Experimental study of phase equilibria in the system PbO-ZnO-SiO2

An experimental study on the ternary system PbO-ZnO-SiO₂ in air by high-temperature equilibration and quenching techniques followed by electron probe X-ray microanalysis was carried out as part of the wider research program on the six-component system PbO-ZnO-SiO₂-CaO-FeO-Fe₂O₃, which combines experimental and thermodynamic computer modeling techniques to characterize zinc and lead industrial slags. Liquidus and solidus data were reported for all primary phase fields in the system PbO-ZnO-SiO₂ in the temperature range 640 °C to 1400 °C (913 to 1673 K).     

Phase relations and thermodynamics of the system Fe-Cr-0 in the temperature range of 1600 °C to 1825 °C (1873 to 2098 K) under strongly reducing conditions

Equilibrium relations involving alloy and oxide phases in the system Fe-Cr-O were determined in the temperature range from 1600 °C to 1825 °C (1873 to 2087 K). Compositions of coexisting alloy and spinel phases were established as a function of oxygen pressure by equilibrating liquid Fe-Cr alloys with iron chromite (Fe_3-xCr_xO₄) solid solutions at 1600 °C and 1700 °C. Combinations of these experimental data and thermodynamic calculations were used to construct composition-oxygen pressure diagrams for the system at 1600 °C and 1700 °C. Additional runs for selected mixtures were made at still higher temperatures (1700 °C to 1825 °C), and thermodynamic parameters were derived for spinel-containing phase assemblages at temperatures up to 1865 °C. The spinel phases occurring in the present system are typically in the high-chromium range of the solid-solution series Fe₃O₄-Cr₃O₄,i.e., in the range between stoichiometric iron chromite (FeCr₂O₄) and Cr₃O₄. The activities of the various oxide components of the spinel solid solution at 1600 °C were calculated from experimentally determined parameters for coexisting alloy and spinel phases, as well as by statistical-mechanical modeling of the same spinel solid solution based on crystal-chemical considerations. The agreement between the two sets of results was excellent. Temperature variation of parameters characterizing the univariant equilibria spinel + Cr₂O₃ + alloy and spinel + alloy + liquid oxide was established. The univariant curves were found to display temperature maxima of 1715 °C ± 5 °C and approximately 1865 °C, respectively. In analogy with relations in the Cr-O system, the increase in divalent chromium of the liquid oxide phase with decreasing oxygen potential was identified as the main cause of the sharp decrease in liquidus temperatures of chromites in contact with Fe-Cr alloys of high Cr contents. Formerly Graduate Research Assistant, Department of Metallurgy, The Pennsylvania State University L.S. DARKEN and ARNULF MUAN, formerly Professors of Geochemistry and Materials Science, The Pennsylvania State University, University Park, PA 16802, are deceased.     

Experimental Investigation of Gas/Slag/Matte/Tridymite Equilibria in the Cu-Fe-O-S-Si System in Controlled Gas Atmospheres: Experimental Results at 1473 K (1200 °C) and <Emphasis Type="Italic">P</Emphasis>(SO<Subscript>2</Subscript>) = 0.25 atm

Experimental studies were undertaken to determine the gas/slag/matte/tridymite equilibria in the Cu-Fe-O-S-Si system at 1473 K (1200 °C), P(SO₂) = 0.25 atm, and a range of P(O₂)’s. The experimental methodology involved high-temperature equilibration using a substrate support technique in controlled gas atmospheres (CO/CO₂/SO₂/Ar), rapid quenching of equilibrium phases, followed by direct measurement of the chemical compositions of the phases with Electron Probe X-ray Microanalysis (EPMA). The experimental data for slag and matte were presented as a function of copper concentration in matte (matte grade). The data provided are essential for the evaluation of the effect of oxygen potential under controlled atmosphere on the matte grade, liquidus composition of slag and chemically dissolved copper in slag. The new data provide important accurate and reliable quantitative foundation for improvement of the thermodynamic databases for copper-containing systems.     

