Study on Combined Vacuum–Mechanical Defoaming Technology for Flotation Froth and Its Mechanism

Abstract

Foam is essential in the flotation process. However, the gas–liquid–solid three-phase froth produced in the flotation process has very strong stability and is difficult to burst spontaneously. The existence of these froths will reduce the transport capacity of the pulp and affect the working efficiency of subsequent processes, such as filtration of the flotation concentrate. In this study, a new defoaming device is designed by combining mechanical impact with depressurized defoaming and its defoaming mechanism is analyzed theoretically. In addition, the liquid level height and pulp overflow method are applied to characterize the defoaming efficiency of the new defoaming device. The effects of impeller structure, pressure drop, impeller rotation frequency, and aeration rate on defoaming efficiency were studied. The results show that when increasing the pressure drop, the defoaming increases, but it will also enhance the generation of bubbles. The efficiency of combined mechanical–vacuum defoaming technology is superior under low-pressure drop using an SC impeller. Under −1 kpa vacuum condition, it only takes 168 s to eliminate 20 cm flotation froth height with combined mechanical impact, while it takes 453 s under ambient pressure, indicating that under vacuum conditions, the mechanical-defoaming method can significantly improve the defoaming efficiency, and the two have a certain synergistic effect.

1. Introduction

Bubbles and froth can often be seen in people’s daily life and work. With the use of soap and surfactant, people can obviously observe and experience the formation and bursting foam through vision and touch. Plato’s time is considered to be the earliest period of foam research [1]. Researchers in various countries have different opinions on the definition of foam, and have not formed a unified understanding for hundreds of years. The bulk gravity of foam is changed by the ratio between the mass of liquid and gas of which the foam consists, which is associated with the different thickness of intervening liquid film. Due to its film structure, foam is a thermodynamically unstable two-phase system that undergoes processes such as gravitational drainage, coalescence, and Ostwald ripening [2,3,4,5].

In the process of forming foam, the liquid turns into a liquid film of bubbles, forming polyhedral irregular bubbles. Most scholars agree that foam itself is a thermodynamic unstable system, but due to the action of surfactants such as most cationic surfactants, a highly stable foam system will be formed. The factors affecting the stability of foam include formation method of foam; the chemical constitution of the “base” solution, including its surfactant concentration, pH value, salt concentration, etc.; and thermodynamic properties, such as temperature, surface tension, etc. [6,7].

Flotation is considered to be the most effective method in the separation of fine particles. Flotation is a separation process that is based on different wettability properties of solid particles. According to the different surface properties of different mineral particles, froth is used to separate minerals and gangue minerals in the flotation feed, so as to enrich the target minerals. However, in order to realize the separation of clean coal, the particles are selectively adsorbed on the bubbles according to the properties of their own surface, pass through the slurry to the froth phase, and form a clean coal layer. The remaining minerals will be deposited at the bottom to form tail coal. Therefore, the formation, properties, and stability of froth play an important role in the flotation process. Clean coal particles, slurry, and foam together form gas–liquid–solid three-phase foam. This froth has strong stability that impedes the subsequent processing of the recovered coal in the froth. Generally, the flotation feed particle size is fine, which makes the three-phase froth more stable. After flotation separation, a large amount of stable froth will remain in the tailing, which will seriously affect the subsequent production process of the coal preparation plant and cause environmental pollution.

Due to the existence of the Marangoni effect, the results of defoaming with impeller alone are poor. Under vacuum conditions, the volume and surface area of the bubbles gradually increase, the liquid film becomes thinner, and the molecular density adsorbed on the surface of froth also decreases, which leads to the increase in the surface tension of the liquid film of the froth. If the froth still cannot burst by itself, the introduction of external mechanical force will make the partial liquid film of the froth thinner.

