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Forecasting the long-term behaviour of municipal solid waste incineration bottom ash: rapid combined tests
Waste Materials in Construction G.R. Woolley,J.J.J.M. Goumans and P.J. Wainwright(Editors) 9 2000 Elsevier Science Ltd. All rights reserved.
475
F o r e c a s t i n g the l o n g - t e r m b e h a v i o u r of m u n i c i p a l solid waste incineration b o t t o m ash: rapid c o m b i n e d tests Franqoise Bod6nan, Mohammed Azaroual, Patrice Piantone BRGM, 3 Av. C. Guillemin, BP 6009, 45 060 Od6ans cedex 2, France (f.bodenan @brgm.fr; m.azaroual @brgm.fr; p.piantone @brgm.fr)
Municipal solid waste incineration bottom ash is a highly reactive material, especially toward atmospheric CO2, which is why we decided to carry out rapid and simple tests combining accelerated ageing and batch leaching to forecast the long-term behaviour of bottom-ash samples of various origins. By speeding up one of the major reactions, i.e. the carbonation that occurs during the natural maturation of bottom ash, it was possible to determine the maximum pollutant release of the elements (metals, sulphates) most detrimental to upgrading. The final products present higher calcite contents and are characterized by a reduction in leachate metals and an increase in leachate sulphates. The laboratory results are in agreement with a full-scale field study carried out elsewhere. Thermodynamic calculations were also undertaken to determine the cause of the high exothermicity evidenced during the tests because, even though the oxidation of metals (Fe, A1) is mainly invoked to explain the general increase of bottom-ash temperature during maturation, the contribution of the carbonation reactions (with portlandite, wairakite, larnite anorthite) cannot be ignored.
1. INTRODUCTION Most waste produced by thermal processes is highly reactive at ambient temperature, and this is particularly so for municipal solid waste (MSW) incineration bottom ash. During maturation, this waste undergoes significant changes that alter the chemical nature of its leachates. Studies carried out on the chemical stability of bottom ash subjected to chemical weathering have, in particular, shown an improvement in leachate quality for soluble salts and metals (Pb, Zn, etc.). Conversely, sulphates are more likely to be released. These conflicting trends are essentially linked to a) the mobility of salts and b) the carbonation process that stabilizes the metals and increases sulphate solubility [ 1,2]. At present, French legislation relies on a batch leaching test (NF X31-210) to evaluate the release potential for a number of elements (SOn, Pb, Cd, CrVI, As, Hg, TOC). However, this prescriptive test only accounts for element release at the time of the measurement. It is therefore inadequate for estimating the long-term behaviour of materials likely to evolve over time.
476
Tests combining accelerated ageing and batch leaching have been developed using the results of a full-scale study. These enable a rapid estimation of the maximum release values for carbonation-aged bottom ash. The study specifically monitored the elements most detrimental to upgrading (Pb, SOn).
2. MATERIALS AND METHOD
2.1. Samples and mechanical preparation The experiments were performed on five samples of bottom ash of various origins, both freshly produced (F1, F2, F3) and stored (S 1 [stored for more than five years] and $2 [stored for two years]). The samples, some 30 kg each, were first dried at 80-100 ~ before magnetically removing any iron particles. Ash particles larger than 4 mm diameter were then reduced to 4 mm and mixed into the <4 mm fraction in a cement mixer. During this mixing, water was progressively sprayed through the bottom ash for a period of 15 minutes so as to obtain a constant water content of 5% w/w in all the samples; this is deliberately well below the initial water content of around 15-20% in order to reduce the kinetics of the carbonation process. The samples were then successively quartered in order to obtain representative and relatively homogenous 1.5 kg splits.
2.2. Ageing tests Five plexiglass percolation columns (one for each bottom ash) connected in series with a CO/bottle were used for the ageing tests. Each column was equipped with a valve at each end, and a 'scatter system' was placed at the bottom to improve the CO2 diffusion during upflow. A manometer was attached to each column to measure the internal pressure. Around 1.5 kg of bottom ash was placed in each column. After air flushing, the samples were subjected to a slight overpressure of CO2 gas upflow for increasing periods of time (20 min, 40 min, 1 h, 3 h, 6 h). At the end of the experiment the samples were dried at 80100 ~ As the reaction appeared to be highly exothermic, the temperature variations of the 6-hour experiment were recorded during the accelerated ageing of the three samples of freshly produced bottom ash (F1, F2, F3).
