Assessment of Valuable and Critical Elements Recovery Potential in Ashes from Processes of Solid Municipal Waste and Sewage Sludge Thermal Treatment

Due to the increasing amount of produced and accumulated wastes, a potential source of elements might be the global waste stream coming from the waste incineration process. As a result of this process, bottom ash, fly ash and air pollution control residues are produced. The goal of this study was to evaluate the raw material potential of the anthropogenic materials which are fly ashes from municipal waste incineration and municipal sewage sludge incineration, and the possibility for the recovery of metallic or other economically valuable elements by comparison of their chemical composition with the chemical composition of Earth materials (ultramafic, mafic and felsic igneous rocks, various sedimentary rocks), and with their lowest content in currently exploited ores. Fly ashes contain more valuable and critical elements when compared to Earth materials; however, they are less concentrated in comparison to the content in currently exploited ores. Since natural resources are becoming depleted, the costs of exploitation, mineral processing and related operations are increasing and the fly ashes are easily accessible. Cheap materials do not demand complicated treatment which might be considered as a future source of P, Zn, Sn, Cr, Pb, Au and Ag, and thus fulfilling the assumptions of close-loop economy and to maximize natural resources protection.


Introduction
Raw material and energy needs are increasing due to population growth, and industrial and technological development. Anthropogenic impact on the environment has been marked strongly during the industrial era, however it reached its biggest peak after World War II, when the population increase threefold and the consumption of resources increased drastically [1,2]. On one hand, there is the increasing consumption of everyday goods still going on, that require raw materials for their production, while on the other we observe a constant decrease in natural raw material resources, which is one of the likely problems in the Anthropocene. We are obliged to protect natural resources and manage them rationally and efficiently to fulfil requirements of the closed-loop economy.
In order to maintain economic growth and improve the quality of life for European Union (EU) citizens, raw materials are indispensable for the European economy [3]. Meanwhile, the demand for industrially important elements will increase, as suggested by [4]. It is thus essential to secure these them without the specific treatment of ashes, which often require additional financial expense or planning for the most effective means of recycling. In particular, we must keep in mind that the amount of FA, even if lower than BA, increases every year and is expected to continue to increase. Because of elements fractionation during the waste incineration process, the degree of concentration of various elements in FA and BA differs significantly. The form of occurrence of BA and FA imply different methods of their processing.
A proper evaluation of the possible recovery of metals or other valuable elements from FA is important before they are used for other applications (e.g., soil treatment, production of building materials and road construction) to avoid their dispersion. It also supports one of the other strategies of the circular economy, which is urban mining, in which as little waste as possible is generated in the Earth system [22].

Materials and Methods
For the comparison, three different FAs produced in three waste-to-energy (WtE) incineration plants in Poland where different technologies are used were taken into consideration. Samples were collected from two municipal waste incineration plants (equipped with grate furnaces) and one sewage sludge incineration plant (equipped with a fluidised bed boiler).

Sewage Sludge Incineration Technology
Sludge from the wastewater treatment plant is transported to the node and the sludge membrane drying system, where the sludge is dried to 36% of the dry mass. The dried sludge is incinerated in the fluidised bed boiler (Pyrofluid™), which provides high turbulence of the fluidised bed at a constant intensity and a stable operating temperature in the range of 850-900 • C to ensure the complete incineration of organic matter. A thermal utilisation station is equipped with a heat exchanger, responsible for pre-cooling the flue gases and the production of saturated steam to both power the drying node and to produce electricity for use in the facility to reduce the cost of operation. Dewatered sludge pumped into the boiler is dried out. As a consequence of the turbulence streams in the fluidised bed, the sludge disintegrates, the organic matter is burnt and heat-resistant and volatile components are released. Heavier components and the incombustible fraction are separated in a multicyclone, captured on an electrostatic precipitator and transported in the form of FA into the ash silo, whereas lighter components and the products of flue gas cleaning (APC residues) are caught up in a bag filter after reactant addition and subjected to further processing. For the purification process of flue gases, NaHCO 3 is added (for more details, see [18]. The annual production of FA in the plant comprises 4452 tonnes [18]. Six samples of FA (referred to as FA1), each with an average weight of 10 kg, were collected during six sampling campaigns.

