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Metals in ash materials filtered from municipal incinerator effluents
Resource Recovery and Conservation, 3 (1978) 19-39 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
19
METALS IN ASH MATERIALS FILTERED FROM MUNICIPAL INCINERATOR EFFLUENTS
STEPHEN L. LAW* U.S. Bureau of Mines, College Park, MD 20740
(U.S.A.)
(Received 24th November 1976)
ABSTRACT Fly-ash and bottom-ash materials were filtered from the aqueous effluents of municipal incinerators and from an incinerator-residue processing plant and were examined for ore grade quantities of metals and for trace metals pollution potential. Several elements, for example Ag, Al, Cu,and Ti, are present in concentrations comparable with low-grade ores presently being mined. Elements such as Ag, As, Cd, Cr, Cu, Hg, Pb, Sb, Sn, and Zn are one to three orders of magnitude more enriched in incinerator ash than they are in average crustal abundances. The fly-ash and bottom-ash materials filtered from incinerator effluents were found to be very similar in relative metal concentrations. Incinerator fly ash is one order of magnitude more enriched in Ag, Cd, Ni, Pb, Sb, and Zn than coal fly ash. No significant elemental concentration differences were observed for samples taken during different time periods and from different geographic locations.
INTRODUCTION
Approximately 65-75 percent of the solid waste disposed of in the United States is amenable to use as a fuel supplement with a resulting 75-95 percent volume reduction in the total refuse [ 11. Municipal incinerators may be converted to, or originally designed as, plants capable of extracting valuable energy during solid waste combustion [ 21. The bottom ash and fly-ash products of solid waste combustion are generally disposed of in landfills. Recovery of metallic values from the residues after conventional incineration, as demonstrated by a U.S. Bureau of Mines pilot plant discussed later, will further reduce the volume sent to a landfill by another 50 percent [l]. The elemental content of the remaining disposed ash material is of considerable interest. If the ash is used for landfill, the possibility exists that leaching facilitated by microbial degradation of unburned carbon in the ash -- may become a problem. The local vegetation may concentrate toxic elements from the water of the landfill and the wild or domestic animals eating the vegetation *This study is taken in part from a dissertation submitted to the Graduate School of the University of Maryland for the Ph.D. degree in chemistry.
20
and drinking the water will be adversely affected. Alternate uses of the ash, for example, as a soil conditioner or a building aggregate, may depend on the concentrations of toxic elements in the material. The corrosion properties of the ash are an important consideration in incinerator-residue processing operations. The possible health effects from inhaled airborne particulates, either windblown from a landfill or escaping from the incinerator stack, are an important consideration requiring a knowledge of the elemental concentrations of the ash. Finally, disposed ash should be considered as a possible source of metal and mineral values. As a part of a program to characterize the aqueous effluents from municipal incinerators [ 3, 41, the ash materials filtered from these effluents were analyzed for elemental concentrations. The primary emphasis of this report is on these filtered solids, which can be expected to represent the finer and less dense materials in the incinerator bottom-ash and fly-ash residues. However, the elemental compositions of the filtered ash materials are compared with the composition of the bulk fly-ash and bottom-ash residues (excluding massive pieces of metal, glass, etc.) and the results indicate that the filtered solids adequately represent the elemental compositions of the total ash materials. The filter-cake solids from the Bureau of Mines incinerator residue processing pilot plant are also similar in elemental composition. Alexandria incinerator The principal emphasis of this study was on the residue from the Alexandria, Virginia, incinerator which recycles approximately 114,000 l(30,OOO gal.) of fly-ash scrubber water and approximately 19,000 1(5,000 gal.) of quench water in completely separate systems. Early Saturday morning, at the end of each operating week, the waters in each system are dumped into the municipal sewer system. The hot residues from the primary combustion chamber drop into a water quench tank. Following quenching, they are carried by drag conveyor to a chute and dropped into disposal trucks. The quench water is screened before going into a holding tank from which it is recycled at a rate of about 1,000 1 (260 gal.) per minute. The ash materials used in the research reported here were filtered from water samples taken after the coarse screen and just prior to entering the holding tank. Upon leaving the primary combustion chamber, the hot gases pass through a flue into a secondary combustion chamber and then into the fly-ash spraybaffle scrubber. Pressurized nozzles spray water down the baffle walls. The combustion gases pass through two 90” turns in the baffle, causing the fly-ash particulates to impinge upon the walls and become entrained in the flowing water. This water is then pumped to a holding tank where the heavy solids are allowed to settle and the water is then recycled at a rate of about 2,000 1 (530 gal.) per minute. The settled fly ash is removed from the holding tank by drag conveyor to trucks for landfill disposal. The fly-ash material used in
21
the research reported here was filtered from samples of this settling tank water. The fly-ash scrubber water becomes highly acidic during the incinerator process because of the various acidic gases being formed [ 51. Soda ash is added continuously and automatically throughout the operating week at an average rate of 135 kg (300 lb) per day to neutralize this acidity to a pH of 4-7. The pH is checked every hour and the rate of soda ash addition is adjusted as needed. Bureau of Mines incinera tar-residue processing plant The U.S. Bureau of Mines designed and built a pilot plant capable of processing 450 kg (0.5 ton) of incinerator residues per hour [6]. This pilot plant relies exclusively on existing mineral engineering technology and is a combination of various mineral beneficiation processes. Continuous screening, shredding, grinding, magnetic-separation, flotation, jigging, and gravity-concentration techniques are applied to produce metallic iron concentrates, highquality aluminum nuggets, mixed colored glass, slag and fine ash tailings. Water is used throughout the plant as the processing medium. Samples for analysis were taken of the filter cake material filtered from this water just prior to discharge into the sewer system. ANALYTICAL
TECHNIQUES
After being dried and powdered, the solids were put into solution using conventional acid digestion with nitric, hydrofluoric, and perchloric acids. Once in solution, most of the elements were determined by conventional flame atomic absorption. Mercury was determined by the cold-vapor atomicabsorption technique. Arsenic and antimony were determined by flameless, graphite furnace, atomic absorption. Conventional semiquantitative emissionspectrographic analyses of the powdered solids were also performed. These analytical methods as applied to incinerator residues have been described previously [ 3, 71. Solids in effluents Fig.1 shows the weights of solids filtered from samples of the Alexandria recycled waters collected during a l-week period of operation. As shown, the amount of solids in the recycled waters fluctuates greatly, probably depending upon the particular operating conditions at the time of sampling. At the time of discharge, the bulk of the undissolved solids have been settled out of the Alexandria incinerator effluents, usually leaving less than 0.5 g/l in the aqueous effluents. Effluents from the Washington, D.C., Solid Waste Reduction Center No. 1 incinerator and the Montgomery County, Maryland incinerator (now inoperative), both of which were also studied in this project, are discharged con-
1
B+ A A B+
D D D G E F+
SRM 1633
Residue processing Alexandria Chicago C+ B
B+ B+
Montgomery Co. Fly ash Bottom ash Combinedb
A B
Al
D D+
Ag
Washington, D.C. Fly ash Combinedb
-
0.3 -3 0.1 -1 0.03-0.3
D E
C+ C D+
Alexandria Fly ash Bottom ash
Samplea
A B+ B
Over 10 Over 5 l-10
E F+
D
E+ E+ E+
E+ E+
E+ F+
B
D E+ E
Letters indicate estimates of concentration
C C
C
C C C
C C
C+ C
Ba
-
G+
-
-
G -
Be
F+ F+
F
E E E
E E+
E+ -
Bi
0.01 -0.1 0.003.--0.03 0.001-0.01
B+ B+
B
B+ A B+
B B
B A
Ca
ranges in percent: F+ F G+
-
-
E+ F+ E+
E+ D
-
Cd
E -
E
E E
E -
E+ -
Co
D D
E+
C C C
D C
D+ D
Cr
0.0003 --0.003 0.0001 -0.001 0.00003-0.0003
D D
E+
D D+ D
D+ D
D E+
Cu
B B
B
B B+ B
B B
C+ C+
Fe
G H -
-
E+ -
Ga
-
F+
-
-
-
--
Ge
C C
C
C+ C+ C+
C C
C+ C
K
E+ E
D
D+ E+ D
D D
D+ E+
Li
0.00001-0.0001 Less than 0.00001 Not detected
Semiquantitative analysis of undissolved solids in incinerator and incinerator-residue processing effluents
TABLE
C C c+ C+ B B B c+ c C+
Alexandria Fly ash Bottom ash
Washington, D.C. Fly ash Combinedb
Montgomery Co. Fly ash Bottom ash Combinedb
SRM 1633
Residue processing Alexandria Chicago D+ D
D+
C C C
D+ C
C D+
Mn
-
E+
E+ F+ E+
E+ E+
E+ F+
MO
C+ C+
C
C+ B C+
C+ C+
B E+
Na
E+ Ei
E+
D D D
E+ D
D E
Ni
D -
-
C C C
C C
-
D+ C
E+
C C C
C C+
C+ D+
-
Ei-
-
-
E+ -
---
-
E+ D D
D D
‘E+ --
A B+
A
A A A
A A
A A
D D
F+
C D+ D+
D+ C
D+ D
D D
C
D+ C D+
D D+
D+ D
-_l__.l P Pb Rb Sb Si Sn Sr .._-_____-..-. _-.-..__. ._--.-_..._ _,.. .__~___
C C
C
B B B
B B
C+ C!
Ti
E+ F+
D
E+ E+ D
D+ D
D E+
V
D+ C
E+
C+ C+ C+
B A
C C
Zn
aAh samples (except the SRM1633 reference coal fly ash) are ash material filtered from the separate aqueous systems of each plant. b”Combined” refers to the solids filtered from the combined fly-ash and bottom-ash water systems prior to discharge to the sewer system.
