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Article

Utilization of an Amide-Based Collector in Fluorite Flotation

by
Peng Liu
1,
Yuhui Tian
2,
Chun Zhang
1,
Mengjie Tian
1,* and
Wei Sun
1
1
School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China
2
Beijing Research Institute of Chemical Engineering and Metallurgy, China National Nuclear Corporation (CNNC), Beijing 101149, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(5), 1609; https://doi.org/10.3390/pr13051609
Submission received: 20 March 2025 / Revised: 4 May 2025 / Accepted: 19 May 2025 / Published: 21 May 2025
(This article belongs to the Section Chemical Processes and Systems)
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Abstract

:
Fluorite is commonly associated with calcite and other Ca-bearing gangue minerals in nature. Fluorite and its associated Ca-bearing gangue minerals share similar surface active sites involving Ca2+ ions and have comparable surface properties, making their flotation separation challenging. Traditional fatty acid collectors, such as oleic acid, suffer from poor selectivity. This study investigates the use of N-hydroxy-N-phenyloctanamide (HPOA) as a novel collector for fluorite, with the goal of improving its flotation separation from Ca-bearing gangue minerals. Flotation tests demonstrate that HPOA provides superior selectivity compared to oleic acid in separating fluorite from calcite. Research on the adsorption capacity of HPOA on mineral surfaces shows that, under equivalent testing conditions, HPOA shows greater adsorption amounts on fluorite than on calcite. As a result, HPOA has a superior collecting capacity in fluorite flotation. First-principles calculations reveal that HPOA adsorbs on the fluorite surface by forming chemical bonds with Ca2+ ions via its hydroxyl and carbonyl groups. Moreover, HPOA exhibits strong adsorption on fluorite surface, as indicated by more significant shifts in the binding energies of Ca2+ ions on fluorite compared to calcite after HPOA adsorption. This study highlights the potential of amide-based collectors to improve fluorite flotation and offers valuable insights into the development of more selective flotation reagents.

1. Introduction

Fluorite (CaF2) holds strategic significance as a non-metallic mineral with broad industrial applications, including use as a flux in the metallurgical industry, a raw material for hydrofluoric acid production, and a mineralizer in cement manufacturing. These extensive applications have earned fluorite the title of “second rare earth”. In natural deposits, fluorite is commonly associated with gangue minerals such as calcite (CaCO3), which results in low fluorite contents in ores [1]. To increase the fluorite contents, fully utilize its fluxing property, and reduce hydrofluoric acid production costs, achieving fluorite purity by separating it from gangue minerals is essential. Since fluorite particles in ores are typically finely disseminated, fine grinding is required to achieve sufficient liberation before separation. This process also produces significant quantities of fine-grained fluorite and gangue mineral particles. Flotation offers the highest efficiency for fine-grained mineral separation among different separation strategies. Flotation is a key physical beneficiation method used to separate minerals based on the differences in hydrophobicities between valuable minerals (e.g., fluorite) and gangue minerals (e.g., calcite). The effectiveness of this process depends on enhancing the differences in hydrophobicities between valuable and gangue minerals. To achieve this, the use of collectors is essential [2,3,4].
Fatty acids represent the most extensively used class of collectors in fluorite flotation, with oleic acid being a typical example [5,6,7]. The attachment of these fatty acids to the fluorite surface is facilitated by their chemical interaction with surface Ca2+ ions. However, they also engage with the calcite surface Ca2+ ions. Since the bonding strengths of fatty acids with the two mineral surface Ca2+ ions are very similar, their adsorption quantities on both minerals are nearly identical. As a result, fatty acid collectors yield comparable flotation recoveries for both fluorite and calcite, highlighting their poor selectivity.
To enhance the selectivity of fatty acid collectors, current approaches include: (1) combining them with depressants, (2) using them alongside other types of collectors, and (3) developing novel collectors based on the molecular structures of conventional fatty acid collectors. In fluorite flotation, sodium silicate stands out as the most prevalent depressant. In slightly acidic to slightly alkaline environments, even small amounts of sodium silicate can strongly inhibit the flotation of siliceous minerals such as quartz (SiO2). However, as the dosage of sodium silicate increases and the slurry pH rises to strongly alkaline levels, sodium silicate also significantly suppresses fluorite flotation. Consequently, when sodium silicate depressant and fatty acid collectors are combined for the selective flotation separation of fluorite and other siliceous minerals, it is crucial to precisely control both sodium silicate depressant dosage and slurry pH [8,9,10]. Beyond siliceous minerals like quartz, chemically modifying sodium silicate through salting and acidification treatments enhances its effectiveness in depressing the flotation of Ca-containing minerals such as calcite [11,12]. Salting treatment typically involves metal ions such as Al3+ or Pb2+ [13,14,15]. Besides sodium silicate, other commonly used depressants in fluorite flotation include organic depressants such as tannic acid, which primarily inhibit the flotation of Ca-bearing gangue minerals [16,17,18].
Compared to using fatty acid collectors alone, mixed collectors offer superior performance. Studies have shown that with an oleic acid/dodecylamine mixed collector, oleic acid initially adsorbs onto the fluorite surface, forming a uniform adsorption layer. Subsequently, dodecylamine intercalates between adsorbed oleic acid molecules on the fluorite surface, extending the apparent length of oleic acid’s hydrophobic carbon chain and increasing its apparent adsorption capacity. This mixed collector, combined with modified sodium silicate, enables effective flotation separation of fluorite from calcite [19]. In fluorite flotation, oleic acid/amide mixed collectors exhibit excellent selectivities [20,21,22], particularly a mixture of oleic acid and erucamide. The mixture retains the strong collecting ability of oleic acid for fluorite while exhibiting significantly reduced collecting ability for calcite. Consequently, oleic acid/erucamide mixed collectors show remarkable selectivity in separating fluorite from calcite [22]. However, the mechanisms underpinning this selectivity remain inadequately understood, especially the reasons for the diminished collecting ability for calcite, which require further investigation.
Introducing new polar groups or modifying existing ones in the molecular structures of current collectors is a crucial strategy for developing collectors with enhanced selectivities [23,24,25,26,27]. In fluorite flotation, oleic acid, a commonly used collector, is modified by replacing its carboxyl group with an N-(2-hydroxyethyl)-amide group and an N-hydroxy-amide group, leading to the synthesis of two novel collectors: the amphoteric collector N-(2-hydroxyethyl)octadec-9-enamide (NOE) [28] and the hydroxamic acid collector N-hydroxy-9-octadecenamide (N-OH-9-ODA) [29,30,31,32]. In practical fluorite ore flotation tests, the NOE collector achieves a higher grade and recovery rate in a flotation concentrate (CaF2 grade 90.28% vs. 84.66%, recovery rate 76.68% vs. 67.53%), while using only half the amount of oleic acid collectors [28]. To address the low solubility of N-OH-9-ODA, its molecular structure is modified by introducing an epoxy group and two hydroxyl groups at the double bond position, resulting in the synthesis of two additional hydroxamic acid collectors: N-hydroxy-9,10-epoxy-octadecanamide (N-OH-9,10-O-ODA) and N,9,10-trihydroxyoctadecanamide (THODA) [26,33,34]. These new hydroxamic acid collectors exhibit excellent selectivities in the flotation separation of fluorite and calcite [26,33]. Beyond these two hydroxamic acid collectors, recent advancements in fluorite flotation have led to the development of hydroxamic acid collectors containing two N-hydroxy amide groups. These molecules, such as decyl-bishydroxamic acid, and octylamino-bis-(butanohydroxamic acid), feature the presence of two N-hydroxy amide groups per molecule [24,27,35].
Building on the design principle of these novel collectors, this study investigates a new collector, N-hydroxy-N-phenyloctanamide (HPOA), for fluorite and calcite flotation separation. The molecular structure of this collector is a modification of the octyl hydroxamic acid collector, with the H atom of the imine group replaced by a phenyl group. Using flotation tests, zeta potential measurements, and other experimental methods, this study explores the selectivity and interaction mechanism of HPOA in fluorite flotation. The findings highlight the potential application of this class of novel collectors, N-hydroxy-N-(phenyl or alkyl)-alkylamides, in fluorite flotation, validate the feasibility of this novel design approach, which involves restructuring the molecular structures of existing collectors, and offer valuable knowledge for future advances in flotation collector development.

