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Mercury re-emission in the smelting flue gas cleaning process: the influence of arsenite Zhilou Liu, Dongli Wang, Bing Peng, Liyuan Chai, Shu Yang, Cao Liu, Cong Zhang, Xiaofeng Xie, and Hui Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01733 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017
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schematic diagram of reaction mechanism 222x120mm (150 x 150 DPI)
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Mercury re-emission in the smelting flue gas cleaning
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process: the influence of arsenite
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Zhilou Liu, a Dongli Wang,b Bing Peng,b, c Liyuan Chai,b, c Shu Yang,b
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Cao Liu,b Cong Zhang,b Xiaofeng Xie,b Hui Liu,b, c*
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a
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Technology, Ganzhou 341000, China
School of Metallurgy and Chemical Engineering, JiangXi University of Science and
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b
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China
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c
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Metal Pollution, Changsha 410083, China
School of Metallurgy and Environment, Central South University, Changsha 410083,
Chinese National Engineering Research Center for Control & Treatment of Heavy
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* Corresponding author. Postal address: School of Metallurgy and Environment,
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Central South University, 932 South Lushan Road, Changsha 410083, China. E-mail:
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[email protected] 1
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ABSTRACT:
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Recently, the Hg0 re-emission from flue scrubbing solutions has become a research
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focus. The objective of this study was to evaluate the influence of arsenite on Hg0
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re-emission by investigating critical important parameters, including the arsenite ion
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concentration, pH values, solution temperature, fluorine ion concentration, and
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chlorine ion concentration. The experimental results indicate that arsenite could
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directly reduce Hg2+ to Hg0 and promote the decomposition of Hg(SO3)22-. High pH,
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temperature and chlorine ion concentration contributed to the inhibition of Hg0
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re-emission, but the fluorine ions had little effect. The mechanism research suggests
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that the formation of unstable HgH2AsO3+ at low arsenite concentration accelerates
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Hg0 re-emission. For the Hg(II)-As(III)-S(VI) system, the formation of unstable
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HgSO3H2AsO3- than Hg(SO3)22- could increase Hg0 re-emission. With an increase in
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arsenite concentration, the formation of Hg(H2AsO3)2 (which comes from the reaction
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between HgSO3H2AsO3- and H2AsO3-) contributes to the suppression of Hg0
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re-emission. The results of this study are conducive to providing new insight into Hg0
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re-emission and mercury control.
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Keywords: Hg0 re-emission; cleaning process; arsenite ions; smelting flue gas
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1. Introduction
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Mercury (Hg), due to its characteristics of long-distance transmission, volatility,
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toxicity and bioaccumulation, has drawn wide attention 1, 2. The atmospheric mercury
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emissions from nonferrous metal industries, especially zinc, lead and copper smelting,
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are regarded as one of the largest anthropogenic sources of mercury in China 3, 4.
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Since Hg is an important trace element associated with heavy metal sulfide ores, the
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concentration of Hg in heavy metal concentrates is relatively high. In roasting and
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smelting processes, nearly all the mercury in the concentrates are transferred to the
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flue gas, leading to the formation of flue gas with high Hg concentration. Therefore,
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understanding how to effectively control mercury emissions from nonferrous metal
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industries has become an urgent question.
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Different air pollution control devices, such as electrostatic precipitators (EP), wet
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flue gas cleanings (WFGC) device and electrostatic demisters (ED), are installed to
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purify nonferrous metal smelting flue gas 5, 6. The WFGC process has been widely
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applied in nonferrous metal smelters due to its high purification efficiency in
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simultaneously removing multiple pollutants and its low operating cost 7-9. In the
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WFGC process, most of the particulate matter and some other harmful elements such
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as fluoride, chloride, oxidized mercury (Hg2+), etc. are removed. Many studies have
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reported that the WFGC process is the most efficient process for Hg removal,
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accounting for 60% to 70% of the total Hg in the flue gas 10-12. However, soluble Hg2+
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in the solution can be deoxidized to elemental mercury (Hg0) by reducible matter,
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which may be followed by the volatilization of Hg0 in the flue gas during the WFGC 3
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process 13-15. After the WFGC process, a sulfuric acid production process is performed
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to recover SO2 16. In such a case, the Hg0 in the flue gas will deteriorate the quality of
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the sulfuric acid product. Therefore, it is very important to understand the reduction
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behavior of Hg2+ to reduce Hg to sulfuric acid and protect the environment.
