The Structure of the Molten FeO-SiO2 System

A structural study of the molten FeO-SiO₂ system has been carried out by means of a high temperature X-ray diffraction technique. The composition range studied was from 0 to 44 mole pct SiO₂. From an analysis of the scattering intensity, the radial distribution function was calculated from which interionic distances and coordination numbers were estimated. The following characteristic features emerge from these results. A constant coordination number of about four is obtained for Si-O pairs in the composition range between 17 and 44 mole pct SiO₂. This supports the existence of SiO₄ tetrahedra in the basic region. The Si-Si distance, which corresponds to the inter SiO₄ distance, gradually decreases as the composition increases from 20 mole pct SiO₂ and then shows a nearly constant value at compositions beyond 39 mole pet SiO₂. This constant value is comparable with the value of the Si-O-Si distance with an angle of less than 180 deg. This indicates that some of the SiO₄ tetrahedra polymerize to form silicate anions. The distance of the Fe-Si and Fe-O varies in a similar manner with the Si-Si correlation.     

Distribution of Boron Between Silicon and CaO-MgO-Al2O3-SiO2 Slags

Production of solar-grade silicon through metallurgical refining methods can be less expensive than current production methods both in terms of energy use and capital cost. Slag refining is a potential metallurgical refining method for removal of boron from silicon. The distribution of boron between silicon and binary CaO-SiO₂ and MgO-SiO₂ slags, and between silicon and ternary CaO-MgO-SiO₂ and CaO-Al₂O₃-SiO₂ slags has been investigated in this work at 1873 K (1600 °C). The distribution coefficient of boron has been found to be between 2 and 2.5 in the binary CaO-SiO₂ and MgO-SiO₂ systems. It has also been found to be in the same range across the entire CaO-MgO-SiO₂ system. In these systems the distribution coefficient is unaffected by slag composition. The distribution coefficient has been found to decrease with increasing Al₂O₃ content in the slag in the ternary CaO-Al₂O₃-SiO₂ system.     

Rate of change of oxygen chemical potential in liquid PbO−SiO2 binary solution

The rate of the chemical potential change of oxygen in a liquid PbO?SiO₂ binary solution, with SiO₂ contents of 10, 20, and 30 mol pct, and in pure PbO, has been measured at temperatures of 900°, 950°, 1000°, 1050°, and 1100°C. The rate increased with temperature according to the Arrhenius type relation and decreased with the increase of the silica content. It is suggested that the rate-controlling step is the counter diffusion rate of Pb²⁺ and Pb⁴⁺ ions, which are considered to be the most easily movable ions in the PbO?SiO₂ solution. The relation between the rate of oxygen chemical potential change and the electrical conductivity is also discussed for the liquid PbO?SiO₂ system.     

Thermodynamic properties of molten PbO−SiO2 systems

Thermodynamic properties of the molten PbO?SiO₂ systems were determined by electromotive force measurements employing solid electrolytes. The experiments were carried out in the temperature range 900° to 1000°C on the galvanic cells \(Pb_{(l)} + PbO_{(l)} \left| \begin{gathered} Electrolyte \\ ZrO_2 \times CaO \\ \end{gathered} \right|Pb_{(l)} + PbO - SiO_{2(l)} \) From the results the activities of PbO and SiO₂, the partial and integral free energies, entropies, and heats of mixing were determined. These experimental results are interpreted and compared with previous experimental data. Based on the present data and according to the model of Toop and Samis,¹the activity coefficients of the free oxygen ion O^2? concentration is reduced.     

