Refractory Aggregates Research Articles

Effects of SiC on the Properties and Microstructures of Closed-Pore Al2O3-MgAl2O4 Refractory Aggregates

Closed-pore Al2O3-MgAl2O4 refractory aggregates have been prepared successfully based on the superplasticity of Al2O3 ceramic by using a mixed ball-milling method and the gelcasting method. The effects of SiC content on phase composition, porosity and microstructures of the refractory aggregates have been investigated. The properties of Al2O3-MgAl2O4 refractory aggregates fabricated by different methods have been compared to our results. We discuss the influence mechanism of SiC on the refractory aggregate. The results showed that the main phase compositions were corundum and MgAl2O4 spinel with accompanying small amounts of mullite phase. The closed porosity increased with increased addition of SiC. SiC facilitated the sinterability of materials resulting in a low level of apparent porosity. When compared to the mixed ball-milling method, the closed pore size distribution in the refractory aggregates fabricated by the gelcasting method was more uniform.

Influence of phosphate binders on the physical and mechanical properties of magnesia carbon monolithics composed of recycled refractory aggregates

In this study, the effect of using phosphate bonding materials such as sodium hexametaphosphate (SHMP) and sodium tripolyphosphate was investigated in the presence of water and recycled magnesia carbon (MgO–C) refractory aggregates as raw materials. For this purpose, different compounds are prepared, and some parameters such as bulk density, volume per cent of apparent porosity and cold crushing strength were measured at different temperatures (200, 500 and 1100°C), and phase and microstructure studies were performed by X-ray diffraction, SEM and energy dispersive spectroscopy. Statistically, effects of the factors were also determined using the analysis of variance method. Results indicated that MgO–C monolithic refractory samples were successfully produced from recycling the spent MgO–C bricks, and use of these phosphate binders especially that of 5 wt-%SHMP produce some phosphate bonds like Mg2P2O7, Mg3(PO4)2 and AlPO4 and improve the physical and mechanical properties.

Preparation and Properties of Porous Mullite Refractory Aggregates Prepared Using Coal Gangue as Starting Material

Porous mullite refractory aggregates were successfully fabricated via recycling of coal gangue and bauxite at temperatures ranging from 1100 to 1350°C, with chaff as the pore former. The phase compositions, microstructures, pore characterization, and cold crushing strength of the aggregates were systematically studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury porosimetry associated with the thermodynamics analysis. The results showed that the firing temperature had a pronounced effect on the phase compositions, the morphology of mullite, the bonding between the mullite crystals, the pore structures (e.g. apparent porosity, average pore size, and distribution of pore sizes), and the cold crushing strength. In addition, secondary mullitization, which occurred at 1100°C, was gradually enhanced with increasing firing temperature. The maximum apparent porosity and highest <8 µm pore volume of 57.5% and 28.2%, respectively, were obtained at 1300°C, and resulted mainly from an increase in the liquid content and the volume expansion effect of the mullitization-crystal-growth process. The firing temperature had a positive effect on the morphology of and the bonding between the mullite crystals. Accordingly, the low porosity and excellent bonding between the mullite crystals resulted in an enhancement of the cold crushing strength.

Experimental Analysis of Hydrothermal Curing Influence on Properties of Fiber-Cement Composite

Special industrial application of fiber-cement composites is currently one important issue of concrete industry and research activity. The field of refractory and high-temperature resistance materials is very large and contains the cement composites too. Hydrothermal curing together with using aluminous cement with refractory basalt aggregates and fibers shows high potential for its applications in high temperature. These composite is characterized by compressive strength over 140 MPa and tensile strength in bending 12 MPa (investigated on specimens 40 x 40 x 160 mm). After exposure to temperature 1000 °C these parameters are 60 MPa in compression respective 6 MPa in bending. Achieved values are significantly higher than in the case of laboratory curing condition and there are suitable especially for prefabricated fire resistance cladding or other special application in the industry.

Effect of MgO micropowder on sintering properties and microstructures of microporous corundum aggregates

Microporous corundum aggregates were fabricated based on superplasticity, with α-Al2O3 micropowder as the main raw material, by adding nano-sized alumina sol and MgO micropowder. The relationship between the superplasticity and the in-situ stress during sintering, the corresponding sintering properties, and the microstructures of the fabricated microporous corundum aggregates were investigated by means of X-ray diffraction (XRD), mercury porosimetry, and scanning electron microscopy (SEM). The experimental results show that the relationship between the superplasticity and the in-situ stress is the main factor that influences the sintering behavior of the microporous corundum aggregates. Various amounts of MgO micropowder were added to the α-Al2O3 micropowder for a fixed content of nano-sized alumina sol. With increasing MgO micropowder, which results in a greater in-situ stress, the total and closed porosity increased initially and decreased afterwards whereas the apparent porosity and bulk density decreased first and then increased. Microporous corundum aggregates with high closed porosity, low apparent porosity and small pore size were obtained with the addition of 1wt% MgO micropowder. Therefore, the relationship between the superplasticity and the in-situ stress should be controlled in order to fabricate microporous refractory aggregates which have high closed porosity and small pore size.

