| A principal obstacle to the direct smelting of copper concentrates to copper in a single reactor is the fact that impurities such as bismuth, antimony and arsenic are collected preferentially in the copper phase. High levels of these elements cannot be tolerated in the subsequent electro-refining step.; This study examined the removal of impurities from molten copper, in particular bismuth, by means of injection refining with sulfur hexafluoride contained in a nitrogen/oxygen flow.; Thermodynamic analysis consisted of the calculation of standard free energy change accompanying the impurity removal reactions, evaluation of control reactions between the impurity elements, the effect of oxygen on removal process and estimation of the thermodynamic equilibrium impurity levels. This analysis showed that the proposed reactions were feasible with antimony being the most susceptible to fluoridation and bismuth the least.; Experimental work on the injection refining of 15 kg melts of copper confirmed that bismuth levels can be reduced to very low values ({dollar}<{dollar}50 ppm) in relatively short periods of time. Analysis of the experimental data showed that lance immersion depth and gas flowrate are extremely important in determining the rate of bismuth removal and the efficiency of SF{dollar}sb6{dollar} utilization. These results also indicate that mass transfer in the liquid phase is the rate controlling step.; A semi-empirical model was developed to describe mass transfer between rising bubbles containing SF{dollar}sb6{dollar} and the surrounding melt. Parameters such as bubble size, mass transfer coefficient, liquid recirculation velocity and bubble rise velocity were calculated from correlations presented in the literature. The results of the model compare fairly well with the experimental data. On this basis, the model was used to predict the effect of various operating parameters. A simple engineering correlation was developed relating the volumetric mass transfer coefficient to depth of immersion and injected gas flowrate. This relation, along with other criteria, was used to design a pilot test for the removal of bismuth in a one-ton pilot furnace. |
