quite better than the others. At the same time, silica sol bonded samples had high strength in medium temperature. This is because that the closer proximity of silica sol and alumina powder and the high activity of nanometer SiO₂ in silica sol are beneficial for the reaction of SiO₂ and Al₂O₃ to generate mullite needed for reinforcement of castables matrix.     

A study on the wetting behavior of liquid iron on forsterite,mullite, spinel and quasi-corundum substrates

Wetting characteristics of liquid iron on magnesia, alumina and silica mixture substrates were studied by sessile drop experiments. Chromium-free forsterite, mullite, spinel and quasi-corundum phases were selected as alternative refractories in MgO-Al₂O₃-SiO₂. Morphological changes of molten electrolytic iron on the oxide substrates were investigated via apparent contact angle measurements. The results showed that the wetting behavior was significantly influenced by FeO compounds that were formed via oxidation of the liquid iron. Morphologies of the reacted layer were studied by Scanning Electron Microscope (SEM)/EDX analysis. The ternary phases FeO-MgO-SiO₂ and FeO-Al₂O₃-SiO₂ improved the wetting of liquid iron on the forsterite and mullite substrates by providing liquid phases at solid (refractory)–liquid (iron) interfaces. However, corrosion by liquid iron was significantly inhibited at spinel phase which did not feature FeO based compounds at the interface. Quasi-corundum (10MgO-25SiO₂-65Al₂O₃) showed a much enhanced resistance to liquid iron compared to forsterite or mullite refractories.     

Silicon carbide refractories with a sialon binder

An investigation concerning molding of silicon carbide refractories with a sialon binder was carried out. It is shown that the alumina-containing component plays an important role in the synthesis of the sialon binder. A corundum addition to the composition of silicon carbide refractories ensures higher parameters for the refractory as compared to a mullite addition. With an increase in the content of crystalline silicon the strength of silicon carbide refractories grows sharply due to the increased amount of -sidalon. Refractories with elevated contents of sialon in the binder have better resistances to alkali, slag, and molten metal than refractories with binders containing silicon nitride.Translated from Ogneupory, No. 4, pp. 2–4, April, 1995.     

A mullite-corundum ramming compound with a carbon-containing component

Conclusions For lining the bottom, hearth, and compression siphon of cupolas a mullite-corundum ramming compound with a carbon-containing component and phosphate binder was developed. In the 25–30 tons/h of iron cupola of the Stankolit Moscow Plant a cupola bottom and hearth lining life of the compound developed of 20 days was obtained and the siphon lining lasted for 2500 tons.In the rammed lining after service in the bottom of the cupola, AlO·Al₂O₃ alumina spinel was observed in addition to corundum, mullite, glassy phase, and graphite.Translated from Ogneupory, No. 7, pp. 50–53, July, 1985.     

Reaction of sintered spinel and corundum refractories with some compounds

Conclusions We investigated the reaction of sintered spinel, spinel-corundum, and corundum refractories with certain oxides at 1500–1750°C. We established conditions in which refractories are stable in respect to the action of individual oxides. We also investigated the mineral compositions of the reaction zones.The least reaction with all the oxides investigated occurs in the spinel refractory, and the greatest in the corundum; that is, there is an increase with rise in alumina content. Magnesia spinel hardly reacts with iron oxides in the test conditions.Magnesia spinel is very resistant to sodium adipates which extensively damage sintered corundum.Carbonates of alkali and alkaline-earth elements, and also fused alkalis at temperatures exceeding their melting points by 200°C seriously damage spinel-corundum refractories, but do not react with sintered corundum or spinel.Metallic fluorides damage corundum but do not react with spinel and spinel-corundum sintered refractories.Thus, spinel sintered refractory should find extensive use in conditions of corrosive action, in particular, alkalis and oxides of iron.Translated from Ogneupory, No. 1, pp. 37–42, January, 1968.     

Phase composition of alumina–mullite–zirconia refractory materials

Refractories in the Al₂O₃–SiO₂–ZrO₂ system are widely used in many applications, for ceramic rollers in particular, and are characterized by high mechanical strength, excellent thermal shock resistance, resistance to corrosion by alkaline compounds and low creep at high temperature. Their performances greatly depend on the amount and chemical composition of crystalline and glassy phases, which were investigated by quantitative XRPD (RIR–Rietveld) and XRF in order to assess the effect of various Al₂O₃/SiO₂ ratios of starting batches and different alumina particle size distributions. Refractories consist of mullite, corundum, zirconia polymorphs and a vitreous phase in largely variable amounts. The mullite percentage, unit cell parameters and composition vary with sintering temperature, being mostly influenced by the Al₂O₃/SiO₂ ratio of the batch. Its orthorhombic unit cell increased its volume from 1400 to 1500 °C, while its stoichiometry became more aluminous. The corundum stability during firing is strongly affected by the Al₂O₃/SiO₂ ratio, but not by the particle size distribution of alumina raw materials. Zirconia raw materials are involved in the high temperature reactions and about one-third of the available ZrO₂ is dissolved in the glassy phase, ensuring excellent resistance to alkali corrosion, mainly depending on the fraction of coarse alumina. The phase composition of the vitreous phase increased with sintering temperature, being over 20% when the fractions of coarse alumina in the starting batch are between 0.2 and 0.5.     