Phase Development During Thermal Treatment of a Fast-setting Cordierite-mullite Refractory

Cordierite based materials are widely used in high temperature applications due to their good thermo-mechanical properties and thermal shock resistance. They are generally employed in the kiln furniture (shelves, brackets, bearing plates) for firing ceramic pieces. Because of its low expansion coefficient and dielectric properties, cordierite is also used in advanced ceramics, catalyst supports automotive, industrial waste gas purification and parts subjected to sudden temperature changes. The low intrinsic strength of the cordierite may be compensated by the presence of mullite, forming a composite material of cordierite-mullite, extending its use to somewhat higher temperatures, at the expense of a slight increase in the thermal expansion coefficient. In this work a cordierite-mullite precursor was prepared from a mixture of magnesium oxide, calcined alumina, silica fume and monoaluminum phosphate solution that produces fast setting at room temperature (∼20minutes) and then, by calcination, cordierite-mullite is generated. The evolution of the mineralogical phases was studied from room temperature to 1350°C by X-ray diffraction, differential thermal analysis and thermogravimetry. A possible sequence of chemicals reactions throughout the heat treatment is also proposed. Adding refractory aggregates to this precursor leads to obtaining a fast-setting concrete, suitable for the formation of individual pieces or repair service at moderately high temperatures. A summary of its main properties is also included.

Studies on densification, mechanical, micro-structural and structure–properties relationship of magnesium aluminate spinel refractory aggregates prepared from Indian magnesite

The present work intends to study the development of magnesium aluminate spinel aggregates from Indian magnesite in a single firing stage. The raw magnesite has been evaluated in terms of chemical analysis, differential thermal analysis, thermogravimetric analysis, infrared spectroscopy, and X-ray diffraction. The experimental batch containing Indian magnesite and calcined alumina has been sintered in the temperature range of 1550°C–1700°C. The sintered material has been characterized in terms of physico-chemical properties like bulk density, apparent porosity, true density, relative density and thermo-mechanical/mechanical properties like hot modulus of rupture, thermal shock resistance, cold modulus of rupture and structural properties by X-ray diffraction in terms of phase identification and evaluation of crystal structure parameters of corresponding phases by Rietveld analysis. The microstructures developed at different temperatures have been analyzed by field emission scanning electron microscope study and compositional analysis of the developed phase has been carried out by energy dispersive X-ray study.

Studies on synthesis and properties of magnesia refractory aggregates prepared from Indian magnesite through plasma fusion

The Indian magnesite has been fused in a plasma reactor for development of synthetic magnesia refractory aggregates. A Novel and low cost indigenously developed plasma reactor has been used to prepare the magnesia aggregates within a few minutes instead of using a long heating schedule of conventional sintering process. Different compositions of magnesia have been prepared by varying zirconia content in the mixture. With addition of zirconia the B.D. of 3.56g/cm3 have been achieved with densification of 96.22%. These fused samples have Hv (hardness) of 9.92GPa and Kic (fracture toughness) of 5.51MPam1/2 with incorporation of zirconia. The microstructure consists of periclase grains surrounded by zirconia having dendritic structure.

Preparation and characterization of porous MgO–Al2O3 refractory aggregates using an in-situ decomposition pore-forming technique

The porous MgO–Al2O3 refractory aggregates containing 30–92wt% Al2O3 were prepared via an in-situ decomposition pore-forming route using magnesite and Al(OH)3 as starting raw materials. The phase compositions, pore characteristics and mechanical strengths were characterised by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM), and mercury porosimetry measurements, etc. The results showed that the Al2O3 contents in the porous refractory aggregates strongly affected the spinel formation, change of the neck bonds between the particles, pore structure (porosity, average pore size and pore size distributions), and then affected the strengths. The porous MgO–Al2O3 refractory aggregates of 62–72wt% Al2O3 showed the best combination of high apparent porosities of 42.1–44.2%, high compressive strengths of 51.1–52.0MPa, high flexural strengths of 17.7–18.6MPa and small average pore size of 10.81–12.07μm.

Studies on densification, mechanical, microstructural and structure–properties relationship of refractory aggregates prepared from Indian magnesite by changing lime–silica ratio

The present work intends to study the development of refractory aggregates from Indian magnesite by modifying the lime–silica ratio. The raw magnesite has been evaluated in terms of chemical analysis, DTA-TG, IR Spectroscopy, XRD analysis. The material has been sintered at temperatures ranging from 1550°C to 1700°C along with modification of the lime–silica ratio. The sintered material has been characterized in terms of bulk density, apparent porosity, true density, relative density, cold modulus of rupture, hot modulus of rupture, thermal shock resistance, structural properties by XRD in terms of phase identification and evaluation of crystal structure parameters of corresponding phases by Rietveld analysis. The microstructures developed at different temperatures have been analyzed by FESEM study and compositional analysis of the developed phases has been carried out by EDAX study.

