Environ. Sci. Technol. 1902, 16, 423-427
Studies of Surface Layers on Single Particles of In-Stack Coal Fly Ash Jeffrey L. Hock" and David Llchtman
Department of Physics and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201 Surface and bulk analytical techniques have been used to study individual fly-ash particles collected in a defined manner and particle size range from within a power plant stack. A sampling system using collection on a dry, softmetal substrate (e.g., indium), which meets the needs of both high-temperature sampling and analysis under clean vacuum conditions, has been employed. Scanning electron microscopy (SEM) and energy-dispersive X-ray fluorescence (EDX) measurements provide morphological and quasi-bulk composition of individual particles, after which the samples are then transferred to another system for single-particle Auger electron spectroscopy (AES) and ion beam depth profiling. The results obtained show clearly a wide chemical and physical disparity between different particles even in this limited sample range. However, there is no evidence of either an organic layer or a surface layer containing high (>1%) concentrations of high atomic number elements in the sample studied. In addition we find 90-95% of the particles are spherical and 10% are electrically conducting. SEM/EDX shows Al, Si, Fe, Ca, S, Cu, C1, Mg, Cr, I, Ni, Zn, Na, Co, P, Mn, Sn, W, Ta, As, Pd, and V in decreasing abundance, whereas AES shows Si, Al, 0, Fe, Ca, K, Na, and Mg.
Introduction When coal is burned at high temperatures, the inorganic material it contains becomes molten. The majority of this material is removed either as slag from the bottom of the furnace or by cyclone filters, electrostatic precipitators, or bag filters. Nevertheless, small quantities of pm-size particles escape in the flue gas and are exhausted from the stack. Previously,a great deal of work has been performed on fly-ash particles which has employed a wide range of analytical techniques; see, for example, ref 1-10. Most of these studies, other than electron microscopy measurements, have used collections of particles, thus obtaining bulk properties averaged over the many particles in the sample. Many questions such as surface chemical segregation, bulk chemical distributions, and surface morphological implications are still unanswered. Thus it seems there may be additional value in obtaining comparative data on individual particles by using surface-sensitiveand bulk-sensitive techniques. The surface-sensitive techniques, in conjunction with ion bombardment surface erosion, have potential to provide elemental and chemical depth-profile information. Toward this end we have employed a number of modern surface-analysis techniques in an attempt to obtain detailed information about the physical and chemical characteristics of individual air particulates in the size range 5-30 pm. The fly-ash particles studied have been collected under defined conditions inside the stack of a power plant, at a temperature of 151 "C, in a manner designed to minimize the number of experimental artifacts. Experimental Section The procedures used to study individual fly-ash particles start with the collection of individual particles under controlled and well-documented conditions. The services of Meteorology Research, Inc. (MRI), were engaged to 0013-936X/82/0916-0423$01.25/0
perform the collection activities under the Plume Validation Study contracted by the Electric Power Research Institute (EPRI). Particles were collected within one of the smokestacks of the Kincaid Generating Station of Commonwealth Edison near Springfield, IL. The collections were performed on dry indium foil, which has the advantage over other substrates of being a very soft metal that is compatible with the ultra-high-vacuum studies discussed below. Impact collectors were employed that used dry indium foils as the collecting stages. These were operated for 2.2 min at an elevation of 405 f t (123.4 m) in the stack. The flue-gas temperature at this elevation was 151 "C, whereas the melting temperature of indium is 156 OC. The impactors were run for 2 min. After collection, the indium foils (which were annular rings of 2.7 cm i.d. and 4.5 cm 0.d.) were sectioned, and orientation markings were pressed into the soft metal by using a scalpel (see Figure 1). Following this preparation, the substrates were placed in a JEOL Model JSM U3 scanning electron microscope (SEM), during which surface morphology was determined and energy-dispersive X-ray analysis (EDX) data were obtained from individual isolated particles by using a Nuclear Diodes 605-X detector and an EDAX Model 504 energy analyzer. Following the initial SEM studies, the samples were then transferred to a Varian 10-keV scanning Auger electron spectroscopy system (AES). The scanning mode of the AES is used for locating the particles of interest. This is done in conjunction with the orientation markings on the indium substrates along with low-magnification photographs produced during the SEM work. Typical AES is performed by using beam currents from 0.2 to 1pA at a beam energy of 8 keV. The spot diameter attainable under these conditions is approximately 15 pm at full width at half maximum. AES, which has a predicted sampling depth of 