The corrosion resistance of microsilica-containing Al2O3–MgO and Al2O3–spinel castables

Microsilica addition in Al₂O₃–MgO and Al₂O₃–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA₆ was formed in the matrix, mainly for the Al₂O₃–spinel composition, which also favored the castable dissolution into the molten slag.     

Corrosion mechanism of cement-bonded Al2O3–MgAl2O4 pre-cast castables in contact with molten steel and slag

In this work, a cement-bonded corundum-spinel (Al₂O₃–MgAl₂O₄) pre-cast refractory brick with two typical Al₂O₃ aggregates was designed as the refractory lining. Corroded microstructure of the used corundum-spinel bricks after industrial trials in a commercial RH refining ladle was analyzed. Degradation processes of two types of alumina aggregates in the same corroded interface also were discussed. Typical corroded microstructure revealed that needle-like calcium hexaaluminate (CA₆) was observed in the matrix of the original layer. The formation of CA₆ was attributed to the reactions with pure calcium aluminate cement and matrix components under a high-temperature gradient during refining. Furthermore, the corrosion process of the used Al₂O₃–MgAl₂O₄ bricks would be discussed based on a post-mortem microstructural characterization, and the corrosion mechanism of the two types of aggregates was also elucidated.     

Effect of carbon black on corrosion resistance of Al2O3–SiC–C castables to SiO2–MgO slag

Metallurgical solid waste recycling is the shape of things to come in green development of Chinese iron and steel industry. Utilization of ironworks slag for producing mineral wool at high temperature is an important approach. However, refractory lining is seriously corroded by the SiO₂–MgO based slag at 1600 °C during the production process. Different production steps need different atmospheres, the changeable service atmospheres (air and reducing atmosphere) put forward high requirements for slag resistance. The Al₂O₃–SiC–C castables containing carbon black are usually used in iron runner, which faces high-temperature service condition of 1450 °C–1500 °C. Nevertheless, the function of carbon black in the Al₂O₃–SiC–C castables at 1600 °C is till essentially unknown. In the current study, the carbon black was introduced to tabular alumina based Al₂O₃–SiC–C castables to improve corrosion resistance to SiO₂–MgO based slag at 1600 °C. The result showed that 0.4 wt% carbon black was suitable for the castables, which the slag resistance of castables was significantly improved. The carbon black had contributed to block slag by wettability resistance. By comparison with the castables without carbon black, the corrosion index and penetration index had been reduced by 20.2% and 28.0%, respectively, under air atmosphere. And there were little corrosion or penetration under reducing atmosphere for castables with 0.4 wt% carbon black. For the mechanical properties, the Al₂O₃–SiC–C castables with 0.4 wt% carbon black could serve production process although the carbon black impaired the physical properties.     

Al2O3-based binders for corrosion resistance optimization of Al2O3–MgAl2O4 and Al2O3–MgO refractory castables

This work addresses the main aspects related to the use of alternative binders [hydratable (HA) or colloidal alumina (ColAlu)] in castables containing different spinel sources (pre-formed or in situ generated), in order to point out: (i) the features that control the corrosion behavior of these materials, and (ii) the key factors to better select a refractory composition. Thermodynamic calculations, corrosion cup-test and SEM analyses were carried out in order to evaluate the slag attack of the designed refractory compositions. According to the attained results, the alumina-based binders (HA or ColAlu) induced a more effective sintering process due to their high specific surface area, improving the physical properties and the binding level of the generated microstructure. The spinel grain size also played an important role in the corrosion behavior of these refractories, as the finer the particles, the greater their dissolution was into the molten liquid, leading to further precipitation of spinel in the solid–liquid interface as a continuous and thick layer. Among the evaluated compositions and considering the presence of silica fume, the most suitable formulation with optimized corrosion resistance was the one with in situ spinel generation and HA as a binder.     

