Environ. Sci. Techno/. 1995, 29, 2429-2436
Evaporation of Heavy Metals durhg the Heat Treatment of Municipal Solid Waste Incinerator Fly Ash A . J A K O B , * S . S T U C K I , AND P . K U H N Paul Scherrer Institute Laboratory of Energy and Process Technology, CH-5232 Villigen, Switzerland
Thermal treatment is a promising way for the decontamination and inertization of residues from waste incineration. The evaporation of heavy metals thereby is of great significance. It is the goal of this work to investigate the fundamental aspects of the evaporation of heavy metals in the heat treatment process and to determine the process parameters leading to complete evaporation of the relevant heavy metals. Evaporation experiments in different atmospheres were carried out with filter ash from municipal solid waste incineration. The quantities of the heavy metals Zn, Pb, Cd, and Cu evaporated as a function of time were measured at temperatures between 670 and 1300 "C; evaporation turned out to be most effective a t temperatures just below the melting range of the residue (Le., a t 1000-1 100 "C)and decreased drastically above this temperature range. The amounts of evaporation (relative to the contents in untreated filter ash) at about 1100 "C were 98-100% of Pb, Cd, and Cu and 50% of Zn in air and 98-100% of Pb, Cd, and Zn and 10% of Cu in argon atmosphere, respectively. Results of experiments using model systems indicate that the decrease in the Zn evaporation at high temperatures is caused by the formation of compounds like ZnZSi04 and ZnA1204. The results of the experiments in argon atmosphere are explained thermodynamically by the reductive potential of the carbon, contained in the residue.
Introduction Landfilling is the cheapest way of disposing of municipal solid waste (MSW), inasmuch as long-term effects and external costs are not considered. The chemical reactions taking place over residence times of years in a reactive multicomponent system such as a MSW landfill cannot be controlled or predicted accurately. Release of toxic compounds is therefore likely to take place, possibly years or decades after deposition. There is a considerable risk that environmental problems may result from contaminated sites in future generations. This is, apart from increasingly scarce landfilling space, the reason why an increasing fraction of the municipal solid waste produced in western * Fax: 056199-21-99; e-mail address:
[email protected].
0013-936X/95/0929-2429$09.00/0
(C 1995 American
Chemical Society
Europe is being incinerated. In comparison to direct landfilling, incineration has two main advantages by reducing the volume of MSW by about 90%and by reducing its reactivity by the nearly complete destruction of organic compounds. In spite of these advantages, the incineration of MSW produces residues, mainly the bottom ash [lo%of the volume of the waste or about 250-300 kg/lOOO kg of waste (1-4)], the hazardous filter ash [about 25-30 kg/ 1000 kg of waste (1- 4 ) ] ,and additional products of the flue gas cleaning processes (dry andlor wet scrubbing). The filter ash is especially a problematic residue because it contains high concentrations of heavy metals (5) beside trace amounts of PCDD and PCDF (polychlordibenzodioxins and -furans) (6, 7). Although bottom ash has been used for road construction, it is expected that in the near future this will no longer be tolerated as more and more stringent regulations regarding reutilization of residues will be put forward. According to Swiss regulations, bottom ash has to be deposited into a landfii that provides tight control of effluents. Filter ashes have to be detoxified or stored in safe but extremely expensive repositories. The costs of dealing with residues as well as the increasing cost of mineral prospection and mining will, on a long term, eventually lead to processes that are able to recover valuable resources, such as rare heavy metals, in a recyclable form. Several new technologies, based on immobilization with cement, wet chemical treatment, or thermal treatment (8, 91, are in development that try to decontaminate toxic residues and/or make them inert in the sense that they could be reused or at least be deposited without any risk. Of great promise are thermal processes (4,8,9),which are melting the residues at temperatures around 1300- 1400 "C and producing a relatively inert glass. The high temperatures and long residence times of such processes lead to complete destruction of toxic organic compounds. The heavy metals are either incorporated in the vitrified residue or separated from the residue by evaporation or by different densities of the melted constituents. Obviously, inherent safety for the glass product is achieved if the heavy metals are extracted quantitatively from the residue. The recovered heavy metal compounds themselves can be reutilized as raw materials in metallurgical processes. Thermal treatment offers a possible way of combining the separation of heavy metals with vitrification of the residues. The major toxic elements in filter residues, which are considered in the relevant Swiss waste disposal regulations, are Cu, Cd, Pb, and Zn, which are volatile to different degrees at elevated temperatures. It is the goal of the present work to contribute to a better understanding of the physicochemical processes underlying the evaporation of heavy metal compounds and to determine the relevant process parameters leading to their complete evaporation. While complete evaporation of Pb and Cd has been observed in the process ofvitrification,the behavior of zinc has been found to be problematic: despite its high volatility, only about 50% of this heavy metal evaporates by heat treatment in air (10). In the present work, the fundamental aspects of the evaporation of the heavy metals zinc, lead, cadmium, and copper in the heat treatment of filter ash from municipal solid waste incineration have been investigated.
