Introduction
Incineration is a common way to dispose municipal solid waste (MSW) and simultaneously generate energy [
1]. This waste-to-energy incineration process is being increasingly used and is a viable management strategy for treating the increasing amounts of MSW that are not recyclable [
1‐
3]. MSW incineration (MSWI) can reduce the waste mass by 70% and waste volume by 90% [
2‐
4]. Consequently, MSWI produces two main solid by-products, which are divided into bottom ash (BA) and fly ash (FA) [
3,
4]. Any metals present in the disposed waste will end up in these fractions, which are currently notably underutilized.
Fly ash is the finer fraction collected from flue gas by air pollution control (APC) devices such as cyclones, electrostatic precipitators, fabric filters, or scrubbers. The composition of the dust is affected by the purification method used, temperature, and additives [
5]. Fly ash is considered toxic waste due to the enrichment of heavy metals, such as Pb, Zn, As, and Cd into this fraction during combustion. According to Jung et al. [
6], approximately one-third of Pb and Zn is distributed to fly ash due to volatilization. The relative amount of chlorides can also be higher in fly ash compared to bottom ash due to the lime scrubber used in the APC system [
4]. To protect the environment and ensure safety for human health, these ash fractions are subjected to different unit processes prior to landfilling. Heavy metals need to be properly cleaned from fly ash due to the risk of metals being released into the environment [
7]. The main treatment technologies used for MSWI fly ash are washing, chemical stabilization, and solidification as well as thermal treatment [
8]. The challenges of landfilling are that the toxic ash material is required to be deposited in specialized sites at high cost, with the risk of environmental pollution and simultaneously the possible loss of valuable metals [
9,
10]. However, it is increasingly difficult to find sites for landfills, and therefore an increasing part of the streams currently classified as waste is becoming a resource. The recovery of metals from fly ash can be considered a source of valuable metals that would otherwise be landfilled; in addition, this would result in an ash residue fraction with lower concentrations of toxic metals [
9]. At the moment, according to the London Metal Exchange [
11], the prices of pure Cu, Ni, Pb, and Zn are as follows: Cu 6 257 US$, Ni 12 895 US$, Pb 1 996 US$, and Zn 2 546 US$. This indicates that for 1 t of fly ash the value of metals is approximately 50 US dollars if counted in terms of pure metal value. Especially, nickel is a valuable battery metal with growing demand [
12,
13], specifically for lithium ion batteries [
14]. Therefore, improved Ni extraction of secondary raw materials is of vital importance. These reasons create not only an environmental but also an economic driving force to recover these metals, thus progressing toward the circular economy of metals.
Hydrometallurgical treatment of fly ash can provide new metal extraction possibilities and should be considered an alternative treatment for recovering valuable metals from fly ash. To achieve a selective and low-cost process, the lixiviant used should be carefully chosen and adapted to the properties of the ash [
9]. Luo et al. [
15] reviews the extraction methods of MSWI ashes, highlighting that mineral acids and alkaline solutions are effective for removal of Cr, Cu, Zn and Pb, whereas organic acids are not as effective, even though soluble metal complexes are forming in these solutions. Mineral acids include hydrochloric acid [
9,
16‐
18], sulfuric acid [
19‐
21], and nitric acid [
19,
22], these being mostly used due to high solubility of several metal compounds at low pH [
23]. Cationic leaching pattern is proposed for most of metals species, such as Ca, Cu, Mn, Zn, Cd and Pb [
15]. Furthermore, organic acids have been seen as potential lixiviants for fly ash leaching, for instance, citric acid [
8,
16,
24‐
26], oxalic acid [
20,
24,
27], and acetic acid [
20,
28]. Additionally, some other lixiviants such as EDTA [
20,
29], sodium acetate [
7], sodium hydroxide [
22,
29], and thiourea [
29] have been investigated for stabilization of fly ash.
