Modelling of material recovery from waste incineration bottom ash

Waste Management 105 (2020) 61–72

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Review

Modelling of material recovery from waste incineration bottom ash Florian Huber ⇑ 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 24 September 2019 Revised 10 December 2019 Accepted 25 January 2020

Keywords: Waste incineration Incineration residues Material flow analysis Bottom ash Metal separation Mineral processing

a b s t r a c t Bottom ash from municipal solid waste incineration is usually treated in order to recover valuable materials like metals and to generate a mineral material for utilisation in construction industry or disposal. At present, different technologies and combinations thereof are used for bottom ash treatment resulting in different quantities and qualities of the final products (metals and minerals). So far, a comparison of these technologies is hardly possible based on the available literature. Hence, the present paper presents and applies a modelling approach that allows predicting the quantities and qualities (in terms of composition) of the final outputs of bottom ash treatment plants. In particular, material flow analysis models of five different bottom ash treatment plants were established on goods, material and element level and the mass and composition of the output flows of these plants were calculated based on an input of 118,000 Mg/a of bottom ash dry matter. The highest recovery of metals (up to 8640 ± 820 Mg/a iron, 1530 ± 220 Mg/a aluminium, 627 ± 73 Mg/a stainless steel and 608 ± 70 Mg/a heavy non-ferrous metals) can be achieved in plants that apply comminution before any ageing processes and are equipped with jiggers, inductive sorting systems and/or a high number of eddy current separators. The iron scrap fractions separated from bottom ash are contaminated by up to 114 ± 44 mg/kg Cd and up to 9900 ± 3300 mg/kg Cu, which might impair their suitability for recycling. Only minor differences in the composition of mineral material generated by different treatment plants could be observed. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction The incineration of municipal solid waste with energy recovery represents a key element for 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, can be found in bottom ash, which represents about 20–25% of the waste input (Morf et al., 2000). While APC residues contain almost only 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

⇑ Corresponding author. E-mail address: [email protected] https://doi.org/10.1016/j.wasman.2020.01.034 0956-053X/Ó 2020 Elsevier Ltd. All rights reserved.

sizes ranging from several mm to m can be found in MSWI bottom ash. MSWI bottom ash is usually treated either directly on site or at a dedicated bottom ash treatment plant where bottom ash from several MSWI plants can be combined and treated. The objectives of bottom ash treatment are (1) recovery of valuable materials for recycling and (2) preparation of a material comprising mainly minerals suitable for utilisation or safe disposal. Most bottom ash treatment plants focus on the recovery of metals, as metals are the bottom ash constituents with the highest economic value. However, about 90% of bottom ash consist of glass and mineral material (Huber et al., 2020), which is used as a construction material in some countries and disposed of on landfills in other countries (Blasenbauer et al., 2020). In order to be suitable as an aggregate for road construction or concrete production, bottom ash has to comply with legal limit values for leachate contents and in some countries (e.g. Austria) also for total contents (Blasenbauer et al., 2020). Furthermore, certain mechanical properties have to be complied with (Lynn et al., 2017; Sormunen et al., 2017). For the utilisation as a replacement for raw meal in cement clinker production, there are only limit values for total contents (Blasenbauer et al., 2020).* The total content of Al, Ca, Fe and Si in bottom ash is a crucial factor for cement clinker production

