Review of flotation reagents for scheelite: collectors and depressants

2.1.2.1 Surface fracture bonds, cleavage and exposed surfaces

The fracture bond properties of mineral surfaces are a key characteristic. Studying these properties can help predict cleavage properties, common exposed surfaces and reaction activity of different atoms on the mineral surface. This information can guide the design of flotation reagents and mineral flotation separation. Mineral surface energy, also referred to as cleavage energy, is the strength of atomic correlation between two newly exposed cleavage surfaces resulting from the cleavage of mineral crystals along a specific direction. The ease of cleavage along the crystal plane is directly proportional to the surface energy of the crystal plane. A lower surface energy of the crystal plane results in a more stable cleavage surface. The relationship between surface fracture bond density and surface energy is often direct. Predicting the cleavage properties of minerals can be done more accurately by using the surface fracture bond density to predict the surface energy of different crystal planes. This can guide grinding and flotation experiments. Studies have shown that the surface fracture bond density (number of fracture bonds per unit area) of scheelite crystal faces follows the order: (103) > (010) > (110) > (101) > (111) > (112) > (001) (Hu et al. 2012). The (112), (001), and (101) crystal planes in the scheelite crystal structure are the most extensively researched both domestically and internationally. Among the crystal planes of scheelite, the (001) plane has the lowest surface fracture bond density and surface energy, and is electrically neutral. Due to its susceptibility to cleavage when subjected to external forces, it is a commonly exposed surface. Research conducted by Chaudhuri and Phaneuf has shown that scheelite crystals tend to cleave along the (001) plane network direction (Mogilevsky 2005). According to Mogilevsky, scheelite crystals tend to grow along the (112) plane to form crystal planes and cleavage planes along the interlayer network of this plane, as revealed by X-ray diffraction research.

2.1.2.2 Surface wettability and electrical properties

When minerals come into contact with liquids, wetting occurs due to mutual adhesion between the solid and liquid. This phenomenon causes a change in the interface free energy before and after wetting the mineral surface. The addition of flotation agents during flotation can alter the free energy of the mineral–water–air three-phase interface, which can expand the difference and achieve separation. Consequently, the wettability and floatability of the mineral surface change. The wettability of minerals can be classified into four types: strong hydrophilicity, weak hydrophobicity, hydrophobicity and strong hydrophobicity. Research has demonstrated that the contact angles of scheelite’s cleavage surfaces (112) and (001) decreased from 62.7° and 73.1° to 31.6° and 42.5°, respectively, after polishing and grinding. The decrease in contact angles may be attributed to the fresh cleavage surfaces formed after polishing and grinding. These surfaces have higher surface reactivity and are more hydrophilic. Scheelite is primarily composed of (101), (112) and (001) cleavage planes, and its surface physicochemical properties are mainly determined by these three types of cleavage planes (Gao 2013). The hydrophobicity of pure scheelite minerals and the flotation recovery rate of scheelite are indirectly determined by the wettability of three common crystal planes. To improve the flotation recovery rate of scheelite, it is important to expose more highly active (101) and (112) surfaces while reducing the exposure ratio of (001) surfaces. Additionally, scheelite exhibits a negative charge across almost the entire pH range. Research has shown that scheelite has an isoelectric point of around pH = 2 and exhibits anisotropic surface charge. As the pH value increases, the charge difference between different crystal planes becomes more pronounced. At natural pH values, scheelite’s three common exposed surfaces exhibit a negative charge. The order of surface negative charge on different crystal surfaces is (101) > (112) > (001) surfaces (Gao et al. 2013; Redolph and Hartmann 2017; Nesset et al. 2012).

2.2.1.1 Anionic collectors

Anion collectors can generally be divided into fatty acids, chelates, sulphonic acids and phosphonic acids, with fatty acids and chelates being the most widely used.

