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Reductive Leaching of Iron and Magnesium out of Magnesioferrite from Victorian Brown Coal Fly Ash Teck Kwang Choo,† John Cashion,‡ Cordelia Selomulya,† and Lian Zhang*,† †

Department of Chemical Engineering and ‡School of Physics and Astronomy, Monash University, Clayton Campus, Melbourne, Victoria 3800, Australia S Supporting Information *

ABSTRACT: This paper for the first time reports the reductive leaching of an iron-rich brown coal fly ash composed principally of a chemically resilient magnesioferrite (MgFe2O4) matrix. The simultaneous mobilization of Fe and Mg out of magnesiorferrite here aims to produce abundant Mg2+ that can convert into high-purity MgCO3, through a mineral carbonation process for CO2 capture, and abundant Fe2+/Fe3+ that can convert into value-added high-purity Fe-rich compounds such as FeOOH. Sulfurbearing compounds, including Na2S, Na2S2O4, and FeS2, were used as reductants, on the basis of the fact that S is one of the inherent elements in fly ash that has a Fe-reductive capability. Synchrotron-based X-ray absorption near-edge spectroscopy was used to quantitatively determine the speciation of Fe (Fe2+ or Fe3+) and S (SO42− or S2−) in the leachate produced. Leaching with Na2S2O4 and FeS2 was found to produce the most Fe2+ (more than 70% of total eluted Fe) in the leachate at 200 °C. Increasing the leaching temperature is beneficial in increasing the reactivity of FeS2, leading to a greater amount of Fe2+ produced at 200 °C, whereas Na2S2O4 reached its best performance at 100 °C. This is due to a quicker dissolution of Na2S2O4 into the leachate to promote the reduction of inherent Fe3+-bearing ash matrix in the liquid phase, whereas FeS2 mainly remains as a solid, which is less reactive. None of the mechanisms involved affected the total Mg2+ cations eluted. Increasing the molar ratio of S to Fe from 0.125 to 0.5 completely reduced all aqueous Fe3+ present to Fe2+ for both reductants. Concurrent with this was an incremental change in total aqueous Fe amount when Na2S2O4 was used. No significant increase in total Fe eluted was observed when FeS2 was used. The fate of S differs for both cases, with S mostly mobilized in the leachate when Na2S2O4 was used while predominantly being in the solid leaching residue in the case of FeS2. In light of this, the use of FeS2 is more promising on a large scale, although it is less active. accompany a leaching reagent to reduce Fe3+ in the form of iron oxide to aqueous Fe2+. Additionally it has been reported that all ferrous chloride/sulfate and ferric chloride/sulfate can serve as a coagulant for the flocculation process in wastewater treatment applications.12,13 For such applications, possibly less manipulation of the Fe-rich leachate produced from Fe mobilization is required. Previously, we have reported that the fly ash from the Yallourn region of Victoria, Australia, is principally composed of Fe and Mg, which are chemically “locked” together to form a magnesioferrite (MgFe2O4) matrix.14 Therefore, the mobilization of Mg via initial leaching in an acid for a subsequent carbonation mineralization reaction inevitably necessitates a simultaneous mobilization of Fe in the leaching step. The Fe content at more than 50 wt % (reported as oxides) in this fly ash is too large to be considered an impurity. Moreover, our HCl leaching tests of the pure fly ash confirmed a very low elution extent of the target Mg2+ out of the MgFe2O4 matrix, even at 200 °C and 10 bar, due to a positive change in Gibbs free energy for the breakage of magnesioferrite in acid. However, upon the presence of impure marcasite (FeS2), this species was found to be cleaved easily. More interestingly, the inherent Fe3+ within it was found to be eluted as Fe2+ out of the

1. INTRODUCTION Iron (Fe) is a metal that contributes to a group of impurities in alkaline industrial waste and natural minerals, both the elemental compositions of which vary broadly. Recovered iron can potentially be further processed for conversion into value-added products. Magnesium (Mg), being an alkaline earth, can be used for mineral carbonation, which is a promising technology for the permanent capture of anthropogenic carbon dioxide (CO2).1,2 Alkaline industrial waste materials, such as steelmaking slag and fly ash rich in Mg and calcium (Ca), have been studied for this purpose. The operating conditions for mineral carbonation can be severe, particularly for natural minerals, which require an energy-intensive comminution prior to the process.3 As an alternative, indirect carbonation, where these metals are first leached out using chemical reagents before being carbonated, can potentially reduce the requirements of this process. As such, the separation of impurities from Mg and Ca to avoid contamination during carbonation is important. Despite its presence in various waste minerals, the mobilization of Fe has not been thoroughly examined. As a result, there is still a shortage of findings relating to the elution, speciation, and fate of Fe upon leaching. The relevant works with regard to mechanisms of structural Fe3+ removal, including smectites,4,5 clay minerals, soil,6 and remediation of acid mine drainage,7,8 do not contain alkaline earths to a significant extent. In those works, Fe-reducing bacteria,9,10 ligand-containing6 compounds, and sulfide-bearing compounds or their mixture11 © XXXX American Chemical Society

