Investigations on the equilibria between Al-Ca-O containing iron melts and CaO-Al2O3-FeOn slags

The equilibria between liquid CaO-Al₂O₃-FeO_n slags and Al-Ca-O-containing iron melts in contact with pure lime (CaO) and pure calcium aluminate (CaO-2Al₂O₃) crucibles were investigated at 1600°C. The iron mass content of the metal phase was varied between 0 and almost 100 %. The results show a strong influence of the miscibility gap in the system Fe-Al-Ca arising from the Fe-Ca binary system on the phase equilibria. By these investigations the conditions of solid alumina reduction by dissolved calcium to liquid calcium aluminate were determined.     

Thermodynamic conditions for inclusions modification in calcium treated steel

The presented paper discussed the fundamental or common thermodynamical relations between calcium-treated aluminium-killed molten steel and non-metallic inclusions. The phase and chemical analyses of inclusions have proven that the correctness of calcium addition can be confirmed and that the analysis of those phenomena can show the effects of previous calcium treatment of aluminium-killed steel. To make the process of manufacturing quality steel successful the factor affecting the necessary calcium addition should be taken into consideration already during the process. Steel, containing too much calcium could have CaS inclusions with a high melting point, while too low contents of calcium cause unsatisfactory modification of solid alumina inclusions to complex liquid calcium-aluminate inclusions. This research included the examination of thermodynamic relations in calcium addition and its reactions with solid Al₂O₃ inclusions. A detailed analysis of the CaO–Al₂O₃ binary system established the modification of solid alumina inclusions via the following intermediate phases: CaO · 6Al₂O₃, CaO · 2Al₂O₃, CaO · Al₂O₃ and liquid phase 12CaO · 7Al₂O₃ and finally again solid CaO, at 1873 K (1600°C). The investigation discusses the further research engaged in consideration of CaO- and Al₂O₃-activities change in each of the quoted intermediate phases. The system as a whole includes details of oxygen activities.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and CaO/SiO<Subscript>2</Subscript> Ratio on Phase Equilbria in the ZnO-“FeO”-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-SiO<Subscript>2</Subscript> System in Equilibrium with Metallic Iron

The phase equilibria and liquidus temperatures in the ZnO-“FeO”-Al₂O₃-CaO-SiO₂ system in equilibrium with metallic iron have been determined experimentally in the temperature range 1383 K to 1573 K (1150 °C to 1300 °C). The experimental conditions were selected to characterize lead blast furnace and imperial smelting furnace slags. The results are presented in a form of pseudoternary sections ZnO-“FeO”-(Al₂O₃ + CaO + SiO₂) with fixed CaO/SiO₂ and (CaO + SiO₂)/Al₂O₃ ratios. It was found that wustite and spinel are the major primary phases in the composition range investigated. Effects of Al₂O₃ concentration as well as the CaO/SiO₂ ratio on the primary phase field, the liquidus temperature, and the partitioning of ZnO between liquid and solid phases have been discussed for zinc-containing slags.     

Effect of Al2O3 and CaO/SiO2 on the Viscosity of Calcium-Silicate–Based Slags Containing 10 Mass Pct MgO

The effect of Al₂O₃ and CaO/SiO₂ on the viscosity of the CaO-SiO₂-10 mass pct MgO-Al₂O₃ slags was studied at fully liquid temperatures of 1773 K (1500 °C) and below. At fixed CaO/SiO₂ between 0.8 and 1.3, higher Al₂O₃ content increased the slag viscosity due to the polymerization of the aluminate structures. At fixed Al₂O₃ of 15 and 20 mass pct, increasing the CaO/SiO₂ from 0.8 to 1.3 resulted in lower viscosity due to the depolymerization of the aluminate structure.     