2.2.1.1.1 Fatty acid collectors

Fatty acid collectors mainly refer to various fatty acids and their soaps, such as oleic acid, linoleic acid, castor oil acid, tar oil, palmitic acid, naphthenic acid, oxidised paraffin soap, etc. The mechanism of action of this type of collector is to coordinate carboxyl groups with calcium active sites on the scheelite surface, forming a collector bilayer film or calcium oleate precipitation. Its advantages are low cost, high collection capacity and high solubility, but its selectivity is poor and it is not resistant to hard water and low temperature. Therefore, in the flotation process, it is necessary to select excellent depressants to suppress gangue minerals. Overall, in response to its shortcomings, the direction of pharmaceutical research and development is combination and modification, such as introducing highly polar groups or unsaturated bonds into fatty acids to improve solubility and low-temperature resistance, or carrying out biological or chemical modification to introduce selective groups to improve selectivity. Yang (2010) investigated the effects of hydrocarbon length, saturation degree, branching and hydroxyl groups on the collection performance of fatty acids. The results showed that the collection performance of fatty acids was greatly affected by the length of hydrocarbon groups. The carbon atoms in fatty acid molecules were more suitable in C12–C20, and minerals were difficult to float below C12. When the solubility was too low and the melting point was too high, the trapping ability was gradually weakened; the trapping effect of unsaturated fatty acids is better than that of saturated fatty acids. As the degree of unsaturation increases, the flotation effect becomes better and the flotation range is wider than that of saturated fatty acids. Ni et al. (2014) conducted flotation experiments on a tungsten mine in Hunan using a ZL collector. The results showed that the ZL collector was chemically adsorbed on the surface of scheelite and calcite, and physically adsorbed on the surface of fluorite. The use of sodium silicate as a depressant can strongly inhibit the collecting effect of ZL on fluorite and calcite, resulting in good selectivity for scheelite. Assuming a raw ore WO₃ content of 0.36 %, a final grade of 7.45 % was obtained, a coarse tungsten concentrate with a recovery rate of 92.35 %. In addition, fatty acid collectors developed by oleic acid modification and combination methods to improve their low temperature resistance, water hardness, etc., also include GYR series, TAB series, BK series, EA series, K series, etc. (Li et al. 2018). For example, Ling (2013) used BK-418 anionic collector to obtain a scheelite concentrate with a grade of 66.04 % and a recovery rate of 81.67 % for a skarn-type copper–sulphur–tungsten polymetallic ore in Jiangxi Province (containing 0.11 % Cu, 2.95 % S and 0.75 % WO₃ in the original ore). A scheelite concentrate with a grade of 66.04 % and a recovery rate of 81.67 % was obtained through the magnetic separation desulphurisation copper–sulphur mixed flotation scheelite flotation process. Meanwhile, the sulphur concentrate contained 30.16 % S with a recovery rate of 77.58 % and the copper concentrate contained 18.28 % Cu with a recovery rate of 76.83 %, achieving the objective of comprehensive recycling. Wu (2022) aimed to overcome the disadvantages of using oxidised paraffin soap (OPS) for flotation of scheelite and calcium-containing gangue minerals. A non-ionic surfactant, octylphenol polyoxyethylene ether (OP-10), and a new agent (DQW) were selected as a combination to improve the dispersion of OPS in the slurry and the adsorption density on scheelite. The mechanism of action is shown in Figures 2–4. The results showed that OPS/OP-10 was chemically adsorbed on the surface of scheelite through Ca sites. Among them, OP-10 had a synergistic effect on OPS, reducing the critical micelle concentration of OPS, improving its water solubility and dispersibility, and making it more suitable for low-temperature flotation; in addition, OP-10 and OPS can co-adsorb on the mineral surface, making OPS/OP-10 densely packed on the mineral surface, increasing the adsorption strength of reagents, thereby improving the flotation of scheelite; the OPS/DQW combination collector chemically adsorbs on the surface of scheelite through Ca sites. Among them, DQW can rely on intermolecular forces to adsorb on the long carbon chains of OPS, making the OPS/DQW combination collector more firmly adsorbed on the mineral surface while significantly increasing its hydrophobicity. As a result, it still has a strong ability to collect minerals in low-temperature slurry environments.

Figure 2:

Contact angles of three minerals, scheelite, calcite and fluorite, with different agents OPS, OPS/DQW, OPS/DQW + water glass (WG). Reprinted with permission from CNKI (copyright 2022) (Wu 2022).

Figure 3:

Atomic force microscopy examination of OPS/DQW combined collector on scheelite before and after treatment: (a) 3D image of a pure scheelite surface; (b) 3D image of the scheelite surface after OPS treatment; (c) 3D image of the scheelite surface after OPS/OP-10 action; (d) 3D image of the scheelite surface after OPS/DQW treatment. Reprinted with permission from CNKI (copyright 2022) (Wu 2022).

Figure 4:

The flotation effect and mechanism of different combinations of collectors OPS, OPS/OP-10 and OPS/DQW on scheelite: (a) the effect of temperature on the recovery rate of scheelite under the action of three collectors; (b) Ca 2p spectra of scheelite treated with different reagents. Reprinted with permission from CNKI (copyright 2022) (Wu 2022).

Although fatty acid collectors all have a common functional group – carboxyl – the effect of collectors under different carbon chains and branches on scheelite and other calcium-containing gangue minerals is significantly different. Therefore, rules need to be summarised to facilitate the selection of fatty acids with better selectivity. In addition, while researching the modification and combination use of fatty acid agents, it is also necessary to thoroughly study the mechanism of action between minerals and such agents, as well as the relationship between the structure of the agent and its capturing ability and selectivity, in order to reduce the cost of the agent, improve the comprehensive utilization rate of resources and reduce the impact of the agent on the environment.

