Selective Separation Behavior and Study on the Interaction Mechanism of 2-Hydroxy-3-Naphthylmethyl Hydroxamic Acid and Cassiterite

Abstract

In this study, the performance of 2-hydroxy-3-naphthylmethyl hydroxamic acid (NHA) in cassiterite flotation was investigated. The aim was to understand the mechanism of action of NHA for the first time. The effect of NHA as a collector in the flotation separation of cassiterite, calcite, and quartz was investigated via microbubble flotation experiments. The experimental results showed that the maximum recovery of cassiterite in the presence of NHA was 91.76%. This was attributed to the selective adsorption of NHA on the cassiterite surface. NHA showed a stronger collection performance for cassiterite than calcite and quartz. Therefore, the mechanism of NHA-cassiterite interaction was investigated using zeta potential, the logarithmic solubility plot (LSD), Fourier Transform Infrared (FT-IR), X-ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscope (SEM) analyses. This study introduces a new adsorption process and mechanism for the NHA-based adsorption of cassiterite. The results show that, under neutral conditions, the solute components of cassiterite surface lattice ions mainly exist in the form of Sn–OH complexes. Chemisorption occurs between cassiterite and NHA which is adsorbed onto the cassiterite surfaces through interaction with O sites, rather than Sn sites as is traditionally expected. Further, the hydroxyl and hydroxyl groups of NHA are chemically coordinated with Sn–OH on the surface of cassiterite to form a six-membered chelate ring. This proposed mechanism can be extended to most systems in which metal ions interact with hydroxamic acids bearing hydroxyl groups. This contributes to a better understanding of the activation mechanism of hydroxamic acid collectors in cassiterite flotation.

1. Introduction

Tin plays a crucial role as a strategic mineral resource, particularly in its significance for national defense and high-tech industries. Additionally, the level of national industrialization can be measured by the presence and utilization of tin [1,2]. There are more than fifty types of tin ores that have been identified, including cassiterite, stannite, herzenberg, thoreaulite, varlamoffite, etc. Among them, cassiterite (SnO₂) is the most economically significant tin-bearing mineral, and is the main source of industrial tin production [3]. According to a special report on the non-ferrous metal industry, with the increasing scarcity of resources and the widening gap between supply and demand, the world’s current tin reserves can only be mined for a further 15 years. Therefore, the effective separation of tin resources has become an urgent problem to be solved in China and the world.

Cassiterite, commonly known as a heavy mineral, exhibits a specific gravity ranging from 6.8 to 7.1 [4,5]. Due to the relatively high specific gravity of cassiterite, the beneficiation of cassiterite along with other heavy minerals can be achieved by means of gravity separation when the gravity property differences between cassiterite and other minerals are significant. It is often found in association with quartz and calcite. The specific gravity of quartz and calcite is relatively low, making gravity separation an important technique for separating cassiterite from quartz and calcite. However, gravity separation methods are only effective when the particle size is suitable. Fine-grained cassiterite present in the raw ore, as well as tin minerals that become muddy during the grinding process, tend to float on the slurry’s surface due to hydraulic and mechanical forces, leading to inevitable loss through overflow. As a result, gravity separation alone is not sufficient for effective recovery, and flotation has emerged as an effective method to recover fine cassiterite and minimize tin metal loss [6].

Beneficiation workers at home and abroad have done a great deal of work and developed various cassiterite beneficiation reagents for flotation, such as salicylhydroxamic acid (SHA), benzohydroxamic acid (BHA), etc. [7,8]. However, there are problems, such as low selectivity, high cost, and environmental pollution.

Cassiterite flotation could be achieved using fatty acids, arsenic acids, hydroxamic acids, phosphoric acids, and alkyl sulfosuccinic acids. Hydroxamic acids, as typical chelating surfactants, have been used in the flotation of metal oxide ores and rare earths [9]. Studies have shown that salicylhydroxamic acid (SHA), benzohydroxamic acid (BHA) and NHA can all be used as collectors for bastnaesite flotation. Before the application of NHA, BHA and SHA have been widely used in the industrial practice of cassiterite flotation and have achieved good flotation indicators [9,10]. However, with the pursuit of environmental protection and economic effects, BHA and SHA cannot well meet the needs of mineral processing. SHA has strong collection performance, but it is expensive and the tail water is red in color, which poses environmental problems; BHA has weak collection performance and requires a large amount of chemicals.

