Flocculation And Settlement Of Coal Slurry In Downflow Fluidized Bed

Coal slurry treatment is an important unit operation in the production process of coal preparation plant,and its treatment effect has a significant impact on the whole coal preparation process.Based on the flocculation kinetics theory,a new type of coal slurry treatment equipment,down-flow fluidized bed,was designed in the study,which can provide a new solution for improving efficiency on the flocculation and sedimentation of coal slurry.First,parameters such as the particle size,ash content,volatile matter,contact angle and surface potential of coal samples were characterized.Next,the dosage of 108 mg/L iron chloride and 4 mg/L anionic polyacrylamide were determined by the jar test.After the test system was built,a single-stage fluidization test was carried out on the PP ball with particle size of 3 mm and the Polyon particles.The PP ball and the coal floc were easily reunited and ran out under the action of water flow.Bed layer,not suitable for fluidized bed particle filling.The results of the three groups of apparent flow rates of 2.75,4.83 and 7.13 cm/s showed that the flocculation index was the best at 2.75 cm/s,and the bed was a fixed bed,complex flow channel and A good residence time gives a good flocculation effect.When the apparent flow velocity is 4.83 cm/s,the fluidization is uneven,and the velocity gradient is small and large,which is easy to cause flocculation.As the filling height increases,the floc size decreases,and the turbidity of the overflow water gradually increases.When the apparent flow rate reaches 7.13 cm/s,the filled particles can be uniformly fluidized,but the velocity gradient is too large,and the residence time is shortened.The overall turbidity of the overflow water is higher,but as the filling height increases,the overflow water becomes turbid.The degree is gradually reduced to 25.6 NTU.Based on single-stage fluidization experiment,Paolilong particles with 3 mm diameter and PP spheres with 5 mm diameter were filled in the equipment respectively in accordance to the differences of fluidization characteristics of filling particles with various properties.The two-stage filling method was used to explore the law of flocculation sedimentation of coal slurry.Compared with single-stage fluidization experiment,the two-stage fluidization experiment had better effect under the same conditions,with larger floc size and lower turbidity of overflow water.The first stage filling particles were small in size and low in density,which were basically in a fixed state at low flow rate and large velocity gradient;the lower half filling particles were large in size and density,with large void fraction after fluidization,and the velocity gradient was small,which met the demand of decreasing velocity gradient of flocculation kinetics,and was conducive to flocculation.From the results of the experiment,the combination of velocity gradient and flocculation time achieved the best flocculation effect when G1 was 526 s-1,G2 was 146 s-1,T1 was 2 s and T2 was 6.6 s.The simplified model of the experimental model was followed by CFD-DEM coupling simulation to explore the effect of the model on the probability of particle collision under different apparent flow rates.The simulation results showed that there was a velocity vortices at the tail of filling particles,and the scale of the vortices increased to a certain extent with the increase of velocity.At the same time,the particle trajectory showed that when the particle was near the filling sphere,its trajectory would deflect and move towards the bottom of the vortex.This was mainly due to the change of the particle trajectory under the combined action of velocity and pressure.The change of flow direction might create conditions for particle collision and flocculation.The cumulative collision times of particles in the vortex region and the lower half of the model were analyzed.The results showed that the cumulative collision times of particles in the two regions increased with the increase of velocity,but the overall difference was not significant.The cumulative collision times of particles in the vortex region and the lower half were 644,669,770 and 4340,respectively,when the apparent velocity was 2.75 cm/s,4.83 cm/s and 7.13 cm/s.The difference between 4792 and 4866 might be due to the difference of scale and pressure distribution of vortices formed at different velocities.