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Article

Sonication-Assisted Surface Erosion and Its Impact on the Flotation of Ultrafine Smithsonite

by
Weiguang Zhou
1,
Weiwei Cao
1,
Haobin Wei
2,
Shulan Shi
1,
Chenwei Li
1 and
Liuyang Dong
3,*
1
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China
2
Shandong Bureau of China Metallurgical Geology Bureau, Jinan 250000, China
3
Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming 650093, China
*
Author to whom correspondence should be addressed.
Metals 2025, 15(7), 731; https://doi.org/10.3390/met15070731
Submission received: 26 May 2025 / Revised: 18 June 2025 / Accepted: 24 June 2025 / Published: 30 June 2025
(This article belongs to the Special Issue State of the Art in Flotation and Separation of Metallic Minerals)
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Abstract

Regulating the dissolution and interfacial behavior of minerals via external force fields is considered a promising strategy for enhancing the flotation of soluble minerals. This study explored the potential of ultrasound-assisted pulp conditioning in improving ultrafine smithsonite flotation. Specifically, we systematically evaluated the effects of ultrasonic pretreatment (UP) on the physicochemical properties of smithsonite suspensions (focusing on surface erosion behavior) and assessed subsequent flotation performance using flotation tests and modern analytical techniques. It has been found that UP can significantly modify smithsonite suspension characteristics, including particle morphology, ionic composition, electrokinetic properties, and pulp pH. Flotation results demonstrate that UP yields higher recovery compared to traditional stirring (TS) conditioning, especially at medium-to-high sodium oleate (NaOL) concentrations. Comparative analysis reveals that ultrasonic-assisted dissolution and ion-selective migration are the main factors driving improved flotation performance. Unlike TS, UP promotes greater zinc ion release, facilitates the dissolution–hydrolysis–precipitation equilibrium, generates more and finer nanoparticles in the bulk phase, and induces the deposition of hydrozincite on smithsonite surfaces. These changes increase active zinc sites for more stable NaOL adsorption, thereby enhancing the flotation of ultrafine smithsonite particles.

1. Introduction

The industrial and economic significance of zinc is attributed to its diverse applications, which span paints, cosmetics, food, pharmaceuticals, detergents, textiles, leather, photography, storage batteries, electrical equipment, and corrosion-resistant coatings [1]. Currently, zinc is predominantly extracted from zinc sulfide minerals. However, as these sulfide ores deplete, the processing of oxidized zinc ores, such as smithsonite (ZnCO3) and hemimorphite (Zn4(H2O)[Si2O7](OH)2), has become increasingly important [2]. Froth flotation is the primary method for recovering oxidized zinc minerals [3]. However, it is more challenging than recovering sulfide minerals due to the lower natural floatability of oxidized minerals [4]. Additionally, slimes can complicate processing and reduce zinc recovery efficiency in industry [5].
Surface dissolution, a crucial surface modification technique, significantly affects the flotation and separation of minerals, especially oxides and silicates. This process can change mineral surface properties and flotation solution chemistry via ion migration and re-adsorption at various interfaces, thus influencing mineral floatability [6,7]. During flotation, stable adsorption of collectors on the mineral surface is essential [8]. Moderate dissolution behavior can be advantageous, as the dissolved ions are hypothesized to provide additional active sites for reagent adsorption on the mineral surface [9]. However, intense dissolution of mineral particles may disrupt stable collector adsorption and even induce collector desorption [10]. This effect is particularly pronounced in fine-grained soluble minerals, where high ion dissolution rates and prolonged dissolution equilibrium times severely undermine the stability of reagent adsorption [11].
Exogenous ions can significantly affect ion migration and transformation at mineral surfaces, serving as a key tool to regulate mineral surface dissolution in practice. In mineral flotation, these ions can either enhance or impede the dissolution of ions from the mineral lattice. This action subsequently modulates the selective adsorption of collectors on the mineral surface, ultimately altering the final flotation response [12]. Recent studies have shown that certain chemical reagents can enhance selective mineral surface dissolution or speed up the dissolution–precipitation equilibrium, aiding stable reagent adsorption and subsequent flotation. For example, it has been reported that NaOH addition can increase Si ion dissolution from spodumene surfaces, leaving more Li ions and enabling more stable fatty acid adsorption [13]. Similarly, adding exogenous acids to smithsonite pulp releases Zn ions into the liquid phase. The reverse adsorption of Zn compounds creates more active sites, boosting collector adsorption [14]. Conversely, certain exogenous ions can inhibit mineral dissolution, thereby enhancing the flotation of target minerals or their separation in specific systems. For instance, sodium carbonate addition can significantly boost the carbonate ion concentration in solutions. Upon saturation, it prevents zinc ion release from smithsonite lattices and promotes zinc ion or zinc hydroxyl compound re-adsorption onto the mineral surface, which is crucial for the flotation and separation of fine smithsonite from other carbonate gangue minerals [15,16]. Thus, given surface dissolution’s crucial impact on soluble salt minerals’ flotation performance, regulating mineral dissolution (enhancing or inhibiting selective dissolution) deserves more research attention.
Beyond chemical regulation, external force fields (e.g., ultrasound and microwave radiation) offer a key alternative to control mineral dissolution and selective ion release [7,17]. Ultrasound, in particular, generates extreme surface vibrations, high pressure, and cavitation, which accelerate particle motion, produce shock waves, and create high-speed microjets. These effects trigger diverse physicochemical reactions, such as surface cleaning and mineral disintegration. These reactions can shorten the dissolution–hydrolysis equilibrium times, which is particularly advantageous for the flotation of highly soluble minerals like carbonates and sulfates [18]. Moreover, external force fields not only regulate ion dissolution but also provide multiple benefits, such as removing slime coatings, generating ultrafine air bubbles via cavitation, producing radicals, and modifying medium properties (e.g., pH, temperature, dissolved oxygen, and conductivity). These factors may enhance the flotation performance of coal and various minerals, as widely reported in the literature [19,20,21].
Over the past 30 years, acoustic waves have been widely used to enhance mineral floatability, increase flotation kinetics, and reduce reagent consumption [22,23]. Ultrasound is applied at various stages, including pulp preconditioning, collector conditioning, and actual flotation [18,24,25]. As a typical semi-soluble salt mineral, smithsonite’s flotation is highly complex due to its intricate solution chemistry, posing challenges for recovering valuable smithsonite particles in industry [1]. Moreover, although ultrasonic pretreatment (UP) is industrially used to enhance ultrafine zinc ore recovery [26,27], its underlying mechanisms remain poorly understood, hindering the advancement of sonication-assisted flotation. To bridge this gap, this study systematically investigated the effects of sonication during pulp preconditioning on ultrafine smithsonite in a sodium oleate (NaOL) flotation system, using flotation tests and advanced detection methods. The study focused on ultrasonic-induced ion dissolution and its impact on pulp physicochemical properties, collector adsorption, and flotation performance in a mildly alkaline solution (pH 8). The findings aim to elucidate the mechanisms of sonication-assisted flotation for oxidized zinc ores and other semi-soluble salt minerals, providing a basis for potential ultrasound applications in their processing.

