Nitrogen solubility in CaO-CaF2-SiO2 melts

The preceding paper^[5] demonstrated that nitrogen dissolves in silicate melts either as a free ion or complex anion, being incorporated into silicate networks. In the present study, the influence of CaF₂ addition to CaO-SiO₂ melts on the nitrogen solubility was investigated along the liquidus at 1573 K and within the liquidus at 1723 K at constant CaF₂ levels. In the latter case, as the SiO₂ content increases from CaO saturation, the total nitrogen content decreases to reach the minimum and then starts to increase up to the SiO₂ saturation. This is in accord with the abovementioned mechanism of nitrogen dissolution, which is supported by the changing behavior of free and incorporated nitrogen contents with the slag composition. The role of CaF₂ is complicated through the formation of fluorosilicates. The CaF₂ seems not to function simply as a diluent but to enhance the dissolution of nitrogen by releasing oxygen from silicate networks, promoting the formation of free nitride ions. Formerly Graduate Student, Department of Metallurgy, The University of Tokyo.     

Sulfide capacity of CaO-CaF2-SiO2 slags

The sulfide capacityC _S ^2- = (pct S^2-) · (P _{O 2}/P _{S 2})^1/2) of CaO-CaF₂-SiO₂ slags saturated with CaO, 3CaO · SiO₂ or 2CaOSiO₂ was determined at 1200 °C, 1250 °C, 1300 °C, and 1350 °C by equilibrating molten slag, molten silver, and CO-CO₂ gas mixtures. Higher sulfide capacities were obtained for CaO-saturated slags. A drastic decrease was observed in those values when the ratio pct CaO/pct SiO₂ is less than 2. The sulfur partition between carbon-saturated iron melts and presently investigated slags was calculated by using the sulfide capacities obtained and the activity coefficient of sulfur in carbon-saturated iron, which was also experimentally determined. For slags saturated with CaO, partitions of sulfur as high as 10,000 were obtained at 1300 °C and 1350 °C. Correlations between the sulfide capacity and other basicity indexes such as carbonate capacity and theoretical optical basicity were also discussed. Formerly with the Department of Metallurgy, The University of Tokyo.     

Thermodynamics of Na2O in the molten CaO-CaF2-SiO2 system

The activity of a few percent of Na₂O in the CaO-CaF₂-SiO₂ system doubly saturated with CaO and 3CaO·SiO₂ has been determined between 1423 and 1623 K by a chemical equilibration technique. The temperature dependence of the activity coefficient of Na₂O may be expressed as follows. {fx553-01} It was found that Henry’s law holds up to 5 mass pct of Na₂O in the flux at 1473 K. The activity of a small amount of Na₂O in the CaO-CaF₂-SiO₂ flux is discussed in comparison with that for the Na₂O-SiO₂ system in terms of their refining characteristics.     

A Critical Evaluation and Thermodynamic Optimization of the CaO-CaF2 System

A critical evaluation and thermodynamic optimization of all the available literature experimental data of the CaO-CaF₂ system was conducted to obtain a set of thermodynamic functions which can reproduce all available and reliable experimental phase diagrams and thermodynamic data. The liquid solution was described using the Modified Quasichemical Model which assumes the mixing of O² and F^? in the pseudo anionic sublattice and CaF₂ solid solution was described using the compound energy formalism. The discrepancies in the CaO and CaF₂ liquidii among the available experimental data were resolved. The recent experimental data on the solubility of CaO in solid CaF₂ phase were well reproduced, which is critical to explain the eutectic temperature of the system.     

