Mechanism of Modified Ether Amine Agents in Petalite and Quartz Flotation Systems under Weak Alkaline Conditions

Abstract

To investigate the flotation separation behavior of petalite and quartz, various methods were employed in this study. These included micro-flotation experiments, a contact angle analysis, zeta potential analysis, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to explore the separation mechanism of a modified ether amine reagent (L0-503) for petalite and quartz under weakly alkaline conditions. The micro-flotation test results indicated that the modified ether amine collector had a higher collecting ability for quartz than for petalite, with a maximum recovery rate of 93.2% for quartz and a recovery rate consistently below 14% for petalite in the presence of L0-503. This indicates that the modified ether amine reagent can be used as a reverse flotation agent for separating petalite and quartz. The separation mechanism results showed that the modified ether amine reagent had a significantly higher adsorption capacity for quartz than for petalite due to a strong reaction between the quartz and the secondary amine (-NH=) on the modified ether amine collector. Additionally, the electrostatic force and hydrogen bonding between the reagent and quartz further enhanced the adsorption, while no reaction occurred between the reagent and petalite.

1. Introduction

With the rapid increase in the penetration of electric vehicles and the rapid development of the energy storage industry, the proportion of demand related to the upstream industry of lithium resources continues to rise [1,2,3,4]. Lithium ores with industrial mining value include spodumene, lepidolite, petalite, zinnwaldite, and phosphoric lithium alumina [5,6,7]. Petalite is a skeletal silicate in which one of the five Si⁴⁺ ions is replaced by Al³⁺. The AlO⁴⁺ tetrahedral sheet is connected to the SiO⁴⁺ tetrahedral sheet parallel to {001} and Li⁺ is in a twisted tetrahedral coordination [8,9]. Due to the previously abundant supply of lithium pyroxene, petalite was not valued or utilized, resulting in a serious waste of assets, but with the dramatic increase in demand for lithium from new energy industries [10] and the continuous discovery of petalite ores around the world, the effective utilization of petalite could alleviate the current shortage of lithium resources [11].

Flotation is a widely used beneficiation method that separates a target mineral from a vein, and most lithium ores in the world are treated using this method [12,13,14,15]. Quartz is the main vein mineral of petalite. Because they have similar crystal structures and surface physicochemical properties, their flotation properties are identical, and achieving the flotation separation of the two has been one of the challenges in the mineral processing community [16,17,18]. Amine traps are the main flotation agents for silicate minerals at this stage [19]. Li et al. [20] synthesized a new amine trap and investigated the trapping performance of the new web on white mica and quartz. The results showed that the agent had adsorption in both minerals, and mechanism analysis showed that electrostatic adsorption had occurred on the surface of the quartz and chemisorption had occurred on the surface of the white mica. Fu et al. [21] quantified the surface active site of dodecylamine-adsorbed quartz and the surface complexation. Liu et al. [22] synthesized an agent with better foaming ability than DDA: N-dodecyl-isopropanolamine using dodecylamine and 1,2-epoxypropane as raw materials in an ethanol solution at 35 °C. The new trap was also found to have excellent floatability and selectivity for quartz, and the most commonly used in practice is the traditional cationic trap dodecyl amine (DDA), which generally traps quartz to achieve the reverse flotation of quartz with other minerals [23,24,25]. DDA suffers from low selectivity, low water solubility, and acidic conditions in the flotation process, resulting in a low concentrate grade, low flotation, low efficiency, and strong acids that are harmful to the environment [26,27]. Therefore, selecting a suitable trapping agent is the critical point of the process [28,29,30].

