Mineral formation in chrome-magnesite refractories in rotary furnaces used to produce defluorinated phosphates

Conclusions Silicates of chrome-magnesite refractories in a large or small degree react with the components of the batch and the decomposition products of apatite. With rise in temperature this reaction sharply increases.The silicate bond in the working zones of chrome-magnesite refractories is located in the liquid or plastic state (depending on the positioning of the brick in the lining and the distance from the working surface of the refractory), since the tricalcium phosphates in contact with the refractory forms a eutectic at 1308°C. The eutectic melt easily penetrates deep down into the refractory, filling its pores and cracks. The bond between the grains and sections of periclase and the chromite is weakened under these conditions, the structure is destroyed with a break in the continuity of the individual grains and the sections which are opened by the firing products moving into the furnace.The service life of the chrome-magnesite refractories can be increased by sharply reducing the quantity of melt formed in the working and partly in the sintering zones of the refractory, which can be obtained by reducing the content of silicates (forsterite and montichellite) in the original brick.The use of denser periclase-spinel refractories should also favor an increase in the life of the linings of rotary furnaces in the firing zone.Translated from Ogneupory, No.2, pp.41–46, February, 1967.     

Wear of high-fired periclase-spinel refractories in the roof of a double-bath steel-melting furnace

Conclusions Periclase-spinel products prepared from magnesite powders which are pure in chemical composition and beneficiated Kempirsaisk chromite possess an increased resistance (30% higher than in ordinary periclase-spinel brick) during service in the roof of a double-bath steel-melting furnace, operating with oxygen blow in the bath. Their wear in general occurs as a result of the fusion of the working surface. The mechanism of this wear is explained by the metasomatic processes which lead almost to complete replacement of the periclase and chrome-spinel by ferritic spinels.The increase in the resistance of the experimental periclase—spinel refractories is helped by the structure with the direct bond between the grains which retards the access of silicates and slags inside the textural elements. This exerts a favorable influence on the change in structure of the refractory during service, especially in the transition zone in which additional sintering and crack formation leading to scaling of the experimental refractory hardly develops.On the basis of the results of the research we recommend that refractory enterprises set up various technological production lines for making periclase-spinel products from pure Satkinsk magnesite powders (94–96% magnesium oxide) and beneficiated Kempirsaisk chromite (59–60% chromium oxide) using high-temperature firing in a tunnel klin. The use of high fired periclase-spinel products with a direct bond between the grains would increase the resistance of the roofs of metallurgical furnaces operating with the use of oxygen and increasing their outputs.Translated from Ogneupory, No. 5, pp. 28–33, May, 1973.     

X-ray phase analysis of open hearths

Conclusions After current and capital repairs, the basic phase in the hearths is periclase. Apart from periclase there is also magnesioferrite, monticellite and forsterite.Hearths surfaced with nine layers of a mixture of magnesite and scale and three layers of pure magnesite with slagging of the scale after capital repairs hardly differ at all in phase composition. When surfacing with nine layers, there is a considerably greater amount of forsterite.The basic laws governing the variation in phase composition according to window and layer are manifested both in the hearth surfaced with nine layers of magnesite and scale, as well as in the ones surfaced in three layers with pure magnesite and slagged with scale.During current repairs, the phase composition of the hearth is basically no different from the phase composition which is obtained during capital repairs. Since there is no substantial difference in the phase composition of the hearth surfaced by the high-speed or conventional methods, the strength of the hearth during high-speed surfacing is not inferior.The composition of the hearths after service is characterized by magnesioferrite, periclase, a solid solution of magnesioferrite in periclase, monticellite, forsterite, merwinite, complex spinel MgO·Al₂O₃, dicalcium silicate and braunmillerite.Typical of the phase distribution and lattice parameter of the periclase according to the level of the hearth is a parallel variation in the content of the periclase and monticellite as well as an inverse relationship between the amount of periclase and its lattice parameter.     