The thermodynamic activity of ZnO in silicate melts

The activity of ZnO in ZnO-SiO₂ and CaO-ZnO-SiO₂ melts has been measured at 1560 °C using a transpiration technique with CO-CO₂ mixtures as the carrier gas. The activities of ZnO in dilute solution in 42 wt pct SiO₂?38 wt pct CaO-20 wt pct A1₂O₃ in the range 1400° to 1550 °C and in 62 wt pct SiO₂?23.3 wt pct CaO?14.7 wt pct A1₂O₃ at 1550 °C have also been measured. The measured free energies of formation of ZnO-SiO₂ melts are significantly more negative than published estimated values and this, together with the behavior observed in the system CaO-Al₂O₃-SiO₂, indicate that ZnO is a relatively basic oxide. The results are discussed in terms of the polymerization model of binary silicate melts and ideal silicate mixing in ternary silicate melts. The behavior of ZnO in dilute solution in CaO-Al₂O₃-SiO₂ melts is discussed in terms of the possibility of the fluxing of ZnO by iron blast furnace slags.     

Activity determinations in the CaF2−CaO−SiO2 system at 1450°C

The silica activities of slags in the CaF₂?CaO?SiO₂ system (N_CaO∶N_{SiO 2}>3∶2) have been determined at 1450°C by measuring the equilibrium partial pressure of carbon monoxide for the reaction (SiO₂)+3C _(s) =SiC _(s) +2CO _(g) The results have been correlated with phase diagram determinations and are not consistent with other activity measurements in the literature. Lime activities have been determined using the ternary Gibbs-Duhem relationship.     

Carbosilisiothermic Reduction of Rutile to Produce Nano-Sized Particles of TiC and Its Composite with SiO2

Ceramic nanoparticles of TiC were successfully synthesized in a matrix of SiO₂ by high-energy ball milling with subsequent heat treatment. The milling procedure includes milling of a mixture of TiO₂, Si, and graphite powders at ambient temperature in an inert gas (Ar) atmosphere. The structural evaluation of powder particles has been accomplished by XRD, TEM, SEM, EDX, and DSC. XRD results suggest that the TiC-SiO₂ nonocomposite was produced after 10 hours of mechanical activation with subsequent heat treatment at 1473 K (1200 °C) for 7 minutes. TEM images reveal that the TiC and SiO₂ crystallites are <14 and 12 nm in size, respectively. The fracture toughness, and Vickers hardness values of the TiC-SiO₂ nanocomposite are measured to be 3.82 MPa m^1/2 and 19.9 GPa, respectively. Dimethylsulfoxide is used to eliminate SiO₂ from the final products.     

Phase Equilibria and Thermodynamic Studies in the System CaO−FeO−Fe2O3−SiO2

Phase equilibria and thermodynamic properties of the system CaO?FeO?Fe₂O₃?SiO₂ were studied at 1450° and 1550°C, over a range ofpO₂ from 1 to about 10^?11 atm. Isothermal phase diagrams and activity-composition diagrams were constructed for 0, 5, 10, 20, and 30 wt pct SiO₂ sections. The data are applicable to further understanding the behavior of simple BOF steelmaking slags.     

The evaluation of the equilibria of displacement reactions involving slags

New relations for the evaluation of displacement reactions in ternary slags,e.g. FeO?CoO?SiO₂, CaO?CaS?Al₂O₃, and CaO?CaS?SiO₂ are derived. In addition, equations for the quarternary system CaO?CaS?MnO?MnS?SiO₂ and analogous systems are derived.     

Phase Relations in CaO-SiO2-Al2O3-15 mass pct CaF2 System at 1523 K (1250 °C)

CaO-SiO₂-Al₂O₃-CaF₂ is a base system of mold flux for high Al steels. Phase equilibrium in CaO-SiO₂-Al₂O₃-15 mass pct CaF₂ system at 1523 K (1250 °C) was investigated using quenching method followed by X-ray diffraction and Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Isothermal section in this system at 1523 K (1250 °C) with Al₂O₃ being less than 25 mass pct and CaO/SiO₂ (mass pct) being between 0.43 and 1.25 was experimentally constructed. The liquidus composition and seven solid-liquid coexistence regions at 1523 K (1250 °C) were determined.     

