Development of a LA-ICP-MS procedure for the spatially resolved analysis of fly ash particles

Abstract

Fly ash and other solid residues are accumulated during every combustion process. The main sectors of industry responsible for the production of incineration waste are: (1) municipal waste incineration and co-incineration of waste in other combustion plants, (2) coal-fired and biomass power plants, (3) the metal producing industry and (4) the cement industry. In addition to the raw material, the combustion technology, operating parameters and the type of emission control system define the properties of the formed ashes. Due to their chemical composition, incineration residues are often considered waste and deposition in landfills is the only possibility. Recycling and reuse of combustion waste poses an alternative and is forced through elevated disposal costs, shrinking availability of landfill space and stricter environmental regulations. If legal standards concerning chemical composition are met, incineration residues are used in the cement and concrete production, in the glass and ceramic industry, for road pavements, in agriculture etc. Furthermore, the resource recovery from waste (urban mining or landfill mining) has become an interesting subject over the past few years. The economic feasibility is hereby of major importance and depends on many factors like the available process technology and the situation on the metal market. Consequently, a careful investigation of certain waste fractions is necessary. Although a lot of information regarding particle composition is available, the spatial distribution of elements within a particle is hardly known. During this work, a method to gain insight into the structure of a single ash particle was developed and the creation of elemental images was fostered using LA-ICP-MS in combination with SEM-EDX. Two different kind of ashes, one obtained from the municipal waste incinerator in Spittelau (Vienna), the other from a blast furnace (voestalpine, Linz) were investigated. First, samples were prepared using a novel sample preparation technique to swirl up a fly ash sample and disperse the particles on a sample carrier. After fixation, LA-ICP-MS allowed the spatially resolved analysis of an isolated specimen. In general, particles with diameters between 60 - 120 µm were selected for analysis. By preparing a cross-section and then scanning the received surface of a single fly ash particle with the laser, the distribution of certain elements was recorded. The aim was to identify shell and core-enriched analytes and visualize the results. Two different approaches of gaining information about elemental distribution were introduced: fast scanning by using only one laser pattern and detailed surface analysis by recording several adjoined laser pattern. The main components as well as the surface structure were analyzed using SEM-EDX. Additionally, EDX mappings served as reference for LA-ICP-MS method validation. Data obtained from both techniques was processed using the ImageLab software. Images with a resolution of 24 x 24 px or 40 x 40 px were obtained depending on the beam diameter and the selected imaging area. By combining information from both measurements a detailed characterization of the fly ash samples was possible. Concerning the two introduced fly ash samples, it was found out that Ca, O, Si and Al formed the basis of FLA fly ash while Ti, Zn, Sn, Sb and Pb were only present in certain particle structures. HZ9 contained iron-based and carbon-based particles. While Al and Ti were found incorporated into carbon particles, iron-based specimen exclusively exhibited surface-enriched analytes like Ti, Mn, Zn and Pb.