Nanobubble Enhances Rutile Flotation Separation in Styrene Phosphoric Acid System

Abstract

Due to the weak hydrophobicity of styrene phosphoric acid (SPA), the amount used as a collector for rutile flotation is too large, resulting in high beneficiation costs. In this study, SPA was modified by nanobubbles to enhance its hydrophobicity. In this paper, the modification of SPA by nanobubbles and the adsorption mechanism of SPA on rutile surface before and after modification were studied by means of nanoparticle tracking analysis, micro-bubble flotation test, contact angle test, zeta potential test, etc. The results show that SPA can significantly increase the concentration of bulk nanobubbles, increase the flotation recovery of rutile from 55% to 69%, and reduce the dosage of SPA from 101 mg/L to 70 mg/L. Nanobubbles interact with SPA in the form of water drainage, significantly reducing the zeta potential of the rutile surface and increasing the solid–liquid interface contact angle of rutile surface. A model of the interaction between nanobubbles, SPA, and rutile surface is established, which is helpful to understand the process mechanism of nanobubble flotation.

1. Introduction

Due to its excellent properties, titanium metal has been widely used in chemical applications, petroleum, electric power, seawater desalination, construction, daily necessities, and other industries [1,2,3]. It is an irreplaceable strategic material in the construction of national defense equipment. Natural rutile has a high content of TiO₂ and few impurities, and is the most ideal raw material for producing titanium dioxide and sponge titanium by chlorination [4]. In view of the characteristics of China’s primary rutile ore, such as complex composition, low grade of raw ore, fine embedded particle size, complex symbiotic relationship, and easy sliming, flotation is the most effective method for recovering low-grade micro-fine embedded rutile ore in many beneficiation and separation methods [5,6]. The innovation of flotation theory and technology of primary rutile, especially the selection of efficient and non-toxic collectors and activators, is the key to realize efficient flotation separation of fine rutile and solve the problem of primary rutile ore beneficiation [5,7,8].

The commonly used flotation collectors for rutile ore include fatty acid, medeca, benzyl arsenic acid, styrene phosphonic acid (SPA), alkylamine dimethyl phosphate, alkyl phosphonate, alkyl hydroxamic acid, and salicyl hydroxamic acid [9,10]. Zhu et al. assert that fatty acid collectors have strong collection capacity and poor selectivity, and are suitable for the flotation of rutile ore with simple gangue mineral structure, and only quartz as the main gangue obtains better flotation indexes [11]. Benzyl arsenate has strong selectivity for rutile. The negatively charged arsenate reacts with the positively charged titanium site to form insoluble salts and adsorb on the surface of rutile. However, benzyl arsenate is highly toxic, and even if mixed with fatty acids and hydroxamic acids, it causes great pollution to the environment [10,12]. Liu et al. [12] used SPA instead of benzyl arsenic acid as a rutile flotation collector and found that the selectivity of SPA on the surface of rutile was stronger than that of benzyl arsenic acid. However, SPA has a certain ability to collect calcium minerals. When the amount of SPA is large, the grade of titanium dioxide in the rutile flotation concentrate is low. In addition, the production cost of SPA is high, and its large-scale promotion and use are limited due to the large dosage of chemicals [9]. In order to realize large-scale industrial application of SPA, it is necessary to solve the problem of weak collection capacity and large amount of SPA. Hydrophobic modification of SPA or addition of an auxiliary collector is an effective way. Wang et al. [13,14] used n-octanol as an auxiliary collector of rutile and used it in combination with SPA. Under the same flotation index, the amount of SPA decreased by about 50%. Ren et al. [15] increased the flotation recovery of fine-grained rutile by adding nanobubbles as auxiliary collectors, and the amount of hydroxamic acid collector decreased by about 30%. Xia et al. [16] discussed the mechanism of nanobubbles enhancing flotation adhesion. On the one hand, interfacial nanobubbles can promote liquid film drainage during particle–bubble collision through boundary slip, and on the other hand, nanobubbles bridge to make particle–bubble interaction appear as long-range gravity. At the same time, the DLVO force between particle–bubble interaction changes from a repulsive force to a gravitational force, thus promoting particle–bubble adhesion. However, although these studies have found that nanobubbles can increase the flotation recovery of micro-fine mineral particles, there are few studies on how nanobubbles affect the flotation process of micro-fine mineral particles. At present, the research on the surface physical-chemical interaction of nanobubbles, surfactants, and oxidized minerals mainly stays at the superficial level, and the microscopic mechanism of the specific interaction is not clear. The interaction form of nanobubbles and surfactants is the key to the chemical-selective adsorption of nanobubbles on the mineral surface. At the same time, research on nanobubble flotation focuses on industrial applications, such as water treatment and coal flotation [17,18,19].

