High-alumina Refractory Castables Research Articles

Effect of Y2O3 additions on low cement high alumina refractory castable

ABSTRACT The effect of adding yttria (Y2O3) up to 2 wt.% on low cement high alumina castable is studied. Castable compositions with self-flowing and vibratable consistency (distribution coefficients, q = 0.21 and 0.29) are prepared using commercial raw materials and processed as per the conventional processing route, cured at 110°C, and fired at 1000 and 1550°C. An increase in yttria content is found to improve the physical and mechanical properties of castables for all temperatures. Phase analysis confirms the formation of the yttrium aluminate phase, and microstructural evaluation at 1550°C indicates enhanced bonding between the matrix and aggregate phase resulting in better density and strength properties.

An accurate correlation between high-temperature performance and cement content of the high-alumina refractory castables

To better guide the composition design of the low-cement high-alumina refractory castables in specific working conditions, in this work, an accurate correlation between their high-temperature performance and cement content was developed and discussed. The results show that both matrix grains and pores show a fine and homogeneous distribution after firing with the cement content in the range of 0–2 wt%. In this range, the liquid distribution is very dispersed, and the matrix is mainly connected by the in situ mullite. Therefore, the maximum working temperature (T0.5) and the creep parameters exhibit a slow linear change with cement content. The liquid amount still shows a linear increase with further increasing the cement content, but it displays a continuous distribution surrounding the matrix grains. This change destroys the direct bonds of mullite and produces many coarse pores, leading to the rapid deformation of the refractories under thermo-mechanical loads. The results suggest that the critical value of cement addition in this refractory is 2 wt% (or ∼10 wt% liquid in the matrix), below which the high-temperature performance can be predicted and kept under control.

Waste rice husk ash derived sol: A potential binder in high alumina refractory castables as a replacement of hydraulic binder

This work provides a new source of binder system, i.e., waste rice husk ash (RHA) derived sol for unshaped refractories. A facile (alkali extraction) route is used to synthesize the sol from the waste RHA. The sol consists of single phase amorphous silica with average particle size around 22 nm. The estimated cost of 30 wt% solid containing RHA derived sol per litre is around 9.35 $. The temperature of mullite (3Al2O3·2SiO2) formation from waste derived sol and reactive alumina containing system is 1250 °C or higher. The sol (30 wt% SiO2) and the dry sol (99.32 wt% SiO2) is used to formulate the high alumina castable samples. The sintering is performed at 1400 °C, 1500 °C and 1550 °C. The different physico-mechanical characterizations and blast furnace slag (BFS) corrosion test at 1500 °C are comprehensively investigated with the prepared castables. The properties of 5 wt% silica (3 wt% from sol and 2 wt% dry sol) containing castable specimens are also equated with the commercially available different silica containing high alumina castable systems. The obtain results are good agreements with the source data. These promising characteristic confirms the possibility of using waste RHA derived sol for manufacturing of cement free high alumina castable and leads to the economic and ecological benefits in the refractory industries.

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).

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.

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.

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.

Effect of Micro-Sized Alumina Powder on the Hydration Process of Calcium Aluminate Cement

Micro-sized alumina powder is widely used in low cement high-alumina refractory castables. The hydration of calcium aluminate cement can be affected by adding micro-sized alumina powder. This work addresses the hydration of a commercial cement at 25°C with 50 wt% micro-sized alumina powder added. The hydration heat was measured by isothermal micro-calorimetry. The phase composition and microstructure of the hydration products at the designated times were studied by XRD and SEM, respectively. The results showed that micro-sized alumina powder accelerated the dissolution of CAC. The induction period of the cement hydration reduced from 13h to 4.5h, and the hydration heat within 50hs was increased from 469J/g to 587J/g with the addition of micro-sized alumina powder. The morphology of the hydrates was flocculent amorphous at the beginning, and then transformed to short rods or cubic shape with micro-sized alumina powder added.

