Chlorination of hafnium dioxide by gaseous chlorine in the presence of carbon

The thermodynamic simulation of chlorination of hafnium oxide by gaseous chlorine in the presence of carbon at T = 450–1000°C is performed. It is shown that the main reaction products are gaseous hafnium tetrachloride, carbon monoxide, and carbon dioxide; as the temperature increases, the partial pressure of CO increases, while that of CO₂ decreases. In the mentioned temperature range, the variations in the enthalpy, entropy, and the Gibbs energy of chlorination reactions are calculated and the composition of the gas phase under the conditions of thermodynamic equilibrium is determined. In the course of studying the kinetics of chlorination of HfO₂ in the presence of carbon in the range T = 600–950°C, it is established that its limiting stage depends on the process temperature. Below 700°C, its rate is limited by the chemical reaction on the surface of nonporous solid spherical particles of the body with the formation of the volatile product; above 700°C, it is limited by the mass transfer of gaseous substances.     

Synthesis of Titanium Oxycarbide from Titanium Slag by Methane-Containing Gas

In this study, reaction steps of a process for synthesis of titanium oxycarbide from titanium slag were demonstrated. This process involves the reduction of titanium slag by a methane-hydrogen-argon mixture at 1473 K (1200 °C) and the leaching of the reduced products by hydrofluoric acid near room temperature to remove the main impurity (Fe₃Si). Some iron was formed by disproportionation of the main M₃O₅ phase before gaseous reduction started. Upon reduction, more iron formed first, followed by reduction of titanium dioxide to suboxides and eventually oxycarbide.     

Reduction of FeO in smelting slags by solid carbon: Experimental results

The reduction of CaO-SiO₂-Al₂O₃-FeO slags containing less than 10 wt pct FeO by solid carbonaceous materials such as graphite, coke, and coal char was investigated at reaction temperatures of 1400 °C to 1450 °C. The carbon monoxide evolution rate from the system was measured using stationary and rotating carbon rods, stationary horizontal carbon surfaces, and pinned stationary spheres as the reductants. The measured reaction rate ranged from 3.25 × 10^?7 mol cm^?2 s^?1 at 2.1 pct FeO under static conditions to 3.6 × 10^?6 mol cm^?2 s^?1 at 9.5 pct FeO for a rotating rod experiment. Visualization of the experiment using X-ray fluoroscopy showed that gas evolution from the reduction reaction caused the slag to foam during the experiment and that a gas film formed between the carbon surface and the slag at all times during experimentation. The reaction rate increased with increased slag FeO contents under all experimental conditions; however, this variation was not linear with FeO content. The reaction rate also increased with the rotation speed of the carbon rod at a given FeO content. A small increase in the reaction rate, at a given FeO content, was found when horizontal coke surfaces and coke spheres were used as the reductant as compared to graphite and coal char. The results of these experiments do not fit the traditional mass transfer correlations due to the evolution of gas during the experiment. The experimental results are consistent, however, with the hypothesis that liquid phase mass transfer of iron oxide is a major factor in the rate of reduction of iron oxide from slags by carbonaceous materials. In a second article, the individual rates of the possible limiting steps will be compared and a mixed control model will be used to explain the measured reaction rates.     

Kinetics of chlorination of tantalum pentoxide in mixture with sucrose carbon by chlorine gas

The mechanism and kinetics of β-Ta₂O₅ chlorination, mixed with sucrose carbon, have been studied by a thermogravimetric technique. The investigated temperature range was 500 °C to 850 °C. The reactants and reaction residues were analyzed by scanning electronic microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller method for surface area (BET). The effect of various experimental parameters was studied, such as carbon percentage, temperature, chlorine partial pressure, and flow, use of the multiple sample method, and carbon previous oxidation. The carbon percentage and previous treatment have an effect on the system reactivity. The temperature has a marked effect on the reaction rate. In the 500 °C to 600 °C temperature interval, the apparent activation energy is 144 kJ/mol of oxide, while at higher temperatures, the activation energy decreases. With high chorine partial pressures, the order of reaction is near zero. The kinetic contractile plate model, X=kt, considering carbon oxidation as the controlling stage, is the one with the best fit to the experimental data. A probable mechanism for the carbochlorination of β-Ta₂O₅ is proposed: (1) activation of chlorine on the carbon surface, (2) chlorination of Ta₂O₅, (3) oxidation of carbon, and (4) recrystallization of β-Ta₂O₅.     

