Article pubs.acs.org/EF
Behavior of Fe2+/3+ Cation and Its Interference with the Precipitation of Mg2+ Cation upon Mineral Carbonation of Yallourn Fly Ash Leachate under Ambient Conditions Teck Kwang Choo,† Barbara Etschmann,‡ Cordelia Selomulya,† and Lian Zhang*,† †
Department of Chemical Engineering, Monash University, Wellington Road, Clayton, Victoria 3800, Australia Department of Geosciences, Monash University, Wellington Road, Clayton, Victoria 3800, Australia
‡
S Supporting Information *
ABSTRACT: A variety of leachates derived from the acid leaching of a unique Fe-rich fly ash, namely Yallourn from the Latrobe Valley of Australia at different temperatures, have been processed to achieve two key goals: synthesis of Fe-rich precipitate and mineral carbonation of alkaline earth metal cations for the storage and utilization of carbon dioxide (CO2). The behavior and interference of unprecipitated Fe-bearing cations on the carbonation stage was for the first time examined by us. The research findings are also applicable to the leachate derived from other industry wastes and even natural minerals, which also contain varying amounts of iron as one impure metal. To precipitate Fe out of the leachate, sodium hydroxide (NaOH) was added to adjust the pH to 4. Subsequently, the pH of the resultant supernatant was further increased to ∼13 and bubbled with CO2 to precipitate the remaining cations. As has been confirmed through the characterization of solid products by synchrotron Fe Kedge X-ray adsorption spectroscopy (XAS), quantitative X-ray diffraction (Q-XRD), and scanning electron microscopy (SEM), Fe was precipitated out as a mixture that was predominantly ferrihydrite with a nanoscale size for its primary nuclei. Its composition and size also varied largely with the leachate, i.e. the acid leaching temperature. For the unprecipitated Fe2+ that is predominant in the high-temperature leachate (i.e., ≥150 °C), it was preferentially oxidized and converted into nanoscale magnetite during the carbonation process, providing seed/nuclei for the crystallization and growth of magnesian calcite (Ca0.2Mg0.8CO3). Accordingly, the carbonation of Mg2+ was enhanced remarkably and reached completion in 20 min at room temperature. In addition, all the resulting products, Fe-precipitate (i.e., ferrihydrite), magnetite, and magnesian calcite, are relatively pure, presenting cheap and suitable precursors for a variety of value-added environmental applications.
1. INTRODUCTION There exists a wide scope for the research of carbon storage technologies, including underground sequestration, ocean storage, and mineral carbonation.1 Of all these methods, mineral carbonation has been shown to be most promising, due to the fact that carbonates are thermodynamically the most stable form of carbon.2 The resultant carbonate products and other byproducts can also be used in a value-added way. Research in this area has attracted increased attention, as it allows a safe disposal of carbon into the environment. Mineral carbonation typically makes use of either waste minerals or natural minerals. Waste minerals provide a potential for not only the capture and storage of CO2 but also the opportunity to reduce any requirement to store them from their production site, and to convert wastes into high-purity carbonates that can be used in value-added applications. There are two routes for carbonation: direct and indirect carbonation.3 Direct carbonation involves the direct contact of a particular mineral with carbon dioxide, normally requiring high temperatures and pressures.4,5 On the other hand, an indirect carbonation process is initiated from the use of either an acid or alkaline leaching reagent to selectively extract alkaline metals out into aqueous solution, which is subsequently bubbled by CO2-laden flue gas to immobilize CO2 into carbonate precipitates. 3 The bulk of research work on indirect carbonation2,6−8 has mainly focused on how to maximize the © XXXX American Chemical Society
yield of carbonates from waste minerals, with parameters for optimum conditions being determined empirically and case-bycase. The largest shortage of knowledge on carbonation processes are the fate and influence (if any) of impure elements during the indirect carbonation process. For any mineral wastes such as fly ash, it consists of a broad range of elements apart from calcium (Ca) and magnesium (Mg). Some metals may compete with calcium and magnesium during the leaching stage, whereas some such as silicon (Si) and aluminum (Al) may host these two target elements in a refractory matrix. All these probabilities exert a remarkable effect on the leaching selectivity of target metals and carbon capture capacity of a mineral waste as well. Unlike existing literature which mostly focuses on fly ashes with Ai and Si as the matrix, and Fe being lean relative to Ca and Mg, we have been working on a unique fly ash which possesses ≥40 wt % Fe and Mg mainly present as magnesioferrite (MgFe2O4) within it.9 Such a fly ash was collected from the Yallourn region of Victoria, Australia. Upon prior multistep acid leaching, Fe is eluted out at a comparable rate with Mg2+ and even Ca2+ into the leachate. It is imperative Received: December 9, 2015 Revised: March 16, 2016
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DOI: 10.1021/acs.energyfuels.5b02867 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Flow diagram of the proposed fly ash utilization process including carbonation, with the scope of this paper contained inside the blue dotted-line border. was water-washed, sieved to less than 10μm and dried. Quantified as oxides, this fly ash is made up of Fe2O3 (48.87 wt %), MgO (27.92 wt %), CaO (7.21 wt %), Al2O3 (5.75 wt %) and SiO2 (5.49 wt %).9 The water-washed sample was then leached with 2 M HCl at 20−200 °C for 5 mins, yielding both solid residue and leachate. The leachates were collected for this study, which is henceforth termed “acid leachate”. Our previous study9 has reported the effect of temperature on the percentage extraction of major metals during the acid leaching of fly ash and hence Table 1 only lists the concentrations by mass of
to clarify if a prior Fe-precipitation stage will be suitable to precipitate Fe entirely out of the leachate, and if not, how the Fe-bearing cations will affect the subsequent carbonation of Mg2+ and even Ca2+. Regarding the precipitation of Fe-bearing cations, it is well-known that Fe3+ can form solid hydroxide/ oxide quickly upon adjusting the pH value of the leachate.6,10 However, it is still unclear how the oxidation state of Fe in the leachate derived from Yallourn fly ash affects these processes. It is also necessary to fine-tune the parameters to maximize the precipitation of Fe at a minimum loss of Mg2+/Ca2+ prior to the carbonation step. This study aims to clarify the feasibility of a prior precipitation step on the removal of Fe from a fly ash leachate, and to clarify the interference of the remaining Fe-bearing cations during the carbonation process of the leachate derived from the acid leaching of Yallourn fly ash. Meanwhile, we also aim to clarify the properties and potential applications of the products derived from Fe precipitate and carbonate. The overall scheme of the research on the use of this specific fly ash for indirect carbonation is illustrated in Figure 1. Following the leaching of Yallourn fly ash in acid, the pH of the leachate was initially adjusted to promote Fe precipitation. Subsequently, the leachate is bubbled with CO2 to precipitate Ca and Mg. To understand the mechanisms underpinning the two major research goals mentioned above, both liquid and solid products have been subjected to a variety of analysis including Q-XRD, inductively coupled plasma optical emission spectroscopy (ICP - OES), SEM coupled with energy-dispersive X-ray analyzer (EDX), and synchrotron XAS. The interpretation of the results here is expected to promote the understanding on the properties of this specific fly ash and the value-added utilization of its mineral carbonation byproducts in the carbon-constrained future.
