| Summary: | MSW slagging-gasification has attracted increasing attention in recent years as a potential alternative to conventional municipal solid waste (MSW) incineration. During slagging-gasification, MSW is gasified to produce combustible syngas, which enables a higher carbon conversion ratio and less generated pollutants such as dioxins than conventional incineration. Most solid residues are melted into slag which could be reused as a construction material directly, and gasification fly ash (GFA) is the only solid residue to be landfilled currently. During the slagging-gasification, high amounts of heavy metals and chlorides volatilized and condensed on GFA, making it a hazardous material. Due to its content of slaked-lime-based air pollution control (APC) residues, GFA is an alkali material that contains a high amount of calcium compounds, which makes it a potential civil engineering material to be applied in fields such as cement replacement, alkali activation, and soil improvement. However, the high contents of heavy metals and chlorides limit the application of GFA in civil engineering. For some similar wastes containing heavy metals and chlorides such as incineration fly ash (IFA), pre-treatments such as washing and carbonation have been tried to improve the performance in civil engineering applications. However, even for IFA which has been widely studied, current methods to apply it in civil engineering are still immature. The addition of treated waste still notably influenced the quality of products in most studies. And for GFA containing higher amounts of heavy metals, related studies are minimal. Therefore, this study aims to investigate the properties of GFA and find proper methods to treat and apply it as a civil engineering material to reduce landfilling and achieve the goal of sustainable development.
In the first part (Chapter 3), a comprehensive characterization was conducted for GFA collected from the Waste-to-Energy Research Facility (WTERF) in Singapore. The physicochemical properties, mineral compositions, and pollutant leaching behaviors of samples were characterized in detail. The results were compared with a conventional IFA sample and available data of IFA in Singapore obtained from the literature. The results showed that GFA and IFA were both mainly composed of calcium compounds and chlorides, while GFA samples contained higher amounts of heavy metals, especially lead (Pb) and zinc (Zn).
In the second part of this study (Chapters 4-6), the effects of different treatment methods on GFA were evaluated and proper optimizations of these treatment methods were suggested. Water washing with different liquid to solid (L/S) ratios and acid washing with different acid concentrations were applied for GFA in Chapter 4. The behaviors of heavy metals including lead (Pb), zinc (Zn), copper (Cu), cadmium (Cd), manganese (Mn), chromium (Cr), and nickel (Ni) during washing processes were studied in a wide pH range. The results showed that heavy metals in GFA were reactive, which tend to dissolve and re-precipitate into different compounds depending on different pH conditions. Water washing could not remove heavy metals but could lead to the change in speciation of some metals such as Zn, while high concentrations (>1 M) of hydrochloride acid (HCl) could effectively extract heavy metals including Pb, Zn, and Cd from GFA. With a preliminary understanding of heavy metal behaviors in GFA, carbonation treatment using carbon dioxide (CO2) was conducted for GFA and water-washed GFA (Chapter 5). The results showed that gaseous carbonation treatment with a proper treatment period could effectively immobilize Pb, Zn, and Cu in GFA. For both GFA and washed GFA, the main changes in weight and mineral composition were completed at a short period after treatment began, with a relatively steady pH condition at this period. Then, the pH of the samples started to decrease, and at pH around 10, the best immobilization effect for most heavy metals was realized. However, continuing carbonation will further decrease the pH and promote the leachabilities of metals such as Zn and Cd. For water-washed GFA, the period to maintain low mobilities of heavy metals was much extended, and the efficiency to adsorb CO2 was also improved. In order to solve problems of heavy metals, chlorides, and sulfates simultaneously, treatment using sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) solutions were tried to treat GFA (Chapter 6). Na2CO3 treatment increased the pH of treated GFA and reduced the leaching of Pb and Cu, while NaHCO3 treatment could slightly reduce the pH of treated GFA and achieve better effects in immobilizing Pb and Zn than Na2CO3 treatment. Both Na2CO3 treatment and NaHCO3 treatment could effectively remove sulfates from GFA and could promote the removal of chlorides compared to water washing.
The last part of this study (Chapter 7) applied GFA, ground granulated blast-furnace slag (GGBS), and Na2CO3 in the stabilization of soft clay. The results showed that although GFA-GGBS stabilized clay could get comparable or higher strength than ordinary Portland cement (OPC) stabilized clay after 56 days, the strength at 14 days and 28 days was low. The addition of a proper amount of Na2CO3 could notably accelerate the hardening of stabilized clay and reduce the leaching of heavy metals. In this system, GFA reacted with Na2CO3 and increased the pH, promoting the hydration of GGBS, and increasing the strength of soft clay. Overall, due to the adsorption effect of soft clay, the stabilization/solidification effect of GGBS, and the carbonation effect of Na2CO3, the heavy metals in GFA were well immobilized.
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