(19) Davis, J. A. Ph.D. Thesis, Stanford University, Stanford, CA, 1977. (20) Davis, J. A.; Leckie, J. 0. J . Colloid Interface Sci. 1980, 74, 32. (21) Anderson, M. A.; Ferguson, J. F.; Gavis, J. J . Colloid Interface Sci. 1976,54, 391.
(22) Pierce, M. L.; Moore, C. B. Enuiron. Sci. Technol. 1980, 14, 214.
Receiued for review May 2,1980. Accepted March 9,1981.
Crystalline Components of Stack-Collected, Size-Fractionated Coal Fly Ash Lee D. Hansen,*t David Silberman, and Gerald L. Fisher* Laboratory for Energy-Related Health Research, University of California, Davis, California 95616
Results of quantitative determinations of quartz, mullite, and magnetic iron oxides (magnetite and y-FezO3) are reported for size-fractionated coal fly ash. The concentrations of these crystalline phases are found to decrease as particle size decreases. Results of chemical analysis of the magnetic phase indicate that it crystallized from molten silicates during ash formation.
Introduction As part of a program to characterize the fly ash which is emitted by coal-fired power plants, qualitative identification and quantitative estimation of the crystalline components of four size-fractionated and one unfractionated fly ash sample are reported. Although fly ash is mostly amorphous to X-rays, the presence of small amounts of quartz, hematite, mullite, gypsum, magnetite, and ferrite have been reported (1-3). However, quantitative determinations of these mineral phases have not been reported, nor have the crystalline phases been studied as a function of particle size. A knowledge of the Crystalline phases is of importance in the consideration of the potential health effects of inhaled particles. Because of the refractory nature of the quartz, mullite, and magnetite phases, these materials will have long residence times in the pulmonary region of the respiratory tract if they are deposited there ( 4 ) .Therefore, it is important to know the particle size distribution and concentrations of these materials in stack-collected coal fly ash. Furthermore, it is generally recognized that crystalline siliceous materials are more toxic than amorphous compounds of the same composition. Such particles are known to have significant effects on lung cells ( 5 )and appear to be important toxicants to the pulmonary macrophage, the primary effector cell for lung immunosurveillance. Magnetite may also be a hazard to health because of its ability t o occlude biologically active transition-metal ions such as Mn and Ni by isomorphous substitution in the spinel crystal lattice (2).Magnetite could thus act as a slow release carrier agent for toxic elements. For this reason we have performed analyses of the magnetic phase for those metals which are likely to be associated with the magnetic fraction of the ash. The crystalline phases are important in determining the physical and chemical properties of the ash. Data on the crystalline phases may be useful in developing methods for resource recovery from, utilization of, and disposal of the ash (2).The mechanisms of formation of the various crystalline + On leave from the Department of Chemistry and the Thermochemical Institute of Brigham Young University, Provo, UT 84602. Present address: Battelle Columbus Laboratories, Toxicology/ Pharmacology Section, 505 King Avenue, Columbus, OH 43201.
0013-936X/81/0915-1057$01.25/0
@ 1981 American Chemical Society
phases are also of interest since a knowledge of these mechanisms may lead to methods for altering the composition of the ash.
