*Article* **A Novel Dry Treatment for Municipal Solid Waste Incineration Bottom Ash for the Reduction of Salts and Potential Toxic Elements**

**Marco Abis 1,\*, Martina Bruno 2, Franz-Georg Simon 3, Raul Grönholm 4, Michel Hoppe 5, Kerstin Kuchta <sup>1</sup> and Silvia Fiore 2,\***


**Abstract:** The main obstacle to bottom ash (BA) being used as a recycling aggregate is the content of salts and potential toxic elements (PTEs), concentrated in a layer that coats BA particles. This work presents a dry treatment for the removal of salts and PTEs from BA particles. Two pilotscale abrasion units (with/without the removal of the fine particles) were fed with different BA samples. The performance of the abrasion tests was assessed through the analyses of particle size and moisture, and that of the column leaching tests at solid-to-liquid ratios between 0.3 and 4. The results were: the particle-size distribution of the treated materials was homogeneous (25 wt % had dimensions <6.3 mm) and their moisture halved, as well as the electrical conductivity of the leachates. A significant decrease was observed in the leachates of the treated BA for sulphates (44%), chlorides (26%), and PTEs (53% Cr, 60% Cu and 8% Mo). The statistical analysis revealed good correlations between chloride and sulphate concentrations in the leachates with Ba, Cu, Mo, and Sr, illustrating the consistent behavior of the major and minor components of the layer surrounding BA particles. In conclusion, the tested process could be considered as promising for the improvement of BA valorization.

**Keywords:** bottom ash; dry treatment; incineration; municipal solid waste; potential toxic elements; salts

### **1. Introduction**

The mining of mineral aggregates is the largest extractive sector in the EU, which, on its own, exceeds the amount of all the minerals produced [1]. Nevertheless, its End-of-Life (EoL) input rate was estimated to be only 8 wt %. Typical EoL materials used as aggregates are construction and demolition waste (CDW) and bottom ash (BA) from municipal solid waste incineration (MSWI). However, the full recycling potential of these waste flows have not been tapped yet due to the existing gap between market prices and extraction, processing, and transportation costs [2].

BA is the main by-product of MSWI and counts approximately 25 wt % of MSW input for thermal valorization [3,4]; 71 Mt of MSW incinerated in Europe in 2018 produced about 18 Mt of BA. Current full-scale material recovery technologies applied to BA mainly focus on the separation of metals, with the most valuable components being aluminum and copper [5,6]; this leaves the mineral fraction unexploited, and which is usually landfilled. The mineral fraction has been estimated at 85–90 wt % of BA [4], resulting in 15–16 Mt/y

**Citation:** Abis, M.; Bruno, M.; Simon, F.-G.; Grönholm, R.; Hoppe, M.; Kuchta, K.; Fiore, S. A Novel Dry Treatment for Municipal Solid Waste Incineration Bottom Ash for the Reduction of Salts and Potential Toxic Elements. *Materials* **2021**, *14*, 3133. https://doi.org/10.3390/ma14113133

Academic Editor: Antonio Gil Bravo

Received: 14 April 2021 Accepted: 21 May 2021 Published: 7 June 2021

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of materials in Europe that could potentially be recycled as aggregates. Several studies have recently explored recycling alternatives for the BA mineral fraction to be used as construction material, e.g., as the sub-base layer for asphalt roads [7], as the source of construction sand [8] and as substitute material for concrete production [9,10]. The recycling of the BA mineral fraction as a secondary aggregate holds the potential of enhancing the profitability of BA management and is fully consistent with EU policy on Circular Economy. Material recovery from BA could entail a significant improvement in the circularity of the management of resources, not only limiting the request for primary aggregates but more particularly reducing the amount of waste sent to the landfill. Despite the low commercial value of secondary mineral aggregates, potential savings from landfill fees could entail significant economic benefits and ensure the profitability of BA mineralfraction management. The production of primary aggregates in Europe in 2018 was 2431 Mt, which was composed of sand, gravel, and crushed rocks [11]. Considering this mass, the mineral fraction of BA could potentially replace 0.7 wt % of primary aggregates produced in Europe. This value was consistent with a recent study [12], which estimated a 0.6% potential substitution rate. Nonetheless, since about 806 Mt of non-hazardous waste generated in Europe are currently disposed in landfills, the recycling of the BA mineral fraction as a secondary aggregate might divert circa 2 wt % of the waste stream directed to landfills.

