Nurni Viswanathan

Studies of dynamic mass transfer at the slag-metal interface - Interfacial velocity measurements

Abstract The dynamics of oxygen transport along the slag–metal interface of pure iron and alumina-saturated CaO–Al2O3–SiO2 slag was studied using high-temperature X-ray image analysis. The oscillations of the metal drop occurring due to the interfacial movement of oxygen atoms driven by Marangoni forces were studied in detail. The change in interfacial area during the oscillations was measured using a digitizing software and MATLAB. It was observed that the interfacial velocity as a function of oxygen exhibits insignificant variation with temperature. Further, the values obtained for the interfacial velocity using oxygen concentration difference at the interface were slightly lower in comparison to those using sulfur. The possible reason for this lower velocity could be that, although oxygen is a smaller atom compared to that of sulfur, the energy barrier at the free iron surface is higher for oxygen, thus hindering its motion along the interface.

Isothermal reduction kinetics of self-reducing mixtures

Isothermal reduction of haematite carbon mixtures was investigated at temperatures 750–1100°C under inert atmosphere. Mass loss curves proved the stepwise reduction of haematite to metallic iron. The non-linear feature of haematite to magnetite reduction kinetics was observed and an activation energy of 209 kJ mol−1 was calculated. Irrespective of carbon-bearing material type, reduction rate of magnetite was linear. Activation energy values were calculated to be 293–418 kJ mol−1. Significant increase in the reduction kinetics in the last step (Wustite reduction) was observed and explained by the catalytic effect of freshly formed metallic iron. During the initial stages of wustite reduction, the activation energy values were calculated to be in the range of 251–335 kJ mol−1 for all carbon-bearing materials.

Recent Trends in Ironmaking Blast Furnace Technology to Mitigate CO2 Emissions : Top Charging Materials

The iron- and steelmaking is the largest energy consuming in the industrial sectors. The high energy consumption is associated with emission of CO2 and other pollutants. The most common ironmaking process used in the world is the blast furnace which contributes around 70 % of the world’s steel production. Recently, blast furnace has undergone tremendous modifications and improvements to reduce the energy consumption and CO2 emissions. The modifications are being focused on two main approaches: (1) development of top charging materials and (2) injections of auxiliary fuels through blast furnace tuyeres. The present chapter will discuss the recent modifications and development in the top charging burden and how it could participate in minimizing the energy consumption and CO2 emissions for more efficient and sustainable iron and steel industry. The injection of auxiliary fuels will be discussed in details in another chapter. The enhancement of burden material quality and its charging mode into the blast furnace has resulted in a smooth and efficient operation. Recently, the usage of nut coke in the modern blast furnace is accompanied by higher production and lower reducing agent rates. An efficient recycling of in-plant fines by its conversion into briquettes with proper mechanical strength is applied in some blast furnaces to exploit the iron- and carbon-rich residues. Nowadays, novel composite agglomerates consist of iron ores and alternative carbonaceous materials represent a new trend for low-carbon blast furnace with lower dependence on the conventional burden materials. The recent investigations demonstrated that the novel composites are able to reduce the thermal reserve zone temperature in the blast furnace and consequently enhance the carbon utilization through its higher reactivity compared to fossil fuels. The top charging of bio-reducers and hydrogen-rich materials into the blast furnace is one of interesting innovations to mitigate the CO2 emissions. Although some of previous approaches are recently applied in the modern blast furnace, others are still under intensive discussions to enhance its implementations.

Recent Trends in Ironmaking Blast Furnace Technology to Mitigate CO2 Emissions: Tuyeres Injection

Minimizing the coke consumption in the blast furnace is the key to achieve both ecological and economic aspects by reducing the CO2 emissions and the overall hot metal production cost. Complementary injection of cheaper auxiliary fuels and waste materials into the blast furnace via tuyeres has been greatly modified in the recent years to reduce the expensive coke consumption. Nowadays, most of the blast furnaces all over the world use pulverized coal at different injection rates. The greatest influence of coal injection on lowering the production cost and enhancement of hot metal production rate has led to further investigations on the injection of various other auxiliary materials including coke oven gas, converter gas, blast furnace dust, waste plastics, charcoal and torrefied biomass. In addition, trials on the injection of iron ore fines, low reduced iron and BOF slag have been recently studied. The injection rate of auxiliary materials into the blast furnace should be optimized to attain the minimum coke consumption and stable operation. The present chapter will discuss the influence of various materials injection on the blast furnace operation. The injection limit and changing of the blast furnace operating conditions, hot metal quality and coke consumption will be explained based on the experimental trials and mathematical modelling.