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Recently, many studies have reported that Hg2+ could be reduced by SO32- to Hg0
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during the scrubbing process, lowering the total Hg capture efficiency 17-19. The
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studies indicated that the formation of unstable HgSO3 was the mechanism of Hg
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re-emission. Moreover, Van Loon 23 reported that some potential intermediates of the
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decomposition reaction of HgSO3 were formed by the transfer of a single electron,
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such as mercurous species (Hg+) and the sulfite radical anion (SO3-). The
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disproportionation of Hg+ is an additional pathway of Hg re-emission. Some water
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soluble ions (e.g., chloride ions, sulfate ions, sulfide ions, selenite ions, and metal ions,
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etc.) could affect Hg2+ reduction or Hg0 emission behaviors that are reported by some
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researchers 20-23. Different ions in solution have various influences on Hg
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re-emission24-26.
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complicated effect on the reduction process: it would benefit Hg re-emission due to
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the formation of HgSO3NO2- at low concentrations which was more unstable than
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HgSO3; it would also play an inhibiting effect on Hg reduction because of the
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formation of Hg(NO2) 42- at high concentrations 22.
For example, the presence of nitrite ions was proven to have a
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Except for traditional gas pollutants such as NOx, HCl and SO3, large amounts of
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arsenic oxide (As2O3) particles can also be captured in the WFGC process 27. During
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the scrubbing process, As2O3 particles, a typical pollutant for nonferrous metal 4
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smelting flue gas, can be dissolved into scrubbing solutions in the form of H3AsO3,
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H2AsO3-, HAsO32- or AsO33- 28, 29. In some smelters, the concentration of arsenite ions
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in the scrubbing solution can reach up to 3.3 mg/L 30. However, there is little research
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that focuses on the effect of arsenite on Hg re-emission. Arsenite ions may play a
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complex role in Hg reduction behavior. On one hand, arsenite ions have an
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intermediate valence state and may deoxidize Hg2+ and accelerate the Hg re-emission.
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On the other hand, arsenite ion may coordinate with Hg2+ to stabilize Hg2+ and reduce
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the Hg0 re-emission. As such, the influence of arsenite ion on Hg0 re-emission is a
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worth studying to elucidate the rule of Hg2+ reduction.
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As a further contribution to the understanding and interpretation of Hg reduction and re-emission behavior, this work studies the influence of arsenite on Hg0
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re-emission in a simulated WFGC system. Variables including the concentration of
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arsenite, sulfite, fluorine and chlorine ions, solution temperature, and solution pH
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were evaluated. In addition, the intermediates of Hg2+ reduction were characterized,
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and a mechanism for Hg0 re-emission under different conditions is proposed.
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2. Experimental section
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2.1 Experimental system and method
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A typical lab-scale device shown in Figure 1 was set up in this study to investigate
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Hg reduction and re-emission behaviors under different conditions. A high purity
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nitrogen gas was used as a carrier gas at a flow rate of 0.6 L/min, and was blown into
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liquid and then used to take the re-emission Hg0 which was released from liquid phase.
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The bubbling reaction equipment included a 500 mL, round bottom, 3-neck flask that
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contained a carry gas inlet, gas outlet, and placement of electrode to measure the
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solution pH. The flask was placed in a constant temperature water bath to control the
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solution
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temperature-controlled heating belt at 120 °C to avoid the condensation of moisture
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and the adsorption of Hg0 on the tube surface. Before mercury levels were tested, a
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solution containing 30% (w/v) NaOH was used to prevent the corrosion by acid gas.