Magnesite-chromite products from high purity magnesite and chromite

Conclusions Products made from magnesite and chromite with low-impurity oxide concentrations possess better thermal factors than ordinary periclase-spinel refractories: refractoriness under load: 1700°C and above; bending strength at 1300°C 2.5–3 times greater; deformation 3–4 times; and the rate of deformation at steady-state creep is about a half.The periclase-spinel products with low quantities of impurities have an increased thermal-shock resistance, which is achieved by adding small quantities of chromite to the granular part of the batch in combination with the additions made to the dispersed constituent.The products develop a direct bond between the crystals of periclase, i. e., periclase-spinel, in which two types of direct bond exist between the periclase and the spinels: the contact between crystal and crystal, and the contacts in the form of shells and edgings of spinels around the crystals of periclase. In the latter case, the development of a direct bond between the crystals is more complete which is contributed to by the replacement in the silicate part of monticellite by dicalcium silicate.Translated from Ogneupory, No. 2, pp. 32–37, February, 1971.     

Production technology of high-density periclase-spinel roof materials

Conclusions High-density periclase-spinel refractories can be produced from a body of magnesite powder containing 94–96% magnesium oxide and Kempirsai chromite by high-pressure molding and high-temperature firing in a tunnel kiln. The product is strong with good thermal stability and high onset temperature of deformation under a load. The structure of the refractory is improved as a result of the formation of a large proportion of direct intergranular bonds between the periclase and spinel.It is planned to produce experimental industrial-scale batches of periclase-spinel roof refractories and to subject them to trials in the roof of open-hearth furnaces in continuous operation.Translated from Ogneupory, No. 8, pp. 39–44, August, 1973.     

Calculation of expenditure norms for periclase-chromite items used to lay slag chamber arches in open-hearth and two-vessel furnaces

Actual data collected during a lengthy operating period are used to construct models giving expenditure norms for periclase — chromite items in slag chamber arches of open-hearth and two-vessel furnaces. The models allow for the furnace capacity and the repair category during the period between cold repairs and give objective estimates of the absolute expenditure of items for this laying element in groups of open-hearth furnaces. For two-vessel furnaces the share of periclase — chromite in the laying of slag chamber arches is taken into account.Translated from Ogneupory, No. 6, pp. 28 – 30, June, 1994.     

The phase composition of chrome-magnesite refractories

Conclusions Study of the phase composition and structure of chrome-magnesite refractories of different composition and degrees of firing led to the development of a range of practical proposals to improve the production technology.The quality of chrome-magnesite refractories can be radically improved mainly by obtaining in them secondary magnesia spinels with energy-stable crystal lattices (mp. above 2100°C) and by getting a phase composition close to equi brium. In this case the intermediate mineral formations will change into stable highly refractory compounds.With existing production methods for chrome-magnesite refractories such mineral-forming processes can be ensured only by using finely ground powders of chromite and magnesite in conjunction with high-temperature firing (of the order of 1750°C and higher).To make high-grade roof and converter magnesite-chromite refractories, the most promising method is the production of periclase-spinel goods by the ordinary or briquette technology.Technological measures for producing roof magnesite-chromite goods from coarsely ground chromite ores (3-0 mm fraction) are of no value, since during firing they develop nonequilibria phases with low melting points and secondary spinels of unstable structure possessing unsatisfactory properties compared with magnesia spinels used in the production of periclase-spinel goods.To prepare high-grade roof and periclase-spinel magnesite-chromite refractories it is necessary to use enriched Kimpersai ores and magnesite powder with a minimum content of impurities, so as to ensure that the silicate bond of the finished goods will contain secondary spinels with a fusion temperature of above 2100°C instead of forsterite and montichellite with eutectic temperatures with spinel of 1670° and 1430° respectively.     

Physical-chemical bases for improving strength of open-hearth bottoms during high speed repairs

Conclusions The physical-chemical basis of high-speed methods of welding on hearths in open-hearth furnaces is intensification of the recrystallization of periclase brought about by high concentrations of iron oxides and high-temperature conditions.An important condition for high speed surfacing methods is the use of scale which intensifies the periclase recrystallization over a relatively short surfacing time.In high-speed methods of surfacing hearths with scale, the periclase recrystallization develops both in the grains as well as in individual fine periclase crystals (magnesiowustite) forming the binder in the hearth together with silicates and glass.The reerystallization of the periclase in the grains (aggregates) and in the binder leads to the magnesiowustite crystals growing together, which increases the strength of the hearth; this is further improved by the increase in the amount of dicalcium silicate in the binder.The strength of the hearth is improved by chromemagnesite powder (the magnesite part in the form of a dispersed fraction) through the formation of solid solutions of complex spinel in the periclase (the spinel evidently dissolves in the silicate melt more slowly) and the strong crystalline concretion of the former dispersed periclase grains in the binding component in the hearth. The use of fine grain metallurgical powder containing 20 – 25% fraction finer than 0.02 or 0.088 mm and a coarse fraction with 7 – 10 mm grains has a favorable effect.     