2. Materials and Methods

2.1. Materials

Fluorite and calcite particulate samples utilized in this investigation were derived from Hunan Province, China. These materials were processed from bulk crystals of the respective minerals. For flotation tests, particles ranging from 38 to 74 μm in size were used, while particles smaller than 5 μm were employed for zeta potential (ZP) measurements and X-ray photoelectron spectroscopy (XPS) analysis. Presented in Figure 1 are the X-ray diffraction (XRD) patterns of the fluorite and calcite samples, which exhibit no impurity peaks, confirming the exceptional purity of the used samples. Reagents used included N-hydroxy-N-phenyloctanamide (HPOA), oleic acid, N,9,10-trihydroxyoctadecanamide (THODA), hydrochloric acid (HCl), sodium hydroxide (NaOH), and terpineol. HPOA and THODA were synthesized in the laboratory. The synthesis method for THODA is described in a previous publication by our research team and is not repeated here [26]. The detailed synthesis procedure is provided below, and its nuclear magnetic resonance (NMR) hydrogen and carbon spectra are depicted in Figure 2. Oleic acid and NaOH were of analytical grade, the HCl solution was purchased with a concentration of 37%, and terpineol had a purity of over 95%.
The synthesis of HPOA was conducted in four steps. In the first step, N-phenylhydroxylamine was synthesized. Initially, 0.20 mol of ammonium chloride and 0.16 mol of nitrobenzene were transferred into a 500 mL three-neck flask at room temperature, followed by 320 mL of water. The reaction mixture was stirred while gradually being heated to 60–65 °C. Then, 0.35 mol of zinc powder was introduced in three portions over 20 min, with continuous stirring for an additional 20 min. The reaction was then halted, and the solution was hot filtered. The filtration temperature was maintained between 60 and 65 °C. The filtrate received 2 mol of sodium chloride, which was mixed thoroughly. After stirring, the solution was allowed to crystallize at a low temperature of 0 to 4 °C for 12 h, followed by filtration. The obtained crystals were recrystallized using dichloromethane and petroleum ether to yield N-phenylhydroxylamine as an intermediate product. In the second step, HPOA was synthesized. The 0.1 mol of N-phenylhydroxylamine from the initial step, along with 0.2 mol of triethylamine, was dissolved in 100 mL of dichloromethane. The solution was cooled to −5 to 0 °C, and stirring was initiated immediately. The addition of 0.15 mol of octanoyl chloride was performed dropwise, followed by 30 min of stirring once the addition was finished. The product was then washed with 50 mL of 1 mol∙L−1 dilute HCl, after which the organic phase was separated. This organic layer was further washed with 50 mL of 10% sodium hydroxide solution, and the solvent was evaporated to obtain crude HPOA. In the third step, the crude product was purified using column chromatography. The fourth step involved acidification. The purified product was dissolved in 1 mol∙L−1 dilute HCl solution and stirred thoroughly for 30 min. Dichloromethane extraction was carried out on the solution, followed by the separation of the organic fraction and its evaporation to give the HPOA product.