Mg
Samplea
-
E -
E
E D E+
E+ E
E+ F+
Zr
-
24
0
Fly
A
Quench
ash
scrubber
waters
waters
0.4
Fig.1. Variation of undissolved solids content in Alexandria incinerator recycled waters during one week of operation.
tinuously into the local sewer systems. These waters likewise seldom contained more than 0.5 g/l of undissolved solids at the point of discharge. Semiquantitative values A semiquantitative elemental analysis was made of samples of the filtered solids from the different aqueous effluents of each incinerator as shown in Table 1. As a reference, similar data are included for the National Bureau of Standards SRM 1633 coal fly-ash standard. Also included are data found for elements in solids from the Bureau of Mines incinerator-residue processing pilot plant obtained during the processing of Alexandria incinerator residues and during the processing of residues from the Northwest Chicago incinerator. This emission spectrographic survey of elements in the solids provides information on elements not determined by atomic absorption. It also served as a guide in preparing dissolved samples and dilutions for the quantitative atomic absorption determinations. Elemental concentrations Table 2 lists the average for elements found in the fly ash and in the quench water solids. An independent instrumental neutron-activation analysis (INAA) of the Alexandria fly ash using samples taken during a different time period, was conducted by Law and Greenberg [7, 81 and the results are included in Table 2. The INAA samples are of the fly ash after settling in the
25
TABLE
2
Elemental composition Element
Ag Al As Ba Be Ca Cd co Cr Cu Fe Rg K Li Mg Mn Na Ni Pb Sb Sn Sr Ti Zn
of solids from Alexandria incinerator
Concentration (mg/kg) AA@ C Scrubber fly ash
INAAb Scrubber fly ash
AA“ Quench water solids
129 + 28 120,000 ?r 12,000 77 f 10 1,500 + 400 <4 23,000 f 10,000 64 f 16 100 f 30 1,160 +_720 510 + 180 24,000 + 8,000 0.88 + 0.72 12,200 +_1,800 342 4 9,700 f 1,700 1,500 + 600 16,000 f 2,000 1,800 + 2,800 7,200 + 3,200 340 f 290 1,250 f 650 210 f 80 < 50 10,100 f 2,000
106 f 111,000 ? 41+ 2,500 i -
18 12,000 15 300
40,000 45 30 1,430 1,000 46,000
6,000 28 4 200 500 7,000
38 49,000 26 1,400 <4 40,000 41 70 520 450 16,000 0.42 6,300 19 12,800 3,100 8,200 210 1,700 120 400 210 < 50 5,500
Wncertainties are standard sample-to-sample variations bResults of an independent analysis and using different CBy atomic absorption.
-
-
12,000 3,400 14,000 740 4,000 260
9,400
i + r i i f
c f f f + i
5,000 700 2,000 1ooc 1,300c 60
i: 1,300
?8 * 8,000 *5 f 600 5 t f f f f
18,000 15 10 240 190 6,000
f 1,400 k3 2 2,600 f 1,700 f 1,800 i- 250 f 800 * 90 * 50 f 1,500
deviations. For most elements the uncertainties result from and not analytical error. study by Greenberg [ 81 using instrumental neutron activation samples from those used for atomic absorption.
settling tank and being removed by drag conveyor to a waiting truck, whereas the samples for analysis by atomic absorption (AA) consisted of solids suspended in the water samples of the settling tank. In spite of these sample differences, the largest disagreement for elemental concentrations is only a factor of three for cobalt and a factor of two or less for the other elements. The values in Table 2 are averages of 11 samples taken on nine days over a period of five months. The uncertainties shown in Table 2 represent the standard deviations and are mainly the result of sample-to-sample variations and not necessarily analytical error. For example, nickel in the quench-water solids shows an average of 210 mg/kg with a standard deviation of + 250 mg/
26
kg. The large standard deviation arises from one sample which had a nickel concentration of 920 mg/kg. If this value is removed, the Ni result becomes 130 + 40 mg/kg. The analytical error for each element of this study was determined by performing analyses of several samples of SRM 1633 coal fly-ash [ 3, 71. For nickel, the result was 100 F 4 mg/kg indicating that the analytical standard deviation is probably at least one order of magnitude less than the sample variation. The elements examined in these solids from day to day, from month to month, and even from year to year appear to remain within a consistent range of concentration values. Table 3 compares the analysis of a sample taken in October 1973 with a sample taken in July 1974. The nickel values are an example of the extremes encountered in all samples for nickel, but the other elements show good agreement. Many more samples taken over several years are needed to establish any trends in element concentrations. TABLE
3
Concentrations of elements in Alexandria incinerator fly ash sample in 1973 and 1974 Element
Ag Al Ba Be Ca Cd co Cr CU Fe K Li Mg Mn Na Ni Pb Sn Sr Zn
Concentration (mg/kg) October 197 3
July 1974
95 126,000 1,700 <4 22,000 56 100 1,140 600 24,000 13,000 31 9,500 1,500 16,700 1,580 10,000 850 245 8,600
130 124,000 1,400 <4 22,000 57 90 730 445 22,000 14,000 34 9,800 1,100 16,500 160 7,200 860 260 8,000
Ash residue comparisons The School of Mines, West Virginia University, performed a series of physical and chemical characterizations of municipal-incinerator fly ash and residues being processed by the Bureau of Mines incinerator-residue processing
27
pilot plant between 1969 and 1972. A comparison of their published results [9, lo] and the Alexandria fly ash and quench-water solids is made in Table 4. The fly ash and bottom ash of the West Virginia University study came from three Washington, D.C., incinerators (not including Solid Waste Reduction Center No. 1); an Arlington, Va., incinerator; the Montgomery County, Md., incinerator; and the Alexandria, Va., incinerator. Their publications did not identify the incinerators, so the,averaged results are specified as “‘Washington, D. C., area” incinerators in Table 4. The earlier report [ 91 used emission spectrography for most of the determinations and the second report [lo] used atomic absorption. The results show no significant differences in the three averaged fly-ash values nor in the comparison of Alexandria quenchwater solids and the average of two samples of fine bottom ash. ‘The uncertainties are standard deviations resulting primarily from sample-to-sample variation. A semiquantitative emission spectrographic analysis of the filtered solids from Bureau of Mines pilot plant processing of Alexandra, Va., incinerator residues and of Chicago Northwest incinerator residues was shown previously in Table 1. A quantitative analysis using atomic absorption and gravimetric techniques was made of filter cake solids collected for this study during the processing of residues sent to the Bureau of Mines from four incinerators in the Chicago, Ill., area: Chicago Southwest; Chicago Northwest; Medill, Ill.; and Calumet, Ill., incinerators. These solids were filtered from the effluents of the Bureau of Mines pilot plant prior to discharge into the sewer system. Averages of the results from the four incinerators are shown in Table 5. Also shown in Table 5 are the averaged analyses of minus &mesh (< 2.4 mm) residues before processing in the pilot plant. These residues came from five Washington, D.C., area incinerators and one Atlanta, Ga., incinerator. Large pieces, including tin cans, massive iron, glass, wire, paper, ceramics, etc., had been handpicked from the residues. All remaining coarse material greater than S-mesh was screened from the dry samples. The minus &mesh materials were passed through a ball mill and screened again through a 100-mesh screen (0.15 mm); only the minus lOO-mesh materials were analyzed to obtain the averages shown in the table. For comparison purposes, the final column of Table 5 again shows the results of the analyses of the solids filtered from the Alexandria, Va., incinerator quench waters. These solids would be expected to represent the fine and less dense materials in the Alexandria incinerator bottom ash residues. The similarity between the three residue materials is noteworthy, especially in view of the fact that they came from different incinerators from geographically separate areas (Chicago and Washington, D.C.) and they were collected during a time span of at least 8 years. Only copper shows a variation, with the residues before processing showing an order of magnitude more copper than the filter cake and quench-water solids. The reason for this difference is not obvious at this time.
4
0.01 13.3 <0.4 4.6 0.05 0.07 2.3 co.03 1.34 1.02 0.06 1.98 <0.03 0.3 0.04 0.5
+ i + f + + f f
1.69 0.98 0.15 1.9 0.2 0.53 0.19 0.87 0.25 0.11 0.10 0.4 0.1 0.47 0.10 0.38
1.7 0.04 1.4 0.019 0.031 1.0
f + + * f *
0.019 11.5 0.14 5.9 0.067 0.083 2.7 -
+ 0.019
Ref.9
Ref.10
Washington, D.C. area fly ash
aAsh materials filtered from the aqueous system. bAverage of two samples only.
f 0.03 + 1.2 f 0.004 t 1.0 f 0.07 + 0.008 f 0.8 + 0.00007 f 0.18 i- 0.17 i 0.06 f 0.2 ?r 0.28 i 0.32 f 0.07 f 0.20
0.013 12.1 0.15 2.3 0.12 0.051 2.4 0.00009 1.22 0.97 0.15 1.6 0.18 0.72 0.13 1.01
Ag
Al Bk Ca Cr CU Fe Hg K Mg Mn Na Ni Pb Sn ZlI
Alexandria scrubber fly asha
‘Element
Concentrations expressed as weight-percent.
0.5 0.03 0.02 0.6
o.o&
0.55
f 0.2
0.0038 4.9 0.14 4.0 0.052 0.045 1.6 0.00004~ 0.63 1.28 0.31 0.82 0.021 0.17
t 0.1 f 0.01
f 0.20 c 0.03 ?: 0.02 + 0.54
f + + +
f 2.6 0.06 1.8 0.024 0.019 0.6
i 0.15
f 0.14 f 0.26 ? 0.17 t 0.14 f 0.025 -t 0.08
f c r f t
f 0.8
i 0.0008
Alexandria bottom asha
Comparison of Alexandria incinerator ashes with averaged Washington, D.C., area ashes
TABLE
3.9 0.2 6.8 0.04 0.23 8.4 < 0.03 0.81 0.81 > 0.1 2.6 < 0.03 0.6 0.08 0.25
< 0.01
Washington, D.C. area bottom ash Ref. 9
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TABLE
5
Solids in incinerator-residue processing Element
Concentration (weight percent) Minus 8-mesh residues before processinga
Al Ba C
7.6 0.3 18.1
Ca Cd co Cr cu Fe K Mg Mn Na Ni P Pb S Si Sn Sr Ti Zn
2.2
f 2.3 f 7.4
0.04 0.35 4.0 1.0 1.0 0.20 2.3 0.01 0.22 0.50 0.30 14.8 0.045 1.2 0.48
* 0.15 rt 1.4
+ 0.06
f 0.17 * 1.7 f 0.022
f 0.23
Filter cake after processingb 4.1 0.88 7.7 5.3 0.0029 <0.005 0.012 0.058 6.1 1.03 1.65 0.21 2.1 0.017 0.25 0.35 0.59 17.6 0.067 0.0082 1.06 0.45
Suspended quenchwater solidsc
+ 0.2 * 0.04 + 2.7
4.9 0.14 -
f 0.8 f 0.06
f 1.1 + 0.0015
4.0 0.0041 0.0070 0.052 0.045 1.60 0.63 1.28 0.31 0.82 0.021 0.17 0.040 0.021 0.55
f + + f f f f f + + f
f f + + r f i. i: i + f f f + t f
0.002 0.017 2.1 0.34 0.61 0.09 0.8 0.010 0.006 0.11 0.21 0.025 0.025 0.0003 0.09 0.025
1.8 0.0015 0.0010 0.024 0.019 0.57 0.14 0.26 0.17 0.14 0.025
* 0.08
+ 0.005 + 0.15
‘Averaged results of the analyses of residues from five metropolitan Washington, D.C., incinerators and one Atlanta, Ga., incinerator [ 11. bFiiter cakes from processing of residues from four Chicago area incinerators: Chicago Southwest, Chicago Northwest, Medill, Ill., and Calumet, Ill. cSolids filtered from Alexandria, Va., incinerator quench waters.
