Municipal Solid Waste Incineration (MSWI) Ash Characterization and Physical Concentration
Escalante Pedraza, Sharon Daiana
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Bottom ash (BA), generated from the incineration of municipal solid waste (MSW), contains valuable elements which present a potential economic incentive to attempt recovery. The first study of this thesis investigated the physical, chemical, and mineralogical characterization of MSWI-BA samples through a number of experiments. To develop a proper physical characterization of the BA material, the sample was ground and subjected to particle size distribution, wet magnetic separation, and a float-sink test. As for the chemical and mineralogical characterization, the sample was subjected to XRD, XRF, SEM-EDX, and elemental composition analysis. Additionally, sequential chemical extraction and acid-leaching tests were conducted. The results from this section revealed that carrying out a combination of beneficiation processes using the MSWI-BA sample previously classified into the coarse, middle, and fine-size fractions could lead to better metal concentration yield and recovery optimization. The wet magnetic separation showed outstanding metallurgy indicators towards Fe, with enrichment ratios close to 2.0 and recovery values near to 80%. Metals such as Cu and Co were also enriched by 1.51 and 1.66, respectively, suggesting that the magnetic separation performance and enrichment are a function of the bound of multi-metallic oxides fractions. The 2.95 SG density test reached enrichment ratios higher than 2.0 in Fe, Cu, Co, and Ni in the coarse fraction of the BA fraction, which decreases when reducing the size fraction. When reducing the density cutoff, the results showed that the sink fraction yield increased as the medium density decreased, and the enrichment ratios of the minor elements (Mn, Co, Ni, Sn, and V) were similar across the different size fractions. Complementary information was obtained by the mineralogical characterization of the enriched streams from the physical concentration test, which explains the results obtained. The Cu speciation and mineral phases identified were copper oxide, copper sulfate, and cupric sulfite. While the main Fe-rich constituents existed in chemical forms of iron oxides, such as magnetite, hematite with substituted varieties, spinel group, and metallic inclusions. The enrichment ratios of Mn, Cr, Cu, and Ni obtained through magnetic separation can be explained by the presence of metallic inclusions, where these elements exhibit an affinity for the iron-bearing particles. The acid leaching test revealed that metals such as Fe, Mn, Co, Cu, and Zn can be efficiently leached using 1M HCl within 30 min of the reaction. The second part of this research study constituted the evaluation of the effect of the particle size reduction, which was performed to assess the intraparticle heterogeneity of MSWI BA. The evaluation consisted of particle size reductions by crushing and grinding for different residence times and then subjecting the sample to a sequence of physical concentration tests, such as particle size distribution, froth flotation, and wet magnetic separation. Additionally, the elemental composition after each test was determined through ICP-MS analysis to compare the particle size effect in the recovery and concentration of the valuable elements. The elemental composition results revealed that the comminution process promotes the interaction of Fe, Zn, and Cu, in the fine fraction, by generating more surface area. In contrast, the minor elements were not significantly enriched by reducing the size fraction, suggesting that the comminution process does not impact the mobility and redistribution of the elements in low concentrations. The froth flotation performed in this study showed that when using 0.338 g/ton diesel as a collector, adjusting and controlling the pH between 8.8 to 9.2 throughout the test, the organic matter content can be efficiently reduced in the BA sample from 14.73% to 4.25% when the sample has been previously ground for 30 min. Slight enrichment ratios were observed in the concentrate stream of the froth flotation, suggesting that these elements are associated with the organic matter in the BA sample. In contrast, the wet magnetic separation results revealed significant enrichment ratios of Fe, Mn, Co, and Ni after 10 min of grinding.
General Audience Abstract
The Bottom Ash (BA) generated by the incineration of household solid waste has been identified as a promising source of valuable elements. However, a comprehensive understanding of the BA sample's properties is required in order to determine the most suitable mineral processing method to enrich the elements. The first study of this research consisted of evaluating BA ash's physical, chemical, and mineralogical properties in the BA sample. Following the characterization study, the effect of particle size, as a function of the grinding time, in the valuable elements' enrichment was evaluated. The results suggest that Ti, Fe, Cu, and Zn are the major and most valuable elements, while Mn, Co, Ni, Sb, and V are valuable elements in a minor concentration in the BA samples. Some elements, such as Ti, Sc, Co, Mn, Ni, Sn, and V, have been declared by the US Department of the Interior as critical minerals due to their economic importance and vulnerability to supply chain disruption. Although Fe and Cu are not considered critical minerals, their consumption in 2022 was 40 and 1.9 million metric tons, respectively. The development of national industry and enhancing the understanding of the alternative sources for the valuable elements present an opportunity to diversify local suppliers, pursue a vertical integration of the economic model, and reduce the third-party international vendors' dependency. Likewise, this research supports the aims to reduce the demand for primary natural resources and contribute to the circular economy model, in which energy, resources, and material are kept in a lifecycle while reducing landfilling disposal.
- Masters Theses