Application of Nanobubbles in the Flotation of Sulfide Minerals from Chilean Copper Porphyry Deposits

Abstract

Nanobubbles have recently been proposed as a promising technology to enhance mineral flotation; however, their behavior in real ores with complex mineralogy remains poorly understood. This study evaluates the effect of nanobubbles on the flotation of copper sulfide ores from Chilean porphyry deposits with contrasting clay contents. Two representative samples were analyzed: a low-clay-content ore (M1) and a high-clay-content ore (M2). Flotation tests were carried out in a 2.7 L forced-air cell, using kinetic experiments with and without nanobubbles and frother addition. The mineralogical composition was characterized by XRD and QEMSCAN, while SEM-EDS was used to analyze surface morphology and particle associations. The results showed that nanobubbles improved copper and molybdenum recoveries in M1 up to 7.5 and 20%, respectively, increasing both kinetics and final recovery, which was supported by SEM observations of clean surfaces and compact agglomerates. In contrast, in M2 the use of nanobubbles decreased flotation efficiency due to enhanced slime coating and the formation of non-selective agglomerates, which reduced the hydrophobicity of sulfide surfaces. Overall, this study demonstrates that the efficiency of nanobubbles strongly depends on ore mineralogy, offering advantages in clean systems but limitations in clay-rich ores, and highlights the need for mineral-specific strategies for their successful industrial application.

1. Introduction

Nanobubbles are often classified in two categories: surface nanobubbles which are bubbles that precipitate at a solid surface and have a height from 10 to 100 nm; and bulk nanobubbles which are spherical gas reservoirs with a diameter less than 1000 nm [1]. Both have gained significant attention over the years for their potential to improve kinetics for the removal of colloidal precipitates [2], oil [3] and solids from wastewater [4] besides improving gas transference and oxidation kinetics in wastewater remediation with Ozone [5,6,7,8] and oxygen [9,10].

It is theorized that nanobubbles have a higher probability of colliding with fine and ultrafine particles than microbubbles or macrobubbles [11], due to their Brownian motion and reduced size. It has been observed that once attached to hydrophobic solids, nanobubbles increase the adhesive forces between the surfaces [11] effectively acting as bridges and forming agglomerates of fine particles [12]. This behavior has brought attention to their applications in coal and mineral flotation for increasing the recovery of fine and ultrafine particles. The effect of nanobubbles on coal flotation has been widely studied, in 2010, Fan et al. [13] designed and tested mechanical cells and columns for nanobubble assisted coal flotation, in their work they found that the use of nanobubble improved coal recovery between 8%–27% depending on product grade and operating conditions while also improving flotation kinetics for all the particle sizes. In 2020, Li et al. [14] used nanobubbles to promote agglomeration and flotation of fine coal particles while also avoiding entrainment of kaolinite in the agglomerates by using polyaluminum chloride to agglomerate and sediment this material.

In the case of mineral flotation, similar concepts have been studied using not naturally hydrophobic particles. In 2018, Tao et al. [15] investigated the effects of nanobubbles in the flotation kinetic and recovery of phosphate ore using a flotation column with integrated nanobubble generation, in their work they found that nanobubbles reduced the collector and frother dosage by about 50% and also increased flotation kinetics and recovery up to 30%. This was attributed to the enhanced collection efficiency and increased surface hydrophobicity of phosphate particles as a result of the unique characteristics of nanobubbles as previously discussed. While similar effects have been found for other not naturally hydrophobic minerals such as apatite [16], pyrite [17], muscovite [18] and quartz [19], the are almost no works regarding the effect of nanobubbles on copper sulfides ore flotation.

