J. Phys. IV France 107 (2003) 463 cEDP Sciences, Les Ulis DOI:10.1051/jp4:20030341 Heavy Metals in MSW Incineration Fly Ashes C. Ferreira 1,3 , A. Ribeiro 2 and L. Ottosen 3 1 CERNAS, Escola Sup. Agrária, 3040-316 Coimbra, Portugal 2 DCEA, Faculdade de Ciências e Tecnologia, Univ. Nova de Lisboa, Portugal 3 Department of Civil Engineering, DTU, Denmark. Abstract. Incineration is a common solution for dealing with the increasing amount of municipal solid waste (MSW). During the process, the heavy metals initially present in the waste go through several transformations, ending up in combustion products, such as fly ash. This article deals with some issues related to the combustion of MSW and the formation of fly ash, especially in what concerns heavy metals. Treatment of the flue gas in air pollution control equipment plays an important role and the basic processes to accomplish this are explained. Fly ash from a semi-dry flue gas treatment system is characterized regarding its physical-chemical properties: pH, solubility, chemical composition, and leaching, amongst others. Results indicate a high alkalinity and the presence of large amounts of calcium, chlorides, sulfates, carbonates, sodium and potassium. Metal concentrations in fly ash are: 6,2 g/kg for zinc, 2,4 g/kg for lead, 1,7 g/kg for iron, and 7,9 g/kg for magnesium. Copper, manganese, chromium and cadmium are also present with 546, 338, 104 and 91 mg/kg of fly ash, respectively. These results are extremely important in subsequent studies on the treatment of fly ash. 1. INTRODUCTION Management of Municipal Solid Waste (MSW) is an important issue in cities around the world because of few available space and environmental concerns. Incineration is a commonly accepted solution throughout the world, with attractive features like an 85-95% volume reduction and the possibility of energy recovery. However, it is not a final solution since it generates combustion products (bottom and fly ash) that must subsequently be properly disposed. In Portugal the first two MSW incinerators were constructed recently. Fly ash resulting from the process may pose a serious environmental problem, and it is important to know its characteristics so that the possibilities of disposal, treatment and reuse can be properly evaluated. 1.1 The combustion process and its sub-products During the combustion, organic compounds are degraded into more simple compounds, most of which will be found in the flue gas: carbon, hydrogen and sulphur will be converted to CO 2 , H 2 O and SO 2 , respectively. Nitrogen will be converted to N 2 , NO and NO 2 . Chlorine is converted to HCl and to some organic chlorides (e.g. dioxins). From the combustion chamber two flows are originated. On the bottom remains the non-combustible material, together with the heavier particulates formed during combustion (bottom-ash); from the top of the chamber leaves a flue gas with the combustion products. 1.2 Transformation of heavy metals during combustion Metals are inorganic substances and are not destroyed during incineration, only oxidized. Therefore, most of the metals submitted to combustion will exit the process as oxides, being partitioned between bottom-ash (heavier particles) and the flue gas (lighter particles). Some very volatile metals such as arsenic, cadmium and mercury, vaporize at furnace temperatures (>850ºC), being transferred to the flue gas as metallic vapour. Other metals, however, vaporize only at temperatures much higher than those occurring in the furnace. These low volatility metals would not be present in the flue gas, if it wasn’t for the fact that they can occur in MSW in the form of salts, which generally have boiling points lower than those of the corresponding metal. During the combustion, these salts are vaporized and transferred to the flue gas. Some metals that are not originally present as salts can also react with existing halogenates, being converted to the corresponding salt, and vaporised. 1.3 Flue gas treatment