Mechanism of grinding effects on coal hydrophobicity during ultra-low ash coal preparation: Raman spectroscopy-based...

Mechanical coal grinding is an essential step in modern clean and efficient coal utilization methods such as ultralow-ash coal preparation and middlings reprocessing. However, the grinding parameters must be appropriately selected to balance the coal-processing efficiency with equipment energy consumption. To rapidly and accurately determine the optimal grinding parameters during the preparation of ultralow-ash coal, this research investigates the relation between grinding time and coal hydrophobicity using Raman spectroscopy. The Raman results are multidimensionally validated using other testing methods. The one-dimensional and two-dimensional Raman spectroscopy results indicate that (a) the anthracite particles primarily undergo structural fragmentation defects during the early grinding stage (before 20 min), (b) repolymerization and condensation of aromatic ring fragments mainly occur in the mid-grinding stage (20-40 min), and (c) secondary oxidation of micro-fine particles occurs during the late grinding stage. Moreover, the aromatic structure of anthracite becomes maximally stable and ordered after 40 min of grinding. X-ray photoelectron spectroscopy, contact angle and surface free energy analyses, mineralogical analyses, and flotation tests verify that 40 min of grinding maximizes the abundance of non-polar hydrophobic groups (i.e., strengthens the hydrophobicity), achieves near-complete mineral liberation, and optimizes the flotation performance (clean coal yield = 18.48%; ash content = 2.43%) of anthracite. Overall, microcrystalline structural analysis based on Raman spectroscopy can quickly and nondestructively identify the relation between the grinding behavior and hydrophobicity of anthracite.