In 2014, Ahmadi et al. [20] studied the effect of micro and nanobubbles on the flotation of fine and ultrafine high purity chalcopyrite particles using Potassium Amyl Xantate (KAX) as collector, in their work they found that the presence of nano-microbubbles increased the chalcopyrite fine and ultrafine particles recovery by 16%–21%, while also reducing the consumption of the collector and frother by 75% and 50%, respectively, being this effect more noticeable for particles in the −14.36 + 5 µm size range. Recently, Heidari et al. [21] proved that this effect may also translate to the micro flotation of chalcopyrite bearing ore, increasing its grade and recovery by 2% and 7%, respectively, when compared to the situation without micro and nanobubbles. Up to date, the work that most closely resembles the actual conditions of current copper sulfide operations is the work of Chipakwe et al. [16] in which real low-grade complex copper bearing ores where concentrated using nanobubble assisted froth flotation, their findings show that nanobubbles can improve the mass recovery and flotation kinetics of Cu, Pb and Zn fine and ultrafine mineral particles at a expense of selectivity. These findings highlight the potential of using nanobubbles in industrial flotation operations for copper and copper–molybdenum ores, such as those found in Chilean porphyry deposits. Therefore, this work addresses this opportunity by studying the effect of water enriched with air nanobubbles on the flotation performance of copper ores from deposits located in northern Chile. Results show a moderate but noticeable effect of nanobubbles in flotation kinetics and recovery of copper and molybdenum for low-clay content ores while showing the opposite trend for the clay-rich ones, highlighting the potential of nanobubbles and the challenges that arise in the presence of clay. In this regard, this work is the first that shows the potential of nanobubbles to enhance the flotation of real copper ores and its possible impact on the copper mining industry.

2. Materials and Methods

2.1. Sample Description and Preparation

Two copper sulfide ores from porphyry copper deposits located in northern Chile were used in this study. The samples exhibited significant differences in clay mineral content; therefore, the ore with low clay content was designated as M1, while the ore with high clay content was designated as M2. Both ores were subjected to comminution, and the optimal grinding conditions were determined to achieve a P80 of 150 µm, which was subsequently used for all flotation tests. Grinding was performed under wet conditions at 67 wt% solids.

All flotation experiments were conducted using fresh water (0.01 M NaCl). The frother used was Methyl Isobutyl Carbinol (MIBC, 98% purity, Sigma-Aldrich, Santiago, Chile) at a dosage of 40 g/t. Commercial-grade Sodium Isobutyl Xanthate (SIBX), thionocarbamate, and diesel were employed as collectors. The SIBX dosage was the only variable optimized for each ore, obtaining maximum recoveries at 52 g/t for M1 and 68 g/t for M2, while the dosages of thionocarbamate (40 g/t) and diesel (15 g/t) were kept constant. The pH modifiers were lime, added as a 10 wt% slurry, and diluted sulfuric acid.

2.2. Flotation Tests

Flotation experiments were conducted in a semi-automated, forced-air flotation cell (EDEMET, Concepción, Chile) with a capacity of 2.7 L. All tests were performed at 30 wt% solids. Conditioning was carried out at 700 rpm for 5 min, during which the pulp pH was adjusted to 9.5. Subsequently, the frother and collector were added at the specified dosage for each test. Flotation was performed at 950 rpm with a constant air flow of 12 L/min.

For all flotation tests, the frother was used as standard in froth flotation. The condition NB + frother refers to experiments in which nanobubbles were produced in water that had been previously treated with frother. For the NB and without NB conditions, the frother was added directly to the flotation cell.

Kinetic flotation tests were conducted for 20 min, with concentrates collected at 0.5, 1, 2, 4, 8, 12, and 20 min. Manual scraping was performed every 10 s to ensure consistent froth recovery. The collected concentrates were dried and weighed, and their elemental contents were determined using X-Ray fluorescence (XRF) and atomic absorption spectroscopy (AAS). The AAS analysis was carried out after sample digestion. Each sample was digested with a suitable acid mixture (HCl–HNO₃) to ensure complete dissolution, and the resulting solutions were analyzed by flame AAS (FAAS). For the tests involving nanobubbles, the dilution water (0.01 M NaCl) was pre-conditioned with nanobubbles and added to adjust the pulp from 67 wt% to 30 wt% solids.

