Environ. Sci. Technol. 2004, 38, 3760-3767
Speciation of Zinc in Municipal Solid Waste Incineration Fly Ash after Heat Treatment: An X-ray Absorption Spectroscopy Study R U D O L F P . W . J . S T R U I S , * ,† CHRISTIAN LUDWIG,† HARALD LUTZ,† AND A N D R EÄ M . S C H E I D E G G E R ‡ PSI, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
Fly ash is commonly deposited in special landfills as it contains toxic concentrations of heavy metals, such as Zn, Pb, Cd, and Cu. This study was inspired by our efforts to detoxify fly ash from municipal solid waste incineration by thermal treatment to produce secondary raw materials suited for reprocessing. The potential of the thermal treatment was studied by monitoring the evaporation rate of zinc from a certified fly ash (BCR176) during heating between 300 and 950 °C under different carrier gas compositions. Samples were quenched at different temperatures for subsequent investigation with X-ray absorption spectroscopy (XAS). The XAS spectra were analyzed using principal component analysis (PCA), target transformation (TT), and linear combination fitting (LCF) to analyze the major Zn compounds in the fly ash as a function of the temperature. The original fly ash comprised about 60% zinc oxides mainly in the form of hydrozincite (Zn5(OH)6(CO3)2) and 40% inerts like willemite (Zn2SiO4) and gahnite (ZnAl2O4) in a weight ratio of about 3:1. At intermediate temperatures (550750 °C) the speciation underlines the competition between indigenous S and Cl with solid zinc oxides to form either volatile ZnCl2 or solid ZnS. ZnS then transformed into volatile species at about 200 °C higher temperatures. The inhibiting influence of S was found absent when oxygen was introduced to the inert carrier gas stream or chloride-donating alkali salt was added to the fly ash.
Introduction In the past two decades many efforts have been undertaken to minimize air pollution from municipal solid waste incineration (MSWI). They have led to considerable developments in flue gas treatment systems, but also accentuated the problem of the disposal of the solid residues (1), as ashes from MSWI plants show high concentrations of toxic heavy metals (1). To prevent groundwater pollution, in Switzerland fly ash has been mixed with cement prior to deposition in landfills (2). In the future, however, production of secondary raw materials from waste will become essential in a sustainable society. Reusable products of interest could then be * Corresponding author phone: +41-56-3104169; fax: +41-563102199; e-mail:
[email protected]. † General Energy Research Department (ENE); Laboratory for Energy and Materials Cycles. ‡ Nuclear Energy and Safety Research Department (NES); Waste Management Laboratory. 3760
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 13, 2004
detoxified mineral additives for building applications and heavy metal concentrates as ore substitutes. The experience of the past decade has shown that a thermal process step is essential for the improvement of the ash quality. Further improvements may be expected by varying the MSWI process conditions (3) or by thermal after-treatment of the fly ash under different carrier gas compositions (inert, reducing, oxidizing) and by adding solid or gaseous additives (4). For this study we heated a certified fly ash (300 f 950 °C) under Ar, respectively, Ar + O2 carrier gas while measuring the evaporation rate of Zn (and other MSWI elements) in the hot carrier gas on-line as a function of time. The interpretation of the Zn evaporation rate as a function of temperature necessitates knowledge of the Zn speciation in the fly ash. Recent progress has been made to understand the influence of the carrier gas composition, the potential of reducing and chlorinating additives (see ref 4 and references therein), and the impact from ash matrix constituents like aluminates and silicates (5). Understanding the element speciation in fly ash from the element composition itself is difficult and necessitates comprehensive analytical approaches (6-8). In our case, conventional X-ray diffraction (XRD) techniques failed due to the low Zn content of the fly ash samples and/or the presence of noncrystalline (or poorly crystalline) Zn phases. In this study we therefore have used XAS to specify the Zn species in the fly ash samples collected at different temperatures.
