Volume 32, Issue 3 - The Ninth International Conference on Molten Slags, Fluxes and Salts (MOLTEN 12)

The thermodynamic behavior of lead in Cu-Fe matte was investigated using a transportation technique where argon gas was bubbled into a bath of copper matte containing approximately 100 ppm lead at temperatures between 1300 and 1400 °C. The effect of flow rate, temperature and matte grade were investigated on the lead transport from a bath containing 100 grams of synthetic copper matte to the gas phase. The concentration of lead in the bath was followed with time. At argon flow rates between 3 l h 1 and 18 l h −1 , it was observed that the concentration change of lead in the matte was found to follow a first order relationship where the calculated concentration at time t , is of the form [Pb] = a.e −b.t , where a is equal to the initial lead content in ppm, [Pb] i , and b is an exponential term and t is time in minutes. The partial pressure of lead species in the gas phase was calculated from the concentration changes in the matte and from the bubbling gas rate. At gas flow rates between 3 and 9 l h −1 , the lead removal appeared to be under equilibrium conditions. At higher gas flow rates, the apparent rate decreased, mainly due to splashing of matte into the cold zone of the furnace. In these experiments with Ar as the carrier gas, the sulphur and oxygen partial pressures of the melt were not controlled. Chemical analysis of the major components in the matte showed only random variation with bubbling time, and so a thermodynamic solution model for copper matte was used to calculate the equilibrium sulphur pressure expected. From this approach the proportion of Pb, PbS and Pb 2 species in the gas could be calculated knowing the relevant reaction constants, e.g., PbS(g) = Pb(g) + ½S 2 (g). From the proportions of the lead species in the gas, the value of the lead activity coefficient with respect to the gas state could be determined. For a 50% copper matte, it was found that the activity coefficient increased with temperature, from a value of 0.8 at 1300 °C to 1.4 at 1400 °C. At the white metal composition, this value was 0.28 at 1300 °C. These results are compared with other relevant studies in the literature.

According to the ion and molecule coexistence theory (IMCT) for molten slags and corresponding phase diagrams, a thermodynamic model for the evaluation of mass action concentrations (designated by N i for structure unit i) for Ce 2 O 3 -Al 2 O 3 , CaO-Al 2 O 3 -Ce 2 O 3 and CaO-Al 2 O 3 -SiO 2 -Ce 2 O 3 slag systems has been formulated. The results show that when the mole fraction of Ce 2 O 3 is at 0.49 in the binary slag, the mass action concentration of Ce 2 O 3 ·Al 2 O 3 achieves its maximum value 0.90. In a composition area of CaO/Al 2 O 3 (mass fraction ratio) from 0.7 to 1.5 and Ce 2 O 3 content (mass fraction) from 0 to 45% of the ternary slag at 1773 K, the results show that trends of calculated N Al 2 O 3 are in good agreement with reported measured trends of a Al 2 O 3 . The iso-curves of N Ce 2 O 3 and N CaO are also drawn from the calculated results. A thermodynamic model of the equilibrium between 55.83%CaO-23%Al 2 O 3 -11.17%SiO 2 -10%Ce 2 O 3 quaternary slag and molten steel containing aluminum has been further discussed. Results indicate that certain content of Ce would be dissolved in steel, which increases with the increasing of temperature and aluminum content, and when the aluminum content is 0.02%, the corresponding cerium content is in the range of 1.87 to 13.30 ppm at 1873 K. In summary, using Ce 2 O 3 to replace Al 2 O 3 can effectively reduce N Al 2 O 3 , certain content of Ce can dissolved in steel.

As a popular thermodynamic calculation method for binary alloys, Miedema's model has been applied in many fields. Chou's Model, a new generation of geometric model for ternary and multicomponent alloy systems, overcomes the intrinsic theoretical defects (including symmetric and asymmetric) existing in some original geometric models. Here, by means of combining Miedema's model and Chou's model as well as including the consideration of the excess entropy we attempted to build the new thermodynamic model to evaluate thermodynamic properties of ternary and multicomponent alloying systems in terms of the physical parameters (molar volume, electronegativity, electronic density and melting point) of constituents. Moreover, the activity and interaction coefficients of a wide of components in iron melt have been discussed in details.

