Complete Heavy Metal Removal from Fly Ash by Heat Treatment

Environ. Sci. Technol. 1996, 30, 3275-3283

Complete Heavy Metal Removal from Fly Ash by Heat Treatment: Influence of Chlorides on Evaporation Rates A. JAKOB,* S. STUCKI, AND R. P. W. J. STRUIS Paul Scherrer Institute (PSI), General Energy Research, Element Cycles CH-5232 Villigen (PSI), Switzerland

Thermal treatment is a promising way for the decontamination and inertization of residues from waste incineration. The evaporation of heavy metal compounds thereby is of great significance. It is the goal of this work to identify, by analyzing evaporation rates, the predominant thermochemical reactions of the heavy metals with other constituents of fly ash, with respect to volatilization. To this end, experiments were performed with fly ash from a municipal solid waste (MSW) incineration plant as well as with synthetic powder mixtures in the temperature range of 670-1000 °C. The rates of Cd, Cu, Pb, and Zn evaporation can be described accurately by a simple first-order rate law and a rate coefficient which itself follows an exponential temperature dependence analogous to the Arrhenius equation. The degrees (completeness) as well as the rates of evaporation of the heavy metals are markedly influenced by chlorides contained in the fly ash, largely as NaCl. Experiments with model substrates indicate that the heavy metals Zn and Cu in fly ash, which are the least volatile among the group investigated, are predominantly present as chlorides. Their evaporation is completed by shifting the oxide/chloride equilibrium if surplus chlorine, e.g., in the form of NaCl, is available. The heavy metal evaporations are probably limited by reactions that form heavy metal silica/alumina compounds.

Introduction The residues from municipal solid waste (MSW) incineration, mainly bottom ash and fly ash (1-5), contain considerable amounts of harmful heavy metals (6) and have hence to be deposited into specially designed repositories. From the point of view of sustainable development, element cycles should be closed as much as possible. Technologies for waste treatment will ultimately have to lead to re-usable residues only. Recycling of incineration residues is, at least in Switzerland, only possible if the toxic heavy metals are * Corresponding author fax: +41 56 310 21 99; e-mail address: [email protected].

S0013-936X(96)00059-4 CCC: $12.00

 1996 American Chemical Society

completely separated from the nontoxic solids and fed back into the metal cycle. In a previous paper, we showed experimentally that the four relevant heavy metals Zn, Pb, Cu, and Cd can be separated nearly completely by thermal treatment of fly ash from MSW incinerators provided that the treatment temperature is chosen high enough but not above the melting range of the fly ash (7). The experimental results have led to the conclusion that the mechanisms leading to an evaporation of the heavy metals compete with chemical reactions between the heavy metal compounds and the aluminium-sodium-silicate matrix of the fly ash to form more stable compounds. Furthermore, thermal treatment tests with fly ash in air as well as first experiments with model substances showed that the chlorine content in the fly ash can influence the evaporation behavior of the heavy metals decisively. In the present work, the influence of chlorides on the volatility of the heavy metals Zn, Pb, Cu, and Cd in air at temperatures between 840 and 1000 °C was investigated. To identify the most important parameters, we attempted to simulate the thermal behavior of the complex mixture of materials present in MSW fly ash with respect to heavy metal evaporation kinetics, by simplified model substances (synthetic fly ashes). Simple first-order rate laws were applied to describe the overall kinetics of the evaporation of the heavy metals. The characteristic parameters of the rate expressions serve as a tool for comparing the real fly ash with the different model substances.

Methods Materials. Fly Ash. Thermal treatment tests were performed using the same fly ash samples from the municipal solid waste incineration plant in Hinwil, Switzerland, as described in our previous investigation (7). One batch of fly ash, in the following referred to as “untreated fly ash” (FU), was homogenized only, whereas the other batch, in the following referred to as “fine-grained fly ash” (FG), was ground and homogenized. The compositions of these two samples are listed in Table 1 as far as it is relevant for this work. Except for the chlorine, the samples were analyzed by EMPA Du ¨ bendorf using X-ray fluorescence (XRF) and inductively coupled plasma atomic emission spectoscopy (ICP-AES). Complete analysis as well as particle size distributions are given in ref 7. Under the assumption that all chlorine in fly ash is present as soluble chlorides, its content was measured by eluation: about 2 g of the fly ash was extracted during 30 min in 100 mL of boiling water. The eluate solution was analyzed by ion chromatography. The uncertainty of the chlorine content is (5%. Synthetic Fly Ash. Model powder mixtures, in the following called synthetic fly ashes (SyF), were prepared by wet mixing and grinding the different powder mixtures with small amounts of water or hexane in a planetary ball mill and subsequent drying. Three groups of synthetic fly ashes were prepared: (I) matrix + NaCl; (II) matrix + heavy metal oxides + NaCl; (III) matrix + heavy metal chlorides (+ NaCl). The quantities of the different compounds were chosen such that they correspond to the absolute concentrations of Cd, Cu, Pb, Zn, and Cl and the relative contents of Al: Ca:Si of fly ash FG, except in the synthetic fly ashes SyF1

VOL. 30, NO. 11, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1

Elementary Compositon of Fine-Grained (FG) and Untreated (FU) Fly Ash element [wt %] Cd Cu Pb

fly ash

Al

Ca

Si

FG FU

6.2 6.8

13.4 13.6

10.5 10.7

a

0.047 0.044

0.1 0.093

0.86 0.77

Zn

Na

Cla

3.0 2.7

2.7 2.9

6.0

Measured at PSI by ion chromatography after chlorine eluation.

