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Complete determination of the material composition of municipal solid waste incineration bottom ash
Waste Management 102 (2020) 677–685
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Review
Complete determination of the material composition of municipal solid waste incineration bottom ash Florian Huber ⇑, Dominik Blasenbauer, Philipp Aschenbrenner, Johann Fellner TU Wien, Institute for Water Quality and Resource Management, Karlsplatz 13/226, 1040 Vienna, Austria
a r t i c l e
i n f o
Article history: Received 2 March 2019 Revised 25 September 2019 Accepted 21 November 2019
Keywords: Bottom ash Waste incineration Combustion residues Sorting analysis Material composition Particle size distribution
a b s t r a c t Bottom ash from waste incineration is heterogeneous and contains different materials. Previous studies on the material composition of bottom ash provide only limited information as to composition, because large pieces present in bottom ash were not investigated nor were all materials were separated and analysed. The objective of the present study is to provide the complete and detailed composition of bottom ash encompassing and extensive range of different materials. Altogether, nine bottom ash samples with a mass of 3000 kg each were sieved to eight size fractions, whereby small particles adhering to larger pieces were separated by water and added to the respective size fractions. In the sorting analysis of all size fractions, the materials enclosed in molten mineral material and materials present as composites (e.g. transformers and batteries) were considered. The material characterisation revealed that the size fraction > 50 mm contains most of the iron (up to 50% of the total iron) and copper (about 20% of the total copper), while batteries, coins, silver and gold are almost exclusively present between 16 and 50 mm. The fractions between 8 and 16 mm show the highest share of aluminium (up to 50% of the total aluminium) and glass (up to 60% of the total glass). While the metal content is underestimated, if large pieces of material are disregarded, the multi-step approach applied in this study enables a complete determination of materials in bottom ash, which is essential for optimising material recovery in bottom ash treatment. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The incineration of municipal solid waste coupled with energy recovery constitutes a key element of sustainable waste management (Brunner and Rechberger, 2015). MSWI (municipal solid waste incineration) significantly reduces the mass and volume of solid waste, but the residues remaining after incineration still have to be managed. Small particles and volatile substances are carried over from the combustion chamber to the APC (air pollution control) system, where they are separated from the flue gas stream. The solid residues generated thereby are referred to as fly ash, wet scrubber residue or dry/semi-dry process residue depending on the APC system used and make up about 3–4% of the waste input in the MSWI process (Chandler et al., 1997). Most solids, however, arise as bottom ash, which represents about 20–25% of the waste input (Morf et al., 2000). While APC residues contain almost exclusively mineral material and have a typical particle size below 1 mm (Buha et al., 2014; Li et al., 2018), pieces consisting of different materials (e.g. minerals, glass, metals) with particle sizes between several mm and m can be found in MSWI bottom ash. ⇑ Corresponding author. E-mail address: [email protected] (F. Huber). https://doi.org/10.1016/j.wasman.2019.11.036 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
Metals can be separated from MSWI bottom ash during bottom ash treatment and subsequently utilised as a secondary raw material in the metal industry (Bunge, 2016; Holm and Simon, 2017). In recent years, there have also been first attempts to separate glass from the bulk of MSWI bottom ash (Makari, 2014; TB Hauer, 2013). The glass fraction could for example be used for foam glass production, while the mineral fraction present in bottom ash is either disposed of on landfills or can be used as an aggregate, mainly in road construction (Huber and Fellner, 2018; Lynn et al., 2017; Olsson et al., 2006). Information about the content of the different materials in MSWI bottom ash is crucial as it determines the resource potential of this residue and can also facilitate the assessment of separate waste collection schemes (del Valle-Zermeño et al., 2017). Several studies have already investigated the different material fractions present in MSWI bottom ash. Chimenos et al. (1999) determined the particle size distribution of MSWI bottom ash from two different MSWI plants in Catalonia, Spain. They took several sample increments with a mass of 2 kg and sorted the material into the fractions glass, minerals, magnetic metals, non-magnetic metals and unburnt organic matter. Later, bottom ash from the same region was characterised again by del Valle-Zermeño et al. (2017). In this study, an increment mass of 5 kg was used and only
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the size fractions between 2 and 30 mm were characterised. As minerals > 30 mm were crushed and added to the material < 30 mm, the samples analysed by del Valle-Zermeño et al. (2017) are not bottom ash as it is generated by MSWI but an intermediate product from bottom ash treatment with a higher content of mineral material. Šyc et al. (2018) characterised MSWI bottom ash from Czech Republic with regard to the content of minerals, glass, magnetic metals, non-magnetic metals and unburnt organic matter. They used sample increments of about 1 kg for their analysis. Muchova (2010) analysed the content of magnetic material, aluminium, heavy non-ferrous metals and unburnt organic matter in the MSWI bottom ash fraction 0–40 mm. Berkhout et al. (2011) analysed the metal concentrate and the metal-depleted bottom ash generated from Dutch MSWI bottom ash by an eddy current separator for the content of aluminium and non-ferrous metals. They used a sample size of 25 kg for the metal concentrate and 100 kg for the metal-depleted bottom ash. Biganzoli et al. (2013) determined the content of metallic aluminium in MSWI bottom ash from Italy by using a sample size of 5–15 kg. Forteza et al. (2004) analysed MSWI bottom ash < 50 mm after the separation of magnetic metals with regard to total and leachate contents of heavy metals. Their results indicate that aged bottom ash can be used in for road construction. Xia et al. (2017) analysed the chemical composition of MSWI bottom ash samples from China with regard to different particle size fractions. According to this study, most of the metals are present in the particle size fraction < 3 mm. However, none of the studies mentioned above provides the total amount of material fractions in MSWI bottom ash, e.g. because large pieces are neglected either directly or indirectly by choosing an increment mass below the mass of large bottom ash pieces (i.e. pieces with a mass of up to 15 kg). Furthermore, metals and unburnt organic matter that are incorporated in the mineral fraction are not fully considered in the literature. Other studies, like the work of Allegrini et al. (2014) determined the metallic content of iron, aluminium and heavy non-ferrous metal scrap in bottom ash taking account of all particle sizes. However, their analysis did not allocate the total content to different size fractions. The objective of the present study is therefore to generate more complete and detailed data on the constitution of MSWI bottom ash with regard to different material fractions by considering all particle sizes and materials and by using sample increments that account also for large pieces present in MSWI bottom ash.
2. Materials and methods 2.1. Sampling of MSWI bottom ash The bottom ash from three MSWI plants in Vienna was sampled. All plants are equipped with grate furnaces and wet bottom ash discharge and they differ in the waste types thermally treated. The mass fractions of different wastes in the feed of the three MSWI plants are given in Table 1. In Plant A a magnet separator
Table 1 Mass fractions of different wastes in the feed of the MSWI plants in 2017. Mass fraction [%]
Plant A
Plant B
Plant C
Mixed municipal waste Materials derived from waste sorting and processing Bulky waste Other waste
52.9 17.1
82.9 9.7
91.9 1.2
14.4 15.6
0.2 7.1
0.1 6.8
is installed downstream of the bottom ash discharge for the removal of iron scrap contrary to Plant B and Plant C. The minimum sample mass M for the bottom ash was calculated according to Eq. (1), which was developed by Skutan and Brunner (2005) based on Bunge and Bunge (1999) in order to account for the low sphericity of most bottom ash particles. The calculation was performed for Cu as analyte and it was assumed that the mass fraction of Cu is identical to the mass fraction of Cu carrier particles, assuming that all Cu carrier particles consist of pure Cu. Furthermore, it was assumed that the average mass fraction xi of Cu is 8 mg/g, mi max is 1000 g and mi 10% is 50 g based on Bunge (2016), Morf et al. (2013), and Skutan and Rechberger (2007). The desired standard deviation si was selected as 15%. The calculation according to Eq. (1) taking into account the above-mentioned assumption results in a minimum sample mass for the bottom ash of 3000 kg, which was used for the sampling. Elements with a higher heterogeneity in bottom ash than Cu have an accepted sampling error higher than 15% (e.g. Ag, Cd, Tl) and elements with a lower heterogeneity have a lower accepted sampling error (e.g. Ca, Fe, Si). 2
M¼
1 ci 1 pffiffiffiffiffiffiffiffiffi mimax mimax xi si 1 þ 3 log p33 ffiffiffiffiffiffiffiffi ffi m
ð1Þ
i10%
M
minimum sample mass [g]
xi mass fraction of carrier particles [g/g]
c i expected mass fraction of the analyte [mg/g] si desired standard deviation [mg/kg] mi max mass of the heaviest carrier particle mi 10% maximal mass of the 10% of the lightest carrier particles For each MSWI plant three random samples with a mass of 20,000 kg were taken, which resulted in a total number of nine bottom ash samples. The sampling took place in different seasons and on different days of the week from November 2017 to November 2018. The material sampled in the present study corresponds to the fresh (i.e. not aged) bottom ash that is transported from the MSWI plant to the bottom ash treatment plant. The sampling procedure is shown in Fig. 1 and is described in the following text. The bottom ash samples were spread on a firm ground in substantially rectangular shape with a height of about 0.3 m by a wheel loader. As the average bulk density of bottom ash is about 1400 kg/m3 (Lynn et al., 2017), the area of the rectangle was approximately 50 m2. Subsequently, shovels and wheelbarrows were used to take subsample increments of about 75 kg from the edge of the spread bottom ash. In order to make the inner part of the bottom ash rectangle also accessible for sampling, a wheel loader was used to decrease the size of the bottom ash rectangles in several steps, so that new edges were generated, from which subsample increments could also be taken. Altogether, 9 subsamples with a total mass of 3000 kg each were produced for further processing in this way. 2.2. Sieving and washing of MSWI bottom ash The total subsample mass of 3000 kg was sieved and washed according to the steps given in Fig. 2, aiming to separate the bottom ash into the following grain sizes: <0.5 mm, 0.5–2 mm, 2– 4 mm, 4–8 mm, 8–12 mm, 12–16 mm, 16–50 mm and > 50 mm. Furthermore, soluble substances were collected in the washing water. In a first step, the subsamples were sieved using a 16 mm sieve. The fraction > 16 mm was sieved again using a 50 mm sieve. The fraction 0–16 mm was spread and increments of about 75 kg were taken to generate a subsample with a total mass of 800 kg. This
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Fig. 1. Sampling procedure from top view. The black area represents the MSWI bottom ash spread in substantially rectangular shape. Sample increments were taken from the edge and segments of bottom ash were moved away by a wheel-loader in several steps. The grey area shows the bottom ash that was removed in the respective step and the white area shows the bottom ash that was removed in a previous step.
Fig. 2. Flow chart of all sieving, washing, sorting and comminution steps applied to the sample of 3000 kg of MSWI bottom ash. The fraction > 50 mm (A) was processed in the same way as the fraction 16–50 mm (B) but separately. The fractions 12–16 mm (C), 8–12 mm (D) and 4–8 mm (E) were processed in the same way but separately. All six washing solutions generated (including the fine fractions) were processed in the same way but separately.
subsample was further sieved by a 12 mm sieve, 8 mm sieve and 4 mm sieve. The masses of all size fractions were recorded and the dry matter content was determined after drying samples at 105 °C until mass constancy was reached.
The size fraction 0–4 mm was further fractionated by a 2 mm sieve and a 0.5 mm sieve under a flow of water from a nozzle. The flow of water was necessary to separate small particles adhering to larger particles. The fine particles were also washed from the
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size fractions > 4 mm in a similar manner. Thereto, the samples were spread on a 4 mm sieve with a 2 mm sieve and 0.5 mm sieve below and sieved under a flow of water from a nozzle. On average about 5–10 L of water were applied per kg of bottom ash. To separate the suspensions into solid particles between 0 and 0.5 mm and liquid solutions, a Büchner funnel equipped with paper filters was used. The masses of the solutions including their TDS (total dissolved solids) content were determined to assess the fraction of MSWI bottom ash dissolved during the process. The dry mass of all size fractions was determined by drying at 105 °C until mass constancy was reached. The procedure of sieving and washing MSWI bottom ash samples described above allowed the mass and particle size distribution of small particles adhering to larger particles in MSWI bottom ash as well as the mass solubilised during washing to be determined. The masses of these fine particles washed from larger particles were added to the masses of their respective grain size fractions. The chemical composition of the different size fractions and also of the easily soluble material dissolved during the washing procedure are described in detail in a subsequent study (Huber et al., 2019). 2.3. Sorting analysis The size fractions of washed bottom ash used for the sorting analyses are summarised in Table 2. All sorting operations were done by hand based on optical appearance, elasticity, hardness, density and magnetism with the assistance of files for removing the oxidised layer on metal pieces and of magnets for detecting magnetic iron. During sorting the following materials were distinguished: minerals, glass, unburnt organic matter, metals and batteries. Shiny, ductile materials were identified as metals. Metals were further sorted into magnetic iron (i.e. mostly unalloyed steel), nonmagnetic iron (i.e. mostly stainless steel), aluminium, copper, brass, coins, silver, gold and pieces made of more than one metal (mostly transformers and electric motors) according to colour and magnetism. Coins were further sorted according to currency and denomination and the chemical composition of every single coin was determined based on information from the respective mints. The transformers and electric motors were cut by an angle grinder and subsequently separated into copper, magnetic iron and aluminium. Batteries were cut open by an angle grinder and separated into minerals, magnetic iron and brass. The minerals found in MSWI bottom ash are either pieces like ceramics, stones or concrete that are not changed by the incineration process or agglomerates of material melted together during incineration (also referred to as secondary glass). The phase composition of agglomerates present in bottom ash was investigated by Alam et al. (2019). These agglomerates contain smaller pieces
Table 2 Masses used for the sorting analysis. The minimum sample masses were calculated as described in Section 2.1 with lower mi max and mi 10%. The average mass of material > 16 mm used for sorting can be calculated based on the particle size distribution (see Fig. 3). Size fraction
Mass used for sorting [kg]
mi
>50 mm
Everything contained in 3.000 kg of bottom ash Everything contained in 3.000 kg of bottom ash 13 6 3 1.5 No sorting possible for this fraction No sorting possible for this fraction
1000
50
110
5.5
3.6 1.5 0.5 0.007 – –
0.18 0.075 0.025 0.00035 – –
16–50 mm 12–16 mm 8–12 mm 4–8 mm 2–4 mm 0.5–2 mm 0–0.5 mm
max
[g]
mi
10%
[g]
of all fractions mentioned above. In order to liberate the metals and unburnt organic matter from these agglomerates, the mineral fractions > 50 mm and 16–50 mm were spread on a steel plate and subsequently crushed by running over the material with a vibrating roller (JCB VMD 70). The crushed material was sieved by a 16 mm sieve. The fractions 0–16 mm were split to receive subsamples with a mass of 13 kg, while the fractions > 16 mm were completely used for further processing. All crushed samples were sorted again as described above and dried at 105 °C. The mineral fractions obtained after sorting of the crushed samples and of the bottom ash size fractions between 0.5 mm and 16 mm were milled using a disc mill (FLSmidth Essa LM201) and subsequently sieved with a 2 mm sieve and a 0.5 mm sieve. The fractions 0–0.5 mm obtained by this procedure were regarded as mineral material. The fractions 0.5–2 mm were mixtures from different metals, but could not be sorted due to their small particle size. The fractions > 2 mm were again sorted to the above mentioned categories (different metals and unburnt material). Thus, the complete amount of metals and unburnt organic matter > 2 mm present in MSWI bottom ash could be determined. Advantageously, applying the procedure described above, all metallic pieces are correctly sorted as metal, even if they are erroneously sorted in the mineral fraction in the first sorting step, since, the material is sorted again after crushing and yet again after milling. This is especially relevant for aluminium as pieces of molten aluminium might sometimes look similar to stones. 2.4. Chemical analysis The size fractions 0.5–2 mm obtained after sieving the milled material, which only contain metals as described in 2.3, could not be sorted. Hence, this material was solubilised by a two-step open digestion without external heating. In the first step, iron and aluminium were dissolved by adding concentrated HCl (mass fraction 0.37) until no further reaction was visible. The solution from the first step was decanted into a volumetric flask. In the second step, the sample was treated with concentrated HNO3 (mass fraction 0.65, added until no further reaction was visible) to dissolve remaining metals like copper and brass. The solution from the second step was decanted into the volumetric flask and thereby combined with the solution from the first step. Then, the volumetric flask was filled to a volume of 1 L. The solutions obtained were analysed by a PerkinElmer Optima 8300 ICP-OES (inductively coupled plasma optical emission spectroscopy) equipped with a SC-2 DX FAST sample preparation system to determine the total content of Al, Fe, Cu and Zn in these fractions. The elements were determined via axial view and triple determination followed by an arithmetic averaging. A customised single-element (Merck, Roth) standard was used for the calibration. As Cu is present as pure copper or as an alloying element in brass, the contents of brass and copper in the samples were calculated according to Eqs. (2) and (3). It was assumed that brass has an average Zn mass fraction of 0.36 and an average Cu mass fraction of 0.63. The remaining mass (mass fraction 0.01) was assumed to be Pb. This composition of brass is based on the information provided by the Deutsches Kupferinstitut (2007) and is in agreement with the results from Taverna et al. (2010). The only metallic material containing Zn that were found in the sorting analysis are brass and coins. It was assumed that all Zn present in the size fractions 0.5–2 mm obtained after sieving the milled material is a constituent of brass, as all coins are considerably larger than 2 mm.
wbrass ¼
wZn 0:36
wcopper ¼ wCu 0:63 wbrass
ð2Þ ð3Þ
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3.2. Material analysis
wZn mass fraction of Zn as determined by ICP-OES wCu mass fraction of Cu as determined by ICP-OES wbrass mass fraction of brass wcopper mass fraction of pure copper As the wear resistance of aluminium is particularly low (Torrance, 2005), abrasion of aluminium particles might take place during treatment in the disc mill. In order to account for the abraded mass of aluminium, the total content of metallic aluminium in all samples milled to 0–0.5 mm was determined by the soda attack method (Aubert et al., 2004). For this purpose, 3 g of the sample were placed in a closed cell. Subsequently, 10 mL of NaOH solution (mass fraction 0.5) were poured onto the powder and the volume of hydrogen gas generated was measured by means of a eudiometer. As each mol of metallic aluminium generates 1.5 mol of H2, the volume of hydrogen gas generated, the exact air pressure and the exact room temperature can be used to calculate the mass fraction of metallic aluminium based on the ideal gas law.