Municipal Waste Incineration Plant 1
In this municipal waste thermal treatment plant, waste collected from a city of a population of almost 2 million inhabitants is mechanically mixed prior to the incineration and transported directly to the furnace where it is thermally treated for 30-120 min on the grate in the shaft furnace (Krüger, Denmark; W-MARK 5 shaft type) at 850-1150 • C (usually >950 • C). BAs are captured at the end of the grate system, cooled with water, processed and stored on a heap for ageing. Flue gas from the furnace resides for 2 sec at 1050 • C in the afterburner chamber just before its transportation to the recovery boiler, where after the non-catalytic reduction of NOx, the thermal energy is recovered, and the gas is cooled to~150 • C. At this stage of the flue gas cleaning system, the first portion of ash is separated (FA). In the next step, in the flue gas collector, fine-grained Ca(OH) 2 is added (10-13 kg/t of waste) to neutralise the SOx, HCl and HF from the fumes. Subsequently, the APC residues are captured using bag filters. In the last stage, toxic metals, dioxins, furans and organic components are absorbed from the flue gases using activated coke in a counter-current adsorber. The annual production of FA in this Resources 2020, 9, 131 4 of 20 facility comprises 225 tonnes Four samples of FA (referred to as FA2), each with an average weight of 10 kg, were collected during four sampling campaigns.

Municipal Waste Incineration Plant 2
The thermal treatment of municipal waste collected from a city of a population of ca. 1 million inhabitants is performed in a grate furnace. In the initial zone of the grate, the waste is heated by radiation or convection to a temperature slightly above 100 • C. This process leads to moisture evaporation. Then, the waste is heated to above 250 • C, while volatile components such as moisture and gases are released. In the third zone of the grate, complete waste incineration occurs. The loss on ignition in this zone is lower than 0.5% of the mass. In the gasification process, volatile products are oxidised by molecular oxygen. Most of the waste is oxidised at 1000 • C in the upper zone of the incineration chamber. In the post-combustion zone, the unburned CO in the exhaust gas is minimised. In this zone, secondary air is provided for complete combustion. The residence time of the flue gas is around 2 s at a minimum temperature of 850 • C. Residues from the APC system were collected from the post-combustion ash after the selective non-catalytic reduction (SNCR) system that contains an economiser, a reactor after hydrated lime and activated carbon injection, and from bag filters. The annual production of FA comprises 15,400 tonnes; however, it is important to mention that in this incineration plant FA and APC residues are mixed together and treated as FA. Therefore, its volume is much larger than in other municipal waste incineration plants where these products are gathered separately. Ten samples of FA (referred to as FA3), with an average weight of 10 kg, were collected during three sampling campaigns. It is important to mention that during the first sampling, the samples were collected after the selective non-catalytic reduction system, the economiser, the reactor after hydrated lime and activated carbon injection, and from the bag filters from two independent production lines; whereas for the two other samplings, four types of ashes were mixed together and stored in a silo.

Analytical Methods
To determine the chemical composition of the FA, inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were performed in Bureau Veritas Minerals (formerly AcmeLabs Analytical Laboratories) in Vancouver, Canada. Analyses allowed the determination of the content of major and trace elements, respectively using LF2000 and AQ200 analyses set. In addition, the total carbon content (Ctot) and total sulphur content (Stot) content were measured using LECO combustion analyses based on infrared spectroscopy (using TC000 analyses set). Loss on ignition was obtained using thermal methods. The results of the chemical analyses of the minimum, maximum and average composition of FA1, FA2 and FA3 (based on 6, 4 and 8 samples, respectively) are listed in Table 1.
The grain size distribution of FA was performed using a Mastersizer 3000 laser diffractometer with a Hydro EV dispersion unit (Malvern, UK). The time of a single measurement was set to 60 s (3 × 10 s for red light with wavelength λ = 632.8 nm and 3 × 10 s for blue light with λ = 470 nm). For calculating the particle size, the Mie theory was used with refractive index = 1.543 and absorption index = 0.01 for quartz as the reference material. Water with refractory index = 1.330 was used as a diffuser. The results were based on the volume distribution at obscurance within the range of 0.1-20% (average 10%).
The results of the chemical analyses were averaged for each FA material and compared to the average composition of ultramafic rock, basalt, high Ca-granite, low Ca-granite, average continental crust composition, tonalite, sandstone, greywacke, shale, carbonate rock and deep-sea clay. The chemical composition of Earth materials was based on the literature [23][24][25]. In addition, the results of the chemical analyses of the averaged FA compositions were compared to the minimum concentrations of ores currently exploited worldwide [26,27] and references therein.
In order to describe the mineral composition of the FA samples, X-ray diffraction (XRD) analyses were applied using a Philips X'Pert (APD type) diffractometer with a PW 3020 vertical goniometer equipped with a curved graphite crystal monochromator (CuKα radiation, analytical range 2-64 • 2Θ, Resources 2020, 9, 131 5 of 20 step 0.02 • , counting time 2 s/step). Phase compositions were identified using Philips X'Pert software (associated with the ICDD database). n-number of samples analysed.    The main minerals in FA1 determined in the XRD analyses were quartz, feldspar, hematite, whitlockite and Fe-PO4 [18]; while anhydrite, calcite, quartz, and minor amounts of halite, periclase, melilite group minerals and larnite [28] were present in FA2; and in FA3, quartz, feldspar, calcite, anhydrite, gypsum, portlandite and mullite were found. Apart from the crystalline phases in all of the FA samples, a high amount of amorphous phase was also present.

