Effect of Frother Type on Surface Properties and Flotation Performance of Galena: A Comparative Study of EH, PPG250, and MIBC

Abstract

The selection of suitable frothers in flotation processes plays a crucial role in controlling bubble formation, foam stability, and ultimately mineral recovery. Therefore, understanding the interfacial behavior of frothers is important to optimize flotation efficiency, especially for valuable sulfide minerals such as galena (PbS). In this study, the interfacial behavior and flotation performance of different frothers in PbS flotation were investigated with a particular focus on surface tension, bubble coalescence, foam stability, and flotation recovery. A high-purity crystalline PbS sample (≈96.65% PbS) obtained from Trabzon, Türkiye, was subjected to systematic experimental analyses including surface tension measurements, critical coalescence concentration (CCC) determination, dynamic foam stability (DFS) tests using the DFA100 analyzer, and micro-flotation experiments. 2-ethylhexanol (EH), polypropylene glycol 250 (PPG250), and methyl isobutyl carbinol (MIBC) were used as frothers, while potassium ethyl xanthate (PEX) was employed as a collector. The results revealed that EH had the highest surface activity (42.67 mN/m at 1000 ppm), and the lowest CCC value (~2 ppm) compared to PPG250 (~3 ppm) and MIBC (~8 ppm). According to the micro-flotation results, the flotation recovery gradually increased with increasing frother dosage; the highest recoveries were obtained with PPG250 (99.45%), EH (98.31%), and MIBC (95.17%). PPG250 and EH achieved higher flotation performance at lower dosages compared to MIBC. These findings highlight the critical role of molecular structure and interfacial properties in the effective selection of frothers for galena flotation.

1. Introduction

Flotation is a widely used separation method in mineral processing, particularly for the beneficiation of sulphide ores, and it also finds applications in sectors such as wastewater treatment and paper recycling [1,2,3]. The process relies on the attachment of hydrophobic particles to air bubbles, while hydrophilic particles remain in the liquid phase, making bubble formation and stability essential for efficiency [4,5,6]. In this context, froth flotation has become the most common method for concentrating sulphide ores [7], with its performance strongly influenced by the properties of reagents, especially frothers, which reduce surface tension, prevent bubble coalescence, and stabilize the froth phase [6,8,9]. Frothers also control bubble size and rise velocity, thereby playing a critical role in flotation kinetics and froth transport [10,11,12,13].

As the frother concentration increases in a flotation cell, the bubble size decreases. This is attributed to the reduction in interfacial tension and the stabilization of liquid films formed between colliding bubbles, preventing them from coalescing [13]. However, froth stability is an important parameter in terms of flotation performance. Studies have shown that even a slight decrease in froth stability can lead to a significant decrease in valuable mineral recovery [14,15].

Frothers are generally classified according to their chemical structures or functional properties [16]. However, frother selection is not limited to promoting foam formation alone; it also significantly affects flotation selectivity and recovery. Indeed, literature indicates that increasing frother dosage beyond a certain threshold can negatively impact flotation performance by reducing selectivity and delaying three-phase contact, thus reducing the probability of bubble–particle attachment [12,17,18,19]. This is especially critical where excessive frother–collector combinations can result in recovery of gangue materials instead of the target mineral, ultimately decreasing overall recovery [19,20,21]. In industrial flotation, a variety of frother types are employed beyond the commonly used alcohols. These include cyclic alcohols such as α-terpineol, phenolic mixtures like cresylic acid, alkoxy paraffins with high selectivity, and polyglycol-based frothers whose properties can be tailored by molecular weight. In some cases, dual-function surfactants are applied, while natural reagents like pine oil and emerging biosurfactants such as rhamnolipids represent cost-effective or environmentally friendly alternatives [16].

