## 2 Materials and Methods
### Summary and Cost Estimation  
A favourable aspect on the use of B.F. sludge as a reducing agent for red mud recovery is the lower temperature need to melt the iron after the hematite reduction. This aspect could be exploited at the industrial level, as a low iron formation temperature would permit lower operational costs. Moreover, mixtures treated by blast furnace sludges have a higher reduction yield (17% versus 13%) than the corresponding samples treated with graphite, even with a lower metallization ratio. This aspect depends on the fact that in addition to the iron contained in the red mud, the iron oxide (around 30% wt.) within B.F. sludges can be also recovered. In addition, the unburnt coke featuring the B.F. sludges is more effective for the reduction. Another aspect playing in favour of B.F. sludge use is a lower unburned carbon fraction at the end of the reduction process. From economic and environmental points of view, the use of blast furnace sludges as a reducing agent for the treatment of the red muds lagooned in Portovesme would have a threefold advantage:  
* high quality of DRI or cast iron can be obtained and used as a raw material in oxygen converters or electric furnaces;  
Figure 9: Compositional map for the slag obtained after reduction at 1500 \({}^{\circ}\)C: (**a**) sample C2 and (**b**) sample A2. * the stockpiled waste amount would be reduced, for both the iron and the aluminum metallurgies, with a clear benefit for the environment;
* the reduction residue (slag) has mineralogical characteristics that can assure safe disposal or virtuous recycling for civil engineering applications. By taking in consideration all the aspects associated with the reduction process, such as residual C, reduction yield, metallization ratio, and slag characteristics, the best mix to have a good compromise between reduction yield and metallization degree is a 1:1 red mud/B.F. sludges, equivalent to 0.85 C/Fe\({}_{2}\)O\({}_{3}\). In order to give a cost estimation of the process, the energy balance of the main involved reduction reaction was conducted in Equation (5). Thus, the enthalpy of reaction was calculated in Equation (6), based on the NIST Chemistry WebBook data for hematite, iron, and carbon monoxide [75; 76; 77]. \[Fe_{2}O_{3}+3C\to 2 Fe+3 CO \tag{5}\]  
\[ \begin{split}\Delta H_{r}=\sum\
u\Delta\cdot H_{r-products}-\sum \
u\Delta\cdot H_{r- reagents}=\sum\
u\cdot\left(H_{r-products}^{0}-\int_{r^{0}}^{T}c_{p-products}dT\right)\\ -\sum\
u\cdot\left(H_{r- reagents}^{0}-\int_{r^{0}}^{T}c_{p- reagents}dT\right)\end{split}  \tag{6}\]  
where \(\Delta H_{r}\) is the reaction enthalpy, \(H_{r}\)\({}^{0}\) is the reaction enthalpy in standard conditions (25 \({}^{\circ}\)C, 1 atm), \(c_{p}\) is the specific heat capacity, \(T^{0}\) is the standard temperature (298.15 K), T is the current temperature and \(\
u\) is the stoichiometric coefficient. The iron melting temperatures defined through TG-DSC analysis in an Ar atmosphere, were used in the computation (1020 \({}^{\circ}\)C for sample C2, 960 \({}^{\circ}\)C for sample A2). Approximately 522.5 kJ/mol was obtained for Fe\({}_{2}\)O\({}_{3}\) reduction by graphite, instead of 520 kJ/mol for reduction by B.F. sludges. The overall reduction reaction of hematite to iron, being endothermic, requires thermal energy to be fulfilled. The required energy can be supplied by methane combustion (802-889 kJ/mol [78]) in a rotary kiln or by the use of air plasma in an arc transferred plasma furnace. In Table 6, a raw estimation of the process profitability is reported referring to three different scenarios: an integrated steel shop (1), a third company (2), or an alumina refinery (3) as possible industrial sites where the process could be conducted. For each site, two options were analysed in relation to the transportation costs; they are supported by the treating plant or by the waste producer.