Monticellite and Forsterite Crystalline Solutions

New data for monticellite crystalline solutions confirm that monticellite (CaMgSiO₄ is unstable at atmospheric pressure. On the join forsterite-monticellite, the estimated range of stable monticellite crystalline solutions is 89 to 93 mol% monticellite at 1200°C and 75 to 92 mol% at 1490°C. The range of forsterite crystalline solution is 93 to 100 mol% forsterite at 1200°C and 81 t0 100 mol% at 1490°C. These results can be applied to the proportions of phases present in impure periclase refractories at service temperatures.     

Retrograde Solubility of Periclase, Forsterite, and Dicalcium Silicate

Published phase equilibria data are used to demonstrate that for certain compositions, including some commercial magnesites, the amount of one crystalline phase present with liquid can increase as the temperature is raised. This effect is important for periclase with forsterite+spinel+liquid and for dicalcium silicate with merwinite+periclase+liquid; it may influence bonding in commercial refractories.     

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.     

Crystalline Solution in the System MgO-Mg,SiO4,-MgAl2,O4,

Crystalline solubility relations in the system MgO-Mg₂SiO₄MgAl₂O₄ (periclase-forsterite-spinel) were studied using coprecipitated gels as starting materials. The substitution 2Al = Mg + Si was investigated along the join Mg₂SiO₄-Mg-Al₂O₄,. At 1720°C the maximum crystalline solution in forsterite is about 0.5 mole % MgAI₂O₄, and in spinel it is slightly more than 5.0 mole % Mg₂SiO₄. The solubility of MgO in forsterite was 0.5 mole % at 1860°C, whereas more than 11 mole % Mg₂SiO₄ can be dissolved in the periclase structure at this temperature. Ternary crystalline solution exists in the periclase structure to a composition of Mg_0.853Al_0.063Si_0.026O at 1710°C.     

Composition and formation of fusions of metallurgical magnesite powder

Conclusions In metallurgical magnesite (periclase) powder obtained in rotary kilns of length 50, 75, 90, and 170 m one encounters no less than eight structural genetic types of granules (fusions) in which are selectively concentrated the impurities CaO, SiO₂, etc. The commonest are diabase and dolomite granules.The formation of diabase and clayey (aluminous) fusions is due to the cementation of periclase particles by the melts, formed deposited from the mechanical impurities in the raw material. Morphologically and genetically similar chamotte, and frequently ferruginous, fusions develop as a result of the granulation of aggregate periclase grains of fusible products from the breaking up of the lining in the rotary kilns.The formation of dolomite and diabase-dolomite fusions with an internal core from the products of the decomposition of the broken primary dolomite is connected with the power of the latter to be preserved during firing in the form of lumps, and not subjected (in contrast to magnesite) to self-dispersion.In the formation of granules chemical reactions occur between the periclase, lime, and molten fusible impurities in the magnesite raw materials. The products of the reaction are spinel, magnesioferrite, forsterite, monticellite, merwinite, cordierite, and silicates and ferrites of calcium. In connection with the increase in the concentration of fusible impurities the fusions (granules) are poor quality constitutents of the magnesite powder.Translated from Ogneupory, No. 5, pp.47–54, May, 1971.     

Microstructure and phase evolution of Indian magnesite‐derived MgAl2O4 as a function of stoichiometry and ZrO2 doping

To aid development of cost‐effective sintered spinel as a refractory raw material, this paper presents an extensive analysis of microstructure and complex phase evolution of Al‐rich, Mg‐rich, and stoichiometric spinel aggregates derived from Indian magnesite and calcined alumina. Pore morphology in Al‐rich spinel was transformed upon sintering at 1650°C and corundum laths embedded in porous Al‐rich spinel matrix was formed. Stoichiometric spinel sintered at 1600°C consisted of mostly direct bonded angular equiaxed spinel grains which incorporated the impurities in solid solution. Mg‐rich spinel was composed of spinel grains with reduced angularity along with intergranular amorphous phase, small round monticellite grains, and periclase. EDS line scan revealed impurity‐free joins existed between direct bonded spinel grains. Mg‐rich spinel containing 0.65 wt% ZrO₂ formed cubic ZrO₂‐CaO‐MgO solid solution located along spinel boundaries, which reduced both intergranular amorphous phase and monticellite. This increased SiO₂ and MgO content in spinel solid solution triggering exsolution of metastable cubic forsterite manifested as split spinel peaks in XRD pattern. A 14.7% reduction in slag penetration was exhibited upon doping Mg‐rich spinel with 0.21% ZrO₂. Stoichiometric and Mg‐rich spinels attained 0.35% and 0.54% apparent porosity at 1600°C, which is better than most commercial sintered refractory spinels.     