Various methods are available in the literature to determine the bubble coalescence. For example, Kracht and Finch [22] used ultrasound to determine bubble coalescence. Moreover, in another study, the effect of salts on bubble coalescence was investigated with the help of light intensities [23]. In another study, a McGill bubble size analyzer unit was used to determine the bubble coalescence in terms of Sauter mean diameter (d32) for commercial and analytical chemicals [24]. Gungoren et al. [25] used a modified bubble–particle attachment timer unit to measure the CCC of MIBC in the presence of NaCl and CaCl₂ salts. And, recently, we modified the method used in the literature [23] and built up a unique setup to determine the CCC values for the frothers using a bubble column based on light adsorption [26].

In this context, comparative investigation of the interfacial behavior and flotation effects of frothers with different chemical structures is of great importance for process optimization. While previous studies have investigated the effects of frothers on bubble size [27,28], foam stability [29,30], or flotation efficiency [31,32], comparative studies that evaluate these interfacial properties together with flotation outcomes remain limited. In particular, most investigations have focused on single aspects of frother performance, often employing different mineral systems, experimental conditions, or measurement techniques, which makes direct comparison difficult. As a result, there is limited understanding of how structurally different frothers simultaneously influence surface tension reduction, suppression of bubble coalescence, stabilization of froth, and mineral recovery when evaluated under identical conditions. Addressing this gap requires a systematic experimental design that combines surface chemistry measurements with flotation testing. Therefore, the present study provides an integrated evaluation of three widely used frothers, namely 2-ethylhexanol (EH), polypropylene glycol 250 (PPG250), and methyl isobutyl carbinol (MIBC), by combining surface tension measurements, determination of the CCC, DFS tests, and micro-flotation experiments on a high-purity galena sample. In this way, the study not only compares the interfacial behavior and flotation performance of these structurally different frothers within a single experimental framework but also helps to provide a clearer understanding of frother selection strategies in galena flotation. In the present study, three different types of frothers were evaluated in galena flotation, focusing on their effects on surface tension, bubble coalescence, froth stability, and mineral recovery. In the present study, three different types of frothers were evaluated in galena flotation, focusing on their effects on surface tension, bubble coalescence, froth stability, and mineral recovery.

2. Materials and Methods

2.1. Materials

The galena (PbS) sample used in the experimental studies was obtained in crystalline form from a mine located in Trabzon, Türkiye. The purity of the sample, determined by chemical analysis using atomic absorption spectrophotometry, revealed a lead content of 83.7%. Based on this result, the mineralogical composition of the sample was calculated to contain approximately 96.65% PbS. The crystalline sample was first ground in an agate mortar and then classified using 74 µm and 38 µm laboratory sieves (Retsch, Haan, Germany). To prevent oxidation, the classified sample was placed in a sealed sample bag filled with N₂ gas and stored in a desiccator. Potassium amyl xanthate (PAX, >90%, Green Chemical, Kocaeli, Türkiye) and potassium ethyl xanthate (PEX, >90%, Green Chemical, Kocaeli, Türkiye), which are commonly used collectors in galena flotation, were used in surface tension experiments to demonstrate their surface activity [33,34,35]. However, PEX was used as the collector only in the micro-flotation experiments. Polypropylene glycol 250 (PPG250) (≥98%), methyl isobutyl carbinol (MIBC) (≥99%), and 2-ethylhexanol (EH) (≥99%) were used as frothers, while analytical grade hydrochloric acid (HCl, 37%) and sodium hydroxide (NaOH, ≥98%) were employed for pH adjustments.

2.2. Methods

2.2.1. Surface Tension Measurements

The surface tension measurements were carried out using a Krüss K6 Force Tensiometer (Krüss, Hamburg, Germany) based on the Du Noüy Ring Method. Solutions prepared at various concentrations were transferred into a 50 mL cylindrical container. A platinum-iridium ring was carefully immersed just below the air-liquid interface and then slowly withdrawn using the tensiometer’s dynamometer. The measurement was recorded at the point where the liquid film detached from the ring. Before each measurement, the platinum-iridium ring was cleaned with analytical grade ethanol and flame-washed for 1 min to ensure complete removal of residues. All measurements were performed at room temperature (23 ± 1 °C). Each experiment was conducted in triplicate to ensure reproducibility.

