The wear of magnesia refractories in electric furnaces for melting cast iron

Conclusions The wear of magnesia refractories, i. e., magnesite, perictase — spinel, and magnesite — chromite bricks and magnesite — phosphate mortar, in the walls of electric furnaces for melting cast iron is the result of the solution predominantly of the periclase crystals and to a lesser extent of the spinel in the lowbasicity ferrosilicate slag melt. Brick of the PShS type proved to be the most durable refractory. The use of high-alumina brick in the roof of the furnaces accelerates the wear of the magnesia bricks in the walls.To increase the durability of the lining of these furnaces trials should be carried out with an all-basic wall lining constructed of high-density PShSP-type brick on MF-1-type magnesite — phosphate mortar and a roof lining constructed of MKhS brick and the same mortar.Translated from Ogneupory, No. 4, pp. 44–49, April, 1976.     

Effect of rare-earth chromites on sintering of magnesia

The effect of neodymium, cerium, lanthanum, and yttrium chromites, added in amounts of not more than 8%, on the sintering of magnesia in air and in vacuum has been studied. On addition of rare-earth chromites, the initial magnesia is sintered to a density of 0.98. By their sintering action, these chromites fall into the following sequence: neodymium chromite — lanthanum chromite — yttrium chromite — cerium chromite. The highest density (0.98 – 0.99) and the lowest porosity (1–2%) are obtained on addition of 2% of neodymium chromite to magnesia. The process of sintering of magnesia is accompanied by an increase in the size of periclase crystals from 12 to 38–56 µm.Translated from Ogneupory, No. 1, pp. 2–4, January, 1994.     

Properties of refractories produced from synthetic magnesia

Conclusions The sinterability, open porosity, and cold-crushing strength of magnesite and magnesite-chromite specimens processed from magnesia powders (96–97% MgO) are improved with a decrease in the size of the periclase grains in the powder and with an increase in the firing temperature and do not depend on the percent and composition of the silicates and on the B₂O₃ content of the magnesia powder.The high-temperature bending strength of both types of refractories increases with a decrease in the B₂O₃ content of the magnesia powder. The creep resistance of the magnesite specimens increases with the ratio CaO/SiO₂ in the magnesia powder while the creep resistance of the magnesite — chromite specimens does not depend on this index.The indices of the open porosity and strength of the magnesite and magnesite — chromite specimens were optimal when they were produced with magnesia obtained by the bicarbonate method from dolomite.To produce dense and strong magnesite refractories from magnesia, they should be fired at a temperature not below 1700°C. The firing temperature of magnesite — chromite refractories should not be below 1750°C.Translated from Ogneupory, No. 6, pp. 53–57, June, 1978.     

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.     

Producing synthetic ceramic magnesia — Alumina spinel for roof refractories

Conclusions Laboratory and technological studies to obtain magnesia spinel from raw materials of different qualities showed that, as regards the degree of activity, the magnesia constituents can be arranged in the following decreasing order: chemically pure magnesia metallurgical magnesite seawater magnesia. The use of technical alumina as the aluminous constituent is preferable to corundum.The minimum degree of sintering occurs in specimens of spinel having a stoichiometric composition.The sintering is improved with an increase both in the excess periclase and in the excess alumina over the stoichiometric ratio.A positive influence on the spinel formation and degree of crystallization is exerted by small (up to 5%) additions of chromite. The industrial significance of small (up to 2%) additions of TiO₂, B₂O₃, and V₂O₅ for the synthesis of spinels is not established.Thus, the spinel used as the batch constituent for making high quality magnesite spinel refractories should contain excess periclase and small quantities of chromite.Translated from Ogneupory, No. 2, pp. 41–49, February, 1972.     

Magnesia roof refractories with fused spinel in the batch

Conclusions Dense highly fired roof refractories based on sintered magnesite powder and electrofused aluminomagnesia spinel are characterized by high volume constancy and a 30–50% lower wear compared with ordinary periclase — spinel products in comparative tests in the roofs of 600 ton open-hearth furnaces working with the use of oxygen.The wear of magnesite — spinel refractories containing 20, 30, and 40% electrofused spinel in the batch during service in the roofs of open-hearth furnaces occurs by fusion.Translated from Ogneupory, No. 1, pp. 24–28, January, 1971.     