Received: September 1, 2015 Revised: January 6, 2016

A

DOI: 10.1021/acs.energyfuels.5b01979 Energy Fuels XXXX, XXX, XXX−XXX

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of water-washed fly ash (pH 7) reported as oxides, as quantified by Xray fluorescence spectroscopy (XRF). It can be seen that the most abundant element is Fe, with 55.6 wt % present, and Mg comes in second at 24.5 wt % (reported as oxides). The mineralogical analysis by X-ray diffraction (XRD) in our previous work14 confirmed the coexistence of metal oxides, including periclase (MgO), maghemite (γFe2O3), and magnesioferrite (MgFe2O4). Since our focus is MgFe2O4, a prior leaching using 2 M HCl at room temperature (pH 1) for 5 min was conducted to remove as much of the first two oxides as possible, leading to the predominance of MgFe2O4 in the residue, which was then used as the feedstock for tests throughout this paper. This starting feedstock will henceforth be termed “first-stage residue”. The elemental composition of the HCl-washed residue is tabulated in Table 1, whereas its mineralogical properties, shown in the Supporting Information (SI), confirmed an exclusive dominance of MgFe2O4. Additionally, the room temperature Mössbauer spectroscopy result of the raw fly ash in Table 2 (spectrum in the SI) strongly supports magnesioferrite as the dominant iron-containing phase, with some unresolvable maghemite and/or hematite containing 5% of the iron. There is also a weak ferric quadrupole split doublet containing 8% of the iron, which is due to a ferric oxyhydroxide or superparamagnetic magnesioferrite or to either, or both, ironsubstituted periclase, MgO:Fe3+, or aluminum-substituted spinel, MgAl2−xFexO4. 2.2. Leaching Experiments. 2.2.1. Control Leaching Experiments (without reductant). Leaching experiments without the use of S-bearing reductants were performed at 20−200 °C. Hydrochloric acid (HCl) of analytical grade (Ajax, 32 wt %) was diluted down to 2 M using Milli-Q water. The liquid/solid ratio of HCl to fly ash was kept constant at 6.67 mL/g, for the addition of 15 g of fly ash to 100 mL of HCl in each run. For 20 °C experiments, HCl and fly ash were mixed in a 250 mL conical flask with a magnetic stirrer bar. The flask was placed on a stirring plate, with the temperature kept constant using a water bath. For experiments at 100 and 200 °C, HCl and fly ash were added into a 300 mL Teflon sample cup that was then placed into an autoclave (Berghof BR-300). Stirring was only started after the mixture has reached the set temperature. For all experiments, the stirring speed used was fixed at 300 rpm. Once the experiment was finished, the resulting slurry was filtered under a vacuum pump, after which the liquid leachate was weighed and bottled for further characterization. The wet fly ash filter cake was rinsed with about 50 mL of Milli-Q water and subsequently dried in an oven at 110 °C. The dry solid residue was finally weighed and analyzed. 2.2.2. Reductive Leaching Experiments. Three different S-bearing compounds were tested, sodium sulfide nonahydrate (Na2S·9H2O, Sigma-Aldrich, ≥ 98% purity), sodium dithionite (Na2S2O4, SigmaAldrich technical grade, 85% assay), and pyrite (FeS2, Sigma-Aldrich, 99.8% trace metals basis). The molar ratio of S in a particular

solid matrix, rather than as it is. It was a strong indication of the reduction of Fe3+ within marcasite. In the follow-up of those findings, this study aims to elucidate the mechanisms governing such a reaction, by further testing and optimizing the leaching conditions of magnesioferrite upon the use of pyrite (FeS2) to maximize the elution of both Mg2+ and Fe3+. For comparison, sodium dithionite (Na2S2O4) and sodium sulfide (Na2S) were also chosen to clarify the effects of the chemical form of sulfur on Fe speciation and elution upon HCl leaching. The leaching tests were conducted at a constant liquid to solid ratio of 6, with temperatures varying from room temperature to 200 °C and the molar ratio of sulfur in reductant to iron in fly ash varying (e.g., from 0.125 to 0.5). To reveal the basic principles underpinning MgFe2O4 mobilization mechanisms, advanced characterization using Mössbauer spectroscopy and synchrotron X-ray absorption near edge structure (XANES) was employed. The results achieved are expected to shed light on advancing the use of Fe-rich Yallourn brown coal fly ash on the mineralization of CO2 and on the synthesis of high-purity, value-added Fe-bearing compounds.