Urea based sols as binder for nano-oxide bonded high alumina refractory castables

Cement free high pure alumina castables are developed for high temperature applications with nano oxide bonding developed by using different sol systems synthesized by using urea as the precipitating and hydrolyzing agent. Bonding of the constables is based on nano oxide powders of alumina, mullite and spinel compositions using their respective sols as bonding agents. The sols are prepared from nitrate precursors using urea and are used in the high alumina castable formulation. The castable compositions are conventionally processed using the synthesized sols as sole binders and characterized for various refractory properties after heat treatment at different temperatures. These synthesized sol containing compositions are also compared with the commercial silica sol containing ones and cement bonded castables, processed under similar conditions. Considerable improvement in terms of hot strength, corrosion resistance and thermal shock resistance are obtained for the synthesized sol containing nano oxide bonded castables.

High-alumina refractory castables bonded with novel alumina-silica-based powdered binders

In recent years, nano-binders (mainly colloidal suspensions) have been proposed as alternative materials for applications that require CaO-free refractory lining or improved mechanical behavior at intermediate temperatures (700 °C< T < 1200 °C). Despite the benefits of these suspensions, nano-bonded castables usually present limited green mechanical strength and different on site logistics to handle the liquid. Considering the availability of novel alumina-silica-based powdered binders, this work investigated the role of submicron alumina and SioxX®-Zero (both supplied by Elkem company) on rheological and mechanical properties of vibratable high-alumina castables, aiming to identify whether they can be suitable options to replace colloidal silica suspensions. Cold and hot mechanical strength and apparent porosity in the range of 110–1400 °C, cyclic thermal shock resistance, creep tests and hot elastic modulus of the designed formulations were evaluated. According to the results, SioxX®-Zero-bonded compositions presented good flowability levels and their sintering process started around 800 °C. Adding boron carbide to the same formulations resulted in transient liquid sintering of the silica-containing refractories, which allowed the development of compositions with improved thermo-mechanical performance in the 600–1400 °C temperature range. Furthermore, the submicron alumina-bonded samples presented fast sintering, resulting in E values close to 200 GPa (the highest value so far registered in our lab for a coarse grain size formulation) after one heating-cooling thermal-cycle up to 1400 °C.

Extended studies on surface‐treated graphite vis‐à‐vis its application in high alumina refractory castable

AbstractThis investigation entails our continued research on calcium aluminate coated graphite for application in alumina‐carbon monolithic refractories. Surface treatment of graphite flakes by a scalable sol‐gel route has been utilized to develop stable bonds between ceramic phases with the functionalized graphite sheets. This study has been primarily conducted to differentiate between the coating evolution pattern at green and calcined conditions. In this regard, C‐H‐N analysis, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), x‐ray diffraction (XRD) with Rietveld analysis, Fourier transformed infrared spectroscopy (FTIR) and zeta potential studies of three kinds of graphites had been compared and critically estimated. These results were supplemented with thermogravimetry (TG), pore size distribution, oxidation resistance and scanning electron microscopy (SEM) of selected samples. A schematic outline of the work has also been proposed. An appreciable improvement of bulk density, apparent porosity and crushing strength of the respective castable has been correlated with the compatibility of the coating to the reactive matrix constituents of the refractory in wide temperature region (110‐1500°C).

Kinetic control of MgO hydration in refractory castables by using carboxylic acids

Based on a kinetic model that describes the hydration reaction via nucleation and growth mechanisms, formic, acetic or propionic acids were analyzed as hydration site activators on magnesia particles with different reactivity and granulometry used as a main binder source in high-alumina refractory castables. These additives induced the hydroxide generation in MgO suspensions and the hydration degree was proportional to the size of the carboxylate chain. Moreover, a more controlled growth of the brucite crystals could be achieved when formic acid was added to the refractory castables. By adjusting the amount of formic acid in the castable mixtures, large crack-free samples could be prepared, which pointed out the role of this additive to prevent the drawbacks associated with MgO hydration. Therefore, the suitable kinetic control of the nucleation and growth of brucite crystals on the magnesia surface allowed for the development of high-alumina magnesia-bonded castables.

