| The hydrated film on the low-rank coal surface caused by the abundant oxygen-containing groups and well-developer pore structure increases the difficulty of adhesion between the particles and bubbles.As a result,it is difficult to separate and refine low-rank coal by conventional flotation methods.Although,the bridging effect of nano-bubbles can effectively promote the adhesion between particles and bubbles and improve the flotation of microfine grained minerals,how to introduce nanobubbles on the surface of hydrophilic low-order coals and improve their sorting effect is yet to be investigated.The nanobubbles adhering low-rank coal cannot be directly characterized by AFM due to the rough and uneven surface of low-rank coal.In this paper,the graphite particles with different wettability were adopted to represent the Coal-Like Hydrophilic Model Minerals(CLHM).The relationship between the hydration film and oil film on the surface of CLHM and its wettability is systematically studied.On the basis of thinning the hydration film on the surface of CLHM by wettability regulation,the adsorption of bulk-phase nanobubbles and the generation of interfacial nanobubbles generated by the decompression and temperature difference,on CLHM surfaces were investigated.Furthermore,flotation experiments were conducted to investigate the mechanism by which the generation of nanobubbles facilitates the flotation of CLHM and low-rank coal,with a view to providing a fundamental understanding of nanobubble-enhanced flotation of low-rank coal.The effect of the surficial wettability of CLHM on the hydration film and oil film(dodecane)properties was investigated using Atomic Force Microscopy(AFM)techniques.The results as follows: The more hydrophilic the surface of the CLHM is,the thicker its surface hydration film will be.The thickness of the hydration film,which shows a trend of decreasing and than,increasing with higher surfactant concentration,on the surface of the CLHM can be effectively reduced by the absorption of the dodecyl alcohol ethoxylate(AEO-15).The thickness of the transition layer in the hydration film will be increased with the excessive absorption of the surfactant on the surface of the CLHM,while the thickness of the ordered layer remains unchanged.The surface,where the hydrophily is enhanced,of the CLHM proves to reduce of the thickness of the oil and Dodecane spreading effect becomes poor.The surficial hydrophobicity of the CLHM is improved,following the absorption of surfactant,which lead to the thickness of the oil film increases and the spreading effect of dodecane on its surface is restored.Simultaneously,the oil film structure on the surface of the hydrophobic surface is similar to the hydrophilic surface hydration film structure.Based on the thinning of hydrated films on the surface of the CLHM,the generation and absorption of nanobubbles on the surface,which are divided into interfacial nanobubble and bulk phase nanobubble,were in investigated,respectively.The location of nanobubble generation and adsorption on the surface of a solid sample is highly related to the surface wettability of the sample.The more hydrophobic the surface is,the denser the nano-bubbles will be on its surface.In addition,the influence of adhesion between mineral particle under the attachment of nanobubbles was investigated.The results are as follows: Hydrophobic forces of gravity are the driving force behind the adhesion of mineral particles to nanobubbles.The more hydrophobic the surface of the model mineral,the greater the interaction between it and the nano-bubbles.When particles and bubbles are in close proximity to each other the distance that particles jump into the surface of the nanobubbles is increased and the adhesion distance between them during separation prove to enlarge.The area of hysteresis loop formed between the extand and retract in the AFM is larger.The hydrophilic graphite oxide particles did not adhere to the nanobubbles.After adsorption of the graphite oxide particles by the surfactant,the distance which the jumping-in phenomenon occurs increase as the particles and bubbles approach each other.In addition,the interfacial nanobubbles produced by the decompression method are more conducive to particle-bubble adhesion than the bulk-phase nanobubbles.Low-rank coal flotation experiments show that the flotation of low-rank coals is greatly enhanced by the introduction of nano-bubbles into the flotation system,which widens the difference in wettability between the organic matter and mineral surfaces of low-rank coals,further increasing the yield of fine coals,combustible recovery and flotation perfection and decreasing the ash content of fine coals.The low-rank coal slurry which is treated by the reduction of the pressure superior to flotation effect which is treated by the temperature difference. |