Ether amines are obtained by introducing ether groups into the hydrocarbons of aliphatic amines, which are more easily dispersed in mineral slurries due to their lower melting points and higher solubility [31,32,33]. Zhu et al. [34] found that the alkyl diamine ether GE-609 had a good trapping ability for rhodochrosite, calcite, and quartz, and by examining the mechanism, it was found that the alkyl diamine ether division acted on the mineral surface via both physical and chemical adsorption. Jiang et al. [35] found that ethylenediamine and polyetheramine in a quartz–feldspar flotation system under neutral pH conditions could preferentially float quartz and achieve the disjunction of feldspar and quartz. Usually, the separation of these two minerals is under the action of hydrofluoric acid and a strong acid, but these methods can cause irreversible harm to the environment as well as the equipment. This study used separation under weak alkaline conditions, which can effectively avoid these problems and provide a green and environmentally friendly beneficiation method. Therefore, a modified ether amine cationic trap (L0-503) was synthesized in the laboratory. It had better trapping performance for minerals due to its lower melting point and higher solubility under weak alkaline conditions, and had better trapping, water solubility, and selectivity compared to common amine traps.

At this stage, researchers have extensively studied the field of the flotation of quartz [17,18], but little research has been conducted in the field of petalite flotation [36]; therefore, this study investigated the selective capturing effect of a modified ether amine reagent (L0-503) in petalite and quartz flotation systems. Using various techniques including micro-flotation experiments, contact angle measurements, zeta potential determination, Fourier transform infrared spectroscopy (FTIR) measurements, and XPS measurements, the flotation performances and adsorption mechanisms of L0-503 in the petalite and quartz flotation systems were revealed. A possible adsorption model of L0-503 in the petalite flotation system is proposed. This study provides new insights and a theoretical basis for improving the separation of petalite and quartz in the flotation process.

2. Materials and Methods

2.1. Materials and Reagents

Pure minerals of petalite were obtained from the Bikita mine in Zimbabwe, Africa, and pure minerals of quartz were obtained from Ye’s mineral specimen house in Huadu District, Guangzhou. After manual selection, mechanical crushing, grinding, and screening using a three-head mill, particles of −74~+38 microns were selected for flotation and particles of −2 µm were selected for mechanistic analysis. Figure 1 shows the results of an X-ray diffraction analysis of petalite and quartz.

Table 1. Chemical compositions of petalite and quartz (wt%).

Contents	SiO2	Li2O	Al2O3	Fe2O3	CaO	MgO
Petalite	78.94	4.44	15.90	0.28	0.074	0.01
Quartz	98.47	-	0.027	0.43	0.12	-

provides a multi-element analysis of the specimens. The purities of the petalite and quartz were 90.98 wt% and 98.47 wt%, respectively.

An analytical-grade dodecyl amine (DDA) agent was obtained from the Tianjin Guangfu Development Co. laboratory for the independent synthesis of a modified ether amine trapping agent (L0-503). All reagents were prepared before use. In order to adjust the pH value of the mineral slurry, we used sodium hydroxide (NaOH) and concentrated sulfuric acid (H₂SO₄) solutions. It should be noted that all of the inorganic reagents that we used were of analytical grade and were purchased from the National Pharmaceutical Group. Furthermore, deionized water was used in all of the experiments to ensure the accuracy and reliability of the experimental results. It is worth mentioning that the adoption of these measures can effectively guarantee the comparability and accuracy of the experimental data and conform to the rigor requirements of scientific research in mineral processing.

2.2. Micro-Flotation Tests

Single-mineral flotation tests were carried out using an XFG hanging tank flotation machine with a flotation temperature of room temperature. Micro-flotation tests of single and artificially mixed minerals were carried out using a 30 mL flotation tank. We added 2.0 g of an ore sample (with an artificial mixing ratio of 1:1) and 20 mL of deionized water to the flotation cell. After stirring the mixture for 2 min at room temperature, we set the rotation speed of the flotation machine to 1400 revolutions per minute and added sufficient deionized water to the flotation cell to start the experiment. The practical steps were as follows: First, the pH adjuster (H₂SO₄ or NaOH), which was prepared in advance, was added, and the slurry was adjusted for 1 min. Then, the trapping agent, the pH adjuster, and the foaming agent (pine alcohol oil) were added in turn, stirring for 3 min. The whole process adopted the manual scraping bubble technique; flotation for 5 min; filtering and drying of the froth product and the bottom of the tank concentrate, respectively; and calculation of the recovery by product weight and grade.

