Surface Properties and Beneficiation of Quartz with Flotation

Abstract

This review aims to advance quartz processing technology by examining the surface properties, flotation behavior, and selective flotation mechanisms of quartz mineral. Characterized by a strong negative charge over a wide pH range and an isoelectric point around pH 2, quartz surfaces allow physical adsorption of cationic collectors, particularly amines, which render the quartz surface hydrophobic and enhance bubble–particle interactions. In contrast, flotation with anionic collectors requires prior surface activation via multivalent metal cations such as Ca²⁺. The pH value of the medium plays a critical role in both collector adsorption and flotation selectivity. Both direct and reverse flotation strategies can be used, depending on whether quartz is targeted as a valuable mineral or a gangue mineral. In direct flotation, depressants like carboxymethyl cellulose and starch are used to depress gangue minerals, while in reverse flotation, quartz is depressed using chemicals such as fluoride ions and cationic polymers. To improve the efficiency and selectivity of quartz flotation, further research is needed on surface chemistry, collector adsorption mechanisms, and the transition from laboratory-scale experiments to industrial applications.

1. Introduction

Silicon (Si) (27.2 wt%) is the second most abundant element in the Earth’s crust after oxygen (45.5 wt%). It is not found in its native form in nature but is usually found in the form of oxygenated compounds. Such minerals containing Si-O-Si bonds are called “silicates”. A total of 95% of the Earth’s crust consists of silicate minerals, including quartz, feldspar, mica, amphibole, pyroxene, olivine, and various clay minerals [1]. Quartz, trigonal, low-temperature α−quartz (SiO₂), is the most important silicon mineral and one of the most common minerals found in the Earth’s crust [2,3]. It is found in all rocks: igneous, metamorphic, and sedimentary [4]. Generalized quartz also includes high-temperature quartz (β−quartz) and several other polymorphs of silica, including tridymite, cristobalite, and coesite [5,6].

The main structure of silicates (orthosilicates) is a silicon atom surrounded by four oxygen atoms in a tetrahedral system (SiO₄)⁴⁻ [7]. The Si-O bond angle in α-quartz is 144°, and the bond length is between 0.16101 and 0.16145 nm. Although quartz is usually transparent or white, the impurities, such as iron oxides, can change its color over a wide range. Its hardness is 7 (very hard) on the Mohs hardness scale [6]. Figure 1 shows a schematic of a quartz molecule and the X-ray diffraction (XRD) spectrum of an analytical-grade quartz [8].

High-purity quartz deposits have been found mainly in various deposits in the United States, Australia, China, Mauritania, Norway, Canada, Russia, and Brazil [6,9]. Since quartz is a common mineral, less pure quartz deposits can be found in many countries, including Austria, Belgium, France, Germany, India, Iran, Italy, Japan, Mexico, South Africa, Spain, Türkiye, and the United Kingdom [10].

Although quartz is found as a gangue mineral in many ores, it is also a critical raw material for many industries [11]. For example, it is used as a bulk product in industries such as glass, ceramics, and foundry. Moreover, high (99.9% SiO₂) [12] and ultra-high (99.999%) purity quartz can be used in high-tech materials such as semiconductors, photovoltaics, and fiber optics due to its unique physical and chemical properties, including resistance to corrosion, low thermal expansion coefficient, high-temperature resistance, exceptional transparency, and effective electrical insulation [13,14,15,16]. When the SiO₂ content of quartz is less than 98%, its analysis can be performed by XRD and X-ray fluorescence (XRF) methods. However, since the analysis of high-purity quartz requires high sensitivity, it is necessary to use Inductively Coupled Plasma—Emission Spectrometry (ICP-ES) [6] or Inductively Coupled Plasma—Mass Spectrometry (ICP-MS) [17] at high SiO₂ grades.

The most important factor determining the application area of quartz is the type and amount of impurities it contains. For example, in glass production, quartz is expected to contain 93%–99% SiO₂, 0.05%–0.1% Al₂O₃, and 0.01%–0.1% Fe₂O₃. The minimum SiO₂ grades demanded for low, medium, and high-grade quartz are 99.95, 99.99, and 99.997%, respectively [18]. Meanwhile, the Al grade of a standard high-purity quartz should not exceed 22 ppm, and other impurities such as B, Ca, Fe, K, and Na should be below 1.5 ppm [6,19,20]. Furthermore, high-purity quartz should contain less than 50 μg/g of impurities (B, Li, Al, Ge, Ti, Fe, Mn, Ca, K, Na, and P) [21]. Therefore, quartz ore needs to be processed after the production [22]. Approximately 15–30 kg of pure quartz is needed to produce 1 kg of high-purity silicon (Si) [18,21].

Common gangue minerals of quartz are feldspar, magnetite, hematite, rutile, mica, pyrite, and tourmaline, and the beneficiation method depends on the type and form of the impurities it contains [23]. Quartz can contain three different types of impurities: independent minerals, inclusions, and lattice impurities. When the elements Fe, Al, and Ti are present as independent minerals, they can be separated from quartz by gravity separation, magnetic separation, fluorine-free acid leaching, and flotation. When these elements are in the form of inclusions, they are calcined and quenched with water to expose the inclusions after grinding. Then, the impurities are removed by magnetic separation, acid leaching, or flotation. When Al and Ti elements are present as lattice impurities, fluorine-containing acid leaching is the main beneficiation method [6]. Since quartz is insoluble in acids except hydrofluoric acid (HF) and concentrated phosphoric acid (H₃PO₄), it can be purified by acid leaching [24,25]. In this method, Fe and Al are dissolved in the acid solution. Mixtures of hydrochloric (HCl), sulfuric (H₂SO₄), nitric (HNO₃), phosphoric (H₃PO₄), oxalic (H₂C₂O₄), and citric (C₆H₈O) acids in various proportions can also be used for this purpose [26]. After acid leaching, a base such as sodium hydroxide (NaOH) can be used to neutralize the acid. High-purity quartz can be obtained by washing quartz with base or directly rinsing it with pure water [8,27,28]. The purity of quartz and the methods used to purify it affect subsequent mineral processing operations, particularly those where surface properties are important, such as flotation [24].