Making magnesite roof refractories with additions of fused spinel

Conclusions The Nikitov Dolomite Combine has produced an industrial batch of magnesite — spinel roof products containing fused spinel in the batch. The experimental refractories have very high quality factors.Testing of these products in the roof of a large open-hearth furnace with intensive oxygen blow in the bath showed that their resistance is 19% higher than the resistance of roofs made from periclase — spinel brick in identical furnaces at the Krivoi Rog Metallurgical Factory (with a simultaneous increase in output of 3.1%)Translated from Ogneupory, No. 4, pp. 1–6, April, 1971.     

Interaction between magnesia and graphite bricks in furnace producing vanadium nitride

Abstract

A cavity between magnesia bricks and graphite slabs was found in a commercial furnace produced vanadium nitride after a 13 months campaign. The magnesia, zirconia–corundum–mullite and hollow sphere Al₂O₃ bricks taken from the dismantled furnace were investigated by chemical analysis, XRD and SEM. It was found that the compositions of magnesia bricks, which contact with graphite slabs, were almost not changed but the thickness of remaining bricks was far lower than that of the original ones. Spinel was detected in used zirconia–corundum–mullite bricks and hollow sphere Al₂O₃ bricks, which were above MgO bricks. A mechanism of cavity formation was proposed. This result may be a new evidence of reaction mechanism between magnesia and graphite via gas phase.     

Chemical corrosion mechanisms of magnesia–chromite and chrome-free refractory bricks by copper metal and anode slag

The penetration and corrosion resistance to copper and anode slag of six magnesia–chromite and six chrome-free refractory brick types were tested using static finger tests at a typical copper-refining temperature (1300 °C). The microstructures of the as-delivered and tested refractory types were investigated by means of electron-probe micro-analysis (EPMA) and scanning electron microscopy (SEM) techniques. The results showed that the overall wear rate of the fingers was very low, with the exception of the alumina-based brick made of fused corundum and magnesia–alumina spinel, and the magnesia-based brick made of sintered magnesia and zircon addition. In all refractory types new phases were formed as a result of slag-refractory interactions. Apart from the samples recovered from the copper zone of the latest generation of direct-bonded magnesia–chromite bricks, all the rest were completely infiltrated by copper and slag components (copper oxide, iron oxide, alumina and silica). However, the amount of infiltrated liquid in the chrome-free types was higher than in the magnesia–chromite bricks. Explanations are provided for the distinct infiltration behaviour. The results show that economically viable chrome-free refractory alternatives are still elusive for anode furnace linings.     

Microstructural characteristics of refractory magnesia produced from macrocrystalline magnesite in China

Sintered and fused magnesia (FM) produced from the macrocrystalline magnesite in China have attracted attention worldwide for the production of various refractories. Herein, dead burnt magnesia (DBM) with varying compositions was investigated. The results revealed that the periclase crystals of the DBM92 sample were subrounded to rounded euhedral, whereas the periclase crystals of the DBM95 sample were subhedral and idiomorphic. In addition, a significant amount of periclase–periclase bonding with straight boundaries was observed in the DBM97 sample. The periclase crystals of the DBM98 sample with no clear boundaries exhibited the densest packing among the samples. The silicate matrix around the periclase grains of the DBM92 sample contained forsterite and monticellite, whereas that around the periclase grains of the DBM95 sample was mainly composed of monticellite and merwinite. Dicalcium silicate and merwinite were observed in small amounts as interstitial phases in the DBM97 sample. In contrast, only dicalcium silicate, which was concentrated in small triangular pockets, was observed in the densely packed periclase grains of the DBM98 sample. The hot modulus of rupture tests revealed that an alkali-resistant DBM95-based brick can be prepared at a lime–silica ratio of <0.5. The best-quality DBM97-based brick can be prepared at a lime–silica ratio of approximately 2.2, which ensures that dicalcium silicate is the only interstitial phase. The periclase crystals of the two-step FM were significantly larger than those of the one-step FM and exhibited remarkably fewer silicate boundaries. The superior structure and large crystal size of the two-step FM can endow magnesia–carbon bricks with slag corrosion resistance.     

Porous magnesia refractories

Summary Permeable, relatively strong, highly refractory material with a porosity of 30–60% and about 98% MgO was produced from magnesia.By varying the grain sizes of the aggregate, the ratio of aggregate and bond, the amount and grading of the combustible, it is possible to regulate the properties of the porous goods over wide limits.Porous periclase refractories have a high refractoriness, gas permeability and low thermal conductivity, and can be used as heat insulation at high temperatures if not subject to heavy loads and sharp changes in temperature; they are suitable for use as filters in aggressive conditions where a basic highly refractory material is needed.     