Fedje et al. [
20] concluded that mineral acids are more effective compared to organic acids in the leaching of Cu, Zn, and Pb from MSWI fly ash. However, leaching of Pb, which is the major toxic element in fly ash, cannot be conducted in sulfuric acid due to the formation of PbSO
4 [
26]. According to Huang et al. [
17], hydrochloric acid appears to be a potential candidate for fly ash leaching, as Pb is able to complex with chloride anions as PbCl
42−, even though the formation of insoluble PbCl
2 is also possible. Fedje et al. [
20] stated that organic acids are not generally effective leaching agents for metals; however, the benefit of organic acids is their biodegradability. Nevertheless, the recovery of metals from organic acids can be challenging, similar to chelating agents [
26].
A new group of lixiviants, deep eutectic solvents or molten salts, have attracted interest in metallurgical research due to their ability to leach refractory ores. The dissolution of metal oxides [
30], electric arc furnace dusts [
31], and selective leaching of metals from mixed metal oxides has been investigated in deep eutectic solutions [
32]. Therefore, deep eutectic solvents may also be interesting and environmentally benign leachants for fly ash.
The objective of this paper is to fill the research gap by building a comparable leaching behavior map of potential lixiviants in metal rich fly ash leaching. The research contains a wide set of lixiviants (HCl, H2SO4, C2H2O4, C6H8O7, C2H4O2 acids, water, and the deep eutectic solvent, ethaline, i.e., choline chloride, C5H14CINO + ethylene glycol HOCH2CH2OH, in a ratio of 1:2). The leaching phenomena related to main valuable and toxic metals of interest (i.e., Zn, Cu, Ni, Pb, and Fe) were investigated, and specifically the focus was in selective leaching vs. Fe, which is the dominating and costly impurity metal in hydrometallurgical processing. Good selectivity (vs. Fe) reports directly to decrease in process costs: (1) decrease in opex, i.e., no need to Fe precipitation chemicals, (2) decrease in capex along the decrease in the required process retention time (i.e., the amount/volume of reactors) needed for Fe precipitation, (3) decrease in capex and opex as no Fe residue filter, residue washing and disposal of iron residue is needed. In the current study, also, the use of ethaline in fly as leaching was investigated for the first time. The comparable map provides new data and observations of leaching phenomena that can grant good basis for the further development of metal recovery from fly ash and thus contribute into the improved circular economy of metals.
Materials and methods
The composition of the raw material (fly ash from waste gas purification) used in this research is depicted in Table
1. The raw material was supplied by a Finnish incineration plant. The main elements of interest were Zn (1.1 wt%), Pb (0.8 wt%), and Fe (1.5 wt%) as well as the minor elements of Cu (0.06 wt%) and Ni (0.03 wt%). The raw material was analyzed by X-ray fluorescence (XRF), as well as by total leaching by atomic absorption spectroscopy (AAS, Varian AA240, USA) and inductively coupled plasma optical emission spectrometry ICP-OES (ICP Perkin Elmer Optima 7100 DV, USA).
Table 1
Chemical composition of the studied fly ash
As | 1537 | 2933 | 3760 | 2743 | 917 |
Cd | < 23 | – | – | – | – |
Co | < 12 | – | – | – | – |
Cr | 385 | < 50 | 246 | 316 | 70 |
Cu | 554 | 553 | 568 | 558 | 7 |
Fe | – | 14,050 | 15,400 | 14,725 | 675 |
Hg | < 20 | – | – | – | – |
Ni | 368 | 268 | 251 | 296 | 52 |
Pb | 8481 | 7100 | 7250 | 7610 | 619 |
S | – | – | 34,050 | – | – |
Sb | 223 | 383 | 377 | 328 | 74 |
Zn | 10,956 | 10,950 | 11,300 | 11,128 | 172 |
Further characterization of the material was performed with an SEM-EDS (scanning electron microscope, LEO 1450 VP, Zeiss, Germany, Energy dispersive X-ray spectroscopy, Oxford Instruments INCA software). The samples were pressed into pellets using a hydraulic press (Compac, Denmark). The pellets were cast into epoxy resin for microanalysis. The SEM-EDS analysis showed that the fly ash is mostly amorphous, which is why analysis could not be conducted adequately by the SEM-EDS method. However, the SEM-EDS results also indicated that the ash particles contained at least Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, and Pb.
