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.).     

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.     

Improving in situ spinel refractory castables using a novel binder

The objective of this paper is to find an efficient binder by investigating binders such as fireclay, white Portland cement and gypsum with different compositions, and ultimately to introduce a suitable binder composition. Evaluation of physical properties and microstructure of castables were performed by mechanical (splitting tensile strength) test, porosity measurements, bulk density measurements, X-ray diffraction spectroscopy and scanning electron microscopy. The results revealed that the performance of the new binder is much better than that of conventional binders such as calcium aluminate cement, because it creates strong bonds between the particles and improves the in situ spinel formation and sintering/diffusion of particles with each other at high temperatures. Analyzing the prepared samples with new binder also demonstrated that the increase of refractory material in binder composition, leads to creation of the strong and cohesive phases and bonds between the particles (such as mullite), and therefore, increases the driving force of in situ spinel phase formation and ultimately improves the performance of refractory masses at high temperatures.     

Sintering effect of calcium carbonate in high-alumina refractory castables

Calcium hexaluminate (CA₆) presents interesting properties and morphology, which can be readily changed depending on specific additives and refractory processing conditions. Aiming to investigate the role of calcium carbonate in inducing the formation of elongated CA₆ grains and also identify its sintering effect during the thermal treatments of high-alumina castables, this work focused on evaluating compositions containing calcium aluminate cement, CaCO₃ or their blend with the help of in situ techniques (hot elastic modulus, assisted sintering) and other traditional methods (mechanical strength, thermal shock resistance, etc.). A sintering effect derived from CaCO₃ addition to alumina castables could be identified during the hot E measurements, pointing out the ability of this compound to undergo a sintering-coarsening-coalescence transformation at 500–900?°C. This process also enhanced the mechanical strength and thermal shock resistance of the designed refractories at intermediate temperatures. Acicular CA₆ grains were formed in all analyzed compositions after firing at 1400?°C.     

Sintering of clay-chamotte ceramic composites for refractory bricks

Clay-chamotte composites were realized for manufacturing refractory bricks. We used two kaolinitic refractory clays mined in Cameroon and two calcined clays (chamottes) with a large grain size (0.1–4 mm). Clay-chamotte composites containing various quantities of chamotte (0–50 wt%) were shaped and sintered at 1200–1350 °C. The structural characteristics of composites indicated the presence of quartz from the initial clay, cristobalite and mullite. SEM observations revealed very heterogeneous microstructures where porosity is weakly distributed and large pores are entrapped at the vicinity of large chamotte and quartz grains. In general, the global porosity increases with the chamotte content. A specific interpretation of the matrix role on the global sintering behaviour reveals that only a part of the matrix acts effectively. Since the most part of the global porosity is within the matrix, it is distributed in matrix zones, which participate effectively to sintering and in inert matrix zones where larger pores occur. The global mechanical strength is controlled by the matrix behaviour, but the high porosity of this phase is unfavourable to high strength values. Besides, the occurrence of larges pores and local cracks at large grain interfaces from thermal stresses are critical flaws, which reduce the mechanical strength.     

Elaboration and characterization of a refractory based on Algerian kaolin

A refractory material was elaborated from kaolin extracted from the region of Djebel Debbagh (Algeria). Kaolin grog was obtained by calcination at a temperature of 1350 °C during 1 h. It was used as aggregates with granulometric distribution composed of fine fraction (mean grain size: 100–250 μm) and coarse fraction (mean grain size: 1000–2500 μm). Crude kaolin (size < 75 μm) was also used as a binder with an amount representing 15% of the dry material. After a 9.28% moistening and a rotting of 1 day, cylindrical samples were shaped by uniaxial pressure at 80 MPa. The samples were submitted to a natural drying during 24 h, a stoving at 100 °C and a calcination at 600 °C during 1 h. They were fired at high temperatures between 1250 and 1450 °C.

