Caratterizzazione mineralogica e chimica delle ceneri pesanti provenienti dalla combustione dei rifiuti

Abstract

Waste generation is, today, a hot topic, strongly linked to environmental problems. Wastes have been produced since the dawn of civilization: the increasing world population and global development occur together with an increasing production of man-made objects, which at the end of their life become waste. Increasing industrial production in the last two centuries pose the problem of their disposal, and in more recent times of their management. Waste management is nowadays a global topic together with that of climatic changes and georesource shortage: it is foreseen to become more and more critical in response to the improvement in life standard in developing countries and in the increased consumption of raw materials. Waste management is a complex issue, aiming to reduce the production and to recycle and reuse the waste, avoiding or minimising any dispersion in the environment. The goal in waste management can be summarised in the mantra “from waste to resource”. Prominent among improvements and innovations concerning waste management are Waste-to-Energy (WtE) plants, systems that generate energy from the combustion of solid waste, mostly municipal. In WTE plants the solid waste is burned, with a positive energy balance, coming from the combustion of the wastes, which is used for the energy production. However, like any anthropogenic activity, this process generates wastes: bottom ashes and fly ashes. Bottom ashes are the residual ashes in the bottom part of the combustion chamber, whereas fly ashes are the residuals blowing out from the chamber. Also, the bottom ashes (BA) from Municipal Solid Waste Incinerator (MSWI) can be recycled. Due to their physical and chemical-mineralogical characteristics, BA can be used as a secondary source of raw material or recycled into construction and cementitious materials. However, their recycling may be compromised due to the presence of Potential Toxic Elements (PTE), i.e. chromium, lead, and zinc, as their release may be harmful to the environment and human health. The aim of this thesis is to provide a systematic and in-depth study on the BA, with special focus on the medium-fine fraction (< 4mm). This work has two goals, the mineralogical characterization of the bottom ashes and the assessment of their potential release. In both, special attention was paid to PTE. The goals are closely related, as the information on the mineralogical phases hosting the PTE is a key point for interpreting and assessing the potential release of the BA in the environment. In this work BA from 5 WtE plants in Northern Italy were sampled and sorted by grain size. Each sampling was analysed by Thermo Gravimetric Analysis (TGA), X-Ray Powder Diffraction (XRPD), X-Ray Fluorescence (XRF), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM - EDS) and X-ray Absorption. Synchrotron based Near Edge Spectroscopy (XANES) and XAF analyses were further conducted, to obtain a full chemical and mineralogical information. The potential release of BA was investigated on particle sizes < 4mm by leaching test and sequential extraction procedure. From the results obtained, we found that several elements exceed the legal limits (chlorides, copper) already in the leaching test, where extraction is conducted only with ultrapure water at neutral pH. The relation between grain size, composition, and leaching potential were assessed in the ashes to discuss the potential of grain size sorting in high value applications of them.

This thesis is organised as follows. In the first chapter the concept of waste is introduced, together with the figures on global and national waste production. The operational procedures and different types of waste-to-energy plants are described, and the characterization of bottom ashes is introduced. A literature review on the current state of the art on chemical, physical and mineralogical characteristics, together with the possible environmental problems and reuses of the bottom ashes is addressed. In the second chapter the materials and methods used in this work are outlined, in what they are common to this investigation. Specific aspects are further discussed in the sub-chapters 3.1, 3.2, 3.3 and 3.4. The chapter 3 shows the results and discussion of the different topics addressed in this investigation. It is divided in four subchapters, each corresponding to a paper published, submitted, or in preparation. In the subchapter 3.1 the paper “Particle size and PTE speciation in MSWI bottom ash”, published in 2021 in the journal Sustainability, is reported, giving a mineralogical characterization of the bottom ashes, with different grain size, from the Parma WtE. In the sub-chapter 3.2 the version submitted to the journal Waste Management of the paper “Toward a deep characterization of bottom ashes from municipal solid waste incineration: new data from 5 plants of Northern Italy” is reported. The bottom ashes from the WtE of Piacenza, Torino, Forlì-Cesena and Ferrara, together with a new sampling from the WtE from Parma were examined. The mineralogy, geochemistry and leaching behaviour of the ashes were compared, and the trend outlined in the sub-chapter 3.1 were discussed in their generalization. In the sub-chapter 3.3 the pre-submission version of the paper “PTE speciation in Bottom Ashes from Municipal Solid Waste Incinerator: a combined SEM-EDS, XRF and XANES by synchrotron radiation study”, is reported, to be submitted to the Journal of Hazardous Materials. This investigation discusses the mineralogy of the PTE elements, combining a SEM-EDS information on the mineralogy of the host phases, with synchrotron based XRF and XANES data, to outline the local distribution and mineralogy of the PTE elements at concentrations of the order of few ppm. In the sub-chapter 3.4 the results of a sequential extraction procedure on the bottom ashes from Parma and Piacenza are reported, and discussed together with a PCA analysis of the correlations between elements and grain size in the bulk material and in the leached products. A new protocol for the sequential extraction procedure in bottom ashes is proposed. Finally, the fourth chapter reports the general conclusions from this work with a final focus on problems in the recycle of the bottom ashes.