In this study, quartz surface properties affecting the flotation process were examined, and the results obtained in previous studies under various conditions and techniques were reviewed and interpreted in detail to contribute to the quartz purification technology. In this context, firstly, the electrochemical and wettability properties of quartz were explained. Then, the interaction of quartz particles with bubbles in flotation was explained with bubble–particle attachment time studies. In terms of beneficiation, physical methods used for the pre-concentration of quartz were explained concisely. Then, the studies on the selective direct and reverse flotation of quartz were examined in detail.

2. Surface Properties of Quartz

2.1. Surface Roughness and Heterogeneity of Quartz

The role of particle morphology, surface heterogeneity, and interfacial forces in governing attachment and deposition dynamics improves understanding of bubble–particle and particle–particle interactions and provides important consequences for colloidal deposition and, in particular, flotation processes. For example, DLVO theory (named after Derjaguin, Landau, Verwey, and Overbeek) describes the overall particle surface interactions in liquids by combining van der Waals attraction and electrostatic double-layer repulsion forces [29,30,31] and is widely applied in fields such as water treatment, emulsions, suspensions, and most importantly, mineral flotation [32]. However, this theory is effective for homogeneous, smooth surfaces and does not account for heterogeneity in real cases [33]. Extended DLVO (XDLVO) theory is an improvement of the classical DLVO theory that includes additional interaction forces to better describe colloidal and interfacial phenomena and considers short-range non-DLVO interactions such as acid-base (Lewis acid/base), hydration, and steric forces [29]. This extended framework allows for more accurate predictions of adhesion, stability, and aggregation behaviors in colloidal systems, particularly when surface chemical heterogeneity and polar interactions are involved [34,35,36]. For example, many studies have indicated that the XDLVO theory better describes bubble–particle and particle–particle interactions in flotation systems and have revealed the limitations of the original DLVO model, which ignores hydrophobic interactions [37,38,39,40,41].

Kuijk et al. [42] developed monodisperse, rod-like silica colloids with tunable aspect ratios. They also introduced a well-characterized model system for studying anisotropic particle behavior and phase transitions in colloidal suspensions. Furthermore, the results obtained in this study indicate that these particles can be used as valuable tools for 3D high-resolution imaging and fundamental studies on shape-dependent assembly.

Meanwhile, Drelich and Wang [32] addressed the limitations of classical DLVO theory in heterogeneous systems, showing that surface heterogeneity at the microscale gives rise to anisotropic interactions, often leading to attractions much stronger than those predicted by mean-field models. They also noted that atomic force microscopy (AFM) can produce detailed maps of surface charge distributions, supporting more accurate modeling and simulation of colloidal interactions.

Additionally, Gomez-Flores et al. [43] used QCM-D and modified DLVO theory to study the deposition behavior of anisotropic silica particles in the presence of different ionic strengths. The results showed that bullet-shaped particles deposited more easily than spherical particles due to orientation effects and lower energy barriers, highlighting the importance of particle geometry.

Furthermore, Gomez-Flores et al. [44] presented a theoretical framework that analyzes how surface roughness, charge heterogeneity, and contact angle heterogeneity affect bubble–particle interactions. Their numerical modeling studies indicated that charge heterogeneity is the strongest influence on interaction energy barriers, followed by surface roughness and contact angle variations.

Gomez-Flores et al. [44] investigated the effect of surface chemical heterogeneity on bubble–particle attachment probabilities in coarse particle flotation. Their results showed that bubble size and hydrophobic dot distribution increased bubble–particle attachment efficiency, with small bubbles preferring densely distributed small dots, while larger bubbles prefer sparsely distributed larger dots.

Many of these studies in the literature advance the understanding of particle anisotropy and surface heterogeneity in colloidal systems, and consequently, the fundamental importance of bubble–particle and particle–particle interactions, particularly as they relate to flotation efficiency. They also demonstrate that experimental techniques, combined with advanced modeling, provide the necessary information to improve flotation efficiency.

2.2. Electrochemical Properties

Most particles in an aqueous solution acquire a surface charge (σ₀) as a result of the ionization of chemical groups or the adsorption of various ions in the solution on their surfaces. The distribution of surrounding ions in the solution is affected by this surface charge. As a result, an inner region (Stern plane) (δ) is formed near the surface, where the ions are strongly bound to the particle. Additionally, just outside this region, a diffuse layer forms in which the ions are more loosely bound. Inside the diffuse layer, there is a boundary where the adsorbed ions move with the moving particle, for example, through Brownian forces. The potential at this boundary, called the surface of shear, is called the zeta (ζ) potential (ψ_δ) [45]. As the distance from the Stern layer to the bulk solution increases, the potential decreases exponentially and reaches zero at the boundary of the bulk solution. The point where the surface charge is zero is called the isoelectric point (IEP) [46]. This structure consists of the charged surface, and the liquid containing the balancing charges is called the electrical double layer (EDL) [46,47,48].

Zeta potential measurements play an important role in understanding the interfacial properties and interactions in flotation. The EDL, and hence, the zeta potential, controls the reagent adsorption, flocculation/dispersion, and slime coating processes of mineral suspensions, as well as the bubble–particle interactions [46]. The zeta potential of air bubbles is negative in the presence of most collectors and frothers. In some exceptional cases, such as in the presence of dodecylamine, it has been measured to be positive over a wide pH range (2.30–11.50) [49,50].

