1. Flotation machine basic functions
A flotation machine is an industrial equipment to achieve flotation. Its key component – the inflatable stirring mechanism, which determines the type, characteristics and effectiveness of the flotation machine. From the point of view of the flotation process, the flotation machine should have the following functions.
1. Flotation machine makes the slurry in turbulent flow to ensure the suspension of the ore particles and the dispersion of the chemicals, and to achieve the adhesion of the ore particles and chemicals with certain kinetic energy movement and collision.
2. Flotation machine introduce air to produce air bubbles of suitable size, sufficient quantity and stability, so that they can be dispersed in the pulp, move with certain kinetic energy, and collide with pharmaceuticals and ore particles to produce selective adhesion and realize mineralization.
3. The mineralization bubbles can rise to the liquid surface, forming a three-phase froth layer and producing secondary enrichment; the froth concentrate and tailings can be discharged in time.
Figure 1 represents the connection between the flotation processes. The dashed boxes indicate the agitator barrel function and the solid boxes indicate the flotation machine function. Figure 2 shows the distribution of the zones of action within a mechanically agitated flotation machine. In the mixing zone, bubble groups and mineral grains carry out a large number of collisions, and some follow the impeller agitation for turbulent motion, whose bubble speed can reach 100cm/s, but the bubble ascending and floating motion is slow. In the flotation zone, the mineralized bubbles gradually increase their flotation speed as the hydrostatic pressure decreases during the flotation process. At the froth area (or scraping bubble area), a large number of mineralized bubbles gather, the bubble floatation speed decreases or even stops, and the bubble merging increases, and the part of water leaked during the merging process when the hydration film ruptures takes away the poor hydrophobicity and weak adsorption of impurity particles caught in the froth layer, playing a secondary enrichment role.
2. Basic requirements and evaluation of flotation machines
In flotation production, a flotation machine with good performance should meet the following requirements.
1. Large and easy to adjust the inflation volume. The number of air bubbles directly affects the flotation speed. The main factor to determine the production capacity of the flotation machine is the inflatable volume due to the large yield of fine coal during coal beneficiation. In addition, different minerals or different process operations (roughing, sweeping, selection) have their own suitable inflatable volume, which requires the inflatable volume to be easy to adjust.
2. The mixing intensity should be sufficient to ensure the uniform distribution of slurry density, particles and bubbles, also the dispersion of the chemicals, and easy to restart directly after full load stop.
3. The slurry passing capacity is large to adapt to the higher processing capacity; the slurry circulation in the tank is large to ensure that the ore particles have more chances to contact with the air bubbles.
4. It forms stable slurry liquid level and froth layer, and discharges froth concentrate in a timely and stable manner.
5. Simple structure, easy maintenance, low energy consumption, long life, and can operate safely for a long time.
6. Adapt to the need of automation, easy and flexible adjustment, reduce the number of detection instruments and actuators.
There is no unified standard for evaluating the performance of flotation machines in various industries, the following indicators are used more often and can be used as a reference basis for evaluation.
1. Inflation performance and flotation index (quality and quantity of concentrate and tailings).
2. Processing capacity (various forms).
3. Power consumption (in tons of raw ore)
4. Price, installation, operation, maintenance costs and installation space, etc.
3. Aeration of the slurry in the flotation machine
One of the main functions of the flotation machine is to add air into the pulp, disperse it into a large number of small bubbles, and then collide with the ore particles to form mineralized bubbles. Therefore, flotation pulp inflating is the premise of flotation, which is the main factor to determine the effect of flotation machine. Inflation of flotation pulp consists of two main processes: generation of bubbles and their incorporation and dissolution.
1. Generation of air bubbles
There are three methods in principle for bubble generation.
(1) Mechanical stirring action crushes the air stream to form bubbles
Mechanical agitation flotation machine bubble generation is mainly through this method. In this case, rotors or impellers of different shapes agitate the pulp fiercely, causing a strong vortex movement of the pulp. Due to the shearing effect of the vortex, the air drawn into the flotation machine is dispersed into bubbles of varying diameters. The greater the relative speed of the pulp and bubbles, the higher the turbulence, the lower the air-liquid interfacial tension, the smaller the diameter and the greater the number of bubbles into which the airflow is divided by the vortex shear of the pulp. However, too strong stirring, energy consumption, unstable liquid level, bubble diameter is too small, also unfavorable to the floating of mineral particles.
The key components of a mechanically agitated flotation machine are the rotor and the stator. The rotor is the impeller, and its working parts can be similar to the shape of a water pump impeller – a turbine, a horizontal disc with upright blades, a hybrid impeller, etc. – or it can be a bar arranged vertically in a cage or a radial plate. When the rotor rotates, cavities are formed behind the blades to draw in air. The pulp around the blades forms two rows of vortices rotating in opposite directions (Carmen vortex), and the air is sucked by these vortices to the vortex center to form bubbles. The air bubbles formed in the turbulent pulsation speed generated by the dynamic pressure and shear force may be split into smaller bubbles. The farther away from the blade, the smaller the bubble splits.
