1. Introduction
Low rank coal, such as lignite and sub-bituminous coal, makes up more than 50% of the world’s proven coal deposits [
1]. With the increase of the world’s energy demand, the utilization of low-rank coal has become an urgent need and received widely attention [
2]. The upgrading of low-rank coal to obtain high-quality coal resources has been conducted by the froth flotation method. This is a surface-based method based on the differences in surface properties between the target minerals and gangue minerals. In the process of froth flotation, the target minerals are often selectively hydrophobic by adding agents called collectors at the liquid/solid interface. The bubbles are injected into the flotation cell, and the hydrophobic minerals in the upper part of the flotation cell are recovered under strong stirring conditions [
3,
4,
5]. In the process of slime flotation, the hydrophobicity of the coal particle surface is one of the most important factors to determine the flotation effect. However, the surface of low-rank coal is abundant with oxygen-containing functional groups (hydroxyl, carboxyl, carbonyl, phenolic hydroxyl), which will form a hydrogen bond with water molecules and lead to the low hydrophobicity of the coal surface [
6]. The low hydrophobicity of low-rank coal limits the adsorption of conventional aliphatic hydrocarbon oil on the surface. It was proved that the introduction of oxygen-containing functional groups into oily collectors could make the collectors stably adsorb on the surface of low-rank coal, thus, changing the surface hydrophobicity of low-rank coal [
7,
8]. Zhou et al. [
9] used Sorbitan Monolete (Span 80) series surfactants with hydroxyl and ester groups to explore the flotation effect of low-rank coal and found that it can significantly improve the hydrophobicity of low-rank coal. Jena et al. [
10] found that alcohol collectors can effectively improve the hydrophobicity of low-rank coal. Xia et al. [
11,
12] investigated that fatty acid methyl esters in biodiesel have a better flotation effect than conventional hydrocarbon oil collectors. Thus, it is valuable to study the adsorption behaviors of the two collectors on low-rank coal and their effects on improving the flotation efficiencies of coal slime to find whether hydroxyl or ester groups is advantageous for reducing the hydrophilicity of low-rank coal.
Molecular dynamics simulation is based on the classical mechanical method to simulate the dynamics and thermodynamics properties of each molecule in the system. At present, the molecular dynamics method has become an important tool to provide the micro and basic understanding of the molecular system, which has been widely used in the field of mineral flotation reagents [
13,
14]. Wang et al. [
15] described the adsorption behavior of dodecyl amine, sodium oleate, and their mixtures on the surface of Muscovite by molecular dynamics simulation. Rai et al. [
16] used molecular dynamics simulation to describe the adsorption process of oleic acid and dodecyl amine hydrochloride on complex aluminosilicate minerals, such as spodumene, jadeite, feldspar, and mica.
All these researches reveal the reactivity and selectivity of collectors and their adsorption behavior on the mineral surface, which is of great significance to further understand the adsorption mechanism of collectors on the mineral surface. However, all these studies were mainly concentrated on minerals with a single structure and chemical composition. Coal is a kind of substance with complex structures and various chemical components. Due to its complex surface properties, the application of molecular simulation in the field of coal flotation is still in its infancy [
17]. Zhang et al. [
18] selected a wiser model to represent bituminous coal in order to simulate the adsorption behavior of three different types of the collector on the coal surface through MD, which provides a basis for the study of the adsorption behavior of collector on the coal surface. Lyu et al. [
19] combined the experiment and molecular dynamics simulation method to study the absorption behavior of NPEO-12 on the surface of Hatcher sub-bituminous coal.
However, due to the limitations of previous coal models (such as Wiser and Hatcher models) in describing the heterogeneous structure of coal, their surface properties may be different from the coal used in the test. The difference between coal surface properties will lead to the difference in collector adsorption behavior. It is necessary to construct a reasonable molecular structure of coal to explore the interaction between coal and collector.
To conduct a comparative study with the experiment, researchers began to construct the coal structure model based on its surface properties used in the experiment. Moreover, it is widely used to explore the adsorption behavior of coalbed methane on the coal surface. Lin et al. [
20] used solid-state
13C nuclear magnetic resonance (NMR), Fourier transform infrared spectra (FTIR), and X-ray photoelectron spectroscopy (XPS) to analyze the surface properties of Indonesian lignite, and constructed the macromolecular structure model of Indonesian lignite. The calculated chemical displacement spectrum of the model is in good agreement with the experimental results. Meng et al. [
21] used a proximate analysis, ultimate analysis, XPS,
13C-NMR to construct the Zhaozhuang coal molecular model. The GCMC method was used to study the adsorption behavior of methane in coal molecules. The relative adsorption error between simulation results and test results is only 3.303%.
In the field of coal flotation, few studies on the adsorption behavior of collectors on the coal surface by constructing the coal model based on the surface properties of the coal used in the experiment. Accurately constructing the molecular structure of coal is the basis of exploring the interaction between coal and collector, which also helps to determine the interaction between different oxygen-containing functional groups (hydroxyl and ester groups) and the coal surface, and then to select an effective collector suitable for sub-bituminous coal.
The purpose of this study is to screen out the suitable non-ionic collector for reducing the strong hydrophilicity of sub-bituminous coal from Bulianta mine by molecular dynamics method and to study the influence on the wettability of Bulianta coal. In this paper, the surface properties of the Bulianta coal were explored, and the corresponding coal structure model was established. A molecular dynamics simulation was adopted to investigate the micro-adsorption behavior of dodecanol and methyl laurate, where the number of nonpolar groups is the same, but with different polar groups. The results of experiments and simulations were compared.
5. Conclusions
In order to select the collector suitable for low-rank Bulianta coal, both methyl laurate and dodecanol (with the same carbon number and different polar groups) are selected in this paper. According to the relevant structural parameters obtained from the experiment, Bulianta coal molecular model is established, and the structure is optimized. The 3D structure model is constructed by molecular dynamics, and the density calculation is carried out, which is in good agreement with the actual density.
The different adsorption behaviors of the two collectors on the surface of Bulitana coal were determined by molecular dynamics. The results show that dodecanol had a higher adsorption capacity near the carboxyl and hydroxyl groups, but less near the carbonyl and ether bonds. Some of the polar groups of dodecanol deviate from the coal surface and tend to the water phase. However, the adsorption of methyl laurate on several oxygen-containing functional groups on the coal surface is relatively uniform. The polar groups of methyl laurate are all toward the coal surface. The methyl laurate containing ester group with weak hydrophilicity can more effectively improve the surface hydrophobicity of Bulianta coal compared with dodecanol containing hydroxyl with strong hydrophilicity.
XPS and flotation results show that methyl laurate can effectively reduce the amount of oxygen-containing functional groups in coal compared with dodecanol. Dodecanol is mainly adsorbed near the carboxyl and hydroxyl groups on the coal surface, but less on the carbonyl group. The adsorption of methyl laurate on the coal surface is more uniform, which makes the oxygen-containing functional groups on the coal surface reduce to a certain extent. This is consistent with the simulation results of the adsorption behavior of the collector on the coal surface.