Experimental Section Characterization of Fly Ash. We studied a fly ash known as BCR176. It had been collected in the electrostatic precipitators of a MSWI plant, sieved (φ e 40 µm), and homogenized extensively. The element composition was certified by the European Community Bureau of Reference Materials in the 1980s (9). The fly ash was used as it was received from Kupper & Co., Bonaduz, Switzerland. Table 1 shows the certified element composition, together with noncertified elements (expressed as the oxides), S and As. The elemental composition of BCR176 is still representative for modern fly ashes (4). A comparison of the thermal behavior of BCR176 with modern fly ashes is given by Lutz (4). Thermal Treatment of Fly Ash. Prior to thermal treatment, each fly ash sample (∼100 mg of BCR176) was placed in an aluminum oxide crucible and first heated to 130 °C by an infrared heater to remove moisture. Fly ash and crucible were then placed in the 300 °C hot quartz tube furnace (see Figure 1) for 10 min to adjust to the temperature and the composition of the carrier gas, either being 0.44 L/min of 99.998% pure Ar or an Ar-O2 gas mixture (98.5:1.5 v/v). In a complete heat treatment experiment, the furnace temperature was increased linearly with time from 300 to 950 °C at a rate of 2 °C/min and was then kept constant at 950 °C for 4 h. Zn (and other elements) vapors from the fly ash in the hot carrier gas stream were detected on-line using an inductively coupled plasma optical emission spectrometer (ICP-OES) from Varian, model Liberty 110. Hereto, the carrier gas was quenched rapidly by generating aerosols at the end of the quartz tube prior to injection into the ICP-OES using an in-house developed condensation interface (10). Under the given experimental conditions the measured ICP-OES intensity is proportional to the rate of evaporation (see first two papers under ref 10). Zn was detected at wavelength λ ) 213.856 nm. Other monitored elements of interest are Na (λ ) 589.592 nm), K (769.896 nm), Cl (837.597 nm), and S (180.731 nm). The heat treatment experiment was carried 10.1021/es0346126 CCC: $27.50
2004 American Chemical Society Published on Web 06/04/2004
TABLE 1. Trace Elements in a City Waste Incineration Ash, BCR176 (9) not certified elements (g/kg dry ash)a
certified elements (mg/kg dry ash) Cd ) 470 ( 9 Hg ) 31.4 ( 1.1 Sb ) 412 ( 18 Zn ) 25770 ( 380 a
Co ) 30.9 ( 1.3 Ni ) 123.5 ( 4.2 Se ) 41.2 ( 2.1 Fe ) 21300 ( 1100
Cu ) 1302 ( 26 Pb ) 10870 ( 170 Tl ) 2.85 ( 0.19 Cr ) 863 ( 30
SiO2 ) 300.3 Al2O3 ) 191.9 Na2O ) 58.0 others:
TiO2 ) 14.2 MgO ) 36.2 K2O ) 54.2 S ) 44.6
P2O5 ) 12.7 CaO ) 123.1 MnO ) 1.8 As ) 0.0933
Expressed as the oxides.
FIGURE 1. Setup for a heat treatment experiment involving the evaporation of heavy metals (HM) from fly ash and their on-line detection by ICP-OES. (CI ) condensation interface.) on until the intensity of the evaporating zinc subsided below 1% of the maximum value recorded. Metal Content Analysis. The fly ashes resulting after complete heat treatment (from repeated experiments, with or without O2 added to Ar) were digested in nitric and sulfuric acid according to the method described by Bajo et al. (11). The Zn contents (and occasionally also those of Na, K, and S) of the digestion solutions were determined with ICP-OES. Collection and Preparation of Fly Ash Samples for XAS. We collected several fly ash samples from individual heat treatment experiments at different temperatures between 300 and 950 °C. This was done by switching off the heating and moving the crucible to the cool part of the quartz tube, while maintaining the carrier gas flow. After reaching room temperature, the samples were transported under the carrier gas atmosphere to a glovebox. The glovebox was purged continuously with water-free, pure N2 (CO2 and O2
