Oxidation of molten impurities in converters by means of combustion flames: Thermodynamic principles. 2. Interaction of flame with metal and slag in converter bath

Thermodynamic analysis is applied to the physicochemical processes in the converter bath when intensifying bath heating by means of gas–oxygen burners. In the converter’s working space, when the combustion flames interact with the liquid bath, the oxygen and natural gas supplied through the burners and the oxygen supplied through the tuyere interact in a bubbling slag–metal emulsion. As a result, iron and the impurities are oxidized. The use of such burners changes the gas composition: not only O₂, CO, and CO₂ are present, but also H₂ and H₂O, which changes the oxidative capacity of the gas phase. The presence of solid carbon (for example, pulverized coal) in the burner flame may be used to control and intensify the combustion process. Combustion is most effective in the oxidation of carbon to CO when the oxygen excess is less than 1.0. The oxidation conditions of carbon in the melt change with variation in its activity as a function of its concentration and the temperature. The equilibrium in the M–O–C system may be described by the oxygen partial pressure \({P_{{O_2}}}\), which may be regarded as a universal characteristic. In addition, the equilibrium may be assessed on the basis of the associated ratios \({P_{CO}}/{P_{C{O_2}}}\) and \({P_{{H_2}}}/{P_{{H_2}O}}\) It is found that iron may be oxidized by oxygen and, to some extent, by carbon dioxide. At 1600–2000 K, there is practically no oxidation of iron by steam. The carbon dissolved in the steel is oxidized relatively effectively by oxygen and carbon dioxide until its concentration is less than 0.1% C. Steam oxidizes carbon very poorly and is not much more effective with manganese and silicon. With increase in temperature, the rate at which carbon dissolved in steel is oxidized by oxygen increases, while the oxidation rate of manganese and silicon falls. Above 1800 K, superoxidized slag with a high FeO content actively oxidizes silicon (to <2% Si), manganese (to <1% Mn), and carbon (to <1.5% C).     

Mathematical model for post combustion in smelting reduction

Post combustion in the top space of iron bath smelting reduction furnaces is analysed with three-dimensional mathematical modelling. Momentum transport and continuity equations in combination with a k-? model of turbulence are numerically solved for the gas flow field. Combustion reactions are modelled by a set of transport equations based on the SCRS combustion model and its extension to the k-?-g model. A two-stage combustion scheme is formulated to include carbon transfer and combustion. Heat transfer to bath and droplets is approximated including radiation. Computation results for rectangular reactors are presented with velocity patterns and combustion fields. The complex shapes of post combustion flames are demonstrated. Process parameters are varied to study their influence on combustion and heat transfer to the bath. Effects of the injection geometry are illustrated.     

Oxidation kinetics of molten copper sulfide

The oxidation kinetics of molten Cu₂S baths, during top lancing with oxygen/nitrogen (argon) mixtures, have been investigated as a function of oxygen partial pressure (0.2 to 0.78), bath temperature (1200 °C to 1300 °C), gas flow rate (1 to 4 L/min), and bath mixing. Surface-tension-driven flows (the Marangoni effect) were observed both visually and photographically. Thus, the oxidation of molten Cu₂S was found to progress in two distinct stages, the kinetics of which are limited by the mass transfer of oxygen in the gas phase to the melt surface. During the primary stage, the melt is partially desulfurized while oxygen dissolves in the liquid sulfide. Upon saturation of the melt with oxygen, the secondary stage commences in which surface and bath reactions proceed to generate copper and SO₂ electrochemically. A mathematical model of the reaction kinetics has been formulated and tested against the measurements. The results of this study shed light on the process kinetics of the copper blow in a Peirce-Smith converter or Mitsubishi reactor.     

