Research On Reactive Oily Bubble Flotation Behavior Of Low Rank Coal And Its Flotation Technique

Low rank coal, with abundant reserve and large production, in China is an important member of production and supply of China’s coal energy. With the improvement in the proportion of mechanical coal mining and dense medium separation of lump and gum coal, the content of fine coal slime and its ash rise rapidly. Flotation of low rank coal slime attracts attentions of numerous experts and scholars in the area of coal preparation. Due to poor flotability and mineralization of low rank coal, entrainment of fine mineral and higher oil consumption in flotation process, the investigation of low rank coal flotation stay in laboratory experiments. In order to solve above problems, two low rank coal samples were adopted in this subject. Based on the analyses of coal properties, the surface characteristics and wetting thermodynamic behavior of low rank coal samples were studied. With the analysis of generation and properties of the oily bubble, the apparatus for oily bubble generation has been designed. Then the effects of surfactants on performance of oily bubble were discussed. Next, the flotability of low rank coal affected by oily bubble and its adhered energy were considered using improved apparatus for contact angle measurements. The flotation selectivity of low rank coal with different flotation technique was studied. Finally, the oil consumption of low rank coal flotation was reduced and the combustible matter recovery was enhanced by means of flotation technique intensification. This work is of guiding significance for enriching flotation theory and performing industrial application of low rank coal flotation.The surface characteristics and wetting thermodynamic behavior of low rank coal samples were investigated by adopting modern measurements, such as XRD, SEM, FT-IR, XPS, pore and specific surface area, and wetting analysis. The results showed that the particle sizes of low rank coal were fine. The contents of particle sizes below 0.045 mm were more than 56% and their ash content were more than 46%, and they were mainly composed by micro-fine particles, ie. fraction of 0.045~0.020 mm. The main minerals in coal surfaces were quartz, kaolinite and muscovite, and an obvious increase of clay minerals in finer coal with size fraction of-0.045 mm. the major pores on coal surfaces were mesopores with width of 3~6 nm. The greater specific surface area, total pore volume and average pore width were presented in finer coal with size fraction of-0.045 mm. The surface of low rank coal with abundant oxygen-containing functional groups is mainly composed with hydroxyl and ether, but less consisted of carbonyl and carboxyl. Compared with full size fractions, the size fraction of-0.045 mm has lower contact angle. Under similar total surface free energy and non-polar component of coal samples, the polar component and surface polarity, especially for base part, of finer particles were higher than full size fractions.A new experimental system for low rank coal flotation has been designed and built. The conventional mineralization between oil and coal particle in flotation was changed. Then the generation and performance of oily bubble was studied. The results showed that the effects of diesel oil, used in oily bubble generation, modified by different type of surfactants were considered. The surface tension of modified diesel oil was reduced significantly under a certain range after addition of surfactants, while the viscosity was increased. When larger amounts of surfactants were added, the non-ionic 2-ethyl hexanol had smaller decreasing amplitude in surface tension and remarkabler in viscosity, while cationic dodecyl amine(DDA) is opposite. The sizes of oily bubble could be reduced by addition of surfactants. The diameter and its discrete degree(standard deviation) of diesel oil modified by 2-ethyl hexanol were increased slightly under larger mixed ratio of surfactant and diesel oil, while it was opposite to DDA.The wettability of low rank coal affected by oily bubble and its adhesion work was considered. In order to measure contact angle of air, oily and reactive oily bubble on coal surface in three air/liquid/solid phase, the current apparatus for contact angle measurement was improved. The contact angles of oily bubbles on coal surface were increased significantly than air bubble. The greater contact angle of reactive oily bubble was modified by proper addition of 2-ethyl hexanol. And the flotability and its adhesion work of low rank coal affected by oily bubble were enhanced by 2-ethyl hexanol. Therefore, it was in favor of adhesion between oily bubble and low rank coal particle.The flotation selectivity of low rank coal under different flotation technique and its intensification was investigated. The results showed that it was required for more than 50 kg/t diesel oil to float coal particles with lower ash content using conventional flotation machine and column. For low rank coal with ash content of 35.78% and +0.045 mm size fraction of 43.59%, the introduced reactive oily bubbles generated from diesel oil modified by 2-ethyl hexanol had obvious intensification for flotation selectivity and collecting power of coarse coal particles. The concentrate with low ash content of 11.16% and combustible matter recovery of 78.83% was achieved. Simultaneously, the oil consumption, 4.43 kg/t, in the flotation process was reduced to no more than one tenth of conventional consumption. In addition, for micro-fine low rank coal with-0.045 mm size fraction of 91.65% and ash content of 43.84%, a certain intensification of flotation selectivity and pronounced improvement of collecting power and obvious reduction of oil consumption was achieved. A concentrate with ash content of 15.35% and combustible matter recovery of 58.61% was obtained.