Particle Petrogenesis and Speciation of Elements in MSW incineration Bottom Ashes

Abstract

The speciation of elements in municipal solids waste incineration bottom ash is important with respect to its impact on leaching behavior and to its treatment for utilization or disposal. We used a variety of techniques to identify the speciation of major, minor and trace elements in both intact bottom ash particles as well as bottom ash powders. Petrography and scanning electron microscopy/x-ray microanalysis (SEM/XRM) were used to classify intact particles and identify ash particle petrogenic sequences. Two distinct features were seen. Particles are comprised of about 15% of materials present in the MSW waste feed to the incinerator. The remaining portion of the particle (85%) is melt structure. A typical particle contains waste glass (10%) waste soil minerals such as pyroxenes, SiO 2 (quartz), and feldspars (2%), waste metals and metal alloys (2%), and waste organics (1%). Particles are also comprised of slag or melt products, derived from the MSW feed material, that include opaque glass (25%), isotropic glass (20%), schlieren (10%), and spinel-group minerals (magnetite, hercynite, chromite) (10%) and melilite group minerals such as CajAljSiO? (gehlenite) and MgCa 2 Si 2 6 7 (akermanite) (20%) which precipitated out of the melt as it cooled. The paragenic sequence is similar to that described for melelite-bearing, igneous rock systems. The system can best be described petrogenically using the CaO-MgO-AljOj-SiOj-NajO-FeO (CMASNF) system. The melt structure was formed at about 1,200°C. Thermodynamically incompatible phases are present in the ash, making it reactive to aging (oxidation, hydrolysis), weathering, and diagenesis. Increasing the silicon content of the ash could result in the formation of more geochemically stable phases. The residue was ground into powders less than 300 μm in size. Magnetic and density separations were performed to segregate powders for further analysis. The residue is comprised of approximate equal fractions of magnetic, high density; non-magnetic, low density; and non-magnetic, high density material. Isodynamic separation of the non-magnetic fraction was also effective in separating minerals. SEM/XRM of powders fractions in thin section was particularly effective in identifying major minerals in identifiable mineral structures as well as minerals associated with “hot spots” of minor and trace elements. These minerals include many pyroxenes, quartz, feldspars, and melilite-group minerals as well as many spinels. Lead appears to largely be incorporated in complex silicate melt structures. X-ray powder diffraction (XRPD) confirmed the presence of minerals seen by petrography and SEM/XRM. X-ray photoelectron spectroscopy (XPS) of powder surfaces also documented the presence of many of these minerals. A number of oxides and carbonates were also seen with XPS, reflecting the role of O 2 (g) and CO 2 (g) in altering the speciation of the particle exterior surface. XPS is particularly well suited for identifying phases associated with leaching at this surface. Solid phases controlling leaching, as determined with the geochemical thermodynamic code MINTEQA2, are not always the same as ones observed with the above mentioned methods. The role of mineral respeciation and diagenesis in controlling leaching is highlighted. The use of such models in predicting leaching behavior is discussed.