Grain Size Distribution and Mineral Composition
The averaged concentration of major and minor elements in the ashes varied in a wide range between samples (Table 1); the variation range for individual samples in each group is however not so wide. Variation of the composition of individual samples from different sampling periods indicates significant temporal variation of the composition of incinerated wastes. The variation in the chemical composition of municipal waste incineration residues from the two towns is probably related to differences in waste collection and segregation processes, and to differences in living standards and consumption habits. Both of these factors make the potential recovery of elements from incineration residues troublesome. The main minerals in FA1 determined in the XRD analyses were quartz, feldspar, hematite, whitlockite and Fe-PO 4 [18]; while anhydrite, calcite, quartz, and minor amounts of halite, periclase, melilite group minerals and larnite [28] were present in FA2; and in FA3, quartz, feldspar, calcite, anhydrite, gypsum, portlandite and mullite were found. Apart from the crystalline phases in all of the FA samples, a high amount of amorphous phase was also present.
The averaged concentration of major and minor elements in the ashes varied in a wide range between samples (Table 1); the variation range for individual samples in each group is however not so wide. Variation of the composition of individual samples from different sampling periods indicates significant temporal variation of the composition of incinerated wastes. The variation in the chemical composition of municipal waste incineration residues from the two towns is probably related to differences in waste collection and segregation processes, and to differences in living standards and consumption habits. Both of these factors make the potential recovery of elements from incineration residues troublesome.
Calculation of the CIPW norms [29] from bulk chemical analyses for comparison with typical rocks indicated the presence of normative apatite, quartz, feldspar and hematite in FA1. In the municipal waste FA (FA2 and FA3), larnite and feldspathoids were calculated as the main normative components. This difference between the normative and modal composition of samples could be related to the fact that the Stot and Ctot contents were excluded from the CIPW calculations, resulting in a lack of carbonates and sulphates that are the main minerals present in municipal waste incineration ashes, in combination with excess Ca with Si (normative larnite), and therefore a lack of normative quartz.

FA in the Rock Classification Scheme
Due to the fact that the FA are characterised by unusual-to-natural rock chemical composition (low SiO 2 content), high Ca content in all samples, and very high P content in FA1 that were not included in the calculation, it was impossible to classify them using the total-alkali-silica (TAS) diagram ( Figure 2). Thus, it was not possible using the main rock classification commonly used in geological studies to classify the FA in any field affiliated to the common igneous rocks. The molar ratio of CaO to the sum of Na 2 O and K 2 O for all samples is between 6.69 and 6.80.
Resources 2020, 9, x FOR PEER REVIEW 7 of 18

FA in the Rock Classification Scheme
Due to the fact that the FA are characterised by unusual-to-natural rock chemical composition (low SiO2 content), high Ca content in all samples, and very high P content in FA1 that were not included in the calculation, it was impossible to classify them using the total-alkali-silica (TAS) diagram ( Figure 2). Thus, it was not possible using the main rock classification commonly used in geological studies to classify the FA in any field affiliated to the common igneous rocks. The molar ratio of CaO to the sum of Na2O and K2O for all samples is between 6.69 and 6.80.