The adsorption of oppositely charged (counter) ions onto the mineral surface by electrostatic and hydrophobic forces, via van der Waals interaction between the hydrocarbon chains, is called physical adsorption or physisorption. In case of physisorption of anionic or cationic collector molecules onto the mineral surface, the mineral surface must be charged opposite to the polar part of the collector, and the magnitude of this charge directly affects the adsorption density of the mineral and, hence, its floatability. A change in the pH can affect the floatability of a mineral because it changes the concentration of H⁺ and OH⁻, the ions that determine the potential for many minerals. Therefore, the change in the zeta potential of the mineral and the pH value of the IEP, if any, are of great importance for such systems [46].

On the other hand, if the surfactant forms covalent bonds with metal atoms on the mineral surface instead of a physical interaction, this process is called chemical adsorption or chemisorption [46]. In chemisorption, the same charge (positive or negative) between the mineral and the collector increases the chemisorption, while a much higher pH than the IEP has a negative effect.

In the case of quartz, which is a mineral composed of Si and O elements, O atoms are shared between two tetrahedra of a continuous SiO₄ structure. As a result of the grinding process, many covalent Si-O bonds are broken (Figure 2a). The broken Si-O bonds adsorb H⁺ or OH⁻ ions in the solution, depending on the pH level; hence, the quartz surface was covered with Si-OH groups. Then, H⁺ dissociates, and negatively charged oxygen atoms (O⁻) remain on the mineral surface, which makes quartz negatively charged at a wide pH range (Figure 2b,c). In this case, the silicate ion (SiO₄⁴⁻) is responsible for the negative charge. At very acidic conditions (pH < 2), beyond its IEP, quartz edges turn positively charged due to the adsorption of H⁺ ions [23,51].

The results for the zeta potential measurements of quartz as a function of pH carried out by several researchers are given in Figure 3. As seen in Figure 3, the zeta potential of quartz was negative over a wide pH range, and its IEP was approximately pH 2 [46,52,53,54,55]. Figure 3 also shows that hematite, one of the main impurities of quartz, has a positive zeta potential up to pH 6.7. Above this pH, the zeta potential was slightly negative [56]. The negative charge on the quartz surface allows it to be easily floated by cationic collectors over a wide pH range. Other impurities within the quartz ore can be selectively depressed according to their IEP values. Therefore, selectivity in flotation can be achieved with attentive engineering.

According to the electrostatic model of flotation, physically adsorbed collectors must contain ions opposite to the surface charge of the mineral. For this reason, oxidized minerals are floated using cationic collectors above the IEP (when the surface is negatively charged) and anionic collectors below the IEP (when the surface is positively charged). Therefore, adjusting the pH in the presence of an ionic collector is of great importance for selective flotation. The results of Smith and Akhtar [58] showed that the magnitude of the zeta potential of quartz (−120 mV) decreased with dodecylammonium acetate (DAA) concentration. Depending on pH (pH 7–11), the IEP reached over 1 × 10⁻⁵ mol/dm³ and was positive at higher concentrations. At lower pH levels, higher DDA concentrations were required to reach the IEP.

Fuerstenau and Pradip [46] reported a strong relationship between the zeta potential, surface coverage, contact angle, and the flotation efficiency of quartz in the presence of DAA (4 × 10⁻⁵ mol/dm³), especially above pH 8. These parameters were maximized at pH 10–11. This is because dodecylamine is present as 50% ammonium ions [NH₄⁺] and 50% amine molecules at pH 10.4. Under these conditions, flotation was maximized due to the co-adsorption of NH₄⁺ ions and amine molecules. As the pH increased to about 12, the flotation efficiency decreased drastically and stopped at pH 12.6. This is because there were not enough NH₄⁺ ions in the medium to bind the collector molecules to the mineral surface. This pH limit is universal for dodecylammonium collectors [46].

The effects of various cations on the zeta potential of quartz have been investigated in previous studies. Qi et al. [59] reported that the zeta potential of quartz became less negative with increasing salt concentration by adding Na⁺, Ca²⁺, and Mg²⁺, chloride salts. The zeta potential results of Zhu et al. [53] at pH 3 are given in Figure 4a as a function of metal ion concentration. Figure 4a shows that Al³⁺, Ca²⁺, and Mg²⁺ had a limited effect on the zeta potential of quartz. On the other hand, the zeta potential of quartz became more positive with increasing Fe³⁺ concentration and changed to positive at 15 mg/dm³ FeCl₃ [53]. This effect of ions on the zeta potential of quartz is of great importance in plant conditions where non-freshwater resources such as borewater or recycled plant water are used, as it will affect the recovery and selectivity of quartz flotation. Therefore, it should be studied specifically for each water chemistry condition.

Ruan et al. [60] investigated the effect of Ca²⁺ ions on the zeta potential of quartz. Their results, shown in Figure 4b, showed that the IEP value of quartz did not change with Ca²⁺ concentration; however, the quartz surface became much less negative in the presence of Ca²⁺ above pH 3.4.

2.3. Wettability

In flotation, the particles of the target mineral must attach to an air bubble that has enough buoyancy to carry them into the froth zone. Therefore, the floatability of a mineral particle depends on the adhesion force between the surface of the particle and the air bubble. The main factor affecting this force is the wettability of the particle surface. While the minerals whose surfaces are wetted easily are called hydrophilic, the minerals with a limited affinity for wetting are called hydrophobic [61].

The most common indicator of wettability is the contact angle, which is the angle that the liquid makes with the solid and gas phases in a three-phase system [62]. For a given solid-liquid-gas system, different contact angles can be measured under various conditions, such as advancing, receding, rest, and equilibrium [63,64]. Furthermore, the contact angle can be measured by various calculation methods such as Young, Wenzel, and Cassie equations [62], and various measurement techniques including sessile drop, captive bubble, Washburn technique, capillary rise, and flotometry [65].

As is known, the contact angles of different liquids on the mineral surface can be measured. In this study, contact angle information of water or aqueous surfactant solutions on quartz is reviewed. Different crystal faces of quartz have different wettability properties. The results of Deng et al. [66], shown in Figure 5, showed that $0001, 10\overline{1}0, \mathrm{a}\mathrm{n}\mathrm{d} 11\overline{2}0$ faces of α-quartz had the approximate average contact angles of 13.35°, 63.75°, and 35.50°, respectively.

