Characterization and Modification of the Secondary Copper Smelting Slag for Smooth Operation and Slag Valorization

An iron silicate (fayalite) based slag is formed in the secondary copper smelting process. This slag contains substantial amount of ZnO, CaO and Al2O3 and minor components such as Na2O, PbO, MgO and Cr2O3. This slag is of great importance to the copper smelting process and the entire production loop. The phase relations and the viscosity of the slag system at the smelting condition influences the process in both kinetic and thermodynamic ways. The cooling process has a great impact on the final microstructure of the solidified slag and further affects the following slag valorization applications. It is reported by the copper recycling industry that the copper smelting slag often has a high viscosity which may lead to problems in slag tapping. In the present study, the copper slag was observed in-situ with confocal laser scanning microscopy (CLSM). It was found that solid precipitation at the tapping temperature is the main reason of the increasing viscosity. In order to solve this problem, the phase relations in the industrial slag system ZnO-“FeO”-SiO2-Al2O3-CaO (CaO/SiO2 = 0.05, Al2O3/SiO2 = 0.15) were investigated by quenching samples after reaching equilibrium at 1423 K (1150 °C) under iron saturation. The equilibrium composition of the phases was determined with electron probe micro-analysis (EPMA). The effect of individual components, such as FeO, ZnO and CaO on the phase equilibrium of the slag system has been quantitatively studied. A high temperature rheometer system has been employed to study the rheological behavior of the industrial slag with iron saturation. Non-Newtonian behavior has been observed and the effect of the major components (FeO, ZnO and CaO) on the rheological property of the slag system has been investigated. The crystallization behavior of both ZnO containing slag (FeO-ZnO-SiO2-Al2O3-CaO, FeO/SiO2 = 2, ZnO = 9 wt.%, Al2O3 = 5 wt.%, CaO = 1 wt.%) and ZnO free slag (FeO-SiO2-Al2O3-CaO, FeO/SiO2 = 1.6, Al2O3 = 5 wt.%, CaO = 1 wt.%) under various cooling rates was observed in-situ by CLSM. Continuous cooling transformation (CCT) diagrams were constructed. The influence of the individual components of the slag, e.g. FeO, CaO and Al2O3 on the crystallization behavior of the fayalite slag was systematically studied via water quenching experiments followed by a detailed EPMA-WDS micro-chemical analysis. Industrial scale experiments were carried out to study the cooling process of the copper smelting slag. The slag was cooled in an iron pot, a copper mold, a steel plate and quenched with water flow. XRF, XRD and EPMA were employed to characterize the slag. The cooling process was simulated with the software COMSOL, the temperature profile and the cooling rate were calculated. The relation between the cooling history and the microstructure of the solidified slag was investigated. Based on the experimental data, suggestions were given to the industry, including the modification of the slag chemical composition and the cooling process, in order to decrease the risk of problematic operation and to adapt the slag in higher added value utilizations.status: publishe