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because these elements make up the clinker phases. For utilisation in cement and concrete the contents of Cl, metallic Al and metallic Zn should be as low as possible, because they deteriorate the quality of the concrete produced (Aubert et al., 2004; Ito et al., 2008; Pera et al., 1997). Cl additionally causes problems in the cement kiln (Ito et al., 2008). Bottom ash treatment plants generally include several processing steps, which can be combined in different manner. The most common equipment in MSWI bottom ash treatment plants are magnetic separators and ECS (eddy current separators) to separate magnetic and non-magnetic metals. As neither of them is able to effectively separate non-magnetic iron (i.e. stainless steel) (Bunge, 2017), inductive sorting systems are necessary for the effective recovery of stainless steel (Egosi and Raabe, 2010; Feil et al., 2019). Heavy non-magnetic metals including stainless steel can also be recovered by density separation (Pfandl et al., 2018, 2019; Stockinger, 2016). Sieves are applied in bottom ash treatment because ECS are more efficient when their feed has a small particle size range (Bunge, 2017) and to generate aggregates with different particles sizes. Crushers are used to liberate metal pieces encapsulated in pieces of mineral material (Bunge, 2017). In order to decrease the leachability of heavy metals, bottom ash is often stored for several weeks to months in an ageing or weathering process (Astrup et al., 2016). The disadvantage of this simple process is, however, that during the ageing period metals like aluminium are partly oxidised (Bunge, 2017; de Vries et al., 2009). Washing processes can be applied to bottom ash in order to remove soluble salts and to enable the separation of small particles sticking on larger pieces (Huber et al., 2020). Washing processes can also be combined with density separation (Pfandl et al., 2018, 2019; Stockinger, 2016). In bottom ash treatment plants, the equipment described above is combined to form cascades. Two modern treatment plants were described in detail by Holm and Simon (2017). One of them applies a wet treatment with bottom ash washing as a crucial step for aggregate cleaning, while the other one applies a dry treatment with two consecutive crushers for maximal metal recovery. The process scheme of a wet treatment plant comprising density separation was provided by Pfandl et al. (2018, 2019) and Stockinger (2016). Another state-of-the-art bottom ash treatment plant was described by Allegrini et al. (2014), who also provided information about the material flows of iron, aluminium and heavy non-ferrous metals through the treatment plant. However, a comparison of these plants with respect to the quantity and quality of outputs is not possible because (1) only for one of them data on the metal flows are available and (2) the bottom ash fed to all plants is different and therefore also the quantity and quality of the outputs is most likely different. Hence, the objective of the present study is to compare different bottom ash treatment plants with regard to the quantity and quality of the output flows produced from one particular bottom ash. The modelling of different MSWI plants is explicitly not within the scope of the present study. As the distance between bottom ash treatment plants applying different technologies might be very big (e.g. located in different countries), large scale experiments aiming to compare their performance for one particular bottom ash are not feasible. To overcome this obstacle, the present study applies a mathematical modelling approach. In particular, the following research questions are addressed: 1. What are the mass flows through the bottom ash treatment plants on goods level? 2. What are the mass flows through the bottom ash treatment plants on substance level for all chemical elements relevant for bottom ash utilisation?

3. What is the quantity and quality of metals recovered by each bottom ash treatment plant? 4. What is the quantity and quality of mineral material generated by each bottom ash treatment plant? 2. Materials and methods 2.1. MSWI bottom ash As material to be treated by the bottom ash treatment plant, bottom ash from three MSWI plants in Vienna equipped with grate furnaces and wet bottom ash discharge was considered. Mixed municipal waste, materials derived from sorting and processing of waste and bulky waste are the main constituents of the feed to these MSWI plants. A detailed characterisation of bottom ash from these MSWI plants was conducted by Huber et al. (2020, 2019). They generated data about the grain size distribution, the different materials present at different grain sizes (e.g. ferrous metals, glass, batteries) and their chemical composition. The information on different materials can be found in Tables S4–S12 in the supplementary information. The data from these two previous studies was used as a basis for the models described hereinafter. It was assumed that 118,000 Mg/a of bottom ash dry matter are treated in the bottom ash treatment plant. This corresponds to approximately 140,000 Mg/a wet matter based on a water content of 0.15 kg/kg. This quantity represents the annually generated amount of bottom ash from the three MSW grate incinerators in Vienna. It was further assumed that the three MSWI plants described by Huber et al. (2020, 2019) contribute in equal parts to the annual mass flow of MSWI bottom ash. 2.2. MSWI bottom ash treatment plants Models of five different MSWI bottom ash treatment plants were established. These modelled plants are inspired by existing plants, but are not necessarily identical to these plants in every aspect. 2.2.1. Plant A The first treatment step in Plant A is sieving by a 50 mm sieve. Iron scrap is removed from the fraction >50 mm by a magnetic separator and manual sorting is applied to the remaining material >50 mm. Bottom ash <50 mm is further sieved by a 4 mm sieve. Metals are separated from the material 4–50 mm by a magnetic separator and an ECS. The process scheme with all processes and material flows of Plant A is illustrated in Fig. 1. 2.2.2. Plant B The design of Plant B is inspired by the plant described by Pfandl et al. (2018, 2019) and Stockinger (2016). The first treatment step in Plant B is sieving by a 50 mm sieve. Iron scrap is removed from the fraction >50 mm by a magnetic separator and manual sorting is applied to the remaining material > 50 mm. Iron is removed from the fraction <50 mm by a magnetic separator and the remainder is fed into a jigger. The jigger is a water bath with a moving plate for separation of metals with high densities (i.e. almost all metals except aluminium). Additionally, soluble salts and small particles are transferred to the water in the jigger. Downstream of the jigger, there are a hydrocyclone and a decanter for the removal of solid particles from the used water and an ECS is used for the recovery of aluminium from the light material fraction. The process scheme with all processes and material flows of Plant B is illustrated in Fig. 1.

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Fig. 1. Material flow analysis of Plants A and B on goods level [Mg/a].

