Mahua Ghosh Chaudhuri

A novel method for synthesis of α-Si3N4 nanowires by sol–gel route

Abstract Silicon nitride (Si3 N4) nanowires have been prepared by carbothermal reduction followed by the nitridation (CTRN) of silica gel containing ultrafine excess carbon obtained by the decomposition of dextrose over the temperature range of 1200–1350 °C. This innovative process involves repeated evacuation followed by purging of nitrogen gas so that the interconnected nanopores of the gel are filled with nitrogen gas prior to heat treatment. During heat treatment at higher temperatures, the presence of nitrogen gas in the nanopores of the gel starts the CTRN reaction simultaneously throughout the bulk of the gel, leading to the formation of Si3 N4 nanowires. The in situ generated ultrafine carbon obtained by the decomposition of dextrose decreases the partial pressure of oxygen in the system to stabilize the nanowires. The nanowires synthesized by this process are of ∼500 nm diameter and ∼0.2 mm length. The product was characterized by scanning electron microscope (SEM), energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD) and infrared (IR) spectra.

Synthesis of pure nickel(III) oxide nanoparticles at room temperature for Cr(VI) ion removal

Ni2O3 nanoparticles of various sizes (∼25.8 to 49.7 nm), obtained by a facile oxidation process using Ni(NO3)2·6H2O, NaOH and sodium hypochlorite as precursor materials at various temperatures (0°, 25°, 50° and 70 °C), are found to remove toxic Cr(VI) from aqueous solution (20 g L−1). The structure, morphology, surface charge and chemical compositions of the synthesized samples were characterized by XRD, TEM, zeta potential and EDX respectively. Adsorption capacity is found to be strongly dependent on the size and surface heterogeneity of the synthesized particles and a plausible mechanism for such significant adsorption efficacy is attributed to the sorbate–sorbent electrostatic interaction and shielding of Cr(VI) ions. The adsorption mechanism fits with the Langmuir isotherm model with maximum 60% Cr(VI) removal capacity (20.768 mg g−1 (calculated) and 20.408 mg g−1 (predicted from isotherms)) corresponding to Ni2O3 nanoparticles, prepared at 70 °C in 3 hours at room temperature. Thermodynamic parameters, obtained from fitting, demonstrate that the adsorption process being endothermic in nature follows a pseudo-second-order kinetic model. The spontaneity of the adsorption process gets reduced with increasing particle size. pH of the solution is observed to have a remarkable effect on the adsorption, giving maximum adsorption at pH = 6.

Statistical optimisation parameter for lean grade self-reducing nuggets by surface response modelling to produce pig iron

Iron ore fines, lean grade coal and coke dust fines have always challenged the metallurgists to develop a suitable process for its optimum use. The aim of this study is to utilise the inferior quality of iron ore fines, coal and plant waste coke dust for reduction. At first mechanical properties of iron ore nuggets are assessed through shatter and abrasion test and subsequent to which cold bonded self-reducing nuggets are directly reduced in standard reducing furnace. The maximum extent of reduction achieved in the present study is 87.2%. The reduced specimens are further characterised using XRD, SEM, EDX and chemical analysis method. Finally, the statistical model of Box Behnken Design (BBD) method is successfully utilised to optimise the process parameter for reduction experiments. The optimised sample thus obtained is subjected to melting for laboratory scale pig iron production. Better slag metal separation is achieved when calcined lime is used as a flux. The microstructure of the metallic iron is studied and it shows ferrite phase with dispersed carbon.

Reduction behaviour of agglomerated iron ore nuggets by devolatisation of high-ash, high-volatile lignite coal for pig iron production

High-quality coking coals all over the world are gradually approaching extinction. These days, steel industries are trying to focus more on the utilisation of non-coking grades of coal. The present work involving high-ash, high-volatile lignite coal can be used indirectly in iron-making processes. Direct use is not possible due to low amount of carbon and high value of ash. High ash content leads to huge sulphur content, and this leads to high cost involvement in secondary processes. On the other hand, huge amount of iron ore fines are generated during mechanised mining, sizing, screening, transportation, beneficiation and sintering processes. Iron ore nuggets are formed from inferior quality iron ore fines using suitable binders with the applied pressure. Mechanical properties of iron ore nuggets are also assessed through shatter and abrasion test. A furnace was designed, to indirectly utilise high-ash, high-volatile lignite coal, for pre-reduction iron ore nuggets. Iron ore nuggets were partly reduced by CO, H2 and fine carbon produced from volatilisation of coal. Optimized pre-reduced nuggets, having high mechanical stability was directly charge in the raising hearth furnace for pig iron production.

Kinetic studies on the reduction of iron ore nuggets by devolatilization of lean-grade coal

An isothermal kinetic study of a novel technique for reducing agglomerated iron ore by volatiles released by pyrolysis of lean-grade non-coking coal was carried out at temperature from 1050 to 1200°C for 10–120 min. The reduced samples were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and chemical analysis. A good degree of metallization and reduction was achieved. Gas diffusion through the solid was identified as the reaction-rate-controlling resistance; however, during the initial period, particularly at lower temperatures, resistance to interfacial chemical reaction was also significant, though not dominant. The apparent rate constant was observed to increase marginally with decreasing size of the particles constituting the nuggets. The apparent activation energy of reduction was estimated to be in the range from 49.640 to 51.220 kJ/mol and was not observed to be affected by the particle size. The sulfur and carbon contents in the reduced samples were also determined.

Phase Evaluation of Pure Nanocrystalline Barium Stannate by Two Different Milling Activations

Pure nanocrystalline BaSnO3 is prepared by two methods: mechanical activation (planetary ball milling) and mechanical hand mixing in agate mortar followed by sintering for both at 1350°C/ 2 h. The phase formations during synthesis are analyzed by x-ray diffraction (XRD) studies and the crystallite size is measured by Scherrer’s formula from the major peaks of the diffractogram. The nanocrystalline barium stannate having single phase simple cubic perovskite structure is synthesized with a crystallite size about 50 nm. Fourier transform infrared spectroscopy (FTIR) is done to determine symmetric and asymmetric stretching of the bonds formed and co-ordination of the ions within crystal structure. FTIR studies justify the phases developed by XRD since the molecular signature and co-ordination of cations are verified. Microstructure and morphology are observed by scanning electron microscopy (SEM), while energy dispersive x-ray analysis (EDX) is done to determine the presence of the required element of composition formation. Ultra violet-visible spectroscopy (UV-VIS) shows absorption spectra of the sample within the UV region, while the band gap is calculated using the Tauc relation. The band gap evaluated for nanocrystalline barium stannate is about 2–2.78 eV for indirect transition, while for direct transition it is about 2.78–3.14 eV. The value is observed to be close to that of semiconductor-based materials.