Properties of spinels formed in highly fired periclase — Spinel refractories

Conclusions In high-temperature firing of periclase-spinel refractories one of the principal aims of a rational technology is achieved: the formation of spinels of magnochromite composition, with the crystal structure of a normal spinel and a high melting point.In such products the modified chrome spinel relates to magnochromite in composition whilst having the structure of normal spinel. The melting point of the modified chrome spinel exceeds 2100°C. The secondary spinels correspond in composition, or are close to, magnochromite. They have the structure of the mixed spinel with a very low degree of rotation, and a melting point of 2050–2070°C.Translated from Ogneupory, No. 8, pp. 41–44, August, 1972.     

水泥含量对高纯铝尖晶石自流浇注料性能的影响

以致密刚玉和尖晶石为主要原料,铝镁尖晶石的含量为19%,在颗粒组成(5～0 mm)不变的情况下,研究了水泥含量分别为3%、8%和13%对高纯铝尖晶石自流浇注料性能的影响.结果显示1)随着水泥含量的增加,浇注料的加水量、干燥后致密度和强度均增加,但烧后致密度和强度都下降;2)水泥含量的变化对自流浇注料热态强度的影响不明显;3)随着水泥含量的增加,自流料烧后线变化增加,但水泥含量达到8%以后变化不明显;4)对于LF精炼炉渣,不管水泥的含量多少,自流料都不被渗透,但随水泥含量增加,自流料的抗熔蚀性变差;5)对于转炉钢包渣,随着水泥含量的增加,自流料的抗渗透性增强,抗熔蚀性变差,而渗透与熔蚀量之和减少;6)对于转炉末期渣,随着水泥含量增加,浇注料的抗渗透性增强,抗熔蚀性变差;7)应用相图分析了铝尖晶石浇注料的侵蚀机理,认为渣中Fe2O3或低熔点相CF的增加是渣渗透的主要原因.     

Melting of Forsterite, Monticellite, Merwinite, Spinel, and Periclase Assemblages

Experimental data which define melting temperatures and crystalline solution compositions show that the assemblage forsterite+monticellite+spinel+periclase melts near 1425°C and that the assemblage monticellite+merwinite+spinel+ periclase melts between 1406° and 1420°C. The melting of the former assemblage is a double reaction point. The forsterite crystalline solution contains 13 mol% monticellite and the monticellite crystalline solution contains 22 mol% forsterite. A projection system shows that the new data agree with much previous data but highlight inconsistencies in previous data for assemblages containing more CaO, such as merwinite+dicalcium silicate+spinel+periclase. These assemblages are encountered in magnesite refractories with CaO:SiO₂<2.0 during firing and in service.     

Thermal Expansion in Air of Ceramic Oxides to 2200°C

Thermal-expansion measurements were made in an air atmosphere to 2200°C on polycrystalline single-phase ceramic oxides. Specimens were fabricated by hot-pressing, slip casting, and isostatic pressing. Magnesium oxide was tested in both air and argon; calcium oxide as well as magnesium aluminate and aluminum oxide specimens were tested in air. Specimens were characterized as to purity, thermal history, apparent density, grain size, and lattice parameter. It was determined that the coefficient of thermal expansion was not affected by changes in grain size or by fabrication techniques. All of the specimens tested in an air atmosphere showed weight and lattice parameter stability, whereas the magnesium oxide tested in argon did not. When tested in an air atmosphere, magnesium oxide, aluminum oxide, and magnesium aluminate spinel showed a permanent expansion with a decrease in density at temperatures well below their melting points.     

Preparation and Properties of Iron-Magnesium-Aluminum Spinel

Lattice parameter, melting point, electrical conductivity, thermal expansion, and thermal diffusivity/conductivity were measured for 0.25Fe₃O₄-0.75MgAl₂O₄ spinel. The electrical conductivity, which appears to be predominantly electronic (>1 S/m above 500 K), and the melting point (2124 K) make this material a candidate for use as an electrode for a magnetohydrodynamic generator.     

Reaction of sintered spinel and corundum refractories with some compounds

Conclusions We investigated the reaction of sintered spinel, spinel-corundum, and corundum refractories with certain oxides at 1500–1750°C. We established conditions in which refractories are stable in respect to the action of individual oxides. We also investigated the mineral compositions of the reaction zones.The least reaction with all the oxides investigated occurs in the spinel refractory, and the greatest in the corundum; that is, there is an increase with rise in alumina content. Magnesia spinel hardly reacts with iron oxides in the test conditions.Magnesia spinel is very resistant to sodium adipates which extensively damage sintered corundum.Carbonates of alkali and alkaline-earth elements, and also fused alkalis at temperatures exceeding their melting points by 200°C seriously damage spinel-corundum refractories, but do not react with sintered corundum or spinel.Metallic fluorides damage corundum but do not react with spinel and spinel-corundum sintered refractories.Thus, spinel sintered refractory should find extensive use in conditions of corrosive action, in particular, alkalis and oxides of iron.Translated from Ogneupory, No. 1, pp. 37–42, January, 1968.     

