Alumina-magnesia Castables Research Articles

Thermomechanical behavior of alumina–magnesia castables and numerical modeling of the steel ladle thermal process

AbstractBy experimental characterization and numerical modeling, the dependence of thermophysical properties on time and temperature as well as its influence on the thermomechanical behavior of alumina–magnesia castables has been studied in this work. The thermal conductivity, thermal expansion coefficient, and Young's modulus are the most important parameters, which evolve with temperature and piecewise change with the thermal processes of drying, preheating, and service. It is mainly related to the sintering degree and amount of formed calcium aluminate and spinel phases. The sensitivity analysis of parallel simulations for multilayer steel ladle with different numbers of temperature‐dependent thermophysical parameters demonstrates that the thermal conductivity is the most influential parameter, as it dominates the temperature distribution and affects the succeeding thermal expansion as well as deformation. The increase of thermal expansion coefficient and decrease of Young's modulus with temperature counterbalance the deformation of linings under thermal process. Meanwhile, compared with the simulation using temperature‐independent parameters, the model with temperature‐dependent thermophysical parameters exhibits more severe damage in outer surface of both the working lining and the permanent lining. This gives new insight into the adoption of temperature‐dependent parameters for actual thermomechanical behavior evaluation of refractories.

Effect of Cr2O3 additive on alumina-spinel and alumina-magnesia castable

Alumina-spinel and alumina-magnesia castables are important for steel ladle applications primarily for the bottom and side wall areas respectively. Presence of spinel in alumina castables enhances the resistances against corrosion and thermal shock and improves the performance and service life of the steel ladles. Development of these type of castables is planned in the present study in presence of Cr2O3 as an additive using high alumina cement as binder. Variation in spinel (10 and 20 wt%) and MgO (2.8 and 5.6 wt%) content was also explored, along with two different particle size distributions (q value 0.21 and 0.29) for different flow consistencies. The castable compositions were processed conventionally and evaluated for various refractory properties after heat treatments at 110, 1000 and 1550 °C. Both the castables showed well developed refractory properties with alumina-magnesia castable having better slag resistance due to greater extent of calcium hexa aluminate phase formation protecting the refractory.

Impact of fast heating and cooling on the mechanical behaviour of alumina-magnesia refractory castables

The alumina-magnesia castables is one kind of monolithic refractories for steel ladle, whose thermo-mechanical properties vary with temperature during the lining pre-heating process and metallurgical service. The understanding of their variation tendency and sensibility to temperature fluctuation is important for the improvement of high-efficiency pre-heating strategy and real-time service performance. In this study, the influence of heat treatment temperature and heating/cooling rate on the thermo-mechanical properties and fracture behaviour of alumina-magnesia castables is investigated using uniaxial compressive tests equipped with an induction heating system and Brazilian test combined with digital image correlation and acoustic emission. The brittleness and mechanical strength of alumina-magnesia castables increase with heat treatment temperature due to the better sintering and formation of calcium hexaluminate and spinel phases. Without full relaxation of residual stresses, the stiffness increases while the strength decreases after fast heating of 200 °C/min. Meanwhile. the same castables after different thermal treatments show disparate mechanical responses to cooling rates.

Investigating the Action Mechanism of Titanium in Alumina–Magnesia Castables by Adding Different Ti-Bearing Compounds

To investigate the action mechanism of titanium, the effects of different Ti-bearing compounds, including CaTiO3, MgTiO3, and nano-TiO2, on the properties of alumina–magnesia castables were studied. By analyzing the phase compositions, microstructures, and physical and mechanical properties of the castables, it was demonstrated that an intermediate product, CaTiO3, was first generated. This was then consumed by solid-solution reactions, and titanium was involved in the liquid formation as the temperature increased. The solid-solution reaction of CA6 (CaAl12O19) was more prominent due to the incorporation of more titanium in the crystal lattice of CA6 instead of spinel (MgAl2O4). Moreover, the liquid formation was strongly promoted when more titanium accompanied the calcium, which finally accelerated the densification and improved the strengths of alumina–magnesia castables. On the whole, castables with CaTiO3 addition presented higher bulk density and excellent strength after the heat treatment. Besides, the castables with 2 wt.% CaTiO3 contents were estimated to possess greater thermal shock resistance.