Hassanzadeh et al. [67] and Mao et al. [68] measured the contact angle of quartz as 12° and 21.2°, respectively. Dai et al. [69] reported that the contact angle of quartz increased from 24.7° to 63.8° after DDA adsorption. Terpilowski [70] investigated the effect of temperature on the advancing and receding contact angles of quartz. They obtained the lowest advancing and receding contact angles as 28.6 ± 2.8° (10 °C) and 19.5 ± 2.9° (20 °C), respectively. Meanwhile, the highest advancing and receding contact angles were 48.7 ± 3.3° (50 °C) and 39.9 ± 3.5° (40 °C), respectively. Like almost all minerals, quartz requires a contact angle of 60–80° to float. Chen et al. [71] reported that the contact angle of quartz could be increased by increasing the ionic strength. Meanwhile, the main reagents used to render quartz hydrophobic are the collectors with 12–18 carbon atoms, such as fatty acids or their salts and amines [62]. In this context, Zhao et al. [72] reported that the contact angle of quartz increased from 45° to 100° after conditioning with 20 mmol/dm³ dodecyl-tri-methylimidazolium chloride. In another study, Wang et al. [73] measured the contact angle of quartz at pH 10 in the presence of DDA and a DDA/sodium oleate (NaOL) mixed collector system at various ratios. As seen from the results of their study in Figure 6a, cationic DDA sharply increased the contact angle of quartz. At a DDA concentration of 1 × 10⁻⁴ mol/dm³, a maximum contact angle of 68° was obtained and reached a plateau. In the case of the mixed collectors, the increasing trend in the contact angle as the collector concentration increased was similar. The contact angle remained at much lower levels as the anionic NaOL ratio increased. Considering that NaOL did not adsorb on the quartz surface in the absence of DDA due to the electrostatic repulsion force, the appropriate collector system can be selected according to whether the quartz should be floated or depressed in a flotation process [73].

Zawala et al. [74] measured the static advancing contact angle of quartz in the presence of cationic n-cetyltrimethylammonium bromide (CTAB), and their results, shown in Figure 6b, indicated that the static advancing contact angle was about 25° at CTAB solutions under 2 × 10⁻⁶ mol/dm³. The static advancing contact angle of quartz increased rapidly with increasing CTAB concentration and reached a maximum of about 50° at 5 × 10⁻⁵ mol/dm³. A decrease in the contact angle was observed as the concentration approached the critical micelle concentration (CMC) value for CTAB.

2.4. Bubble–Particle Interactions

Although the contact angle is the most important indicator of the wettability of minerals, it is a static method in terms of measurement techniques. However, in the flotation medium, bubbles and particles remain in contact for a limited time, and the successful attachment of mineral particles to air bubbles is vital for flotation technology [8,75,76]. In order for particles to float, they must attach to the bubble within this short time [77,78,79,80]. For successful bubble–particle attachment, the liquid between the bubble and the particle must first thin to a critical thickness, then the liquid film must rupture to form a three-phase contact (TPC) nucleus, and finally, this nucleus must expand sufficiently to form a stable contact line [62]. Therefore, particles that can attach to the bubble in a shorter time have a higher probability of being transported to the froth zone. The relationship between the TPC and various affecting parameters can be found in the literature [37,81,82,83].

Bubble–particle attachment time is measured stochastically with a device specifically designed for this purpose. As a result of bubble–particle attachment time experiments performed at different bubble–particle contact times in milliseconds (ms), it is observed whether the particle attaches to the bubble or not, and the contact time at which 50% attachment occurs is accepted as the bubble–particle attachment time [61,82,84].

Yoon and Yordan [81] reported that the attachment time for the quartz particles in the quartz-DAH system decreased with increasing DAH concentration up to the CMC of about 1.5 × 10⁻² mol/dm³, and then the attachment time increased significantly. Moreover, at a certain DAH concentration, the attachment time was minimum around pH 10.5. At this pH level, the collector was hydrolyzed to form neutral amine, and flotation efficiency was maximized.

Zawala et al. [74] measured the TPC formation (t_TPC) time for quartz in the presence of CTAB and compared it with the flotation recovery. As seen in Figure 7, quartz flotation recovery increased with the decrease of t_TPC. A decrease in the t_TPC means faster bubble–particle attachment due to the rupture of the liquid film on the quartz surface. Therefore, the probability of the bubble–particle attachment increases, resulting in a higher flotation recovery.

3. Mineral Processing Technology

3.1. Pre-Concentration of Quartz with Physical Beneficiation Techniques

As with many industrial minerals, run-of-mine quartz, which is usually produced by open-pit mining, is crushed with the jaw and cone crushers. The crushed material is then ground to 840–590 μm (20–30 mesh) in rod mills in a closed circuit using sieves or classifiers [19]. The advantages of various methods are utilized in the beneficiation of quartz [6,9]. For example, desliming is a commonly used method for clay removal [85,86]. Clay minerals in quartz ores are subjected to a “dispersion” process in scrubbers at >75% solids ratio by a high-speed rotating impeller, thus separating them from the quartz surfaces. Fine clay particles are then removed as slime (<20 μm) by particle size separation (spiral classifier, hydrocyclone, sieve, etc.).

Moreover, gravity separation is another technique used for quartz purification [87,88]. While the specific gravity of quartz is 2.65 [89], the specific gravity of some gangue minerals found in a quartz ore may have sufficient differences to allow an efficient gravity separation (rutile: 4.2, ilmenite: 4.4–4.7, anatase: 3.9 [90], magnetite: 5.20 [89], hematite: 5.26). Therefore, relatively large-sized liberated particles can be removed using gravity separators. However, several minerals, such as feldspar (2.50–2.80) [91] and muscovite (2.72–2.80) [92], have a specific gravity close to quartz, and these minerals may not be separated by gravity separation effectively.