TABLE 2

Nominal Compositions of Synthetic Fly Ashes weights (g) components/samples

SyF1

Al2O3 CaO Ca(OH)2 SiO2

SyF2

1.994 3.193 3.824

SyF3

Matrix 3.903 5.419 8.253 7.476

SyF4

SyF5

1.754

1.833

3.710 3.364

3.877 3.515

Heavy Metal Oxides 0.015 0.035 0.252 1.701 1.014

CdO CuO PbO ZnO

Mm,T(t) ) am,T (1 - e-km,T t)

Heavy Metal Chlorides

CdCl2‚H2O CuCl2‚2H2O PbCl2 ZnCl2

0.0096 0.0272 0.116 0.643 Alkaline Chloride 0.989 2.689 0.887

NaCl total amount

adjusted by bubbling the argon gas stream through a gas washing flask that was filled with water and thermostated to 21.3 °C. The gas flow was in all experiments kept at 20 L/h. Experimental Techniques. Evaporation experiments were carried out using a tube furnace with a water-cooled cooling finger, which could be replaced during an experiment, and a control unit for the purge gas flow. A detailed description of the experimental setup as well as of the principal interpretation of the heavy metal evaporation experiments is given in ref 7. The analysis of the chlorine content followed that described earlier for fly ash, now using sample amounts of about 0.5 g. Investigations concerning the speciation of the heavy metal compounds in the cooling finger condensate as well as in the annealed specimen were carried out by X-ray powder diffraction (Philips X’Pert). Data Analysis for Quantifying Rates of Heavy Metal Evaporation. It turns out that, in all experiments using fly ash or synthetic mixtures, the amount M of evaporation of the heavy metal m as a function of time t at a given temperature T can be approximated by the following simple first-order rate law:

0.0128 0.0266 0.115 0.646

0.380

10.000 23.637 8.007 10.004

10.025

TABLE 3

Measured Heavy Metal Contents of Synthetic Fly Ashes

where am,T denotes the asymptotic value and km,T means the rate coefficient. The asymptotic value am,T describes the maximum amount of evaporation of the heavy metal m that will be reached after an infinitely long annealing time at temperature T. M is given in percent, i.e., the amount of heavy metal evaporated is taken relative to the total heavy metal content in the fly ash before thermal treatment. The temperature dependence of the evaporation rates is given by the following exponential of the rate coefficient km,T, resembling the Arrhenius equation, where k0,m and Em are constant parameters and R is the universal gas constant:

measured heavy metal contents (wt %) fly ash SyF1 SyF2 SyF3 SyF4 SyF5

Cd

Cu

Pb

Zn

0.051

0.104

3.081

0.053 0.068

0.099 0.097

0.870 20.407 0.815 0.811

2.837 2.789

and SyF3. Table 2 shows the nominal compositions of the different synthetic fly ashes as weighed. To determine the heavy metal contents in the synthetic fly ash samples after homogenization, about 100 mg of each was completely dissolved in 2 mL of HNO3 (65%) + 2 mL of HF (40%) in an acid digestion bomb, either at 190 °C during 6 h in an oven or in a microwave digestion unit during 30 min. After the digestion, 40 mL of boric acid (H3BO3, 2.5%) was added and diluted with water to 100 mL. The heavy metal analysis of the solutions was performed by ICP-AES using standard solutions of the same acid mixtures as reference. The results are shown in Table 3; the medium value of three separate digestions are tabulated. The scatter in heavy metal contents of the separate digestions (1-2%) were smaller than the uncertainty of the ICP-AES measurement (5%). Evaporation experiments were performed in air (compressed laboratory air), argon, and in an argon/2.5 vol % water vapor mixture. The water content in the argon was

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(1)

km,T ) k0,me-Em/RT

(2)

For determining Em, the linearized form of eq 2 is useful. Em thereby is derived from the slope in the diagram ln (km,T) vs 1/T. It has to be mentioned that the equations listed above serve only as a tool to condense and compare the numerous experimental data; no definite conclusions about reaction mechanisms are possible on the basis of curve fitting.

Results Fly Ash. Evaporation experiments with untreated (FU) and fine-grained (FG) fly ash were performed in an airstream at temperatures of 670-1000 °C. Figure 1 shows the relative amounts of the heavy metals Cd, Cu, Pb, and Zn, and Figure 2 shows the relative amounts of chloride evaporating from fine-grained fly ash, as a function of time at temperatures of 750, 840, and 920 °C, respectively (amounts present in the starting materials ) 100%). The experimental data of heavy metal and chloride evaporation are fitted by the simple first-order rate law according to eq 1. In analogy to the heavy metal evaporation, the chloride evaporation gets faster with increasing temperature (Figure 2). The results shown in Figures 1a-c and 2 reveal that the experimental data of the heavy metal evaporation is well fitted by the exponential function (1); furthermore, it can

FIGURE 2. Relative amount of Cl evaporation as a function of time by the thermal treatment of fine-grained fly ash (FG) in air at 750, 840, and 920 °C.

