Inventory of MSWI Fly Ash in Switzerland: Heavy Metal Recovery Potential and Their Properties for Acid Leaching

: From the year 2021 on, heavy metals from Swiss municipal solid waste incineration (MSWI) ﬂy ash (FA) must be recovered before landﬁlling. This is predominantly performed by acid leaching. As a basis for the development of deﬁned recovery rates and for the implementation of the recovery process, the authorities and plant operators need information on the geochemical properties of FA. This study provides extended chemical and mineralogical characterization of all FA produced in 29 MSWI plants in Switzerland. Acid neutralizing capacity (ANC) and metallic aluminum (Al 0 ) were additionally analyzed to estimate the e ﬀ ort for acid leaching. Results show that all FA samples are composed of similar constituents, but their content varies due to di ﬀ erences in waste input and incineration conditions. Based on their geochemical properties, the ashes could be divided into four types describing the leachability: very good (6 FA), good (10 FA), moderate (5 FA), and poor leaching potential (8 FA). Due to the large di ﬀ erences it is suggested that the required recovery rates are adjusted to the leaching potential. The quantity of heavy metals recoverable by acid leaching was estimated to be 2420 t / y Zn, 530 t / y Pb, 66 t / y Cu and 22 t / y Cd.


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][2][3][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 CaO + H 2 O → Ca(OH) 2 (1) Below a pH of~7.3, calcite is dissolved by consuming protons and releasing CO 2 in the process (Equation (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 (Al 0 ) 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 Al 0 is at the expense of metals such as Pb and Cu (Equation (4)), which are reductively cemented and removed from the leachate [8].
To prevent reductive precipitation, an oxidizing agent (e.g., H 2 O 2 ) is added during acid leaching. It is speculated that other metals such as Fe 0 and Zn 0 in FA may reduce Cu 2+ and Pb 2+ during the FLUWA process. This seems, however, to be unlikely as both elements are less reactive than Al 0 [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.

Sampling and Sample Processing
The sampling campaign was launched in December 2016 at all 29 MSWI plants in Switzerland. For three weeks, 100-200 g of FA was sampled daily to obtain a representative sample of several kilograms of FA. This timespan was chosen to avoid bias from daily fluctuation [9]. All but three MSWI plants were sampled as requested in January 2017. The others performed the sampling in the following months. One MSWI plant was not included in the study because it was shut down in 2020. The samples were split, stored in sealed bags and aliquots for XRD and XRF, and were dried at 35 and 105 • C, respectively. All FA samples were anonymized.

Chemical Analysis
Elemental composition was determined by energy dispersive X-ray fluorescence analysis (ED-XRF) using a Xepos spectrometer (SPECTRO, Kleve, Germany); with matrix adjusted calibration. The measurement was performed on pressed powder pellets (32 mm diameter) using 4.0 g of ground sample material and 0.9 g of Hoechst wax C from Merck as binder. Trace elements and rare metals were analyzed by Actlabs (Canada) by the Ultratrace5 program. Actlabs uses INAA and ICP-MS to determine the respective concentrations in the lower ppm and ppb range.

Mineralogical Analysis
Dried FA samples were mixed with 20 wt.% corundum (internal standard) and milled for 6 min using an XRD-McCrown Mill Retsch GmbH, Haan, Germany). The powder was then filled in a glass capillary (0.3 mm diameter, Hilgenberg Glass no. 10). The measurement was done at the Swiss-Norwegian Beamline (SNBL) (at the European Synchrotron Radiation Facility (ESRF) in Grenoble using a Pilatus 2M (Dectris, Baden, Switzerland) detector measuring from 0.0051 to 34.3751 [ • 2θ] with a step size of 0.01 [ • 2θ] and a scan step time of 1 s by radiating the capillary using a focused beam (diameter 100 µ) with a wavelength of 0.69264 Å and 17.9 keV. Phase identification and quantification was done using TOPAS Academics V6 (Coelho Software, Brisbane, Australia) and HighScorePlus 4.6 (Malvern Panalytical, Malvern, UK) using the Rietveld method.

Acid Neutralizing Capacity
FA (2 g) was added into 20 mL Mili-Q water (L/S ratio of 10). The suspension was then titrated in 40 steps using a 785 DPM Titrino device (Metrohm, Herisau, Switzerland) by adding every 10 min 1 mL 1M HCl under constant stirring. This procedure was determined in a previous study to best describe the behavior of FA for acid leaching conditions [14].

Metallic Aluminum
Metallic Aluminum was measured by oxidation of Al 0 with H 2 O at high pH conditions (Equation (5)) by measuring the amount of produced gas and calculating Al 0 using the ideal gas law.
Theoretically, all base metals can from hydrogen in contact with water. A solution of 100 mL 0.5 M NaOH(Merck & Co., Kenilworth, NJ, USA) was filled in a Schott laboratory bottle (1 L) from Smilax and flushed with Ar gas for~5 min to create an inert atmosphere. FA (25 g) and a magnetic stirring device (300 rpm) were added to the solution, and the bottle was immediately closed and sealed. Temperature and pressure in the thermally isolated bottle were recorded every 10 s by a P/T logger (HOBO U20-001-01, Onset, Cape Cod, MA, USA) for at least 8 h until reaction equilibrium was reached.