2.2. Micro-Flotation Experiment

Oleic acid serves as the predominant fatty acid collector in fluorite flotation. To validate the superior selectivity of the novel collector HPOA, comparative flotation separation tests of fluorite and calcite were conducted using HPOA and oleic acid collectors, respectively. Flotation experiments were performed using an XFG-Ⅱ type hanging-cell flotation machine (Changchun Exploration Machinery Factory, Changchun, China). The flotation procedure was as follows: A flotation cell (40 mL) was filled with 2 g of either fluorite or calcite particles, followed by the introduction of 35 mL of water. Stirring commenced immediately, and the mineral particle suspension was subjected to pH adjustment. After stabilizing the pH for 3 min, either HPOA or oleic acid was added. Following 3 min of stirring, terpineol frother was introduced. Following a one-minute interval, particles that were buoyed up in flotation were manually collected with a scraper over a period of 3 min, constituting a flotation concentrate. Following filtration of the suspension, the solids retained on filter paper were collected as a flotation tailing. Both the concentrate and tailing were subjected to drying, followed by mass determination through weighing. The mineral flotation recovery was computed by calculating the ratio of the concentrate weight to the total weight of the concentrate plus the tailing, multiplied by 100%.

2.3. ZP Tests

Under neutral and alkaline conditions, hydroxamic acids predominantly exist in their anionic forms within the slurry. Upon adsorption onto mineral surfaces, hydroxamate anions lower their zeta potentials [36,37,38]. These shifts vary in magnitude according to the amounts of hydroxamic acids adsorbed, with a larger shift indicating a higher degree of adsorption. Consequently, this study evaluates the adsorption amounts of HPOA on fluorite and calcite surfaces by measuring the extents of the shifts in their ZPs post HPOA adsorption. ZP measurements of mineral surfaces were conducted with a ZEN3690 potential meter (Malvern Instruments Ltd., Malvern, UK). The following steps were employed to measure the ZP values: fluorite or calcite particles (35 mg) were incorporated into a beaker filled with 35 mL of water. The pH modification was performed while stirring the suspension. Once a stable pH level was achieved, stirring continued for an additional 3 min, followed by the addition of a specified amount of HPOA. The suspension was stirred for another 3 min and then left undisturbed for 10 min. This standing treatment step allows mineral particles larger than 10 μm to settle under gravity, ensuring that the suspended particles in the upper clear liquid meet the instrument’s detection criteria (i.e., particles smaller than 10 μm). Finally, the supernatant was extracted using a syringe for ZP measurements.

2.4. X-Ray Photoelectron Spectroscopic (XPS) Tests

The study employs XPS to semi-quantitatively measure the adsorption of HPOA on fluorite and calcite surfaces, as well as to characterize its adsorption strengths [39,40,41]. The ESCALAB Xi+ spectrometer (Thermo Fisher Scientific, East Grinstead, UK) was employed for XPS characterization. Operational parameters included: monochromatic Al Kα X-ray source (1486.6 eV photon energy), 12.5 kV accelerating voltage, 16 mA filament current, and ultrahigh vacuum environment (8 × 10−10 Pa) in the analysis chamber. The methodology employed in the sample preparation for XPS testing is analogous to that utilized for the ZP tests, with two key differences: first, the stirring time of the slurry after adding HPOA for XPS testing was extended to one hour, compared to the ZP tests, to ensure sufficient adsorption of HPOA onto fluorite and calcite surfaces, and the mineral particles collected on the paper were washed three times with deionized water. Following the washing process, vacuum drying was applied to the particles prior to XPS testing.

2.5. First-Principles Calculations

First-principles calculations are employed to optimize and compare the adsorption configurations of HPOA molecules on fluorite and calcite surfaces, as well as to calculate and compare their adsorption energies. These calculations are performed using VASP (version 5.4.4) software. The calculation parameters and methodology for constructing the initial configurations follow those outlined in the authors’ previous work and, therefore, will not be repeated [26,42].

