Refractory Castables Research Articles

MgO Influence on the Fundamental Characteristics of Corundum Refractory Castables

The paper focuses on the determination of how periclase content in the raw material influences the final properties of no-cement refractory castables. The motivation for this research was the continued pressure on increasing the utility properties of refractory castables, especially their corrosion resistance. The influence of MgO was observed in vibration-compacted refractory castables; the mixtures were made so that the consistency of each one was the same. There were only minimal differences in the water content of the mixtures. The mineral composition of the raw materials used in the manufacturing of the refractory castables was determined and the influence of the firing temperature on their compressive strength, flexural strength, apparent porosity, mineralogy and pore structure was described. The corrosion resistance was determined by means of the crucible test according to CSN P CEN/TS 15418 (726022) Method of test for dense refractory products – Guidelines for testing the corrosion resistance of refractories caused by liquids.

Effect of Multi Walled Carbon Nanotubes Incorporated on Refractory Castables

Two different types of multi walled carbon nanotubes (MWCNT) were introduced in a calcium aluminate refractory castable used in steelmaking process. The first MWCNT was purchased from Cheap Tubes (USA), a hydroxyl (-OH) functionalized, outer diameter of 50-80 nm, length of 10-20 μm, and, the second one was supplied by Nanocyl (Belgium), not functionalized, outer diameter of 9.5 nm, and length of 1.5 μm. Both types of MWCNT were incorporated into the dry castable mixture before water addition. The workability and properties of the final product, such as apparent density, apparent porosity and hot modulus of rupture were evaluated. The nanocarbons were characterized by SEM, TEM, TGA and Raman spectroscopy. In both cases, the incorporation of MWCNT had a negative effect on the rheology of wet castables, decreasing their workability. No significant improvements on mechanical properties of the castables were observed with the incorporation until MWCNT concentration reached 0.15 % (m/m), except for the increase of hot modulus of rupture observed for Cheap Tubes MWCNT introduction.

Effect of B4C on the properties and microstructure of Al2O3 -SiC-C based trough castable refractories

The effect of B4C addition (0.5wt%, 1wt%, 1.5wt%, 2wt%) on properties and microstructure of Al2O3-SiC-C (ASC) based trough castables was studied in this paper. The results showed that the bulk density of samples after firing at 1450°C decreased when the addition of B4C was over 1wt%. The cold modulus of rupture of samples improved significantly with the addition of B4C when the treating temperature was over 1100°C. Among them, sample with 1.5wt% of B4C had the best anti-slag performance and thermal shock resistance. The SiC fibers were formed in the non-oxidized zone of ASC castables with addition of B4C after firing at 1450°C in air condition. The obtained fibers rooted in the walls of inner micropores of samples with the diameter of 50–200nm, and the lengths of 10–50µm. The experiment revealed the nucleation of SiC started at about 1000°C, and then the growth of SiC fibers happened at higher temperature in a favorable condition. A stepped twin structure perpendicular to the growth direction of fibers could be observed. Finally, the growth mechanism was interpreted by thermodynamic calculations.

The role of MgO in the thermal behavior of MgO–silica fume pastes

The compounds of MgO–silica fume (SF) pastes constitute magnesium silicate hydrate (M–S–H) in a new generation of basic castables. However, Mg(OH)2 is a common reaction product with the formation of M–S–H. This study aims to reduce the formation of Mg(OH)2 in MgO–SF pastes. In this study, MgO powders were prepared by calcining magnesite at different temperatures and then mixed with SF and water to prepare MgO–SF pastes. The properties of MgO powders were characterized, and the pH values in the pore solutions of MgO–SF pastes were measured. The MgO–SF pastes cured for 90 days were calcined at 500, 700, 900 and 1200 °C, and the microstructure was characterized afterward. The results showed that both the reactivity of MgO powders and the pH value of the pore solution of MgO–SF pastes were diverse, which essentially depended on the grain sizes and the crystalline degree of MgO. Increasing the calcination temperature of MgO was beneficial to reduce the formation of Mg(OH)2 or even stop it when using MgO calcined at 1450 °C. Enstatite and forsterite formed for all MgO–SF pastes after calcination. However, the microstructure of MgO–SF paste with MgO calcined at 1450 °C was denser than others. MgO–SF pastes were suitable for the new-generation refractory castables. Notably, using MgO calcined at 1450 °C is more appropriate.