2.3. Leaching tests/leachate analysis As described in the French standard batch leaching test (NF X31-210), the dry equivalent quantity of a 100 g wet sample reduced to 4 mm is mixed with water to a liquid/solid ratio of 10 and agitated for 24 h. A simplified and arbitrary procedure of mixing 90 g dry solid with 1 litre of water was used for the present leaching tests on both the untreated and CO2-treated samples. The leachates were then filtered at 0.45 lum and, after measurement of their pH, analysed for lead and sulphate content by ICP and ion-exchange chromatography, respectively.
2.4. Infrared spectroscopy Infrared spectroscopy was used to verify that the carbonation process had taken place in the bottom ash. KBr pellets were prepared for both the non-carbonated sample and the 6-hour
477 aged samples (maximum treatment). Special care was taken to obtain representative subsamples by successive quartering, then crushing to 80 lum and grinding to lower granulometry.
3. RESULTS
3.1. Monitoring the CO2 treatment The upflow of CO2 in the columns containing the freshly produced bottom ash (F1, F2, F3) showed that the bottom-ash reactivity towards carbonation is exothermic, with an initial rapid heating (Fig. l a). The same pattern was noted with the two-year-old stored bottom ash $2. The stored bottom ash S1 (more than five years old), however, showed no evidence of temperature variation throughout the 6-hour treatment.
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Figure l a. Temperature profile of the freshly produced bottom ash (F1, F2, F3) during the CO2 treatment.
t (h)
Figure lb. Weight variation of the five bottom ash samples during the CO2 treatment.
The bottom ash reactivity towards CO2 also resulted in a rapid weight variation for F1, F2, F3 and $2, the maximum being reached after about one hour, whereas once again no significant change was noted for S 1 (Fig. lb).
3.2. SO4 and Pb leaching behaviour As expected from the full-scale study results, sulphate leaching increased with the accelerated CO2 treatment. Apart from S 1, a sulphate release plateau was identified after a few minutes of carbonation in the bottom ash leachates (Fig. 2a). This reaction was accompanied by a decrease in the pH values (Fig. 2b). The fresh bottomash (F1, F2, F3) leachates, initially at around pH 11.5-13.0, fell to pH 9.0-9.5 after ageing. The change in pH of the leachates from the stored bottom ash (S 1, $2) was significantly less: from pH 10 to pH 8.5 for $2, and no significant variation for S 1 (steady at pH 8.0-8.5). The leachates of the non-carbonated F2 and F3 bottom ash also contained 0.57 and 4.87 mg/kg of Pb, respectively. The Pb concentrations for all the other samples (both noncarbonated and carbonated) were below the detection limit (0.02 mg/kg).
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Figure 2a. 504 release in the leachates of the five bottom ash samples during C O 2 accelerated ageing with time.
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60
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300
360
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Figure 2b. pH evolution of the leachates.
3.3. Solids description The Fourier transform infrared (FTIR) spectra of the bottom ash samples both before and after carbonation made it possible to monitor the chemical evolution of the solids content during the treatment. This was especially significant for the two extreme cases, i.e. the F3 fresh bottom ash, which showed the highest temperature rise (up to 42 ~ and the S 1 stored bottom ash, which showed no significant change during carbonation. Before accelerated ageing, the F3 bottom ash spectrum showed the main characteristic peaks of calcite (1430, 875 and 713 cm -l) and a small peak at 3645 cm -I corresponding to the OH vibration band of portlandite. After the 6-hour treatment the portlandite peak disappeared and a higher calcite content is detected (Fig. 3a). The spectra of S 1 bottom ash before and after treatment are superimposable, in agreement with no reactivity towards CO2. The portlandite peak is absent and the peaks relative to the presence of calcite are identical (Fig. 3b). Calibration from ternary mixtures (decarbonated bottom ash/ portlandite/calcite) is consistent with a 2-5% portlandite content in the F3 bottom ash. For the intermediate samples (F1, F2, $2) the portlandite peak was not distinguished on either the treated or untreated sample spectra. Nevertheless, the samples contained a higher calcite content after treatment than before (as shown by the increased intensity of the three main characteristic peaks). Thus if portlandite is present in these samples, its content is necessarily below 1-2%. Minerals other than portlandite (non detectable by FFIR) may also be carbonated. Even if complementary calibrations and special bottom ash preparation (for example fine particle separation) were to be developed, infrared spectroscopy still appears to be an efficient tool for determining the maturation state of bottom ash and its evolution during carbonation.
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Figure 3. FTIR spectra of freshly produced (F3) and stored (S 1) bottom ash before (0 h) and after (6 h) COa treatment (c: calcite; p: portlandite).