Some results of the installation of a technology for periclase-spinel refractories

The characteristics of periclase-spinel refractories and mixtures with an additive of synthesized spinel produced by the Magnezit Company are presented. The best refractories are produced from spinel synthesized by block melting and casting into molds and by sintering caustic magnesite and aluminum-containing slag. The possibility of manufacturing spinel-containing mixtures using waste periclase-spinel refractories is considered. Replacement of part of the mixture by a spinel-containing material improves considerably the physicomechanical parameters of the mixture.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 9, pp. 31 – 33, September, 1996.     

On the Melting Point of AlMgB 14

The melting (decomposition) point for AlMgB14 aluminum magnesium boride containing alumomagnesium spinel as an impurity (5.5 wt %) is experimentally evaluated. The melting is accompanied by the release to the gas phase. The decomposition process covers the temperature range of (2400–2573) ± 30 K. A partial loss of Mg is observed at 2073 K. The structure and element composition of crystallized objects are also studied.     

Articles with sintered spinel based on bauxite and periclase

Conclusions Laboratory-production investigations were made to develop a technology for periclasespinel refractories using sintered spinel based on bauxite and periclase. It is shown that bauxite is an acceptable replacement for alumina in synthesizing magnesia-alumina spinel. Control batches of ladle brick were made, and these had satisfactory properties.Translated from Ogneupory, No. 4, pp. 12–13, April, 1992.     

残炭对高镁煤灰熔融特性影响机制研究

为研究残炭对高镁煤灰熔融特性的影响,选取典型宁东气化用煤——梅花井煤为原料进行了不同残炭质量分数煤灰的灰熔融温度(t_AF)测试。利用FactSage7.3热力学软件对煤灰熔融过程进行模拟计算,探究了不同残炭质量分数煤灰在一定温度区间内的矿物转变。利用X射线衍射(XRD)和扫描电子显微镜结合能量色散光谱仪(SEM-EDS)分别对煤灰的矿物质组成及微观形貌进行了表征。结果表明:在煤灰熔融过程中,钙长石和辉石类矿物质在1275℃开始熔融为液相,其含量明显降低,煤灰的熔融是熔融-溶解机制;煤灰在熔融过程形成大量的低温共熔物(橄榄石、尖晶石和辉石)主导了煤灰的熔融温度,从而使得煤灰的熔融温度较低;随着残炭质量分数增加,低熔点矿物尖晶石含量急剧增加,煤灰的熔融温度呈降低趋势,这是由于氧化镁的离子势较低(3.0);在高温条件下MgO对煤灰中其他组分的作用是氧给予体,而残炭的存在能够剥夺氧化镁中的氧,从而阻止多聚物聚集,引起煤灰熔融温度降低。     

Synthesis and Densification of Transparent Magnesium Aluminate Spinel by SPS Processing

Mixtures of elementary oxides, MgO–Al₂O₃, were used to fabricate transparent polycrystalline magnesium aluminate spinel specimens by means of the spark plasma sintering technique. A sintering aid, 1 wt% of LiF, was added to the mixed powder. The presence of the additive promotes the synthesis of spinel that starts at 900°C and is completed at 1100°C. The LiF additive wets spinel on its melting and promotes densification, which is completed at 1600°C. LiF vapor plays a cardinal role in eliminating residual carbon contamination and in the fully dense state, allows attaining a 78% level of optical transmittance. The optimal conditions for achieving adequate transparency were determined and the role of the LiF addition in the various stages of the process is discussed.     