Reaction mechanism for the CaO-Al and CaO-CaF2 desulfurization of carbon-saturated iron

The desulfurization of carbon-saturated iron with CaO is inefficient due to the formation of solid reaction products on the CaO particles which inhibit sulfur transfer. It has been shown in full scale and pilot plant studies that Al improves desulfurization with CaO. The present results indicate that Al affects the rate at 1450 °C, but not at 1350 °C, and that zirconium, which has a stronger affinity for oxygen than Al, does not increase the rate as much as aluminum. Alternate methods of producing a liquid reaction surface, such as desulfurization with CaO-CaF₂ or a premelted calcium aluminate flux, also increase the rate. From these results, it is concluded that it is critical to have a liquid reaction surface for effective desulfurization. Aluminum in the metal, reacting with oxygen and CaO, provides a liquid calcium aluminate surface. An optimum aluminum level of about 0.1 to 0.3 pct was found. At lower levels, the Al is consumed and eventually solid calcium silicate forms, and at higher levels, possibly an alumina-rich solid phase forms.     

Reaction mechanism for the CaO-Al and CaO-CaF2 desulfurization of carbon-saturated iron

The desulfurization of carbon-saturated iron with CaO is inefficient due to the formation of solid reaction products on the CaO particles which inhibit sulfur transfer. It has been shown in full scale and pilot plant studies that Al improves desulfurization with CaO. The present results indicate that Al affects the rate at 1450 °C, but not at 1350 °C, and that zirconium, which has a stronger affinity for oxygen than Al, does not increase the rate as much as aluminum. Alternate methods of producing a liquid reaction surface, such as desulfurization with CaO-CaF₂ or a premelted calcium aluminate flux, also increase the rate. From these results, it is concluded that it is critical to have a liquid reaction surface for effective desulfurization. Aluminum in the metal, reacting with oxygen and CaO, provides a liquid calcium aluminate surface. An optimum aluminum level of about 0.1 to 0.3 pct was found. At lower levels, the Al is consumed and eventually solid calcium silicate forms, and at higher levels, possibly an alumina-rich solid phase forms. Formerly a Graduate Student with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

Mechanism Study on the Formation of Liquid Calcium Aluminate Inclusion from MgO‐Al2O3 Spinel

A study on the formation mechanism of liquid calcium aluminate inclusions from MgO‐Al₂O₃ spinel was carried out. It was found that spinel reacted with dissolved Ca in liquid steel forming liquid calcium aluminate phase. Stability calculations showed good agreement with the experimental result. According to the thermodynamic calculation, liquid calcium aluminate inclusions would form from spinel even at very low level of dissolved Ca content at 1873 K. At lower temperatures, the compound CaO‐2Al₂O₃ was found to be the stable phase at the spinelmetal interface. Potential sources of dissolved Ca during ladle processes were also discussed.     

Thermodynamic investigations and substantiation of the aluminothermic fabrication method of calcium

To search for ways to solve the problem of decreasing the production prime cost of metallic calcium, it is proposed to consider its aluminothermic production by the example of the CaO–Al system. The thermodynamic analysis implemented for this system showed that the aluminothermic reduction of calcium from its oxide is technically performable under a pressure of 5–10 Pa and temperature of 1200–1500°C. It is revealed that the implementation of reduction under the residual pressure lower than 1 atm (101.3 Pa) considerably lowers the thermodynamic temperatures of the onset of the reaction. It is established that only three reactions, in the course of which calcium aluminates 3CaO ? Al₂O₃, 5CaO ? 3Al₂O₃ (12CaO ? 7Al₂O₃), and CaO ? Al₂O₃ are formed, can be used for the practical purposes. It is proposed, depending on the final state, to separate the process into a “low-temperature” one (up to 1200°C, the calcium yield is no higher than 64.3%) and a “high-temperature” one (up to 1500°C, up to 75% Ca). It is planned tofurther confirm these data experimentally.     

Experimental Determination of the Liquidus in the High-Basicity Region in the Al2O3(25?mass?pct)-CaO-MgO-SiO2 and Al2O3(35?mass?pct)-CaO-MgO-SiO2 Systems

Liquidus in the Al₂O₃(25 mass pct)-CaO-MgO-SiO₂(<20 mass pct) and Al₂O₃(35 mass pct)-CaO-MgO-SiO₂(<20 mass pct) systems were determined experimentally in the high-CaO-containing region at 1873 K (1600 °C). For the Al₂O₃(35 mass pct)-CaO-MgO-SiO₂(<20 mass pct) system, liquidus data were also determined for 1773 K (1500 °C). The equilibrating and quenching technique with subsequent electron probe microanalyzer (EPMA) microanalysis were employed. Based on the data, liquidus lines were constructed for the 25 and 35 mass pct alumina planes at silica contents generally below 20 mass pct. The current results showed a slightly lower solubility of CaO and a higher solubility of MgO at 1873 K (1600 °C) for the 25 mass pct Al₂O₃ section compared with the existing phase diagram. At 1773 K (1500 °C), the result showed a slightly lower solubility of both CaO and MgO in the 35 mass pct Al₂O₃ section compared with the existing phase diagram. In addition, the activities of MgO, CaO, and Al₂O₃ were estimated at 1773 K and 1873 K (1500 °C and 1600 °C) using the phase diagram information.     