4. INTERPRETATION/DISCUSSION Apart from sample S 1 (bottom ash stored for more than five years), the tested samples of MSW incineration bottom ash all showed a similar evolution during the combined tests. In particular, maximum sulphate release from the leachate was attained in less than 1 h of treatment with CO2 gas. The results also showed that contained lead was trapped during the carbonation treatment. Sample S 1 did not react to the accelerated carbonation. Moreover, the pH of its leachate was constant at around 8, which signifies that it had already attained peak maturation during its storage. The initial quantities of leached Pb and SOn obviously depend on the nature, and thus the origin, of the bottom ash. The sulphate content of sample $2 was higher than that of sample F1, which in tum was higher than that of F2, again higher than that of F3. During the
480
accelerated maturation, the quantity of released sulphates was multiplied by a factor of 2 for sample $2 (already naturally aged over two years), by a factor of 4 for samples F1 and F2, and by a factor of 84 for sample F3. The last, which gave the most basic leachates (pH 13) corresponds to the 'youngest' of the fresh bottom waste samples.
Table 1. Enthalpy of the carbonation reactions of certain calcic minerals in bottom ash and of the oxidation of iron metal (Fe(s)) and aluminium metal (Al(s)). Mineral
Reaction
Portla'ndite Ca(OH)2(s) + CO2(aq) r Calcite + H20 Ettringite* 0.33 Ett. + CO2(aq) r Calcite + Gypsum + 0.33 A1203 + 8.67 H20 Wairakite Ca(AI2Si4)OI2:2H20+ CO2(aq)r Calcite + + Kaolinite* + 2 Chalcedony Prehnite 0.5 CaEAIESiaOlo(OH)2 + CO2(aq) + 0.5 H20 r Calcite + 0.5 Chalcedon~ + 0.5 Kaolinite* Diopside CaMgSi206 + CO2(aq) + H20 r Calcite + 2 Chalcedony + Brucite Anhydrite CaSO4 + C O 3 -2 r Calcite + 5 0 4 -2 Larnite 0.5 Ca2SiO4 + CO2(aq) r Calcite + 0.5 Chalcedony Anorthite Ca(A12Si2)O8 + CO2(aq) + 2 H20 r Calcite + Kaolinite* Fe(s) 1.333 Fe(s) + O2(aq) r 0.667 Fe203(s) Al(s) 1.333 Al(s) + O2(aq) + 2 H20 r 1.333 Al(OH)3(s) Pyrite 0.211 FeS2(s) + O2(aq) + 0.7'37 H20 r 0.211 Fe(OH)3(s) + 0.421 5042- § 0.482 H+ Pyrrhotite 0.444 FeS(s) + O2(aq) + 1.111 H20 r 0.444 Fe(OH)3(s) + 0 . 4 4 4 5042 § 0.889 H + ,,,
log K (25~
1~-IR (kcal)
,dk][-IR pH=13.46
l~kI-IR pH=12.23
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-22.10 -0.85
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10.53
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-9.05
- 12.20
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-14.42
3.65
0.50
3.93
-11.91
6.17
3.01
4.17 12.95
-1.81 -22.42
-1.65 -4.46
-1.39 -7.61
11.58
-27.67
-9.60
-12.76
121.49 -175.36 -125.31 189.64 -274.45 -237.19
-111.41 -223.29
47.03
-71.17
-47.45
-32.11
61.03
-94.49
-68.69
-53.28
The AHR (kcal) values are given for 25 ~ under standard conditions. AHR (pH = 13.46) corresponds to the enthalpy of the reaction in a bottom waste leachate (I = 0.47, log fCO2 = -11.78, log fO2 = -29.28, log aCO2(aq) = -13.29, log aO2(aq) = -32.22), and AHR(pH = 12.23) is calculated for a less saline leachate (I = 0.19, log fCO2(g) = -9.47, log fO2(g) = -39.46, log aCO2(aq) = -10.94, log aO2(aq) =-42.36). *Detailed chemical formula: Ettringite Ca6A12(SO4)3(OH)I226H20; Kaolinite A12SiEOs(OH)4.
The results of the laboratory study reflect the observations of a full-scale study made in the field on the alteration mechanisms of a large bottom ash heap subjected to atmospheric weathering and where CO2 consumption was evidenced and calcite found to be the predominant neoform mineral generated. A 22-month monitoring of species concentrations in the leachates indicated three major stages in the maturation process: 1) chloride leaching, 2) carbonation with Pb, Zn trapping, and 3) late sulphate leaching [ 1,2]. Meima and Comans (1999) came to similar conclusions in a separate study [3].
