Research On Emulsion Flotation Of Low-rank/Oxidized Coal: Mechanism And Application

The reserves of low-rank/oxidized coal is abundant in China.It has been a very important part of the coal energy production and supply,especially in the field of power system,and chemical engineering.The amount of lowrank/oxidized coal slime has been increasing with the development of mechanized mining and the enlargement of gravity concentrator equipment.Coal flotation is widely applied to recover fine coal.However,due to its poor floatability and large consumption of oil collectors,a large amount of fine coal is not recovered effectively.To address the above problems,the effect of oxygen-containing functional groups on the interfacial water structure at the coal surface was studied.In addition,the flotation performance of the high internal phase HIP W/O emulsion was investigated.Based on the theoretical analysis of the enhanced emulsion flotation,the emulsion stability was improved by the optimization of the composition of the internal water phase.The low-rank/oxidized coal can be economically recovered by flotation with the optimized HIP W/O emulsion.These researches will provide a new way to improve the flotation performance of oxidized coal and contribute to the efficient,clean development and utilization of coal resources.(1)The interfacial water structure at the coal surface was studied by molecular dynamics simulation to reveal the mechanism of large consumption of oil collectors for low-rank/oxidized coal flotation.In our simulation,the interfacial water structure at coal surfaces with different content of oxygencontaining functional groups were studied,including the simulated contact angle,distribution and mobility of water molecules,hydrogen bond structure,as well as the interaction energy between water molecules and surface sites.The molecular dynamics simulation results were also verified by advancing/receding contact angle measurements.Both advancing and receding contact angles notably decreased when the hydroxyl content of the coal surface increased.The coal surface with higher hydroxyl content was strongly hydrophilic,resulting in more water molecules being accommodated at the coal surface and the mobility of water molecules being restricted.Compared with other oxygen-containing functional groups(-COOH,-C-O-C-,-C=O),the hydroxyl group sites could hydrate more water molecules through hydrogen bonds and exhibited a much higher interaction energy.Therefore,an extended understanding of the effect of oxygen-containing functional groups on the wetting of coal surfaces by water is provided at the atomic/molecular level.(2)Surface chemistry and floatability experiments were carried out for coal samples of different rank obtained from coal mines in Shanxi province and for oxidized coal samples prepared in the laboratory at 185 °C in air.The surface properties were investigated by scanning electron microscopy with an energy dispersive spectrometer(SEM/EDS),X-ray photoelectron spectroscopy(XPS),electrokinetic studies,and contact angle measurements.The flotation response was evaluated by micro-flotation experiments.The results indicate that lower rank coal surfaces have lower contact angles and a reduced flotation response due to the presence of more hydroxylated oxygencontaining functional groups at the coal surface.The kerosene dosage is greater than 8 kg/t for low rank coal flotation.The results from the surface chemistry study and flotation experiments indicated that the surface properties and flotation response of coal samples with different degrees of oxidation correspond to the results for the coal samples of different rank.For highly oxidized coal,more oxygen-containing functional groups were detected at the coal surface,making the point of zero charge shift to negative.Highly oxidized coal surfaces have lower contact angles and a reduced flotation response.(3)HIP W/O emulsion with 85 vol% internal dispersed water phase was selected for the micro-flotation of fine oxidized coal.In this study,optical microscopy and high speed video microscopy were employed to study the dispersion behavior and microstructure of the HIP W/O emulsion in water,as well as the emulsion droplets attachment at the coal surface.In addition,the interfacial adsorption state of Span 80 on the flat oxidized coal surface was investigated by molecular dynamics simulation.Our micro-flotation results showed a significant oil dosage reduction when a HIP W/O emulsion was used as a collector for coal flotation.The improved flotation performance may benefit from internal water space filling and the oil like surface of HIP W/O emulsion.This study found that the HIP emulsion tends to be dispersed as a W1/O/W2 double emulsion during mixing in water because of the migration of the surplus Span 80 in the oil phase to the O-W2 interface.More importantly,the Span 80 emulsifier at the O-W2 interface seems to facilitate hydrogen bond formation between the hydrophilic moieties of Span 80 and the oxygen containing functional groups at the coal surface,which is verified by the results from molecular dynamics simulation(MDS).The hydrogen bond formation plays a leading role in the attachment and/or spreading of the emulsion droplets at the oxidized coal surface.(4)The effect of mixing speed,emulsifier concentration,Na Cl concentration and xanthan gum concentration on the emulsion properties were investigated.The centrifuge stability,droplet size,viscosity,and the stability in water of HIP W/O emulsion were evaluated.The relationship between the emulsion properties and flotation performance were studied by micro-flotation.The results showed that better flotation performance of low rank coal was achieved as the stability of HIP W/O emulsion was improved.The HIP W/O emulsion stability can be improved with higher mixing speed,larger emulsifier concentration,and appropriate Na Cl concentration.More importantly,the gelation of the internal water phase with the addition of 3 wt‰ xanthan gum tends to hinder the welling rate of HIP W/O emulsion in water.Thus,the flotation performance of emulsion with 3 vol% emulsifier is equivalent to the flotation performance of the emulsion with 4 vol% emulsifier.A coal yield of 88.54% was obtained for the optimized HIP W/O emulsion at a dosage of 1.6 kg/t.The ratio of oil saving is 80%.(5)The effect of HIP W/O emulsion on the separation of coal and mineral was studied by micro-flotation and batch flotation.The micro-flotation results of the separation of coal particles with different size and kaolinite showed that kaolinite slightly influenced the flotation performance.A coal yield of 86.43% was obtained for the optimized HIP W/O emulsion at a dosage of 2 kg/t.The ratio of oil saving is 83.33%.In addition,the flotation performance of HIP W/O emulsion prepared with kerosene from the coal preparation plant was studied by batch flotation.A coal yield of 82.57% was obtained for the HIP W/O emulsion prepared with kerosene from the coal preparation plant at a dosage of 1.2 kg/t.The ash content of the concentrate is 10.67%.The ratio of oil saving is 70%.The flotation results show that the coal and mineral can be separated with the HIP W/O emulsion as flotation collector.The emulsion flotation of low-rank/oxidized coal has good adaptability.The HIP W/O emulsion could be a potential economical and efficient replacement for the traditional oily collectors as a promising collector for the flotation of low rank/oxidized coal.The oil dosage can be reduced dramatically,leading to economic benefit improvement and industrial development of coal preparation.