METAL VALUES
A U.S. Bureau of Mines report on the composition and characteristics of municipal incinerator residues concluded that “From the standpoint of reclaiming mineral values, the fine ash fraction is the most promising.” [ 11. The School of Mines, West Virginia University, study of municipal incinerator residues found that “The elements aluminum, silver, copper, chromium, gallium, lead, tin, tantalum, and zinc are present in several of the refuse materials in amounts which appear to be near-competitive with ore concentrations currently considered mineable.” [ 91. Table 6 compares some of the metal concentrations in incinerator ash materials with the concentrations in surfacemineable materials. Although some metal values are comparable with the
30
TABLE
6
Comparison of metal values in incinerator ash and in surface mined ore@ Element
Material grade (%) (surface mines)b _
Ag Ai CU Fe Hg Ni Pb Ti zn
0.007 17 0.15 22. 0.009 1.4c 5.5 2.0 4.3
Incinerator ash concn. (%) Bottom ash
Fly ash
0.004 9. 0.05 2.3 0.00004 0.03 0.2 1.2 0.6
0.01 23 0.05 3.4 0.00009 0.2 0.7 -
__--
1.9
-__
__-
eThe chemical-mineral form of the metal in the ash materials may be different from the form occurring in a surface mine. For example, Al is given as 17% bauxite in the surface mine, whereas the bottom ash and fly ash percentages are for Al,O,. bRef.11. CRef.12.
average or low mineable materials, conventional mineral processing for the metal content of ash may not be practical. Most mineral-separation techniques rely on the surface properties of microcrystalline solids, and this type of order is not common in ash materials. For example, aluminum is probably not present as bauxite but rather as a refractory oxide or silicate requiring an expensive chemical treatment for extraction. Another problem is the rather insignificant volume of ash material at any single location compared with the volume of ore present at a surface mine. The Alexandria incinerator produces less than 300 metric tons per week of combined fly ash and bottom ash. However, the study of metal-recovery systems should be expanded because of the possible noneconomic incentive of environmental contamination from landfills, and the trend to use municipal solid waste as a fuel supplement. ENRICHED
ELEMENTS
Comparison with crustal abundances To obtain a better understanding of the possible environmental significance of the elemental concentrations found in the solids removed from the various effluents, a comparison of concentrations with crustal abundances is given in Table 7. Mean crustal concentrations can only be used as a crude approximation of the relative composition of the environment into which the ash materials may be introduced to serve as a landfill, soil conditioner, or in some other function. The differing types of crustal materials and soils in various
31 TABLE
7
Comparison of elements from the effluents of the Alexandria incinerator and the Bureau of Mines pilot plant with crustal abundances Element
Ag Al As Ba Ca Cd co cr cu : K Li Mg Mn Na Ni Pb S Sb Si Sn Sr Ti Zn
CrustaI abundance, (mg/kg) [I31 0.06 78,300 1.7 590 28,700 0.1 12 70 30 35,400 0.03 28,200 30 13,900 690 24,500 44 15 310 0.2 305,400 3 290 4,700 60
Ratio of absolute concentration to crustal abundance Fly ashe
Bottom ashc
Filter-cake solids 5
2,200 1.5 45 2.5 0.80 640 8.3 17 17 0.68 29 0.43 1.1 0.70 2.2 0.66 55 480 -
630 0.63 15 2.4 1.4 410 5.8 7.4 15 0.45 14 0.22 0.63 0.92 4.5 0.33 5.7 110 -
-
1,700 -
600 -
-
420 0.72 170
130 0.72 92
-
0.52
15 1.8 290 -
-
1.7 19 1.7 0.37
1.2 3.0 0.86 3.9 230 19 0.58 220 0.28 2.3 75
aFiltered from Alexandria incinerator processing waters. 5Filtered from effluents of the Bureau of Mines pilot plant during the processing of incinerator residues from the Chicago, Ill., area.
areas will require more specific comparisons to determine the suitability of the incinerated residues for a particular application. Table ‘7, however, does indicate that silver, cadmium, lead, antimony, tin, and zinc are up to two or three orders of magnitude higher in concentration than might be expected in average crustal materials. Concentrations up to one order of magnitude above crustal abundances are also observed for arsenic, barium, cobalt, chromium, copper, mercury, manganese, nickel, sulfur, and titanium. The more soluble alkali and alkaline-earth metals are often lower in concentration than the mean crustal abundances. A higher mercury ratio was expected because of reported observations of mercury plumes from incinerators [ 14, 151. The mercury is presumed to re-
32
main in the vapor phase rather than attached to the solids. In studies of a furnace fired with pulverized coal, 90 percent of the mercury was apparently discharged in the vapor phase and only 10 percent remained with the furnace residual ash [ 161. If arsenic, lead, antimony, and other elements or their chlorides are also sufficiently volatile at stack temperatures (% 290” C) to escape in the vapor phase, incinerators could be significant contributors of these elements to the urban atmosphere. Further research on vapor phase emissions from incinerators is necessary. The observed enrichments of elements in the incinerator solids results in part from the composition of refuse. Many components of refuse, for example photographic waste, solder, colored inks, paints, batteries, plastics, etc., are enriched above crustal abundance in one or more elements. However, if it were simply a matter of total composition, aluminum would also be highly enriched. As Table 7 shows, aluminum is essentially the same as found in crustal materials. The form of the elements in the refuse is important. For example, silver from photographs or lead from printing inks in magazines and newspapers can be expected to appear in the fine ash, aqueous, and atmospheric emissions more readily than aluminum from cans. The high temperatures throughout an incinerator, and the high concentrations of acid-forming gases are additional factors definitely involved in enrichment processes. A closer look at sources for the metals observed in the ash is currently underway at the Bureau of Mines, College Park, Maryland. Enrichment factor Absolute concentrations are of prime interest from a public health point of view. However, they are difficult to use in attempting to identify the sources of pollutants in the environment. For example, the fly ash and the bottom ash analyzed in Table 7 are from the same incinerator, but the absolute elemental concentrations in each would indicate that they could have come from different sources. Incinerators in other locations, or even the same incinerator, may have different operating parameters producing ash materials containing varying amounts of carbonaceous components. Thus, the intercomparison of incinerator residues based on absolute concentrations may reveal little in common. It would be very difficult to identify an airborne particle as coming from an incinerator residue landfill if only absolute concentrations were compared. Because environmental studies are costly and timeconsuming, it is essential that the results be analyzed in ways that make the data as widely applicable as possible to other situations. A very promising approach to this ideal has been proposed by individuals involved in atmospheric particulate studies. To remove the effects of variation of absolute concentrations, normalization of all concentrations to that of a nonvolatile major element as well as to crustal abundances has been found useful [ 171. Because of the prominence of aluminum in aluminosilicates and the availability of data for aluminum concentrations, this element is the one
33
principally used for the normalizations. This double normalization is illustrated in the definition of the enrichment factor, EF, [ 171
procedure
where C represents the concentrations of element x and aluminum in medium i (in this study, ash materials from the incinerators) and in the average crustal materials of the earth. Elements with EF values close to 1 are similar to crustal materials. Elements showing large, positive EF values will come from sources having higher concentrations of those elements than found in the average crustal materials. Aluminum was also chosen as the element for normalized EF comparisons of the ash materials. Aluminosilicates derived from crustal materials are plentiful in municipal solid wastes in the form of clay fillers in paper products, in glass and ceramic materials, and in other materials in the refuse. The boiling temperature of aluminum metal is 2,467” C and for Al,03 is 2,980” C. The furnaces of the Alexandria incinerator operate at about 780” C and are not allowed to exceed 1,000” C. Thus, aluminum and many of its compounds will meet the nonvolatile criterion. The halides of aluminum (not including AlFs)
Fig. 2. Enrichment of elements relative to crustal abundances in Alexandria incinerator fly ash and quench water solids.
34
are volatile at incinerator temperatures; A1C13sublimes at 180” C. But in this respect, aluminum is no different from many other lithophile elements and any halide formation should not seriously interfere with the use of aluminum as a normalizing element for EF values. Perturbations arising from the significant volume of aluminum cans and other aluminum metal scrap in the refuse might be expected. However, examination of the values listed in Table 7 and the EF results shown graphically in Figs.2 and 3 indicate that these bulk aluminum metal sources do not result in an aluminum-enriched ash. Many of the lithophile elements would be depleted relative to aluminum = 1.0 in Figs.2 and 3 if excess aluminum were a problem. Depletion because of solubility is noticeable for elements such as calcium and magnesium in solids filtered from the acidic fly-ash scrubber waters. However, EF values for these same elements are not depleted in the quench waters, which have a high pH, indicating that solubility is the probable cause of depletion and not excess aluminum. Apparently, physical separation because of the bulk size keeps it from appearing as an EF perturbation in the ash materials.
Fig. 3. Enrichment of elements relative to crustal abundances in filter cake solids obtained during pilot plant processing of Chicago area incinerator residues and in Alexandria incinerator quench-water solids.
