## 2 Theoretical Considerations and Thermodynamic Calculations  
The potential of recovering valuable metals from residual materials by means of chlorination has been demonstrated repeatedly. Indium can be recovered from spent LCDs using polyvinyl chloride (PVC) [41; 42] or aqueous HCl [43]; furthermore, the contained metals in CIGS-solar cells (copper-indium-gallium-selenium) can be extracted to noteworthy extents using chlorine gas or ammonium chloride [44]. [PERSON] et al. proposed the possibility of recovering Au and Ag from cyanide tailings using CaCl\({}_{2}\)[45]. Pyrrite (FeS\({}_{2}\)) addition positively influences the chlorination of Au as the formation of CaSO\({}_{4}\) results in the liberation of chlorine [46]. NH\({}_{4}\)Cl was stated to extract Zn via chlorination from steel mill dusts (BOS) [47]; a similar approach was developed for recovering Zn and also Pb from EAFD by treatment in an Ar-Cl\({}_{2}\) atmosphere. Comprehensive investigations were carried out in the field of chlorination of valuable elements from municipal solid waste (MSW) using HCl for Zn and Pb [48], PVC for Cu, Zn, Pb, and Cd [49]. Furthermore, metal chlorides such as NaCl, CaCl\({}_{2}\), and MgCl\({}_{2}\) were investigated, leading to noteworthy extraction of Cd, Cr, Cu, Ni, Pb, or Zn [50; 51]. In the course of this work, the reactivity of various solid metal chlorides for multimetal recovery from an excess iron matrix was investigated. Metal chlorides are present in huge extents worldwide and used in various applications. The six compounds NaCl, KCl, CaCl\({}_{2}\), MgCl\({}_{2}\), AlCl\({}_{3}\), and FeCl\({}_{3}\) were investigated regarding their reactiveness for chlorinating valuable elements from residual materials. It can be stated that the chlorination of a metal of interest is highly dependent on the present form of the element chlorine itself. Hereby, three different ways for supplying chlorine by metal chlorides can be cited, being the liberation of gaseous chlorine, the liberation of gaseous hydrochloric acid, and the direct reaction of the solid, liquid, or gaseous metal chloride compound. For example, the chlorination of Zn from ZnO using gaseous HCl proceeds according to the following reaction: ZnO + 2 HCl (g) \(\leftrightarrow\) ZnCl\({}_{2}\) (g) + H\({}_{2}\)O (g). The demand of chlorination agent depends on the valency of the respective valuable elements. It is noteworthy in this context that all mentioned chlorides except NaCl and KCl are strongly hygroscopic ([PERSON], 2013; [PERSON] et al., 2007) and therefore are generally present in a partially hydrated  
Figure 1: Concept flow sheet of the chlorination process for treating precipitation residues. form. Thermodynamic calculations which were carried out using the software packages FactSage ([PERSON] et al., 2016) and HSC Chemistry V10 (Metso Outotec) revealed that NaCl and KCl are significantly more stable than the other compounds, showing no or very little tendency for reaction with valuable metal oxides. Calculations using HSC chemistry V10 show that the chlorides CaCl\({}_{2}\), MgCl\({}_{2}\), AlCl\({}_{3}\), and FeCl\({}_{3}\) in their hydrated form (MeCl\({}_{x}\)-nH\({}_{2}\)O) show a different behavior regarding their ability to liberate gaseous hydrochloric acid. For the theoretical interpretation of the equilibrium composition of chemical reactions in dependence of temperature, the Free Gibbs Energy (Delta-G) can be considered, whereby negative values indicate that the reaction proceeds in the given direction. For CaCl\({}_{2}\) and MgCl\({}_{2}\), there are three different types of hydrates (nH\({}_{2}\)O with n being 2, 4, or 6), while for the trivalent chlorides AlCl\({}_{3}\) and FeCl\({}_{3}\), solely the hexahydrate compounds are available in the used database.