NEW REFRACTORY COMPOSITIONS RESISTANT TO MOLTEN ROCK PHOSPHATE*

The Tennessee Valley Authority has carried out small-scale and large-scale studies during the past five years, directed toward the development of refractories more resistant to corrosion by molten rock phosphate than the present commercial types. Small-scale tests were made on refractories containing varying proportions of zirconium silicate and oxides of aluminum, beryllium, calcium, cerium, chromium, magnesium, thorium, and zirconium. Promising compositions were then tested by fabrication into standard 13l½-in. and 9-in. brick and by determining their resistance to corrosion by molten rock phosphate, basic open-hearth slag, and electric phosphate-smelting furnace slag. The most promising of the compositions tested were (1) Cr₂O₃ 67%, CaO 30%, and Al₂O₃ 3% and (2) ZrO₂ 33.5%, Cr₂O₃ 33.5%, CaO 30%, and Al₂O₃ 3%. A 25-kw. Detroit rocking furnace was lined successively with brick of these two compositions, as well as a commercial superduty firebrick and a commercial unburned magnesite brick, and each lining was tested against molten rock phosphate until failure occurred. The tests show that the two new types of refractories are superior to the commercial refractories and that they may have promising possibilities for use where basic refractories are now extensively used.     

Cast refractories of the Al2O3-Cr2O3-CaO system

Conclusions In order to develop stable materials for glassmaking, we studied the physical and technological properties of the refractories belonging to the Al₂O₃-Cr₂O₃-CaO system that contain 5–20% CaO, 15–35% Cr₂O₃ and 45–80% Al₂O₃.The glass resistance of the refractories of the experimental systems (compositions) exceeds that of the BKCh-33 baddeleyite-corundum products by 3–5 times and their thermal shock resistance is superior to that of the well known chromium-containing refractories at comparable levels of mechanical properties.The developed refractories are recommended for the top or the bottom structures of the glassmaking furnaces depending on their glass resistance and thermal shock resistance and for making the refractory components of ferrous metallurgical units that are in contact with highly basic slags.Translated from Ogneupory, No. 3, pp. 23–26, March, 1989.     

Combustion synthesis of B4C/Al2O3/C composite powders and their effects on properties of low carbon MgO-C refractories

To improve the dispersity and oxidation resistance of nano carbon black (CB) in low carbon MgO-C refractories, B₄C/Al₂O₃/C composite powders were prepared by a combustion synthesis method using B₂O₃, CB and Al powders as the raw materials. The phase compositions and microstructures of the synthesized products were characterized by X-ray diffraction (XRD), Raman spectroscopy, and a scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The results show that an 80 wt% excess of CB is the maximum amount of CB that can be added under the condition of a self-propagating combustion wave, and the phase compositions of the products are B₄C, α-Al₂O₃ and CB. B₄C particles with uniform sizes and cubic polyhedral structures are embedded in the Al₂O₃ matrix. The combustion-synthesized B₄C/Al₂O₃/C powders and mechanically mixed B₄C/Al₂O₃/C powders were added to the low carbon MgO-C refractories, and their corresponding properties were compared. The apparent porosity (AP) of the refractories with the synthesized powders (labelled as M3) is lower than those of the refractories with mechanically mixed powders (labelled as M2) and without composite powders (labelled as M1). The oxidation ratio and slag erosion depth of M3 were lower than those of M2 and M1. The thickness of the decarburized layer of M3 was 10.2% and 22.4% less than that of M2 and M1, respectively. The penetration depth of M3 was 12.0% and 27.9% less than that of M2 and M1, respectively. The thermal shock resistance of M3 was better than that of M2 and M1. The residual strength ratio of M3 was 15.8% and 17.2% more than that of M2 and M1, respectively. These results suggest that the combustion-synthesized B₄C/Al₂O₃/C composite powders can be used as new and promising additives for low carbon MgO-C refractories.     