Magnesite-Spinel products from fused materials with by-products from chemical processes

Conclusions The feasibility was demonstrated of producing chrome-containing magnesia spinel from a mixture of the by-products of chemical processes and magnesite. The composition of this spinel is similar to that of spinel fused from a mixture of commercial alumina, magnesite, and chromite. The fused spinel from the by-products contains a large proportion of SiO₂ and correspondingly less FeO + Fe₂O₃ than spinel fused from commercial alumina.The properties of magnesite-spinel refractories based on fused and sintered spinels from the by-products were investigated and compared with those of specimens based on fused and sintered spinels from alumina. The properties of the products containing fused spinels from the by-products proved to be good and similar to those of products containing fused spinel from alumina.Fused spinel from the by-products can be used also as starting material for the production by the ceramic method of spinel brick with good properties.Translated from Ogneupory, No. 6, pp. 33–39, June, 1974.     

Enhanced mechanical properties of lightweight Al2O3-C refractories reinforced by combined one-dimensional ceramic phases

We prepared a new lightweight Al₂O₃-C refractory material with a higher strength by using microporous corundum aggregates instead of dense tabular corundum aggregates, which was reinforced by in situ formed SiC whiskers, multi-walled carbon nanotubes (MWCNTs), and mullite rods. A comparative study of the microstructure, mechanical properties, and fracture behavior was carried out for dense and lightweight Al₂O₃-C refractories coked at 1200°C and 1400°C, respectively. By using the microporous corundum aggregates, a better aggregate/matrix interface bonding and an optimized distribution of SiC whiskers were obtained. The SiC whiskers formed inside the microporous corundum aggregates and simultaneously in the matrix by a vapor-solid reaction mechanism, resulting in an enhancement at the microporous aggregate/matrix interface. Furthermore, the in situ formed MWCNTs and well-developed mullite rods at 1200°C in the matrix also contributed to the better interface structure. Thus, due to the improved microporous aggregate/matrix interface, the crack propagation along the aggregate/matrix interface was suppressed, resulting in an increased crack propagation within the aggregates. Consequently, the synergy between microporous corundum aggregates and combined one-dimensional ceramic phases caused a lower bulk density but a markedly higher strength, a higher fracture energy, and a higher toughness of lightweight Al₂O₃-C refractories compared to the dense ones. Overall, our study allows to overcome the traditional concept that a higher strength of refractories is reached by a higher density.     

Synthesis and Properties of Fusion-Cast Refractories in the Al2O3 – ZrO2 System

The synthesis and properties of fusion-cast refractories in the Al₂O₃ – ZrO₂ system studied by x-ray phase analysis and petrography are reported. The phase composition of synthetic materials is represented by corundum and ZrO₂ monoclinic modification. Adding ZrO₂ results in a decrease in the size of corundum crystals. The corrosion resistance of the synthetic refractories increases with ZrO₂ concentration; still, further effort is needed to improve the fabricability of fusion-cast Al₂O₃ – ZrO₂-based refractories.     

Synthesis of Al2O3-SiC powder from electroceramics waste and its application in low-carbon MgO-C refractories

Composite additives are an efficient means to improve the high-temperature stability and slag resistance of low-carbon MgO-C refractories. In this work, Al₂O₃-SiC powder was firstly synthesized from electroceramics waste by carbon embedded method at 1500°C, 1550°C, and 1600°C for 4 h, and then the as-synthesized Al₂O₃-SiC powder was used as an additive to low-carbon MgO-C refractories. The effects of its addition amounts of 0, 2.5 wt.%, 5.0 wt.%, and 7.5 wt.% on the properties of the refractories were investigated in detail. It was found that increasing the heat treatment temperature is beneficial to the phase conversion of mullite and quartz to alumina and silicon carbide in the electroceramics waste. Furthermore, the addition of Al₂O₃-SiC powder effectively improves the performance of low-carbon MgO-C samples, and the formation of spinel dense layer and high-viscosity isolation layer is the internal reason for the improvement of the oxidation resistance and slag resistance of low-carbon MgO-C samples. This work provides ideas for the reuse of electroceramics waste and presents an alternative strategy for the performance optimization of low-carbon MgO-C refractories.     

Reactions between Synthetic Mica and Simple Oxide Compounds with Application to Oxidation-Resistant Ceramic Composites

Reaction-couple experiments have been pursued in order to evaluate the potential of a phyllosiiicate to act as a chemically protective, fracture-deflecting, oxidation-resistant interphase for oxide fiber–oxide matrix composites. The synthetic mica fluorophiogopite (KMg₃[AlSi₃]O₁₀F₂) was reacted with single-phase substrates of alumina (Al₂O₃), mullite (3Al₂O₃·2SiO₂), forsterite (Mg₂SiO₄), or enstatite (MgSiO₃). X-ray spectroscopy, X-ray diffraction, and scanning electron and transmitted polarized light microscopy were applied to the analysis of the reaction couples. Fluorophlogopite reacts strongly with alumina, mullite, and enstatite, resulting in substantial damage to the substrate as well as the breakdown of the mica. The chemical reactions between mica–alumina and mica–mullite are examined critically. In the case of alumina, the reaction results in the formation of a planar spinel (MgAl₂O₄) layer separating the substrate from the breakdown products of the mica. This unvarying result suggests, therefore, that a spinel diffusion barrier would prove effective in protecting alumina from fluorophlogopite. Experiments revealed such effectiveness: local equilibrium is established in the layer sequence alumina–spinel–fluorophlogopite; i.e., planar interfaces are established amongst these phases that are stable under conditions of high temperature and high oxygen fugacity. A similar chemical approach for protection of mullite is not obvious. Based on an understanding of its intrinsic fracture energy, the fluoromica interphase is expected to be effective in mechanically protecting adjacent oxides from propagating cracks, a behavior qualitatively demonstrated by indentation experiments on the kinetically persistent alumina–spinel–fluorophlogopite–spinel–alumina laminates.