Alumina Refractories Research Articles

Additive manufacturing of alumina refractories by binder jetting

In this work, refractory components based on alumina were produced by binder jetting using a large-scale 3D printer. The formulation contained several particle fractions up to a grain size of 3 mm, equal to the printer resolution. The binder system contained fine dead burnt magnesia, milled citric acid and reactive alumina, which were added to the aggregate mixture to create the powder bed. Deionized water was deposited from the printer's nozzles and triggered the binding reaction between the magnesia and citric acid. After 24 h, the printed samples were removed from the powder bed, dried and sintered at 1600 °C for 5 h. Reactive alumina contributed to the in situ creation of magnesium aluminate spinel at high temperature. The samples were characterized in terms of Young's modulus of elasticity, bending and compressive strength in 2 directions (parallel and perpendicular to the printing direction). The broken parts were used to investigate physical properties such as the open porosity and bulk density. The microstructure was studied by means of computed tomography. Finally, powder samples were used to determine the phase composition at different stages of production by means of XRD.

Synthesis of Alumina Sol Binder for Pure Alumina Refractory Castable and its Comparison with Other Commercially Available Binders

Synthesis of Alumina Sol Binder for Pure Alumina Refractory Castable and its Comparison with Other Commercially Available Binders

In-situ investigations via confocal video-processing analyses of mullite vs. alumina refractories for application to biomass-fueled gasifiers

The National Energy Technology Laboratory (NETL) targets modular gasifiers that can accommodate diversified carbon feedstock such as biomass and waste plastics. Towards this goal, it is necessary to better understand interactions of candidate refractory ceramics with feedstock ashes. In this work, confocal scanning laser microscopy in conjunction with a variety of image-processing techniques was utilized to assess these interactions in-situ with biomass ashes. We found that various ash components followed different timelines. A key factor which further affected these timelines was proximity to chemical and/or topological boundaries, including but not limited to other ash or slag, pores, cracks, crevices, grain boundaries, and matrix-aggregate boundaries. Alignment techniques such as Scale-Invariant Features Transformation (SIFT) enabled increased comparability of features as ZY or XZ planes could be aligned fully onto a consistent location of an ash. In tandem with thresholding techniques and assessment of useful features, SIFT allows potential for future rapid analyses of otherwise difficult to quantify videography.

Characterization of Sintered Niobium–Alumina Refractory Composite Granules Synthesized by Castable Technology

Aggregates of niobium–alumina refractory composites are characterized, which are produced by a castable technology followed by a crushing process. Phase separation between niobium and corundum is found for the obtained aggregate classes and the impurity phases NbO and β–Nb2C are present according to X‐ray diffraction measurements. Among the studied aggregate classes, niobium shows differences in chemistry expressed by the corresponding lattice parameters. The determined morphology and roughness parameters show that the synthesized aggregates are not substantial different independent of their density and porosity.

High‐Temperature Compressive Behavior of Refractory Alumina–Niobium Composite Material

The development of coarse‐grained refractory composites combining refractory ceramics with refractory metals provides new approaches for high‐temperature applications. In particular, coarse‐grained Nb‐Al2O3 composites are widely interested due to their promising functional properties such as high thermal shock resistance, low shrinkage and good electrical conductivity. In order to identify and release the potential of these materials in advanced applications, their mechanical behavior should be evaluated. This study presents room‐ and high‐temperature compression behavior up to 1500 °C of Nb‐Al2O3 refractory composites manufactured via castable and extrusion technology. The influences of production method, particle size, open porosity as well as temperature and strain rate are discussed. For comparison, pure niobium and alumina specimens are used as reference materials. The results indicate that Nb‐Al2O3 refractory composites show high ductility at high temperatures. This behavior is also verified with microstructural investigations by scanning electron microscope (SEM).