Quartz ores generally contain various magnetic minerals such as magnetite (Fe₃O₄), hematite (Fe₂O₃), and ilmenite (FeTiO₂). Therefore, magnetic separation is also a frequently used method in quartz purification to separate iron-containing minerals [93,94,95]. Meanwhile, electrostatic separation is also an alternative method that can be used in the separation of conductive minerals such as magnetite, hematite, and ilmenite [96,97,98]. However, it is not widely preferred in the industry due to high energy costs. All these physical separation methods are generally used to prepare pre-concentrates for flotation.

3.2. Flotation of Quartz

Flotation is a separation method based on the difference in wettability of mineral surfaces used in the beneficiation of various ores. With a few exceptions, most minerals are naturally hydrophilic. Therefore, in flotation, the surfaces of target mineral particles are selectively made hydrophobic by the adsorption of collectors onto their surfaces, while other minerals remain hydrophilic. During the flotation process, air bubbles used as a separation medium collide with mineral particles. Sufficiently hydrophobic particles attach to the air bubbles due to hydrophobic forces and are transported to the froth zone and collected in the concentrate launder. Meanwhile, the hydrophilic particles remain in the pulp zone [65,99].

The flotation process can be investigated in six groups according to the type of mineral to be floated: natural hydrophobic, sparsely soluble, soluble (salt-type), sulfide, oxide, and silicate minerals [100,101]. The flotation of oxide minerals formed by the combination of oxygen with other elements is controlled by many factors, such as surface potential, solubility of the mineral, type and concentration of inorganic species in the flotation medium, collector properties, pH level of the medium, ionic strength, and temperature. The main mechanisms effective in oxide flotation are electrostatic interactions, chemical adsorption, chain-chain interactions of collector molecules adsorbed on the mineral surface, and modification of mineral surfaces by inorganic substances [102]. Silicate minerals are formed as a result of the combination of oxygen and silicon. In the flotation of silicate minerals with cationic collectors, the electrostatic interaction between the mineral surface and the collector molecule is of great importance. The selectivity of collectors is low [58,103,104,105,106]. Even if the contact angle is high, the attachment force between the collector and the mineral is limited [58,81,82]. Therefore, long-chain collectors consisting of at least 10 carbons are preferred, and these collectors may also exhibit frother properties. Flotation is usually carried out at moderate collector concentration and is sensitive to slime-sized material [58].

3.2.1. Cationic Flotation of Quartz

Almost all oxide and silicate minerals can be floated with cationic collectors [58]. In the physisorption of ionic collector molecules on an oppositely charged mineral surface, electrostatic interactions between the collector, the mineral, and other charged species in the medium play an important role [102].

The flotation of quartz with cationic collectors has a complex mechanism, in which chain-chain reactions and hemimicelle formation play an important role [84,102,107]. The theoretical background and experimental validations on this phenomenon can be found in various previous studies [108,109,110,111,112]. In the presence of a cationic collector such as an amine, at low collector concentrations or in the early stages of adsorption, the polar head of the collector is physically adsorbed (by electrostatic attraction) onto the mineral surface. In this case, the hydrocarbon chains of the collector molecules are oriented toward the aqueous solution (Figure 8a). Over time, the number of collector molecules adsorbed on the mineral surface increases. The hydrophobic chains of the adsorbed molecules are pulled from the aqueous solution into the inner parts of the collector clusters and form hemimicelles on the mineral surface. These hemimicelles provide the formation of hydrophobic sites on the particle surfaces (Figure 8b). As adsorption increases, the number of hemimicelles increases. This results in a decrease in the magnitude of the zeta potential of the particle and the EDL strength. As a consequence, the hydrophobicity of the particle surface increases. In addition, the cationic collector molecule is adsorbed on the air/water interface with the hydrocarbon chain placed in the air and the polar group remaining in the water phase. This causes the zeta potential and EDL force of the air bubble to decrease. Under these conditions, when a particle and an air bubble collide in the flotation medium, the repulsive EDL forces between them are minimal, and the hydrophobic attractive force is strong; therefore, bubble–particle attachment occurs in a very short time. This results in increased flotation efficiency. At this stage, although bilayer formations may occur partially on the mineral surface, they are in small amounts, and their negative effects on flotation are limited (Figure 8c). However, continued adsorption to the mineral surface increases bilayer formation. As the number of collector molecules with polar groups facing the aqueous solution side increases, the magnitude of the zeta potential and EDL force of the particle increases. As a result, the hydrophobicity of the mineral decreases, bubble–particle attachment time increases, and, therefore, flotation efficiency is negatively affected (Figure 8d).

The length of the hydrocarbon chain of the collector molecule has an important effect on the formation of hemimicelles, as it increases adsorption. When longer hydrocarbon chain collectors are used, the collector dosage required for similar flotation recovery decreases [58]. Supporting this fact, the results of a study for quartz in the presence of alkylammonium acetate collectors with various hydrocarbon chain lengths showed that the collector concentration required for flotation decreased systematically as the chain length of the aminium salt increased [46].

In the cationic flotation of quartz, long-chain collectors, which are generally chloride and acetate salts of n-dodecylamines, are preferred [113,114,115,116]. The hydrocarbon chains of these collectors are long enough to render the quartz surface hydrophobic, and they are also water-soluble. DDA can exist in various forms, such as RNH₃⁺, (RNH₃)₂²⁺, RNH₂·RNH₃⁺, RNH₂, or RNH₂ precipitates, depending on its concentration and the pH of the flotation medium, which affects the selectivity of flotation. The balance among different species of DDA is given in Figure 9.