Studies on Synthesis and Characterization of Magnesia Based Refractory Aggregates Developed from Indian Magnesite

The present work intends to study the properties of magnesia based refractory aggregates developed from Indian magnesite by changing lime/silica ratio. The material has been sintered in the temperature range of 1550C - 1700C. The sintered samples are characterized in terms of bulk density, apparent porosity, true density, percentage densification, mechanical, thermo-mechanical properties like cold modulus of rupture, hot modulus of rupture and thermal shock resistance and structural properties by XRD. The developed microstructures at different temperatures are studied through FESEM study and compositional analysis of the developed phases is done by EDX study.

MgAl2O4 spinel as an effective ceramic bonding in a MgO–CaZrO3 refractory

Abstract The influence of MgAl2O4 spinel addition as a ceramic bonding in the MgO–CaZrO3 refractory was established by the evaluation of physical and microstructural characteristics in terms of density, porosity, crystalline phases, phase distribution and morphology. X-ray diffraction analyses and scanning electron microscopy with microanalysis have been used. The mechanical behavior has been evaluated in terms of cold crushing strength at room temperature and modulus of rupture at 25 and 1260 °C. Static and dynamic resistances tested by chemical attack of clinker raw constituents have been carried out at 1450 °C. Results showed that thermo-mechanical properties significantly improved with increasing the content of spinel. Microstructural analysis revealed that spinel phase aided to develop a strong bond between MgO and CaZrO3 refractory aggregates. Finally, the refractory bodies exhibited a good thermal stability and an excellent chemical resistance against the clinker raw material.

Study on the Coating of Lost Foam Casting for Steel Casting

The coating is one key technology of lost foam casting (LFC). This paper discussed the influences of refractory aggregate contents on strength, viscosity and other performances of LFC coating, and develops a high performance coating which can meet the requirement of LFC for steel casting. The results shown that with the content of the high bauxite increasing and the content of zircon powders decreasing, the strength, permeability and attached quantity of the coating increased. The best ratio of refractory aggregate is 70% high bauxite and 30% zircon powders. The optimal formula in the experiments is 70% high bauxite, 30% zircon powders, 3% sodium bentonite, 0.5% CMC and 5% ludox. This coating has good comprehensive performances to meet the requirements of LFC steel casting.

Hercynite and magnesium aluminate spinels acting as a ceramic bonding in an electrofused MgO–CaZrO3 refractory brick for the cement industry

Innovative chrome-free basic refractory bricks have been design based on electrofused magnesia–calcium zirconate (MgO-CaZrO3) technology using as ceramic bonding magnesium aluminate spinel (MgAl2O4) and hercynite spinel (FeAl2O4) in order to improve their properties. Industrial refractory bricks have been manufactured by solid state sintering of magnesium and calcium zirconate aggregates with MgAl2O4 and FeAl2O4 spinels at 1650°C in a tunnel kiln. Physical and microstructural characteristics of new refractory bricks have been characterised in terms of density, porosity, crystalline phases, phase distribution and morphology. X-ray diffraction (XRD) analyses and scanning electron microscopy (SEM) with microanalysis (using energy dispersive spectroscopy analysis -EDS) have been used. The mechanical behaviour has been evaluated in terms of cold crushing strength (CCS) at room temperature and three point bending modulus of rupture (MOR) at 25 and 1260°C. Static and dynamic resistance test by chemical attack of clinker raw meal constituents have been carried out at 1450°C. Results have shown that thermo-mechanical properties of new refractory bricks significantly improved with increasing both type of spinel in content. Microstructural analysis revealed that spinel phases aided to develop a strong bond between the magnesia and calcium zirconate refractory aggregates. Finally, these refractory matrixes exhibit a good thermal stability and an excellent chemical resistance against clinker raw meal.

Dehydration investigations of a refractory concrete using DTA method

The base mix refractory concrete is corundum-based, containing corundum as refractory aggregate and CAC as hydraulic binder, with a spinel as an additive. The authors investigated the dehydration reactions which occur from the moment when water is added (at the beginning of components mixing), to the moment when installed refractory concrete lining is put into the service. Sintering process kinetic of low-cement content refractory concrete was investigated by means of differential thermal analysis at four different heating rates (5, 10, 20, and 30 °C/min). Thus, temperature was increased from 20 to 1100 °C. It was noticed that first dehydration step occurs at lower temperatures, indicating at a desorption of physically adsorbed and interlayer water molecules. Second dehydration step, at higher temperatures is due to dehydroxylation of the lattices and decomposition of the interlayer anions.