2. EXPERIMENTAL SECTION 2.1. Fly Ash Preparation and Properties. The fly ash was collected from the electrostatic precipitator (ESP) of Energy Australia’s Yallourn power plant, Latrobe Valley, Australia. The ash sample was first wet-sieved to remove the water-soluble species at a liquid-to-solid ratio of 10 mL/g and sieved to less than 38 μm simultaneously. The wet slurry was oven-dried for a few days at 110 °C. The second column of Table 1 shows the elemental composition

Table 1. Elemental Analysis of Water-Washed and 2 M HClLeached Yallourn Fly Ash (wt %) As Quantified by XRF Yallourn fly ash (wt %) element oxide

water-washed

2 M HCl-leached

SiO2 Al2O3 Fe2O3 CaO MgO MnO SO3

3.2 2.9 55.6 9.3 24.5 0.8 3.7

3.2 2.9 60.9 5.2 24.8 0.9 2.1

Table 2. Parameters from Fits to the Mössbauer Spectra MS1−MS4 and Literature Values for Selected Phasesa doublet sample ID

IS (mm/s)

QS (mm/s)

MS1 MS2 MS3 MS4

0.33(1) 0.28(3) 0.25(3) 0.32(1)

0.75(1) 0.71(6) 0.78(4) 0.62(1)

hematite Fe3+ oxyhyd27 SPM MagFerr27 MgO:Fe27 Fe3+ spinel27 pyrite

0.36 0.2−0.3 0.36 0.2−0.3 0.30

0.4−1.0 0.51−0.68 0.82−0.58 0.5−0.9 0.61

hematite area (%) 8.4 1.7 2.1 6.5

IS (mm/s) 0.30(1) 0.38(1) 0.37(1) 0.37(1) Literature 0.37

MagFerr

QS (mm/s)

HFF (T)

area (%)

area (%)

−0.23(2) −0.12(1) −0.11(1) −0.12(1)

50.0(1) 50.2(1) 49.9(1) 49.7(1)

5.3 17.3 5.0 3.8

86.5 81.0 92.8 89.6

−0.10

51.8

a The MgO:Fe3+ QS range is for 0−8% Fe substitution. Abbreviations: IS = isomer shift, QS = quadrupole splitting, HFF = hyperfine field, MagFerr = magnesioferrite (MgFe2O4), MS1 = water-washed fly ash (unleached with HCl), MS2 = control, MS3 = Na2S2O4 as reductant, MS4 = FeS2 as reductant.

B


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Figure 1. (a) Proportion of Fe and Mg in first-stage leaching residue eluted and (b) speciation of eluted Fe, for different reductants at 200 °C.

Figure 2. (a1) Effect of temperature on Fe and Mg eluted using Na2S2O4 and (a2) its Fe speciation. (b1) Effect of temperature on Fe and Mg eluted using FeS2 and (b2) its Fe speciation. Proportions normalized to the first-stage residue amounts as in Figure 1. reductant to Fe in fly ash was first fixed at 0.125, corresponding to the theoretical ratio determined from eq 1.14

XANES was performed on selected leaching residues to determine the oxidation state of S. HSC Chemistry 7.1 was used to quantify the change in Gibbs free energy for two iron oxide reactions with HCl. Details regarding the use of XRF, ICP-OES, MS, and XANES can be found in the SI.

4MgFe2O4 (s) + 24HCl(aq) + S2 −(aq) → 4Mg 2 +(aq) + 24Cl−(aq) + 8Fe2 +(aq) + 12H 2O(l) + SO4 2 −(aq)

3. RESULTS AND DISCUSSION 3.1. Yields and Speciation of Fe and Mg. Figure 1a gives the elution yields of Mg and total Fe (Fe2+ + Fe3+) in the leachate at 200 °C. The pH value for all leachates was 1. The amount of Mg extracted was between a minimum of 53 wt %, shown in the Na2S experiment, and a maximum of 59 wt %, when Na2S2O4 was used, with the control and FeS2 experiments between these boundaries, both producing 56 wt % Mg. The total Fe elution increases remarkably when moving in the following sequence: control, Na2S, Na2S2O4, and FeS2. Any Fe that came from the dissolution of FeS2 was found to be insignificant compared to the amount extracted from the fly ash. Figure 1b further shows the corresponding amounts of Fe2+/3+ in the leachates. The amounts of these two cations in

(1)

