Pct SiO2 Research Articles

Reduced-Pressure Foaming of Aluminum Alloys

We developed a novel process for foaming aluminum and its alloys without using a blowing agent. The process involves a designated apparatus in which molten aluminum and its alloys are first foamed under reduced pressure and then solidified quickly. Foaming was done for pure aluminum (99.99 pct) and AlMg5 alloy not containing stabilizing particles and AlMg5 and AlSi9Mg5 alloys containing 5 vol pct SiO2 particles. We discuss the foaming mechanism and develop a model for estimating the porosity that can be achieved in this process. The nucleation of pores in foams is also discussed.

Dissolution Mechanism of Indium in CaO-Al2O3-SiO2 Slag at Low Silica Region

The solubility of indium was measured in the low-silica region (<20 mass pct SiO2) of the CaO-Al2O3-SiO2 system by a thermochemical equilibration technique. The dissolution mechanism of indium into the CaO-Al2O3-SiO2 slag at 1773 K (1500 °C) under a reducing atmosphere was elucidated. The indium solubility increases in the calcium silicate-based flux and decreases in the calcium aluminate-based flux with increasing oxygen partial pressure. Also, the solubility was found to decrease initially with increasing slag basicity until the basicity reached a critical level after which the solubility increases. This behavior is believed to indicate that the indium dissolution mechanism changes according to the basicity of the slag.

Confocal Scanning Laser Microscopy Studies of Crystal Growth During Oxidation of a Liquid FeO-CaO-SiO2 Slag

The oxidation of FeO in 30 wt pct FeO-35 wt pct CaO-35 wt pct SiO2 slag was investigated as part of a wider study on the recovery of Fe units through magnetic separation. A confocal scanning laser microscopy (CSLM) technique was used to visualize the oxidation of FeO in the liquid slag. The formation event was observed in situ under the CSLM and the onset of precipitation on a surface of the slag liquid was recorded at various temperatures in an oxidizing atmosphere. A Time-Temperature-Transformation (TTT) diagram was constructed based on the CSLM results. Samples obtained from the CSLM heating chamber were analyzed by a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS).

Thermodynamics of Arsenic in FeOx-CaO-SiO2 Slags

The distribution of arsenic between calcium ferrite slag and liquid silver (wt pct As in slag/ wt pct As in liquid silver) with 22 wt pct CaO and between iron silicate slag with 24 wt pct SiO2 and calcium iron silicate slags was measured at 1573 K (1300 °C) under a controlled CO-CO2-Ar atmosphere. For the calcium ferrite slags, a broad range of oxygen partial pressure (10–11 to 0.21 atm) was covered, whereas for the silicate slags, the oxygen partial pressure was varied from 10–9 to 3.1 × 10–7 atm. The measured relations between the distribution ratio of As and the oxygen partial pressure indicates that the oxidation state of arsenic in these slags is predominantly As3+ or AsO1.5. The measured distribution ratio of arsenic between the calcium ferrite slag and the liquid silver was about an order of magnitude higher than that of the iron silicate slag. In addition, an increasing concentration of SiO2 in the calcium-ferrite-based melts resulted in decreases in the distribution of arsenic into the slag. Through the use of measured equilibrium data on the arsenic content of the metal and slag in conjunction with the composition dependent on the activity of arsenic in the metal, the activity of AsO1.5 in the slags was deduced. These activity data on AsO1.5 show a negative deviation from the ideal behavior in these slags.