Sintering behavior of periclase–doloma refractory mixes

Steelmaking electric arc furnaces (EAF) hearth is usually built up with dry vibratable refractory mixes. During furnace operation, the refractory mix undergoes sinterization, slag attack, and chemical interaction with steel. Material sintering is needed in the upper part of the EAF hearth, in order to develop enough mechanical strength and wear resistance during operation. Densification of the upper layer also reduces steel infiltration from the hot surface.In this work, the sintering behavior of three dry vibratable commercial refractory materials for EAF hearth is reported. The characterization of the mixes includes chemical, mineralogical, and granulometric analyses. The study of sintering behavior is carried out by dilatometric analysis and microstructural and mechanical evaluation of sintered specimens.Results showed that the analyzed mixes mainly contain periclase and doloma, with a wide granulometric distribution and different content of minor components (mainly, iron oxides). Material sintering began at temperatures higher than 1200 °C and was associated to liquid phase formation. Differences in sintering mechanisms with distinct amounts of liquid phase involved were determined in the analyzed materials and related to their iron oxides contents. Well-sintered specimens with higher room temperature mechanical strength and lower porosities were obtained from the mix with highest iron oxide content.     

Resistance of magnesite refractories to smelting products of blister copper

Conclusions An improved method of assessing the results and testing refractories for slag resistance was described.It was established that the most resistant materials to the action of converter slags are periclase-spinel refractories.Magnesia articles are destroyed at the bond owing to the passage of thin periclase constituent into the fusible magnesia-iron silicates.To reduce the rate of wear and increase the resistance of refractories in these melting conditions for blister copper, it is necessary in the production of magnesia articles to use magnesite and chromite ore with a minimum content of SiO₂ and iron oxides.     

Production and service-testing of refractories based on flotation-concentrated magnesite

Conclusions Flotation-concentrated magnesite and pure and concentrated chromite were used for producing high-temperature fired magnesite, magnesite-chromite, and periclase-spinel refractories which, compared with ordinary types, contained less silicates and, more direct bonds between the high-refractoriness minerals so that their refractoriness under a load and their thermal strength were higher.In the lining of 100-ton converters for steel, the durability of the experimental magnesite refractory produced from concentrated magnesite and tar-impregnated was 19% better than that of ordinary tarbonded magnesite brick.In the lining of the tuyere zone and of the zone above it in 20-ton and 30-ton converters for copper and nickel, the durability of the experimental magnesite-chromite and periclase-spinel refractories produced from concentrated magnesite and concentrated and pure chromite was 30–35% better than that of ordinary MKhS and PShS type refractories.Translated from Ogneupory, No. 1, pp. 11–15, January, 1976.     

The action of slags of varied basicity on spinel - Periclase refractories

Conclusions In the action of slags of the Fe₂O₂-CaO-SiO₂ system on spinel-periclase refractories consisting of a filler and a binder the mechanism of erosion depends on the composition of the slag and the degree of erosion on the composition of the spinel party of the binder of the refractory.The essence of the chemical interaction of the refractory with the penetrating melt consists in the formation of solid solutions of periclase and spinels with the ferric component of the slag, and in the interaction of the periclase and silica in the presence of acid slags which results in the formation of forsterite; in the presence of basic slags it is primarily the spinel which is eroded and interacts with the calcium oxide, the result being the formation of calcium monoaluminate and chromite, and periclase.The erosion of the refractory depends largely on the stability of its bond; the periclase grains are affected only superficially by the processes of the interaction.When slags of different basicity act by turns on spinel-periclase refractories, the nature of the processes developing in the refractory as a result of the penetration of the melt is quite different. The chemical compounds formed in the interaction with the previous slag are dissolved in the melt penetrating into the refractory, i.e., the forsterite in the subsequent interaction with basic slag and the alumino- and chromocalcium compounds in the subsequent interaction with acid slag. The result is that the ratio CaOSiO₂ in the melt approaches two so that the chemical activity and solution capacity of the melt decrease. In this case the principal product of the crystallization of the melt is represented by monticellite.This investigation showed that in contact with melts in the Fe₂O₃-CaO-SiO₂ system a marked advantage lies with compositions which contain high-alumina spinels, the reason being the volumetric stability of these spinels to iron oxides.Translated from Ogneupory, No. 2, pp. 39–47, February, 1977.     