Thermodynamic study of Na2O-SiO2 melts at 1300° and 1400 °C

The vapor pressures of Na above stirred Na₂O-SiO₂ melts in equilibrium with graphite and CO were determined at 1300° and 1400 °C using the transpiration technique. Compositions studied ranged from about 60 mole pct SiO₂ to close to SiO₂ saturation. Activities of components Na₂O and SiO₂ were calculated from the data. Log a_Na2_O (pure liquid as standard state) varies from about −8.7 and −8.5 at silica saturation to −6.3 and −6.1 at 40 mole pct Na₂O at 1300° and 1400 °C, and the molar Gibbs energy of mixing, ΔG _m, at the disilicate composition (X_Na2_O = 0.33) at each of these temperatures is −83.0 and −85.4 kJ, respectively. The Toop and Samis, Yokokawa and Niwa, and Lin and Pelton solution models for binary silicates were applied to the ΔG _m data at 1350 °C and parameters for the models were estimated to give best fits. All three models show good correspondence with the measured ΔG _m curve. The capabilities of the models in predicting activity data in this system have been compared. D. N. Rego, Formerly Graduate Student at Carnegie-Mellon University, G.K. Sigworth, Formerly with Carnegie-Mellon University,     

A thermodynamic study of dephosphorization using BaO-BaF2, CaO-CaF2, and BaO-CaO-CaF2 systems

Phosphorus partition ratios between BaO-BaF₂ fluxes and copper-phosphorus alloys have been measured as a function of slag composition at 1400 °C. A molybdenum sleeve has been used to avoid contact between the slag and the crucible in order to prevent the absorption of slag by the graphite crucible. The effects of additions of BaO to the CaO-CaF₂ system have been investigated by measuring the phosphorus partition ratios between these fluxes and Fe-C_sat-P alloy as a function of slag composition at 1400 °C. Also, the activity of BaO as a function of slag composition at 1400 °C has been determined by equilibrating a silver-barium alloy with the BaO-CaO-CaF₂ fluxes and CO in a graphite crucible. The results indicate that phosphorus partition ratios with carbon-saturated iron, L_P, for the BaO-BaF₂ system are relatively high, going up to 400, even for oxygen partial pressure as low as 7.8 × 10⁻¹⁶ atm. The phosphate capacity and the activity coefficient of PO_2.5 for this system were calculated from the experimental results using the available thermodynamic data. No effects of barium oxide addition in lime-based fluxes were observed for BaO contents less than 40 pct. One of the reasons for this phenomenon is that BaO activity in a lime-based flux is very small for BaO content less than 46 pct. However, for high BaO contents, increases significantly, as does the activity of BaO. Formerly Research Associate, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

Thermodynamics of phosphate and phosphide in CaO-CaF2 melts

The thermodynamics of phosphate and phosphide in CaO-CaF₂ melts was studied by equilibrating the melts in a graphite boat with CO-Ar mixtures and Ag-P alloys between 1400 and 1550 ‡C. The dependence of phosphate and phosphide concentrations on the partial pressure of phosphorus, the partial pressure of oxygen, and slag composition confirmed the behavior of P³ and PO ₄ ³ in the melts as thermodynamically expected, giving 0.52 and 8.5 x 10⁻⁴ as the values for the activity coefficients of Ca_3/2PO₄ and Ca_3/2P, respectively.     

An interacting pair model for alkaline binary and ternary liquid silicates: Application to the systems Na2O-K2O-SiO2 and Na2O-Cs2O-SiO2

Among all the available models describing the structure of molten silicates, that of Pelton et al. allows accurate fitting of the experimental data of the free energies in the binary systems MO-SiO₂ with M=Fe, Mn, Pb, and Ca. An extension to the ternary systems MO-M′O-SiO₂, where M and M′ are alkaline metals, is proposed. A statistical treatment of the configurations of the solution gives a meaning to the new parameters. The mixed alkaline effect is quite well shown by the application of the model to the ternary systems K₂O-Na₂O-SiO₂ and Cs₂O-Na₂O-SiO₂, in particular, in the calculation of the configurational entropy.     