The aim of the present study is the CALPHAD modeling of the systems CaO-SiO 2 -M 2 O (M=K,Na) based on a careful review of the available literature data – phase diagram and thermodynamics – as well as own experiments. The heat capacities (Cp) of three compounds, CaSiO 3 , K 2 Ca 2 Si 2 O 7 and K 8 CaSi 10 O 25 (determined using drop calorimetry), were included in the optimization of the ternary phase diagram CaO-SiO 2 -K 2 O.

Reduction of titano-magnetite powder containing 56.9 mass% of iron and 9.01 mass% of TiO 2 with H 2 -Ar gas mixtures was investigated in isothermal experiments using thermo-gravimetric analyzer (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reduction of titano-magnetite was proved to proceed via a dual reactions mechanism. The first reaction is reduction of titano-magnetite to wüstite and ilmenite and the second one is reduction of wüstite and ilmenite to iron and titanium-containing phase. It was found that the dual reactions occurred simultaneously during the reduction. The reduction kinetics of titano-magnetite was analyzed according to a dual reactions kinetic model and the results indicated that the gaseous species diffusion in product layer was the rate controlling step for the first reaction, and interfacial chemical reaction was that for the second reaction. The apparent activation energies were extracted to be 98 kJ/mol and 115 kJ/mol for the first and second reaction, respectively.

The published phase diagrams of some key P 2 O 5 -containing systems which are relevant to the steelmaking slag and the available experimental data on phosphorus partitioning between liquid iron and slags consisting of SiO 2 -Al 2 O 3 -Fe 2 O 3 -FeO-MnO-MgO-CaO-Na 2 O have been reviewed and assessed. A set of data under carefully controlled experimental conditions, which was considered to be more reliable based on the assessment, was selected for optimising the generalised central atom (GCA) model parameters of phosphorus-containing slag systems. The developed model database is proved to be able to represent the liquidus temperature of some key P 2 O 5 -containing systems and the phosphorus distribution ratio between the steelmaking slags and liquid iron reasonably well. With the developed GCA model database, the dephosphorization reaction in the steelmaking process was modelled under various operating conditions such as slag chemistry and temperature. The results show that the phosphorus distribution ratio between the slags and liquid iron displays a maximum point with variation of the FeOx content in the slag. It also shows that the phosphorus deportment to the slag is favored by decreasing the operating temperature and MgO content, and increasing the CaO/SiO 2 ratio in the slag. Comparison with the model of the phosphorus distribution data from a commercial BOS furnace shows that operating conditions do not permit to reach P equilibrium contents. The dis-equilibrium degree of P was found to be increased with increasing slag viscosities.

The Na 2 O-MgO-CaO-FeO-Fe 2 O 3 -Al 2 O 3 -SiO 2 -P 2 O 5 system is a basic oxide system for the Basic Oxygen Furnace (BOF) process as well as the hot metal dephosphorization process. Numerous experimental investigations on this oxide system are being carried out to find out an advanced process route for P removal from molten iron. In spite of their industrial importance, however, phase equilibria in oxide systems containing P 2 O 5 have not been well investigated due to the complexity of their chemistry. No systematic thermodynamic modeling of these systems has been conducted to date, either. In order to meet the strong demands of steelmaking and other industries, new systematic thermodynamic modeling of the P 2 O 5 -containing oxide systems (Na 2 O-MgO-CaO-FeO-Fe 2 O 3 -Al 2 O 3 -SiO 2 -P 2 O 5 ) and key phase diagram experiments have been carried out over the past years. In the present study, the results of the thermodynamic modeling of unary, binary and ternary P 2 O 5 -containing systems and the applications of the thermodynamic database to the dephosphorization by multi-component slag in BOF process are presented in comparison with experimental data. All thermodynamic calculations were performed using FactSage thermodynamic software.

The dissolution of chromite from the Bushveld Complex of South Africa in liquid slags was studied in the temperature range 1550° to 1665°C under argon gas. The slag compositions were similar to those of ferrochromium production and stainless steel making. Empirical relations between the slag composition and the dissolution of chromite were established through the use of a statistical model. The dissolution process was investigated by using the rotating cylinder technique and measured by the chemical analysis of the samples taken from the melt and the SEM-EDAX analysis of the reacted chromite cylinder samples. The chromite grains were depleted in iron and chromium as the dissolution progressed, leaving behind an alumina and magnesia rich spinel. The experimental data was evaluated by using kinetic models and mass transfer coefficients of chromium, iron and oxygen ions through the phase boundary between the solid chromite cylinder and the liquid slag were determined. The dissolution of chromite in liquid slags increases with increasing stirring rate. SEM-EDAX studies on the reacted chromite cylinders showed that coring took place within the chromite grains subjected to dissolution for sufficient length of time. Chromium and iron concentrations in the chromite were decreasing from the centre towards the surface of the grains while aluminum and magnesium contents were increasing at the edges compared to the centre of the chromite grains. Furthermore, the slag rich in alumina and magnesia diffuses in bulk into the chromite with a net result of increase in the concentration of these elements. The rate of dissolution of chromite in liquid slags was found to be controlled by the mass transfer of oxygen ions (O 2− ) through the liquid phase boundary between the solid chromite and the liquid slag. The activation energy for the mass transfer of O 2− ions was calculated as 30.61 kCal/mol (128.07 kJ/mol).