Designing low-carbon MgO–Al2O3–La2O3–C refractories with balanced performance for ladle furnaces

In order to achieve high-quality and stable production of special steel, the performance of low-carbon MgO-C refractories needs to be further optimized. For this purpose, low-carbon MgO–Al₂O₃–La₂O₃–C refractories with enhanced thermal shock resistance and slag resistance were designed and successfully prepared by introducing Al₂O₃ as a reinforcer and La₂O₃ as a modifier. The results showed that the refractory samples with additives show better overall performance than those without additives. When 10 wt% of Al₂O₃ and La₂O₃ were added, the oxidation resistance, thermal shock resistance and slag resistance of the refractory samples coked at 1400 °C are increased by 13.57%, 17.75% and 43.09%, respectively. The analysis found that this can be mainly attributed to the formation of MgAl₂O₄, Mg₂SiO₄, and 2CaO·4La₂O₃·6SiO₂ and the consequent volume expansion effect and intergranular phase enhancement effect. Therefore, a low-cost and enforceable reinforcement strategy for low-carbon MgO-C refractories is proposed, which is expected to be applied in steelmaking.     

Optimization of corrosion resistance and mechanical properties of Al2O3-Cr2O3 refractories for waste incinerator

The corrosion resistance and mechanical properties directly affects the operation and service life of Al₂O₃-Cr₂O₃ refractories used in waste incinerators. In this study, ZrO₂ particles were introduced via vacuum impregnation to adjust microstructure and properties of Al₂O₃-Cr₂O₃ refractories. The results showed that the impregnated ZrO₂ particles and increasing impregnation times resulted in the decreased median pore size and increased compactness, and mechanical strengths of refractories were elevated from the inhibited cracks propagation by ZrO₂ particles. The decreased amounts of large pores and increased amounts of small pores from the filled ZrO₂ particles inhibited penetration of low melting point phases, and the formed CaZrO₃ phase from the reactions between corrosion reagent and ZrO₂ particles increased the viscosity of penetrated corrosion reagent, resulting in the decreased penetration index from 8.57% to 2.58%. Meanwhile, the filled ZrO₂ particles around alumina particles prevented reactions between molten corrosion reagent and alumina, leading to the decreased corrosion index from 3.78% to .74%. The decreased pore size and formation of CaZrO₃ phase were primary factors that enhanced the penetration resistance. And formation of wrapped layers from ZrO₂ particles around alumina particles presented prominent effects on the strengthened corrosion resistance of Al₂O₃-Cr₂O₃ refractories.     

Nanoscale toughening of low carbon Al2O3-C refractories by means of SiCnw/Al2O3 composite reinforcement

To improve the thermal shock resistance of low carbon Al₂O₃-C refractories, SiC nanowires (SiC_nw) containing SiC_nw/Al₂O₃ composite reinforcement were introduced. The specific fracture energy of the Al₂O₃-C refractory matrix was obtained by statistical grid nano-indentation. The reinforcement mechanism of SiC_nw/Al₂O₃ on thermal shock resistance of refractories was investigated. The results revealed that the matrix-specific fracture energy of A6 (6 wt% SiC_nw/Al₂O₃ added) was 217 N/m, which was 58.4% higher than reference sample A0 (137 N/m) and 18.6% higher than MA6 (183 N/m, 6 wt% SiC/Al₂O₃ added). A6 showed the highest residual strength ratio of 49.8%, which was 114.7 % higher than A0 (23.2%) and 82.4 % higher than MA6 (27.3%). The components with different morphology in SiC_nw/Al₂O₃ cluster, especially SiC nanowires, promote the generation of microcracks, crack multi-deflection, and branching, which toughen the matrix and improve the thermal shock resistance of refractories. In comparison to the literature, A6 showed a higher rising in residual strength ratio than those with higher graphite content (4 wt% and 20 wt%), which will greatly reduce the consumption of carbon-containing refractories and contribute to the reduction of CO₂ emission.     