Corrosion of MgO—MgAl2O4 Spinel Refractory Bricks by Calcium Aluminosilicate Slag

Microstructural analysis of MgO—MgAl₂O₄ refractory bricks corroded at 1400–1450°C by calcium aluminosilicate slag reveals secondary spinel, monticellite, merwinite, and MgO as microscopic corrosion products, generally forming in this sequence as the brick is penetrated. The secondary spinel forms an incomplete layer close to (but not at) the MgO grain. Thermodynamic calculations are used to support a detailed model of the corrosion mechanism.     

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.     

Mineralogy of western fly ash

Twenty-six fly ash specimens from North Dakota, Wyoming and Montana lignite and sub-bituminous source coals have been studied in detail by X-ray diffraction. Chemically, these western fly ashes are characterized by higher CaO+MgO+SO₃ contents and lower Al₂O₃+SiO₂ contents than eastern bituminous fly ashes. These western fly ashes have greater proportions of crystalline material. The characteristic phases are quartz, lime, periclase, anhydrite, ferrite spinel, tricalcium aluminate, merwinite and melilite. Alkali sulfates, a sodalite structure phase and hematite also occur in some fly ashes.     

Reactions Occurring in Basic Brick

Possible reactions occurring in basic brick were first explored by studying ( a ) binary mixtures of sesquioxides or magnesia spinels and ( b ) three silicates, namely forsterite, monticellite, and di-calcium silicate. Compositions ranging from 10 to 90% of each constituent were pressed into cylinders and heated at 100°C intervals between 800°C and a maximum of 1700°C or until melting occurred. Bulk density and volume changes were recorded and X-ray diffraction was used to explore chemical reactions. Reactions between the three silicates and three chrome ores were also studied. Since the most refractory specimens were those containing magnesiochromite as the spinel constituent, the effect of free magnesia as a third component in these series was finally explored.     

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。     

Melting Equilibria of the Bi-Sr-Ca-Cu-O (BSCCO) System in Air: The Primary Crystallization Phase Field of the 2212 Phase and the Effect of Silver Addition

The subsolidus and primary crystallization phase field of the "2212" (Bi:Sr:Ca:Cu) phase in the BSCCO system have been investigated. At 830°C, this phase was observed to be in equilibrium with ten phases. Sixteen four-phase equilibrium regions that surround the 2212 phase have been identified at this temperature. The melting events as-sociated with these four-phase regions were studied with differential thermal analysis, quenching experiments, and wicking experiments, in which samples of the liquid for analysis were absorbed by capillary action. The melt com-positions were obtained by using quantitative energy-dis-persive X-ray spectrometry. The 2212 phase melts incon-gruently, beginning at ∼825°C. Its primary crystallization field encompasses a compositional range of 24–42 mol% BiO^1.5, 7–33 mol% SrO, 2–27 mol% CaO, and 19–43 mol% CuO. The compositions of melts in equilibrium with the 2212 phase were mostly on the calcium-deficient side of the 2212 solid solution. An approximate polygonal model of the 2212 primary crystallization field has been presented. The addition of silver depresses the melting temperatures in the vicinity of the 2212 phase, from 4°C to 22°C. Silver entered the melt at a saturation level of 2–8 mol%. In the more copper-rich liquids, silver replaced some of the copper, which resulted in a shift of the 2212 crystallization field away from the copper oxide corner.     

Mineralogical characterization of a lignite gasification ash from a low-BTU fixed-bed gasifier I. X-ray phase analysis

A lignite coal ash produced in a low-Btu atmospheric pressure gasifier has been studied by X-ray diffraction. The ash was dominantly crystalline and consisted of CaOMgONa₂O-silicates (melilite, merwinite, Ca₂SiO₄-bredigite, nepheline/carnegieite), oxides (ferrite spinel, quartz, periclase, hematite), phases formed by reaction with water or precipitation from solutions used in ash handling (calcite, portlandite, brucite, thenardite, ettringite, gypsum, goethite) and several additional trace phases (plagioclase, pyroxene, cristobalite, serpentine and talc).     

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.     

Mineralogical transformations of calcareous rich clays with firing: A comparative study between calcite and dolomite rich clays from Algarve, Portugal

Mineralogical transformations during firing of two extremely calcareous clays, one calcite and other dolomite rich, and relatively poor in silica were studied. Original clays were mineralogical and chemically characterized with X-ray diffraction (XRD) and X-ray fluorescence (XRF). Firing of both clays was carried out in the temperature range 300–1100 °C under oxidizing conditions and the mineralogical transformations were investigated with XRD, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy associated with energy dispersive X-ray spectroscopy (SEM-EDS).Important compositional differences in the neoformed phases were observed between calcite and dolomite rich clays. In the Ca-rich clay the assemblage gehlenite + wollastonite + larnite was observed. In the Mg(Ca)-rich clay the reaction products included akermanite, diopside, monticellite, forsterite, periclase and spinel.XRD and SEM-EDS showed the presence, in both clays, of a potassium–calcium sulfate in samples fired between 900 and 1100 °C.     