Kinetics of oxidation of carbon in liquid iron-carbon-silicon-manganese-sulfur alloys by carbon dioxide in nitrogen

The oxidation of carbon with the simultaneous oxidation of silicon, manganese, and iron of liquid alloys by carbon dioxide in nitrogen and the absorption of oxygen by the alloys from the gas were studied using 1-g liquid iron droplets levitated in a stream of the gas at 1575 °C to 1715 °C. Oxidation of carbon was favored over oxidation of silicon and manganese when cast iron (3.35 pct C, 2.0 pct Si, 0.36 pct Mn, and 0.05 pct S) reacted with CO₂/N₂ gas at 1635 °C. An increase in the flow rate of CO₂/N₂ gas increased the decarburization rate of cast iron. The rate of carbon oxidation by this gas mixture was found to be independent of temperature and alloying element concentrations (in the range of silicon = 0 to 2.0 pct manganese = 0 to 0.36 pct and sulfur = 0 to 0.5 pct) within the temperature range of the present study. Based on the results of a kinetic analysis, diffusion of CO₂ in the boundary layer of the gas phase was found to be the rate-limiting step for the reactions during the earlier period of the reaction when the contents of carbon, silicon, and manganese are higher. However, the limiting step changed to diffusion of the elements in the metal phase during the middle period of the reaction and then to the diffusion of CO in the gas phase during the later period of the reaction when the content of the elements in the metal were relatively low. For the simultaneous oxidation reactions of several elements in the metal, however, the diffusion of CO₂ in the gas phase is the primary limiting step of the reaction rate for the oxidation of carbon during the later period of reaction. Formerly Visiting Assistant Research Scientist, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109     

The chlorination kinetics of zirconium dioxide in the presence of carbon and carbon monoxide

Chlorination of zirconium dioxide, an important step in the commercial production of reactor grade zirconium metal, has been studied using carbon and carbon monoxide as reductants. Zirconium tetrachloride was produced when the oxide was reacted with chlorine alone above 1000°C; by introducing carbon the reaction temperatures could be lowered 200°C, and when carbon monoxide was used the reaction temperatures necessary for similar rates were even lower. With carbon monoxide the chlorination of zirconium dioxide is described by two regions differentiated by temperature and the dependence of reaction rate upon chlorine and carbon monoxide pressures.     

Electrochemical Characterization of a Solid Oxide Membrane Electrolyzer for Production of High-Purity Hydrogen

A laboratory-scale solid oxide membrane (SOM) steam electrolyzer that can potentially use energy value in waste or any source of carbon or hydrocarbon to produce high-purity hydrogen has been fabricated and evaluated. The SOM electrolyzer comprises an oxygen-ion-conducting yttria-stabilized zirconia (YSZ) electrolyte with a Ni-YSZ cermet cathode coated on one side and liquid-metal anode on the other side. The SOM electrolyzer is operated at 1000 °C by providing a steam-rich gas feed to the Ni-YSZ cermet cathode and feeding a reductant source into the liquid-metal anode. The steam is reduced over the cathode, and oxygen ions are transported through the YSZ electrolyte and are oxidized at the molten metal electrode by the reductant feed. The advantage of SOM electrolyzer over the state-of-the-art solid oxide electrolyzer is its ability to use solid, liquid, and gaseous reductant feed in the liquid-metal anode to reduce the oxygen chemical potential and drive the reaction for hydrogen production. In this study, an electrochemical process model for a SOM electrolyzer was developed. The condition of the liquid-metal anode with reductant was simulated by bubbling humidified hydrogen (3 pct H₂O) in the liquid metal, and the electrochemical performance of the SOM electrolyzer was modeled. The experimental data were curve-fitted into the model to identify the various polarization losses. It showed that the performance of the SOM electrolyzer was dominated by the ohmic resistance of the YSZ membrane. Based on the results of this study, future work is needed toward increasing the performance efficiency of the SOM electrolyzer.     

Vacuum Carbothermic Reduction of Panzhihua Ilmenite Concentrate: A Thermodynamic Study

Electric arc furnace is mainly used in the production of high titania slag; however, since impurities cannot be eliminated, this causes difficulty in the production of titania pigment with chlorination process. Consequently, removing impurities is the crucial way to deal with low-grade ilmenite, especially for the Panzhihua ilmenite concentrate in China. This article studied the theoretical calculation of vacuum carbothermic reduction of Panzhihua ilmenite concentrate. Thus, when the temperature was higher than 1600°C and the carbon amount was greater than 12%, all of the Fe almost entered into the gas phase. When the temperature was higher than 1300°C and the carbon amount was greater than 14%, magnesium also entered the gas phase. When the temperature was higher than 1100°C, most of the element manganese was volatilized in the gas phase. The TiO₂ grade increased with the increase in carbon amount (14%). When the temperature was higher than 1600°C and the carbon amount was less than 14%, the TiO₂ grade in the slag phase could reach the maximum value, which can be used for the chlorination process to prepare titanium dioxide.     