FIGURE 3. Rate coefficients k of the evaporation of Cd, Cu, Pb, and Zn by the thermal treatment of untreated (FU) and fine-grained (FG) fly ash in air.

FIGURE 1. Relative amount of evaporation of the heavy metals Cd, Cu, Pb, and Zn as a function of time by the thermal treatment of fine-grained fly ash (FG) in air at (a) 750, (b) 840, and (c) 920 °C.

be observed that the time when the heavy metal evaporation curves level off coincides rather well with the disappearence of residual chlorine. The respective annealing times are at 750 °C, >6 h; at 840 °C, 3-4 h; and at 920 °C, about 1.5 h. Note, however, that the evaporation curves are normalized with respect to the amounts present in the starting materials. In absolute numbers (i.e., multiplying the relative amounts with the contents in the fly ash) the molar amount of chlorine (taken as Cl2) evaporating exceeds the total molar amount of heavy metals evaporating during the same time interval by a factor of 3-3.5.

In addition to the experiments with fine-grained fly ash, evaporation experiments with untreated fly ash were performed and analyzed in the same way. Table 4 shows the asymptotic values am,T, the rate coefficients km,T, and the correlation coefficients R 2 of the curve fittings of the different experiments. The values set in parantheses are neglected for further evaluations. For these data points (element/temperature) at low temperatures, the evaporation duration was too short to produce reliable information about the asymptotic value am,T, and alternatively, at high temperatures, it was impossible to set the interval of measurement, i.e., the changing of the cooling finger, short enough to measure the fast evaporation of the heavy metals at the beginning of the annealing experiment. In general, one can see that the maximum amounts of heavy metal evaporation (am,T) are lower with untreated fly ash than with fine-grained fly ash. In Figure 3, the natural logarithms of the evaporation rate coefficients in fine-grained and untreated fly ash are shown as a function of the inverse absolute temperature. It turns out that the data points of the separate heavy metals lie, within the measurement uncertainty, on a straight line. The slopes Em obtained by


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TABLE 4

Parameters of Simple First-Order Rate Law of Heavy Metal Evaporation with FG and FU parameters of the M vs t curve fitting

a (%)

k (h-1)

R2

temp (°C)

element

670

Cd Cu Pb Zn Cd Cu Pb Zn Cd Cu Pb Zn Cd Cu Pb Zn

Fine-Grained Fly Ash (FG) 95.00 0.18 (2071) (0.003) 95.82 0.98 11.09 0.40 95.87 0.82 102.36 0.26 97.22 3.42 19.88 1.06 96.15 4.15 93.80 1.27 98.06 14.48 34.16 2.93 (97) (8) 96.97 3.17 (98) (15) (43) (4)

0.9885 (0.9921) 0.9958 0.9975 0.9948 0.9970 0.9946 0.9949 0.9971 0.9993 0.9998 0.9913 (0.9997) 0.9854 (0.9998) (0.9965)

Cd Cu Pb Zn Cd Cu Pb Zn Cd Cu Pb Zn Cd Cu Pb Zn Cd Cu Pb Zn

Untreated Fly Ash (FU) (152) (0.06) (231) (0.01) 92.16 0.82 (4) (0.4) 95.24 1.15 76.02 0.34 92.96 5.69 12.97 1.60 96.73 5.65 63.20 1.50 92.73 9.64 23.79 3.10 95.88 9.71 68.28 3.03 (94) (11) 32.63 4.74 (97) (7) 80.93 8.34 (96) (14) 38.04 9.08

(0.9993) (0.9948) 0.9981 (0.9967) 0.9987 0.9994 0.9988 0.9944 0.9985 0.9914 0.9971 0.9922 0.9989 0.9983 (0.9992) 0.9977 (0.9996) 0.9995 (0.9999) 1.0000

750

840

920

670

750

840

920

1000

TABLE 6

TABLE 5

Parameters of Linear Regression ln (k) vs 1/T of Fly Ash parameters of the linear regression (FG + FU) element

E (kJ/mol)

ln (k0)

R2

Cd Cu Pb Zn

154.25 ( 10.97 141.64 ( 4.61 132.81 ( 14.34 90.35 ( 6.05

18.08 ( 1.25 15.49 ( 0.49 16.92 ( 1.69 10.79 ( 0.67

0.9779 0.9883 0.9531 0.9736

linear regression are given in Table 5, together with the values of the constant ln (k0) and the correlation coefficients R 2. Synthetic Fly Ash. The experimental conditions of temperature and atmosphere of the evaporation tests with synthetic fly ashes are summarized in Table 6. (I) Matrix + NaCl, SyF1. The simple synthetic fly ash SyF1 was prepared to simulate the evaporation of chlorides in fly ash. The chloride evaporations are shown in Figure 4 as a function of time, together with the respective values of the fine-grained fly ash. At 840 °C the chloride evaporation is faster with FG than with SyF1. Especially in the beginning of the evaporation experiment, SyF1 is able to simulate the FG at 920 and 1000 °C.

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FIGURE 4. Relative amount of Cl evaporation in fine-grained fly ash (FG) and synthetic fly ash SyF1 as a function of time by the thermal treatment in air at 840, 920, and 1000 °C.