Elastic and Mechanical Fatigue at High Temperatures of High-Alumina Castables with Addition of Partially Stabilized Zirconia

This work evaluates the impact of added zirconia on the thermomechanical and thermoelastic fatigue properties of high-alumina refractory castables. Three testing scenarios are presented in the study: 1) Resonant Frequency Damping Analysis (RFDA) after cyclic temperature changes with and without mechanical load, 2) Refractoriness under Load (RuL) tests, and 3) results of abrupt temperature shifts between two high temperatures in a special thermal shock furnace. The tested refractory formulations were a reference castable based on tabular alumina and two castables each containing 13.75 mass-% of zirconia. The formulations of the zirconia castables had an addition of either fully-stabilized zirconia doped with 8 mol-% yttria (Y-FSZ) or partially-stabilized zirconia doped with 3 mol-% calcia (Ca-PSZ). The Y-FSZ castable displayed elastic and mechanical behaviour by means of RFDA and RuL similar to the reference sample and had good recovery after thermal shock testing. The Ca-PSZ formulation showed some beneficial toughening through energy dissipation of progressing cracks, but after cooling exhibited the matrix fracturing influence of the martensitic zirconia phase transformation which would negatively impact service life of the refractory.

The relationship between the pore size distribution and the thermo-mechanical properties of high alumina refractory castables

AbstractA series of high alumina refractory castables were prepared via casting with tabular alumina aggregates (0.088 – 5 mm), fine powders, and calcium aluminate cement as starting materials. The effect of the median diameter of alumina (D50 = 7.26 μm, 5.33 μm, 2.37 μm) on the thermo-mechanical properties of the refractory castables was investigated. The results indicate that the decrease in the alumina particle size from 7.26 μm to 2.37 μm has little influence on both the apparent porosity and bulk density of castables. However, the median pore size of castables fired after 1 600 °C decreased drastically from 4.7 μm to 2.4 μm correspondingly, which led to significant growth in the strength and thermal shock resistance of castables. The cold modulus of rupture and crushing strength were increased by 201 % and 120 %, respectively. At the same time, the hot modulus of rupture and elastic modulus were also increased by 143 % and 127 %, respectively. The residual elastic modulus was enhanced twice after three thermal cycles.

Synthesis and Characterization of Alumina Sol and Its Use as Binder in No Cement High‐Alumina Refractory Castables

Alumina sol was prepared from inorganic precursors and characterized for its solid content, particle size analysis, DSC‐TG, phase formation on calcination, etc. The sol was used as sole binder in high‐alumina refractory castables, with different particle size distributions (PSDs), by varying distribution coefficients as per Dinger and Funk model. The castables were processed conventionally and characterized for physical and mechanical properties after heat treatment at various temperatures. Reasonable strength was obtained for the sintered castables, and no impurity phase was found in the matrix phase of the castables.

Incorporating Nano-silica as a Binder to Improve Corrosion Resistance of High Alumina Refractory Castables

In this study, four types of castables as calcium-aluminate cement (CAC)-bonded and nano-silica (NS)-bonded castables based on tabular-alumina and bauxite aggregates were prepared to investigate the replacement of the calcium-aluminate cement by NS. All samples were allowed to dry at 110 °C then fired at 800 and 1200 °C. Bulk density and apparent porosity of samples were measured. The molten aluminum static corrosion test (cup test) results showed that NS-bonded specimens had higher resistance to corrosion compared to CAC-bonded samples (based on the measured average aluminum penetration depth into the refractory texture). However no penetration was observed in bauxite NS-bonded samples. The results were consistent with dynamic corrosion test in aluminum melt carried out at 800 °C for 100 h. The small pore size in NS-bonded castables was found to be the main cause for high corrosion resistance as micro-pores prevented the melt to penetrate into the refractory.