At present, there are two main defoaming methods: chemical defoaming and physical defoaming. In short, the chemical defoaming method can reduce the surface tension of the froth and the viscosity of the liquid film, so as to burst the froth; the method of physical defoaming is to make the liquid film of the froth disturbed by an external mechanical force, and then destroy the force balance of the liquid film and burst the froth. In terms of chemical defoaming, Guo shows that the foaming capacity of the liquid is inversely proportional to its surface tension [4]. Bashevaes et al. proposed the mechanism of bridging stretching [8]. Dippenaar first proposed the bridging dewetting mechanism by studying the defoaming mechanism [9]. The defoaming mechanism of defoamer in the defoaming process mainly depends on the dehumidification and deformation speed of defoamer droplets, but there is no ready-made theoretical model to prove it.

The mechanical defoaming method is to eliminate froth by using the sharp change in pressure, such as shear force, compression force, and impact force. According to the characteristics of its effect on froth, it can be divided into the centrifugal method, hydrodynamic method, and pneumatic method [10,11]. The centrifugal method is to use a high-speed rotating centrifugal blade device to eliminate the froth or impact with the fixed surface to break the froth. The hydraulic method and pneumatic method use the liquid flow or air flow under pressure to eliminate the froth. The air pressure method relies on the change in pressure to eliminate bubbles. Garrett introduced in detail the theory and method of defoaming with a rotating device and ultrasonic wave [12,13]. Rodriguez et al.’s [14] system studied the influence of ultrasonic operating parameters on the defoaming effect. The research results show that the intensity of the ultrasonic wave and time are the two main factors affecting the ultrasonic defoaming effect. In order to obtain the ideal defoaming effect, it is necessary to maintain a certain strength and a certain duration. Liu et al. developed a novel froth defoamer with perforated plates to enhance defoaming; the results showed that the perforated plate could significantly strengthen bubble coalescence or bubble collapse and promote the defoaming percentage [15].

Some scholars have studied the effect of acoustic sound on the stability of froth in the flotation process [16,17,18]. Ng et al. found that the change in froth stability caused by acoustic sound only occurs in the sound field, which indicates that this method can solve the problem of overfrothing without affecting the flotation efficiency [19].

In order to further improve the defoaming effect of flotation froth in a coal preparation plant, a new defoaming device is designed by combining the mechanical defoaming method with vacuum-defoaming technology. The effects of structural parameters and operating parameters of the device on the defoaming efficiency were studied.

2. Materials and Methods

2.1. Equipment

The schematic diagram of vacuum mechanical defoamer (VMD) is shown in Figure 1. The VMD used in the laboratory is made of plexiglass. The total length of the defoamer is 1455.5 mm, which is cylindrical, and the inner diameter of the column is 140 mm. The specific structural model is shown in Figure 1. From bottom to top, the main structure of the defoamer is composed of lower structure (LS), middle structure (MS), defoamer impeller, upper structure (US), etc. The middle structure and defoaming impeller are the core parts of defoamer.

The defoaming impeller is the core part of the defoamer. It uses the high-speed rotation of the impeller to achieve defoaming, and also makes the slurry circulate. Effect of two different impellers on defoaming efficiency was studied in the paper. The two impellers used in the test are shown in Figure 2.

Impeller DC is composed of six blades with a certain inclination angle. This kind of impeller has a wide speed range and has the ability of high shear force and turbulent diffusion. The scattered circulating flow is formed at the lower part of the disc, and the froth is thrown around, which is especially suitable for the shear dispersion operation.

Impeller SC consists of upper and lower impellers. The lower impeller is composed of four spiral conical curved blades, which have a certain shear force, and can obtain a high flow field in transition flow and turbulent operation. Through high-speed rotation, it will have a strong attraction to froth. The upper impeller is of high-speed type. The centrifugal action generated by its high-speed rotation makes froth throw out with the liquid centrifugally. At the same time, the blade generates shear force on the gas phase and liquid phase, breaking the bubbles into a large number of tiny bubbles and dispersing them in the liquid phase.

2.2. Working Principle

The slurry enters the defoamer from the feed inlet and is extracted through the discharge outlet. The gas and slurry are mixed through the bubble generator and filled into the bottom of the slurry from the air inlet to form a mineralized froth layer. The froth gradually rises along the induced inner cylinder to the high-speed rotating impeller. The impeller throws the froth towards the induced inner cylinder wall. The froth breaks due to impact and turns into liquid and flows along the inner cylinder wall.