3. Results and Discussion

3.1. Micro-Flotation Experimental Results

To investigate the flotation separation of fluorite and calcite, the research initially fixed HPOA collector dosages at 10−4 mol∙L−1 and examined the separation results under varying pH conditions. As illustrated in Figure 3a, increasing fluorite pulp pH from 6 to 7 leads to a slight improvement in flotation recovery, reaching a maximum of over 90%. However, further increases in pH cause a sharp decline in recovery, dropping below 40% at pH 12. In contrast, calcite flotation recoveries show little variation, staying close to 30% within the pH range of 6 to 10, but decrease significantly to around 10% at pH 12. At pH 7, the recovery difference of close to 60% between fluorite and calcite reflects the high selectivity of the HPOA collector for fluorite over calcite. Since fluorite recovery peaks at this pH, subsequent experiments were conducted at pH 7 to evaluate the flotation separation results under varying HPOA dosages.
As depicted in Figure 3b, increasing HPOA dosage from 5 × 10−5 mol∙L−1 to 10−4 mol∙L−1 gradually enhances fluorite recovery from approximately 60% to over 90%. Beyond this concentration, the recovery rate plateaus, showing only a marginal further increase. Unlike fluorite, calcite recovery improves steadily with higher HPOA collector dosages, showing no signs of plateauing even at elevated concentrations. At HPOA dosages of 5 × 10−5 mol∙L−1, 10−4 mol∙L−1, and 1.5 × 10−4 mol∙L−1, calcite recoveries reach approximately 10%, 30%, and over 40%, respectively. Using 10−4 mol∙L−1 of HPOA, fluorite and calcite show a recovery difference of almost 60%, showcasing the outstanding selectivity of the HPOA collector.
When the oleic acid collector is used, fluorite and calcite recovery curves intersect at pH 8. Below pH 8, fluorite recoveries are slightly higher than those of calcite, whereas above pH 8, calcite recoveries marginally exceed those of fluorite (Figure 3a). Both mineral recoveries are consistently at high levels across the full pH range under investigation, with fluorite exceeding 70% and calcite surpassing 60%. Both minerals exhibit similar trends: their recoveries initially increase, peak at pH 10, and subsequently decline. At pH 10, the maximum flotation recovery of fluorite is approximately 80%, while that of calcite reaches around 90%. Since fluorite recovery peaks at pH 10, subsequent experiments investigating the effect of oleic acid dosage on fluorite−calcite separation results are conducted at a fixed pH of 10 for both minerals. When oleic acid collector is employed, the largest difference in flotation recoveries between fluorite and calcite across the studied pH range is about 10%, indicating the limited selectivity of oleic acid; a commonly used collector in fluorite flotation. This finding underscores the urgent need to develop new, highly selective collectors to replace oleic acid.
As shown in Figure 3b, increasing oleic acid dosage results in similar flotation recovery trends for both fluorite and calcite: initial gradual increases, followed by slower rises, with turning points at a dosage of 10−4 mol∙L−1 oleic acid, where the increase rates decline. When oleic acid dosages are less than or equal to 10−4 mol∙L−1, calcite exhibits slightly higher flotation recoveries than fluorite, with differences of approximately 5%. Moreover, when the dosages exceed 10−4 mol∙L−1, fluorite and calcite recoveries become nearly identical. With oleic acid dosed at 5 × 10−5 mol∙L−1, fluorite recovery is at its lowest, slightly below 80%. As the dosage increases to 10−4 mol∙L−1, fluorite recovery rises to nearly 90%. When the dosage reaches 1.5 × 10−4 mol∙L−1, fluorite recovery further rises to approximately 100%. The data in Figure 3b indicate that oleic acid collector by itself fails to achieve effective flotation separation of fluorite and calcite, highlighting its poor selectivity.
THODA is a novel hydroxamic acid collector developed by our research team [26]. Its molecular structure is derived from oleic acid through two modifications: (1) replacing the carboxyl group with an N-hydroxy amide group, and (2) introducing two hydroxy groups into the carbon chain. Flotation test results (Figure 3c) demonstrate that, under neutral pH conditions, THODA exhibits an excellent selectivity in the fluorite-calcite separation system, similar to HPOA. As depicted in Figure 3d, at an HPOA dosage of 4.5 × 10−5 mol∙L−1, the flotation recovery of fluorite reaches nearly 90%, while the calcite recovery is less than 20%, with a recovery difference of approximately 70%. In comparison, under optimal experimental conditions, using the HPOA collector results in a flotation recovery difference of approximately 60% between fluorite and calcite. These results clearly demonstrate that the selectivity of THODA collector in the fluorite-calcite flotation separation system slightly exceeds that of HPOA.