Experimental Conditions of Evaporation Tests with Synthetic Fly Ashes experimental conditions group I II III

synthetic fly ash

evaporation temp (°C)

atmosphere

SyF1 SyF2 SyF3 SyF4 SyF5

840, 920, 1000 840, 920, 1000 840, 920 840, 920, 1000 840, 920

air air Ar, Ar/2.5H2O air air, Ar

(II) Matrix + Heavy Metal Oxides + NaCl, SyF2 and SyF3. The synthetic fly ash SyF2 was mixed supposing that all heavy metals in fly ash are present as oxides. Interpretation of the heavy metal evaporation experiments followed that of FG. The asymptotic values am,T, rate coefficients km,T, and the correlation coefficients R 2 of curve fitting the data points are summarized in Table 7. Except for the Zn evaporation at 840 °C, the correlations are good. The data for Zn at 840 °C (R 2 ) 0.894) are not used further. The maximum amount of Cl evaporation increases from 92% at 840 °C to 98% at 1000 °C of the Cl content in untreated SyF2. At 920 and 1000 °C, the curves for chlorine and heavy metal evaporation level off at roughly the same time; at 840 °C the coincidence is not as well expressed. Figure 5 shows the natural logarithms of the rate coefficients of heavy metal evaporation for SyF2 as a function of the inverse absolute temperature. From the linear regression of ln (km,T) vs 1/T, the values of Em and ln (k0) are computed and listed in Table 8 with the respective correlation coefficients R 2. To examine more closely the chemistry leading to volatile heavy metal species in synthetic fly ash SyF2, the simple synthetic fly ash SyF3 was prepared. The Pb content was chosen distinctly higher than in FG, and the molar proportion of Pb:Cl was taken to be 1:2. To exclude humidity in the samples, the Al2O3 powder was dried in an argon stream at 950 °C and NaCl in air at 120 °C during 2 h before use. The data of the Pb and Na evaporation are presented in Figure 6. It turns out that the water content in the argon stream has no effect on the evaporation of Pb and Na.

FIGURE 5. Rate coefficients k of the evaporation of Cd, Cu, Pb, and Zn by the thermal treatment of synthetic fly ash SyF2 in air.

TABLE 9

TABLE 7

Parameters of Simple First-Order Rate Law of Heavy Metal Evaporation with Synthetic Fly Ash SyF2 parameters of the M vs t curve fitting (SyF2) temp (°C)

element

840

Cd Cu Pb Zn Cd Cu Pb Zn Cd Cu Pb Zn

920

1000

FIGURE 6. Relative amounts of evaporation of the elements Na and Pb as a function of time by the thermal treatment of synthetic fly ash SyF3 in Ar and Ar/2.5% H2O at 840 and 920 °C.

a (%)

k (h-1)

R2

51.26 43.91 88.10 (6) 64.44 49.31 90.47 12.33 77.42 63.59 94.10 5.47

0.73 0.56 1.14 (2) 2.06 1.87 3.33 2.74 9.29 7.61 11.56 8.04

0.9994 0.9963 0.9983 (0.8922) 0.9931 0.9895 0.9978 0.9929 0.9994 0.9997 0.9996 0.9977

Parameters of Simple First-Order Rate Law of Heavy Metal Evaporation with Synthetic Fly Ash SyF4 parameters of the M vs t curve fitting (SyF4) temp (°C)

element

a (%)

k (h-1)

R2

840

Cd Cu Pb Zn Cd Cu Pb Zn Cd Cu Pb Zn

94.63 53.15 94.05 12.69 97.04 74.23 (95) 15.65 (99) (84) (98) 18.94

7.43 1.58 17.63 2.96 19.23 4.34 (26) 6.26 (22) (6) (21) 10.39

0.9992 0.9882 0.9993 0.9876 1.0000 0.9676 (1.0000) 0.9925 (0.9999) (0.9916) (0.9995) 0.9998

920

1000

TABLE 8

Parameters of Linear Regression ln (k) vs 1/T of Synthetic Fly Ash SyF2 parameters of the linear regression (SyF2) element

E (kJ/mol)

ln (k0)

R2

Cd Cu Pb Zn

186.20 ( 26.05 191.55 ( 15.62 169.74 ( 14.63 169.91 ( 38.56

19.71 ( 2.64 20.06 ( 1.58 18.43 ( 1.48 18.14 ( 3.77

0.9795 0.9934 0.9931 1a

a

Trivial case: line through two points.

Powder diffraction investigations of the condensates deposited on the cooling finger show clearly that the condensates are composed of PbCl2 and NaCl (8). The diffractograms of the annealed samples (nonvolatile part) show broadened peaks, which can be well explained by superimposing the diffractograms of R-Al2O3 and diverse sodium aluminates (Na2O‚5Al2O3, Na2O‚7Al2O3, Na2O‚11Al2O3). (III) Matrix + Heavy Metal Chlorides (+ NaCl), SyF4 and SyF5. The synthetic fly ash SyF4 was mixed assuming that all heavy metals contained in the real fly ash are present as chlorides. The interpretation of the heavy metal evaporation data followed that of the FG. The nominal

FIGURE 7. Rate coefficients k of the evaporation of Cd, Cu, Pb, and Zn by the thermal treatment of synthetic fly ash SyF4 in air.

values of the curve fittings according to eq 1 are given in Table 9. Figure 7 shows the natural logarithms of the rate coefficients as a function of the inverse absolute temperature. From the linear regression of ln (km,T), the values of Em and ln (k0) as well as the correlation coefficients R 2 are computed and listed in Table 10.