The reductant compound is added to the fly ash−acid mixture just before stirring commenced. For Na2S2O4 and FeS2, additional experiments were conducted for two other S to Fe molar ratios, 0.25 and 0.50, at the leaching temperatures of 100 and 200 °C. 2.3. Sample Characterization. The elemental quantification of water-washed fly ash and its first-stage leaching residue was performed using XRF. The quantification of total Fe and total Mg in leachates was accomplished using inductively coupled plasma optical emission spectroscopy (ICP-OES). X-ray diffraction (XRD) was used to specify the mineral phases in fly ash and all its leaching residues, with the results shown in the SI. Mössbauer spectroscopy (MS) was used to determine the specific Fe-bearing phases present in fly ash and selected leaching residues. Fe K-edge XANES was performed on selected leachates to determine the oxidation state of Fe, while S K-edge C


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Figure 3. (a1) Effect of S:Fe ratio on Fe and Mg eluted using Na2S2O4 and (a2) its Fe speciation. (b1) Effect of S:Fe ratio on Fe and Mg eluted using FeS2 and (b2) its Fe speciation. Proportions normalized to first-stage residue amounts as in Figure 1.

°C, 51 wt % of Mg was dissolved using Na2S2O4 and 45 wt % using FeS2. The larger amount of Mg extracted using Na2S2O4 continued at 200 °C by 6 wt %, with 57 wt % eluted. On the other hand, the leaching of total Fe only reached 5 and 4 wt % at 20 °C using Na2S2O4 and FeS2, respectively. Although an increase in Mg eluted was observed between 100 and 200 °C using Na2S2O4, there was no significant change in total Fe eluted, which remained nearly the same at 47 wt % for the two temperatures. For both reductants, increased elution of Fe was observed from 100 to 200 °C. The speciation of Fe in the leachates of 100 and 200 °C is shown in parts a2 and b2 of Figure 2. Note that the original XANES spectra and least common factor (LCF) fitting curves are illustrated in Figure S3 in the SI. Analysis of the room-temperature leachate was not conducted, due to the trivial amount of Fe within it. One can see a significant dependence of the elution of Fe2+ on the leaching temperature. Upon the increase from 100 to 200 °C, the elution of Fe2+ was increased from 23 to 34 wt % in the case of using Na2S2O4 as the reductant, which was accompanied by a decrease in Fe3+ from 24 to 13 wt %. The elution of Fe2+ cation upon temperature rise increased more significantly when FeS2 was used as the reductant, rising from 13 wt % at 100 °C to 41 wt % at 200 °C. This is a clear sign of the stronger reduction capability of FeS2 for the elution of the inherent Fe3+ out of magnesioferrite matrix. Parts a1 and b1 of Figure 3 further indicate the elution amounts of Mg and total Fe in the leachate at 200 °C as a function of S to Fe molar ratios for the use of two reductants, Na2SO4 and FeS2, respectively. Parts a2 and b2 of Figure 3 give the Fe speciation for those conditions, derived from their respective Fe XANES spectra shown in Figure S4 of the SI. When the ratio was increased from 0.125 to 0.25 using Na2S2O4 as a reductant, there was only an increase in the total amount of Fe eluted by 4 wt % (which is more or less due to experimental error), whereas the amount of Fe2+ produced increased remarkably from 33.7 to 49 wt %. At a ratio of 0.5, Fe2+ was the only speciation of Fe observed, with 55 wt % produced. The only significant change in Mg eluted was from a ratio of 0.125 to 0.25, where it increased from 57 to 62 wt %, after which it

leachate were quantified on the basis of the Fe K-edge XANES spectra in the SI. As can be seen, the elution of Fe2+ is the major source contributing to the gradual increase of the Fe elution within the aforementioned sequence. The amount eluted increased from 11 wt % for the control case to 41 wt % in the case of using FeS2 as the reductant. In contrast, the elution of Fe3+ was reduced considerably, although the primary speciation of Fe is present as Fe3+ in the form of MgFe2O4 in the starting feedstock. Its proportion of the eluent from Figure 1b decreases from 65% to 29%, 28%, and 20% as one moves through the sequence. This is also confirmed by independent measurement of the frozen eluent by Mössbauer spectroscopy, which gave 55% of Fe in the form of Fe3+ for the control and 19% for the FeS2 sample (the spectrum for frozen sample is shown in Figure MS5 in the SI). MS was performed on solid leaching residues to determine the type of Fe-bearing species present, the parameters of which are shown in Table 2. Their respective spectra can be seen in Figures MS1−MS4 of the SI. Mössbauer analysis of those residues showed that almost all of the paramagnetic periclase/ spinel species was removed by all three leaching procedures. There were clear hematite signals comprising approximately 17% of the Fe in the control sample and 5% and 3% for the Na2S2O4 and FeS2 leached samples, respectively. The FeS2 leached sample contained approximately 5% of the iron as pyrite. All the remaining iron in all samples was identified as MgFe2O4. This will be discussed in detail later. With regard to the use of Na2S, the action of Na2S is less pronounced due to a portion of HS− (the conjugate base of S2−)15 reacting with H+ to form gaseous H2S,16 indicating that Na2S is the least stable of the reductants. Due to this and also because Na2S2O4 and FeS2 produced the largest amounts of Fe2+, their performances upon variation in temperature were examined, without any further consideration for NaS2. Parts a1 and b1 of Figure 2 give the amounts of Mg and Fe eluted for temperature variation experiments using Na2S2O4 and FeS2, respectively. There is a comparable increase in Mg with increase in temperature using both reductants. At 20 °C, around 14% Mg was eluted using either reductant, while at 100 D


