| Cyclonic-static micro-bubble flotation column is a kind of large-scale equipment with independent intellectual property rights in China.Because of its unique step-optimized separation structure,it has showed obvious advantages on the fine-grained mineral separation.The column flotation unit of this flotation column was packed with sieve plate to maintain its static separation environment.The structure of sieve plate has a great impact on the flotation flow field,its specific rule and mechanism has been lack of comprehensive and systematic research,which limits the improvement of flotation efficiency in cyclonic-static micro-bubble flotation column.In this paper,using electrical resistance tomography(ERT),particle image velocimetry(PIV)and high-speed dynamic system,the effect of sieve plates different in aperture,thickness and packing position on the bubble motion parameters including gas holdup,bubble velocity,bubble size in the column flotation unit was studied to reveal its effect on the flotation flow field.After that,the flotation kinetics was discussed.The main work and conclusions were as follows.The effect of sieve plate on gas holdup in column flotation unit was explored by ERT.First of all,ERT was used to measure gas holdup at different liquid fluxes and air inflows in cyclonic-static micro-bubble flotation column without packing.The results showed that the distribution of gas holdup along the radial direction was uneven,its value was high in the central region and low near the wall region.Under a certain air inflow,gas holdup decreased with the increase of liquid flux.After that,sieve plate which was different in aperture,thickness,or packing position was packed in column flotation unit to explore the effect of sieve plate structure on gas holdup.The results showed that fixing opening percentage,the larger the diameter of sieve plate was,the more uniform the distribution of gas holdup was in the radial direction.The thicker the sieve plate was,the stronger the inhibition of upward dispersed bubbles was,and the lower the gas holdup upstream of the sieve plate was.From the view of the uniformity distribution of bubble,the closer sieve plate was to cyclone separation unit,the better packing effect was.Finally,the distribution and value of gas holdup in sieve plate packing flotation column was compared with open flotation column at the same operating conditions.The results showed that sieve plate was lower gas holdup in the central region and higher gas holdup near the wall region,which revealed that sieve plate effectively dispersed the group of bubbles and made bubbles evenly distributed in radial direction.It was more conducive to the collision between bubbles and the ore particles evenly distributed in the pulp,so as to improve the mineralization efficiency.By the combination of endoscope,PIV and CCD camera,the tangential velocity of bubbles was measured at different liquid fluxes such as 1 m~3/h,1.5 m~3/h,2 m~3/h in sieve plate packing flotation column and open flotation column.And the results showed that the addition of sieve plate made the radial distribution of bubble tangential velocity more uniform.And sieve plate had a strong effect on weakening the tangential velocity of bubbles,which was conducive to collide with the mineral particles feeding vertically downward and strengthen the countercurrent mineralization in column flotation unit.For example,the 5 mm thick sieve plate reduced the peak value of the tangential velocity of the bubble from 0.41 m/s to 0.046m/s,which weakened 89%of it.A high-speed dynamic measurement system on bubble size was established to explore the effect of sieve plate on bubble size.At first,fixing liquid flux as 1.5 m~3/h,the size of bubble was photographed in open flotation column under three air inflows of 0.7 L/min,1.1 L/min and 1.5 L/min.The results showed that the diameter of bubble increased with the increase of air inflow.Secondly,sieve plate which was different in aperture,thickness,or packing position was packed in column flotation unit to explore the effect of sieve plate structure on bubble size.The results showed that the average diameter of bubbles in column flotation unit increased with the increase of its aperture and thickness.When sieve plate packed at 95 mm(0.5D)from cyclone separation unit,the average diameter of bubbles was minimum,following by the cyclone separation unit and 190 mm(1D)from cyclone separation unit.On the basis of the previous research,in order to directly reveal the influence of sieve plate on bubble size,the size of bubble in sieve plate packing flotation column was compared with open flotation column.The results showed that sieve plate maintained the uniformity of bubble size by forcing small bubbles to gather and merge while cutting and breaking large bubbles.By analyzing the effects of sieve plate on bubble motion parameters,the conclusions were as follows.(1)Considering the influence of sieve plate packing position on gas holdup and the average diameter of bubbles,it was considered that the packing effect of sieve plate was better at 95 mm(0.5D)from cyclone separation unit.(2)Sieve plate had a strong inhibition on the tangential velocity of bubbles,and the thicker sieve plate was,the lower gas holdup and the larger the average diameter of bubbles was.Therefore,while ensuring the packing effect,the thinner sieve plate should be selected as far as possible.If the swirl was too strong,it could be considered to use a multi-layer thin sieve plate for packing.(3)Considering the influence of sieve plate aperture on gas holdup and the average diameter of bubbles,it was considered that after ensuring sieve plate would not be blocked,try to select the sieve plate with small aperture.Based on the flow field characteristics of sieve plate packing cyclonic static micro bubble flotation column,the formula of bubble particle collision,adhesion,desorption probability,flotation rate constant and residence time were established.And the formula of recovery rate which was suitable for column flotation unit in this flotation column was derived.The paper includes 26 figures,13 tables and 81 references. |
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