From the thermodynamic point of view, the adhesion of the ore particles to the bubbles must be achieved by discharging the hydration layer under the action of external force, and the free energy of the ore particles and bubble system before and after adhesion can be reduced to form a stable bubble mineralization system. Thermodynamic analysis can determine the direction and results of the mineralization process, but the mechanism and dynamics of the process cannot be determined, so the two should be combined.

Adhesion work

Figure 4-6-34 Pre- and post-state of ore particles and bubbles

The most basic behavior in the flotation process is bubble mineralization, which is based on the difference in mineral wettability. Some minerals can adhere to the bubbles, while others do not, allowing separation of the different minerals. Whether the process spontaneously obeys the second law of thermodynamics. That is, under constant temperature and constant pressure conditions, if the free energy of the system decreases after the behavior occurs, the process proceeds spontaneously, and vice versa, it cannot proceed spontaneously. This system can be simplified to the state shown in Fig. 4-6-34 by thermodynamic analysis, (a) indicating that the ore particles adhere to the bubbles; and (b) indicating that the ore particles are attached to the bubbles. If the system is regarded as an isothermal isobaric system, then the free energy of the system before the particles are attached to the bubbles is

ΔW is the change of the free energy of the system before and after attaching a unit surface area. The flotation is called the floatability index or the adhesion work. If W1>W2, then ΔW>0, indicating that the free energy of the adhesion process system is reduced. According to the second law of thermodynamics, this process can be carried out spontaneously. It can be seen from the formula (4 - 6 - 13) that the floatability index is a function of the surface free energy of the gas-liquid interface and the contact angle θ of the mineral surface. For absolutely hydrophilic minerals, θ = 0. , ΔW =0, the ore particles cannot spontaneously adhere to the bubble; θ>0. ΔW=0, the larger the θ, the larger the ΔW value, so the more spontaneous the mineral particles adhere to the bubble, the more significant the trend. The phenomenon that hydrophobic mineral particles can adhere to bubbles while hydrophilic mineral particles cannot adhere is explained here.

2. Colloidal particles collide with bubbles

(1) Thinning of hydration layer

As shown in Figure 4-6-35. During the flotation process, the ore particles and the bubbles are close to each other, and the ordinary water between the two is first excluded.

Since the molecules of ordinary water are disordered and free, they are easily squeezed away. When the ore particles are further approached to the bubbles, the hydrated film on the surface of the ore particles is thinned by the displacement of the bubbles. The change in free energy of the hydration film thinning process is related to the hydration characteristics of the mineral surface:

1 The surface of the mineral is highly hydrated (hydrophilic surface). As the bubble approaches the ore, the free energy of the surface of the hydrated membrane increases, as shown in curve 1 of Figure 4-6-35. When the ore is close to the bubble Table

2 medium hydration surface, as shown in curve 2 of Figure 4-6-35, this is a common situation of flotation.

3 weakly hydrated surface, ie hydrophobic surface, as shown in curve 3 of Figure 4-6-35. The hydration film on the hydrophobic surface is relatively fragile, and some of it spontaneously ruptures, and the free energy is reduced. However, a layer of hydration layer that is very close to the surface is still difficult to rule out, and the sharp rise of curve 3 on the left side illustrates this point.

The common minerals of flotation are neither completely hydrophilic nor absolutely hydrophobic, and are often in an intermediate state, as is the case of curve 2 in Figure 4-6-35. The process of attaching the ore particles to the bubbles can be divided into four stages in detail, and the distance between the corresponding ore particles and the bubbles is h1, h2, h3, and h4, respectively.

The a stage is the close proximity of the ore particles and the bubbles, which are caused by the aeration mixing of the flotation machine, the movement of the slurry, the attraction between the surfaces, the size of the ore particles and the bubbles. At this time, the free energy does not change much. The b stage is the contact between the ore particles and the hydration layer of the bubble. The ordinary water layer between the original ore and the bubble is gradually squeezed away from the nip to the hydration layer on the surface of the ore and the hydration layer on the surface of the bubble. Since the water molecules of the hydration layer are within the range of the force field of the surface bond energy, the water molecule dipoles are oriented, which is different from the disordered arrangement of ordinary water molecules. Therefore, to squeeze out the water molecules in the hydration layer, if the curve 2 is pushed from b to c, it is necessary to work outside the system to overcome the energy peak of b to c. The c stage is the thinning or cracking of the hydrated film. The hydration layer is thinned to a certain extent by the action of the external energy, and becomes a hydrated film with a separation distance of h3, and the corresponding curve 2 is from c to d. At this point, the hydrated film shows instability.

