Refractory Castables Research Articles

Development of waste derived nano-lakargiite bonded high alumina refractory castable for high temperature applications

Abstract This work provides a new binder system, i.e., nano-lakargiite (NL) [CaZrO3] for unshaped refractories. NL powder formulation is done through a green and facile route, i.e., solution mixing technique, which is easily capable of recycling the byproducts. The waste eggshells are practiced as a source of CaO for the preparation of NL. The characteristics such as differential thermal/thermogravimetric (DTA-TGA) analysis, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) of the synthesized NL powder are accomplished. The single phase orthorhombic crystal structure with the Pnma space group of nano CaZrO3 (avg. crystallite size ∼19 nm) is attained at 1100 °C. High alumina castable specimens are formulated through the replacement of calcium aluminate cement (CAC) by the NL and heat treated at 1600 °C. The different physico-mechanical properties of the prepared castables are comprehensively investigated. The corrosion resistance in the molten glass at 1400 °C is also examined. The properties are also equated with the different advanced bonding systems of high alumina refractory castables. Significant progress in terms of densification, hot strength, and thermal shock resistance is achieved for the NL bonded refractory castables.

Binding additives with sintering action for high-alumina based castables

Abstract 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.

Characteristics of castables incorporating highly porous alumina aggregates fabricated by high-temperature evaporation method

Porous alumina aggregates were prepared for use in adiabatic refractories via a high-temperature evaporation method. The mechanical and thermal characteristics of the porous alumina bodies and of castable refractories made using the alumina as an aggregate were investigated. Specimens were produced by adding boron hydroxide, sodium carbonate and water glass to aluminum hydroxide powder compacts in varying proportions, followed by heating at 1700 °C for 1 h in a reducing atmosphere to form porous alumina bodies. During heating, these additives formed a Na 2 O-B 2 O 3 -SiO 2 liquid glass phase that promoted the growth of alumina platelets. These platelets subsequently formed a “house of cards” structure. The porosity and compressive strength of the resulting porous alumina were 69.8% and 1.8 MPa, while the thermal conductivity of this material was 0.23 W m −1 ·K −1 at 1000 °C. A castable incorporating the porous alumina aggregate exhibited improved mechanical strength, higher heat resistance and greater FeO corrosion resistance than a sample made using a CA 6 (CaO∙6Al 2 O 3 ) aggregate. The porous alumina castable exhibited similar thermal conductivity to that of the CA6 castable at high temperatures, with a value of 0.85 W m −1 ·K −1 at 1400 °C. Hence, the porous alumina aggregate fabricated in this study can be expected to have practical applications.

Investigation the Effect of Water Ratio and Additives on the Properties of Castable Refractory

Castable refractories are resistant to the high temperatures just like the other refractories. Significant progress has been made after calcium aluminate cement has been started using in castable refractory technology. The properties of these refractories are vastly dependent on the selected mixture and the amount of water added. In this study, the effects of water amount and additives on setting time, fluidity and mechanical properties was investigated by using 25% CAC (calcium aluminate cement) and fused silica. As a result of the studies, it was observed that the as the amount of water decreased, the setting time is also decreased, the mechanical properties improved and the fluidity was variable depending to the composition. Keywords : CAC, castable refractories, fused silica DOI : 10.7176/JSTR/5-4-07

Evolution of Thermal Microcracking in Refractory ZrO₂-SiO₂ after Application of External Loads at High Temperatures.

Zirconia-based cast refractories are widely used for glass furnace applications. Since they have to withstand harsh chemical as well as thermo-mechanical environments, internal stresses and microcracking are often present in such materials under operating conditions (sometimes in excess of 1700 °C). We studied the evolution of thermal (CTE) and mechanical (Young’s modulus) properties as a function of temperature in a fused-cast refractory containing 94 wt.% of monoclinic ZrO2 and 6 wt.% of a silicate glassy phase. With the aid of X-ray refraction techniques (yielding the internal specific surface in materials), we also monitored the evolution of microcracking as a function of thermal cycles (crossing the martensitic phase transformation around 1000 °C) under externally applied stress. We found that external compressive stress leads to a strong decrease of the internal surface per unit volume, but a tensile load has a similar (though not so strong) effect. In agreement with existing literature on β-eucryptite microcracked ceramics, we could explain these phenomena by microcrack closure in the load direction in the compression case, and by microcrack propagation (rather than microcrack nucleation) under tensile conditions.

Correction to: Formation Mechanisms and Leachability of Hexavalent Chromium in Cr2O3-Containing Refractory Castables of Electric Arc Furnace Cover

Due to an error introduced during the production process, the corresponding author’s e-mail address was incorrect. It should have read: songs@wust.edu.cn.