Table 1. Colors of Acid Leachates and Concentrations of Individual Metals, as Quantified by ICP-OES (g/L)
individual metals Al, Ca, Fe, Mg and Si in the leachates. The colors of selected leachates are also shown. Clearly, upon the increase of leaching temperature, the leachate turns dark brownish due to the enhanced elution of Fe, reaching 6.17−21.9 g/L in the leachates. Mg is the second most abundant, followed by Ca, Al and Si in a descending sequence that is in line with the contents of these metals in the original fly ash. Additionally, Figure 2 shows that although Fe3+ is the predominant cation in the room temperature leachate, the concentration of Fe2+ increases dramatically upon the rise of the leaching temperature. 2.2. Precipitation of Fe2+/3+ Cations Prior to Carbonation. For each run 50 mL of leachate was measured and transferred into a 250 mL conical flask containing a magnetic stirrer bar. Subsequently, NaOH (diluted from pellets purchased from Merck, Reagent grade) solution was added dropwise from a buret into the leachate until the final pH reached a set pH value ranging from 4 to 8.11 A precalibrated pH electrode was placed into the flask to continuously monitor the pH
2. EXPERIMENTAL SECTION 2.1. Fly Ash Leachate Properties. Prior to the carbonation stage, the fly ash sample obtained from the Yallourn power plant in Victoria B
DOI: 10.1021/acs.energyfuels.5b02867 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
carbonate samples was also examined to clarify the association of different species within one another. For such a purpose, a small amount of the sample was put into an epoxy resin mix and allowed to solidify at room temperature. The hardened resin was placed under vacuum overnight before being polished with silicon carbide and diamond papers. The resin was finally coated with iridium prior to the SEM analysis. 2.4.3. Synchrotron XAS. The Fe K-edge XANES and EXAFS characterization of Fe precipitates and carbonates was conducted at the Beamline BL16A1, National Synchrotron Radiation and Research Centre (NSRRC). A small amount of sample powder was mixed with boron nitride (BN) before being dusted onto Kapton tape mounted on a sample holder. For the diffraction of X-ray beam, a fixed exit double crystal Si (111) monochromators was used.9 Pure metal Fe with a white-line position of 7112 eV was used for calibration and this value was used as the base position (0 eV) for the analysis of all real samples. The XANES analysis was performed from 200 eV below to 250 eV above this energy value. Additionally, EXAFS analysis was performed up to 792 above this base position for selected samples. A model compound iron(III) chloride hexahydrate (FeCl3.6H2O, Sigma-Aldrich, ≥98.0% purity) was also analyzed. The XANES spectra for ferrihydrite, goethite and lepidocrocite used for comparison were obtained from Prof. Enzo Lombi from the University of South Australia, while the XANES spectrum for magnetite was obtained from Matthew Marcus from the Lawrence Berkeley National Lab. The software ATHENA was used for the processing of XANES spectra where a normalization order of two was used. The energy range for the normalization of the pre-edge region was −150 to −30 eV, while the energy range was +77.7 to +280.7 eV for postedge normalization. Additionally, Linear Combination Fitting (LCF) was also performed on sample spectra using the aforementioned model compounds. For this purpose, the fitting was done from −20 to +50 eV relative to the peak energy position of Fe. XANES and EXAFS data were analyzed with the HORAE package,14 using FEFF version 9.15 1-σ errors are reported from EXAFS analyses (HORAE package14). The data were refined in the range from 2 ≤ k ≤ 12 (Å−1) and 1 ≤ r ≤ 5.5 (Å) with a Hanning window and multiple kn (n = 1,2,3) weights. The data were fitted with two models (1) using a combination of hematite (cif file from ref16) and magnetite (cif from ref.17) and (2) using a combination of hematite and magnesioferrite (cif from ref18). The fraction of hematite and the second component was refined. It should be noted that paths for magnetite and magnesioferrite were calculated for both octahedral and tetrahedral Fe and weighting these paths by a factor of 0.5, this gave a better fit than just using paths around octahedral Fe. 2.4.4. HSC Chemistry Simulation. HSC Chemistry simulation was performed to see if the Fe(OH)2 can be converted to Fe3O4 without the use of oxygen at mild temperatures (