Effect of partial substitution of alumina–chromium slag for Al2O3 on microstructures and properties of Al2O3–SiC–C trough castables

Alumina–chromium slag (ACS), a cheap and abundant refractory raw material comprising aluminum–chromium oxides and β-Al₂O₃, is a byproduct of ferrochrome smelting. For this reason, we investigated the relationships between composition and mechanical properties, abrasion resistance, oxidation resistance, and resistance to iron slag erosion for Al₂O₃–SiC–C trough castables in which ACS was substituted for alumina. Due to the presence of β-Al₂O₃ in ACS, the aluminum-chromium slag reacted with SiO₂ to form a low-melting phase of albite and promoted the formation of mullite, which filled the pores at high temperatures and reduced the porosity, thereby promoting densification and strengthening of the sample. The cold mechanical properties of the sample and the normal temperature wear resistance were enhanced, but the high-temperature mechanical properties and the resistance to iron slag corrosion of the sample were impaired. According to the results of the anti-oxidation experiment, the presence of β-Al₂O₃ in the ACS reduced the porosity and made the sample more dense, which remarkably improved oxidation resistance of the sample. For industrial production requirements, ACS substitution should not exceed 48?wt% due to of thermomechanical properties and anti-slag corrosion performance in Al₂O₃–SiC–C trough castables.     

Preparation of CaCO3 coated corundum aggregates by dip-coating and heat treatment and its effects on the properties and microstructures of Al2O3–MgO castables

The CaCO₃ coated corundum aggregates were prepared by impregnating tabular corundum aggregates with sizes of 1–5 mm in calcium hydrogen citrate solution and heat treatment at 430 °C, which were also used in Al₂O₃–MgO castables. The effects of Ca²⁺/Cit^3? mole ratio in precursor solution on coating characteristics of CaCO₃ coated corundum aggregates as well as the effects of CaCO₃ coatings on properties and microstructure of castables were investigated. It is found that the thickness and continuity of CaCO₃ coating is increased and the size of CaCO₃ particles in coatings decreases first and then increases as Ca²⁺/Cit^3? mole ratio is decreased. High-temperature properties of castables are improved by in-situ formation of calcium hexaaluminate (CA₆) layer at aggregate/matrix interface after sintering at 1600 °C. The Al₂O₃–MgO castables exhibit the best thermal shock resistance when Ca²⁺/Cit^3? mole ratio is 1/3. It is contributed by deflections of cracks and consumptions of fracture energy in a continuous platelet CA₆ layer with thickness of 10 μm, which is in-situ formed through reaction between Al₂O₃ and CaO derived from CaCO₃ coatings. The present investigation provides a novel approach to enhance thermal shock resistance of the Al₂O₃–MgO castables.     

Preparation and characterization of Al2O3–25 mol% ZrO2 composites

Al₂O₃–ZrO₂ (AZ_x), with 25 mol% ZrO₂ content, was prepared using the co-precipitation method. Synthesized powders were characterized by thermal reaction using a differential thermal analysis technique (TG–DTA) and were investigated by phase formation using X-ray diffraction. It indicated that the reaction occurred at 850 °C; cubic (c)-ZrO₂ phase and Al₂O₃ were obtained. By increasing temperature to 1100 °C, tetragonal (t)-ZrO₂ phase was detected. The Al₂O₃–25 mol% ZrO₂ was sintered for 2 h in the temperature range of between 1300 and 1600 °C. The majority phases of ceramics were m-ZrO₂ and α-Al₂O₃, although a t-ZrO₂ phase also appeared as a minor phase and decreased with higher temperature. Moreover, morphology and particle size evolution have been determined via the SEM technique. SEM showed that the particles of powder are agglomerated and basically irregular in shape. An SEM micrograph of ceramics exhibits uniform microstructure without abnormal grain growth.     