One‐step synthesis of in situ nanocarbon‐containing calcium aluminate cement in reducing atmosphere

AbstractIn order to overcome the problems caused by poor water wettability/dispersion and oxidation resistance of carbons for improving properties of carbon‐containing refractory castables, we prepare in situ nanocarbon‐containing cement through the carbon‐bed sintering method with calcium citrate tetrahydrate (CCT, C12H10Ca3O14·4H2O) and Al2O3 as raw materials, and FeCl3 as a catalyst. In this paper, the carbonization process of CCT was analyzed by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetric analysis coupled with a FTIR. In addition, the synthesized cement was characterized through XRD, Raman spectroscopy, field‐emission scanning electron microscopy, and high‐resolution transmission electron microscopy. The results show that the in situ carbon was generated through carbonization of calcium aconitic acid which was generated by pyrolysis of CCT, accompanying with overflowing of H2O, CO2, CO, and CH4. Interestingly, as the sintering temperature increases, the freshly generated carbons were transformed into nanofiber and nanoflake carbons under the action of Fe catalyst. After sintering at 1500°C for 4 hours, the phase compositions of the synthesized product approached to that of the commercial cement Secar71, and the in situ carbons with nanofiber and nanoflake morphologies dispersed in calcium aluminate grains.

Corrosion of alumina-magnesia castable by high manganese steel with respect to steel cleanness

High manganese steel has been widely used in the national defense, automobile and petrochemical industry fields due to its excellent wear resistance and low temperature toughness. Alumina-magnesia refractory plays a significant role in high manganese steel smelting. In this paper, the interaction between alumina-magnesia castable and high manganese steel, and its effects on the quality of high manganese steel were analysed. The Fe and Mn of the molten steel transferred into the castable during the early smelting stage, so a reacted metamorphic layer with lower melting point was formed between the refractory castable and high manganese steel. With the prolongation of smelting time, the thickness of the reaction layer and number of molten steel droplets entrained into the refractory castable-high manganese steel reaction layer were increased under dynamic conditions. Some refractory particles were loosed and dropped into the reaction layer, which accelerated the erosion of the refractory castable, causing an increase in the total [O] content in the steel. An isolation layer of (Fe, Mg, Mn)Al2O4 composites began to form after 30 min and the total [O] content in the steel decreased gradually to below 10 ppm after 60 min. The formation of an isolation layer prevented the corrosion of the refractory castable and guaranteed steel cleanness within 90 min of smelting, but resulted in a 2% decrease of the Mn content in the high manganese steel during the smelting process.

Nanotechnologie in feuerfesten Gießmassen: Ein Überblick

Abstract: In recent times, nanotechnology has gained significant attention in the field of refractory research. As the unshaped refractory, especially the castable, is becoming of prime importance in refractory research, a good amount of work is going on globally to study the effect of nanotechnology on castables. The effect of different nano-oxide binders, formed from colloidal bonding agents and nano-additives is also important. Conventional bonding materials, like, high-alumina cement, have drawbacks in the processing steps (mainly drying) and with respect to properties developed. The use of colloidal silica as an alternative binder has improved this condition and is being practiced commercially, but restrictions on high temperature applications have encouraged the use of other colloidal systems, namely alumina, mullite, spinel, sols, etc. Nanoparticles are also present in the castable system from binders like hydratable alumina, additives like microsilica, etc. are also present in the castable system and influence the properties developed. Nano scaled additives are also added to reduce the energy consumption and to improve the densification process at lower temperatures. In this paper, various aspects of the contribution and effect of nanotechnology, on the development of refractory castables are discussed.

Drying behavior optimization of dense refractory castables by adding a permeability enhancing active compound

New challenges have been identified over recent decades regarding the increasing complexity of the dry out step of refractory castables, mainly due to the advances on the particle size distribution design (and consequent reduction in the ceramic linings' permeability) and growing demand for shorter equipment maintenance time, which generally results in pressures to speed up the drying. Some efforts focused on the design of novel permeability enhancing active compounds (PEAC) have been receiving great attention, as such products may be an interesting alternative for producers and end-users. This work evaluates the addition of a permeability enhancing active compound (AC) to high-alumina based castables and also analyzes the effect of different parameters (i.e., particle packing models, hydraulic binders, drying temperature, etc.) which could affect the dry out behavior of such refractories. Initially AC-containing castables, additive-free and mixtures with a polymeric fiber with low melting point were analyzed. Further investigations of the selected AC action mechanism were carried out in a second moment, where novel castables containing distinct binders (Secar 51, Secar 71 or Alphabond 300) and with or without this drying agent were characterized regarding their flexural strength, apparent porosity, permeability and explosion resistance at different temperatures and heating rates. The obtained results indicated that adding this active compound into the alumina-based castables favored a change in the cement hydration path, giving rise to gel-like hydrates (instead of AH3 and C3AH6 phases) that underwent decomposition at low temperatures (100–140 °C). Consequently, such transformations resulted in the samples' permeability increase, which inhibited their explosion during the heating up step. However, other important properties such as flowability, setting time and green mechanical strength of the AC-containing compositions were also affected, due to the interaction of this additive with water and the fine refractories' raw materials and the generation of a CAH-gel phase in the castables’ microstructure.

