Arc melting plant for synthesizing and producing fusion-cast refractories

Data are given on arc melting plant for synthesizing and producing fusion-cast refractories. The technical characteristics of experimental plant type EDP-600 with moving electrodes are such as to allow one to synthesize fused materials on the basis of the oxides Al₂O₃, MgO, Cr₂O₃, and ZrO₂, and also allows one to produce refractory components for industrial glass-melting furnaces. The DSPM-1.5 arc furnace is recommended for the industrial production of fusion-cast baddeleyite-corundum, high-alumina, and chromium-bearing refractories.     

In situ hot elastic modulus evolution of MgO–C refractories containing Al,Si or Al–Mg antioxidants

Metals and alloys (such as Al, Si, Al–Mg and Al–Si) are commonly added to MgO–C refractory bricks as antioxidants due to their effectiveness to prevent carbon oxidation (in the 600–1400 °C range) and their low cost. These additives act at different temperatures and react with refractory components and gases in the environment, inducing significant changes in the resultant microstructure and affecting the overall thermo-mechanical performance of these products. This work addresses the evaluation of physical properties, cold and hot mechanical resistance, as well as in situ hot elastic modulus (E) measurements in the temperature range of 30–1400 °C for MgO–C bricks containing antioxidants (Al, Si or Al–Mg alloy) in a reducing atmosphere. Cured and fired samples of the designed formulations were evaluated throughout 1 or 2 heating-cooling cycles. Despite the improved mechanical behavior (higher cold crushing strength and hot modulus of rupture) of the antioxidant-containing formulations, compared to the additive-free MgO–C sample, the interaction of the selected additives with the refractory components and CO_(g) led to a generation of phases (i.e., Al₄C₃, Al₂O₃, SiC, SiO₂, MgAl₂O₄) that could not be well accommodated in the microstructure. Consequently, the in situ E drop was observed during cooling (mainly below 600 °C) of the antioxidant-containing sample due to crack and flaw formations. Si and Al–Mg were the most promising antioxidants, whereas the Al-containing composition showed the highest E damage level after two heating/cooling cycles up to 1400 °C for cured samples. Based on the elastic modulus profiles with the temperature, the results also indicated the best working conditions for these ceramic materials.     

Hot pressing of oxides in the presence of a liquid phase

Conclusions By incorporating certain substances, which, at low temperatures develop a liquid phase, it is possible to prepare by hot pressing at 1000–1300°C high-density (98–99.8% of the theoretical density) products from highly refractory oxides (MgO and Al₂O₃), including their simultaneous synthesis from their compound (Mg Al₂O₄).The role of the liquid phase with this method of hot pressing consists in partially facilitating the grain slip at the initial stages of the process, but mainly in the fact that the liquid phase is a medium through which diffusion occurs, contributing to the densification of the material and its recrystallization. In this case the correspondences between the melting point and the pressing temperature, the structure of the liquid phase, and its composition are decisive.The method of hot pressing highly refractory oxides at low temperatures and in the presence of additives, forming a melt and volatilizing during prolonged subsequent heating, is promising for pressing pure and high purity materials.Translated from Ogneupory, No. 8, pp. 36–40, August, 1971.     

Basic Zeolites: Characterization and Uses in Adsorption and Catalysis

The presence of basic centers in some oxides has been recognized for a long time as being important in catalysis [1-4]. Usually both basic and acid sites exist simultaneously. The two centers may work independently or in a concerted way. For instance, in alcohol transformation, dehydration is favored on acidic sites and dehydrogenation on basic centers [3,5]. A large variety of materials are cited as having basic character. They include single-metal oxides (MgO, CaO, ZnO), supported alkali metals (Na/MgO, K/K₂CO₃), mixed-metal oxides (MgO-A1₂O₃, ZnO-SiO₂, MgO-TiO₂), zeolites (X and Y saturated with alkaline cations of low electronegativity), hydrotalcite-type anionic clays, asbestoslike materials, carbon-supported basic catalysts, and basic organic resins.

     

The influence of B4C and MgB2 additions on the behavior of MgO–C bricks

Carbon-containing refractory materials have received great attention over the last years due to their importance in the steelmaking process. The oxidation of carbon present in refractory materials at temperatures above 500 °C is usually accompanied by the decrease of their mechanical strength and chemical resistance. Aiming to improve the oxidation resistance of carbon-oxide refractories, the use of materials known as antioxidants has been extensively studied. In this work we evaluated the performance of MgB₂ and B₄C antioxidants when incorporated into MgO–C bricks. We observed that the co-addition of metallic antioxidants and B₄C or MgB₂ leads to refractory bricks with enhanced hot modulus of rupture and resistance against oxidation and slag corrosion. However, the excessive addition of these antioxidants could impair the performance of the obtained bricks. Thus, when determining the optimum concentration of MgB₂ and B₄C to be added into MgO–C refractories, one must take into consideration this behavior.     