That is, the energy peak has been crossed again, and the distance h3 is reduced to h4, the free energy is reduced, and the thickness of the hydrated film is spontaneously thinned. According to the measurement, the approximate interval is (1~100)×10-9m. At this point, the ore particles and the bubbles spontaneously approach.

The d stage is the contact of the ore particles with the bubbles. From c spontaneously to d, the ore particles begin to come into contact with the bubbles. After the contact occurs, if it is a hydrophobic mineral surface, the contact perimeter may continue to expand.

According to some studies, there may be a “residual hydration film” in the contact area between the ore particles and the bubble. As far as the film properties are concerned, it is nearly semi-solid. To remove the film, a large applied energy is required, if residual water is present. When the membrane is formed, the ore particles and the bubbles are only in two phases, that is, only one solid solution and one liquid and one gas interface. Then the balance of the three-phase contact should be rewritten as two phases.

(2) Collision process and influencing factors

The collision and adhesion process of the ore particles and bubbles are related to the nature of the fluid in the flotation machine, the relative motion of the two, the size of the bubbles and the size of the ore particles, and the former two are mainly related to the type of the flotation machine. . When the size of the bubble and the ore particle is within a certain range, the collision attachment can be divided into a laminar collision and a turbulent collision according to the movement form of the slurry and the relative movement trajectory of the ore and the bubble.

One-Layer Flow Collision—A flow with a Reynolds number less than 103 is generally referred to as a laminar flow. In the case of laminar flow, the particles move along the flow line and the inertia of the fluid is negligible. Depending on the type of flotation machine, the areas where laminar collisions occur in the flotation machine are not the same. Figures 4-6-36 are different sections of a conventional mechanically agitated flotation machine and are generally divided into four zones, namely a foam zone, a separation zone, a mineralized transport zone, and a mixing zone. In the flotation column, it is completely different from the above partitions, and there is basically no mixing zone, mainly a separation zone.

The gas-solid liquid three-phase is vigorously mixed in the mixing zone. The gas stream is cut and dispersed into bubbles, and the bubble group collides with the suspended particle group in a large amount. The movement of bubbles and ore particles in this area does not have a certain direction, does not belong to laminar flow, and its collision belongs to turbulent collision. In the separation zone and the mineralized transportation zone, the bubbles move upward with the slurry flow or by buoyancy. The ore particles mainly move downward by gravity or move upward with the slurry flow. The movement of bubbles and ore particles belongs to the laminar flow range. Collisions in this zone are laminar collisions. Most of the photographs that study the collision process between the flotation bubble and the ore particle are taken under such conditions and can be used for the analysis of the collision process under laminar flow conditions. In order to simplify the whole system, it is assumed that one ore particle falls from above the bubble, as shown in Fig. 4-6-37, the radius of the ore particle is RP, and the radius of the bubble is RB. When there is no other relative motion between the bubble and the water medium, above the bubble, the ore particles in the radius of RP ten RB water column can be brought into contact with the bubble by gravity. When the bubble and the water flow have relative motion, when the water passes over the bubble, the streamline is bent. The ore particles located in the water are affected by the viscous force of the medium, and the motion trajectory will deviate from the bubble, and only the flow tube in the radius b is The ore particles may be in contact with the bubbles, ie b<RP ten RB. The size of b depends on the viscous force of the ore particles and the inertial force of the ore particles themselves. The viscous force causes the ore particles to bend along the streamline and leave the bubble; the inertial force causes the ore particles to deviate from the streamline and may collide with the bubble. The part where the ore particles collide with the bubbles is on the bubble hemisphere where the ore particles and the bubbles move toward each other.

2 Endstream collisions Generally, a flow with a Reynolds number between 104 and 106 is called an end flow, and an end flow collision occurs in a stirred mixing zone of a flotation machine. In this region, the direction of movement of the ore particles and bubbles is variable, and the collision can occur in any The location, and the intensity of the turbulence has a certain randomness on the impact of the collision, the speed of the floating and the probability of falling off.

Figure 4-6-37 Ore particle trajectory

In turbulent flow, the collision probability of particles and bubbles is affected by the turbulence intensity, and also by the concentration of the slurry, the number of particles in the slurry, and the number of bubbles. If the turbulence intensity, the concentration of the slurry, the number of particles in the slurry, and the number of bubbles increase, the collision probability can be increased. However, as described above, after the turbulence intensity is increased, the shedding force between the bubbles and the particles is also increased. Moreover, the coarser the particle size, the higher the density, and the stronger the hydrophilicity of the surface of the particles, the more significant the side effects of such turbulence. Therefore, after the turbulence intensity is increased, the floating rate of the particles does not necessarily increase as the turbulence intensity increases. Under industrial conditions, to ensure maximum recovery of coarse particles, the appropriate turbulence intensity should be selected. In order to overcome the effects of the flow around, a higher turbulence intensity can be selected for the fine material, which is advantageous for adhesion. After the turbulence intensity is increased, the mechanical agitation flotation machine can generate more small bubbles, which helps to increase the collision probability and flotation speed of the fine particles (-10 μm).