Phase Equilibria Studies of the Cu-Fe-O-Si System in Equilibrium with Air and with Metallic Copper

Phase equilibria of the Cu-Fe-O-Si system have been investigated in equilibrium: (1) with air atmosphere at temperatures between 1373?K and 1673?K (1100?°C and 1400?°C) and (2) with metallic copper at temperatures between 1373?K and 1573?K (1100?°C and 1300?°C). High-temperature equilibration/quenching/electron-probe X-ray microanalysis (EPMA) techniques have been used to accurately determine the compositions of the phases in equilibrium in the system. The new experimental results are presented in the form of ??Cu₂O??-??Fe₂O₃??-SiO₂ ternary sections. The relationships between the activity of CuO_0.5(l) and the composition of slag in equilibrium with metallic copper are discussed. The phase equilibria information of the Cu-Fe-O-Si system is of practical importance for industrial copper production processes and for the improvement of the existing thermodynamic database of copper-containing slag systems.     

Experimental study of phase equilibria in the PbO-ZnO-“Fe2O3”-(CaO + SiO2) system in air for the lead and zinc blast furnace sinters (CaO/SiO2 weight ratio of 0.933 and PbO/(CaO + SiO2) ratios of 2.0 and 3.2)

Experimental studies on phase equilibria and liquidus in the multicomponent system PbO-ZnO-CaO-SiO₂-FeO-Fe₂O₃ in air have been conducted over the temperature range between 1323 K (1050 °C) and 1623 K (1350 °C) to characterize the phase relations of the complex slag systems encountered in lead and zinc blast furnace sinters. The liquidus in two pseudoternary sections ZnO-“Fe₂O₃”-(PbO + CaO + SiO₂) with the CaO/SiO₂ weight ratio of 0.933 and PbO/(CaO + SiO₂) weight ratios of 2.0 and 3.2 have been constructed.     

Experimental study of phase equilibria in the systems PbO x -CaO and PbO x -CaO-SiO2

The reported experimental work on the systems PbO _x -CaO and PbO _x -CaO-SiO₂ in air is part of a wider research program that combines experimental and thermodynamic computer modeling techniques to characterize zinc/lead industrial slags. Extensive experimental investigation by high-temperature equilibration and quenching techniques followed by electron probe microanalysis was carried out in the temperature range 640 °C to 1500 °C (913 to 1773 K) and in the composition ranges 0 to 65 mol pct SiO₂ and 0 to 42 mol pct CaO. Liquidus and solidus data were reported for most of the primary phase fields. Liquidus surfaces in the systems CaO-Pb-O and PbO _x -CaO-SiO₂ in air were completely reconstructed. Extensive solid solutions of PbO in α′ dicalcium silicate and Ca₂Pb₃Si₃O₁₁ were measured.     

Phase diagram and thermodynamic properties of the yttrium-zinc system

The phase relations of the yttrium zinc system have been reexamined. A total of eight compounds have been identified by differential thermal analysis, metallographic and X-ray diffraction techniques. The three compounds YZn, YZn₂ and YZn_8.5 melt congruently at 1105, 1080 and 890°C, respectively. The compounds YZn₃, YZn_3.67, YZn_4.46, YZn₆ and YZn₁₂ all decompose peritectically at 905, 896, 882, 872 and 685°C, respectively. Three eutectics were found at 875°C and 23.2 wt pct Zn (29.12 at. pct), 1015°C and 51 wt pet Zn (59 at. pct) and at 863°C and 82 wt pct Zn (86 at. pct). A polymorphic transformation was observed in the YZn₂ compound that proceeds at 750°C in the yttrium rich region but drops to approximately 630°C in the zinc rich region. The thermodynamic properties of the intermetallic compounds have been determined using the dewpoint method to measure the vapor pressure of zinc. The standard free energy relations vary from δG° (YZn) = −24,020 + 8.76T to δG° (YZn₁₂) = -85,940 + 48.61T cal/mole.     

Thermodynamic Analysis of Zinc Ferrite Decomposition in Electric Arc Furnace Dust by Lime

Scientific basis of pyrometallurgical processing technology of dusts of electric steelmaking containing zinc ferrites was investigated. The thermodynamic analysis of the decomposition of zinc ferrite with lime was carried out. The analysis of calculated data has shown that, in order to decompose more than 90% ZnFe₂O₄, it is necessary to add no less than 46% CaO for dust, while to decompose more than 95% ZnFe₂O₄, no less than 60% CaO is necessary. The results of the calculation were verified experimentally using a laboratory furnace. The experimental calcination of dust in air with the addition of lime in an amount of 60% of the dust weight at 1000°C with a holding time of 4 h confirmed that the decomposition of zinc ferrite with calcium oxide with the formation of zinc oxide and dicalcium ferrite occurs. In addition, sublimates were also formed in an amount of 50 kg per 1 t of dust containing 29% of lead and 15% of zinc. Dust calcination with lime can be applied to transform zinc from ferrite into a soluble oxide form. Intermediate products for the recovery of zinc and lead can be obtained by the calcination. After zinc leaching, it is possible to obtain an iron-containing product, which can be used in ferrous metallurgy. This approach has a series of process advantages compared with the well-known Waelz technology. In particular, calcination with lime requires lower temperature (1000°C) than the known technology (1250°C), it eliminates the second stage of the Waelz treatment necessary to purify zinc oxide fed for leaching from halides, considerably reduces coke consumption, and significantly simplifies gas cleaning from dust due to a decrease in the amount of sublimates by a factor of 6–8.     