481 The temperature increase noted in the columns during the accelerated tests is unmistakably linked to the carbonation of highly reactive phases and notably to the presence of portlandite. This carbonation reaction also probably plays a role in the temperature rise noted during the natural maturation of bottom waste, despite the fact that the reactions generally quoted in the literature to explain this rise correspond to the oxidation of the metals Fe ~ A1~ [4]. At this stage of the study, therefore, we considered it essential to determine the relative importance of the carbonation reaction in the exothermic stage undergone by all bottom ash on a maturation site. Because of the variety of mineral species present in MSW incineration bottom ash, numerous reactions, such as carbonation, oxidation-reduction and hydration, are likely to influence the maturation (Table 1). The minerals that we have taken into consideration are typical minerals found in MSW incineration bottom ash (portlandite, ettringite, larnite, anhydrite), minerals associated with zeolites (wairakite, prehnite) present in the first stages of ageing, high-temperature minerals derived from melts (diopside, anorthite), metals that are always present at the end of combustion (A1~ Fe~ and sulphides (pyrrhotite, pyrite). More than 40 reactions were calculated. As a first approximation, with regard to the aluminium oxides, the reaction product has been defined as close to A1203 even where present as A1203.nH20 gels [5]. Under standard conditions, all the reactions are theoretically exothermic (AH~ as shown by the calculations made using the SUPCRT92 software [6]. If one considers the effect of salinity on ion activity (calculations done with the EQ3NR software [7]) it is seen that in fact only a few reactions are likely to be sufficiently efficient to explain the temperature rise. The remaining possibilities are i) the carbonation of portlandite, larnite and anhydrite, ii) the carbonation of anorthite and wairakite accompanied by the formation of kaolinite, iii) oxidation of the metals, and iv) hydrolysis of the sulphides. Of these, the last two are the most exothermic. The carbonation of calcium sulphates, such as ettringite, anhydrite and gypsum, is not an exothermic process. Moreover, the reactions are highly dependent on the ionic strength. In general, reaction enthalpy increases by 1 kcal per gram of salinity, and if salinity is too high it can inhibit the reactions. In bottom waste from which iron particles have been magnetically removed, the mass percentages of native metals are of the order of 3% for Fe ~ and 0.5% for A1~ with sulphides about 0.5% and portlandite about 6%. The amount of freeable energy is associated mainly with oxidation reactions of the metals and of some sulphides, rather than with the carbonation reaction. However, this observation must be modulated by parameters such as specific surface area of the minerals and/or size of the crystallites, which play a not insignificant role in the level of reactivity. All the observations show that the cited metals are present in forms with limited surface areas, initial manufactured goods (Fe~ melt drops (A1~ or globules (sulphides) commonly included in glass, unlike the portlandite that is present as fine crystallites highly accessible to the fluids. The high calorific potential of the metals and sulphides will thus be compensated by the specific surface area of the minerals susceptible to carbonation and each of the components will play a role in the temperature rise associated with the natural maturation of the bottom waste.
482 5. C O N C L U S I O N Conducting leaching tests on artificially-aged MSW incineration bottom ash is proving to be a valuable tool in forecasting its long-term behaviour, because the physico-chemical characteristics of the obtained leachates correspond to those of fully-matured bottom ash. Ageing can therefore be achieved in a few hours in the laboratory, as against months and even years, depending on storage conditions, for the natural ageing process. Moreover, in cases where the threshold values of toxic elements in the bottom ash are exceeded, this test enables one to determine the most suitable treatment to upgrade the bottom ash on its leaving the incineration plant, such as improving its physico-chemical characteristics through the addition of mineral substances. ACKNOWLEDGEMENTS This work was supported by the MATE (French Ministry of the Environment). The authors would like to thank Sir Patrick Skipwith and Ms Rowena Stead.
REFERENCES 1. P. Freyssinet, Y. Itard, M. Azaroual, B. Clozel, P. Piantone and D. Guyonnet, Proceedings of the International Conference Waste stabilization and environment, Lyon, France, (1999) 92. 2. B. Clozel, F. Bod6nan and P. Piantone, Proceedings of the International Conference Waste stabilization and environment, Lyon, France, (1999) 46. 3. J.A. Meima and N.J. Comans, Appl. Geochem., 14 (1999) 171. 4. C.A. Johnson, G.A. Richner, T. Vitvar, N. Schittli and M. Eberhard, J. Contam. Hydrol., 33 (1998) 376. 5. L.C. Lange, C.D. Hills and A.B. Poole, Environ. Sci. Technol., 30 (1996) 25. 6. J.W. Johnson, E.H. Oelkers and H.C. Helgeson, Computers Geosci., 18 (1992) 899. 7. T.J. Wolery, EQ3NR 7.0, UCRL-MA-110662-PT-I, Lawrence Livermore National Laboratory, Livermore, California (1992) 246.