Thermal shock resistance of a refractory castable containing andalusite aggregate

One of the most important aspects to be evaluated when predicting the performance of refractory materials is their thermal shock resistance, especially considering the thermomechanical loads to which they are subjected. The methods currently used to predict thermal shock resistance behavior are based on Hasselman's studies, which take into account various experimental parameters. Thermal shock resistance can also be evaluated experimentally by applying thermal shock cycles and measuring the residual elastic modulus after each cycle. To predict and evaluate this resistance, test specimens of a commercial castable were cast, cured, dried and fired at 1000°C and 1450°C for five hours. Using Hasselman's parameters, the refractory fired at 1450°C was predicted to present lower thermal shock resistance, which was confirmed by experimental thermal shock tests. The results indicated that the modulus of elasticity and modulus of rupture decreased by about 72% in specimens fired at 1000°C, and by 82% in those fired at 1450°C. Therefore, the specimens fired at 1450°C showed lower thermal shock resistance. This finding underscores the importance of evaluating the behavior of castables with different microstructures resulting from different firing temperatures.

Carbon/Air Thermal Battery: New Materials and Performance

Elemental carbon may be used to generate electrical power through the reaction C + O2 = CO2 in galvanic cells. There is particular interest in carbon conversion in primary cells because of potentially high coulombic capacity (8.9 kAh/kg-C) and efficiency above 60%- ΔH° in molten carbonate electrolyte at 725 °C (1,2). We report two key technical developments: [1] Novel composites based on calcium aluminate were used to make low-cost battery and laboratory components resistant to molten carbonate corrosion. [2] Efficiency vs. power for cells fueled by renewable charcoal and charcoal/fossil-carbon composites were determined by simultaneousmeasurement of anode off-gas composition and anode potential. Concrete bound by calcium aluminate (CA) cement has long been used in crucibles to transport molten aluminum from smelter to casting plant. These materials resist corrosion by sodium- and potassium carbonates that result from oxidation of trace alkali metals that are co-reduced with aluminum. We modified the structure and composition of industrial materials proven to resist corrosion and with minimal dimensional change at 1250 °C to allow production of battery components by low-cost casting techniques (3). This entailed some sacrifice of strength. We confirmed corrosion resistance of our modified structure using the tri-eutectic (Li, Na, K)2CO3 at 800 °C by prolonged exposure (175 h) and rapid temperature cycling. Our composite accepts common additives (e.g., corn starch) to increase porosity. Permeability and connectivity may also be adjusted; overall strength may be increased with the use of reinforcing materials. (3) We report Darcy's-law constants, porosity, conductivities, microstructure and other properties important to component design. The raw material costs for crucibles for carbon batteries is below $2.00/kg. Such low cost offsets the disadvantage of low power density (50-150 mW/cm2-cross-section) typically found for carbon/air cylindrical cells. Total energy efficiency of a C/O2(air) cell is the product of coulombic efficiency (effective carbon valence (n) divided by 4) and cell voltage (4). Assuming carbonate ion transference number is unity, effective valence is a simple function of mole fraction, xCO2 = CO2/(CO2 +CO), given by the equation, n = 2 (1+x)/(2-x) (5). Many researchers measure off-gas composition and cell voltage to project efficiencies greater than 60%, but rarely are such determinations made simultaneously, as previously reviewed (6). In our approach, a sample of off-gas is collected in a flow-through syringe and transferred into an inverted graduated cylinder initially filled with 1 M NaOH. Anode potential is continuously monitored. Mole fraction of CO2 is inferred from the reduction of volume (corrected for CO solubility) associated with selective hydrolysis of the CO2 to form sodium carbonate. The extraction is linear with time for excess base and CO2, the reaction being controlled by the first (slow) step in hydrolysis: CO2 + OH- = HCO3 - (7) A similar process is used in the industrial separation of CO from CO2. Base-catalyzed hydrolysis of CO is known, but is far too slow to interfere at ambient temperature. We examine applications in off-grid communities for powering cell phones and household electronics, LED lighting and small appliances (e.g., hand tools or thermoelectric refrigerators), using modular units of approximately 50 W each. Discharge is continuous; unused energy is stored as phase-change ice packs or in large appliance batteries. Potential is stepped from parallel-connected cells to an output of 5.1 VDC. No separate storage batteries are required. Applications are also envisioned for remote or off-grid sensors (e.g., SCADA, LIDAR), where the power source is available continuously regardless of climate, latitude or time of year. References J. F. Cooper, J. R. Selman, K. Witt and C. Jacobson, "Refuelable Carbon/Air Battery for Off-Grid Power," Tech Connect Innovations and Expo; June 2015, Washington DC; (Recipient of National Innovation Award).K. Hemmes, J. F. Cooper, J.R. Selman, I. J. Hydrogen Energy 38 8503 (2013).C. Wohrmeyer, C. Alt, N. Kreuels, C. Parr, M. Vialle, "Calcium aluminate aggregates for use in refractory castables", (Paper TP-GB-RE-LAF-016, 35th American Ceramic Soc. Symposium, St. Louis, MO, March 1999). J. F. Cooper and J. R. Selman, I. J. Hydrogen Energy, 37 19319 (2012).J. F. Cooper and J. R. Selman, ECS Trans. 19(14) 15 (2009). J. F. Cooper and J. R. Selman, I. J. Hydrogen Energy 39 12361 (2014).A. F. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (John Wiley & Sons, NY (1966)).