An X-ray diffraction (XRD) analysis showed that the refractory samples are composed of mullite and silica. Silica is a mixture of a vitreous phase and cristobalite at 1300, 1350 and 1400 °C and becomes completely amorphous when the samples are fired at higher temperature (1450 °C). The sample porosity is about 30%. The mechanical tests carried out as a function of temperature revealed different behaviours of the material. From the ambient up to 600 °C, the refractory behaviour is pseudo-plastic caused by micro-cracking. Between 700 and 900 °C, the samples become more rigid. At 1000 °C, the material exhibits a visco-plastic behaviour. The amorphous phase governs the sample properties variation with temperature increasing. Its content varies between 28% and 34% according to the firing temperature. Thermal shock tests realized in water showed that the refractory samples present good thermal shock resistance.     

Hot-erosion of nano-bonded refractory castables for petrochemical industries

Cold-erosion tests are usually applied for refractory castable evaluation in petrochemical industries, mainly for materials used in cyclones and risers of fluidized catalytic cracking units (FCCU). Due to the fact that in the operation, the refractory material is subjected to high temperature, at first, hot-erosion tests could be more representative. Erosion wear has a great influence on the FCCU refractory working life and the lost income associated with a production halt can reach values close to US$500,000 per day. These aspects are strong enough to justify hot-erosion resistance characterization. The test viability, which could result in selecting better material for the application, was evaluated using commercial products and nano-bonded castables from 200 up to 815 °C. At this temperature range, a significant difference between the cold and hot-erosion loss was not observed. Therefore, it indicates that the cold-erosion test is suitable and sufficient for selecting refractory castables for FCCU applications. The cold-erosion test also presented a higher data correlation with the splitting tensile strength and the hot modulus of rupture. Regarding the different evaluated materials, the nano-bonded castables showed a higher erosion resistance and were less susceptible to the temperature effect than the calcium aluminate cement bonded commercial products.     

Preparation of refractory cordierite using amorphous rice husk silica for thermal insulation purposes

This study aims to investigate the effect of sintering temperatures on the phase formation and physical characteristics of refractory cordierite prepared from rice husk silica, Al₂O₃, and MgO powders. The samples were subjected to sintering temperatures of 1050–1350 °C, and development of structures was characterized using Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD) coupled with Rietveld analysis, scanning electron microscopy (SEM) and dilatometry. The results obtained indicated the significant role of sintering temperatures on phase transformation of spinel and cristobalite into cordierite, in which at sintering temperatures of 1230–1350 °C the cordierite emerges as a dominant phase, while spinel and cristobalite are practically undetected. Formation of cordierite was followed by decrease in density, porosity, and thermal expansion coefficient, while for hardness and bending strength the opposite was true. Thermal expansion coefficient of the sintered sample at 1350 °C is 3.3×10⁻⁶/°C and the XRD analysis demonstrated that the main crystalline phase is cordierite. Based on these characteristics, the samples are considered as insulator, suggesting their potential use in refractory devices.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.     

水泥窑用耐火浇注料的开发及应用

本文介绍了水泥窑用耐火浇注料的研究和应用情况。实际使用表明，在水泥窑的某些部位，特别是在那些由于结构复杂或窑体变形而不适宜采用砖的部位，采用浇注料更适宜。     

Mullite-based cellular ceramics obtained by a combination of direct foaming and reaction bonding

Mullite-based cellular ceramics were prepared via the polymer precursor route using poly(silsesquioxane) in combination with particulate alumina or alumina/aluminum mixtures. The multi-functional preceramic polymer was used as pore-forming agent by employing a self-foaming process during the polymer cross-linking step, as well as a precursor for reactive silica, one of the reagents in mullite formation. The size of filler particulates was found to strongly affect foaming of the polymer/filler mixtures, with coarser particles facilitating an improved foaming performance. Thermal conversion in air at 1600 °C resulted in the formation of cellular ceramics with high mullite contents. The partial substitution of alumina with aluminum in the initial mixtures resulted in improved mechanical properties at comparable porosities, resulting in compressive strengths of 0.3 MPa at total porosities of 93%. A correlation between thermal analysis data and crystalline phase development during the thermal treatment allowed for the clarification of processes taking place during heat treatment, yielding information for a future process optimization approaches.     