3.2. ZP Measurements

Figure 4a illustrates the ZP values of fluorite/calcite surfaces at varying pH values. Compared to fluorite slurry without HPOA, the addition of HPOA induces significant negative alterations in the fluorite surface ZPs. With rising slurry pH, the negative shift becomes less pronounced. Fluorite surface zeta potentials experience negative shifts of about 30 mV in absolute terms at pH 6 and 7. Moreover, when the pH rises to 10, the absolute value of the negative shift drops to 10 mV. As an amide reagent, HPOA predominantly exists in its anionic form under neutral and alkaline conditions. Upon adsorption of HPOA anions on the fluorite surface, negative alterations in the ZPs occur, with the magnitudes of these alterations being directly linearly correlated with the surface-bound HPOA concentrations. These results suggest that, for slurries with pH values above 7, the adsorption of HPOA on fluorite decreases as the pH increases. This trend aligns with the lowered fluorite flotation recovery as the slurry pH increases when using an HPOA collector (Figure 3a).
Within the pH range of 6 to 10, the addition of HPOA to calcite slurry causes slight negative shifts in the zeta potentials of calcite surface versus the slurry without HPOA (Figure 4a). The sizes of these shifts are similar across the pH range, with their absolute values near 5 mV. This observation points to the fact that, in this pH interval, the adsorption levels of HPOA on the calcite surface remain relatively constant. Consequently, with the HPOA collector, calcite flotation recoveries show little variation within this pH range (Figure 3a). Within this pH range, HPOA introduction induces much greater shifts in the fluorite surface zeta potentials relative to the calcite surface. This indicates that the levels of HPOA adsorbed on fluorite exceed those on calcite. As such, with the HPOA collector, fluorite flotation recoveries surpass those of calcite by a significant margin within the specified range (Figure 3a). The distinct contrast in HPOA adsorption between fluorite and calcite surfaces facilitates their selective flotation separation at the equivalent settings. When the slurry pH exceeds 10, the negative shifts in the ZPs of the calcite surface are relatively smaller compared to the pH range below 10. As a result, when using HPOA collector, the flotation recovery of calcite decreases significantly at pH values above 10 (Figure 3a).
At pH 6–9, the fluorite surface exhibits strong positive charges, while the calcite surface shows much weaker positive charges. Consequently, HPOA anions are more strongly attracted to the fluorite surface via electrostatic interactions, which likely accounts for the greater adsorption of HPOA on fluorite compared to calcite in this pH range. When the slurry pH exceeds 10, the fluorite surface becomes strongly negatively charged, while the calcite surface exhibits weaker negative charges. Despite the considerable electrostatic repulsion between HPOA anions and fluorite, HPOA adsorption amounts on fluorite still outperform those on calcite within this pH range. This suggests that electrostatic repulsion does not significantly reduce HPOA adsorption on fluorite. Furthermore, when an HPOA collector is used, fluorite flotation recoveries consistently outperform those of calcite within this pH range. These observations indicate that the attachment of HPOA to both fluorite and calcite surfaces is not solely governed by electrostatic attraction. The attachment pathway of hydroxamic acids to mineral surfaces involves both direct adsorption through the formation of chemical association with surface metal ions and indirect adsorption via the formation of complexes with dissolved metal ions in slurry, followed by their attachment to mineral surfaces [43,44,45].
As depicted in Figure 4b, with the slurry pH fixed at 7, the ZP values of the surfaces of fluorite and calcite were measured at varying HPOA concentrations. The results indicate that as HPOA concentration increases to 10−4 mol∙L−1, the fluorite surface zeta potential undergoes a rapid reduction, falling from about 25 mV to nearly −5 mV. Beyond this point, further increases in HPOA concentration led to slower, continued decreases in the zeta potential. This suggests that in the range of HPOA concentrations, from 0 to 10−4 mol∙L−1, HPOA adsorption on fluorite increases rapidly as its concentration rises. At 10−4 mol∙L−1, HPOA adsorption reaches saturation, and additional increases in HPOA concentration result in only marginal increases in adsorption. Consequently, when an HPOA collector is used, the flotation recovery of fluorite increases sharply as HPOA concentration rises within this concentration range. As HPOA concentration continues to rise, the rate of recovery improvement slows. The point at which this deceleration occurs is with HPOA at 10−4 mol∙L−1 concentration (Figure 3b). In contrast, over the complete range of HPOA concentrations tested (0 to 1.5 × 10−4 mol∙L−1), the calcite surface zeta potential steadily decreases with increasing HPOA concentration, without exhibiting signs of deceleration. This implies that within this concentration range, HPOA adsorption on calcite continues to increase without reaching saturation as HPOA concentration rises. As a result, when an HPOA collector is used, calcite flotation recovery shows a continuous rise with higher HPOA concentrations, without a slowdown in the rate of improvement (Figure 3b). Under identical HPOA concentration conditions, the negative shifts in fluorite surface zeta potentials are significantly larger versus calcite, indicating that, at the same concentrations, HPOA adsorption amounts on the fluorite surface are much greater. This leads to far greater fluorite recoveries as opposed to calcite in flotation using an HPOA collector (Figure 3b).

3.3. X-Ray Photoelectron Spectroscopic (XPS) Tests Results

HPOA, a hydroxamic acid, contains abundant C and N elements in its molecular structure. When adsorbed onto fluorite and calcite surfaces, it enhances the C content and introduces N on these surfaces [46,47,48]. Figure 5a presents the C 1s XPS spectra of fluorite surfaces pre- and post-HPOA adsorption. In the absence of HPOA adsorption, two distinct C 1s peaks are recorded at binding energies (BEs) of 284.80 eV and 288.35 eV, typical of C-containing contaminants, such as carbon dioxide, adsorbed on the fluorite surface. After HPOA adsorption, a noticeable enhancement in the C 1s XPS line intensity at 286.41 eV is observed, indicating the emergence of a new peak. Peak fitting reveals the emergence of a new signal at this binding energy, indicative of HPOA adsorption on fluorite. This analysis suggests that the addition of HPOA to fluorite slurry results in significant adsorption of HPOA molecules onto the fluorite surface.
Figure 5c displays comparative C 1s XPS spectra profiles for calcite surfaces before and after HPOA adsorption. Peak fitting of the C 1s XPS line of the calcite surface prior to HPOA adsorption reveals three distinct peaks, located at BEs of 284.80 eV, 286.62 eV, and 289.53 eV. The two peaks at lower binding energies correspond to C-based pollutants adsorbed on the calcite surface, while the peak at the highest binding energy is associated with carbonate groups in calcite crystal. Following HPOA adsorption, the intensity of the C 1s XPS spectrum shows no significant change, indicating that only a few HPOA molecules adsorb on the calcite surface after HPOA is added to the calcite slurry. As a result, no distinct peaks indicative of HPOA adsorption on calcite are detected in the XPS analysis.
Table 1 presents the elemental compositions on fluorite and calcite surfaces pre- and post-HPOA adsorption. The results clearly demonstrate that, under the same testing condition, the C content rises by 5.71% on the fluorite surface upon HPOA adsorption, in contrast to a mere 3.26% increase on calcite. Additionally, N is detected on both the fluorite and calcite surfaces, with the N content on the fluorite surface being 0.77%, higher than the 0.39% observed on the calcite surface. HPOA adsorbs in greater quantity on fluorite than on calcite under the given conditions, as confirmed by these observations. Thus, HPOA proves to be more effective in collecting fluorite versus calcite in flotation tests.
Figure 5b,d display the Ca 2p XPS spectra of fluorite and calcite surfaces pre- and post-HPOA adsorption. Both surfaces contain the metal ion Ca2+. Upon adsorption of a hydroxamic acid, it donates electrons to the mineral surface Ca2+ ions, forming chemical bonds. The electron transfer lowers the electronic BEs of the Ca2+ ions on the mineral surfaces. The degree of the shift correlates with the number of electrons donated by the hydroxamic acid, which reflects the strength of its adsorption [49,50,51,52]. After HPOA adsorption on the fluorite surface, negative shifts of ~0.2 eV are observed for both Ca 2p peaks. In contrast, the negative shifts of both Ca 2p peaks on calcite are under 0.1 eV. This suggests that HPOA adsorption is enhanced on fluorite versus calcite. Consequently, the stronger adsorption of HPOA on fluorite results in elevated adsorption levels on fluorite under identical testing conditions.