2. Experimental

2.1. Materials and Reagents

The smithsonite sample was collected from Yunnan Province, China. Characterization of the smithsonite through XRD analysis (Figure 1) and chemical analysis (Table 1) confirms the high purity of the ore sample. The sample underwent a series of preparatory steps: it was first comminuted in a porcelain mortar, followed by grinding in an agate mortar, and subsequently separated using a dry classifier. The ultrafine fraction (<5 μm) obtained from the classifier was utilized in this study.
Sodium oleate (NaOL, CP) purchased from Beijing Dingguo Biotechnology Co., Ltd. (Beijing, China) was used as the collector. Analytical-grade hydrochloric acid (HCl) and sodium hydroxide (NaOH), both sourced from Aladdin Reagent Company, were used as pH modifiers. Deionized water with a resistivity of 5 MΩ was employed for all the experimental work.

2.2. Morphological Characterizations of Ultrafine Smithsonite Particles

In this study, an ultrasonic cleaner (KMD-II, KeMeida Co., Ltd., Shenzhen, China; 28 kHz, 60 W) was used for the initial UP of ultrafine smithsonite suspensions (5 wt%). The tank-type ultrasonic device, which emits parallel ultrasonic waves, offers uniform energy distribution and higher throughput compared to probe-type devices [28,29]. The traditional stirring (TS; 600 rpm) or UP conditioning lasted for 3 min, a duration short enough to keep the slurry temperature virtually unaffected. After treatment, the smithsonite slurry or the smithsonite powder (pre-subjected to low-temperature vacuum drying) was promptly transferred for flotation testing or another analysis.
To assess the impact of UP on the morphology of smithsonite particles, a range of analytical techniques were employed at room temperature (22 ± 2 °C) under natural pH conditions. More precisely, the morphological characteristics of ultrafine smithsonite particles were comprehensively characterized through size analysis, porosity analysis, and morphological appearance analysis. Specifically, the size distribution of smithsonite particles treated with TS and UP was measured using a Mastersizer 2000 (Malvern Instrument Ltd., Malvern, UK). The porosity of the samples was determined by an Autosorb-iQ (Quantachrome, Boynton Beach, FL, USA) through N2 adsorption–desorption isotherms at 77 K, yielding specific surface area (SSA), pore volume, and mean pore diameters (MPD). Morphological changes were examined using a Nova NanoSEM230 (FEI, Hillsboro, OR, USA), while an FE-SEM Zeiss Supra TM55 coupled with an Oxford-Inca EDS was used for local chemical analysis. Collectively, these techniques provided a comprehensive understanding of the morphological alterations of smithsonite particles in dissolution.

2.3. ICP Analysis

To investigate the dissolution behavior of smithsonite particles under various pretreatments, the concentration of dissolved zinc ions was measured using an ICP optical emission spectrometer (model PS-6, Bird, Santa Monica, CA, USA) at room temperature under natural pH conditions (pH 8 ± 0.2). Given the high purity of the smithsonite, the dissolution of other metal ions was considered negligible. Therefore, monitoring the zinc ion concentration effectively reflects the surface dissolution of smithsonite particles under different conditions. For each analysis, the smithsonite suspension (5 wt% and natural pH) was initially preconditioned through TS or UP for a predetermined period, after which it was processed using a 1 μm pore-sized microporous filter membrane to yield the supernatant. Ultimately, the concentration of zinc ions in each supernatant was determined via the ICP optical emission spectrometer, with three replicate measurements taken to ensure accuracy. The mean value of these replicates was then calculated and reported as the final result.

2.4. Flotation of Ultrafine Smithsonite

Single mineral flotation tests were conducted using an XFG flotation machine at a spindle speed of 1700 rpm and room temperature. A mineral suspension was prepared by dissolving 2.5 g of smithsonite minerals in 47.5 mL of deionized water, with the pH adjusted to a predetermined value using NaOH or HCl. The pulp was subsequently preconditioned for a specified duration, utilizing either TS or UP. This was followed by the addition of the NaOL collector, after which the pulp underwent an additional 3 min conditioning period. The flotation process lasted for 3 min, yielding floated and unfloated products. These products were then filtered and dried to determine the flotation recovery based on the solid weight distribution between the two fractions.

