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Treatment and storage of residues from incineration plants
... The residues from municipal solid waste (MSW) incineration, mainly bottom ash and fly ash (1)(2)(3)(4)(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. ...
... k m,T ) k 0,m e -E m /RT (2) 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. ...
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.
Through thermal waste treatment, organic components should be mineralized to a high degree. Inorganic components are transferred to the output fractions with and without transformation, depending on the type of particle and the technical system. The technological possibilities of today's waste incinerators (grate systems) permit specific treatment of hazardous compounds and therefore a disposal in landfills without negative environmental impact. Efforts to new thermal waste treatment processes (melting processes) are primarily justified in a better resource management. The article demonstrates how to combine methodological approaches regarding hazardous compounds and resource management and gives a vision to future landfill design.
Several glasses of compositions similar to those of real incineration residues were prepared and then characterised in terms of both their mechanical and chemical stabilities. Microhardness, optical microscopy and thermal analysis measurements were correlated with leaching tests in order to determine if glasses with a suitable microhardness also present good chemical stability as they contain heavy metal oxides which are pollutants for the environment. The mechanical and chemical resistances of a glass are influenced by its bulk structure as well as the reactions occurring on the surface. Addition of small quantities of appropriate oxides can improve chemical stability if their mutual influences are established.
Types and Quantities of ResiduesChemical Composition of Residues from IncinerationPhysical and Geotechnical PropertiesAgeing Processes in Residues from IncinerationLeaching Properties of Residues from IncinerationTreatment of ResiduesReferences
Total carbon (TC), carbonate carbon (CC) and total organic carbon (TOC) were determined in bottom ash, filter dust and flue gas of Swiss municipal solid waste (MSW) incinerators. The highest TOC load was found in the bottom ash (2–3 g kg1 MSW), followed by the filter dust (0.1–1.0 g kg −1 MSW) and the flue gas (0.05–0.3 g kg−1 MSW). The composition and behaviour of the bulk of TOC in these products is not yet known. In order to minimize the risk of leaching organic substances as well as metals due to biological, chemical and physical reactions of the products of incineration in a landfill, it is suggested the incineration process be optimized towards complete combustion.
This paper highlights the uses and limitations of thermodynamic calculations in the planning of leach tests in the laboratory or for research in the field. Heavy metal solubility has been studied in leachate from Landfill Lostorf, AG, Switzerland. Also, the influence of pH on the solubility of Cu, Pb, Cd and Zn has been determined in the laboratory. The results have been compared with the maximum allowable heavy metal concentrations in equilibrium with the appropriate (hydr)oxides and carbonates. Copper is supersaturated with respect to Cu(OH)2 in both laboratory and field studies. Complexation with organic ligands is a probable explanation for this observation. Both Zn and Pb are undersaturated with respect to pure (hydr)oxides and carbonates, though agreement between calculations and measurements are close enough, that PbCO3 could be controlling the solubility in the laboratory experiments. The markedly lower concentrations of Pb in the field in comparison with the laboratory data could be explained by the affinity for Pb to bind to solids and the higher solid:solution ratio in field conditions. The solubility of Cd could be controlled by the formation of CdCO3. The relatively high concentrations of Mo (up to 10−5m) in the landfill leachate could be limited by the precipitation of CaMoO4.
Field and laboratory experiments were carried out to elucidate the geochemical and hydrological mechanisms that are important to understand the binding mechanisms of heavy metals in landfills with cement based waste materials. The focus of the work was on Zn(II), firstly in sorption experiments with calcium-silicate-hydrate, and secondly as a component in the leachate from a field lysimeter experiment. The leachate of the lysimeter containing comented electrofilter ashes was sampled during rain events in order to determine the leaching processes.It was found that most of the rainwater was in intimate contact with the waste material in this field system and that while hydrological factors caused changes in concentrations of up to 100% (8–16μM), the concentration range was controlled by geochemical factors. The residence time of the water was sufficiently long to be able to describe Zn concentrations by thermodynamic calculations. The pH values in the leachate ranged between 12.5 and 13.1 where Zn2SiO4(s) appeared to be the most stable phase. Comparison with laboratory experiments suggested that alternative mechanisms could be important. In the laboratory experiments Zn eppeared to be incoporated into the calcium-silicate-hydrate frame forming solid solutions that have varying solubilities depending on the Ca/Zn ratio in the Ca1−x-Zhx-silicate-hydrates. The field data agreed well with this alternative model. Thermodynamic, and kinetic factors are discussed and compared with respect to the geochemical hydrological contributions.
The leachate composition of the Landfill Lostorf, Buchs, Switzerland has been examined as a function rain events and dry periods between November 1994 and November 1996. Discharge and electrical conductivity of the central drainage discharge were monitored continuously, whilst samples for chemical analysis were taken at discrete intervals. The average total concentrations of Na, Cl, K, Mg, Ca and SO4 are 44.5, 47.1, 11.8, 0.63, 8.2 and 12.4 mM, respectively. During rain events, the leachate is diluted by the preferential flow of rainwater into the drainage discharge. Drainage discharge pH values range between 8.68 and 11.28, the latter under dry conditions. Thermodynamic calculations indicate that CaSO4, ettringite (3CaOAl2O3CaSO4·32H2O) and Al(OH)3 may control the concentrations of the components Ca, SO4 and Al. Dissolved Si may be in thermodynamic equilibrium with either Ca silicate hydrate or imogolite. Cadmium, Mo, V, Mn and Zn are also diluted during rain events and concentration changes agree with those of conductivity (representing the major constituents). Average concentrations are 0.012, 5.4, 2.3, 0.085, and 0.087 μM, respectively. Components such as Al, Cu, Sb and Cr increase in concentration with increased discharge. Average concentrations are 1.6, 0.27 and 0.21 μM, respectively. For Cu, the explanation lies in its affinity for total organic carbon (TOC). Thermodynamic calculations indicate that whilst dissolution/precipitation reactions with metal hydroxides and carbonates can explain the observed concentrations of Cd, sorption and complexation reactions probably influence the concentrations of Cu, Pb (average measurable concentration 0.013 μM), Zn and Mn. For the oxyanion species such as MoO4 and WO4 (average concentration 0.61 μM), it is probable that Ca metallate formation plays a dominant role in determining concentration ranges. Geochemical processes appear to determine concentration ranges and the hydrological factors, the fluctuations in concentration.
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