3.4. First-Principles Calculations

This study utilizes first-principles calculations to optimize the adsorption configurations of HPOA molecules on the (111) surface of fluorite and the (104) surface of calcite and to calculate their adsorption energies. The results are used to characterize the adsorption strengths of HPOA on both mineral surfaces and to explain the differences in its adsorption amounts. Fig. 6a illustrates the optimized adsorption configuration of HPOA on fluorite (111) surface. The results show that the HPOA molecule forms two chemical bonds with the two adjacent Ca2+ ions on the fluorite surface via its hydroxyl and carbonyl O atoms, with bond lengths of 2.36 Å and 2.26 Å, respectively. In this configuration, the adsorption energy of HPOA on the fluorite surface is −78.51 kJ∙mol−1 (Table 2). The charge density difference map (Figure 6a) demonstrates significant electron accumulation between the O atoms of the HPOA molecule and the Ca2+ ions on the fluorite surface, suggesting strong bonding interaction. The Bader charge analysis further supports this conclusion: both the bonded Ca2+ ions and O atoms show varying increases in electron density, indicating significant electron localization in the Ca–O bond region, thereby forming stable chemical bonds. Figure 6b shows the optimized adsorption configuration of one HPOA molecule on the (104) surface of calcite. The closest distances between the hydroxyl and carbonyl O atoms of the HPOA molecule and the Ca2+ ions on the calcite surface are 2.65 Å and 2.56 Å, respectively, indicating that the HPOA molecule cannot easily form direct bonds with calcite surface Ca2+ ions. Moreover, the adsorption energy on calcite is positive (Table 2), which suggests that the molecule is less likely to adsorb via chemical bonding with the Ca2+ ions. Comparison of the optimized adsorption configurations and calculated adsorption energies of HPOA on the fluorite and calcite surfaces reveals more robust HPOA interaction with fluorite. Consequently, under identical experimental conditions, more HPOA molecules attach to the fluorite surface than to calcite. Additionally, the greater adsorption amounts of HPOA on fluorite surface are associated with the greater availability of Ca2+ ions on fluorite compared to calcite. Table 2 presents a Ca2+ ion surface density of 7.61 nm−2 for the (111) plane of fluorite, compared to 4.66 nm−2 for the (104) plane of calcite. The greater availability of Ca2+ ions on fluorite surface provides more adsorption sites for HPOA.

4. Discussion

In oxide mineral flotation, oleic acid serves as a commonly used fatty acid collector, recognized for its strong collecting ability. However, it suffers from poor selectivity [53,54,55,56]. To address the limited selectivity of traditional oleic acid collectors, the preliminary work of this study focuses on designing new, highly selective collectors to replace oleic acid by modifying its molecular structure. In one of our published papers, the carboxyl group in the oleic acid molecule is replaced with an N-hydroxy amide group, leading to the design and synthesis of a novel hydroxamic acid collector, N-hydroxy-9-octadecenamide (N-OH-9-ODA). This collector displays significant selectivity advantages in the flotation of oxide minerals, such as bastnaesite and spodumene [29,30,31,32]. However, like oleic acid, N-OH-9-ODA exhibits poor solubility. To overcome this limitation, further modifications are made to its molecular structure by introducing two hydroxyl groups at the C9–C10 double-bond position, resulting in the synthesis of a new collector, N,9,10-trihydroxyoctadecanamide (THODA). Compared to N-OH-9-ODA, THODA shows markedly improved solubility and superior selectivity as a collector in fluorite flotation [26]. These findings demonstrate that by introducing polar groups and other modifications to the existing collector molecular structures, it is possible to design new, highly selective collectors.
In oxide mineral flotation, octyl hydroxamic acid (OHA) is a commonly used collector [57,58,59]. Inspired by previous studies, this research designs a new highly selective collector, N-hydroxy-N-phenyloctanamide (HPOA), based on the molecular structure of OHA. Specifically, the hydrogen atom directly attached to the nitrogen atom in the OHA molecule is replaced with a phenyl group. The introduction of the phenyl group creates an extended π-electron conjugation system, which significantly reduces the electron cloud density of the N-hydroxy amide group through electron delocalization, thereby lowering its reactivity. The reactivities of the polar groups in collector molecules are often inversely related to their reaction selectivities. Therefore, the introduction of the phenyl group enhances the selectivity of the N-hydroxy amide group in the HPOA molecule.
In the flotation separation of fluorite and calcite, HPOA collector demonstrates a superior selectivity compared to oleic acid and a similar selectivity to THODA. Under the optimal conditions for each of the three collectors, the flotation recovery of fluorite with an HPOA collector is approximately 60% higher than that of calcite, whereas the recovery of fluorite using oleic acid is slightly lower than that of calcite, with a difference of about 10% (Figure 3b). When the THODA collector is used, the flotation recovery of fluorite is about 70% higher than that of calcite (Figure 3d).
The results of ZP measurements and XPS analysis show that, under identical experimental conditions, the adsorption levels of HPOA on the fluorite surface are significantly higher than on the calcite surface. This difference directly contributes to the selective flotation of fluorite by the HPOA collector. Experimental data show that, when the HPOA concentration ranges from 1 × 10−4 mol∙L−1 to 1.5 × 10−4 mol∙L−1 and slurry pH is 7, the ZP of fluorite surface shifts negatively by approximately 30 mV following the implementation of HPOA, while the ZP of calcite surface shifts negatively by only 5–10 mV (Figure 4b). XPS analysis further confirms this difference: The carbon content on the fluorite surface after HPOA adsorption increases by 5.71%, and the N content reaches 0.77%; in contrast, the carbon content on the calcite surface increases by only 3.26%, with a N content of 0.39% (Table 1). These data collectively demonstrate that HPOA exhibits selective adsorption behavior on both fluorite and calcite surfaces, which explains its selective collecting ability in the fluorite/calcite flotation separation system.
The difference in the adsorption quantities of HPOA on the surfaces of fluorite and calcite is primarily attributed to the varying adsorption strengths on these two mineral surfaces. XPS analysis shows that, after HPOA adsorption, the binding energies of Ca2+ ions on the fluorite surface shift negatively by approximately 0.2 eV, while the binding energies of Ca2+ ions on the calcite surface shift by less than 0.1 eV (Figure 5b,d). This observation suggests that the interaction between HPOA and Ca2+ ions on fluorite surface is stronger, involving greater electron transfer. First-principles calculations further demonstrate that the adsorption energy of HPOA on the fluorite surface is significantly higher than on the calcite surface (Table 2), which aligns with the XPS results and confirms that HPOA exhibits stronger adsorption on the fluorite surface.
Common collectors used in fluorite flotation include fatty acid-based and amide-based collectors. The novel HPOA collector is classified as an amide compound. From a sourcing perspective, fatty acid-based collectors are primarily derived from animal and plant sources, making them natural products, while amide-based collectors must be synthetically produced. In terms of environmental friendliness, amide-based collectors have distinct disadvantages: first, their inherent toxicity is significantly higher than that of fatty acid-based collectors; second, the raw materials used in their synthesis (such as hydroxylamine, acyl chloride, etc.) also possess some toxicity. To treat residual amide-based collectors in flotation wastewater, the current process primarily employs a “flocculation precipitation–Fenton oxidation” treatment, which effectively degrades these collectors [60,61].

5. Conclusions

This paper investigates the use of a novel amide-based collector, N-hydroxy-N-phenyloctanamide (HPOA), in fluorite flotation. Under optimal experimental conditions, flotation recovery rates of fluorite and calcite differ by nearly 60% when using an HPOA collector, whereas with oleic acid collectors, the recovery rates are very similar, with a maximum difference of about 10%. These results highlight that HPOA provides superior selectivity over oleic acid.
Zeta potential tests show that, after adding the same amounts of HPOA to fluorite and calcite slurries, more pronounced negative shifts in fluorite surface zeta potentials are an indication of increased HPOA adsorption on fluorite. X-ray photoelectron spectroscopic tests further confirm this by showing a greater increase in C content and the appearance of a higher N content on the fluorite surface after treatment with HPOA versus calcite. The greater adsorption amounts of HPOA on fluorite correlate with its stronger collecting ability in flotation.
First-principles calculations indicate that HPOA can form chemical bonds with Ca2+ ions on fluorite surface through its hydroxyl and carbonyl O atoms, facilitating its adsorption. In contrast, HPOA has difficulty interacting directly with Ca2+ ions on calcite. The optimized adsorption energy calculations show that HPOA has a negative adsorption energy on fluorite and a positive value on calcite, indicating that direct adsorption on calcite is thermodynamically infeasible.
This study highlights the potential application of an HPOA collector in fluorite flotation. A key future direction is to explore its feasibility in practical ore flotation. The superior selectivity of HPOA underscores the value of developing similar amide-based collectors, and further research into such collectors could advance fluorite flotation technology.
Hydroxamic acid collectors (e.g., octyl hydroxamic acid) are significantly more expensive than conventional fatty acid-based collectors (e.g., oleic acid), typically costing five to six times more. Moreover, HPOA is even more expensive than the commonly used octyl hydroxamic acid collector. The design of the novel HPOA collector is based on the molecular structure of octyl hydroxamic acid. HPOA synthesis is more complex than that of octyl hydroxamic acid, involving two key reactions: (1) the reduction of nitrobenzene to N-phenylhydroxylamine using a Zn-NH4Cl reduction system, and (2) nucleophilic substitution of N-phenylhydroxylamine with octanoyl chloride to form HPOA. In contrast, octyl hydroxamic acid synthesis requires only one nucleophilic substitution reaction (octanoyl chloride + hydroxylamine). Regarding raw material costs, HPOA synthesis involves multiple reagents (e.g., nitrobenzene, ammonium chloride, zinc powder, octanoyl chloride), while octyl hydroxamic acid synthesis requires only two main reagents (octanoyl chloride and hydroxylamine). The purification of HPOA is more challenging due to the formation of by-products, leading to a lower yield and increased purification difficulty compared to octyl hydroxamic acid. Given the complexity of HPOA synthesis, it can be inferred that its synthesis cost is higher. This high synthesis cost limits the industrial applicability of HPOA, with its theoretical research value currently outweighing its practical application potential. To enhance its practical use, breakthroughs should be pursued in the following areas: expanding application scenarios, where its use in high-value mineral flotation could reduce the overall cost of HPOA; developing mixed collectors that combine HPOA with less expensive auxiliary collectors to reduce the required amount of HPOA and its associated cost; creating more cost-effective synthesis processes for HPOA; and developing alternative flotation collectors with similar structures and performance to HPOA, but at lower cost.

