Material analysis of Bottom ash from waste-to-energy plants

Characterization of lightweight aerated mortars using waste-to-energy bottom ash (WtE-BA) as aerating agent

The management of Waste-to-Energy Bottom Ash (WtE-BA), generated during the incineration of waste, poses a global challenge. Presently, the majority of WtE-BA is disposed of in landfills due to the lack of alternatives. Meanwhile, the construction industry remains the primary consumer of raw materials and significantly contributes to Greenhouse Gas Emissions. This study attempts to address these issues by utilizing the fine fraction of WtE-BA (<2 mm) as a raw material for aerated mortar production. Thanks to its metallic aluminum content, WtE-BA is utilized as an aerating agent. The study investigates how the quantities of water and WtE-BA, as well as its granulometric sub-fractions, impact the properties of the final product. An analysis of properties such as density, compressive strength, and thermal conductivity was conducted. Additionally, the environmental impact of each raw material (i.e. WtE-BA, cement and sand) was assessed through leaching tests and elemental content analysis enabling the determination of their individual contribution to the presence of trace elements in the produced mortars. The aforementioned properties are discussed using microstructure and porosity analyses. The findings demonstrate that the quantity of water is a crucial factor in controlling the aeration of mortars, whereas the granulometry of the WtE-BA particles did not significantly affect their macro-properties. Furthermore, this study highlights that WtE-BA based mortars has the potential to exhibit better environmental and insulating performances than standard aerated mortar of equal density and strength. The differences in pore size and type between WtE-BA and aerated mortars can account for the variation in performance. Thus, WtE-BA proves to be an effective substitute for aerating agent in the production of aerated mortars.

Wet shaking table operating parameters optimization for maximizing metal recovery from incineration bottom ash fine fraction

Municipal solid waste incineration bottom ash has emerged as a secondary source of valuable metals, including aluminum and copper. Specifically, the fine fraction, with a particle size less than two millimeters, exhibits average grades of 2.5 g/kg for copper and 10.0 g/kg for elemental aluminum. Gravimetric concentration equipment, such as wet shaking tables, is widely used in the mining and recycling industries to concentrate materials based on density. However, the optimization of these devices typically relies on a trial-and-error approach. This paper presents a statistical model that optimizes the crucial working parameters of the wet shaking table for recovering elemental aluminum in a light product mineral matrix and copper in a high-density product. The statistical analysis highlights that upper values of shaking amplitude are beneficial for both the recovery and grade of copper and aluminum in the obtained products. Conversely, variations in other parameters, such as wash water or desk tilt, yield contrasting effects on grade and recovery. By precisely adjusting the working parameters of the device, the analyses demonstrate that copper enrichment of up to 45 times can be achieved in the product smaller than 500 µm and up to 15 times in the 500-2000 µm product, resulting in the recovery of approximately 65 % of the total copper.

Glass recovery and production of manufactured aggregate from MSWI bottom ashes from fluidized bed and grate incineration by means of enhanced treatment

Enhanced treatment of incineration bottom ashes (IBA) from municipal solid waste incineration can contribute to a circular economy since not only metals can be recovered but also glass for recycling. Moreover, the remaining mineral fraction can be utilized in concrete as manufactured aggregate. To evaluate the effects of an enhanced treatment, three IBAs from fluidized bed combustion (FB-IBAs) and three grate incineration bottom ashes (G-IBAs) were standardly treated in a jig and further processed on a pilot scale, including improved metal recovery and sensor-based glass separation. The removed glass fractions were weighed and their composition was assessed by means of manual sorting. The manufactured aggregate was also sorted manually and its total and leachate contents were determined before and after aging. Results showed general differences between FB-IBAs and G-IBAs. For G-IBAs, higher contents of heavy metals and residual metal pieces were determined, while the share of glass removed was low compared to FB-IBA. The treated mineral fractions from G-IBA contained more mineral agglomerates, whereas FB-IBAs contained more glass. However, the glass-fractions removed from FB-IBAs need further treatment to be accepted in glass recycling. Austrian limit values for utilization in concrete were met by all manufactured aggregates produced from FB-IBA, but only by one from G-IBA. Overall, the enhanced treatment in the study performed well compared to the literature. Nevertheless, further investigations are necessary to improve the recyclability of the recovered glass fractions and to determine the technical suitability of manufactured aggregates produced from IBAs.