Preparation and characterization of equimolar SiO2–Al2O3–TiO2 ternary aerogel beads

Crack-free mesoporous equimolar SiO₂–Al₂O₃–TiO₂ ternary aerogel beads have been synthesized and characterized. Silica sol, alumina sol, and titania sol were synthesized individually to prevent the formation of inhomogeneous structure due to the different hydrolization and polymerization rate of individual precursor. After mixing these three types of acidic sols, SiO₂–Al₂O₃–TiO₂ ternary beads were prepared by the ball dropping method. The ternary aerogel beads were typically mesoporous, showing high surface area (305 m² g^?1), large pore volume (1.32 cm³ g^?1), and high surface acid amount (0.884 mmol NH₃ g^?1). Moreover, the acid sites of the ternary aerogel beads showed higher thermal stability than those of binary aerogel beads. Gradient drying (GD), supercritical drying (SD), ambient drying (AD), extended aging (EA) and hydrophobic modifying drying (HM) have been employed to investigate the effects of drying method on the characteristics of the aerogel beads. The surface areas of the ternary aerogel beads obtained by different drying methods decrease in the sequence EA > HM > GD > SD > AD. The ternary aerogel beads have been characterized by scanning electron microscopy, nitrogen adsorption, X-ray powder diffraction, Fourier-transform infrared spectroscopy (FTIR), solid-state NMR, temperature-programmed desorption measurements, pyridine adsorption FTIR, and differential scanning calorimetry.     

Preparation,characterization and hydrotreating performances of ZrO2–Al2O3-supported NiMo catalysts

A series of Al₂O₃–ZrO₂ composite supported NiMo catalysts with various ZrO₂ contents were prepared. Several techniques including XRD, SEM, N₂ physisorption, H₂-TPR, and UV–vis DRS were used for typical physico-chemical properties characterization of the ZrO₂–Al₂O₃ composite supports and their NiMo/ZrO₂–Al₂O₃ catalysts. The test results showed that the composite supports prepared by the chemical precipitation method existed as amorphous phase in the samples with insufficient contents of ZrO₂, and the incorporation of ZrO₂ into supports provided a better dispersion of NiMo species, which made their reductions become easier. The pyridine-adsorbed FT-IR results indicated that the Lewis acid sites of catalysts increased significantly by the introduction of ZrO₂ into the supports. The activities of these catalysts for diesel oil hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) were evaluated in a high pressure micro-reactor system. The results showed that the ZrO₂–Al₂O₃-supported NiMo catalysts with suitable ZrO₂ contents exhibited much higher catalytic activities than that of Al₂O₃-supported one, and when the ZrO₂ contents were 15% and 5%, the NiMo/Al₂O₃–ZrO₂ catalysts presented the highest HDS and HDN activities, respectively.     

Preparation and characterization of Al2O3/TiC micro–nano-composite ceramic tool materials

An Al₂O₃-based composite ceramic tool material reinforced with micro-scale and nano-scale TiC particles was fabricated by a hot-pressing technology with cobalt additive in different sintering processes. The microstructure, indention cracks and phase composition of composites were characterized with scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The experimental results showed that Al₂O₃/TiC_μ/TiC_n micro–nano-composite containing 6 vol% nano-scale TiC and 35 vol% micro-scale TiC, which were sintered under a pressure of 32 MPa at a temperature of 1650 °C in vacuum for 20 min, had optimum mechanical properties. The addition of both nano-scale TiC and Co contributed to the microstructure evolution and the improvement of mechanical properties. Effects of nano-scale TiC on mechanical properties were investigated. The toughening and strengthening mechanisms of micro–nano-composites were discussed.     