Ceramics of good thermal strength from fused powders of yttrium and scandium oxides

Conclusions An investigation was carried out of the conditions for producing refractories from electrofused Y₂O₃ by casting. Components of intricate shape were produced at a firing shrinkage of about 6%.The conditions were refined for producing refractories with good thermal strength from fused powders of Y₂O₃ and Sc₂O₃ by semidry molding.Translated from Ogneupory, No. 5, pp. 57–59, May, 1976.     

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.     

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.     

Aluminosilicate Refractories Based on High-Alumina HCBS.

Ceramic concrete specimens are prepared based on HCBS of mullite-silica composition (58% Al₂O₃) and fillers of similar and chamotte compositions, having markedly better properties than for traditional refractories with the same Al₂O₃ content. An ultimate strength in compression of 150 – 250 MPa with porosity of 6 – 11%, and a temperature for deformation under load T _g of 1540 and 1460 MPa are obtained correspondingly for ceramic concrete specimens with an Al₂O₃ content of 58 and 42%. With respect to T _g, mullite-silica ceramic concretes (58% Al₂O₃) correspond to traditional mullite (62 – 72% Al₂O₃), chamotte (42% Al₂O₃) and mullite-silica (45 – 62% Al₂O₃) refractories.     

Effect of Alumina Reactivity on the Densification and Properties of Al2O3–Cr2O3 Refractories

Alumina‐chrome (Al₂O₃–Cr₂O₃) refractories with Al₂O₃:Cr₂O₃ molar ratio 1:1 were synthesized in the temperature range of 1400–1700°C by conventional solid–oxide reaction route. The effect of different aluminas (viz., hydrated and calcined) on the densification, microstructure, and properties of Al₂O₃–Cr₂O₃ refractories was investigated without changing the Cr₂O₃ source. The starting materials were analyzed to determine the chemical composition, mineralogy, density, surface area, and particle size. Sintered materials were characterized in terms of densification, phase assemblage, and mechanical strength at room temperature and at higher temperatures. Microstructural evolution at different sintering temperature was correlated with sintering characteristics. It can be concluded that the Al₂O₃–Cr₂O₃ refractories prepared with hydrated alumina as Al₂O₃ source show better densification and hot mechanical strength than corresponding calcined variety.     

A novel approach to lightweight alumina-carbon refractories for flow control of molten steel

Functional refractory materials for flow control devices of molten steel in continuous casting used to be prepared from Al₂O₃–C refractories containing dense corundum aggregates. According to the traditional concept, the denser the refractories, the higher the strength of refractories. However, we prepared a new lightweight Al₂O₃–C refractory material using microporous corundum aggregates instead of dense corundum aggregates, which were reinforced by in situ formed SiC whiskers. A comparative analysis of microstructures and properties was carried out for conventional and lightweight Al₂O₃–C refractories with and without Si powder addition. We showed that microporous aggregates formed a better aggregate/matrix interface bonding and an improved distribution of SiC whiskers. The SiC whiskers formed not only in the matrix, but also inside of the microporous aggregates and at the aggregate/matrix interface by a vapor-solid reaction mechanism. Due to the formation of a microporous aggregate/matrix interface reinforced by SiC whiskers, the crack propagation along the aggregate/matrix interface was suppressed, whereas the percentage of cracks propagating within the aggregates was enhanced. Thus, the synergy between in situ formed SiC whiskers and microporous aggregates resulted in a significant higher strength of lightweight Al₂O₃–C refractories compared to conventional ones. The results therefore provide an original strategy to strengthen Al₂O₃–C refractories.     