475
F o r e c a s t i n g the l o n g - t e r m b e h a v i o u r of m u n i c i p a l solid waste incineration b o t t o m ash: rapid c o m b i n e d tests Franqoise Bod6nan, Mohammed Azaroual, Patrice Piantone BRGM, 3 Av. C. Guillemin, BP 6009, 45 060 Od6ans cedex 2, France (f.bodenan @brgm.fr; m.azaroual @brgm.fr; p.piantone @brgm.fr)
Municipal solid waste incineration bottom ash is a highly reactive material, especially toward atmospheric CO2, which is why we decided to carry out rapid and simple tests combining accelerated ageing and batch leaching to forecast the long-term behaviour of bottom-ash samples of various origins. By speeding up one of the major reactions, i.e. the carbonation that occurs during the natural maturation of bottom ash, it was possible to determine the maximum pollutant release of the elements (metals, sulphates) most detrimental to upgrading. The final products present higher calcite contents and are characterized by a reduction in leachate metals and an increase in leachate sulphates. The laboratory results are in agreement with a full-scale field study carried out elsewhere. Thermodynamic calculations were also undertaken to determine the cause of the high exothermicity evidenced during the tests because, even though the oxidation of metals (Fe, A1) is mainly invoked to explain the general increase of bottom-ash temperature during maturation, the contribution of the carbonation reactions (with portlandite, wairakite, larnite anorthite) cannot be ignored.
1. INTRODUCTION Most waste produced by thermal processes is highly reactive at ambient temperature, and this is particularly so for municipal solid waste (MSW) incineration bottom ash. During maturation, this waste undergoes significant changes that alter the chemical nature of its leachates. Studies carried out on the chemical stability of bottom ash subjected to chemical weathering have, in particular, shown an improvement in leachate quality for soluble salts and metals (Pb, Zn, etc.). Conversely, sulphates are more likely to be released. These conflicting trends are essentially linked to a) the mobility of salts and b) the carbonation process that stabilizes the metals and increases sulphate solubility [ 1,2]. At present, French legislation relies on a batch leaching test (NF X31-210) to evaluate the release potential for a number of elements (SOn, Pb, Cd, CrVI, As, Hg, TOC). However, this prescriptive test only accounts for element release at the time of the measurement. It is therefore inadequate for estimating the long-term behaviour of materials likely to evolve over time.
476
Tests combining accelerated ageing and batch leaching have been developed using the results of a full-scale study. These enable a rapid estimation of the maximum release values for carbonation-aged bottom ash. The study specifically monitored the elements most detrimental to upgrading (Pb, SOn).
2. MATERIALS AND METHOD
2.1. Samples and mechanical preparation The experiments were performed on five samples of bottom ash of various origins, both freshly produced (F1, F2, F3) and stored (S 1 [stored for more than five years] and $2 [stored for two years]). The samples, some 30 kg each, were first dried at 80-100 ~ before magnetically removing any iron particles. Ash particles larger than 4 mm diameter were then reduced to 4 mm and mixed into the <4 mm fraction in a cement mixer. During this mixing, water was progressively sprayed through the bottom ash for a period of 15 minutes so as to obtain a constant water content of 5% w/w in all the samples; this is deliberately well below the initial water content of around 15-20% in order to reduce the kinetics of the carbonation process. The samples were then successively quartered in order to obtain representative and relatively homogenous 1.5 kg splits.
2.2. Ageing tests Five plexiglass percolation columns (one for each bottom ash) connected in series with a CO/bottle were used for the ageing tests. Each column was equipped with a valve at each end, and a 'scatter system' was placed at the bottom to improve the CO2 diffusion during upflow. A manometer was attached to each column to measure the internal pressure. Around 1.5 kg of bottom ash was placed in each column. After air flushing, the samples were subjected to a slight overpressure of CO2 gas upflow for increasing periods of time (20 min, 40 min, 1 h, 3 h, 6 h). At the end of the experiment the samples were dried at 80100 ~ As the reaction appeared to be highly exothermic, the temperature variations of the 6-hour experiment were recorded during the accelerated ageing of the three samples of freshly produced bottom ash (F1, F2, F3).
2.3. Leaching tests/leachate analysis As described in the French standard batch leaching test (NF X31-210), the dry equivalent quantity of a 100 g wet sample reduced to 4 mm is mixed with water to a liquid/solid ratio of 10 and agitated for 24 h. A simplified and arbitrary procedure of mixing 90 g dry solid with 1 litre of water was used for the present leaching tests on both the untreated and CO2-treated samples. The leachates were then filtered at 0.45 lum and, after measurement of their pH, analysed for lead and sulphate content by ICP and ion-exchange chromatography, respectively.
