Study On Adsorption Behavior Of Solid-liquid Separation Chemicals At Coal Tailings Interface

Coal is the main energy source in China and coal preparation serves as the source of clean utilization of coal.Solid-liquid separation is one of difficulties and usually carried out by flocculation-sedimentation-dewatering processes in washing,processing and utilization industries.Flocculation with inorganic salts or high-molecule chemicals is the main method to strengthen solid-liquid separation.However,the traditional industrial application and theoretical research work mainly focuses on the macro-application effect and influencing factors of these chemicals.The research on the adsorption behavior and mechanism of the chemicals at the coal tailings interface is not perfect enough to meet the requirements of the basic theory of solid-liquid separation at present.In this paper,the adsorption and desorption behavior,kinetics and adsorption layer configuration of different inorganic salts and high-polymers on the surface of model coal(amorphous carbon)and montmorillonite were studied in situ by QCM-D.The influence of water quality conditions on the adsorption of high polymers and the bridging effect of polymers were also investigated.On this basis,the micro-structure characteristics of coal,nano-scale wettability,water/polymers adsorption on the surface of coal and montmorillonite were studied by molecular simulation.The analysis is performed in combination with equilibrium configuration,energy contribution,density distribution,radial distribution function,and interaction energy.The main conclusions are as follows:(1)The adsorption mass of inorganic salts on amorphous carbon surface increases with the increase of concentration or valence.NaCl,KCl,MgCl₂,CaCl₂and AlCl₃ show single-stage adsorption.Adsorption is almost completely reversible;FeCl₃ solution The FeCl₃ solution is characterized by multi-stage adsorption,and the density of each adsorption layer is different,and the adsorption is irreversible.Under alkaline conditions,the adsorption mass of CaCl₂ and FeCl₃ on the amorphous carbon surface is more than that under acidic conditions.CaCl₂ solution exhibits multi-stage and irreversible adsorption characteristics as FeCl₃ solution,indicating that the irreversible adsorption behavior of inorganic salts is related to the presence of metal hydroxides such as Ca(OH)₂ and Fe(OH)₃;Debye length and double-layer thickness decrease with the increase of valence or concentration of inorganic salts,and charge inversion phenomenon exists in trivalent AlCl₃/FeCl₃ environment.The concentration which makes the potential of particles zero is the optimum concentration.The electrostatic repulsion energy is the lowest and the coagulation of particles is most favorable.When the concentration is lower than the optimum concentration,the reduction effect of trivalent inorganic salts on electrostatic repulsion energy is much better than that of monovalent and bivalent inorganic salts;when the concentration is higher than the optimum concentration.The zeta potential becomes positive.With the increase of concentration,the reduction effect of trivalent inorganic salts on electrostatic repulsion energy will gradually weaken then the effect of bivalent ions.(2)Polyacrylamide(PAM)exhibits two-stage adsorption characteristics on the amorphous carbon surface.After adsorption in a short time,the adsorption rate decreases as the effective adsorption site decreases,and the adsorption layer became loose.PAM molecules can form intermolecular association through Van der Waals.The first layer of PAM adsorbed on the surface can further adsorb PAM molecules from the solution,so the adsorption mass increase continuously and the adsorption equilibrium cannot be achieved.The adsorption of cationic polyacrylamide(CPAM)reaches equilibrium within 5 min.The configuration of adsorption layer first becomes loose,and then remains unchanged with the increasing adsorption mass;CPAM has the strongest effect on amorphous carbon,followed by polyacrylamide,and the adsorption is completely irreversible.The anionic polyacrylamide(APAM)can hardly interact with the surface of amorphous carbon.(3)Inorganic salts have little effect on the adsorption mass of PAM on the surface of amorphous carbon,while they can increase its adsorption rate;The inorganic salt inhibits the adsorption of CPAM,and the inhibition effects increases with the increase of the valence or concentration of the inorganic salt.However,the divalent and trivalent inorganic salts greatly promote the adsorption of APAM.With the increase of the valence or concentration of inorganic salts,the adsorption mass of APAM increases.The adsorption layer first becomes loose,and then becomes dense.These results indicate that APAM adsorb on amorphous carbon surface by the"metal cationic bridging"action,forming the phenomenon of alternate adsorption of metal cations and APAM.Therefore,in the presence of high valent metal cations,the adsorption of APAM on amorphous carbon surface hardly reach equilibrium even after 15 h.(4)Decreasing pH results in the increase of adsorption mass of montmorillonite on alumina surface,which is contributed to the enhancement of electrostatic attraction between positive alumina surface and negative montmorillonite particles.On the alumina and pre-adsorbed montmorillonite particles alumina surface,the adsorption mass of polymers is ranked as:CPAM>PAM>APAM.Further research indicates that CPAM has the strongest bridging effect on montmorillonite particles,followed by PAM,while APAM can hardly bridge montmorillonite particles.(5)The molecular simulation results show that the adsorption energy of water and oxygen-containing functional groups(especially hydroxyl and carboxyl group)on coal surface is the highest,followed by nitrogen-containing functional groups,and the weakest interaction with sulfur-containing functional groups and hydrocarbon functional groups;In the liquid phase,coal has a strong coordination tendency with PAM.The preferential adsorption of PAM reduces the coal-water coordination tendency.The adsorption of PAM on the coal surface mainly depends on Van der Waals interaction,followed by electrostatic interaction and hydrogen bonding.Van der Waals can contribute about 80%of the total adsorption energy.Although hydrogen bonding has a certain contribution,it accounts for a relatively low proportion(1.95%).(6)Among the polyacrylamides and their derivatives,the cationic functional group(N(CH₃)³⁺)has the strongest interaction with montmorillonite surface,followed by the amide functional group(CONH₂),and the anionic functional group(COO^-)has the weakest effect.The first two are characterized by adsorption and the anionic functional group is repulsive;CPAM with different charge can be adsorbed on montmorillonite surface through the cationic and amide functional groups,but at higher charge levels,the cationic functional groups are mutually exclusion and steric hindrance make the adsorption layer loose;For APAM,when the charge is low,the amide functional group is dominant,and it can be adsorbed on montmorillonite.When the charge is high,the anionic functional group is dominant,thus the adsorption is weakened.This research work provides a theoretical basis for the selection and design of solid-liquid separation chemicals,as well as a new way to study the adsorption behavior and mechanism of metal ions and polymer chemicals.It is not only of great scientific importance for deepening the theory of solid-liquid separation,but also of certain practical significance for the efficient recycling of water resources and comprehensive utilization of mineral resources.