Chemical Composition of FA in Comparison with Rocks And ores
The average content of the SiO2 in FA (37.5 wt%, 18.2 wt% and 30.9 wt% for FA1, FA2 and FA3, respectively) is lower than in most of the rocks ( Table 2). The lower concentrations were only observed for carbonate rocks and shales. The MgO, MnO, Na2O and K2O average concentrations in FAs are low, and within the range of the average concentration in the rocks. The Al2O3 concentration is approximately 8 wt% for each FA. The content of Fe2O3 is much higher in the municipal waste FA than in most of the considered rocks, whereas in the sewage sludge FA, the iron concentration (14.3 wt%) is as high as that found in ultramafic rocks and basalts, and three times higher than in the FA from municipal waste incineration. Moreover, [30] suggested that Fe and Al oxides present in FA bind potentially toxic elements such as Sb, As, Be, Cd, Pb, Hg and Se, which can have a negative environmental impact due to mobilisation. Furthermore, the TiO2 average content is higher than in most rocks, and close to the average content in basalts, even though the overall content is 1 wt%, 1.9 wt% and 1.4 wt% for FA1, FA2 and FA3, respectively. CaO content in the FA is high (12.1 wt% in FA1, 36.6 wt% in FA2, and 28.3 wt% in FA3), lower only than the Ca concentration in carbonate rocks (42.3 wt%). The P2O5 content is four times higher in the FA2 and FA3 than in the rocks, whereas in the FA1, this concentration is 20 times higher.

Chemical Composition of FA in Comparison with Rocks And ores
The average content of the SiO 2 in FA (37.5 wt%, 18.2 wt% and 30.9 wt% for FA1, FA2 and FA3, respectively) is lower than in most of the rocks ( Table 2). The lower concentrations were only observed for carbonate rocks and shales. The MgO, MnO, Na 2 O and K 2 O average concentrations in FAs are low, and within the range of the average concentration in the rocks. The Al 2 O 3 concentration is approximately 8 wt% for each FA. The content of Fe 2 O 3 is much higher in the municipal waste FA than in most of the considered rocks, whereas in the sewage sludge FA, the iron concentration (14.3 wt%) is as high as that found in ultramafic rocks and basalts, and three times higher than in the FA from municipal waste incineration. Moreover, [30] suggested that Fe and Al oxides present in FA bind potentially toxic elements such as Sb, As, Be, Cd, Pb, Hg and Se, which can have a negative environmental impact due to mobilisation. Furthermore, the TiO 2 average content is higher than in most rocks, and close to the average content in basalts, even though the overall content is 1 wt%, Resources 2020, 9, 131 8 of 20 1.9 wt% and 1.4 wt% for FA1, FA2 and FA3, respectively. CaO content in the FA is high (12.1 wt% in FA1, 36.6 wt% in FA2, and 28.3 wt% in FA3), lower only than the Ca concentration in carbonate rocks (42.3 wt%). The P 2 O 5 content is four times higher in the FA2 and FA3 than in the rocks, whereas in the FA1, this concentration is 20 times higher.
The average concentrations of industrially important metals and elements of typical environmental concerns are higher in the FA than in the rocks, except for Ba, Ni, V, Tl, Co, Ga, Nb, Ta and Hg, where the average concentrations are within the range of the average in different rocks (Table 3). A high proportion of Hg was measured in FA3, which was 100-fold higher than in the ores (Figure 3). Elevated concentrations of Hg can be related to the presence of used or broken thermometers, batteries or electronic components [31] not being separated from the municipal waste before incineration.     Ten times higher concentrations were measured for Cr, Mo, Pb and Be in the FA compared to the rocks, and elevated concentrations of As in comparison to the rocks except for the deep-sea clay and shell. The Au and Sb concentrations vary for different FA but are 10-100 times higher than in the rocks. Cd, Cu, Se, Sn and Ag concentrations are 100 times higher in the FA than in the rocks, whereas the Zn content is 1000 times higher than in the rocks (Table 3). Zn is not only used as a corrosion protection layer covering metal products [32], but it is also one of the main alloy components in brass [33], a commonly used household material. Besides the usages of metallic zinc, zinc oxide is used for various purposes, such as animal feed, ceramics, chemicals, pharmaceuticals (in particular, sunscreens and ointment), pneumatic tyres, and so forth [33].
The concentration of rare earth elements (REE) in the FA is quite low. The total content of these elements in municipal waste incineration does not exceed 75 ppm in FA3 and 107 ppm in FA2 ( Table 4). The highest total content of REE was measured in FA1 at over 120 ppm, with these concentrations only higher in the deep-sea clays, carbonate rocks and ultramafic and mafic rocks. Since the main components of FA are aluminium silicates (glass), silicates (zircon), phosphates (apatite, monazite, xenotime) and (hydr)oxides (Fe-(hydr)oxides), we can assume that these phases are the main carriers for REE. Additionally, [34] found a positive correlation between P 2 O 5 , SiO 2 , Al 2 O 3 and Fe 2 O 3 and the occurrence of REE in municipal waste incineration products, which supports this assumption. Table 3. Average content of other metals and semimetals in FA compared to the average content in rocks and currently exploited ores.