Application Performance of Microporous Sintered Alumina in Alumina Magnesia Castables for Ladles

Application Performance of Microporous Sintered Alumina in Alumina Magnesia Castables for Ladles

Corrosion Mechanism of A Density-Reduced Steel Ladle Lining Containing Porous Spinel-Calcium Aluminate Aggregates

Refractory monolithics for steel ladle linings are typically products with low porosities and high bulk densities. They achieve high temperature, penetration, and corrosion resistance. Despite the high density of these products, which is due to the low porosity of the aggregates, their matrices still exhibit a high amount of pores. Since calcium magnesium aluminate (CMA) has already proven its resistance to penetration and corrosion as a binder in the matrix, this paper investigated if alumina spinel refractories containing microporous calcium magnesium aluminate aggregates can withstand conditions that occur in a steel ladle wall. The objective was to reduce the castable density with the advantage of a lower material requirement for a ladle lining and reduced heat and energy losses. This was achieved by replacing dense alumina aggregates by up to 38% of porous CMA aggregates (grains with 30 vol% porosity), which resulted in a bulk density reduction from 3.1 g/cm3 for the dense alumina castable to 2.8 g/cm3 for the 38% CMA aggregates containing castable. However, the despite the higher porosity, penetration, and corrosion resistance and thermomechanical properties were not impacted negatively for a model alumina spinel castable. A postmortem investigation was conducted on a newly developed dry-gunning mix that was installed in a steel ladle wall on top of a slag penetrated castable and that achieved a service life of 31 heats versus only 18 heats for the reference mix that contained dense alumina and spinel aggregates. This new repair mix contained the newly designed porous CMA aggregates, which in this case partly replaced the dense alumina and spinel aggregates. These porous aggregates consisted of magnesium aluminate and calcium aluminate micro-crystals. The postmortem study revealed two important phenomena that can explain the improved performance: at the hot face in contact with steel and slag, a thin densified zone was observed that blocked the slag penetration into the porous matrix and the porous aggregates. Iron oxides were almost completely blocked from penetration, and only some manganese oxide was observed in the penetrated zone together with some silica and lime from the slag. Clusters of calcium aluminate (CA6) and magnesium aluminate (MA) spinel build the refractory back-bone on the hot side of the material and gussets filled with mostly glassy calcium aluminum silicates close to the hot face and gehlenite further inside the penetrated zone. Alumina grains had a reaction rim consisting of CA2 or CA6 and a very intimate connection to the surrounding matrix unlike the CMA-free mix that showed micro cracks around the alumina grains. At the colder side, the gunning mix with CMA aggregates showed a very good connection to the substrate, supported by a hercynite formation in the gunning mix resulting from a cross-reaction with remains of iron oxide on the CMA containing repair mix. Furthermore, macroscopic observations of a CMA aggregate containing alumina magnesia castable in the metal zone of a steel ladle revealed that macro cracks developed only very slowly, which resulted in a superior service life.

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.

The Interfacial Behavior of Alumina-Magnesia Castables and Molten Slag under an Alternating Magnetic Field

Electromagnetic metallurgical technology has become an important method for the smelting of high purity steel with the development of metallurgical technology. The electromagnetic fields existing extensively in the steelmaking process can not only affect the smelting efficiency and the cleanness of steel, but also the interfacial behavior between the refractory and the molten slag. Focusing on alumina-magnesia castables, an important lining material in ladles, the interfacial behavior between the castable and different ladle slags with or without an alternating magnetic field (AMF) is studied in this paper. The results showed that the slag corrosion and especially the slag penetration of the alumina-magnesia castable are more severe under an AMF. Moreover, an AMF accelerates the migration of Fe, Mn and their oxides from the molten slag to the castable. Meanwhile, some low melting point phases such as diopside and anorthite form in the corrosion layer of the castable, which was deduced to accelerate the slag corrosion and penetration of the alumina-magnesia castable.