2.2.3. Plant C The design of Plant C is inspired by the plant described by Allegrini et al. (2014).The first treatment step in Plant C is crushing followed by sieving by a 50 mm sieve. Iron scrap is removed from the fraction > 50 mm by a magnetic separator and manual sorting is applied to the remaining material > 50 mm. Mineral material > 50 mm is fed back to the crusher. The material < 50 mm undergoes an ageing process and is subsequently sieved by a 16 mm, 8 mm and 2 mm sieve. Non-ferrous metals are removed by three ECS from the size fractions 16–50 mm, 8–16 mm and 2–8 and the material 16–50 mm is fed

to an induction sorting system downstream of the ECS for removal of stainless steel. The process scheme with all processes and material flows of Plant C is illustrated in Fig. 2. 2.2.4. Plant D The design of Plant D is inspired by the plant described by Holm and Simon (2017) and referred to as ‘‘plant A” in the cited publication. The first treatment step in Plant D is sieving by a 50 mm sieve. Iron scrap is removed from the fraction > 50 mm by a magnetic separator and manual sorting is applied to the remaining material > 50 mm. The material < 50 mm is washed and sieved

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Fig. 2. Material flow analysis of Plants C and D on goods level [Mg/a].

by a 4 mm sieve. A hydrocyclone is used for separation of solid particles (0–0.5 mm) from the washing water. Non-ferrous metals are removed from the fraction 0.5–2 mm by an ECS and ferrous metals are removed from the fraction 4–50 mm. Subsequently, this fraction is further sieved by a 16 mm sieve and non-ferrous metals are removed by two ECS (one for each size fraction). The process scheme with all processes and material flows of Plant D is illustrated in Fig. 2.

downstream of the second crusher. The remaining material is sieved by a 16 mm, 4 mm and 2 mm sieve. The material 16– 50 mm is fed to a magnet separator followed by an ECS, while the two fractions 2–4 mm and 4–16 mm are fed to two subsequent ECS each. The process scheme with all processes and material flows of Plant E is illustrated in Fig. 3.

2.2.5. Plant E The design of Plant E is inspired by the plant described by Holm and Simon (2017) and referred to as ‘‘plant B, alpha line” in the cited publication. The first treatment step in Plant E is an ageing process (alike to the ageing process of Plant C) followed by a first crusher and a sieving step by a 50 mm and a 2 mm sieve. Iron scrap is removed from the fraction > 50 mm by a magnetic separator and manual sorting is applied to the remaining material > 50 mm. Contrary to Plant C, mineral material > 50 mm is not fed back to the first crusher. The material 2–50 mm is fed to a second crusher and magnetic metals are separated from the material by a magnet

2.3.1. Material flow analysis An MFA (material flow analysis) was conducted according to Brunner and Rechberger (2004) using the software STAN (Cencic and Rechberger, 2008). The MFA was conducted on goods level and on substance level for the chemical elements Ag, Al, As, Au, Ba, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sn, Ti, Tl, V, W and Zn. Furthermore, the material level for the metals aluminium, brass, coins, copper, iron and stainless steel as well as the mineral material, glass and unburnt organic matter were considered in the MFA. The mass fractions of each material in the different size fractions were derived from Huber

2.3. Modelling approach

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Fig. 3. Material flow analysis of Plant E on goods level [Mg/a].

et al. (2020) and are given in Tables S4–S12 in the supplementary information. The mass fractions of each chemical element in the different materials for each size fraction were derived from Huber et al. (2019). The coins fraction consists of magnetic and non-magnetic coins as determined by Huber et al. (2020), which were considered separately. The system boundary is the treatment of 118,000 Mg of bottom ash dry matter and one year, which represents the quantity of bottom ash generated in the city of Vienna. The mass of each material flow and the mass of each element in each material flow were calculated separately nine times. Each of the nine calculation corresponds to one bottom ash sample analysed by Huber et al. (2020, 2019). For each mass flow the average values and the standard error of the nine calculations were determined according to Altman and Bland (2005). These average values and standard errors were used as input for STAN. STAN was used for data reconciliation and for the calculation of the composition of the output flows. The material flows were calculated based on the composition of the input material (see Huber et al. (2020, 2019)) and on transfer coefficients derived from literature or expert judgements. 2.3.2. Modelling of MSWI bottom ash treatment processes Plants A, C and E apply a dry bottom ash treatment, which means that small particles adhering to larger pieces cannot be separated by sieving. On the contrary, Plant B and D apply a wet treatment, which enables the removal of the abovementioned sticking particles during sieving. The results of dry and wet sieving of bottom ash (e.g. transfer coefficients for different grain sizes and materials) were determined by Huber et al. (2020) and applied in the models of Plants A, C, E and Plants B and D, respectively. It