During this process, the escaping air also carries part of the froth to escape upward, which is further thrown to the outer cylinder wall by the high-speed impeller, collided with the outer cylinder wall and converted into liquid again, so as to be secondary captured. This part of liquid is collected in the capture chamber and recycled through the return pipe flow to the collection tank. The slurry is extracted through the discharge port, and a new froth layer is generated through aeration circulation. The gas discharged after bubble rupture escapes from the upper part of the impeller and enters the diffusion chamber. At the same time, very few bubbles and droplets are finally captured in the diffusion chamber. When the air extraction at the exhaust port is greater than the amount of gas overflowed after the froth burst, a vacuum is generated in the defoamer, which causes the bubble to expand and burst, or the froth bursts with the help of the external mechanical force of the impeller.

2.3. Experimental System

The vacuum mechanical defoaming system (VMD) is shown in Figure 3. The system was mainly composed of defoaming circulation system and air extraction system. It should be ensured that the froth in the slurry eliminated by the defoamer is consistent with the froth in the actual production of the coal preparation plant as far as possible. Therefore, the defoamer is inflated in the collection tank at the lower part of the structure, so as to simulate the environment of slurry aeration rate in the actual production of the coal preparation plant. However, the defoamer circulation system was designed considering the repeatability and operability of the test.

In the defoaming cycle system, the slurry is fed from the feed port (11); then, the valve at the feed port is closed to start the defoaming cycle test. The circulating pump (10) extracts the slurry from the defoamer (8), passes through the bubble generator (9), mixes the slurry and air, and feeds them into the collection tank through the air inlet. The aeration environment of the slurry is simulated in the collection tank, resulting in a large amount of froth. With the continuous rise of the froth layer, the motor (7) drives the defoaming impeller for defoaming. The frequency of the defoaming impeller is adjusted by the frequency converter (5), and the froth is converted into slurry after defoaming by the defoamer. The collecting tank flows along the inner wall of the defoamer and is pumped out by the bottom circulating pump for reaeration rate to generate froth, so as to realize the process of circulating defoaming.

In the air extraction system, the water ring vacuum pump (1) extracts the gas in the defoamer through the steam water separator (2) to form a vacuum. The air extraction volume of the water ring vacuum pump is controlled by adjusting the size of the valve, so as to control the vacuum degree inside the defoamer. The vacuum degree inside the defoamer can be read out by the vacuum pressure gauge (6). When the system is tested for defoaming under normal pressure and low vacuum, the system is pumped by small air pump (3), and the vacuum degree of the system is controlled by gas rotameter (4).

In the test, the aeration rate is 200, 240, 280, 320, 360, and 400 L/h, respectively. The impeller rotation frequency is 0, 20, 28, and 36 Hz. The vacuum degree is −1.0, −2.0, and −3.0 kpa.

2.4. Measurement Method of Defoaming Efficiency

Due to the complex distribution of froth inside the equipment, there are many influencing factors. In addition to the amount of aeration rate, the outlet pressure of the circulating pump and the nature of the pulp itself, the vacuum inside the equipment is generated due to the need to pump air through the vacuum pump, which has many limiting factors on the determination of defoaming efficiency. In this test, liquid level height method and slurry overflow method are proposed to measure defoaming efficiency.

The height of froth layer under atmospheric pressure and that under vacuum were recorded with a scale. By comparing the changes in froth layer height under different environments, the effects of aeration rate and vacuum on froth layer height were studied.

Under certain aeration rate and vacuum conditions, the rotation frequency of the impeller is slowly reduced to observe whether there is pulp overflow at the air extraction port. When the pulp begins to overflow slowly, the rotation frequency is recorded at this time. Then, the corresponding defoaming frequency is recorded successively by improving the aeration rate and pressure drop. The relationship between defoaming frequency, aeration rate, and pressure drop is established.

