Effect of molten basic open-hearth slag on the composition of the MgO-MgAl2O4-MgCr2O4 system

Conclusions We employed x-ray diffraction and petrographic analysis to study the reactions of molten basic open-hearth slag with compositions in the MgO-MgAl₂O₄-MgCr₂O₄ system.The phase transformations that occur during this interaction consist in solid solution of the iron and manganese oxides in the spinelides and periclase and a subsequent slight reaction between these solutions and the lime-silicate components of the melt.When combined with periclase, high-chromium spinelides absorb larger amounts of iron and manganese oxides than do high-alumina spinelides.Spinelides rich in magnesiochromite have the lowest slag resistance in the MgAl₂O₄-MgCr₂O₄ system.Addition of periclase to spinelides has a favorable effect on their chemical stability with respect to molten slag. However, combinations in which the spinel component consists of high-chromium spinelides are less stable.The degree of refractory breakdown produced by exposure to molten basic open-hearth slags is governed principally by the depth to which the melt penetrates. The chemical processes involved merely facilitate such breakdown.Translated from Ogneupory, No. 4, pp.48–52, April, 1969.     

Phase relationships and properties of the phase composition periclase-spinel

Conclusions We investigated the phase composition and properties of the compositions periclase-spinel bond in the system MgO-Mg (Al_1–xCr_x)₂O₄, where x=0–1, using spinel-phase concentrations of from 5 to 30% by weight.The essential feature of the phase inversions consists in the development and decomposition of solid solutions of spinel constituent in the periclase. With an increase in the content of magnesiochromite in the spinels, their absorption by the periclase increases. After cooling, the concentration of spinel phase in the solid solution with the periclase is about 5%.An increase in the concentration of the spinel phase from 5 to 30%, and also change in its composition in the direction from magnesio-alumina spinel to magnesiochromite (x1) impairs the thermalshock resistance, strength and porosity.To improve the properties of magnesite articles, it is desirable to use 5–15% by weight of magnesio-alumina spinel as the bond, or chromitic ore, the spinel of which contains a large quantity of magnesia-alumina constituent (MgAl₂O₄).Translated from Ogneupory, No. 5, pp. 42–50, May, 1967.     

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.     

The physicochemical processes in the reactions of the components of system Mg(Al1−XCrx)2O4-SiO2-iron oxide

Conclusions The phase transition in the firing of compositions based on the system Mg(Al_1–XCr_X)₂O₄-(Fe₂O₃, SiO₂) consist essentially of the formation of solid solutions of spinels and the ferrous component of the reagent, and the solution of the spinel in the iron-silica melt.In compositions with magnesia-alumina spinel the solution of the spinel in the melt is the dominant process in every case while in compositions with a magnesite-chromite spinel the dominance of a given process depends on the composition of the reagent.The solubility of spinels in an iron-silica melt depends on the composition of the spinel and the percent iron oxides in the primary melt while the total percent melt depends mainly on the percent silica in the original mixture. The solubility of high-alumina spinels in melts of Fe₂O₃ and SiO₂ is greater than that of high-chromite spinels in every case.Deceased.Translated from Ogneupory, No. 7, pp. 45–49, July, 1974.     

Action of iron melts on spinel-periclase refractories

Conclusions Using methods of phase, chemical, x-ray, and petrographic analysis we investigated the reaction between melts of Fe₃O₄ and MgFe₂O₄ with certain compositions on the basis of the systems MgO-Al₂O₃-Cr₂O₃.The phase inversions with the action of the melt consist in the development of solid solutions between the spinel phase and the iron reagent.Destruction by the iron melt of the spinel phases in the system MgO-Al₂O₃-Cr₂O₃ is due to the laminating scaling of the solid solutions, expecially those enriched by MgCr₂O₄ which in a large degree is expressed in the MgCr₂O₄ itself. The cause of this is that owing to the sharp differences in the composition of the zones as a result of bad wetting of MgCr₂O₄ by the iron melt, apparently, we get substantial stresses which produce lamination of the scaly growths.In the system MgO-Al₂O₃-Cr₂O₃ the lower resistance to iron melts is possessed by compositions enriched with MgCr₂O₄.The free magnesia exerts a positive effect on the resistance of the spinels to the action of iron melts. However the worst properties, as previously, are shown by compositions enriched with MgCr₂O₄.The melt of MgFe₂O₄ gives rise to substantial volume changes in the reaction zone, and to a lower degree, the same destruction of the refractories as the magnetite melts.Our practical conclusions for the technology of refractories is to express a preference for the compositions with substantial preponderence of periclase over spinels, of which, in turn, the magnesia-alumina spinel should be preferred to compositions of the type periclase-chromite, when we are considering the thermal resistance of magnesia refractories working in conditions of iron-reagent action.     