was assumed that wet processing transfers all soluble salts (as determined by Huber et al. (2020, 2019)) and also insoluble particles 0–0.5 mm to the washing water. In Plants B and D, hydrocylcones and decanters for the separation of insoluble particles from the washing water are installed. It was assumed that the hydrocyclones and decanters remove all of these particles. Furthermore, it was assumed that at the magnetic separators the transfer coefficient to the magnetic scrap is 95% for magnetic materials (i.e. iron, iron containing composites like transformers and electric motors and magnetic coins) and 0% for non-magnetic materials from bottom ash. The separation efficiency of magnetic separators is higher for larger particle sizes. However, as less than 5% of the total iron present in bottom ash are smaller than 4 mm, it seems appropriate to use the same transfer coefficient for all particles sizes. For manual sorting, it was assumed that all sorting fractions contain 100% of the respective materials. According to Bunge (2017), the concentrate grade (as defined by Bunge (2017)) of non-ferrous metal concentrates generated by ECS is the lower the higher the metal recovery (as defined by Bunge (2017)). For example, a metal recovery close to 100% can result in a concentrate grade of only 45% and a concentrate grade close to 100% can lead to a recovery of only 60%. Furthermore, ECS only provide good separation results, if the largest particles are not more than a factor 3 larger than the smallest particles (Bunge, 2017). Hence, for plants that fulfil this requirement (Plant C, D and E), the recovery of the ECS was assumed as 80% and the concentrate grade of the concentrate was assumed as 85% based on the data from Bunge (2017). As the ECS in Plants A and B are fed with material having a wide particle range, a lower recovery of only 50% with a concentrate grade of 85% was assumed.

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Consequently, the transfer coefficient of non-ferrous metals (i.e. aluminium, brass, copper, gold, silver and non-magnetic coins) was 0.8 and 0.5, respectively and the transfer coefficient of all other materials present in MSWI bottom ash was set in order to achieve the abovementioned purities of the non-ferrous metal concentrates. Plant E uses two ECS in series. According to Bunge (2017), it can be expected that the recovery of the second ECS is 65% of that of the first ECS. The jig of Plant B has a transfer coefficient to the metal concentrate of 0.95 (based on own assumptions) for all metals except aluminium and a transfer coefficient of 0 for all other materials. The jig additionally separates swimming materials. The data about the share of each material separated by floating on water from Major (2019) and Huber et al. (submitted) was used in the model of Plant B. The transfer coefficient of the inductive sorting system was assumed as 0.9 for stainless steel and as 0 for all other materials. During ageing processes, metals present in bottom ash are partly corroded. According to Bunge (2017) and de Vries et al. (2009) about 15% of aluminium are corroded during ageing of bottom ash. It was further assumed that iron is corroded to the same degree, while stainless steel, copper, brass, silver and gold are not affected by corrosion due to their higher nobility. For the modelling this means that in plants applying ageing processes, 15% of the metallic aluminium and iron are transformed to their oxides and, hence, constitute mineral material. The particle size of this newly formed mineral material was assumed to be 0–0.5 mm. In practice, corrosion depends of course on a number of factors like ageing time, available surface area or pH. As iron particles in bottom ash are typically larger than aluminium particles (Huber et al., 2020), the corrosion of iron might be overestimated in the models. The crushers of Plants C and E were modelled according to King (2001) applying Eqs. (1)–(3).

x n1 x n2 Bðx; yÞ ¼ Kð Þ þ ð1  KÞð Þ y y

ð1Þ

bjj ¼ 1  BðDj ; dpj Þ

ð2Þ


bij ¼ B Di1 ; dpj  BðDi ; dpj Þ

ð3Þ

bij bjj B(x;y) K n1 n2 dpj

Di

fraction of particles breaking in size class j that end up in size class i fraction of particles of size class j staying in size class j fraction of daughter particles smaller than size  that result from a single particle of the size y empirical constant, assumed as 0.3 according to King (2001) empirical constant, assumed as 0.45 according to King (2001) empirical constant, assumed as 3.2 according to King (2001) particle size of size class j, defined as the geometric mean of the largest and the smallest particle of size class j particle size of the largest particle size in size class i

This calculation was conducted for all pairs of the following size fractions > 50 mm, 16–50 mm, 12–16 mm, 8–12 mm, 4–8 mm, 2–4 mm, 0.5–2 mm and 0–0.5 mm. As largest particle size in the fraction > 50 mm, 250 mm were assumed. The results of bjj and bij for all pairs of size classes can be found in Tables S2 and S3 in the supplementary information. It was assumed that 10% of metals

1.E+04

Mass flow of recovered metal [Mg/a]

1.E+03

1.E+02

1.E+01

1.E+00

1.E-01

1.E-02

1.E-03 Silver

Aluminium Plant A

Gold Plant B

Copper Plant C

Brass

SS

Plant D

Plant E

Iron

Coins

Fig. 4. Amount of metal scrap recovered form MSWI bottom ash by Plants A to E. The total amounts of each metal in the input of the bottom ash treatment plants can be found in the supplementary information. SS. . ..stainless steel. Note that the axis is logarithmic.

