## 2 Theoretical Considerations and Thermodynamic Calculations
The values for Delta-G in dependence of the temperature for both the solo or multiple-step dehydration (splitting of water leaving the pure metal chloride) and the hydrolysis (splitting of HCl and H\({}_{2}\)O leading to the formation of the metal oxide) are given in Figure 2. When assuming an actual trial mixture to heat up to processing temperature, the depicted data suggest that for the three chlorides MgCl\({}_{2}\)\(\cdot\)6H\({}_{2}\)O, FeCl\({}_{3}\)\(\cdot\)6H\({}_{2}\)O, and AlCl\({}_{3}\)\(\cdot\)6H\({}_{2}\)O, the chemical driving force is higher for liberating gaseous hydrochloric acid than for splitting of solely the water groups (values of dashed lines are more negative than the ones of solid lines). For MgCl\({}_{2}\)\(\cdot\)6H\({}_{2}\)O, this accounts for all three possible steps of dehydration or hydrolysis. This effect cannot be stated for CaCl\({}_{2}\)\(\cdot\)6H\({}_{2}\)O, where the values for liberating water are more negative at given temperatures, indicating that the compound is unlikely to set free the highly reactive gaseous hydrochloric acid. Figure 2: Calculated Delta-G values for hydrolysis or dehydration of the hydrated metal chlorides. The main aspect concerning the recovery of valuable metals by selective chlorination and evaporation is the low evaporation temperature \(\mathrm{T_{vap}}\) of the formed metal chlorides compared to the elements in metallic or oxide form. Figure 3a shows the values for \(\mathrm{T_{vap}}\) of the elements of interest in various compound forms. Sulfate compounds are not included because they would generally react to oxides under the liberation of sulfur-oxygen compounds. All elements except silver evaporate completely as chlorides at temperatures below 1000 \({}^{\circ}\)C. In this context, it must be pointed out that all chlorides have significant vapor pressures even at lower temperatures, and extraction is effectively possible even for silver, as will be shown below. In addition to the individual values for \(\mathrm{T_{vap}}\), the range of the averaged values of all metals and oxides as well as those of the chlorides is shown. The difference between the evaporation temperatures of the compound types is significant, leading to the general conclusion that elements of value are effectively extractable in their chloridic form. Furthermore, the software package FactSage with the database FactPS was used to calculate the values for Delta-G for the reactions of the elements of interest as well as iron in the form of oxides with gaseous HCl under formation of the metal chlorides, as given in Figure 3b. Since the software considers the compounds in their most stable form at given temperatures, the slopes of the curves change due to melting and evaporation. The dashed red line shows the values of Delta-G for the reaction of Fe\({}_{2}\)O\({}_{3}\). It can be stated that the curve is in the positive area for the majority of the temperature range. This is the theoretical basis for the selective mulitmetal separation from the iron matrix, which accounts for 30-40% (elemental Fe) in the iron precipitation residues, depending on the actual residue. The only other elements that show positive values are copper and nickel. The reaction of indium is in equilibrium at temperatures over 600 \({}^{\circ}\)C (Delta-G = 0).