Ind. Eng. Chem. Res. 2001, 40, 5465-5468
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Neutralization of Toxic Components in Ashes and Slags from Waste Incineration M. Modigell and J. Pro1 lss* Lehr- und Forschungsgebiet Mechanische Verfahrenstechnik (IVT), RWTH Aachen, 52064 Aachen, Germany
The most effective treatment of toxic ashes and slags from waste incineration is the melting process. Organic substances are destroyed and heavy metals partially vaporized. The product is a glassy material which is mostly resistant to leaching. Activity and mass-transfer coefficients of minority components in synthetic slags, necessary to calculate the vaporization process, were determined using a top-blowing reactor. Different experiments were carried out with varying slag composition, oxygen partial pressure, and temperature. Additionally, the slag leaching resistance was tested. The aim of the work was the development of a basis for a melting process operating calculation. Introduction Ashes and slags from waste incineration cannot be recycled without pretreatment because of a high concentration of noxious components. Hence, incineration residues are usually washed. Because leaching only affects the material surface and the washing time is limited, the total amount of noxious substances is not significantly lowered by this treatment. A more effective treatment is the melting process whereby organic substances are destroyed and heavy metals partially vaporized. The product of the melting process is a glassy material in which the remaining heavy metals are mostly fixed in a tight bond within the structure of glass, attaining a resistance to leaching. Though the melting process has been known for years as the most efficient technique for the neutralization of ashes and slags, it is not utilized. Different proposals for a technical-scale melting process have been made, and some were tested in small-scale plants.1-3 Up to now the process is not yet sufficiently understood for use in an optimized operating strategy. In particular, the minority components’ activities in the multicomponent mixtures are not known exactly. In this paper the experimental determination of activity and mass-transfer coefficients of volatile minority components (Zn, Pb, Cr, and K) in synthetic slags is presented. The experiments were conducted in a topblowing reactor, which is often used for the refining of liquid metals and slags. Experimental Section Apparatus. The apparatus as shown in Figure 1 was used for the experiments. A microbalance was used to measure the sample weight continuously. Temperatures of up to 1500 °C were adjusted by a furnace in which the whole setup was placed. A ceramic tube surrounds the sample chamber, in which a homogeneous atmosphere can be maintained. The sample itself is stirred by a top-blowing gas jet, which acts simultaneously as a carrier gas. The gas was fed through a lance in which * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (++49) 241-80-92252.
Figure 1. Apparatus for the determination of the partial pressure.
it is heated to temperatures near the furnace temperature. Experimental Conditions. Although the composition of the residue from waste incineration changes in a wide range, the main components are always silica, calcium, and aluminum oxides.4 For all experiments the same basic slag consisting of 42 wt % SiO2, 38 wt % CaO, and 20 wt % Al2O3 was used. To this mixture was added one of the minor components, such as ZnO, PbO2, Cr2O3, or KCl. For some experiments, further components (Fe2O3 and MgO) were admixed. The powdery samples were first molten to receive a homogeneous liquid slag. During the melting process, small quantities of minority components were vaporized. According to equilibrium calculation, PbO2 is transferred to PbO during melting.5 The composition of the resulting slag before and after the transpiration experiments were analyzed by an atomic absorption spectrophotometer. All experiments were conducted at temperatures between 1290 and 1430 °C. The slag weight was kept
10.1021/ie010271y CCC: $20.00 © 2001 American Chemical Society Published on Web 10/23/2001
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Table 1. Properties, Operating Conditions, and Results of the Evaporation Experiment with Zn, Pb, and K T ) 1300 °C PbO/Pb
carrier gas
n˘ 0 [mol/min]
K7
N2 + H2
0.0403 0.0667 0.0403 0.0521 0.0667
0.04843
ZnO/Zn
13.7
ω [1/min] 0.01 0.0262 0.0072 0.0169 0.0233
T ) 1430 °C
k′ [mol/min
m2]
1.902 6.077 1.902 4.517 6.077
K7
ω [1/min]
k′ [mol/min m2]
0.1375
0.0478 0.0869 0.0207
9.68 19.17 9.68
0.0411
24.17
35
T ) 1290 °C n˘ 0 [mol/min]
carrier gas PbO/PbO
air
0.0291 0.0379 0.0518 0.0291 0.0379 0.0518
K2O/K
a
K7
ω [1/min]
0.025
0.0086a
11.7
T ) 1415 °C
k′ [mol/min
m2]
1.5 1.71 1.88 1.5 1.71 1.88
0.0098a 0.0124a 0.0074 0.0085 0.0091
K7
ω [1/min]
0.097
0.0263b
13.6
k′ [mol/min m2] 7
0.0564b 0.0044 0.0074 0.0106
16.41 7 11.22 16.41
0.74-1.35 mol %. b 0.37-0.7 mol %.