High-temperature corrosion of aluminium nitride ceramics by coal ashes

New high-efficiency gasification reactors with higher temperatures and pressures and with changed temperature cycles require new types of refractory linings. Besides the oxide materials currently in use, nonoxide materials such as SiC and AlN ceramics show great promise as refractories in the reducing atmospheres found in gasifiers. One of the main corrosion mechanisms occurring involves the interaction between the lining and the slag. Hence, AlN materials (without additives and with MgO or CaO additives) were tested in basic and acidic coal ashes at temperatures of 900–1300 °C in a tube furnace with an atmosphere containing H₂, CO and H₂O. After dwell times of up to 200 h the cross sections of the materials were analysed by SEM and EDS. The results showed the high stability of the materials in acidic ashes, but interaction with alkaline and calcium oxides along the grain boundaries in basic ashes.     

Molten cast materials of the Al2O3-MgO system

Molten cast alumomagnesia refractory materials containing up to 28.3% MgO (MgAl₂O₄) are investigated. It is shown that materials with 10–15% MgO have a complex structure and phase composition. The latter can be interpreted as a solid solution of corundum and spinel or as a combination of MgO · 2.5Al₂O₃. The behavior of these materials in molten TK16 and K8 glass is investigated.     

Wear-resistant refractories in aluminum pyrometallurgy

Service conditions for refractories and physicochemical processes at their contact with molten aluminum are provided. Thermodynamic calculations establish the relative wear resistance of a number of oxides, silicates, and oxygen-free compounds towards aluminum in the range 700–1200°C (937–1473 K). The role is revealed of contact physicochemical processes at the boundary of a refractory with molten aluminum. The high resistance towards aluminum is demonstrated for high-alumina calcium aluminates, particularly bonite CaO · 6Al₂O₃ and spinel MgO · Al₂O₃. The most practicable wear-resistant fuzed materials for preparing lining of melting furnaces are lime-aluminate slags of OAO Klyuchevsk Ferroalloy Plant. __________ Translated from Novye Ogneupory, No. 9, pp. 15–19, September 2007.     

Cathode materials modified by surface coating for lithium ion batteries

Recent research results confirm the importance of structural surface features of cathode materials for their electrochemical performance. Modification by coating is an important method to achieve improved electrochemical performance, and the latest progress was reviewed here. When the surface of cathode materials including LiCoO₂, LiNiO₂, LiMn₂O₄ and LiMnO₂ is coated with oxides such as MgO, Al₂O₃, SiO₂, TiO₂, ZnO, SnO₂, ZrO₂, Li₂O·2B₂O₃-glass and other materials, the coatings prevent the direct contact with the electrolyte solution, suppress phase transition, improve the structural stability, and decrease the disorder of cations in crystal sites. As a result, side reactions and heat generation during cycling are decreased. Accompanying actions such as suppression of Mn²⁺ dissolution, increase in conductivity and removal of HF in electrolyte solutions have been observed. Consequently, marked improvement of electrochemical performance of electrode materials including reversible capacity, coulomb efficiency in the first cycle, cycling behavior, rate capability and overcharge tolerance has been achieved. In conclusion, further directions are suggested for the surface modification of electrode materials. With further understanding of the effects of the surface structure of cathode materials on lithium intercalation and de-intercalation, better and/or cheaper cathode materials from surface modification will come up in the near future.     

Influence of calcium aluminate cement in magnesia castable on total oxygen content of interstitial free steel

Abstract

Samples of interstitial free (IF) steel buried in MgO castable bonded by calcium aluminate cement (CA) in graphite crucibles were heated at 1600°C for 90 min. Total oxygen content (TOC) of the steel was examined after heating and the refractory was investigated by SEM and EDS. It was found that TOC was higher in IF steel samples in contact with MgO castables containing 3 or 5 wt-% CA than with castables containing 7 wt-% CA or without CA. A liquid layer formed between refractory oxide and molten steel separates the refractory oxides from molten steel and inhibits direct dissolution of oxides in the molten steel. Transfer of oxygen between the refractory oxide and molten steel occurs by the formation of CaO.Fe₂O₃ at the boundary between the refractory oxide and the liquid layer, diffusion of CaO.Fe₂O₃ in the liquid phase layer, decomposition of CaO.Fe₂O₃ and dissolution of FeO into the molten steel. W ith increasing CA content in MgO based castables the CaO content in molten steel increases, but iron oxide content decreases, leading to the result mentioned above.     