2.2.1.1.2 Chelating collectors

Chelating collectors refer to surfactants that can form stable chelates with metal ions on mineral surfaces, mainly including hydroxamic acid or isohydroxamic acid collectors. Examples include benzohydroxamic acid (BHA), salicylhydroxamic acid, benzylarsenic acid, toluenearsenic acid, etc. Common industrial applications include the GY series (hydroxamic acids) and CF series (nitroso phenyl hydroxylamine ammonium salts). Most studies have shown that hydroxamic acid collectors can form relatively stable quaternary or pentagonal ring chelates with the surface localised ions Ca²⁺ on scheelite, resulting in co-adsorption of ion molecules. They are also one of the commonly used collectors for scheelite. Yu and Pan (2017) conducted flotation experiments on a low-grade scheelite ore in Hubei using sodium hydroxamate decanoate as a collector. The experimental results showed that sodium hydroxamate decanoate had a stronger collecting ability for scheelite than 731 oxidised paraffin soap, and the amount used was less. Under the condition of 0.40 % WO₃ content in the ore, after a rough selection stage, a tungsten concentrate with a tungsten grade of 3.16 % and a recovery rate of 80 % can be obtained. Qiu et al. (2001) found that at pH 4.7–13.7, BHA chelates with the localised ion Ca²⁺ on the surface of scheelite through O–O chelation, forming a five-membered ring chelate, which undergoes co-adsorption of ion molecules, thereby increasing the adsorption amount of BHA on the surface of scheelite and achieving the best flotation effect. The above research methods usually require pre-activation of metal ions, such as Pb²⁺, Ca²⁺ and Fe²⁺, followed by the addition of chelating agents to improve the selectivity and capture ability of reagents for scheelite. Currently, some researchers are also using metal ions and chelating agents to pre-react to form a complex, which is then added to the slurry to capture tungsten. This technology is more effective than the traditional method of activating with metal ions before adding chelating agents and has been successfully applied in industry in Shizhuyuan. Han et al. (2018) modified hydroxamic acid to synthesise BHA (benzoisohydroxamic acid) and investigated the flotation mechanism of scheelite using BHA as a collector and Pb(NO₃)₂ as an activator. The adsorption model of BHA on the surface of scheelite is shown in Figure 5: Model 1 shows the stepwise catalysis, where Pb ions adsorb on the surface of scheelite and create active sites for BHA adsorption; Model 2 shows the formation of Pb²⁺–BHA composite colloids in solution, which are then adsorbed on the surface of scheelite, giving it strong hydrophobicity. Therefore, Pb²⁺ can promote and catalyse the adsorption of BHA on the surface of scheelite, change the surface energy of scheelite and thus improve the recovery rate of scheelite. The –CONHOH group O and N atoms in BHA enhance its ability to bind minerals by chelating metal ions exposed on the mineral surface. Zhao et al. (2013) compared cyclohexylhydroxamic acid (CHA) and BHA to determine which hydroxamic acid is suitable as a collector for scheelite flotation. The results showed that at alkaline pH, CHA and BHA would ionise to a nonyl form and combine with Ca²⁺ ions to form a five-membered ring structure, as shown in Figure 6. CHA has a stronger adsorption capacity for Ca²⁺ and a greater negative binding energy and therefore has a better scavenging ability than BHA. BHA itself has a weaker ability to capture scheelite than sodium oleate. However, the covalent bond between sodium oleate and Ca²⁺ is weaker than that between BHA and Ca²⁺ (Yin et al. 2015). Lu et al. (2021) designed and synthesised a derivative of N-substituted phenyl octyl hydroxamic acid (NPOHA) by introducing a phenyl group into the structure of octyl hydroxamic acid (OHA), as shown in Figure 7. Compared with OHA and benzo hydroxamic acid (BHA), NPOHA has not only excellent electron donating ability and chemical reactivity but also stronger hydrophobicity, which contributes to its application in the flotation of wolframite. Sun et al. (2024) carried out flotation experiments and mechanism studies on the separation of scheelite from gypsum by lead–BHA (Pb–BHA) complex traps. The flotation experiments and mechanism analysis showed that the selective separation of scheelite from gypsum was achieved by using Pb–BHA traps, and the Pb-BHA traps were chemically adsorbed on the surfaces of both scheelite and gypsum, and the adsorption level of scheelite was significantly higher than that of gypsum. The new type of collector, Pb–BHA complex, showed stronger flotation selectivity compared with the traditional collector, providing a new way for the separation of scheelite from gypsum. Yao et al. (2023) synthesized a novel cinnamic hydroxamic acid trap (CIHA) by the hydroxylamine method using methyl cinnamate as raw material. The performance of hydroxamic acid as a trap for scheelite and vein calcite was investigated, and flotation separation tests of scheelite and calcite were carried out using CIHA as a trap. The single-mineral flotation test and artificial mixing test showed that CIHA had good trapping effect and selectivity. Zhu et al. (2025) designed N-(3-(octylamino))hydroxamic acid (NOHA) as a strong capture agent to enhance scheelite enrichment without lead nitrate preactivation. The results of microflotation showed that NOHA had good trapping ability for scheelite at pH 8.0 and 2.0 × 10⁻⁴ mol/L, and the recovery of scheelite reached more than 95.0 %. Dai et al. (2024) optimized the flotation pharmaceutical system of scheelite by using a new type of collector, 4-methoxybenzoic acid (4-MBA), in combination with BHA. Individual 4-MBA exhibits a weak ability to collect scheelite. However, the combination of 4-MBA and BHA optimized the flotation system of scheelite while ensuring the recovery of scheelite and reducing the dosage of the chemicals.

