Study On Selective Adsorption Of PEO On Kaolinite And Coal Surfaces

With the improvement of coal mining mechanization and the deterioration of resources and geological conditions in China,the raw coal slime becomes more fine and high-mud,resulting in a large amount of high-mud coal slime water.In high-mud coal slime water,there is a large amount of clay minerals,which can reach over 60%of the ash forming minerals in coal.Most of these clay minerals exist in the form of micro nano particles,with large specific surface area and strong electronegativity,which makes it difficult to settle naturally in coal slime water.Kaolinite,montmorillonite and illite are three kinds of common clay minerals in coal.Among them,the different cleavage planes of kaolinite represent all the surface and interface properties of other common clay minerals.Polyethylene oxide（PEO）is widely used as a flocculant in coal flotation and coal slime water clarification operations.Taking kaolinite and coal as objects,this thesis uses the methods of experiment,density functional theory and molecular dynamics simulation to reveal the adsorption mechanism of PEO on the surface of coal and kaolinite,and clarify the selectivity difference of PEO.The main conclusions include:（1）The results of adsorption test show that PEO reaches adsorption equilibrium on the surface of kaolinite and coal after 4 hours.The adsorption process of PEO on the surface of coal and kaolinite is rapid in the early stage and then tends to be balance gradually.Compared with coal,the adsorption rate of PEO-600K on kaolinite surface is faster and the adsorption amount is larger.The adsorption process conforms to the quasi second order reaction kinetic equation and Langmuir model.（2）Density functional theory calculation of coal particle surface shows that-CH₂-CH₂-of PEO and groups on the coal surface are mainly adsorbed on the coal surface through hydrophobic interaction.The adsorption energy is:-COOH>-C=O>Ph-OH>-O-.Water molecules are mainly adsorbed on different oxygen-containing functional groups of on the surface of coal through hydrogen bonds,with adsorption energies of-COOH>-C=O>Ph-OH>-O-.The adsorption energy of PEO on coal surface is less than that of water,indicating that PEO can remove water and adsorb on the coal surface.When PEO interacts with oxygen-containing functional groups on the coal surface,there are three different adsorption configurations:horizontal,vertical,and diagonal configurations.Diagonal adsorption is the most stable adsorption configuration.（3）The density functional theory calculation of kaolinite particle surface shows that the adsorption energy of PEO on kaolinite（001）surface（-112.88～-91.28k J/mol）is less than that on kaolinite（00 1?）surface（-87.15～-75.80k J/mol）,that is,PEO preferentially adsorbs on kaolinite（001）surface.The ether oxygen bond on PEO is easy to form two strong C-O…H hydrogen bonds with the hydroxyl group on the kaolinite（001）surface.The hydrogen bond length is 1.907?and 2.030?,while the interaction distance between kaolinite（001?）surface and PEO is greater than 3?,indicating that it is difficult to form hydrogen bonds,and the hydrophobic force or van der Waals force is the main force.Water molecules are mainly adsorbed on the（001）and（001?）surfaces of kaolinite by hydrogen bonding.The adsorption energy of water molecules on the（001）surface of kaolinite（-51.64～-39.82k J/mol）is less than that on the（001?）surface of kaolinite（-39.90～-21.08k J/mol）,that is,water molecules hinder the adsorption of PEO on the（001）surface of kaolinite.（4）According to density functional theory,the lowest adsorption energy of PEO on the kaolinite（001）surface is-112.88k J/mol,the lowest adsorption energy on the（001?）surface is-87.15k J/mol,and the lowest adsorption energy on the coal surface is-57.49k J/mol.It indicates that PEO will be more likely to adsorb on the kaolinite surface to form a stable structure.（5）Molecular dynamics simulation results show that PEO on（001）and（001?）surfaces of kaolinite and coal tends to be monolayer and flat adsorption configurations.As the concentration of PEO increases,the molecular chain of PEO undergoes a slight twist.The increase in PEO concentration causes a small portion of PEO to move away from the clay surface,forming clusters with other PEO molecules,and the molecular chain skeleton is significantly twisted.The large angle tilting and twisting of PEO on the coal surface results in a portion of alkyl groups moving away from the coal surface,weakening the hydrophobic effect.（6）The molecular dynamics calculation results indicate that when the number of PEO chains on the surface of kaolinite is the same,the binding effect of PEO on the（001）and（001?）surfaces of kaolinite is stronger than that of coal on PEO and the number of water molecules discharged from the（001?）surface of kaolinite is greater than that from the（001）surface.Therefore,there is a selective adsorption difference between PEO on the surface of coal and kaolinite,which makes it easier for PEO to be adsorbed on the surface of kaolinite.