Computational fluid dynamic modelling of an electric smelting furnace in the platinum recovery process

[摘要] The electric smelting furnace is found at the heart of the platinum recovery process where the powerinput from the electrodes produces a complex interplay between heat transfer and fluid flow. Afundamental knowledge of the dynamic system hosted by the electric furnace is valuable formaintaining stable and optimum operation. However, describing the character of the system hostedby the electric furnace poses great difficulty due to its aggressive environment. A full-scale threedimensionalComputational Fluid Dynamics (CFD) model was therefore developed for the circular,three-electrode Lonmin smelting furnace.The model was solved as time dependent to incorporate the effect of the three-phase AC current,which was supplied by means of volume sources representing the electrodes. The slag and mattelayers were both modelled as fluid continuums in contact with each other through a dynamic interfacemade possible by the Volume of Fluid (VOF) multi-phase model. CO-gas bubbles forming at electrodesurfaces and interacting with the surrounding fluid slag were modelled through the Discrete PhaseModel (DPM).To account for the effect of concentrate melting, distinctive smelting zones were identified within theconcentrate as assigned a portion of the melting heat based on the assumption of a radiallydecreasing smelting rate from the centre of the furnace. The tapping of slag and matte was neglectedin the current modelling approach but compensation was made for the heating-up of descendingmaterial by means of an energy sink based on enthalpy differences.Model cases with and without CO-gas bubbles were investigated as well as the incorporation of a thirdphase between the slag and matte for representing the 'mushy' chromite/highly viscous slagcommonly found in this region. These models were allowed to iterate until steady state conditions hasbeen achieved, which for most of the cases involved several weeks of simulation time.The results that were obtained provided good insight into the electrical, heat and flow behaviourpresent within the molten bath. The current density profiles showed a large portion of the current toflow via the matte layer between the electrodes. Distributions for the electric potential and Joule heatwithin the melt was also developed and showed the highest power to be generated within theimmediate vicinity of the electrodes and 98% of the resistive heat to be generated within the slag.Heat was found to be uniformly distributed due the slag layer being well mixed. The CO-gas bubbleswas shown to be an important contributor to flow within the slag, resulting in a order of magnitudedifference in average flow magnitude compared to the case where only natural buoyancy is at play.The highest flow activity was observed halfway between electrodes where the flow streams from theelectrodes meet. Consequently, the highest temperatures are also observed in these regions. Thetemperature distribution within the matte and concentrate layers can be characterized as stratified.Low flow regions were identified within the matte and bottom slag layer which is where chromite andmagnitite deposits are prone to accumulate.The model results were partially validated through good agreement to published results where actualmeasurements were done while also falling within the typical operating range for the actual furnace.The modelling of the electric furnace has been valuably furthered, however for complete confidence inthe model results, further validation is strongly recommended.

[发布机构] Stellenbosch University