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After measuring the Hg0 concentration in the flue gas, the tail gas treated with a
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solution containing 5% (w/v) KMnO4 and activated carbon (AC) to absorb Hg0.
temperature.
Teflon
tubing
was
used
and
heated
with
a
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At the beginning of every experiment, the simulated solution, which contained a
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certain arsenite concentration, was put into a flask. Then the solution was heated to a
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setting temperature and adjusted for pH. Afterwards the solutions with a
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pre-determined concentration of Hg2+ or SO32- were quickly injected. The total
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reaction solution volume was 300 mL. During the experiment, airtightness was
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ensured to avoid Hg0 emission.
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Figure 1. Schematic diagram of simulate Hg0 re-emission experimental device
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2.2 Measurement Method
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The Hg0 from Hg2+ reduction in outlet gas was continuously monitored using a
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Lumex Zeeman Mercury Spectrometer (RA-915M, detection limit = 2 ng/m3), which
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employed the resonance absorption of mercury atoms at the wavelength of 253.7 nm.
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The Mercury Spectrometer had to be calibrated by pure nitrogen before taking
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measurements. The solution containing Hg2+ was diluted to a suitable concentration
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that was determined using atomic fluorescence spectrometry. The sulfite, fluorine and
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chlorine concentrations in solution were determined using an ion chromatograph (883
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basic IC plus, Metrohm Ltd.). A S220-Basic Seven Compact Mettler Toledo apparatus
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was used for pH measurements of the solution.
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To identify the mechanism of Hg2+ reduction in the mixed solution, the UV-visible
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spectrum (UV-vis DRS: UV2450, China) was employed in this research. The solution
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containing Hg2+ was prepared by dissolving HgO in concentrated perchloric acid
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(HClO4) or nitric acid (HNO3), and then diluted to the required concentration. The
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arsenic and sulfite solutions were obtained by dissolving pure sodium arsenite and
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sodium sulfite in the dilute HClO4 respectively. Before every measurement, the
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baseline spectrum of a solution with ultrapure water was collected and would be
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subtracted from the spectrum of sample solutions to ensure the accuracy and
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correctness of the detector response. After obtaining a uniform mixture, the prepared
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solutions were placed in quartz cuvettes with the volume of 4 mL. The pH of the
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scanning solution was adjusted by the addition of NaOH and HClO4 solutions. The
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range of scanning wavelengths was from 200 to 300 nm, with a scanning speed of 100 7
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nm/min.
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2.3 Materials
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Sodium sulfite anhydrous, potassium permanganate, sodium chloride, sodium
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fluoride and sodium arsenite were obtained from Sinopharm Chemical Reagent Co.,
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Ltd. Perchloric acid (70 %), stannous chloride, sulfuric acid, hydrochloric acid and
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nitric acid were purchased from Kemiou Chemical Reagent Co., Ltd. Sodium sulfite
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anhydrous, sodium chloride, sodium fluoride are guarantee reagent while others are
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analytical grade. Mercuric oxide (the purity is higher than 99.9 %) as the source of
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divalent mercury are analytical grade and obtained from Kaihua Chemical Reagent
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Co., Ltd.
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3. Results and discussion
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3.1 Mercury re-emission in presence of arsenite
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The arsenite ion is one of the most common ions in the scrubbing solutions for the
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nonferrous metal smelting industry and easily accumulates at high levels due to the
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recycling of scrubbing solutions. To test the effects of arsenite content on Hg0
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re-emission, a series of corresponding experiments were conducted under different
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arsenite concentrations ranging from 0 to 0.48 mM and the results are shown in
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Figure 2 (the online date shown in Figure 2 are provided in the Supporting
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Information). Individual arsenite ions can result in Hg0 re-emission. The presence of
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arsenite at low concentrations could significantly increase Hg0 re-emission. As the
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arsenite concentration increased from 0.03 to 0.12 mM, the total amount of
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re-emission Hg0 over one hour rose from 15.86 to 48.92 µg. However, the Hg0
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concentration in the outlet was inhibited obviously by a further increase in the arsenite
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concentration. Additionally, to further determine mercury transfer paths in the
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cleaning solution, the mass balances of mercury were performed based on the
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determined results of the mercury concentration in solution before and after the
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experiments. The results in Table S1 show that the measured mercury amount in the
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flue gas is in good agreement with the difference value in solution between before and
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after experiments, with a relative error of less than 2%, which further proves the
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experimental accuracy.