Dynamic interaction of refractory and molten steel: Effect of alumina-magnesia castables on alloy steel cleanness

High quality alloy steel is an important material needed for social and economic development. It is of great significance to major national projects and defence security. Refractories are used in the smelting process of steel; they are some of the main sources of impurities which have an important effect on the quality of steel. As alumina-magnesia refractories are the main lining materials used for steel refining, the influence of these refractories on the cleanliness of molten steel under dynamic smelting conditions has been studied. The size, quantity, composition, and structure evolution of inclusions in steel are analysed. The results show that after smelting, the content of alloy elements in the steel is stable, and that the total oxygen content and inclusions in the steel are increased by the corrosion of the alumina-magnesia castables. However, the maximum average particle size of the inclusions in the steel was limited to 20 µm, which did not cause large inclusions in the steel or seriously affect the quality of steel. During the dynamic melting process, because of the presence of Si and Mn in the alloy steel, the inclusions changed from homogeneous CaS wrapped Al2O3-MgO composite sphere to MnS wrapped egg-shaped structure. The alloy elements in steel were found to be beneficial, as they reduced the effect of alumina and magnesia inclusions on the quality of steel. The results indicate that it is feasible to smelt high quality alloy steel using alumina-magnesia carbon-free castable, and that it would be better to limit the refining time to 45 min during smelting.

Effects of fine reactive alumina powders on properties of alumina-magnesia castables with TiO2 additions

The design of alumina-magnesia castables was optimized based on the action mechanism of raw materials. In this study, the effects of fine reactive alumina powders on the properties of alumina-magnesia castables with TiO2 additions were investigated. The phase composition and microstructure of castables with different contents of TiO2 and two fine reactive alumina powders were characterized by X-ray diffraction and scanning electron microscopy. The results of these studies showed that the Na2O/SiO2 ratio of reactive alumina affected on the phase evolution and properties of castables. The changes in the spinel and CA6 content and their solid solubility could account for the comprehensive effect of TiO2 and reactive alumina on the castables. The difference in the expansion of castables with 1wt% TiO2 addition after calcination at 1350°C was significant. The permanent linear changes and the apparent porosity of castables first increased and then decreased with increasing the calcination temperature. Castables containing reactive alumina with lower Na2O/SiO2 ratio had higher cold moduli of rupture.

Improvement of Durability of Purging Plugs Using MgO Micropowder for Refining Ladles

To modulate the matrix of purging plugs, MgO micropowder was introduced as a replacement to magnesia powder in alumina–magnesia castables, and the effect of MgO micropowder on the properties of alumina–magnesia castables and the possibility of developing chrome‐free castables were investigated. Experimental results showed that the introduction of MgO micropowder resulted in an improvement in the volume stability, strength, and thermal shock resistance of alumina–magnesia castables due to its high surface energy and small particle size. However, excessive amounts of MgO micropowder led to a lower densification, and there was a slight degradation in the performance of the alumina–magnesia castables. The slag resistance of the prepared alumina–magnesia castables was significantly better than that of the alumina–chrome castables. Microstructure and energy spectrum analysis showed that the formation of a solidified reaction layer, mainly consisting of spinel and CaAl12O19, was the major cause of the observed difference in slag resistance. In addition, the alumina–magnesia castables had a lower linear thermal expansion coefficient than that of the alumina–chrome castables at each experimental temperature, which effectively decreased the thermal stress during its service period, thus exhibiting good thermal shock resistance.

Towards slag-resistant, anti-clogging and chrome-free castable for gas purging

In this study, a chrome-free refractory slot plug was prepared by adding micro-magnesia powder to alumina-magnesia castable. The currently used alumina-chrome castables and improved alumina-magnesia castables were used to investigate slag infiltration and corrosion of plugs with slots of different widths at different temperatures. In addition, dynamic and thermodynamic calculations were performed to determine the causes of slag infiltration and clogging. The results showed that the slag-infiltration depths in the slots of the alumina-chrome castable were significantly higher than those in the slots of the alumina-magnesia castable. In addition, increasing the slot width, soaking time, and temperature all led to increases in the depth of slag infiltration in the slots of the purging plugs. Owing to the wettability of (Al,Cr)2O3 with respect to the molten slag being poor, the molten slag exhibited low viscosity and infiltrated deep into the slots of the alumina-chrome castable. However, as the slag infiltrated further into the slots, the dissolution of (Al,Cr)2O3 led to the formation of extensive high-melting-point phases, which existed deep in the slots and resulted in significant blockage of the purging plugs. The addition of micro-magnesia to the alumina-chrome castable resulted in the formation of a homogeneously distributed spinel with fine grains, which could absorb the FeOx present in the slag and increased the slag viscosity. As a result, the slag-infiltration rate was reduced significantly. The slag in the slots exhibited an invariant point at a temperature lower than 1600°C. Therefore, its oxygen burning should be easy, leading to the higher service life of such castables than that of alumina-chrome ones.