Structure characteristics of fluoride-Free mold flux containing simultaneously B 2 O 3 and TiO 2 have been investigated by Raman spectroscopy in this work. Raman spectra for glass samples with different basicities, different contents of TiO 2 and B 2 O 3 were recorded during the experiments. According to the experiments results, increase of TiO 2 content leads to the appearance of [TiO 4 ] and [TiO 6 ] structure groups, and [TiO 4 ] becomes the main structure unit in the system. TiO 2 produces a certain destructive effect on Si-O-Si network structure as well as large borate group and conducive to the formation of some other complex structure groups, such as (Si,Ti) coupling in sheet unit. It can be concluded that, with the increase of B 2 O 3 , the ratio of mixing of the Q 0 structure unit and [TiO 4 ] structural group decrease and the ratio of sheet structure unit increase, and there forms large borate group. Existence of B 2 O 3 increases polymerization degree of the slag system. In addition, increasing basicity causes to the decrease of Q 2 and sheet structure unit and increase of mixing of the Q 0 structure unit and [TiO 4 ] structural group, and weakening the large borate group. It could be concluded that the increase of basicity reduces the degree of polymerization of the system.

Hexagonal BN powder with average particle size of 1.2 µm was exposed at 1273 K in H 2 O/air gas mixture with the range of mass ratio from 0:100 to 6.5:100 at a total pressure of 1 atm. Thermalgravimetry (TG) method was used to measure the reaction behavior of BN powder in water vapor. XRD and SEM were employed to analyze the phases and morphological revolution during reaction. The TG curves showed that the overall reaction process could be divided into two stages. In first stage, weight gained rapidly and the main reaction was the oxidation of BN. The weight gain turning point of the curves with different water-vapor contents varied, indicating that water-vapor had an impact on oxidation rate to some degree. In second stage, weight loss due to B 2 O 3 reacting with H 2 O to form volatile product. The rate of weight loss increased with increasing the water vapor content. The reaction rate was described by a linear rate law, i.e., α  = − k l t . The relationship between k l and partial pressure of water vapor p was described by a function of k l  = 0.4 p . The determination of the activation energy was complicated due to variations in the reaction mechanism.

Liquidus temperatures of mould fluxes for casting of Ti-stabilized stainless steel were determined by differential thermal analysis (DTA) techniques in the present work. Multiple DTA curves were employed to guarantee better accuracy for liquidus temperature determination. To investigate effect of compositions change on solidification of mould fluxes during casting of Ti-stabilized stainless steel, changes of liquidus and solidus temperatures with TiO 2 substitution for SiO 2 2 in mould fluxes were studied. It was found that TiO 2 substitution for SiO 2 2 in mould fluxes would lead to decrease of liquidus temperature. Thermodynamic package FactSage was also employed to calculate the liquidus temperature of mould fluxes. The calculated results were compared with experimental data, and discrepancy between calculated and experimental liquidus temperatures was discussed.

Detailed investigation has been performed on tensile work hardening behaviour in terms of the variations of instantaneous work hardening rate (θ = dσ/dε p , where σ is true stress and ε p is true plastic strain) with stress and true plastic strain rate (ε̇ p ) for temperature range 300–873 K in two different material conditions, (i) normalised and tempered plate and (ii) quenched and tempered tubeplate forging of 9Cr-1Mo ferritic steel. Both plate and tubeplate forging exhibited two-stage work hardening and three different temperature regimes in the variation of θ with σ. The variations of θ with respect to ε̇ p exhibited unified work hardening in terms of a single master curve independent of temperature and initial microstructure. θ varied linearly with reciprocal of plastic strain rate, i.e. 1/ε̇ p , and as a consequence, linear correlation between the rate of change of true stress and true plastic strain rate independent of temperature and microstructure has been obtained.