Binding additives with sintering action for high-alumina based castables

Great efforts have been made recently to totally or partially replace calcium aluminate cement (CAC) by alternative materials in refractory castables, in order to attain an enhanced thermomechanical performance of these ceramic linings at intermediate temperatures (600–1200 °C). Besides that, using additives that induce earlier sintering/densification of the refractory microstructure may also reduce the energy costs derived from the production of pre-formed pieces. Based on these aspects, this work investigated the viability of replacing CAC by calcium carbonate (CaCO₃) or calcium hydroxide [Ca(OH)₂] to ensure a suitable binding action and effective sintering/densification of the designed compositions at intermediate temperatures. Six high-alumina castables containing these alternative additives or their blend were prepared and their green mechanical strength, apparent porosity and Young's modulus evolution with temperature were evaluated within the 30–1400 °C range. After that, the most promising compositions were characterized via X ray diffraction and thermomechanical tests, such as cold and hot modulus of rupture, thermal shock resistance, etc. Although the selected binders did not result in specimens with green mechanical strength values as high as the ones for the cement-bonded materials (2–8 MPa versus ∼18 MPa, respectively), they could be demolded and handled without any problems. CaCO₃ and/or Ca(OH)₂-bonded compositions presented a sintering effect at intermediate temperatures (600–1000 °C) due to the so-called “sintering-coarsening-coalescence” phenomenon. These transformations favored the faster sintering/densification of the tested castables, resulting in samples with improved cold and hot mechanical strength at 900 °C, reaching values within the range of 28–30 MPa instead of 10–13 MPa for the CAC-bonded one. After firing the evaluated compositions at higher temperatures (up to 1500 °C), all compositions presented similar results regarding their modulus of rupture or thermal shock resistance.     

Porous mullite blocks with compositions containing kaolin and alumina waste

In the production of alumina by the Bayer process, the calcination step generates a waste containing ~90% aluminum oxide (Al₂O₃). Due to the high content of this oxide, this waste can be used as a source of alumina in porcelain formulations, especially those used in the synthesis of mullite. The purpose of this study was to produce porous mullite blocks using compositions containing kaolin and alumina waste. The compositions were formulated based on a mullite stoichiometry of 3:2. Heat treatments were carried out in a conventional furnace at temperatures of 1450 to 1500 °C, applying a heating rate of 5 °C/min and a 1-h hold time at the firing temperature threshold. The powders were characterized by means of X-ray fluorescence (XRF); X-ray diffraction (XRD); thermal analysis (TGA-DTA); scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The physic mechanical properties of the test specimens: water absorption, apparent porosity, linear shrinkage and flexural strength were also evaluated. The XRD results revealed the formation of mullite as the major phase. The morphological analysis by SEM revealed typical mullite needles originating from clay minerals. The size of the mullite needles was calculated based on the TEM analysis, which indicated diameters smaller than 400 nm, confirming the nanometric dimensions of the needles. The flexural strength test of the specimens indicated that this parameter ​​tends to increase as the temperature is raised.     

Nanotechnology in castable refractory

In recent times nanotechnology has drawn significant attention in the field of refractory research. Different nano-powders and colloidal suspensions have been utilized to improve the properties of refractory castables. Various studies have been carried out worldwide with nano scaled binders; such as, hydratable alumina (HA), colloidal alumina (CA), colloidal silica (CS), micro silica, etc.; to improve the thermo mechanical properties of refractory materials. Nano scaled additives are also being applied to reduce the energy consumption and to improve the densification process at lower temperatures. In this paper, the contributions of nanotechnology in selection of raw materials, the binders and choice of additives to improve the quality of refractory materials, and the future of nanotechnology in refractory research are reviewed.     

耐火浇注料衬里受冻试验研究

研究了常用的重质和轻质耐火浇注料在正常养护以及加入普通混凝土防冻剂后经受低温处理的性能变化 ,分析了耐火浇注料的低温冻害机理。认为只要在施工和养护期间采取适当的保暖措施 ,使其强度得以正常发展后即可解除保暖而不会影响衬里的性能。普通混凝土常用的防冻剂不利于耐火浇注料的高温性能 ,不能盲目使用。     

Nano mullite bonded refractory castable composition for high temperature applications

Development of high pure alumina castable is studied by using synthesized mullite sol as the sole binder. Mullite sol is prepared by wet chemical route and is characterized by its solid content, particle size, thermal analysis, phase development with temperature, microstructure, etc. This sol is used at two different percentages in high alumina castable compositions with two different particle size distribution patterns. Conventional castable processing is done on the compositions and the characterizations are done after heat treatment at three different temperatures. Finally, the best composition is also compared with the commercially available silica sol containing high alumina castable with similar particle size distribution. Considerably improved hot strength, high corrosion resistance and flexural strength (hot modulus of rupture) are obtained for the mullite sol containing composition but with relatively lower thermal shock resistance.     

