## 1 Introduction  
Steel mill dusts with high zinc content, occurring during the production of carbon steel, are a well-known secondary resource for the zinc industry. Because of the heavy metal content in the dust (Pb, Cr, Cd, etc. ), the EAFD is categorized as a hazardous waste and, therefore, creates a major issue for the steel producer. Due to environmental concerns, it is important to recycle the dust in a proper way. Despite considerable disadvantages, such as low product quality and high amounts of residual waste, the Waelz process has dominated the recycling of these dusts for several decades. Throughout Europe, more than 90% of the EAFDs are treated in Waelz kilns. Decreasing landfill capacities and increasing disposal costs, along with global rethinking towards a circular economy, have led to significant interest in establishing a new, innovative process concept. The desire for a zero-waste approach and high product quality provides strong motivation for a new recycling route. Therefore, research is focused on multi-metal recovery and a possible application of the final slag in the building industry. A suitable concept could be the recycling of the EAFD via an iron bath containing dissolved carbon. Process developments such as the PIZO, ESRF, and 2 sDR processes make use of this technology [1, 2, 3, 4, 5]. In characterization studies of electric arc furnace dusts, it should be noted that a large part of the compounds occurring are iron oxide (Fe\({}_{2}\)O\({}_{3}\)), zinc oxide (ZnO), and zinc ferrite (ZnO*Fe\({}_{2}\)O\({}_{3}\)). Depending on the concentration of the individual elements or the influence of temperature, the ratios between these three species can change, even though the total mass of all compounds remains almost unchanged. It can also be considered that in a neutral atmosphere zinc ferrite decomposes to ZnO and Fe\({}_{2}\)O\({}_{3}\) at \(\sim\)1200 \({}^{\circ}\)C, as shown in (1). Therefore, ZnO*Fe2O3 can be neglected at the trial process temperature described in this paper (1400 \({}^{\circ}\)C) [1; 2; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. \[\mathrm{ZnO}\times\mathrm{Fe_{2}O_{3(s)}}=\mathrm{ZnO_{(s)}}+\mathrm{Fe_{2}O_{3 (s)}} \tag{1}\]  
Furthermore, the following experiments focus on the reduction behavior of ZnO, while iron is only considered as an additional reducing agent. For this reason, the reduction reactions of zinc oxide with carbon (C) dissolved in the iron melt, as well as the interactions with iron oxide and metallic iron, are described below. The system can be divided into three areas:  
* The metal bath containing the dissolved reducing agent (carbon);
* The slag, including the zinc and iron oxide compounds;
* The gaseous phase above the slag. At the metal-slag interface, direct reduction occurs with the dissolved carbon in the iron bath as well as with metallic iron. ZnO reacts with Fe from the melt and forms FeO as shown in (2). A similar behavior can be observed in the reaction with dissolved carbon listed in (3). A high C content in the iron bath increases the carbon activity and, therefore, the driving force for the reduction of ZnO [6; 7; 9; 10; 17; 18; 19; 20]. \[(\mathrm{ZnO})+\mathrm{Fe_{(I)}}=\mathrm{Zn_{(g)}}+(\mathrm{FeO}) \tag{2}\]  
\[(\mathrm{ZnO})+[\mathrm{C}]=\mathrm{Zn_{(g)}}+\mathrm{CO_{(g)}} \tag{3}\]  
The reduction of zinc oxide by iron is confirmed by the Richardson-Ellingham diagram (RED), which is limited to standard conditions (1 mol of oxygen, ambient pressure of 1 atm, and an activity of 1 for solid and liquid substances). Nevertheless, the RED gives a good overview of the stability of the individual metal oxides, plotting their Gibbs free energy over temperature. Low reaction enthalpies indicate a higher stability of the oxides. Metals that form stable oxides have the characteristics of reducing less stable oxide compounds. In terms of ZnO, the stability is lower than that of FeO at temperatures over 1200 \({}^{\circ}\)C. Above this temperature, the reduction of ZnO by iron is thermodynamically possible.