2.5. Nanoparticle Tracking in Smithsonite Suspensions

Laser particle size analyzers are mainly useful for detecting micrometer-sized particles in slurries. However, in our study, UP can fragment sub-micron mineral particles and cause surface erosion, generating nanoscale particles. Moreover, the re-hydrolysis of smithsonite dissolution products and their interaction with NaOL are likely to produce nanoscale colloidal substances containing zinc hydroxides and zinc oleate. These nanoscale substances can adsorb, aggregate, and deposit at various interfaces, affecting reagent–mineral interactions and flotation performances. Therefore, it is necessary to reveal the properties of nanoparticles in smithsonite suspensions with differing treatments for a deeper analysis. In this experiment, we employed the dynamic light scattering (DLS) technique to detect and quantify nanoparticles in smithsonite suspensions. The slurries were prepared following the flotation test protocol, with a consistent conditioning time of 3 min. Subsequently, they were filtered through a 1-micrometer syringe filter to obtain the supernatant enriched with nanoscale particles. Afterwards, using the nanoparticle tracking analysis (Particle Metrix’ ZetaView, Malvern Instrument, Malvern, UK) and the associated analytical software (Version 8.06.01), we determined the size–concentration distribution of the nanoscale particles and measured their concentration three times, reporting the average value as the final data.

2.6. Zeta Potential Measurements

Zeta potential measurements were conducted using a Nano ZS90 zeta potential analyzer (Malvern Co., Malvern, UK) at room temperature. For each sample, a dilute mineral suspension was prepared by adding 0.02 g of ultrafine smithsonite particles to 40 mL of a 0.01 M KNO3 solution (serving as the background electrolyte solution during the tests). The suspension was first preconditioned by either TS or UP for 3 min, followed by pH adjustment and measurement. After preconditioning, the suspension was magnetically stirred for an additional 10 min and allowed to stand for 5 min. The supernatant was then extracted for measurement. Each experimental condition was measured at least three times, and the average zeta potential and standard deviation were calculated and recorded as the final data.

2.7. XPS Tests

XPS analysis was utilized to examine the surface changes of smithsonite under various conditions. The samples for XPS analysis were prepared in a flotation cell, following a procedure analogous to that employed in flotation experiments. The minerals were subsequently dried in a vacuum oven at 40 °C, and the resulting powders were analyzed using XPS. The XPS measurements were conducted using a K-Alpha+ X-ray photoelectron spectrometer (Thermo Fisher, Waltham, MA, USA), equipped with an Al Kα X-ray source operating at 200 W and a pass energy of 20 eV. During the measurements, the test chamber pressure was maintained below 10−9 mbar, and the takeoff angle was set at 90°. The binding energy scale was calibrated using the C 1s peak from adventitious carbon (around 284.8 eV) as an internal standard. All data were processed using Thermo Scientific Advantage 4.52 software.

3. Results and Discussion

3.1. Morphological Characterization Results

Figure 2 presents a comparative analysis of the size distribution and porosity characteristics of smithsonite samples subjected to different treatments. As illustrated in Figure 2a, the application of energetic ultrasound treatment results in a notable reduction in particle size, with the average particle size (D[4,3]) decreasing from 2.916 μm to 2.395 μm. This reduction is indicative of particle disintegration induced by sonication [30]. However, for the ultrafine smithsonite particles used in this study, the particle size reduction induced by sonication is unlikely to cause significant changes in final flotation performance. Furthermore, the nitrogen adsorption–desorption isotherms of the smithsonite samples consistently exhibit Type IV behavior with hysteresis loops [31], characteristic of mesoporous materials (Figure 2b–d). Notably, the SSA (using the BET method) of the smithsonite sample treated with UP is significantly higher than that of the TS-treated sample (6.414 m2·g−1 vs. 2.667 m2·g−1). This enhancement in surface area is likely attributed to two primary factors: the grinding and surface denudation effects of ultrasound and the formation of additional nano-sized zinc hydroxide compounds facilitated by ultrasonic dissolution. In addition, the pore size distribution of the tested smithsonite samples was analyzed using the Barrett–Joyner–Halenda method. As shown in Figure 2c,d, the sample subjected to UP exhibits a smaller MPD value compared to the untreated sample. Given the nanometer-scale surface pores of smithsonite, we proposed that the reduction in the average mesopore size is mainly due to ultrasonic-assisted ion dissolution and hydrolysis on the smithsonite surface, leading to subsequent surface deposition that occludes some mesopores. This is likely because ions near the pores have higher surface energy, facilitating their dissolution and hydration precipitation.
Figure 3a,b shows the surface characteristics of smithsonite particles via SEM. The sample subjected to TS displays a rough, stepped fracture surface with visible fissures and parallel features, and occasional minor aggregation among fine particles. This occurs because smithsonite, a semi-soluble salt, can release Zn(OH)2(aq) upon dissolution, promoting surface adsorption and particle aggregation through deprotonation in weakly alkaline conditions [32,33,34]. In contrast, the UP-treated sample exhibits slight surface dissolution and loss of adhering substances, disrupting the original patterns. This results in minimal new cavities or cracks, producing a smoother surface with smaller, cake-like particles and flocculate aggregates that coalesce into larger structures. These findings highlight sonication’s significant impact on smithsonite’s morphological transformation, particularly in enhancing surface dissolution and erosion.
Figure 3c,d shows the EDS analysis of a selected point on the smithsonite surface. After UP, the zinc-to-oxygen (wt%) ratio increased from 1.209 to 1.573. This increase is likely attributed to ion migration, ion hydration, and adsorption of ionic or colloidal substances onto smithsonite surfaces within the aqueous suspension [35,36]. Although the mechanisms of smithsonite surface dissolution, ion migration at the multiphase interface, and physicochemical reactions under various pretreatment conditions are not fully understood, SEM-EDS data preliminarily show that UP can significantly boost the surface zinc abundance on smithsonite. Such changes in surface inhomogeneity are likely to affect smithsonite’s surface wettability, electrical properties, reagent adsorption, and flotation performance [8,10,11].