Effect of the Heat Treatment on the Chromium Partition in CaO-MgO-SiO2-Cr2O3 Synthetic Slags

Mg-spinel phase is known to be important for control of Cr leaching from Cr-containing slags. The objective of the present study is to get an understanding of the phase relationships in the CaO-MgO-SiO₂-Cr₂O₃ system with a view to control the precipitation of Cr-spinel in the slag phase. The equilibrium phases in CaO-MgO-SiO₂-Cr₂O₃ slag system in the range of 1673 K to 1873 K (1400 °C to 1600 °C) have been investigated experimentally and compared with the results from thermodynamic calculations. The slag compositions close to the industrial slag systems were chosen. The Cr₂O₃ and MgO contents in the slag were fixed to be 6 and 8 wt pct, respectively. The basicity (CaO/SiO₂) of the slag was varied in the range of 1.0 to 2.0. The slags were synthesized at a pre-determined oxygen partial pressure (10^?4) or air (2.13 × 10⁴ Pa) at a temperature above the liquidus temperature. The samples were then soaked at targeted temperatures for 24 hours in controlled atmosphere in order to achieve the equilibrium state before quenching in water. Four different heat-treatment regimes (defined as Ia, Ib, II.a and II.b) in Section II–D) were used in the present experiments. The lower oxygen partial pressure was maintained by a suitable mixture of CO and CO₂ gases. Phases present and their compositions in the quenched slags were studied using scanning electron microscopy coupled with energy-dispersive spectroscopy and X-ray diffraction techniques. The chromium content in the phases present was analyzed using wavelength-dispersive spectrometer. The experimental results obtained are compared with the calculation results from Factsage software. The size of spinel crystals increased drastically after slow-cooling from 1873 K (1600 °C) followed by annealing at 1673 K (1400 °C) for 24 hours (heating regimes II) compared to samples being quenched directly after soaking at 1873 K (1600 °C) (heating regime I.a). It was found that the amount of foreign elements in the spinel phase, and other phases decreased after soaking at oxygen partial pressure of 10^?4 Pa resulting in phases with less defects and foreign oxide contents compared to those treated in air. The size of spinel crystals was found to be larger in samples with lower basicity.     

Critical thermodynamic evaluation and optimization of the FeO-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO-SiO<Subscript>2</Subscript> system

A complete literature review, critical evaluation, and thermodynamic modeling of the phase diagrams and thermodynamic properties of all oxide phases in the FeO-Fe₂O₃-MgO-SiO₂ system at 1 bar total pressure are presented. Optimized model equations for the thermodynamic properties of all phases are obtained, which reproduce all available thermodynamic and phase-equilibrium data within experimental error limits from 25 °C to above the liquidus temperatures at all compositions and oxygen partial pressures. The complex phase relationships in the system have been elucidated, and discrepancies among the data have been resolved. The database of the model parameters can be used along with software for Gibbs-energy minimization in order to calculate all thermodynamic properties and any type of phase-diagram section. The modified quasichemical model was used for the liquid-slag phase. Sublattice models, based upon the compound-energy formalism, were used for the olivine, spinel, pyroxene, and monoxide solid solutions. The use of physically reasonable models means that the models can be used to predict thermodynamic properties and phase equilibria in composition and temperature regions where data are not available.     

Thermodynamic analysis of interaction processes in the Pb-Ag-Zn system

The values of thermodynamic parameters that allow one to calculate the equilibrium compositions of liquid lead and related intermetallic phases in the Pb-Ag-Zn system are calculated at t = 325–425°C. Via thermodynamic calculations, the solubility surface of components in metal is constructed for this system. The diagram can be used for analysis of the known literature data and evaluation of technical processes associated with interaction of zinc with silver in the lead melt. The results of calculation are compared with the experimental data.     