Boron removal from silicon melt by gas blowing technique

Abstract Due to the detrimental effects of boron (B) on the efficiency of silicon (Si) photovoltaic cells, complete boron removal from Si is necessary to produce solar grade Si (SoG–Si, with a maximum limit of 0.1 ppmw boron). Gas refining is a promising technique for boron removal from Si, in which the thermodynamic equilibrium never establishes. Hence, by starting from any B concentration in the melt, the required limit for SoG–Si will be achieved. This research is devoted to studying the refractory interactions’ effect with melt and the chamber atmosphere on boron removal. For this purpose, gas refining experiments were carried out in alumina and graphite crucibles with H2 and H2–3% H2O refining gases. Gas refining in Ar, He, and continuous vacuuming conditions were also carried out to study the effect of chamber atmosphere. The gas refining results are supported by the characterization of the evaporated species by molecular beam mass spectroscopy (MBMS) technique. The MBMS measurements indicated that the boron evaporation occurs by the formation of the volatile species BH x , BO y , and B z H x O y compounds. Most of these compounds are already known in the literature. However, HBO, HBOH, and AlBO (in the case of alumina refractories) were measured experimentally in this work. Results indicate that the evaporation of B in the form of AlBO x compounds leads to higher mass transfer coefficients for boron removal in alumina crucibles. Density-functional theory (DFT) and coupled cluster calculations are carried out to provide a thermodynamic database for the gaseous compounds in the H–B–O–Al system, including enthalpy, entropy, and C P values for 21 compounds.

Effect of silicon powder-modified tabular alumina aggregates on the mechanical properties and microstructures of carbon-bonded alumina refractories

Effect of silicon powder-modified tabular alumina aggregates on the mechanical properties and microstructures of carbon-bonded alumina refractories

An experimental study on compressive properties of high density polyethylene-nano alumina-groundnut shell hybrid composites

In recent years, bio-fibers have become increasingly popular as reinforcements for thermoplastics and thermosets. Research into the use of natural fibers such flax, bamboo, sisal, hemp, and jute as reinforcing elements in the production of various composites has been considerable. Groundnut shell particles and refractory alumina powder were combined with high density polyethylene (HDPE) polymer matrix to create new bio-based composites in this study. The results were promising. Composite boards were synthesized by with HDPE with randomly distributed nano alumina and groundnut shell particles with volume percentages of 95:0:5, 95:2.5:2.5 and 95:5:0. ASTM standards were used to evaluate the compressive, shore hardness and water intake characteristics of the composites. The highest compressive load, compressive strength, increase in area, maximum displacement, displacement at break and compressive strain were analyzed for the specimen having HDPE, nano alumina and groundnut shell particles proportion 95:2.5:2.5. Using HDPE, nano alumina, and groundnut shell particles, Possibly a composite might be produced that could replace wood in many settings.

Graphene-Reinforced Carbon-Bonded Coarse-Grained Refractories.

Carbon-bonded alumina refractories offer excellent thermal shock performance but are lacking in terms of mechanical strength. In the present contribution, the influence of the particle packing and the addition of graphene oxide (GO) to carbon-bonded alumina refractories on the physical and mechanical properties before and after thermal shock was investigated. Coarse tabular alumina grains were coated by a GO suspension and used to prepare dry-pressed compacts. The included graphite fraction (15 wt%) was either regarded as a lubricating matrix component or as a quasi-spherical component of a calculated density-optimized aggregate size distribution. During coking, the GO was reduced to thermally reduced graphene. The porosity, true density and thermal shock behavior in terms of the cold modulus of rupture (CMOR) and Young’s modulus were compared. Samples with a higher density were obtained when the irregularly shaped graphite was considered as the matrix component (lubricant). The results showed that the use of GO had a positive impact on the mechanical properties of the graphene-reinforced Al2O3–C refractories, especially in the case of a less optimized packing, due to the bridging of delamination gaps. In addition, the thermal shock only had a minor impact on the Young’s modulus and CMOR values of the samples. SEM investigation revealed very similar microstructures in coked as well as thermally shocked samples.

Effects of Post-Treatment to Improve the Surface Quality of 3D Printing Cement Mold Casting

Powder bed 3D printing can be applied to sandcasting mold manufacturing to ensure high quality and economy through process innovation. In this study, refractory alumina cement was used as an aqueous binder to ensure high-temperature thermal stability to minimize the addition of organic matter to reduce gas generation. In addition, spherical silica sand, the study material, was selected to a size of 30 µm to improve the casting mold resolution. To improve the surface quality through the post-treatment process, we confirmed the change in the surface roughness of the mold depending on the surface treatment of colloidal silica and the presence or absence of heat treatment, and finally made the mold through actual casting. Changes in the surface roughness and flowability of the cast body after mold post-treatment were confirmed. For aluminum castings, the shrinkage rate and surface roughness were confirmed in a box-shaped mold via gravity casting, and the flowability of the molten metal in the mold was confirmed in a hand-shaped mold. There was a change in the roughness and porosity of the mold, owing to the post-treatment, and the influence of the surface roughness and flowability of the cast body during actual casting was confirmed.