Distributions of trace elements within MSWI bottom and combined ash components: Implications for reuse practices

Concentrations of 25 inorganic elements were measured in both bulk ash and individual ash components from residuals at three municipal solid waste incineration (MSWI) facilities in the US (two combined ash (CA) and one bottom ash (BA)). Concentrations were assessed based on particle size and component to understand the contribution from each fraction. The results found that among facilities, the finer size fractions contained elevated concentrations of trace elements of concern (As, Pb, Sb) when compared to the coarse fraction, but concentrations varied among facilities depending on the type of ash and differences in advanced metals recovery processes. This study focused on several constituents of potential concern, As, Ba, Cu, Pb, and Sb, and found that the main components of MSWI ash (glass, ceramic, concrete, and slag) are sources of these elements in the ash streams. For many elements, concentrations were significantly higher in CA bulk and component fractions opposed to BA streams. An acid treatment procedure and scanning electron microscopy/energy-dispersive x-ray spectroscopy analysis revealed that some elements, such as As in concrete, are result of the inherent properties of the component, but other elements, such as Sb, form on the surface during or after incineration and can be removed. Some Pb and Cu concentrations were attributed to inclusions in the glass or slag introduced into the material during the incineration process. Understanding the contributions of each ash component provides critical information for developing strategies to reduce trace element concentrations in ash streams to promote reuse opportunities.

Comparing the quantity and quality of glass, metals, and minerals present in waste incineration bottom ashes from a fluidized bed and a grate incinerator

Bottom ash is the primary solid residue arising from municipal solid waste incineration. It consists of valuable materials such as minerals, metals and glass. Recovering these materials from bottom ash becomes evident when integrating Waste-to-Energy within the circular economy strategy. To assess the recycling potential from bottom ash, detailed knowledge of its characteristics and composition is required. The study at hand aims to compare the quantity and quality of recyclable materials present in bottom ash from a fluidized bed combustion plant and a grate incinerator, both located in the same city in Austria and receiving mainly municipal solid waste. The investigated properties of the bottom ash are grain-size distribution, contents of recyclable metals, glass, and minerals in different grain size fractions, and the total and leaching contents of substances in minerals. The study results reveal that most recyclables present are of better quality for the bottom ash arising at the fluidized bed combustion plant. Metals are less corroded, glass contains fewer impurities, minerals contain fewer heavy metals, and their leaching behavior is also favorable. Furthermore, recoverable materials, such as metals and glass are more isolated and not incorporated into agglomerates as observed in grate incineration bottom ash. Based on the input to the incinerators more aluminum and significantly more glass can potentially be recovered from bottom ash from fluidized bed combustion. On the downside, fluidized bed combustion produces about five times more fly ash per unit of waste incinerated, which is currently disposed of in landfills.

Utilization of Different Grain Size of Municipal Solid Waste Bottom Ash in High-Performance Mortars

Globalization is bringing increased industrialization and municipal solid waste (MSW). This is a major concern in heavily populated areas. In order to reduce MSW generation, incineration is commonly used, resulting in two types of ashes: bottom and fly ash. Bottom ash is gathered at the incineration bed and is larger in mass than fly ash. To test the qualities of high-performance mortar, MSW-BA in three sizes (fine, medium, and coarse) was replaced with sand at three replacement levels of 10%, 20%, and 30%. The high-performance mortar integrating MSW-BA was tested for hardened density, mechanical properties such as compressive and flexural strength, resistance to NaOH solution, and heavy metal leaching. The substitution level of MSW-BA increased the hardened density of the mortar mixes. The volume change and residual strength of the mortar mixes were measured following exposure to the NaOH solution. Fine-particle mortar mixes shrank whereas medium- and coarse-particle mortar mixes expanded. The largest loss in flexural and compressive strength was recorded when 20% of sand was replaced with a fine fraction of MSW-BA. Heavy metals including cadmium and copper were not leached from MSW-BA combinations of any size. The minuscule amounts of lead and zinc discovered were well below acceptable limits. The present study illustrates the MSW-BA can be utilized as a substitute for sand in the development of high-performance mortar.