Effect of zirconia particle size on the properties of alumina-spinel castables

The industrial application of alumina-spinel refractory castables has crucial requirements on the service performance. Thus, the effects of different sized desilicated zirconia particles on the castables microstructure, thermal-mechanical properties and high temperature elastic modulus have been investigated. The zirconia particle sizes were varied from 1000µm to 2.5µm (d50). It was observed that the finer (below 88µm) zirconia particles were beneficial to improve the cold modulus of rupture (CMOR) and the hot modulus of rupture (HMOR), but could not effectively enhance the thermal shock resistance. Fine zirconia particles can homogeneously disperse in the matrix and significantly promote the sintering process. Accompanied with the phase transformation of zirconia, both the high density of matrix cracks and the strong ceramic bonding (between the coarse grains and the matrix) were found in the refractory castables, which was responsible for an increase of CMOR. However, the binding characteristic could also give rise to the high stored elastic energy that was adverse to the thermal shock resistance, and the excessive amount of preexisting matrix cracks could induce more microdamage during the thermal shock. Additionally, it was proposed that the second-phase dispersion reinforcement and the highly ceramics bonding resulted in the superior HMOR when introducing fine ZrO2 particles.

Influence of Contents of Pre-Synthesized Spinel on the Microstructures and Mechanical Properties of the Lightweight Corundum-Spinel Refractory Castables

Five kinds of lightweight refractory castables were prepared by using lightweight corundum-spinel aggregates (LCSA), pre-synthesized spinel (PSS), magnesia and corundum powders as raw materials. Effects of the PSS contents on the microstructures and mechanical properties of the refractory castables were studied by means of X-ray diffractometer (XRD), elastic modulus tester and scanning electron microscopy (SEM), etc. It is found that with the PPS content increase from 0 to 15.3 wt %, the permanent linear change (PLC) of castables sintered at 1600 °C decreases from 1.02% to -0.56%; the bulk density (BD) and apparent porosity (AP) are in the range of 29.6 %~27.8 % and 2.71~2.79 g/cm3, respectively, changing a little; the cold compressive strength (CCS) decreases from 106.0 MPa to 94.4 MPa; the thermal shock resistance improves; the hot modulus of rupture (HMOR) decreases from 6.7 MPa to 4.3 MPa. Considering these aspects, sample with 7.6 wt% PPS (LA7.6) has the best performance.