3.2. Mineral Dissolution and Solution Chemical Analysis

Smithsonite, a semi-soluble carbonate mineral of the calcite group, can release substantial quantities of carbonate and zinc ions from its surface into aqueous solutions. Here, to investigate its surface dissolution behavior more directly, we quantified the concentration of zinc ions in the supernatant of smithsonite suspensions using ICP spectroscopy. It has also been shown that sonication can alter slurry pH, which is critical for mineral dissolution, solution chemistry, and flotation outcomes [37,38]. Therefore, we simultaneously monitored pH changes in smithsonite suspensions during the conditioning. The pH value of the smithsonite pulp was monitored in real time, with measurements repeated three times to ensure accuracy. The corresponding data are shown in Figure 4.
Figure 4a presents the concentration of zinc ions in aqueous suspensions subjected to TS and UP. Results indicate that, regardless of the pretreatment method, the zinc ion concentration initially increases and then decreases with conditioning time, eventually stabilizing after approximately 12 min. This stabilization reflects the equilibrium among ion dissolution, ion hydrolysis, and hydration precipitation during pretreatment. Both methods attain peak zinc ion concentrations within 3 min. However, the UP groups achieve higher concentrations, attributed to their more concentrated energy output, which significantly enhances the dissolution of zinc ions from the mineral lattice. Additionally, UP shows greater concentration fluctuations, as seen in the error analysis. This indicates that the intense ultrasonic field promotes a dynamic equilibrium among mineral dissolution, ion hydrolysis, precipitation, and re-adsorption within a wider range.
Figure 4b shows a significant initial drop in solution pH for both smithsonite samples, likely due to the rapid release and hydrolysis of zinc ions during the early stages of smithsonite dissolution [8,16]. The literature indicates that zinc ions have a higher hydration-free energy (−1949 kJ/mol) than carbonate ions (−1416 kJ/mol), leading to their preferential release and hydrolysis [16,39]. Subsequently, the pH gradually increases as carbonate ions in the solution undergo progressive hydrolysis [8]. Once the solution reaches saturation with dissolved components, further carbonate dissolution may restrict zinc ion dissolution, which in turn, facilitates the re-adsorption of zinc ions or zinc hydroxyl compounds onto the smithsonite surface [40]. Moreover, for the TS-treated suspension, dynamic equilibrium is achieved more slowly, with pH stabilization at around 7.94 only after 15 min of pulp adjustment. In contrast, the UP-treated suspension reaches a metastable state with pH stabilization at about 7.95 within the first three minutes. These results highlight that UP is more effective than TS in stabilizing pulp pH within a short treatment duration. Given the critical role of pulp pH in mediating mineral dissolution and hydrolysis, these findings suggest that UP is superior to TS in establishing a stable and balanced smithsonite dissolution–hydrolysis–precipitation environment under short-time treatment conditions, which is more favorable for stable and consistent reagent–mineral interactions [8]. Additionally, with extended UP treatment, the suspension pH gradually decreases, reaching a dynamic equilibrium within a broader fluctuation range and stabilizing at approximately 7.58 after 12 min of sonication. This trend is likely due to the multi-level hydrolysis of zinc ions and pre-formed zinc components as sonication time increases [41,42]. These results demonstrate that UP promotes more zinc ion dissolution compared to TS, resulting in a lower pulp pH at the final dissolution–hydrolysis–precipitation equilibrium.
Mineral dissolution can significantly impact solution chemistry by releasing ions and modifying suspension properties. However, conventional ICP analysis only reveals elemental composition, not ionic concentration, while pH measurements lack detailed insights into dissolved ion actions. Thus, integrating theoretical solution chemistry analysis is crucial for a deeper understanding of pulp association’s with mineral dissolution.
Smithsonite, a semi-soluble salt with a solubility product constant of 1.46 × 10−10 M, exhibits complex dissolution behavior due to the unique hydrolysis characteristics of zinc ions [43]. According to the species distribution diagram of smithsonite dissolution developed by Hu et al., zinc ions liberated from the smithsonite surface are expected to primarily exist in the forms of Zn2+, ZnOH+, Zn(OH)2(aq), and other species under near-neutral pH conditions [44,45]. The species Zn2+ and ZnOH+ are believed to enhance the subsequent adsorption of the fatty acid collector by serving as reactive sites. In contrast, colloidal Zn(OH)2(aq), predominantly formed via hydrolysis, poses significant challenges in modulating the hydrophobicity of smithsonite in weakly alkaline conditions [15]. Although the impact of sonication on dissolved components is complex, ultrasonic surface cleaning can largely reduce the adsorption and deposition of colloidal Zn(OH)2(aq) on smithsonite [42]. Additionally, a lower equilibrium pH in the UP system promotes higher Zn2+ and ZnOH+ concentrations in the bulk phase [15].
The data indicate that sonication can significantly accelerate ion dissolution from ultrafine smithsonite surfaces and quickly establishes a metastable equilibrium among ion dissolution, hydrolysis, and hydroxyl precipitation within about 3 min. In contrast, TS has a weaker effect on ion dissolution, leading to a longer time to reach equilibrium. Sonication’s ability to rapidly accelerate ion release and establish equilibrium highlights its economic and operational advantages for pretreating ultrafine smithsonite in flotation. Next, we will examine the impact of short-term ultrasonic pretreatment on smithsonite flotation.