Redistribution of mineral phases of incineration bottom ash by size and magnetic separation and its effects on the leaching behaviors

Size and magnetic separation of incineration bottom ash (IBA) are common for ferrous metals recovery, however, their influences on the mineral phase and the element redistribution, and subsequently the induced variation of metal leaching potential herein remain limited understanding. The lack of research in this field may misunderstand IBA performances, cause confused results for comparison among various studies, and potentially lead to biased conclusions. We herein quantitatively investigate the effects of size and magnetic separation on the IBA based on element distribution, leaching behavior, morphology, and mineralogy with statistical analysis. For preparation, sieving was performed with the original IBA (to obtain 7 size-fractions termed as OR1-7, respectively), followed by magnetic separation of each, to further yield magnetic fractions (MF1-7) to discriminate nonmagnetic fractions (NF1-7). In this study, we show that size and magnetic separation may pose significant yet different impacts on different fractions, which would affect their leaching potential concerning their respective downstream applications.

Leaching characteristics of heavy metals in MSW and bottom ash co-disposal landfills

Bottom ash (BA) management is often implemented through its co-disposal with municipal solid waste (MSW) in landfills. However, BA co-disposal may lead to heavy metal leaching in landfills. In this study, the effect of BA co-disposal on heavy metal leaching behavior under different scenarios, specifically, MSW, low BA co-disposal (BA_L), high BA co-disposal (BA_H), and BA monofill were investigated. The heavy metal concentrations in the leachate decreased in landfills over time. The leached metals primarily included Zn, Cu, Mn, Pb, Cr, and Cd. The discharge concentration ratio of heavy metals in the leachates exhibited the following decreasing order: MSW, BA_L, BA_H, and BA. In particular, the discharge concentration ratio of Cu in the MSW, BA_L, BA_H, and BA cases ranged from 7.1 × 10^-3 to 8.8 × 10^-1 (mean = 3.0 ×10^-1), 2.8 × 10^-4 to 2.0 × 10^-1 (mean = 5.4 ×10^-2), 9.1 × 10^-5 to 3.0 × 10^-2 (mean = 5.9 ×10^-3), and 4.4 × 10^-4 to 7.9 × 10^-3 (mean = 1.8 ×10^-3), respectively. Moreover, the leaching of the heavy metals could be attributed to waste contents, properties of the heavy metals, and leachate characteristics, such as the pH, chemical oxygen demand (COD), alkalinity, and Cl^- content. The presented findings can help clarify the leaching characteristics of heavy metals in BA co-disposal landfills, thereby facilitating the optimization of practical landfills.

Characteristic properties and recyclability of the aluminium fraction of MSWI bottom ash

The increasing use of aluminimum in packaging applications results in many different aluminium-based products ending up in consumer mixed-waste bins. This waste is typically incinerated, generating an aluminium-containing bottom ash. The current work investigates the recyclability of the aluminium fraction in the bottom ash from waste incineration plants in the USA, UK and Denmark. Incinerated Al-samples from different size fractions (2-6 mm, 6-12 mm and 12-30 mm) were characterized in terms of inherent oxide thickness, re-melting yield/coagulation and composition. The measured average oxide thickness on Al particles was 68 µm (SD=100), with the metal yield and coagulation efficiency measured to between 76 and 92% and 87-99% respectively. Larger particle size fractions resulted in a higher metal yield due to their higher mass to surface ratio. A simplified model correlating metal yield and particle size was proposed. The aluminium content of the melted material was determined to between 95.6 and 98.5% with main impurities being Fe, Si, Mn, Zn, Mg and Cu, corresponding to major aluminium alloying elements and waste charge components.