VOL. 29, NO. 9,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
2429
TABLE 1
Elementary Composition of Filter Ash As Received (FU) and Fine-Grained Filter Ash (FGP content (weight %) Mb
Eb
A9 AI As Ba Bi C Ca Cd CI co Cr cs cu F Fe
B
ABC
B BC B D ABC
B B B AB
B B B ABC
content (weight 70)
FU
FG
E
-a? ! a CI
0
1
2
3
4
5
6
7
E
io0
700
900
600
1000
1100
1200
1300
temperature ["C]
time of evaporation [h]
-
b
b
c
&? 100 Y
C 0
'E 80
P
0 P 0
5
60
c 0 U
C
3
40
0
E,
._a> 0)
20
c
2
0
0
1
2
3
4
5
6
7
E
600
700
800
t h e of evaporation [h]
Y
E0
1200
1300
TABLE 2
a
Composition of Synthetic Filter Ashes
60
synthetic filter ash a
c 0
composition
CI
3 0
1100
80
P
5
1000
FlGURE6. Maximum amount of heavy metal evaporationas a function of temperature by thermal treatment of (a) filter ash as received in an Arm2 atmosphere (7% H2) and (b) fine-grained filter ash in pure Ar.
g 100 .-:
-
900
temperature ["C]
40
species
!l20 .-e
-a? ! o
Si02 CaO A1203
U
0
1
2
3
4
5
6
7
time of evaporation [h]
FIGURE 5. Relative amount of evaporation of the heavy metals Cd, Cu, Pb, and Zn as a function of time by heat treatment of filter ash as received in an Ar/H2 atmosphere (7% H2) at (a) 760, (b) 940,end (c) 1130 "C.
chloride content of the material (see Table l),it is most probable that in air the heavy metals are evaporating as chlorides. This may happen in two ways: either the heavy metals exist as chlorides in the filter ash or the heavy metal chlorides are formed in the filter ash by the heat treatment. A hint that the chlorides can actually be formed from the heavy metal oxides by heat treatment is given by the evaporation experiments with the synthetic filter ashes a and b. In these experiments, evaporation is detectable only if chlorine (in the synthetic filter ash b as CaC12)is added to the heavy metal oxides (Figure 7 ) . However, Figure 7 2434
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 2 9 , NO. 9.1995
CdO CUO PbO ZnO CaClz.2HzO
(9)
(rno1/10-2)
8.373 6.884 4.711 0.052 0.1 18 1.053 3.366
13.935 12.275 4.620 0.040 0.148 0.472 4.147
synthetic filter ash b composition (gl (mo1/10-2) 8.353 6.961 4.697 0.010 0.023 0.166 0.674 3.092
13.902 12.413 4.607 0.008 0.029 0.074 0.828 2.104
shows that, unlike the results with fine-grained filter ash at the same temperature (Figure3b),the evaporation is not complete for either of the metals with synthetic filter ash b. Zn is found to show incomplete evaporation in air, at all temperatures, and with alldifferent samples investigated. This behavior may be explained qualitatively by two hypotheses: (1)The oxides react with the added chloride to form volatile compounds as well as with matrix compounds (Le.,Si02 orAlzOs)to form more stable compounds. The two competing reactions taking place at the same time could explain the limited degree of evaporation. (2) The effect is caused by the degrees of dispersion of the heavy
100
,
.