35
Enrichments
in ash
The EF values for fly ash filtered from the spray-chamber waters and bottom ash from the quench waters are strikingly similar as shown in Fig.2 This is in contrast to the conclusion one would obtain from examination of absolute concentrations only and indicates the value of using EF data to identify sources of materials. The few differences in EF (for example, barium, calcium, magnesium, and manganese) between the two types of solids can be attributed primarily to differences in solubility at around pH 5 for the scrubber waters and pH 11 for the quench waters. The EF values in the scrubber waters are generally somewhat lower, as would be expected from solubility considerations. The more dense ash materials falling into the quench waters from the furnace had settled out before the water samples for this study were taken. Therefore, bulk pieces of metal, glass, ceramics, etc., were not included in the analyses. Although more coarse and containing more carbonaceous material,, the light solids remaining suspended in the quench waters are apparently very similar in metal-concentration ratios to the fly ash scrubbed from the flue gases downstream in the incinerator spray-chamber baffles. Fig.3 shows a similar agreement of EF values between the Alexandria incinerator quench water solids and the filter cake solids from the Bureau of Mines pilot plant obtained during processing of bottom-ash residues from Chicago area incinerators. Again, all bulk metal, glass, ceramic, and other large pieces had been removed from the residues by the pilot plant processes before the filter cake solids were sampled. Therefore, it is not too surprising that the filter cake solids from the pilot plant are similar to the solids filtered from the incinerator quench waters. The important message of Fig.3 is the similarity in enrichment factors for metal composition of incinerator residues from Alexandria,, Va., and Chicago, Ill. Similarities in absolute concentrations for these and other solids were noted earlier in Table 5. The greatest enrichments (lo2 to over 103) are seen for silver, cadmium, lead, antimony, tin, and zinc and, to a lesser extent (10 to 102), for arsenic, chromium, copper, mercury, and nickel. Even greater enrichment factors (up to 105) are seen for these same elements (except mercury which was not determined) in Greenberg’s INAA study of particles being emitted from the stack of the Alexandria incinerator [S] . Greenberg, by using an in-stack cascade impactor [7], found that many of these stack-emitted elements are preferentially concentrated on small particles. Small particle size could indicate that the particles were formed by vaporization followed by condensation upon cooling. He further found that aluminum was distributed predominantly on larger particles typical of those released by mechanical processes. The fly ash and quench-water solids in this study would be in the category of the larger, mechanically released particles. Thus, the aluminum content would be higher and the resulting enrichment factors based on aluminum could be expected to be lower in fly ash and in quench-water solids than in the stack-emitted particles formed by condensation as observed by Greenberg.
36
Incinerator versus coal fly ash Enrichment factors were calculated for elements in the National Bureau of Standards’ Coal Fly Ash Standard Reference Material 1633, and they are listed in Table 8. This reference material is a blend of coal fly ashes supplied by five electric power plants: Tennessee Valley Authority, Stevenson, Alabama; Commonwealth Edison, Chicago, Illinois; Baltimore Gas and Electric Co., Baltimore, Maryland; Carolina Light and Power Co., Roxboro, North Carolina; and Potomac Electric Power Co., Washington, D.C. [19]. Comparison of the enrichment factors for elements in the coal fly ash with EF values for elements in the incinerator fly ash shows the incinerator fly ash to be enriched above the coal fly ash EF values by one order of magnitude or more in silver, cadmium, nickel, lead, antimony, and zinc. Only the alkali and alkaline-earth elements are less enriched in the incinerator fly ash than in the TABLE
8
Comparison of elemental enrichment factors in coal fly ash and in incinerator fly ash Element
Coal fIy ash (mg/kgja
Ratio incinerator/coal
Fly ash, enrichment factor coal
Incinerator
34 1.00 22. 2.8 1.0 8.8 2.2 1.2 2.7 1.1 3.7 0.35 1.3 0.80 0.44 0.08 1.4 2.9 21 0.42 56 3.6 0.97 2.2
1,400 1.00 31 1.6 0.54 400 5.7 11 11 0.45 19 0.28 0.73 0.45 1.5 0.43 28 310 1,100 1.1 280 0.46 4.3 110
-
& Al As Ba Ca Cd co Cr CU Fe Rg K Li Mg Mn Na Ni Ph Sb Si Sn Sr Ti Zn
3 127,000 61 2,700 47,000 1.45 41.5 131 128 62,000 0.14 16,100 62 18,000 493 3,200 98 70 6.9 210,000 270 1,700 7,400 216
41 1.00 1.4 0.57 0.54 45 2.6 9.2 4.1 0.41 5.1 0.80 0.56 0.56 3.4 5.4 20 107 52 2.6 5.0 0.13 4.4 50
aNational Bureau of Standards Coal Fly Ash SRM 1633. Certified values used where given [19]. Other values from refs.3, 7, and 18.
37
coal fly ash. This is probably due to their solubility in the incinerator processing waters. The absolute concentrations of aluminum in the coal fly ash and in the incinerator fly ash are essentially identical, 127,000 mg/kg and 121,000 mg/kg, respectively. Thus, the absolute observed enrichments and the aluminum normalized enrichments are the same in this particular case. A study of incinerated sewage sludge ash indicates that heavy metals leach out of the ash even with neutral distilled water [20]. Natural waters are often acidic, and together with microbial degradation of the incinerator ash in a landfill, high leaching rates are a possibility. The potential of environmental contamination by heavy metals warrants a thorough examination of ash disposal methods. CONCLUSIONS
The economic value of incinerator ash materials was reviewed by Sullivan and Makar [ 211. Recovered values from the Bureau of Mines incinerator residue processing pilot plant, which they describe, include ferrous products, aluminum products, nonferrous products, glass products, and waste products. The carbonaceous pilot plant filter-cake materials, analyzed in the research described in this report, were found by the Bureau of Mines to contain from 1 to 3 percent each of potash and phosphorus. Use of this fine ash as a nutritipus soil conditioner on highway embankments or median strips was suggested by Sullivan and Makar. However, extensive leach tests of these ash materials should be conducted before considering incinerator ashes as soil conditioners for produce or forage crops, and perhaps even for highway soils. Elements such as silver, arsenic, cadmium, chromium, copper, mercury, lead, antimony, tin, and zinc are from 10 to 1,000 times more concentrated relative to aluminum in the ash materials than they are in the average crustal material. Comparison of the incinerator fly ash with coal fly ash shows the incinerator fly ash to be more highly enriched in all metals except the alkali and alkaline-earth metals. The coal ash would probably be as effective as a soil conditioner and have less environmental pollution potential. Some metal values in the incinerator ash materials are comparable with low grade mineable materials. They are probably present as intractable oxides or bound by refractory silicates and alumina, making recovery difficult and uneconomical. The potential of environmental contamination, however, provides a noneconomic incentive to develop viable alternatives to landfill disposal of incinerator ash materials. The fly ash and the fine bottom-ash materials from incinerators are similar in relative metal concentrations. The elemental enrichments observed seem to be established in the initial raw refuse input and during the incineration process. The resulting fly ash and quenched bottom ash seem to differ mainly in physical size, flue gas buoyancy, and in unburned carbonaceous content, but not in metal concentrations relative to aluminum.
38
The ash materials were also found to be very similar regardless of geographic origin or period of time. However, more samples will need to be taken to establish any definite trends. ACKNOWLEDGEMENTS
The author wishes to thank Harry L. Dodson, manager of the Alexandria, Va., incinerator; R. Cecil Allnutt, manager of the Montgomery County, MD., incinerator; Robert A. Holbrook, manager of Solid Waste Reduction Center No. 1, Washington, D.C.; and Paul M. Sullivan and Harry V. Makar, U.S. Bureau of Mines, College Park Metallurgy Research Center, for their cooperation and assistance in obtaining samples and in supplying information. Assistance from the analytical staff of the College Park Metallurgy Research Center, Bureau of Mines, is also gratefully acknowledged.
REFERENCES
7 8 9
10
11 12 13 14 15
Kenahan, C.B., Sullivan, P.M., Ruppert, J.A. and Spano, E.F., 1968. Composition and characteristics of municipal incinerator residues. BuMines RI 7204. Jackson, F.R., 1974. Energy From Solid Waste. Noyes Data Corporation, Park Ridge, N.J. Law, S.L., 1976. Metals in the aqueous effluents from municipal incinerators and an incinerator-residue processing plant. Ph.D. Dissertation, Chemistry Department, University of Maryland, College Park, Maryland. Law, S.L., 1977. Dissolved metals in the aqueous effluents from municipal incinerators. J. Water Pollut. Control Fed., 49: 2454. Carotti, A.A. and Smith R.A., 1974. Gaseous emissions from minicipal incinerators. Report SW-l&, U.S. Environmental Protection Agency, Washington. Sullivan, P.M. and Stanczyk, M.H., 1971. Economics of recycling metals and minerals from urban refuse. BuMines TPR 33. Law, S.L. and Greenberg, R.R., 1976. Characterization of municipal incinerator effluents. Prog. Anal. Chem., 8: 55. Greenberg, R.R., 1976. A study of trace elements emitted on particles from municipal incinerators. Ph.D. Dissertation, Chemistry Department, University of Maryland, College Park, Maryland. Wilson, E.B. and Akers, D.J., 1970. Characterization of metropolitan incinerator refuse and fly ash. In: Proceedings of the Second Mineral Waste Utilization Symposium, U.S. Bureau of Mines and IIT Research Institute, Chicago, p. 313. Buttermore, W.J. Lawrence, W.F. and Muter, R.B., 1972. Characterization, beneficiation and utilization of municipal incinerator fly ash. In: M.A. Schwartz (Editor), Proceedings of the Third Mineral Waste Utilization Symposium, U.S. Bureau of Mines and IIT Research Institute, Chicago, pi 397. 1973 Minerals Yearbook, Vol. I. U.S. Bureau of Mines, Washington, D.C., 1975, p. 80. 1970 Minerals Yearbook, Vol. I. U.S. Bureau of Mines, Washington, D.C., 1973, p. 74. Wedepohl, K.H., 1968. In: L.H. Ahrens (Editor), Origin and Distribution of the Elements, Pergamon, London, pp. 999-1916. Mercury in air (1971). Sci. News, 99: 280. Washington Star-News, Washington, D.C., 1974. Incinerator still belching mercury into D.C. skies, Wednesday, March 13, p. B-3.
39
16 11 18
19
20 21
Billings, C.D. and Matson, W.R., 1972. Mercury emissions from coal combustion. Science, 176: 1232. Zoller, W.H., Gladney, E.S. and Duce, R.A., 1974. Atmospheric concentrations and sources of trace metals at the South Pole. Science, 183: 198. Ondov, J.M., Zoller, W.H., Olmez, I., Aras, N.K., Gordon, G.E., Rancitelli, L. A., Abel, K.H., Filby, R.H., Shah, U.R. and Ragaini, R.C., 1976. Elemental concentrations in the National Bureau of Standards’ environmental coal and fly ash standard reference materials. Anal. Chem., 47: 1102. Office of Standard Reference Materials. Standard Reference Material 1633 - Trace Elements in Coal Fly Ash, Dept. of Commerce, National Bureau of Standards, Washington, D.C. 20234. Diosady, L.L. Recycling of incinerator ash. Research Report No. 19, Ontario Ministry of the Environment, Toronto, Ontario, Canada. Sullivan, P.M. and Makar, H.V., 1976. Quality of products from Bureau of Mines resource recovery systems and suitability for recycling. In: Proceedings of the Fifth Mineral Waste Utilization Symposium, U.S. Bureau of Mines and IIT Research Institute, Chicago, p. 223.