Distribution of recoverable metal resources and harmful elements depending on particle size and density in municipal solid waste incineration bottom ash from dry discharge system

Although municipal solid waste incineration bottom ash (BA) has the potential to be used as a metal resource, it raises concerns about the potential release of harmful elements into the environment. Element distribution in terms of particle size and density should be assessed to determine the fractions for the metal resources' recovery and to remove harmful elements. For this purpose, this study proposed a series of sorting processes based on the distribution of 25 elements in the sorted fractions by sieving, magnetic separation, air table sorting, and milling from dry BA < 8 mm. The Ca, Na, Mg, P, S, Cl, and Ti contents exhibited a decreasing tendency with increasing particle density and could affect the formation of low-density particles. The highest density fraction of non-magnetic components of 0.5-8 mm had abundant metal particles and recorded high Cu, Zn, Cr, Ni, Mo, Fe, Pb, Sb, and Au contents. In particular, the Cu (132000 mg-Cu/kg) and Zn (43000 mg-Zn/kg) contents demonstrated potential as metal resources. The fraction contained considerable proportions of Mo (77%), Cd (46%), Cu (39%), Zn (34%), Pb (26%), Au (40%), and Ag (18%) of the total amount. After milling and sieving of the highest density fraction, a substantial amount of Cd (44%), Cu (18%), Zn (12%), Pb (13%), and Ag (11%) were found in residual minerals; they could become harmful elements when recycled for construction purposes. The results show that air table sorting can separate metal resources and harmful elements before milling of BA.

Performance of structural concrete using Waste-to-Energy (WTE) combined ash

In the U.S., about 27 million metric tons of municipal solid waste are used as fuel in Waste-to-Energy (WTE) power plants, generating about seven million tons of mixed bottom ash and fly ash (combined ash) annually, which are disposed of in landfills after metal separation. This study assessed the effect of using combined ash as a substitute of mined stone aggregates on the mechanical properties and leachability of cement mortar and concrete. The as-received combined ash was separated into three fractions: fine (<2 mm), medium (2-9.5 mm), and coarse (9.5-25 mm). The substitution of up to 100% of stone aggregate by the coarse and medium fractions of combined ash produced concrete with compressive strength exceeding 28 MPa after 28 days of curing. Similar results were obtained when the fine combined ash was used as a sand substitute, at 10 wt%, in mortar. The concrete specimens were subjected to several days of curing and mechanical testing. The results were comparable to the properties of commercial concrete products. The mechanical test results were supplemented by XRD and SEM analysis, and leachability tests by EPA Method 1313 showed that the optimal concrete products effectively immobilized the heavy metals in the combined ash.

Metal recovery from incineration bottom ash: State-of-the-art and recent developments

Municipal solid waste incineration (MSWI) is one of the leading technologies for municipal solid waste (MSW) treatment in Europe. Incineration bottom ash (IBA) is the main solid residue from MSWI, and its annual European production is about 20 million tons. The composition of IBA depends on the composition of the incinerated waste; therefore, it may contain significant amounts of ferrous and non-ferrous (NFe) metals as well as glass that can be recovered. Technologies for NFe metals recovery have emerged in IBA treatment since the 1990s and became common practice in many developed countries. Although the principles and used apparatus are nearly the same in all treatment trains, the differences in technological approaches to recovery of valuable components from IBA - with a special focus on NFe metals recovery - are summarized in this paper.

Determination of metal-abundant high-density particles in municipal solid waste incineration bottom ash by a series of processes: Sieving, magnetic separation, air table sorting, and milling