The interaction between nanobubbles, surfactants, and mineral surfaces is a complex multiphase system with both physical and chemical effects. The modification of nanobubbles by surfactants enable nanobubbles to have selective adsorption function on the active sites of the mineral surface, and this realizes the efficient flotation recovery of micro-fine primary rutile ore. On the basis of an in-depth investigation of the current situation of primary rutile flotation in China, this paper aims to solve the problems of flotation recovery of fine minerals and low concentrate grade and carries out flotation research on fine primary rutile ore modified by the surface of nanobubble minerals.

2. Materials and Methods

2.1. Materials and Reagents

The rutile and garnet samples used in the flotation tests were obtained from Weihai city in Jiangsu, China. After the rutile concentrate was re-separated in a shaking table,—0.038 + 0.074 mm pure rutile mineral was obtained. The garnet concentrate was passed through a 100-mesh screen for magnetic separation, and the products on the screen passed through dry magnetic separation, vibration mill, and wet screen to obtain—0.038 + 0.074 mm pure garnet mineral for testing. After splitting, an analysis of the sample was performed to investigate the principal chemical components, and the results are presented in Table 1. Table 1 shows that the purity of the rutile single mineral is more than 97%, and that of garnet single mineral is more than 92%, which meets the requirements of single mineral flotation.

The ultra-pure water with a conductivity of 18.2 MΩ·cm used in the experiments was provided by an USF-ELGA Maxima water purification system. The reagents used in this paper were analytically pure, with certain exceptions.

2.2. Micro-Flotation

The flotation test of the rutile and garnet pure minerals was carried out on a 100 mL RK/FGC35 hanging cell flotation machine, with a rotating speed of 2000 r/min, and the feed rate of the rutile or garnet pure minerals for each test was 3 g. During the test, the pulp was stirred for 2 min, the pH was adjusted with hydrochloric acid or sodium hydroxide, and SPA and methyl isobutyl methanol (MIBC) were added in sequence as required and stirred for 3 min after adding one agent. Then, the foam was scraped once every 6 s, water was added once every 30 s, and the foam was scraped for 3 min in total. The products in the tank and the foam products were dried and weighed, respectively, and the flotation recovery rate was calculated.

2.3. Generation and Detection of Nanobubbles

Nanobubbles were produced in a machine named the spring and summer with a power of 0.7–1.0 m³/L. The principle of a nanobubble generator is to make water and gas highly miscible, and ultrasonic cavitation dispersion releases high-density and uniform ultra-micron bubbles, forming a “milky” gas–liquid mixture. The “milky white” solution is let stand for 10 min until micron bubbles disappear and the solution becomes clear.

The concentration and size distribution of nanobubbles were determined using a NS300 nanoparticle tracking analyzer (NTA) produced by Marvin Company, (Malvern City, UK). The measurement of the NTA is based on the principle of Brownian motion and dynamic light scattering, and its working principle is detailed in the literature [20]. The number of particles (nanobubbles) is counted by the NTA, so the concentration of particles (nanobubbles) can be obtained by dividing the volume of the field of view. The size of each particle (nanobubble) can be calculated from the diffusion of Brownian motion using Stokes–Einstein equation [21]:

$\overline{{(x,y)}^2}=\frac{2k_{B}T}{3r_h\pi \eta }$

Here, k_B is the Boltzmann constant, $\overline{{(x,y)}^2}$ represents the mean square velocity of particles, T is the temperature (°C), η is the viscosity of solution, and r_h is the hydraulic radius of particles.

