## 3 Results
### Effect of Basicity on Phase Chemistry  
One factor which determines Cr leaching behavior is slag basicity. As can be seen in Figure 4, increasing the basicity (BI\({}_{4}\)), the Cr leached during standard testing increases as well. This result agrees with previous studies, since at high basicity other phases than spinels tend to form [5; 6; 7; 23; 26]. On the other hand, on milled slag, the more basic slag resulted in less leaching. This means that other factors contribute to determining the leaching behavior of milled slag. Figure 3: Cr-bearing phases fraction (**a**) and Cr concentration in Cr-bearing phases (**b**) as a function of Cr\({}_{2}\)O\({}_{3}\) concentration in the slag. Figure 2: Example of SEM-BSE pictures of analyzed slag (samples 1, 4, 8, 9, 13, 18). B: brownmillerite; CaCr: Ca-chromite; G: gehlenite; H: haturrite; K: kirschsteinite; L: larnite; Sp: spinel; W: wustite. Basicity plays a key role in the characteristics of slag, especially in spinel formation and evolution. For instance, increasing the slag basicity, the concentration of non-spinel-forming elements (Ca, Si) increases in the slag spinel; as is also worthy of note is the fact that the higher the basicity, the higher the Cr concentration in the spinel is (Figure 5a). When Cr concentration in a spinel increases, the probability to form unstable spinel also increases, since more Cr requires more spinel-forming elements (Al, Mg, Fe) (Figure 5a) [8]. However, by increasing the basicity, the opposite occurs. In fact, the spinels are enriched by destabilizing species (Ca and Si) as reported in Figure 5c. In addition, a cut-off value can be identified (BI\({}_{4}\) > 2), which leads to the formation of calcium-chromium compounds. When basicity is in the range of 2-3, one can identify the coexistence of Cr-spinel and Ca-chromate (CaCrO\({}_{4}\)) (Figure 5b). If basicity is increased to over 4, Ca-chromate evolves in Ca-chromite (CaCr\({}_{2}\)O\({}_{4}\)). This is due to a decrease of Ca in Ca-Cr compounds and a collateral enrichment in Cr. These results are in line with previous researches [5,6,15,28]. The Cr enrichment in Ca-Cr compounds also corresponds with a dilution of other elements, i.e., Fe, Si, Mg, which can be considered as impurities of the above-mentioned phases (Figure 5d). Figure 4: Effect of slag basicity on Cr leaching for (**a**) bulk slag and (**b**) slag powder. Marked samples represent remarkable behavior slag. Figure 5: Elements distribution within the Cr-bearing phases: forming-elements in (**a**) spinel and (**b**) Ca-Cr compounds; impurities in (**c**) spinel and (**d**) Ca-Cr compounds. As reported by [PERSON] et al. [20] and [PERSON] et al. [5], a slag with high basicity is interested by a (Mg,Ca)Cr2O3 solid solution precipitation from the liquid slag with a high amount of impurities and irregular lattice structure and morphology. This seems attributable to the presence of Cr6+ which induces spinel instability. Cr6+ formation is favored in high basicity slag, since it should have a high fraction of non-combined CaO that could react with Cr to yield Cr6+ [29]. Despite an increase in basicity leading to a higher concentration of impurities, these do not seem to have a marked influence on the release of chromium. The balance of the constituent elements of a spinel itself seems to be the parameter controlling Cr leaching, in line with the study carried out by [PERSON] et al. [30]. Among these elements, the most influential with regards to Cr elution seems the Cr itself, as can be seen from the graph in Figure 6a. An increase in Cr concentration seems to favor its leaching. Nevertheless, most studies assessed that if Cr is fixed in spinels, the leaching does not depend on the Cr2O3 content in the slag. This aspect should be analyzed in relation to the stoichiometry of spinel composition. The general formula used to indicate the spinel group is XY2O4, where X is a bivalent cation and Y is a trivalent cation. In the system under examination, the elements that can appear as trivalent cations are Fe, Cr and Al.