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Figure 4. (a) S XANES spectra plotted with those of S-bearing standards. (b) Synchrotron S K-edge XANES spectra for leaching residues from different reductant experiments; dots represent actual data and solid lines represent LCF curves.

did not seem to increase further at a ratio of 0.5. Similar phenomena were observed for the use of FeS2 as the reductant. That is, there was little change in the amount of total Fe extracted at about 50 wt %, with similar proportions of Fe2+ at 81% of total Fe at S/Fe ratios of 0.125 and 0.5. However, upon increasing the S to Fe ratio to 0.5, the elution of total Fe reached 55 wt %, which is entirely in the form of Fe2+ in the leachate. This is an indication of a lower reduction capability of FeS2 than Na2S2O4, which could be due to a slow dissolution of the former reductant in HCl. 3.2. Mechanisms of Reductive Leaching. 3.2.1. Variation in Reductant Type. In the control experiment without reductant, the Mössbauer spectra showed that there was a greater amount of Fe in the form of solid hematite (17.3% of total Fe) compared to raw fly ash (5.3%) and it was also greater than when Na2S2O4 and FeS2 were used (5.0% and 3.8% of total Fe, respectively), as shown in Table 2. This suggests that the breaking of the primary MgFe2O4 matrix took place when there was no reductant, resulting in the formation of solid hematite, Fe2O3,17 and MgO (eq 2). Simultaneously, both two oxides were dissolved into HCl.18,19 MgFe2O4 (s) → Fe2O3(s) + MgO(s)

Regarding the gaseous SO2 formed from eq 4, it further oxidizes to form SO42−, which is distributed between both solid and aqueous phase, as confirmed by the S XANES peak at 2481 eV in Figure 4 (solid samples), which matches the white-line positions of both standard compounds FeSO4 and CaSO4 and also Figure 5 (leachate). The unleached Fe remains as its original form of MgFe2O4, as shown by Mössbauer spectroscopy results (Table 2, sample ID MS3), where 92.8% of Fe is in that form.

(2)

Na2S2O4 is a well-known Fe3+-reducing compound.20 In the aqueous phase, the structure of S2O42− disproportionates to form free sulfoxylate radicals (SO2−•),4 an intermediate containing an unpaired electron, as shown in eq 3.5,21 During the leaching process, a lone electron moves from SO2− to solid Fe3+ via eq 4,5,21 reducing it to Fe2+ while simultaneously producing an overall negative charge within the MgFe2O4 framework. This network instability would trigger point defects in parts of the matrix, releasing additional electrons that further reduce the surrounding Fe3+ and also that within the matrix.5 S2 O4 2 −(aq) → 2SO2−•(aq)

(3)

SO2−•(aq) → SO2 (g) + e−

(4)

Figure 5. Typical synchrotron S K-edge XANES spectra for leachates from all experiments.

For the use of FeS2 as the reductant, previous studies have shown that pyrite in aqueous ferric chloride systems can reduce Fe3+ with the formation of elemental S from pyritic S, as shown in eq 5, which can also act to further reduce Fe3+ (eq 6).22 These equations suggest that FeS2 will only participate in the reduction reaction after an initial Fe3+ elution into the aqueous phase. E


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Figure 6. S K-edge XANES spectra of solid residues from experiments using (a) Na2S2O4 at 100 and 200 °C along with spectrum for FeSO4 and (b) FeS2 at 100 and 200 °C along with spectra for FeSO4 and FeS2. Dots represent actual data and solid lines represent LCF curves.

2Fe3 +(aq) + FeS2(s) → 3Fe2 +(aq) + S(s)

resulting in slightly less Fe2+ than Fe3+. An increment of temperature from 100 to 200 °C did not significantly increase the total Fe eluted but increased the proportion of Fe2+, suggesting that the additional heat predominantly decomposed S2O42− into SO2−• radicals,21 proposed in eq 7, rather than having an effect on Fe3+ in the solid MgFe2O4 matrix.