Catalytic combustion of soot over Ru-doped mixed oxides catalysts

We employed modified substrates as outer heterogeneous catalysts to reduce the soot originating from the incomplete diesel combustion. Here, we proposed that ceria（CeO2）-based catalysts could lower the temperature at which soot combustion occurred from 610 oC to values included in the operation range of diesel exhausts（270–400 oC）. Here, we used the sol-gel method to synthesize catalysts based on mixed oxides（ZnO：CeO2） deposited on cordierite substrates, and modified by ruthenium nanoparticles. The presence of ZnO in these mixed oxides produced defects associated with oxygen vacancies, improving thermal stability, redox potential, sulfur resistance, and oxygen storage. We evaluated the morphological and structural properties of the material by X-ray diffraction（XRD）, Brumauer-emmett-teller method（BET）, temperature programmed reduction（H2-TPR）, scanning electron microscopy（SEM）, and transmission electron microscopy（TEM）. We investigated how the addition of Ru（0.5 wt.%） affected the catalytic activity of ZnO：CeO2 in terms of soot combustion. Thermogravimetric analysis（TG/DTA） revealed that presence of the catalyst decreased the soot combustion temperature by 250 oC, indicating that the oxygen species arose at low temperatures, which was the main reason for the high reactivity of the oxidation reactions. Comparative analysis of soot emission by diffuse reflectance spectroscopy（DRS） showed that the catalyst containing Ru on the mixed oxide-impregnated cordierite samples efficiently oxidized soot in a diesel stationary motor： soot emission decreased 80%.     

Oxidation of low carbon steel in multicomponent gases: Part I. Reaction mechanisms during isothermal oxidation

The article describes rates of oxidation of low carbon steel in various nitrogen-based atmospheres of O₂, CO₂, and H₂O in the temperature range 800 °C to 1150 °C. In characterizing the oxidation process, the weight gains of the samples per unit surface area vs time data were analyzed. Reaction rates during oxidation in the binary atmospheres of CO₂-N₂ and H₂O-N₂ followed a linear rate law and were found to be proportional to the partial pressures of CO₂ or H₂O. These rates were controlled by rate of reactions at the oxide surface and were highly dependent on oxidation temperature. The activation energies of the phase boundary reactions obtained were approximately 274 and 264 kJ/mole, for oxidation in CO₂ and H₂O atmospheres, respectively. Oxidation in gases containing free oxygen showed that the main oxidizing agent was the free oxygen and that additions of CO₂ and H₂O had little effect on the magnitude of the initial oxidation rates. Experiments for oxidation in multicomponent gases showed that the overall oxidation rates were the additions of rates resulting from oxidation with the individual gaseous species O₂, CO₂, and H₂O. Oxidation in these atmospheres exhibited an initial linear rate law which gradually transformed into a parabolic. Examination of scale microstructure after 1 hour of oxidation showed that, for oxidation in carbon dioxide and water vapor atmospheres, only wustite was present, while in atmospheres containing free oxygen, all three iron oxides, wustite, magnetite, and hematite, were present.     

Analysis of bath smelting processes for producing iron

A simulation model on bath smelting processes for the production of iron was developed which predicts the coal, flux, ore, and oxygen consumptions and the off gas volume, temperature and composition. The model is comprehensive in that it takes into account all of the important variables including coal composition, metal composition, ore composition, slag basicity, post combustion ratio, (PCR), prereduction degree (PRD), heat transfer coefficient (HTC), flux, scrap charge, and heat losses. Four basic cases were considered: I. 30% PRD–50% PCR; II. 90% PRD–0% PCR; III. 60% PRD–30% PCR; and IV. 0% PRD–50% PCR. Several different coals were considered and a sensitivity analysis of the critical variables was performed. The model also estimates the sulfur content of the metal. The major conclusions are: Post combustion siginificantly reduces coal consumption but above 20% PCR little reduction of FeO to Fe can be performed with the off gas. Prereducing to FeO (case I) and having as much post combustion as consistent with good heat transfer is an attractive process. This process only requires a simple prereducer, uses less coal, and is relatively insensitive to the type of coal used. High off-gas temperatures may pose a potential problem. The off-gas temperature can be reduced by using an O₂–air mixture for post combustion, limiting post combustion or adding water to the gas. The use of CaCO₃ in place of CaO or of supplemental electricity does not appear attractive. The melting unit is theoretically an energy efficient scrap melter. For case I using 200 kg of scrap as part of the charge the coal consumption decreases by about 80 kg. With PCR > 30% the FeO content of the slag is expected to be 2–5%, and the metal will not be saturated with carbon. These factors and the increased sulfur load since coal is the fuel indicate the sulfur content of the metal may exceed 0.25%.     

