| Coal is one of the most important energy sources in China and has made great contributions to the development of national economy.In the premise of energy efficient utilization,environmental protection and sustainable development,coal preparation and processing become an indispensable key procedure.Currently,the degree of mining mechanization is higher and the content of fine particle slimes in raw coal is increasing.However,there are data show that in some coal preparation plants,the content of coarse coal slimes in raw coal accounts for even more than 40%,which could not get effective separation and certainly will cause the waste of coal resources,and have an impact on the overall economic benefit of coal preparation plants.As for heavy medium cyclone which is the core equipment in dense medium coal preparation plants,the effective separation granularity is 1-50 mm,while the upper limit of separation granularity in conventional flotation is-0.25 mm.Thus,for the coal particles,whose size fractions are between the lower limit of dense medium separation and the upper limit of flotation equipment,they are not efficiently processed.Therefore,the separation of coarse coal slimes is the bottleneck that restricts the development of current coal processing technology,and is also the technical hot spot and difficult point to overcome in coal preparation.Flotation is the most efficient method of mineral fine-grained separation,which has been widely used in separating slime coal,and one of the problems faced by flotation field is how to widen the upper limit of particle size.In a conventional flotation machine,increasing the rotating speed of impellers does improve the efficiency of coarse coal particles,but high-speed rotating impeller will also increase the detachment probability of particle-bubble aggregation.If the flotation reagents are added to improve the yield of coarse coal particles,then the cost will accordingly increased.With the emergence of HydroFloat separator,a new idea for coarse particles flotation is provided,namely fluidized flotation.By using liquid-solid-gas three-phase fluidized bed made by the laboratory and experimental system,this thesis focused on-2.0mm fraction of coarse coal slime and made a systematic comparison research on the settlement regularity of particles groups and particles of different properties.Also,a contrast experimental research was carried out on the bed expansion degree and escape velocity under different conditions of upward water flow.On this basis,series separation experiments are completed centering on the coarse coal slimes of narrow size fraction with low density and wide size fraction with full density.The main research contents and innovative results are as follows:(1)A solid-liquid-gas three-phase fluidized bed separation appliance was designed to complete the construction of the coarse slime separation experimental system.(2)Experiment research on particle settling velocity adopted the method of single particle suspension.The introduction of air bubbles in the rising upward water flow can reduce the flow rate required for particles suspension and the "lifting" effect of air bubbles on particles make up for the energy loss caused by reduction of the upward water flow rate.The "lifting" effect of bubbles decreased with the increase of particle density.(3)On the experimental research of bed expansion degree of narrow size fraction and narrow density particles,it was found that in the same size fraction,the bed expansion degree of low density particles under the same upward water flow rate was always greater than that of the high density particles.Compared to upward water flow,the introduction of bubbly flow increased the bed expansion degree,and the effect of air bubbles to the expansion degree of low density particles was more remarkable than that of high density particles.The effect will be enhanced with the increase of the introduced gas volume fraction.As for the similar expansion degree of different size and density coal particles in the upward water flow,when upward bubbly flow was introduced,the expansion degree of the large size and low density particles was higher than that of the small size and high density particles.Thus,the existence of bubbles widened the difference in the expansion degree of particle groups with similar settling velocity.(4)On the research of escape velocity regularity in upward flow of narrow fraction and narrow density particles groups,the results showed that under the same upward water flow rate,the required escape velocity of same size fraction coal particles at adjacent density was relatively close to each other.After the introduction of air bubbles,the yield of escape particles at adjacent density decreased with the increase of the density fraction,and the existence of bubbles reinforced the effect of particle density on the separation.Meanwhile,with air bubbles,the required upward water flow for escape particles was reduced and this effect would be weakened with the increase of particle density fraction.For particle groups with similar escape velocity,when introducing bubbles in upward water flow,the effect of equal settling velocity was weakening.The yield of escape particles of low density is less affected by bubbles while the yield of high density escape particles suffered a greater reduction.The introduction of air bubbles narrowed the gap between upward water flow rates required for low density escape particles at different size fractions and weakened the effect of particle granularity on the escape velocity.(5)When adding reagents in the upward bubbly flow in the flotation experiment of coarse coal slime in fluidized bed,the results showed that with the action of collector,the low density coal particles attached bubbles in the upward flow and formed particle-bubble aggregations.Thus,apparent density of particles was greatly reduced,which made the large size particles at low density available to be effectively separated under the flotation condition of weak upward bubbly flow.In the flotation experiment of coal particles at-2.0+0.5mm,when the superficial velocity of upward water flow was 1.39 cm/s,the superficial velocity of upward bubbly flow 0.125 cm/s with collector concentration of 250 g/t,frother concentration of 75 g/t and ash of flotation concentration coal 11.3%,the yield of concentration coal reached 48.7% and the ash of tailings coal was 64.4%.The efficiency of concentration coal of the-2.0+1.5mm、-1.5+1.0mm、-1.0+0.71mm、-0.71+0.5mm size fractions was 96.24%,92.57%,95.64% and 96.25%,respectively.When conducting the floatation experiment of-2.0mm particles at full size fractions with the same flotation conditions,the ash of flotation concentration coal was 21.8%,yield reached 78.5% and the ash of tailings coal was 84.0%.The reason for the high ash content was caused by the size fraction of-0.25 mm.The ash of fine fraction concentration coal was 27.3%,yield reached 94.0% and the ash of tailings was 66.2%.Through the research on the fluidization separation regularity of coarse coal slime and flotation experiments,this paper proves that using the rising upward bubbly flow fluidization flotation as the separation method of wide size fraction coal slime is feasible,and this provides a theoretical basis for further broaden the slime separation size fraction limit. |
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