Low-rank coals are difficult to float using common hydrocarbon oily collectors, such as dodecane and diesel. In this investigation, a mixture of dodecane and n-valeric acid was used as a collector to enhance low-rank coal flotation. The changes in the induction time and surface functional groups of low-rank coal were measured before and after different collector’s adsorption to indicate its absorption mechanism. The surface free energy was calculated to identify the interaction mechanism quantitatively between coal and collectors. The results showed that the flotation performance using the mixture as a collector was much better than that using dodecane or n-valeric acid solely. The induction time of coal conditioned with the mixed collector was the shortest, which meant the better hydrophobicity and floatability. When used the mixture as collector, the polar group of the n-valeric acid interacting with the oxygenated functional group on the coal surface by hydrogen bond while dodecane primarily covers the hydrophobic sites on the coal surface. Additionally, the mixed collector can minimize the surface free energy of raw coal, the dodecane primarily reduces the nonpolar part of the coal surface free energy while the n-valeric acid reduces the polar part of the coal surface free energy.

In this work, the assembly behavior and structure of a compound collector with different carboxyl group positions at the low-rank coal (LRC)–water interface were investigated through coarse-grained molecular dynamics simulation (CGMD) combined with sum-frequency vibration spectroscopy (SFG). The choice of compound collector was dodecane +decanoic acid (D-DA) and dodecane +2-butyl octanoic acid (D-BA). CGMD results showed that the carboxyl group at the carbon chain’s middle can better control the assembly process between carboxylic acid and D molecules. SFG research found that the carboxyl group at the carbon chain’s termination had a greater impact on the displacement of the methyl/methylene symmetric stretching vibration peak, while the carboxyl group at the carbon chain’s middle had a greater impact on the displacement of the methyl/methylene asymmetric stretching vibration peak. The spatial angle calculation results revealed that the methyl group’s orientation angle in the D-BA molecule was smaller and the carboxyl group’s orientation angle in the BA molecule was bigger, indicating that D-BA spread more flatly on the LRC surface than D-DA. This meant that the assembled structure had a larger effective adsorption area on the LRC surface. The flotation studies also verified that the assembly behavior and structure of D-BA with the carboxyl group at the carbon chain’s middle at the LRC–water interface were more conducive to the improvement of flotation efficiency. The study of interface assembly behavior and structure by CGMD combined with SFG is crucial for the creation of effective compound collectors.

The study on the effect of fatty acid saturation on low-rank coal (LRC) flotation is still limited. In this investigation, density functional theory (DFT) combined with Zeta potential and Fourier transform infrared spectroscopy (FTIR) was used to study the mechanism of intermolecular weak interaction at the LRC-water interface of fatty acids (decanoic acid (DA), undecylenic acid (UA), and phenyl propionic acid (PA)) with different saturations and different dodecane (D) composition hydrocarbon oil-fatty acid mixed collectors (D-DA, D-UA, D-PA). The findings demonstrated that the hydrogen bond interaction and electrostatic interaction between the UA/PA with unsaturated bonded carbon chains and the LRC molecular fragments/water molecules were stronger than DA without a saturated bond carbon chain, and UA/PA strengthened its interaction with water molecules on the whole, even PA molecules would preferentially interact with water molecules. The unsaturated bond had a minimal impact on the adsorption of the LRC hydrophobic site, and the strength of the hydrogen bond between the mixed collector and LRC is D-DA > D-UA > D-PA. In the actual flotation process, the strong hydrogen bonding and electrostatic interaction between UA/PA and water molecules weaken the collection performance of the mixed collector D-UA/D-PA for LRC, which also confirmed the research results of DFT, FTIR, and Zeta.

The efficient utilization of low-rank coal (LRC) is of great strategic significance to achieve the goal of carbon emission proposed by the Chinese government. Flotation is the most economical and e...