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A recyclable naturally derived magnetic pyrrhotite for elemental mercury recovery from the flue gas Yong Liao, Dong Chen, Sijie Zou, Shangchao Xiong, Xin Xiao, Hao Dang, Tianhu Chen, and Shijian Yang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03288 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016
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A recyclable naturally derived magnetic pyrrhotite for elemental
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mercury recovery from the flue gas
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Yong Liao,┼ Dong Chen, ╪ Sijie Zou, ┼ Shangchao Xiong, ┼ Xin Xiao, ┼ Hao Dang, ┼ Tianhu Chen,
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Shijian Yang ┼, *
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┼
Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of
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Environmental and Biological Engineering, Nanjing University of Science and Technology,
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Nanjing, 210094, P. R. China
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╪
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Environmental Engineering, Hefei University of Technology, Hefei, 230009, China
Laboratory for Nanomineralogy and Environmental Material, School of Resources and
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Abstract:
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Magnetic pyrrhotite, which was derived from the thermal treatment of natural pyrite, was
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developed as a recyclable sorbent to recover elemental mercury (Hg0) from the flue gas as a
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co-benefit of the wet electrostatic precipitators (WESP). The performance of naturally derived
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pyrrhotite for Hg0 capture from the flue gas was much better than those of other reported magnetic
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sorbents for example Mn-Fe spinel and Mn-Fe-Ti spinel. The rate of pyrrhotite for gaseous Hg0
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capture at 60 oC was 0.28 µg g min-1 and its capacity was 0.22 mg g-1 with the breakthrough
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threshold of 4%. After the magnetic separation from the mixture collected by the WESP, the spent
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pyrrhotite can be thermally regenerated for the recycle. The experiment of 5 cycles of Hg0 capture
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and regeneration demonstrated that both the adsorption efficiency and the magnetization were not
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degraded notably. Meanwhile, the ultra-low concentration of gaseous Hg0 in the flue gas was
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concentrated to high concentrations of gaseous Hg0 and Hg2+ during the regeneration process,
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which facilitated the centralized control of mercury pollution. Therefore, the control of Hg0
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emission from coal-fired plants by the recyclable pyrrhotite was cost-effective and did not have
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the secondary pollution.
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Table of Content
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1.
Introduction
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Minamata Convention, an international and legally-binding convention to prevent mercury
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emission, has been delivered in 2013.1 The emission of mercury from coal-fired utility boilers is
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an important source of anthropogenic mercury emission,2-4 so nations agree to reduce mercury
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emission from coal-fired plants in the treatment. Hg emits from coal combustion mainly as
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elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp).5 Hgp and
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Hg2+ can be effectively removed by the particulate control device and wet flue gas desulfurization
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(FGD), respectively. However, Hg0 cannot be removed by currently available control devices in
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coal-fired plants. Therefore, Hg emits from coal-fired plants to atmosphere mainly as gaseous
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Hg0.6
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Now, the control of Hg0 emission from coal-fired plants mainly falls into the conversion of
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gaseous Hg0 to Hgp and the catalytic oxidation of Hg0 to water soluble Hg2+. The conversion of
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gaseous Hg0 to Hgp can be achieved through the adsorption by sorbents for example brominated
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activated carbon (PAC), and the catalytic oxidation of Hg0 to Hg2+ can be realized as a co-benefit
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of the selective catalytic reduction (SCR) units. The catalytic oxidation of Hg0 to Hg2+ is currently
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restricted for at least three reasons: the lack of HCl in the flue gas, the interference of NH3 with
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Hg0 oxidation, and the re-release of Hg0 in the FGD unit due to the reduction of Hg2+ by
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sulphite.7-9 Meanwhile, the adsorption of Hg0 by sorbents is limited for at least three reasons: the
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high operation cost due to the non-recycle of the sorbents, the negative effect of the sorbents on
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the utilization of fly ash, and the interference of SO2 and H2O with Hg0 adsorption.10, 11 Moreover,
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there is a common major disadvantage for the two methods that gaseous Hg0 is not collected for
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the centralized control and it is converted to more toxic HgCl2 or HgBr2 in the
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desulfurization gypsum and fly ash.
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ash as building materials may cause a persistent exposure of the residents to mercury.