Research on Transformation Mechanism of Oxide Inclusion in Tire Cord Steel

In order to investigate the relation and transformation mechanism of two type oxide inclusions (MnO–Al₂O₃–SiO₂ and CaO–Al₂O₃–SiO₂ system) in tire cord steel the experiment was carried out. It’s found that oxide inclusions’ transformation has experienced MnO–Al₂O₃–SiO₂ type at RH stage, CaO–Al₂O₃–SiO₂ type at LF stage and soft blow argon stage, and finally MnO–Al₂O₃–SiO₂ type in casting blank and billet during the production process of test steel. According to the analysis of reaction among slag/steel/inclusion it’s found that Ca content in steel is a major factor for transformation of inclusion composition. With the rising of Ca content in liquid steel the formation of CaO becomes easier and it is good for the formation of CaO–Al₂O₃–SiO₂ complex inclusion. When the [O] content in steel is less than 0.002 %, the Al₂O₃–CaO–2SiO₂ inclusions may be generated if Ca content in steel is more than 0.0001 %.     

Thermodynamic activity of Na2O in Na2O-CaO-SiO2, Na2O-MgO-SiO2, and Na2O-CaO-SiO2-Al2O3 melts at 1400°C

A gas phase equilibration technique was used to generate Na₂O isoactivity data at high-silica compositions in the systems Na₂O-CaO-SiO₂, Na₂O-MgO-SiO₂, and Na₂O-CaO-SiO₂-(10 and 20 wt pct) A1₂O₃ at 1400 °. Loga _{Na 2 O} values referenced to pure liquid Na₂O at 1400 ° as standard state ranged from about ?8.0 to ?7.0. Silica activities were calculated in the Na₂O-CaO-SiO₂ system using the Gibbs-Duhem equation. Richardson’s model for ideal mixing of basic oxides in silica was applied to the Na₂O-CaO-SiO₂ system. The model shows best fit when cationic mixing is assumed to occur on divalent sites. The alkali retention in slags has been described using a defined “sodium capacity”. The temperature variation of alkali retention was estimated, and the resulting sodium capacity was used to evaluate alkali stability in blast furnace slags.     

The self diffusion of iron in Fe2SiO4 and CaFeSiO4 melts

The self diffusion of iron in Fe₂SiO₄ and CaFeSiO₄ melts has been measured in the temperature range 1250° to 1540°C using Fe⁵⁹ as the radio tracer and the capillary-liquid reservoir method of diffusion measurement. The results obtained are represented by $$log D_{Fe} = - \frac{{3800 \pm 500}}{T} - 2.74 \pm 0.29$$ for Fe₂SiO₄, and $$log D_{Fe} = - \frac{{5450 \pm 620}}{T} - 1.93 \pm 0.37$$ for CaFeSiO₄. Excellent agreement is obtained with the self-diffusivity of iron calculated from the measured interdiffusivity of iron and oxygen in iron oxide melts.     

The sulfide capacities of CaO-SiO2 melts containing MgO,FeO, TiO2, and Al2O3

The sulfide capacities of CaO-SiO₂ melts containing MgO, FeO, TiO₂, and A1₂O₃ have been measured by simultaneous equilibration within quartz capsules at 1776 K (1503 °C). The compositions of the melt samples and static gas phase within each capsule adjusted to establish a mutual equilibrium. The sulfide capacities, C_s, of all equilibrated samples must satisfy the following equation: $$C_{{\text{S(}}j{\text{)}}} = \frac{{{\text{pctS}}_j }}{{pctS_i }} \bullet C_{S(i)} $$ The inclusion of several reference slags for which values of C_s are known from previous measurements enabled the calculation of the sulfide capacities of unknown samples. The influences of basic oxide substitutions (FeO and MgO) for CaO and acidic oxide substitutions (TiO₂ and A1₂O₃) for SiO₂ on the sulfide capacities of CaO-SiO₂ melts have been measured. When a basic oxide/acidic oxide ratio of 1.21 is maintained, MgO additions decrease sulfide capacity, while FeO, TiO₂ and A1₂O₃ additions increase sulfide capacity. The results are compared with earlier measurements on the influences of CaF₂ and B₂O₃, and the relative effects are discussed in terms of the activity of free oxide ions.     

