Investigation Of Low-rank Coal Particle-oily Bubble Interaction And Characteristics Of Oily Bubble In Flotation

The reserves and output of low-rank coal resources are abundant and huge in China.It plays an important role in the coal energy production and supply.However,due to its poor floatability,low attachment efficiency,flotation entrainment phenomenon and large consumption of oil collectors,the flotation process of low-rank coal particles is still at the laboratory stage.To address above problems,low-rank coal samples from Da Liuta and Mei Zhiyou coal preparation plants in Shendong mining area were selected.Based on the analysis results of low-rank coal assay,the surface characteristic and wetting thermodynamic behavior of low-rank coal particles were studied.Moreover,a camera-analysis system for measuring the spreading behavior of air/oily bubbles on the surface of low-rank coal particles was designed.The three-phase contact(TPC)formation process of air/oily bubbles on different roughness low-rank coal surfaces was analyzed by high-speed camera and atomic force microscopy(AFM)techniques and fitted using the fluid dynamics and molecular-dynamic models.Based on the induction time result of low-rank coal particle-air/oily bubble interaction,non-DLVO theory and Stefan-Reynolds liquid film thinning equation were used to calculate the hydrophobic force constant(K132)between the low-rank coal particle and air/oily bubble surfaces.From the flotation rate test results of low-rank coal particle-air/oily bubble,the induction times between low-rank coal particles and air/oily bubbles were calculated using the Sutherland's theory about the relationship between the total probability(E)of solid particles entering the foam product and the flotation rate constant(?).In addition,the high-speed camera technology was used to measure the angle velocity of the lowrank coal particles sliding on the surface of air or oily bubble.Moreover,the rising speed and lifetime time of an air or oily bubble were measured in the surfactant solution.Meanwhile,using high-speed camera technology,the sliding times of low-rank coal particles moving on the surface of a rising air or oily bubble were measured.The theoretical basis on the low-rank coal flotation using oily bubble has been improved and the industrial application of low-rank coal flotation is developed by these researches.In this paper,XRD,SEM,BET,FT-IR,XPS and IT(induction time instrument)were used to analyze the mineral composition,surface morphology,specific surface pores,specific surface area,and oxygen-containing functional groups of low-rank coal particles.Moreover,the surface hydrophobicity,wettability and flotability of low-rank coal particles were also analyzed.The results showed that the dry ash-free oxygen(Odaf)content of low-rank coal samples was 27.24%,which indicated that the oxygencontaining functional groups on the low-rank coal surfaces were abundant.A lot of hydrophilic oxygen-functional groups such as-OH,>C=O,and C-O,and ash-forming minerals such as Si-O-Si,Si-O-Al,and Fe S2 were found on the low-rank coal surfaces.The oxygen-containing functional groups on the whole size low-rank coal surfaces were mainly presented as carbonyl(C=O)and carboxyl(O=C-O)forms.The contents of carbon and oxygen on the surface of low-rank coal particles with-1.3 g/cm3 were 82.56% and 15.86%,respectively,while the total amount of aluminum and silicon was only 1.58%.Moreover,the ash content of the-1.3 g/cm3 low-rank coal samples was only 1.91%.Therefore,there were few hydrophilic minerals on the surface of-1.3 g/cm3 low-rank coal samples.It reduced the influence of low-rank coal heterogeneity on the subsequent gas/oil bubble spread tests on low-rank coal surfaces and the effect of particle-gas/oil bubble adhesion experiments.The differences in the spreading process of gas/oily bubbles on the surface of lowrank coals with different roughness were compared using high-speed camera and atomic force microscopy(AFM)techniques.It was found that the spreading rate of oily bubbles on the smooth surface of low-rank coal was significantly higher than that of air bubbles.Before the first 20 ms,the spreading process of oily bubbles and bubbles on the surface of low-rank coals completed 97.58% and 70.73% of the entire spreading process,respectively.The fitting results of hydrophobic force constant(K132)showed that the hydrophobic force constant(K132)between low-rank coal particles and oily bubbles was three times than that between low-rank coal particles and air bubbles while the concentration of DAH solution was 5×10-5 mol/L.The hydrophobic force constant(K132)between low-rank coal particles and oily bubbles was in the order of 10-16 while the concentration of DAH solution is 10-6 mol/L.The hydrophobic force constant(K132)between low-rank coal particles and oily bubbles was 15 times than that between lowrank coal particles and air bubbles.Therefore,the surface hydrophobicity of the oily bubble was stronger than that of the air bubble.Besides the concentration of DAH solution,10-3 mol/L,the ratio of electrostatic force to hydrophobic force(Eedl/Ehyd)between low-rank coal particles and oily bubbles was lower than that between low-rank coal particles and air bubbles.This indicated that the hydrophobic force(Eedl)between the low-rank coal particles and oily bubbles was higher than that between the low-rank coal particles and air bubbles.It is also proved that the surface of oily bubbles is more hydrophobic than the surface of air bubbles.With the same collector consumption,the maximum yields of low-rank coals in air and oily bubble flotation process were 25.99% and 95.69%,respectively.The flotation rate constants between low-rank coal particles and air or oily bubbles were 1.35 min-1 and 2.30 min-1,respectively.It demonstrated that the surface hydrophobicity of oily bubble is stronger than that of air bubble.Therefore,the flotation performance between low-rank coal particles and oily bubbles was much better than that between low-rank coal particles and air bubbles.The rising speed of oily bubbles in deionized water and DAH solution was obviously lower than that of oily bubbles in DAH solution.Moreover,the lifetime of oily bubbles in deionized water solution and DAH solution was also significantly shorter than that of oily bubbles in deionized water and DAH solution.The sliding angle velocity of the low-rank coal particles on the fixed surface of oily bubble was significantly higher than that of the low-rank coal particles on the fixed surface of air bubble.Meanwhile,the induction time between the low-rank coal particle bed and oily bubble was significantly lower than that between the low-rank coal particle bed and air bubble.Therefore,the surface hydrophobicity of the oily bubble in the flotation process was more than that of the air bubble.The results of the low-rank coal particles sliding time on the rising air/oily bubble surface showed that while the collision contact angle between the low-rank coal particle and air/oily bubble increased from about 3° to 61°,it can be observed that the sliding times of low-rank particles on the oily surface was always lower than that of the low-rank coal particles on the air bubble surface.No attachment phenomenon between low-rank coal particles and air bubbles was observed whlie the collision contact angle was greater than 61°.However,the attachment process between low-rank coal particles and the oily bubbles was found.On the one hand,it indicated that the surface hydrophobicity of low-rank coal particles was lower.On the other hand,it also demonstrated that the surface hydrophobicity of oily bubbles was higher than that of air bubbles.The half time for the low-rank coal particles to slide from the collision contact angle of 0° to the bottom of the oily bubble was about 7 ms higher than the time for the low-rank coal particles to slide from the collision contact angle of 90° to the bottom of the oily bubble.This is mainly due to the fact that the concentration of collector kerosene at the top of the rising oily bubble is lower than that of collector kerosene at the tail of the rising oily bubble.