Chemical characterization and leaching of treated fly ash from a MSWI plant

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1 Chemical characterization and leaching of treated fly ash from a MSWI plant I. M. Martins & A. M. Esteves Laboratório Nacional de Engenharia Civil, Lisboa, Portugal J. P. Forth University of Leeds, Leeds, United Kingdom ABSTRACT: With the aim of reducing landfill deposition the incorporation of industrial waste into the cement matrix of building materials is becoming common practice. An assessment of the elements and compounds present in the waste that have a detrimental effect on cement hydration or on the environment, such as lead, zinc, chromium and sulphates, should always be performed. Fly ashes from the heat recovery and air pollution control systems of MSWI plants are classified as hazardous wastes owing to the concentration of potentially hazardous heavy metals as well as their high salt content. Waste pre-treatment represents a possible approach to prepare these residues for incorporation into building materials as it will reduce their pollution potential. This paper presents the results of the chemical characterization of two fly ash mixtures, collected at different locations from a Portuguese MSWI plant, before and after a selected pretreatment, and an evaluation of their leaching behaviour. 1 INTRODUCTION Nowadays, there are growing pressures to use waste materials within the construction sector. This use of wastes in civil engineering requires an evaluation of both the environmental and technical suitability of the waste. From the environmental point of view the main concern is the leaching out of hazardous substances, such as heavy metals, to soil and water supplies while from the technical point of view issues like changes in cement hydration, durability analysis and economic evaluation should be addressed. Extensive research has been performed in to the use of solid residues from municipal solid waste incineration (MSWI) as cement or aggregate replacement in building materials (Al-Rawas et al., 2005, Aubert et al., 2006, Collivignarelli & Sorlini, 2002, Huang & Chu, 2003, Juric et al., 2006, Lin et al., 2003, Reijnders, 2007, Wainwright, 2000). Municipal solid waste (MSW) generation in Europe increased at an annual rate of 2% from 1995 to 2003 and, in 2003, 17.3% of these wastes were incinerated (Eurostat, 2005). Solid residues from MSWI have different chemical compositions and can be broadly classified as bottom ash and fly ash. Bottom ash is recovered from the base of the combustion chamber and fly ash from the heat recovery and air pollution control systems. Incineration of a tonne of municipal solid waste generated, on average, 200 to 300 kg of bottom ash and 25 to 50 kg of fly ash (Chandler A. J., 1997). Owing to stringent air emission policies, fly ash which contains high contents of soluble salts, such as calcium carbonate, sodium and potassium chlorides and sulphates, heavy metals and residual amounts of organic composites are classified as hazardous waste (Ferreira et al., 2005, Wan et al., 2006). In order to upgrade MSWI fly ash to a usable product it is essential to use preliminary treatments that minimize the prejudicial effects of some of these compounds. The composition of MSWI fly ash varies from plant to plant and it is not possible to establish a universal treatment for them (Derie, 1996). At present, suggested treatments can be divided into