Characterization and sintering of gels in the system MgO–Al2O3–SiO2

Cordierite aerogels, made by supercritical drying, and xerogels, formed by ambient pressure drying, have been prepared by combining two different recipes. The chemical composition of the gels varied from stoichiometric cordierite 2MgO·Al₂O₃·5SiO₂ to 0·5MgO·1·4Al₂O₃·5SiO₂ due to different procedures for washing of the gels. The crystallization of nearly stoichiometric cordierite gels was shown to be relatively complex involving the formation of several metastable phases such as μ-cordierite (Mg₂Al₄Si₅O₁₈), spinel (Al₆Si₂O₁₃) and sapphirine (Mg₄Al₈Si₂O₂₀) before the equilibrium phase composition was obtained at around 1350°C. On the other hand, during crystallization of gels with stoichiometry close to 0·5MgO·1·4Al₂O₃·5SiO₂ the equilibrium phases mullite, cristobalite and α-cordierite were the major phases formed during heat treatment. A lower densification rate was observed for aerogels compared to xerogels due to a larger pore size. A lower crystallization temperature in aerogels probably due to heterogeneous nucleation reduced the densification. For gels with a composition near 0·5MgO·1·4Al₂O₃·5SiO₂ nucleation and densification occur simultaneously and large differences in the densification behavior was observed. ©     

Preparation and pore-forming mechanism of MgO–Al2O3–CaO-based porous ceramics using phosphorus tailings

A simple strategy for preparing MgO–Al₂O₃–CaO-based porous ceramics (MACPC) with high strength and ultralow thermal conductivity has been proposed in this work based on the raw material of phosphorus tailings. The effects of phosphorus tailings content, carbon black addition and heat treatment temperature on the properties of MACPC were studied, and their pore-forming mechanism during sintering was revealed. The results showed that the main phase composition of MACPC was magnesia alumina spinel and calcium aluminate after sintering at 1225 °C. Furthermore, the MACPC exhibited excellent comprehensive properties when 60 wt% phosphorus tailings and 40 wt% alumina were added, whose apparent porosity was 62.8%, cold compressive strength was 14.8 MPa, and the thermal conductivity was 0.106 W/(m·K) at 800 °C. The synchronously enhanced strength and thermal insulation properties of MACPC were related to the formation of uniformly distributed micropores (<2 μm) and passages in the matrix, which originated from the decomposition of phosphorus tailings and the burnt out of carbon black during the sintering process. The preparation of MACPC with high temperature resistance and excellent mechanical and thermal insulation properties with the raw material of phosphorus tailings provided an effective method for the high-value utilization of phosphorus tailings.     

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.     

Modification of glass network and crystallization of CaO–Al2O3–MgO–SiO2 based glass ceramics with addition of iron oxide

Modification of glass network and crystallization process of a CaO–Al₂O₃–MgO–SiO₂ (CAMS) based glass ceramic to form diopside through addition of iron oxide were investigated using differential thermal analysis (DTA), Raman spectrum, X-ray diffraction, SEM and EBSD techniques. The experimental results showed that addition of Fe₂O₃ led to remarkable reductions in both the glass transition temperature (Tg) and crystallization temperature (Tp) of the CAMS glass ceramic. At addition level below 5 wt%, the Tg and Tp temperatures were 651°C and 903°C, respectively, and the crystallization only occurred on the surface of the glass ceramic samples. Increasing the addition level to 10 wt% and 15 wt%, not only led to reduction in the Tg and Tp temperatures to 643-641°C and 892-876°C, respectively, but also promoted the formation of crystalline diopside throughout the CAMS samples. Based on the results of Raman spectrums, it was confirmed that Fe₂O₃ addition reduced the strength of glass connection as a result of chemical reactions between the isolated Si–O tetrahedron and Fe³⁺ ion, forming Fe³⁺O₄–SiO₄, which can be regarded as Q₂ unit. And this is the first experimental evidence that proving the approach of Fe³⁺ mending glass network. Microstructural examination also identified the formation of large numbers of spherical Fe-enriched regions within the CAMS glass matrix as a result of the amorphous phase separation due to the Fe₂O₃ addition. The interfaces between the Fe-enriched regions and the glass matrix acted as preferred nucleation sites for the diopside, facilitating the crystallization. Crystallographic analysis using EBSD technique determined the <001> as the most favorite growth direction for the diopside crystals in the CAMS based glass ceramic.     