Molten salt synthesis of a SiC coating on graphite flakes for application in refractory castables

A silicon carbide coating was formed on the surface of graphite flakes by reaction of molten Si with carbon at 1100–1300°C in a 95%KCl-5%NaF molten salt under Ar atmosphere. The effect of temperature and Si/graphite ratio in the initial mixture on the quality and the amount of SiC were investigated by XRD and SEM/EDS analyses. Also, the water wettability, oxidation resistance and zeta potential of un-coated and coated graphite were examined by TGA analysis and sedimentation test. The results show the amount of coating to increase in the coated flakes with increasing temperature and Si/graphite ratio. The SiC coating improves water wettability of graphite and acts as a protective layer to enhance oxidation resistance. The zeta potential of coated graphite was also increased which indicated a better dispersion in water based systems. These improvements in both the water dispersivity and oxidation resistance of SiC coated graphite would make it as promising candidate raw materials for application in C-containing refractory castables.

Microstructure evolution during the heating process and its effect on the elastic properties of CAC-bonded alumina castables

In this paper, the alumina-based refractory castables were prepared using calcium aluminate cement (CAC) as binder. The phase composition and microstructural evolution of the castable samples during the heating process were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), and thermal expansion analysis. The relationship between the microstructural characteristics of castables and the properties of elastic modulus was investigated. The results showed that, after the castables were cured at room temperature for 24h and dried at 110°C for 24h, the main phases present after hydration were Al2O3, C3AH6, and AH3. C3AH6 and AH3 would be transformed into C12A7 and AlO(OH) or Al2O3, as temperature increased from 110°C to 800°C, the microcrack caused by decomposition of the hydrates will led to the lower values of both the elastic modulus and strength of the castable samples. From 900°C to 1000°C, CA was recrystallized through the reaction between C12A7 and A, resulting in an increase in elastic modulus and strength. Between 1100°C and 1200°C, CA2 would be formed, which resulted in a volume expansion of the material and a slight increase in elastic modulus. Finally, the CA6 phase was formed as the temperature increased to 1400°C. The enhancement of the strength and elastic modulus of the castable samples could be attributed to their strong interlocking bonds between platelet CA6 and matrix particles at high temperature.

Enhancing the green mechanical strength of colloidal silica-bonded alumina castables using a silane coupling agent

The use of colloidal silica as a binder for refractory castables has attracted great interest in recent years. Nevertheless, the resultant low green mechanical strength of colloidal silica-bonded castables has hindered their application in relevant areas. A silane coupling agent, 3-methacryloxypropyltrimethoxysilane (CH2=C(CH3)COOC3H6Si(OCH3)3), normally called KH-570, has been added to colloidal silica-bonded alumina castables to enhance their green mechanical strength. The results show that KH-570 has a positive effect on the green mechanical strength improvement of colloidal silica-bonded alumina castables. The reaction between hydration products of KH-570 and the solid particles of colloidal silica (confirmed by Fourier transform infrared spectroscopy (FTIR)) could promote the formation of an organic-inorganic interpenetrating network structure, which is the main reason for the green mechanical strength improvement. Meanwhile, the water consumption by KH-570 hydration and the hydration products of methanol accelerate the gelling process of colloidal silica. This also has some positive influences on the early strength enhancement.

Elastic and Mechanical Fatigue at High Temperatures of High-Alumina Castables with Addition of Partially Stabilized Zirconia

This work evaluates the impact of added zirconia on the thermomechanical and thermoelastic fatigue properties of high-alumina refractory castables. Three testing scenarios are presented in the study: 1) Resonant Frequency Damping Analysis (RFDA) after cyclic temperature changes with and without mechanical load, 2) Refractoriness under Load (RuL) tests, and 3) results of abrupt temperature shifts between two high temperatures in a special thermal shock furnace. The tested refractory formulations were a reference castable based on tabular alumina and two castables each containing 13.75 mass-% of zirconia. The formulations of the zirconia castables had an addition of either fully-stabilized zirconia doped with 8 mol-% yttria (Y-FSZ) or partially-stabilized zirconia doped with 3 mol-% calcia (Ca-PSZ). The Y-FSZ castable displayed elastic and mechanical behaviour by means of RFDA and RuL similar to the reference sample and had good recovery after thermal shock testing. The Ca-PSZ formulation showed some beneficial toughening through energy dissipation of progressing cracks, but after cooling exhibited the matrix fracturing influence of the martensitic zirconia phase transformation which would negatively impact service life of the refractory.