2.3. Contact Angle Measurements

A JC2000A contact angle tester was utilized in order to carry out the task of contact angle determination. In the beginning, chunks of quartz ore and petalite ore were chopped apart and polished until they had flat surfaces. After the surfaces had been washed and dried, they were subjected to a treatment with a collecting solution for ten minutes. Finally, the processed samples were transferred to a measurement platform, where they were subjected to three separate measurements.

2.4. Zeta Potential Measurements

The zeta potentials of the minerals were measured using a Nano-ZS90 Malvern zeta potentiometer. The specific operation involved grinding the mineral sample to −2 μm using a three-head grinder, adding distilled water, adding a certain amount of reagent, stirring for half an hour, adding a pH adjuster to adjust the pH, stirring for 10 min, and injecting the upper dilution into the test tank of the zeta potential analyzer to measure the zeta potential. Each test was measured three times, and the average value was taken as the final test value.

2.5. Fourier Transform Infrared Spectroscopy (FTIR) Measurements

To characterize the interaction between the mineral surface and the reagent, a Nicolet 380 FTIR spectrometer (Thermo Scientific, Inc.) (81 Wyman Street, Waltham, MA, USA) was used to detect the FTIR spectra of the mineral samples before and after the reagent treatment. Prior to the measurement, the mineral samples were ground to below −2 µm using a three-headed grinder and then mixed with a specific concentration of the reagent adjustment solution to meet the experimental standard. The mixture was stirred for 30 min under pH 9 conditions and filtered using a filter press. The filtered samples were air-dried naturally to obtain the samples for the FTIR analysis.

2.6. XPS Measurements

In order to analyze the elemental compositions of the mineral surfaces and further understand the adsorption mechanisms of the agents on the mineral surfaces, XPS spectra were measured using a Thermo Fisher Scientific Escarabo 250Xi X-ray Photoelectron Spec Gauge. During the analysis, XPS Peak (version 4.1) software was used, and a C1s peak calibration spectrum with a binding energy of 248.70 eV was used.

3. Results and Discussion

3.1. Micro-Flotation Tests

Figure 2 depicts the relationships between the petalite and quartz recoveries with the L0-503 and DDA concentrations. The flotation recovery of quartz increased from 25.65% to 94.54% with increasing L0-503 engagement and reached stability at 4 × 10⁻⁴ mol/L. However, the flotation recovery of petalite was only 10.43%, regardless of the increase in the amount of L0-503. With the increase in the DDA concentration, the flotation recovery of quartz increased from 44.22% to 79.52% and stabilized at 4 × 10⁻⁴ mol/L with 72.75% quartz recovery. When the DDA concentration reached 4 × 10⁻⁴ mol/L, the flotation recovery of petalite reached 60.6%. Therefore, at pH = 9, the trapping performance of L0-503 was better than that of the conventional trapping agent DDA.

The pulp pH affects the surface properties of minerals, the flotation activity of the agent, the form of adsorption of the agent on the mineral surface, etc. [37]. The effect of pH on the flotation of petalite and quartz was investigated at the same concentration of L0-503 and DDA (both 4 × 10⁻⁴ mol/L). As shown in Figure 3, with the addition of L0-503, the recovery of quartz reached 79.1% and that of petalite reached 89.3% under acidic conditions at pH = 5. This indicates that the agents had strong trapping effects on both minerals under acidic conditions, but could not achieve their separation purpose. The recovery of petalite reached the lowest point, and as the pH reached 10, the recovery of quartz reached 80.3% and the recovery of petalite reached 20.5%. Thus, at this concentration, 9 is the ideal pH for the separation of these two minerals. Based on the literature [38], it is known that amine trapping agents mainly rely on electrostatic adsorption on the mineral surface, and we speculate that under acidic and alkaline conditions, the pulp pH changes the surface electrical energy of the minerals, which changes the trapping capacity of the trapping agent, and the trapping capacity of the trapping agent changes. The recovery of quartz after the addition of DDA first increased and subsequently declined as the pH rose, peaking at pH 7. Petalite recovery increased initially, reduced as the pH climbed, and reached its peak at pH 8. Despite the similar recoveries of the two minerals at this concentration, the separation efficiency did not reach the desired target.

