# Pyrometallurgical Technology for Extracting Iron and Zinc from Electric Arc Furnace Dust
## 3 Theoretical Background for the Pyrometallurgical Processing of EAFD  
The pyrometallurgical processing of EAFD involves the reduction of iron and zinc in a charge from their oxygen-containing forms with active reagents (C and CO) to obtain granulated pig iron and a zinc-containing product [28]. The reduction of metal oxides with carbon monoxide is based on the reversible reaction:  
\[\mathrm{MeO}+\mathrm{CO}\leftrightarrow\mathrm{Me}+\mathrm{CO}_{2}. \tag{1}\]  
Reaction (1) proceeds to the right when the Gibbs energy decreases during the transition from the initial state to equilibrium. The diagram of the temperature dependence of the equilibrium concentrations of CO mixed with CO\({}_{2}\) for the reduction of metal oxides with carbon monoxide shows that a gas containing CO \(\geq\) 50% at 600 \({}^{\circ}\)C is a reducing agent for FeO and an oxidizing one for ZnO, i.e., an excess of carbon is required to initiate reducing ZnO:  
\[\mathrm{C}+\mathrm{CO}_{2}=2\mathrm{CO}. \tag{2}\]  
Increasing the temperature shifts the CO formation equilibrium towards an increase in the CO content in the gas mixture, according to [PERSON]'s principle, since this reaction proceeds with heat absorption. Zinc oxide is reduced by carbon monoxide according to the reaction:  
\[\mathrm{ZnO}+\mathrm{CO}=\mathrm{Zn}\dagger+\mathrm{CO}_{2}. \tag{3}\]  
Zinc starts reducing from its oxide at \(\sim\)1000 \({}^{\circ}\)C. The Zn reduction rate is a function of carbon dioxide deoxidation where CO is continuously regenerated. Based on [PERSON]'s principle of the sequence of transformations, iron reduces from oxides gradually from higher oxides to lower ones according to the scheme: Fe\({}_{2}\)O\({}_{3}\)\(\rightarrow\)Fe\({}_{3}\)O\({}_{4}\)\(\rightarrow\)FeO\(\rightarrow\)Fe. CO acts as a reducing agent during metallization in a rotary hearth furnace:  
\[\mathrm{3 Fe}_{2}\mathrm{O}_{3}+\mathrm{CO}=\mathrm{2 Fe}_{3}\mathrm{O}_{4}+ \mathrm{CO}_{2}, \tag{4}\]  
\[\mathrm{Fe}_{3}\mathrm{O}_{4}+\mathrm{CO}=\mathrm{3 FeO}+\mathrm{CO}_{2}, \tag{5}\]  
\[\mathrm{FeO}+\mathrm{CO}=\mathrm{Fe}+\mathrm{CO}_{2}. \tag{6}\]  
Along with the aforementioned reactions, metallic iron is carburized in the system, leading to the formation of iron carbide [29]:  
\[\mathrm{3 Fe}+\mathrm{C}=\mathrm{Fe}_{3}\mathrm{C}. \tag{7}\]  
Carburized iron has a lower melting point compared to pure iron. Thus, if pure iron has a melting point of 1539 \({}^{\circ}\)C, then with a carbon content of 2 and 4%, this temperature decreases to 1380 and 1170 \({}^{\circ}\)C, respectively. Melting transforms carburized iron into a liquid state, and due to cohesive forces, small particles combine into larger ones, forming drops of molten iron. The cohesion of reduced iron proceeds under conditions when the slag-forming components of the charge are in a solid state due to their refractoriness. The resulting slag melting temperature should be within 1300-1380 \({}^{\circ}\)C. The slag composition is defined based on the CaO-MgO-SiO\({}_{2}\) system [30]; in this temperature range, melting occurs in the pseudo-wollastonite region. In the molten state, metal and slag are immiscible liquids that separate into independent phases due to the difference in densities and have an interface with different surface energies (interfacial tension). Upon cooling, the melt crystallizes while forming pig iron and slag. In general, the granulated pig iron production stages are shown in Figure 4.  
EAFD contains 25.8% iron; however, to obtain granulated pig iron, the iron content in the raw material should be at least 50% (according to preliminary laboratory studies). Therefore, a raw material with a high iron content should be added to EAFD to obtain an iron-zinc-containing feedstock meeting this requirement. Another technogenic material, an oily scale from Magnitogorsk Iron and Steel Works PJSC, was chosen as such raw material for additional charging. Oily scale is an iron oxidation product, mainly formed in rolling and pipe-rolling industries during the heat treatment of metal.