## 2 Materials and Methods
### Oxy-Cutting Fines  
Pursuant to its name, these fines originated from the cutting of billets by an oxygen lance. At SIDENOR, the oxy-cutting fines were collected in a filter house and stored in big bags. Figure 4 shows the sample and Table 4 gives the chemical analysis of the oxy-cutting fines. Due to the high content of iron in the sample, it was analysed specifically for the iron oxide and metallic iron content. Table 5 gives the results of the physical analysis regarding moisture and density. The physical analysis was in good agreement with the iron forms within the samples. For instance, practically no mass variations were registered during the heat treatment for moisture and LOI evaluation. This was due to the fact that most of the sample was in oxide form and thus remained unaltered during roasting. The particle size distribution of the oxy-cutting fines sample is shown in Figure 5.  
Figure 3: SEM micrographs of (**a**,**b**) as-received, (**c**) 550 \({}^{\circ}\)C and (**d**) 1050 \({}^{\circ}\)C treated grinding sludge samples.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**SUBSTANCE** & **CONCENTRATION (\%)** & **METHOD** \\ \hline Al\({}_{2}\)O\({}_{3}\) & 0.170 & \\ CaO & 1.295 & \\ Cr\({}_{2}\)O\({}_{3}\) & 0.654 & \\ MgO & 0.368 & WD-XRF \\ MnO & 0.815 & \\ P2O\({}_{5}\) & 0.530 & \\ S & 0.188 & \\ SiO\({}_{2}\) & 1.065 & \\ C & 0.31 & Elemental analysis \\ Fe total & 68.2 & DIN EN ISO 11885 \\ Fe\({}^{2+}\) & 16.4 & AM\_EG.26 (Titration) \\ Fe\({}^{3+}\) & 51.5 & Calculated \\ Fe met & 0.3 & ISO 5416 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Chemical analysis of SIDENOR oxy-cutting fines samples.  
Figure 4: Picture (**a**) and stereo-micrography (**b**) of SIDENOR oxy-cutting fines samples.  
Figure 5: Particle size distribution of SIDENOR oxy-cutting fines.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**CHARACTERISTIC** & **VALUE** & **TEST TEMPERATURE (\({}^{\circ}\)C)** & **METHOD** \\ \hline Moisture & 4.730\% & 105 & EN 14346:2006 \\ LOI & 0.115\% & 550 & EN 15169:2007 \\ Carbonates & N.A. & 1050 & EN 459-2\_2010 \\ Bulk density & 1.824 g/cm\({}^{3}\) & & ONORM EN 1097-3: 1998 08 01 \\ True density & 4.739 g/cm\({}^{3}\) & & ONORM EN 1097-7: 2009 01 01 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Physical analysis of SIDENOR oxy-cutting fines samples.  
From the crystallographic point of view, the oxy-cutting fines were composed of magnetite (Fe\({}_{3}\)O\({}_{4}\)), as determined by the titration analysis shown in Table 4 (Figure 6).  
Oxy-cutting fines appeared dusty, easily compactable and brown. No coarse particles were detected (Figure 4). At high magnification, the fines appeared to be formed by slightly oxidised spherical metallic particles, ranging from 1 to 10 \(\upmu\)m, with some coarse particles reaching 100 \(\upmu\)m (Figure 5) and blocky compounds of 20-30 mm length (Figure 7). Metallic spheres were mainly composed of iron with traces of copper. The blocky compounds were magnesite residues (Table 6).  
Figure 6: XRD pattern of as-received oxy-cutting fines (M = magnetite).  
Figure 7: SEM–secondary electron (SEM-SE) (**a**) and SEM–back-scattered electron (SEM-BSE) (**b**) micrographs of as-received oxy-cutting fines.  
TG-DSC analysis of the grinding sludges did not show any significant transformation, and for this reason, the results are not reported.