Removal of phosphorus from CaO-SiO2-MgO-Al2O3-P2O5 melts to the gas phase

The behavior of phosphorus in CaO-SiO₂-MgO-Al₂O₃-P₂O₅ melts is studied experimentally, so as to determine the potential for the removal of phosphorus from oxide melts to the gas phase. To identify the factors that affect the removal of phosphorus from oxide melts to the gas phase, experiments are conducted with different basicity CaO/SiO₂ and with the injection of N₂, Ar, and CO + CO₂ mixture (with different CO/CO₂ ratios). It is established that the basicity is the main factor affecting the transfer of phosphorus from the oxide melt to the gas. When the basicity is 1.0 or less, phosphorus is removed from oxide melts. At higher basicity, no transfer of phosphorus from the oxide melt to the gas is observed. These findings are confirmed by calculations based on the theory of regular ionic solutions for acidic slag, at different basicity values. Calculations show that, when the basicity is 1.05 or less, the activity of SiO₂ exceeds that of CaO, and correspondingly SiO₂ displaces P₂O₅ from its strong compound with CaO. That creates favorable conditions for the transfer of phosphorus from the oxide melt to the gas.     

The Wear Behavior of In-Situ Al–AlN Metal Matrix Composites

Al–AlN composites are synthesized using NH₄Cl + CaO powder as a nitrogenation precursor in the melt of pure aluminum. In-situ formation of AlN to varying volume fraction is attempted using different proportion of NH₄Cl + CaO precursor into the aluminum melt held at 700 °C. Mechanical properties of synthesized metal matrix composites are evaluated for different volume fraction and distribution of AlN particles in aluminum matrix. Agglomeration of AlN is noticed with increasing precursor addition and synthesis time into the aluminum matrix. Due to heterogeneous distribution of AlN particles in aluminum matrix, marginal changes in hardness are observed. Pin on disc, dry sliding wear of metal matrix composites is carried to study wear behavior of synthesized composites. Composite with good dispersion of AlN particulates has shown higher hardness and wear resistance. Present paper discusses wear behavior of composites with different weight fraction of AlN tested under load and sliding distance as wear parameters. The shearing mechanism of agglomerate due to friction and its correlation with the wear loss is also highlighted in the present paper.     

Desulfurization of liquid steel containing aluminum or silicon with lime

The reaction mechanisms for the desulfurization of iron containing from 0.04 to 1.5 pct aluminum or 1.1 to 3.7 pct silicon by CaO at 1600° have been examined. The desulfurization of Fe-Al by CaO is considerably faster than that of Fe-Si. The basic difference between the two processes is that Fe-Al alloys can be desulfurized by the formation of AI₂O₃, whereas for Fe-Si melts it is necessary to form Ca₂SiO₄. The rate of desulfurization of Fe-Si alloys by CaO is controlled by the formation of the gaseous intermediates, SiS and S, and is the same as that for desulfurization in vacuum. The rate of desulfurization of Fe-Al melts is fast, and is apparently controlled by the diffusion of sulfur to the liquid metal—CaO interface. Experiments were also conducted to demonstrate that sulfur could be transferred to CaO by the gaseous intermediates SiS and S.     