Experimental Section Materials. The ash samples used in this study have been described in detail in previous papers (6-11). The size-fractionated samples were obtained from the stack breeching, downstream from the electrostatic precipitator (ESP), of a large coal-fired power plant in the southwestern U S . which was burning low-sulfur, high-ash coal. A sample collected from the ESP hopper was also analyzed. Min-u-sil216,obtained from Whittaker, Clark and Daniels, Inc., South Plainfield, NJ, was used as an a-quartz standard. The X-ray diffraction (XRD) pattern obtained from this material is given in Table I. A mullite standard was prepared by crushing and grinding a high-temperature furnace liner tube, dissolving the amorphous material in 1%HF, and washing with concentrated HN03, concentrated HC1,0.25 M EDTA, concentrated NHdOH, and methanol according to a recently reported procedure (12).The mullite standard was analyzed by atomic absorption spectrometry (AAS) (8),which gave 30 f 3% SiOz, 64 ii 1%A1203, 2.2 f 0.1% CaO, 0.22 ic 0.02% MgO, 330 f 50 ppm NazO, and 630 f 23 ppm KzO. The standard sample thus appears to be 89 f 1%mullite. The XRD pattern of the mullite standard is given in Table I. The acids used for trace-element analyses were HF (Baker Analyzed Reagent, 48%) and HC1 (G. Frederick Smith, 6 M, redistilled). Equipment. The X-ray diffraction patterns were obtained with a Diano XRD-8000 X-ray diffractometer using Ni filtered Cu K a radiation. AAS analyses were done with a Perkin-Elmer Model 306 AA spectrophotometer. Procedures. Specimens for identification by X-ray diffraction were mounted on flat glass slides either (a) by slurrying the solid with HzO, spreading on the slides, and drying at room temperature or (b) by sprinkling an excess of the dry ash onto double-stick Scotch tape on the slide and then gently tapping to remove the excess. Diffraction patterns were collected at 1.6' (28)/min from 2 to 70'(20) with a strip chart recorder speed of 1.6 in./min. Positions of peaks in the diffraction pattern could be located with an accuracy of f0.05'( 20). Estimations of quartz and mullite were performed by comparison of diffraction peak heights at 4.26 and 5.39 A to those obtained from a series of standard quartz-mullite mixtures. These wavelengths were chosen because there is no overlap between these peaks and those of any other component. Fly ash samples and the standards were prepared by spreading 25 mg of dry solid into a 1-cm diameter circle on double-stick Scotch tape attached to a glass slide. The dry solids were pressed into place with a second glass slide and Volume 15, Number 9, September 1981
1057
Table I. X-ray Diffraction Pattern from Coal Fly Ash Samples re1 inlenslty lor magnetlc
obsd d spacing, A
5.35-5.41 4.25-4.28 3.41-3.43 3.36-3.39 3.34-3.36 2.94-2.95 2.88-2.89 2.77 2.68-2.70 2.51-2.55 2.45-2.48 2.40-2.43 2.28-2.29 2.24 2.20-2.21 2.11-2.15 2.08 1.98-1.99 1.82-1.84 1.69-1.70
1.60 1.54 1.52-1.53 1.47-1.48 1.44-1.46 1.38 a
rei lntenslty for ESP ash
concentrate Of cut 1
31 20 36 38
9 9 13
>loo
34 19
11
21 20
4 16 54
5
4 13 4 26 10
7 6 9 11
4 7 9
8 7 14
5
6
11 5 15
7 7
identity a
mullite (5.39, 50) quartz (4.26, 35) mullite (3.43, 95) mullite (3.39, 100) quartz (3.34, 100) magnetite (2.97, 30) mullite (2.88, 30) y-Fep03(2.78, 19) mullite (2.69, 40) mullite (2.54, 50) magnetite (2.53, 100) quartz (2.46, 12) magnetite (2.42, 8) quartz (2.28, 12) quartz (2.24, 6) mullite (2.21, 60) quartz (2.13, 9) magnetite (2.10, 20) quartz (1.98, 6) quartz (1.82, 7) quartz (1.66, 3) quartz (1,67, 7) magnetite (1.72, 10) magnetite (1.62, 30) quartz (1.54, 15) mullite (1.52,35) magnetite (1.49, 40) quartz (1.45, 3) mullite (1.44, 18) quartz (1.38, 7 and 1 1)
a-quartz (min-U-Si1 216)
4.27, 23
3.36,100
*
mullite (synthetic)
5.40, 34 4.04,5 3.77, 3 3.43,57 3.39,100 2.89,15 2.69, 32 2.69, 32
2.46, 7 2.29, 5 2.24,3
2.42,1 1 2.29,13
2.13, 3
2.21,45 2.12,17
1.98, 2 1.82,8 1.67,2 1.66, 1
1.88, 6 1.84, 8 1.71, 6 1.70,13
1.54, 4
1.60,1 1 1.58,6 1.52,28
1.45, 1 1.38, 4
1.46, 6 1.44,1 1 1.42,4 1.40, 4
Numbers in parethenses are spacings and relative intensities from ref 22. Values are spacings followed by relative intensities.