Wettability of Silicon Carbide by CaO-SiO2 Slags

The wettability of silicon carbide by liquid CaO-SiO2 slags that contain 47 to 60 wt pct SiO2 was studied using the sessile drop wettability technique. The experiments were carried out in Ar and CO atmospheres. A small piece of slag was melted on SiC substrates under different heating regimes up to 1600 °C. It was found that the wetting is not significantly dependent on the temperature and the heating rate. However, the wettability is relatively high, and the wetting is higher for slags that contain lower SiO2 concentrations. Moreover, the wettability between the slags and SiC is dependent on the gas phase composition, and it is higher in Ar than that in CO. When the SiO2 concentration changes from 47 pct wt to 60 pct wt, the wetting angle changes from 20 deg to 73 deg in Ar and from 58 deg to 87 deg in a CO atmosphere. The formation and bursting of gas bubbles also was observed after some contact time, which indicates that the wetting system is a reactive type. However, microscopic studies indicated that no metal phase exists at the slag/silicon–carbide interface. Therefore, it was concluded that chemical reactions between the slag and SiC take place and that SiO2 is slowly reduced to form CO and SiO gases. Based on the experimental data, the dependence of the Girifalco–Good coefficient on the slag composition and the relationship between the interfacial tension of CaO-SiO2 slags and SiC also were estimated.

Kinetics of Spinel Formation and Growth during Dissolution of MgO in CaO-Al2O3-SiO2 Slag

The formation and growth of MgAl2O4 spinel crystals on a single-crystal MgO substrate submerged in a 40 pct CaO, 40 pct SiO2, and 20 pct Al2O3 slag were directly observed using high-temperature microscopy. This showed that the crystals initially form on the MgO surface, but may break off and be carried out into the liquid slag. Still pictures extracted from digitally recorded images were used to measure the size of these crystals at 1420 °C, 1440 °C, and 1460 °C as a function of time. Growth of the crystals was found to follow the parabolic rate law, with rates increasing with temperature. An activation energy of 564 kJ mol−1 was estimated from the experimental data. This was found to be comparable with previously published results from different types of experiments on spinel formation.

Observation of the crystallization behavior of a slag containing 46 wt pct CaO, 46 wt pct SiO2, 6 wt pct Al2O3, and 2 wt pct Na2O using the double hot thermocouple technique

In this work, isothermal crystallization of a synthetic slag containing 46 wt pct CaO, 46 wt pct SiO2, 6 wt pct Al2O3, and 2 wt pct Na2O has been investigated by means of double hot thermocouple technique (DHTT). The effect of Na2O content on crystallization time was confirmed. Two different types of calcium silicate crystals were observed. Calcium di-silicate forms at temperatures above 1150 °C and calcium tri-silicate precipitate at temperatures below 1050 °C. A mixture of the two types of calcium silicate has been observed between the two temperatures. The tendency of crystals to become richer in calcium at low temperatures that has also been observed in previous published works has been confirmed. No effect of the cooling rate on crystallization start time was confirmed in the range of cooling rates applied in this investigation.

Interaction between refractory crucible materials and the melted NiTi shape-memory alloy

Attempts have been made to quantify the amount of contaminants absorbed by liquid metal from commercial ZrO2-, Al2O3-, and SiC-base crucibles used for vacuum melting of Ni-45 wt pct Ti alloy. The molten alloy was held under vacuum for 90 minutes at 1450 °C to become homogenized. Reactions between the liquid metal and the crucible were investigated by visual observation, chemical analysis, scanning electron microscopy (SEM) image processing, and X-ray mapping. The relative degree of contamination declined in the following sequence: commercially pure SiC>SiC-5 wt pct Al2O3-5 wt pct SiO2>slurry cast alumina>recrystallized alumina>zircon type A>oxygen deficient high-purity zirconia. Thermodynamic calculations showed a difference between the equilibrium and the experimental data, indicating that except for commercially pure SiC crucible, the amount of the crucible elements entering the melt is greater than the calculated equilibrium values. This discrepancy seems to be due to the immersion into the melt of the undissolved chemical compounds formed due to the reactions between the crucible and the liquid phase.