3.3. Flotation Results

Smithsonite exhibits high floatability with NaOL at pH 7.0–8.0 [1]. Figure 5a shows that the flotation recovery of ultrafine smithsonite increases rapidly with NaOL concentration and stabilizes at higher levels, with UP consistently outperforming TS, especially at moderate-to-high NaOL concentrations (i.e., 4–6 × 104 M). In this study, a low-frequency ultrasound is used primarily for dispersion, minimizing Bjerknes and aggregation effects [46]. Smithsonite’s strong hydrophilicity limits interactions between possible fine cavitation bubbles and mineral particles during pretreatment [47,48]. UP does induce a modest reduction in the particle size of smithsonite, but given the relatively minor extent of this change compared to the original particle size, its influence on flotation performance is deemed negligible. Simultaneously, UP significantly enhances the specific surface area of smithsonite, with a slight reduction in the average interparticle pore size. These changes may affect reagent adsorption during flotation, as fine minerals generally have stronger adsorption affinities for organic molecules [49]. However, these physical alterations are unlikely to be the primary drivers of the substantial flotation improvement. Instead, the release and transfer of metal ions during preconditioning, which can activate NaOL adsorption onto smithsonite, are probably a critical factor.
Considering the significant correlation between conditioning time and mineral dissolution, an investigation into the flotation behavior of ultrafine smithsonite under different pretreatment durations was conducted. As shown in Figure 5b, flotation recovery initially increases and then decreases with extended conditioning time, regardless of NaOL concentration. This trend is likely related to the changes in zinc ion and zinc component concentrations in the solution over time. Specifically, at shorter conditioning times, the concentration of dissolved zinc ions in the solution increases gradually. These ions adsorb onto the smithsonite surface, providing active sites for NaOL adsorption and thereby enhancing the flotation of smithsonite particles [14]. Conversely, at longer treatment times, the concentration of dissolved ions and zinc hydrolysis products in the pulp increases. These components can precipitate the collector or inhibit NaOL adsorption by pre-adsorbing onto the smithsonite surface, leading to poorer flotation [1,50]. Furthermore, UP consistently achieves a higher flotation recovery in a shorter treatment time compared to TS at a fixed NaOL concentration. This suggests that UP accelerates smithsonite surface dissolution and the zinc ion’s hydrolysis–precipitation equilibrium, allowing more oleate ions to stably adsorb onto the mineral solids, consequently. The results in Figure 5b align well with those in Figure 4, highlighting that the differences in surface dissolution between TS and UP are critical for subsequent smithsonite flotation performance.

3.4. Nanoparticle Tracking and Analysis

Zinc, a divalent transition metal ion, can form nanoscale hydroxyl zinc colloids under neutral and alkaline conditions, which can suspend or anchor on smithsonite surfaces. Ultrasonic disintegration can meanwhile generate numerous nanoscale smithsonite particles. However, conventional ICP analysis only reveals total elemental composition, not individual component concentrations. The unclear proportion of zinc in an ionic versus a colloidal/particle forms across different pretreatments thus limiting our understanding of the sonication-enhanced dissolution mechanism of ultrafine smithsonite. Moreover, NaOL has been extensively known to enhance the hydrophobicity of salt-oxidized minerals through two primary mechanisms: direct interaction with surface cations and the formation of insoluble oleate colloids that adsorb onto the mineral surface [50,51]. To elucidate the mechanisms by which NaOL improves the flotation of ultrafine smithsonite across pretreatment systems, it is essential to investigate changes in nanoscale particles/colloids within the suspensions during pulp conditioning. Thus, DLS was employed to profile nanoparticle concentrations as a function of size under different conditions (Figure 6).
Figure 6a illustrates that all pretreated smithsonite suspensions contain nanoscale particles, predominantly within the 0–300 nm range, with only a few larger particles. In the absence of NaOL, the UP-treated sample exhibits a significantly higher proportion of fine nanoparticles (<100 nm) compared to the TS-treated sample. As shown in Figure 6b, UP results in a smaller average nanoparticle size and a significantly higher total particle count. Specifically, the UP-treated suspension contains approximately 15 times more nanoparticles than the TS-treated suspension. However, ICP analysis reveals that the total zinc ion concentration in the UP-treated supernatant is only about 1.2 times that of the TS system. These results suggest that UP has significantly increased the proportion of fine zinc-containing colloidal particles while reducing the proportion of free zinc ions, compared to the TS condition.
At the low NaOL concentration (10−4 M), the TS-treated suspension demonstrates a modest decrease in nanoparticle size and a slight increase in total particle number. This can be attributed to oleate adsorption on nanoparticle surfaces, which causes larger particles to aggregate and precipitate, while free zinc ions react with oleate to form smaller oleate zinc colloids. Meanwhile, for the UP-treated pulp, the introduction of NaOL leads to a significant reduction in the concentration of nanoparticles in the suspension, highlighting NaOL’s role in promoting nanoscale colloid aggregation and precipitation. This also underscores the importance of the surface precipitation mechanism in oleate adsorption on the smithsonite surface [52], as shown by improved flotation performance. Under this low NaOL concentration condition (10−4 M), nanoparticles in the UP-treated pulp are smaller with a broader size distribution compared to those in the TS-treated pulp. This implies that sonication has facilitated the formation of more and finer nanoscale nuclei for NaOL adsorption, and those newly formed zinc oleate nuclei have undergone some degree of growth.
At the higher NaOL concentration (8 × 10−4 M), the TS and UP systems show a reduced average nanoparticle size and a significant increase in particle count compared to the lower NaOL concentration group under the same pretreatment conditions. Notably, the nanoparticle concentration in the UP-treated pulp peaks at nearly 2 × 1011 particles/mL, the highest among all tested groups. Meanwhile, the supernatant turns milky, indicating a complete reaction between NaOL and zinc-containing components, and even micellization of some free NaOL molecules [53,54]. Given that oleate colloids are much smaller than zinc-containing colloids under experimental conditions [55,56], this transformation increases nanoparticle count while reducing the average size. Importantly, the UP-treated pulp consistently exhibits smaller nanoparticles and a higher particle count than the TS-treated pulp, reconfirming that UP enhances the release of zinc ions from smithsonite as free ions or nanoscale zinc-compound colloids. Besides, previous studies have established that nanoparticle size is a critical factor influencing the ability of nanoparticle collectors to hydrophobize mineral surfaces and enhance flotation [57]. At a fixed NaOL concentration, the UP-treated pulp consistently has smaller nanoparticles than the TS-treated pulp, which indicates that UP is more conducive to the effective and stable adsorption of NaOL on the smithsonite surface than TS conditioning.