Microsilica influence on the phase consTITUTION and properties of spinel-forming composition

Experience in using low cement alumina-magnesia castables, containing spinel-forming reagents, in monolithic ladle linings identified their significant advantages in terms of thermomechanical properties, corrosion resistance and slag resistance. A particular feature of these castables is the synthesis of «in situ» magnesium aluminate spinel at high operating temperatures, which provides increased operational lifetime of the lining.The microsilica influence on the phase constitution of the composite mixture, containing calcium aluminate cement and spinel-forming reagents - calcined alumina and sintered periclase after firing at 17000C was investigated in the paper. It was found that as a result of the interaction of cement calcium aluminates with microsilica, fusible compounds - anorthite and helenite that, at increased microsilica content, impair physical and technical properties of the composite are formed. It was determined that the ratio of spinel-forming reagents and content of the silica-containing material in a matrix component of alumina-magnesia castables is a more important factor of the synthesis of high-melting-point crystalline phases than the microsilica content. Directed regulation of the phase constitution of the matrix component of alumina-magnesia castables by adjusting the grain-size composition of spinel-forming reagents and their optimal ratio will ensure achieving a set of the given physical and technical characteristics of concrete and increasing the operational lifetime of monolithic ladle linings.

Effects of TiO2 addition on the expansion behavior of alumina–magnesia refractory castables

Cement-bonded alumina–magnesia refractory castables present excellent properties because of the in situ spinel and calcium hexaluminate formation. However, controlling the expansion derived from in situ reactions is essential for keeping the integrity of castables. The present work addresses the effects of TiO2 addition on the expansion behavior of alumina–magnesia refractory castables by the assisted sintering technique. The mechanism was discussed by comparing the apparent porosity, phase composition and the resultant microstructure. The change of the expansion with the temperatures for castables containing 0–2 wt% of TiO2 presented a trend of first increase and then decrease. The expansion level of alumina–magnesia refractory castables after firing at 1500 °C for 5 h decreased gradually with TiO2 amount from 0.5 to 2 wt%. The results indicated that TiO2 acted in a greater extent in accelerating the spinel and CA6 formation at 1300 °C, while the effect of TiO2 was reduced with the increase of the temperature. The microstructure development of alumina–magnesia castables with TiO2 addition was more complex. The more liquid phase with lower viscosity formed by adding TiO2, resulted in the drastic decrease of the overall expansion for alumina–magnesia refractory castables.

Possible improvements of alumina–magnesia castable by lightweight microporous aggregates

This paper focuses on the properties of lightweight alumina–magnesia castable prepared with homemade microporous corundum aggregate, as well as on the investigation of the effects due to the introduction of microporous corundum aggregate on alumina–magnesia castable. The results showed that, in comparison to common alumina–magnesia castable, due to the small pore size and low apparent porosity of microporous corundum aggregate, its introduction leads to an improvement in volume stability, strength, heat insulation and thermal shock resistance of alumina–magnesia castable. The slag resistance of lightweight alumina–magnesia castable is significantly better than that of common alumina–magnesia castable. Microstructure and energy dispersive analyses show that the formation of conically crystallizing CA2 and CA6 is the main reason for the difference in slag resistance. The conical crystals, interlaced and distributed around the aggregate, prevent the sample from further corrosion and penetration of the slag. In addition, since the CaO content of the slag is largely absorbed by the refractory, the viscosity of slag increases, and a solidified layer is formed and adhered on the hot face of sample, thus further deterring the penetration of the slag.