Atomic Force Microscopy Investigation of the In Situ‐Formed Oxide Layer at the Interface of Al2O3−C/Steel Melt in Terms of Adhesion Force and Roughness in a Model System

Herein, the in situ‐formed layers on carbon‐bonded alumina refractories immersed in liquid 42CrMo4 steel at 1650 °C are investigated with regard to their macro‐ and microroughness, wetting properties, as well as adhesive interactions. Colloidal probe (CP) atomic force microscopy (AFM) are used to describe the adhesion in a water‐based model system. To take the interaction between melt and the in situ layer into account, the adhesive force is measured via drop probe AFM using a model system with mercury droplets. For poorly wetted in situ layer samples, adhesion forces increase with smaller hold times in the melt due to a less homogeneous layer and vice versa if only van der Waals forces act. In contrast, the influence of the immersion time in the melt and the lattice parameter of the crystals on the layer is of minor importance for melt droplet−layer adhesion.

Kinetic Study of the Reactions of AlO with H2O and H2; Precursors to Stellar Dust Formation

AlO is relatively abundant around oxygen-rich Asymptotic Giant Branch (AGB) stars, where it can react with major gas-phase species such as H2 and H2O to form AlOH. These Al-containing species are the likely precursors of refractory alumina nanoparticles, which may provide the nuclei for dust formation. In the present study, the kinetics of these AlO reactions were measured from 295 to 780 K using the pulsed laser photolysis of Al(C5H7O2)3, with time-resolved laser induced fluorescence detection of AlO. The experimental results were interpreted using quantum chemistry calculations and a Master Equation solver for reactions with multiple energy wells. For the recombination reaction AlO + H2O (+ N2) → Al(OH)2, log10(krec,0/cm6 molecule–2 s–1) = −32.9185 + 8.80276 log10(T) – 2.4723(log10(T))2; log10(krec,∞/cm3 molecule–1 s–1) = −19.4686 + 7.62037 log10(T) – 1.47214(log10(T))2; Fc = 0.28 (uncertainty ±13% from 295 to 760 K). For the bimolecular reactions, k(AlO + H2O → AlOH + OH) = (3.89 ± 0.47) × 10–10 exp(−(1295 ± 150)/T) and k(AlO + H2 → AlOH + H) = (5.37 ± 0.52) × 10–13 (T/300)(2.77 ± 0.19) exp(−(2190 ± 110)/T) cm3 molecule–1 s–1. Rate coefficients for Al + H2O → AlOH + H, AlOH + H → AlO + H2 or Al + H2O, and the absorption cross sections of AlOH and AlO were calculated theoretically. Al chemistry around an O-rich AGB star was then investigated using a β-trajectory model, which predicts that AlOH is the major gas-phase Al species beyond two stellar radii and shows that the relative AlO abundance is very sensitive to the AlOH photolysis rate.

Synthesis of porous silicate materials using sodium fluride as fluxing agent

Foam glass products are very actively used for thermal and chemical protection of oil and gas infrastructure instead of polymer materials. The main trend in foam glass technology is a search for new raw materials. A presence of refractory alumina in the raw materials necessitates the introduction of substances that reduce the melting temperature of silicate masses (fluxes). The aim of this work was to study the processes that occur during the heat treatment of foam glass batches when sodium fluoride NaF is added as a flux. The synthesis of porous materials was carried out by powder method according to the hydrate mechanism based on the interaction between silicate raw materials and an alkali solution. The initial batch compositions were developed, and their heat treatment was performed. TG/DSC analysis showed that NaF does not affect mass change processes, including the removal of physically bound water, crystalline hydrated water from hydrosilicates, chemically bound water, and decomposition of organic and carbonate impurities. There is also an endothermic effect of the “β-quartz→α  -quartz” transition for both compositions. However, there is a sharp endothermic jump in the composition with the NaF addition at temperatures above 600 оC due to exposure to sodium fluoride. It was shown that ions from fluoride dissociation are concentrated in a less polymerized microphase, sharply changing its viscosity properties and the viscosity of the melt as a whole. It was set that most of the silicate mass is in an X-ray amorphous glassy state. The main crystalline phase is α -quartz in an amount of not more than 25%, and albite crystals can also be distinguished. As a result of studying the internal structure and density of the synthesized samples, it was found that the introduction of sodium fluoride significantly accelerates the process of silicate mass melting, which makes it possible to obtain samples with a density of 313 kg/m3 at 750 оC and subsequently leads to the breakthrough of pore walls, pore coalescence, and severe sedimentation of the foam, which complicates manufacturing process of high-quality materials. Thus, the use of sodium fluoride in the synthesis of porous silicate materials can be recommended in a small amount as an active fluxing agent.This research was carried out by the Platov South-Russian State Polytechnic University (NPI) and funded by the Russian Science Foundation under the Agreement No. 19-79-00015 “Understanding some general regularities related to the synthesis of porous materials on the basis of man-made and natural silicates” (led by B. M. Goltsman).