In addition, artificial mixed-mineral flotation tests were conducted to verify the selectivity of L0-503 at a pH of 9. The flotation results of the mixed ore before and after using L0-503 are shown in Figure 4.

The histogram in Figure 4 shows that after adding L0-503 at pH 9, quartz exhibited fairly good floatability, with a recovery of over 92.9%, when using L0-503 as a trapping agent. However, the recovery of petalite was consistently around 13%. The folded graph shows the Li₂O grade of the concentrate at the bottom of the tank, which gradually increased with the increasing chemical concentration up to a maximum of 4.5%. The mixed ore test showed that L0-503 could effectively separate petalite and quartz.

3.2. Contact Angle Analysis

The contact angle can be used as a physical quantity to characterize the wettability and surface free energy of solid surfaces, and it has a wide range of functions in the field of flotation [27]. Different reagents can be used to increase the wettability of minerals. Figure 5 depicts the contact angles of quartz and petalite under different conditions.

According to the results shown in Figure 5, the contact angles of quartz and petalite before treatment with the reagent were 36.5° and 25.9°, respectively. This was in good agreement with the contact angle of quartz in the literature [38]. As can be seen in the figure above, using the same concentration of the treatment agent, the contact angles of quartz and petalite changed. The difference was that the contact angle of quartz changed more, from 36.5° to 69.5°, while the contact angle of petalite did not change much, only increasing from 25.9° to 37°. From these phenomena, it can be seen that the hydrophobic effect of the addition of L0-503 on quartz was significantly greater than that on petalite.

Therefore, it can be inferred that L0-503 may be adsorbed more on the quartz surface than on the petalite surface, thus creating the necessary conditions for the flotation recovery of quartz particles.

3.3. Zeta Potential Analysis

By measuring the zeta potentials of mineral surfaces, information on the distribution of surface charges can be obtained, thereby exploring the interaction mechanism between minerals and reagents. This testing method is widely used in fields such as mineral processing, mineral flotation, and mineral adhesion, and plays an important role in optimizing mineral processing and efficiently utilizing mineral resources [28]. The pH range of the possible zeta test falls within the same parameters as the flotation pH range. Figure 6 illustrates the differences in the zeta potentials of quartz and petalite before and after treatment with 4 × 10⁻⁴ mol/L L0-503 in a variety of pH slurry conditions.

This suggests that the mineral samples may have been able to satisfy the requirements of the test. As can be seen in Figure 6, the zeta potentials of petalite and quartz consistently showed decreasing trends when the pH of the slurry was increased throughout the tested range. This phenomenon was observed across the whole pH spectrum. The zeta potential curves of petalite show that following the L0-503 treatment, there was not much of a change in the surface charge of the petalite, and the two curves tended to overlap. According to the findings, L0-503 either did not have any adsorption on the surface of the petalite or had extremely poor adsorption there. According to the analysis of the zeta potential curve of quartz, it was noticed that the surface potential of quartz underwent significant changes after treatment with L0-503, indicating the important influence of L0-503 on the surface charge of quartz. This is the explanation for the outstanding floatability of quartz when L0-053 was used as a trapping agent. In the range of pH values over 10, the zeta potential difference in the quartz before and after the L0-503 treatment decreased, which may have been the reason for the decrease in quartz flotation recovery at pH values over 10 in the micro-flotation tests, and we speculate that this was due to a change in the action between the agent and the mineral caused by increasing the pH [34].