2 three main categories: stabilization/solidification (S/S), thermal treatments and extraction treatments (Alba et al., 2001, Chan et al., 2000, Chimenos et al., 2005). Experiments involving a combination of the aforementioned processes have been applied to MSWI fly ash in order to upgrade them for incorporation in building materials (De Casa et al., 2007, Mangialardi, 2001). Stabilization/solidification (S/S) of MSWI fly ash is the most common treatment and is successful due to the chemical and physical processes that occur between the residue and a binder. When cement is used as a binder it produces changes in the speciation and solubility of the contaminants as they interact with the cement hydration products; physical adsorption and encapsulation can take place (Batchelor, 2006, Malviya & Chaudhary, 2006, Qiao et al., 2007). A major disadvantage of S/S is the increase in porosity and permeability of the solidified structure with time and the consequent reduction in strength, due to the leaching of large amounts of soluble salts. Another drawback of S/S lies on the increase of weight and volume of the residue after treatment (Geysen et al., 2004). Thermal treatments are not as common as S/S owing mainly to the large input of energy required that make them very costly. By heating below or at the melting temperature the chemical and structural properties of the residue are changed and the vitreous or crystalline slag formed can be usefully applied as a construction material (Sakai & Hiraoka, 2000). Extraction treatments involving different media lead to an efficient reduction in salts content (Abbas et al., 2003, Ferreira et al., 2005, Hong et al., 2000). Water-based extraction is the simplest process to treat MSWI fly ash. The efficiency of this treatment relies on the type of residue and on the liquid to solid ratio (L/S). According to Derie a liquid to solid ratio between 5 and 10 and a washing time of 1 hour will remove soluble alkali chlorides and some calcium sulphate without solubilising to a great extent the undesirable metals (Derie, 1996). Wilewska-Bien et al prooved the feasibility of water extraction treatment with an L/S equal to 3, 5 and 10 in a pilot plant (Wilewska-Bien et al., in press). Abbas et al. reported the removal of salts in MSW fly ash from fluidized bed incinerators using L/S of 1,2 and 4 with extraction times of up to 60 minutes (Abbas et al., 2003). The main drawback of water extraction is the generation of an effluent whose characteristics need to be evaluated before it is discharged to surface waters. The aim of this research is to define a preliminary treatment, using water as the extractant media that will reduce the level of salts in MSWI fly ash to a level that does not impair their incorporation into concrete and reinforced concrete. This paper reports the chemical and mineralogical characterisation of two fly ashes from a Portuguese MSW incinerator according to their leaching behaviour and the effects of the selected treatment on the content of chloride, sulphate, chromium, copper, lead and zinc. 2 MATERIALS AND METHODS 2.1 Fly ash The fly ashes used in this study were collected from two distinct locations, depicted in Figure 1 as I and II, of a Portuguese MSW incineration plant. The unit is a mass burn waste-to-energy facility, with a capacity of 2000 tons/day, using semidry scrubbers and fabric bag-house filters for emissions control. During three days, ash identified as Ash I (comprising a combination of ashes from the hopper of the boiler and of the economiser) and Ash II (a mixture enclosing Ash I, semidry scrubber ash and fabric filter ash) were sampled and stored in dry and sealed containers. 2.2 Chemical and mineralogical characterisation To evaluate the chemical composition of the fly ash samples they were first ground with a HERZOG HSM 50 vibrating grinding mill. They were then passed through a 90 µm sieve, mixed with wax (wax/ash ratio of 0.6) and pressed into pellets using a HERZOG press by applying a 200kN force for 30 s. The oxide composition was obtained using a wavelength X-ray fluorescence spectrometry, with an AXIOS/PANANALYTICAL spectrometer. Sulphate content, expressed as SO3, was determined by gravimetry and chloride content by titrimetry.

3 Figure 1. MSW incinerator: 1 Furnace; 2 Boiler; 3 Superheater; 4 Economiser; 5 Scrubber; 6 Fabric filter baghouse; 7 Stack The crystalline structure of the fly ash was assessed using an X-ray difractometer at 35 kv and 45 ma using Co Kα radiation and 10-60º 2θ range. Before the X-ray diffraction (XRD) analysis was performed the fly ash was passed through a 106 µm sieve. The water content, Wc, of the fly ashes was tested by weighing 10 g samples and drying at 105 ºC until a constant mass was reached. Loss on ignition, LOI, was determined from samples of 1 g at 950 ºC until constant mass was reached. 2.3 Leaching test The European compliance leaching test EN (CEN, 2002) for granular waste materials was performed on untreated fly ashes. Demineralised water was used as the leachant at a liquid to solid ratio (L/S) of 2 l / kg and a leaching time of 24 h. The leachate was recovered after filtration over a 0.45 µm filter and acidified with ultra-pure nitric acid. Heavy metals concentration was determined using an inductively coupled plasma atomic emission spectrometer, ICP- AES Jobin Yvon 24, and a graphite furnace atomic absorption spectrometer, GBC 904 GFAAS. Chlorides and sulphates were quantified by titrimetric and gravimetric methods. 2.4 Fly ash water extraction The soluble salts content of the fly ashes was reduced by mixing the residues with distilled water and following three different stirring regimes: One washing step with L/S ratio 2 l / kg and extraction time of 15 minutes; Three washing steps with L/S ratio 2 l / kg and extraction times of 5 minutes; Two washing steps with L/S ratio of 5 l / kg and extraction times of 5 minutes. In sequential washing steps, the solids were allowed to settle and the supernatant liquid was filtered. The material retained on the filter was recovered and mixed with the solid. The chlorides and sulphates concentration in the water extracts were obtained by titration and gravimetry, respectively. A quantitative analysis for the heavy metals was performed by ICP- AES and GFAAS.