Rh/CeO2 composites prepared by combining dealloying with calcination as an efficient catalyst for CO oxidation and CH4 combustion

In this paper, a series of Rh/CeO₂ catalysts with three-dimensional porous nanorod frameworks and large specific surface area were prepared by chemical dealloying Al–Ce–Rh precursor alloys and then calcining in pure O₂. The effects of the Rh content and calcination temperature on CO oxidation and CH₄ combustion were studied, and the results reveal that the Rh/CeO₂ catalysts produced by dealloying melt-spun Al_91.3Ce₈Rh_0.7 alloy ribbons and then calcining at 500 °C exhibit the best catalytic activity, the reaction temperatures for the complete conversion of CO and CH₄ are as low as 90 and 400 °C, respectively. Furthermore, after 150 h of continuous testing at high concentrations of H₂O and CO₂, the nature of the catalyst is not irreversibly destroyed and can still return to its initial level of activity. This excellent catalytic activity is attributed to a portion of Rh being uniformly distributed on the CeO₂ nanorod surface in the form of nanoparticles, forming strong Rh–CeO₂ interfacial synergy. Another portion of Rh permeated into the CeO₂ lattice, which results in a significant increase in the number of oxygen vacancies in CeO₂, thus allowing more surface active oxygen to be adsorbed and converted from the gas phase. Moreover, the catalytic reaction can proceed even in an oxygen-free environment due to the excellent oxygen storage performance of the Rh/CeO₂ catalyst.     

Computational fluid dynamics and combustion modelling of HIsarna incinerator

HIsarna technology combines the cyclone converter furnace (CCF) technology owned by Tata Steel and the HIsmelt technology owned by RioTinto. The CCF is mainly a prereduction vessel that prereduces and melts the iron ore particles, while the final reduction to metallic iron takes place in the smelt reduction vessel. The off-gases from the smelt reduction vessel undergo post-combustion in the CCF. Depending on the operating conditions of the HIsarna process, the off-gases may still contain small amounts of unburnt carbon and hydrogen; hence, for process safety and environmental reasons, they are passed through an incinerator that needs to be operated within a temperature window that will guarantee full combustion of the off-gases. In one of the HIsarna campaigns, it was observed that the temperatures in the incinerator dropped significantly during short periods of production. In order to avoid this phenomenon, the HIsarna process can be adjusted to meet the design conditions in the incinerator, but this is not a preferred option. A computational fluid dynamics study of the incinerator was carried out with different compositions of off-gases from the CCF with varying degrees of post-combustion and flowrates, in order to improve its design and operation. The combustion model predicted complete burn out of CO, CH₄ and H₂ when sufficient air/O₂ was injected. The computational fluid dynamics study showed that in all the cases, the flow pattern of the gases remained asymmetric. The temperature in the incinerator was generally higher if natural gas was mixed with the cyclone off-gas and much higher if oxygen was also injected. Modifications to the incinerator layout were recommended. In subsequent HIsarna trials, the new design was successfully implemented.     

Redox Properties of Gas Phases

In the H₂–O₂–C system, in the general case, two reversible reactions of carbon gasification and the water gas reaction, the gas mixture H₂–H₂O–CO–CO₂ is formed at high temperatures. In this mixture, the very low content of oxygen formed by the dissociation of H₂O and CO₂ is represented by the oxygen potential log (\({p_{{O_2}}}\), atm). Thus, the redox properties may be assessed in terms of the oxygen potential. In any gas mixture containing H₂O and/or CO₂, it may be calculated from the equations

$${\log [{p_{{O_2}}},atm] = 2\log (\frac{{{x_{{H_2}O}}}}{{{x_{{H_2}}}}}) - \frac{{25708}}{T} + 5.563}$$

;

$$\log [{p_{{O_2}}},atm] = 2\log (\frac{{{x_{C{O_2}}}}}{{{x_{CO}}}}) - \frac{{29529}}{T} + 9.149$$

