Dinabandhu Ghosh

Behavior of fluxed lime iron oxide pellets in hot metal bath during melting and refining

Lump lime as a flux material in a basic oxygen furnace (BOF) often creates problems in operation due to its high melting point, poor dissolution property, hygroscopic nature, and fines generation tendency. To alleviate these problems, fluxed lime iron oxide pellets (FLIP) containing 30% CaO were developed in this study using waste iron oxide fines and lime. The suitable handling strengths of the pellet (crushing strength: 300 N; drop strength: 130 times) of FLIP were developed by treating with CO2 or industrial waste gas at room temperature, while no separate binders were used. When the pellet was added into hot metal bath (carbon-containing molten iron), it was decomposed, melted, and transformed to produce low melting oxidizing slag, because it is a combination of main CaO and Fe2O3. This slag is suitable for facilitating P and C removal in refining. Furthermore, the pellet enhances waste utilization and use of CO2 in waste gas. In this article, emphasis is given on studying the behavior of these pellets in hot metal bath during melting and refining along with thermodynamics and kinetics analysis. The observed behaviors of the pellet in hot metal bath confirm that it is suitable and beneficial for use in BOF and replaces lump lime.

Smelting characteristics of fluxed chromite sinter and its performance assessment in electric arc furnace to produce high carbon ferrochrome

More than 80% of the high grade chromite ores are fragile and tend to form fines during their handling. In order to utilise these chromite ore fine, in ferrochrome production, agglomeration is necessary. In the present study, the direct sintering of chromite ore fines in the presence of coke breeze has been carried out, which does neither require further grinding of ore fines ( − 10 mm) nor binder. It uses suitable fluxes and coke breeze as heat source to raise temperature up to 1600°C and produces a semifused mass (20% molten phases) with good strength. The developed sinter showed very good strength properties suitable for cold handling. Since it contains higher amount of fluxes than conventional pellet, the study on its smelting characteristics is necessary to assess its suitability in Fe–Cr production. In the present paper, smelting reduction characteristics and assessment of its performance with respect to the lump ore in a 50 kVA electric arc furnace have been studied in 10 kg scale. Different smelting parameters such as coke and flux requirement, energy consumption, etc. has been optimised through both thermodynamically and experimentally to get maximum extent of reduction, metallic yield and chromium recovery. Coke (21%), quartzite (7.52%) and bauxite (10%) addition with 45 kWh of heat input was found to be optimum to achieve 76% metallic yield and 91%, chromium recovery. In the comparative study in identical condition, the chromite sinter showed much better metallic yield (76%) and higher chromium content (54.6%) in the produced ferrochrome than the lump ore (70 and 51. 9% respectively) of the same grade.

Dissolution Characteristics of CO2-Treated Fluxed Pellets in Hot Metal Bath

Lump lime and iron ore are generally used in the basic oxygen furnace as flux and cooling material, respectively. Owing to high melting point, poor dissolution property, fines generation tendency, and hygroscopic nature of lump lime, delay in process and operational complexities are generally encountered. On the other hand, iron ore charging creates slag foaming. In order to alleviate the above problems and to utilize waste materials, fluxed lime–iron oxide pellets (FLIP) containing waste iron oxides and lime fines (10%–40%) were prepared and subsequently strengthened with CO2 gas treatment. FLIP may have the potential to partially replace scrap and lump lime in the conventional basic oxygen furnace charge. In order to assess the applicability of FLIP in steelmaking, the dissolution characteristics of these pellets were studied in a high-temperature pot furnace equipped with a charge-coupled device (CCD) camera under varying experimental conditions. It was found that the dissolution time decreased with increasing hot metal temperature, increasing specific surface area of the pellet, and decreasing lime content of the pellet. The melting of the pellet in the absence of hot metal took much higher time than its presence.

A Model for Dissolution of Lime in Steelmaking Slags

In a previous study by Sarkar et al. (Metall. Mater. Trans. B 46B:961 2015), a dynamic model of the LD steelmaking was developed. The prediction of the previous model (Sarkar et al. in Metall. Mater. Trans. B 46B:961 2015) for the bath (metal) composition matched well with the plant data (Cicutti et al. in Proceedings of 6th International Conference on Molten Slags, Fluxes and Salts, Stockholm City, 2000). However, with respect to the slag composition, the prediction was not satisfactory. The current study aims to improve upon the previous model Sarkar et al. (Metall. Mater. Trans. B 46B:961 2015) by incorporating a lime dissolution submodel into the earlier one. From the industrial point of view, the understanding of the lime dissolution kinetics is important to meet the ever-increasing demand of producing low-P steel at a low basicity. In the current study, three-step kinetics for the lime dissolution is hypothesized on the assumption that a solid layer of 2CaO·SiO2 should form around the unreacted core of the lime. From the available experimental data, it seems improbable that the observed kinetics should be controlled singly by any one kinetic step. Accordingly, a general, mixed control model has been proposed to calculate the dissolution rate of the lime under varying slag compositions and temperatures. First, the rate equation for each of the three rate-controlling steps has been derived, for three different lime geometries. Next, the rate equation for the mixed control kinetics has been derived and solved to find the dissolution rate. The model predictions have been validated by means of the experimental data available in the literature. In addition, the effects of the process conditions on the dissolution rate have been studied, and compared with the experimental results wherever possible. Incorporation of this submodel into the earlier global model (Sarkar et al. in Metall. Mater. Trans. B 46B:961 2015) enables the prediction of the lime dissolution rate in the dynamic system of LD steelmaking. In addition, with the inclusion of this submodel, significant improvement in the prediction of the slag composition during the main blow period has been observed.