3. Results and discussion 3.1. Particle size distribution The cumulative particle size distribution of bottom ash from the three MSWI plants investigated is shown in Fig. 3. The mass fractions of the individual size fractions are also given in Table S1 in the supplementary information. The results reveal no significant difference in the particle size distribution of bottom ash from Plant B and C, but Plant A has a lower share of particles < 8 mm (45% compared to 60% of Plant B and C) probably because of the different waste input into the MSWI plant. About 1% of the bottom ash has been solubilised and therefore transferred to the washing solution during the washing processes shown in Fig. 2. However, the mass fraction of solubilised material could also be considerably higher, if more water is used and more intense washing procedures are applied. If only dry sieving of the bottom ash samples (i.e. treatment without washing) had been applied, the mass fractions of smaller particle size would have been lower. This is due to the fact that small particles adhering on larger particles cannot be separated during dry sieving and are therefore misleadingly understood to be part of the larger particle size fractions.
The cumulative particle size distributions for mineral material, magnetic iron, non-magnetic iron, aluminium, brass and copper are shown in Fig. 4. The mass fractions of all individual materials (i.e. the materials listed above plus non-metallic battery contents, coins, silver and gold) in each size fraction are given in Tables S2– S9 in the supplementary information. The results indicate that the size fraction > 50 mm contains most of the magnetic and non-magnetic iron (up to 50% of total magnetic and non-magnetic iron) and copper (about 20% of total copper). Therefore, large particle size fractions are most important for recovery of metal scrap. Glass and aluminium are highly enriched in the size fractions between 8 and 16 mm. The content of glass and aluminium in this size fraction reaches up to 50% and 6%, respectively, (with 100% being the dry matter of the respective size fraction). Hence, this size fraction is most interesting for the separation and recovery of these materials. In contrast, the content of brass is similar in all size fractions. The content of mineral material increases with decreasing particle size. Batteries, coins, silver and gold are almost exclusively present in the size fraction 16–50 mm with some minor contents in the fraction 12– 16 mm. Thus, separating these size fractions could remove most batteries from bottom ash and at the same time generate a material with a high content of coins and valuable materials. The enrichment of glass in the fractions between 8 and 16 mm and the enrichment of magnetic iron in fractions with particle sizes > 16 mm corresponds to the results of Šyc et al. (2018). Del Valle-Zermeño et al. (2017) and Chimenos et al. (1999) also find the highest mass fraction of glass in the same size fractions, but according to their studies the highest mass fraction of magnetic iron is present in the smaller size fractions < 4 mm. The total amount of magnetic iron and copper present in bottom ash determined in the present study is in agreement with Allegrini et al. (2014). However, they report a significantly lower amount of aluminium present in bottom ash (1.4 kg/100 kg bottom ash wet matter in comparison to about 3–4 kg/100 kg bottom ash dry matter). Mitterbauer et al. (2009) (as cited by Rechberger (2010)) also report an aluminium content of 1.5 kg/100 kg bottom ash wet matter. This difference could be explained either by a higher rate of source separation of Al or by the negligence of very small aluminium particles that are not recovered in the bottom ash treatment plant investigated by Allegrini et al. (2014) and not
100
Mass fracon [% dry maer]
90 80 70 60 50 40 30 20 10 0
TDS
<0.5 mm
<2 mm Plant A
<4 mm Plant B
<8 mm
<12 mm
<16 mm
<50 mm
Plant C
Fig. 3. Particle size distribution of bottom ash from three different MSWI plant. TDS. . .fraction of bottom ash solubilised during the washing procedure, BADM. . ..bottom ash dry matter. The error bars contain twice the standard deviation.
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Fig. 4. Mass fractions of aluminium, magnetic iron, non-magnetic iron, brass, copper, glass and mineral material in different size fractions of bottom ash from three different MSWI plants. BADM. . ...bottom ash dry matter. The error bars contain twice the standard deviation.
detected by the procedure of Mitterbauer et al. (2009). According to Mitterbauer et al. (2009), 100 kg of wet bottom ash contain additionally 3.2 kg iron, 0.33 kg brass, 0.28 kg stainless steel and 0.24 kg copper. While their results for brass, stainless steel and copper are in agreement with the present study, the iron content is significantly higher in the present study compared to Mitterbauer et al. (2009). Differences between results in the previous literature and the present study can either be explained by the different waste feed and operation of MSWI plants or the different characterisation methodology applied. An obvious difference between the MSWI plants is the lower content of magnetic iron in Plant A due to the magnet separator present directly on site. A lower content of magnetic iron causes a higher content of all other materials, which is especially relevant for the size fraction > 50 mm because it consists mainly of magnetic iron. The content of batteries, coins and silver (cutlery and jewellery) is highest in Plant C and lowest in Plant A. These results correspond to the fraction of mixed municipal waste in the input of the plants as shown in Fig. S1 in the supplementary information and therefore suggest that mixed municipal waste is the main source of these materials. Furthermore, the results presented in Fig. 4 reveal a significantly higher content of copper for plant A
(0.6 kg Cu/100 kg bottom ash) in comparison to plant B and C (0.25 kg Cu/100 kg bottom ash), which might results for some commercial and industrial wastes in the feed of plant A (such as car shredder residues). The sorting of coins showed that bottom ash from Plant A and B contains about 0.5 EUR/100 kg and bottom ash from Plant C contains about 1.2 EUR/100 kg, whereby the mass fraction of coins is considerable higher in the size fractions between 12 and 50 mm (up to 7 EUR/100 kg). The distribution of coins between Euro, foreign currencies, coins with collector’s value and obsolete currencies is shown in Fig. 5 and in more detail in Table S10 in the supplementary information. The chemical composition of coins is also shown in Fig. 5. Only about 60% of coins in bottom ash are Euro and small denominations were much more frequent than large denominations. Therefore, coins in bottom ash have a high average content of Fe, which is the main constituent of 1, 2 and 5 EUR cent coins, as shown in Fig. 5. In general, it can be presumed that coins were discarded together with municipal waste by accident. As already mentioned in 2.1, the uncertainty of the results is higher for materials with a lower total content and lower for materials with a higher content in MSWI bottom ash. As the samples
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Fig. 5. Distribution of coins between Euro, foreign currencies, coins with collector’s value and obsolete currencies and chemical composition of coins in bottom ash. Average values from all nine samples. BADM. . ...bottom ash dry matter. The error bars contain twice the standard.
were taken at different times of the year, seasonal variations of waste composition are covered by the present study. Of course, a switch in the waste feed of the MSWI plants would change the composition of bottom ash. For example, the present study shows that the different input in Plant A (less mixed MSW) leads to differently constituted bottom ash (fewer small particles, coins, silver and gold). Furthermore, dry bottom ash discharges produces bottom ash with a different composition and different grain sizes compared to quenched bottom ash like the one investigated in the present study (Fierz and Bunge, 2007). Fig. 6 shows the share of the total metallic aluminium, magnetic iron, non-magnetic iron, brass and copper determined by different operations performed in the present study. Simple sorting of
washed MSWI bottom ash fractions reveals only about 50% of the aluminium, brass and copper. The rest of the material is not accessible by sorting because it is embedded in agglomerates probably generated due to partial melting of the material in the MSWI plant. Hence, for MSWI bottom ash treatment, crushing of bottom ash is necessary in order to access and recovery these embedded metal pieces. The share of magnetic and non-magnetic iron that can be determined by simple sorting is about 85% and 95%, respectively, most likely because iron pieces are mainly present in the particle size fractions > 16 mm. As illustrated in Fig. 6, about 10% of the total aluminium is present in the 0–0.5 mm material generated by milling due to abrasion of aluminium pieces in the disc mill. Furthermore, about 20% of the total copper and about 3% of the
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0% Aluminium
Magnec iron
Non-magnec iron
Brass
Copper
Sorng of original material
Sorng of material >2 mm aer milling
Acid digeson of material 0.5-2 mm aer milling
Analysis of metals in the fracon 0-0.5 aer milling
Disassembling of transformers, electric motors etc.
Disassembling of baeries
Fig. 6. Share of the total metallic aluminium, magnetic iron, non-magnetic iron, brass and copper determined by different operations. Average values from all nine samples.