Valuable non-ferrous (NFe) metals are contained in municipal solid waste incineration (MSWI) bottom ash (BA). The applicability of an air table for separating NFe metal-abundant particles in MSWI BA (<8 mm) was studied. A stepwise separation procedure was developed based on the performance tests of the air table conducted by changing three variables (air injection, vibration, and end slope). As a result of the stepwise separation, six bulk density ranges (<0.7 to >1.1 g/cm³ at intervals of 0.1 g/cm³) were prepared from non-magnetic fractions with four size ranges (4-8, 2-4, 1-2, and 0.5-1 mm). The effectiveness of air table sorting was evaluated based on dry particle density and proportion of metals. NFe metals were obtained by sieving (>0.5 mm) after a ball mill process, confirming that NFe metals accounted for 3.2% of the < 8 mm MSWI BA. The highest particle density of each size fraction and the proportion of NFe metals in the fractions were 3.19 g/cm³ and 59.9% (4-8 mm), 2.97 g/cm³ and 28.8% (2-4 mm), 2.78 g/cm³ and 10.4% (1-2 mm), and 2.87 g/cm³ and 4.4% (0.5-1 mm). In the 0.5-2 mm particles, the highest density fraction contained 68.7% of the NFe metals, which were expected to be recovered by applying an air table separator to MSWI BA 0.5-2 mm.

Complete determination of the material composition of municipal solid waste incineration bottom ash

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.

Melt Conditioned Direct Chill (MC-DC) Casting of AA-6111 Aluminium Alloy Formulated from Incinerator Bottom Ash (IBA)

The melt conditioned direct chill (MC-DC) casting process has been used for the production of billets of AA-6111 alloy formulated from recycled aluminium derived from incinerator bottom ash (IBA). The billets were homogenised and then extruded into planks. Optical metallography of the MC-DC billets showed equiaxed refined grains in comparison to DC and grain refined (DC-GR) billets formulated from the same scrap source. Microstructure evaluation for the extruded planks showed a less extensive peripheral coarse grain (PCG) for the MC-DC sample.

Chemical composition and leachability of differently sized material fractions of municipal solid waste incineration bottom ash

The chemical composition and leachability of municipal solid waste incineration bottom ash are important parameters determining its suitability for utilisation. The objective of the present study is to investigate the chemical composition of individual size and material fractions and their contribution to the total elemental contents of bottom ash. Nine bottom ash samples with a mass of 3000 kg each were sieved to eight size fractions and sorted into different materials. The materials (mineral material, glass, batteries) were separately analysed by inductively coupled plasma optical emission spectrometry after acid digestion. Additionally, x-ray fluorescence measurements and leaching tests were performed. Metals were further analysed by sorting analysis. The chemical analysis revealed that large particles have higher contents of Fe and Si, but lower contents of Ca and S compared to smaller particles. All mineral fractions exceed the legal limit values for utilisation in Austria mainly because of the total contents of Pb and Tl and the leachate contents of Cr and Sb. Glass from bottom ash is enriched in As, Na, Si and Tl compared to the mineral material. Although battery contents contribute only 0.2% to the total mass of bottom ash, they contribute at least 30% to the total content of Cd. Most previous studies neglected large metallic pieces and batteries, which contain most of the Cd, Cr, Cu and Ni present in bottom ash. This practice can result in an underestimation of the total contents of these elements by up to about 70%.

Geopolymers produced from drinking water treatment residue and bottom ash for the immobilization of heavy metals

Drinking water treatment residue (DWTR) and municipal waste incineration bottom ash (BA) have been traditionally considered as solid waste. With the development of urbanization, their subsequent treatment and resource regeneration need to be further researched. In this work, a composite geopolymer with BA and DWTR was successfully synthesized and applied in the immobilization of Cd, Pb and Zn. The analysis of the geopolymers with different ratios of BA and DWTR, curing times and heavy metals was performed through chemical analysis, SEM, FTIR, XRD, XPS, ICP-AES and compressive strength tests. The results show that the geopolymer samples based on BA and DWTR (BWG) presented higher compressive strength than the samples with single BA material. The sample with 20% DWTR and 80% BA (BWG20) possesses the highest compressive strength (24.10 MPa) among the materials ratios. Furthermore, the microstructure and characterization results indicate that the geopolymer matrix was successfully formed in BWG and was significantly changed by the ratio, curing time and addition of heavy metals. The immobilization efficiency for different categories and dosages of heavy metals by BWG20 were all higher than 99.43%. Moreover, the XPS results demonstrate that the heavy metals were immobilized in geopolymer mainly by divalent state forms.