An experimental study and thermodynamic evaluation of the Fe-Mo-Ti system at 1000 °C

The phase equilibria in the Fe-Mo-Ti system at 1000 °C have been studied using the diffusion couple technique. The various phases formed during annealing at 1000 °C for 480 hours were examined by X-ray diffraction, and no new ternary phase was observed. The equilibrium com-positions of these phases and some tie-lines were measured by means of electron probe micro-analysis. The results show that the solubility of Mo in the á phase Fe₂Ti ranges from 0 to 25.03 at. pct following the formula Fe₂(Mo,Ti) and solubility of Ti in the μ phase ranges from 0 to 14.55 at. pct following Fe₇(Mo,Ti)₆, whereas the solubility of Mo in the FeTi phase is negligible. It is expected that Fe₂Mo and Fe₂Ti can form a complete series of solid solutions below 900 °C. On the basis of the sublattice models, the thermodynamic properties of the Fe-Mo-Ti system at 1000 °C have been evaluated from the present results. An isothermal section calculated at 1000 °C is presented, and it shows good agreement with the experiments. Formerly Graduate Student, Department of Physical Metallurgy and Ceramics, Royal Institute of Technology, 10044, Stockholm, Sweden.     

Experimental study of phase equilibria in the systems Fe-Zn-O and Fe-Zn-Si-O at metallic iron saturation

The reported experimental work on the systems Fe-Zn-O and Fe-Zn-Si-O in equilibrium with metallic iron is part of a wider research program that combines experimental and thermodynamic computer modeling techniques to characterize zinc/lead industrial slags and sinters in the system PbO-ZnO-SiO₂-CaO-FeO-Fe₂O₃. Extensive experimental investigations using high-temperature equilibration and quenching techniques followed by electron probe X-ray microanalysis (EPMA) were carried out. Special experimental procedures were developed to enable accurate measurements in these ZnO-containing systems to be performed in equilibrium with metallic iron. The systems Fe-Zn-O and Fe-Zn-Si-O were experimentally investigated in equilibrium with metallic iron in the temperature ranges 900 °C to 1200 °C (1173 to 1473 K) and from 1000 °C to 1350 °C (1273 to 1623 K), respectively. The liquidus surface in the system Fe-Zn-Si-O in equilibrium with metallic iron was characterized in the composition ranges 0 to 33 wt pct ZnO and 0 to 40 wt pct SiO₂. The wustite (Fe,Zn)O, zincite (Zn,Fe)O, willemite (Zn,Fe)₂SiO₄, and fayalite (Fe,Zn)₂SiO₄ solid solutions in equilibrium with metallic iron were measured.     

Experimental study of phase equilibria in the Al-Fe-Zn-O system in air

The phase equilibria in the Al-Fe-Zn-O system in the range 1250 °C to 1695 °C in air have been experimentally studied using equilibration and quenching techniques followed by electron probe X-ray microanalysis. The phase diagram of the binary Al₂O₃-ZnO system and isothermal sections of the Al₂O₃-“Fe₂O₃”-ZnO system at 1250 °C, 1400 °C, and 1550 °C have been constructed and reported for the first time. The extents of solid solutions in the corundum (Al,Fe)₂O₃, hematite (Fe,Al)₂O₃, Al₂O₃*Fe₂O₃ phase (Al,Fe)₂O₃, spinel (Al,Fe,Zn)O₄, and zincite (Al,Zn,Fe)O primary phase fields have been measured. Corundum, hematite, and Al₂O₃*Fe₂O₃ phases dissolve less than 1 mol pct zinc oxide. The limiting compositions of Al₂O₃*Fe₂O₃ phase measured in this study at 1400 °C are slightly nonstoichiometric, containing more Al₂O₃ then previously reported. Spinel forms an extensive solid solution in the Al₂O₃-“Fe₂O₃”-ZnO system in air with increasing temperature. Zincite was found to dissolve up to 7 mole pct of aluminum in the presence of iron at 1550 °C in air. A meta-stable Al₂O₃-rich phase of the approximate composition Al₈FeZnO_14+x was observed at all of the conditions investigated. Aluminum dissolved in the zincite in the presence of iron appears to suppress the transformation from a round to platelike morphology.     