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TABLE 10

Parameters of Linear Regression ln (k) vs 1/T of Synthetic Fly Ash SyF4 parameters of the linear regression (SyF4) element

E (kJ/mol)

ln (k0)

Cd Cu Pb Zn

131.29 ( 6.16 139.51 ( 35.62

16.19 ( 0.64 15.53 ( 3.75

92.61 ( 5.78

11.12 ( 0.58

a

R2 1a 1a 0.9949

Trivial case: line through two points.

a

experimental conditions, heavy metal evaporation is only possible as long as there is residual chloride in the sample. Taking this point of view, it shall be discussed how the evaporation of chloride during the heat treatment of fly ash can be described. In the next sections, the interaction of chlorine and the heavy metal evaporation are discussed. Two different mechanisms shall be examined more closely: does the chlorine contained in the fly ash effect the heavy metal evaporation by converting non volatile heavy metal compounds into volatile heavy metal chlorides, or does the chlorine contained in the fly ash prevent the transformation and incorporation of volatile heavy metal compounds into the matrix. Concerning the rates of heavy metal evaporation, in particular the question of what are the rate-determining processes, no definite conclusions are possible at this moment. Only speculative conclusions can be given about possible mechanisms leading to the exponential shape of the evaporation curves and to the observed exponential temperature dependence of the rate coefficient. Chloride Evaporation. The most simple explanation for the simultaneous chloride and heavy metal evaporation by annealing would be that the whole amount of volatilized chlorine evaporates in the form of heavy metal chlorides MCl2 (M ) Cd, Cu, Pb, Zn). Considering the absolute amounts of element evaporation, it is obvious that this scenario does not reflect the behavior of real fly ash. For example with fine-grained fly ash in air at a temperature of 920 °C, the molar amount of chlorine (calculated as Cl2) evaporating exceeds the total molar amount of heavy metals evaporated during the same time interval by a factor of 3-3.5. Investigations about element speciation in fly ash have shown that chlorine is present in measurable, i.e., distinct, concentrations as NaCl (9, 10). It was because of these results that the simple synthetic fly ash SyF1 (matrix + NaCl) was chosen to simulate the decrease of the chlorine content. The experimental data of SyF1 compared with FG in Figure 4 show that it is useful to weigh the chorine content in model powder mixtures as NaCl. In addition to the evaporation of NaCl itself, the following chemical reactions may potentially lead to the Cl evaporation by forming gaseous Cl2 or HCl (11, 12):

4xNaCl + ySiO2 + xO2 f 2xNa2O‚ySiO2 + 2xCl2 (3a)

b

FIGURE 8. Relative amount of evaporation of the heavy metals Cd, Cu, Pb, and Zn as a function of time by the thermal treatment of synthetic fly ash SyF5 at 920 °C in (a) air and (b) argon.

To investigate the evaporation of heavy metal chlorides themselves, i.e., without additional alkali chloride, the synthetic fly ash SyF5 was mixed. Figure 8 shows the heavy metal evaporation at 920 °C in air and argon streams. The evaporation of Cu and Zn is clearly more complete in argon than in air nevertheless, huge amounts of Zn and trace amounts of Cu and Pb do not volatilize and stay in the synthetic fly ash.

Discussion The obvious synergy in the rates of heavy metal and chloride evaporations in fine-grained fly ash in air (Figures 1 and 2) leads to the conclusion that, at least under the given

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2xNaCl + ySiO2 + xH2O f xNa2O‚ySiO2 + 2xHCl (3b) Analogous reactions are possible with alumina (13) or aluminium silicates (14, 15) respectively instead of the SiO2 producing sodium aluminate and sodium aluminium silicate. In dry air, reaction 3a is predominant. In humid air, both reactions take place; however, reaction 3b will dominate. The velocity of the hydrolysis of NaCl according to eq 3b increases if, instead of SiO2, a mixture of 2SiO2 + Al2O3 and more so if 2SiO2‚Al2O3 (metakaolinite) is present (16). The rate-determining chemical reaction is the formation of sodium silicate with Na2O, formed by the hydrolysis, and SiO2 (17). Can Alkali Chlorides in Fly Ash Lead to Formation of Volatile Heavy Metal Chlorides? The oxides of Zn, Cd, Pb, and Cu are sparingly volatile under the given experimental conditions. This has been shown in previous experiments with powder mixtures consisting only of matrix + heavy metal oxides (7). However, by adding NaCl to this mixture of matrix + heavy metal oxides, i.e., the synthetic fly ash

SyF2, distinct heavy metal evaporation occurs under the same experimental conditions. The following type of chemical reaction can account for the conversion of heavy metal oxides (MO) into volatile heavy metal chlorides (MCl2):

xMO + 2xNaCl + ySiO2 f xMCl2 + xNa2O‚ySiO2 (4) In analogy to reactions 3a and b, it is possible to replace SiO2 by alumina, aluminium silicates, or sodium aluminium silicates. As was shown for reactions 3a and b, NaCl can form either HCl in the presence of water or Cl2 in the presence of oxygen. Therefore, it is possible that the overall reaction 4 proceeds, for example, with water, via two seperate steps:

