Waste incineration is one of the most common treatment methods in waste management. It allows waste mass and volume reduction. Nevertheless, it causes growth in the worldwide production of waste incineration residues. During thermal treatment, waste is detoxified and transformed into three solid materials: bottom ash (BA), fly ash (FA) and air pollution control (APC) residues. As concentrates of inflammable waste fraction, they are composed of newly formed phases and components inherited from waste such as metallic fragments, fragmented glass or ceramic (Chandler et al., 1997). Elements inherited from waste are entrapped in incineration residues and excluded from their natural cycles; therefore, procedures for the post-treatment management or recycling of incineration residues need to be developed. Most metallic elements concentrate in BAs thus, they might be considered as a source of valuable components or waste-based raw material in industrial applications (Funari et al., 2015; Lam et al., 2010; Kowalski et al., 2016).

BAs are heterogeneous grainy materials. Their composition is mainly dependent on the local waste stream composition, applied waste management system and technology of incineration. Besides amorphous and crystalline components formed during incineration, BAs are rich in residual components inherited from waste (often barely affected by high temperature). Also, metallic components are present in the BAs, even if the recycling of metal products is performed prior to the thermal treatment (Chandler et al., 1997; Saffarzadeh et al., 2009; Wei et al., 2011). After thermal processing, the next stage of recycling in the incineration plant is usually performed by using single magnets or eddy-current separators. The process is effective for the separation of large fragments of metal products released in the furnace from waste. Nevertheless, it is not possible to recover single or small metal fragments without fragmentation of the BA grains.

Common metallic elements such as Fe, Al and Mg are present in the BAs in the highest amount, while up to a few wt % of other metallic elements is also often present (e.g. Ti, Zn, Cu, Cr). Currently, it is considered that the recovery of single elements might not be economically reasonable because of the presence of easy accessible natural or secondary resources. Nevertheless, incineration residues could be treated as easily accessible, produced continuously in large amounts, and a waste-based source of at least a few metallic elements (Kuo et al., 2007). From this point of view, the value of elements also present in minority might be encouraging for developing procedures of their separation and recovery. For the estimation of BAs' resource potential, the basic studies of minor metallic elements are of great importance. Based on this presumption, the main focus of this study was on BAs' minor metallic elements, especially those present as side elements in alloys and dispersed within various BA phases. We also focus on the distribution of minor metallic elements within the phases in two types of BA produced from waste generated in households and industry.

The BAs were materials rich in Si, Ca, Fe, Na and Al. In lower amounts, Mg, Ti, C and S were present (Table 1). In comparison to the MSWI BA, the IHWI BA had less Si and Ca, similar content of Fe, Mg and usually slightly higher content of other main elements. In both types of BAs, the content of particular elements varied in some range when comparing single samples collected in different sampling campaigns (standard deviation values for the analysed group of samples are given for each value in Tables 1 and 2). The IHWI BA chemical composition was less uniform, which could be expected due to the fact that the output waste mixture was much more diversified than the MSWI BA. Additionally, Chandler et al. (1997) reported the variability of BA chemical composition mostly as a result of changes in the incinerated waste composition, but also as a result of the effectiveness of the waste management system and the used techniques of waste pre-treatment and BA valorisation in the incineration plant. In contrast to the variability of chemical composition, BA from the industrial waste was macroscopically more homogeneous and contained smaller amounts of residual components (only 0.5 wt %–2.0 wt % of metallic fragments). The MSWI BA contained 25 wt %–35 wt % of fragmented glass, 2.5 wt %–5.5 wt % of metal and 2.0 wt %–7.0 wt % of ceramic products.

The chemical composition of the BAs was reflected in their phase composition. Both types of BA contained a high amount of amorphous phase, mostly represented by silicate glass, although its content was higher in the IHWI BA (55 wt %–65 wt % in MSWI BA vs. 65 wt %–80 wt % in IHWI BA) (Kowalski et al., 2017). Crystalline phase was composed of several phases in both types of BA. The main minerals were silicates and aluminosilicates containing Ca and alkalis, carbonates and oxides. In smaller amounts phosphates, sulphates and chlorides were present. The MSWI BA contained more quartz (SiO2), and melilite group minerals (Ca2Mg[Si2O7] - Ca2Al[AlSiO7]), whereas the IHWI BA was rich in sulphate minerals (Ca[SO4] ⋅ nH2O; Na2Ca[SO4]2; Na2[SO4]), calcite (Ca[CO3]) and Fe-oxides (FexOy).

In both types of BA, the group of minor elements was dominated by metallic elements. Ti, Cu, Zn, Mn and Cr were the most important (Table 2). Kuo et al. (2007) concluded that Cu, Zn, Cr and Pb from the group of minor metallic elements were concentrated mostly in the BA and are promising from the point of view of their recovery. Similar to the major elements, the content of minor components was diversified, which made the estimation of averaged values difficult, and for the average data to be reliable it was necessary to analyse a significant amount of the material. In general, the IHWI BA had remarkably higher content of minor metallic elements, and despite the level of variability this material seems to be more promising from the point of view of the metallic elements' recovery. Also in this material, in some samples a much higher content of particular elements was detected (e.g. in IHWI BA >1 wt % of Zn) (Table 2).