The effect of CaF2 on the viscosities and structures of CaO-SiO2(-MgO)-CaF2 slags

The viscosities of CaO-SiO2(-MgO)-CaF2 slags were measured to clarify the effect of CaF2 on the viscous flow of molten slags at high temperatures and the solidification behavior of slags. Furthermore, the infrared (IR) spectra of the quenched slags were analyzed to understand the structural role of CaF2 in the modification of slag structure. The CaF2 affects the critical temperature (T CR) of the slags; that is, the higher the content of CaF2, the lower the T CR of the slags. It is suggested that some extent of undercooling as a driving force is needed for the precipitation of solid particles in the melt. In the composition of B (≡(mass pct CaO)/(mass pct SiO2)) = 1.0, the T CR was decreased about 150 to 200 K by addition of 10 mass pct MgO, while the T CR was increased about 100 K by MgO addition at B = 1.3. The effect of CaF2 on the viscous flow of molten slags can be understood based on a decrease in the degree of polymerization by F− as well as by O2− ions and this was confirmed by the IR spectra of the quenched slags. The relative intensity of the IR bands for [SiO4]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO4]-tetrahedra with high NBO/Si increased with increasing CaF2 content. The decrease in viscosity of the CaO-SiO2-MgO-CaF2 (B = 1.0) system by CaF2 addition was negligible, while the effect of CaF2 on the viscosity was significant in the more basic system (B = 1.3).

A thermodynamic study on cobalt containing calcium ferrite and calcium iron silicate slags at 1573 K

Redox equilibria, activities of cobalt, iron and their oxides in calcium ferrite and calcium ironsilicate slags, were measured through metal-slag-gas equilibrium experiments under controlled oxygen potentials (10−7 to 3 × 10−7 atm) at 1573 K. Results on the redox equilibria show that addition of CoO to calcium ferrite slag increases the equilibrium Fe3+/Fe2+ ratio in these melts. Measured activities of CoO and FeO showed positive deviations from ideal behavior, while that of Fe2O3 showed negative deviation. Partial substitution of CaO by SiO2, by up to 4 wt pct SiO2 in the calcium ferrite based melts, resulted in increases in the activity coefficients of CoO and Fe2O3. Phase equilibria studies on the cobalt containing CaO-FeO-Fe2O3-SiO2 slags were also carried out using the drop-quench technique. Good agreement between the activity data and the liquidus temperature with respect to magnetite solid solution containing CoO was observed.

Interfacial phenomena in some slag-metal reactions

In the present work, the change of the interfacial tension at the slag-metal interface for sulfur transfer between molten iron, slag, and gas phases was monitored by X-ray sessile drop method in dynamic mode in the temperature range of 1830 to 1891 K. The experiments were carried out with pure iron samples immersed partly or fully in the slag phase. The slag consisted of 30 wt pct CaO, 50 wt pct Al2O3, and 20 wt pct SiO2 (alumina saturated at the experimental temperatures) with additions of FeO. Metal and slag samples contained in alumina crucibles were exposed to a CO-CO2-SO2-Ar gas mixture with defined oxygen and sulfur partial pressures, and the change of the shape of the metal drop was determined as a function of time. The equipment and the technique were calibrated by measurements of the surface tensions of the pure Cu, Ni, and Fe containing two different amounts of dissolved oxygen. A theoretical model was developed to determine the sulfur content of the metal as a function of time on the basis of sulfur diffusion in the slag and metal phases as well as surface tension-induced flow on the metal drop surface. Attempts were made to compute the interfacial tensions on the basis of force balance.

