High temperature phase equilibria in the Fe-Co-Cu-Si system pertinent to slag cleaning
Date
2006-12
Authors
Banda, Wezi
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
In the smelting of copper waste slags to recover cobalt and copper, the prediction of the metal
liquidus temperature and the associated superheat for liquid metal handling for subsequent
treatments cannot be done with certainty, making the management of furnace integrity very
difficult. By studying the phase equilibria and solution thermodynamics in liquid ferrocobalt new
experimental data on the liquidus temperature and phase equilibria of the quaternary system can
contribute to the improvement of existing copper slag smelting processes. This will alleviate the
operational uncertainties and difficulties associated with ferrocobalt production in electric arc
furnaces.
There is no specific literature available that describes the physicochemical and thermochemical
properties of the ferrocobalt produced from smelting of waste copper slags. Therefore, the
quaternary system Fe-Co-Cu-Si has been characterised by studying and reviewing the binary and
ternary subsystems with respect to the high temperature phase equilibria.
The ferrocobalt metal has been modelled on the Fe-Co-Cu-Si quaternary system. The liquidus
temperatures and phase equilibria in the Fe-Co-Cu-Si system, within the composition and
temperature regimes pertinent to smelting of slag, were investigated by differential thermal
analysis and metallography. Drop-quench techniques coupled with scanning electron microscopy
were used to study the phase equilibria. The activity of silicon in liquid Fe-Co-Cu-Si at 1450 °C
was calculated from gas-alloy-silica equilibrium experiments conducted in controlled oxygen
partial pressure atmospheres at 10-13
P, 10P
-14
P, and 10P
-15
atmosphere (absolute) corresponding to the
conditions found in the industrial application.
The liquidus temperature of the quaternary Fe-Co-Cu-Si is influenced by the content of silicon in
the system. When silicon is added to the Fe-Co-Cu ternary the liquidus temperature is lowered in
the new system (Fe-Co-Cu-Si). In the range of silicon content studied (0 0.1) < XSi ≤ , the
liquidus temperature decreased by over 70 °C. The liquidus temperature profiles of the
subsystems of the quaternary Fe-Co-Cu-Si, show large composition dependence too, except in
the Fe-Co system. In the ternary Fe-Co-Cu, the liquidus temperature decreases with increasing
copper content and is characteristic of the profiles of the liquidus lines in the binary subsystems
Fe-Cu and Co-Cu.In the dilute concentrations of silicon, it is shown that the phase equilibria in the quaternary
system have attributes of the Fe-Si and Fe-Cu-Si systems. Silicon is associated more with the
iron rich phase than it is with the copper rich phase. It stabilises the metastable liquid
immiscibility when added to the Fe-Cu, Co-Cu, and Fe-Co-Cu in the corresponding ternary
systems Fe-Cu-Si, Co-Cu-Si and quaternary Fe-Co-Cu-Si system.
The activity of silicon in liquid Fe-Co-Cu-Si at 1450 °C, in the composition range 1 to 5 wt. %Si
exhibits a negative deviation from ideal liquid solution behaviour. The activity coefficient
approaches a constant value of 0.2×10P
-3
P, with pure liquid silicon as reference state, as the silicon
concentration approaches zero implying a Henrian solution behaviour. This information should
be useful in the thermodynamic modelling of the system.
Description
Thesis (PhD (Process Engineering))--University of Stellenbosch, 2006.
Keywords
Pyrometallurgy, Slag, Dissertations -- Metallurgical engineering, Theses -- Metallurgical engineering