The forms that the minerals take in the pulp are relevant to the adsorption that takes place between the mineral surface and the capturing agent. Under varying pH conditions, the electronegativity and potential of the surfaces of quartz and petalite shift. After coming into contact with the agent surface, the zeta potential analysis made it abundantly evident that the quartz and petalite surfaces showed significant amounts of strong negative electronegativity on the quartz surface and a comparatively low amount of relatively weak negative electronegativity on the petalite surface. After the L0-503 treatment, the zeta potential of the quartz was positively shifted, and the zeta potential of the petalite was negatively shifted to a small extent, which indicates that the presence of L0-503 had a significant effect on the zeta potential of the quartz, with a maximum difference of 24, while the L0-503 had no significant effect on the zeta potential of the petalite. L0-503 had little effect on the zeta potential of the petalite, and in light of these results, we hypothesize that the trapping agent interacted electrostatically on the surface of the quartz thereby causing a significant change in its zeta potential [37].

3.4. Infrared Spectra Analyses

Figure 7 displays the results of an FTIR analysis performed on petalite and quartz before and after the L0-503 treatment. Using the necessary databases and published studies [28], the infrared spectra of the surfaces of both minerals were analyzed before and after the action of L0-503. These studies focused on the changes that occurred in the functional groups of the two minerals.

Several unique absorption peaks are seen at various wavenumbers in the infrared spectra of L0-503. The stretching vibration of the secondary amine (-NH=) is specifically responsible for the absorption peak at 3360 cm⁻¹, and the peaks at 2960 cm⁻¹, 2925 cm⁻¹, 2854 cm⁻¹, and 1406 cm⁻¹ correspond to the asymmetric stretching vibration of the methyl groups in the aliphatic hydrocarbon chain, the symmetric stretching vibration of the methylene groups, and the bending vibration of the methylene groups. Peaks at 1164 cm⁻¹, 1054 cm⁻¹, 775 cm⁻¹, and 692 cm⁻¹ in the spectra of quartz are, respectively, typical peaks of Si-O, Si-O-Si, and O-Si-O [28,29,30]. Therefore, the distinctive vibrations of Si-O, Si-O-Al, Si-O-Si, and Al-O-Al are responsible for the peaks in petalite at about 1180 cm⁻¹, 1066 cm⁻¹, 1007 cm⁻¹, and 730 cm⁻¹, respectively.

After interacting with L0-503, the quartz surface exhibited two distinct firm absorption peaks at 2964 cm⁻¹ and 2927 cm⁻¹, demonstrating that solid adsorption occurred on the material. These peaks were associated with the -CH₂ and -CH₃ functional groups and were found on the surface of the quartz. These two peaks that had not undergone significant displacement were natural for L0-503, which indicates that the interaction between L0-503 and quartz had more of a physical nature than a chemical one. This was indicated by the fact that the shifts took place. On the other hand, as shown in Figure 7b, the spectrum of the petalite was practically unaffected by the L0-503 treatment, in contrast to the spectrum of the quartz. This suggests that there was almost no adsorption of L0-503 on the petalite, which was the cause of the weak trapping effect that L0-503 had on the petalite.

3.5. XPS Analysis

The XPS data are displayed in

Table 2. XPS characterization of reference compounds on the quartz sample.

Ingredients	Element Content (%)
	C	O	N	Si
Quartz	25.47	48.68	-	25.01
Quartz with L0-503	38.21	40.51	1.03	20.25

and Figure 8 before and after the quartz was treated with 4 × 10⁻⁴ mol/L L0-503 at pH = 9. Table 2 and Figure 8 show that the N element signal peak appeared on the surface of the quartz following the reaction with 4 × 10⁻⁴ mol/L L0-503, demonstrating the substance’s successful adsorption. The relative concentration of the elements on the quartz surface was also altered. Specifically, following treatment, the concentration of the element C rose from 25.47% to 38.21% on the surface of the quartz, whereas the concentrations of the elements O and Si fell from 48.68% to 40.51% and from 25.01% to 20.25%, respectively. These findings support L0-503’s adsorption on the surface of quartz.

