Utilization of Multiphase Fluxes for the Dephosphorization of Hot Metal

Although steelmaking slags have been usually treated and studied as homogeneous liquids, they are actually mixtures of a liquid and solids in practical processes. CaO‐based refining flux that does not contain fluxing agents such as CaF₂ inevitably forms a heterogeneous slag in normal cases, and hence, it is defined as a “multiphase flux.” Efficient utilization of this type of flux would decrease the consumption of resources and the emission of CO₂, and thus, would reduce the load on the environment. Metallurgical studies on multiphase fluxes are limited, however, the physical chemistry and reaction kinetics of the same are important for the development of advanced refining processes. The reaction mechanism of dephosphorization using a multiphase flux at hot metal temperatures was investigated in this study. The reaction of a P₂O₅‐containing slag with solid CaO was studied by immersing a CaO disc in the slag. A CaO‐FeO layer was formed near the interface, and a solid solution of Ca₂SiO₄‐Ca₃P₂O₈ was observed in this layer. The Fe‐P‐Si alloy reacted with calcium ferrites at 1673 K, and the samples were analysed by XMA. The same solid solution (Ca₂SiO₄‐Ca₃P₂O₈) was observed near the slag‐metal interface, which suggests that the phosphorus removed from the metal gets concentrated in the solid phase. The experimental results were reproduced with a kinetic simulation model. The simulation program was also applied to the reaction of the CaO‐FeO droplet in a hot‐metal bath.     

Interfacial phenomena in the reactions of Al-B,Al-Ti-B,and Al-Zr-B alloys with KF-AIF3 and NaF-AIF3 melts

The interfacial phenomena occurring during the contacting of liquid Al-B, Al-Ti-B, and Al-Zr-B melts with KF-AlF₃ liquid fluxes have been investigated by optical examination of quenched metal drops previously immersed in the liquid fluxes. Reactions in the Al-B/KF-AlF₃ system involve the formation of metastable AlB₁₂ at the metal/flux interface. At high KBF₄ levels in the flux, the AlB₁₂ is dispersed in the flux and also at low KBF₄ levels in the metal. Reactions in the Al-Ti-B/KF-AlF₃ system involve the formation of TiB₂, which may be dis-persed in either the metal or the flux depending upon the composition of the flux. The results obtained for the Al-Ti-B/NaF-AlF₃ and Al-Zr-B/KF-AlF₃ systems were similar to those ob-served for the Al-Ti-B/KF-AlF₃ system. M. S. LEE, formerly Graduate Student, Department of Materials, Imperial College, London     

Phase Equilibrium for the CaO-SiO₂-FeO-P₂O₅ System at 1673K for Dephosphorization With Multi Phase Flux

Development of the efficient hot metal dephosphorization processes during steelmaking process is one of the most essential topics for the production of high grade clean steels. Since the formation of solid solution composed of tricalcium phosphate and dicalcium silicate could obtain a considerable mass transfer of phosphorus from liquid slag into solid phase during hot metal dephosphorization, itcould obviously sustain a high phosphatecapacityof theliquid slag without huge consumption of lime or addition of fluxes, such as fluorite. The above outlines are the main idea of multi phase flux dephosphorization. For the last few decades, many studies have been done towards the scientific principles and the commercial utilization of this technique. However, the reaction mechanism by using multi phase fluxes remains unclear even by now due tolack of evidence. Based on those previous works, providing a reliable and available phase diagram for the fundamental understanding of the reaction mechanism of multi phase flux dephosphorization has become the main purpose of present research. As well known, the CaO-SiO2-FeO-P2O5 slag is the main component of current steelmaking process. Hence the CaO-SiO2-FeO-P2O5 system at equilibrium has been studied at 1673K with low oxygen partial pressure. The solid phase coexisting with liquid flux is approved to be the solid solution composed of CaO, SiO2 and P2O5. Phosphorus distributes mainly in solid solution rather than liquid phase.     

