Enhancement in floatability of sub-bituminous coal by low-temperature pyrolysis and its potential application in coal cleaning

Sub-bituminous coal is one type of low rank coal, which is difficult to upgrade using flotation cleaning technology because of its high hydrophilic properties. Low-temperature pyrolysis is widely used to convert low rank coal to gas/liquid components and the coal char is burned for power generation. It is friendlier to the environment if the coal char is forwarded to the cleaning process prior to the burning/combustion. This investigation aimed to assess the possibility of upgrading the coal char obtained from the low-temperature pyrolysis of sub-bituminous coal. The scanning electron microscopy, X-ray photoelectron spectroscopy, attachment time, and flotation tests were used to reveal the changes of surface properties and floatability of sub-bituminous coal during low-temperature pyrolysis with different pyrolysis times, i.e. 30, 60, 90, and 120 min, respectively. The results indicated that many pores and cracks were created on the coal char compared to raw coal surface. The content of hydrophobic functional groups on coal surface was increased whereas the content of hydrophilic oxygen-containing functional groups on the coal surface was reduced after the pyrolysis. The attachment time of coal particle-bubble was significantly decreased while the flotation recovery of coal was increased after the pyrolysis. Throughout this paper, the pyrolysis time of 30 min may be suitable for the enhancement of coal floatability by considering the gas/liquid production as well as economy and time saving. It is also inferred that the floatability of coal char should be governed by both its surface morphology and its surface composition of functional groups.