Advanced boron-containing refractory castables bonded with calcium-free binders

The replacement of calcium aluminate cement (CAC) and/or the addition of specific agents (i.e., mineralizers) have been proposed as alternative routes to develop refractory products suitable for intermediate working temperatures (700–1000 °C). In this sense, the use of calcium-free binders, such as hydratable alumina or silica-based compounds, has gained some attention. Additionally, the incorporation of sintering additives to castables can speed up the expected sintering reactions, due to the generation of liquid phase in the designed microstructure during the first heating treatment of these products. Therefore, this work evaluates self-flowing high-alumina castables bonded with 4 wt.% of calcium-free binders: hydratable alumina, a novel powdered microsilica-alumina-based one or their blend. Besides that, it was also analyzed the influence of adding 0 or 1 wt.% of boron carbide to the castables, as this compound can act as a transient liquid sintering agent. The designed compositions were characterized via flowability, thermogravimetric tests, hot elastic modulus, erosion resistance, cold and hot mechanical strength and other techniques. The castable bonded with the blend of the selected binders showed shorter setting time and lower drying profile performance, as the samples’ explosion for these compositions could be detected even for small samples (50 × 50 mm) when using a heating rate of 5 °C/min. On the other hand, the refractory containing the microsilica-alumina-based binder presented enhanced thermo-mechanical behavior. Furthermore, boron addition favored faster sintering of the castables, resulting in optimized performance around 815 °C or even at 1100 °C for the mixture containing only the silica-based additive.

Formation Mechanisms and Leachability of Hexavalent Chromium in Cr2O3-Containing Refractory Castables of Electric Arc Furnace Cover

Formation Mechanisms and Leachability of Hexavalent Chromium in Cr2O3-Containing Refractory Castables of Electric Arc Furnace Cover

The Effect of Nano-Alumina Particles Generated <i>in Situ</i> in the Matrix of Ultralow and No Cement 85 wt% Al<sub>2</sub>O<sub>3</sub> Refractory Castables

Advanced high alumina refractory castables of ultra-low and to cement types, are well-known because of their ability on developing similar and/or superior thermal and mechanical properties. Following the recent trend of including nanoparticles in refractory castables, in this work, it is presented a novel way to obtain the benefit effects on the thermal and mechanical properties, promoted by the development in situ, of alumina’s nanoparticles in the matrix of castable (85 wt% Al2O3). The alumina nanoparticles were originated in situ after firing, due to the pyrolysis and oxidation of an aqueous resin, produced by the Pechini process. The resin played a double role, one as mixing liquid vehicle and the other as the aluminum oxide nanoparticles precursor. The results indicate a strong increase in flexural strength and elastic modulus as well as leading to a higher residual strength after thermal shock.

Effect of reclaimed bauxite on andalusite-based refractory castables for tundish applications

Effect of reclaimed bauxite on andalusite-based refractory castables for tundish applications

Vpliv dodatka MnO na lastnosti ognjevarnih materialov na osnovi Al2O3-MgO

Vpliv dodatka MnO na lastnosti ognjevarnih materialov na osnovi Al2O3-MgO

Quantitative X-ray microtomography technique to evaluate high-temperature transient diffusion of Iron diffusants in high alumina-silicate porous refractory media

Diffusion tests were performed with solid diffusants of Iron oxides in contact with porous silico-aluminous refractory castables in high-temperature conditions. A non-destructive X-ray computer microtomography technique with digital 3D reconstruction (3DμCT) was used for spatial monitoring the diffusion into the media. A particle tracking (PT) method was applied to evaluate diffusion through porous materials and to quantify its diffusive properties based on 3D images over time. The influence of temperature was examined in the range from 1100°C to 1300°C. Each sample was heat treated for 100h in the investigated temperature. The normalized concentration of diffusants as a function of the penetration was obtained by 3DμCT images and compared with the concentration profile determined by elementary microanalysis with energy dispersive spectroscopy and surface chemical mapping (MAP/EDS). Thus, the diffusivity of the porous media was quantified from the experimental data. It was observed a variation less than 3.52% between the MAP/EDS and 3DμCT, i.e., between the destructive and non-destructive methods, suggesting that the 3DμCT method may be extended to other media and diffusants.

Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables

The microstructural evolution and comprehensive properties of alumina refractory bonded with calcium aluminate cement and silica sol have been studied. Results have been correlated with the microstructural and phase evolutions using X-ray diffraction and scanning electron microscopy, as function of the pre-formed spinel powders. Matrix samples were obtained for phase and microstructural characterization in details, and the results were compared with those corresponding to the refractory castables. The room temperature and high temperature properties, including permanent of linear change, mechanical properties, hot modulus of rupture (HMOR) and slag resistance were measured. The castables exhibited a microstructural optimization and properties enhancement due to the addition of pre-formed spinel. A lot of secondary spinel and CA6 (CaO·6Al2O3) with small size were produced in the castables, and the contents of micro pores were greatly increased. As a result, the permanent of linear change of castables was decreased by 61%, while cold modulus of rupture (CMOR) and HMOR were increased more than 45% and 100%, respectively. The penetration indexes in the static slag resistance were decreased from 28.8% to 12.2%.

Evaluation of Chemical and Physico-Mechanical Properties of some Nigeria Natural Clays Samples for Foundry Applications

Studies have shown that Nigerian clays have not been fully utilized for foundry applications, either as refractories or even bonding clays, and are still being imported into the country. There is need to effectively exploit and adequately utilize the available natural resources to improve Nigeria economic activities, most especially during the present economic recession. Samples of clays were obtained from Auchi, Dada-Okelele (Ilorin), Kaba, Maraba-Rido, Mokwa and New-Bussa in Nigeria. The samples’ physico-mechanical properties were examined for its suitability for foundry/refractory applications. The results of the chemical analysis revealed that the samples belong to Alumino-Silicate refractories. The samples exhibited 35.3-96.2%, 17.1-28.7%, 1.72-2.34 g/cm3, 2.2-10.1%, 73-86 and >1300oC as values for percentage clay contents, apparent porosity, bulk density, permeability, linear shrinkage and refractoriness respectively. These values were within the standard range values of >35%, 22-30%, 17-2.4%, 2.0-10.1% and 25-90 required as percentage clay contents, apparent porosity, bulk density, permeability and linear shrinkage values respectively, for refractory clays/brick lining or alumina-silicates, kaolin and fireclays. Hence, the clays could suitably replace imported clays / ceramics in refractory applications, such as in production of earthen wares, chalk; as insulating refractories for casting and melting of low and medium temperature iron and steel. Appropriate utilization of these clays for local foundry applications will assist in addressing the problem of inadequate job, and over dependence on foreign goods.Keywords— Alumino-Silicate, Clay, Foundry, Moisture and Refractoriness

Microstructural and mechanical investigation of high alumina refractory castables containing nano-titania

In this study, mechanical and structural properties of two high alumina refractory castables, i.e. without nano-titania and with 0.4 wt% TiO2, are investigated. Bulk density, apparent porosity, cold compressive strength, cold modulus of rupture, permanent linear changes, hot modulus of rupture and shock resistance tests were performed on the samples. The microstructure and phase analyses were conducted by field emission scanning electron microscope equipped with EDX analyzer and X-ray diffraction, respectively. Results showed that hibonite phase formation in refractory is dependent on the temperature which could be decreased by addition of nano-titania, leading to the improvement of refractory strength. On the other hand, the decrease of hot modulus of rupture and shock resistance was observed for the refractory castable containing nano-titania. Also, Ca3Ti8Al12O37 phase was produced after firing at 1000 °C in the presence of nano-titania. Probable mechanisms are proposed for phase and microstructural changes in high alumina refractory castable containing nano-titania.

Molten salt synthesis and characterization of SiC whiskers containing coating on graphite for application in Al2O3-SiC-C castables

The wettability of graphite was significantly ameliorated by applying a molten liquid phase method using silicon powder as a raw material to form a SiC whisker coating on a graphite surface. The effect of replacing the ball pitch with coated graphite (CG) on the microstructure, apparent porosity, physical properties, oxidation resistance, and slag corrosion resistance of Al2O3–SiC–C (ASC) refractory castables was investigated by a scanning electron microscope, an energy-dispersive X-ray spectroscope, a transmission electron microscope, and other dedicated equipment. The results show that a uniform and continuous SiC whisker coating was formed on the surface of the graphite, and the contact angle with water changed from 89° to only 6.5°. The apparent porosity of the ASC specimen with 2 wt.% of CG decreased by 12.7%, and the cold modulus of rupture increased from 7.08 to 13.30 MPa with increasing CG content from 0 to 2 wt.%. Further, the sample containing 2 wt.% of CG exhibited the largest force and displacement values. Owing to the presence of SiC whiskers on the surface, the ASC castable samples with 2 wt.% of CG have the best antioxidation performance, and those with 1 wt.% of CG have the best anti-slag performance.