Citric acid as anti-hydration additive for magnesia containing refractory castables

Magnesia hydration is a key concern in refractory castable processing. The volumetric expansion that follows this reaction can result in cracks or even explosion during the first heating-up. Citric acid (CA) and other chelants can significantly reduce MgO hydration rate in aqueous suspensions by forming an insoluble magnesium citrate protective coating on the magnesia particles’ surface. In the present work, the performance of CA as an anti-hydration additive in refractory castables was evaluated by hydration tests, mechanical strength and apparent volumetric expansion (AVE) measurements and thermogravimetry. The results attained have shown that CA effectiveness depends strongly on the amount added and by the interaction with other raw materials in the composition, in particular calcium aluminate cement.     

Low-melting-point polymeric fiber performance as drying additives for refractory castables

Polymeric fibers can be used as drying additives for refractory compositions as their softening and decomposition at the beginning of the heating procedures of those ceramics induces the generation of permeable pathways, providing safer steam release out of the dense microstructure. Thus, it is required to select fibers that favor the preparation of castables with suitable permeability levels at temperatures as low as possible to enhance their explosion resistance during drying. Therefore, this work addresses the production of low-melting-point polymeric fibers (T_m ～ 70–150 °C) and the evaluation of their performance when incorporated into dense high-alumina calcium aluminate cement- or hydratable alumina-bonded refractory castables. EVA (ethylene vinyl acetate copolymer), PCL (polycaprolactone) and PLA (polylactic acid) fibers were produced and added to the designed compositions. Based on the collected results, changes in fiber morphology during their softening and melting process had a great impact on the drying behavior of the castables under different heating rates. Hydratable alumina-bonded compositions were more susceptible to explosions due to such transformations, whereas the CAC-bonded ones exhibited explosion resistance at the hardest heating schedule (20 °C/min). Micrographic analysis of the fiber morphology during heating provided relevant information that helped to better understand the drying process and the permeability evolution of the evaluated dense refractory castables.     

耐火浇注料用分散剂进展

简要介绍了分散剂在耐火浇注料中的作用及作用机理,以及无机分散剂与有机分散剂的不同分散作用机理。着重介绍了有机分散剂的进展及各分散剂的化学结构式、性质与使用效果,并提出耐火浇注料用分散剂的发展方向。     

Novel drying additives and their evaluation for self-flowing refractory castables

The drying step of dense refractory castables containing hydraulic binders is a critical process, which usually requires using slow heating rates due to the high explosion trend of such materials during their first thermal treatment. Thus, this work investigated the performance of alternative additives to induce faster and safer drying of self-flowing high-alumina refractory castables bonded with calcium aluminate cement (CAC) or hydratable alumina (HA). The following materials were analyzed for this purpose: polymeric fibers, a permeability enhancing compound (RefPac MIPORE 20) and an organic additive (aluminum salt of 2-hydroxypropanoic acid). The drying behavior and explosion resistance of the cured samples were evaluated when subjecting the prepared castables to heating rates of 2, 5 or 20 °C/min and the obtained data were then correlated to the potential of the drying agents to improve the permeability and mechanical strength level of the refractories at different temperatures. The collected results attested that the selected additives were more efficient in optimizing the drying behavior of the CAC-bonded compositions, whereas the HA-containing castables performed better when the aluminum-based salt was blended with a small amount of CAC (0.5 wt%), which changed the binders hydration reaction sequence and optimized the permeability level of the resulting microstructure. Consequently, some of the designed compositions evaluated in this work showed improved drying behavior and no explosion was observed even during the tests carried out under a high heating rate (20 °C/min).