2xNaCl + xH2O + ySiO2 f 2xHCl + xNa2O‚ySiO2 (5a) xMO + 2xHCl f xMCl2 + xH2O

(5b)

Once again, SiO2 can be replaced by the previously mentioned compounds. Analogous reactions can be written replacing H2O by 1/2O2 and HCl by 1/2Cl2. The formation of highly volatile oxychlorides, as is well known for chromium, is not known for the heavy metals discussed in this work. The results of the experiments with SyF3 (the PbO:NaCl: Al2O3 system) indicate that in fact volatile PbCl2 is being formed from PbO and NaCl in the presence of a sodium oxide “acceptor”, i.e., the alumina matrix. The data presented in Figure 6 also shows that, under the given experimental conditions, the evaporation rates of Na and Pb are not affected by the presence or absence of H2O vapor. This implies two possible explanations: either the reaction via the gas phase (eqs 5a and b) is not relevant, i.e., slow in comparison with the direct solid state reaction of NaCl with PbO (reaction 4), or both reaction paths have a common rate-determining step. The only common step is the reaction of Na2O with the alumina matrix to form the sodium aluminates detected by X-ray analysis. The latter seems plausible considering the discussion of reaction 3b above. The amount of Na evaporation presented in Figure 6, i.e., that amount of NaCl, that does not react according to the reaction 4 but evaporates as NaCl, is found to be 30%, irrespective of the temperature. This is in good agreement with measurements by Kiyoura and Ito in the system NaCl-SiO2 at 900-1000 °C in steam (18). One would expect that the amount of Pb found in the condensate should be limited by the available chlorine in the system, i.e., with 30% of the sodium evaporating Pb should level off at 70% (if no Cl leaves the system as HCl/Cl2) or below 70% (if HCl/Cl2 can escape). According to Figure 6, however, more Pb and Na are evaporated than can be accounted for by the total chloride content in the system. At present it is unclear whether this discrepancy is caused by experimental uncertainties or by additional reactions not involving Cl. Comparing the evaporation kinetics of synthetic fly ash SyF2 with fly ash from an incinerator, heat treated under the same experimental parameters, a number of differences are noticed. While for real fly ash the value of Em for Zn is clearly lower than the values for Cd, Cu, and Pb, the respective values for the synthetic fly ash SyF2 are closer to each other and at a higher level. Further, the values of k0,m are different, especially for Zn and Cu. The ranking of the maximum amount of evaporation am,T for the different heavy metals is the same with real fly ash and the synthetic

SyF2 (aZn < aCu < aCd < aPb), but the absolute values are different. The striking conformity of the ln (k) vs 1/T curves obtained for the different heavy metals in the SyF2 system indicates that the evaporation of the different heavy metals is likely to be controlled by one common rate-determining reaction. The only reaction step that all individual heavy metal oxides have in common is once again the formation of the sodium aluminate or silicate, i.e., the release of Cl from NaCl. Apart the volatilization of heavy metals according to reaction 4, the mobilization of heavy metals from ternary or polynary matrices can be an important reaction in fly ash. Additional experiments with a mixture for example of Zn2SiO4 (Willemite) and NaCl are expected to give some more detailed insigths into reactions 6a and b, which are described in the following section. Does Chlorine Prevent Transformation and Incorporation of Volatile Heavy Metal Compounds? All experiments carried out using model systems with heavy metal chlorides (SyF4 and SyF5) show that in these systems complete evaporation, especially in the cases of Zn and Cu, was not achieved (see Figure 8). Obviously an immobilization reaction transforms the chlorides into less volatile species. This could be either a diffusion of heavy metal chlorides into the matrix or a transformation of heavy metal chlorides into oxides (MO) or, alternatively, aluminates and silicates. As far as we know, immobilization by diffusion of heavy metal chlorides into the matrix can be neglected. The formation of heavy metal aluminates/silicates is, compared to the formation of simple oxides, thermodynamically favored and has been observed (19). Therefore, analogous to the chemical reactions of the NaCl transformation (eqs 3a and b), the following incorporation reactions are proposed, which could account for the formation of heavy metal-matrix compounds:

2xMCl2 + ySiO2 + xO2 f 2xMO‚ySiO2 + 2xCl2

(6a)

xMCl2 + ySiO2 + xH2O f xMO‚ySiO2 + 2xHCl (6b) SiO2 can accordingly be replaced by substances like alumina, aluminium silicate, or sodium aluminium silicate. If these reactions (6a and 6b) are decisive in determining the maximum amount of evaporation of a given metal, then the gas atmosphere, i.e., the partial pressures of oxygen species as well as of chlorine species, should be important in influencing the overall reaction by shifting the equilibrium of reactions (6a,b) to either side. To verify the proposed reactions, evaporation experiments with the synthetic fly ash SyF5 were performed in air and argon. Comparing Figure 8a and b shows that the evaporation is found to be clearly enhanced by excluding oxygen, although complete evaporation, as would be expected considering reactions 6a and b, is only observed for Cd. A total of 6% of the copper and lead and 70% of the zinc was retained in the fly ash; however, at 920 °C, the boiling point of ZnCl2 (732 °C) is clearly exceeded. The small amounts of copper and lead persisting in the fly ash could be explained by the fact, that some part of the heavy metal chloride vapor was physically adsorbed and by that immobilized at the surface of the matrix, as found for NaCl and KCl at 800-900 °C on Al2O3 (20). However, in the case of zinc (the most abundant of the heavy metals in the system investigated), the total quantity immobilized in the residue has to be explained otherwise. Thermogravimetric investigations with the synthetic fly ash SyF5 with simultaneous


