Reduction of molybdenite with carbon in the presence of lime

The thermodynamics of the MoS₂-C-CaO system has been studied in order to understand the carbothermic reduction of molybdenite in the presence of CaO. Kinetic studies were also conducted with mixtures of MoS₂+C+CaO in the temperature range of 900 °C 1200 °C. The reduction of MoS₂ with carbon in the presence of lime proceeds through the direct oxidation of MoS₂ by CaO to form intermediate molybdenum oxidized species, MoO₂ and CaMoO₄, which subsequently undergo reduction by CO to yield mixtures of Mo, Mo₂C, and CaS. Complete conversion of MoS₂ can be obtained at 1200 °C in less than 20 minutes for molar concentrations of MoS₂:C:CaO=1:2:2. The kinetic model ln (1−X)=kt was used to determine the rate constants. The activation energy found for the temperature range studied was 218.8 kJ/mol.     

Influence of pellet composition and structure on carbothermic reduction of silica

The melting zone in a cupola has temperatures greater than 1773 K and a reducing atmosphere. This condition is suitable for the carbothermic reduction of silica. The key to the applicability of carbothermic reduction of silica for ferroalloy production is rapid in situ production of SiC and its subsequent dissolution in the hot metal. The main objective of this investigation was to study the kinetics of the carbothermic reduction process and determine the optimum parameters for rapid and complete in situ conversion of silica to SiC. At temperatures above 1773 K, the key reactions in the carbothermic reduction process are (1) SiO₂ (s)+CO (g)=SiO (g)+CO₂ (g), (2) SiO (g)+2C (s)=SiC (s)+CO (g), (3) C (s)+CO₂ (g)=2CO (g). To meet the objective of this study, conditions must be such that the surface reactions occurring at the carbon and silica surfaces are rate limiting and the entire silica is converted to SiC. Pellet composition and structure in terms of carbon to silica ratio, their particle sizes, and compaction pressure that ensure surface reaction is rate controlling were determined. The gas-solid reaction kinetics was mathematically modeled in terms of the process parameters. The reaction kinetics improved by reducing both carbon and silica particle sizes. However, below a certain critical particle size, there was no significant improvement in the reaction kinetics. For complete conversion of SiO₂ (s) to SiC (s), excess carbon and critical porosity are necessary to ensure that the entire SiO (g) generated by Reaction [1] is consumed via Reaction [2] within the pellet.     

Determination of the standard gibbs energies of formation of Ba3P2 and Ba3(PO4)2

The standard Gibbs energies of formation of barium phosphide and barium orthophosphate were determined by a chemical equilibration technique yielding the following results: 3Ba(1)+P₂(g)=Ba₃P₂ (s) ΔG°=−732,000+156.1T(±12,800) (J/mol) 3BaO (s)+P₂(g)+5/2O₂(g)=Ba₃(PO₄)₂(s) ΔG°=−2,523,000+580.0T(±16,600) (J/mol) The stability and the thermodynamic behavior of barium compounds as reaction products of dephosphorization of steel were discussed in terms of the oxygen partial pressure and the activity coefficient of Ba_1.5P in molten Ba saturated with CaO. D.J. MIN, formerly Graduate Student, Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan     

Reforming of Blast Furnace Gas with Methane,Steam, and Lime for Syngas Production and CO2 Capture: A Thermodynamic Study

The production of iron and steel by the blast furnace process is a major source of CO₂ release, with blast furnace gas contributing about 5% to the anthropogenic greenhouse gas emission. Its main components are CO₂, CO, N₂, and H₂. Chemical equilibrium calculations are made to determine the thermodynamic constraints for converting these components into valuable syngas for producing hydrogen, methanol, Fischer–Tropsch hydrocarbons, or ammonia, by either reforming with CH₄, water-gas shift reaction, partial oxidation, or CaO carbonation—while achieving partial or complete CO₂ capture. By a two-step thermochemical cycle, the CaCO₃ formed by lime carbonation could be calcined back to CaO, while releasing relatively pure CO₂ for utilization. The implications of such reactions with respect to hydrogen production, CO₂ emission avoidance, and process efficiency are examined.     