3. Microbubble precipitation

Under normal conditions, a certain amount of air is dissolved in the water, and the amount of dissolution is subject to Henry's law. The amount of dissolved air changes with changes in pressure and temperature. For example, when the temperature is lowered, the pressure is lowered, or the pressure is constant, the gas originally dissolved in the slurry is precipitated in the form of microbubbles. Floating mineral slurry precipitated microbubbles are precipitated at a constant temperature down conditions.

The diameter of the microbubble core suspended in water is on the order of 10-4 μm. Theoretical studies suggest that water containing such microbubble nuclei is more susceptible to fracture than water in which it is dissolved, which facilitates the precipitation and expansion of microbubble nuclei on the surface of the ore. The more hydrophobic the mineral surface, the faster the bubble grows, see Figure 4-6-38. Microbubble nucleation can also be observed in any turbulent vortex center (speed midsection and local depressurization zone). The higher the supersaturation of water and the larger the bubble content, the development of microbubble nucleus. The probability of being a microbubble is also higher.

The microbubbles attached to the surface of the fine particles can carry the particles floating up, or the microbubbles and the particles form an air floc with each other to cause them to float. The former's floating speed is much slower than the large bubble, and the latter can effectively float by the buoyancy of the microbubble group. Microbubbles attached to the surface of the coarse grain, due to insufficient buoyancy, cannot directly float the coarse material, but the microbubbles can become a bridge between the particles and the large bubbles, promoting adhesion between them. The adhesion of the particles through the microbubbles and the large bubbles is stronger than the adhesion between the general particles and the bubbles. This is because the adhesion contact area between them is large, and the adhesion between the particles and the microbubbles is a gas-solid direct contact without a residual hydration layer.

The microbubble is the gas core of the hydrophobic region on the surface of the particle, which can increase the adhesion probability of uneven hydrophobic particles and difficult-to-float particles, especially improve the recovery rate and flotation rate of coarse and heavy particles, and improve the selection of fine minerals. Sex. Krasin used three different aeration methods to float the 0.074 -0.147mm oleic acid-treated fluorite , as shown in Figure 4-6-39. The results show that when the microbubbles and large bubbles exist simultaneously, the flotation effect is the most. it is good.

4. Induction time and contact time

When the colloidal particles collide with the air bubbles and begin to enter the attachment stage, the hydration layer should be thinned or even broken, that is, from collision to adhesion, the hydration layer is thinned, cracked, the expansion of the three-phase wetting periphery, and the particles are stably stuck. Attached to the four processes above the bubble, this time is called the induction time, and the over-sensing time. A particle that may adhere to a bubble will generally slide down the surface of the bubble after it collides with the bubble until the lower half or tail is attached to the bubble.

During the attachment of particles and bubbles, the time from the onset of collision of particles and bubbles to the detachment of particles from the bubbles is referred to as contact time. A particle is attached to the bubble and its induction time must be less than the contact time. That is, the particles and bubbles should be able to complete the hydration layer thinning, cracking, three-phase wetting peripheral expansion and the particles adhere stably to the bubble during the contact time; otherwise, the induction time is greater than the contact time, the contact time The above stages cannot be completed, and particles and bubbles cannot adhere. The shorter the induction time, the easier the particles adhere to the bubbles. The induction time is as short as a few thousandths of a second, up to a few tenths of a second, depending on the degree of hydrophobicity of the surface of the particles, which is shortened as the degree of hydrophobicity of the surface of the particles increases. Flotation agents, mineral particle size, etc. also affect the induction time.

Flotation agents can alter the hydrophobicity or hydrophilicity of the surface of the particles, thus affecting the induction time. The adsorption of the collector on the surface of the ore particles can increase the hydrophobicity and shorten the induction time. The increase of the dosage can shorten the induction time; the adsorption of the inhibitor on the surface of the particles increases the hydrophilicity of the surface of the particles, thus prolonging the induction time.

The induction time increases as the mineral size increases, but when the particle size is greater than 50 μm, the induction time tends to be constant. The bubble size, the surface roughness of the particles, the wetting retardation effect, the particle shape, and the like all affect the rupture and induction time of the hydrated film more or less.

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