Thermodynamics of Ca-CaF2 and Ca-CaCI2 systems for the dephosphorization of steel

Calcium is soluble in halide salts which can be used to remove phosphorus from steel as a phosphide ion. The activity and activity coefficient of calcium phosphide, and the equilibrium phosphorus distribution ratio between Ca-CaF₂ and Ca-CaCl₂ fluxes and pure solid iron were measured as a function of the Ca composition in the flux at 1350 °, 1400 °, and 1450 °. The Ca-Ca halide fluxes were equilibrated with pure solid iron and a Ag-Ca alloy in an iron crucible under an Ar atmosphere. The Ag-Ca alloy was used to maintain a constant chemical potential of calcium. Phosphorus distribution between between these fluxes and solid pure iron increased with increasing calcium activity and decreasing temperature. The activity coefficient of γ_{Ca 1.5 P} was calculated to be 36.6 at 1350 ° and 11.0 at 1450 ° for a calcium activity of 0.2 (wt pct Ca = 2.5) in the Ca-CaF₂; the activity coefficient increases with increasing Ca in the flux. In addition, the activity of Ca in the Ca-Ca halide fluxes was determined. The equilibrium phosphorus distribution ratio between Ca-Ca halide systems and molten chromium steel was calculated as functions of Cr and C contents of the metal and calcium activity in the flux at 1600 °C by using γ_{Ca 1.5 P} obtained in the present work. This ratio was found to be about 20 for 18 pct Cr stainless steel at 1600 °. Formerly a Graduate Student at Carnegie Mellon University     

Thermodynamics of Ca-CaF2 and Ca-CaCI2 systems for the dephosphorization of steel

Calcium is soluble in halide salts which can be used to remove phosphorus from steel as a phosphide ion. The activity and activity coefficient of calcium phosphide, and the equilibrium phosphorus distribution ratio between Ca-CaF₂ and Ca-CaCl₂ fluxes and pure solid iron were measured as a function of the Ca composition in the flux at 1350 °, 1400 °, and 1450 °. The Ca-Ca halide fluxes were equilibrated with pure solid iron and a Ag-Ca alloy in an iron crucible under an Ar atmosphere. The Ag-Ca alloy was used to maintain a constant chemical potential of calcium. Phosphorus distribution between between these fluxes and solid pure iron increased with increasing calcium activity and decreasing temperature. The activity coefficient of γ_{Ca 1.5 P} was calculated to be 36.6 at 1350 ° and 11.0 at 1450 ° for a calcium activity of 0.2 (wt pct Ca = 2.5) in the Ca-CaF₂; the activity coefficient increases with increasing Ca in the flux. In addition, the activity of Ca in the Ca-Ca halide fluxes was determined. The equilibrium phosphorus distribution ratio between Ca-Ca halide systems and molten chromium steel was calculated as functions of Cr and C contents of the metal and calcium activity in the flux at 1600 °C by using γ_{Ca 1.5 P} obtained in the present work. This ratio was found to be about 20 for 18 pct Cr stainless steel at 1600 °. Formerly a Graduate Student at Carnegie Mellon University     

Corrosion of Refractory Alloys in Molten Lithium and Lithium Chloride

Abstract

Several refractory materials have been considered over the years as containment material for lithium and lithium halides. Surface modified refractory metals are being extensively investigated for containment of reactive metals, radionuclides and their compounds. An overview of experimental observations and results of liquid lithium corrosion of selected engineering refractory materials are presented. The nature of the degradation and its mechanism has been explained. The influence of temperature, microstructure, stress, impurities and service time on the corrosion behavior for various refractory alloys have been discussed. Selection rules for materials of containment for liquid lithium and lithium compounds have been suggested. Recent experimental observations on the behavior of tantalum and niobium-based refractory metal alloys in a specific molten salt environment comprising LiCl/Li₂O/Li/Li₃N at 725^°C have been included in an effort to select suitable materials for molten salt equipment. It has been observed that oxygen contamination is particularly harmful for the refractory metal alloys where as nitrogen is deleterious to iron-based alloys.     

Interfacial tension and flotation characteristics of liquid metal-sodium flux systems

Interfacial tensions between two liquid phases are discussed based on Dupre’s and Good’s equations. Thermodynamically, it is argued that the interfacial tension will always be less than the sum of the surface tensions of the two pure phases. The interfacial tensions between different metals and sodium fluxes were measured using the sessile drop technique combined with X-ray radiography. The metal-flux systems studied were Ag, Bi, Cu, Pb, and Sn with NaF, NaCl, Na₂CO₃, Na₃AlF₆, and Na₂OSiO₂. The interfacial tension decreased with temperature for all the systems studied. For a given flux, the highest value of the interfacial tension was obtained for the system with the largest value of the surface tension of the metal. The average value of Good’s interaction parameter was 0.31 for metals and sodium fluxes. The lowest value of the interaction parameter was obtained when using cryolite as flux.     

