Leaching Behaviour And Mechanism Of MSWI Fly Ashes

In recent years, there is a growing concern for environmental impact of hazardous waste in disposal and utilization. As a kind of typical hazardous waste, municipal solid waste incinerator (MSWI) fly ashes also were given the same receptions duo to the rapid development of the technology of municipal solid waste incinerators. To understand the changes in MSWI fly ash under different scenes is therefore important for predicting its environmental impact. The emphasis of this study of MSWI fly ashes is on leaching behaviors and mechanism and dynamics of major compositions and heavy metals under different leaching conditions and on different time scales.A sequential chemical extraction procedure was used to accelerate the degradation process of mineralogical structures, and XRD and XRF were used respectively to determine compositions and mineralogical structures. Using these methods, the leaching process of major compositions and heavy metals in the fly ashes was studied. The results indicated that there were different control processes under on different time scales. The dissolutions happened in the surface and diffusions were rapid in the initial stages, which were controlled by mineralogical dissolutions. With time passing, neutralization of the acid-soluble fraction controlled the leaching course. The materials of leaching were carbonate and Fe-Mn oxides. And chemical weathering of residues controlled the long-term leaching behavior.A series of batch experiments were used to investigate that how the major controlling factors affected the leaching behavior of the major compositions and heavy metals. The results indicated that different extractants, the pH of the extractant, liquid/solid ratio and leaching time demonstrated a little depending relation with the major compositions (Ca, K, Na, and Cl). However, the leaching behaviors of heavy metals were affected by the above four factors. The different extractants and the pH of the extractant controlled the characteristic of leaching system of batch experiments, and affected the degradation and diffusions of acid-soluble fractions.A column experiment and a column-like experiment were used to simulate the leaching process of fly ashes in flow-through system. It can be expected that there were a high release peak of the salts associated with surface phases in initial stage. The front increased gradually with the depth of fly ashes layer. Simultaneously with time passing, the leaching front decreased rapidly. Heavy metals in fly ashes also might form the release peaks in the flow-through system, which exhibited the similar variation rules with the major compositions. The long-term leached fly ashes showed the stronger leaching abilities than the initial fly ashes duo to lower acid neutralization capacity.Acid neutralization capacity was studied using a static batch pH titration experiment. And Acid neutralization process in flow-through system was compared with that in a batch experiment. The neutralising reactions were fast due to the rapid dissolution of surface and soluble phase. The material which contained Ca consumed a lot of protons. Acid neutralization capacity went to stabilization when the degradation processes of mineralogical structures were restricted; the leaching process stepped into the stage of matrix glass phase breaking and dissolution. Moreover, the concept of the ANC for a batch reaction system is not suitable for a flow-through.Based on the above study, the leaching mechanism and dynamics of major compositions and heavy metals on different time scales were analyzed with combining the experimental investigations and geochemical simulations. Leaching processes were described in the framework of heterogeneous non-catalytic reactions in conjunction with the shrinking core model. A new kinetic model of self inhibiting heavymetals leachingwasformed. Taking Pb as an example, the exactness of Leaching processes was approved. The chemical weathering mainly involved the matrix of combustion residues consisting mostly of glass phases. The dissolution kinetics of waste glass and other possible processes involved in the chemical weathering had been investigated.We might consider several aspects, which included: the solute transport that results in mass transfer by the flowing water, The neutralising reactions, the precipitation of secondary minerals when the aqueous solution becomes saturated to the corresponding minerals, carbonation reactions that are the most important buffering mechanisms, and the interaction among the aquatic species, i.e. complexing reactions.In conclusion, the experimental investigations and geochemical simulations were combined in the paper. The leaching behaviors and mechanism and dynamics of major compositions and heavy metals under different leaching conditions and on different time scales were studied, which was interesting to the systemic and scientific environmental risk assessment of fly ash in disposal and utilization.