CaO-MgO-Al_2O_3-SiO_2精炼渣组分活度热力学研究

为探索精炼渣-钢液-夹杂物三者之间的热力学平衡关系,采用Factsage热力学计算软件对GCr15轴承钢CaO-MgO-Al_2O_3-SiO_2精炼渣系各组分活度进行计算。结果表明:1600℃温度下,wMgO=5%时,aAl_2O_3、aCaO、aMgO及aSiO_2的范围分别为0.003~0.04、0.35~0.9、0.49~0.9、0.00008~0.0005,活度波动范围比较大。随着MgO含量增加,精炼渣系的液相区域有所减少,各组分活度波动范围明显减小。当wMgO=10%时,在高碱度范围内,降低碱度、增加Al2O3含量可增大aAl_2O_3,增加碱度及降低Al_2O_3含量可增大aCaO、aMgO和aSiO_2。与试验结果对比,应用热力学软件计算精炼渣的活度是可行的。     

Thermodynamic and Kinetic Behavior of B and Na Through the Contact of B-Doped Silicon with Na2O-SiO2 Slags

Boron (B) is the most problematic impurity to be removed in the processes applied for the production of solar grade silicon. Boron removal from liquid silicon by sodium-silicate slags is experimentally studied and it is indicated that B can be rapidly removed within short reaction times. The B removal rate is higher at higher temperatures and higher Na₂O concentrations in the slag. Based on the experimental results and thermodynamic calculations, it is proposed that B removal from silicon phase takes place through its oxidation at the slag/Si interfacial area by Na₂O and that the oxidized B is further gasified from the slag through the formation of sodium metaborate (Na₂B₂O₄) at the slag/gas interfacial area. The overall rate of B removal is mainly controlled by these two chemical reactions. However, it is further proposed that the B removal rate from silicon depends on the mass transport of Na in the system. Sodium is transferred from slag to the molten silicon through the silicothermic reduction of Na₂O at the slag/Si interface and it simultaneously evaporates at the Si/gas interfacial area. This causes a Na concentration rise in silicon and its further decline after reaching a maximum. A major part of the Na loss from the slag is due to its carbothermic reduction and formation of Na gas.     

Sulfide capacities of CaO-CaF2-CaCl2 melts

The sulfide capacity of CaO-CaF₂-CaCl₂ slag was determined at temperatures from 1000 °C to 1300 °C by equilibrating molten slag, molten silver, and CO-CO₂-Ar gas mixture. The sulfide capacity increases with replacing CaCl₂ by CaF₂ in slags of constant CaO contents. The sulfide capacity also increases with increasing temperature as well as with increasing CaO content at a constant ratio of CaF₂/CaCl₂ of unity. A linear relationship between the sulfide capacity and carbonate capacity in literature was observed on a logarithmic scale. SIMEON SIMEONOV, formerly Visiting Research Fellow, Institute of Industrial Science, University of Tokyo. TOSHIHIKO SAKAI, formerly Research Fellow, Institute of Industrial Science, University of Tokyo.     

Messung der Aktivität der Sauerstoffionen in den flüssigen Systemen Na2O-SiO2 und K2O-SiO2

Erörterung der Anwendung von Festelektrolytzellen zur Ermittlung thermodynamischer Eigenschaften von Schlackenschmelzen. Verwendung einer Festelektrolytzelle zur Bestimmung der Schlackenbasizität durch Berechnen des negativen Logarithmus der Sauerstoffionenaktivität (pO-Wert) aus den EMK-Messungen an den flüssigen Systemen Na₂O–SiO₂ und K₂O–SiO₂ in Abhängigkeit von Zusammensetzung und Temperatur. Ermittlung der Kieselsäureaktivitäten der Schlacken. Aussagen über die Bildung von Anionenkomplexen.     

Thermodynamic behaviour of phosphorus in Mn-Si-Ca-P melts

Thermodynamic experiments of Mn-Si-Ca-P melts were carried out in Mo-wire-heated furnace. With these experimental data, the first and second order activity interaction coefficients of Ca, Si and P upon P under the conditions of the same activity and the same concentration method were evaluated. The standard free energy of solution of P in liquid Mn based on 1 wt.% solution standard formulated in temperature was given and it only holds near 1623 K.     