Preparation of Microporous Corundum Aggregates and their Applications in Lightweight Alumina-Magnesia Castables

The industrialization process all over the world has brought with it problems of energy being consumed too fast and a serious energy crisis has developed. The use of heat insulation refractories is a main factor used to reduce industrial furnace energy. Heat insulation material that is nearer the work layer provides better insulation. Two areas work are discussed in this paper. Firstly, in order to prepare the kind of high-performance microporous aggregate used for the alumina-magnesia castable in ladle linings, with α-Al2O3 micropowder as the main raw material, a microporous corundum aggregate was prepared by the method of wet grinding. The distribution of pore size, the thermal conductivity and the microstructure of the microporous corundum aggregate were tested by means of mercury porosimetry, a laser flash thermal analyzer and scanning electron microscopy, etc. The results show that: Secondly, a lightweight alumina-magnesia castable was obtained using this microporous corundum aggregate, fused magnesia powder and corundum powder as the main raw materials. In terms of the alumina-magnesia castable, the sintering properties, the strength after baking and the heat treatment at different temperatures, the thermal conductivity coefficient and the resistance to slag were tested in the experiment. The results show that:

Slag Resistance of Al2O3–MgO Refractory Castables in Different Environmental Conditions

Although the corrosion performance of spinel‐containing castables has been extensively investigated in recent years, no previous studies accessed the different conditions present in the ladle bottom. In this region, strong variations in the atmospheric environment are often detected, which could drastically change the interactions between refractory and molten slag. In the present work, the main corrosion mechanisms of an alumina–magnesia castable in two environmental conditions (oxidizing – pO2 = 0.21 atm—or reducing – pO2 = 10−15 atm— atmosphere) were evaluated by means of scanning electron microscopy and EDS analyses of the corroded samples and thermodynamic simulations. The attained results showed that the slag penetration was suppressed in the presence of oxygen due to the precipitation of a great amount of calcium monoaluminate (CA) crystals as the refractory interacted with slag. Conversely, the CA phase was not stable under reducing conditions and, therefore, many more refractory components (Al2O3, MgO, and MgAl2O4) had to be dissolved to precipitate calcium dialuminate (CA2) by reacting with infiltrating slag. Thus, besides providing a suitable and more realistic understanding of the castable performance in service conditions, the results also indicated that the prediction of the environmental conditions is of utmost importance for the design of high performance refractories.

Al2O3–MgO castables performance in an expansion constrained environment

The present work addresses the importance of considering the working structural conditions when evaluating the expansive behavior of alumina–magnesia castables under chemical corrosion. Slag resistance tests were conducted on spinel castables with different expansion levels, inserted in cement-bonded high alumina blocks, to limit their expansion, and compared to the results attained after performing the same experiments without any constraint. The results pointed out that in an expansion controlled condition, the penetration indexes were reduced as a result of the lower volume of pores in the castable microstructure. Moreover, this benefit was enhanced for the coarser MgO-containing materials, contradicting the trends usually observed when no structural constraint is considered.

Fundamentals and applications on in situ spinel formation mechanisms in Al2O3–MgO refractory castables

Although the physical expansion associated with the in situ formation of magnesium–aluminate spinel (MgAl2O4) is well-reported, some questions related to this behavior, such as the different volume change values experimentally attained when compared to theoretical one and the pore generation after the reaction, remain open. Thus, the main objective of this work is to shed some light on these questions by evaluating a cement-bonded alumina–magnesia castables, designed using dead-burnt magnesia of different particle size ranges. Microstructural observations suggested that the faster Mg2+ migration during the spinel formation led to vacancy accumulation and, consequently, to pore generation, as a direct result of the Kirkendall effect. Additionally, the overall expansion of alumina–magnesia castables seemed to be ruled by two main factors: its sintering efficiency and the different possibilities of the Al2O3 and MgO interactions in the mixture. Those consequences, however, do not usually affect the castable corrosion behavior in industrial applications, due to the benefits imposed by the structural constraint.

Reaction behavior of slag and spinel in steel ladle castables

The difference in corrosion behavior between alumina-spinel and alumina-magnesia castables in a steel slag was investigated with emphasis on the composition of spinel phase. The spinel primarily-added to the alumina-spinel castable had the stoichiometric composition, whereas the spinel forming secondarily in the alumina-magnesia castable was in an alumina-rich composition. The model reaction between sintered spinel and slag revealed that the stoichiometric spinel was highly wettable to slag and the molten slag deeply penetrated into the sintered body. In contrast, the alumina-rich spinel was less wettable. It was found that the alumina-rich spinel changed into stoichiometric one at the interface between spinel and slag, and that an iron-alumina spinel formed following to dissolution of the excessive alumina into slag. The increase in alumina content seemed to increase the viscosity of the slag, leading to retardation of penetrative wetting of slag to the alumina-rich spinel.