Figure 3 shows the impeller dispersing the air into bubbles. Experiments show that there is a limit to the rotor’s ability to disperse air, and the ability of the impeller self-priming flotation machine to draw in air depends on the dispersion ability of the rotor and will not exceed this limit. Increase the number of blades or increase the blade speed can increase the inflatable volume, but the power consumption and wear will increase, and the liquid level may also be unstable, which is not good for coarse particle sorting.
(2) Air forms bubbles through the tiny eyelets of porous media
This method is used for bubble generation in press-in flotation machines (e.g. flotation columns). The pressed-in air passes through the porous media with tiny holes, such as microporous ceramics, canvas and other special inflators to produce tiny bubbles, as shown in Figure 4. But the air pressure to be appropriate: too small is not conducive to air through the bubble number is small; too large and easy to form a jet stream and not bubble, and cause the liquid level instability. Pressure size by the inflator to decide. In addition, the size of the hole on the inflator and the interval also affects the bubble generation, the size of the hole affects the resistance of air through; interval is too small when the bubbles formed by adjacent holes are easy to meet and merge, the interval is too large inflatable volume is too small.
(3) Air precipitates from the slurry to form micro bubbles
In the standard state, the solubility of air in water is about 2%, when the pressure is reduced, the dissolved gas will be precipitated from the solution in the form of tiny bubbles. The precipitated microbubbles are characterized by small diameter, high dispersion, large gas-liquid interface and selective precipitation on the surface with high hydrophobicity, thus they are called active microbubbles. Modern flotation machines are designed to ensure a large number of microbubbles and to enhance the flotation process. The precipitation of microbubbles on the surface of the particles facilitates the breakthrough of the hydration layer between the particles and the bubbles. When there are microbubbles on the surface of the coarse grains, other bubbles can be attached to the grains through the microbubbles, forming a residue-free hydration film, or many microbubbles can be attached to the coarse grains to form a bubble floc and float, enhancing the flotation of the coarse grains.
The main factors affecting the precipitation of microbubbles are
A. The degree of saturation of air in the slurry at the beginning, i.e. the solubility of air. There are many opportunities to dissolve the pulp in the air, such as stirring, pumping, etc. can promote the dissolution of the pulp. In order to increase the solubility, the pulp pressure is an important measure, which is also the basis for the use of pressure pulp in modern jet flotation machines and so on.
B. The subsequent degree of depressurization of the ore slurry. There are many reasons for pulp depressurization, such as the injection of pressure pulp, the movement of pulp from the bottom of the trough to the top of the trough, the reduction of pressure at the trailing edge of the impeller blade, and the instantaneous pressure fluctuation and reduction caused by the impeller throwing out the pulp.
C. Whether there is a “core” for microbubble precipitation. When there are “cores” (such as hydrophobic surfaces of minerals), microbubbles can be precipitated in these “cores” in preference. The more hydrophobic surfaces, the more favorable the precipitation of microbubbles, especially if the micro-pores, cracks and gaps on the surface of hydrophobic mineral grains are filled by gas molecules (called gas buds), they become the “core” for microbubble precipitation. Therefore, the presence of hydrophobic particles in the pulp improves the conditions for microbubble precipitation, which in turn enhances the flotation of hydrophobic particles.
The number of microbubbles precipitated by different structures of flotation machines is different. Increasing the initial pressure of the pulp (e.g. stirring, pressurization) and the degree of pressure reduction (e.g. jetting, etc.), i.e. increasing the pressure difference before and after bubble precipitation, is an effective measure to obtain a large number of microbubbles.
The first of the above three bubble generation methods is the most used. In a flotation machine, more than two methods of bubble generation can be used at the same time.
2. Incorporation and dissolution of air bubbles
The final degree of pulp inflation depends not only on the process of inflation and dispersion, but also on the process of merging and dissolution of the generated bubbles. The merging of bubbles makes them larger in diameter, unstable and easy to break, reduces the bubble size and the total bubble area, which is not good for mineralization. The dissolution of bubbles makes them disappear, but the effect is not as obvious as the merger. Generally the smaller the bubble, the lower the degree of gas saturation in the liquid phase, the greater the solubility. However, the dissolution will also be precipitated by microbubbles when the pressure is lowered, so the annexation of bubbles should be mainly controlled. Adding frothing agent can improve the stability of bubbles and slow down the merging of bubbles. The generation and disappearance of bubbles in the flotation machine are two opposite processes, and quickly reach a dynamic equilibrium. Increasing stirring strength and adding frothing agents can make the balance develop in the direction of generation. According to the measurement, in the commonly used mechanical agitation flotation machine, the bubble diameter is between 0.05-1.5mm, of which 0.5-1.2mm is the majority, and its surface area accounts for about 80% of the total surface area of the bubble.