3.5. Zeta Potential Results

In semi-soluble mineral flotation, various ions undergo hydrolysis, and their hydrolysis products can re-adsorb onto the mineral surface, thereby altering the surface charge through interactions with crystal lattice ions [15,58]. The electrical double layer at the mineral–water interface also affects the adsorption of reagents during oxide mineral flotation [58]. Consequently, zeta potential measurements can serve as an effective tool for characterizing interfacial phenomena in smithsonite flotation systems.
Figure 7 shows the zeta-potential–pH profiles of smithsonite particles subjected to TS and UP, with and without varying concentrations of NaOL. TS-treated smithsonite particles exhibit an isoelectric point (IEP) near pH 8, consistent with Shi’s findings [15]. In contrast, UP shifts the zeta potential curve to the right, with the IEP moving slightly below pH 9. This shift indicates that sonication enhances the entry of positively charged ions or polar groups into the Helmholtz layer, rendering the smithsonite surface positively charged at pH 8 [39]. Beyond the reverse adsorption of dissolved cations and their hydrolysis products, research has revealed a correlation between particle size and surface charge density. As the size of smithsonite particles decreases, the density of exposed zinc active sites increases, leading to a greater number of positive charges being exposed [11], which contributes to the elevated IEP of smithsonite as well.
The introduction of NaOL induces significant negative shifts in the zeta potential of smithsonite particles, reflecting the specific adsorption of anionic oleate ions into the Helmholtz layer, primarily via chemisorption onto zinc active sites [15]. However, a comparative analysis of the effects of TS and UP on the surface potential of smithsonite reveals that the concentration of NaOL is a critical determinant. At low concentrations, NaOL causes a negative shift in zeta potential in both TS and UP systems, yet smithsonite particles in TS exhibit stronger electronegativity. Although UP enhances the interaction between NaOL and smithsonite particles, the low concentration of oleate ions in the solution implies that NaOL adsorption on the smithsonite surface has not reached saturation. The increased exposure of zinc cation active sites in the case of UP results in a reduced overall negative charge on the particles [11]. In contrast, at high concentrations, zeta potential values converge in both systems, indicating near saturation of NaOL adsorption and a surface dominated by zinc oleate components. Zeta potential data further confirm that UP enhances smithsonite dissolution more effectively than TS. The dissolved zinc components and their hydrolysis products affect surface potential and NaOL adsorption, necessitating higher NaOL concentrations for adsorption saturation due to the enhanced dissolution behavior induced by UP. However, it is important to note that, while unsaturated NaOL adsorption does influence the electrostatic properties of mineral particles, its effect on mineral flotation may not be as pronounced. This is because studies have demonstrated that, even with significant enhancement in the floatability of smithsonite at optimal NaOL dosages, many zinc sites on the smithsonite surface remain unoccupied [16,39]. Thus, unsaturated NaOL adsorption does not impede the adequate hydrophobization and floatability of smithsonite.

