B4C mineralizing role for mullite generation in Al2O3-SiO2 refractory castables

Based on refractory end-users’ requirements, continuous efforts have been made to design engineered products able to withstand high temperatures (800–1500 °C) and severe thermal gradients. One alternative to enhance the mechanical properties of alumina-based compositions consists of inducing in situ generation of phases with platelet or acicular shape within their matrix fraction, which may improve crack deflection and grain bridging mechanisms. Mullite and Al₁₈B₄O₃₃ are some compounds that present such interesting features. Thus, this work addresses the evaluation of alumina refractory castables bonded with SioxX-Zero and/or microsilica, containing 0 or 1 wt% of B₄C (sintering additive), aiming to: (i) induce transient liquid sintering, (ii) point out which silica source could favor a more effective mullite formation at high temperatures, and (iii) analyze the influence of B₄C in the phase transformation and thermo-mechanical properties of the designed compositions. Comparing SioxX-Zero and microsilica-bonded refractories, the latter showed more likelihood to give rise to the mullite phase during the samples’ thermal treatments. Moreover, adding B₄C to the castables containing 3 wt% of SiO₂ induced the generation of a boron-rich liquid phase with transient features during the samples’ firing step, favoring earlier sintering and faster mullite and Al₁₈B₄O₃₃ formation. These transformations resulted in refractories with enhanced creep, thermal shock resistance and HMOR behavior in a broader temperature range (600–1550 °C), which may enable them to be used in various industrial applications (petrochemical, steel-making, etc.).     

The physical and mechanical properties of alumina-based ultralow cement castable refractories

In this study, the effects of the type of alumina on the physical, chemical and mechanical properties of the ultralow cement castable (ULCC) refractories were investigated. Brown fused alumina, tabular alumina and rotary bauxite-based ULCC refractories were prepared by mixing each type of alumina with silicon carbide, carbon, cement, metallic silicon and microsilica. The density, porosity and cold crushing strength (CCS) of the refractory castables were measured after drying at 110 °C for 24 h and firing at 1450 °C for 5 h. The slag penetration resistance of the refractory castables was determined using slag corrosion tests. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD) were used to characterize the castables. It was found that all three refractory castables had strong slag penetration resistance and that the tabular alumina-based refractory castable had the largest specific cold crushing strength with an acceptable percent of porosity among the refractory castables.     

Monoaluminum phosphate-bonded refractory castables for petrochemical application

Phosphate-bonded refractories may be applied as repairing materials due to their short setting time and good thermo-mechanical properties in the 30–1000 °C temperature range. This works addresses the development of vibratable high-alumina castables containing commercial monoaluminum phosphate (MAP) solutions (Fosbind 151 and Fosbind 50) and dead-burnt magnesia (d<212 µm, setting agent) as the binder systems. Flowability, setting time, X-ray diffraction, cold erosion, thermal shock resistance, cold and hot modulus of rupture, thermogravimetric measurements and hot elastic modulus tests were carried out in order to understand the phase evolution and thermo-mechanical behavior of the refractories. Furthermore, the effect of adding a boron source (sintering additive) to the phosphate-bonded compositions was also investigated. According to the attained results, the reaction of MAP with MgO and the reactive aluminas of the compositions resulted in setting times of the mixtures around 90–120 min at 30 °C, which was associated with the in situ generation of magnesium and aluminum phosphates [MgHPO₄·3H₂O, Mg(H₂PO₄)₂(H₂O)₂ and AlPO₄.2H₂O]. The boron-containing castables presented Al₁₈B₄O₃₃ around 650–800 °C and this phase favored the increase of the samples' stiffness, mechanical strength, erosion and thermal shock resistance. The refractoriness under load measurements indicated that the maximum working temperature for the evaluated refractories was in the range of 1400–1500 °C.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Effect of spinel content on hot strength of alumina-spinel castables in the temperature range 1000–1500°C

