| Coal is the main fossil energy source in China and its dominant position in China's primary energy will not be replaced for a long period of time to come.With the gradual depletion of high-quality coal resources,the exploitation and consumption of low-quality coal,such as low-rank coal and oxidized coal,have become increasingly.However,low-rank coal with low coalification has a characteristic of poor hydrophobicity and low floatability.Therefore,it is difficult to achieve its efficient and economical recovery by the conventional flotation method.Focusing on these technical problems,this study,taking a typical long-flame coal as the research object and closely following the research route that is“the surface and interface characteristics of low-rank coal?reactive oily-bubbles enhancing the flotation of low-rank coal?the mechanism of attachment between low-rank coal particles and bubbles/oily bubbles”,firstly explored its floatability characteristics and mechanism of difficult floatation.Secondly,aiming at the floatability characteristics of coal sample,the suitable surfactants were used to enhance the surface/interfacial activity of oily bubbles,and the effects of surfactants on the surface and interface behaviors of oily bubbles,such as the surface tension,viscosity,zeta potential of the oily bubbles and the wetting thermodynamics of collector for coal samples,were investigated.The influence of surfactants on the adsorption behavior of collector on the surface of low-rank coals was studied,and the mechanism of surfactant enhancing the low-rank coal flotation was thoroughly discussed.Thirdly,organically combining the extended DLVO theory and the Stefan-Reynolds model,the hydrophobic interaction energy constant between coal particles and air/oily bubbles was estimated,the interaction energies intervening air/oily bubbles and coal particles were calculated and compared,and the thinning characteristics of wetting film intervening air/oily bubbles and coal particles was investigated.As a result,the internal mechanism of oily-bubble flotation technology to enhance the separation of low-rank coal was revealed.Finally,combining high-speed camera and the software Image Pro Plus,the effect of surfactants and hydrophobicity of particles on the dynamic process of attachment between particles and air/oily bubbles was studied,and the differences in dynamic process of attachments of particles on air bubbles and oily bubbles were discussed.Consequently,the strengthening effect of oily bubbles/reactive oily bubbles on the mineralization process of flotation was clarified.Through the research of this subject,the high-efficiency flotation of the low-rank coal was achieved and the collector consumption in the flotation of low-rank coal was significantly reduced;the mechanism of oily bubbles enhancing the mineralization process of the flotation of low-rank coal was revealed from the aspects of the macro thermodynamics and the micro dynamic attachment process,which is significant to promote the development of low-rank coal flotation technology and further enrich and improve the basic theory for low-rank coal flotation.Ultimate analysis and surface functional group measurement for coal sample indicated the oxygen in the coal was 22.07%in content,and it mainly existed in the form of oxygen-containing functional groups,such as hydroxyl,carbonyl and/or carboxyl groups,with a total content of 30.32%.The coal sample was extremely poor in floatability,and it was required to consume 60 kg/t or more of collector in the conventional flotation to attain a satisfied recovery of the coal sample.The abundant oxygen-containing functional groups on low-rank coal surface is the decisive factor that causes its poor floatability,which is manifested by that the strong combination,in the form of hydrogen bonds,of polar groups in these oxygen-containing groups with water molecules will form a thick wetting film on the coal surface,inhibiting the mineralization stage of flotation.Moreover,rough surface morphology and developed pore structure were another incentive that make low-rank coal difficult to flotation,which is mainly manifested by that the filling of water molecules in surface pores stabilizes the hydration film formed on the coal surface.The modification of diesel oil using surfactants was performed,and it indicated that the surfactant,2-ethylhexanol,can improve the wettability or spreading of diesel on the coal sample,and the wettability reached a maximum of 8.92×10-4 g2/s when the mass ratio of 2-ethylhexanol to diesel oil was 10%.The opposite is true for the surfactant didodecyldimethylammonium bromide?DDAB?.The wetting ability of modified diesel oil for the coal sample did not entirely increase with the decreasing surface tension,and liquid viscosity was also a relevant factor.In this study,a high-efficiency