Effect of sieve plate packing on bubble size distribution in a cyclonic flotation column
Graphical abstract
Introduction
Flotation column has been widely studied because of its excellent flotation performance for fine minerals since 1980s [1,2]. The high column structure can provide a static separation environment to obtain high-grade concentrate. However, the high column reduces the flotation efficiency and restricts the large-scale equipment. The application of packed technology effectively solves this problem and promotes the development of column flotation technology [3].
Cyclonic-static microbubble flotation column(FCSMC) is a successful fine particle flotation equipment [4,5]. It has a three-stage separation structure, including column flotation zone, cyclone flotation zone and pipe flotation zone, as shown in Fig. 1 [4]. In practice, the cyclone force field in cyclone flotation zone will inevitably move up and expand, which will destroy the static environment in column flotation zone. A common method is to pack the sieve plates to weaken this effect.
The effect of sieve plate on the fluid flow inside the column section has been studied extensively. Through LDV test and computational fluid dynamics (CFD) simulation, Liu [6] found that the sieve plate can reduce the tangential velocity of fluid by 50% - 60%, and improve the dispersion of radial fluid and bubbles; Zhang [7]compared the relationship between flow characteristics and flotation recovery of open and packed flotation columns by CFD simulation. The results show that the packed medium can stabilize the fluid flow in the column unit, and greatly reduce the probability of particle detachment; Yan [8] studied the influence of the sieve plate on the hydrodynamic characteristics of the column flotation with particle image velocimetry (PIV) and CFD simulation, and the results show that the arrangement of the sieve plate can effectively straightens the rotation flow and weaken the turbulence; Zhang's [9] study on sieve plate packed shows that the sieve plates can reduce the turbulent kinetic energy, turbulent dissipation and axial fluid velocity, which is more conducive to flotation separation.
These studies mainly focus on the influence of sieve plate on flow field in flotation column. However, the influence of the packed sieve on the motion characteristics of the bubbles in the flotation column cannot be ignored. In the previous work [10], we studied the influence of the sieve plate on the bubble movement, and this work will focus on the influence of the sieve plate packing on the bubble size distribution.
In flotation, bubbles as carriers collide and adhere to particles. The size and velocity of bubbles greatly affect the flotation recovery [11,12]. The flotation efficiency is determined by the collision probability, adhesion probability and detachment probability, as shown in Eq. (1):
In column flotation unit, fluid flow is close to laminar flow, bubble Reynolds number is less than 1, collision probability can be described as [13]:
When the contact time between particles and bubbles is longer than the induction time, adhesion will occur. The adhesion probability can be described as [14]:
Particles will detach form the surface of bubble in a highly turbulent environment, and the probability of detachment can be described as [15]:
Where, θ is the contact angle of particles, σ is the surface tension of liquid, α is the polar angle of particles on the surface of bubble. According to Eqs. (2) and (3), the smaller the bubble diameter, the higher the collision probability and the adhesion probability. According to equations (4), the lager bubble diameter, the higher detachment probability. Combining Eqs. (1), (2), (3) and (4), reducing bubble diameter will improve flotation efficiency.
The opening hole size and thickness of the sieve plate determine the passability of the bubbles, and the packed position of the sieve plate determines the flow of column unit. These will affect the velocity and size of the bubbles, thereby affecting the flotation performance of the column flotation unit. In this study, through particle image velocimetry(PIV) and high-speed dynamic camera measurement, the size and velocity of bubbles in open and packed flotation column were compared. Meanwhile, the effect of opening hole size, thickness and packed position of sieve plate on bubble size distribution was studied, which provides guidance for the design of packed method in flotation column.
Section snippets
Experimental set-up
The experimental flotation column system was shown in Fig. 2(a). The FCSMC used in experiment was made of transparent plexiglass. Its inner diameter is190mm(D), the total height is 1400 mm, and the column section height is 800 mm. In order to minimize the deformation of bubbles caused by the reflection of the cylinder surface, a square optical compensation box was added to the outside of the upper column section. Before the experiment, the water was injected into the box to compensate the
Effect of sieve plate on size and velocity of bubble
In order to study whether the sieve plate can effectively weaken the influence of the rotation flow, and change the bubble size distribution, the tangential velocity and size of bubbles in open and packed flotation column unit are measured. The structure and installation position of the sieve plate are as follows: the opening hole size is 6 mm, the thickness is 5 mm, and the packed position is at 0.5D.
Effect of sieve plate on bubble size distribution
The effect of the thickness, opening hole size and packed position on the bubble size distribution was studied in this section. The structural parameters of the sieve plate are shown in Tables 1, 2 and 3.
Conclusion
In this study, the tangential velocity and size of bubbles in open and packed flotation column were compared with PIV measurement and high-speed dynamic measurement system. The effect of opening hole size, thickness and packed position of sieve plate on bubble size distribution was be studied. The mainly conclusion are as follows:
- (1)
Sieve plate can weaken the influence of rotation flow, which is helps to reduce the detachment of the particles. The tangential velocity of bubbles in open flotation
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This project is supported by the Fundamental Research Funds for the Central Universities (2019QNA29).
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Equal contribution as the first author.