Bio-Slag High-Temperature Corrosion on an Alumina Refractory under the Reducing Environment

The alkali and alkaline earth metals in the biomass are one of the major challenges that restrict the biomass/low-rank-coal entrained-flow gasification in the way of ash slagging and corrosion. To ...

Effect study of an amount of active alumina addition on the properties of alumina refractories with a complex additive of chromium oxide and zirconium dioxide

An effect of active alumina amount introduced into a batch (7.5 - 22.0 %) on the properties of alumina samples containing a complex additive of chromium oxide (10 %) and zirconium dioxide (3 %) was studied. The optimal amount of introduced active alumina (12.5 %) was established, which ensures the production of samples fired at a temperature of 1580 ºС, with the lowest open porosity (18.2 %), the highest apparent density (3.25 g/cm3) and the highest cold crushing strength (117 N/mm2), refractoriness under a load of 0.2 N/mm2 > 1700 ºС, thermal shock resistance (in the mode of 1300 ºС — water) not less than 7 thermal cycles.The results of carried out studies were used in the development of production technology of domestic refractory products based on fused corundum, α-form alumina, active alumina, containing the complex addition of chromium oxide and zirconium dioxide, by semi-dry pressing method. These refractory products characterized by high properties and intended for service at a temperature of 1850 ºС in the combustion chamber of carbon black production reactors.The use of these refractories in the lining of above mentioned high-temperature industrial units will allow to increase a duration of their operation due to an increase in the refractory durability in service, as well as to save material and energy resources associated with a relining of the reactors.

Influence research of temperature heat treatment on the microstructure of alumina ceramics

The JSC "URIR named after A. S. Berezhnoy" developed and introduced the technology of highly refractory alumina extra-dense ceramics. It is known that, the structure and properties of alumina ceramics in many cases depend on the temperature of its heat treatment, the holding time at the final temperature, the thermal unit, and a number of other factors. In order to study the processes of structure formation during of heat treatment the microstructure of alumina ceramic samples of two compositions (composition 1 with a moisture content of 30 %, as well as with dispersing and hardening additives, and composition 2 with a moisture content of 20 % only with dispersing additive) which were made by slip casting aqueous alumina slips based on reactive alumina α–from with Al2O3 content over 99 %, specific surface ~ 9 m2/g by electron microscopic analysis was studied. It was established that, after heat treatment at 1000 °C the beginning of sintering and recrystallization of corundum grains are observed, after heat treatment at 1200 °C take place packing of structure and recrystallization of α–Al2O3. After firing at 1400 °C the microstructure of samples composition 1 is represented to a large extent by crystals of tabular corundum with a particle size ~ 0.5—5.5 μm, predominant — ~ 2.5—3.0 μm. After firing at 1400 °C the microstructure of samples composition 2 is represented — by individual particles of a rounded and oval form with a size of ~ 0.5—3.5 μm, predominant — ~ 1.5—2.0 μm which are interconnected by intercrystalline layers. The microstructure of alumina ceramic samples of both compositions after firing at 1500 °C is similar and is composed of recrystallized corundum grains with a size of ~ 0.5—5.5 μm, predominant — ~ 1.5—2.0 μm, among which there are also larger α–Al2O3 grains, with a size of ~ 2—8 μm. After firing at 1580 °C the microstructure of samples is represented by a dense fine-grained structure of well-formed practically defect-free corundum grains, with a size of ~ 0.5—3.0 μm, among which grains of ~ 5—10 μm.

Study of the silica–magnesia relationship in silicate slags on electrical conductivity at high temperature

Slags are metallurgical aggregates with beneficial minerals and in some processes, they constitute the most abundant mass of the operation and for this reason their control is necessary. These materials have physicochemical behaviors that serve to model the process variables. A particular condition of silicate slags in processes for obtaining some ferroalloys is that there are relationships between compounds that allow predicting the behavior of other variables, these relationships are maintained during production. This study aims to know the electrical behavior of a slag so that the data will contribute to have an additional control in the operation of an electric arc furnace. An alumina content of up to 50% was gradually added to one sample and the other was remelted on three occasions, for all cases the electrical conductivity was determined at a temperature between 1400 °C and 1600 °C. For the electrical resistance value, a Miller 400 type resistivity measurement advice was used, and the temperature was achieved by melting the samples in a high-frequency induction furnace with a collapsible silicon carbide crucible, lined with high-grade refractory alumina. Analysis of the electrical conductivity data for silicate slags made it possible to demonstrate that they are proportional to the values of the major oxides of the slag in their silica-magnesia ratio.