The procedure was as follows: use a glass syringe to inject the generated nanobubble aqueous solution into the NTA sample cell, suck 2 mL of solution each time, push the syringe slowly during the measurement process, and start the measurement after the image is stable.

2.4. Contact Angle Test

Before and after the action of minerals and agents (collectors and nanobubbles), the wettability of the mineral surface was tested on the JC2000C contact angle tester produced by Shanghai Zhongchen Digital Technology Co., Ltd (Shanghai, China). First, a massive single mineral sample was cut into 12 mm × 12 mm × 2 mm cube and was then polished using diamond sand with a roughness of 100 µm, 40 µm, and 9 µm, respectively, to obtain a flat surface. Then, 1.0 µm, 0.3 µm, and 0.05 µm of alumina powder solution were continuously polished by a polishing cloth. The polished sample was cleaned with deionized water first, and then ultrasonically cleaned in an ultrasonic cleaner for 30 min to remove residual aluminum oxide powder on the surface of the sample. Finally, the polished surface was cleaned with anhydrous ethanol and washed in a Plasma Cleaner for 2 min to remove residual organic pollutants on the surface. After the sample was prepared, the polished sample was immersed in a solution containing the reagent (the concentration was the same as the flotation condition) for 10 min in the order of reagent addition; then, it was taken out, washed with deionized water three times, and, finally, blew dry with high-purity N₂ flow. Using a graduated syringe, deionized water drops were added to the sample surface, and, finally, the contact angle of deionized water on the mineral surface was measured with a contact angle instrument. Four drops were added at different places, and the average value was taken as the result record. The whole measurement process was carried out at 25 °C.

2.5. Zeta Potential Test

The surface electrokinetic potential of the single mineral was measured on a JS94H micro-electrophoresis apparatus. The procedure was as follows: Fine grind the single mineral sample to −5 µm. Weigh 20 mg of finely ground sample each time and put it into a 100 mL beaker; add 40 mL of 0.1% KNO₃ solution as the electrolyte solution; add the corresponding reagents as needed; and use dilute HCl and NaOH solutions to adjust the pH after magnetic stirring for 5 min. Then, extract the supernatant after the pH value is stable, inject it into the sample tank, measure the mineral surface potential under this condition, repeat the measurement three times for each condition, and take the average value as the result record.

3. Results and Discussion

3.1. Effect of Collectors on the Properties of Nanobubbles

3.1.1. Effect of Collector Concentration on the Properties of Nanobubbles

SPA is a typical surfactant, which can significantly change the surface tension of water. The change in the surface tension of water affects the energy barrier of the formation of nanobubbles [22]. Figure 1 shows the variation in the concentration and diameter of nanobubbles with the concentration of SPA.

It can be seen from Figure 1a that the concentration of nanobubbles in the solution gradually increases with an increase in the concentration of SPA. When a small amount of SPA exists in the solution, the concentration of bulk nanobubbles in the solution increases from 2.36 × 10⁷/mL to 65.36 × 10⁷/mL. When the concentration of SPA increases to 80 mg/L, the concentration of bulk nanobubbles increases to 85.62 × 10⁷/mL. However, as the concentration of SPA continues to increase, the concentration of nanobubbles does not increase significantly. This change trend of the concentration of nanobubbles may be determined by the surface tension of the solution.

It can be seen from Figure 1b that with an increase in the concentration of SPA, the diameter of nanobubbles in the solution shows a trend of decreasing first and then increasing. When the concentration of SPA in the solution is 40 mg/L, the diameter of bulk nanobubbles in the solution reaches a minimum value of 129.52 nm. From the error analysis results, it can be seen that the smaller the diameter of nanobubbles in the solution, the more stable the nanobubbles are, and the better the repeatability of the measurement is. It may be that the smaller the bubbles, the lower the collision probability between them, so smaller bubbles are more stable than larger bubbles [23].

3.1.2. Effect of Standing Time on the Properties of Nanobubbles

According to the classical thermodynamic theory, the theoretical existence time of nanobubbles is very short. However, some existing experiments had proved that nanobubbles could exist stably for a long time [24,25]. Figure 2 shows the relationship between the concentration and diameter of nanobubbles and the standing time when the concentration of SPA is 60 mg/L.