2.4. Infrared spectroscopy Infrared spectroscopy was used to verify that the carbonation process had taken place in the bottom ash. KBr pellets were prepared for both the non-carbonated sample and the 6-hour
477 aged samples (maximum treatment). Special care was taken to obtain representative subsamples by successive quartering, then crushing to 80 lum and grinding to lower granulometry.
3. RESULTS
3.1. Monitoring the CO2 treatment The upflow of CO2 in the columns containing the freshly produced bottom ash (F1, F2, F3) showed that the bottom-ash reactivity towards carbonation is exothermic, with an initial rapid heating (Fig. l a). The same pattern was noted with the two-year-old stored bottom ash $2. The stored bottom ash S1 (more than five years old), however, showed no evidence of temperature variation throughout the 6-hour treatment.
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3
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4
,.............
5
6
6:00
Figure l a. Temperature profile of the freshly produced bottom ash (F1, F2, F3) during the CO2 treatment.
t (h)
Figure lb. Weight variation of the five bottom ash samples during the CO2 treatment.
The bottom ash reactivity towards CO2 also resulted in a rapid weight variation for F1, F2, F3 and $2, the maximum being reached after about one hour, whereas once again no significant change was noted for S 1 (Fig. lb).
3.2. SO4 and Pb leaching behaviour As expected from the full-scale study results, sulphate leaching increased with the accelerated CO2 treatment. Apart from S 1, a sulphate release plateau was identified after a few minutes of carbonation in the bottom ash leachates (Fig. 2a). This reaction was accompanied by a decrease in the pH values (Fig. 2b). The fresh bottomash (F1, F2, F3) leachates, initially at around pH 11.5-13.0, fell to pH 9.0-9.5 after ageing. The change in pH of the leachates from the stored bottom ash (S 1, $2) was significantly less: from pH 10 to pH 8.5 for $2, and no significant variation for S 1 (steady at pH 8.0-8.5). The leachates of the non-carbonated F2 and F3 bottom ash also contained 0.57 and 4.87 mg/kg of Pb, respectively. The Pb concentrations for all the other samples (both noncarbonated and carbonated) were below the detection limit (0.02 mg/kg).
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Figure 2a. 504 release in the leachates of the five bottom ash samples during C O 2 accelerated ageing with time.
'
360
0
60
120
180
240
300
360
t (min)
Figure 2b. pH evolution of the leachates.
3.3. Solids description The Fourier transform infrared (FTIR) spectra of the bottom ash samples both before and after carbonation made it possible to monitor the chemical evolution of the solids content during the treatment. This was especially significant for the two extreme cases, i.e. the F3 fresh bottom ash, which showed the highest temperature rise (up to 42 ~ and the S 1 stored bottom ash, which showed no significant change during carbonation. Before accelerated ageing, the F3 bottom ash spectrum showed the main characteristic peaks of calcite (1430, 875 and 713 cm -l) and a small peak at 3645 cm -I corresponding to the OH vibration band of portlandite. After the 6-hour treatment the portlandite peak disappeared and a higher calcite content is detected (Fig. 3a). The spectra of S 1 bottom ash before and after treatment are superimposable, in agreement with no reactivity towards CO2. The portlandite peak is absent and the peaks relative to the presence of calcite are identical (Fig. 3b). Calibration from ternary mixtures (decarbonated bottom ash/ portlandite/calcite) is consistent with a 2-5% portlandite content in the F3 bottom ash. For the intermediate samples (F1, F2, $2) the portlandite peak was not distinguished on either the treated or untreated sample spectra. Nevertheless, the samples contained a higher calcite content after treatment than before (as shown by the increased intensity of the three main characteristic peaks). Thus if portlandite is present in these samples, its content is necessarily below 1-2%. Minerals other than portlandite (non detectable by FFIR) may also be carbonated. Even if complementary calibrations and special bottom ash preparation (for example fine particle separation) were to be developed, infrared spectroscopy still appears to be an efficient tool for determining the maturation state of bottom ash and its evolution during carbonation.
479
F3
C
035
C
! /
~--'~-21" 3500
I [ i
2500
i
i
i
i
, 1 1
1500
500
(r "~)
W avelength $1
o,
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,
,
i
i
3500
,
,
,
i
,
,
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i
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1
I
i
1500
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i
i
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500
Figure 3. FTIR spectra of freshly produced (F3) and stored (S 1) bottom ash before (0 h) and after (6 h) COa treatment (c: calcite; p: portlandite).
