Computational fluid dynamic modelling of zinc slag fuming process

Slag fuming is a reductive treatment process for molten zinciferous slags for extracting zinc in the form of metal vapour by injecting or adding a reductant source such as pulverized coal or lump coal and natural gas. The process has been operative since 1930’s for recovering zinc from lead blast furnace slag. Though slag fuming is a well-established process and has been industrially operative for over eighty years, there is only limited understanding of the process kinetics and fluid flow behaviour inside the slag fuming furnace. The purpose of this study is detailed fluid dynamic analysis including combustion behaviour, gas-liquid momentum interaction, generation of splashing due to gas injection process in slag fuming furnace, analysis of fuming behaviour at different locations of the furnace by using computational fluid dynamic (CFD) modelling technique. This PhD thesis focuses on the Computational Fluid Dynamic (CFD) modelling study of the zinc slag fuming process. In the first stage of the present research, a Computational Fluid Dynamic (CFD) modelling study of the Top Submerged Lance (TSL) gas injection process was carried out in a laboratory scale isothermal air-water model. The multiphase flow simulation, based on Euler-Euler approach, elucidated the effect of swirl and non-swirl flow inside the bath. The effects of lance submergence level and air flow rate were also investigated in that phase. The simulation results for velocity fields and generation of turbulence in the bath were validated against existing experimental data from the previous water model experimental study of Morsi et al. [1]. In the next stage of the research, a Computational Fluid Dynamic (CFD) model of the pilot plant scale top submerged lance slag fuming furnace was developed to investigate details of fluid flow, combustion behaviour, reaction kinetics and heat transfer in the furnace. The model integrates submerged CH4 combustion at the lance tip and chemical reactions with the heat, mass and momentum interfacial interaction between the phases present in the system. Commercial CFD package AVL Fire 2009.2 (AVL, Graz, Austria) coupled with a number of user defined subroutines in FORTRAN programming language were used to develop the model. The model is based on 3-D Eulerian multiphase flow approach and it predicted the velocity and temperature field of the molten slag bath, generated turbulence, vortex and plume shape at the lance tip. The model also predicted the mass fractions of slag and gaseous components inside the furnace. The model was validated by adopting the macro – step validation approach by using the zinc fuming rate against the pilot plant scale experimental study on top submerged lance zinc fuming process carried out by Waladan et al. [2]. Finally, the developed CFD model for TSL furnace was extended for submerged coal combustion instead of CH4 combustion and applied to a conventional tuyere blown slag fuming furnace. The model considered a thin slice of a conventional tuyere blown slag fuming furnace consisting two opposing set of tuyere. The model was developed in Eulerian multiphase flow approach by employing 3D hybrid unstructured orthographic grid system. The aim was to investigate details of fluid flow, submerged coal combustion dynamics, coal utilization behaviour, jet penetration behaviour, bath interaction conditions and generation of turbulence in the bath. The model was developed by coupling the CFD with the kinetics equations developed by Richards et al. for a zinc slag fuming furnace. The model predicted the velocity, temperature field of the molten slag bath, generated turbulence and vortex, coal utilization behaviour from the slag bath. The jet penetration depth at the tuyere tip was validated against the experimental study carried out by Hoefele and Brimacombe [3].