4 3 RESULTS AND DISCUSSION 3.1 Chemical and mineralogical characterization Table 1 shows the composition of the untreated fly ashes expressed as metal oxides; calcium is the major metal. The main difference between the ashes is the higher content of silica in Ash I, which is twice the value of Ash II, and the level of chlorides in Ash II, which is about five times more than Ash I. As stated in the Introduction, the high chloride and sulphate content make these ashes unsuitable for use in concrete and so they must be subjected to preliminary treatment. Regarding the heavy metals, zinc is the most concentrated element in the two ashes. Loss on ignition is high on both ashes, especially in Ash I. Table 1. Semi-quantitative analysis of untreated fly ashes. Oxides I II % % Al 2 O CaO SiO Na 2 O MgO K 2 O Fe 2 O TiO Cl SO CrO MnO NiO CuO ZnO SrO Sb 2 O BaO PbO CdO LOI 9 20 W c The mineralogical composition of Ash I and II is very complex. The low intensity of the peaks in the X-ray diffraction analysis show that most of the compounds in the ashes are mainly in the amorphous form. The main differences between the ashes observed in the semiquantitative analysis are confirmed by the XRD results. The higher height of the quartz (α-sio 2 ) peak in the X- ray diffractograms show the high level of SiO 2 in Ash I in relation to Ash II. The high concentration of chlorides in Ash II in relation to Ash I is observed through the higher height of the crystalline forms of halite (NaCl), sylvite (KCl) and calcium chloride hydroxide. The heights of the halite and sylvite peaks confirm that Na and K were present in large quantities in Ash II when compared with Ash I. The main crystalline forms of calcium, the major element in the ashes, are calcite (CaCO3) and anhydrite (CaSO4), however, it is possible to observe the presence of calcium as portlandite (Ca(OH)2), lime (CaO) and calcium silicate. Iron and titanium have been identified by the XRD analysis in the forms of hematite and titanium oxide. Regarding the presence of heavy metals, it was not possible to match any of the peaks for these elements, probably owing to their low content.

5 a) b) Figure 2. X-ray pattern for raw and washed fly ashes a) Ash I b) Ash II: Q-quartz, C-calcite, L-lime, P- portlandite, A-anhydrite, CS-calcium silicate, CCh-calcium chloride hydroxide, S- Sylvite, Ha-halite, H- hematite, T-titanium oxide, Pe-perovskite, MS-calcium monosulphoaluminate, E-ettringite 3.2 Leaching test The leaching test used in this investigation is the recommended European Union compliance leaching test for granular waste. Table 2 shows the leachate content of chlorides, sulphates, and of selected heavy metals for untreated fly ashes as well as the regulatory limits for nonhazardous wastes (Nh) which are to be disposed of to landfill, according to Council Decision 2003/33/CE.

