Study On Collision And Attachment Behavior Between Particle And Bubble In Coal Slime Flotation

Based on flotation micro model, the flotation process could be divided into three independent sub-processes i.e., collision, attachment and detachment. In the pulp, particle first collide the bubble, and the water film of bubble surface thins and ruptures, then wetting surrounding of solid-liquid-gas three phrases forms and expands to stabilize the particle upon the bubble surface. The particle-bubble combination rises and doesn’t detach from the bubble surface, then goes into froth layer and becomes concentrates. Therefore, the probability of mineral flotation can be explained using three parameters in corresponding independent sub-processes, i.e. collision probability attachment probability and detachment probability.In flotation micro model, the collision probability model and attachment probability model which are the foundation of flotation gain more researches and emphasis. Sutherland simplified the flotation process and made some assumptions that the velocity and mass of particle are neglected, the movement of particle is totally dependent on the surrounding fluid and movement trail of particle could be expressed using fluid line of the center of position flowing around the particle, therefore, the particle-bubble collision probability model and attachment probability model are attained. Sutherland made groundbreaking contribution and laid the development direction of flotation micro model. Thereafter, on a basis of these models, many other researchers enrich and perfect the particle-bubble collision and attachment modelsThe collision probability model and adsorption probability model have been detaile reviewed in this paper,the probability of collision with the fluid flow around the line and the equation is based on the adsorption probability model has been found in-depth analysis on the basis of existing models,that’s limited to-50m particles and the larger the particle size of the applicability is unknown.The actual coal flotation include coarse level of 0.25-0.5mm,intermediate grain size of 0.075-0.25mm, and fine-grained level of-0.075mm,among which the middle class is the best grain processing range, master the rules of the various size fractions of flotation has practical significance to establish the mathematical model.This paper aiming at the collision and attachment process between particle and bubble, the kinetics of collision and attachment was analyzed, experiment system measuring collision probability and attachment probability was designed, and the effect of particle size、particle density and bubble size on collision probability and attachment probability was intensively studied with WuHai coal slime. And in consider of the high gas holdup in flotation cell, bubble size observation system was designed, experiment was conducted with the XDF2L laboratory flotation machine. The effect of four frother and aeration rate on bubble size distribution was studied, and the main conclusions are as follows:Determination system of particle-bubble collision probability was designed to investigate the collision efficiency between bubble and particle which settles in different relative positions. Results show that the particle-bubble collision probability almost reaches 100% when particle settles down above the center of bubble; and turbulent flow occurs due to wall effect of bubble when particle position departs from the center of bubble. The determining method of collision probability was confirmed, that’s, if particle-bubble collision efficiency is 50% when particle settles down with distance Re between particle and bubble, Re was defined as critical collision radius.Collision efficiency increases gradually with an increase of particle density. For bubble with diameter 1.44mm and particles with size 0.15-0.18mm, when particles density increases in an order of 1.25 g/cm3,1.35 g/cm3,1.45 g/cm3,1.45 g/cm3 and 1.70 g/cm3, the collision efficiency is 58.10%,67.79%,68.78%,76.76% and 79.21%, respectively.The collision probability between particle and bubbles with size 0.81mm,1.05mm, 1.44mm and 1.86mm was also determined, and results show that the collision probability almost increases with an increase in bubble size. For particle with density 1.35g/cm3 and size 0.16mm, the collision probability is at an increasing order of 60.14%, 64.47%,67.79% and 73.96%.It is demonstrated from the collision experiment of particles with different size fraction, the profile of collision efficiency curve of particles with size 0.16mm and 0.26mm and with different density looks similarly, that’s, when collision efficiency is almost 100% when particle is above the center of bubble exactly, and collision efficiency reduces when particle is far away from the center. It is showed from the comparison of collision efficiency curve of particles with same density fraction that, for any particle positions, the particle with size 0.26mm has the greater collision probability than that with 0.16mm at the same position. For particles with size 0.16mm and 0.26mm and with different density, when bubble size increases from 0.81mm to 1.86mm, the collision probability increases, but with different increment. For particles with 0.15-0.18mm, with an increase in density, the increments are 6.29%,8.08%,9.49%, 8.52% and 9.48%, respectively; and for particles with 