Effects of pore compression pretreatment on the flotation of low-rank coal

The poor flotation efficiency of low-rank coal in the flotation process is due to the large and rich pore structure on its surface. In this study, the changes in the pore structure before and after pore compression pretreatment were analyzed by a scanning electron microscope and Brunauer–Emmett–Teller analysis. The effects on the flotation were then evaluated by analyzing the results of collector adsorption experiments and flotation kinetics tests. After pore compression, the pore state on low-rank coal surface was from open to half-closed and then closed. Further, the pore volume, specific surface area, and average pore diameter of the particles decreased from 0.02497 to 0.01214 cm3/g, 17.383 to 5.006 m2/g, and 9.6992 to 5.7466 nm, respectively. In addition, the adsorption rate of the collectors improved and the equilibrium adsorption time of n-dodecane decreased from 436 s to 250 s, whereas the adsorption rate of water decreased with the porosity of the coal after pore compression. Meanwhile, according to the flotation results, the maximum clean coal yield (74.44%) was obtained within 400 MPa of pressure pretreatment using n-dodecane as a collector; this value is 20.46% points higher than that of raw coal flotation using n-hexane as the collector. Furthermore, it was also found that a collector with too high a viscosity or too short a carbon chain causes the total cumulative yield of the low-rank coal to decrease. It is anticipated that the results of this work can provide guidance in low-rank coal flotation applications.