separation of low-rank coal was achieved through the oily-bubble flotation technology.When the collector dosage for the oily-bubble flotation was 7 kg/t,a good flotation efficiency that is the combustible matter recovery of 76.82%,and the concentrate ash content of 7.21%was attained,which is comparable to that of the conventional flotation at a collector dosage of 40 kg/t.It was equivalent to reducing the collector consumption by 82.50%.Moreover,the surface/interface activity of oily bubbles can be enhanced by 2-ethylhexanol and DDAB,and therefore the flotation effect of the coal sample can be further improved.The combustible matter recovery respectively reached to 89.27%and 88.08%in the oily-bubble flotation using the reactive oily bubbles,C24 and CS1,and it increased by 16.21%and 15.99%,respectively,compared with that of using ordinary oily bubbles.Combining the extended DLVO theory and the Stefan-Reynolds model,the hydrophobic interaction energy constant between air bubbles and coal particles was estimated to be about-4.19 mJ/m2,which is about 1/7 of that between oily bubbles and coal particles.The time required for the wetting film to be thinned to a critical thickness between coal particles and air/oily bubbles was about 193.44 and 40.00 ms,respectively,while the time required for liquid film rupture,three-phase contact line?TPC?formation and expansion was about 1/5 of the attachment time.Compared to air bubbles,the energy barrier during the adhesion of coal particles onto oily bubbles was much smaller,and it was about 1/15 of that for the adhesion of coal particles to air bubbles.Moreover,the oily bubble was much larger than air bubbles in terms of the critical film thickness in their attachment with coal particles,and it was about 11.23 nm for oily bubbles.Hydrophobic attraction existing between hydrophobic surfaces was the decisive factor that drove the attachment and mineralization between coal particles and air/oily bubbles.The oily bubble greatly enhanced the hydrophobic interaction between the traditional flotation carrier?i.e.,air bubbles?and hydrophobic particles,which was the fundamental reason for the oily bubble to strengthen the mineralization process of low-rank coal in flotation.It was found from the research on the dynamic process of attachment of particles onto air bubbles and oily bubbles that an obvious film thinning can be observed after the hydrophobic glass sphere?M2?collided with air bubbles,and the film was ruptured at about 36 ms.Then,the TPC formed and extended to the maximum at about 58 ms,eventually adhering to the bottom of air bubbles due to the stable TPC.For oily bubbles,the thinning rate of wetting film was more than air bubbles,the films were ruptured at about 28 ms,and TPC extended to the maximum at about 42 ms.Compared with air bubbles,the M2 formed a greater final TPC when adhering on oily bubbles,forming a stronger attachment at the bottom of oily bubbles.Moreover,the collecting power for hydrophobic particles instead of hydrophilic particles was enhanced,which ensured the excellent selectivity during the oily-bubble flotation.Besides,the reactive oily bubbles,C24 and CS1,can furthermore accelerate the rupture of wetting film and the formation and expansion of TPC,which meant the surfactants further shortened the induction time for the attachment between M2 and oily bubbles,enhanced the adhesion power of oily bubbles for hydrophobic particles,and thus strengthened the mineralization process of oily-bubble flotation.Moreover,the differences in the attachment behavior of air and oily bubbles on the solid surface were also studied,and it suggested that the time at which TPC between air bubbles and the smooth plexiglass plate?Y3000?surface started to form was 173ms,which was much less than that between air bubbles and Y3000 surface?353 ms?.Meanwhile,the time required to complete the TPC expansion for oily bubbles?32 ms?was also less than that of air bubbles?50 ms?.In addition,the final TPC length between the oily bubble and Y3000 surface was about 3.15 mm,which is 37.55%greater than that for air bubble and Y3000 surface.These findings indicated the oil film covered on air bubbles enhanced the attachment force between the air bubble and Y3000 surface,therefore accelerated the thinning of the hydration film and the formation and expansion of TPC,and also increased the attachment strength of air bubbles to the Y3000 surface.Similarly,it was found that the surfactants can further promote the spreading and attachment of oily bubbles on the solid surface.However,the excessive DDAB will greatly hinder the spread of the collector on Y3000 surface,and thereby decreased the efficiency of attachment between solid surface and oily bubbles.There exist 121 figures,16 tables as well as 221 references in this dissertation. |
PDF Full Text Request