Immobilization of lead and zinc in scrubber residues from MSW combustion using soluble phosphates
Introduction
In Flanders, municipal wastes are separated by the citizens in different fractions: glass, paper, hazardous waste, vegetables-fruit-greenery (GFT), Plastic-bottles-Metals-packaging-and-Beverage-carton (PMD), non-separable mixed fraction and a bulky fraction further fractionated after collection. Due to good municipal waste separation practice, in 2000, 65.8% of municipal wastes were collected separately and were reused or recycled. For the total Flemish population of 6 million people only 1 million tons of municipal solid waste has to be incinerated giving 250,000 tons of bottom ashes and 40,000 tons of flue gas cleaning residues (boiler ashes, fly ashes and scrubber residues) (Mira-T, 2001). All types of ashes and residues contain minor or trace concentrations of heavy metals. In contrast to bottom ashes, flue gas cleaning residues contain the more volatile metals such as Pb and Zn, in relatively high concentrations and As, Cd, Cr and Hg in trace amounts (IAWG, 1994). The high concentration of these metals together with the high concentration of soluble salts and of some organic pollutants make that flue gas cleaning residues cannot be recycled in construction application as is the case for bottom ashes (Vrancken, 2001). Disposal of flue gas cleaning residues is the only option. It seems that, even in the future, disposal will remain the main waste management option for these ashes. Reducing the heavy metal concentration of ashes from waste incineration by improving the separation of wastes before combustion is not easy. Pb not only occurs in the non-ferrous metal fraction, but also in brick and ceramics. The major Pb source has not been identified yet. The Zn concentration of waste incinerator ashes can still be reduced by improving the separation of batteries, PMD and GFT fractions (Hensels, 2002). Improving the separate collection and treatment of Hg-containing lamps and batteries can reduce the Hg concentration.
To prevent leaching of heavy metals, flue gas cleaning residues have to be treated before landfilling. The treatment mainly consists of mixing the residue with cement and water at a cement to residue mass ratio of 0.1–0.3. It was shown that cement decreases heavy metal leaching by the incorporation of metals in a C-S-H network in which calcium is substituted by the metal (Rose, 2001, Ziegler, 2001, Moulin, 1999, Johnson, 1999, Gougar, 1996).
In Belgium and Germany, DIN 38414-S4 leaching test results have to be used to compare leaching with landfill acceptance criteria (Vlarem II, 1995, TA Abfall, 1991). The test consists of leaching at L/S=10 (l/kg) during 24 h with DW water (DIN 38414-S4, 1984). A comparable test (X 31-210) is used in France (van der Sloot, 1997). The leachate pH is only controlled by the composition of the waste or of the treated waste in contrast to the TCLP and NEN 7341 test where acid is dosed (van der Sloot, 1997).
The heavy metals with the highest concentration in MSWI flue gas cleaning residues are Pb (1–7 mg/g) and Zn (3–10 mg/g). Of the three types of flue gas cleaning residues (boiler ash, fly ash and scrubber residue), scrubber residues usually do not comply with landfill acceptance criteria for Pb (2 mg/l) and in some cases for Zn (10 mg/l), the other residues in general do not constitute a problem. Scrubber residues are alkaline due to the high concentrations of e.g. residual Ca(OH)2 and leachate pH usually exceeds 12. An increased leachability is observed for amphoteric metals such as Pb and Zn at such high pH value. Using cement as additive for immobilization does not significantly decrease the pH of the leachate, so that, although a large fraction of the heavy metal concentration is incorporated in the C-S-H network, the leaching of Pb at high pH still exceeds the limit of 2 mg/l. The same problem is observed in Japan where the JLT-13 leaching test (Uchida, 1996) is used.
Therefore, other additives are required that form less soluble metal containing minerals at a pH above 12. In several studies it was shown that phosphates could be used for immobilization of divalent heavy metals from wastewaters, solid wastes and contaminated soils (Eusden J.D., 2002, Yang, 2001, Crannell, 2000, Lyons, 2000, Boisson, 1999, Eighmy, 1997, Uchida, 1996, Ma, 1993). The decreased heavy metal leachability can be attributed to the formation of minerals based on an apatite structure e.g. hydroxy-pyromorphite [Pb5(PO4)3OH] and chloro-pyromorphite [Pb5(PO4)3Cl] or to the formation of solid solutions or mixed precipitates of divalent or trivalent metals in minerals based on such an apatite structure (Eighmy, 1997, Zhang, 1997, Zhang, 1998).
The objective of this study was to evaluate the use of soluble PO43− for the immobilisation of scrubber residues and compare Pb and Zn leaching with results obtained for cement-treated residues. Attention had to be given to leaching at pH above 12.
To evaluate PO43− treatment, four different scrubber residues were solidified/stabilised (S/S) by mixing with water and Na2HPO4 at a dosage of 0, 0.1, 0.2, 0.3 and 0.4 mg/g residue and leached following the DIN 38414-S4 leaching protocol after 1 week curing. Pb and Zn leachate concentrations were compared with leachate concentrations of samples of the four residues solidified/stabilised with cement (dosages of 0, 0.1, 0.2, 0.3, 0.4). Also pH dependent leaching (pH ranging from 0.3–12.5) was used to compare both treatments.
For one scrubber residue, the efficiency of Na2HPO4, NaH2PO4 and H3PO4 as soluble PO43− source are compared.
Section snippets
Scrubber residues
Immobilization with cement is compared to immobilization with Na2HPO4 for four different scrubber residues (SR1–SR4). SR1 is a scrubber residue obtained from a municipal solid waste incinerator (MSWI) equipped with Seghers grate furnaces and steam boilers. The facility consists of two identical lines with a total annual capacity of 200,000 ton of waste. Flue gas cleaning includes injection of lime (semi dry system) and activated carbon in the scrubber, followed by a fabric filter to separate
Characterisation of the scrubber residues
Table 1 gives total concentrations of major and minor elements of the four scrubber residues (SR1, SR2, SR3 and SR4). The results show that the lime based scrubber residues (SR1, SR2, SR3) contain 260 mg/g to 350 mg/g Ca and 19.2 to 24.6 mg/g Na. The NaHCO3-based scrubber residue (SR4) contains of course less Ca (90.8 mg/g) but much more Na (296.4 mg/g). Pb and Zn are the heavy metals in highest concentrations (3.05 mg/g- 4.84 mg/g for Pb and 3.65 mg/g- 12.28 mg/g for Zn). Table 2 shows
Summary and conclusions
Comparing immobilization of different scrubber residues with cement and with soluble PO43− showed that with soluble PO43−, Pb leaching was lower than with cement. With three out of four different residues, Pb was below the landfill limiting value of 2 mg/l when 0.2 g PO43−/g residue was used. For all four residues, Pb could be reduced to below 2 mg/l with 0.27 g PO43−/g residue. Pb exceeded the landfill limit for the cement-treated residues. Based on these limited data, curing cement-treated
Acknowledgements
Grateful acknowledgement is made to Prof. R. Ottenburgs and Prof. R. Swennen for the use of the XRD-equipment, to Bradley Crannell and Taylor Eighmy for data on solubility products and to Jon Petter Gustafsson for many updates of the Visual Minteq version.
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