Refractory Castables Research Articles

Effect of Cr2O3 addition on corrosion mechanism of refractory castables for waste melting furnaces and concurrent formation of hexavalent chromium

Cr2O3 containing refractories are used as the linings of various waste melting furnaces. Formation of hexavalent chromium (Cr(VI)) has been a major concern despite its admirable corrosion resistance properties. Al2O3-CaO-Cr2O3 refractory castables (0, 4%, 8%, and 12wt% Cr2O3) were fabricated in reducing condition to inhibit Cr(VI) formation while subsequent corrosion tests using waste slag was performed in air atmosphere to simulate the operating conditions. The corrosion mechanism between Cr2O3-containing corundum castables and synthetic waste slag were investigated by XRD and SEM-EDS, whereas concurrent formations of Cr(VI) were assessed effectively by leaching tests. Corrosion of castables (0wt% Cr2O3) occurs via dissolution of Al2O3 and CA6 phases, and formation of Ca2(Al,Mg)(Al,Si)2O7 and (Ca,Na)(Al,Si)2Si2O8, which were significantly restricted by the formation of Al2O3-Cr2O3 (III) solid solution (4%, 8%, and 12wt% Cr2O3). Most of the specimens exhibited less than 5mg/L (EPA limit) of Cr(VI). Only specimen M3 (12wt% Cr2O3) from slag-refractory interface (stage C) leached out 5.41mg/L of Cr(VI) which is above the EPA limit. Specimen M2 (8wt% Cr2O3) exhibits optimum properties in terms of corrosion and Cr(VI) formation.

Self-reinforced high-alumina refractory castables

Among the available routes to enhance the mechanical behavior and toughening degree of ceramics, in situ formation of acicular or platelet grains in the resulting microstructure has gained special attention. Various studies focused on investigating the self-reinforced role of these elongated phases (acting as bridging sites in the wake of a crack), as well as the use of very reactive raw materials to favor faster and more effective sintering/densification, have been carried out for compositions comprised mainly by fine raw materials. Nevertheless, there is a lack of studies considering more complex ceramics, such as refractory castables, which commonly present heterogeneous microstructures with coarse and fine components. Based on that, this work addresses the evaluation of self-reinforced high-alumina refractories containing reactive aluminas (AloxX spheres and/or submicron alumina) and calcium aluminate cement as binders. Boron carbide and microsilica were also added to some of the prepared compositions to favor the in situ formation of Al18B4O33 or CA6 phases with needle-like morphology. Various experimental evaluations (cold and hot mechanical strength, hot elastic modulus, SEM, XRD, thermal shock resistance, refractoriness under load, etc.) were carried out to identify the influence of the new phases’ formation in the overall performance of such materials. According to the attained results, both mineralizing agents (B4C and SiO2) induced the formation of Al18B4O33 or CA6 grains in the designed compositions. However, a remaining liquid content was still present in these samples at a high temperature, resulting in some drawbacks concerning their hot properties. Additionally, submicron alumina proved to be an interesting binder as it induced improved sintering and led to a refractory with high stiffness (~200GPa), high thermal shock resistance and mechanical strength.

Matrix microstructure optimization of alumina-spinel castables and its effect on high temperature properties

In this paper, submicron spinel-containing calcium aluminate (SP-CA) cements were prepared at temperatures of 1200°C (RA12), 1300°C (RA13), and 1450°C (RA14) using reactive alumina, magnesium hydroxide, and calcium carbonate as raw materials. The microstructural characteristics and high temperature properties of refractory castables bonded by SP-CA cement were investigated. The results indicated that the mineralogical compositions of SP-CA were spinel (MgAl2O4, MA) and CaAl2O4 (CA). The spinel phases presented no definite borders or shapes, with average size ranging from 0.1μm to 1.2µm at different sintering temperatures. After heating alumina-spinel castables, the submicron spinel particles bonded with corundum and hibonite (CA6) grains could strengthen the castable body. However, the large sized (< 88µm) spinel did not form links between spinel and alumina in the castable with added preformed spinel (castable AS). Submicron spinel can decrease the pore sizes of refractory castables from 2µm to 0.5µm. Extended finite element method (XFEM) calculations showed that smaller pores prevent stress concentration and result in increased deformation with limited crack propagation. Hence, large numbers of 0.5µm pores can act as crack-stoppers to improve the thermal shock resistance of castables with added SP-CA cements. Thermal shock resistance tests showed that R'''' of castable RA12 was 39.7m, which is significantly higher than that of castable AS (19.8m). The corrosion rate of castable RA12 was 5mm2/min, which is significantly lower than that of castable AS (7.4mm2/min). Small pore size and submicron spinel played relevant roles in corrosion behavior.