Bonding phase formation in eco-friendly periclase−spinel−Al bricks used in RH degassing process

Mere unburnt periclase–spinel–Al bricks have been accepted by steel mills in the chromium-free campaign of the lining materials for Ruhrstahl ?Heraeus (RH) degassers, in terms of comparable/optimistic performance to traditional material, low carbon emission due to unburnt manufacturing process and chromium-free material for eco-friendly steel-making process. Investigations are made on the used periclase–spinel–Al bricks for the thermal evolution of their components and the formation of novel phase and bonding structure. Under the working atmosphere of RH degasser, metallic Al particles got molten above its melting point, leaving Al rim around their circumference, and AlN formed in the gaseous state dispersing into overall matrix of periclase–spinel–Al bricks with rising temperature. AlN formed and Mg reduced in their gaseous state germinated MgAlON whisker initially in the original space of metallic Al particles, and MgAlON whisker grew further all over the matrix. A whisker-interwoven network has been full of the matrix behind the hot face and toward the cold face of the used bricks, which is a completely novel type of bond and distinguished from traditional ceramic one. The whisker-interwoven network is somewhat like the stripe graphite containing microstructure of magnesia–carbon brick, which results in low wettability and high flexibility. The superior performance of periclase–spinel–Al bricks is attributed to such a bonding structure of whisker-interwoven network, which could reduce slag penetration and facilitate thermomechanical stress resistance.     

不同添加剂对镁质浇注料性能的影响

以电熔镁砂和高纯镁砂为主要原料制备了镁质浇注料,研究了不同添加剂对镁质浇注料性能的影响。研究结果表明,5种添加剂中,加入氧化铝微粉的镁质浇注料经1 100℃和1 500℃烧后的体积密度最大,浇注料基质中形成的方镁石/镁铝尖晶石复相结构有利于提高试样的抗热震性。加入氧化铬、铝铬渣和焦宝石的镁质浇注料,其抗热震性均高于未加入添加剂的镁质浇注料的该项性能。     

添加浮选镁砂颗粒和Al2O3细粉对方镁石-尖晶石砖性能的影响

以烧结镁砂、浮选镁砂、电熔镁铝尖晶石和煅烧Al₂O₃细粉为主要原料,以木质素磺酸钙为结合剂,经混练、成型、1 650℃烧成制备了方镁石-尖晶石砖,主要研究了以浮选镁砂颗粒替代普通烧结镁砂颗粒以及在此基础上外加不同量煅烧Al₂O₃细粉（质量分数分别为0、3%和5%）对方镁石-尖晶石砖性能的影响。结果表明：1)以浮选镁砂颗粒替代普通烧结镁砂颗粒制备的方镁石-尖晶石砖,显气孔率和常温耐压强度降低,体积密度、抗热震性、高温强度和高温韧性提高。2)在以浮选镁砂颗粒替代普通烧结镁砂颗粒制备的方镁石-尖晶石砖中,随着煅烧Al₂O₃细粉引入量的增多,显气孔率和常温耐压强度降低,体积密度、抗热震性、高温强度和高温韧性提高。     

Experience with the use of mullite-corundum refractories in the roofs of electric-steel furnaces

Conclusions The resistance of mullite-corundum roof articles, as for periclase chromite, is due to the absorption of melt products from the working region of the furnace and the action of one-sided, high-temperature heating, but in view of the considerable difference in the physicochemical properties the use of the first in the central zone of the roof, subjected to the most severe working conditions, is more desirable than the second.The use of mullite corundum roof bricks, structurally reinforced with nonisometric grains of fused mullite, instead of periclase-chromite refractories will provide an increased roof life, both during the melting of stainless and ballbearing steels, without a change in the structure design; the increase amounts to 25–35%.Translated from Ogneupory, No. 4, pp. 43–46, April, 1986.     

Behavior of magnesia bricks in furnaces for the production of manganese alloys

Conclusions The application of dense magnesia bricks contributes to increased durability of the lining in metal manganese and manganese slag producing furnaces.The life of magnesia brick linings with an 11 to 13% porosity amounts to 45 days.The employment of large-size bricks hightens the resistance of the furnace bottom.Periclase-spinel brick proved unsuitable in the production of manganese slag.It is suggested that bricks with a porosity of 14 to 16% be tested for the purpose of determining optimal porosity.     

