Mechanism of pore wetting degree affecting the floatability of lignite

Pore wetting degree is one of the important factors affecting the floatability of porous fine coal. However,the intrinsic relationship between the pore wetting degree of coal and its floatability is rarely reported. It is difficult to accurately detect the behavior of its pore wetted by water due to the small size and large number of fine coal particles.Thus,in this paper,the porousactive carbon material with a cylindrical of regular shape is used to study the mechanism of pore wetting degree. This study aims to explain the mechanism of pore wetting degree affecting the floatability of lignite. The porous active carbons are modified using tetraethyl orthosilicate and cetyltrimethoxysilane. The contact angle and Fourier transform infrared spectroscopy (FTIR) are used to characterize the surface hydrophobicity of the active carbon before and after modification. The results show that the content of hydrophilic group decreases and the hydrophobic groupisgenerated,and the contact angle is improved by 67° after the modification of the active carbon,which improve its hydrophobicity. Low-field nuclear magnetic resonance (LF-NMR) is used to determine the pore wetting degree of hydrophilic and hydrophobic active carbons at different immersion times. The results show that the signal amplitude of the T₂ spectrums of both hydrophilic and hydrophobic samples increases with the increase of immersion time. The signal amplitude of the hydrophilic samples is much larger than that of the hydrophobic samples at each immersion time. The T_2g value of the hydrophilic samples is larger than that of the hydrophobic samples at each immersion time,indicating that the pore wetting degree of the hydrophilic samples is higher than that of the hydrophobic samples at the same immersion time. However,the difference in the pore wetting degree of the two samples is the smallest and the pores in the hydrophilic and hydrophobic samples are both wetted by water well when the immersion time is 10 min. The relationship between T_2g value of hydrophilic/hydrophobic samples and immersion time is nonlinearly fitted to the exponential function,which is the same as the first-class kinetic model of flotation. The wetting rate of hydrophilic samples is about 3.85 times of that of the hydrophobic samples,further indicating that the pores of hydrophilic samples are easily wetted by water compared with the hydrophobic samples. The results of lignite flotation experiments at different pre-wetting times show that the flotation recovery of concentrate decreases as the pre-wetting time increases. The long wetting time increases the pore wetting degree of lignite to form a thick and stable hydration film on its surface,resulting in a decrease in the adhesion efficiency of coal particles and bubbles.The flotation results of lignite are consistent with the behavior of pore wetting of active carbons,which also better reveals the intrinsic relationship between the floatability of porous fine coal and its pore wetting degree.