On the hot metal desulfurization

Although there is no thermodynamic limitation to the desulfurization of carbon-saturated iron with CaO, the process is well-known to be slow. Consequently, the desulfurization of carbon-saturated Fe-S, Fe-S-Si, Fe-S-Si-Al, Fe-S-Zr alloys by CaO, and Fe-S-Si alloys by CaO-10 % CaF₂ were investigated to determine the reaction mechanism. For this purpose, dense CaO and CaO-10 % CaF₂ discs were cemented to the bottoms of graphite crucibles containing the carbon saturated alloys. The desulfurization experiments were run at 1450°C in an SiC resistance furnace under argon gas atmosphere. The results indicate that the desulfurization of hot metal by CaO is greatly improved by prior addition of aluminum to the hot metal. The addition of 10 % CaF₂ to CaO also increases the rate of desulfurization. Zirconium, which has a stronger affinity for oxygen than aluminum did not increase the rate. SEM and X-ray diffraction studies on the surfaces of CaO discs used to desulfurize carbon-saturated Fe-S, and Fe-S-Si alloys showed that solid CaS, and solid CaS plus small amount of solid 2 CaO · SiO₂ compounds form on the lime surface, respectively. The slow reaction rate with CaO is attributed to the solid reaction products which block the reaction by preventing the occurrence of interfacial turbulence.     

Modeling of Noncatalytic Hydrogen Reduction of Molybdenum Disulfide in the Presence of Lime,by Complex Multistep Gas–Solid Reactions

Direct reduction of molybdenum disulfide (MoS₂) by hydrogen in the presence of lime is an alternative method for producing molybdenum metal. Such a process can be envisaged to have many advantages, including decreased number of operating stages and elimination of SO₂ emissions. A mathematical model for predicting behavior of reduction of MoS₂ with hydrogen in the presence of lime is presented in this work. And finally, the results predicted by the model have been evaluated and validated using experimental data.     

Lime-enhanced carbon monoxide reduction of cuprous sulfide

Kinetic studies were conducted on the carbon monoxide reduction of cuprous sulfide powder in the presence of lime as a function of quantity of lime in the charge, CO flowrate, temperature, and time of reduction and particle size of the sulfide. Lime was found to enhance drastically the rate of reduction as well as reduce the COS emission into the off-gas to negligible levels. Both temperature and flowrate of the reducing gas were found to influence the reduction rate, and best results were obtained at 1273 K and at a CO flowrate of 3.33 cm³ s⁻¹. The overall reaction seems to be governed by the intrinsic kinetics of the Cu₂S-CO reaction. Kinetic analysis reveals the observance of the Valensi’s equation, indicating diffusional control through the product layer formed over reacting Cu₂S particles. The calculated experimental activation energy of 169.6 kJ/mole in the termperature range of 1123 to 1273 K is in good agreement with that reported in the literature for sulfur diffusion in copper. A critical comparison has been made of the lime-scavenged reduction of Cu₂S by different reagents, namely, hydrogen, carbon monoxide, and carbon.     

Reduction equilibria of the calcium ferrites in the Fe-Fe2O3-CaO system as a function of oxygen concentration and temperature

The results of isothermally conducted reduction tests on the calcium ferrites CF₂+F (= 20 mol. % CaO), CF₂ (= 35 mol. % CaO), CF (= 50 mol. % CaO) and C₂F (= 66.7 mol. % CaO) with CO/CO₂-bearing gases of changing composition in the temperature range between 700 and 1100°C lead both to a plot of the specimen's weight loss as a function of the test duration with the reduction potential CO'₂ of the inlet gases as parameters, and to a preciser determination of the reduction stages by means of a plot of the reduced iron oxide oxygen as a function of the CO'₂ concentration of the reducing gas. The weight losses are attributable to the three-phase equilibria with the ternary compounds CWF and CW₃F contained in the Fe-Fe₂O₃-CaO phase diagram. Further evaluation taking account of temperature-dependent reduction reactions leads, on the one hand, to a complete reduction diagram for iron oxides with lime in the form of log CO₂/CO as a function of MT with largely linear equilibrium curves and, on the other hand, to the familiar Baur-Glaessner diagram, the reduction potential CO'₂ as a function of temperature. The main finding compared with previous literature is the existence of the ternary ferrite CW₃F extending into the liquid phase, and the fresh determination of the temperatures for the four-phase equilibria in the Fe-Fe₂O₃-CaO system below 1100°C.     

