## 3 Results and Discussion
### Mineralogical Results of Laboratory and Pilot Scale Experiments
#### 3.2.1 Laboratory Scale  
The back scattered electron (BSE) images of the original sample A and the remelted sample AR2 (moderate cooling, laboratory scale) in Figure 3a,b show that remelting the slag without quartz addition already had an effect on the mineralogical phase distribution. The spinel grain size increased from  
Figure 2: Back scattered electron (BSE) images. (**a**) Sample A, (**b**) Sample B, and (**c**) Sample C showing the main mineral phase composition derived from x-ray diffraction (XRD) and, if possible, electron microprobe (EMP) measurements. Abbreviations according to [PERSON] and [PERSON] (2010) [28]: wuestite solid solution (Wus), spinel solid solution (Spl), larnite (Lrm), brownmillerite (Brm), olivine group (OI), and melilite group (Mill). approximately 10 \(\upmu\)m to approximately 100 \(\upmu\)m, the dendritic wuestite shape was more dominant than the spherical shape, and instead of olivine, bredigite (Ca\({}_{7}\)Mg(SiO\({}_{4}\))\({}_{4}\)) was formed. Nevertheless, the abundance of calcium silicate phases was still low. A comparison of the different cooling rates revealed that the size of the mineral phases, especially the spinel size, increased with decreasing cooling rate (see Supplementary Materials Figure S1). A possible explanation for the higher abundance of spinel phases compared to the original sample due to solely remelting the material might be an interaction between the crucible material and the melt, since MgO is known to foster spinel formation [5; 29]. However, chemical analyses (Table 1, Section 3.3) showed that the material input (MgO) for the remelted slag samples was not significant. Quantitative EMP analyses performed on individual spinel grains (see Supplementary Materials Tables S2-S21) showed that the MgO concentrations per phase were comparable for the original sample A (11.1 \(\pm\) 0.53 mass-%) and the samples AR1-3 (11.8 \(\pm\) 1.5 mass-%, 14.1 \(\pm\) 0.58 mass-%, and 12.5 \(\pm\) 1.2 mass-%, respectively). Additionally, EMP analyses revealed that fast cooling led to an inhomogeneous distribution of FeO (24.2-63.0 mass-%) and Cr\({}_{2}\)O\({}_{3}\) (0.36-39.8 mass-%) in spinel, resulting in either Fe-rich or Cr-rich spinel phases. However, the sum of FeO and Cr\({}_{2}\)O\({}_{3}\) concentrations was rather constant (62.6 \(\pm\) 1.9 mass-%). In contrast, the FeO and Cr\({}_{2}\)O\({}_{3}\) concentrations measured in spinel phases in sample AR2 (moderate cooling) and in the original sample A were rather homogenous within the range of standard deviations of the performed EMP analyses (30.0 \(\pm\) 3.0 mass-% and 30.8 \(\pm\) 1.8 mass-% for sample A, respectively, 38.5 \(\pm\) 3.0 mass-% and 23.7 \(\pm\) 1.6 mass-% for sample AR2). The increased abundance of larger spinels in sample AR3 (slow cooling) led to reduced Cr\({}_{2}\)O\({}_{3}\) concentrations of 0.32-8.1 mass-% per analyzed phase, but to increased FeO concentrations of 45.9-56.1 mass-%. The sum of FeO and Cr\({}_{2}\)O\({}_{3}\) was again quite homogeneous with 54.4 \(\pm\) 2.7 mass-%. In contrast to the original sample A, V was only in few spinel analyses above the detection limit of the applied method with concentrations ranging from 0.05 mass-% to 0.08 mass-%. The same applied to V in the measured wuestite solid solutions (0.06-0.07 mass-% V), however, in all measured phases, Cr was detected with concentrations of 0.6 \(\pm\) 0.1 mass-% to 1.1 \(\pm\) 0.3 mass-%. Quantitative analyses on calcium silicate phases was only possible in sample AR3, since the two phases identified in samples AR1 and AR2 were too small and no discrete grain boundaries were found. For sample AR3, XRD analyses (Figure S2) confirmed the formation of bredigite, but the Al-bearing calcium silicate phase could not be unequivocally assigned to melilite or pyroxene by XRD or EMP analyses. In all remelted samples, neither Cr nor V were detected in the measured calcium silicate phases via EMP analyses.