It can be seen from Figure 2a that the concentration of nanobubbles in the solution gradually decreases with an increase in the standing time, and the rate of decrease is faster and faster. When the residence time is 10 min, the concentration of nanobubbles in the solution reduces from 82.35 × 10⁷/mL to 71.25 × 10⁷/mL, and the reduction is only 13.48%, indicating that nanobubbles in the solution are relatively stable in a short time. The flotation time of general minerals is also within 10 min, which meets the requirements for the stability of nanobubble system. However, as the residence time continues to extend, the concentration of nanobubbles in the solution decreases rapidly. When the residence time reaches 25 min, the solubility of nanobubbles in the solution decreases to 11.25 × 10⁷/mL, with a reduction of more than 86%. Therefore, in the flotation process, micro- and nanobubbles need to be generated in sections to ensure the concentration of nanobubbles in the whole flotation system. Xiao et al. [14] also drew a corresponding conclusion when studying the change rule of the surface tension of an aqueous solution with concentration of SPA. With the addition of SPA, the surface tension of the solution decreases rapidly, resulting in a decrease in the nucleation barrier of nanobubbles in the solution and an increase in the concentration of nanobubbles.

It can be seen from Figure 2b that the diameter of nanobubbles in the solution gradually increases with the prolongation of the standing time. In the first 10 min, the diameter of nanobubbles slowly increases. After more than 10 min, the diameter of nanobubbles increases sharply. With the prolongation of the standing time, the change trend of the diameter of the nanobubble is opposite to that of its concentration. This may be caused by the collision and fusion between nanobubbles.

3.2. Micro-Flotation

Due to the special properties of their surface, nanobubbles are often considered to have a significant enhancement effect on the flotation of fine minerals [15]. The relationship between the flotation recovery of rutile and garnet with the pulp pH value and the amount of collector in the presence or absence of nanobubbles is shown in Figure 3.

It can be seen from Figure 3a that when there are no nanobubbles, the flotation recovery of rutile increases first and then decreases with an increase in pH value. When the pH value reaches about 3.5, the flotation recovery of rutile reaches the maximum value of 49.25%. In the nanobubble system, the recovery rate of rutile is higher than that of the system without nanobubbles in the whole pH range. With an increase in pH value, the difference of recovery rate increases gradually. This may be due to the formation of nanobubbles under acidic conditions. When the pH value is about 4.2, the flotation recovery of rutile reaches the maximum value of 56.26%. The addition of nanobubbles causes the optimal flotation pulp pH of rutile to move to a high pH. If the pH value continues to increase, the flotation recovery of rutile should also be significantly reduced, which indicates that nanobubbles only act as auxiliary collectors in the process of rutile flotation.

It can be seen from Figure 3b that the flotation recovery of rutile increases rapidly at first and then tends to be stable with an increase in the amount of SPA, regardless of the presence of nanobubbles. When there is no nanobubble, the concentration of SPA required for rutile flotation to reach the best flotation recovery rate (55%) is 101 mg/L. In the nanobubble system, the concentration of SPA required for rutile flotation to reach the best flotation recovery rate (69%) is only about 70 mg/L. The existence of nanobubbles not only improves the recovery rate of rutile flotation, but it also greatly reduces the amount of SPA.

3.3. Zeta Potential

Because of the negative charge on the surface of nanobubbles, the adsorption of nanobubbles may lead to a change of zeta on the mineral surface. Figure 4 shows the effect of nanobubbles and SPA on the zeta potential of rutile and garnet surfaces.