Interfacial phenomena in the reactions of AI and Al-Ti melts with KF-AIF3 and NaF-AIF3 melts

The interfacial phenomena occurring during the contacting of Al and Al-Ti melts with KF-AlF₃ liquid fluxes have been investigated by optical examination of quenched metal drops previously immersed in the liquid fluxes. A range of temperatures between 690 °C and 840 °C was em-ployed. Reactions in the Al-Ti/KF-AlF₃ system involve the formation of TiAl₃ at the metal/ flux interface. This TiAl₃ is wetted by the KF-AlF₃ melt, and at 690 °C the TiAl₃ is dispersed in the KF-AlF₃. At 720 °C and above, a metal/flux emulsion is found in which there are high levels of K₂TiF₆ in the flux as a result of the dispersion of the TiAl₃ surface layer in the flux. At low K₂TiF₆ levels, the TiAl₃ is dispersed in the metal. Addition of Mg or Ca to the system in the form of MgF₂ or CaF₂ has been found to inhibit the formation of TiAl₃, thus also inhibiting the metal/flux emulsification. A certain critical level of Mg or Ca is required to prevent emul-sification. The results obtained for the Al-Ti/NaF-AlF₃ system were in all cases similar to those found for the Al-Ti/KF-AlF₃ system. M.S. LEE, formerly Graduate Student, Department of Materials, Imperial College, London.     

Rapid melting and solidification of a surface layer

A one-dimensional computer heat flow model is used to investigate the effect of high intensity heat fluxes,e.g. those achieved via continuous CO2 laser radiation, on the important surface layer melting and subsequent solidification variables of three substrate materials: aluminum, iron, and nickel. Temperature profilesvs time, melting, and solidification interface velocities, heating, and cooling rates in the surface layers of the three metals are calculated. Results are presented in a general form to permit determination of these variables for large ranges of absorbed heat fluxes and times. General trends established show that temperature gradients in the liquid and solid phases and interface velocities are directly proportional to the absorbed heat flux, whereas melt depth is inversely proportional to the absorbed heat flux. Average cooling rates comparable to splat cooling can be achieved by increasing the heat flux and reducing the dwell time of the incident radiation. An order of magnitude increase in the absorbed heat flux results in a corresponding two orders of magnitude increase in average cooling rates in the liquid during solidification of crystalline and noncrystalline structures. Formerly Research Associate, Formerly Research Associate, Formerly Research Associate,     

Density measurements of the lithium fluoride/lithium sulfide eutectic at high temperature

A straightforward and reliable method to determine densities of molten salts at high temperatures was de-veloped by Janz and Lorenz several years ago.^[1] This method was followed in order to determine the density of the LiF/Li₂S eutectic^[2] over the temperature range of 1176 to 1355 K in which the eutectic is liquid. The rel-ative lack of data for this eutectic is surprising given its potential usefulness in the study of advanced batteries'31 and electrowinning of metals from molten sulfides.^[41] The method is based on the fact that a solid piece of metal of known volume suspended from a pan balance into a molten salt will weigh less than if it were sus-pended in air at the same temperature. This difference in weight measured in grams will be equal to the buoyant force of the liquid at that temperature. The density of the salt bath can then readily be determined by dividing this difference by the volume of the solid piece of metal that is immersed in the bath. The procedure can be re-peated to give density values over a range of temperatures.     

Viscosity of casting fluxes used during continuous casting of steel

Molten flux viscosity of eighteen commercially available casting fluxes has been measured as a function of temperature in the range of 1223 to 1723 K. Results show that, at a constant temperature, the flux viscosity is primarily dependent on the mole fractions of SiO₂ and A1₂O₃. The calculated activation energy for viscous flow at 1573 K varies from 59 to 215 kJ/mol depending on the flux viscosity and the flux basicity ratio. The observed “breakpoints” in the viscosity vs temperature plots are explained in terms of precipitation of crystalline phases. The tendency for crystal precipitation is related to the relative viscosity and the basicity of the flux. Using X-ray diffraction techniques the precipitating crystal phases have been identified. Metallographic structures of quenched and air-solidified casting fluxes are reported.     