6 According to Table 2, untreated Ash II has higher contents of all the selected elements. The heavy metal most leached out was lead (0.5 % and 11.4% of the total concentration for Ash I and Ash II, respectively). For Ash I, only the chloride content exceeded the limit for nonhazardous waste though this ash is classified as hazardous waste. Ash II is also categorized as hazardous owing to the presence of Pb, Zn and Cl. The ph of the leachates was 12.5 and 12.2 for Ash I and II respectively. At these levels of ph there is an increase in solubility of hydroxides of zinc and lead (Chimenos et al., 2005). Table 2. Leachate contents and limits for non-hazardous waste Element Ash I Ash II Nh limits mg / kg dry wt. Cd < 0.05 < Cr Cu Pb Zn Cl SO Fly ash water extraction As can be seen from Table 3 the ph of the studied ashes during extraction is always very high. The higher content of lime in Ash I is responsible for the higher ph of the extracts when compared with Ash II. Water extraction results in significant removal of chlorides from the fly ashes (Tab. 3). For Ash II, the highest amount of chloride was removed using an L/S of 2 l / kg with 5 minute extraction times. After the second stage of extraction 95% of the chlorides present in Ash II were removed. For Ash I all chlorides were removed at the end of the second step using an L/S = 5. However, owing to the fact that this experimental regime involved the major consumption of water and that the total chloride content of Ash I is much lower than that of Ash II, the use of an L/S = 2 with a two step extraction of 5 minutes will be sufficient to take away most of the chlorides and allow the use of this ash in building materials. Longer extraction times at L/S = 2 are no more effective than the shorter times. Both ashes generated hydrogen during extraction (in a greater amount for Ash I), corroborating the presence of metallic aluminium, as stated by other authors (Aubert et al., 2004, Chan et al., 2000). This evolution of hydrogen must be further evaluated before MSWI fly ash can be considered for use in concrete. Sulphates were not removed to the same extent as the chlorides. However, combining the results for sulphate solubilization with those obtained for the chlorides it was concluded that the best extraction regime occurs at L/S = 2 with two extraction times of 5 minutes. Table 3. Results of extraction experiments Extraction condition Ash I Ash II Final ph SO 3 (%) Cl (%) Final ph SO 3 (%) Cl (%) L/S = 2 15 min L/S = 2 5 min min min L/S = 5 5 min min The presence of Cd, Cr, Cu, Pb and Zn found in the extracts during the multistage treatment with a L/S = 2 was also evaluated (Fig. 3). The quantity of Cd and Cu released was insignificant and is not shown in Figure 3. Generally, the release of Cr and Zn from the MSWI fly ashes decreases from stage to stage. For Pb a different behaviour was observed as there was an increase

7 in removal during the second washing step. In relation to the total content it was found that there was only a small release in heavy metals, with Cr being the most leachable metal in Ash I, (0.06%), and Pb in Ash 2, (0.15%). Release of heavy metals (mg/kg) Ash I Cr Pb Zn stage 1 stage 2 stage 3 Release of heavy metals (mg/kg) Ash II Cr Pb Zn stage 1 stage 2 stage 3 Figure 3. Release of heavy metals in the multistage treatment at L/S = 2 From the results of the XRD analysis of the treated fly ashes following the multistage treatment with L/S=2 it was also possible to see the evolution of chlorides due to the extraction treatment (Fig. 2). In the washed Ash I sample the disappearance of the peaks of halite and sylvite (the chloride crystalline forms) could be observed. In the Ash II sample there is also a significant change in the pattern of the diffractogram; the disappearance of the peaks of halite, sylvite and calcium chloride hydroxide confirm the efficient removal of chlorides. Other peaks from the XRD analysis indicate a reduction in their intensities and there were also some new peaks detected (i.e. ettringite). 4 CONCLUSIONS This study has shown differences in the chemical and mineralogical composition of Ash I and Ash II which must be further investigated in order to confirm the use of MSWI fly ashes in the construction sector. According to the regulatory thresholds of the Council Decision 2003/33/CE, the two untreated fly ashes are classified as hazardous wastes. The high chloride level in the leachate of Ash I is the only reason for this classification. Remarkably, the removal of chlorides in fly ashes is achieved by the simple process of washing the residues with distilled water, at the ph of the wastes. The best compromise between low water/time consumption and chloride removal was a two step washing at L/S of 2 l / kg using 5 minute extraction times; 83% of chlorides from Ash I and 95% of chlorides from Ash II are removed. Additional research is being carried out on the leaching behaviour of treated fly ashes in order to define possible percentages of incorporation for both ashes in concrete. 5 ACKNOWLEDGMENTS The authors wish to thank Valorsul, S.A. for their kind cooperation in supply and delivery of the fly ashes. REFERENCES Abbas, Z., et al Release of salts from municipal solid waste combustion residues. Waste Management, 23 (4):

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