It can be seen from Figure 4a that the zero point of zeta potential on the surface of rutile is 4.3, which is consistent with the value in the literature [15]. When the optimum flotation range is about pH 3.5, the surface of rutile has a strong positive charge, which is beneficial for the adsorption of anionic surfactants. When SPA is present in the system, SPA is adsorbed on the surface of rutile in the form of anion, resulting in a significant reduction in zeta potential on the surface of rutile. When the pH value of the pulp exceeds the zero point of the rutile surface, the rutile surface is charged with a negative charge. However, after the addition of SPA, the zeta potential on the rutile surface also decreases significantly, which indicates that the adsorption of SPA on the rutile surface includes both physical adsorption based on electrostatic adsorption and chemical adsorption [5]. From the decrease in zeta potential, it can be seen that the adsorption of SPA on the surface of rutile may be mainly chemical adsorption. When there is SPA in the system, the zeta potential on the surface of rutile decreases to a certain extent compared with that in the presence of SPA only, which may be caused by the negative charge on the surface of nanobubbles [25]. This also shows that SPA and nanobubbles are adsorbed on the surface of rutile together, resulting in a synergistic effect [14].

It can be seen from Figure 4b that the zero point of the garnet surface is 4.1, which is close to the zero point of the rutile surface. In the SPA system, when the pulp pH value is less than the isoelectric point, SPA is adsorbed on the garnet surface in the form of electrostatic force, resulting in a decrease in zeta potential on the garnet surface. At the same pH value, compared with the adsorption behavior of SPA on the surface of rutile, it can be found that the reduction in zeta potential on the surface of rutile is much greater than that on the surface of garnet, which indicates that, under similar electrostatic force conditions, there is another adsorption mode of SPA on the surface of rutile besides electrostatic adsorption. The literature [14] shows that SPA chelates with titanium particles on the surface of rutile in the form of five rings, which proves the chemisorption method of SPA on the surface of rutile. When the pulp pH value is close to or greater than the isoelectric point of garnet, the zeta potential of garnet surface does not change significantly, which indicates that SPA in the solution is hardly adsorbed on the garnet surface. There is no significant difference in the zeta potential of the garnet surface in the nanobubble solution with SPA and in the solution with SPA alone.

3.4. Contact Angle

The floatability of mineral particles is determined by the hydrophobicity of the mineral surface, and the hydrophobicity of the mineral surface is determined by the contact angle between the mineral and the water solution interface [7,26]. The influence of nanobubbles on the contact angle of the solid–liquid interface on the mineral surface is shown in Figure 5.

As can be seen from Figure 5a, in pure water, the solid–liquid interface contact angle on the surface of rutile is only 37.8°, indicating that the natural hydrophobicity of rutile mineral is poor. This is also consistent with the reports of most literatures [5,27]. In the separate SPA solution, the solid–liquid interface contact angle of rutile surface reaches 54.6°, but in the separate nanobubble aqueous solution, the solid–liquid interface contact angle of rutile surface is only 38.1°, and the solid–liquid interface contact angle of rutile surface in pure water is almost unchanged. These experimental results show that SPA could adsorb on the surface of rutile and improve the hydrophobicity of rutile. However, single nanobubbles could hardly adsorb on the surface of rutile. This also confirms the conclusion that nanobubbles are formed on a strongly hydrophobic surface, but not easily on a hydrophilic surface [28,29]. When in the SPA solution with nanobubbles, the solid–liquid interface contact angle on the surface of rutile reaches 72.5°, which is 17.9° higher than that in the single SPA solution. This indicates that there is some interaction between nanobubbles and SPA, which makes SPA and nanobubbles adsorb on the surface of rutile at the same time and increases the hydrophobicity of the surface of rutile.

It can be seen from Figure 5b that in pure water, the solid–liquid interface contact angle of garnet surface is 32.3°, which is similar to the contact angle of rutile surface, indicating that rutile and garnet have the same natural hydrophobicity, and it is difficult to separate the two minerals by conventional flotation methods. In the SPA solution, the solid–liquid interface contact angle on the surface of garnet is only 38.1°, which is similar to that in pure water, indicating that the adsorption capacity of SPA on the surface of garnet is weak, which is consistent with the results shown by zeta potential. In the nanobubble solution, the solid–liquid interface contact angle of garnet surface is 33.7°, which is almost the same as that of the garnet surface in pure water. In the SPA solution with nanobubbles, the solid–liquid interface contact angle of garnet surface is 40.1°. The results of the solid–liquid interface contact angle on the surface of garnet show that the hydrophobicity of the garnet surface cannot be effectively improved by either SPA or nanobubbles.

