Behavior of oxyanions forming heavy metals in municipal solid waste incineration

A number of metals respectively half-metals of high environmental concern have the capability to form oxyanionic compounds. The purpose of the present work is to investigate the behavior of selected elements out of this group, namely arsenic, antimony, chromium and molybdenum, during the waste combustion process and the environmental properties of the solid combustion residues. As a first investigation step, the concentration and the chemical speciation of the investigated metals in municipal solid waste are assessed on the base of literature review. The thermal behavior of the prevailing input species of the elements in question is successively estimated by the minimization of the thermodynamic function Gibbs' free enthalpy. Thermogravimetric and calorimetric measurements in a thermobalance as well as X-ray diffractometric analyses of mixtures of metal salts and calcium oxide provide evidence that oxyanionic salts are formed in the temperature range around 500-900 C. The results of spiking tests in the TAMARA pilot plant for waste incineration show that the primary air number has a distinct effect on the transfer of the elements out of the fuel bed into the flue gas and on their partitioning among the solid residues. The effect is particularly strong for the two volatile metals arsenic and antimony. Sampling and analysis of fly ash samples at different locations along the flue gas path allow the characterization of the ashes as a function of the deposition temperature. The analysis of samples of filtered gas show the insignificance of gaseous metallic species at temperatures below 200 C and hence the low stack emission potential for the investigated metals. The last part of this work is concerned with the characterization of the TAMARA grate and fly ashes and with the assessment of the leaching potential of the selected metals from the residues. Different regulatory tests have been performed, the results are compared and interpreted in view of the speciation of the elements in question in these residues. All elements showed clearly an oxianionic pattern in their elution from the grate and boiler ashes. The elution from filter ashes instead showed a cationic pattern, especially for arsenic and antimony. The leachate pH appears to be the dominant factor controlling the mobilization of the metals. Molybdenum shows a very high potential for leaching over the entire pH range as a consequence of the formation of highly soluble molybdates. Chromium exhibits a leaching behavior which is strongly dependent on the combustion conditions. For low air number values the amounts released by means of leaching processes from the grate ashes are nearly two orders of magnitude lower than for the reference air number value. Thus, the oxygen concentration in the furnace seems to play an important role in the formation of highly toxic and highly soluble hexavalent chromates. (orig.)Available from TIB Hannover: ZA 5141(6755) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman