Phase Equilibria in the System NiO–Al2O3–SiO2

Equilibrium diagrams for the systems NiO-SiO₂, NiO-Al₂O₃, NiAl₂O₄-SiO₂, Ni₂SiO₄-NiAl₂O₄, and NiAl₂O₄-Al₆Si₂O₁₃ were drawn from data obtained by quenching and direct observational techniques. The only intermediate compound in the binary system NiO-SiO₂ is Ni₂SiO₄, which has the olivine structure. Unlike other olivines which melt congruently, nickel olivine has an upper temperature of stability (1545°C) and at temperatures between 1545° and 1650°C, NiO and SiO₂ coexist in equilibrium. The only compound in the binary system NiO-Al₂O₃ is NiAl₂O₄, which has a spinel structure. The nickel aluminate spinel varies in composition from 50 to 35 mole % Al₂O₃ at 1800°C, and the stoichiometric NiAl₂O₄ composition has a melting point near 2110°C. Of the joins within the ternary system NiO-Al₂O₃-SiO₂ which were studied, only Ni₂SiO₄-NiAl₂O₄ is not binary. In this join, crystals of NiO exist in equilibrium with liquid and a ternary assemblage of NiO + NiAl₂O₄+ liquid is stable to 1775°C. The decomposition temperature of Ni₂SiO₄ is decreased from 1545°C in the binary system to approximately 1490°C, presumably the result of solubility of NiAl₂O₄ in Ni₂SiO₄. The join NiAl₂O₄-SiO₂ is binary in that the compositions of crystalline phases can be expressed in terms of the chosen components. The eutectic temperature in the system is 1495°C. The join NiAl₂O₄-Al₆Si₂O₁₃ is binary for the same reasons and has a eutectic temperature at 1720°C. Using the data obtained in this study and those published for the well-known system Al₂O₃-SiO₂, a liquidus surface diagram for the system NiO-Al₂O₃-SiO₂ is proposed. Nickel olivine, even though it has an upper limit of stability in the binary system, has a primary field in the ternary system NiO-Al₂O₃-SiO₂. This is the only refractory oxide system known to illustrate this so-called “typical case,” the governing principles of which have been clearly presented in discussions of phase equilibria.     

Reactions Between Aluminum Oxide and Carbon The Al2O3—Al4C3 Phase Diagram

A phase diagram of the system A1₂O₃-A1₄C₃ is proposed. Two intermediate oxycarbides, A1₄O₄C and A1₂OC, were established. Eutectic melting between alumina and A1₄O₄C occurred at 1840° C. No other low melting was observed. The alumina phase was not corundum but was similar to delta-alumina. Because of the high reactivity of aluminum carbide and all the intermediate compounds with moisture and oxygen, use of refractories based on the system A1₂O₃-A1₄C₃ must be limited to applications where these agents are excluded. The behavior of high-alumina refractories in the presence of carbon is explained.     

Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

Solid-state compatibility and melting relationships in the subsystem Al₂O₃—MgAl₂O₄—CaAl₄O₇ were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al₂O₃. The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca₂Mg₂Al₂₈O₄₆ and CaMg₂Al₁₆O₂₇) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl₄O₇, CaAl₁₂O₁₉, MgAl₂O₄, Ca₂Mg₂Al₂₈O₄₆, and CaMg₂Al₁₆O₂₇ were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al₂O₃—MgO—CaO.     

Phase Equilibrium Diagram CaO—CaF2—2CaO.SiO2

Differential thermal analysis and quenching experiments were used to establish the ternary phase diagram CaO-CaF₂-2CaO.SiO₂. Hermetically-closed platinum capsules were used to prevent fluorine loss in the form of HF, SiF₄, and CaF₂ by reaction of the CaF₂ with water vapor or SiO₂, or by evaporation. The melting point of pure CaF₂ was 1419°± 1°C. There is one binary eutectic in the system CaO-CaF₂ and there are two ternary eutectics in the system CaO-CaF₂-2CaO.SiO₂. The results of the present study were combined with literature data to construct the phase diagram CaO-CaF₂-SiO₂.     

Determination of 1600° and 1700°C. Liquidus Lines in CaO-2Al2O3 and AI2O3 Stability Fields of the System CaO—Al2O3—SiO2

The 1600° and 1700°C. liquidus lines in the CaO·2Al₂O₃ and A1₂O₃ stability fields of the system CaO-Al₂O₃-SiO₂ are determined from the chemical analyses of saturated slags at these temperatures.     

The Al2O3-Nd2O3 Phase Diagram

A tentative phase diagram for the system Al₂0₃-Nd₂O₃ is presented. Three compounds were obtained: a β -A1₂O₃-type compound, the perovskite NdAlO₃, and Nd₄Al₂O₉. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al₂O₃, mp 1900°C; Nd₄Al₂O₉, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al₂O₃, 1750°C; 23 mol% Al₂O₃,1800°C. Nd₄Al₂O9 decomposes in the solid state at 1780°C.     

The Binary System Nb2O5— SiO2

The binary system Nb₂O₅— SiO₂ has been shown to include an extensive two-liquid region over the range 5 to 80% Nb₂O₅. The minimum temperature of the two-liquid area is 1695°C. A eutectic composition occurs at 95% Nb₂O₅ and 1448°C. and another at approximately 5% Nb₂O₅ and 1695°C. The experimental results were obtained by the cone-fusion method.     

Liquid us Curves in the Quaternary System NaF—AIF3—CaF2—Al2O3

THE electrolyte that is employed in the commercial production of aluminum generally consists of a fused mixture of cryolite, aluminum fluoride, calcium fluoride, and alumina. The aluminum fluoride is added to alter the cryolite ratio (3NaF/AlF₃)* from the stoichiometric value of 1.5 to some lower value. Calcium fluoride is added to permit lower operating temperatures. Typically, the calcium fluoride content of the commercial electrolyte is usually between 7 and 9%. Alumina, of course, is continually being consumed in the production of aluminum. Periodic additions of alumina to the cell maintain the level between 2 and 50%.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Phase Equilibria in High-Alumina Part of the System CaO–MgO–Al2O3–SiO2

Results are presented of a study of phase equilibria among crystalline and liquid phases in the quaternary system CaO–MgO-Al₂O₃–SiO₂ at Al₂O₃ contents greater than 35%. Equilibrium diagrams shown are for the five triangular joins CaAl₂Si₂O₃-Ca₂Al₂SiO₇-MgAl₂O₄, Ca₂Al₂SiO₇-MgAl₂O₄-Al₂O₃, CaAl₂Si₂O₈-MgO-Al₂O₃, CaAl₂Si₂O₈-Mg₂SiO₄-MgAl₂O₄, and CaAl₂Si₂O₈-MgO-Mg₂SiO₄. The composition and nature of the four quaternary peritectic points and the relationships of univariant lines and primary phase volumes are discussed.     

Influence of SO3 on the Phase Relationship in the System CaO–SiO2–Al2O3–Fe2O3

The influence of the additive SO₃ on the phase relationships in the quaternary system CaO-SiO₂-Al₂O₃-Fe₂O₃ was investigated by observing the change of volume ratio of 3CaOSiO₂ (C₃S) to 2CaOSiO₂ (C₂S) + CaO (C) in the sintered material with the increase of SO₃ content. The primary phase volume of C₃S in the quaternary phase diagram shrank with the increase of SO₃ and disappeared when the SO₃ content exceeded 2.6 wt% in the sintered material. Changes in the peritectic reaction relationship between CaO (C), 2CaOSiO₂ (C₂S), 3CaOSiO₂ (C₃S), 3CaOAl₂O₃ (C₃A), 4CaOAl₂O₃Fe₂O₃ (C₄AF), and liquid were also observed and discussed.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

THE SYSTEM: Al2O3.SiO2

This paper deals with a study of the equilibrium relations of mixtures of pure alumina and silica at high temperatures. The results are expressed concisely in the form of an equilibrium, diagram and their bearing on ceramic problems is discussed. The principal feature of the diagram is the absence of the compound Al₂O₃.SiO₂, the only compound being 3Al₂O₃.2SiO₂. Crystals of this latter compound occur in all alumina-silica refractories. The optical properties of these crystals have been determined and are compared with those of sillimanite, Al₂O₃.SiO₂, which has hitherto been regarded as the crystalline compound occurring in refractories and clay bodies in general. The behavior of natural sillimanite on heating is discussed.     

Phase Equilibrium Relationships at Liquidus Temperatures in the System FeO–Fe2O3–Al2O3–SiO2

Phase equilibrium data at liquidus temperatures are presented for mixtures in the system FeO–Fe₂O₃–Al₂O₃–SiO₂. The volume located between the 1 and 0.2 atm. O₂ isobaric surfaces of the tetrahedron representing this system was studied in detail. Scattered data were obtained at lower O₂ pressures. Results obtained in the present investigation were combined with data in the literature to construct a phase equilibrium diagram, at liquidus temperatures, for the entire system FeO–Fe₂O₃–Al 2 O₃–SiO₂. Methods for interpretation of the diagram are explained.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.