Refractory Castables Research Articles

Effect of Reactive Alumina on the Physico-Mechanical Properties of Refractory Castable

The effect of reactive alumina and its different proportions on physico-mechanical properties of conventional high alumina castables were studied. The castables with different percentages of reactive alumina were prepared by using calcium aluminate cement as a binder and various refractory properties of the prepared castables, i.e. apparent porosity, bulk density, cold crushing strength, hot modulus of rupture were studied. The mineralogical analysis was done by X-ray diffractometry, whereas morphological analysis was carried out by scanning electron microscopy. The results showed that the castable with high percentage of reactive alumina hydrated early as well as developed early sintered strength due to its faster rate of hydration and higher reactivity at lower temperature. The hot modulus of rupture was also found to be higher in the castable with more reactive alumina. Mineralogical analysis showed well developed mullite and corundum phases in the refractory concrete which was supported by morphological analysis.

Mullite-based refractory castable engineering for the petrochemical industry

Abstract 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.

Research and Application of Corundum Based Self-Flowing Castable Refractories

The effects of different deflocculants, fume silica, reactive micro-alumina powder and high alumina cement on the flowability of corundum based self-flowing castables were investigated using brown corundum as the main starting materials. The behavior of the castables was evaluated according to the self-flow value of the castables. The results showed that the best self-flow characteristics of corundum based self-flowing castables is obtained using sodium hexametaphosphate as deflocculant, suitable amount of alumina micropowder, fume silica, and high alumina cement as binders. The resultant castables were successfully used in the bottom of a refining ladle with capacity of 80 tons. The service life could match that of well blocks and meet the production requirements.

Weighted Poisson Cells as Models for Random Convex Polytopes

We introduce a parametric family for random convex polytopes in ℝ d which allows for an easy generation of samples for further use, e.g., as random particles in materials modelling and simulation. The basic idea consists in weighting the Poisson cell, which is the typical cell of the stationary and isotropic Poisson hyperplane tessellation, by suitable geometric characteristics. Since this approach results in an exponential family, parameters can be efficiently estimated by maximum likelihood. This work has been motivated by the desire for a flexible model for random convex particles as can be found in many composite materials such as concrete or refractory castables.

Influence of Spinel on the Fracture Energy of Refractory Castable

The specific fracture energy of alumina-spinel refractory castables was studied by the wedge-splitting method in this paper. The influence of synthetic spinel on the specific fracture energy of aluminamagnesia refractory castables was investigated. In-situ formation spinel was replaced by sintering spinel as the synthetic spinel. The experimental results indicated that the specific fracture energy and the compressive flesural mechanical properties had the same changing trend with the increase of the synthetic spinel, but the thermal shock resistance of the materials showed the opposite trend. However, the thermal shock resistance was closely associated with the specific fracture energy according to one theory raised by Hasselman.

Thermal shock damage evaluation of refractory castables via hot elastic modulus measurements

Abstract When refractory castables are submitted to continuous thermal changes, crack nucleation and/or propagation can take place resulting in a loss of mechanical strength and overall degradation of such materials. This work evaluates the thermal shock damage cycling of high-alumina and mullite refractory castables designed for petrochemical application. Hot elastic modulus analyses were carried out after 0, 2, 4, 6, 8 and 10 thermal cycles (Δ T =800 °C) in order to investigate the microcracking evolution due to the temperature changes. Additionally, apparent porosity, hot modulus of rupture, erosion and work of fracture measurements were also performed. According to the attained results, it was detected at which temperature range the stiffening or embrittlement took place in the mullite-based refractory (M-SA) microstructure. Nevertheless, the damage induced by the thermal shock tests was not permanent, as further increase of the elastic modulus results was observed for all evaluated samples after annealing. On the other hand, the alumina-based composition containing a sintering additive (TA-SA) presented enhanced mechanical strength, high thermal stability and E values. Simulations indicated that refractories with high E values (∼140 GPa, such as those attained for alumina-based castable) showed a reduced amount of stored elastic strain energy even under severe thermal stresses, which seems to be a key aspect for the engineered design of thermal shock resistance materials.

Dryout temperature–vapor pressure profile of polymeric fiber containing refractory castables

Different types of polymeric fibers are added to refractory castables' formulations as drying additives to make dewatering processes easier and reduce the risks of explosive spalling of these low permeability materials. Many works describe how these fibers control the permeability increase in castables before and after their melting-decomposition. The water vapor pressure profile developed inside the structure and how it is modified by the fiber presence has not yet been explored systemically. In the present paper, thin K-type thermocouples were inserted (at different depths) in polymeric fibers (olefin copolymer, polypropylene and aramidic ones) containing castable samples. Their temperature profile and mass loss were recorded during the first heat-up. After combining internal heating and mass loss rate, vapor pressure levels calculated using Antoine's equation, hot air permeametry (HAP) and work of fracture (γWOF) results, novel insights into the mechanisms by which polymeric fibers avoid explosive spalling during castables dewatering were attained.