2.3. Nanobubble Generation

Nanobubbles were generated using an NB-3000 nanobubble generator (Xi’an Benan Technology Co., Ltd., Llanquihue, Chile), which introduces nanometric bubbles into aqueous solutions by hydrodynamic cavitation combined with pressurized gas injection. This method produces stable nanobubbles with diameters below 200 nm. Distilled water was conditioned for 30 min at a gas pressure of 4 bar under controlled liquid flow to ensure continuous nanobubble generation. The presence and size distribution of nanobubbles were verified using an Anton Paar PSA 990 laser diffraction particle size analyzer (Anton Paar GmbH, Graz, Austria). During flotation, the nanobubble-enriched water was used as the liquid phase, while conventional air was simultaneously supplied through the impeller to maintain standard aeration conditions.

2.4. X-Ray Diffraction (XRD) Analysis

The mineralogical composition of the flotation samples and the proportion of clay minerals were determined by X-Ray diffraction (XRD). Analyses were conducted on a Bruker D8 Advance diffractometer (Bruker, Bremen, Germany), operating with Cu-Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA. Diffractograms were recorded in the 2θ range of 5–70°, with a step size of 0.02° and a counting time of 1 s per step. Mineral phases were identified by comparison with the International Centre for Diffraction Data (ICDD) database using DIFFRAC.EVA software Version 6.

2.5. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS)

Morphological and elemental characterization of mineral surfaces was performed using scanning electron microscopy (SEM) coupled with energy-dispersive X-Ray spectroscopy (EDS). Samples corresponded to flotation concentrates obtained with and without nanobubble treatment for both mineral types M1 and M2.

Samples were dried at room temperature, mounted on aluminum stubs, and sputter-coated with a thin gold film. Observations were carried out using a Zeiss Gemini SEM 360 operating at 32 kV (Carl Zeiss AG, Oberkochen, Germany). EDS analyses were performed on selected regions of concentrate particles to determine the elemental composition of their surfaces.

3. Results

3.1. Chemical and Mineralogical Characterization of the Samples

The mineralogical analyses revealed marked differences in clay content between the two samples investigated: the low-clay-content ore contained 3.8% (M1) and the high-clay-content ore contained 13.2% (M2). The clays identified were mainly kaolinite, smectite, illite, and biotite, which have been extensively reported in the literature as detrimental to flotation due to their swelling capacity, the formation of high-viscosity suspensions, and the surface coating of valuable particles that reduces their hydrophobicity [22,23]. Such characteristics are particularly relevant in fine particle systems, where clay minerals can impair particle–bubble attachment and modify froth stability. In the context of nanobubble-assisted flotation, recent studies have highlighted that the presence of clays may lead to different interaction mechanisms, influencing both the kinetics of recovery and the selectivity of flotation [24].

XRD analyses, complemented by QEMSCAN, confirmed the presence of these clay minerals and additionally identified their association with quartz, feldspars, pyrite, and various copper sulfides. These results suggest that clay minerals are not only present as discrete phases but may also occur as surface coatings on sulfide particles, increasing their potential impact on flotation performance. The chemical assays further confirmed that the head grades of the ores differed slightly: copper grades were 0.55% (M1) and 0.73% (M2), while molybdenum grades were 0.012% and 0.030%, respectively, as determined by atomic absorption spectroscopy (AAS). Although these differences in metal content are relatively modest, they are sufficient to indicate distinct metallurgical responses, particularly when combined with the contrasting clay mineralogy (Figures S5 and S6).

3.2. Flotation Test with and Without Nanobubbles

The flotation process was evaluated through kinetic tests; each performed in duplicate to ensure reproducibility. The results (Figure 1a) show that the introduction of nanobubbles enhanced copper recovery across the entire flotation time range, yielding an increase of 7.86 percentage points in the maximum recovery of sample M1. These findings demonstrate that nanobubbles not only accelerate flotation kinetics but also improve the overall recovery of the ore.

To confirm that the observed effect was not due to mechanical entrainment, additional tests were carried out in the flotation cell without the addition of frother. Under these conditions, copper recovery did not exhibit significant variation, as the differences observed (≈1.3% higher recovery with frother and nanobubbles) were within the experimental error (Figure 1a).