Improving the heating system and structure of shaft furnaces for roasting limestone

Improvements in the thermal system of counterflow shaft furnaces—including the heating systems, burners, gas-distribution units, and control algorithms—are described, for new and reconstructed furnaces producing lime containing 92–96% (CaO + MgO)_act. The burnout is no more than 1%, and the quantities of CO and NO _x are significantly below the permitted values.     

Electrode polarization in the DC electroslag melting of pure iron

It is evident from the known ionic properties of the slags used in electroslag melting, that the dc melting process must be accompanied by Faradaic reactions on the slag/ingot and slag/electrode interfaces. The present work has determined the magnitude of the overpotentials resulting from concentration polarization at these interfaces, in the case of pure iron/CaF₂+Al₂O₃, CaF₂+CaO slags using a galvanostatic pulsing technique in an electrolytic cell. The polarization overpotential existing on an electrode in an operating ESR unit has been measured by the same technique. It is found that the potentials observed on the ESR electrode agree well with the results from the electrolytic cell. The primary anodic process is postulated to be the corrosion of iron, leading to an Fe²⁺-saturated layer on the anode surface at sufficiently high current densities. The cathodic process is suggested to be the Faradaic reduction of Al³⁺ or Ca²⁺, to give a concentration of [Al]_Fe or (Ca)_slag in the cathode interface region. This observation is supported by the fact that the cathodic potentials with respect to a C/CO reference electrode are close to those predicted from the reactions: (Al₂O₃)+3C=3CO(g)+2Al(l) or (CaO)+C=CO(g)+Ca(g) At very high current densities both the anodic and cathodic processes may convert to arcs, leading to process instability. The chemical and thermal effects of the overpotentials are briefly discussed and compared with the present results on ESR ingots of pure iron.     

Design and Fabrication of Pellets for Magnesium Production by Carbothermal Reduction

For carbothermal reduction (CTR) to be an economic and clean process for magnesium metal production, operational challenges must be overcome. Strong and reactive precursor pellets are necessary to effectively and selectively produce Mg_(g) from any feedstock. In this study, the effects of ore (magnesia and dolime), carbon (petroleum coke, charcoal, algal char, and carbon black), and binder (organic and inorganic) on pellet strength and reactivity, product yield and purity, and reduction selectivity were analyzed. Theoretically and experimentally, the CTR of dolime (MgO·CaO) favored MgO reduction over CaO reduction; however, with enough carbon and heat, both oxides could be reduced. CaO carbothermal reduction produced CaC₂ and Ca_(g). The selectivity to CaC₂ remained constant (7 ± 4 pct) for all C/MgO·CaO ratios analyzed, while the selectivity to Ca_(g) increased (5 pct → 40 pct) when C/MgO·CaO was increased from 0.5 to 2.0. As the overall metal yield decreased (77.6 pct → 59.7 pct) with increasing CaO reduction (38.2 pct → 78.1 pct), Ca_(g) reverted faster than Mg_(g). Heavy metal impurities primarily remained in the residue (< 30 pct volatilized) and, when volatilized, condensed at high temperatures (700 °C to 1450 °C), relative to light metal impurities (350 °C to 1000 °C, > 78 pct volatilized). Organic binders added reducing power to the pellets but produced frail pellets (radial crush strength = 9.1 ± 0.7 N) after pyrolysis, relative to pellets with inorganic binders (15.1 ± 3.2 N). Kinetic parameters were determined for extruded pellets to predict the reaction rate as a continuous function of pressure and temperature.     

Reaction Kinetics of FeO Containing Molten Slags

The kinetic study of FeO‐containing slag is valuable if we consider slag‐gas and slag‐metal reactions in steelmaking process. In the present work, the reduction rate of Fe_tO‐SiO₂–TiO₂–MO_x (MO_x = CaO, MgO, AlO_1.5, PO_2.5) melts in equilibrium with solid iron by CO gas was measured using the thermobalance system at 1673 K. A rate equation was developed based on the results obtained. The mechanisms of the reaction and the effect of P₂0₅ as a surfactant were discussed. Solid CaO was reacted with FeO‐containing slag at 1573 to 1673 K. The CaO–slag interface was analyzed by SEM and EDX, and the reacted layers were identified. The rate of solid CaO dissolution into a stagnant FeO‐containing slag at hot‐metal temperatures was explained by the FeO diffusion in slag phase.     