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Therefore, the usages of desulfurization gypsum and fly
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To recover Hg0 in the flue gas for the centralized control and decrease the cost for Hg0 capture,
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many researchers now aim to develop recyclable sorbents. The magnetization of the sorbents
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makes it possible to separate spent sorbents from the mixture with fly ash, so a series of magnetic
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sorbents, for example MagZ-Ag0,14, 15 Fe-based spinels 16-21 and Co-MF 22 have been developed to
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capture gaseous Hg0 from the flue gas. However, the recovery of the spent magnetic sorbents is 4
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still difficult and impractical as they are generally collected by electrostatic precipitators (ESPs) as
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a mixture with greater than 99.9% of fly ash particles.12 Now, wet electrostatic precipitators
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(WESP) are installed downstream of the FGD in some coal-fired plants to control the emissions of
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ultrafine particulates and gypsum rain. Because most of the fly ash particulates (>99%) in the flue
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gas have been collected by the ESP, the particulates collected by the WESP are mainly small
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amounts of desulfurization gypsum and ultrafine particulates. Therefore, the recovery of spent
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magnetic sorbents from the mixture collected by the WESP will be practicable. In our previous
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study, magnetic Mn-Fe spinel was developed to recover gaseous Hg0 from the flue gas as a
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co-benefit of the WESP.12 However, approximately 50% of Hg species formed on Mn-Fe spinel
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were water soluble HgSO4. Then, they would be leached during the collection of the particulates
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from the WESP by water washing, resulting in a low efficiency for mercury recovery and a
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secondary pollution. Furthermore, Mn-Fe spinel was an artificial material, resulting in a high cost
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for mercury recovery.
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In this work, a novel magnetic pyrrhotite (Fe1-xS), which was derived from the thermal treatment
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of natural pyrite (FeS2), was developed to recover Hg0 in the flue gas as a co-benefit of the WESP.
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As pyrite is widely dispersed in the world with a huge geological reserve,23, 24 naturally derived
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pyrrhotite is very cheap and easy of access. Furthermore, Hg species formed over pyrrhotite was
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mainly HgS, so the leaching of Hg species from pyrrhotite after Hg0 capture cannot be observed.
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Therefore, naturally derived pyrrhotite was a promising magnetic sorbent to recover Hg0 in the
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flue gas as a co-benefit of the WESP.
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2.
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2.1 Sample preparation
Experimental Section
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Natural pyrite used in this study was collected from Tongling ore cluster area, Anhui Province,
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China. Meanwhile, pyrrhotite was obtained through the calcination of natural pyrite under N2
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atmosphere at 700 oC for 1 h. 25 The chemical compositions of natural pyrite and naturally derived
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pyrrhotite, which were determined by an electron probe microanalysis (JXA-8100), were shown in
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Table S1 in the Supporting Information.
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2.2 Characterization
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The measurements of BET surface area, X-ray diffraction pattern (XRD) and saturation 5
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magnetization were performed on a nitrogen adsorption apparatus (Quantachrome, Autosorb-1),
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an X-ray diffractionmeter (Bruker-AXS D8 Advance) and a vibrating sample magnetometer (VSM,
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LakeShore 735), respectively. Thermogravimetric analysis and differential scanning calorimetry
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(TG-DSC) were performed on a Netzsch STA 409PC Instrument, and SO2 concentration in the
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outlet was simultaneously measured online by a Fourier transform infrared spectrometer (FTIR,
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Thermo SCIENTIFIC, ANTARIS, IGS Analyzer). The morphology was characterized by a
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transmission electron microscopy (TEM, FEI Tecnai 20) and a scanning electron microscopy
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(SEM, SU8010). X-ray photoelectron spectra (XPS) over the spectral regions of Fe 2p, S 2p, O 1s
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and Hg 4f were recorded on an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250).
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2.3 Elemental mercury capture
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Hg0 capture was performed on a fixed-bed quartz tube microreactor with the internal diameter
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of 6 mm 26 and the reaction temperature varied from 40 to 100 oC (i.e. the temperature range of the
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flue gas downstream of the FGD). The mass of sorbent with 40-60 mesh was 200 mg, and the flow
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rate of simulated flue gas was 500 mL min-1 (room temperature), resulting in a gas hourly space
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velocity (GHSV) of 1.5×105 cm3 g-1 h-1 (i.e. 150000 h-1). The simulated flue gas contained
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approximately 0.12 mg m-3 of gaseous Hg0, 5% of O2, 80 ppm of SO2 (when used), 8% of H2O
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(when used) and balance of N2. The concentrations of gaseous Hg0 in the inlet and in the outlet
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were determined online by a cold vapor atomic absorption spectrophotometer (Lumex R-915+).