2.2.2. Bubble Coalescence Measurements

The critical coalescence measurements were conducted using the method proposed by Guven et al. [26], which is widely used in the literature [36,37]. In this method, a micro-flotation cell with a total volume of 155 cm³ (30 × 220 mm) and frit pore sizes ranging from 10 to 16 µm was utilized. Nitrogen gas was introduced into the cell at a controlled flow rate of 50 cm³/min. The experimental setup included a cold light source (Soif Optical Instrument Co., Shanghai, China), a light intensity detector (Thorlabs, Newton, NJ, USA), and a computer equipped with Optical Power Monitor software (Version 6.1) for data acquisition and analysis. The experimental setup operates on the principle that light emitted from the source passes through the solution within the micro-flotation cell, during which a portion of the light is absorbed by the solution before reaching the detector. The intensity of the transmitted light is then measured. The light beam was directed approximately 10 cm above the frit, which corresponds to the midpoint of the solution column. All measurements were repeated three times, and the average values were reported.

2.2.3. Dynamic Foam Stability Measurements

In this study, foam stability measurements were carried out using the KRÜSS Dynamic Foam Analyzer DFA100 (KRÜSS GmbH, Hamburg, Germany). This instrument is designed to measure foam formation and the stability of the generated foam in liquid systems. The device consists of a measuring cell, optical sensors that monitor the foam height, and a computer system for data acquisition and analysis. In the experiments conducted with the DFA100, the maximum foam height was used to determine the dynamic foam stability (DFS) of the frothers using Equation (1):

$$\Sigma ={\displaystyle \frac{v_f}{Q}}={\displaystyle \frac{H_{max}\cdot A}{Q}}$$

(1)

Ʃ represents the dynamic foam stability (min), V_f is the foam volume (cm³), Q is the gas flow rate (cm³/min), H_max is the maximum foam height (cm), and A denotes the cross-sectional area of the column (cm²).

In the experiments, 50 mL of the prepared solution was transferred to a tempered glass column with an inner diameter of 40 mm and a height of 250 mm, and the system was started. Air was introduced at a flow rate of 0.2 L/min to generate foam and dispersed through a porous filter plate with pore sizes ranging from 12 to 25 μm. The foam generation phase was set to 60 s, and the foam height was recorded continuously during this time. After the 60 s foam formation period, the system automatically switched to the decay phase, where foam collapse was monitored for another 40 s. After all measurements, the data were processed using the integrated software. All experiments were conducted at room temperature (23 ± 1 °C). Each test was performed in three repetitions, and the average values were reported.

2.2.4. Micro-Flotation Experiments

Micro-flotation experiments were carried out using a 155 mL volume column with a ceramic frit with pore sizes ranging from 10 to 16 μm. In these experiments, a purified galena sample with a particle size range of −74 + 38 μm was used.

The flotation experiments were carried out at a 1% solids concentration (w/w) by adding an appropriate amount of galena sample to water. The suspension pH was adjusted to 9.00 ± 0.1 using NaOH and conditioned using a magnetic stirrer at 600 rpm for 1 min. This pH value was chosen because it represents an optimum condition for both xanthate–galena interactions and effective action of frothers [38,39]. Then, the desired amount of PEX was added, and the suspension was conditioned for 3 min. Finally, the frother was added and mixed for another 1 min. At the end of the conditioning period, the suspension was transferred to the flotation cell, and air was supplied at a flow rate of 50 cm³/min for 1 min to ensure mineral flotation. Each flotation test took approximately 6 min in total.

In the flotation experiments, the frother dosage was selected within the range of 1–1000 ppm. This range enabled observation of flotation behavior from insufficient frother levels to very high concentrations, helping to evaluate the performance of each frother under both typical and extreme conditions. It is known from the literature that frothers may be insufficient to significantly reduce bubble coalescence at very low concentrations (1 ppm), leading to larger bubble size, low flotation recovery, and poor froth stability [26,40,41]. Furthermore, at higher concentrations (100 ppm), frothers can cause excessive froth stability, leading to entrainment of gangue particles and reduced selectivity. On the other hand, extending the range to 1000 ppm provides a safety margin for determining potential overdose effects.