If considering the total content of metallic elements (Al, Fe, Mg, Ti, Mn, Cr, Ni, Mo, Cu, Pb, Zn and Sn), they constituted >12.7 wt % (8.8 wt %–17.1 wt %) of the MSWI BA and ∼17.6 wt % (11.5 wt %–23.1 wt %) of the IHWI BA. In both materials, 2/3–4/5 of the given amounts were Fe and Al. Metallic elements in both BA types were mostly concentrated in the form of metallic fragments, mainly as metallic inclusions (in the size of 1–15 µm) and separated metallic grains (in the size of 50–300 µm). Those occurrences were dominated by Fe- and Al-rich fragments, whereas others (Cu, Zn, Ni and very few Pb, Ti, Cr, Sn-rich metallic fragments) were present less frequently. Similar forms and types of metallic occurrences were described by Wei et al. (2011). Also Funari et al. (2015) concluded that metallic elements are mostly concentrated in the fine-grained fraction of BAs and concentrations of some of them (e.g. Cu, Zn, Pb, Ni) are close to values in low-grade ores. Most of the minor metallic elements in the BA were admixtures in multi-element metallic occurrences and other non-metallic phases. In the MSWI BA (based on WDX analysis), the usual content of Pb in the BA components was <0.15 wt %. Pb usually co-occurred with Fe, Sn or Zn and was rarely present in the glass. From 25 wt % to 85 wt % of the Pb contained Pb-rich occurrences and sulphides. Cu was mostly present in a form of metallic fragments containing 35 wt %–85 wt % of the element (also several wt % in sulphides). Rarely, 0.1 wt %–0.5 wt % of Cu was measured in the composition of Fe- and Al-rich grains. From 0.1 to 0.5 wt % of Mn/Cr were usually present in Fe-rich occurrences. Several percent of them were present in alloys with Al, Fe and Cr (Mn) or Fe, Pb and Mn (Cr). Zn was usually measured in Fe-rich grains in low amounts (0.1 wt %–0.3 wt %), but 30 wt %–90 wt % was present in Zn-rich occurrences and sulphides.

In the IHWI BA components (based on WDX analysis), Cu (0.2 wt %–4.0 wt %) was present mainly in multi-element metallic occurrences. Higher content was detected in the composition of sulphides (20 wt %–40 wt %) and Cu-dominated fragments (50 wt %–80 wt %). Pb (0.1 wt %–0.3 wt %) was present in Fe-, Al- and Sn-rich metallic fragments. In Pb-rich occurrences, its content reached 50 wt %–90 wt %. Zn usually occured in multi-element occurrences and its content was in a range of 0.2 wt %–2.0 wt %. Higher values of Zn (20 wt %–50 wt %) were present in sulphides and occurrences dominated by Fe. Cr (0.1 wt %–1.0 wt %) was usually measured within Fe-, Al- or Ti-rich fragments. Higher amounts of the element (15 wt %–25 wt %) were present in multi-element occurrences. Mn was commonly present in various metallic fragments but its content was usually below 0.5 wt %.

As presented in Figs. 1 and 2, microprobe WDX maps were generated in areas of glassy matrix containing grains rich in metallic inclusions. The results indicated that Al and Fe were concentrated mostly in discrete grains and large inclusions often containing several other elements in low amounts. Mn and Cu were present as minor components of metallic occurrences. Ti occurred in marginal zones of metallic Fe-rich fragments, Cr was concentrated in the form of micro-inclusions, while Sn and Pb were dispersed evenly with local spots of enrichment. This type of pattern was common among the glassy matrix of the BA grains where metallic inclusions were present. Areas of local enrichment in metallic elements were usually surrounding groups of metallic inclusions. As suggested by Saffarzadeh et al. (2009), metallic components could be fixed within the silica-rich matrix as a result of complex reactions in the furnace. This reaction seems to immobilise metals and make their recovery difficult, since silica is known to be a stabilising agent for heavy metals (Rodella et al., 2017). Minor metallic elements occurred in various types of BAs; they were present in similar forms and occurred as multi-element associations. Allegrini et al. (2014) estimated the recovery efficiency for Fe and nonferrous metals to be 85 % and 61 %, respectively, which enables us to consider BA as an easily accessible and cheap waste-based source of important elements. Nevertheless, the recovery processes could be impeded because advanced recovery techniques need to be applied for dispersed elements or those entrapped within a number of phases (often as trace components), while the multiplicity of metallic fragments' chemical composition might be problematic, since a number of them contained a high amount of Al and Fe.

Despite the differences in composition and type of waste they originate from, a number of similarities occurred during the analysis of both types of BA. They were composed of similar mineral phases, types of grains and had remarkably similar internal structures. Metallic elements represent 8.8 wt %–23.1 wt % of the BA (more metallic elements were concentrated in the IHWI BA). Besides high content of Fe and Al (ca. 10.8 wt %–12.1 wt %), the elevated content of Ti (∼3.0 wt % in the IHWI BA), Cu (0.1 wt %–0.4 wt %) and Zn (∼0.6 wt % in the IHWI BA) was measured. The BAs' chemical composition was diversified, but these variations did not affect the forms of metallic occurrences. Metallic elements were concentrated in both types of BA in the form of metallic inclusions (in the size of 1–15  µm) and separate metallic grains (in the size of 50–300 µm), mostly in the form of a few elements' occurrences. More metallic grains were present in the IHWI BA, despite the material being more diversified.

Minor metallic elements present in the BAs might be interesting from the point of view of their recovery. The most promising in terms of their recovery are occurrences dominated by a particular element, where its content is usually much higher than 50 wt %, and when the metallic fragment is in the form of a separate grain or a component weakly bonded in the grains' aggregate.