The Interfacial Characteristics Of Low-Rank Coal-Water/Hydrocarbon Oil And Its Floatability Enhancement

Abundant oxygen-containing groups and well-developed pore structure limit the adsorption and spreading of hydrocarbon oil collectors on the surface of low-rank coal.As a result,the recovery of low-rank coal particles by flotation process has become an industry problem and a research hotspot.Deepening the understanding of low-rank coal floatability and strengthening the interaction between collectors and the surface of low-rank coal are the premise and key to improve the flotation efficiency of low-rank coal.The application of polar/compound collectors and the regulation of surface wettability of low-rank coal are alternatives to solve this problem.However,the current research lacks molecular-level analysis of the floatability of low-rank coal,and some important issues such as the microscopic interaction mechanism between the reagents and the surface of low-rank coal,its structure-activity relationship and screening principles have not been completely clarified.In this study,to this end,the internal causes for the difficulty of efficient flotation recovery of low-rank coal were fully analyzed.The interfacial mechanism of polar/compound collectors and surface wettability regulation to enhance the floatability of low-rank coal was investigated in depth and the structure-activity relationship and screening criteria were established.The micro-interface strengthening mechanism of the adsorption and spreading of hydrocarbon oil collectors on the surface of low-rank coal aided by surfactant was also discussed.The internal reasons of poor floatability and low flotation separation efficiency of low-rank coal were comprehensive differentiated and analyzed.From the chemical structure point of view,the oxygen-containing groups,being rich on the surface of low-rank coal,have strong hydrogen bonding and electrostatic attraction to water molecules,thus hindering the adsorption and spreading of non-polar oil droplets on the surface of low-rank coal in the water environment.Carboxyl and hydroxyl groups are more attractive to water molecules than other O-containing groups,so the oil droplets hardly adsorb on these surfaces.In terms of physical structure,the pores in low-rank coals are the main space for water molecules.The adsorption kinetics and thermodynamics of water molecules in the pores of low-rank coal are in accordance with the second-order kinetic equation and Langmuir adsorption isotherm model,respectively.Compared with the bulk water,the atomic arrangement of O-O,H-H and H-O in pore water does not change,but the orderliness of the arrangement increases.Besides,the diffusivity of pore water is significantly weakened,indicating that the presence of pores increases the strength and stability of the hydration film.The H₂O adsorption intensity in pore could be affected by pore size and oxygen content.Small aperture would result in reduced adsorption capacity and weakened strength whereas the interaction strength of a single water molecule with low-rank coal is increased.Under the same porosity,the model with low oxygen content weakens the adsorption strength of water molecules.Both the oxygen-containing organic matter surfaces and minerals surfaces have a strong attraction to water molecules,which weakens the natural surface wettability difference between them,resulting in low flotation separation efficiency.The interfacial mechanism and structure-activity relationship of polar/compound collectors on the enhancement of low-rank coal floatability were extensively investigated,and a new standard for screening polar collectors was established.The combination of polyoxyethylene laureth ether-4(C₁₂EO₄)and n-dodecane has a certain degree promoting effect in improving the flotation efficiency of low-rank coal.The combined effect of the adsorption of C₁₂EO₄ on the surface of oxygen-containing groups of low-rank coal and the improvement of non-polar oil dispersion and surface electronegativity of oil droplets by C₁₂EO₄ makes the C₁₂EO₄-n-dodecane compound collector improve the flotation efficiency of low-rank coal.The extremely strong foaming ability of C₁₂EO₄ makes it unable to improve the flotation performance of low-rank coal when used alone.The-COOH of n-octanoic acid can replace the water molecules pre-adsorbed on the oxygen-containing group in water environment.N-octanoic acid can form a stable adsorption layer on the surface of oxygen-containing groups in a reclining or lay-flat configuration,enhancing the surface hydrophobicity of low-rank coals,and improving the floatability and flotation behavior.The flotation performance of n-octanoic acid can be further improved by compounding n-octanoic acid and n-dodecane.Due to the excellent collecting and foaming ability of n-octanoic acid,so no additional foaming agent can be used during flotation process.The use of methyl ester compound alone or in combination with n-dodecane can improve the floatability of low-rank coal.The use of methyl decanoate alone can obtain the best flotation recovery rate and flotation perfection index.When the volume fraction of methyl ester is 25%,a better low-rank coal flotation performance can be obtained.The length and structure of the hydrophobic chain in the methyl ester collector and the surface properties of low-rank coal also have a certain influence on the collection performance.Sec-octanol also has a certain ability to collect low-rank coal,and the addition methods（one time addition and step-by-step addition）basically do not affect the flotation behavior.The molecular polarity index of collector can be used as the preliminary screening standard,taking the molecular structures and physicochemical properties of the four types of collectors of alkanes,alcohols,acids,and esters into consideration,it can be found that the molecular polarity index of collector in the range of 6.5～8.0 kcal/mol can effectively promote the floatability of low-rank coal.The regulation and control performance of polyoxyethylene ether-based nonionic surfactants on the surface wettability difference of low-rank coal organic matter and gangue and its structure effect were discussed in detail.Among polyoxyethylene laureth-9(C₁₂EO₉),polyoxyethylene laurate-9（LAE-9）,and polyoxyethylene octylphenol-9（OP-9）three surfactants,the C₁₂EO₉ adsorption amount difference between organic matter and gangue surface of low-rank coal is the largest.And the adsorption of C₁₂EO₉ can significantly reduce the surface hydrophilicity of low-rank coal,but its ability to reduce the hydrophilicity of the gangue surface is poor,that is to say,the linear alkyl alcohol polyoxyethylene ether type non-ionic surfactant has excellent surface wettability differential control performance.The flotation effect of low-rank coal obtained by surfactant pretreatment is better than that by surfactant solution flotation,and the floatability of the low-rank coal is promoted by the both methods follow:C₁₂EO₉>LAE-9>OP-9.The interfacial mechanism of surfactants promoting the adsorption and spreading of hydrocarbon oil collectors on the surface of low-rank coal is inductively explored.The"head-on"adsorption configuration of surfactants on the surface of low-rank coals increaases its surface atomic composition（reduces the content of hydrophilic oxygen atoms and increases the content of hydrophobic carbon atoms at the same time）,which weakens the strength of the interaction between water molecules and the surface of low-rank coals,and enhances the adsorption and spreading performance of non-polar oil on the surface of low-rank coals,changes the adsorption configuration of oil droplets.The enhanced dispersion of hydrocarbon oil collectors by surfactants is helpful for the full exertion of the action efficiency of single non-polar oil molecules.This paper provides molecular-level insights for the in-depth understanding of the floatability of low-rank coal,deepens the microscopic mechanism of difficult to float of low-rank coal,and introduces the evaluation parameter of collector performance—molecular polarity index,and provides data support and theoretical guidance for the selection of low-rank coal flotation collectors.