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FIGURE 9. Modeling of the Cd evaporation in the thermodynamic equilibrium of SiO2, Al2O3, and CdCl2 in air (79% Ar, 21% O2) plus alternatively NaCl or NaCl and H2O.

analysis of the escaping gases by FTIR spectroscopy have shown that, upon annealing, considerable amounts of water (10 wt %) escape from the model fly ash. This water escaping at temperatures between 100-250 and 350-500 °C results mainly from dehydratation of metal chlorides and decomposition of calcium hydroxide (Ca(OH)2). The different components were not dried before mixing the synthetic fly ash. Zinc chloride as well as copper chloride most probably reacted with water during the heating-up period of the samples in the oven according to reaction 6b. Comparing the experimental data of synthetic fly ash SyF4 and SyF5, one can conclude that the presence of chlorine in the form of NaCl does prevent the transformation and incorporation of volatile heavy metal chlorides to a certain degree. The maximum amounts of heavy metal evaporation that were measured in air with SyF4 (am,T in Table 9) are significantly higher than with SyF5 (Figure 8a). Note that the only difference between these two fly ashes is the additional NaCl in SyF4. A plausible explanation for this observation could be that NaCl, by reacting with the alumina/silica matrix (reaction 3), enhances the local partial pressure of chlorine species and by that shifts the MCl2MO equilibria (reaction 6) toward the volatile chlorides. This equilibrium explanation of the influence of NaCl on evaporation data is confirmed by a simple thermodynamic model of the evaporation process. The model consists of calculating the equilibrium compositions of the resulting gas and solid phases for a given system in contact with a given volume of air. The gas phase, including the species evaporated, is then removed from the system, replaced by the same air volume, and the equilibrium composition with the new inventory of substances computed again (equilibration steps q). The equilibrium computations were carried out using the program STANJAN (21), which essentially minimizes the free energy of a given multicomponent system. In Figure 9, the results of a calculation simulating the Cd evaporation from fly ash in air at 920 °C is shown. The components of the system were Al2O3, SiO2, CdCl2, and occasionally NaCl in proportions analogous to SyF4 plus dry or humidified “air” (a mixture of 79% Ar + 21% O2). The air volume, in subsequent steps q equilibrated with the solids, was chosen to be 1 L, which is the quantity of air passing through the tube of the experimental oven in 3 min. Using this approach, a time

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axis is created in a purely thermodynamic calculation. The following compounds have been considered for the thermodynamic equilibrium calculations: gas phase species: Ar, Cd, CdCl2, Cl2, HCl, H2O, NaCl, O2; condensed phases: Al2O3, Al2O3‚SiO2, Cd, CdO‚Al2O3, CdCl2, CdO, CdO‚SiO2, NaCl, Na2O‚Al2O3, Na2O‚SiO2, Na2O‚2SiO2, SiO2. The model calculations have been carried out in air (Ar + O2) for the separate cases of Al2O3 + SiO2 + CdCl2 with and without NaCl as well as for Al2O3 + SiO2 + CdCl2 with NaCl + H2O. In the last case, the total amount of H2O was chosen to be 10 wt % of the powder mixture (cf. TG-FTIR measurement of the synthetic fly ash SyF5). Furthermore, the total amount of water was distributed in equal amounts to the first three sequences q1-q3 to simulate the initial release of water in the heating-up phase of the samples. The curves in Figure 9 illustrate qualitatively the experimental findings with SyF4: Cd evaporation in dry air is found to be increased by the presence of NaCl in the thermodynamic system analyzed. The influence of water is very marked: if humidity is present, cadmium is preferentially bound in silicate compounds and immobilized. The model calculations, far from presenting a quantitative picture, illustrate well the importance of the composition of the gas atmosphere. Clearly, thermodynamics cannot explain all features of the evaporation curves; for that a more detailed model, taking into consideration the kinetics of the rate determining steps, will have to be developed. The best quantitative fit between the thermal behavior of real and synthetic fly ashes is achieved experimentally with SyF4: the characteristic data measured with model mixture SyF4 in air (Tables 9 and 10) correspond very well with the data obtained from experiments with real fly ash (Tables 4 and 5). Comparing the values of Em and ln (k0) shows that especially the rates of the Zn and Cu evaporation are nearly identical. The absolute differences of the respective values for the Cd evaporation with SyF4 compared to those of the real fly ash are in the same range as the respective absolute differences between SyF2 and the real fly ash. To be able to discuss the behavior of lead, additional evaporation experiments were necessary in the temperature range of 900-1000 °C. The maximum amounts of evaporation am,T of the synthetic fly ash SyF4 are in better agreement with those of real fly ash than are the respective values of the synthetic SyF2 (Table 7). With these findings, it is reasonable to conclude that the metals Cu and Zn most probably are present in fly ash as chlorides, while Cd might be present in equal amounts of oxides and chlorides. It emerges from Figure 7 that the rates of heavy metal evaporations with the synthetic fly ash SyF4 are different for the separate heavy metals. The above modeling of the heavy metal evaporation by thermodynamic equilibrium calculations confirms that the modeled evaporation curve has an approximated exponential shape (Figure 9), although there are quantitative discrepancies. Especially in the presence of water the thermodynamic model yields, in contrast to the experimental data, virtually no volatilization but nearly complete incorporation of the heavy metals into the matrix. Therefore, the rates of MCl2 evaporations must be controlled by the rates of MCl2 transformation or incorporation, respectively, according to the chemical reactions 6a and b. In analogy to the systems matrix + NaCl and matrix + MO + NaCl, where the formation of sodium silicate seemed to be rate determining, it is reasonable to conclude that, in the system MCl2 + H2O/O2 (+NaCl), the formation of silicate is the rate-determining

