Environ. Sci. Technol. 2000, 34, 5030-5037
Morphological and Chemical Characterization of Calcium-Hydrate Phases Formed in Alteration Processes of Deposited Municipal Solid Waste Incinerator Bottom Ash C. SPEISER, T. BAUMANN, AND R. NIESSNER* Institute for Hydrochemistry, Technical University of Munich, Marchioninistrasse 17, D-81377 Munich, FRG
During a study which investigates the exothermal heating of municipal solid waste incinerator bottom ash (MSWI) in a landfill samples were taken at a solid waste incinerator in southern Germany operating at ∼1000 °C. The chemical and mineralogical bulk composition was determined by X-ray fluorescence (XRF) and X-ray diffraction (XRD). Single bottom ash particles were investigated by optical microscopy and scanning electron microscopy with quantitative energy-dispersive X-ray microanalysis (SEM/ EDX). The fresh bottom ash consists of ash (42%), melting products (40%), metallic components (8%), usually aluminum, iron, and copper, and residual parts (10%). The main phase of the bottom ash is glass (∼ 40%) with relics (e.g. quartz) and quench phases (e.g. gehlenite). The main crystalline phases are silicates (e.g. gehlenite, augite, diopside, quartz), oxides (e.g. magnetite, spinel, hematite), carbonates (e.g. calcite, metal-carbonates), and salts (e.g. chlorides and sulfides). In the deposited bottom ash endothermic and exothermic alteration processes are observed (dissolution/precipitation of salts, glass corrosion, hydration and oxidation reactions of metals, slaking of lime, cementation and carbonation processes). In the course of these processes new mineralogical phases are formed. Among these are e.g. anhydrite, portlandite, calcite, iron oxides and hydroxides, or gibbsite. These minerals are always accompanied by different calcium-hydrate phases with different mineralogical and chemical properties. The hydrate phases were morphologically and chemically characterized by SEM/EDX. The single crystals have a fibrous, ribbonlike, or tabular habit. Ca, Al, Si, and Fe were found as main components, with minor amounts of Na, K, Mg, Cu, Zn, Ti, Mn, and Cl, S, P.
Introduction At the current state the threshold values for total organic carbon (TOC) given in the Technical Instruction for Municipal Waste (TASi (1)) can only be achieved by treatment in a Municipal Solid Waste Incinerator (MSWI). The low TOC threshold should ensure a minimum of organic decomposition reactions within municipal waste disposals. A maximum inertization of the municipal waste is also indispensable for * Corresponding author phone: +49 (89) 7095 7981; fax: +49 (89) 7095 7999; e-mail:
[email protected]. 5030
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 23, 2000
a future use of the waste residues in the sense of recycling. Landfills without aftercare must be established with inert, mineralized, and homogenized waste residues. However, the municipal solid waste incinerator (MSWI) bottom ash is generally highly reactive (2-7). During the embedding of the ash residues into the landfill, endothermic and exothermic alteration processes occur. The temperatures in the studied landfill increased up to 90 °C which may cause a destabilization of flexible membrane liner (FML) and a desiccation of the clay barrier; these two form the landfill bottom liner system. As a consequence shrinkage cracks may be formed in the clay liner, affecting the function of the baseliner as a security system (8-12). Therefore a thorough knowledge of the exothermic weathering processes inside the MSWI bottom ash during its deposition is essential to predict the security of landfill. After 2 years deposition five vertical drillings (each 8 m) were brought down into the studied landfill body to investigate the alteration processes. Due to differences in the local composition, in the grainsizes of the material, and in other physical and chemical parameters no complete thermodynamic equilibrium is usually reached during the combustion of waste within the incinerator. The high-temperature phases (glass and quench phases, etc.) and different metal oxides formed in the bottom ash are of primary importance for the alteration processes because of their potential instability in the waste deposit (4). The alteration processes start immediately after the quenching of the hot bottom ash. Depending on the ambient chemical and physical parameters the alteration reactions can persist either for days or for years (2). According to different studies (2, 13-17) the following major alteration reactions can be distinguished in bottom ash: dissolution/ precipitation of salts, glass corrosion, hydration and oxidation reactions of metals or metal oxides, slaking of lime, and hardening reactions (cementation and carbonation). The reported alteration products are anhydrite, ettringite, calcite, iron oxides and hydroxides as well as gibbsite to name a few of the resulting weathering phases. Usually, the formation of these phases has been interpreted in terms of chemical and mineralogical concepts of aging processes (like rusting). In most previous studies the observed temperature increase has been attributed to hydration and oxidation reactions of metallic components (2, 3, 13). Bottom ash is a very heterogeneous material, but this study shows clearly that the observed alteration processes are closely combined with the formation of Ca-hydrate phases. The formation of these hydrate phases is considerably exothermic (-40 to -140 kJ/mol) (18). Previous studies reported on single Ca-hydrate phases (e.g. ettringite, portlandite) but did not systematically investigate their formation, chemical composition, morphology, and alteration-microstructures. The aim of this paper is to characterize these Ca-hydrate phases and to supply an insight into the participation of these phases formed in the alteration and hardening processes of the weathered MSWI bottom ash. Have the newly formed Ca-hydrate phases an effect on the alteration processes in the bottom ash landfill? Are reaction shells formed that could impede further alteration? Does the composition of Ca-hydrate phases and the microstructure of the reaction sites give hints on the participation of metal particles in the Ca-hydrate formation reactions? Are there indications of metal-fixation in the hydration products? This information is necessary to assess the development of the MSWI bottom ash under various conditions and to optimize the landfill operation. 10.1021/es990739c CCC: $19.00
2000 American Chemical Society Published on Web 10/21/2000
TABLE 1. Chemical Composition of Fresh Grain-Size Fractionated Bottom Ash Measured by X-ray Fluorescence Spectrometrya CaO, wt % Na2O, wt % K2O, wt % SiO2, wt % Al2O3, wt % Fe2O3, wt MgO, wt % TiO2, wt % Zn, mg/kg Ba, mg/kg S, mg/kg Cl, mg/kg Cu, mg/kg Pb, mg/kg Mn, mg/kg Ni, mg/kg Cr, mg/kg a
>20 mm
4-20 mm
1-4 mm
0.2-1 mm
10 wt %) are SiO2, CaO, Fe2O3, Al2O3. Na2O, K2O, MgO, and TiO2 are found in minor concentrations (0.4-5 wt %). Ba, Zn, Cl, Mn, and Pb are trace elements (