These results indicate that the improvement in recovery is attributable to the specific effect of nanobubbles. This is consistent with mechanisms reported in the literature, which suggest that nanobubbles interact directly with hydrophobic sites on mineral surfaces, enhancing particle hydrophobicity and flotation response. Such an effect can even substitute for the role of a secondary collector, highlighting the potential of nanobubbles to optimize reagent consumption in sulfide flotation [14,16].

In contrast, Figure 1b shows the flotation kinetics of sample M2. In this case, a negative effect of nanobubble application was observed in ores with high clay content, reflected in a decrease of approximately 5 percentage points in the final copper recovery. This behavior may be associated with the interaction of clays, which promotes the formation of slime coating structures on the surfaces of valuable minerals, thereby reducing their hydrophobicity and flotation response [23].

When comparing flotation tests with and without frother in the presence of nanobubbles, no significant differences in recovery were observed during the initial minutes of flotation. However, at the end of the test, a difference of about 3 percentage points was detected, suggesting that smaller bubble sizes could enhance the recovery of fine particles. This finding is consistent with previous studies that investigated the role of frothers in flotation and their ability to modify bubble size distribution, directly influencing collection efficiency [24,25].

The results of copper recovery as a function of weight recovery are presented in Figure 2. For sample M1 (Figure 2a), the increase in copper recovery is also associated with an increase in weight recovery, rising from a maximum of 5.9% under the condition without nanobubbles to 7.5% when nanobubbles were combined with MIBC. This behavior indicates that, although the improvement in final copper recovery is evident, part of this increase may be attributed to two complementary mechanisms.

On the one hand, there is mechanical entrainment, derived from the higher gas flow in the system, which increases the probability of transporting fine particles and even gangue fractions into the froth [22,23,26]. On the other hand, the increase may also be attributed to the recovery of copper particles associated with other minerals, where small exposed hydrophobic surfaces are sufficient to enable their capture in the presence of nanobubbles. These mechanisms have been previously discussed in the literature, where it is recognized that the use of nanobubbles can intensify both the adhesion efficiency of valuable particles and the non-selective transport of associated gangue particles [9,24,25].

For sample M2, the results are shown in Figure 2b. In this case, unlike sample M1, the condition with nanobubbles and MIBC did not lead to an increase in either weight recovery or copper recovery. On the contrary, a tendency toward lower values was observed, which can be attributed to the slime coating effect generated by clay minerals, which cover the surfaces of copper sulfides and hinder their effective interaction with bubbles.

Furthermore, when comparing the condition without nanobubbles to that with nanobubbles only, an increase of approximately 1.5 percentage points in weight recovery was observed, suggesting a greater degree of fine particle entrainment. This behavior reinforces the idea that, in systems with high clay content, nanobubbles may intensify non-selective transport mechanisms rather than enhance the effective recovery of valuable particles, in clear contrast to the behavior observed in sample M1.

Molybdenum recovery as a function of time is shown in Figure 3a. For sample M1, a significant increase in recovery was observed, with a variation of up to 20 percentage points when comparing the maximum recovery obtained without nanobubbles to that achieved under nanobubbles combined with frother. In this case, the condition without frother is positioned at an intermediate level between both extremes.

For sample M2 (Figure 3b), a behavior similar to that observed for copper was identified. The effect is relatively symmetrical, as the difference between the lowest recovery (nanobubbles without frother) and the highest recovery is approximately 6 percentage points, with higher values obtained under flotation without nanobubbles.

Comparatively, in terms of weight recovery, sample M1 shows a higher weight recovery as a function of molybdenum recovery (Figure 4a). In contrast, for the ore with higher clay content (M2), as observed in the case of copper, the increase in weight recovery is directly associated with the increase in molybdenum recovery (Figure 4b).