含镁铝尖晶石的铝酸盐水泥的制备及其抗侵蚀性

以白云石和工业Al2O3为原料,采用烧结法制备了含镁铝尖晶石的铝酸盐新型水泥,利用X射线衍射检测了合成产物的物相组成,采用扫描电子显微镜观察了新型铝酸盐水泥中各物相的形貌和能谱分析了成分分布,测量了这种铝酸盐水泥的凝结时间、耐火度以及其所结合的高铝矾土制成的耐火浇注料的早期强度.选择静态坩埚法进行抗渣性实验,对比了新型铝酸盐水泥和纯铝酸钙水泥结合刚玉浇注料的抗渣性差异.结果表明:这种水泥的物相组成为镁铝尖晶石、一铝酸钙和二铝酸钙;物相分布较为均匀.与纯铝酸钙水泥比较,凝结时间正常,新型铝酸盐水泥结合刚玉浇注料与纯铝酸钙水泥结合刚玉浇注料的抗弯强度相当,耐火度较高和抗侵蚀性较好,其原因在于水泥中存在镁铝尖晶石相,而镁铝尖晶石有较高的熔点和抗熔渣侵蚀能力.     

Testing the bond of refractory concretes in zinc scrap melts

Conclusions Compositions of bonding or refractory concretes with high contents of chrome spinel are stable inregard to molten clinker produced in the reprocessing of zinc-containing scrap.The bonds of refractory concretes made from slags obtained in aluminothermal production are resistant to molten clinker.It is desirable to use slags obtained from melting metallic chromium, and to a less degree titanoaluminous slag as finely milled constituents of the bond in refractory concretes for lining rotary furnaces that are used in zinc production.The final conclusions will be made after we have investigated refractory concretes containing bonds that are more resistant in molten clinker.Translated from Ogneupory, No. 1, pp. 26–31, January, 1968.     

Content and diffusion of water and gases in MgAl2O4 spinel crystals synthesized to produce ceramics

Content, evolution and diffusion characteristics of water and gases in fine-crystalline spinel MgAl₂O₄ were studied by kinetic thermodesoption mass spectrometry. Water is the main volatile component by quantity in the spinel structure. From the spinel crystals with an average size of 0.52 μm, water is released in vacuum in three temperature ranges: at 100–200 °C due to desorption from micropores, at 300–600 °C due to near-surface dehydroxylation and at 500–800 °C due to diffusion of water from the crystal bulk. The content of structural water, diffusively released from the crystals, is about 3000 ppm. The coefficients and activation energy of diffusion of water from spinel crystals in the range 500–700 °C were calculated. This allows us to estimate at any temperature the degassing time of the spinel with a certain degree of dispersion and ceramics made of it, and thereby promote the production of high-quality ceramics.     

Phase transformations in periclase-spinel roofs of open-hearth furnaces with blowing through of the bath by compressed air

Conclusions The crystallochemical properties of the periclase and the spinels of the periclase-spinel bricks were radically changed during service in open-hearth furncces with blowing through of the bath with compressed air; new types of ferrite spinels and their solid solutions in periclase were formed. Periclase forms magnesiowüstite and magnesioferrite with ferric oxides resulting in a lowering of the melting point from 2800 to 1600°C. This type of transformation of periclase is one of the main causes of wear of periclase-spinel roofs by fusion.Under the service conditions chrome spinel is transformed into magnesioferrite and magnetite, with a lowering of the melting point from 2100 to 1750°C.In the working zone the lattice parameters of the solid solutions in periclase (magnesiowüstite) increased from 4.204 to 4.218 Å, indicating a considerable amount of divalent iron and manganese cations in the periclase crystal lattice.In the periclase-spinel bricks we identified independent types of spinel, represented by inverted (magnetite), almost inverted (magnesioferrite), partially inverted (chrompicotite) and ordinary (magnesiospinel) structures. In the spinels the cations are variously distributed at tetrahedral and octohedral lattice sites. In the working zone ferrite spinels (up to 90%) — magnesioferrite and magnetite — predominated; they have an inverted structure and low melting points. The content of easily fusible ferrite spinels increased with increasing intensity of blowing of air through the bath. The formation of a large amount of ferrite spinels is one of the main causes of the fusion and rapid wear of periclase-spinel roofs of open-hearth furnaces.Translated from Ogneupory, No. 2, pp. 25–31, February, 1969.     

The effects of in situ formation of Y3Al5O12 on property improvement of magnesium aluminate spinel refractories

Magnesium aluminate spinel is widely used in cement rotary kilns, in the iron and steel industries, as well as in glass melting furnaces due to its excellent performance and chemical stability at both room temperature and elevated temperatures. In spite of these advantages, there are some practical problems during production of magnesium aluminate spinel refractories due to their poor sinterability: poor mechanical properties and poor creep resistance. These issues can cause problems during service. This study improved the sinterability of spinel refractories and in turn improved mechanical properties while decreasing the creep rate. This was done by forming a second low creep rate phase of yttrium aluminum garnet in the matrix structure. The addition of Y₂O₃ and reactive Al₂O₃ accelerated the densification process and increased the cold strength. There was a significant increase in the hot modulus of rupture due to the formation of YAG or the solid solution with spinel.     