Wear mechanisms of Al2O3-MgO spinel-forming refractories used in steel ladle impact pads

Refractory castables containing alumina-magnesia/self-forming spinel (MgAl2 O4 ) are used in impact pads of steel ladles in steelmaking processes. In order to understand the wear mechanisms of refractory materials, several recipes were tested from a corrosion, slag resistance and thermal shock point of view. The results show that the corrosion is extremely limited at the slag/refractory interface for all cases. Nevertheless, for higher cement alumina content castables, the formation of cracks is observed in refractory castables into which slag can penetrate. The slag reacts with the alumina to form a new phase such as hibonite (CA6) and calcium dialuminate (CA2). The volumetric change of these reactions involving CA2 and CA6 lead to the apparition of macro-cracks. Thus, the penetration of slag and steel are increased, causing hot mechanical properties to degrade. For lower cement alumina castables, the formation of micro-cracks is avoided by controlling volume expansion. Thus, the slag deposit reacts with alumina grains and the matrix at the slag/refractory interface to produce a monomineral layer of hibonite. In this way, the monomineral layer acts as a barrier and limits the penetration of slag and steel into the refractory lining. Thus, to increase the lifetime of refractory castables containing alumina-magnesia/self-forming spinel, it is advised to control volume expansion in order to avoid the formation of cracks and limit the penetration of secondary metallurgy steel ladle slag.

The Influence of B<sub>4</sub>C on Mechanical Properties of MgO-SiC Based Refractory Castables

The effects of B4C addition on the room temperature physical properties and hot mechanical properties of MgO-SiC based refractory castables were investigated using magnesia and SiC fines as starting materials and silica fume as a binder. The microstructure was characterized by scanning electron microscopy (SEM). The results showed that drying strength of MgO-SiC based castables decreased with the increase in B4C addition, the immediate temperature strength and hot temperature strength increased. The HMOR at 1400°C for 0.5h decreased. This is because B4C oxidized and produced a liquid phase during heating, contributing to sintering and making the material denser. So the cold strength increased. On the other hand, due to the formation of liquids, direct binding reduced and the HMOR decreased

Influence of Silicon Nitride on Properties of Corundum Based Castable

The effects of Si3N4 addition on the room temperature physical properties and thermal shock resistance properties of corundum based refractory castables were investigated using brown corundum, white corundum and alumina micropowder as the starting materials and pure calcium aluminate as a binder. The phase composition, microstructure, mechanical properties of corundum based castables were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that as the increase in Si3N4 addition, the bulk density decreased and apparent porosity increased, the cold strength deduced. However, the residual strength rate increased. That is, the thermal shock resistance was improved. This is because even though the introduction of Si3N4 inhibited the sintering of material and deduced the compactness, microcracks were produced in the materials due to a difference in thermal expansion coefficient. So the thermal shock resistance of corundum based castable was improved.

Innovative Superplastic Forming Based on <i>In Situ</i> Infra-Red Sheet Heating

This paper describes the research work that was performed in order to propose an innovative and low cost process route for superplastic forming of aluminum and titanium alloy sheets. The driving idea was to heat only the metallic sheet using heating elements included in the pressure chamber. Several heating configuration have been tested and equipment designs investigated. Based on experimental results and numerical thermal analysis, it was found that halogen heating lamps with a high reflective thermal insulation was the best for the upper pressure chamber, whereas low effusivity Refractory Castables materials seem optimal as low cost forming die. Energy consumption evaluation shows more than 80% energy saving in nominal titanium alloy forming. A pilot forming equipment was developed and first aluminum sheet forming trials give interesting results.

Improved spinel-containing refractory castables for slagging gasifiers

In gasifiers the reaction of a carbon feedstock with oxygen and water takes place to produce synthetic gas. The decrease of the operation temperature at the refractory lining by an innovative gasification technology permits the development of new chrome oxide free materials fulfilling economical as well as ecological aspects. Considering this aspect, spinel-containing refractory castables with increasing spinel content have been investigated according to their thermomechanical properties, phase formation as well as their resistance against corrosion attack by molten coal slag under gasification-similar conditions. In order to determine the influence of spinel and cement in the alumina-spinel castables, increasing spinel amounts and cement types has been added to alumina. In particular, MA-spinel cement has been applied. It has been shown, that the addition of MA-spinel cement to spinel-rich alumina improves the corrosion resistance against molten slag remarkably.

