Acetic Acid Role on Magnesia Hydration for Cement‐Free Refractory Castables

Different approaches to master magnesia hydration in refractory castables have been recently proposed. Among them, the use of hydrating agents can change the Mg(OH)₂ crystals morphology and its distribution in the resultant microstructure, minimizing the drawbacks related to the reaction expansion. In this work, the hydrating effect of acetic acid in Al₂O₃–MgO cement‐free castable properties during curing, drying, and firing steps was evaluated by elastic modulus, thermogravimetric, apparent porosity, and SEM analyses. Based on the attained results, adding acetic acid resulted in hydroxide crystals with distinct morphology and flexibility leading to a better accommodation of Mg(OH)₂ in the designed microstructure, which inhibited the samples' cracking during curing. In addition, the drying behavior of the evaluated compositions was further optimized by incorporating polypropylene fibers. Thus, this study highlights a novel perspective for fine MgO powders application, indicating that brucite morphology engineering may be a key aspect for the development of advanced Al₂O₃–MgO cement‐free refractory castables.     

Systemic analysis of MgO hydration effects on alumina–magnesia refractory castables

The use of magnesia sources with high specific surface area and small particle size in the Al₂O₃–MgO system can induce faster in situ spinel (MgAl₂O₄) formation in castable compositions, improving the slag corrosion resistance. However, the higher reactivity of these raw materials lead to an intensive brucite formation (followed by volumetric expansion), spoiling the castable's properties during the curing and drying steps. Considering these aspects, a systemic analysis of three magnesia sources (dead-burnt and caustic ones) was carried out in order to evaluate: (1) their hydration impact on the refractory castables properties, and (2) their bonding ability in cement-free compositions. Mechanical strength, thermogravimetric and Young's modulus tests were conducted during the castables’ curing and drying steps. According to the results, the elastic modulus measurement is an efficient tool to evaluate the magnesia hydration. The addition of proper amounts of calcium aluminate cement and/or silica fume to the castables can inhibit the crack formation and provide suitable mechanical properties. The results also show that under certain conditions, MgO can be used as a binder, replacing calcium aluminate cement and leading to a significant reduction in the castables costs with no drawbacks to their refractoriness.     

Crack-free caustic magnesia-bonded refractory castables

A growing interest in designing high-alumina MgO-bonded refractory castables has been identified in recent years due to the magnesia ability to react: (i) with water at the initial processing stages of these materials (inducing the precipitation of brucite phase) or (ii) with alumina, giving rise to in situ MgAl₂O₄ generation at high temperatures. Nevertheless, despite the great potential of caustic magnesia to be used as a binder in such systems due to its high reactivity, it is still a challenge to control the hydration reaction rate of this oxide and the negative effects derived from the expansive feature of Mg(OH)₂ formation. Thus, this work evaluated the incorporation of different contents of aluminum hydroxyl lactate (AHL) into caustic magnesia-bonded castables, aiming to control the brucite precipitation during the curing and drying steps of the prepared samples, resulting in crack-free refractories. The designed compositions were characterized via flowability, setting behavior, X-ray diffraction, cold flexural strength, porosity, permeability and thermogravimetric measurements. According to the results, instead of Mg(OH)₂, hydrotalcite-like phases [Mg₆Al₂(OH)₁₆(OH)₂.4.5H₂O and Mg₆Al₂(OH)₁₆(CO₃)·4H₂O] were the main hydrated phases identified in the AHL-containing compositions. The addition of 1.0 wt% of aluminum hydroxyl lactate to the designed castable proved to be, so far, the best option for this magnesia source, resulting in the development of a crack-free refractory with enhanced properties and greater spalling resistance under heating.     