Figure 5:

Adsorption model of BHA on the surface of scheelite. Reprinted with permission from Elsevier (copyright 2018) (Han et al. 2018).

Figure 6:

Adsorption model of BHA on the surface of scheelite: (a) optimized molecular and ionic models of BHA and CHA; (b) optimized binding model of BHA (left) and CHA (right) ions and calcium; (c) the effect of pH on scheelite floatability at 150 mg/L CHA or BHA; (d) the influence of initial concentration of CHA or BHA on their adsorption capacity. Reprinted with permission from Elsevier (copyright 2013) (Zhao et al. 2013).

Figure 7:

Adsorption mechanism of NPOHA on the surface of scheelite. Reprinted with permission from Elsevier (copyright 2021) (Lu et al. 2021).

Composite collectors can form stable chelates on the surface of target minerals, with good selectivity but weak collecting ability. Usually, metal ions such as Pb²⁺, Fe²⁺ and Co²⁺ are required to activate the target minerals in advance to achieve enhanced flotation; in addition, the synthesis of the reagents is complex, production costs are high and it has an impact on the environment.

2.2.1.2 Cationic collectors

According to the crystal structure characteristics of scheelite, [WO₄]²⁻ and Ca²⁺ on the mineral surface will undergo dissolution imbalance in solution, and [WO₄]²⁻ will undergo ion exchange with other anions, making the electronegativity of the scheelite surface greater than that of gangue minerals such as calcite and fluorite. Therefore, cationic collectors are widely used as efficient agents in flotation separation of scheelite and calcareous gangue minerals. Cationic collectors are mainly amine collectors such as fatty amines, aromatic amines and dioctyldimethylammonium bromide (BDDA). Hu et al. (2011) found that BDDA was adsorbed on the surface of scheelite by electrostatic interaction, and its flotation effect was better than that of oleic acid. Hicyilmaz et al. (1993) experimentally demonstrated that the electrostatic adsorption capacity of dodecylamine acetate on the surface of scheelite is much greater than that of calcite, which can achieve the separation of scheelite and calcite and is suitable for the selection process of scheelite. Li and Wang (2010) found that amine collectors with different polar groups have different collecting abilities for scheelite. The study found that the three collectors, secondary dodecyldimethylbenzylammonium chloride, have the strongest collecting ability for scheelite. Atademir et al. (1979) studied only the effect of octadecylpyridinium bromide (ODPB) on scheelite. ODPB can flotate scheelite, but when the pH exceeds 10, the adsorption of free alkali forms of scheelite by ODPB becomes weaker, but the selectivity of ODPB is lower. Wang et al. (2016) mixed dodecylamine with sodium oleate, and when DDA and sodium oleate were added in a molar ratio of 2:1, using water glass as a promoter, they achieved a stronger capture capability and higher selectivity than a single collector. The author believes that this is because the pre-adsorption of sodium oleate weakens the electrostatic repulsion between RNH³⁺; this increases the hydrophobic binding of side carbon chains, thereby increasing the hydrophobicity of scheelite. Conversely, on the surface of calcite, water glass has a strong chemical adsorption effect on Ca²⁺ in calcite, preventing the adsorption of oleates on calcite and improving the selectivity of the collector. Liu (2021) synthesised hydroxypropyl propanolamine and hydroxypropyl dipropylamine collectors by addition reactions using dodecylamine/tetradecylamine/hexadecylamine and epichlorohydrin as raw materials. Based on the results of single mineral flotation experiments, N-tetradecylpropanolamine (NTIA) was selected as the collector with the best flotation separation performance for scheelite. As shown in Figure 8, the study found that NTIA adsorbs to the surface of scheelite mainly through physical electrostatic and hydrogen bonding. The positively charged amine functional groups in NTIA adsorb on the negative surface of scheelite through electrostatic attraction and hydrogen bonding. The –OH, –CH₂ and –CH₃ groups in NTIA interact with the W–O bond on the negative surface of scheelite through hydrogen bonding, which greatly enhances the adsorption capacity of NTIA on the scheelite surface. After NTIA is adsorbed on the surface of scheelite, the two hydrophobic long carbon chains –C₁₄H₂₉ and –CH₂CH(OH)CH₃ greatly increase the buoyancy of scheelite. This promotes the separation of the scheelite concentrate with good hydrophobicity in scheelite–calcite mixed ore.

