Cladding and corrosion characteristics of magnesium-aluminum spinel refractory by alkaline slag during alkali recovery

The alkaline slag produced during alkali recovery might cause harm to the furnace's refractory components. The seat-drop technique and static dry pan method are used to explore the cladding features and corrosion characteristics of alkaline slag on the surface of magnesia-alumina spinel refractory in this paper. SEM-EDS and XRD are used to study the microstructure, fundamental changes, and compound composition of the molten cladding and interfacial layers, as well as the trends of slag column alterations. Factsage 7.2 software is used to model the interaction between the molten slag and the refractory. The results demonstrate that the wettability process of the slag column is impeded in a reducing environment. Combine with the results of the thermodynamic simulate, it is found that the formation and transformation of a large number of high melting point compounds in a reducing atmosphere is the decisive reason for the inhibition of wettability. Temperature increases promote the production of high melting points substances such as magnesium silicate and sodium metal aluminate, which alters the micro-morphology of the materials and improves slag resistance and permeability resistance of refractories.     

Effects of different particle size of spinel and Y2O3 additive of the periclase-spinel refractory on the sintering densification and corrosion resistance to copper smelting slag

In order to analyze the sintering densification and copper smelting slag corrosion resistance of periclase-spinel refractories, the periclase-spinel refractories were prepared with fused magnesia, magnesia-rich spinel, industrial alumina, and yttrium oxide as the main raw materials. The different particle sizes of spinel in material and with or without Y₂O₃ additive were studied. The study demonstrated that: (1) The different particle sizes of spinel in periclase-spinel refractories can result in different effects. Adding particle spinel to the refractory can improve the strength and corrosion resistance of the periclase-spinel refractories. The addition of spinel and magnesia powders in the matrix resulted in cracks due to the great difference of coefficient of thermal expansion between magnesia and spinel. The reduction in bulk density and strength of the material decreased slag penetration resistance because of its poor sintering properties. While adding the alumina in the matrix can further fill the crack and prevent slag penetration by the volume expansion of in-situ reaction to form spinel. (2) The periclase-spinel refractories can be reacted with Cu slag to form a Mg₂FeO₄ insulating layer as the iron ion becomes oxidized. Adding Y₂O₃ in periclase-spinel refractories can result in grain boundary phase reconstruction, which can promote sintering densification, improve the slag physical infiltration resistance, and improve the chemical corrosion resistance of materials.     

Corrosion modeling of magnesia aggregates in contact with CaO–MgO–SiO2 slags

Ladle refining is an efficient process for improvement of quality of steel on secondary metallurgy under harsh conditions. Magnesia refractories with high purity are important raw materials for ladle lining in high-quality steel production. However, the penetration by CaO–MgO–SiO₂ slags damages magnesia refractories, which considerably limits their service life. Abundant grain boundaries in magnesia create channels for slag penetration and lead to the destruction of the structure. The effect of the microstructure on the slag corrosion behavior of magnesia aggregates requires further systematic investigation. In this study, a corrosion model was established to describe the slag penetration process of magnesia aggregates. The effects of the grain-boundary size and slag CaO/SiO₂ mass ratio (C/S ratio) on slag penetration were investigated, and the possibility of the microstructure optimization of magnesia aggregates was discussed. The results indicated that magnesia aggregates exhibited excellent slag resistance for slag with a C/S ratio above 1.5 or even 2.0. When the slag C/S ratio was lower than 1.5, the dissolution rate of magnesia decreased more rapidly with the increase in the slag C/S ratio. In addition, the much smaller grain-boundary size increased the slag penetration resistance by promoting the formation of a dense isolation layer and inhibiting further penetration processes. The calculation results agreed well with the experimental results, suggesting that the corrosion model is promising for predicting slag corrosion.     

Key features of alumina/magnesia/graphite refractories for steel ladle lining

The corrosion resistance of resin bonded alumina/magnesia/graphite refractories containing different kinds of aggregates were investigated when submitted to the action of slags of several CaO/SiO₂ ratios. The laboratory testing was performed by means of the rotary slag attack test. Specifically evaluated was the influence of alumina/carbon ratio and magnesia and silica contents on the refractories corrosion resistance. It was found that this property could be improved by increasing the refractory Al₂O₃/SiO₂ ratio as well as by choosing the appropriate Al₂O₃/C ratio.     