Sintering kinetics and alumina yield in lime-soda sinter process for alumina from coal wastes

An investigation of the application of the lime-soda sinter process to alumina extraction from coal wastes has been carried out. In the sintering stage, the optimal operating conditions have been obtained for the highest yield of alumina. The kinetics of sodium aluminate formation have also been studied in the sintering stage. The sinter mixes have been fired isothermally in air in the temperature range 1100 to 1350 °C. Alumina recovery of about 80 pct has been obtained by sintering coal-waste mixes having molar ratios of Na₂O/Al₂O₃ = 1.3 and CaO/SiO₂ = 1.8 at 1200 to 1250 °C for 20 to 30 minutes. Insoluble alumina compounds are responsible for the incomplete recovery. The major sinter components are identified as sodium aluminate and β-dicalcium silicate. The nucleation and growth kinetics equation is used to correlate the experimental data of sodium aluminate formation obtained under atmospheric pressure in the temperature range 1000 to 1200 °C. An activation energy of 286 kJ/mol has been calculated for fine coal-waste powder mixtures. Formerly Graduate Student in Metallurgy.     

Production of calcium metal by aluminothermic reduction of Egyptian limestone ore

Calcium metal was successfully produced from Egyptian limestone ore under vacuum using aluminium as a reducing agent. The aluminothermic reduction process of CaO by aluminium metal was followed up by collected amounts of produced calcium metal. The progress of the reduction process was investigated by the phase analysis of produced reaction residues through X-ray diffraction. The technical parameters affecting the reduction progress were investigated such as reductant stoichiometry, reduction temperature and reduction time. Maximum metallic yield (30%) was obtained by using charge containing 19.4% aluminium powder reduced at 1300°C for 90?min under vacuum (10^?3?mbar). The reduction residues composed mainly of unreacted calcium oxide, tri-calcium aluminate (3CaO·Al₂O₃), mayenite (12CaO·7Al₂O₃) and traces of calcium aluminide (CaAl₄). The probable reaction sequences of the aluminothermic reduction of calcium oxide were also discussed.     

Evaluation of Desulphurization Flux in CaO‐Al2O3‐BaO‐CeO2‐MgO System

The flux of the CaO‐Al₂0₃‐BaO‐CeO₂‐MgO system as a desulphurization flux containing no fluorine for the secondary metallurgy process was evaluated in this study. The flux composition was designed using the eutectic compositions of the binary systems. The melting and desulphurization abilities of the fluxes were evaluated by measuring their liquidus temperatures and the distribution ratios of sulphur between the fluxes and the carbon‐saturated iron or stainless steel. The lowest liquidus temperature of 1325°C was obtained by adding 5.7 mass% MgO to the 80mass%A‐20mass%B flux. (A: 12CaO‐7Al₂O₃, B: BaCeO₃+12mass%Al₂O₃). The distribution ratios of sulphur and sulphide capacities of the fluxes in this study were higher than those of the commercial product of calcium aluminate flux. This means that the CaO‐Al₂O₃‐BaO‐CeO₂‐MgO fluxes developed in this study have higher desulphurization and melting abilities compared with the commercial product of calcium aluminate flux.     