Preparation and Properties of Spinel Made by Vapor Transport and Diffusion in the System MgO-Al2O3

In a hydrogen atmosphere, MgO was vaporized by heating MgO (periclase) in the range 1500° to 1900° C, and the vapor products diffused into Al₂O₃ (sapphire) to form a uniform outer layer of spinel on all surfaces. At 1900°C. a spinel layer 48 mils thick could be obtained in 16 hours. The rates of spinel formation were determined and the activation energy was calculated to be about 100 kcal. per mole. In the spinel layer, the lattice constant, Vickers hardness, and the refractive index varied from the outer surface to the inner boundary in a fairly uniform manner, indicating a continuous change in composition from 1MgO: 1Al₂O₃ to 1MgO: 2–3Al₂O₃. In conversion to spinel a volume increase of 47% over that of the original sapphire took place. Transparent clear shapes of spinel such as disks and rods were obtained from clear sapphire and from translucent polycrystalline alumina. Clear rods of spinel with polished ends acted as thin lenses, round rods as convex lenses, and flat rods as planocylindrical lenses owing to the increase in refractive index from the outer surface to the inner central portion. Objects made of opaque polycrystalline alumina were also converted to spinel. The MgO periclase blocks were etched in the hydrogen atmosphere, and the vapor products of Al₂O₃ entered the MgO to form tiny spinel droplets in an opalescent border. In an oxidizing atmosphere, spinel was formed only on the surface of the sapphire directly in contact with periclase, in the range 1500° to 1900°C.     

Synthesis and densification of magnesium aluminate spinel: effect of MgO reactivity

Stoichiometric magnesium aluminate spinel was synthesized by reaction sintering of alumina with caustic and sintered magnesia. The volume expansion of 5–7% during MgAl₂O₄ formation was utilized to identify the starting temperature of spinel formation and densification by high temperature dilatometry. The magnesia reactivity was determined by measurement of crystallite size and specific surface area. Caustic magnesia and sintered magnesia behave differently vis-à-vis phase formation and densification of spinel. Densification of stoichiometric Mag-Al spinel was carried out between 1650 and 1750 °C. Attempts were made to correlate the MgO reactivity with microstructure and densification of spinel.     

电磁场作用下高铁渣对MgO-C耐火材料的侵蚀

电磁场会促使熔渣中的铁、锰离子与MgO-C耐火材料反应形成锰掺杂镁铁尖晶石相,为进一步研究锰掺杂镁铁尖晶石的生成形貌及特征,采用Fe2O3质量分数为53.62%、CaO与SiO2质量比为0.8的高铁渣,分别在有、无电磁场环境下,对碳质量分数为14%的MgO-C耐火材料进行渣蚀试验。结果表明:感应炉存在电磁场,使熔渣中部分Fe2+/Fe3+与镁砂中Mg2+发生置换形成镁铁尖晶石,其含有少量Mn2+离子;镁铁尖晶石中铁含量从渣蚀层到渗透层急剧降低,锰含量几乎维持不变;侵蚀后试样渗透层较明显。电阻炉无电磁场,则侵蚀后试样没有形成镁铁尖晶石,熔渣中Si、Ca渗透到方镁石晶格中,形成钙镁榄橄石低熔相,将镁砂溶解到熔渣中;渣蚀层有明显的MgAl2O4生成。     

Enhanced fracture toughness in nonstoichiometric magnesium aluminate spinel through controlled dissolution of second phase alumina

Dissolution of Al₂O₃ particles into a MgAl₂O₄ (spinel) matrix is accompanied by a volumetric expansion that is predicted to lead to a compressive stress field upon cooling, resulting in a promising microstructure for enhanced toughening of transparent spinel. This study explores the conditions to form such a microstructure by hot‐pressing powders of Al₂O₃ and spinel, at temperatures that promote dissolution of the Al₂O₃. Tough, particulate‐reinforced composites were formed under lower temperatures and shorter times, but single phase, cubic spinel was formed at 1700°C for 10 hours. The single phase spinel made in this way exhibited a toughness of 2.26 ± 0.17 MPa√m, significantly higher than the equivalent nonstoichiometric spinel made by traditional methods, 1.72 ± 0.06 MPa√m. X‐ray diffraction measurements revealed lattice parameter changes consistent with the dissolution of Al₂O₃ into spinel. Mechanics modeling reveals that toughening arises due to the volume expansion as Al₂O₃ dissolves into the spinel matrix.