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total magnetic iron are present in transformers, electric motors and similar devices that had to be disassembled with great effort. The magnetic iron and brass present in batteries amounts to less than 1% of the total amount of these metals. 4. Conclusion A detailed characterisation of nine bottom ash samples from three different MSWI plants located in Vienna was performed. Thereby, a multi-step procedure was developed in order to completely determine the material composition of different size fractions of bottom ash. As the application of this procedure demonstrated that most of the magnetic iron, non-magnetic iron and copper are present in the size fraction > 50 mm, disregarding the relevance of large pieces present in bottom ash (e.g. by using sample increments < 2 kg) leads to a significant underestimation of the total content of these materials in bottom ash (e.g. by up to about 50% if only bottom ash 0–50 mm is considered). Furthermore, about 50% of aluminium, brass and copper are captured within pieces of molten mineral material and can therefore not be detected by conventional sorting analysis. Consequently, the sampling and sample preparation procedure presented in this study can improve the accurateness of bottom ash analysis and allows better assessment of the overall resource potential of bottom ash. Information about the latter is essential for evaluating the recovery rates achieved by bottom ash treatment plants. Another implication of the results presented is that crushing of MSWI bottom ash is necessary to recover the large share of enclosed metals. The present study also gives detailed information about material fractions such as coins or batteries, which have not been considered in the literature so far. The uneven distribution of different materials among different size fractions can be used to generate bottom ash fractions with different compositions and to direct these fractions to different treatment processes. For example, glass recovery should focus on bottom ash particles between 8 and 16 mm with a mass fraction of glass of about 50%. At the same time, the separation of batteries, coins, gold and silver from the size fraction 16–50 mm only could lead to an almost complete removal of these materials. Thereby, precious metals can be recovered and batteries separated from MSWI bottom ash can be sent to a specialised battery recycling facility. The mass fractions of coins, batteries and silver in bottom ash correspond with the mass fraction of mixed municipal waste in the feed of the incineration plant. As about 20% of the total copper present in bottom ash is found in transformers and electric motors, which are present only in the size fractions > 16 mm, the separation of these composites from the magnetic fraction is crucial to improve non-ferrous metal recycling, on the one hand, and to enhance the quality of iron scrap (as Cu present in iron deteriorates its properties) on the other. Both would lead to increased revenues from the treatment of MSWI bottom ash. In addition to the material quantities present in MSWI bottom ash, it is crucial to know the composition (i.e. quality) of these materials to foster their utilisation. Hence, this question was answered in a subsequent study (Huber et al., 2019). The results from the latter and the present study can be used for a detailed investigation on the material and substance flows in different MSWI bottom ash treatment plants (Huber, submitted). This investigation will provide information on recovery yields and product qualities for different process chains comprising different bottom ash treatment processes. Acknowledgements The work presented is part of a large-scale research initiative on anthropogenic resources (Christian Doppler Laboratory for Anthro-
pogenic Resources). The financial support of this research initiative by the Federal Ministry of Digital, Business and Enterprise and the National Foundation for Research, Technology and Development is gratefully acknowledged. Industry partners co-financing the research centre on anthropogenic resources are Altstoff Recycling Austria AG (ARA), Borealis group, voestalpine AG, Wien Energie GmbH, Wiener Kommunal-Umweltschutzprojektgesellschaft GmbH (WKU), and Wiener Linien GmbH & Co KG. The authors want to express their particular gratitude to the municipal department 48 of the City of Vienna for not only co-financing this project via its subsidiary WKU, but also for its essential contribution to the experiments in the form of facilities and staff. The support of Jost Gadermaier, Manuel Hahn, Helene Lutz, Zsombor Major, Ole Mallow and Klaus Stücklschwaiger with sampling, sample preparation and chemical analysis is gratefully acknowledged. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.11.036. References Alam, Q., Schollbach, K., van Hoek, C., van der Laan, S., de Wolf, T., Brouwers, H.J.H., 2019. In-depth mineralogical quantification of MSWI bottom ash phases and their association with potentially toxic elements. Waste Manage. 87, 1–12. https://doi.org/10.1016/j.wasman.2019.01.031. Allegrini, E., Maresca, A., Olsson, M.E., Holtze, M.S., Boldrin, A., Astrup, T.F., 2014. Quantification of the resource recovery potential of municipal solid waste incineration bottom ashes. Waste Manage. 34, 1627–1636. https://doi.org/ 10.1016/j.wasman.2014.05.003. Aubert, J.E., Husson, B., Vaquier, A., 2004. Metallic aluminum in MSWI fly ash: quantification and influence on the properties of cement-based products. Waste Manage. 24, 589–596. https://doi.org/10.1016/j.wasman.2004.01.005. Berkhout, S.P.M., Oudenhoven, B.P.M., Rem, P.C., 2011. Optimizing non-ferrous metal value from MSWI bottom ashes. J. Environ. Protect. 02, 564. https://doi. org/10.4236/jep.2011.25065. Biganzoli, L., Grosso, M., Forte, F., 2013. Aluminium mass balance in waste incineration and recovery potential from the bottom ash: a case study. Waste Biomass Valor 5, 139–145. https://doi.org/10.1007/s12649-013-9208-0. Brunner, P.H., Rechberger, H., 2015. Waste to energy – key element for sustainable waste management. Waste Manage. 37, 3–12. https://doi.org/10.1016/j. wasman.2014.02.003. Buha, J., Mueller, N., Nowack, B., Ulrich, A., Losert, S., Wang, J., 2014. Physical and chemical characterization of fly ashes from swiss waste incineration plants and determination of the ash fraction in the nanometer range. Environ. Sci. Technol. 48, 4765–4773. https://doi.org/10.1021/es4047582. Bunge, R., 2016. Recovery of metals from waste incinerator bottom ash. https:// www.umtec.ch/fileadmin/user_upload/umtec.hsr.ch/Dokumente/DokuDownload/Publikationen/Metals_from_MWIBA_2017.pdf (accessed 11.02.2019) Bunge, R., Bunge, K., 1999. Probenahme auf Altlasten: Minimal notwendige Probenmasse. altlasen spektrum 3, 174–179. Chandler, A.J., Eighmy, T.T., Hartlén, J., Hjelmar, O., Kosson, D.S., Sawell, S.E., van der Sloot, H.A., Vehlow, J., 1997. Municipal Solid Waste Incineration Residues. Studies in Environmental Science, Elsevier, Amsterdam. Chimenos, J.M., Segarra, M., Fernández, M.A., Espiell, F., 1999. Characterization of the bottom ash in municipal solid waste incinerator. J. Hazard. Mater. 64, 211– 222. https://doi.org/10.1016/S0304-3894(98)00246-5. del Valle-Zermeño, R., Gómez-Manrique, J., Giro-Paloma, J., Formosa, J., Chimenos, J. M., 2017. Material characterization of the MSWI bottom ash as a function of particle size. Effects of glass recycling over time. Sci. Total Environ. 581–582, 897–905. https://doi.org/10.1016/j.scitotenv.2017.01.047. Deutsches Kupferinstitut, 2007. Kupfer-Zink-Legierungen (Messing und Sondermessing). https://www.kupferinstitut.de/fileadmin/user_upload/ kupferinstitut.de/de/Documents/Shop/Verlag/Downloads/Werkstoffe/i005.pdf (accessed 03.05.2018). Fierz, R., Bunge, R., 2007. Trockenaustrag von KVA-Schlacke. https://www.umtec. ch/fileadmin/user_upload/umtec.hsr.ch/Dokumente/Doku-Download/ Publikationen/Trockenaustrag_von_KVA-Schlacke.pdf (accessed 29.05.2019) Forteza, R., Far, M., Seguí, C., Cerdá, V., 2004. Characterization of bottom ash in municipal solid waste incinerators for its use in road base. Waste Manage. 24, 899–909. https://doi.org/10.1016/j.wasman.2004.07.004. Holm, O., Simon, F.-G., 2017. Innovative treatment trains of bottom ash (BA) from municipal solid waste incineration (MSWI) in Germany. Waste Manage. 59, 229–236. https://doi.org/10.1016/j.wasman.2016.09.004. Huber, F., submitted. Modelling of material recovery from waste incineration bottom ash. Waste Management.
