The FA sample used for the leaching experiments was obtained from a Colombian thermal power plant that uses mixtures of bituminous coal for its operation. The specific location, date of generation of the ashes, and their storage characteristics were not provided by the company.

Leaching methods and reactives: To carry out the leaching tests, conventional inorganic reagents were used, such as sulfuric acid (H₂SO₄), hydrochloric (HCl), and nitric acid (HNO₃), as well as other alternatives such as ferric chloride (FeCl₃) and sodium citrate (Na₃C₆H₅O₇.2H₂O) at a concentration of 0,5 M. All solutions were prepared with JT Baker and Milli-Q deionized water. In some tests, it was necessary to modify the concentration of the solutions in order to determine its effect on the dissolution of the studied metals. The extraction process was carried out via stirring leaching (500 rpm), using mechanical stirrers without baffles (Model 50006-03, Cole-Parmer) with paddle-type propellers. An S/L ratio of 50 g of FA per liter of solution was used for 3 hours, at room temperature and pressure (1 022 bar; 17 °C). In citrate solutions, the pH was adjusted with the addition of sodium hydroxide (NaOH) and HNO₃. The potential was monitored using a saturated Ag/ AgCl reference electrode (Oakton pH ORP 700 Benchtop Meter). Subsequently, the values were adjusted to the standard hydrogen electrode (SHE) for the corresponding thermodynamic analyses. Predominant area diagrams were elaborated using the MEDUSA software (Eriksson, 1979; Puigdomenech, 2004). The thermodynamic data of the metallic species in solution were contrasted with the NIST 46 database (NIST, 2004). The metal contents of the solutions were determined by microwave plasma atomic emission spectrophotometry (Agilent MP-AES), using the indications and calibration standards recommended by the manufacturer.

Sample characterization: The FA sample was chemically and mineralogically characterized. The chemical composition was determined by digestion with aqua regia (HCl: HNO₃, 3:1). The results showed that iron was the most present element in the study sample (Table 1).

Table 1

Source: Authors

On the other hand, the content of trace elements such as Zn and Pb is due to their association with the inorganic fraction of coal, which, during its combustion process, vaporizes, condenses, and finally concentrates in fine ash particles in the form of oxides.

Mineralogical characterization was performed via X-ray diffraction (Panalytical X'pert Pro), using Bragg-Brentano geometry with a cobalt cathode. The ICDD database (International Center for Diffraction Data) made it possible to contrast the peaks shown in the diffraction pattern (Figure 1), which indicated that the FA was mainly composed of silica (SiO₂) and alumina (Al₂O₃). The presence of these two was expected due to the dehydration suffered by kaolin in the coal combustion process. The iron content of the sample (Table 1) comes from both the pyrite (FeS₂) inherent in the formation of the fuel and the iron oxide (Fe₂O₃), a product of the little oxidation of this sulfide during the combustion process:

Both the considerable presence of species such as SiO₂ and Al₂O₃ and the absence of CaO in the FA sample indicate its silico-aluminous classification (F) (ASTM International, 2014), as well as the fact that the concentration of its relatively inert crystalline components is in a slow reaction phase, rich in iron and silica (Santaella, 2001).

Zn and Pb oxides are found in low concentrations, so they cannot be identified in the spectrum. However, it is possible to know its presence in FA due to its formation process:

Figure 1 Source: Authors

It was necessary to conduct a thermodynamic analysis for the carboxylic agent to identify the pH and potential conditions of the soluble and metallic species. The information provided by the prevalence diagrams indicates that, for Pb, at pH values 1 to 9, the formation of soluble species with potentials greater than -200 mV is favored. Under more alkaline conditions, the species formed are in a solid state. Zn, for its part, shows independence from the potential for species formation, while the favorable pH is within a range from 1 to 11. This can be seen in Figure 2.

Figure 2 Source: Authors

The above shows the citrate leaching ability of FA heavy metals across a wide pH range. Thus, three different values of pH were tested (4, 8, and 10) in order to determine their effect on the extraction of the metals of interest.

Leaching using HCl, HNO ₃ , FeCl ₃ and H ₂ SO ₄ : In hydrometallurgy, the capabilities of inorganic acids to chemically attack metallic elements is well known. Similarly, it has been reported that salts such as FeCl₃ are favorable for eliminating heavy metals such as Zn and Pb (Guo et al., 2016). Figure 3 shows the performance of HCl, HNO₃, H₂SO₄, and FeCl₃ in metals extraction. Regarding Zn dissolution, it is evident that acidic media provide an adequate amount of hydronium ions, which is favorable for metal dissolution. However, the dissolution kinetics are different for the reagents tested. For the three acids, a greater rate of leaching was observed in the first hour, during which between 80 to 85% of Zn was recovered. The extraction with HCl and H₂SO₄ showed an asymptotic behavior, while HNO₃ exhibited a progressive metals extraction of up to 98%. This can be attributed to the fact that, in the Zn-HCl and Zn-H₂SO₄ systems, the acid is quickly consumed, thus requiring a higher concentration of the solution to improve recovery.