constant at about 4 g. The carrier gas consisted of air, pure nitrogen, or a nitrogen/hydrogen mixture, respectively. Modified Transpiration Method. With a modified transpiration method, the activity coefficients of volatile components in a liquid mixture can be determined with high precision.6 Thereby, the concentration xi(t) of an evaporating substance is approximated by
x(t) ) x0e-ωt
(1)
Figure 2. Overall transport coefficient: measurement vs calculation (- - -).
with
ω) ns
(
γ
1 1 + k′A n˘ 0K
)
(2)
depending on the overall transport coefficient k′ and the activity coefficient γ. K is the equilibrium constant, A the phase boundary area, ns the amount of slag, and n˘ 0 the carrier gas flow rate. If the experiment is carried out with an evaporating substance “1” at particular transport conditions “a” the mole fraction x1a(t) can be calculated from the measured weight difference over time. The system is described by one equation with two unknown values k′a and γ1 depending on the material system and blowing conditions, respectively. When transport conditions (k′b) change, e.g., in the case of different gas flow rates at constant γ1, a second equation can be derived. Further experiments conducted on a second material system with a different activity coefficient γ2 but the same transport conditions (k′a and k′b) provide two additional equations. Considering all experiments, four equations with four unknown values (k′a, k′b, γ1, and γ2) are available. Hence, the system is soluble analytically. Overall Transport Coefficient Pb and Zn evaporation in a reactive carrier gas at 1300 and 1430 °C and PbO and K vaporization at 1290 and 1415 °C using a nonreactive carrier gas were evaluated. The evaporation kinetics can be described with the exponential factors ω given in Table 1. Evaluation with eq 2 leads to the specified transport coefficients k′.
To estimate the limiting mass transfer in the liquid phase, the mass-transfer coefficient of a top-blowing reactor can be derived from8
Shl ) 1.074Scl0.33Rel1.13
(3)
with
Rel ≈ Reg0.63
( ) ( ) () () We* Reg
-0.362
Fr* Reg
0.0535
ηg ηl
1.89
Fg Fl
-1.22
The correlation was developed on the basis of experiments in a reactor of semi technical scale at ambient and high temperatures. The experimentally found transport coefficients are in good agreement with this correlation, as shown in Figure 2. Activity Coefficient of Volatile Components Activity coefficients of slag components containing Zn, Pb, Cr, and K were measured at different temperatures and oxygen partial pressures (Table 2). The obtained values are the apparent activity coefficient under the assumption that the volatile species form oxide compounds in the slag. In experiments with a nitrogen/ hydrogen carrier gas, the vaporization of pure Zn and Pb was considered. In the case of an air or nitrogen carrier gas, it is expected that Pb and Cr vaporize in an oxidized state. Reyes and Gaskell9 found the activity coefficient for ZnO to be 0.30 at 1430 °C, which is in good agreement with our experimentally determined value of 0.354. The evaporation rate of lead in air deviates from the exponential trend by changing PbO fractions in the melt. This is accompanied with a change of the activity
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5467 Table 2. Apparent Activity Coefficients of Slag Components Containing Zn, Pb, Cr, and Ka T ) 1300 °C PbO ZnO
T ) 1430 °C
carrier gas
mole fraction [%]
γ
mole fraction [%]
γ
N2 + H2 N2 + H2
0.43-1.9 0.7-1.9
1.15 0.617
0.036-0.16 1.3-2.9
1.32 0.354
T ) 1290 °C PbO Cr2O3 K2O K2O + 10 wt % Fe2O3 K2O + 2 wt % MgO a
T ) 1415 °C
carrier gas
mole fraction [%]
γ
mole fraction [%]
γ
N2 + O2 N2 N2
0.1-1.35 0.15-0.72 0.8-1.25
see Figure 3 1.8 ( 0.45 687
N 2 + O2
0.8-1.6
0.811 0.715 0.638
0.03-0.7 0.1-0.3 1.2-1.6 0.8-1.2 0.8-1.6
see Figure 3 0.969 ( 0.44 198 351 0.109
Initial slag composition: 42 % SiO2, 38 % CaO, and 20 % Al2O3-slag.
