A Study of the Behavior of Foaming Slag in Steelmaking

The thesis presents a fundamental study on some aspects regarding foaming slag. In today’s society, the requirements on environmental impact and energy consumption are increasing. It is essential for the steel industry to better understand the fundamentals of their processes to be able to develop both the process but also process control to reach the demands. In this work, the basic BOF furnace is the main focus, even though foaming slag may also occur in some electric arc furnaces. The BOF is a chaotic process with the purpose to decarburize and dephosphorize hot metal from the blast furnace. The foaming slag plays a profound role in the kinetics of the reactions, maintains the temperature in the furnace and protects the lining material of the furnace.The first part of the thesis includes a study of the structure of different foams and some viscosity measurements. Both room temperature and high temperature experiments are conducted. It is shown that the apparent viscosity of a foam is not only depending on the dynamic viscosity of the liquid phase and the gas fraction of the foam, but also the surface tension of the liquid and the bubbles size of the foam. The foam appears to show a shear thinning non-Newtonian behavior with apparent viscosities up to 5 times as high as for the pure liquid. The reason is found in the structure of the foams.The behavior of particles falling through foams generated from different sugar solutions is studied in the second part of the thesis. The particle behavior differs a lot depending on the size and density of the particle, and the viscosity and surface tension of the liquid phase. Some particles get stuck in the foam, some heavy particles fall through the foam crushing the bubbles in its path, and some particles fall slowly in a zick-zack pattern towards the bottom. The residence time is also measured, and a semi-empirical model is developed to describe the apparent velocity of the particles falling in a zick-zack pattern.The decarburization is the source of the foam in the furnace, and is studied in the third part of the thesis. Pig iron is dropped into liquid slag and the reaction rate is studied by quenching of samples at different times. Small droplets are decarburized faster than big droplets, and the incubation time of the reaction is found to play a big role in the decarburization process for different slag compositions.ivThe last part of the thesis is a comparison of lime dissolution in liquid versus foaming BOF slags. The controlling dissolution mechanism in liquid slag is the removal of interfacial layers of calcium silicates. In the foam on the other hand, the contact area between liquid slag and lime seem to be the controlling mechanism. In total, the rate seem to be similar in some liquid and foaming slag compositions while in others, the lime is dissociated into small pieces in the foaming slag due to the chaotic environment in the experiment.QC 20180522