Elsevier

Fuel

Volume 272, 15 July 2020, 117737
Fuel

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

https://doi.org/10.1016/j.fuel.2020.117737Get rights and content

Highlights

  • The method of converting the T2 spectrum to the PSD was introduced.

  • Porosity of water-saturated coal is 23.22%, dominating by mesopores and macropores.

  • Water wetting degree of the coal pores depended mainly on the macropores.

  • Water wetting of macropores had strongest negative effect on coal floatability.

  • Correlation between water wetting of pores and flotation recovery was determined.

Abstract

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.

Introduction

Low rank coal is usually difficult to recover directly via conventional flotation [1]. The surface of low rank coal is hydrophilic due to its hydrophilic functional groups, such as C-O, C=O, and O-C=O, which interact strongly with water. Water-covered low rank fine coal particles do not attach efficiently to the surface of bubbles. However, good flotation recovery of low rank coals such as lignite can be achieved using ultrasonic pre-treatment [2], [3], [4], [5]. The mechanical effects of ultrasonic pre-treatment, such as surface cleaning and surface crushing, that occur due to the collapse of cavitation bubbles are considered to be the main reason for the improved flotation. Additionally, the mechanical methods, such as attrition, high intensity conditioning and surface grinding also can improve the surface of hydrophobicity of oxidized coal through the removal of oxidized layer from coal particle surface [6], [7], [8], [9]. Furthermore, microwave [10], [11], [12] and heating pretreatments [13], [14], [15] have both been found to enhance the floatability of low rank coal by reducing the moisture in the coal pores and removing hydrophilic functional groups from the coal surface. Several effective flotation reagents have also been used in the flotation of low rank coal [16], [17], [18], [19]. These reagents usually contain hydrophilic functional groups that can form bonds with the hydrophilic functional groups on the surface of the low rank coal.

The surface of low rank coal is porous. The pores on the surface of low rank coal can be filled with water or air [20]. Air entrapped in the pores can form micro/nano-bubbles on the coal surface, which can accelerate the attachment between the coal particles and the flotation bubbles. However, water-filled pores form water-pockets on the coal surface, which prevent the attachment of bubbles to the particle, resulting in decreased floatability [21], [22]. The effects of entrapped air in the pores of mineral surfaces have been widely investigated. However, the role of water filled pores in mineral flotation has rarely been studied. As the low rank coal surface is highly porous, the effects of the water filling degree of its pores on its flotation requires in-depth mechanistic analysis.

In previous studies of the flotation of low rank/oxidized coal, long pre-conditioning times have been found to cause the pores on the surface of oxidized coal to be wetted completely with water, decreasing the floatability of the coal [23], [24]. Thus, oxidized coal may develop a thick hydration layer on its surface at long pre-conditioning times. Additionally, long pre-conditioning processes have also been reported to evict the air trapped in the pores of minerals and consequently reduce their floatability. Recently, Chen et al. [25] investigated the flotation of molybdenite after stirring or ultrasonic pre-treatments of various lengths. The flotation recovery of the samples after long-duration pre-treatment (stirring or ultrasound) was reduced by more than 60% at 11 min compared with that of the untreated samples. They concluded that the bubbles trapped in the pores were discharged due to the long duration of the pre-treatment process, reducing the floatability of the molybdenite. In order to eliminate the negative effects of long pre-conditioning processes on the flotation of oxidized coal, a new flotation process, namely, the zero-conditioning flotation method, was designed by Xia et al. [26]. In their method, the contact time between the coal particles and water/reagents was very short (only about 5 s), which improved the combustible matter recovery and decreased the ash content of the concentrate. In another study, Piskin and Akgun also found that a short pre-conditioning time (1 min) could enhance the floatability of oxidized coal by removing the oxidized layer on the coal surface [27]. The above-mentioned investigations focused on improving the flotation recovery of coal. However, it is still unclear which kinds of pores on the surface of coal play a major role in determining the floatability of coal particles, and how the wetting degree of water in the different types of coal pores affects flotation.

In this investigation, pre-conditioning times of 0, 1, 3, 5, 7, and 10 were utilized. Low-field nuclear magnetic resonance (LF-NMR) was employed to detect the pore size distribution (PSD) of the coal surfaces treated for different pre-conditioning times, which can reflect the percentage of coal pores wetted by water. The degree of water in the coal pores during the pre-conditioning process was studied by analyzing the in-situ wetting percentage/rate. Additionally, the correlation between the water wetting percentage of the pores on the coal surface and the flotation recovery was established for the first time to reveal the effects of water wetting of the coal pores on the coal flotation behavior.

Section snippets

LF-NMR theory

Currently, the PSD of coal can be obtained using a variety of qualitative and quantitative methods. The qualitative methods include scanning electron, transmission electron, and optical microscopy. The quantitative methods are low-field nuclear magnetic resonance (LF-NMR), micro-X-ray computed tomography (μ-CT), gas adsorption (nitrogen gas/carbon dioxide), mercury intrusion porosimetry (MIP), and small angle X-ray/neutron scattering (SAXS/SANS). Some of the methods used to measure PSD have

Laser particle size analyzer results

The particle size distribution of the coal that was subjected to 10 min of pre-conditioning was nearly the same as that of raw coal, as shown in Fig. 2. This indicated that the coal particles were not crushed after a long pre-conditioning period. In this study, a magnetic stirrer operating at a speed of 500 rpm was used to pre-condition the coal particles; this low impeller speed did not significantly affect the particle size of coal. Thus, the effect of the change in the particle size of the

Conclusions

The water wetting percentage of coal pores after different pre-conditioning times and its influence on the coal flotation behavior were investigated. Since low speed magnetic stirring (500 rpm) was used to pre-condition the coal particles, the difference in the particle size of the coal before and after pre-conditioning as determined using a laser particle size analyzer was slight, and therefore the influence of the pre-conditioning treatment on the particle size of coal was ignored in this

CRediT authorship contribution statement

Yuqiang Mao: Writing - original draft, Methodology, Software. Wencheng Xia: Conceptualization, Supervision, Writing - review & editing. Yaoli Peng: Writing - review & editing. Guangyuan Xie: Writing - review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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