3.6. XPS Analysis

XPS was employed to investigate the elemental composition and surface characteristics of smithsonite, with a focus on zinc ions, their hydroxide derivatives, and their interactions with NaOL (Figure 8). Figure 8a displays the full-spectrum scan of the smithsonite surface after different pretreatments. Only C, Zn, and O were detected, indicating no extraneous impurities were introduced. Deconvolution fitting of narrow XPS scans (Figure 8b) revealed that the relative atomic concentration of Zn 2p increased with UP compared to TS, suggesting more carbonate ions dissolved relative to zinc ions during smithsonite dissolution. This aligns with previous findings of lower carbonate density and higher zinc atom density on the ultrafine smithsonite surface after dissolution [11]. The surface inhomogeneity of smithsonite has dual effects: increased zinc site exposure promotes hydroxyl compound formation, hindering hydrophobic layers of organic collectors, while more active zinc sites may benefit NaOL adsorption and mineral floatability [11,16]. In the presence of NaOL, the relative C 1s content increases significantly, while Zn 2p decreases, attributed to the formation of a hydrophobic zinc oleate layer. Specifically, Zn 2p decreased from 19.31% to 18.4% in the TS system and from 19.66% to 19.31% in the UP system. The smaller decrease in the UP system is due to the low NaOL concentration used in the experiment, resulting in unsaturated adsorption. Overall, XPS results align well with SEM-EDS data and zeta potential trends.
Extensive research has demonstrated that hydrozincite [Zn5(CO3)2(OH)6(s)], smithsonite [ZnCO3(s)], zinc hydroxide [Zn(OH)2(s)], and amorphous [Zn(OH)2(s)] collectively influence the solubility of zinc-bearing species in neutral-to-weakly alkaline aqueous solutions [35,59,60]. To explore the chemical state changes of elements on the smithsonite surface after differing pretreatments in detail, narrow-scan XPS spectra of the Zn element were acquired (Figure 8c). Notably, the binding energy of the Zn 2p3/2 orbital exhibited a distinct shift, decreasing progressively with the assistance of UP and NaOL. After TS treatment, the Zn 2p3/2 peak appeared at 1022.37 eV, positioned between the Zn 2p3/2 peaks of ZnCO3 (1022.23 eV) and Zn(OH)2 (1022.54 eV) [61,62]. This indicates the hydroxylation of zinc on the smithsonite surface, resulting in a mixture of zinc carbonate and zinc hydroxide. In contrast, UP treatment shifted the Zn 2p3/2 peak to 1022.03 eV (−0.34 eV shift from TS), suggesting a slight change in the chemical state of Zn. Compared to database entries, this peak value lies between the Zn5(CO3)2(OH)6 (1021.70 eV) and ZnCO3 [63,64]. Thus, it is inferred that the smithsonite surface primarily consists of Zn5(CO3)2(OH)6 and ZnCO3 after UP treatment. This phase transformation is likely due to the high-temperature, high-pressure environment from ultrasonic cavitation, promoting [Zn5(OH)6(CO3)2(s)] colloidal particle formation and adsorption on the smithsonite surface. Hydrozincite, with a variable {n(OH)/n(CO3)} ratio, shows enhanced stability under standard CO2 partial pressure compared to smithsonite and zinc hydroxide [16,59]. Though less hydrophobic than zinc carbonate, hydrozincite exhibits stronger hydrophobicity than zinc hydroxide due to lower surface energy from carbonate components.
With the addition of NaOL, the Zn 2p3/2 peak shifted to 1021.97 eV in the TS system. However, in the UP system, the Zn 2p3/2 peak shifted to 1021.95 eV, a value much closer to the classical Zn 2p3/2 binding energy of zinc oleate (1021 eV) [14,65]. These results confirm that UP induces more intense smithsonite–NaOL interaction, leading to more hydrophobic zinc oleate formation. This aligns well with reports of sonication-assisted enhancement of reagent adsorption and various oxide minerals’ flotation.
The data and analysis presented herein demonstrate that UP induces substantial modifications to the physicochemical properties of smithsonite suspensions, as evidenced by alterations in particle geometry and morphology, ionic compositions, electrokinetic properties of suspended particles, and pulp pH. Comparative analysis indicates that enhanced collector adsorption, driven by ultrasonic-assisted dissolution and ion-selective migration, is the primary factor improving the flotation of ultrafine smithsonite particles, rather than other physical modifications. Specifically, UP enhances smithsonite surface dissolution, releasing more Zn ions (predominantly as Zn2+, Zn(OH)+, or zinc-containing colloids) into the bulk phase under neutral-to-weakly alkaline conditions. This provides additional active sites for NaOL adsorption. Relative to TS, UP can meanwhile accelerate the dissolution–hydrolysis–precipitation equilibrium of smithsonite, a process critical for the stable adsorption of NaOL on the ultrafine smithsonite particles. Also, unlike TS, which is hypothesized to release a higher concentration of free zinc ions during the dissolution, UP results in a significantly higher concentration of smaller nanoparticles in the smithsonite solution. These nanoparticles can facilitate NaOL adsorption on the smithsonite surface via surface precipitation, thereby increasing the surface hydrophobicity and floatability of smithsonite. Furthermore, unlike TS conditioning, which results in the formation of zinc hydroxide on the smithsonite surface, UP conditioning forms hydrozincite instead. Hydrozincite is more stable and naturally hydrophobic, and it enhances the surface hydrophobization of smithsonite. Collectively, these factors contribute to the superior flotation performance of UP-treated smithsonite. A schematic diagram illustrating the interaction mechanisms of smithsonite and NaOL under different pretreatments is presented in Figure 9.

4. Conclusions

The present study investigates the effects of UP on the property transformations of ultrafine smithsonite suspensions, particularly focusing on surface dissolution behavior and its subsequent impact on flotation performance. The following conclusions are drawn from the results:
  • UP significantly modifies the physicochemical properties of smithsonite suspensions, including particle geometry and morphology, ionic composition, electrokinetic properties, and pulp pH. Among these changes, ultrasonic-assisted selective surface dissolution is the key factor in enhancing NaOL adsorption and improving the flotation of ultrafine smithsonite.
  • Compared with TS, UP significantly enhances the release of Zn ions (mainly as Zn2+, Zn(OH)+, or zinc-containing colloids) from the smithsonite lattice under neutral-to-weakly alkaline conditions, creating more active sites for NaOL adsorption. Additionally, within the tested short-term conditioning time, a metastable dissolution–hydrolysis–precipitation equilibrium is established in UP cases rather than the TS cases, which is conducive to the stable NaOL adsorption on ultrafine smithsonite particles.
  • TS conditioning creates more free zinc ions during the dissolution, while UP conditioning leads to a significantly higher concentration of smaller nanoparticles in the smithsonite suspension. These nanoparticles can facilitate NaOL adsorption on the smithsonite surface via surface precipitation, thereby increasing the surface hydrophobicity and floatability.
  • TS conditioning typically forms some zinc hydroxide on the smithsonite surface, whereas UP conditioning forms some hydrozincite. Hydrozincite is more stable and inherently hydrophobic, enhancing the surface hydrophobization of smithsonite.
The large specific surface area, significant surface dissolution, and hydration capacity of ultra-fine smithsonite pose substantial challenges to conventional flotation processes. To address these challenges, we developed an innovative approach utilizing sonication to modulate mineral surface dissolution. This technique enhances the flotation performance of fine smithsonite by improving collector adsorption and surface reactivity. Initial laboratory tests on single minerals under idealized conditions demonstrate the potential of sonication for commercial application in processing complex smithsonite ores. However, further research is necessary to confirm its selectivity with common gangue minerals like quartz, goethite, calcite, etc.