Hot modulus of rupture of Al₂O₃-spinel castables containing 5–15 wt% alumina-rich magnesia alumina spinel and 1·7 wt% CaO generally increases with increase in spinel content and temperature from 1000 to 1500°C. The magnitudes of hot modulus of rupture of castables containing 15 wt% spinel and 1·7 wt% CaO are 14·3 MPa at 1400°C and 15·6 MPa at 1500°C, while those of castables containing 20 wt% spinel and 1·7 wt% CaO are 12·5 MPa at 1400°C and 14·7 MPa at 1500°C. The former castables contained 15 wt% spinel of −75 μm size, while the latter contained 10 wt% spinel of +75 μm size and another 10 wt% spinel of −75 μm size. The bond linkage between the CA₆ and spinel grains in the matrix is believed to cause both the spinel content and temperature dependence of hot strength of Al₂O₃-spinel castables, as well as fine grain spinel even in amount less than coarser grain spinel to be more effective for enhancing hot strength. The trend of the magnitude of thermal expansion under load (0·2 MPa) above 1500°C of the castables is not necessarily indicative of the magnitude of hot modulus of rupture at 1400 or 1500°C. ©     

Effect of micro-sized MgCO3 addition on properties of MgAl2O4 spinel containing castables

Micro-sized MgCO₃ was used in castables as the MgO-precursor for generating sub-micro sized MgO and for producing subsequent in-situ magnesium aluminate (MgAl₂O₄, MA) spinel. The influence of 0–2.0 wt% micro-sized MgCO₃ addition on the volumetric stability and thermo-mechanical properties of castables after firing at 1000 °C and 1550 °C was investigated. The in-situ spinel formation and its influence on the microstructure evolution in castable matrices with different micro-sized MgCO₃ contents after heat-treatment were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The dependence of the volume stability and thermo-mechanical properties of castables on the micro-sized MgCO₃ addition was discussed with respect to the in-situ spinel formation.     

Boron carbide coatings on diamond particles

Boron carbide (B₄C) coatings on diamond offer potential for obtaining homogeneous B₄C-diamond composites with improved properties. A method was developed for coating diamond particles with B₄C at 1150 °C under argon atmosphere for dwell times of 2–6 hours in a powder mixture of boric acid (H₃BO₃) and amorphous boron. The B₄C coating showed very good adhesion to the diamond substrate, and an unusual five-fold symmetry thought to be due to a twinned growth mechanism. The sudden onset of nucleation at T > 1000 °C is ascribed to the stabilising effect of hydrogen from the decomposition of H₃BO₃ on the diamond surface reactivity.     

Gel-cast hierarchical porous B4C/C preform and its role in fabricating reaction bonded boron carbide composites

The resorcinol-formaldehyde (RF) gel-casting system is employed for the first time to fabricate a hierarchical porous B₄C/C preform, which was subsequently used for the fabrication of reaction bonded boron carbide (RBBC) composites via a liquid silicon infiltration process. The effect of the carbon content and carbon structures of this perform on the microstructures and mechanical properties of B₄C/C preform and the resultant RBBC composites is reported. The B₄C/C preform (16 wt% carbon) exhibit a strength of 34±1 MPa. The obtained RBBC composites shown uniform microstructure is consisted of SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase. The Vickers hardness, flexural strength and fracture toughness of the RBBC composites (16 wt% carbon) are 24 GPa, 452 MPa and 4.32 MPa m^1/2, respectively.     

The dispersion of boron carbide powder in aqueous media

In the present work, the dispersion of boron carbide powder in aqueous media was studied in terms of surface treatment, surface characterization, the particle stability as well as rheological behavior, etc. Initially, the influence of the impurity and boron oxide from commercial B₄C powder on the slurry dispersion was studied. The surface compositions, bonding and surface charge behavior of acid-treated B₄C powder were investigated systematically. The positive effect of TMAH (tetramethylammonium hydroxide) on the dispersion of as-treated B₄C powder was studied and the content was optimized based on rheological measurement. Results showed that well dispersed B₄C slurries could be developed with the solid loading of 55 vol.% using 0.18 wt% TMAH as the dispersant. The dispersion mechanism was also investigated before and after TMAH addition.     