The Impact of &lt;i&gt;In Situ&lt;/i&gt;-Formed Spinel on the Porosity of No-Cement Refractory Castables

The paper focuses on the determination of how in-situ formed magnesia-alumina spinel (MA spinel) content in corundum refractory castables influences the final properties and pore size distribution of no-cement refractory castables. The motivation for this research was the continued pressure on increasing the utility properties of refractory castables, namely their corrosion resistance to blast-furnace slag. The influence of MA spinel content was observed in vibration-compacted refractory castables (three mixtures). The corrosion test was performed to determine the dependence of the size of the slag-penetrated area on MA spinel content in the specimen. The pore structure of a slag-penetrated NCC and an unaffected castable was described.

B4C mineralizing role for mullite generation in Al2O3-SiO2 refractory castables

Abstract Based on refractory end-users’ requirements, continuous efforts have been made to design engineered products able to withstand high temperatures (800–1500 °C) and severe thermal gradients. One alternative to enhance the mechanical properties of alumina-based compositions consists of inducing in situ generation of phases with platelet or acicular shape within their matrix fraction, which may improve crack deflection and grain bridging mechanisms. Mullite and Al 18 B 4 O 33 are some compounds that present such interesting features. Thus, this work addresses the evaluation of alumina refractory castables bonded with SioxX-Zero and/or microsilica, containing 0 or 1 wt% of B 4 C (sintering additive), aiming to: (i) induce transient liquid sintering, (ii) point out which silica source could favor a more effective mullite formation at high temperatures, and (iii) analyze the influence of B 4 C in the phase transformation and thermo-mechanical properties of the designed compositions. Comparing SioxX-Zero and microsilica-bonded refractories, the latter showed more likelihood to give rise to the mullite phase during the samples’ thermal treatments. Moreover, adding B 4 C to the castables containing 3 wt% of SiO 2 induced the generation of a boron-rich liquid phase with transient features during the samples’ firing step, favoring earlier sintering and faster mullite and Al 18 B 4 O 33 formation. These transformations resulted in refractories with enhanced creep, thermal shock resistance and HMOR behavior in a broader temperature range (600–1550 °C), which may enable them to be used in various industrial applications (petrochemical, steel-making, etc.).

The effect of carbon and polypropylene fibers on thermal shock resistance of the refractory castable

This work investigates medium cement refractory castable with additives of carbon and polypropylene fibers. The peculiarities of microstructure changes in the fiber and castable matrix contact zone, channel formation, and changes of cold crushing strength of fiber additives, which have a refractory castable matrix under temperature treatment, were investigated. The investigation results allowed to predict that using a mix of fibers more effective than using them indi­vidually. The influence of fiber additives on the mechanical characteristics and thermal shock resistance of the refractory castable with fiber additives was tested. It was found that the addition of carbon fiber has a positive impact on the ther­mal shock resistance of the investigated castable, which is confirmed by the results obtained by thermal cycling, as well as by the values calculated for thermal shock resistance R4 and Rst. In addition, the results of the investigation of thermal cycling show that the value of the thermal shock resistance was highest when a mixed fiber additive (CF+PP) was used.