Interactions between Ti-47Al-2Cr-2Nb (at.%) alloy and alumina, zirconia and yttria moulds were studied under high-temperature and long-time condition, and the reaction mechanisms were explored. The moulds were prepared by traditional precision casting technique, and the experiments were carried out in a carbon resistance furnace at 1550°C for 30 min. The results indicated that the interfacial reactions between TiAl alloys and alumina and zirconia moulds were serious. Obvious reaction layers composed of Ti-rich phases and Al 2 O 3 inclusions were formed on the alloy-mould interfaces, and a large amount of dark Al 2 O 3 inclusions were found in the alloy matrixes. Interaction between TiAl and yttria mould was mild. No obvious reaction layer was formed on the alloy-mould interface, and only a small amount of white inclusions inferred as Al-Y oxide were found in the alloy matrix. Al 2 O 3 and ZrO 2 cannot be used as mould materials for the directional solidification of TiAl alloys. Y 2 O 3 mould possesses a higher stability, which can be used in the directional solidification of TiAl alloys by the proper choice of more stable binder.

Microwave and conventional heating were employed to synthesize micro-sized hematite powder from hydrous ferrous sulfate, a by-product in the process of titanium pigment manufacture. The TG/DTG analysis was used to study thermal behavior of the industrial ferrous sulfate. Therefore the temperature which was significant to product was defined. The calcination experiments were carried out to assess the effect of calcination temperature covering three different temperatures (650°C, 750°C and 850°C, respectively). X-ray powder diffraction (XRD) was used to characterize the structure of the product, while the scanning electron microscopy (SEM) was utilized to characterize the morphology of the product. Microwave calcination process resulted in a mixture of hematite and minor magnetite while a considerable amount of magnesium sulfate was detected with the conventional calcination process. The transformation from periclase to magnesium was discussed during the calcinations as well. The conventional heating resulted in porous product while the microwave heating resulted in smaller particle size.

Al 2 O 3 /TiO 2 eutectics containing 62–64 wt% Al 2 O 3 were fabricated by a laser floating zone method. Raman spectrometry was used to determine the phase composition. The compressive creep strength of AT36 (Al 16 Ti 5 O 34 /Al 2 O 3 ) crystal at 1500°C under the strain rate of 1.0 × 10 −4  s −1 has 372 MPa, which is about higher than β-Al 2 TiO 5 and β-Al 2 TiO 5 /Al 2 O 3 crystals. High-temperature flexural strength, elastic modulus, hardness and fracture toughness of the AT36 crystals were analyzed. The highest flexure strength at ambient temperature was found in the material with 36 wt% TiO 2 . In addition, we briefly discussed the relationships between microstructure and mechanical properties in AT36 crystal.

Hot corrosion behavior of pure Co, Co-10Cr and Co-10Cr-5Al alloys in the presence of a salt coat of Na 2 SO 4 +NaCl in air at 1173 K was studied. The results indicated that among the three samples, the mass gain of Co-10Cr-5Al alloy is the smallest, while that of Co-10Cr is biggest. The corrosion products of Co-10Cr-5Al can be divided into three layers, of which the outside layer is CoO, the middle layer consists of Cr 2 O 3 and Al 2 O 3 , and the inside layer is a small amount of Chromium sulfide and aluminum sulfide. The corrosion products of Co-10Cr can also be divided into three layers, from the outermost to the innermost layer are CoO, Cr 2 O 3 , and small amount of Cr 2 S 3 . The corrosion resistance of Co-10Cr-5Al alloy is better than that of Co-10Cr alloy.

The features of inclusion and microstructure for Ti-added carbon structural steel were studied through the scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). From the SEM-EDS mapping images, two kinds of titanium-contained oxides are observed. The type I is the Al-Ti-O phase, in which the elements of Al and Ti coexist. The type II is the titanium-contained oxides, which precipitate on the Al 2 O 3 . Through the statistical analysis, it can be seen that the former type is dominating in number and smaller in size. After etching, the microstructure of intragranular acicular ferrites (IAF) is observed, which is totally different from that in the comparison sample without Ti addition. Furthermore, it’s found that the type I inclusion can act as nucleation sites for the IAF. However, the similar nucleation phenomenon is complex for the type II inclusion. This complexity is attributed to the diversity of precipitation of Ti and Mn.