Thermodynamics of TiO x in blast furnace-type slags

Equilibrium studies between CaO-SiO2-10 pct MgO-Al2O3-TiO1.5-TiO2 slags, carbon-saturated iron, and a carbon monoxide atmosphere were performed at 1773 K to determine the activities of TiO1.5 and TiO2 in the slag. These thermodynamic parameters are required to predict the formation of titanium carbonitride in the blast furnace. In order to calculate the activity of titanium oxide, the activity coefficient of titanium in carbon-saturated iron-carbon-titanium alloys was determined by measuring the solubility of titanium in carbon-saturated iron in equilibrium with titanium carbide. The solubility and the activity coefficient of titanium obtained were 1.3 pct and 0.023 relative to 1 wt pct titanium in liquid iron or 0.0013 relative to pure solid titanium at 1773 K, respectively. Over the concentration range studied, the effect of the TiO x content on its activity coefficient is small. In the slag system studied containing 35 to 50 pct CaO, 25 to 45 pct SiO2, 7 to 22 pct Al2O3, and 10 pct MgO, the activity coefficients of TiO1.5 and TiO2 relative to pure solid standard states range from 2.3 to 8.8 and from 0.1 to 0.3, respectively. Using thermodynamic data obtained, the prediction of the formation of titanium carbonitride was made. Assuming hypothetical ‘TiO2,’ i.e., total titanium in the slag expressed as TiO2, and using the values of the activity coefficients of TiO1.5 and TiO2 determined, the equilibrium distribution of titanium between blast furnace-type slags and carbon-saturated iron was computed. The value of [pct Ti]/(pct ‘TiO2’) ranges from 0.1 to 0.2.

Kinetics studies on the dissolution of nitrogen in the CaO-Al2O3-SiO2 and CaO-Al2O3-TiO x melts

The rate of nitrogen dissolution in CaO-Al2O3-SiO2 and CaO-Al2O3-TiO x melts was measured by 14N–15N isotope exchange reaction. The rate constant for the CaO-Al2O3-SiO2 melts at the ratio of mass pct CaO/mass pct Al2O3 = 1 increases as SiO2 content increases, whereas the rate constant for the same melts at the ratio of mass pct CaO/mass pct SiO2 = 1 increases as Al2O3 content increases. The rate constant for the CaO-Al2O3-TiO x melts at the ratio of mass pct CaO/mass pct Al2O3 = 1 decreases as the TiO x content increases. The activation energies of nitrogen dissolution in CaO-Al2O3-SiO2 melts are about 1.5 to 3 times larger than that of molten pure iron. Moreover, the rate constant of nitrogen dissolution is independent of the ratio of Ti3+/Ti4+.

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

The reported experimental work on the systems PbOx-CaO and PbOx-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 PbOx-CaO-SiO2 in air were completely reconstructed. Extensive solid solutions of PbO in α′ dicalcium silicate and Ca2Pb3Si3O11 were measured.

Thermodynamics of chromium oxides in CaO-SiO2-CaF2 slag

The distribution ratio of chromium between a CaO-SiO2-CaF2 slag and liquid silver under the oxygen partial pressure used in practical hot-metal dephosphorization treatment was measured at 1623 K. The distribution ratio was minimal when the basicity index of a slag, wt pct CaO/wt pct SiO2, was about 2. The redox equilibrium between CrO (Cr2+) and CrO1.5 (Cr3+) in the slag was also measured as a function of slag composition. The calculated activity coefficient of CrO had a maximum value at wt pct CaO/wt pct SiO2=2, whereas that of CrO1.5 decreased monotonously with the increase in slag basicity.

A high-accuracy, calibration-free technique for measuring the electrical conductivity of molten oxides

A high-accuracy, calibration-free technique to measure the electrical conductivity of molten oxides has been developed—the coaxial cylinders technique. Because the melt under investigation comes in contact only with metal and not with anything dielectric, the new technique enables the measurement of the electrical properties of liquids inaccessible by classical high-accuracy techniques. Two coaxial cylindrical electrodes are immersed in the melt to an arbitrary initial depth, and ac impedance is measured over a wide range of frequency. The electrodes are then immersed deeper, and ac impedance is again measured over the same range of frequency. This process is repeated at multiple immersions. The electrical conductivity is calculated from the change in measured conductance with immersion. The temperature dependence of electrical conductivity has been measured for two oxide melts: (M) 50.95 pct CaO, 12.51 pct MgO, 36.54 pct SiO2, 1733 K<T<1843 K; and (S) 24.59 pct CaO, 26.15 pct MgO, 49.26 pct SiO2, 1763 K<T<1903 K, where concentration is expressed in mole percent. Both melts exhibited behavior characteristic of ionic liquids.