Simulation and experimental investigation of preferred β-Sialon growth and its effects on thermo-mechanical properties of Al2O3–C refractories

β-Sialon bonded Al₂O₃–C refractories possess high strength and superior thermal shock performance. In this study, the growth of preferred β-Sialon (Si₃Al₃O₃N₅) and its effects on thermo-mechanical properties of Al₂O₃–C refractories were investigated via simulations and experiments. The results indicate that the additive Fe₂O₃ contributed to the formation of β-Sialon and helped its column structure become plate-like. Transmission electron microscopy confirmed that the (101) crystal plane was a growth plane of plate-like β-Sialon. The growth mechanism of β-Sialon was suggested by density functional theory; calculation results revealed that the key step for the formation and growth of β-Sialon was the adsorption of the gaseous molecule Al₂O on the Si₃N₄ (101) crystal plane. It was found that the existence of Fe atoms could significantly reduce the adsorption energy. Additionally, Al₂O₃–C refractories containing plate-like β-Sialon possessed a high cold modulus of rupture and crushing strength, which increased by 40% and 15%, respectively, compared with the specimens containing column β-Sialon. It was also found that the formation of plate-like β-Sialon resulted in significantly better thermal shock resistance for the Al₂O₃–C refractory specimens, and the residual strength loss ratio of the sintered specimens was only 4% after five thermal shock cycles.     

Enhancement of oxidation resistance at 1000–1400 °C of low carbon Al2O3–C refractories with pre-synthesized SiCnw/Al2O3

The oxidation resistance of low carbon Al₂O₃–C refractories with the addition of SiC_nw/Al₂O₃ composite powders and the enhancement mechanisms were investigated. The oxidation resistance was evaluated by oxidation index (O.I.) and oxidation rate constant (k). The enhancement mechanisms of SiC_nw/Al₂O₃ on oxidation resistance were analyzed based on the phases and microstructures. The results showed that the SiC_nw/Al₂O₃ can improve the oxidation resistance of Al₂O₃–C refractories, the O.I. and k of A6 (6 wt% SiC_nw/Al₂O₃ addition) were 26.0% and 34.5% lower than those of reference sample A0, respectively. The oxidation resistance of refractories was improved in a range of 1000–1400 °C due to the introduction of SiC_nw/Al₂O₃. The enhancement mechanisms can be explained that SiC_nw is more susceptible to be oxidized due to its high specific surface area, which expanded the action temperature range of other antioxidants and itself. The mullite and dense protective layer generated during oxidation is also beneficial to impede the diffusion of O₂.     

Effect of ceramic bonding phases on the thermo-mechanical properties of Al2O3–C refractories

Ceramic bonding phases of non-oxide whiskers can enhance the hot strength and the thermal shock resistance of Al₂O₃–C refractories. In this paper, the effect of different metals on the microstructure and thermo-mechanical properties of Al₂O₃–C refractories has been investigated. Thermodynamic calculation of Al–Si–O–C–N systems shows that Al₄C₃, AlN, SiC and β-Sialon are stable at elevated temperature. AlN with the shape of short column can be generated in Al₂O₃–C refractories with metallic Al, which leads to high hot modulus of rupture (HMOR) and poor resistance to thermal shock. SiC whiskers formed in Al₂O₃–C refractories with metallic Si give rise to low HMOR and good resistance to thermal shock. When metallic Si and Al are added together in the refractories, β-Sialon (z=2) with plane structure can be generated under the action of catalyst (nano-sized Ni). The existence of the catalyst promotes the diffusion of Al and O in Si₃N₄ crystals and contributes to the generation of plane-shaped β-Sialon. The corresponding HMOR and residual cold modulus of rupture respectively increase to about 20 MPa and 10.3 MPa. The plane-shaped β-Sialon can significantly enhance both hot strength and thermal shock resistance of Al₂O₃–C refractories.     