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Meanwhile, the concentration of the total Hg species (i.e. HgT, including Hg2+ and Hg0) in the
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outlet was simultaneously determined after the reduction of Hg2+ to Hg0 by a SnCl2 solution.
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Temperature programmed desorption of Hg (Hg-TPD) from the sorbent after Hg0 capture was
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carried out on the microreactor with the N2 gas flow of 100 mL min-1 at a heating rate of 10 oC
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min-1.
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The cycle of Hg0 capture and regeneration was performed on the microreactor at 60 oC. 800 mg
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of naturally derived pyrrhotite with 40-60 mesh and 500 mL min-1 (room temperature) of the
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simulated flue gas containing approximately 0.12 mg m-3 of gaseous Hg0, 5% of O2, 80 ppm of
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SO2 and 8% of H2O were used for the test. Pyrrhotite after Hg0 capture for 3 h was first washed by
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20 mL of deionized water in the microreactor to simulate the collection of the mixture of fly ash,
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desulfurization gypsum and pyrrhotite from WESP by water washing. Then, pyrrhotite was 6
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thermally treated at 500 oC for 40 min with a N2 gas flow of 100 mL min-1 followed by a new
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capture.
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3.
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3.1 Characterization
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3.1.1 XRD and BET surface area
Results and Discussion
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XRD patterns of natural pyrite and naturally derived pyrrhotite corresponded very well to the
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standard cards of cubic pyrite phase (JCPDS: 42-1340) and monoclinic pyrrhotite phase (JCPDS:
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89-1549), respectively. Meanwhile, the peaks corresponding to any other iron oxides, iron sulfides
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and iron sulfates cannot be clearly observed (shown in Figure 1A). Figure 1A also shows that the
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full wave at half maximum (FWHM) of natural pyrite was much less than that of naturally derived
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pyrrhotite. It suggests that the crystal size of naturally derived pyrrhotite was much less than that
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of natural pyrite. TEM and SEM images (shown in Figures S1 and S2 in the Supporting
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Information) both demonstrate that the particle size of naturally derived pyrrhotite was much
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smaller than that of natural pyrite. Therefore, the BET surface area of pyrrhotite (19.7 cm2 g-1) was
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much high than that of pyrite (close to zero).
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3.1.2 Magnetization
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Natural pyrite showed a neglectable magnetization and its saturation magnetization was only
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0.1 emu g-1 (shown in Figure 1B). However, naturally derived pyrrhotite showed an excellent
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magnetization with the saturation magnetization of 7.0 emu g-1. Pyrrhotite exhibited an obvious
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magnetization hysteresis with the coercivity of 31000 A m-1 (shown in Figure 1B). Although
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pyrrhotite did not show a super-paramagnetism, it can be well dispersed due to its lower
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permanent magnetization of 3.7 emu g-1.
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3.1.3 Thermal transformation of natural pyrite to naturally derived pyrrhotite
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DSC curve of natural pyrite under N2 shows three obvious endothermic peaks at approximately
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118, 536 and 635 oC, which corresponded well to three weight losses (shown in Figure 2a). The
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first endothermic peak at 118 oC was assigned to the dehydroxylation and dehydration.27 The
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second endothermic peak at 536 oC corresponded well to the emission of gaseous SO2, so it was
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attributed to the decomposition of amorphous ferrous sulfate to amorphous hematite and gaseous
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SO2.25 The third endothermic peak at 635 oC was attributed to the phase transformation of pyrite 7
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to pyrrhotite.25 During the phase transformation, some sulfur species emitted as gaseous sulfur,
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resulting in a notable weight loss.