Oxidation Studies of SiAlON/MgAlON Ceramics with Fe2O3 and CaO Impurities, Part II: Phase Evolution

The oxidation behavior of composite SiAlON/MgAlON phases, synthesized from the leaching residue after the aqueous treatment of salt cake from aluminum remelting, is compared with the oxidation of corresponding synthetic samples. The samples were subjected to oxidation under air as the oxidant atmosphere in the temperature range of 1373 K to 1773 K (1100 °C to 1500 °C). The phases present were analyzed by scanning electron microscopy (SEM)-electron-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) to arrive at the evolution of the various phases formed during oxidation. From the experimental results, especially by the characterization of the oxidation products, the mechanism of the oxidation reaction was deduced as follows: With the progress of oxidation, the composition of the material being oxidized moved toward the Al₂O₃-rich corner of MgO-Al₂O₃-SiO₂ and CaO-MgO-Al₂O₃-SiO₂ phase diagrams relevant to the SiAlON/MgAlON composite. At lower temperatures, the addition of Fe₂O₃ and CaO facilitated the formation of cordierite and anorthite, respectively. With increasing temperature, islands of silicate melt were formed dissolving these oxides, with the liquidus temperature getting lowered as a consequence. The liquid phase formed engulfed the adjacent solid phases providing strong mobility for the cations and enabling the crystal growth. As a result, intermediate products, i.e., cordierite, anorthite, and spinel, which were formed earlier during oxidation, are found to get dissolved in the liquid phase.     

Formation of hexavalent chromium by reaction between slag and magnesite-chrome refractory

The goal of this work was to understand how Cr⁶⁺ formation is affected by the interaction between chromite phases present in magnesite-chrome refractory and different slag compositions. In addition, the formation of Cr⁶⁺ as a function of chromite particle size and cooling rate due to the chromite phase/slag interaction was investigated. The following slag compositions were studied: calcium aluminate, calcium aluminum silicate, and calcium silicate. It was found that the presence of uncombined CaO in the calcium aluminate slags is responsible for a higher yield of Cr⁶⁺. However, the replacement of Al₂O₃ by SiO₂ in calcium aluminate slags decreases the formation of Cr⁶⁺. SiO₂ reacts with CaO to form stable 2CaO·Al₂O₃·SiO₂ and CaO·SiO₂ phases, consequently decreasing the amount of uncombined CaO available to react with the chromite phase to form Cr⁶⁺. Moreover, the content of Cr⁶⁺ is decreased by increasing chromite particle size and increasing the cooling rate of magnesite-chrome refractory.     

The utilization of galvanic cells using Caβ″-Alumina solid electrolytes in a thermodynamic investigation of the CaO- AI2O3 system

Caβ″-alumina solid electrolytes have been used in calcium concentration electrochemical cells to determine the standard free energies of formation of the calcium aluminates, from their constituent oxides, in the temperature ranges specified: (1) CaO(s) + 6Al₂O₃(s) → CaO6Al₂0₃(s) ΔG° =-4270.9 - 9.4r(K)(±200)cal = -17869.4 - 39.3T (±840)J; 1100 to 1500 K. (2) CaO(s) + 2Al₂O₃(s) → CaO.2Al₂O₃(s) ΔG° = -3087.1 - 6.39HK) (±300)cal = -12916.4 -26.74T (±1260)J; 1100 to 1500 K. (3) CaO(s) + Al₂O₃(s)→ CaO-Al₂O₃(s) ΔG° = -3612.1 -4.35T(K) (±200)cal = -15113.0 - 18.2r(±840)J; 1050 to 1500 K. (4) 3CaO(s) + Al₂O₃(s) → 3CaO-Al₂O₃(s) ΔG° = -1868.7 - 7.05T(K)(±200)cal = -7818.6 - 29.57(±840)J; 1050 to 1320 K.     

Investigation on the removal efficiency of inclusions in Al-killed liquid steel in different refining processes

Industry trials were carried out to study the removal efficiency of inclusions in Al-killed liquid steel in the processes of BOF–LF–RH–CC and BOF–RH–CC. It was found that the removal efficiency of inclusions has a high dependence on inclusion types. Solid inclusions are more easily to be removed than liquid inclusions. The removal efficiency of solid Al₂O₃ inclusions is higher than that of solid CaO–Al₂O₃–MgO inclusions. As liquid CaO–Al₂O₃–MgO inclusions coexisted with solid CaO–Al₂O₃–MgO inclusions in the liquid steel, the low removal efficiency of inclusions in RH degassing process was found in BOF–LF–RH–CC process. However, high removal efficiency and ultra-low total oxygen (T.O) content were obtained in BOF–RH–CC process because the inclusions were mainly composed of solid Al₂O₃ although initial T.O content before RH degassing was relatively high. This is due to the fact that solid Al₂O₃ tends to form cluster-shaped inclusions which have both a higher contact angle and a lower work of adhesion with steel than calcium aluminate, resulting in easier removal by RH degassing. Therefore, it is proposed to weaken steel–slag reaction and calcium treatment before RH degassing to retain solid Al₂O₃ inclusions in the steel.     