Figure 8:

Adsorption mechanism of NTIA on the surface of scheelite: FTIR spectra changes of scheelite (a) and calcite (b) treated with NTIA at pH 7.0; (c) schematic diagram of the flotation process. Reprinted with permission from CNKI (copyright 2021) (Liu 2021).

In summary, the flotation separation of scheelite can theoretically be achieved by physical adsorption using the difference in dynamic potential between the scheelite and the gangue minerals. This type of reagent has made some progress in laboratory research. However, due to the fact that cationic collectors can select both tungsten and silicon, if the ore contains a large amount of siliceous minerals, this type of agent will prioritise the flotation of siliceous minerals. In addition, its high price makes it difficult to meet the requirements of many processing plants and also makes it difficult to use in industrial production.

2.2.1.3 Amphoteric collectors

Amphoteric collectors refer to collectors with both anionic and cationic properties. Common amphoteric collectors include aminocarboxylic acids, aminophosphates, aminosulphonic acids and amidocarboxylic acids. Their characteristic is that they exhibit different ionic properties at different pulp pHs, making them suitable for flotation of different minerals over a wide pH range from strong acid to strong alkaline.

To overcome the shortcomings of anionic and cationic collectors in scheelite flotation, researchers have studied collectors with both types of charge (anionic and cationic) simultaneously. Xu et al. (1987, 1986) systematically investigated the performance of zwitterionic collectors in scheelite flotation systems. At pH 11.2, zwitterionic collectors can effectively separate scheelite (including fine-grained minerals) and calcium-containing gangue minerals in Shizhuyuan. Hu and Xu (2003) studied a phenylaminobenzyl phosphate (BABP) which, when applied to the mineral surface, can change the surface electrokinetic potential of the mineral. However, for scheelite to be well separated from calcite and fluorite, the pH of the slurry and the depressant must be carefully controlled and selected. According to literature reports (Tian et al. 2012), a zwitterionic collector Medialan is obtained by condensation of fatty acids and sarcosine, which can be applied to the flotation of scheelite. The binary artificial mixed scheelite–cassiterite can achieve good separation performance at pH 9–9.5. Wang et al. (2021a,b,c) developed a new combination collector (Pb–BHA–NaOL) to overcome the shortcomings of traditional collectors (Pb–BHA), which can selectively separate scheelite and fluorite without the addition of depressants. The combination collector reduces the amount of collector used while achieving the same ore dressing indicators. Analysis shows that the combination collector has a synergistic effect on the surface of scheelite, with NaOL promoting the selective adsorption of Pb²⁺ and BHA on the scheelite.

Amphoteric collectors have good water solubility and low-temperature resistance due to their combination of negatively and positively charged polar groups. They are less affected by hard water and can undergo electrostatic and chemical adsorption or form chelating rings with some metal ions. However, due to the high cost and complex synthesis process, research on zwitterionic collectors is limited to theoretical studies and has limited industrial applications.

2.2.1.4 Combination collectors

By combining several identical or different types of collectors, such as polar–non-polar combination collectors (Filippov et al. 2012; Geng et al. 2017), polar–polar combination collectors (Filippov et al. 2018; Wang et al. 2021a,b,c), etc., it is often possible to obtain better flotation indicators than by using individual collectors. This phenomenon is known as a synergistic effect. Combination reagents can exert synergistic effects between reagents and improve the application deficiencies of single reagents. By adjusting the order of reagent addition, group distribution ratio, slurry pH, reagent concentration and reagent type, combination reagents can produce positive synergistic effects and improve the performance of single reagents (Zhang 1990).

The synergistic mechanisms primarily include co-adsorption, charge compensation, chelation and functional complementarity. Common co-adsorption models include the layered and intercalated types. In the layered mechanism, the mineral surface first interacts with highly surface-active collectors, followed by interaction with less active ones. The intercalated type refers to different types of collectors successively adsorbing onto the mineral surface and the remaining space, achieving the desired effect by increasing the reagent density on the mineral surface. Liu (1992) studied the mechanism of combined collectors in the flotation of scheelite and found that the reagents were easily adsorbed in the high active energy areas at the beginning. However, when the concentration of the reagents was high, especially when there was interaction between the reagents, low-energy sites could occur. It is therefore speculated that there may be two different adsorption sites for scheelite. The adsorption capacity of the mineral surface increases with the addition of benzyl arsenate, followed by the addition of butyl yellow. When benzyl arsenate:butyl yellow = 2:1, the degree of activation is highest and a stable supramolecular beam can be formed at this time. Due to the good selectivity of chelating agents towards metal ions, chelating agents can combine with metal ions on mineral surfaces, undergo chemical reactions and form insoluble and stable chelates. Xia et al. (2004) used the ab initio algorithm to find that benzohydroxamic acid is more stable than benzohydroxamic acid, and the electrons lost by the latter can react with metal ions on the mineral surface to form a chelate of O,O five-membered ring. A study by Bai (2014) found that these two agents chemically adsorb on the surface of scheelite, with stronger adsorption capacity than a single agent, but weaker adsorption capacity for fluorite and calcite. Using a combination of OXB (mainly composed of 733) and MES as a collector, a WO₃ grade of 65 was obtained for the raw ore with a WO₃ grade of 0.57–76 % scheelite concentrate with a recovery rate of 66.04 %.