(5)

22

6Fe3 +(aq) + S(s) + 4H 2O(l) → 6Fe2 +(aq) + HSO4 −(aq) + 7H+(aq)

(6)

SO2−•(aq) + Fe3 +(aq) → SO2 (g) + Fe 2 +(aq)

22

The observation in Figure 1 indeed substantiates the superior reductive ability of FeS2 in the reduction of Fe3+ than that of Na2S or Na2S2O4 at 200 °C, producing an additional 10 wt % in Fe2+, despite FeS2 being far less soluble.23,24 Moreover, as seen in eq 5, solid FeS2 directly participates in the reaction with aqueous Fe3+, in contrast to SO2−• radicals in the aqueous phase, indicating a solid−liquid reaction. This implies that a portion of solid FeS2 acts as a physical support that helps in the reduction reaction.25 This phenomenon also explains the occurrence of unreacted FeS2 present in leaching residues at both temperatures (indicated by a peak at 2471 eV in Figure 4 and the Mössbauer result in Table 2) confirming that FeS2 is in excess. Studies5 have shown that there is far less contribution by unpaired electrons in sulfides than dithionites when partaking in a reduction reaction. Nevertheless, the combined action of elemental S and FeS2 has proven superior to that of free radicals formed from S2O42−. 3.2.2. Variation in Temperature. The close similarities between extraction yields of Fe and Mg using both reductants (Na2S2O4 and FeS2, respectively) at 20 °C seen in parts a1 and b1 of Figure 2 imply that these two reductants are not reactive enough at such a low temperature. It is likely that a certain threshold of thermal energy needs to be overcome for both reductants to be chemically activated during leaching of this fly ash with HCl. On the basis of results shown in Figure 2a2, it is inferred that the breaking down of MgFe2O4 to hematite and MgO17 happened at 100 °C instead of 200 °C. At 100 °C, the leachable portion of the Fe contributes to slightly over half (51.4%) of leached Fe in the form of Fe3+, likely by a protonation reaction,14 dominating over the reductive action of S2O42−,

at 200 °C

(7)

Electron Spin Resonance (ESR) studies21 have shown that at 180 °C and above, SO2−• decays at a higher rate, which meant that the reaction in eq 7 could have taken place to a larger extent at 200 °C, corroborating the larger amount of Fe2+ observed at 200 °C than 100 °C. Furthermore, as there was no significant increase in total Fe in the leachate at 200 °C compared to 100 °C and given the fact that hematite comprised only 5% of total Fe (MS result in Table 2), which is far less than the 17.3% observed in the control residue, it is inferred that the reduction mechanism by SO2−• radicals may have inhibited the simple acid dissolution of hematite14 that produces Fe3+ observed at 100 °C, leading to only a slight increase in total Fe eluted. However, the increase in Mg extraction from 100 to 200 °C seen in Figure 2a1 indicates that there is additional Mg2+ eluted, although reaction between HCl and solid Fe2O3 has been inhibited. This suggests that SO2−• radicals do not affect Mg elution. Moreover, this observation suggests that when the possible inhibition of solid Fe3+ took place, solid Mg2+ was preferentially leached by H+. In contrast to the action by S2O42−, incrementing to 200 °C increases the amounts of both total Fe and Fe2+, along with Mg2+, when FeS2 was used as the reductant, as shown in Figure 2b2. A 3-fold increase in the concentration of Fe2+ was observed at 200 °C from 100 °C, comparatively larger than that produced by S2O42−, indicating a stronger dependency of FeS2 reductive potential on temperature. This could have been caused by an increase in reactivity of pyrite between 100 and 200 °C due to the conditions applied, which may have changed the surface properties25 of pyrite. F


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Figure 7. S K-edge XANES spectra of solid residues from experiments at 200 °C using three S to Fe molar ratios with (a) Na2S2O4 along with spectrum for FeSO4 and CaSO4 and (b) FeS2 along with spectra for FeSO4, CaSO4, and FeS2. Dots represent actual data and solid lines represent LCF curves.