As shown in Figure 9, ionic RNH₃⁺ and (RNH₃)₂²⁺ are dominant between pH 2.0 and 9.0. Meanwhile, at pH 10, the neutral molecule RNH₂ precipitates. At pH 10.5, the ion-molecular complex RNH₂·RNH₃⁺ is maximum. When pH is above 10, the primary species are molecular and precipitate forms of RNH₂. The active collecting species of RNH₃⁺, (RNH₃)₂²⁺, and RNH₂·RNH₃⁺ tend to decrease significantly. The concentrations of (RNH₃)₂²⁺ and RNH₂·RNH₃⁺ are lower by more than two orders of magnitude than those of RNH₃⁺ and RNH₂. Other cationic collectors have the same tendency. Therefore, collectors are defined as protonation forms at low pH and molecules at high pH [117]. With dodecylamine (1 × 10⁻⁵ mol/dm³), quartz flotation efficiency reaches a maximum (~90%) around pH 10 [102].

The results of a study using dodecylamine (Figure 10) showed that the flotation recovery of quartz (212 × 150 μm) increased as a function of DAH concentration and reached over 90% at 1 × 10⁻⁴ mol/dm³ at a natural pH of 6.0–6.5 [8]. Furthermore, Gungoren et al. [118] stated that the temperature can increase the activity and, hence, the adsorption of amine molecules onto the quartz surface; therefore, the flotation can be improved.

Meanwhile, there are also studies using short-chain amines. Kowalczuk [63] investigated the effect of hexylamine on the surface properties and flotation behavior of quartz. Their flotation results with the Hallimond tube (Figure 11a) and mechanical flotation machine (Figure 11b) showed that quartz did not float without a collector, but flotation recovery increased with hexylamine concentration. In addition, the authors stated that hexylamine also acts as a frother [63].

In another study, Ren et al. [52] obtained a flotation recovery of over 80% in the presence of alkyl ether amine (Clariant Flotigam EDA-C). The results of Duan et al. [51], given in Figure 12, showed that n-dodecylenediamine (ND), which can be obtained by adding a secondary amino group to dodecylamine (DDA), exhibited better collector properties than DDA for quartz over a wide pH range, thus increasing the flotation recovery.

The type of amine also affects the flotation recovery. At a fixed amine concentration, the highest flotation recoveries were obtained in the presence of quaternary amines, followed by primary, secondary, and tertiary amines, respectively [119].

Li et al. [57] used a mixture of various amines as collectors in the single mineral flotation of quartz (45 × 75 μm). Their results, shown in Figure 13, indicated that the flotation recovery increased with collector concentration and reached 90.05% at 300 mg/dm³.

3.2.2. Anionic Flotation of Quartz

The presence of oxygen atoms in oxide and silicate minerals is the most important reason why these minerals have a negative zeta potential over a wide pH range and, therefore, have an affinity for water (hydrophilicity). Therefore, quartz is also a highly hydrophilic mineral. Quartz cannot be rendered hydrophobic directly with anionic collectors due to the strong electrostatic repulsion force between the mineral surface and the collector molecule. Moreover, sulfiding cannot be applied to quartz because it has no affinity for sulfur. On the other hand, since the solubility of quartz is very limited and silicon is the only cation constituting the mineral, its surface can be activated for the chemisorption of anionic collectors such as sodium oleate (NaOL) and sodium dodecylsulfate (SDS) [60]. For this purpose, multivalent metal cations are added to the system at a pH range where hydrolysis of the ions to hydroxy complexes (Metal-OH cations) occurs [120,121].

Various multivalent cations such as Ba²⁺, Ca²⁺, Cu²⁺, Mg²⁺, Mn²⁺, Pb²⁺, Sr²⁺, Al³⁺, and Fe³⁺ are used for the activation of quartz [23,102,122]. For each multivalent cation, there is a concentration and pH value that maximizes the quartz flotation recovery. This pH value is between 9 and 10 for Mg²⁺, Al³⁺, and Fe³⁺, while it is slightly higher for Ca²⁺ (Figure 14a). Furthermore, it is seen in Figure 14b that under the desired pH for activation, the suitable concentration of Ca²⁺ and Fe³⁺ was 2.5 × 10⁻³ mol/dm³, while it was 1.0 × 10⁻³ mol/dm³ for Mg²⁺ and Al³⁺. The activation abilities (adsorption affinity/strength) of multivalent cations were in the order of Ca²⁺ ≥ Mg²⁺ > Fe³⁺ > Al³⁺ [60].

Therefore, the most common cation used in quartz activation is Ca²⁺, particularly in the presence of NaOL (Figure 15a). As seen in Figure 15a, Ca²⁺ is adsorbed on the quartz surface in the form of CaOH⁺. Then, the hydroxyl group in the CaOH⁺ molecule combines with the hydrogen on the quartz surface to form water. After dehydration, Ca²⁺ is stably adsorbed on the quartz surface. Therefore, an active site is formed on the quartz surface for chemisorption of the collector molecule added subsequently. Figure 15b shows the distribution of calcium species as a function of pH at 3 × 10⁻⁴ mol/dm³ total calcium concentration. As seen in Figure 15b, more Ca²⁺ exists in the form of Ca(OH)⁺ at pH 12–13, which facilitates the formation of -Si-O-Ca⁺ [22,23,123,124]. It should be noted that flotation does not occur in pH ranges where there are no hydroxy complexes [102].

Li et al. [125] observed that the flotation recovery of quartz increases significantly with increasing Ca²⁺ concentration in the presence of NaOL. The maximum recovery of over 80% was obtained at pH 12 [23]. Furthermore, the results of Zhu et al. [123] showed that surface roughness can increase the Ca²⁺ and, therefore, NaOL adsorption on quartz particles.

It should also be noted that even after activation, quartz cannot be floated with sulfhydryl or short-chain carboxyl collectors [120]. Furthermore, cationic collectors such as amines cannot be adsorbed on quartz after cation activation [126]. In Figure 16, the negative effect of Ca²⁺ ions in the presence of alkyl ether amine (Flotigam EDA-C from Clariant) as a collector is seen from the results of Ren et al. [52]. Although the recovery increased slightly with the increase in collector concentration, it could not reach the recovery in the absence of Ca²⁺.