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3.1.4 XPS
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The Fe 2p 3/2 spectra over natural pyrite mainly appeared at 707.4, 709.2, 711.3 and 713.4 eV
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(shown in Figure 3a), which were assigned to Fe2+ bonded with S22-, Fe2+ bonded with O2-, Fe3+
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bonded with -OH and Fe2+ bonded with SO42-, respectively.20, 28, 29 The O 1s spectra mainly
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appeared at 531.5 and 532.3 eV (shown in Figure 3b), which were attributed to O in -OH and O in
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SO42-, respectively.20 Furthermore, another binding energy at 533.5 eV can be clearly observed in
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the O 1s spectral region, which was assigned to O in SiO2. SiO2 was the impurity in natural pyrite.
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The S 2p spectra mainly appeared at 162.8, 164.0, 168.8 and 169.8 eV (shown in Figure 3c),
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which were assigned to S22-, polysulfur, SO42- and HSO4-, respectively. 20, 28, 29 The presences of O
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species and SO42- on natural pyrite suggest that the surface of pyrite was partly oxidized.
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The Fe 2p 3/2 spectra over naturally derived pyrrhotite mainly appeared at 707.1, 709.4, 711.1
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and 713.3 eV (shown in Figure 3d), which were assigned to Fe2+ bonded with S2-, Fe2+ bonded
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with O2-, Fe3+ bonded with -OH and Fe2+ bonded with SO42-, respectively.
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529.9 eV corresponding to O2- in iron oxides
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Figure 3e). The S 2p spectra over pyrrhotite mainly appeared at 161.4, 162.6, 163.8 and 168.7 eV
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(shown in Figure 3f), which were attributed to S2-, S22-, polysulfur and SO42-, respectively. 20, 28-30
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20, 28-30
A new peak at
can be clearly observed on pyrrhotite (shown in
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The percentages of O and S species, S/Fe and O/Fe on natural pyrite and naturally derived
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pyrrhotite, which were resulted from the XPS analysis, were shown in Table 1. As most of SO42- in
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pyrite were decomposed during the thermal treatment (hinted by Figure 2a), SO42- percentage on
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pyrrhotite (1.7%) was much less than that on pyrite (14.9%). Table 1 also shows that the
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percentages of S22- and polysulfur on pyrite obviously decreased after the thermal treatment at 700
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o
C. Instead of this, S2- became the major S species on pyrrhotite. As a lot of S in pyrite emitted as
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SO2 and gaseous sulfur during the thermal treatment, S/Fe on pyrrhotite was only one fifth of that
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on pyrite (shown in Table 1). Meanwhile, O/Fe on pyrrhotite was much less than that on pyrite due
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to the decomposition of SO42- to gaseous SO2.
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3.2 Elemental mercury capture
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3.2.1 Performances of pyrite and pyrrhotite for elemental mercury capture 8
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Figures S3a and S3b in the Supporting Information show the breakthrough curves of Hg0
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capture by natural pyrite and naturally derived pyrrhotite. The average rates of Hg0 capture by
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pyrite and pyrrhotite in the first 3 h, which were resulted from the breakthrough curves, were
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listed in Table 2. Although the BET surface area of pyrite was close to zero, it exhibited an
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excellent performance for gaseous Hg0 capture and its rate for gaseous Hg0 capture varied from
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0.21 to 0.31 µg g-1 min-1 at 40-100 oC. Pyrrhotite also showed an excellent performance for
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gaseous Hg0 capture at 40-100 oC and its rate for gaseous Hg0 capture was close to that of pyrite
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(shown in Table 2). As the magnetization made it possible to reclaim spent sorbent for the recycle,
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pyrrhotite was preferred to capture gaseous Hg0 in the flue gas than pyrite.
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The presence of O2 caused to the oxidation of metal sulfides, which would show an inhibition on
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Hg0 capture. To investigate the effect of O2, Hg0 capture by pyrrhotite was performed under N2
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atmosphere (shown in Figure S3c in the Supporting Information). Table 1 shows that the rate of
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pyrrhotite for gaseous Hg0 capture under 5% O2/N2 was slightly higher than that under N2. It
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suggests that the presence of O2 did not show a notable inhibition on Hg0 capture by pyrrhotite.
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The emission of SO2 during Hg0 capture by pyrrhotite was a serious concern as the process was
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downstream of the FGD. Figure S4 in the Supporting Information shows that the concentrations of
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SO2 in the outlet during Hg0 capture by pyrrhotite at 40-100 oC were very low (
