Study On Flotation Performances Of Low Rank Coal Using Nonionic Surfactant As Collectors

The reserves of low-rank coal(LRC)in China account for more than 40%of the coal resources that have been discovered.However,due to the high oxygen content and poor hydrophobicity on the surfaces of LRC,it is difficult to be floated.In addition,LRC has large water content,high impurity content and low calorific value.If the direct combustion of LRC is not only energy-consuming,but also the amount of flue gas generated during combustion is 1～2 times higher than that of high-rank coal.Thus,this greatly limits the mining value of LRC.Therefore,the washing reform of LRC is imminent,and flotation is one of the most effective means to separate fine coal slime.However,due to the low metamorphism degree of LRC,its surface is rich in a large number of oxygen-containing functional groups,resulting in poor hydrophobicity.Traditional non-polar hydrocarbon oil collectors are difficult to effectively improve the surface hydrophobicity of LRC,resulting in unsatisfactory flotation results and thus plaguing the flotation upgrading of LRC slime.In addition,if LRC is not fully utilized,it will not only cause waste of resources,but also cause environmental pollution around the mining area.Therefore,it is imperative to explore new flotation reagents and improve the flotation effect of LRC.In this paper,a typical sample of LRC(long-flame coal)was selected as the research object.Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,particle-bubble adhesion desorption test and foam flotation test were used to study the flotation performances of LRC.At the same time,combined with molecular simulation software,the microscopic mechanism and wetting mechanism of different molecular types of collectors on the surface of LRC were studied.In the molecular simulation process,the classical lignite of Wender model was selected.Combined with molecular dynamics,density functional theory and Monte Carlo simulation,the microscopic mechanism of the effect of nonionic collectors(linoleic acid methyl ester,oleic acid and ricinoleic acid)on the flotation effect of LRC particles was explored.The mutual adsorption behavior and adsorption performance between different oxygen-containing functional groups in the collector molecule and different oxygen-containing functional groups on the surfaces of LRC were clarified.The main conclusions are as follows:(1)Study on the hydrophobic improvement effect of non-ionic collector on the surface of LRC.Under the condition of single reagent,the flotation effects of linoleic acid methyl ester,oleic acid,castor oil acid and diesel oil as collectors on LRC surfaces were compared by flotation experiments.The yield of LRC flotation concentrates under the action of the above four collectors from high to low is:linoleic acid methyl ester(86.95%),oleic acid(60.06%),ricinoleic acid(43.41%)and diesel(30.43%),respectively.It can be seen that fatty acid collectors can greatly improve the surface hydrophobicity of LRC.XPS test results show that the changes of surface functional groups of coal samples modified by three nonionic collectors(linoleic acid methyl ester,oleic acid and ricinoleic acid)are as follows.After modification of linoleic acid methyl ester,the content of non-polar functional groups on the surface of LRC samples is the highest,followed by oleic acid,and the effect of ricinoleic acid is the worst.In addition,the oxygen-containing functional groups on the surface of LRC after pretreatment with different collectors are as follows:linoleic acid methyl ester(78.88%),oleic acid(74.60%),castor oil acid(71.23%),diesel(67.91%)and raw coal(64.54%).According to the analysis of XPS test results,the hydrophobic functional group(C-C/C-H)on the surfaces of LRC pretreated by methyl linoleate was 14.34%higher than that on the surface of raw coal.The bubble-low rank coal particle adhesion and desorption experiments show that the bubble adhesion ability and adhesion strength are improved after different collector treatments,but the effects are different.Among them,the improvement effect of linoleic acid methyl ester is the most obvious,followed by oleic acid,and the improvement effect of castor oil acid is the weakest.This shows that fatty acid collectors have the most obvious effect on improving the hydrophobicity of LRC.When the mixture of dodecane and ricinoleic acid(3:1)is used as collector,the flotation efficiency and dispersion are much better than that of dodecane and ricinoleic acid.The highest yield of flotation concentrate can reach 82.89%,which is second only to methyl linoleate.(2)Through molecular dynamics simulation and quantum chemical calculation,it is found that the unsaturated bonds and polar groups on the nonionic collector are the main reasons for the collector to adsorb on the surfaces of LRC.The surface of LRC contains a large number of oxygen-containing groups,which can effectively form hydrogen bonds and weak van der Waals forces with polar groups on the collector,while the alkyl chain of the collector extends to the aqueous phase,thereby covering the hydrophilic water level point on the surfaces of LRC and excluding water molecules.Therefore,non-ionic collectors are more likely to adsorb and spread on the surfaces of LRC.The average maximum adsorption capacities of methyl linoleate,oleic acid,ricinoleic acid and dodecane were 148.44,122.00,118.00 and 117.65,respectively.The corresponding interaction energies are-183.13,-144.62,-126.62 and-70.53kJ/mol,respectively.It can be seen that the above three nonionic collector molecules can form effective adsorption on the surfaces of LRC,and the improvement of the surface hydrophobicity oflow rank coal from strong to weak is:linoleic acid methyl ester>oleic acid>ricinoleic acid.In addition,through relative concentration and MSD analysis,it was found that the dispersion of linoleic acid methyl ester system was the strongest,and that of ricinoleic acid system was the weakest.