Some special features of open-hearth furnace bottoms

Conclusions The peculiar feature of the formation of hearths fettled with fine-grained chromemagnesite powder compared with those fettled with metallurgical magnesite powder is the formation of a more clearly expressed crystalline concretion of periclase as a result of the fine grain size composition of the powder and the presence in it of grains of chromite which help the process of hearth formation.Wear of the hearths made of chromemagnesite and magnesite powders is due mainly to one and the same factor — destruction of the crystalline concretion of periclase under the action of iron-silicate melt in the slag. The process of destruction of the hearths made from chromemagnesite powder occurs more slowly owing to the more complete formation of the concretion of crystals of periclase in these hearths, which is probably the basic cause of the higher resistance.Investigation of samples of hearth taken from the site of an accidental escape of metal through the bottom, showed that they contained crystals of iron spinel which is probably due to the local supersaturation of the hearth with iron oxides.     

Effect of composition of metallurgical powder on the slopes and hearths of electric arc furnaces

Summary To reduce the sliding of fettling powders from the slopes of electric furnaces, it is desirable to use powders with not more than 10% of the fraction finer than 0.088 mm, and the SiO₂ content should be about 4.5–6%. Addition to the magnesite powder of coarse dolomite fraction 15-0.5 mm, and also 6–7% coal tar pitch, reduces the mobility of the powders.The rational composition of metallurgical powders used for fettling slopes and hearths of electric arc furnaces largely depends on the grade of steel being melted.The increase in the life of the slopes and hearths of an electric furnace in which stainless steel was being melted, fettled with MPMZ powder, was due to the increase in the content of periclase bonded with melilite and mervinite in the slopes and melilite and forsterite in the hearths.During the melting of steel of changing sorts, the increase in the life of the slopes and hearths, fettling with magnesite-dolomite powders is due to the presence of crystals of periclase bonded mainly with highly refractory dicalcium silicate.When fettling is done with MPMZ powder the structure of the slopes and hearths is identical. The use of chromemagnesite, magnesite-chromite and magnesite-dolomite powders give rise to the formation of a heterogeneous structure in the slopes and hearths which leads to their irregular wearing away.To prolong the service of the hearths and slopes of electric furnaces it is necessary to continue investigating the wear resistance of fettling materials in furnaces of different capacities, where steels of different types are being melted, typifying the life of the powders by the consumption per ton of steel melted, the burn-out profile of the lining, the interrepair periods and other factors.     

Refractories based on fused pe riclase — Chromite material for vacuum steel refining plants

Conclusions The relevant technology was developed and production has started of high-quality refractories based on a high-density fused periclase —chromite material.The refractories are characterized by a low content in silica and silicates, well-developed direct bonds between the high-refractoriness minerals, and high indices for density, cold-crushing strength, thermal strength, refractoriness under a load, and in-vacuo stability to the action of slag and steel melts and the gases. The most favorable combination of properties was achieved when using starting materials of high-purity, fused periclase-chromite with a maximum grain size of 5 mm, batches with a low content of fine-ground components, high specific molding pressures, high firing temperatures, and long holding times at the peak temperature.The refractories are designed for the lining of the pipe, bottom, and walls of the chamber of equipment for vacuum steel refining and can be used with good results in the lining of converters, electric-arc and plasma furnaces, and other installations for melting high-temperature materials.Translated from Ogneupory, No. 8, pp. 5–12, August, 1976.     