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encapsulated inside of mineral material are liberated by the crushing process and for simplicity it was further assumed that 100% of these liberated metals have a particle size of 8–16 mm. 3. Results and discussion 3.1. Material flow analysis The results of the material flow analysis on goods level are shown in Fig. 1 to Fig. 3. The results of the material flow analysis on material and elemental level are shown in Figs. S1–S205 in the supplementary information. The mass flows of the input to the treatment plants and their uncertainties are slightly different in some cases because of the data reconciliation applied in STAN. Fig. 1 to Fig. 3 show that by far the largest output flow for all plants is the mineral material < 50 mm, which includes considerable amounts of glass. The material > 50 mm is removed in an early stage in all plants. In Plant C all mineral material > 50 mm is crushed and therefore forms a part of the smaller particle size fractions and in Plant E the crushing of mineral material > 50 mm results in particles < 50 mm, which are mixed with the other particle size fractions, and in particles remaining > 50 mm after crushing. The mass of metallic aluminium and iron is decreased by oxidation and the mass of mineral material is increased by oxidation of metals. However, a decrease or increase in mass cannot be integrated in the software STAN. Hence, the decrease and increase of material during the ageing process is illustrated in the form of output and input flows, respectively. These flows are 0 on goods and element level, and for all materials except mineral material, metallic aluminium and metallic iron. A comparison between the output of non-ferrous metal scrap generated by the parallel ECS in Plants C and E shows that the size fraction 8–16 mm has the highest potential for recovery of non-ferrous metal scrap, while almost no metals can be separated from the fraction 2–4 mm. All of the abovementioned findings correspond with the results of Allegrini et al. (2014). The amount of filter cake (insoluble material < 0.5 mm) in Plant D amounts to 12% of the input, which corresponds precisely to the data of Holm and Simon (2017). This agreement confirms the suitability of the model established in the present study. The mass flows of Plant E are also in rough agreement with Holm and Simon (2017). The minor deviations can be explained by the special design of the crusher described by Holm and Simon (2017).

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ing the ageing process. Plant B shows the highest recovery of copper, brass, silver, gold and coins because of the high separation efficiency of the jig compared to ECS. Because of the high separation efficiency of the jig, the recovery of stainless steel is the highest of all plants too. However, it has to be noted that the transfer coefficient used for the jig is not based on experimental data. Plants D and E have a similarly high recovery of heavy nonferrous metals as Plant B because the ECS are fed with material with a narrow particle size range. Plant C also has a high recovery of stainless steel because of the use of an inductive sorting system for the size fraction 16–50 mm. With regard to aluminium, the highest recovery can be achieved by Plants D and E because of the parallel ECS for different particle sizes. The almost identical recovery of aluminium of these two plants is an interesting result because in Plant E 15% of the metallic aluminium are lost due to corrosion. This loss is compensated by the liberation of metallic

3.2. Quantity and quality of metal scrap The quantity of scrap recovered by each of the bottom ash treatment plants under investigation is illustrated in Fig. 4 and in Table S1 in the supplementary information. It has to be noted that the materials shown in Fig. 4 include all metals, which were accessible in the sorting analysis conducted by Huber et al. (2020). As particle fractions < 2 mm are not accessible to manual sorting, the gold content in these fractions (mainly derived from electronic devices) is not included in Fig. 4. However, the Au content in the smaller particle size fractions and also in the mineral fractions is included in Figs. S66–S70 in the supplementary information. Recovered scrap is the amount of metallic iron and magnetic coins in the magnetic fraction, the amount of metallic aluminium, brass, copper, coins, silver and gold in the non-ferrous metal fractions and the amount of stainless steel separated by inductive sorting systems, jigs or manual sorting. The highest recovery of iron scrap is achieved by Plants B and C, because all bottom ash particles are fed to magnet separators, while in Plants A and D bottom ash < 4 mm is not fed to magnet separators. The separation of iron scrap in Plant E is the lowest because of the corrosion of iron dur-

Fig. 5. Mass fractions of Al (total), Ca, Fe, Si, Cl and Al (metallic) in the mineral material generated by Plants A to E based on dry matter. In Plants C to E, different particle size fractions are first separated and then mixed again after metal separation. Hence, the mass flows of the separate fractions as well as the mass flows of the mixed fractions are shown.