Author Contributions

Data curation, W.S.; Writing—original draft, P.L.; Writing—review & editing, Y.T.; Visualization, C.Z.; Supervision, M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China (2024YFC2909600), the Youth Program of the Natural Science Foundation of Hunan Province, China (2025JJ60314), the Joint Innovation Fund Project of China Uranium Corporation Ltd. and the National Key Laboratory of Nuclear Resources and Environment at Donghua University (2023NRE-LH-20), and the Stable Support for Scientific Research Project of the R&D Platform of China National Nuclear Corporation (2023-431).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Yuhui Tian was employed by the company Beijing Research Institute of Chemical Engineering and Metallurgy, China National Nuclear Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The company Beijing Research Institute of Chemical Engineering and Metallurgy, China National Nuclear Corporation-companies in affiliation and funding had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. The XRD patterns of (a) fluorite and (b) calcite particles.
Figure 1. The XRD patterns of (a) fluorite and (b) calcite particles.
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Figure 2. (a) Proton NMR spectrum and (b) carbon-13 NMR spectrum of the synthesized HPOA sample.
Figure 2. (a) Proton NMR spectrum and (b) carbon-13 NMR spectrum of the synthesized HPOA sample.
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Figure 3. Flotation recoveries of fluorite and calcite using HPOA, oleic acid, and THODA collectors under varying pH values and collector dosages. (a,b) HPOA and oleic acid at a fixed dosage (10−4 mol∙L−1) and at varying dosages (pH fixed at 7); (c,d) THODA at a fixed dosage (2.5 × 10−5 mol∙L−1) and at varying dosages (pH fixed at 7).
Figure 3. Flotation recoveries of fluorite and calcite using HPOA, oleic acid, and THODA collectors under varying pH values and collector dosages. (a,b) HPOA and oleic acid at a fixed dosage (10−4 mol∙L−1) and at varying dosages (pH fixed at 7); (c,d) THODA at a fixed dosage (2.5 × 10−5 mol∙L−1) and at varying dosages (pH fixed at 7).
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Figure 4. ZPs of fluorite and calcite surfaces (a) at different pH values and (b) at varying HPOA dosages.
Figure 4. ZPs of fluorite and calcite surfaces (a) at different pH values and (b) at varying HPOA dosages.
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Figure 5. The XPS spectra of fluorite (a,b) and calcite (c,d) surfaces before and after HPOA adsorption.
Figure 5. The XPS spectra of fluorite (a,b) and calcite (c,d) surfaces before and after HPOA adsorption.
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Figure 6. (a) The optimized adsorption configuration of one HPOA molecule on the (111) surface of fluorite and its charge density difference map, and (b) the optimized adsorption configuration of one HPOA molecule on the (104) surface of calcite.
Figure 6. (a) The optimized adsorption configuration of one HPOA molecule on the (111) surface of fluorite and its charge density difference map, and (b) the optimized adsorption configuration of one HPOA molecule on the (104) surface of calcite.
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Table 1. Elemental analysis of fluorite and calcite surfaces prior to and following HPOA adsorption.
Table 1. Elemental analysis of fluorite and calcite surfaces prior to and following HPOA adsorption.
SampleElemental Content/%
C 1sO 1sCa 2pN 1sF 1s
Fluorite23.438.2123.53-44.82
Fluorite, HPOA29.1410.3520.790.7738.95
Calcite39.3746.6713.96--
Calcite, HPOA42.6344.3312.650.39-
Table 2. Adsorption energies (kJ∙mol−1) of HPOA molecules and Ca2+ ion densities (nm−2) on fluorite (111) and calcite (104) surfaces.
Table 2. Adsorption energies (kJ∙mol−1) of HPOA molecules and Ca2+ ion densities (nm−2) on fluorite (111) and calcite (104) surfaces.
Fluorite (111) SurfaceCalcite (104) Surface
HPOA Molecule Adsorption Energy−78.512.36
Ca2+ Ion Density7.614.66
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Liu, P.; Tian, Y.; Zhang, C.; Tian, M.; Sun, W. Utilization of an Amide-Based Collector in Fluorite Flotation. Processes 2025, 13, 1609. https://doi.org/10.3390/pr13051609

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Liu P, Tian Y, Zhang C, Tian M, Sun W. Utilization of an Amide-Based Collector in Fluorite Flotation. Processes. 2025; 13(5):1609. https://doi.org/10.3390/pr13051609

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Liu, Peng, Yuhui Tian, Chun Zhang, Mengjie Tian, and Wei Sun. 2025. "Utilization of an Amide-Based Collector in Fluorite Flotation" Processes 13, no. 5: 1609. https://doi.org/10.3390/pr13051609

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Liu, P., Tian, Y., Zhang, C., Tian, M., & Sun, W. (2025). Utilization of an Amide-Based Collector in Fluorite Flotation. Processes, 13(5), 1609. https://doi.org/10.3390/pr13051609

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