Magnesite refractory with a high CaO content

Conclusions Chromite, added in amounts of 10% to dolomitic magnesite containing 8.35% CaO by bonding with it completely during firing, is an effective stabilizer.An increase in the chromite content of the batch of more than 10% lowers the quality of the refractory.The hydrothermal treatment of the calcined dolomitic magnesite with 10% chromite accelerates the process of hydrating the free CaO and disperses the material, which helps the refractory to sinter during firing.The proposed technique provides for combined grinding and firing of a mixture of dolomitic magnesite with chromite, excludes aging of the body, reduces the firing temperature of the body and increases the quality of the refractory compared with the periclase-spinel technique.The calcium oxide in dolomitic magnesite with the addition of 10% chromite is bonded mainly with theCr₂O₃ into calcium oxychromite 9CaO · 4CrO₃ · Cr₂O₃ and the ferric oxide is introduced into the lattice of the periclase with the formation of a solid solution. With 30% chromite in the batch 3CaO · 2CrO₃ · 2Cr₂O₃ is formed, and the ferric oxide enters the magnesio-ferrite.Calcium oxychromite, existing in the refractory with 10% chromite and being a secondary phase, at its fusing temperature (>1250–1290°C) reversibly converts to calcium monochromite with a fusing temperature of 2170°C, which explains the increase in the refractoriness under load.     

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al₂O₃ slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al₂O₃ slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.     

Effect of viscosity and phase composition of converter slags on the destruction of refractories

Conclusions Converter slags formed in the first 10–12 min of the blowing of the iron, after cooling consist mainly of manganese-iron monticellite, glassy, and cryptocrystalline substances, free calcium oxide, and metallic iron. The primary slags are characterized by a low basicity, refractoriness, and viscosity, and rapidly react with the refractory during the period of their formation in the converter at 1150–1360°C. The primary slags are changed into the truly liquid state at 1256–1312°C.In the middle of the iron blow the slags show a substantial increase in their quantities of calcium silicates, which increase their refractoriness and viscosity. The slags are formed, change into the truly liquid state, and begin to react with the refractory at higher temperatures with the formation of smaller quantities of melt, compared with what is observed during the reaction of the primary slags and the refractory.At the end of the blow the slags show substantial increases in the concentrations of iron oxides, on account of which ferrites and aluminoferrites of calcium are formed, reducing the refractoriness and the viscosity of the slags.The presence of ferrites and aluminoferrites of calcium increases the corrosiveness of the final slag with respect to periclase-spinel refractories, compared with the corrosiveness of slags formed in the middle of the blowing of the iron.Translated from Ogneupory, No. 3, pp. 40–45, March, 1972.     

Grain-Boundary Reactions in Magnesia-Chrome Refractories: Application of the Electron Probe, I

The technique for the preparation of polyphase refractories for electron probe investigations is described. Chemical changes occurring during heating of an unused chemically bonded magnesia-chrome refractory were studied and the spatial distribution of Fe, Cr, Al, Mg, Ca, and Si at different temperatures was determined. The development of the reaction layer around the chromite grains, the migration of ions into the periclase grains, and the distribution of ions in silicates were followed qualitatively using Kα X-ray and electron back-scattering displays.     

Service of the magnesia-spinellide lining of a melting furnace roof

Conclusions On the basis of a study of the magnesia-spinellide roof of a melting furnace it was established that the refractory consists of four zones, the least changed, the working, the zone of contact with the lining slag, and in the lining slag. A nonuniform distribution of the components in specimens of lining slag taken from different portions of the roof was observed.The lining slag and the zone of contact with the lining slag consist of crystals of picrochromite, ferrite, and magnesioferrite, between which there is located a silicate binder including forsterite, hedenbergite, and the impurity phases bunsenite, hematite, and magnetite. The working zone consists of chromite, picrochromite, monticellite, and the introduced melting products copper, chalcosite, and cuprite.On the basis of analysis of the data on failure of refractories as the result of chemical interaction with the melting products, penetration of the low-melting components, and thermomechanical cracking at the boundaries of the zones it was established that one of the methods of protection of the roof is cooling.Translated from Ogneupory, No. 2, pp. 51–55, February, 1989.