Kinetics of Lime-Enhanced Hydrogen Reduction of Solid Nickel Sulphide

Investigations have been carried out on the hydrogen reduction of solid nickel sulphide (β-NI₃S₂) in the presence of lime. The effects of the charge composition, temperature (500-700°C). hydrogen flow rate, time of reduction and particle size of the sulphide have been studied. Lime was found to tremendously enhance the reduction process and drastically stifle H₂S emission into the off-gas. Temperature as well as hydrogen flow rate were found to affect the reduction process and best results were achieved (in static bed experiments) with 200% CaO addition at 630°C in 2 hr employing a hydrogen flow rate of 0.2 l/min. Thermodynamic considerations and several experimental findings indicate that the progress of the Ni₃S₂-CaO-H₂ reaction is governed by the intrinsic kinetics of the Ni₃S₂-H₂ reaction. Kinetic analysis reveals the observance of Jander's linear rate equation indicative of phase boundary control at the sulphide/gas interface. Scanning electron microscopic studies on the reduced nickel sulphide pellets show that like in solid state transformations, discontinuous precipitation (cellular morphology) is exhibited.     

Possible failure of emf oxygen sensor in liquid iron containing dissolved calcium or magnesium

The solubility products of CaO and MgO in liquid iron, measured using the emf oxygen sensor, are several orders of magnitude greater than those calculated from the thermochemical data. This would imply that the values of Δ and Δ derived from the solubility products are, by a large amount, less negative than those given in the compiled thermochemical tables. As is shown in this paper, the values of Δ derived indirectly from numerous experimental data on various gas-solid (-liquid) reactions involving pure CaO, are in general accord with the compiled thermochemical data. It is surmised from these observations that the oxygen activities in liquid iron saturated with CaO or MgO, measured by the emf cell, giving high solubility products of CaO and MgO, may be attributed to the failure of the emf oxygen sensor in liquid iron containing dissolved calcium or magnesium. Two reaction mechanisms are discussed in the paper to account for the malfunction of the oxygen sensor under highly reducing conditions, which will prevail in liquid iron containing even a small amount of dissolved calcium or magnesium. These two mechanisms are (i) redox reaction at the (Y₂O₃)ThO₂ electrolyte/melt interface and (ii) oxygen flux through the electrolyte from the reference electrode Cr–Cr₂O₃ to the melt/electrolyte interface. Suggestions are made for some experimental work to confirm or refute the argument presented in this communication.     

Activities in CaO-SiO2-Al2O3 slags and deoxidation equilibria of Si and Al

By using the data in previous and present slag-metal equilibrium experiments, the activities of SiO₂ along the liquidus lines in CaO-SiO₂-Al₂O₃ slags were determined at 1823 and 1873 K from the reaction Si+2O=SiO₂ (s), in which the oxygen activities were estimated from the measured oxygen contents or from the combination of nitrogen distribution ratios (L _N) and nitride capacities (C _N). The activities of Al₂O₃ were also determined from the reaction 2Al+3O=Al₂O₃ (s), in which the oxygen activities were estimated from the values for L _N and C _N, or from the reaction 3SiO₂ (s)+4Al=2Al₂O₃ (s)+3Si, in which the activities of SiO₂ and the contents of Al and Si along with the respective interaction coefficients were used. The activities of Al₂O₃ and CaO in the entire liquid region were estimated from the Rein and Chipman’s activities of SiO₂ by using the method of Schuhmann. On the basis of these activities, the deoxidation equilibria of Si and Al in steels were discussed.     

Production and properties of burnt lime coated with dicalcium ferrite

Abstract

Ferruginous lime is the term applied to burnt lime (CaO) coated with dicalcium ferrite (2CaO.Fe₂O₃). Its high degradation strength, its resistance to hydration, and its capacity for fast and complete solution of fluxes in the basic oxygen and electric arc furnaces make it extremely attractive as a steelmaking flux. The objectives of the present work were to assess the feasibility of producing ferruginous lime in a rotary kiln type reactor, and to determine the operating conditions favourable for both the formation of a hydration resistant product and the minimisation of problems such as accretions and agglomerations within the reactor. With respect to processing conditions, the trials suggested that ferruginous lime be generated using a peak reaction temperature of 1260°C and allowing 45 min at greater than 1200°C. The optimal oxide addition is 10% by weight of the limestone charge to the rotary lime kiln. When subjected to short duration hydrating conditions, i.e. 30 min in contact with steam at 100°C (according to ASTM specification X6), the ferruginous lime product exhibited good resistance to hydration (relative to pure CaO) and moderate physical degradation. Laboratory tests demonstrated the significantly enhanced rate and degree of dissolution of ferruginous lime (compared with uncoated lime) in a steelmaking slag.     