0.25-0.28mm, the increments at different density fractions are 12.72%,7.78% ,5.29% ,4.51% and 3.63%, respectively. It’s showed clearly that, for any density fractions except density fraction - 1.3g/cm3, particle with 0.25-0.28mm has smaller collision probability increment than particle with 0.15-0.18mm when bubble size varies.The BBO equation is applied to the analysis of particles’collision with bubbles. Resistance coefficient fr and fθ are introduced to depict the radial and axial resistance exerted on particles by bubbles. We also use the fourth order Ronge-Kutta method to calculate the kinematic equation of particles, discovering that the numerical calculating results agree well with the experimental results when the particle size is 0.16mm, bu the prediction effect for size 0.26mm is bad.An attachment probability test system has been established to investigate the contact between particles and bubbles. As the results show, the attachment probability reach maximum if the particle falls from the central location and then collides with the bubble, and the adsorption probability gradually decreases with the particle’s deviation from the centre of bubble. The process of attachment between particles and bubbles is the thinning of hydration shell followed by its breakdown. As the hydration shell cracks, the particle is found to rotate on the surface of bubble because of the cavity produced by the breakdown of hydration shell, and the cavity makes the bubble inhale the particle. We also find that the hydration shell can crack on the lower hemisphere as well as on the upper one. A stable attachment may not be formed due to the breakdown of hydration shell.When the bubble collides with a hydrophilic surface, the hydration shell does not thin. As the particle slides along the surface of the bubble, the particle separates from the bubble once the particle surmounts the centre of the bubble, with the separation angle generally being between 100 and 120 degrees. There is a breakdown of hydration shell during the contact between a bubble and a hydrophobic particle, thus an adsorption force is exerted on the particle, which increases the particle’s sliding time on the bubble. If an adsorption cannot take place between a particle and a bubble, the separation angle is usually greater than 130 degrees.Contact angle of particles whose density are 1.25 g/cm3、1.35 g/cm3、1.45 g/cm3、 1.55 g/cm3and 1.70g/cm3 respectively were measured. It was found that the contact angle decrease lineally with the increase of particle density. Attachment experiment result of particles with different density show that attachment probability increases lineally with the increase of contact angle. Hence attachment probability shows lineally relationship with particle density.Attachment experiment of particles at different bubble size shows that with the increase of bubble size, attachment probability increase accordingly. But the influence of bubble size on particles varies with particle density. For example, attachment probability of particle with size 0.16mm and density 1.25g/cm3 on bubbles with diameter of from 0.81mm to 1.86mm, are 81.69%、85.79%、85.04%、and 84.64% accordingly. And the increment is only 4.10%. With the increase of particle density, the increment of which are 11.979%、25.97%、23.46% and 16.52% and shows that attachment probability of denser particles benefit more from the increase of bubble diameter. And meanwhile, the increment of particles with density 1.70g/cm3 is bigger than that with density 1.25 g/cm3 and 1.35g/cm3, which illustrate the sacrifice of selectivity for enhancement of attachment probability of denser particles. Hence, small bubbles are fevered to increase selectivity of flotation.It was concluded that the contact angle of the coal particles of 0.26mm diameter is the same with that of 0.16mm diameter of the same density through the contact angle determination of the coal particle of 0.26mm diameter. The adsorption rate of particles of 0.26mm diameter is lower than that of 0.16mm diameter through the comparison of their adsorption rate.Bubble size measurement system based on image technich was built, and the size distribution of the bubble of different content of octanol, terpilenot, MIBC and 2-hexanol and different inflating volume are researched.The result showed that:Frother can narrow the range of the size of bubbles and make the bubbles smaller at the same time. The distribution of size of bubbles in the flotation cell can be described through a Upper-Limit function.The forming property of frother is affected by its molecular structure, and octanol has the best forming proterty for its chain is the longest. Though terpilenol has a structure of 6 C,its forming property was hindered by the 6-C-ring.MIBC and 2-hexanol both has a structure of 6 C, and MIBC is inear chain while 2-hexanol has a branch.But their forming property was alike. The forming property of the four frother can be arrange as octanol>terpineol>MIBC≈2-hexanol.There is a critical value for the dosage of the frother.The critical value for octanol terpinol and MIBC are 11.97mg/L,21.40mg/L and 21.89mg/L respectively. The Sauter diameter of bubbles from the three kinds of frother when the dosage is beyond their critical value are 0.54mm,0.54mm and 9.58mm respectively.The relationship of the Sauter diameter of bubble, inflating volume and frothe content is as follow...