Ultrasonic treatment of municipal solid waste fly ash for utilization as construction material

• Main Author: Phua, Zheng Hui
• Other Authors: Dong Zhili
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/138170
• Institution: Nanyang Technological University

In recent years, the use of municipal solid waste incineration for waste management is becoming more and more popular due to the potential of energy recovery and high-volume reduction. However, the use of incineration gives rise to the generation of more hazardous by-products such as fly ash (FA) and air pollution control (APC) residues. In Singapore, the FA and APC were not treated but instead disposed of at an offshore landfill. Due to the rapid increase in waste generation, this offshore landfill is only expected to last till 2035 and there is an urgent need to divert away some of these by-products for utilization purposes. However, these by-products contained high salt and heavy metal contents that could result in many problems during utilization. Therefore, proper treatment must be done before these by-products could be utilized and diverted away from the landfill. This research proposed using ultrasonication as a form of treatment for the FA before partial cement replacement. The ultrasound method has not been studied widely yet in the past but as ultrasound technology advances and lowering in cost of ultrasonic devices, this method showed great potential to become a sustainable and economically feasible method for ash treatment in the future. During ultrasonication, both stable and transient cavitation can occur to give rise to various physical and chemical effects. The hotspot theory postulates that when the bubble undergoes transient cavitation, localized hotspots of high pressure and high temperature in excess of 500 atm and 5000 K are formed to increase the rate of reaction and also generate OH and H radicals. Also, ultrasound has been known to increase the rate of dissolution in many solid-liquid systems due to the increase in the driving force for mass transfer due to the formation of micro-jets and other microstreaming effects. Before ultrasonication, detailed characterizations such as TCLP leaching, variable pH leaching, total elemental composition, morphology studies and mineralogy studies were conducted. The characterization results agreed with other literature, highlighting the concern over leaching of Cr, Cd, Pb and Zn. It was shown that both FA and APC had both high natural pH but FA had low acid-neutralizing capacity thus making it more susceptible to enhanced leaching of heavy metal if pH decreases. The results from the variable pH leaching test indicated that the leaching of most metals was highly dependent on the pH based on the respective dominant mineral phase. After the in-depth characterization and variable pH leaching study, ultrasound treatment was applied to both FA and APC with washing treatment as a control. It was found that washing treatment could only remove mostly soluble metal chlorides while ultrasound showed enhanced extraction and particle size reduction. Further characterization of the sonicated FA using SEM and XRD revealed the presence of needle-like micro-ettringite which was a natural weathering product in FA capable of immobilizing various heavy metals within its matrix. These results showed that treated FA exhibited reduced heavy metal leaching and increased homogeneity of the cementitious contents. An optimization study applying ultrasound with different extracting agents at various concentrations on FA was also carried out. It was found out that 5 mins of sonication were enough to promote the accelerated mineral formation and enhanced leaching from FA. 15 to 30 mins was the optimal duration for better performance. However, re-adsorptions of heavy metals were observed at a prolonged sonication duration of 60 mins. FA with 60 mins sonication showed the best leaching results indicating relatively high stability of re-adsorbed metals, possibly due to chemisorption. Acidic leaching agents such as citric and nitric acid showed good enhanced leaching yields, but the lowering of pH affected the formation of ettringite and increased the mobility of Pb and Zn significantly. EDTA was found to be the best extracting agent due to high extraction yield independent of pH which gave relatively good leaching results. However, at a higher concentration of EDTA at 0.15 M, the formation of ettringite was also affected and no characteristic peaks were detected from the XRD. The final phase of this study was to use the ultrasound treated FA (FA Probe) as a partial replacement for cement in construction material. It was found that the FA Probe performed much better than the untreated FA (FA Raw) and washing treated FA (FA Wash) in terms of compressive strength reaching a maximum of 36.2 MPa at 28 days (>75% strength of pure cement) even at 30% replacement. It was revealed that the concrete samples with FA Raw and FA Wash had much lower compressive strength due to the high amount of large air void due to the reaction of non-ferrous metallic content with highly alkaline cement powder. Samples with FA Raw showed even lower compressive strength due to the presence of high chloride content delaying the hydration process and the recrystallization cycle of salts disintegrated the integrity of the structure. Assessment of the long-term environmental impact of sonicated FA samples shows low leachability and indicated that it was suitable for utilization.