Corrosion of Carbon Free and Bonded Refractories for Application in Steel Ingot Casting

This phenomenological study investigates the corrosion of refractories by a highly corrosive steel (1.6587, 18CrNiMo7‐6) with a high aluminum content and casting temperature of 1580 °C. The applied refractory castables with matrices based on alumina, mullite, and zirconia/titania doped alumina (AZT) are carbon free or low carbon (4 wt%) containing with and without nanoscaled additives. The corrosion is analyzed mainly by microscopy after the corrosion tests. The carbon containing samples are negligibly corroded due to inhibited wetting. The nanoadditives in the carbon containing samples show no influence on the corrosion. The carbon free AZT is attacked most strongly with a corrosion layer of about 14 mm. In the alumina (corrosion layer about 6 mm) and AZT sample, compositions corresponding to manganese aluminates form with manganese from the steel. When also silicon diffuses into the refractory, compositions referring to manganese aluminosilicates form. In the mullite matrix crucible (corrosion layer about 1 mm) compositions corresponding to manganese aluminosilicates form directly with manganese from the steel resulting in a highly viscous melt at the interface which retard the further attack. For a future final evaluation, however, also the steel quality has to be taken into account as will be studied by an Aspex‐SEM.

The Preparation of Heat Insulating Moulded Refractories

The thesis deals with the study of influence of the foaming agent Schäumungsmittel W 53 addition on the specific weight and mechanical characteristics of mullite based heat insulating refractory castables. The effect of dosing and mixing time was observed in this work. The influence of partial substitution of mullite matrix by kaolin was also studied. Kaolin was added in dry matter or in the foamed state. Optimal dosage of the agent was 0,25 % and the optimal mixing time was 10 min. The addition of kaolin was the most effective in the foamed state.

The relationship between the pore size distribution and the thermo-mechanical properties of high alumina refractory castables

AbstractA series of high alumina refractory castables were prepared via casting with tabular alumina aggregates (0.088 – 5 mm), fine powders, and calcium aluminate cement as starting materials. The effect of the median diameter of alumina (D50 = 7.26 μm, 5.33 μm, 2.37 μm) on the thermo-mechanical properties of the refractory castables was investigated. The results indicate that the decrease in the alumina particle size from 7.26 μm to 2.37 μm has little influence on both the apparent porosity and bulk density of castables. However, the median pore size of castables fired after 1 600 °C decreased drastically from 4.7 μm to 2.4 μm correspondingly, which led to significant growth in the strength and thermal shock resistance of castables. The cold modulus of rupture and crushing strength were increased by 201 % and 120 %, respectively. At the same time, the hot modulus of rupture and elastic modulus were also increased by 143 % and 127 %, respectively. The residual elastic modulus was enhanced twice after three thermal cycles.

Formation, leachability and encapsulation of hexavalent chromium in the Al2O3–CaO–Fe2O3–Cr2O3 system

Refractory castables can contain Cr2O3, either as a specific addition or as a impurity element in the raw materials. When calcium aluminate cement is used as a binder in such castables, Cr(III) can transform to Cr(VI) during use, which can cause subsequent problems with the disposal of the used refractory lining. The formation of Cr(VI) in Al2O3–CaO–Fe2O3–4wt% Cr2O3 mixtures, representing a simplified refractory castable system, was investigated as well as its leachability. Samples were prepared from reagent grade chemicals, equilibrated at 1300°C in air and quenched in liquid nitrogen. A Cr(VI)-containing phase Ca4Al6CrO16 (hauyne) formed in all the examined samples. During the ASTM D3987-85 leach test the co-existing C3A, C12A7, Ca2(Al,Fe)2O5 and CA phases encapsulated the Cr(VI) by forming Cr(VI) analogue phases 3CaO·Al2O3·CaCO4·nH2O. These phases decreased the leachability of Cr(VI) to below the EPA limit of 5mg/l. Decreasing the pH and increasing the temperature can however remobilise the Cr(VI).

