**1. Introduction**

About 4 million tons of waste are incinerated in Switzerland each year in 29 municipal solid waste incineration (MSWI) plants to reduce the mass and volume of waste, destroy organic compounds, and to recover energy. After incineration about 20 wt.% and 2 wt.% of the waste input remains as bottom ash and fly ash (FA). FA precipitates from the flue gas by passing through boiler and electrostatic precipitator. FA has been characterized by numerous studies [1–4]. The major chemical components are Ca, Na, K, Cl, and S. The elevated Cl concentration in FA (often above 10 wt.%) results mostly from the incineration of plastics (PVC). Chlorine forces the volatilization of heavy metals with high vapor pressure by the formation of Cl-complexes [5]. In addition, some heavy metals (e.g., Zn, Pb, Cu, Sb, Sn, and Cd) are chalcophile, and the high S concentration in the waste input additionally supports the transfer into the flue gas. This results in the increased weight percent of several toxic metals in FA. Thus, direct disposal in landfills in Switzerland without previous treatment is prohibited. Furthermore, disposal also means that the metals in FA reach their end of life and are lost as valuable resources. In the current trend towards a circular economy, where urban mining is prominent, FA has become an interesting source for metal recovery. Therefore, the Swiss authorities released the Ordinance of the Avoidance and Disposal of Waste (ADWO), which prescribes the recovery of heavy metals from FA prior to disposal [6]. Currently, FA is either disposed in underground storage of neighboring countries,

treated with a neutral leaching, and cemented afterwards, or it is treated by the so-called FLUWA process, which is an acidic leaching process that was established in Switzerland in 1997. The FLUWA process is currently the only feasible state-of-the-art process that achieves the demands of the ADWO. Heavy metals from FA are recovered at varying rates [4,7,8] depending on the type of metal. As a basis for the development of defined recovery rates and for the implementation of the recovery process, the mass flow of metals in FA and their geochemical properties must be known. The metal content in FA depends heavily on the waste input (industrial or household waste), and elemental concentrations differ not only from plant to plant but also on a daily and seasonal basis [9]. The acid arising at the plants' wet flue gas cleaning systems is used as leaching agent. During the FLUWA process, the acid and alkaline scrub water is mixed with FA and reacts in two- to three-stage cascade reactors. After 40–60 min of leaching, vacuum filtration separates the solid metal depleted filter cake from the filtrate (leachate) with dissolved metals. This leachate is used for direct metal recovery [7], or the metals are precipitated as hydroxide sludge by the addition of lime. The sludge is exported, and the metals are recovered by smelting plans. The depleted filter cake is disposed in a Swiss landfill of type D.

The efficiency of the process depends mainly on the pH, Eh, liquid-to-solid ratio (L/S ratio), temperature, and leaching time. The content of heavy metals as well as the mineralogical composition of the FA are important additional factors that influence the efficiency. The FLUWA process is performed at a low pH (3–4) to successfully dissolve the heavy metals from FA. The addition of acid leads to the dissolution of lime (CaO) and calcite (CaCO3) (among other minor phases), which buffer the pH. Lime reacts in a first step with water to form portlandite (Ca(OH)2) before it dissolves, and two hydroxide ions are released (Equations (1) and (2)).

$$\text{CaO} + \text{H}\_2\text{O} \to \text{Ca(OH)}\_2\tag{1}$$

$$\text{Ca(OH)}\_{2} \rightarrow \text{Ca}^{2+}\_{\text{(aq)}} + 2\text{OH}^{-} \tag{2}$$

Below a pH of ~7.3, calcite is dissolved by consuming protons and releasing CO2 in the process (Equation (3)).

$$\text{CaCO}\_3 + 2\text{H}^+ \rightarrow \text{Ca}^{2+}\_{(\text{aq})} + \text{CO}\_2 + \text{H}\_2\text{O} \tag{3}$$

If the acid neutralizing capacity of the FA is larger than the amount of acid scrub water, additional acid (e.g., 32% HCl) must be added to achieve low pH conditions, causing additional cost. The oxidation of metallic aluminum (Al0) in FA forces reducing conditions [10]. Aluminum is usually present in FA as aluminum foil particles, which are entrained with the flue gas. Despite their low content in FA, their presence diminishes the leaching efficiency during the FLUWA process [10]. The oxidation of Al0 is at the expense of metals such as Pb and Cu (Equation (4)), which are reductively cemented and removed from the leachate [8].

$$2\text{Al}^{0} + 3\text{Cu}^{2+} \rightarrow 2\text{Al}^{3+} + 3\text{Cu}^{0} \downarrow \tag{4}$$

To prevent reductive precipitation, an oxidizing agent (e.g., H2O2) is added during acid leaching. It is speculated that other metals such as Fe0 and Zn0 in FA may reduce Cu2<sup>+</sup> and Pb2<sup>+</sup> during the FLUWA process. This seems, however, to be unlikely as both elements are less reactive than Al<sup>0</sup> [11,12]. The addition of an oxidizing agent at low pH conditions is crucial for Cu and Pb recovery, as it enhances the yield greatly ([13], Table 1).

Regarding the currently limited capacity of only 12 FLUWA facilities and the capacities to be expanded (either by new construction or by external treatment at other plants), an inventory of Swiss FA was made. Knowledge about the FA composition and its properties will help FLUWA operators and authorities in the implementation of the guidelines according to the ADWO. This study therefore presents an overview of all forms of Swiss FA and their chemical and mineralogical composition, acid neutralizing capacity, and content of metallic aluminum. The FA types were divided into exemplary groups (clusters) regarding combined FLUWA processing. In addition, the recovery potential of heavy and valuable metals in FA was calculated.


**Table 1.** Average metal recovery achieved by acidic leaching with the FLUWA process [13].
