## 2 Materials and Methods  
Based on the knowledge described above, a series of laboratory-scale trials were executed to investigate the influence of the initial carbon concentration in the metal bath on the reduction kinetics of zinc oxide. For this reason, reduction experiments were performed on iron baths with defined starting concentration of carbon. A synthetic slag with basicity B\({}_{2}\) = 1 was used to simulate process conditions as they occur during the recycling of steel mill dust. The separate addition of zinc oxide defined the initial concentration of ZnO in the slag. The resulting slag and iron samples were analyzed by using SEM-EDX and spark spectrometry.  
The tests were carried out in the \"Indutherm MU/700\" induction furnace with a maximum power input of 12 kW at the Chair of Nonferrous Metallurgy of the Montanuniversitat Leoben. Heating was provided by a graphite crucible with the following dimensions:  
* External dimensions (height: 170 mm; diameter: 110 mm);
* Internal dimensions (height: 140 mm; diameter: 56 mm);
* Thermal isolation of the crucible was secured by a ceramic shell of 7 mm.  
Since the experiments were investigating the influence of the carbon concentration in the molten iron on zinc oxide reduction, the experimental setup had to be designed to prevent any contact between the metal bath and the graphite crucible. For this reason, a spinel crucible (70% Al\({}_{2}\)O\({}_{3}\), 30% MgO; height: 85 mm; external diameter: 56 mm; thickness: 3 mm) was placed within the graphite crucible. This setup provided uniform, indirect heating of the spinel crucible through the graphite, and protected against additional carbon input. The temperature was measured using a thermocouple placed at the bottom of the graphite crucible. The schematic experimental setup is shown in Figure 2.  
The required metal bath was created by melting a pre-produced iron-carbon alloy with a defined C content. After heating the metal bath to 1400 \({}^{\circ}\)C, the previously produced synthetic slag was added. The composition of the synthetic slag was based on the slag components occurring in steel mill dusts and thermodynamic calculations ensuring the lowest possible melting point of the slag. After 10 min of temperature homogenization, the zinc oxide was added. The charging of ZnO into the liquid slag was also defined as the starting point of the experiments. The measured temperature at the bottom of the crucible  
Figure 2: Experimental setup.  
was set to 1400 \({}^{\circ}\)C throughout the entire test period. The compositions of the synthetic slag and zinc oxide are shown in Tables 1 and 2, respectively. Since the crucible was not sealed, the produced gaseous zinc was directly re-oxidized in the furnace atmosphere, and ZnO was transferred into the off-gas system.  
To show the temporal progression of ZnO concentration, the fully reproducible experiments were stopped at defined process times. After completion of the individual trials, the compositions of the slag and the iron-carbon alloy were analyzed. The series of tests included 4 different carbon concentrations in the iron alloy (0.85%. 2.16%, 2.89%, and 4.15%), and the time intervals of the experiments were set to 2, 4, 6, 10. and 15 min. The amounts of the individual components used for the experimental procedures are given in Table 3, and resulted in an initial concentration of ZnO in the slag of 10.55%.