.In the present work, possible compositions of the H₂–O₂–C system at 700–1500 K and a total pressure of 1 atm are considered: H₂–H₂O, CO–CO₂, CO–CO₂–C, H₂O–CO₂–O2, H₂–CO–C, H₂–H₂O–CO–CO₂, and H₂–H₂O–CO–CO₂–C. Analysis yields two nomograms in the following coordinates: log(\({x_{{H_2}O}}\)/\({x_{{H_2}}}\))–log\({p_{{O_2}}}\)–T and log(\({x_{C{O_2}}}\)/xCO)–log\({p_{{O_2}}}\)–T. Using the nomograms and reference information regarding the dissociation pressure of metal oxides, the redox properties of the gas mixtures with respect to those oxides may be assessed. In CO–CO₂ systems without hydrogen that are obtained in the combustion of CO, carbon may be formed as soot. This explains the existence of a limited region of gas-phase compositions and log\({p_{{O_2}}}\) in the corresponding nomogram and hence the limited potential for the reduction of some metal oxides in CO–CO₂–C systems. The introduction of hydrogen permits the creation of gas mixtures with extremely low oxygen pressure and hence increases the thermodynamic probability of reduction for any metal oxide. Hydrogen may be introduced in the system by methods that differ in economic expediency: from the use of pure hydrogen to the production of gas mixtures as a result of the reaction between water vapor and carbon. In the first case, the reduction of the oxide by hydrogen in the MeO–C–H₂ system activates the gasification of carbon by water vapor, the water gas reaction, the reduction of carbon monoxide, and the gasification of carbon dioxide. In the second case, practically pure H₂–CO mixture may be obtained above 1300 K. The utility of representing the results on a three-dimensional diagram based on the H₂–O₂–C concentration triangle is analyzed. If methane formation is taken into account, the equilibrium parameters of gas mixtures are changed markedly only at temperatures below about 900 K.

     

Influence of Direct Reduction Condition of Hematite Pellets with H2/CO on the Oxidation Behaviour of DRI in Air

Direct reduced iron (DRI) is the product of some commercial direct reduction (DR) of iron ore on base of natural gas. DRI tends to oxidize in air generally above 300 °C and then follows spontaneous combustion. To control the oxidation mechanism, several investigators have used different iron samples and methods. This paper gives the results of experimental work carried out for determination of DRI oxidation. The behaviour of DRI oxidation in air after isothermal reduction of hematite pellets with different size, temperature and H₂ / CO mixture is investigated.     

Kinetics and Reaction Mechanisms of High-Temperature Flash Oxidation of Molybdenite

The kinetics and reaction mechanism of the flash oxidation of +35/–53 μm molybdenite particles in air, as well as in 25, 50, and 100 pct oxygen higher than 800 K, has been investigated using a stagnant gas reactor and a laminar flow reactor coupled to a fast-response, two-wavelength pyrometer. The changes in the morphology and in the chemical composition of partially reacted particles were also investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and electron microprobe. High-speed photography was also used to characterize the particle combustion phenomena. The effects of oxygen concentration and gas temperature on ignition and peak combustion temperatures were studied. The experimental results indicate that MoS₂ goes through a process of ignition/combustion with the formation of gaseous MoO₃ and SO₂ with no evidence of formation of a molten phase, although the reacting molybdenite particles reach temperatures much higher than their melting temperature. This effect may be a result of the combustion of gaseous sulfur from partial decomposition of molybdenite to Mo₂S₃ under a high gas temperature and 100 pct oxygen. In some cases, the partial fragmentation and distortion of particles also takes place. The transformation can be approximated to the unreacted core model with chemical control and with activation energy of 104.0 ± 4 kJ/mol at the actual temperature of the reacting particles. The reaction was found to be first order with respect to the oxygen concentration. The rate constant calculated at the actual temperatures of the reacting particles shows a good agreement with kinetic data obtained at lower temperatures. The ignition temperature of molybdenite shows an inverse relationship with the gas temperature and oxygen content, with the lowest ignition temperature of 1120 K for 100 pct oxygen. Increasing the oxygen content from 21 to 100 pct increases the particle combustion temperature from 1600 K to more than 2600 K. A high oxygen content also resulted in a change of the reaction mechanism from relatively constant combustion temperatures in air to much faster transient combustion pulses in pure oxygen.     

