## 3 Materials and Methods
### Campaigns with Industrial Iron Precipitation Residues  
Various iron precipitation residues were investigated and processed in defined mixtures in a muffle furnace (Nabertherm LT9/12) in a scale of 80 g. The main purposes for the trial campaign consisting of 108 experiments are at first the identification of the most effective chlorination agents for the extraction of specific metals as well as for the multi-metal recovery from various industrial residues with partly low concentrations of valuable elements. Furthermore, the influence of increasing the concentration of the respective chlorination agent as well as higher process temperatures are evaluated. The raw materials in use were javorie and goethite residues of the zinc industry as well as a javorie residue originating in the nickel production. Each material was tested in its original form as well as thermally pre-treated (caliched) in a muffle furnace at 800 \({}^{\circ}\)C for five hours using air as atmosphere in order to decompose the mineral structures to simpler oxidic and sulfatic compounds. Chemical analyses were conducted for the untreated as well as for the pre-calcined materials at two accredited laboratories; the results are shown in Table 1.  
The chlorides AlCl\({}_{3}\cdot\)6H\({}_{2}\)O, FeCl\({}_{3}\cdot\)6H\({}_{2}\)O, and MgCl\({}_{2}\cdot\)6H\({}_{2}\)O were used as chlorination agents, which is premised on the thermodynamic calculations and STA trials. The trial setup is the treatment of the mixture in an open crucible which is charged into the furnace, leading to the evaporation of a share of chloride without taking part in the targeted reactions. Furthermore, it has to be pointed out that chemical reactions do not occur ideally. Therefore, the trials were carried out with an excess of chlorine supply. The 2-fold of the total calculated stoichiometric chlorine demand for full chlorination of considered valuable elements and 5% of iron was set as the base chlorine addition. As it is of interest if the extraction of certain elements can be significantly increased by the higher addition of the respective chlorides, the experiments were carried out with 2-fold and 3-fold of the base value chlorine addition for all three chlorides. It was assumed in the course of the calculations that the valuable element chlorides that are formed during the treatment of jarosite, goethite, calcined jarosite, and calcined goethite are AgCl, BiCl\({}_{3}\), CuCl\({}_{2}\), InCl\({}_{3}\), KCl, NaCl, PbCl\({}_{2}\), SnCl\({}_{2}\), and ZnCl\({}_{2}\). The chlorination of iron in all trials was considered to an extent of 5%, forming the trivalent compound FeCl\({}_{3}\). For the trials with nickel jarosite (original and calcined), the chlorine demand was calculated for the full chlorination of nickel and in turn 5% of Fe. The elements Na and K are considered due to their partly high concentration in precipitation residues, which would lead to significant chlorine consumption due to their affinity for chloride formation. In reference to these suppositions, the calculation of the mixture ratios for the trials was executed. Each jarosite and goethite material, untreated as well as pre-treated, was mixed with each of the mentioned chlorination agents in the three proposed mixing ratios. The resulting practically used mixtures are shown in Table S1 (Supplementary Materials). R represents the amount of residual material, and CA represents the mass of the respective chlorination agent. After the preparation of a mixture of milled dry residual material and chlorination agent, an amount of 80 g was filled into a silica crucible. The crucible was charged into a muffle furnace at trial temperature where the mass decrease was tracked via an integrated balance. Each mixture was tested at two different temperatures, 900 \({}^{\circ}\)C and 1100 \({}^{\circ}\)C, with air as atmosphere. After 30 min, the crucible was removed from the furnace to cool down at ambient air temperature, and the remaining solid residue was milled and analyzed.