For the difficult-to-select scheelite–calcite–fluorite-type scheelite ore, the use of combined collectors in the industry can reduce the reagent dosage, and the collecting ability and selectivity are relatively stronger, improving flotation, increasing efficiency and protecting the environment.

2.2.2.1 Inorganic depressants

2.2.2.2 Organic depressants

Organic depressants generally contain several polar groups in their molecular structure, located at the ends or in the middle of the molecule. Polar groups will undergo hydrophilic interactions on the surface of the ore, hindering the adsorption of the collector on the surface; it will also form a hydrophilic film on the surface of the ore, increasing the hydrophilicity of the mineral, thereby achieving the effect of inhibiting the floatability of the mineral. Organic depressants can be divided into small-molecule organic depressants and large-molecule organic depressants based on their molecular weight. Small molecule depressants generally have functional groups such as –OH and –COOH which can form chelates with Ca²⁺ dissolved in calcium-containing gangue minerals, effectively avoiding the adsorption of anionic collectors on the surface of calcium-containing gangue minerals. Their inhibition capacity decreases with increasing carbon chain length. Small-molecule organic depressants includes oxalic acid, citric acid, tartaric acid and other hydroxyl carboxylic acids (Li 2019; Xu 2015; Yu et al. 2013). Macromolecular organic depressants include polysaccharides such as tannins, modified starch, CMC, sodium humate, xanthan gum and sodium alginate, and synthetic polymers (Dong et al. 2019; Fu et al. 2018; Wei et al. 2020a,b). Chen (1998) investigated the inhibitory ability and mechanism of various small molecule depressants on calcareous gangue minerals. The results indicate that the ability of depressants to act is related to the type and quantity of functional groups carried by the reagent itself. The stronger the polarity and quantity of functional groups, the easier it is to integrate with calcium gangue minerals and thus achieve better inhibitory effects on calcium gangue minerals. Meanwhile, the effectiveness of depressants is also influenced by the length of the carbon chains, with longer carbon chains resulting in weaker effects. Zhang (2012) compared the separation effect of sodium silicate and sodium polyacrylate on scheelite and calcium-containing gangue, and found that sodium polyacrylate can adsorb preferentially on the surfaces of fluorite and calcite. Sodium polyacrylate with a molecular weight of 8–10 million has the most ideal inhibiting effect, and qualified scheelite concentrate cannot be obtained if the molecular weight is too large or too small. Ren (2013) investigated the mechanism of tannin action on calcium gangue minerals through single mineral flotation experiments. The experimental results showed that Ca–O, hydrogen bonding and electrostatic interactions formed between the phenolic hydroxyl groups at the molecular end of tannin and the surface of calcium gangue minerals can cause chemical and physical adsorption of tannin on calcium gangue minerals, hinder the adsorption of collector DW-1 on the mineral surface and inhibit the action of calcium gangue minerals. Wang and Cui (2025) used sulphonated phenolic resin (SPR) as a depressant to investigate its effect on the flotation behaviour of scheelite and calcite in a system with NaOL as a collector. The results showed that scheelite and calcite could be effectively separated at pH 9.5 and an SPR of 60.0 mg/L with a recovery difference of 51.33 %. The mechanism study showed that SPR was adsorbed on the mineral surface through the interaction between sulphonate oxygen atoms and calcium ions, and the adsorption energy of SPR on calcite was −81.05 kJ/mol and that on scheelite was −57.89 kJ/mol. Therefore, SPR occupied more calcium adsorption sites on the calcite surface and significantly hindered the adsorption of NaCl sites on the calcite surface, significantly hindering the subsequent adsorption of NaOL on calcite while minimally interfering with the adsorption of NaOL on scheelite. This differential adsorption behaviour enhanced the floatability contrast between the two minerals. Shen et al. (2025) synthesized carboxymethyl sulphonated organosolv lignin (CSOL) with different degrees of carboxylation as organic depressants. The results showed that the recovery rates of calcite and fluorite decreased significantly from 70.98 % and 91.8 % to 9.34 % and 8.00 %, respectively, and the recovery rate of scheelite decreased slightly from 94.9 % to 91.62 % with the addition of CSOL. The hydrophilic CSOL can be adsorbed on the calcite sites on the surface of calcite and fluorite through carboxyl and sulphonate groups, which improves the hydrophilicity of calcite and fluorite and reduces their floatability. The negatively charged CSOL had a small amount of adsorption on the negatively charged scheelite surface due to strong electrostatic repulsion and thus had little effect on the further adsorption of the trap NaOL and the flotation of scheelite. Similarly, Yang et al. (2025) designed a bio-based environmentally friendly depressant sulphomethylated depolymerized alkali lignin (SMDL). It was synthesized by oxidation, hydrolysis, electrocatalytic depolymerization and sulphomethylation modification of alkali lignin (AL). The sulphonation degree and molecular weights were determined by FTIR, GPC and NMR under different depolymerization strategies. SMDL-2 has the lowest molecular weight and the highest degree of sulphonation (2.2 mmol/g) and has a strong inhibition effect on fluorite and calcite. The flotation recovery rate of SMDL-2 for scheelite, calcite and fluorite were 85.27 %, 13.34 % and 12.76 %, respectively. SMDL possesses abundant calcium-binding interaction sites, and these sites were positively correlated with the degree of sulphonation. SMDL interacted with the calcium sites of fluorite and calcite through hydrophilic-SO₃ to achieve selective inhibition of fluorite and calcite. Chen et al. (2025) used a novel depressant, poly(sodium 4-styrenesulphonate) (PSS), with the NaOL system to separate scheelite and calcite. Under the optimal conditions of NaOL 10 mg/L, PSS 10 mg/L and pH = 8, the recovery rate of scheelite monoflotation was 87.18 %, the recovery rate of calcite was 5.57 % and the recovery rate of WO₃ in artificial mixed ore flotation was 70.53 % with a grade of 61.08 %. PSS could not be adsorbed on scheelite by chemical reaction, and PSS had no effect on NaOL on the surface of scheelite. adsorption on scheelite surface. Peng et al. (2024) developed a novel hexakisic acid inhibitor – hexaethylenediaminetetramethylenephosphonic acid (HDTMP or H6L). The experimental results showed that HDTMP had better adsorption effect on calcite, and the main active component of HDTMP was HL5-anion at pH 10.0. Density functional theory (DFT) calculations further revealed that HDTMP has stronger interactions with calcite through chemical binding with Ca sites and hydrogen bonds (P–O⋯Hwater and P–O H⋯Ocalcite), which are formed by phosphate groups in HDTMP and H/O atoms on the calcite surface.

