Oil aggregated behavior for coal recovery and combustion characteristics of their aggregates from different grade coals

Abstract

Large amounts of waste fine coals are very difficult to be treated due to the presence of ash and inorganic sulphur compounds. In order to use waste fine coal efficiently, a retrieval technique is necessary for the recovery of combustible contents of coal from fine waste coals. Nowadays, a flotation process has been used for the treatment, but it is impractical for developing countries due to its higher costs. Therefore, oil agglomeration process has been used to deal with these problems. In this study, the factors affecting the coal cleaning efficiency of the oil agglomeration process were investigated with the element contents and chemical structure of three different grade coals. Chemical contents in three different grade coals were determined by proximate and ultimate analyses and the differences in chemical structure of carbonaceous contents of different grade coals were investigated by a Fourier transform-infrared spectrometry. In free coals or their mixed samples, the ratio of ash and carbonaceous contents were maintained to make it homogeneous. From the results of oil agglomeration experiments, it was concluded that the characteristics of agglomerate and the coal cleaning efficiency of oil agglomeration were not only influenced by the type of oils but also by the oxygen contents and the aromatic and aliphatic chemical structures in different grade coals. The oxygenic functional groups of carbonaceous contents in coal samples prevented oil from attaching the carbonaceous surface and form the bulky aggregate. This depressed the combustible matter recovery, but this was resolved by changing the oil types. Oxygen contents in oils such as vegetable oil played a role in bridging material to the oxygenic functional groups of carbonaceous contents in coal samples. Meanwhile, it was observed that aromatic functional groups in carbonaceous contents interacted badly with the aliphatic functional groups in oil due to the resonance inspection of delocalized π electrons. Comparatively carbonaceous contents consisting of more aliphatic series or graphite-reinforced structures were tended to form aggregates easily. Even when different types of coals were used in the oil agglomeration, it was possible to achieve a better efficiency by taking these factors into account. Moreover, by thermogravimetric-derivative thermogravimetric (TG-DTG) analyses, it was found that the combustion characteristics of carbonaceous contents were improved due to oil attachment.