2.5. Sample and Regents

The coal sample used in the test comes from Gujiao City, Shanxi Province, and the particle size of the coal sample is −0.25 mm. Proximate analysis of coal sample is given in

Table 1. Proximate analysis of coal sample (air-dried).

Mad (%)	Aad (%)	Vad (%)	FCad (%)
6.27	10.12	18.64	64.97

where M_ad is the moisture content, V_ad is the volatile content, FC_ad is the fixed carbon content, A_ad is the ash content.

. The particle size composition of the coal sample is shown in

Table 2. Particle size distribution of LRC sample.

Size Fraction (mm)	Percentage (%)	Ash Content (%)
0.25–0.125	8.08	7.68
0.125–0.075	11.12	8.67
0.075–0.045	25.34	8.89
−0.045	55.46	11.34
Total	100.00	10.12

. It can be seen from Table 2 that the particle content of −0.045 mm in the coal sample is the highest. In the flotation process, this part of fine particles will promote the formation of highly stable froth, affecting the subsequent production process of the coal preparation plant. In the test, n-dodecane and methyl isobutyl methanol were used as collectors and foaming agents, respectively. Both agents were purchased from Aladdin Reagent Co., Ltd., and the agents used in the test were of analytical purity.

In the test, the pulp concentration of coal was fixed at 100 g/L, the vacuum degree was −0.5 kPa, −1 kPa, and −1.5 kPa, respectively, and the dosage of n-dodecane was 1000 g/t; the dosage of MIBC is 100 g/t and the aeration rate is 200, 240, 280, 320, 360 and 400 L/h, respectively.

3. Results

3.1. Effect of Aeration Rate and Applied Pressure Difference on the Height of Froth

The froth in the system is mainly generated by the incoming air, so the aeration rate has an important impact on the height of the froth in the system. For the internal pressure of the device, on the one hand, the reduction in the internal pressure causes more froth to be generated in the pulp; on the other hand, it also accelerates the rupture of the froth. Therefore, it is necessary to study the impact of the vacuum inside the device on the height of the froth.

The effect of aeration rate on the height of the froth is shown in Figure 4. It can be seen from Figure 4 that the height of the froth increases with the increase in aeration rate, because the number of bubbles in the pulp increases with the increase in aeration rate. Comparing the height of the froth under normal pressure and vacuum, it can be seen that the height of the froth under vacuum is higher than that under normal pressure. This is because the solubility of the gas in the pulp decreases under vacuum, and some gas evolves and forms froth. When the aeration rate is 240 L/h, the corresponding froth layer height under the two conditions is 63.8 cm and 78.3 cm, respectively, and the height difference of the froth layer reaches 14.5 cm.

3.2. Effect of Pressure Drop on Defoaming

The relationship between the impeller frequency and aeration rate under different vacuum degrees is shown in Figure 5. It can be seen from Figure 5 that with the increase in aeration rate, the impeller frequency also increases gradually, and the relationship between them is basically a linear function. Under the same aeration rate, with the increase in pressure drop, the amount of bubbles and the rising speed of bubbles increase, so the required impeller frequency is also higher. This is conducive to the faster elimination of bubbles in the pulp, that is, with the increase in vacuum, the defoaming efficiency and defoaming volume also gradually increase. However, under high-vacuum conditions, more froth needs to be eliminated, which means higher impeller speed and higher energy consumption. Therefore, in order to obtain a better defoaming effect and consider saving energy consumption, the defoaming should be carried out under low-vacuum conditions. As can be seen from Figure 5, when the system is under vacuum, the frequency of the impeller is lower than that under normal pressure.

In the defoaming process under vacuum conditions, the defoaming effect is related to two factors. The first factor is that the change in vacuum degree leads to the formation of new froth in the froth system. The amount of froth produced is related to the vacuum degree and the amount of aeration rate; the second factor is that the change in pressure drop changes the properties of the bubbles, weakens the stability of the bubble liquid film, and makes the froth easier to burst. Under the condition of high vacuum, the first influencing factor is more significant, so a higher impeller speed is needed to eliminate bubbles. Under the condition of low vacuum, the second influencing factor is more significant, so the required impeller speed is low, and the result is the most ideal.