Organic depressants have a wide variety of types, are highly adaptable and are relatively easy to degrade and less polluting than inorganic depressants. They come from a wide range of sources and are easy to synthesise, making them increasingly favoured by researchers (Liu et al. 2016a,b; Qian et al. 2021).

2.2.2.3 Combined depressants

Compared with the single addition of depressants, the combination of inorganic and organic depressants, as well as organic and inorganic depressants, can significantly enhance the selective inhibition of gangue minerals, improve foam mineralisation, reduce the cycle load of intermediate ore and improve the flotation index of scheelite. It has wide application prospects (Feng et al. 2019; Tian 1991). Ai et al. (2017) used a combination depressant Na₂SiO₃ + Al₂(SO₄)₃ to treat a complex and difficult-to-select low-grade scheelite ore in Jiangxi, effectively suppressing gangue minerals and achieving good indicators. Kang et al. (2016) investigated the inhibitory effects of three depressants, namely, water glass, acidified water glass and ATM, on the low grade and high content of calcium-containing gangue minerals in a certain scheelite mine in Henan province. The experimental results show that when the three depressants are used alone, ATM has the best inhibitory effect on gangue minerals, followed by acidified water glass, which is the worst. The combined use of ATM and acidified water glass has a better effect than a single depressant. Wang (2022) investigated the inhibitory effects of eight depressants, including sodium hexametaphosphate, three different moduli of water glass (moduli 2.17, 2.50 and 3.30), carboxylated chitosan, sodium lignosulphonate, sodium carboxymethylcellulose and sodium humate on the flotation separation of scheelite from fluorite, calcite and quartz. A new combination depressant, DWC-1, was prepared by screening sodium lignosulphonate with strong selectivity and sodium humate with strong inhibition in a mass ratio of 4:1. As shown in Figures 9 and 10, DWC-1 can selectively adsorb on the surface of gangue minerals, and electrostatic adsorption occurs on the surfaces of fluorite, calcite and quartz, while there is almost no electrostatic adsorption on the surface of scheelite. Ions have a significant effect on the adsorption capacity of reagents, with calcium ions weakening the selectivity of depressants and increasing the inhibitory effect on scheelite. Zheng et al. (2024) investigated the synergistic effect of sodium tripolyphosphate (STPP) and sodium ethylenediamine tetramethylene phosphonate (EDTMPS) in the flotation separation of fluorite and calcite. Flotation tests showed that the combined depressant of STPP and EDTMPS significantly improved the separation efficiency of scheelite from fluorite and calcite. STPP and EDTMPS were more easily adsorbed on the surface of fluorite and calcite due to their unique electrostatic repulsive force and site resistance. The co-adsorption of STPP and EDTMPS covered a wider surface area of the minerals, which enhanced the surface of fluorite and calcite’s hydrophilicity. This, in turn, increased the resistance of the mineral surfaces to subsequent NaOL adsorption. This widens the difference in floatability of the three calcium-bearing minerals and optimizes the sorting efficiency.