Interfacial phenomena in the liquid copper-calcium ferrite slag system

The surface and interfacial tensions of the copper-calcium ferrite slag system were determined at 1573 K as a function of oxygen and sulfur pressures and CaO content in the slag. The effect of a 2.9 mass pct SiO2 addition to the calcium ferrite slag was also investigated. The measurements were carried out using the sessile drop technique with a high-temperature X-ray setup. The surface tension of copper was found to be a strong function of oxygen and sulfur partial pressures. The surface tension of the calcium ferrite slag decreased slightly with CaO addition. On the other hand, CaO had a negligible effect on the interfacial tension of the system. Surface tension of the slag and the interfacial tension were influenced by the oxygen partial pressure in the system. Sulfur partial pressure also altered the interfacial tension between the copper and the ferrite slag. Based on the experimental data, filming and flotation coefficients were calculated to predict the entrainment behavior of copper, by bubble attachment in the ferrite slag.

Viscosity of PbO-SiO2 melts

Viscosities (η) of PbO-SiO2 melts that contained 25.0 to 48.8 mole pct SiO2 were measured at temperatures 928 to 1273 K by a rotating cylinder method. The data were analyzed in terms of the conventional polymer theory. The results followed Arrhenius behavior in the different temperature ranges despite the general belief of non-Arrhenius behavior of viscosity in slag melts. The calculated activation energies were a function of temperature and composition of the melts. The activation energies for viscous flow, in general, at above approximately 850 °C were lower than those below 850 °C and varied between 25 and 150 kJ per mole.

Viscosity of copper slags from chalcocite concentrate smelting

The viscosity of smelting slags from the Glogow copper plant in Poland was measured using a concentric cylinder viscometer. These slags contain typically 45 pct SiO2, 16 pct CaO, 8 pct MgO, 11 pct Al2O3, and only 5 to 7 pct total iron. The viscosity was measured as a function of the CaO, MgO, SiO2, Cu2O, Cr2O3, and Fe3O4 contents in the temperature range from 1473 to 1623 K. Silica and chromium oxide additions increased the viscosity, while small additions of the other oxides decreased the viscosity. However, at large additions of CaO or MgO, cooling resulted in a rapid increase in the viscosity upon reaching the transition temperature. This critical transition temperature increased with increasing additions of CaO and MgO. This was explained by the precipitation of solid particles upon reaching the saturation limit. Depending on the slag composition, the activation energy for viscous flow was found to be in the range from 200 to 370 kJ/mol.

Determination of refractive index and absorption coefficient of iron- oxide-bearing slags

The refractive index and absorption coefficient of amorphous Fe2O3-SiO2-CaO-Al2O3 slags have been determined at room temperature as functions of the basicity of [mass pct CaO]/[mass pct SiO2] and the Fe2O3 concentration. The refractive index has been measured by an ellipsometer at 546 nm, and the absorption coefficient has been determined from transmittance measured by a spectrophotometer in the wavelength range of 350 to 1000 nm. The refractive index increases with increasing basicity of [CaO]/[SiO2] and the Fe2O3 concentration. On the other hand, with decreasing basicity of [CaO]/[SiO2], the absorption coefficient is increased in the near-infrared region, and the absorption edge by the charge transfer band is shifted to a shorter wavelength. With increasing Fe2O3 concentration, the absorption coefficient is increased in the measured wavelength range, and the absorption edge by the charge transfer band is shifted to a longer wavelength. These dependences of refractive index and absorption coefficient on the basicity and the Fe2O3 concentration are discussed.