Figure 6 shows the S XANES spectra of leaching residues at 100 and 200 °C when FeS2 was used. Pyritic S is indicated by the peak at 2471 eV, implying that excess pyrite is present in the leaching residue, while a portion of S is in the form of SO42−, as indicated by the peak at 2481 eV. These observations show that temperature had no effect on the speciation of S in the residue. 3.2.3. Variation in S to Fe Molar Ratio. In Figure 3a2 it was shown that complete reduction of Fe in the leachate was observed at an S to Fe ratio of 0.5, indicating that the greater presence of free SO2−• radicals ensures an excess supply of electrons that reduce the aqueous Fe3+ into its ferrous counterpart. When FeS2 was used (Figure 3b2), there was no significant changes in the amount of total Fe eluted, although at the highest tested S to Fe molar ratio of 0.5, all eluted Fe was reduced. Figure 7 shows the S XANES spectra for leaching residues from FeS2 experiments at the three ratios. It can be seen that individual peaks at 2471 eV were observed for all three conditions, corresponding to a white-line peak for FeS2. This implies that there was excess unreacted S2− from the leaching experiments, with the excess increasing with the proportion of FeS2. In Figure 8, the quantification of S in leachates is shown. At all S to Fe ratios tested, there was consistently more dissolved S in experiments with Na2S2O4 than with FeS2. As expected, the concentration of S increases with increasing amounts of Na2S2O4, confirming that all S2O42− added readily dissolves in HCl to form soluble S,26 with some S possibly liberated as SO2 or H2S. This is also validated from the fact that no characteristic XANES peak corresponding to Na2S2O4 was observed in any of the solid leaching residues. The S concentration in leachates from FeS2 experiments does not change significantly from ratios of 0.125 to 0.25, corroborating the fact that S remains in the form of insoluble, solid FeS2 and there is only a small consumption of S by the Fe3+ reduction reactions. 3.3. Summary of Proposed Mechanisms. When no reductant is present during the leaching process at 200 °C,

Figure 8. Effect of S to Fe ratio on concentrations of S in leachate.

there are two main steps that take place. The provided heat thermally decomposes MgFe2O4 matrix, as proposed in eq 8, allowing solid Fe3+ species to come into contact with protons (H+) from HCl, where Fe3+ and Mg2+ are then eluted via protonation (eqs 9 and 10, respectively). MgFe2O4 (s) → Fe2O3(s) + MgO(s)

(8)

Fe2O3(s) + 6H+(aq) → 2Fe3 +(aq) + 3H 2O(l)

(9)

+

2+

MgO(s) + 2H (aq) → Mg (aq) + H 2O(l)

(10)

The breaking down of MgFe2O4 into Fe2O3 and MgO is an important step that needs to take place for the reduction of Fe3+ and elution of Mg2+ cations, even if an external reductant is present. This has been proven in the variation of temperature experiments, where at 20 °C, both the use of Na2S2O4 and FeS2 yielded practically the same low Fe and Mg extractions. G


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The electron transfer from disulfide (S22−) to aqueous Fe3+ comes entirely from solid FeS2, rather than a prior release of S from FeS2, as seen in eq 6, although the governing reaction as an intermediate reaction is proposed by eq 14 (after reactions in eqs 8 and 9 have taken place):

The critical step where the reductant plays a role is in depleting aqueous Fe3+ concentrations in their respective reduction reactions by electron donation, in accordance with eq 11, which shifts the chemical equilibrium to increase Fe3+ concentrations in the mixture, hence dissolving Fe from the solid MgFe2O4 matrix. 3+

−

2+

Fe (aq) + e → Fe (aq)

S2 2 −(s) + 2Fe3 +(aq) → 2S(s) + 2Fe 2 +(aq)

(11)

As a consequence of eq 14 taking place, aqueous Fe3+ concentrations would be diminished locally, allowing the protonation of Fe3+ from the fly ash matrix14 in accordance with chemical equilibrium principles. The continuing conversion of Fe3+ to Fe2+ permits solid Fe3+ to keep dissolving until the leaching is intervened by terminating stirring. Mg2+ elution seem unaffected by variation in S/Fe ratio when FeS2 was used as a reductant, but it increased with temperature between 100 and 200 °C, concurrent with Fe3+ reduction. The former indicates that Mg extraction is not dependent on FeS2 presence as a reductant but depends more strongly on additional thermal energy supplied, which aided in breaking down MgFe2O4 into MgO and Fe2O3, exposing individual MgFe2O4 crystallites, which permits H+ access for Mg2+ cations to be mobilized. Finally, it is noteworthy that the use of FeS2 is superior over that of Na2S2O4 in maintaining a low SO42− concentration in the leachate and even a low gaseous SO2 emission, as demonstrated in Figure 8. The unreacted FeS2 mainly remains in solid residue, which is easily separated and reused. In the light of this, it is obviously a desirable reductant that can be used in an environmentally friendly way on a large scale.