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.     

Effect of reaction of periclase and iron-chrome spinel on the properties of periclase-spinel refractories

Conclusions Using a typical composition of the type MgO-Mg(Al_1–x–yCr_xFe_y)₂O₄ a study was made of the effect of the processes involved in the processing and decomposition of solid solutions of periclase and spinels on the technical properties of the fired products.During cooling from 1600–1750°C decomposition occurs in the solid solutions of periclase and spinel; the crystals of spinel separating under these conditions are able to grow to large sizes which inevitably should lead to the development of stresses inside the periclase grains.The decomposition of the solid solution of periclase and spinel adversely affects the strength and thermal-shock resistance of the fired specimens.The best technical properties are possessed by combinations of periclase with small (up to 15%) quantities of high-alumina (x=0.16 and 0.46) spinels.The use of compositions of periclase with high-alumina spinels of the type x=0.16 and 0.46 enables us to obtain denser and more thermal-shock resistant refractories.Translated from Ogneupory, No.6, pp. 44–47, June, 1970.     

Some properties of highly refractory ceramics in the system MgO-MgAl2O4

Conclusions A region for compositions in the system MgO-MgAl₂O₄was detected which has great interest for practical use.Compositions containing up to 15% Al₂O₃ permit a big improvement to be obtained in the spalling resistance of periclase ceramics with a simultaneous reduction in the sintering temperature.Spinel and compositions close to it are distinguished from periclase by a lower volatilization capacity. This makes it possible to use the corresponding ceramic materials at temperatures that are much higher than with the ceramics containing a predominant content of periclase, despite the fact that in terms of deformation the spinel is much inferior to the periclase. The limiting service temperature of periclase ceramics (especially in vacuum) is limited not by the thermomechanical and refractory properties but by the high volatilization of the magnesia.It should be mentioned that there is a reduction in the sintering temperature and that bodies close in composition to spinel have high strengths.Bodies of intermediate composition containing from 20 to 50% Al₂O₃ apparently are of no practical interest as highly refractory ceramics.     

Phase composition and some properties of the fired products of composition Mg (Al1-xCrx)2O4+MgFe2O4

Conclusions The phase composition of the fired products of the composition Mg (Al_1–xCr_x)₂O₄+MgFe₂O₄ is determined not only by the temperature and gaseous atmosphere, but by the capacity of the system for sintering-open porosity. The secondary phase which separates, and is of variable composition-magnesio-wustite to magnomagnetite -oxidizes during cooling, and is absorbed by the spinels.Solid solutions of magnesia-aluminous spinel containing magnesioferrite possess better properties than solid solutions of chrome-containing spinels and magnesioferrite which are destroyed with a content of 40–50% MgFe₂O₄.The results of the investigation suggest that destruction is due to oxidation of the products, preventing reduction of magnesioferrite.The possibility of destruction is determined also by the degree of reduction of the magnesioferrite, the rate of oxidation, and the temperature range in which it occurs.Translated from Ogneupory, No. 2, pp. 44–50, February, 1968.     

Reactions between MgAl2O4 and MgCr2O4 spinels and phosphate bonds

Conclusions A study was made of the reaction between spinels MgAl₂O₄ and MgCr₂O₄ with orthophosphoric acid and sodium chromophosphate bond. The setting of the material is related to the formation of various hydrated phosphates and hydrates. During heating complex phase inversions take place, with the formation of intermediate compounds. At elevated temperatures the stable phases are spinels.Translated from Ogneupory, No. 2, pp. 53–55, February, 1973.     