MgO fumes as a potential binder for in situ spinel containing refractory castables

MgO is pointed out as an alternative binder for refractory materials, mainly for systems where the presence of CaO might not be desired. Selecting the most suitable magnesia source is an important step as its purity and reactivity should influence the hydration reaction, leading to binding effect or cracks. This work investigated the design of vibratable high-alumina compositions bonded with MgO fumes [which is a very fine powdered oxide (d < 3?µm) resulting from the production process of electrofused magnesia] and/or dead-burnt magnesia (d < 212?µm). Acetic and formic acids were added to the castables during their processing steps in order to adjust the density of active sites for Mg(OH)₂ formation and control the crystal growth of this phase. The green mechanical strength and thermomechanical performance (cold and hot mechanical strength, thermal shock, refractoriness under load, corrosion, etc.) of designed MgO-bonded compositions were analyzed. Improved green mechanical strength and crack-free samples were obtained when adding up to 6?wt% of MgO fumes to the refractories and processing them with aqueous solutions with 3?wt% of formic acid. The compositions with 6?wt% of magnesia fumes resulted in samples with flexural strength in the range of 12.0?MPa after curing at 50?°C/24?h and similar green mechanical strength (12.9?MPa) as the ones bonded with 4.0?wt% of calcium aluminate cement after drying at 110?°C for 24?h, which highlights the great potential of this MgO source. Despite the enhanced green mechanical strength, alumina-based castables containing 6?wt% of MgO (fumes, dead-burnt or their blend) showed low mechanical strength at intermediate temperatures and high linear expansion, as a consequence of the in situ spinel phase formation above 1200?°C. Thus, better densification, improved HMOR, thermal shock resistance and corrosion behavior were obtained for the castables prepared with less MgO fume contents.     

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.     

Microsilica or MgO grain size: Which one mostly affects the in situ spinel refractory castable expansion?

Microsilica is commonly added to alumina–magnesia castables to counterbalance the in situ spinel expansion. This effect is attained by the generation of a low-melting temperature phase, which also affects the expansive reaction kinetics. Additionally, the MgAl₂O₄ formation depends on the grain size of the reactants. The use of coarse magnesia grains results in lower Mg²⁺ dissolution and could lead, at 1500 °C, to forsterite development (Mg₂SiO₄). For finer MgO, silica was detected at the edge of the spinel grains. Considering these aspects, this work evaluated the effect of microsilica content for different magnesia grain sizes (<45 or <100 μm). Due to a faster spinel formation for the fine MgO source, microsilica counterbalanced the MgAl₂O₄ expansion. Conversely, for the coarser MgO, silica increased the Mg²⁺ dissolution, speeding up the spinel formation and expansion. Therefore, microsilica presented opposite roles, pointing out that it does not always counterbalance the spinel expansion. This work also indicated the need for a systemic approach for the expanding design of alumina–magnesia refractory castables.     

Influence of magnesia surface on the setting time of magnesia-phosphate cement

The following paper presents the influence of magnesia reactivity in magnesia-phosphate cement. When water is added to cement, monoammonium dihydrogen phosphate (NH₄H₂PO₄ or MAP) goes in solution till saturation while magnesia (MgO) is wetted and starts to dissociate. This dissociation only depends on MgO surface, except when magnesium carbonate is present. In this case, it accelerates the dissolution process, independently of the surface state. For magnesia, the higher the amount of surface defect sites, the higher the MgO wetting. Wetting and nucleation are promoted by a large interface between MgO and MAP, because adsorption probabilities are more important. Therefore, grinding a powder allows a better reactivity. On the contrary, calcination, in a first step, enhances the surface state and then reduces MgO reactivity due to the melting of grains, which reduces the total developed surface.     

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.     

Mechanical properties and microstructure evolution of MgO–Al–C slide plate refractories in presence of Al powder-modified magnesia aggregates

MgO–Al–C slide plate refractories were fabricated using sintered magnesia and modified sintered magnesia as aggregates, fused magnesia aggregates and fines, Al powder and carbon black (N220) as fines, and thermosetting phenolic resin as the binder. Al powder-modified magnesia aggregates were prepared and characterized and were introduced into the MgO–Al–C slide plate refractories. The effects of the modified aggregates on the properties, phase composition, and microstructure were investigated. 1) The Al powder-modified magnesia aggregates exhibited considerably high bonding strengths and low Al powder shedding ratios, thus meeting the preparation requirements of MgO–Al–C slide plate refractories. 2) At high temperatures, more needle-like and fibrous Al₄C₃, AlN and octahedral MgAl₂O₄ were generated on the surface of the modified magnesia aggregates, which enhanced the bond between the matrix and the aggregates and increased the hot modulus of rupture of the material. 3) Non-oxide Al₄C₃ and AlN phases were formed in situ and had high thermal conductivity and low coefficient of expansion; this could relieve the internal thermal stress of the material and create a toughening effect, improving the thermal shock resistance of the material.     

