High temperature phase equilibria in the Fe-Co-Cu-Si system pertinent to slag cleaning

Banda, Wezi (2006-12)

Thesis (PhD (Process Engineering))--University of Stellenbosch, 2006.


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.

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