Critical assessment of activity coefficients of oxides in molten binary and ternary silicates,aluminates and aluminosilicates

Critical assessment is made of the activity coefficients of CaO, MgO, MnO, FeO, Al₂O₃ and SiO₂ in molten silicates, aluminates and aluminosilicates. In this assessment due consideration is given to the consistency of the free energies of formation of the interoxide compounds derived from the oxide activity data, compared with those derived from the compiled thermochemical data. For most oxides, it is found that by using an empirical formulation of the melt composition in ternary systems, the composition dependence of the oxide activity coefficient, γOX, can be represented by a single curve. For example, over a wide composition range in the FeO‐CaO‐SiO₂ system, the log(γCaO) is a single function of the melt composition in mol fraction as (?SiO₂ + 0.3xFeO); this relation being similar to that in the binary CaO‐SiO₂ melts. Therefore, with respect to γCaO, this ternary system is reduced to a pseudo binary system as xCaO ‐ (?SiO₂ + 0.3xFeO). With respect to γCaO, the ternary system CaO‐Al₂O₃‐SiO₂ is reduced to a pseudo binary system as xCaO ‐ (?SiO₂ + 0.4 xAl₂O₃). With γSiO₂, this ternary system is reduced to a pseudo binary system as (?CaO + xAl₂O₃) ‐ xSiO₂.     

Dissolution of Lime in Synthetic ‘FeO’‐SiO2 and CaO‐‘FeO’‐SiO2 Slags

Dissolution of different CaO samples into molten synthetic ‘FeO’‐SiO₂ and ‘FeO’‐SiO₂‐CaO slags was carried out in a closed tube furnace at 1873 K. The slag was kept stagnant. It was found that the dissolution rate was very fast when CaO rod was dipped into ‘FeO’‐SiO₂ slag. In the case of ‘FeO’‐SiO₂‐CaO slag, the dissolution of CaO rod in the stagnant slag was retarded after the initial period (2 minutes). Only less than 16 percent CaO reacted with the slag, irrespective of the type of lime. Three phase‐regions were identified in the reacted part of the lime rod by SEM‐EDS analysis. The formation of these regions was explained thermodynamically. A dense layer of 2CaO · SiO₂ was found to be responsible for the total stop of the dissolution. It could be concluded that constant removal of the 2CaO · SiO₂ layer would be of essence to obtain a high dissolution rate of lime. In this connection, it was found necessary to study the dissolution of lime in moving slag to reach a reliable conclusion regarding the relevance of the reactivity obtained by water ATSM test to the real reactivity of lime in high temperature slag.     

Kinetic studies of the reaction between Cr23 C6 particles and Cr2O3 particles

The kinetics of reaction between Cr₂₃C₆ particles and Cr₂O₃ particles which yields metallic chromium was studied. This reaction is one of the basic reactions in the Simplex process and is also related to the carbon reduction of metal oxide. The rate of reaction between these particles in a pellet of the mixture was measured by thermogravimetry in vacuum at 1050, 1075, and 1100°C. The molar ratio of Cr₂₃C₆ to Cr₂O₃ in the pellet was maintained at 1:8. At the earlier stages of the reaction, a rate equation for interfacial reaction control was applied and the rate constant α was observed to be inversely proportional to the reciprocal effective radius of Cr₂₃C₆ particles. The indirect gaseous reactions are predominant: 1/6 Cr₂₃C₆ + CO₂ = 23/6 Cr + 2CO 1/3 Cr₂O₃+ CO = 2/3 Cr + CO₂ The reduction of Cr₂O₃ with CO gas is presumed to proceed so rapidly and the equilibrium is attained instantaneously. The reaction of CO₂ with Cr₂₃C₆ particles occurs in the sequence co₂ (g) +c< ⇋ c(o) + co(g) c(o) → CO(g) +c< The latter reaction was thought to be the rate determining step, and the activation energy of this reaction was estimated to be approximately 60 kcal per mole.