Thermodynamic studies of oxygen behavior in tantalum-based alloys

An experimental study of equilibrium thermodynamic properties of oxygen in pure tantalum and tantalum alloyed with V, Nb or Mo was made by EMF measurements on solid electrolytic cells over the temperature range of 873–1373 K (600–1100°C). The solubility of oxygen in pure tantalum in equilibrium with Ta₂O₅ was determined to be C_s = 12.4exp[−(16 kJ/mol)/(RT)] over the experimental temperature range. It was concluded that oxygen obeys Henry's law for concentrations up to the terminal solubility in tantalum for the temperature range 873–1373 K (600–1100°C). The oxygen activity coefficient increased with Mo content in TaMo alloys and decreased with V content in TaV alloys. The equilibrium results showed virtually no change in thermodynamic behavior of the oxygen solution by adding Nb to TaO alloys. The positive deviation of the oxygen activity coefficient in TaMo alloys is evidence of a repulsive interaction between molybdenum and oxygen atoms. The interaction energy was calculated assuming that the interaction extends to third nearest neighbors. It may, in fact, extend to fourth or higher neighboring shells of atoms.     

Aqueous Oxidation of Iron Monosulfide (FeS) by Molecular Oxygen

Aqueous oxidation of iron monosulfide (FeS) by oxygen at initial pH between 2.5 and 5 was investigated in a closed system at different temperatures (25, 35, and 40°C). It was found that the rate of aqueous oxidation of FeS increases when initial [H⁺] and temperature increase. The reaction order with respect to [H⁺] was 0.16 ± 0.02 at 25°C. The activation energy was found to be 23 ± 5 kJ mol^–1 at initial pH 2.5. This value suggests that aqueous oxidation of FeS by oxygen is controlled by a mixed regime of diffusion and surface reaction control. FTIR analysis of the initial and reacted FeS samples has shown that during the aqueous oxidation of FeS by O₂ a sulfur-rich layer is formed on the mineral surface. The experimental results indicate that the protons adsorb on the mineral surface and catalyze Fe²⁺ release into solution (by Couloumbic repulsion) and S(-II) oxidation to higher oxidation states.     

One-step solution combustion synthesis of micro-nano-scale porous Cu/CeO2 with enhanced photocatalytic properties

The Cu/CeO₂ nanoporous composite material was prepared via a one-step and energy-saving method of solution combustion synthesis(SCS).The phase composition,surface morphology and optical characteristics of Cu/CeO₂ were studied.The results show that the SCS products are composed of cubic fluorite CeO₂ and Cu.Due to the generation and escape of gas during the synthetic reaction,the SCS CeO₂ shows porous structure,in which the mesopores(diameter 10-17 nm) ...     

Preparation and characterization of Ce-Fe-Zr-O(x)/MgO complex oxides for selective oxidation of methane to synthesize gas

A series of Ce-Fe-Zr-O(x)/MgO (x denotes the mass fraction of Ce-Fe-Zr-O, x=10%, 15%, 20%, 25%, 30%) complex oxide oxygen carriers for selective oxidation of methane to synthesis gas were prepared by the co-precipitation method. The catalysts were characterized by means of X-ray diffraction (XRD) and H₂-TPR. The XRD measurements showed that MgFeO₄ particles were formed and Fe₂O₃ particles well dispersed on the oxygen carriers. The reactions between methane diluted by argon (10% CH₄) and oxygen carriers were investigated. Suitable content of CeO₂/Fe₂O₃/ZrO₂ mixed oxides could promote the reaction between methane and oxygen carriers. There are mainly two kinds of oxygen of carriers: surface lattice oxygen which had higher activity but lower selectivity, and bulk lattice oxygen which had lower activity but higher selectivity. Among all the catalysts, Ce-Fe-Zr-O(20%)/MgO exhibited the best catalytic performance. The conversion of the methane was above 56%, and the selectivity of the H₂ and CO were both above 93%, the ratio of H₂/CO was stable and approached to 2 for a long time.     

Core-shell MnCeOx catalysts for NO oxidation and mild temperature diesel soot combustion

Environmental contamination such as soot particles and NO_x has aroused extensive attraction recently.However,the main challenge lies in the oxidation of soot at mild temperature with the assistance of NO_x.Here,a series of core-shell MnCeO_x catalysts were successfully synthesized by hydrothermal method and employed for low-temperature catalytic oxidation of soot in the presence of NO_x.X-ray diffraction(XRD),inductively coupled plasma-optical emission sp...     