Boron sources as sintering additives for alumina-based refractory castables

Abstract Sintering additives (SA) have great potential to be used in refractories to enhance their thermo-mechanical properties at intermediate temperatures (600–1200 °C). Boron-based compounds are pointed out as suitable options for this purpose as they undergo phase transformations (i.e., decomposition, oxidation, etc.) at relatively low temperatures, which may lead to boron-rich liquid phase generation and transient liquid phase sintering. Considering these aspects, the present study aims to evaluate the role of five different SA [boron oxide (B 2 O 3 ), boric acid (H 3 BO 3 ), sodium borosilicate (BS), magnesium borate (BM) and boron carbide (B 4 C)] when 0.5, 1.0 and 2.0 wt% of these materials were added to alumina-based castable compositions bonded with hydratable alumina. Flowability, apparent porosity, XRD, hot elastic modulus, thermal shock resistance and mechanical tests were carried out in order to identify whether (i) transient liquid phase sintering would be favored in these refractories, and (ii) at which temperature range such compositions might be submitted during service and still present an improved performance when compared to an additive-free castable (reference). Based on the results, incorporating boron oxide into the alumina-based castables was not recommended as this hygroscopic material reduced the flowability of the compositions, preventing a suitable preparation of the samples. The other boron-containing compounds could be directly added to the high-alumina mixtures without resulting in major changes in their processing steps. Nevertheless, the paper highlights the importance of selecting a proper SA source and optimizing its content in the castable composition as these parameters may affect its overall rheology and thermo-mechanical properties. The formulations containing 0.5 wt% of H 3 BO 3 or B 4 C, 1 wt% of sodium borosilicate and 2 wt% of magnesium borate presented enhanced erosion and thermal shock resistances and hot mechanical strength at 1000 °C and 1200 °C, which makes them potential refractory castables to be used in petrochemical and aluminum industries.

Effect of surface‐treated graphites on the properties of carbon‐containing monolithic refractory materials—A comprehensive comparison

AbstractThe performance of alumina‐ and magnesia‐based carbon‐containing refractory castables, suitable for different sectors of steel plants, has been compared. Natural flaky graphites have been modified by their surface treatment via sol‐gel route. The sustainability of graphite in oxidizing atmosphere was noticeably increased by this method with a synergistic possibility of pollution abatement. The hydrophilic nanocoatings of ceramic oxides derived by inorganic precursors reduced the water demand of the composite batch and simultaneously enhanced its oxidation resistance. The oxide coating compositions together with the precursor chemistry have been varied to substantiate their common beneficial features. Scanning electron microscopy (SEM), Raman spectroscopy, Differential thermal analysis (DTA), Thermogravimetry (TG), X‐ray diffraction, thermal conductivity, and zeta‐potential studies have been carried out to compare the influence of heat treatment on as‐received and surface‐modified graphites. The properties of refractory castables, namely, bulk density, apparent porosity, and cold crushing strength have been measured within wide temperature region (110–1500°C), to confirm the positive influence of nanoengineering on graphite surface. The sequence of important phase reactions during sintering of both the composite materials has been discussed. Finally the slag resistance behavior at aggressively fired condition has been investigated to corroborate the improved matrix characteristics due to coated graphites.

Effect of submicron spinel powders on the microstructure evolution and properties of alumina‐based refractory castables

AbstractThe physical properties, including flow value, apparent porosity, and mechanical strength, increased significantly with submicron spinel contents in this study. The excellent physical properties can be attributed to the close‐packed microstructure and high bonding strength between spinel and aggregates/matrix. Additionally, the submicron spinel powders promoted the formation of tabular CA6 grains that generated a cross‐linked structure and enhanced the mechanical properties. The microstructures of corroded castables suggested that submicron spinel powders contributed to the formation of a continuous and thick spinel layer at the slag–refractory interface and subsequently improved the slag resistance of the castables.

Maximum working temperature of refractory castables: do we really know how to evaluate it?

Abstract Refractoriness under load (RUL) tests are usually indicated to evaluate the maximum working temperature that refractory materials can withstand by measuring their linear dimensional change when subjected to compressive stress. Nevertheless, ASTM C832 and ISO 1893 standards do not point out which sort of samples (pre-fired or not) should be used during the measurements of this property. Thus, this study addresses the evaluation of different refractory castable systems, focusing on analyzing calcined (fired up to 600 °C for 5 h for the decomposition of the hydrated phases) or pre-fired (1550 °C for 5 h) samples in order to identify the effect of the material's thermal treatment on the RUL results. Additional properties (i.e. linear expansion change, apparent porosity and cold flexural strength after firing at different temperatures) of the designed compositions were also evaluated. Based on the results, it can be concluded that the maximum working temperature of monolithics should not be assessed via one single test (i.e. RUL), but only a set of techniques (RUL, dilatometry, assisted sintering, creep and others) can provide useful data to better understand the refractories performance. RUL measurements should be carried out after a preliminary sintering step of the castables (where the selected firing temperature might be as close as possible to the expected T0.5) to enable most of the microstructural changes resulting from heating up and sintering could already have taken place. Furthermore, it is most likely that numerical solutions (modeling) could help to better understand and interpret the multivariable aspects of this complex issue. However, reliable and accurate values of the refractory properties (based on well-selected tests) must be fed to the software to permit calculations that yield meaningful and accurate results, and conclusions.