19
METALS IN ASH MATERIALS FILTERED FROM MUNICIPAL INCINERATOR EFFLUENTS
STEPHEN L. LAW* U.S. Bureau of Mines, College Park, MD 20740
(U.S.A.)
(Received 24th November 1976)
ABSTRACT Fly-ash and bottom-ash materials were filtered from the aqueous effluents of municipal incinerators and from an incinerator-residue processing plant and were examined for ore grade quantities of metals and for trace metals pollution potential. Several elements, for example Ag, Al, Cu,and Ti, are present in concentrations comparable with low-grade ores presently being mined. Elements such as Ag, As, Cd, Cr, Cu, Hg, Pb, Sb, Sn, and Zn are one to three orders of magnitude more enriched in incinerator ash than they are in average crustal abundances. The fly-ash and bottom-ash materials filtered from incinerator effluents were found to be very similar in relative metal concentrations. Incinerator fly ash is one order of magnitude more enriched in Ag, Cd, Ni, Pb, Sb, and Zn than coal fly ash. No significant elemental concentration differences were observed for samples taken during different time periods and from different geographic locations.
INTRODUCTION
Approximately 65-75 percent of the solid waste disposed of in the United States is amenable to use as a fuel supplement with a resulting 75-95 percent volume reduction in the total refuse [ 11. Municipal incinerators may be converted to, or originally designed as, plants capable of extracting valuable energy during solid waste combustion [ 21. The bottom ash and fly-ash products of solid waste combustion are generally disposed of in landfills. Recovery of metallic values from the residues after conventional incineration, as demonstrated by a U.S. Bureau of Mines pilot plant discussed later, will further reduce the volume sent to a landfill by another 50 percent [l]. The elemental content of the remaining disposed ash material is of considerable interest. If the ash is used for landfill, the possibility exists that leaching facilitated by microbial degradation of unburned carbon in the ash -- may become a problem. The local vegetation may concentrate toxic elements from the water of the landfill and the wild or domestic animals eating the vegetation *This study is taken in part from a dissertation submitted to the Graduate School of the University of Maryland for the Ph.D. degree in chemistry.
20
and drinking the water will be adversely affected. Alternate uses of the ash, for example, as a soil conditioner or a building aggregate, may depend on the concentrations of toxic elements in the material. The corrosion properties of the ash are an important consideration in incinerator-residue processing operations. The possible health effects from inhaled airborne particulates, either windblown from a landfill or escaping from the incinerator stack, are an important consideration requiring a knowledge of the elemental concentrations of the ash. Finally, disposed ash should be considered as a possible source of metal and mineral values. As a part of a program to characterize the aqueous effluents from municipal incinerators [ 3, 41, the ash materials filtered from these effluents were analyzed for elemental concentrations. The primary emphasis of this report is on these filtered solids, which can be expected to represent the finer and less dense materials in the incinerator bottom-ash and fly-ash residues. However, the elemental compositions of the filtered ash materials are compared with the composition of the bulk fly-ash and bottom-ash residues (excluding massive pieces of metal, glass, etc.) and the results indicate that the filtered solids adequately represent the elemental compositions of the total ash materials. The filter-cake solids from the Bureau of Mines incinerator residue processing pilot plant are also similar in elemental composition. Alexandria incinerator The principal emphasis of this study was on the residue from the Alexandria, Virginia, incinerator which recycles approximately 114,000 l(30,OOO gal.) of fly-ash scrubber water and approximately 19,000 1(5,000 gal.) of quench water in completely separate systems. Early Saturday morning, at the end of each operating week, the waters in each system are dumped into the municipal sewer system. The hot residues from the primary combustion chamber drop into a water quench tank. Following quenching, they are carried by drag conveyor to a chute and dropped into disposal trucks. The quench water is screened before going into a holding tank from which it is recycled at a rate of about 1,000 1 (260 gal.) per minute. The ash materials used in the research reported here were filtered from water samples taken after the coarse screen and just prior to entering the holding tank. Upon leaving the primary combustion chamber, the hot gases pass through a flue into a secondary combustion chamber and then into the fly-ash spraybaffle scrubber. Pressurized nozzles spray water down the baffle walls. The combustion gases pass through two 90” turns in the baffle, causing the fly-ash particulates to impinge upon the walls and become entrained in the flowing water. This water is then pumped to a holding tank where the heavy solids are allowed to settle and the water is then recycled at a rate of about 2,000 1 (530 gal.) per minute. The settled fly ash is removed from the holding tank by drag conveyor to trucks for landfill disposal. The fly-ash material used in
21
the research reported here was filtered from samples of this settling tank water. The fly-ash scrubber water becomes highly acidic during the incinerator process because of the various acidic gases being formed [ 51. Soda ash is added continuously and automatically throughout the operating week at an average rate of 135 kg (300 lb) per day to neutralize this acidity to a pH of 4-7. The pH is checked every hour and the rate of soda ash addition is adjusted as needed. Bureau of Mines incinera tar-residue processing plant The U.S. Bureau of Mines designed and built a pilot plant capable of processing 450 kg (0.5 ton) of incinerator residues per hour [6]. This pilot plant relies exclusively on existing mineral engineering technology and is a combination of various mineral beneficiation processes. Continuous screening, shredding, grinding, magnetic-separation, flotation, jigging, and gravity-concentration techniques are applied to produce metallic iron concentrates, highquality aluminum nuggets, mixed colored glass, slag and fine ash tailings. Water is used throughout the plant as the processing medium. Samples for analysis were taken of the filter cake material filtered from this water just prior to discharge into the sewer system. ANALYTICAL
TECHNIQUES
After being dried and powdered, the solids were put into solution using conventional acid digestion with nitric, hydrofluoric, and perchloric acids. Once in solution, most of the elements were determined by conventional flame atomic absorption. Mercury was determined by the cold-vapor atomicabsorption technique. Arsenic and antimony were determined by flameless, graphite furnace, atomic absorption. Conventional semiquantitative emissionspectrographic analyses of the powdered solids were also performed. These analytical methods as applied to incinerator residues have been described previously [ 3, 71. Solids in effluents Fig.1 shows the weights of solids filtered from samples of the Alexandria recycled waters collected during a l-week period of operation. As shown, the amount of solids in the recycled waters fluctuates greatly, probably depending upon the particular operating conditions at the time of sampling. At the time of discharge, the bulk of the undissolved solids have been settled out of the Alexandria incinerator effluents, usually leaving less than 0.5 g/l in the aqueous effluents. Effluents from the Washington, D.C., Solid Waste Reduction Center No. 1 incinerator and the Montgomery County, Maryland incinerator (now inoperative), both of which were also studied in this project, are discharged con-
1
B+ A A B+
D D D G E F+
SRM 1633
Residue processing Alexandria Chicago C+ B
B+ B+
Montgomery Co. Fly ash Bottom ash Combinedb
A B
Al
D D+
Ag
Washington, D.C. Fly ash Combinedb
-
0.3 -3 0.1 -1 0.03-0.3
D E
C+ C D+
Alexandria Fly ash Bottom ash
Samplea
A B+ B
Over 10 Over 5 l-10
E F+
D
E+ E+ E+
E+ E+
E+ F+
B
D E+ E
Letters indicate estimates of concentration
C C
C
C C C
C C
C+ C
Ba
-
G+
-
-
G -
Be
F+ F+
F
E E E
E E+
E+ -
Bi
0.01 -0.1 0.003.--0.03 0.001-0.01
B+ B+
B
B+ A B+
B B
B A
Ca
ranges in percent: F+ F G+
-
-
E+ F+ E+
E+ D
-
Cd
E -
E
E E
E -
E+ -
Co
D D
E+
C C C
D C
D+ D
Cr
0.0003 --0.003 0.0001 -0.001 0.00003-0.0003
D D
E+
D D+ D
D+ D
D E+
Cu
B B
B
B B+ B
B B
C+ C+
Fe
G H -
-
E+ -
Ga
-
F+
-
-
-
--
Ge
C C
C
C+ C+ C+
C C
C+ C
K
E+ E
D
D+ E+ D
D D
D+ E+
Li
0.00001-0.0001 Less than 0.00001 Not detected
Semiquantitative analysis of undissolved solids in incinerator and incinerator-residue processing effluents
TABLE
C C c+ C+ B B B c+ c C+
Alexandria Fly ash Bottom ash
Washington, D.C. Fly ash Combinedb
Montgomery Co. Fly ash Bottom ash Combinedb
SRM 1633
Residue processing Alexandria Chicago D+ D
D+
C C C
D+ C
C D+
Mn
-
E+
E+ F+ E+
E+ E+
E+ F+
MO
C+ C+
C
C+ B C+
C+ C+
B E+
Na
E+ Ei
E+
D D D
E+ D
D E
Ni
D -
-
C C C
C C
-
D+ C
E+
C C C
C C+
C+ D+
-
Ei-
-
-
E+ -
---
-
E+ D D
D D
‘E+ --
A B+
A
A A A
A A
A A
D D
F+
C D+ D+
D+ C
D+ D
D D
C
D+ C D+
D D+
D+ D
-_l__.l P Pb Rb Sb Si Sn Sr .._-_____-..-. _-.-..__. ._--.-_..._ _,.. .__~___
C C
C
B B B
B B
C+ C!
Ti
E+ F+
D
E+ E+ D
D+ D
D E+
V
D+ C
E+
C+ C+ C+
B A
C C
Zn
aAh samples (except the SRM1633 reference coal fly ash) are ash material filtered from the separate aqueous systems of each plant. b”Combined” refers to the solids filtered from the combined fly-ash and bottom-ash water systems prior to discharge to the sewer system.