3.2.3. Selectivity in Quartz Flotation

Direct Flotation of Quartz. Most oxide and silicate minerals can be floated in the presence of cationic collectors, mainly amines [120]. Because of the electrostatic adsorption of amines, selectivity in mineral-amine systems is not as good as in chemical adsorption systems [58].

In the presence of DDA as a collector and magnesite as a gangue mineral, DDA adsorbs on the surface of both quartz and magnesite, both minerals float, and there is no separation (Figure 17a). Meanwhile, when carboxymethyl cellulose (CMC) is added first as a depressant, very few CMC molecules are adsorbed on the quartz surface due to the electrostatic repulsion force. Under the condition of adding DDA after CMC pretreatment, DDA molecules find sufficient space to adsorb on the quartz surface and render the particle hydrophobic. On the other hand, CMC molecules can easily adsorb on the magnesite surface and prevent DDA adsorption. As a result, quartz floats and magnesite are depressed, which provides a selective separation (Figure 17b) [127].

In the separation of quartz from hematite, quartz can be floated with amines at pH 6–7. Cationic amine molecules are adsorbed on negative quartz surfaces but are generally not adsorbed on uncharged hematite surfaces [102]. In addition, the use of depressants is an alternative. Starch, while depressing iron oxides by increasing surface hydrophilicity, is not very effective on quartz [58]. Furthermore, quartz can be activated with multivalent metal cations and then floated with anionic collectors at pH 11 and 12, while iron-bearing minerals (hematite and magnetite) are depressed by starch [128]. If starch is not used under these conditions, hematite will also float [102].

Reverse Flotation of Quartz. The silica content in quartz ores is usually quite high compared to other impurities. Therefore, reverse flotation, in which quartz is depressed and other impurities are floated, is technically easier and is often the preferred method [120].

In quartz–feldspar flotation, feldspar is floated with low dosages of short-chain cationic collectors. In this technique, quartz is depressed with fluoride ions in the presence of hydrofluoric acid [23,129]. During this process, care should be taken not to activate quartz by metal ions [120]. In another study to remove impurities from feldspar and quartz, Malayoglu and Ozkan [85] employed ultrasonic energy during the desliming and flotation stages and obtained promising results.

In the presence of amines, the use of cationic polymers is another method for the depression of quartz. In this case, the effective mechanism is the masking of amine molecules by polymer species rather than the inhibition of collector adsorption [102].

In the separation of hematite from quartz, hematite can be floated with amines in the presence of hydrochloric and sulfuric acid at pH 1.5. Quartz remains hydrophilic at this pH level because of its positive surface charge [102].

Single mineral flotation tests of Bu et al. [114] showed that effective separation of quartz from kaolin was possible at pH 3 in the presence of dodecyl amine and starch as a depressant.

Medjahed et al. [130] first removed the clay minerals by desliming (−20 μm) from the ore ground below 100 μm. The authors first floated mica with DDA+H₂SO₄, then iron minerals (hematite and ilmenite) using NaOL+pine oil. In the final stage of flotation, feldspar was floated using DDA+HF+pine oil. As a result, a high-purity quartz concentrate containing 99.65% SiO₂ was obtained.

pH plays an important role in the anionic reverse flotation of quartz. Quartz activated with copper and iron floats in slightly alkaline environments, while it is depressed in acidic and strongly alkaline media. Sodium sulfide is a useful depressant in the depression of activated quartz. Under these conditions, the metal compound that activates quartz decomposes. This enables the separation of quartz from other non-sulfide minerals with carboxyl collectors [120]. Han et al. [131] reported that in the presence of NaOL, citric acid could be used for the depression of iron-activated quartz by covering and desorbing the active sites of Fe³⁺ and causing the steric hindrance to prohibit the adsorption of NaOL. Sodium silicate (Na₂O(SiO₂)) is another widely used chemical for the depression of quartz [132,133]. As seen from the results of Jin et al. [134] in Figure 18a, the flotation recovery of quartz decreased sharply with respect to sodium silicate concentration.

The results of Zhu et al. [53], seen in Figure 18b, indicate that andalusite can be separated from quartz by flotation at around pH 3 in the presence of 2 g/dm³ sodium petroleum sulfonate (RSO₃Na).

In the flotation of hematite from quartz, Somasundaran and Lou [102] reported that hematite can be floated using sulfonate-type collectors between pH 2 and 4. Anionic sulfonate collector molecules adsorb on positively charged hematite surfaces, while negatively charged quartz remains hydrophilic at this pH level. Moreover, the results of Vidyadhar et al. [55] showed that when quartz was floated, over 75% recovery at the natural pH in the presence of 1 × 10⁻⁴ mol/dm³ NaOL or SLS, the recovery of quartz remained under 30% under these conditions.

3.3. High-Purity Quartz Processing Technology

The mineralogical properties of the ore determine the upper limits of the purity of the quartz that can be achieved as a result of purification processes. The type, amount, and distribution patterns of impurities in quartz necessitate the use of different purification methods for different quartz ores. High-purity quartz was produced with purification techniques including calcination-water quenching, leaching, and chlorination roasting [15].

In calcination-water quenching, inclusion impurities are ruptured and exposed. This method enhances acid leaching through volume expansion during quartz phase transition and allows for the exposure of mineral inclusions and fluid inclusions in quartz due to the thermal stress [9,15,135].