Correlation between changes in mechanical strength and damping of a high alumina refractory castable progressively damaged by thermal shock

Resistance to thermal shock damage is an important characteristic in refractory materials, since it determines their performance and service life in many applications. Therefore, the use of more sensitive techniques is desirable to improve the evaluation of thermal shock damage and monitoring of nucleation and propagation cracks and microcracks.The aim of this work was to evaluate the potential of damping change characterization to quantify thermal shock damage and to estimate the retained mechanical strength in complement to the dynamic Young's modulus test. Variations in damping and retained Young's modulus and their correlation with the mechanical strength of a high alumina refractory castable were evaluated at different thermal shock temperatures and number of cycles. The changes in damping were proportional to the retained mechanical strength, similarly to the retained Young's modulus. Changes in damping were also detected which were not indicated by the Young's modulus measurements.

The impact of pre-formed and in situ spinel formation on the physical properties of cement-bonded high alumina refractory castables

Alumina–magnesia and alumina–spinel castables present some unique characteristics. Whereas the in situ spinel (MgAl 2O 4) formation provides an enhanced corrosion and thermal-shock resistance to the castable, the composition with pre-formed grains shows higher volumetric stability and do not have the inherent MgO hydration problems. As these distinct ways of spinel incorporation result in particular properties to the refractory material, a castable containing both in situ and pre-formed spinel might present balanced properties and a suitable performance in steel ladle applications. Considering these aspects, cement-bonded high alumina castables containing both pre-formed and in situ spinel were developed and evaluated by means of assisted sintering tests, microstructural analyses, thermodynamics simulation and mechanical and thermal-mechanical properties evaluation. The CA 6 formation presented different features according to the content of in situ spinel generation in the matrix, which affected the castable properties. Additionally, compositions containing both in situ and pre-formed spinel seemed to be a feasible way of attaining a designed linear dimensional change without affecting the spinel and CA 6 contents.

Formação in-situ ou adição de espinélio pré-formado: o que é melhor para concretos refratários aluminosos?

O desenvolvimento do setor siderúrgico está intimamente relacionado aos progressos obtidos na produção de refratários. Nas panelas de siderurgia, uma das principais exigências são revestimentos que apresentem excelentes propriedades termomecânicas para suportar os ciclos térmicos, além de uma boa resistência à corrosão e à penetração de escórias básicas. Em concretos refratários aluminosos, essas características são obtidas pela incorporação de espinélio pré-formado ou pela espinelização in-situ, onde nesta segunda rota os óxidos de alumínio e de magnésio reagem entre si em temperaturas elevadas durante o uso do revestimento. O objetivo do presente trabalho foi a análise comparativa e sistêmica destas duas classes de concretos aluminosos espinelizados, visando-se avaliar as diferenças em suas propriedades principalmente após sinterização. Foi observado que os concretos contendo espinélio in-situ apresentaram propriedades mecânicas superiores após queima a 1500 ºC, além de maior variação dimensional. Adicionalmente, o efeito da adição de frações grosseiras de espinélio pré-formado indicou a influência dos agregados sob a estabilidade volumétrica de concretos espinelizados in-situ.

The effect of nano-size additives on the electrical conductivity of matrix suspension and properties of self-flowing low-cement high alumina refractory castables

Influence of two nano-size additives on electrical properties of suspension matrix of self-flowing low-cement high alumina refractory castable is investigated. For this purpose, castament FS 10 and FS 20 on the basis of polycarboxylate ether were considered. The self-flow value, workability and mechanical strength of the castable are evaluated and their relations with electrical conductivity are determined. Using these relations, the type and optimum amount of proper additive for these refractory castables are determined. It was shown that if the electrical conductivity of matrix suspension is less than 0.71 mS/cm, high alumina low-cement self-flowing refractory castable can be obtained. The best self-flow, sufficient working time and adequate mechanical strength in the castables are obtained with 0.08 wt.% FS 20.