Determination of the Phase Transformation Temperatures of the Fe-Co-Cu-Si System in the Fe-Rich Corner by Thermal Analys
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I.
INTRODUCTION
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. The liquid metal phase produced in this operation, ferrocobalt, is primarily a Fe-CoCu-Si alloy with minor components such as sulfur and carbon (Table I). The copper waste slags being treated are fayalitic slags, high in iron and silica. This, combined with the reducing conditions encountered in these furnaces (pO2 ± 10-13 to 10-15 atm), results in the reduction of silica from the slag into the ferrocobalt alloy. There are no experimental data available in the literature on the physical properties of ferrocobalt. Jones et al.[1] investigated the recovery of cobalt using DC arc furnaces and assumed a liquidus temperature of ferrocobalt value between 1500 C and 1600 C based on the melting point of the individual metallic elements iron, cobalt, and copper. Nelson et al.[2] reported a value of the liquidus temperature of ferrocobalt at 1370 C but how this value was determined is not reported, and no compositional data are given either. The lack of published information relating the composition of ferrocobalt and the liquidus temperatures makes it W. BANDA, Postdoctoral Student, G.A. GEORGALLI, Researcher, and J.J. EKSTEEN, Extraordinary Professor, are with Department of Process Engineering, University of Stellenbosch, Matieland 7602, South Africa. Contact e-mail: [email protected] C. LANG, Professor, is with the Department of Mechanical Engineering, University of Cape Town, Cape Town 7700, South Africa. Manuscript submitted June 22, 2007. Article published online October 7, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
difficult to quantify the values given by Jones et al. and Nelson et al. Nelson et al.[2] reported operational problems in DC arc furnace that are related the slag and alloy superheat and that put the furnace integrity at risk. This problem causes increased heat flux on the side walls of the furnace resulting in thermal stress on the refractory lining and subsequent premature loss of the lining. Again it can be observed that the operating temperatures of furnace are based on temperatures that are supposed to be the liquidus temperatures of the alloy and slag. The operating philosophy of the furnace is to operate above these liquidus temperatures inducing a superheat in the slag of about 400 C in order to keep the alloy liquid. Now, in a case where the thermodynamics of the alloy and slag are not well understood, the energy supplied to the melt might be overestimated, resulting in problems as mentioned by Nelson et al. In the post tap hole operations of slag cleaning the metal fluidity must be sustained. Plasma arcing is used for the purpose of maintaining the ferrocobalt temperatures above the liquidus. The temperature reported by Jones et al.[1] of 1650 C to achieve this is again based on a guess val
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