In essence, froth is an unstable system. When the external pressure of the froth decreases, the bubble structure is affected. By measuring the time required for the discharge of froth under different pressures, it was found that there was a linear relationship between the pressure and the discharge time of the froth. This is because as the external pressure of the bubble gradually decreases until reaching the negative pressure environment, the bubble begins to expand because the external pressure of the bubble is lower than the internal space pressure of the bubble; since the quality of froth is constant, the larger the radius of the froth, the larger the surface area of the froth, and the thinner the liquid film. As a result, the liquid between the bubble liquid films begins to drain downward under the pressure and gravity, and the liquid film begins to thin and eventually leads to bubble coalescence and the rupture of bubbles.

3.3. Influence of Impeller Structure on Defoaming

The equipment is filled with froth; then, the air inlet valve is closed, and the time required for two impellers with different structures to eliminate 20 cm high froth is measured. The influence of the rotation frequency of the impeller DC on the defoaming time under different vacuum conditions is shown in Figure 6. As can be seen from Figure 6, with the increase in impeller rotation frequency, the time to eliminate 20 cm of froth gradually becomes shorter, and the defoaming speed increases, indicating that increasing the impeller rotation frequency can effectively improve the defoaming effect. When the impeller is closed, it takes 531 s to eliminate 20 cm of froth under normal pressure. With the increase in vacuum, the time is gradually shortened. It only takes 205 s under −3 kpa vacuum, saving 326 s. This shows the advantages of vacuum defoaming.

The influence of the rotation frequency of the impeller SC on the defoaming time under different vacuum conditions is shown in Figure 7. With the increase in impeller rotation frequency, the time to eliminate 20 cm of froth gradually becomes shorter, and the defoaming speed increases. However, comparing Figure 6 and Figure 7, it can be seen that under all test conditions, the defoaming time with the SC impeller is lower than that with the DC impeller. This is due to the special structure of the SC impeller. When rotating at high speed, a negative pressure zone is formed at the lower part of the impeller to attract froth, so that more froth can be eliminated at the same time. Therefore, the defoaming effect is better than that of the DC impeller. In addition, under normal pressure, the defoaming effect of the SC impeller is more significant. This is because the defoaming under normal pressure mainly depends on the stirring and crushing effect of the impeller.

3.4. Mechanism of Vacuum Mechanical Combined Defoaming

Under vacuum, the density of the surfactant molecules in the liquid film is very low. When the liquid film is locally thinner, the surrounding surfactant molecules cannot be supplemented in time, and the liquid membrane cannot be restored to the original thickness, resulting in the reduction in the mechanical strength of the liquid membrane. Therefore, at this time, bubbles are easy to rupture under the action of external mechanical force.

For the gas–liquid–solid three-phase froth system, the decay mechanism of the froth mainly includes three aspects: the discharge of the liquid film, the diffusion of gas through the liquid film, and the disappearance of the framework of the three-phase froth system. The drainage of the liquid film is composed of two processes. The first process is gravity drainage when the liquid film is thick, and the second process is curved-surface-pressure drainage when the liquid film is thin. Under vacuum conditions, it accelerates the drainage speed of the liquid film, so as to make the bubbles burst faster.

The surface pressure drainage process is mainly caused by the plateau boundary, as shown in Figure 8. As described above, as the pressure outside the bubble gradually decreases, the liquid surface tension gradually increases, and the volume of froth continues to grow. According to Laplace equation:

$${\mathrm{P}}_{\mathrm{A}}={\mathrm{P}}_0-\frac{2\mathsf{\sigma }}{{\mathrm{R}}_1}$$

(1)

$${\mathrm{P}}_{\mathrm{B}}={\mathrm{P}}_0-\mathsf{\sigma }\left(\frac{1}{{\mathrm{R}}_1}+\frac{1}{{\mathrm{R}}_2}\right)$$