The influence of talc as a nucleation agent on the nonisothermal crystallization and morphology of isotactic polypropylene: The application of the Lauritzen–Hoffmann,Avrami, and Ozawa theories

A new method is proposed, which can be used to analyze the influence of different additives and fillers on the nonisothermal crystallization of polymers. The composites of talc in isotactic polypropylene (i‐PP) were prepared using a corotating twin‐screw extruder. The compounds were subsequently dried and injection molded. PP morphology and talc dispersion were visualized using optical microscopy and computed tomography. Wide‐angle X‐ray scattering and small‐angle X‐ray scattering measurements provided an insight into the crystal structure of PP. The data obtained from nonisothermal DSC measurements were fitted to the Avrami model for the nonisothermal case. The calculated Avrami's exponent (n), which takes into account the influence of talc on the nucleation and growth of the PP crystals, was used in the combination of Lauritzen–Hoffman and Ozawa models to calculate the nucleation parameter (K_g). A good agreement was found between the model predictions and literature values. The examination shows that the developed model extension gives an expected trend in the case of i‐PP filled with talcs from the same origin but with different particle sizes. Furthermore, it is shown that delaminated talc with a higher specific surface is more efficient in nucleation of i‐PP. Thus, the introduced model extension could be a useful tool for comparing of nucleation ability of different additives in the crystallization of polymers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis of chemically bonded phosphate ceramic coatings with tailored curing behaviour and enhanced oxidation resistance

The objective of this study was to develop chemically bonded phosphate ceramic (CBPC) coatings with relatively low curing temperature, properly prolonged curing time, and enhanced oxidation resistance. The coating was synthesised using aluminium phosphate as binder, surface modified MgOw/MgO as curing agent, and spherical microscaled Al particle as filler. The MgOw/MgO curing agent was surface coated by an Al₂O₃ layer via sol-gel routes. The Al₂O₃-coated MgOw/MgO agent facilitated a controllable curing process owing to the sustained-release effect of the surrounding microstructure. The weight loss profiles of all CBPCs with a surface modified curing agent exhibited a quasi-parabolic trend. The curing time decreased with the addition of the curing agent, and the Al₂O₃-coated MgOw curing agent was more effective owing to the good dispersion and larger particle number/volume percentage. The thermogravimetric and differential scanning calorimetry curves proved that the Al element exhibited similar effect as Mg²⁺, which could also enhance the curing process via substitution reaction between the basic metal ions with hydrogen in phosphate. Therefore, the addition of Al filler resulted in further curing and densification, exhibiting a decrease in the curing time (30–50 min) and increase in the weight loss (~40%). Proper binder-to-curing agent-to-filler mass (B:C:F) ratio was very important, and the CBPCs with improved surface roughness, hardness, and free of cracks were synthesised at a B:C:F ratio of 10:1:0.05. During oxidation at 800 °C, the Al filler in the CBPCs transformed into a continuous Al₂O₃ layer, which protected the Ti6Al4V alloy from further oxidation.     

Interfacial reactions between magnesia refractory and electric arc furnace (EAF) slag with use of direct reduced iron (DRI) as raw material