The choice of operating variables and their levels was based on optimum values reported in the literature for galena micro-flotation studies [35,42,43,44,45]. After flotation, the float and sink products were collected separately and subjected to gravimetric analysis, and the flotation recovery was calculated using Equation (2):

$${\displaystyle \frac{m_{ft}}{m_{fd}}}\times 100$$

(2)

where m_ft denotes the mass of the floated product, while m_fd represents the mass of feed. All flotation experiments were performed in triplicate to ensure statistical reliability.

3. Results and Discussion

3.1. Effect of Reagent Type on Surface Tension

The surface tension experiments were carried out for the reagents as a function of concentration from 1 to 1000 ppm (Figure 1). As seen in Figure 1, the results revealed distinct trends depending on the functional group of each reagent.

Among the reagents tested, EH showed the most significant decrease in surface tension. The surface tension, which started from 64.8 mN/m at 1 ppm, decreased sharply to 59.83 mN/m at 10 ppm and reached 42.67 mN/m at 1000 ppm. This rapid decrease suggests that EH has a high affinity for the air–water interface, likely due to its short-chain, branched alcohol structure. Such compounds are known to adsorb rapidly and intensively at the interface, effectively reducing surface tension and increasing foamability in flotation systems [46,47].

Similarly, PPG250 gradually decreased surface tension as its concentration increased, and the values decreased from 69.4 mN/m at 1 ppm to 44.87 mN/m at 1000 ppm. Its amphiphilic and flexible polymeric chain enables the formation of stable interfacial films that delay bubble coalescence. Various studies [6,48] reported that PPG-type frothers provide moderate foam stability and a stable drainage performance. Furthermore, the study by Finch and Zhang [49] showed that the hydrophilic–lipophilic balance (HLB) and the H-ratio are the main factors affecting the CCC, and PPG frothers occupy an intermediate position on the HLB–CCC curve.

MIBC, on the other hand, showed a more moderate and delayed effect. The surface tension remained relatively unchanged up to 100 ppm (70.7 mN/m), while it gradually decreased at higher concentrations, reaching 57.4 mN/m at 1000 ppm. Using molecular dynamics simulations, Alvarado et al. [50] showed that MIBC molecules tend to adsorb less densely and more irregularly at the air–water interface compared to short-chain alcohols [46], leading to a slower and less efficient surface tension reduction. However, this feature can be useful in controlling froth height and mobility in industrial flotation applications.

In contrast, the xanthate collectors PAX and PEX showed minimal effect on surface tension, with values remaining almost unchanged across the entire concentration range. This is consistent with the known behavior of xanthates, which prefer to adsorb onto mineral surfaces rather than accumulate at the air-water interface. Several studies [51,52,53] have confirmed that xanthates show poor surface activity in aqueous systems and that their main role in flotation is due to chemical interactions with sulfide mineral surfaces rather than surfactant-like behavior [54]. As seen in Figure 1, no significant change was observed in the surface tension of PAX/PEX even at higher concentrations, indicating that the selectivity of PAX/PEX towards the mineral/water interface is quite low. Furthermore, as is known from the literature, there are a few carbon atoms in the hydrocarbon chain structure of xanthate molecules to reduce the surface tension [54].

3.2. Determination of CCC Values of Different Types of Frothers

Figure 2 shows the percentage of bubble coalescence as a function of frother concentration. Using the conventional 50% coalescence threshold to define the CCC [26], EH again emerged as the most efficient frother, with a CCC of approximately 2 ppm, followed by PPG250 (~3 ppm) and MIBC (~8 ppm).

These CCC trends parallel the surface tension results, indicating that frothers that reduce surface tension also tend to suppress coalescence more effectively. This correlation has been confirmed in previous studies showing that high surface activity promotes rapid formation of stable interfacial films, thereby enhancing bubble-to-bubble repulsion and preventing film rupture [49,55].