Figure 3. Activity coefficient of PbO as a function of the PbO concentration.
coefficient of PbO. The dependence of γ from the concentration is shown in Figure 3. The influence of Fe2O3 and MgO addition on the activity of minority components is studied exemplary with potassium. The experiments performed at 1290 °C with air as the carrier gas indicate that the activity coefficient of potassium is decreased by both components (Table 2). Experiments with a Technical Slag Mixture To prove the transferability of evaporation relations to complex slag mixtures with several volatilizable components, an experiment with a mixture similar to typical ashes from waste incineration was performed at 1290 °C. On the basis of the previously derived activity coefficients, the evaporation was calculated and compared then with the experimental results. For the calculation, the interaction between different minority components and the influence of Fe2O3 and MgO on evaporation rates were disregarded. It is found that the calculation results are in good agreement with the experimental data. The leaching behavior of a molten slag was tested at standard leaching conditions (DIN 38 414 S4). The slag composition was identical to the initial composition of the sample treated in the above-described evaporation process. All heavy-metal concentrations in the leaching solution have been found to be lower than the detection limit of 0.005 mg/l (0.025 mg/l for Zn). The pH value changes from 9.47 at the beginning to 7.4 at the end. It is assumed that KCl dissolves, which has no verifiable effect on the solubility of the heavy metals. The leaching resistance of the glassy slag was found to be much better in comparison to that of treated and untreated ashes from waste incineration.6 Conclusion The melting of oxide mixtures comparable to ashes from waste incineration in a melting process leads to
vaporization of heavy metals and produces a glassy slag. The aim of this work was the determination of activity and mass-transfer coefficients of volatile components in such a slag treated in a top-blowing reactor. The activities of Zn, Pb, Cr, and K contained in a slag in the form of oxides were determined for different slag compositions at different oxygen partial pressures and temperatures varied from 1290 to 1430 °C. The experiments indicate that the activity coefficients of the volatile metal oxides are independent from the concentrations of the minority components. Only the activity coefficient of lead oxide is found to vary with the Pb concentration. Activity coefficient data given in the literature9 are in accordance with the experimentally determined values. In addition to activity coefficients, the mass-transfer coefficients prevailing in the smelting reactor were also determined. The derived values can be approximated by a correlation given in the literature.8 The results my serve as a basis for the calculation of the melting process and its optimization. Finally the leaching behavior of the glassy slag produced in a melting process was tested. The slag was found to be leaching-resistant in reference to the metals mentioned above. Nomenclature A ) phase boundary area [m2] D ) diffusion coefficient [m2/s] g ) gravitational constant [m/s2] k′ ) transfer coefficient [mol/m2 min] K ) equilibrium constant L ) distance nozzle-surface [m] n ) amount of substance [mol] n˘ ) flow rate [mol/min] p˘ ) impulse flow rate [kgm/s2] t ) time [min] T ) temperature [°C] x ) mole fraction [mol %] γ ) activity coefficient η ) viscosity [Pa s] F ) density [kg/m2] σ ) surface tension [kg/s2] ω ) exponential coefficient Fr* ) p˘ /L3gFg ) modified Froude number Reg ) p˘ Fg/ηg2 ) Reynolds number of the jet Sc ) η/DF ) Schmidt number Sh ) k′L/D ) Sherwood number We* ) p˘ /Lσ ) modified Weber number
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Indices g ) gas i ) component i l ) liquid s ) slag 0 ) carrier gas
Literature Cited (1) Hirth, M.; Wieckert, Ch.; Jochum, J.; Jodeit, H. Ein thermisches Entgiftungsverfahren fu¨r Filtersta¨ube aus Mu¨llverbrennungsanlagen. Mu¨ llverbrennung und Umwelt 3; EF-Verlag fu¨r Energie und Umwelttechnik: Berlin, 1989. (2) Klein, H. Plasmatechnik der Krupp MaKsReststoffe aus der Rauchgasreinigung. Mu¨ ll Abfall., Beih. 1990, 29, 119. (3) Schumacher, W.; Gugat, J.-A. EloMelt- und FosMelt-Verfahren Thermische Behandlungskonzepte fu¨r Reststoffe aus der Mu¨llverbrennung. Mu¨ ll Abfall., Beih. 1994, 31, 152. (4) Reimann, D. O. Menge, Beschaffenheit und Verwertungsmo¨glichkeiten von MV-SchlackensGesamtu¨bersicht. Mu¨ ll Abfall., Beih. 1994, 31, 30.
(5) Ericson, G.; Hack, K. ChemSagesA Computer Program for the Calculation of Complex Chemical Equilibrium. Metall. Trans. B 1990, 21B, 1013. (6) Modigell, M.; Pro¨lss, J. Melting process for the treatment of toxic ashes and slags from waste incineration. Proc. ECCE 2001 2001. (7) Barin, I. Thermodynamical Data of Pure Substances; VCH: Weinheim, Germany, 1989. (8) Modigell, M.; Barin, I. Rate Phenomena in Top-Blowing Reactors. Proceedings of Technological Advances in Metallurgy, Lulea, 1988. (9) Reyes, R. A.; Gaskell, D. R. The Thermodynamic Activity of ZnO in Silicate Melts. Metall. Trans. B 1983, 14B, 725.
Received for review March 27, 2001 Revised manuscript received August 21, 2001 Accepted August 23, 2001 IE010271Y