Cluster Analyis
To analyze similarities among FA, a data set with the obtained values of Zn, Al 0 , ANC, and the amount of produced FA was compiled. The cluster analysis was done in MATLAB (R2018a, MathWorks, Natick, MA, USA, 2018) using the linkage function to calculate an agglomerative hierarchical cluster tree.

Chemical Composition
The average chemical composition of all FA samples ( Table 2) is dominated by the oxides CaO (270,000 mg/kg), SO 3 (13,000 mg/kg), Na 2 O (100,000 mg/kg), SiO 2 (80,000 mg/kg), K 2 O (70,000 mg/kg), and Cl (130,000 mg/kg) (full details of chemical data can be found in Tables A1 and A2 and in the Supplementary Materials). Further major constituents are Al 2 O 3 (35,000 mg/kg), Fe 2 O 3 (25,000 mg/kg), P 2 O 5 (10,000 mg/kg), MgO (12,000 mg/kg), and TiO 2 (17,000 mg/kg). Of the recoverable elements, Zn (average 36,000 mg/kg) is the most abundant followed by Pb (8000 mg/kg), Cu (2000 mg/kg), and Cd (~200 mg/kg). Precious metals (e.g., Au and Ag) as well as the total content of rare earth elements (REE) show low mg/kg concentration. FA samples have similar constituents, but the content varies heavily due the different waste input ( Figure 1). Ca, the dominating contributor to ANC, varies from 150,000 to almost 400,000 mg/kg. S and Cl, which promote the transfer of heavy metals into the flue gas, scatter from 75,000 to 200,000 and 60,000 to 250,000 mg/kg, respectively. Of the total metal content (60,000 to 140,000 mg/kg), the recoverable metals are Zn (15,000-70,000 mg/kg), Pb (2500-16,000 mg/kg), Cu (1000 to 3000 mg/kg), and Cd (100-350 mg/kg). The large concentration range of Zn, Pb, Cu, and Cd indicates again the large differences in the waste input.

Mineralogical Composition
Phase analyses show that the major solid phases occur in all FA samples. On average, all samples contain an amorphous part of~41 wt.% including the minor and unidentified phases (Table 3,  a complete table can  phases in FA is indicated by the large boxes in Figure 2. Beside the main mineralogical constituents, FA samples have many minor phases (<1 wt.%), which cannot be identified or quantified properly. M NaOH(Merck & Co., Kenilworth, NJ, USA) was filled in a Schott laboratory bottle (1 l) from Smilax and flushed with Ar gas for ~5 min to create an inert atmosphere. FA (25 g) and a magnetic stirring device (300 rpm) were added to the solution, and the bottle was immediately closed and sealed. Temperature and pressure in the thermally isolated bottle were recorded every 10 s by a P/T logger (HOBO U20-001-01, Onset, Cape Cod, MA, USA) for at least 8 h until reaction equilibrium was reached.

Cluster Analyis
To analyze similarities among FA, a data set with the obtained values of Zn, Al 0 , ANC, and the amount of produced FA was compiled. The cluster analysis was done in MATLAB (R2018a, MathWorks, Natick, MA, USA, 2018) using the linkage function to calculate an agglomerative hierarchical cluster tree.

Chemical Composition
The average chemical composition of all FA samples ( Table 2) is dominated by the oxides CaO (270,000 mg/kg), SO3 (13,000 mg/kg), Na2O (100,000 mg/kg), SiO2 (80,000 mg/kg), K2O (70,000 mg/kg), and Cl (130,000 mg/kg) (full details of chemical data can be found in Tables A1 and A2 and in the Supplementary Materials). Further major constituents are Al2O3 (35,000 mg/kg), Fe2O3 (25,000 mg/kg), P2O5 (10,000 mg/kg), MgO (12,000 mg/kg), and TiO2 (17,000 mg/kg). Of the recoverable elements, Zn (average 36,000 mg/kg) is the most abundant followed by Pb (8000 mg/kg), Cu (2000 mg/kg), and Cd (~200 mg/kg). Precious metals (e.g., Au and Ag) as well as the total content of rare earth elements (REE) show low mg/kg concentration. FA samples have similar constituents, but the content varies heavily due the different waste input ( Figure 1). Ca, the dominating contributor to ANC, varies from 150,000 to almost 400,000 mg/kg. S and Cl, which promote the transfer of heavy metals into the flue gas, scatter from 75,000 to 200,000 and 60,000 to 250,000 mg/kg, respectively. Of the total metal content (60,000 to 140,000 mg/kg), the recoverable metals are Zn (15,000-70,000 mg/kg), Pb (2500-16,000 mg/kg), Cu (1000 to 3000 mg/kg), and Cd (100-350 mg/kg). The large concentration range of Zn, Pb, Cu, and Cd indicates again the large differences in the waste input.