Figure 9:

3D images of different mineral surface morphologies before and after depressant adsorption: (a) and (b) represent the depressant before and after adsorption on the surface of scheelite; (c) and (d) represent the depressant before and after adsorption on the surface of calcite; (e) and (f) represent the depressant before and after adsorption on the surface of fluorite, respectively. Reprinted with permission from CNKI (copyright 2022) (Wang 2022).

Figure 10:

XPS energy spectrum analysis before and after the action of different minerals and depressants: (a) denotes scheelite; (b) denotes calcite; (c) denotes fluorite; (d) denotes the changes in the types and contents of surface elements of scheelite, calcite and fluorite before and after the action of depressants. Reprinted with permission from CNKI (copyright 2022) (Wang 2022).

In recent years, the modification of organic depressants by metal ions to promote their inhibitory effects has also received greater attention. Zheng et al. (2025) investigated the mechanism of the synergistic effect of iron ions (Fe²⁺) on the selective adsorption capacity and stability of citric acid (CA). The flotation separation of scheelite and cassiterite was successfully realized by using Fe²⁺/CA mixed depressant. The results of the microflotation test showed that the recovery rate of scheelite in the froth concentrate was 84.60 % and the cassiterite recovery rate was 23.64 % with grades of 66.26 % and 17.73 %, respectively, after the addition of Fe²⁺/CA. The strong inhibition of Fe²⁺/CA on cassiterite resulted in the decrease of cassiterite recovery rate by 69.30 % and the decrease of grade by 20.33 %. This indicates the effective separation of scheelite from cassiterite. The FeOH + complex formed by the pre-reaction of Fe²⁺ with CA showed obvious chemisorption at the Sn active site, thus inhibiting the activity of cassiterite. Qiao et al. (2024) investigated the mixed depressant of iron ions and gallic acid to improve the separation efficiency of scheelite from calcite. The flotation performance of scheelite and calcite was evaluated under the molar ratio of iron ions to gallic acid (1:5), concentrations (2 × 10⁻⁵ mol/L and 1 × 10⁻⁴ mol/L) and NaOL dosage of 1 × 10⁻⁴ mol/L. The results showed that the recovery rate of scheelite was 81 % and that of calcite was 8.3 %. The mixed ore test proved that the addition of a mixed depressant could increase the grade of scheelite from 37.08 % to 58.56 %, an increase of 21.48 %, compared with the addition of the collector alone. The experiment shows that the mixed depressant has better inhibition effect on calcite. Surface characterization analysis showed that the inhibition effect of the mixed depressant on scheelite was weak, but the inhibition effect on calcite was better, and the mechanism was that the iron ions promoted the adsorption of gallic acid on the surface of calcite, thus inhibiting the enrichment of sodium oleate. Dong et al. (2023) explored the effect of mixed depressant Cu²⁺/CA on the flotation separation of scheelite and calcite in response to the poor selectivity of single copper ion (Cu²⁺) and poor inhibition of citric acid (CA). The flotation experiments and mechanism analysis showed that the separation effect of the mixed depressant on the two minerals was better than that of the single depressant, and the laboratory-scale experiments verified the efficient separation effect of the mixed depressant. Solution chemical analysis showed the main components in the slurry environment. The hybrid depressant selectively co-adsorbed on the calcite surface as copper species complexes. Thermodynamic calculations of complex formation and calculations of conditional stability constants demonstrated that copper complexes (CuL-) are more easily formed and more stable than calcium complexes. The mixed depressant Cu²⁺/CA was effective in separating scheelite and calcite and in enriching scheelite.

In summary, research into new highly selective depressants remains one of the most important directions for scheelite flotation, with the aim of optimising flotation conditions, improving flotation indicators, reducing the problem of high energy consumption in sorting operations and reducing the entry of non-target minerals into the concentrate. The combination of reagents is beneficial for handling complex co-existing ores in actual production, increasing the planktonic difference between target minerals and gangue minerals, and improving the adaptability and selective inhibition of reagents to ores. It has important practical implications for improving the working environment, reducing costs and promoting the development of the mining industry (Ai et al. 2018; Chen et al. 2018a,b; Chen et al. 2019; Wei et al. 2020a,b).