The phase conversions and properties of refractories based on the MgO-Al2O3-Cr2O3system

Conclusions An investigation of the phase conversions in the MgO-Al₂O₃-Cr₂O₃ system at a below-unity ratio MgO/R₂O₃ and varied content of sesquioxides showed that the magnesiochromite initially being formed in all mixtures interacts with the Al₂O₃ at temperatures above 1000°C with the result that two kinds of solid solutions are formed, viz., a spinel one Mg(Cr_xAl)_1–x)₂O₄ and (Al_xCr_1–x)₂O₃. The predominance of a given component in the original composition remains preserved in both phases.The presence was established of a solid solution of Al₂O₃ in the spinel phase which contained a signficant amount of MgAl₂O₄. No solid solutions of R₂O₃ are formed in the spinel when the chromite component predominates in both phases.The changes in the properties of the specimens correspond to the phase conversions. After high-temperature firing materials of the type Mg(Al, Cr)₂O₄-(Al, Cr)₂O₃ possess adequate property indices.The analysis is concerned with that part of the system for which MgO/(Al₂O₃ + Cr₂O₃) is less than unity.Translated from Ogneupory, No. 8, pp. 48–53, August, 1976.     

Electrochemical performance of quaternary (1-x)ZnMn2O4/(x)MgFe2O4 solid solution as supercapacitor electrode

Novel nano quaternary metals solid solutions from MgFe₂O₄ and ZnMn₂O₄ were synthesized using a sol-gel procedure. The development of a solid solution and the formed phase were examined by phase analysis utilizing the X'Pert High Score Plus program and Fourier transform infrared (FTIR) spectroscopy technique. Rietveld analysis of X-ray diffraction data (XRD), scanning electron microscope (SEM/EDS) and transmission electron microscope (TEM) were applied to determine the lattice parameters, crystallite size, different cations distribution and elemental analysis of the formed solid solutions. The measured dielectric properties of obtained solid solutions are found to be affected by the composition ratio (x). The solid solution (1-x)ZnMn₂O₄/(x)MgFe₂O₄ samples exhibited a ferroelectric–paraelectric transition at ferroelectric Curie temperature (T_c). The different hopping mechanisms in the different samples were also examined. The electrochemical performance was tested and influence of composition ratio (x) on the cathodic and anodic potential was investigated. The specific capacitance (C_s) value of electrodes depended on the composition ratio (x) beside the type of cations forming oxides. The (0.9)MgFe₂O₄/(0.1)ZnMn₂O₄ sample showed the best performance as a supercapacitor material. The outstanding electrochemical property of the (0.9)MgFe₂O₄/(0.1)ZnMn₂O₄ electrode was further confirmed by EIS inspection. The super stability of MgFe₂O₄/ZnMn₂O₄ solid solution could be attributed to the activation of the solid solution materials during the CV cycling and the synergistic effects.     

Phase formation during the sintering process of a chromium ore-concentrate having a titanium-containing additive and during the firing process of the periclase-spinellide products based on it

Conclusions It was shown that the interaction occurring between the titanium-containing sintering additive and the chromium ore-concentrate during the firing process leads to the formation of the solid solutions of titanium dioxide in spinellide and the titanates of the trivalent metals [(Cr_1–x–yAl_xFe_y)₂TiO₅], that are localized mainly in the form of films and inclusions at the surface of the spinellide grains.The disappearance of the titanates during the firing process of the periclase-spinellide products obtained using the aforementioned chromium ore-concentrate and periclase can be attributed to the interaction of the free magnesium oxide with the titanates and the subsequent dissolution of the intermediate spinellide in the original spinellide.Thus, a complete and uniform appropriation (distribution) of the titanium dioxide additive in the form of a solid solution in the spinellide is ensured by introducing an excess quantity (with respect to the composition of the spinellide) of magnesium oxide into the charge.It is advisable to carry out a detailed study of the effect of the content of titanium dioxide in the spinellide on the physical and technological properties and the service characteristics of the refractories.Translated from Ogneupory, No. 7, pp. 2–5, July, 1993.     

