Article pubs.acs.org/EF
Mechanisms Underpinning the Mobilization of Iron and Magnesium Cations from Victorian Brown Coal Fly Ash Teck Kwang Choo, Yi Song, Li Zhang, Cordelia Selomulya, and Lian Zhang* Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia S Supporting Information *
ABSTRACT: This paper reported the leaching of Yallourn brown coal fly ash uniquely rich in magnesioferrite (MgFe2O4) using hydrochloric acid, by varying acid concentration, liquid-to-solid (L/S) ratio, temperature, and time, to understand the mechanisms underpinning the extraction of iron and magnesium. Results indicate that Yallourn fly ash is composed of a densely packed, crystalline ash matrix, mainly in the form of MgFe2O4 and maghemite (γ-Fe2O3). Thermal cleaving at 100 °C and above liberates embedded octahedrally coordinated Fe3+, a controlling step for the leaching process. Increments from 100 to 200 °C did little to enhance extraction of Fe and Mg in single-stage leaching. Except at 70 °C, the thermal decomposition for the elution of both metals was complete within 1 min, while increasing the L/S ratio and acid concentration had negligible influence. The application of two-stage leaching gave protons a larger driving force to access embedded unreacted species. Subsequently, the extraction yields of both Fe and Mg went beyond 80%. The reductive leaching of Fe3+ with the assistance of a small amount of marcasite (FeS2) is the crucial reaction improving the breakage of framework at 100 °C and above. Consequently, the concentration of eluted Fe2+ accounted for 60% and up to 90% in 100 and 200 °C leachates, respectively.
1. INTRODUCTION
extraction: MO(s) + 2HCl(aq)
Mineral carbonation provides a solution for the permanent capture of carbon dioxide (CO2) without geological leakage,1,2 as resulting carbonate products are thermodynamically stable.3 The carbonation process involves spontaneous liberation of heat energy, which typically falls in the ranges of 64−90 and 118−179 kJ/mol for use of silicates and metal oxides as feedstocks, respectively.4 Magnesium (Mg) and calcium (Ca) in minerals are two principal elements that form stable carbonate precipitates for the storage of CO2 with little being decomposed over 1000 years.5 The acceleration in natural weathering process of Mgcontaining minerals has been extensively studied with regards to the permanent and safe disposal of CO2. Earliest and ongoing work involves minerals in the form of magnesium silicates such as serpentine, by virtue of them having large reserves in countries such as USA, Lithuania, and Finland.6 Mg has been chosen as the agent for CO2 capture because in its soluble form, it provides the necessary alkalinity that is central to the binding of CO2.7 The past decade has seen the development of mineral carbonation processes utilizing extreme conditions such as temperatures up to 450 °C8 and CO2 partial pressures as high as 185 atm9 for a direct, single-step capture of gaseous CO2 into a mineral. An alternative process using milder conditions is indirect carbonation, whereby the mineral is first allowed to react with a chemical reagent for initial mobilizing of Ca or Mg into an aqueous phase, which is subsequently subjected to a carbonation reaction, resulting in purer carbonate products that can be used in a value-added manner.4 The reactions involved in an indirect carbonation process are shown below, where hydrochloric acid is taken as an example, and the symbol M refers to the element Ca or Mg:4,10 © 2014 American Chemical Society
→ M2 +(aq) + 2Cl−(aq) + H 2O(l)
(1) 2+
Dissolved CO2 forms carbonate ions that react with M : carbonation: M2 +(aq) + CO32 −(aq) → MCO3(s)
(a)
In principle, minerals rich in Ca and/or Mg have the potential to be adopted in mineral carbonation processes. Compared to natural minerals such as serpentine and olivine, waste minerals discharged from industrial processes have been attracting much attention for their potential to drastically reduce the cost of mineral carbonation processes. These industrial wastes include cement waste, mine tailings, oil shale ash,11 fly ash derived from municipal solid waste incinerators (MSWI)12,13 and those derived from the combustion of lignite, i.e., brown coal.10 Use of industrial wastes eliminates costs relating to mineral quarrying and procurement.4 It can also help convert wastes into value-added carbonates and other byproducts. For more discussion on the use of alkaline waste minerals, please refer to the Supporting Information. In this study, we examined the acid leaching performance of a local brown coal fly ash rich in both iron (Fe) and Mg oxides. Fly ash was collected from a pulverized coal-fired boiler for the combustion of Yallourn brown coal in the Latrobe Valley, Victoria, Australia. The fly ash has an annual yield of approximately 1.3 million tonnes,10 little of which is being used because it is a class C fly ash that is unsuitable for use as a cement additive. Brown coal fly ash has been shown to potentially sequester up to 264 kg of CO2 per tonne of coal fly Received: March 19, 2014 Revised: May 19, 2014 Published: May 19, 2014 4051
dx.doi.org/10.1021/ef500618r | Energy Fuels 2014, 28, 4051−4061
Energy & Fuels
Article
ash.10 This study aims to elucidate the mechanisms associated with the mobilization of both Mg and Fe cations upon acid leaching of Yallourn brown coal fly ash used. The resulting metallic cations in the leachate can be employed for carbonation or the production of pure Fe-bearing oxide/ hydroxide via precipitation at higher pH values, which will be discussed in an accompanying paper of this research. Given the broad variation of fly ash properties and the unusual abundance of Fe-bearing species within the fly ash sample studied here, it is also of interest to clarify how Fe will interfere with the extraction of target Mg2+ cation during acid leaching. The acid used was 2 M hydrochloric acid and the acid to solid ratio was fixed at 6.67 mL/g, whereas the dependency of percentage extractions for both Mg2+ and Fe2+/3+ cations with temperature and time were investigated. Apart from singlestage leaching, two-step leaching was also employed to elucidate the dissolution of target metals from different mineral species within the fly ash matrix. For the characterization of solid residues and leachates, a variety of advanced instruments have been employed to reveal mechanisms underpinning the mobilization of two major metals, including X-ray fluorescence spectroscopy (XRF), quantitative X-ray diffraction (Q-XRD), Mö ssbauer spectroscopy (MS), and synchrotron X-ray adsorption near edge spectroscopy (XANES).
Figure 1. XRD spectra of water-washed Yallourn fly ash (a) and its sequential leaching residues (b) ammonium acetate-insoluble residue, (c) acetic acid-insoluble residue, and (d) nitric acid-insoluble species; solid curves represent original data points after background removal, while dotted lines represent fitted curves as quantified by Siroquant 3.0. M refers to magnesioferrite (MgFe2O4), maghemite or magnetite; P for periclase (MgO), C for calcite (CaCO3), Q for quartz (SiO2), G for gypsum (CaSO4.H2O), and A for anhydrite (CaSO4).
2. EXPERIMENTAL SECTION 2.1. Yallourn Fly Ash Properties. Fly ash procured from Energy Australia (Yallourn) power plant’s electrostatic precipitator (ESP) was water-washed to remove water-soluble sodium compounds and unburnt carbon, dried in an oven at 110 °C, and sieved. The particle size