Structural, microstructural, and thermal characterizations of a chalcopyrite concentrate from the Singhbhum shear zone, India

The structural and morphological characterizations of a chalcopyrite concentrate, collected from the Indian Copper Complex, Ghatshila, India, were carried out by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The concentrate powder was composed mainly of free chalcopyrite and low quartz in about 3:1 weight ratio. The particle size was about 100 μm. Spectroscopic studies (FTIR, Raman, UV-visible) of the concentrate supported the XRD findings, and also revealed a marginal oxidation of the sulfide phase. The energy band gap of the sulfide was found to be 3.4 eV. Differential thermal analysis and thermogravimetry of the concentrate showed a decomposition of chalcopyrite at 658 K with an activation energy of 208 kJ·mol−1, and two successive structural changes of silica at 848 K and 1145 K.

Structural, Spectroscopic, Magnetic, and Thermal Characterizations of a Magnetite Ore from the Nagaland Region, India

An Indian magnetite ore from the Nagaland region was characterized by powder X-ray diffraction, spectroscopies (ultraviolet-visible and Raman), high temperature magnetization and thermal analysis. A Rietveld refinement of the diffraction data revealed that the principal phase present in the sample is magnetite with partial substitution of Fe with Mg (mainly) and Cr, leading to the chemical formula of MgCr0.2Fe1.8O4. The UV-visible spectra of the sample in MeCN solution showed three distinct peaks. The lower region bands are due to metal-oxygen charge transitions and the other band is for d-d transition of iron. Raman spectral study at room temperature gave four characteristics bands of magnetite. The sample underwent a magnetic transition at high temperature and the thermomagnetization curve shows hysteresis during cooling cycle which indicates the possibility of first order structural transition at high temperature. The thermal property of the concentrate was studied by differential scanning calorimetric technique.

A new way of constructing the stability diagram of the system Fe–C–O with the help of a C/Fe2O3 vs. T plot and the related issue of direct and indirect reduction

A new type of Fe–C–O stability diagram, which is a plot of the molar ratio vs. temperature (T), is drawn for 1 atm total pressure in the temperature range 300–1300 K. Apart from delineating the stability fields of Fe and its oxides, this diagram directly furnishes the minimum number of moles of carbon required to reduce 1 mol of the starting Fe2O3 (pure) to Fe, FeO or Fe3O4, or their mixtures, at a given temperature and 1 atm total pressure. Besides, the confusing issue of the direct reduction (DR) vs. indirect reduction (IR) of iron oxide by carbon is reviewed and clarified, and used in the construction of the diagram. The general representations of 100 pct DR and 100 pct IR of Fe2O3, which will be applicable at any temperature and pressure within the stability domain of Fe (solid), are proposed as follows: The combined DR and IR has the same representation as the 100 pct DR. However, in the combined case, the molar ratio CO/CO2 in the product gas is dictated by the equilibrium with Fe–FeO and in the case of 100 pct DR by the carbon saturation. For the 100 pct IR, the ratio is, again, dictated by the Fe–FeO equilibrium; in addition, the ratio C/O2 on the reactant side is governed by the carbon-saturated CO–CO2 mixture obtained from the burning of C in O2.

Reduction Kinetics and Characterization Study of Synthetic Magnetite Micro Fines

The present work deals with the characterization of pure magnetite microfines (<5 μm) and its hydrogen reduction. The structural and morphological properties of magnetite powder were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption analysis by five points isotherm BET method, and scanning electron microscopy along with energy dispersive X-ray spectroscopy (SEM-EDS). The hydrogen reduction of the magnetite powder was carried out in a thermogravimetric analyzer (TGA) under a steady flow of hydrogen or hydrogen-argon mixture (to produce different partial pressures of hydrogen). The variables studied were reduction temperature (973–1273 K), hydrogen partial pressure (0.25–1 atm) and sample bed height (0.184–0.68 cm). The apparent activation energy was obtained as 22 kJ mol−1. The rate equations developed for the reaction system under study were applied to determine the rate controlling step. The reduction was found to be rate controlled by diffusion through the stagnant gas film enclosed above the sample inside the crucible. The true activation energy was calculated to be 9 kJ mol−1.