1
'
1
'
1
'
l
'
l
'
l
'
I
I 0
1
2
3
4
5
6
.700 400
7
time of evaporation [h]
FIGURE 7. Relative amount of evaporation of the heavy metals Cd, Cu, Pb, and Zn as a function of time by heat treatment of the synthetic filter ash b in air at 940 "C.
metal compounds within the matrix, i.e., by transport limitations of reaction partners. The quantitative differences in the maximum evaporation levels observed with the filter ash as received, the finegrained filter ash, and the synthetic mixture b are most likely due to a dispersion effect (hypothesis 2). On the other hand, the formation of zinc orthosilicate (ZnzSi04, willemite) and zinc spinell (ZnAl2O4),which we found after heating ZnO and the respective oxides to 1150 "C, is clearly a hint that in fact solid-state chemical reactions leading to highly stable compounds may play an important role in determining the maximum degree of evaporation at elevated temperatures (hypothesis 1). The formation of Zn2Si04and ZnAlzO4 could explain the drastic reduction in the Zn evaporation at temperatures equal or higher than the melting range of the filter ash. In this temperature range, the reacting species become more mobile, which results in an enhanced rate of formation of Zn2Si04 and ZnAlzO4, and by this to a reduced evaporation of Zn. The evaporation characteristics of Cu (Figure 3) may, most probably, be understood in the same way as the Zn evaporation, but in order to clarify the behavior of the Cu evaporation, specific model experiments have to be performed. More model experiments with synthetic filter ashes are necessary to investigate in detail the chemical and physical processes that lead to the evaporation of volatile heavy metal species in the temperature range of interest. Evaporation Experiments in ArlHz and Ar Atmospheres. Evaporation experiments in the Ar/Hz atmosphere were carried out in order to find out whether Zn could be evaporated completely by reducing it to the metallic state. Indeed, the evaporation experiments performed in Ar/H2 atmosphere show nearly complete Zn evaporation if the temperature was high enough (Figure 6a). Surprisingly, the same result was obtained when no HZwas added to the atmosphere, Le., with the evaporation experiments in pure Ar (Figure 6b). This finding can be explained by the reducing potential of carbon (most probably the largest part of the 4% carbon in the filter ash is in a reduced form), which in the evaporation tests performed in air is burning off. The Gibbs free enthalpy diagram of Figure 8 shows that, at temperatures over about 660 "C, the curve of the reaction 2H2 0 2 = 2H20is placed at higher levels of A C than the curve of the reaction 2C 0 2 = 2CO. This means
+
+
'
'
600
'
800
1000
1200
1400
1600
1800
2000
temperature ["C]
FIGURE 8. Gibbs free enthalpy of oxide formations as a function of temperature.
that, at temperatures over 660 "C, carbon has a stronger reducing potential than hydrogen. Thus, in the temperature range of interest, the predominant reducing agent is the carbon within the filter ash, and hence the two thermal treatment experiments, untreated filter ash in Ar/H2 and fine-grained filter ash i n k , respectively, are essentially the same with respect to reducing properties and can be compared directly. Whereas Pb and Cd are both found to evaporate completely irrespective of the gas atmosphere (oxidizing or inert/reducing), there are very marked differences for Cu and Zn. In the discussion of the evaporation of Cu and Zn, let us first consider the behavior of Cu. Our experiments reveal that Cu hardly evaporates under reducing conditions (Figure6). Cu compounds are readilyreduced to elemental Cu (boiling point at 2573 "C!) by either H:! or C in the temperature range of interest (cf. the relative position of the AGO lines for the oxidation of Cu, C, and H2 in Figure 8).