The compound 2-hydroxy-3-naphthylmethyl hydroxamic acid (NHA) is a di-active group collector containing hydroxyl and hydrochloric acid. It has been used in the flotation separation of bastnaesite and calcium-containing gangue minerals [11,12]. It has the advantages of low cost, environmental protection, etc. [13]. Applying NHA to cassiterite flotation is a new approach. The flotation mechanism of naphthalene hydroxamic acid and cassiterite has not been well studied and no detailed reports have been provided.

In this study, the flotation mechanism of NHA on fine cassiterite was investigated, employing zeta potential experiments, chemical analysis of metal ion flotation solution, infrared spectroscopy, the logarithmic graph dissolved components of cassiterite, SEM, and XPS analyses.

2. Materials and Methods

2.1. Materials

The cassiterite samples used in this study were obtained from Wenshan, Yunnan province, China. The samples were first crushed manually, and relatively high-grade particles were hand-selected by visual observation. Then, each sample was ground using a ceramic ball mill to obtain −74 + 37 µm size fractions for the flotation experiments. Finally, the ground cassiterite sample was soaked in hydrochloric acid (HCL), rinsed repeatedly with deionized (DI) water, dried, and transferred to a jar for use (list of experimental reagents is shown in

Table 1. List of experimental reagents.

Agent Name	Molecular Formula	Purity	Manufacturer
Deionized (DI)	H2O	Analytically pure.	BGRIMM Technology Group, Beijing, China
Waterhydrochloric acid	HCI	Analytically pure.	Sinopharm Chemical Reagent Co. Ltd., Shanghai, China
Sodium hydroxide	NaOH	Analytically pure.	Sinopharm Chemical Reagent Co. Ltd.
2-hydroxy-3-naphthylmethyl hydroxamic acid	C11H8O3	Analytically pure.	BGRIMM Technology Group
Pure terpenic oil	C10H18O2	Analytically pure.	Sinopharm Chemical Reagent Co. Ltd.

). The −37 µm size fraction was further ground to −5 µm (d 90) for the zeta potential measurements, FT-IR analysis, and XPS characterization studies.

The endowment of tin resources in China is relatively poor, with approximately 90% of these resources being multi-component co-associated tin resources. These tin resources are typically accompanied by gangue minerals such as carbonate mineral calcite and silicate mineral quartz. The presence of these gangue minerals ultimately results in a low recovery rate and poor grade of cassiterite.

For our experiments, the purity of several minerals was determined using X-ray diffraction analysis. As seen in Figure 1a, the pattern of the diffraction peak recorded from the cassiterite is very similar to the standard diffraction peak, and there are no impurity peaks, indicating that the cassiterite sample had high purity. Chemical composition analysis also verified that the grade of cassiterite was 97.40% (

Table 2. Chemical composition of cassiterite, calcite, and quartz.

Element (%)	Sn	CaCO3	SiO2	MgO	Fe	K2O	Cu
Cassiterite	76.72	–	–	0.010	0.010	0.011	<0.005
Calcite	–	99.42	0.22	0.052	–	–	<0.005
Quartz	–	–	99.21	–	–	–	–

). The XRD results for calcite and quartz samples, seen in Figure 1b,c, also indicated that the purity of both samples was high. The elemental content of several minerals was measured by titration. The chemical composition analysis showed that the mass content of calcite was 99.42% and the quartz was 99.21%, both of which were above 95%. They were thus qualified to be used for the experiments.

As a collector, 2-hydroxy-3-naphthylmethyl hydroxamic acid (NHA) with more than 90% purity was used. Analytical-grade hydrochloric acid (HCI) and sodium hydroxide (NaOH) were used to adjust the pH of the suspensions, and analytical pure terpenic oil was used as a frother. DI water with a conductivity of 2.0 × 10⁻⁵ s·m⁻¹ was used for all experiments.

2.2. Methods

2.2.1. Micro-Flotation Experiments

The pure mineral flotation experiments were carried out as shown in Figure 2. First, the flotation experiments were performed using a XFG 150 flotation machine (JILIN EXPLORATION MACHINERY, Changchun, China) at 1499 rpm. In each test, 2.0 g mineral particles were cleaned using an ultrasonic cleaner, then put into a 40 mL plexiglass cell filled with an appropriate amount of DI water, and mixed using a magnetic stirrer for 1 min. The pH of the slurry was maintained at the expected value using HCl and NaOH. After 2 min of continuous conditioning, the collector was added into the suspension at a pre-determined concentration and mixed for an additional 2 min. Finally, the frother was added and agitated for 2 min. The froth was scraped for 4 min (scraped once every 15 s, for a total of 16 times). After the flotation experiment was over, the floated products and sinks were dried and weighed, and the flotation recovery was calculated.

