Mathematical leach model for leaching behavior of solidification of incinerator fly ash
Date Issued
2008
Date
2008
Author(s)
Liu, Kuen-Shan
Abstract
Natural resources are short recently. Waste has been widely use in civil engineering work instead of sending it to the landfill. From an environmental aspect, more accurate assessments of constituent leaching of solidified waste are necessary.ne semi-dynamic leaching test aimed at predominately evaluating the release of metals in diffusion-controlled environments. We are limited to adding 20% cement to solidified waste and the testing specimen volume to surface area ratio 10. he diffusion behavior of a solidified waste matrix is a non-homogeneous diffusion phenomenon, it can be divided into a set of simple problems, solved by using the separation of variables method. The mathematical diffusion model for the solidified waste matrix cylinder is set up with homogeneous diffusion and non-homogeneous generation rate. Accordingly, Duhamel’s theorem with the time-dependent boundary condition and/or time-dependent condition provides a convenient approach for mass diffusivity and mass generation rate constant measurements. The experimental set-up design and development of the cylindrical diffusion model in this study was based on Duhamel’s Theorem. The diffusion model is mainly controlled by diffusion. ifferential equations are used to solve this initial value and boundary value problem. In this study, the physical and chemical processes are combined into one lumped parameter that can be applied to a variety of disposal/reusable scenarios to predict the environmental impact. wo types of solidified waste forms (ratio of the cement to waste was 20%, 40% and 11%, 13%, 22%, respectively) including lead isotope and lead oxide added, were produced by solidifying the fly ash using cement as a binder, respectively. In this recipe, the solidified waste forms were molded into a cylindrical form with a diameter of 1 cm and a height of 4 cm. Before the solidified test specimen was subjected to the leach test, it was allowed to harden for two weeks. The molded solidified waste matrix was placed in a cylindrical container. Extensive leaching tests were applied to this study for six hour leaching using pH 0.89 phosphoric acid and pH 2 acetic acid as the leachant, respectively. The leachant volume to the specimen’s external geometric surface area (VL/S) ratio was maintained at 7.5 cm. A γ detector instrument and ICP-MS were applied to measure the lead isotope and lead leached out of the solidified waste matrix, respectively. he empirical mass diffusivity ( )and empirical mass generation rate constant( ) were obtained using the least square method for the first, second, third , and fourth leaching result intervals , and a range of Bessel function roots were used to establish the cylindrical diffusion model . An appropriated Bessel function root lead the simulated leaching results compare with the experiment. The lead released from the waste specimen in the fifth and sixth intervals of the mass diffusion model were used to verify the mass diffusion model .The diffusion model was verified with the extended simulated leaching results within one standard derivation of experiment. The best fitted Bessel function root in the diffusion model depends on the leachant, the amount of cement added to the waste. Increasing the solidified agent in the waste matrix acquired a more accurate diffusion model. In this study, the best fitted root of Bessel function for the diffusion model of the solidified of incinerator fly with 20% to 40% cement addition is 11 using phosphoric acid as leachant. The best fitted root of Bessel function for the diffusion model of the solidified of incinerator fly with 11% to 22% cement addition is 2 using acetic acid as leachant. The amount of cement added to the waste carried a very distinguished mass diffusivity and mass generation rate constant that demonstrated the effectiveness of the cylindrical diffusion model in this study. Using acetic acid as the leachant is more stable than phosphoric acid.he leachability index was derived from the verified diffusion model used simulated leaching values and lumped- parameters. The long-term leachability index can be obtained by extrapolating the leachability index profiles (straight line). Simulated leaching can be obtained only with a graph and simple mathematical calculation. Using the proposed leachability index, it is possible to estimate the incremental probability of some harm occurring. The leachability index derived from this cylindrical diffusion method for investigating the diffusion behavior developed in this study, can be used as an effective tool for establishing the design of the solidification facility, waste resources recovery, and regulatory standards.
Subjects
Isotope leaching test
chemical leaching test
diffusion model
lead
fly ash
solidification
Type
thesis
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