The CCC values of polyglycol ether frothers have been reported in the literature to range from 6.0 to 6.6 ppm [8,18,56,57,58], based on bubble size-concentration measurements using image analysis techniques. In the present study, however, the CCC value of PPG250 was determined as 3 ppm using the light scattering method, indicating that the measurement technique can influence the reported values. PPG-based frothers are well known to generate persistent foams owing to their ability to form sterically stabilized interfacial films that effectively resist drainage and coalescence [18].

Although MIBC is one of the most widely used frothers in flotation due to its moderate selectivity and cost-effectiveness, it exhibits a relatively high CCC, typically reported in the range of 6–10 ppm [26,56]. A higher CCC indicates that higher dosages of MIBC are needed to achieve froth stability similar to more surface-active frothers such as short-chain alcohols or PPG-based polyglycol ethers, which generally have lower CCC values [8,18]. This requirement arises from the fact that frothers with lower CCCs are more efficient in suppressing bubble coalescence at lower concentrations by forming stable interfacial films.

3.3. Dynamic Foam Stability Measurements

Figure 3 illustrates the evolution of foam height over time for various frothers and highlights the performance of the commonly used medium-strength aliphatic alcohol MIBC. The results show that increasing the MIBC concentration leads to a gradual increase in foam height, and maximum foam stability is observed at 1000 ppm. This trend is consistent with the findings of Alsafasfeh et al. [59], who reported that both foam height and dynamic froth stability increased with MIBC dosage in flotation systems. Furthermore, the foam decay curve for MIBC indicates a controlled and continuous drainage pattern, suggesting the formation of moderately stable, yet permeable interfacial films. This observation agrees with the work of Wang et al. [60], who showed that MIBC-stabilized foams exhibited relatively rapid drainage and shorter foam half-lives compared to those formed by more surface-active frothers, such as polypropylene glycols.

In the case of EH, at 1 ppm, the foam height stabilized around 4–5 mm, while 10 ppm and 100 ppm yielded higher and more stable foam layers (~8–10 mm). Further increases up to 1000 ppm resulted in only a marginal additional increase (~11–12 mm). This plateau behaviour is consistent with surface tension measurements reported by Yuan and Herold [61], who found that EH rapidly decreases surface tension to below 45 mN m⁻¹ at sub-ppm levels, approaching a solubility-limited plateau near 800–900 ppm.

In the case of PPG250, the foam height was significantly dependent on the concentration, and particularly pronounced effects were observed at 100 and 1000 ppm. These observations are in agreement with previous studies showing that increasing the frother concentration leads to increased foam height and dynamic stability [21,62]. Only a thin and unstable foam layer was observed at the lowest concentration of 1 ppm, while moderate improvements in foam stability occurred at 10 ppm. The most significant foaming behavior was noted at 1000 ppm, where a thick and persistent foam layer developed, consistent with the findings that polyglycol-type frothers promote the formation of smaller and more stable bubbles, thereby strengthening the foam structure as the concentration increases [11,21]. In this study, it was found that at higher concentrations, a residual foam layer of approximately 8 mm thick remained throughout the decaying phase. This type of persistent foam is mechanistically attributed to the suppression of bubble coalescence and the stabilization of the air-water interface through surface tension gradients; this is often described in terms of Gibbs or Marangoni effects [11,63]. However, the prolonged presence of this residual foam layer can lead to operational difficulties as it can inhibit the efficient transport and removal of froth from the flotation cell, a factor that plays a critical role in overall flotation performance [64,65]. This issue has also been highlighted in recent studies, where excessive froth stability resulting from high frother dosages has been identified as a practical concern in industrial flotation systems [16].