The maximum amount of evaporation of Zn from filter ash under reducing conditions has been found to increase from lower than20% at 750 "C to 100%at about 950-1130 "C (Figure 6). The complete evaporation of Zn at a temperature above 950 "C (Fig.6b) suggests that ZnO is the predominant Zn species in filter ash and that the formation of ZnzSiO4 and ZnAl2O4is not relevant below 950 "C. The following thermodynamic argumentation will illustrate this point: In Figure 8, the AGO vs temperature curve of the reaction 2Zn 02 = 2Zn0 crosses the curve of the reaction 2C O2 = 2CO at a temperature of about 950 "C. This means that ZnO can be reduced by carbon at temperatures above 950 OC to elemental Zn, which will be volatilized. If Zn was present in the form of willemite or spinell, not 100% evaporation of the Zn would be possible at 950 "C as the corresponding AG" curves cross the carbon line at 1010 and 1070 "C, respectively. Combining the results of the experiments performed in oxidizing and reducing atmospheres allows the design of a thermal process, which will eliminate by more than 99% all of the Cu, Pb, Cd, and Zn contents of filter asheswithout vitrification, leading to a clean and inherently safe residue and to a small fraction of heavy metal condensates that should be suitable for recycling.
+
+
Acknowledgments This work was performed within the Priority Programme Environment of the Swiss National Science Foundation. VOL. 29, NO. 9. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The financial assistance of this foundation is gratefully acknowledged. The authors owe a thanks to Mr. R. Keil (PSI)and to Mrs. M. Guecheva and Dr. H. Vonmont (EMPADubendorf) for the ICP-AES measurements as well as to Mr. A. Schuler (PSI) for technical assistance with the evaporation experiments.
Literature Cited (1) Belevi, H. In Techniken der Restmiillbehandlung: kalte una7 oder thermische Verfahren;WiirzburglVeitshochheirn,April 2021, 1993: VDI Gesellschaft Energietechnik, VDI-Verlag: Diisseldorf, 1993, p p 261-276. (2) Baccini, P.; Brunner, P. H. Gas-Wasser-Abwasser1985,65,403409. (3) Brunner, P. H. Mull Abfall 1989, 21 (41, 166-180. (4) Stark, R. In Umweltschutz, Wie? ReststofPverwertung a m der thermischen Abfallbehandlung; Gutke, K., Ed.; Nov 4-5, 1993; K. Gutke Verlag: Hagen, 1993; pp 299-337. (5) Law, S. L.; Gordon, G. E. Environ. Sci. Technol. 1979, 13, 432438. ( 6 ) Olie, K.; Verrneulen, P. L.; Hutzinger, 0. Chemosphere 1977, 6, 455-459. (7) Buser, H. R.; Bosshardt, H. P.; Rappe, Ch. Chemosphere 1978, 7, 165-172.
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(8) Faulstich, M. In Muterialrecycling durch Abfallaufbereitung; Thorn6-Kozrniensky, K.-J., Ed.; EF-Verlag fiir Energie- und Urnwelttechnik Berlin, 1992; pp 465-483. (9) Faulstich, M. In Umweltschutz,Wie?Reststoffverwertungausder thermischen Abfallbehandlung Gutke, K., Ed.; Nov 4-5, 1993; K. Gutke Verlag: Hagen, 1993; pp 49-79. (10) Hirth, M.; Jochurn, J.; Jodeit, H.; Wieckert, Ch. In Proceedings of Envirotech; Zirrn, K. L., Mayer, J., Ed.; Vienna 1989; Vol. 2 (Part 2) Urnweltbundesarnt Wien; Elsevier Science Publishers: Amsterdam, 1989; pp 267-281. (11) Tafel, V.; Sille, G. Metall Erz 1930, 27, 338-341. (12) Kitchener, J. A,; Ignatowicz, S. Trans. Faraday Soc. 1951, 47, 1278-1286. (13) Baud, P.; Brusset, H.; Joussot-Dubien,J.; Larnure, J.; Pascale, P. Nouveau Trait6 de Chimie Minkrale, Tomme V,Zinc - Cadmium - Mercure; Pascal, P., Ed.; Masson et Cie: Paris, 1962. (14) Barin, I.; Knacke, 0. Thermochemical properties of inorganic substances; Springer-Verlag: Heidelberg and New York, 1973.
Received for review March 1, 1995. Accepted June 3, 1995.@
ES950140M @Abstractpublished in Advance ACS Abstracts, August 1, 1995.
