## 2 Materials and Methods
### Reduction Tests  
Reduction tests in the muffle furnace allow us to assert that blast furnace sludges are an effective reducing agent that also provides better results than simple graphite. In fact, from Figures 5 and 6 it is clear that, although both products have a compact and solid aspect, sample A2 shows the clear presence of a well separated slag from the metallic fraction; in sample C2, instead, such an aspect is not so evident. In addition, the appearance of the slag obtained through blast furnace sludge addition is more vitrified than the reciprocal obtained by pure graphite. This aspect will be better described with the XRD analysis of the obtained slag. Iron was obtained for all of the samples at all the temperatures, however, the chemistry and the morphology of the reduced products change. At 1200 and 1300 \({}^{\circ}\)C, the typical spongy morphology of direct reduced iron was obtained Figure 5; Figure 6a,c. Also, the non-metallic fraction has a different morphology as a function of the roasting temperature. For instance, at 1200 \({}^{\circ}\)C, the non-metallic fraction appears only sintered, with porous morphology. At 1500 \({}^{\circ}\)C the metal melts. This is confirmed by the drop morphology of the metallic fraction embedded in the slag Figure 6b,d. Increasing the temperature, the non-reduced oxides can form eutectic compounds and melt, leading to the formation of a compact slag. Unfortunately, the viscosity of the slag, due to a high amount of glass-network oxides, was too high to allow a complete separation between slag and cast iron at 1500 \({}^{\circ}\)C Figure 6b,d [66]. The properties of the final product strongly depend on the initial carbon concentration. The samples with 45% of carbon added (Figure 5--C4), have a powdered aspect, after roasting at 1500 \({}^{\circ}\)C. This characteristic confirms the hypothesis made through the thermal analysis where it was found that the  
Figure 4: Comparison of the calculated and measured mass loss (**a**), specific energy of CO/CO\({}_{2}\) formation peak (**b**), total mass loss (**c**), and peak temperature of the C combustion (**d**) as a function of C/Fe\({}_{2}\)O\({}_{3}\) ratio. Air atmosphere. aforementioned mix has the lowest values of specific energy and therefore would show few metallic fractions with high unburned graphite and not compacted slag. Also, the samples with the highest specific energy associate to the melting of iron (C3 and A3) showed a less compact morphology, whereas the samples at 25% of equivalent C (A2, C2) showed a good compromise between reduction and compactness. At this point, it is possible to confirm that from the visual analysis of the products, the best initial carbon concentrations of the mixes are 25% and 35% However, since 35% C is resulted in less compacted slag, this means that 25% seems to be the ideal initial carbon concentration. Figure 5: Overview of some of the samples obtained after reduction in a muffle furnace. The amount of metallic material that can be obtained from each sample is of great importance. A comparison between the theoretical maximum amount of cast iron that should be obtained at 1500 \({}^{\circ}\)C and the actual fraction measured according to ISO 2597 was performed Figure 7a. For the computation of the expected amount of metal, all the Fe\({}_{2}\)O\({}_{3}\) (reaction c) and reducible oxides (reactions d to j) were considered completely reduced to metal (by considering the carbon available). The comparison was made for the samples roasted at 1500 \({}^{\circ}\)C. At this temperature Mn and V oxides were considered completely reduced. Through computation, the metallic fraction was overestimated by a systematic difference between measured and calculated of about 5%. This is likely due to the inability to forecast the amount of post-oxidized metal that actually forms during the cooling step of the roasting cycle. Moreover, the amount of non-reacted carbon cannot be estimated, because the partial pressure of CO and CO\({}_{2}\) cannot be measured in the furnace.