## 2 Materials and Methods
### Thermogravimetric Analysis in Air  
The same analysis was led on TG-DSC test results in air atmosphere. The samples reduced through blast furnace sludges show the same behaviour as the one reduced though graphite. Figure 3 reports the DSC, TG and dTG lines. From Figure 3a,b, four peaks can be determined. The reactions related to each peak are:  
* 200-400 \({}^{\circ}\)C \(\circ\) **Al(OH)\({}_{2}\)\(\rightarrow\)Al\({}_{2}\)O\({}_{3}\)+3H\({}_{2}\)O** \(\circ\) **AlOOH \(\rightarrow\)Al\({}_{2}\)O\({}_{3}\)+H\({}_{2}\)O** \(\circ\) **Ca(OH)\({}_{2}\)\(\rightarrow\)CaO+ H\({}_{2}\)O**. * 600 \({}^{\circ}\)C-800 \({}^{\circ}\)C: \(\circ\) **C+ O\({}_{2}\)\(\rightarrow\)CO\({}_{2}\)** \(\circ\) **2C+ O\({}_{2}\)\(\rightarrow\)2 CO** \(\circ\) but also 3 Fe\({}_{2}\)O\({}_{3}\)+CO\(\rightarrow\) 2 Fe\({}_{3}\)O\({}_{4}\)+ CO\({}_{2}\). The two valleys at 200-400 \({}^{\circ}\)C in the heat flow lines (not well defined in TG and dTG signals) are the same already discussed for TG-DSC experiments in Ar. A great exothermic peak, starting at around 600 \({}^{\circ}\)C and ending at 800 \({}^{\circ}\)C is visible in Figure 3, for both graphite and B.F. sludges reduced samples. The first reaction that takes place is the formation of CO\({}_{2}\), however at 700 \({}^{\circ}\)C, CO\({}_{2}\) is unstable and reacts with C to form CO, in addition C directly reacts with oxygen, giving CO.  
At this temperature also trace of hematite to magnetite reduction can happen as detected in similar experiments [53, 57]. Complete reduction of magnetite to iron is not possible in this experimental condition due to the fact that the CO formed was carried away by the continuously renewed air atmosphere. Thus, only a partial reduction of hematite into magnetite would be expected since this reaction can develop completely at 750 \({}^{\circ}\)C with few ppm of CO.  
To validate the previous deductions, the theoretical loss of mass was computed, taking into account the following assumptions, as the sum of:  
* the loss of all the carbon contained in each sample (reaction a, Section 2.3);
* the loss of the all oxygen contained in the Fe\({}_{2}\)O\({}_{3}\) content when it is reduced to Fe\({}_{3}\)O\({}_{4}\) (reaction b, Section 2.3);
* no further reductions to FeO or Fe were counted, as the CO atmosphere was not enough to carry out such reactions;
* the loss of all the oxygen contained in the oxides owing a standard oxygen potential higher than CO at 1200 \({}^{\circ}\)C (ZnO, PbO, P\({}_{2}\)O\({}_{5}\), SO\({}_{3}\), and Cr\({}_{2}\)O\({}_{3}\)) (reactions d to h, Section 2.3). The comparison between the actual loss of mass and the computed one is shown in Figure 4a: all the points are almost overlapped on the control line at 45\({}^{\circ}\). This means that the calculation confirms that the loss of mass in air atmosphere is due to carbon combustion and the reduction stage from Fe\({}_{2}\)O\({}_{3}\) to Fe\({}_{3}\)O\({}_{4}\). As for the argon tests, the analysis of the carbon oxidation peak area increases as well as the initial carbon concentration of the sample Figure 4b. As the initial carbon concentration increases (and as the C/Fe\({}_{2}\)O\({}_{3}\) increases), the specific energy associated with the peak increases with a nearly linear behaviour. This is because (in air atmosphere) the more carbon there is in the samples, the more there is a release of energy through the formation reaction of carbon monoxide. With regard to the reducing agent, it is clear that, when the initial carbon concentration is the same (for example in samples C2 and A2), and when blast furnace sludges are used, other elements are present to influence the process. As a result, this lowers the energy of the peak. The mass loss trend shown in Figure 4c coincides, as expected, with the carbon monoxide formation energy. The peak related to the C combustion does not always take place at the same temperature Figure 2d. In particular, it occurs at a lower temperature for the samples with a higher carbon concentration.