Phase equilibrium in binary La2O3-Dy2O3 and ternary CeO2-La2O3-Dy2O3 systems

Cerium oxide doped with oxides of rare earth elements is a multifunctional material, a wide range of uses which is associated with its unique physicochemical properties. Phase diagrams of multicomponent systems are the physicochemical basis for the creation of new materials with improved characteristics.In this work, phase equilibria in ternary CeO₂–La₂O₃–Dy₂O₃ and binary La₂O₃–Dy₂O₃ systems in the whole concentration range were studied. No new phases have been identified in these systems. An isothermal section of the phase diagram of the CeO₂–La₂O₃–Dy₂O₃ system at a temperature of 1500 °С is constructed. No new phases have been detected in the system. It was found that in the studied ternary system solid solutions are formed on the basis of (F) modification of CeO₂ with structure of fluorite type, monoclinic (B), cubic (C) and hexagonal (A) modifications of Ln₂O₃.In the La₂O₃–Dy₂O₃ binary system (1500–1100 °С) three types of solid solutions are formed: based on hexagonal modification A-La₂O₃, monoclinic modification B-Dy₂O₃ and cubic modification C-Dy₂O₃ separated by two-phase fields (A+B) and (B+C), respectively. The boundaries of the regions of homogeneity of solid solutions based on A-La₂O₃ are determined by compositions containing 35–40, 20–25, 15–20 mol% Dy₂O₃ at 1500, 1250, 1100 °C, respectively. From the obtained data it follows that the solubility of Dy₂O₃ in the hexagonal modification of lanthanum oxide is 39 mol% at 1500 °C, 23 mol. % at 1250 °C and 16 mol% at 1100 °C. The limits of existence of solid solutions based on monoclinic B-modification are determined by compositions containing 30–35, 65–60 (1250 °С), 35–40, 55–60 (1100 °С) 40–45, 70–75 (1500 °C) mol% Dy₂O₃.In the studied system, with a decrease in temperature from 1500° to 1100°C, there is a decrease in the solubility of La₂O₃ in the crystal lattice of cubic solid solutions of C-type from 16 to 10 mol%.     

Computer modeling of the local structure and mixing properties of a solid solution of MgAl2O4-MgCr2O4

An original method of the atomistic computer modeling of substitutional solid solutions is applied to the noble spinel MgAl₂O₄-magnesia chromite MgCr₂O₄ binary system. The parameters of the interatomic potentials in the partially ionic approximation are optimized using the experimental values of the structural, elastic, and thermodynamic characteristics of these minerals. The point defect formation energy is estimated. The modeling of Mg(Al _x Cr_{1 ? x} )₂O₄ solid solutions is carried out in a supercell of the 4 × 4 × 4 structural type of spinel that contains 3584 atoms; the GULP 3.4.9 program, which is implemented in the SKIF Chebyshev supercomputer (Moscow State University), is used. The mixing properties of the solid solution such as the mixing enthalpy, the interaction parameters, the mixing volumes, the deviations of the compression moduli from additivity, and the oscillatory entropy were determined. Calculations of the Gibbs energy have yielded the assessments of the stability fields of the MgAl₂O₄-MgCr₂O₄ solid solution and the comparisons of these assessments with the experimental data. Histograms of interatomic spacings are plotted and the values of the relaxation parameters of the Cr-O and Al-O bonds are estimated; these values agree well with the experimental values obtained by measuring the optical absorption spectra on a Cr³⁺ ion.     

Synthesis and characterization of nano-LiMn2O4 powder by tartaric acid gel process

Polycrystalline nano-LiMn₂O₄ spinel powder was synthesized by the tartaric acid gel process and developed without any detectable minor phase at 300 °C. The powders synthesized by the tartaric acid gel process had a relatively smaller particle size, larger specific surface area and narrower particle size distribution than those prepared by the conventional solid state reaction. The average valence of manganese decreased with increasing synthesis temperature and resulted in an increase of the lattice parameter with calcination temperatures. As temperatures were increased to above 500 °C, LiMn₂O₄ underwent phase transition from a cubic to a tetragonal phase by removing oxygen ion in the lattice. From the results of MAS NMR, LiMn₂O₄ spinels formed at 300–800 °C had a large Knight shift of ∼520 and 560 ppm in reference to LiC1.     