Figure 4 demonstrates a significant concentration-dependent trend in the dynamic foam stability (DFS) of the frothers. While all frothers increased foam life by increasing the concentration from 1 to 1000 ppm, the degree of this improvement varied significantly. Among these, PPG250, a polypropylene glycol-based frother, exhibited the most pronounced and nonlinear increase in DFS, increasing from approximately 25 s at 1 ppm to approximately 65 s at 10 ppm, and exceeding 190 s at 1000 ppm. This trend agrees with previous findings showing that medium-molecular-weight PPGs form sterically stabilized interfacial films that effectively inhibit bubble coalescence when they exceed their CCC (3 ppm) [6,48]. In contrast, MIBC showed a more gradual, almost linear increase in DFS, from approximately 22 to 75 s, reflecting its higher CCC (typically 6–10 ppm) and the lower surface elasticity of its aliphatic films. Similar moderate trends for MIBC were reported by Melo and Laskowski [8] and Finch and Zhang [49]. The lowest foam stability was observed for EH, a branched short-chain alcohol, whose DFS only increased from ~18 to ~50 s. This observation supports previous studies suggesting that EH forms loosely packed films resulting in thin lamellae prone to rapid drainage, despite rapid interface adsorption [47]. Taken together, the overall order of foam stability, PPG250 > MIBC > EH, coincides with well-known relationships between frother molecular structure, hydrophilic–lipophilic balance, and foam persistence reported in flotation literature [49,66].

3.4. Effect of Frother Type on Flotation Recovery

Figure 5 presents the flotation recoveries of three structurally different frothers, polypropylene glycol ether (PPG250), a branched aliphatic primary alcohol (EH), and a branched aliphatic secondary alcohol (MIBC) over a wide concentration range. All frothers exhibited characteristic sigmoidal recovery curves with three distinct regions based on concentration.

In the initial region (≤1 ppm), the flotation recoveries remained below 25% for all frothers, indicating that flotation was under sub-CCC conditions, approximately 2 ppm for EH, 3 ppm for PPG250, and 8 ppm for MIBC, where bubble coalescence predominates due to insufficient interface stabilization [26,49]. In the transition region (1–10 ppm), both PPG250 and EH rapidly exceeded their CCCs, achieving 89% and 86% flotation recoveries, respectively. In contrast, MIBC gave only 74% recovery at the same dosage, reflecting its relatively lower surface activity and the need for higher concentrations to effectively suppress bubble coalescence [8,18].

In the saturation region (≥100 ppm), all frothers approached their maximal flotation recoveries (>95%). The increases in concentration up to 1000 ppm led to insignificant improvement, indicating that the surface tension had already reached its minimum level, and beyond this, the additional frother had no further effect on bubble stabilization [6,58]. These findings highlight the critical role of molecular structure in governing frother performance, emphasizing that PPG250 and EH in the presence of 1 ppm PEX exhibit superior flotation efficiency at lower dosages due to their enhanced ability to suppress bubble coalescence and reduce surface tension.

4. Conclusions

• In this study, the interfacial behavior and flotation performance of three structurally distinct frothers (EH, PPG250, and MIBC) were systematically investigated using a high-purity galena sample. The results revealed the critical role of frother molecular structure and surface activity in determining surface tension, bubble coalescence, foam stability, and flotation efficiency.
• EH exhibited the highest surface activity, reached the lowest surface tension and CCC value (~2 ppm), indicating rapid interface adsorption and effective coalescence suppression.
• PPG250 provided the best dynamic foam stability and flotation recovery by forming persistent interfacial films even at low dosages.
• MIBC, despite its widespread industrial use, showed moderate performance; higher concentrations (~8 ppm CCC) were required to achieve comparable stability and recovery due to its weaker surface activity and lower film elasticity.
• The results of the micro-flotation experiments confirmed that all frothers followed a typical flotation recovery curve, with recovery increasing gradually as frother dosage increased; the highest recoveries were obtained with PPG250 (99.45%), EH (98.31%), and MIBC (95.17%).
• In this study, the experiments were carried out using a single high-purity galena mineral. However, industrial flotation processes often involve complex ore bodies and the presence of various gangue minerals, which can significantly affect reagent behavior and overall process performance. Therefore, future research should aim to validate these findings in multi-mineral flotation systems to better represent industrial conditions.
• The results also carry important practical implications for mineral processing operations. Selecting frothers with lower CCC values and higher interfacial activity (such as EH and PPG250) allows reduced reagent consumption. In addition to reducing operational costs and increasing process efficiency, lower dosages contribute to more sustainable flotation practices by minimizing chemical usage, reducing waste generation, and limiting environmental impacts, a critical factor especially in large-scale operations.