Removal of phosphorus from synthetic oxide melts

The behavior of phosphorus in FeO-Fe₂O₃-CaO-MgO-SiO₂-Al₂O₃-P₂O₅ oxide melts is experimentally studied. To determine the factors affecting the removal of phosphorus from the oxide melt to the gas phase, experiments are conducted with different melt basicity (CaO/SiO₂) and with different CO/CO₂ ratios in injection. The removal of phosphorus is greatest when the basicity is 0.5. At higher basicity, thermodynamic factors hinder the removal of phosphorus; at lower basicity, the viscosity of the melt is a hindrance. In kinetic calculations, the apparent activation energy of the process is found to be E = 140 ± 56 kJ/mol. That suggests that the process is limited by the diffusional mass transfer of phosphorus pentoxide in the liquid phase.     

The thermodynamic properties of liquid Fe−Si alloys

The thermodynamic properties of liquid Fe?Si alloys have been determined electrochemically by use of the following galvanic cells: $$\begin{gathered} Cr - Cr_2 O_3 (s)|ZrO_2 (CaO)|Fe - Si(l), SiO_2 (s) \hfill \\ Cr - Cr_2 O_3 (s)|ThO_2 (Y_2 O_3 )|Fe - Si(l), SiO_2 (s) \hfill \\ \end{gathered} $$ The free energy of formation of SiO₂ was measured and is ?139.0 and ?134.3 kcals per mole at 1500° and 1600°C, respectively. The activity coefficients of iron and silicon for the atom fraction of siliconN _Si<0.35 at 1600° and 1500°C can be represented by the quadratic formalism. $$\begin{gathered} \left. {\begin{array}{*{20}c} {log \gamma _{Fe} = - 2.12 N_{Si}^2 } \\ {log \gamma _{Si} = - 2.12 N_{Fe}^2 - 0.22} \\ \end{array} } \right\}1600^ \circ C (2912^ \circ F) \hfill \\ \left. {\begin{array}{*{20}c} {log \gamma _{Fe} = - 2.50 N_{Si}^2 } \\ {log \gamma _{Si} = - 2.50 N_{Fe}^2 - 0.13} \\ \end{array} } \right\}1500^ \circ C (2732^ \circ F) \hfill \\ \end{gathered} $$ The results indicate that an excess stability peak occurs at about the equimolar composition. Combining the heats of solution determined in this study with previous data indicates that the heats also follow the quadratic formalism. The partial molar heats, \(\bar L_{Si} \) and \(\bar L_{Fe} \) , are represented by $$\begin{gathered} \bar L_{Si} = - 31 N_{Fe}^2 - 4 kcals per mole \hfill \\ \bar L_{Fe} = - 31 N_{Si}^2 kcals per mole \hfill \\ \end{gathered} $$ ForN _Si less than 0.35 and by $$\begin{gathered} \bar L_{Si} = - 22 N_{Fe}^2 \hfill \\ \bar L_{Fe} = - 22 N_{Fe}^2 - 7.0 \hfill \\ \end{gathered} $$ forN _Fe less than 0.35. There is an inflection point in the transition region similar to an excess stability peak for the excess free energies. At 1600°C the ThO₂(Y₂O₃) electrolyte exhibited insignificant electronic conductivity at oxygen partial pressures as low as that in equilibrium with Si?SiO₂ (2×10^?16 atm).     

Rates and mechanisms of FeO reduction from slags

The rate of FeO reduction from CaO?SiO₂ slags has been determined in a stationary magnesia crucible with a graphite rod as the reductant. At 1650°±35°C, with a slag basicity CaO/SiO₂=1.2±0.3, and in the total iron concentration,C, range of 10 pct<C<40 pct, the specific rate,Q _E , of reduction in g-moles per min per sq cm was found to beQ _E =8.25×10^?6 C ^1.77 According to this and previous literature results, the reduction rates span three possible stages. The order of reaction in regard to concentration varies from first order in the low concentrations (0.1 pct<C<1.5 pct) to second order in the intermediate concentrations (1.5 pct<C<15 pct) and finally to 1.77th order in the high concentrations (15 pct<C<40 pct). The reduction reaction can best be interpreted by convective mechanisms because the observed rates fall between the rates of the chemically limited graphite gasification and the rates predicted from molecular diffusion.