Effect of low-temperature pyrolysis on surface properties of sub-bituminous coal sample and its relationship to flotation response

- Many pores and cracks were created on coal surface after the pyrolysis.
- A porous surface of coal has negative effects on its hydrophobicity and floatability.
- Hydrophilic functional groups reduced while hydrophobic functional groups increased.
- The attachment time of bubble-coal particle reduced after the pyrolysis.
- The flotation recovery of coal increased after the pyrolysis.

Coal spontaneous combustion makes coal suffer a high-temperature heating process (similar to a low-temperature pyrolysis). Additionally, low rank coal is considered to be friendly utilized through two steps, i.e. low-temperature pyrolysis of low rank coal to gain gas/liquid components and then coal char forwarded to the burning or other chemical applications. Therefore, it is necessary to investigate the effect of low-temperature pyrolysis on the surface properties of low rank coal and its role in the floatability of coal particles because coal already suffering spontaneous combustion should be upgraded before usage. In this investigation, SEM, XPS, attachment time and flotation tests were employed to reveal the changes of surface properties and floatability of sub-bituminous coal before and after the pyrolysis. After the pyrolysis, a significant mass loss was observed and many pores/cracks were newly created as well as the content of hydrophobic functional groups on coal surface was increased whereas the content of hydrophilic oxygen-containing functional groups was reduced. The attachment time of coal-bubble was significantly decreased after the pyrolysis, which directly made an increase in the hydrophobicity and floatability of sub-bituminous coal. The findings of this paper may be useful for a better use of sub-bituminous coal resources because sub-bituminous coal is well known as difficult to float and sub-bituminous coal fines are usually wasted in coal preparation plants. This paper proposes that coal char from sub-bituminous coal may be forwarded to a further upgrading process (i.e. flotation) because the floatability of sub-bituminous coal is significantly improved by the pyrolysis.