The results presented on this paper are validated for the specific type of fly ash provided by Finnish industrial incineration plant from specific waste gas purification stage from the incineration of hazardous type of waste. The material used in this study contains similar amounts of Fe and Pb as described by Chandler et al. [
33] The zinc content in this material is lower compared to values observed by Huang et al. [
17,
26] Cu and Ni corresponded to concentrations presented by Chandler et al. [
33] The concentrations of Cu, Ni, Pb and Zn are in range of general fly ash compositions reported by Jiao et al. [
34] This suggests that the leaching behavior discussed in this paper can also be adapted to other fly ashes. However, it needs to be noted that the other metals, such as Na, Si, and Ca can complicate the dissolution of valuable metals.
The leaching tests for fly ash were conducted at 33 °C in ambient pressure and in the absence of oxygen purging. The studied lixiviants were water, sulfuric acid, hydrochloric acid, citric acid, acetic acid, oxalic acid, and ethaline (choline chloride, C
5H
14CINO + ethylene glycol HOCH
2CH
2OH, 1:2). The chemicals used and investigated concentrations are listed in Tables
2 and
3, respectively. The concentrations of 0.2–7 M were investigated for mineral acids. The concentrations for organic acids were limited by the solubility of organic acids into water.
Table 2
Chemicals used suppliers and grade
HCl (37%) | Merck | Analytical grade |
H2SO4 (95–97%) | Merck | Analytical grade |
C2H4O2 (96%) | VWR | Analytical grade |
C2H2O4·2H2O | Merck | Emplura® |
C5H14CINO | MP Biomedicals | Cell culture reagent |
C6H8O7 (99.8%) | VWR | Pharmacopoeia grade |
HOCH2CH2OH (99.5%) | Merck | Analytical grade |
Table 3
Studied concentrations of the lixiviants investigated
HCl | 0.2, 0.5, 1, 2, 3, 5, 7 |
H2SO4 | 0.2, 0.5, 1, 2, 3, 5, 7 |
C2H2O4 | 0.5, 1 |
C2H4O2 | 0.5, 1, 2, 3, 5, 7 |
C6H8O7 | 0.5, 1, 2, 3, 4 |
Water | |
Ethaline | 1 part C5H14CINO and 2 parts of 1,2-ethanediol |
Ethaline solution was prepared by measuring one part of choline chloride, C
5H
14CINO, and two parts of 1,2-ethanediol into a beaker [
30]. The beaker was heated up to 80 °C and stirred with a magnetic stirrer (IKA
® RT10) until the solution was clear, after which the solution was allowed to cool down.
In each test, the solid-to-liquid ratio used was 50 g/L, and mixing was conducted with a magnetic stirrer at 300 RPM for 24 h. The pH and redox were measured before and after the 24 h leaching time using a Mettler Toledo Inlab® saturated calomel electrode (SCE) vs. platinum wire for redox potential and Hanna Instruments edge® Multiparameter pH Meter-HI2020 for pH. At the end of each leaching experiment, the leach residue was filtered, dried, and the metal extractions were then calculated based on solution analysis by a commercially accredited laboratory (Labtium Oy, Espoo, Finland). Filtration was performed at room temperature under atmospheric conditions with a vacuum filter using double filter paper (Munktell, grade 10, size Ø 90 mm) in a Büchner funnel. The filtrated solution samples were analyzed by AAS and ICP-OES.
The selectivity coefficient was calculated by comparing the ratio of the dissolved elements in solution, for example c(Cu)/c(Fe). The main interest was to evaluate the selective dissolution of valuable metals compared to iron, which precipitates back into the neutralization residue of hydrometallurgical operations, if dissolved.
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