Partitioning of hazardous trace elements during coal preparation

Abstract

The samples of feed coal, cleaned coal, middlings, slime and gangue were collected from Xiqu, Antaibao and Heidaigou coal preparation plants, China. The analyses of minerals, ash, sulfur and 20 hazardous trace elements were performed on the samples by SEM/EDX, INAA, ICP-AES, CV-AAS, GAAS and also on the samples from experimental products (by heavy-media, electrostatic separation and forth-floatation). Partitioning of sulfur and these trace elements under the conditions of different coal ranks and coal preparation types were discussed. The results show that the partitioning behavior of a given element during the cleaning processes is mainly controlled by its modes of occurrence, the distribution forms of its carrier minerals and the cleaning technique types. Although hazardous trace elements have diverse degrees of removal due to different coal ranks and coal preparation types, the elements associated with fine minerals (e.g. Pb, U and Be) and organic constituents (e.g. Br and Cl ) will not be effectively reduced by physical coal cleaning and may still be enriched in the cleaned coal. As to the coarse sized feeds (particle size >0.5 mm), the elements associated with coarse/epigenetic minerals can be removed more easily by gravity separation methods and report to the gangue and slime. As to the finer-grained feeds (<0.5 mm), the elements associated with minerals with a high dielectric coefficient are easily removed by electrostatic separation; in comparison with the larger-sized fractions (74–150 µm), the smaller-sized fractions (<74 µm) show a high removability of elements by forth-floatation as a whole. The physical coal cleaning is not only effective in the removal of ash and sulfur, but also in reducing the concentration of most hazardous trace elements. However, as byproducts, the middlings, especially the slimes are preferentially enriched in many hazardous elements (e.g. Cr, Ni and Mo), so that these fractions have a potential of environmental risk and should not be directly used as fuel.