## 3 Results  
### Description of the Slag Samples  
In Figure 2, an example of the most representative microstructure of the analyzed slag is reported. This picture, showing only six slag samples, is suitable to compare the phases' morphology as well as their arrangement as a function of the slag group. For instance, the slag belonging to group A (re-bar steel), are characterized by relative low basicity, high fraction of wustite and low fraction of Cr-spinel. This is due to the operative conditions of the furnace, i.e., low quality and strong scraps oxidation. The production of high quality and high-alloyed steels (group B and C) requires a better control of the scrap quality together with a low oxygen consumption in order to maximize the steel yield. This implies high basicity and a very low amount of wustite. A high basicity, augmented primarily by the copious addition of lime, induces the formation of high CaO-bearing phases, like 3 CaO.SiO\({}_{2}\) (H in Figure 2), brownmillerite (B in Figure 2) or Ca-chromite (CaCr in Figure 2). Nonetheless, for the samples belonging to group B, due to the use of high-alloyed steel scraps, the average amount of CrO\({}_{\text{x}}\) in the slag is significantly more than for the other two categories.  
Increasing the chromium oxide (Cr\({}_{2}\)O\({}_{3}\)) concentration in the slag, the weight fraction of Cr-bearing phases increases (Figure 3a). From Figure 3a it can be noted that most of the samples are grouped in a limited range of Cr\({}_{2}\)O\({}_{3}\) concentration (0-5 wt.%). Those slag are related both to re-bar and quality steels. Usually, the scraps used for this kind of productions are poor in alloying elements such as Cr and, for this reason, the amount of oxidized Cr migrating to the slag is relatively low. Nevertheless, re-bar steel slag presents a higher concentration of Cr\({}_{2}\)O\({}_{3}\) compared to the quality steel slag since the former requires strong oxidation to remove the impurities within the scraps. Generally, these slag are characterized by small and dispersed Cr-spinels (max 20 \(\upmu\)m), as can be seen in Figure 2. On the contrary, only a few samples (8, 9, 10, 19) feature very high (>20% wt.%) concentration of Cr oxide and all of them belong to the B-group, i.e., highly alloyed steel production. These slag are generally characterized by very coarse spinels (>50-100 \(\upmu\)m) and, if the basicity is high, by needle-like Ca-chromite as well. An increase in Cr concentration in a slag also induces an enrichment of Cr in its  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**Chemical**} & \multicolumn{2}{c}{**Phase Fraction (wt.\%)**} & \multicolumn{2}{c}{**Cr Leaching**} \\  & & \multicolumn{2}{c}{**Composition (wt.\%)**} & & \multicolumn{2}{c}{**Phase Fraction (wt.\%)**} & \multicolumn{2}{c}{**(ug/L)**} \\ \hline \multirow{2}{*}{**Sample ID**} & \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Cr\({}_{2}\)O\({}_{3}\)**} & \multirow{2}{*}{**BI\({}_{4}\)**} & \multirow{2}{*}{**Wustite**} & \multicolumn{2}{c}{**Ca-Cr**} & \multirow{2}{*}{**Cr-Spinel**} & \multirow{2}{*}{**Bulk**} & \multirow{2}{*}{**Powder**} \\  & & & & & & & & \\ \hline
1 & A & 4.28 & 0.829 & 29.7 & - & 19.6 & n.d. & 168 \\
2 & A & 2.87 & 0.614 & 21.4 & - & 16.2 & n.d. & 10 \\
3 & A & 2.84 & 1.103 & 31.6 & - & 12.6 & n.d. & 169  
bearing phases (Figure 3b). This aspect must be taken into consideration since an excess of Cr needs other elements, i.e., Al, Mg, Fe, to form stable and safe phases. This condition is not always respected, and thus, in particular situations, the risk of leaching a high concentration of Cr can be relevant, even if Cr is fixed in apparently safe phases.