Abnormal Growth Transport in Oxide Scales on Fe-16Cr Steels in Water Vapor

The oxidation behavior of Fe-16Cr steels in N₂-12 vol pct H₂O was studied at 850 °C. The oxide scale was compact and had excellent adhesion to the substrate; moreover, there were three layers of different compositions existing in the scale. To gain an insight into the transport mechanism, two-stage oxidation was carried out in N₂-12 vol pct H₂¹⁶O and followed in N₂-12 vol pct H₂¹⁸O gas mixtures. The oxygen isotope profiles in oxide scales were determined by secondary ion mass spectrometry. The results showed that oxidation in water vapor proceeded by outward chromium transport, especially, the oxidation involved inward transport of water molecules.     

Thermodynamic fundamentals of ferrous cake sulfitization

The Pourbaix diagrams of the systems SO₄^2??SO₃^2??H₂O and iron hydroxide (oxide)–H₂O are refined. The E(pH) dependence of the sulfitization of iron(III) hydroxide is refined with allowance for the regions of predominant phase constituents of the systems. The potential E–pH electrochemical equilibrium diagrams of the systems Fe(OH)₃–H₂SO₄–SO₃^2?–H₂O, FeOOH–H₂SO₄–SO₃^2?–H₂O, and Fe₂O₃–H₂SO₄–SO₃^2?–H₂O are plotted. These diagrams can be considered as a thermodynamic basis for the sulfite conversion of the ferrous cake of copper–nickel production.     

Ce–Ti catalysts modified with copper and vanadium to effectively remove slip NH3 and NOx from coal-fired plants

In this work, V/Ce–Ti catalysts were modified with different kinds of transition metals (Cu, Fe, Co, Mn) by sol–gel and impregnation methods. The NH₃ oxidation performance of them was tested to select the most active catalyst in NH₃-selective catalytic oxidation (NH₃–SCO). The effect of NO, SO₂ and H₂O was also investigated. The experimental results indicate that 1% Cu–V/Ce–Ti catalyst exhibits the most significant ability to remove slip ammonia discharged from coal-fired plants and its NH₃ conversion efficiency reaches 90% at 300 °C. In addition, 97% NO_x can be removed when NO is introduced in the gas. Cu–V/Ce–Ti catalyst also obtains good resistance to H₂O and SO₂. Based on the characterization experiment, the introduced Cu and V are highly dispersed on Ce–Ti catalyst and they can increase the redox properties and the number of acidic sites. Besides, the redox cycles among Cu, V and Ce species on Cu–V/Ce–Ti catalyst surface are conducive to generating more active oxygen and promoting the oxidation capacity of the catalyst.     

Promotional effects of cerium doping and NOx on the catalytic soot combustion over MnMgAIO hydrotalcite-based mixed oxides

A series of MnMgA10 samples with different amounts of Ce doping were facilely prepared using coprecipitation method and their catalytic soot combustion activity was evaluated by temperature programmed oxidation reaction （TPO）. The methods of X-ray diffraction （XRD）, Brumauer-Emmett-Teller （BET）, H2-TPR, NO-TPO and in situ 1R were used to characterize the physio- chemical properties of these samples. Dopant Ce improved the soot combustion performance of MnMgA10 catalyst due to the en- hanced redox ability. Introduction of NOx led to the further increase of catalytic soot oxidation activity on these samples. Over Ce-containing samples, the catalytic activity was slightly decreased as the amount of dopant Ce increased in 02. Diftbrently, in NO＋O2, a certain amount of dopant Ce was much more favorable and excess amount of Ce resulted in a sharp drop of the catalytic soot combustion activity. Both NO： and nitrates were found to have great contributions to the effects of NOx on the soot combustion activity of Ce-doped catalysts. More NO2 was generated as dopant Ce increased. When appropriate amount of Ce was introduced, the as-formed NO2 was stored as bridging bidentate nitrate on Mn-Ce site, which was confirmed to have higher reactivity with soot than nitrite or monodentate nitrate on Mn and/or Ce sites. Overall, Mno.sMg2.sCeo.lAlo.90 was considered as the most potential catalyst for soot combustion.