Three bond modes of basic refractories used for Ruhrstahl Heraeus degassing process

Magnesia–spinel brick and unburnt periclase–spinel–Al brick are being employed as a substitution of traditional magnesia–chrome brick in the chromium-free campaign of lining materials in Ruhrstahl Heraeus (RH) degasser. These three materials are investigated, in terms of physical properties, corrosion resistance and flexibility by wedge splitting test. Tracking their physical alterations and chemical reactions through burning or heating, three bond modes are discovered. Magnesia–chrome brick is subject to a series of phase transformation with rising temperature to yield a liquid envelop around chromite-ore particles, to further form porous rim while liquid is gradually absorbed by surrounding magnesia and eventually to precipitate secondary chromite spinel lied between magnesia particles by thoroughly dissociating chrome ore. The precipitated chromite spinel functions as the featured bond that enhances hot strength and corrosion resistance to slag, and additionally liquid coexistence improves the flexibility. The direct bond mode of magnesia particles in magnesia–spinel brick endures slag penetration by immanent character of MgO. Spinel incorporation in magnesia effectively improves thermal shock resistance. Due to minor negative value of permanent linear change after reheating, further sintering (densifying) in using at high temperature would bring a risk of loosening and open joints of magnesia–spinel lining. While used in RH degasser, unburnt periclase–spinel–Al bricks undergo a miraculous process of metallic Al melting, gaseous AlN and AlON formation, MgAlON whiskers germination combined with gaseous Mg reduced, and micron-size whisker network bond domination in their matrix. Such a whisker-network bond renders the material a successful eco-friendly alternative to magnesia–chrome refractory.     

Sighting tubes for temperature measurements in open-hearth roofs

Conclusions Service tests of sighting tubes made from corundum and magnesia spinel showed that a gas permeability of 0.0009–0.00006 liter·m/m²·h·mm water guarantees the measurement of temperature with optical pyrometers without interference from the gaseous atmosphere.The maximum life of sighting tubes placed in the roof of open-hearth furnaces is obtained only when the articles are fixed at a distance not exceeding 40–50 mm in front of the roof surface; the minimum life is possessed by those products protruding into the furnace space by a magnitude equal to 3 diameters. The greatest resistance is shown by articles made from magnesia spinel.The sighting spinel, thin-walled tubes can be used for short control temperature checks of the roof of open-hearth furnaces; corundum and spinel tubes can be used for automatic measurements of temperatures in periodic kilns for firing refractories.Translated from Ogneupory No. 6, pp. 7–10, June, 1968.     

Effect of composition,quantity, and synthesis temperature of spinels,and some properties of periclase — Spinel compositions

Conclusions The incorporation of spinel (5–10%) into the composition of the solid solution leads to periclase grain growth and to its sintering. Excess spinel in combinations with periclase over this quantity prevents recrystallization sintering of the specimens.A reduction in the synthesis temperature of most the spinels from 1750 to 1450°C contributes the sintering of the compositions under investigation, and improves their technical properties.In the periclase—spinel compositions the maximum influence on the grain growth and the properties of the specimens is exerted by the temperature conditions used for synthesizing the high-alumina spinels, while the conditions used to obtain the highly chromic spinels, and especially the magnesiochromite, have little or no effect on the technical properties of the fired product.With an increase in the spinel phase concentration from 5 to 30%, and also when there is a change in its composition in the direction from magnesia-alumina spinel to magnesiochromite, such factors as strength, porosity, and spalling are impaired.In the MgO-Mg(Al_1–X, Cr_X)₂O₄ system it is possible to obtain strong, dense, and simultaneously spalling-resistant refractories by using small additions of alumina spinel, synthesized at low temperatures ensuring the completion of the spinel-forming reaction.Translated from Ogneupory, No. 3, pp. 53–57, March, 1971.     

Crystallization of sublimation products in periclase refractories in high-temperature processes

Conclusions During the melting, firing, and service of magnesia refractories crystallization occurs in the sublimation products in the form of thin-walled tubes, fine films, dendrites, needles and threads of periclase, periclase-forsterite, and spinel compositions. The presence of carbon in these newly formed compounds indicates development mainly in a reducing atmosphere.Translated from Ogneupory, No. 5, pp. 39–42, May, 1982.