However, the potential reuse of the BA mineral fraction as a secondary aggregate is hindered by the presence of Potential Toxic Elements (PTEs) [13], which have negative environmental impacts [14,15]. Among the PTEs, cadmium, chromium, and molybdenum were the most found in MSWI BA [16,17]. Most European countries set threshold limits for recycled aggregates due to PTEs and chloride and sulphate leaching. A detailed analysis of the different leaching tests applied to BA and related limits in Europe were presented in a recent study [12]. In this framework, special attention should be devoted to studying BA composition, not only to detect the mineral phases for further geochemical dissolution modelling, but also to map their occurrence and locations in the solids. X-ray diffraction analyses (XRD), energy dispersive X-ray spectroscopy (EDX), and Scanning Electron Microscopy (SEM) were some analytical techniques adopted for such investigations. The key result of those characterization studies [18] was that the presence of chloride and sulphate salts is mostly limited to the surface layer coating the coarser BA particles. This observation justified the strong drops in chloride concentration observed by different authors [5,14,19] while performing percolation leaching tests on BA after washout and dissolution treatments. Similarly, the presence of other sparingly soluble salts, such as calcium sulphate and their similar releasing mechanisms, also suggests their accumulation in a layer coating the coarser BA particles. Finally, the existence of several PTEs was linked with the presence of calcite (CaCO3), melilite, and iron oxides [16], and weathering products such as gypsum, ettringite, and zeolite have been proven to contain high amounts of PTEs.

Several efforts have been devoted to reducing the amounts of salts [20,21] and of the metals Zn, Cu, and Ni [22] from BA through the application of intensive washing processes, which were proven to be rather effective in the removal; however, run-off waters resulted as contaminated from the presence of heavy metals, chlorides, and sulphates [23]. Particularly, concern arose from the leaching of copper [24] and antimony [25], exceeding wastewater discharge limits. Therefore, wet processes aimed at reducing the leaching of salts and PTEs from the mineral fraction of BA, even if effective, still presented several critical downstream issues (e.g., wastewater treatment, sludge thickening, and disposal) in need of optimization. To our knowledge, no literature is available on dry treatment processes applied to the mineral fraction of BA; a dry process, if effective in reducing the release of salts and PTEs, could avoid any wastewater in need of further treatment. In this study, a novel dry treatment process was explored, based on the findings that most of the salts and PTEs released from BA are located on a thin, superficial layer coating coarser BA particles, which can be selectively removed by controlling the mutual abrasion of the particles in a tumbling mill. More precisely, while processing aggregates in a tumbling mill,

the grinding of the material occurs. Grinding is driven by three main mechanisms [26]: impact (compression), chipping (attrition), and abrasion [27] when applied with normal, oblique, and parallel forces, respectively (Figure 1). The main objective of this study was to avoid and minimize the compression and chipping forces, and to maintain BA coarseness, while promoting the abrasion on the particles' surfaces.

**Figure 1.** Grinding mechanisms: (**a**) compression, (**b**) chipping, (**c**) abrasion (adapted from Wills and Finch 2015 [27]).

The forces acting in a tumbling mill can be controlled by varying the operating rotational speed. High rotational speeds are associated with impact and compression forces, generated by particles that are cataracting and cascading (Figure 2).

**Figure 2.** Aggregate motion in a tumbling mill (adapted from Ali et al., 2019 [28]; Wills 2016 [29]; Wills and Finch 2015 [27]).

Alternatively, abrasion is promoted, operating at rotational speeds below 30% of the critical speed Cs, above which particles are only centrifuged without being ground [30]. For low rotational speeds, particles slide on the drum surface remaining in the abrasion zone (Table 1).

**Table 1.** Empirical description of the grinding actions at different percentages of Cs. Numbers from 1 to 3 indicate the ascending degree of action (adapted from Gupta and Yan 2016 [30]).


Consequently, the dry treatment process presented in this work did not rely simply on dissolution and mass transfer, which divert the contaminants to wastewater in need of further treatment (as in wet processes); instead, it took advantage of the natural abrasive behavior of BA particles, which scrub off each other's external contamination. Moreover, abrasion allows BA particles to preserve their original size, and therefore remain suitable for recycling as secondary aggregates. The output streams of the treatment investigated in this study were a cleansed coarse fraction of inert material and a fine fraction easily removable by sieving. In particular, the intensity of the abrasion process was controlled to limit its effect on the superficial abrasion of the particles' external layers, avoiding comminution and subsequently, particle-size reduction. This work was aimed at exploring the feasibility of a dry abrasion treatment for the reduction of the BA leaching potential for salts and contaminants, in the light of BA reuse as building materials. Furthermore, the effect of the applied treatment was assessed, comparing the leaching behaviors of treated and untreated BA samples. This approach was an advancement with regard to the current industrial trends, where there is no industrial alternative to the wet treatment of BA. To our knowledge, there are still no literature studies investigating abrasion processes applied to BA at the laboratory scale. The work in the present study was performed with batch experiments only within a laboratory setting. However, tumbling mills were applied in the comminution process of ores in continuous operation mode with throughputs in the range of thousands t/h [27]. In industrial processing plants, it is possible to increase the residence time in abrasion units, as well as install liners on the inner surface of rotating trommels in order to decrease the dimension of the processing unit and optimize its filling ratio.
