Effect Of Ca²⁺ On The Flotation Effect Of Low-rank Coal And Mechanistic Study Of Its Improvement Measures

Flotation is an effective method for upgrading low-rank coal.However,the presence of large amounts of Ca²⁺in the slurry can have an adverse impact on the flotation of low-rank coal（LRC）.Currently,the mechanism of action of Ca²⁺on the surface of LRC and its improvement measures have not been clearly reported.Studies have found that adding aromatic compounds to the hydrocarbon collector can effectively improve the flotation performance of LRC in high-calcium environments.In this study,flotation experiments,contact angle,zeta potential,XRD,ICP,FTIR,XPS and molecular dynamics simulation were used to investigate the interaction between Ca²⁺,hydrocarbons,aromatic hydrocarbons,and composite collectors with the surface of LRC at both macro and micro levels,and to reveal their mechanisms of action.The LRC used in this study had a high ash content of about 25%,with quartz as the main impurity mineral,followed by kaolinite,illite,and other clay minerals.The particle size distribution of-75+25μm was relatively large,accounting for about 53.47%,indicating that the LRC sample was suitable for separation and upgrading through flotation experiments.As the concentration of Ca²⁺in the slurry increased,the flotation performance of LRC gradually deteriorated.It was found that Ca²⁺could adsorb on the coal surface.When the Ca²⁺concentration in the slurry was 600 mg/L,the adsorption of Ca²⁺on the coal surface reached its maximum,and all flotation evaluation indexes gradually stabilized.This was because LRC surface has abundant oxygen-containing functional groups such as C-O-C/C-OH,carbonyl groups（C=O）,and carboxyl groups（O=C-O）.Ca²⁺in the slurry could react chemically with the phenol hydroxyl and carboxyl groups on the LRC surface to generate calcium phenol and calcium carboxylate,which could then be adsorbed on the LRC surface and shield the O element on the coal surface.Molecular dynamics simulation results showed that the surface of LRC,which had hydrophilic oxygen-containing functional groups,could interact with water molecules through van der Waals force,forming a stable hydration film on the LRC surface;Ca²⁺could adsorb around the phenol hydroxyl and carboxyl groups on the LRC surface,enhancing the interaction between water molecules and the LRC surface,increasing the hydrophilicity of coal particles,and leading to poor adhesion-mineralization effects between LRC particles and oil droplets,ultimately resulting in difficult flotation of the coal sample;After Ca²⁺adsorption,dodecane molecules changed from being micro-aggregated and spread on the LRC surface to aggregating into spherical structures,and the adsorption strength was significantly weakened.The calculation results proved that Ca²⁺weakened the adsorption effect of diesel on the surface of LRC.To address the adverse impact of Ca²⁺in LRC flotation,this study added aromatic compounds to the hydrocarbon collector.The results showed that the flotation performance of LRC in high-calcium environments was significantly improved after the addition of aromatic compounds.The selected aromatic compound used in this study was naphthalene,and the mass fraction of naphthalene in the composite collector with diesel was 10%,which produced the best flotation performance.The combustible recovery increased from 40.64%to 44.05%,the flotation improvement coefficient increased from 14.58%to 18.31%,and the yield of clean coal increased from 36.89%to 39.62%;the ash content of clean coal decreased from18.09%to 16.00%.The experimental and molecular dynamics simulation results showed that the benzene ring structure in naphthalene played an important role in its adsorption on the LRC surface.One benzene ring structure in the naphthalene molecule interacted with the aromatic skeleton on the coal molecule through aπ-πstacking interaction,while the other benzene ring covered the LRC surface,thereby shielding the oxygen-containing functional groups near the aromatic skeleton.This effectively increased LRC surface hydrophobic sites,enhanced the hydrophobicity,and promoted the adsorption of the hydrocarbon collector,significantly improving the flotation performance of LRC in high-calcium environments.The molecular dynamics simulation calculation results were consistent with the experimental phenomena at the macro level and laid a theoretical foundation for the further design of new LRC collectors.