More efficient utilization of alumina in refractories production

Conclusions An investigation was carried out of ways of utilizing commercial alumina more efficiently in refractories production by improving the composition of the products and the technology of their manufacture. Commercial alumina should be used mainly for the production of mullite refractories from synthetic mullite and of corundum refractories containing 90–99% Al₂O₃. Commercial alumina must be used for the production of spinel and spinel-containing refractories.More extensive use should be made of the alumina-containing waste from chemical plants for the production of refractories with a low mullite content, of ramming compounds, fusioncast refractories, and electrofused spinel as a substitute for chromite in roof bricks. To produce refractories with a higher Al₂O₃ content than can be achieved with kaolin more extensive use should be made of natural alumina-containing starting materials.Some mullite and corundum refractories should be produced with low porosity by using high molding pressures, actively sintering starting materials, and hydrostatic molding.Translated from Ogneupory, No. 8, pp. 33–39, August, 1978.     

MgO−Al2O3−Cr2O3 refractories of increased corrosion resistance for nonferrous metallurgical furnaces

The paper sums up a study that had as its objective to develop MgO−Al₂O₃−Cr₂O₃ refractories of increased corrosion resistance for harsh service conditions. The refractories thus developed, designated PShKhM-1 and PShKhM-2, have better high-temperature strength and abrasion resistance than PKhS refractories manufactured commercially. The experimental refractories will enhance the durability of furnace and converter linings, extend their campaigns, step up their productivity, and reduce consumption of refractories and repair costs. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 1, pp. 33–37, January, 1998.     

Improved thermal shock resistance of low-carbon Al2O3–C refractories fabricated with C/MgAl2O4 composite powders

The addition of C/MgAl₂O₄ composite powders can improve the thermal shock resistance of low-carbon Al₂O₃–C refractories attribute to the formation of microcracks in the agglomerated structure, thus consuming more thermal stress and strain energy. Moreover, C/MgAl₂O₄ composite powders additive promote the formation of short fibrous ceramic phases in the refractories, which suggest a bridging role in the interior of the refractories and increase its toughness. Furthermore, the C/MgAl₂O₄ composite powders also result in a remarkable enhancement of the slag corrosion resistance in the refractories.     

Effect of CaCO3 addition on the performance of lightweight Al2O3-MgAl2O4 refractories with gradient density

In this study, to improve the comprehensive performance of lightweight Al₂O₃-MgAl₂O₄ refractories with gradient density, CaCO₃ was added in the matrix to develop platelet structures by adding different amounts (1, 2, 3, and 4 wt%). The effect of CaCO₃ addition on the phase composition, microstructure, and properties of lightweight refractories was investigated. The results showed that CaO·6Al₂O₃ and Ca₂Mg₂Al₂₈O₄₆ formed. Meanwhile, platelet structure is distributed at the junction between aggregate and matrix. With the increase of CaCO₃ content, although there were no apparent changes in apparent porosity and bulk density, both the compressive strength and refractoriness under load improved significantly. Furthermore, the results of three-point bending tests showed thermal shock resistance was improved in terms of the value of residual strength ratio from 14.2% to 22.5% for specimens before and after 4 wt% CaCO₃ addition. The findings showed that the introduction of CaCO₃ is feasible and effective for performance improvements of lightweight refractories.     

Oxidation resistance,residual strength,and microstructural evolution in Al2O3-MgO–C refractory composites with YAG nanopowder

The decisive role of nanostructured yttrium aluminium garnet (YAG;Y₃Al₅O₁₂) powder addition on oxidation resistance, residual strength and microstructural evolution were studied in Al₂O₃-MgO–C refractory composites. Oxidation index and rate constant calculations indicated that the oxidation resistance was almost 70 % improved for the nano-YAG containing refractories oxidized in air at 1600 °C. Residual compressive strength (R_c) estimations showed that there was nearly 75 % strength retained in these oxidized refractories fortified with nano-YAG. Residual bending strength (R_b) estimations based on cyclic thermal shock, exhibited that there was almost 70 % thermal shock resistance enhancement in refractories reinforced with nano-YAG, showed a good agreement between R_b and R_c values. These beneficial properties were attributed to the formation of a well-sintered framework of YAG/Spinel bonding grains throughout the dense oxidized layer microstructure of these new class of refractories. The concept of interfacial toughening and implications of these results to practical applications are discussed.