Preliminary micro-characteristics analysis of municipal solid waste incineration fly ash treated by geocasting and calcination

Municipal solid waste (MSW) generation is increasing rapidly since 20th century. For reducing volume and weight of MSW, incineration is one of the major treatments. Major products of municipal solid waste incineration (MSWI) are bottom ash and fly ash. MSWI Fly ash contains heavy metals like Pb and Zn. In order to immobilize hazardous heavy metals in fly ash, some treatments are used such as chelate treatment. However, chelate treatment has possibility of chemical/biological decomposition and cause long-term high concentration of organic hydrocarbons in landfill site as well as leaching of heavy metals to the environment. In this study, the authors focus on the alternative treatment. Fly ash can be replaced fluidized bed sand as heat carrier and it might provide thermal treatment like calcination during combustion operations. To replace sand by fly ash, however, fly ash is too fine particle so it should be aggregated to bigger particles in order to comply with physical requirements of heat carrier in fluidized bed combustors. Therefore, geocasting might be a promising solution for both fly ash application and fly ash thermal treatment. Materials used in this study are fluidized bed type MSWI fly ash, potassium silicate and potassium hydroxide. Geopolymer slurry was produced using 15M KOH solution, which should be used 24 hours after the solution curing. Then KOH solution was mixed with potassium silicate and distilled water in a mixer (200 rpm, 20 min) according to the following weight ratio: KOH (15 M): potassium silicate: H2O = 1:2: 0.5. In this study, the authors added 20 g of KOH solution, 40 g of potassium silicate and 10 g of distilled water. Fly ash was added to this solution with equal amount to 15 M KOH solution. They were stirred at 200 rpm for 30 min at room temperature. Accelerating geopolimerzation reaction, geopolymer slurry was placed into an oven at 80 °C for 2 hours. After geopolymer foam was casted, some samples were calcined at 600 °C for 4 hours to simulate similar situation in a fluidized bed combustor. After sample treatments, raw fly ash, geocasted fly ash and geocasted-calcinated fly ash were analyzed by scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX). From SEM observations, raw fly ash mainly consisted of Al, Na, Ca and Cl. They showed sphere particles with smooth surface. On the other hand, geo casted fly ash consisted of more Si and K than raw fly ash. Geocasted fly ash also showed porous characteristic and irregular shape than raw fly ash. Lastly, geocasted-calcinated fly ash consisted of similar elements with geocasted fly ash particles. After calcination, however, porous characteristic disappeared. These results imply that geocasting makes porous particle and following calcination might melt the surface and degrade porous structure. Further research is necessary to check physical stability of goecasted fly ash and toxic element immobilization after geocasting and calcination.