High‐temperature calorimetric study of oxide component dissolution in a CaO–MgO–Al2O3–SiO2 slag at 1450°C

Blast‐furnace slags are formed, as iron ore is reduced to metal, as a molten a mixture of refractory and not easily reducible oxides, largely silica, alumina, lime, and magnesia. Their relatively low silica content makes them basic and poor glass formers. Their thermodynamic properties, though important for modeling their formation and reactivity, as well as furnace heat balance, are poorly known. Solution calorimetry of small amounts of solid oxides in a molten oxide solvent at high temperature (up to about 1500°C) permits direct assessment of energetics of dissolution. The enthalpies of solution of slag forming oxides: CaO, SiO₂, Al₂O₃, MgO, and Fe₂O₃ in a simplified model slag of composition: CaO (45.9 mol%), SiO₂ (35.1 mol%), Al₂O₃ (8.3 mol%), MgO (10.7 mol%) were measured by high‐temperature drop solution calorimetry at 1450°C. For this slag composition, enthalpies of solution become more exothermic in the order: Fe₂O₃ (279.3 ± 20.8 kJ/mol), MgO (56.7 ± 9.1 kJ/mol), Al₂O, (41.6 ± 11.3 kJ/mol), CaO (?4.3 ± 2.3 kJ/mol), and SiO₂, (?20.4 ± 4.4 kJ/mol), reflecting the relatively basic character of this low‐silica melt. Within these fairly large experimental errors, characteristic of calorimetry at this high temperature, there is little or no discernible concentration dependence for these heats of solution. The trends seen for these five solutes parallel those seen for heats of solution of the same oxides in other melts at various temperatures, with changes in magnitude reflecting the differences in acid‐base character of the melts. The new data for quartz show systematic behavior which extends the range of basicity studied for the enthalpy of dissolution of silica. The results provide reliable data for future modeling of the thermal balance of steel‐making furnaces and geologic and ceramic systems.     

Preparation and characterisation of closed-pore Al2O3-MgAl2O4 refractory aggregate utilising superplasticity

ABSTRACT

A novel high closed porosity Al₂O₃-MgAl₂O₄ refractory aggregate has been successfully fabricated by utilising superplasticity with Al₂O₃ and MgO as raw materials, SiC as high temperature pore-forming agent. The effects of the addition amounts of MgO and SiC on porosity, sintering behaviours, phase composition, pore size distribution and microstructure of the refractory aggregate have been investigated. The formation mechanism of the closed pore in the refractory aggregate has been discussed. The results showed that the MgO can improve the superplastic deformation ability of Al₂O₃-based ceramic at high temperature. With the content of MgO and SiC increased, the closed porosity and the pore size increased. The oxidation of SiC improved the sinterability of materials at the initial stage of sintering, and then the released gases due to the further oxidation of SiC promoted the formation of closed pores by motivating the superplastic deformation ability of Al₂O₃-based materials.     

Some thermomechanical properties of pure oxide ceramics

Conclusions The proposed vacuum furnace makes it possible to determine the deformation temperature under load and the coefficient of thermal expansion to high temperatures.Ceramics of pure oxides, Al₂O₃, ZrO₂, MgO, and BeO, have high temperatures of softening under load. The temperature of initial softening of Al₂O₃ ceramics containing additives lies in the range 1860–1930°C. The magnesia and beryllia specimens show high softening temperatures under load, but in vacuum at high temperatures they are very volatile. The initial softening temperature of ZrO₂ is about 2250°C.The linear expansion of pure-oxide ceramics reaches 2–3% at 1800–2000°C. Values obtained for the average coefficients of expansion for Al₂,O₃, ZrO₂, MgO, and BeO are little different from those in the literature.The compressive strength and bending strength of pure oxides at high temperatures are relatively low. The highest o_bend at high temperatures is possessed by specially pure zirconia, stabilized with MgO. Magnesia and beryllia in compressive strength at high temperatures exceed the other oxides.The highest spalling resistance is shown by beryllia ceramics. The combined addition to alumina of 1% TiO₂ and 5% ZrO₂ leads to a reduction in sintering temperature and an increase in thermal shock resistance. Ceramics based on specially pure zirconia stabilized with an optimum amount of CaO and MgO show a high thermal shock resistance.     

How effective is the addition of nanoscaled particles to alumina–magnesia refractory castables?

The addition of nanoscaled alumina and magnesia particles to the matrix of alumina–magnesia refractory castables drastically reduces the residual expansion related to the in situ spinel formation. Nonetheless, as their benefits on other relevant properties have not been reported so far, the effectiveness of such nanoengineering design for castables applied in steel ladles is still uncertain. In the present work, not only the expansion level, but also the corrosion resistance, the hot modulus of rupture and the creep deformation of different nanoparticle-containing castables were evaluated and compared with the results attained by refractory materials designed only by micrometric-scaled Al₂O₃ and MgO. Although the addition of a nanoalumina and nanomagnesia mixture ensured the best results regarding to the expansive behavior, thermo-mechanical and thermo-chemical properties, its performance was only slightly superior to the castable containing micrometric alumina and magnesia particles. Therefore, as the cost–benefit ratio is one of the main requirements for the end users, the nanotechnology use in the refractory production must be previously carefully analyzed.     