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The primary species of arsenite in the acidic scrubbing solution is H2AsO3- (see
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Figure S1). At low arsenite concentrations, a portion of Hg2+ would be deoxidized to
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Hg0 by H2AsO3-. In that case, the reaction path is listed as R1. Meanwhile, it has been
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widely recognized that arsenite ions can easily combine with heavy metal ions to form
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coordination complexes. Therefore, H2AsO3- could react with Hg2+ to form a stable
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Hg(II)-As(III) complex at high arsenite concentrations. This may be the reason why
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high arsenite concentrations inhibit Hg0 re-emission. The detailed intermediate
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products and reaction mechanisms are presented in the following sections.
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Hg 2+ + H 2 AsO 3 − + H 2 O = Hg 0 + HAsO 4 2 − + 3H +
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(R1)
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2500 0 mM NaAsO2 0.03 mM NaAsO2
Hg0 con. (µg/m3)
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0.06 mM NaAsO2 0.12 mM NaAsO2 0.24 mM NaAsO2
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0.48 mM NaAsO2
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0 0
10
20
30
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50
60
T (min)
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Figure 2. Effect of arsenite concentrations ranging from 0 to 0.48 mM on Hg0 re-emission.
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Experimental conditions: [Hg2+] = 0.02 mM; solution volume= 300 mL; solution temperature =
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35 °C; initial pH value = 9.0; N2 flow rate = 600 mL/min.
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3.2 Effect of solution temperature
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Generally, the temperature of the actual scrubbing solution is between 30~60 °C.
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To evaluate the effect of temperature on Hg0 re-emission, several tests were conducted
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with the solution temperature ranging from 25 to 65 °C. The results shown in Figure 3
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demonstrate that the Hg0 re-emission could markedly increase with the solution
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temperature. As the temperature increased from 25 to 65 °C, the Hg0 concentration
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increased by several times, from approximately 352 to 950 µg/m3. This consequence
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can be explained by the following reasons. The proton diffusion in liquid-gas could be
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accelerated with the temperature increase. The redox reaction rate is also improved
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due to the reduction of energy barriers at high temperature. Additionally, Littel R 31
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reported that the increase of temperature could speed up the deprotonation process of
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arsenite in accordance to R2, leading to the increase of Hg re-emission.
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H 2 AsO 3− → HAsO 32 − + H + → AsO 33− + 2H + 10
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(R2)
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Hg0 con. (µg/m3)
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2000
1500
1000
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0 0
10
20
30
40
50
60
T (min)
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Figure 3. Effect of solution temperatures on Hg0 re-emission. Experimental conditions: [Hg2+] =
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0.02 mM; [H2AsO3-] = 0.06 mM; solution volume = 300 mL; N2 flow rate = 600 mL/min; initial
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pH value= 9.0.
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3.3 Effect of solution pH
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Figure 4 shows the Hg0 re-emission under different pH values. The results indicate
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that the Hg0 re-emissions increased as the pH values increased from 2 to 12.
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Interestingly, the outlet Hg0 concentrations differed greatly under acidic and alkaline
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conditions. When the pH values were below 6, the Hg0 concentrations were less than
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50 µg/m3, whereas the Hg0 concentrations exceeded 450 µg/m3 under alkaline
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conditions. This outcome illustrates that pH has a great influence on Hg0 re-emission
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in the presence of arsenite. This may be because the pH can greatly affect the
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distribution of arsenite species in solution (see Figure S1). Under acidic conditions,
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the H3AsO3 molecule is the primary species. With the increase of pH (6
