Modelling the thermal, electrical and flow profiles in a 6-in-line matte melting furnace

[摘要] The furnace at Polokwane is designed to treat high chromium containing concentrateswhich requires higher smelting temperatures to prevent or limit the undesirableprecipitation of chromium spinels. The furnace has therefore been designed to allow fordeep electrode immersion with copper coolers around the furnace to permit theoperation with the resulting higher heat fluxes.Deep electrode immersion has been noted to result in dangerously high mattetemperatures. Matte temperatures however can be influenced by a number of furnacefactors which emphasize the need to understand the energy distribution inside thefurnace. Computational fluid dynamics (CFD) has therefore been identified to analyzethe flow and heat profiles inside the furnace. The commercial CFD software code Fluentis used for the simulations.Attention has been given only to a slice of the six-in-line submerged arc furnacecontaining two electrodes or one pair while focusing on the current density profiles, slagand matte flow profiles and temperature distribution throughout the bath to ensure themodel reflects reality. Boundary conditions were chosen and calculated from actual plantdata and material specifications were derived from previous studies on slag and matte.Three dimensional results for the current, voltage and energy distributions have beendeveloped. These results compare very well with the profiles developed by Sheng, Ironsand Tisdale in their CFD modelling of a six-in-line furnace. It was found the current flowmainly takes place through the matte, even with an electrode depth of only 20%immersion in the slag, but the voltage drop and energy distribution still only take placein the slag.Temperature profiles through-out the entire modelling domain were established. Thevertical temperature profile similar to Sheng et al. 1998b was obtained which shows aspecifically good comparison to the measured temperature data from the Falconbridgeoperated six-in-line furnace. The temperature in the matte and the slag was found to beuniform, especially in the vertical direction.It has been found that similar results with Sheng et al. (1998b) are obtained for the slagand matte velocity vectors. Different results are, however, obtained with differentboundary conditions for the slag/matte interface and matte region; these results are stillunder investigation to obtain an explanation for this behaviour.The impact of the bubble formation on the slag flow was investigated and found to be asignificant contributor to the flow. With the bubble formation, it is shown that possible'dead zones' in the flow with a distinctive V-shape can develop at the sidewalls of thefurnace with the V pointing towards the centre of the electrode. This behaviour can havea significant impact on the point of feed to the furnace and indirectly affect the feed rateas well as the settling of the slag and matte. These results are not validated though.Different electrode immersions were modelled with a constant electrical current input tothe different models and it was found that the electrode immersion depth greatly affectsthe stirring of the slag in the immediate vicinity of the electrode, but temperature (whichdetermines the natural buoyancy) has a bigger influence on the stirring of the slagtowards the middle and sidewall of the slag bath.The sensitivity of the model to a different electrode tip shape with current flowconcentrated at the tip of the electrode was also modelled and it was found that theelectrode shape and electrical current boundary conditions are very important factorswhich greatly affect the voltage, current density and temperature profiles through thematte and the slag. A detailed investigation to determine the electrode tip shape atdifferent immersions, as well as the boundary conditions of the current density on the tipof the electrode is necessary as it was proven that the model is quite sensitive to theseconditions.Several recommendations arose from this modelling work carried out in thisinvestigation. Time constraints, however, did not allow for the additional work to becarried out and although valuable results were obtained, it is deemed to be a necessity ifa more in-depth understanding of furnace behaviour is to be obtained. Future work willinclude the validation of the results, understanding the liquid matte model, investigatingthe MHD effects and modelling different furnace operating conditions.

[发布机构] Stellenbosch University