2.2.2. Zeta Potential Measurements

The zeta potential measurements were performed using a Nano ZS90 analyzer from Malvern Instruments (Malvern, Worcestershire, UK). 40 mg of −5 μm (d 90) sample was mixed with 40 mL of 1 × 10⁻³ mol/L KCl as a background electrolyte solution. The system was magnetically stirred for 2 min to thoroughly disperse the samples. The pH of the slurry was adjusted with HCl or NaOH, and then the required concentration of the reagent NHA was added to the system. After the adjustment, the particles in the slurry were allowed to settle down for 5 min and the pH was recorded. The supernatant was collected for the zeta potential measurement. In this study, the zeta potential of each sample was measured three times and the average value was reported as the final value. The experimental error for the measurements is ±0.5 mV.

2.2.3. FT-IR Spectroscopy Analysis

A Nicolet 6700 FT-IR Spectrometer (Thermo Scientific, Waltham, MA, USA) was used to determine the infrared spectra of beneficiation agents and minerals. The KBr tablet pressing method was used to complete the infrared spectroscopy measurement in the wavelength range of 400–4000 cm⁻¹. The pure mineral particles were ground to −5 μm in an agate mortar and a suspension was prepared by adding 1.0 g of the pure mineral particles to 40 mL DI water in the absence and presence of chemicals at the desired pH, which was conditioned for 30 min. After the action was over, the ore pulp was filtered, rinsed with DI water 4 times, dried naturally, and sent for infrared spectroscopy analysis.

2.2.4. XPS Analysis

The XPS analysis before and after the interaction of cassiterite and NHA was performed on an ESCALAB 250Xi XPS analyzer (Thermo Scientific, Waltham, MA, USA). All elements in the measured samples were detected by a survey scan, and the high-resolution XPS spectra of a targeted element were then collected. All spectra were calibrated according to the C1s spectrum at a binding energy of 284.8 eV for charge compensation.

For each test, 2.0 g of the mineral samples were dispersed into the aqueous solution, then stirred according to the flow of the flotation test, adjusted to the required pH, and the collectors were added to extend the action test of minerals and agents to half an hour. Next, the samples were filtered and dried naturally at room temperature. Finally, the XPS tests were carried out.

3. Results and Discussion

3.1. Flotation Experiments

As shown in Figure 3a, the recovery of cassiterite flotation was over 80% in the pH range of 5.0 to 9.0. Specifically, the highest recovery rate of cassiterite was obtained at 91.76% at pH 7.0. This indicates that NHA has a wide practical range regarding the pH of cassiterite flotation. At the same time, it has a strong ability to collect cassiterite. However, there is also evidence that NHA has a strong ability to capture quartz at pH = 7.0. Under alkaline conditions, the changes in pH showed little effect on the recovery of calcite.

In further trials, Figure 3b showed that the recovery of cassiterite flotation increases from 35.90% to 89.68% within the dosage range of 0~10 mg/L collector, which is significantly different from the recovery of quartz and calcite.

As a novel collector, NHA introduces a fresh perspective on achieving efficient flotation of cassiterite. In this study, the mechanism of action between NHA and cassiterite was focused on.

3.2. Zeta Potential Measurements

Cassiterite initially forms a hydroxylated surface in an aqueous solution. The adsorption or dissociation of H⁺ on the mineral surface also produces a protonated surface (Sn–OH₂⁺) and a deprotonated surface (Sn–O⁻), which makes the mineral surface charge different (Figure 4).

Figure 5 reveals the zeta potential of cassiterite as a function of pH. The purpose of this measurement was to determine the interaction between flotation reagents and mineral surfaces. It can be seen from Figure 5 that, in the DI water system, the experimentally measured zero point of charge (ZPC) of cassiterite is approximately 4.0, which is consistent with the results in the literature [14]. After the addition of NHA, the zero point of charge and the negative shift of the zeta potential appear. In other words, the agent adsorbs to the cassiterite. When pH > 4.0, the surface of cassiterite is negatively charged. The presence of this reagent leads to a negative shift of the zeta potential on the surface of cassiterite, indicating that the agent and cassiterite have a bonding effect, which is chemical adsorption. When the pH was in the range of 6.0–9.0, the dynamic potential decreased sharply, indicating that more drugs were adsorbed on the cassiterite surface at this time, which improved the flotation recovery rate. This is consistent with the results of the mineral flotation test.

