*3.5. Cluster Analysis*

The dendrogram of the cluster analysis is shown in Figure 5. The cophenetic correlation coefficient, a measure of how faithfully a tree represents the dissimilarities among observation, is 0.71 (the maximum would be 1), which is acceptable. The red cluster on the right has eight samples (FA01, FA04, FA02, FA06, FA14, FA10, FA03, and FA29). On the left side is a branch with a yellow cluster of five samples (FA20, FA21, FA23, FA28, and FA26), a green cluster of six samples (FA16, FA18, FA24, FA22, FA27 and FA25), and finally a blue cluster representing 10 samples (FA13, FA17, FA19, FA11, FA12, FA08, FA11, FA15, FA05, and FA07).

**Figure 5.** Cluster tree of the FA samples according to their content of Zn and Al0, the ANC, and the amount of FA produced in 2016. The y-axis shows the distances between the calculated values and hence is a mathematical value that expresses the dissimilarity.

Every cluster represents FA samples with similar properties. The average value of each property and cluster is shown in Figure 6. The green cluster representing FA with very good leaching potential contains FA with the highest Zn concentration of 5.7 wt.% and the lowest content of Al0 (0.05 wt.%). Each MSWI plant produces almost 4000 tons of FA per year. This cluster is the most interesting for economic metal recovery due to the high Zn recovery with the lowest H2O2 consumption and low acid consumption (7.4 mL mol H<sup>+</sup> per kg FA to reach pH 2). The blue cluster representing FA with good leaching potential is the largest cluster with an average Zn concentration of almost 4 wt.% and a content of Al0 of 0.2 wt.%. These plants produce on average 2200 t of FA per year. The ANC is the lowest because the FA samples required only 7.3 mol H<sup>+</sup> per kg FA to reach a pH of 2 during the titration and a relatively low amount of hydrogen peroxide. The yellow cluster representing FA with moderate leaching potential contains only five MSWI plants. Their FA shows low averaged Zn concentration of 2.6 wt.% but a rather high Al0 concentration of 0.4 wt.%. Since some MSWI plants are among the largest in Switzerland, the amount of produced FA is 4300 t per year on average. The metal recovery of these ashes requires higher amount of acid (8.6 moL H<sup>+</sup> per kg FA to reach pH 2) and a high amount of H2O2. The red cluster containing FA with poor leaching potential shows the lowest Zn concentration of all clusters (2.2 wt.%) but by far the highest concentration of Al0 (0.8 wt.%). On average, plants in this cluster produces only 1300 t of FA per year. However, this cluster has the highest ANC of 10 mol H<sup>+</sup> per kg FA to reach pH 2.

**Figure 6.** Average Zn and Al0 concentration as well as average produced FA per plant and the average ANC for each cluster.

### **4. Discussion**

### *4.1. Mass Flow of Metals in Swiss FA*

The estimated annual mass flow of metals in Swiss FA is illustrated in Table 4. The mass flow was calculated based on concentrations and amount of FA from all plants in 2016 [15]. The total quantity of recoverable metals is: 3052 t/y Zn, 667 t/y Pb, 172 t/y Cu, and 20 t/y Cd. Other base metal contributions are: 1392 t/y Al, 1340 t/y Fe, 791 t/y Ti, and the chalcophile elements Sb 194 t/y and Sn 140 t/y. The annual mass flow for REE (including Sc and Y) is 4.8 t/y, mainly represented by the light-REE Ce (1.8 t/y), La (1 t/y), Y (0.7 t/y), and Nd (0.6 t/y). Other notable metals are Ni (10 t/y), As (6 t/y), Co (4 t/y), Ag (3.1 t/y), and W (1.4 t/y). Gold is a minor constituent in FA, and thus only 12 kg is landfilled each year from FA. The low mass flow of REE and other precious metals (Ag, Au) in FA is due to their low vapor pressure, expressed with low partitioning coefficients < 0.1 [16]. Preferentially chalcophile elements such as Zn, Pb, Cu, Sb, and Sn are expected to be enriched in FA. The origin of the metals in the waste input was not investigated in this study. It can be assumed that abundant metals are mainly from alloys (e.g., Zn, Sn), color pigments (e.g., Ti), or additives in plastic (e.g., Sb). Table 4 shows the annual technically possible acid leaching potential of Zn (2420 t/y), Pb (530 t/y), Cu (66 t/y), and Cd (21.8 t/y) considering the FLUWA process is optimized by using HCl and H2O2 as additives [14]. The recovery potential of metals is very high compared to the unwrought metal imported in Switzerland in 2017 [17]. Approximately 30% Zn, 16% Pb, and 1% Cu of the annual import could be replaced by metal recovery from FA. Metal prices fluctuate frequently, and future changes in waste input, e.g., by enhanced metal separation prior to combustion, could drastically change the quantity of heavy metals recoverable by FA and the economic aspects. Ecologically it is, however, beneficial to recover other metals as well, such as Sb and Sn. Heavy metals in landfills are a constant threat to the surrounding environment, especially from a long-term perspective, and primary production (mining, excavation, and extraction) have dramatic impacts on the environment and its inhabitants [18].


**Table 4.** Total amount of Zn, Pb, Cu, and Cd that is recoverable by the FLUWA process. The data of the raw import in Switzerland in 2017 only refers to the unwrought metal (no. 7901, 7801, 7403, and 8104 of the Swiss Explanatory notes of the Customs Tariff—Tares [19].