The potential use of steel slag in refractory concrete

Abstract Steel slags are by-products in the steel production industry and can be used for different purposes; e.g. as aggregates in concrete or asphalt, and for blended cements. In the present study the possible use of electric arc furnace slag from carbon steel production in refractory concrete was studied. It was found that slag undergoes a mineralogical transformation of wustite into magnetite when it is heated to temperatures higher than 800 °C, this transformation being accompanied by volumetric expansion, which introduces cracks in the the refractory concrete and drastically worsens its mechanical properties. This transformation is irreversible, so that when slag is heated up to a temperature of 1000 °C prior to its use for refractory concrete, the final products exhibit mechanical properties which are comparable to concrete with conventional refractory aggregate, e.g. bauxite.

Theory and Applications of High-Power Nanosecond Pulses to Processing of Mineral Complexes

This paper reviews current research in high pulsed power technologies for processing of precious metals containing refractory ores and natural mineral aggregates. This is a branch of experimental engineering physics that critically depends on national priority research projects for its dynamic development. The aim of the manuscript is to show progress in the study of nanosecond processes involved in the disintegration and breaking-up of mineral complexes with fine disseminated precious metals. The manuscript presents results of theoretical and experimental studies of the mechanisms of the nonthermal action of high-power electromagnetic pulses with nanosecond leading edge, pulse duration, and high electric field strength on natural mineral media. Experimental data are presented to confirm the formation of breakdown channels and selective disintegration of mineral complexes as a result of pulse irradiation. This makes for efficient access of lixiviant solutions to precious metal particles and enhances precious metal recovery into lixivia during leaching. The paper shows the advantages of high-energy pulse treatment that provides a stable gain in valuable components recovery (5–80% gain for gold and 20–50% for silver) and at the same time helps to reduce energy consumption and cost of products in the processing of resistant gold-containing ores and beneficiation products from Russian deposits. X-ray photoelectron spectroscopy was used for determining the relationship between electromagnetic pulses energy and the surface chemical composition for pyrite and arsenopyrite. It has been concluded that impulsive treatment influences oxidation and hydrophobicity of the minerals and, therefore, it allows for the control of the hydrophobic–hydrophilic mineral surface balance.

Characterization, microstructure and corrosion behavior of magnesia refractories produced from recycled refractory aggregates

This paper aims to report the results of some investigations carried out in Iranian steel industries to reuse the spent magnesia graphite refractory bricks in the forms of the new shaped and unshaped magnesia refractories. Economical aspects of recycling and minimizing the environmental effects of spent refractories landfills were the basic goals of this research. The spent MgO–C refractory bricks from electric arc (EAF) and ladle (LF) furnaces were analyzed in terms of microstructural and chemical properties. Different samples were prepared from natural sintered magnesia and 10–30 wt.% of recycled aggregates in the forms of magnesia refractory brick and ramming mix and their physical and mechanical properties were evaluated. Also the slag corrosion behavior and microstructural properties of corroded samples were investigated. The results showed that the addition of up to 30 wt.% of recycled aggregates had no negative effects on the properties of magnesia refractories.

A New Phosphate-Bonded Investment Material for Rapid Ceramic Molding of Medium-Size Castings

Phosphate-bonded investment（PBI）ceramic mold materials have outstanding properties, such as rapid setting rate , moderate green strength and high temperature strength, and could be widely utilized in dental restoration and micro-casting of artistic products. However, fast hydration and gel rate of the phosphate-bonded ceramic slurry has been an obstacle to the wide applications of ceramic mold material for medium and large-size precision castings. This paper presents a new phosphate-bonded ceramic mold material by modifying the composition of refractory aggregates and additive agents, which is promising to be used for rapid ceramic mold casting for medium-size super alloy components. The results indicate that the properties of ceramic mold slurry including initial setting time and fluidity are suitable for complete embedding medium-scale patterns and get accurate duplicate of external configurations when the amount of boric acid is 1.11wt% and sodium tri-poly phosphate (STP) is 0.92wt%. The strength of ceramic mold material can resist impact force when pouring molten metal and easy take casting out from ceramic mold.

Synthesis of mullite aggregates from fly ash: effect on thermomechanical behaviour of low cement castables

Refractory aggregates were synthesised from beneficiated fly ash by reaction sintering with calcined alumina at 1600°C, and 83% mullite yield was achieved. The aggregates had low porosity, low thermal expansion and good refractoriness. To study compatibility in castable refractories, the aggregates were used in high alumina cement based low cement castables and their thermomechanical behaviour was studied. Microstructural characterisation revealed that the emergence of new bond phases such as mullite and calcium hexa-aluminate had a beneficial effect on the hot modulus of rupture and creep resistance of castables. An attempt was made to establish a structure-property relationship.