## 5 Discussion  
To cope with the drastic climate situation and security of supply for technology metals in Europe, processes are needed that make valuable metals recoverable from residues with the lowest possible CO2 emissions. In the course of thermodynamic calculations and experiments using STA, as well as in comprehensive test campaigns in a resistance-heated muffle furnace, the extraction of several valuable metals via selective chlorination was demonstrated. However, the calculations and experimental investigations carried out only provide information on the feasibility on a limited scale in a small experimental setting. In experiments with pure substances by means of STA, the three metal chlorides MgCl2.6H2O, AlCl3.6H2O, and FeCl3.6H2O that release chlorine in the form of gaseous hydrochloric acid at increasing temperatures were identified as effective for chlorination. FeCl3.6H2O has practically a lower tendency for hydrolysis; this finding is in agreement with thermodynamic calculations. MgCl2.6H2O reacts in three hydrolysis and dehydration steps to form MgO. AlCl3.6H2O splits off the bound crystal water and chlorine in one single decomposition step. From thermodynamic calculations, it appears that the three  
Figure 8: Extraction rates of nickel in trials with (**a**) original and (**b**) calcined jarosite from the nickel industry. Figure 7: Individual extraction rates in percentage in trials at 1100 \({}^{\circ}\)C with (**a**) jarosite and goethite from the zinc industry in original as well as in (**b**) calcined form. other chlorides NaCl, KCl, and CaCl\({}_{2}\cdot\)6H\({}_{2}\)O have lower chlorine liberation tendencies. This fact was confirmed by the practical evaluations, which showed that the chlorination of In\({}_{2}\)O\({}_{3}\) is not or is insufficiently possible with those three reactants. In trials with pure valuable metal compounds, silver was proven to have a high chlorination tendency, but the extraction is aggravated by the high evaporation temperature. The chlorination and extraction of In, Zn, and Pb from oxides is feasible in mixtures with MgCl\({}_{2}\cdot\)6H\({}_{2}\)O, AlCl\({}_{3}\cdot\)6H\({}_{2}\)O, and FeCl\({}_{3}\cdot\)6H\({}_{2}\)O, Ni shows poor chlorination behavior. In the case of sulfate compounds, chlorination is more difficult as the chlorination agents react under the formation of sulfate compounds which are subsequently decomposed. In general, lower mobility of the formed intermediate phases SO\({}_{2}\) or SO\({}_{3}\) can also be mentioned as a possible reason for the lower chlorination tendency. In the tests with real residual materials, a different behavior is observed when abruptly heating the mixture by charging into the hot furnace. AlCl\({}_{3}\cdot\)6H\({}_{2}\)O seems to release chlorine completely already at low temperatures due to rapid hydrolysis, where the reaction kinetics are still too low for chlorination. Only MgCl\({}_{2}\cdot\)6H\({}_{2}\)O and FeCl\({}_{3}\cdot\)6H\({}_{2}\)O could be identified as effective for chlorination from residual mixtures. Concerning a recycling process for iron precipitation residues, the addition of FeCl\({}_{3}\cdot\)6H\({}_{2}\)O is the preferred option, since no additional impurities would be introduced. It is worth mentioning that silver can be almost entirely recovered from goethite and calcined goethite by chlorination with MgCl\({}_{2}\cdot\)6H\({}_{2}\)O and FeCl\({}_{3}\cdot\)6H\({}_{2}\)O (extraction up to 97%). Zinc, lead, gold, and bismuth are extracted in high extents from all residues of the zinc industry using the two mentioned chlorides. Nickel extraction from nickel jarosite is comparably poor, yields are even lower when the material is thermally decomposed before the experiment. MgCl\({}_{2}\cdot\)6H\({}_{2}\)O is the most effective chlorination agent followed by FeCl\({}_{3}\cdot\)6H\({}_{2}\)O.