| As the deterioration of coalfield geological conditions, the improvement of automation degree of coal mining, and the aggravation of mechanical crushing and argillization during coal separation, the primary slime and secondary slime content has increased greatly. The quality of the final cleaned coal and highly efficient utilization of coal resources will be severely restricted if this part of slime can’t be separated effectively. Compared with advanced separation technology for lump coal, the fine coal separation technology remains to be further perfected. In this paper, by using the combination of gravity&flotation separation technology which is different from the separation principle of existing fine coal separation equipments, a combination of gravity&flotation separation equipment was developed for separation of fine coal. The equipment has two parts:one is the jet device for bubble generation, and the other is the cyclone designed on the basis of the Cyclone Column Separator, a patented invention. A centrifugal separation system for fine coal separation in gas-liquid-solid multiphase system was built based on the combination of gravity&flotation separation equipment. The investigations mainly focused on the gas-liquid ratio and chemical reagents affecting the separation performance of gravity separation and the combination of gravity&flotation separation for the separation of fine coal in gas-liquid-solid multiphase system, the flash flotation of the overflow of the air-injected water-only cyclone and the regulators affecting the separation performance of the centrifugal separator for the separation of fine coal.The aeration performance of jet aerator and the cyclone structure influencing of fine coal separation were studied. The optimized structure of the combination of gravity separation and flotation equipment involves a nozzle of Φ5-6mm,a vortex finder of Φ22mm,an apex of Φ8mm, a complicated positive and negative cone of90°and a vortex finder length of70mm. The proper solid concentration of the feed and the inlet pressure are100g/L and0.15MPa, respectively.The separation tests were performed using the gas-liquid ratio values of0%,10%,30%and50%with a nozzle diameter of6mm. The results show that the effects of the gas-liquid ratio on the separation change with the particle size. An increase in the gas-liquid ratio decreases the overflow yield and the ash content of+0.5mm fraction in the overflow and the underflow. For the0.5-0.25mm and0.25-0.125mm fractions, the ash content of the overflow is minimum at the gas-liquid ratio of30%, while the ash content of the underflow increases compared with the gas-liquid ratio of10%. The mass yield and the ash content of the overflow and underflow of-0.125mm fraction do not change obviously with the gas-liquid ratio. The separation performance of the air-injected water-only cyclone was evaluated through the float-sink test of the separation products. The δpand Ep values for different size fractions clearly decrease with the increasing of the gas-liquid ratio. Theδp of the+0.5mm fraction decreases from1.725to1.591g/cm, and the Ep value decreases from0.166to0.050before and after air injection. The difference in δp and Ep values of different size fractions is gradually decreased as the gas-liquid ratio increases. By referring to the organic efficiencies and the total misplaced material of different size fractions, there is an optimum gas-liquid ratio of30%that results in the highest separation efficiency. Additionally, the density cut-point of a water-only cyclone can be adjusted in real time by changing the gas-liquid ratio to meet variable market requirements.The Computational Fluid Dynamics software Ansys-Fluent6.3.26was used to perform numerical simulations of the flow field inside the cyclone under different gas-liquid ratios. The simulation results indicate that the static pressure in the cyclone’s separation flow field as well as the tangential, axial, and radial velocity component values increase with increasing of the gas-liquid ratio. The gas volume fraction in the flow field inside the cyclone increases with the increasing of the gas-liquid ratio, so the separating medium density will reduce gradually. It is verified that the separation performance of the air injected water-only cyclone can be adjusted in real time by changing the gas-liquid ratio. When the gas-liquid ratio ranges from20to30%, the cyclone’s flow field characteristics are favorable to separation efficiency improving.The apparent density of the coal particles decreased due to the formation of bubble-coal particle agglomerates and the size of the particle agglomerates increased because of the air bubbles that were introduced and attached to the hydrophobic particles. Therefore, the influence of particle size on the separation was diminished and the efficiency of the separation of fine particles was improved.In order to increase the dispersion, uniformity and stability of the bubbles introduced into the cyclone, the2-octyl alcohol as frother is added to the pulp. The results show that2-octyl alcohol has negative effect on+0.5mm fraction, nearly no effect on-0.074mm fraction, but positive effect on0.5-0.074mm fraction. The effects of2-octyl alcohol dosage, gas-liquid ratio and their interaction on the separation of0.25-0.125mm and0.125-0.074mm size fractions are highly significant. The ash content of+0.125mm fraction of the overflow can be controlled within10%when gas-liquid ratio,2-octyl alcohol and kerosene dosage are40%,50g/t and600g/t, respectively. At the same time, the ash content of the underflow is above60%. So the combination of gravity&flotation separation can be realized. The effects of flotation reagents on separation mechanism for coarse and fine coal particles in centrifugal field is analyzed using the collision theory of particles and bubbles and the probability formula for detaching in the turbulent flow field.To make full use of the kinetic energy and gas holdup in the overflow a flowsheet was designed.to further separate the overflow of the air-injected cyclone employing flash flotation technology. The results of the laboratory test and continuous test in coal preparation plant show that the mineralized bubbles in the overflow of the air-injected cyclone can only be partly recycled using conventional flotation column. So the flash flotation device of the overflow needs to make further research.In order to improve the gravity separation efficiency of fine particles using the surface properties of them, the effect of a flocculant PAM, a coagulant MgCl2, a dispersant (NaPO3)6and two surfactants Span80and PEG-200as the modifying agents on the centrifugal separation of fine coal was investigated with the presence and the absence of air-injection. The results show that:PAM and MgCl2can improve the separation efficiency of+0.074mm fractions but has no obvious influence on the separation efficiency of-0.074mm fraction. The effects of (NaPO3)6on-0.074mm size fraction was more apparent. Span80and PEG-200can improve the separation efficiency of-0.25mm fractions gradually with the increase of dosage, but decrease the separation efficiency of+0.25mm fractions gradually. The effects of different types of regulators on the gravity separation mechanism of fine coal were investigated by size composition analysis and zeta potential measurements.This paper may provide a new idea about the development and application of new equipments and new technique for the separation of fine coal. |
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