Enhancement of bonding network for silica sol bonded SiC castables by reactive micropowder

Silica sol bonded castables have obvious advantages over low cement or hydratable alumina bonded castables in drying performance and sintering properties for SiC castables. However, they are not widely used due to their weak strength at low temperature. The efficiency of bonding network for silica sol bonded SiC castable in the presence of different reactive micropowder such as SiO₂ micropowder and α-Al₂O₃ micropowder was evaluated through oscillatory tests, sintered properties and microstructural analysis. Results show that the polymerization reaction between SiO₂ micropowders enhanced the siloxane network and reinforced the bonding strength, furthermore, the addition of α-Al₂O₃ micropowder contributed to accelerating the formation of the siloxane network and hardening of the silica sol at lower temperatures and shorter time. Silica sol performed well as a binder agent for SiC castables with an addition content of 3 wt% SiO₂ micropowder and 2 wt% α-Al₂O₃ micropowder, which showed high strength and good workability at room temperature. And Silica sol bonded SiC castable with the above micropowder contents possessed the best mechanical behavior after heat treatment due to combined binding of SiC whiskers and mullite.     

Effect of SiC–graphite–Al-metal addition on low- and ultra-low cement bauxite castables

Eight batches of low- and ultra-low cement castables were prepared from calcined Chinese bauxite and high alumina cement (HAC). The effect of alumina-cement replacement by SiC, graphite and aluminum metal on the sinterability and properties of these castables was investigated. Physical properties such as bulk density and apparent porosity of hydrated and sintered castables were studied. The sintered castables were also characterized for their solid phase compositions and microstructure using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. In the castables containing SiC, new phases such as mullite (3Al₂O₃·2SiO₂), SiC, and quartz (SiO₂) were formed at the expense of calcium aluminate phases (i.e. CA and CA₂; the main cement phases). Generally, the bulk density of the control castable sample was the highest among all prepared samples, while the batches containing graphite showed the lowest bulk density. The presence of Al-metal reduced the oxidation of SiC and consequently increased the densification of the castables compared with castables containing graphite only. Cold crushing strength (CCS) of the hydrated specimens i.e. green castables, decreased as the additives contents increased at the expense of HAC which is responsible for the bonding at room temperature. The highest CCS value of the sintered castable was obtained for the sample containing 6 wt.% SiC, 3 wt.% CA and 0.5 wt.% Al-metal.     

Self-flowing high-alumina phosphate-bonded refractory castables

Phosphate refractories have a great potential to be applied in petrochemical industries as they present suitable properties at the temperature range used in fluid catalytic cracking units. This study addresses the development of high-alumina self-flowing castables bonded with H₃PO₄ solution (48 wt% concentration) or a mixture of phosphoric acid and monoaluminum phosphate (MAP) solutions, using MgO as a setting agent. Two polyphosphates (Budit 3H and 6H) and citric acid were evaluated as dispersant additives for these castables. The compositions were characterized by measuring their free-flow and temperature evolution over time, working and setting times, cold and hot mechanical strengths, drying behavior and explosion resistance, eroded volume and thermal shock resistance. The results indicated that high flowability (free flow >100%) could be attained when adding the selected polyphosphates to the mixtures, whereas citric acid acted mainly as a retarder agent for the castables’ setting. Moreover, free-flowing compositions with a suitable working time were obtained when combining H₃PO₄+MAP solutions as main binders. The thermo-mechanical tests pointed out that the most promising designed refractory (containing mixture of H₃PO₄+MAP and 0.5 wt% of Budit 3H) presented similar or even a better performance than a benchmark commercial vibratable product used in petrochemical units.     

The influence of Al2O3 nanoparticles addition on the microstructure and properties of bauxite self–flowing low-cement castables

In this research, the impact of Al₂O₃ nanoparticles addition on microstructure, mechanical, and physical properties of bauxite self–flowing low-cement castables were investigated. Also, the optimum amount of Al₂O₃ nanoparticles is determined. For this propose, up to 3 wt% Al₂O₃ nanoparticles were added to the bauxite castable compositions. The physical and mechanical properties of castable compositions such as bulk density (BD), apparent porosity (AP), self-flow values (SFV), and cold crushing strength (CCS) were examined. Also, the X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDX) techniques were used for detection the ceramic phase's formation and microstructural analysis of the castables compositions, respectively. Results show that addition of Al₂O₃ nanoparticles up to 1 wt% improved the properties of bauxite self–flowing low-cement castables. As well as, the use of Al₂O₃ nanoparticles led to the formation of the platy and needle crystalline phases such as hibonite (CaO·6Al₂O₃), calcium dialuminate (CaO·2Al₂O₃), and mullite (3Al₂O₃·2SiO₂), between the grain boundaries of the bauxite particles. Also, Al₂O₃ nanoparticles addition led to aforementioned phase formation occur at the lower temperatures.     