Al, Cr, Fe, and Ti are removed from quartz through the leaching method. For this purpose, HF, H₂SO₄, HCl, HNO₃, and H₃PO₄ are the most commonly used inorganic acids, while ethylenediaminetetraacetic acid (EDTA), citric acid, thioacetic acid, and oxalic acid are the most commonly used organic acids [136,137,138]. HF is useful for removing feldspar, mica, and kaolin from quartz. H₂SO₄ is good at dissolving sulfide minerals, including pyrite. Meanwhile, HCl works well for breaking down calcite, dolomite, and galena. HNO₃ is efficient in the dissolution of pyrite, siderite, and arsenopyrite. However, the acids used in this technique result in the formation of solutions with corrosive properties, which results in environmental pollution [15].

The impurity elements such as Na, K, Fe, and Li within the quartz lattice can be removed by chlorination roasting below the melting point of quartz (1723 °C). NaCl, CaCl₂, and NH₄Cl are the common chlorinating agents for solid-state chlorination roasting; meanwhile, Cl₂ and HCl are generally used in gaseous chlorination roasting. When using this method, attention should be paid to the released harmful chlorine gases [15,26,139].

3.4. Present Challenges, Recent Advancements, and Future Perspectives in Quartz Flotation

The formation of high-quality natural quartz minerals occurs only under specific geological, physical, and chemical conditions; hence, they are extremely rare in the Earth’s crust. Therefore, the beneficiation of quartz resources is of great importance for obtaining high-purity quartz. However, each beneficiation method typically only removes certain types of impurities. Therefore, different beneficiation methods should be used in quartz processing. In addition, current beneficiation techniques present challenges such as high energy and significant chemical consumption and long processing times [140]. Furthermore, complex processes, inflexible conditions, low beneficiation recoveries, and unstable product qualities restrict the industrial applicability of several beneficiation techniques.

Considering the diversity of quartz ore types and impurities found in the ore, overcoming these challenges in quartz beneficiation will be possible with increased scientific studies in this area. Therefore, future studies should focus on the exploration of suitable mineral deposits for high-purity quartz production, increasing the efficiency and concentrate grade of quartz beneficiation processes and devices in an environmentally friendly manner.

Quartz is usually found with feldspar in nature, and they are difficult to separate effectively through flotation owing to their similar physicochemical properties. Flotation with hydrofluoric acid (HF) is the most efficient method. However, since HF may cause considerable health and environmental problems, the effective and environmentally friendly separation of quartz and feldspar is still a tough challenge [14,23].

A quartz ore containing feldspar and muscovite as main impurities was processed by a series of applications (ultrasonic scrubbing-desliming, magnetic separation, fluorine-free flotation, high-temperature calcination, water quenching, hot-press acid leaching, and deionized water cleaning) [14]. As a result of their study, they were able to increase the SiO₂ content from 99.06% to 99.9972%.

Both mono- and diamines can be used in quartz flotation [125]. Calgaroto et al. [141] investigated the effect of nanobubbles on quartz flotation in the presence of alkyl ether monoamine. The authors reported that the quartz flotation in the presence of nanobubbles (200–720 nm) was not effective due to the low buoyancy of nanobubbles. However, the use of nanobubbles together with coarse bubbles increased the flotation recovery of fine (8–74 μm) quartz particles by 20%–30%. This is because nanobubbles increase the contact angle of quartz and the aggregation of fine particles.

In conclusion, this review provides a comprehensive and in-depth analysis of scientific efforts aimed at the beneficiation of pure quartz from ore minerals, with special emphasis on the structural, compositional, and physico-chemical properties of quartz and their influence on flotation behavior and selectivity. Since each quartz ore has different mineralogical properties, surface impurities, crystallinity, particle morphology, surface charge properties, and impurities of different types and formations, and the final concentrate grade desired on an industrial scale is determined by the specific industries, it is not possible to recommend a single universal beneficiation method. Furthermore, unlike controlled laboratory conditions, industrial flotation processes are affected by complex, large-scale variables such as hydrodynamics, slurry rheology, process water chemistry, and multi-phase mineral interactions. Therefore, there is a significant gap between laboratory findings and their practical implementation on an industrial scale. While this review makes a significant contribution to the theoretical understanding of quartz flotation, future studies should bridge the gap between laboratory science and industrial application. To address this gap, future studies should prioritize (a) incorporating pilot-scale testing under industrially relevant conditions, (b) developing detailed process flow diagrams, (c) using process simulation and modeling tools, and (d) evaluating feasible, efficient, and environmentally friendly reagent systems for large-scale operations.

4. Conclusions

This review aimed to contribute to the quartz processing technology by explaining surface properties, flotation behavior, and selective flotation of quartz. The following conclusions can be drawn from this review:

• The purity of quartz determines its industrial use and economic value.
• Physical separation methods (desliming, magnetic separation) are partially effective; flotation is usually used for final beneficiation.
• Grinding breaks Si–O bonds, causing quartz surfaces to become negatively charged at most pH levels (IEP ≈ pH 2).
• Depending on measurement methods, a hydrophilic surface with a contact angle of 10–45° is obtained.
• Reduced attachment time (via increased hydrophobicity via collector adsorption) increases flotation recovery.
• Cationic amine collectors (C12–C18) adsorb onto quartz surfaces above IEP via physical adsorption and hemimicelle formation.
• At high concentrations, collector bilayers may form, reducing efficiency.
• Amines precipitate (as RNH₂(s)) above pH 10 and inhibit adsorption.
• Anionic collectors (e.g., NaOL) are ineffective on negatively charged quartz unless activated with multivalent cations such as Ca²⁺.
• At a certain pH, CaOH⁺ forms chemisorption sites through surface dehydration and oxygen binding.
• Carboxymethyl cellulose (CMC) and starch selectively depress magnesite and hematite in quartz flotation.
• It is effective when quartz is the major component; impurities are floated while quartz is depressed.
• Fluoride ions, cationic polymers, and starch are common quartz depressants.
• pH control is critical to prevent quartz activation.

Finally, systematic studies are needed to understand surface/interface chemistry and collector interactions. Most importantly, bridging the gap between lab-scale studies and industrial processes is essential for practical application.