The relative stabilities of Cr23C6/ Cr7C3/ and Cr3C2 and the phase relationships in ternary Cr-Mo-C system

The thermodynamic properties of CrC_6/23, CrC_3/7, CrC_2/3, and MoC_1/2 reported in the literature have been correlated with the relative stabilities of these carbide phases derived from ternary phase diagrams. On the basis of the free energy of formation for one of the carbides and the free energy changes for the various equilibria occurring in ternary Mo-Cr-C system at 1300°C, free energies of formation for the stable and unstable carbide phases have been obtained. Using this free energy data, a calculated ternary Mo-Cr-C phase diagram is presented which agrees well with the experimentally determined one.     

Reaction mechanisms for the coatings formed during the hot dipping of iron in 0 to 10 Pct Al-Zn baths at 450° to 700°C

The reaction mechanisms and the structures of the phases formed during the hot dipping of iron in 0 to 10 pct Al-Zn alloy baths at temperatures of 450° to 700°C were studied by X-ray diffraction and electron microprobe analysis techniques. A new mechanism for the inhibition reaction between iron and zinc is proposed. At bath temperatures below 600°C, a thin layer of an Fe-Al-Zn ternary compound forms on the iron surface and inhibits the growth of Fe-Zn phases. Breakdown of inhibition occurs during the dipping process when the ternary compound becomes rich in aluminum and transforms to a more stable structure which is isomorphous with Fe₂Al₅. While this breakdown is occurring, the zinc atoms react with iron and form the conventional Fe-Zn phases. In 1 to 10 pct Al-Zn baths at temperatures≥600°C a very violent, highly exothermic reaction occurs during hot dipping. This type of process is due to the electronic bond rearrangements which occur during the formation of the intermetallic Fe₂(AlZn)₅. This intermetallic forms from the reaction of aluminum-bearing FeZn₇ with the Zn-Al alloy bath.     

Experimental Investigation of Gas/Slag/Matte/Tridymite Equilibria in the Cu-Fe-O-S-Si System in Controlled Gas Atmosphere: Experimental Results at 1523 K (1250 °C) and <Emphasis Type="Italic">P</Emphasis>(SO<Subscript>2</Subscript>) = 0.25 atm

To assist in the optimization of copper smelting and converting processes, accurate new measurements of the phase equilibria of the Cu-Fe-O-S-Si system have been undertaken. The experimental investigation was focused on the characterization of gas/slag/matte/tridymite equilibria in the Cu-Fe-O-S-Si system at 1523 K (1250 °C), P(SO₂) = 0.25 atm, and a range of P(O₂)s. The experimental methodology, developed in PYROSEARCH, includes high-temperature equilibration of samples on substrate made from the silica primary phase in controlled gas atmospheres (CO/CO₂/SO₂/Ar) followed by rapid quenching of the equilibrium condensed phases and direct measurement of the phase compositions with electron-probe X-ray microanalysis (EPMA). The data provided in the present study at 1523 K (1250 °C) and the previous study by the authors at 1473 K (1200 °C) has enabled the determination of the effects of temperature on the phase equilibria of the multicomponent multiphase system, including such characteristics as the chemically dissolved copper in slag and Fe/SiO₂ ratio at silica saturation as a function of copper concentration in matte. The new data will be used in the optimization of the thermodynamic database for the copper-containing systems.     

Phase relations in the Mo-Si-C system relevant to the processing of MoSi2-SiC composites

Phase relations in the Mo-Si-C system were evaluated at 1200 °C and 1600 °C within the composition range delimited by the phases Mo₅Si₃, MoSi₂, Mo_≤5Si₃C_≤1, and SiC. The evaluation included estimation of possible equilibria from known thermodynamic data of the binary phases as well as experimental work. For the experimental evaluation, high-purity powders were hot-pressed, heat-treated, and characterized by X-ray diffraction and electron microprobe analysis. It is shown that MoSi₂ is in equilibrium with Mo_≤5Si₃C_≤1 at 1600 °C, as previously established by Nowotnyet al, (Monatsh. Chem., 1954, vol. 85, pp. 255-72), and at 1200 °C, in contrast to the Mo₅Si₃-SiC equilibrium reported by van Looet al. (High Temp.- High Press., 1982, vol. 14, pp. 25-31). The thermodynamic estimation suggests that these phase relations should extend to lower temperatures in the range of compositions investigated. Thus, the third phase in silicon-lean MoSi₂-SiC composites should be the Nowotny phase (Mo_≤5Si₃C_≤1) instead of Mo₅Si₃. The Gibbs free energy of formation at 298 K of the idealized compound Mo₅Si₃C is estimated as -40.2 kJ/mol.