Interfacial reactions between the electric arc furnace (EAF) slag, i.e., CaO–SiO₂–FeO–MgO–Al₂O₃–MnO system, and the magnesia refractory as a function of direct reduced iron (DRI) addition (0, 10, 20, 30 wt%) were investigated at 1550 °C under an Ar atmosphere. MgO solubility increases with increasing DRI content by decreasing basicity (i.e., CaO/SiO₂ ratio), which is due to an increase in SiO₂ supplied from DRI. The measured MgO content was always lower than the theoretical MgO saturation level irrespective of DRI content because the magnesiowüstite (MW) intermediate layer, which formed at the slag/refractory interface, retarded the direct dissolution of the refractory by acting as a self-protective layer. The thickness of the MW intermediate layer and dissolution depth were proportional to DRI content. However, the penetrativity decreased with increasing DRI content by decreasing the fluidity of the slag. Several kinetic parameters were estimated, including the dissolution rate constant of the MW intermediate layer, the dissolution rate of the MgO refractory, and the rate constant of MW growth. Dissolution of MgO refractory is controlled by the dissolution of the MW intermediate layer. Increasing the growth rate is very important for protecting refractory after the formation of a MW intermediate layer. In addition, we provided a schematic diagram of the slag/refractory interfacial reaction phenomena that compares situations of low and high DRI content. The results of the present study show that it is necessary to control DRI content to minimize refractory degradation during the EAF process. If a large amount of DRI must be used in the EAF process, then MgO content in the slag should be at the saturation limit at first, which accelerates growth of the MW intermediate layer.     

The influence of composition and temperature on hydrated phase assemblages in magnesium oxychloride cements

Due to their nonhydraulic nature, magnesium oxychloride cements (MOCs) are susceptible to degradation following contact with water. Improving the water resistance of these materials requires better understanding of hydrated phase relations and the sensitivity of hydrated phases to water. Toward this end, a series of targeted experiments and complementary thermodynamic calculations were carried out to assess hydrated phase assemblages in the system MgO‐MgCl₂‐H₂O across a range of compositions. Focus is placed on appraising the effects of composition and reaction temperature. In broad agreement with literature data, under ambient conditions, hydrated MOCs are noted to contain Phase 3 (P3), Phase 5 (P5), and brucite, but the mass partitioning of these phases is highly dependent on H₂O/MgCl₂ and MgO/MgCl₂ molar ratio as well as curing temperature and age. At room temperature, P3 is favored at lower water contents, however, P5 and/or brucite are favored to form as water availability increases. Thermodynamic calculations indicate that P3 is “more stable” than P5 at lower temperatures—an outcome which impacts the engineering properties, for example, strength and volume stability. The impacts of the accuracy and self‐consistency of currently available thermodynamic data and their implications on predicted phase assemblages are discussed.     

PHYSICAL PROPERTIES OF SOME HIGH-TEMPERATURE REFRACTORY COMPOSITIONS*

In the course of an investigation that involved a study of pyrochemical reactions, it was necessary to develop a refractory that could be used satisfactorily at temperatures in the range of 1800° to 2200°C. It was found that calcined magnesia (96%, MgO) or electrically fused magnesia (98% MgO) could be bonded adequately by mixing sized aggregates with 2.5%, by weight of calcined sea-water magnesia and wetting with a 24° Bé. solution of magnesium chloride Large shapes of these compositions fired at 1450°C. were satisfactory for use in the required temperature range. A small-scale study OF the properties of various refractory bodies showed that compositions containing relatively pure limestone or dolomite readily hydrated in water even after firing to 2100°C. and were unsuitable for refractory use. The addition of silica, alumina, zirconia, chromic oxide, or combinations of these oxides to dolomite or limestone resulted in a refractory stable against hydration. The inversion of zirconia was reduced appreciably by the addition of 5% magnesia. Bodies containing BaO·ZrO₂ and CaO·ZrO₂ were found to be stable after firing to 2100°C. with no inversion up to 1200°C. and with a coefficient of expansion less than that of electrically fused magnesia. Small- and large-scale tests of an MgO·Cr₂O₃ spinel composition showed this material to be highly refractory with a low coefficient of expansion; the compound, however, dissociates and loses Cr₂O₃ above 1700°C. While the small-scale tests disclosed a number of compositions which show promise as high-temperature refractories, their full evaluation for use on a large scale was not made.     

