Effect of low-temperature pyrolysis on the surface hydrophobicity of low rank coal and the analysis of floatability of semi-coke

Low-grade coal is difficult to obtain high efficiency by conventional flotation due to its poor surface hydro- phobicity. In addition,the reactivity between low-grade coal and oxygen is high,so it is prone to spontaneous combus- tion. However,there is less oxygen in coal bed or internal dump,so the coal mainly suffers high temperature baking ( similar to low temperature pyrolysis environment). Low temperature pyrolysis has gradually become one of the ways of efficient transformation and utilization of low-grade coal. Therefore,it is of great significance to explore the effect of low temperature pyrolysis on the hydrophobicity of low-grade coal and to reveal the change of the floatability of pyrolysis solid products. In this paper,non-caking coal was used as the object of study,and the weight loss characteristics of coal sample pyrolysis were depicted by thermo-gravimetric analyzer(TGA). Gas / liquid chromatography(GC / LC) was used to detect the composition of volatile products during pyrolysis. X-ray photoelectron spectroscopy( XPS) and scanning electron microscopy(SEM) were used to characterize the functional groups and microstructure of coal samples before and after pyrolysis. The induction time and unit flotation tests were used to analyze the changes of hydrophobicity and floatability of coal samples before and after pyrolysis. The results show that the coal sample is deprived by low tempera- ture pyrolysis,and a large number of volatile components(hydrogen,methane and other gaseous products,phenol-based liquid products) escapes,which leads directly to the content of hydrophilic oxygen-containing functional groups on the coal surface. The content of hydrophobic functional groups increased,the surface porosity and cracks increased signifi- cantly and the roughness increased. Compared with the raw coal,the pyrolysis half-coke induction time was shortened and the flotation yield was significantly increased. The low temperature pyrolysis enhances the hydrophobicity of the non-stick coal surface,and improves the flotation recovery rate,which promotes the technology development of “ low temperature pyrolysis-semi-coke flotation and upgrading”.