| It is technically and economically essential to solve the high ash problem of coal flotation concentrate caused by slime contamination. This thesis aims to reveal the contamination mechanism of fine slime to clean coal in the flotation process, thereby providing guidance to solving the contamination problem with column flotation. A series of works were conducted following this goal, like analyzing the characteristics of contamination minerals in industrial flotation column concentrate, studying the effect of fine slime on hydrophobic coal particle flotationas well as the recovery behavior of fine slime in concentrate, exploring the spatial distribution of coal particles and fine slime in flotation column and the mechanism of slime contamination to clean coal. Based on these works, several methods on reducing fine slime contamination in column flotation clean coal were put forward. The main research results are as follows:The size distribution, mineral composition of fine slime in concentrate of industrial flotation column were identified. According to the results, ash content of +0.045 mm fine particles wereusually lower than the product requirement while that for-0.045 mm fine particleswere much higher than the requirement. The contamination of these high density particles, though just composite less than 5 % in weight of the flotation concentrate, is the chief reason for the high ash problem. The mean size of slime particles in the flotation concentrate was around 10-20 ?m, where the fraction of-20 ?m particles was higher than 60%. A general trend was that the problem of concentrate contamination would be remarkably serious with finer slime. The mineral compositions of fine slime were mainly hydrophilic clay minerals and quartz minerals.The influence of fine slime on coal particle flotation as well as the recovery behavior of fine slime in concentrate were explored in this research. The effect of fine slime on coal flotation varied with feed coal particle size and fine slime proportion: at a small fine slime percentage, the flotation recovery of coal particles with different size were all enhanced compared to that without fine slime; however, a detrimental effect in coal recovery was seen in all experiments when the percentage of fine slime was too high in the feed, and the decreasing trend was much significant for large particles. For 0.5-0.25 mm particles, a small amount of fine slime would increase the flotation speed while a large proportion may decrease the flotation speed. In comparison, a monotonically increasing trend and a monotonically decreasing trend were found for 0.25-0.125 mm particle and 0.125-0.074 mm fine coal respectively. However, for low ash coal particles at a particular size fraction, the decrease of flotation rate constant was obtained with the increase proportion of fine slime, which indicate that the increase of fine slime percentage could lead to the drop of flotation speed constant. The increase of cumulative recovery of low ash coal particles with time can be described by the first order matrix distribution model, while the relationship for fine slime recovery and time can be described by the two-stage matrix distribution model. In flotation of mixedsamples of fine slime and coal particles, the recovery of fine slime is mainly due to entrainment, and the recovery of fine slime particlesincreases with the increase of fine slime percentage in the feed.The spatial distribution of coal particles and fine slime in flotation column is also revealed in this research. The axis distribution of low ash fine coal and fine slime was explored in both batch flotation and pilot continuous flotation with a self-designed flotation column. In batch flotation, low ash fine coal was evenly distributed in froth zone from the bottom to the top while the concentration of fine slime particles at the bottom was higher than that in the top. In the pulp zone, the concentration of fine coal showed an upward trend from the top to the middle and then decreased from the middle to the bottom, but the maximum concentration was obtained at the bottom. While the concentration of fine slime increased clearly from the top to the bottom. In continuous flotation, the change of low ash coal particle concentration in the froth zone from the top to the bottom wassmall, while the concentration of fine slime increased significantly. From the top to the bottom of the pulp zone, the coal particle concentration increased first and then decreased while the slime concentration showed an increase trend. When compared the results obtained in batch flotation and continuous flotation, a similar trend was seen for low ash particles and fine slime in the froth zone, but in the pulp zone, the distribution of coal particles in the column was much different in these two experiment conditions while the distribution of fine slimewas still the same. Samples were collected from industrial cyclonic micro-bubble flotation column(FCMC) in both axis and radius direction. And the ash content, yield and mass percentage of fine slime were analyzed. In axis direction, the mass fraction of fine slime particles generally increased from the froth zone to the collection zone and cyclone separation zone, and the content differences in different zones are obvious, but the slime content changed little within the collection zone. In radius direction, the slime content decreased from the center to outside in the froth zone, while a reverse trend can be seen in the cyclone separation zone, and the concentration change of slime in the collection zone was not significant. It is noteworthy that, a similar result of the fine slime distribution in axis direction was obtained in both laboratory flotation column and industrial scale flotation column.The mechanism of slime contamination to clean coal was illustrated here. With the help of high speed camera, particle video microscope(PVM) and induction timer, we confirmed the slime entrainment recovery mechanisms like bubble trailing vortex, the bubble boundary layer adhesion recovery mechanism and slime coating recovery mechanism. Furthermore, the multibubble liquid flow carrying recovery mechanism was proposed base on the observation results. Among these mechanisms, the bubble trailing vortex would recover fine slime and coal particles with no selectivity, and finer particles are easier to be carried by the vortex. The induction time of those micro-fine slime particles are in the same order with those of fine coal at the same size. This indicates that fine slime can attach to the bubbles. In contrast, the induction time of particles larger than 0.125 mm would be larger than 5000 ms, which means coarse slime particles would not attach to the bubbles. However, there is a fundamental difference between hydrophilic mineral particle-bubble attachment and hydrophobic coal particle-bubble attachment, the hydrophilic particle would be entrained by the thin water film at the bubble surface while coal particles would achieve true attachment with air bubbles by penetrating the thin liquid film. In the flotation process where volume of bubbles flow upward continuously, fine slime can be carried upward by coordination of several bubbles, which is named the multi-bubble liquid flow carrying recovery mechanism here. This is actually a reality example of entrainment mechanism. Slime coating of fine slime particles would also exist at the lump coal surface. In specifically, the coating of kaolinite at the lump coal surface was the most significant, followed by calcite, while quartz particles can hardly attach to the lump coal surface.Four methods were proposed to reduce the slime contamination to clean coal, namely: introducing incline plates at the froth zone, aerating at multi locations within the flotation column, adding fresh water into the pulp zone where tailing discharge speed should be increased correspondingly, and adding appropriate amount of spray water at the top of the froth layer. According to preliminary results, all these four methods would decrease flotation concentrate coal ash by the mean time of enhancing coal flotation. |
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