Author Contributions

Conceptualization, W.Z., C.L. and L.D.; methodology, W.Z., S.S. and W.C.; software, W.Z., W.C. and L.D.; validation, W.C., H.W., and S.S.; formal analysis, W.Z., W.C. and C.L.; investigation, W.Z. and L.D.; resources, W.Z. and L.D.; data curation, H.W. and L.D.; writing—original draft preparation. W.Z. and H.W.; writing—review and editing, W.Z., C.L. and L.D.; supervision, W.C., H.W., C.L. and L.D.; project administration, W.Z. and L.D.; funding acquisition, S.S., C.L. and L.D. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (52204297 and 52204291) and Yunnan Fundamental Research Projects (No. 202301BE070001-012 and 202401CF070130).

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

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. XRD results for the smithsonite samples.
Figure 1. XRD results for the smithsonite samples.
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Figure 2. The size distribution and porosity characteristics of smithsonite samples with different treatments. (a) Particle size distribution; (b) N2-absorption and desorption isothermal curves; (c) pore size distribution; (d) statistical values of SSA and MPD for different smithsonite samples.
Figure 2. The size distribution and porosity characteristics of smithsonite samples with different treatments. (a) Particle size distribution; (b) N2-absorption and desorption isothermal curves; (c) pore size distribution; (d) statistical values of SSA and MPD for different smithsonite samples.
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Figure 3. SEM-EDS analysis of the smithsonite particles: (a,c) smithsonite + TS; (b,d) smithsonite + UP.
Figure 3. SEM-EDS analysis of the smithsonite particles: (a,c) smithsonite + TS; (b,d) smithsonite + UP.
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Figure 4. Dissolved zinc ion concentration and pH of smithsonite suspension under different pretreatment conditions as a function of conditioning time (a) Variation of dissolved zinc ion concentration in smithsonite suspensions with conditioning time; (b) Measured pH variations in smithsonite suspensions over conditioning time.
Figure 4. Dissolved zinc ion concentration and pH of smithsonite suspension under different pretreatment conditions as a function of conditioning time (a) Variation of dissolved zinc ion concentration in smithsonite suspensions with conditioning time; (b) Measured pH variations in smithsonite suspensions over conditioning time.
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Figure 5. Flotation of ultrafine smithsonite particles with different treatments. (a) Flotation recovery vs. NaOL concentration; (b) flotation recovery vs. conditioning time.
Figure 5. Flotation of ultrafine smithsonite particles with different treatments. (a) Flotation recovery vs. NaOL concentration; (b) flotation recovery vs. conditioning time.
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Figure 6. Nanoparticles counting in smithsonite suspensions under different conditions (a) Size distribution of nanoparticles in suspensions; (b) The average particle size and total particles concentration of nanoparticles in suspensions.
Figure 6. Nanoparticles counting in smithsonite suspensions under different conditions (a) Size distribution of nanoparticles in suspensions; (b) The average particle size and total particles concentration of nanoparticles in suspensions.
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Figure 7. Zeta potential of smithsonite particles under different treatments and NaOl concentrations.
Figure 7. Zeta potential of smithsonite particles under different treatments and NaOl concentrations.
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Figure 8. XPS analysis of smithsonite particles with different treatments. (a) Full spectra. (b) Relative atomic concentration. (c) Narrow-scan spectra of Zn 2p3/2; C(NaOL) = 10−4 M.
Figure 8. XPS analysis of smithsonite particles with different treatments. (a) Full spectra. (b) Relative atomic concentration. (c) Narrow-scan spectra of Zn 2p3/2; C(NaOL) = 10−4 M.
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Figure 9. A schematic diagram illustrating the interaction mechanisms of smithsonite and NaOL under different pretreatments.
Figure 9. A schematic diagram illustrating the interaction mechanisms of smithsonite and NaOL under different pretreatments.
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Table 1. Chemical composition of the smithsonite sample.
Table 1. Chemical composition of the smithsonite sample.
ZnOFeAl2O3SiO2CaOL.O.I
64.10.100.0150.750.0534.98
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Zhou, W.; Cao, W.; Wei, H.; Shi, S.; Li, C.; Dong, L. Sonication-Assisted Surface Erosion and Its Impact on the Flotation of Ultrafine Smithsonite. Metals 2025, 15, 731. https://doi.org/10.3390/met15070731

AMA Style

Zhou W, Cao W, Wei H, Shi S, Li C, Dong L. Sonication-Assisted Surface Erosion and Its Impact on the Flotation of Ultrafine Smithsonite. Metals. 2025; 15(7):731. https://doi.org/10.3390/met15070731

Chicago/Turabian Style

Zhou, Weiguang, Weiwei Cao, Haobin Wei, Shulan Shi, Chenwei Li, and Liuyang Dong. 2025. "Sonication-Assisted Surface Erosion and Its Impact on the Flotation of Ultrafine Smithsonite" Metals 15, no. 7: 731. https://doi.org/10.3390/met15070731

APA Style

Zhou, W., Cao, W., Wei, H., Shi, S., Li, C., & Dong, L. (2025). Sonication-Assisted Surface Erosion and Its Impact on the Flotation of Ultrafine Smithsonite. Metals, 15(7), 731. https://doi.org/10.3390/met15070731

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