3.3. The Logarithmic Graph Dissolved Components of Cassiterite (LSD)

Figure 6 demonstrates the logarithmic solubility plot (LSD) of cassiterite. It can be seen noticeably that under different pH conditions, the dissolution of cassiterite surface lattice ions in the solution produces different component forms. Under acidic conditions, the dissolved components of cassiterite mainly exist in the form of Sn⁴⁺, Sn(OH)³⁺, Sn(OH)₂²⁺, Sn(OH)₃⁺, etc.. Under alkaline conditions, there are mainly dissolved cassiterite components in the form of Sn(OH)₅⁻ and Sn(OH)₆²⁻. Under neutral conditions, the cassiterite dissolved components mainly exist in the form of Sn(OH)_4(a), accompanied by a small amount of dissolved cassiterite components of Sn(OH)₃⁺ and Sn(OH)₅⁻ [15].

3.4. FT-IR Results

To analyze the changes in surface functional groups and the adsorption mechanism, FT-IR spectroscopy analysis was carried out between the different reagent conditions. Figure 7 shows the FT-IR spectra of NHA, cassiterite, and cassiterite treated with NHA. In the FT-IR spectrum of NHA, the peak of 3276.33 cm⁻¹ can be attributed to the overlap stretching vibrations of N–H and O–H groups, which is the characteristic peak of hydroxamic acid [16,17]. Additionally, 1636.65 cm⁻¹ is the asymmetric stretching vibration peak of carbonyl C=O [18,19]. There are two peaks at 1610.21 cm⁻¹ and 1535.20 cm⁻¹, which are the peaks of the conjugate structure in NHA; at 1355.54 cm⁻¹ is the phenol O–H in-plane variable-angle oscillation absorption peak [20]. In the case of cassiterite, the one at 639.45 cm⁻¹ corresponds to the characteristic peak of SnO₂ [21]. The absorption peaks at 3442.39 cm⁻¹ and 1637.94 cm⁻¹ are mainly due to the stretching vibration and bending vibration of water molecules OH on the surface of cassiterite [22].

Figure 7 presents the FT-IR spectra of cassiterite in the absence and presence of NHA. When cassiterite was treated with NHA alone, several new peaks appeared at 1637.60 cm⁻¹ and 1543.64 cm⁻¹, corresponding to the peak at 1610.21 cm⁻¹ and 1535.20 cm⁻¹. The overlapping absorption peak of N–H and O–H at 3276.33 cm⁻¹ disappeared, which may be related to the chemical adsorption of NHA on the surface of cassiterite. At the same time, the C=O peak at 1636.65 cm⁻¹ widened and the stretching vibration was weakened. This may be related to providing a lone pair of electrons with the O atom in the C=O bond to chemically coordinate with the Sn on the surface of the cassiterite, thereby reducing the electron cloud density of the C=O conjugated valence bond [20]. In addition, the in-plane variable-angle oscillation absorption peak of the O–H bond in NHA migrated from 1354.54 cm⁻¹ to 1384.96 cm⁻¹, and the migration amount was 30.42 cm⁻¹. This may be attributed to the phenolic hydroxyl groups of NHA being involved in chemical bonding.

3.5. XPS Analysis

The cassiterite samples were subjected to the XPS analysis under various reagent conditions to further study the interaction mechanism between NHA and cassiterite. The molar concentrations of elements on the surface of cassiterite after different chemical treatments are shown in Figure 8.

The molar percentage concentrations of cassiterite pure mineral and cassiterite + NHA are displayed in order from left to right. Compared with pure cassiterite minerals, treated with NHA, the N element content on the surface of cassiterite increases from 0% to 1.83%, indicating that NHA is adsorbed on the cassiterite surface.

It can be seen from Figure 9a that the narrow-range scanning spectrum of Sn element on the surface of cassiterite is composed of the characteristic double peaks of Sn3d. The binding energies differ by 8.41 eV, which indicates that Sn⁴⁺ is in a normal oxidation state in SnO₂ crystals [23,24].

Figure 9b is a high-resolution scanning pattern of Sn element on the surface of cassiterite after adding NHA. It can be assured that the peak position of Sn3d 5/2 is at the position of the binding energy of 486.36 eV, and the peak position of Sn3d 3/2 is at the position of the binding energy of 494.73 eV [25]. The peaks of Sn3d 3/2 and Sn3d 3/2 are not shifted, designating that the chemical environment of Sn on the surface of cassiterite has not changed after the addition of NHA [26]. It may be that the oxygen atoms on the surface of cassiterite are chemically bonded to NHA.