Size-dependent hydration mechanism and kinetics for reactive MgO and Al2O3 powders with respect to the calcia-free hydraulic binder systems designed for refractory castables

This work investigates the time-dependent hydration behavior of 1:1 molar ratio mixture of reactive α-Al2O3 nano- or micro-powder and reactive MgO in relation to the development of new hydraulic binder systems for in situ spinel refractory corundum castables. Mg–Al–CO3 hydrotalcite-like phases formed in the cementitious matrices cured in the corresponding curing condition up to 280 days of age at 50 °C were subjected to morphological, chemical and structural characterization using XRD, FT-IR, NMR, SEM and TEM techniques. The observations indicate that both the short-term cured MgO–nano-Al2O3 cementitious matrix and the long-term cured one containing micro-Al2O3 contain mainly OH–(Mg3) hydroxyl group, whereas the long-term cured MgO–nano-Al2O3 cementitious matrix is dominated by OH–(Mg2Al) hydroxyl group. The thermal stability of hydrotalcite-like compounds (Mg0.667Al0.333(OH)2(CO3)0.167(H2O)0.5, Mg6Al2CO3(OH)16·4H2O) (HTlcs) through combined DTA–TGA–EGA–MS analyses shows that HTlcs undergo dehydration first and then dehydroxylation overlapping with decarbonization processes. According to EGA–MS and FT-IR measurements, the Mg–Al cementitious matrices were progressively carbonated over curing time and then interlayer water molecules were preferably coordinated by interlayer CO32− ions that exist in a complex network of hydrogen bonds between octahedral layers. In the near-fully-hydrated binder paste, dehydration of crystalline water molecules occurs at 262 °C and the thermal stability of hydroxyl groups in HTlcs is OH–(Mg2Al) (422 °C) > OH–(Mg3) (337 °C) at a ramping rate of 10 °C/min. The basic information provided by the microcalorimetric measurement is such that the starting grain size significantly affects the heat flow curve. From the SEM and TEM images, the hydrates are found to be hexagonal in shape.

High Temperature Erosion of Dense Refractory Castables for CFBC Boilers

The erosion resistance of dense refractory bricks or castables used in CFBC boilers is tested as per ASTM C 704. This method of testing determines the relative erosion of a refractory at room temperature only. Hence, solid particle erosion at elevated temperatures is desired in selecting the suitable refractory quality for boiler applications. The dense refractory materials for CFBC boilers are usually eroded at operating temperature (∼900°C) by hot circulating solids at high temperatures. In this work, low cement castable−45, low cement castable−80 and an 80% alumina-based plastic refractory castable, three varieties of dense refractory castables used in CFBC boilers, were selected for testing of erosion properties. The erosion tests were conducted at room temperature and boiler operating temperature of 900°C at three different angles of impingement (30°, 45° and 90°) using specially developed high temperature erosion test equipment following the ISO 16349 (2015) standard. The erosion loss result values (in cm3) were compared, and this comparison is useful in selection of suitable material for various erosion prone zones of the boiler.

Lightweight Preheating Stands and Lids Used in Steelwork Trinec

In a wide range of metallurgical operations, steel casting or filling ladles and tundishes must be dried or preheated. Many types of preheating and covering lids are used in these processes. Technological facilities of this type are used in the Trinec melt shops for production and processing of steel. Refractory castables, dense or lightweight refractory bricks, or refractory fibre module systems can be used to line various types of lids. The article focuses on the particular advantages of fibre module systems applied in the construction of these types of linings. Fibre ensures low weight, easy maintenance and satisfactory service lifetime, which enables streamlined usage under busy steel production conditions. Indeed, fibre can be envisioned as a solution for an even larger range of lining systems.