Phase transitions during reducing roasting of a leucoxene concentrate with carbon

The phase transitions during the reducing roasting of a leucoxene concentrate with carbon are studied to obtain an anosovite product. Thermodynamic modeling of the reducing roasting is performed, and the influence of the temperature and the amount of a reducing agent on the reduction of rutile to Ti₃O₅-based anosovite is studied. Almost complete reduction of rutile to anosovite occurs in a temperature range of 1350–1400°C in the presence of 2.5–5.0% carbon. The Magnéli phases of various compositions are predominantly formed at lower temperatures and smaller amounts of the reducing agent. At temperatures higher than 1400°C and a reducing agent amount >2.5%, rutile reduction results in the formation of anosovite along with an undesirable titanium carbide phase.     

Reduction of Norwegian and Indian ilmenite with carbon monoxide and hydrogen gas blends

Synthesis gas, produced from natural gas, can be used to minimise coal consumption and carbon dioxide emissions during the processing of ilmenite. Two different ilmenite ores have been reduced with carbon monoxide and hydrogen mixtures to investigate the effects of temperature and gas composition on the final product. Thermogravimetric analysis was used to observe the reaction progress. Experimental work revealed that between 900 and 1000?°C the hydrogen content in the gas has an equally significant effect as the temperature. Statistical analysis determined that the source of the ore did not have a significant effect on the reaction rate. Armalcolite is one of the main products of pre-reduction in addition to metallic iron and rutile from these ilmenite concentrates. There is also some indication that titanium dioxide is reduced. Optical microscope images revealed that increasing amounts of hydrogen resulted in smaller more uniformly distributed metallic iron particles.     

Kinetics of carbon combustion in packings of casting powder

The kinetics of carbon combustion in packings of casting flux have been investigated in isothermal laboratory experiments. At low temperature (300, 400°C) combustion is controlled by chemical reaction, and at high temperatures (600 to 800°C) by mass transfer within the powder packing and in the gaseous space above the packing. Inbetween, there is a range of mixed control. The paper intends to demonstrate how the relevant kinetic parameters can be deduced with rather simple experimental methods. These parameters can then be used in the modelling of the more complex combustion process occurring in real continuous casting.     

The kinetics of reduction of hematite by carbon

Reduction kinetics of mixtures of hematite and carbon powders were investigated in the temperature range of 850° to 1087°C. Experiments were carried out under argon atmosphere and the isothermal weight loss of the samples was determined as a function of time. The effects of carbon particle size, hematite/carbon ratio of the mixture, and addition of promotive or inhibitive reagents were also investigated. The results were summarized in the form of fractional reaction vs time plots. A kinetic model developed on the basis of carbon solution-loss reaction as rate-controlling represented the results fairly well. An enthalpy of activation of 72 kcal/per mole was calculated, within the range of 957° to 1087°C. The observed effects of Li₂O and FeS on the reduction kinetics are consistent with the influence these reagents are known to exercise on the solution-loss reaction.     

Kinetics of chloridization of nickel oxide with gaseous hydrogen chloride

The kinetics of chloridization of finely divided as well as granulated nickel oxide by gaseous HCl were studied in the temperature range 150 °C to 400 °C. The rate of chloridization depended upon temperature, partial pressure of HCl, and granule size. The conversion of NiO to NiCl₂ follows the logarithmic rate law or a pore-blocking model, which is attributed to the large increase in molar volume of the product phase. The blocked pores decreased the diffusion of gaseous reactant through the product layer. This observation has been supported by the low value of activation energy, the high order of reaction with respect to the partial pressure of HCl (g), and a slope value of −2 for the log-log plot of rate constant vs grain size. The morphological characteristics of both unreacted and chloridized nickel oxide have been studied with the help of a scanning electron microscope.     

A study of the mechanisms of the salt catalyzed carbochlorination of kaolin

Continuous weighing of kaolin during carbochlorination has been used to show the superiority of NaCl as a catalyst for increasing the reaction rate and selectivity at 625° to 650 °C, and the possibility of using different types of carbon as a reductant. Initial distribution of NaCl in the kaolin was found to be of importance. A reaction mechanism involving the formation of NaAlCl₄ and alumina solubility in this salt is used to explain the effect of reactor configuration and kaolin particle geometry.     