Mullite-based refractory castable engineering for the petrochemical industry

Refractory castables used in fluid catalytic converter (FCC) risers should present suitable particle erosion and thermal shock resistances at temperatures below 900 °C. Considering that calcium aluminate cement (CAC)-bonded refractories usually start their densification above 1200 °C, the use of sintering additives to induce faster densification is a promising technological alternative. Therefore, this work addresses the evaluation of mullite-based castables containing a boron-based sintering additive and CAC and/or hydratable alumina as the binder sources. Hot elastic modulus, cyclical thermal shock, hot modulus of rupture and cold erosion resistance measurements were carried out to evaluate the compositions. According to the attained results, adding 1.5 wt% of the evaluated sintering additive to the designed castables led to a remarkable increase of the hot modulus of rupture (maximum of 40.4 MPa at 800 °C for the CAC-containing refractory) and high erosion resistance (1.5–2.9 cm³) after pre-firing at 800 °C for 5 h. Moreover, the combination of CAC and hydratable alumina gave rise to an improved refractory (M–2CAC–2HA–S) showing a transient liquid formation at an increased temperature, high thermal shock resistance (no E decay after 8 thermal cycles, ΔT=800 °C) and high mechanical strength at 800 °C and 1000 °C.     

Pressureless sintering of boron carbide with Cr3C2 as sintering additive

In this study, chromium carbide (Cr₃C₂) was selected as the sintering additive for the densification of boron carbide (B₄C). Cr₃C₂ can react with B₄C and form graphite and CrB₂ in situ, which is considered to be effective for the sintering of B₄C composites. The sintering behavior, microstructure development and mechanical properties of B₄C composites were studied. The density of B₄C composite increased with the increase of Cr₃C₂ content and sintering temperature. The formation of liquid phase could effectively improve the densification of B₄C composites. The abnormal grains began to appear at 2080 °C. The bending strength could reach 440 MPa for the 25 wt% and 30 wt% Cr₃C₂ samples after sintering at 2070 °C.     

ZnO and MgO as inducers of spinel-like phase formation on alumina-based castables

Environmental issues regarding Cr⁶⁺ formation lead to replacing chrome-containing refractories with greener alternatives. MgO-containing compositions have been extensively investigated for this purpose, however, few studies evaluated the likelihood of using other chemical elements as inducers of spinel-like phase formation in refractory castables. In this study, the addition of zincite in alumina-based castables was evaluated and compared with its MgO-counterpart. In-situ elastic modulus, assisted sinterability and differential scanning calorimetry pointed out that the gahnite (ZnAl₂O₄) formation took place at lower temperatures (～ 1100 °C) than MgAl₂O₄ (～ 1300 °C). On one hand, this feature induces anticipated strengthening of the Zn-containing compositions, giving rise to the possibility of firing these compositions at lower temperatures. On the other, the faster kinetics of gahnite formation led to a significant Kirkendall effect, changing the morphology of the pores created during sintering, which became preferentially located at the interface of alumina aggregates, negatively affecting some mechanical properties of the castable.     

Effect of boron oxide on the microstructure of mullite-based glass-ceramic glazes for floor-tiles in the CaO–MgO–Al2O3–SiO2 system

The effect of increasing replacement of Al₂O₃ by B₂O₃ in a parent glass on the sintering and further crystallization of mullite was investigated. The composition of the parent glass was chosen in the mullite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system. Glass powder pellets were heated under standard (10 °C/min and 2 h of hold time) and fast heatings (25 °C/min and 5 min of hold time) at different temperatures from 700 to 1190 °C. Sintering of B₂O₃-containing glasses took place in the range between 850 and 1050 °C. X-ray diffraction results showed that mullite formed as unique crystalline phase for glasses containing amounts of B₂O₃ larger than 6 wt%. For lower amounts of boron oxide cordierite was formed as secondary crystalline phase. Quantitative determination of mullite by Rietveld analysis indicated that the higher amount of mullite present in the glass-ceramic fast heated at 1160 °C was 19.5 wt% for the glass containing 9 wt% of B₂O₃. The final microstructure of the glass-ceramic glazes showed the presence of well shaped, long acicular mullite crystals dispersed within the residual glassy phase. Results of glass-ceramic glazes when applied as slurry and under industrial heating conditions pointed out promising mechanical properties.     