Some Aspects of Interfacial Phenomena in Steelmaking and Refining

Unique experiments were designed to study the surface phenomena in steelmaking reactions. The concept of surface sulfide capacities and an understanding of the surface accumulation of surface-active species, based on experimental results, are presented. In order to understand the flow phenomenon at slag/metal interface, experiments were designed to measure the interfacial velocity of S on the surface of an iron drop immersed in an aluminosilicate slag using the X-ray sessile drop method. The oscillation of the iron drop in the slag due to the change in the surface concentration of sulfur at the slag–metal interface was monitored by X-ray imaging. From the observations, the interfacial velocity of sulfur was evaluated. Similar experiments were performed to measure the interfacial velocity of oxygen at the interface as well as the impact of oxygen potential on the interfacial velocity of sulfur. The interfacial shear viscosity and the dilatational modulus were also evaluated. In a study of the wetting of alumina base by iron drop at constant oxygen pressure under isothermal condition, the contact angle was found to be decreased with the progress of the reaction leading to the formation of hercynite as an intermediate layer creating non-wetting conditions. In the case of silica substrate, an intermediate liquid fayalite layer was formed.

Modelling and Experimental Studies of Diffusivity of Sulfur and Its Relevance in Observing Surface Oscillations at the Slag Metal Interface Through X‐Ray Imaging

A generic model was conceived for predicting the diffusion coefficient of species in slag. The diffusion coefficient of sulfur in CaO-Al2O3-SiO2 slag with low silica was measured using a combinational technique of the generic model and experiments. The uniqueness of the experiments was in the method of collecting samples. Another milestone was that the diffusion coefficient of sulfur in the slag was obtained through the sulfur levels in the metal (silver). Later the order of magnitude of the diffusion coefficient of sulfur in slag was used to estimate the time required for sulfur to reach the slag- metal interface of an iron drop immersed in CaO-Al2O3-FeO-SiO2 slag. This estimated time for arrival of sulfur at the interface was comparable to the actual observation. The current paper describes the challenges in measuring the diffusion coefficient of sulfur. It also describes the time estimates calculated based on the X-ray image for sulfur to reach the slag-metal interface.

Modelling of Physico-Chemical Phenomena between Gas inside a Bubble and Liquid Metal during Injection of Oxidant Gas

Gas liquid reactors are extensively used in many metallurgical processes involving the refining of liquid metals. In these processes, reactions leading to the oxidation of various solutes in liquid metal often compete with each other, which ultimately determine the liquid metal composition. In the present paper, a model has been proposed to simulate the evolution of solute contents in a metallic melt considering mass transfer of solutes in the melt in the vicinity of the bubble, equilibrium at the gas-metal interface and gas composition evolution in the bubble during its ascent through the melt. The composition of solutes at the metal-gas interface in principle can be altered by changing the injected gas composition.The model was applied to the case of oxygen injection through a lance into liquid steel-containing C and Cr, aiming sufficient decarburization without much oxidation of Cr to the slag. The Cr loss to the slag by oxidation is generally much more than that expected based on equilibrium thermodynamics applied to the bulk metal and gas. The actual Cr loss, as shown by the present model, is determined by the composition of solutes at the metal-gas interface rather than in the bulk. The effect of change of the partial pressure of oxygen in the bubble by replacing oxygen by carbon dioxide in the injected gas and the corresponding evolution of C and Cr contents in the melt was simulated. Some preliminary experiments were conducted to validate the model predictions. The frame work of the model is generic and can be extended to many gas-liquid metal reactors in liquid metal processing.