Oleg Ostrovski

Effect of preoxidation and sintering on properties of ilmenite concentrates

Abstract The paper examines the effects of preoxidation and sintering on the phase composition, specific surface area, morphology and reducibility of ilmenite concentrates. Samples were preoxidised in air or sintered in argon by temperature-programmed heating to temperatures of 600–1400°C at a ramping rate of 300°C/h. In the concentrates preoxidised at temperatures up to 800°C, ilmenite formed an intermediate product, Fe 2 O 3 ·2TiO 2 , which decomposed into Fe 2 O 3 and TiO 2 . At temperatures above 800°C, pseudorutile was transformed into ferric pseudobrookite and rutile. Haematite recombined with rutile to form pseudobrookite at temperatures above 1000°C. When samples were heated in an argon atmosphere, ferric–ferrous pseudobrookite solid solution was formed from ilmenite and pseudorutile. Preoxidation and sintering at temperatures above 600°C caused a sharp decrease in the specific surface area of samples. The specific surface area decreased by more than 97% when samples were heated to 1200°C. Preoxidation enhanced while sintering retarded the reduction of iron oxides in ilmenite concentrates by methane–hydrogen gas mixtures in the temperature range of 550–800°C. Both preoxidation and sintering increased the temperature required to reduce titanium oxides. At high temperatures, such as 1200°C, the effects of preoxidation and sintering on isothermal reduction of ilmenites were negligible.

Slag-graphite wettability and reaction kinetics. Part 1 Kinetics and mechanism of molten FeO reduction reaction

Abstract The present paper reports results relating to the kinetics and mechanism of FeO reduction by graphite, the data being obtained from experimental investigations into the wettability of graphite by molten slag containing FeO. The rate of FeO reduction was determined by measuring the volume of CO gas formed as a result of the reduction of FeO in experiments conducted in the same sessile drop apparatus. The reduction reaction initiated by direct slag–graphite contact produces CO gas which spreads into the molten slag droplet causing foaming of the slag; further reduction of FeO proceeds mostly via indirect reduction. The rate of reduction was found to depend directly on the initial FeO content. An increase in temperature improves the rate of reaction, which has an activation energy of 112·18 kJ mol-1. These results indicate that transport of FeO (Fe2+, O2- ) in the liquid slag phase is probably the slowest step.

Molecular-dynamic simulation of the thermophysical properties of liquid uranium

The procedure for the calculation of the embedded atom model (EAM) potential, which involves the use of data on the structure of liquid metal in the vicinity of the melting temperature and of the results of impact tests, is applied to uranium. The use of the method of molecular dynamics and of the EAM potential produces good agreement with experiment as regards the structure, density, and potential energy of liquid metal at temperatures up to 5000 K, as well as along the shock adiabat up to pressures of ≈360 GPa. The thermodynamic properties of solid (bcc) and liquid uranium are determined at pressures up to 470 GPa and temperatures up to 12 000 K. The predicted value of bulk modulus of liquid at 1406 K is close to the actual value. The self-diffusion coefficient under isobaric heating increases with temperature by the power law with exponent of ≈2.103. The Stokes—Einstein relation is used to determine the dynamic viscosity at temperatures up to 6000 K. The obtained potential is not quite adequate for describing crystalline uranium under normal conditions. The melting temperature of uranium with EAM potential is equal to 1455 ± 2 K and somewhat higher than real. The melting temperature monotonically increases with pressure and reaches the value of 7342 K at 444 GPa. For obtaining agreement with experimental data for energy of uranium along the p = 0 isobar, it is assumed that an additional contribution to energy emerges at elevated temperatures, which is due to excitation of atomic electrons and leads to a high heat capacity: it may be as high as almost 100 kJ/mol at 5000 K. This contribution further causes a high heat capacity of highly compressed states of uranium.

Effect of Gas Atmosphere on Carbothermal Reduction and Nitridation of Titanium Dioxide

This article examined the reduction/nitridation of rutile in the He-N2, Ar-N2, and He (Ar)-H2-N2 gas mixtures, as well as pure nitrogen, in the temperature-programmed and isothermal experiments in a fixed-bed reactor. The extents of reduction and nitridation were determined from the off gas composition and LECO analysis. The off-gas composition was monitored using the infrared sensor (CO, CO2, and CH4) and dew point analyzer (H2O). The phase composition of the reduced samples was analyzed using X-ray diffraction (XRD). The temperature and gas composition had a strong effect on the rutile reduction. The reduction was the fastest in the H2-N2 gas mixture, followed by a reduction in nitrogen; the rate of reduction/nitridation in the He-N2 gas mixture was marginally higher than in the Ar-N2 gas. The rate of titania reduction/nitridation in the He (Ar)-H2-N2 gas increased with the replacement of He (Ar) with hydrogen. The article also discusses the mechanisms of reduction/nitridation in different gas atmospheres.