Thermodynamic behavior of Na2O-B2O3 melt

The activity of Na₂O in a Na₂O-B₂O₃ melt was measured at 1373 K by a chemical equilibration technique to understand the thermodynamic behavior of this slag system. Also, the activity coefficient of Na in Ag was preliminarily measured as fundamental thermodynamic data, to estimate the activity of Na₂O in the slag. Sodium in the silver melt, within the present concentration range, exhibits the Henrian behavior, and the Henrian activity coefficient of Na in Ag (γ ⁰ _Na) is estimated to be 37.5 at 1373 K, indicating a positive deviation from ideality. The activity of Na₂O in the slag varies from 1.86×10⁻⁶ to 4.99×10⁻⁴ when its content is increased from 11.0 to 64.9 mol pct; this indicates a significant negative deviation from ideality. The excess stability does not exhibit any pronounced peak, despite being drastically changed at specific slag compositions. Comparing the molar Gibbs free energy of mixing in various binary slags, Na₂O was considered to be more basic than BaO (CaO) was, followed next by MgO; also, P₂O₅ would be significantly more acidic than SiO₂ would be, followed next by B₂O₃. Comparing the heats of formation of solid compounds in binary slags, the larger the differences in the electronegativity values between slag components, the more the relative ionic characters of the melts seemed to be.     

Distribution of Phosphorus between CaO-CaF2 Slag and Fe-C-P Melt

SymbolList　　a(i) ———Activityofconstituentiinslag ;　　CP———Phosphatecapacityofslag ;　　eji———Interactioncoefficientofconstituentjtoconstituenti;　　f[i] ———Activitycoefficientofconstituentiinmelt;　　Km———Equilibriumconstantofreactionm ;     

Dissolution Behavior of Rhodium in the Na2O-SiO2 and CaO-SiO2 Slags

To understand the behavior of rhodium during its recovery process, the dissolution behaviors of rhodium in Na₂O-SiO₂ and in CaO-SiO₂ slags at temperatures ranging from 1423 K to 1623 K (from 1150 °C to 1350 °C) and from 1773 K to 1873 K (from 1500 °C to 1600 °C), respectively, in an oxidizing atmosphere were investigated. The solubility of rhodium in the slags was found to increase with increasing oxygen partial pressure, temperature, and the basic oxide content. The correlation between the solubility of rhodium and the oxygen partial pressure suggested that rhodium dissolved into the slags as RhO_1.5. The dissolution of rhodium was slightly endothermic: the enthalpy change of the dissolution of solid rhodium was determined to be 50 ± 10 kJ/mol for the 50(mass pct)Na₂O-50SiO₂; and 188 ± 94 kJ/mol for the 56(mass pct)CaO-44SiO₂ slag systems. The increase in the solubility of rhodium with the basic oxide content indicated that rhodium exhibits acidic behavior in slags. The correlation between the solubility of rhodium and the sulfide capacity of the slags suggested that the ionic species of rhodium in slags is the rhodate ion, RhO ₂ ^? . The rhodate capacity of the slags was defined, and its application to estimate the possible rhodium content in various slag systems was proposed.     

CaO-CaF2对钢包精炼顶渣性能的影响

为减少密封合金化精炼钢包浸渍罩粘渣,采用按m(CaO)/m(CaF<,2>)=2:1配制的混合调质剂对钢包顶渣进行调质处理,研究调质剂对钢包顶渣性能的影响.熔化性能研究结果表明:调质剂能显著降低钢包顶渣的熔化温度和黏度,当其加入量为钢包下渣量的10%时,渣熔化温度从调质前的1 439℃降至1 400℃;在1 500℃时,未调质的钢包顶渣黏度约为6.5 Pa·s,而调质后渣的黏度低于2 Pa·s.渣金反应实验表明,调质后的渣可使钢液脱硫,但同时会引起钢液少量回磷.工业试验表明,对钢包渣调质处理后,浸渍罩粘渣得以控制,使用寿命提高1倍以上.