Experimental Liquidus Studies of the Pb-Cu-Si-O System in Equilibrium with Metallic Pb-Cu Alloys

Phase equilibria of the Pb-Cu-Si-O system have been investigated in the temperature range from 1073 K to 1673 K (800 °C to 1400 °C) for oxide liquid (slag) in equilibrium with solid Cu metal and/or liquid Pb-Cu alloy, and solid oxide phases: (a) quartz or tridymite (SiO₂) and (b) cuprite (Cu₂O). High-temperature equilibration on silica or copper substrates was performed, followed by quenching, and direct measurement of Pb, Cu, and Si concentrations in the liquid and solid phases using the electron probe X-ray microanalysis has been employed to accurately characterize the system in equilibrium with Cu or Pb-Cu metal. All results are projected onto the PbO-“CuO_0.5”-SiO₂ plane for presentation purposes. The present study is the first-ever systematic investigation of this system to describe the slag liquidus temperatures in the silica and cuprite primary phase fields.     

Heat-Transfer Behavior of Mold Fluxes for Continuous Casting of Invar Alloy

The heat-transfer behavior across mold fluxes for Invar alloy Fe-36Ni would introduce significant influence on the slab surface quality. A study on the heat-transfer property of mold flux film for Invar alloy Fe-36Ni was carried out by an interaction between laboratory simulation and field trial. The study results indicate that great effect on heat transfer across flux film is caused by chemical compositions of mold fluxes. An increase of basicity and CaF₂ content suppresses heat transfer across flux film; heat transfer across flux film increases when the Al₂O₃ content increases from 4 pct to 8 pct but decreases when Al₂O₃ content is above 8 pct. The crystalline phases of both the conventional mold fluxes and the improved mold fluxes are all cuspidine phases. However, crystallization capability of the improved mold fluxes decreases as the result of the increase of basicity and CaF₂ content as well as the decrease of Al₂O₃ content. The average thickness of flux film taken from mold is about 1.6 mm, and the crystalline fraction is only 21.4 pct. All these promote heat transfer across the flux film. The field trial of the improved mold fluxes shows that the properties of liquid slag are steady during continuous casting; comprehensive heat transfer across flux film meets the needs of continuous casting of Fe-36Ni. Border solidification structures of solidified shell are refined remarkably, and hot cracking gets avoidance eventually.     

SYNTHESIS OF COPPER METAL/SALTS FROM COPPER BLEED SOLUTION OF A COPPER PLANT

Copper bleed solution (CBS) generated during the electrorefining of anode copper contains 40 g/L Cu, 10–20 g/L Ni, and 170–200 g/L H₂SO₄, along with other impurities in different quantities. In order to get the valuable metals from this CBS, studies have been carried out to crystallize and recover metal salts on bench scale with/without partial decopperization from the point of view of reuse and the recycling of the sulphate salts and the acid recovered back to the system. Studies showed the possibility of recovering copper sulphate salt without affecting its purity with respect to nickel by controlling the extent of evaporation. In an alternative approach, a part of copper metal of purity 99.85% produced during partial decopperization of copper from 39.8 to 9.64 g/L Cu without affecting the current efficiency at a current density of 100 A/m². Subsequently, mixed salt containing 9.80% Cu and 13.19% Ni has also been produced from the partially decopperized solution. The mixed salt could be processed for metal recovery by solvent extraction. The mother and wash liquor could be recycled in the electrorefining plant for acid and copper make-up.     

A new process for the separation of hafnium from zirconium

A new process for the separation of hafnium from zirconium has been developed. This process is based on the repeated reaction of the mixed tetrachloride vapors of zirconium and hafnium with alkali metal chloride salts in a solid form; hafnium tends to remain in the condensed phase while zirconium concentrates in the vapor phase. This process has been investigated experimentally as a function of the alkali chloride salt, the number of reaction steps and the mass ratio of vapor-to-condensed phases. The mechanism of the separation process is discussed in terms of thermodynamic considerations and the calculation of the theoretical separation efficiency is in agreement with the experimental results.     

