Advances and Perspectives of Green and Sustainable Flotation of Low-Rank/Oxidized Coal: A Review

Flotation is a mature technology for the commercial processing of fine coals. However, due to the difficult flotation characteristics of low-rank/oxidized coals, conventional flotation is often inefficient, resulting in massive low-rank/oxidized coal resources not being utilized efficiently and thus going to waste. In addition, the large quantities of conventional oil collectors used to meet satisfactory flotation objectives adversely affect the environment. Under the current huge energy demand and environmental pressures, the efficient, green, and sustainable upgrading of low-rank/oxidized coals is an urgent need for the coal industry. In recent years, a number of attractive techniques to improve separation accuracy have been proposed. However, few ideas have been reported about the application of different types of technologies throughout the flotation process which can improve the flotation process objectives and thus reduce the formation of contaminants. In order to explore an ideal flotation technology for low-rank/oxidized coals, this paper provides a systematic review of the research progress in low-rank/oxidized coal flotation, including the updates in three aspects: the development of pretreatment processes, selection and combination of flotation reagents, as well as optimization of flotation conditions. The synergy of multiple technologies, that maximize flotation objectives and thus reduce the dosage of non-renewable collectors, is the key to achieving green separation of low-rank/oxidized coals. According to the advantages of existing technologies, a green and sustainable flotation process for low-rank/oxidized coal combining pretreatment process, green flotation reagents, and flotation optimization process is proposed to obtain positive process objectives while maximizing economic savings and environmental friendliness.