Flow induced phase inversion agglomeration: Fundamentals and batch processing

A novel agglomeration technique, based on flow induced phase inversion (FIPI) is described and applied to the batch preparation of polyethylene-bound abrasive calcite agglomerates. Water soluble polymers are used to agglomerate the needle-like crystals of tetraacetylethylene diamine and also sodium chloride crystals. In a typical isothermal FIPI agglomeration process primary particles are dispersed in the molten binder, which is subsequently inverted by the addition of sufficient amount of primary particles, which also defines the critical filler concentration at phase inversion, C_c. Agglomerate particle size is primarily a function of C_p – C_c where C_p is the mean concentration of filler. C_c decreases with increasing binder molecular weight and primary particle surface area. Agglomerate size distribution is affected by processing, mainly by the mixing time after phase inversion. For the non-isothermal FIPI agglomeration process, phase inversion is induced locally, by the addition of fine particles in the molten binder. Phase inversion is then propagated by cooling the dispersion during mixing. Agglomerate characteristics such as particle size, particle size distribution, binder concentration distribution in each agglomerate size range, agglomerate topology, binder morphology in the agglomerates, agglomerate strength, and agglomerate dissolution rate in water were evaluated. These agglomerate characteristics are related to the binder and filler properties as well as to the processing conditions.     

Formation mechanism of whiskers in Al–MgAl2O4–MgO refractories at 1400 °C under N2 atmosphere

Al–MgAl₂O₄–MgO refractories were prepared using fused magnesia, metal Al, fused spinel and sintered high purity magnesia as raw materials. The phase composition and microstructure of Al–MgAl₂O₄–MgO refractories treated at 1400 °C under N₂ atmosphere were investigated by means of XRD, SEM and EDS. The results showed that magnesia (MgO) whiskers and MgAlON whiskers were formed on the surface and in the inner area of the Al–MgAl₂O₄–MgO refractories, respectively. The MgO whiskers grew preferentially along the axial direction, forming cylindrical shape MgO whiskers. Then the cylindrical MgO whiskers further absorbed Mg(g) and O₂(g), and grew along the radial direction to form the square columnar shape MgO whiskers. The MgAlON whiskers firstly grew in one-dimensional direction, forming whisker shape MgAlON, then some whisker shape MgAlON gradually developed and grew into two-dimensional flake shape MgAlON. The sintering and thermal shock resistance was significantly improved by the whiskers. The growth process of magnesia whiskers and MgAlON whiskers were dominated by a vapor-solid (VS) mechanism.     

Development of ZrO2‐MgO nanocomposite powders by the modified sol‐gel method

The ZrO₂‐MgO nanocomposites were synthesized using a new sol‐gel method with sucrose and tartaric acid as a gel agent. The samples were characterized by thermal analysis (TG/DTA), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy‐dispersive X‐ray mapping (EDX mapping), and Ultraviolet‐visible spectroscopy (UV‐vis). The results showed that the cubic phase of ZrO₂‐MgO was formed in the presence of both gel agents. The average particle size of the samples synthesized with sucrose was lower (30 nm) than that of tartaric acid (50 nm). Finally, the formation mechanism and the optical properties of zirconia‐magnesia have been discussed.     

Effect of MgO Grain Size on Thermal Expansion Behavior of Alumina–Magnesia–Carbon Refractory

The effect of MgO grain size on the permanent linear change behavior of resin bonded alumina–magnesia–carbon refractory has been studied in relation to the formation of spinel phase in these refractories as a function of firing time and temperature. From scanning electron microscopic studies, spinelization at the interface of Al₂O₃ and MgO grains has also been studied to determine the reaction kinetics for gaining insight into the process.     

Quantification of gypsum crystal nucleation,growth, and breakage rates in a wet flue gas desulfurization pilot plant

The aim of this work is to study the influence of nucleation, growth and breakage on the particle size distribution (PSD) of gypsum crystals produced by the wet flue gas desulfurization (FGD) process. The steady state PSD, obtained in a falling film wet FGD pilot plant during desulfurization of a 1000 ppm(v) SO₂ gas stream, displayed a strong nonlinear behaviour (in a ln(n(l)) vs. l plot) at the lower end of the particle size range, compared to the well‐known linear mixed suspension mixed product removal model. A transient population balance breakage model, fitted to experimental data, was able to model an increase in the fraction of small particles, but not to the extent observed experimentally. A three‐parameter, size‐dependent growth model, previously used for sodium sulphate decahydrate and potash alum, was able to describe the experimental data, indicating either size‐dependent integration kinetics or growth rate dispersion. © 2009 American Institute of Chemical Engineers AIChE J, 2009