were found to be stable if handled gently. The solid mixtures for standards were prepared by slurrying the quartz and mullite with water (100 mg of solids/mL), suspending the solids by placing the slurry in an ultrasonic bath, withdrawing 0.25 mL of the slurry with an Eppendorf pipet, drying the slurry aliquot on a glass slide, and transferring the dry powder to the double-stick tape on another glass slide. Because of the low intensities of the magnetite lines in the X-ray diffraction patterns of the original ash samples, no attempt was made to determine magnetite from the XRD data. The total amount of magnetic material in the ash was determined by placing a tared, Teflon-coated stirring bar in a water slurry of the ash, stirring on a magnetic stirrer for several hours, retrieving the stirring bar, rinsing with water, drying, and weighing. Because the magnetic crystals are fused to siliceous and other nonmagnetic materials ( I O ) , the above determination does not give an accurate estimate for magnetite. Too little material was available of cuts 2-4 to determine total magnetic material by this method. In order to determine the oxidation state of Fe in the magnetic material, we placed the stirring bar and attached magnetic material into HF solution containing vanadium(V). The Fe(I1) present in the sample was determined according to the colorimetric procedure of Wilson (13).Reduction of another aliquot of the solution with hydroxylamine hydrochloride then gave total magnetic Fe by the same colorimetric 1058
Environmental Science & Technology
method. An estimate of the weight percent of magnetic iron oxides in the ash was obtained from these data by multiplying the weight percent of total magnetic iron in the ash by 1.40 (1.40 is the average of the gravimetric factors for Fez03 (1.43) and Fe304 (1.38)).The ratio of Fe(I1) to Fe(II1) was used to determine the proportions of the ?-Fez03 or magnetite, FeaO4, in the crystalline magnetic material. For the purpose of determining the extent of association of various elements with the magnetic phases, separate samples of magnetic material from ESP hopper ash and cut 1 were collected by a dry process. A magnet was enclosed in a plastic bag, brought into contact with the dry ash, tapped vigorously several times to remove any nonmagnetic and weakly magnetic particles, and then removed from the bag to release the particles. This procedure was repeated at least 3 times on each sample of the magnetic material in order to obtain a sample as free as possible from nonmagnetic and weakly magnetic particles. Optical photomicrographs of the magnetic material collected in this way and of the bulk ash from which it was separated are shown in Figure 1.The magnetic material collected (0.5 g) was then dissolved in a mixture of H F (10 mL of 48%) and HCl(10 mL of 6 M), diluted to 50.0 mL, and analyzed by AAS (8).A standard electrolytic Fe, NBS-SRM 365, was analyzed in parallel for most of the same trace elements as a check on the accuracy of the procedures. The results of the analysis of the Fe standard, given as (certified concen-
Table II. Calibration Data for Quantitative Determination of Quartz and Mullite by X-ray Diffraction ,*IInl.lnl1y
rrt%
quart.
c
Flgure 1. (A) ESP hopper ash !E, Dry-separated magnetic concenbale from ESP hopper ash 801ri pholoqrapn~are a1 500X magnhcalion.
tration f uncertainty; measured concentration f standard deviation of four determinations), are as follows: Fe, 9o (99.90 f 0.02; 97.7 f 2.5 a t 248.3 nm); Si, ppm (80f 5; 81 f 7 at 251.6 nm); Ni, ppm (410 f 10;427 f 9 at 352.6 nm); Cu, ppm (58 f 1;58 f 2 at 324.8 nm); Zn,ppm (