Comparative study of B4C,Mg2B2O5, and ZrB2 powder additions on the mechanical properties,oxidation, and slag corrosion resistance of MgO–C refractories

Boron-containing additives are used to improve the oxidation resistance of carbon-containing refractories; however, their effects on the mechanical properties and slag corrosion resistance of the refractories have rarely been studied. In this work, B₄C, Mg₂B₂O₅, and ZrB₂ powders were incorporated into low-carbon MgO–C refractories to study their effects on the mechanical properties, oxidation resistance, and slag corrosion resistance of the refractories. The relationships between these properties and the microstructure and phase evolution were also studied. The results show that the flexural strengths of the MgO–C refractories at high temperatures are closely related to the apparent porosity and formation of an Mg₃B₂O₆ phase. The oxidation resistances are greatly improved after the introduction of boron-containing additives into the MgO–C refractories in terms of both thermodynamical aspects and the filling of voids and pores. The most effective antioxidant is B₄C, followed by the ZrB₂ and Mg₂B₂O₅ powders. The mechanisms through which the vanadium-containing slag attacks the MgO–C refractories mainly include the dissolution of magnesia to form melting phases, penetration through pores, and redox reaction with carbon.     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al₂O₃ slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al₂O₃ slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.     

Corrosion mechanism of magnesia-chrome refractory bricks with FetO-SiO2-Cr2O3 copper converter slag

The paper investigates the effect of Cr₂O₃ on the resistance of magnesia-chrome refractory bricks to copper converter slag. The static crucible method was employed to carry out the slag resistance experiment. The corrosion of magnesia-chrome refractory bricks under the action of Fe_tO-SiO₂-xCr₂O₃ (x = 0–5 wt%) slag at 1300 °C was discussed. The microstructure of the corroded sample was analyzed by XRD and SEM-EDS to elucidate the corrosion mechanisms of magnesia-chrome refractory bricks with Fe_tO-SiO₂-Cr₂O₃ slag. The results indicated that the permeability index of the slag-resistant samples gradually decreased with increasing Cr₂O₃ content in the Fe_tO-SiO₂-Cr₂O₃ slag. Combined with SEM and XRD characterization, the MgO in the refractory reacted with FeO and SiO₂ in the molten slag, leading to dissolution and reaction corrosion of the refractories. Meanwhile, forming a (Mg, Fe)O solid solution layer in corroded samples can prevent further chemical reactions and high-temperature dissolution between the Fe_tO-SiO₂-Cr₂O₃ slag and refractories. With the addition of Cr₂O₃ in the Fe_tO-SiO₂-Cr₂O₃ slag, the corrosion effect of slag on refractories was weakened, and the (Mg, Fe)O solid solution layer became thinner. The magnesia-chrome refractory bricks showed excellent slag resistance when the Cr₂O₃ content in the copper converter slag was 5 wt%.     

Corrosion Resistance and Microstructure of High-Alumina Refractories, Based on the Rotary Slag Test

The corrosion resistance and microstructure of four highalumina and two chromia-containing high-alumina refractories were evaluated. Analysis of the refractories after exposure in the rotary slag test showed that the chromia-containing products had better slag resistance than the chromia-free products. Three factors contributed to this conclusion, including formation of a Cr-spinel layer at the slag/refractory interface, formation of an impermeable mullite layer just below the interface, and improved matrix bonding from chromia-alumina solid-solution formation.     

富铝尖晶石对镁质耐火材料抗侵蚀性的影响

研究了富铝尖晶石对镁质耐火材料抗钢渣与抗钙处理钢侵蚀性的影响。结果表明 :随着富铝尖晶石加入量的增加 ,镁质耐火材料的抗钙处理钢和钢渣熔蚀性逐渐减弱 ,而抗钢渣渗透性逐渐增强 ;纯镁质和镁尖晶石质耐火材料在抗钢渣与抗钙处理钢侵蚀方面远远优于铝锆碳质材料     

Cr7C3: A potential antioxidant for low carbon MgO–C refractories

Magnesia carbon (MgO–C) refractory, one of the most commonly used refractories in the steelmaking system, relies on graphite to improve the thermal shock resistance and slag corrosion resistance. The oxidation of graphite carbon in a MgO–C brick usually leads to the destruction of the carbon network in the brick, which causes the structure of the brick to become loose and easily eroded. At present, metal powders, carbides, and borides are used as antioxidants to prevent the oxidation of carbon in MgO–C bricks. The metal carbide Cr₇C₃ can be prepared from aluminum chromium slag through a simple synthetic process and at a low cost. In this work, we investigated the oxidation resistance of low carbon MgO–C refractories with different amounts of Cr₇C₃ powder (1, 2, 3, and 4 wt%). The refractories with 3 wt% Cr₇C₃ powder showed optimal resistance to oxidation. The microstructure indicated that oxygen reacts with Cr₇C₃ preferentially over carbon to form chromium oxide and magnesium chromium spinel, blocking the pores and hindering oxygen diffusion. Carbon arising from the reduction of carbon monoxide by Cr₇C₃ can act as a supplementary carbon source. The better oxidation resistance also contributed to the improvements in slag corrosion and thermal shock resistance of the refractories.     