Influence of refractory oxide additives on the stability of alumium titanate

Conclusions Batch additives incorporated in the form of various oxides as a means of improving the physicochemical properties of aluminum titanate can be divided into two groups. The first consists of additives which prevent the decomposition of the titanate at temperatures below 1200°C, and stabilize its high-temperature modification. In terms of the degree of activity these oxides are arranged in the series: MgO, TiO₂, SiO₂, Cr₂O₃, La₂O₃. It is possible to assume that their influence is due mainly to the formation of limited solid solutions, and also the development during sintering of impurity grain boundary phases. The second group of oxides (SnO₂, NiO, excess Al₂O₃, etc.) contributes to the sintering of aluminum titanate by forming secondary crystalline phase on the grain boundaries of the ceramic. In this connection in obtaining sintered ceramic materials based on aluminum titanate it is preferable to consider additives of the first group or their combinations with additives of the second group.Translated from Ogneupory, No. 4, pp. 29–31, April, 1987.     

Investigation of CuO/mesoporous SBA-15 sorbents for hot gas desulfurization

Metal oxides, are widely used as sorbents to remove H₂S from hot gases produced by gasification processes. These oxides, however, suffer from several problems such as low capacity, sintering, evaporation, low duration and mechanical strength. Pure metal oxides or their admixtures loaded on inert materials have been used to overcome these problems.     

INVESTIGATION OF REACTIONS OF SIMPLE MAGNESIA SPINELS WITH ALKALINE EARTH ORTHOSILICATES IN THE SOLID STATE *

The spinels MgO Al₂O₃, MgO Cr₂O₃., and MgO Fe₂O₃ and the orthosilicates 2MgO SiO₂, 2CaO SiO₂, 2SrO SiO₂, and 2BaO SiO₂ were synthesized by solid-state reaction of the component oxides or compounds yielding the oxides on ignition. Each of the spinels was tested at various temperatures with each orthosilicate in turn by placing a pellet of each in contact and also by intimately mixing the two. Petrographic microscopy and X-ray diffraction were employed in the identification of reaction products.     

Interaction of Al2O3-rich slag with MgO–C refractories during VOD refining—MgO and spinel layer formation at the slag/refractory interface

The interaction mechanisms between a pitch-bonded MgO–C refractory and an Al₂O₃ rich (～15 wt%) stainless steelmaking slag were investigated by rotating finger tests in a vacuum induction furnace. A porous MgO layer (instead of a dense MgO layer) was observed at the hot face of the MgO–C bricks. This implies that under the present low oxygen pressure conditions, the oxygen supply from the slag is insufficient to meet the demand of reoxidising the entire amount of Mg vapor generated from the MgO–C reaction to form a fully dense MgO layer. A Mg(Al,Cr)₂O₄ spinel layer with zoning was found at the slag/brick interface in the top slag zone specimen of Test 3 (CHS3). Based on the thermodynamic analyses with and experimental data, a mechanism of Mg(Al,Cr)₂O₄ spinel formation is proposed. Initially, hot face periclase grains take up Cr₂O₃, and to a much lesser extent, Al₂O₃ from the slag. The further diffusion of Cr₂O₃ and Al₂O₃ from the slag establishes a spinel layer of three distinct compositions of the type MgAl_2(1?x)Cr_2xO₄, with x decreasing when moving from the interior to the exterior spinel layer. Due to the low oxygen pressures, the thermodynamically less stable, dissolved Cr₂O₃ in the hot face periclase decomposes and forms chromium-rich metal droplets.     

PHYSICAL PROPERTIES OF SOME HIGH-TEMPERATURE REFRACTORY COMPOSITIONS*

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

Effect of MgO and PVA on the Synthesis and Properties of Negative Thermal Expansion Ceramics of Zr2(WO4)(PO4)2

A green synthesis of Zr₂(WO₄)(PO₄)₂ ceramics from ZrO₂, WO₃ and P₂O₅ is presented. It is shown that the ceramics can be synthesized by one‐step sintering within 60 min. The relative density of the ceramics can be enhanced from about 75% without sintering additives to 99.8% of the theoretical value with 1.0 wt% MgO and 2.0 wt% polyvinyl alcohol. The grain sizes of the ceramics are smaller and more uniform with MgO added in the raw materials than with MgO added in the Zr₂(WO₄)(PO₄)₂ powder. The coefficients of thermal expansion are about ?2.325 × 10^?6, ?1.406 × 10^?6, ?1.509 × 10^?6 and ?1.384 × 10^?6°C^?1 for the samples without MgO, with MgO added in the raw materials, with MgO added in the Zr₂(WO₄)(PO₄)₂ powder and with MgO and PVA added in the raw materials, respectively.