Ag
As   The average content of metals and critical elements were also compared to the minimal profitable content in the currently exploited ores. As seen in Figures 3-5, in the studied FA, only the content of Ba, Cr, Sb and Ce are above values typical for ores. For the other elements, the average concentrations in the studied FA are much lower, indicating that their recovery is problematic. Nevertheless, their fine-grained nature allows us to consider their direct use in the enrichment process, which might raise the content of valuable elements in the concentrates. In addition, easy access to already produced material, which will cause no additional production or preparation costs, makes it reasonable to consider them for direct usage. Resources 2020, 9,   The phosphorus content is very high in the sewage sludge ash (~17 wt% of P2O5), not only in comparison to the average rock composition, but also in comparison to the minimal profitable content and maximal content in the currently exploited raw materials ( Figure 6). The P2O5 content in phosphate ores varies from 2-6% to 25-34%, depending on the processing methods, mining and geological conditions, and other factors that place sewage sludge ash at the level of the medium grade ores if used directly without processing for enrichment. The phosphorus content is very high in the sewage sludge ash (~17 wt% of P 2 O 5 ), not only in comparison to the average rock composition, but also in comparison to the minimal profitable content and maximal content in the currently exploited raw materials ( Figure 6). The P 2 O 5 content in phosphate ores varies from 2-6% to 25-34%, depending on the processing methods, mining and geological conditions, and other factors that place sewage sludge ash at the level of the medium grade ores if used directly without processing for enrichment. Resources 2020, 9, x FOR PEER REVIEW 12 of 18

Possible Applications of FA
Due to the fact that FA, and especially those from sewage sludge incineration, contain elements which are micro-(e.g., Fe) and macro-nutrients (P, Ca, Mg, K, Al), these could be utilised for plant growth [35]. However, exceeding the concentration limits of Cr and Zn excludes this material from direct use in agriculture, in accordance with the requirements of the EU directive ( Figure 7A; Table 5 [36]). For solid organic-mineral fertilisers, the standards for Cr and Ni are exceeded ( Figure 7C, Table  5 [37]), but all standards for solid mineral fertilisers are fulfilled in the case of FA in accordance with Polish legislation ( Figure 7B; Table 5 [37]).  n.d-no data. The high content of Fe2O3 and Al2O3 enables us to consider them as a base material for zeolite synthesis [38]. The chemical composition (major components) and the particle size allow FA to be

Possible Applications of FA
Due to the fact that FA, and especially those from sewage sludge incineration, contain elements which are micro-(e.g., Fe) and macro-nutrients (P, Ca, Mg, K, Al), these could be utilised for plant growth [35]. However, exceeding the concentration limits of Cr and Zn excludes this material from direct use in agriculture, in accordance with the requirements of the EU directive ( Figure 7A; Table 5 [36]). For solid organic-mineral fertilisers, the standards for Cr and Ni are exceeded ( Figure 7C, Table 5 [37]), but all standards for solid mineral fertilisers are fulfilled in the case of FA in accordance with Polish legislation ( Figure 7B; Table 5 [37]). The high content of Fe 2 O 3 and Al 2 O 3 enables us to consider them as a base material for zeolite synthesis [38]. The chemical composition (major components) and the particle size allow FA to be treated as sorbents [35]. Moreover, their application in cement production, road construction and brick material is possible [39], although the utilisation of FA in the aforementioned applications may be limited because of the content of minor components with a negative environmental impact or which influence the properties of the products. The recovery of valuable elements or other applications for FA could be competitive and the method of FA utilisation should be based on a comprehensive evaluation of the benefits.
Resources 2020, 9, x FOR PEER REVIEW 14 of 18 treated as sorbents [35]. Moreover, their application in cement production, road construction and brick material is possible [39], although the utilisation of FA in the aforementioned applications may be limited because of the content of minor components with a negative environmental impact or which influence the properties of the products. The recovery of valuable elements or other applications for FA could be competitive and the method of FA utilisation should be based on a comprehensive evaluation of the benefits.