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Fig. 6. Mass fractions and limit values for utilisation in clinker production of As, Cd, Co and Sb in the mineral material generated by Plants A to E based on dry matter. In Plants C to E, different particle size fractions are first separated and then mixed again after metal separation. Hence, the mass flows of the separate fractions as well as the mass flows of the mixed fractions are shown.

aluminium pieces in the two crushers and the application of five ECS in parallel and in series. The results of the MFA on Cd and Cu (c.f. Figs. S81–S85 and S101–S105 in the supplementary information) show that a significant share of these elements is transferred from the bottom ash input to the magnetic scrap (i.e. iron scrap) during bottom ash

treatment. The mass fraction of Cd and Cu in the magnetic scrap in different size fractions generated by different plants is shown in Table S2 in the supplementary information. Cadmium is enriched in the magnetic scrap of size fraction < 50 mm and in particular in the size fraction 16–50 mm because this contains most batteries. The mass fraction of Cd in

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the present study is lower than the results of Skutan et al. (2009), who determined a Cd content of 320 ± 100 mg/kg for magnetic scrap < 50 mm from bottom ash treatment. In the meantime, the use of Cd in batteries was restricted according to Directive 2006/66/EC (European Parliament, 2006). Hence, it is no surprise that the number of Cd containing batteries in Austria has decreased in the last 10 years. The presence of copper in the magnetic scrap separated from MSWI bottom ash can be explained by the presence of transformers, electric motors and other devices in bottom ash. These devices consist of a magnetic iron core wrapped with copper wire. The presence of copper in magnetic scrap is in agreement with the data of Rem et al. (2012). This figure is relevant, because copper cannot be separated from iron in the metallurgic recycling process and deteriorates the quality of secondary iron products (Björkman and Samuelsson, 2014; Brunner and Ma, 2009; Daehn et al., 2017; Theo, 1998). According to the model established in the present study, the Cu content in magnetic scrap from MSWI bottom ash is up to 10,000 mg/kg, which is considerably above the average value for secondary steel of 1,500 mg/kg given by Daehn et al. (2017). According to Björkman and Samuelson (2014), a Cu content of 5,000 mg/kg is too high for the production of secondary steel. Consequently, magnetic scrap from MSWI bottom ash can only be used in secondary steel production if it is mixed with scrap with a lower content of contaminants or if Cu is removed from the scrap prior to recycling. 3.3. Quantity and quality of mineral fractions The mass flows of mineral fractions generated by Plants A to E are shown in Figs. 1–3. The mass fractions of the different particle size fractions of mineral material depend on the sieves

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and crushers used in bottom ash processing. Particularly, the use of crushers increases the mass of mineral material with small particle sizes, which cannot be used in road construction (Zentar et al., 2008). The mass fractions of Al, Ca, Fe and Si in the mineral material generated by Plants A to E are shown in Fig. 5. The mass fractions of these four elements are relevant as they are necessary to form cement phases, if bottom ash is used as a raw material for clinker production. While Al and Si are often widely available in primary and secondary raw materials, the availability of Ca and Fe may be limited. The highest mass fraction of Ca can be found in the mineral material < 0.5 mm generated in the wet treatment plants B and D. The highest Fe content is on the contrary achieved in the mineral material >50 mm. As more than 50% of this Fe are present in metallic form, it may be worthwhile to crush the mineral material to liberate and subsequently separate the metals, even if this bottom ash fraction is used in clinker production. The mass fractions of Cl and metallic Al, which both can cause problems in cement production (Ito et al., 2008; Pan et al., 2008; Pera et al., 1997), are shown in Fig. 5. Unsurprisingly, the Cl content is lower if wet bottom ash treatment is applied. Similarly, the higher the recovery of metallic aluminium the lower is the residual content of metallic aluminium in the mineral fraction. Furthermore, smaller particle size fractions have a lower content of metallic aluminium compared to larger particle size fractions. The Austrian Ministry for the Environment set limit values for the mass fractions of As, Cd, Co, Cr, Hg, Ni, Pb, Sb and Tl (BMLFUW, 2016) in secondary raw material for clinker production. Materials complying with these limit values may be used in unlimited amounts in the production of cement clinker. Hence, the total contents of these elements are shown in Figs. 6–8. Most mineral

Fig. 7. Mass fractions and limit values for utilisation in clinker production of Hg and Tl in the mineral material generated by Plants A to E based on dry matter. In Plants C to E, different particle size fractions are first separated and then mixed again after metal separation. Hence, the mass flows of the separate fractions as well as the mass flows of the mixed fractions are shown.