Formation of liquid-phase isolation layer on the corroded interface of MgO/Al2O3-SiC-C refractory and molten steel: Role of SiC

Formation of a dense layer on corroded interface to suppress corrosion is always desired, but it is controlled by numerous environmental conditions. In this work, corroded microstructures of MgO/Al₂O₃-SiC-C refractories in metal bath area of ladle furnace were investigated after industrial trails. A liquid-phase isolation layer in which MgO islands and liquid phases was established on the corroded interface of refractories with 6 wt% coarse/fine SiC-additive. The formed isolation layer against steel/slag attacks led to an approximate 30% improvement in corrosion resistance than that of refractory with 3 wt% fine SiC-additive. More importantly, the liquid-phase isolation layer blocked the direct mass transfer between molten steel and refractories while it decreased exogenous pollution from refractories. SiC-additive affected the formation process of isolation layer by controlling the generation/migration of Mg(g) on refractory' surface. A further formation mechanism of liquid-phase isolation layer was discussed in detail and role of SiC was elucidated.     

Role of ZrO2 in sintering and mechanical properties of CaO containing magnesia from cryptocrystalline magnesite

As main components of magnesia-based refractories, magnesia exhibits excellent properties such as high refractoriness and good basic slag corrosion resistance. However, magnesia produced from CaO containing cryptocrystalline magnesite has limited application owing to the low hydration resistance and poor thermal shock resistance (TSR). This work aimed to investigate the reinforcing effects of microscale monoclinic ZrO₂ on free CaO containing magnesia for optimizing mechanical properties, TSR and hydration resistance. The results showed that adding ZrO₂ could promote the removal of the open pores, strengthen the interface bonding between various grains and produce crack deflection, which improved flexural strength and fracture toughness. As a result, the TSR of the specimens was enhanced effectively due to increased strength and toughness and reduction in the thermal expansion coefficient. Besides, as the ZrO₂ was introduced, hydration resistance of the specimens improved significantly, mainly attributing to the decrease in apparent porosity and elimination of the free CaO by forming CaZrO₃ and cubic ZrO₂ phases.     

The development of a thermodynamic model for Al2O3–MgO refractory castable corrosion by secondary metallurgy steel ladle slags

Alumina magnesia in situ spinel castables are used as ladle refractory lining in the steel industry. In contact with slag, they suffer degradations which limits their performance. The purpose of this article is to predict the thermochemical attack of a slag on alumina magnesia refractory using Factsage^® thermodynamic modeling. To evaluate the reliability of the thermodynamic results, a validation step was carried out, which supported that the database was well adapted to the alumina magnesia spinel system. The corrosion phenomenon was then computed for a simple to a complete system to understand the mechanism and the influence of specific oxides. The model was also compared to corroded microstructures from a steel ladle to evaluate the contribution of each constituent in the castable. The aggregates of alumina react with slag to produce monomineral layers of lime aluminates (CA6 and CA2), while complex spinels (Mg, Fe, Mn)O (Fe₂, Al₂)O₃ are formed from the reaction of the slag with the matrix of the castable. Several oxides (MnO, FeO, Fe₂O₃) from the slag contribute to the formation of the spinel structures. The microstructures of refractories used in steel ladles confirm the main conclusions and the thermodynamic approach.     

Corrosion behaviour of MgO-based refractories by different existence states of manganese-containing volatile phases

Medium/high manganese steel is great interest because of its excellent energy absorption and strength. Further, it has emerged as a potential advanced high-strength steel in automobiles. In this study, the corrosion behaviour of MgO-based refractories by different existence states of manganese-containing volatile phases was systematically investigated via laboratory experiments of the vacuum treatment of medium/high manganese steel. The dissection of a magnesia crucible indicated that the manganese-containing volatile phases exist in three states: manganese vapour, liquid manganese and manganese oxides. The corrosion mechanism models of the MgO-based refractories were established as follows: (1) For the manganese vapour, the periclase was dissociated into the fine blocks when the magnesia-carbon brick was continuously exposed to the manganese vapour. (2) For the liquid manganese, a MnO layer was detected near the manganese metal, which was resulted from the MgO oxidation, and a further reaction took place to form a MnO·MgO solid solution layer at elevated temperatures, which led to the poor binding ability of the MgO crucibles. (3) For the manganese oxides, the Al₂O₃ crucibles were preferentially corroded by the molten manganese oxides, which are attributed to the superior thermodynamic driving force for the formation of MnAlO₄. Moreover, a reaction layer composed of a MnO·MgO solid solution and a low-melting-point silicic acid phase was formed at the interface between the manganese oxides and the magnesia-carbon brick along the decarburized infiltrated zone. In addition, the molten manganese oxides infiltrated along the periclase grain boundaries into the magnesia-carbon brick and to form a reactive metamorphic layer. Consequently, under the attack of physical and chemical corrosion, the refractory particles were loosened and dropped into the corrosive manganese oxides, causing the irreversible damage of the refractory. This work provides a strong theoretical basis for the qualitative evaluating damage of MgO-based refractories by manganese-containing volatile phases.     