(2)

$${\mathrm{P}}_{\mathrm{A}}-{\mathrm{P}}_{\mathrm{B}}=\mathsf{\sigma }\left(\frac{1}{{\mathrm{R}}_1}+\frac{1}{{\mathrm{R}}_2}\right)$$

(3)

Generally, R₁ is much larger than R₂, so it can be ignored, and Equation (3) is transformed into Equation (4):

$${\mathrm{P}}_{\mathrm{A}}-{\mathrm{P}}_{\mathrm{B}}=\frac{\mathsf{\sigma }}{{\mathrm{R}}_2}$$

(4)

where P₀ is the pressure in the bubble, P_A is the liquid pressure between two adjacent bubbles, P_B is the liquid pressure in the plateau boundary, σ is the liquid surface tension, R₁ is the radius of curvature of the bubble, and R₂ is the radius of curvature of the plateau boundary.

As the pressure outside the bubble decreases gradually, the liquid surface tension increases gradually. With the continuous expansion of the froth system, the radius of curvature R₁ in the P₀ region gradually decreases. From Equation (4), it can be seen that the pressure at P_A is greater than that at P_B, and with the gradual decrease in the external pressure of the bubble, the pressure difference between P_A and P_B also gradually increases.

Under the action of this pressure difference, the liquid in the liquid film converges from P_A to P_B, and the vacuum condition accelerates the speed of liquid converging from P_A to P_B, so the thinning speed of the liquid film is accelerated, and the mechanical strength of the bubble is also accelerated to decline. At the same time, there are a large number of coal particles on the froth wall; when the thickness of the liquid film becomes thinner to a certain extent, the liquid film breaks through the external mechanical force and the gravity of solid particles.

In the process of bubble rising, with the gradual decrease in pressure, more small bubbles surround the periphery of the large bubbles, and the phenomenon of mutual merger between bubbles is more obvious. The gas in the small bubbles diffuses into large bubbles through the liquid film, resulting in the gradual reduction in and disappearance of small bubbles. The large bubbles gradually expand and eventually burst under the combined action of the vacuum and small bubbles.

Due to the existence of hydrophobic coal particles in the froth, a system similar to a skeleton is formed on the outer wall of the froth. As the bubble is wrapped inside, the froth does not break under the external mechanical force, and it also supports the solid particles that are easy to fall off.

Under the action of pressure, the discharge of the liquid film is accelerated, so that the fluidity of liquid is increased, and the liquid discharge channel is prevented from being blocked by solids in the bubble gap, so that the process of froth liquid-film thinning is accelerated. Because the coal particles covered on the bubble’s surface prevent the bubble from merging, and the vacuum condition can promote the particles adhered on the surface of the froth to fall off, this accelerates the interaction between bubbles, and makes the gas in the bubble diffuse outward through the liquid film, resulting in fracture. In addition, under vacuum conditions, although most of the liquid films in the froth break, the crisscross froth does not collapse immediately but continues for a certain period of time due to the existence of the skeleton structure. However, the remaining skeleton becomes very fragile. After being subjected to an external mechanical force, the skeleton breaks and collapses rapidly until it finally disappears. When the whole framework system disappears completely, it means that the three-phase froth is completely burst.

4. Conclusions

In this investigation, a vacuum mechanical combined-defoaming technology for flotation froth and its mechanism were studied. The principal conclusions are:

(1)

The combining the vacuum-defoaming method with mechanical-defoaming technology, a new vacuum-mechanical-defoaming device is designed, which has high defoaming efficiency.

(2)

Increasing the pressure drop of the defoaming device can improve the defoaming efficiency, but it also increases the generation of bubbles. The effect of using the vacuum-mechanical-defoaming device under low vacuum is the best.

(3)

The structure of the impeller has an important influence on the defoaming efficiency, and the hollow impeller with induction function has a higher defoaming efficiency.

(4)

Similarly, to eliminate the 20 cm high flotation froth, the time consumed under −1 kpa pressure is only one-third of that under normal pressure.