Application of Magnesia-Phosphate Cement for Chromia/Alumina Castables

Refractory castables containing calcium aluminate cement (CAC) are widely used in a range of furnace lining applications in the iron and steel, cement, glass, ceramic, and petrochemical industries. However, magnesia-phosphate cement (MPC) based material could be a new types of cement material, with many advantages such as rapid hydration, high early strength and circumstance suitability, which has very important value and wide application. In this study, MPC was used at Chromia/Alumina castable as binder addition instead of conventional calcium aluminate cement. Meanwhile, it also explains the relationship between the micro-mechanism and performance by micro methods such as SEM. The results shows that MPC based castables have good corrosion resistance, interface adhesiveness and abrasion resistance.

Nano-bonded refractory castables

Abstract Calcium aluminate cement (CAC) contents higher than 3 wt% in refractory castables can have some drawbacks in the various processing steps (mainly drying) and also in their refractoriness when in contact with SiO 2 . The use of colloidal silica as an alternative binder has been studied by many researchers in recent years and recently reports have also explored the use of colloidal alumina for the same purpose. This article reviews the recent developments in nano-bonded refractory castables focusing on the use of colloidal silica or alumina. In the first part of the paper, a comparison of different binding systems for refractory castables is shown. The benefits of replacing CAC or hydratable alumina by colloidal binders are discussed. In the second part, the advantages of colloidal silica/alumina as a refractory binder are highlighted. Meanwhile, the characterization techniques and functional mechanisms of these binders are presented in order to understand the behavior of these systems. Finally, in the last section, the challenges for suitable use of colloidal binders are discussed and the future direction of nano-structured refractory castables is outlined.

Effect of Binders on the Properties of Chromia/Alumina Refractory Castables

Refractory castable specimens were prepared using industrial white fused alumina as aggregates, chromium oxide green and ultrafine alumina powder as matrix materials, solid aluminum dihydrogen phosphate (Al(H2PO4)3) or liquid AlPO4 as binder materials. Effect of different binder additions on the propertiSubscript textes of high chromium oxide IGCC refractory castables, such as bulk density, apparent porosity, strength was evaluated and their microstructures were observed by SEM and mercury porosimetry.

Effect of fine andalusite on the properties and microstructure of low-cement cordierite-mullite castables

Results are presented from the development and study of cordierite-mullite panels which are based on low-cement refractory castables (LCC) and can be used as kiln furniture. Cordierite powder was synthesized from talc, kaolin, and alumina. Synthetic cordierite aggregate was used as the main component in addition to andalusite, Kerphalite, cement Secar 71, calcined alumina, and microsilica. It was established that samples of the cordierite-based material with a mullite binder had a satisfactorily low CLTE (3.72 × 10–6 1/K), which is indicative of their excellent thermal shock resistance.

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.

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.

Ceramic fibre reinforced refractory castables for very high temperature superplastic forming

AbstractIn the frame of the development of very high temperature die material solutions, two grades of alumina based ceramic fibres with a diameter close to 12 μm have been investigated as reinforcement solutions for an andalusite refractory castable. High temperature mechanical tests were performed up to 1200°C in four‐point bending conditions: rupture tests at high temperature and anisothermal creep tests. After testing, microstructural and rupture surface were observed in order to get information on deformation mechanisms and rupture processes. Results show that significant mechanical improvement were obtained up to 1100°C when compared to non reinforced refractory castables.

Módulo elástico como ferramenta para avaliação da hidratação da magnésia e dos processamento de concretos refratários

Considerando-se as dificuldades para a utilização de MgO em concretos refratários, técnicas anti-hidratação têm sido propostas com o objetivo de minimizar os efeitos deletérios causados pela reação da magnésia com a água. Em paralelo, surge a necessidade de investigar novas metodologias que permitam melhor avaliar os danos causados pela hidratação. Neste sentido, uma técnica promissora consiste no acompanhamento do módulo de Young durante as etapas de cura e secagem dos concretos. No presente trabalho, concretos refratários contendo magnésia cáustica ou sínter de MgO foram avaliados por meio do acompanhamento do módulo elástico utilizando-se o método de ressonância de barras. A avaliação das etapas iniciais de processamento (cura e secagem) dos concretos por meio do uso desta técnica apresentou-se como uma alternativa adequada para detecção dos efeitos causados pela hidratação do MgO. Os resultados obtidos também destacam que, em comparação com outras técnicas comumente empregadas, as medidas do módulo elástico permitem delinear com maior precisão a evolução da microestrutura em função do tempo, além de detectar antecipadamente os danos relacionados à formação de brucita, garantindo-se assim maior sensibilidade no monitoramento de tal reação.