Enhanced flotation of long⁃flame coal by reactive oily bubbles

Low⁃rank coal accounts for a large proportion in China’s coal supply.The flotation problem of low⁃rank coal severely restricts the clean and efficient utilization of fine low⁃rank coal particles，and it is a major technical prob⁃ lem that urgently needs to be solved by the coal industry.Aiming at the problem of low⁃rank coal flotation，the existing oily bubble flotation method in which the air bubbles were covered by a thin oily⁃collector layer to enhance its ability for capturing low⁃rank coal particles was improved in this paper so as to achieve the purpose of strengthening the min⁃ eralization process of low⁃rank coal.Additionally，the suitable surfactants were used to improve the surface / interface ac⁃ tivity of oily bubbles，and the effect of surfactant type and dosage on the performance of oily⁃bubble flotation was dis⁃ cussed from the perspective of thermodynamics.The feasibility of the reactive oily⁃bubble flotation technology was com⁃ prehensively evaluated through the induction time measurement and the oily bubble flotation experiment，and the strengthening mechanism of reactive oily⁃bubbles to the low⁃rank coal flotation was discussed. The results indicated that there existed as high as 29.21% of oxygen⁃containing functional groups on the coal surface，which resul⁃ ted in the extremely poor floatability of the coal sample.Correspondingly，a collector dosage of 60 kg / t was required for obtaining a combustible matter recovery of 82. 06%. The oily⁃bubble flotation method proposed in this paper realized the high⁃efficiency recovery of low⁃rank coal.A good flotation efficiency with a combustible matter recovery of 76.82% and a concentrated ash content of 7.21% was attained when the collector dosage for the oily⁃bubble flotation was7 kg/t，which is better than that of the conventional flotation at a collector dosage of 40 kg/t.In other words，the collector consumption was reduced by 82.50% using the oily⁃bubble flotation technology.The surfactants 2⁃ ethylhexanol and DDAB can reduce the surface tension of diesel oil.At the same time，2⁃ethylhexanol has a very limit⁃ ed effect on the viscosity of diesel oil，while DDAB can greatly increase the viscosity of diesel oil.Wetting thermody⁃ namic studies indicated that the two surfactants can promote the adsorption and spread of diesel molecules on the low⁃ rank coal surface to a certain extent，improving the thermodynamic properties of the collector.Moreover，the wetting a⁃ bility of modified diesel oil for the coal sample does not entirely increase with the decreasing surface tension，and the liquid viscosity is also a very relevant factor.The results of induction time measurement and flotation experiments con⁃ firmed that the two surfactants can further enhance the surface / interface activity of oily bubbles and improve the flota⁃ tion effect of the coal sample.However，there is an optimal critical amount for the surfactants，and the flotation effect of the coal sample is more sensitive to the DDAB dosage.Consequently，the dosage of surfactant needs to be strict⁃ ly controlled according to the properties of the coal sample.