## 4 Full-Scale Production Verifications  
### Controlling Basicity in Full-Scale Experiments  
Full-scale trials targeting basicity 2.2 were conducted at the EAF in Hofors. The SiO\({}_{2}\)-based sand, composition shown in Table 7, was added in the second scrap basket to the EAF. The sand used as a SiO\({}_{2}\) carrier was not optimal, as it contained about 13 wt% Al\({}_{2}\)O\({}_{3}\). Additional Al\({}_{2}\)O\({}_{3}\) content in the slag may promote the formation of brownmillerite or increase the Al\({}_{2}\)O\({}_{3}\) content in brownmillerite. Increased Al\({}_{2}\)O\({}_{3}\) content in brownmillerite produces a more reactive brownmillerite [25], the opposite of what was intended. The SiO\({}_{2}\) sand was added directly to the furnace instead of during slag tapping to promote an even distribution and integration of the slag and the sand. Since the scrap composition is not fixed, the basicity varies; therefore, different amounts of SiO\({}_{2}\)-sand addition, from 300 to 500 kg, were tested. Single batches of slag were collected and were kept isolated to avoid contamination by ordinary slag batches. Therefore, the modified slag batches did not experience the normal cooling and reheating cycle. The sampling of cooled slag was done from different areas of the slag pile after the slag had been transported off tipping site. The samples of the same slag were mixed and crushed to less than 4 mm before autoclave treatment and the following EN 12457-2 leaching test.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **CaO** & **MgO** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **FeO** & **Na\({}_{2}\)O** & **K\({}_{2}\)O** \\ \hline
**Sand 1** & 1.6 & 1.4 & 71.2 & 13.9 & 3.46 & 2.6 & 4.1 \\
**Sand 2** & 1.76 & 0.87 & 73.5 & 12.7 & 2.33 & 3.05 & 3.73 \\ \hline \end{tabular}
\end{table}
Table 7: Composition in wt% of the SiO\({}_{2}\) sand used to modify the slag in full scale.  
Figure 7: Chromium leaching from autoclave-treated slag with basicity 2.1 and 2.8 cooled by different methods. The leaching of slag at basicity 2.1 is below detection limit in the reference and hold temperature experiment.  
The basicity of the collected modified batches was below the targeted 2.2. The highest chromium leaching from the single batches was 0.23 mg/kg (see Figure 8) and therefore leached below the limit of inert material of 0.5 mg/kg.  
Brownmillerite was detected by XRD analysis in the batch with the highest chromium leaching (see Figure 9) where the second brownmillerite peak in was the largest in that diffractogram. This was not seen earlier when brownmillerite was detected in the other slag samples. In Figures 3 and 6 the second brownmillerite peak was in the same magnitude as the first brownmillerite peak at 12 2-theta. There is a possibility that another mineral was present and adding intensity to the two larger brownmillerite peaks seen in the diffractogram. The presence of another mineral needs to be confirmed by SEM-EDS analysis. The Al\({}_{2}\)O\({}_{3}\) content in the slag could not be connected to the formation of brownmillerite or the leaching of chromium. The Al\({}_{2}\)O\({}_{3}\) content of the slag that leached 0.23 mg/kg chromium was at 9.1 wt%, which was close to the 8.3 wt% Al\({}_{2}\)O\({}_{3}\) of the slag that leached 0.04 mg/kg. The slag leaching 0.09 mg/kg had an Al\({}_{2}\)O\({}_{3}\) content of 5.2 wt% but a similar diffractogram to the slag leaching 0.04 mg/kg. The results from the full-scale single batches verified the results of the laboratory-scale tests. Decreased basicity led to less brownmillerite formation, which resulted in less chromium leaching.  
Figure 8: Chromium leaching from autoclave-treated slag from full-scale single batches from Ovako Hofors EAF.  
Figure 9: XRD diffractograms of the modified slag batches. The letters representing the minerals found in the analysis—: spinel; b: brownmillerite; m: magnesiumbsite; l: larnite.