F. Huber et al. / Waste Management 102 (2020) 677–685 Huber, F., Blasenbauer, D., Aschenbrenner, P., Fellner, J., 2019. Chemical composition and leachability of differently sized material fractions of municipal solid waste incineration bottom ash. Waste Manage. 95, 593–603. https://doi.org/10.1016/j. wasman.2019.06.047. Huber, F., Fellner, J., 2018. Quality Criteria for the Utilisation of MSWI bottom ash in Europe. 4th Symposium on Urban Mining. Bergamo, Italy. Li, W., Ma, Z., Huang, Q., Jiang, X., 2018. Distribution and leaching characteristics of heavy metals in a hazardous waste incinerator. Fuel 233, 427–441. https://doi. org/10.1016/j.fuel.2018.06.041. Lynn, C.J., Ghataora, G.S., Dhir, O.B.E.R.K., 2017. Municipal incinerated bottom ash (MIBA) characteristics and potential for use in road pavements. Int. J. Pavement Res. Technol. 10, 185–201. https://doi.org/10.1016/j.ijprt.2016.12.003. Makari, C., 2014. Optical Sorting for the Recovery of Glass from WIP Slags, in: Waste Management 4. TK Verlag, Neuruppin, pp. 245–254. http://www.vivis.de/ phocadownload/Download/2014_wm/2014_wm_345_354_makari.pdf (accessed 30.08.2017) Mitterbauer, G., Skutan, S., Rechberger, H., 2009. Charakterisierung der Rostasche der MVA Dürnrohr im Hinblick auf die Rückgewinnung von Metallen. Institute for Water Quality and Resource Management, TU Wien. Morf, L.S., Brunner, P.H., Spaun, S., 2000. Effect of operating conditions and input variations on the partitioning of metals in a municipal solid waste incinerator. Waste Manage. Res. 18, 4–15. https://doi.org/10.1034/j.1399-3070.2000.00085. x. Morf, L.S., Gloor, R., Haag, O., Haupt, M., Skutan, S., Lorenzo, F.D., Böni, D., 2013. Precious metals and rare earth elements in municipal solid waste – Sources and fate in a Swiss incineration plant. Waste Manage. 33, 634–644. https://doi.org/ 10.1016/j.wasman.2012.09.010.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Review
Complete determination of the material composition of municipal solid waste incineration bottom ash Florian Huber ⇑, Dominik Blasenbauer, Philipp Aschenbrenner, Johann Fellner TU Wien, Institute for Water Quality and Resource Management, Karlsplatz 13/226, 1040 Vienna, Austria
a r t i c l e
i n f o
Article history: Received 2 March 2019 Revised 25 September 2019 Accepted 21 November 2019
Keywords: Bottom ash Waste incineration Combustion residues Sorting analysis Material composition Particle size distribution
a b s t r a c t Bottom ash from waste incineration is heterogeneous and contains different materials. Previous studies on the material composition of bottom ash provide only limited information as to composition, because large pieces present in bottom ash were not investigated nor were all materials were separated and analysed. The objective of the present study is to provide the complete and detailed composition of bottom ash encompassing and extensive range of different materials. Altogether, nine bottom ash samples with a mass of 3000 kg each were sieved to eight size fractions, whereby small particles adhering to larger pieces were separated by water and added to the respective size fractions. In the sorting analysis of all size fractions, the materials enclosed in molten mineral material and materials present as composites (e.g. transformers and batteries) were considered. The material characterisation revealed that the size fraction > 50 mm contains most of the iron (up to 50% of the total iron) and copper (about 20% of the total copper), while batteries, coins, silver and gold are almost exclusively present between 16 and 50 mm. The fractions between 8 and 16 mm show the highest share of aluminium (up to 50% of the total aluminium) and glass (up to 60% of the total glass). While the metal content is underestimated, if large pieces of material are disregarded, the multi-step approach applied in this study enables a complete determination of materials in bottom ash, which is essential for optimising material recovery in bottom ash treatment. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The incineration of municipal solid waste coupled with energy recovery constitutes a key element of sustainable waste management (Brunner and Rechberger, 2015). MSWI (municipal solid waste incineration) significantly reduces the mass and volume of solid waste, but the residues remaining after incineration still have to be managed. Small particles and volatile substances are carried over from the combustion chamber to the APC (air pollution control) system, where they are separated from the flue gas stream. The solid residues generated thereby are referred to as fly ash, wet scrubber residue or dry/semi-dry process residue depending on the APC system used and make up about 3–4% of the waste input in the MSWI process (Chandler et al., 1997). Most solids, however, arise as bottom ash, which represents about 20–25% of the waste input (Morf et al., 2000). While APC residues contain almost exclusively mineral material and have a typical particle size below 1 mm (Buha et al., 2014; Li et al., 2018), pieces consisting of different materials (e.g. minerals, glass, metals) with particle sizes between several mm and m can be found in MSWI bottom ash. ⇑ Corresponding author. E-mail address: [email protected] (F. Huber). https://doi.org/10.1016/j.wasman.2019.11.036 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
Metals can be separated from MSWI bottom ash during bottom ash treatment and subsequently utilised as a secondary raw material in the metal industry (Bunge, 2016; Holm and Simon, 2017). In recent years, there have also been first attempts to separate glass from the bulk of MSWI bottom ash (Makari, 2014; TB Hauer, 2013). The glass fraction could for example be used for foam glass production, while the mineral fraction present in bottom ash is either disposed of on landfills or can be used as an aggregate, mainly in road construction (Huber and Fellner, 2018; Lynn et al., 2017; Olsson et al., 2006). Information about the content of the different materials in MSWI bottom ash is crucial as it determines the resource potential of this residue and can also facilitate the assessment of separate waste collection schemes (del Valle-Zermeño et al., 2017). Several studies have already investigated the different material fractions present in MSWI bottom ash. Chimenos et al. (1999) determined the particle size distribution of MSWI bottom ash from two different MSWI plants in Catalonia, Spain. They took several sample increments with a mass of 2 kg and sorted the material into the fractions glass, minerals, magnetic metals, non-magnetic metals and unburnt organic matter. Later, bottom ash from the same region was characterised again by del Valle-Zermeño et al. (2017). In this study, an increment mass of 5 kg was used and only
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the size fractions between 2 and 30 mm were characterised. As minerals > 30 mm were crushed and added to the material < 30 mm, the samples analysed by del Valle-Zermeño et al. (2017) are not bottom ash as it is generated by MSWI but an intermediate product from bottom ash treatment with a higher content of mineral material. Šyc et al. (2018) characterised MSWI bottom ash from Czech Republic with regard to the content of minerals, glass, magnetic metals, non-magnetic metals and unburnt organic matter. They used sample increments of about 1 kg for their analysis. Muchova (2010) analysed the content of magnetic material, aluminium, heavy non-ferrous metals and unburnt organic matter in the MSWI bottom ash fraction 0–40 mm. Berkhout et al. (2011) analysed the metal concentrate and the metal-depleted bottom ash generated from Dutch MSWI bottom ash by an eddy current separator for the content of aluminium and non-ferrous metals. They used a sample size of 25 kg for the metal concentrate and 100 kg for the metal-depleted bottom ash. Biganzoli et al. (2013) determined the content of metallic aluminium in MSWI bottom ash from Italy by using a sample size of 5–15 kg. Forteza et al. (2004) analysed MSWI bottom ash < 50 mm after the separation of magnetic metals with regard to total and leachate contents of heavy metals. Their results indicate that aged bottom ash can be used in for road construction. Xia et al. (2017) analysed the chemical composition of MSWI bottom ash samples from China with regard to different particle size fractions. According to this study, most of the metals are present in the particle size fraction < 3 mm. However, none of the studies mentioned above provides the total amount of material fractions in MSWI bottom ash, e.g. because large pieces are neglected either directly or indirectly by choosing an increment mass below the mass of large bottom ash pieces (i.e. pieces with a mass of up to 15 kg). Furthermore, metals and unburnt organic matter that are incorporated in the mineral fraction are not fully considered in the literature. Other studies, like the work of Allegrini et al. (2014) determined the metallic content of iron, aluminium and heavy non-ferrous metal scrap in bottom ash taking account of all particle sizes. However, their analysis did not allocate the total content to different size fractions. The objective of the present study is therefore to generate more complete and detailed data on the constitution of MSWI bottom ash with regard to different material fractions by considering all particle sizes and materials and by using sample increments that account also for large pieces present in MSWI bottom ash.