The System MgO–MgAl2O4

In the determination of the liquidus, solidus, and subsolidus of the system MgO-MgAl₂O₄ the limits of the solid solution of A1 ions in periclase and Mg ions in spinel were measured. By using both X-ray diffraction and optical techniques, the maximum periclase solid solution was found at 82 wt% MgO, 18 wt% A12O3 (9.5% A1³⁺) and maximum spinel solid solution at 39% MgO, 61 % A1203 (6% Mg⁺⁺). Periclase and spinel solid solutions existed stably in easily detectable amounts at temperatures above approximately 1500°C.     

Synthesis of Magnesium Aluminate Spinels from Bauxites and Magnesias

The synthesis of refractory Mg-Al spinel aggregates from bauxites and magnesias was addressed by applying the alumina–magnesia–silica ternary equilibrium phase diagram as the fundamental basis. Four different alumina sources (bauxites) and four different magnesia sources were investigated. These were fired in air through 1700°C and their reactions were monitored using X-ray diffraction. Evolution of their microstructure was observed by scanning electron microscopy. Initially, the periclase reacts with the free corundum of the bauxite to produce Mg-Al spinel. The periclase then reacts with the mullite in the bauxites to yield additional spinel and also some forsterite. Spinels were produced for all sixteen different raw-material combinations.     

Liquidus Relationships on 10% MgO Plane of the System Lime-Magnesia-Alumina-Silica

The system CaO-MgO-Al₂O₃-SiO₂ is well known for its importance in many fields of ceramic technology, especially in the field of industrial refractories. The present study, although providing additional data on the liquid phases in basic refractory materials, was undertaken primarily to determine the composition-melting- point behavior of iron blast-furnace slags. About 95% of the composition of blast-furnace slags generally can be expressed in terms of CaO, MgO, Al₂O₃, and SO₂, the additional constituents being FeO, MnO, and S. The MgO content of these slags depends on the type of "stone" used as a flux, which may be essentially pure limestone, dolomitic limestone, or a mixture of limestone and dolomite as determined by economic factors related to sources of raw materials. The amount of MgO usually varies between 2 and 10% in the final slag, and it is often important to know the effect on the melting point of a substitution of MgO for CaO. In the present study one hundred and two glass compositions were prepared for liquidus and in some cases secondary-phase temperature determinations by the quenching method. The investigation was confined chiefly to the central part of the plane adjacent to the lime-silica-magnesia face of the tetrahedron. Primary fields of s'lica, diopside, anorthite, pseudowollastonite, cordierite, melilite, spinel, mullite, merwinite, dicalcium silicate, periclase, and corundum were encountered in the course of the work. Liquidus temperatures ranged from a minimum of 1230°C. at the diopside-anorthite-tridymite intersection to a maximum of about 1650°C. in the dicalcium silicate field, which was the upper safe limit for the equipment. Petrographic and X-ray diffraction methods were used to identify phases.     

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

When a magnesia-chrome refractory is heated in air a reaction layer develops around the chromite grains. This layer is magnesioferrite at 800°C; above 800° it comprises a solid solution of spinel of the type Mg(Al,Cr,Fe³⁺)₂O₄ and a magnesiowustite solid solution. As the temperature increases, the composition of the spinel in the reaction layer changes toward enrichment in chromium and aluminum and impoverishment in iron. A direct-bond chromite-periclase, well defined at about 1750°C, is formed essentially by diffusion. In slowly cooled specimens the average composition of the spinel in the reaction layer in the direct bond approximates to Mg(Al_0.05Cr_0.40-Fe_0.10)₂O₄. The order of diffusion of the individual ions from chromite to periclase is: Fe ≫ Cr > Al. This order can be explained by considering the charge and size of ions involved and the energy required to create cation vacancies. In specimens quenched from high temperatures the concentration of the magnesium-rich phases in the silicate pockets increases in the direction of the periclase grains whereas the calcium-rich phases are concentrated at the center of the pockets.     

Kinetics of Precipitation and Measurements of Strain in the System MgO-Al2O3-Cr2O3

A quantitative X-ray technique for measuring precipitation strains has not been previously applied in metallic or oxide systems. The Warren-Averbach analysis of strain was used to determine the buildup of elastic strain energy in the spinel crystalline solution matrix (gross composition = 60 mol% MgAl₂O₂+ 40 mol% Cr₂O₃) during the isothermal (1135°C) precipitation of a metastable (coherent) monoclinic phase. The elastic strain energy of the spinel crystalline solution matrix increased to a maximum of about 3.1 × 10⁷ ergs/cm³ for a reaction time of 8 h. There was a marked decrease in the elastic strain energy during the initial precipitation of the equilibrium corundum crystalline solution with the composition (Al³⁺_0.72 Cr³⁺_0.25)O₃. An overall diffusion activation energy for precipitation of the mono-clinic phase was approximately 86 kcal/mol.