Influence of magnesia on iron ore sinter properties and productivity

Abstract

Dolomite and other MgO bearing materials are being increasingly used as basic flux constituents for production of fluxed sinters. Addition of flux materials in sinter influences the resultant sinter microstructure and chemical properties. The physical and metallurgical properties of sinter mainly depend on mineralogy of the sinter. Dolomite is the source of double carbonate of calcium and magnesium. Recent studies reveal that, apart from the additional fuel needed, the addition of dolomite and MgO bearing material greatly influences the magnetite content and the properties of the sinter produced. The increasing use of MgO bearing fluxes in the blast furnace burden, and the trend to incorporate a major part of fluxes in the sinter mix led to an investigation of the influence of MgO on sinter properties and productivity. In this study, the systematic investigation has been made on the influence of MgO% (1·4 to 2·6) on sinter mineralogy and sinter properties with dolomite. Microstructural examination of dolomite sinter revealed that hematite and calcium ferrite phases decreased whereas magnetite phase increased with increase in MgO percentage in sinter. From the laboratory pot grate sintering results it was found that sinter reduction degradation index improved whereas tumbler index and reducibility decreased with increase in MgO%.     

Slip Transfer Across Hetero-Interfaces in Two-Phase Titanium Aluminum Intermetallics

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned (PST) crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity in both the γ and the α ₂ phases. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study details of the hard mode of the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl.     

液渣成分和硅酸二钙颗粒含量对铁水预处理非均相脱磷剂脱磷能力的影响简

 高磷铁水预处理脱磷的难题是脱磷剂用量太大、温降太多，急需研究脱磷能力强的脱磷剂。含有固体颗粒和液渣的非均相脱磷剂比仅含液渣的均相脱磷剂的脱磷能力强很多。为此，针对磷的质量分数为0. 5%的高磷铁水，应用FactSageTM热力学软件优选出脱磷能力强的3种液渣，添加不同数量的硅酸二钙颗粒配制非均相脱磷剂试样，脱磷剂和熔铁在1560℃下反应6h，测定熔铁中的平衡磷含量，用以评价其脱磷能力，然后在1400℃下进行了铁水脱磷预处理试验。研究结果表明，随着硅酸二钙颗粒含量的增加，非均相脱磷剂的脱磷能力明显改善；采用非均相脱磷剂有助于减少渣量和控制反应器内衬的侵蚀；采用非均相脱磷剂对铁水脱磷，仍然需要控制较高的渣铁界面FetO浓度。     

Effects of crystallisation behaviour of mould fluxes on properties of liquid slag film

Abstract

During the continuous casting of medium carbon steel slabs, stable liquid slag film performance is necessary to ensure that the process runs smoothly. In this paper, the effects of mould flux crystallisation behaviour on the properties of leftover liquid slag films were examined. First, the compositions of the leftover liquid slags of high fluorine, CaO–SiO₂–TiO₂ and CaO–SiO₂–Na₂O based mould fluxes after a single crystalline phase was separated were calculated. Then, the viscosity, melting temperature, break temperature and degree of crystallisation of each leftover liquid flux were measured. The results revealed that the properties of leftover liquid slag films of CaO–SiO₂–TiO₂ and CaO–SiO₂–Na₂O based mould fluxes are not as stable as those of high fluorine films after crystallisation. This paper provides a new method for developing fluorine free mould fluxes and for determining the most suitable type of crystalline phase that should be utilised in the continuous casting of medium carbon steel slabs.     

Phase relationships and thermodynamic properties of the Pd-S system

Sulfur pressures in the Pd-S system have been measured for liquid mattes and PdS/liquid matte mixtures by a Knudsen effusion technique, and for Pd/matte, Pd4S/matte, and Pd/Pd4S mixtures by an emf method employing a solid oxide electrolyte. The phase diagram has also been determined by conventional DTA methods from 0 to 50 at. pct sulfur, using microprobe analysis to confirm the identification of equilibrated phases. From this information, the standard free energies of formation of solid and liquid Pd₄S, Pd₃S, and Pd₁₆S₇ and solid PdS have been derived. For the reaction of solid Pd with S₂ gas to form solid sulfide, the values are ΔG°_TPd₄S = -184400 + 84.10T, ΔG°_TPd₃S =-172430 + 83.68T, ΔG°_TPd_16/7S = -182340 + 105.66T, and ΔG°_TPd_16/7S = -145180 + 88.12T, all in joules. The results for Pd₄S differ considerably from the previously accepted values.