Improving in situ spinel refractory castables using a novel binder

The objective of this paper is to find an efficient binder by investigating binders such as fireclay, white Portland cement and gypsum with different compositions, and ultimately to introduce a suitable binder composition. Evaluation of physical properties and microstructure of castables were performed by mechanical (splitting tensile strength) test, porosity measurements, bulk density measurements, X-ray diffraction spectroscopy and scanning electron microscopy. The results revealed that the performance of the new binder is much better than that of conventional binders such as calcium aluminate cement, because it creates strong bonds between the particles and improves the in situ spinel formation and sintering/diffusion of particles with each other at high temperatures. Analyzing the prepared samples with new binder also demonstrated that the increase of refractory material in binder composition, leads to creation of the strong and cohesive phases and bonds between the particles (such as mullite), and therefore, increases the driving force of in situ spinel phase formation and ultimately improves the performance of refractory masses at high temperatures.

The impact of bonite aggregate on the properties of lightweight cement-bonded bonite–alumina–spinel refractory castables

The lightweight bonite–alumina–spinel (CA6–Al2O3–MA) refractory castables with bonite aggregate and different spinel sources (pre-formed and in situ formation) were prepared in this study. The phase composition, microstructural features, and mechanical and thermo-mechanical properties of CA6–Al2O3–MA castables treated at various temperatures were investigated by techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending method, and thermal shock test. The results indicated that the incorporation of bonite aggregate had a positive influence on the strength, thermal shock resistance and slag corrosion resistance. It especially decreased the thermal conductivity and had a slightly negative influence on the refractory under load and slag penetration resistance of the castables. For the in situ spinel-containing castable, the formation of in situ spinel with finer particle sizes and acicular CA6 grains led to higher overall volume expansion, resulting in higher thermal expansion (∆L/L0), linear change and the apparent porosity of castables. Also, the heat insulation, thermal shock and slag penetration resistance of castables with in situ spinel improved, while the strength, displacement, refractory under load and slag corrosion resistance decreased sharply.

Mg(OH)2 Nucleation and Growth Parameters Applicable for the Development of MgO‐Based Refractory Castables

MgO is a very attractive raw material for refractory applications. However, its use has been mainly aimed at developing high‐magnesia and magnesia–carbon bricks due to the marked hydration likelihood of this oxide and the related drawbacks (volumetric expansion, crack formation) associated with this transformation. This work aims to evaluate some critical aspects that affect the MgO reaction with water (magnesia source, particle size, concentration of available sites for brucite nucleation, influence of a hydrating additive – acetic acid, and others) during the curing and drying steps of Al2O3–MgO binder‐free refractory castables. The attained experimental results were associated with the boundary nucleation and growth model proposed in the literature. According to the in situ elastic modulus measurements carried out at 110°C, the MgO particle size and reactivity present an important effect on the nucleation and growth of brucite crystals, highlighting that a proper site activation should be induced during the castables' curing process in order to effectively allow the use of magnesia as a binder source for refractories. Faster Mg(OH)2 nuclei generation also helps to minimize further growth of these crystals, leading to a decrease in the samples' porosity and, consequently, a continuous increase in the overall refractory stiffness (E values).

Development of Multi-Walled Carbon Nanotube and Processed Graphite Incorporated Hydraulically Bonded Refractory Castable

The incorporation of carbon materials is known to significantly enhance both corrosion and thermal shock resistance of refractories. For mixing and placement of refractory castables, water is the most suitable liquid media due to its availability, cost, non-hazardous characteristics and ease of handling. The main factors limiting the development and use of carbon-containing castables are the inferior wettability of carbon by water. Dispersion of carbon-containing materials in water with and without addition of surfactant was tested. By varying the carbon source and keeping the quality and quantity of all other raw materials constant, zero cement castable (ZCC) compositions were designed. The samples were coked and physical properties, oxidation resistance, and pore sizes were measured and FESEM studies carried out. The properties of castables containing processed graphite (PG) and multi-walled carbon nanotubes (MWCNT) were found to be most favourable for the iron and steel industry.