3.5. Discussion on Adsorption Model

In the flotation system, due to the diversity of pulp environment, the interaction of bubbles, collectors, and mineral particles is very complex [30,31]. In order to simplify the slurry environment, the surfactant was mixed with the nanobubble solution. The interaction between the surfactant and nanobubbles was analyzed via a zeta potential test and dynamic light scattering, and then the contact angle test revealed the mechanism of nanobubbles being modified by the surfactant on the surface of the rutile. Finally, single mineral flotation was used to verify that the addition method after the interaction of nanobubbles and collectors can significantly improve the recovery of rutile flotation. Based on the results of the above tests, the interaction model between nanobubbles, surfactants, and rutile mineral particles is inferred, as shown in Figure 6.

It is generally believed that the adsorption behavior of nanobubbles on the mineral surface does not have the selectivity of chemical elements, and their adsorption capacity is only related to the physical properties of the mineral surface (such as surface hydrophobicity and surface roughness) [28,29,32]. However, after interacting with the collector, nanobubbles have a strong selective adsorption capacity for metal ions on the mineral surface, and their selectivity is determined by the hydrophilic group of the surfactant. If the hydrophilic group of the surfactant has strong coordination reaction ability to a certain metal ion on the mineral surface, but does not react with other metals on the mineral surface, then the selective adsorption of nanobubbles on the target mineral can be realized. The realization of selective adsorption of nanobubbles on the surface of target minerals is of great significance to enhanced flotation of fine minerals. First, after the selective adsorption of nanobubbles, the hydrophobic difference of the mineral surface increases, which is beneficial to the flotation process of minerals. Secondly, due to the bridging effect between nanobubbles, the mineral particle solution with nanobubbles adsorbed will agglomerate. Because the adsorption of nanobubbles is selective, the agglomeration behavior of all mineral particles is also selective. The realization of selective agglomeration will lead to an increase in the surface particle size of the target mineral particles, and its flotation recovery will also become relatively enhanced. Finally, after the target mineral particles adsorb the nanobubbles, because the collision and fusion between the nanobubbles and the macro-flotation bubbles are easier than the collision between the mineral fine particles and the macro-bubbles, the selective adsorption of the nanobubbles on the surface of the target mineral is conducive to improving the collision probability of the target mineral particles and the macro-flotation bubbles and improving the mineralization efficiency.

4. Conclusions

• (1). SPA can significantly increase the concentration of bulk nanobubbles, reduce the diameter of nanobubbles, and increase the existence time of bulk nanobubbles.

• (2). After the pre-interaction of SPA and nanobubbles, the flotation recovery of rutile can be significantly improved, and the amount of SPA can be reduced.

• (3). SPA acts as a bridge to adsorb nanobubbles on the surface of rutile particles, significantly improving the solid–liquid interface contact angle of rutile surface.

Author Contributions

Methodology, W.Q.; software, Y.W.; formal analysis, Y.W.; investigation, W.X.; data curation, Y.W.; writing—original draft preparation, Y.W.; writing—review and editing, W.X. and W.Q. and supervision, W.X. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. The concentration (a) and diameter (b) of nanobubbles vary with the concentration of SPA.

Figure 2. The relationship between the concentration (a) and diameter (b) of nanobubbles and the standing time.

Figure 3. The effect of pulp pH (a) and SPA concentration (b) on the flotation recovery of rutile and garnet: (a) the concentration of SPA is 50 mg/L, and (b) the pH of the pulp is 4.2.

Figure 4. Effects of SPA and nanobubbles on zeta potential of rutile (a) and garnet (b) surfaces.

Figure 5. Effects of SPA and nanobubbles on the solid–liquid interface contact angle of rutile (a) and garnet (b) surfaces.

Figure 6. Adsorption model of SPA and nanobubbles on rutile surface.

Table 1

Chemical analysis results of rutile and garnet.

Sample	Component Content
	Na2O	MgO	Al2O3	SiO2	TiO2	CaO	MnO	Fe2O3
Rutile	-	-	-	0.73	97.59	0.32	0.61	0.51
Garnet	0.75	5.26	16.76	38.15	1.72	9.12	0.51	26.94

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