Furthermore, when performing a comparative analysis between weight recovery and the expected metal recovery for the high-clay-content ore (Figure 2b and Figure 4b), it is observed that for weight recoveries below 7%, higher yields are obtained in the recovery of the valuable metals, both copper and molybdenum. This type of comparative analysis is particularly relevant when projecting operational scenarios, in which weight recovery can be controlled with greater accuracy. In industrial conditions, flotation kinetics can in fact be regulated by managing the froth levels in flotation cells, allowing the flotation rate to be adjusted through froth height and thereby enabling a more controlled optimization of recovery.

Overall, the results show that the use of nanobubbles enhances the flotation of sulfide ores with low clay content (M1), increasing both kinetics and final copper and molybdenum recovery. In contrast, in ores with high clay content (M2), their effect is limited or even negative due to slime coating phenomena, which decrease hydrophobicity and promote non-selective entrainment. These findings demonstrate that the efficiency of nanobubbles strongly depends on mineralogy, and that their industrial application requires complementary control strategies to maximize effectiveness in ores of varying complexity (Figures S1–S4).

3.3. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS)

SEM-EDS analysis revealed clear differences between samples M1 and M2 under conditions with and without nanobubbles.

For M1, the surfaces without nanobubbles (Figure 5a,b) showed exposed sulfides with little adherence of fine particles (green arrows), whereas samples processed with nanobubbles (Figure 5c,d) more compact and cohesive agglomerates (encircled in blue) were observed. This effect favors the capture and transport of particles to the froth, consistent with the higher copper and molybdenum recoveries recorded in the flotation tests and to what has been found in the literature for synthetic samples [20,24].

For M2, the micrographs without nanobubbles (Figure 6a,b) revealed laminar coatings enriched in Si and Al, characteristic of clays (red arrows), which became more pronounced when nanobubbles were introduced (Figure 6c,d) with some particles (encircled in red) being almost completely coated by clay. Under these conditions, sulfide particles appeared to be covered by more stable clay-rich matrices, explaining the lower hydrophobicity and the reduction in final recovery observed in flotation.

Overall, the combined flotation and SEM results demonstrate that nanobubbles can significantly enhance the recovery of sulfide minerals when clay content is low (M1), by promoting selective agglomeration and more effective particle–bubble attachment. However, in ores with higher clay content (M2), nanobubbles intensify slime coating and non-selective aggregation, leading to reduced flotation efficiency. As found in kaolinite by Lei et al. the presence of bulk nanobubbles causes entrainment of clay particles during flotation. In their work, they hypothesize that the coating of clay with nanobubbles may induce and stabilize E-E contacts [27], which in this case is causing the formation of coatings around the copper sulphides. These findings highlight the strong dependence of nanobubble performance on mineralogy and underline the need for tailored strategies when applying this technology on an industrial scale.

4. Conclusions

This study aimed to be the first to analyze the effect of nanobubbles in the froth flotation of real copper sulphides, considering the presence of clay which is a common cause of reduced flotation kinetics and recovery in Chilean flotation plants. In this context, this study demonstrated that clay content decisively conditions the performance of nanobubble-assisted flotation in copper sulfide ores from Chilean porphyry deposits.

In low-clay-content samples, nanobubbles enhanced copper and molybdenum recovery by improving both kinetics and final recovery. This effect was corroborated by SEM-EDS analyses, which revealed clean surfaces and more compact agglomerates. In contrast, in high-clay-content samples, a decrease in process efficiency was observed, associated with the reinforcement of slime coating and the formation of non-selective agglomerates, which reduced sulfide hydrophobicity and recovery.

From an operational perspective, nanobubbles may influence froth stability by decreasing bubble coalescence and enhancing froth persistence. Although this effect could be beneficial for recovery, excessive stabilization may affect froth drainage in long-term operation, which warrants further study. Moreover, since nanobubble generation requires minimal energy and no additional reagents, its use represents a potentially sustainable approach for improving flotation performance while reducing chemical consumption.

These findings indicate that the efficiency of nanobubbles strongly depends on ore mineralogy, showing clear benefits in clean systems but limitations in clay-rich ores. Therefore, their industrial application requires a careful assessment of mineralogical composition and the design of complementary strategies to mitigate clay-related effects.