Molecular dynamics simulation of oxides with ionic–covalent bonds

Abstract A “semi-classical” method was developed for molecular dynamics simulation of a system with ionic–covalent bonds like silica. The ionic charges were calculated by minimization of the potential energy on each step of molecular dynamics simulation. Ionic–covalent potential was used in modeling of SiO2 molecule, non-crystalline silica, and calcium metasilicate. The internal energy of a system includes energies of silicon ionization, affinity of oxygen to electrons, Coulomb interactions and repulsion of ions, and covalent SiO energy. Calculated properties of glassy and liquid silica and SiO2 molecule, such as density, internal energy, compressibility, distances between ions and vibration frequencies, are close to experimental values.

Slag–graphite wettability and reaction kinetics Part 2 Wettability influenced by reduction kinetics

Abstract The present paper reports results of experimental investigations into the wettability of graphite by molten slag containing FeO. The wettability was determined by measurement of the graphite–slag contact angle. A higher initial FeO content in the slag phase and higher temperature lead to better wettability of graphite owing to the chemical reaction between the molten FeO in the slag and the graphite surface. The presence of FeO also reduces the surface tension of the slag. The free energy of reaction released per unit area ΔG r has been estimated and correlated with the wettability parameters. The overall contribution of this factor to lowering the interfacial tension has also been examined.

Kinetic Modelling of MnO Reduction from Manganese Ore

Abstract This paper analyzes the results of MnO reduction from manganese ore and ferromanganese slag obtained at NTNU (Trondheim) and UNSW (Sydney). Manganese ore upon melting consists of two phases: solid MnO phase which may be a pure MnO oxide or a MnO-MgO solid solution and liquid slag. The process of manganese ore reduction includes MnO phase dissolution into the molten slag and MnO reduction from the slag. It is suggested that the reduction rate of MnO is controlled by the intrinsic kinetic step. The proposed kinetic model takes into account changes in the interfacial area and manganese oxide activity in the slag in the course of reduction. The kinetic model is used to examine effects of temperature, ore composition, CO partial pressure, coke size and reaction time on the extent of MnO reduction. Kinetic constraints in electric furnace and blast furnace ferromanganese making are also discussed.

Carbothermal reduction of ilmenite concentrates and synthetic rutile in different gas atmospheres

Abstract Carbothermal reduction of ilmenite concentrates and synthetic rutile was studied in isothermal experiments in hydrogen, argon and helium in a tube reactor. Concentrations of CO, CO2 and CH4 in the off gas were measured online using an infrared gas analyser. The reaction products were analysed by X-ray diffraction. Pseudorutile and ilmenite were the main phases in ilmenite concentrates. Reduction of ilmenite concentrates and synthetic rutile in hydrogen was significantly faster than that in inert atmosphere. The effect of gas atmosphere became stronger for lower grade ilmenite containing more iron oxides. The conversion rate of titania to titanium oxycarbide in hydrogen decreased with increasing grade of ilmenite concentrate. In inert gas, the reduction rates of secondary ilmenite and HYTI70 were close to that of primary ilmenite. Reduction of synthetic rutile was faster in comparison with ilmenite concentrates.

Molecular Dynamics Modelling of Liquid Fe-C Alloys

Properties and structure of liquid Fe-C alloys with carbon concentration up to 20 at% at temperatures up to 2500 K were calculated using molecular dynamics modelling. Interaction between Fe-Fe and Fe-C atoms was described using the embedded atom potential (EAM). The Morse potential was chosen for Fe-C pair interaction, while C-C pair interaction was presented in the form of the repul- sive potential. Parameters of potentials were adjusted using experimental data for density, internal energy, bulk modulus and distance between Fe and C atoms in liquid Fe-C alloys near the liquidus temperature. Calculated density and molar volume of Fe-C alloys at 1873 K decreased with increasing carbon concentration; a temperature coefficient of density of liquid alloys was, practically, independent of the carbon concentration. Carbon content of the alloys had a negligible effect on the distance between Fe-Fe and Fe-C atoms and on a sum of coordination numbers Fe-Fe and Fe-C. Distribu- tion of atoms in the first coordination sphere in the Fe-C alloys was close to statistical.

Iron ore reduction/cementation: experimental results and kinetic modelling

Abstract Iron ore reduction and iron cementation by H2-CH4-Ar gas mixtures were investigated in a laboratory isothermal fixed bed reactor in the temperature range 600-925°C. Iron ore was first reduced to metallic iron by hydrogen, then metallic iron was carburised to cementite by methane. Increasing temperature and hydrogen content accelerated the reduction process. However, for >55 vol.-%, the effect of H2 content was not significant. Methane had almost no effect on the reduction process. Increasing temperature increased the rate of iron cementation and also the rate of free carbon deposition. Optimum conditions for cementite formation were: temperature 750°C and reducing/carburising gas contents of 40-55 vol.-%H2 and 35 vol.-%CH4. Under these conditions, reduction of iron ore to cementite was completed in ~15 min. A two interface grain model and a volume reaction model were used to simulate the process of iron ore reduction and iron cementation. The simulated results for both reduction and cementation were consistent with the experimental data.