According to the results shown in Figure 9a, the presence of C1s peaks could be observed. These peaks were located at 288.3 eV, 284.7 eV, and 285.7 eV, corresponding to the C elements in C-C, C-H, and C-N, respectively. Additionally, according to the results shown in Figure 9b, the presence of O1s peaks could be observed. These peaks were located at 533 eV, 532.3 eV, and 531.9 eV, corresponding to the O elements in Si-OH, C-O-C, Si-O-Si, and Si-O⁻. It is important to note that no new peaks were detected on the quartz surface after treatment with L0-503, indicating that chemical bonding was not involved, which was consistent with the FTIR results. Furthermore, the C1s spectra showed overall shifts of −0.3 eV, 0.6 eV, and 0 eV, while the O1s spectra showed overall shifts of 0.6 eV, −0.1 eV, and −0.3 eV. These results indicate that the oxygen atoms on the quartz surface are involved in this process, and hydrogen bonding may be the reason for the energy shifts observed in the C1s and O1s photoelectron spectra.

Figure 10 shows a schematic representation of the supposed adsorption process. This picture was created by integrating the findings of the micro-flotation, contact angle, zeta potential, FT-IR, and XPS tests. The main reason for the adsorption of the collector on the quartz surface is electrostatic action, and a part of it is hydrogen bonding, which is shown in the figure. It is hypothesized that when weak alkaline conditions are present after the agent first enters the pulp, a portion of the secondary amino group (-NH=) on the agent molecule is electrostatically adsorbed with the O on the quartz surface. Additionally, it is possible that, with an increase in dosage, a portion of the trapping agent (L0-503 molecule) can also be adsorbed on the quartz surface via the hydrogen bonding force.

4. Conclusions

In this study, the flotation performance and the selective separation mechanism of the laboratory-synthesized modified ether amine L0-503 trap were characterized for quartz and were further assessed and analyzed by means of micro-flotation tests as well as contact angle analysis, zeta potential analysis, infrared spectroscopy, and XPS analysis.

(1)

The modified ether amine L0-503 trap, which was synthesized in the laboratory, achieved good trapping performance for quartz flotation recovery, while it had little effect on the flotation of petalite, indicating that L0-503 is an efficient selective trap.

(2)

The flotation results of single minerals showed that L0-503 had good catching ability and selectivity for quartz, and the recovery of pure quartz ore reached more than 94.0% at a pulp pH of 9.0 and an L0-503 concentration of 4 × 10⁻⁴ mol/L. Furthermore, the flotation results of the artificially mixed minerals showed that the recovery of quartz exceeded 92.9% when the concentration of L0-503 was 4 × 10⁻⁴ mol/L and the pulp pH was 9; however, the recovery of petalite remained at about 13%. The separation of quartz and petalite was better, and the results of single-mineral flotation and artificial mixed ore flotation showed that the petalite and quartz could be effectively separated under weak alkaline conditions.

(3)

According to the findings of the contact angle test, L0-503 adheres to the quartz surface more effectively than the petalite surface. The trapped L0-503 is adsorbed on the quartz surface via electrostatic forces, according to the zeta potential studies, it is presumed that the trapping agent L0-503 is adsorbed on the quartz surface by electrostatic force, and the adsorption of L0-503 molecules with the quartz surface is physical rather than chemical adsorption according to FTIR spectra. The XPS data indicate that electrostatic and hydrogen-bonded adsorption are both involved in the adsorption of L0-503 on the quartz surface. The results of the flotation tests are supported by the zeta potential, FTIR, and XPS measurements.