0.5-3 nm, is used to obtain data that upon sensitivity factor corrections, provide the elemental composition of the exposed surface of the particles, which had previously been characterized in the SEM. In addition to the AES system, the vacuum chamber is equipped with a rasterable sputter ion gun (Varian Model 981-2043). This is used in conjunction with the AES system to obtain depth-profile data on individual particles (see Figure 2 for the relative beam orientations and beam size). It is to be noted that the electron beam overlaps the edge of the particle and interacts with some of the background substrate material. This results in some component of indium in the Auger spectra obtained from single particles. The method used to center the particle under the electron beam is to move the particle by using a precision manipulator in such a manner as to minimize the residual indium signal. Another aspect of this arrangement is the size of the sputtering beam. The sputter ion beam has a fwhm of approximately l/z cm, much larger than the particle. This resulb in a significant fraction of the substrate being sputtered and a possibility for substrate material to be deposited onto the particle being studied. This problem has been observed by others (IO,11) and is discussed in detail elsewhere (12).So that accurate depthprofile information can be obtained, the sputtering rate
@ 1982 American Chemical Soclety
Environ. Sci. Technol., Vol. 16, No. 7, 1982 423
Table I. SEM Particle Data 5% particles
with this
element Fe Si
AI Ca c1
S cu Cr I
Mg Zn
Ta
P Na Ni
co W
Mn Sn As Pd
V
av atomic % population element at std dev concn > 15% comp 84 35 46 90 20 27 97 18 22 71 10 6.4 41 13 5.2 55 4.5 8.7 43 4.5 10 20 1.1 4.7 14 1.1 4.3 26 0.68 1.6 0.55 2.1 8.7 0.49 4.1 1.5 0.43 2.8 5.8 0.39 1.7 8.7 0.36 12 1.1 0.25 1.1 7.3 0.16 0.98 2.9 0.13 0.65 4.4 0.10 2.9 0.61 1.5 0.10 0.85 0.06 0.48 1.5 0.03 0.24 1.5
Results Optics. In addition to the SEM and AES work to be discussed below, preliminary work has been performed by using visible light microscopy. Similar work has been performed by others (14),and our investigations have also Flpure 1. Section of indlurn foil showing orientation markings and several indivaual isolated particles of the type discussed in this w a k . revealed a range of colors for the particles from clear to black, the predominant colors seen being yellow, red, and orange. These colors are undoubtedly related to chemical composition and trace impurities in the glassy material. However, to date no correlations between color and chemical composition have been made. In addition to the variety of colors present, it has been observed that a large fraction, -95%, of the particles are spherically shaped and lie on the top of the indium foil, imbedded less than 20% of their diameter. Scanning Electron Microscopy. The studies performed in the SEM verified the usefulness of the collecting \ \ techniques employed. In the size range of interest for these studies ( 5 1 5 fim) many single isolated particles have been found on the sections of foil examined, along with clusters of smaller single particles (see Figure 1). The general nclun2. s c h e m a t i c & a ~ ~ l h e ~ U v e a n ( 1 8 . o f i n d d o l l c . nature of the particles that have been collected is that they fw the electron beam and the Ion beam. A of the UIIPUSOI are 9(t95% spherical with surface morphology that varies with an indlun sutatrateand patilde mourted to I is shorn fdoq with from g h y smooth to rough surfaces (thisis as viewed with a s ~ c t b nof the cylindrical rnlrror analyzer (note: the particle size is the secondary electron image and does not necessarily infer greatly exaggerated fw chrlty). optical morphology). of the ion gun must be known for the particle being anaEDX analysis shows a large variety of elements, which lyzed. This information is not available for each particle, are listed in Table I in the order of decreasing average so standards of known thickness that match the matrix abundance (EDX is not routinely sensitive to elements of composition of the particles must be used. Fly ash derived atomic number less than 11). As can be seen from examfrom the combustion of coal in a conventional power plant ining the population standard deviation shown in Table is composed primarily of an impure aluminosilicate glass I, the percent abundance of a particular element varies together with small amounts of several crystalline minerals widely from particle to particle. Four of the particles in (13). Thus, a standard of a SiO, layer of known thickness the group of 69 studied had iron concentration (by EDX) on pure Si was used. (The SiOz/Si samples were prepared of over 90%, whereas ten particles contained no measurby Bell Labs under part of Round Robin depth-profile able iron. It is to be noted that, due to the limitation in study.) From operation of the sputter ion gun in 5 X 10” detecting low atomic number elements by EDX, it is torr (1.3 X l0-S Pa) of argon and at an ion energy of 3 keV, psaible that the particles that indicated large iron content a sputtering rate of 2.8 i 0.2 nm/min is obtained for this may actually have been largely composed of lighter atomic standard. This sputtering rate is used as a calibration for number elements (atomic number