Water Wettable Graphite through Nanotechnology and its Application in Refractories

Nanocarbon-coated graphite with high resin and water wettability characteristics can expand the application of refractories and carbon-carbon composites in different fields. Flaky graphite was made water wettable by coating nanocarbon on the surface of graphite by using suitable surfactants and dispersants. The coated graphite characteristics were confirmed by XRD, FESEM and HRTEM, which show that nanocarbon is coated on the graphite and it has the same nature as natural graphite. This developed coated graphite has wide applications in refractory brick, castables and masses for application in the steel industry. MgO-C bricks with lower (3 mass-%) carbon content, with nanocarbon-coated grains and coated graphite, have led to better properties than normal higher (10 mass-%) graphite containing bricks. Uniform distribution of graphite in the matrix is possible by using this developed graphite.

Refractory Monolithics versus Shaped Refractory Products

The share of the world’s gross refractory production due to monolithics has progressively increased. This trend can be attributed primarily to improved properties of new products and the introduction of new installation techniques. Development of advanced refractory castables is briefly described and the properties of monolithic refractories are compared with the properties of similar shaped products. The use of monolithics as a substitute for refractory bricks in different industrial high-temperature processes is discussed.

Effects of rare earth oxides additions on microstructure and properties of alumina-magnesia refractory castables

To accommodate the glaring requirements of castables with high performances for the iron and steel making industry, effects of rare earth oxides additions on microstructure and properties of alumina-magnesia refractory castables were investigated. The properties of castables including phase compositions, apparent porosity, permanent linear change and strength were compared. It was demonstrated that yttria and ceria presented varying degrees of influence on in-situ reactions and volumetric stability of castables. The solid-soluted content of spinel and calcium hexaluminate (CA6) was both dependent on the additions.

Self-flowing high-alumina phosphate-bonded refractory castables

Phosphate refractories have a great potential to be applied in petrochemical industries as they present suitable properties at the temperature range used in fluid catalytic cracking units. This study addresses the development of high-alumina self-flowing castables bonded with H3PO4 solution (48wt% concentration) or a mixture of phosphoric acid and monoaluminum phosphate (MAP) solutions, using MgO as a setting agent. Two polyphosphates (Budit 3H and 6H) and citric acid were evaluated as dispersant additives for these castables. The compositions were characterized by measuring their free-flow and temperature evolution over time, working and setting times, cold and hot mechanical strengths, drying behavior and explosion resistance, eroded volume and thermal shock resistance. The results indicated that high flowability (free flow >100%) could be attained when adding the selected polyphosphates to the mixtures, whereas citric acid acted mainly as a retarder agent for the castables’ setting. Moreover, free-flowing compositions with a suitable working time were obtained when combining H3PO4+MAP solutions as main binders. The thermo-mechanical tests pointed out that the most promising designed refractory (containing mixture of H3PO4+MAP and 0.5wt% of Budit 3H) presented similar or even a better performance than a benchmark commercial vibratable product used in petrochemical units.

Effects of Matrix Densification Based on Particle Packing on Lightweight Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-MgO Castables

The property of refractory matrix was dominated by both the maineral composition and particle packing behaviors. In this study, the theoretical packing density of refractory castables was calculated to design the particle size distribution (PSD) of its matrix. Four lightweight Al2O3-MgO castables with different matrix PSD (represented by q-value) were prepared and examined. Results show that a suitable q-value was needed to ensure acceptable properties including sintering characteristics, strength and slag resistance, which deteriorated distinctly at high q ( &gt; 0.31). For the sample with q = 0.28, the matrix showed dense and uniform mirostructure, and the properties of castable reached a favorable compromise among sintering characteristics (apparent porosity = 14.8%, bulk density = 3.02g∙cm-3, permanent linear change &lt; 0.6%), strength (cold modulus of rapture = 12.4MPa, cold crashing strength=155.5MPa), and resistance against both slag corrosion (Ic = 22.4%) and penetration (Ip = 11.5%). The sample with q = 0.25 showed the highest strength and resistance against slag corrosion, but its slag penetration resistance was lower due to the existence of cracks between aggregates and matrix.

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.