Ordering in the carbide W2C and phase equilibria in the tungsten — carbon system in the region of its existence

We have studied alloys in the W - C system with compositions 24 to 48 at.% C, melted in an arc furnace, homogenized and quenched from temperatures of 1450°C to 2650°C or cooled stepwise from a subsolidus temperature down to 850°C, and annealed for a prolonged period at this temperature or at temperatures close to it. We studied the specimens by x-ray single-crystal and powder diffraction, metallography, and differential thermal analysis. We have shown that the carbide W₂C exists in two modifications: an hexagonal phase with structure of type L′₃ crystallizes from the melt, which at T ≈ 1850°C transforms to an ordered hexagonal phase of the ε-Fe₂N type. The latter, for a carbon content of about 32 at.% and T ≈ 1250°C, also decomposes according to a eutectoid reaction to 〈W〉 and WC. This decomposition is accompanied by formation of a metastable phase of the ξ-Fe₂N type as an intermediate. Taking the results obtained and literature data into consideration, we have corrected the phase diagram for the W - C system.     

Kinetics of reduction of lead oxide in liquid slag by carbon in iron

The reduction of lead oxide in dilute solution in CaO-Al₂O₂-SiO₃ slag by carbon dissolved in iron was investigated using a composite crucible as a container so as to exclude graphite from the system. The variables studied to elucidate the reaction mechanism were pressure inside the crucible, carbon content of the metal, lead oxide concentration in slag, and slag composition. The experimental results are best explained by postulating the existence of a gas film at the slag metal interface. It is suggested that the rate controlling step for the lead oxide reduction by carbon is a chemical reaction at the gas/slag interface. The rate constant for up to 3 wt pct PbO in the slag and 2.0 to 4.3 pct C in iron at 1400 °C as calculated from the present study is 4.6 x 10^-4 mol/cm²/min/atm.     

Interaction between nonstoichiometric titanium carbide and Fe-C alloys

This article describes a thermodynamic analysis, and the experimental verification, of the chemical interaction between nonstoichiometric titanium carbide and molten Fe-C alloys. The calculation and the analysis of the isothermal sections of the ternary Fe-Ti-C phase diagram in the 1500 °C to 1600 °C temperature range show that variations in composition and in the metal-ceramic ratio can occur in the course of the TiC _x -(Fe-C) composite preparation. These changes, in particular, those of the carbon content in the interacting phases, can affect the microstructure and properties of the resulting composite. The experimental results support the predictions of the equilibrium calculations, according to which nonstoichiometric titanium carbide tends toward the stoichiometric composition by absorbing carbon dissolved in the metallic binder or by releasing titanium to the melt. The microstructural observations also support the prediction that a significant amount of the carbide phase with a low carbon content can dissolve in the melt. The results of this study suggest that it is possible to design the microstructure of both the metallic and the ceramic phases in the composite by proper choice of the carbon content in the carbide phase. This allows the mechanical properties of the composites to be designed on the basis of thermodynamic considerations.     

Kinetics of the reaction of SiO(g) with carbon saturated iron

The reaction of SiO(g) with carbon saturated iron plays an important role in silicon transfer in the iron blast furnace. In the tuyere zone SiO(g) is generated from the reaction of coke with its ash, and the SiO(g) reacts with carbon saturated iron droplets as they pass through the furnace. This reaction may also play a role as a gaseous intermediate reaction for the reduction of SiO₂ from slags by carbon dissolved in iron. The rate and controlling mechanism for the SiO reaction with carbon dissolved in liquid iron was determined in the temperature range 1823 to 1923 K. A constant pressure of SiO(g) was generated by the reaction of CO with SiO₂; the reaction of carbon with SiO₂ gave higher but decreasing pressures of SiO with time. The SiO(g) generated was then reacted with carbon saturated iron under conditions for which the gas phase mass transfer conditions are clearly defined. By a systematic variation of the appropriate variables it was demonstrated that the rate was controlled by gas phase transfer of SiO to the gas-metal interface. The rate changed with gas diffusional distance but was not a strong function of temperature or of melt composition. The rate calculated for gas phase mass transfer was in good agreement with the experimental results. An erratum to this article is available at .     

Carbon deposition on iron surfaces in CO–CO2 atmosphere

Thermodynamic calculations and thermogravimetric (TG) analysis were performed in order to understand the mechanism of carbon deposition on the surfaces of iron particles during the reduction of iron ore in a CO–CO₂ atmosphere. The results of the thermodynamic equilibrium phase analysis indicate that the phases of the carbon deposition process can be predicted on the basis of the carbon potential, reaction temperature and gas pressure. The optimal thermodynamic conditions for carburisation are a low temperature (T?<?T_m) and a high carbon potential (α_c>1). TG analysis is performed in a gas mixture of 65 vol.-% CO and 35 vol.-% CO₂ at 650, 706 and 750°C. Cementite (Fe₃C) is generated as an intermediate product, which acts as a catalyst for carbon deposition. Carbon deposition is inhibited at high temperatures (T>791°C) owing to the high stability of Fe₃C. When the reaction temperature is higher than the thermodynamic limit for the formation of Fe₃C, carbon deposition cannot occur. A mechanism for carbon deposition is proposed based on the experimental results.