## 4 Discussion  
### Characteristics of Reduction Processes with Carbon  
As can be seen from Figure 1, the EAFD is composed of spherical particles, and the mixture of reducing agents and the EAFD is a dispersion system with good contact. According to the thermodynamics principle, the chemical reactions of EAFD containing zinc reduced by carbon powder are shown in Equations (3)-(11) [11; 17; 18], which comprise the direct reduction of solid carbon and the indirect reduction of CO:  
\[\mathrm{ZnO(s)+C(s)=Zn(g)+CO(g)\ \ \ \Delta G_{1}^{\theta}=348480-286.1T} \tag{3}\]  
\[\mathrm{3 Fe_{2}O_{3}(s)+C(s)=2 Fe_{3}O_{4}(s)+CO(g)\ \ \ \Delta G_{2}^{\theta}=45369 09-224.37T} \tag{4}\]  
\[\mathrm{Fe_{3}O_{4}(s)+C(s)=3 FeO(s)+CO(g)\ \ \ \Delta G_{3}^{\theta}=207510-217.62T} \tag{5}\]  
\[\mathrm{FeO(s)+C(s)=Fe(s)+CO(g)\ \ \ \Delta G_{4}^{\theta}=158970-160.25T} \tag{6}\]  
\[\mathrm{ZnO(s)+CO(g)=Zn(g)+CO_{2}(g)\ \ \ \Delta G_{5}^{\theta}=178020-111.6T} \tag{7}\]  
\[\mathrm{3 Fe_{2}O_{3}(s)+CO(g)=2 Fe_{3}O_{4}(s)+CO_{2}(g)\ \ \Delta G_{6}^{ \theta}=35380-40.16T} \tag{8}\]  
\[\mathrm{Fe_{3}O_{4}(s)+CO(g)=3 FeO(s)+CO_{2}(g)\ \ \ \Delta G_{7}^{\theta}=71940-73.62T} \tag{9}\]  
\[\mathrm{FeO(s)+CO(g)=Fe(s)+CO_{2}(g)\ \ \Delta G_{8}^{\theta}=-13160+17.21T} \tag{10}\]  
\[\mathrm{C(s)+CO_{2}(g)=2 CO(g)\ \ \Delta G_{9}^{\theta}=172130-177.46T} \tag{11}\]  
According to the above equations, the chemical reaction of EAFD containing zinc reduced by carbon powder is a complicated heterogeneous reaction process. As shown in Figure 7, when the temperature is less than 900 \({}^{\circ}\)C, a direct reduction reaction is the main reaction; the reactions are of the solid-solid type. When the temperature is higher than 900 \({}^{\circ}\)C, the reaction is principally indirect, belonging to a gas-solid reaction. The reaction rate of the multiphase reaction depends predominantly on the diffusion rate of the reaction components and the interfacial chemical reaction rate. Since the  
Figure 6: The mineral components of the samples obtained at 1200 \({}^{\circ}\)C.  
solid reactants are in direct contact with each other before the start of the reaction, the reaction can be started primordially. However, a solid product layer (metal phase and slag phase) is formed between the dust particles and the solid carbon during the reduction procedure. Consequently, further progress of the reaction will depend on the diffusion of the reactants through the layer. Under the influence of the Boudouard reaction, Equation (11), the carbon-reduction process is transformed from a solid-solid reaction to a gas-solid phase reaction. The general steps for the corresponding process include the external diffusion of gas reactants and the products in the boundary layer on the solid surface, the internal diffusion of gas reactants, and products through the solid product layer, as well as the interfacial chemical reactions between gas and solid reactants. According to the microstructure of the dust samples (A5) reduced by carbon at 1100 \({}^{\circ}\)C, the particles in the sample still maintain a relatively independent shape and a certain number of pores between the particles, as shown in \[\mathrm{3 Fe_{2}O_{3}(s)+C(s)=2 Fe_{3}O_{4}(s)+CO(g)\ \ \ \Delta G_{2}^{\theta}=45369 09-224.37T} \tag{4}\]0 This provides a good condition for the diffusion of the reducing agent, CO, and reduction product, Zn (g). Since the boiling point of zinc is 908.47 \({}^{\circ}\)C, the zinc vapor could be separated from EAFD and discharged as a gas. The evaporation rate of zinc is governed by an interfacial chemical reaction, the diffusion of the zinc vapor, and the temperature. In the low temperature stage (900 \({}^{\circ}\)C), the evaporation rate of zinc in the sample is very low because of the boiling point of zinc.