Catalytic graphitization of novolac resin for refractory applications

This study investigated how to induce graphite generation from the carbonization process of novolac resins using conditions that can be adopted for carbon-containing refractories (CCRs) production. The effect of boron oxide or boric acid (graphitizing agents), cross-linking additive (hexamethylenetetramine) and some processing parameters (mixing technique, vacuum degassing, heating rate and thermal treatments) on carbon graphitization from a commercial novolac resin were evaluated. The X-ray diffraction (XRD) technique was selected to measure the graphitization level and crystal parameters of the prepared samples. Based on the attained results, adding graphitizing agents prior to the pyrolysis of resin resulted in carbon crystallization. The best graphitization level was obtained when the mixtures containing 6 wt% B₂O₃ or 10 wt% H₃BO₃ were fired up to 1000 °C for 5 h using a heating rate of 3 °C/min. Although the reproducibility of the obtained results was ascertained, heterogeneous graphitization could be observed based on the XRD profiles, as well as some discrepancies in the calculated graphitization level values. This phenomenon was attributed to the additives susceptibility to agglomeration, preferential graphitization starting from lower binding energy sites and heat treatment temperature, among others.     

Incorporating Zr to achieve self-protecting and enhancement of silica sol bonded SiC castables at active oxidation condition

SiC castables exhibit degraded properties in static air at 1700 °C, due to the formation of gaseous products. The efficiency of different contents of Zr in SiC castables was evaluated by considering sintered properties, mechanical performance, isothermal oxidation behavior, and microstructural analysis of the SiC castables. Specimens with more Zr exhibited enhanced mechanical behavior and anti-oxidation capability. The addition of Zr decreased the evaporation of SiO₂ by reducing its equilibrium partial pressure (g), and formed a dense ZrO₂-SiO₂ protective layer (e.g., the sample with 0.9 wt% Zr) to prevent further degradation of the SiC castable. The Zr that was preferentially oxidized to ZrO₂ reduced the partial pressure of the oxidizing gases (O₂ and CO₂) in the matrix, and increased SiO (g) content, which facilitates formation of SiC fibers, which, in turn, improves the anti-oxidation capability and mechanical behavior of SiC castables, preventing their degradation in static air at 1700 °C. The addition of Zr created a ZrO₂-SiO₂ protective layer on the surface and prevented the decrease in SiC content, by forming SiC fibers. This made the silica sol bonded SiC castable a self-protecting refractory.     

Towards chrome-free of high-temperature solid waste gasifier through in-situ SiC whisker enhanced silica sol bonded SiC castable

Refractory containing Cr₂O₃ was widely used in solid waste gasifier due to its excellent slag resistance. However, hexavalent chrome compounds (formed during the preparation and the use of refractory containing Cr₂O₃) will give rise to detrimental effect on environment and human's health. In addition, the Al₂O₃-Cr₂O₃ materials acted as the lining materials. Serious exfoliation occurred after about twenty days. For the purpose of chrome-free and service longevity of lining materials for solid waste gasifier, in-situ SiC whisker enhanced SiC castable and spinel castable containing 20%wt of Cr₂O₃ were prepared. The mechanical properties and corrosion resistance after heat treatment in different temperature of the castables were determined. The strength of SiC castable rised with the increasing of the temperature. And the nano SiC/SiO₂ core-shell whiskers was formed at 1500 °C. In comparison to the spinel castable containing 20 wt% of Cr₂O₃, the better volume stability and the reinforcement of the nano whiskers led to excellent resistance to crack propagation at high temperature. In addition, SiC castable showed lower apparent porosity because of the forming of SiO₂ through the oxidation of SiC over 1300 °C, the viscosity of slag increased since that the SiO₂ dissolve into the slag, which caused excellent penetration resistance of SiC castable compared with spinel-Cr₂O₃ castable. Excellent mechanical properties and slag resistance at high temperature indicated that SiC castable had the application prospect for high-temperature solid waste gasifier.