4. Determination and evaluation of the degree of pulp inflation in the flotation machine
The degree of pulp inflation refers to the air content in the pulp, the degree of bubble dispersion and the uniformity of bubble distribution in the pulp. The degree of pulp inflation is related to the type of flotation machine, the structure of the aerator, the method of dispersing airflow, the intensity of agitation, the size and shape of the flotation tank, the concentration of the pulp, the type and amount of frothing agent, etc., and is interrelated and influential. The degree of pulp inflation directly affects the bubble mineralization process, flotation speed, process index and the amount of flotation chemicals. Intensive inflating can accelerate the flotation speed and increase the production capacity of the flotation machine, and can also reduce the dosage of chemicals to a certain extent, especially the dosage of frothing agent.
1. Inflation volume and its determination
In terms of the three types of flotation machines used at home and abroad, the inflatable flotation machine inflatable volume is the largest, mechanical agitation type is the smallest, inflatable agitation type in the middle. When the impeller is immersed in a certain depth, the faster the impeller speed, the faster the speed of the pulp, the greater the kinetic energy formed by the impeller, the more air intake, and the impeller line speed and work between the pressure head is 2 times the relationship. However, when the speed increases, the resistance of the blades is also proportional to the impeller circumferential linear speed squared, so it is bound to lead to increased power consumption and stirring device wear. Therefore, the impeller line speed of mechanical agitation flotation machine generally does not exceed 10m / s. Reduce the depth of the flotation tank or impeller dip depth can improve the inflatable volume, because at this time the impeller rotation formed by the working pressure head and the trough pulp on the impeller static pressure head difference increases, the pulp dumping speed increases, the inflatable volume also increased. Reduce the depth of the tank can also reduce electricity consumption. Therefore, under the premise of ensuring the normal operation of the flotation machine (keeping the liquid level stable), the depth of the tank should be reduced as much as possible. Shallow tanks are an important trend in the development of flotation machines.
In addition, the structural parameters of the inflator, such as impeller diameter, shape, blade height and number, the distance between the impeller and the bottom of the tank, the shape of the stator blade, inclination angle, the gap between the impeller, etc. will affect the inflatable volume and bubble dispersion. Mechanical stirred flotation machine in the inflatable amount of good, the average volume of air is about 20%-30%, the inflatable amount will increase significantly bubble merging; excessive inflatable will also be a large number of mineral particles entrained in the foam, reducing the quality of concentrate, while causing the pulp level instability, increased energy consumption, especially for mechanical stirred flotation machine will also increase impeller wear and mineral mud.
The amount of air can be measured by measuring cylinder method or inflatable measuring instrument, calculate the unit time, unit flotation machine area or unit flotation machine volume can be sucked into the air volume.
2. The degree of air diffusion in the slurry
The filling volume indicates the quantity of filling, while the degree of air diffusion refers to the degree of air dispersion into bubbles at a certain filling volume, indicating the quality of filling. The higher the degree of diffusion, the smaller the bubbles generated, the more bubbles, the larger the total area of bubbles provided, and therefore the greater the chance of collision and adhesion between ore particles and bubbles. However, when the bubbles are too small, the floating speed is slowed down, which is not conducive to carrying the ore particles to the surface. Adding foaming agent can improve the degree of bubble diffusion: within a certain degree, the amount of foaming agent increases, the bubble diameter decreases. If 20mg/L pine oil is added to the pure water, the average diameter of bubble will be reduced from 4.5-8mm to 0.38mm. Increase the degree of agitation can also improve the degree of diffusion.
3. Uniformity of distribution of air bubbles in the slurry
The uniformity of bubble distribution in the pulp is also used to indicate the quality of the inflatable. The amount of inflatable volume varies in different parts of the flotation machine, and the uniformity of bubble distribution varies greatly with the type of flotation machine structure. The uniformity of bubble distribution in the pulp affects the “effective volume” or “inflatable volume” of the flotation machine tank. Not all volumes of the flotation machine have bubbles, only the part of the volume with bubbles can realize the collision and adhesion of ore particles and bubbles, so the part of the volume containing bubbles is called “effective volume” or “inflatable volume”. Increase the mixing intensity can improve the uniformity of bubble distribution.
The uniformity of bubble distribution in the pulp directly affects the efficiency of the flotation machine. The greater the uniformity of inflation or the utilization coefficient of the inflatable volume, the greater the capacity of the flotation machine as measured by the unit tank.