Potentiality of “orujillo” (olive oil solid waste) to produce hydrogen by means of pyrolysis

The extraction of olive oil generates great amounts of solid waste and wastewater. The objective of this work is to evaluate the potentiality of slow pyrolysis of the solid oil waste, known as “orujillo”, to produce hydrogen rich gases. The effect of temperature and of different treatments of pyrolysis vapors on the yield and composition of the gases has been experimentally studied. “Orujillo” was pyrolyzed at 500 °C and 700 °C, and the pyrolysis vapors produced were directly fed to a second reactor where they were treated at 800 °C in different ways: just thermally, thermally treated through a refractory alumina bed and thermo-catalytically treated through a three layer bed (alumina + SiC mixed with Ni catalyst + alumina). The catalyst used was a commercial prereduced nickel-catalyst which contains 44 wt% Ni over calcium aluminate support (CaO/Al2O3). Concerning the effect of temperature, it has been proved that the raise in temperature leads to a decrease in the liquid yields and an increase in the gas yields, as well as to an increase in H2 and a decrease in CO2 content of the gases. With respect to the vapors treatments it has been observed that as a general rule any of the treatments reduces liquid and increases gas yields, and concerning gas composition, H2 content increases, CO slightly increases and CO2 decreases. These effects are more pronounced when the Ni-catalyst is used. With the Ni-catalyst about 50 wt% gas yield is obtained, with ≈ 50 vol% H2 both at 500 and at 700 °C.

Recycling of carbon fiber composites in carbon-bonded alumina refractories

This study investigates the feasibility of recycling carbon fiber reinforced polymer (CFRP) composites in carbon-bonded alumina refractories. For this purpose, different procedures for pretreatment and separation of recyclable CFRP composites were examined. The influence of pretreatment, dimensions and condition of the CFRPs on physical, mechanical, and thermo-mechanical properties of carbon-bonded alumina refractories were evaluated.The CFRP fractions were classified by sieving and partly treated by means of mechanical and thermal methods. The mechanical approach describes a simple method to obtain short fibers along with a reusable fiber dust. By thermal treatment in an oxidizing atmosphere, the carbon fibers can be easily separated from the polymer matrix. Carbon-bonded alumina bricks of industry standard with CFRPs of different length and treatment and partly silicon as antioxidant were produced and compared to the respective reference without CFRPs.The addition of recycled CFRPs to carbon-bonded alumina refractories resulted in a modified microstructure consistent with open porosity and altered mechanical properties at room temperature. The microstructural change in the refractory material provoked by recycled CFRPs < 1mm increased the high-temperature Young's modulus up to 200%. The addition of mechanically treated and short CFRPs resulted in enhanced creep resistance at high temperature. Thus, different treatment methods of CFRPs exhibit promising recycling approaches for reusing carbon composites in carbon-bonded refractories by improving thermo-mechanical properties and simultaneously operating as final repository.

Analysis of deposits from combustion chamber of boiler for dendromass

This article records the degradation of the refractory alumina-silicate lining in a boiler furnace combusting dendromass after ten years of use. The deterioration of refractory alumina material was evaluated from phase and chemical analyses of accretions. The fusion temperatures of accretion were measured with a high-temperature microscope and the results were compared with the solidus (point of first liquid formation) temperature predicted by FactSage 7.3 thermodynamic simulation software. Content of SiO2 (50–65%) in accretions confirms that fine silicate particles of ash adhere to the lining. These silicates react with subliming alkalis from the dendromass and form aggressive eutectic melts on surface of lining at operating temperatures. Increase in operating temperature, inhomogeneity (porosity) and alkali content in accretions are the main factors influencing accretion viscosity, melt convection and lining corrosion. For this reason accretions on the vertical walls lower down are much thicker than on the walls in the upper part and the arch. The Al2O3 concentration is higher in the arch accretions; there the refractory material corrodes intensively. The fusion temperatures of the glassy accretions (measured at the furnace atmosphere: accretion boundary) are around 1150 °C. However, the calculated temperatures of slag formation stated by FactSage are about 150 °C lower, and these correspond to the operating temperature in the upper section of the combusting chamber. The fusion temperatures increase with the Al2O3 content in accretions closer to the lining. The obtained results will be applied to define the requirements for the development of boiler furnace refractories.