A study of slag freezing in metallurgical furnaces

Many smelting and slag-cleaning furnaces operate with cooling systems designed to freeze a slag layer over the refractory to protect it. The fluid flow and heat transfer conditions associated with the freeze layer and mushy zones are poorly understood. This study was conducted to understand the chill layer formation and heat transfer that is required to design cooling systems in pyrometallurgical operations where a slag layer is required to protect the furnace wall.; The freeze layer formation and heat transfer in mushy zones were experimentally study at room temperature in a 2-dimensional square cavity differentially heated, using an aqueous solution of calcium chloride to simulate the slag. Reasonable similarity with conditions encountered with copper and nickel smelting systems was achieved (Pr ≈ 50 and Ra ≈ 108, in the laminar-turbulent transition). Measurements of velocities were made with the Particle Image Velocimetry (PIV) technique. The freeze layer development was tracked using a digital camera.; Direct Numerical Simulations (DNS) of the mean flow using a finite control volume technique with a fixed domain method were also made of the unsteady fluid flow and heat transfer problem. It was found that the macro solidification process is well described using an improved model for high molecular viscosity in the mushy zone. Solid front growth, isothermal profiles, velocity profiles and heat transfer through the walls showed good agreement between the PIV and DNS results. Experimental and numerical velocity profiles close to the freeze layer show a parabolic behaviour in the vertical velocity profile which is completely different from the calculation of heat transfer using a sharp interface model. The reason for this is attributed to the effects of the mushy zone with a high viscosity and high shear stresses acting on that area.; In Part III of this Thesis, effects of slag viscosity temperature relationship were analysed with a two-dimensional mathematical model of an electric smelting furnace. The model was focused on the fluid dynamics of the molten slag and the effects over the formation of magnetite-rich slag layer over the walls. The results of the previous experimental and mathematical work, Part I and II, were used to describe mathematically the freeze layer formation on the furnace walls using a fixed-grid model from a highly viscous liquid. Chemical composition of the slag was taken into account through the effect of the viscous activation energy as well the solidus and liquidus temperatures. The results show that the flow pattern is strongly affected in the areas of high viscosity. The results are discussed in terms of heat flux over the refractories and their effects on cooling system design.