Abstract
Fly ash from the combustion of coal shows potential as a source of valuable metals like the rare earth elements (REEs). These are present in the fly ash at low concentrations. Thus, simple dry pre-processing methods that can produce REE-enriched fractions from the fly ash could improve the feasibility of subsequent extraction processes. The performed air classification experiments with a coal combustion fly ash showed a certain size dependence of the REE concentrations. In the different size fractions the REEs were enriched or depleted. Thereby, the different groups of REEs showed differences in their behavior. In the finest size fraction all REEs were enriched by a factor of 1.05 to 1.65. In contrast, in the coarsest size fraction and in the second finest size fraction all REEs were depleted (0.63 to 0.78 and 0.58 to 0.92). In the medium size fraction the concentrations were close to those in the fly ash and in the second largest size fraction the light REEs (La, Ce, Pr, Nd and Sm) were enriched by a factor of 1.19 to 1.35, while the concentrations of the other REEs were similar as in the fly ash. Thus, air classification shows some potential as a simple dry pre-processing step for the separation of REE-enriched fractions from the bulk of a coal fly ash.
Highlights
The increasing demand for rare earths elements (REEs) in a wide range of applications and the export quota reductions by China as the majority producer of rare earth elements (REEs) has increased the research interest in alternative resources for REEs as coal, coal by-products and coal combustion ash as well as expanded recycling
The REE concentration of the investigated fly ash was on the lower end of the reported range
In SF1 and SF4 the REE concentration was higher compared to the original fly ash sample while in the other size fractions (SF2, SF3 and SF5) the concentration was lower
Summary
The increasing demand for rare earths elements (REEs) in a wide range of applications and the export quota reductions by China as the majority producer of REEs has increased the research interest in alternative resources for REEs as coal, coal by-products and coal combustion ash as well as expanded recycling Fly ash is a fine grained material which makes it ideal for processing by leaching saving the energy required for the comminution of ore. The typically range of the REE concentration of fly ash is 200 to 550 ppm (Yao et al, 2015; Iyer and Scott, 2001; Font et al, 2005; Dai et al, 2018; Fang and Gesser, 1996). The suggested cut-off grade is 1000 ppm (Dai and Finkelman, 2018). This makes recovery of the REEs from fly ash a complex task for which hydrometallurgical leaching techniques can be used (Sahoo et al, 2016). An important parameter in the leaching process is the efficiency of the recovery of the REEs (Kolker et al, 2017). The feasibility of such leaching processes might be improved by low-cost pre-processing steps in which the valuable elements are enriched
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