Sukomal Ghosh

Development of fluxed micropellets for sintering utilising iron oxide waste fines

Abstract Ultrafine iron oxide wastes such as slime, blue dust and Linz–Donawitz (LD) converter sludge have very limited use in sintering of iron ore due to their excessive fineness (−50 μm). Pelletisation of these ultrafine materials for use in blast furnace involves high temperature curing, which is a highly energy intensive process. Briquetting of LD sludge requires costly binders and contains high moisture, which creates problem at high temperature of the downstream process. In order to alleviate these problems, the current study has developed a process for preparing micropellets of waste iron oxide fines (2–6 mm size) without using any binder. The strength of the micropellet has been increased by a novel CO2 treatment process at room temperature. Developed micropellets exhibit very suitable drop strength (125 Nos), tumbler properties and cold compressive strength (∼9 kg/pellet) to withstand cold handling. Low lime containing micropellets have the possibility of being used as a mixed material in usual sinter making, and high lime containing micropellets may be exploited for making super fluxed sinter that can be used as synthetic flux in the basic oxygen furnace process towards the formation of low melting oxidising slag at the early stage of blow.

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.

Investigation of sub-surface cracks in continuous cast billets

Sub-surface cracks in the continuous cast product of steel were studied to understand the causes of such defects and to suggest remedial measures. Even after hot rolling the cracks were not welded. The diameter of the as cast billets were 225 mm and most of the cracks were located around l cm away from the edges. The cracks were visible with the naked eye at transverse sections. The cracks were not continuous but scattered. The objective of this work was to identify the root cause of the formation of the sub-surface defects on the concast product and to ascertain whether it was due to any material related problem or due to any other cause such as processing. The existing cracks were opened through mechanical testing. The fracture surfaces were observed by SEM. Metallography and SEM observations revealed that the cracks were generated due to preferential segregation of sulphur, Cu and subsequent formation of various types of sulphide inclusions. The presence of a higher amount of Cu, particularly in the cracked region is of great concern. Cu is very harmful in steel if it exceeds the tolerable limit, particularly with the presence of high amount of S. The sulphide inclusions are found at the existing cracked regions. Inclusions at the crack is indicative of a correlation between inclusions and cracks. The inclusions act as a second phase and may influence the solidification of the matrix by restriction in directional solidification, differential contraction and even acting as a source of stress concentration—hence the appearance of cracks. The location-specific segregation of the inclusions is however surprising. Had there been dispersion of inclusions, probably the cracks would not have resulted.

Performance Assessment of Partially Pre-fused Synthetic Flux in Basic Oxygen Steel Making

Lump lime as the most common flux and iron ore as a coolant are used in basic oxygen steel making. However, high melting point, poor dissolution property, fines generation tendency and hygroscopic nature of lump lime often create problems in operation. As the combination of both iron oxide (Fe2O3) and CaO shows eutectic at 1230 °C, a combined mass of iron oxide and lime melts at lower temperature and dissolves faster in a molten bath. A partially pre-fused synthetic flux (PSF) was prepared through an innovative way in combination of iron oxide fines viz. Linz Donawitz converter sludge and blast furnace flue dust and lime fines by micro-pelletization of the mix followed by coke breeze free sintering. The developed PSF shows good cold handling strength, low melting point (1 180 ºC), good thermal shock resistance, etc. As a low melting synthetic flux, its performance was assessed through dissolution/melting study in hot metal bath and refining of hot metal in a simulated bottom blown converter using (i) PSF, (ii) only lump lime and (iii) lump lime with iron ore when keeping other conditions identical. Very fast dissolution (27–80 s for 1–3 g lumps), enhanced removal of C and P (11–12 min), controlled slag foaming, and reduced oxygen consumption was obtained for using PSF.

Development of Activated Alumina and its Effect on the Properties of Cement Free Castables Refractories

Activated alumina is prepared by thermal dehydration (300°-600°C) of chemically pure aluminium hydroxide. The prepared materials have been characterized by residual water content, BET surface area and hydraulic activity. These aluminas are incorporated in cement free castable batch. Important properties like bulk density, apparent porosity, linear change, cold compressive strength, strength retainment after thermal shock have been studied. X-ray diffraction analysis has been performed to identify the phases formed while SEM studies have been used to get an idea about the microstructure of the castable matrix.

Effect of Titania on the Stability of Monocalcium Aluminate Hydrate

Monocalcium aluminate, a major hydraulic phase of high-alumina cements, hydrates to form metastable CAH10 which finally converts to stable C3AH6 and AH3. The thermal stability of monocalcium aluminate hydrate in relation to titania was studied by thermogravi- metry. The dehydration rate curves obtained at different temperatures in the range 100°–400°C followed formal first order kinetics. The activation energy values of the dehydration reactions were determined and found to be dependent on the titania content. Six per cent titania has beneficial role on the thermal stability of the hydrated monocalcium aluminate.