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.     

HfO2-based refractory compounds and solid solutions: 2. Kinetics and mechanism of compound formation in the systems HfO2M2O3 (MO)

In a previous communication¹ we considered the papers published in the USSR on studies of phase diagrams for the systems hafnia-alkali-earth oxides and hafnia — rare-earth oxides. The purpose of many investigations was the preparation of new refractory compounds and solid solutions which could create the basis of high-refractory materials. In addition to the great analogy between binary systems with hafnia and zirconia, our interest in hafnium-containing compounds was promoted by their higher melting temperatures compared with those of zirconia-based compounds and by several peculiar physico-chemical properties characteristic of hafnia-based compounds and solid solutions. It has been shown¹ in the HfO₂MO systems, strontium forms the greatest number of compounds: SrHfO₃, Sr₂HfO₄, Sr₃Hf₂O₇ and Sr₄Hf₃O₁₀; calcium forms two compounds: CaHfO₃ and CaHf₄O₉; barium and magnesium each produce one compound: BaHfO₃ and Mg₂Hf₅O₁₂, respectively. Rare earth oxides with large ionic radii usually form pyrochlore-type (P) compounds of composition Ln₂Hf₂O₇, whereas rare-earth oxides with small ionic radii combine with hafnia to form wide regions of fluorite-like solid solutions. In the region of compositions corresponding to the pyrochlore-type compounds rare-earths with the smallest ionic radii (Sc, Yb) form compounds with rhombohedral structures (R) of the composition M₄Hf₃O₁₂(M₇O₁₂). Together with the latter, compounds M₇O_11.5 and M₇O₁₁ were found for gadolinium and erbium and M₉O₁₇ and M₇O₁₃ for scandium. Most of these compounds are stable only over a certain temperature range and transform into disordered F-type cubic solid solutions at elevated temperatures. In addition, there exist limited regions of solid solutions based on the monoclinic (M) and tetragonal (T) modifications of HfO₂ as well as on various structural modifications (C, B, A, H, X) of rare-earth oxides.     

Synthesis of Ga2O3–Al2O3 Catalysts by a Coprecipitation Method for CH4-SCR of NO

Synthesis of the γ-Ga₂O₃–Al₂O₃ solid solutions by the coprecipitation method was examined. Precipitates were obtained by adding at once an aqueous solution of aluminum and gallium nitrates to an excess base solution (reverse strike precipitation). The precipitates were calcined at 700 °C to give γ-Ga₂O₃–Al₂O₃ solid solutions. When an ammonium carbonate solution was used as the precipitant, ammonium aluminum carbonate hydroxide (ammonium dawsonite, AACH)–ammonium gallium carbonate hydroxide (AGCH) solid solutions were obtained. Calcination of the AACH–AGCH solid solutions at 700 °C gave the γ-Ga₂O₃–Al₂O₃ solid solutions which exhibited high activity for selective catalytic reduction of NO with methane.     

Cast refractories of the Al2O3-Cr2O3-CaO system

Conclusions In order to develop stable materials for glassmaking, we studied the physical and technological properties of the refractories belonging to the Al₂O₃-Cr₂O₃-CaO system that contain 5–20% CaO, 15–35% Cr₂O₃ and 45–80% Al₂O₃.The glass resistance of the refractories of the experimental systems (compositions) exceeds that of the BKCh-33 baddeleyite-corundum products by 3–5 times and their thermal shock resistance is superior to that of the well known chromium-containing refractories at comparable levels of mechanical properties.The developed refractories are recommended for the top or the bottom structures of the glassmaking furnaces depending on their glass resistance and thermal shock resistance and for making the refractory components of ferrous metallurgical units that are in contact with highly basic slags.Translated from Ogneupory, No. 3, pp. 23–26, March, 1989.