2. Materials and methods 2.1. Sampling of MSWI bottom ash The bottom ash from three MSWI plants in Vienna was sampled. All plants are equipped with grate furnaces and wet bottom ash discharge and they differ in the waste types thermally treated. The mass fractions of different wastes in the feed of the three MSWI plants are given in Table 1. In Plant A a magnet separator
Table 1 Mass fractions of different wastes in the feed of the MSWI plants in 2017. Mass fraction [%]
Plant A
Plant B
Plant C
Mixed municipal waste Materials derived from waste sorting and processing Bulky waste Other waste
52.9 17.1
82.9 9.7
91.9 1.2
14.4 15.6
0.2 7.1
0.1 6.8
is installed downstream of the bottom ash discharge for the removal of iron scrap contrary to Plant B and Plant C. The minimum sample mass M for the bottom ash was calculated according to Eq. (1), which was developed by Skutan and Brunner (2005) based on Bunge and Bunge (1999) in order to account for the low sphericity of most bottom ash particles. The calculation was performed for Cu as analyte and it was assumed that the mass fraction of Cu is identical to the mass fraction of Cu carrier particles, assuming that all Cu carrier particles consist of pure Cu. Furthermore, it was assumed that the average mass fraction xi of Cu is 8 mg/g, mi max is 1000 g and mi 10% is 50 g based on Bunge (2016), Morf et al. (2013), and Skutan and Rechberger (2007). The desired standard deviation si was selected as 15%. The calculation according to Eq. (1) taking into account the above-mentioned assumption results in a minimum sample mass for the bottom ash of 3000 kg, which was used for the sampling. Elements with a higher heterogeneity in bottom ash than Cu have an accepted sampling error higher than 15% (e.g. Ag, Cd, Tl) and elements with a lower heterogeneity have a lower accepted sampling error (e.g. Ca, Fe, Si). 2
M¼
1 ci 1 pffiffiffiffiffiffiffiffiffi mimax mimax xi si 1 þ 3 log p33 ffiffiffiffiffiffiffiffi ffi m
ð1Þ
i10%
M
minimum sample mass [g]
xi mass fraction of carrier particles [g/g]
c i expected mass fraction of the analyte [mg/g] si desired standard deviation [mg/kg] mi max mass of the heaviest carrier particle mi 10% maximal mass of the 10% of the lightest carrier particles For each MSWI plant three random samples with a mass of 20,000 kg were taken, which resulted in a total number of nine bottom ash samples. The sampling took place in different seasons and on different days of the week from November 2017 to November 2018. The material sampled in the present study corresponds to the fresh (i.e. not aged) bottom ash that is transported from the MSWI plant to the bottom ash treatment plant. The sampling procedure is shown in Fig. 1 and is described in the following text. The bottom ash samples were spread on a firm ground in substantially rectangular shape with a height of about 0.3 m by a wheel loader. As the average bulk density of bottom ash is about 1400 kg/m3 (Lynn et al., 2017), the area of the rectangle was approximately 50 m2. Subsequently, shovels and wheelbarrows were used to take subsample increments of about 75 kg from the edge of the spread bottom ash. In order to make the inner part of the bottom ash rectangle also accessible for sampling, a wheel loader was used to decrease the size of the bottom ash rectangles in several steps, so that new edges were generated, from which subsample increments could also be taken. Altogether, 9 subsamples with a total mass of 3000 kg each were produced for further processing in this way. 2.2. Sieving and washing of MSWI bottom ash The total subsample mass of 3000 kg was sieved and washed according to the steps given in Fig. 2, aiming to separate the bottom ash into the following grain sizes: <0.5 mm, 0.5–2 mm, 2– 4 mm, 4–8 mm, 8–12 mm, 12–16 mm, 16–50 mm and > 50 mm. Furthermore, soluble substances were collected in the washing water. In a first step, the subsamples were sieved using a 16 mm sieve. The fraction > 16 mm was sieved again using a 50 mm sieve. The fraction 0–16 mm was spread and increments of about 75 kg were taken to generate a subsample with a total mass of 800 kg. This
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Fig. 1. Sampling procedure from top view. The black area represents the MSWI bottom ash spread in substantially rectangular shape. Sample increments were taken from the edge and segments of bottom ash were moved away by a wheel-loader in several steps. The grey area shows the bottom ash that was removed in the respective step and the white area shows the bottom ash that was removed in a previous step.
Fig. 2. Flow chart of all sieving, washing, sorting and comminution steps applied to the sample of 3000 kg of MSWI bottom ash. The fraction > 50 mm (A) was processed in the same way as the fraction 16–50 mm (B) but separately. The fractions 12–16 mm (C), 8–12 mm (D) and 4–8 mm (E) were processed in the same way but separately. All six washing solutions generated (including the fine fractions) were processed in the same way but separately.
subsample was further sieved by a 12 mm sieve, 8 mm sieve and 4 mm sieve. The masses of all size fractions were recorded and the dry matter content was determined after drying samples at 105 °C until mass constancy was reached.
The size fraction 0–4 mm was further fractionated by a 2 mm sieve and a 0.5 mm sieve under a flow of water from a nozzle. The flow of water was necessary to separate small particles adhering to larger particles. The fine particles were also washed from the
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size fractions > 4 mm in a similar manner. Thereto, the samples were spread on a 4 mm sieve with a 2 mm sieve and 0.5 mm sieve below and sieved under a flow of water from a nozzle. On average about 5–10 L of water were applied per kg of bottom ash. To separate the suspensions into solid particles between 0 and 0.5 mm and liquid solutions, a Büchner funnel equipped with paper filters was used. The masses of the solutions including their TDS (total dissolved solids) content were determined to assess the fraction of MSWI bottom ash dissolved during the process. The dry mass of all size fractions was determined by drying at 105 °C until mass constancy was reached. The procedure of sieving and washing MSWI bottom ash samples described above allowed the mass and particle size distribution of small particles adhering to larger particles in MSWI bottom ash as well as the mass solubilised during washing to be determined. The masses of these fine particles washed from larger particles were added to the masses of their respective grain size fractions. The chemical composition of the different size fractions and also of the easily soluble material dissolved during the washing procedure are described in detail in a subsequent study (Huber et al., 2019). 2.3. Sorting analysis The size fractions of washed bottom ash used for the sorting analyses are summarised in Table 2. All sorting operations were done by hand based on optical appearance, elasticity, hardness, density and magnetism with the assistance of files for removing the oxidised layer on metal pieces and of magnets for detecting magnetic iron. During sorting the following materials were distinguished: minerals, glass, unburnt organic matter, metals and batteries. Shiny, ductile materials were identified as metals. Metals were further sorted into magnetic iron (i.e. mostly unalloyed steel), nonmagnetic iron (i.e. mostly stainless steel), aluminium, copper, brass, coins, silver, gold and pieces made of more than one metal (mostly transformers and electric motors) according to colour and magnetism. Coins were further sorted according to currency and denomination and the chemical composition of every single coin was determined based on information from the respective mints. The transformers and electric motors were cut by an angle grinder and subsequently separated into copper, magnetic iron and aluminium. Batteries were cut open by an angle grinder and separated into minerals, magnetic iron and brass. The minerals found in MSWI bottom ash are either pieces like ceramics, stones or concrete that are not changed by the incineration process or agglomerates of material melted together during incineration (also referred to as secondary glass). The phase composition of agglomerates present in bottom ash was investigated by Alam et al. (2019). These agglomerates contain smaller pieces
Table 2 Masses used for the sorting analysis. The minimum sample masses were calculated as described in Section 2.1 with lower mi max and mi 10%. The average mass of material > 16 mm used for sorting can be calculated based on the particle size distribution (see Fig. 3). Size fraction
Mass used for sorting [kg]
mi
>50 mm
Everything contained in 3.000 kg of bottom ash Everything contained in 3.000 kg of bottom ash 13 6 3 1.5 No sorting possible for this fraction No sorting possible for this fraction
1000
50
110
5.5
3.6 1.5 0.5 0.007 – –
0.18 0.075 0.025 0.00035 – –
16–50 mm 12–16 mm 8–12 mm 4–8 mm 2–4 mm 0.5–2 mm 0–0.5 mm
max
[g]
mi
10%
[g]
of all fractions mentioned above. In order to liberate the metals and unburnt organic matter from these agglomerates, the mineral fractions > 50 mm and 16–50 mm were spread on a steel plate and subsequently crushed by running over the material with a vibrating roller (JCB VMD 70). The crushed material was sieved by a 16 mm sieve. The fractions 0–16 mm were split to receive subsamples with a mass of 13 kg, while the fractions > 16 mm were completely used for further processing. All crushed samples were sorted again as described above and dried at 105 °C. The mineral fractions obtained after sorting of the crushed samples and of the bottom ash size fractions between 0.5 mm and 16 mm were milled using a disc mill (FLSmidth Essa LM201) and subsequently sieved with a 2 mm sieve and a 0.5 mm sieve. The fractions 0–0.5 mm obtained by this procedure were regarded as mineral material. The fractions 0.5–2 mm were mixtures from different metals, but could not be sorted due to their small particle size. The fractions > 2 mm were again sorted to the above mentioned categories (different metals and unburnt material). Thus, the complete amount of metals and unburnt organic matter > 2 mm present in MSWI bottom ash could be determined. Advantageously, applying the procedure described above, all metallic pieces are correctly sorted as metal, even if they are erroneously sorted in the mineral fraction in the first sorting step, since, the material is sorted again after crushing and yet again after milling. This is especially relevant for aluminium as pieces of molten aluminium might sometimes look similar to stones. 2.4. Chemical analysis The size fractions 0.5–2 mm obtained after sieving the milled material, which only contain metals as described in 2.3, could not be sorted. Hence, this material was solubilised by a two-step open digestion without external heating. In the first step, iron and aluminium were dissolved by adding concentrated HCl (mass fraction 0.37) until no further reaction was visible. The solution from the first step was decanted into a volumetric flask. In the second step, the sample was treated with concentrated HNO3 (mass fraction 0.65, added until no further reaction was visible) to dissolve remaining metals like copper and brass. The solution from the second step was decanted into the volumetric flask and thereby combined with the solution from the first step. Then, the volumetric flask was filled to a volume of 1 L. The solutions obtained were analysed by a PerkinElmer Optima 8300 ICP-OES (inductively coupled plasma optical emission spectroscopy) equipped with a SC-2 DX FAST sample preparation system to determine the total content of Al, Fe, Cu and Zn in these fractions. The elements were determined via axial view and triple determination followed by an arithmetic averaging. A customised single-element (Merck, Roth) standard was used for the calibration. As Cu is present as pure copper or as an alloying element in brass, the contents of brass and copper in the samples were calculated according to Eqs. (2) and (3). It was assumed that brass has an average Zn mass fraction of 0.36 and an average Cu mass fraction of 0.63. The remaining mass (mass fraction 0.01) was assumed to be Pb. This composition of brass is based on the information provided by the Deutsches Kupferinstitut (2007) and is in agreement with the results from Taverna et al. (2010). The only metallic material containing Zn that were found in the sorting analysis are brass and coins. It was assumed that all Zn present in the size fractions 0.5–2 mm obtained after sieving the milled material is a constituent of brass, as all coins are considerably larger than 2 mm.