Mg
Samplea
-
E -
E
E D E+
E+ E
E+ F+
Zr
-
24
0
Fly
A
Quench
ash
scrubber
waters
waters
0.4
Fig.1. Variation of undissolved solids content in Alexandria incinerator recycled waters during one week of operation.
tinuously into the local sewer systems. These waters likewise seldom contained more than 0.5 g/l of undissolved solids at the point of discharge. Semiquantitative values A semiquantitative elemental analysis was made of samples of the filtered solids from the different aqueous effluents of each incinerator as shown in Table 1. As a reference, similar data are included for the National Bureau of Standards SRM 1633 coal fly-ash standard. Also included are data found for elements in solids from the Bureau of Mines incinerator-residue processing pilot plant obtained during the processing of Alexandria incinerator residues and during the processing of residues from the Northwest Chicago incinerator. This emission spectrographic survey of elements in the solids provides information on elements not determined by atomic absorption. It also served as a guide in preparing dissolved samples and dilutions for the quantitative atomic absorption determinations. Elemental concentrations Table 2 lists the average for elements found in the fly ash and in the quench water solids. An independent instrumental neutron-activation analysis (INAA) of the Alexandria fly ash using samples taken during a different time period, was conducted by Law and Greenberg [7, 81 and the results are included in Table 2. The INAA samples are of the fly ash after settling in the
25
TABLE
2
Elemental composition Element
Ag Al As Ba Be Ca Cd co Cr Cu Fe Rg K Li Mg Mn Na Ni Pb Sb Sn Sr Ti Zn
of solids from Alexandria incinerator
Concentration (mg/kg) AA@ C Scrubber fly ash
INAAb Scrubber fly ash
AA“ Quench water solids
129 + 28 120,000 ?r 12,000 77 f 10 1,500 + 400 <4 23,000 f 10,000 64 f 16 100 f 30 1,160 +_720 510 + 180 24,000 + 8,000 0.88 + 0.72 12,200 +_1,800 342 4 9,700 f 1,700 1,500 + 600 16,000 f 2,000 1,800 + 2,800 7,200 + 3,200 340 f 290 1,250 f 650 210 f 80 < 50 10,100 f 2,000
106 f 111,000 ? 41+ 2,500 i -
18 12,000 15 300
40,000 45 30 1,430 1,000 46,000
6,000 28 4 200 500 7,000
38 49,000 26 1,400 <4 40,000 41 70 520 450 16,000 0.42 6,300 19 12,800 3,100 8,200 210 1,700 120 400 210 < 50 5,500
Wncertainties are standard sample-to-sample variations bResults of an independent analysis and using different CBy atomic absorption.
-
-
12,000 3,400 14,000 740 4,000 260
9,400
i + r i i f
c f f f + i
5,000 700 2,000 1ooc 1,300c 60
i: 1,300
?8 * 8,000 *5 f 600 5 t f f f f
18,000 15 10 240 190 6,000
f 1,400 k3 2 2,600 f 1,700 f 1,800 i- 250 f 800 * 90 * 50 f 1,500
deviations. For most elements the uncertainties result from and not analytical error. study by Greenberg [ 81 using instrumental neutron activation samples from those used for atomic absorption.
settling tank and being removed by drag conveyor to a waiting truck, whereas the samples for analysis by atomic absorption (AA) consisted of solids suspended in the water samples of the settling tank. In spite of these sample differences, the largest disagreement for elemental concentrations is only a factor of three for cobalt and a factor of two or less for the other elements. The values in Table 2 are averages of 11 samples taken on nine days over a period of five months. The uncertainties shown in Table 2 represent the standard deviations and are mainly the result of sample-to-sample variations and not necessarily analytical error. For example, nickel in the quench-water solids shows an average of 210 mg/kg with a standard deviation of + 250 mg/
26
kg. The large standard deviation arises from one sample which had a nickel concentration of 920 mg/kg. If this value is removed, the Ni result becomes 130 + 40 mg/kg. The analytical error for each element of this study was determined by performing analyses of several samples of SRM 1633 coal fly-ash [ 3, 71. For nickel, the result was 100 F 4 mg/kg indicating that the analytical standard deviation is probably at least one order of magnitude less than the sample variation. The elements examined in these solids from day to day, from month to month, and even from year to year appear to remain within a consistent range of concentration values. Table 3 compares the analysis of a sample taken in October 1973 with a sample taken in July 1974. The nickel values are an example of the extremes encountered in all samples for nickel, but the other elements show good agreement. Many more samples taken over several years are needed to establish any trends in element concentrations. TABLE
3
Concentrations of elements in Alexandria incinerator fly ash sample in 1973 and 1974 Element
Ag Al Ba Be Ca Cd co Cr CU Fe K Li Mg Mn Na Ni Pb Sn Sr Zn
Concentration (mg/kg) October 197 3
July 1974
95 126,000 1,700 <4 22,000 56 100 1,140 600 24,000 13,000 31 9,500 1,500 16,700 1,580 10,000 850 245 8,600
130 124,000 1,400 <4 22,000 57 90 730 445 22,000 14,000 34 9,800 1,100 16,500 160 7,200 860 260 8,000
Ash residue comparisons The School of Mines, West Virginia University, performed a series of physical and chemical characterizations of municipal-incinerator fly ash and residues being processed by the Bureau of Mines incinerator-residue processing
27
pilot plant between 1969 and 1972. A comparison of their published results [9, lo] and the Alexandria fly ash and quench-water solids is made in Table 4. The fly ash and bottom ash of the West Virginia University study came from three Washington, D.C., incinerators (not including Solid Waste Reduction Center No. 1); an Arlington, Va., incinerator; the Montgomery County, Md., incinerator; and the Alexandria, Va., incinerator. Their publications did not identify the incinerators, so the,averaged results are specified as “‘Washington, D. C., area” incinerators in Table 4. The earlier report [ 91 used emission spectrography for most of the determinations and the second report [lo] used atomic absorption. The results show no significant differences in the three averaged fly-ash values nor in the comparison of Alexandria quenchwater solids and the average of two samples of fine bottom ash. ‘The uncertainties are standard deviations resulting primarily from sample-to-sample variation. A semiquantitative emission spectrographic analysis of the filtered solids from Bureau of Mines pilot plant processing of Alexandra, Va., incinerator residues and of Chicago Northwest incinerator residues was shown previously in Table 1. A quantitative analysis using atomic absorption and gravimetric techniques was made of filter cake solids collected for this study during the processing of residues sent to the Bureau of Mines from four incinerators in the Chicago, Ill., area: Chicago Southwest; Chicago Northwest; Medill, Ill.; and Calumet, Ill., incinerators. These solids were filtered from the effluents of the Bureau of Mines pilot plant prior to discharge into the sewer system. Averages of the results from the four incinerators are shown in Table 5. Also shown in Table 5 are the averaged analyses of minus &mesh (< 2.4 mm) residues before processing in the pilot plant. These residues came from five Washington, D.C., area incinerators and one Atlanta, Ga., incinerator. Large pieces, including tin cans, massive iron, glass, wire, paper, ceramics, etc., had been handpicked from the residues. All remaining coarse material greater than S-mesh was screened from the dry samples. The minus &mesh materials were passed through a ball mill and screened again through a 100-mesh screen (0.15 mm); only the minus lOO-mesh materials were analyzed to obtain the averages shown in the table. For comparison purposes, the final column of Table 5 again shows the results of the analyses of the solids filtered from the Alexandria, Va., incinerator quench waters. These solids would be expected to represent the fine and less dense materials in the Alexandria incinerator bottom ash residues. The similarity between the three residue materials is noteworthy, especially in view of the fact that they came from different incinerators from geographically separate areas (Chicago and Washington, D.C.) and they were collected during a time span of at least 8 years. Only copper shows a variation, with the residues before processing showing an order of magnitude more copper than the filter cake and quench-water solids. The reason for this difference is not obvious at this time.
4
0.01 13.3 <0.4 4.6 0.05 0.07 2.3 co.03 1.34 1.02 0.06 1.98 <0.03 0.3 0.04 0.5
+ i + f + + f f
1.69 0.98 0.15 1.9 0.2 0.53 0.19 0.87 0.25 0.11 0.10 0.4 0.1 0.47 0.10 0.38
1.7 0.04 1.4 0.019 0.031 1.0
f + + * f *
0.019 11.5 0.14 5.9 0.067 0.083 2.7 -
+ 0.019
Ref.9
Ref.10
Washington, D.C. area fly ash
aAsh materials filtered from the aqueous system. bAverage of two samples only.
f 0.03 + 1.2 f 0.004 t 1.0 f 0.07 + 0.008 f 0.8 + 0.00007 f 0.18 i- 0.17 i 0.06 f 0.2 ?r 0.28 i 0.32 f 0.07 f 0.20
0.013 12.1 0.15 2.3 0.12 0.051 2.4 0.00009 1.22 0.97 0.15 1.6 0.18 0.72 0.13 1.01
Ag
Al Bk Ca Cr CU Fe Hg K Mg Mn Na Ni Pb Sn ZlI
Alexandria scrubber fly asha
‘Element
Concentrations expressed as weight-percent.
0.5 0.03 0.02 0.6
o.o&
0.55
f 0.2
0.0038 4.9 0.14 4.0 0.052 0.045 1.6 0.00004~ 0.63 1.28 0.31 0.82 0.021 0.17
t 0.1 f 0.01
f 0.20 c 0.03 ?: 0.02 + 0.54
f + + +
f 2.6 0.06 1.8 0.024 0.019 0.6
i 0.15
f 0.14 f 0.26 ? 0.17 t 0.14 f 0.025 -t 0.08
f c r f t
f 0.8
i 0.0008
Alexandria bottom asha
Comparison of Alexandria incinerator ashes with averaged Washington, D.C., area ashes
TABLE
3.9 0.2 6.8 0.04 0.23 8.4 < 0.03 0.81 0.81 > 0.1 2.6 < 0.03 0.6 0.08 0.25
< 0.01
Washington, D.C. area bottom ash Ref. 9
29
TABLE
5
Solids in incinerator-residue processing Element
Concentration (weight percent) Minus 8-mesh residues before processinga
Al Ba C
7.6 0.3 18.1
Ca Cd co Cr cu Fe K Mg Mn Na Ni P Pb S Si Sn Sr Ti Zn
2.2
f 2.3 f 7.4
0.04 0.35 4.0 1.0 1.0 0.20 2.3 0.01 0.22 0.50 0.30 14.8 0.045 1.2 0.48
* 0.15 rt 1.4
+ 0.06
f 0.17 * 1.7 f 0.022
f 0.23
Filter cake after processingb 4.1 0.88 7.7 5.3 0.0029 <0.005 0.012 0.058 6.1 1.03 1.65 0.21 2.1 0.017 0.25 0.35 0.59 17.6 0.067 0.0082 1.06 0.45
Suspended quenchwater solidsc
+ 0.2 * 0.04 + 2.7
4.9 0.14 -
f 0.8 f 0.06
f 1.1 + 0.0015
4.0 0.0041 0.0070 0.052 0.045 1.60 0.63 1.28 0.31 0.82 0.021 0.17 0.040 0.021 0.55
f + + f f f f f + + f
f f + + r f i. i: i + f f f + t f
0.002 0.017 2.1 0.34 0.61 0.09 0.8 0.010 0.006 0.11 0.21 0.025 0.025 0.0003 0.09 0.025
1.8 0.0015 0.0010 0.024 0.019 0.57 0.14 0.26 0.17 0.14 0.025
* 0.08
+ 0.005 + 0.15
‘Averaged results of the analyses of residues from five metropolitan Washington, D.C., incinerators and one Atlanta, Ga., incinerator [ 11. bFiiter cakes from processing of residues from four Chicago area incinerators: Chicago Southwest, Chicago Northwest, Medill, Ill., and Calumet, Ill. cSolids filtered from Alexandria, Va., incinerator quench waters.