Basic slag corrosion of alumina–magnesia–carbon refractory bricks containing Al and Si antioxidants

The slag-corrosion behavior of two alumina–magnesia–carbon refractories (AMC) with different antioxidants (Si and Al) has been comparatively studied by means of thermodynamic simulation and laboratory tests. A cup test (static) at 1723 K and 1873 K and dipping test (dynamic) at 1873 K were carried out using a steelmaking ladle slag. An iterative method that considers the change of the liquid's composition while it penetrates the refractory was employed for the thermodynamic calculation. The simulation as well as the static tests showed similar performance between both refractories (5% of wear at 1723 K and 6% at 1873 K). In spite of the type of antioxidant, the material with smaller particles of magnesia exhibited better performance in the dynamic test, with 55% less wear than the other AMC brick. This behavior was attributed to the faster MgAl₂O₄ spinel formation, which increased the material's cohesion, especially the matrix, during the heating stage. The Si antioxidant improved oxidation resistance, although it did not have a positive effect on the corrosion resistance of the material in any of the performed tests.     

Degradation of low‐carbon MgO‐C refractory by high‐alumina stainless steel slags in VOD ladle slagline

A new type of low‐carbon magnesia carbon refractory (LCMCR) substituting for MgO‐Cr₂O₃ refractory was successfully used in vacuum oxygen decarburization (VOD) ladle slagline, and the composition and microstructure of the used LCMCR were investigated. The results indicated that the decarburizing reaction (MgO‐C reaction) in the LCMCR under the VOD refining condition (high temperature, low pressure) was inhibited due to the low carbon content in the MgO‐C refractory and the dense layer formed between slag and original layer. The dense layer prevented the penetration of the external O₂ into the LCMCR inside because of the lower permeability of this layer, and thus, the direct burnout of the C in the LCMCR was substantially restrained. On the other hand, the large size crystal and the ultra‐low inclusions (SiO₂ and Fe₂O₃) content of the fused magnesia in the LCMCR made the magnesia more slag resistance, because the grain boundary in magnesia had higher slag penetration resistance and the contact area between the slag and the magnesia was reduced. The two aspects of the inhibited decarburizing reaction and the high quality magnesia synthetically contributed to the high slag resistance of the LCMCR.     

Determination of the fracture behaviour of MgO-refractories using multi-cycle wedge splitting test and digital image correlation

Refractories with reduced brittleness show a pronounced deviation from linear elastic behaviour and an enhanced thermal shock resistance. This paper aims to study the influence of microstructure on the fracture behaviour of magnesia refractories. The wedge splitting test(WST), which enables stable crack propagation for quasi-brittle materials, was used to identify the fracture behaviour and evaluate the energy dissipation. The evaluation of the crack lengths of the magnesia and magnesia spinel materials during the entire cyclic WST is based on the localized strain evaluated using the digital image correlation (DIC). A significant fracture process zone develops in the magnesia spinel material. The relationship between the dissipated energy and the actual crack length, which was used to characterize the crack growth resistance, was determined. The refractory materials that showed reduced brittleness consume a small amount of energy for fracture initiation but a large amount of energy for further crack propagation.     

Influence of chrome-bearing sols vacuum impregnation on the properties of magnesia-chrome refractory

Two kinds of nanosized chrome-bearing sols, i.e. Cr₂O₃ precursor sol and MgCr₂O₄ spinel precursor sol, were obtained by homogeneous precipitation. Properties of the sol vacuum impregnated magnesia-chrome refractory, such as bulk density, cold crushing strength, pore distribution, and chemical composition, etc., are superior to those of the un-impregnated sample. SEM micrographs show a different microstructure of the impregnated sample as compared to the un-impregnated one. The influence of vacuum impregnation on copper slag corrosion resistance of magnesia-chrome refractories has also been evaluated. The results show that both sols could improve the magnesia-chrome bricks corrosion resistance in impregnation.     

Novel phenomenon of quasi-volcanic corrosion on the alumina refractory-slag-air interface

A novel phenomenon of quasi-volcanic corrosion at refractory – slag – air interface under simulated normal smelting conditions was discovered when alumina refractory came in contact with CaO-Al₂O₃-SiO₂ slags and soaked at 1600°C. Based on thermodynamic and microstructure analysis, the mechanism of quasi-volcanic corrosion was revealed and the effect of CaO/SiO₂ mass ratio (C/S ratio) on corrosion was discussed. The results indicate that a two-stage intensified convection led to the severe upwelling corrosion of the refractory at the triple-phase interface. Furthermore, the critical intensity of the corrosive convection at the first stage on the formation of “slag volcano” was determined. This can serve as guidance for prolonging the service life of alumina refractory.