wbrass ¼
wZn 0:36
wcopper ¼ wCu 0:63 wbrass
ð2Þ ð3Þ
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3.2. Material analysis
wZn mass fraction of Zn as determined by ICP-OES wCu mass fraction of Cu as determined by ICP-OES wbrass mass fraction of brass wcopper mass fraction of pure copper As the wear resistance of aluminium is particularly low (Torrance, 2005), abrasion of aluminium particles might take place during treatment in the disc mill. In order to account for the abraded mass of aluminium, the total content of metallic aluminium in all samples milled to 0–0.5 mm was determined by the soda attack method (Aubert et al., 2004). For this purpose, 3 g of the sample were placed in a closed cell. Subsequently, 10 mL of NaOH solution (mass fraction 0.5) were poured onto the powder and the volume of hydrogen gas generated was measured by means of a eudiometer. As each mol of metallic aluminium generates 1.5 mol of H2, the volume of hydrogen gas generated, the exact air pressure and the exact room temperature can be used to calculate the mass fraction of metallic aluminium based on the ideal gas law.
3. Results and discussion 3.1. Particle size distribution The cumulative particle size distribution of bottom ash from the three MSWI plants investigated is shown in Fig. 3. The mass fractions of the individual size fractions are also given in Table S1 in the supplementary information. The results reveal no significant difference in the particle size distribution of bottom ash from Plant B and C, but Plant A has a lower share of particles < 8 mm (45% compared to 60% of Plant B and C) probably because of the different waste input into the MSWI plant. About 1% of the bottom ash has been solubilised and therefore transferred to the washing solution during the washing processes shown in Fig. 2. However, the mass fraction of solubilised material could also be considerably higher, if more water is used and more intense washing procedures are applied. If only dry sieving of the bottom ash samples (i.e. treatment without washing) had been applied, the mass fractions of smaller particle size would have been lower. This is due to the fact that small particles adhering on larger particles cannot be separated during dry sieving and are therefore misleadingly understood to be part of the larger particle size fractions.
The cumulative particle size distributions for mineral material, magnetic iron, non-magnetic iron, aluminium, brass and copper are shown in Fig. 4. The mass fractions of all individual materials (i.e. the materials listed above plus non-metallic battery contents, coins, silver and gold) in each size fraction are given in Tables S2– S9 in the supplementary information. The results indicate that the size fraction > 50 mm contains most of the magnetic and non-magnetic iron (up to 50% of total magnetic and non-magnetic iron) and copper (about 20% of total copper). Therefore, large particle size fractions are most important for recovery of metal scrap. Glass and aluminium are highly enriched in the size fractions between 8 and 16 mm. The content of glass and aluminium in this size fraction reaches up to 50% and 6%, respectively, (with 100% being the dry matter of the respective size fraction). Hence, this size fraction is most interesting for the separation and recovery of these materials. In contrast, the content of brass is similar in all size fractions. The content of mineral material increases with decreasing particle size. Batteries, coins, silver and gold are almost exclusively present in the size fraction 16–50 mm with some minor contents in the fraction 12– 16 mm. Thus, separating these size fractions could remove most batteries from bottom ash and at the same time generate a material with a high content of coins and valuable materials. The enrichment of glass in the fractions between 8 and 16 mm and the enrichment of magnetic iron in fractions with particle sizes > 16 mm corresponds to the results of Šyc et al. (2018). Del Valle-Zermeño et al. (2017) and Chimenos et al. (1999) also find the highest mass fraction of glass in the same size fractions, but according to their studies the highest mass fraction of magnetic iron is present in the smaller size fractions < 4 mm. The total amount of magnetic iron and copper present in bottom ash determined in the present study is in agreement with Allegrini et al. (2014). However, they report a significantly lower amount of aluminium present in bottom ash (1.4 kg/100 kg bottom ash wet matter in comparison to about 3–4 kg/100 kg bottom ash dry matter). Mitterbauer et al. (2009) (as cited by Rechberger (2010)) also report an aluminium content of 1.5 kg/100 kg bottom ash wet matter. This difference could be explained either by a higher rate of source separation of Al or by the negligence of very small aluminium particles that are not recovered in the bottom ash treatment plant investigated by Allegrini et al. (2014) and not
100
Mass fracon [% dry maer]
90 80 70 60 50 40 30 20 10 0
TDS
<0.5 mm
<2 mm Plant A
<4 mm Plant B
<8 mm
<12 mm
<16 mm
<50 mm
Plant C
Fig. 3. Particle size distribution of bottom ash from three different MSWI plant. TDS. . .fraction of bottom ash solubilised during the washing procedure, BADM. . ..bottom ash dry matter. The error bars contain twice the standard deviation.
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Fig. 4. Mass fractions of aluminium, magnetic iron, non-magnetic iron, brass, copper, glass and mineral material in different size fractions of bottom ash from three different MSWI plants. BADM. . ...bottom ash dry matter. The error bars contain twice the standard deviation.
detected by the procedure of Mitterbauer et al. (2009). According to Mitterbauer et al. (2009), 100 kg of wet bottom ash contain additionally 3.2 kg iron, 0.33 kg brass, 0.28 kg stainless steel and 0.24 kg copper. While their results for brass, stainless steel and copper are in agreement with the present study, the iron content is significantly higher in the present study compared to Mitterbauer et al. (2009). Differences between results in the previous literature and the present study can either be explained by the different waste feed and operation of MSWI plants or the different characterisation methodology applied. An obvious difference between the MSWI plants is the lower content of magnetic iron in Plant A due to the magnet separator present directly on site. A lower content of magnetic iron causes a higher content of all other materials, which is especially relevant for the size fraction > 50 mm because it consists mainly of magnetic iron. The content of batteries, coins and silver (cutlery and jewellery) is highest in Plant C and lowest in Plant A. These results correspond to the fraction of mixed municipal waste in the input of the plants as shown in Fig. S1 in the supplementary information and therefore suggest that mixed municipal waste is the main source of these materials. Furthermore, the results presented in Fig. 4 reveal a significantly higher content of copper for plant A
(0.6 kg Cu/100 kg bottom ash) in comparison to plant B and C (0.25 kg Cu/100 kg bottom ash), which might results for some commercial and industrial wastes in the feed of plant A (such as car shredder residues). The sorting of coins showed that bottom ash from Plant A and B contains about 0.5 EUR/100 kg and bottom ash from Plant C contains about 1.2 EUR/100 kg, whereby the mass fraction of coins is considerable higher in the size fractions between 12 and 50 mm (up to 7 EUR/100 kg). The distribution of coins between Euro, foreign currencies, coins with collector’s value and obsolete currencies is shown in Fig. 5 and in more detail in Table S10 in the supplementary information. The chemical composition of coins is also shown in Fig. 5. Only about 60% of coins in bottom ash are Euro and small denominations were much more frequent than large denominations. Therefore, coins in bottom ash have a high average content of Fe, which is the main constituent of 1, 2 and 5 EUR cent coins, as shown in Fig. 5. In general, it can be presumed that coins were discarded together with municipal waste by accident. As already mentioned in 2.1, the uncertainty of the results is higher for materials with a lower total content and lower for materials with a higher content in MSWI bottom ash. As the samples
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Fig. 5. Distribution of coins between Euro, foreign currencies, coins with collector’s value and obsolete currencies and chemical composition of coins in bottom ash. Average values from all nine samples. BADM. . ...bottom ash dry matter. The error bars contain twice the standard.
were taken at different times of the year, seasonal variations of waste composition are covered by the present study. Of course, a switch in the waste feed of the MSWI plants would change the composition of bottom ash. For example, the present study shows that the different input in Plant A (less mixed MSW) leads to differently constituted bottom ash (fewer small particles, coins, silver and gold). Furthermore, dry bottom ash discharges produces bottom ash with a different composition and different grain sizes compared to quenched bottom ash like the one investigated in the present study (Fierz and Bunge, 2007). Fig. 6 shows the share of the total metallic aluminium, magnetic iron, non-magnetic iron, brass and copper determined by different operations performed in the present study. Simple sorting of
washed MSWI bottom ash fractions reveals only about 50% of the aluminium, brass and copper. The rest of the material is not accessible by sorting because it is embedded in agglomerates probably generated due to partial melting of the material in the MSWI plant. Hence, for MSWI bottom ash treatment, crushing of bottom ash is necessary in order to access and recovery these embedded metal pieces. The share of magnetic and non-magnetic iron that can be determined by simple sorting is about 85% and 95%, respectively, most likely because iron pieces are mainly present in the particle size fractions > 16 mm. As illustrated in Fig. 6, about 10% of the total aluminium is present in the 0–0.5 mm material generated by milling due to abrasion of aluminium pieces in the disc mill. Furthermore, about 20% of the total copper and about 3% of the
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0% Aluminium
Magnec iron
Non-magnec iron
Brass
Copper
Sorng of original material
Sorng of material >2 mm aer milling
Acid digeson of material 0.5-2 mm aer milling
Analysis of metals in the fracon 0-0.5 aer milling
Disassembling of transformers, electric motors etc.