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Fig. 8. Mass fractions and limit values for utilisation in clinker production of Cr, Ni and Pb in the mineral material generated by Plants A to E based on dry matter. In Plants C to E, different particle size fractions are first separated and then mixed again after metal separation. Hence, the mass flows of the separate fractions as well as the mass flows of the mixed fractions are shown.

material fractions exceed the limit values for Cd, Cr, Pb and Tl, independent of the treatment plant and size fraction considered. Size fractions < 4 mm also exceed the limit value for Sb. The mass fractions of Cr and Ni depend largely on the share of stainless steel that is not recovered during bottom ash processing and therefore remains in the mineral fraction, as can be seen in Fig. 8. The highest values for Cr and Ni (up to 4,370 mg/kg and 1,910 mg/kg, respectively) can be found in the mineral material 16–50 mm of Plant E, because this size fraction is enriched in stainless steel and no recovery of stainless steel < 50 mm is conducted in Plant E. Bottom ash to be used as aggregate in road construction in Austria has to comply with the limit values set in the federal waste management plan (BMNT, 2018). There are limit values for total and leachate contents. As the leachate contents are not assessed in the present study, only a comparison of the mineral material with the limit values for total contents (for Cd, Cr, Ni and Pb) can be conducted. Only the mineral material > 50 mm from Plants A to E and the swimming material from Plant B comply with all limit values for total contents. However, the swimming materials contain only about 50% mineral material and about 50% unburnt organic matter, which consists mainly of C, H, O and N. Hence, the mineral material is diluted in the swimming fraction, which can definitely not be utilised in road construction.

4. Conclusion In the present study, detailed data about the composition of MSWI bottom ash was used to successfully establish MFA models of five different bottom ash treatment plants. Thereby, the performance of each bottom ash treatment plant with regard to the separation efficiency of different materials (magnetic metals, nonferrous metals, stainless steel, etc.) could be assessed. It has to be noted that the validity of the conclusions can only be guaranteed based on the processes described in this manuscript with the transfer coefficients given. Other processes with different transfer coefficients could lead to different conclusions. From the results on the quantity of metal scrap separated for recycling by each bottom ash plant, the following conclusions can be drawn. Ageing processes may decrease the leachability of heavy metals, but they also lead to partial oxidation of metals in bottom ash (up to about 15%). Therefore, ageing of bottom ash should be performed after the separation of metals in order to maximise the metal yield. Furthermore, crushers have to be used to further increase the amount of metal recovered (up to about 35%) because they liberate metal pieces encapsulated in mineral material. The price for this increased recoverability of the metals is the higher share of smaller mineral particles after crushing,

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which cannot be used as aggregate for road construction and other applications. E.g. the two-stage crusher of Plant E causes an increase in fine fraction (0–4 mm) mass flow by about 270% compared to Plant A. Therefore, an optimum of comminution has to be found depending on the value of metals encapsulated in bottom ash (based on their mass fraction and scrap prices) and the value of the mineral material for use as an aggregate compared to the value of the mineral material as cement raw material and the costs of bottom ash disposal on landfills. In case of the utilisation of mineral material for clinker production or as an aggregated in reinforced concrete, wet treatment might be advantageous as it decreases the Cl content. E.g. the decrease in Cl in Plant B is up to about 20% compared to the bottom ash feed. However, more thorough washing with more water could lead to an even better Cl removal. As iron scrap recovered from bottom ash is significantly contaminated with Cd (up to 200 mg/kg) and Cu (up to 10,000 mg/ kg), the separation of batteries, transformers and electric motors prior to recycling of this iron scrap is crucial to increase secondary steel quality and decrease the risk of heavy metal emissions. It would be even more advantageous to improve separate collection so that these devices do not end up in the MSWI plant. The recycling of high quantities and qualities of metals and minerals from bottom ash is not only an economic issue but also important with regard to legal circumstances. On the one hand the Industrial Emissions Directive (European Parliament, 2010) of the European Union states that residues from waste incineration shall be recycled, where appropriate, and on the other hand according to the Waste Framework Directive (European Parliament, 2018) metals recovered from bottom ash can be taken into account to achieve the recycling targets. The mineral material fractions generated by different treatment plants show only minor differences regarding composition, whereby the main factor determining the total heavy metal content is the recovery of metals like stainless steel. E.g. about 50% of Cr and almost all Ni are removed in Plant B. Mineral material fractions with the exception of the material > 50 mm do not comply with the Austrian limit values for unlimited use for clinker production and for use in road construction. Nevertheless, a limited amount of bottom ash may still be used in clinker production, if the limit values for total contents of heavy metals in the cement produced thereby are complied with. However, certain fractions of mineral material could comply with the regulations of other countries, as in some countries (e.g. the Netherlands) no limit values for total contents but only for leachate content have to be complied with to be able to utilise MSWI bottom ash. Although the models established in the present study already help to understand the material flows through bottom ash treatment plants, further efforts are necessary to validate these models. Furthermore, future research should strive for the integration of other advanced and promising bottom ash treatment techniques like the separation of glass in the models. This is important as glass makes up about 10–20% of bottom ash and most of it is packaging glass that can potentially be recycled if separated from the bottom ash. Even if waste glass from separate collection has a higher quality than glass from bottom ash, recovery of glass from ashes can still contribute to higher recycling rates because separate collection does not cover 100% of waste glass. Additionally, bottom ash from different locations should be considered. However, hardly any literature is available that shows the characteristics of bottom ash to such a high level of detail as necessary for the use of the model developed. Hence, there is also a demand for more data on the detailed grain sized specific composition of MSWI bottom ashes. For the complete assessment of bottom ash treatment plants, data on leaching and mechanical properties are necessary as well. If models precisely representing real bottom ash treatment

