## 2 Materials and Methods
### Cast Iron and Slag Characterization  
An optical microscope (OM) and a scanning electron microscope (SEM) were used to analyse the reduced iron obtained with the aim to understand its properties and characteristics and foresee its possible utilization. The cast iron obtained is mainly formed by a perlite matrix with cementie at the grain boundaries. Non-significant fraction of a second non-metallic phase, i.e., sulphides, were observed. Lamellar graphite was observed only in specific regions of the samples, probably where C is concentrated more. The chemical composition was determined by SEM-EDS. Table 5 reports the chemical composition of some metallic spheres collected from the different samples. The main elements detected were Si, Cr, and V. Mn was practically absent since no Mn source is present, in the red muds or in the blast furnace sludge. Traces of this elements were observed only in the sulphides precipitated in the iron matrix. Sulphur was only detected in the sample with higher amount of blast furnace sludges (A3) and this is consistent with the fact that more coke in the mixture can drive more S to the iron. V was mainly detected in graphite reduced samples because in the B.F. sludges reduced samples, this element undergoes a strong dilution and the residual amount solved in the iron is not detectable by the EDS probe. The same for Cr. With respect to the cast iron obtained by the reduction of jarosite sludges from zinc hydrometallurgy, the cast iron formed by the treatment of red muds appear less polluted by harmful elements (Cu, S, P, As) and thus, of a greater quality, leading to a wider possibility of its utilization as raw material for foundries and steel shops. Figure 8: Computed versus measured mass loss at 1500 \({}^{\circ}\)C (**a**), metal-to-charge ratio as a function of C/Fe\({}_{2}\)O\({}_{3}\) ratio (**b**). Regarding the slag, some differences can be highlighted as a function of the roasting temperature. The main constituents of the slag, detected in all the samples by means of XRD, were gehlenite (Ca\({}_{2}\)Al(AlSi)O\({}_{7}\)), diaoyudaoite (Al\({}_{11}\)NaO\({}_{17}\)), and perovskite (CaTiO\({}_{3}\)) [58]. Gehlenite was already formed during the red mud roasting by solid state reaction between CaO, Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), and traces of Na\({}_{2}\)O; diaoyudaoite was probably the reaction product of Al\({}_{2}\)O\({}_{3}\) and the Na\({}_{2}\)O set free from thermonatrite melting (at 850-860 \({}^{\circ}\)C [69]); whereas perovskite forms by the reaction between CaO and anatase at 1100 \({}^{\circ}\)C. The latter transformation is slightly visible also in the dTG curves (Figure 1). At lower temperature (1200 \({}^{\circ}\)C) peaks belonging to Na-Al-silicate were detected. By increasing the temperature, such a phase starts to convert to nepheline and at 1500 \({}^{\circ}\)C all the silicate is converted into nepheline. Likely this transformation is strictly related to the decomposition of sodalite. After melting at 1064 \({}^{\circ}\)C, sodalite starts to decompose into NaCl and nepheline, in the range 1090-1320 \({}^{\circ}\)C [70]. In the samples at 1500 \({}^{\circ}\)C sodalite is completely decomposed into nepheline, and for this reason in the reduced red muds mixtures, only their peaks are visible. Such compounds reflect a fully crystalline development of the slag structure. Other components of the slag were the iron oxides, with some differences in the samples from 1200 to 1500 \({}^{\circ}\)C. At 1200 \({}^{\circ}\)C, magnetite was observed as the only oxide form of iron. At 1300 \({}^{\circ}\)C very weak peaks associated to a mixture of Fe\({}_{3}\)O\({}_{4}\) and FeO might be detected but not indexed, since their intensity was not enough to be matched with the reference patterns. At 1500 \({}^{\circ}\)C, the oxide form of iron was FeO, with no more traces of Fe\({}_{3}\)O\({}_{4}\). This behaviour agrees with the iron oxide reduction happening in a blast furnace or DRI plants [71].