METAL VALUES
A U.S. Bureau of Mines report on the composition and characteristics of municipal incinerator residues concluded that “From the standpoint of reclaiming mineral values, the fine ash fraction is the most promising.” [ 11. The School of Mines, West Virginia University, study of municipal incinerator residues found that “The elements aluminum, silver, copper, chromium, gallium, lead, tin, tantalum, and zinc are present in several of the refuse materials in amounts which appear to be near-competitive with ore concentrations currently considered mineable.” [ 91. Table 6 compares some of the metal concentrations in incinerator ash materials with the concentrations in surfacemineable materials. Although some metal values are comparable with the
30
TABLE
6
Comparison of metal values in incinerator ash and in surface mined ore@ Element
Material grade (%) (surface mines)b _
Ag Ai CU Fe Hg Ni Pb Ti zn
0.007 17 0.15 22. 0.009 1.4c 5.5 2.0 4.3
Incinerator ash concn. (%) Bottom ash
Fly ash
0.004 9. 0.05 2.3 0.00004 0.03 0.2 1.2 0.6
0.01 23 0.05 3.4 0.00009 0.2 0.7 -
__--
1.9
-__
__-
eThe chemical-mineral form of the metal in the ash materials may be different from the form occurring in a surface mine. For example, Al is given as 17% bauxite in the surface mine, whereas the bottom ash and fly ash percentages are for Al,O,. bRef.11. CRef.12.
average or low mineable materials, conventional mineral processing for the metal content of ash may not be practical. Most mineral-separation techniques rely on the surface properties of microcrystalline solids, and this type of order is not common in ash materials. For example, aluminum is probably not present as bauxite but rather as a refractory oxide or silicate requiring an expensive chemical treatment for extraction. Another problem is the rather insignificant volume of ash material at any single location compared with the volume of ore present at a surface mine. The Alexandria incinerator produces less than 300 metric tons per week of combined fly ash and bottom ash. However, the study of metal-recovery systems should be expanded because of the possible noneconomic incentive of environmental contamination from landfills, and the trend to use municipal solid waste as a fuel supplement. ENRICHED
ELEMENTS
Comparison with crustal abundances To obtain a better understanding of the possible environmental significance of the elemental concentrations found in the solids removed from the various effluents, a comparison of concentrations with crustal abundances is given in Table 7. Mean crustal concentrations can only be used as a crude approximation of the relative composition of the environment into which the ash materials may be introduced to serve as a landfill, soil conditioner, or in some other function. The differing types of crustal materials and soils in various
31 TABLE
7
Comparison of elements from the effluents of the Alexandria incinerator and the Bureau of Mines pilot plant with crustal abundances Element
Ag Al As Ba Ca Cd co cr cu : K Li Mg Mn Na Ni Pb S Sb Si Sn Sr Ti Zn
CrustaI abundance, (mg/kg) [I31 0.06 78,300 1.7 590 28,700 0.1 12 70 30 35,400 0.03 28,200 30 13,900 690 24,500 44 15 310 0.2 305,400 3 290 4,700 60
Ratio of absolute concentration to crustal abundance Fly ashe
Bottom ashc
Filter-cake solids 5
2,200 1.5 45 2.5 0.80 640 8.3 17 17 0.68 29 0.43 1.1 0.70 2.2 0.66 55 480 -
630 0.63 15 2.4 1.4 410 5.8 7.4 15 0.45 14 0.22 0.63 0.92 4.5 0.33 5.7 110 -
-
1,700 -
600 -
-
420 0.72 170
130 0.72 92
-
0.52
15 1.8 290 -
-
1.7 19 1.7 0.37
1.2 3.0 0.86 3.9 230 19 0.58 220 0.28 2.3 75
aFiltered from Alexandria incinerator processing waters. 5Filtered from effluents of the Bureau of Mines pilot plant during the processing of incinerator residues from the Chicago, Ill., area.
areas will require more specific comparisons to determine the suitability of the incinerated residues for a particular application. Table ‘7, however, does indicate that silver, cadmium, lead, antimony, tin, and zinc are up to two or three orders of magnitude higher in concentration than might be expected in average crustal materials. Concentrations up to one order of magnitude above crustal abundances are also observed for arsenic, barium, cobalt, chromium, copper, mercury, manganese, nickel, sulfur, and titanium. The more soluble alkali and alkaline-earth metals are often lower in concentration than the mean crustal abundances. A higher mercury ratio was expected because of reported observations of mercury plumes from incinerators [ 14, 151. The mercury is presumed to re-
32
main in the vapor phase rather than attached to the solids. In studies of a furnace fired with pulverized coal, 90 percent of the mercury was apparently discharged in the vapor phase and only 10 percent remained with the furnace residual ash [ 161. If arsenic, lead, antimony, and other elements or their chlorides are also sufficiently volatile at stack temperatures (% 290” C) to escape in the vapor phase, incinerators could be significant contributors of these elements to the urban atmosphere. Further research on vapor phase emissions from incinerators is necessary. The observed enrichments of elements in the incinerator solids results in part from the composition of refuse. Many components of refuse, for example photographic waste, solder, colored inks, paints, batteries, plastics, etc., are enriched above crustal abundance in one or more elements. However, if it were simply a matter of total composition, aluminum would also be highly enriched. As Table 7 shows, aluminum is essentially the same as found in crustal materials. The form of the elements in the refuse is important. For example, silver from photographs or lead from printing inks in magazines and newspapers can be expected to appear in the fine ash, aqueous, and atmospheric emissions more readily than aluminum from cans. The high temperatures throughout an incinerator, and the high concentrations of acid-forming gases are additional factors definitely involved in enrichment processes. A closer look at sources for the metals observed in the ash is currently underway at the Bureau of Mines, College Park, Maryland. Enrichment factor Absolute concentrations are of prime interest from a public health point of view. However, they are difficult to use in attempting to identify the sources of pollutants in the environment. For example, the fly ash and the bottom ash analyzed in Table 7 are from the same incinerator, but the absolute elemental concentrations in each would indicate that they could have come from different sources. Incinerators in other locations, or even the same incinerator, may have different operating parameters producing ash materials containing varying amounts of carbonaceous components. Thus, the intercomparison of incinerator residues based on absolute concentrations may reveal little in common. It would be very difficult to identify an airborne particle as coming from an incinerator residue landfill if only absolute concentrations were compared. Because environmental studies are costly and timeconsuming, it is essential that the results be analyzed in ways that make the data as widely applicable as possible to other situations. A very promising approach to this ideal has been proposed by individuals involved in atmospheric particulate studies. To remove the effects of variation of absolute concentrations, normalization of all concentrations to that of a nonvolatile major element as well as to crustal abundances has been found useful [ 171. Because of the prominence of aluminum in aluminosilicates and the availability of data for aluminum concentrations, this element is the one
33
principally used for the normalizations. This double normalization is illustrated in the definition of the enrichment factor, EF, [ 171
procedure
where C represents the concentrations of element x and aluminum in medium i (in this study, ash materials from the incinerators) and in the average crustal materials of the earth. Elements with EF values close to 1 are similar to crustal materials. Elements showing large, positive EF values will come from sources having higher concentrations of those elements than found in the average crustal materials. Aluminum was also chosen as the element for normalized EF comparisons of the ash materials. Aluminosilicates derived from crustal materials are plentiful in municipal solid wastes in the form of clay fillers in paper products, in glass and ceramic materials, and in other materials in the refuse. The boiling temperature of aluminum metal is 2,467” C and for Al,03 is 2,980” C. The furnaces of the Alexandria incinerator operate at about 780” C and are not allowed to exceed 1,000” C. Thus, aluminum and many of its compounds will meet the nonvolatile criterion. The halides of aluminum (not including AlFs)
Fig. 2. Enrichment of elements relative to crustal abundances in Alexandria incinerator fly ash and quench water solids.
34
are volatile at incinerator temperatures; A1C13sublimes at 180” C. But in this respect, aluminum is no different from many other lithophile elements and any halide formation should not seriously interfere with the use of aluminum as a normalizing element for EF values. Perturbations arising from the significant volume of aluminum cans and other aluminum metal scrap in the refuse might be expected. However, examination of the values listed in Table 7 and the EF results shown graphically in Figs.2 and 3 indicate that these bulk aluminum metal sources do not result in an aluminum-enriched ash. Many of the lithophile elements would be depleted relative to aluminum = 1.0 in Figs.2 and 3 if excess aluminum were a problem. Depletion because of solubility is noticeable for elements such as calcium and magnesium in solids filtered from the acidic fly-ash scrubber waters. However, EF values for these same elements are not depleted in the quench waters, which have a high pH, indicating that solubility is the probable cause of depletion and not excess aluminum. Apparently, physical separation because of the bulk size keeps it from appearing as an EF perturbation in the ash materials.
Fig. 3. Enrichment of elements relative to crustal abundances in filter cake solids obtained during pilot plant processing of Chicago area incinerator residues and in Alexandria incinerator quench-water solids.