Fundamental thermodynamic aspects of the CaO–Al2O3–SiO2 system

The most important metallurgical effects of ladle treatment of aluminium-killed steels with calcium, are associated with the modification of alumina inclusion. For the development of the deoxidation-control model for inclusions, the thermodynamic slag model, based on the Gibbs energy minimization and modelling approaches postulated from J. Hastie et al., was used to calculate component oxide activities in the system CaO–Al₂O₃ and part of systems 3CaO · Al₂O₃ – SiO₂, 12 CaO · 7Al₂O₃ – SiO₂ and CaO · Al₂O₃ – SiO₂ at 1600°C.     

Effect of temperature and alumina/caustic ratio on precipitation of boehmite in synthetic sodium aluminate liquor

In Bayer process gibbsite (alumina trihydrate, Al₂O₃·3H₂O) is precipitated from sodium aluminate liquor for production of alumina by calcination. Precipitation of boehmite or alumina monohydrate (Al₂O₃·H₂O) is an alternative method for production of alumina. Boehmite is also a good precursor material for production of different activated aluminas. At present this boehmite precipitation process is in the early stage of development. This process if developed would reduce the energy consumption for making alumina. The present work is aimed to carry out boehmite precipitation under atmospheric pressure conditions. Parameters such as alumina/caustic (A/C) ratio and temperature are varied to precipitate out boehmite from synthetically prepared sodium aluminate liquor. It was found that boehmite precipitation could be possible at normal pressure only when boehmite seed was added to the supersaturated sodium aluminate solution keeping the alumina/caustic ratio at either 1.1 or 1.0 and temperature at ≥ 85 °C. By decreasing the A/C ratio to 0.95 and below, it was possible to precipitate out boehmite at 60 °C.     

Thermodynamics of manganese distribution between liquid iron and the CaO–Al2O3–SiO2–FeO–MnO system

The thermodynamics of distribution of constituents between liquid iron and the CaO–Al₂O₃–SiO₂–FeO–MnO system at 1600°C was studied using electrochemical indication of the equilibrium partial pressure of oxygen in both phases. The results show that oxidation potential of the Fe(l)–CaO–Al₂O₃–SiO₂–FeO–MnO system, expressed in terms of log p(O₂), is directly proportional to log (x(MnO) · x(FeO)/w| Mn |). Manganese distribution coefficient, L'_mn, in intersection CaO/Al₂O₃ = 1 decreases with increasing slag basicity expressed in terms of activity a(CaO) or 1/γ(MnO). Experimentally determined equilibrium constant K_Mn/Fe is equal to 2.7 for 1600°C. The number of exchanged electrons between Fe-O-Mn-Si electrode and the slag approaches the theoretical value.     

Effect of Iron Oxides on Activity of Calcium Aluminate Clinker in CaO-Al_2O_3-SiO_2 System

The sintering characteristics at 1350℃ and leaching property at 80℃ of calcium aluminate clinkers in the CaO-Al2O3-SiO2 （C-A-S） system with different additions of FeO and Fe2O3 were investigated. FeO inhibits the conversion of β-Ca2SiO4 to γ-Ca2SiO4 and makes the clinker not pulverizable. FeO and Fe2O3 inhibit the formation of CaAl2O4, but promote the formation of Ca12Al14O33. The interplanar spacing at （2 1 1） crystal face of Ca12 Al14O33 in the clinker increases with the increase of FeO addition, which indicates that FeO forms solid solutions in Ca12Al14O33. The clinkers with Fe2O3 addition form a new phase Ca2Fe2O5, and the amount of Ca2Fe2o5 increases with the in crease of Fe2O3 addition. Both FeO and Fe2O3 do not affect the Al2O3 leaching rate of calcium aluminate clinker in sodium carbonate solution, but they increase the molar ratio of caustic Na2O to Al2O3 in the leached liquor.