Wetting of coal pores characterized by LF-NMR and its relationship to flotation recovery

The wetting of coal pores by water has an observable influence on the flotation recovery of combustible matter from fine coal resources. In this investigation, low-field nuclear magnetic resonance (LF-NMR) was employed to measure the porosity of coal pores after various pre-conditioning times; the water wetting degree of coal pores was then characterized using this porosity data. The water wetting percentage of coal pores was calculated by using the ratio of the measured porosity to the total porosity (obtained from a water-saturated coal sample). The water-saturated coal sample had a total porosity of 23.22%, which was dominated by mesopores and macropores. The measured porosity of the coal surface increased from 11.33% to 15.37% as the pre-conditioning time was increased from 0 to 10 min, and the flotation recovery was reduced from 65.23% to 21.58%. The water wetting degree of the macropores was found to have a stronger negative effect on the coal flotation than that of the mesopores. The larger pores were more easily wetted by water than the smaller pores, resulting in the formation of a thick hydration layer on the coal surface, which reduced the floatability of coal particles. Additionally, the negative linear correlation between the water wetting percentage of the coal pores and the flotation recovery was determined to elucidate the effects of the water wetting of the coal pores on its flotation behavior.