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plants are established, the results of these models can also be validated by analysis of products generated by these plants. Declaration of Competing Interest The author declares that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work presented is part of a large-scale research initiative on anthropogenic resources (Christian Doppler Laboratory for Anthropogenic 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 author wants to express his 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. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2020.01.034. References 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. Altman, D.G., Bland, J.M., 2005. Standard deviations and standard errors. BMJ 331, 903. https://doi.org/10.1136/bmj.331.7521.903. Astrup, T., Muntoni, A., Polettini, A., Pomi, R., Van Gerven, T., Van Zomeren, A., 2016. Chapter 24 - Treatment and Reuse of Incineration Bottom Ash, in: Prasad, N.M. V., Shih, K. (Eds.), Environmental Materials and Waste. Academic Press, pp. 607– 645. https://doi.org/10.1016/B978-0-12-803837-6.00024-X 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. Björkman, B., Samuelsson, C., 2014. Chapter 6 - Recycling of Steel. In: Worrell, E., Reuter, M.A. (Eds.), Handbook of Recycling. Elsevier, Boston, pp. 65–83. https:// doi.org/10.1016/B978-0-12-396459-5.00006-4. Blasenbauer, D., Huber, F., Lederer, J., Quina, M.J., Blanc-Biscarat, D., Bogush, A., Bontempi, E., Dahlbo, H., Blondeau, J., Fagerqvist, J., Chimenos, J.M., GiroPaloma, J., Hjelmar, O., Hykš, J., Keaney, J., Lupsea-Toader, M., O’Caollai, C.J., Orupõld, K., Paja˛k, T., Simon, F.-G., Svekova, L., Šyc, M., Ulvang, R., Vaajasaari, K., Van Caneghem, J., van Zomeren, A., Vasarevicˇius, S., Wégner, K., Fellner, J., 2020.. Legal situation and current practice of waste incineration bottom ash utilisation in Europe. Waste Manage. 102C, 868–883. https://doi.org/10.1016/j. wasman.2019.11.031. BMLFUW, 2016. Technische Grundlagen für den Einsatz von Abfällen als Ersatzrohstoffe in Anlagen zur Zementerzeugung. https://www.bmnt.gv. at/dam/jcr:db653bd3-f77e-41d2-afad-fdf4ddd63abd/Technische% 20Grundlagen%20f%C3%BCr%20den%20Einsatz%20von%20Abf%C3%A4llen% 20als%20Ersatzrohstoffe%20in%20Anlagen%20zur%20Zementerzeugung_ Dezember%202017_2.%20Auflage.pdf (accessed 28.05.2018). BMNT, 2018. Bundesabfallwirtschaftsplan 2017. https://www.bmnt.gv.at/dam/jcr: 33627731-8a48-4b78-aadd-8a5c7b48e3e3/BAWPL_2017_Teil_1_Ver%C3% B6ffentlichung%202018-01-17_BMNT.pdf (accessed 14.03.2018). Brunner, P.H., Ma, H.-W., 2009. Substance flow analysis. J. Ind. Ecol. 13, 11–14. https://doi.org/10.1111/j.1530-9290.2008.00083.x. Brunner, P.H., Rechberger, H., 2015. Waste to energy – key element for sustainable waste management. Waste Manage. Special Thematic Issue: Waste Energy Process. Technol. 37, 3–12. https://doi.org/10.1016/j.wasman.2014.02.003. Brunner, P.H., Rechberger, H., 2004. Practical handbook of material flow analysis. Lewis Publ, Boca Raton, Florida. 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

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