At 100 °C, it has been shown that the total amount of Fe eluted when Na2S2O4 was used was larger (47 wt %) than when FeS2 was used (41 wt %). This showed that S2O42− acts more rapidly than FeS2 at 100 °C, due to the fact that it easily dissolves in the aqueous phase to release electrons that have a quick and intimate reaction with the Fe3+ cation derived from the breaking down of magnesioferrite. When Na2S2O4 is used at 200 °C, compared to 100 °C, there was an increase in Mg elution and Fe2+ production but no significant change in the total amount of Fe eluted. This indicates that the action of SO2− at 200 °C is targeted toward Fe3+ in the aqueous phase, with the possibility of H+ acting as catalyst specifically for this reaction, as H+ usually attacks species in the solid matrix. In doing so, there would be less H+ available locally for elution of Fe from the solid MgFe2O4 matrix. Any excess H+ available for protonation were then directed toward the MgO portion of the matrix, hence allowing a larger amount of Mg extraction (preferential leaching of Mg). HSC Chemistry simulation shown in Figure 9 for eqs 12 and 13

4. CONCLUSIONS We have critically examined the reactions associated with the reductive leaching of brown coal fly ash that was largely composed of a chemically resilient MgFe2O4 matrix that only dissolves significantly in strong acids. Using 2 M HCl, a preleached fly ash from the Yallourn region of Victoria was acid-leached using three different S-bearing compoundsNa2S, Na2S2O4, and FeS2all at 200 °C. Variations in temperature and S to Fe ratio were also investigated for experiments involving Na2S2O4 and FeS2. The leaching residues and leachates were characterized using ICP-OES, XRF, and synchrotron K-edge XANES. The main conclusions for this study are as follows: (1) The strength at which the three reductants act upon acid leaching of fly ash varies, with amounts of Fe2+ in leachate increasing in the following order: Na2S, Na2S2O4, and FeS2. The mechanisms governing the interaction between these reductants and the fly ash−HCl mixture vary. As a result of the instability of Na2S in HCl resulting in the least amount of Fe2+ produced, no further investigation was performed using this compound. Na2S2O4 dissolves in HCl to form SO2−• radicals that reacts with aqueous Fe3+, in contrast to solid-state FeS2 during the reduction of Fe3+ in a solid−aqueous reaction. There was no specific relationship between the presence of reductant compound and the amount of Mg2+ cations eluted. Although FeS2 is less active than the other two counterparts, it is supervisor in inducing a minimum concentration of SO42− in leachate and SO2 in gas emission as well. (2) A thermal activation of the S-bearing compounds is essential for them to express their reductive potentials, which was confirmed for temperatures 100 and 200 °C. While SO2−• radicals released using Na2S2O4 in HCl increased the reduction

Figure 9. ΔG values for eqs 12 and 13.

has shown that the reaction between HCl and Fe2O3 to produce Fe3+ is not spontaneous (indicated by positive ΔG values) from 100 °C onward, but is spontaneous for Mg2+ production (always negative ΔG values). Fe2O3(s) + 6HCl(aq) → 2Fe3 +(aq) + 6Cl−(aq) + 3H 2O(l) MgO(s) + 2HCl(aq) → MgCl2(aq) + 3H 2O(l)

(14)

(12) (13)

Compared to Na2S2O4, the action of FeS2 is more sensitive toward temperature between 100 and 200 °C, where there was an increase in Mg2+, Fe2+, and total Fe. This indicates that the additional heat assists the action of solid FeS2 in carrying out the solid−liquid reaction between FeS2 and aqueous Fe3+ after the elution of Fe3+ by HCl. H


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Energy & Fuels of aqueous Fe3+, it inhibited the acid dissolution of solid Fe3+, causing no significant change in total Fe cations eluted between 100 and 200 °C. On the other hand, the same temperature variation with FeS2 as a reductant increased both total Fe eluted and Fe2+, which can be attributed to thermal changes in surface properties of solid FeS2 allowing greater accommodation of aqueous Fe3+. For both reductants, higher temperatures led to larger amounts of Mg2+ cation eluted. (3) Changes in S to Fe molar ratio for Na2S2O4 caused incremental changes in the amount of Fe eluted, while simultaneously increasing the amount of Fe2+, but it did little to increase the amount of Mg eluted. On the other hand, changes in S to Fe molar ratio for FeS2 did not significantly increase either Fe or Mg elution amounts, although at the maximum ratio tested, the speciation of Fe was completely Fe2+.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01979. Further information on characterization of samples using XRF, XRD, ICP-OES, Mössbauer spectroscopy, synchrotron K-edge XANES, and HSC Chemistry 7.1 simulation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61-3-9905-2592. Fax: +61-3-9905-5686. E-mail: lian. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to express appreciation to Yorrick Nicholson of Energy Australia (Yallourn) for provision of fly ash samples. T.K.C. acknowledges the State Government of Victoria for the Victorian International Research Scholarship (VIRS). The authors are also grateful to beamline scientists Ling-Yun Jang and Bing-Jian Su of NSRRC for support in operating Synchrotron XANES.

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REFERENCES

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