Figure 3 Source: Authors

On the other hand, the acidity of the chloride medium (pH 2,6) also allowed for a progressive recovery of the metal. However, after three hours, HNO₃ was the one that achieved the best extraction of the non-ferrous metal. Due to the high presence of FeS₂ in FA, the chemical systems of the three inorganic reagents have a lower lead leaching capacity. The presence of sulfur in the acidic medium gives way to the formation of solid-state sulfate species, which leads to the inhibition of the dissolution of the metal.

Pb chlorides resulting from leaching with FeCl₃ are of low solubility, and their dissolution takes place at temperatures equal to or greater than 100 °C (Aguilar-Pérez et al., 1997). The low extraction of Pb with these agents is interesting, as it implies selectivity in leaching. Subsequent treatments can be proposed to detoxify the leached solid while obtaining a liquor rich in Zn.

Leaching using sodium-citrate

pH effect.: The influence of pH on the extraction of the metals of interest in the citrate solution was evaluated (Figure 4). With oxidation potentials in ranges from 400 to 600 mV that were recorded by the leaching tests, it was found that the dissolution of metals is independent of pH values.

Figure 4 Source: Authors

Likewise, it was observed that Pb is leached in the same way in the three media (acid, basic, or neutral). This characteristic creates the need for adequate FA treatment in order to avoid possible toxic effects on the environment caused by this heavy metal. Regarding Zn, tests at pH 8 indicate a slight improvement in metal dissolution, albeit only 6%. Due to economic and environmental factors, it is advisable to work under neutral conditions. Thus, a pH value of 8,0 was selected to continue the study.

Metal dissolution: The graphs indicate that the extraction of Zn and Pb combine the dissolution of two phases: the first corresponds to the spontaneous leaching of the oxides ZnO and PbO, while the second represents the slow dissolution of the sulfides inherent to coal, which fail to oxidize in the fuel combustion stage. For this reason, both Pb-Cit and Zn-Cit systems exhibit rapid dissolution kinetics, achieving beneficial extractions within the first hour. After this time, metal recovery becomes too slow (more for Zn at pH 4 and 10).

Sulfide leaching is more complex due to both their refractory nature and the formation of a passivating layer during the process (Santaella, 2001; Borda and Torres, 2022). Research has shown that it is possible to dissolve divalent Zn and Pb ions with citrate when these are associated with sulfides. However, additional time is necessary to obtain a better dissolution of the metals (Torres and Lapidus, 2020), as well as a higher molar concentration of citrate and the presence of hydrogen peroxide (Zárate et al., 2015). However, it has been shown that, in processes that combine the dissolution of two phases (in this study: oxides and sulfides), overall leaching kinetics can be affected (Borda et al., 2022).

HNO₃ at 0,5 M achieved the highest Zn extraction (98%) compared to citrate under the same conditions (Figure 5). Note that the low dissolution of Pb is advantageous, as the selectivity towards Zn in the process facilitates its subsequent recovery. Leaching was required to detoxify the leached solid by removing the Pb from the residue. To this effect, the solution was filtered after three hours in order to subject the solid to a second leaching, this time using a fresh solution of sodium citrate with the initially established conditions (pH 8, 0,5 M).

The results show that the post-treatment reaches a 40% elimination of the Pb from the leached FA. As explained above, under these conditions, citrate can form soluble complexes only with the Pb ions present in oxide form. However, after previously extracting the Zn from the FA sample, the amount of reagent available to react with Pb sulfide compounds increases, which is reflected on a 10% improvement in metal removal in comparison with direct citrate leaching (Table 2).

Table 2

	Leaching I	Leaching II
Zn (%)	98	98
Pb (%)	0,04	39,45

Source: Authors

Iron extraction

The results of the chemical digestion revealed that the iron content in the study sample was 20,6%. Its presence in the FA sample is high compared to the percentage of trace elements such as Zn and Pb (Table 1). However, its extraction is not relevant in this study, as it is not considered as a contaminating or interfering element in the mechanical properties of the secondary material after FA treatment (Yao et al., 2015). The main issue with the extraction of iron lies the difficulty of its electrodeposition (Torres and Lapidus, 2017). Thus, in order to avoid drawbacks in the electrolytic recovery of Zn, it is preferable to minimize iron extraction.

Figure 6 shows the low dissolution of iron in the direct leaching with HNO₃ and citrate, as well as the secondary leaching from the solid leached. The extracted metal content can come from the two iron phases present in FA: an easy to leach Fe₂O₃ and a somewhat more complex Fe₂S (Figure 1). The fast iron extraction in the first hour is attributed to the dissolution of the oxide in the solutions, and, while the citrate leaching stops, HNO₃ continues with very slow kinetics due to the dissolution of the sulfide species.

Figure 6 Source: Authors