4. INTERPRETATION/DISCUSSION Apart from sample S 1 (bottom ash stored for more than five years), the tested samples of MSW incineration bottom ash all showed a similar evolution during the combined tests. In particular, maximum sulphate release from the leachate was attained in less than 1 h of treatment with CO2 gas. The results also showed that contained lead was trapped during the carbonation treatment. Sample S 1 did not react to the accelerated carbonation. Moreover, the pH of its leachate was constant at around 8, which signifies that it had already attained peak maturation during its storage. The initial quantities of leached Pb and SOn obviously depend on the nature, and thus the origin, of the bottom ash. The sulphate content of sample $2 was higher than that of sample F1, which in tum was higher than that of F2, again higher than that of F3. During the
480
accelerated maturation, the quantity of released sulphates was multiplied by a factor of 2 for sample $2 (already naturally aged over two years), by a factor of 4 for samples F1 and F2, and by a factor of 84 for sample F3. The last, which gave the most basic leachates (pH 13) corresponds to the 'youngest' of the fresh bottom waste samples.
Table 1. Enthalpy of the carbonation reactions of certain calcic minerals in bottom ash and of the oxidation of iron metal (Fe(s)) and aluminium metal (Al(s)). Mineral
Reaction
Portla'ndite Ca(OH)2(s) + CO2(aq) r Calcite + H20 Ettringite* 0.33 Ett. + CO2(aq) r Calcite + Gypsum + 0.33 A1203 + 8.67 H20 Wairakite Ca(AI2Si4)OI2:2H20+ CO2(aq)r Calcite + + Kaolinite* + 2 Chalcedony Prehnite 0.5 CaEAIESiaOlo(OH)2 + CO2(aq) + 0.5 H20 r Calcite + 0.5 Chalcedon~ + 0.5 Kaolinite* Diopside CaMgSi206 + CO2(aq) + H20 r Calcite + 2 Chalcedony + Brucite Anhydrite CaSO4 + C O 3 -2 r Calcite + 5 0 4 -2 Larnite 0.5 Ca2SiO4 + CO2(aq) r Calcite + 0.5 Chalcedony Anorthite Ca(A12Si2)O8 + CO2(aq) + 2 H20 r Calcite + Kaolinite* Fe(s) 1.333 Fe(s) + O2(aq) r 0.667 Fe203(s) Al(s) 1.333 Al(s) + O2(aq) + 2 H20 r 1.333 Al(OH)3(s) Pyrite 0.211 FeS2(s) + O2(aq) + 0.7'37 H20 r 0.211 Fe(OH)3(s) + 0.421 5042- § 0.482 H+ Pyrrhotite 0.444 FeS(s) + O2(aq) + 1.111 H20 r 0.444 Fe(OH)3(s) + 0 . 4 4 4 5042 § 0.889 H + ,,,
log K (25~
1~-IR (kcal)
,dk][-IR pH=13.46
l~kI-IR pH=12.23
14.20 10.08
-22.10 -0.85
- 10.91 16.51
- 14.05 13.40
10.53
-27.10
-9.05
- 12.20
6.73
-14.42
3.65
0.50
3.93
-11.91
6.17
3.01
4.17 12.95
-1.81 -22.42
-1.65 -4.46
-1.39 -7.61
11.58
-27.67
-9.60
-12.76
121.49 -175.36 -125.31 189.64 -274.45 -237.19
-111.41 -223.29
47.03
-71.17
-47.45
-32.11
61.03
-94.49
-68.69
-53.28
The AHR (kcal) values are given for 25 ~ under standard conditions. AHR (pH = 13.46) corresponds to the enthalpy of the reaction in a bottom waste leachate (I = 0.47, log fCO2 = -11.78, log fO2 = -29.28, log aCO2(aq) = -13.29, log aO2(aq) = -32.22), and AHR(pH = 12.23) is calculated for a less saline leachate (I = 0.19, log fCO2(g) = -9.47, log fO2(g) = -39.46, log aCO2(aq) = -10.94, log aO2(aq) =-42.36). *Detailed chemical formula: Ettringite Ca6A12(SO4)3(OH)I226H20; Kaolinite A12SiEOs(OH)4.
The results of the laboratory study reflect the observations of a full-scale study made in the field on the alteration mechanisms of a large bottom ash heap subjected to atmospheric weathering and where CO2 consumption was evidenced and calcite found to be the predominant neoform mineral generated. A 22-month monitoring of species concentrations in the leachates indicated three major stages in the maturation process: 1) chloride leaching, 2) carbonation with Pb, Zn trapping, and 3) late sulphate leaching [ 1,2]. Meima and Comans (1999) came to similar conclusions in a separate study [3].