step in the chemical reactions 6a and b. Because in the case of SyF4 different heavy metal oxides besides Na2O are incorporated in forming silicates, it is plausible that the rates of the separate silicate reactions are different like the linked evaporation rates shown in Figure 7. In future work, the rates of silicate formation and the diffusion of oxide species in the oxide matrix will require closer investigation.

Acknowledgments This work was performed within the Priority Programme Environment of the Swiss National Science Foundation. The financial assistance of this foundation is gratefully acknowledged. Thanks go to Mr. A. Schuler and Mrs. M. Quintilii for helping with the experimental work and the numerous ICP analysis as well as to Dr. P. Kuhn for fruitful discussions and reading the manuscript critically.

Literature Cited (1) Belevi, H. In Techniken der Restmu ¨ llbehandlung: kalte und/ oder thermische Verfahren, Wu ¨ rzburg/Veitsho¨chheim, Apr 2021, 1993; VDI Gesellschaft Energietechnik; VDI-Verlag: Du ¨ sseldorf, 1993, pp 261-276. (2) Baccini, P.; Brunner, P. H. Gas Wasser Abwasser 1985, 65, 403409. (3) Brunner, P. H. Mu ¨ ll Abfall 1989, 21 (4), 166-180. (4) Stark, R. In Umweltschutz, Wie? Reststoffverwertung aus der thermischen Abfallbehandlung; Gutke, K., Ed.; Gutke Verlag: Ko¨ln, 1993, pp 299-337. (5) Reimann, D. O. In Entsorgung von Schlacken und sonstigen Reststoffen; Reimann, D. O., Ed.; Beiheft zu Mu ¨ ll und Abfall Heft 31; Erich Schmidt Verlag: Berlin, 1994; pp 30-37.

(6) Law, S. L.; Gordon, G. E. Environ. Sci. Technol. 1979, 13, 432438. (7) Jakob, A.; Stucki, S.; Kuhn, P. Environ. Sci. Technol. 1995, 29, 2429-2436. (8) Jakob, A. In preparation. (9) Eighmy, T. T.; Eusden, J. D.; Krzanowski, J. E.; Domingo, D. S.; Sta¨mpfli, D.; Martin, J. R.; Erickson, P. M. Environ. Sci. Technol. 1995, 29, 629-646. (10) Amann, P.; Nu ¨ esch, R. In SPP Umwelt, Modul 6/3A, Zusammenfassender Bericht zur mineralogischen Charakterisierung der Referenzmaterialien; Bericht No. 4436/2; Institut fu ¨r Geotechnik, ETH Zu ¨ rich, Zu ¨ rich: Sep 27, 1994. (11) Clews, F. H.; Thompson H. V. J. Chem. Soc. 1922, 121, 14421448. (12) Iler, R. K.; Tauch, E. J. Am. Inst. Energy 1941, 37, 853-877. (13) Clews, F. H. J. Chem. Soc. 1925, 127, 735. (14) Scandrett, L. A.; Clift, R. J. Inst. Energy 1984, 57, 391-397. (15) Uberoi, M.; Punjak, W. A.; Shadman F. Prog. Energy Combust. Sci. 1990, 16, 205-211. (16) Briner, E. C. R. Hebd. Seances Acad. Sci. 1948, 227, 703-705. (17) C ˇ irkov, S. K. Zh. Khim. Prom. 1937, 14, 845-846. (18) Kiyoura, R.; Ito, Y. J. Ceram. Assoc. Jpn. 1952, 60, 325-328. (19) Uberoi, M.; Shadman, F. AIChE J. 1990, 36, 307-309. (20) Luthra, K. L.; LeBlanc, O. H., Jr. J. Phys. Chem. 1984, 88, 18961901. (21) STANJAN chemical equilibrium solver, version 3.95; Wm. C. Reynolds, Stanford University, CA: Stanford, 1987.

Received for review January 19, 1996. Revised manuscript received June 14, 1996. Accepted June 14, 1996.X ES960059Z X

Abstract published in Advance ACS Abstracts, September 15, 1996.


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Complete Heavy Metal Removal from Fly Ash by Heat Treatment

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