In incineration processes, Heavy Metals (HM) are not destroyed but concentrated in the residues and it is of great interest to control the distribution of HMs between the different residues. Because of the respective quantities of bottom ashes (300 kg per ton of waste) and fly ashes (25 kg per ton of waste) it is advantageous to recover HMs in fly ashes and rather than in the bottom ashes in order to allow their use in road construction.
MELODI gives a good prediction of HMs distribution in the incineration process as well as any thermal process. The Vitrification of bottom fly ashes - as it is a thermal treatment and thus a relevant application of the MELODI approach - was also considered in the project.
It is based on equilibrium calculations and consists in a software tool aimed at describing the equilibrium state (temperature, gas and condensed phases flows and compositions). A thermodynamical database established on purpose comprises the physical properties of all compounds possibly existing in the successive elementary steps of the incineration process.
- For each elementary step of the process, MELODI describes for each compound and especially for HMs:
-The speciation of the HMs and other elementary species: the nature of the compound to which the HM can be bounded, has a great influence on the leachability of the material.
-The physical state of each compound (gas or condensed) thus its distribution between the different streams thus between the different residues.
MELODI allows also the identification of operating parameter influence on the HMs behaviour. As an example the effect of air/ratio on zinc behaviour in the combustion step and on distribution between gas and solid is shown on the attached figure: at low air ratio (1.2 max), zinc is volatised as it is under metallic state. At higher air ratio (above 1.3) - even for temperature greater than 1 200°C - zinc is condensed in the bottom ashes under its oxide form ZnO which is not volatile.
To recover zinc in the fly ashes at high air-ratio it is necessary to use high enough chlorine concentrations in the fed mixture; zinc is then converted into volatile chloride ZnCl(2). The same calculation for Lead shows that lead volatilisation is mainly arisen by the temperature as all lead compounds possible present (Pb, PbS, PbO) are vaporised for air ratios greater than 0.55.
- Operation conditions aimed at removing HMs from the bottom ashes can thus be identified and selected in respect with initial waste composition.
- MELODI also showed that HT filtration in presence of chlorine in the gas effluent cannot ensure a complete confinement of HMs as some of them are converted into chlorides which are volatile and cross through the HT filters.
- In vitrification of bottom or fly ashes, MELODI allows to identify the HMs possible leak from the molten bath with the gas flow.
The MELODI APPROACH and associated model can thus be used in a broad range of possible applications related to problems caused by HMs hazardousness in the materials produced by thermal processes.