Effect of surface properties of MgO on its dissolution and spinel growth in CaO–SiO2–Al2O3 slag

The dissolution behavior of MgO in CaO–SiO₂–Al₂O₃ ternary slag at the interface of single-crystal, dense poly-crystal, and porous poly-crystal MgO was investigated to evaluate the effect of the surface properties of the MgO. The experimental results revealed that a detached spinel layer formed at the MgO interface due to the change in thermodynamic condition of the slag, which was independent of the surface properties. On the other hand, it was also confirmed that the growth rate and morphology of the detached spinel layer strongly depended on the surface properties, such as porosity and curvature of MgO. During the formation of the spinel layer at the interface during MgO dissolution, a kinetic approach adopting parabolic relation theory was employed to determine the correlation between the surface properties and the spinel growth mechanism.     

Synthesis of CaO·2MgO·8Al2O3 (CM2A8) and its slag resistance mechanism

CM₂A₈ is a solid solution of MA and CA₆. As a component in the ternary phase diagram of CaO-Al₂O₃-MgO, it was not reported in detail. So in this work, analytically pure CaO, MgO and Al₂O₃ were adopted as starting materials and batched stoimotrically according to CM₂A₈. The raw materials were ground, mixed, shaped and reaction-sintered at different temperatures (1550 °C, 1650 °C, 1700 °C, and 1750 °C) to synthesize CM₂A₈. The synthesized specimens were analyzed by XRD, SEM, and TEM and the corrosion mechanism against LF slag was also researched. The results show that high-purity CM₂A₈ can be synthesized by reaction sintering at 1750 °C. During synthesis, granular MA and flake CA₆ form and grow together; at 1650 °C, they solid-solve together to form C₂M₂A₁₄. As the temperature rises, solid solution reaction goes on, which results in the disappearance of CA₆, C₂M₂A₁₄, and MA in succession and forms high-purity CM₂A₈ growing towards hexagonal column crystals. In the high temperature reaction between CM₂A₈ and steel slag, Ca²⁺ in the slag reacts with CM₂A₈ to form CA₂, damaging the structure of CM₂A₈ and releasing excessive Mg²⁺ and Al³⁺ which move towards the molten slag. As the reaction between Ca²⁺ and CM₂A₈ proceeds, a dense CA₂ coating forms on the surface of CM₂A₈. The slag viscosity increases as well because of the entrance of Mg²⁺ and Al³⁺. Thus, the formation of the CA₂ coating and the enhancement of the slag viscosity restrain the penetration and corrosion of slag towards CM₂A₈.     

Preparation and characterization of ordered mesoporous Mg–Al–Co hydrotalcite based catalyst for decarboxylation of jatropha oil

This paper reported preparation of novel order mesoporous Mg–Al–Co hydrotalcite based catalysts through sol–gel procedure using precursors such Mg(NO₃)₂, Al(NO₃)₃ and Co(NO₃)₂ and Na₂CO₃. The catalyst also contained both acidity and basicity being very convenient for decarboxylation process of vegetable oil to green hydrocarbons. The alkaline media was maintained at pH 10 during the processes. Molar ratio of metal cations and temperature of the sol–gel processes were investigated for their effect in the mesoporous structure formation. The results showed that the procedure should be established at 70 °C with the molar Mg/Al/Co ratio of 1/5/0.2. Acidity and basicity of the mesoporous hydrotalcite based catalyst were demonstrated for their co-existence. The as-synthesized material at the suitable conditions was used as catalyst for decarboxylation of jatropha oil to obtain green hydrocarbons mainly belonging to diesel fraction. The decarboxylation was carried out at 400 °C for 3 h in closed auto-pressurized reactor exhibiting a yield of diesel involving hydrocarbons of over 70% after distillation and analysis. The result also confirmed that the acidity and basicity greatly accelerated the activity of the catalyst. Some techniques were used to characterizing the catalyst including XRD, SEM, TEM, TGA, NH₃-TPD, CO₂-TPD and BET, and GC–MS was also used to analyze the main product composition.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.