Modelling the thermal, electrical and flow profiles in a 6-in-line matte melting furnace
Date
2008-12
Authors
Snyders, Cornelius Albert
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
The furnace at Polokwane is designed to treat high chromium containing concentrates
which requires higher smelting temperatures to prevent or limit the undesirable
precipitation of chromium spinels. The furnace has therefore been designed to allow for
deep electrode immersion with copper coolers around the furnace to permit the
operation with the resulting higher heat fluxes.
Deep electrode immersion has been noted to result in dangerously high matte
temperatures. Matte temperatures however can be influenced by a number of furnace
factors which emphasize the need to understand the energy distribution inside the
furnace. Computational fluid dynamics (CFD) has therefore been identified to analyze
the flow and heat profiles inside the furnace. The commercial CFD software code Fluent
is used for the simulations.
Attention has been given only to a slice of the six-in-line submerged arc furnace
containing two electrodes or one pair while focusing on the current density profiles, slag
and matte flow profiles and temperature distribution throughout the bath to ensure the
model reflects reality. Boundary conditions were chosen and calculated from actual plant
data and material specifications were derived from previous studies on slag and matte.
Three dimensional results for the current, voltage and energy distributions have been
developed. These results compare very well with the profiles developed by Sheng, Irons
and Tisdale in their CFD modelling of a six-in-line furnace. It was found the current flow
mainly 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 place
in the slag.
Temperature profiles through-out the entire modelling domain were established. The
vertical temperature profile similar to Sheng et al. 1998b was obtained which shows a
specifically good comparison to the measured temperature data from the Falconbridge
operated six-in-line furnace. The temperature in the matte and the slag was found to be
uniform, especially in the vertical direction.
It has been found that similar results with Sheng et al. (1998b) are obtained for the slag
and matte velocity vectors. Different results are, however, obtained with different
boundary conditions for the slag/matte interface and matte region; these results are still
under investigation to obtain an explanation for this behaviour.
The impact of the bubble formation on the slag flow was investigated and found to be a
significant 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 the
furnace with the V pointing towards the centre of the electrode. This behaviour can have
a significant impact on the point of feed to the furnace and indirectly affect the feed rate
as 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 to
the different models and it was found that the electrode immersion depth greatly affects
the stirring of the slag in the immediate vicinity of the electrode, but temperature (which
determines the natural buoyancy) has a bigger influence on the stirring of the slag
towards the middle and sidewall of the slag bath.
The sensitivity of the model to a different electrode tip shape with current flow
concentrated at the tip of the electrode was also modelled and it was found that the
electrode shape and electrical current boundary conditions are very important factors
which greatly affect the voltage, current density and temperature profiles through the
matte and the slag. A detailed investigation to determine the electrode tip shape at
different immersions, as well as the boundary conditions of the current density on the tip
of the electrode is necessary as it was proven that the model is quite sensitive to these
conditions.
Several recommendations arose from this modelling work carried out in this
investigation. Time constraints, however, did not allow for the additional work to be
carried out and although valuable results were obtained, it is deemed to be a necessity if
a more in-depth understanding of furnace behaviour is to be obtained. Future work will
include the validation of the results, understanding the liquid matte model, investigating
the MHD effects and modelling different furnace operating conditions.
Description
Thesis (MScEng (Process Engineering))--Stellenbosch University, 2008.
Keywords
Computational fluid dynamics, Melting furnaces, Nickel matte, Dissertations -- Process engineering, Theses -- Process engineering