Disassembling of baeries
Fig. 6. Share of the total metallic aluminium, magnetic iron, non-magnetic iron, brass and copper determined by different operations. Average values from all nine samples.
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total magnetic iron are present in transformers, electric motors and similar devices that had to be disassembled with great effort. The magnetic iron and brass present in batteries amounts to less than 1% of the total amount of these metals. 4. Conclusion A detailed characterisation of nine bottom ash samples from three different MSWI plants located in Vienna was performed. Thereby, a multi-step procedure was developed in order to completely determine the material composition of different size fractions of bottom ash. As the application of this procedure demonstrated that most of the magnetic iron, non-magnetic iron and copper are present in the size fraction > 50 mm, disregarding the relevance of large pieces present in bottom ash (e.g. by using sample increments < 2 kg) leads to a significant underestimation of the total content of these materials in bottom ash (e.g. by up to about 50% if only bottom ash 0–50 mm is considered). Furthermore, about 50% of aluminium, brass and copper are captured within pieces of molten mineral material and can therefore not be detected by conventional sorting analysis. Consequently, the sampling and sample preparation procedure presented in this study can improve the accurateness of bottom ash analysis and allows better assessment of the overall resource potential of bottom ash. Information about the latter is essential for evaluating the recovery rates achieved by bottom ash treatment plants. Another implication of the results presented is that crushing of MSWI bottom ash is necessary to recover the large share of enclosed metals. The present study also gives detailed information about material fractions such as coins or batteries, which have not been considered in the literature so far. The uneven distribution of different materials among different size fractions can be used to generate bottom ash fractions with different compositions and to direct these fractions to different treatment processes. For example, glass recovery should focus on bottom ash particles between 8 and 16 mm with a mass fraction of glass of about 50%. At the same time, the separation of batteries, coins, gold and silver from the size fraction 16–50 mm only could lead to an almost complete removal of these materials. Thereby, precious metals can be recovered and batteries separated from MSWI bottom ash can be sent to a specialised battery recycling facility. The mass fractions of coins, batteries and silver in bottom ash correspond with the mass fraction of mixed municipal waste in the feed of the incineration plant. As about 20% of the total copper present in bottom ash is found in transformers and electric motors, which are present only in the size fractions > 16 mm, the separation of these composites from the magnetic fraction is crucial to improve non-ferrous metal recycling, on the one hand, and to enhance the quality of iron scrap (as Cu present in iron deteriorates its properties) on the other. Both would lead to increased revenues from the treatment of MSWI bottom ash. In addition to the material quantities present in MSWI bottom ash, it is crucial to know the composition (i.e. quality) of these materials to foster their utilisation. Hence, this question was answered in a subsequent study (Huber et al., 2019). The results from the latter and the present study can be used for a detailed investigation on the material and substance flows in different MSWI bottom ash treatment plants (Huber, submitted). This investigation will provide information on recovery yields and product qualities for different process chains comprising different bottom ash treatment processes. Acknowledgements The work presented is part of a large-scale research initiative on anthropogenic resources (Christian Doppler Laboratory for Anthro-
pogenic Resources). The financial support of this research initiative by the Federal Ministry of Digital, Business and Enterprise and the National Foundation for Research, Technology and Development is gratefully acknowledged. Industry partners co-financing the research centre on anthropogenic resources are Altstoff Recycling Austria AG (ARA), Borealis group, voestalpine AG, Wien Energie GmbH, Wiener Kommunal-Umweltschutzprojektgesellschaft GmbH (WKU), and Wiener Linien GmbH & Co KG. The authors want to express their particular gratitude to the municipal department 48 of the City of Vienna for not only co-financing this project via its subsidiary WKU, but also for its essential contribution to the experiments in the form of facilities and staff. The support of Jost Gadermaier, Manuel Hahn, Helene Lutz, Zsombor Major, Ole Mallow and Klaus Stücklschwaiger with sampling, sample preparation and chemical analysis is gratefully acknowledged. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.11.036. References Alam, Q., Schollbach, K., van Hoek, C., van der Laan, S., de Wolf, T., Brouwers, H.J.H., 2019. In-depth mineralogical quantification of MSWI bottom ash phases and their association with potentially toxic elements. Waste Manage. 87, 1–12. https://doi.org/10.1016/j.wasman.2019.01.031. Allegrini, E., Maresca, A., Olsson, M.E., Holtze, M.S., Boldrin, A., Astrup, T.F., 2014. Quantification of the resource recovery potential of municipal solid waste incineration bottom ashes. Waste Manage. 34, 1627–1636. https://doi.org/ 10.1016/j.wasman.2014.05.003. Aubert, J.E., Husson, B., Vaquier, A., 2004. Metallic aluminum in MSWI fly ash: quantification and influence on the properties of cement-based products. Waste Manage. 24, 589–596. https://doi.org/10.1016/j.wasman.2004.01.005. Berkhout, S.P.M., Oudenhoven, B.P.M., Rem, P.C., 2011. Optimizing non-ferrous metal value from MSWI bottom ashes. J. Environ. Protect. 02, 564. https://doi. org/10.4236/jep.2011.25065. Biganzoli, L., Grosso, M., Forte, F., 2013. Aluminium mass balance in waste incineration and recovery potential from the bottom ash: a case study. Waste Biomass Valor 5, 139–145. https://doi.org/10.1007/s12649-013-9208-0. Brunner, P.H., Rechberger, H., 2015. Waste to energy – key element for sustainable waste management. Waste Manage. 37, 3–12. https://doi.org/10.1016/j. wasman.2014.02.003. Buha, J., Mueller, N., Nowack, B., Ulrich, A., Losert, S., Wang, J., 2014. Physical and chemical characterization of fly ashes from swiss waste incineration plants and determination of the ash fraction in the nanometer range. Environ. Sci. Technol. 48, 4765–4773. https://doi.org/10.1021/es4047582. Bunge, R., 2016. Recovery of metals from waste incinerator bottom ash. https:// www.umtec.ch/fileadmin/user_upload/umtec.hsr.ch/Dokumente/DokuDownload/Publikationen/Metals_from_MWIBA_2017.pdf (accessed 11.02.2019) Bunge, R., Bunge, K., 1999. Probenahme auf Altlasten: Minimal notwendige Probenmasse. altlasen spektrum 3, 174–179. Chandler, A.J., Eighmy, T.T., Hartlén, J., Hjelmar, O., Kosson, D.S., Sawell, S.E., van der Sloot, H.A., Vehlow, J., 1997. Municipal Solid Waste Incineration Residues. Studies in Environmental Science, Elsevier, Amsterdam. Chimenos, J.M., Segarra, M., Fernández, M.A., Espiell, F., 1999. Characterization of the bottom ash in municipal solid waste incinerator. J. Hazard. Mater. 64, 211– 222. https://doi.org/10.1016/S0304-3894(98)00246-5. del Valle-Zermeño, R., Gómez-Manrique, J., Giro-Paloma, J., Formosa, J., Chimenos, J. M., 2017. Material characterization of the MSWI bottom ash as a function of particle size. Effects of glass recycling over time. Sci. Total Environ. 581–582, 897–905. https://doi.org/10.1016/j.scitotenv.2017.01.047. Deutsches Kupferinstitut, 2007. Kupfer-Zink-Legierungen (Messing und Sondermessing). https://www.kupferinstitut.de/fileadmin/user_upload/ kupferinstitut.de/de/Documents/Shop/Verlag/Downloads/Werkstoffe/i005.pdf (accessed 03.05.2018). Fierz, R., Bunge, R., 2007. Trockenaustrag von KVA-Schlacke. https://www.umtec. ch/fileadmin/user_upload/umtec.hsr.ch/Dokumente/Doku-Download/ Publikationen/Trockenaustrag_von_KVA-Schlacke.pdf (accessed 29.05.2019) Forteza, R., Far, M., Seguí, C., Cerdá, V., 2004. Characterization of bottom ash in municipal solid waste incinerators for its use in road base. Waste Manage. 24, 899–909. https://doi.org/10.1016/j.wasman.2004.07.004. Holm, O., Simon, F.-G., 2017. Innovative treatment trains of bottom ash (BA) from municipal solid waste incineration (MSWI) in Germany. Waste Manage. 59, 229–236. https://doi.org/10.1016/j.wasman.2016.09.004. Huber, F., submitted. Modelling of material recovery from waste incineration bottom ash. Waste Management.
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