Acid Neutralizing Capacity
The titration curves of selected FA samples are shown in Figure 3. Seven FA samples had a starting pH below 8 and reached a pH of 2 before 8 moL H + was added. Twelve samples started at pH 10-12 but dropped to a pH below 8 after the addition of a 1 moL H + . The remaining 10 samples started at pH 12 and required up to 3 moL H + before dropping below pH 8, and an additional 5-10 moL H + was needed to reach pH 2. There are three major plateaus apparent. The first one is assigned to the dissolution of portlandite (Ca(OH) 2 ). Lime (CaO) reacts immediately with water, forms portlandite, and elevates the pH above 12 (see Equation (2)). The next major step is assigned to the dissolution of calcite (CaCO 3 ,  (4)). The final major step around pH 4 marks the start of the dissolution of Ca-Si phases like belite (Ca 2 SiO 4 ) and (partly) gehlenite (Ca 2 Al 2 SiO 7 ), and other minor phases start to dissolve. Of all phases, calcite is the dominating phase of the ANC. Ca-Si phases as well as the amorphous part only dissolve partially below pH 4. The required amount of H + per kg FA to reach pH 2 with an LS of 10 is shown in Figure 4 as boxplot (a complete table can be found in the Supplementary Materials).    dissolution of calcite (CaCO3, see Equation (4)). The final major step around pH 4 marks the start of the dissolution of Ca-Si phases like belite (Ca2SiO4) and (partly) gehlenite (Ca2Al2SiO7), and other minor phases start to dissolve. Of all phases, calcite is the dominating phase of the ANC. Ca-Si phases as well as the amorphous part only dissolve partially below pH 4. The required amount of H + per kg FA to reach pH 2 with an LS of 10 is shown in Figure 4 as boxplot (a complete table can be found in the Supplementary Materials).

Metallic Aluminum Al 0
The content of Al 0 in FA is shown as a boxplot in Figure 4

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 dissolution of Ca-Si phases like belite (Ca2SiO4) and (partly) gehlenite (Ca2Al2SiO7), and other minor phases start to dissolve. Of all phases, calcite is the dominating phase of the ANC. Ca-Si phases as well as the amorphous part only dissolve partially below pH 4. The required amount of H + per kg FA to reach pH 2 with an LS of 10 is shown in Figure 4 as boxplot (a complete table can be found in the Supplementary Materials).

Metallic Aluminum Al 0
The content of Al 0 in FA is shown as a boxplot in Figure 4

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

Metallic Aluminum Al 0
The content of Al 0 in FA is shown as a boxplot in Figure 4

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). (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). 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 Al 0 (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 + 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 Al 0 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 + 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 Al 0 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 + 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 Al 0 (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 + per kg FA to reach pH 2. 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 Al 0 (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 H 2 O 2 consumption and low acid consumption (7.4 mL mol H + 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 Al 0 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 + 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 Al 0 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 + per kg FA to reach pH 2) and a high amount of H 2 O 2 . 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 Al 0 (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 + per kg FA to reach pH 2.

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].

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 H 2 O 2 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].

Characterization of FA Regarding the FLUWA Treatment
To estimate implementation planning and effort for FA leaching, the properties of all FA samples were grouped with similar properties into four different clusters. Of all MSWI plants, 16 out of 29 produce FA that shows very high or high leaching potential, and 13 MSWI plants produce FA that shows moderate or low leaching potential. FA with moderate or poor leaching potential require higher effort and has smaller return through the recovery of Zn. Thus, mixing of different FA types could be an expedient means of diminishing the effects of poor leaching potential. For optimized processing, FA composition is suggested to be monitored regularly by simple tests of ANC, Zn (Pb, Cu), and Al 0 . The cluster, however, provides preliminary information for general planning and FLUWA design.

Situation in Switzerland from 2021 on
As of 2021, there will be 29 MSWI plants in Switzerland producing~80,000 tons of FA per year that must be treated prior to disposal. Although the type of process to recover metals from FA will not be prescribed, all FA is expected to be treated by the FLUWA process, as it represents the state-of-the art process. In 2018, only 12 plants individually conducted acid leaching. The remaining plants produce approximately 43,000 t/y of FA, which has to be treated in external or newly constructed facilities. Considering the costs of 350-450 CHF per ton of FA [4], there is a market potential of CHF 15-20 million per year.

Conclusions
This study shows the wide range of chemical and mineralogical differences of the FA in Switzerland and the properties influencing acid leaching. Consequently, the effort to recover heavy metals from FA varies widely is mainly dependent on metal and Al 0 content and on the ANC. It is especially unfavorable for FA with low Zn content but high ANC to recover a fixed rate of heavy metals. This fact was considered in the implementation of the guidelines of the ADWO, which prescribes that the recovery rate of Zn and Pb to achieve is based on the initial concentration in FA [20]. After complete implementation, the FLUWA treatment of all Swiss FA will produce a significant quantity of recovered heavy metals. These quantities are otherwise disposed as pollutants in landfills and removed from the raw metal cycle. However, the remaining filter cake that is deposited still contains a significant load of contaminants, such as Sb and Sn. Further development is therefore needed to increase the recovery of heavy metals and to extend it to less easily recoverable elements.