481 The temperature increase noted in the columns during the accelerated tests is unmistakably linked to the carbonation of highly reactive phases and notably to the presence of portlandite. This carbonation reaction also probably plays a role in the temperature rise noted during the natural maturation of bottom waste, despite the fact that the reactions generally quoted in the literature to explain this rise correspond to the oxidation of the metals Fe ~ A1~ [4]. At this stage of the study, therefore, we considered it essential to determine the relative importance of the carbonation reaction in the exothermic stage undergone by all bottom ash on a maturation site. Because of the variety of mineral species present in MSW incineration bottom ash, numerous reactions, such as carbonation, oxidation-reduction and hydration, are likely to influence the maturation (Table 1). The minerals that we have taken into consideration are typical minerals found in MSW incineration bottom ash (portlandite, ettringite, larnite, anhydrite), minerals associated with zeolites (wairakite, prehnite) present in the first stages of ageing, high-temperature minerals derived from melts (diopside, anorthite), metals that are always present at the end of combustion (A1~ Fe~ and sulphides (pyrrhotite, pyrite). More than 40 reactions were calculated. As a first approximation, with regard to the aluminium oxides, the reaction product has been defined as close to A1203 even where present as A1203.nH20 gels [5]. Under standard conditions, all the reactions are theoretically exothermic (AH~ as shown by the calculations made using the SUPCRT92 software [6]. If one considers the effect of salinity on ion activity (calculations done with the EQ3NR software [7]) it is seen that in fact only a few reactions are likely to be sufficiently efficient to explain the temperature rise. The remaining possibilities are i) the carbonation of portlandite, larnite and anhydrite, ii) the carbonation of anorthite and wairakite accompanied by the formation of kaolinite, iii) oxidation of the metals, and iv) hydrolysis of the sulphides. Of these, the last two are the most exothermic. The carbonation of calcium sulphates, such as ettringite, anhydrite and gypsum, is not an exothermic process. Moreover, the reactions are highly dependent on the ionic strength. In general, reaction enthalpy increases by 1 kcal per gram of salinity, and if salinity is too high it can inhibit the reactions. In bottom waste from which iron particles have been magnetically removed, the mass percentages of native metals are of the order of 3% for Fe ~ and 0.5% for A1~ with sulphides about 0.5% and portlandite about 6%. The amount of freeable energy is associated mainly with oxidation reactions of the metals and of some sulphides, rather than with the carbonation reaction. However, this observation must be modulated by parameters such as specific surface area of the minerals and/or size of the crystallites, which play a not insignificant role in the level of reactivity. All the observations show that the cited metals are present in forms with limited surface areas, initial manufactured goods (Fe~ melt drops (A1~ or globules (sulphides) commonly included in glass, unlike the portlandite that is present as fine crystallites highly accessible to the fluids. The high calorific potential of the metals and sulphides will thus be compensated by the specific surface area of the minerals susceptible to carbonation and each of the components will play a role in the temperature rise associated with the natural maturation of the bottom waste.
482 5. C O N C L U S I O N Conducting leaching tests on artificially-aged MSW incineration bottom ash is proving to be a valuable tool in forecasting its long-term behaviour, because the physico-chemical characteristics of the obtained leachates correspond to those of fully-matured bottom ash. Ageing can therefore be achieved in a few hours in the laboratory, as against months and even years, depending on storage conditions, for the natural ageing process. Moreover, in cases where the threshold values of toxic elements in the bottom ash are exceeded, this test enables one to determine the most suitable treatment to upgrade the bottom ash on its leaving the incineration plant, such as improving its physico-chemical characteristics through the addition of mineral substances. ACKNOWLEDGEMENTS This work was supported by the MATE (French Ministry of the Environment). The authors would like to thank Sir Patrick Skipwith and Ms Rowena Stead.
REFERENCES 1. P. Freyssinet, Y. Itard, M. Azaroual, B. Clozel, P. Piantone and D. Guyonnet, Proceedings of the International Conference Waste stabilization and environment, Lyon, France, (1999) 92. 2. B. Clozel, F. Bod6nan and P. Piantone, Proceedings of the International Conference Waste stabilization and environment, Lyon, France, (1999) 46. 3. J.A. Meima and N.J. Comans, Appl. Geochem., 14 (1999) 171. 4. C.A. Johnson, G.A. Richner, T. Vitvar, N. Schittli and M. Eberhard, J. Contam. Hydrol., 33 (1998) 376. 5. L.C. Lange, C.D. Hills and A.B. Poole, Environ. Sci. Technol., 30 (1996) 25. 6. J.W. Johnson, E.H. Oelkers and H.C. Helgeson, Computers Geosci., 18 (1992) 899. 7. T.J. Wolery, EQ3NR 7.0, UCRL-MA-110662-PT-I, Lawrence Livermore National Laboratory, Livermore, California (1992) 246.