In order to further explore the mechanism of action of NHA on the surface of cassiterite, a high-resolution scanning analysis of O1s element on the surface of cassiterite was carried out, as shown in Figure 9c. It can be ascertained that the pure cassiterite mineral is composed of three peaks, which correspond to the positions of the binding energies of 530.2 eV, 531.49 eV, and 532.52 eV in the map, respectively [27,28]. The peak at the binding energy of 530.2 eV is attributed to the lattice oxygen (O₃²⁻) on the surface of cassiterite, and the peaks at 531.49 eV and 532.52 eV are, respectively, attributed to the oxygen in the water molecule on the surface of cassiterite; that is, the bridge oxygen (O_bri²⁻) and Sn–OH (O²⁻) [29,30].

Figure 9d is a high-resolution scanning pattern of O element on the surface of cassiterite after the action of NHA. The increased peak of 532.18 eV is the characteristic peak of hydroxamate –C(=O)NHO [31], bespeaking that NHA is adsorbed on the surface of cassiterite. Meanwhile, the three peaks of the pure O1s element of cassiterite are shifted from the positions of binding energies of 530.2 eV, 531.49 eV, and 532.52 eV to the positions of 530.26 eV, 531.33 eV, and 532.85 eV. This indicates that the oxygen atoms’ chemical environment of the surface-treated cassiterite has changed. In particular,, 0.3 eV shift of Sn–OH(O²⁻) attributed to 532.52 eV indicates that the oxygen sites of Sn–OH on the cassiterite surface are the main active sites. The characteristic peaks of phenol–OH in NHA were not found in the narrow-range scanning spectrum of O1s after the action of NHA, which may be due to cassiterite Sn–OH and phenol–OH dehydration. That caused the deviation of Sn–OH oxygen sites to shift.

3.6. Scanning Electron Microscopy (SEM) Analysis

Figure 10a,b displays images captured by the Scanning Electron Microscope (SEM) magnified 1000 times on the surface of cassiterite before and after NHA treatment. It was evident that after the NHA treatment, some tiny particles can be observed on the surface of the cassiterite. Further magnification of 10,000 times of the images before and after NHA treatment revealed that prior to the application of NHA, the surface of the cassiterite was exceptionally smooth, once again confirming its exceptionally high quality with almost no impurities. Following the NHA treatment, the absorption of NHA onto the surface of the cassiterite can be clearly observed.

3.7. Adsorption Model and Discussion

Based on these analyses, we present an adsorption model to elucidate the impact of NHA on cassiterite flotation (Figure 11). NHA chemically adsorbs onto the surface of cassiterite at pH 6.0–9.0. Therein, the oxygen atom in the carbonyl group (C=O bond) in NHA is capable of providing a lone pair of electrons for chemical coordination with tin (Sn) on the surface of cassiterite. Additionally, the phenolic hydroxyl group (–OH) is combined with the Sn–OH group (where oxygen is the active site) on the cassiterite surface, resulting in the formation of a new chelate ring, leading to a firm adsorption of NHA on the cassiterite surface.

4. Conclusions

In this study, 2-hydroxy-3-naphthylmethyl hydroxamic acid (NHA) was applied to cassiterite flotation, with its underlying mechanism also first under discussion. The flotation experiment results showed that the collecting ability of NHA towards cassiterite was strong in the pH range of 5.0–9.0, working best in neutral conditions and needing only a small amount of NHA to achieve a good collection effect on cassiterite. Zeta potential measurements indicated the chemical adsorption between NHA and cassiterite, which was supported by the FT-IR analysis results. The logarithmic graph of dissolved components of cassiterite (LSD) shows that under neutral pH conditions, the dissolved components of cassiterite surface lattice ions in solution mainly exist in the form of Sn–OH complexes. The XPS analysis further proved that the active site for the chemisorption of cassiterite and NHA is the O site of Sn–OH. The hydroxyl and hydroxamic acids groups of NHA are chemically coordinated with Sn–OH on the surface of cassiterite to form a six-membered chelate ring. Finally, through SEM analysis, the adsorption between cassiterite and NHA is observed and confirmed at the microscopic level. This study contributes to enriching the understanding of the activation mechanism of hydroxamic acid for the flotation of cassiterite. The proposed mechanism can be generalized to most systems where metal ions interact with hydroxamic acid-bearing hydroxyl groups.