35
Enrichments
in ash
The EF values for fly ash filtered from the spray-chamber waters and bottom ash from the quench waters are strikingly similar as shown in Fig.2 This is in contrast to the conclusion one would obtain from examination of absolute concentrations only and indicates the value of using EF data to identify sources of materials. The few differences in EF (for example, barium, calcium, magnesium, and manganese) between the two types of solids can be attributed primarily to differences in solubility at around pH 5 for the scrubber waters and pH 11 for the quench waters. The EF values in the scrubber waters are generally somewhat lower, as would be expected from solubility considerations. The more dense ash materials falling into the quench waters from the furnace had settled out before the water samples for this study were taken. Therefore, bulk pieces of metal, glass, ceramics, etc., were not included in the analyses. Although more coarse and containing more carbonaceous material,, the light solids remaining suspended in the quench waters are apparently very similar in metal-concentration ratios to the fly ash scrubbed from the flue gases downstream in the incinerator spray-chamber baffles. Fig.3 shows a similar agreement of EF values between the Alexandria incinerator quench water solids and the filter cake solids from the Bureau of Mines pilot plant obtained during processing of bottom-ash residues from Chicago area incinerators. Again, all bulk metal, glass, ceramic, and other large pieces had been removed from the residues by the pilot plant processes before the filter cake solids were sampled. Therefore, it is not too surprising that the filter cake solids from the pilot plant are similar to the solids filtered from the incinerator quench waters. The important message of Fig.3 is the similarity in enrichment factors for metal composition of incinerator residues from Alexandria,, Va., and Chicago, Ill. Similarities in absolute concentrations for these and other solids were noted earlier in Table 5. The greatest enrichments (lo2 to over 103) are seen for silver, cadmium, lead, antimony, tin, and zinc and, to a lesser extent (10 to 102), for arsenic, chromium, copper, mercury, and nickel. Even greater enrichment factors (up to 105) are seen for these same elements (except mercury which was not determined) in Greenberg’s INAA study of particles being emitted from the stack of the Alexandria incinerator [S] . Greenberg, by using an in-stack cascade impactor [7], found that many of these stack-emitted elements are preferentially concentrated on small particles. Small particle size could indicate that the particles were formed by vaporization followed by condensation upon cooling. He further found that aluminum was distributed predominantly on larger particles typical of those released by mechanical processes. The fly ash and quench-water solids in this study would be in the category of the larger, mechanically released particles. Thus, the aluminum content would be higher and the resulting enrichment factors based on aluminum could be expected to be lower in fly ash and in quench-water solids than in the stack-emitted particles formed by condensation as observed by Greenberg.
36
Incinerator versus coal fly ash Enrichment factors were calculated for elements in the National Bureau of Standards’ Coal Fly Ash Standard Reference Material 1633, and they are listed in Table 8. This reference material is a blend of coal fly ashes supplied by five electric power plants: Tennessee Valley Authority, Stevenson, Alabama; Commonwealth Edison, Chicago, Illinois; Baltimore Gas and Electric Co., Baltimore, Maryland; Carolina Light and Power Co., Roxboro, North Carolina; and Potomac Electric Power Co., Washington, D.C. [19]. Comparison of the enrichment factors for elements in the coal fly ash with EF values for elements in the incinerator fly ash shows the incinerator fly ash to be enriched above the coal fly ash EF values by one order of magnitude or more in silver, cadmium, nickel, lead, antimony, and zinc. Only the alkali and alkaline-earth elements are less enriched in the incinerator fly ash than in the TABLE
8
Comparison of elemental enrichment factors in coal fly ash and in incinerator fly ash Element
Coal fIy ash (mg/kgja
Ratio incinerator/coal
Fly ash, enrichment factor coal
Incinerator
34 1.00 22. 2.8 1.0 8.8 2.2 1.2 2.7 1.1 3.7 0.35 1.3 0.80 0.44 0.08 1.4 2.9 21 0.42 56 3.6 0.97 2.2
1,400 1.00 31 1.6 0.54 400 5.7 11 11 0.45 19 0.28 0.73 0.45 1.5 0.43 28 310 1,100 1.1 280 0.46 4.3 110
-
& Al As Ba Ca Cd co Cr CU Fe Rg K Li Mg Mn Na Ni Ph Sb Si Sn Sr Ti Zn
3 127,000 61 2,700 47,000 1.45 41.5 131 128 62,000 0.14 16,100 62 18,000 493 3,200 98 70 6.9 210,000 270 1,700 7,400 216
41 1.00 1.4 0.57 0.54 45 2.6 9.2 4.1 0.41 5.1 0.80 0.56 0.56 3.4 5.4 20 107 52 2.6 5.0 0.13 4.4 50
aNational Bureau of Standards Coal Fly Ash SRM 1633. Certified values used where given [19]. Other values from refs.3, 7, and 18.
37
coal fly ash. This is probably due to their solubility in the incinerator processing waters. The absolute concentrations of aluminum in the coal fly ash and in the incinerator fly ash are essentially identical, 127,000 mg/kg and 121,000 mg/kg, respectively. Thus, the absolute observed enrichments and the aluminum normalized enrichments are the same in this particular case. A study of incinerated sewage sludge ash indicates that heavy metals leach out of the ash even with neutral distilled water [20]. Natural waters are often acidic, and together with microbial degradation of the incinerator ash in a landfill, high leaching rates are a possibility. The potential of environmental contamination by heavy metals warrants a thorough examination of ash disposal methods. CONCLUSIONS
The economic value of incinerator ash materials was reviewed by Sullivan and Makar [ 211. Recovered values from the Bureau of Mines incinerator residue processing pilot plant, which they describe, include ferrous products, aluminum products, nonferrous products, glass products, and waste products. The carbonaceous pilot plant filter-cake materials, analyzed in the research described in this report, were found by the Bureau of Mines to contain from 1 to 3 percent each of potash and phosphorus. Use of this fine ash as a nutritipus soil conditioner on highway embankments or median strips was suggested by Sullivan and Makar. However, extensive leach tests of these ash materials should be conducted before considering incinerator ashes as soil conditioners for produce or forage crops, and perhaps even for highway soils. Elements such as silver, arsenic, cadmium, chromium, copper, mercury, lead, antimony, tin, and zinc are from 10 to 1,000 times more concentrated relative to aluminum in the ash materials than they are in the average crustal material. Comparison of the incinerator fly ash with coal fly ash shows the incinerator fly ash to be more highly enriched in all metals except the alkali and alkaline-earth metals. The coal ash would probably be as effective as a soil conditioner and have less environmental pollution potential. Some metal values in the incinerator ash materials are comparable with low grade mineable materials. They are probably present as intractable oxides or bound by refractory silicates and alumina, making recovery difficult and uneconomical. The potential of environmental contamination, however, provides a noneconomic incentive to develop viable alternatives to landfill disposal of incinerator ash materials. The fly ash and the fine bottom-ash materials from incinerators are similar in relative metal concentrations. The elemental enrichments observed seem to be established in the initial raw refuse input and during the incineration process. The resulting fly ash and quenched bottom ash seem to differ mainly in physical size, flue gas buoyancy, and in unburned carbonaceous content, but not in metal concentrations relative to aluminum.
38
The ash materials were also found to be very similar regardless of geographic origin or period of time. However, more samples will need to be taken to establish any definite trends. ACKNOWLEDGEMENTS
The author wishes to thank Harry L. Dodson, manager of the Alexandria, Va., incinerator; R. Cecil Allnutt, manager of the Montgomery County, MD., incinerator; Robert A. Holbrook, manager of Solid Waste Reduction Center No. 1, Washington, D.C.; and Paul M. Sullivan and Harry V. Makar, U.S. Bureau of Mines, College Park Metallurgy Research Center, for their cooperation and assistance in obtaining samples and in supplying information. Assistance from the analytical staff of the College Park Metallurgy Research Center, Bureau of Mines, is also gratefully acknowledged.
REFERENCES
7 8 9
10
11 12 13 14 15
Kenahan, C.B., Sullivan, P.M., Ruppert, J.A. and Spano, E.F., 1968. Composition and characteristics of municipal incinerator residues. BuMines RI 7204. Jackson, F.R., 1974. Energy From Solid Waste. Noyes Data Corporation, Park Ridge, N.J. Law, S.L., 1976. Metals in the aqueous effluents from municipal incinerators and an incinerator-residue processing plant. Ph.D. Dissertation, Chemistry Department, University of Maryland, College Park, Maryland. Law, S.L., 1977. Dissolved metals in the aqueous effluents from municipal incinerators. J. Water Pollut. Control Fed., 49: 2454. Carotti, A.A. and Smith R.A., 1974. Gaseous emissions from minicipal incinerators. Report SW-l&, U.S. Environmental Protection Agency, Washington. Sullivan, P.M. and Stanczyk, M.H., 1971. Economics of recycling metals and minerals from urban refuse. BuMines TPR 33. Law, S.L. and Greenberg, R.R., 1976. Characterization of municipal incinerator effluents. Prog. Anal. Chem., 8: 55. Greenberg, R.R., 1976. A study of trace elements emitted on particles from municipal incinerators. Ph.D. Dissertation, Chemistry Department, University of Maryland, College Park, Maryland. Wilson, E.B. and Akers, D.J., 1970. Characterization of metropolitan incinerator refuse and fly ash. In: Proceedings of the Second Mineral Waste Utilization Symposium, U.S. Bureau of Mines and IIT Research Institute, Chicago, p. 313. Buttermore, W.J. Lawrence, W.F. and Muter, R.B., 1972. Characterization, beneficiation and utilization of municipal incinerator fly ash. In: M.A. Schwartz (Editor), Proceedings of the Third Mineral Waste Utilization Symposium, U.S. Bureau of Mines and IIT Research Institute, Chicago, pi 397. 1973 Minerals Yearbook, Vol. I. U.S. Bureau of Mines, Washington, D.C., 1975, p. 80. 1970 Minerals Yearbook, Vol. I. U.S. Bureau of Mines, Washington, D.C., 1973, p. 74. Wedepohl, K.H., 1968. In: L.H. Ahrens (Editor), Origin and Distribution of the Elements, Pergamon, London, pp. 999-1916. Mercury in air (1971). Sci. News, 99: 280. Washington Star-News, Washington, D.C., 1974. Incinerator still belching mercury into D.C. skies, Wednesday, March 13, p. B-3.
39
16 11 18
19
20 21
Billings, C.D. and Matson, W.R., 1972. Mercury emissions from coal combustion. Science, 176: 1232. Zoller, W.H., Gladney, E.S. and Duce, R.A., 1974. Atmospheric concentrations and sources of trace metals at the South Pole. Science, 183: 198. Ondov, J.M., Zoller, W.H., Olmez, I., Aras, N.K., Gordon, G.E., Rancitelli, L. A., Abel, K.H., Filby, R.H., Shah, U.R. and Ragaini, R.C., 1976. Elemental concentrations in the National Bureau of Standards’ environmental coal and fly ash standard reference materials. Anal. Chem., 47: 1102. Office of Standard Reference Materials. Standard Reference Material 1633 - Trace Elements in Coal Fly Ash, Dept. of Commerce, National Bureau of Standards, Washington, D.C. 20234. Diosady, L.L. Recycling of incinerator ash. Research Report No. 19, Ontario Ministry of the Environment, Toronto, Ontario, Canada. Sullivan, P.M. and Makar, H.V., 1976. Quality of products from Bureau of Mines resource recovery systems and suitability for recycling. In: Proceedings of the Fifth Mineral Waste Utilization Symposium, U.S. Bureau of Mines and IIT Research Institute, Chicago, p. 223.